High-electric-field quantum yield roll-off in efficient europium chelates-based light-emitting diodes

The europium chelates-based light-emitting-diodes (LEDs) have been fabricated showing the maximum electroluminescence (EL) quantum efficiency (QE) up to 5%photons∕carrier at electric fields slightly below F=1MV∕cm and current density j≅0.01mA∕cm2. Their line-like emission QE drops, however, rapidly within the high drive voltage range. This roll-off effect, exceeding one order of magnitude at F≅1.6MV∕cm(j≅15mA∕cm2), is shown to be underlain by the electric field-assisted dissociation of electron-hole pair precursors of europium ion-localized emissive states. The high field dependence of EL QE fits the Onsager model of geminate recombination well. This is at variance with triplet-triplet annihilation mechanism assumed previously to reduce QE in such LEDs.