In this paper, we have synthesized a series of carbazole-based copolymers, poly(N-9 ''-heptadecanyl-2,7-carbazole-alt-bithiophene) (PCzT2),poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5'-bis(3'-hexyl-2 '' thienyl)-2,2'-bithiophenyl-5 ''-5 ''-diyl] (PCzT4), and poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2'-,1',3'-benzothiadiazole)] (PCz3TB), by Suzuki coupling polymerization. The optical and electrochemical properties of copolymers were effectively tuned by changing conjugation lengths of comonomer units from bithiophene to quaterthiophene or by introducing the electron-accepting benzothiadiazole into the polymer main chain. Finally, the organic photovoltaic cells (OPVs) were fabricated by blending of the synthesized polymers as donor and PCBM as acceptor, and the photovoltaic properties of the copolymers such as a power conversion efficiency (PCE), a short-circuit current (J(SC)), an open-circuit voltage (V-OC), and a fill factor (FF) were evaluated. The relationship between molecular structures and device performances could be explained on the basis of the optical and electrochemical properties of the copolymers.