Of the many materials and methodologies aimed at producing low-cost, efficient photovoltaic cells, inorganic-organic lead halide perovskite materials(1-17) appear particularly promising for next-generation solar devices owing to their high power conversion efficiency. The highest efficiencies reported for perovskite solar cells so far have been obtained mainly with methylammonium lead halide materials(1-10). Here we combine the promising-owing to its comparatively narrow bandgap-but relatively unstable formamidinium lead iodide (FAPbI(3)) with methylammonium lead bromide (MAPbBr(3)) as the light-harvesting unit in a bilayer solar-cell architecture(13). We investigated phase stability, morphology of the perovskite layer, hysteresis in current-voltage characteristics, and overall performance as a function of chemical composition. Our results show that incorporation of MAPbBr(3) into FAPbI(3) stabilizes the perovskite phase of FAPbI(3) and improves the power conversion efficiency of the solar cell to more than 18 per cent under a standard illumination of 100 milliwatts per square centimetre. These findings further emphasize the versatility and performance potential of inorganic-organic lead halide perovskite materials for photovoltaic applications.