Efficient red organic electroluminescent devices based on trivalent europium complex obtained by designing the device structure with stepwise energy levels

- Electroluminescent performances of europium complex were further improved.
- Device structure with stepwise energy levels was designed.
- Better carriers' balance was realized by improving the injection and transport of holes.
- The selection of bipolar host caused the broadening of recombination zone.

In this study, we aim to further enhance the electroluminescence (EL) performances of trivalent europium complex Eu(TTA)3phen (TTA=thenoyltrifluoroacetone and phen=1,10-phenanthroline) by designing the device structure with stepwise energy levels. The widely used bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) was chosen as host material, while the doping concentration of Eu(TTA)3phen was optimized to be 4%. To facilitate the injection and transport of holes, MoO3 anode modification layer and 4,4′,4′′-Tris(carbazole-9-yl)triphenylamine (TcTa) hole transport layer were inserted in sequence. Efficient pure red emission with suppressed efficiency roll-off was obtained attributed to the reduction of accumulation holes, the broadening of recombination zone, and the improved balance of holes and electrons on Eu(TTA)3phen molecules. Finally, the device with 3 nm MoO3 and 5 nm TcTa obtained the highest brightness of 3278 cd/m2, current efficiency of 12.45 cd/A, power efficiency of 11.50 lm/W, and external quantum efficiency of 6.60%. Such a device design strategy helps to improve the EL performances of emitters with low-lying energy levels and provides a chance to simplify device fabrication processes.