The effects of nitrogen ion bombardment on TiO2 films prepared by the Cat-CVD method have been studied to improve the optical and electrical properties of the material for use in Si thin film solar cells. The refractive index n and the dark conductivity of the TiO2 film increased with irradiation time. The refractive index n of the TiO2 film was changed from 2.1 to 2.4 and the electrical conductivity was improved from 3.4 x 10(-2) to 1.2 x 10(-1) S/cm by the irradiation. These results are due to the formation of Ti-N bonds and oxygen vacancies in the film. (c) 2007 Elsevier B.V. All fights reserved.