Elevating the triplet level of carbazolyl benzonitrile-based dendritic hosts by suppressing intramolecular charge transfer for solution-processed blue thermally activated delayed fluorescence OLEDs

Realizing appropriate and excellent solution-processed thermally activated delayed fluorescence host materials is of great consequence to the development of low-cost, large-area OLED displays, as the doped devices mostly yield better efficiency than the undoped devices. Herein, two carbazolyl benzonitrile-based dendritic host materials were designed and synthesized by inserting phenyl bridges to 2,4,6-tri(9H-carbazol-9-yl) benzonitrile (3CzBN) and encapsulating flexible alkyl chains. The phenyl bridge extended the backbone of acceptor unit and donor unit respectively, allowed for a better separation of the HOMO and LUMO orbital and weakened intramolecular charge transfer, which endowed high triplet level of 3.01 eV, 2.85 eV for 3CzPhCN-O-Cz and 3PhCzCN-O-Cz. The introduction of the peripheral alkyl chains resulted in better solubility and film-forming ability. These features made the host materials suitable for fabricating blue solution-processed devices. The experimental results proved that the device performance was better when the phenyl bridge was inserted between the accepter unit and the center benzene ring than when it was introduced between the donor unit and the center benzene ring, which paved the way toward achieving high triplet level host materials for solution-processed OLEDs.