Vapor Deposition of FAI on SnO₂ Provides Interface Modification and Crystallization Control for High-Performance and Stable Perovskite Solar Cells

The modification of electron transport layers (ETLs) is an important strategy to optimize the performance of organic–inorganic hybrid perovskite solar cells (PSCs). In this study, the components of the perovskite precursor, formamidinium iodide (FAI), were introduced to form an ultra-thin uniform FAI film on the surface of the prepared SnO 2 film by a novel vapor deposition method. This method helped fill up the defects of the ETL, reduce excess PbI 2 in the perovskite film, and enlarge the grain size to improve the quality of the ETL and the perovskite film. Therefore, better interfacial contact of the ETL-perovskite, reduced trap-state density, increased electron extraction and transport ability, and improved energy level matching were achieved. By optimizing the FAI, the power conversion efficiency (PCE) presented an impressive increase from 17.15 to 20.54%, with negligible hysteresis and improved long-term device stability. In addition, the corresponding flexible PSCs also exhibited an excellent PCE of 17.78% and an enhanced bending durability.