Oxygen plasma treatment of TiO2 films has been used to improve the efficiency of dye sensitized solar cells. Both a commercial TiO2 sample and a TiO2 thin film synthesized by a sol-gel technique were treated using a custom built inductively coupled plasma apparatus. X-ray photoelectron spectroscopy revealed that oxygen-plasma treatment increased the number of oxygen functional groups (hydroxyl groups) and introduced some Ti3+ species on the surface of TiO2. A sample solar cell with plasma treated TiO2 showed an overall solar-to-electricity conversion efficiency of 4.3%, about a 13% increase over untreated TiO2. The photon conversion efficiency for the plasma treated TiO2 was 34% higher than untreated TiO2. This enhanced cell-performance is partly due to increased dye adsorption from an increase in surface oxygen functional groups and also may be partly due to Ti3+ states on the surface of TiO2. (C) 2013 Elsevier B.V. All rights reserved.