Dye-sensitized solar cells with solvent-free ionic liquid electrolytes

We systematically studied the temperature-dependent physicochemical properties, such as density, conductivity, and fluidity, of 1,3-dialkylimidazolium iodides. In combination with the amphiphilic Z907Na sensitizer, we have found that it is important to use low-viscosity iodide melts with small cations to achieve high-efficiency dye-sensitized solar cells. By employing high-fluidity eutectic-based melts the device efficiencies considerably increased compared to those for cells with the corresponding state of the art ionic liquid electrolytes. We propose a modified Stokes-Einstein equation by correlating ion mobility and fluidity to quantitatively depict the triiodide transport in ionic liquid electrolytes. These studies reveal that the viscosity-dependent transport of triiodide in ionic liquid electrolytes with high iodide concentration can be explained by two parallel processes. Apart from the normal physical diffusion, the coupling process of physical diffusion and bond exchange is responsible for the observed abnormally high diffusion coefficients. This work has provided useful insight for further improvement of solvent-free electrolytes based on rational design of their constituents, facilitating the large-scale practical application of lightweight, flexible dye-sensitized solar cells.