Bis(imidazolium) iodides (bis-Im(+)I(-)s) are synthesized with different substituents and used as electrolytes in dye-sensitized solar cells (DSSCs). Three kinds of low-volatility electrolytes are prepared by using 1,1'-methylene bis(3-imidazolium) diiodide (MIDI), 1,1'-methylene bis(3-n-methylimidazolium) diiodide (MMIDI), and 1,1'-methylene-bis(3-n-ethylimidazolium) diiodide (MEIDI) as the iodide sources. The effects of these substituents on the photovoltaic performance of the cells are investigated. It is found that the device shows a lower short-circuit photocurrent (Jsc), higher open-voltage (Voc) and fill factor (FF) with the increased cation size in electrolyte. These results are explained by electrostatic interactions between the solvated Im(+) and the negatively charged species. Meanwhile, the explanation is supported by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), open circuit voltage decay (OCVD), and dark current measurements.