Two-dimensional (2D) halide perovskites have drawn attention for their unique optoelectronic properties. When introduced into three-dimensional (3D) perovskite films, 2D perovskites can stabilize them against thermal- and moisture-induced degradation. This improved photostability has led to the incorporation of 2D perovskites into photovoltaic and light emitting display devices. However, these 2D perovskite films are relatively unstable and are susceptible to halide ion migration. Anions of mixed halide 2D perovskite filmsbecome mobile under steady state irradiation to form I-rich and Br-rich domains. Upon ceasing illumination, film reverts to their original, mixed-halide compositions, establishing the reversibility of the phase segregation phenomenon. If these films are in contact with dichloromethane, light irradiation causes the sequential expulsion of iodide and bromide and introduces irreversible changes to 2D films. The sequential disappearance of I- and Br- from pristine films (BA2Pb2Br4, and BA2Pb2I4) under photoirradiation, as observed from variances in expulsion rates, reflect differences in halide ion mobilities in these films. We will discuss how such a photoinstability of 2D films raise the question on its use in stabilizing bulk, three-dimensional (3D) perovskite solar cells through 3D/2D interfaces.

Photoinduced transformations in Ruddlesden-Popper 2D perovskite films

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