Driving voltage reduction and efficiency increase by narrow bandgap host materials in phosphorescent organic light-emitting diodes

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/m2, power efficiency is enhanced at least by twenty percent compared to general CBP based green and red PHOLEDs.