Blue Organic Light-Emitting Diodes with External Quantum Efficiencies over 20% Based on Europium(II) Emitters
The realization of blue electroluminescence with high efficiency and lifetime remains a long-standing hurdle for organic light-emitting diode (OLED) technology to overcome. Divalent Europium [Europium(II)] complexes offer a fundamentally distinct pathway toward this goal, as their atomic 4f-5d transitions yield single-Gaussian, spectrally pure emission with theoretical 100% exciton utilization in electroluminescence and no involvement of fragile organic bonds in the emissive process. Here, we present a rigid aza-crown europium(II) complex (Eu5NHCrown) that achieves near-unity photoluminescence quantum yield with bright, pure-blue emission and its incorporation in OLEDs. The emitter complex sublimes without decomposition and can be processed by industry-standard vacuum deposition. A bottom-emitting, single-host OLED architecture delivers an external quantum efficiency (EQE) of 20.7% with minimal roll-off (19.3% at 1000 cd m${}^{-2}$ ) and a narrowband electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.12, 0.25). This ligand design provides enhanced steric shielding of the Eu(II) center, enabling the highest reported efficiency among vacuum-deposited Eu(II)- based blue OLEDs while maintaining performance at high luminance. These results reveal the true potential of divalent Europium 4f-5d transitions for high-efficiency blue OLEDs, establishing a molecular design concept that bridges atomic-transition efficiency with the processability of organic materials.