Elucidation of the suppression of photoinduced segregation in 2D mixed halide, APbIBr: Critical role of APbBr photostability.

2D lead halide perovskites are used to improve device operational stability due to increased environmental stability and reduced ion migration compared to 3D perovskites. However, the relationship between the 2D perovskite stability under illumination and spacer cation is still not well understood. Thus, we examine photoinduced halide segregation (PHS) in different 2D mixed-halide perovskites and show that PHS is suppressed in the materials which have photostable bromide halide phase. As the spacer cations provide the barrier to ion migration in 2D perovskites, PHS would be facilitated by the loss of spacer cations through their interactions with various mobile oxidized halide species, resulting in organic ammonium deprotonation and spacer cation vacancy formation. The existence of a photostable APbBr phase, which does not exhibit spacer cation loss, results in the suppression of PHS in 2D mixed halide perovskites due to reduced spacer vacancy formation and consequently reduced halide ion migration under illumination.