Effects of Varying Deposition Rates of a Carbazole and Triazine Derivative on Its Tunable Emission and Charge‐Transporting Properties Beneficial for Efficient Red Electroluminescent Devices

The discovery of tunable emission and charge‐transporting properties via varying deposition rates of a new derivative of carbazole and triazine is a novel and intriguing aspect of this research, leading to a shortened electroluminescence lifetime in red organic light‐emitting diodes. Well‐balanced hole and electron transport, along with efficient thermally activated delayed fluorescence (TADF) with an average lifetime of 101 ns in thin films of 9,9'‐bis(4,6‐diphenyl‐1,3,5‐triazin‐2‐yl)‐9H,9'H‐2,2'‐bicarbazole ( 2Cz2TAZ ), is demonstrated. These properties of 2Cz2TAZ support the rapid harvesting of normally long‐lived triplet states. This results in reduced efficiency roll‐off in organic light‐emitting diodes using emitters that exhibit TADF and phosphorescence, when 2Cz2TAZ is employed as the host, compared to a reference device with a conventional host. A maximum external quantum efficiency of 26.2% is achieved for devices using 2Cz2TAZ as the host. Replacing the conventional host with 2Cz2TAZ in the same device structure decreases the average electroluminescence lifetime from 249.6 to 10.7 µs. Balanced hole and electron mobility values exceeding 1 × 10 −4 cm 2 Vs −1 and the short TADF lifetime support fast electroluminescence. Theoretical and experimental approaches are employed to investigate the tunable electronic properties of the trans and cis conformers of 2Cz2TAZ .