Ion Gel-Gated Polymer Thin-Film Transistors: Operating Mechanism and Characterization of Gate Dielectric Capacitance, Switching Speed, and Stability

Abstract

We report comprehensive characterization of electrolyte-gated polymer thin-film transistors (TFTs) incorporating solution processable polymer semiconductors and high capacitance "ion gel" gate dielectrics. The ion gel dielectrics comprise self-assembled networks of triblock copolymers such as poly(styrene-b-methylmethacrylateb-styrene) [PS-PMMA-PS] that are swollen with ionic liquids, e.g., (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]). The capacitance of the gels is exceptionally large (>10 µF/cm 2 at 10 Hz), which is derived from the high concentration of mobile ions and facilitates operation of ion gelgated organic TFTs (GEL-OTFTs) at very low voltages (< 2.5 V). Gate-induced hole densities in GEL-OTFTs employing different polythiophene semiconductors in the channel are on the order of 10 14 carriers/ cm 2 , with associated saturation hole mobilities that are also remarkably large, ∼1 cm 2 /(V s), likely because of the large gate-induced carrier densities. Examination of the frequency response of GEL-OTFTs indicates that increases in the OFF current with frequency ultimately limit switching speed; the cutoff frequency correlates with the ionic conductivity versus frequency response of the gel dielectric. Further, attenuated total internal reflection infrared (ATR-IR) spectroscopy of the ion gel/polymer semiconductor gate stack reveals that the conductance switching mechanism in GEL-OTFTs spans both electrochemical and electrostatic (field effect) regimes. Specifically, modeling of the time dependence of the near-infrared polaron absorption in gated GEL-OTFTs indicates that the [TFSI]anion diffusivity in regioregular poly(3-hexylthiophene) is on the order of 10 -12 cm 2 /s at room temperature. This diffusivity implies that, for time scales greater than 1 ms, there is significant penetration (>1 nm) of [TFSI]anion into the polymer semiconductor at the gel/polymer semiconductor interface, corresponding to an electrochemical doping process. On the other hand, for time scales shorter than 1 ms (i.e., for GEL-OTFT switching frequencies >1 kHz), the device switching mechanism can be viewed as primarily electrostatic as average ion penetration depths are less than 1 nm.

A strain-absorbing design for tissue–machine interfaces using a tunable adhesive gel

“…This trend is also observed in electrical double-layer capacitors 22,23 . The increase in the conductivity at higher frequency is similar to what is noticed in ionic gels 24 . The resistance of the 5 wt% PVA gel was higher than that of the PVA-free gel by a factor of two at 5 mV.…”

Hopping transport and the Hall effect near the insulator–metal transition in electrochemically gated poly(3-hexylthiophene) transistors

“…S1. In essence, the device is a four-terminal bottom-contacted van der Pauw geometry P3HT thin film transistor, gated electrochemically using an ion gel based on an ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoro-methylsulfonyl) imide ((EMIM)(TFSI))) and a triblock copolymer (poly(styrene-b-methylmethacrylate-b-styrene; PS-PMMA-PS)) [19][20][21] . Such ionic liquids and gels have been used recently to electrostatically induce two-dimensional charge densities B10 14 cm À 2 in a variety of materials (for example, refs [22][23][24][25].…”

“…In the case of semi-crystalline semiconducting polymers such as P3HT, the open structure results in deep penetration of the dopant ions, providing uniform 3D electrochemical doping (Fig. 1b) 21,26 . That the P3HT thickness employed here (60 nm) can be safely assumed to be in the 3D doping limit is well established 21,26 .…”

“…1b) 21,26 . That the P3HT thickness employed here (60 nm) can be safely assumed to be in the 3D doping limit is well established 21,26 . Although quantification of P3HT crystallinity remains a challenge, it is generally regarded to be quite low 6 , and heavy electrochemical doping is expected to further reduce it.…”

High performance photolithographically-patterned polymer thin-film transistors gated with an ionic liquid/poly(ionic liquid) blend ion gel

“…Relatively slow switching times (< 1 kHz) are typical for unoptimised electrolyte-gated OTFTs limited by ionic diffusion. 6,36,37 A straightforward way to improve the operating frequency of the devices would be to use a different PIL/IL system with a higher ionic conductivity, or that allows for a higher IL content without loss of the mechanical properties of the resulting ion gel. Pushing the IL content in the current system to 75 wt% resulted in a slight improvement in switching speed (around 10 ms), at the cost of the gel's mechanical properties, making the deposition of the PEDOT:PSS gate electrode difficult and resulting in a high number of short-circtuited devices and larger disparities between the characteristics of working devices.…”