Copolymers of diketopyrrolopyrrole and thienothiophene for photovoltaic cells

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 [60]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 [60]PCBM correlates with the photocurrent and the maximum external quantum efficiency.