We demonstrated that driving voltages, external quantum efficiencies, and power conversion efficiencies of organic light-emitting diodes (OLEDs) are improved by inserting a wide-energy-gap interlayer of (4,4′-N,N′-dicarbazole)biphenyl (CBP) between a hole-transport layer of N,N-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (α-NPD) and a light-emitting layer of tris(8-hydroxyquinoline)aluminum. By optimization of CBP thicknesses, the device with a 3-nm-thick CBP layer had the lowest driving voltage and the highest power conversion efficiency among the OLEDs. We attributed these improvements to enhancement of a carrier recombination efficiency and suppression of exciton-polaron annihilation. Moreover, we found that the degradation of the OLEDs is caused by decomposition of CBP molecules and excited-state α-NPD molecules.
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