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

The mechanism for energy transfer leading to electroluminescence (EL) of a lanthanide complex, Eu(TTA)(3)phen (TTA=thenoyltrifluoroacetone,phen=1,10-p henanthroline), doped into 4,4'-N,N'-dicarbazole-biphenyl (CBP) host is inv estigated. With the device structure of anode/hole transport layer/Eu(TTA)( 3)phen(1%):CPB/electron transport layer/cathode, we achieve a maximum exter nal EL quantum efficiency (eta) of 1.4% at a current density of 0.4 mA/cm(2 ). Saturated red Eu3+ emission based on D-5(x)-F-7(x) transitions is center ed at a wavelength of 612 nm with a full width at half maximum of 3 nm. Fro m analysis of the electroluminescent and photoluminescent spectra, and the current density-voltage characteristics, we conclude that direct trapping o f holes and electrons and subsequent formation of the excitons occurs on th e dopant, leading to high quantum efficiencies at low current densities. Wi th increasing current between 1 and 100 mA/cm(2), however, a significant de crease of eta along with an increase in CBP host emission is observed. We d emonstrate that the decrease in eta at high current densities can be explai ned by triplet-triplet annihilation. (C) 2000 American Institute of Physics . [S0021-8979(00)05011-8].