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(x)MA(y)Cs(1-x-y)PbI(3) [FA(+) = (NH2)(2) CH+, MA(+) = CH3NH3+] active layers with mixed orthorhombic, post-perovskite (delta(ortho)-CsPbI3), and cubic perovskite (alpha) phases (i.e., alpha/delta-phase FA(x)MA(y)Cs(1-x-y)PbI(3)) exhibit improved cation stability against applied bias relative to pure alpha-phase perovskites (i.e., FA(0.85)Cs(0.15)PbI(3) and FA(0.76)MA(0.15)Cs(0.09)PbI(3)). Infrared photothermal heterodyne imaging and time-of-flight secondary ion mass spectrometry are used to visualize exclusive alpha-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; alpha/delta-phase FA(x)MA(y)Cs(1-x-y)PbI(3) lateral devices/solar cells, by contrast, show improved stabilities. Enhanced a/S-F Csi-x-yPbIl stability is rationalized by delta(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(x)MA(y)Cs(1-x-y)PbI(3) perovskites and suggests a materials design strategy toward suppressing cation instabilities in hybrid perovskites.