The double layer capacitance of ionic liquids for electrolyte gating of ZnO thin film transistors and effect of gate electrodes

Abstract

Electrolyte gated thin film transistors (TFTs) based on sol-gel processed zinc oxide (ZnO) are investigated using imidazolium-based ionic liquids (ILs), namely [bmim][BF4] and [bmim][PF6], as electrolytes. The capacitance of the ILs is determined by means of electrochemical impedance spectroscopy. The frequency dependence of the capacitance measurements indicates that the electric double layers (EDLs) form below 1 kHz. Impedance measurements are also acquired at different gate voltages and the effect of the bias is discussed in detail. The experimental data suggest that the double layer capacitance of the two ILs depends on the polarization voltage. In terms of transistor characteristics, minimum hysteresis is observed in the case of [bmim][BF4]. Being smaller in size out of the two, a higher ionic mobility is expected. Therefore, faster formation of the EDL can account for suppressed hysteresis in the I-V characteristics. The estimated capacitance was 1.17 μF cm-2 for [bmim][PF6] and 4.05 μF cm-2 for [bmim][BF4], which resulted in a TFT field-effect mobility as high as 6.0 and 1.4 cm2 V-1 s-1, respectively. Also the on-off ratio of the EG-TFT is particularly high, being 104. Moreover, transient current measurements showed that the two ILs behave differently under continuous bias stress. The effect of the metal gate electrode on the device characteristics is also investigated. Five metals of varying work functions are employed for establishing a gate/IL interface. The data do not support that there is a correlation between the threshold voltage and the work function of the metals. On the other hand, the capacitance seems to be more susceptible to changes of the metal gate

Electric Double Layer Field-Effect Transistors Using Two-Dimensional (2D) Layers of Copper Indium Selenide (CuIn7Se11)

“…Several techniques have been used to estimate capacitance directly at the ionic liquid-semiconductor interface. For example, electrochemical impedance spectroscopy (EIS) was used in a ZnO thin film transistor [44], Hall-effect measurement was used in MoS 2 thin flake transistors [41], the interconnection between an SiO 2 back gate and ionic liquid top gate (change in threshold voltage of EDL-FET with application of back gate voltage) in dual gating FET [16] and the Mott-Schottky method was used for organometal perovskite solar cells [45]. We have used the Mott-Schottky method to estimate the EDL capacitance which we used for the mobility calculation.…”

Organic Electrochemical Transistors Based on Room Temperature Ionic Liquids: Performance and Stability

“…Room Temperature Ionic Liquids (RTILs) such as C2MIM EtSo4 are an alternative to water‐based electrolytes . So far, ionic liquids have been used extensively in electrolyte gated organic field‐effect transistors . In general, the slow polarization response of electrolytes limits the switching speed of these transistors to a few hertz at room temperature.…”

Ink‐Jet Printable, Self‐Assembled, and Chemically Crosslinked Ion‐Gel as Electrolyte for Thin Film, Printable Transistors

“…Equation (1) is generally used to calculate the C eff based on the impedance results In Equation (1), f is the frequency, A is the surface area of electrode, and Z im is the imaginary part of the impedance results. [39][40][41] Figure 4b shows that the phase angle of the ion-gel is non-ideal, where the lowest phase angle (<400 Hz) is close to −75°. [39][40][41] Figure 4b shows that the phase angle of the ion-gel is non-ideal, where the lowest phase angle (<400 Hz) is close to −75°.…”