Three novel low-bandgap (LGB) conjugated polythiophenes (PThBTDmCzn) incorporating separate and content-tunable benzothiadiazole and carbazole moieties have been designed and synthesized for application in bulk heterojunction polymer solar cells (PSCs). The absorption spectral, thermal, electrochemical and photovoltaic properties of the random copolymers were investigated. Broad absorption from a single polymer covering the visible region from 300 to 800nm was observed, which was ideal for highly efficient harvesting of the solar spectrum. DSC analysis showed that the polymers readily crystallized, indicating highly ordered intermolecular packing, which is beneficial for efficient charge-carrier transport. Electrochemical studies indicate desirable HOMO/LUMO levels that enable a high open-circuit voltage while blending them with fullerene derivatives as electron acceptors. Polymer solar cells using 1:1 or 1:2wt/wt polymer: PC61BM (methanofullerene [6,6-phenyl C61-butyric acid methyl ester] blends as the photoactive layers were fabricated and characterized. The preliminary investigation on the photovoltaic device of the PThBTDmCzn polymers gave similar power conversion efficiency of 1.1–1.2% with Voc of 0.64–0.68V under simulated solar light AM 1.5 G (100mW/cm2).