A series of donor-acceptor low band gap polymers (DINIBT-C8, DINIBT-C10 and DINIBT-EH) containing 5,10-dihydroindolo[3,2-b]indole (DINI) as an electron donating unit and benzothiadiazole (BT) as an electron accepting unit with various kinds of alkyl chains (octyl; C8, decyl; C10 and ethyl hexyl; EH) on the donor unit have been designed, synthesized, and characterized for bulk heterojunction polymer solar cell applications. Due to the presence of long alkyl chains on the donor unit, all the polymers showing good solubility in common organic solvents. The frontier orbital energy levels of polymers were calculated using cyclic voltametry analysis. The influence of alkyl chains on the photo-physical properties of polymers were carried out. Among the three polymers, the DINIBT-C8 shows higher absorption coverage and good energy level matching with acceptor PC71BM than that of the other two polymers. Thus, the bulk-heterojunction polymer solar cells (BHJ PSCs) were fabricated using DINIBT-C8:PC71BM as an active layer with the configuration of ITO/PEDOT/ DINIBT-C8:PC71BM/LiF/Al. The DINIBT-C8 shows the maximum PCE of 0.16%.