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.