The ion transport properties of electrolytes based on room temperature ionic liquids (RTILs) used in dye-sensitized solar cells (DSCs) have been studied by electrochemical voltammetry experiments and molecular dynamics (MD) simulations. Fully atomistic models based on the Lennard-Jones potential with atomic point charges have been developed for imidazolium and pyrrolidinium imides (Tf2N). MD simulations with the proposed force fields reproduce accurately the density and the self-diffusion coefficient of the anion and cation of the RTIL, as well as their temperature dependence and the effect of the length of the alkyl chain. The diffusion coefficients of iodide/tri-iodide redox mediator in mixture of RTIL and acetonitrile have been studied by voltammetry and MD. Both experiment and simulation show a 2 orders of magnitude decrease of the diffusion coefficient of the redox mediator between pure acetonitrile and pure RTIL. However, the variation of the diffusion coefficient with the RTIL/acetonitrile mixing ratio shows that a small amount of acetonitrile is sufficient to induce an improvement of the ion transport properties of the electrolyte, which can be very beneficial to design efficient and stable electrolytes for DSCs.