benzothiophene (C8-BTBT) - ACS Publications

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Organic Electronic Devices

Scalable Directed Assembly of Highly Crystalline 2,7dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) Films Zhimin Chai, Salman A. Abbasi, and Ahmed Busnaina ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01433 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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ACS Applied Materials & Interfaces

Scalable Directed Assembly of Highly Crystalline 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) Films Zhimin Chai, Salman A. Abbasi, Ahmed A. Busnaina* NSF Nanoscale Science and Engineering Center for High-Rate Nanomanufacturing (CHN), Northeastern University, Boston, Massachusetts 02115, United States KEYWORDS: Dip coating, C8-BTBT, pulling speed, concentration, transistor

ABSTRACT

Assembly of organic semiconductors with ordered crystal structure has been actively pursued for electronics applications such as organic field-effect transistors (OFETs). Among various film deposition methods, solution based film growth from small molecule semiconductors is preferable because of its low material and energy consumption, low cost, and scalability. Here, we

show

scalable

and

controllable

directed

assembly of

highly

crystalline

2,7-

dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films via a dip coating process. Selfaligned stripe patterns with tunable thickness and morphology over a centimeter scale are obtained by adjusting two governing parameters, the pulling speed of a substrate and the solution concentration. OFETs are fabricated using the C8-BTBT films assembled at various conditions.

ACS Paragon Plus Environment

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A field-effect hole mobility up to 3.99 cm2V−1s−1 is obtained. Owing to the highly scalable crystalline film formation, the dip coating directed assembly process could be a great candidate for manufacturing next-generation electronics. Meanwhile, the film formation mechanism discussed in this paper could provide a general guideline to prepare other organic semiconducting films from small molecule solutions.

1. INTRODUCTION Owing to merits of low cost, low energy consumption, easy fabrication and flexibility, organic field effect transistors (OFETs) have attracted a great deal of attention to applications such as active-matrix displays,1-5 complementary integrated circuits,6,

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radio frequency identification

tags,8, 9 and bio and chemical sensors.10-13 Polymers and small molecules are commonly used semiconducting channel materials for the OFETs. The polymer semiconducting materials usually exhibit a high molecular disorder and a low degree of crystallinity,14, 15 which limits the charge transport and results in a low mobility of polymer-based transistors.16-19 The small molecules on the other hand can reach a high degree of crystallinity and thus show better performance than polymers.20-22

As

a

small

molecule

semiconductor,

2,7-dioctyl[1]benzothieno[3,2-

b][1]benzothiophene (C8-BTBT) has attracted great interest because of its strong intermolecular packing force and outstanding carrier mobility (as high as 43 cm2V-1s-1).23 C8-BTBT films can be deposited using vacuum vapor deposition.24-26 However, vacuum vapor deposition is a costly method which is contrary to the inexpensive merit of the OFETs. Another candidate for C8-BTBT film deposition is a solution-based process. Virtues of vacuum free, low cost and high scalability have made the solution-based process a method pursued for depositing organic semiconducting layers in the OFETs. Spin coating is the most commonly used solutionbased process to deposit C8-BTBT films.27-29 Uniform C8-BTBT films can be prepared by spin


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coating in a few seconds. However, fast solvent evaporation during spin coating shortens the time for crystallization, which leads to low crystallinity and low field-effect mobility of ascoated films.29,

30

Although post annealing in solvent vapor could improve crystallinity and

mobility,23, 31, 32 it takes a few hours and at the same time adds an additional step to the spin coating process. Drop casting, a slow evaporation process, can produce highly crystalline films.33-35 However, these films typically show poor uniformity due to evaporation of solvent induced rotational flow and “coffee ring” effect.36 Additionally, crystals in the films grow radially inward from a droplet edge as the droplet shrinks toward its center.37,

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The radial

crystal alignment of the drop casted films hampers their practical application in large scale functional devices. Uniaxial crystal alignment can be realized by dip coating process. In the dip coating process, a substrate is dipped into a solution vertically and then pulled out at a controlled speed. Straight three-phase (air-liquid-substrate) contact line combined with unidirectional pulling lead to uniaxial crystal growth along the pulling direction. Although self-aligned crystalline organic films have been deposited using the dip coating process,39, 40 C8-BTBT films deposited through dip coating are rarely reported.41 Here, we report a directed assembly of self-aligned and highly crystalline C8-BTBT films over a square centimeter sized area via the dip coating process. Films with tunable thickness and morphology are assembled by adjusting two governing parameters, the pulling speed of a substrate and the solution concentration. Bottom-gate bottom-contact transistors are constructed using C8-BTBT films assembled at various parameters. Thicker films exhibit higher field-effect hole mobilities, and a mobility of up to 3.99 cm2V−1s−1 has been obtained. 2. EXPERIMENTAL METHODS 2.1 Film Assembly and Characterization


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The C8-BTBT purchased from Sigma–Aldrich was dissolved in toluene. Toluene has a high boiling point of 110.6 oC and evaporates slowly in ambient air. The low evaporation rate of toluene facilitates the crystallization of C8-BTBT films as well as allows the solution concentration to remain stable. To assemble C8-BTBT thin films, substrates were vertically immersed in the as-prepared C8-BTBT solution, and then pulled out at a constant speed using a dip coater (KSV instruments). A schematic illustration of the dip coating directed assembly process is shown in Figure 1 (a). All the experiments were conducted under ambient conditions (temperature 20 oC, relative humidity 40-50%). To control the film morphology and thickness, a wide range of pulling speeds (from 0.5 mm/min to 90 mm/min) and various solution concentrations (from 1 mg/mL to 15 mg/mL) were utilized.

Figure 1. (a) A schematic illustration of the directed assembly of aligned C8-BTBT film utilizing the dip coating process. The inset is the molecular structure of C8-BTBT. The C8BTBT film shows a stripe pattern, and the stripe formation mechanism is shown in (b). When pulling the substrate out of the solution, continuous line of C8-BTBT molecules at the air-liquidsubstrate contact line segregates into periodically distributed domains because of ‘fingering


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instability’. These domains work as nucleation sites and direct the crystallization of C8-BTBT into stripes parallel to the pulling direction. (c) and (d) show the status of the air-liquid-substrate contact area at different pulling speeds. In the low speed regime (c, meniscus controlled), a meniscus climbs up the substrate. Toluene at the meniscus evaporates, which leads to C8-BTBT molecule accumulation at the air-liquid-substrate contact line via the upward convective flow. In the high speed regime (d, entrainment controlled), a liquid film containing C8-BTBT molecules is entrained on the substrate above the meniscus. The morphology and microstructure of the C8-BTBT films were captured using an optical microscope (Optiphot 200 D, Nikon) equipped with a high-resolution color digital camera. Film topography was observed utilizing an atomic force microscope (AFM, Park Systems NX20) in a tapping mode. An AFM probe (PPP-NCHR, NanoSensors) with a spring constant of ~42 N/m and a resonant frequency of ~300 kHz was used. Film thickness was subsequently determined using an off-line analysis software. Crystal structure of the films was measured using an X-ray diffractometer (XRD, PANalytical X’Pert) with a Cu Kα radiation at a scan rate of 0.02 2θ/s. 2.2 Device fabrication Highly doped p-type silicon wafers (0.001-0.005 Ω cm, University Wafer) were used as substrates to fabricate OFETs. The substrates were cleaned using piranha solution (sulfuric acid/hydrogen peroxide, 2:1) for five minutes, rinsed with deionized (DI) water for ten minutes and then dried with nitrogen. A silicon dioxide/silicon nitride (SiO2/SiN, 150 nm/150 nm) gate dielectric bilayer was deposited on the substrates by plasma-enhanced chemical vapor deposition (PECVD, Surface Technology Systems). The capacitance per unit area for the dielectric film is 15.2 nFcm2. Photoresist (MICROPOSIT S1800, MicroChem) was spun coated on the dielectric film and then patterned using optical photolithography. A 5 nm titanium adhesion layer and a 25


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nm gold layer were deposited using E-beam evaporation on the patterned photoresist which was then lifted off using acetone to form source and drain electrodes with a channel width of 100 µm and a channel length of 80 µm. The C8-BTBT film was then assembled on the patterned substrates to complete the OFETs fabrication. The transfer and output characteristics of the fabricated OFETs were probed using a semiconductor parameter analyzer (HP 4156C, Agilent Technologies) at ambient conditions. The hole mobility µ and the threshold voltage Vth were extracted from the transfer curve in the saturation regime using the following equation: =

(1)

where W and L are channel width and length, respectively, C is the capacitance per unit area of the gate dielectric layer, Id is the drain current, and Vg is the gate voltage. 3. RESULTS AND DISCUSSION In this section, we first characterize the C8-BTBT films assembled on the dielectric film (SiO2/SiN) coated substrates (without electrodes). The change in film morphology and thickness due to the two governing parameters, the pulling speed and the solution concentration, is investigated, and the film growth mechanism is discussed. OFETs are fabricated using the C8BTBT films as p-type semiconductors and the electrical properties of the OFETs are investigated. 3.1 C8-BTBT films Assembled at Low Rates Figure 2 shows optical and AFM images of the C8-BTBT films assembled at low rates (pulling speed < 5 mm/min) with a solution concentration of 15 mg/mL. The film assembled at 5 mm/min (Figure 2 (k) and (l)) shows stripe patterns parallel to the pulling direction. The stripes have uniform thickness and inter-stripe distance over a square centimeter sized area. When the substrate is immersed into the C8-BTBT solution, a meniscus is formed at the three-phase (air-


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liquid-substrate) contact line (Figure 1 (c)). Toluene at the meniscus evaporates, which induces an upward convective flow toward the contact line to compensate for the lost toluene. Meanwhile, C8-BTBT molecules are carried by the convective flow to the contact line and deposited on the substrate. At the initial stage of film deposition, the continuous line of C8BTBT at the contact line segregates into periodically distributed domains due to ‘fingering instability’ (Figure 1 (b)).42-44 These domains work as nucleation sites and direct the crystallization of C8-BTBT into stripes parallel to the pulling direction. At lower pulling speeds, horizontal stripes as well as vertical stripes are formed on the substrate. The optical image of the C8-BTBT film deposited at 2 mm/min (Figure 3 (a)) clearly shows this phenomenon. The formation of horizontal stripes is attributed to a ‘stick-slip’ motion in the dip coating process.45-47 A schematic illustration of the ‘stick-slip’ motion is shown in Figure S1. At low pulling speeds, toluene evaporates fast and the contact line recedes slowly, which leads to excess C8-BTBT molecules that convect to and pin the contact line. When the substrate moves upward, the meniscus near the contact line is dragged by the solution and becomes stretched. Eventually, the meniscus is too heavy to be retained by the pinned contact line. It tears off and the contact line recedes to a new pinning position. The pinning and depinning processes is then repeated. The horizontal stripes are formed when the C8-BTBT molecules accumulate at the pinned contact line (‘stick’ state) and the vertical stripes are formed when the pinned contact line recedes (‘slip’ state). The spacing between two adjacent horizontal stripes is determined by the distance of the substrate travelling during a meniscus lifetime. The shorter travel distance at the lower pulling speed results in a smaller inter-stripe distance, as seen in Figure S2. As discussed above, the ‘stick-slip’ motion originates from evaporation induced over assembly of C8-BTBT molecules at the contact line. A solvent with a lower boiling point


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evaporates faster, which would intensify the ‘stick-slip’ motion and result in more horizontal stripes. This hypothesis is validated by optical images of the C8-BTBT films assembled with a 15 mg/mL solution using chloroform as the solvent (Figure S3). Because of the low boiling point (61.2 oC) of chloroform, the C8-BTBT films assembled using the chloroform solution show denser horizontal stripes than those assembled using the toluene solution at the same dip coating parameters.

Figure 2. Optical (top) and AFM (bottom) images of the C8-BTBT films assembled in the low pulling speed regime with the solution concentration of 15 mg/mL. Cross-sectional profiles of the films are shown as well with all the y axes ranging from -40 to 40. The pulling speeds are 0.5 mm/min for (a) and (b), 1 mm/min for (c) and (d), 2 mm/min for (e) and (f), 3 mm/min for (g) and (h), 4 mm/min for (i) and (j), and 5 mm/min for (k) and (l).


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Figure 3. (a) Optical image of the C8-BTBT film assembled at the pulling speed of 2 mm/min with the solution concentration of 15 mg/mL. Because of the ‘stick-slip’ motion, horizontal stripes are formed as well as vertical stripes. The horizontal stripes are formed during the ‘stick’ state due to excess C8-BTBT molecules accumulation at the pinned contact line, while the vertical stripes are formed in the ‘slip’ state. The topography of the C8-BTBT film at the ‘stick’ and ‘slip’ interface is shown in (b). The horizontal stripe exhibits a bump, as seem in the green cross-sectional profile. The thickness of the C8-BTBT films is measured from the AFM images. Figure 4 (a) shows the thickness of the films assembled at pulling speeds from 0.5 mm/min to 5 mm/min. With the increase of the pulling speed, the thickness of the C8-BTBT films decreases, which is consistent with the meniscus controlled film growth.45, 48 In the meniscus controlled film growth mode, the deposition of the films originates from evaporation induced convective flow. At lower pulling up speeds, the substrate stays at one position for a longer time, causing longer evaporation time and more accumulation of C8-BTBT molecules on the substrate and thus thicker films. Besides film thickness, thickness variation also increases with the decreasing pulling speed, which is visible from different colors in the optical images and steps in the AFM images. Nonuniform nucleation of C8-BTBT molecules at the contact line, resulting from inhomogeneous wet property of the


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substrate and nonuniform convective flow,40 may contribute to the variation of the film thickness. When the film thickness increases, the thickness variation becomes more significant. Another reason for the thickness variation will be discussed later.

Figure 4. The thickness of the C8-BTBT films as a function of the pulling speed (a) and (b) and the solution concentration (c). 3.2 C8-BTBT Films Assembled at High Rates Figure 5 shows optical and AFM images of the C8-BTBT films assembled at high rates (pulling speed > 10 mm/min) with a solution concentration of 15 mg/mL. The film assembled at 10 mm/min (Figure 5 (a) and (b)) shows uniform vertical stripes. Increasing the pulling speed to 30 mm/min, the C8-BTBT film still shows vertical stripe patterns (Figure 5 (c) and (d)). However, the distance between stripes (dark blue region) decreases. Further increasing the pulling speed to 50 mm/min or above, the stripe patterns vanish and the films tend to be continuous. The morphology variation of the C8-BTBT films results from changing of the domain distribution at the contact line. When the pulling speed is increased, the C8-BTBT molecules do not have enough time to migrate and form domains, resulting in wide stripes and eventually continuous two-dimensional (2D) films.


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Figure 5. Optical (top) and AFM (bottom) images of the C8-BTBT films assembled in the high pulling speed regime with the solution concentration of 15 mg/mL. Cross-sectional profiles of the films are shown as well with all the y axes ranging from -20 to 20. The pulling speeds are 10 mm/min for (a) and (b), 30 mm/min for (c) and (d), 50 mm/min for (e) and (f), 70 mm/min for (g) and (h), and 90 mm/min for (i) and (j). Figure 4 (b) shows the thickness of the C8-BTBT films assembled at pulling speeds from 10 mm/min to 90 mm/min. With the increase of the pulling speed, the film thickness increases, which shows the same behavior as entrainment controlled film growth. In the entrainment controlled film growth mode, a uniform liquid film is entrained on the substrate above the meniscus (Figure 1 (d)). The thickness of the entrained film can be calculated by Landau–Levich equation49, 50 = 0.944

⁄ ⁄ ⁄

(2)

where µ, σ and ρ are the viscosity, surface tension and density of the solution, respectively, V is the pulling speed, and g is the acceleration due to gravity. Equation 2 shows that the entrained film thickness t is proportional to the pulling speed V at the power of 2/3. That explains why the thickness of the C8-BTBT films increases with the pulling speed.


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Similar to the film thickness, the thickness variation of the C8-BTBT films also increases with the pulling speed. As mentioned above, when increasing the pulling speed, the time for domain formation at the contact line is shortened. C8-BTBT molecules do not have adequate time to migrate and form domains with uniform thickness. Instead, terrace shaped domains are formed, resulting in films with steps and terraces. The height of the terraces is around 3 nm (see Figure S4), which corresponds to the monolayer thickness of C8-BTBT.21 Because of the terraces, the C8-BTBT films grown at higher pulling speed show larger thickness variation. High speed induced nonuniform nucleation also explains the large thickness variation in the low pulling speed regime. Based on the discussion above, low pulling speeds should provide adequate time for uniform nucleation at the contact line. However, even if the total time is long, most of the time elapses in the ‘stick’ state and the ‘slip’ state happens promptly.51 High slipping speed leads to high thickness nonuniformity for the vertical stripes in the low speed regime. 3.3 C8-BTBT Films Assembled at Different Solution Concentrations Besides the pulling speed, solution concentration is another key parameter in the assembly process. Usually, higher concentration gives more solute around the contact line and thus more deposition. Figure 6 shows optical and AFM images of the C8-BTBT films assembled at the pulling speed of 10 mm/min with solution concentrations from 1 mg/mL to 10 mg/mL. At 1 mg/mL, only few discontinuous dots appear on the substrate, as seen in Figure 6 (a). AFM image (Figure 6 (b)) shows some branch patterns as well as dots. The branch patterns are also not continuous. Increasing the solution concentration to 5 mg/mL, we start to see continuous stripe patterns. Further increasing the concentration to 10 mg/mL, uniform vertical stripes are obtained. Figure 4 (c) shows the thickness of the C8-BTBT films as a function of the solution concentration. As expected, the film thickness increases with the solution concentration.


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Figure 6. Optical (top) and AFM (bottom) images of the C8-BTBT films assembled at pulling speed of 10 mm/min with various solution concentrations. Cross-sectional profiles of the films are shown as well with all the y axes ranging from -10 to 10. The solution concentrations are 1 mg/mL for (a) and (b), 5 mg/mL for (c) and (d), and 10 mg/mL for (e) and (f). 3.4 Crystal Structure of the Assembled C8-BTBT Films The crystal structure of the C8-BTBT films is measured using XRD. Figure 7 shows the outof-plane XRD pattern of the C8-BTBT film assembled at the pulling speed of 10 mm/min with the 15 mg/mL solution. C8-BTBT films assembled at other parameters show similar XRD patterns (Figure S5). A series of peaks located at n × 2.93° (n = 1, 2 and 3) appear in the pattern,


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which corresponds to (00l) planes of the C8-BTBT crystal.20, 24 The strong and sole (00l) texture indicates that the C8-BTBT films are highly crystallized.

Figure 7. Out-of-plane XRD pattern of the C8-BTBT film assembled at the pulling speed of 10 mm/min with the 15 mg/mL solution. 3.5 OFETs characterization The OFETs are fabricated and characterized in ambient air, and all the devices show typical pchannel behavior. Figure 8 (a) and (b) show transfer and output characteristics of the OFET prepared from the C8-BTBT film assembled at a pulling speed of 0.5 mm/min with a 15 mg/mL concentration. The device exhibits an on/off ratio of over 104 and a threshold voltage of approximately 0.086 V. A hole mobility of 2.54 cm2V−1s−1 in the saturation regime is calculated. The calculated mobility value is comparable to those of the transistors with C8-BTBT films fabricated using vacuum vapor deposition,52 spin coating,27, 28, 30 and drop casting

34

techniques.

The high mobility can be attributed to the high crystallinity of the C8-BTBT film. 30 devices are measured in order to inspect the variation of the mobility. The distribution of the mobility is shown in Figure 8 (c). An average mobility of 1.65 cm2V−1s−1 is obtained, with a highest mobility of 3.99 cm2V−1s−1, a lowest mobility of 0.43 cm2V−1s−1, and a standard deviation of 0.92


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cm2V−1s−1. The high variation of the mobility is due to the high variation of the film thickness in the low pulling speed regime. The device shows a good saturation behavior in the high drain voltage regime, as seen in the output curve. In the low drain voltage regime, the drain current does not increase linearly with the drain voltage, indicating a prominent contact resistance between the C8-BTBT film and the gold source and drain electrodes. The work function of polycrystalline gold film deposited in vacuum is 4.6 eV,53 while the highest occupied molecular orbital (HOMO) of the p-type C8BTBT is 5.39 eV.54 The energy level mismatch results in an energy barrier for hole injection from gold to C8-BTBT and in turn contributes to the high contact resistance. A hole injection layer (HIL) with a large work function, such as iron(III) chloride (FeCl3)

22, 55

, molybdenum

trioxide (MoO3)21 and tetracyanoquinodimethane (F4-TCNQ),56 could be utilized to decrease the contact resistance.

Figure 8. Transfer (a) and output (b) characteristics of the OFET prepared from the C8-BTBT film assembled at the pulling speed of 0.5 mm/min with the 15 mg/mL solution. (c) Distribution


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of the hole mobility for 30 devices. (d), (e) and (f) show the mobility of the C8-BTBT films as a function of the pulling speed and the solution concentration. The transfer curves of the OFETs fabricated from C8-BTBT films assembled at other dip coating parameters are shown in Figure S6. Figure 8 (d), (e) and (f) show the mobility of the C8BTBT films as a function of the pulling speed and the solution concentration. The dependence of the mobility follows the same trend as the dependence of the film thickness on the pulling speed and the solution concentration. Larger film thickness gives higher mobility and vice versa. The reason lies in that thicker films give better contact to the electrodes. The OFETs in our investigation employ a bottom-gate bottom-contact configuration. During the film assembly process, C8-BTBT films need to coat and bridge the existing source and drain electrodes. When the thickness of the C8-BTBT films is much smaller than that of the electrodes (30 nm thick), the films could break at the electrode and film interface. Figure S7 (a) and (b) show the OFET prepared from the 5 nm thick C8-BTBT film grown at the 10 mm/min pulling speed with the 5 mg/mL solution. Only few stripes (enclosed in the green dashed box in the AFM image) make extremely thin connections with the electrodes, and most of the other stripes (enclosed in the red dashed box in the AFM image) do not connect the electrodes. The poor contact quality results in very low average mobility of 4.5 × 10-4 cm2V−1s−1. When the thickness of the C8-BTBT film (10 mm/min pulling speed using 15 mg/mL concentration) increases to 16 nm, the contact quality is greatly improved. Even though thin connection still exists, no disconnected stripes can be seen. At the same time, the average mobility increases 100 times and reaches 0.088 cm2V−1s−1. It should be mentioned that only ~1/4 of the channel region has C8-BTBT stripes, which reduces the effective channel width to 1/4 of its nominal value. If the effective channel width is taken into account, the actual mobility would be four times larger. When the thickness of the C8-BTBT


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film (3 mm/min pulling speed with the 15 mg/mL concentration) equals to the height of the electrodes, wide stripes are formed between the electrodes, which alleviates the contact problem and leads to the increase of the average mobility to 0.45 cm2V−1s−1. Further increase in the film thickness to 57 nm (0.5 mm/min pulling speed with the 15 mg/mL concentration), the electrodes are covered by the C8-BTBT film and the average mobility increases to 1.65 cm2V−1s−1. At low pulling speeds, ‘stick’ and ‘slip’ states exist and film thickness uniformity is poor, which results in large variation of the mobility. Based on the above discussion, we can conclude that the mobility of the OFETs depends on the contact at the film-electrode interface rather than the film thickness. By employing a top-contact configuration, that is to say, assembling C8-BTBT films on the gate dielectric film first and then depositing source and drain electrodes, the C8-BTBT films would have a good contact with the electrodes. In this case, the mobility of the OFETs may not change with the film thickness because only few molecular layers of the semiconductor next to the dielectric interface contribute to the charge transport.57-59 4. CONCLUSION In this paper, we present a directed assembly of self-aligned and highly crystalline C8-BTBT films over a square centimeter sized area using a dip coating process. Two governing parameters, the pulling speed and the solution concentration, are used to control the film morphology and thickness. Uniform stripe patterns parallel to the pulling direction are obtained in a pulling speed range from 5 mm/min to 10 mm/min. At lower or higher pulling speeds, nonuniform nucleation at the solution-substrate contact line takes place, resulting in a nonuniform film thickness and morphology. It is worth noting that stripe patterns perpendicular to the pulling direction as well as parallel to the pulling direction are formed at low pulling speeds due to the ‘stick-slip’ motion. Solution concentration determines the amount of solute feeding to the contact line. Continuous


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stripe patterns are formed only at a high enough solution concentration (> 5 mg/mL). Bottomgate bottom-contact transistors are fabricated using the C8-BTBT films grown at different assembly parameters. The transistors show good field-effect property, and a hole mobility up to 3.99 cm2V−1s−1 is obtained. The mobility changes with the channel film thickness. Thicker films give larger mobilities because of better contact at the film and electrode interface. The presented directed assembly process demonstrates a great potential for fabricating next generation electronics with its merits of low material and energy consumption, low cost, and scalability. Meanwhile, the film formation mechanism discussed in this paper could provide a better understanding to help depositing other organic semiconducting films from small molecule solutions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. A schematic illustration of the ‘stick-slip’ motion, optical images of the C8-BTBT films with both vertical and horizontal stripes, optical images of the C8-BTBT films assembled using chloroform as the solvent, AFM image showing monolayer C8-BTBT film, XRD patterns of the C8-BTBT films assembled at various parameters, transfer curves of the C8-BTBT films assembled at various parameters, optical and AFM images showing the interface between the gold electrodes and the C8-BTBT films (PDF) AUTHOR INFORMATION Corresponding Author


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*

E-mail addresses: [email protected]

ACKNOWLEDGEMENT This work was mostly conducted at the George J. Kostas Research Center at Northeastern University. The silicon dioxide/silicon nitride gate dielectric bilayer was deposited by plasmaenhanced chemical vapor deposition at the Harvard University Center for Nanoscale Systems (CNS). The work is funded by the Massachusetts Technology Collaborative and The Advanced Nanomanufacturing Cluster for Smart Sensors and Materials (CSSM). REFERENCES (1) Braga, D.; Erickson, N. C.; Renn, M. J.; Holmes, R. J.;

Frisbie, C. D. High-

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