Lattice Strain Regulation Enables High‐Performance Formamidinium Perovskite Photovoltaics

Abstract Formamidinium lead iodide (FAPbI₃) perovskite possesses an ideal optical bandgap and is a potential material for fabricating the most efficient single‐junction perovskite solar cells (PSCs). Nevertheless, large formamidinium (FA) cations result in residual lattice strain, which reduces the power conversion efficiency (PCE) and operational stability of PSCs. Herein, the modulation of lattice strain in FAPbI₃ crystals via a π‐conjugated organic amine, i.e., 4‐pyrene oxy butylamine (PYBA), is proposed. PYBA pairs at the grain boundary serve as a template for the crystallization of FAPbI₃ perovskite, thereby inducing a highly oriented crystal and a pure α‐phase film. The PYBA pairs with strong π–π interactions provide a solid fulcrum for external compression strain, thus compensating for the inherent tension strain of FAPbI₃ crystals. The strain release elevates the valence band of the perovskite crystals, thereby decreasing the bandgap and trap density. Consequently, the PYBA‐regulated FAPbI₃ PSC achieves an excellent PCE of 24.76%. Moreover, the resulting device exhibits improves operational stability and maintains over 80% of its initial PCE after 1500 h under maximum power point tracking conditions.