Thermally activated delayed fluorescent (TADF) emitters are one of the most promising candidates for developing low-cost organic light-emitting devices (OLEDs) that can achieve an internal quantum efficiency of 100%. However, the power efficiency values are still significantly lower than that of their phosphorescent counterparts. To achieve high power efficiency, both a high external quantum efficiency and low drive voltage are required. Here, TADF OLEDs with an ultrahigh power efficiency of 133 lm W−1 and an unprecedented low turn-on voltage of 2.22 V are successfully developed via the strategic use of electron-transport materials (ETMs). 9-[4-(4,6-Diphenyl-1,3,5-triazin-2-yl)phenyl]-N,N,N′,N′-tetraphenyl-9H-carbazole-3,6-diamine as a TADF emitter is used. A series of green TADF OLEDs are fabricated using two different types of pyrimidine-based ETMs as an exciplex host partner and electron-transport layer to minimize the drive voltages. The optimized device shows an external quantum efficiency of 29.2% and maximum power efficiency of 133.2 lm W−1 without any light-outcoupling enhancement. The device also shows 124.0 lm W−1 at 100 cd m−2 and 95.2 lm W−1 at a high brightness of 1000 cd m−2. The drive voltages are 2.48 V at 100 cd m−2 and 2.88 V at 1000 cd m−2.
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