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• Four TADF materials with carbazole donor, and benzonitrile-substituted pyrimidine or triazine as acceptor core were developed. • OLEDs with m-CzPym, p-CzPym, m-CzTrz, and p-CzTrz as host and 4CzIPN were fabricated. • An impressive external quantum efficiency of 31.5% and a superior power efficiency of 116.5 lm W−1 were achieved. • Impressive efficiency roll-off performance (approximately 90% of EQE max at 5000 cd m−2). Four bipolar molecules, namely m-CzPym , p-CzPym , m-CzTrz , and p-CzTrz , with carbazole (Cz) donor and a benzonitrile-substituted pyrimidine (Pym) or triazine (Trz) acceptor core were synthesized and characterized. The electron deficiency of heteroaryl cores together with the substitution pattern of benzonitrile were employed to tune the energy levels as well as the thermally activated delayed fluorescence (TADF) characteristics. The four molecules exhibited TADF behavior with inferior photoluminescent quantum yields (PLQYs) that limit their applications as emitters. These bipolar molecules were employed as TADF host materials for the benchmark TADF emitter 4CzIPN to achieve high-performing green TADF organic light-emitting diodes (OLEDs). Among the molecules, m-CzPym -hosted TADF-OLEDs achieved a maximum external quantum efficiency (EQE max) of 31.5%, maximum power efficiency (PE max) of 95.6 lm/W, and maximum current efficiency (CE max) of 100.2 cd/A. Notably, p-CzPym -hosted TADF-OLEDs also achieved a PE max of 116.5 lm/W, turn-on voltage of 2.5 V, and impressive low efficiency roll-off performance (>89% of EQE max at 5000 cd/m2), representing one of the highest efficiencies ever reported in 4CzIPN-doped devices. The high device efficiency can be ascribed to the balanced ambipolar carrier-transporting character of the host materials and high PLQY as well as the outstanding light outcoupling efficiency of the emitting layer. [ABSTRACT FROM AUTHOR]