We designed and synthesized two new ambipolar host materials, namely CzPO and Cz3PO, which contain electron-donating carbazole and electron-accepting triphenylphosphine oxide moieties. Thermally activated delayed fluorescence (TADF)-based OLEDs employing CzPO and Cz3PO as host materials and the 9-(4-(4,6diphenyl-1,3,5-triazin-2-yl)phenyl)-1,3,6,8-tetramethyl-9H-carbazole (CzTRZ2) as the emitter resulted in improved maximum external quantum efficiencies, EQEmax, of 13.1% and 13.2%, respectively, together with small efficiency roll-offs, while the device based on bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) showed a much more pronounced efficiency roll-off. The reduced efficiency roll-off in the devices based on CzPO and Cz3PO can be ascribed to their improved ambipolar charge transport capacity compared to that of DPEPO, which results in a broader recombination zone in the emitting layer using these ambipolar hosts.
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