Perovskite/perovskite/silicon triple-junction solar cells hold promise for surpassing their two-junction counterparts in performance. Achieving this requires monolithic integration of a ∼2.0 eV band-gap perovskite subcell, characterized by a high bromide:iodide ratio (>7:3), and with low-temperature processability and high optoelectronic quality. However, light-induced phase segregation in such perovskites remains a challenge. To address this, we propose modifying the wide-band-gap perovskite with potassium thiocyanate (KSCN) and methylammonium iodide (MAI) co-additives, where SCN− increases the perovskite grain size, reducing the grain boundary defect density; K+ immobilizes the halide, preventing the formation of halide vacancies; and MA+ eliminates the residual light-destabilizing SCN− in the perovskite films via double displacement reactions. Our co-additive strategy enables enhanced photostability, whereas individual usage of MAI and KSCN would result in adverse effects. Triple-junction tandem solar cells, incorporating co-additive-modified 2.0 eV perovskites as top cell absorbers, reach a 3.04 V open-circuit voltage and a PCE of 26.4% over a 1 cm2 area.