Regulation of the backbone structure and optoelectrical properties of bis-pyridal[2,1,3]thiadiazole-based ambipolar semiconducting polymers via a fluorination strategy

Abstract

Polymer semiconductors with coplanar, π-extended confirmation and high electron affinity are regarded as the promising candidates for high-mobility ambipolar organic field-effect transistors. Herein a highly electron-deficient, coplanar, π-extended bis-pyridal[2,1,3]thiadiazole (BPT) acceptor is embedded into two novel D–A type conjugated polymers (PBPT-TT and PBPT-FTT), in which alkyl-substituted terthiophenes (TT) and alkyl-substituted difluoroterthiophenes (FTT) are used as the donor segments, respectively. Moreover, a facile fluorination strategy is adopted to regulate the backbone coplanarity, optoelectrical properties, and film organization coupled with the charge transport properties of polymers. It is found that, compared with PBPT-TT, the attachment of electron-deficient fluorine substituents to the main chain of PBPT-FTT achieves not only improved electron affinity, but also enhanced backbone coplanarity owing to the formation of the F⋯S noncovalent conformation lock. Such good backbone coplanarity and fluorine substituents endow PBPT-FTT with improved interchain organization ability, thereby achieving a more uniform lamellar structure and a smaller π–π stacking distance than those of PBPT-TT. Benefiting from these merits, PBPT-FTT based organic field-effect transistors exhibit significantly improved ambipolar transport performance relative to PBPT-TT. The highest hole (μ_h) and electron (μ_e) mobilities of PBPT-FTT are determined to be 0.332 and 1.602 cm² V⁻¹ s⁻¹, respectively; both of them are much higher than those of PBPT-TT (μ_h/μ_e = 0.0135/0.0191 cm² V⁻¹ s⁻¹). Our findings suggest that the introduction of accessible fluorine substituents in the polymer main chain is a feasible and effective pathway to enhance the backbone coplanarity, film organization, and charge transport ability.