Large-Scale Alignment of Polymer Semiconductor Nanowires for

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Organic Electronic Devices

Large-Scale Alignment of Polymer Semiconductor Nanowires for Efficient Charge Transport via Controlled Evaporation of Confined Fluids Gyounglyul Jo, Jae Won Jeong, Solip Choi, Hyungwoo Kim, Jongjin Park, Jaehan Jung, and Mincheol Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18055 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Applied Materials & Interfaces

Large-Scale Alignment of Polymer Semiconductor Nanowires for Efficient Charge Transport via Controlled Evaporation of Confined Fluids Gyounglyul Jo,1 Jae Won Jeong,1 Solip Choi,1 Hyungwoo Kim,1,2 Jongjin Park,1,2 Jaehan Jung,3,* and Mincheol Chang1,2,* 1School

of Polymer Science and Engineering and 2Alan G. MacDiarmid Energy Research Institute Chonnam National University, Gwangju 61186, South Korea 3Department of Materials Science and Engineering, Hongik University, Sejong 30016, South Korea KEYWORDS. alignment, confined fluid, charge transport, nanowires, poly(3-hexylthiophene)

ABSTRACT: Long-range alignment of conjugated polymers is as critical as polymer chain packing for achieving efficient charge transport in polymer thin films used in electronic and optoelectronic devices. Here, the present study reports a facile, scalable strategy that enables the deposition of macroscopically aligned polymer semiconductor nanowire (NW)-array films with highly enhanced charge carrier mobility, using a modified controlled evaporative self-assembly (MCESA) technique. The organic field-effect transistors (OFETs) based on highly oriented poly(3-hexylthiophene) (P3HT)-NW films exhibit a more than 10-fold enhancement in the carrier mobility, with the highest mobility of 0.13 cm2 V-1 s-1, compared to the OFETs based on pristine P3HT films. Significantly, the large-area aligned P3HT NW-films, which are deposited over 12 arrays of transistors on a 4-in wafer by the MCESA coating, result in a lower device performance variation (i.e., standard deviation   0.0172 (16 %) cm2 V-1 s-1) as well as an excellent average device performance (i.e., average charge mobility  0.11 cm2 V-1 s-1), compared to those obtained using the conventional CESA coating, overcoming a critical challenge in the field of OFETs.

1. INTRODUCTION Over the past decade, conjugated polymers have attracted great attention in diverse research fields such as organic field effect transistors (OFETs),[1] organic light-emitting diodes (OLEDs),[2] and organic photovoltaic cells (OPVs)[3] as well as chemical sensors[4] and biosensors,[5] due to the benefits arising from low temperature, solution-based processability.[6-7] However, polymer semiconductors exhibit relatively inferior charge transport properties compared to their inorganic counterparts, owing to a low crystallinity (i.e., intra- and interchain interactions), high grain boundary density, and poor orientation of crystal grains, resulting in low device performance.[8-10] Typically, charge transport in these polymers takes place preferentially along the direction of p-orbital overlap, which can occur within and/or between individual polymer chains.[10] Therefore, it is of great importance to organize polymer chains into a desired configuration such as a well-ordered one-dimensional (1-D) nanostructure, and further, to align these 1-D nanostructures in the desired direction. To date, a number of reports have demonstrated that crystalline 1-D nanostructures of conjugated polymers, which have excellent charge transport characteristics, can be readily prepared via various processing strategies such as solubility tuning,[11] solution aging,[12] sonication,[13] and UV irradiation.[14] Conjugated polymers have a tendency to selfassemble into 1-D nanostructures such as nanowires and nanofibers via strong  interactions along the axes of the nanostructures.[11-15]

In contrast, much less effort has been focused on the alignment of 1-D polymer semiconductor nanostructures, although aligned 1-D nanostructures are expected to further facilitate high charge transport. Recently, several research groups have attempted to align 1-D nanostructures using barcoating,[16] capillarity on pre-engraved substrates,[17] LangmuirBlodgett techniques,[18] gravitational force-driven alignment,[19] electric or magnetic field-assisted alignment,[20-22] and electrostatic alignment.[23] However, these techniques present limitations for the large-area deposition of highly-aligned polymer thin films because of the use of external facilities, scale-up problems arising from uncontrolled solvent evaporation, and complicated processes regarding additional steps such as substrate pre-patterning and surface modifications of substrates.[16-23]

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In recent years, some alternative strategies that could more simply enable large-area ordered patterns using controlled evaporative self-assembly (CESA) have been reported.[24-26] When a drying droplet is confined in a space, evaporation is restricted to the edges, so that an outward capillary flow carries the solvent and solute from the interior to the contact line to compensate for the loss of solvent, leaving behind a wellordered pattern subject to the confined geometry. For instance, Lin et al. demonstrated macroscopically well-organized gradient patterns of conjugated polymers such as poly(3butylthiophene) (P3BT), poly(3-hexylthiophene) (P3HT), and P3BT-b-P3HT (P3BHT) by evaporating the corresponding polymer solutions in an axially symmetric cylinder on a Si substrate.[24] Zhang et al. reported one-step growth and patterning of N,N-dimethylquinacridone nanowires (NWs) by simply evaporating the solution in a spherical lens on the substrate. Further, the density, length, and periodicity of the patterns could be tuned by controlling the evaporation rate.[25] Wang et al. developed a one-step alignment, self-assembly, and patterning process for P3HT by using mixed solvents in the same confined geometry that Lin et al. used.[26] Consequently, the large-area ordered patterns prepared in this manner exhibited significantly enhanced electrical properties, indicating that the CESA approach could be a powerful strategy for aligning and patterning organic semiconductors for the fabrication of low-cost, large-area organic electronic devices with high performance. Although very intriguing, these techniques are limited in that they typically result in a striped pattern or thickness-gradient film rather than a uniform film; this is attributed to the balance between pinning and depinning forces at the contact line where solvent evaporation primarily occurs.[24-26] In addition, these approaches require a long solvent drying time during film formation because a polymer solution is covered with a static plate.[24-26] The large-scale deposition of uniform organic semiconductor films is a prerequisite for fabricating low-cost, large-area electronic devices with high reliability and performance. Herein, we report a facile and robust strategy to obtain largescale aligned conjugated polymer NW films with a uniform morphology using a modified CESA (MCESA) technique that is assisted by mechanically-driven meniscus motion. This technique can also provide a reduced solvent drying time, compared to the conventional CESA method, enabling a more rapid process for the fabrication of electronic devices. The aligned polymer films exhibit a remarkable 10-fold enhancement in charge carrier mobility as compared to pristine films. We systematically investigated how the meniscus motion affects the molecular ordering, orientation, and charge transport properties of the resultant films. The motion rate of the meniscus was revealed to profoundly influence the molecular ordering, morphologies, and charge transport characteristics of the aligned NW films. Significantly, our approach has proven facile, effective, and scalable, allowing us to create uniform films on a large-scale device substrate, resulting in highly reliable device performance over 12 device arrays. 2. RESULTS AND DISCUSSION P3HT was selected as a model polymer in our study and is widely used in a variety of electronic device applications due to its good solubility in a range of organic solvents, high chemical stability, and excellent charge transport properties.[27,28] The

Figure 1. (a) Schematic illustration of the procedure for aligning conjugated polymer NWs on a FET device substrate via CESA assisted by a mechanically-driven meniscus movement. The comparison of solution drying process of the (b) conventional CESA and (c) modified CESA (MCESA) method. Photographs of P3HT stripped patterns and films obtained via the MCESA method with different dragging speeds of (d) 0.00, (e) 0.01, and (f) 0.03 mm/s.

solutions of P3HT-NW in chloroform were readily prepared by following the procedures described in a previous report.[14] Figure 1a presents a schematic illustration of the MCESA process, through which polymer semiconductor NWs are oriented along the direction of meniscus movement, resulting in a large-area aligned NW-array film. The polymer-NW dispersion is trapped by capillary forces in the gap between a tilted cover plate and an electrode-patterned device substrate; the cover plate is tilted at an angle of 3˚ with respect to the device substrate. Subsequently, the cover plate is continuously and mechanically lifted up on one side to the vertical at a constant speed, in contrast to the standard CESA method that confines the fluid in a stationary space. Consequently, highly aligned polymer semiconductor NW films are obtained over a large area. When the solvent drying process took place while the cover plate was fixed, striped patterns were deposited due to the “stick-slip” motion arising from the competition between pinning (i.e., friction) and depinning forces (i.e., capillary force) (Figure 1b,d). Interestingly, the stripe-patterns evolved into stripe-patterned films as the cover plate was dragged up at a low speed, during which the depinning force may be slightly dominant, but still comparable to the pinning force, as seen in Figure 1c,e and Figure S1. P3HT NW films with relatively continuous uniform morphologies began to appear with further increases in dragging speed (Figure 1f and Figure S1); the depinning force increases and thus exceeds the pinning force as the cover plate begins to be dragged up, indicative of preferred “slip” motion. Noticeably, the film thickness gradually decreased from 75 to 34 nm as the dragging speed increased


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ACS Applied Materials & Interfaces from 0.01 to 0.03 mm/s (Figure S2), which indicates that solvent evaporation occurs primarily at the meniscus surface (evaporation regime).[29] In contrast, the thickness increased up to 65 nm with further increases in speed up to 0.20 mm/s, which implies that the solvent evaporates from an entire liquid film which is dragged out by viscous forces (Landau-Levich regime).[29] Similar trends were observed in the change of film roughness with dragging speed (Figure S2).

Figure 2. AFM images (a,b), orientation maps (c,d), and POM images (e,f) of P3HT NW films obtained by spin-coating and MCESA, respectively. The spin-coating was conducted at 2000 rpm and the MCESA process was performed while one part of the cover plate was lifted up at 0.03 mm/s. The orientation map produced by using GTFiber software represents the orientation of NWs. In Figures 2c and 2d, the inset (left, bottom) is the orientation distribution and the inset (right, bottom) is the color wheel. P3HT NW films deposited onto substrates using the MCESA coating method were revealed to be well-oriented along the direction of contact line movement, compared to those obtained using the conventional spin-coating method (Figure 2). The thin films spin-coated from a P3HT NW suspension showed randomly oriented P3HT NWs, as shown in Figure 2a. On the other hand, the films obtained via the MCESA method showed a significantly higher degree of orientation of the NWs (Figure 2b). The orientational order of the fibrillar morphologies observed via atomic force microscopy (AFM) shown in the images above (Figure 2a,b) was quantified by an automated image analysis protocol (GTFiber software) that could extract fiber backbones and their orientations, as illustrated in Figure 2c,d.[30] The orientation distribution (inset in Figure 2c) indicates that the average orientation of the NWs deposited by spin-coating was ~30˚ from the horizontal with an orientational order parameter S2D of 0.31. On the contrary, the orientation distribution of the NWs deposited via MECSA (inset in Figure

2d) shows that the average orientation was ~0˚ with a S2D of 0.72. The order parameter S2D quantifies the alignment of a population of NWs, with 0 being isotropic and 1 being perfectly anisotropic.[30] It should be noted that the orientation of the NWs significantly decreased as the dragging speed increased in to the Landau-Levich regime (> 0.03 mm/s), due to relative increase in viscose force dragging a liquid film where capillary force is released and thus orientation of the NWs decreases during solvent evaporation (Figure S3).[29] Consistently, the birefringent texture of the P3HT NW films prepared by the MCESA displayed significantly higher brightness compared to the spin-coated films (Figure 2e,f and Figure S4), which is attributed to the enhanced anisotropy and orientation.[12,13] P3HT has a tendency to self-assemble into highly ordered aggregates with strongly anisotropic features as depicted in Figure 3a.[27,28,31] UV-vis absorption spectroscopy was used to elucidate the effect of the MCESA method on the intra- and intermolecular ordering of the resultant films. As shown in Figure 3b, the P3HT films prepared by the MCESA method exhibited considerable enhancement in the lower energy vibronic bands at ~558 and 608 nm relative to the higher-energy band at 520 nm, compared to the spin-coated films, which is indicative of improved intermolecular interactions; the higherenergy features (* intraband transition) arise from disordered single chains while the lower-energy bands are related to interchain coupling and ordered aggregates.[31-33] Noticeably, this enhancement increased with decreasing dragging speed, suggesting that slow evaporation of the solvent facilitates strong intermolecular interactions between polymer chains. The analysis of intramolecular ordering, namely, determining the effective conjugation length of the polymer chains, was performed by means of UV-vis spectroscopy using Spano’s model assuming that the polymer crystalline region consists of weakly interacting H-aggregates.[32-34] The theoretical absorption contribution of P3HT aggregates can be described by Eq 1. 𝑒 ―𝑠𝑆𝑚

𝑊𝑒 ―𝑠

𝑚!

2𝐸𝑝

( )(1 ―

[Eq. (1)] 𝐴 ∝ ∑𝑚 = 0

(𝐸 ― 𝐸0 ― 0 ― 𝑚𝐸𝑝 ― × exp( ―

2𝜎2

2

)

𝐺𝑚 𝐴 1 2𝑊𝑆𝑚𝑒

2

) ―𝑆 )

where A is the absorbance as a function of the photon energy (E), W is the exciton bandwidth, S is the Huang-Rhys factor (~1.0),[32-34] 𝐸𝑝 is the intermolecular vibrational energy of the symmetric vinyl stretching mode (~0.18 eV) [32-34], Gm is a vibrational level dependent constant and is given by the 𝑆𝑛/𝑛!(𝑛 ― 𝑚), where n is the equation, 𝐺𝑚 = ∑ 𝑛( ≠ 𝑚)

vibrational quantum number, E0-0 is the 0-0 transition energy, and σ is the Gaussian linewidth. The exciton band width (W) was obtained by fitting the experimental spectra of P3HT films to the theoretical absorption spectra of P3HT aggregates (Figure 3c and Figure S5). W is inversely related to the length of the cofacially packed chain segments in P3HT aggregates. A decrease in W represents an increase in the conjugation length, i.e., intramolecular ordering; decreasing W corresponds to increasing 0-0 energy transition strength relative to the 0-1 energy transition strength.[32-34] The P3HT films deposited via the modified CESA technique showed relatively lower W values compared to spin-coated or randomly oriented P3HT



films (Figure 3d), suggesting that the MCESA technique would effectively extend or coplanarize the individual polymer chains, indicative of enhanced intramolecular ordering. Specifically, the W value of spin-coated P3HT NWs was found to be 66.9 meV, whereas the W value of P3HT NWs aligned by the MCESA method under a dragging speed of 0.01 mm /s was found to be 30.4 meV. Interestingly, the W value gradually increased to 66.6 meV as the dragging speed was varied from 0.01 to 0.20 mm/s, indicative of less planarization of the polymer backbone. It is believed that a higher dragging speed relative to solvent evaporation leaves a liquid film rather than a solid film behind the moving meniscus, so solvent evaporation occurs through the entire surface of the liquid film, resulting in a more disordered P3HT film.

Grazing incidence X-ray diffraction (GIXRD) measurements provided additional insight into the molecular packing of the P3HT films. The intensity of the (100) peak at around 2θ = 5.30˚, which corresponds to lamellar stacking along the crystallographic direction perpendicular to the backbone,[34,35] was enhanced by the MCESA coating compared to that in the spin-coating method (Figure 3e). This enhancement could be ascribed to either an increase in the size of individual crystallites, the number of crystallites, or both.[31] However, the peak intensity gradually decreased with increasing dragging speed from 0.01 to 0.20 mm/s. These results are in good agreement with the results obtained from UV-vis spectroscopy analysis (Figure 3b,d). A discernable change in the d-spacing of the lamellar packing was not observed with increasing dragging speed; the value remained at ~ 1.62 nm upon the variation of dragging speed from 0.01 to 0.20 mm/s, which suggests no change in the interdigitation and tilt of P3HT alkyl side chains.[11,36] The average coherence length (i.e., crystal grain size) corresponding to P3HT lamellar packing was calculated using the Scherrer equation.[34,37] Significantly, crystal size was increased by the MCESA method compared to that in conventional spin-coating. In particular, a dragging speed of 0.01 mm/s resulted in the largest coherence length (~12.4 nm). However, the coherence length gradually decreased with increasing dragging speed up to 0.20 mm/s. A relatively slow evaporation rate of the solvent is preferred for the formation of large polymer crystallites owing to their slow nucleation process.[7,11,31] A series of bottom gate, bottom contact OFETs with different channel lengths (channel length = 30, 50, 100, 200, and 300 μm, channel width = 2000 μm) were fabricated to investigate the charge transport properties of pristine P3HT films and P3HT NW films deposited by spin-coating or MCESA coating (Figure 4a). Pristine P3HT films deposited by MCESA coating

Figure 3. (a) Schematic packing behavior of P3HT: a, P3HT backbone; b,  stacking; c, lamellar stacking. (b) Normalized UV-vis absorption spectra of spin-coated P3HT pristine film and NW film and P3HT NW film deposited via the MCESAcoating under different dragging speeds (i.e., 0.01, 0.03, 0.05, 0.10, and 0.20 mm/s). P1 = pristine spin-coated, P2 = pristine deposited via the MCESA at 0.03 mm/s, N1 = P3HT NWs spincoated, N2 = P3HT NWs deposited via the MCESA at 0.01 mm/s, N3 = P3HT NWs deposited via the MCESA at 0.03 mm/s, N4 = P3HT NWs deposited via the MCESA at 0.05 mm/s, N5 = P3HT NWs deposited via the MCESA at 0.10 mm/s, N6 = P3HT NWs deposited via the modified CESA at 0.20 mm/s. (c) Absorption spectrum of the film deposited at 0.01 mm/s subjected to Spano analysis using Eq 1. The blue line depicts the spectra of the aggregates, and the red line depicts the absorption spectra associated with amorphous P3HT chains in the respective films. The black line indicates the experimental absorption spectra. (d) Calculated exciton bandwidth (W) of pristine P3HT and P3HT NW films. (e) GIXRD profiles and plots of the (100) layer spacing (left axis) and (f) crystal grain size (right axis) along the [100] direction of the corresponding films.

Figure 4. (a) Schematic structure of OFET device. (b) Mobility comparison of spin-coated P3HT pristine films and NW thin films and P3HT NW thin films deposited via the MCESA coating under a dragging speed of 0.03 mm/s, respectively. (c) Average and maximum mobilities of P3HT NW films coated as a function of dragging speed. (d) Total device resistance Rtotal of NWs films spin-coated and sheared at 0.03 mm/s as a function of channel length. The contact resistance, Rc is obtained by extrapolating the value of Rtotal to L = 0.


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ACS Applied Materials & Interfaces exhibited an improved charge carrier mobility (μavg = 6.0  10-2 cm2 V-1 s-1) compared to spin-coated pristine P3HT films (μavg = 1.7  10-2 cm2 V-1 s-1), which is attributed to the planarization and alignment of the polymer chains and the increase in grain size. Particularly, P3HT NW films aligned by MCESA coating exhibited an improvement in the average charge carrier mobility (μavg = 0.12 cm2 V-1 s-1), attributed to improved molecular ordering, orientation of the polymer NWs, and increased grain size (i.e., reduced grain boundaries), as evidenced by UV-vis and GIXRD data (Figures 3d and f); the highest mobility of 0.13 cm2 V-1 s-1 was recorded in aligned P3HT-NW films deposited with a dragging speed of 0.03 mm/s, which corresponds to a 10-fold enhancement relative to the mobility of randomly oriented films deposited by spin coating (Figure 4b,c and Figure S6). It should be noted that the mobility of the spin-coated P3HT-NW films presented in this research (~ 0.076 cm2/Vs) was higher than that of single P3HT NWs reported in previous studies (~ 0.056 cm2/Vs).[38,39] it is generally known that single P3HT NWs exhibit a mobility higher than that of P3HT-NW films.[38,39] It is believed that the single P3HT NWs prepared by UV irradiation intrinsically possess superior charge transport properties compared to the P3HT NWs prepared by whisker formation in a solution. This is because the structural coherent length of the former NWs (~ 11.5 nm) is larger than that of the latter NWs (~ 5.9 nm).[39] The OFET devices exhibited on/off-current ratios of ~ 103. Meanwhile, the devices based on P3HT-NW films exhibited a higher threshold voltage (Vth), compared to those based on pristine P3HT films, which is presumably due to the effects of unintentional doping and charge trapping at the polymer/oxide and/or the grain boundary interfaces (Figure S6).[40,41] Note that the variation of the dragging speed led to no discernable change in both the on/off-current ratio and Vth. P3HT-NW films deposited at 0.01 mm/s exhibited a higher degree of intra- and intermolecular interactions and orientation, whereas a more uniform surface was observed for P3HT-NW films deposited at 0.03 mm/s, as evidenced by the UV-vis, polarized optical microscopy (POM), and AFM data, compared to P3HT films deposited at 0.05, 0.10, and 0.20 mm/s. Consequently, the highest mobility was measured from P3HT-NW films aligned at 0.03 mm/s, indicating that a complex interplay between molecular ordering and morphology must govern the charge carrier mobility of the P3HT thin-films. The contact resistance, RC, between the electrodes and polymer semiconductor film becomes more important as the intrinsic mobility of the polymer layer increases. We extrapolated the measured total device resistance (Rtot = ∂𝑉𝐷/∂ 𝐼𝐷) to a channel length (L) of 0 to obtain the contact resistance, RC, as shown in Figure 4d.[42-44] The contact resistance appeared lower in the devices based on the spin-coated P3HT-NW films (RC = 1.96  10-1 MΩ) compared to those based on the pristine P3HT films (RC = 9.57  10-1 MΩ). Further, the value of RC slightly decreased to 1.22  10-1 MΩ for the aligned P3HT-NW film devices, which is attributed to the superior alignment of well-ordered P3HT NWs near the metal electrodes.[44] To demonstrate the process scalability of the MCESA technique, large-area integrated OFET arrays were fabricated by depositing a P3HT-NW solution using the MCESA technique on a 4-in wafer substrate patterned with gold electrodes, as shown in Figure 5a. The space between the device substrate and cover glass (52 mm  152 mm  1 mm) tilted at

Figure 5. Photographs of (a) MCESA coating process performed to fabricate large-area integrated OFET arrays on a 4-in SiO2/Si wafer and (b) device arrays assigned to “1” ~ “12”, on which a P3HT-NW film was deposited by MCESA coating at a dragging speed of 0.03 mm/s. (c) Average charge carrier mobility profile over the OFET device arrays. Each array contains three single transistors.

an angle of 3˚ with respect to the substrate was filled with the polymer solution, and then one side of the cover plate was vertically dragged at a speed of 0.03 mm/s. As shown in Figure 5b, a uniform, large-area P3HT-NW film was readily deposited over 12 device arrays (~ 50 mm  40 mm) within ~60 s after beginning to drag up the cover plate. The aligned film exhibited a mostly uniform morphology on the device arrays while some edge regions showed non-uniform features. Consequently, the charge carrier mobility was revealed to be reliably consistent over all 12 arrays of the film-based devices (Figure 5c). The device arrays exhibited a high average mobility of ~ 0.11 cm2 V-1 s-1 with a low standard deviation of  0.0172 (16 %) cm2 V-1 s-1. In contrast, the device arrays composed of aligned films obtained by a conventional CESA-coating (cover plate is fixed) showed a relatively lower average mobility of ~ 0.0175 cm2 V1 s-1 with a higher standard deviation of  0.00172 (57 %) cm2 V-1 s-1 (Figure S7). These results support the conclusion that the MCESA technique provides excellent charge transport properties with low device performance variation over an array of film-based devices because it produces highly aligned uniform polymer films over large areas. 3. CONCLUSIONS In conclusion, we demonstrated a facile, scalable strategy to fabricate large-scale aligned conjugated polymer NW films using a CESA process with mechanically-driven meniscus motion. It was revealed that the charge transport properties of the aligned films can be maximized by controlling the molecular ordering and morphology of the film, which can be achieved by varying the rate of meniscus motion. In particular, the highest carrier mobility of the aligned P3HT-NW film devices was recorded at 0.13 cm2 V-1 s-1, which is 10-fold higher than that of pristine P3HT film devices. Further, there was little variation in device performance over a large-area device array because of the uniform morphology of the aligned films used as



the active layer in OFET devices: average mobility, ~ 0.11 cm2 V-1 s-1 and standard deviation,  0.0172 (16 %) cm2 V-1 s-1. The MCESA technique would serve as a powerful approach for creating highly aligned conjugated polymer semiconductor films over a large area, which have uniform morphologies and excellent electrical properties, in a variety of applications such as photonics, optoelectronics, and biosensors.

Page 6 of 9 𝐼𝐷𝑆 =

𝑊𝐶𝑂𝑋

(𝑉 ― 𝑉𝑇)2 2𝐿 μ 𝐺𝑠

where W and L refer to channel width and length, respectively, VT is the threshold voltage, and Cox represents the capacitance of the SiO2 gate dielectric (~ 1.15×10-8 F/cm2).

ASSOCIATED CONTENT

4. MATERIALS AND METHODS Materials. 96% regioregular P3HT (MW of 43.7 kDa, Mn of 19.7 kDa) was obtained from Rieke Metals Inc. and chloroform (anhydrous grade) was purchased from Sigma Aldrich Chemical Co., and both were used without additional purification. Preparation of P3HT Thin Films and FET Devices. Ten milligrams (10 mg) of P3HT was introduced in 2 mL of chloroform in a 20-mL glass vial and sealed with a cap. Successively, the solution was heated to 55 ˚C for at least 60 min to allow for complete polymer dissolution. To prepare a P3HT-NW solution, UV irradiation of the as-prepared P3HT solution was conducted by following the procedure described in a previous paper.[14] The bottom-gate bottom-contact FET devices were constructed by following the reported procedure.[34] P3HT thin films were coated onto the precleaned device substrates using either spin-coating at a spin rate of 2000 rpm for 60 s or modified controlled evaporative self-assembly (MCESA) coating while dragging the cover glass at various speeds (i.e., 0.01, 0.03, 0.05, 0.10, and 0.20 mm/s). For the typical MCESA coating process, a glass cover plate (10 mm 

Optical microscopy images; film thickness and roughness profiles; AFM images; orientation maps; UV-vis spectra; output and transfer curve; average field-effect mobilities of P3HT films.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected]

Author Contributions Mincheol Chang and Jaehan Jung conceived and designed the experiments and supervised the project. Gyounglyul Jo, Jae Won Jeong, and Solip Choi carried out the experiments. Jongjin Park helped with FET fabrication and electrical characterization. Hyungwoo Kim helped with UV-vis spectroscopy and GIXRD characterization. Gyounglyul Jo and Mincheol Chang contributed to the interpretation of the results. Mincheol Chang, Jaehan Jung, and Gyounglyul Jo wrote the manuscript. All authors discussed the results and commented on the manuscript.

Notes The authors declare no competing financial interest.

50 mm  1 mm) was tilted at an angle of 3˚ with respect to the FET device substrate; then the P3HT-NW solution was injected between the glass cover plate and device substrate. Subsequently, MCESA coating was performed by lifting up one part of the glass plate at a different speed using a stepper motor (Zaber A-LSQ150A) while fixing the other side which was in contact with the device substrate. Residual solvent was removed by placing the OFETs in a vacuum oven (1 Torr)

ACKNOWLEDGMENT

overnight at 55 ˚C.

(1) Son, S. Y.; Kim, Y.; Lee, J.; Lee, G. Y.; Park, W. T.; Noh, Y. Y.; Park, C. E.; Park, T. High-Field-Effect Mobility of Low-Crystallinity Conjugated Polymers with Localized Aggregates. J. Am. Chem. Soc. 2016, 138, 8096-8103. (2) Sandstrom, A.; Dam, H. F.; Krebs, F. C.; Edman, L. Ambient Fabrication of Flexible and Large-Area Organic Light-Emitting Devices Using Slot-Die Coating. Nat. Commun. 2012, 3, 1002 (3) Subbiah, J.; Mitchell, V. D.; Hui, N. K. C.; Jones, D. J.; Wonh, W. W. H. A Green Route to Conjugated Polyelectrolyte Interlayers for High‐Performance Solar Cells. Angew. Chem. Int. Ed. 2017, 56, 84318434. (4) Khim, D.; Ryu, G. S.; Park, W. T.; Kim, H.; Lee, M.; Noh, Y. Y. Precisely Controlled Ultrathin Conjugated Polymer Films for Large Area Transparent Transistors and Highly Sensitive Chemical Sensors. Adv. Mater. 2016, 28, 2752-2759. (5) Han, S.; Zhuang, X.; Shi, W.; Yang, X.; Li, L.; Yu, J. Poly (3hexylthiophene)/Polystyrene (P3HT/PS) Blends Based Organic FieldEffect Transistor Ammonia Gas Sensor. Sens. Actuators, B 2016, 225, 10-15. (6) Chang, M.; Lim, G. T.; Park, B.; Reichmanis, E. Control of Molecular Ordering, Alignment, and Charge Transport in SolutionProcessed Conjugated Polymer Thin Films. Polymers 2017, 9, 212. (7) Ahmed, E.; Kim, F. S.; Xin, H.; Jenekhe, S. A. Benzobisthiazole− Thiophene Copolymer Semiconductors: Synthesis, Enhanced Stability,

Characterization. Solid state UV-vis spectra of P3HT thin films were obtained by using a UV-vis spectrometer (Agilent 8510). To record the UV-vis spectra, P3HT thin films were deposited onto precleaned glass slides by either spin-coating or MCESA coating. POM images of the corresponding films were obtained using a LEICA DM 750PT polarized optical microscope equipped with an iCM 3.0 IMT i-Solution Inc. digital camera. GIXRD measurements were conducted at a low incident angle of 0.2˚ using a Panalytical X’Pert Pro system with a Cu K X-ray source operated at 45 kV and 40 mA. The surface morphologies of the thin films deposited onto the FET devices were imaged using an atomic force microscope (XE100, Park systems) operated in tapping mode with a silicon tip (OMCL-AC160TS, Park systems). The charge carrier mobilities of the OFET devices were evaluated in an argonfilled glovebox by using a semiconductor analyzer (Agilent 4155C). The mobility was calculated from the saturation regime of transistor operation (VDS = - 80 V) by extrapolating the slope of the drain current (IDS) versus gate voltage (VGS) using the following equation:

This work was financially supported by Chonnam National University (Grant number: 2018-0914) and the National Research Foundation of Korea (NRF) grant by the Korea government (MSIT, Ministry of Science and ICT) (NRF-2017R1C1B1004605 and NRF-2017R1C1B5017856).

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ACS Applied Materials & Interfaces


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Large-Scale Alignment of Polymer Semiconductor Nanowires for

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