Cyclohexane-coupled bipolar host materials with high triplet energies for organic light-emitting diodes based on thermally activated delayed fluorescence

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 highefficiency 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 electrondonating 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 glasstransition 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 chargerecombination zone within the emission layer arising from the bipolar charge transport ability of these host materials.