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.