Solid polymer electrolyte gate dielectric based on cross-linked poly(ethylene oxide) (CPEO) was developed and employed for organic thin-film transistors (OTFTs). Mechanical stability, high areal capacitance, and amorphous morphology of CPEO were achieved via the use of polyhedral oligomeric silsesquioxane (POSS) as cross-linker, dissolved ion [EMIM][TFSI] as electrolyte, and PEO with low molecular weight as polymer matrix, respectively. The resulting solid polymer electrolyte showed excellent insulating properties with low leakage current density (1.8 X 10-7 A cm-2 at 1 V) and high capacitance per area (~1 mF cm-2 at 100 Hz). Furthermore, dielectric properties of the developed polymer electrolytes including ionic conductivity as well as segmental relaxation time were investigated. The polyelectrolyte dielectric was employed for bottom-gate/top-contact organic thin-film transistors and the resulting devices showed decent electrical performance with a carrier mobility of 0.12 (±0.03) cm2 V-1 s-1 and a current on/off ratio of 103 at low operating voltage of 5 V.