Organic Electrical Double Layer Transistors Based on Rubrene Single

ARTICLE pubs.acs.org/JPCC

Organic Electrical Double Layer Transistors Based on Rubrene Single Crystals: Examining Transport at High Surface Charge Densities above 1013 cm
2 Wei Xie and C. Daniel Frisbie* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. SE, Minneapolis, Minnesota 55455, United States

bS Supporting Information ABSTRACT: We report comprehensive electrical characterization of electrolyte-gated field-effect transistors (FETs) incorporating organic single crystals of rubrene and several types of high capacitance ionic liquids (ILs). The specific capacitance associated with the liquid gate is exceptionally large, in the range of 1
10 μF/cm2, which facilitates the operation of devices at gate voltages below one volt. Gate-induced hole densities in rubrene single crystals are therefore on the order of 1013 cm
2, as determined from displacement current measurements (DCM) and confirmed by AC impedance measurements. Importantly, we observe a pronounced maximum in channel conductance with all ionic liquid gates which we attribute to a carrier localization effect at the semiconductor/liquid interface. Effective carrier mobility is a nonmonotonic function of gate voltage and depends on the choice of the IL. By gating with a tris(pentafluoroethyl)trifluorophosphate (FAP) containing IL, maximum carrier mobility in rubrene can be enhanced up to 3.2 cm2 V
1 s
1 at room temperature. Extensive efforts have been made to maximize the charge densities accumulated in rubrene crystals. At lower temperatures, higher gate bias can be applied before device breakdown, and up to 6  1013 cm
2 carriers can be accumulated at the rubrene/IL interface (0.3 holes per rubrene molecule), which doubles the amount of accumulated charge achieved at room temperature.

’ INTRODUCTION Free carrier density is a key parameter controlling the electronic properties and phase behavior of insulators, semiconductors, and superconductors.1,2 By tuning charge density levels, novel transport phenomena have been observed in a wide category of materials, for example, superconductivity in alkali-doped C60 and picene, and field-induced superconductivity in SrTiO3.3
7 Electric-field tuning of charge density in a field-effect transistor (FET) platform is becoming an increasingly attractive option compared with chemical doping for exploring electronic phase transitions and transport because it is convenient and minimizes structural and chemical disorder. In an FET structure, the surface conductivity of a material can be controlled over many orders of magnitude with a gate electrode separated by a dielectric layer. However, it is important to be able to achieve as high a carrier density as possible. For conventional dielectrics with dielectric constants less than 10, it is difficult to obtain carrier densities above 1013 cm
2.8 Several research groups have employed ultrathin selfassembled monolayers (SAMs) or cross-linked polymer films as gate insulators (because capacitance is inversely related to dielectric thickness),9
13 but still charge densities above 5  1013 cm
2 are difficult to reach before a dielectric breakdown. Another strategy has been to employ complex oxide ferroelectrics with field-dependent dielectric constants near 300 at room r 2011 American Chemical Society

temperature, and in these cases, carrier concentrations in the vicinity of 1014 cm
2 have been realized in favorable circumstances.1,14 Recently, electrolyte gating in FETs has become a promising technique to raise the charge density level into a much higher regime, on the order of 1013
1015 cm
2.7,15
21 The reason for ultrahigh charge accumulation with electrolyte gates is that, upon the application of gate bias, electrolyte ions will migrate toward the gate/electrolyte and electrolyte/semiconductor interfaces, forming two electrical double layers (EDLs). The bulk region of the electrolyte remains charge neutral, while the potential only drops at the nanometer-thick double layers. The capacitance associated with the EDLs is therefore enormous, on the order of μF/cm2, which far exceeds the maximum attainable capacitance for conventional dielectric materials. Indeed, with charge densities larger than 1014 cm
2 through electrolyte gating, researchers have been able to control the insulator-to-metal transition in ZnO and NdNiO3 electric double-layer transistors (EDLTs),17,22 and other groundbreaking observations include field-effect induced superconductivity on the surface of SrTiO3 and ZrNCl.7,15 There is also Received: May 4, 2011 Revised: June 3, 2011 Published: July 01, 2011 14360

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’ EXPERIMENTAL SECTION

Figure 1. (a) Chemical structures of rubrene and ionic liquids. Combinations of cations and anions are used as the gate insulators. (b) Cross section of a rubrene electrical double-layer transistor (EDLT).

substantial research interest in extending electrolyte gating to other systems. One example is organic semiconductors, which have versatile electronic functionality23 and potential applications in printable and flexible electronics.24
26 However, studies of transport physics at high charge densities in organic semiconductors gated with electrolyte are limited, which significantly impedes the realization of novel electronic phenomena on organic surfaces and interfaces. In this paper, we have carried out a systematic study of transport behavior of rubrene single crystal EDLTs gated with different ionic liquids (ILs), particularly at charge densities larger than 1013 cm
2 (see Figure 1). Ionic liquids are room-temperature molten salts that typically have a large ionic conductivity (10
3 S/cm) but negligible electronic conductivity.27 Unlike polymer semiconductors that are susceptible to electrochemical doping in which ions can penetrate into the film and induce structural disorder, organic single crystal surfaces are not compromised by electrolyte gating and charge accumulation on the rubrene surface is via the field effect.21,28,29 Organic single crystal devices gated with an ionic liquid can thus be described properly as EDLTs. We expand upon our previous work in which we addressed a significant mobility lowering effect in electrolyte gated rubrene EDLTs.30 Specifically, we characterize and compare the gate-induced hole densities, carrier mobility, and dielectric capacitance of rubrene EDLTs gated with different ionic liquids via the displacement current measurement (DCM). We also demonstrate high performance of rubene EDLT devices with minimized gate leakage, excellent device stability and reproducibility, and remarkable room-temperature carrier mobility up to 3.2 cm2 V
1 s
1. Further efforts have been made to study low temperature transport and especially to maximize attainable charge densities induced on rubrene surfaces. A charge accumulation of 6.3  1013 cm
2 can be achieved at 210 K with applied gate voltage of
3.5 V, which is the highest charge concentration achieved in organic single crystal EDLT devices.18,31
34 Collectively, these results imply that organic EDLTs are promising testbeds for fundamental transport physics at ultrahigh charge densities in organic semiconductors.

Materials. Rubrene powders (sublimed grade) were purchased from Sigma Aldrich and were purified by multiple (at least two) sublimations under flowing argon as described previously.35 Single crystals of rubrene were grown from the purified source material using horizontal physical vapor transport (PVT) with a source temperature of 280
300 °C.35 Plate-like rubrene crystals several millimeters in length and width and tens of micrometers in thickness were obtained after a few days' growth. Crystals were observed under an optical microscope, and only those with uniform crystalline regions and smooth surfaces were selected for further examination. High-resolution X-ray diffraction (Philips X'pert) and atomic force microscopy (Veeco Instruments, Multimode) were carried out to characterize typical rubrene crystals from each growth batch. We found that repeated sublimations of rubrene produced small decreases in the threshold voltage and increases in carrier mobility for vacuum gap rubrene FETs (parameter extraction explained in the following section). Different from chemical impurities, no significant differences in structural and surface homogeneity were found in crystals from successive purifications. Polydimethylsiloxane (PDMS) prepolymer was purchased from Ellsworth Adhesives, and stamps with suitable relief structures were fabricated according to previously reported procedures in the Nanofabrication Center at University of Minnesota.36,37 The 2.5 nm ( 0.2 nm (0.1 nm/s) Ti and 17.5 nm ( 0.2 nm (0.1 nm/s) Au were sequentially evaporated with a CHA electron beam system onto the patterned PDMS, forming electrically isolated source, drain, and gate electrodes. The recession region (known as the air gap) was estimated to be 4.6 ( 0.1 μm in depth using surface profilometry (KLA-Tencor P-16 surface profiler). High purity (99.9%) ionic liquids (except [P13][TFSI] which was from Koei Chemical Co., Ltd., (Osaka, Japan)) were purchased from EMD Chemicals, Inc. (New Jersey, USA). Cations include 1-ethyl-3methyl imidazolium ([EMI]), 1-butyl-1-methyl pyrrolidinium ([P14]), and 1-propyl-1-methyl pyrrolidinium ([P13]); anions include bis(trifluoromethylsulfonyl)imide ([TFSI]) and tris(pentafluoroethyl)trifluorophosphate ([FAP]); ionic liquids with different combinations of these cations and anions were employed in this study (Figure 1a). To minimize the water and oxygen contamination, all liquids were dried in a vacuum oven at 70 °C for three days and were kept in a N2-filled glovebox afterward. Water concentrations for all dried liquids were around 20
50 ppm (by weight) measured by a Denver Instruments Karl Fischer titrator. Device Fabrication. Rubrene air gap FETs were fabricated by laminating crystals with their long axis (b-axis) aligned across source and drain electrodes and contacts were formed by spontaneous adhesion of the crystals to the rubber stamp. The air gap (∼5 μm) for these devices served as the gate dielectric. Electrical double-layer transistors (EDLTs) were fabricated by putting an ionic liquid drop close to the crystal, which could then quickly fill the channel area by capillarity.28 No optical fringes were observed after liquid filling, indicating a good wetting front between the ionic liquid and the crystal surface. In response to a negative gate voltage, cations and anions moved toward the gate/ liquid and liquid/rubrene interfaces, respectively, forming two EDLs with nanometer thicknesses, and a large concentration of holes can thus be accumulated on the rubrene surface (Figure 1b). In a similar way, a calibration structure was fabricated by laminating a piece of silicon wafer (coated with 3 nm Cr/ 30 nm Au) on a PDMS substrate which had the same relief 14361

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Figure 2. (a) Capacitance
frequency (C-F) relation of pure ionic liquids in an Au/IL/Au test structure. (b) Temperature dependence of C-F relation in an Au/[EMI][FAP]/Au structure. 5 μm gaps are used in both cases.

patterns as in rubrene devices, and ionic liquids were filled beneath, forming an Au/liquid/Au (MIM) structure used in impedance measurements. Electrical Measurements. Current
voltage (I-V) characteristics of rubrene air-gap transistors and EDLTs were measured in a Desert Cryogenics (Lakeshore, Inc.) vacuum probe station in a N2-filled glovebox with Keithley 236 and 6517 electrometers. In the four-probe measurement, two 10 μm-wide voltage-sensing probes were connected with Keithley 6517 electrometers and were inserted into the transport channel that had a length ranging from 300 to 1000 μm. The low temperature measurement employed a Lakeshore temperature controller and liquid nitrogen to vary the temperature from 300 K to ∼200 K. Displacement current measurements were performed with a Keithley 2612 source meter with two channels, which swept gate voltage and measured displacement current through the source/drain contacts (both grounded), respectively. For the impedance measurements, an HP 4192A LF impedance analyzer (with AC amplitude of 10 mV) was used to measure the capacitance
voltage (C-V) and capacitance
frequency (C-F) responses in both metal
insulator
metal (MIM) and metal
insulator
semiconductor (MIS) structures in a frequency range of 10
106 Hz. All measurements were carried out in vacuum at pressure 0) has been observed at a critical carrier density, and at this critical density, the sheet resistance was found to be temperatureindependent and of the order of quantum resistance, h/e2∼ 25.6 kΩ. 62 However, the minimum sheet resistance in the [P14][FAP] gated rubrene EDLT studied here is 125 kΩ, which implies that the crossover point between insulating state and metallic state requires further increases in surface conductivity. A larger charge concentration and higher carrier mobility are needed to further increase the maximum attainable conductivity in organic EDLTs. Furthermore, a better understanding of the factors leading to mobility (conductivity) lowering at high gate voltages is also required. High Density Charge Accumulation at Low Temperature. The maximum attainable charge densities at room temperature can be limited by gate leakage and the electrochemical window of the ILs so that only up to
1 V or
1.5 V gate bias can be applied. However, at lower temperatures where the IL electrochemical activity is suppressed, higher gate biases can be applied, and potentially more charge carriers can be accumulated. It is of great importance to investigate the maximum charge density that can be obtained in organic EDLTs. To explore the possibility of inducing more charges, we first fabricated rubrene EDLTs with gate areas much larger than the channel area. In this experiment, a larger drop of ionic liquid was used to fill the channel area, while another piece of clean glass slide was laminated close to the crystal to hold the excess liquid. We expect that if the capacitance of gate/IL interface (with larger area) dominates over IL/ rubrene interface, the majority of gate potential drop would be at the IL/rubrene interface, resulting in larger carrier accumulations on the rubrene surface. To our surprise, we did not find any substantial difference between normal gate and large gate EDLT devices in terms of charge density and channel current (not shown). In hindsight, this result is fully consistent with our previous finding that the Au/IL interface has a higher capacitance (because of the larger DOS of Au) than the rubrene/IL interface; thus, increasing the gate/IL area is not necessary. Figure 8 shows the gate voltage dependence of charge density at 210 and 300 K (used as a comparison) as well as displacement 14366

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novel electronic phenomena in organic semiconductors at high charge densities, such as field-induced superconductivity.

’ ASSOCIATED CONTENT

bS

Supporting Information. Electrical characteristics of rubrene EDLTs gated with different ionic liquids, capacitance
voltage (C-V) measurements, device stability, and sweep rate dependence of charge densities. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Charge accumulation of the [P14][FAP] gated rubrene EDLT at 210 K. The displacement current (open symbols, left axis) is taken at 0.12 V/s. Charge densities (solid symbols, right axis) are plotted versus VG at both 210 and 300 K (as a comparison).

current taken at 210 K. It can be seen that at lower temperature up to
3.5 V of gate bias can be applied while the displacement current can be measured in a stable and reversible manner. Charge densities as high as 6.3  1013 cm
2 are accumulated at VG =
3.5 V, corresponding to 0.33 holes per rubrene molecule. This doubles the amount of the charge density induced at room temperature (3.1  1013 cm
2) and is also the highest measured charge concentration for organic single crystal EDLTs gated with electrolyte.18,31
34 A further increase in gate voltage beyond
3.5 V is prevented by increased leakage current and device degradation. Unlike in ZnO EDLTs where up to 6 V of gate bias could be applied at lower temperature,16 the organic surface may be more vulnerable to large electric fields or trace amounts of water contamination which prevent the rubrene EDLT device from being operated at such high biases. The situation could be potentially improved by extensive purification of organic crystals and further dehydration of ionic liquids to reduce the interface trap densities45 and enlarge the IL electrochemical window.59

’ CONCLUSIONS We have systematically investigated the electrical transport properties of organic EDLTs based on rubrene single crystals and several imidazolium and pyrrolidinium ionic liquids. Displacement current measurement, which is a powerful approach to study the dynamic charging and discharging behavior in a FET structure, provided us with a detailed characterization of charge density and dielectric capacitance, and thus carrier mobility in rubrene EDLTs at high charge densities above 1013 cm
2. We identified the ionic liquid that gives the best EDLT performance so far and investigated variable temperature transport and high charge carrier accumulation in rubrene EDLTs. We demonstrated that a conductivity maximum occurs near a surface charge density at (1.5
2.5)  1013 cm
2, reaffirming our prior results. This conductance peak appears to be a general phenomenon for organic EDLTs when gating with ILs. The liquid-gated rubrene EDLTs also exhibited thermally activated transport behavior, and the electrostatically accumulated charge decreases with decreasing temperature while the charge density at the channel conductance maximum remains the same. The maximum induced charge concentration of 6.3  1013 cm
2 occurs at the freezing point of the ionic liquid (∼210 K) with gate bias of
3.5 V, when more than one-third of rubrene molecules are oxidized. These findings are potentially important for ongoing efforts to realize

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the MRSEC program of the National Science Foundation at University of Minnesota under Award Number DMR-0819885. W.X. would like to thank Dr. Shimpei Ono at the Center Research Institute of Electric Power Industry (CRIEPI) for providing the ionic liquid [P13][TFSI] and invaluable discussions. ’ REFERENCES (1) Ahn, C. H.; Triscone, J. M.; Mannhart, J. Nature 2003, 424, 1015–1018. (2) Ahn, C. H.; Di Ventra, M.; Eckstein, J. N.; Frisbie, C. D.; Gershenson, M. E.; Goldman, A. M.; Inoue, I. H.; Mannhart, J.; Millis, A. J.; Morpurgo, A. F.; Natelson, D.; Triscone, J.-M. Rev. Mod. Phys. 2006, 78, 1185–1212. (3) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991, 350, 600–601. (4) Kelty, S. P.; Chen, C.-C.; Lieber, C. M. Nature 1991, 352, 223– 225. (5) Kortan, A. R.; Kopylov, N.; Glarum, S.; Gyorgy, E. M.; Ramirez, A. P.; Fleming, R. M.; Thiel, F. A.; Haddon, R. C. Nature 1992, 355, 529–532. (6) Mitsuhashi, R.; Suzuki, Y.; Yamanari, Y.; Mitamura, H.; Kambe, T.; Ikeda, N.; Okamoto, H.; Fujiwara, A.; Yamaji, M.; Kawasaki, N.; Maniwa, Y.; Kubozono, Y. Nature 2010, 464, 76–79. (7) Ueno, K.; Nakamura, S.; Shimotani, H.; Ohtomo, A.; Kimura, N.; Nojima, T.; Aoki, H.; Iwasa, Y.; Kawasaki, M. Nat. Mater. 2008, 7, 855–858. (8) Veres, J.; Ogier, S.; Lloyd, G.; de Leeuw, D. Chem. Mater. 2004, 16, 4543–4555. (9) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schutz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature 2004, 431, 963–966. (10) Hur, S.-H.; Yoon, M.-H.; Gaur, A.; Shim, M.; Facchetti, A.; Marks, T. J.; Rogers, J. A. J. Am. Chem. Soc. 2005, 127, 13808–13809. (11) Yoon, M.-H.; Facchetti, A.; Marks, T. J. Proc. Natl. Acad. Sci. U. S.A. 2005, 102, 4678–4682. (12) Yoon, M.-H.; Yan, H.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 10388–10395. (13) Kim, C.; Wang, Z.; Choi, H.-J.; Ha, Y.-G.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 6867–6878. (14) Ahn, C. H.; Gariglio, S.; Paruch, P.; Tybell, T.; Antognazza, L.; Triscone, J.-M. Science 1999, 284, 1152–1155. (15) Ye, J. T.; Inoue, S.; Kobayashi, K.; Kasahara, Y.; Yuan, H. T.; Shimotani, H.; Iwasa, Y. Nat. Mater. 2009, 9, 125–128. (16) Yuan, H.; Shimotani, H.; Tsukazaki, A.; Ohtomo, A.; Kawasaki, M.; Iwasa, Y. Adv. Funct. Mater. 2009, 19, 1046–1053. 14367

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