Synthesis and characterization of low bandgap π-conjugated copolymers incorporating 4,7-bis(3,3′/4,4′-hexylthiophene-2-yl)benzo[c][2,1,3]thiadiazole units for photovoltaic application

4,7-Bis(3,3'/4,4'-hexylthiophene-2-yl)benzo[c][2,1,3]thiadiazoles (HT-BT-HT) were used as building blocks to construct a series of low bandgap pi-conjugated copolymers for photovoltaic applications. The desired copolymers were obtained by incorporating the HT-BT-HT comonomers together with donor or acceptor units, such as 3,4-ethylenedioxythiophene (EDOT), bis-EDOT, thieno[3,4-b]pyrazine (TP), and 2,3-dimethyl-TP, via a palladium-catalyzed Stille cross-coupling method. A facile synthetic method has also been developed for the synthesis of several EDOT- and TP-based copolymers via direct C-H arylation of EDOT, bis-EDOT, and TP derivatives using the commercially available catalyst Pd(OAc)(2) under Heck-type experimental conditions (Jeffery method). For all of the synthesized copolymers, moving the hexyl side chains of the HT unit in the HT-BT-HT comonomers from 3,3'-positions (close to BT, as in P1-P4) to 4,4'-positions (away from BT, as in P5-P8) led to a significant red shift of the UV-vis absorption spectrum, a decrease of the energy bandgap, an increase of the glass transition temperature, and more promising photovoltaic performances. The thin-film copolymer P7 incorporating TP units (-TP-HT-BT-HT-)(n) exhibited the most extended absorption (beyond 1000 nm) and the lowest optical bandgap (1.24 eV) among the synthesized copolymers. According to time-dependent density functional theory calculations, the TP unit, in contrast to EDOT, has its lowest unoccupied molecular orbital (LUMO) at the same level as BT. An extended pi-conjugation along the TP and BT units leads to low-lying LUMO levels of the resulting copolymer P7 and in turn its reduced bandgap. The power conversion efficiencies (PCEs) of organic photovoltaic devices employing copolymers P1-P8 were measured in the configuration of ITO/PEDOT:PSS/copolymer (P1-P8) : PC60BM (1 : 1 w/w)/Al. Copolymer P7 in particular showed the highest PCE of 3.32%.