Enabling Ultrasonic Spray Coating of Uniform and Compact 1.6 eV CsFAPbIBr Films for Efficient Perovskite Solar Cells by Controlling the Wet-to-Dry Film Transition

In this study, uniform and compact ultrasonically spray-coated 1.6 eV CsFAPbIBr films were developed by precisely controlling the wet-to-dry film transition. Achieving both fast nucleation─essential for film compactness─and a smooth wet-to-dry transition to enhance uniformity is particularly challenging in the ultrasonic spray coating of perovskite (PSK). The rapid solvent evaporation via gas quenching, required for achieving fast nucleation, can induce fluid flows that disrupt the film’s uniformity, especially in ultrasonic spray coating processes, where the wet film often exhibits thickness variations. Key parameters─including substrate temperature, N2 gun distance, natural drying time, nozzle path speed, and cosolvent ratio─were adjusted to control the wet-to-dry film transition and suppress unwanted fluid flows. These fluid flows are triggered by the impact of the N2 flow during gas quenching and are also potentially driven by surface tension gradients. The optimized conditions resulted in a uniform and pinhole-free PSK layer. Integrating the developed PSK film in a completely scalable device stack resulted in uniform performance across the sample’s area. The devices showed reproducible power conversion efficiency (PCE) values centered around 17%.