Photoinduced Self-healing of Halide Segregation in Mixed-halide Perovskites

Photoinduced halide segregation (PIHS) is an inevitable issue in the deterioration of the phase stability of band-gap-tunable mixed-halide perovskites (MHPs). Herein, under continuous irradiation of sunlight, we first discover that the polycrystalline CsPbI3-xBrx films always experience a phase transformation from halide mixing to demixing and then to halide remixing (namely photoinduced self-healing of PIHS). Particularly, the obtained film will no longer demix after the mixing-demixing-remixing cycle. We then demonstrate that the specific positions for halide ions migration gradually approach their demixing terminal (here a strain-tolerance threshold of the perovskite lattice) driven by photoexcited carriers. However, once the threshold is exceeded, the intolerable strain breaking the lattice leads to the emergence of nanoscale cracks for strain release, acting as direct evidence for the popular strain-induced PIHS models. In addition, the visible cracks, located at grain boundaries and interiors, can clarify the controversial issue of iodide ions migrating positions. These findings can contribute to an in-depth understanding of the PIHS phenomenon for stable MHPs.