Lanthanide complexes with d-f transition: new emitters for single-emitting-layer white organic light-emitting diodes

White organic light-emitting diodes (WOLEDs) is a new generation of lighting technology and has stimulated wide-ranging studies. Despite the advantage of simple device structure, single-emitting-layer WOLEDs (SEL-WOLEDs) still face the challenges of difficult material screening and fine energy level regulation. Herein, we report efficient SEL-WOLEDs with a sky-blue emitting cerium(III) complex Ce-TBO^2Et and an orange-red emitting europium(II) complex Eu(Tp^2Et)₂ as the emitters, showing a maximum external quantum efficiency of 15.9% and Commission Internationale de l'Eclairage coordinates of (0.33, 0.39) at various luminances. Most importantly, the electroluminescence mechanism of direct hole capture and hindered energy transfer between the two emitters facilitate a manageable weight doping concentration of 5% for Eu(Tp^2Et)₂, avoiding the low concentration (<1%) of the low-energy emitter in typical SEL-WOLEDs. Our results indicate that d-f transition emitters may circumvent fine energy level regulation and provide development potential for SEL-WOLEDs.