Well-aligned undoped and Co-doped nanorod ZnO films were grown by electrochemical deposition onto p-Si substrates from an aqueous route. Aqueous solution of Zn(NO3)(2)center dot 6H(2)O and hexamethylenetetramine (HMT) were prepared using triple distilled water. Two different atomic ratios of Co(NO3)(2)center dot 6H(2)O were used as a dopant element. Electrodepositions were carried out in a conventional three electrode cell for the working electrode (p-Si), reference electrode (Ag/AgCl, sat.) and counter electrode (platin wire). The effects of Co doping on the structural, morphological and electrical properties of ZnO films were investigated. X-ray diffraction (XRD) measurement showed that the undoped ZnO nanorod film was crystallized in the hexagonal wurtzite phase and presented a preferential orientation along the c-axis. Only one peak, corresponding to the (002) phase, appeared on the diffractograms. The lattice parameters and texture coefficient values were calculated. The nanorods were confirmed by the field emission scanning electron microscopy (FE-SEM) measurements. The FE-SEM image showed that the ZnO nanorods grow uniformly on the substrates, providing a surface with fairly homogeneous roughness. The surface morphology was transformed into uniform multi-oriented rods with incorporation of Co. Co-doped ZnO nanorod films showed a multi-oriented spear-like structure. The diffuse reflectance spectra of the films were measured and the optical band gap values were determined using Kubelka-Munk theory. The van der Pauw method was used to measure the sheet resistance of the films. The sheet resistance was affected significantly by Co content. The pn heterojunction diodes were fabricated and the diode parameters were determined from the analysis of the measured dark current-voltage curves. Rectifying behavior was observed from the I-V characteristics of these heterojunction diodes. (C) 2013 Elsevier B. V. All rights reserved.