Vertical Compositional Heterogeneity Induces Instability in All-Inorganic CsPbIBr₂ Perovskites

Understanding the vertical compositional homogeneity and defect distribution is of paramount importance in elucidating and maximizing the performance of halide-perovskite-based optoelectronic devices. This work reports the depth-dependent study of the chemical composition and metallic Pb⁰ content of all-inorganic CsPbIBr₂ perovskite films undertaken using lab-based hard X-ray photoelectron spectroscopy and soft X-ray photoelectron spectroscopy. The presence of elemental or metallic Pb (Pb⁰), in the bulk and at the surface of the perovskite films highlights the formation of defect or recombination centers throughout the analyzed depth. The Pb⁰ content was found to be of higher concentration in the bulk of the CsPbIBr₂ films compared to that at the surface. Engineering the CsPbIBr₂ film growth using appropriate antisolvents resulted in the overall reduction and/or complete elimination of Pb⁰ at the surface and at the bulk of the perovskite films. However, the effect of antisolvent treatment was significantly pronounced in the bulk-like region as compared to that at the surface. Pb⁰ is synonymous with defect states/recombination centers in perovskite films and this reduction in defect density due to the antisolvent treatment corroborates the enhanced phase stability and improved solar cell performance of the corresponding CsPbIBr₂ devices.