We have fabricated white organic light-emitting diodes (OLEDs) by co-doping of red and blue phosphorescent guest emitters into the single host layer. Tris(2-phenyl-1-quinoline) iridium(III) [Ir(phq)3]and iridium(III)bis[(4,6-di-fluorophenyl)-pyridinato-N,C2`]picolinate (FIrpic) were used as red and blue dopants, respectively. The effects of dopant concentration on the emission, carrier conduction and external quantum efficiency characteristics of the devices were investigated. The emissions on the guest emitters were attributed to the energy transfer to the guest emitters and direct excitation by trapping of the carriers on the guest molecules. The white OLED with 5% FIrpic and 2% Ir(phq)3 exhibited a maximum external quantum efficiency of 19.9% and a maximum current efficiency of 45.2 cd/A.

We have fabricated white organic light-emitting diodes (OLEDs) using several thicknesses ofelectron-transport layer. The multi-emission layer structure doped with red and blue phosphorescent guestemitters was used for achieving white emission. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) wasused as an electron-transport layer. The thickness of BCP layer was varied to be 20, 55, and 120 nm. The current efficiency, emission and recombination characteristics of multi-layer white OLEDs wereinvestigated. The BCP layer thickness variation results in the shift of emission spectrum due to therecombination zone shift. As the BCP layer thickness increases, the recombination zone shifts toward theelectron-transport layer/emission-layer interface. The white OLED with a 55 nm thick BCP layerexhibited a maximum current efficiency of 40.9 cd/A.