Direct hole injection in to 4,4′-N,N′-dicarbazole-biphenyl: A simple pathway to achieve efficient organic light emitting diodes

The conventional carrier-blocking design of the exciton formation zone used in nearly all organic light emitting diodes is shown to be problematic, due to exciton quenching from accumulated radical cations. To reduce exciton quenching, a single layer of 4,4'-N,N'-dicarbazole-biphenyl (CBP) is used as hole transport layer, resulting in a dramatically improved device efficiency even at high luminance (e.g., 20.5 cd/A at 100 000 cd/m(2) for fluorescent green). Various high work function transition metal oxides (WO(3), V(2)O(5), and MoO(3)) coated on indium tin oxide anodes have been shown to enable direct hole injection into the deep highest occupied molecular orbital of CBP (6.1 eV). (C) 2010 American Institute of Physics. [doi:10.1063/1.3456513]