Thermal Management of OLEDs Based on TADF Materials: Implication of Heat Upconversion to the Visible Light

High temperatures normally deteriorate the performance of light-emitting diodes, leading to excessive leakage currents, material degradation, and device failure. However, a thermal activation step is necessary for proper operation of OLEDs based on thermally-activated delayed fluorescence (TADF) compounds; therefore, heat might be exploited as a useful factor and as a side energy converted to the OLED emission. Here, we show that the performance of OLEDs based on two different TADF materials can be significantly improved by heating, where thermal management is extremely dependent on the applied biases. Device heating by ~30 K above room temperature causes enhancement of the OLED’s electroluminescence by 5-6 times when operating near the turn-on voltage, while emission enhancement decreases at higher applied biases. At the same time, temperature dependencies of currents, current differential sensitivity, and activation energy for carrier injection behave differently, showing normal and abnormal regions, where currents increase and decrease with temperature, respectively. The discrepancy of temperature dependence of electroluminescence and current in the abnormal region provides increased external quantum efficiency and improved power efficiency of the device with increasing temperature. These findings suggest potential exploitation of TADF-based OLEDs in energy-saving applications and as energy upconverters pumping waste thermal energy to light emission.