Detailed electro-optical modeling of thermally-activated delayed fluorescent OLEDs with different host-guest concentrations

Abstract

OLED device optimization often relies on time-consuming trial-and-error experiments. While the photoluminescence quantum yield can serve as a first indicator to find the best performing host-guest ratio, this quantity does not consider the impact of the latter on the charge transport in the full device. Herein, we analyse four thermally activated delayed fluorescence OLEDs with varied host-guest ratio in the emissive layer. These devices were characterized and modelled in steady-state, under transient conditions and in the frequency domain. In this set of devices charge injection into and transport inside the emissive layer plays a crucial role in the performance. Evidenced by a particular transient electroluminescence turn-off overshoot, we show that for the 5% guest concentration device electron and hole transport occurs mainly on the host molecules, with guest molecules acting as trap states. For the other devices with higher guest concentration, we find that transport occurs mainly on the guest molecules. As a second step, a fit of the luminance efficiency is performed, with which we can extract the triplet-triplet annihilation and triplet-polaron annihilation rates. By comparing the extracted parameters, we found that they increase with increasing concentration of guest molecules. Moreover, we were able to identify triplet-polaron quenching from holes to be limiting the luminance efficiency at low current, while triplet-polaron quenching from electrons and triplet-triplet annihilation are the dominant non-radiative decay processes in the high current regime. Overall, we demonstrate that model-based analysis of steady-state, transient and frequency domain data obtained for a thermally activated delayed fluorescence OLED allows to get a deeper understanding of the efficiency limiting factors for various host-guest concentrations and driving currents. • Electro-optical characteristics of various OLEDs is well described by the simulation. • Charge transport occurs on TADF guest molecules only at large guest concentrations. • The polarity of TPBi can be captured with impedance and CELIV techniques. • Simulations of the efficiency roll-off allow to quantify quenching processes. • Triplet quenching processes scale empirically with the log of guest concentration.