Polymer/fullerene bulk heterojunctions have recently generated a lot of scientific interest due to their potential in low-cost photovoltaic applications. In this paper we detail the synthesis and characterization of two new low-band-gap polythiophenes, poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-4,7-bis(2-thienyl)-2,1,3-benzothiadiazole-5′,5′′-diyl] (PCPDTTBTT) and poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,3-dioctylquinoxaline-5,8-diyl] (PCPDTQ), for use in these applications. The PCPDTQ polymer did not produce efficient solar cells. A high power efficiency of 2.1% under one sun was found for a PCPDTTBTT/fullerene mixture. The high efficiency was achieved by alteration of the morphology using a solvent additive. Analysis of atomic force microscopy phase images shows that material phases with distinct mixing ratios are formed and altered with the addition of the solvent additive.