Multi-walled carbon nanotubes (MWCNTs) placed under nitrogen (N(2)) and argon (Ar) microwave plasma in order to functionalize covalently their side walls with nitrogen containing groups. X-ray photoelectron spectroscopy (XPS) study shows surface modification of the MWCNTs with imine, amine, nitride and amide groups grafted on the side walls. Due to the functional groups.. homogenous distribution of MWCNTs in solvent could be obtained. For photovoltaic device fabrication MWCNTs film was casted over n-Si wafer and poly(3-octylthiophene) solution was infiltered. Devices with functionalized MWCNTs show short circuit current density (J(sc)), open circuit voltage (V(oc)), fill factor (FF) and power conversion efficiency (eta) as 1.8 mA/cm(2), 0.20 V, 24% and 0.086%, respectively. In the composite film functionalized MWCNTs facilitate photo induced charge separation and efficient holes transportation, suppressing recombination of photo generated charges. (C) 2008 Elsevier B.V. All rights reserved.