Macroscopic Alignment of One-Dimensional Conjugated Polymer

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Macroscopic Alignment of One-Dimensional Conjugated Polymer Nanocrystallites for High-Mobility Organic Field-Effect Transistors Mincheol Chang,† Dalsu Choi,‡ and Eilaf Egap*,†,§,⊥ †

Department of Chemistry and §The Wallace H. Coulter Department of Biomedical Engineering, Emory University, Atlanta, Georgia 30322, United States ‡ School of Chemical and Biomolecular Engineering ⊥The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Controlling the morphology of polymer semiconductors remains a fundamental challenge that hinders their widespread applications in electronic and optoelectronic devices and commercial feasibility. Although conjugated polymer nanowires (NWs) are envisioned to afford high charge-carrier mobility, the alignment of preformed conjugated polymer NWs has not been reported. Here, we demonstrate an extremely simple and effective strategy to generate well-aligned arrays of one-dimensional (1D) polymer semiconductors that exhibit remarkable enhancement in charge transport using a solution shear-coating technique. We show that solution shear coating of poly(alkylthiophene) NWs induces extension or coplanarization of the polymer backbone and highly aligned network films, which results in enhanced intra- and intermolecular ordering and reduced grain boundaries. Consequently, highly aligned poly(3-hexylthiophene) NWs exhibited over 33-fold enhancement in the average carrier mobility, with the highest mobility of 0.32 cm2 V−1 s−1 compared to pristine films. The presented platform is a promising strategy and general approach for achieving well-aligned 1D nanostructures of polymer semiconductors and could enable the next generation of high-performance flexible electronic devices for a wide range of applications. KEYWORDS: poly(alkylthiophene), molecular ordering, nanowires, solution shearing, charge-carrier mobility, organic-field-effect transistors enhance the intermolecular interactions and maximize π-orbital overlaps. While the molecular engineering approach has successfully resulted in novel and high-carrier-mobility conjugated polymers, the crystallinity, molecular packing, and anisotropy of conjugated polymer chains are largely dependent on the processing and fabrication conditions.7−9,12,17 Processing techniques such as thermal annealing,18 filmdeposition methods (e.g., spin coating, dip coating, and drop casting),19 and solution treatments (e.g., tuning solubility, solution aging, sonication, and UV irradiation)7,20−24 have proven to be effective strategies in enhancing and controlling the nanoscale morphology and intra- and intermolecular interactions of polymer semiconductors and improving charge transport. While such approaches are promising, they are limited in their ability to align crystalline domains and polymer chains between domains to maximize charge-transport characteristics.11−13,25 Because charge transport is governed by crystal packing, grain boundary, and intercrystalline morphology,8,9,11−13,26 the orientation and anisotropy of

1. INTRODUCTION Solution-processable polymer semiconductors are of significant fundamental and technological interest for portable and flexible electronic and optoelectronic devices including flat-panel displays,1 chemical and biological sensors,2 and integrated circuits.3 They are pursued as active materials in flexible and printed electronics because of their low cost and large-area processability.4−9 However, charge transport in polymer semiconductors has generally been limited by the low degree of intra- and intermolecular ordering and poor polymer interchain alignment and connectivity in thin films.10−13 Solution processing of polymer semiconductors often results in semicrystalline polymer films that may contain crystalline domains often randomly embedded within a largely disordered matrix. This type of morphology typically leads to grain boundaries, trap sites, and inefficient charge hopping between domains.10−13 Significant effort has been focused on the design of new materials in attempts to control the crystallinity, namely, intraand intermolecular order, and to induce anisotropic alignment of polymer chains. Various strategies have been utilized including the incorporation of fused rigid rings14 and donor− acceptor monomers15 and engineering of side chains16 to © XXXX American Chemical Society

Received: February 22, 2016 Accepted: May 10, 2016

A

DOI: 10.1021/acsami.6b02216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of conjugated polymer NW thin films deposited on a FET device substrate via a solution shear-coating technique.

of 1D NW crystals is generally mediated by strong noncovalent interactions such as π−π interactions along the long axes of the fiber, which coincides with the optimum direction for charge transport.20−24 The NWs often exhibit high aspect ratios and form in solution with up to a few micrometers in length. Even though conjugated polymer NWs are envisioned to facilitate high charge transport, there are no reports that demonstrate the alignment of preformed NWs. Herein, we demonstrate that the use of a polymer solution containing NWs for solution shear coating can be an extremely simple strategy to form well-aligned arrays of polymer NWs, which results in a remarkable enhancement in the charge-carrier mobility by a factor of 33-fold. The molecular ordering, morphologies, and charge-transport properties of the aligned 1D NWs were profoundly influenced by the solution shearcoating speed. The correlation of molecular ordering, morphologies, and electrical properties of aligned and randomly oriented NW films was systematically studied by means of static absorption spectroscopy, X-ray diffraction (XRD), atomic force microscopy (AFM), polarized optical microscopy (POM), and charge-carrier mobility measurements as a function of the solution shear-coating speed.

polymer crystals and chains inevitably play a crucial role in determining the efficiency of the carrier mobility. Solution shear-coating techniques such as zone casting,27 doctor blading,28 strain stretching,29 and template-guided8,9,12 have been demonstrated to be effective strategies for the alignment of crystalline domains and polymer chains. Typically, the alignment of polymers or small molecules is achieved through the use of template-guided alignment or confined fluid flows on preengraved substrates.8,9,11,12,29,30 While solution shear coating of small molecules has resulted in well-controlled and aligned single-crystalline morphology,30 it still leads to locally ordered crystalline domains in polymer semiconductors, presumably, in part, due to the large entropy in polymers.8,9,12,29 Hence, moderate enhancement in the chargecarrier mobility has been observed from polymer thin films prepared by solution shear coating compared to thin films obtained via conventional spin-coating methods.8,9,11,12,29 For example, a common solution shear-coating approach resulted in a 5-fold enhancement in the carrier mobility of poly[bis(thiophene-2-yl) tetrathienoacene] (P2TDC17FT4) thin films.29 Recent strategies that rely on capillary action and nanogrooved substrates resulted in the unidirectional alignment of polymer chains and enhanced charge-carrier mobilities by factors of up to 10.7-fold in poly(cyclopentadithiophen-2-yl-altthiadiazolopyridine) (PCDTPT) and poly(cyclopentadithiophene-alt-benzothiadiazole) (CDTBTZ) derivatives.9 A 6-fold enhancement in the mobility was observed from poly[2,5-bis(3-hexadecylthiophene-2-yl)thieno(3,2-b)thiophene] (PB16TTT) films prepared by applying mechanical compression to the polymer solution on the substrate of an ionic liquid.11 Micropatterned polymer prisms composed of diketopyrrolopyrrole (DPP), thiophene, and thienothiophene units, which is formed by the guidance of a patterned poly(dimethylsiloxane) template under shear force, showed improved carrier mobility by 2.6-fold.12 Despite these great efforts to align domains in polymer semiconductor thin films, charge transport is still limited by grain boundaries. Therefore, simple and scalable platforms that provide controlled morphology, solid-state molecular packing, and alignment over a large area are essential. The incorporation of one-dimensional (1D) nanowires (NWs) or nanofibers presents a successful and rational strategy to control the solid-state morphology, molecular packing, and crystallinity of polymer semiconductor films.7,20−24 The growth

2. RESULTS AND DISCUSSION To understand the effect of solution shear coating on the molecular ordering, morphology, and charge transport, we used poly(3-hexylthiophene) (P3HT) and poly(3-butylthiophene) (P3BT) as model polymers. Highly crystalline NWs or nanofibers of poly(alkylthiophene)s can readily be prepared by a variety of solution treatment methods including solvent exchange,7,21 solution aging,20 sonication,22 and UV irradiation.24 We used two different methods, namely, UV irradiation and solution aging, to prepare P3HT and P3BT NWs/ nanofibers to demonstrate the generality of this platform. The macroscopic alignment of polymer semiconductor NWs is schematically represented in Figure 1. The suspension of polymer NWs is initially drop-cast onto a substrate in which no source and drain electrodes are patterned and covered with a glass plate. Subsequently, the polymer NW dispersion is continuously and mechanically sheared by the glass plate across the substrate at a constant velocity and deposited onto an electrode-patterned device substrate, generating a macroscopically aligned 1D NWs along the direction of shear (Figure 1). The impact of the film deposition method on the morphology B


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Figure 2. AFM images of P3HT thin films from a pristine solution: (A) spin-coated and (B) shear-coated at 2.0 mm/s. P3HT NW films: (C) spincoated and shear-coated at (D) 0.5, (E) 1.0, (F) 2.0, and (G) 4.0 mm/s. The arrows in the figures indicate the shearing direction.

Figure 3. (A) Normalized UV−vis absorption spectra of P3HT thin films. (B) Absorption spectrum of P3HT NW thin films sheared at 1.0 mm/s. The lines with filled red and open blue squares indicate the Gaussians corresponding to the (0−0) and (0−1) bands, respectively. The lines with black open circles depict the experimental absorption spectra. (C) Calculated exciton bandwidth (W) of P3HT of pristine and NW thin films spincoated and sheared at 0.5, 1.0, 2.0, and 4.0 mm/s.

speed was increased from 0.5 to 2.0 mm/s (Figures 2D−F and S1A−C), likely due to an increase in the shear force along the shearing direction. It is also likely that the mechanical force of shearing leads to stretching and coplanarization of the polymer chains along the shearing direction.11,12 However, a further increase in the shearing speed of up to 4.0 mm/s results in a random morphology with no alignment relative to the shearing direction (Figures 2G and S1D). This lack of preferential orientation or isotropic morphology as a result of an increase in the shearing speed has been attributed to generation of a temperature gradient during the film formation.31 The solution-sheared films were examined using a crosspolarized optical microscope (Figure S2). When both spincoated and shear-coated pristine P3HT films were rotated with respect to the crossed polarization axes, the film textures appear dark in all directions, thus suggesting an isotropic morphology (Figure S2A,B).24,32 This isotropic morphology or lack of anisotropy is consistent with the AFM images (Figure 2A,B). In contrast, both randomly oriented and shear-aligned P3HT

was investigated by AFM (Figure 2). Parts A and B of Figure 2 show spin-coated and shear-coated pristine P3HT films, respectively. AFM images of pristine films obtained via a spin-coating method show typical phase separation with nanostructured domains that presumably formed during the spin-coating process and are randomly embedded in a largely disordered amorphous region. Upon shearing of the pristine P3HT solution, there is some alignment of the nanodomains.8,9,12,29 Thin films deposited by spin coating of a P3HT NW suspension clearly lead to randomly oriented P3HT NWs, as shown in Figure 2C. On the other hand, P3HT NWs deposited onto substrates using a solution shear-coating method give rise to well-aligned NWs (Figures 2D−G and S1). The orientation and morphology of the NWs were significantly influenced by variation of the shearing speed. An improvement and preferential alignment of P3HT NWs along the shearing direction were observed as the shearing speed was increased from 0.5 to 2.0 mm/s (Figure 2D−F). In addition, the NWs gradually became linearly stretched and sparse as the C


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(∼0.18 eV).33,36 Shear-aligned P3HT films show relatively lower W values compared to spin-coated or randomly oriented P3HT films (Figure 3C). For example, the W values of pristine P3HT thin films and randomly oriented P3HT NWs appear at 127.9 and 115.9 meV, respectively, whereas the W values of shear-coated pristine P3HT films and shear-aligned P3HT NWs appear at 115.8 and 83.3 meV, respectively. These results suggest that solution shear coating would effectively extend or coplanarize the individual polymer backbone and therefore facilitate enhanced intramolecular ordering. The intramolecular ordering was also significantly influenced by the shearing speed. The W value decreased from 77.1 to 68.5 meV with an increase in the shearing speed from 0.5 to 1.0 mm/s. However, the W value began to gradually increase to 110.3 meV as the shearing speed was varied from 1.0 to 4.0 mm/s. A gradual increase of the shearing speed to ∼1.0 mm/s facilitates an efficient coplanarization of the single polymer chain; however, a further increase in the shearing speed leads to less planarization of the polymer backbone and an isotropic morphology. Grazing-incidence XRD (GIXRD) studies were performed to provide further insight into the molecular packing of conjugated polymer NWs and pristine thin films. The effect of solution shear coating on the P3HT microstructure was systematically investigated. The XRD spectra of pristine P3HT thin films, randomly oriented P3HT NWs, and shear-aligned P3HT NWs are shown in Figure 4A. The XRD patterns of all thin films showed strong diffraction peaks in the range of 2θ = 5.14−5.43° corresponding to a lamellar ordering with d

NWs exhibit bright textures under crossed polarizers, suggesting that the films are crystalline and anisotropic (Figure S2C−G).24,32 However, the birefringent textures of shearaligned P3HT NWs display higher brightness compared to the randomly oriented P3HT NWs, which is primarily due to enhancement in the anisotropy. The birefringent brightness of shear-aligned P3HT NWs increases as a function of the shearing speed in the following order: 4.0 mm/s < 0.5 mm/s < 2.0 mm/s < 1.0 mm/s. This trend is consistent with the results observed in the AFM images (Figure 2D−G). UV−vis spectroscopy was used to elucidate the effect of shearing on the inter- and intramolecular ordering of the polymer semiconductor thin films.33 Figure 3A presents the spectra of pristine P3HT films and P3HT NWs deposited by either spin coating or solution shear coating. The higher-energy features (π−π* intraband transition) arising from disordered single polymer chains appear at ∼520 nm, while the lowerenergy vibronic bands associated with interchain coupling and ordered aggregates appear at ∼555 and 605 nm, which correspond to the (0−1) and (0−0) transitions, respectively.33,34 In general, the intensity of the lower-energy bands increases relative to that of the higher-energy bands as the crystallinity of the P3HT films increases.33,34 Shear-aligned P3HT NWs exhibit significant enhancement in the lowerenergy vibronic structure band at 605 nm compared to randomly oriented P3HT NWs, indicative of improved intermolecular interactions and long-range order. Interestingly, shear-coated pristine P3HT films show enhanced lower-energy bands compared to randomly oriented P3HT NWs. In general, a decrease in the shearing speed resulted in an enhancement in the lower-energy bands at 555 and 605 nm relative to the higher-energy band at 520 nm. This suggests that a relatively low shearing speed can facilitate strong intermolecular interactions of polymer chains. The slow shearing speed presumably allows for the slow evaporation rate of the solvent and provides polymer chains and NWs to reach an equilibrium rate.7,21,32,35 Quantitative modeling of the absorption spectra was employed to probe the intramolecular ordering and effective conjugation length of the polymer chains.22−24,36 Two phases are observed in the P3HT absorption spectrum that correlate with a crystalline phase and an amorphous phase. The lowerenergy bands are associated with the crystalline phase (ordered chains), while the higher-energy bands are correlated with the amorphous phase (disordered chains).33,35 The Spano model assumes that the crystalline phase consists of weakly interacting H-aggregates that exhibit distinct vibronic bands.33,36 The conjugation length of an individual chain within the aggregates and the intramolecular ordering can be associated with these vibronic bands, which correlates with the exciton bandwidth (W).33,36 An increase in W represents a decrease in the intramolecular ordering.33,36 The W values are calculated according to eq 1, using ratios of the band intensities between the (0−0) and (0−1) transitions. Gaussian peak fittings to the experimental spectra are performed to obtain the intensities of the (0−0) and (0−1) transitions (Figures 3B and S3).33,36 2 I0 − 0 ⎛ 1 − 0.24W /Ep ⎞ ⎜ ⎟ ≈ I0 − 1 ⎜⎝ 1 + 0.073W /Ep ⎟⎠

Figure 4. (A) GIXRD profiles of pristine P3HT and P3HT NW films spin-coated and sheared at 0.5, 1.0, 2.0, and 4.0 mm/s. (B) Plots of the corresponding (100) layer spacing (left axis) and crystal grain size (right axis) along the [100] direction. P1 = pristine spin-coated, P2 = pristine sheared at 2.0 mm/s, U1 = P3HT NWs spin-coated, U2 = P3HT NWs sheared at 0.5 mm/s, U3 = P3HT NWs sheared at 1.0 mm/s, U4 = P3HT NWs sheared at 2.0 mm/s, and U5 = P3HT NWs sheared at 4.0 mm/s.

(1)

I0−0 and I0−1 are the intensities of the (0−0) and (0−1) transitions, respectively. Ep corresponds to the vibrational energy associated with the symmetric vinyl stretching mode D


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Figure 5. (A) Schematic representation of organic FET devices. (B) Typical transfer curves of spin-coated P3HT pristine, P3HT NW spin-coated, and P3HT NWs sheared at 2.0 mm/s, respectively. (C) Typical output curve of P3HT NW films shear-coated at 2.0 mm/s. (D) Mobility comparison of P3HT pristine films, P3HT NWs spin-coated and shear-coated at 2.0 mm/s, respectively. (E) Average and maximum FET carrier mobilities of P3HT NW films shear-coated as a function of the shearing speed. (F) Distribution of the carrier-mobility-aligned P3HT NWs at 2.0 mm/s.

diffraction peaks, the crystal grain size corresponding to the interlayer packing along the [100] direction is increased by the solution shear coating compared to the conventional spin coating. In particular, a shearing speed of 0.5 mm/s provided the largest coherence length (∼9.1 nm). The large grain size observed at lower shearing speed may be attributed to expansion in the lamellar spacing and the relatively slow evaporation rate of the solvent. It is well established that the slow evaporation rate of the solvent leads to large polymer crystallites because of the slow nucleation process.7,21 The charge-transport properties of pristine P3HT thin films and P3HT NW films deposited by spin coating or solution shear coating were investigated by field-effect transistors (FETs) using a bottom-contact/bottom-gate geometry; gold source/drain electrodes were patterned on top of silicon (gate) and silicon dioxide (dielectric) substrates (transistor channel width = 2000 μm and length = 50 μm; Figure 5A). Typical output and transfer characteristics of pristine P3HT and P3HT NW films show clear current modulation and saturation, as shown in Figure 5B,C. The transfer curves show high turn-on voltage (VON), presumably due to residual doping and traps at the polymer/oxide and/or at the grain boundary interfaces.21,23−25 Figure 5D summarizes the charge-transport properties of the following four different processing conditions of P3HT thin films: (i) pristine P3HT thin film deposited by spin coating; (ii) pristine P3HT thin films sheared; (iii) randomly oriented P3HT NWs; (iv) shear-aligned P3HT NWs at a speed of 2.0 mm/s. Solution shear coating of pristine P3HT thin films resulted in an improvement in the average charge-carrier mobility (μavg = 0.1 cm2 V−1 s−1) compared to spin-coated P3HT thin films (μavg = 0.006 cm2 V−1 s−1). This improvement could be attributed to the planarization and alignment of polymer chains and NWs by the shear force. On the other

spacings of 16.3−17.7 Å that can be assigned to the (100) reflection.7,21,37 In general, a decrease in the (100) reflection peak was observed as a result of solution shear coating compared to the spin-coating method, indicative of an increase in the d spacing of the lamellar packing by shearing. Figure 4B shows the change in the lamellar spacing as a function of the shearing speed. For example, spin-coated pristine P3HT thin films showed a diffraction peak at 2θ = 5.35° corresponding to a d spacing of 16.5 Å compared to the 2θ = 5.31° peak correlated to a d spacing of 16.6 Å for the pristine film sheared at 2.0 mm/s. P3HT NWs sheared at a speed of 1.0 mm/s exhibited an increase of up to 1 Å in the lamellar d spacing compared to spin-coated or sheared NWs at a speed of 0.5 mm/s. At a faster speed of 4.0 mm/s, the (100) reflection increased to 2θ = 5.14°, suggesting that the lamellar spacing had decreased compared to a lower shearing speed of 0.5 mm/ s. Although an increase in the lamellar d spacing may suggest some disorder along the (100) plane, the UV−vis spectrum suggests that there is an enhancement in the molecular ordering, presumbly along the π−π-stacking direction, that coincides with the (010) plane.21,37,38 The full width at half-maximum (fwhm) of the (100) peak correlates with the average coherence length (grain size) corresponding to P3HT lamellar packing, according to Scherrer’s equation (eq 2).32,38 The grain size was calculated using Scherrer’s equation in Figure 4B: Dhkl =

0.9λ βhkl cos θ

(2)

where Dhkl is the apparent crystal size along the (hkl) direction, λ is the X-ray wavelength, and βhkl is the fwhm of a (hkl) diffraction (in radians). On the basis of Scherrer analysis of the E


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Figure 6. AFM images of P3BT NW films (A) spin-coated and (B) shear-coated at 1.5 mm/s. (C and D) POM images, (E) UV−vis absorption spectra, and (F) a plot of the corresponding measured FET carrier mobilities.

2.0 mm/s seem to show higher degree of intra- and intermolecular ordering and alignment as evidence from the UV−vis, POM, and AFM data, compared to aligned P3HT NWs sheared at 0.5 and 4.0 mm/s. The highest charge carrier mobility measured is from aligned P3HT NWs films sheared at 2.0 mm/s. These results would implicate that charge carrier mobility is governed by complicate interplay between molecular ordering and morphology of conjugated polymer thin films. The dependence of the carrier mobility in which the NWs were perpendicular to the channel length was examined to further probe the impact of anisotropic alignment of P3HT NWs on the charge transport (Figure S5). The devices with source-drain oriented perpendicular to P3HT NWs exhibited a mobility of 0.1 cm2 V−1 s−1, which is relatively lower compared to that of devices with the source-drain oriented parallel to P3HT NWs (μ = 0.2 cm2 V−1 s−1). This clearly demonstrates that polymer chain alignment is an important factor for achieving high-performance electronic devices. Our strategy to controlling the morphology of polymer semiconductors should broadly be applicable to various conjugated polymers. We applied this platform to an alternate polymer, P3BT NWs, to investigate the generality of this approach. The P3BT NWs were prepared from chloroform using a solvent aging method.25 P3BT NWs deposited onto substrates using solution shear coating provided well-oriented polymer thin films compared to a conventional spin-coating method, as is clearly seen from the AFM and POM images in Figure 6A−D. UV−vis spectra of aligned P3BT NW thin films show enhanced lower-energy features at 550 and 605 nm and a red shift of the higher-energy feature (π−π* intraband transition)

hand, a 33-fold improvement in the average carrier mobilities was measured in shear-aligned P3HT NWs (μavg = 0.20 cm2 V−1 s−1) compared to randomly oriented P3HT NWs. The highest calculated carrier mobility of 0.32 cm2 V−1 s−1 was measured in shear-aligned P3HT NWs at a speed of 2.0 mm/s, which corresponds to a 53-fold enhancemeent in the average mobility compared to the mobility in randomly oriented films deposited by spin coating (Figures 5E,F). We attribute the significant enhancement in the charge-carrier mobility to improved molecular ordering, alignment of the polymer NWs and polymer chains, and reduced grain boundaries. Both aligned single polymer chains and 1D crystallites NWs act as an effective conduit for charge transport in the macroscale due to strong π−π interactions and reduced grain boundaries along the direction of the fiber.13,18,23,24 Furthermore, the shearaligned P3HT NWs are oriented parallel to the source-drain electrodes, which provide an efficient pathway for charge transport due to bridging chains compared to randomly oriented P3HT NWs, where the grain boundaries are randomly oriented.13,28,32 We evaulated the impact of the shearing speed on the electrical properties of aligned P3HT NWs at different shearing speeds of 0.5, 1.0, 2.0, and 4.0 mm/s (Figures 5E and S4). The average mobility along the shearing direction increased up to 0.20 cm2 V−1 s−1 for films obtained from P3HT NWs dispersion as the shearing speed increased from 0.5 to 2.0 mm/s. An increase in the shearing speed up to 4.0 mm/s resulted in an overall decrease in the average carrier mobility (μavg = 0.1 cm2 V−1 s−1). The same trend was observed in the carrier mobility of P3HT pristine films as a function of shearing speed (Figure S4). Aligned P3HT NWs films sheared at 1.0 and F


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ACS Applied Materials & Interfaces to a longer wavelength (∼518 nm) compared to that of randomly oriented P3BT NW films (∼504 nm) deposited by a conventional spin-coating method (Figure 6E). These results indicate that the shear-coated P3BT NW thin films show enhanced intra- and intermolecular ordering and suggest that chains are also more extended and coplanarized as a result of shearing.23,24,33 Consequently, the carrier mobility of the shearcoated P3BT NW films was measured to be about 8-fold higher compared to that of spin-coated P3BT NW films (1.6 × 10−2 cm2 V−1 s−1 vs 2.1 × 10−3 cm2 V−1 s−1; Figure 6F).

IDS =

WCOX μ(VGS − VT)VD L

(3)

where W, L, and VT are the transistor channel width, length, and threshold voltage, respectively. COX represents the capacitance per unit area of the SiO2 gate dielectric (∼1.15 × 10−8 F/cm2). Spectroscopy and Morphology Characterizations. An Agilent 8510 UV−vis spectrometer was used to record the solid-state UV−vis spectra of thin films. The GIXRD measurements were performed using a Panalytical X’Pert Pro system with a copper X-ray source operated at 45 kV and 40 mA. The scanning was conducted from 3° to 20°, while the grazing incidence was fixed at an angle of 0.2°. The ICON dimension scanning probe microscope (Bruker) was operated in tapping mode with a silicon tip (RTESP, Bruker) for the AFM measurements.

3. CONCLUSIONS In conclusion, we presented an extremely simple and effective strategy to generate well-aligned arrays of conjugated polymer NWs using a solution shear-coating strategy that results in tremendous enhancement in charge transport and molecular packing. We found that the shearing speed profoundly influences the intra- and intermolecular ordering as well as the morphology of shear-coated films. Highly aligned P3HT NWs exhibited over 33-fold enhancement in the average carrier mobility compared to randomly oriented P3HT NW films or pristine films. This significant improvement in the charge transport is attributed to the alignment of 1D domains, coplanarization of the polymer backbone, and reduced grain boundaries as a result of solution shear coating. We demonstrated that this approach is applicable to other conjugated polymer NWs including P3BT. The presented design is a general strategy to fabricate highly aligned conjugated polymer NW films to enable high-performance, large-area, flexible electronic devices for a wide range of commercial applications.

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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.6b02216. AFM images of shear-coated P3HT films obtained from a P3HT NW solution under different shearing speeds, POM images of spin- and shear-coated P3HT films, peak fittings to UV−vis absorption spectra of P3HT films subjected to Spano’s model, charge-carrier mobility of the films shear-coated from a pristine solution as a function of the shearing speed, and an AFM image and transfer curve of the P3HT NW film obtained by a solution shear coating that was conducted vertically to the channel length (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

4. MATERIALS AND METHODS

Notes

The authors declare no competing financial interest.

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Materials. Commercially available materials used in this study were used as is and without any further purification. Regioregular P3HT [regioregularity (RR) ≈ 96%, Mw ≈ 90 kDa, and polydispersity (PDI) ≈ 2.3] and P3BT [RR ≈ 87%, Mw ≈ 29 kDa, and PDI ≈ 1.4] were purchased from Rieke Metals Inc. Chloroform (anhydrous grade) was bought from Sigma-Aldrich. Sample Preparation. All polymer solutions were prepared in chloroform. P3HT and P3BT NWs were prepared by two different methods, UV irradiation24 and solvent aging,25 following previously reported procedures. In both methods, 10 mg of the polymer (P3HT or P3BT) was dissolved in 2 mL of a chloroform solution in a 20 mL clean glass vial in ambient atmosphere. The solution was then heated at 60 °C for at least 60 min in a sealed vial. Electrical Characterizations. The FET device structures used for electrical characterization were constructed following the procedures previously reported.24 Polymer thin films were coated onto precleaned FET devices using either spin coating (Laurell, model WS-650MZ23NPP) at a spinning speed of 1500 rpm for 60 s or solution shear coating at various shearing speeds (i.e., 0.5, 1.0, 2.0, and 4.0 mm/s) in air, followed by thermal annealing at 120 °C for 10 min in a nitrogenfilled glovebox. For typical solution shear coating, a device substrate was stuck onto a homemade vacuum chuck equipped on the microstage (Zaber A-LSQ150A). Approximately 40 μL of the polymer solution was drop-cast onto the substrate with a pipet, and then the solution was immediately covered by a clean glass slide (14 × 25 × 1 mm). Subsequently, the microstage was translated at a different speed by a stepper motor, between 0.5 and 4.0 mm/s, while the glass slide was fixed. Finally, as-prepared devices were tested in the glovebox using a semiconductor analyzer (Agilent 4155C). The carrier mobility was extracted from the linear regime of transistor operation (VDS = −3 V) by extrapolating the slope of the drain current (IDS) versus gate voltage (VGS) using the following equation:

ACKNOWLEDGMENTS This research was supported by Emory University. The authors thank Rui Chang and Nabil Kleinhenz for their help with POM analyses.

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REFERENCES

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Research Article

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Macroscopic Alignment of One-Dimensional Conjugated Polymer

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