Exciton and Polaron Quenching in Doping-Free Phosphorescent Organic Light-Emitting Diodes from a Pt(II)-Based Fast Phosphor

By introducing a neat Pt(II)-based phosphor with a remarkably short decay lifetime, a simplifi ed doping-free phosphorescent organic light-emitting diode (OLED) with a forward viewing external quantum effi ciency (EQE) and power effi ciency of 20.3 ± 0.5% and 63.0 ± 0.4 lm W − 1 , respectively, is demonstrated. A quantitative analysis of how triplet-triplet annihilation (TTA) and tripletpolaron annihilation (TPA) affect the device EQE roll-off at high current densities is performed. The contributions from loss of charge balance associated with charge leakage and fi eld-induced exciton dissociation are found negligible. The rate constants k TTA and k TPA are determined by time-resolved photoluminescence experiments of a thin fi lm and an electrically-driven unipolar device, respectively. Using the parameters extracted experimentally, the EQE is modeled versus electric current characteristics of the OLEDs by taking both TTA and TPA into account. Based on this model, the impacts of the emitter lifetime, quenching rate constants, and exciton formation zone upon device effi ciency are analyzed. It is found that the short lifetime of the neat emitter is key for the reduction of triplet quenching.