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.