Thienothiophene-2,5-Dione-Based Donor–Acceptor Polymers: Improved Synthesis and Influence of the Donor Units on Ambipolar Charge Transport Properties

A series of donor–acceptor polymers incorporating the thieno[3,2-b]thiophene-2,5-dione (TTD) acceptor unit and different donor units are synthesized. The synthesis of a TTD-based key monomer, 3,6-bis(5-bromo-4-alkylthiophen-2-yl)thieno[3,2-b]thiophene-2,5-dione, is successfully improved to afford higher total yield with less reaction steps (42%/4 steps) than those previously reported (14%/7 steps). The polymers exhibit low-lying lowest unoccupied molecular orbital (LUMO) energy levels of around −3.8 eV and highest occupied molecular orbital (HOMO) energy levels ranging from −5.49 to −5.14 eV. Organic field-effect transistors based on the polymers exhibit ambipolar characteristics with high hole and electron mobilities in the order of 10⁻¹ cm² V⁻¹ s⁻¹ in air. These high mobilities can be attributed to the formation of highly crystalline lamellar structure with preferential edge-on orientation of the polymer thin films. Interestingly, the ratio of the electron to hole mobilities decreases with the extension of the donor units. This can be explained by the distribution of HOMOs and LUMOs along the backbones. Additionally, complementary inverters using the polymers with well-balanced ambipolar characteristics exhibit sharp switching characteristics with high gain of ≈140 at the supply voltage of 40 V.