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Tandem solar cells coupling narrow- and wide-band-gap thin-film polycrystalline absorbers are attractive for achieving ultrahigh efficiency with low manufacturing cost. For established narrow-band-gap CIGS thin-film bottom cells, a challenge is to develop highly efficient polycrystalline wide-band-gap top cells. Here, we demonstrate a 1.68-eV (FA_0.65MA_0.20Cs_0.15)Pb(I_0.8Br_0.2)₃ wide-band-gap perovskite solar cell with an efficiency of ∼20% enabled by using PEAI and Pb(SCN)₂ complementary additives in the perovskite precursor. The coupling of PEA⁺ and SCN⁻ provides a synergistic effect that overcomes growth challenges with either additive individually and improves perovskite film quality with enhanced crystallinity, reduced formation of excess PbI₂ (in comparison to using Pb(SCN)₂ additive alone), lower defect density and energetic disorder, and an improved carrier mobility (∼47 cm² V⁻¹s⁻¹) and …