Robust universal bipolar host materials with thermally activated delayed fluorescence for high-performance OLEDs

Abstract

Robust host materials play a critical role in improving efficiency and stability for organic light-emitting diodes (OLEDs). To address the challenges of exciton quenching and imbalanced charge transport in traditional unipolar host systems, herein, we design two V-shaped bipolar host materials, v-CzTRZ and v-InCzTRZ, consisting of a carbazole (Cz)/7,7-dimethyl-5,7-dihydroindeno[2,1-b]carbazole (InCz) donor, a triazine (TRZ) acceptor and a o-terphenyl bridge. Their thermal stability, electrochemical properties, electrical structures, optical characteristics, charge transport capabilities, and applicability as host materials are systematically investigated. v-InCzTRZ exhibits pronounced thermally activated delayed fluorescence (TADF) property and superior charge transport ability, rendering it an efficient and versatile host material for a wide range of luminescent materials, including noble metal-containing phosphors, and purely organic classical TADF and multi-resonance (MR) TADF emitters. The OLED using v-InCzTRZ as host for phosphorescent Ir(tptpy)₂acac exhibits an excellent maximum external quantum efficiency (η_ext,max) of 37.24 % and an ultrahigh luminance of 248500 cd m⁻² with a minimal efficiency roll-off of only 2.8 % at 1000 cd m⁻² luminance. Besides, the OLED utilizing v-InCzTRZ as host for MR-TADF emitter of BN2 achieves an outstanding η_ext,max of 38.78 %, much better than those using common host materials. These results demonstrate the high potential of V-shaped bipolar hosts for high-performance OLEDs.