Preventing phase segregation in mixed-halide perovskites: a perspective

Mixed-halide perovskites are ideal materials for the demanding applications of tandem solar cells and emission-tunable light-emitting diodes (LEDs) because of their high compositional flexibility and optoelectronic performance. However, one major obstacle to their use is the compositional instability some mixed-halide perovskites experience under illumination or charge-carrier injection, during which the perovskite material demixes into regions of differing halide content. Such segregation of halide ions adversely affects the electronic properties of the material and severely limits the prospects of mixed-halide perovskite technology. Accordingly, a considerable amount of research has been performed aiming to uncover the underlying mechanisms and mitigating factors of the halide segregation process. Here we present a perspective of strategies designed to reduce the effects of halide segregation in working mixed-halide perovskite devices, based on recent literature reports. We discuss a multitude of mitigating techniques, and conclude that a combination of stoichiometric engineering, crystallinity control and trap state passivation is clearly imperative for abating halide segregation. In addition, the reduction of halide vacancies and control over illumination and temperature can, to a certain extent, mitigate halide segregation. Less direct approaches, such as a change in atmospheric environment, perovskite incorporation into a nanocrystalline composition, or direct control over the crystallographic structure of the perovskite, may however prove too cumbersome to be of practical use. This perspective paves the way for the design and creation of phase-stable, mixed-halide perovskite materials for photovoltaic and LED applications.