Vertical Halide Segregation and Stability of Two-Dimensional Layered Perovskite Nanostructures: Implications for Optoelectronic Devices

Two-dimensional (2D) layered perovskites have garnered widespread attention due to their exceptional environmental stability and adaptable optical properties. However, quasi-2D layered perovskites typically undergo ion migration and phase segregation when illuminated, presenting a significant challenge in achieving stable optoelectronic devices. In this study, we employ time-dependent photoluminescence spectroscopy to investigate the photoinduced phase segregation of 2D perovskites and to explore the dependence of film stability on perovskite phases. Our results reveal that phase segregation is more likely to occur at the top side (film/air interface) than at the bottom side (film/substrate interface) of the 2D mixed halide perovskite film. We attribute this phenomenon to the 3D perovskite phase located at the top side of the film, which promotes the phase segregation, while the formation of the small- n phase at the bottom side inhibits the photoinduced phase segregation. The vertical alignment of different phases has a decisive effect on photoinduced phase segregation and, therefore, on the vertical stability of the film. These findings provide guidelines for optimizing the vertical stability of 2D mixed halide perovskite structures toward high-performance optoelectronics.