Light-Induced Reversal of Ion-Segregation in Mixed-Halide Perovskites

Bandgap-instability in mixed-halide perovskites, owing to light-induced phase-segregation, is poorly understood and presents a major challenge for their future commercial use. Here we demonstrate that photoinduced halide-ion segregation can be completely reversed at sufficiently high illumination intensities. With control over halide segregation we tune the optical bandgap of a single mixed-halide perovskite crystal by simply optimizing the input photogenerated carrier density. We develop a polaron-based 2D lattice model that rationalises the experimentally observed phenomena by assuming that the driving force for photoinduced halide segregation is dependent on carrier-induced strain gradients that vanish at high carrier densities. Using illumination sources with different excitation intensities we demonstrate write-read-erase experiments, showing that it is possible to store information in the form of latent images over several minutes. This new ability to control the local halide-ion composition could become key to solving the issue of halide-ion segregation in these materials, and presents new opportunities for their use in concentrator and tandem solar cells, as well as in high-power light-emissive devices and optical memory applications.