Organic-inorganic lead halide perovskites are intensively studied for their large potential in low-cost and high-resolution light-emitting devices due to the advantages of solution-processability, good color purity, and facile emission wavelength tunability. We study the perovskite materials containing a mixture of methylammonium (MA ⁺) and ethylammonium cations (EA ⁺), which show the cation-ratio dependent emission wavelength. Introduction of EA ⁺ cations increases the photoluminescence quantum yield and photo-stability simultaneously, reduces the density of defect states significantly. Related green-emitting devices show a maximum external quantum efficiency and power efficiency of 7.7% and 25.1 lm·W ^-1, which are 1.9 and 1.7 times as those of the MAPbBr₃-based analogs, respectively, along with improved operational stability.