Here we summarize recent progress in the development of electrolyte-gated transistors (EGTs) for organic and printed electronics. EGTs employ a high capacitance electrolyte as the gate insulator; the high capacitance increases drive current, lowers operating voltages, and enables new transistor architectures. Although the use of electrolytes in electronics is an old concept going back to the early days of the silicon transistor, new printable, fast-response polymer electrolytes are expanding the potential applications of EGTs in flexible, printed digital circuits, rollable displays, and conformal bioelectronic sensors. This report introduces the structure and operation mechanisms of EGTs and reviews key developments in electrolyte materials for use in printed electronics. The bulk of the article is devoted to electrical characterization of EGTs and emerging applications.

A free-standing polymer electrolyte called an ion gel is employed in both organic and inorganic thin-film transistors as a high capacitance gate dielectric. To prepare a transistor, the free-standing ion gel is simply laid over a semiconductor channel and a side-gate electrode, which is possible because of the gel's high mechanical strength.

The effects of composition, temperature, and polymer identity on the electrical and viscoelastic properties of block copolymer-based ion gels were investigated. Ion gels were prepared through the self-assembly of poly(styrene-b-ethylene oxide-b-styrene) (SOS) and poly(styrene-b-methyl methacrylate-b-styrene) (SMS) triblock copolymers in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([EMI][TFSI]). The styrene end-blocks associate into micelles, whereas the ethylene oxide and methyl methacrylate midblocks are well-solvated by this ionic liquid. The properties of the ion gels were examined over the composition range of 10−50 wt % polymer and temperature range of 25−160 and 25−200 °C for the SOS- and SMS-based gels, respectively. The response of the ion gels to ac electric fields below 1 MHz can be represented by a resistor and constant phase element (CPE) series circuit, with a characteristic time corresponding to the establishment of stable electrical double layers (EDLs) at the gel/electrode interfaces. The ionic conductivity and specific capacitance were found to range from 3 × 10−5 to 3 × 10−2 S/cm and 0.3 to 10 μF/cm2, respectively. For 1 mm thick gels, the corresponding RC time constants ranged from 2 × 10−5 to 5 × 10−3 s. Notably, at high polymer concentrations, the ionic conductivity is much higher in SOS than SMS due to the higher glass transition of the methyl methacrylate block. Two relaxation modes have been observed in the ion gels under oscillatory mechanical shear. The faster mode corresponds to the relaxation of the midblocks in the ionic liquid, while the slow mode reflects motion of the end-blocks within their micellar cores. The plateau modulus of the gels was found to vary from 0.5 to 100 kPa over the measured composition and temperature ranges. While the ionic conductivity generally decreases as the modulus increases, it is possible to achieve conductivities greater than 0.01 S/cm with moduli above 10 kPa in the SOS system.

The electrical properties (capacitance, resistance, and conductivity) of ion gel films were examined as a function of film geometry and temperature by using electrical impedance spectroscopy. Ion gel films, which consist of a triblock copolymer, poly (styrene-b-methyl methacrylate-b-styrene) [SMS], and an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMI][TFSI], were deposited by spin coating from ethyl acetate solution. The thickness (2.2-13.4 μm) and the area (0.01-0.06 cm(2)) of the film sandwiched between two gold electrodes were varied systematically to investigate the relation between the electrical properties and the geometry of the film. The resistance (R) was directly proportional to the thickness and the reciprocal area, as expected, whereas the specific capacitance (C') was insensitive to the film geometry. Importantly, the gel polarization time constants (RC, where C = C' × area) were as small as 2.8 μs for 2.2 μm thick ion gel films. Conductivity and capacitance of the film both increase with increasing temperature, with conductivity following the Vogel-Fulcher-Tamman equation, indicating entropically activated behavior, and capacitance at 10 Hz showing Arrhenius-type activation.

Self-assembly of ABA triblocks in ionic liquids provides a versatile route to highly functional physical ion gels, with promise in applications ranging from plastic electronics to gas separation. However, the reversibility of network formation, so favorable for processing, restricts the ultimate mechanical strength of the material. Here, we describe a novel ABA system that can be chemically cross-linked in a second annealing step, thereby providing greatly enhanced toughness. The ABA triblock is a poly(styrene-b-ethylene oxide-b-styrene) polymer in which about 25 mol % of the styrene units have a pendant azide functionality. After self-assembly of 10 wt % triblock in the ionic liquid [EMI][TFSA], the styrene domains are cross-linked by annealing at elevated temperature for ca. 20 min. The high ionic conductivity (ca. 10 mS/cm) of the physical ion gels is preserved in the final product, while the tensile strength is increased by a factor of 5.

We report low-temperature, high-field magnetotransport measurements of SrTiO 3 gated by an ionic gel electrolyte. A saturating resistance upturn and negative magnetoresistance that signal the emergence of the Kondo effect appear for higher applied gate voltages. This observation, enabled by the wide tunability of the ionic gel-applied electric field, promotes the interpretation of the electric field-effect-induced 2D electron system in SrTiO 3 as an admixture of magnetic Ti 3þ ions, i.e., localized and unpaired electrons, and delocalized electrons that partially fill the Ti 3d conduction band.The Coulomb interaction amongst electrons and ions in a solid can spontaneously generate internal magnetic fields and effective magnetic interactions. Unexpected magnetic phenomena may emerge whenever we consider a new system where interactions are important. In recent years, predictions for and observations of magnetism originating in the two-dimensional (2D) system of electrons at the interface between SrTiO 3 (STO) and LaAlO 3 (LAO) have attracted much attention [1][2][3][4][5][6][7], particularly the prediction of charge disproportionation and the emergence of þ3-valent Ti sites with unpaired spin [1] and direct measurements of in-plane magnetization [6,7]. The conducting electrons at the LAO=STO interface are believed to be induced by polar LAO's strong internal electric fields, and to reside on the Ti sites on the STO side of the interface, partially filling the lowest-lying Ti 3d bands [8-10]. Questions remain, however, over the role of the growth process, in particular, whether oxygen vacancy formation or cation intermixing are in fact responsible for the observed n-type conduction [11][12][13][14].Other than growing a polar overlayer, a 2D system of electrons in STO can be made by chemical doping with Nb, La, or oxygen vacancies [15][16][17][18], or purely electrostatic charging in an electric double layer transistor (EDLT) [19,20]. If electronic reconstruction in response to overlayer polarity is an accurate description for LAO=STO, then that system can be closely modeled by field-effectinduced electrons in undoped STO, where confounding questions over growth conditions do not arise, and the applied electric field can be widely tuned.In this Letter, we expand on the body of evidence for Ti 3þ magnetism in STO that conducts in two dimensions. We demonstrate a gate-controlled Kondo effect in the 2D electron system in undoped STO formed beneath the bare surface by the electric field from an ionic gel electrolyte, and interpret this system as an admixture of magnetic Ti 3þ ions (unpaired and localized electrons) and delocalized electrons partially filling the Ti 3d conduction band, as predicted theoretically [2,21]. The Kondo effect is an archetype for the emergent magnetic interactions amongst localized and delocalized electrons in conducting alloys [22,23], and the ability to produce and tune the effect by purely electrostatic means in any conducting system is of interest in its own right [24,25]. The observed appear...

The viscoelastic properties and ionic conductivity of ion gels based on the self-assembly of a poly(styrene-b-ethylene oxide-b-styrene) (SOS) triblock copolymer (M n,S = 3 kDa, M n,O = 35 kDa) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]) were investigated over the composition range of 10–50 wt % SOS and the temperature range of 25–160 °C. The poly(styrene) (PS) end-blocks associate into micelles, whereas the poly(ethylene oxide) (PEO) midblocks are well-solvated by this ionic liquid. The ion gel with 10 wt % SOS melts at 54 °C, with the longest relaxation time exhibiting a similar temperature dependence to that of the viscosity of bulk PS. However, the actual values of the gel relaxation time are more than 4 orders of magnitude larger than the relaxation time of bulk PS. This is attributed to the thermodynamic penalty of pulling PS end-blocks through the PEO/[EMI][TFSA] matrix. Ion gels with 20–50 wt % SOS do not melt and show two plateaus in the storage modulus over the temperature and frequency ranges measured. The one at higher frequencies is that of an entangled network of PEO strands with PS cross-links; the modulus displays a quadratic dependence on polymer weight fraction and agrees with the prediction of linear viscoelastic theory assuming half of the PEO chains are elastically effective. The frequency that separates the two plateaus, ωc, reflects the time scale of PS end-block pull-out. The other plateau at lower frequencies is that of a congested micelle solution with PS cores and PEO coronas, which has a power law dependence on domain spacing similar to diblock melts. The ionic conductivity of the ion gels is compared to PEO homopolymer solutions at similar polymer concentrations; the conductivity is reduced by a factor of 2.1 or less, decreases with increasing PS volume fraction, and follows predictions based on a simple obstruction model. Our collective results allow the formulation of basic design considerations for optimizing the mechanical properties, thermal stability, and ionic conductivity of these gels.

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