New carbazole-based host material for low-voltage and highly efficient red phosphorescent organic light-emitting diodes

A new carbazole-based host material for red emitters, BBTC, was designed, synthesized and characterized with the phosphorescent organic light-emitting diodes (PHOLEDs). With the molecular design strategy of maintaining the large triplet energy and a good hole transporting ability of carbazole while increasing the morphological and electrochemical stability, the C3 and C6 positions of carbazole are blocked with biphenyl groups and the C9 position is terminated with a terphenyl group. Red PHOLEDs employing a conventional dopant material, bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) ((piq)2Ir(acac)), in the emissive layer showed nearly 100% internal quantum efficiency (corresponding to the external quantum efficiency of 19.3%) with reduced efficiency roll-off. We attribute these results to the good electron–hole balance resulting from the good hole mobility of BBTC and low hole injection barrier from the hole transport layer to BBTC host in the emission layer. In addition, owing to its good hole transporting property, it can be utilized as a hole transport layer in organic light-emitting devices enabling low voltage operation of the devices.