Efficient electrophosphorescence is demonstrated using single-layer organic light-emitting devices (OLEDs) containing fac-tris(2-phenylpyridine) iridium (III) [Ir(ppy)3], bis(1-phenylisoquinoline)-(acetylacetonate) iridium (III) [PQIr], and iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C 2′]picolinate [FIrpic] for emission in the green, red, and blue, respectively. Peak forward-emitted external quantum (ηEQE) and power efficiencies (ηP) of ηEQE (16.9 ± 0.4) and ηP (65.0 ± 1.1)lm/W, ηEQE (12.0 ± 0.4) and ηP (8.3 ± 1.1) lm/W, and ηEQE (10.9 ± 0.3) and ηP (28.1 ± 1.1)lm/W, are obtained for optimized green, red, and blue OLEDs, respectively. Devices are doped uniformly with the phosphorescent guest, and contain a continuously-varying host composition from predominately hole-transporting material (HTM) at the anode to predominately electron transporting material (ETM) at the cathode. The highly tunable composition gradient allows for the optimization of electron-hole charge balance and low-voltage operation while maintaining charge and exciton confinement. For each emitter, the optimum composition gradient is understood by considering the dependence of the electron and hole charge carrier mobilities on HTM:ETM composition.