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

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• 作者：Jiyoul Lee ; Loren G. Kaake ; Jeong Ho Cho ; X.-Y. Zhu ; Timothy P. Lodge ; C. Daniel Frisbie
• 刊名：Journal of Physical Chemistry C
• 出版年：2009
• 出版时间：May 21, 2009
• 年：2009
• 卷：113
• 期：20
• 页码：8972-8981
• 全文大小：435K
• 年卷期：v.113,no.20(May 21, 2009)
• ISSN：1932-7455

文摘

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 μF/cm² 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 10¹⁴ carriers/cm², with associated saturation hole mobilities that are also remarkably large, 1 cm²/(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 cm²/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.