Investigation of the conduction properties of ionic liquid crystal electrolyte used in dye sensitized solar cells

To investigate the conduction properties of ionic liquid crystal electrolytes and to correlate them to their molecular structure, two electrolytes based on imidazolium iodides have been chosen for a comparative study: one ionic liquid crystal (1-dodecyl-3-methylimidazolium iodide) (C12) and one ionic liquid (1-hexyl-3-methylimidazolium iodide) (C6). For the electrical characterization of the samples the electrochemical impedance spectroscopy technique (EIS) has been used in the frequency range 1 MHz ÷ 10 mHz. For both electrolytes three frequency domains have been singled out. At large frequencies a hopping mechanism ensures the relative high conductivity of ionic liquids. A good fit of the conductivity data with respect to temperature with a Vogel-Tamman-Fulcher (VTF) equation has been done. The interesting result is that although C12 has a higher viscosity than C6 and thus a smaller conductivity was expected the observed conductivity is actually larger, this fact being attributed to the smectic A liquid crystal order of C12. At medium frequencies and especially at low frequencies the buildup of double layers by the electrodes is well observed. Assuming that in this region (10 Hz ÷ 10 mHz) the cell is equivalent to an electric circuit formed by the parallel of a resistance, R0, and a capacitance, C0, the simple Debye relaxation model, characterized by one relaxation time, has been modified taking into account the adsorption phenomenon in the Langmuir approximation. A theoretical interpretation of the experimental results based on the double layer model is presented.