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

Hyung Jin Yang, Ho Won Lee, Seok Jae Lee, Dong Hyung Lee, Ja Ryong Koo, Jhin Yeong, Seung Soo Yoon, Young Kwan Kim
Journal of Nanoscience and Nanotechnology 2014, 14 (11): 8337-41
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%.

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