Synergistic solvent and composition engineering of perovskites for tandems on industrial silicon

Wide‐bandgap perovskites based on mixed formamidinium−cesium cation and iodide−bromide halide are promising materials in the top cells that are well‐matched with crystalline silicon bottom cells to construct efficient tandem photovoltaics. Nevertheless, mixed cation−halide perovskite films with sub‐micron film thickness suffer from poor crystallinity with inhomogeneous and undesirable phases, owing to the presence of multiple pathways of crystal nucleation and phase transition. Herein, we propose a synergistic solvent and composition engineering (SSCE) strategy to regulate the solvated phases and manipulate the transition pathways simultaneously. The resultant mixed cation−halide perovskite film shows optimizing crystallization and desired phase structure with suppressed non‐radiative recombination and improved phase stability under aging stresses. Consequently, the SSCE strategy enables the tandem cells based on industrially ultra‐thin silicon wafers (120 µm) to achieve a certified stabilized power conversion efficiency of 31.0%. Those encapsulated devices maintain 90% of their initial performance after 1200 h continuous operation.