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=1 MV/cm and current density j congruent to 0.01 mA/cm(2). 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 congruent to 1.6 MV/cm (j congruent to 15 mA/cm(2)), 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.