Rationalizing Light-Induced Phase Segregation Reversal by Halide Oxidation and Diffusion in Mixed Halide Perovskites

Lead halide perovskites are one of the most promising materials as active layers of optoelectronic devices. Phase segregation under illumination in mixed halide perovskites is one of the major issues in stable device operation. Herein, we rationalize illumination power dependent phase segregation phenomena, including two thresholds between which phase segregation occurs, and the reversal of phase segregation. Our experimental observation combining confocal photoluminescence mapping with in situ Raman spectroscopy supports the halide oxidation model. We observed phase segregation beyond the illuminated area, while the illuminated area remained mixed. Reversal of phase segregation under illumination was also observed. We propose that the spatial distribution of phase segregation is driven by halide oxidation and diffusion of the products through mass flow, as verified by light- and spatial-dependent lattice halide vibrations. Our insights into phase segregation may provide new perspectives for manipulating phase segregation by local light intensity for dynamically tunable optoelectronics.