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 CsFAPbI x Br 3− x WBG perovskite thin films: one involved reacting CsFAPbI 3 in FABr vapor (I‐based IE perovskite), while the other used CsFAPbBr 3 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°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.