Investigating the Role of I2SCN− on the Fermi Level of Electrolyte for Dye-Sensitized Solar Cells

We investigated the electrochemical effect of the formation of a halogen-pseudohalogen complex ion, I2SCN− (diiodothiocyanate), on the Fermi level of the conventional I−/I3−-based electrolyte when thiocyanate was added as an additive into the electrolyte of dye-sensitized solar cells. It was found that the formation of I2SCN− induced a negative shift in the Fermi level of the electrolyte (positive in potential scale), resulting in a net enhancement of the open circuit voltage (Voc), up to ca. 30 mV, for the cells with 1-ethyl-3-methylimidazolium thiocyanate as additive. A significant negative shift of the conduction band edge of TiO2, up to ca. 115 meV, upon adsorption of guanidinium cation on TiO2 surface was almost counterbalanced by the retardation of charge recombination at the TiO2|electrolyte interface due to the adsorbed cation itself and therefore, the net enhancement of the Voc was mostly associated with the formation of the new redox level represented by (I−, SCN−)/I2SCN− with a standard electrochemical potential more positive than that of I−/I3−.