Researchers have developed a novel analytical model that reveals the kinetics of exciton dynamics in thermally activated delayed fluorescence (TADF) materials.
Organic light-emitting diodes, or OLEDs, are photoluminescence devices that use organic compounds to generate light. Compared to traditional LEDs, OLEDs are more efficient, can be made into ultra-thin, flexible materials, and deliver higher dynamic range in image quality. To continue improving OLED performance, researchers around the world are studying the fundamental chemistry and physics that drive the technology.
At Kyushu University, a research team has developed a new analytical model that captures the kinetics of exciton dynamics in OLED materials. Their findings, published in Nature Communications, could help extend the lifetime of OLED devices and speed up the development of more advanced, efficient materials.
The Role of Excitons in Fluorescence
OLEDs produce light through the action of excitons, which are excited electrons. When energy is applied to atoms, electrons absorb it and jump to higher energy levels. As they return to their original state, they release light through fluorescence. Excitons can exist in either a singlet state, labeled S1, or a triplet state, labeled T1. Fluorescence occurs only when excitons transition from the singlet state.
“Thankfully, excitons can transfer between the triplet and singlet states. Therefore, if we can convert triplet excitons into singlets, the efficiency of fluorescence drastically improves,” explains Professor Chihaya Adachi of Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA), who led the study. “One of the major breakthroughs of OLED research was in the development of thermally activated delayed fluorescence, or TADF, materials. These materials would close the ‘gap’ between S1 and T1, so that T1 excitons more easily transfer to S1, thus producing more fluorescence.”
Challenges in Measuring the Energy Gap
Understanding the gap between S1 and T1 in TADF materials is fundamental in both evaluating the efficiency of OLED materials and in testing the efficacy of new materials. However, the standard method of testing this gap has been occasionally unreliable due to its inherent subjectivity and conditional assumptions.
“When developing new TADF materials, we employ quantum calculations to forecast this gap, denoted as ΔEst. However, it’s not feasible to theoretically calculate the behavior of all electrons to determine the accurate excitation state configuration. So, to reduce computation costs, we usually work with certain assumptions. But this results in different values between experimental and estimated data,” explains first author of the study, Research Associate Professor Youichi Tsuchiya.
“To close the gap between theoretical and experimental methods, our team worked to developed a model that can more accurately estimate ΔEst. Our new analytical method employed several fundamental theories of physical chemistry and put into account the exciton transfer between the triplet energy states.”
Accurately describing the excited-state structures of organic molecules was something that had been difficult to explore in detail until now. The team hopes their work will not only contribute to the research and development of high-performance luminescent materials but also pave the way for further advances in photochemistry.
“This new analytical method will be utilized on other types of TADF materials as well, helping us to clarify exciton dynamics in future OLED research,” concludes Adachi. “We also want to explore the use of AI to accurately predict the properties of new materials.”
Funding: Kyulux Inc, Japan Science and Technology Agency, Japan Society for the Promotion of Science, Kyushu University, Sumitomo Basic Science Research Projects
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