A quasi-solid-state, dye-sensitized photoelectrochemical solar cell employing a gel electrolyte obtained by sol-gel chemistry is described. The gel electrolyte is based on a ureasil precursor (i.e., a poly(propylene oxide) oligomer end-capped by triethoxysilane groups through urea bridges) and sulfolane and it incorporates the I-3(-)/I-redox couple. It is shown that the combination of these two reagents prevents crystallization of KI, thus ensuring a long life for the cell and a satisfactory overall efficiency that surpasses 5%. Cell efficiency increases with temperature. Optimization of gel-electrolyte performance has been obtained by studying mobility with fluorescence-quenching techniques complemented by direct-current conductivity measurements.