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Triple external quantum efficiencies — a new material TADF was developed

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Credit: Osaka University

An international joint research group succeeded in developing a novel thermally activated delayed fluorescence (TADF) material which displays emission of light in colors from green to deep-red through Intersystem Crossing from the singlet to the triplet excitons, a world first. The results will contribute to R&D in white TADF light emitting devices for indoor and outdoor use through combination with TADF materials that emit light in shorter wavelengths (deep blue to yellow).

Over the last few decades, research on organic light-emitting diodes (OLEDs) has greatly advanced. Indeed, many types of OLEDs can already be encountered in prototypical and commercial applications such as smartphones, lighting, and flat panel displays. Currently, almost all the commercial OLEDs contain the rare metals such as platinum or iridium because of their efficiency and stability. Due to their high costs, the development of inexpensive and highly efficient emitters is desired.

In OLEDs, the recombination of electrons and holes in an active material leads to the formation of various excited states such as singlet and triplet excitons, with the statistical probability of 25% and 75%, respectively (Fig.1). With a conventional fluorescent emitter, the generated triplet excitons are dissipated through non-radiative (NR) processes. Therefore, these triplets should be efficiently converted into emissive singlets to develop efficient OLEDs, and without using a heavy metal atom-containing phosphorescent emitters, the phenomena of delayed fluorescence (DF), either via the process of triplet-triplet annihilation (TTA) yielding a maximum 62.5% internal quantum efficiency, or much better via TADF attaining theoretically 100% harvesting of excitons can be used.

A research group led by Youhei Takeda, Associate Professor and Masato Okazaki, a graduate student at Graduate School of Engineering, Osaka University together with Dr. Przemyslaw Data and Andrew P. Monkman, Professor at the Department of Physics, Durham University discloses the development of a new family of efficient TADF emitters comprised of a new A unit core, dibenzo [a,j] phenazine (DBPHZ), and two Ds (Figure 2a). Through a detailed optical property investigation of this material, this group also confirmed that TADF was achieved through ISC from the singlet excited state (1CT) to the triplet excited state (3LEA) (Fig. 2b). Furthermore, these new emitters were found to yield green to deep-red/NIR OLED devices with high external quantum efficiencies up to 16%, greatly surpassing 5%, the limiting value obtained by conventional fluorescent materials.

These results were reported in "Angewandte Chemie International Edition" on April 6, 2016 published by an internationally acclaimed German publisher, Wiley-VCH Verlag GmbH.

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