LAUSR

A facile and eco‐friendly dimethyl sulfoxide‐mediated solution aging (DMSA) treatment is presented to control the crystallization dynamics of methylammonium (MA)‐free wide‐bandgap (WBG) perovskite films, enhancing film quality, and morphology for high‐performance tandem solar cells. The comprehensive structural, morphological, and characterization analyses reveal that the DMSA treatment significantly enhances composition and morphology homogeneity while suppressing halide segregation. Consequently, opaque, and semi‐transparent MA‐free WBG perovskite solar cells (PSCs) exhibit remarkable power conversion efficiencies (PCEs) of 18.28% and 17.61%, respectively. Notably, the unencapsulated DMSA‐treated devices maintain 95% of the initial PCE after 900 h of continuous operation at 55 °C ± 5 °C. Furthermore, stacking semi‐transparent DMSA‐treated PSCs as top cells in a 4T tandem configuration, along with silicon heterojunction (SHJ), lead–tin (Pb–Sn) alloyed PSCs, and organic photovoltaics (OPV) as bottom cells, yields impressive PCEs of 28.09%, 26.09%, and 25.28%, respectively, for the fabricated tandem cells. This innovative approach opens new avenues for enhancing the photo‐stability and photovoltaic performance of perovskite‐based tandem solar cells.