Three semiconducting polymers containing benzodithiophene with decylphenylethynyl substituents have been synthesized for which the spacing between alkyl substituents has been systematically increased by introduction of thiophene and bithiophene units in the backbone of the polymer. As the spacing between the side chains increased, an increase of the weight ratio of polymer to fullerene acceptor was required for achieving the highest power conversion efficiency of bulk heterojunction solar cells. The spacing between the side chains of the polymers was estimated from DFT calculations. The TMAFM analysis of polymer PCBM blends showed that the smooth films exhibited the highest solar cell performance.