Studies on the Light-Induced Phase Transition of CsPbBr Metal Halide Perovskite Materials.

We investigate the internal mechanism of the light-induced phase transition of CsPbBr perovskite materials via density functional theory simulations. Although CsPbBr tends to appear in the orthorhombic structure, it can be changed easily by external stimulus. We find that the transition of photogenerated carriers plays the decisive role in this process. When the photogenerated carriers transit from the valence band maximum to conduction band minimum in the reciprocal space, they actually transit from Br ions to Pb ions in the real space, which are taken away by the Br atoms with higher electronegativity from Pb atoms during the initial formation of the CsPbBr lattice. The reverse transition of valence electrons leads to the weakening of bond strength, which is proved by our calculated Bader charge, electron localization function, and integral value of COHP results. This charge transition releases the distortion of the Pb-Br octahedral framework and expands the CsPbBr lattice, providing possibilities to the phase transition from the orthorhombic structure to tetragonal structure. This phase transition is a self-accelerating positive feedback process, increasing the light absorption efficiency of the CsPbBr material, which is of great significance for the widespread promotion and application of the photostriction effect. Our results are helpful to understand the performance of CsPbBr perovskite under a light irradiation environment.