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

摘要

Direct arylation was used to synthesize a series of novel terthiophene (T3)-based D–A _(1) –D–A _(2) polymers and D–A _(2) –D monomers in fewer synthetic steps. In these T3-based D–A _(1) –D–A _(2) polymers, pyrrolo[3,4- c ]pyrrole-1,4-dione (DPP) was selected as the first acceptor A _(1) , 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 A _(2) . 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 (A _(2) ), 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:PC _(71) BM morphology and PSC performances between the T2-based and T3-based D–A _(1) –D–A _(2) polymers. Varying the secondary acceptor (A _(2) ) from BT to TPD also promotes the crystallinity and backbone planarity leading to enhanced PSC performances of the T3-based D–A _(1) –D–A _(2) 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:PC _(71) BM 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%.