Mechanistic insights into defect-governed ion migration and phase instability in mixed-halide perovskites

Abstract

Ion migration critically underpins the operational instability of metal halide perovskites, yet the fundamental mechanisms by which defect chemistry and physics govern ion migration in mixed-halide perovskites, and their critical role in driving the distinct instability pathways under dark versus illuminated conditions, remain inadequately understood. Here, we investigate ion-migration dynamics coupled with defect and phase segregation in FAPb(I_1–xBr_x)₃, uncovering a defect-governed evolution of ion migration across different Br compositions. Under dark conditions, ion migration is revealed to be dominated by the specific characteristics of halide interstitial defects, whereas illumination triggers FA-cation migration that strongly correlates with the deep-level defect density. Despite exhibiting comparable halide-migration barriers, high-Br compositions undergo a two-state phase segregation process, comprising an initial thermodynamically driven halide separation followed by a kinetically accelerated regime facilitated by a substantial reduction in the halide migration barrier. These findings provide essential insights for stabilizing wide-bandgap perovskites in high-efficiency tandem photovoltaics.