Ion Gel-Gated Polymer Thin-Film Transistors: Operating Mechanism

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Ion Gel-Gated Polymer Thin-Film Transistors: Operating Mechanism and Characterization of Gate Dielectric Capacitance, Switching Speed, and Stability Jiyoul Lee, Loren G. Kaake, Jeong Ho Cho, X.-Y. Zhu, Timothy P. Lodge, and C. Daniel Frisbie* Departments of Chemistry and Chemical Engineering & Materials Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: February 16, 2009; ReVised Manuscript ReceiVed: April 7, 2009

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/cm2 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 GELOTFTs 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 GELOTFTs 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. Introduction Organic thin-film transistors (OTFTs) have been investigated extensively due to their potential importance as critical current and voltage modulating components of organic electronic circuitry.1-5 In an OTFT, the organic semiconductor is insulated from the gate electrode by a dielectric layer, and the gate/ dielectric stack is responsible for inducing mobile charges in the semiconductor channel.6 It is desirable for the gate dielectric to have a high specific capacitance, because higher capacitance results in greater induced charge in the semiconductor and thus higher ON currents at lower gate voltages. Generally, a high capacitance dielectric layer can be attained by reducing its thickness, and several research groups have employed ultrathin cross-linked polymer films7-9 or self-assembled monolayers (SAM)2,10-12 as dielectrics. The capacitances of these systems have reached 1 µF/cm2, which allows low voltage OTFT operation. Solid polymer electrolytes consisting of a salt dissolved in a polymer matrix offer another promising approach for achieving high capacitance values.13 The exceptionally high capacitance (e.g., Ci 10-100 µF/cm2) of solid polymer electrolytes results from the formation of an electrical double layer only nanometers * To whom correspondence should be addressed. E-mail: frisbie@ cems.umn.edu.

in thickness at the electrolyte/gate electrode interface, when cations and anions move in response to the gate bias.14,15 Several groups have demonstrated solid polymer electrolyte-gated transistors based on organic single crystals,16-19 organic semiconductor thin films,20-26 or carbon nanotubes.27-30 These devices display good static device characteristics such as high ON currents and low operating voltages. However, they generally show poor dynamic characteristics, especially low switching frequency (e.g., 1 Hz), resulting from the slow polarization response of the polymer electrolyte. (A recent exception is ref 26, which reports 10 µs switching times.) In order to improve the dynamic behavior of electrolyte-gated transistors, we have fabricated polymer thin-film transistors (TFTs) using high capacitance “ion gels” as the gate dielectric, Figure 1a.31-33 Ion gels can be prepared from mixtures of an ionic liquid and a block copolymer and can provide specific capacitances in excess of 10 µF/cm2. In this work, we used a combination of two different imidizolium-based ionic liquids, (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), and two ABA type triblock copolymers, poly(styrene-b-methylmethacylate-b-styrene) [PSPMMA-PS] and poly(styrene-b-ethylene oxide-b-styrene) [PSPEO-PS], to form ion gels, Figure 1b. The ionic liquids of the imidazolium family were selected due to their high capacitance

10.1021/jp901426e CCC: $40.75  2009 American Chemical Society Published on Web 04/23/2009

Ion Gel-Gated Polymer Thin-Film Transistors

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Figure 1. (a) Cross-sectional scheme of an ion gel-gated organic thin-film transistor (GEL-OTFT). The devices have channel lengths of 20 µm and channel widths of 400 µm. (Not drawn to scale: the gate is several times larger than the channel length). Typical gel thicknesses in this work are on the order of 100 µm; the polymer semiconductor thickness is ∼20 nm. Source (IS), drain (ID), and gate (IG) currents were measured simultaneously while applying the source-gate (VG) and source-drain (VD) voltages. (b) Molecular structures of the polymer semiconductors (left) and ion gel dielectrics (right).

and fast polarization response.34 Ion gels are formed at low concentrations (∼5 wt %) of PS-PMMA-PS or PS-PEO-PS; these triblock copolymers self-assemble to form physically cross-linked networks in the ionic liquid in which the nonpolar PS blocks form glassy micelles that are interconnected by the polar PMMA or PEO blocks.35,36 Because the midblocks are polar, the network is readily swelled by the ionic liquid. Importantly, the triblock copolymer network infuses mechanical strength into the ionic liquid with little reduction in ionic mobility.37-39 In this report, we have significantly expanded upon our previous work31-33 in which we demonstrated that ion gel-gated OTFTs (GEL-OTFTs) operated at very low voltages and could be switched at frequencies >1 kHz. Specifically, we have (i) characterized both the capacitance and conductivity of ion gels incorporating two different block copolymers and related these electrical characteristics to device performance, particularly the device OFF current and cutoff frequency; (ii) demonstrated that GEL-OTFTs exhibit remarkable robustness in both steady-state and cyclic bias stress tests; and (iii) explicitly addressed the mechanism of GEL-OTFT operation (electrostatic versus electrochemical) and shown that it depends on the operation frequency. We have also confirmed the high charge density and

hole mobilities reported earlier for GEL-OTFTs.31-33 Collectively, these new results demonstrate that ion gels are extremely promising dielectric materials for solution processable, low voltage polymer TFTs, and they provide insight into the fundamental mechanisms that limit the ultimate switching speed of GEL-OTFTs. Experimental Section Materials. Poly(3,3′′′-didodecylquaterthiophene) (PQT-12) (American Dye Source) and poly(3-hexylthiophene) (P3HT) (Rieke Metals) were purified by successive Soxhlet extractions with methanol, acetone, and hexane. 1,2-Dichlorobenzene solutions (3 mg of polymer/mL of solvent) of PQT-12 and P3HT were prepared in a glovebox. A symmetric PS-PMMA-PS (SMS) triblock copolymer was synthesized, as previously described,38 with block molecular weights of Mn(PS) ) 6 kg/ mol and Mn(PMMA) ) 125 kg/mol (overall polydispersity Mw/ Mn ) 1.5). PS-PEO-PS (SOS) triblock copolymer (Mn(PS) ) 9.5 kg/mol and Mn(PEO) ) 48 kg/mol with Mw/Mn ) 1.2) was purchased from Polymer Source, Inc. and used as received. For convenience, we denote these triblock copolymers SMS and SOS, where S represents a styrene unit, O an oxyethylene unit, and M a methyl 2-methylpropanoate unit. [EMIM][TFSI] was

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Figure 2. (a) p-Si/ion gel/Cu test capacitor (left) and the electric double layer formation in the test device (right). (b) C-V characteristics of the capacitor at four frequencies. The C-V curves indicate large frequency dependent hole accumulation in the Si at negative bias on the top contact. (c) Frequency dependence of the maximum capacitance for four different ion gels. Gel thickness in each case was 120 µm.

synthesized as recently described,39 and [BMIM][PF6] was purchased from Solvent Innovation GmbH (Germany). Ionic liquids are moisture sensitive, so solution preparation and electrical measurements were, therefore, carried out in a glovebox. Triblock copolymers and ionic liquids (1:9 by wt) were dissolved in methylene chloride. The solution was stirred for 12 h vigorously and poured into a Petri dish at ambient temperature. The ion gels were formed by evaporating the residual solvent from the solution overnight under vacuum. Capacitance and Ionic Conductivity Measurements. Capacitance-voltage (C-V) measurements were performed on the ion gels using metal-insulator-semiconductor (MIS) structures with a HP 4192A LF impedance analyzer as a function of frequency (10-105 Hz). A 120 µm thick ion gel layer was sandwiched by hand between p-type silicon (〈100〉 orientation, 25 Ω cm) and a laminated top copper contact (area: ∼4 × 10-3 cm2). The ionic conductivity was determined in a homemade cell by means of complex impedance measurements using a Solartron 1255B frequency response analyzer at an AC amplitude of 10 mV. The frequency range employed for the measurements was typically 1 Hz to 100 kHz. The cell is made of two stainless steel discs (electrodes) separated by a Teflon spacer. Device Fabrication and Electrical Measurements. GELOTFT fabrication began with the thermal evaporation of ∼40 nm thick Au source and drain contacts through a silicon stencil mask onto an oxidized Si wafer to define a channel length of 20 µm and a channel width of 400 µm. The polymer semiconducting layer (PQT-12 or P3HT) was then spin cast. Film thicknesses were typically 20 nm. On top of the semiconductor layer, the ion gel was pasted by hand over the channel region, and the gate contact was prepared by laminating a polyestersupported 400 µm wide and 5 mm long copper strip onto the top of the gel overlapping the source and drain electrodes. Gel thickness was approximately 100 µm. Current-voltage (I-V) characteristics were measured in a Desert Cryogenics vacuum probe station with a temperature-controlled stage and Keithley 236 and 6517 electrometers. In the response time measurement, the input gate bias was supplied by an Agilent 33120A arbitrarywaveform generator, and a Keithley 2612 source meter was connected in series with the devices to detect the output currents.

The output voltage signal of the GEL-OTFT inverter was monitored with a Tektronix TDS1002B digital oscilloscope. All electrical measurements were performed in vacuum at 500 Hz).

SMS/[EMIM][TFSI], SOS/[BMIM][PF6], and SMS/[BMIM][PF6] ion gels, respectively, at 10 Hz. All capacitance values of the ion gels decrease with increasing frequency, as expected, because the ion mobilities limit the polarization response time. In particular, [BMIM][PF6]-based ion gels showed a stronger dependence on frequency than the [EMIM][TFSI]-based gels, which correlated with differences in ionic conductivity (see below). The capacitance of [EMIM][TFSI]-based gels remains >1 µF/cm2 at 100 kHz, suggesting that operation of GELOTFTs at this frequency should be possible. However, the potential switching speed of GEL-OTFTs is not solely determined by the capacitance-frequency characteristic; it also depends on the conductivity-frequency response as discussed below. Figure 3 displays a log-log plot of the frequency dependence of ionic conductivity for ion gels based on SMS/[BMIM][PF6], SMS/[EMIM][TFSI], SOS/[BMIM][PF6], and SOS/[EMIM][TFSI]. The plot can be divided into a frequency dependent power law region (f < ∼1 kHz) and an almost frequency independent plateau region (f > ∼1 kHz).41,42 The lower ionic conductivity of ion gels at low frequency is likely due to interfacial impedance (capacitance). At low frequency, ionic species accumulate near the electrode-ion gel interfaces, leading to a cancelation of the electric field in the bulk and, thus, an apparent drop in conductivity. However, at higher frequencies, only a limited number of ions can accumulate at the interfaces, and thus, the majority of ions in the bulk drift in response to the electric field that persists in the bulk. As shown, the ionic conductivity increases with frequency and eventually plateaus, which is typical of polymer electrolytes.43,44 The high frequency conductivity of all four ion gels was in the range of (1-8) × 10-3 S/cm at room temperature, far higher than that of common solid polymer electrolytes, which have maximum conductivities on the order of 10-5-10-4 S/cm. The high ionic conductivity of ion gels correlates with a shorter dielectric polarization time in the sub kilohertz regime, which is advantageous for use of the gels as gate dielectrics in transistors, but at higher frequencies, the conductivity of the gel limits switching speeds. We will return to this point later in the context of device measurements. We note that the ionic conductivities are more strongly affected by the choice of ionic liquid rather than the choice of triblock copolymer. This is expected, because the polymer weight fraction has intentionally been kept low (∼5%) to maximize ionic conductivity while providing mechanical integrity.35,36 2. Electrical Properties of GEL-OTFTs. Figure 4a shows the drain current (ID)-drain voltage (VD) plot for a SMS/ [EMIM][TFSI]-gated PQT-12 transistor. The output characteristics of the transistor display reasonable saturation behavior,

Figure 4. Performance of a representative GEL-OTFT employing PQT-12 as the active layer. (a) Output characteristics of a SMS/ [EMIM][TFSI]-gated OTFT. (b) Corresponding OTFT transfer characteristics (ID vs VG). (c) ID-VG and IG-VG plot for a PQT-12 OTFT with an SOS/[EMIM][TFSI] ion gel at a gate voltage sweep rate of 75 mV/s. All devices have a channel length of 20 µm and a channel width of 400 µm. Gel thickness was ∼100 µm.

and the output current, |ID|, is greater than 1.0 × 10-3 A at VG ) -2.0 V and VD ) -2 V, a direct result of the large capacitance of the ion gel gate dielectric. The drain current as a function of gate voltage for SMS/[EMIM][TFSI]-based PQT12 transistors is shown in Figure 4b. The curve was measured by sweeping VG from +1.0 to -3.0 at a rate of 75 mV/s. Using the slope of the VG - (|ID|)1/2 curves of more than five devices, average charge carrier mobilities were calculated in the saturation regime (VD ) -1 V) from the following equation:6

|ID| )

W Cµ(VG - Vth)2 2L

(1)

where C is the capacitance of the gate dielectric taken at 10 Hz, W is the channel width (400 µm), L is the channel length (20 µm), Vth is the threshold voltage, and µ is the charge carrier mobility. It has been shown that eq 1 applies to electrochemically gated transistors45,46 as well as traditional field effect devices; the linear dependence of (|ID|)1/2 οn VG also supports the use of eq 1. All PQT-12- and P3HT-based data summarized

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TABLE 1: GEL-OTFT Characteristics for PQT-12- and P3HT-Based Transistors (W ) 400 µm, L ) 20 µm) with Different Ion Gels polymer semiconductor

ionic liquids

triblock copolymer

mobility (cm2/(V s))

max. on current (mA) at VG ) -2.5 V

ON/OFF ratio

subthreshold voltage swing (V/dec)

turn on voltage (V)

PQT-12

[EMIM][TFSI]

SMS SOS SMS SOS SMS SOS SMS SOS

1.77 ( 0.69 0.35 ( 0.17 0.30 ( 0.21 0.30 ( 0.05 1.52 ( 0.78 0.74 ( 0.58 0.50 ( 0.29 0.50 ( 0.43

1.05 0.19 0.17 0.11 0.38 0.25 0.17 0.24

∼5 × 104 ∼1 × 105 ∼1 × 105 ∼5 × 104 ∼1 × 105 ∼5 × 104 ∼5 × 105 ∼5 × 104

0.11 0.27 0.13 0.14 0.38 0.17 0.28 0.13

-0.2 ( 0.12 -0.5 ( 0.42 -0.7 ( 0.21 -0.8 ( 0.11 -0.3 ( 0.05 -0.6 ( 0.12 -0.7 ( 0.08 -0.8 ( 0.10

[BMIM][PF6] P3HT

[EMIM][TFSI] [BMIM][PF6]

in Table 1 show that the GEL-OTFT mobilities are quite large (∼1 cm2/(V s)) compared to the hole mobilities obtained for these polymer semiconductors using TFTs with typical inorganic or organic dielectric layers.47-50 Moreover, the turn-on voltages are less than a volt, and the subthreshold voltage swing parameter (8 min (not shown). There could be several reasons for this deviation from simple one-dimensional diffusion. For example, hole diffusion and ion diffusion are coupled; i.e., an excess hole concentration creates a net positive charge which exerts an electrostatic force upon the negatively charged [TFSI]- ions. Also, when the concentration of the negatively charged [TFSI]- ions in the P3HT is sufficiently high, we expect structural changes in the P3HT film.59,60 To quantitatively analyze the experimental uptake curve, we employed a linear combination of two solutions for eq 4, each with a different diffusion constant. Such a two-component diffusion model provides an excellent description of the experimental data, as shown in Figure 8. Naturally, the faster term dominates at the initial stage. This diffusion constant is represented as a function of gate voltage in Figure 8. At high gate voltages, the diffusivity is on the order of 10-12 cm2/s, and the diffusivity decreases as |VG| becomes smaller. For comparison, Kaneto et al. reported ClO4- diffusion constants of 10-12-10-10 cm2/s in a polythiophene film in contact with liquid electrolyte.61 To further explore the role of ion diffusion on the switching speeds of a GEL-OTFT (i.e., in the short diffusion time regime), the diffusion equation was solved using boundary conditions which more accurately reflect the situation near the dielectric/ semiconductor interface at short time scales. The boundary condition given in eq 3 is changed such that the P3HT film is approximated as semi-infinite and then requiring that [TFSI](zf∞,t) ) 0. Given this, the solution to eq 2 is

(

[TFSI](z,t) ) [TFSI](z)0,t) 1 - erf

[ ]) z 2√Dt

(7)

The right side of eq 7 can be integrated with respect to z over the limits of zero to infinity to yield the following:

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[TFSI](z,t) 2 dz ) √Dt ∫0∞ [TFSI](z)0,t) √

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(8)

π

The concentration profile specified in the lefthand side of eq 8 is commonly approximated as linear with a thickness, l, that evolves at a rate of (Dt)1/2.62 l [TFSI](z,t) z l dz w ∫0 1 - dz ) ∫0∞ [TFSI](z)0,t) l 2

(9)

Thus, the thickness of the electrochemically doped layer is given by the following:

l)

4 √Dt √π

(10)

To determine the time it takes for electrochemical doping to occur, it is useful to specify a definition for time dependent electrochemical doping. For example, electrochemical doping can be considered to occur in the time it takes for the depth of the concentration profile to exceed the lattice parameter of the organic semiconductor in question. Referring to the previously published XRD parameters for semicrystalline P3HT,63 we set l ) 1 nm (approximately the c-axis dimension for crystalline domains) in eq 10 and solve for the time required to electrochemically dope a single molecular layer of P3HT at the gel/ P3HT interface. This characteristic time is ∼2 ms at VG ) -2 V. Thus, for times longer than 2 ms, electrochemical doping can be assumed to occur, but for times less than 2 ms, the mechanism is essentially electrostatic. This characteristic time can be inverted to obtain the maximum frequency for which a GEL-OTFT can be thought to act as an electrochemical device. This result is plotted as a function of gate voltage in panel c of Figure 8, which can be regarded as a map of different operating regimes. High frequencies and lower gate voltages tend to favor an electrostatic switching mechanism, while low frequencies and higher gate voltage favor electrochemical doping. Importantly, even at high gate voltages (VG ) -2.5 V), operation at speeds greater than 1 kHz favors the electrostatic charging mechanism. Collectively, the IR experiments demonstrate that for GEL-OTFTs employing permeable polymer semiconductors, either electrochemical or electrostatic switching regimes can be dominant depending on the operating conditions. Conclusions We have investigated ion gel-gated PQT-12 and P3HT transistors utilizing four different ion gel compositions based on two imidazolium ionic liquids ([EMIM][TFSI] and [BMIM][PF6]) and two ABA type triblock copolymers (SMS and SOS). The capacitances for the [EMIM][TFSI]-based ion gels are ∼30 µF/cm2 at 10 Hz and >1 µF/cm2 at 100 kHz. The cutoff frequency for GEL-OTFTs based on these materials is determined primarily by the increase in the OFF current, which in turn reflects the conductivity-frequency characteristic of the ion gel dielectric and the device footprint. Thus, while capacitance of the ion gel is very large at frequencies up to 100 kHz, such high frequency switching of GEL-OTFTs has so far not been obtainable due to the steep rise in OFF current. Future efforts to improve the switching frequency must focus on shrinking the device dimensions and on improving the capacitance-frequency and conductivity-frequency responses of the gel materials. An ATR-NIR/IR spectroscopy technique was used to investigate the switching mechanism in P3HT GEL-OTFTs, in particular to distinguish between electrochemical and electrostatic charge accumulation regimes in the channel. Ions from

the gel are able to diffuse into the polymer semiconductor under an applied gate bias, a process that results in electrochemical doping of the transistor channel. The time scale for this process is on the order of 1 ms at the largest gate voltages applied to these devices (-2.5 V). Thus, gate bias switching rates less than 1 kHz favor an electrochemical charging mechanism, while frequencies above this threshold favor electrostatic charging. These considerations are important for ongoing efforts to understand and to improve the performance of ion gel-gated transistors. Acknowledgment. This work was supported by the Korean Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-214-D00061 for J.L. and KRF-2006352-D00107 for J.H.C.) and by the University of Minnesota Materials Research Science and Engineering Center funded by the National Science Foundation (NSF) (DMR-0819885). Additional funding was provided by NSF through Award DMR0406656 (T.P.L.) and by the Department of Energy through Award DE-FG02-05ER46252 (X.Y.Z. and C.D.F.). References and Notes (1) Forrest, S. R. Nature (London, U.K.) 2004, 428, 911–918. (2) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature (London, U.K.) 2007, 445, 745–748. (3) Gundlach, D. J. Nat. Mater. 2007, 6, 173–174. (4) Someya, T.; Kato, Y.; Sekitani, T.; Iba, S.; Noguchi, Y.; Murase, Y.; Kawaguchi, H.; Sakurai, T. Proc. Natl. Acad. Sci. U.S.A. 2005, 102 (35), 12321–5. (5) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126 (26), 8138–8140. (6) Sze, S. M. Physics of Semiconductor DeVices; Wiley: New York, 1999. (7) Yoon, M.-H.; Yan, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127 (29), 10388–10395. (8) Kim, S. H.; Yang, S. Y.; Shin, K.; Jeon, H.; Lee, J. W.; Hong, K. P.; Park, C. E. Appl. Phys. Lett. 2006, 89, 183516. (9) Kim, C.; Wang, Z.; Choi, H.-J.; Ha, Y.-G.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130 (21), 6867–6878. (10) Yoon, M.-H.; Facchetti, A.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102 (13), 4678–4682. (11) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schu¨tz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature (London, U.K.) 2004, 431, 963–966. (12) Hur, S.-H.; Yoon, M.-H.; Gaur, A.; Shim, M.; Facchetti, A.; Marks, T. J.; Rogers, J. A. J. Am. Chem. Soc. 2005, 127 (40), 13808–13809. (13) Panzer, M. J.; Frisbie, C. D. AdV. Mater. 2008, 20, 3177–3180. (14) Gray, F. M. Solid Polymer Electrolytes: Fundamentals and Technological Applications; VCH Publishers: New York, 1991. (15) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (16) Takeya, J.; Yamada, K.; Hara, K.; Shigeto, K.; Tsukagoshi, K.; Ikehata, S.; Aoyagi, Y. Appl. Phys. Lett. 2006, 88, 112102. (17) Shimotani, H.; Asanuma, H.; Takeya, J. Appl. Phys. Lett. 2006, 89, 203501. (18) Shimotani, H.; Asunuma, H.; Takeya, J.; Iwasa, Y. Appl. Phys. Lett. 2006, 88, 073104/1. (19) Panzer, M. J.; Frisbie, C. D. Appl. Phys. Lett. 2006, 88, 203504. (20) Panzer, M. J.; Newman, C. R.; Frisbie, C. D. Appl. Phys. Lett. 2005, 86, 103503. (21) Panzer, M. J.; Frisbie, C. D. J. Am. Chem. Soc. 2005, 127, 6960– 6961. (22) Panzer, M. J.; Frisbie, C. D. AdV. Funct. Mater. 2006, 16, 1051– 1056. (23) Dhoot, A. S.; Yuen, J. D.; Heeney, M.; McCulloch, I.; Moses, D.; Heeger, A. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 11834–11837. (24) Panzer, M. J.; Frisbie, C. D. J. Am. Chem. Soc. 2007, 129 (20), 6599–6607. (25) Herlogsson, L.; Crispin, X.; Robinson, N. D.; Sandberg, M.; Hagel, O.-J.; Gustafsson, G.; Berggren, M. AdV. Mater. 2007, 19, 97. (26) Herlogsson, L.; Noh, Y.-Y.; Crispin, X.; Sirringhaus, H.; Berggren, M. AdV. Mater. 2008, 20, 4708. (27) Rosenblatt, S.; Yaish, Y.; Park, J.; Gore, J.; Sazanova, V.; McEuen, P. L. Nano Lett. 2002, 2, 869. (28) Siddons, G. P.; Merchin, D.; Back, J. H.; Jeong, J. K.; Shim, M. Nano Lett. 2004, 4, 927–931.

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