Thiophene-containing polymers blended with fullerenes have recently demonstrated impressively high photovoltaic efficiencies. One drawback of this class of polymers is their relatively low ionization potential, which leads to rather low open-circuit voltages. Polyterthiophenes belong to a material class that has recently captured a large amount of interest for polymer electronic applications because of its excellent transport properties. Because of the slightly lower ionization potential, this material class appears more attractive for photovoltaic applications than polythiophenes. In this work, the photovoltaic performance of bulk heterojunction solar cells from polyterthiophene/fullerene composites is discussed and compared to the polymer/fullerene blend morphology. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.