Resonance-driven dynamically bipolar organic semiconductors for high-performance optoelectronic applications

Abstract

Organic semiconductors with bipolar charge transporting character are promising for optoelectronic applications, but the molecular design strategies of bipolar materials mostly rely on donor–acceptor systems, requiring both donor and acceptor functional units joined together to respond to the hole and electron injection/transport, respectively. Here, we show that the bipolar character can be alternatively achieved by facilitating resonance enantiomer transitions in donor-resonance-donor (D-r-D) architecture for a very small variation energy (0.4 eV) based on a newly developed resonance linkage, N–CO. Without the intrinsic participation of acceptor units, the D–r–D molecule exhibits a high triplet energy (3.01 eV) and excellent charge transport balancing ability, where one carbazole responds for hole transport and the other positively charged carbazole in the enantiotropic N⁺C–O⁻ canonical form for electron transport. Benefiting from the in situ dynamic response of this facile resonance variation, the blue phosphorescent organic light emitting diodes (OLEDs) and thermally activated delayed fluorescence OLEDs employing the D–r–D molecule as the host show high maximum external quantum efficiencies of 31.2% and 20.5%, respectively. These outstanding device performances with the aid of the new concept of resonance-driven acceptor-free bipolar D–r–D host molecule illustrate exciting progress in designing dynamically bipolar organic semiconductors for high-performance optical and electronic applications.