Oxygen ion-beam irradiation of TiO 2 films reduces oxygen vacancies and improves performance of dye-sensitized solar cel
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Md. Khaled Parvez Department of Polymer Science and Engineering, Chungju National University, Chungju 380-702, Republic of Korea; and Synthetic Chemistry Group, Max Planck Institute for Polymer Research, D-55021 Mainz, Germany (Received 25 October 2010; accepted 21 January 2011)
Oxygen vacancy-related surface defects on porous TiO2 films were reduced by oxygen ion-beam treatment, and the effect of such defects on the performance of dye-sensitized solar cells was examined. An oxygen ion-beam treatment of a TiO2 film caused a significant decrease in particle agglomeration and an increase in surface area of the resulting TiO2 film. In addition, the increased hydrophilicity of the TiO2 film by the ion beam treatment led to an increase in dye adsorption. The oxygen ion beam treatment at 500 and 1000 eV caused a significant decrease in oxygen vacancies and increase in the open-circuit voltage (Voc). Oxygen ion beam–treated TiO2 film electrodes showed the maximum solar-to-electricity conversion efficiency (g%) of 8.04% compared to the 6.15% obtained from an untreated TiO2 electrode.
I. INTRODUCTION
The dye-sensitized solar cell (DSSC) is a promising candidate for low-cost photovoltaic devices. Porous TiO2 film and its huge internal surface area play an important role in both charge separation and transport, and the efficiency of DSSCs is strongly dependent on the existence of surface defects on the porous TiO2 network.1 Long relaxation times and charging effects provide evidence for the existence of a large number of surface defects in the TiO2/electrolyte system.2,3 Because of the large surface-to-volume ratio, these surface defects are expected to affect the cell performance significantly. Although TiO2 has high lattice energy, a photoelectron-conducting TiO2 film usually suffers from small oxygen loss,4 which results in the formation of dominant surface defect states. These surface defect states (also known as oxygen vacancies) are located in the interband region and cause slow charge transport in a TiO2 film owing to the trapping and detrapping of electrons.5 Oxygen vacancies, which act as electron scavengers, facilitate the recombination of injected conduction-band electrons with triiodide ions (I3 ) in the electrolyte. Interband surface defects can also increase the amount of light absorption in the visible region. The oxygen vacancy states exist at approximately 0.75–1.18 eV below the conduction band edge of TiO26 and can be excited from the valence band by visible wavelengths of ;506 and 614 nm. However, the optical a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.34 1012
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
http://journals.cambridge.org
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absorption by these surface states does not contribute to the photocurrent in a DSSC. Therefore, to reduce the recombination of injected electrons and the light absorption by the surface states, it is essential to reduce the number of oxygen vacancies. It was known that the oxygen vacancies in TiO2 can be reduced
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