New model to boost performance of OLEDs

Researchers from Kyushu University have developed an analytical model that could help develop better organic light emitting diodes (OLEDs) by improving the understanding of the fundamental chemistry and physics behind the technology. The analytical model details the kinetics of the exciton dynamics in OLED materials; the research findings, published in Nature Communications, have the potential to enhance the lifetime of OLED devices and accelerate the development of more advanced and efficient materials.

OLEDs are a type of photoluminescence device that utilise organic compounds to produce light. Compared to traditional LEDs, OLEDs have shown to be more efficient, with a higher dynamic range in image quality. They can also be built into ultra-thin and flexible materials. Fluorescence devices like OLEDs light up because of excited electrons, or excitons. When energy is added into atoms, their electrons get excited and jump to a higher energy state. When they come back down to their regular energy state, they produce fluorescence. Excitons can also go into different states, namely a singlet state, denoted as S1, or a triplet state, denoted as T1. Fluorescence can only happen when excitons drop from the singlet state.

“Thankfully, excitons can transfer between the triplet and singlet state. Therefore, if we can convert triplet excitons into singlets, the efficiency of fluorescence drastically improves,” said Professor Chihaya Adachi of Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA).

“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,” Adachi said.

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 can be unreliable due to its inherent subjectivity and conditional assumptions.

First author of the study, Professor Youichi Tsuchiya, said the researchers used quantum calculations to forecast this gap when developing the new TADF materials. This was denoted as ΔEst. However, it’s not feasible to theoretically calculate the behaviour 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,” Tsuchiya said.

To close this gap between theoretical and experimental methods, the researchers developed a model that can 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,” Tsuchiya said.

Accurately describing the excited-state structures of organic molecules that had been difficult to explore in detail until now. The researchers hope their findings will contribute to the development of high-performance luminescent materials while also paving the way for further advances in photochemistry.

“This new analytical method will be utilised on other types of TADF materials as well, helping us to clarify exciton dynamics in future OLED research. We also want to explore the use of AI to accurately predict the properties of new materials,” Adachi said.

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