Resolving Nanoscale Phases Purity and Lattice Strain in Multi‐Cationic Perovskite Solar Cells for Enhanced Stability

A-site cations in organic-inorganic lead halide perovskites (OLHPs) critically stabilize phases, inhibit ion migration, and reinforce interfaces, ensuring device operational stability. However, the mechanistic links between A-cation engineering and phase purity, and lattice strain in determining perovskite solar cell stability remain elusive. Here, optimized hybrid perovskite A-site cation ratios (FA:70-90%, MA:5-15%, Cs:5-25%) are achieved using an orthogonal L₉ (3^⁴) array, reducing experimental permutations by 66.7% while maintaining statistical rigor through balanced phase-space sampling and orthogonal factor matching, revealing composition-dependent linkages among phase purity, lattice strain, and stability in OLHPs. The results demonstrate that cation coordination hierarchy (FA⁺>MA⁺>Cs⁺>Rb⁺) governing phases purity. Further, Rb⁺ substitution markedly suppresses defect density in quaternary FA_0.82MA_0.05Cs_0.05Rb_0.08PbI₃ films. The geometric phase analysis (GPA) demonstrates that stark contrasts between ternary (FA_0.80MA_0.15Cs_0.05PbI₃) and quaternary (FA_0.82MA_0.05Cs_0.05Rb_0.08PbI₃) perovskites: the former exhibits pronounced compressive (ɛ_xx = -3.3%) and tensile (ɛ_yy = 27.5%) strains, whereas the latter shows significantly reduced strain magnitudes (ɛ_xx = -1.2%, ɛ_yy = 1.6%), indicative of decreased lattice strain. Photovoltaic devices incorporating optimized FA_0.80MA_0.15Cs_0.05PbI₃ achieve a PCE of 24.65%, while the FA_0.82MA_0.05Cs_0.05Rb_0.08PbI₃ device reaches 25.53%. Stability assessments reveal exceptional durability: the quaternary device retains ≈88% of its initial PCE after 500 h under humidity exposure.