The optimization of TiO2 compact layer in dye-sensitized solar cell by the analysis of performance and internal impedance

The working electrode of a Dye-sensitized Solar Cell (DSC) is a fluorine-doped tin oxide (FTO) glass substrate pasted with nano-porous TiO2. Under the illumination, dye molecules are excited and electrons are emitted. The electrons pass through two interfaces of dye/TiO2 and TiO2/FTO. Therefore, electron of the TiO2 conduction band combines with I-3-ion of the electrolyte. A compact layer between the FTO and nano-porous TiO2 layer makes adherence of TiO2 to FTO surface improved, and the electron recombination reduced because it blocks the direct contact between the redox electrolyte and the FTO surface. Although some researches have studied the insertion of compact layer so far, only materials and the coating methods were studied. To the best of our knowledge, there was no study of a correlation between thickness and efficiency. Therefore, in this study, we investigated on the change in the output characteristics of DSCs according to thickness of TiO2 compact layer fabricated by coating in TiCl4 aqueous solution. The thickness of compact layer was controlled by repetitive coatings and measured by field emission scanning electron microscopy. The effects in terms of the I-V characteristics and the electrochemical impedance spectroscopy (EIS) were examined for the analysis of the change in the thickness of TiO2 compact layer. As a result, the optimum thickness which has the highest output in the IV characteristics and the smallest area of EIS semi-circle was derived. And the photovoltaic performance of DSC with an optimum compact layer was maximized. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim