New bipolar host materials for high power efficiency green thermally activated delayed fluorescence OLEDs

Four bipolar molecules, namely m-CzPym, p-CzPym, m-CzTrz, and p-CzTrz, with carbazole (Cz) donor and a benzonitrile-substituted pyrimidine (Pym) or triazine (Trz) acceptor core were synthesized and characterized. The electron deficiency of heteroaryl cores together with the substitution pattern of benzonitrile were employed to tune the energy levels as well as the thermally activated delayed fluorescence (TADF) characteristics. The four molecules exhibited TADF behavior with inferior photoluminescent quantum yields (PLQYs) that limit their applications as emitters. These bipolar molecules were employed as TADF host materials for the benchmark TADF emitter 4CzIPN to achieve high-performing green TADF organic light-emitting diodes (OLEDs). Among the molecules, m-CzPym-hosted TADF-OLEDs achieved a maximum external quantum efficiency (EQE(max)) of 31.5%, maximum power efficiency (PEmax) of 95.6 lm/W, and maximum current efficiency (CEmax) of 100.2 c4/A. Notably, p-CzPym-hosted TADF-OLEDs also achieved a PEmax of 116.5 lm/W, turn-on voltage of 2.5 V, and impressive low efficiency roll-off performance (>89% of EQE(max) at 5000 cd/m(2)), representing one of the highest efficiencies ever reported in 4CzIPN-doped devices. The high device efficiency can be ascribed to the balanced ambipolar carrier-transporting character of the host materials and high PLQY as well as the outstanding light outcoupling efficiency of the emitting layer.

Automated summary

Four bipolar molecules with carbazole donor and benzonitrile-substituted pyrimidine or triazine acceptor were synthesized to tune energy levels and TADF characteristics. These molecules as TADF host materials achieved high-performing green OLEDs, with m-CzPym-hosted devices reaching a maximum EQE of 31.5% and p-CzPym-hosted devices showing impressive efficiency roll-off performance. The high efficiency was attributed to the balanced carrier-transporting character, high PLQY, and outstanding light outcoupling efficiency of the host materials.