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

Abstract 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.