Cation and Octahedral Synergistic Regulation for Stable FAPbI₃ Perovskite Solar Cells

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.