Energy conversion efficiency of the dye-sensitized solar cell is improved from 3.5% to 4.5% by adding a small amount of CuI into an ionic liquid electrolyte. It is found that other copper-I salts, for example, CuBr, have the same effect for the dye-sensitized solar cell. Experimental results show that no Cu2+ ions exist in this electrolyte.It is suggested that this improvement is caused by the adsorption of Cu+ onto the TiO2 porous film.