Increases in photovoltaic performances for dye-sensitized solar cells having ionic liquid type electrolytes are reported. These results are explained by diffusion coefficient for I-3(-), charge transfer resistances on counter electrodes, flat band potentials of TiO2, redox potentials for I-/I-3(-), electron diffusion constants, electron life time, and diffusion length in TiO2 layers. Methylpropylimidazolium iodide is selected because of the lowest viscosity and the highest conductivity. Increases in the photovoltaic performance are observed when a small amount of water was added into the ionic liquid consisting of both LiI and t-butylpyridine as the additives. These improvements are brought about by enhancements of all of J(sc), ff and V-oc. The increases in J(sc) and ff are associated with the decrease in charge transfer resistances on counter electrodes and increases in ionic conductivities. V-oc may be explained by an increase in the difference between redox potentials of I-/I-3(-) and Fermi level. (C) 2004 Elsevier. All rights reserved.