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

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-methylmethacrylate-b-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 mu F/cm(2) at 10 Hz), which is derived from the high concentration of mobile ions and facilitates operation of ion gel-gated 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, similar to 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. Oil the other hand, for time scales shorter than I ins (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 I nm.