Investigation Of Energy Transfer Mechanisms Between Two Adjacent Phosphorescent Emission Layers

The investigation of energy transfer mechanisms between two adjacent phosphorescent emission layers comprising the green emitter molecule fac-tris(2-phenly-pyridin) iridium (Ir(ppy)(3)) and the red emitter molecule iridium(III) bis(2-methyldibenzo[f,h]quinoxaline(acetylacetonate) (Ir(MDQ)(2)(acac)) is presented. We show that the performance can be enhanced by a variation of the emission layer thickness and the emitter concentration. By inserting different interlayer materials between the emission units, we demonstrate that triplet excitons are formed on the Ir(ppy)(3) and subsequently transferred to the Ir(MDQ)(2)(acac) molecules via the hole transporting host material N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine of the red emission layer. The variation of the interlayer thickness shows that the triplet diffusion length is several tens of nanometers. After optimization of the guest-host system an efficiency enhancement by 15% was achieved and the lifetime of the red-green emissive unit could be enhanced by 55%. Additionally, it is shown that this improved red-green unit can be combined with a fluorescent blue emitter in a state-of-the-art stacked white emissive organic light emitting diode. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4724346]