New perspectives for more stable perovskite materials 

Researchers from the Adolphe Merkle Institute’s Soft Matter Physics group have developed a new type of highly efficient next-generation perovskite solar cell that is more stable than previous models, offering prospects for future commercialization.

Until now, solar cell technology has been almost exclusively based on silicon, which is currently the leading solar cell material. Costs have dropped over the years, while efficiency has increased. However, further improvements have become more and more difficultas the theoretical limit is approached. There are many hurdles that need to be overcome in order to improve silicon solar cells, such as the material’s complexity and the energy requirements.

Researchers are increasingly turning their efforts towards perovskite solar cells (PSCs), one of the most promising developments for photovoltaic energy in recent years. Despite their novelty, the efficiency of PSCs, which can absorb the high-energy blue photons of sunlight particularly well, is already approaching the performances of silicon solar cells. However, there are still a number of challenges to overcome before perovskites can effectively challenge silicon, most notably long-term stability.

The commercialization of perovskite solar cells needs enormous efforts to develop new perovskite materials that are not only highly efficient in terms of power conversion, but also nontoxic, low cost, and most importantly stable” says AMI postdoctoral researcher Somayyeh Gholipour. The latter point is a pivotal step, which requires overcoming the stability issues of PSCs related to thermal stress, ultraviolet light, and moisture. “The components used in perovskite materials can degrade in just hours, sometimes even minutes, under normal light conditions or in a humid environment,” explains Photovoltaics Group Leader Dr. Michael Saliba.

“It is a huge challenge to make them stable for years or even decades, like silicon-based cells.” Saliba has specifically addressed the problem of the highly volatile and heat-sensitive methylammonium (MA) molecule. The most efficient perovskite solar cells contain unstable MA, mainly because of their capacity to provide high-performance values. By exchanging the organic MA with the inorganic elements rubidium and cesium, the researchers have shown that it is possible to avoid unstable MA while maintaining a similarly high efficiency. This allows for more stable solar cells, which is a key step towards eventual commercial use. “These new perovskites can also harvest more sunlight, meaning they are more efficient and therefore more profitable,” says Saliba, whose results were published in the prestigious journal Science. “In addition, the new materials are compatible with flexible substrates, making them prime contenders for a wide variety of applications.”

“Essentially, this sets perovskites on the path of becoming a profitable, long-term solution for a sustainable energy future,” Saliba continues.  “With small additional improvements, perovskite solar cells can be-come a commercial game changer within a short time.”Among  the  potential  improvements  is  the  com-bination  of  a  layer  of  perovskites,  which  absorbs  high-energy  blue  photons  in  sunlight,  with  standard  silicon,  which  more  efficiently  absorbs  lower-energy  red photons. “The option of a perovskite on silicon tandem, where innovative perovskite materials boost established, reliable silicon, has the potential to become a disruptive technology,” Saliba states. The biggest challenge is to make the perovskite-silicon tandem work together efficiently in order to gen-erate maximum current.  Get it wrong, and the weakest element will limit power output.  Saliba reckons, though, that eliminating silicon from the equation altogether could be the future for some forms of solar power. “Perovskites even have the potential to replace the silicon layer, thus enabling a perovskite-on-perovskite tandem which is compatible with a flexible, light-weight substrate,” he explains. “Such technology does not currently exist and could have a major impact in future innovations in the photovoltaics field.”

Reference: Turren-Cruz, S.-H., Hagfeldt, A., Saliba, M. Methylammonium-Free, High-Performance, and Stable Perovskite Solar Cells on a Planar Architecture, Science, 2018, 362, 6413, 449