Binary ionic liquid composed of 1-propyl-3-methylimidazolium iodide (PMII) and 1-butyl-3-methylimidazolium thiocyanate (BMISCN) were mixed at four different ratios to study its effect on the dye-sensitized solar cells' (DSSCs) efficiency. The addition of low viscosity ionic liquid which is BMISCN is to overcome the low mass transportation of iodide/triiodide faced by pure PMII. Therefore, the effect of mixing different ratios of these ionic liquids on the viscosity and triiodide diffusion coefficient were studied and the best ratio was identified. Based on the experimental work, the mixture of BMISCN with PMII of higher viscosity has allowed reducing the mass transport limitation of tri-iodide in the electrolyte as the effect of lowering the overall ionic liquid's viscosity. The binary electrolyte PMII: BMISCN (1:0.75) give the highest power conversion efficiency of 1.89, with a fill factor of 0.47, open circuit voltage of 0.62, and short circuit current density of 6.52 mA/cm2 and triiodide diffusion coefficient of 8.47 x 10-7 cm2 s-1. This work has provided valuable insight for further development of binary ionic liquid electrolyte for DSSCs. © 2016 The Authors. Published by Elsevier Ltd.