Regulation of Backbone Structure and Optoelectrical Properties of Bis-Pyridal[2,1,3]thiadiazole-Based Ambipolar Semiconducting Polymers via a Fluorination Strategy

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 the 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, film organization coupled with the charge transport properties of the polymers. It is found that, compared with PBPT-TT, attachment of electron-deficient fluorine substituents in the main chain of PBPT-FTT achieves not only the improved electron affinity, but also the enhanced backbone coplanarity owing to the formation of 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 as relative to PBPT-TT. The highest hole (μh) and electron (μe) mobilities of PBPT-FTT are determined as 0.332 and 1.602 cm2 V–1 s–1, respectively, both of them are much higher than those of PBPT-TT (μh/μe = 0.135/0.191 cm2 V–1 s–1). Our findings suggest that the introduction of the 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.