Impact of Halide Anions in CsX (X = I, Br, Cl) on the Microstructure and Photovoltaic Performance of FAPbI₃-Based Perovskite Solar Cells

The key role of Cs cation and X halide anions (X = I, Br, Cl) on the microstructure, crystal structure, structural defects, optoelectronic properties, and photovoltaic parameters of FAPbI₃-based perovskite solar cells is investigated. The CsCl–FAPbI₃ perovskite film shows the highest photoluminescence (PL) intensity, longest PL lifetime, and highest power conversion efficiency compared with the CsI–FAPbI₃ and CsBr–FAPbI₃ perovskite films. The morphology and crystallography of (Formula presented.) _c nanotwins and stacking faults of perovskite films are studied using transmission electron microscopy and selected-area electron diffraction. The microstructure, crystallography, and atomic structure model of intersecting twin boundaries are presented. Finally, the degradation pathways and the mechanism behind the formation of FAPbI₃-based perovskites under ambient conditions are systematically studied. The grain boundaries of the perovskite films are nonuniformly damaged, resulting in many black nanoparticles after 4 weeks. Electron diffraction analyses of the black nanoparticles confirm the hexagonal PbI₂ phase formation in all CsX–FAPbI₃ perovskite samples after 4 weeks of aging.