Molecular Origin of the Asymmetric Photoluminescence Spectra of CsPbBr at Low Temperature.

CsPbBr has received wide attention due to its superior emission yield and better thermal stability compared to other organic-inorganic lead halide perovskites. In this study, through an interplay of theory and experiments, we investigate the molecular origin of the asymmetric low-temperature photoluminescence spectra of CsPbBr. We conclude that the origin of this phenomenon lies in a local dipole moment (and the induced Stark effect) due to the preferential localization of Cs in either of two off-center positions of the empty space between the surrounding PbBr octahedra. With increasing temperature, Cs ions are gradually occupying positions closer and closer to the center of the cavities. The gradual loss of ordering in the Cs position with increasing temperature is the driving force for the formation of tetragonal-like arrangements within the orthorhombic lattice.