Massachusetts Institute of Technology researchers are among dozen who have studied the lifespan of perovskite solar cells in the past few years. Image: MIT.

“It’s pretty clear that you need to keep oxygen and moisture out,” says Saif Haque, professor at Imperial College London and lead author of a study on the why and how of perovskite, a much-discussed material that is one of the few new solar cell technologies, so dismantle quickly.

Given that solar parks are exposed to the elements, this doesn’t sound like good news. But Haque says his team is satisfied, even excited, with the results of their tests, which focused on tin-based perovskite.

The structure is less toxic and less stable than lead-based versions, the structure reacts with oxygen to produce tin (IV) iodide and then iodine, which in a “vicious cycle” of decomposition produces even more iodine, meaning the perovskite cell much shorter shelf life compared to traditional versions.

While it shows the problems manufacturers are currently facing, Haque believes the results are promising.

“This discovery helps explain why this material is unstable, it decomposes in the presence of oxygen and moisture and under ambient conditions,” he says. From there we can find a way to stop this and come up with a new technology that could revolutionize the solar sector.

Next generation modules

Thin film perovskite solar cells (PSCs) have attracted worldwide attention in recent years thanks to their unique crystallographic structure that makes them highly effective at converting light photons into usable electricity. In addition, they are comparatively inexpensive to manufacture.

Researchers in Japan began using perovskite material for solar cells for the first time in 2009, although their conversion efficiency at that time was around 3.8% and would take no more than a few seconds because the electrolyte dissolved the perovskite crystals almost instantly. Today, however, several teams have made great strides in improving efficiency. At the end of 2020, perovskite solar cell developer Oxford Photovoltaics (PV) collapsed its own industrial cell efficiency record after months of research on tandem silicon heterojunction / perovskite 2T (terminal) solar cells, which were certified by the US National Renewable Energy Laboratory (NREL) with 29.52%. The company said it was increasing conversion efficiency by about 1% per year and in 2019 partnered with module maker Meyer Burger to build a 200 MW heterojunction line from Meyer Burger for the production of tandem solar cells at its German plant in Brandenburg to buy the Havel. The Polish company Saule Technologies announced last month that it had commissioned the world’s first industrial production line of perovskite-lined solar modules.

The material has caught the attention of policy makers looking to build their own domestic solar production sites. Both European Commission and the U.S. Department of Energy (DOE) have pledged to help grow their own manufacturing sectors, with the DOE providing $ 40 million for 22 research and development projects in perovskite solar technologies. The DOE said last year that these cells “have the potential to produce highly efficient thin-film solar cells with very low production costs.”

Developers who want to work with perovskites as the active layer in solar cells, however, still face significant hurdles, hence the research and development push. Stability and durability are still a core issue of the material, which degrades faster than conventional silicone cells.

Cells with pervoskit foils in production at Oxford PV in the UK. Image: Oxford PV.

Research and Development

In the past year, a number of new studies have been published that address this key hurdle. Scientists at the Queensland University of Technology (QUT) last month used haircuts from a barber shop in Brisbane to create “armor” that increases the power conversion efficiency of the material in solar cells, while a team from the Massachusetts Institute of Technology (MIT) along with five other universities around the world used a data fusion process to manufacture and test various perovskite formulations and assess their longevity. Recently, scientists at the University of Sheffield found that storing perovskite at low temperatures can extend the life of the material by up to three months.

Universities have also started to set up spin-out companies to use their findings to support the development of a commercial market. The start-up Evolar, which has prototype line equipment for the scale-up and testing of perovskite cells, has been spun off from the research cluster for thin-film solar cells at Uppsala University and has been run by the same research team behind Solibro, the copper-indium-gallium-de-selenide ( CIGS) solar module specialist, which was taken over by Q CELLS in 2006. Since then, the company has received significant funding from Norwegian investor Magnora to expand its technological developments and has announced that it will seek partnerships with solar manufacturers to scale and test the technology to generate revenue through design, engineering, software and licensing fees.

CEO Mats Ljunggren tells PV Tech that Magnora’s latest investment will be used to scale the startup’s PV power booster technology to full module size and complete the plant design.

“Like everyone else in the industry, we are refining the composition of the perovskite cell stack,” says Ljunggren, but declined to provide further details on the company’s commercial efforts to improve stability. “As we are dealing with the perovskite-silicon tandem area, it is critical to match the perovskite durability level with that of the silicon layer.”

Lead halide models are some of the most successful in terms of stability, but pose more complicated problems in terms of their environmental impact. A 2020 study published in Nature communication found that while many confirm that lead perovskite used in solar cells poses a small risk of lead leakage into plants and, consequently, into the food cycle, compared to other electronics that use the metal, “Ten times more effective than other lead”. Contaminants that are already present as a result of human activity ”.

“The issue of stability remains a problem,” says Haque, but adds that “very encouraging” studies are emerging around the world on extending service life. Having lead in the system itself is also problematic, he adds, because of its toxicity and potential environmental damage. “Trying to replace the lead with something less toxic, more environmentally friendly is important.”

Therefore, alternative metals have become increasingly important in the development of perovskite solar cells. Oxford PV’s tandem Perovskite cells are tin-based, and Ljunggren says Evolar’s facilities are not limited to just lead-based materials.

Haque tells PV Tech that tin-based structures have shown the most promise in terms of conversion efficiency and durability, but they are still less stable than lead-based versions. Researchers from his department and the University of Bath published their own results earlier this month. In Nature Communications, too, the study identifies the reasons for the rapid destabilization of tin-based perovskite and possibly sheds light on how deterioration can be prevented during the production process.

“[Tin-based perovskites] react very quickly with oxygen to form a number of products and one of those products is tin (IV) iodide, ”he says. “Tin (IV) iodide can very quickly convert to iodine in the presence of moisture and oxygen. Iodine acts as an oxidizing agent, it reacts with the perovskite to produce more tin (IV) iodide … It facilitates the further breakdown of this perovskite into tin (IV) iodide, and then this vicious circle ends. “

Haque warns that this reaction has to be taken into account when making the film. P-type self-doping and the presence of excess holes in tin perovskite are problematic, he says, and that self-doping is detrimental to device performance. However, a focus on the intermediate layer in the component stack has produced perovskites with better durability.

“If you process the perovskite film on what is known as a hole transport layer or a hole extractor, it can remove some of these holes from the tin perovskite, and the net effect is to improve stability.”

Another study published earlier this week by New York University researchers found that making perovskite solar cells using carbon dioxide in the doping process under ultraviolet light not only shortens manufacturing and processing time, but also improves stability and conductivity elevated.

Studies like this are invaluable to companies looking to capitalize on mass production of perovskite solar cells. Aside from durability issues, however, keeping production costs down remains a competitive challenge. According to Ljunggren, his start-up’s business model is focused on selling equipment to produce perovskite-silicon tandem panels, which means that “keeping capital expenditures low enough to be attractive to our customers is critical”.

Ljunggren says Evolar benefits from working directly with R&D groups and its own association with Uppsala University, but “we always review and evaluate published R&D performance and are open to including the best”.