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Nonmonotonic photostability of BA2MAn-1PbnI3n+1 homologous layered perovskites

Layered lead halide A'2An-1PbnI3n+1 perovskites (2D LHPs) are attracting considerable attention as a more stable alternative with respect to APbI3 parent phases - a promising material for a new generation of solar cells. However, a critical analysis on the photostability of 2D perovskites homologous row is still lacking. In this work, we perform a comparative study of BA2MAn-1PbnI3n+1 (BA - butylammonium, MA - methylammonium) 2D LHPs with different layer number n = 1-3, considered as study-case systems, and MAPbI3, as a reference. We discuss a stability testing protocol with general validity, comparing photometrical determination of iodine-containing products in nonpolar solvents, X-ray diffraction, and photoluminescence spectroscopy. The resulting photostability trends in inert atmosphere demonstrate a nonmonotonic dependence of degradation rate on the perovskite layer thickness n with a "stability island" at n > 3 which is caused by a combination of antibate factors of electronic structure and chemical composition in the family of 2D perovskites. We also identify a critical oxygen concentration in the surrounding environment that affects the mechanism and strongly enhances the rate of layered perovskites photodegradation.