Spatially Uniform Thin-Film Formation of Polymeric Organic

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Spatially Uniform Thin-Film Formation of Polymeric Organic Semiconductors on Lyophobic Gate Insulator Surfaces by SelfAssisted Flow-Coating Kirill Bulgarevich,†,‡ Kenji Sakamoto,*,† Takeo Minari,† Takeshi Yasuda,† and Kazushi Miki†,‡ †

National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan

‡

S Supporting Information *

ABSTRACT: Surface hydrophobization by self-assembled monolayer formation is a powerful technique for improving the performance of organic field-effect transistors (OFETs). However, organic thin-film formation on such a surface by solution processing often fails due to the repellent property of the surface against common organic solvents. Here, a scalable unidirectional coating technique that can solve this problem, named self-assisted f low-coating, is reported. Producing a specially designed lyophobic−lyophilic pattern on the lyophobic surface enables organic thin-film formation in the lyophobic surface areas by flow-coating. To demonstrate the usefulness of this technique, OFET arrays with an active layer of poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2b]thiophene) are fabricated. The ideal transfer curves without hysteresis behavior are obtained for all OFETs. The average field-effect hole mobility in the saturation regime is 0.273 and 0.221 cm2·V−1·s−1 for the OFETs with the channels parallel and perpendicular to the flow-coating direction, respectively, and the device-to-device variation is less than 3% for each OFET set. Very small device-to-device variation is also obtained for the on-state current, threshold voltage, and subthreshold swing. These results indicate that the self-assisted flow-coating is a promising coating technique to form spatially uniform thin films of polymeric organic semiconductors on lyophobic gate insulator surfaces. KEYWORDS: organic field-effect transistors, polymeric organic semiconductors, lyophobic−lyophilic patterns, self-assisted flow-coating, device-to-device variation

1. INTRODUCTION Solution-processed organic field-effect transistors (OFETs) are actively investigated because of their potential applications in large-area, low-cost, lightweight, and flexible electronics, such as flexible displays, radio frequency identification tags, smart cards, and sensors.1−7 The field-effect charge carrier mobility is one of the important parameters that determine OFET performance. Highly hydrophobic gate insulator surfaces are often used to improve the charge carrier mobility. Such a hydrophobic surface induces an edge-on orientation of organic semiconducting molecules, which aligns the π−π stacking direction parallel to the surface plane. Since the channel current of organic thin-film transistors flows parallel to the gate insulator surface, the edge-on orientation is preferable for the charge transport. Indeed, significant mobility improvement of both small-molecule and polymeric organic semiconductors was achieved by depositing them on highly hydrophobic surfaces.8 However, the use of such a highly hydrophobic (lyophobic) surface often induces the dewetting of common organic solvents, which prevents the stable formation of organic thin films by solution process. Thus, the ease of coating and the device performance improvement is in a trade-off relationship, which is a serious problem in fabricating OFETs by solution © XXXX American Chemical Society

processing. Therefore, the development of a solution coating technique to form organic thin films on lyophobic surfaces is strongly desired.9 Since solution processes to form organic semiconductor films are mainly developed for fabricating large-area flexible electronic devices, they are required to be scalable and applicable to the practical industry. Although spin coating is widely used to form a spatially uniform organic thin film from solution in the laboratory, spinning a substrate is not practical for industrial scale-up. In this sense, we have focused on a flowcoating method10−15 as a scalable unidirectional coating technique. In this method,13 a slightly inclined movable blade is set to a few hundred micrometers above a device substrate surface, and an organic solution is kept by capillary force in the gap space between the device substrate and the movable blade. The organic thin film is formed by moving the blade parallel to the substrate surface at a constant speed. The flow-coating method is compatible with roll-to-roll processing and possesses a potential ability of molecular orientation control. Recently, we Received: December 1, 2016 Accepted: January 24, 2017 Published: January 24, 2017 A

DOI: 10.1021/acsami.6b15398 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces reported that spatially uniform, highly oriented crystalline films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) could be formed by flow-coating13 and also that the OFET array with a flow-coated TIPS-PEN active layer showed very small device-to-device variation.16 In this article, we report the self-assisted flow-coating method that can form spatially uniform thin films of organic semiconductors on lyophobic gate insulator surfaces. In this method, the flow-coated organic semiconductor film itself acts as the lyophilic surface for the semiconductor solution, and the continuous film growth on lyophobic substrate surfaces is achieved by pinning the contact line of solution, which is defined by the boundary among the substrate surface, solution, and air, to the film growth front with the assistance of the own film. To accomplish that, a specially designed lyophobic− lyophilic pattern is formed on a gate insulator surface by vacuum ultraviolet (VUV) light exposure through a photomask. A useful pattern of lyophilic areas is a rectangle followed by stripe lines aligned parallel to the coating direction. The top rectangular region acts as the initial nucleation area, and the stripe lines contribute to the stabilization of the contact line pinning in the lyophobic areas. By starting the flow-coating from the top lyophilic rectangular region, the organic semiconductor thin film continuously grows on both lyophilic and lyophobic surfaces in the stripe pattern region. This coating technique was named self-assisted f low-coating. Poly(2,5-bis(3-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT-C16) was selected as a model semiconductor material for this study. This is because pBTTT-C16 is a promising polymeric semiconducting material for solutionprocessed OFETs showing a field-effect hole mobility comparable to that of amorphous silicon (∼0.5 cm2·V−1· s−1)17−20 and also because it is a typical polymeric semiconducting material that shows the trade-off relationship mentioned above.19,20 We succeeded in forming an ∼20 nm thick pBTTT-C16 film on a substrate surface with a 0.7 mm width lyophobic line and 1.3 mm width lyophilic space pattern by self-assisted flow-coating. High spatial uniformity in a centimeter scale was realized, which was confirmed by evaluating the device-to-device variation of 30 parallel and 30 perpendicular OFETs fabricated in the lyophobic surface area. Here, the “parallel (perpendicular)” specifies the channel current direction with respect to the coating direction. Anisotropic charge transport was observed as expected. Identical transfer characteristics with negligible device-to-device variation were observed for each set of the parallel and perpendicular OFETs. These results suggest that the selfassisted flow-coating is a promising scalable unidirectional coating technique from a practical application perspective. We believe that the self-assisted coating concept is also applicable to other unidirectional coating techniques in which the contactline movement is controlled by translating a blade or nozzle, such as zone-casting,21 solution shearing,22 continuous edgecasting,23 and blade-coating.24

Figure 1. Schematic illustrations of (a) the homemade flow-coater used in this study and (b) process of self-assisted flow-coating.

wt % solution (20 μL) of pBTTT-C16 in o-dichlorobenzene (oDCB) was performed under the following conditions; the gap height between the movable blade and the substrate surface, the substrate temperature, and the coating speed were 200 μm, 80 °C, and 100 μm/s, respectively. The process of self-assisted flow-coating is shown in Figure 1b. Here, octadecyltrichlorosilane (ODTS)-treated Si substrate (20 × 20 mm2) with a thermally grown SiO2 layer was used as a highly hydrophobic and lyophobic surface, which showed a water contact angle of greater than 110° and an o-DCB contact angle of 50°. On such a surface, no thin-film formation of pBTTT-C16 by flow-coating occurred due to the dewetting of solution as shown in Figure 2a. Self-assisted flow-coating of pBTTT-C16 was realized by forming a specially designed lyophobic−lyophilic pattern on the surface by VUV light exposure through a photomask.25−28 Although surface energy patterns are usually used for area-selective coating of organic semiconductors and conductive inks in the lyophilic and hydrophilic surface areas, respectively,24−26,29−37 here the lyophobic−lyophilic pattern was used to form a semiconductor film on the lyophobic surfaces. A useful lyophobic−lyophilic pattern is the 0.7 mm width lyophobic line and 1.3 mm width lyophilic space pattern connected to the top lyophilic rectangular region, as shown in the bottom part of Figure 2b. By starting the flow-coating from the top lyophilic rectangular region, the continuous film growth was achieved on both lyophilic and lyophobic surfaces as shown in the top part of Figure 2b. The surface area in which high-performance OFETs can be fabricated is limited to the lyophobic areas, but the remaining lyophilic areas can be used for circuit wiring and fabrication of passive components. The film thickness measured with a stylus-type step profiler was 23 ± 2 nm. The thickness

2. SELF-ASSISTED FLOW-COATING OF PBTTT-C16 Self-assisted flow-coating was performed using a homemade flow-coater13 illustrated in Figure 1a. It consists of a movable blade, which is a 14 mm wide fluorine-coated glass plate (2 mm thick), and a stationary sample stage with a ceramic heater, which enables a temperature-controlled coating up to 140 °C. The inclination angle of the movable blade is ∼1.5° against the substrate surface plane. In this study, the flow-coating of a 0.7 B


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Figure 2. Optical microscope images (upper) of the pBTTT-C16 films formed on SiO2 surfaces with different lyophobic−lyophilic patterns (lower) by flow-coating. (a) Large solid lyophobic pattern and (b) 0.7 mm width lyophobic line and 1.3 mm width lyophilic space pattern connected to the top lyophilic rectangular region. The black and white areas correspond to lyophobic (ODTS-covered) and lyophilic (bare) surfaces, respectively. The picture (b) was taken after depositing source/drain electrodes.

Figure 3. Contact lines of pBTTT-C16 solution on the lyophobic− lyophilic surface pattern, when the flow-coating was started from different positions. The lyophobic−lyophilic surface pattern and the starting positions (SPs) of flow-coating for (a, b) are shown in the top part of this figure. The meanings of the black and white areas are the same as those of Figure 2. The pictures were taken when the movable blade came at the center of the region bounded by the rectangular box.

can be controlled by varying the concentration of semiconductor solution, the coating speed, and the substrate temperature during flow-coating. To show the importance of the presence of the top lyophilic rectangular region for realizing self-assisted flow-coating, the contact line shape was examined when the flow-coating was started from two different positions: the top lyophilic region and the middle of the line-and-space pattern. On the one hand, by starting the flow-coating from the top lyophilic region, the adequate spread of organic solution toward the starting line was kept in both lyophobic and lyophilic surface areas during flowcoating. Consequently, the contact line became straight and was located at ∼0.14 mm from the leading edge of movable blade as seen in Figure 3a. In this case, a pBTTT-C16 thin film was formed in both lyophobic and lyophilic surface areas. On the other hand, when the flow-coating was started from the middle of the line-and-space pattern, the contact line could be seen only in the lyophilic surface areas as shown in Figure 3b. This result indicates that the contact line in the lyophobic surface areas is located beneath the movable blade. In this situation, the thin-film formation occurred in the lyophilic surface areas but did not occur in the lyophobic surface areas. From these observations, we found that the uniform and continuous thin-film formation in the lyophobic surface areas is thanks to the adequate spread of organic solution originating from the film formation in the top lyophilic rectangular region. The mechanism of self-assisted flow-coating will be discussed later.

observation (Figure S1) and the polarization dependence of the Raman spectra (Figure S2), we confirmed that the pBTTT-C16 backbone structures aligned on average along the flow-coating direction. Thus, to evaluate the spatial uniformity, a bottomgate/top-contact (BG/TC) type of OFET array composed of 30 parallel and 30 perpendicular OFETs was fabricated on the lyophobic (highly hydrophobic) gate insulator surface over an area of ∼1 cm2, as shown in Figure 4a. The channel length (L) and width (W) were 50 and 300 μm, respectively. The deviceto-device variation was evaluated for each set of the parallel or perpendicular OFETs. Typical transfer and output characteristics of the parallel OFETs are shown in Figure 4b,c, respectively. The transfer curves were measured at a constant drain-source voltage (Vds) of −30 V. The gate-source voltage (Vgs) was varied from +5 to −30 V in the forward sweep and consecutively varied from −30 to +5 V in the reverse sweep. For all parallel and perpendicular OFETs (totally 60 OFETs), good p-channel transistor behavior was observed without drain current (Id) hysteresis between the forward and reverse sweeps, and the maximum current on/off ratios were greater than 1 × 107. The output characteristics were measured by varying Vds from +1 to −30 V in the forward sweep and from −30 to +1 V in the reverse sweep at a constant Vgs, which was varied from 0 to −30 V at intervals of 5 V. No Id hysteresis was observed, as shown in Figure 4c. Typical transfer and output characteristics of the perpendicular OFETs are shown in Figure S3. The field-effect hole mobility μ, threshold voltage Vth, and subthreshold swing SS were determined from the transfer curves in the forward Vgs sweep. The μ and Vth were extracted using the following equation: W |Id| = Ciμ(Vgs − Vth)2 (1) 2L

3. SPATIAL UNIFORMITY OF PBTTT-C16 FILMS FORMED BY SELF-ASSISTED FLOW-COATING Since the target application of the present study is OFETs, the spatial uniformity of the pBTTT-C16 film formed by selfassisted flow-coating was examined from the aspect of OFET properties. The device-to-device variation of OFET properties is closely related to the spatial uniformity of the active layer; that is, the higher spatial uniformity leads to the smaller deviceto-device variation. From the polarizing optical microscope C


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Figure 5. Histograms of (a) field-effect hole mobility, (b) on-state current, (c) threshold voltage, and (d) subthreshold swing of the OFET array. The filled and open bars show the results for the parallel and perpendicular OFETs, respectively. The bin width of each histogram is (a) 0.005 cm2·V−1·s−1, (b) 0.4 μA, (c) 0.1 V, and (d) 0.05 volts per decade.

standard deviations (σ/Av) are summarized in Table 1. The field-effect hole mobilities for the parallel and perpendicular OFET sets (μ// and μ⊥) were 0.273 ± 0.007 and 0.221 ± 0.006 cm2·V−1·s−1, respectively. The in-plane anisotropy (= μ///μ⊥) was found to be 1.24. Although the difference between μ// and μ⊥ was small (Δμ = μ// − μ⊥ = 0.052 cm2·V−1·s−1), the distribution of μ// was completely separated from that of μ⊥, as shown in Figure 5a. This is thanks to very small device-todevice variation of both OFET sets, which is less than 3%. The complete separation clearly shows that the charge transport in the flow-coated pBTTT-C16 film is anisotropic. The observed mobility was slightly lower than that reported by Umeda et al.,20 even though the OFET structures were similar. To check the causes, we fabricated 30 spin-coated OFETs on the lyophobic gate insulator surface. (The spincoated pBTTT-C16 film was formed using a different lyophobic−lyophilic pattern shown in Figure S4. The mechanism of spin-coating on such a patterned surface is probably different from that of flow-coating. It is very interesting, but we will not further discuss it, because we focus on unidirectional coating methods.) The mobility of the spin-coated OFETs was 0.262 ± 0.006 m2·V−1·s−1, which was almost the same as, but a little bit smaller than, that of the parallel flow-coated OFETs. This result shows that the quality of flow-coated films is comparable to that of the spin-coated films. Therefore, the relatively smaller mobility is probably due to the quality variation of commercially available polymer reagents or due to the inevitable air exposure in the OFET fabrication process conducted in this study. In addition to μ, the device-to-device variation of IdON was also less than 3% for both parallel and perpendicular OFET sets, as shown in Figure 5b. This is attributed to the narrow distribution of Vth shown in Figure 5c, because the device-todevice variation of IdON is determined by those of μ and Vth, as seen from eq 1. Furthermore, the OFET array showed very narrow distribution of SS as shown in Figure 5d. The very small device-to-device variation of these four quantities should result in the identical transfer characteristics for each OFET set. To

Figure 4. (a) Schematic illustration of the array composed of 30 parallel and 30 perpendicular pBTTT-C16 OFETs fabricated by selfassisted flow-coating, and typical (b) transfer and (c) output characteristics of the parallel OFETs with a channel length (L)/ width (W) of 50/300 μm. The “parallel” and “perpendicular” specify the channel current direction with respect to the flow-coating direction. (b, c) Data in the forward and reverse sweeps are shown by the solid and dotted curves, respectively. The relationship between the square root of |Id| and Vgs is also shown in (b), and the field-effect hole mobility and threshold voltage were derived from the broken straight line.

where Ci (= 36.0 nF/cm2) is the insulator capacitance per unit area. This equation is derived under the gradual channel approximation and describes Id in saturation regime. The onstate current IdON was defined by |Id| at Vgs = Vds = −30 V. From the transfer characteristics shown in Figure 4b, we obtained: μ// = 0.273 cm2·V−1·s−1, Vth = −4.25 V, SS = 0.45 volts per decade, and IdON = 19.5 μA. The device-to-device variation of the OFET array was evaluated by conducting the same analysis for all OFETs. Figure 5 shows the histograms of μ, IdON, Vth, and SS of the OFET array. The filled and open bars show the results for the parallel and perpendicular OFET sets, respectively. Their average values (Av), standard deviations (σ), and relative D


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Table 1. Summary of the Average Values (Av), Standard Deviations (σ), and Relative Standard Deviation (σ/Av) of the FieldEffect Hole Mobility (μ), On-State Current (IdON), Threshold Voltage (Vth), and Subthreshold Swing (SS) for the Parallel and Perpendicular OFET Sets parallel OFETs μ (cm2·V−1·s−1) Idon (μA) Vth (V) SS (volts per decade)

perpendicular OFETs

Av

σ

σ/Av

Av

σ

σ/Av

0.273 19.7 −4.1 0.50

0.007 0.5 0.2 0.05

0.024 0.027

0.221 16.2 −3.9 0.53

0.006 0.4 0.1 0.06

0.026 0.027

0.10

0.11

two: lyophobic and lyophilic areas, and the boundary line is located at ΔLpho from the leading edge of coating blade. The lyophilic area during the continuous film growth corresponds to the area covered with the pBTTT-C16 film, as shown in Figure 7b. When the contact line is located at x (>0) from the leading edge of coating blade, the total surface energy per unit width E/ w in the lyophobic line pattern area is given by

clearly demonstrate that, all the transfer curves of 30 parallel and 30 perpendicular OFETs were overlaid in Figure 6a,b,

E − E0 = γlgLg + (γslb − γsgb)Lb + (γslpho − γsgpho)Lpho w + (γslphi − γsgphi)Lphi

(2)

where w is the line width of lyophobic area; E0/w is the total energy per unit width of the system without solution; γlg, γisg, and γisl are the surface energy of liquid, the solid−air interface energy, and the solid−liquid interface energy, respectively, and i = b, pho, and phi represent the movable blade and the “lyophobic” and the “lyophilic” surfaces, respectively; and Lb, Lpho, Lphi, and Lg (= Lg1 + Lg2) are the interface lengths between the solution and the blade, between the solution and the lyophobic surface, between the solution and the lyophilic surface, and between the solution and air, respectively. Using the Young’s equations (γisl = γisg − γlg cos θi), we can remove all the interface energies from eq 2: ΔE = Lg − Lb cos θ b − Lpho cos θ pho − Lphi cos θ phi w·γlg (3)

Figure 6. Overlaid transfer curves of (a) 30 parallel and (b) 30 perpendicular OFETs. Both transfer curves in the forward and reverse sweeps are plotted. (insets) Optical microscope images of one of the parallel or perpendicular OFETs.

where ΔE = E − E0, and θi are the contact angles. Since Lb, Lpho, Lphi, and Lg are geometrically calculated from the experimental conditions for given values of x and ΔLpho (Equations S1−S4), now we can calculate ΔE/w·γlg using the experimentally determined contact angles. For calculation, we used the contact angles of the pure solvent (o-DCB) instead of those of the pBTTT-C16 solution: that is, θb = 51°, θpho = 50°, and θphi ≤ 35° (depending on the degree of dryness). Figure 7c shows the calculated results of ΔE/w·γlg as a function of x for different values of ΔLpho, where θphi was assumed to be equal to the contact angle (35°) of o-DCB on the dry pBTTT-C16 film. From Figure 7c, we found that ΔE/w·γlg has minimums at xpho (∼0.057 mm) and/or xphi (∼0.101 mm), depending on the value of ΔLpho. For ΔLpho ≥ xphi, a single minimum appears at xpho, which corresponds to the contact line position on the fully lyophobic surface (ΔLpho = ∞) as shown in Figure 7d. When the contact line is located at xpho, the spread of organic solution toward the open air side is not enough to induce the concentration gradient sufficient for film growth near the contact line by solvent evaporation. In this case, no film formation occurs. This is the case of the lyophobic surface areas in Figure 3b. For ΔLpho < xphi, ΔE/w·γlg has at least a minimum at xphi, which corresponds to the contact line position during the coating on

respectively. In these figures, the transfer curves in both forward and reverse sweeps are plotted. Here, we would like to emphasize that the excellent overlapping in the transfer characteristics indicates very small device-to-device variation not only of the mobility but of the whole electrical properties. From these results, we confirmed that the spatial uniformity of the pBTTT-C16 film formed by self-assisted flow-coating was very high.

4. MECHANISM OF SELF-ASSISTED FLOW-COATING To understand the mechanism of self-assisted flow-coating, the surface energy calculation was performed assuming a simplified solution profile shown in Figure 7a. The solution surfaces exposed to air are assumed to be flat. Since the contact angles of o-DCB droplets on the fluorine-coated glass blade (51°) and the ODTS-treated substrate (50°) are almost the same, the flat solution surface in the substrate−blade gap is assumed to make the same angle (= 90° − α/2) against both blade and substrate surfaces. Here, α is the inclination angle of the blade against the substrate surface plane. The substrate surface is divided into E


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Figure 7. (a) Simplified model for the surface energy calculation, where Lb, Lpho, Lphi, and Lg (= Lg1 + Lg2) are the lengths of solution in contact with the blade, the lyophobic surface, the lyophilic surface, and air, respectively; h is the gap height between the blade and the substrate surface; α is the inclination angle of the blade with respect to the substrate surface plane; x and ΔLpho are the locations of the contact line and the boundary line between the lyophobic and lyophilic surfaces, respectively. (b) Solution profile during self-assisted flow-coating (ΔLpho < xphi). (c) Calculated results of ΔE/w·γlg as a function of x for different of ΔLpho. (d) Solution profile for fully lyophobic surface (ΔLpho = ∞). In this case, no film formation occurs.

polymeric organic semiconductors on highly hydrophobic and lyophobic gate insulator surfaces, which was named self-assisted flow-coating. Forming a suitable lyophobic−lyophilic pattern on the device substrate enabled the thin-film formation of pBTTT-C16 in both lyophobic and lyophilic surface areas by flow-coating. This film formation is achieved by the pinning of the contact line of organic semiconductor solution to the film growth front, which is realized as the metastable state with the assistance of the own film. This mechanism was confirmed by a simple surface energy calculation. The spatial uniformity of the pBTTT-C16 film formed by self-assisted flow-coating was evaluated from the aspect of OFET properties. In the lyophobic surface area, we fabricated a BG/TC-type OFET array, which was composed of 30 parallel and 30 perpendicular OFETs with respect to the flow-coating direction. The device-to-device variations in the OFET properties were evaluated for each set of the parallel and perpendicular OFETs. The average hole mobility was 0.273 cm2·V−1·s−1 for the parallel OFETs and 0.221 cm2·V−1·s−1 for the perpendicular ones. The device-todevice variations of the field-effect hole mobility and the onstate current were less than 3% for both OFET sets. In addition, the very narrow distributions of the threshold voltage and the subthreshold swing were obtained. As a result, the transfer curves of all OFETs overlapped for each parallel and perpendicular OFET set. The excellent overlapping in the transfer characteristics showed that the device-to-device variation not only of the mobility but of the whole electrical properties was very small. These results clearly show that the spatial uniformity of the pBTTT-C16 film formed by self-

the lyophilic surface. This calculation result indicates that the adequate spread of organic solution on the lyophobic surface can be realized with the assistance of the own film as the metastable (xpho < ΔLpho < xphi) or stable (ΔLpho ≤ xpho) state. We believe that the condition of ΔLpho < xphi is satisfied by the covering of the lyophobic substrate surface in contact with solution with a very thin wet pBTTT-C16 layer, as shown in Figure 7b. This is plausible, because the pBTTT-C16 solution near the contact line becomes supersaturated when the film is continuously growing. ΔLpho would be nearly equal to but slightly smaller than xphi. Therefore, once the film formation occurs in the top lyophilic rectangular area, the contact line on the lyophobic (lyophilic) surface is pinned to the film growth front in the metastable (stable) state, and the film growth is continued even in the lyophobic substrate surface area. The main role of the top lyophilic rectangular area is the initiation and stabilization of the film formation. This is the mechanism of self-assisted flow-coating. Finally, we would like to mention the role of the lyophilic space areas in the line-and-space pattern region. The presence of the lyophilic space areas is not essential for self-assisted flowcoating, as can be seen from the fact that the lyophilic space areas are not considered in the above discussion. The important role of the lyophilic space areas is to stabilize the contact line pinning in the lyophobic areas.

5. CONCLUSION We have reported a scalable unidirectional solution coating technique that can form spatially uniform thin films of F


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electrode and the gate insulator, respectively. After self-assisted flowcoating of pBTTT-C16, the sample was annealed at 150 °C for 15 min to obtain a terrace-phase pBTTT-C16 film.19,20,41 The terraced morphology of the pBTTT-C16 film was confirmed by atomic force microscope (AFM; Hitachi AFM5100N and AFM5000II; Figure S5). To fabricate a BG/TC-type OFET array, the source/drain electrodes were deposited on the pBTTT-C16 film by sequential thermal evaporation of MoO3 (25 nm thick) and Au (63 nm thick) through a shadow mask in vacuum.42 The source/drain electrodes were placed in the lyophobic (highly hydrophobic) surface area. The shadow mask produced a 12 × 5 array of OFETs with a channel length (L)/width (W) of 50/300 μm, in which the channel direction of neighbor OFETs was orthogonal to each other. Thirty parallel and 30 perpendicular OFETs were fabricated over an area of ∼1 cm2, as shown in Figure 4a. 6.5. Electrical Characterization. The electrical characteristics of OFETs were measured with a combined system of a vacuum probe station (VIC International, Inc. VMP-100) and a semiconductor parameter analyzer (Keithley 4200-SCS). This measurement was performed at room temperature under vacuum condition less than 1 × 10−4 Pa in the dark, after isolating each OFET from the others by removing the surrounding pBTTT-C16 film with a probe of the vacuum probe station. First the transfer characteristics were measured for all OFETs, and then the output characteristics of the 12 OFETs in the center row of the array were measured. The field-effect hole mobility, on-state current, threshold voltage, and subthreshold swing were extracted from the transfer characteristics measured in the saturation regime. 6.6. Surface and Film Characterization. The contact angles of water and o-DCB droplets were measured with a contact angle meter (Kyowa DM 500), and the thickness of pBTTT-C16 films and source/ drain electrodes were measured with a stylus-type step profiler (Kosaka ET200). The optical microscope observation of pBTTT-C16 films was performed with an OLYMPUS BX51 optical microscope. The molecular orientation of the pBTTT-C16 active layer of the OFET array was examined by measuring the polarized Raman spectra using a micro-Raman spectrometer system (Lambda Vision MicroRAM-300).

assisted flow-coating was very high and suggest that the selfassisted flow-coating is a promising coating technique from a practical application perspective.

6. EXPERIMENTAL SECTION 6.1. Preparation of Highly Hydorophobic Surfaces. Heavily doped n-type Si wafers (20 × 20 mm2) with a thermally grown SiO2 layer (thickness: 94 or 300 nm) were used as substrates. The substrate was cleaned by immersion for 10 min three times in a piranha solution, which is a mixture of sulfuric acid and hydrogen peroxide, rinsed several times with deionized water, and then dried with nitrogen gas blow. Subsequently, it was treated with ODTS vapor at 120 °C for 3 h,38 as follows. The substrate was put in a Petri dish with three drops of ODTS. The Petri dish was covered and placed in a well-closed container, which was placed on a hot plate at 120 °C. After 3 h, the substrate was rinsed with anhydrous toluene several times. These treatments were processed in a nitrogen-purged glovebox to prevent undesired polymerization of ODTS by reaction with water in air. Then, the substrate was subjected to sonication sequentially in toluene for 10 min and in 2-propanol for 10 min twice and then dried with nitrogen gas blow. After the substrate was kept in clean air for 16 h, it was annealed on a hot plate at 150 °C for 1 h in nitrogen atmosphere. This ODTS treatment produces routinely a highly hydrophobic surface showing a water contact angle greater than 110°. The contact angle of o-DCB on the ODTS-treated surface was 50°. 6.2. Producing Lyophobic-Lyophilic Patterns. Before proceeding to the patterning process, the ODTS-treated surface was rubbed with a rayon cloth (Yoshikawa Chemical Co., YA-18-R) by using a homemade rubbing machine39 as a precaution. Polymerized ODTS particles can be effectively removed by this rubbing treatment,40 if they exist. A lyophobic−lyophilic surface pattern was formed by exposing the ODTS-treated surface to VUV light (wavelength 172 nm) through a photomask,25−28 which was placed at 20 μm above the substrate surface to produce an air layer. The presence of the 20 μm thick air layer enables efficient removal of the ODTS self-assembled monolayer by VUV light irradiation. An excimer lamp irradiation unit (Ushio SUS06) was used as the light source, and the exposure time was 60 s. After the VUV light treatment, the substrate was subjected to sonication sequentially in toluene and 2-propanol for 5 min each and then dried with nitrogen gas blow. The area exposed to VUV light becomes lyophilic, showing a contact angle of o-DCB of less than 8°. 6.3. Self-Assisted Flow-Coating of pBTTT-C16. pBTTT-C16 was purchased from Merck KGaA and used without further purification. The thin-film formation of pBTTT-C16 was conducted with a homemade flow-coater illustrated in Figure 1a.13 The movable blade was inclined at ∼1.5° against the substrate surface plane. The gap height between the movable blade and the substrate surface was set to 200 μm. The flow-coating process adopted in this study was as follows: the substrate was placed on the stationary sample stage at 80 °C so that the rubbing direction was parallel to the flow-coating direction; to perform a round-trip coating, the leading edge of the movable blade was set to the ending line of flow-coating (15 mm from the top edge of the substrate); then, 20 μL of a 0.7 wt % solution of pBTTT-C16 in o-DCB, which was heated on a hot plate at 120 °C, was poured into the gap between the blade and the substrate; after waiting for 1 min to stabilize the temperature of both substrate and solution, the movable blade was moved to the starting line of flowcoating (0.5 mm from the top edge of the substrate) at a constant speed of 1000 μm/s, and immediately thin-film formation of pBTTTC16 was started by moving the blade at a constant speed of 100 μm/s toward the ending line. This round-trip motion of the blade was effective for removing pBTTT-C16 particles, which might be generated in the solution bottle and/or a pipet, from the thin-film formation area. This flow-coating process was performed in a nitrogenpurged glovebox. 6.4. Fabrication of OFET Arrays with a BG/TC Structure. A heavily doped n-type Si substrate (20 × 20 mm2) with a 94 nm thick thermally grown SiO2 insulating layer was used as an OFET array substrate. The Si substrate and SiO2 layer serve as a common gate

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15398. Additional figures and equations as mentioned in the text, including polarizing optical microscope images, polarized Raman spectra, OFET transfer and output characteristics, mathematical expressions, and an AFM image (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kenji Sakamoto: 0000-0002-1379-874X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Prof. Y. Furukawa and Mr. R. Iwasaki of Waseda Univ. for performing preliminary Raman measurement. They also thank Prof. D. Kumaki of Yamagata Univ. for helpful discussion on the sample preparation. This work was supported in part by JSPS KAKENHI Grant No. 25286045. G


Research Article


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(19) Umeda, T.; Tokito, S.; Kumaki, D. High-Mobility and Air-Stable Organic Thin-Film Transistors with Highly Ordered Semiconducting Polymer Films. J. Appl. Phys. 2007, 101, 054517. (20) Umeda, T.; Kumaki, D.; Tokito, S. Surface-Energy-Dependent Field-Effect Mobilities up to 1 cm2/Vs for Polymer Thin-Film Transistor. J. Appl. Phys. 2009, 105, 024516. (21) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb, U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Müllen, K. A Zone-Casting Technique for Device Fabrication of Field-Effect Transistors Based on Discotic Hexa-peri-hexabenzocoronene. Adv. Mater. 2005, 17, 684− 689. (22) Becerril, H. A.; Roberts, M. E.; Liu, Z.; Locklin, J.; Bao, Z. HighPerformance Organic Thin-Film Transistors through Solution-Sheared Deposition of Small-Molecule Organic Semiconductors. Adv. Mater. 2008, 20, 2588−2594. (23) Soeda, J.; Uemura, T.; Okamoto, T.; Mitsui, C.; Yamagishi, M.; Takeya, J. Inch-Size Solution-Processed Single-Crystalline Films of High-Mobility Organic Semiconductors. Appl. Phys. Express 2013, 6, 076503. (24) Pierre, A.; Sadeghi, M.; Payne, M. M.; Facchetti, A.; Anthony, J. E.; Arias, A. C. All-Printed Flexible Organic Transistors Enabled by Surface Tension-Guided Blade Coating. Adv. Mater. 2014, 26, 5722− 5727. (25) Liu, X.; Kanehara, M.; Liu, C.; Sakamoto, K.; Yasuda, T.; Takeya, J.; Minari, T. Spontaneous Patterning of High-Resolution Electronics via Parallel Vacuum Ultraviolet. Adv. Mater. 2016, 28, 6568−6573. (26) Minari, T.; Kanehara, Y.; Liu, C.; Sakamoto, K.; Yasuda, T.; Yaguchi, A.; Tsukada, S.; Kashizaki, K.; Kanehara, M. RoomTemperature Printing of Organic Thin-Film Transistors with πJunction Gold Nanoparticles. Adv. Funct. Mater. 2014, 24, 4886−4892. (27) Sugimura, H.; Nakagiri, N. Force Microscopy Imaging of Photopatterned Organosilane Monolayers: Application to Probe Alignment in AFM Patterning Following Photolithography. Appl. Phys. A: Mater. Sci. Process. 1998, 66, S427−S430. (28) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Micropatterning of Alkyl- and Fluoroalkylsilane Self-Assembled Monolayers Using Vacuum Ultraviolet Light. Langmuir 2000, 16, 885−888. (29) Kagan, C. R.; Breen, T. L.; Kosbar, L. L. Patterning Organic− Inorganic Thin-Film Transistors Using Microcontact Printed Templates. Appl. Phys. Lett. 2001, 79, 3536−3538. (30) Ando, M.; Kawasaki, M.; Imazeki, S.; Sasaki, H.; Kamata, T. SelfAligned Self-Assembly Process for Fabricating Organic Thin-Film Transistors. Appl. Phys. Lett. 2004, 85, 1849−1851. (31) Lee, K. S.; Smith, T. J.; Dickey, K. C.; Yoo, J. E.; Stevenson, K. J.; Loo, Y.-L. High-Resolution Characterization of Pentacene/Polyaniline Interfaces in Thin-Film Transistors. Adv. Funct. Mater. 2006, 16, 2409−2414. (32) Park, S. K.; Mourey, D. A.; Subramanian, S.; Anthony, J. E.; Jackson, T. N. Non-Relief-Pattern Lithography Patterning of Solution Processed Organic Semiconductors. Adv. Mater. 2008, 20, 4145−4147. (33) Avnon, E.; Paz, Y.; Tessler, N. Photopatternable Self-Assembled Monolayers as Micron Scale Templates for Polymer Based Field Effect Transistors. Appl. Phys. Lett. 2009, 94, 013502. (34) Chang, K.-J.; Yang, F.-Y.; Liu, C.-C.; Hsu, M.-Y.; Liao, T.-C.; Cheng, H.-C. Self-Patterning of High-Performance Thin Film Transistors. Org. Electron. 2009, 10, 815−821. (35) Lee, M.-W.; Lee, M.-Y.; Choi, J.-C.; Park, J.-S.; Song, C.-K. Fine Patterning of Glycerol-Doped PEDOT:PSS on Hydrophobic PVP Dielectric with Ink Jet for Source and Drain Electrode of OTFTs. Org. Electron. 2010, 11, 854−859. (36) Acton, O.; Hutchins, D.; Á rnadóttir, L.; Weidner, T.; Cernetic, N.; Ting, G. G.; Kim, T.-W.; Castner, D. G.; Ma, H.; Jen, A. K. -Y. Spin-Cast and Patterned Organophosphonate Self-Assembled Monolayer Dielectrics on Metal-Oxide-Activated Si. Adv. Mater. 2011, 23, 1899−1902. (37) Kim, Y.-H.; Yoo, B.; Anthony, J. E.; Park, S. K. Controlled Deposition of a High-Performance Small-Molecule Organic Single-

REFERENCES

(1) Gelinck, G.; Heremans, P.; Nomoto, K.; Anthopoulos, T. D. Organic Transistors in Optical Displays and Microelectronic Applications. Adv. Mater. 2010, 22, 3778−3798. (2) Baude, P. F.; Ender, D. A.; Haase, M. A.; Kelley, T. W.; Muyres, D. V.; Theiss, S. D. Pentacene-Based Radio-Frequency Identification Circuitry. Appl. Phys. Lett. 2003, 82, 3964−3966. (3) Rotzoll, R.; Mohapatra, S.; Olariu, V.; Wenz, R.; Grigas, M.; Dimmler, K.; et al. Radio Frequency Rectifiers Based on Organic ThinFilm Transistors. Appl. Phys. Lett. 2006, 88, 123502. (4) Zhu, Z. T.; Mason, J. T.; Dieckmann, R.; Malliaras, G. G. Humidity Sensors Based on Pentacene Thin-Film Transistors. Appl. Phys. Lett. 2002, 81, 4643−4645. (5) Roberts, M. E.; Mannsfeld, S. C. B.; Queralto, N.; Reese, C.; Locklin, J.; Knoll, W.; Bao, Z. Water-Stable Organic Transistors and Their Application in Chemical and Biological Sensors. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 12134−12139. (6) Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. A Large-Area, Flexible Pressure Sensor Matrix with Organic FieldEffect Transistors for Artificial Skin Applications. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 9966−9970. (7) Zhang, Q.; Subramanian, V. DNA Hybridization Detection with Organic Thin Film Transistors: Toward Fast and Disposable DNA Microarray Chips. Biosens. Bioelectron. 2007, 22, 3182−3187. (8) Lee, W. H.; Cho, J. H.; Cho, K. Control of Mesoscale and Nanoscale Ordering of Organic Semiconductors at the Gate Dielectric/Semiconductor Interface for Organic Transistors. J. Mater. Chem. 2010, 20, 2549−2561. (9) Ikawa, M.; Yamada, T.; Matsui, H.; Minemawari, H.; Tsutsumi, J.; Horii, Y.; Chikamatsu, M.; Azumi, R.; Kumai, R.; Hasegawa, T. Simple Push Coating of Polymer Thin-Film Transistors. Nat. Commun. 2012, 3, 1176. (10) Meredith, J. C.; Smith, A. P.; Karim, A.; Amis, E. J. Combinatorial Materials Science for Polymer Thin-Film Dewetting. Macromolecules 2000, 33, 9747−9756. (11) Stafford, C. M.; Roskov, K. E.; Epps, T. E., III; Fasolka, M. J. Generating Thickness Gradients of Thin Polymer Films via Flow Coating. Rev. Sci. Instrum. 2006, 77, 023908. (12) DeLongchamp, D. M.; Kline, R. J.; Jung, Y.; Germack, D. S.; Lin, E. K.; Moad, A. J.; Richter, L. J.; Toney, M. F.; Heeney, M.; McCulloch, I. Controlling the Orientation of Terraced Nanoscale “Ribbons” of a Poly(thiophene) Semiconductor. ACS Nano 2009, 3, 780−787. (13) Sakamoto, K.; Ueno, J.; Bulgarevich, K.; Miki, K. Anisotropic Charge Transport and Contact Resistance of 6,13-Bis(triisopropylsilylethynyl) pentacene Field-Effect Transistors Fabricated by a Modified Flow-Coating Method. Appl. Phys. Lett. 2012, 100, 123301. (14) Diao, Y.; Tee, B. C-K.; Giri, G.; Xu, J.; Kim, D. H.; Becerril, H. A.; Stoltenberg, R. M.; Lee, T. H.; Xue, G.; Mannsfeld, S. C. B.; Bao, Z. Solution Coating of Large-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nat. Mater. 2013, 12, 665− 671. (15) Li, B.; Zhang, C.; Jiang, B.; Han, W.; Lin, Z. Flow-Enabled SelfAssembly of Large-Scale Aligned Nanowires. Angew. Chem., Int. Ed. 2015, 54, 4250−4254. (16) Sakamoto, K.; Bulgarevich, K.; Miki, K. Small Device-to-Device Variation of 6,13-Bis(triisopropylsilylethynyl)pentacene Field-Effect Transistor Arrays Fabricated by a Flow-Coating Method. Jpn. J. Appl. Phys. 2014, 53, 02BE01. (17) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Liquid-Crystalline Semiconducting Polymers with High ChargeCarrier Mobility. Nat. Mater. 2006, 5, 328−333. (18) Hamadani, B. H.; Gundlach, D. J.; McCulloch, I.; Heeney, M. Undoped Polythiophene Field-Effect Transistors with Mobility of 1 cm2V−1s−1. Appl. Phys. Lett. 2007, 91, 243512. H


Research Article

ACS Applied Materials & Interfaces Crystal Transistor Array by Direct Ink-Jet Printing. Adv. Mater. 2012, 24, 497−502. (38) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. A Stable SolutionProcessed Polymer Semiconductor with Record High-Mobility for Printed Transistors. Sci. Rep. 2012, 2, 754. (39) Sakamoto, K.; Arafune, R.; Ito, N.; Ushioda, S.; Suzuki, Y.; Morokawa, S. Molecular Orientation of Rubbed and Unrubbed Polyimide Films Determined by Polarized Infrared Absorption. Jpn. J. Appl. Phys. 1994, 33, L1323−L1326. (40) Ito, Y.; Virkar, A. A.; Mannsfeld, S.; Oh, J. H.; Toney, M.; Locklin, J.; Bao, Z. Crystalline Ultrasmooth Self-Assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors. J. Am. Chem. Soc. 2009, 131, 9396−9404. (41) Lee, M. J.; Gupta, D.; Zhao, N.; Heeney, M.; McCulloch, I.; Sirringhaus, H. Anisotropy of Charge Transport in a Uniaxially Aligned and Chain-Extended, High-Mobility, Conjugated Polymer Semiconductor. Adv. Funct. Mater. 2011, 21, 932−940. (42) Kano, M.; Minari, T.; Tsukagoshi, K. Improvement of Subthreshold Current Transport by Contact Interface Modification in p-Type Organic Field-Effect Transistors. Appl. Phys. Lett. 2009, 94, 143304.

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Spatially Uniform Thin-Film Formation of Polymeric Organic

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