Efficient bilayer phosphorescent organic light-emitting diodes: Direct hole injection into triplet dopants

In phosphorescent organic light-emitting diodes (OLEDs), a hole transporting layer is traditionally thought to be required to facilitate hole injection into the host molecule. It is found that fac-tris(2-phenylpyridine)iridium [Ir(ppy)(3)] doped into 4,4(')-N,N-'-dicarbazole-biphenyl can be used to directly inject and transport holes from an indium tin oxide anode, and thus simplify the device structure and selection of materials. The efficiencies of the simplified bilayer OLEDs exceed 41 lm/W and 57 cd/A at a brightness of 100 cd/m(2). We attribute the excellent performance to direct hole injection from the anode to Ir(ppy)(3) dopant.