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We report on a new metal halide perovskite photovoltaic cell that exhibits both very high solar-to-electric power-conversion efficiency and intense electroluminescence. We produce the perovskite films in a single step from a solution containing a mixture of FAI, PbI₂, MABr, and PbBr₂ (where FA stands for formamidinium cations and MA stands for methylammonium cations). Using mesoporous TiO₂ and Spiro-OMeTAD as electron- and hole-specific contacts, respectively, we fabricate perovskite solar cells that achieve a maximum power-conversion efficiency of 20.8% for a PbI₂/FAI molar ratio of 1.05 in the precursor solution. Rietveld analysis of x-ray diffraction data reveals that the excess PbI₂ content incorporated into such a film is about 3 weight percent. Time-resolved photoluminescence decay measurements show that the small excess of PbI₂ suppresses nonradiative charge carrier recombination. This in turn …