Simple synthesis of novel terthiophene-based D–A1–D–A2 polymers for polymer solar cells

Abstract

Direct arylation was used to synthesize a series of novel terthiophene (T3)-based D–A₁–D–A₂ polymers and D–A₂–D monomers in fewer synthetic steps. In these T3-based D–A₁–D–A₂ polymers, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) was selected as the first acceptor A₁, octyl-thieno[3,4-c]pyrrole-4,6-dione (TPD) or 2,1,3-benzothiadiazole (BT) or fluorinated benzothiadiazole (FBT) was selected as the secondary acceptor A₂. T2-based polymer with the bithiophene segments (T2) as the donor was synthesized for comparison, too. UV-vis absorption, electrochemical properties, blend film morphology, and photovoltaic properties of the polymers were studied to explore the effects of the oligothiophene unit and secondary acceptor moiety (A₂), meanwhile, the fluorine substitution effect was also discussed. It is shown that the change of donor segment from T2 to T3 introduces a difference in the energy levels, crystallinity, polymer:PC₇₁BM morphology and PSC performances between the T2-based and T3-based D–A₁–D–A₂ polymers. Varying the secondary acceptor (A₂) from BT to TPD also promotes the crystallinity and backbone planarity leading to enhanced PSC performances of the T3-based D–A₁–D–A₂ polymer. Although the effectiveness of fluorine substitution for tuning the UV-vis absorption, energy levels and degree of crystallinity has been demonstrated, the insufficient E^DONOR_LUMO − E^PCBM_LUMO energy offset and poor miscibility of polymer:PC₇₁BM limit the short circuit current (J_sc). In addition, the highest J_sc of 12.98 mA cm⁻² is achieved for P1, while the higher HOMO level limits the open circuit voltage (V_oc) and leads to a power conversion efficiency (PCE) of 4.36%.