Room Temperature Ionic Liquid Capping Layer for High Efficiency FAPbI3 Perovskite Solar Cells with Long‐Term Stability

Abstract Ionic liquid salts (ILs) are generally recognized as additives in perovskite precursor solutions to enhance the efficiency and stability of solar cells. However, the success of ILs incorporation as additives is highly dependent on the precursor formulation and perovskite crystallization process, posing challenges for industrial‐scale implementation. In this study, a room‐temperature spin‐coated IL, n‐butylamine acetate (BAAc), is identified as an ideal passivation agent for formamidinium lead iodide (FAPbI 3 ) films. Compared with other passivation methods, the room‐temperature BAAc capping layer (BAAc RT) demonstrates more uniform and thorough passivation of surface defects in the FAPbI 3 perovskite. Additionally, it provides better energy level alignment for hole extraction. As a result, the champion n–i–p perovskite solar cell with a BAAc capping layer exhibits a power conversion efficiency (PCE) of 24.76%, with an open‐circuit voltage (Voc) of 1.19 V, and a Voc loss of ≈330 mV. The PCE of the perovskite mini‐module with BAAc RT reaches 20.47%, showcasing the effectiveness and viability of this method for manufacturing large‐area perovskite solar cells. Moreover, the BAAc passivation layer also improves the long‐term stability of unencapsulated FAPbI 3 perovskite solar cells, enabling a T80 lifetime of 3500 h when stored at 35% relative humidity at room temperature in an air atmosphere.