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Electrophosphorescent organic light-emitting diodes (PHOLEDs) with alternate multilayer structures, which consist of N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) and 5,6,11,12-tetraphenylnaphthacene (rubrene) as hole-transporting layers have been fabricated. The effect of the alternate multilayer structures upon ther performance of PHOLEDs with fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) doped into 4,4′-N, N′-dicarbazole-biphenyl (CBP) as the emitting layer has been investigated. Compared with the conventional diode only with NPB as hole-transporting layer, high efficiency and long lifetime diodes have been obtained with the alternate structures. Such an improvement in the device performance was attributed to the improved hole-electron balance, which can be further attributed to the introduction of the alternate multilayer structure into PHOLEDs.

It is generally recognized that hole-transport layers (HTLs) with thickness on the order of tens nanometer are indispensable to high luminance and quantum efficiency in small molecule organic light-emitting diodes (OLEDs). However, in this work, without conventional HTLs, a thin polytetrafluoroethylene (Teflon) layer was introduced as hole-injection layer, followed by deposition of an emissive layer (EML), Alq3 doped with 0.7wt% DMQA. And simultaneously, a blocking and electron transport layer of aluminum (III) bis (2-methyl-8-quinolinato) 4-phenylphenolate (BAlq) ETL was employed to confine holes and excitons in Alq3 layer. The obtained OLEDs exhibited a luminance of over 88000 cd/m2 and external quantum efficiency of 5.4% (21.1 cd/A) at voltage of 19.8 V and current density of 418 mA/cm2. When the Teflon thickness was 10.0 nm under our conditions, well-balanced charge injection could be obtained in OLEDs. It was also found that the high performance was related to triplet-triple annihilation, in which triplet excited tris (8-hydroxyquinoline) aluminum (3Alq3*) excitons go through annihilative reactions with other 3Alq3* excitons to form singlet excited molecules. It provides a new way of obtaining highly efficient and stable OLEDs with well-balanced charge injection.