Differential capacitance in ion-gel-gated organic transistors investigated by impedance spectroscopy

Ionic liquid-gated organic transistors have been reported to have high mobility and low operational voltage; they mainly benefit from the capacitance of electric double layers (EDLs) at the interface of an electrolyte and a semiconductor. Although the capacitance of EDLs is usually considered a fixed value, contrary to the theory of EDLs, EDL capacitance is tunable by the electrode potential. In this study, we prepared organic transistors with ionic liquid mixed with polymer as the dielectric. We found that the conductance parameter of the semiconductor channel decreased with decreasing gate voltage. Then, to examine the changes in interfacial capacitance under various gate voltages, the impedance of the whole transistor was measured, and an equivalent circuit model was applied to extract the differential capacitance. The decreased EDL capacitance with the gate potential is ascribed to the finite ion volume in the ionic liquid gel based on Kornyshev’s theory.