High performance solution-processed red phosphorescent organic light-emitting diodes by co-doping europium complex as sensitizer

In this work, efficient red phosphorescent OLEDs (PHOLEDs) were designed and fabricated by solution pro-cessing method. Based on well optimized doping concentrations of emitter and ratio of mixed host materials, Eu (DBM)3Phen (DBM = 1,3-diphenylpropane-1,3-dion, Phen = 1,10-phenonthroline) was doped into the light -emitting layer (EML) which plays the role of sensitizer. Experimental results indicated that the incorporation of Eu(DBM)3Phen molecules can effectively trap the superfluous electrons within EML, which is helpful for balancing charges' distribution and widening the recombination zone. Besides, the presence of Eu(DBM)3Phen molecules can accelerate the transfer of triplet energy from hosts to red emitters, therefore remarkably enhances device performance. By optimizing the doping concentrations of emitter and sensitizer, ratio of co-hosts and the functional layer thickness, recombination center was accurately adjusted to further broaden recombination zone, therefore causing decreased excitons density and suppressed excitons quenching. Eventually, the optimal solution-processed red PHOLED obtained the maximum current efficiency (47.12 cd/A) and external quantum efficiency (27.3%).