Three triazatruxene-based donors Tr-Me, Tr-Ph, and Tr-Tol were intermixed with three acceptors 3P-T2T, 3P-T2P, and 3P-Pyr equipping with different heteroarene cores to generate an array of nine blends to probe the feasibility of exciplex formation.

In 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 (EQE max ) 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 EQE max 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.