Abstract
Red-light emitting organic electroluminescent devices using a trivalent europium complex (Eu 3+ ) 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 5 D 1 state relative to the dominant 5 D 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.
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