Molecular engineering by σ-Bond spacer enables solution-processable host materials for TADF emitter towards high-performance OLEDs

High-performance solution-processable host materials for thermally activated delayed fluorescence (TADF) emitters are of great significance to the development of organic light-emitting diodes (OLEDs). In this study, three solution-processable hosts, namely mCP-Ph, mCP-Py, and mCP-BmPy featuring a basic structure paradigm of donor-σ-acceptor (D-σ-A) are reported for green TADF OLEDs. Attributing to the well-designed structure, the three host materials exhibited high triplet energy levels and excellent solution processability. It was found that the host materials had a significant impact on the excited state characteristics of the TADF emitter, and the mCP-Ph host with the lowest dipole moment showed the highest intersystem crossing rate constants (kISC and kRISC). In consequence, solution-processed TADF OLEDs based on the emitter 4-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2,6-diphenylpyridine-3,5-dicarbonitrile (DMAC-PCN) and the host 9-(3-(9H-carbazol-9-yl)phenyl)-3-(2,7-di-tert-butyl-9-phenyl-9H-fluoren-9-yl)-9H-carbazole (mCP-Ph) secured a maximum external quantum efficiency of 21.6%, demonstrating the effectiveness of the host engineering by σ-spacer. This work reveals the important principle for designing bespoke host for TADF emitters.