A novel benzothienothiophene isoindigo (BTTIID) acceptor unit was designed and synthesized by extending the conjugated system along the isoindigo. This new building block was endowed with greatly high molar extinction coefficient in the visible region due to the extended pi conjugated length. Donor-Acceptors polymers based on BTTIID as acceptor unit were designed and synthesized for photovoltic application. These Polymers exhibited a narrow bandgap of about 1.56 eV and a relative deep HOMO energy level, solar cells based on these polymers showed high power conversion efficiency. High photo-current of 17.08 mA/cm2 and PCE of 8.72 % were obtained in a single junction polymer solar cell device without any additive for one of the polymers.