Herein, we present a versatile way to increase the photopic contrast of electrochromic devices (ECDs), approximately 14%, through the use of binary mixtures of propylene carbonate, diethyl carbonate, and ethylene carbonate that serve the role as plasticizers in the gel electrolyte. The binary plasticizer mixture changed the physical properties of the gel electrolyte medium, including the viscosity and dielectric constant, and also facilitates the diffusion of ions and monomer in the medium leading to an increase in ionic conductivity and the photopic contrast. ECDs were fabricated using a one-step lamination procedure, based on poly(2,2-dimethyl-3,4-propylenedioxythiophene) (PProDOT-Me-2) with different polymer gel electrolyte compositions where the electrochromic polymer was electrochemically obtained in situ. Gel electrolytes based on unary or binary plasticizer systems and poly(ethylene glycol) diacrylate (PEG-DA) with varying lithium trifluoromethanesulfonate (LiTrif) loading were investigated for the effect of solvent on ionic conductivity and the resulting photopic contrast of the ECD. This study demonstrates a relationship between photopic contrast and ionic conductivity, where the system reaches a maximum in photopic contrast the system is also at the maximum ionic conductivity. (C) 2014 Elsevier B.V. All rights reserved.