Grain Transformation and Degradation Mechanism of Formamidinium and Cesium Lead Iodide Perovskite under Humidity and Light

Mixed cation perovskites have improved stability, but cation alloying enables the formation of separated phases, as well as leading to new decomposition pathways. Here, using photothermal infrared microscopy, we report degradation mechanisms in FA0.85Cs0.15PbI3, a study enabled by our ability to resolve the location of formamidinium (FA) cations with <100 nm spatial resolution. We find that upon exposure to either light or water alone, no decomposition nor change in the spatial distribution of FA was observed. However, upon exposure to both light and high humidity simultaneously, FA0.85Cs0.15PbI3 degrades through a multistep pathway within 30 h. First, the FA0.85Cs0.15PbI3 phase separates to δo-CsPbI3, δ-FAPbI3, and PbI2 and the spatial distribution of FA becomes increasingly heterogeneous. Second, FA evaporates from δ-FAPbI3 and PbI2 remained. Understanding the degradation pathways will aid in the design of more stable perovskites with, for example, the suppressing of grain transformation during the phase separation.