Tailoring Tensile Strain in Pb–Sn Perovskite Film for Efficient and Stable Narrow‐Bandgap Perovskite Solar Cells

Pb–Sn perovskites (PSPs) with narrow bandgap and low toxicity show great promise in next‐generation perovskite‐based photovoltaics, especially in constructing all‐perovskite tandem solar cells as the rare subcell. However, partially replacing Pb by Sn with smaller ionic size in PSPs is shown to reduce their crystal lattice symmetry, which leads to intrinsic compressed lattice strain and reduced structural stability, resulting in the poor device performance of PSPs‐based solar cells. Herein, the effect of lattice tensile strain on the optoelectronic properties of FA_0.7MA_0.3Pb_0.5Sn_0.5I₃ film is investigated and the tensile strain is finely adjusted by introducing monovalent‐cation chlorides with different cation sizes as additive. It is found that a moderate tensile strain in the film is beneficial for stabilizing the crystal structure, promoting oriented crystal growth and reducing the defect density. After optimization, FA_0.7MA_0.3Pb_0.5Sn_0.5I₃‐based perovskite solar cells using FACl as additive can produce the device with champion power conversion efficiency (PCE) of 19.30% together with improved device reproducibility. Importantly, the FACl‐based device shows robust stability that exhibits no PCE loss after being stored in N₂ for 1000‐h. This work provides new insight into the key role of tensile strain in PSPs, which can facilitate the development of PSP‐based optoelectronic devices.