Dye-sensitized TiO2 solar cells using imidazolium-type ionic liquid crystal systems as effective electrolytes

A novel ionic liquid crystal (ILC) system (C(12)MImI/I-2) with a smectic A phase used as an electrolyte for a dye-sensitized solar cell (DSSC) showed the higher short-circuit current density (J(SC)) and the higher light-to-electricity conversion efficiency than the system using the non-liquid crystalline ionic liquid (C(11)MImI/I-2), due to the higher conductivity of ILC. To investigate charge transport properties of the electrolytes in detail, the exchange reaction-based diffusion coefficients (D-ex) were evaluated. The larger D-ex value of ILC supported that the higher conductivity of ILC is attributed to the enhancement of the exchange reaction between iodide species. As a result of formation of the two-dimensional electron conductive pathways organized by the localized I-3(-) and I- at S-A layers, the concentration of polyiodide species exemplified by I-m(-) (m = 5, 7, ...) was higher in C(12)MImI/I-2. However, as the increment of the concentration of polyiodide species is less than that of D-ex, the contribution of a two-dimensional structure of the conductive pathway through the increase of collision frequency between iodide species was proposed. Furthermore, a quasi-solid-state ionic liquid crystal DSSC was successfully fabricated by employing a low molecular gelator. Addition of the 5.0 g/L gelator to ILC improved light-to-electricity conversion efficiency through the increase of J(SC) due to the enhancement of the conductivity in C(12)MImI/I-2-gel.