Local A-Site Phase Segregation Leads to Cs-Rich Regions Showing Accelerated Photodegradation in Mixed-Cation Perovskite Semiconductor Films
Zixu Huang, Fangyuan Jiang, Zhaoning Song, Kshitiz Dolia, Tao Zhu, Yanfa Yan, David S. Ginger
Abstract
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 x Br 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.