Elucidating the mechanisms underlying PCBM enhancement of CH₃NH₃PbI₃ perovskite solar cells using GIXRD and XAFS

In this study, the stability mechanism of the optimized methyl ammonium lead triiodide (CH₃NH₃PbI₃, MAPbI₃)/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) perovskite active layer upon exposure to moisture, light, and heating was investigated. For the first time, the existence of high-valence lead arising from PCBM is confirmed by X-ray absorption fine spectroscopy (XAFS) based on synchrotron radiation, which helps to form stronger Pb-I bonds and facilitate carrier transport. Moreover, the degree of disorder of perovskites and the [PbI₆]^4- octahedral distortion are significantly improved, as well as the crystallization both on the surface and in the bulk. The result of this study provides a fundamental understanding of the functions of additives with electron-withdrawing properties (e.g., PCBM) in perovskite materials and could specifically be extended to systematically design and fabricate high-efficiency and stable solar cells.