The effect of succinonitrile on cell efficiency, interfacial resistance, and dye regeneration time of [0.5 poly(ethylene oxide):0.5 succinonitrile] blend/LiI-I2 solid polymer electrolyte is reported for the first time. The addition of succinonitrile increased the electrical conductivity of poly(ethylene oxide)/LiI-I2 by more than an order in magnitude to ∼3 × 10−4 S cm−1 due to the plasticizing and molecular diffusing properties of succinonitrile. This resulted in an enhancement of the cell efficiency from 0.5% to 2%. The electrochemical impedance spectroscopy revealed a large decrease of interfacial resistances. The transient absorption spectroscopy showed an improvement of the dye regeneration time from 15 to 5 µs.