2005 Volume 38 Issue 8 Pages 588-592
White organic light-emitting diodes (OLEDs) based on a multilayer structure were fabricated, in which the red emission is from [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo [ij] quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene]propane-dinitrile (DCM2) doped 4,4′-bis[N-1-naphthyl-N-phenyl-amino]biphenyl (NPB) layer and the blue-green emission from 4,4′-bis[2-(3-N-ethylcarbazoryl)vinyl]biphenyl (BCzVBi) doped 4,4′-bis(2,2-diphenylvinyl)biphenyl (DPVBi) layer. White light emission was obtained by controlling the doping concentration of each dopant material in a hole-transporting layer and an emitting layer. But in this white device, electroluminescence spectra depend on the applied voltage. By inserting a tris-(8-hydroxyquinoline)aluminum (Alq3) layer as an electron-transporting layer, voltage-independent white emission of the CIE coordinates (0.31, 0.32) was obtained. The luminance of this device was 10,600 cd/m2 at 15 V and the power efficiency was 1.50 lm/W at 7.75 V. This voltage independence is believed to result from efficient electron injection and a narrowing effect of the width of the recombination zone by insertion of an Alq3 layer.
