Klaus Weber

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 FAPbI 3 ‐based perovskite solar cells is investigated. The CsCl–FAPbI 3 perovskite film shows the highest photoluminescence (PL) intensity, longest PL lifetime, and highest power conversion efficiency compared with the CsI–FAPbI 3 and CsBr–FAPbI 3 perovskite films. The morphology and crystallography of 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 FAPbI 3 ‐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 PbI 2 phase formation in all CsX–FAPbI 3 perovskite samples after 4 weeks of aging.