Phase segregation and sequential expulsion of iodide and bromide in photoirradiated ruddlesden-popper 2D perovskite films

Two-dimensional (2D) Ruddlesden–Popper mixed-halide perovskite films, BA2PbBr2I2, undergo phase segregation when excited with visible light to generate bromide- and iodide-rich regions, as marked by absorption and emission changes. Upon stopping illumination, the process reverses, allowing original film compositions to be restored. However, if films are in contact with dichloromethane, light irradiation causes the sequential expulsion of iodide and bromide and introduces irreversible changes to the 2D films. The sequential disappearance of I– and Br– from pristine films (BA2Pb2Br4 and BA2Pb2I4) under photoirradiation, as observed from variances in expulsion rates, reflects differences in halide ion mobilities in these films. The photoinstability of 2D films raises questions about their use in stabilizing bulk, three-dimensional (3D) perovskite solar cells through 3D/2D interfaces.