Vapor Post‐treatment for High‐Performance Perovskite Solar Cells via Vapor–Solid Reaction

Manufacturing perovskite thin films via vapor–solid reaction processes enables good compatibility with existing photovoltaic device fabrication workflows, facilitating the development of tandem solar cells. However, during vapor–solid reactions, gaseous organic ammonium molecules must diffuse into the solid lead halide film to react chemically. This diffusion is inherently inhomogeneous, which can compromise the crystallinity and stoichiometry control of the perovskite films and lead to a high density of defects both within the crystal lattice and at the film surface. To address this, introducing a gas‐phase post‐treatment step for vapor–solid grown perovskite films can further improve film quality, thereby enhancing process stability and reproducibility. In this work, we explored different guanidinium salts as gas‐phase post‐treatment materials. The GA + cation provides surface passivation, while we focused on the effects of various anions during the post‐treatment. Our results show that guanidine thiocyanate (GASCN) promotes a secondary crystallization process in the perovskite, significantly improving crystallinity and reducing defect density in the film. Consequently, small‐area devices treated with GASCN achieved a peak power conversion efficiency of 20.96%. The unencapsulated device retained 91.4% of its initial efficiency after storage at room temperature for 1202 h, demonstrating good stability and commercial potential.