Reduced geminate recombination in iridium-based electrophosphorescent materials

We demonstrate reduced geminate recombination of excitons in electrophosphorescent organic light emitting devices using the green phosphor fac-tris(1-(9′,9′-dimethyl-2′-fluorenyl)pyrazolato,N,C2)iridium(III). At low phosphor doping concentrations (<20 wt.%) in a N,N′-dicarbazolyl-3,5-benzene host, transient electroluminescence spectra show no bimolecular quenching at the highest excitation densities used. At higher doping concentrations (>60 wt.%), the transient decay reflects the presence of phosphor molecular aggregates. The roll-off in external quantum efficiency at high current densities is attributed to the field-assisted dissociation of charge pairs prior to exciton formation.