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

We use hyperspectral photoluminescence microscopy to study compositional heterogeneity and its influence on the stability of mixed-cation (formamindinium (FA) and cesium) lead halide perovskite solar cells of the composition FA_1-yCs_yPb(I_xBr_1-x)₃ at different concentrations of Cs. By correlating PL maps with time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging, we show that the redder regions of the film are associated with Cs-rich clusters. XRD and confocal Raman microscopy provide evidence for the presence of -phase CsPbI_xBr_3-xclusters in these regions. Photo aging studies show that Cs-rich regions tend to undergo faster photoinduced PL degradation than the rest of the film. These observations highlight the importance of local heterogeneities and their influence on the stability of halide perovskite semiconductors being studied for solar cell applications.