White organic light-emitting diodes emitting from both hole and electron transport layers

Some white emitting organic light-emitting diodes (LED) have been made by inserting a carrier block layer, diphenyl-p-(t-butylphenyl-1,3,4-oxadiazole (PBD), between a blue emissive (N-p-methoxylphenyl-N-phenyl)-p-methoxylphenyl-styrylamine (SA) layer and a green emissive 8-hydroxyquinoline aluminium chelate layer or a red emissive laser dye 4-dicyanomethylene-2-methyl-6-(p-dimenthyaminostyryl)-4H-pyran (DCM)-doped Alq3 layer. The electroluminescent (EL) behaviours of the diodes have been investigated. It is found that the emission colour of the diode varies depending on the thickness of PBD layer in the devices. By choosing the thickness of PBD film, we can obtain near white emitting diodes. The emissions from the triple-layer devices are found be emitted from both SA and Alq3 or Alq3:DCM layers, which are situated in the two sides of PBD, by analysing the spectra of the devices with different thicknesses at different applied voltage.