Achieving High-Quality Wide Bandgap Perovskite Thin Films via Regulating the Halide Ion Exchange Order in Vapor-Solid Reaction

Wide-bandgap (WBG) perovskite films are vital for advancing high-efficiency silicon/perovskite tandem technology. However, the performance of WBG perovskite films produced using vapor deposition techniques often lags behind that of solution-based methods due to challenges in accurately controlling the halide ions and crystallization quality, particularly the Br/I ratio in vapor-deposited perovskite films. In this study, we investigated the halide ion exchange (IE) process in vapor-solid reaction and developed two methods for producing CsFAPbIxBr3-x WBG perovskite thin films: one involved reacting CsFAPbI3 in FABr vapor (I-based IE perovskite), while the other used CsFAPbBr3 in FAI vapor (Br-based IE perovskite). Our findings demonstrate that the Br-based IE perovskite exhibits superior crystallization quality and lower defect density throughout the ion exchange process. As a result, this approach has facilitated the development of WBG perovskite solar cells with a maximum power conversion efficiency of 19.51%. Additionally, unencapsulated devices were able to retain 88.9% of their initial efficiency after being stored for 1500 hr under atmospheric conditions (25 degrees C, 18 +/- 5% RH). This research provides a novel strategy and methodology for fabricating high-performance WBG perovskite solar cell via vapor-based techniques, which is crucial for the industrialization of both perovskite solar cells and silicon/perovskite tandem solar cells.