Iodine-rich mixed composition metal–halide perovskites were developed to improve the performance of perovskite solar cell devices incorporating tin(IV) oxide substrates for electron transport layers by optimizing the I/Br halide ion ratio. Device performance was further enhanced by exposing to ambient atmosphere over several days, which correlated with a shift of the energy levels in the perovskite and significant suppression of charge carrier recombination. A high power conversion efficiency of 20.6% was obtained for the aged device.