Ionic Liquid‐assisted Defect Passivation for Efficient Carbon‐based Perovskite Solar Cells with Enhanced Filling Factor

Abstract The preparation of high‐quality perovskite thin films with long‐term stability is the prerequisite for realizing efficient perovskite solar cells (PSCs). In this work, the effect of the bifunctional additive 1‐ethyl‐3‐methylimidazolium acetate (EMIMAc) ionic liquid on defect passivation in perovskite films was systematically investigated. Both theoretical simulations and experimental results reveal that EMIMAc has a strong coordination interaction with the undercoordinated Pb 2+ through the lone electron pairs of carboxyl functional groups and the electron‐rich imidazole moieties, leading to a decreased deep defect density of MAPbI 3 system. Besides, EMIMAc treatment realizes energy band alignment. As a result, the photoelectric conversion efficiency (PCE) of optimized PSCs reaches 17.07 %, and the filling factor (FF) exceeded 74.91 % which is the highest FF for hole transport layer (HTL)‐free carbon‐based MAPbI 3 devices based on TiO 2 electron transport layer. Moreover, the unencapsulated EMIMAc‐modified device maintains approximately 89 % of its initial PCE after 30 days, which demonstrates much better air stability than control devices. These results provide effective strategies for improving the efficiency and long‐term stability of HTL‐free carbon‐based PSCs (H‐C‐PSCs).