Efficient and color-stable white organic light-emitting diodes based on exciton management and phosphorescent sensitization

We report efficient and color-stable white organic light-emitting diodes (WOLEDs) by an approach combining the concepts of exciton management and phosphorescent sensitization to achieve 100% internal quantum efficiency and reduce efficiency roll-off. The approach employs a layer of 4-bis-(1-naphthyl-N-phenylamino)-biphenyl (NPB) as a fluorescent blue emitter to collect all singlet excitons for blue emission, and fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)3) as a phosphorescent emitter and sensitizer in other layer to harvest all triplet excitons for generating green light and sensitizing a fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4Hpyran (DCJTB) for orange–red emission. By carefully adjusting the concentration of DCJTB and the thickness of the NPB layer, efficiency roll-off has been reduced and efficient balanced white light has been produced from the device ITO/PEDOT:PSS (30 nm)/poly(vinylcarbazole) (PVK):Ir(ppy)3:DCJTB (100:5:0.4 in wt.) (60 nm)/NPB (4 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/tris(8-hydroxyquinoline)aluminum (Alq3) (20 nm)/LiF (1 nm)/Al. The operating principles have been studied and the results show that the long diffusion length of triplet excitons and long-range nonradiative energy transfer between the triplets of the phosphorescent emitter and the singlets of the fluorescent dye are crucial to realize white light emission.