High efficiency electroluminescence devices using a series of Ir(III)-tetrazolate phosphors: Mechanisms for the drive current evolution of quantum yield

We demonstrate high-brightness and high-efficiency blue-green to yellow-green electrophosphorescent organic light-emitting diodes employing a series of organic Ir complexes [Ir-((CN)-N-and)(2)((NN)-N-and)]. Three different complexes have been synthesized showing high photoluminescence solid blend efficiencies up to 44%. A low current density increase of the electroluminescence (EL) external quantum efficiency (phi((ext))(EL)) is observed and a maximum of phi((ext))(EL)=10.6%+/- 0.8% photon/e and power efficiency eta=27 +/- 2 lm/W are achieved at a current density of j=0.01 mA/cm(2). We examine various electronic processes that underlie a nonmonotonous current density dependence of the EL quantum efficiency of electrophosphorescent light-emitting diodes. The shape of phi((ext))(EL) versus j is shown to reflect a trade off between electron-hole encounter and charge carrier transit times, electric field effect on electron-hole pair dissociation time, and current driven triplet molecular exciton lifetime.