Multilayer organic electrophosphorescent white light-emitting diodes without exciton-blocking layer

We demonstrate high-efficiency white organic light-emitting diodes with two emissive layers, in which different hosts are employed. With the energy gap of the two hosts, there was no exciton-blocking layer between the emissive layers to confine the diffusion of excitons. The phosphorescent dye, bis[(4,6-difluorophenyl)-pyridinato-N, C 2 ′ ] (picolinato) Ir(III) and bis (1-phenyl-isoquinoline) (acetylacetonate) iridium (III) [ Ir ( pic ) 2 acac ] were employed as guests, while N, N ′ -dicarbazolyl-1, 4-dimethene-benzene and 4, 4 ′ -N, N ′ -dicarbazole-biphenyl were employed as hosts, respectively. The device exhibited white emission by controlling the thickness of the emissive layers and the maximum current efficiency and luminance were 10.5 cd ∕ A and 22 000 cd ∕ m 2 , respectively.