Wide-bandgap perovskite with adjustable bandgaps can be integrated with organic solar cells to form tandem solar cells (TSCs), thereby surpassing the Shockley-Queisser limit. However, increasing Br content to elevate the bandgap above 1.8 eV complicates crystallization, leading to inferior film quality and defects due to the unmanageable evolution of intermediate phases. Surface passivation improves crystallization but hard to moderate the inhomogeneous component distributions and defects in the bulk phase. Here, we introduce a diammonium salt as an additive to regulate the homogeneity and crystallization of perovskite film, eliminating the low-dimensional intermediate phase for orientated crystallization of 1.85 eV perovskite, resulting in efficient wide-bandgap perovskite solar cells with an impressive open-circuit voltage (VOC) of 1.379 V and operational stability remaining 85% of their initial efficiency after illumination for 1200 h. Furthermore, perovskite-organic TSCs achieve a champion power conversion efficiency of 24.03% and a high VOC of 2.108 V, one of the highest VOC for perovskite-organic TSCs.