Using solid state polymer electrolyte, the overall energy conversion efficiency of solid state DSSCs was improved up to 5-6 % recently. In particular, PEO/PEGDME/XI/I2 electrolyte system showed a good solar cell performance, where X is either K or 1-methyl-3-propylimidazolium (MPI). The overall energy conversion efficiency was not different very much (6.20 % for KI and 5.79 % for MPII with the cell size of 0.09 cm2 at 1.5 AM and one sun conditions) although the large difference was expected in the electrochemical properties including the electron diffusion coefficient in the TiO₂ layer, the diffusion coefficient of triiodide, and charge transfer resistance at the counter electrode side. The electron diffusion coefficients through the TiO₂ semiconductor layer were 1~2*E-4 and 7~8*E-5 cm2/s, respectively, for KI and MPII. The electron diffusion length values measured were thus obtained 19.29um for KI and 7.70um for MPII. While the electron diffusion length value of polymer electrolyte employing KI was detected 3 times higher, the diffusion coefficients of iodide/triiodide redox couple measured by symmetric cells were 2.19x10-6 cm2/s for KI and 5.00x10-6 for MPII. It was thus concluded that the effects of the diffusion coefficient of electrons and ions on the overall energy conversion efficiency in the solid state DSSCs were compensated each other, resulting in not very different overall energy conversion efficiency.