Wide-bandgap perovskite solar cells are essential as top cells in all-perovskite tandem solar cells but remain limited by nonradiative recombination at defects and interfaces. Here, we developed a comprehensive multifunctional dual-interface passivation strategy to improve crystallinity, passivate defects, and tune band alignment in Cs0.2FA0.8Pb(I0.6Br0.4)3 with a 1.77 eV bandgap. An ethylenediammonium diiodide (EDAI2) treatment before perovskite spin coating promotes α-phase crystallization with preferred (100) orientation and reduces series resistance. Additional EDAI2 top passivation further improves device quality, leading to a higher open-circuit voltage (VOC). Deep-level transient spectroscopy (DLTS) reveals defects associated with I– vacancies and A-site cations. Compared to the control sample, the defect density linked to I– vacancies decreased to about 30% and A-site cation-related defects are completely removed by EDAI2 top passivation, consistent with the observed increase in VOC. Finally, a subsequent dodecylammonium iodide (DDAI) treatment shifts the perovskite energy levels to reduce the conduction band offset with the C60 electron transport layer, facilitating electron extraction and raising the short-circuit current density (JSC) and fill factor (FF). Multifunctional dual-interface passivation, combining these three passivation strategies, showed improvements in all photovoltaic parameters, VOC, JSC, and FF, relative to the control, achieving a champion power conversion efficiency of 19.16% (VOC = 1.31 V, JSC = 18.19 mA cm–2, and FF = 80.19%) with high long-term stability. This low-temperature, solution-processed strategy resolves the conventional VOC/FF trade-off in wide-bandgap single-junction devices and establishes a generalizable strategy for efficient and stable top cells for all-perovskite tandem solar cells.