In Situ Pseudo‐Halide Diffusion Enables Buried Interface Regulation and Crystallinity Enhancement in Perovskite Solar Cells

ABSTRACT Regulating buried interfaces is pivotal for suppressing interfacial defects and facilitating crystallization toward compact, high‐quality perovskite films. Here, we propose a thermally activated lithium formate (LiHCOO) buried interface strategy that triggers in situ pseudo‐halide diffusion, thereby simultaneously reconstructing the SnO ₂ /perovskite interface and regulating perovskite nucleation and growth. High‐temperature treatment transforms LiHCOO from its low‐temperature hexagonal polymorph to a metastable monoclinic phase with a more open molecular packing structure, thereby enabling deeper HCOO ⁻ diffusion into the buried perovskite. Diffused HCOO ⁻ strongly interacts with undercoordinated Pb ²⁺ sites, inhibiting pinhole formation, reducing trap density, compensating for halide vacancy related defects, and releasing residual tensile strain. Meanwhile, LiHCOO modification passivates the SnO ₂ surface by reducing oxygen vacancies and hydroxyl defects, improving interfacial electrical properties. This enhances the built‐in potential of perovskite devices from 0.91 to 1.00 V while optimizing energy level alignment. Ultimately, the optimized perovskite solar cell achieved a champion efficiency of 25.48%, with an open‐circuit voltage of 1.213 V and a fill factor of 82.57%, while also demonstrating outstanding long‐term stability. This work reveals polymorph‐mediated pseudo‐halide diffusion as a novel approach to low‐loss and robust perovskite photovoltaics.