Energy distribution in white organic light-emitting diodes with three primary color emitting layers

Two types of organic light-emitting diodes with structures of ITO/N,N?-bis(1-naphthyl)-N,N?-diphenyl,1,1′-biphenyl-4,4′-diamine (NPB)/tris(8-hydroquinolinato)aluminum(Alq₃)/2,9-dimethyl-4,7- diphenyl-l,10-phenanthroline(BCP)/Alq₃:4-dicyanome-thylene-2-(tert- butyl)-6-methyl-4H-pyran(DCJTB)/Alq₃/Al and ITO/NPB/BCP/Alq ₃/Alq₃:DCJTB/Alq₃/Al were studied. NPB was chosen as a hole-transporting/blue-emitting layer. Alq₃ adjacent to BCP acted as a green emitting layer while that adjacent to the Al cathode acted as an electron-transporting layer. Alq₃ doped with 2 wt.% DCJTB was used as a red emitting layer. The operating principles of the devices were explained by the mechanism of Förster energy transfer and the hole and exciton blocking effect of BCP. It was found that the spectral characteristics of the devices strongly depended on the relative location between the green emitting Alq₃ layer and the BCP layer, as well as their thickness. Pure white emission with the CIE coordinates of (0.33, 0.33) was achieved by mixing the three primary colors in the device with the structure of ITO/NPB(30 nm)/ BCP(6 nm)/Alq₃(30 nm)/Alq₃:DCJTB(30 nm)/Alq ₃(30 nm)/Al. The BCP layer played an important role in distributing the exciton energy among the three emitting layers to achieve a balanced white light. The white emission of this device was largely insensitive to the driving voltage (15-27 V) with the insertion of the green emitting Alq₃ layer.