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Solar photovoltaic technology: A review of different types of solar cells and its future trends

Mugdha V Dambhare 1 , Bhavana Butey 1 and S V Moharil 2

Published under licence by IOP Publishing Ltd Journal of Physics: Conference Series , Volume 1913 , International Conference on Research Frontiers in Sciences (ICRFS 2021) 5th-6th February 2021, Nagpur, India Citation Mugdha V Dambhare et al 2021 J. Phys.: Conf. Ser. 1913 012053 DOI 10.1088/1742-6596/1913/1/012053

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1 G H Raisoni College of Engineering, Nagpur, India

2 Department of Physics, PGTD, Nagpur

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The Sun is source of abundant energy. We are getting large amount of energy from the Sun out of which only a small portion is utilized. Sunlight reaching to Earth's surface has potential to fulfill all our ever increasing energy demands. Solar Photovoltaic technology deals with conversion of incident sunlight energy into electrical energy. Solar cells fabricated from Silicon aie the first generation solar cells. It was studied that more improvement is needed for large absorption of incident sunlight and increase in efficiency of solar cells. Thin film technology and amorphous Silicon solar cells were further developed to meet these conditions. In this review, we have studied a progressive advancement in Solar cell technology from first generation solar cells to Dye sensitized solar cells, Quantum dot solar cells and some recent technologies. This article also discuss about future trends of these different generation solar cell technologies and their scope to establish Solar cell technology.

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Researchers improve efficiency of next-generation solar cell material

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Perovskites are a leading candidate for eventually replacing silicon as the material of choice for solar panels. They offer the potential for low-cost, low-temperature manufacturing of ultrathin, lightweight flexible cells, but so far their efficiency at converting sunlight to electricity has lagged behind that of silicon and some other alternatives.

Now, a new approach to the design of perovskite cells has pushed the material to match or exceed the efficiency of today’s typical silicon cell, which generally ranges from 20 to 22 percent, laying the groundwork for further improvements.

By adding a specially treated conductive layer of tin dioxide bonded to the perovskite material, which provides an improved path for the charge carriers in the cell, and by modifying the perovskite formula, researchers have boosted its overall efficiency as a solar cell to 25.2 percent — a near-record for such materials, which eclipses the efficiency of many existing solar panels. (Perovskites still lag significantly in longevity compared to silicon, however, a challenge being worked on by teams around the world.)

The findings are described in a paper in the journal Nature by recent MIT graduate Jason Yoo PhD ’20, professor of chemistry and Lester Wolfe Professor Moungi Bawendi, professor of electrical engineering and computer science and Fariborz Maseeh Professor in Emerging Technology Vladimir Bulović, and 11 others at MIT, in South Korea, and in Georgia.

Perovskites are a broad class of materials defined by the fact that they have a particular kind of molecular arrangement, or lattice, that resembles that of the naturally occurring mineral perovskite. There are vast numbers of possible chemical combinations that can make perovskites, and Yoo explains that these materials have attracted worldwide interest because “at least on paper, they could be made much more cheaply than silicon or gallium arsenide,” one of the other leading contenders. That’s partly because of the much simpler processing and manufacturing processes, which for silicon or gallium arsenide requires sustained heat of over 1,000 degrees Celsius. In contrast, perovskites can be processed at less than 200 C, either in solution or by vapor deposition.

The other major advantage of perovskite over silicon or many other candidate replacements is that it forms extremely thin layers while still efficiently capturing solar energy. “Perovskite cells have the potential to be lightweight compared to silicon, by orders of magnitude,” Bawendi says.

Perovskites have a higher bandgap than silicon, which means they absorb a different part of the light spectrum and thus can complement silicon cells to provide even greater combined efficiencies. But even using only perovskite, Yoo says, “what we’re demonstrating is that even with a single active layer, we can make efficiencies that threaten silicon, and hopefully within punching distance of gallium arsenide. And both of those technologies have been around for much longer than perovskites have.”

One of the keys to the team’s improvement of the material’s efficiency, Bawendi explains, was in the precise engineering of one layer of the sandwich that makes up a perovskite solar cell — the electron transport layer. The perovskite itself is layered with a transparent conductive layer used to carry an electric current from the cell out to where it can be used. However, if the conductive layer is directly attached to the perovskite itself, the electrons and their counterparts, called holes, simply recombine on the spot and no current flows. In the researchers’ design, the perovskite and the conductive layer are separated by an improved type of intermediate layer that can let the electrons through while preventing the recombination.

This middle electron transport layer, and especially the interfaces where it connects to the layers on each side of it, tend to be where inefficiencies occur. By studying these mechanisms and designing a layer, consisting of tin oxide, that more perfectly conforms with those adjacent to it, the researchers were able to greatly reduce the losses.

The method they use is called chemical bath deposition. “It’s like slow cooking in a Crock-Pot,” Bawendi says. With a bath at 90 degrees Celsius, precursor chemicals slowly decompose to form the layer of tin dioxide in place. “The team realized that if we understood the decomposition mechanisms of these precursors, then we’d have a better understanding of how these films form. We were able to find the right window in which the electron transport layer with ideal properties can be synthesized.”

After a series of controlled experiments, they found that different mixtures of intermediate compounds would form, depending on the acidity of the precursor solution. They also identified a sweet spot of precursor compositions that allowed the reaction to produce a much more effective film.

The researchers combined these steps with an optimization of the perovskite layer itself. They used a set of additives to the perovskite recipe to improve its stability, which had been tried before but had an undesired effect on the material’s bandgap, making it a less efficient light absorber. The team found that by adding much smaller amounts of these additives — less than 1 percent — they could still get the beneficial effects without altering the bandgap.

The resulting improvement in efficiency has already driven the material to over 80 percent of the theoretical maximum efficiency that such materials could have, Yoo says.

While these high efficiencies were demonstrated in tiny lab-scale devices, Bawendi says that “the kind of insights we provide in this paper, and some of the tricks we provide, could potentially be applied to the methods that people are now developing for large-scale, manufacturable perovskite cells, and therefore boost those efficiencies.”

In pursuing the research further, there are two important avenues, he says: to continue pushing the limits on better efficiency, and to focus on increasing the material’s long-term stability, which currently is measured in months, compared to decades for silicon cells. But for some purposes, Bawendi points out, longevity may not be so essential. Many electronic devices such as cellphones, for example, tend to be replaced within a few years anyway, so there may be some useful applications even for relatively short-lived solar cells.

“I don’t think we’re there yet with these cells, even for these kind of shorter-term applications,” he says. “But people are getting close, so combining our ideas in this paper with ideas that other people have with increasing stability could lead to something really interesting.”

Robert Hoye, a lecturer in materials at Imperial College London, who was not part of the study, says, “This is excellent work by an international team.” He adds, “This could lead to greater reproducibility and the excellent device efficiencies achieved in the lab translating to commercialized modules. In terms of scientific milestones, not only do they achieve an efficiency that was the certified record for perovskite solar cells for much of last year, they also achieve open-circuit voltages up to 97 percent of the radiative limit. This is an astonishing achievement for solar cells grown from solution.”

The team included researchers at the Korea Research Institute of Chemical Technology, the Korea Advanced Institute of Science and Technology, the Ulsan National Institute of Science and Technology, and Georgia Tech. The work was supported by MIT’s Institute for Soldier Nanotechnology, NASA, the Italian company Eni SpA through the MIT Energy Initiative, the National Research Foundation of Korea, and the National Research Council of Science and Technology.

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Article Contents

Introduction, 1 installed capacity and application of solar energy worldwide, 2 the role of solar energy in sustainable development, 3 the perspective of solar energy, 4 conclusions, conflict of interest statement.

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Solar energy technology and its roles in sustainable development

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Ali O M Maka, Jamal M Alabid, Solar energy technology and its roles in sustainable development, Clean Energy , Volume 6, Issue 3, June 2022, Pages 476–483, https://doi.org/10.1093/ce/zkac023

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Solar energy is environmentally friendly technology, a great energy supply and one of the most significant renewable and green energy sources. It plays a substantial role in achieving sustainable development energy solutions. Therefore, the massive amount of solar energy attainable daily makes it a very attractive resource for generating electricity. Both technologies, applications of concentrated solar power or solar photovoltaics, are always under continuous development to fulfil our energy needs. Hence, a large installed capacity of solar energy applications worldwide, in the same context, supports the energy sector and meets the employment market to gain sufficient development. This paper highlights solar energy applications and their role in sustainable development and considers renewable energy’s overall employment potential. Thus, it provides insights and analysis on solar energy sustainability, including environmental and economic development. Furthermore, it has identified the contributions of solar energy applications in sustainable development by providing energy needs, creating jobs opportunities and enhancing environmental protection. Finally, the perspective of solar energy technology is drawn up in the application of the energy sector and affords a vision of future development in this domain.

With reference to the recommendations of the UN, the Climate Change Conference, COP26, was held in Glasgow , UK, in 2021. They reached an agreement through the representatives of the 197 countries, where they concurred to move towards reducing dependency on coal and fossil-fuel sources. Furthermore, the conference stated ‘the various opportunities for governments to prioritize health and equity in the international climate movement and sustainable development agenda’. Also, one of the testaments is the necessity to ‘create energy systems that protect and improve climate and health’ [ 1 , 2 ].

The Paris Climate Accords is a worldwide agreement on climate change signed in 2015, which addressed the mitigation of climate change, adaptation and finance. Consequently, the representatives of 196 countries concurred to decrease their greenhouse gas emissions [ 3 ]. The Paris Agreement is essential for present and future generations to attain a more secure and stable environment. In essence, the Paris Agreement has been about safeguarding people from such an uncertain and progressively dangerous environment and ensuring everyone can have the right to live in a healthy, pollutant-free environment without the negative impacts of climate change [ 3 , 4 ].

In recent decades, there has been an increase in demand for cleaner energy resources. Based on that, decision-makers of all countries have drawn up plans that depend on renewable sources through a long-term strategy. Thus, such plans reduce the reliance of dependence on traditional energy sources and substitute traditional energy sources with alternative energy technology. As a result, the global community is starting to shift towards utilizing sustainable energy sources and reducing dependence on traditional fossil fuels as a source of energy [ 5 , 6 ].

In 2015, the UN adopted the sustainable development goals (SDGs) and recognized them as international legislation, which demands a global effort to end poverty, safeguard the environment and guarantee that by 2030, humanity lives in prosperity and peace. Consequently, progress needs to be balanced among economic, social and environmental sustainability models [ 7 ].

Many national and international regulations have been established to control the gas emissions and pollutants that impact the environment [ 8 ]. However, the negative effects of increased carbon in the atmosphere have grown in the last 10 years. Production and use of fossil fuels emit methane (CH 4 ), carbon dioxide (CO 2 ) and carbon monoxide (CO), which are the most significant contributors to environmental emissions on our planet. Additionally, coal and oil, including gasoline, coal, oil and methane, are commonly used in energy for transport or for generating electricity. Therefore, burning these fossil fuel s is deemed the largest emitter when used for electricity generation, transport, etc. However, these energy resources are considered depleted energy sources being consumed to an unsustainable degree [ 9–11 ].

Energy is an essential need for the existence and growth of human communities. Consequently, the need for energy has increased gradually as human civilization has progressed. Additionally, in the past few decades, the rapid rise of the world’s population and its reliance on technological developments have increased energy demands. Furthermore, green technology sources play an important role in sustainably providing energy supplies, especially in mitigating climate change [ 5 , 6 , 8 ].

Currently, fossil fuels remain dominant and will continue to be the primary source of large-scale energy for the foreseeable future; however, renewable energy should play a vital role in the future of global energy. The global energy system is undergoing a movement towards more sustainable sources of energy [ 12 , 13 ].

Power generation by fossil-fuel resources has peaked, whilst solar energy is predicted to be at the vanguard of energy generation in the near future. Moreover, it is predicted that by 2050, the generation of solar energy will have increased to 48% due to economic and industrial growth [ 13 , 14 ].

In recent years, it has become increasingly obvious that the globe must decrease greenhouse gas emissions by 2050, ideally towards net zero, if we are to fulfil the Paris Agreement’s goal to reduce global temperature increases [ 3 , 4 ]. The net-zero emissions complement the scenario of sustainable development assessment by 2050. According to the agreed scenario of sustainable development, many industrialized economies must achieve net-zero emissions by 2050. However, the net-zero emissions 2050 brought the first detailed International Energy Agency (IEA) modelling of what strategy will be required over the next 10 years to achieve net-zero carbon emissions worldwide by 2050 [ 15–17 ].

The global statistics of greenhouse gas emissions have been identified; in 2019, there was a 1% decrease in CO 2 emissions from the power industry; that figure dropped by 7% in 2020 due to the COVID-19 crisis, thus indicating a drop in coal-fired energy generation that is being squeezed by decreasing energy needs, growth of renewables and the shift away from fossil fuels. As a result, in 2020, the energy industry was expected to generate ~13 Gt CO 2 , representing ~40% of total world energy sector emissions related to CO 2 . The annual electricity generation stepped back to pre-crisis levels by 2021, although due to a changing ‘fuel mix’, the CO 2 emissions in the power sector will grow just a little before remaining roughly steady until 2030 [ 15 ].

Therefore, based on the information mentioned above, the advantages of solar energy technology are a renewable and clean energy source that is plentiful, cheaper costs, less maintenance and environmentally friendly, to name but a few. The significance of this paper is to highlight solar energy applications to ensure sustainable development; thus, it is vital to researchers, engineers and customers alike. The article’s primary aim is to raise public awareness and disseminate the culture of solar energy usage in daily life, since moving forward, it is the best. The scope of this paper is as follows. Section 1 represents a summary of the introduction. Section 2 represents a summary of installed capacity and the application of solar energy worldwide. Section 3 presents the role of solar energy in the sustainable development and employment of renewable energy. Section 4 represents the perspective of solar energy. Finally, Section 5 outlines the conclusions and recommendations for future work.

1.1 Installed capacity of solar energy

The history of solar energy can be traced back to the seventh century when mirrors with solar power were used. In 1893, the photovoltaic (PV) effect was discovered; after many decades, scientists developed this technology for electricity generation [ 18 ]. Based on that, after many years of research and development from scientists worldwide, solar energy technology is classified into two key applications: solar thermal and solar PV.

PV systems convert the Sun’s energy into electricity by utilizing solar panels. These PV devices have quickly become the cheapest option for new electricity generation in numerous world locations due to their ubiquitous deployment. For example, during the period from 2010 to 2018, the cost of generating electricity by solar PV plants decreased by 77%. However, solar PV installed capacity progress expanded 100-fold between 2005 and 2018. Consequently, solar PV has emerged as a key component in the low-carbon sustainable energy system required to provide access to affordable and dependable electricity, assisting in fulfilling the Paris climate agreement and in achieving the 2030 SDG targets [ 19 ].

The installed capacity of solar energy worldwide has been rapidly increased to meet energy demands. The installed capacity of PV technology from 2010 to 2020 increased from 40 334 to 709 674 MW, whereas the installed capacity of concentrated solar power (CSP) applications, which was 1266 MW in 2010, after 10 years had increased to 6479 MW. Therefore, solar PV technology has more deployed installations than CSP applications. So, the stand-alone solar PV and large-scale grid-connected PV plants are widely used worldwide and used in space applications. Fig. 1 represents the installation of solar energy worldwide.

Installation capacity of solar energy worldwide [ 20 ].

1.2 Application of solar energy

Energy can be obtained directly from the Sun—so-called solar energy. Globally, there has been growth in solar energy applications, as it can be used to generate electricity, desalinate water and generate heat, etc. The taxonomy of applications of solar energy is as follows: (i) PVs and (ii) CSP. Fig. 2 details the taxonomy of solar energy applications.

The taxonomy of solar energy applications.

Solar cells are devices that convert sunlight directly into electricity; typical semiconductor materials are utilized to form a PV solar cell device. These materials’ characteristics are based on atoms with four electrons in their outer orbit or shell. Semiconductor materials are from the periodic table’s group ‘IV’ or a mixture of groups ‘IV’ and ‘II’, the latter known as ‘II–VI’ semiconductors [ 21 ]. Additionally, a periodic table mixture of elements from groups ‘III’ and ‘V’ can create ‘III–V’ materials [ 22 ].

PV devices, sometimes called solar cells, are electronic devices that convert sunlight into electrical power. PVs are also one of the rapidly growing renewable-energy technologies of today. It is therefore anticipated to play a significant role in the long-term world electricity-generating mixture moving forward.

Solar PV systems can be incorporated to supply electricity on a commercial level or installed in smaller clusters for mini-grids or individual usage. Utilizing PV modules to power mini-grids is a great way to offer electricity to those who do not live close to power-transmission lines, especially in developing countries with abundant solar energy resources. In the most recent decade, the cost of producing PV modules has dropped drastically, giving them not only accessibility but sometimes making them the least expensive energy form. PV arrays have a 30-year lifetime and come in various shades based on the type of material utilized in their production.

The most typical method for solar PV desalination technology that is used for desalinating sea or salty water is electrodialysis (ED). Therefore, solar PV modules are directly connected to the desalination process. This technique employs the direct-current electricity to remove salt from the sea or salty water.

The technology of PV–thermal (PV–T) comprises conventional solar PV modules coupled with a thermal collector mounted on the rear side of the PV module to pre-heat domestic hot water. Accordingly, this enables a larger portion of the incident solar energy on the collector to be converted into beneficial electrical and thermal energy.

A zero-energy building is a building that is designed for zero net energy emissions and emits no carbon dioxide. Building-integrated PV (BIPV) technology is coupled with solar energy sources and devices in buildings that are utilized to supply energy needs. Thus, building-integrated PVs utilizing thermal energy (BIPV/T) incorporate creative technologies such as solar cooling [ 23 ].

A PV water-pumping system is typically used to pump water in rural, isolated and desert areas. The system consists of PV modules to power a water pump to the location of water need. The water-pumping rate depends on many factors such as pumping head, solar intensity, etc.

A PV-powered cathodic protection (CP) system is designed to supply a CP system to control the corrosion of a metal surface. This technique is based on the impressive current acquired from PV solar energy systems and is utilized for burying pipelines, tanks, concrete structures, etc.

Concentrated PV (CPV) technology uses either the refractive or the reflective concentrators to increase sunlight to PV cells [ 24 , 25 ]. High-efficiency solar cells are usually used, consisting of many layers of semiconductor materials that stack on top of each other. This technology has an efficiency of >47%. In addition, the devices produce electricity and the heat can be used for other purposes [ 26 , 27 ].

For CSP systems, the solar rays are concentrated using mirrors in this application. These rays will heat a fluid, resulting in steam used to power a turbine and generate electricity. Large-scale power stations employ CSP to generate electricity. A field of mirrors typically redirect rays to a tall thin tower in a CSP power station. Thus, numerous large flat heliostats (mirrors) are used to track the Sun and concentrate its light onto a receiver in power tower systems, sometimes known as central receivers. The hot fluid could be utilized right away to produce steam or stored for later usage. Another of the great benefits of a CSP power station is that it may be built with molten salts to store heat and generate electricity outside of daylight hours.

Mirrored dishes are used in dish engine systems to focus and concentrate sunlight onto a receiver. The dish assembly tracks the Sun’s movement to capture as much solar energy as possible. The engine includes thin tubes that work outside the four-piston cylinders and it opens into the cylinders containing hydrogen or helium gas. The pistons are driven by the expanding gas. Finally, the pistons drive an electric generator by turning a crankshaft.

A further water-treatment technique, using reverse osmosis, depends on the solar-thermal and using solar concentrated power through the parabolic trough technique. The desalination employs CSP technology that utilizes hybrid integration and thermal storage allows continuous operation and is a cost-effective solution. Solar thermal can be used for domestic purposes such as a dryer. In some countries or societies, the so-called food dehydration is traditionally used to preserve some food materials such as meats, fruits and vegetables.

Sustainable energy development is defined as the development of the energy sector in terms of energy generating, distributing and utilizing that are based on sustainability rules [ 28 ]. Energy systems will significantly impact the environment in both developed and developing countries. Consequently, the global sustainable energy system must optimize efficiency and reduce emissions [ 29 ].

The sustainable development scenario is built based on the economic perspective. It also examines what activities will be required to meet shared long-term climate benefits, clean air and energy access targets. The short-term details are based on the IEA’s sustainable recovery strategy, which aims to promote economies and employment through developing a cleaner and more reliable energy infrastructure [ 15 ]. In addition, sustainable development includes utilizing renewable-energy applications, smart-grid technologies, energy security, and energy pricing, and having a sound energy policy [ 29 ].

The demand-side response can help meet the flexibility requirements in electricity systems by moving demand over time. As a result, the integration of renewable technologies for helping facilitate the peak demand is reduced, system stability is maintained, and total costs and CO 2 emissions are reduced. The demand-side response is currently used mostly in Europe and North America, where it is primarily aimed at huge commercial and industrial electricity customers [ 15 ].

International standards are an essential component of high-quality infrastructure. Establishing legislative convergence, increasing competition and supporting innovation will allow participants to take part in a global world PV market [ 30 ]. Numerous additional countries might benefit from more actively engaging in developing global solar PV standards. The leading countries in solar PV manufacturing and deployment have embraced global standards for PV systems and highly contributed to clean-energy development. Additional assistance and capacity-building to enhance quality infrastructure in developing economies might also help support wider implementation and compliance with international solar PV standards. Thus, support can bring legal requirements and frameworks into consistency and give additional impetus for the trade of secure and high-quality solar PV products [ 19 ].

Continuous trade-led dissemination of solar PV and other renewable technologies will strengthen the national infrastructure. For instance, off-grid solar energy alternatives, such as stand-alone systems and mini-grids, could be easily deployed to assist healthcare facilities in improving their degree of services and powering portable testing sites and vaccination coolers. In addition to helping in the immediate medical crisis, trade-led solar PV adoption could aid in the improving economy from the COVID-19 outbreak, not least by providing jobs in the renewable-energy sector, which are estimated to reach >40 million by 2050 [ 19 ].

The framework for energy sustainability development, by the application of solar energy, is one way to achieve that goal. With the large availability of solar energy resources for PV and CSP energy applications, we can move towards energy sustainability. Fig. 3 illustrates plans for solar energy sustainability.

Framework for solar energy applications in energy sustainability.

The environmental consideration of such applications, including an aspect of the environmental conditions, operating conditions, etc., have been assessed. It is clean, friendly to the environment and also energy-saving. Moreover, this technology has no removable parts, low maintenance procedures and longevity.

Economic and social development are considered by offering job opportunities to the community and providing cheaper energy options. It can also improve people’s income; in turn, living standards will be enhanced. Therefore, energy is paramount, considered to be the most vital element of human life, society’s progress and economic development.

As efforts are made to increase the energy transition towards sustainable energy systems, it is anticipated that the next decade will see a continued booming of solar energy and all clean-energy technology. Scholars worldwide consider research and innovation to be substantial drivers to enhance the potency of such solar application technology.

2.1 Employment from renewable energy

The employment market has also boomed with the deployment of renewable-energy technology. Renewable-energy technology applications have created >12 million jobs worldwide. The solar PV application came as the pioneer, which created >3 million jobs. At the same time, while the solar thermal applications (solar heating and cooling) created >819 000 jobs, the CSP attained >31 000 jobs [ 20 ].

According to the reports, although top markets such as the USA, the EU and China had the highest investment in renewables jobs, other Asian countries have emerged as players in the solar PV panel manufacturers’ industry [ 31 ].

Solar energy employment has offered more employment than other renewable sources. For example, in the developing countries, there was a growth in employment chances in solar applications that powered ‘micro-enterprises’. Hence, it has been significant in eliminating poverty, which is considered the key goal of sustainable energy development. Therefore, solar energy plays a critical part in fulfilling the sustainability targets for a better plant and environment [ 31 , 32 ]. Fig. 4 illustrates distributions of world renewable-energy employment.

World renewable-energy employment [ 20 ].

The world distribution of PV jobs is disseminated across the continents as follows. There was 70% employment in PV applications available in Asia, while 10% is available in North America, 10% available in South America and 10% availability in Europe. Table 1 details the top 10 countries that have relevant jobs in Asia, North America, South America and Europe.

List of the top 10 countries that created jobs in solar PV applications [ 19 , 33 ]

Solar energy investments can meet energy targets and environmental protection by reducing carbon emissions while having no detrimental influence on the country’s development [ 32 , 34 ]. In countries located in the ‘Sunbelt’, there is huge potential for solar energy, where there is a year-round abundance of solar global horizontal irradiation. Consequently, these countries, including the Middle East, Australia, North Africa, China, the USA and Southern Africa, to name a few, have a lot of potential for solar energy technology. The average yearly solar intensity is >2800 kWh/m 2 and the average daily solar intensity is >7.5 kWh/m 2 . Fig. 5 illustrates the optimum areas for global solar irradiation.

World global solar irradiation map [ 35 ].

The distribution of solar radiation and its intensity are two important factors that influence the efficiency of solar PV technology and these two parameters vary among different countries. Therefore, it is essential to realize that some solar energy is wasted since it is not utilized. On the other hand, solar radiation is abundant in several countries, especially in developing ones, which makes it invaluable [ 36 , 37 ].

Worldwide, the PV industry has benefited recently from globalization, which has allowed huge improvements in economies of scale, while vertical integration has created strong value chains: as manufacturers source materials from an increasing number of suppliers, prices have dropped while quality has been maintained. Furthermore, the worldwide incorporated PV solar device market is growing fast, creating opportunities enabling solar energy firms to benefit from significant government help with underwriting, subsides, beneficial trading licences and training of a competent workforce, while the increased rivalry has reinforced the motivation to continue investing in research and development, both public and private [ 19 , 33 ].

The global outbreak of COVID-19 has impacted ‘cross-border supply chains’ and those investors working in the renewable-energy sector. As a result, more diversity of solar PV supply-chain processes may be required in the future to enhance long-term flexibility versus exogenous shocks [ 19 , 33 ].

It is vital to establish a well-functioning quality infrastructure to expand the distribution of solar PV technologies beyond borders and make it easier for new enterprises to enter solar PV value chains. In addition, a strong quality infrastructure system is a significant instrument for assisting local firms in meeting the demands of trade markets. Furthermore, high-quality infrastructure can help reduce associated risks with the worldwide PV project value chain, such as underperforming, inefficient and failing goods, limiting the development, improvement and export of these technologies. Governments worldwide are, at various levels, creating quality infrastructure, including the usage of metrology i.e. the science of measurement and its application, regulations, testing procedures, accreditation, certification and market monitoring [ 33 , 38 ].

The perspective is based on a continuous process of technological advancement and learning. Its speed is determined by its deployment, which varies depending on the scenario [ 39 , 40 ]. The expense trends support policy preferences for low-carbon energy sources, particularly in increased energy-alteration scenarios. Emerging technologies are introduced and implemented as quickly as they ever have been before in energy history [ 15 , 33 ].

The CSP stations have been in use since the early 1980s and are currently found all over the world. The CSP power stations in the USA currently produce >800 MW of electricity yearly, which is sufficient to power ~500 000 houses. New CSP heat-transfer fluids being developed can function at ~1288 o C, which is greater than existing fluids, to improve the efficiency of CSP systems and, as a result, to lower the cost of energy generated using this technology. Thus, as a result, CSP is considered to have a bright future, with the ability to offer large-scale renewable energy that can supplement and soon replace traditional electricity-production technologies [ 41 ]. The DESERTEC project has drawn out the possibility of CSP in the Sahara Desert regions. When completed, this investment project will have the world’s biggest energy-generation capacity through the CSP plant, which aims to transport energy from North Africa to Europe [ 42 , 43 ].

The costs of manufacturing materials for PV devices have recently decreased, which is predicted to compensate for the requirements and increase the globe’s electricity demand [ 44 ]. Solar energy is a renewable, clean and environmentally friendly source of energy. Therefore, solar PV application techniques should be widely utilized. Although PV technology has always been under development for a variety of purposes, the fact that PV solar cells convert the radiant energy from the Sun directly into electrical power means it can be applied in space and in terrestrial applications [ 38 , 45 ].

In one way or another, the whole renewable-energy sector has a benefit over other energy industries. A long-term energy development plan needs an energy source that is inexhaustible, virtually accessible and simple to gather. The Sun rises over the horizon every day around the globe and leaves behind ~108–1018 kWh of energy; consequently, it is more than humanity will ever require to fulfil its desire for electricity [ 46 ].

The technology that converts solar radiation into electricity is well known and utilizes PV cells, which are already in use worldwide. In addition, various solar PV technologies are available today, including hybrid solar cells, inorganic solar cells and organic solar cells. So far, solar PV devices made from silicon have led the solar market; however, these PVs have certain drawbacks, such as expenditure of material, time-consuming production, etc. It is important to mention here the operational challenges of solar energy in that it does not work at night, has less output in cloudy weather and does not work in sandstorm conditions. PV battery storage is widely used to reduce the challenges to gain high reliability. Therefore, attempts have been made to find alternative materials to address these constraints. Currently, this domination is challenged by the evolution of the emerging generation of solar PV devices based on perovskite, organic and organic/inorganic hybrid materials.

This paper highlights the significance of sustainable energy development. Solar energy would help steady energy prices and give numerous social, environmental and economic benefits. This has been indicated by solar energy’s contribution to achieving sustainable development through meeting energy demands, creating jobs and protecting the environment. Hence, a paramount critical component of long-term sustainability should be investigated. Based on the current condition of fossil-fuel resources, which are deemed to be depleting energy sources, finding an innovative technique to deploy clean-energy technology is both essential and expected. Notwithstanding, solar energy has yet to reach maturity in development, especially CSP technology. Also, with growing developments in PV systems, there has been a huge rise in demand for PV technology applications all over the globe. Further work needs to be undertaken to develop energy sustainably and consider other clean energy resources. Moreover, a comprehensive experimental and validation process for such applications is required to develop cleaner energy sources to decarbonize our planet.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Researchers take major step toward developing next-generation solar cells

A technician installs solar panels on the roof of the building which houses the University of Colorado Center for Innovation and Creativity in Boulder.  (Credit: Glenn Asakawa/University of Colorado)  

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The solar energy world is ready for a revolution. Scientists are racing to develop a new type of solar cell using materials that can convert electricity more efficiently than today’s panels. 

In a new paper published February 26 in the journal Nature Energy, a CU Boulder researcher and his international collaborators unveiled an innovative method to manufacture the new solar cells, known as perovskite cells, an achievement critical for the commercialization of what many consider the next generation of solar technology.

Today, nearly all solar panels are made from silicon, which boast an efficiency of 22%. This means silicon panels can only convert about one-fifth of the sun’s energy into electricity, because the material absorbs only a limited proportion of sunlight’s wavelengths. Producing silicon is also expensive and energy intensive.

Enter perovskite. The synthetic semiconducting material has the potential to convert substantially more solar power than silicon at a lower production cost.

Michael McGehee

“Perovskites might be a game changer,” said Michael McGehee , a professor in the Department of Chemical and Biological Engineering and fellow with CU Boulder’s Renewable & Sustainable Energy Institute. 

Scientists have been testing perovskite solar cells by stacking them on top of traditional silicon cells to make tandem cells. Layering the two materials, each absorbing a different part of the sun’s spectrum, can potentially increase the panels’ efficiency by over 50%.

“We're still seeing rapid electrification, with more cars running off electricity. We’re hoping to retire more coal plants and eventually get rid of natural gas plants,” said McGehee.  “If you believe that we're going to have a fully renewable future, then you're planning for the wind and solar markets to expand by at least five to ten- fold from where it is today.” 

To get there, he said, the industry must improve the efficiency of solar cells.

But a major challenge in making them from perovskite at a commercial scale is the process of coating the semiconductor onto the glass plates which are the building blocks of panels. Currently, the coating process has to take place in a small box filled with non-reactive gas, such as nitrogen, to prevent the perovskites from reacting with oxygen, which decreases their performance.  

“This is fine at the research stage. But when you start coating large pieces of glass, it gets harder and harder to do this in a nitrogen filled box,” McGehee said. 

McGehee and his collaborators set off to find a way to prevent that damaging reaction with the air. They found that adding dimethylammonium formate, or DMAFo, to the perovskite solution before coating could prevent the materials from oxidizing. This discovery enables coating to take place outside the small box, in ambient air. Experiments showed that perovskite cells made with the DMAFo additive can achieve an efficiency of nearly 25% on their own, comparable to the current efficiency record for perovskite cells of 26%. 

The additive also improved the cells’ stability. 

Commercial silicon panels can typically maintain at least 80% of their performance after 25 years, losing about 1% of efficiency per year. Perovskite cells, however, are more reactive and degrade faster in the air. The new study showed that the perovskite cell made with DMAFo retained 90% of its efficiency after the researchers exposed them to LED light that mimicked sunlight for 700 hours. In contrast, cells made in the air without DMAFo degraded quickly after only 300 hours. 

While this is a very encouraging result, there are 8,000 hours in one year, he noted. So longer tests are needed to determine how these cells hold up overtime. 

“It’s too early to say that they are as stable as silicon panels, but we're on a good trajectory toward that,” McGehee said. 

The study brings perovskite solar cells one step closer to commercialization. At the same time, McGehee’s team is actively developing tandem cells with a real-world efficiency of over 30% that have the same operational lifetime as silicon panels. 

McGehee leads a U.S. academic–industry partnership called Tandems for Efficient and Advanced Modules using Ultrastable Perovskites (TEAMUP). Together with researchers from three other universities, two companies and a national laboratory, the consortium received $9 million funding from the U.S. Department of Energy last year to develop stable tandem perovskites that can feasibly be used in the real world and are commercially viable. The goal is to create tandem more efficient than conventional silicon panels and equally stable over a 25-year period. 

With higher efficiency and potentially lower price tags, these tandem cells could have broader applications than existing silicon panels, including potential installation on the roofs of electric vehicles. They could add 15 to 25 miles of range per day to a car left out in the sun, enough to cover many people’s daily commutes. Drones and sailboats could also be powered by such panels.  

After a decade of research in perovskites, engineers have built perovskite cells that are as efficient as silicon cells, which were invented 70 years ago, McGehee said. “We are taking perovskites to the finish line.  If tandems work out well, they certainly have the potential to dominate the market and become the next generation of solar cells,” he said. 

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Researchers take major step toward developing next-generation solar cells

by Yvaine Ye, University of Colorado at Boulder

The solar energy world is ready for a revolution. Scientists are racing to develop a new type of solar cell using materials that can convert electricity more efficiently than today's panels.

In a paper published February 26 in the journal Nature Energy , a University of Colorado Boulder researcher and his international collaborators unveiled an innovative method to manufacture the new solar cells, known as perovskite cells, an achievement critical for the commercialization of what many consider the next generation of solar technology.

Today, nearly all solar panels are made from silicon, which boasts an efficiency of 22%. This means silicon panels can only convert about one-fifth of the sun's energy into electricity because the material absorbs only a limited proportion of sunlight's wavelengths. Producing silicon is also expensive and energy-intensive.

Enter perovskite. The synthetic semiconducting material has the potential to convert substantially more solar power than silicon at a lower production cost.

"Perovskites might be a game changer," said Michael McGehee, a professor in the Department of Chemical and Biological Engineering and fellow with CU Boulder's Renewable & Sustainable Energy Institute.

Scientists have been testing perovskite solar cells by stacking them on top of traditional silicon cells to make tandem cells. Layering the two materials, each absorbing a different part of the sun's spectrum, can potentially increase the panels' efficiency by over 50%.

"We're still seeing rapid electrification, with more cars running off electricity. We're hoping to retire more coal plants and eventually get rid of natural gas plants," said McGehee. "If you believe that we're going to have a fully renewable future, then you're planning for the wind and solar markets to expand by at least five to ten-fold from where it is today."

To get there, he said, the industry must improve the efficiency of solar cells.

But a major challenge in making them from perovskite at a commercial scale is the process of coating the semiconductor onto the glass plates which are the building blocks of panels. Currently, the coating process has to take place in a small box filled with non-reactive gas, such as nitrogen, to prevent the perovskites from reacting with oxygen, which decreases their performance.

"This is fine at the research stage. But when you start coating large pieces of glass, it gets harder and harder to do this in a nitrogen-filled box," McGehee said.

McGehee and his collaborators set off to find a way to prevent that damaging reaction with the air. They found that adding dimethylammonium formate, or DMAFo, to the perovskite solution before coating could prevent the materials from oxidizing.

This discovery enables coating to take place outside the small box, in ambient air. Experiments showed that perovskite cells made with the DMAFo additive can achieve an efficiency of nearly 25% on their own, comparable to the current efficiency record for perovskite cells of 26%.

The additive also improved the cells' stability.

Commercial silicon panels can typically maintain at least 80% of their performance after 25 years, losing about 1% of efficiency per year. Perovskite cells, however, are more reactive and degrade faster in the air. The new study showed that the perovskite cell made with DMAFo retained 90% of its efficiency after the researchers exposed them to LED light that mimicked sunlight for 700 hours. In contrast, cells made in the air without DMAFo degraded quickly after only 300 hours.

While this is a very encouraging result, there are 8,000 hours in one year, he noted. So longer tests are needed to determine how these cells hold up over time.

"It's too early to say that they are as stable as silicon panels, but we're on a good trajectory toward that," McGehee said.

The study brings perovskite solar cells one step closer to commercialization. At the same time, McGehee's team is actively developing tandem cells with a real-world efficiency of over 30% that have the same operational lifetime as silicon panels. The goal is to create tandem that are more efficient than conventional silicon panels and equally stable over a 25-year period.

With higher efficiency and potentially lower price tags, these tandem cells could have broader applications than existing silicon panels, including potential installation on the roofs of electric vehicles. They could add 15 to 25 miles of range per day to a car left out in the sun, enough to cover many people's daily commutes. Drones and sailboats could also be powered by such panels.

After a decade of research in perovskites, engineers have built perovskite cells that are as efficient as silicon cells, which were invented 70 years ago, McGehee said. "We are taking perovskites to the finish line. If tandems work out well, they certainly have the potential to dominate the market and become the next generation of solar cells," he said.

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• Review Article
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• Published: 05 January 2023

Next-generation applications for integrated perovskite solar cells

• Abdulaziz S. R. Bati   ORCID: orcid.org/0000-0001-6346-4396 1 ,
• Yu Lin Zhong   ORCID: orcid.org/0000-0001-6741-3609 2 ,
• Paul L. Burn 1 ,
• Mohammad Khaja Nazeeruddin 3 ,
• Paul E. Shaw   ORCID: orcid.org/0000-0002-3326-3670 1 &
• Munkhbayar Batmunkh   ORCID: orcid.org/0000-0002-7493-4186 2  

Communications Materials volume  4 , Article number:  2 ( 2023 ) Cite this article

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• Devices for energy harvesting
• Nanoscale devices
• Solar energy

Organic/inorganic metal halide perovskites attract substantial attention as key materials for next-generation photovoltaic technologies due to their potential for low cost, high performance, and solution processability. The unique properties of perovskites and the rapid advances that have been made in solar cell performance have facilitated their integration into a broad range of practical applications, including tandem solar cells, building-integrated photovoltaics, space applications, integration with batteries and supercapacitors for energy storage systems, and photovoltaic-driven catalysis. In this Review, we outline notable achievements that have been made in these photovoltaic-integrated technologies. Outstanding challenges and future perspectives for the development of these fields and potential next-generation applications are discussed.

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Introduction

Over the past decade, metal halide perovskites with the chemical structure ABX 3 (A = methylammonium (MA), formamidinium (FA), or cesium (Cs); B = Pb, Sn; and X = I − , Br − , or Cl − , or combinations thereof) have emerged as promising photovoltaic (PV) materials due to their extraordinary optical and electrical properties such as high absorption coefficients, low exciton binding energy, bandgap tunability, ambipolar transport characteristics, excellent charge-carrier mobilities, long charge-carrier lifetimes, long carrier diffusion lengths and high defect tolerance 1 , 2 , 3 . These remarkable properties have underpinned the rapid development of PV devices based on perovskite absorbers, which is illustrated by the improvement in power conversion efficiencies (PCEs) from 3.8% to 25.7% 4 . This significant advance in PV performance has placed perovskite solar cells (PSCs) in the front-of-line for realizing next-generation low-cost PV and integrated technologies. PSCs are slated to hold several advantages over established and emerging PV technologies. For instance, silicon solar cells require pure silicon, produced by heating sand at elevated temperatures (>1000 °C), have complicated manufacturing processes (e.g., texturing, anti-reflective coatings) that are usually carried out using special facilities, and greenhouse gases in their fabrication, all of which add to the fabrication cost. In contrast, perovskite materials can be solution processed, enabling low-embedded energy manufacturing using commercial coating technologies. Compared to silicon solar cells, some emerging solar cells, such as organic solar cells (OSCs), tend to be more cost-effective and wet-processable. However, efficient OSCs need to overcome some intrinsic properties such as low relative dielectric constants (2–4, meaning free charge carriers are not directly formed upon photoexcitation), low effective carrier mobility (10 −5 to 10 −4  cm 2  V −1  s −1 ), and low charge-carrier diffusion length at open circuit (≈20 nm) 5 . In contrast, PSCs exhibit a larger relative dielectric constant in the range of 20–50, more effective charge-carrier mobility of 0.1–10 cm 2  V −1  s −1 , and large charge-carrier diffusion length at open circuit (>500 nm) 5 .

In general, PSCs are fabricated with a layered device structure that consists of a transparent conductive oxide (TCO), electron transporting layer (ETL), perovskite absorption layer, hole transporting layer (HTL), and a counter electrode. However, in common with cadmium-telluride thin-film solar cells, plans will need to be put in place to recover the heavy metals in perovskite solar cells. Furthermore, it is important to note that all solar types require encapsulation. Depending on the position of the charge-selective layer, PSCs can be classified as standard ( n – i – p ) or inverted ( p – i – n ) configurations 6 . The operational mechanism of PSCs can be described briefly as follows: upon light absorption, electron-hole pairs are generated in the perovskite layer, which are then extracted through the charge-selective HTL and ETL materials to the corresponding conductive electrodes 7 .

Motivated by the unprecedented advancement in the PCEs of PSCs over the past few years, a relatively new and growing area of research has been recently explored where PSCs are utilized as an energy source for integrated systems such as energy conversion and storage devices. Although these research areas are still in their infancy, early activities in integrating PSCs into a wide range of applications have already shown significant promise.

In this review, we explore the integration of state-of-the-art PSCs into a comprehensive range of next-generation applications, including tandem solar cells, building-integrated PVs (BIPVs), space applications, PV-powered batteries, supercapacitors, and energy sources for catalytic synthesis of high-value chemicals (Fig.  1 ). Finally, we present a brief outlook highlighting the challenges and future perspectives in this vibrant research field.

The figure outlines the development of halide perovskite materials-based applications.

Tandem solar cells

The PCEs of single-junction PSCs are approaching the maximum of 25.7% under one sun illumination. Further enhancing the PCE to the theoretical Shockley–Queisser limit (~33%), requires the thermalization of high-energy carriers and photon transmission losses to be reduced 8 . In order to minimize these energy losses and overcome the Shockley–Queisser limit for a single junction device, designing multiple junctions (tandem or greater solar cells) composed of a wide-bandgap absorber (top layer) and a low-bandgap absorber (bottom layer) have been proposed and implemented 9 . Such a device configuration allows absorption of the fraction of incident photons with energy higher than the wide-bandgap absorber, while the low energy photons pass through to the bottom subcell where they are harvested by the low-bandgap active layer 10 . There are two general structures for tandem devices—two-terminal (2 T, also called monolithic) and four-terminal (4 T) tandem solar cells (see Fig.  2 ). In the former, a single substrate is used to construct both subcells (stacked together with an interconnection layer) with a transparent front electrode and a non-transparent back electrode. In the latter case (4 T), two separate cells are fabricated individually and then physically connected together to form a full device. Due to the lower fabrication cost of the 2 T architecture (i.e., only two electrodes are involved and no extra external circuit is required) and the absence of a physical gap between the two connected subcells, which in turn reduces the optical loss, the 2T device configuration is more appealing for practical applications than the 4 T tandem structure 10 . Theoretical analysis has predicted that stacked cell configurations fabricated from two-junction (tandem) and three-junction architectures could achieve power conversion efficiencies as high as 42% and 49%, respectively. Furthermore, if an infinite number of solar cells could be stacked, then the upper limit efficiency can be further increased to reach 68% and 86% under unconcentrated and concentrated sunlight, respectively 11 . However, from a manufacturing perspective, the cost of fabricating multi-junction stacked devices increases significantly, which can outweigh the efficiency gains. It should be noted that there are several different classes of multi-junction (tandem) solar cells including III–V semiconductor based devices 12 , but their commercialization pathways are limited due to their high production cost and complicated fabrication process.

The figure shows two-terminal and four-terminal device configurations. It should be noted that in the literature, the positions of the top and bottom subcells are defined differently. In this Review, we define the top subcell as the cell in which the light initially passes through to the bottom subcell.

With their lower fabrication cost, low-temperature solution processability, roll-to-roll manufacturing, and wide-bandgap tunability, PSCs have the potential to become the candidate of choice for high-efficiency tandem solar cells 13 . Importantly, the ability to tailor the optical properties of the perovskite materials by tuning their chemical composition provides a means to optimize the light absorption for different device architectures, and hence perovskite materials can be potentially used to form either/or the top and/or bottom subcells in a tandem device 14 . In addition to the tandem device structures made of perovskite-organic or perovskite–perovskite subcells, the integration of a wide-bandgap perovskite with well-established low-bandgap materials such as Si and CIGS to build tandem solar cells is an attractive proposition and has received considerable attention from the PV community. In the following sub-sections, the major advancements that have been made in perovskite-based tandem solar cells will be discussed in detail.

Perovskite/organic tandem solar cells

Organic solar cells (OSCs) are an attractive option for next-generation photovoltaics due to their low-cost, tunable optical properties, solution processability, mechanical flexibility and lightweight form-factors 15 . The best OSCs have now been reported to have PCEs of over 18%. Despite achieving high efficiencies, OSCs generally feature low V oc values (<1 V). To overcome this limiting factor, tandem devices comprising a wide-bandgap perovskite cell and a small optical gap organic cell have promise. In addition to improving the device performance, the hydrophobic nature of the organic layers can potentially play a role in stabilizing the perovskite subcell, which is particularly sensitive to moisture. Another distinct advantage of perovskite/organic tandem solar cells is that the absorbing layers can be deposited from orthogonal solvents, which ensures that the coating of the organic layer on top of the perovskite layer does not cause the underlying layer to dissolve. Thus, these types of tandem devices can in principle be fabricated as all-solution-processed tandem devices, which are compatible with large-scale roll-to-roll production coating techniques.

Yang and colleagues pioneered 2 T perovskite/organic tandem devices, which were found to have a PCE of 10.2% and an V oc of 1.52 V. The tandem device used CH 3 NH 3 PbI 3 (MAPbI 3 ) as the perovskite absorber and an IR-sensitive block copolymer PBSeDTEG8:fullerene blend as the organic semiconductor absorber 16 . Although this work was the first demonstration of integrating perovskite and organic semiconductor polymer subcells into a tandem structure, a number of challenges remain. To avoid damaging the polymer subcell underneath during the fabrication of the upper perovskite subcell (i.e., during thermal annealing and chemical treatment), Liu et al. 17 inverted the order of the layers in the tandem device structure by employing a very thin MAPbI 3 layer (~90 nm) as the top subcell and an organic layer as the bottom subcell. The resulting tandem devices exhibited a PCE of 16% and an V oc of 1.63 V, with the PCE being higher than a single-junction perovskite device assembled with an identical perovskite layer thickness (9.1%) and the single-junction organic device (9.7%). By adopting the FA 0.8 MA 0.02 Cs 0.18 PbI 1.8 Br 1.2 perovskite with a wide-bandgap of 1.77 eV as the top subcell and a PBDBT-2F:Y6:PC 71 BM blend with a small optical gap of 1.41 eV as the bottom subcell, Yang and co-workers were able to fabricate 2 T perovskite/organic tandem solar cells that delivered a PCE of 20.6% and an V oc of 1.90 V (Fig.  3a, b ) 18 . It was also shown that the perovskite subcell acted as a UV filter eliminating the UV sensitivity of the organic subcell, leading to enhanced photostability of the tandem device. Based on the semi-empirical device model developed in this study, perovskite/organic tandem solar cells were predicted to be able to achieve PCEs exceeding 30%, although at this time these have yet to be realized.

a Tandem cell structure highlighting the interconnection layer (ICL) design. b Simulated distribution of photon absorption in tandem cells using the transfer matrix method. a , b adapted with permission from ref. 18 , Copyright 2020 Elsevier Inc. c Energy-level diagram of an inorganic perovskite/organic tandem solar cell (TSCs). c adapted with permission from ref. 10 , Copyright 2021 American Chemical Society. d Schematic illustration of the trimethylammonium chloride (TACl) and IPA synergistically induced surface reconstruction (SR) processes. d adapted with permission from ref. 21 , Copyright 2021 Wiley-VCH. e EQE spectra of a NiOx/PTAA tandem device measured for the individual subcells and the total reflection ® spectra depicted as 1-R. Perovskite, CIGSe, and 1-R spectra are denoted with red, blue, and gray lines and areas. Integrated photocurrents and reflection losses from the EQE and 1-R spectra values are also shown. e adapted with permission from ref. 30 , Copyright 2019 American Chemical Society. f J–V curve of a monolithic CIGSe/perovskite-tandem solar cell (active area of 1.034 cm 2 ), with MeO-2PACz2PACz as a hole-selective contact (HSC) that is used to conformally cover the rough CIGSe bottom cell. The orange circle indicates the MPP at a PCE of 23.3% PCE. Inset: SEM image of the cross-section of a representative tandem device. f adapted with permission from ref. 31 , Copyright 2019 Royal Society of Chemistry. g Cross-sectional scanning electron micrograph of an all-perovskite tandem. g adapted with permission from ref. 35 , Copyright 2019 Elsevier Inc. h J–V curves of the current champion all-perovskite-tandem solar cells (all-PTSCs) fabricated with a formamidine sulfinic acid additive scanned in the reverse and forward directions. i Continuous MPP tracking of encapsulated all-PTSCs over a period of 500 h under 1 Sun (100 mW cm −2 ) light illumination without an ultraviolet filter in ambient air with a humidity of 30–50%. ( h , i ) adapted with permission from ref. 36 Copyright 2020 Springer Nature. j , k SEM images of perovskite films prepared ( j ) without and ( k ) with 7% of a GuaSCN additive. j , k adapted with permission from ref. 1 μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells. Science 364, 475–479 (2019)." href="/articles/s43246-022-00325-4#ref-CR37" id="ref-link-section-d3742168e884">37 , Copyright 2019 American Association for the Advancement of Science.

The inorganic large-bandgap CsPbI 2 Br perovskite has also been demonstrated to be an excellent candidate for integration with organic subcells due to its superior UV and high thermal stability 19 , 20 . Wang et al. 10 demonstrated that a hole transporting material (HTM) in the interconnecting layers was essential for monolithic perovskite/organic tandem solar cells to reduce the charge accumulation at the interface, and therefore minimize the voltage loss. By employing the wide-bandgap CsPbI 2 Br as the top subcell, the narrow-optical gap PM6:Y6-BO blend as the bottom subcell, and a 4-butyl- N , N -diphenylaniline homopolymer (polyTPD) as the HTL in the interconnecting layer, a tandem device was found to achieve a PCE of 21.1% and a V oc of 1.96 V. Notably, the full name of the PM6 polymer is poly[(4,8-bis{5-[2-ethylhexyl]-4-fluoro-2-thienyl}]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-2,5-thiophenediyl(5,7-bis{2-ethylhexyl}-4,8-dioxo-4 H ,8 H -benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl)-2,5-thiophenediyl], while that of Y6-BO is defined as 2,2′-[(2 Z ,2′ Z )-({12,13-bis[2-butyloctyl]-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3′:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl}bis{methanylylidene})bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)]dimalononitrile. The energy diagram of the tandem device is depicted in Fig.  3c . At around the same time, Li and co-workers reported a PCE of 21.0% using a 2 T perovskite/organic tandem solar cell with a V oc of 2.05 V 21 . Their strategy relied on passivating the defects of CsPbI 1.8 Br 1.2 perovskite surface using trimethylammonium chloride (Fig.  3d ), which resulted in the growth of high-quality pinhole-free perovskite films and the suppression of surface nonradiative charge recombination. Importantly, those devices showed enhanced operational and UV stability relative to the individual subcells. Despite the progress made over the past several years, the PV performance of perovskite/organic tandem solar cells is still far from the maximum potential efficiency. The main limitations in this class of tandem devices are thought to be the V oc loss from wide-bandgap perovskite subcells and the non-ideal interconnecting layers. Chen et al. 22 employed a nickel oxide (NiO x ) based HTL in combination with benzylphosphonic acid to suppress the interfacial recombination within the devices, and achieved a V oc of 1.26 V for the 1.79-eV-bandgap perovskite subcell. The authors also developed an interconnecting layer structure comprising a 4-nm-thick sputtered indium zinc oxide layer, which provided enhanced electrical properties and transmittance in the near-infrared region (NIR). These improvements resulted in perovskite/organic tandem solar cells with maximum and certified PCEs of 23.6% and 22.95%, respectively. This work demonstrates that there are further opportunities to enhance the PV performance of perovskite/organic based systems.

Recently, the emergence of non-fullerene acceptors (NFAs) with their facile synthetic routes and wide energy-level adjustment enabled the fabrication of OSCs with a significant reduction in V oc loss 23 , 24 . With the recent advances in developing novel NFAs, single-junction OSCs with a certified PCE of 19.2% have been reported. This large performance enhancement is beneficial for more efficient perovskite/organic devices. In a recent study, Brinkmann et al. 25 demonstrated perovskite–organic tandem solar cells with a certified PCE of 23.1% and a high V oc of 2.15 V based on a Y6-containing ternary system for the OSC component. We anticipate that with the continuous significant advances in OSCs subcells, more efficient tandem devices will be achieved. Future studies on perovskite/organic tandem solar cells should focus on developing narrow-optical gap organic semiconductors with excellent environmental stability and suppressing phase segregation in wide-bandgap perovskites.

Perovskite/CIGS tandem solar cells

Polycrystalline thin-film copper indium gallium selenide (CIGS) based solar cells are well-established and commercially available. The record efficiency of single-junction CIGS solar cells has reached 23.4%, which makes this class of solar cells very attractive for integration into perovskite containing tandem solar cells 26 . CIGS-based absorbers have an adjustable direct bandgap that can be tuned to 1 eV 27 , and high absorption coefficient of around 10 5  cm −1 . The latter property means that it is possible to significantly reduce the absorber thicknesses required, and hence the costs associated with fabricating tandem devices 28 .

The first 2 T perovskite/CIGS tandem solar cells were reported by Todorov et al. 29 in 2015, and these device had a maximum PCE of 10.9% and an V oc of 1.45 V, although it should be noted that the PCE was lower than the efficiencies of the individual subcells. This reduced efficiency was ascribed to the optical losses resulting from the top Al electrode and the high series resistance in the device. Furthermore, the high surface roughness of CIGS layers represents a major obstacle for obtaining high-quality and uniform perovskite films when they are deposited on top of the CIGS layer. In addition, the high surface roughness can significantly increase the probability of device shunting. Various strategies have been developed to fabricate a smoother CIGS-based bottom subcell surface. For instance, Albrecht and co-workers found that the deposition of dual p-type HTLs consisting of atomic layer deposited NiO x and spin-coated PTAA on a rough CIGS bottom subcell not only prevented device shunting, but also improved the performance of the monolithic perovskite/CIGSe tandem solar cells 30 . Using this approach, a device with an active area of 0.78 cm 2 yielded a PCE of 21.6%. Despite the good PCE, the recorded photocurrent showed a large mismatch between the two subcells, which was attributed to the parasitic absorption and the rough interfaces of the device (Fig.  3e ). In 2019, the same group demonstrated how employing a self-assembled monolayer (SAM) that binds to the oxide layer of the CIGS rough surface could boost the PCE of monolithic perovskite/CIGS tandem solar cells. The devices had a larger active area of 1.03 cm 2 and a PCE of 23.3% (Fig.  3f ) 31 . It was proposed that the SAM forms an energetically favorable interface with the perovskite, acting as an efficient hole-selective contact without introducing nonradiative losses. Considering the negligible amount of SAMs required for device fabrication, SAMs and other surface passivators may be a realistic and cost-effective strategy to realize high efficiency and low-cost PV technologies. It should be noted that MiaSolé Hi-Tech and the European Solliance Solar Research (Solliance) have announced the development of perovskite/CIGS tandem solar cells with a record efficiency of 26.5%. However, the exact details of their discovery are still unknown 32 .

Very recently, Jošt et al. 33 reported monolithic perovskite/CIGS tandem solar cells with a certified PCE of 24.2% utilizing a large bandgap perovskite (1.68 eV) containing a PEAI additive, Me-4PACz monolayer as the HTM, and a LiF interlayer. This work demonstrates the high potential of perovskite/CIGS tandem solar cells.

Perovskite/perovskite-tandem solar cells

All-perovskite-tandem solar cells (all-PTSCs) are also attractive although there are challenges that need to be addressed. In an all-PTSC, a wide-bandgap perovskite (~1.7 eV) and a narrow-bandgap (~1 eV) perovskite are required as the top and bottom subcells, respectively. In a single-junction configuration, PSCs are typically fabricated with a bandgap of 1.5–1.7 eV (e.g., MAPbI 3 ), which meets the requirement for the wide-bandgap subcell. However, obtaining a narrow-bandgap perovskite is challenging and usually requires the partial replacement of Pb 2+ with Sn 2+ . This substitution creates several undesirable issues, which include the tendency of Sn 2+ to oxidize to Sn 4+ resulting in pinholes and/or a non-uniform perovskite surface with high defect density, both of which are detrimental to device performance. When compared with pure Pb-based perovskites, Sn-containing perovskites suffer from a shorter carrier lifetime and diffusion length, and small near-infrared absorption coefficient, which means that the perovskite film thickness needs to be increased to ensure that the long wavelength light is sufficiently absorbed 34 .

Despite the aforementioned obstacles, advances in efficiency of all-PTSCs have been achieved. It is worth mentioning at this stage that the solution-processed fabrication of 2 T all-PTSCs represents a challenge as the deposition of the top subcell can easily dissolve/damage the bottom subcell given the materials are often soluble in the same processing solvents. Hence, an interconnecting layer with orthogonal solubility between the subcells can play and important role in protecting the bottom perovskite layer. In 2019, Palmstrom et al. 35 reported an effective surface treatment of a C 60 interconnecting layer using a 1 nm thick poly(ethylenimine) ethoxylated (PEIE) layer and an atomic layer deposited aluminum zinc oxide (AZO) film (Fig.  3g ). The incorporation of the PEIE improved the nucleation of the AZO and also protected the modified layer from damage by water or N , N -dimethylformamide (DMF). Using this strategy, 2 T all-PTSCs with PCEs of 23.1% and 21.3% on rigid and flexible substrates, respectively, were obtained. This work not only demonstrates that efficient all-PTSCs can be formed, but that they can be lightweight and have a flexible form factor.

To inhibit the oxidation of Sn 2+ and passivate the defects on the mixed Pb–Sn perovskite surface, Xiao et al. 36 incorporated zwitterionic antioxidant additives, achieving an excellent PCE of 25.6% for a 2 T all-PTSC with an active area of 0.049 cm 2 (Fig.  3h ). The encapsulated devices showed good operational stability at the maximum power point (MPP), preserving 88% of their initial PCEs after 500 h of continuous illumination at a temperature of 54-60 °C under ambient atmosphere (Fig.  3i ). Although this all-PTSC fabricated with antioxidant additives exhibited promising operational stabilities, the longer-term stability of these devices is yet to be determined. Tong et al. 1 μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells. Science 364, 475–479 (2019)." href="/articles/s43246-022-00325-4#ref-CR37" id="ref-link-section-d3742168e1106">37 integrated guanidinium thiocyanate (GuaSCN) into the perovskite films in order to reduce the density of defects and improve the carrier lifetime and diffusion lengths. The SEM images of the perovskite films with and without GuaSCN additive shown in Fig.  3j, k reveal the structural changes in the perovskite film. The use of GuaSCN has led to the current record PCE of 25.4% for a 4 T all-PTSC configuration, as well as an efficiency of 23.1% for a 2 T all-PTSC. Lin et al. 38 were able to fabricate a thick Pb–Sn mixed perovskite subcell (1.2 μm) with the aim of increasing the photocurrent in monolithic all-perovskite-tandem solar cells. In order to reduce losses associated with the short carrier diffusion length relative to the perovskite film thickness, the Pb–Sn perovskites were passivated using 4-trifluoromethylphenylammonium (CF3-PA), resulting in a significant increase in the carrier diffusion length which exceeded 5 μm. Using this strategy, the authors fabricated all-perovskite-tandem solar cells with a certified PCE of 26.4% that maintained over 90% of the initial PCE after 600 h under illumination at the maximum power point in an ambient environment. It is worth mentioning that a monolithic perovskite–perovskite–silicon based triple-junction tandem solar cell with an efficiency of over 20%, a V oc of 2.74 V, and a FF of 86% was recently demonstrated 39 . However, to compensate for the increased cost of such a complicated device structure, the PCEs would need to increase further.

Perovskite/silicon tandem solar cells

With a large market share of more than 90%, low fabrication cost, suitable bandgap, exceptional performance, and life span of over 20 years, Si solar cells are the most mature candidate to combine with PSCs in a tandem device. Indeed, the integration of PSCs with silicon cells to form tandem devices has provided a great opportunity to realize high-efficiency PV systems 40 , 41 . One of the challenges in the development of perovskite/silicon tandem solar cells (PSTSCs) is the requirement for transparent and conductive electrodes to allow for the transmittance of the near-infrared (NIR) part of the incident light through the semitransparent perovskite top subcell to the bottom Si subcell. Typically, transparent conducting oxides (TCOs) such as indium tin oxide (ITO) are employed as the electrode of the semitransparent perovskite cell. This is problematic as the electrode material needs to be deposited directly onto the perovskite, for example, via magnetron sputtering in the case of ITO, which can degrade the quality of the underlying perovskite layer. To address this issue, a buffer layer can be inserted to protect the perovskite, although this adds complexity to the fabrication process. As such, a wide range of semitransparent electrodes made from different materials have been explored, such as silver nanowires, for their suitability for PSTSCs 42 . Recently, Wang et al. 43 employed a thermally evaporated semitransparent electrode composed of a MoO 3 /Au/MoO 3 multilayer for the perovskite subcell. The champion 4 T perovskite/Si tandem device using this transparent conducting electrode exhibited a PCE of 27%, which was higher than that of the individual subcells.

The interconnection layer (ICL) within the 2 T tandem configuration plays an important role in optically and electrically connecting the top and bottom subcells and facilitates the balanced recombination of photogenerated carriers to ensure the flow of current throughout the entire tandem device. Under operation, the overall photocurrent of the 2 T tandem structure relies on matching the photocurrent of both subcells and is limited by the subcell with the lower current. In this regard, the ICL acts as a recombination site to facilitate the current flow and inhibit the formation of a p–n junction. Moreover, the quality of the ICL directly impacts the V oc , as the incorporation of a poorly performing ICL can lead to the accumulation of charge carriers at both ICL interfaces, introducing a reverse electric field that reduces the overall output voltage. Therefore, optimizing the properties of the ICL such as transmittance, thickness, resistivity, and refractive index is very important 44 . In 2015, Mailoa et al. 45 were the first to report 2 T PSTSCs employing a n ++ /p ++ Si tunnel junction between the Si bottom subcell and the perovskite top subcell, delivering a PCE of 13.7%. Since this study, the development of efficient ICLs has become the focus of many research groups leading to the creation of several effective ICLs materials, including ITO 46 . Recently, Mazzarella et al. 47 described an interlayer consisting of nanocrystalline silicon oxide between the perovskite and Si subcells to reduce the infrared reflection losses. After optimizing the thickness and refractive index of the interlayer by varying the oxygen content, 2 T PSTSCs with a certified PCE of 25.2% were obtained. Despite this promising PCE, the performance of this tandem device was limited by the bottom Si cell, which had a slightly lower current density (J sc ) (18.8 mA cm −2 ) than the top perovskite cell (19.9 mA cm −2 ), as determined from the EQE spectrum. Therefore, it is reasonable to expect further enhancements in the PV performance of this class of tandem device by matching the J sc values for both the bottom and top subcells.

Enhancing the hole extraction process and minimizing nonradiative recombination at the HTL interface with the perovskite is also important for improving the performance of PSTSCs. Al-Ashouri et al. 29% efficiency by enhanced hole extraction. Science 370, 1300–1309 (2020)." href="/articles/s43246-022-00325-4#ref-CR48" id="ref-link-section-d3742168e1187">48 showed that a HTL composed of the SAM with methyl group substitution [4-(3,6-dimethyl-9H-carbazol-9-yl)butyl]phosphonic acid Me-4PACz in conjunction with a p–i–n perovskite subcell can significantly enhance the hole extraction and transporting efficiency. This strategy has led to the current world record certified PCE of 29.15% for a 2 T PSTSC (Fig.  4a, b ). In addition to the p–i–n configuration for the perovskite subcell, improving the charge-selective layers for the n–i–p architecture is also of great interest 49 . The strategy of incorporating 2D with a 3D perovskite to improve stability has attracted a lot of interest. Duong et al. 50 designed a 2D/3D mixed perovskite system by coating n -butylammonium bromide onto the surface of a 3D perovskite, which improved the charge-carrier lifetime and PV performance (PCE of 27.7%) and lifetime. It is worth noting that the surface passivation of the perovskite film has also been reported for the p–i–n PCSs device structure by Isikgor et al. 51 , who showed that incorporating phenformin hydrochloride (PhenHCl) into the perovskite solution can passivate the perovskite surface and suppress light-induced phase segregation (Fig.  4c ). The authors of this work were able to increase the V oc of the perovskite cells by 100 mV as compared to the control device and achieved a maximum PCE of 27.4% for their 2 T PSTSCs. Importantly, the fabricated devices showed no V oc losses after thermal aging at 85 °C for 3000 h in a nitrogen atmosphere. This stability is promising for the development of high efficiency and stable tandem cells, which is currently the key challenge for their commercialization. In a separate study, a thermally evaporated CsBr thin-layer was used between the perovskite and HTL, which led to the development of fully-textured PSTSCs with a PCE of 27.48%. The unencapsulated CsBr based device stored in the dark inside a N 2 -filled glove box showed excellent stability for over 10,000 h maintaining about 95% of its initial PCE as compared to only 74% for the control device without the CsBr (Fig.  4d, e ) 52 .

a Schematic illustration for the layered structure of a monolithic perovskite/silicon tandem solar cell. b Certified J–V curve measured at the Fraunhofer ISE, including the MPP value and the device parameters (red), in comparison to a tandem cell with PTAA (gray) as the HTL measured in-house. a , b adapted with permission from ref. 29% efficiency by enhanced hole extraction. Science 370, 1300–1309 (2020)." href="/articles/s43246-022-00325-4#ref-CR48" id="ref-link-section-d3742168e1250">48 , Copyright 2020 American Association for the Advancement of Science. c Illustration of the p–i–n device structure, grain boundary passivation (GBP) and top surface passivation (TSP) strategies, and the molecular structure of the PhenHCl passivation molecule used in the study. c adapted with permission from ref. 51 , Copyright 2021 Elsevier Inc. d , e Comparison of the original efficiency of the PSTSC device stored in a nitrogen atmosphere ( d ) without CsBr after 8760 h and (e) with CsBr after 10488 h. d , e adapted with permission from ref. 52 , Copyright 2021 Wiley-VCH. f Cross-sectional SEM images of a textured c-Si with an average pyramid size of 2 μm (top) and corresponding substrates covered by solution-processed perovskite crystals (bottom). f adapted with permission from ref. 53 , Copyright 2020 American Association for the Advancement of Science. g Device architecture of a perovskite/CZTSSe tandem solar cell. h J–V curves of various types of the sub- and tandem cells based on a perovskite and CZTSSe. g , h adapted with permission from ref. 59 , Copyright 2022 Wiley-VCH.

Depositing a perovskite layer onto fully-textured Si bottom cells provides a strategy to improve light trapping and reduce the cost of single-side textured Si wafers (i.e., the cost associated with polishing the front side of the Si wafers and the additional requirement for antireflection foils), which is the commonly used configuration with 2 T PSTSCs 53 . However, obtaining high-quality perovskite films with full coverage on a rough surface while avoiding electrical shunting is challenging. In order to achieve compact micrometer-thick perovskite films with full coverage on the Si pyramids, Hou et al. 53 proposed spin-coating a concentrated perovskite precursor solution followed by passivation of the perovskite surface using 1-butanethiol (Fig.  4f ). The corresponding monolithic 2 T PSTSCs achieved a certified PCE of 25.7%, and had excellent thermal and operational stability under MPP tracking over 400 h of testing. In addition to this strategy, deposition of perovskite films on textured Si with full coverage by blade coating has been demonstrated 54 , which paves the way for high-throughput commercial-scale production of PSTSCs. Very recently, an exceptional PCE of 29.2% for a 4 T PSTSCs was reported in the popular literature 55 . However, while the exact device structure and the full experimental details were not described, it demonstrates that it may be possible to reach the 30% PCE milestone, which would catalyze the potential commercialization of tandem devices.

Other emerging perovskite-based tandem solar cells

In addition to the above-mentioned perovskite-based tandem solar cells, there have been other approaches to perovskite-tandem cells employing emerging photovoltaic materials. These include combinations such as perovskite/CZTSSe, perovskite/colloidal quantum dots (QDs), and perovskite/CdTe. The application of Cu 2 ZnSn(S,Se) 4 (CZTSSe) as the bottom subcell in perovskite-based tandem solar cells holds specific promise relative to the use of CIGS due to its low-cost, high abundancy of the chemical components, excellent absorption coefficient across the visible wavelengths, solution processibility, and the fact that it has a tunable bandgap 56 . Despite the lower reported efficiencies of earlier perovskite/CZTSSe tandem devices (<17%) 57 , 58 , significant progress in their PV performances continues to be made. For example, Wang et al. 59 showed that a 1.66 eV semitransparent perovskite top subcell and a 1.1 eV CZTSSe bottom subcell can form a 4 T tandem device with a PCE of 22.27% (Fig.  4g ). It should be noted that in these devices the light absorption of the bottom cell is limited as it is filtered by the semitransparent perovskite film. As shown in Fig.  4h , the J sc of the CZTSSe bottom subcell decreased from 35.24 mA cm −2 (single-junction) to 15.43 mA cm −2 , while the perovskite top subcell had a much higher J sc (18.82 mA cm −2 ).

The absorption properties of colloidal quantum dots (CQDs) can be manipulated through control of their size, and this alongside their low-cost and solution processibility makes them excellent candidates for bottom subcells of tandem applications. Although the efficiency of perovskite/CQDs tandem devices is far from the theoretically estimated value of 43% 60 , recent studies have already demonstrated their feasibility. Chen et al. 61 reported that integrating PbS CQDs with non-fullerene acceptors (NFA) to complement the CQD absorption and connecting this bottom hybrid subcell with a semitransparent perovskite top subcell could give 4 T tandem devices with a PCE of 24%. Recently, Tavakoli et al. 62 employed surface passivation to reduce the surface defects of PbS QDs and ZnO nanowires (used in the ETL) with CdCl 2 and SnO 2 , respectively. After optimizing the thickness and matching the current density of both subcells, 2 T and 4 T tandem perovskite/PbS QDs devices with PCEs of 17.1% and 21.1%, respectively were obtained. The 2 T tandem device showed excellent stability when tested at MPP under continuous light illumination in a nitrogen environment maintaining 94% of its initial PCE. Furthermore, no changes in the PCE were observed when exposing the device to a high RH of 65% over 72 days as compared to 37% efficiency loss observed with the single junction PSC device, which further highlights the important role of PbS QDs in protecting the perovskite from degradation under high humidity conditions.

Cadmium-telluride (CdTe) solar cells are currently among the most successful low-cost thin-film technology in the PV market with an installed capacity of over 25 GW 63 . The certified record PCE of a CdTe cell is 22.1% 4 . The bandgap of CdTe is ~1.5 eV, which makes them unsuitable to be used with the conventional wide-bandgap perovskites. However, the bandgap of CdTe can be reduced to 1.36 eV when doped with selenium 64 . Nevertheless, this bandgap is too wide to be used as the bottom subcell—the optimum being 1 eV. In the scenario where CdTe is used as the top subcell, perovskites with wider bandgaps (>2 eV) are needed. However, wide-bandgap perovskites usually suffer from low efficiencies and poor stability. For instance, Siegler et al. 65 reported the use of MAPbBr 3 (bandgap of 2.3 eV) to fabricate 4 T perovskite/CdTe tandem cells, but these were found to have a very poor PCE of 3.5%. This was attributed to the optical haze in the perovskite film causing a significant optical loss. Therefore, optimizing the bandgaps of both subcells is needed before this tandem device configuration is viable.

The above-mentioned results demonstrate the exciting rapid improvement in the efficiency and stability of perovskite-based tandem solar cells, which have now surpassed those of single-junction perovskite devices. Table  1 shows a summary of the best-performing perovskite-based tandem solar cells. It is anticipated that further efficiency and stability enhancement will compensate for the additional costs derived from constructing tandem structures. A recent cost estimation analysis of several tandem devices was conducted by Li et al. 66 using the levelized cost of electricity (LCOE). Based on their calculations and assumptions, an LCOE of 4.34 US cents kWh −1 for a single-junction planar PSCs was obtained, which was found to be 21% lower than that of a silicon solar cell. Their findings also revealed that the LCOE increases to 5.22 US cents kWh −1 with silicon/perovskite-tandem cells, which is still about 5% lower than that of a conventional silicon solar cell. Surprisingly, the LCOE was found to be considerably reduced to 4.22 US cents kWh −1 with perovskite/perovskite-tandem devices. The lower LCOE was attributed to the high efficiency and reduced cost of perovskite devices. It was also predicted that the LCOE values could be further decreased by improving the PCE and stability of the devices presented in the study. These studies clearly demonstrate the appeal of perovskite-tandem devices for commercialization.

Challenges for perovskite-based subcells

Stability of perovskite solar cells.

The long-term stability of PSCs represents a key obstacle for their commercial deployment. Perovskite materials typically used in solar cells have been shown to be unstable when exposed to oxygen, water, heat, and light. In addition to these external factors, some studies have also shown that the inherent properties of perovskite materials, such as ion migration and low defect formation energy, play a significant role in the rapid decomposition of perovskite films. With 25 years of an outdoor operational lifetime required by the marketplace, PSCs currently lag behind this target. To overcome the stability issues, many strategies have been developed, such as compositional engineering, interface engineering, and surface/bulk defect passivation 67 , 68 , 69 , 70 , 71 , 72 . For instance, by fully or partially substituting the highly volatile A site MA cation with formamidinium (FA) and/or cesium (Cs) in the perovskite, the device stability was found to be enhanced 73 . Another strategy involves the incorporation of various materials, such as 1D and 2D materials 1 , 74 , polymers 75 , and fullerene derivatives 76 into the perovskite film to passivate its defects and hence improve the overall device stability. The replacement of doped HTMs with undoped ones is also a promising route to prevent ion migration and interaction between the dopants and perovskite, but their PV performance remains questionable 77 . The migration of ionic species in halide perovskite films can also be largely suppressed by the formation of 2D/3D multidimensional perovskite structures, leading to greater device stability 78 . It is worth mentioning that in common with all PV technologies, encapsulating PSCs can prolong their lifetime by protecting them against external environmental factors 79 .

Toxicity of lead

To date, state-of-the-art PSCs are constructed of lead-based halide perovskites due to their excellent optical and optoelectronic properties . However, the potential hazard of lead leaking into the environment and potential human health risks means that encapsulation and end of life recycling needs to be considered. The European Commission (EU) has restricted the maximum amount of lead that can be used in electrical and electronic equipment to 0.1 wt%, which is significantly lower than the amount of lead currently needed to fabricate PSCs (5–10 wt%) 80 . As such, tremendous efforts have been made to design low-toxic/lead-free metal halide perovskites for PV applications. The complete replacement of Pb using other metal halides such as tin (Sn), germanium (Ge), bismuth (Bi), and antimony (Sb) have been attempted. However, PSCs based on these alternative metal-based perovskites show inferior performance to their Pb-based counterparts. For instance, while the PCE of Pb-based PSCs exceeded 25%, the highest PCE of Sn-based PSCs has only recently achieved slightly over 14% 81 , 82 . These poor PCEs are currently below that required for single-junction and tandem solar cell applications. Another practical approach to reducing the content of Pb while maintaining the high performance of PSCs is to partially substitute a portion of the Pb with less-toxic metal cations. Indeed, Pb–Sn mixed perovskite absorbers with their close-to-ideal bandgap of ~1.2–1.3 eV enabled the fabrication of efficient devices with PCEs exceeding 21% 83 . This narrow bandgap, coupled with the high PCEs, makes Pb–Sn mixed PSCs desirable candidates as the bottom subcell in all-perovskite-tandem solar cells. For instance, Xiao et al. 36 demonstrated excellent performance of a single-junction Pb–Sn mixed PSCs with a certified PCE of 20.7%. By employing a wide-bandgap perovskite of 1.77 eV (Cs 0.2 FA 0.8 PbI 1.8 Br 1.2 ) and a narrow-bandgap perovskite of 1.22 eV (FA 0.7 MA 0.3 Pb 0.5 Sn 0.5 I 3 ), the group was able to fabricate monolithic all-perovskite-tandem cells with a certified PCE of 24.2% and an aperture area of 1.041 cm 2 . It is anticipated that with further tuning the optoelectronic properties of Pb–Sn mixed perovskites and developing a novel method to passivate the surface of Pb–Sn mixed perovskite films, exciting opportunity lies ahead in realizing more efficient all-perovskite-tandem devices. However, the trade-off is that at this stage Sn-containing perovskites are less stable.

Building-integrated photovoltaic

Electricity-generating solar panels are generally mounted on the building rooftops. However, PV systems can be building-integrated (BIPV) and are increasingly employed in new ways during the construction of buildings. BIPV includes inclusion of panels on or as parts of the building envelope such as the windows, skylights, exterior walls, or facades. The requirements for BIPV are therefore different for rooftop-mounted systems with a greater emphasis on the optical properties, such as color and transparency, weight and form factor. In addition to color tunability, PSCs can be fabricated on transparent, conductive and flexible substrates, making them attractive for BIPV applications. In the context of perovskite-based BIPV, three major categories have been developed and explored. The first category is semitransparent PSCs, which have been explored for use in building windows and glass roofs. The second category of perovskite-based BIPVs are colorful PSCs, which can be applied in building fences, walls and car park roofs. The third category is smart PV windows (SPWs), which are dual-functional BIPVs created by combining the solar harvesting function with electrochromic/thermochromic functionalities. SPWs are interesting in that they can harvest sunlight to produce electricity while blocking sunlight entering the building on a hot summer day (Fig.  5a ). However, before BIPV becomes widespread there are manufacturing and performance related challenges to be addressed 84 , 85 , 86 , 87 . In the following sub-sections, we outline and discuss some of the recent advances that have been made in these three categories of BIPV that use PSCs.

a Schematic illustration of the key components of a self-powered smart photovoltaic window in hot and cold conditions. b Colors of MAPb(I 1−x Br x ) 3 perovskite inks at temperatures of 25 °C, 60 °C, 90 °C and 120 °C. b Adapted with permission from ref. 91 Copyright 2017 American Chemical Society. c Optical transmittance of CH 3 NH 3 PbI 3 • x CH 3 NH 2 complex based PSCs in the bleached and colored states. d J–V curves of switchable PV devices in the dark and under illumination. c , d Adapted with permission from ref. 92 , Copyright 2017 Springer Nature. e Schematic illustration of the phase transitions of CsPbI 3−x Br x perovskite caused by thermal heating and exposure to moisture. e Adapted with permission from ref. 93 Copyright 2018 Springer Nature. f A schematic illustration of the first PVCC device using a PSC as an external electrical supply. f Adapted with permission from ref. 95 , Copyright 2015 Royal Society of Chemistry. g Repeatability of PVCC fabricated from a transparent PSC and ion-gel based electrochromic components. g Adapted with permission from ref. 97 , Copyright 2021 Springer Nature.

Smart PV windows

A smart window is a glass whose optical transmission is altered when an external stimuli (e.g., heat, voltage, or light) is applied. In general, smart windows are constructed using switchable films such as thermochromic, photochromic and electrochromic layers. Integration of smart windows with PV devices has the promise to reduce cooling/heating costs and ventilation loads, improve privacy, and harvest excess solar energy as electricity, thus maximizing the overall energy efficiency of the building. In this regard, emerging PV systems, including organic solar cells, dye-sensitized solar cells (DSSCs), and PSCs have been considered as candidates for SPWs due to the high degree of tunability of their properties. Both organic molecules and dye sensitizers can exhibit photochromic properties, allowing them to be integrated into photochromic solar windows, but they typically exhibit poor PCEs 88 , 89 , 90 . Interestingly, the temperature required to crystallize perovskite light absorbers opens new avenues for research in SPWs as the temperature can adjust the color of perovskite films. Overall, considerable progress has been made in the development of integrated SPW systems involving PV device and an electrochromic layer using each type of emerging PV cell, including organic solar cells, DSSCs and PSCs. Since the output voltage plays a vital role in operating the electrochromic windows, PSCs with their high voltage are particularly attractive. Perovskite-based SPWs can be categorized into dual-function thermochromic solar cells and photovoltachromic cells (PVCCs) depending on their functionality. It is still early days for these two categories of SPWs and while there are many challenges remaining to be addressed the technology is ripe for further research and development.

Interestingly, a group of researchers led by Bakr demonstrated the temperature-dependent thermochromic properties of hybrid halide perovskites 91 . The authors prepared perovskite inks based on MAPb(I 1−x Br x ) 3 with varying x. At room temperature, the ink was yellow in color, but it changed to orange upon heating to 60 °C, bright red at 90 °C, and finally black at 120 °C (Fig.  5b ). They found that this temperature-induced thermochromic variation was reversible in the presence of solvent. It should be highlighted that the halogen components in the perovskite plays the key role in this unusual crystallization behavior. Wheeler et al. 92 reported the practical use of the thermochromic properties of perovskites in switchable photovoltaic windows. In this work, the thermochromic layer was made of a halide perovskite with differing amounts of methylamine (CH 3 NH 3 PbI 3 • x CH 3 NH 2 ). The working principle of this SPW device can be described as follows. Upon illumination (solar photothermal heating), the thermochromic film switches from a transparent state (68% visible transmittance) to an opaque colored state (<3% visible transmittance) due to the dissociation of CH 3 NH 2 from the perovskite-CH 3 NH 2 complex (Fig.  5c ). After cooling, the CH 3 NH 2 complex is re-formed in the absorber layer, making the device transparent (bleached state) to visible light. This switchable PSC device exhibited a PCE of 11.3% in the colored state, while the control cell, which did not show switching behavior, had an average efficiency of 16.3% (Fig.  5d ). Despite the promising PCE, the PV device performance decreased over time due to the loss of CH 3 NH 2 and disruption of the film morphology during the cycling process. In a separate study, the structural phase transitions of an all-inorganic perovskite, CsPbI 3−x Br x , were used to obtain a thermochromic smart solar window with improved stability 93 . Thermal annealing at a temperature of 105 °C and exposure to moisture were used to control the reversible transition between the transparent (81.7% transmittance) and colored (35.4% transmittance) phases (Fig.  5e ). Importantly, no significant color fading and efficiency reduction were observed for this all-inorganic perovskite-based SPW during the phase transition cycles, showing the potential of perovskite-based smart solar windows. However, the efficiencies of this switchable device during the phase transition cycles were only around 4–6%, suggesting further improvements in the PV performance are required for this class of SPWs. In addition to the low efficiency, the 105 °C heating requirement to generate the colored phase means that the strategy was not practical, as the temperature is much higher than the temperature expected as a result of solar illumination. Indeed, it is important to design photoactive perovskites that can switch colors at low temperatures. Recently, highly robust and stable SPWs with rapid reversibility were constructed using a 2D perovskite ((C 6 H 4 (CH 2 NH 3 ) 2 )(CH 3 NH 3 )[Pb 2 I 7 ]), but their PV efficiencies were less than 1% 94 . Currently, the key challenges in thermochromic perovskite-based smart solar windows include low device efficiency, poor reversibility and stability. Moreover, studying the thermochromic properties of lead-free perovskites for SPWs would be an interesting research direction.

The final category of perovskite-based SPWs is PVCC, which combines a PV cell as the power supply and an electrochromic coated glass as the smart window. The first work integrating PSCs and electrochromic layers was reported by Cannavale et al. 95 who used two separate glass sheets for the PV device and electrochromic layer. A schematic of this device architecture is illustrated in Fig.  5f . The PV device was made of a semitransparent perovskite layer coated on the top TCO substrate, while the electrochromic layer was made of WO 3 deposited on the bottom TCO glass. This semitransparent system exhibited an AVT of 15.9% when bleached, which changed to 5.5% upon darkening. However, the PCE of the device was only 5.5% when colored, suggesting that considerable enhancement in the PV performance should be made for this class of SPWs. Moreover, the transparency of the device in the neutral state was still low at 15.9%, which is undesirable. One possible strategy to overcome this issue for this class of SPWs would be to integrate the PV cells onto the frame of the electrochromic windows.

Current SPWs have at least one of the following limitations: low PV efficiency, poor operational stability, and/or long response time. Efforts have been made to address some of these issues, but usually at a cost to the others. Xia et al. 96 aimed to overcome these challenges by coupling multiresponsive liquid crystal/polymer composite (LCPC) films and semitransparent PSCs. The strategy involved using the PSCs as a power source, with the LCPC films used to adjust the transparency of the windows. The semitransparent PSCs had an AVT higher than 10%, with the PV device having a PCE of >16%. Thus the SPW exhibited excellent power output, energy saving, and privacy protection. Recently, Liu et al. 97 developed a PVCC integrating a transparent PSC with ion-gel based electrochromic components. The device was constructed in a vertical tandem architecture without an intermediate electrode. The authors were able to adjust the halide-exchange period precisely and achieved a high transmittance of up to 76% for the PVCC module (Fig.  5g ). Further impressive parameters such as a color-rendering index of up to 96, a wide contrast ratio of >30%, and a self-adaptable transmittance adjustment were also obtained for their PVCC. Due to the simple architecture and scalable manufacturing, this particular PVCC device architecture shows great promise in the development of future energy-saving smart technologies. In addition to combining PV devices with electrochromic films, there have been efforts on integrating PSCs with both energy storage systems and electrochromic layers 98 , 99 . These types of integrated systems are expected to provide novel green technologies that can not only produce and store power, but also automatically control their optical transparencies. These initial results show significant technological promise, and are a fruitful area for further research and development.

Semitransparent PSCs

Among perovskite-based BIPVs, semitransparent PSCs are the most widely studied because of the tunability of the perovskite film transparency. Efficient semitransparent solar cells should have high PV performance at the highest possible optical transmittance. Important optical factors include color-rendering index, average visible transmittance (AVT), and average near-infrared (NIR) transmittance. It should be noted that the theoretical Shockley–Queisser (SQ) limit for a single-junction wavelength-selective transparent solar cell with an AVT of 100% is around 20.6% 100 , although this has yet to be realized. Promisingly, the state-of-the-art semitransparent devices with organic layers have achieved PCEs of around 13% with AVT values of ⁓20% 101 , 102 , 103 , 104 . Semitransparent DSSCs tend to exhibit lower efficiencies as compared to organic solar cells due to the device architecture 105 . On the other hand, researchers have been making rapid developments in the area of semitransparent PSCs with improved performance and design 106 . Recently, a PCE of over 13% with an AVT of 27% was achieved using plasmonic gold nanorod integrated perovskite-based PSCs 13% efficiency and 27% transparency using plasmonic Au nanorods. ACS Appl. Mater. Interfaces 14, 11339–11349 (2022)." href="/articles/s43246-022-00325-4#ref-CR107" id="ref-link-section-d3742168e2776">107 , suggesting a bright future for transparent PV devices using perovskite light absorbers. A typical PSC (high-efficiency device) has an average thickness of 500-600 nm, which is too thick for semitransparent devices. In 2014, two independent research groups reduced the thickness of the perovskite layers to obtain semitransparent films for solar cells 108 , 109 . Devices fabricated by Bolink et al. 108 with a 180 nm thick perovskite film delivered a PCE of 7.31% and an AVT of 22%, whereas a semitransparent PSC with a 135 nm perovskite film prepared by Qi et al. 109 exhibited a PCE of 9.9%. However, no AVT value was reported for the latter semitransparent cell. An ideal semitransparent device should exhibit a high PCE while also maintaining a high AVT (25% is the current benchmark) 84 . Therefore, it is critical to investigate both the PV efficiency and AVT to determine the overall performance of semitransparent solar cells. Since these two pioneering studies, researchers have further improved both the efficiency and AVT of the devices using various strategies. For instance, Jen’s group used transparent CuSCN as a HTM in an inverted (p–i–n) device with different perovskite film thicknesses ranging from 60 nm to 300 nm (Fig.  6a ) 110 . They found that a device with a 180 nm thick perovskite film displayed a PCE of over 10% and an AVT of 25%.

a Photographs of CH 3 NH 3 PbI 3 films with different thicknesses on CuSCN treated ITO-coated glass substrates. a Adapted with permission from ref. 110 , Copyright 2015 Wiley-VCH. b A layered structure of the dewetting strategy applied to a PSC. c Cross-sectional SEM image of a neutral colored semitransparent device. b , c Adapted with permission from ref. 111 , Copyright 2014 American Chemical Society. d SEM images of perovskite film formations within (i) TiO 2 and (ii) SiO 2 honeycomb scaffolds. d Adapted with permission from ref. 113 , Copyright 2015 Royal Society of Chemistry. e (i) Tilted-view and (ii) top-view SEM images of MEIS-ITO. f Photographs of planar (reference) and MEIS based semitransparent PSCs. e , f adapted with permission from ref. 114 , Copyright 2021 Wiley-VCH.

Meanwhile, a novel strategy (dewetting) was introduced by Eperon et al. 111 to fabricate neutral colored semitransparent PSCs (Fig.  6b ). The key attraction of this strategy was to create microstructured arrays of perovskite “islands” to enable unattenuated transmission of light between the islands (Fig.  6c ). The fabricated semitransparent device showed a good AVT of 30%, but its efficiency was only 3.5% due to the lower geometric fill factor of the active perovskite sections of the film. These authors further improved both PCE and AVT of color neutral semitransparent devices to 5.2% and 28%, respectively, using FAPbI 3 112 . Despite these improvements, it can be observed from Fig.  6c that the direct contact of the ETL and HTL in the perovskite-free region leads to poor device performance. Therefore, depositing an extra layer as a shunt-blocking layer on the uncovered surface could be used to improve the performance.

Improving/modifying the microstructure of the perovskite film is another method for obtaining semitransparent PSCs. Snaith and his colleagues used a highly ordered metal oxide honeycomb structure to control the size and structure of the perovskite (Fig.  6d ) 113 . The honeycomb structure allowed them to control the growth of the perovskite crystal. In this device design, the honeycomb region was transparent, which allowed them to fabricate semitransparent PSCs with an efficiency of 9.5% and an AVT of 37%. Fan’s group recently reported a PCE of 10.5% with an AVT of 32.5% for semitransparent PSCs using a moth-eye-inspired structure (MEIS) (Fig.  6e ) 114 . The incorporation of MEIS into the devices resulted in light manipulation, which improved the performance and visual appearance of the devices. Figure  6f compares photographs of the planar (control) and MEIS based PSCs. These studies clearly show that controlling the structure of perovskite films is a promising approach for the development of efficient semitransparent PSCs. However, there is still room for the development of techniques that can be used to accurately control the growth of the perovskite film, which can then be used for the construction of semitransparent PV cells.

In addition to optimization of the properties of the perovskite films, the choice and structure of the metal electrodes acting as the charge collectors, such as Au and Ag, are of great importance for semitransparent devices. A dielectric–metal–dielectric (DMD) electrode is one strategy for ultrathin metal-electrode based semitransparent PSCs 115 due to their excellent electrical conductivity and suitable energy band alignment. Carbon-based electrode materials such as carbon nanotubes (CNTs) and graphene are also promising candidates for PSCs 116 . You et al. 117 have used stacked multilayer graphene as the top electrode of semitransparent PSCs. With a transmittance of around 26%, the semitransparent device exhibited a PCE of 6.6% when illuminated from the graphene side. This work was one of the first examples that showed graphene electrodes are candidates for use in semitransparent PSCs. Recently, a collaborative research team led by Shi and Grätzel introduced an innovative strategy to construct semitransparent PSCs using carbon materials 118 , 119 . The key innovation was assembling a semi-cell (thin carbon layer coated perovskite film) with a charge collector (carbon electrode). Using this approach, a steady-state PCE of over 20% was achieved for carbon-based semitransparent PSCs, while maintaining excellent operational stability under 1 sun illumination at 25 °C and 60 °C 119 . The semitransparent PSC fabricated using this strategy was also used to construct a tandem solar cell with a CuInSe 2 based bottom subcell, delivering an efficiency of 27.1% 120 . Devices fabricated using carbon electrodes have the potential benefit of low production costs, but their hydrophobic nature could also provide enhancement of the stability of PSCs through reducing the ingress of moisture. Future work should aim to improve the hole selectivity and/or enhance p-type conductivity of the carbon electrodes. The same group has recently reported the use of CNT based electrodes as an alternative to the top metal electrode to fabricate semitransparent PSCs. When multi-walled CNTs (MWCNTs) were used, they were able to achieve a PCE of more than 22% for the semitransparent PSC. Other carbonaceous materials such as MXene (Ti 3 C 2 T x ) are also expected to be promising electrode candidates as they show high conductivity and excellent transparency. Until now, progress in semitransparent PSCs has mainly focused on devices with lead-based perovskites, leaving the development of semitransparent lead-free PSCs as a fruitful area to explore.

Colorful PSCs

Colorful PV devices including PSCs have drawn considerable attention for various applications where esthetics are important. The color of PSCs and their esthetic properties can be tuned by controlling the light absorption properties or using external layers. A wide range of colors can be achieved in PSCs by adjusting the elemental components of the perovskites to change their bandgap. A great example of tunable device colors is demonstrated by changing the content of iodine and bromine in the perovskite, which is depicted in Fig.  7a that shows the appearance in reflected light 121 . Noh et al. 122 studied the chemical tunability of inorganic-organic hybrid perovskites with the basic composition of MAPb(I 1–x Br x ) 3 and showed that the onset of the absorption band of the MAPb(I 1–x Br x ) 3 perovskite could be tuned from 786 nm (1.58 eV) to 544 nm (2.28 eV) (Fig.  7b ). The authors were able to observe the direct changes in the perovskite bandgap as a function of bromide composition (x) – the higher the bromide content the higher the gap (Fig.  7c ). The devices fabricated with the perovskite containing a small amount of bromide (x = 0.2) were found to have an average PCE of over 10% with the best-performing device exhibiting a PCE of 12.3%. It should be noted that a PCE of 12.3% was a remarkable PV performance for PSCs in 2013. Interestingly, the cells with higher Br content exhibited greatly improved stability under humid conditions. In 2015, Snaith and colleagues fabricated PSCs with tunable structural colors across the visible spectrum (from red to blue) using a porous photonic crystal scaffold within the photoactive layer 123 . Inspired by this pioneering work, Huang’s group designed vividly colorful PSCs using a doctor blade coating technique 124 . In this work, the photonic structures on the perovskite film form instantly by Rayleigh-Bénard convection and the “coffee-ring effect”, resulting in a tunable domain pattern and concentric rings in each domain with near equal ring spacing (Fig.  7d ). These structures were responsible for the appearance of vivid colors. However, this type of architecture leads to an increased number of grain boundaries within the film, which can be clearly observed, and these lead to increased charge recombination and reduced performance. Therefore, reducing the number of grain boundaries while maintaining the photonic structure is important for obtaining high PV efficiency using this strategy.

a A photo of the fabricated PSCs, with Spiro-MeOTAD as the HTM and gold as the current collector, showing the appearance in reflected light. a adapted with permission from ref. 121 , Copyright 2016 Royal Society of Chemistry. b Changes in the UV-vis absorption spectra and ( c ) the corresponding bandgap of perovskites (MAPb(I 1–x Br x ) 3 ) as a function of Br composition (x). b , c Adapted with permission from ref. 122 , Copyright 2013 American Chemical Society. d The precursor solution concentration dependent perovskite film morphologies for vividly colorful PSCs. d adapted with permission from ref. 124 , Copyright 2015 Royal Society of Chemistry. e Different pigment materials spin-coated on semitransparent perovskites. e adapted with permission from ref. 125 , Copyright 2016 Wiley-VCH. f A photo of PSCs with different thicknesses (40–160 nm) of PEDOT:PSS. f adapted with permission from ref. 126 Copyright 2016 American Chemical Society.

In addition to the compositional engineering of perovskites, simple pigment materials with different colors can be coated on fabricated devices to obtain colorful PSCs. Guo et al. 125 have created semitransparent PSCs with a PCE of 5.36% and an AVT of 34% using polyvinylpyrrolidone (PVP) as a dopant material in the perovskite. Then the authors spin-coated different pigments (yellow, red and green) on top of fully fabricated devices to obtain colorful PSCs that were also semitransparent (Fig.  7e ). However, while devices with any color of choice can be formed, this class of solar cells suffer from low cell efficiencies due to the parasitic absorption of the pigment filters. Colorful PSCs with PV efficiencies of up to 16% were successfully fabricated by Zhou’s group using the transparent conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), as both the top electrode and as a spectrally selective antireflection coating 126 . By adjusting the thickness of the PEDOT:PSS layer, they were able to effectively tune the reflectance of the devices (Fig.  7f ) and thus the perceived color. These initial studies provide the foundations for approaches to methods and modifications that can be made to produce PCEs that are colorful.

Space applications

PSCs are promising candidates for space applications due to their distinctive features such as their superior gamma-ray radiation resistance and high power-to-weight (also known as specific power) 127 , 128 , 129 . In addition, the instability issues of PSCs that arise from the exposure to oxygen and moisture in the atmosphere on earth do not exist in the space environment, which further enhances the potential of PSCs for space applications. A reasonable basis on which to evaluate the performance of solar cells for space applications is to consider the AIAA-S111 standard for the qualification of space solar cells. A solar cell system must satisfy the requirements associated with the performance and stability before being considered for space applications 130 . For instance, solar cells need to withstand 1 MeV electrons with a fluence of 1 × 10 16 electrons per square centimeter and 3 MeV protons with a fluence of 1 × 10 13 protons per square centimeter. In addition, solar cells should be characterized over a temperature range from −150 °C to 150 °C. Currently, the highest PCE of 47.1% was achieved using six-junction inverted metamorphic solar cells under 143 suns 12 . Although this PCE is higher than the state-of-the-art single-junction PSCs, two-junction perovskite-based tandem devices, such as perovskite-Si, have already approached ~30% and are more cost-effective. However, the feasibility of using perovskite-based tandem devices for space applications has not been practically determined yet. Investigations are needed to assess the impact of exposing PSCs to the vacuum of space, different temperatures, and UV radiation. It is worth mentioning that according to the IEC-61345 industrial standard, a solar cell system needs to preserve over 95% of its initial PCE after 15 kWh m −2 of UV exposure 131 . For space applications, the UV level is even more important, considering that it is much higher than under AM0 conditions. Therefore, testing PSCs under high UV irradiance is important. Although UV-light stable PSCs (CsPbBr 3 ) for 120 h have been reported 131 , extending the time measurement window and varying the perovskite compositions to maximize the device efficiency is critical for future space applications.

Given the difficulty and complexity of undertaking PSCs performance testing under real space conditions, simulated space environments are generally used. The capability of the solar cells to survive different space conditions such as high-energy particle irradiation (e.g., protons, electrons, and gamma-rays), high vacuum, and elevated temperatures is of great importance. There have been some promising test results on single-junction PSCs 132 , 133 . For example, Lang et al. 134 were the first to study the operational stabilities of two types of perovskite-based tandem solar cells under the harsh radiation conditions of 68 MeV proton irradiation at a dose of 2 × 10 12  p + /cm 2 (see Fig.  8a for the device structures). They found that monolithic perovskite/Si solar cells became severely degraded, maintaining only 1% of their initial PCE, which compared poorly to perovskite/CIGS tandem solar cells that retained 85% of the initial PCE under space solar illumination conditions (AM0). The poor device stability of monolithic perovskite/Si solar cells was ascribed to the radiation-induced formation of recombination centers in the Si. It was also found that the primary reason for the PCE loss in perovskite/CIGS tandem solar cells was due to increased recombination in the CIGS subcell. Following this pioneering work, the same group recently reported the hardness of all-perovskite-tandem devices when exposed to high-energy proton irradiation (68 MeV at an accumulated dose of 1 × 10 13  p + /cm 2 ) 135 . Remarkably, over 94% of the initial PCE was maintained, clearly indicating that perovskite materials are resilient to high irradiation exposure and thus suitable candidates for the space industry. It is worth noting that an accumulated dose of 1 × 10 13  p + /cm 2 is equivalent to the accumulated dose after >100 years in near-earth and >10 years in geostationary orbit.

a Device architecture for two different types of perovskite-based tandem solar cells. These two devices were tested against harsh radiation conditions (68 MeV high proton irradiation). a Adapted with permission from ref. 134 , Copyright 2020 Elsevier Inc. b Solar cell mounting structure, and schematics of the flight altitude. b Adapted with permission from ref. 136 , Copyright 2018 Elsevier. c Schematic Overview of the MAPHEUS-8 Sounding Rocket Flight. c Adapted with permission from ref. 137 , Copyright 2020 Elsevier Inc. d Representative schematic showing the relative position of the Sun, the high-altitude balloon, and the Earth (courtesy of National Aeronautics and Space Administration). e The photograph of launch site. f The photograph of the high-altitude balloon with a pod in near space. d – f adapted with permission from ref. 138 , Copyright 2019 Springer Nature.

Although the development of PSCs for space applications is still in its infancy, there have also been a few studies carried out under real space conditions. To the best of our knowledge, Cardinaletti et al. 136 were the first to track the changes in PSC performance attached to stratospheric balloons that reached an altitude of 32 km (Fig.  8b ). The output of the MAPbI 3 based devices in a near-space environment were recorded over the 3 h of stratospheric flight. Although this work was a great demonstration, longer testing times are required in a space environment. In subsequent work, Reb et al. 137 fabricated both mesoporous (TiO 2 ) and SnO 2 based standard PSCs that were mounted on a suborbital rocket. The device performance was evaluated after the rocket attained the apogee of 239 km under temperatures ranging from 30 °C to 60 °C for a 6 min period (Fig.  8c ). Despite this short tracking time, the devices showed satisfactory performance (power densities exceeded 14 mW cm −2 ) under strong solar irradiation. Furthermore, Tu et al. 138 used a high-altitude balloon to carry an FA 0.81 M A0.10 Cs 0.04 PbI 2.55 B r0.40 based large-area PSC (1.00 cm 2 ) to an altitude of 35 km for 2 h (Fig.  8d, f ). The TiO 2 -based PSC maintained 95% of its initial PCE during the test. The authors also found that using an ultraviolet (UV) filter could further improve the stability of the devices. These findings have laid the foundation for additional research to promote the applications of PSCs in space. However, these advances in exploring the feasibility of perovskite-based devices under real space environments have only been made using single-junction PSCs. Comparable efforts on the exploration of perovskite-based tandem solar cells for practical space applications have not yet been reported.

PV-integrated energy storage systems

Solar energy will continue to be a leading source of renewable energy. However, conventional solar cells are instantaneous photoelectric conversion devices and the electrical output has to be consumed immediately or stored 139 . To address the need of uninterrupted energy availability it is therefore important to develop integrated energy conversion-storage systems. In this regard, integrating solar cells as an energy conversion unit with energy storage units has become a promising solution for developing renewable and clean technologies. Supercapacitors (SCs), lithium-ion batteries (LIBs) and other rechargeable batteries are the most promising energy storage units owing to their high energy and power density and long lifetime. It should be noted that considerable attention has been given to integrated systems based on energy storage devices (batteries and supercapacitors) and a range of solar cells technologies, such as DSSCs and organic photovoltaic devices (OPVs) 140 , 141 , but the overall performance of these integrated systems are still unsatisfactory mainly due to the limited PCEs of the solar cells. This has led to recent advances being focused on employing PSCs in integrated systems. When integrating energy conversion and storage units, voltage matching is of great importance. In this context, PSCs with their high V oc values are expected to be a lead candidate for energy conversion/storage capability. Furthermore, their maximum power point can be close to the charge/reaction voltage plateau, which is vital for avoiding metal plating in battery technologies. In PV-integrated energy storage systems, the cost-benefit has been regarded as one of the key factors for the investment. For the analysis of cost-effectiveness, factors that should be accounted for include system architecture, size of the components (e.g., energy storage devices, PV modules, electric cables, inverters, etc.), operation and maintenance costs, and replacements. Importantly, the cost benefits of integrated systems must outweigh the costs of the technology to deliver their advantages. In this regard, the manufacturing cost and PCE of individual PV cells will play critical roles in determining the final cost benefits of PV-integrated energy technologies.

For a broad perspective of the field, Fig.  9 shows a schematic illustration of PV-integrated energy storage devices and PV-cell-driven catalysis reactions, highlighting the advantages of integrated systems. The average voltage outputs required to drive supercapacitors, water splitting, CO 2 reduction, and batteries are also provided. The following sub-sections outline and evaluate the recent progress on integrated systems based on PSCs and energy storage devices such as supercapacitors and batteries.

Integrated PV and energy storage devices or catalysis systems. The overpotential windows required to drive different solar energy conversion and storage, particularly supercapacitors, water splitting, CO 2 reduction, are provided. It should be noted that the voltage outputs required to drive these systems vary depending on the performance of the electrode materials and catalysts.

PSCs–supercapacitors

Of the different types of energy storage devices, supercapacitors exhibit unique advantages including ultralong cycling stabilities, rapid charging/discharging, and high power densities. Importantly, the voltage demands for supercapacitors are relatively low compared to other energy storage devices, making them attractive for integration with solar cells. Xu et al. 142 fabricated the first integrated system using a CH 3 NH 3 PbI 3 -based PSC connected with a supercapacitor assembled from a cellulose membrane/polypyrrole (PPy) nanofibers/MWCNTs combination. The integrated device displayed an energy storage efficiency of 10% and high output voltage of 1.45 V under AM 1.5 G illumination (Fig.  10a ). The overall efficiency was calculated considering the light density, the device active area and the charging time. Notably, this performance was much higher than that of other integrated systems constructed from other types of solar cells. However, active area mismatch between the PSCs and supercapacitors was the key limitation in this work, causing a relatively long charging time of 300 s. A device with a large active area is expected to shorten the charging time of the capacitor. Later, Liu’s group designed a self-powered wearable sensing device by integrating a flexible PSC, a flexible lithium-ion capacitor (LIC) module, and a graphene-based strain sensor (Fig.  10b ) 143 . For the flexible PSC module, the authors connected four individual PSCs in series to achieve a voltage output of 3.95 V for LIC charging. The flexible module was able to display an overall efficiency of 8.41% and an output voltage of 3 V at a discharge current density of 0.1 A g −1 . In a slightly different approach from the above configurations, a flexible all-solid-state wire-connected integrated system based on self-stacked solvated graphene films was also developed by Du et al. 144 , which achieved a gravimetric specific capacitance of 245 F g −1 and stability over 10,000 cycles. By avoiding the use of aqueous electrolytes, the solid electrolyte significantly improved the stability of the device, suggesting that this strategy has great potential to satisfy the technical requirements for integrated energy systems.

a J–V curves of an integrated system using a CH 3 NH 3 PbI 3 based PSC and a polypyrrole-based supercapacitor (supercapacitor was charged at 0.6 V). Inset: Connection of the integrated system. a Adapted with permission from ref. 142 , Copyright 2015 American Chemical Society. b Schematic diagram of a self-powered wearable electronic sensing device (PSC-LIC integrated system). b Adapted with permission from ref. 143 , Copyright 2019 Elsevier Ltd. c Schematic illustration of energy harvesting and storage ribbon consisting of a PSC and a symmetric supercapacitor with a shared copper electrode. c Adapted with permission from ref. 146 , Copyright 2016 Springer Nature. d Schematic diagram of the first PSC–LIB integrated system. e J–V curves of the connected PSCs before and after various cycles. f Discharge capacity of of the PSC-LIB integrated device. d – f adapted with permission from ref. 152 , Copyright 2015 Springer Nature. g Schematic diagram of the integrated PSC–Li-S battery. g adapted with permission from ref. 153 Copyright 2015 Wiley-VCH. h A layered structure of the PSTSC designed for an aqueous solar flow battery. h Adapted with permission from ref. 157 , Copyright 2020 Springer Nature.

Besides these wire-connected integrating strategies, the design of the shared electrode has drawn much interest due to its lower integration cost and better technological features. An integrated energy conversion and storage device was constructed using a PSC with a PEDOT-carbon-based shared electrode 145 . In this design, the carbon electrodes played dual roles in collecting holes from the perovskite layer and that could be used by the redox supercapacitors. The hybrid device showed an overall energy conversion and storage efficiency of 4.7% and 74%, respectively. A highly conductive metal electrode has been used by Li et al. 146 for an all-solid-state, energy harvesting and storage ribbon that integrates a PSC with a symmetric supercapacitor via a copper (Cu) ribbon, which acts as a shared electrode (Fig.  10c ). Upon illumination, the PSC achieved a PCE of >10% and the supercapacitor exhibited an energy density of 1.15 mWh cm −3 and a power density of 243 mW cm −3 . However, it is worth mentioning that the supercapacitors in integrated systems are typically constructed with carbon-based electrodes such as CNTs, graphene, and composites due to the need to achieve low-cost and highly conductive electrodes 147 . Carbon-based materials have also shown promise for use in stable PSCs owing to their hydrophobic characteristics and chemical stability 148 . Therefore, developing high-efficiency PSCs with carbon back-electrodes for integrated energy storage devices is a promising research direction. A novel integrated system based on a PSC with a MoO 3 /Au/MoO 3 transparent electrode and electrochromic supercapacitor has also been reported 98 . Despite the functionality of smart coloring, the PCEs of these perovskite photovoltachromic supercapacitor cells was less than 4% with the colored electrodes.

Devices consisting of a PSC and a supercapacitor are known as photo-supercapacitors and have attracted attention over the past few years due to their potential for being green portable power supply technologies. This class of integrated device does not need an external wire connection, but the challenge is the requirement for high operating and output voltages. Liu et al. 149 developed a system using four individual photo-supercapacitors assembled in series, and was able to obtain a stable output voltage of ∼ 3.8 V. This power pack was able to drive light-emitting diodes (LEDs) after being photo-charged, demonstrating the potential of this innovative technology. Given that supercapacitors require high voltages, their combination with PSCs to form high-efficiency PTSCs are expected to be promising candidates for solar rechargeable supercapacitors.

PSCs–batteries

Smart electronic devices, electric vehicles and smart grids have received a lot of attention and seem set to become an integral part of our day-to-day life. Currently, these advanced technologies depend on rechargeable batteries as the key energy storage device. Due to their high-energy density and excellent chemical stabilities, metal-ion batteries (e.g., lithium-ion batteries (LIBs)) are expected to be energy storage units for solar rechargeable batteries. Indeed, LIBs have been integrated with Si-based multi-junction solar cells in early reports and with DSSCs 150 , 151 . However, the output voltages of individual energy conversion units (solar cells) are often less than 0.8 V, which is insufficient to drive power storage devices. To provide sufficient output voltages, multiple PV units need to be connected in series, but this strategy is undesirable for the development of compact integrated systems. In this regard, PSCs with their high voltages are promising candidates for solar rechargeable batteries. An early study on integrating PSCs with LIBs was by Dai’s group 152 , where LIBs with a voltage range of 1.0-2.6 V were constructed, with four CH 3 NH 3 PbI 3 based solar cells connected in series to allow for direct photo-charging (Fig.  10d ). The connected PSCs delivered a V oc value (3.84 V) for photo-charging the LIBs (Fig.  10e ). Although promising cycling stability was demonstrated ( ∼ 2.05% decay per cycle) (Fig. 10f ), these PSCs-LIB integrated systems still require significant improvements in their operational stabilities and more testing conditions need to be applied, including thermal, long-term, repeated cycling and humidity tests.

Lithium–sulfur (Li-S) batteries are expected to be one of the leading technologies due to their high-energy density and weight, and with a cut-off charge voltage of 2.8 V, they are well suited for integration with a serially connected PSC pack for solar-driven batteries. Chen et al. 153 designed an integrated solar-driven rechargeable Li-S battery using three CH 3 NH 3 PbI 3 based PSCs connected in series. The connected PSC unit had a PCE of 12.4% and V oc of 2.79 V, which were sufficient to photo-charge the Li-S battery. As a result, an overall energy conversion efficiency of 5.1% was achieved for the integrated battery with a specific capacity of 750 mAh g −1 . Notably, in this integrated system, the sulfur-based electrode was connected with the joint carbon electrode of the three PSC units (Fig.  10g ). The use of carbon materials can be beneficial for integrated systems due to their low-cost, high stability and simple structure. In addition to the state-of-the-art Li-based batteries, emerging metal-based batteries such as Al-ion 154 , Na-ion 155 and aqueous zinc batteries 156 have been integrated with PSCs as demonstrators for solar rechargeable battery systems.

It should be emphasized that voltage matching between the solar cell and the battery device is critical for integrated systems. In this context, PTSCs show particular promise as they not only exhibit high PCEs, but also suitable photovoltage outputs due to the bandgap tunability of the perovskite top layer. In 2020, Li et al. 157 developed a tandem solar cell constructed using a (FAPbI 3 ) 0.83 (MAPbBr 3 ) 0.17 based PSC as the top subcell and Si as the bottom subcell (Fig.  10h ) with a suitable photovoltage for an aqueous solar flow battery. During the operation of the solar flow battery system, more than 90% of the PCE of the PSTSC was effectively utilized, suggesting that good photovoltage matching was achieved in this integrated device. Despite this advance, more effort should still be made to develop high-efficiency integrated systems using PTSCs. Noticeably, the majority of studies on integrated PSC-battery systems have employed simple perovskites such as MAPbI 3 . However, as discussed earlier there are many different perovskite materials developed that could be used in conjunction with batteries. Considering the technical requirements for commercialization of the integrated systems, a comprehensive range of lifetime tests including thermal, moisture and light stabilities under harsh testing conditions over extended durations should be conducted. Furthermore, although excellent progress has been made on integrated PSC-battery systems, the wire connection should be minimized in future work to reduce energy losses and device fabrication costs.

PV cell-driven catalysis

Solar-driven catalytic reactions are regarded as an emerging sustainable chemical production route. Advanced catalytic reactions such as water splitting and carbon dioxide (CO 2 ) reduction have the potential for green, sustainable and cost-effective routes for energy and feedstocks for industry. There are several categories of solar-driven catalysis, including photocatalytic, photoelectrochemical catalytic, photothermal catalytic and photosynthetic processes. For a broad perspective of the field, there are several reviews on these solar-driven catalysis processes available 158 , 159 , 160 , and in this review we will focus on the foundational processes of water splitting and carbon dioxide reduction. Of particular interest in this section is PSC cell-driven catalysis of water splitting and CO 2 reduction.

PSCs–driven water splitting

Hydrogen (H 2 ) energy (known also as H 2 fuel) needs no introduction as a zero-carbon fuel that can be used in internal combustion engines and fuel cells. H 2 energy can be stored as a gas under high pressure and can even be delivered through natural gas pipelines. H 2 production from water (H 2 O) has been considered as a promising green strategy. By applying an electric current to a suitable electrode, splitting of H 2 O into H 2 and oxygen (O 2 ) is achieved. Of particular importance in water electrolysis is the selection of an efficient electrocatalyst and the use of a high voltage PV-electrolyzer. Theoretically, a thermodynamic equilibrium potential of 1.23 V is required as minimum energy for water-splitting, but the practical operating potential can be varied between 1.5 and 2.0 V 161 . Due to the demand of such high operating voltages, several junctions and/or individual cells need to be connected in series for the electrolyzer 126 . Since PSCs typically display V oc values of more than 1.0 V, connecting only two individual PSC units is expected to meet the electrochemical potential required for water splitting. In 2014, Grätzel’s group reported the use of PSCs as an electrolyzer for water splitting for the first time (Fig.  11a ) 162 . They connected two individual CH 3 NH 3 PbI 3 based PSCs that each had a PCE of 17.3% and an V oc of 1.06 V. When the two cells were connected in series, the module deliver a V oc of 2.00 V, which was sufficient for efficient water splitting. Importantly, these authors designed a novel bifunctional catalyst (efficient for both H 2 and O 2 evolution) by directly growing a NiFe layered double hydroxide on a Ni foam. By integrating the connected PSCs and NiFe/Ni foam electrode, a solar-to-hydrogen (STH) efficiency of 12.3% was achieved (Fig.  11b ), which even at this early stage approached the theoretical limit (17.8%) of H 2 generation for this type of system, as defined by a 1.5 eV bandgap and the solar flux. Further improvements in the STH efficiency should be achievable by applying a shared electrode strategy to form an integrated system and/or by employing other efficient perovskite light absorbers. However, the stability issues associated with the PSCs at the time impacted the viability of this approach. The same authors made considerable improvements in both STH efficiency and stability of PSC-driven water-splitting system by employing (FAPbI 3 ) 1−x (MAPbBr 3 ) x based PSCs, while using a cobalt phosphide (CoP) catalyst for the H 2 evolution and NiFe/Ni foam for the O 2 evolution 163 . In that work, two individual PSC units were also connected in series to provide a potential of over 2.0 V, and the authors were able to achieve an STH efficiency of 12.7%. The system was found to retain more than 70% of its initial STH efficiency after 16 h of operation (Fig.  11c ), which was significantly better than their first integrated device reported in 2014. Despite this, further improvements in both STH performance and stability are required to make this approach economically viable since the power (electricity) consumed is currently more valuable than the H 2 produced.

a Schematic diagram of the water-splitting device powered by CH 3 NH 3 PbI 3 based PSCs (two cells connected in series). b J–V curves of the connected PSCs measured under dark and simulated illumination of 100 mW cm −2 , and the NiFe/Ni foam electrodes in a two-electrode configuration. a , b Adapted with permission from ref. 162 , Copyright 2014 American Association for the Advancement of Science. c The stability test of the integrated PSC-driven water splitting system with CoP and NiFe/Ni foam catalysts. c adapted with permission from ref. 163 , Copyright 2016 Wiley-VCH. d Energy diagram of the PSTSC-driven water-splitting system, and ( e ) the corresponding performance evaluation. d , e Adapted with permission from ref. 165 , Copyright 2019 Elsevier Inc.

One of the key strategies to construct an efficient and cost-effective PV cell-driven water-splitting system is to use high voltage solar cells. In this regard, single-junction all-inorganic PSCs and bromine (Br) based cells are good candidates. Building perovskite-based tandem PV devices (in particular 2 T) would also be appealing for use in integrated systems. Indeed, the first hybrid PTSC-driven water-splitting was reported by Bin et al. 164 who used graphene-based materials as the catalysts for both H 2 and O 2 evolution. Their tandem cell was able to deliver an V oc value of 1.86 V, which was sufficient for water splitting, but the obtained STH efficiency was <10%. Recently, Luo and colleagues constructed a 2 T PSTSC with an V oc of 1.76 V as a low-cost alternative to III–V multi-junction solar cells to drive water splitting 165 . The authors used TiC/Pt as the H 2 evolution catalyst and a NiFe/Ni foam for O 2 evolution, and powered the water splitting process using a mixed halide-based perovskite (Cs 0.19 FA 0.81 Pb(Br 0.13 I 0.87 ) 3 ) and monocrystalline-Si tandem solar cell (Fig.  11d ). The integrated system showed a peak STH efficiency of 18.7% (Fig.  11e ). This performance is the highest reported value to date among halide perovskite-based PV cell-driven and non-III–V-class light absorber-based water-splitting systems. Remarkably, after operating for over 2 h, the STH conversion efficiency dropped to only 18.02%, demonstrating promising stability of the system. Despite this significant milestone, further improvements are still expected in this class of integrated systems by maximizing the PV performance of the tandem solar cells and by designing efficient bifunctional electrocatalysts.

PSC-driven CO 2 reduction

The conversion of CO 2 into valuable chemical feedstocks and fuels has been the focus of catalysis research for many years. This approach is not only important for producing high-value chemicals, but has the potential to reduce the global greenhouse effect caused by CO 2 . The CO 2 reduction reaction (CO 2 RR) powered by renewable electricity generated from solar energy is an ideal approach to effectively utilize these abundant resources to produce high-value chemicals. However, CO 2 RR demands driving voltages that are considerably higher than supplied by single-junction solar cells. In particular, the power supply unit (PV cell) should provide an output voltage of >2.0 V 161 , which again requires that single-junction PV cells are connected in series. Schreier et al. 166 used three single-junction PSCs connected in series to achieve a V oc of 3.10 V for the reduction of CO 2 to carbon monoxide (CO). In this work, iridium oxide (IrO 2 ) was used as the oxygen-evolution catalyst, while gold (Au) was for the cathodic CO evolution. The integrated system delivered a solar-to-CO (STC) efficiency of over 6.5% with excellent stabilities (Fig.  12a ). When the system was operated without any external bias under constant illumination, no significant changes in the current density, Faradaic yield (CO%) and STC efficiency was observed over at least 18 h (Fig.  12b ), highlighting the stable operation of both the catalyst and the PV cells. A schematic illustration of the energy diagram for converting CO 2 into CO using this series of three PSCs is shown in Fig.  12c . Although this work is an excellent demonstration of integrating PSCs with CO 2 RR, further work is required to achieve efficiency improvements. Similarly, two series of three individual triple cation PSCs connected in parallel were used to convert CO 2 to hydrocarbons 167 . More recently, for the purpose of light-driven CO 2 conversion to methane (CH 4 ), four series-connected PSCs (delivering an V oc of 4.20 V) were electrically coupled to an electrochemical cell that had copper (Cu) and RuO 2 electrodes, providing a 5% solar-to-fuel conversion efficiency 168 .

a J–V curves of three series-connected PSCs, overlaid with the matched J–V characteristics of the CO 2 RR and oxygen-evolution electrodes. b Current density, CO% and STC efficiency of the integrated system during a stability test without any external bias under constant illumination for 18 h. c Schematic diagram of the CO 2 RR system powered by CH 3 NH 3 PbI 3 based PSCs (three cells connected in series). a – c adapted with permission from ref. 166 , Copyright 2015 Springer Nature. d J–V curve of monolithic all-perovskite triple-junction cells with the structure illustrated in the inset. d adapted with permission from ref. 169 , Copyright 2020 American Chemical Society.

Considering the need of high driving voltages for CO 2 RR, it is reasonable to expect that achieving high-efficiency CO 2 conversion at low-cost will utilize multi-junction (notably triple) PSCs. As far as we are aware, until now, there has been no effort in designing perovskite multi-junction (tandem) solar cells for CO 2 RR despite many groups having reported high-efficiency perovskite triple-junction solar cells. For example, Tan’s group fabricated monolithic all-perovskite triple-junction solar cells with an efficiency of 20.1% and an V oc value of 2.80 V (Fig.  12d ) 169 . This triple-junction device was constructed using three perovskites with different bandgaps (1.22 eV, 1.60 eV and 1.99 eV) and is a good example of how the characteristics of a single perovskite device might be tuned towards an application with very specific requirements, such as CO 2 RR.

Metal halide perovskites are exciting PV materials with fascinating properties including high absorption coefficients, bandgap tunability, excellent charge-carrier mobilities and solution processability. PV devices fabricated using these materials have demonstrated the steepest growth in terms of PCE of any PV technology in history. Considering the rapid progress in PV performance, PSCs have been considered to be ideal candidates for integrating with other systems to realize new innovative technologies. The next-generation applications of perovskite-based solar cells include tandem PV cells, space applications, PV-integrated energy storage systems, PV cell-driven catalysis and BIPVs. Herein, we have discussed the major advances towards integrating PSCs with these innovative technologies, highlighting the key advantages and challenges with some potential ways forward. We now summarize the perspectives and provide potential ways forward for the development of these exciting research areas.

(i) The integration of PSCs with other PV cells to form tandem solar cells has provided an opportunity to realize high-efficiency PV systems and leverage existing PV technologies. Although excellent progress has been made, there are several critical issues that need urgent attention. When integrating with wide-bandgap semiconductors based cells, perovskites with low-bandgap should be employed. However, the preparation of low-bandgap perovskites is not an easy task and generally requires the partial replacement of Pb 2+ with Sn 2+ . This process not only causes detrimental issues associated with the perovskite films such as large defect density, pinholes and non-uniform surfaces, Sn-based perovskites also exhibit lower carrier lifetime, diffusion length and poorer stability. Therefore, alternative strategies to design low-bandgap perovskites should be explored including the replacement of Sn with other stable metals and surface passivation techniques. In contrast, wide-bandgap perovskites are needed when low-bandgap materials such as CdTe are employed as the top subcell. However, wide-bandgap perovskites generally suffer from poor efficiencies, which should also be addressed to obtain high PV efficiencies.

One of the major requirements for high-efficiency tandem solar cells is highly conductive transparent electrodes, which play important roles not only in electrically conducting the charge carriers, but controlling the transmittance of the incident light through the top subcell to the bottom subcell. Carbon materials such as graphene, CNTs and MXene, with their high electrical conductivity and excellent optical transparencies are expected to be ideal electrode materials for semitransparent top subcells. The advantages of employing carbon electrodes in solar cells include low-cost, high efficiencies and enhanced device stabilities due to their hydrophobic nature. However, very limited efforts have been made on fabricating semitransparent PSCs using carbon electrodes for tandem solar cells. Carbon electrodes with improved hole selectivity and conductivity should be explored for HTM-free and metal-free PSCs.

It is well understood that the photocurrent matching of top and bottom subcells plays a critical role in achieving high PV performance for 2-T tandem solar cells. Therefore, particular attention should be paid to obtain strongly matched J sc values for the two subcells when designing tandem devices. Although there are many reports demonstrating the excellent operational stabilities of tandem solar cells, testing conditions and duration should be extended to harsher environments for longer times, respectively, to better reflect the real-world and improve the prospects for commercialization.

(ii) The high output voltage values of PSCs open a new technology avenue for integrated energy storage systems. A single integrated device made up of a PSC and a battery (or a supercapacitor) is known as a solar rechargeable power system. Although these types of integrated systems are highly attractive, high operating and output voltages are required. Therefore, voltage matching between the energy conversion unit (solar cell) and the battery device is critical. However, the output voltage of single-junction PV cells including PSCs is insufficient to drive energy storage devices. In this regard, perovskite-based multi-junction tandem solar cells would be excellent candidates to power integrated energy storage systems. For example, the V oc value can exceed 2.2 V for two series connected PSCs with higher voltages obtained by adding more series-connected cells.

It is well known that carbon-based materials such as CNTs, graphene and carbon particles can play significant roles in the construction of energy storage devices due to their low-cost and high conductivity. Therefore, designing integrated systems using carbon electrode based PSCs and energy storage devices would be of great value. Furthermore, on the basis of current literature, it is noticeable that the majority of available reports on PSCs-integrated energy storage systems have used the conventional perovskite type (CH 3 NH 3 PbI 3 ), whereas there are a comprehensive range of other perovskite structures available for use.

(iii) PV-driven conversion of CO 2 and water splitting have gained increasing attention as emerging sustainable routes to produce high-value chemicals. Electrolysis of water theoretically requires a minimum potential of 1.23 V, but in practice the requirement can be up to 2.0 V, which can be achieved by connecting two single-junction PSC units in series. However, instead of connecting two individual cells, employing perovskite-based tandem devices as an electrolyzer for water splitting can be a better approach. Currently, a STH efficiency of 18.7% is the best reported value among halide perovskite-based PV cell-driven water-splitting systems. Other important factors for achieving high-efficiency PV-integrated catalysis systems include the activity of the electrocatalysts. It is important to utilize catalyst materials that are readily available for use and exhibit outstanding catalytic activity for the proposed reactions. Therefore, it is reasonable to expect a STH efficiency of 20% by employing high-efficiency tandem solar cells and designing catalysts with outstanding activities for both H 2 and O 2 evolution. Considering the demand of much higher driving voltages for CO 2 RR (>2.0 V) as compared to water electrolysis, the use of multi-junctions PSCs and serially connected tandem solar cells is a powerful strategy. However, based on current performance, the power consumed would be more valuable than the converted CO 2 using PV cells.

(iv) Among the different types of perovskites based BIPVs, major progress has been made on semitransparent PSCs. However, the energy benefits of BIPVs must outweigh the costs of the technology to deliver the advantage of integrating semitransparent PSCs into buildings. As such, significant improvements are needed in the PCE of semitransparent PSCs while enhancing their optical transparencies. Typically, the benchmark AVT value is considered to be 25% for solar windows. However, it is difficult to generate a high photocurrent if 25% of the incident light is transmitted through the window. Therefore, future studies on semitransparent and colorful PSCs should aim to achieve high photovoltage values to compensate for the low photocurrent. This can be achieved through multiple strategies such as compositional engineering and bandgap engineering of the perovskites.

There is no doubt that PSCs have appealing characteristics for the development of BIPVs. However, the key challenge lies in realizing all of the following features in a single system: high PCE, excellent device stability and rapid response characteristics. Some of these features can be achieved, but at a cost to the others. For the potential commercialization of BIPVs, the devices should be scaled up with window-like dimensions. Moreover, lead-free perovskites should be explored for different types of BIPVs. Perovskite-based SPWs are innovative technologies that show many attractive features. Despite their great promise, thermochromic perovskite-based smart solar windows suffer from several issues including fast decay in the PCE during cycling and response delays in switching between bleached and colored states. The key issue for temperature based photochromics is the temperature requirement (>100 °C) to crystallize perovskite, which is well above the temperature reached from solar radiation (<100 °C). Therefore, designing perovskites that can be crystallized at low temperature while showing rapid response characteristics would be a valuable research direction for perovskite photochromic windows.

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Acknowledgements

This work was financially supported by the Australian Research Council (DE220100521). A. S. R. B acknowledges support from King Abdullah University of Science and Technology (KAUST) through the Ibn Rushd Postdoctoral Fellowship Award. M.B. acknowledges the support of Griffith University internal grants. P.L.B. is a University of Queensland Laureate Fellow.

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M.B. proposed the idea. A.S.R.B. and M.B. conceptualized the proposal. A.S.R.B. wrote the first draft of the review under the supervision of M.B. and P.E.S. The review received critical feedback from Y.L.Z., P.L.B. and M.K.N.

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Bati, A.S.R., Zhong, Y.L., Burn, P.L. et al. Next-generation applications for integrated perovskite solar cells. Commun Mater 4 , 2 (2023). https://doi.org/10.1038/s43246-022-00325-4

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Photovoltaic Cell Generations and Current Research Directions for Their Development

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The purpose of this paper is to discuss the different generations of photovoltaic cells and current research directions focusing on their development and manufacturing technologies. The introduction describes the importance of photovoltaics in the context of environmental protection, as well as the elimination of fossil sources. It then focuses on presenting the known generations of photovoltaic cells to date, mainly in terms of the achievable solar-to-electric conversion efficiencies, as well as the technology for their manufacture. In particular, the third generation of photovoltaic cells and recent trends in its field, including multi-junction cells and cells with intermediate energy levels in the forbidden band of silicon, are discussed. We also present the latest developments in photovoltaic cell manufacturing technology, using the fourth-generation graphene-based photovoltaic cells as an example. An extensive review of the world literature led us to the conclusion that, despite the appearance of newer types of photovoltaic cells, silicon cells still have the largest market share, and research into ways to improve their efficiency is still relevant.

1. Introduction

Concerns about climate change and the increase in demand for electricity due to, among other things, an ever-growing population, necessitate efforts to move away from conventional methods of energy production. Rising carbon dioxide levels in the atmosphere caused by the use of fossil fuels is one of the factors causing ongoing climate change. Switching to renewable energy will produce energy with a smaller environmental footprint compared to fossil fuel sources. We are able to harness the full potential of sunlight energy to develop the best possible energy harvesting technologies capable of converting solar energy into electricity [ 1 ].

The currently used solar energy is very marginal—0.015% is used for electricity production, 0.3% for heating, and 11% is used in the natural photosynthesis of biomass. In contrast, about 80–85% of global energy needs are met by fossil fuels. The difficulty with fossil fuels is that their resources are limited and hostile to the environment due to their CO 2 emissions. For instance, for every ton of coal burned, one ton of carbon dioxide is released into the atmosphere. This emitted carbon dioxide is toxic to the environment and is a primary cause of global warming, the greenhouse effect, climate change, and ozone depletion [ 2 ].

The necessity of finding new renewable energy forms is extremely relevant and urgent today. That is why mankind must find alternative sources of energy to provide a clean and sustainable future. Within this context, solar energy is the best option among all alternative renewable energy sources due to its widespread accessibility, universality, and eco-friendly nature [ 3 ].

The most common metric used to evaluate the performance of photovoltaic technologies is conversion efficiency, which expresses the ratio of solar energy input to electrical energy output. The efficiency combines multiple component characteristics of the system, such as short-circuit current, open-circuit voltage, and fill factor, which in turn are dependent upon basic material features and manufacturing defects [ 4 ].

The cost-effectiveness of making a photovoltaic cell and its efficiency depend on the material from which it is made. Much research in this field has been carried out to find the material that is the most efficient and cost-effective for building photovoltaic cells. The specifications for an ideal material for PV solar cells include the following [ 5 ]:

• The cells are expected to have a band gap between 1.1 and 1.7 eV;
• Should have a direct band structure;
• Need to be easily accessible and non-toxic; and
• Should have high photovoltaic conversion efficiency [ 5 ].

A key problem in the area of photovoltaic cell development is the development of methods to achieve the highest possible efficiency at the lowest possible production cost. Improving the efficiency of solar cells is possible by using effective ways to reduce the internal losses of the cell. There are three basic types of losses: optical, quantum, and electrical, which have different sources of origin. Reducing losses of any kind requires different, often advanced, methods of cell manufacturing and photovoltaic module production. An upper efficiency limit for commercially accessible technologies is determined by the well-known Shockley–Queisser (SQ) limit, taking into account the balance between photogeneration and radiative recombination [ 6 ].

However, the greatest potential lies in the ability to reduce quantum losses, as they are intimately connected with the material properties and internal structure of the cell. Relevant here is the concept of band gap, which defines the minimum required energy of a photon incident onto the cell surface for it to take part in the photovoltaic conversion process. There is a relationship between the efficiency of the cell and the value of the band gap, which in turn is highly dependent on the material from which the photovoltaic cell is made. The basic, commonly used material for solar cells is silicon, which has a band gap value of about 1.12 eV, but by introducing modifications in its crystal structure, the physical properties of the material, especially the band gap width, can be affected [ 7 ].

The dominant loss mechanisms in conventional photovoltaic cells are the inability to absorb photons below the band gap and the thermalization of solar photons with energies above the band gap energy. Third-generation solar cell concepts have been proposed to address these two loss mechanisms in an attempt to improve solar cell performance. These solutions aim to exploit the entire spectrum by incorporating novel mechanisms to create new electron–hole pairs [ 8 ].

Major development potential among these concepts for improving the power generation efficiency of solar cells made of silicon is shown by the idea of cells whose basic feature is an additional intermediate band in the band gap model of silicon. It is located between the conduction band and the valence band, and its function is to allow the absorption of photons with energies below the width of the energy gap, resulting in higher quantum efficiency (a higher number of excited electrons in relation to the number of photons incident onto the surface of the cell) [ 9 ]. Currently, many directions of research development on the introduction of intermediate bands in semiconductors can be identified. One of them is the use of ion implantation, where two methods can be distinguished: introduction of dopants with extremely high concentrations to the substrate of the semiconductor, and implantation of the layer of silicon with high-dose metal ions [ 10 ].

The improvement of solar cell efficiency involves reducing various types of losses affecting the resultant cell efficiency. The National Renewable Energy Laboratory (NREL) runs a compilation of the highest verified research cell conversion efficiencies for different photovoltaic technologies, compiled from 1976 to the present ( Figure 1 ). Cell efficiency results are given for each semiconductor family: multi-junction cells; gallium arsenide single-junction cells; crystalline silicon cells; thin film technologies; emerging photovoltaic technologies. The latest world record for an individual technology is indicated by a flag across the right edge containing the efficiency and technology symbol [ 11 ].

NREL Best Research-Cell Efficiencies chart [ 11 ].

Photovoltaic cells can be categorized by four main generations: first, second, third, and fourth generation. The details of each are discussed in the next section.

2. Photovoltaic Cell Generations

In the past decade, photovoltaics have become a major contributor to the ongoing energy transition. Advances relating to materials and manufacturing methods have had a significant role behind that development. However, there are still numerous challenges before photovoltaics can provide cleaner and low-cost energy. Research in this direction is focused on efficient photovoltaic devices such as multi-junction cells, graphene or intermediate band gap cells, and printable solar cell materials such as quantum dots [ 12 ].

The primary role of a photovoltaic cell is to receive solar radiation as pure light and transform it into electrical energy in a conversion process called the photovoltaic effect. There are several technologies involved with the manufacturing process of photovoltaic cells, using material modification with different photoelectric conversion efficiencies in the cell components. Due to the emergence of many non-conventional manufacturing methods for fabricating functioning solar cells, photovoltaic technologies can be divided into four major generations, which is shown in Figure 2 [ 13 ].

Various solar cell types and current developments within this field [ 14 ].

The generations of various photovoltaic cells essentially tell the story of the stages of their past evolution. There are four main categories that are described as the generations of photovoltaic technology for the last few decades, since the invention of solar cells [ 15 ]:

• First Generation: This category includes photovoltaic cell technologies based on monocrystalline and polycrystalline silicon and gallium arsenide (GaAs).
• Second Generation: This generation includes the development of first-generation photovoltaic cell technology, as well as the development of thin film photovoltaic cell technology from “microcrystalline silicon (µc-Si) and amorphous silicon (a-Si), copper indium gallium selenide (CIGS) and cadmium telluride/cadmium sulfide (CdTe/CdS) photovoltaic cells”.
• Third Generation: This generation counts photovoltaic technologies that are based on more recent chemical compounds. In addition, technologies using nanocrystalline “films,” quantum dots, dye-sensitized solar cells, solar cells based on organic polymers, etc., also belong to this generation.
• Fourth Generation: This generation includes the low flexibility or low cost of thin film polymers along with the durability of “innovative inorganic nanostructures such as metal oxides and metal nanoparticles or organic-based nanomaterials such as graphene, carbon nanotubes and graphene derivatives” [ 15 ].

Examples of solar cell types for each generation along with average efficiencies are shown in Figure 3 .

Examples of photovoltaic cell efficiencies [ 16 ].

2.1. First Generation of Photovoltaic Cells

Silicon-based PV cells were the first sector of photovoltaics to enter the market, using processing information and raw materials supplied by the industry of microelectronics. Solar cells based on silicon now comprise more than 80% of the world’s installed capacity and have a 90% market share. Due to their relatively high efficiency, they are the most commonly used cells. The first generation of photovoltaic cells includes materials based on thick crystalline layers composed of Si silicon. This generation is based on mono-, poly-, and multicrystalline silicon, as well as single III-V junctions (GaAs) [ 17 , 18 ].

Comparison of first-generation photovoltaic cells [ 18 ]:

• Solar cells based on monocrystalline silicon (m-si)

Efficiency : 15 ÷ 24%; Band gap : ~1.1 eV; Life span : 25 years; Advantages : Stability, high performance, long service life; Restrictions : High manufacturing cost, more temperature sensitivity, absorption problem, material loss.

• Solar cells based on polycrystalline silicon (p-si)

Efficiency : 10 ÷ 18%; Band gap : ~1.7 eV; Life span : 14 years; Advantages : Manufacturing procedure is simple, profitable, decreases the waste of silicon, higher absorption compared to m-si; Restrictions : Lower efficiency, higher temperature sensitivity.

• Solar cells based on GaAs

Efficiency : 28 ÷ 30%; Band gap : ~1.43 eV; Life span : 18 years; Advantages : High stability, lower temperature sensitivity, better absorption than m-si, high efficiency; Restrictions : Extremely expensive [ 18 ].

The first generation concerns p-n junction-based photovoltaic cells, which are mainly represented by mono- or polycrystalline wafer-based silicon photovoltaic cells. Monocrystalline silicon solar cells involve growing Si blocks from small monocrystalline silicon seeds and then cutting them to form monocrystalline silicon wafers, which are fabricated using the Czochralski process ( Figure 4 a). Monocrystalline material is widely used due to its high efficiency compared to multicrystalline material. Key technological challenges associated with monocrystalline silicon include stringent requirements for material purity, high material consumption during cell production, cell manufacturing processes, and limited module sizes composed of these cells [ 19 ].

A picture showing ( a ) the Czochralski process for monocrystalline blocks and ( b ) the process of directional solidification for multicrystalline blocks [ 21 ].

Multicrystalline silicon blocks are produced through melting high-purity silicon and crystallizing it in a big crucible by directional solidification process ( Figure 4 b). There is no reference crystal orientation in this process, as in the Czochralski process, and therefore, silicon material with different orientations is produced. The most commonly used base material for solar cells are p-type Si substrates doped with boron. The n-type silicon substrates are also used for the fabrication of high-efficiency solar cells, but they present additional technical challenges, such as achieving uniform doping along the silicon block in comparison to p-type substrates [ 20 ].

In the production of crystalline solar cells, six or more steps need to be carried out sequentially. These typically include surface texturing, doping, diffusion, oxide removal, anti-reflective coating, metallization, and firing. At the end of the process, the cell efficiency and other parameters are measured (under standard test conditions). The efficiency of photovoltaic cells is determined by the material quality that is used in their manufacture [ 21 ].

The theoretical efficiency threshold for first-generation PV cells appears to have been estimated at 29.4%, and a sufficiently close value was reached as early as two decades ago. At the laboratory scale, reaching 25% efficiency was recorded as early as 1999, and since then, very minimal improvements in efficiency values have been achieved. Since the appearance of crystalline silicon photovoltaic cells, their efficiency has increased by 20.1%, from 6% when they were first discovered to the current record of 26.1% efficiency. There are factors that limit cell efficiency, such as volume defects. Breakthroughs in the production of these cells include the introduction of an aluminum back surface field (Al-BSF) to reduce the recombination rate on the back surface, or the development of Passivated Emitter and Rear Cell (PERC) technology to further reduce the recombination rate on the back surface [ 22 ].

2.1.1. Al-BSF Photovoltaic Cells

Silicon solar cells with distributed p-n junctions were invented as early as the 1950s, soon after the first semiconductor diodes. Originally, boron diffusion in arsenic-doped wafers was used to form p-n junctions, but now, the industry standard is phosphor diffusion in boron-doped wafers. After the transition in the 1960s from n-type wafers to p-type wafers, the implementation of an aluminum back-surface field (Al-BSF) by fusing the back contact to the substrate made it possible to reduce recombination on the back side ( Figure 5 ). This fairly simple contact screen printing design held a dominant position, with 70–90% of the market share for the past several decades [ 23 ].

Silicon solar cell structure: Al-BSF [ 1 ].

Standard aluminum back surface field (Al-BSF) technology is one of the most widely used solar cell technologies due to its relatively simple manufacturing process. It is based on depositing Al entirely on the full rear-side (RS) in a screen-printing process and forming a p+ BSF, which helps repel electrons from the rear-side of the p-type substrate and improves the cell performance. The process flow of Al-BSF solar cell fabrication is shown in Figure 6 . Standard commercial solar cell design consists of a front side with a grid and a rear-side with full area contacts [ 24 ].

Al-BSF solar cell manufacturing process [ 21 ].

2.1.2. PERC Photovoltaic Cells

The efficiency of the industrial Al-BSF cell, however, reached about 20% around 2013. It has therefore become attractive to replace the fully contacted Al-BSF cell with a PERC (Passivated Emitter and Rear Cell) structure with local back contacts to achieve enhanced electrical and optical properties ( Figure 7 ). The passivated emitter and rear contact (PERC) solar cell improves the Al-BSF architecture by the addition of a passivation layer on the rear side to improve passivation and internal reflection. Aluminum oxide has been found to be a suitable material for rear side passivation [ 25 ].

Silicon solar cell structure: PERC [ 1 ].

The capability of this cell structure was demonstrated as early as the 1980s, although it was limited to laboratory processing because of its high cost relative to the yield gain. Moving the PERC technology into mass industrial production in theory involved a comparatively small industry threshold, as only two steps needed to be added to the Al-BSF process, i.e., passivation of the back surface and precise calibration of local back contacts. Nevertheless, decades passed before a profitable PERC process could be developed. A number of reasons led to the implementation of PERC in low-cost, high-volume production, and the increase in productivity to levels ranging from 22% to 23.4% [ 26 ]:

• Introduction of aluminum oxide back surface passivation by plasma-enhanced chemical vapor deposition (PECVD) and formation of local back surface field (BSF) by laser ablation of back passivation layer and Al alloy;
• Introduction of a selective emitter process in low-cost manufacturing, a “back-etching” process, or through a laser doping process;
• Reducing the width of front metallization fingers from about 100 μm to less than 30 μm in high-volume production while reducing contact resistance for lightly phosphorus-doped silicon;
• Adding a low-cost hydrogenation step at the end of the cell formation process to passivate volume defects and inactivate boron–oxygen complexes responsible for light-induced degradation (LID); and
• Reappearance of monocrystalline silicon wafers as a result of cost reduction in silicon ingot production by the Czochralski method and the introduction of diamond wire cutting [ 27 ].

2.1.3. SHJ-Type Photovoltaic Cells

In parallel with PERC cells, other high-performance cell designs such as interdigitated back contact (IBC) solar cells and heterojunction solar cells (SHJ) have been introduced to mass production. Silicon heterojunction solar cells (SHJ), otherwise referred to as HIT cells, use passivating contacts based on a stack of layers of intrinsic and doped amorphous silicon ( Figure 8 ). Among the major technological challenges associated with this promising cell structure is that once the amorphous silicon layer is deposited, processes above 200 °C cannot be used. This rules out the well-known burned-in screen-printed metal contacts, and thus demands alternative methods using low-temperature pastes or galvanic contacts [ 28 ].

Silicon solar cell structures: heterojunction (SHJ) in rear junction configuration [ 1 ].

There are currently intensive efforts to develop high-capacity production lines that could be competitive with present production standard lines. For SHJ technology to become widespread, there will be a need to overcome the challenges of increased cost of cell manufacturing tools, reducing the use of silver or replacing it with copper by developing Cu electroplating technology, as well as reducing the use of indium in the transparent conductive oxide (TCO) layer [ 29 ].

Moreover, as shown in Figure 9 , the HIT solar cell has a symmetric structure, which has two advantages. One is that the cell can be used in what is known as a bifacial module, which can generate more electricity than a regular module, and the other is that the structure is less stressed, which is important when processing thinner wafers [ 30 ].

Structure of an HIT solar cell [ 30 ].

2.1.4. Photovoltaic Cells Based on Single III-V Junctions

GaAs-based single III-V junctions are reviewed at the end of this section. The III-V materials give the greatest photovoltaic conversion efficiency, achieving 29.1% with a GaAs single junction under single sunlight and 47.1% for a six-junction device under concentrated sunlight. These devices are also thinner (absorption layers typically being 2 to 5 µm thick) and thus could be fabricated as lightweight, flexible devices capable of being placed on curved surfaces. The III-V devices have high stability and have a history of high performance for challenging applications such as space [ 31 ].

The dominant III-V layer deposition process, metal–organic vapor phase epitaxy (MOVPE), holds the responsibility behind practically every performance record for III-V devices. Yet, historically, this process has been considered as a costly growth technique because of the high cost of precursors, the comparatively low usage of these precursors, and batch growth cycles that require many hours to be completed. Latest studies have significantly improved the growth rate and demonstrated much greater use of precursor chemicals using both MOVPE and hydrogen vapor phase epitaxy (HVPE) techniques, with HVPE also solving the precursor cost problem. Finishing currently includes a great number of labor-intensive, high-priced, and comparatively inefficient process steps, involving photolithography, manual application of spin coating, contact alignment, and metal evaporation and lifting [ 32 ].

2.2. Second Generation of Photovoltaic Cells

The thin film photovoltaic cells based on CdTe, gallium selenide, and copper (CIGS) or amorphous silicon have been designed to be a lower-cost replacement for crystalline silicon cells. They offer improved mechanical properties that are ideal for flexible applications, but this comes with the risk of reduced efficiency. Whereas the first generation of solar cells was an example of microelectronics, the evolution of thin films required new methods of growing and opened the sector up to other areas, including electrochemistry [ 33 ].

The second-generation photovoltaic cell comparison [ 18 ]:

• Solar cells based on amorphous silicon (a-si)

Efficiency : 5 ÷ 12%; Band gap : ~1.7 eV; Life span : 15 years; Advantages : Less expensive, available in large quantities, non-toxic, high absorption coefficient; Restrictions : Lower efficiency, difficulty in selecting dopant materials, poor minority carrier lifetime.

• Solar cells based on cadium telluride/cadium sulfide (CdTe/CdS)

Efficiency : 15 ÷ 16%; Band gap : ~1.45 eV; Life span : 20 years; Advantages : High absorption rate, less material required for production; Restrictions : Lower efficiency, Cd being extremely toxic, Te being limited, more temperature-sensitive.

• Solar cells based on copper indium gallium selenide (CIGS)

Efficiency : 20%; Band gap : ~1.7 eV; Life span : 12 years; Advantages : Less material required for production; Restrictions : Very high-priced, not stable, more temperature-sensitive, highly unreliable [ 18 ].

2.2.1. CIGS Photovoltaic Cells

A key aspect that needed improvement was reducing the high dependence on semiconductor materials. This was the driving force that led to the emergence of the second generation of thin film photovoltaic cells, which include CIGS. In terms of efficiency, the record value for CIGS is 23.4%, which is comparable to the best silicon cell efficiencies. It should be noted, however, that the efficiency of the research cells does not directly translate to industrially achievable efficiency due to the nature of large-scale processing. Nevertheless, module efficiencies above 20% are already a reality. There has been a significant increase in the efficiency of CIGS cells in recent years and further increases are expected, for example, as a result of further research into alkaline treatment after deposition [ 34 ].

Group I-III-VI semiconducting chalcopyrite alloys (Ag,Cu)(In,Ga)(S,Se) 2 , commonly known as CIGS, are particularly favorable absorber materials for solar cells. They have direct band gaps ranging from ~1 to 2.6 eV, high absorption coefficients, and favorable internal defect parameters that allow high minority carrier lifetimes, and solar cells made from them are inherently stable in operation. The first recorded yield was 12% in a monocrystalline device in the mid-1970s. Subsequently, CIGS thin film absorbers, processing, and contacts were greatly improved, resulting in thin film cells with a small area and an efficiency of 23.4%. Current record module efficiencies are 17.6% on glass and 18.6% on flexible steel [ 35 ].

CIGS solar cells have been developed in a standard substrate configuration; however, deposition of CIGS at comparatively low temperatures on metal or polymer substrates to form flexible solar products is also possible. CIGS thin films are mainly being deposited by co-evaporation/devaporation or sputtering, and to a minor extent by electrochemical deposition as well as ion beam-assisted deposition. Since these are quaternary compounds, it is critical to control the stoichiometry of the thin film during fabrication. Work is also underway to produce fully or partially solution-deposited CIGS solar cells, and some predict that they could be the ultimate path to ultra-thin, coiled, and flexible PV modules [ 36 ].

The steps to improve the efficiency of CIGS cells may be described in the following way: (1) evaporation of CIS compound; (2) reactive elemental bilayer deposition; (3) selenization of sputtered metal precursors; (4) chemical bath deposition of CdS with ZnO:Al as emitter; (5) gallium alloying; (6) sodium alkali incorporation; (7) three-step co-deposition; (8) post-deposition treatment involving heavy alkali ion exchange; and (9) sulfurization after selenization (SAS). Progress is far from linear, with the complete potential for the optimization of the complex interactions between those techniques, along with others under development (e.g., silver alloys), yet to be achieved. A large number of scientists who specialize in CIGS think that efficiencies of 25% can be reached [ 37 ].

CIGS is a versatile material that can be produced by many processes and used in a variety of forms. There are currently four main categories of depositing methods used to fabricate CIGS films: (1) metal precursor deposition followed by sulfo-selenization; (2) reactive co-deposition; (3) electrodeposition; and (4) solution processing. All recent world records and the greatest commercial successes have been achieved by two-step sulfo-selenization of metal precursors or reactive co-deposition. CIGS can be deposited on a variety of substrates, including glass, metal films, and polymers. Glass is suitable for making rigid modules, while metal and polymer films allow applications that require lighter or flexible modules. With the evolution of global energy markets toward an appreciation of greenhouse gas reduction and circular economy aspects, the comparatively benign environmental impact of CIGS (especially without CdS) in comparison to different photovoltaic technologies is becoming the next competitive advantage [ 38 ].

Photovoltaic cells based on CIGS technology are composed of a pile of thin films deposited on a glass substrate by magnetron sputtering: a bottom molybdenum (Mo) electrode, a CIGS absorbing layer, a CdS buffer layer, and a zinc-doped oxide (ZnO:Al) top electrode. The co-evaporation and CdS buffer layer deposit the CIGS active layer by means of a chemical bath in a regular procedure ( Figure 10 ) [ 38 ].

Demonstration of the CIGS-based standard solar cell stack [ 38 ].

2.2.2. CdTe Photovoltaic Cells

Second-generation photovoltaic cells also include CdTe-based solar cells. An interesting property of CdTe is the reduction in cell size—due to its high spectral efficiency, the absorber thickness can be reduced to about 1 μm without much loss in efficiency, although further work is needed ( Figure 11 ). Super-thin cells are particularly attractive for flexible applications, particularly in building-integrated photovoltaics (BIPV) due to their lighter weight, and transparent photovoltaic panels with CdTe can be developed due to the choice of transparent coating. Their transparency varies from about 10% to 50%, with the disadvantage that an increase in transparency necessarily decreases efficiency. Still, the transparent panels could replace window panels in buildings, not only generating electricity that could be used to power itself, but also contributing to noise reduction and thermal insulation, since most panels are encased in double glass [ 39 ].

Schematic of a CdTe solar cell [ 1 ].

The technology of CdTe solar cells has developed considerably with the passage of time. In the 1980s, the efficiency of certified cells reached 10%, and in the 1990s, the efficiency was above 15% with the use of a glass/SnO 2 /CdS/CdTe layer structure and annealing in a CdCl 2 environment, and subsequent Cu diffusion. By the 2000s, efficiency of the cells hit 16.7% using sputtered Cd 2 SnO 4 and Zn 2 SnO 4 as transparent conductive oxide (TCO) layers. Over the past decade, new cell efficiency records have reached 22.1%. CdTe technology is increasingly used in rooftop systems and building-integrated photovoltaics [ 40 ].

In 2001, NREL produced a cell with an efficiency of 16.5%, which remained the benchmark for about 10 years. The record efficiency has been improved several times in the past 2 years by First Solar and GE Global Research. Currently, CdTe thin films account for less than 10% of the global PV market, with capacity expected to increase. Most of the commercial CdTe cells are manufactured by First Solar, which has achieved record cell efficiencies of 22.1% and average commercial module efficiencies of 17.5–18% [ 41 ].

The history of research and development and production of CdTe-based PV cells begins several decades beyond the first studies conducted by Bell Labs (Murray Hill, NJ, USA) in the 1950s on Si crystalline cells. The leading companies have been working on the commercialization of the underlying technology: Matsushita (Kadoma, Osaka, Japan), BP Solar (Madrid, Spain), Solar Cells Inc.—predecessor to First Solar (Tempe, AZ, USA), Abound Solar (Loveland, CO, USA) and GE PrimeStar (Denver, CO, USA). The top manufacturer of thin film CdTe PV is currently First Solar Solar (Tempe, AZ, USA), having fabricated 25 GW of PV modules since 2002 [ 42 ].

A range of comparatively easy and inexpensive approaches have been used to produce solar cells with 10–16% efficiency. Examples of several promising cheap deposition techniques include (1) close-space sublimation, (2) spray deposition, (3) electrodeposition, (4) screen printing, and (5) sputtering [ 43 ].

Recently, a record efficiency of 16% was reported in a CdS (0.4 μm)/CdTe (3.5 μm) thin film solar cell in which CdS and CdTe layers are deposited using metal–organic CVD (MOCVD) and CSS deposition techniques, respectively. Most of the high-performance solar cells use a device configuration of the superstrate type, where CdTe is deposited on a window layer of CdS. Typically, the structure of the device is composed of glass/CdS/CdTe/Cu-C/Ag. Most of the time, post-deposition heat treatment of the CdTe layer in the presence of CdCl 2 is necessary to optimize device performance [ 44 ].

The recent increase in efficiency is due partly to almost maximum photocurrent by optimizing the optical properties of the cell, deleting parasitically absorbing CdS and introducing CdSe x Te 1−x with a lower band gap. CdSe x Te 1-x extends the bandwidth of the absorber from ~1.4 to 1.5 eV and increases the carrier lifetime, thus improving photocurrent collection with no proportional loss of photocurrent. The use of ZnTe in the rear contact also improves the contact ohmicity significantly, and thus the efficiency [ 45 ].

2.2.3. Kesterite Photovoltaic Cells

In recent years, kesterite thin film materials have attracted more interest than CdTe and CIGS chalcogenide materials. Cu 2 ZnSnS x Se 4−x (CZTSSe) thin film photovoltaic material is attracting worldwide attention for its exceptional efficiency and composition derived from the Earth. A lot of research is being conducted on material engineering or designing new architecture to achieve high-performance CZTSSe thin film solar cells. Until recently, the most advanced thin film CZTSSe solar cells have been limited to 11.1% power conversion efficiency (PCE), with these efficiency levels reached using the hydrazine suspension method. Further vacuum and non-vacuum deposition techniques also proved effective in producing CZTSSe solar cells that had a PCE above 8%. Yet still, even record equipment with a PCE of 11% is significantly below the physical limit, generally referred to as the Shockley–Queisser (SQ) limit, which is around 31% efficiency under the Earth’s conditions [ 46 ].

A hydrazine-based pure solution method is used to prepare CZTSSe layers, and a Cu-poor and Zn-rich stoichiometry is adopted in the starting solution (Cu/(Zn + Sn) = 0.8 and Zn/Sn = 1.1). Multiple layers of components are spin-coated onto Mo-coated soda-lime glass and annealed at temperatures above 500 °C. Regarding the fabrication of devices, CZTSSe layers are deposited on Mo-coated glass substrates, then 25 nm CdS is deposited in a standard chemical bath and sputtered with 10 nm ZnO/50 nm ITO. A 2 μm thick Ni/Al top metal contact and 110 nm MgF 2 should be deposited on top of the devices by electron beam evaporation. The area of the device should be determined by mechanical scribing [ 47 ].

2.2.4. Photovoltaic Cells Based on Amorphous Silicon

The last type of cells classified as second-generation are devices that use amorphous silicon. Amorphous silicon (a-Si) solar cells are by far the most common thin film technology, whose efficiency is between 5% and 7%, rising to 8–10% for double and triple junction structures. Some varieties of amorphous silicon (a-Si) are amorphous silicon carbide (a-SiC), amorphous germanium silicon (a-SiGe), microcrystalline silicon (μ-Si), and amorphous silicon nitride (a-SiN). Hydrogen is required to dope the material, leading to hydrogenated amorphous silicon (a-Si:H). The gas phase deposition technique is typically used to form a-Si photovoltaic cells with metal or gas as the substrate material [ 48 ].

A typical manufacturing process for a-Si:H cells is the roll-to-roll process. First, a cylindrical sheet, usually stainless steel, is rolled out to be used as a deposition surface. The sheet is washed, cut to the desired size, and coated with an insulating layer. Next, a-Si:H is applied to the reflector, after which a transparent conductive oxide (TCO) is deposited on the silicon layer. Finally, laser cuts are made to join the different layers and the module is closed [ 49 ].

Amorphous silicon is usually deposited by plasma-enhanced vapor phase deposition (PECVD) at comparatively low substrate temperatures of 150–300 °C. A 300 nm thick a-Si:H layer is capable of absorbing about 90% of photons above the passband in a single pass, allowing the fabrication of lighter and more flexible solar cells [ 2 ].

Figure 12 shows the step-by-step fabrication process of an a-Si-based photovoltaic cell. Photovoltaic cells based on thin films are cheaper, thinner, and more flexible compared to first generation photovoltaic cells. The thickness of the light absorbing layer, which was 200–300 µm in first-generation photovoltaic cells, is 10 µm in second-generation cells. Semiconductor materials ranging from “micromorphic and amorphous silicon” to quaternary or binary semiconductors such as “cadmium telluride (CdTe) and copper indium gallium selenide (CIGS)” are used in thin films of photovoltaic cells [ 50 ].

Manufacturing process of a-Si-based solar PV cell [ 2 ].

2.3. Third Generation of Photovoltaic Cells

The third generation of solar cells (including tandem, perovskite, dye-sensitized, organic, and emerging concepts) represent a wide range of approaches, from inexpensive low-efficiency systems (dye-sensitized, organic solar cells) to expensive high-efficiency systems (III-V multi-junction cells) for applications that range from building integration to space applications. Third-generation photovoltaic cells are sometimes referred to as “emerging concepts” because of their poor market penetration, even though some of these have been studied for more than 25 years [ 51 ].

The latest trends in silicon photovoltaic cell development are methods involving the generation of additional levels of energy in the semiconductor’s band structure. The most advanced studies of manufacturing technology and efficiency improvements are now concentrated on third-generation solar cells.

One of the current methods to increase the efficiency of PV cells is the introduction of additional energy levels in the semiconductor’s band gap (IBSC and IPV cells) and the increasing use of ion implantation in the manufacturing process. Other innovative third-generation cells that are lesser-known commercial “emerging” technologies include [ 52 ]:

• Organic materials (OSC) photovoltaic cells;
• Perovskites (PSC) photovoltaic cells;
• Dye-sensitized (DSSC) photovoltaic cells;
• Quantum dots (QD) photovoltaic cells; and
• Multi-junction photovoltaic cells [ 52 ].

Third-generation photovoltaic cell comparison [ 18 ]:

• Solar cells based on dye-sensitized photovoltaic cells

Efficiency : 5 ÷ 20%; Advantages : Lower cost, low light and wider angle operation, lower internal temperature operation, robustness, and extended lifetime; Restrictions : Problems with temperature stability, poisonous and volatile substances.

• Solar cells based on quantum dots

Efficiency : 11 ÷ 17%; Advantages : Low production cost, low energy consumption; Restrictions : High toxicity in nature, degradation.

• Solar cells based on organic and polymeric photovoltaic cells

Efficiency : 9 ÷ 11%; Advantages : Low processing cost, lighter weight, flexibility, thermal stability; Restrictions : Low efficiency.

• Solar cells based on perovskite

Efficiency : 21%; Advantages : Low-cost and simplified structure, light weight, flexibility, high efficiency, low manufacturing cost; Restrictions : Unstable.

• Multi-junction solar cells

Efficiency : 36% and higher; Advantages : High performance; Restrictions : Complex, expensive [ 18 ].

2.3.1. Organic and Polymeric Materials Photovoltaic Cells (OSC)

Organic solar cells (OSCs) are beneficial in applications related to solar energy since they have the potential to be used in a variety of prospects on the basis of the unique benefits of organic semiconductors, including their ability to be processed in solution, light weight, low cost, flexibility, semi-transparency, and applicability to large-scale roll-to-roll processing. Solution-processed organic solar cells (OSCs) that absorb near-infrared (NIR) radiation have been studied worldwide for their potential to be donor:acceptor bulk heterojunction (BHJ) compounds. In addition, NIR-absorbing OSCs have attracted attention as high-end equipment in next-generation optoelectronic devices, such as translucent solar cells and NIR photodetectors, because of their potential for industrial applications. With the introduction of non-fullerene acceptors (NFAs) that absorb light in the NIR range, the value of OSC is increasing, while organic donor materials capable of absorbing light in the NIR range have not yet been actively studied compared to acceptor materials that absorb light in the NIR range [ 53 ].

The most advanced BHJ structure by combining organic donor and acceptor materials showed tremendous hope for low-cost and lightweight organic solar cells. Over the past decade, enormous progress was made, with power conversion efficiencies reaching more than 14% for a single-junction device and more than 17% for a tandem device through the design of new NIR photoactive materials with low bandwidth. Compared to wide-band organic photovoltaic materials, low-band donor and non-fullerene acceptor materials with wide-range solar coverage extended to the NIR region typically exhibit more tightly superimposed electronic orbitals, easier delocalization of π electrons, higher dielectric constant, stronger dipole moment, and lower exciton binding energy. These properties make low-bandwidth photovoltaic materials play an important role in high-performance organic solar cells, including single-junction and tandem devices [ 54 ].

A clever strategy in active layer design could be summed up as optimizing the weight ratio of donor to acceptor materials, using ultra-low band gap materials as a third component to improve NIR light utilization efficiency, and adjusting the thickness of the active layer to achieve a compromise between photon collection and charge accumulation. Much effort has gone into optimizing the translucent top electrode: well-balanced conductivity and transmittance in the visible light range, increased reflectance in the NIR or ultraviolet (UV) light range, and better compatibility with active layers. In terms of device engineering, photon crystal, anti-reflection coating, optical microcavity, and dielectric/metal/dielectric (DMD) structures have been placed to realize selective transmission and reflection for simultaneous improvement of power conversion efficiency and average transmission of translucent OSC visible light [ 55 ].

2.3.2. Dye-Sensitized Photovoltaic Cells (DSSC)

Conjugated polymers and organic semiconductors have been successful in flat panel displays and LEDs, so they are considered advanced materials in the current generation of photovoltaic cells. A schematic representation of dye-sensitized organic photovoltaic cells (DSSCs) is shown in Figure 13 . Polymer/organic photovoltaic cells can also be divided into dye-sensitized organic photovoltaic cells (DSSCs), photoelectrochemical photovoltaic cells, and plastic (polymer) and organic photovoltaic devices (OPVDs), differing in mechanism of operation [ 56 ].

Schematic representation of a DSSCs [ 2 ].

Dye-sensitized solar cells (DSSCs) represent one of the best nanotechnology materials for energy harvesting in photovoltaic technologies. It is a hybrid organic–inorganic structure where a highly porous, nanocrystalline layer of titanium dioxide (TiO 2 ) is used as a conductor of electrons in contact with an electrolyte solution also containing organic dyes that absorb light near the interfaces. A charge transfer occurs at the interface, resulting in the transport of holes in the electrolyte. The power conversion efficiency has been shown to be about 11%, and commercialization of dye-sensitized photovoltaic modules is underway. A novel feature in DSSC solar cells is the photosensitization of nanosized TiO 2 coatings in combination with optically active dyes, which increases their efficiency to more than 10% [ 57 ].

DSSCs hold promise as photovoltaic devices because of their simple fabrication, low material costs, and their benefits in transparence, color capability, and mechanical flexibility. The main challenges in commercializing DSSCs are poor photoelectric conversion efficiency and cell stability. The highest attainable theoretical energy conversion efficiency was estimated at 32% for DSSCs; however, the highest efficiency reported to date is only 13%. Intensive work is underway to understand the parameters governing the DSSC to improve its efficiency. Numerous attempts have been made to optimize the redox pair and absorbance of the dye, modify a wide band gap semiconductor as a working electrode, and develop a counter electrode (CE). In addition to increasing the efficiency of DSSC, the cost of materials is another major issue that needs to be solved in future work [ 58 ].

2.3.3. Perovskite Photovoltaic Cells

Perovskite solar cells (PSCs) are a revolutionary new photovoltaic cell concept that relies on metal halide perovskites (MHPs), e.g., methylammonium iodide as well as formamidine lead iodide (MAPbI 3 or FAPbI 3 , respectively). MHPs integrate a number of features favored in photovoltaic absorbers, including a direct band gap with a high absorption coefficient, long carrier lifetime and diffusion length, low defect density, and ease of tuning the composition and band gap. In the year 2009, MHP was first described as a sensitizer in a dye cell based on liquid electrolyte conducting holes. In 2012, MHP demonstrating ~10% efficiency of PSCs based on a solid-state hole conductor sparked an explosion of PSC studies. In about a decade of research, the efficiency of a single PSC junction increased to a certified level of 25.2% [ 59 ].

The development of PSCs has been heavily influenced by the improvement of material quality through a broad range of synthetic methods designed under the guidance of a fundamental understanding of MHP growth mechanisms. Comprehension of the complex and correlated processes of perovskite growth (e.g., nucleation, grain growth, as well as microstructure evolution) has aided in the development of a broad range of high-efficiency growth modes (for example, single-step growth, sequential growth, dissolution process, vapor process, post-deposition processing, non-stoichiometric growth, additive-assisted growth, and fine-tuning of structure dimensions). The latest efforts were concentrated on interface engineering, focusing on reducing open-circuit voltage losses and improving stability, particularly by introducing a two-dimensional perovskite surface layer. With progress in synthetic control, the perovskite composition is becoming simpler, mainly toward FAPbI 3 . This will undoubtedly contribute to the simplification of scale deposition methods and a basic understanding of the properties of these cells [ 60 ].

2.3.4. Quantum Dots Photovoltaic Cells

Solar cells made from these materials are called quantum dots (QDs) and are also known as nanocrystalline solar cells. They are fabricated by epitaxial growth on a substrate crystal. Quantum dots are surrounded by high potential barriers in a three-dimensional shape, and the electrons and electron holes in a quantum dot become discrete energy because they are confined in a small space ( Figure 14 ). Consequently, the ground state energy of electrons and electron holes in a quantum dot depends on the size of the quantum dot [ 61 ].

( a ) A scheme of a solar cell based on quantum dots, ( b ) solar cell band diagram [ 64 ].

Nanocrystalline cells have relatively high absorption coefficients. Four consecutive processes occur in a solar cell: (1) light absorption and exciton formation, (2) exciton diffusion, (3) charge separation, and (4) charge transport. Due to the poor mobility and short lifetime of excitons in conducting polymers, organic compounds are characterized by small exciton diffusion lengths (10–20 nm). In other words, excitons that form far from the electrode or carrier transport layer recombine and the conversion efficiency drops [ 62 ].

The development of thin film solar cells with metal halide perovskites has led to intensive attention to the corresponding nanocrystals (NCs) or quantum dots (QDs). Today, the record efficiency of QD solar cells was improved to 16.6% using mixed colloidal QDs with perovskites. The universality of these new nanomaterials regarding ease of fabrication and the ability to tune the band gap and control the surface chemistry allows a variety of possibilities for photovoltaics, such as single-junction, elastic, translucent, controlled cells with heterostructures and multi-junction tandem solar cells which would push the field even further. However, a narrower size distribution has the potential to enhance the performance of QD solar cells through more ways than one. Firstly, electron transport might be better in smaller QDs, as larger QDs function as a band tail or shallow trap that makes transport more difficult. Secondly, the open-circuit voltage (V OC ) of QD solar cells could be limited by the smallest band gap (largest size) QD near the contacts. Enhancing the homogeneity and uniformity of QD size would also improve PV performance by the minimization of such losses. Although controlled experiments such as these have not yet been reported, it is possible that more controlled synthesis might provide benefits to QD cells [ 63 ].

2.3.5. Multi-Junction Photovoltaic Cells

Multi-junction (MJ) solar cells consist of plural p-n junctions fabricated from various semiconductor materials, with each junction producing an electric current in response to light of a different wavelength, thereby improving the conversion of incident sunlight into electricity and the efficiency of the device. The concept to use various materials with different band gaps has been suggested to utilize the maximum possible number of photons and is known as a tandem solar cell. An entire cell could be fabricated from the same or different materials, giving a broad spectrum of possible designs [ 65 ].

Usually, the cells are integrated monolithically and connected in series through a tunnel junction, and current matching between cells is obtained through adjusting each cell’s band gap and thickness. The theoretical feasibility of using multiple band gaps was examined and was found to be 44% for two band gaps, 49% for three band gaps, 54% for four band gaps, and 66% for an infinite number of gaps. Figure 15 illustrates a scheme of an InGaP/(In)GaAs/Ge triple solar cell and presents crucial technologies to enhance efficiency of conversion [ 66 ].

Schematic illustration of a triple-junction cell and approaches for improving efficiency of the cell [ 65 ].

Grid-matched InGaP/(In)GaAs/Ge triple solar cells have been widely used in space photovoltaics and have achieved the highest true efficiency of over 36%. Heavy radiation bombardment of various energetic particles in the space environment inevitably damages solar cells and causes the formation of additional non-radiative recombination centers, which reduces the diffusion length of minority carriers and leads to a reduction in solar cell efficiency. The sub-cells in multi-junction solar cells are connected in series; the sub-cell with the greatest radiation degradation degrades the efficiency of the multi-junction solar cell. To improve the radiation resistance of (In)GaAs sub-cells, measures such as reducing the dopant concentration, decreasing the thickness of the base region, etc., can be used [ 66 ].

2.3.6. Photovoltaic Cells with Additional Intermediate Band

The National Renewable Energy Laboratory (NREL) estimates that multi-junction and IBSC photovoltaic cells have the highest efficiency under experimental conditions (47.1%). The main feature of these cells is precisely the additional intermediate band in the band gap of silicon. Currently, two types of these cells are specified in the world literature: IBSC (Intermediate Band Solar Cells) and IPV (Impurity Photovoltaic Effect) [ 67 ].

Impurity Photovoltaic Effect (IPV) is one of the solutions used to increase the infrared response of PV cells and thus increase the solar-to-electric energy conversion efficiency. The idea of the IPV effect is based on the introduction of deep radiation defects in the structure of the semiconductor crystal structure. These defects ensure a multi-step absorption mechanism for photons with energies below the band gap width. The addition of IPV dopants into silicon solar cell structure, under certain conditions, increases the spectral response, short circuit current density, and conversion efficiency [ 68 ].

A major direction of study with great potential for development is Intermediate Band Solar Cells (IBSCs). They represent a third-generation solar cell concept and involve not only silicon, but also other materials. The idea behind the intermediate band gap solar cell (IBSC) concept is to absorb photons with an energy corresponding to the sub-band width in the cell structure. These photons are absorbed by a semiconductor-like material that, in addition to the conduction and valence bands, has an intermediate band (IB) in the conventional semiconductor’s band gap ( Figure 16 ). In IBSCs, the silicon layers are implanted with very high doses of metal ions to create an additional energy level [ 69 ].

Energy band diagram of an intermediate band solar cell (IBSC) [ 69 ].

Based on the research conducted on the effect of defects introduced into the silicon structure, a model was developed according to which introducing selected deep defects into the charge carrier capture region results in improved PV cell efficiency. Of particular interest are defects that facilitate the transport of majority carriers and defects that counteract the accumulation of minority carriers. This contributes significantly to reducing the recombination process at the charge carrier capture site. Finally, by introducing defects into the structure of the silicon underlying the solar cell, we combine effective surface passivation with simultaneous reduction in optical losses [ 70 ].

The introduction of intermediate bands in semiconductors, using ion implantation, can be executed using two methods: by introducing dopants of very high concentration into the semiconductor substrate, or by implanting the silicon layer with high-dose metal ions. The increasing use of ion implantation in the photovoltaic cell manufacturing process has the potential to reduce the cost of deployment and increase the cost-effectiveness of silicon cells by increasing their efficiency. The use of ion implantation technology provides increased precision of silicon layer doping and generation of additional levels of energy in the band gap, as well as shortening the individual stages of cell fabrication, which ultimately translates into improved quality and lower production costs [ 71 ].

Lately, the technique of ion implantation is gaining popularity in the solar industry, gradually displacing the diffusion technique that has been used for many years. As can be seen in Figure 17 , cell performance is expected to continue to improve as the technology evolves toward higher efficiencies. In addition to local and reference doping, the major benefits of this technology involve high precision control of the amount and distribution of dopant doses, which results in high uniformity, repeatability, and increased efficiency (above 19%), with a significantly narrower distribution of cell performance [ 72 ].

Stabilized cell efficiency trend curves [ 72 ].

In the method of ion implantation, chosen ions with the required impurity are inserted into the semiconductor by accelerating the impurity ions to a high energy level and implanting the ions into the semiconductor. The energy given to the impurity ions defines the depth of ion implantation. Contrary to the diffusion technology (where the impurity ion dose is introduced only at the surface), in the ion implantation technique, a controllable dose of impurity ions can be placed deeply into the semiconductor [ 73 ].

2.4. Fourth Generation of Photovoltaic Cells

Fourth-generation photovoltaic cells are also known as hybrid inorganic cells because they combine the low cost and flexibility of polymer thin films, with the stability of organic nanostructures such as metal nanoparticles and metal oxides, carbon nanotubes, graphene, and their derivatives. These devices, often referred to as “nanophotovoltaics”, could become the promising future of photovoltaics [ 74 ].

Graphene-Based Photovoltaic Cells

By using thin polymer layers and metal nanoparticles, as well as various metal oxides, carbon nanotubes, graphene, and their derivatives, the fourth generation provides excellent affordability and flexibility. Particular emphasis was placed on graphene because it is considered a nanomaterial of the future. Due to their unique properties, such as high carrier mobility, low resistivity and transmittance, and 2D lattice packing, graphene-based materials are being considered for use in PV devices instead of existing conventional materials. However, to achieve adequate device performance, the key to its practical applications is the synthesis of graphene materials with appropriate structure and properties [ 75 ].

Since the properties of graphene are fundamentally related to its fabrication process, a judicious choice of methods is essential for targeted applications. In particular, highly conductive graphene is suitable for use in flexible photovoltaic devices, and its high compatibility with metal oxides, metallic compounds, and conductive polymers makes it suitable for use as a selective charge-taking element and electrode interlayer material [ 76 ].

In the past two decades, graphene has been combined with the concept of photovoltaic material and is showing a significant role as a transparent electrode, hole/electron transport material, and interfacial buffer layer in solar cell devices. We can distinguish several types of graphene-based solar cells, including organic bulk heterojunction (BHJ) cells, dye-sensitized cells, and perovskite cells. The energy conversion efficiency exceeded 20.3% for graphene-based perovskite solar cells and reached 10% for BHJ organic solar cells. In addition to its function of extracting and transporting charge to the electrodes, graphene plays another unique role—it protects the device from environmental degradation through its packed 2D lattice structure and ensures the long-term environmental stability of photovoltaic devices [ 77 ].

Semi-metallic graphene having a zero band gap creates Schottky junction solar cells with silicon semiconductors. Even though graphene was discovered for the first time in 2004, the first graphene–silicon solar cell was not characterized as an n-silicon cell until 2010. Figure 18 schematically shows a graphene–silicon solar cell with a Schottky junction. Graphene sheets (GS), cultured by chemical vapor deposition (CVD) on nickel films, were wet deposited on pre-patterned Si/SiO 2 substrates with an effective area of 0.1–0.5 cm 2 . The graphene sheet forms a coating on the exposed n-Si substrate, creating a Schottky junction. The graphene sheet was contacted using Au electrodes [ 78 ].

Graphene–silicon Schottky junction solar cell. ( a ) Cross-sectional view, ( b ) schematic illustration of the device configuration [ 75 ].

Graphene synthesis uses mainly two methodologies, which are the bottom-up and top-down methods. In the top-down approach, graphite is the starting material, and the goal is to intercalate and exfoliate it into graphene sheets by solid, liquid, or electrochemical exfoliation. Another approach under this categorization is the exfoliation of graphite oxide into graphene oxide (GO), after which chemical or thermal reduction takes place. A bottom-up approach is to produce graphene from molecular precursors by chemical vapor deposition (CVD) or epitaxial growth. The structure, morphology, and attributes of the resulting graphene, including the layer numbers, level of defects, electrical and thermal conductivity, solubility, and hydrophilicity or hydrophobicity, are dependent on the manufacturing process [ 78 , 79 ].

Graphene can absorb 2.3% of incident white light even though it is only one atom thick. Incorporating graphene into a silicon solar cell is a promising platform since graphene has a strong interaction with light, fulfilling both the optical (high transmittance) and electrical (low layer resistance) requirements of a typical transparent conductive electrode. It is important to note that both the layer resistance and the transmittance of graphene change with the number of layers. As the layer resistance decreases as the number of graphene layers increases, the optical transparency decreases as well [ 80 ].

For PV technology, graphene offers a lot more because of its flexibility, environmental stability, low electrical resistivity, and photocatalytic features, while having to be carefully and deliberately designed for the targeted applications and specific requirements [ 78 , 80 ].

One problem for graphene application is the absence of a simpler, more reliable way to deposit a well-ordered monolayer with low-cost flakes on target substrates having various surface properties. The other problem is the adhesion of the deposited graphene thin film, a subject that has not yet been studied properly. Large-area continuous graphene layers with high optical transparency and electrical conductivity may be fabricated by CVD. As an anode in organic photovoltaic devices, graphene holds great promise as a replacement for indium tin oxide (ITO) because of its inherently low-cost manufacturing process and excellent conductivity and transparency properties [ 81 ].

Graphene’s major disadvantage is its poor hydrophilicity, which negatively affects the design of devices processed in solution, but that fact may be overcome through modifying the surface by non-covalent chemical functionalization. Given graphene’s mechanical strength and flexibility, as well as its excellent conductivity properties, it can be anticipated that new applications in plastic electronics and optoelectronics will soon emerge involving this new class of CVD graphene materials. The discovery paves the way for low-cost graphene layers to replace ITO in photovoltaic and electroluminescent devices [ 82 ].

3. Prospects and Research Directions

Since the beginning of photovoltaic cells, crystalline silicon-based photovoltaic technology has played a dominant role in the market, with crystalline PV modules accounting for about 90% of the market share in 2020. In recent years, there has been a rapid development of thin film solar cells (such as cadmium telluride (CdTe) and indium–gallium selenium compounds (CIGS) cells) and new solar cells (such as dye-sensitized solar cells (DSSCs), perovskite solar cells (PSCs), quantum dot solar cells (QDSCs), etc.) [ 83 ].

The growing interest in BIPV systems has contributed to the overall development of photovoltaic technology, which has led to lower costs, increasing the feasibility of investment. Most of the standard second-generation technologies show efficiencies of 20–25%, and while they are expensive, the cost of silicon cells has come down and it is the improvement of silicon technologies that is now one of the key research directions [ 84 ].

Graphene and its derivatives are a promising area of research as they are in the early stages of research and development. The goal of using carbon nanostructures is to produce energy-efficient products that combine transport, active, and electrode layers. Many researchers in contemporary graphene research are now focusing on new graphene derivatives and their novel applications in manufacturing devices [ 85 ].

Nevertheless, the technologies used for third- and fourth-generation cells are still in the prototyping stage. Production-scale prototypes have also been built and have been successful (10–17% efficiency). In contrast, third-generation multi-junction cells are already commercially available and have achieved exceptional conversion factors (from 40% to over 50%) that place this alternative as the best [ 85 ]. Considering the market trends of increasing use of intermediate energy levels in PV cell production, it makes perfect sense to conduct research in this direction, which is exactly what our research team is doing.

The practical realization of the idea of energy-efficient IBSC-type silicon solar cells with intermediate energy levels in the band gap of the semiconductor, produced by ion implantation, needs more studies directed at the search for the optimal implantation parameters, which is the energy, type, and dose of ions, adjusted to the substrate material properties, particularly the level and type of dopant [ 86 ].

It appears that implantation can also lead to a reduction in the optical losses present in the cell. Impurities and defects introduced into the silicon crystal lattice under the right conditions can create additional intermediate band gaps, which realistically contributes to the reduction in the energy gap width. As a result, some photons with energies lower than the band gap value cause the formation of additional electron–hole pairs. The existence of this additional energy band contributes to the increase in the value of the photoelectric current, which results from the absorption of photons not previously involved in the photovoltaic conversion process. The range of absorbed light radiation increases toward the infrared, and after absorbing a photon from this range, the electron goes first to the intermediate band and then to the conduction band [ 87 ].

Our long-standing studies on changing the electrical parameters of silicon through the use of neon ion implantation have resulted in the development of the authorial methodology for the generation and identification of additional levels of energy in the silicon band structure, improving the efficiency of photovoltaic cells made based on it [ 88 ].

The research has been directed at determining the effect of the degree and type of silicon defect in terms of the possibility of producing intermediate energy levels in the semiconductor’s band gap, thereby increasing the efficiency of solar cells by enabling a multi-step transition of electrons from the valence band to the intermediate band and then to the conduction band.

The object of our research is a method of producing intermediate energy levels in the band gap of n- and p-type silicon, with a specific resistivity ρ ranging from 0.25 Ω·cm to 10 Ω·cm, by generating deep radiation defects in the crystal structure of the semiconductor by implantation of Ne + neon ions. The research material is doped with elements such as boron, phosphorus, and antimony.

Neon ions were chosen because the ions primarily produce point defects, the deliberate introduction of which into the crystalline lattice of silicon in the process of implantation makes it possible to alter its fundamental electrical parameters, including energy gap width and resistivity. The parameters significantly affect internal losses in photovoltaic cells [ 89 ]. Experimental studies were conducted to provide details for determination of the optimal dose of implanted neon ions because of their ability to generate intermediate energy levels in the semiconductor band gap.

The Results of the Author’s Research

The silicon samples were implanted with neon ions of energy E = 100 keV and different doses D using a UNIMAS 79 ion implanter and then isochronically annealed at 598 K for 15 min in a resistance furnace. The electrical parameters of the silicon samples were tested using a Discovery DY600C climate chamber using the proprietary PV Cells Meter computer program and the Winkratos software. A GW Instek LCR-8110G Series LCR meter was used to measure capacitance and conductance values, while sample temperature values were measured using Fluke 289 and Lutron TM-917 multimeters ( Figure 19 ).

Silicon samples laboratory stand. ( a ) Schematic diagram of the laboratory stand: 1—solar cell, 2—supporting construction, 3—temperature sensor, 4—pyranometer, 5—light source, V1—Fluke 289, V2—The LCR-8110G Series LCR meter, RC—shunt resistor, RL—adjustable load. ( b ) Special measuring holders inside the climate chamber to hold silicon samples. ( c ) Discovery DY600C climate chamber [ 90 ].

The resulting capacitance and conductance measurements allowed us to determine the position values of the additional energy levels in the band gap. Two methods were used for this purpose. The first is the Thermal Admittance Spectroscopy (TAS) method, by which it was possible to determine the e t ( T p ) rate that determines the thermal emission, followed by the Arrhenius curves. By using the Arrhenius equation, it was possible to determine the activation energies of the deep energy levels by approximating the experimental data with a linear function [ 86 ]. An example of the results obtained by the TAS method is shown in Figure 20 a.

The Arrhenius law approximation ranges for silicon implanted with neon Ne + ions of energy E = 100 keV ( a ) P-type silicon doped with boron, ρ = 0.4 Ω·cm, D = 2.2 × 10 14 cm −2 , Δ E = 0.46 eV. ( b ) N-type silicon doped with phosphorus, ρ = 10 Ω·cm, D = 4.0 × 10 14 cm −2 , Δ E = 0.23 eV [ 86 , 87 ].

Another method of determining the activation energy is the approximation of selected parts of the course C p = f(1000/ T p ) with the function of the equation ln(y) = Ax + B, where C p is the unit capacitance of the tested sample, and T p is the temperature of the sample during the measurements performed at the frequency of the measuring signal f = 100 kHz. This in turn allowed the calculation of the conduction activation energy Δ E , which determines the depth of the additional intermediate energy level [ 87 ]. An example of the results obtained by the Arrhenius curve approximation method is shown in Figure 20 b.

On the basis of the conducted research, it was possible to identify radiation defects that create additional energy levels in the silicon band gap, with corresponding activation energies, where the results are shown in Table 1 . Our research proved that the implantation of Ne+ ions results in generating radiation defects in the crystal lattice of silicon as a photovoltaic cell base material and enables the generation of intermediate levels of energy in the band gap, improving the efficiency of photovoltaic cells made on its basis.

Determination of intermediate energy levels for boron and phosphorus doped silicon samples implanted with Ne + ions and energy E = 100 keV, isochronically annealed at 598 K [ 86 , 87 ].

4. Conclusions

Solar energy is one of the most demanding renewable sources of electricity. Electricity production using photovoltaic technology not only helps meet the growing demand for energy, but also contributes to mitigating global climate change by reducing dependence on fossil fuels. The level of competitiveness of innovative next-generation solar cells is increasing due to the efforts of researchers and scientists related to the development of new materials, particularly nanomaterials and nanotechnology.

It is noted that the solar cell market is dominated by monocrystalline silicon cells due to their high efficiency. About two decades ago, the efficiency of crystalline silicon photovoltaic cells reached the 25% threshold at the laboratory scale. Despite technological advances since then, peak efficiency has now increased very slightly to 26.6%. As the efficiency of crystalline silicon technology approaches the saturation curve, researchers around the world are exploring alternative materials and manufacturing processes to further increase this efficiency. Polycrystalline and amorphous thin film silicon cells are seen as a serious competitor to monocrystalline silicon cells. However, their disadvantage is their disordered nature which results in low efficiency.

In this paper is a comprehensive overview of various PV technologies that are currently available or will be available in the near future on a commercial scale. A comparative analysis in terms of efficiency and the technological processes used is presented. Over the past few decades, many new materials have emerged that provide an efficient source of power generation to meet future demands while being cost-effective. This paper is a comprehensive study covering the generations of photovoltaic cells and the properties that characterize these cells. Photovoltaic cell materials of different generations have been compared based on their fabrication methods, properties, and photoelectric conversion efficiency.

First-generation solar cells are conventional and based on silicon wafers. The second generation of solar cells involves thin film technologies. The third generation of solar cells includes new technologies, including solar cells made of organic materials, cells made of perovskites, dye-sensitized cells, quantum dot cells, or multi-junction cells. With advances in technology, the drawbacks of previous generations have been eliminated in fourth-generation graphene-based solar cells. The popularity of photovoltaics depends on three aspects—cost, raw material availability, and efficiency. Third-generation solar cells are the latest and most promising technology in photovoltaics. Research on these is still in progress. This review pays special attention to the new generation of solar cells: multi-junction cells and photovoltaic cells with an additional intermediate band.

Recent advances in multi-junction solar cells based on n-type silicon and functional nanomaterials such as graphene offer a promising alternative to low-cost, high-efficiency cells. Currently, multi-junction cells, which benefit from advances enabled by nanotechnology, are breaking efficiency records. They are still quite expensive and represent a complex system, but there are simpler alternatives that may eventually provide a path to the competitiveness of the highest efficiency devices. Another significant advance is being made in the generation of additional energy levels in the band structure of silicon. In both cases, more research evidence, policies, and technology are needed to make them accessible. Therefore, it remains crucial to develop silicon-based technologies. The use of these new solar cell architectures would provide a new direction toward achieving commercial goals. Multi-junction based solar cells and new photovoltaic cells with an additional intermediate energy level are expected to provide extremely high efficiency. The research in this case focuses on a low-cost manufacturing process. Therefore, commercialization of these cells requires further work and exploration.

Nanotechnology and newly developed multifunctional nanomaterials can help overcome current performance barriers and significantly improve solar energy generation and conversion through photovoltaic techniques. Many physical phenomena have been identified at the nanoscale that can improve solar energy generation and conversion. However, the challenges associated with these technologies continue to be an issue when they are incorporated into PV manufacturing. Thanks to initial successes in recent years, nanomaterials are one of the most promising energy technologies of the future and are expected to significantly reform the future energy market. Carbon nanoparticles and their allotropic forms, such as graphene, are expected to offer high efficiency compared to conventional silicon cells in the near future and thus contribute to new prospects for the solar energy market.

Funding Statement

This research was funded by the Lublin University of Technology, grant number FD-20/EE-2/708.

Author Contributions

P.W. proposed a study on photovoltaic cell generations and current research directions for their development and guided the work. J.P. conducted a literature review and wrote the paper. J.P. and P.W. described further prospects and research directions and outlined conclusions based on the collected literature. P.W. reviewed and edited the work. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Informed consent statement, data availability statement, conflicts of interest.

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Dye-Sensitized Solar Cells: Fundamentals and Current Status

• Nano Review
• Open access
• Published: 28 November 2018
• Volume 13 , article number  381 , ( 2018 )

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• Khushboo Sharma 1 ,
• Vinay Sharma 2 &
• S. S. Sharma 3  

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Dye-sensitized solar cells (DSSCs) belong to the group of thin-film solar cells which have been under extensive research for more than two decades due to their low cost, simple preparation methodology, low toxicity and ease of production. Still, there is lot of scope for the replacement of current DSSC materials due to their high cost, less abundance, and long-term stability. The efficiency of existing DSSCs reaches up to 12%, using Ru(II) dyes by optimizing material and structural properties which is still less than the efficiency offered by first- and second-generation solar cells, i.e., other thin-film solar cells and Si-based solar cells which offer ~ 20–30% efficiency. This article provides an in-depth review on DSSC construction, operating principle, key problems (low efficiency, low scalability, and low stability), prospective efficient materials, and finally a brief insight to commercialization.

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Introduction

Dye-sensitized solar cells (DSSCs) have arisen as a technically and economically credible alternative to the p-n junction photovoltaic devices. In the late 1960s, it was discovered that electricity can be generated through illuminated organic dyes in electrochemical cells. At the University of California at Berkeley, chlorophyll was extracted from spinach (photosynthesis). First chlorophyll-sensitized zinc oxide (ZnO) electrode was synthesized in 1972. For the first time, through electron injection of excited dye molecules into a wide band gap of semiconductor, photons were converted into electricity [ 1 ]. A lot of research has been done on ZnO-single crystals [ 2 ], but the efficiency of these dye-sensitized solar cells was very poor, as the monolayer of dye molecules was able to absorb incident light only up to 1%. Thus, the efficiency was improved by optimizing the porosity of the electrode made up of fine oxide powder, so that the absorption of dye over electrode could be enhanced and as a result light harvesting efficiency (LHE) could also be enhanced. As a result, nanoporous titanium dioxide (TiO 2 ) electrodes with a roughness factor of ca.1000 were discovered, and in 1991, DSSCs with 7% efficiency were invented [ 3 ]. These cells, also known as Grätzel cells, were originally co-invented in 1988 by Brian O’Regan and Michael Grätzel at UC Berkeley [ 3 ] and were further developed by the aforementioned scientists at Ecole Polytechnique Fédèrale de Lausanne (EPFL) till 1991.

Brian O’Regan and Michael Grätzel fabricated a device based on a 10-μm-thick, high surface area and optically transparent film of TiO 2 nanoparticles, coated with a monolayer of a charge transfer dye with ideal spectral characteristics to sensitize the film for light harvesting. The device harvested a high proportion of the incident solar energy flux of 46% and showed exceptionally high efficiencies, even more than 80% efficiencies for the conversion of incident photons to electrical current. The overall incident photon to current conversion efficiency (IPCE) yield was 7.1–7.9% in simulated solar light and 12% in diffuse daylight. A large short circuit current density J SC (greater than 12 mAcm − 2 ) and exceptional stability (sustaining at least five million turnovers without decomposition) and low cost made the practical application feasible [ 3 ]. In 1993, Grätzel et al. reported 9.6% efficiency of cells, and then in 1997, they achieved 10% at the National Renewable Energy Laboratory (NREL). The sensitizers are usually designed to have functional groups such as –COOH, –PO 3 H 2 , and –B(OH) 2 for stable adsorption onto the semiconductor substrate [ 4 , 5 ]. Recently in 2018, an efficiency of 8.75% was reported for hybrid dye-titania nanoparticle-based DSSC for superior low temperature by Costa et al. [ 6 ]. In a traditional solar cell, Si provides two functions: acts as source of photoelectrons and provides electric field to separate the charges and create a current. But, in DSSCs, the bulk of semiconductor is only used as a charge transporter and the photoelectrons are provided by photosensitive dyes. The theoretically predicted power conversion efficiency (PCE) of DSSCs was approximately 20% [ 7 , 8 ]; thus, an extensive research has been made over the years on DSSCs to improve the efficiency and to augment its commercialization. However, in the last few decades, a lot of experiments were carried out to improve the performance of DSSCs. For instance, if one goes through the review articles or papers published around 1920 and 1921, a remarkable difference may be observed in the performance as well as fabrication of these cells. Few review papers are discussed below with the objective and main results shown in a respective article to get an idea how the performance of these cells has been improved and, thus, how the DSSCs became a hot topic for researchers.

Anandan reviewed the improvements and arising challenges in dye-sensitized solar cells till 2007 [ 9 ]. The main components of his review study were light harvesting inorganic dye molecules, p-CuO nanorod counter electrodes, and self-organization of electroactive polymers, and he showed how these materials perform in a rationally designed solar cell. However, the maximum IPCE of 7% was discussed in the review paper for naphthyridine coordinated Ru complex [ 10 ] which was good till 2007 but is almost half to the efficiencies shown in later work.

The main emphasis of the review paper published by Bose et al. [ 11 ] was the current state and developments in the field of photoelectrode, photosensitizer, and electrolyte for DSSCs till 2015. They have included an interesting study of comparing the performance of the DSSC module with that of the Si-based module by the graph shown in Fig.  1 [ 12 ] and concluded that the performance of the DSSC module is far better than that of the Si module. Also, the highest efficiency discussed in this review paper was 11.2% for N719 dye-based DSSC.

The performance of dye PV modules increases with temperature, contrary to Si-based modules [(Web reference [available online at http://www.sta.com.au/downloads/DSC%20Booklet.pdf ] [ 11 , 12 ]

Shalini et al. [ 13 ] emphasized on sensitizers, including ruthenium complexes, metal-free organic dyes, quantum-dot sensitizer, perovskite-based sensitizer, mordant dyes, and natural dyes. However, this article provides a great knowledge about the different types of sensitizers, but lacks the information regarding other important components of the DSSCs. Again, apart from discussing all different components of DSSCs, the review article by Jihuai Wu et al. [ 14 ] was concentrated over the counter electrode part. They have discussed the study of different types of counter electrodes based on transparency and flexibility, metals and alloys, carbon materials, conductive polymers, transition metal compounds, and hybrids. A highest efficiency of 14.3% was discussed for the DSSC fabricated with Au/GNP as a counter electrode, Co 3+/2+ as a redox couple, and LEG4 + ADEKA-1 as a sensitizer [ 15 ] and was shown in the review article. Similarly, Yeoh et al. and Fan et al. [ 16 , 17 ] have given a brief review over the photoanode of DSSC. They have classified modification of photoanode into three categories, namely interfacial modification through the introduction of blocking and scattering layer, compositing, doping with non-metallic anions and metallic cations, interfacial engineering, and replacing the conventional mesoporous semiconducting metal oxide films like with 1-D or 2-D nanostructures.

Thus, by comparing different review articles published earlier, it can be easily seen that the present review article “Dye Sensitized solar Cells: Fundamentals and Current Status” gives the in-depth study of different components and their application in DSSCs as well as construction and working of these cells.

Construction and Working of DSSCs

The working electrode, sensitizer (dye), redox-mediator (electrolyte), and counter electrode are four key parameters for a DSSC. DSSC is an assembly of working electrode soaked with a sensitizer or a dye and sealed to the counter electrode soaked with a thin layer of electrolyte with the help of a hot melt tape to prevent the leakage of the electrolyte (as shown in Fig.  2 ). The components as well as the construction and working of DSSCs are shown below:

Construction and working principle of the dye-sensitized nanocrystalline solar cells

Transparent and Conductive Substrate

DSSCs are typically constructed with two sheets of conductive transparent materials, which help a substrate for the deposition of the semiconductor and catalyst, acting also as current collectors [ 18 , 19 ] There are two main characteristics of a substrate being used in a DSSC: Firstly, more than 80% of transparency is required by the substrate to permit the passage of optimum sunlight to the effective area of the cell. Secondly, for the efficient charge transfer and reduced energy loss in DSSCs, it should have a high electrical conductivity. The fluorine-doped tin oxide (FTO, SnO 2 : F) and indium-doped tin oxide (ITO, In 2 O 3 : Sn) are usually applied as a conductive substrate in DSSCs. These substrates consist of soda lime glass coated with the layers of indium-doped tin oxide and fluorine-doped tin oxide. The ITO films have a transmittance > 80% and 18 Ω/cm 2 of sheet resistance, while FTO films show a lower transmittance of ~ 75% in the visible region and sheet resistance of 8.5 Ω/cm 2 [ 18 ].

Working Electrode (WE)

The working electrodes (WE) are prepared by depositing a thin layer of oxide semiconducting materials such as TiO 2 , Nb 2 O 5 , ZnO, SnO 2 (n-type), and NiO (p-type) on a transparent conducting glass plate made of FTO or ITO. These oxides have a wide energy band gap of 3–3.2 eV. The application of an anatase allotropic form of TiO 2 is more commendable in DSSCs as compared to a rutile form due to its higher energy band gap of 3.2 eV whereas the rutile form has a band gap of about 3 eV [ 20 , 21 ], although alternative wide band gap oxides such as ZnO and Nb 2 O 5 have also given promising results [ 22 , 23 ]. Due to being non-toxic and less expensive and its easy availability, TiO 2 is mostly used as a semiconducting layer. However, these semiconducting layers absorb only a small fraction of light in the UV region; hence, these working electrodes are then immersed in a mixture of a photosensitive molecular sensitizer and a solvent. After soaking the film within the dye solution, the dye gets covalently bonded to the TiO 2 surface. Due to the highly porous structure and the large surface area of the electrode, a high number of dye molecules get attached on the nanocrystalline TiO 2 surface, and thus, light absorption at the semiconductor surface increases.

Photosensitizer or Dye

Dye is the component of DSSC responsible for the maximum absorption of the incident light. Any material being dye should have the following photophysical and electrochemical properties:

Firstly, the dye should be luminescent.

Secondly, the absorption spectra of the dye should cover ultraviolet-visible (UV-vis) and near-infrared region (NIR) regions.

The highest occupied molecular orbital (HOMO) should be located far from the surface of the conduction band of TiO 2 and the lowest unoccupied molecular orbital (LUMO) should be placed as close to the surface of the TiO 2 , and subsequently should be higher with respect to the TiO 2 conduction band potential.

HOMO should lie lower than that of redox electrolytes.

The periphery of the dye should be hydrophobic to enhance the long-term stability of cells, as it results in minimized direct contact between electrolyte and anode; otherwise, water-induced distortion of the dye from the TiO 2 surface can appear which may reduce the stability of cells.

To avoid the aggregation of the dye over the TiO 2 surface, co-absorbents like chenodeoxycholic acid (CDCA) and anchoring groups like alkoxy-silyl [ 24 ], phosphoric acid [ 25 ], and carboxylic acid group [ 26 , 27 ] were inserted between the dye and TiO 2 . This results in the prevention of dye aggregation and thus limits the recombination reaction [ 28 ] between redox electrolyte and electrons in the TiO 2 nanolayer as well as results in the formation of stable linkage.

Electrolyte

An electrolyte (such as I − /I − 3 , Br − /Br −   2 [ 29 ], SCN − /SCN 2 [ 30 ], and Co(II)/Co(III) [ 31 ]) has five main components, i.e., redox couple, solvent, additives, ionic liquids, and cations. The following properties should be present in an electrolyte:

Redox couple should be able to regenerate the oxidized dye efficiently.

Should have long-term chemical, thermal, and electrochemical stability.

Should be non-corrosive with DSSC components.

Should be able to permit fast diffusion of charge carriers, enhance conductivity, and create effective contact between the working and counter electrodes.

Absorption spectra of an electrolyte should not overlap with the absorption spectra of a dye.

I − /I −   3 has been demonstrated as a highly efficient electrolyte [ 32 ], but there are certain limitations associated with its application in DSSCs. I − /I −   3 electrolyte corrodes glass/TiO 2 /Pt; it is highly volatile and responsible for photodegradation and dye desorption and has poor long-term stability [ 33 , 34 ]. Acetonitrile (ACN), N -methylpyrrolidine (NMP), and solvent mixtures, such as ACN/valeronitrile, have been used as a solvent having high dielectric constants. 4-Tert-butylpyridine (TBP) is mostly used as an additive to shift the conduction band of TiO 2 upwards, which results in an increase in the value of open circuit voltage ( V OC ), reduced cell photocurrent ( J SC ), and less injection driving force. It is believed that TBP on a TiO 2 surface reduces recombination through back transfer to an electrolyte [ 35 ]. However, the biggest drawback allied with the ionic liquid is their leakage factor. Thus, solid-state electrolytes are developed to avoid the drawbacks associated with ionic liquid (IL) electrolytes [ 36 ]. Also, to test the failure of the redox electrolyte or the sealing under long-term illumination, long-term light soaking tests on sealed cells have also progressed significantly over the years [ 37 ].

Counter Electrode (CE)

CE in DSSCs are mostly prepared by using platinum (Pt) or carbon (C). Both working and counter electrodes are sealed together, and subsequently, an electrolyte is filled with a help of a syringe. Counter electrode catalyzes the reduction of I − /I −   3 liquid electrolyte and collects holes from the hole transport materials (HTMs). Pt is used mostly as a counter electrode as it demonstrates higher efficiencies [ 38 ], but the replacement of Pt was much needed due to its higher cost and less abundance. Thus, several alternatives have developed to replace Pt in DSSCs, such as carbon [ 39 ], carbonylsulfide (CoS) [ 40 ], Au/GNP [ 15 ], alloy CEs like FeSe [ 41 ], and CoNi 0.25 [ 42 ], although the different types of the CEs are also discussed by Jihuai Wu et al. [ 14 ].

Working Principle

The working principle of DSSC involves four basic steps: light absorption, electron injection, transportation of carrier, and collection of current. The following steps are involved in the conversion of photons into current (as shown in Fig.  2 ):

Firstly, the incident light (photon) is absorbed by a photosensitizer, and thus, due to the photon absorption, electrons get promoted from the ground state (S + /S) to the excited state (S + /S*) of the dye, where the absorption for most of the dye is in the range of 700 nm which corresponds to the photon energy almost about 1.72 eV.

Now, the excited electrons with a lifetime of nanosecond range are injected into the conduction band of nanoporous TiO 2 electrode which lies below the excited state of the dye, where the TiO 2 absorbs a small fraction of the solar photons from the UV region [ 43 ]. As a result, the dye gets oxidized.

These injected electrons are transported between TiO 2 nanoparticles and diffuse towards the back contact (transparent conducting oxide [TCO]). Through the external circuit, electrons reach at the counter electrode.

The electrons at the counter electrode reduce I −   3 to I − ; thus, dye regeneration or the regeneration of the ground state of the dye takes place due to the acceptance of electrons from I − ion redox mediator, and I − gets oxidized to I −   3 (oxidized state).

Again, the oxidized mediator (I −   3 ) diffuses towards the counter electrode and reduces to I ion.

Evaluation of Dye-Sensitized Solar Cell Performance

The performance of a dye-sensitized solar cell can be evaluated by using incident photon to current conversion efficiency (IPCE, %), short circuit current ( J SC , mAcm − 2 ), open circuit voltage ( V OC , V ), maximum power output [ P max ], overall efficiency [ η , %], and fill factor [FF] (as shown in Fig.  3 ) at a constant light level exposure as shown in Eq.  1 [ 44 ].

I–V curve to evaluate the cells performance

The current produces when negative and positive electrodes of the cell are short circuited at a zero mV voltage. V OC ( V ) is the voltage across negative and positive electrodes under open circuit condition at zero milliampere (mA) current or simply, the potential difference between the conduction band energy of semiconducting material and the redox potential of electrolyte. P max is the maximum efficiency of the DSSC to convert sunlight into electricity. The ratio of maximum power output ( J mp  ×  V mp ) to the product ( V OC  ×  J SC ) gives FF.

Also, the overall efficiency (%) is the percentage of the solar energy (shining on a photovoltaic [PV] device) converting into electrical energy, where ɳ increases with the decrease in the value of J SC and increase in the values of V OC , FF, and molar coefficient of dye, respectively.

External quantum efficiency (also known as IPCE) is the ratio of number of electrons flowing through the external circuit to the number of photons incident on the cells surface at any wavelength λ . It is given as follows:

IPCE values are also related to LHE, φE1, and ηEC. As shown in Eq. 3 [ 45 ],

where LHE is the light harvesting efficiency, φE1 is electron injection quantum efficiency, and ηEC is the efficiency of collecting electrons in the external circuit.

Limitations of the Devices

In the recent years, comparable efficiencies have been demonstrated for the DSSCs, but still they need a further modification due to some of the limitations associated with these cells. In terms of limitations, stability failure can be characterized in two different classes: (i) limitation towards extrinsic stability and (ii) limitation towards intrinsic stability. Also, a huge amount of loss in energy of oxidized dye takes place during the process of regeneration, due to the energy mismatch between the oxidized dye and an electrolyte. Thus, in the queue to enhance the efficacy of these cells, different electrolytes have been developed. Grätzel et al. showed over 900-mV open circuit voltages and short circuit currents I SC up to 5.1 mA by blending the hole conductor matrix with a combination of TBP and Li[CF 3 SO 2 ] 2 N, yielding an overall efficiency of 2.56% at air mass 1.5 (AM 1.5) illuminations [ 46 ]. Also, the sheet resistance of FTO glass sheet is about 10 Ω/sq.; thus, this makes scaling of the device difficult and acts as a limiting factor for an active cell area > 1 cm 2 . Therefore, to increase the sheet resistance as well as to maintain spacing between working and counter electrode, the short circuiting of the solar cell is required or either the spacing should be increased by 25 to 50 μm [ 47 ] in small modules (where these modules consist of small stripes of an active cell area (1 cm 2 ) with adjacent silver lines [ 47 , 48 ]). As a consequence, a drop in the IPCE value from 10.4 to 6.3% for a 1-cm 2 cell was observed for a submodule of 26.5 cm 2 [ 49 ].

To upscale the cell performance, silver fingers can be used to collect the current and using a sealant material like hotmelt tape, for the protection from the leakage of the electrolytes. Although due to the chemically aggressive nature of the electrolyte, the use of silver fingers is less feasible. And due to the small modules, the chances of leakage increase which results in reduced active cell area by 32% [ 47 ]. Another factor is the conductivity of the glass sheet that affects the performance of the DSSCs. Therefore, the conductivity of the transparent conducting oxide (TCO) can be improved by combining the indium-doped tin oxide (ITO, highly conductive but less chemically stable) and fluorine-doped tin oxide (FTO, highly chemically stable but less conductive) together. This results in the reduction of the sheet resistance of TCO glass to 1.3 Ω/sq. [ 50 ].

Limitation Towards the Stability of the Devices

The DSSCs need to be stable extrinsically as well as intrinsically as to be comparable to that of Si-solar cells, so that they can fulfill market needs, and thus, their commercialization can be increased. The limitations towards the stability are discussed below:

Limitation Towards Extrinsic Stability (Stability of Sealant Material)

Sealant materials like Surlyn® and Bynel® hotmelt foils are used in DSSCs to seal the cells [ 48 ]. Their sealing capability decreases when the pressure builds up inside the cell [ 51 ] and also if exposed within a cyclic or regular temperature variation [ 52 ]. But due to their low cost and easy processing, their utilization cannot be neglected. Thus, it is required to increase their adhesion with glass by pretreatment of the glass with metal oxide particles. As an alternative, sealants based on low melting glass frits [ 53 ] were also developed which offer more stability than the hotmelt foils, but these sealants are not suitable for the large area module production.

Limitation Towards Intrinsic Stability

To examine the intrinsic stability of the cell, accelerated aging experiments were performed. These accelerated aging experiments lasts for 1000 h to show the thermal stability of the dye, electrolyte, and Pt-counter electrode at 80 °C of temperature. Through these experiments, it was found that small test cells can maintain 90% of the initial efficiency under elevated temperatures and the observed initial efficiencies were 7.65% [ 54 ] and 8% [ 55 ], respectively. Also, under AM 1.5 and 55–60 °C moderate temperatures, the device was stable for 1000 h. But when both the stress factors, i.e., temperature about 80 °C and light soaking, were combined, a rapid degradation in the performance of the cell was observed [ 52 ]. Therefore, improvement in the intrinsic stability of the cell is required as 80 °C temperature can be easily attained during sunny days.

Different Ways to Augment the Efficiency of DSSCs

To enhance the efficiency as well as the stability of the DSSCs, researchers have to focus on fundamental fabrication methods and materials, as well working of these cells. Different ways to improve the efficiency of these solar cells (SCs) are discussed below:

To increase the efficiency of DSSCs, the oxidized dye must be firmly reduced to its original ground state after electron injection. In other words, the regeneration process (which occurs in the nanosecond range [ 56 ]) should be fast as compared to the process of oxidation of dye [the process of recombination (0.1 to 30 μs)]. As the redox mediator potential (I − ion) strongly effects the maximum photovoltage, thus the potential of the redox couple should be close to the ground state of the dye. To carry out this viable repeated process, about 210 mV driving force is required (or ca. 0.6 V [ 56 ]).

By increasing the porosity of the TiO 2 nanoparticles, the maximum dye absorption takes place at WE.

Reducing or prohibiting the formation of the dark current by depositing a uniform thin layer or under layer of the TiO 2 nanoparticles over the conduction glass plate. Thus, the electrolyte does not have a direct contact with the FTO or back contact and hence not reduced by the collector electrons, which restricts the formation of the dark current.

Preventing the trapping of nanoporous TiO 2 nanoparticles by TBP molecules or by an electrolyte solvent. Thus, uniform sensitization of the WE by a sensitizer is required. If the entire surface of the nanoporous TiO 2 electrode is not uniformly covered by the sensitizer, then the naked spots of nanoporous TiO 2 can be captured by TBP molecules or by an electrolyte solvent.

Co-sensitization is another way to optimize the performance of DSSC. In co-sensitization, two or more sensitizing dyes with different absorption spectrum ranges are mixed together. to broaden the spectrum response range [ 57 ].

By promoting the use of different materials in the manufacture of electrodes like nanotubes, nanowires of carbon, graphene; using varied electrolytes instead of a liquid one like gel electrolyte and quasi-solid electrolytes; providing different pre-post treatments to the working electrode like anodization pre-treatment and TiCl 4 treatment; using different types of CEs [ 14 ] and by developing hydrophobic sensitizers, the performance as well as the efficiency of these cells can be tremendously improved.

By inserting phosphorescence or luminescent chromophores, such as applying rare-earth doped oxides into the DSSC [ 58 , 59 , 60 ], coating a luminescent layer on the glass of the photoanode [ 60 , 61 , 62 ], i.e., using plasmonic phenomenon [ 63 ] and adding energy relay dyes (ERDs) to the electrolyte [ 57 , 64 , 65 ].

Previous and Further Improvements in DSSCs

To fabricate low cost, more flexible, and stable DSSCs with higher efficiencies, new materials that are light weight, thin, low cost, and easy to synthesize are required. Thus, previous as well as further improvement in the field of DSSCs is included in this section. This section gives a brief account on the work done by the different researchers in the last 10–12 years and the results they observed for respective cells.

Working and Counter Electrodes

Grätzel and co-workers showed drastic improvements in the performance of DSSCs. They demonstrated efficiency of 7–10% under AM 1.5 irradiation using nanocrystalline (nc) TiO 2 thin-film electrode with nanoporous structure and large surface area, and used a novel Ru bipyridyl complex as a sensitizer and an ionic redox electrolyte at EPFL [ 3 , 26 ]. The conduction band level of TiO 2 electrode and the redox potential of I − /I −   3 as − 0.7 V versus saturated calomel electrode (SCE) and 0.2 V versus SCE has been evaluated [ 66 , 67 ]. A binary oxide photoelectrode with coffee as a natural dye was demonstrated, in 2014 [ 68 ]. SnO 2 ( x )–ZnO (1 −  x ) binary system with two different SnO 2 composition ( x  = 3, 5 mol%) were prepared by solid-state reaction at high temperature and employed as a photoanode. An improved efficiency was demonstrated for the larger SnO 2 composition and an overall power conversion efficiency (PCE) observed for SnO 2 : ZnO device was increased from 0.18% (3:97 mol%) to 0.26% for a device with SnO 2 :ZnO (5:95 mol%) photoanode. Hu et al. observed that the performance of the DSSCs with graphite-P25 composites as photoanodes has been significantly enhanced by 30% improvement of conversion efficiency compared with P25 alone. They found an enhancement in the value of J SC from 9.03 to 12.59 mA/cm 2 under the condition of 0.01 wt% graphite amount and attained the conversion efficiency of 5.76% [ 69 ]. Figure  4 shows the SEM images of the photoanodes. Apart from TiO 2 , carbon and its different allotropes are also widely applied in DSSCs to fulfill future demand and arisen as a perfect surrogate material for DSSCs. Some reports have shown that incorporating carbon nanotube (CNT) in TiO 2 by hydrothermal or sol–gel methods greatly improved the cell’s performance [ 70 , 71 , 72 ]. Also, by improving the interconnectivity between the TiO 2 and CNT, an increase in the IPCE can be found [ 70 ]. Sun et al. reported that the DSSCs incorporating graphene in TiO 2 photoanode showed a PCE of 4.28%, which was 59% higher than that without graphene [ 73 ]. Sharma et al. has shown the improvement in the PCE value from 7.35 to 8.15% of the co-sensitized solar cell using modified TiO 2 (G-TiO 2 ) photoanode, instead of pure TiO 2 photoanode [ 74 ]. In 2014, it was shown that the electronically and catalytically functional carbon cloth works as a permeable and flexible counter electrode for DSSC [ 75 ]. The researchers have found that the TiN nanotube arrays and TiN nanoparticles supported on carbon nanotubes showed high electrocatalytic activity for the reduction of triiodide ions in DSSCs [ 76 , 77 ]. Single-crystal CoSe 2 nanorods were applied as an efficient electrocatalyst for DSSCs by Sun et al. in 2014 [ 78 ]. They prepared single-crystal CoSe 2 nanorods with a facile one step hydrothermal method. By drop-casting the CoSe 2 nanorod suspension onto conductive substrates followed by simple drying without sintering, they fabricated the thin CoSe 2 films and used as a highly efficient electrocatalyst for the reduction of I −   3 . They showed a power conversion efficiency of 10.20% under AM1.5G one-sun illumination for DSSCs with the standard N719 dye. Park et al. prepared a mesoporous TiO 2 Bragg stack templated by graft copolymer for dye-sensitized solar cells [ 79 ]. To enhance dye loading, electron transport, light harvesting and electrolyte pore-infiltration in DSSCs, they prepared organized mesoporous TiO 2 Bragg stacks (om-TiO 2 BS) consisting of alternating high and low refractive index organized mesoporous TiO 2 (om-TiO 2 ) films. They synthesized om-TiO 2 films through sol-gel reaction using amphiphilic graft copolymers consisting of poly(vinyl chloride) backbones and poly(oxyethylene methacrylate) side chains, i.e., PVC- g -POEM as templates. They showed that a polymerized ionic liquid (PIL)-based DSSC fabricated with a 1.2-μm-thick om-TiO 2 BS-based photoanode exhibited an efficiency of 4.3%, which was much higher than that of conventional DSSCs with a nanocrystalline TiO 2 layer (nc-TiO 2 layer) with an efficiency of 1.7%. An excellent efficiency of 7.5% was demonstrated for a polymerized ionic liquid (PIL)-based DSSC with a heterostructured photoanode consisting of 400-nm-thick organized mesoporous TiO 2 interfacial (om-TiO 2 IF) layer, 7-μm-thick nc-TiO 2 , and 1.2-μm-thick om-TiO 2 BS as the bottom, middle, and top layers, respectively, which was again much higher than that of nanocrystalline TiO 2 photoanode with an efficiency of 3.5%. Lee et al. reported platinum-free, low-cost, and flexible DSSCs using graphene film coated with a conducting polymer as a counter electrode [ 80 ]. In 2014, Banerjee et al. demonstrated nickel cobalt sulfide nanoneedle-array as an effective alternative to Pt as a counter electrode in dye-sensitized solar cells [ 81 ].

SEM images of a P25 film, b 1 wt% graphite-P25 composite film, and ( c ) the cross section of P25 film on FTO [ 69 ]

Calogero et al. invented a transparent and low-cost counter electrode based on platinum nanoparticles prepared by a bottom-up synthetic approach. They demonstrated that with such a type of cathode, the observed solar energy conversion efficiency was the same as that obtained for a platinum-sputtered counter electrode and even was more than 50% obtained with a standard electrode, i.e., one prepared by chlorine platinum acid thermal decomposition, in similar working condition [ 82 ]. By using a special back-reflecting layer of silver, they improved upon the performance of a counter electrode based on platinum sputtering and achieved an overall η of 4.75% under 100 mWcm − 2 (AM 1.5) of simulated sunlight. They showed that, for the optical transmittance at different wavelengths of platinum-based films, i.e., Pt nanoparticles, Pt thermal decomposition, and Pt sputtered deposited onto FTO glass, the platinum nanoparticle-based cathode electrode (CE) prepared by Pt sputtering deposition method appeared more transparent than the platinum CE prepared using the Pt acid thermal decomposition method. Meanwhile, when Pt nanoparticle deposition method was employed, the transmittance was very poor (as shown in Fig.  5 ). Anothumakkool et al. showed a highly conducting 1-D aligned polyethylenedioxythiophene (PEDOT) along the inner and outer surfaces of a hollow carbon nanofiber (CNF), as a counter electrode in a DSSC to enhance the electrocatalytic activity of the cell [ 83 ]. They showed that the hybrid material (CP-25) displayed a conversion efficiency of 7.16% compared to 7.30% for the standard Pt counter electrode, 4.48% for bulk PEDOT and 5.56% for CNF, respectively. The enhanced conversion efficiency of CP-25 was accredited to the accomplishment of high conductivity and surface area of PEDOT through the 1-D alignment compared to its bulk counterpart. Further, through a long-term stability test involving efficiency profiling for 20 days, it was observed that CP-25 exhibited extraordinary durability compared to the bulk PEDOT. Recently, Huang et al. improved the performance of the device by inserting a H 3 PW 12 O 40 layer between the transparent conductive oxide layer and the compact TiO 2 layer [ 84 ]. They observed the reduction in the recombination of the electrons upon the addition of H 3 PW 12 O 40 layer, resulting in longer electron lifetime and obtained a η  = 9.3%, respectively.

Optical transmittance of platinum-based films (Pt nanoparticles, Pt thermal decomposition, Pt sputtered) deposited onto FTO glass [ 82 ]

Li et al. reported that the transition metal nitrides MoN, WN, and Fe 2 N show Pt-like electrocatalytic activity for dye-sensitized solar cells, where MoN showed superior electrocatalytic activity and a higher PV performance [ 85 ]. Characteristic J–V curves of DSSCs using different metal nitrides and Pt counter electrodes showed that the cell fabricated with the MoN counter electrode achieved a FF = 0.66, which was higher than that of the Pt electrode (as shown in Fig.  6 ). However, J SC  = 11.55 mAcm − 2 was relatively high and the V OC of 0.735 V was almost same to the V OC  = 0.740 V offered by Pt electrode. In the case of WN, V OC and J SC were relatively low, indicating a low efficiency of 3.67%. DSSC with the Fe 2 N electrode attained lower values of V OC and FF, i.e., 0.535 V of V OC and 0.41 of FF, resulting in a poor η  = 2.65%. Thus, above data shows superior performance of MoN-based DSSC among all other metal nitrides as CE material. Gokhale et al. showed a laser-synthesized super-hydrophobic conducting carbon with broccoli-type morphology as a CE for dye-sensitized solar cells in 2012 [ 86 ]. In 2014, plasmonic light harvesting of dye-sensitized solar cells by Au nanoparticle-loaded TiO 2 nanofibers was demonstrated by Naphade et al. [ 87 ] because the surface morphology of a WE and a CE play a key role in the performance of DSSC. Usually, mesoporous TiO 2 nanoparticle films are used in WE fabrication because they provide large surface area for efficient dye adsorption. However, there are certain limitations associated with them as short electron diffusion length (10–35 μm) and random electrical pathway induced by the substantial trapping and detrapping phenomena that take place within excessive surface states, defects, and grain boundaries of nanoparticles [ 88 ] and disorganized stacking of TiO 2 films which limits the electron transport [ 89 ]. Thus, doping of metallic cations and non-metallic anions in TiO 2 , treating FTOs [ 90 ], applying 1-D nanostructures like nanowires, nanorods, nanosheets, nanoplates [ 16 ], and hollow spheres are approaches to modify the WE. However due to the low surface area, these 1-D nanostructures show poor dye loading. In 2015, Zhao et al. studied the influence of the incorporation of CNT-G-TiO 2 NPs into TiO 2 NT arrays and attained an efficiency of 6.17% for the DSSC based on CNT-G-TiO 2 nanoparticles/TiO 2 nanotube double-layer structure photoanode [ 91 ]. An efficiency of 8.30% was demonstrated by Qiu et al. for the DSSC based on double-layered anatase TiO 2 nanospindle photoanodes [ 92 ].

Characteristic J – V curves of DSSCs using different metal nitrides and Pt counter electrodes, measured under simulated sunlight at 100 mWcm − 2 (AM 1.5) [ 85 ]

Apart from NTs, bilayer TiO 2 hollow spheres/TiO 2 nanotube array-based DSSC also showed an effective efficiency of 6.90% [ 93 ]. Efficiency can also be improved by incorporating SnO 2 as a shell material on a photoanode [ 94 ]. The integration of SnO 2 as a shell material on ZnO nanoneedle arrays results in a larger surface area and reduced recombination rate [ 94 ], thus increasing the dye adsorption which plays a crucial role in the performance of a cell. Huang and co-workers synthesized mesoporous TiO 2 spheres of high crystallinity and large surface area and applied it as a WE in the device. An excellent efficiency of 10.3% was achieved for the DSSC-employed TiO 2 spheres with long-term stability due to the terrific dye-loading and light-scattering abilities as well as attenuated charge recombination. Further, the efficiency was improved by performing the TiCl 4 treatment [ 95 ].

Maheswari et al. reported various DSSCs employing zirconia-doped TiO 2 nanoparticle and nanowire composite photoanode film. They demonstrated highest η  = 9.93% for Zirconia/TNPW photoanode with a hafnium oxide (HfO 2 ) blocking layer and observed that the combination of zirconia-doped photoanode with blocking layer possibly restrains the recombination process and increases the PCE of the DSSCs effectively [ 96 ]. However, many ideas do not achieve a great efficiency initially but at least embed different ideas and aspects for the synthesis of new materials. For instance, by using carbon-coated stainless steel as a CE for DSSC, Shejale et al. demonstrated a η  = 1.98%, respectively [ 97 ]. Recently in 2018, a study was carried out to determine the effect of microwave exposure on photoanode and found an enhancement in the efficiency of the cell upon exposure. For the preparation of the DSSC, a LiI electrolyte, Pt cathode, TiO 2 photoanode, and Alizarin red as a natural sensitizer were used. An efficiency of 0.144% was found for the cell, where 10 min of microwave exposure was carried upon the photoanode [ 98 ].

Similarly, varied materials as mentioned earlier are synthesized as CE for efficient DSSCs. Last year, Guo et al. synthesized an In 2.77 S 4 @conductive carbon (In 2.77S4 @CC) hybrid CE via a two-step method and achieved η  = 8.71% for the DSSC with superior electrocatalytic activity for the reduction of triiodide and, also, comparable to the commercial Pt-based DSSC that showed PCE of 8.75%, respectively [ 99 ]. The doping of an organic acid, 1S-(+)-camphorsulfonic acid, with the conductive polymer poly(o-methoxyaniline) to form a hybrid (CSA/POMA) and its application in DSSCs as CE has been examined by Tsai et al. This CE showed increased surface roughness, decreased impedance, and increased crystallinity [ 100 ]. In 2017, Liu et al. fabricated DSSCs employing Co(bpy) 3 3+/2+ as the redox couple and carbon black (CB) as the CE [ 101 ]. The observation revealed superior electrocatalytic activity of a well-prepared CB film compared to that of conventional sputtered Pt. Due to the flexible nature of Cu foil substrates, Cu 2 O has also been employed as a CE in DSSC [ 102 ]. The fabrication of different samples by varying the sintering temperature of the CEs and obtaining the maximum efficiency of 3.62% at 600 °C of temperature has been reported [ 102 ]. Figure  7 shows the I – V characteristics and IPCE curves of DSSCs employing different Cu 2 O CEs. In 2013, by replacing the FTO with Mo as the conductor for the counter electrode, an increase in the value of FF as well as η was found [ 103 ]. The EIS Nyquist plots (as shown in Fig.  8 ) showed the difference in R s between the devices employed FTO (15.11 Ωcm 2 ) and Mo (7.25 Ωcm 2 ) due to the dissimilarity of the sheet resistance between FTO (8.2 Ω/sq) and Mo (0.16 Ω/sq). Also, by replacing FTO with Mo, a decrease in the R ct1 value from 6.87 to 3.14 Ωcm 2 was induced by the higher redox reactivity of Pt on Mo than that on FTO. In the queue of developing new materials, Maiaugree et al. fabricated DSSCs employing carbonized mangosteen peel (MPC) as a natural counter electrode with a mangosteen peel dye as a sensitizer [ 104 ]. They observed a typical mesoporous honeycomb-like carbon structure with a rough nanoscale surface in carbonized mangosteen peels and achieved the highest value of η  = 2.63%. By analyzing the Raman spectra (shown in Fig.  9 ), they found a broad D-peak (130.6 cm − 1 of FWHM) located at 1350 cm − 1 indicating the high disorder of sp3 carbon and a narrower G peak (68.8 cm − 1 of FWHM) at 1595 cm − 1 which correlated with a graphite oxide phase observed in 2008 [ 105 ]. Thus, it was concluded the graphite oxide from MPC was a highly ordered sp2 hexagonal carbon oxide network. Furthermore, I–V characteristics of DSSCs employing different WE and CE are summarized in Table  1 .

The a current density–voltage ( J – V ) and b incident monochromatic photon-to-current conversion efficiency (IPCE) curves of DSSCs using various Cu 2 O CE [ 102 ]

Nyquist plots of the Device_FTO and Device_Mo. The inset indicates an equivalent circuit model used for the devices [ 103 ]

Raman spectra of mangosteen peel carbon [ 104 ]

To improve and study the performance of DSSCs, different electrolytes like gel electrolytes, quasi-solid-state electrolytes, ionic liquid electrolytes etc. have been applied as mediators so far. However, a different trend to optimize the performance of the DSSCs has been initiated by adding the energy relay dyes to the electrolyte.

Liquid Electrolyte

The cells efficiency through liquid electrolyte can be augmented by introducing iodide/triiodide redox couple and high dielectric constant organic solvents like ACN, 3-methoxypropionitrile (MePN), propylene carbonate (PC), γ-butyrolactone (GBL), N -methyl-2-pyrrolidone (NMP), ethylene carbonate (EC), and counter ions of iodides, where solvents are the key component of a liquid electrolyte. On the basis of their stability, organic solvents can be sequenced as imidazolium < picolinium < alkylpyridinium. Among various characteristics of solvents like donor number, dielectric constants, and viscosity, the donor number shows manifest influence on the V OC and J SC of DSSCs. Adding the small amount of electric additives like N -methylbenzimidazole (NMBI), guanidinium thiocyanate (GuSCN), and TBP hugely improves the cell performance. Just like solvents, a coabsorbent also plays a key role in the functioning and performance of an electrolyte. The addition of coabsorbents in an electrolyte trims down the charge recombination of photoelectrons in the semiconductor with the redox shuttle of the electrolyte. Secondly, a coabsorbent may alter the band edge position of the TiO 2 -conduction band, thus resulting in an augmentation in the value of V OC of the cell. This suppresses the dye aggregation over the TiO 2 surface and results in long-term stability of the cell as well as increase in V OC . Although the best regeneration of the oxidized dye is observed for iodide/triiodide as a redox couple for a liquid electrolyte, its characteristic of severe corrosion for many sealing materials results in a poor long-term stability of the DSSC. Thus SCN − /SCN 2 , Br − /Br 2 , and SeCN − /SeCN 2 bipyridine cobalt (III/II) complexes are some of the other redox couples applied in DSSCs. The ionic liquids (IL) or room temperature ionic liquids (RTIL) are stand-in material for organic solvents in a liquid electrolyte. Despite the many advantages, i.e., negligible vapor pressure, low flammability, high electrical conductivity at room temperature (RT), and wide electrochemical window, they are less applicable in DSSCs. Because of their higher viscosity, restoration of oxidized dye restricts due to the lower transport speed of iodide/triioide in solvent-free IL electrolytes. Thus, the performance of the dye-sensitized solar cells can also be enhanced by modifying the TiO 2 dye interface, i.e., by reducing vapor pressure of the electrolyte’s solvent. In 2017, Puspitasari et al. investigated the effect of mixing dyes and solvent in electrolyte and thus fabricated various devices. They have used two types of gel electrolyte based on PEG that mixed with liquid electrolyte for analyzing the lifetime of DSSC. They also changed solvents as distilled water (type I) and ACN (type II) with the addition of concentration of KI and iodine, and achieved better efficiency for the electrolyte type II [ 106 ].

As low-viscous solution can cause leakage in the cell, thus, application of solidified electrolytes obtained by in situ polymerization of precursor solution containing monomer or oligomer and the iodide/iodine redox couple results in a completely filled quasi-solid-state electrolyte within the TiO 2 network with negligible vapor pressure [ 107 ]. Komiya et al. obtained initial efficiency of 8.1% by applying the aforementioned approach [ 107 ]. But still a question arises whether the polymer matrix will degrade under prolonged UV radiations or not. The effect on the addition of SiO 2 nanoparticles to solidify the solvent was also studied as to increase the cell efficiency [ 108 ], where only inorganic materials were applied in this technique. However, there are certain limitations associated with the addition of organic solvents within a liquid electrolyte, i.e., this leads hermetic sealing of the cell and the evaporation of solvents at higher temperature, and thus the cells do not uphold long-term stability. Therefore, more research was carried over the developments and implementation of gel, polymer, and solid-state electrolytes in the DSSCs with various approaches, such as the usages of the electrolytes containing p-type inorganic semiconductors [ 109 ], organic hole transporting materials (HTMs) [ 110 ], and polymer gelator (PG) [ 111 ]. Chen et al. fabricated a solid-state DSSC using PVB-SPE (polyvinyl butyral-quasi-solid polymeric electrolyte) as an electrolyte. They measured the efficiency approximately 5.46%, which was approximately 94% compared to that of corresponding liquid-state devices, and the lifetime observed for the devices was over 3000 h [ 112 ]. Recently, a study explained the stability of the current characteristics of DSSCs in the second quadrant of the I–V characteristics [ 113 ]. The study explains the continuous flow of the forward current and the operating voltage point that gradually shift towards more negative voltages in the second quadrant of the I–V characteristics. The increase in the ratio of iodide to tri-iodide in the electrolyte rather than to the decomposition or the coupling reactions of the constituent materials was considered to be the reason behind it. According to the studies, these changes were also considered as reversible reactions that can be detected based on the changes in the color of the electrolyte or the I–V measurements.

However, ILs with lower viscosity and higher iodine concentration are needed as to increase J SC by increasing iodine mass transport. Laser transient measurements have been attempted and revealed that the high iodide concentration present in the pure ILs leads to a reductive quenching of the excited dye molecule [ 114 ]. Due to the low cost, thermal stability, and good conductivity of the conductive polymers based on polytiophenes and polypyrroles, they can be widely applied in DSSCs despite using ILs [ 115 ]. For the application point of view, the IL should have a high number of delocalized negative charge and counterions with a high chemical stability. Also, the derivatives of imidazolium salts are one of the best applicable in DSSCs. When 1-ethyl-3-methylimidazolium dicyanamide [EMIM] [DCA] with a viscosity of only 21 mPa s [ 116 ] was combined with 1-propyl-3-methylimidayolium iodide (PMII, volume ratio 1:1), an efficiency of 7.4% was observed and, after prolonged illumination, some degradation was also found. A cell with a binary IL of 1-ethyl-3-methylimidazolium tetracyanoborate in combination with PMII showed a stable efficiency of 7% that retained at least 90% of its initial efficiency after 1000 h at 80 °C in darkness and 1000 h at 60 °C, at AM 1.5 [ 117 ]. Moudam et al. studied the effect of water-based electrolytes in DSSC and demonstrated a highly efficient glass and printable flexible dye-sensitized solar cells upon application [ 118 ]. They used high concentrations of alkylamidazoliums to overcome the deleterious effect of water. The DSSCs employed pure water-based electrolyte and were tested under a simulated air mass 1.5 solar spectrum illumination at 100 mWcm − 2 and found the highest recorded efficiency of 3.45% and 6% for flexible and glass cells, respectively. An increase in the value of V OC from 0.38 to 0.72 V on the addition of TBP to the electrolyte has been observed [ 26 ]. Thus, to improve the efficiencies of DSSC, new materials were synthesized and applied in DSSCs. L-cysteine/L-cystine redox couple was employed in DSSC by Chen et al. which showed a comparable efficiency of 7.70%, as compared to the cell using I − /I 3 − redox couple (8.10%) [ 119 ]. In 2016, Huang et al. studied the effect of liquid crystals (LCs) on the PCE of dye-sensitized solar cells. They observed that the addition of minute amounts of LC decreases the J SC because it reduces the electrochemical reaction rate between the counter electrode and an electrolyte. Also, it delays the degradation rates of the cell because of the interaction between cyano groups of the doped LCs and organic solvent in the liquid electrolyte [ 120 ]. Main components of different kinds of electrolytes are discussed below:

Pyridine Derivatives (Like 4-Tert-Butylpyridine [TBP], 2-Propylpyridine, N -Methylbenzimidazole [NMBI])

The improved efficiency for a DSSC can be achieved by adding about 0.5 M of pyridine derivative within the electrolyte, due to which an increase in the value of V OC occurs. This improved V OC can be attributed to the positive band edge movement and slightly affected charge recombination rate on the basis of intensity-modulated photovoltage spectroscopy (IMVS) [ 121 ]. The study showed that after the adsorption of pyridine ring on TiO 2 surface, the pyridine ring induced electron density into the TiO 2 creating a surface dipole. But, the band edge movement results in the slight decrease in J SC as compared to the untreated cell, due to the diminution in the driving force for electron injection [ 122 ]. Further, application of NMBI over TBP was studied in 2003, due to its long-term stability under elevated temperature [ 54 ].

Alkyl Phosphonic/Carboxylic Acids (Like Decylphosphonic Acid [DPA], Hexadecylmalonic Acid [HDMA])

An improved V OC with slight decrease in the J SC have been observed when DPA [ 54 ] and HDMA [ 123 ] were combined. This was due to the presence of self-assembled long alkyl chain on the surface of TiO 2 , which is responsible for the formation of densely packed hydrophobic monolayer and reduction in recombination rate too, as these long alkyl chains repel iodide from TiO 2 surface.

Guanidinium Derivatives (GuSCN)

The addition of guanidium thiocyanate as a co-absorbent in an electrolyte results in enhanced V OC by ca. 120 mV with a downward shift in the conduction band by ca. 100 mV [ 124 ] at the same time due to the suppression in the recombination rate by a factor of 20 and a difference of 20 mV gained for V OC . By limiting the downward shift in the conduction band, an improvement in the overall efficacy can be attained.

Solid-State Electrolyte (SSE)

The SSE falls in two subcategories: (1) where hole transport materials are used as a transport medium and (2) SSE containing iodide/triiodide redox couple as a transport medium. Both kinds of SSEs are discussed below:

Hole Transport Materials (HTMs)

HTMs fall in the category of solid-state electrolytes, where HTMs are used a medium. These materials have set a great milestone in DSSCs and effectively applicable in cells because iodine/iodide electrolytes are highly chemically aggressive by nature and corrodes other materials easily, mostly metals. Most of the HTMs are chemically less-aggressive inorganic solids, organic polymers, or p-conducting molecules, although the results are still unmatched with the one obtained for iodine/iodide redox electrolytes because of the following reasons:

Due to their solid form, an incomplete penetration of solid HTMs within nanoporous TiO 2 -layer leads to poor electronic contact between HTMs and the dye. Thus, incomplete dye regeneration takes place.

The high frequencies of charge recombination from TiO 2 to HTMs.

Due to the presence of organic hole conducting molecules, the series resistance of the cell increases due to the low hole mobility in the organic HTMs as compared to IL electrolytes.

HTM results in a drop in V OC , as the recombination rate of electrons of CB with HTM becomes higher as compared to iodine/iodide redox electrolytes.

Low intrinsic conductivities of HTMs.

Thus, researchers need to synthesize and focus on HTMs whose VB energy should be slightly above the energy of the oxidized dye, should not absorb light, and must be photochemically stable, so that they can keep a healthy contact with the dye. Among a number of HTMs, some of the HTMs are discussed below:

Inorganic CuI Salt

CuI halogens and pseudohalogens are two classes of inorganic CuI salts that can be applied as HTMs in a DSSC. Copper bromide (CuBr), copper iodide (CuI), and copper thiocyanate (CuSCN) are some copper-based compounds which work as a hole conductor and are more effective due to their good conductivity. Although CuSCN is one of the best pseudohalogen HTMs and despite its high hole mobility, its application results in high series resistance and does not support high current and also shows poor electronic contact between CuSCN and the dye, and poor pore filling due to their fast crystallization rates, which resulted in low η of < 4% for the corresponding solid-state DSSCs [ 125 ]. Thus, to reduce the high recombination rate of electrons, additional blocking layers of insulating materials like SiO 2 or Al 2 O 3 can be applied or coated around the TiO 2 particles which enhance the V OC due to the suppressed recombination rate. With respect to halogens, CuBr showed an efficiency of 1.53% with thioether as an additive [ 126 ] and demonstrated high stability under prolonged irradiation of about 200 h at RT and the application of nickel oxide (NiO) showed moderate PCEs of 3% [ 127 ]. But, due to the easy poor solubility as well as crystallization of these materials, their application became a challenge and, thus, pseudohalogens have proven to be more stable and efficient in DSSCs. But the devices were found to be highly unstable and the reproducibility became dubious.

Hole-Conducting Molecules

spiro-OMeTAD {2,2′,7,7′-tetrakis(N,N′-di-p methoxyphenylamine)-9,9′-spirobifluorene} is one of the most suitable candidate in the prospect of hole conducting molecules and thus also widely applicable in integrated devices [ 128 ]. It was first introduced in 1998 [ 110 ] with a high glass transition temperature of ca. 120 °C. The researchers observed the formation of amorphous layers that are necessary for the complete pore filling and showed an IPCE of 33%, yielding overall efficiency to about 0.74% [ 110 ], and finally 4% of efficiency with an ambiphillic dye Z907 was demonstrated [ 129 ]. Some other triphenylamine detrivatives also demonstrated sufficient efficiencies in DSSCs [ 130 ]. Again, spiro-OMeTAD has certain limitations as it has low charge carrier mobility, ca. 104 cm 2 /Vs [ 130 ], that limits the thickness of the TiO 2 layer up to 2 μm and thus leads to incomplete light harvesting efficiency (LHE) of dye. Also, a high recombination rate between TiO 2 and FTO leads to low efficiencies in DSSCs.

Triphenylamine (TPA)

Phenylamines demonstrate a remarkable charge transporting property which makes them great hole transporting materials in organic electroluminescent devices [ 131 ]. However, despite a huge range of non-conjugated polymers of di- and triphenylamine which are synthesized and used efficiently as HTMs in organic electroluminescent devices, their conjugated polymers are still rare. Polyaniline (PANI) is the only well-recognized conjugated diphenylamine polymer [ 132 ] due to its highly electrical conductive property and is environmentally stable in the doped state. In 1991, triphenylamine (TPA)-conjugated polymers were synthesized by Ni-catalyzed coupling polymerization [ 133 ]. Okada et al. reported dimer (TPD 9), trimer (TPTR 10), tetramer (TPTE 11), and pentamer (TPPE 12) of TPA with the aid of Ullmann coupling reaction between the corresponding primary or secondary arylamines and aryl iodides [ 134 ].

SSE Containing Iodide/Triiodide Redox Couple

These SSEs have larger applications than those of HTMs, because interfacial contact properties of these solid-state electrolytes are better than those of HTMs. Fabrication of a DSSC based on solid-state electrolyte was reported by adding TiO 2 nanoparticle into poly(ethylene oxide) (PEO) and the overall light-to-electricity conversion efficiency of 4.2% for the cell was obtained under irradiation of AM 1.5100 mWcm − 2 [ 135 ].

Quasi-Solid-State Electrolyte (QSSE)

QSSE has a hybrid network structure, because it consists of a polymer host network swollen with liquid electrolytes, thus showing the property of both solid (cohesive property) and liquid (diffusive transport property), simultaneously. Thus, to overcome the volatilization and leakage problems of liquid electrolytes, ILs like 1-propargyl-3- methylimidazolium iodide, bis(imidazolium) iodides and 1-ethyl-1-methylpyrrolidinium and polymer gel-like PEO, and poly(vinylidinefluoride) and polyvinyl acetate containing redox couples are commonly used as QSSEs [ 136 , 137 ]. In 2015, Sun et al. fabricated a DSSC employing wet-laid polyethylene terephthalate (PET) membrane electrolyte, where PET is a commonly used textile fiber used in the form of a wet-laid non-woven fabric as a matrix for an electrolyte. According to their observation, this membrane can better absorb electrolyte turning into a quasi-solid, providing excellent interfacial contact between both electrodes of the DSSC and preventing a short circuit. The quasi-solid-state DSSC assembled with an optimized membrane exhibited a PCE = 10.248% at 100 mWcm −2 . To improve the absorbance, they plasma-treated the membrane separately with argon and oxygen, which resulted in the retention of the electrolyte, avoiding its evaporation, and a 15% longer lifetime of the DSSC compared to liquid electrolyte [ 138 ]. Figure  10 shows the polarization curves of DSSCs with various electrolytes under simulated AM 1.5 global sunlight (1 Sun, 100 mWcm −2 ).

Polarization curves of DSSCs with various electrolytes under simulated AM 1.5 global sunlight (1 sun, 100 mWcm −2 ) [ 138 ]

Hole-Conducting Polymers

IPCE of 3.5% by the application of C60/polythiphene derivative in DSSCs has been achieved for pure organic solar cells [ 139 ]. However, this field is developing slowly, as its deposition by standard methods (like CBD) is difficult, because solid polymer does not penetrate the TiO 2 -nanoporous layer. Hence, there are only few groups applied as conducting polymers in DSSCs. Ravirajan et al. demonstrated a monochromatic efficiency of 1.4% at 440 nm by applying fluorene-thiophene copolymer [ 140 ]. Researchers are working hard so long to develop new efficient materials for electrolytes. Jeon et al. reported that the addition of alkylpyridinium iodide salts in electrolytes enhanced the performance of the dye-sensitized solar cells. They observed better J–V characteristics, 7.92% efficiency with V OC  = 0.696 V, J SC  = 17.74 mA/cm 2 , and FF = 0.641 for the cell applying EC6PI (pyridinium salts) as compared to EC3ImI (imidazolium salts), whose η  = 7.46% with V OC  = 0.686 V, J SC  = 16.99 mA/cm 2 , and FF = 0.64 [ 141 ]. For a comparison, they added UV spectra for C6ImI and observed that the higher quantum efficiencies from the cell with EC6PI were obtained within the wide range from 460 to 800 nm. The quantum efficiencies were almost the same in the range of shorter wavelengths, may be due to the ability of C6PI to absorb more incident light than C3ImI at shorter wavelengths. Even so, the absorption coefficients for C6PI were higher than those for C6ImI over all the range, but the cell efficiencies are quite comparable (as shown in Fig.  11 ) [ 141 ]. Lee et al. developed and utilized the conjugated polymer electrolytes (CPEs) like MPF-E, MPCZ-E, MPCF-E, and MPCT-E containing quaternized ammonium iodide groups in polymer solution and gel electrolytes for DSSCs. They observed, as the polymer content in the electrolyte solution increased, the electrochemical impedance also increased for the cells based on CPE containing polymer solution electrolytes, whereas the PV performances showed the reverse trend [ 142 ]. Table  2 shows the FF and efficiencies for the DSSCs employing various dyes and mediators.

UV-vis spectroscopy selected pyridinium and imidazolium salts. The inset is the IPCE data for the cells with EC3ImI and EC6PI, which are the best cells among each series [ 141 ]

Developments in Dye Synthesis

As dyes play a key role in DSSCs, numerous inorganic and organic/metal-free dyes/natural dyes, like N3 [ 26 ], N719 [ 143 ], N749 (black dye) [ 144 ], K19 [ 145 ], CYC-B11 [ 146 ], C101 [ 32 ], K8 [ 147 ], D102 [ 148 ], SQ [ 149 ], Y123 [ 101 ], Z907 [ 150 ], Mangosteen [ 106 ], and many more have been utilized as sensitizers in DSSCs. Few of them will be discussed below briefly:

Metal (Ru) Complexes

Metal complex dyes produced from the heavy transition metals such as the complexes of ruthenium (Ru), Osmium (Os), and Iridium (Ir) have widely been used as inorganic dyes in DSSCs because of their long excited lifetime, highly efficient metal-to-ligand charge transfer spectra, and high redox properties. ML2(X)2 is the general structure of the sensitizer preferred as a dye, where M represents a metal, L is a ligand like 2,2′-bipyridyl-4,4′-dicarboxylic acid and X presents a halide, cyanide, thiocyanate, acetyl acetonate, and thiocarbamate or water substituent group [ 151 ]. Due to the thermal and chemical stability and wide absorption range from visible to NIR, the ruthenium polypyridyl complexes show best efficiencies and, thus, have been under extensive use so far.

Ru Complexes

In 1991, O’Regan and Grätzel reported the efficiency of 7.12% for the very first DSSC based on the ruthenium dye (black dye) [ 3 ]. Later, an efficiency of about 10% was reported by them using Ru-based dye (N749) which has given this topic a new sight. Most of the Ru complexes consist of Ru(II) atoms coordinated by polypyridyl ligands and thiocyanate moieties in octahedral geometery, and because of the metal to ligand charge transfer (MLCT) transitions, they exhibit moderate absorption coefficient, i.e., < 18,000 M − 1  cm − 1 . Ru (II) complexes lead the inter crossing of excited electron to the long lived triplet state and augmentation in the electron injection. Further, to improve the absorption and emission as well as electrochemical properties of Ru complexes, bipyridyl moieties can be replaced by the carboxylate polypyridine Ru dyes, phosphate Ru dyes, and poly nuclear bipyridyl Ru dyes. Table  3 and Fig.  12 show the molecular structure, the absorption spectra, and photoelectric performance for DSSCs based on different metal complex [polypyridyl (RuII)] dyes.

N3/N719/N712 Dyes

In 1993, Nazeeruddin et al. reported DSSC based on Ru-complex dye known as N3 dye {cis-di(thiocyanato)bis(2,2-bipyridine-4,4-dicarboxylate)ruthenium}, which contained one Ru center and two thiocyanate ligands (LL’) with additional carboxylate groups as anchoring sites and absorbed up to 800 nm radiations [ 26 ]. They obtained 10.3% efficiency for a system containing N3 dye and treated the dye covered film with TBP. At 518 and 380 nm wavelength, this dye attained maximum absorption spectra with respective extinction coefficients as 1.3 × 10 4  M − 1  cm − 1 and 1.33 × 10 4  M − 1  cm − 1 , respectively. The dye has showed the 60 ns of excited state lifetime and sustained for more than 10 7 turnovers without the significant decomposition since the beginning of the illumination [ 26 ]. Further, the absorption of the dye can be extended into the red and NIR by substituting the ligands such as thiocyanate ligands and halogen ligands. For example, a device containing acetylacetonate showed η  = 6.0% [ 152 ], followed by a pteridinedione complex with 3.8% efficiency [ 153 ] and a diimine dithiolate complex with 3.7% efficiency [ 154 ].

It has been investigated that during esterification, the dye gets bounded to the TiO 2 chemically which results in the partial transformation of protons of the anchoring group to the surface of the TiO 2 . Thus, it was concluded that the photovoltaic (PV) performance of the cell gets influenced by the presence of the number of protons on the N3 photosensitizer or, in other words, the modification in protonation level of N3 (N712, N719) affects the performance of the device [ 155 , 156 ], in two major aspects. Firstly, the increase in the concentration of the protons results in the positively charged TiO 2 surface and the downward shift in the Fermi level of TiO 2 . Hence, a drop in the V OC takes place due to the positive shift of the conduction band edge induced by the surface protonation. Secondly, the electric field associated with the surface dipole enhances the absorption of the anionic Ru(II) complexes and, thus, insists the electron injection from the excited state of the dye to the conduction band of the TiO 2 . In 2001, Nazeeruddin et al. reported a 10.4% of efficiency for the DSSCs using a ruthenium dye, i.e., “black dye” [ 157 ], where its wide absorption band covers the entire visible range of wavelengths. Grätzel and group demonstrated the PCE of 9.3% for the monoprotonated sensitizer N3 [TBA] 3 closely followed by a diprotonated sensitizer N3[TBA] 2 or N719 with a conversion efficiency of 8.4% [ 156 ]. Later, Wang et al. and Chiba et al. reported a η  = 10.5% [ 158 ] and η  = 11.1% [ 159 ], for the devices that used black dye as a sensitizer in DSSCs.

A new dye “N719” was reported by Nazeeruddin et al. by replacing four H + counterions of N3 dye by three TBA + and one H + counterions and achieved η  = 11.2% for the respective device [ 155 ]. Despite having almost the same structure to the N3 dye, the higher value of η for N719 was accredited to the change in the counterions, as they altered the speed of adsorption onto the porous TiO 2 electrode, i.e., N3 is fast (3 h) whereas N719 is slow (24 h). The dye-sensitized solar cell database (DSSCDB) yields around 329 results assembled from over 250 articles when queried as “N719,” where the reported efficiencies range between 2 and 11% [ 160 ].

Recently, Shazly et al. fabricated the solid-state dye-sensitized solar cells based on Zn 1-x Sn x O nanocomposite photoanodes sensitized with N719 and insinuated with spiro-OMeTAD as a solid hole transport layer [ 161 ] and achieved highest efficiency of 4.3% with J SC  = 12.45 mAcm − 2 , V OC  = 0.740 V, and FF = 46.70. Similarly, by applying different techniques, like post treatment of photoanode, optimizing the thickness of the nc-TiO 2 layer, and the antireflective filming, Grätzel group reported η  = 11.3% [ 32 ] for the device containing the dyes C101 and η  = 12.3% [ 162 ] for Z991 dye-based DSSCs (molecular structure shown in Fig.  13 ). Again, if a sensitizer does not carry even a single proton, the value expected for V OC will be high but the value for J SC becomes low. Thus, there should be an optimal amount of protonation of the sensitizer required, so that the product of both J SC and V OC can determine the conversion efficiency of the cell as a maximum. And thus, deprotonation levels of N3, N719, and N712 in solar cells were investigated, where the doubly protonated salt form of N3 or N719 showed higher PCE as compared to the other two sensitizers [ 143 ]. Figure  14 shows the effect of dye protonation on the I–V characteristics of TiO 2 photoanode sensitized with different Ru dyes as N3 (4 protons), N719 (2 protons), N3[TBA] 3 (1 proton), and N712 (0 protons) dyes, measured under AM 1.5 [ 156 ]. However, the main limitations of N3 sensitizers are their relatively low molar extinction coefficient and less of absorption in the red region of the visible spectrum.

Molecular structure of Ruthenium complex based dye sensitizers

Molecular structure of C101 and Z991 sensitizers

π-System Extension (N945, Z910, K19, K73, K8, K9)

As compared to the other organic dyes, standard Ru complexes have significantly lower absorption coefficient and thus a thick layer of TiO 2 was required, which results in the higher electron recombination probability. Thus, two carboxylic acid groups of N3 can be replaced by the ligands containing conjugated π-systems to enhance the absorption and the cell efficiency, simultaneously. Thus, the reason behind the π-system extension in dyes is to create sensitizers with higher molar extinction coefficients ( ε ), so that the LUMO of the dye can be tuned to get directionality in the excited state and to introduce hydrophobic side chains that repel water and triiodide from the TiO 2 surface. Recently, Rawashdeh et al. have demonstrated an efficiency of 0.45% by modifying the photoanode as graphene-based transparent electrode sensitized with 0.2 mM N749 dye in ethanolic solution [ 163 ].

Styryl-ligands attached to the bipyridil ring showed the utmost results. The ε  = 1.69 × 10 4  M − 1  cm − 1 for the Z910 dye [ 164 ], ε  = 1.82 × 10 4  M − 1  cm − 1 for the K19 dye [ 145 ], and ε  = 1.89 × 10 4  M − 1  cm − 1 for the N945 dye [ 165 ] have been found, which were at least 16% more as compared to the standard N3 dye. An efficiency of 10.2% was demonstrated by Wang et al. for Z910 dye [ 166 ]. 10.8% of the efficiency was observed for the N945 dye [ 167 ] in 2007, on thick electrodes and with volatile electrolytes which was about the same as for the N3 as reference, but, when applied on thin electrodes and with non-volatile electrolytes, the observed PCE was significantly higher. At the same time, a remarkable stability at 80 °C (in darkness) and 60 °C temperature (under AM 1.5) was observed [ 168 ], and between − 0.71 V and − 0.79 V vs. normal hydrogen electrode (NHE) [ 145 , 165 , 166 , 168 ], the excited state of these dyes has been reported and was observed sufficiently more negative than the conduction band of TiO 2 (ca. − 0.1 V vs. NHE) to ensure the complete charge injection. In terms of higher molar extinction coefficient, Nazeeruddin et al. synthesized K8 and K9 dyes that showed even better results as compared to the previous ones. K8 and K9 complexes showed broad and intense absorption bands between 370 and 570 nm. In DMF solution, the K9 complex showed the maxima at 534 nm ( λ max ) with a ε  = 14,500 M − 1  cm − 1 which was blue shifted by 22 nm compared to K8 complex which showed maxima at 556 nm with a ε  = 17,400 M − 1  cm − 1 , respectively. Thus, due to the substitution of 4, 4′-bis (carboxylvinyl)-2, 2′-bipyridine by 4,4′- dinonyl-2,2′-bipyridine, ε observed for K9 complex was ~ 20% less than that of the K8 complex. The overall PCE observed for K8 and K9 complexes were 8.46% with J SC  = 18 mA/cm 2 and V OC  = 640 mV and 7.81% with J SC  = 16 mA/cm 2 and V OC  = 666 mV [ 169 ], respectively. Grätzel group synthesized K19 as a second amphiphilic dye and demonstrated that K19 shows 18,200 M − 1  cm − 1  M extinction coefficient, 7.0% overall conversion efficiency and a low energy metal-to-ligand transition (MLCT) absorption band at 543 nm, which was higher than the corresponding values for the first amphiphilic dye Z907 with a molar extinction coefficient of 12,200 M − 1  cm − 1 , 6.0% overall conversion efficiency, and standard N719 dye with a molar extinction coefficient of 14,000 M − 1  cm − 1 with 6.7% overall PCE under the same fabrication and evaluation conditions. They appraised the performance of the device using N719, Z907, and K19 as sensitizers during thermal aging at 80 °C and observed a lower stability for N719 dye may be due to the desorption of the sensitizer at higher temperature; however, K19 and Z907, both retained over 92% of their initial performances under the thermal stress at 80 °C for 1000 h [ 145 , 169 ].

Thiophene ligands containing Ru sensitizers also showed good efficiencies. In 2006, Yanagida et al. reported a Ru complex, by replacing a phenylvinyl group of K19 by thienylvinyl group in HRS-1 [ 170 ] and an improved stability along with respectable LHE in vis-NIR and a reversible one electron oxidation process was reported. They found a η up to 9.5% for HRS-1 (substituted thiophene derivatives). Several thiophene containing sensitizers have been developed without conjugation, such as C101 [ 32 ] and CYC-B1 [ 171 ]. After the development of C101 dye, Ru (II) thiophene compounds gained special attention as having set a new DSSC efficiency record of 11.3–11.5% and became the first sensitizer to triumph over the well-known N3 dye [ 32 ].

Amphiphilic Dyes with Alkyl Chains

Two of the four carboxylic groups of N3 dye are replaced by long alkyl chains because the ester linkage of the dye to the TiO 2 was prone to hydrolyze, if water gets adsorbed on the TiO 2 surface [ 172 ], thus resulting in usually lower absorption spectrum in these sensitizers due to the smaller conjugated π-system of the bipyridil-ligand. Even though the PCE offered by these sensitizers were appreciable, ranging from 7.3% for Z907 (with 9 carbon atoms) [ 54 ] to 9.6% for N621 (with 13 carbon atoms) [ 155 ] and were highly stable, Z907 sensitized DSSCs passed 1000 h at 80 °C in darkness and at 55 °C under illumination without any degradation [ 173 ]. It has been found that by coadsorption of decylphosphonic acid on the TiO 2 NPs, the hydrophobicity of the surface could be even enhanced and, thus, stable cells have been demonstrated [ 54 ].

Different Anchoring Groups

Most of the sensitizers in DSSCs have carboxylic acid groups as an anchor on the surface of TiO 2 . But, the dye molecules get desorbed at the semiconductor surface at a pH > 9, due to the shifting of the equilibrium towards the reactant side. Thus, dyes with different anchoring groups are much needed. Again, most of the research focuses on phosphonic acid and the credit goes to its binding strength to a metal oxide surface, as the binding strength to a metal oxide surface decreases in the order, phosphonic acid > carboxylic acid > ester > acid chloride > carboxylate salts > amides [ 174 ]. Z955 is a Ru-complex containing phosphonic acid as an anchoring group and demonstrated a η  = 8% accompanied by good stability under prolonged light soaking for about 1000 h at AM 1.5 and 55 °C [ 175 ]. Triethoxysilane [ 176 ] and boronic acid are some other anchoring groups. π-Extended ferrocene with varied anchoring groups (–COOH, –OH, and –CHO) has been applied as photosensitizers in DSSCs [ 177 ]. Chauhan and co-workers has synthesized and characterized two new compounds as FcCH=NC 6 H 4 COOH (1) and FcCH=NCH 2 CH 2 OH (2), where Fc = C 5 H 4 FeC 5 H 5 and FcCHO are used as the starting material [ 177 ]. By cyclic voltammetry (CV) in dichloromethane solution and using density functional theory (DFT) calculations, they have explained the quasi-reversible redox behavior of the dyes. The redox-active ferrocenyl group exhibited a single quasi-reversible oxidation wave with E′ = 0.34, 0.44, and 0.44 V for 1, 2, and 3, respectively. In 2017, a study was carried out to inspect the influence of a cyano group in the anchoring part of the dye on its adsorption stability and the overall PV properties like electron injection ability to the surface and V OC [ 178 ]. The results indicated that the addition of the cyano group increased the stability of adsorption only when it adsorbs via CN with the surface and it decreased the photovoltaic properties when it was not involved in binding.

However, in the race of improved efficiency and efficient DSSCs, Ru (II) dyes are still an ace. The most vital reason following usage of Ru dyes in DSSCs is their extraordinary stability when being absorbed on the TiO 2 surface. N749 and Z907 are the two important Ru dyes, although N749 which shows broad absorption and high efficiency, in contrast, has low absorption coefficient about ~ 7000 M − 1  cm − 1 and the stability of this dye was not so good as compared to other Ru sensitizers. PCE of 10.4% has been observed for black dye, under AM 1.5 and full sunlight [ 48 ]. It achieved sensitization over the whole visible range extending into the NIR up to 920 nm with 80% IPCE and 10.4% overall efficiency, when anchored on TiO 2 nanocrystalline film. At NREL, black dye (N749)-sensitized DSSC showed efficiency of 10.4% with J SC  = 20.53 mA/cm 2 , V OC  = 0.721 V, and FF = 0.704, where the active area of the cell was 0.186 cm 2 [ 157 , 179 ]. Nazeeruddin et al. reported a comparative study between the spectral response of the photocurrent of the two dyes, N3 and N749 [ 26 ], as shown in Fig.  15 . In the vis-range, both chromophores showed very high IPCE values. The response of N749 dye was observed to be extended 100 nm further into the IR compared to that of N3. The recorded photocurrent onset was close to 920 nm and there on the IPCE rose gradually until at 700 nm it reached to a plateau of ca. 80%. From overlap integral of the curves in Fig.  8 with the AM 1.5 solar emission, it could be predicted that J SC of the N3 and black dye-sensitized cells to be 16 and 20.5 mA/cm 2 , respectively [ 37 ]. In Z907 sensitizer, one of the dicarboxy bipyridine ligands in N3 molecule was replaced by a nonyl bipyridine, which resulted in the formation of a hydrophobic environment on the device. However, the dye has set a precedent for hundreds of tris-heteroleptic Ru complexes with isothiocyanate ligands that were developed in the last 15 years, but provides efficiencies rarely comparable to N719 [ 180 ].

Effect of dye protonation on photocurrent-voltage characteristics of nanocrystalline TiO 2 cell sensitized with N3 (4 protons), N719 (2 protons), N3[TBA] 3 (1 proton), and N712 (0 protons) dyes, measured under AM 1.5 sun using 1 cm 2 TiO 2 electrodes with an I − /I 3 − redox couple in methoxyacetonitrile [ 156 ]

Metal-Free, Organic Dyes

Despite the capability to provide highly efficient DSSCs, the range of application of Ru dyes are limited to DSSCs, as Ru is a rare and expensive metal and, thus, not suitable for cost-effective, environmentally friendly PV systems. Therefore, development and application of new metal-free/organic dyes and natural dyes is much needed. The efficiency of DSSCs with organic dyes has been increased significantly in the last few years and an efficiency of 9% [ 181 ] was shown by Ito and co-workers. The molecular structure and the efficiency for DSSCs based on different metal-free organic dyes are shown in Table  4 and Fig.  16 .

Thus, metal-free organic dyes are developing at a fast pace to overcome the limitations discussed above and especially promising is the fast learning curve, which raises hope of the further synthesis of new materials with higher stability and, thus, designing highly efficient DSSCs at much lower prices. Though the efficiencies offered by these dyes are less comparable to those by Ru dyes, their application is vast as they are potentially very cheap because of the incorporation of rare noble metals in organic dyes; thus, their cost mainly depends on the number of synthesis steps involved. Other advantages associated with organic dyes are their structure variations, low cost, simple preparation process, and environmental friendliness as compared to Ru complexes; also, the absorption coefficient of these organic dyes is typically one order of magnitude higher than Ru complexes which makes the thin TiO 2 layer feasible. Thus, there is a huge demand to develop new pure organic dyes, so that the commercialization of DSSCs becomes easier.

However, there are certain limitations associated with these dyes too, as under high elevated temperatures the observed stability of the organic dyes were not as good as expected. Therefore, to get a larger photocurrent response for newly designed and developed organic dyes, it is essential to attain predominant light-harvesting abilities in the whole visible region and NIR of dyes, with a sufficiently positive HOMO than I − /I −   3 redox potential and sufficiently negative LUMO than the conduction band edge level of the TiO 2 semiconductor, respectively [ 182 , 183 ]. In 2008, Tian et al. fabricated DSSCs based on a novel dye (2TPA-R), containing two triphenylamine (TPA) units connected by a vinyl group and rhodanine-3-acetic acid as the electron acceptor to study the intramolecular energy transfer (E n T) and charge transfer (ICT) [ 184 ]. They found that the intramolecular E n T and ICT processes showed a positive effect on the performance of DSSCs, but the less amount of dye was adsorbed on TiO 2 which may make it difficult to improve the efficiency of DSSCs [ 184 ]. An efficiency of 2.3% was attained for the DSSC used 2TPA-R dye and an imidazolium iodide electrolyte, whereas η  = 2% was achieved for TPA-R dye. This improved efficiency for 2TPA-R device was possibly due to the contribution of the E n T and ICT. They studied the effect of 2TPA-R via absorption spectrum (as shown in Fig.  17 ) and found that the two absorption bands, i.e., λ abs at 383 nm and 485 nm obtained for 2TPA-R, are almost the same as those observed for 2TPA ( λ abs at 388 nm) and TPA-R ( λ abs at 476 nm) in CH 2 Cl 2 solution (2 × 10 − 5  M). Thus, the study on intramolecular E n T and ICT could help in the design and synthesis of efficient organic dyes. Hence, a suitable anchoring group which can chemically bind over the TiO 2 surface with a suitable structure and effective intramolecular E n T and ICT processes, is also required for synthesis.

Photocurrent action spectra obtained with the N3 (ligand L) and the black dye (ligand L_) as sensitizer. The photocurrent response of bare TiO 2 films is also shown for comparison [ 26 ]

The construction of most of the organic dyes is based on the donor- acceptor (D-A)-like structure linked through a π-conjugated bridge (D–π–A) and usually has a rod-like configuration. Moieties like indoline, triarylamine, coumarin, and fluorine are employed as an electron donor unit, whereas carboxylic acid, cyanoacrylic acid, and rhodamine units are best applicable as electron acceptors to fulfill the requirement. The linking of donor and acceptor is brought about by adding π spacer such as polyene and oligothiophene [ 185 , 186 ]. This type of the structure results in a higher photoinduced electron transfer from the donor to acceptor through linker (spacer) to the conduction band of the TiO 2 layer, where the π-conjugation can be extended either by increasing the methine unit or by introducing aromatic rings such as benzene, thiophene, and furan or in other words by adding either electron donating or withdrawing groups, which results in the enhanced light harvesting ability of the dye, and by using different donor, linker, and acceptor groups, the photophysical properties of the organic dyes can also be tuned [ 187 , 188 ]. Whereas the photophysical properties change with the expansion of π-conjugation due to the shift of the both HOMO and LUMO energy levels, thus, D–π–A structure was considered to be the most promising class of organic dyes in DSSCs as they can be easily tuned [ 189 ]. Moreover, in 2010, the encouraging efficiency up to 10.3% was reported using organic dyes [ 190 ]. Fuse et al. demonstrated a one-pot procedure to clarify the structure–property relationships of donor–π–acceptor dyes for DSSCs through rapid library synthesis [ 191 ]. Four novel organic dyes IDB-1, ISB-1, IDB-2, and ISB-2, based on 5-phenyl-iminodibenzyl (IDB) and 5-phenyliminostlbene (ISB) as electron donors and cyanoacrylic acid moiety as an electron acceptor connected with a thiophene as a π-conjugated system, were designed by Wang et al. in 2012 [ 192 ]. The highest efficiencies for the devices based on ISB-2 were observed due to the larger red shifts of 48 nm for ISB-2, indicating the more powerful electron-donating ability due to the increased linker conjugation. The absorption peaks for IDB-1, ISB-1, IDB-2, and ISB-2 were obtained at 422, 470, 467, and 498 nm in dichloromethane-diluted solution, respectively.

Tetrahydroquinolines [ 193 , 194 ], pyrolidine [ 195 ], diphenylamine [ 196 ], triphenylamine (TPA) [ 27 , 197 ], coumarin [ 198 , 199 ], indoline [ 200 , 201 ], fluorine [ 202 ], carbazole (CBZ) [ 203 ], phenothiazine (PTZ) [ 204 , 205 ], phenoxazine (POZ) [ 206 ], hemicyanine dyes [ 207 ], merocyanine dyes [ 208 ], squaraine dyes [ 209 ], perylene dyes [ 210 ], anthraquinone dyes [ 211 ], boradiazaindacene (BODIPY) dyes [ 212 ], oligothiophene dyes [ 213 ], and polymeric dyes [ 214 ] are widely used in DSSCs and are still under development. Jia et al. designed quasi-solid-state DSSCs employing two efficient sensitizers FNE55 and FNE56, based on fluorinated quinoxaline moiety, i.e., 6, 7-difluoroquinoxaline moiety, and an organic dye FNE54 without fluorine was designed for comparison [ 215 ]. From the studies, it was concluded that the absorption properties of the dye enhanced bathochromically from 504 nm for FNE54 to 511 nm and 525 nm for FNE55 and FNE56 sensitizers upon the addition of fluorine into the dye. The addition of fluorine resulted in the improved electron-withdrawing ability of the quinoxaline and, thus, enhanced the push–pull interactions and narrowed the energy band gap. Due to the high polarizability, spectroscopically and electrochemically tunable properties and high chemical stability, π-conjugated oligothiophenes were well applied as spacers in DSSCs [ 194 , 204 ]. To induce a bathochromic shift and augment the absorption, a number of thiophene units could be increased in the spacers, and by controlling the length of these thiophene units or chain, higher efficiencies up to two to three units can be achieved [ 216 , 217 ], as the π-conjugated spacers used previously were thiophenes linked directly or through double bonds to the donor moiety [ 218 ].

As good electron injection is one of the parameters for higher efficiency in the DSSCs, cyanoacetic acid and cyanoacrylic acid are well employed as acceptor units due to their strong electron withdrawing capability. Yu et al. concluded cyanoacrylic acid as a strong electron acceptor for D–π–A-based dyes because the dye incorporating cyanoacrylic acid as an electron acceptor showed the best results and, due to the maximum absorption spectrum and the highest molar excitation coefficient, the DSSC achieved η  = 4.93% [ 219 ]. Wang and co-workers designed organic dyes based on thienothiophene as π conjugation unit, where they used triphenylamine as donor and cyanoacetic acid as an acceptor. They substituted different alkyl chains on the triphenylamine unit and found the best efficiency of about 7.05% for the sensitizers with longer alkoxy chains due to the longer electron lifetime [ 220 ]. Acceptors based on rhodanine-3-acetic acid were also used as an alternative, but due to the low lying molecular LUMO, the results obtained were not pleasing [ 221 , 222 ].

Coumarin Dyes

Coumarin is a synthetic organic dye and is a natural compound found in many plants like tonka bean, woodruff, and bison grass (molecular structure shown in Fig.  18 a). In 1996, Grätzel et al. found the efficient electron injection rates of 200 fs from C343 into the conduction band of the TiO 2 , where for the first time the transient studies on a coumarin dye in DSSCs were performed [ 223 ]. But the narrow absorption spectrum of C343, i.e., lack of absorption in the visible region, resulted in the lower conversion efficiency of the device. This can be altered by adding more methane groups that result in expanding the π-conjugation linkers and an increased efficiency of the DSSC [ 224 ]. Giribabu and co-workers synthesized RD-Cou sensitizers and obtained the conversion efficiency of 4.24% using liquid electrolyte, where coumarin moiety was bridged to the pyridyl groups by thiophene, which resulted in the extended π-conjugation and broadening of the metal-to-ligand charge transfer spectra [ 225 ]. They found that the absorption spectrum of RD-Cou dye was centered at 498 nm with a ɛ  = 16,046 M − 1  cm − 1 . Despite the lower efficiency offered by these cells, the thermal stability of the sensitizer make its rooftop applications possible because the dye showed stability of up to 220 °C during thermal analysis.

Molecular Structure of metal-free organic dyes

Indole Dyes

Indole occurs naturally as a building block in amino acid tryptophan, and in many alkaloids and dyes too (molecular structure shown in Fig.  18b ). It is substituted with an electron withdrawing anchoring group on the benzene ring and an electron donating group on the nitrogen atom, and these dyes have demonstrated good potential as a sensitizer. Generally, the D–A structure of an indole dye is such that the indole moiety acts as an electron donor and is connected to a rhodanine group that acts as an electron acceptor. Also by introducing the aromatic units into the core of the indoline structure, the absorbance in the infrared (IR) region of the visible spectra as well as the absorption coefficient of the dye can be enhanced significantly [ 226 ]. An efficiency of 6.1% was demonstrated for DSSCs with D102 dye, and by optimizing the substituents, 8% of the efficiency was attained with D149 dye [ 200 ]. Another dye “D205” was synthesized by controlling the aggregation between the dye molecules, as an indoline dye with an n-octyl substituent on the rhodanine ring of D149 [ 227 ]. They investigated that n-octyl substitution increased the V OC without acknowledging the presence of CDCA too much. However, the increase in the V OC of D205 due to the CDCA was approximately 0.054 V but showed little effect on D149 with an increase of 0.006 V only. But the CDCA and n-octyl chain (D205) together improved the V OC by up to 0.710 V significantly, which was 0.066 V higher (by 10.2%) than that of D149 with CDCA.

Further in 2012, η  = 9.4% was shown by Wu et al. with the observed J SC  = 18 mAcm − 2 , V OC  = 0.69 V, and FF = 0.78, by employing indoline as an organic dye in the respective DSSC [ 228 ]. Suzuka et al. fabricated a DSSC sensitized with indoline dyes in conjunction with the highly reactive but robust nitroxide radical molecules as redox mediator in a quasi-solid gel form of the electrolyte. They obtained an appreciable efficacy of 10.1% at 1 sun. To suppress a charge-recombination process at the dye interface, they introduced long alkyl chains, which specifically interact with the radical mediator [ 229 ]. Recently in 2017, Irgashev et al. synthesized a novel push-pull thieno[2,3-b]indole-based metal-free dyes and investigated their application in DSSCs [ 230 ]. They designed IK 3–6 dyes based on the thieno[2,3-b]indole ring system, bearing various aliphatic substituents such as the nitrogen atom as an electron-donating part, several thiophene units as a π-bridge linker, and 2-cyanoacrylic acid as the electron-accepting and anchoring group. An efficiency of 6.3% was achieved for the DSSCs employing 2-cyano-3-{5-[8-(2-ethylhexyl)-8H-thieno[2,3-b]indol-2-yl]thiophen-2-yl}acrylic acid (IK 3), under simulated AM 1.5 G irradiation (100 mWcm − 2 ), whereas the lower values of η  = 1.3% and 1.4%, respectively, were shown by the dyes IK 5 and IK 6. The LUMO energy levels are more negative than the conduction edge of the TiO 2 (− 3.9 eV), and their HOMO energy levels of all four dyes were found to be more positive than the I − /I 3 − redox couple (− 4.9 eV), making possible regeneration of oxidized dye molecules after injection of excited electrons into TiO 2 electrode (as shown in Fig.  19 ) [ 230 ]. The less efficiency of other dyes was contributed by the intermolecular π-stacking and aggregation processes in these dyes, proceeding on the photoanode surface.

Absorption spectra of 2TPA, TPA-R, and 2TPA-R in CH 2 Cl 2 solutions [ 184 ]

Porphyrin shows strong absorption and emission in the visible region and has a long lifetime in its excited singlet state (> 1 ns), very fast electron injection rate (femtosecond range) [ 231 ], millisecond time scale electron recombination rate [ 232 ], and tunable redox potentials [ 13 ]. In 1987, the first paper was published on DSSCs based on efficient sensitization of TiO 2 with porphyrins [ 233 ]. This led researchers in the direction to make efforts for the synthesis of novel porphyrin derivatives with the underlying idea to mimic nature’s photosystems I and II, so that the large molar extinction coefficient of the Soret bands and Q bands can be exploited. In 2007, a Zn-porphyrin dye-based DSSC was fabricated by Campbell et al. and has given the exceptional PCE of 7.1% [ 195 ]. Krishna and co-workers investigated the application of bulky nature phenanthroimidazole-based porphyrin sensitizers in DSSCs [ 234 ]. The group designed a novel D–π–A-based porphyrin sensitizer having strong electron-donating methyl phenanthroimidazole ring and ethynylcarboxyphenyl group at meso-position of porphyrin framework (LG11). They have attached the hexyl phenyl chains to the phenanthroimidazole moiety to reduce the unwanted loss of V OC caused by dye aggregation and charge recombination effect, thus achieving an increase in V OC to 460 and 650 mV. The energy level diagram and the absorption–emission spectra for the sensitizers (LG11-14) are shown in Fig.  20 [ 234 ].

Molecular structure of a Coumarin and b indole

Wang et al. have synthesized zinc porphyrins in a series bearing a phenylethynyl, naphthalenylethynyl, anthracenylethynyl, phenanthrenylethynyl, or pyrenylethynyl substituent, namely LD1, LD2, LD3a, LD3p, and LD4, as photosensitizers for DSSCs (as shown in Fig.  21 ). The overall efficiencies of the corresponding devices resulted as LD4 (with η  = 10.06%) > LD3p > LD2 > LD3a > LD1. The higher value of η and V OC  = 0.711 V was achieved for LD4 due to the broader and more red-shifted spectral feature; thus, the IPCE spectrum was covered broadly over the entire visible region [ 235 ]. Later for a push–pull zinc porphyrin DSSCs, changes in the structural design were carried out and structures with long alkoxyl chains enveloping the porphyrin core were built. By following the process, a η  = 12.3% was achieved by Yella et al. for DSSC with cobalt as the mediator [ 236 ].

HOMO and LUMO energy level diagram of dyes IK 3–6 [ 230 ]

Giovannetti et al. investigated the free base, Cu(II) and Zn(II) complexes of the 2,7,12,17-tetrapropionic acid of 3,8,13,18-tetramethyl-21H,23H porphyrin (CPI) in solution and bounded to transparent monolayer TiO 2 nanoparticle films to determine their adsorption on the TiO 2 surface, to measure the adsorption kinetics and isotherms, and to use the obtained results to optimize the preparation of DSSC PVCs (photovoltaic cells) [ 237 ]. The absorption spectra study of CPI, CPIZn, and CPICu molecules onto the TiO 2 surface (as shown in Fig.  22 ) revealed the presence of typical strong Soret and weak Q bands of porphyrin molecules in the region 400–450 nm and 500–650 nm, which were not changed with respect to the solution spectra. They observed no modification in the structural properties of the adsorbed molecules.

Energy level diagram of LG-11 to LG-14 porphyrins, electrolyte and TiO 2 ( a ) and absorption (left, solid line) and emission (right, dashed line) spectra of porphyrin sensitizers LG-13 and LG-14 in the THF solvent ( b )

Triarylamine Dyes

Due to the good electron as well as transporting capability and its special propeller starburst molecular structure with a nonplanar configuration, the triarylamine group is widely applied as a HTM in various electronic devices. Triarylamine derivative distributes the π–π stacking and, thus, improves the cells performance by reducing the charge recombination, minimizing the dye aggregation and enhancing the molar extinction coefficient of the organic dye [ 202 , 217 , 238 ]. By the addition of alkyl chains or donating groups, the structural modification of the triarylamine derivatives could be performed [ 218 , 220 , 239 ]. The performance of a basic D–π–A organic dye can be improved by simply binding donor substitutions on the π-linker of the dye [ 240 ]. Thus, Prachumrak and co-workers have synthesized three new molecularly engineered D–π–A dyes, namely T2–4, comprising TPA as a donor, terthiophene containing different numbers of TPA substitutions as a π-conjugated linker and cyanoacrylic acid as an acceptor [ 240 ]. To minimize the electron recombination between redox electrolyte and the TiO 2 surface as well as an increase the electron correction efficiency, the introduction of electron donating TPA substitutes on the π-linker of the D–π–A dye can play a favorable game, leading to improved V OC and J SC , respectively [ 240 ]. In 2006, Hagberg et al. published a paper on TPA-based D5 dye [ 241 ], where the overall PCE demonstrated for D5 dye was 5.1% in comparison with the standard N719 dye with an efficiency of 6.40% under the similar fabrication conditions. Thus, D5 appeared as an underpinning structure to design the next series of TPA derivatives.

In 2007, a series L0-L4 of TPA-based organic dyes were published by extending the conjugation in a systematic way [ 218 ]. By increasing the π conjugation, the absorption spectra and molar extinction coefficients of L0-L4 were increased. The observed IPCE spectra for L0 and L1 dyes were high, but the spectra of these dyes were not broad; as a result, lower conversion efficiencies were obtained for L0 and L1, whereas the broad absorption spectrum as well as the broad IPCE was obtained for L3 and L4 by the augmentation of linker conjugation, but the efficiencies observed were less than the L0 and L1 due to the amount of dye loading, i.e., with the increase in the size of dye there appears a decrease in the dye amount. Thus, the lower IPCE obtained for longer L3 and L4 may be accredited to unfavorable binding with the TiO 2 surface. Higher efficiencies were obtained for solar cells based on L1 and L2, 2.75% and 3.08%, respectively. Baheti et al. synthesized DSSCs based on nanocrystalline anatase TiO 2 and simple triarylamine-based dyes containing fluorene and biphenyl linkers [ 242 ]. They reported that the fluorene-based dyes showed better solar cell parameters than those of the biphenyl analogues. In 2011, Lu et al. reported the synthesis and photophysical/electrochemical properties of three functional triarylamine organic dyes (MXD5-7) as well as their application in dye-sensitized solar cells. They used the nonplanar structures of bishexapropyltruxeneamino as an electron donor [ 243 ] and investigated the impact of addition of chenodeoxycholic acid (CDCA) in the respective dyes, as MXD5-7 without CDCA showed lower photocurrent and efficiency as compared to the dyes MXD5-7 with 3 mM CDCA. However, the highest efficiency of 6.18% was observed for MXD7 (with 3 mM CDCA) with electron lifetime ( τ ) = 63 ms, under standard global AM 1.5 solar conditions (molecular structure is given in Table  4 , where R = propyl).

Using furan as a linker, different TPA-based chromophores were studied by Lin and co-workers [ 244 ]. When D5 and its furan counterpart were compared, the results were exciting, still the light harvesting abilities observed for D5 were higher ( λ abs  = 476 nm with ε  = 45,900 M − 1  cm − 1 in ACN) than those for the furan counterpart ( λ abs  = 439 nm with ε  = 33,000 M − 1  cm − 1 in ACN). However, the performance of the solar cells based on the furan counterpart (ɳ max  = 7.36%) was better as compared to the one based on D5 (ɳ max  = 6.09%) because of the faster recombination lifetimes in D5. Again, the tendency of trapping of charge from the TPA moeity was higher in thiophene than the furan. In 2016, Simon et al. reported an enhancement in the photovoltage for DSSCs that employed triarylamine-based dyes, where halogen-bonding interactions existed between a nucleophilic electrolyte species (I − ) and a photo-oxidized dye immobilized on a TiO 2 surface. They found larger rate constants for dye regeneration ( k reg ) by the nucleophilic electrolyte species when heavier halogen substituents were positioned on the dye. Through the observations, they concluded that the halogen-bonding interactions between the dye and the electrolyte can boost the performance of DSSC [ 245 ]. However, the most efficient metal-free organic dye-based DSSC has shown PCE of 10.3% in combination with a cobalt redox shuttle, by using the phenyl dihexyloxy-substituted triphenylamine (TPA) (DHO-TPA) Y123 dye [ 246 ]. In 2018, Manfredi and group have designed di-branched dyes based on a triphenylamino (TPA) donor core with different aromatic and heteroaromatic peripheral groups bonded to TPA as auxiliary donors [ 247 ]. Thus, due to the improved strategic interface interactions between the dye sensitized titania and the liquid electrolyte, better optical properties were achieved.

Phenothiazine (PTZ) Dyes

Phenothiazine is a heterocyclic compound containing electron-rich sulfur and nitrogen heteroatoms, with a non-planar and butterfly conformation in the ground state, which can obstruct the molecular aggregation and the intermolecular excimer formation. Thus, PTZ results as a promising hole transport semiconductor in the organic devices, presenting unique electronic and optical properties [ 248 ].

Tian and co-workers investigated the effect of PTZ as an electron-donating unit in DSSCs, and because of the stronger electron donating tendency of PTZ unit than the TPA unit (0.848 and 1.04 V vs. the normal hydrogen electrode (NHE), respectively) [ 249 ], they found efficient results for the sensitizers based on PTZ rather than those based on the TPA [ 250 ]. In 2007, a new series of PTZ-based dyes as T2–1 to T2–4 was demonstrated [ 251 ]. In these dyes, PTZ unit acted as an electron donor, cyanoacrylic acid or rhodanine-3-acetic acid was used as an electron acceptor, and alkyl chains were used to increase the solubility. They found a red shift in the absorption spectra of T2–3 ( η  = 1.9%) and T2–4 ( η  = 2.4%) dyes with low IPCE values for rhodanine-3-acetic acid as an anchoring group, as compared to T2–1 ( η  = 5.5%) and T2–2 ( η  = 4.8%) dyes with cyanoacrylic acid as an anchoring group. This proved the use of the cyanoacrylic acid is more viable than a rhodanine-3-acetic acid. In 2010, Tian et al. reported modified phenothiazine (P1-P3) dyes [ 252 ] with the molecular structure containing the same acceptor and conjugation chain but different donors. Due to the presence of two methoxy groups attached to TPA, a red shift was observed in the absorption spectra of P1 as compared to P2 and P3. This resulted in an increment in the extent of electron delocalization over the whole molecule and, thus, a little red shift in the maximum absorption peak was observed. Xie et al. synthesized two novel organic dyes (PTZ-1 and PTZ-2) using electron-rich phenothiazine as electron donors and oligothiophene vinylene as conjugation spacers. They employed 13 μm transparent and 1.5 μm scattering TiO 2 electrode and used an electrolyte composed of 0.6 M butylmethylimidazolium iodide (BMII), 0.03 M I 2 , 0.1 M GuSCN, 0.5 M 4-tert-butylpyridine in acetonitrile (TBP in ACN), and valeronitrile. They demonstrated that the (2E)-2-cyano-3-(5-(5-((E)-2-(10-(2-ethylhexyl)-10H-phenothiazin-7-yl)vinyl)thiophen-2-yl)thiophen-2-yl)acrylic acid (PTZ-1) and (2E)-3-(5-(5-(4,5-bis((E)-2-(10-(2-ethylhexyl)-10Hphenothiazin-3-yl)vinyl)thiophen-2-yl)thiophen-2-yl)thiophen-2-yl)-2cyanoacrylic acid (PTZ-2)-based DSSC showed V OC  = 0.70 V, J SC  = 11.69 mAcm − 2 , FF = 65.3, and η  = 5.4% and V OC  = 0.706 V, J SC  = 7.14 mAcm − 2 , FF = 55.6, and η  = 2.80% [ 150 ] under AM 1.5100 mWcm −2 illumination, respectively. The effect of hydrophilic sensitizer PTZ-TEG together with an aqueous choline chloride-based deep eutectic solvent (used as an electrolyte) has been reported [ 253 ]. In the study, glucuronic acid (GA) was used as a co-absorbent because it as has a simple structure and polar nature and is also able to better interact with hydrophilic media and components and possibly participates to the hydrogen bind interaction operated in the DES medium. PCE of 0.50% was achieved for the 1:1 dye/coabsorbent ratio.

Carbazole Dyes

It is a non-planar compound and can improve the hole transporting ability of the materials as well as avert the dye aggregate formation [ 235 ]. Due to its unique optical, electrical, and chemical properties, this compound has been applied as an active component in solar cells [ 254 , 255 ]. Even with the addition of carbazole unit into the structure, the thermal stability and glassy state durability of the organic molecules were observed to be improved significantly [ 256 , 257 ]. Tian et al. reported an efficiency of 6.02% for the DSSCs using S4 dye as a sensitizer, with an additional carbazole moiety to the outside of the donor group and found that the additional moiety facilitated the charge separation thereby decreasing the recombination rate between conduction band electrons and the oxidized sensitizer [ 185 ].

A series of MK-1, MK-2, and MK-3 dyes based on carbazole were reported by Koumura et al., where MK-1 and MK-2 have alkyl groups but MK-3 had no alkyl group. They showed that the presence of alkyl groups increased the electron lifetime and consequently V OC in MK-1 and MK-2 [ 203 , 258 , 259 ], and due to the absence of alkyl groups, lower electron lifetime values could be responsible for the recombination process between the conduction band electrons and dye cations in MK-3. New structured dyes, i.e., D–A–π–A-type and D–D–π–A-type organic dyes, have been developed by inserting the subordinate donor–acceptor such as 3,6-ditert-butylcarbazole-2,3-diphenylquinoxaline to facilitate electron migration, restrain dye aggregation, and improve photostability [ 260 ]. Thus, by further extending the π conjugation of the linkers, mounting the electron-donating and electron-accepting capability of donors and acceptors, and substituting long alkyl chains, more stable DSSCs with lower dye aggregation and higher efficiency can be achieved.

Phenoxazine (POZ) Dyes

Phenoxazine is a tricyclic isoster of PTZ. The PTZ and POZ units display a stronger electron donating ability than the TPA unit (0.848, 0.880, and 1.04 V vs. normal hydrogen electrode (NHE), respectively) [ 261 ]. However, DSSCs based on POZ dyes show better cell performance as compared to PTZ dye-based DSSCs [ 261 ]. In 2009, two POZ-based dyes were demonstrated by Tian et al., i.e., a simple POZ dye TH301 and triphenylamine attached to TH301, named as TH305. Due to the insertion of TPA unit in TH305, a red shift in the absorption band was seen because of the higher electron donating capability of POZ. The efficiencies obtained for TH301 and TH305 were 6.2% and 7.7%, respectively, where standard N719 sensitizer showed an efficiency of 8.0% under similar conditions [ 206 ]. Thus, in 2011, Karslson reported a series of dyes MP03, MK05, MK08, MK12, and MK13, based on POZ unit, to increase the absorption properties of the sensitizers [ 261 ]. Further, two novel metal-free dyes (DPP-I and DPP-II) with a diketopyrrolopyrrole (DPP) core were synthesized for dye-sensitized solar cells (DSSCs) by Qu et al. [ 262 ]. They demonstrated the better photovoltaic performance with a maximum monochromatic IPCE of 80% and η  = 4.14% with J SC  = 9.78 mAcm − 2 , V OC  = 605 mV, and FF = 0.69, for the DSSC based on dye DPP-I.

Singh et al. have demonstrated nanocrystalline TiO 2 dye-sensitized solar cells with PCE of 4.47% successfully designed two metal-free dyes (TPA–CN1–R2 and TPA–CN2–R1), containing triphenylamine and cyanovinylene 4-nitrophenyls as donors and carboxylic acid as an acceptor [ 263 ].

Semiconductor quantum dots (QDs) are another attractive approach to being sensitizers. These are II–VI and III–V type semiconductor particles whose size is small enough to produce quantum confinement effects. QD is a fluorescent semiconductor nanocrystal or nanoparticle typically between 10 and 100 atoms in diameter and confines the motion of electrons in conduction band, holes in valence band, or simply excitons in all three spatial directions. Thus, by changing the size of the particle, the absorption spectrum of such QDs can be easily varied. An efficiency of 7.0% has been recorded by collaborating groups from the University of Toronto and EPFL [ 264 ]. This recorded efficiency was higher than the solid-state DSSCs and lower than the DSSCs based on liquid electrolytes. A high performance QDSSC with 4.2% of PCE was demonstrated by Li et al. This cell consisted of TiO 2 /CuInS 2 -QDs/CdS/ZnS photoanode, a polysulfide electrolyte, and a CuS counter electrode [ 265 ]. In 2014, a conversion efficiency of 8.55% has been reported by Chuang et al. [ 266 ]. Recently, Saad and co-workers investigated the influence on the absorbance peak on N719 dye due to the combination between cadmium selenide (CdSe) QDs and zinc sulfide (ZnS) QDs [ 267 ]. The cyclic voltammetry (CV) of varying wt% of ZnS found that the 40 wt% of ZnS is an apposite combination for a DSSC’s photoanode and has produced the higher current. However, 50 wt% of ZnS was found to be the best concerto to increase the absorbance peak of the photoanode.

Natural dyes

New dye materials are also under extensive research, due to the intrinsic properties of Ru(II)-based dyes, and as a result to replace these rare and expensive Ru(II) complexes, the cheaper and environmentally friendly natural dyes overcome as an alternative [ 268 ].

Natural dyes provide low-cost and environmentally friendly DSSCs. There are various natural dyes containing anthocyanin [ 268 ], chlorophyll [ 269 ], flavonoid [ 270 ], carotenoid [ 271 ], etc. which have been used as sensitizers in DSSCs. Table  5 provides the general characteristics of these dyes, i.e., their availability and color range.

Molecular Structure

Anthocyanin : The molecular structure of anthocyanin is shown in Fig.  23 a. In anthocyanin molecule, the carbonyl and hydroxyl groups are bound to the semiconductor (TiO 2 ) surface, which stimulates the electron transfer from the sensitizer (anthocyanin molecules) to the conduction band of porous semiconducting (TiO 2 ) film. Anthocyanin can absorb light and transfer that light energy by resonance energy transfers to the anthocyanin pair in the reaction center of the photosystems [ 272 ].

Molecular structures of LD porphyrins

Flavonoid : Flavonoid is an enormous compilation of natural dyes which shows a carbon framework (C 6 –C 3 –C 6 ) or more particularly the phenylbenzopyran functionality, as shown in Fig.  23 b [ 273 ]. It contains 15 carbons with two phenyl rings connected by three carbon bridges, forming a third ring, where the different colors of flavonoids depend on the degree of phenyl ring oxidation (C-ring). Its adsorption onto mesoporous TiO 2 surface is quite fast by displacing an OH counter ion from the Ti sites that combines with a proton donated by the flavonoid [ 274 ].

Carotenoid : Andanthocyanin, flavonoids, and carotenoids are often found in the same organs [ 275 ]. Carotenoids are the compounds having eight isoprenoid units that are widespread in nature (as shown in Fig.  23 c). Beta-carotene dye has an absorbance in wavelength zones from 415 to 508 nm, has the largest photoconductivity of 8.2 × 10 − 4 and 28.3 × 10 − 4 (Ω.m) − 1 in dark and bright conditions [ 276 ], and has great potential as energy harvesters and sensitizers for DSSCs [ 277 ].

Cholorophyll: Among six different types of chlorophyll pigments that actually exist, Chl α is the most occurring type. Its molecular structure comprises a chlorine ring with a Mg center, along with different side chains and a hydrocarbon trail, depending on the Chl type (as shown in Fig.  23d ).

In 1997, antocyanins extracted from blackberries gave a conversion efficiency of 0.56% [ 268 ]. The roselle ( Hibiscus sabdariffa containing anthocyanin) flowers and papaw ( Carica papaya containing chlorophyll) leaves were also investigated as natural sensitizers for DSSCs. Eli et al. sensitized TiO 2 photoelectrode with roselle extract ( η  = 0.046%) and papaw leaves ( η  = 0.022%), respectively and found better efficiency for roselle extract-sensitized cell because of the broader absorption of the roselle extract onto TiO 2 [ 278 ]. Tannins have also been attracted as a sensitizer in DSSCs due to their photochemical stability. DSSCs using natural dyes tannins and other polyphenols (extracted from Ceylon black tea) have given photocurrents of up to 8 mAcm − 2 [ 168 ]. Haryanto et al. fabricated a DSSC using annato seeds ( Bixa orellana Linn ) as a sensitizer [ 279 ]. They demonstrated V OC and J SC for 30 g, 40 g, and 50 g as 0.4000 V, 0.4251 V, and 0.4502 V and 0.000074 A, 0.000458 A, and 0.000857 A, respectively. The efficiencies of the fabricated solar cells using annato seeds as a sensitizer for each varying mass were 0.00799%, 0.01237%, and 0.05696%. They observed 328–515 nm wavelength range for annato seeds with the help of a UV-vis spectrometer. Hemalatha et al. reported a PCE of 0.22% for the Kerria japonica carotenoid dye-sensitized solar cells in 2012 [ 280 ].

In 2017, a paper was published on DSSCs sensitized with four natural dyes (viz. Indian jamun, plum, black currant, and berries). The cell achieved highest PCE of 0.55% and 0.53%, respectively, for anthocyanin extracts of blackcurrant and mixed berry juice [ 281 ]. Flavonoid dye extracted from Botuje ( Jathopha curcas Linn ) has been used a sensitizer in DSSCs. Boyo et al. achieved η  = 0.12% with the J SC  = 0.69 mAcm − 2 , V OC  = 0.054 V, and FF = 0.87 for the flavonoid dye-sensitized solar cell [ 282 ]. Bougainvillea and bottlebrush flower can also be used as a sensitizer in DSSCs because both of them show a good absorption level in the range of 400 to 600 nm as a sensitizer, with peak absorption at 520 nm for bougainvillea and 510 nm for bottlebrush flower [ 283 ]. A study of color stability of anthocyanin (mangosteen pericarp) with co-pigmentation method has been conducted by Munawaroh et al. They have found higher color retention for anthocyanin-malic acid and anthocyanin-ascorbic acid than that of pure anthocyanin [ 284 ]. Thus, the addition of ascorbic acid and malic acid as a co-pigment can be performed to protect the color retention of anthocyanin (mangosteen pericarp) from the degradation process. The I – V characteristics of DSSCs employing different natural dyes are shown in Table  6 .

Organic Complexes of Other Metals

  Os, Fe and Pt complexes [ 285 , 286 , 287 ] are considered to be some other promising materials in DSSCs. Besides the fact that Os complexes are highly toxic, they are applied as a sensitizer in DSSCs due to its intense absorption (α811nm = 1.5 × 103 M− 1 cm− 1) and for the utilization of spin forbidden singlet-triplet MLCT transition in the NIR. Higher IPCE values were obtained in this spectral region; however, the overall conversion efficiency was only 50% of a standard Ru dye. Pt complexes have given modest efficiencies of ca. 0.64% [ 286 ] a and iron complexes, which are very interesting due to the vast abundance of the metal and its non-toxicity; the solvatochromism of complexes like [FeIIL2(CN)2] can be used to adjust their ground and excited state potentials and increase the driving force for electron injection into the semiconductor conduction band or for regeneration of the oxidized dye by the electrolyte couple [ 287 ].

Thus, a number of metal dyes, metal-free organic dyes, and natural dyes have been synthesized till today. Many other dyes like K51 [ 288 ], K60 [ 289 ], K68 [ 290 ]; D5, D6 (containing oligophenylenevinylene π-conjugated backbones, each with one N , N -dibutylamino moiety) [ 291 ]; K77 [ 292 ]; SJW-E1 [ 293 ]; S8 [ 294 ]; JK91 and JK92 [ 295 ]; CBTR, CfBTR, CiPoR, CifPoR, and CifPR [ 296 , 297 ]; Complexes A1, A2, and A3 [ 298 ]; T18 [ 299 ]; A597 [ 300 ]; YS-1–YS-5 [ 301 ]; YE05 [ 302 ]; and TFRS-1–3 [ 303 ] were developed and applied as sensitizers in DSSCs.

Latest Approaches and Trends

However, a different trend to optimize the performance of the DSSCs has been started by adding the energy relay dyes (ERDs) to the electrolyte [ 57 , 304 ]; inserting phosphorescence or luminescent chromophores, such as applying rare-earth doped oxides [ 58 , 59 , 60 ] into the DSSC; and coating a luminescent layer on the glass of the photoanode [ 61 , 62 ]. In the process of adding the ERDs to the electrolyte or to the HTM, some highly luminescent fluorophores have to be chosen. The main role of ERD molecules in DSSCs is to absorb the light that is not in the primary absorption spectrum range of the sensitizing dye and then transfer the energy non-radiatively to the sensitizing dyes by the fluorescence (Forster) resonance energy transfer (FRET) effect [ 305 ]. An improvement in the external quantum efficiency of 5 to 10% in the spectrum range from 400 to 500 nm has been demonstrated by Siegers and colleagues [ 306 ]. Recently, Lin et al. reported the doping of 1,8-naphthalimide (N-Bu) derivative fluorophore directly into a TiO 2 mesoporous film with N719 for application in DSSCs [ 307 ], in which the N-Bu functioned as the FRET donor and transferred the energy via spectral down-conversion to the N719 molecules (FRET acceptor). An improvement of the PCE from 7.63 to 8.13% under 1 sun (AM 1.5) illumination was attained by the cell. Similarly, Prathiwi et al. fabricated a DSSC by adding a synthetic dye into the natural dye containing anthocyanin (from red cabbage) in 2017 [ 308 ]. They prepared two different dyes at different volumes, i.e., anthocyanin dye at a volume of 10 ml and combination dyes at a volume of 8 ml (anthocyanin): 2 ml (N719 synthetic dye), respectively. They observed an enhancement in conversion efficiency up to 125%, because individually the anthocyanin dye achieved a conversion efficiency of 0.024% whereas for the combination dye 0.054% conversion efficiency was achieved. This enhancement was considered due to the higher light absorption. Thus, greater photon absorption took place and the electrons in excited state were also increased to enhance the photocurrent. Thus, cocktail dyes are also developing as a new trend in DSSCs. Chang et al. achieved a η  = 1.47% when chlorophyll dye (from wormwood) and anthocyanin dye (from purple cabbage) as natural dyes were mixed together at volume ratio of 1:1 [ 309 ], whereas the individual dyes showed lower conversion efficiencies. Puspitasari et al. fabricated different DSSCs by mixing the three different natural dyes as turmeric, mangosteen, and chlorophyll. The highest efficiency of 0.0566% was attained for the mixture of the three dyes, where the absorbance peak of the mixed dyes was observed at 300 nm and 432 nm [ 106 ]. Similarly, Lim and co-workers have achieved a 0.085% of efficiency when mixing the chlorophyll and xanthophyll dyes together [ 310 ]. In 2018, Konno et al. studied the PV characteristics of DSSCs by mixing different dyes and observed highest ɳ  = 3.03% for the combination dye “D358 + D131,” respectively [ 311 ]. Figure  24 shows the IPCE of mixed pigments and single pigments.

UV-vis spectra and in insert Q-band magnification for CPI, CPICu, and CPIZn incorporated into the TiO 2 films [ 237 ]

An approach used to enhance the performance of DSSCs is plasmonic effect. Surface plasmon resonance (SPR) is resonant oscillation of conduction electrons at the interface between negative and positive permittivity material stimulated by incident light. In 2013, Gangishetty and co-workers synthesized core-shell NPs comprising a triangular nanoprism core and a silica shell of variable thickness. SPR band centered at ~ 730 nm was observed for the nanoprism Ag particles, which overlapped with the edge of the N719 absorption spectrum very well. They found the incorporation of the nanoprism Ag particles into the photoanode of the DSSCs yielded a 32% increase in the overall PCE [ 312 ]. Hossain et al. used the phenomenon of plasmonic with different amounts of silver nanoparticles (Ag NPs) coated with a SiO 2 layer prepared as core shell Ag@SiO 2 nanoparticles (Ag@SiO 2 NPs) and studied the effect of SiO 2 -encapsulated Ag nanoparticles in DSSCs. They found the highest PCE of 6.16% for the photoanode incorporated 3 wt% Ag@SiO 2 ; the optimal PCE was 43.25% higher than that of a 0 wt% Ag@SiO 2 NP photoanode [ 313 ]. However, a simultaneous decrease in the efficiency with further increases in the wt% ratio, i.e., for 4 wt% Ag@SiO 2 and 5 wt% Ag@SiO 2 , was observed. This decrease for the excess amounts of Ag@SiO 2 NPs was attributed to three reasons: (i) reduction in the effective surface area of the films, (ii) absorption of less amount of the dye, and (iii) an increase in the charge-carrier recombination [ 314 ]. After analyzing the nyquist plots (as shown in Fig.  25 ), they have found a decreased diameter of Z 2 monotonically as the Ag@SiO 2 NP content increased to 3 wt% and R2 decreased from 10.4 to 6.64 Ω for the conventional DSSC to the 3 wt% Ag@SiO 2 NPs containing DSSC. Jun et al. used quantum-sized gold NPs to create plasmonic effects in DSSCs [ 315 ]. They fabricated the TiO 2 photoanode by incorporating the Au nanoparticles (Au NPs) with an average diameter of 5 nm into the commercial TiO 2 powder (average diameter 25 nm) and used N749 black dye as a sensitizer. Thus, due to the SPR effect, the efficiency for the DSSC (incorporating Au NPs) was enhanced by about 50% compared to that without Au nanoparticles. Effect of incorporating green-synthesized Ag NPs into the TiO 2 photoanode has been investigated in 2017 [ 316 ]. Uniform Ag NPs synthesized by treating silver ions with Peltophorum pterocarpum flower extract at room temperature showed the Ag NPs as polycrystalline in nature with face centered cubic lattice with an approximate size in the range of 20–50 nm [ 316 ]. The PCE of the device was improved from 2.83 to 3.62% with increment around 28% after incorporation of the 2 wt% of the Ag NPs due to the plasmonic effect of the modified electrode. Bakr et al. have fabricated Z907 dye-sensitized solar cell using gold nanoparticles prepared by pulsed Nd:YAG laser ablation in ethanol at wavelength of 1064 nm [ 63 ]. The addition of synthesized Au NPs to the Z907 dye increased the absorption of the Z907 dye, thus achieving ɳ  = 1.284% for the cell without Au NPs and 2.357% for the cell incorporating the Au NPs. Recently, in 2018, a novel 3-D transparent photoanode and scattering center design was applied as to increase the energy conversion efficiency from 6.3 to 7.2% of the device [ 317 ] because the plasmonics plays an important role in the absorption of light and thus, the application is developing at a very fast pace and grabbing a lot of attention worldwide in the last few years. Recently, a study on incorporation of Mn 2+ into CdSe quantum dots was carried out by Zhang and group [ 318 ]. An improved efficiency from 3.4% (CdS/CdSe) to 4.9% (CdS/Mn-CdSe) was achieved for the device upon the addition of Mn 2+ into CdSe because when Mn 2+ is doped into the CdSe (as shown in Fig.  26 ), the QDs on the surface of the film became compact and the voids among the particles were small, thus reducing the recombination of photogenerated electrons. Also with the loading of Mn 2+ into the CdSe, the size of the QD clusters was increased. However, in QDSCs (quantum dot-sensitized solar cells), there is an inefficient transfer of electrons through the mesoporous semi-conductor layer [ 319 ], because their application on a commercial level is still far off. Thus, Surana et al. reported the assembling of CdSe QDs, tuned for photon trapping at different wavelengths in order to achieve an optimum band alignment for better charge transfer in QDSC [ 319 ]. TiO 2 hollow spheres (THSs) synthesized by the sacrifice template method was reported as a scattering layer for a bi-layered photoanode for DSSCs by Zhang and co-workers [ 320 ]. They used the mixture of multi-walled carbon nanotubes with P25 as an under layer and THSs as an overlayer for the photoanode which showed good light scattering ability. The cross-sectional FESEM images revealed the disordered mecroporous network for the scattering layer containing THSs which was supposed to be responsible for the enhanced light absorption and the transfer of electrolyte. Thus, ɳ  = 5.13% was achieved for P25/MWNTs-THSs, whereas 4.49% of efficiency was reported for a pure P25 photoanode-based DSSC. Also, the electron lifetime ( τ e ) estimated for pure P25 by Bode phase plots of EIS spectra was 5.49 ms; however, 7.96 ms was shown for P25/MWNTs-THSs.

Chemical structures of a anthocyanin, b flavonoid, c β,β-carotene, and d chlorophyll

IPCE of mixed pigment and single pigments, where single pigment were Eosin Y, D131, and D358 and mixed pigments were D358 and Eosin Y; D358 and D131; D131 and Eosin Y [ 311 ]

a Nyquist plots obtained from the EIS of DSSCs with varying Ag@SiO 2 content (inset shows the equivalent circuit). b R2 ohm with respect to the Ag@SiO 2 NPs content [ 313 ]

SEM images of a CdS/CdSe and b CdS/Mn:CdSe QD sensitization on TiO 2 surface. c TEM image of CdS/Mn:CdSe QDs [ 318 ]

John and group reported the synthesis and application of ZnO-doped TiO 2 nanotube/ZnO nanoflake heterostructure as a photoanode in DSSCs for the first time in 2016 [ 321 ]. They used different characterization techniques to investigate the layered structure of the novel nanostructure. The Rutherford backscattering spectroscopy revealed that during the doping process, a small percentage of Zn was doped into TONT in addition to the formation of ZnO nanoflakes on the top, which led to a preferential orientation of the nanocrystallites in the tube on annealing. Back in 2017, Zhang et al. reported paper on low-dimensional halide perovskite and their applications in optoelectronics due to the ~ 100% of photoluminescence quantum yields of perovskite quantum dots [ 322 ]. The main emphasis of their paper was on the study of halide perovskites and their versatile application, i.e., in optoelectronics in spite of PV applications only. The main role of perovskite nanoparticles in solar cells is being applied as sensitizers. Similarly, in the queue of developing highly efficient DSSCs, Chiang and co-workers fabricated DSSCs based on PtCoFe nanowires with rich {111} facets exhibiting superior I −   3 reduction activity as a counter electrode, which surpassed the previous PCE record of the DSSCs using Ru(II)-based dyes [ 323 ]. Recently, in accordance with enhancing the charge collection efficiencies ( η coll ) as well as PCE of DSSCs, Kunzmann et al. reported a new strategy of fabricating low-temperature (lt)-sintered DSSC and demonstrated the highest efficacy reported for lt-DSSC to date [ 324 ]. They have integrated TiO 2 -Ru(II) complex (TiO 2 _Ru_IS)-based hybrid NPs into the photoelectrode. Due to a better charge transport and a reduced electron recombination, devices with single-layer photoelectrodes featuring blends of P25 and TiO 2 _Ru_IS give rise to a 60% η coll relative to a 46% η coll for devices with P25-based photoelectrodes. Further, for usage of a multilayered photoelectrode architecture with a top layer based on TiO 2 _Ru_IS only, devices with an even higher η coll (74%) featuring a η  = 8.75% and stabilities of 600 h were shown. The two major rewards obtained for such devices were the dye stability due to its amalgamation into the TiO 2 anatase network and, secondly, the enhanced charge collection yield due to its significant resistance towards electron recombination with electrolytes.

Conclusions

The main aim of this study was to put a comprehensive review on new materials for photoanodes, counter electrodes, electrolytes, and sensitizers as to provide low-cost, flexible, environmentally sustainable, and easy to synthesize DSSCs. However, a brief explanation has been given to greater understand the working and components of DSSCs. One of the important emphases in this article has been made to establish a relation between the photosensitizer structure, the interfacial charge transfer reactions, and the device performance which are essential to know as to develop new metal and metal-free organic dyes. In terms of low stability offered by DSSCs, two major issues, i.e., low intrinsic stability and the sealing of the electrolytes (extrinsic stability), have been undertaken in this study. To fulfill huge demand of electricity and power, we have two best possible solutions: this demand should be compensated either by the nuclear fission or by the sun. Even so, the nuclear fission predicted to be the best alternative has great environmental issues as well as a problems associated with its waste disposal. Thus, the second alternative is better to follow. DSSCs are developed as a cheap alternative but the efficiency offered by DSSCs in the field is not sufficient. Thus, we have to do a wide research on all possible aspects of DSSCs. We proposed to develop DSCCs based on different electrodes viz. graphene, nanowires, nanotubes, and quantum dots; new photosensitizers based on metal complexes of Ru or Os/organic metal-free complexes/natural dyes; and new electrolytes based on imidazolium salts/pyridinium salts/conjugated polymers, gel electrolytes, polymer electrolytes, and water-based electrolytes. In summary, so far, extensive studies have been carried out addressing individual challenges associated with working electrode, dye, and electrolytes separately; hence, a comprehensive approach needs to be used where all these issues should be addressed together by choosing appropriate conditions of electrolyte (both in choice of material and structure), optimum dye, and the most stable electrolyte which provides better electron transportation capability.

In terms of their commercial application, a DSSC needs to be sustainable for > 25 years in building-integrated modules to avoid commotion of the building environment for repair or replacement and a lifespan of 5 years is sufficient for portable electronic chargers integrated into apparel and accessories [ 325 ]. However, DSSCs are being quite bulky due to their sandwiched glass structure, but the flexible DSSCs (discussed elsewhere) that can be processed using roll-to-roll methods may came as an alternative but then has to compromise with the shorter lifespan. Although the stability and lifetime of a DSSC most probably depend on the encapsulation and sealing as discussed above. Apart from the usage of expensive glass substrates in the case of modules and panels, one of the biggest hurdles is to manufacture glass that is flat at the 10 μm length scale over areas much larger than 30 × 30 cm 2 [ 326 ] and the humidity. Another challenge is to choose which metal interconnects in the cells that are more or less corroded to the electrolyte, and high degree of control over cell-to-cell reproducibility is required to achieve same current and/or voltage for all the cells in the module. If the abovementioned challenges would be overcome, then there is no roadblock for the commercial applications of DSSCs, which has been restricted up to an amicable extent. G24i has introduced a DSC module production of 25 MW capacity in 2007 in Cardiff, Wales (UK), with extension plans up to 200 MW by the end of 2008 (http://www. g24i. com), and afterwards, many DSSC demonstration modules are now available. However, the maximum outdoor aging test of DSSCs is reported for 2.5 years up to now [ 327 ].

Abbreviations

Acetonitrile

Silver nanoparticles

Air mass 1.5

Gold nanoparticles

Boradiazaindacene

4,5-Bis(4-methoxyphenyl)-1H-imidazole

Chenodeoxycholic acid

Cadmium selenide

Carbon nanofiber

Carbon nanotube

Conjugated polymer electrolytes

Copper bromide

Copper iodide

Copper thiocyanate

Cyclic voltammetry

Diketopyrrolopyrrole

Dye-sensitized solar cell database

Dye-sensitized solar cells

Ethylene carbonate

Electron impedance spectroscopy

Ecole Polytechnique Fédèrale de Lausanne

Energy relay dyes

Fill factor

Fluorescence (Forster) resonance energy transfer

Fluorine-doped tin oxide

Glucuronic acid

γ-Butyrolactone

Guanidinium thiocyanate

Hafnium oxide

Highest occupied molecular orbital

Hole transport materials

Ionic liquid

Intensity-modulated photovoltage spectroscopy

Incident photon to current conversion efficiency

Indium-doped tin oxide

Short circuit current

Liquid crystals

Light harvesting efficiency

Lowest unoccupied molecular orbital

3-Methoxypropionitrile

Metal to ligand charge transfer

Nanocrystalline

Normal hydrogen electrode

Near-infrared region

N -Methylbenzimidazole

N -Methylpyrrolidine

Nanoparticles

Polyaniline

Propylene carbonate

Power conversion efficiency

Poly(ethylene oxide)

Polyethylene terephthalate

Maximum power output

Phenoxazine

Phenothiazine

Photovoltaic

Quantum dots

Quasi-solid-state electrolyte

Room temperature

Saturated calomel electrode

Solar cells

Surface plasmon resonance

Solid-state electrolyte

4-Tert-butylpyridine

Transparent conducting oxide

Titanium dioxide

Triphenylamine

Ultraviolet-visible

Open circuit voltage

Working electrode

Zinc sulfide

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Acknowledgements

The authors would like to acknowledge the Department of Science & Technology, SERB Division, Govt. of India (Award # SR/FTP/PS-112/2012 Dated 1.11.2013), for providing financial support to carry out research on dye-sensitized solar cells (DSSCs).

This work was supported by Science and Engineering Research Board, Department of Science & Technology, Govt. of India (SR/FTP/PS-112/2012).

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It is a review article that gives a comprehensive study about the materials including the photoanode, sensitizer, electrolyte and counter electrode, device architecture, and fabricating techniques used in the fabrication of dye-sensitized solar cells (DSSCs). It emphasizes the role of the sensitizer and the strategies to improve the performances of the dye as well as some recent development aiming to answer specific issues till date.

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Department of Physics, Bhagwant University, Ajmer, 305004, India

Khushboo Sharma

School of Materials Science and Engineering, Nanyang Technological University, Singapore, 639798, Singapore

Vinay Sharma

Department of Physics, Govt. Women Engineering College, Ajmer, 305002, India

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SSS developed the concept. VS collected the study materials required for the preparation of the manuscript entitled “Dye Sensitized Solar Cells: Fundamentals and Current Status”. SSS is a supervisor of one author. KS drafted the article and SSS polished the content to present form. All authors reviewed the paper. All authors read and approved the final manuscript.

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Khushboo Sharma is a Research Scholar in Physics at Bhagwant University, Ajmer, India, and currently working in the field of dye-sensitized solar cells. She did her Master’s degree in 2013 from MDS University, Ajmer, India. She had been working as a project fellow for project on “Development of New Materials for Dye-Sensitized Solar Cells” of Department of Science and Technology, SERB Division, New Delhi at Government Women Engineering College, Ajmer, India.

Vinay kumar Sharma did his M. Tech in Material Science from the Centre for Converging Technologies, University of Rajasthan, Jaipur, India. Presently, he is working at the School of Materials Science and Engineering, Nanyang Technological University, Singapore. Vinay does his research in Materials Physics, Solid State Physics and Materials Science. His current project is on “Magnetocaloric effect in iron based systems”.

Shyam S. Sharma is a faculty in Physics at the Govt. Women Engineering College, Ajmer, India. He obtained his Ph.D. in 2010 at the University of Rajasthan, Jaipur, India, in the field of Organic Solar Cells. His research interest is in the area of organic semiconductor materials and devices for electronic and optoelectronic technology. He has about 50 scientific publications in international journals and proceedings of international and national conferences, and has published a book on Synthesis and characterization of organic photovoltaic cells. He has been honored for his research work with an Innovative Engineer Award from United Engineers Council. He is a life member of the Indian Physics Association (IPA), Indian Association of Physics Teacher (IAPT), Material Research Society of India (MRSI), and The Indian Science Congress Association. He is also associated with the Material Research Society of Singapore, Synchrotron Radiation Center, Italy, and UGC-DAE CSR, Indore. Presently, he is the Chief Coordinator of the World Bank funded project TEQIP (Technical Education Quality Improvement Programme) Phase-III in his institute.

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Sharma, K., Sharma, V. & Sharma, S.S. Dye-Sensitized Solar Cells: Fundamentals and Current Status. Nanoscale Res Lett 13 , 381 (2018). https://doi.org/10.1186/s11671-018-2760-6

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Received : 22 September 2017

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Published : 28 November 2018

DOI : https://doi.org/10.1186/s11671-018-2760-6

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The researchers were interested in finding out if solar cells, devices that turn sunlight into electricity, could be tweaked to perform better in different parts of the world, where concentration of solar light may be higher. To examine this, they used machine learning models and neural networks (AI) to understand how the sun's radiation would behave in different spots on Earth.

They integrated this data into an electronic model to calculate the solar cells' output. By simulating various scenarios, they could predict how much energy the solar cells could produce at various locations worldwide.

Their findings published in Joule , however, revealed a surprising twist. "Making solar cells super-efficient turns out to be very difficult. So, instead of just trying to make solar cells better, we figured some other ways to capture more solar energy," said Dr. Tomi Baikie, first author of the study and Research Fellow at the Cavendish Laboratory and at Lucy Cavendish College. "This could be really helpful for communities, giving them different options to think about, instead of just focusing on making the cells more efficient with light."

Imagine solar panels that can flex and fold like origami or become partially transparent to blend seamlessly into surroundings and make them easy to install. By enhancing the durability and versatility of these panels, they could be integrated into a wide range of settings, promising longevity and efficiency.

"We suggest a different plan that can make solar panels work well in lots of different places around the world," said Baikie. "The idea is to make them flexible, a bit see-through/semi-transparent, and able to fold up. This way, the panels can fit into all kinds of places."

Furthermore, the researchers advocate the use of patterning the solar capture devices with the aim to optimise their arrangement for maximum sunlight absorption. This approach holds the potential to improve the design of solar arrays, increasing their effectiveness in harnessing solar energy.

"This realisation means that we can now focus on different things instead of just making solar cells work better. In future, we're going to examine solar harvesting pathwaysthat includes tessellation. It's like a puzzle pattern that could help us capture even more sun power," concluded Baikie.

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Materials provided by University of Cambridge . Note: Content may be edited for style and length.

Journal Reference :

• Tomi K. Baikie, Benjamin Daiber, Emil Kensington, James Xiao, Neil C. Greenham, Bruno Ehrler, Akshay Rao. Revealing the potential of luminescent solar concentrators in real-world environments . Joule , 2024; 8 (3): 799 DOI: 10.1016/j.joule.2024.01.018

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Perovskite solar cell with self-assembled organic electron transport layer achieves 21.5% efficiency

Developed by scientists in Saudia Arabia, the device is reportedly the most efficient perovskite solar cell ever reported to date with an organic electron transporting layer. The cell achieved an open-circuit voltage of 1.13 V, a short-circuit current density of 24.7 mA cm2, and a fill factor of 77%

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Schematic of the solar cell

Image: KAUST, ACE Energy Letters, Creative Commons License CC BY 4.0

A group of researchers led by Saudi Arabia's King Abdullah University of Science and Technology (KAUST) has fabricated a perovskite solar cell based on an organic electron transport layer (ETL) using self-assembled monolayer (SAM) molecules containing phosphonic acid as an anchoring group.

“All self-assembled monolayers were designed to collect holes, which works well for perovskite solar cells in the p-i-n polarity architecture,” the research's lead author, Stefaan De Wolf, told pv magazine . “Here, we tested a range of electron-selective SAMs designed and synthesized by Kaunas University of Technology and found that these work well in p-i-n polarity perovskite solar cells. So essentially the concept of SAM decoration of metal oxide to tune the charge selectivity has now been proven to work well for both polarities. Overall, this has quite some advantages, as for example low-temperature processibility of the contacts.”

The scientists used non-fullerene semiconductors composed of molecules known as anthraquinone (AQ) and naphthalenediimide (NDI), which they said allow covalent binding with indium tin oxide (ITO) surfaces within the cell and energetic matching with the perovskite. They used, in particular, two modified versions of the molecules, which they called PANDI and PAAQ.

The research group built the cell with a substrate made of glass and ITO, the electron-selecting SAMs, a perovskite absorber, a hole transport layer (HTL) based on Spiro-OMeTAD , a molybdenum oxide (MoOx) layer, and a silver (Ag) metal contact.

The academics conducted thermogravimetric analysis (TGA) on these modified molecules and found they are thermally stable with only 5% weight loss at 356 C and 268 C, respectively.

“UV–vis transmittance results of SAM functionalization on the ITO surface show negligible optical losses compared to bare ITO and ITO/SnO 2 films,” they stated, noting that these SAMs also showed relatively suitable energetic alignment with the perovskite absorber and ITO contact. “In particular, PANDI-based SAMs demonstrate a higher surface homogeneity on the ITO surface than PAAQ SAMs. We found that the increasing surface homogeneity on the ITO/PANDI can effectively suppress nonradiative interfacial recombination through the field-effect passivation, as indicated by longer charge carrier lifetimes and higher QFLS values.”

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Tested under standard illumination conditions, the cell achieved a power conversion efficiency of 21.5%, which the research team is the highest efficiency ever reported to date for a perovskite solar cell relying on an organic electron transporting layer. The device also achieved an open-circuit voltage of 1.13 V, a short-circuit current density of 24.7 mA cm 2 , and a fill factor of 77%

“We also tested the operational stability of our SAM-based devices at 65 C with above 90% retention of their initial performance for 1000 h,” the researchers also explained.

They identified the PANDI SAMs are the most suitable candidates for future perovskite solar cells with p-i-n structures, as they can be more easily applied to flexible substrates. “The PANDI-based device also showed improved operational long-term thermal stability, confirming that the PANDI SAM has a potential future to be utilized as ETL materials,” they concluded.

The solar cell was introduced in the study “ Nonfullerene Self-Assembled Monolayers As Electron-Selective Contacts for n-i-p Perovskite Solar Cells ,” which was recently published in ACS Energy Letters .

This content is protected by copyright and may not be reused. If you want to cooperate with us and would like to reuse some of our content, please contact: editors@pv-magazine.com .

Emiliano Bellini

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Mar. 27, 2024 Press Release Engineering

A solar cell you can bend and soak in water

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Researchers from the RIKEN Center for Emergent Matter Science and collaborators have developed an organic photovoltaic film that is both waterproof and flexible, allowing a solar cell to be put onto clothes and still function correctly after being rained on or even washed.

One of the potential uses of organic photovoltaics is to create wearable electronics—devices that can be attached to clothing that can monitor medical devices, for example, without requiring battery changes. However, researchers have found it challenging to achieve waterproofing without the use of extra layers that end up decreasing the flexibility of the film.

Now, in work published in Nature Communications , a group of scientists have been able to do precisely that. They took on the challenge of overcoming a key limitation of previous devices, which is that it is difficult to make them waterproof without reducing the flexibility. Photovoltaic films are typically made of several layers. There is an active later, which captures energy of a certain wavelength from sunlight, and uses this energy to separate electrons and “electron holes” into a cathode and anode. The electrons and holes can then reconnect through a circuit, generating electricity. In previous devices, the layer transporting the electron holes was generally created sequentially by layering.

For the current work, however, the researchers deposited the anode layer, in this case a silver electrode, directly onto the active layers, creating better adhesion between the layers. They used a thermal annealing process, exposing the film to air at 85 degrees Celsius for 24 hours. According to Sixing Xiong, the first author of the paper, “It was challenging to form the layer, but we were happy to have accomplished it, and in the end were able to create a film that was just 3 micrometers thick, and we looked forward to seeing the results of tests.”

What the group saw from the testing was very encouraging. First, they immersed the film completely in water for four hours and found that it still had 89 percent of its initial performance. They then subjected a film to stretching by 30 percent 300 times underwater, and found that even with that punishment, it retained 96 percent of its performance. As a final test, they ran it through a washing machine cycle, and it survived the ordeal, something that has never been achieved before.

According to Kenjiro Fukuda, one of the corresponding authors of the paper, “What we have created is a method that can be used more generally. Looking to the future, by improving the stability of devices in other areas, such as exposure to air, strong light, and mechanical stress, we plan to further develop our ultrathin organic solar cells so that they can be used for really practical wearable devices.”

In addition to RKEN CEMS, members of the research group were from the University of Tokyo and the Huazhong University of Science and Technology in China.

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Kenjiro Fukuda, Senior Research Scientist Emergent Soft System Research Team RIKEN Center for Emergent Matter Science

Jens Wilkinson RIKEN International Affairs Division Tel: +81-(0)48-462-1225 Email: jens.wilkinson [at] riken.jp

Waterproof and flexible organic photovoltaic film developed in this study

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ASU researcher Meng Tao and his collaborators aim to improve solar panel design to increase material recovery during recycling

by TJ Triolo | Mar 11, 2024 | Features , Research

With climate change becoming an increasingly dire problem, solar, or photovoltaic, power generation can help to remedy the problem as a zero-emission source of electricity. Despite providing green energy, solar panels aren’t without their environmental drawbacks: They’re difficult and expensive to recycle.

“For the last 40 years, the focus has been on solar panel efficiency, cost and reliability,” says Meng Tao , a professor of electrical engineering in the Ira A. Fulton Schools of Engineering at Arizona State University. “I was one of the pioneers who said, ‘We need recycling technologies.’”

After a panel’s life expectancy, which ranges from about 25 to 30 years , is reached and the panel is no longer useful, it’s typically added to the large amounts of trash that go into landfills and incinerators every year. As solar technology use continues to increase, the International Renewable Energy Agency estimates that by the 2050s, end-of-life solar panels will result in 5.5 to 6 million metric tons of waste per year.

Tao, a faculty member in the School of Electrical, Computer and Energy Engineering , part of the Fulton Schools, set out to enhance economical circularity — the reusability of products and their base materials — by recycling them. While his efforts thus far have focused on recycling methods for existing panel designs, he found limitations to how much of the material recycled from existing solar panels can be.

Tao then decided to increase panel circularity by collaborating with experts in a variety of areas. The result was five recommended changes to panel designs that will make them easier and more cost-efficient to recycle that are detailed in a paper in the One Earth scientific journal.

A circular economy that manufactures sustainability  

Among the recommendations Tao suggests is to eliminate lead from solar panels. The toxic metal is part of an alloy, or combination of metals, used with tin in the solder that connects solar panels’ conductive pieces together.

Current recycling techniques are uneconomical for lead recovery, costing about 100 times the value of the salvaged metal. Tao and his collaborators recommend replacing the solder with a lead-free alternative.

Along with lead, he sets his sights on replacing a more valuable metal found in solar panels: silver. Over time, solar panel producers have reduced their panels’ silver content to save on manufacturing costs. The silver in a panel is now low enough that recovery costs more than the material is worth.

So instead of using silver, Tao and his colleagues recommend using copper, which is already incorporated in solar panel parts, to reduce manufacturing costs and eliminate the need for silver recycling.

While there are technical challenges to overcome due to chemical reactions such as oxidation and interactions between copper and solar panels’ silicon cells, Tao consulted collaborator Thad Druffel , a researcher and expert in solar technology at the University of Louisville, to confirm the idea as worthy of exploration.

Replacing current materials for successful solar recycling

Adding to the difficulty of solar panel recycling is encapsulant, a compound used as a glue to stick solar panels’ glass casing to the silicon cells that convert light into electricity.

“You’ll want encapsulant to be stable under UV radiation, hot and cold temperatures, day and night, in moisture and all kinds of weather conditions,” he says. “But when it comes to end of life, you want an encapsulant that is very easy to degrade or destroy. It’s two opposite requirements.”

Tao recommends the use of a new encapsulant that can firmly hold panels together but detach easily when exposed to certain conditions, such as chemicals or heat, during the recycling process.

For accuracy and expertise in recommendations for new encapsulant materials, he consulted Kim McLoughlin, a principal engineer with chemical company Braskem .

A related problem to encapsulants is modern dual-glass panel designs. Dual-glass solar panels use glass to cover the front and back of a panel and are more rigid than older designs, which use a softer material known as a fluoropolymer , to cover the back.

While dual-glass panels are more durable and have higher power output than a traditional module, current technology can’t separate the glass from both sides of the panels. The result is that no metals in the cells can be recovered, and the glass will have to be recycled into lower quality raw material such as concrete aggregate or sandblasting.

Tao worked with Paul W. Leu , a University of Pittsburgh industrial engineering professor and B.P. Faculty Fellow, to develop ideas to make delamination between silicon and glass easier, such as a new encapsulant material.

Tao and his colleagues see even better sustainability potential for dual-glass modules if a recycling method can be developed for them: Fluoropolymers aren’t recyclable, meaning less material would need to be trashed for good when recycling dual-glass panels.

New ID system gives solar panels new life

Last on the list of recommendations is creating a system that can trace and identify solar panels for up to 25 years. Because different kinds of solar panels use distinct materials and designs, recycling methods that work well for one kind of panel may not perform as well for another.

After a solar panel’s life is spent being exposed to the elements, labels showing model numbers and specifications are often faded enough to be unidentifiable. The information on the labels is also limited and not provided in a consistent format. Recyclers can make only educated guesses as to the best ways to break the panels down into their components, resulting in wasted material if the wrong method is chosen based on appearance alone.

The current ID and label system can also present a problem when attempting to extend the life of an individual panel. When an older solar power generation system is replaced, some panels may still work for other systems. However, without being able to determine if a functional used panel has the proper specifications, it’s useless.

Tao and his colleagues suggest a new traceability system, featuring durable Smart Tags, which would consist of technology such as barcodes, QR codes and radio frequency identification tags, with industry standard unique IDs mapped to data to reveal make, model, constituent materials and electrical properties such as wattage. The system would work similarly to how digits in vehicle identification numbers on cars and trucks correspond to mechanical and cosmetic specifications.

Tao turned to Alicia Farag, co-founder and president at Locusview , a company that makes construction management and asset traceability software to create a digital thread from manufacturer to end user. Farag has expertise in standardized IDs for industries and consulted on the One Earth paper.

Additionally, Farag is working with Tao and Dwarak Ravikumar , a Fulton Schools assistant professor of civil, environmental and sustainable engineering and an expert in the circular economy and sustainability of photovoltaic systems, to develop such a solar panel ID system.

“Professor Meng Tao is a true expert, and it has been very insightful to combine his deep knowledge of solar panel design with our knowledge of asset tracking and traceability to provide a novel and innovative solution to solar panel traceability for reuse and recycling,” Farag says.

Tao and Ravikumar, a faculty member in the School of Sustainable Engineering and the Built Environment , part of the Fulton Schools, have a history of collaboration. Tao was on the committee for Ravikumar’s doctoral dissertation on improving solar panel circularity, and they have co-authored and published scientific studies on improving the sustainability of photovoltaic systems.

“The solar panel itself is good for the environment,” Ravikumar says. “But we have to understand that it still requires a lot of raw materials and energy to manufacture solar panels.”

He says much of solar panel manufacturing happens in regions heavy on fossil fuel use such as China, making it essential to use the panels for as long as possible and recover a maximum amount of material from end-of-life panels.

With so many recommendations for solar panel design and recycling to explore, Tao encourages students to assist in his research in the field. Students of all levels — from those studying for undergraduate to doctoral degrees — who are passionate about sustainability and the environment are eligible to apply. Interested students majoring in electrical engineering, chemistry, chemical engineering or materials science can contact Tao at [email protected] .

About The Author

TJ Triolo is the embedded communications specialist for the Ira A. Fulton Schools of Engineering's School of Electrical, Computer and Energy Engineering. He's a 2020 graduate of ASU's Walter Cronkite School of Journalism and Mass Communication. After starting his career in marketing and communications with a car wash company in Arizona, he joined the Fulton Schools' communications team in 2022.

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How Tiny Crystals Are Giving Solar Panels A Glow-Up

As the global conversation around renewable energy continues to heat up, Juan Guio ‘26 is looking at ways to keep things cool, at least where solar panels are concerned.

Guio spent the spring 2023 semester working on research pertaining to efficiency in photovoltaics as a member of Dr. Matthew Sheldon’s laboratory in the Department of Chemistry at Texas A&M University. While Guio’s part of the project was small — measuring and mapping how different solutions impacted the absorbance of certain wavelengths of light — the work thrilled him.

“The way I see it, small things are what make up the big things, so that makes chemistry very exciting,” he said.

The small things he was working with are a type of tiny crystals known as halide perovskite nanocrystals. “Think of them as little light boosters,” Sheldon said. Students worked on synthesizing these crystals and embedding them into thin, film-like structures to make them suitable for coating solar panels.

“Solar panels, which many of us see on rooftops or large solar farms, convert sunlight into electricity,” Sheldon explained. “One way to make these solar panels even more efficient is to use special coatings that can capture more light. In this research project, undergraduate students explored the development of coatings that can glow under light, essentially amplifying the amount of sunlight that reaches the solar panel.”

This work was very interesting to Guio, who was eager to try new solutions in the lab. “I got excited about figuring out what we could do next,” he said. “One of the properties of these nanocrystals is that when you shine a light at them at a certain energy, they emit light at a higher energy. They do that by taking in some heat.” Absorbing heat as well as light to create energy via a solar panel would be especially useful because it provides cooling as well as light, he explained.

Guio, who is originally from Bogota, Columbia, and currently resides in Houston, says he has long been attracted to chemical engineering because he loves working with chemical reactions. In addition to the science and hands-on lab work for the nanocrystals project, Guio and the other students in the Sheldon Research Group also had opportunities to analyze their results and share them through presentations, practicing the essential work of science communication.

Guio made up solutions of nanocrystals and solvents and then used a spectrophotometer to analyze how changes affected which wavelengths of light were absorbed. He also graphed results of different solutions to create an absorbance map. By the end of the semester, he was able to present the range of results to others in the lab working on different elements of nanocrystal research. He appreciated hearing about others’ research findings, as he built on their work to create a presentation that would provide value to the whole group.

Learning Through Research

While the work he was doing was minor in the larger field of photovoltaic research, Guio says it was a great introduction to the process of investigative lab work.

“I learned that research is not a linear process,” he reflected. “You work on ideas all over the place that open up room for creativity and lead you where you want to go next. You can take a risk and see ‘will this bear fruit and be helpful or not?’ and try other options.”

The experience cemented Guio’s desire to pursue a career in chemical engineering with an emphasis on energy.

“I’m fascinated by energy, in finding ways to improve the oil and gas industry, as well as renewable energy, maybe create something new that hasn’t been thought of yet,” he said.

A key takeaway from the experience for Guio was the importance of reaching out to faculty members to explore collaboration. Sheldon, one of Guio’s favorite teachers, had mentioned his research on photovoltaics in class, and Guio’s curiosity was piqued. He sent Sheldon an email asking if he could be involved in some way and was invited to participate in the lab work, though he was a freshman at the time.

“Don’t be afraid to reach out to a teacher if the topic is interesting to you,” he admonished other students. “Let them know that you’re interested and that you want to work with them, because they love working with motivated students.”

Media contact: Shana K. Hutchins, [email protected]

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