Cation and Octahedral Synergistic Regulation for Stable FAPbI3 Perovskite Solar Cells

Abstract Formamidinium lead iodide (FAPbI 3 ) perovskite, one of the most promising light‐absorbing materials, faces substantial stability issues, including FA + organic component volatilization and undesirable phase transition between corner‐sharing and face‐sharing [PbI 6 ] 4 ̶ octahedra. Especially, the asymmetric hydrogen bonding, arising from oriented and irregularly spinning FA + cation, accelerates these transformations, compromising both the efficiency and long‐term stability of FAPbI 3 PSCs. Herein, a robust strategy is reported to stabilize FAPbI 3 perovskite by using tricyclohexylphosphine trifluoromethanesulfonate (Cy 3 PH + SO 3 CF 3 − ) to strengthen hydrogen bonds within FA + and alleviate octahedral deformation. The hydrogen‐bonding capacity of Cy 3 PH + effectively constrains and stabilizes orientated FA + through strong hydrogen bonds (F─H, N─H), while the strong electronegative SO 3 CF 3 − ion modifies [PbI 6 ] 4 − octahedral deformation by diversified covalent bonds (Pb─F, Pb─O) and releases the internal stress of the lattice. As such, the resulting FAPbI 3 demonstrates mitigated organic volatilization and suppressed phase transition, significantly enhancing phase stability under thermal/humidity stress conditions. Moreover, because of co‐regulated FA + cation and octahedral lattice, FAPbI 3 perovskite exhibits improved carrier dynamics and better matched energy‐level alignment with carrier transport layers. The optimized FAPbI 3 ‐based PSCs deliver an impressive efficiency of 25.93% and exhibit exceptional stability, retaining 97% of initial efficiency after over 1500 h maximum power point tracking.