Lowering the δ-to-α Phase-Transition Barrier via a Transient Vapor Mediator in Perovskite Formation

Hybrid sequential deposition (HSD) remains a practical pathway for the scalable fabrication of perovskite photovoltaics, though it is frequently constrained by the formation of dense evaporated precursor frameworks and the associated kinetic limitations of the subsequent solid interdiffusion. These constraints often manifest as incomplete conversion and vertical inhomogeneity in the resulting perovskite film. Herein, we report a transient vapor-deposited mediator (TVM) strategy that translates the principles of liquid-phase mediator engineering to a vapor-phase process. By incorporating Pb(SCN)2 during the coevaporation of inorganic precursors, the resulting framework exhibits modified porosity that facilitates the infiltration of organic salts and lowers the activation energy for the solid-state δ-to-α phase transition. The elimination of the mediator under ambient humidity is accompanied by the egress of FASCN-related volatile species, promoting more complete and uniform conversion through the full film thickness with reduced residual PbI2 and lower trap density. This TVM process enables solar cells' power-conversion efficiency to increase from 22.3% to 25.2% and yields enhanced operational robustness, retaining 91% of initial performance after 1200 h of thermal aging at 85 °C and 95% after 1600 h of continuous maximum power point tracking under illumination.