High-triplet-energy bipolar host materials based on phosphine oxide derivatives for efficient sky-blue thermally activated delayed fluorescence organic light-emitting diodes with reduced roll-off

We designed and synthesized two new ambipolar host materials, namely CzPO and Cz3PO. Combining CzPO and Cz3PO with CzTRZ2 as the emitter resulted in improved maximum external quantum efficiencies 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) as host showed a much more pronounced efficiency roll-off