Molecular Design of Highly Efficient Thermally Activated Delayed Fluorescence Hosts for Blue Phosphorescent and Fluorescent Organic Light-Emitting Diodes

Recently, thermally activated delayed fluorescence (TADF) materials have become the most promising hosts for realizing high-performance phosphorescent and fluorescent organic light-emitting diodes (OLEDs) because of their ability to upconvert triplet excitons to singlet excitons. However, despite a few TADF hosts having been introduced for low energy phosphorescent and fluorescent dopants, developing host materials with TADF properties for blue phosphorescent and TADF OLEDs is still a great challenge to date. In this study, bipolar hosts exhibiting TADF behavior and high triplet energy, consisting of the carbazole group as the donor, diphenylsulphone moiety as the acceptor, and m-bitolyl as the pi-conjugated bridge, are synthesized and applied for the first time to blue devices. The Delta E-ST value of the TADF host is tuned via the introduction of a cyano group in the carbazole moiety due to the increase of the LE contribution in the CT excited state. Detailed photophysical studies confirm the efficient TADF properties of bipolar hosts. The blue phosphorescent and TADF devices using BT-01 as the host give external quantum efficiencies of 31.8% and 25.5%, respectively. The blue devices based on the BT-01 host exhibit superior electroluminescence performance and more reduced efficiency roll-off compared with those hosted by BT-02, ascribed to the faster reverse intersystem crossing process on the BT-01 host. These excellent results manifest that the use of the bipolar host with TADF behavior is a promising approach for the realization of highly efficient blue phosphorescent and TADF devices in the future.