Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from similar to 20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of similar to 1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)(2)](0.83)Cs0.17Pb(I0.6Br0.4)(3), with an optical band gap of similar to 1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm(2) cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.