In-depth understanding of ionic liquid assisted perovskite film formation mechanism for two-step perovskite photovoltaics

Ionic liquids (ILs) have been widely applied in the one-step fabrication of perovskite with noticeable enhancement in the device performance. However, in-depth mechanism of ionic-liquid-assisted perovskite film formation is not well understood for also important two-step perovskite fabrication method, with better control of crystallization behavior. In this work, we introduced ionic liquid methylammonium formate (MAFa) into organic salt to produce perovskite film via a two-step method. Systematic investigations on the influence of MAFa on the perovskite thin film formation mechanism were performed. Ionic liquid is shown to assist lowering the perovskite formation enthalpy upon the density functional theory (DFT) calculation, leading to an accelerated crystallization process evidenced by in-situ UV-Vis absorption measurement. A gradient up-down distribution of ionic liquid has been confirmed by time-of-flight SIMS. Importantly, besides the surface passivation, we found the HCOO− can diffuse into the perovskite crystals to fill up the halide vacancies, resulting in significant reduction of trap states. Uniform perovskite films with significantly larger grains and less defect density were prepared with the help of MAFa IL, and the corresponding device efficiency over 23% was obtained by two-step process with remarkably improved stability. This research work provides an efficient strategy to tune the morphology and opto-electronic properties of perovskite materials via ionic-liquid-assisted two-step fabrication method, which is beneficial for upscaling and application of perovskite photovoltaics.