Exciton quenching in highly efficient europium-complex based organic light-emitting diodes

Red-light emitting organic electroluminescent devices using a trivalent europium complex (Eu3+) as emitter were fabricated. A simple three-layer device, consisting of triphenyldiamine derivative (TPD) as hole transport layer, a 1,2,4-triazole derivative (TAZ) as electron injection layer, and europium(dibenzoylmethane)(3)bathophenylphenathroline (Eu(DBM)(3)BPhen) as electron transport and emission layer, exhibits efficient emission at very low currents. We attain peak external quantum efficiency of 7.5 +/- 0.5% and a corresponding power efficiency of 10 +/- 1 lm/W. For higher driving currents, we observe drastic decrease of efficiencies, accompanied by changes in the emission spectrum, i.e., an increase of emission from the D-5(1) state relative to the dominant D-5(0) emission. We show that both effects are caused by exciton-exciton annihilation. By co-evaporation of the Eu complex with TPD in the emitting layer, we can reduce quenching effects and color changes and achieve efficient electroluminescence also at higher currents. (C) 2005 Elsevier B.V. All rights reserved.