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 simplified doping-free phosphorescent organic light-emitting diode (OLED) with a forward viewing external quantum efficiency (EQE) and power efficiency 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 triplet-polaron 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 field-induced exciton dissociation are found negligible. The rate constants k(TTA) and k(TPA) are determined by time-resolved photoluminescence experiments of a thin film 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 efficiency are analyzed. It is found that the short lifetime of the neat emitter is key for the reduction of triplet quenching.