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

Abstract

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 π-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 π-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%.