Ionic Liquid-Assisted Defect Passivation for Efficient Carbon-Based Perovskite Solar Cells with Enhanced Filling Factor

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 Pb²⁺ through the lone electron pairs of carboxyl functional groups and the electron-rich imidazole moieties, leading to a decreased deep defect density of MAPbI₃ 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 MAPbI₃ devices based on TiO₂ 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).