Introducing alkylthio side chains into acceptor units to improve the photovoltaic performance of a quinoxaline based D-A polymer

Two new donor-acceptor (D-A) polymers, namely PNDT-QX and PNDT-QXS, with naphtho [l,2-b:5,6-b']dithiophenes (NDT) as donor unit and quinoxaline (QX) as acceptor unit, were designed and synthesized. The difference in the structures of the two polymers is that, in PNDT-QXS the side chains on the QX acceptor units are (2-ethylhexyl) thio thienyl groups, whereas in PNDT-QX the side chains are 2-ethylhexyl thienyl groups, which allow us to systematically compare the effects of alkylthio group on optical, electrochemical, molecular packing properties and PSCs performance of polymers. Our study demonstrated that, relative to alkyl group, alkylthio substituents tune down the HOMO and LUMO energy levels, red-shift and broaden the absorption, and enhance the interchain interaction, therefore significantly affect the BHJ morphology of the copolymer with PC71BM. As a result, the polymer solar cell based on the blend of PNDT-QXS and PC71BM got a highest PCE of 5.03% without using extra post-treatment steps, whereas the PNDT-QX cell only had a 1.93% highest PCE. The results prove for the first time that introducing alkylthio side chains into acceptor moiety would also be an effective strategy for improving the photovoltaic performance of the D-A copolymers.