Efficiency Enhancement in Thermally Activated Delayed Fluorescence Organic Light-Emitting Devices by Controlling the Doping Concentration in the Emissive Layer

Efficiency roll-off defined as the rapid efficiency drop by increasing the electrical current density is a major issue in the design of novel organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF). In this paper, it is shown that efficiency enhancement and suppression of efficiency roll-off in TADF OLEDs can be accomplished by broadening the exciton generation zone by precisely tuning the doping concentration of the TADF emissive material in the host matrix. This universal and simply tunable parameter can be applied to any TADF OLED to further enhance its performance characteristics. In this paper, a typical TADF OLED with realistic parameters is numerically investigated by solving the charge carrier transport and excitonic equations and taking into account all relevant processes such as triplet-triplet annihilation, triplet-polaron, and singlet-triplet quenching, and (reverse) intersystem crossing. It is shown that the internal quantum efficiency (IQE) decreases at very low and very high doping densities due to exciton distribution nonuniformity and charge balance factor reduction, respectively. Power efficiency depends on the IQE as well as the potential drop across the emissive layer and decreases at very low doping densities even for a perfectly balanced device; however, the reduction for an unbalanced device is much more substantial. For a perfectly balanced device with an emissive layer thickness of 30 nm, at a current density of 10 mAcm ^-2 , the IQE can be increased from 18% to more than 58% by decreasing the doping concentration from c= 15% to 0.5%. Power efficiency reaches its peak value of 38 lm/W at 1.4% doping.