New generation solar: developing more stable, ecological and commercially competitive perovskites
“It’s pretty clear that you need to keep the oxygen and humidity out,” says Saif Haque, professor at Imperial College London and lead author of a study on the why and how of perovskites, a much talked about material that is one of the few new solar cell technologies, degrades so quickly.
Given that solar farms are exposed to the elements, this doesn’t sound like good news. But Haque says his team were satisfied, if not excited, with the results of their tests, which focused on the tin-based perovskite.
Both less toxic and less stable than the lead-based versions, the structure reacts with oxygen to create tin (IV) iodide, then iodine, which generates even more iodine in a “vicious cycle” of deterioration, which means that the perovskite cell has a shorter shelf life compared to traditional versions.
While this shows the issues that manufacturers are currently facing, Haque believes the results are promising.
“This discovery helps explain why this material is unstable, it degrades in the presence of oxygen and humidity and in ambient environmental conditions,” he says. From there, we can find a way to stop it and create new technology that could revolutionize the solar industry.
New generation modules
Thin Film Perovskite (PSC) solar cells have gained worldwide attention in recent years due to their unique crystallographic structure which makes them very efficient at converting light photons into usable electricity. They are also relatively cost effective to produce.
Japanese researchers started using perovskite material for solar cells in 2009, although at the time their conversion efficiency was only around 3.8% and lasted no more than a few seconds because the The electrolyte dissolved the perovskite crystals almost instantly. Today, however, several teams have made huge strides in improving efficiency. In late 2020, perovskite solar cell developer Oxford Photovoltaics (PV) broke its own industry cell efficiency record after months of research into 2T silicon / perovskite heterojunction tandem solar cells (Terminal) , which have been certified by the National Renewable Energy Laboratory in the United States. (NREL) at 29.52%. The company said it was increasing the conversion efficiency at a rate of around 1% per year and partnered with module maker Meyer Burger in 2019 to purchase a 200 MW heterojunction line from Meyer Burger for the production of solar cells in tandem at its German factory in Brandenburg an der Havel. Polish company Saule Technologies announced last month that it had launched the world’s first industrial production line of solar panels coated with perovskite film.
The material has caught the attention of policymakers looking to develop their own national solar power manufacturing bases. The European Commission and the US Department of Energy (DOE) have pledged to support the growth of their own manufacturing sectors, with the DOE setting aside $ 40 million for 22 perovskite solar technology research and development projects. The DOE said last year that these cells have the “potential to make very efficient thin-film solar cells with very low production costs.”
However, developers who hope to work with perovskites as the active layer of solar cells still face significant hurdles, hence the R&D surge. Stability and durability continue to be a critical issue with the material, which degrades faster than traditional silicone cells.
Research and development
A series of new studies have been published over the past year dedicated to addressing this key hurdle. Scientists at the Queensland University of Technology (QUT) used hair clippings from a Brisbane barber shop to form a “weave” that increases the energy conversion efficiency of the material in the solar cells. last month, while a team from the Massachusetts Institute of Technology (MIT) alongside five other universities around the world applied a data fusion process to produce and test different formulations of perovskite and assess their longevity. More 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 also began to create spin-off companies to help their discoveries support the development of a commercial market. The start-up Evolar, which has prototype line equipment for scaling and testing perovskite cells, was formed out of the thin-film solar cell research hub at Uppsala University. and was launched by the same research team behind Solibro, the Copper Indium Gallium de-Selenide (CIGS) specialist in solar modules acquired by Q CELLS in 2006. It has since received substantial funding from the Norwegian investor Magnora to scale up its technological developments and said it would seek partnerships with solar manufacturers to evolve and test the technology, with a view to generating design, engineering, software and royalty income.
Managing Director Mats Ljunggren tells Photovoltaic technology that Magnora’s latest investment will be used to expand the start-up’s PV Power Booster technology to full module size and to finalize the equipment design.
“Like any other industry, we are refining the composition of the perovskite cell stack,” Ljunggren said, but declined to elaborate on the company’s business efforts to improve stability. “As we approach the perovskite-silicon tandem zone, it is crucial to successfully match the durability level of the perovskite with that of the silicon layer. “
The lead halide models have been among the most successful in terms of stability, but raise more complex issues related to their environmental impact. A 2020 study published in Nature Communication found that while many attest that the lead perovskite used in solar cells carries a low risk compared to other electronic devices that use the metal, lead seeping into the soil can enter plants, and therefore in the food cycle, “ten times more efficient than other lead contaminants already present as a result of human activities”.
“The issue of stability remains a problem,” Haque says, but adds that there are “very encouraging” studies emerging around the world on how to extend their lifespan. The presence of lead in the system itself is also problematic, he adds, due to its toxicity and potential damage to the environment. “Trying to replace lead with something less toxic, more environmentally friendly is important.”
This is why 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 equipment on Evolar’s line is not limited to use for lead-based materials alone.
Hack tells Photovoltaic technology that the tin-based structures have shown the most promise in terms of conversion efficiency and durability, but that they are even less stable than the lead-based versions. Researchers from his department and the University of Bath published their own findings earlier this month. Also in Nature Communication, the study identifies the reasons why tin-based perovskite rapidly destabilizes, potentially shedding light on how to prevent degradation early in 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 turn into iodine in the presence of moisture and oxygen. The iodine acts as an oxidant, it reacts with the perovskite to generate more tin iodide (IV)… It facilitates this further decomposition of this perovskite into tin iodide (IV), and you end up with this circle vicious.
Haque warns that this reaction must be taken into account during the production process of the film. P-type autodoping and the presence of excess holes in the tin perovskite are problematic, he says, and this autodoping is detrimental to the performance of the device. But the emphasis on interleaving in the stack of devices has produced perovskites with better durability.
“If you process the perovskite film on what we call a hole transport layer or hole extractor, this is able to remove some of those holes from the tin perovskite and the net effect of that is to improve stability. “
Another study published earlier this week by researchers at New York University found that making perovskite solar cells using carbon dioxide in the doping process under ultraviolet light not only shortened the time to workmanship and processing, but also increased stability and conductivity.
Such studies have become invaluable for companies looking to capitalize on the mass production of perovskite solar cells. However, apart from sustainability issues, keeping production costs low remains a competitive challenge. Ljunggren says his start-up’s business model focuses on selling equipment for the production of perovskite-silicon tandem panels, which means that it is “crucial … to show investments low enough to be attractive to our customers. clients “.
Ljunggren says Evolar benefits from direct collaborations with R&D groups and its own connection with Uppsala University, but “we are still digitizing and evaluating published R&D achievements and are open to including the best.”