Cation and Octahedral Synergistic Regulation for Stable FAPbI3 Perovskite Solar Cells

Abstract Formamidinium lead iodide (FAPbI3) 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 [PbI6]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 FAPbI3 PSCs. Herein, a robust strategy is reported to stabilize FAPbI3 perovskite by using tricyclohexylphosphine trifluoromethanesulfonate (Cy3PH+SO3CF3−) to strengthen hydrogen bonds within FA+ and alleviate octahedral deformation. The hydrogen‐bonding capacity of Cy3PH+ effectively constrains and stabilizes orientated FA+ through strong hydrogen bonds (F─H, N─H), while the strong electronegative SO3CF3− ion modifies [PbI6]4− octahedral deformation by diversified covalent bonds (Pb─F, Pb─O) and releases the internal stress of the lattice. As such, the resulting FAPbI3 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, FAPbI3 perovskite exhibits improved carrier dynamics and better matched energy‐level alignment with carrier transport layers. The optimized FAPbI3‐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.