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Exciplex emitters have emerged as an important class of thermally activated delayed fluorescence (TADF) materials for highly efficient OLEDs. A TADF exciplex emitter requires an intermolecular donor/acceptor pair. We have synthesized a bipolar donor-type material, DPSTPA, which was used to pair with known acceptor materials (2CzPN, 4CzIPN, or CzDBA). The OLEDs based on the exciplex emitters, DPSTPA/X, where X = 2CzPN and CzDBA, give green and orange-red colors with record-high external quantum efficiencies (EQEs) of 19.0 ± 0.6 and 14.6 ± 0.4%, respectively. In contrast, the exciplex pair DPSTPA/4CzIPN gave a very low photoluminescence quantum yield (PLQY) and a very low EQE value of the device. The DFT calculations indicate that the intermolecular distance between the donor and the acceptor plays a key factor for the PLQY and EQE. The observed low PLQY and the poor device performance for the DPSTPA/4CzIPN pair are probably because of the relatively long distance between the DPSTPA and 4CzIPN in the thin film caused by the four congested carbazole (Cz) groups of 4CzIPN, which effectively block the interaction of the nitrile acceptor with the triphenylamino donor of DPSTPA.

Recently, thermally activated delayed fluorescence (TADF) materials have become the most promising hosts for realizing high-performance phosphorescent and fluorescent organic light-emitting diodes (OLEDs) because of their ability to upconvert triplet excitons to singlet excitons. However, despite a few TADF hosts having been introduced for low energy phosphorescent and fluorescent dopants, developing host materials with TADF properties for blue phosphorescent and TADF OLEDs is still a great challenge to date. In this study, bipolar hosts exhibiting TADF behavior and high triplet energy, consisting of the carbazole group as the donor, diphenylsulphone moiety as the acceptor, and m-bitolyl as the π-conjugated bridge, are synthesized and applied for the first time to blue devices. The ΔEST value of the TADF host is tuned via the introduction of a cyano group in the carbazole moiety due to the increase of the LE contribution in the CT excited state. Detailed photophysical studies confirm the efficient TADF pr...