Numerical Device Model for Organic Light‐Emitting Diodes Based on Thermally Activated Delayed Fluorescence

Abstract

AbstractIn 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). To model the operation of TADF‐based OLEDs, quantification of the triplet population is therefore a prerequisite. A numerical drift‐diffusion model is presented for TADF OLEDs that next to singlet and triplet generation also includes the positional dependence of intersystem crossing, rISC and triplet‐triplet annihilation (TTA). As experimental model system, a single‐layer OLED is used based on the TADF emitter 9,10‐bis(4‐(9H‐carbazol‐9‐yl)−2,6‐dimethylphenyl)−9,10‐diboraanthracene that possesses nearly trap‐free transport and a high photoluminescence quantum yield. The 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. The model reveals that the steep increase in EQE at low voltage originates from emissive trap states, whereas the efficiency decrease at high voltage 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.