A low-cost phenylbenzoimidazole containing electron transport material for efficient green phosphorescent and thermally activated delayed fluorescent OLEDs
A new phenylbenzoimidazole-based electron-transport material (ETM), 2,4,6-tris(2-phenyl-1H-benzo[d]imidazol-1-yl)benzonitrile (iTPBI-CN), is designed and synthesized through a simple low-cost one-step C–N coupling reaction by using 2,4,6,-trifluorobenzonitrile and 2-phenyl-1H-benzo[d]imidazole as the starting materials. In comparison with the four step synthesis of commercial ETM of 2,2,2-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TBPI), the introduction of a cyano moiety into iTPBI-CN greatly simplifies the synthetic procedure and allows for an isomerized linkage of phenylbenzoimidazole. The glass transition temperature increases from 124 °C of TPBI to 139 °C of iTPBI-CN. Both compounds show similar HOMO levels of ∼5.9 eV and a triplet energy of ∼2.6 eV. The deeper LUMO level of iTPBI-CN (2.79 eV) than TPBI (2.38 eV) allows for more efficient electron-injection and a much higher device efficiency. Solution-processed green phosphorescent OLEDs with the structure of ITO/PEDOT:PSS/host:Ir(mppy)3/iTPBI-CN versus TPBI/LiF/Al show maximum current and power efficiencies of 37.7 cd A−1 and 29.0 lm W−1versus 26.1 cd A−1, 12.2 lm W−1 in the CBP host and 31.3 cd A−1 and 23.9 lm W−1versus 20.6 cd A−1, 7.4 lm W−1 in the mCP host. Furthermore, the superior device performance of iTPBI-CN over TPBI is also found in both CBP and mCP hosted green thermally activated delayed fluorescence (TADF) devices by using 2,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy) as a dopant.