Fabrication of Electrochemical Transistor Based on π−Conjugate

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Fabrication of Electrochemical Transistor Based on π-Conjugate Polymer Langmuir-Blodgett Film Jun Matsui,*,†,‡ Yoshitaka Sato,† Takeshi Mikayama,† and Tokuji Miyashita*,† Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira 2-1-1, Aoba-ku, Sendai, 980-8577, Japan, and Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8, Honcho, Kawaguchi 332-0012 ReceiVed December 6, 2006. In Final Form: April 28, 2007 We fabricated an efficient organic electrochemical transistor (OECT) composed of polymer Langmuir-Blodgett (LB) film. The π-conjugated polymer LB film, which was constructed from a poly(N-dodecylacrylamide) (pDDA) and poly(3-hexylthiophene) (PHT) mixture, was used as a conduction channel layer to connect source and drain electrodes. The mixed-polymer LB film was characterized using UV-vis spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), and cyclic voltammetry. Subsequent UV spectra measurements, XRD measurements, and AFM measurements show that PHT forms a crystalline lamellar domain in the layered structure of pDDA. The OECT included 10 layers of the mixed-polymer LB film as the conduction channel layer. The OECT showed an on/off ratio of 1.1 × 104 and mobility of 7.5 × 10-2 cm2 V-1 s-1 at low gate (VG ) -1.2 V) and source-drain voltages (VDS ) -0.5 V). Moreover, the necessary charge to operate the OECT was 1.1 × 10-9 mol of e-1 cm-2, which was 2 orders smaller than the value reported using a similar device structure. The relatively high on/off ratio and low charge consumption suggest that this OECT, which is fabricated from π-conjugated polymer LB films, is applicable to macroelectronic devices.

Introduction In the past several years, research into organic materials that are applicable to electric devices has aroused considerable interest because of such materials’ advantages such as low fabrication cost, high mechanical flexibility, and low weight.1,2 Recently, several flexible electronic devices that utilize the advantage of organic materials have been proposed and fabricated.3,4 Organic transistors are important components in flexible electronic devices. Therefore, the fabrication of organic field-effect transistors (OFETs) has become an active area of research in organic electronics.5,6 In an OFET, organic semiconductors, such as aromatic molecules and π-conjugated polymers are used to create the conduction channel. The source-drain current (IDS) is modulated by the application of an electric field from the gate electrode across an insulating layer. In conventional OFETs, high gate voltage (several tens of volts) is necessary to operate the OFET because of poor capacitive coupling through low gate dielectric SiO2. Therefore, to decrease the operation voltage, application of high-capacitance gate dielectric materials or ultrathin gate materials as an insulating layer has been reported.7-9 On the other hand, several groups have reported organic electrochemical transistors (OECTs) with modulation of IDS by * Author to whom correspondence should be addressed. E-mail: [email protected] (T.M.); [email protected] (J.M.). † Tohoku University. ‡ Japan Science and Technology Agency. (1) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581-2590. (2) Pron, A.; Rannou, P. Prog. Polym. Sci. 2002, 27, 135-190. (3) Katz, H. E. Chem. Mater. 2004, 16, 4748-4756. (4) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Chem. Mater. 2004, 16, 4413-4422. (5) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99117. (6) Horowitz, G. J. Mater. Res. 2004, 19, 1946-1962. (7) 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. (8) Majewski, L. A.; Schroeder, R.; Grell, M. AdV. Mater. 2005, 17, 192-196. (9) Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, J.; Callegari, A.; Shaw, J. M. Science 1999, 283, 822-824.

electrochemical doping and de-doping reactions of conjugated polymers.5,10-20 Because the OECT uses an electrochemical reaction, a high on/off ratio can be achieved using a low gate voltage (several volts). However, the electrochemical doping gives a higher charge consumption in operating OECT than in operating OFET.21 Consequently, the amount of the polymer to connect the source and drain channel should be as small as possible to create a highly efficient OECT. The Wrighton group created source and drain electrodes with separations of micrometer to nanometer length22 to reduce the amount of polymer in use. Another strategy is application of an ultrathin conductive layer to connect the channel. We have reported that several acrylamide polymers form a stable monolayer at the air/water interface, and that the polymer ultrathin film prepared by successive deposition of the monolayers using Langmuir-Blodgett (LB) technique has a well-defined structure with nanometer thickness.23 Recently, we fabricated ultrathin π-conjugated polymer LB films by mixing poly(3hexylthiophene) (PHT) with poly(N-dodecylacrylamide) (pDDA), (10) Nilsson, D.; Kugler, T.; Svensson, P. O.; Berggren, M. Sens. Actuators B 2002, 86, 193-197. (11) Nilsson, D.; Chen, M. X.; Kugler, T.; Remonen, T.; Armgarth, M.; Berggren, M. AdV. Mater. 2002, 14, 51-54. (12) Taniguchi, M.; Kawai, T. Appl. Phys. Lett. 2004, 85, 3298-3300. (13) Chao, S. H.; Wrighton, M. S. J. Am. Chem. Soc. 1987, 109, 6627-6631. (14) Chao, S.; Wrighton, M. S. J. Am. Chem. Soc. 1987, 109, 2197-2199. (15) White, H. S.; Kittlesen, G. P.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 5375-5377. (16) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 7389-7396. (17) Mabeck, J. T.; DeFranco, J. A.; Bernards, D. A.; Malliaras, G. G.; Hocde, S.; Chase, C. J. Appl. Phys. Lett. 2005, 87, 013503. (18) Lin, F. D.; Lonergan, M. C. Appl. Phys. Lett. 2006, 88, 133507. (19) Saxena, V.; Shirodkar, V.; Prakash, R. J. Solid State Electrochem. 2000, 4, 234-236. (20) Rani, V.; Santhanam, K. S. V. J. Solid State Electrochem. 1998, 2, 99101. (21) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 5133-5140. (22) Jones, E. T. T.; Chyan, O. M.; Wrighton, M. S. J. Am. Chem. Soc. 1987, 109, 5526-5528. (23) Mitsuishi, M.; Matsui, J.; Miyashita, T. Polym. J. 2006, 38, 877-896.

10.1021/la063526r CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007

Fabrication of OECT from π-Conjugated LB Films

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because it was difficult to fabricate a stable monolayer with pure PHT.24 The mixed-polymer LB film of PHT/pDDA becomes conductive by chemical oxidation; an OFET was fabricated using the PHT/pDDA mixed-polymer LB film. For this study, the π-conjugated polymer LB film was applied to create a highly efficient electrochemical transistor. The mixed monolayer was transferred onto a solid substrate using the LB technique, and the resulting π-conjugated polymer LB film was characterized by absorption spectroscopy, X-ray diffraction (XRD), and atomic force microscopy (AFM). The OECT was fabricated using the π-conjugated polymer LB film. Then, the OECT efficiency was assessed. Moreover, the effect of layer thickness on the OECT performance was studied. Experimental PHT was prepared using oxidative polymerization with FeCl3 as an oxidative reagent according to reported procedures.25 Briefly, in a 100-mL three-necked flask, 18.8 mmol of iron trichloride (Merck) was dissolved in 50 mL of dry chloroform under nitrogen, and then 3-hexylthiophene was added dropwise to the solution. The mixture was allowed to react at 0 °C for 1 h and at room temperature for 24 h. Then the solution was precipitated into methanol (200 mL), and the black precipitate was collected on a filter. The polymer was washed by Soxhlet extraction using methanol (24 h) and extracted by Soxhlet extraction using chloroform (24 h). Then chloroform solution was evaporated, and the polymer film was dried under vacuum. The molecular weights were determined to be Mw ) 8.3 × 104 and Mn ) 2.8 × 104, respectively, using gel permeation chromatography (Tosoh Corp.) using a polystyrene standard. The head-to-tail ratio was determined to be 83% by taking the intensity ratio of R-methylene protons between δ ) 2.81 (HT) and δ ) 2.57 (HH) in the 1H NMR spectrum.26 Elemental analysis showed a very low amount of chloride (Cl < 0.3%) residue in the polymer. p(DDA) was prepared as described previously.27-29 Then PHT and p(DDA) were dissolved in a chloroform solution with the ratio of 2:1 in each monomer unit. The water used as a subphase was purified using an ultrapure water system (CPW-101; Advantec). The deposition of monolayers was carried out using an automatic Langmuir trough (LB lift controller, FSD-51; USI) with a Wilhelmy-type film balance at 20 °C and compressed at a rate of 15 cm2/min. The monolayer was transferred onto a substrate at a surface pressure of 20 mN/m with a dipping speed of 10 mm/min. The silicon and glass substrates that were used as a deposition substrate were cleaned by treatment with a UV-O3 cleaner (NL-UV253; Nippon Laser and Electronics Laboratory) and hydrophobicized by immersion of the substrates into a ca. 1 × 10-6 M octadecyltrichlorosilane (Tokyo Chemical Industry Co., Ltd.) chloroform solution. A cyclic voltammogram was conducted with an electrochemical analyzer (ALS 600; BSA) using a three-electrode cell and a two-electrode cell. The threeelectrode cell was equipped with the Au electrode covered by the π-conjugated polymer LB film (working electrode), a Pt plate (counter electrode), a silver wire (quasi-reference electrode), and a 0.1 M LiPF6/acetonitrile as the supporting electrolyte. The quasi-reference electrode was calibrated versus the saturated calomel electrode (SCE) through the addition of ferrocene as an internal standard using 0 E(Fc/Fc+) ) 0.32 versus the SCE. In the two-electrode cell, the Au electrode covered by the π-conjugated LB film was used as a working electrode, and an indium tin oxide (ITO) electrode was used as the (24) Matsui, J.; Yoshida, S.; Mikayama, T.; Aoki, A.; Miyashita, T. Langmuir 2005, 21, 5343-5348. (25) Leclerc, M.; Diaz, F. M.; Wegner, G. Makromol. Chem. 1989, 190, 31053116. (26) Chen, T. A.; Wu, X. M.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233-244. (27) Matsui, J.; Mitsuishi, M.; Miyashita, T. J. Phys. Chem. B 2002, 106, 2468-2473. (28) Matsui, J.; Mitsuishi, M.; Aoki, A.; Miyashita, T. J. Am. Chem. Soc. 2004, 126, 3708-3709. (29) Matsui, J.; Mitsuishi, M.; Aoki, A.; Miyashita, T. Angew. Chem., Int. Ed. 2003, 42, 2272-2275.

Figure 1. Surface pressure-area isotherms of mixed monolayers containing PHT and pDDA with 2:1 mixture ratios. Inset: Chemical structures of pDDA and PHT. reference and counter electrode. The electrochemical cell and the electrolyte solution were maintained inside a dry box to avoid contamination by oxygen or water. Planar and interdigitated array (IDA) electrodes were fabricated using thermal deposition of Cr/Au with a metal mask. The IDA electrode has two sets of comb-type Au arrays; each array has eight electrode elements, which are 0.5mm wide and 8.5-mm long and are separated by 0.2 mm from adjacent elements. The electrochemical transistor current-voltage characteristics were measured using a source unit (2400; Keithley Instruments, Inc.) for measuring the source-drain current and a DC power supply (PA110-0.6A; Kenwood) for application voltage to the gate electrode. XRD measurement was carried out using an MAC Science M18XF22-SRA instrument with CuKa (0.154 nm) as a target.

Results and Discussion Characterization of π-Conjugated Polymer LB Film Composed of PHT/pDDA Mixture. The monolayer behavior of a mixture of PHT and pDDA with 2:1 mixture ratios on the water surface was investigated using π-A isotherm measurements (Figure 1). Recently, we reported that a stable mixed-polymer monolayer is formed with mixing regioregular PHT (RR-PHT) with pDDA, and the maximum ratio of RR-PHT is RR-PHT/ pDDA ) 2:1 to transfer the mixed LB film effectively onto a solid substrate.24 Although the PHT used in this work is prepared using oxidative polymerization and is regioirregular (head-totail ratio is 83%), the PHT/pDDA mixed-polymer monolayer on the water surface shows a steep rise in the surface pressure with decreasing surface area (Figure 1), indicating that the stable monolayer is formed irrespective of the regularity of the PHT. The average limiting surface area is estimated by extrapolating the linear portion of the condensed state in the π-A isotherm to zero. From the average limiting surface area, the surface area occupied by the PHT monomer unit is calculated to be almost 0 nm2/monomer unit with the assumption that the area for the DDA unit is 0.28 nm2/monomer unit.23 This indicates that PHT was squeezed out from the air/water interface, which is supported by the following AFM measurements. The mixed monolayer can be transferred onto a solid substrate using the LB technique, yielding a stable polymer LB film. The transfer ratios were 1.0 ( 0.1 in both the downward and upward strokes, creating a Y-type LB film layer structure. Deposition of the mixed film was monitored by an optical absorption spectrum of the deposited film (Figure 2a). The mixed-polymer LB film showed broad UV spectra with a maximum around 493 nm,

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Figure 4. (a) AFM image of 10 layers of a PHT/pDDA LB film deposited on a hydrophobic silicon substrate. (b) Film thickness of a PHT/pDDA LB film with different numbers of layers, as measured using AFM.

Figure 2. (a) UV-vis spectra of PHT/pDDA LB film with different numbers of layers. (b) Absorption intensity at 493 nm as a function of the number of layers. Figure 5. Cyclic voltammogram for 10 layers of a PHT/pDDA mixed LB film in 0.1 M LiPF6/CH3CN solution at a 200 mV s-1 scan rate.

Figure 3. XRD pattern of 40 layers of a PHT/pDDA LB film.

which is characteristic of the π-π* absorption band of PHT. The optical absorbance of the LB film at 493 nm increases linearly with increasing number of deposited layers, indicating that regular deposition of the monolayer was carried out (Figure 2b). The XRD spectrum shows two Bragg diffraction peaks at 2θ ) 2.5° (d ) 3.6 nm) and 5.2° (d ) 1.7 nm) (Figure 3). The former peak was attributed to the diffraction from the bilayer periodical structure of pDDA, and the latter peak was attributed to the crystalline lamellar structure of PHT by comparing the peaks with reported values (d ) 3.44 nm for the bilayer structure of pDDA30 and d ) 1.60-1.68 nm for the lamellar structure of PHT26,31,32). On the other hand, the surface morphology of PHT/ pDDA polymer LB film with 10 layers shows a relatively rough surface with rms ) 7 nm (Figure 4 a). Moreover, the thickness of the PHT/pDDA multilayer film was measured using AFM. A part of the multilayer film deposited onto the silicon substrate (30) Miyashita, T.; Mizuta, Y.; Matsuda, M. Br. Polym. J. 1990, 22, 327-331. (31) Mccullough, R. D.; Tristramnagle, S.; Williams, S. P.; Lowe, R. D.; Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910-4911. (32) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. Macromolecules 1992, 25, 4364-4372.

was scratched out, and the thickness of the multilayers was evaluated from the difference in height between the film and the substrate. The thickness increased linearly with increasing deposited layer number (Figure 4b), and the mixed monolayer thickness was determined as 2.7 nm from the slope of the thickness versus the number of layers. The mixed monolayer thickness was 1.5 times larger than the layer spacing of pDDA measured by the XRD measurement (1.8 nm). These measurement results indicate that PHT forms domains on top of the layered pDDA by squeezing out from the air/water interface and depositing onto a solid substrate. Electrochemical properties of PHT/pDDA mixed-polymer LB film were characterized using cyclic voltammetry (CV) measurements. Figure 5 shows the cyclic voltammogram of a PHT/ pDDA mixed LB film with 10 layers deposited onto the Au substrate. The cyclic voltammogram of PHT in the mixed-polymer LB film shows a reversible redox peak from +0.5 V to +1.2 V (vs the SCE). The important features of the CV waves for the PHT/pDDA LB film were similar to those of poly(alkylthiophene) films,33 which indicates that pDDA has no effect on electrochemical PHT properties. Fabrication of OECT Using a PHT/pDDA Mixed LB Film. The procedure to fabricate OECT is shown in Figure 6. First, the PHT/pDDA mixed LB film with 10 layers was deposited onto the glass substrate with Au source-drain electrodes. Then, silicone rubber was used to create a rectangular well (width × length × height ) 10 mm × 10 mm × 0.5 mm), and electrolyte was poured into the space. Finally, the well was covered with (33) Sato, M.; Tanaka, S.; Kaeriyama, K. Makromol. Chem. 1987, 188, 17631771.

Fabrication of OECT from π-Conjugated LB Films

Figure 6. Fabrication process of an OECT using π-conjugated polymer LB film as a channel layer. (a) Gold IDA electrodes were fabricated onto the glass substrate using thermal deposition of gold through a metal mask. (b) Ten layers of PHT/pDDA LB film were deposited onto the substrate using the LB technique. (c) An electrolyte solution was poured into a rectangular well made of silicone rubber and covered by the ITO substrate, which acts as a gate electrode.

Figure 7. Transistor characteristics, IDS vs VDS, at various gate voltages. The gate voltage was swept from 0 to -1.2 V at 0.1 V steps. Inset: IDS vs VG at constant VDS (VDS ) -0.5 V).

an ITO gate electrode. The preparation of OECT was carried out in a dry box. After cell preparation, the OECT cell was taken from the dry box, and measurements of OECT performance were carried out in air. Figure 7 shows the current-voltage (I-V) characteristics of a representative transistor using 0.1 M LiPF6/ CH3CN as an electrolyte. The source-drain voltage was swept from 0 to -0.5 V, and the gate voltage was swept from 0 to -1.2 V. In OECT measurements, the source electrode was grounded, and the voltages were applied against the source electrode. When VG ) 0 V, the transistor was in its “off” state, and IDS was very low (on the order of 10-8 A). The IDS showed a small change: even the VG was increased to -0.8 V. However, when the VG became greater than -0.9 V, IDS showed a remarkable increase, which indicates that the PHT/pDDA mixed-polymer LB film was doped around -0.9 V in the present condition. The on/off ratio was calculated as 1.1 × 104 using IDS (VDS ) -0.5 V) at VG ) -1.2 V versus that at VG ) 0 V. The on/off ratio is 2 orders smaller than that of a state of the art OFET. The inset in Figure 7 shows the source-drain current as a function of the gate voltage for the VDS ) -0.5 V. The plot shows that the transconductance gm ) ∂IDS/∂VG (VDS ) constant) can be extracted as 0.04 mS mm-1 of the gate width. The low transconductance is derived from the large gate length (0.2 mm). It takes a few seconds to switch the OECT to a high conduction state from a low conduction state. Moreover, once the OECT was in a high conduction state, it takes several tens of minutes to switch to a low conduction state at VG ) 0 V, because this process is similar to the discharge process of a polymer battery.

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Figure 8. Cyclic voltammogram for 10 layers of pDDA/PHT LB film in a 0.1 M LiPF6/CH3CN solution at a 100 mV s-1 scan rate measured in a two-electrode cell configuration. The crosses show the starting points of the sweeps; the initial directions of the sweeps are denoted by triangles.

The amount of charge to obtain this high on/off ratio in the OECT is calculated from the CV measurement.34 In the OECT, gate voltages were applied against the source electrode (not against the reference electrode); therefore, a two-electrode cell was fabricated to perform CV measurements (Figure 8). The cell structure resembles that of the OECT except for the gold electrode structure, which uses a planar gold electrode in the CV measurement. The CV spectrum shows the onset of oxidation around +0.8 V, which was close to the onset voltage (absolute value) of the OECT. The similarity of the onset oxidation voltage indicates that the cell conditions between the OECT and the two-electrode cell are comparable. Consequently, the amount of charge to operate the OECT can be estimated from the twoelectrode cell CV measurement; the amount is calculated by integrating the current of the CV result versus time as 1.1 × 10-9 mol of e-1 cm-2. The value is 2 orders smaller than the reported value with similar device structure21 because the amount of PHT required to connect the channel becomes very low using a nanometer-thick π-conjugated polymer LB film. The charge carrier mobility (µ) is calculated using the following equation:

µ ) σ/(e × n) where σ (S cm-1) is the conductivity of the film, e is the electron charge (1.6 × 10-19 C), and n (cm-3) is the density of the charge carrier. The film conductivity was calculated from the linear region of the I-V curve of OECT as 2.4 S cm-1. The charge carrier density was calculated from the two-electrode CV result as 2.0 × 1020 cm-3. Using these values, the mobility of the OECT fabricated from a π-conjugated polymer nanosheet is calculated as 7.5 × 10-2 cm2 V-1 s-1, which is comparable to that of an OFET fabricated from RR-PHT.35,36 As discussed in the former section, PHT forms a crystalline lamellar structure in the mixed LB film, which is a reason for the relatively high mobility of the OECT. Consequently, the OECT fabricated from PHT/pDDA mixed LB films showed a good on/off ratio, high mobility, and very low charge consumption; even the source-drain channel was wide (0.2 mm). Effect of the Number of Layers on OECT Performance. Reportedly, the OECT performance is dependent on the volume (34) Panzer, M. J.; Frisbie, C. D. AdV. Funct. Mater. 2006, 16, 1051-1056. (35) Bao, Z.; Dodabalapur, A.; Lovinger, A. J. Appl. Phys. Lett. 1996, 69, 4108. (36) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 17411744.

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Matsui et al. Table 1. Effect of Deposition Number on the On Current, Off Current, and the On/Off Ratio of the OECT layer number

on current (mA)

off current (nA)

on/off ratio

6 10 20 50

0.34 0.9 1.4

30 90 200

1.1 × 104 1.0 × 104 0.7 × 104

current, which is defined as IDS at VG ) 0 V, VDS ) -0.5 V also increased with an increasing number of layers. The off current arose from unintentionally doped PHT during polymer synthesis or device fabrication process. Consequently, the on/off ratio was the highest in the OECT fabricated from 10 layers of a PHT/ pDDA mixed LB film (Table 1).

Conclusion

Figure 9. Transistor characteristics, IDS vs VDS, of OECTs constructed from PHT/pDDA mixed LB films with different numbers of layers. The numbers of PHT/pDDA LB film layers were (a) 6 layers, (b) 10 layers, (c) 20 layers, and (d) 50 layers. The gate voltage was swept from 0 V to -1.2 V at 0.1 V steps.

of polymer that connects the channel.22 The polymer volume in OECT fabricated from the PHT/pDDA LB film is controlled easily by changing the number of deposition layers. OECTs of PHT/pDDA mixed-polymer LB films with different numbers were fabricated, and the current-voltage (I-V) characteristics of each OECT were measured (Figure 9). The OECT constructed from six layers of the PHT/pDDA mixed LB film shows no current between the source and drain electrodes because the amount of PHT is insufficient to connect the channel. Upon increasing the number of layers, the “on” current of OECT at VG ) -1.2 V, VDS ) -0.5 V was increased. However, the “off”

An OECT was fabricated using a π-conjugated polymer LB film composed of PHT/pDDA. The π-conjugated polymer LB film was characterized using π-A isotherm, UV-vis spectra, and AFM measurements. The π-conjugated polymer mixture forms a stable monolayer at the air/water interface; then the monolayer can be transferred onto a solid substrate regularly using the LB technique. An OECT was fabricated from the π-conjugated polymer LB film with 10 deposited layers. The transistor operates as an accumulation p-type FET. The relatively high on/off ratio (1.1 × 104) and high mobility (7.5 × 10-2 cm2 V-1 s-1) with low charge consumption (1.1 × 10-9 mol of e-1 cm-2) were obtained from the OECT, even in a long channel (0.2 mm). This high performance indicates that the OECT can be useful for macroelectronics applications. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research (No. 17105006) and Priority Area “Super-Hierarchical Structures” from the Ministry of Education, Culture, Sports, Science and Technology, Japan and PRESTO from the Japan Science and Technology Agency. LA063526R