The preparation and characteristics of flexible indium tin oxide (ITO) electrodes grown on polyethylene terephthalate (PET) substrates using a specially designed roll-to-roll sputtering system for use in flexible organic solar cells are described. It was found that both electrical and optical properties of the flexible ITO electrode were critically dependent on the Ar/O(2) flow ratio in the continuous roll-to-roll sputter process. In spite of the low substrate temperature (<50 degrees C), we can obtain the flexible ITO electrode with a sheet resistance of 47.4 Omega/square and an average optical transmittance of 83.46% in the green region of 500-550 nm wavelength. Both X-ray diffraction and field emission scanning electron microscopy analysis results showed that all flexible ITO electrodes grown on the PET substrate were amorphous with a very smooth and featureless surface, regardless of the Ar/O(2) flow ratio due to the low substrate temperature, which is maintained by a cooling drum. In addition, the flexible ITO electrode grown on the At ion-beam-treated PET substrates showed more stable mechanical proper-ties than the flexible ITO electrode grown on the wet-cleaned PET substrates, due to an increased adhesion between the flexible ITO and the PET substrates. Furthermore, the flexible organic solar cell fabricated on the roll-to-roll sputter-grown flexible ITO electrode at an optimized condition exhibited a power conversion efficiency of 1.88%. This indicates that the roll-to-roll sputtering technique is a promising continuous sputtering process in preparing flexible transparent electrodes for flexible solar cells or displays. (C) 2009 Elsevier B.V. All rights reserved.