Electrical impedance of spin-coatable polymer electrolyte based on ionic liquid

Ionic liquids have attracted considerable attention due to their unique properties including high thermal and electrochemical stability, negligible vapor pressure, and large ionic conductivity and specific capacitance. Such properties of ionic liquids make them suitable electrolytes for electrochemical and electrical device applications such as fuel cells, solar cells, electrical double layer capacitors and thin film transistors. From an applications perspective it is desirable to harness the outstanding characteristics of ionic liquids noted above in a solid film. This can be achieved by adding structuring polymers to an ionic liquid. We have explored the use of the solid polymer electrolyte called an ion gel as a gate dielectric material for thin film transistors. Due to remarkably high capacitance of an ion gel, low-voltage operation (∼1?2 V) and high ON/OFF current ratio (∼10 ₅) can be realized. To understand working mechanisms and improve the device performance the thickness, area, and temperature of the ion gel were systematically varied. From the impedance measurements, the resistance depends linearly on the thickness, and is inversely proportional to the area. However, the interfacial capacitance and conductivity of the ion gel remains almost the same. Importantly, a gel polarization time constant of a few microseconds is achieved by reducing the ion gel thickness. Additionally, both the interfacial capacitance and conductivity of an ion gel increase with increasing temperature; the mechanism for the former dependence is not yet clear.