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.