Regulating Intermediate State for Low‐Strain Perovskite Films

ABSTRACT Defect‐induced lattice distortion and thermally induced lattice expansion result in lattice strains in the perovskite film. Suppressing the lattice strain is significant for improving efficiency and stability of perovskite solar cells (PSCs) but remains challenging. Herein, we report a strain inhibition strategy by treating the intermediate‐state perovskite (Inter‐PVK) film pre‐annealed at a low temperature using a boronic acid‐based molecule. The boronic acid group interacts with ionic defects on the surface of the Inter‐PVK film, in favor of reducing defect‐induced lattice distortion in the final‐state perovskite film during its downward growth process. The interaction also immobilizes organic components, preventing their volatilization and consequently inhibiting the lattice expansion. Both aspects synergistically mitigate the lattice strain throughout the entire perovskite film. The resultant low‐strain perovskite film enables an efficiency of 26.32% for inverted PSCs (certified 26.20%) and 25.57% for regular PSCs, respectively. Moreover, the devices retain 94% of their initial efficiency after 1500 h of continuous 1‐sun light soaking. Our work provides a new strategy for regulating strains in perovskite films to enhance PSC performances.