Rest Potentials of Binary Mixtures of Ionic Liquids and Threshold Voltages of Electric-Double-Layer Transistors

Abstract

There is a growing attention to the electric-double-layer (EDL) transistors, in which electrolytes are adopted as gate dielectrics, because a strong electric field produced by EDLs improves transistor performance. In our previous study, we investigated the relation between the rest potentials and the threshold voltages of EDL transistors, using two kinds ambipolar phthalocyanines and various ionic liquids. It was found that the rest potential is a unique property of ionic liquid, and the threshold voltages of the EDL transistors are proportional to the rest potentials of the ionic liquids. This means that the threshold voltages of the EDL transistors can be tuned by controlling the rest potentials, though what determines the rest potentials of the ionic liquids is still unclear. In the present study, we tried to understand the rest potentials of the ionic liquids and to control the threshold voltages of the EDL transistors, by making the binary mixtures of ionic liquids, because their mixing ratios can be continuously changed. We prepared the binary mixtures of the ionic liquids, combining DEME-TFSI and five kinds of ionic liquids, shown in the inset of Figs. 1 and 2. These combinations were selected, because these ionic liquids are miscible each other and the rest potential of DEME-TFSI is higher than those of the others. The rest potentials of the binary ionic liquids were examined with an Ag/AgCl reference electrode and a Pt working electrode. The rest potentials of the binary ionic liquids were found to change non-linearly with respect to the mixing ratio of the ionic liquids (Fig. 1). This dependence can be well fit to the equation of y = a + b*log(x), where x is the mixing ratio and, a and b are the fitting parameters. This strongly suggests that the rest potential of ionic liquid is governed by its chemical potential μ, because this equation has the same form as that of the chemical potential, μ = μ 0 + RTln(x), where R is the gas constant. Then, we tested the EDL transistors of platinum phthalocyanine (PtPc) which showed a high stability even in various ionic liquids. It was found that there was a linear relation between the rest potentials and the threshold voltages of the EDL transistors (Fig. 2), and the threshold voltages can be continuously controlled by changing the mixing ratio of the ionic liquids. Figure 1