In this report, the potential of diethyl carbonate (DEC) as a plasticizer influence the conductivity and dye-sensitized solar cells (DSSCs) efficiency. PVA-PC-EC-DMSO-TBAI-KI-DEC based gel polymer electrolytes (GPEs) have been prepared and utilized in DSSCs. The GPEs contain DEC have been characterized using X-ray diffraction and electrical impedance spectroscopy (EIS). X-ray diffraction indicates the amorphousness of the prepared GPEs. The highest conductivity derived from the EIS measurement is (6.64 ± 0.07) mS cm−1 exhibited GPE containing 5.47 wt.% PVA: 10.95 wt.% PC: 8.21 wt.% EC: 60.22 wt.% DMSO: 8.05 wt.% TBAI: 3.45 wt.% KI: 1.08 wt.% I2: 2.57 wt.% DEC gel polymer electrolyte. The highest conducting electrolyte was dominated by the highest charge carrier mobility and diffusivity. The conductivity-temperature plotted is Arrhenius. The DSSCs with the highest conducting GPE exhibits the highest efficiency of 7.50%. Other parameters are short-current density (Jsc) of 16.05 mA cm−2, open-circuit voltage (Voc) of 0.64 V and fill factor (FF) of 0.73. In short, the DEC plasticizer improves the conductivity and DSSCs efficiency precisely.