Accelerated Degradation of FAPbI₃ Perovskite by Excess Charge Carriers and Humidity

Excess charge carriers in metal halide perovskite layer have been known to accelerate degradation of the film and devices to cause poor operational stability of perovskite solar cells (PSCs). While mechanisms for such degradation have been predominantly studied for methylammonium-based perovskites, effects of excess charge carriers and their interplays with other degradation causes are barely studied for widely used formamidinium-based perovskites. Herein, a possible decomposition mechanism of the formamidinium lead tri-iodide (FAPbI₃) perovskite in the presence of excess charge under different humidity levels is investigated. The operating condition with excessive charges is simulated by placing half devices with either electron-transporting layer (ETL) or hole-transporting layer (HTL) under 1 sun illumination. FAPbI₃ in contact with ETL degrades more rapidly than the one with HTL, which is attributed to excess hole charge carriers in the film. Under higher humidity, the synergetic effect of excess charge carriers and humidity is found and thus degradation pathway and kinetics are strongly dependent on the humidity level. The fundamental understanding of degradation pathways for formamidinium perovskites should provide a useful insight toward the development of efficient and operationally stable PSCs toward practical usage.