Pinning Bromide Ion with Ionic Liquid in Lead‐Free Cs2AgBiBr6 Double Perovskite Solar Cells

Lead‐free Cs2AgBiBr6 double perovskite has received widespread attention because of its non‐toxicity and high thermal stability. However, intrinsic bromide ion (Br–) migration limits continuous operation of Cs2AgBiBr6‐based perovskite solar cells (PSCs). Herein, an operational and simple strategy is carried out to improve the power conversion efficiency (PCE) and long‐term stability of Cs2AgBiBr6‐based PSCs by introducing 1‐butyl‐1‐methylpyrrolidinium chloride (BMPyrCl) and 1‐butyl‐3‐methylpyridinium chloride (BMPyCl) ionic liquids (ILs). The higher binding energy between Br– in Cs2AgBiBr6 and cation in IL containing pyrrole can inhibit Br– migration effectively, thereby reducing film defects and improving energy level matching. The optimized PCE of 2.22% is obtained for hole transport layer‐free, carbon‐based PSC, which hardly degrades at 40% ± 5% relative humidity and 25 °C for 40 days. This work highlights an effective method to mitigate the halide migration in Cs2AgBiBr6 perovskite, thus providing an effective route in promoting the development of lead‐free double PSCs. Br− migration in Cs2AgBiBr6 perovskite is effectively mitigated through the interaction between ionic liquid and perovskite. Thus, reduced defect states leads to improved energy level alignment and enhanced efficiency. Efficiency hardly degrades at 40 ± 5% relative humidity and 25 °C for fully inorganic perovskite solar cells without a usual hole transport layer.