A semiconductor that can be processed on a large scale with a bandgap around 1.8 eV could enable the manufacture of highly efficient low cost double-junction solar cells on crystalline Si. Solution-processable organic-inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells, and though it is possible to tune the bandgap of (CH3NH3)Pb(BrxI1-x)3 between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photoinduced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, it is possible to synthesize a mixed halide absorber material with improved optical and thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV.