Suppression of Efficiency Roll-off in Co-doped Electroluminescent Devices Based on Europium Complex

By co-doping alq(3)[tris(8-hydroxyquinoline) aluminum] and eu(tta)(3)phen (tta=thenoyltrifluoroacetone, phen =1,10-phenanthroline) into cbp (4,4' -n,n' dicarbazole -biphenyl), a series of electroluminescent (el) devices were fabricated and investigated. after optimizing the co-doping concentration of alq(3), significant suppression of el efficiency roll-off and almost doubled maximum brightness were realized without deteriorating the color purity. the presence of alq(3) molecules in light-emitting layer (eml) not only facilitates the injection and transport of electrons but also retards the transport of holes. alq(3) molecules function as ladders of hole transfer from cbp to eu(tta)(3)phen molecules as supported by the el spectroscopy results, thus facilitating the balance of holes and electrons on eu(tta)(3)phen molecules. therefore, two possible reasons are suggested to be responsible for the suppression of el efficiency roll-off: one is the broadening of recombination zone, the other is the improved balance of holes and electrons on eu(tta)(3)phen molecules.