Effects of the electrolyte of DSCs on impedance spectra were evaluated by changing concentration of redox couple, viscosity, and additives to electrolyte. The relation with current-voltage characteristics (I-V characteristics) was investigated. In many cases, the impedance component attributed to charge transfer at TiO2 vertical bar electrolyte interface demonstrated strong relation with the I-V characteristics. The recombination of electrons in TiO2 with I-3(-) in electrolyte was a key factor in determining performance of DSCs. To evaluate the effect of I-3(-), diffusion-limiting current in the electrolyte for various viscosities was evaluated by cyclic voltammetry. When the short circuit current (SCC) was almost equal to the diffusion-limiting current, strong influence of the diffusion coefficient on the impedance spectra was observed: impedance arcs were enlarged as the diffusion coefficient was decreased. On the other hand, when the diffusion-limiting current was larger than the SCC, photo-excitation and electron injection processes became dominating factors in the DSCs performance. The SCC was regulated by the charge recombination process at TiO2 vertical bar electrolyte interface, and thus the impedance component omega(3) was related to the performance in such condition. (c) 2006 Elsevier Ltd. All rights reserved.