It has been shown recently, that the presence of alkyl side chains at the 3-positions on the thiophene rings placed next to 2,1,3-benzothiadiazole core in the backbone of several conjugated polymers results in severe steric hindrance and prevents efficient planarity of the thiophene-2,1,3-benzothiadiazole-thiophene (TBzT) segment. Both properties have a strong influence on the optoelectronic properties of the polymer and need to be considered when the polymer is to be used for organic electronics applications. In this work, we modified a previously synthesized oligothiophene copolymer, consisting of two 3,4'-dialkyl-2,2'-bithiophene units attached to a 2,1,3-benzothiadiazole unit (TBzT segment) and a thieno[3,2-b]thiophene unit, by optimizing the lateral alkyl side chains following a density functional theory investigation. It is demonstrated that eliminating the alkyl side chains from the 3-positions of the TBzT segment and anchoring them onto the thieno[3,2-b]thiophene, using an efficient synthesis of the 3,6-dihexylthieno[3,2-b]thiophene unit, allows us to reduce the energy band gap. In addition, the chemical modification leads to a better charge transport and to an enhanced photovoltaic efficiency of polymer/fullerene blends. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012