Selective Solvent Polishing Coupled With Post‐Passivation Yields Ultra‐Smooth Single‐Crystal Perovskite Film for Solar Cells

Metal halide perovskite single crystals have emerged as promising materials for photovoltaics due to their exceptional optoelectronic properties. However, solution‐grown single crystals encounter significant challenges, including surface contamination and high densities of trap states. These issues lead to nonradiative recombination, which negatively affects device performance. Herein, we introduced selective chemical polishing coupled with post‐passivation (CPP) to address these limitations. This approach effectively removes surface impurities from single crystals, resulting in an ultra‐smooth surface with a roughness of only 0.46 nm. The single‐crystalline films have a lead‐iodide stoichiometric balance, significantly reduce iodide vacancies, and suppress trap‐assisted recombination. Compared to the control device, the CPP treatment increased device efficiency from 12.01% to 20.05%, while enhancing operational stability by more than 4 times. Our strategy solves the surface issues of single‐crystal perovskite films, paving the way for single‐crystal perovskite solar cells.