Device Operation of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence

In organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF), non-emissive triplet excitons are converted to emissive singlet excitons via reverse intersystem crossing (rISC), enabling a 100% internal quantum efficiency. Despite the impressive progress in efficiency, understanding of the device physics of TADF OLEDs is still in early development. To model the operation of TADF based OLEDs, quantification of the triplet population is a prerequisite. We present a numerical drift-diffusion model for TADF OLEDs that next to singlet and triplet generation also includes the positional dependence of intersystem crossing (ISC), rISC and triplet-triplet annihilation (TTA). As experimental model system, we use a single-layer OLED based on the yellow TADF emitter 9,10-bis(4-(9H-carbazol-9-yl)−2,6-dimethylphenyl)−9,10-diboraanthracene (CzDBA) that possesses nearly trap-free transport and a high photoluminescence quantum yield. Our model accurately describes the voltage dependence of the current density and external quantum efficiency (EQE), both as a function of temperature and active layer thickness. Our model reveals that the steep increase in EQE at low voltage originates from emissive trap states, whereas the efficiency decrease at high voltage (roll-off) is dominated by TTA, with a temperature independent rate constant of 7±3 × 10^-18 m³ s^-1. The model allows us to quantitatively disentangle the various contributions of direct and trap-assisted recombination as well as recombination following rISC to the EQE, providing a useful tool for further optimization of TADF OLEDs.