The mechanism has been discovered that causes solar cells made with organic lead halide perovskites to rapidly deteriorate when exposed to oxygen and light. The discovery could provide the basis for solving issues related to long-term stability of perovskite cells. Perovskite cells are more flexible and cheaper to manufacture than traditional solar cells made with silicon, but break down quickly in natural conditions. Fresh (left) and degraded (right) solar cells are pictured. Courtesy of Imperial College London. Currently, the only way to protect perovskite cells from degradation by air and light is to encase them in glass. Since perovskite solar cells are made from flexible material designed to be used in a range of settings, a glass encasement severely limits their function. A research team at Imperial College London had previously discovered that the breakdown in perovskite efficiency was due to the formation of superoxides that attack the perovskite material. The team has further identified how superoxides develop and how they cause damage to the perovskite material. The team discovered that superoxide formation was stimulated by spaces in the perovskite structure normally filled by iodide molecules, a component of perovskite. By dosing the material with extra iodide during the manufacturing process, the stability of the cell was improved. The team considers that a more permanent solution would be to remove the defects in the perovskite where the iodide molecules are missing. “After identifying the role of iodide defects in generating superoxide, we could successfully improve the material stability by filling the vacancies with additional iodide ions," said researcher Nicholas Aristidou. "This opened up a new way of optimizing the material for enhanced stability by controlling the type and density of defects present.” The present study, although not exhaustive, highlights critical areas for further work, which could include probing the combined effect of oxygen and water on perovskite material as well as large-scale atomistic simulations of oxygen diffusion and iodine interstitials. “We have now provided a pathway to understand this process at the atomic scale and allow the design of devices with improved stability,” said researcher Saif Haque. The team believes that the understanding of degradation phenomena gained from this study could be important for the future design and optimization of stable perovskite solar cells. The researchers hope to next test the stability of the cells in real-world settings, where the cells will be exposed to a combination of both oxygen and moisture. The research was published in Nature Communications (doi:10.1038/ncomms15218).
Photosensitizers