625 www.MaterialsViews.com wileyonlinelibrary.com . IntroductionPaper is a cheap, ubiquitous, and biofriendly material that is used as a fl exible substrate for many applications in daily life. Modern methods of pulp and paper processing have also substantially reduced the energy consumption associated with paper production. [ 1 ] Hence, using paper as a substrate for lowcost, printable, and fl exible electronics has been an attractive goal for some time. [2][3][4][5][6][7] The absorbency and roughness of ordinary paper make it a challenging substrate compared to smooth but more expensive and non-biodegradable plastic sheets, such as polyethylene terephthalate (PET) and polyimide (PI). However, these issues may be overcome by using a recently developed multilayer-coating structure [ 8 ] which holds great potential for applications in fl exible electronics. [ 9,10 ] Furthermore, special micro-and nanocellulose papers, which are transparent and have a much smoother surface than regular paper and a low thermal expansion coeffi cient, have attracted considerable attention recently as substrates for circuits [11][12][13][14][15] and actuators [ 16,17 ] In general, paper is seen as a renewable, biofriendly material whose value and utility is increased substantially by adding extra functionalities such as energy storage and (opto-)electronic circuits. [ 18 ] In order to build electronic circuits on paper, suitable electrode materials, semiconductors and dielectrics are required, which should ideally reproduce the advantageous properties of the paper, e.g., biodegradability, low cost, and fl exibility, while maintaining high performance. They should not only be compatible with paper in terms of their mechanical, thermal, and adhesion properties, but should also be processable in the same way. High temperature and vacuum processes are therefore less suitable. One crucial component of an electronic circuit containing many thin-fi lm fi eld-effect transistors (FETs) is the gate dielectric, which determines to a large degree the operating voltage and switching speed. High capacitance dielectrics are ideal for lowvoltage FETs. [ 19 ] However, most polymeric printable dielectrics (e.g., Cytop, PMMA, PVA) result in relatively low capacitances and are not particularly compatible with paper substrates, while high-permittivity inorganic dielectrics (e.g., Al 2 O 3 or HfO 2 ) require vacuum deposition or sputtering. Here we introduce a new class of cellulose-based ionogels as high capacitance gate dielectrics for electrolyte-gated FETs on paper. These ionogels are produced from microcellulose thin fi lms and tailor-made methylphosphonate ionic liquids, which result in fl exible, transferable, and high capacitance dielectrics, that match the advantageous properties of paper while allowing for low-voltage (<2 V) operation of solution-processed inorganic and organic FETs.Electrolyte-gating has recently emerged as a promising method to obtain low voltage, high performance FETs with solution-processed organic, inorganic, or colloidal se...

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...

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A new type of silane-based ionogel is demonstrated that can be used in low-voltage electrolyte-gated polymer field-effect transistors.