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AC impedance measurement was found as an efficient tool to elucidate the charge storage and transfer in dye-sensitized solar cells. The imaginary impedance of the apparent space charge region at the dye adsorbed TiO2/electrolyte interface was inversely dependent on the density of photoexcited electrons. For a given TiO2 material, the thicker the thickness of the substrate, the higher the charge storage and the slower the charge transfer of the resulting DSC. Accordingly, the existence of an optimized thickness of TiO2 substrate is inherent for the highest overall conversion efficiency of a DSC fabricated with a given TiO2 material. The combination of the charge storage or the density of photoexcited electrons in the porous TiO2 film and the prompt charge transfer between it and TiO2/TCO glass interface was worthwhile to note for improvement on the overall conversion efficiency of DSC.