Buried Interface Modification by Multifunctional Ionic Liquids for Triple-Cation Perovskite Solar Cells Made in a Fully Ambient Air

Triple-cation CsFAMA perovskite solar cells (PSCs) have gained great attention because of their excellent stability. Buried interface management is one of the most effective methods to further improve device performance; however, such a strategy has not been extensively studied in air-processed triple-cation PSCs. Herein, we report an effective management strategy that incorporates ionic liquids (ILs) of 1-butyl-3-methylimidazole hexafluorophosphate (BMIMPF6) to modify the buried interface. The synergistic effect of BMIMPF6 is revealed by employing several characterization methods, including time-of-flight secondary ion mass spectrometry and inductively coupled plasma mass spectrometry. It was found that BMIMPF6 can significantly coordinate with Sn4+ and fill the oxygen vacancy defects from one side of the electron transport layer, and simultaneously bond with uncoordinated Pb2+ from the other side of the perovskite layer, leading to reduced nonradiative recombination losses and enhanced carrier extraction. More importantly, such an IL "bridge" enabled BMIMPF6 to exist at the buried interface, which avoids damage to the passivating layer in the next spin-coating process for achieving the perovskite film. Moreover, BMIMPF6 also penetrates the perovskite film and regulates the crystallinity of perovskite. As a result, the BMIMPF6-modified device fabricated in ambient air without glove boxes or dry rooms delivers a champion power conversion efficiency (PCE) of 21.74%. The unencapsulated device maintains 77.5% of its initial PCE after being stored in ambient air for more than 3500 h. Additionally, the lead leakage rate from the perovskite film is reduced. This work provides a facile and low-cost method to fabricate high-performance all-air-processed PSCs. The diagram of the chemical structure of BMIMPF6 and device structure in this work.