Printed electrolyte-gated transistors and circuits for flexible electronics

Abstract

Printed electronics has broad potential applications due to its low fabrication cost, compatibility with flexible substrates, and its suitability for applications where large footprints are required. However, the supply voltages of printed circuits are high in general due to limitations of both the electronic properties of printable materials and the coarse dimensions of printed transistors. This thesis aims to demonstrate low-voltage operation of printed circuits by employing a printable electrolyte, a so-called ion gel, with very large specific capacitance (on the order of μF/cm<super>2<super>) as the gate insulator. Ion gels are composites formed by the self-assembly of triblock copolymers, e.g. poly(styrene-b-methyl methacrylate-b-styrene) (PS-PMMA-PS), in an ionic liquid, e.g. 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]). These ion gels and other functional materials, such as semiconductors and conductive polymers were fabricated by aerosol jet printing to form electrolyte-gated transistors (EGTs) and circuits on plastic substrates. This thesis demonstrates that these printed devices can achieve low-voltage operation, fast switching speed, remarkable operational stability, and can realize electronic functions of logic gates, capacitor, electrochromic device, etc. To explore the trade-off between supply voltage and switching speed, this thesis studied EGTs consisted of high capacitance ion gel and high mobility carbon nanotubes (CNTs), which together enable the fast switching speed at low voltages. The CNT EGTs were ambipolar and could be used to make complimentary-like inverters and circuits. Five-stage ring oscillators printed on flexible substrate achieved above 2 kHz frequency, corresponding to less than 50 μs delay time. The impact of key parameters on delay times were studied, including the EGT channel length, ionic conductivity of the ion gel, parasitic capacitance and resistance. With these understandings, the architecture of EGT was optimized, and ring oscillators with stage delay as short as 1.2 μs time was successfully demonstrated at voltages < 3 V. These results represent a significant improvement in the performance of printed electronics. Fabrication and characterization of inverters and NAND gates, device operational stability and power consumption were also discussed. To demonstrate integration of EGTs, a flexible circuit with 23 EGTs, 12 capacitors, 20 resistors and an electrochromic (EC) display pixel operates at a voltage as low as 1 V was fabricated on plastic substrates. All of the key components were aerosol jet printed from liquid inks, such as the ion gel, poly (3-hexylthiophene) (P3HT), a semiconductor, and the conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Characteristics and operation mechanisms of each device were discussed respectively. The circuit operated continuously for 100 min with no degradation. Overall, this thesis demonstrates that high reproducibility of device fabrication is possible and that EGTs may be used to achieve conventional electronic function at low voltage on plastics.