Hu Y.-CChen L.-MLin Z.-LLee J.-WWei W.-CKo T.-YLo C.-YHung W.-YKEN-TSUNG WONG2022-12-142022-12-14202200094536https://www.scopus.com/inward/record.uri?eid=2-s2.0-85129730524&doi=10.1002%2fjccs.202200094&partnerID=40&md5=54bb97209400eb864779c84921332c4fhttps://scholars.lib.ntu.edu.tw/handle/123456789/626368In this study, a new triazatruxene-based donor Tr-iBu bearing branched alkyl groups is reported to suppress the aggregation found in a previously reported methyl-substituted counterpart Tr-Me. The effects of a steric factor on the propensity of exciplex formation were examined by respectively mixing these triazatruxene-based donors with PO-T2T and two new acceptors PO-T2P and PO-Pyr, all bearing phosphine oxide as the peripheral groups and N-heteroarene cores with different numbers of nitrogen. Based on the steric hindrance of side groups and their corresponding molecular orbitals alignments of donors and acceptors, the characteristics of exciplex-forming systems were analyzed. Among them, the exciplex-forming blends Tr-iBu:PO-T2P and Tr-iBu:PO-Pyr were selected to serve as the emitting layer (EML) to achieve OLED devices with a moderate maximum external quantum efficiency (EQEmax) of 8.27 and 7.54%, respectively. Adopting the Förster resonance energy transfer strategy, the exciplex cohost was doped with a fluorescence emitter, DPy2CN, to realize a deep-red OLED device with the emission peak centered at 674 nm while retaining EQEmax of 6.28%. Our results highlight the importance of suppressing the donor aggregation, adequate donor: acceptor combination, and a suitable triplet state of composing the materials for giving efficient exciplex-based OLEDs. © 2022 Chemical Society Located in Taipei and Wiley-VCH GmbH.deep-red fluorescence emitter; diphenylphosphine oxide; exciplex; organic light-emitting diode; thermally-activated delayed fluorescence; triazatruxene[SDGs]SDG7journal article10.1002/jccs.2022000942-s2.0-85129730524