A regioregular conjugated oligomer (MF) and its polymer counterpart (PF) containing [3,4-b] thiophene moiety have been developed. The existence of thieno[3,4-b]thiophene extends the absorption of the molecules to longer wavelengths and increases the current density of solar cell devices using these materials. The regioregularity of the polymers from the incorporation of regioregular oligothiophene fragments also enhances hole mobility. Consequently, the polymers show higher solar energy conversion efficiencies in bulk heterejunction (BHJ) solar cells than the low molecular weight oligomers. Spectroscopic and Structural Studies reveal that composite films prepared from the polymer exhibit a larger charge carrier density and smaller domain sizes for the electron donor and acceptor than the oligomer counterpart. These results rationalize the origin for the higher solar cell efficiency.