Abstract
Thermally activated delayed fluorescence-based organic light-emitting diodes (TADF-OLEDs) have recently attracted tremendous research interest as next-generation optoelectronic devices. However, there are a limited number of host materials with an appropriately high lowest-excited triplet energy (ET) and bipolar charge transport properties for high-efficiency TADF-OLEDs. Moreover, these host materials should have high thermal and morphological stabilities. In this study, we develop novel bipolar host materials consisting of an electron-donating 9-phenylcarbazole unit and an electron-accepting triphenylphosphine oxide, triphenylphosphine sulfide, or 2,4,6-triphenyl-1,3,5-triazine unit linked by a nonconjugated cyclohexane core. These bipolar host materials possess high glass-transition temperatures of over 100 °C and high ET values of approximately 3.0 eV. TADF-OLEDs employing these bipolar host materials could achieve high external electroluminescence quantum efficiencies of up to 21.7% together with reduced efficiency roll-off characteristics, because of expansion of the charge-recombination zone within the emission layer arising from the bipolar charge transport ability of these host materials.
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