Nanoscale Heterogeneity in CsPbBr₃ and CsPbBr₃:KI Perovskite Films Revealed by Cathodoluminescence Hyperspectral Imaging

The nanoscale morphology of solar cell materials strongly affects their performance. We report direct evidence for the existence of multiple length scales of heterogeneity in halide perovskites such as CsPbBr3 and CsPbBr3:KI. Contrary to the general notion of two distinct phases, our study suggests the presence of multiple phases in mixed halide perovskites. Highly spatially resolved (≈50 nm) cathodoluminescence maps reveal that the length scale of heterogeneity is composition-dependent: smaller (≈200 nm) for CsPbBr3 and larger (≈500–1000 nm) for CsPbBr3:KI. Moreover, these nano-/micro-scale heterogeneities exist both laterally and vertically in mixed halides and correlate with high densities of carrier traps and fast trap-assisted recombination. The observed heterogeneities also lead to reduced power conversion efficiency of solar cells, higher hysteresis loss, and faster degradation. These insights argue for advanced nanoscale characterization of halide perovskites to guide reduction of heterogeneity and so improve device performance and stability.