The effect of the iodide/triiodide redox electrolyte in various organic solvents on the photoelectrochemical properties of bis(tetrabutylammonium) cis-bis(thiocyanato)bis(4-carboxy-2,2'-bipyridine-4'-carboxylato)ruthenium(II)-sensitized nanocrystalline TiO2 solar cells was studied. Solvents with large donor numbers dramatically enhanced the open-circuit voltage (V-oc), but usually reduced the short-circuit photocurrent density (J(sc)). For a mixed solvent of tetrahydrofuran (THF) and acetonitrile, V-oc increased and the fill factor decreased with increasing THF concentration, but J(sc) remained relatively constant. As the partial charge of the N or O atom of the solvent molecule increased, V-oc increased, but J(sc) was unchanged up to a certain value of the partial charge (for THF, -0.46). For cells using 0.3 M 4-tert-butylpyridine and 20 vol% THF in the electrolyte, a short-circuit photocurrent density of 18.23 mA cm(-2), an open-circuit voltage of 0.73 V, a fill factor of 0.73, and an overall conversion efficiency of 9.74% were obtained. (c) 2005 Elsevier B.V. All rights reserved.