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

Abstract 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.