Poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester bulk hetero-junction photovoltaic cells doped with indigo dye as a p-type solar collector were studied. Doping indigo increases the absorption of the visible light spectrum, as determined using ultraviolet-visible spectrophotometry analysis. The photocurrent was measured in air under a solar simulator with AM 1.5G illumination (100 mW/cm(2)). The best photovoltaic efficiency obtained was 2.55% after doping indigo into the cell. (C) 2012 Elsevier B.V. All rights reserved.