Mr. Chi Hyun Ryoo Profile

Chi Hyun Ryoo

at Seoul National Univ

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Publications (2)

Proceedings Article | 3 October 2022 Presentation

Breaking bad organic materials: how do thermally activated delayed fluorescence emitters break the rules of photophysics? (Conference Presentation)

Illia Serdiuk, Michal Monka, Vladyslav Ievtukhov, Daria Grzywacz, Karol Kozakiewicz, Estera Hoffman, Chi Hyun Ryoo, Beata Liberek, Soo Young Park, Piotr Bojarksi

Proceedings Volume PC12208, PC122080X (2022) https://doi.org/10.1117/12.2646613

KEYWORDS: Luminescence, Organic materials, Resonance energy transfer, Organic light emitting diodes, Materials processing, Liquids, Fluorescence resonance energy transfer, Computing systems

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Proceedings Article | 10 September 2019 Presentation

Shifting TADF of phenyl-s-triazine derivatives to deep-blue: the approaches increasing triplet and singlet excited state energies (Conference Presentation)

Illia Serdiuk, Chi Hyun Ryoo, Karol Kozakiewicz, Beata Liberek, Ji Eon Kwon, Soo Young Park

Proceedings Volume 11093, 1109305 (2019) https://doi.org/10.1117/12.2528673

KEYWORDS: Organic light emitting diodes, Polishing, Luminescence, Optoelectronic devices, Excitons

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During the last few years, the phenomenon of thermally activated delayed fluorescence (TADF) finds more and more applications in numerous fields of technology, especially in organic light emitting diodes (OLED) giving rise to variously shaped and flexible optoelectronic devices and displays. Further development of products using OLED technology requires emitters exhibiting deep-blue TADF with relatively short emission lifetimes and high chemical stability. The state-of-the-art pure organic TADF emitters comprise donor and acceptor fragments separated spatially. This enables very small gap between the lowest singlet and triplet excited states of charge-transfer (CT) character. However, as these states differ only by multiplicity, the transition between them is forbidden by the selection rules. According to the current understanding of TADF mechanism, the efficient conversion of nonradiative triplet excitons to the emissive singlet ones occurs when a locally excited triplet state (3LE) is energetically close to the ¹CT and ³CT states. The photophysical principles described above work very well for design of efficient yellow, green, and sky-blue TADF materials. To achieve deep-blue TADF, the emitter should comprise highly energetic CT and ³LE states, which is a challenging requirement. To face this problem we investigated two approaches of increasing excited states’ energies: (i) reduction of conjugation of the acceptor fragment by its deplanarization and (ii) introduction of electron-releasing substituents in the acceptor fragment. 2-Phenyl-s-triazine and 9,10- dihydro-9,9-dimethylacridine were selected as acceptor and donors fragments, respectively. Our results show that both approaches efficiently increase energies of the CT and acceptor ³LE states, maintaining the rate constant of reverse intersystem crossing in the 10⁵–10⁶ s^–1 range. This presentation will focus on details of the above mentioned investigations including characteristics of the OLED devices.

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