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A new low band gap copolymer containing dialkylfluorene and 4,7-dithienyl-2,1,3-benzothiadiazole (TBT), poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PF-TBT) was synthesized by Heck cross-coupling polymerization. The copolymer is soluble in common organic solvents such as chloroform, tetrahydrofuran and chlorobenzene. The TGA result indicated that the copolymer possesses good thermal stability. The absorption, electrochemical and photovoltaic properties of PF-TBT were investigated and compared with those of poly(fluorenevinylene-alt-4,7-diphenyl-2,1,3-benzothiadiazole) (PF-DBT) whose structure is similar to PF-TBT. The copolymer exhibited a broad absorption band with an absorption edge close to 700 nm and an optical band gap of 1.82 eV. Cyclic voltammetry studies indicated that the relatively low HOMO energy level assured a higher open circuit voltage (Voc) when PF-TBT is used as the donor material in a photovoltaic cell. The bulk heterojunction (BHJ) solar cell using PF-TBT as the donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor with the structure of ITO/PEDOT : PSS/copolymer : PCBM/LiF/Al, exhibited a Vocof 0.86 V, short-circuit current density (Jsc) of 3.97 mA cm−2, fill factor (FF) of 0.35, and a power conversion efficiency (PCE) of 1.18% under one sun of AM 1.5 solar simulator illumination (100 mW cm−2). [ABSTRACT FROM AUTHOR]