Three copolymers of thieno[3,2-b]thiophene (TT) and diketopyrrolo[3,4-c]pyrrole (DPP) with a varying number of interconnecting thiophene units (nT) were synthesized by Suzuki polymerization. The PDPPnT-TT polymers display optical band gaps in the range of 1.33 to 1.52 eV and show ambipolar charge transport when applied in a field-effect transistor (FET) with hole and electron mobilities up to 3 × 10-2 and 2 × 10-3 cm2 V-1 s-1, respectively. Organic solar cells based on the PDPPnT-TT polymers as donors and the fullerene derivative PCBM as acceptor achieve power conversion efficiencies up to 3.4% when the layer is deposited with the use of a co-solvent. The co-solvent strongly affects the morphology of the active layer and improves the performance. For these polymers the LUMO–LUMO offset with PCBM correlates with the photocurrent and the maximum external quantum efficiency.