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Direct Uniaxial Alignment of a Donor-Acceptor Semiconducting Polymer using Single-Step Solution Shearing Leo Shaw, Pascal Hayoz, Ying Diao, Julia Antonia Reinspach, John W. F. To, Michael F. Toney, R. Thomas Weitz, and Zhenan Bao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01607 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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ACS Applied Materials & Interfaces

Direct Uniaxial Alignment of a Donor-Acceptor Semiconducting Polymer using Single-Step Solution Shearing Leo Shaw,1 Pascal Hayoz,2* Ying Diao,1,3† Julia Antonia Reinspach,1 John W. F. To, 1 Michael F. Toney,4 R. Thomas Weitz,5‡ and Zhenan Bao1* 1

Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA. 2

3

BASF Schweiz AG, GMV/BE, R-1059.5.09, Mattenstrasse, 4058 Basel, Switzerland.

Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA.

4

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 5

BASF SE, GMV/T, J542, 67056 Ludwigshafen, Germany.

Keywords: organic semiconductors, polymer semiconductors, polymer alignment, solution processing, solution shearing, donor-acceptor copolymers, field-effect transistors.

Abstract:

The alignment of organic semiconductors (OSCs) in the active layers of electronic devices can confer desirable properties, such as enhanced charge transport properties due to better ordering, 1 ACS Paragon Plus Environment


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charge transport anisotropy for reduced device crosstalk, and polarized light emission or absorption. The solution-based deposition of highly aligned small molecule OSCs has been widely demonstrated, but the alignment of polymeric OSCs in thin films deposited directly from solution has typically required surface templating or complex pre- or post-deposition processing. Therefore, a single-step solution processing and the charge transport enhancement afforded by alignment continue to be attractive. We report here the use of solution shearing to tune the degree of alignment in poly(diketopyrrolopyrrole-terthiophene) thin films by controlling the coating speed. A maximum dichroic ratio of ~7 was achieved on unpatterned substrates without any additional pre- or post-deposition processing. The degree of polymer alignment was found to be a competition between the shear alignment of polymer chains in solution and the complex thin film drying process. Contrary to previous reports, no charge transport anisotropy was observed because of the small crystallite size relative to the channel length, a mesh-like morphology, and the likelihood of increased grain boundaries in the direction transverse to coating. In fact, the lack of aligned morphological structures coupled with observed anisotropy in X-ray diffraction data suggests the alignment of polymer molecules in both the crystalline and the amorphous regions of the films. The shear speed at which maximum dichroism is achieved can be controlled by altering deposition parameters such as temperature and substrate treatment. Modest changes in molecular weight showed negligible effects on alignment, while longer polymer alkyl sidechains were found to reduce the degree of alignment. This work demonstrates that solution shearing can be used to tune polymer alignment in a one-step deposition process not requiring substrate patterning or any post-deposition treatment.

1. Introduction 2 ACS Paragon Plus Environment

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Organic semiconductors have attracted significant attention because of their ability to fill an application space underserved by conventional, silicon-based technologies. While much intellectual activity has focused on the chemical design and synthesis of both small molecule and polymeric organic semiconductors (OSCs), the way by which these materials are ultimately processed in the final device architecture can have a very important, and sometimes dramatic, effect on overall performance. To this end, advancing our understanding of how different processing techniques and parameters influence the microstructure, morphology, and crystallographic texture of OSC thin films is crucial to realizing their real-world potential in electronics applications. The design of polymer OSCs in recent years, while diverging in terms of specific chemical functionalities, have converged in terms of addressing their processing – the vast majority of semiconducting polymers possess linear or branched alkyl chains that facilitate specifically solvation of the polymer in common laboratory solvents. To this end, much of the recent push has been toward solution processing because of the potential for lower cost processing vis-à-vis low temperature, roll-to-roll deposition on flexible substrates.1-2 A plethora of solution-based processing techniques has been developed, ranging from laboratory-scale batch processes like dropcasting to continuous deposition techniques amenable to real industrial application. Among the latter, solution shearing has emerged as a versatile, facile processing method capable of (1) depositing high-quality single-crystalline small molecule OSC thin films capable of record performance as transistor active layers;3 (2) tuning the charge transport and molecular packing of small molecule OSCs via lattice strain;4 (3) enhancing nucleation to control the domain size in bulk heterojunction solar cells;5 (4) forming large-area arrays of self-aligned single-crystalline OSC domains;6 (5) controllably selecting for metastable packing polymorphs of small molecules;7 (6) inducing relative molecular orientation at the interfaces of binary OSC

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systems;8 (7) creating densely aligned films of carbon nanotubes;9 and (8) altering the crystalline packing of polymer lamellae by inducing structural rearrangements.10 In this study, we add to this list the ability to tune the in-plane alignment of a donor-acceptor semiconducting polymer – PDPP3T (Figure 1, top left) – using solution shearing as a one-step process not requiring any surface templating or additional pre- or post-deposition treatments. The alignment of organic semiconductors may enhance charge transport preferentially along a particular direction and also results in the emission or absorption of polarized light.11-12 In small molecule systems, there likely exists a fastest charge transport direction corresponding to a particular crystallographic direction, which behooves attempts to process such materials so that this direction is oriented along the channel length of a thin film transistor. For polymer OSC systems, charge transport along the conjugated backbone is believed to be a major path for charge transport, together with hopping between chains.13-14 Inducing the alignment of the polymer molecules can potentially enhance the performance of field-effect transistors, although the improvement may not always be along the chain alignment direction. Small molecule OSCs are generally easier to align than polymers, especially when they possess a fast-growth direction during crystallization from a given solvent. Meniscus-guided processing techniques15 take advantage of this natural tendency during crystallization for evolving films to grow crystals aligned along this direction and are sometimes capable of forming single-crystalline thin films with substrate patterning and control of crystal growth.16 However, polymers are, by design, chemically heterogeneous, and the intrinsic disorder arising from the inability to fully crystallize the backbone and side-chains have made it difficult to align them without the aid of sometimes complex pre- or post-deposition treatments. Earlier efforts focused on polymers exhibiting intrinsic liquid-crystalline (LC) phases


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accessible by heating and involved templating by a substrate (by friction transfer17 or mechanical alignment

via

abrasion18-19)

with

an

aligned

and

well-ordered

polymer

(i.e.

poly(tetrafluoroethylene), poly(p-phenylene), polyimides, etc.20). Atop this layer, polymer solutions were cast with21-24 or without25-26 additional heat treatment to induce their LC phase transition. This type of alignment, as well as variations on this procedure,12 is effective but requires multi-step processing and restricts the generality of the method to only those polymers with LC phases. While polymers can be tailored to specifically exhibit intrinsic liquid crystallinity,27 the utility of these methods is severely hampered by their complexity or specificity. Even though this design approach can be extended to directly enable alignment without substrate patterning,28 a technique widely applicable to polymers is desirable. The friction transfer method has been applied directly to conjugated polymers,29-30 but even then, the overall process still requires additional steps for alignment. Because each of these techniques requires steps before and/or after the deposition of the semiconducting material, they can be considered part of a single category of processing methods that we will denote as “ex-situ alignment,” where the alignment is actually achieved separate from the deposition step. For example, Finnberg et al. observed that the application of uniaxial mechanical rubbing with subsequent thermal annealing achieved a dichroic ratio of up to 5.1 in P3HT films and a factor of 8 mobility enhancement in transistors for which the rubbing direction is parallel to the charge transport direction.31 Bäcklund et al. demonstrated the use of a PEDOT:PSS layer spun atop an intermediate poly(4-vinylphenol) (PVP) layer to align a regioregular poly(3-hexylthiophene) (P3HT) film.32 The mechanical stress induced by shrinking the PEDOT:PSS film facilitated the radial alignment of the underlying P3HT film. Further work in this category includes high-temperature rubbing,33-35 poly(dimethylsiloxane) (PDMS)



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lamination and uniaxial mechanical stretching,36-37 and magnetic alignment during thermal annealing of block co-polymer supramolecular assemblies with grafted mesogens,38 as well as solvent-assisted nanoimprint lithography.39 Methods facilitating the single-step alignment of the semiconducting polymer – “in-situ alignment” – are attractive from many standpoints and have garnered much research attention. For example, Watanabe et al. achieved a mobility anisotropy ratio of 7.5 and an absorption ratio of 2.5 in poly[2-methoxy-5-(2’-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PVV) films five monolayers thick using a modified Langmuir-Blodgett (LB) technique.40 Zone casting of the ribbon-phase poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) at 140°C and 0.015 mm s-1 was able to achieve a dichroic ratio of 6.6, although an annealing step at 275°C was used as well.41 Other methods have been demonstrated to produce optical dichroism in the resulting films and include electrospinning,42-43 zone casting of a liquid crystalline OSC (albeit a small molecule),44 LB film formation using polythiophenes,45-47 dipcoating,48 magnetic-fieldenhanced49 or capillary-guided50-51 dropcasting,

ionic liquid thin film compression,52 and

epitaxial crystallization using binary solvents.53 Based on these previous studies, attaining inplane alignment requires surface patterning, low deposition speeds, or specific properties of the polymers (exhibiting liquid crystallinity, etc.). In contrast, we report here the deposition via solution shearing of diketopyrrolopyrrole-terthiophene polymer thin films exhibiting optical dichroic ratios as high as 7 without substrate patterning or post-deposition treatments. We characterize film morphology, microstructure, and charge transport behavior and investigate various material and processing factors influencing the in-situ alignment mechanism underpinning solution shearing. This work provides valuable insights into both solution-based deposition and into the chemical design of polymeric OSCs.


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Figure 1. UV-vis absorption spectra for PDPP3T films sheared at 0.05, 0.2, and 0.6 mm s-1 polarized in the direction parallel (dark blue) and perpendicular (light blue) to the shearing direction. The normalized, dilute solution-phase spectrum at room temperature is shown as a black dashed line in each plot. As shearing speed increases, the absorption in perpendicular direction significantly decreases relative to that in the parallel direction. (top left) PDPP3T and a schematic of the solution shearing process.

2. Polymer Alignment 2.1 Polarized UV-vis Absorption Donor-acceptor polymers with low bandgaps usually exhibit absorption in the near-IR region, which corresponds to charge transfer between donor and acceptor groups. By using polarized UV-vis spectroscopy, we can probe optical absorption anisotropies when the incident beam is polarized parallel and perpendicular to the coating direction. Because the transition dipole 7 ACS Paragon Plus Environment


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moment for these polymers is typically aligned along the long axis of their backbones, measured differences in polarized absorption provides a metric to probe how preferentially aligned the polymer chains are. Equal absorption for both polarization directions suggests a random average orientation of polymer chains in the film, while unequal absorption indicates greater alignment of chains in the direction where there is higher absorption. For all tested conditions, PDPP3T has two absorption peaks of interest around 850 nm and 725 nm, which we call the 0-0 and 0-1 peaks respectively. We define the dichroic ratio R for a film to be the absorption of the 0-0 peak in the direction parallel to shearing over the absorption in the perpendicular direction (R = A||/A ). ⟂

Similar trends for the dichroic ratios calculated from the 0-1 peaks were seen, although the ratios were generally lower than the corresponding 0-0 ratios. This is likely because the dipole moment for this and higher order transitions have a nonzero vector component normal to the plane of the backbone, which reduces the maximum attainable dichroic ratio. Figure 1 shows the UV-vis spectra for films deposited at selected shearing speeds onto glass treated with n-octadecyltrimethoxysilane (OTS). A clear trend can be seen – as shearing speed increases, the overall absorption in the direction perpendicular to the shearing direction decreases, indicating that the polymer chains are increasingly aligned along the shearing direction. Figure S2 summarizes the calculated dichroic ratios, along with the film thicknesses as measured by profilometry. Only a few measurements could be taken for a small range of shear speeds because the strongly lyophobic OTS surface created fluid dynamical instabilities in the meniscus. Specifically, when the shear speed was too high, the solution would dewet, completely interrupting film deposition. Selective surface modification (see Methods) was able to increase the meniscus stability and expand the accessible range of shear speeds. However, to avoid these issues altogether and to capture the full behavior of the dichroic ratio as a function of shear


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Figure 2. Optical dichroic ratio, film thickness, and shear rate as a function of shear speed for films sheared from tetralin on untreated glass. The dichroic ratios (bottom) peak at a critical shear speed of 0.2 mm s-1 before dropping back down to 1 at higher speeds. The film thicknesses (top) measured from profilometry and AFM, and the fitted curves (red for the evaporative regime, purple for the Landau-Levich regime, both separated by a vertical dashed line) are shown for comparison. On the same plot is the average shear rate across through the solution cross-section leaving the blade (black dashed line). While the solution also experiences high shear rates in the Landau-Levich regime (grey short-dashed line), the polymer relaxes before fixing its orientation in the dry film. speed, we deposited PDPP3T films onto glass substrates without OTS modification, which allowed us to access four orders of magnitude of shear speeds. Figure 2 shows the dichroic ratio (bottom), the film thickness (top, solid lines), and the approximate shear rate (top, dashed and



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dotted lines) as a function of shear speed for films deposited at 140°C. The shear rate dependence was derived using lubrication theory54 (see Supporting Information), which is valid here for our small gap height (40 µm) relative to the size of the solution droplet. For slow speeds, thick films are deposited with hardly any alignment because the low shear speeds impose correspondingly low shear strain in the solution. As the speed is increased, the dichroic ratio curve exhibits a single peak with a maximum of about 7 that occurs at a “critical” shear speed of 0.2 mm s–1. We propose the high shear strain at this combination of shear speed and temperature is the primary factor facilitating polymer alignment. At speeds above the critical value, the dichroic ratio remains steady as the speed is increased until it drops toward 1 (isotropy) as a shear speed of 1 mm s–1 is approached. The manifestation of this dichroic ratio peak is likely due to a competition between shear-induced polymer alignment in the solution and the actual film formation mechanism – i.e. solvent evaporation from the meniscus, polymer crystallization, etc. We believe that at low speeds, insufficient shear strain limits any alignment. After maximal alignment occurs at the critical shear speed, the effect of further increasing the coating speed marginally increases alignment at the expense of also changing the nucleation and crystallization kinetics. As the shear speed increases, the meniscus lengthens, increasing the overall area exposed to the ambient and thus increasing the solvent evaporation rate. Such increase can lead to an increased nucleation density at the air-liquid interface. Given this higher number of nuclei, the drop in the dichroic ratio can be caused by (1) the formation of more nuclei that are themselves misaligned (especially those formed away from the drying front), (2) the inability to reorient formed crystallites in the solution because of their greater number, and (3) the frustration of any alignment of the polymer chains in the amorphous regions existing outside the crystalline grains. It is also possible that the thinner films deposited at speeds around 5 mm s-1 create a


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vertical confinement effect that interrupts the alignment mechanism, possibly by inhibiting the growth of aligned polymer crystallites. The elucidation of this mechanism using quantitative, as well as in-situ,55-56 X-ray diffraction methods is forthcoming. The thickness data (Figure 2, top) reveals two regimes of film deposition – the “evaporative” regime, which is also known as “convective assembly” in colloidal systems, and the LandauLevich regime.57-58 In the former, the characteristic time scales for solvent evaporation and solid film deposition are similar, in effect producing films under the direct influence of the fluid flow induced by the coating apparatus. The evaporative regime is characterized by the thinning of films as coating speed is increased. On the other hand, the mechanism for the Landau-Levich regime involves first dragging out a wet film via viscous forces before subsequent drying. For this polymer-solvent system, the transition shear speed separating the two regimes occurs around 5 mm s–1. Near these speeds, the films are quite thin and transparent, yielding optical absorption too low for analysis. In the Landau-Levich regime, alignment is neither observed nor expected since the deposition process becomes similar to dropcasting or rapid blade coating, whereby polymer deposition results from the drying of a thin liquid layer, and film morphology is mostly controlled by the drying speed (which is influenced by the substrate temperature). Despite experiencing the highest shear rates in this regime (Figure 2, top, grey dotted line), the polymer – either as aggregates or as free chains – relax before their orientation can be fixed in the final film. When comparing our thickness curves with the expected behavior for these two regimes, we see slight differences. In the evaporative regime, our data shows a power law dependence on shear speed with an exponent of -1.18 (R2 = 0.977). This is slightly more negative than the -1 exponent expected with a simple mass balance around the meniscus.57 Our exponent thus 11 ACS Paragon Plus Environment


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suggests that the films are growing thinner than expected, which is consistent with the fact that several factors – such as concentration gradients,59 Marangoni flow,60-61 polymer nucleation,62-67 crystallite growth,68 edge effects, and vitrification near the contact line – are not accounted for in the idealized mass balance model. For the Landau-Levich regime, the exponent is 1.80 (R2 = 0.951), much greater than the 0.667 expected.58 We attribute this deviation to very ostensible solution dewetting during the drying process. For trials sheared on OTS-treated substrates, only the evaporative regime was accessible, and the thickness dependence indicated a power law exponent of -1.24 (R2 = 0.882, Figure S2), comparable to that for the untreated glass substrates. The films sheared on OTS all have at least a thickness of about 20 nm. For the untreated glass, the steep dropoff in dichroic ratio occurs when the films approach 10 nm in thickness. It is possible that the confinement effect mentioned previously contributes to the drop in observed optical anisotropy. 2.2 Optical and Atomic Force Microscopy Cross-polarized optical micrographs were taken of the films sheared onto OTS-treated SiO2/Si substrates at various magnifications. In Figure 3 (left), we can see while the morphology of the films sheared at 0.05, 0.1, and 0.3 mm s-1 hints at some alignment along the shearing direction, no such features can be seen for the films sheared at 0.5 mm s-1 (and beyond – not shown). Because the films at the highest speeds have the highest dichroic ratios of this set of films, we conclude that the film morphology does not have a strong influence on the observed optical dichroism. The lack of any discernable features in the micrographs suggests that the semicrystalline films possess many small crystallites embedded in an amorphous matrix. Atomic force micrographs (Figure 3, right) reveal no ostensible micron-scale polymer fibers. For the films sheared at 0.05 and 0.1 mm s-1, the film morphology shows hills and valleys, the latter


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Figure 3. Cross-polarized optical micrographs (left) and AFM height maps (right) of films sheared at selected speeds onto OTS-treated substrates. Both the optical micrographs and the AFM images show no significant morphological features indicating obvious alignment, especially at the higher shear speeds. The shearing direction is vertically upwards in all images. of which are about micron-sized in diameter. At 0.3 mm s-1, the size of the holes has decreased, while their number have increased. For the fastest shear speed of 0.5 mm s-1, the trend persists. Furthermore, the pores in this case penetrate the entire thickness of the film. The morphology is 13 ACS Paragon Plus Environment


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reminiscent of surface dewetting, perhaps indicating that the strongly dewetting OTS layer on the substrate also induces the polymer to form a mesh-like network of polymer bundles; such morphology was not observed in films deposited on glass. The polymer mesh appears isotropic and does not seem to exhibit any alignment in the shearing direction, as indicated by the circular, rather than ellipsoidal, pores. Again, the polymer alignment and consequent optical dichroism are not a direct consequence of film morphology. This is in contrast to previous studies where well-aligned fibers are observed – those polymers were either mechanically abraided34 or of high molecular weight (Mw ≥ 100 kDa),69-70 which may explain why none are observed in this system (Mw ~ 23 kDa). We discuss the morphology further in the next section. 2.3 Grazing-incidence X-ray Diffraction X-ray diffraction images taken at grazing incidence for selected shear speeds are shown in Figure 4. All samples were measured such that the shearing direction of the film was parallel or perpendicular to the incident X-ray beam, and the intensities were scaled with exposure time, Xray path length, and approximate film thickness so that the color scales are normalized, linear, and thus comparable across all diffraction images. The samples were measured in the two orientations to investigate how the crystalline regions of the film are aligned: within grains, πstacked polymer chains aligned along the direction parallel to shearing would diffract most strongly when the incident beam is also aligned along the shearing direction and vice versa (Figure S3). At all speeds, strong out-of-plane reflections from the (h00) reflections can be seen up to fourth order, indicating crystalline and ordered lamella with a d-spacing of 20.3 Å corresponding to a Q(100) value of 0.31 Å-1. The higher order reflections show progressive peak broadening, indicating paracrystalline disorder expected for such semicrystalline OSC films. At lower speeds, these lamella (h00) peaks form rings with the majority of the intensity oriented on


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Figure 4. Grazing-incidence X-ray diffraction images of films sheared onto OTS-treated SiO2/Si substrates with the incident beam oriented parallel (left) and perpendicular (right) to the shearing direction. All images were scaled for exposure time and illuminated volume to provide a qualitative comparison across different samples. The films at higher speeds are more crystalline, and the intensity difference between the two orientations further shows strong structural anisotropy. the meridian, whereas at higher speeds, the (h00) texture is strongly out-of-plane; the polar angle 15 ACS Paragon Plus Environment


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widths are only within a few degrees. This shows that while the majority of the polymer lamella is edge-on, as the shear speed increases, the minority population of lamella that is not edge-on decreases (see Figure S4 for more detail). This is likely the result of vertical confinement resulting from the thinner dry films deposited.71 Scattering from the branched alkyl side-chains produce the broad, diffuse halo centered at Q ≈ 1.4 Å-1. At the outer edge of this ring is the πstacking peak centered around 1.71 Å-1, or 3.67 Å in real space. This value for the π-stacking distance is comparable to reported literature values for other semiconducting polymers and does not change with shear speed. The polar angle width of the π-stacking peak shows that as shearing speed increases, polymer crystallites oriented in the direction parallel with shearing become increasingly edge-on beginning at a speed of 0.2 mm s-1. The overall diffraction intensity also increases with shear speed, suggesting the thinner films coated at higher speeds are more crystalline. Despite the lower signal over the diffuse background produced by the alkyl sidechains at higher speeds, the π-stacking reflection can be clearly discerned from its higher intensity near the horizon. When we compare the images taken parallel versus perpendicular to the shearing direction, we can see that at all speeds greater than 0.3 mm s-1, the intensity of the πstacking peak in the perpendicular direction decreases and becomes comparable to that of the alkyl scattering. This is clearly seen in the in-plane linecut of the horizon shown in Figure S4. Together, this indicates that the crystalline domains composed of π-stacked polymer chains are strongly oriented in-plane with the chains along the shearing direction, with the π-stack direction perpendicular to the shearing direction and the lamella perpendicular to the substrate surface. The diffraction images for the film sheared at 0.2 mm s-1 appears to show weaker πstacking intensity in the parallel direction as compared to the perpendicular, suggesting more crystallites exist in the film such that their polymer chains are aligned perpendicular to the


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shearing direction. However, the optical dichroic ratio for this shearing condition is greater than one, meaning there is more polymer oriented along the shearing direction than not. Since X-ray diffraction techniques only probe the crystalline regions of the film, the polymer chains existing in the amorphous regions of the semicrystalline film are not detected, even though they would still absorb UV-vis light. These two metrics for gauging polymer alignment – the optical dichroic ratios (which depends on the ensemble average of the cos2 of the angle between the incident light polarization and the polymers’ transition dipole moments) and the ratio of scattering intensity from the π-stacking peaks in the two measurement directions – are clearly unequal. It is possible that the observed optical dichroism results from two separate contributions: the aligned, πstacked crystallites and the aligned polymer chains in the amorphous regions of the film. The relative impact of these two factors will be determined by a quantitative treatment of these data, which is the subject of future research. However, the lack of clearly aligned fibrils in the crosspolarized optical micrographs and the AFM images, as well as the qualitative analysis of the dichroism observed in the GIXD images presented here, may suggest a nonnegligible contribution from the amorphous regions of the film. Previously published reports involving liquid crystalline polymers have relied on dichroism that can be attributed primarily to the aligned crystalline domains of the thin films as observed via cross-polarized microscopy. Here, it is likely both microstructural components of the whole film, the crystalline and the amorphous regions, are aligned to differing degrees. Furthermore, together with the information regarding the out-of-plane texture, we can conclude that as the shear speed increases, the polymer lamella become increasingly edge-on, the crystallites themselves become more oriented along the shearing direction, and the individual chains become more edge-on in relation to the substrate.



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Figure 5. Average hole mobilities extracted from bottom-gate, top-contact field-effect transistors operating in saturation mode. The modest increase in mobilities comes partly from the reduction in active layer thickness, which reduces contact resistance. The relative isotropy between transistors whose charge transport direction is parallel and perpendicular to the shearing direction suggests that the channel length used (50 μm) is too large to probe the intrinsic charge transport anisotropy for the different crystallographic directions of the polymer. The error bars indicate the mobility values one standard deviation above and below the mean, which was obtained from 3 to 4 devices for each direction. 2.4 Charge Transport Behavior Figure 5 plots the saturation-regime hole mobilities of films sheared onto OTS-treated SiO2/Si substrates. Contrary to others’ reports, the high optical anisotropies observed in the solution sheared films do not exhibit a corresponding charge transport anisotropy. The mobilities in both the direction parallel and perpendicular to the shearing direction follow the same trend: 18 ACS Paragon Plus Environment

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increasing as the shear speed increases until about 0.6 mm s-1. We attribute this to the reduction of contact resistance caused by the decreasing film thickness. At 0.7 mm s-1, the film likely becomes too thin, thus reducing both tie-chain density and effective charge transport. We can define a charge transport anisotropy as a ratio similar to that described for the UV-vis absorption analysis. The result is plotted as a function of shear speed in Figure S2. It is clear that there is no trend, and there is no charge transport anisotropy. Given that the polymer is of relatively low molecular weight and that the transistor channel length is much greater than the length of an individual polymer molecule, we believe charge transport is primarily intermolecular along the π-stacking direction rather than along the backbone. Moreover, the low molecular weight polymer molecules here may lead to a density of tie chains connecting nearby crystallites that is lower than what would be present in films of higher molecular weight polymer simply by virtue of having potential tie chains that are shorter. We thus expect that charge transport should be more effective in the direction perpendicular to shearing (along the π-stacking direction), but this is clearly not observed. The transistor devices measure the averaged electrical properties of many crystallites in the channel, as well as of those of the amorphous chains that connect them. Based on the optical micrographs presented earlier, the crystallites in the film are too small to be probed for charge transport anisotropy in the π-stacking versus backbone directions using a transistor channel length of 50 μm. Transmission electron microscopy (TEM) images (Figure S5) reveal relatively isotropic features with length scales on the order of nanometers. Since charge transport depends on the complete film morphology – which includes polymer orientation, polymerpolymer orientation, polymer domain orientation, and aggregated domain connectivity, the lack of charge transport anisotropy here despite a high degree of polymer alignment is not completely unexpected.



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While the macroscopic morphology of these films does not reveal significant anisotropy, the alignment of the nanoscale crystalline domains may also be responsible for the charge transport isotropy. Given that the three crystallographic directions of the polymer are quite different chemically, we expect the relative crystallization rates for these directions to differ, producing aggregates in solution that are anisotropic in shape. In fact, it is likely that the high optical dichroism observed for this polymer is the ultimate result of aligned, rod-like aggregates in solution, where the long axis corresponds to the polymer backbone direction and is much longer than the short axis. During deposition, the shearing process may align the long axis of these aggregates along the shearing direction, which then act as nuclei during crystallite formation. While the TEM images (Figure S5) suggest isotropic domains in the solid film, this does not preclude the existence of anisotropic polymer aggregates in the solution phase since polymer crystallization during solvent evaporation – a non-equilibrium process – can dramatically influence and change the ultimate shape of the crystalline domains in the solid phase. If we assume faster intermolecular over intramolecular transport and low overall tie-chain density, the π-stacking direction (which is parallel to the short axis) is thus transverse to shearing, and we would expect that charge transport should be faster in this direction. However, there are many more grain boundaries in this direction. As a result, a tradeoff exists between high molecular alignment and fast charge transport in particular crystallographic directions. Alignment of aggregates that are anisotropic only in solution may be sufficient to impart this effect in the solid film. It is thus possible that the chemical constituents and deposition parameters of polymer semiconductors that can enhance their alignment may also reduce observed field-effect mobilities in the direction transverse to alignment. Such insights can provide valuable design principles to guide efforts in polymer synthesis, and we expect to further probe this hypothesis in


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future work. 3. Factors Influencing Alignment To study the effect of deposition temperature on alignment, the dichroic ratio curve also was measured for samples deposited at 80°C and 185°C, corresponding to 74% and 95% of the boiling point, in absolute temperature, of tetralin (b.p. 207°C, or 408 K) (Figure 6). The transition shear speed between the evaporative and Landau-Levich regimes is increased with an

Figure 6. The effect of temperature on the dichroic ratio of films sheared on untreated glass. The critical shear speed shifts higher as temperature is increased, but the maximum dichroic ratio does not monotonically increase with the higher shear strains accessible at higher shear speeds because of the influence of increased nucleation. The highest observed dichroic ratio occurred for an intermediate temperature at around 85% of the solvent’s boiling point (140°C, or 480 K). Dashed curves are drawn only to guide the eye. The black dashed line indicates isotropy. increase in temperature, while decreasing the temperature shifts it lower. The maxima for the 21 ACS Paragon Plus Environment


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dichroic ratio curves occurred at different critical shear speeds, and the values for the dichroic ratio in both cases were lower than for 140°C. It appears that going from 80°C to 140°C, both the maximum optical dichroic ratio and the critical shear speed at which it occurs both increase with increasing temperature. Above 140°C, the maximum dichroic ratio drops while the critical shear speed continues to increase. Our model of the primary competing processes during deposition as shear molecular alignment and solvent evaporation is supported by these results. As increasing temperature enhances solvent evaporation, we are able to move the critical shear speed to higher speeds where the shear strain exerted on the solution is higher. But when the solvent evaporation is greatly increased closer to the boiling point, stochastic nucleation greatly depresses the alignment process, leading to low dichroic ratios even though the critical shear speed and shear strain is higher. A comparison was also made among different surface treatments and solution concentrations. Figure S6 superimposes the dichroic ratio curves for samples deposited on untreated glass, O2-plasma-cleaned glass, and OTS-treated glass. The curves for the first two surfaces are nearly identical, and the OTS curve appears similar in nature but shifted to higher shear speeds substantially. Because the OTS-treated surface is quite dewetting, we expect the dynamic contact angle during shearing to be much higher than when shearing on untreated or plasma-cleaned glass, reducing the relative evaporation rate. The slower evaporation rate is likely the controlling factor and explains the shift. A full mapping of the dichroic ratio curve on OTS at higher speeds, unfortunately, is not possible. Altering the solution concentration also does not dramatically affect the resulting films’ dichroic ratio. Figure S7 shows that the dichroic ratio is about constant for films coated from 11 mg mL-1 solutions down to 3 mg mL-1, only dropping at 1 mg mL-1. Because the shear speed and temperature are held constant, the drop in


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optical anisotropy could be the result of the much smaller thickness at 1 mg mL-1. The lack of a concentration dependence is not unexpected because one of the primary effects of decreasing the polymer concentration is a decrease in the solution viscosity. According to lubrication theory, viscosity primarily influences the pressure term of the shear rate equation, but the contribution of the pressure-driven flow is still very much less than that of the boundary-driven flow for our configuration. To determine the effect of longer polymer side chains, we tested a polymer of similar

Figure 7. The effect of side-chain length on the dichroic ratio of films sheared on untreated glass at 140°C. The dichroic ratio function for PDPP3T-10/12 reaches a maximum of ~5 before plateauing, which contrasts with the maximum of ~7 and subsequent drop-off for PDPP3T. The greater entropic difficulty of efficiently packing longer alkyl side-chains likely frustrates the polymer alignment process. The black dashed line indicates isotropy. 23 ACS Paragon Plus Environment


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molecular weight and polydispersity but with longer side-chains. In comparison with PDPP3T’s branched C6 and C8 side-chains (a.k.a. 2-hexyldecane), we chose to study a polymer consisting of C10 and C12 side-chains with the same β branching position (2-decyltetradecane), which we will refer to as PDPP3T-10/12. Figure 7 compares the obtained optical dichroic ratios for the two polymers sheared with exactly the same processing conditions. At the critical shear speed of 0.2 mm/s, PDPP3T-10/12 achieves a maximum dichroic ratio of about 5, which is ~70% of PDPP3T’s maximum. While the PDPP3T-10/12 data indicates similar behavior before the critical shear speed of PDPP3T, the subsequent dichroic ratios plateau and are consistently below those of PDPP3T. Because longer alkyl side-chains have a greater number of conformational degrees of freedom, the greater entropic disorder introduced by the longer side-chains influences polymer crystallization during film deposition. The lower dichroic ratios are likely the result of the frustration of the polymer alignment process caused by this enhanced disorder, suggesting that shorter side chains may be amenable to better alignment. Furthermore, the longer sidechains may change the shape of the polymer aggregates in solution because of differences in solubility. More isotropic aggregates would be more difficult to align in solution under shear stress. Lastly, we tested the effect of a different polymer molecular weight on the optical dichroic ratio. The polymer was synthesized with a ~60% greater molecular weight (36,082 g mol-1 versus 22,850) but with similar PDI (1.82 versus 1.77), which we refer to as PDPP3T-2. The two polymers exhibited very similar optical dichroic ratio trends (Figure S8). While for PDPP3T-2 the weight-average number of monomers per chain has increased from 27 to 43, such a change has little effect on polymer alignment via solution shearing. Given that these molecular weights are much less than conventionally studied, non-conducting polymers, it is unlikely that factors


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such as polymer entanglement density are at play in these systems. The data suggests that this difference in polymer length is effectively negligible. We would, however, expect to see some difference with much higher molecular weights where chemical differences – like higher aggregation energies – and fluid mechanical considerations – such as higher viscosity or nonNewtonian behavior – can influence the deposition. 4. Conclusion In summary, the degree of alignment in polymeric OSC thin films deposited with solution shearing was observed to change by altering the shear speed. A maximum dichroic ratio as high as ~7 was achieved on untreated glass substrates without any additional pre- or post-processing steps. In-situ polymer alignment is a competition between two processes – shear alignment of polymer chains and the thin film drying process, which is a complex interplay between solvent evaporation and polymer nucleation and growth, among others. Contrary to previous reports, no charge transport anisotropy was observed, likely because of (1) small crystallite size, (2) the lack of well-defined fibers stemming from the lower molecular weight polymers studied here, (3) contributions from the amorphous portions of the films, and (4) the possibility of a greater density of grain boundaries transverse to the alignment direction. X-ray diffraction data and morphology images indicate the alignment of both crystalline and amorphous regions of the film, which has not been explicitly shown previously for semiconducting polymers. The shear speed at which maximum dichroism is achieved can be manipulated by altering deposition parameters such as temperature and substrate treatment. Bulkier alkyl side-chains reduce the degree to which polymer chains can be aligned, while modest differences in molecular weight have negligible effect on alignment. The work presented here represents one of the few instances where polymer alignment in OSC thin films can be achieved in a one-step, in-situ deposition process.



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Furthermore, the general principles uncovered here can provide a better understanding of the crucial factors needed for any solution-based coating technique to induce tunable control of polymer alignment in the resulting thin film. 5. Experimental Methods 5.1

Materials.

Anhydrous

1,2,3,4-tetrahydronaphthalene

(tetralin)

and

anyhydrous

trichloroethylene (TCE) were purchased from Sigma-Aldrich (St. Louis, USA). noctadecyltrimethoxysilane (OTS) was purchased from Gelest (Morrisville, PA, USA). Degenerately doped n-type, Si wafers with 300-nm-thick thermal oxide layers (R < 0.005 Ω·cm) were used for transistor fabrication and X-ray diffraction experiments. Plain microscope slides (3 in. × 1 in. × 1 mm) were purchased from Fisher Scientific and used after cleaning in toluene, acetone, and isopropyl alcohol (IPA). Poly(dimethylsiloxane) (SYLGARD® 184 Silicone Elastomer) stamps were cured overnight at 80°C. PDPP3T (Mw = 22,850 g mol-1, PDI = 1.77), PDPP3T-2 (Mw = 36,082 g mol-1, PDI =1.82), PDPP3T-10/12 (Mw = 43,420 g mol-1, PDI = 1.93) were synthesized according to previous reports.72 Differential scanning calorimetry curves for PDPP3T are shown in Figure S1 in the Supplementary Information. 5.2 Substrate preparation. Plain, glass microscope slides were subjected to oxygen plasma cleaning (150 W, 200 mtorr O2) for 5 min and then spincoated with a 0.1 vol.% solution of OTS in TCE at 3000 rpm for 10 sec. The slides were annealed with ammonium hydroxide vapor overnight under vacuum and then sonicated for 10 minutes in toluene. Finally, they were rinsed with toluene, acetone, and IPA. Each glass slide was cut into four 2-by-1.5 cm samples. PDMS stamps cut into 1.5-by-0.5 cm strips were placed on each substrate near its center. These samples were then oxygen plasma cleaned again for 1.5 min to remove the OTS in the exposed perimeter. The substrates were rinsed again with the same solvent series as before prior to deposition. For 26 ACS Paragon Plus Environment

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field-effect transistors, the same OTS deposition treatment was applied to whole silicon wafers, with the latter part of the process performed on individual samples cut from the wafers. 5.3 Solution shearing. PDPP3T was solution sheared onto each sample from an 11 mg mL-1 tetralin solution at 130°C (OTS samples) or 140°C (untreated glass samples) with a gap height of ~40 μm and a blade tilt angle of 8°. The blade used was a dewetting, OTS-treated silicon wafer. Each sample was allowed to sit on the heating stage for ~2 min to drive off excess solvent after shearing. 5.4 Polarized UV-vis spectroscopy. UV-vis data was obtained in transmission geometry through a polarizer crystal. The incident beam (spot size: 5 mm × 5 mm) was polarized parallel and perpendicular to the shearing direction for each measurement. Three separate sections of each film were measured, and their dichroic ratios were averaged, making sure to select portions representing the equilibrium film morphology for the given shear speed and to avoid regions of concentration instability. Error bars shown in Figures 2, 6, 7, S6, S7, and S8 denote the standard deviation of the dichroic ratios measured in this way. An OTS-treated glass slide without PDPP3T was used to obtain the background absorption, which was subtracted from each raw spectrum. The absorption in the region around 1150 to 1300 nm was averaged and then used to zero the spectra, since none of the materials of interest have absorption in these regions. Discontinuities caused by changing the detector and the grating mid-scan were removed by manually shifting the absorption values. These corrected absorption values were used without further adjustment because deconvolution of the 0-0 and 0-1 peaks was not possible. The dichroic ratios were calculated by obtaining the maximum value in the 800-to-900-nm region, which corresponds to the 0-0 absorption peak. Data analysis was performed in MATLAB. Solution spectra were taken without polarization using a very dilute tetralin solution in a quartz



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cuvette. 5.5 Field-effect transistor fabrication. After polymer film deposition onto the OTS-treated Si/SiO2 substrates, 40-nm-thick Au electrodes were thermally evaporated through a shadow mask to yield transistor arrays whose channel lengths and widths were 50 and 1000 μm respectively. These masks produced differently oriented arrays of electrodes such that charge transport could be measured in the direction parallel and perpendicular to the shearing direction. Electrical characterization was performed using a Keithley 4200-SCS semiconductor parameter analyzer. All transistors were in the bottom-gate, top-contact geometry – i.e. the silicon substrate acted as a common gate electrode, with the SiO2 as gate insulator. Each electrode array was electrically isolated by gently scratching the semiconductor film with tweezers. Measurements were carried out in an N2-filled glovebox at room temperature, and hole mobilities were extracted in the saturation regime using a drain-source voltage Vds of -100 V. 5.6 Cross-polarized microscopy. A Leica DM4000M microscope with cross-polarizer was used to record optical micrographs of the sheared films. All images were taken in regions close to the transistor channel so as to reflect its morphology. 5.7 Atomic force microscopy. A Multimode Nanoscope III (Digital Instruments/Veeco Metrology Group) was used in tapping mode to record 5 µm × 5 µm height and phase maps. 5.8 Grazing-incidence X-ray diffraction. Grazing-incidence X-ray diffraction (GIXD) was performed at the Stanford Synchrotron Radiation Lightsource on beamline 11-3 using a MAR345 image plate as the 2-D area detector. The X-ray energy was 12.73 keV, and the incidence angle was 0.12°. Samples were held in a helium chamber during measurements to reduce background scattering and sample damage. They were positioned so that the incident beam was oriented parallel and perpendicular to the shearing direction for each sample.


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Furthermore, each parallel and perpendicular scan for a given speed was taken from the same film. Data analysis was performed using WxDiff and MATLAB. Linecuts provided in the Supporting Information were taken by plotting the integrated intensity between qz = 0 Å-1 and qz = 0.01 Å-1 for the horizon as a function of qxy and by plotting the integrated intensity between qxy = -0.005 Å-1 and qxy = 0.005 Å-1 for the meridian as a function of approximate qz. Intensity near the beamstop was ignored. 5.9 Transmission Electron Microscopy. A 200 kV FEI Tecnai G2 F20 X-TWIN transmission electron microscope (TEM) was used in brightfield mode to capture images of a PDPP3T film sheared onto a OTS-treated substrate from tetralin at 140°C and 0.3 mm s-1. Using a PDMS stamp and a water-soluble, sacrificial polystyrene sulfonate (PSS) layer, the film was transferred onto a 300-mesh Cu grid (Ted Pella, Inc.), which consisted of an orthogonal array of 1.2-μmdiameter square holes with 1.3-μm separation supporting a QUANTIFOIL® holey carbon film.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Differential scanning calorimetry curves Dichroic ratio plots GIXD linecuts X-ray diffraction schematics TEM images Lubrication theory derivations

AUTHOR INFORMATION 29 ACS Paragon Plus Environment


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Corresponding Authors * Correspondence and requests for materials should be addressed to P. H. (email: [email protected]) or Z. B. (email: [email protected]). Present Addresses † Department of Chemical and Biomolecular Engineering, University of Illinois at UrbanaChampaign, 600 South Mathews Avenue, 213 Roger Adams Laboratory, Urbana, IL 61801 (Y. D.). ‡ Physics of Nanostructures, Faculty of Physics, Ludwig-Maximilians-Universität München, Amalienstraße 54, 80799 München, Germany (R. T. W.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources National Science Foundation (DMR-1303178), BASF, National Science Foundation Materials Genome Program (Grant no. 1434799). Notes The authors declare no competing financial interest.

Acknowledgments L. S. gratefully acknowledges support from the Kodak Graduate Fellowship. The authors 30 ACS Paragon Plus Environment

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would also like to acknowledge support from the National Science Foundation (DMR-1303178) and BASF. M. T. and Z. B. acknowledge support from the National Science Foundation Materials Genome Program (Grant no. 1434799). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC0276SF00515. L.S. is grateful for support from the Kodak Graduate Fellowship, and J.R. acknowledges support from the Swedish Knut and Alice Wallenberg Foundation.

ABBREVIATIONS AFM, atomic force microscopy; UV-vis, ultraviolet-visible; PDI, polydispersity index.

REFERENCES (1) Helgesen, M.; Sondergaard, R.; Krebs, F. C. Advanced Materials and Processes for Polymer Solar Cell Devices. J. Mater. Chem. 2010, 20, 36-60. (2) Krebs, F. C.; Tromholt, T.; Jorgensen, M. Upscaling of Polymer Solar Cell Fabrication Using Full Roll-to-Roll Processing. Nanoscale 2010, 2, 873-886. (3) Diao, Y.; Tee, B. C. K.; Giri, G.; Xu, J.; Kim, D. H.; Becerril, H. A.; Stoltenberg, R. M.; Lee, T. H.; Xue, G.; Mannsfeld, S. C. B.; Bao, Z. Solution Coating of Large-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nat. Mater. 2013, 12, 665-671. (4) Giri, G.; Verploegen, E.; Mannsfeld, S. C. B.; Atahan-Evrenk, S.; Kim, D. H.; Lee, S. Y.; Becerril, H. A.; Aspuru-Guzik, A.; Toney, M. F.; Bao, Z. Tuning Charge Transport in Solution-Sheared Organic Semiconductors Using Lattice Strain. Nature 2011, 480, 504-508. (5) Diao, Y.; Zhou, Y.; Kurosawa, T.; Shaw, L.; Wang, C.; Park, S.; Guo, Y.; Reinspach, J. A.; Gu, K.; Gu, X.; Tee, B. C. K.; Pang, C.; Yan, H.; Zhao, D.; Toney, M. F.; Mannsfeld, S. C. B.; Bao, Z. Flow-Enhanced Solution Printing of All-Polymer Solar Cells. Nat. Commun. 2015, 6, 7955. (6) Park, S.; Giri, G.; Shaw, L.; Pitner, G.; Ha, J.; Koo, J. H.; Gu, X.; Park, J.; Lee, T. H.; Nam, J. H.; Hong, Y.; Bao, Z. Large-Area Formation of Self-Aligned Crystalline Domains of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Organic Semiconductors on Transistor Channels Using Connect. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 5561-5566. (7) Giri, G.; Li, R.; Smilgies, D.-M.; Li, E. Q.; Diao, Y.; Lenn, K. M.; Chiu, M.; Lin, D. W.; Allen, R.; Reinspach, J.; Mannsfeld, S. C. B.; Thoroddsen, S. T.; Clancy, P.; Bao, Z.; Amassian, A. One-Dimensional Self-Confinement Promotes Polymorph Selection in LargeArea Organic Semiconductor Thin Films. Nat. Commun. 2014, 5, 3573. (8) Ma, W.; Reinspach, J.; Zhou, Y.; Diao, Y.; McAfee, T.; Mannsfeld, S. C. B.; Bao, Z.; Ade, H. Tuning Local Molecular Orientation–Composition Correlations in Binary Organic Thin Films by Solution Shearing. Adv. Funct. Mater. 2015, 25, 3131-3137. (9) Park, S.; Pitner, G.; Giri, G.; Koo, J. H.; Park, J.; Kim, K.; Wang, H.; Sinclair, R.; Wong, H. S. P.; Bao, Z. Large-Area Assembly of Densely Aligned Single-Walled Carbon Nanotubes Using Solution Shearing and Their Application to Field-Effect Transistors. Adv. Mater. 2015, 27, 2656-2662. (10) Giri, G.; DeLongchamp, D. M.; Reinspach, J.; Fischer, D. A.; Richter, L. J.; Xu, J.; Benight, S.; Ayzner, A.; He, M.; Fang, L.; Xue, G.; Toney, M. F.; Bao, Z. Effect of Solution Shearing Method on Packing and Disorder of Organic Semiconductor Polymers. Chem. Mater. 2015, 27, 2350-2359. (11) Zhu, R.; Kumar, A.; Yang, Y. Polarizing Organic Photovoltaics. Adv. Mater. 2011, 23, 4193-4198. (12) Yoshida, Y.; Tanigaki, N.; Yase, K.; Hotta, S. Color-Tunable Highly Polarized Emissions from Uniaxially Aligned Thin Films of Thiophene/Phenylene Co-Oligomers. Adv. Mater. 2000, 12, 1587-1591. (13) Zhang, X.; Bronstein, H.; Kronemeijer, A. J.; Smith, J.; Kim, Y.; Kline, R. J.; Richter, L. J.; Anthopoulos, T. D.; Sirringhaus, H.; Song, K.; Heeney, M.; Zhang, W.; McCulloch, I.; DeLongchamp, D. M. Molecular Origin of High Field-Effect Mobility in an Indacenodithiophene–Benzothiadiazole Copolymer. Nat. Commun. 2013, 4, 2238. (14) Noriega, R.; Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.; Toney, M. F.; Salleo, A. A General Relationship between Disorder, Aggregation and Charge Transport in Conjugated Polymers. Nat. Mater. 2013, 12, 1038-1044. (15) Diao, Y.; Shaw, L.; Bao, Z.; Mannsfeld, S. C. B. Morphology Control Strategies for Solution-Processed Organic Semiconductor Thin Films. Energy Environ. Sci. 2014, 7, 21452159. (16) Shaw, L.; Bao, Z. The Large-Area, Solution-Based Deposition of Single-Crystal Organic Semiconductors. Isr. J. Chem. 2014, 54, 496-512. (17) Wittmann, J. C.; Smith, P. Highly Oriented Thin Films of Poly(Tetrafluoroethylene) as a Substrate for Oriented Growth of Materials. Nature 1991, 352, 414-417. (18) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.; Nothofer, H.-G.; Scherf, U.; Yasuda, A. Blue Polarized Electroluminescence from a Liquid Crystalline Polyfluorene. Adv. Mater. 1999, 11, 671-675. (19) Toney, M. F.; Russell, T. P.; Logan, J. A.; Kikuchi, H.; Sands, J. M.; Kumar, S. K. NearSurface Alignment of Polymers in Rubbed Films. Nature 1995, 374, 709-711. (20) Tanigaki, N.; Kyotani, H.; Wada, M.; Kaito, A.; Yoshida, Y.; Han, E.-M.; Abe, K.; Yase, K. Oriented Thin Films of Conjugated Polymers: Polysilanes and Polyphenylenes. Thin Solid Films 1998, 331, 229-238. (21) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu, W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Mobility Enhancement in Conjugated Polymer Field-Effect 32 ACS Paragon Plus Environment

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Transistors through Chain Alignment in a Liquid-Crystalline Phase. Appl. Phys. Lett. 2000, 77, 406-408. (22) Chi, C.; Lieser, G.; Enkelmann, V.; Wegner, G. Packing and Uniaxial Alignment of Liquid Crystalline Oligofluorenes. Macromol. Chem. Phys. 2005, 206, 1597-1609. (23) Knaapila, M.; Kisko, K.; Lyons, B. P.; Stepanyan, R.; Foreman, J. P.; Seeck, O. H.; Vainio, U.; Pålsson, L.-O.; Serimaa, R.; Torkkeli, M.; Monkman, A. P. Influence of Molecular Weight on Self-Organization, Uniaxial Alignment, and Surface Morphology of HairyRodlike Polyfluorene in Thin Films. J. Phys. Chem. B 2004, 108, 10711-10720. (24) Toshiyuki, E.; Takashi, N.; Takashi, K.; Hiroyoshi, N. Highly Oriented Polymer FieldEffect Transistors with High Electrical Stability. Jpn. J. Appl. Phys. 2013, 52, 121601. (25) Nishizawa, T.; Lim, H. K.; Tajima, K.; Hashimoto, K. Highly Uniaxial Orientation in Oligo(P-Phenylenevinylene) Films Induced During Wet-Coating Process. J. Am. Chem. Soc. 2009, 131, 2464-2465. (26) Schott, S.; Gann, E.; Thomsen, L.; Jung, S.-H.; Lee, J.-K.; McNeill, C. R.; Sirringhaus, H. Charge-Transport Anisotropy in a Uniaxially Aligned Diketopyrrolopyrrole-Based Copolymer. Adv. Mater. 2015, 27, 7356-7364. (27) Kim, B.-G.; Jeong, E. J.; Chung, J. W.; Seo, S.; Koo, B.; Kim, J. A Molecular Design Principle of Lyotropic Liquid-Crystalline Conjugated Polymers with Directed Alignment Capability for Plastic Electronics. Nat. Mater. 2013, 12, 659-664. (28) Hoogboom, J.; Swager, T. M. Increased Alignment of Electronic Polymers in Liquid Crystals Via Hydrogen Bonding Extension. J. Am. Chem. Soc. 2006, 128, 15058-15059. (29) Era, M.; Tsutsui, T.; Saito, S. Polarized Electroluminescence from Oriented P‐Sexiphenyl Vacuum‐Deposited Film. Appl. Phys. Lett. 1995, 67, 2436-2438. (30) Misaki, M.; Ueda, Y.; Nagamatsu, S.; Yoshida, Y.; Tanigaki, N.; Yase, K. Formation of Single-Crystal-Like Poly(9,9-Dioctylfluorene) Thin Film by the Friction-Transfer Technique with Subsequent Thermal Treatments. Macromolecules 2004, 37, 6926-6931. (31) Heil, H.; Finnberg, T.; von Malm, N.; Schmechel, R.; von Seggern, H. The Influence of Mechanical Rubbing on the Field-Effect Mobility in Polyhexylthiophene. J. Appl. Phys. 2003, 93, 1636-1641. (32) Bäcklund, T. G.; Sandberg, H. G. O.; Österbacka, R.; Stubb, H.; Torkkeli, M.; Serimaa, R. A Novel Method to Orient Semiconducting Polymer Films. Adv. Funct. Mater. 2005, 15, 1095-1099. (33) Biniek, L.; Pouget, S.; Djurado, D.; Gonthier, E.; Tremel, K.; Kayunkid, N.; Zaborova, E.; Crespo-Monteiro, N.; Boyron, O.; Leclerc, N.; Ludwigs, S.; Brinkmann, M. HighTemperature Rubbing: A Versatile Method to Align Π-Conjugated Polymers without Alignment Substrate. Macromolecules 2014, 47, 3871-3879. (34) Biniek, L.; Leclerc, N.; Heiser, T.; Bechara, R.; Brinkmann, M. Large Scale Alignment and Charge Transport Anisotropy of Pbttt Films Oriented by High Temperature Rubbing. Macromolecules 2013, 46, 4014-4023. (35) Jandke, M.; Strohriegl, P.; Gmeiner, J.; Brütting, W.; Schwoerer, M. Polarized Electroluminescence from Rubbing-Aligned Poly(P-Phenylenevinylene). Adv. Mater. 1999, 11, 1518-1521. (36) Gargi, D.; Kline, R. J.; DeLongchamp, D. M.; Fischer, D. A.; Toney, M. F.; O’Connor, B. T. Charge Transport in Highly Face-on Poly(3-Hexylthiophene) Films. J. Phys. Chem. C 2013, 117, 17421-17428.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(37) O'Connor, B.; Kline, R. J.; Conrad, B. R.; Richter, L. J.; Gundlach, D.; Toney, M. F.; DeLongchamp, D. M. Anisotropic Structure and Charge Transport in Highly Strain-Aligned Regioregular Poly(3-Hexylthiophene). Adv. Funct. Mater. 2011, 21, 3697-3705. (38) Tran, H.; Gopinadhan, M.; Majewski, P. W.; Shade, R.; Steffes, V.; Osuji, C. O.; Campos, L. M. Monoliths of Semiconducting Block Copolymers by Magnetic Alignment. ACS Nano 2013, 7, 5514-5521. (39) Wei, S.; Zhang, Y.; Liu, J.; Li, X.; Wu, Y.; Wei, H.; Weng, Y.; Gao, X.; Li, Y.; Wang, S.D.; Hu, Z. Large Modulation of Charge Transport Anisotropy by Controlling the Alignment of Π–Π Stacks in Diketopyrrolopyrrole-Based Polymers. Adv. Mater. Interfaces 2015, 2, 1500153. (40) Shun-ichiro, W.; Hisaaki, T.; Shin-ichi, K.; Akio, T.; Haruki, T.; Shusaku, N.; Takahiro, S. Charge Transport Anisotropy Due to Interfacial Molecular Orientation in Polymeric Transistors with Controlled in-Plane Chain Orientation. Appl. Phys. Express 2012, 5, 021602. (41) Lee, M. J.; Gupta, D.; Zhao, N.; Heeney, M.; McCulloch, I.; Sirringhaus, H. Anisotropy of Charge Transport in a Uniaxially Aligned and Chain-Extended, High-Mobility, Conjugated Polymer Semiconductor. Adv. Funct. Mater. 2011, 21, 932-940. (42) Li, D.; Wang, Y.; Xia, Y. Electrospinning of Polymeric and Ceramic Nanofibers as Uniaxially Aligned Arrays. Nano Lett. 2003, 3, 1167-1171. (43) Kakade, M. V.; Givens, S.; Gardner, K.; Lee, K. H.; Chase, D. B.; Rabolt, J. F. Electric Field Induced Orientation of Polymer Chains in Macroscopically Aligned Electrospun Polymer Nanofibers. J. Am. Chem. Soc. 2007, 129, 2777-2782. (44) Pisula, W.; Tomović, Ž.; Stepputat, M.; Kolb, U.; Pakula, T.; Müllen, K. Uniaxial Alignment of Polycyclic Aromatic Hydrocarbons by Solution Processing. Chem. Mater. 2005, 17, 2641-2647. (45) Olivati, C. A.; Gonçalves, V. C.; Balogh, D. T. Optically Anisotropic and Photoconducting Langmuir–Blodgett Films of Neat Poly(3-Hexylthiophene). Thin Solid Films 2012, 520, 2208-2210. (46) Xu, G.; Bao, Z.; Groves, J. T. Langmuir−Blodgett Films of Regioregular Poly(3Hexylthiophene) as Field-Effect Transistors. Langmuir 2000, 16, 1834-1841. (47) Watanabe, S.-i.; Tanaka, H.; Kuroda, S.-i.; Toda, A.; Nagano, S.; Seki, T.; Kimoto, A.; Abe, J. Electron Spin Resonance Observation of Field-Induced Charge Carriers in Ultrathin-Film Transistors of Regioregular Poly(3-Hexylthiophene) with Controlled in-Plane Chain Orientation. Appl. Phys. Lett. 2010, 96, 173302. (48) Tsao, H. N.; Cho, D.; Andreasen, J. W.; Rouhanipour, A.; Breiby, D. W.; Pisula, W.; Müllen, K. The Influence of Morphology on High-Performance Polymer Field-Effect Transistors. Adv. Mater. 2009, 21, 209-212. (49) Pan, G.; Chen, F.; Hu, L.; Zhang, K.; Dai, J.; Zhang, F. Effective Controlling of Film Texture and Carrier Transport of a High-Performance Polymeric Semiconductor by Magnetic Alignment. Adv. Funct. Mater. 2015, 25, 5126-5133. (50) Takuya, H.; Naoyuki, Y.; Hideyuki, U.; Hiroyuki, Y.; Akihiko, F.; Masanori, O. Anisotropic Properties of Aligned Π-Conjugated Polymer Films Fabricated by Capillary Action and Their Post-Annealing Effects. Appl. Phys. Express 2011, 4, 091602. (51) Naoyuki, Y.; Yasuo, M.; Hiroyuki, Y.; Akihiko, F.; Masanori, O. Solution Flow Assisted Fabrication Method of Oriented Π-Conjugated Polymer Films by Using GeometricallyAsymmetric Sandwich Structures. Jpn. J. Appl. Phys. 2011, 50, 020205. 34 ACS Paragon Plus Environment

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(52) Soeda, J.; Matsui, H.; Okamoto, T.; Osaka, I.; Takimiya, K.; Takeya, J. Highly Oriented Polymer Semiconductor Films Compressed at the Surface of Ionic Liquids for HighPerformance Polymeric Organic Field-Effect Transistors. Adv. Mater. 2014, 26, 6430-6435. (53) Müller, C.; Aghamohammadi, M.; Himmelberger, S.; Sonar, P.; Garriga, M.; Salleo, A.; Campoy-Quiles, M. One-Step Macroscopic Alignment of Conjugated Polymer Systems by Epitaxial Crystallization During Spin-Coating. Adv. Funct. Mater. 2013, 23, 2368-2377. (54) Leal, L. G., Advanced Transport Phenomena: Fluid Mechanics and Convective Transport Processes. Cambridge University Press: 2007. (55) Smilgies, D.-M.; Li, R.; Giri, G.; Chou, K. W.; Diao, Y.; Bao, Z.; Amassian, A. Look Fast: Crystallization of Conjugated Molecules During Solution Shearing Probed in-Situ and in Real Time by X-Ray Scattering. Phys. Status Solidi Rapid Res. Lett. 2013, 7, 177-179. (56) Wan, J.; Li, Y.; Ulbrandt, J. G.; Smilgies, D.-M.; Hollin, J.; Whalley, A. C.; Headrick, R. L. Transient Phases During Fast Crystallization of Organic Thin Films from Solution. APL Mater. 2016, 4, 016103. (57) Le Berre, M.; Chen, Y.; Baigl, D. From Convective Assembly to Landau−Levich Deposition of Multilayered Phospholipid Films of Controlled Thickness. Langmuir 2009, 25, 2554-2557. (58) Landau, L.; Levich, B. Dragging of a Liquid by a Moving Plate. Acta Physicochimica U.R.S.S. 1942, XVIII. (59) Doumenc, F.; Guerrier, B. Drying of a Solution in a Meniscus: A Model Coupling the Liquid and the Gas Phases. Langmuir 2010, 26, 13959-13967. (60) Hu, H.; Larson, R. G. Marangoni Effect Reverses Coffee-Ring Depositions. J. Phys. Chem. B 2006, 110, 7090-7094. (61) Fanton, X.; Cazabat, A. M.; Quéré, D. Thickness and Shape of Films Driven by a Marangoni Flow. Langmuir 1996, 12, 5875-5880. (62) Hoffman, J. D.; Lauritzen, J. I.; Passaglia, E.; Ross, G. S.; Frolen, L. J.; Weeks, J. J. Kinetics of Polymer Crystallization from Solution and the Melt. Kolloid-Zeitschrift und Zeitschrift für Polymere 1969, 231, 564-592. (63) Keller, A.; Hikosaka, M.; Rastogi, S.; Toda, A.; Barham, P. J.; Goldbeck-Wood, G. An Approach to the Formation and Growth of New Phases with Application to Polymer Crystallization: Effect of Finite Size, Metastability, and Ostwald's Rule of Stages. J. Mater. Sci. 1994, 29, 2579-2604. (64) McHugh, A. J. Flow-Induced Crystallization from Solution: The Relative Effects of Extension and Shearing Flow Fields. J. Appl. Polym. Sci. 1975, 19, 125-140. (65) McHugh, A. J.; Spevacek, J. A. The Kinetics of Flow-Induced Crystallization from Solution. J. Polym. Sci. Part B Polym. Phys. 1991, 29, 969-979. (66) Pennings, A. J.; Mark, J. M. A. A.; Kiel, A. M. Hydrodynamically Induced Crystallization of Polymers from Solution. Kolloid-Zeitschrift und Zeitschrift für Polymere 1970, 237, 336358. (67) Welch, P.; Muthukumar, M. Molecular Mechanisms of Polymer Crystallization from Solution. Phys. Rev. Lett. 2001, 87, 218302. (68) Rogowski, R. Z.; Darhuber, A. A. Crystal Growth near Moving Contact Lines on Homogeneous and Chemically Patterned Surfaces. Langmuir 2010, 26, 11485-11493. (69) Luo, C.; Kyaw, A. K. K.; Perez, L. A.; Patel, S.; Wang, M.; Grimm, B.; Bazan, G. C.; Kramer, E. J.; Heeger, A. J. General Strategy for Self-Assembly of Highly Oriented



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Nanocrystalline Semiconducting Polymers with High Mobility. Nano Lett. 2014, 14, 27642771. (70) Tseng, H.-R.; Ying, L.; Hsu, B. B. Y.; Perez, L. A.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. High Mobility Field Effect Transistors Based on Macroscopically Oriented Regioregular Copolymers. Nano Lett. 2012, 12, 6353-6357. (71) Jimison, L. H.; Himmelberger, S.; Duong, D. T.; Rivnay, J.; Toney, M. F.; Salleo, A. Vertical Confinement and Interface Effects on the Microstructure and Charge Transport of P3ht Thin Films. J. Polym. Sci. Part B Polym. Phys. 2013, 51, 611-620. (72) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G. J.; Wienk, M. M.; Turbiez, M.; de Leeuw, D. M.; Janssen, R. A. J. Poly(Diketopyrrolopyrrole−Terthiophene) for Ambipolar Logic and Photovoltaics. J. Am. Chem. Soc. 2009, 131, 16616-16617.


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Table of Contents Graphic



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UV-vis absorption spectra for PDPP3T films sheared at 0.05, 0.2, and 0.6 mm s-1 polarized in the direction parallel (dark blue) and perpendicular (light blue) to the shearing direction. The scaled, dilute solution-phase spectrum at room temperature is shown as a black dashed line in each plot. As shearing speed increases, the absorption in perpendicular direction significantly decreases relative to that in the parallel direction. (top left) PDPP3T and a schematic of the solution shearing process. 95x73mm (300 x 300 DPI)

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Optical dichroic ratio, film thickness, and shear rate as a function of shear speed for films sheared from tetralin on untreated glass. The dichroic ratios (bottom) peak at a critical shear speed of 0.2 mm s-1 before dropping back down to 1 at higher speeds. The film thicknesses (top) measured from profilometry and AFM, and the fitted curves (red for the evaporative regime, purple for the Landau-Levich regime, both separated by a vertical dashed line) are shown for comparison. On the same plot is the average shear rate across through the solution cross-section leaving the blade (black dashed line). While the solution also experiences high shear rates in the Landau-Levich regime (grey short-dashed line), the polymer relaxes before fixing its orientation in the dry film. 273x266mm (300 x 300 DPI)



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Cross-polarized optical micrographs (left) and AFM height maps (right) of films sheared at selected speeds onto OTS-treated substrates. Both the optical micrographs and the AFM images show no significant morphological features indicating obvious alignment, especially at the higher shear speeds. The shearing direction is vertically upwards in all images. 95x165mm (300 x 300 DPI)


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Grazing-incidence X-ray diffraction images of films sheared onto OTS-treated SiO2/Si substrates with the incident beam oriented parallel (left) and perpendicular (right) to the shearing direction. All images were scaled for exposure time and illuminated volume to provide a qualitative comparison across different samples. The films at higher speeds are more crystalline, and the intensity difference between the two orientations further shows strong structural anisotropy. 97x152mm (300 x 300 DPI)



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Average hole mobilities extracted from bottom-gate, top-contact field-effect transistors operating in saturation mode. The modest increase in mobilities comes partly from the reduction in active layer thickness, which reduces contact resistance. The relative isotropy between transistors whose charge transport direction is parallel and perpendicular to the shearing direction suggests that the channel length used (50 µm) is too large to probe the intrinsic charge transport anisotropy for the different crystallographic directions of the polymer. The error bars indicate the mobility values one standard deviation above and below the mean, which was obtained from 3 to 4 devices for each direction. 258x201mm (300 x 300 DPI)


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The effect of temperature on the dichroic ratio of films sheared on untreated glass. The critical shear speed shifts higher as temperature is increased, but the maximum dichroic ratio does not monotonically increase with the higher shear strains accessible at higher shear speeds because of the influence of increased nucleation. The highest observed dichroic ratio occurred for an intermediate temperature at around 85% of the solvent’s boiling point (140°C, or 480 K). Dashed curves are drawn only to guide the eye. The black dashed line indicates isotropy. 273x208mm (300 x 300 DPI)



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The effect of side-chain length on the dichroic ratio of films sheared on untreated glass at 140°C. The dichroic ratio function for PDPP3T-10/12 reaches a maximum of ~5 before plateauing, which contrasts with the maximum of ~7 and subsequent drop-off for PDPP3T. The greater entropic difficulty of efficiently packing longer alkyl side-chains likely frustrates the polymer alignment process. The black dashed line indicates isotropy. 273x208mm (300 x 300 DPI)


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