Composites for a photo-active layer in an organic photovoltaic device are fabricated using homogeneously dispersed carbon nanotubes (CNTs) in a polymer:fullerene bulk-heterojunction matrix. CNTs are functionalized by alkyl-amide groups for high dispersion in organic media and by homogeneously mixing them with the polymer and fullerene in solution. In a Fourier transform infrared spectrometry analysis, the formation of functional groups is confirmed. Scanning electron microscopy images indicate the high dispersion of the CNTs. These composites show considerable improvement of their optical and electrical properties due to effects of the wideband photo-absorption and high charge carrier mobility of the CNTs. These effects were investigated by ultraviolet-visible spectroscopy and by the fabrication of an organic thin film transistor. An organic solar cell was fabricated from these composites as a photo-active layer, showing a remarkable 40% (3.2-4.4%) increase of the power conversion efficiency compared to an organic solar cell using a photo-active layer without CNTs. (C) 2011 Elsevier Ltd. All rights reserved.