A new naphthodifuran (NDF) building block has been designed and synthesized. The distinguishing feature of the monomer is two dialkyl substituted thieno groups which are placed in the middle of the ring-fused structure as solubilizing groups. The steric interactions between repeating units in the polymer can be reduced by placing the solubilising groups in the middle of the ring-fused structure. Those dialkyl substituted thieno groups not only improved the solubility of the polymer but also provided additional conjugation in the second dimension. The new monomer (NDFT) was applied to the synthesis of the conjugated polymer (PNDFT-DTBT). The photovoltaic properties of the polymer were evaluated in bulk hetero-junction polymer solar cell devices with PC61BM as acceptors under AM 1.5G illumination at 100 mW cm−2. The device showed an open circuit voltage (Voc) of 0.89 V, a fill factor (FF) of 0.72, a short-circuit current density (Jsc) of 8.21 mA cm−2, and a power conversion efficiency (PCE) of 5.22%.