Local A-Site Phase Segregation Leads to Cs-Rich Regions Showing Accelerated Photodegradation in Mixed-Cation Perovskite Semiconductor Films

Phase stability under illumination is important for applications of halide perovskite semiconductors in both solar photovoltaics and light-emitting diodes (LEDs). We use hyperspectral photoluminescence microscopy to study compositional heterogeneity and its influence on the photostability of formamidinium (FA)-cesium (Cs) mixed-cation lead halide perovskites. We observe increased lateral photoluminescence heterogeneity of perovskite films made with higher Cs concentrations. We correlate photoluminescence maps with time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling and show that the redder-photoluminescence regions of the perovskite films are associated with vertical A-site segregation with Cs-rich compositions on top and FA-rich compositions underneath. X-ray diffraction and confocal Raman spectroscopy indicate the presence of nonphotoemissive δ-phase CsPbI_xBr_3–x in these regions of redder photoluminescence. Under illumination, these Cs-rich cluster regions undergo a faster degradation of their photoluminescence emission. These observations highlight the importance of local A-site compositional heterogeneities on the stability of halide perovskite semiconductors being studied for optoelectronics.