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

Abstract 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.