Ambient-humidity degradation pathways in mixed-cation perovskite films probed by in situ grazing-incidence scattering

Abstract

Mixed-cation lead halide perovskites offer improved thermal stability over their methylammonium-only predecessors, but their degradation under ambient humidity remains the principal barrier to deployment. We tracked structural evolution in formamidinium-caesium films at 45% and 75% relative humidity using in situ grazing-incidence wide-angle X-ray scattering over 400 hours. Two distinct pathways emerged: at moderate humidity, degradation proceeded through a hydrated intermediate that remained reversible for approximately 120 hours, whereas at high humidity an irreversible transition to the delta phase began within 18 hours and bypassed the intermediate entirely. Caesium content above 15% suppressed the irreversible pathway without affecting the reversible one, suggesting the two mechanisms have distinct compositional dependencies. Encapsulation strategies that target only bulk moisture ingress may therefore be addressing the less consequential of the two.