We demonstrate a high efficiency organic electrophosphorescent device comprised of a 4,4 ' ,4 ' ' -tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) hole transport layer and a 4,4 ' -N,N ' -dicarbazole-biphenyl (CBP) host doped with the metallorganic phosphor, fac-tris(2-phenylpyridine)iridium (Ir(ppy) 3 ) as the green light-emitting layer. The device exhibits peak external quantum and power efficiencies of (12.0+/-0.6)% and (45+/-2) lm/W, respectively, corresponding to ~60% internal quantum efficiency. A luminance of 1850 cd/m 2 is observed at a current density of 10 mA/cm 2 . The device operating properties are controlled by electron injection into, and transport by the CBP layer along with hole injection from m-MTDATA directly into the Ir(ppy) 3 highest occupied molecular level, leading to direct carrier recombination and exciton formation on the phosphor dopant. Ambipolar conduction properties of the Ir(ppy) 3 :CBP layer are established by analysis of triplet-triplet annihilation, exciton formation and the luminance-current-voltage characteristics.