Ionic Liquids for Electrolyte-Gating of ZnO Field-Effect Transistors

Article pubs.acs.org/JPCC

Ionic Liquids for Electrolyte-Gating of ZnO Field-Effect Transistors S. Thiemann,† S. Sachnov,‡ S. Porscha,† P. Wasserscheid,‡ and J. Zaumseil†,* †

Institute of Polymer Materials, Friedrich-Alexander Universität Erlangen-Nürnberg, Martensstraße 7, D-91058 Erlangen, Germany Department of Chemical and Bioengineering, Friedrich-Alexander Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany

‡

S Supporting Information *

ABSTRACT: We investigate the influence of the chemical structure of a range of imidazolium-based ionic liquids (IL) on their properties as electrolytes and the device characteristics of electrolyte-gated field-effects transistors (FETs) based on spray-deposited polycrystalline zinc oxide (ZnO). We find a decrease of electron field-effect mobility that correlates with the capacitance of the ionic liquids and not only with the size of the IL-cation. The device stability depends significantly on testing conditions. While they are reasonably stable in nitrogen, ZnO-FETs degrade rapidly in ambient air due to absorption of water by the IL and the resulting ZnO surface reactions. Replacement of the most acidic hydrogen atom of the imidazolium cation and surface passivation of ZnO with hexamethyldisilazane improve environmental stability.

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INTRODUCTION Field-effect transistors (FETs) based on solution processable semiconductors, for example, organic semiconductors, semiconducting nanoparticles, and transparent conducting oxides (TCOs), are one crucial building block for flexible electronics. To achieve true flexibility not only the semiconductor but also all other components must be flexible and processable at low temperatures including the gate dielectric. Polymer gate dielectrics, for example polyvinylalcohol (PVA), polymethylmetacrylate (PMMA), and parylene have been employed, but they all have relatively low dielectric constants and require thick layers to avoid leakage currents resulting in low capacitances and high operating voltages.1 Hence, over the past decade the development of new dielectric materials with high capacitances and low processing temperatures has been an important area of research for flexible electronics. An ideal dielectric should have a high capacitance for low voltage operation and exhibit low leakage currents and a high breakthrough voltage. This is difficult to achieve with traditional organic insulating materials. Panzer et al. demonstrated that solid polymer electrolytes based on LiClO4 or lithium bis(trifluoromethylsulfonyl)imide in polyethylene oxide could be used as dielectrics in organic field-effect transistors (OFET) exhibiting very high specific capacitances (>10 μF·cm−2) and very low operating voltages.2,3 This so-called electrolyte gating is based on the redistribution of ions within the electrolyte when applying a voltage. For example, when a positive bias is applied to the gate electrode anions will move toward the gate and cations toward the semiconducting layer where the ions form an electric double layer (EDL) at each interface. The applied gate voltage drops almost entirely over the few nanometer thick EDLs leading to a very high capacitance (several μF·cm−2). Thus, accumulation of large charge carrier concentrations at the semiconductor interface even at low voltages takes place. © 2012 American Chemical Society

The main disadvantage of solid electrolytes was the low switching speed (few Hz) of the OFETs due to the slow diffusion of ions within the polymer. To increase the switching speed, ionic liquid gels were first introduced by the Frisbie group.4−6 Ionic liquids (IL) are molten (organic) salts that are liquid below 100 °C. Many ILs are liquid at room temperature. They are very interesting electrolytes due to their electrochemical stability, large specific capacitance (∼10 μF·cm−2), and high ionic conductivity. This last property leads to switching speeds of several kilohertz for OFETs with IL electrolyte dielectrics. Mechanically stable ionic liquid gels are formed by blending a triblock-co-polymer with an ionic liquid forming an open polymer network where ions can move without hindrance. These gels are printable while retaining the favorable properties of the IL.7,8 Cho et al. applied ionic liquid gels as dielectrics for all-printed OFETs on flexible substrates with high on-currents and switching speeds.9 Ionic liquids also have been used by several groups to gate a range of other semiconducting materials including single-walled carbon nanotubes,10,11 lead selenide nanoparticle assemblies,12 and ZnO films and single crystals.13,14 The most commonly used ionic liquid for electrolyte gating is 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI], also often abbreviated as [EMIM][Tf2N]). However, [EMIM][TFSI] has its drawbacks, in particular, its electrochemical window is limited and, as for many ILs, its properties change with water content.15 These properties are problematic when charge transport at extremely high carrier concentrations is to be investigated16 or the semiconductor is sensitive to water as, for example, some metal oxides.17 Received: March 13, 2012 Revised: May 1, 2012 Published: June 1, 2012 13536

dx.doi.org/10.1021/jp3024233 | J. Phys. Chem. C 2012, 116, 13536−13544

The Journal of Physical Chemistry C

Article

Since the introduction of [EMIM][TFSI] a few years ago, the field of ionic liquid synthesis has progressed and many ILs are nowadays available that have a larger electrochemical window, higher ionic conductivity, or larger hydrophobicity. Here, we investigate a wide range of new ionic liquids with respect to their ionic conductivity, specific capacitance, electrochemical properties, and, most importantly, their performance as electrolytes in FETs with the objective to find an alternative to [EMIM][TFSI]. Cho et al. and Xie et al. investigated a few ILs based on different anions and cations in comparison to [EMIM][TFSI].4,16 They showed that the chemical structure of the IL influences its viscosity and therefore the ion mobility (i.e., ionic conductivity) as well as the charge transport behavior in OFETs. However, because of the limited number of ionic liquids general conclusions were not yet possible. Our semiconductor of choice is spray-coated zinc oxide. ZnO and other metal oxides have recently been in the focus of research for flexible and transparent electronics due to their large bandgap, high electron mobility, relative environmental harmlessness, and robustness, which are important for potential applications.18,19 The general advantages of electrolyte gating also apply to ZnO, but even small amounts of water in the IL can change the device performance as shown by Yuan et al. for single-crystal ZnO,17 clearly limiting its applicability. Our ultimate goal is thus to find an ionic liquid that could be used with transparent metal oxide semiconductors under ambient conditions without degradation of performance. In particular, we are investigating eight ILs (shown in parts a and b of Figure 1) with cations based on 1-alkyl-3methylimidazolium with different alkyl chain lengths (ethyl, butyl and hexyl) and 1-alkyl-2-methyl-3-methylimidazolium cations where the acidic hydrogen in position 2 is replaced by a methyl group. Anions include [TFSI]− as well as the highly mobile [TCB]− (tetracyanoborate, [B(CN)4]−) and the bulky, very hydrophobic and noncoordinating [FAP] − (tris(pentafluoroethyl)trifluorophosphate, [PF3(C2F5)3]−), which is supposed to increase the electrochemical window compared to the [TFSI]− anion.20,21 We will show that by changing the chemical structure of the ionic liquid we can tune the threshold voltage for electron transport and the electron field-effect mobility in ZnO FETs. We find that long-term stability of electrolyte-gated ZnO FETs under ambient conditions is improved by certain ILs and can be increased further by passivation of the ZnO surface.

Figure 1. Molecular structures of cations (a) and anions (b) of ionic liquids (ILs). (c) Schematic illustration of IL-gated zinc oxide fieldeffect transistor (S − source electrode, D − drain electrode).

For cyclic voltammetry (CV) and impedance measurements, a conventional small volume three-electrode electrochemical cell with a glassy carbon (GC) working electrode, a platinum counter electrode (both 2 mm in diameter), and a silver wire as a pseudoreference electrode was employed. The cell was continuously purged with argon. The scan speed for all CVmeasurements was 100 mV·s−1. For impedance analysis at frequencies from 1 MHz to 0.1 Hz, two platinum electrodes with 2 mm diameter and a silver wire as pseudoreference were used. Measurements were performed at an oscillation amplitude of 10 mV using an Autolab potentiostat PGSTAT30 (Metrohm) with an impedance analyzer unit. Additional frequency and bias dependent capacitances were measured with an Agilent LCR parameter analyzer 4980A (using Cp-mode) with the IL confined between two platinum electrodes by a polydimethylsiloxane (PDMS, Corning Sylgard 184) frame (approximate height 5 mm). The platinum electrodes consisted of stainless steel sheets with 100 nm of electron-beam evaporated platinum. This configuration will be referred to as Pt/IL/Pt. Frequency-dependent specific capacitances were measured at a dc bias of 0 V. For voltagedependent capacitance measurements the frequency was kept at 20 Hz and the voltage was scanned from negative to positive. ZnO thin films were fabricated by spray coating based on procedures established by Adamopoulos et al.22 and Faber et al.23 For the precursor solution, 439 mg zinc acetate dehydrate

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EXPERIMENTAL SECTION Ionic liquids were purchased as high purity grade ILs from Merck KGaA ([EMIM][TFSI], [EMIM][TCB], [EMIM][FAP], [BMIM][FAP], and [HMIM][FAP]) or synthesized ([EMMIM][TFSI], [BMMIM][TFSI], and [BMIM][TFSI]) using an anion exchange method with imidazolium halide salts from Merck KGaA as starting materials as described in the literature.20,21 All ILs were dried in vacuum (∼5 × 10−2 mbar) at 60 °C for 12 h before being transferred into a nitrogen glovebox for measurements and storage. In all cases, the water content was less than 200 ppm. Karl Fischer-Titration was performed with a Metrohm 756 KF Coulometer to determine the water content of all ILs after drying and after 7 to 14 days of exposure to air. Conductivities of all ILs (after drying) were in the range of few mS·cm−1 as measured directly with a conductivity meter (GLF 100, Greisinger) in the glovebox. 13537



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Table 1. Physical Properties (Ionic Conductivity, Capacitance, Anodic, and Cathodic Limit and Water Content) of Ionic Liquidsa cation

anion

condb [mS·cm−1]

condc [mS·cm−1]

capd [μF·cm−2]

cape [μF·cm−2]

capf [μF·cm−2]

anodic limit [V]

cathodic limit [V]

H2O-contentg [ppm]

H2O-contenth [ppm]

[EMIM] [BMIM] [HMIM] [EMIM] [BMIM] [EMIM] [EMMIM] [BMMIM]

[FAP] [FAP] [FAP] [TFSI] [TFSI] [TCB] [TFSI] [TFSI]

5.2 2.8 1.9 9.3 4.2 16.3 4.0 2.2

3.6 2.3 1.3 7.4 n.a. 13.0 n.a. n.a.

5.5 2.0 3.9 7.8 3.9 5.2 3.9 3.7

2.4 1.3 3.3 3.8 3.3 8.4 3.5 3.1

3.4 3.2 2.4 7.0 5.9 8.7 4.7 4.0

2.2 2.2 2.0 2.5 2.2 2.4 2.2 2.2

−2.2 −2.2 −2.0 −2.2 −2.6 −2.2 −2.7 −3.0

175 38 63 76 193 148 87 147

361 358 243 1577 4769 (*) 19 700 3742 (*) 3137 (*)

a

cond = conducitivity, cap = capacitance, n.a. = not available. bConductivity measured with a conductivity meter within a temperature range of 24 to 30 °C. cConductivity as quoted by supplier. dCapacitance fitted to the RC-RC-model by Yuan et al.27 eCapacitance calculated from impedance analysis according to Lee et al.26 fMaximum capacitance (Cp - mode) measured with LCR meter on a Pt/IL/Pt configuration at 20 Hz, dc bias 0 V. g H2O-content of vacuum-dried ILs. hH2O-content after 7 days in ambient air or after 14 days(*).

(ZnAc2·2 H2O, 99.9%, Sigma-Aldrich) were dissolved in 20 mL of methanol (Sigma-Aldrich). Highly doped silicon (100) wafers with 300 nm of thermal SiO2 served as the substrate and were placed on a hot plate at 390 °C. The precursor solution was sprayed twice onto the hot substrates using an air-brush gun with intervals of 15 s of spraying and 1 min of heating to achieve 20−30 nm thick ZnO layers with an average crystallite size of 19.4 nm as determined from the (002) diffraction peak (Phillips Xpert-MPD PW3040 diffractometer). During spraying, the temperature dropped to 377 °C. The mean roughness of the produced ZnO layers was 1.1 nm as determined by atomic force microscopy (AFM, Veeco diDimension 5000). Surface passivation was achieved by dropcasting HMDS (hexamethyldisilazane) twice onto the spraycoated ZnO layer at room temperature followed by heating to 150 °C over 2 min. Subsequently, the samples were kept at 150 °C for 2 h to remove unreacted HMDS residue from the surface. The resulting −Si(CH3)3 surface termination led to an increased contact angle of water on the HMDS-modified ZnO surface (105°) compared to pristine ZnO (51°). All devices were completed by deposition of 35 nm aluminum on top of the ZnO by thermal evaporation through a polyimide shadow mask to pattern source and drain electrodes (channel lengths L = 10 to 80 μm, channel widths W = 500 to 4000 μm with constant W/L = 50). The current−voltage characteristics of ZnO-FETs were recorded with an Agilent 4155C semiconductor parameter analyzer. A PDMS frame on top of the Si/SiO2/ZnO/Al devices kept the ionic liquid in place for all measurements. An immersed platinum wire served as the gate electrode. Unless otherwise noted, all samples were stored and all measurements were carried out in a dry nitrogen glovebox.

other hand results in lower ionic mobility as, for example, [EMIM][FAP] with only 5.2 mS·cm−1. As expected, increasing the alkyl chain length of the imidazolium cation from ethyl to hexyl reduces the ionic conductivity as does introducing an additional methyl group at the C2 position of the imidazolium ring as shown for [EMMIM][TFSI] (2.2 mS·cm−1) versus [EMIM][TFSI]. Nevertheless, all conductivity values are high and in a range suitable for FET electrolyte dielectrics. The second important feature of a good electrolyte is its electrochemical window. Part a of Figure 2 shows the cyclic voltammograms (CVs) of [EMIM][TFSI], [EMIM][TCB], and [EMIM][FAP] using a silver-wire as a quasi-reference, glassy carbon as the working electrode, and platinum as the counter electrode under inert conditions. The [EMIM] cation has a cathodic potential of −2.2 V independent of the anion. For glassy carbon as the working electrode, the anodic potentials of the anions are 2.2 V for [FAP]−, 2.4 V for [TCB]−, and 2.5 V for [TFSI]−. Contrary to our expectations, we do not find a larger anodic limit for ILs with [FAP]− anions. Ignat’ev et al. and Bejan et al. who reported an increased electrochemical window for [EMIM][FAP] performed measurements in an acetonitrile (CH3CN) solution with a glassy carbon working electrode, a platinum auxiliary electrode, and a Ag/AgNO3 (CH3CN) reference electrode.20,21 These different measurement conditions may explain the deviation from our values. All other cathodic and anodic potentials are listed in Table 1. Note that replacing the acidic C2-hydrogen of the imidazolium cation24,25 with a methyl group significantly increases its cathodic potential to up to −3.0 V, for example for [BMMIM][TFSI]. The total electrochemical windows of our ILs vary from 4.0 to 5.2 V. As previously observed by O’Mahny et al.,15 the electrochemical windows shrink substantially with water absorption under ambient conditions. For the determination of the field-effect mobility of IL-gated ZnO-FETs, it is necessary to know the number of mobile charges present in the channel (carrier concentration). This number is usually derived using the areal capacitance of the dielectric and the applied gate voltage. For standard dielectrics, this capacitance is easily calculated from the thickness and dielectric constant of the insulating layer. For electrolyte-gated devices, this is more difficult due to the frequency and bias dependent formation of the EDL. In the following, we apply and compare different techniques and theoretical models for the evaluation of the specific capacitance of ionic liquid gate dielectrics that are found in the literature.

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RESULTS AND DISCUSSION Characterization of Ionic Liquids. All ionic liquids in this study were characterized with respect to their electrochemical window, ionic conductivity, specific capacitance, and water content. The results are summarized in Table 1. The ionic conductivities of these ILs offer a first indication of whether they are useful for fast switching FETs, that is whether they can fully form an electric double layer within a switching cycle. [EMIM][TFSI] (9.3 mS·cm−1) and [EMIM][TCB] (16.3 mS·cm−1) show the highest conductivities of all investigated ILs. [TCB]− is a very small anion and thus a high ionic mobility is expected. The bulky and hydrophobic [FAP]− ion on the 13538



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capacitance. We apply this model to our data but without the Warburg impedance. By fitting the data using the NovaSoftware (Metrohm) toolkit both capacitances are obtained. Table 1 shows the specific capacitance of the EDLd extracted with this method. Because at low frequencies the bulk capacitance is much smaller than the EDL capacitance, the equivalent circuit can be simplified to a resistor (IL bulk) and capacitor (EDL) connected in series29,30 and the same impedance data as above is analyzed with this model. The impedance Z is the alternating current resistance and is the sum of the real part (Z′) and imaginary part (Z″) with Z = Z′ + iZ″, where i is the imaginary unit. For a nonreactive electrolyte (i.e., within its electrochemical window) exhibiting capacitive behavior at low frequencies and resistive behavior at high frequencies (part b of Figure 2) the total impedance depends on the ohmic resistance (R) and the electric double layer capacitance (CEDL), with Z = R + i2πf CEDL, where f is the frequency of the ac perturbation and CEDL is the capacitance of the electric double layer. The imaginary part of the impedance is inversely proportional to the specific capacitance according to Z″ = −

1 C 2πfA

(1)

with A as the electrode area. Specific capacitances of all ILs calculated with eq 1 at the maximum of the phase angle are listed in Table 1 as cape. The Nyquist plots (i.e., Z′ vs Z″ plot, Figure S2 of the Supporting Information) of all ILs show a straight line with a slight curvature, which is common for ionic liquids due to high charge-transfer at the metal−dielectric interface compared to the ion diffusion.26,30,31 Eq 1 reproduces the linear part of the Nyquist-plot but not the small curvature at high frequencies. Furthermore, the Z″ versus frequency plots of all ILs (Figure S4 of the Supporting Information) show a slope of −0.89 instead of the expected slope of −1 for an ideal serial RC-element. Thus, the obtained capacitance values are not very reliable. More detailed models of ionic liquid EDLs use a constant phase element instead of an ideal capacitive EDL and are discussed in the literature.29,31 Finally, the frequency-dependent capacitance of an IL can be measured with a Pt/IL/Pt sandwich structure and a precision LCR-meter using the Cp-mode, which assumes a resistor and a capacitor in parallel. This configuration is very similar to the situation in the actual FET. Part c of Figure 2 shows the specific capacitance versus frequency from 1 MHz to 20 Hz for [EMIM][TFSI], [EMIM][TCB], and [EMIM][FAP]. We observe the typical decrease of specific capacitance with frequency for all ILs due to the slow formation of the EDL depending on the mobility of the ions in the electrolyte. [EMIM][TCB] shows a specific capacitance of 8.6 μF·cm−2, [EMIM][TFSI] 7.0 μF·cm−2, and [EMIM][FAP] 3.0 μF·cm−2 at 20 Hz. The capacitance of [EMIM][TFSI] is somewhat lower than reported in literature (10 μF·cm−2),16 possibly due to high electrode roughness and contact resistance between the platinum film and the steel plates. Maximum specific capacitances close to zero bias for all ILs at 20 Hz are listed in Table 1 as capf. The insert of part c of Figure 2 shows the specific capacitance versus the dc bias voltage at 20 Hz. Variations of capacitance with applied bias are commonly observed and depend on the shape, size, and chemical structure of the electrolyte ions.31,32 Furthermore, the roughness of the electrode surface plays an important role in the formation of the EDL.33 Both the evaporated platinum layer and the ZnO

Figure 2. (a) Electrochemical window of ILs under inert conditions with glassy carbon as a working electrode and a Ag-wire as quasireference electrode. (b) phase-( f) plot for [EMIM][TFSI], [EMIM][TCB], and [EMIM][FAP] between two Pt-electrodes. (c) Frequency dependence of the specific capacitance (Cp) measured with a Pt/IL/Pt sandwich configuration for [EMIM][TFSI], [EMIM][TCB], and [EMIM][FAP] at 0 V bias. Inset: voltage dependence of the specific capacitance for [EMIM][TFSI], [EMIM][TCB], and [EMIM][FAP] at a frequency of 20 Hz.

First, we use electrochemical impedance spectroscopy (EIS) with a three-electrode configuration. Part b of Figure 2 displays the phase angle versus frequency (f) behavior for [EMIM][TFSI], [EMIM][TCB], and [EMIM][FAP] (see Figures S1− S5 of the Supporting Information for phase angle versus frequency plots, Nyquist-plots, Z′(f), and Z″(f) plots of all ILs). At higher frequencies, the phase angle is small corresponding to a resistor behavior. For low frequencies, the phase angle is nearly 90° as expected for a capacitor.26 In this frequency range, the phase angle for [EMIM][TFSI] decreases with increasing dc bias (part a of Figure S5 of the Supporting Information) similar to observations by Yuan et al.27 Simultaneously, the phase angle peak shifts toward higher frequencies and the curve shape becomes more symmetric. Various different models for the calculation of the capacitance of an ionic liquid from impedance data are discussed in the literature.28,29 In general, the employed equivalent circuits consist of a capacitor and a resistor in parallel or in series and variations thereof. Yuan et al. applied a model with two resistor-capacitor (RC) circuits connected in series including an additional Warburg impedance.27 The EDL is considered as a capacitor with a resistive part (in parallel) and the bulk ionic liquid is considered as a resistor with a small capacitive component (also in parallel). At high frequencies, the behavior is dominated by the bulk IL with a low capacitance and at low frequencies the EDL dominates with a very high 13539



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transfer (ID vs VG) characteristic of FETs with [EMIM][TFSI], [EMIM][TCB], [BMIM][TFSI], and [HMIM][FAP] as the electrolyte measured in dry nitrogen atmosphere. All FETs exhibit good transfer characteristics at low applied voltages with little current hysteresis and on/off ratios of 103 to 104. The gate leakage currents within the range of 10−7−10−8 A are typical for electrolyte-gated FETs with an unpatterned semiconductor. The maximum on-currents for FETs with W/L = 50 are around 1 mA. Complete transfer and output characteristics of all ILgated FETs and for bottom-gate FETs with 300 nm SiO2 as the gate dielectric is shown in Figures S6−S14 of the Supporting Information and extracted parameters are summarized in Table TS1 of the Supporting Information. All ZnO-FETs show negative turn-on and threshold voltages indicating n-doping of the ZnO. Oxygen vacancies used to be seen as the origin, but recently hydrogen impurities are assumed to account for most of the n-type conductivity in ZnO. Hydrogen can be introduced easily by humidity under ambient conditions and acts as a shallow donor.34−36 The estimated field-effect mobilities for IL-gated ZnO-FETs are about 10 times higher than those of ZnO-FETs with a SiO2 dielectric in a bottom-gate configuration (Figure S14 of the Supporting Information). This might be an effect of the grain size of the polycrystalline ZnO at the semiconductor−dielectric interface compared to the top surface. According to TEM studies by Faber et al., larger crystallites are formed in the middle of the spray-coated layer than at the bottom and top.23 A similar difference between mobilities has been observed previously by Bong et al. for ZnO-FETs. They argued that the higher charge density induced by the electrolyte-gating leads to the filling of shallow traps that usually reduce carrier mobilities.13 For all ZnO-FETs the ID−VD plots exhibit a linear regime for small drain voltages (80 transfer measurements) in nitrogen atmosphere. The transfer characteristics for both devices are reasonably stable over time. For [EMIM][TCB], we observe a positive threshold shift of 0.5 V and a decrease of the off-current mainly due to reduced gate current. The on−off ratio even increases slightly. For [EMIM][FAP], there is hardly any threshold shift but hysteresis and on-current are reduced. The on−off ratio decreases by almost an order of magnitude and the linear field-effect mobility falls slightly. However, in ambient atmosphere (parts c and d of Figure 5) the long-term behavior is quite different. For [EMIM][FAP], the on- and off-currents

Figure 4. a) Linear field-effect mobility vs the specific capacitance (Cp) for all electrolyte-gated FETs and for bottom-gate FET with 300 nm SiO2 as the dielectric. b) Threshold voltage (black squares) and onvoltage (red circles) vs inverse of the specific capacitance (Cp) for FETs gated via different ILs and SiO2.

capacitance of the EDL also changes with bias as shown in part c of Figure 2, whereas here we only use the capacitance value at a bias of 0 V. However, we also see a decreasing transconductance ∂ID/∂VG (VD = const.) with increasing specific capacitance of the ILs, which indicates a reduced mobility independently from any uncertainties of the capacitance. A similar dependence of mobility on capacitance has been observed for IL-gated p-type rubrene single-crystal FETs.16,37 Xie et al. argued that due to the larger radius of the [FAP]− ion compared to the [TFSI]− ion, the holes may be less strongly bound at the interface with the organic semiconductor (rubrene).16 Our data show mainly a dependence on capacitance and only partially on the size of the cations forming the EDL at the semiconductor−dielectric interface (part a of Figure 4). If the anion remains the same, an increase of the cation size, for example from [EMIM] to [HMIM], leads to an increase in field-effect mobility as expected due to its size and the lower dielectric constant.38 However, for the same cation the field-effect mobility also depends strongly on the anion, that is increasing in the sequence of [EMIM][TCB], [EMIM][TFSI], to [EMIM][FAP]. The nature of the ion pair may have a larger impact on the formation and capacitance of the EDL than the cations alone. The dependence of charge carrier mobilities in organic semiconductors on the dielectric constant of the insulator is generally attributed to the higher dipolar and thus energetic disorder at the semiconductor− dielectric interface.39,40 The charge transport in ZnO is different from that in organic semiconductors and the dielectric constants of ILs are all quite similar (8−15).38,41 However, the 2D ionic mobility of ion pairs at the interface, which scales roughly with the measured capacitance (Table 1), could lead to local potential fluctuations and thus a broadening of the distribution of tail states in ZnO that would correlate with a reduction of carrier mobility. More experimental data on n-type and p-type semiconductors and theoretical modeling of the 13541



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replaced with a methyl group (e.g., [BMMIM][TFSI] and [EMMIM][TFSI]). The device characteristics and timedependent on−off ratios are shown in Figure 6 and Figure

Figure 5. Transfer characteristics of ZnO-FETs with [EMIM][TCB] (VD = 1.0 V) and [EMIM][FAP] (VD = 1.0 V) after 0 and 7 h in dry nitrogen (a) and in air (c). On−off ratios vs time for [EMIM][TCB] (VD = 1.0 V) and [EMIM][FAP] (VD = 1.0 V) in nitrogen (b) and in air (d).

Figure 6. Transfer characteristics of ZnO-FETs with [BMIM][TFSI] (VD = 1.5 V) and [BMMIM][TFSI] (VD = 1.0 V) after 0 and 7 h in dry nitrogen (a) and in air (c). On−off ratios vs time for [EMIM][TFSI] (VD = 1.5 V), [EMMIM][TFSI] (VD = 1.0 V), [BMIM][TFSI] (VD = 1.5 V), and [BMMIM][TFSI] (VD = 1.0 V) in nitrogen (b) and in air (d).

drop quickly, hysteresis increases, and the threshold voltage shifts toward positive values. For [EMIM][TCB], the offcurrents remain stable and only the on-current decreases. Similar to [EMIM][FAP], the hysteresis increases whereas the threshold voltage shifts. Both devices show significantly degraded device performances as the ILs absorb water from the atmosphere although the estimated amounts are quite different (Table 1). Yuan et al. investigated the influence of small amounts of water on the device characteristics of single-crystal ZnO-FETs with different surface terminations.17 They argued that chemisorption of H+ on oxygen-terminated ZnO surfaces causes a large threshold shift toward negative voltages, whereas zinc-terminated surfaces remain unaffected. The surface of our spray-coated ZnO is not well-defined. It probably contains some residual acetate groups and hydroxyl groups due to the deposition process in air at high temperatures. Hence, it is difficult to postulate a mechanism that would explain the observed threshold shifts and reduced currents. We can exclude electrochemical dissolution of the ZnO by the IL as there is no trace of zinc in the IL as determined by inductively coupled plasma mass spectrometry (ICP-MS) after extended stressing. In the model by Yuan et al., the protons in ILs with sufficient water content (∼500 ppm) chemisorb onto the O-terminated ZnO surface due to a reduction reaction including protons and electrons at positive gate voltages (the O-ZnO surface acts as the anode) resulting in an abrupt increase of the gate current.17 The slightly acidic C2-hydrogen of the imidazolium ring (pKa = 22)25 may dissociate in a protic environment and thus increase the number of available protons for surface reactions. To test this, we compare the stability of ZnO-FETs with ILs with the acidic C2-hydrogen (e.g., [BMIM][TFSI] and [EMIM][TFSI]) with those for which this most acidic hydrogen was

S16 of the Supporting Information. In nitrogen atmosphere, we observe similar behavior as described before for all ILs except [BMIM][TFSI]. The on−off ratios are relatively stable. However, all devices show a reduction in off- and on-currents with time. The on-current for [BMIM][TFSI] drops quickly. For [BMIM][TFSI] and [BMMIM][TFSI], the threshold voltage shift is about 0.5 V, for [EMIM][TFSI] and [EMMIM][TFSI] only 0.2 V. All devices exhibit reduced hysteresis after 7 h. The gate current versus gate voltage curves indicate a typical charge injection and charge extraction behavior for [EMIM][TFSI], [EMMIM][TFSI], and [BMMIM][TFSI] (Figure S17 of the Supporting Information). This is not observed for [BMIM][TFSI]. Devices with [BMIM][TFSI] degrade even under inert conditions, although the acidities of [EMIM]+ and [BMIM]+ are similar.25 In summary, replacing the acidic H-atom increases ambient stability of IL-gate ZnO-FETs but it is not sufficient for possible applications. As an alternative, we also tested ZnO-FETs that were passivated with HMDS using [EMIM][TFSI] as the electrolyte in air and dry nitrogen. The device characteristics of pristine and passivated ZnO-FET in nitrogen are very similar, both are reasonably stable and show a voltage shift and reduction of the on- and off-currents as described above. Figure 7 displays ID− VG-characteristics and on−off ratios of [EMIM][TFSI]-gated ZnO-FETs with and without surface treatment at the beginning and after 13 h in air. Transfer measurements were repeated every hour for the first 5 hours and then every 20 min for the 13542



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ASSOCIATED CONTENT

S Supporting Information *

Phase vs frequency plots (including dc bias dependence), Nyquist-plots, Z′ vs frequency plots from 104 Hz to 106 Hz and Z″ vs frequency plot from 0.1 Hz to 104 Hz, the transfer and output characteristics for ZnO-FETs with all ionic liquids used in this study, the dependence of saturation field-effect mobility on capacitance and additional stability data for devices in nitrogen and air. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for financial support by the Deutsche Forschungsgemeinschaft (DFG) via the Research Training Group ’Disperse Systems for Electronic applications’ (GRK 1161) and the Cluster of Excellence ‘Engineering of Advanced Materials’ (EXC 315) and Evonik Degussa GmbH, Germany. The authors thank H. Faber for help with ZnO thin film fabrication, M. Halik for access to the thermal evaporator and U. Marten-Jahns for XRD-measurements.

Figure 7. (a) Transfer characteristics at start and after 13 h and (b) on−off ratio vs time for nonpassivated (pristine) ZnO-FETs with [EMIM][TFSI] (VD = 1.0 V) in air. (c) Transfer characteristics at start and after 13 h and (d) on−off ratio vs time for passivated ZnO-FETs with [EMIM][TFSI] (VD = 1.0 V) in air.

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REFERENCES

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next 8 hours. The ZnO-FETs without HMDS treatment degrade over time as shown before. The ZnO-FETs with surface treatment show a slight decrease in on−off ratio with time, but devices still exhibit transfer characteristics after 13 h and even after 21 h. The threshold voltage shifts toward positive values by about 0.5 V but hysteresis remains similar. The linear field-effect mobility drops to a tenth of its initial value. Clearly, passivation of the ZnO-surface improves the stability of IL-gated FETs quite efficiently by suppressing detrimental surface reactions. However, the devices are still not stable over weeks. Further improvement of the passivation of the ZnO-surface is necessary and could be achieved using alkylphosphonic acids or alkylsilanes.42,43

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CONCLUSIONS In summary, we investigated the influence of the chemical structure of various imidazolium-based ionic liquids on the device characteristics of electrolyte-gated ZnO-FETs. We could not identify an ionic liquid that combines all desirable properties for fast-switching, high field-effect mobility and airstable FETs. The field-effect mobilities showed a nonlinear decrease with increasing capacitance of the ILs probably due to energetic disorder in the electric double layer at the semiconductor−dielectric interface. Transistor performances were largely stable over time in nitrogen atmosphere but degraded quickly in air due to water absorption and surface reactions of the ZnO. Replacement of the acidic H-atom of the imidazolium cation and passivation of the ZnO surface with HMDS improved device stability in air. These findings are likely to be applicable to other oxide semiconductors44 that are of interest for flexible electronics. 13543



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Ionic Liquids for Electrolyte-Gating of ZnO Field-Effect Transistors

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