A molecular design strategy to facilitate electron injection and to reduce driving voltage was proposed to reach high power efficiency in blue phosphorescent organic light-emitting diodes (PHOLEDs) doped with a phenylimidazole ligand based triplet emitter. The host materials were designed to keep the hole transport properties and triplet energy of a parent molecule. One or two CN units were attached to 3- or 3,6-positions of carbazole of 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) to manage electron transport properties of the parent mCBP host. The CN unit donated electron accepting character to the mCBP host and increased electron current density in the device, which improved power efficiency of the blue PHOLEDs from 41.8 lm W⁻¹ to 57.1 lm W⁻¹.