Layered lead halide A2An-1PbnI3n+1 perovskites (2D LHPs) are attracting considerable attention as a more stable alternative with respect to APbI3 counterparts, a workhorse material for a new generation of solar cells. However, a critical analysis on the photostability of 2D perovskites comparing n = 1 to n > 3 and to APbI3 system is still missing. In this work, we perform a comparative study of BA$_2$MA$_{n-1}$Pb$_n$I$_{3n+1}$ (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. We identify oxygen concentration as a critical factor affecting 2D perovskites photostability. This leads to a photocorrosion of LHPs that becomes highly dependent on the perovskite dimensionality and the chemical origin of atmosphere at the aging stage as confirmed by joint experimental and theoretical analyses. This mechanism, based on redox equilibriums with internal (I-/I2, Pb2+/Pb, RAH+/RA+H2) and external (O2/H2O) species, explains both a nonmonotonic dependence of 2D LHPs photostability in an inert atmosphere on the number n and a strong enhancement of photocorrosion rate under oxidizing environment.
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