A low-cost phenylbenzoimidazole containing electron transport material for efficient green phosphorescent and thermally activated delayed fluorescent OLEDs

Journal of Materials Chemistry C Pub Date: 2015-04-27 DOI: 10.1039/C5TC00350D

Abstract

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.

Graphical abstract: A low-cost phenylbenzoimidazole containing electron transport material for efficient green phosphorescent and thermally activated delayed fluorescent OLEDs
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