Interfacial Topochemical Fluoridation of MAPbI₃ by Fluoropolymers

Herein it is demonstrated that, under conditions relevant to perovskite synthesis (>140 °C in air), fluoride can topochemically react across the interface between a halide perovskite and a fluoropolymer when in close contact, thereby creating a small quantity of strongly bonded lead fluoride species. The quantity increases with temperature and processing duration. Photoinduced charge carrier lifetime provides a metric for the resulting changes in electronic structure of the perovskite. Under short-duration and/or moderate temperature processing, fluoride transfer to the perovskite yields increased carrier lifetimes, up to 3-fold longer than control samples, which is attributed to passivation of surface defects. Under more forcing conditions, the trend reverses: excessive fluoridation leads to shortened carrier lifetimes, which is ascribed to substantial interfacial formation of PbF₂. It is demonstrated that an interface with bulk crystalline PbF₂ quenches perovskite photoluminescence, likely due to PbF₂ serving as an electron acceptor for the conduction band of MAPbI₃.