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Droplet Manipulation by an External Electric Field for Crystalline Film Growth Takeshi Komino,† Hirokazu Kuwabara,†,‡ Masaaki Ikeda,‡ Masayuki Yahiro,# Kazuo Takimiya,∥ and Chihaya Adachi*,†,#,§ †

Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ‡ Research & Development Group (R&D Planning Division), Nippon Kayaku Company Ltd, 3-31-12 Shimo, Kita, Tokyo 115-8588, Japan # Institute of System, Information Technologies and Nanotechnologies (ISIT), 2-1-22 Momochi-hama, Sawara, Fukuoka 814-0001, Japan § International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan ∥ Emergent Molecular Function Research Group, RIKEN Center for Emergent Matter Science (CEMS), 2-1 Hirosawa, Wako, Saitama 341-0198, Japan S Supporting Information *

ABSTRACT: Combining droplet manipulation by the application of an electric field with inkjet printing is proposed as a unique technique to control the surface wettability of substrates for solution-processed organic field-effect transistors (FETs). With the use of this technique, uniform thin films of 2,7-dioctyl[1]benzothieno[2,3,-b][1]benzothiopene (C8− BTBT) could be fabricated on the channels of FET substrates without self-assembled monolayer treatment. High-speed camera observation revealed that the crystals formed at the solid/liquid interface. The coverage of the crystals on the channels depended on the ac frequency of the external electric field applied during film formation, leading to a wide variation in the carrier transport of the films. The highest hole mobility of 0.03 cm2 V−1 s−1 was obtained when the coverage was maximized with an ac frequency of 1 kHz.

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INTRODUCTION Solution processes for organic field-effect transistors (OFETs) are of great importance to achieve low temperature, large area, and low-cost fabrication of electronic devices.1−3 In particular, a printing process for OFETs would expand their applications greatly compared with other competiting field-effect transistors (FETs), such as silicon thin film transistors. Inkjet printing is a promising choice for this role.4,5 Numerous studies have been devoted to enhancing the carrier mobility of organic semiconductors. Minemawari et al. recently reported OFETs with a hole mobility as high as 30 cm2 V−1 s−1 that were fabricated using inkjet-printed 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8−BTBT) crystals.6 Given that they realized a very wide molecular terrace of over 100 μm by controlling the crystal growth speed and direction, wet processes might be useful for the fabrication of OFETs. However, inkjet printing has some unavoidable problems associated with patterning (e.g., surface wettability of substrates). A self-assembled monolayer (SAM) can be used to overcome these issues,7−10 but this approach requires complex optimization for fabrication of OFETs. To simplify this process, we used an electric method instead of SAM formation. Hasegawa et al. carried out some innovative work in 2003.11 They used an electrochemical crystal growth method to fabricate conductive nanowires of an © 2013 American Chemical Society

axially substituted cobalt phthalocyanine tetraphenylphosphonium salt within the micrometer-sized gap between Au electrodes on a glass substrate. While they used a conductive charge-transfer complex, thin films, even of organic semiconductors, can be fabricated using a similar technique. In fact, Saji et al. fabricated thin films of phthalocyanines from solutions containing trifluoroacetic acid by electrophoretic deposition.12,13 The important point of these techniques for us is that charged molecules can be forced to migrate between two electrodes. These studies focused on the migration of solutes in solutions, but droplets of a charged solution (or solvent) might possibly be manipulated, even in inkjet printing, allowing the surface wettability of substrates to be improved. Here, we report that a droplet can be manipulated on a substrate by an electric field to fabricate OFETs from a C8−BTBT solution.

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EXPERIMENTAL SECTION

Heavily doped n-type silicon wafer substrates with a 300 nm thick layer of SiO2 were cleaned by sequential immersion in piranha solution, sonication in pure water, soaking in boiling 2-propanol, and UV− Received: May 7, 2013 Revised: June 12, 2013 Published: June 26, 2013 9592

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ozone treatment. A bottom gate/bottom contact structure was fabricated on the substrates. A 50 nm thick Au layer with a 3 nm thick Cr underlayer was deposited on precleaned Si/SiO2 substrates by thermal evaporation and patterned by photolithography to produce source and drain electrodes with a channel length L of 50−100 μm. Organic semiconductor C8−BTBT was synthesized as described elsewhere14 and then purified by sublimation before use. Thin films of C8−BTBT were fabricated on the prepared FET substrates from a solution of cyclohexanone (1 wt %, 150−1500 pL) by dropwise addition of the solution using an inkjet head (PIJ-25NSET; Cluster Technology Company). The volume of solution was controlled by the number of droplets (the volume of each droplet was approximately 15 pL). The average thickness of the prepared films was estimated to be approximately 5 μm from the concentration of the solution (1 wt %), volume of the solution (150 pL), relative density of the solution (1), density of the crystal (1.1 g cm−3), and area of the prepared films (r ∼ 250 μm). The experimental setup for film fabrication is depicted in Figure 1. During the fabrication, dc (Vdc = 0−300 V) or ac rectangular

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RESULTS AND DISCUSSION Control of the Wettability of Substrates. While it is difficult to control the surface free energy of substrates without surface modification, it should be possible to modify their wettability by manipulating a droplet on a substrate by an external electric field, E. One might consider that there is no interplay between E and a droplet. However, E possibly affects the positive electricity of a solvent, making it possible to manipulate a droplet. To maintain a droplet within a channel, we applied Vdc to source-drain electrodes. Because of its appropriate boiling point (156 °C), we used cyclohexanone as the solvent in our experiments. Figure 2 shows the dynamic

Figure 2. Behavior of droplets on channels under applied voltages of (a) Vdc = 0 V and (b) Vdc = 300 V. Both droplets undergo the processes of (i) formation, (ii) dropping, (iii) spreading, and (iv) drying.

Figure 1. Experimental setups for (a) high-speed camera observation in a “dc mode” and (b) inkjet printing of C8-BTBT films in the “ac mode”.

behavior of solvent droplets on FET channels (L = 50 μm) with/without Vdc = 300 V. In the case of Vdc = 0 V, a drop of solvent spread monotonically on the substrate across the channel and Au electrodes to form a circular pool of solvent, and then dried from the edge of the pool toward the center. However, when Vdc = 300 V was applied, a droplet showed quite different behavior; it was selectively maintained within the channel and spread along the Au electrodes. This modification of wettability is attributed to the static electricity of the solvent (cyclohexanone); a droplet is accelerated toward the edge of the cathode by an applied electric field F = q(−grad ϕ), where q is charge and ϕ is the electric potential. This result indicates that the motion of droplets can be actively controlled by using this phenomenon. Tuning of surface wettability with an applied voltage was reported for a dodecylbenzenesulfonate-doped polypyrrole using a redox reaction.15 However, this technique is not suitable for the fabrication of OFETs because the droplet and substrate surface would need to be exposed to an electrolyte. In contrast, our proposed technique can be applied to substrates in air. Using this technique, we attempted to form a crystalline film of C8−BTBT selectively into the channel. Figure 3 (panels a and b) show an OM image and atomic force microscope

voltage (Vac = 140 V, peak-to-peak) with a frequency of 1−10 kHz was applied to the source and drain electrodes. The dynamic behavior of droplets after dropping was observed by a high-speed camera system (CW-9000; Kyence) with a frame rate of 23000 fps. The surface morphology and root-mean-square (rms) roughness of films were investigated using a scanning probe microscope (JSPM-5400; JEOL) in ac mode. The probes (OMCL-AC160TS-C2; Olympus) were operated with a resonant frequency of 280−330 kHz. All images were collected at a scan rate of 0.05−0.70 Hz, with a scan resolution of 1024 × 1024 pixels and a scan size of 10 × 10 μm2. Crystalline shapes were examined by an optical microscope (OM, BX51P; Olympus), using a charge-coupled device camera (DP-72; Olympus) with a resolution of 4140 × 3096 pixels. The output and transfer characteristics of all devices were measured using a pair of picoammeter/voltage source units (B1500A; Agilent Technologies) under a vacuum of less than 1 × 10−3 Pa. For the transfer characteristics, field-effect hole mobilities were extracted in the saturation regime (Vd = −100 V) using the relation μ = (2IdL)/ [WCi(Vg − Vth)2], where Vd is the drain voltage, Id is the drain current, Ci is the capacitance per unit area of the SiO2 gate dielectric (11.5 nF cm−2), Vg is the gate voltage, and Vth is the threshold voltage. 9593

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z, where n(z) = n(0)/10. Because the d spacing and channel height are almost the same, this result indicates that a monolayer of C8−BTBT crystals was not formed in the vicinity of the anode, suggesting that this method cannot be used to fabricate FETs using the current condition. Film Fabrication by ac Voltage. To improve the coverage of C8−BTBT crystals (the effect of the repulsive and attractive force between the electrodes and droplets), the dc power supply was replaced by an ac power supply, as shown in Figure 1, by combination of a function generator and bipolar amplifier. Using the frequency modulation of the applied voltage Vac = 140 V, we attempted to prevent asymmetric crystal growth. Figure 4 shows the motion of a droplet of C8−BTBT solution

Figure 4. The motion of droplets of C8−BTBT solution cyclohexanone on channels when ac 140 V was applied to source and drain electrodes with a frequency of 10 Hz. The droplet dried, repeating the oscillation with a synchronized frequency of 10 Hz.

captured by a high-speed camera with an ac frequency of 10 Hz. After a droplet was formed on a channel (L = 100 μm), the droplet immediately started to oscillate between the two parallel electrodes with a frequency of 10 Hz. This oscillation was continued, decreasing in range, until the solution dried. Similar movement was observed using other ac frequencies, and the dwelling time of a droplet on a channel (or electrodes) seemed to depend on frequency. It comes as no surprise that the movement is dependent on the ac frequency because the motion would be partially governed by the viscoelasticity of the solution; change of the ac frequency may lead to variation of the phase difference between the ac frequency and that of an oscillating solution. Figure 5 (panels a−f) shows polarized optical microscope (POM) images of the fabricated C8−BTBT crystals on FET substrates. Comparison of the results obtained using ac and dc voltages clearly demonstrated that the coverage of the crystal was improved using an ac voltage. Interestingly, the coverage was dependent on ac frequency. This trend is ascribed to the shape of the droplets during the drying process and is consistent with the dependence of the movement of droplets under ac frequency. The frequency that gave the highest coverage was found to be 1 kHz. Figure 6 (panels a−d) shows the output and transfer characteristics of transistors fabricated with ac frequencies of 1 Hz and 1 kHz. Channel widths were 300−420 μm. While the extracted field-effect hole mobility μ in the case of an ac frequency of 1 Hz was as low as 4 × 10−5 cm2 V−1 s−1, a relatively high μ of 0.03 cm2 V−1 s−1 was achieved using an ac frequency of 1 kHz. This value of μ is comparable to those in films fabricated by a spin-coating technique.14 The dependence of μ on ac frequency is illustrated in Figure 6e. The value of μ increased with ac frequency until at 1 kHz and then decreased rapidly. This trend is similar to that for the dependence of coverage of C8−BTBT crystals on ac frequency (Figure 5g),

Figure 3. (a) OM image and (b) AFM image of crystalline films of C8−BTBT fabricated on a channel. Red lines in the AFM image (b) correspond to the three cross-sectional profiles shown in (c), (d), and (e).

(AFM) images, respectively, of a crystalline film fabricated from 1 wt % C8−BTBT solution in cyclohexanone (1500 pL), applying Vdc = 100 V. Cross-sectional profiles of the AFM images revealed that the fabricated crystalline film formed a relatively wide molecular terrace of 5 μm with a step height of 3 nm (Figure 3, panels c−e). Because the d-spacing of C8-BTBT is 2.9 nm,14 this result indicates that our fabrication method could form single molecular steps. In addition, some step bunching was also observed, especially near the cathode (Figure 3d). This trend is shown clearly in the OM image (Figure 3a), which shows that crystals tended to accumulate near the cathode. This is possibly because the crystals grew from the cathode. In contrast, the counter electrode was not covered by crystals because of repulsion between the solvent and anode. Because of the poor coverage at the anode, the FET characteristics of this film were unstable, and we could not confirm electric conduction in most cases. The channel height where charges n(z) are accumulated (against perpendicular to the substrate z) was estimated to be ∼1 nm (Debye length LD is ∼0.5 nm) from n(z) = (CiVg)2(2kTεrε0)−1(1 + z/21/2LD)−2 with LD = (21/2kTεrε0)(qCiVg) −1, using Vg = 100 V and temperature T = 300 K,16 where εr is relative permittivity (3.00), ε0 is vacuum permittivity (8.85 × 10−12 F m−1), and k is the Boltzmann constant. Here, we defined the channel height as 9594

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structure,14 resulting in isotropic carrier transport in the lateral direction. An obvious dependence of the crystal growth direction on ac frequency could not be confirmed from the POM observations (Figure 5a), so we conclude that the improvement of μ mainly originated from better crystal coverage. Minemawari et al. reported that a crystal growth of C8− BTBT occurs at the liquid/air interface when the crystal is formed by a combination of antisolvent crystallization17 and inkjet printing.6 However, high-speed camera observation demonstrated that separation of the solute occurred as the solution was drawn across the channel in this method (see the Supporting Information), indicating that crystals formed at the solid/liquid interface. This might be the reason why relatively large domains could form on the channel, despite the fact that the shape of the air/liquid interface was continuously changed during film formation. Thus, we consider that this technique is a promising strategy for the active control of wettability in the formation of OFETs from solution without any surface modifications. Figure 7 (panels a and b) shows schematic models for crystal growth at liquid/air and solid/liquid interfaces, respectively. In the case of crystal growth at a liquid/air interface, organic molecules diffuse and self-organize with the fluidic nature of the droplet to form a single crystal (Figure 7a).6 By contrast, at a position where the solute concentration reached the saturation concentration, the separation of the solute occurred to form a nucleus. At the edge of a droplet (or pinned contact points), the condensed phase generally forms deposits (Figure 7b).5,18 In our proposed method, such positions were created in the middle of the channels in the case of frequencies of 10−1000 Hz. Once the

Figure 5. POM images of C8−BTBT films fabricated with ac frequencies of (a) 1 Hz, (b) 10 Hz, (c) 100 Hz, (d) 500 Hz, (e) 1 kHz, and (f) 10 kHz. (g) Coverage of C8−BTBT crystals on channels. The curve is a visual guide.

indicating that μ can be improved by optimization of a crystal coverage using an optimal ac frequency. The crystal growth direction (size of a monodomain) might also be increased at an ac frequency of 1 kHz compared with lower ac frequencies. However, it is impossible to determine whether the crystal growth direction was affected by ac frequency because C8− BTBT molecules adopt herringbone packing in their crystal

Figure 6. Output characteristics of FET measurements for films formed at ac frequencies of (a) 1 Hz and (c) 1kHz. Curves were obtained from Vg = 0−100 V in 10 V intervals. Transfer characteristics for films formed at ac frequencies of (b) 1 Hz and (d) 1kHz. Both characteristics showed hysteresis loops, but no clear dependence of the hysteresis on ac frequency was observed. 9595

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST) and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). T.K. thanks Mr. Hideo Matsutani and Mr. Yuichi Yabe from Cluster Technology Company, Ltd. helping to construct the special setup for high-speed camera observations of inkjet printing.

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Figure 7. Crystal growth at the (a) liquid/air and (b) solid/liquid interfaces. Nuclei form on the substrate at positions where the concentration of the solution is high and initiate crystal growth.

(1) Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Siegrist, T.; Li, W.; Lin, Y.-Y.; Dodabalapur, A. A Soluble and Air-Stable Organic Semiconductor with High Electron Mobility. Nature 2000, 404, 478− 481. (2) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. High-Resolution Inkjet Printing of All-Polymer Transistor Circuits. Science 2000, 290, 2123−2126. (3) Sirringhaus, H. Device Physics of Solution-Processed Organic Field-Effect Transistors. Adv. Mater. 2005, 17, 2411−2425. (4) Singh, M.; Haverinen, H. M.; Dhagat, P.; Jabbour, G. Inkjet Printing-Process and Its Applications. Adv. Mater. 2010, 22, 673−685. (5) Ryu, G. S.; Lee, M. W.; Jeong, S. H.; Song, C. K. Thermally Dried Ink-Jet Process for 6,13-Bis(triisopropylsilylethynyl)-Pentacene for High Mobility and High Uniformity on a Large Area Substrate. Jpn. J. Appl. Phys. 2012, 51, 051601. (6) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364−367. (7) Li, Y.; Liu, C.; Kumatani, A.; Darmawan, P.; Minari, T.; Tsukagoshi, K. Patterning Solution-Processed Organic Single-Crystal Transistors with High Device Performance. AIP Adv. 2011, 1, 022149. (8) Minari, T.; Kano, M.; Miyadera, T.; Wang, S. D.; Aoyagi, Y.; Tsukagoshi, K. Surface Selective Deposition of Molecular Semiconductors for Solution-Based Integration of Field-Effect Transistors. Appl. Phys. Lett. 2009, 94, 093307. (9) Minari, T.; Kano, M.; Miyadera, T.; Wang, S. D.; Aoyagi, Y.; Seto, M.; Nemoto, T.; Isoda, S.; Tsukagoshi, K. Selective Organization of Solution-Processed Organic Field-Effect Transistors. Appl. Phys. Lett. 2008, 92, 173301. (10) Janssen, D.; Palma, R. D.; Verlaak, S.; Haremans, P.; Dehaen, W. Static Solvent Contact Angle Measurements, Surface Free Energy and Wettability Determination of Various Self-Assembled Monolayers on Silicon Dioxide. Thin Solid Films 2006, 515, 1433−1438. (11) Hasegawa, H.; Kubota, T.; Mashiko, S. Fabrication of Molecular Nanowire Using an Electrochemical Method. Thin Solid Films 2003, 438−439, 352−355. (12) Yamanouchi, H.; Irie, K.; Saji, T. Electrophoretic Deposition of Copper Phthalocyanine from Trifluoroacetic Acid-Dichloromethane Mixed Solution. Chem. Lett. 2000, 29, 10−11. (13) Shrestha, N. K.; Kohn, H.; Imamura, M.; Irie, K.; Ogihara, H.; Saji, T. Electrophoretic Deposition of Phthalocyanine in Organic Solutions Containing Trifluoroacetic Acid. Langmuir 2010, 26, 17024−17027. (14) Ebata, H.; Izawa, T.; Miyazaki, E.; Takimiya, K.; Ikeda, M.; Kuwabara, H.; Yui, T. Highly Soluble [1]benzothieno[3,2-b]benzothiophene (BTBT) Derivatives for High-Performance, Solution-Processed Organic Field-Effect Transistors. J. Am. Chem. Soc. 2007, 129, 15732−15733. (15) Tsai, Y. T.; Choi, C. H.; Gao, N.; Yang, E. H. Tunable Wetting Mechanism of Polypyrrole Surface and Low-Voltage Droplet Manipulation via Redox. Langmuir 2011, 27, 4249−4256.

nuclei formed, the kinks might grow to form a crystalline film at the solid/liquid interface. This is consistent with high-speed camera observations and POM images (Figure 5). The obtained carrier mobility was comparable to those reported for polycrystalline films,14 suggesting that the generated crystal cannot cover the channel with a monodomain. However, this mechanism highlights the potential of our method because the controlled crystal growth speed and direction can create a uniform crystal.19−26 In the case of over 10 kHz (or dc mode), deposits might be generated near the electrodes (or cathode). The resulting crystalline shape depends on the film formation process, leading to a wide variety of shapes. One might consider that other ac voltage waveforms create a different shape of the crystals. However, the effect is expected to be small since the crystal growth speed and direction are not strongly dependent on the waveforms. We emphasize that this method allows the film formation process to be controlled simply by modulation of an applied electric field.

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CONCLUSION We developed a method to fabricate OFETs by combining droplet manipulation by an external electric field with inkjet printing. Although droplets were stirred in the vicinity of the air/liquid interface during film formation, relatively large domains with molecular terraces as wide as 5 μm could be formed from 1 wt % C8−BTBT solution in cyclohexanone. This is because the crystals grew at the solid/liquid interface. The crystalline film formed with an ac frequency of 1kHz exhibited a field-effect hole mobility of 0.03 cm2 V−1 s−1. Because droplet manipulation can be controlled simply with ac frequency, film fabrication can be easily and comprehensively optimized. As a result, our technique will allow wettability control to assist fabrication of OFETs.

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ASSOCIATED CONTENT

S Supporting Information *

Dynamics of droplets on channels with dc and ac voltages (Figures 2 and 4). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 9596

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(16) Horowitz, G. Organic Thin Film Transistors: From Theory to Real Devices. J. Mater. Res. 2004, 19, 1946−1962. (17) Tung, H.-H.; Paul, E. L.; Midler, M. ; McCaruley, J. A. Crystallization of Organic Compounds: An Industrial Perspective; WileyVCH: Weinheim, 2009, 179−205. (18) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Capillary Flow as the Cause of Ring Stains from Dried Liquid Drops. Nature 1997, 389, 827. (19) Lee, W. H.; Kim, D. H.; Jang, Y.; Cho, J. H.; Hwang, M.; Park, Y. D.; Kim, Y. H.; Han, J. I.; Cho, K. Solution-Processable Pentacene Microcrystal Arrays for High Performance Organic Field-Effect Transistors. Appl. Phys. Lett. 2007, 90, 132106. (20) Minakata, T.; Natsume, Y. Direct Formation of Pentacene Thin Films by Solution Process. Synth. Met. 2005, 125, 1. (21) Natsume, Y.; Minakata, T.; Aoyagi, T. Crystalline Structure of Solution-Processed Pentacene Thin Films. Synth. Met. 2009, 159, 338. (22) Natsume, Y.; Minakata, T.; Aoyagi, T. Pentacene Thin Film Transistors Fabricated by Solution Process with Direct Crystal Growth. Org. Electron. 2009, 10, 107. (23) Duffy, C. M.; Andreasen, J. W.; Breiby, D. W.; Nielsen, M. M.; Ando, M.; Minakata, T.; Sirringhaus, H. High-Mobility aligned Pentacene Films Grown by Zone-Casting. Chem. Mater. 2008, 20, 7252. (24) Karthaus, O.; Imai, T.; Sato, J.; Kurimura, S.; Nakamura, R. Control of Crystal Morphology in Dewetted Films of Thienyl Dyes. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 903. (25) Hashimoto, Y.; Karthaus, O. Preparation of an Ordered Array of Cyanine Complex Microdomes by a Simple Dewetting Method. J. Colloid Interface Sci. 2007, 311, 289. (26) Karthaus, O.; Kawatani, Y. Self-Assembly and Aggregation Control of Cyanine Dyes by Adsorption onto Mesoscopic Mica Flakes. Jpn. J. Appl. Phys. 2003, 42, 127.

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