The addition of the plasticizer poly(ethylene glycol)methyl ether to the polymer electrolyte based on poly(epichlorohydrin-co-ethylene oxide), NaI and I2 increased the ionic conductivity by one order of magnitude (1.7 × 10−4 S cm−1) without compromising its electrochemical, thermal and dimensional stabilities. The plasticized polymer electrolyte presented an estimated diffusion coefficient of 2 × 10−6 cm−2 s−1, ca. five times higher than the diffusion coefficient estimated for the polymer electrolyte without plasticizer. Solid-state dye-sensitized TiO2 solar cells (1 cm2) were assembled with the plasticized polymer electrolyte and presented an open circuit potential of 0.64 V, short-circuit current of 0.60 mA cm−2 and an energy conversion efficiency of 1.75% under light intensity of 10 mW cm−2. This efficiency remained unchanged for 30 days, showing that cell efficiency and stability can be improved using a plasticized polymer electrolyte.