A-site cation mixing can enhance the photovoltaic performance of a wide-bandgap (WBG) perovskite, but rubidium (Rb) cation mixing generally forms a nonperovskite phase. We report that lattice strain locks Rb ions into the α-phase of the lattice of a triple-halide WBG perovskite, preventing phase segregation into a nonperovskite Rb-cesium–rich phase. This process cooperates with chloride accommodation and promotes halide homogenization across the entire film volume. The resulting 1.67–electron volt WBG perovskite exhibits photoluminescence quantum yields exceeding 14% under 1-sun-equivalent irradiation, corresponding to a quasi–Fermi level splitting of ~1.34 electron volts. A WBG perovskite solar cell with an open-circuit voltage ( V OC ) of 1.30 volts was prepared, corresponding to 93.5% of the radiative V OC limit and representing the lowest photovoltage loss relative to the theoretical limit observed in WBG perovskites.