Performance improvement of green phosphorescent organic light emitting diodes with partial bulk heterojunctioned emitting layer.

Green phosphorescent organic light emitting diodes (PHOLEDs) were developed using a mixed layer system. They were fabricated with 4,4'-N,N'-dicarbazolyl-biphenyl (CBP) and typical charge transporting materials, which are 4,4'-bis[N-(naphthyl)-N-phenylamino]biphenyl (NPB) as a hole transporting material and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) as an electron transporting material, mixed at each interface in a stacked organic layer to reduce the hole injection barrier and extend the recombination zone. We introduced a mixed layer for the hole transporting layer side, the electron transporting layer side, and on both sides to make a bulk heterojunction. This reduced the driving voltage, and the luminous efficiency (LE) was increased to 500 cd/m2. The optimized device showed a maximum LE of 59.87 cd/A and a maximum external quantum efficiency of 17.52%.