A tris β-diketonate europium(III) complex based OLED fabricated by thermal evaporation method displaying efficient bright red emission

A new tris β-diketonate europium(III) complex [Eu(btfa)₃Py-Im] (Eu-1) (4,4,4-trifluoro-1-phenyl-1,3-butanedione (btfa) and 2-(2-pyridyl)benzimidazole (Py-Im)] has been synthesized and structurally characterized. A single crystal X-ray diffraction analysis shows that Eu-1 is octacoordinated and the coordination sphere is composed of a EuO₆N₂ core with a trigonal dodecahedral (D_2d) geometry. The photophysical properties of Eu-1 were analysed in detail and with the help of the experimental PL data and theoretical modelling, energy transfer rates were calculated, and an energy transfer mechanism is proposed for Eu-1. The complex has been used as the emitting layer (EML) to fabricate organic light emitting diodes (OLEDs). Eight OLEDs, of which four single-EML and four double-EML with varying doping concentration, were fabricated via a thermal evaporation method using Eu-1 as the EML. Under the optimum conditions a highly monochromatic bright red emission (CIE_x,y = 0.640, 0.311) with brightness (B) = 896 cd/m², current efficiency (η_c) = 2.26 cd/A, power efficiency (η_p) = 1.92 lm/W and external quantum efficiency (EQE) = 1.6% at very low V_turn-on = 3.4 V was obtained.