We report efficiency increase and voltage reduction by bipolar host materials, 4,4'-N,N'-dicarbazoleterpheyl and 4,4'-N,N'-dicarbazolequaterpheyl, in green and red phosphorescent organic light-emitting diodes (PHOLEDs). These host materials are designed to have good electron transporting characteristics with narrow band-gap by adding more phenyl moieties in a 4,4'-N,N'-dicarbazolebiphenyl (CBP) chemical structure. At a given constant luminance of 1000 cd/m(2), power efficiency is enhanced at least by twenty percent compared to general CBP based green and red PHOLEDs. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.