Ionic-Liquid Component Dependence of Carrier Injection and Mobility

Article pubs.acs.org/JPCC

Ionic-Liquid Component Dependence of Carrier Injection and Mobility for Electric-Double-Layer Organic Thin-Film Transistors Takuya Fujimoto,† Michio M. Matsushita,† and Kunio Awaga*,†,‡ †

Department of Chemistry, Graduate School of Science and Research Center for Materials Science, Nagoya University, Chikusa−ku, Nagoya 464−8602, Japan ‡ CREST, JST, Nagoya University, Chikusa−ku, Nagoya 464−8602, Japan S Supporting Information *

ABSTRACT: Electrostatic carrier injection and electrochemical doping of octathio[8]circulene thin films is examined for six kinds of ionic liquids using in situ cyclic voltammetry (CV) and conductivity measurements. The frequency dependence of the capacitance measurements indicates that the ionic liquids form electric-double-layers (EDLs) below 102 Hz. The performance of the EDL-organic thin film transistors (OTFTs) of octathio[8]circulene demonstrates that the transistor carrier mobility shows a linear decrease with an increase in the capacitance of the ionic liquids. In contrast, the electrochemical oxidation potentials and the threshold voltage of the EDL-OTFT are governed only by one component of the ionic liquid; namely, the electrostatic and electrochemical hole injections are significantly affected by the anions.

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INTRODUCTION Ionic liquids have attracted much attention in electronics due to their potential applications in various devices, such as capacitors, batteries, solar cells, fuel cells, and actuators.1−7 They are also applicable to organic thin film transistors (OTFTs), which have seen much progress in recent years.8−14 OTFTs have various advantages, including their flexibility, processability, the possibility for large-area production, and low cost.15−23 In contrast to the large variety of organic materials studied for the active semiconductive layers in OTFTs, the gate dielectric materials have been mostly limited to inorganic solid-state compounds, despite their many drawbacks, such as low capacitance, large surface roughness effects, carrier injection barriers, and/or thermal shrinkage mismatch, which cumulatively result in low-density carrier accumulation. Recently, several groups have proposed utilizing ionic liquids as gate dielectrics for OTFTs8,10,13,24 because high-density carrier accumulation can be realized through the electric-double-layers (EDLs) formed at the interface between the organic semiconductor and the ionic liquid.25,26 It is also expected that ionic liquids would produce additional benefits for OTFTs, such as low power operation, flexibility, easy device fabrication, transparency, and thermal and chemical stability, in addition to the fact that ionic liquids exhibit no crack and dielectric breakdown, which often spoil the performance of conventional solid-state transistors.27,28 Scheme 1 shows the electrostatic and electrochemical carrier injections into a thin film of planar organic molecules in an ionic-liquid electrolyte. In this scheme, the molecular planes are assumed to be nearly perpendicular to the substrate, as can be seen in the thin-film structures for most aromatic organic molecules.29,30 At zero bias voltage (V = 0 V), there is no © 2012 American Chemical Society

Scheme 1. Electrostatic and Electrochemical Model of the Thin Filma

a

V1 and V2 are applied voltages, which have the relationship of V1 < V2.

interaction between the counterions and the thin film. When a voltage of V1 (>0) is applied, the anions concentrate at the surface of the thin film and effectively induce an electrostatic carrier injection at the interface through the formation of an EDL. Once a higher voltage of V2 (V1 < V2) is applied, however, electrochemical doping takes place; the anions penetrate into the thin film and produce carriers in the whole thin film. This process is associated with irreversible penetration of the counteranions, spoiling the transistor performance. Received: December 20, 2011 Revised: February 6, 2012 Published: February 7, 2012 5240

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Octathio[8]circulene was prepared as described in ref 31. Thin films with a thickness of 50 nm were prepared on interdigitated array electrodes (Pt) by vacuum vapor deposition at 400 °C under 3 × 10−4 Pa, with a deposition rate of 2−6 nm min−1, using an ULVAC VPC260FN vacuum coater. The film thickness was monitored during deposition by a quartz crystal microbalance located adjacent to the sample position within the bell jar. The interdigitated array electrodes were prepared by photolithography on surface-oxidized n-doped Si substrates (SiO2: 300 nm), for which the channel dimensions correspond to w = 100 cm width and L = 2 μm length. The coupled electrochemical measurements, which include cyclic voltammetry (CV) and in situ EDL-OTFT performance testing, were carried out at room temperature under dark and vacuum conditions in the ionic liquids. A platinum wire was used as a counter electrode and a Ag/AgCl electrode as a reference electrode, which were immersed in the ionic liquid electrolyte (see Figure 1(a)). The in situ measurements were performed using a Hokuto HAB151 electrochemical analyzer and a digital lock-in amplifier. The EDL-OTFT performance was examined by the alternating current method38 for the source-drain current measurements to avoid a capacitance effect induced by a direct current voltage between the source and the drain electrodes. In situ UV/vis absorption measurements were carried out using an ALS420 electrochemical analyzer and an Ocean Optics HR4000CG-UV-NIR spectrometer, in the photon energy range 1.6−2.6 eV for the thin films (500 nm) of octathio[8]circulene on the ITO substrates (see Figure S1, Supporting Information).

The choice of organic semiconductors for ionic-liquid EDLOTFTs has been limited, due to their high solubility in ionic liquids. Nevertheless, octathio[8]circulene, called “sulflower” (see Figure 1(a)), is a promising candidate since this molecule

Figure 1. (a) Molecular structure of octathio[8]circulene and a schematic illustration of the EDL-OTFT, consisting of a thin film deposited on interdigitated array electrodes (Pt) and an electrolyte of ionic liquid. (b) Molecular structures of the ionic liquids.

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RESULTS AND DISCUSSION EDL Formation. To confirm the formation of EDLs at the interface between the ionic liquids and the electrodes, the frequency dependence of the capacitance CIL was examined for 1−6, in the range from 10−1 to 105 Hz. The results are shown in Figure 2(a), where the frequency is plotted in a logarithmic

forms robust thin films due to the strong intermolecular interactions through the sulfur atoms exposed at the outside of the molecular ring.31−34 Octathio[8]circulene has also attracted attention as a high-performance semiconductor, with a hole mobility of 9 × 10−3 cm2 V−1 s−1, which was obtained using a SiO2 gate dielectric.35 This is probably due to large intermolecular π-overlaps and S···S contacts developed in the crystal structure.31,32,36 In our previous works, we studied the electrochemistry and the EDL-OTFT performance of thin films of octathio[8]circulene in an ionic liquid, N,N-diethyl-Nmethyl (2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl)imide (abbreviated as DEME-TFSI).37,38 In the present work, we reexamined these processes, using six kinds of ionic liquids, and elucidated the crossover between electrostatic and electrochemical doping and to what extent the nature of the ionic liquid governs the transistor performance and the carrier injection.

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EXPERIMENTAL SECTION The ionic liquids, DEME-TFSI (1), N,N-diethyl-N-methyl(2methoxyethyl) ammonium tetrafluoroborate (DEME-BF4, 2), 1-butyl-3-methylimidozolium bis(trifluoromethylsulfonyl) imide (BMIM-TFSI, 3), BMIM-BF4 (4), 1-butyl-3-methylimidozolium trifluoromethane sulfonate (BMIM-TFS, 5), and 1-butyl-3-methylimidozolium hexafluorophosphate (BMIM-PF6, 6) (see Figure 1(b)), were purchased from Kanto Reagents and used without further purification. The capacitances CIL of these ionic liquids were measured using a sandwich structure, consisting of two Al films and a layer of ionic liquid in between. The frequency dependence of the capacitance was measured using an NF ZM2371 LCR meter in the range of 0.1 Hz−100 kHz with an AC voltage amplitude of 50 mV.

Figure 2. Capacitance (a) and tan δ (b) of the ionic liquids as a function of frequency. 5241

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frequencies. In addition, the decrease in CIL and increase in tan δ at high frequencies suggests that the EDLs are formed by a spatial arrangement of the component cations and anions in the ionic liquids at the electrode interfaces, and the deformation cannot follow high-frequency field alternation. In the frequency range where the EDL formation is expected, the capacitances are of the order of 10−6 F cm−2, which can lead to a high carrier density of ∼1015 cm−2. This is much larger than those of usual solid-state gate dielectrics; their typical capacitances are of the order of ∼10−7 F cm−2, and the induced carrier density is ∼1013 cm−2. It is worth noting the material dependence of CIL for the ionic liquids, 1−6, which have been confirmed with good reproducibility. It is clear that they are not governed by the constituent components, namely, cation or anion. For example, the ionic liquids, 1 and 2, commonly include the DEME cation, but their CIL values are significantly different. It is hard to rationalize this material dependence because the capacitance of an ionic liquid is probably governed by various factors, such as volume, shape, and polarizability of the component ions and the interactions between them. In the following sections, we will present the carrier injection and transport processes which are operated by the ionic liquids and will discuss what aspect of the ionic liquids governs them. Electrostatic Carrier Injection. The EDL-OTFT performance of the thin films of octathio[8]circulene was examined in vacuum condition, using the ionic liquids, 1−6, as gate dielectric materials. These measurements were done using a three-electrode system, consisting of the working, counter, and reference electrodes. The experimental setup is shown in Figure 1(a), where VG and −VC are the potentials of the working electrode against the counter and reference electrodes, respectively. The difference between VG and −VC was regulated to be smaller than the width of the electrochemical window of the ionic liquids ( 2 ≈ 4 > 6, namely, TFS > TFSI > BF4 > PF6. It can reasonably be thought that the polarizabilities of the former two are larger than the latter two, so that the former two anions would be adsorbed in greater number on the semiconductor interfaces and would form larger EDLs, which bring about more effective carrier injections. The difference between the two groups, (TFS, TFSI) and (BF4, PF6), can be rationalized by polarizability. Another possible factor is the size of the anions; TFSI is the largest, followed by TFS, PF6, and BF4 in this order. The electrostatic and electrochemical processes need a close contact between anions and semiconductor and an anion penetration into the semiconductor, respectively (see Scheme 1). The difference between TFS and TFSI, or between BF4 and PF6, could be explained by this size effect.

because the molecular volume of the anion is much smaller than VU. It is found that dOX in the first scan is rather large (13 nm), but this is followed by a gradual, linear increase by 3−5 nm/scan after the second scan. Figure 5 (b) shows a gradual increase in |ID| at VC = 0.0 V with the cycles, as indicated by the gray arrow. This is probably caused by trapped counteranions in the thin film, even at VC = 0.0 V. To confirm the doping effects, in situ UV/vis absorption measurements were carried out in the photon energy range 1.6−2.6 eV for a thin film (500 nm) of octathio[8]circulene on an ITO substrate. The results are shown in Figures S3 and S4 (Supporting Information). The absorption spectra indicate two peaks at hν = 2.07 and 2.21 eV in the oxidation process, which correspond to the triplet biradical and doublet monoradical species, respectively, as discussed in detail in our previous report.37 The peak intensity at hν = 2.21 eV shows an increase even after dedoping (VC = 0 V), with an increase upon cycling (see Figure S5, Supporting Information). This is consistent with the presence of residual anions in the thin films. The electrochemical oxidation processes of the octathio[8]circulene thin films in the other ionic liquids, 2−6, were also studied (see Figure S6, Supporting Information). They are essentially the same as that in 1, but interestingly, the oxidation potentials depend on the ionic liquids significantly. Table 1 shows a comparison of the oxidation potentials (VOX) of 1−6 that is defined as a CV-curve peak voltage in the forward scan. Anion Control of Carrier Injection. In the previous sections, it was revealed that the mobility of the EDL-OTFTs of the ionic liquids, 1−6, was governed by their capacitance, and the values of VT and VOX depend on the ionic liquids. In this section, we discuss the controlling elements of the ionic liquids for VT and VOX. Figure 7 shows the relation between VT and VOX for 1−6. There is approximately a linear relation between them; the dotted line, which serves as a guide, has a slope of 1. This indicates a strong correlation between the electrostatic and electrochemical hole injections in the thin films of octathio[8]circulene. Furthermore, it is recognized that the values of VT and VOX for the ionic liquid 1 are close to the corresponding ones of 3, and the values for 2 are very similar to those of 4. Note that 1 and 3

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CONCLUSIONS

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ASSOCIATED CONTENT

We have examined the operation of EDL-OTFTs, and the electrochemical doping of the octathio[8]circulene thin films, using six kinds of ionic liquid as a gate dielectric material, or a solvent. The mobilities of the EDL-OTFTs show a linear relationship between the capacitance of the ionic liquids. In contrast, it was experimentally demonstrated that the threshold voltage of EDL-OTFTs and the electrochemical ionization potential are governed by the anion component of the ionic liquids.

S Supporting Information *

The parasitic resistance of the ionic liquids 1−6, the in situ UV/vis absorption spectra of the octathio[8]circulene thin films in 1, and the CV curves for 2−6. This material is available free of charge via the Internet at http://pubs.acs.org. 5244

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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 We thank Simon Dalgleish for helpful discussion and proofreading the manuscript. This work was supported by a Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and the Japan Society for the Promotion of Science (JSPS). T.F. thanks the JSPS for a Research Fellowship for Young Scientists.

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