Organic-inorganic halide perovskites are promising semiconductors to mate with silicon in tandem photovoltaic cells due to their solution processability and tunable complementary bandgaps. Herein, we show that a combination of two additives, MACl and MAH2PO2, in the perovskite precursor can significantly improve the grain morphology of wide-bandgap (1.64–1.70 eV) perovskite films, resulting in solar cells with increased photocurrent while reducing the open-circuit voltage deficit to 0.49–0.51 V. The addition of MACl enlarges the grain size, while MAH2PO2 reduces non-radiative recombination through passivation of the perovskite grain boundaries, with good synergy of functions from MACl and MAH2PO2. Matching the photocurrent between the two sub-cells in a perovskite/silicon monolithic tandem solar cell by using a bandgap of 1.64 eV for the top cell results in a high tandem Voc of 1.80 V and improved power conversion efficiency of 25.4%. Display omitted •Synergistic effects of additives create large grains and low defect density•1.80 V tandem device enabled by a 1.64-eV perovskite with Voc deficit of 0.49 V•25.4%-efficient monolithic perovskite/Si tandem device The efficiency of organic-inorganic halide perovskite solar cells skyrocketed in the past 6 years, reaching 23.3%. Their pairing with silicon in tandem solar cells offers a promising path for further reducing the levelized cost of electricity of photovoltaics. Strategies such as compositional engineering and charge-transport-layer optimization have been reported to improve the tandem efficiency. However, the large open-circuit voltage deficit of wide-bandgap perovskite cells still limits the tandem performance. Here, we utilize combined additives to smooth the perovskite film, increase its grain size, and lower its defect density. The synergistic effect of the additives leads to increased photocurrent and reduced open-circuit voltage deficit for wide-bandgap perovskite solar cells. When additives are used to form a top cell with a bandgap of 1.64 eV, the perovskite and silicon sub-cells are current matched and yield a perovskite/silicon tandem device with an efficiency of 25.4%. Grain engineering through combined MACl and MAH2PO2 additives in perovskite precursors improves the photovoltaic performance of perovskite/silicon tandem cells. MACl increases the grain size of wide-bandgap perovskite films and also produces smooth films. MAH2PO2 suppresses non-radiative recombination sites at grain boundaries. The synergetic effects of MACl and MAH2PO2 further promote grain growth and prolong the carrier recombination lifetime. This enables a power conversion efficiency of 25.4% for a perovskite/silicon tandem device.