Low band-gap donor–acceptor copolymers based on dioxocyclopenta[c]thiophene derivatives as acceptor units: synthesis, properties, and photovoltaic performances

New donor-acceptor type copolymers containing dihexyldioxocyclopenta[c]thiophene or (dihexylmethylidene)dioxocyclopenta[c]thiophene as acceptor units have been designed and synthesized for the application as hole-transporting (p-type) organic semiconducting materials in organic photovoltaics (OPVs). The investigation of photophysical and physicochemical properties revealed that these copolymers featured low optical band gaps (1.56-1.73 eV) and low-lying HOMO energy levels (-5.40 to -5.02 eV). Bulk heterojunction OPV devices based on these copolymers and [6,6]-phenyl-C sub(61)-butyric acid methyl ester as active layers showed moderate power conversion efficiencies (PCEs) of between 1.36 and 2.68% under air mass 1.5 simulated solar illumination. Space-charge-limited current measurements and atomic force microscopy measurements of the blend films revealed that both charge-transporting characteristics and film morphologies have significant influences on the photovoltaic performances. OPV devices based on the copolymers with [6,6]-phenyl-C sub(71)-butyric acid methyl ester showed a PCE of up to 5.17% with a short circuit current of 10.1 mA cm super(-2), an open circuit voltage of 0.80 V and a fill factor of 0.64.