Photoinduced Anion Segregation in Mixed Halide Perovskites

Alloyed lead halide perovskites have taken a dominant role in the quest for third-generation solar cells. This is due to optimal light-harvesting properties, which can be tuned across the visible spectrum by mixing halide (X = Cl–, Br–, and I–) anions and A+ cations (A+ = FA+, MA+, and Cs+). Durability issues related to ion movement within the perovskite lattice, however, impede large-scale commercialization. Uniformly mixed halide perovskites [e.g., APb(I1–xBrx)3] reversibly segregate into narrow bandgap I-rich and wide bandgap Br-rich domains during continuous visible illumination. Subsequent I-rich domains reduce local open circuit voltages and decrease mixed halide perovskite solar cell power conversion efficiencies. In this review, we assess the known effects of halide segregation on the structural and optical properties of mixed halide materials, discuss ongoing research to suppress the phenomenon, and provide a mechanistic overview of its underlying origins.