Suppressing Cation Migration in Triple-Cation Lead Halide Perovskites

Ion migration represents an intrinsic instability of metal halide perovskite solar cells. Here we show that triple-cation FA_xMA_yCs_1–x–yPbI₃ [FA⁺ = (NH₂)₂CH⁺, MA⁺ = CH₃NH₃⁺] active layers with mixed orthorhombic, post-perovskite (δ_ortho-CsPbI₃), and cubic perovskite (α) phases (i.e., α/δ-phase FA_xMA_yCs_1–x–yPbI₃) exhibit improved cation stability against applied bias relative to pure α-phase perovskites (i.e., FA_0.85Cs_0.15PbI₃ and FA_0.76MA_0.15Cs_0.09PbI₃). Infrared photothermal heterodyne imaging and time-of-flight secondary ion mass spectrometry are used to visualize exclusive α-phase perovskite lateral device A⁺ cation accumulation (depletion) at perovskite negative (positive) electrode interfaces. The resulting compositional heterogeneities lead to degradation. Operational stability testing of solar cells reveals similar degradation behavior; α/δ-phase FA_xMA_yCs_1–x–yPbI₃ lateral devices/solar cells, by contrast, show improved stabilities. Enhanced α/δ-FA_xMA_yCs_1–x–yPbI₃ stability is rationalized by δ_ortho-phase inclusions, acting as barriers through which A⁺ cations do not easily migrate. This study thus provides new insights into cation migration in FA_xMA_yCs_1–x–yPbI₃ perovskites and suggests a materials design strategy toward suppressing cation instabilities in hybrid perovskites.