Improving Quality of Vapor-Processed Perovskite Thin Film via Methylamine Post-Treatment

While the development of methylammonium lead iodide (MAPbI₃) perovskite in solar harvesting research has matured for solution-based fabrication methods, the growth of high-quality material from vapor processes continues to be difficult due to the challenges of vapor chemistry control. Methylamine (MA) vapor post-deposition treatments are approaches to improve the quality of MAPbI₃perovskites that are compatible with vapor-processed films. Exposure to MA vapor causes a collapse of the MAPbI₃ crystal structure, becoming a transparent liquid phase in MA ambient, recrystallizing back to the perovskite structure on the removal of MA gas. The treatment improves film morphology and crystallinity, producing highly oriented, large-grain perovskite films. In this work, characterization of the liquefaction and recrystallization processes during MA post-treatments of vapor-grown MAPbI₃ perovskite films is performed. The impacts of major treatment factors, including substrate temperature, MA partial pressure, and MA exhaust flow are identified. Enhancement of material electronic properties through defect healing following treatment is confirmed from steady-state and time-resolved photoluminescence spectroscopy. Solar cells from the films treated at optimal conditions showed increased open circuit voltage to over 1 V and fill factor up to 70%. Statistical data on solar cell performance is provided as additional evidence to support the conclusion. The result of this study is a useful pathway for the fabrication of high-quality MAPbI₃films from all-vapor process manufacturing.