Research Article www.acsami.org
Parasitic Capacitance Effect on Dynamic Performance of Aerosol-JetPrinted Sub 2 V Poly(3-hexylthiophene) Electrolyte-Gated Transistors Fazel Zare Bidoky†,‡ and C. Daniel Frisbie*,‡ †
Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Printed, low-voltage poly(3-hexylthiophene) (P3HT) electrolyte-gated transistors (EGTs) have favorable quasi-static characteristics, including sub 2 V operation, carrier mobility (μ) of 1 cm2/(V s), ON/OFF current ratio of 106, and static leakage current density of 10−6 A/cm2. Here we study the dynamic performance of P3HT EGTs in which the semiconductor, dielectric, and gate electrode were deposited using aerosol-jet printing; the source and drain electrodes were patterned by conventional microlithography. With a source-to-drain separation of 2.5 μm, the highest theoretical achievable switching frequency is ∼10 MHz, assuming the movement of charge through the semiconductor is the limiting step. However, the measured maximum switching frequency of P3HT EGTs to date is ∼1 kHz, implying that another process is slowing the response. By systematically varying the device geometry, we show that the frequency is limited by the capacitance between the gate and drain (i.e., parasitic capacitance). The traditional scaling of switching time with the square of channel length (L) does not hold for P3HT EGTs. Rather, minimizing the size of the drain electrode increases the maximum switching speed. We achieve 10 kHz for P3HT EGTs with source/drain electrode dimensions of 2.5 μm × 50 μm and channel dimensions of 2.5 μm × 50 μm. Further improvements will require additional shrinkage of electrode dimensions as well as consideration of other factors such as ion gel thickness and carrier mobility. KEYWORDS: ion gel, poly(3-hexylthiophene), electrolyte-gated transistors, aerosol-jet printing, dynamic performance, cut-off frequency, parasitic capacitance
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advantage for printed circuits that are powered by thin film batteries. Here we examine dynamic performance of aerosol-jetprinted, top-gated EGT inverters featuring a poly(3-hexylthiophene) (P3HT) channel, an ion gel dielectric, and a poly(3,4ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) gate in series with a 100 ± 25 kΩ PEDOT:PSS resistor. Figure 1 shows the structure for the resistor-loaded EGT inverter. To fabricate this structure, the P3HT, ion gel, and PEDOT:PSS were aerosol-jet-printed on conventionally microfabricated gold source and drain electrodes and contact pads. The p-type P3HT EGTs investigated here display hole mobilities (μ) of ∼1 cm2/(V s).4,20 Thus, for a 2.5 μm channel length, the switching frequency (fswitch) for a P3HT EGT should be on the order of 10 MHz, as shown by eq 1.
INTRODUCTION Electrolyte-gated transistors (EGTs, Figure 1) have favorable characteristics for printed electronics, such as solution processability, low-voltage switching,1,2 and relaxed layer-tolayer alignment requirements.3,4 Consequently, they are being developed for applications in strain sensors,5 biosensors,6−12 printed circuitry,13−16 e-textiles,17,18 and antennas.19 EGTs can, in principle, operate at speeds higher than 1 MHz but currently are known to operate only at a few kHz with polymer semiconductors.14,20 As we demonstrate here, the limitations are not due to intrinsic materials properties but rather parasitic effects. The crucial aspect of EGTs is the use of electrolytes, such as an ion gel dielectric, as a high-capacitance gate insulator.4 Ion gels are mixtures of an ionic liquid such as 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) and a triblock copolymer matrix material such as poly(styrene-b-methyl methacrylate-b-styrene) (PS-PMMAPS).4 Sandwiched between electrodes, these materials (and other electrolytes) exhibit very high specific capacitances on the order of 10 μF/cm2, due to the formation of nanometer-thick electrical double layers at the gel/electrode interfaces. When employed as gate dielectrics in EGTs, the high capacitance of ion gels provides devices that can be switched ON and OFF with very low gate voltages, typically
