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Increasing energy demand, environmental issues, and limited availability of fossil fuels are demanding research on sustainable and renewable energy resources. The sun is the ultimate source to accomplish clean energy demand and photovoltaics, also known as solar PV, have been growing exponentially to harness it. The global solar photovoltaics (PV) industry has entered a new era, as since 2018 the electricity generation from PV has become one of the cheapest, or in some cases, the cheapest, energy harvesting technology available to date. Many electricity generation scenarios are pointing towards a massive PV implementation in the coming decades, projecting PV to be the most important electricity generation technology in 2050.
The global PV Market has witnessed an annual growth rate of 24% between 2010 and 2017 . To fulfill the strategic climate goals the PV-markets both in Europe and the rest of the world will continue to grow between 10% and 30% up to 2030. . In order to enable the realization of this enormous PV deployment, improved, new and widely accepted PV technologies will be required besides the current existing PV technologies.
The, traditional solar cells, are bulky and expensive to manufacture, plus they are inflexible and cannot be made transparent, which can be useful for temperature-monitoring sensors placed on windows and car windshields.
There is a need for cheaper, more efficient solar cells than the traditional silicon solar cells so that more people may have access to this technology. One of the promising technology for efficient establishment of solar cell technology on a global scale is organic-inorganic halide perovskites as solar cells because of it’s high power conversion efficiency (PCE) and the low-cost fabrication.
Perovskites are a family of materials with a specific crystal structure, named after the mineral with that structure. When used to create solar cells, they have shown potential for high performance and low production costs.
Perovskites comprise a class of minerals, named after a Russian mineralogist (L.A. Perovski, 1792-1856), that are based on calcium titanate, CaTiO3. The terminology is loosely used these days to refer to any compound of ABX3 stoichiometry — where A and B are cations and X is the bonding anion- that exhibits the perovskite crystal structure. Within solar, lead is often the dominant metal used in perovskites. The perovskites that researchers are most excited about are the organometal trihalides, the most commonly studied of which is CH3NH3PbI3, where A = CH3NH3 (methylammonium), B = Pb (lead) and X = I (iodide). Other mixed halides such as CH3NH3Pb(I1-xClx)3 and CH3NH3Pb(I1-xBrx)3 have been studied as well.
Of the many different perovskites formulations that can be used in solar cells, the methylammonium lead iodide perovskite (MAPbI3) has been the most widely studied. Solar cells made of this material have been able to reach efficiencies exceeding 20% and are cheaper to manufacture than silicon.
Dependant on which atoms/molecules are used in the structure, perovskites can have an impressive array of interesting properties including superconductivity, giant magnetoresistance, spin dependent transport (spintronics) and catalytic properties.
Perovskite solar cells have shown remarkable progress in recent years with rapid increases in conversion efficiency, from reports of about 3% in 2006 to over 25% today.
For in-depth understanding on Perovskite Solar cells technology and applications please visit: The Rise of Perovskite Solar Cells: An Introduction to Their Advantages and Challenges
There are many existing applications where even disposable low-cost, high-efficiency solar cells could be attractive, such as use in disaster response, device charging and lighting in electricity-poor regions of the world. Perovskite solar cells also have the highest power to weight ratio amongst viable photovoltaic technologies.
Perovskites have generated huge interest in recent years because of their potential for solid-state lighting and displays, despite lagging behind other state-of-the-art technologies in efficiency and longevity.
While perovskite solar cells have become highly efficient in a very short time, a number of challenges remain before they can become a competitive commercial technology.
Pervoskite Solar Cell types: Perovskite silicon solar cell
There are two types of perovskite-based solar cells. One is a silicon cell with a layer of perovskite on top of it, known as a tandem solar cell. Perovskite silicon solar cells are now almost at their limit in terms of conversion efficiency, and that extra perovskite layer on top of it can add a huge boost to efficiency, without drastically changing the production process.
In fact, labs have found promising success with perovskite when it works in tandem with other solar technologies. Perovskites absorb more of the light spectrum, so that layer is placed on top of a successfully stable material. CIGS-perovskite tandem cells are a popular testing choice and have made big gains, from 17.8% efficiency in late 2016 to 21.5% in January 2019. And since CIGS thin-film already has success at scale, improving perovskite’s performance in tandem may be easier than working on the new technology alone. Pairing perovskite with silicon is also not out of the question. Researchers at imec think a silicon-perovskite stacked cell could easily push 30% efficiency.
The silicon-perovskite tandem cells will be more expensive than the current silicon cells because extra process steps and material are needed compared to the standard cells. However, the gain in efficiency will compensate considerably for this extra cost, resulting in a lower cost/watt peak. Tandem solar cell modules will certainly win the race with current cheap silicon modules.
This type of cell will mainly be used in solar cell parks. For solar cell parks in less sunny areas—where the light intensity and clouds are more variable-bifacial silicon solar cells can also be used with a layer of perovskite on top. This allows you to also capture light that is reflected from the ground.
Pure thin-film perovskite cells
The other option—pure thin-film perovskite cells—will be used in different integrated applications, such as in cars, building materials, windows, and clothing. As this requires a completely new approach to the integration process and business model, this type of perovskite-based cells is likely to appear on the market a little later.
Thin-film perovskite cells have the potential to be very cheap. Of course, the final price will depend on the material used, the design of the stack and the type of process used, as well as the application and market size of this application. But as a guide, the EPKI White Paper mentions about 20 eurocents/Wp for the next 5 to 10 years with a reduction to 10 and maybe even 4 eurocents/Wp as further progress is made in the development and efficiency of the cells.
Perovskites have demonstrated a meteoric rise compared to most other technologies over a relatively short period of time. In the space of three years perovskite solar cells have managed to achieve power conversion efficiencies comparable to Cadmium Telluride which has been around for nearly 40 years.
Anita Ho-Baillie, a Senior Research Fellow at the Australian Centre for Advanced Photovoltaics (ACAP), announced that her team at UNSW has achieved the highest efficiency rating with the largest perovskite solar cells to date.The 12.1% efficiency rating was for a 16 cm2 perovskite solar cell, the largest single perovskite photovoltaic cell certified with the highest energy conversion efficiency, and was independently confirmed by the international testing centre Newport Corp, in Bozeman, Montana. The new cell is at least 10 times bigger than the current certified high-efficiency perovskite solar cells on record. Her team has also achieved an 18% efficiency rating on a 1.2 cm2 single perovskite cell, and an 11.5% for a 16 cm2 four-cell perovskite mini-module, both independently certified by Newport.
In addition, Perovskite based solar cells are fast approaching the same level of photon energy utilisation as the current leading monolithic crystalline technologies such as silicon and GaAs. Large charge-carrier diffusion lengths (~one micrometer), 100X higher than other photovoltaics result in high photoconversion efficiencies. These photo-conversion efficiencies over this short period, as well as their higher open-circuit voltage (1.1 V) versus silicon (0.6 V), have attracted world-wide attention. Furthermore, They’re flexible, cheap to produce and simple to make – which is why perovskites are the hottest new material in solar cell design.
A group of researchers from the National University of Science and Technology (MISIS) in Moscow, in cooperation with their colleagues from the University of Texas at Dallas, managed to develop a flexible solar battery which is at least three times cheaper than its silicon analogues, the MISIS website reported. Led by Professor Anvar Zakhidov, MISIS scientists created a thin-filmed photoelectric cell based on hybrid metal-organic compounds called perovskites, which in turn made it possible to hammer out an advanced solar battery, according to the website.It said that “these perovskites can convert solar energy radiation into electric energy with a performance coefficient of more than 15 percent, and with a planned rate of 20 percent.” “Before long, this new technology will give way to light, flexible, and cheap solar panels based on perovskites to be used for charging and powering a bevy of electronic devices, from tablets to buildings’ electrical grids,” the website said.
Issues
The stability of perovskite solar cells has long been a problem, and the PV industry was very skeptical about this new kid on the block. But here, too, a lot of progress has been made, and the first perovskite solar cells are expected to roll off the belt within 2 to 3 years. The EPKI White Paper states that the production capacity (worldwide) will increase from 0.4 to 1.3 GWp in that period. Dozens of companies around the world are preparing their production processes and expanding their production capacity, and both Europe and China are expected to play an important role in this new technology.
There is a standard test for (silicon) solar cells to guarantee a service life of 20 years. By 2020, perovskite cells will be able to pass this test. The question is whether this test can offer the same guaranteed 20-year lifespan for perovskite cells, since it was specifically developed for silicon cells. Therefore, in parallel, the research community needs to gain more insights into degradation mechanisms in perovskite cells in order to develop new, more suitable tests in the long run
Although it has shown excellent performance potential in the lab (efficiency has increased from 12% to over 23% in just four years), the issue with perovskite is that its efficiency declines quickly as the module size increases. NREL has spent considerable research time on perovskite to prove the material’s effectiveness at solar conversion. The research lab attributes the decline in performance to the non-uniform coating of chemicals in the cell and conversion losses when perovskite is layered with other solar cell technologies.
However, instability of perovskite materials restricted their scaling up and commercialization. The current research work on the PSCs mostly focuses on ways of getting high efficiency and stability via different fabrication methods and material engineering. The control on recombination reactions is a key factor for achieving high performance in PSCs.
The environmental degradation of PCE is one of the vital issues that warrant serious attention. Currently, the only major unknown in the field of perovskite research is the stability of devices over their operational lifetime, they are currently prone to fluctuating temperatures and moisture, making them last only a few months without protection. Although lifetime studies of actual devices are limited, research into the stability of these films has shown that there are several reaction pathways leading to degradation that involve water, oxygen, and even the diffusion of electrode materials.
Current leading research is focused upon reproducing the high power conversion efficiencies but with the addition of stabilising agents such as Ceasium and Rubidium. Another issue is that toxic lead (Pb) in the most-used perovskite material (CH3NH3PbI3) may leach out of solar panels into the environment and presents a barrier to commercialization. The best efficiencies are achieved today with lead-containing perovskite cells. Alternatives are being considered, but they do not score very well for the time being. Several recent attempts have been made to replace the lead with tin (Sn) in these perovskite materials. Unfortunately, low photoconversion efficiencies (five to six percent) were observed and Sn (unlike Pb) has a strong tendency to convert to its more stable 4+ oxidation state, which is promoted by moisture (to yield SnO2, tin oxide), and degrade overall cell performance.
The amount of lead in the cells is very low: the lead-containing layers in a perovskite cell are typically about 0.3 µm thick, which translates into 1 g of lead iodide/m². This is in accordance with the RoHS directive. In addition, solutions are being sought to further minimize the very small chance that lead would end up in the environment as a result of damage to the PV cell. For example, materials can be integrated into the cell that bind with the lead in case of exposure and form water-insoluble components.
A key development has been the improvement in processing techniques used. Previously, vacuum based techniques offered the highest efficiency devices but lately improvements in solution based deposition through the use of solvent quenching techniques has shifted the record breaking devices to solution based processing. In addition, the ability of these materials to be solution-processed via liquid-phase chemical reactions and material coating/deposition by methods such as spraying, roll-to-roll printing and spin-coating make it possible for solar-cell manufacturers to eventually replace expensive clean rooms and sophisticated vapor-deposition equipment with simple, inexpensive benchtop processes.
Manufacturability
Most of the world’s commercial solar cells are made from a refined, highly purified silicon crystal and, like the most efficient commercial silicon cells, need to be baked above 800 deg C in multiple high-temperature steps. They’re also really only designed to efficiently harvest energy from powerful sunlight, not low indoor light.
Perovskites, on the other hand, are made at low temperatures and 200 times thinner than silicon cells. Perovskite cells, on the other hand, can be printed using easy roll-to-roll manufacturing techniques for a few cents each; made thin, flexible, and transparent; and tuned to harvest energy from any kind of indoor and outdoor lighting.
In lab settings, perovskite cells are made by depositing chemicals by spin-coating, spraying or “painting” them onto a substrate. The perovskite material forms as the chemicals crystallize. Easy application by painting onto substrates opens perovskite’s potential in markets wanting flexible, lightweight and non-uniform solar generation options.
Scaling up perovskite manufacturing is required to enable production of perovskite solar cells. Making the processes scalable and reproducible could increase manufacturing and allow perovskite PV modules to meet and potentially exceed the office’s levelized cost of electricity targets.
The cells are thin-film devices built with layers of materials, either printed or coated from liquid inks or vacuum-deposited. Producing uniform, high-performance perovskite material in a large-scale manufacturing environment is difficult and there is a substantial difference in performance between small-area cell efficiency and large-area module performance. The future of perovskite manufacturing will depend on solving this challenge, which remains an active area of work within the PV research community.
Various methods have been used to produce lab-scale perovskite devices. Many of these methods are not easily scalable, but there are significant efforts to apply highly scalable approaches to perovskite fabrication. For thin-film technologies, these can be split into two major types of production line:
Sheet-to-Sheet: Device layers are deposited on a rigid substrate, which typically acts as the front surface of the completed solar module. This approach is commonly used in the cadmium telluride thin-film industry.
Roll-to-Roll: Device layers are deposited on a flexible substrate, which can then be used as either an interior or exterior portion of the completed module. Researchers have tried this approach for other PV technologies, but it did not gain significant commercial traction owing to barriers to obtaining high solar conversion efficiency (independent of the fabrication approach). It is, however, widely used to produce photographic and chemical film and paper products, such as newspapers.
The scalability of these fabrication approaches give perovskites the potential to enable faster capacity expansion relative to silicon photovoltaics. The processes under consideration are well established in the film and display industry, making the knowledge and supply chains around the tooling and components easily leveraged to further reduce scaling costs and risk.
“The versatility of solution deposition of perovskite makes it possible to spray-coat, print or paint on solar cells,” said Dr. Anita Ho-Baillie, a Senior Research Fellow at the Australian Centre for Advanced Photovoltaics at UNSW. “The diversity of chemical compositions also allows cells be transparent, or made of different colours. Imagine being able to cover every surface of buildings, devices and cars with solar cells.”
Additional barriers to commercialization are the potential environmental impacts related to the perovskite absorber, which is lead-based. As such, alternative materials are being studied to evaluate, reduce, mitigate, and potentially eliminate toxicity and environmental concerns.
New perovskite fabrication technique could lead to large-scale solar cell production reported in April 2021
The mass production of high-performance perovskite solar cells could soon become easier now that researchers in Taiwan and the US have discovered a simple alteration to the manufacturing process. The technique was developed by Leeyih Wang at National Taiwan University and colleagues, who showed that it boosts both the power conversion efficiency and operational lifetime of a perovskite mini-module. Their innovation could soon open new routes towards the large-scale manufacture of perovskite solar cells, making them a strong competitor to existing silicon-based cells.
Currently, the fabrication process usually involves dripping an antisolvent onto a perovskite precursor that has been spin-coated onto a substrate. Ideally, this technique can create films with uniform, high-quality crystal structures. However, the conditions of the process must be tightly controlled, and the antisolvent must be applied within a time window of just 9 s following the initial deposition. Otherwise, the resulting perovskite film could be rough and uneven – diminishing its performance as a solar cell. As films become larger, it becomes increasingly difficult to implement this process.
They found that hydrogen bonding between sulfolane molecules and perovskite precursor ions slowed down the crystallization process significantly, thereby extending the time window for antisolvent addition to 90 s. This enabled compact, highly uniform crystal structures to form in far less stringent processing conditions. To demonstrate this improvement, Wang and colleagues fabricated a perovskite solar cell mini-module with an active area of 36.6 cm2.
Their device achieved a very respectable power conversion efficiency of over 16%, and retained around 90% of its initial performance after operating for 250 hours at 50 °C – the point at which it extracted the maximum possible amount of power. This high efficiency and long operational lifetime set the stage for large-scale perovskite solar cell production, in far more flexible manufacturing conditions. Wang’s team hope that the technology could soon become widely available commercially and may even become a viable competitor to silicon-based solar cells – boosting the outlook for renewable solar energy.
The research is described in Joule.
To combat this issue, Wang’s team, which also included researchers at Los Alamos National Laboratory, introduced a technique that significantly broadened the post-deposition time window. They did this using sulfolane as an antisolvent, which enabled them to fabricate uniform, high-quality, and large-area perovskite films in their experiment. To investigate the molecular mechanisms responsible for this improvement, they studied the chemical reactions involved using a combination of X-ray diffraction and infrared spectroscopy.
Perovskite solar cells with decent energy conversion efficiency – similar to solar panels – and that are also see-through, reported in April 2020
Research supported by ARENA and led by Professor Jacek Jasieniak from the ARC Centre of Excellence in Exciton Science and Monash University has managed to produce perovskite solar cells that have a decent energy conversion efficiency – similar to solar panels – and that are also see-through. The researchers said two square metres of the semi-transparent solar glass will generate about as much electricity as a standard rooftop solar panel. “Rooftop solar has a conversion efficiency of between 15 and 20 per cent,” said Professor Jasieniak. “The semi-transparent cells have a conversion efficiency of 17 per cent, while still transmitting more than 10 per cent of the incoming light, so they are right in the zone.”
He said that when tinted to the same degree as glazed commercial windows the solar glass would generate about 140 watts of electricity per square metre. Investigations are underway to see if the new technology can be built into commercial products made by Viridian Glass, an Australian glass manufacturer. The goal is to have the product available to purchase in under 10 years. Professor Jasieniak expects the economics to work well in multistorey buildings because large windows are already expensive and the added cost of using the semi-transparent solar cells will be marginal. “But even with the extra spend, the building then gets its electricity free,” he said.
The researchers expect the glass to be a “game changer” for planners and designers, and might alter the way buildings are designed. Buildings might have to be reorientated, for example, so that glass walls face the sun. The only downside is that the darker the glass, the more energy it’s capable of producing. This means architects will need to make decisions about how much light comes into a building versus how much energy the glass produces. There are already semi-transparent solar products on the market but the researchers said they tend to be “very expensive, unstable or inefficient”. The new glass design replaces a commonly used solar cell component, Spiro-OMeTAD, with an organic semiconductor that can be made into a polymer. The problem with Spiro-OMeTAD is that it “shows very low stability because it develops an unhelpful watery coating”. Researchers said the organic replacement produced “astonishing results”.
Breakthrough Self-Assembly Innovation Enables Cheaper Solar Energy Production reported in March 2020
Material, synthesized by Kaunas University of Technology (KTU), Lithuania scientists, which self-assemble to form a molecular-thick electrode layer, presents a facile way of realizing highly efficient perovskite single-junction and tandem solar cells. According to scientists, achieving perovskite-based solar cells, combining low price and high efficiency, has proven to be hard endeavor in the past. The particular challenge in large-scale production is the high price and limited versatility of the available hole-selective contacts. KTU chemists have addressed this challenge.
“Solar element is akin to a sandwich, where all of the layers have its function, i.e. to absorb the energy, to separate the holes from electrons, etc. We are developing materials for the hole-selective contact layer, which is being formed by the molecules of the materials self-assembling on the surface of the substrate,” explains Artiom Magomedov, PhD student at the KTU Faculty of Chemical Technology, co-author of the invention.
Developed monolayers can be called a perfect hole transporting material, as they are cheap, are formed by a scalable technique and are forming very good contact with perovskite material. The self-assembled monolayers (SAMs) are as thin as 1-2 nm, covering all the surface; the molecules are deposited on the surface by dipping it into a diluted solution. The molecules are based on carbazole head groups with phosphonic acid anchoring groups and can form SAMs on various oxides.
According to the scientists, the use of the SAMs helped to avoid the problem of the rough surface of the CIGS cell. By integrating a SAM-based perovskite solar cell into a tandem architecture, a 23.26%-efficient monolithic CIGSe/perovskite tandem solar was realized, which is currently a world record for this technology. Moreover, one of the lately developed SAMs used in the Si/perovskite tandem cell achieved the nearly record-breaking efficiency of 27.5%.
“Perovskite-based single-junction and tandem solar cells are the future of solar energy, as they are cheaper and potentially much more efficient. The limits of efficiency of currently commercially used silicon-based solar elements are saturating. Moreover, the semiconductor-grade silicon resources are becoming scarce and it is increasingly more difficult to extract the element,” says Professor Vytautas Getautis, the head of the KTU research group behind the invention.
China’s perovskite solar module efficiency exceeds 20% reported in April 2021
Wuxi Utmost Light Technology Co Ltd (UtmoLight), announced in April 2021 that it has reached a new world record of 20.5% power conversion efficiency for its mini-module with a designated area of 63.98 cm2. The result was certified by the internationally recognised test center – Japan Electrical Safety & Environment Technology Laboratories (JET). Such an efficiency currently is the highest record generated by perovskite solar modules in the world, and is already comparable to the efficiency of mainstream crystalline silicon products. This indicates that China has mastered the core technology in the field of perovskite solar modules and it is only a matter of time before the country achieving mass production.
Moreover, the Jiangsu-province based UtmoLight plans to build production lines for manufacturing large-area perovskite solar modules to accelerate the commercialization of the perovskite photovoltaic technology. Large-area preparation is considered to be the biggest obstacle to the commercialization and mass production of perovskite technology, this explains why the technological breakthrough achieved by UtmoLight has attracted widespread attention across the industry.
Even though the efficiency of perovskite solar cells is fairly close to that of the crystalline silicon solar cells, its efficiency drops rapidly as the area enlarges, due to the difficulty of preparing high-quality perovskite films over large areas by using conventional methods. UtmoLight has mastered a large-area perovskite preparation technology which enables large-area module production without sacrificing high conversion efficiency. The perovskite solar module efficiency exceeding 20% is a big leap forward for perovskite solar technology, symbolising the commencement of a new photovoltaic era in which perovskite will be momentous.
Conclusion
Since the technology of perovskite solar cell was introduced in 2009, the efficiency has evolved fast from 3.8 percent to 25.5 percent in 2020. With the potential of achieving even higher efficiencies and lower production costs, it has showed a huge prospect for commercialization. Therefore it has received extensive attention from the industry. Based on the opinion of many industry insiders, the overall situation of the development of perovskite technology is now quite clear, and large-scale commercial mass production is only a matter of time.
Perovskite-based solar cells—tandem cells with silicon or thin-film cells for integrated applications—will hit the market very soon. The first perovskite solar cells are expected to roll off the belt within 2 to 3 years. The EPKI White Paper states that the production capacity (worldwide) will increase from 0.4 to 1.3 GWp in that period. Dozens of companies around the world are preparing their production processes and expanding their production capacity, and both Europe and China are expected to play an important role in this new technology. Both the R&D community and the PV industry will play a major role in making this a success by further improving efficiency and stability, and finding new business models to enter new markets with solar-based cars, building elements, clothing, and more.
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