Direct Observation of Suppressing Ion Migration at Surface and Buried Interface toward Stable Perovskite Solar Cells

Ion migration can lead to detrimental consequences, including hysteresis effects, interfacial reactions, etc., which degrades the stability and efficiency of perovskite solar cells (PSCs). Ionic liquid has been introduced to enhance the stability of PSCs, yet the detailed mechanism is still under debate. To address the question, in situ wide-field photoluminescence microscopy is employed to characterize the ion migration, which is found more obviously suppressed at the perovskite buried interface than the surface after 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) modification. The experimental results show that BF4 - is distributed mainly at the buried interface, while BMIM exists throughout the perovskite film and accumulates at the surface. BF4 - can suppress ion migration through filling the iodine vacancies and passivating undercoordinated Pb2+, thus reducing the defect density. Meanwhile, BMIM+ can passivate lead vacancies (VPb) and undercoordinated Pb2+ across the whole perovskite film, effectively decreasing the Pb-related defects. Consequently, PSCs incorporated with BMIMBF4 exhibit enhanced power conversion efficiency and stability. This study provides a comprehensive understanding of the role of ionic liquids in the ion migration of perovskite interfaces and its impact on the performance of PSCs.