Electroluminescence mechanisms in organic light emitting devices employing a europium chelate doped in a wide energy gap bipolar conducting host

The mechanism for energy transfer leading to electroluminescence (EL) of a lanthanide complex, ${\mathrm{Eu(TTA)}}_{\mathrm{3}}\mathrm{phen}$ $(\mathrm{TTA=thenoyltrifluoroacetone,phen=1,10-phenanthroline}\mathrm{),}$ doped into ${\text{4,4}}^{\prime }{\mathrm{-N,N}}^{\prime }$ -dicarbazole-biphenyl (CBP) host is investigated. With the device structure of anode/hole transport ${\mathrm{layer/Eu(TTA)}}_{\mathrm{3}}\mathrm{phen}\text{(1\%):}\mathrm{CPB/electron}$ transport layer/cathode, we achieve a maximum external EL quantum efficiency (η) of 1.4% at a current density of 0.4 mA/cm². Saturated red ${\mathrm{Eu}}^{\mathrm{3+}}$ emission based on ${}^5{D}_x{–}^7{F}_x$ transitions is centered at a wavelength of 612 nm with a full width at half maximum of 3 nm. From analysis of the electroluminescent and photoluminescent spectra, and the current density–voltage characteristics, we conclude that direct trapping of holes and electrons and subsequent formation of the excitons occurs on the dopant, leading to high quantum efficiencies at low current densities. With increasing current between 1 and 100 mA/cm², however, a significant decrease of η along with an increase in CBP host emission is observed. We demonstrate that the decrease in η at high current densities can be explained by triplet–triplet annihilation.