Improving electroluminescence efficiency and operational lifetime of multi-resonance emitters with bipolar host materials†

Abstract

Multi-resonance (MR) thermally activated delayed fluorescence (TADF) materials often face the challenges of concentration-caused emission quenching and severe exciton loss at high voltages. They have to be dispersed in proper host materials to alleviate these problems, but their electroluminescence (EL) performances are not satisfactory in commonly used host materials without the aid of phosphorescence or TADF sensitizers. Herein, we develop three new host materials by introducing a cyano group on the different positions of the diphenyl moiety of a widely used host 3,3′-di(9H-carbazol-9-yl)-1,1′-biphenyl (mCBP). The generated materials exhibit bipolar charge transport properties, with greatly improved electron-transporting ability relative to mCBP. They can serve as efficient host materials for a series of MR-TADF emitters, providing significantly enhanced EL efficiencies and elongated operational lifetimes than mCBP. The MR-TADF emitters in these new host materials can have higher photoluminescence efficiencies, accounting for the improved EL performances and device stability. These results reveal that the exploration of bipolar host materials could be a promising alternative to maximize EL performance of MR-TADF materials without employing sensitizers.