Highly efficient, low-voltage OLEDs comprising bis(2-phenylpyridinato-N, C-2')iridium(acetylacetonate) ((ppy)(2)Ir(acac)) as the host and tris(1-phenylisoquinolinolato-C-2,N)- iridium(III) (Ir(piq)(3)) as a red phosphorescent guest are fabricated. Reduced driving voltage, increased maximum power efficiency (see figure), and improved durability of the OLEDs are demonstrated.