How Ternary Cations and Binary Halogens Stabilize Trigonal FA(1-x-y)MA(x)Cs(y)PbI(3-z)Br(z) Perovskites: From a Single Crystal Perspective

FA(1-x-y)MA(x)Cs(y)PbI(3-z)Br(z) perovskites exhibit excellent stability and optoelectronic performances in solar cells. However, the detailed contributions of each component toward preventing decomposition and phase transition are not understood. Here, we link the contributions of each component to lattice stability and carrier transport from a single crystal perspective, which eliminates the interference of grain boundaries and amorphous perovskite. The exquisite long-term stable trigonal FA(1-x-y)MA(x)Cs(y)PbI(3-z)Br(z) SCs under 90% humidity proves that Br is indispensable, which shrinks the lattice and brings contributive distortions for both octahedron (Oh) and lattice framework, consequently weakening the lattice strain against phase transition. The Br content should be controlled within 15% to avoid phase segregation. Less than 5% content of MA adjusts Oh distortion and promotes carrier transport, but excessive MA increases the extraction risk by H2O. Introduction of Cs (5%) assists Br in bringing sufficient distortion to maintain lattice stability, and exceeding 10% can lead to the phase segregation.