We investigate the influence of the chemical structure of a range of imidazolium-based ionic liquids (IL) on their properties as electrolytes and the device characteristics of electrolyte-gated field-effects transistors (FETs) based on spray-deposited polycrystalline zinc oxide (ZnO). We find a decrease of electron field-effect mobility that correlates with the capacitance of the ionic liquids and not only with the size of the IL-cation. The device stability depends significantly on testing conditions. While they are reasonably stable in nitrogen, ZnO-FETs degrade rapidly in ambient air due to absorption of water by the IL and the resulting ZnO surface reactions. Replacement of the most acidic hydrogen atom of the imidazolium cation and surface passivation of ZnO with hexamethyldisilazane improve environmental stability. ■ INTRODUCTIONField-effect transistors (FETs) based on solution processable semiconductors, for example, organic semiconductors, semiconducting nanoparticles, and transparent conducting oxides (TCOs), are one crucial building block for flexible electronics. To achieve true flexibility not only the semiconductor but also all other components must be flexible and processable at low temperatures including the gate dielectric. Polymer gate dielectrics, for example polyvinylalcohol (PVA), polymethylmetacrylate (PMMA), and parylene have been employed, but they all have relatively low dielectric constants and require thick layers to avoid leakage currents resulting in low capacitances and high operating voltages. 1 Hence, over the past decade the development of new dielectric materials with high capacitances and low processing temperatures has been an important area of research for flexible electronics. An ideal dielectric should have a high capacitance for low voltage operation and exhibit low leakage currents and a high breakthrough voltage. This is difficult to achieve with traditional organic insulating materials. Panzer et al. demonstrated that solid polymer electrolytes based on LiClO 4 or lithium bis(trifluoromethylsulfonyl)imide in polyethylene oxide could be used as dielectrics in organic field-effect transistors (OFET) exhibiting very high specific capacitances (>10 μF·cm −2 ) and very low operating voltages. 2,3 This so-called electrolyte gating is based on the redistribution of ions within the electrolyte when applying a voltage. For example, when a positive bias is applied to the gate electrode anions will move toward the gate and cations toward the semiconducting layer where the ions form an electric double layer (EDL) at each interface. The applied gate voltage drops almost entirely over the few nanometer thick EDLs leading to a very high capacitance (several μF·cm −2 ). Thus, accumulation of large charge carrier concentrations at the semiconductor interface even at low voltages takes place.The main disadvantage of solid electrolytes was the low switching speed (few Hz) of the OFETs due to the slow diffusion of ions within the polymer. To increase the switching speed, ionic liquid gels we...