The design of novel low-band-gap conjugated polymers with appropriate frontier orbital energy levels and good charge transport is needed to improve the conversion efficiency of organic photovoltaic devices. In this article, we describe the synthesis and structure property relationships of a series of photovoltaic copolymers with a common conjugated backbone and differing solubilizing side chains. The copolymer optoelectronic properties and the related photovoltaic device performances are reported. Our results clearly show that the side chains have a major impact on the material and device properties. The electronic band gap can be varied by more than 0.3 eV, the charge mobilities by orders of magnitude, and the optimized fullerene content of photovoltaic devices by a factor of 4 by barely changing the side-chain positioning and/or by switching from linear to branched alkyl chains. A power conversion efficiency of 2.7% could be achieved with devices using the most promising polymer.