A dramatic improvement in dye-sensitized solar cell (DSSC) performance and stability has been achieved by using polyethylene oxide (PEO) as an additive to organic liquid electrolyte containing 1-propyl-3-methylimidazolium iodide (PMImI). The high efficient with long lifetime is strongly related to the chemical composition of electrolyte. Comparison to non-polymeric system, the ionic liquid electrolyte containing 10 wt. % PEO has shown a greater short-circuit photocurrent density (Jsc), consequently light-to-electricity conversion efficiency ( ). Photoelectrochemical performance of DSSC based PEO-PMImI electrolyte boosts by a factor of 1.2, 1.3, and 1.4 on the 1st, 7th, and 15th day of measurement, respectively. The advantage of PEO-PMImI electrolyte for the photovoltaic performance may stem from an increase of ionic conductivity and a decrease of interfacial resistance. The role of PEO to the lifetime of the photoelectrochemical cell could be explained in terms of viscosity.