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 degrees 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.