Efficient Orange-Red Thermally Activated Delayed Fluorescence Emitters Feasible for Both Thermal Evaporation and Solution Process

Development of red thermally activated delayed fluorescence (TADF) emitters has been lagging behind when compared with those of blue and green fluorophores, especially for solution-processable ones. In this work, two novel orange-red TADF emitters 3,6-di(10H-phenoxazin-10-yl)dibenzo[a,c]phenazine (DBPZ-DPXZ) and 10,10'-(11,12-bis(3,5-di-tert-butylphenyl)dibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (tDBBPZ-DPXZ) are developed. A high-performance orange-red TADF emitter, DBPZ-DPXZ, is first prepared by connecting a rigid acceptor and two rigid donor segments. While this design strategy endows DBPZ-DPXZ with an excellent TADF performance leading to a vacuum-processed organic light-emitting diode (OLED) with a high external quantum efficiency (EQE) of 17.8%, the rigid segments limit its solubility and applications in solution-processed devices. Based on this prototype, tDBBPZ-DPXZ is designed with the addition of 3,5-di-tert-butylphenyl groups to boost its solubility with barely an influence on the photophysical properties. In particular, tDBBPZ-DPXZ maintains nearly an identical photoluminescence quantum yield of 83% and singlet-triplet energy splitting of 0.03 eV with EQE of 17.0% in a vacuum-processed orange-red OLED. Furthermore, it can be applied on the orange-red solution-processed OLED realizing an EQE as high as 10.1%, representing one of the state-of-the-art results of the reported orange-red solution-processed TADF-OLEDs. This work provides an effective strategy to address the conflicting requirements between high efficiency and good solubility and develop efficient soluble orange-red TADF emitters.