Thermal energy upconversion using thermally-activated delayed fluorescence materials

Heat and thermal radiation are large components of energy provided by direct sun radiation (where infrared constitutes a significant portion), terrestrial radiation, byproducts of industrial and agricultural activity, physical, chemical and biological processes in nature as well. However, a subsequent conversion of thermal energy into other useful forms such as electricity or mechanical work is associated with significant irreversible losses and generally low efficiencies if operating at modest temperature differences. Therefore, novel principles of thermal energy conversion into convenient forms, which are less dependent on high temperatures and temperature gradients as well as on spontaneous discharge, are still of great demand. Thermally activated delayed fluorescence (TADF) is an interesting option for thermal energy upconversion into the visible light, where the latter can be further used for solar cell operation to produce electricity or for direct lighting. In TADF process, the thermal activation step is needed to fulfill an electronic transition from the triplet to singlet energy level of the TADF emitter, from which radiative relaxation of the excited electron occurs, where heat might be exploited as a side energy assisting luminescence. In this work, the potential of heat upconversion to visible light through thermal management of the TADF-based OLEDs is demonstrated. We show that the performance of OLEDs based on the yellow (Y) and green (G) TADF emitters can be effectively tuned by temperature. In particular, enhancement of the OLED’s electroluminescence by 5-6 times was achieved when operating near the turn-on voltage and heating the device to 50 °C, while enhancement decreases at higher applied biases. Upon heating, enhancement of the external quantum efficiency and power efficiency of the devices was achieved by factors of up to 7.4 and 1.13 for the Y device and by 3.8 and 1.16 for the G device, respectively. From the practical point of view, exploiting OLEDs at high temperatures may still be a challenging task due to the potentially negative long-term impact of heat on device performance, which should be prevented as well. By overcoming the related problems, the thermal management of emissions and the energy upconversion of TADF-based OLEDs promise potential application of this phenomenon for heat harvesting and energy-saving applications.

Dr. Dimitriev is a physicist specializing in photo physics of organic dyes and conjugated polymers, also working in other related fields such as material science of organic and hybrid hetero structures, energy conversion, photoi nduced charge transfer, ecology issues as well. He obtained his Ph.D. degree in solid state physics from Donetsk State University in 1992 and currently is a Senior Scientist at V. Lashkaryov Institute of Semiconductor Physics, National Academy of Science of Ukraine. He also works as Assistant Professor at Igor Sikorsky Kyiv Polytechnic Institute. He has been a recipient of international awards obtained from Swedish Institute, Fulbright Visiting Scholar Program, German Academic Exchange Service (DAAD), and Japan Society for the Promotion of Science (JSPS). He is the author of over 100 publications and two popular science books.