Highly efficient photovoltaic devices, constructed using polythiophene derivatives blended with fullerene, are described. The photovoltaic behavior of the sandwich-structure devices (current-voltage characteristic, conversion efficiency) is reported and the electrical, optical (optical absorption, photoluminescence), and morphological properties of the blends are presented. The importance of the morphology of the films is pointed out: it can determine the photovoltaic properties of the devices.