New Bipolar Host Materials Based on Indolocarbazole for Red Phosphorescent OLEDs

We designed and synthesized new indolocarbazole-triazine derivatives, 9-di-tert-butyl-5,7-bis(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5,7-dihydroindolo[2,3-b]carbazole (2TRZ-P-ICz) and 3,9-di-tert-butyl-5,7-bis(5'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1':3',1″-terphenyl]-2'-yl)-5,7-dihydroindolo[2,3-b]carbazole (2TRZ-TP-ICz), as new bipolar host materials for red phosphorescent OLEDs. In the film state, 2TRZ-P-ICz and 2TRZ-TP-ICz exhibited photoluminescence maxima at 480 nm and 488 nm, respectively. The dipole moment characteristics of the new compounds under various solvent conditions were investigated using the Lippert-Mataga equation. The results showed that the dipole moment of 2TRZ-P-ICz is 26.9D, while that of 2TRZ-TP-ICz is 21.3D. The delayed fluorescence lifetimes were 0.188 μs for 2TRZ-P-ICz and 2.080 μs for 2TRZ-TP-ICz, with 2TRZ-TP-ICz showing TADF characteristics. Additionally, 2TRZ-TP-ICz was found to have a ΔEST of less than 0.2 eV. The triplet energy levels of the newly synthesized bipolar host materials were found to be 2.72 and 2.75 eV, confirming their suitability for use in red phosphorescent OLEDs. To investigate the carrier mobility of the synthesized materials, hole-only devices and electron-only devices were fabricated and tested. The hole mobility value at 1V was found to be 3.43 × 10^-3 cm²/Vs for 2TRZ-P-ICz and 2.16 × 10^-3 cm²/Vs for 2TRZ-TP-ICz. For electron mobility at 1V, 2TRZ-P-ICz showed a value of 4.41 × 10^-9 cm²/Vs, while 2TRZ-TP-ICz exhibited a value of 9.13 × 10^-9 cm²/Vs. As a result, when the new material was used as a host in red phosphorescent OLEDs, 2TRZ-TP-ICz achieved a current efficiency of 9.92 cd/A, an external quantum efficiency of 13.7%, CIE coordinates of (0.679, 0.319), and an electroluminescence maximum wavelength of 626 nm.