Effects of the alkyl chain length of imidazolium iodide in the electrolyte solution on the performance of black-dye-based dye-sensitized solar cells

Five kinds of electrolyte solutions containing the imidazolium iodide with a different alkyl chain length (DMImI, EMImI, MPImI, DMPImI and HMImI; DMImI = 1,3-dimethylimidazolium iodide, EMImI = 1-ethyl-3-methylimidazolium iodide, MPImI = 1-methyl-3-n-propylimidazolium iodide, DMPImI = 1,2-dimethyl-3-n-propylimidazolium iodide, HMImI = 1-n-hexyl-3-methylimidazolium iodide) have been prepared to investigate the effects of the alkyl chain length of the imidazolium iodide on the performance of dye-sensitized solar cells (DSCs) with Black dye and D131. The photocurrent density (Jsc) increased and the fill factor (FF) decreased with increasing the alkyl chain length of the imidazolium iodide. Electrochemical impedance spectroscopic measurements of the DSCs revealed that the conduction band energy of TiO₂ shifted to the positive direction and the diffusion resistance of the redox couple in the electrolyte solution increased with increasing the alkyl chain length of the imidazolium iodide. Therefore, the observed increment of the Jsc value seems to be attributed mainly to the conduction band energy shift of TiO₂, and the observed decrement of the FF value would be caused by increasing the diffusion resistance of the redox couple in the electrolyte solution.The conversion efficiency of the DSCs was found to depend on the ion conductivity of the electrolyte solution, which is closely related to the alkyl chain length of the imidazolium iodide. This study demonstrated that the conversion efficiency could be improved by using EMImI instead of the conventional imidazolium iodide (DMPImI) due to the larger ion conductivity with a slightly higher conduction band energy of TiO₂.