Efficient carrier and exciton confinement in white phosphorescent organic light-emitting diodes

We demonstrated that the luminous and the external quantum efficiencies of white phosphorescent organic light-emitting diodes (PHOLEDs) were improved by using a device architecture that confines the carriers and the excitons inside an emitting layer (EML). Two white PHOLEDs were fabricated with a commercially available red-phosphorescence emitter bis(2-phenylquinolinato)-acetylacetonate iridium III (Ir(pq)(2)acac) doped in N,N'-dicarbazolyl-3,5-benzene (mCP), and a blue-phosphorescence emitter bis(3,5-Difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl) iridium III (FIrpic) doped in p-bis(triphenylsilyly)benzene (UGH2), as red/blue emitting layers. In addition, both devices (devices A and B) had a triplet exciton blocking layer (TEBL) on the hole-transportinglayer (HTL) side, and a high triplet state material was used to get a high efficiency. Device B had one more buffer layer of p-type 4,4',4aEuro(3)-tri(N-carbazolyl) triphenylamine (TCTA) material between the HTL and the TEBL. The properties of device B, which exhibited a maximum luminous efficiency and a maximum external quantum efficiency of 22.77 cd/A and 11.5%, respectively, was found to be superior to the device A and showed white emission with CIE (X,Y) coordinates of (x = 0.33, y = 0.37) at 8 V.