Effects of substituents of imidazolium cations on the performance of dye-sensitized TiO2 solar cells

Imidazolium iodides (Im+I−s) were synthesized with different substituents of the cation and used as electrolytes in dye-sensitized solar cells (DSSCs), and the effects of such substituents were investigated in terms of the photovoltaic performance of the cells. Synthesized iodides were verified by 1H-NMR. Among the iodides, 1,3-diethylimidazolium iodide enabled a solar energy conversion efficiency of 4.8% for its DSSC, while 1-(4-acetophenyl)-3-ethylimidazolium iodide rendered an efficiency of 3.1% for its cell. In all cases the short-circuit photocurrent (J sc) was found to increase with decrease in size of the substituent, which was also verified to be valid in the case of a quasi-solid state DSSC. Results are explained by the electrostatic interactions between solvated Im+ and negatively charged species based on the correlation between diffusion coefficients of I− and I 3 − and J sc values. These explanations are supported by steady-state voltammetry and electrochemical impedance spectroscopy (EIS).