In Situ Pseudo-Halide Diffusion Enables Buried Interface Regulation and Crystallinity Enhancement in Perovskite Solar...

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 SnO₂/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 Pb²⁺ sites, inhibiting pinhole formation, reducing trap density, compensating for halide vacancy related defects, and releasing residual tensile strain. Meanwhile, LiHCOO modification passivates the SnO₂ 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.