Europium(II) Complex with d‐f Transition: New Emitter for Blue Light‐Emitting Electrochemical Cells with an External Quantum Efficiency of 19.8%

Light-emitting electrochemical cells (LECs) have good prospects in the solid-state lighting field due to their simple single-layer structure and low manufacturing cost. However, the lack of high-efficiency blue LECs limits their development and further application. To solve this problem, many luminescent materials with various mechanisms such as fluorescence, phosphorescence, and thermally-activated delayed fluorescence are studied as emitters in LECs. Besides these materials, herein lanthanide europium(II) (Eu(II)) complex with d-f transition is demonstrated as a new type of emitter for LECs. In detail, a blue emitting Eu(II) complex bis[hydrotris(3-tert-butylpyrazolyl)borate]europium(II) is dispersed into 9-(3-(triphenylsilyl)phenyl)-9H-3,9'-bicarbazole host and tetrahexylammonium tetrafluoroborate electrolyte to make a blend film, which attains a high photoluminescence quantum yield of ≈100%. The LEC using the blend film as an active layer achieves a blue emission with Commission Internationale de L'Eclairage coordinates of (0.12, 0.18) and a record-breaking external quantum efficiency of 19.8%, corresponding to an exciton utilization efficiency of ≈100%. This work reveals that Eu(II) complex with d-f transition is a promising emitter toward high-performance blue LECs, which can inspire further research on Eu(II) complex based LECs.