Synthesis and optoelectronic properties of new D–A copolymers based on fluorinated benzothiadiazole and benzoselenadiazole

Two donor–acceptor (D–A) copolymers (PBDTBT and PBDTBSe) based on bithienyl-benzodithiophene (BDT) as the donor (D) unit, fluorinated benzothiadiazole (BT) or benzoselenadiazole (BSe) as the acceptor (A) unit, were designed and synthesized. The optoelectronic properties of the two polymers, including the absorption spectra, electronic energy levels, hole mobility and photovoltaic properties, were fully studied and compared. The effects of the replacement of S by Se in the BT unit and F substitution on their optoelectronic properties were investigated in detail. The replacement of S by Se in the benzothiadiazole unit red-shifted the absorption spectra and increased the hole mobility of the D–A copolymers. Polymer solar cells (PSCs) based on the polymers as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) as the acceptor were fabricated, and the photovoltaic performance of the PSCs was optimized by optimizing the device fabrication conditions, including the solvent, donor–acceptor blend ratio, thermal annealing and solvent additive. The highest power conversion efficiency (PCE) of the optimized PSCs based on the polymers reached 5.06% and 2.20% for PBDTBT and PBDTBSe, respectively. The lower photovoltaic performance of the PSCs based on PBDTBSe is due to the poorer morphology of its active layer.