2,2′-Bis(trifluoromethyl)biphenyl as a building block for highly ambient-stable, amorphous organic field-effect transistors with balanced ambipolarity

Abstract

Novel conjugated polymers, PBPV–TPA, PBPV–CBZ, PBPV–MEH and PBPV–FLO, consisting of trifluoromethyl-substituted biphenyl (2,2′-bis(trifluoromethyl)biphenyl, CF₃-BP) as the acceptor with various aromatic donors (triphenylamine TPA, carbazole CBZ, 2-(2-ethylhexyloxy)-5-methoxybenzene MEH and fluorene FLO, respectively) have been successfully synthesized. Their amorphous features, confirmed by wide-angle X-ray scattering, small-angle X-ray scattering and atomic force microscopy, are attributed to lateral side chains on the CBZ, MEH and FLO units, and the propeller geometry of the TPA unit. By integration of CF₃-BPs into the polymer backbone, the ambipolarities and well-balanced charge mobilities of OFETs based on these polymers were successfully demonstrated. OFETs based on PBPV–FLO exhibit the highest hole mobility of 0.0152 cm² V⁻¹ s⁻¹ and electron mobility of 0.0120 cm² V⁻¹ s⁻¹. In addition, these OFETs also exhibit annealing-free characteristics and ambient stability. The OFET performance without encapsulation remained nearly unchanged in ambient conditions for up to 90 days. This could be attributed to the enhanced oxidative stability from their relatively deep HOMO energy levels and better moisture resistance from the trifluoromethyl substitution. Considering the ambipolar, annealing-free and ambient-stable properties of these CF₃-BP-based amorphous conjugated polymers, the electron-accepting CF₃-BP unit can be considered as a promising building block in preparing the easily processable conjugated polymers used in high-stability optoelectronic applications.