Efficient blue-to-violet organic light-emitting diodes

Organic light-emitting diodes emitting in the range of 400nm (violet) to 460nm (blue) are reported. The basic device structure consists of indium-tin oxide/N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4, 4′-diamine (TPD)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/lithium fluoride (LiF)/aluminum. Offset of the energy levels at the TPD/BCP interface favors blocking of holes on the TPD side of the interface. Voltage-induced color change is observed and explained in terms of a switching from emission dominated by interfacial exciplex-induced recombination at low applied bias to one dominated by bulk exciton-induced recombination at high applied bias. With the addition of copper(II) phthalocyanine (CuPc) as an anode buffer layer and tris-8-(hydroxyquinoline) aluminum (Alq₃) as a cathode buffer layer, external quantum efficiencies as high as 0.5% at blue emission and 0.4% at violet emission have been obtained.