Structural tuning of quinoxaline-benzodithiophene copolymers via alkyl side chain manipulation: synthesis, characterization and photovoltaic properties

Abstract

We report here the synthesis and characterization of two novel semiconducting quinoxaline (FQ)-benzodithiophene (BDT) based copolymers, PFQBDT-TR₁ and PFQBDT-T2R₂, in which the BDT unit is substituted with either 2-octylthienyl (-TR₁) or 2,3-dihexylthienyl (-T2R₂), respectively, as side groups. The effect of the alkyl side chain(s), linked to the thienyl side groups, on the optical, electronic and morphological properties of the resulting polymers is investigated and correlated with the photovoltaic performance. Solution-processed BHJ solar cells, using these copolymers as electron donor materials and PC₆₁BM (or PC₇₁BM) as an electron acceptor counterpart, are prepared by a blade-coating technique under ambient conditions. As a result, power conversion efficiencies (PCEs) of ∼5.7% and ∼3.4% have been achieved for PFQBDT-TR₁ and PFQBDT-T2R₂ based devices, respectively, highlighting the crucial role of the alkyl portion of the π-conjugated side segment in the optoelectronic properties of this class of copolymers.