Elucidating degradation mechanisms of mixed cation formamidinium-based perovskite solar cells under device operation conditions

Organic cation-based metal halide perovskites suffer from volatile material decomposition, and the approach to ensure stability under device operation conditions is still far from commercialization. Formamidinium (FA)-based perovskite solar cells (PeSCs) have been widely used, showing efficient power conversion efficiencies (PCEs). Despite the high PCE, FA-based perovskites lack stability, but the related degradation mechanisms are quite indefinite. In this study, we compared two different systems by introduction of inorganic and organic cations to increase FA-based perovskite stability. To relate the device operational stability, we exposed the mixed cation perovskite films under 1 sun illumination at 60 °C in ambient air and investigated the difference in the phase transition of inorganic-mixed and organic-mixed FA perovskites. Morphological and structural characterizations were collected to confirm that the type of the secondary cation plays a major role in determining whether perovskite degradation is triggered at the grain boundary or perovskite grain surfaces. Although the PCEs of the two device systems are similar, the degradation mechanisms varied significantly depending on the chemical properties of perovskite components.