A series of fluorene-thiophene-based pi-conjugated copolymers, PFT2, PFT4, and PFT2BP, have been synthesized using the Suzuki coupling reaction. Compared to PFT2 and PFT2BP, PFT4 film showed clearly red-shifted UV-visible absorption and photoluminescence (PL) spectra with maxima at 474 and 597 nm, respectively. The optical and electrochemical properties of copolymers could be tuned by changing conjugation lengths of comonomer units from bithiophene (PFT2) to quaterthiophene (PFT4). Moreover, PFT4 containing quaterthiophene showed relatively lower optical band gap of 2.16 eV. The organic photovoltaic cells (OPVs) were fabricated using the copolymers as donor with a sandwich configuration of ITO/PEDOT:PSS/copolymer:PC71BM/LiF/Al. PFT4 showed the best device performance with the optimized power conversion efficiency (PCE) of 2.26% (J(SC) = 7.67 mA/cm2, V(OC) = 0.81 V, FF = 0.36). The higher OPV performance of PFT4 can be explained by its relatively low energy band gap and improved UV absorption property.