Electroluminescence of ruthenium(II)(4,7-diphenyl-1,10-phenanthroline)3 from charge trapping by doping in carrier-transporting blend films

Light-emitting devices, using ruthenium(II)(4,7-diphenyl-1,10-phenanthroline)3 ([Ru(dphphen)3]2+) as dopant and emitting center have been investigated, in a structure of indium-tin-oxide glass (ITO)/poly(N-vinylcarbazole) (PVK):2-(4-biphenyl)-5-(4-ter-butylphenyl)-1,3,4-oxadiazole (PBD):[Ru(dphphen)3]2+/tris-(8-hydroxyquinoline) aluminum (Alq3)/Al. Instead of the long response time observed at high doping concentration, instantaneous light emission from [Ru(dphphen)3]2+ is obtained at a doping concentration at which the electronic coupling between the [Ru(dphphen)3]2+ molecules is eliminated by the host matrix. This is attributed to the direct exciton formation at [Ru(dphphen)3]2+ molecule by charge trapping from PVK and PBD. The emission is isolated to [Ru(dphphen)3]2+ by inserting a layer of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) between PVK:PBD:[Ru(dphphen)3]2+ and Alq3 layers.