  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.
