Dissected MACl Involved Reaction Pathway for Low Temperature Fabrication of High‐Efficiency Flexible Perovskite Solar Cells

FAPbI 3 ‐based perovskite solar cells (PVSCs) are highly promising due to their excellent power conversion efficiency (PCE). However, the formation of stable α‐FAPbI 3 requires annealing at a temperature of at least 150 °C, which can induce significant residual stress and make the process incompatible with plastic substrates, resulting in poor crystal quality and compromised mechanical properties. To address these issues, we present a novel approach using methylammonium chloride (MACl) vapor‐assisted lead iodide (PbI 2 ) to form homogeneous intermediate phases (MAPbCl 3 and Pb 3 I 8 ). This method optimizes the reaction pathway; accordingly pure α‐phase perovskite can be completely formed at 100 °C, contrasting with conventional MACl component engineering in organic amine salt solution. We have intensively deciphered the reaction pathway leading to intermediate phase formation, as well as the nucleation and phase transition processes into the photoactive perovskite at 100 °C. Additionally, we have conducted a comprehensive study on temperature‐dependent residual stress and cation reaction competition, meanwhile extensively dissecting the competitive implication of interface mismatch and lattice defects leading to residual stress and cation component inhomogeneity. Consequently, the modified flexible‐PVSCs have achieved a remarkable efficiency of 24.46% (25.50% for rigid PVSCs). The devices also exhibit significantly improved long‐term and mechanical stability.