Oxygen plasma and ion-beam treatments of nano-structured TiO2 were carried out to enhance the efficiency of dye-sensitized solar cells (DSSCs). Relative changes in surface states and recombination reactions were examined. Comparative studies showed that the oxygen ion-beam treatment was more effective in reducing the number of oxygen vacancies in TiO2 and the recombination reactions between TiO2 conduction band electrons and I-3 ions in the electrolyte than the plasma treatment. Oxygen ion-beam treated TiO2 film in the DSSC showed a 23 mV increase in the open-circuit voltage (V-oc) and an improvement in the photo-conversion efficiency from 5.05% to 6.64%. (C) 2010 Elsevier B.V. All rights reserved.