Nucleation, Growth, and Alignment of Poly(3-hexylthiophene

Article pubs.acs.org/accounts

Nucleation, Growth, and Alignment of Poly(3-hexylthiophene) Nanofibers for High-Performance OFETs Nils E. Persson,† Ping-Hsun Chu,† Michael McBride,† Martha Grover,† and Elsa Reichmanis*,†,‡,§ †

School of Chemical & Biomolecular Engineering, ‡School of Chemistry & Biochemistry, and §School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

CONSPECTUS: Conjugated semiconducting polymers have been the subject of intense study for over two decades with promising advances toward a printable electronics manufacturing ecosystem. These materials will deliver functional electronic devices that are lightweight, flexible, large-area, and cost-effective, with applications ranging from biomedical sensors to solar cells. Synthesis of novel molecules has led to significant improvements in charge carrier mobility, a defining electrical performance metric for many applications. However, the solution processing and thin film deposition of conjugated polymers must also be properly controlled to obtain reproducible device performance. This has led to an abundance of research on the process− structure−property relationships governing the microstructural evolution of the model semicrystalline poly(3-hexylthiophene) (P3HT) as applied to organic field effect transistor (OFET) fabrication. What followed was the production of an expansive body of work on the crystallization, self-assembly, and charge transport behavior of this semiflexible polymer whose strong π−π stacking interactions allow for highly creative methods of structural control, including the modulation of solvent and solution properties, flow-induced crystallization and alignment techniques, structural templating, and solid-state thermal and mechanical processing. This Account relates recent progress in the microstructural control of P3HT thin films through the nucleation, growth, and alignment of P3HT nanofibers. Solution-based nanofiber formation allows one to develop structural order prior to thin film deposition, mitigating the need for intricate deposition processes and enabling the use of batch and continuous chemical processing steps. Fiber growth is framed as a traditional crystallization problem, with the balance between nucleation and growth rates determining the fiber size and ultimately the distribution of grain boundaries in the solid state. Control of nucleation can be accomplished through a sonication-based seeding procedure, while growth can be modulated through supersaturation control via the tuning of solvent quality, the use of UV irradiation or through aging. These principles carry over to the flow-induced growth of P3HT nanofibers in a continuous microfluidic processing system, leading to thin films with significantly enhanced mobility. Further gains can be made by promoting long-range polymer chain alignment, achieved by depositing nanofibers through shearbased coating methods that promote high fiber packing density and alignment. All of these developments in processing were carried out on a standard OFET platform, enabling us to generalize quantitative structure−property relationships from structural data sources such as UV−vis, AFM, and GIWAXS. It is shown that a linear correlation exists between mobility and the in-plane orientational order of nanofibers, as extracted from AFM images using advanced computer vision software developed by our group. Herein, we discuss data-driven approaches to the determination of process−structure−property relationships, as well as the transferability of structural control strategies for P3HT to other conjugated polymer systems and applications.

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photovoltaics,5 biomedical devices6 and sensors.7 Considerable efforts were placed on the design and synthesis of new, often complex molecular structures that might afford enhanced

INTRODUCTION

Since the late 1980s, when the semiconducting properties of soluble polyalkylthiophenes were first reported,1,2 interest has surged in active polymer materials for lightweight, flexible and solution-processable devices including organic field effect transistors (OFETs),3 organic light-emitting diodes,4 organic © 2017 American Chemical Society

Received: December 22, 2016 Published: February 24, 2017 932

DOI: 10.1021/acs.accounts.6b00639 Acc. Chem. Res. 2017, 50, 932−942

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Accounts of Chemical Research semiconducting performance.3,8 Today, both hole and electron transport conjugated polymers have been developed and their charge carrier mobility, a property crucial to the switching frequency attainable in transistors, can reach as high as 10 cm2/ (V s).9−12 While still 2 orders of magnitude below doped crystalline silicon, this value places conjugated polymers in direct competition with amorphous silicon, bringing them to the brink of commercialization in flexible displays and organic photovoltaic devices.13−15 Once a candidate polymer has been synthesized, process protocols must be developed to enable applications. Precise control of a material’s electrical performance depends on device architecture decisions as well as its microstructure on length scales from the intramolecular to the device scale. This challenge is no better represented than by the design of OFETs using P3HT, the “fruit fly” of conjugated polymers, as their active layer material. In this system, reported field effect mobilities range from 10−6 to 10−1 cm2/(V s),16 while it is theorized that the intrinsic intracrystalline mobility of polythiophenes could be over 1 cm2/(V s).17 Assuming a standard device architecture has been selected, numerous solution processing strategies exist to obtain thin film microstructures commensurate with high field effect mobility. These strategies can be separated into three categories: (1) solution-state preprocessing, (2) controlled deposition methods, and (3) solid-state postprocessing. Solution-state preprocessing leverages the ability of some conjugated polymers to organize into semicrystalline π−π-stacked nanofibers directly in solution.18,19 Controlled deposition methods utilize a combination of shear-aligning coating techniques and slow solvent evaporation to obtain ordered microstructures.20,21 Postprocessing in the solid state can involve methods ranging from thermal annealing to mechanical deformation.22,23 This Account relates our recent progress in solution-state preprocessing and controlled deposition techniques as applied to the fabrication of P3HT-based OFETs. To achieve a thin film microstructure yielding high mobility, one must obtain a highly interconnected network of aligned crystalline material with few electronic trap states. Solution-state self-assembly of nanofibers allows one to form as much of that structural order as possible prior to deposition, enabling the use of highly scalable continuous flow process architectures, and mitigating the need for postdeposition heat treatment or mechanical processes. Self-assembly of P3HT is framed as a crystallization problem here, encompassing the control of nucleation, growth, and alignment. Nucleation can be initiated through regimens such as sonication or cooling. Growth of nanofibers can be controlled by varying supersaturation through a tunable solvent, using UV irradiation, or through flow-induced crystallization methods. Alignment of the resulting nanofibers arises from physical ties formed between fibers and the shear forces acting during deposition. Successful implementation of these techniques in combination with one another yields highly crystalline thin films with long-range alignment, producing some of the highest values of field effect mobility observed for P3HT. Herein, we review facile techniques for the characterization of P3HT nanofibers’ structure, experimental methods for the control of P3HT nanofiber nucleation, growth, and alignment, and general trends in the performance of transistors fabricated using these methods. Finally, we discuss the applicability of these methods to more diverse applications of P3HT and its structural analogues, as well as future challenges

in the quantification of conjugated polymers’ process− structure−property relationships.

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CHARACTERIZATION OF THE P3HT NANOFIBER IN SOLUTION AND THIN FILMS Understanding the assembly of P3HT nanofibers must start with its idealized crystal lattice. The P3HT nanofiber has been extensively characterized experimentally and computationally, with structural metrics of note illustrated in Figure 1. The π−π

Figure 1. Semicrystalline packing behavior of P3HT, lattice and repeat unit spacings, and quantities that determine fiber length and width.

stacking (010) axis forms a fiber’s long backbone, and with a dspacing of approximately 0.38 nm, a fiber only 1 μm in length can be comprised of thousands of polymer chains along this axis.24−26 The polymer backbone runs along the width of a fiber and the repeat unit spacing is also reported as 0.38 nm. Verilhac et al. found that increasing polymer molecular weight led to correlated increases in fiber width, but only up to a width of 20 nm, corresponding to a chain with a degree of polymerization of 53 and molecular weight of 8.7 kDa. Beyond this chain length, fiber width increased more slowly and plateaued at 30 nm,24,27 indicating that longer chains tend to fold into adjacent layers in the (100) axis.28 The (100) axis corresponds to lamellar stacking and has a reported lattice spacing of 1.6−1.8 nm, depending on the assumed hexyl side chain packing motif.29 Due to the low interchain electronic transfer integral in this direction it has limited relevance as a charge transport pathway, but this axis is frequently used to calculate Herman’s orientation factor, with a value of 1 indicating a fully edge-on orientation, favorable to charge transport in OFETs.30−32 Characterization of conjugated polymer thin films has progressed remarkably in sophistication and standardization. While recent studies have rightly increased focus on the semiconductor/dielectric interface largely responsible for charge transport, the bulk and surface characterization methods laid out in Figure 2 have nonetheless yielded valuable knowledge on the process-structure and structure−property relationships for P3HT. The aggregation of P3HT has a wellcharacterized effect on its UV−vis absorbance spectrum in both the solution and solid state, as depicted in Figure 2a. The percentage of P3HT involved in aggregates is directly correlated with the percentage of the spectrum accounted for by a fitted Franck−Condon progression, with amorphous chains making up the balance.33 The ratio of the 0−0 vibronic transition peak (ca. 610 nm) to its neighboring 0−1 peak is used to calculate a sample’s exciton bandwidth. A stronger 0−0 933


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Figure 2. Summary of common structural characterization of thin films of P3HT. (a) UV−vis spectra analyzed using a Franck−Condon fit to extract percent aggregates (shaded area) and exciton bandwidth (related to the ratio of the lowest energy peaks). (b) AFM images of the surface can be analyzed to extract mesoscale fiber packing and orientational order. (c) XRD peak analysis yields information on d-spacing and grain size.

size of each axis using the full-width-half-max of each peak, and Herman’s orientation factor quantifies the alignment of a given axis with the substrate normal vector by integrating an angular cut of the GIWAXS spectrum. The reader is directed to Rivnay et al. for a more detailed explanation of thin film characterization techniques.32 The combination of UV−vis, AFM, and X-ray data offer a comprehensive characterization of the microstructure of P3HT thin films and are used throughout this account to link the effects of solution processing to the developed structures and electrical properties of fabricated OFET devices.

peak yields a lower exciton bandwidth, which indicates a higher conjugation length and increased planarization of the P3HT backbone, as well as a strengthening of J-aggregate behavior relative to H-aggregate.34 This can also be interpreted as intramolecular electronic interactions becoming stronger relative to intermolecular interactions. Major differences in both percent aggregates and exciton bandwidth can be observed in solutions and films. The long-range packing behavior and morphology of P3HT nanofibers can be observed in atomic force microscopy (AFM) images, which provide information about their assembly mechanisms as well as the aligning effects of various deposition methods. To quantify the orientational order of the fibrillar morphologies observed via AFM, an automated image analysis protocol was developed to extract fiber backbones and their orientations, illustrated in Figure 2b.35 Two structural parameters provided a highly descriptive feature space: Sfull, the full-image value of the orientational order parameter S2D, and λC, the length scale of the decay of S2D across the image. These parameters can be used to analyze the extent of local fiber alignment and bundling, which can be induced by solution-based phenomena and by shear forces during deposition. Fiber length, width, and packing density can also be measured directly from AFM. Finally, grazing incidence wide-angle X-ray scattering (GIWAXS) can be used to analyze the spacing, size and texture of crystalline grains in the bulk of the thin film. The earlier studies covered by this account utilized grazing incidence X-ray diffraction (GIXRD), which is a line cut subset of GIWAXS data. Bragg’s Law is used to extract the d-spacing of each axis from in-plane or out-of-plane line cuts, as illustrated in Figure 2c. Scherrer’s equation allows estimation of the grain

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NUCLEATION AND GROWTH OF P3HT NANOFIBERS

An aggregating polymer solution undergoes a nucleation and growth process, similar to other crystallizing systems.30 Control of crystallization is traditionally accomplished by controlling the solution’s supersaturation, a driving force for both the nucleation and growth of the crystalline phase. Reducing temperature or reducing solvent quality both cause increases in supersaturation. Adding preformed nuclei to the solution, or seeding, is another common control strategy, intended to replace the sometimes-unpredictable nucleation process. Nucleation and growth both compete for supersaturated material, so at constant supersaturation, more nucleation leads to smaller crystals, while more growth leads to larger crystals. It is shown here that these strategies have analogues in the control of P3HT nanofiber crystallization, and that a balance must be struck between nucleation and growth to obtain thin films with optimal electrical properties. 934


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Accounts of Chemical Research Sonication

attributed to an activated nucleation and growth mechanism, illustrated in Figure 3i: at 0% 2-MP, the activation energy for the formation of stable nanofiber nuclei is high enough to prevent nucleation on short time scales. The addition of a nonsolvent lowers this activation barrier enough that a minimal number of nuclei form. Since the fibers reached a length of over 1 μm (≈2600 P3HT chains), it is apparent that the fibrillar phase is energetically favorable if provided stable nuclei as growth fronts. Sonication of the solutions of P3HT in chloroform and 2-MP resulted in the morphologies depicted in Figure 3g and h. For the solution with 0% 2-MP, the result of sonication was simply the formation of many short fibers. In the case of 15% 2-MP, many fibers of substantial length were formed. The energetics are illustrated again in Figure 3i: in both cases, sonication provides the energy to surmount the activation barrier to nucleation. However, with 15% 2-MP, the increased supersaturation provides more P3HT chains for crystal growth, resulting in longer fibers. It should be noted that the (100) grain size and field effect mobility of the thin films peaked at 15% 2-MP and 2 min sonication, suggesting that this process point produced nanofibers with optimal crystallinity and morphology. A remarkably similar situation arose when sonication was combined with UV irradiation as a solution preprocessing regimen.37 UV irradiation was shown in a previous study to effectively induce both nucleation and growth of elongated P3HT nanofibers within 8 min of irradiation time.38 Similar to nonsolvent addition without sonication (Figure 3b), UV irradiation alone produced long fibers (>2 μm), but sonication followed by UV produced a high density of medium-length fibers (≈ 1 μm), comparable to sonication +15% 2-MP (Figure 3e). In both the sonication + 2-MP and sonication + UV studies, an intermediate amount of seeding via sonication yielded the film with the highest charge carrier mobility; 2 min sonication plus either 15% 2-MP or 6 min UV irradiation were commensurate with maximum mobility. Too much sonication creates an excess of fibers, resulting in many grain boundaries in the thin film. Too little sonication likely results in insufficient crystalline material to cross the percolation threshold necessary for macroscale charge transport.39

Sonication was initially demonstrated as an effective seeding method for the nucleation of P3HT nanofibers. Aiyar et al. found that micrometer-size assemblies could be formed in regioregular P3HT solutions through the application of low intensity ultrasound.18 The 0−0 vibronic transition peak in solution and film UV−vis absorbance spectra increased in intensity as sonication time was increased from 0 to 10 min, indicative of increased conjugation length. Similarly, the intensity of the (100) peak from XRD profiles increased monotonically with sonication time. AFM images revealed the presence of elongated fibrillar objects embedded in the amorphous phase. Sonication was combined with nonsolvent addition to study the interplay of seeded nucleation with supersaturation-driven crystallization.36 Nonsolvent 2-methylpentane (2-MP) was added to solutions of P3HT and chloroform at 0 to 40% (v/ v), and fibrillar morphology after spin coating was tracked via AFM phase imaging with and without subsequent sonication of the solutions, presented in Figure 3. Consider the morphologies observed in Figure 3c and d, in which no sonication was applied. In the film spin-cast from P3HT in chloroform with no treatment, fibers were absent. Upon addition of 15% 2-MP, a sparse population of long fibers appeared. This behavior can be

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ALIGNMENT OF P3HT NANOFIBERS It is widely recognized in conjugated polymer processing that long-range alignment of polymer chains leads to enhanced charge transport.23,40 This has been accomplished using external mechanical stimuli and directional crystallization techniques,41,42 but solution-based assembly of nanofibers offers a potentially faster and more scalable route to longrange polymer alignment. Since nanofibers are self-contained domains of aligned polymer chains, aligning nanofibers in a thin film essentially accomplishes the goal of aligning the polymer chains. Here, we examine various solution processing and deposition strategies that lead to nanofiber alignment, as well as their underlying mechanisms.

Figure 3. (a,b) Schematic of solution behavior with only 2-MP addition. Blue polymer chains indicate nuclei. (c,d) AFM tapping mode phase images (2 × 2 μm) of fibrillar morphologies observed from the solutions in (a) and (b). (e,f) Schematic of solution behavior with sonication and 2-MP addition. (g,h) AFM images of films cast from the solutions in (e) and (f). (i) Schematic of the energetics of P3HT crystallization, with the x-axis indicating the crystal radius, r, and the y-axis indicating the Gibbs free energy change of the system, ΔG. Increased nonsolvent concentration lowers the activation barrier for the formation of a stable nucleus. Sonication provides the energy to cross this barrier.

Flow-Aligned Crystallization

The solution processing methods laid out in previous sections utilized batch-to-batch processing, which is potentially inefficient for industrial-scale production. A continuous processing strategy to induce oriented polymer aggregation under flow was demonstrated as a solution to this problem. Solutions of P3HT in chloroform were introduced into a 935


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Accounts of Chemical Research microfluidic system that contained a flow-cooling-enhanced nucleation step and a UV-assisted growth step, illustrated in Figure 4a.19 With constant residence times in each step, the

solution over days to weeks.46−48 Kleinhenz conducted a series of studies demonstrating that long-term aging of P3HT leads to observable increases in both the solution and solid-state orientational order. Aged solutions of P3HT loaded into capillaries possessed long-range birefringence, as evaluated by polarized optical microscopy, and increasing orientational order, as evaluated by polarized micro-Raman spectroscopy. Orientational order rose in concert with the percent aggregates in solution.49 In a follow-up study, sonicated and nonsonicated solutions of P3HT in chloroform were aged alongside each other for 8 days and deposited on OFET substrates to study their structural and electrical properties.50 For nonsonicated solutions, birefringence was observed in solution and in the thin film as early as 24 h, but sonicated solutions took up to 4 days to develop long-range order. After 4 days of aging, however, the sonicated solutions yielded spin-cast thin films with superior structural and electrical properties: they displayed a higher percent aggregates in solution, lower thin film exciton bandwidth, greater edge-on orientation, and higher field effect mobility. It was hypothesized that the disentangling effect of sonication led to the formation of more defect-free crystals.51 More evidence for solution-based fiber bundling emerged in the aging study as well. Figure 5a shows a high resolution 10

Figure 4. (a) Experimental diagram of the microfluidic-cooling-UV continuous processing system.19 Adapted with permission from ref 19. Copyright 2015 American Chemical Society. (b) AFM phase image of thin film from microfluidic-cooling-UV processed solution. Fibers were extracted and their orientations labeled using GTFiber [gtfiber.github. io].35

flow rate was shown to modulate nanofiber growth and alignment. An intermediate flow rate of 0.25 m/s yielded optimal values of mobility, percent aggregates in solution, film exciton bandwidth, edge-on orientation, and long-range fiber alignment. Both higher and lower flow rates yielded suboptimal results. Increasing the shear rate in a polymer solution has been shown to increase the formation of activated nuclei by favoring polymers’ stretched conformation, aligning polymers along the axis of flow, and increasing the frequency of their collisions.43,44 It is likely that the microfluidic crystallization system followed the same principles as the supersaturation-driven crystallization model introduced in Figure 3, with a moderate nucleation rate yielding optimal results. The mesoscale morphology of the optimal microfluidic process point displayed a previously unseen level of orientational order, presented in Figure 4b alongside its colored orientation map. This was especially noteworthy because the thin film was deposited via spin coating, a high shear rate technique that previously led to more disordered morphologies. Tie chains between neighboring nanofibers could have formed as growth proceeded in the aligning laminar flow (Figure 4a), providing physical bonds between fibers that held them in alignment during spin coating. Alternatively, the shish-kebab nucleation mechanism commonly observed during the flow crystallization of polymers could have been responsible for interfiber connections and increased local alignment.21,45

Figure 5. (a) High resolution 10 μm AFM phase image of a thin film deposited from a solution of P3HT in chloroform that was aged for 48 h. (b) Orientation map produced using GTFiber in which fibers have been extracted and their orientations labeled; (inset) orientation distribution of fiber segments on a polar plot, with the calculated value of Sfull.

μm AFM image taken from a thin film deposited from a solution that was aged for 48 h with no sonication. Figure 5b shows the corresponding orientation map with accompanying orientation distribution.35 While the overall morphology does not possess a strong preferential orientation as per the low value of Sfull, fibers are clearly packed in tight local bundles. Any given fiber is accompanied by at least five or six neighboring fibers of similar orientation, yet there are stark boundaries between these locally oriented domains. It is possible that this strong level of local orientational order was induced by the formation of physical connections between fibers during the aging process. The aligning effects of the spin coating, however, cannot be ignored. Analysis of 10 μm images surrounding the center of a spin-cast film indicated that the centrifugal flow field biases fiber orientations radially, which accounts for the slight overall bias toward 60° in the orientation distribution of Figure 5b.35 A similar effect was observed by Bielecka et al. with solutions of fibers aged for 1 week.46

Ordering of Fibers in Solution

One of the simplest reported methods to grow nanofibers of P3HT and other conjugated polymers is simply to age them in 936


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Figure 6. (a−d) AFM tapping mode phase images (4 μm) of thin film morphology resulting from slide coating solutions of UV-irradiated P3HT nanofibers at various coating velocities. (e) Image order parameter Sfull as a function of coating velocity for images (a)−(d). (f) Image order parameter Sfull as a function of solution aging time in images (g)−(j). (g−j) AFM phase images (5 μm) of thin film morphology resulting from blade coating solutions of P3HT in which nanofibers were formed through UV irradiation and aging. Coating direction was horizontal in all images.

Table 1. Top Performing Devices in the Studies Reviewed in This Account author

year

57

Chu et al. Chang et al.55 Wang et al.19

2016 2016 2015

Kleinhenz et al.50

2016

Chang et al.37

2014

Choi et al.36

2014

Aiyar et al.18

2011

annealing time/ temp (°C)/ vacuum

chan. len. (μm)e

transfer curve model

μ (cm2/(V s))

blade slide spin

12 h/25/Vac 10 min/120 12 h/25/vac

50 50 50

saturation linear saturation

0.2 0.2 0.16

5

spin

12 h/50/vac

50

saturation

0.15

CHCl3

5

spin

12 h/25/vac

50

saturation

0.12

CHCl3/ 2-MPf CHCl3

5

spin

12 h/50/vac

50

linear

0.1

3.5

spin

50

saturation

0.03

parameters studied

Mn (kD)

PDI

aging blade velocity microfluidic flow rate sonication, aging sonication, UV sonication, solvent sonication

41 41 20

2.2 2.3 2.2

96 96 96

CHCl3 CHCl3 CHCl3

5 5 5

32

2.2

96

CHCl3

20

2.2

96

41

2.3

96

31

2.0

98

a

RR (%)b

solvent

init. concn (mg/mL)c

deposition

d

a

Polydispersity Index. bRegioregularity. cInitial concentration of polymer solution. dDeposition method: Spin = spin coated; slide = slide coated; blade = blade coated. eChannel length. f2-Methylpentane.

Alignment through Blade Coating

irradiation technique. The degree of alignment was tunable by the coating velocity, as illustrated in Figure 6a−e.55 Exciton bandwidth, (100) d-spacing, fiber alignment, and mobility were optimized at intermediate coating rates between 1 and 2 mm/s. The coating velocity likely modulated the film thickness as well, which can play an important role in the measured charge transport.56 A similar study by Chu et al. demonstrated that mobility of up to 0.2 cm2/(V s) could be obtained by combining UV irradiation, aging, and blade coating at 3 mm/s.57 The evolution

Shear-coating with micropatterned blades has received a fair amount of attention as a mechanism to align polymer chains.52−54 However, when coating solutions of preformed nanofibers, simpler blade designs can be used to achieve macroscale alignment and enhanced charge transport. Chang et al. obtained aligned films of P3HT and poly(3-butylthiophene) nanofibers by using a glass coverslip as the shearing device, and growing nanofibers using the previously described UV 937


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Accounts of Chemical Research of morphology and alignment are shown in Figure 6f−j. As aging proceeded, alignment increased, as measured by Sfull from AFM images and the dichroic ratio of the 0−0 transition peak from UV−vis spectra. It is possible that as fibers increased in length and number, entropic constraints forced them into greater alignment. Interestingly, anisotropic mobility was observed when measured parallel and perpendicular to the coating direction. Mobility was higher in the perpendicular direction, indicating that paths along P3HT chain backbones were more favorable than along π−π stacked fibers. The ability to precisely control fiber alignment should enable new developments in the theory of fibers under shear flow as well as charge transport in semicrystalline thin films.58,59

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RATIONAL DESIGN OF OFETs Thus, far, we have focused primarily on the process-structure relationships governing P3HT nanofiber formation and deposition. These experimental efforts were largely aimed at achieving higher field effect mobility. However, many factors influence the measured mobility in a given OFET beyond the thin film microstructure of P3HT. In examining the structure− property trends observed in our OFET fabrication efforts, it is important to provide context on the processing and device architecture decisions that were made. Table 1 compiles the process and performance information for the highest mobility devices from each of the studies reviewed here. We can infer that differences in electrical performance are due largely to differences in the thin film microstructure because our other processing decisions have remained largely consistent. All device substrates are bottom-gate, bottomcontact with an unmodified 300 nm SiO2 dielectric, gold source and drain electrodes and a 50 μm channel length and 2000 μm channel width. Spin coating was performed in air at 1500 rpm for 60 s, with the exception of sonication + aging, for which 800 rpm and 30 s were used. Saturation regime transfer curve models were used in most cases. Almost every study included an overnight vacuum step for solvent removal at either room temperature or 50 °C, while the slide coating study utilized a brief period of thermal annealing. It is important to note that the number-average molecular weight (Mn) of P3HT used varied between 20 and 41 kDa. While early studies indicated a strong dependence between molecular weight and mobility,60 this was due in large part to the lack of intergrain connectivity in samples with Mn < 20 kDa.61−63 Our devices fall into the regime of interconnected aggregates, thus the differing molecular weights have a more limited impact. These are simply factors to keep in mind while comparing the performance of the processes under examination. It is still very difficult to generalize structure−property relationships across studies. Characterization methods change, analytical models are refined, and raw data becomes buried in complex published figures. Among all of the structural data sources introduced in Figure 2, however, the most consistently collected and published was AFM. AFM images are also presented in a format not far removed from their raw form, enabling us to retrieve them and perform cross-cutting historical analysis. Figure 7 demonstrates a general, linear correlation that arises between mobility and fiber alignment (Sfull) when a standardized analysis procedure is applied across a collection of our group’s AFM images using GTFiber software.35 The limitations of this relationship should be noted: these are images of the thin film surface, rather than the buried semiconductor/dielectric interface, they have been collected at

Figure 7. General correlation between mobility and Sfull, the imagebased order parameter for in-plane fiber alignment. Data from each study is represented by colors indicated in the legend.

a variety of length scales, and the underlying crystallinity of the fibers may differ. Nonetheless, it is an informative relationship that reflects the progress our group has made in controlling the crystallization and alignment of P3HT nanofibers, and the positive impact that has had on device performance. In general, however, an outstanding value of a single structural parameter is a necessary but not sufficient criterion for high mobility. Performance will be maximized only when all structural properties are optimized, including a low thin film exciton bandwidth, large (100) grain size, and smaller d-spacing, Herman’s orientation factor indicating edge-on orientation, and a tightly packed and aligned fibrillar morphology.

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PERSPECTIVE AND OUTLOOK Poly(3-hexylthiophene) has played the role of the Drosophila melanogaster of semiconducting polymers to significant benefit across multiple fields. In this Account, we have detailed advances in our understanding and control of its solution-based self-assembly mechanisms, and the application of these strategies to the fabrication of high-performance OFET devices. Effective control of nucleation and growth can be accomplished through crystal seeding by sonication and supersaturation control by tuning solvent quality. However, the amount of nucleation must be limited, lest supersaturated P3HT is overconsumed by nuclei. UV irradiation also provides the necessary energy for expedited fiber growth. Once fiber formation is controlled, long-range alignment is desirable for further gains in charge carrier mobility. A flow crystallization system was demonstrated for the continuous production of aligned P3HT nanofibers, and simple aging of a sonicated P3HT solution can yield comparable results. Deposition-based alignment techniques such as slide coating and blade coating can further tune fiber alignment to achieve long-range polymer chain alignment, yielding field effect mobility in the upper echelon of that reported for P3HT. There is every indication that the theory and methods introduced in this article are extensible to the broader class of semicrystalline conjugated polymers beyond P3HT. Analogues and copolymers of P3HT have proliferated, especially in the design of donor species for organic photovoltaics. With the intention of increasing open circuit voltage, Bronstein et al. added thiazole units to polythiophenes, resulting in deeper 938



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HOMO levels.64,65 Heeney et al. synthesized a selenophenebased analogue with a reduced band gap but a HOMO level similar to P3HT,66 as well as a selenophene-diketopyrrolopyrrole copolymer displaying ambipolar performance in transistors.67 Work by the Thompson group includes the side-chain engineering of P3HT to control both open circuit voltage and surface energy.68,69 Some of these analogues have been shown to form the same fibrillar structures as P3HT,70,71 indicating that our nucleation, growth and alignment strategies could be beneficial to the control of charge transport in these polymers, as well as the control of morphology in organic photovoltaics. For example, the microfluidic crystallization technique was demonstrated on the electron transporting poly(NDI2OD-T2),19 and the slide coating technique was effective on poly(3-butylthiophene).55 Blending conjugated polymers with insulating polymers is another attractive strategy to reduce raw material usage while maintaining charge transport and improving barrier properties;72 as such, sonication, UV irradiation, and blade coating have been demonstrated on blends of P3HT with polystyrene 7 3 , 7 4 and poly(dimethylsiloxane)75 with similar aligning effects and improvements in charge transport. As process scale-up for organic electronics manufacturing becomes a stronger focus of research, precise control of electronic properties such as mobility will be necessary to meet tight performance specifications. This will require quantitative process−structure−property relationships that facilitate the design of process recipes to achieve targeted microstructures with a high degree of repeatability. Crucial questions to be answered include: What are the principal sources of variance in mobility for devices that were otherwise processed identically? In what stage of processing is the greatest amount of structural variability introduced, and can it be mitigated? Even for the emerging class of high-performance conjugated polymers with weaker π−π stacking interactions, long-range alignment is likely to induce charge transport anisotropy, which will be important to control in device applications.12 While the specific design rules of P3HT-based devices may not transfer directly to the next generation of conjugated polymers, what must be carried over are the standardized reporting practices and most reproducible lab-scale techniques.76 It is essential that transistor processing, structural and performance characterization data files be made available from each study so that results can be contextualized and crosscutting meta-analysis can be easily performed. Such a database would hold value for the modeling community and as inputs to informatics algorithms for future process design and optimization, as per the goals of the Materials Genome Initiative.77 The solution-based nanofiber growth and alignment strategies presented here represent a decidedly different paradigm than that of solid-state techniques such as heat treatment, surface modification and mechanical deformation. Solution-phase control of P3HT assembly is potentially transferrable to the growing number of applications beyond OFETs: P3HT has found use as a hole transport layer for perovskite solar cells,78 as well as in biomedical applications as a light-sensitive scaffold for cell growth and an interface material for cellular stimulation.79,80 A fundamental understanding of the crystallization and structural ordering mechanisms of P3HT will surely be of benefit for the growing number of fields that take advantage of the unique properties of this versatile polymer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Elsa Reichmanis: 0000-0002-8205-8016 Notes

The authors declare no competing financial interest. Biographies Nils E. Persson was born in Edina, Minnesota and grew up in nearby Minnetonka before moving to Minneapolis to pursue his B.S.E. in chemical engineering from the University of Minnesota. He is currently an Institute for Materials graduate fellow coadvised by Professor Martha Grover and Professor Elsa Reichmanis in Chemical & Biomolecular Engineering at Georgia Tech, with research interests in organic electronics, computer vision, and materials data science. Ping-Hsun Chu was born and grew up in Changhua, a city located in the middle of Taiwan. He graduated with B.S. degree in Chemical Engineering from Chuang Yuan Christian University and earned his M.S. degree in Chemical Engineering from National Taiwan University of Science and Technology. He is currently pursuing his Ph.D. degree in Chemical & Biomolecular Engineering at Georgia Tech. His research in the Reichmanis group is focused on conjugated polymer and organic electronics. Mike A. McBride was born in Gilbert, Arizona and attended the University of Arizona for his B.S. degree in chemical engineering. He is a current fellow in the NSF NESAC IGERT program and is coadvised by Professor Martha Grover and Professor Elsa Reichmanis. His research interests include organic electronics and energy policy. Martha A. Grover was born and raised in Belvidere, Illinois, and received her B.S. degree in mechanical engineering from the University of Illinois, Urbana−Champaign. She earned her M.S. and Ph.D. degrees from Caltech, before joining the faculty of Chemical & Biomolecular Engineering at Georgia Tech, where she is now a professor. Her research group uses systems engineering approaches to study supramolecular organization. Elsa Reichmanis was born in Melbourne, Australia and moved to Syracuse, New York when she was 8. She received her B.S. and Ph.D. degrees in Chemistry from Syracuse University, after which she joined Bell Laboratories in Murray Hill, New Jersey. In 2008, she joined the faculty of the School of Chemical & Biomolecular Engineering at Georgia Tech, where she is now Pete Silas Chair in Chemical Engineering and Brook Byers Professor of Sustainability. Her research interests include the chemistry, properties and application of materials technologies for photonic and electronic applications.

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ACKNOWLEDGMENTS All authors acknowledge support from NSF Grant No. 1264555: Morphology and Mobility Control for Functional Robust Flexible Electronics and Photovoltaics. N.E.P. also gratefully acknowledges financial support from the NSF FLAMEL IGERT Traineeship program, NSF Grant No. 1258425, IGERT-CIF21: Computation-Enabled Design and Manufacturing of High Performance Materials. M.M. acknowledges an NSF IGERT NESAC Traineeship, NSF Grant No. 1069138: Nanostructured Materials for Energy Storage and Conversion. 939


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(19) Wang, G.; Persson, N.; Chu, P.; Kleinhenz, N.; Fu, B.; Chang, M.; Deb, N.; Mao, Y.; Wang, H.; Grover, M. A.; Reichmanis, E. Microfluidic Crystal Engineering of π-Conjugated Polymers. ACS Nano 2015, 9, 8220−8230. (20) Shaw, L.; Hayoz, P.; Diao, Y.; Reinspach, J. A.; To, J. W. F.; Toney, M. F.; Weitz, R. T.; Bao, Z. Direct Uniaxial Alignment of a Donor−Acceptor Semiconducting Polymer Using Single-Step Solution Shearing. ACS Appl. Mater. Interfaces 2016, 8, 9285−9296. (21) Brinkmann, M.; Chandezon, F.; Pansu, R. B.; Julien-Rabant, C. Epitaxial Growth of Highly Oriented Fibers of Semiconducting Polymers with a Shish-Kebab-like Superstructure. Adv. Funct. Mater. 2009, 19, 2759−2766. (22) Cho, S.; Lee, K.; Yuen, J.; Wang, G.; Moses, D.; Heeger, A. J.; Surin, M.; Lazzaroni, R. Thermal Annealing-Induced Enhancement of the Field-Effect Mobility of Regioregular poly(3-Hexylthiophene) Films. J. Appl. Phys. 2006, 100, 114503. (23) 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. (24) Verilhac, J.-M.; LeBlevennec, G.; Djurado, D.; Rieutord, F.; Chouiki, M.; Travers, J.-P.; Pron, A. Effect of Macromolecular Parameters and Processing Conditions on Supramolecular Organisation, Morphology and Electrical Transport Properties in Thin Layers of Regioregular poly(3-Hexylthiophene). Synth. Met. 2006, 156, 815−823. (25) Dag, S.; Wang, L. Packing Structure of Poly(3-Hexylthiophene) Crystal: Ab Initio and Molecular Dynamics Studies. J. Phys. Chem. B 2010, 114, 5997−6000. (26) Cheung, D. L.; McMahon, D. P.; Troisi, A. Computational Study of the Structure and Charge-Transfer Parameters in LowMolecular-Mass P3HT. J. Phys. Chem. B 2009, 113, 9393−9401. (27) Zhang, R.; Li, B.; Iovu, M. C.; Jeffries-EL, M.; Sauvé, G.; Cooper, J.; Jia, S.; Tristram-Nagle, S.; Smilgies, D. M.; Lambeth, D. N.; McCullough, R. D.; Kowalewski, T. Nanostructure Dependence of Field-Effect Mobility in Regioregular Poly(3-Hexylthiophene) Thin Film Field Effect Transistors. J. Am. Chem. Soc. 2006, 128, 3480−3481. (28) Baghgar, M.; Labastide, J. A.; Bokel, F.; Hayward, R. C.; Barnes, M. D. Effect of Polymer Chain Folding on the Transition from H- to JAggregate Behavior in P3HT Nanofibers. J. Phys. Chem. C 2014, 118, 2229−2235. (29) Alexiadis, O.; Mavrantzas, V. G. All-Atom Molecular Dynamics Simulation of Temperature Effects on the Structural, Thermodynamic, and Packing Properties of the Pure Amorphous and Pure Crystalline Phases of Regioregular P3HT. Macromolecules 2013, 46, 2450−2467. (30) Lim, J. A.; Liu, F.; Ferdous, S.; Muthukumar, M.; Briseno, A. L. Polymer Semiconductor Crystals. Mater. Today 2010, 13, 14−24. (31) Salleo, A. Charge Transport in Polymeric Transistors. Mater. Today 2007, 10, 38−45. (32) Rivnay, J.; Mannsfeld, S. C. B.; Miller, C. E.; Salleo, A.; Toney, M. F. Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale. Chem. Rev. 2012, 112, 5488−5519. (33) Niles, E. T.; Roehling, J. D.; Yamagata, H.; Wise, A. J.; Spano, F. C.; Moulé, A. J.; Grey, J. K. J-Aggregate Behavior in Poly-3Hexylthiophene Nanofibers. J. Phys. Chem. Lett. 2012, 3, 259−263. (34) Clark, J.; Chang, J. F.; Spano, F. C.; Friend, R. H.; Silva, C. Determining Exciton Bandwidth and Film Microstructure in Polythiophene Films Using Linear Absorption Spectroscopy. Appl. Phys. Lett. 2009, 94, 163306. (35) Persson, N. E.; McBride, M. A.; Grover, M. A.; Reichmanis, E. Automated Analysis of Orientational Order in Images of Fibrillar Materials. Chem. Mater. 2017, 29, 3−14. (36) Choi, D.; Chang, M.; Reichmanis, E. Controlled Assembly of Poly(3-Hexylthiophene): Managing the Disorder to Order Transition on the Nano- through Meso-Scales. Adv. Funct. Mater. 2015, 25, 920− 927. (37) Chang, M.; Lee, J.; Chu, P.-H.; Choi, D.; Park, B.; Reichmanis, E. Anisotropic Assembly of Conjugated Polymer Nanocrystallites for

REFERENCES

(1) Rughooputh, S. D. D. V.; Nowak, M.; Hotta, S.; Heeger, A. J.; Wudl, F. Soluble Conducting Polymers: The poly(3-Alkylthienylenes). Synth. Met. 1987, 21, 41−50. (2) Assadi, A.; Svensson, C.; Willander, M.; Inganäs, O. Field-effect Mobility of poly(3-hexylthiophene). Appl. Phys. Lett. 1988, 53, 195− 197. (3) Holliday, S.; Donaghey, J. E.; McCulloch, I. Advances in Charge Carrier Mobilities of Semiconducting Polymers Used in Organic Transistors. Chem. Mater. 2014, 26, 647−663. (4) Sekine, C.; Tsubata, Y.; Yamada, T.; Kitano, M.; Doi, S. Recent Progress of High Performance Polymer OLED and OPV Materials for Organic Printed Electronics. Sci. Technol. Adv. Mater. 2014, 15, 034203. (5) Earmme, T.; Hwang, Y.-J.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A. All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor. J. Am. Chem. Soc. 2013, 135, 14960−14963. (6) Pu, K.; Shuhendler, A. J.; Jokerst, J. V.; Mei, J.; Gambhir, S. S.; Bao, Z.; Rao, J. Semiconducting Polymer Nanoparticles as Photoacoustic Molecular Imaging Probes in Living Mice. Nat. Nanotechnol. 2014, 9, 233−239. (7) Knopfmacher, O.; Hammock, M. L.; Appleton, A. L.; Schwartz, G.; Mei, J.; Lei, T.; Pei, J.; Bao, Z. Highly Stable Organic Polymer Field-Effect Transistor Sensor for Selective Detection in the Marine Environment. Nat. Commun. 2014, 5, 2954. (8) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Integrated Materials Design of Organic Semiconductors for Field-Effect Transistors. J. Am. Chem. Soc. 2013, 135, 6724−6746. (9) Bittle, E. G.; Basham, J. I.; Jackson, T. N.; Jurchescu, O. D.; Gundlach, D. J. Mobility Overestimation due to Gated Contacts in Organic Field-Effect Transistors. Nat. Commun. 2016, 7, 10908. (10) Venkateshvaran, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.; Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.; Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil, J.; Beljonne, D.; Sirringhaus, H. Approaching Disorder-Free Transport in High-Mobility Conjugated Polymers. Nature 2014, 515, 384−388. (11) Caironi, M.; Bird, M.; Fazzi, D.; Chen, Z.; Di Pietro, R.; Newman, C.; Facchetti, A.; Sirringhaus, H. Very Low Degree of Energetic Disorder as the Origin of High Mobility in an N-Channel Polymer Semiconductor. Adv. Funct. Mater. 2011, 21, 3371−3381. (12) 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. (13) Sirringhaus, H. 25th Anniversary Article: Organic Field-Effect Transistors: The Path Beyond Amorphous Silicon. Adv. Mater. 2014, 26, 1319−1335. (14) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153−161. (15) Krebs, F. C.; Hoesel, M.; Corazza, M.; Roth, B.; Madsen, M. V.; Gevorgyan, S. a.; Sondergaard, R. R.; Karg, D.; Jorgensen, M. Freely Available OPV-The Fast Way to Progress. Energy Technol. 2013, 1, 378−381. (16) Persson, N.; McBride, M.; Grover, M.; Reichmanis, E. Silicon Valley Meets the Ivory Tower: Searchable Data Repositories for Experimental Nanomaterials Research. Curr. Opin. Solid State Mater. Sci. 2016, 20, 338−343. (17) Salleo, A.; Chen, T. W.; Völkel, A. R.; Wu, Y.; Liu, P.; Ong, B. S.; Street, R. A. Intrinsic Hole Mobility and Trapping in a Regioregular Poly(thiophene). Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 115311. (18) Aiyar, A. R.; Hong, J.-I.; Izumi, J.; Choi, D.; Kleinhenz, N.; Reichmanis, E. Ultrasound-Induced Ordering in Poly(3-Hexylthiophene): Role of Molecular and Process Parameters on Morphology and Charge Transport. ACS Appl. Mater. Interfaces 2013, 5, 2368− 2377. 940


Article

Accounts of Chemical Research Enhanced Charge Transport. ACS Appl. Mater. Interfaces 2014, 6, 21541−21549. (38) Chang, M.; Lee, J.; Kleinhenz, N.; Fu, B.; Reichmanis, E. Photoinduced Anisotropic Supramolecular Assembly and Enhanced Charge Transport of Poly(3-Hexylthiophene) Thin Films. Adv. Funct. Mater. 2014, 24, 4457−4465. (39) Duong, D. T.; Ho, V.; Shang, Z.; Mollinger, S.; Mannsfeld, S. C. B.; Dacuña, J.; Toney, M. F.; Segalman, R.; Salleo, A. Mechanism of Crystallization and Implications for Charge Transport in Poly(3Ethylhexylthiophene) Thin Films. Adv. Funct. Mater. 2014, 24, 4515− 4521. (40) Brinkmann, M.; Hartmann, L.; Biniek, L.; Tremel, K.; Kayunkid, N. Orienting Semi-Conducting π-Conjugated Polymers. Macromol. Rapid Commun. 2014, 35, 9−26. (41) Skrypnychuk, V.; Wetzelaer, G.-J. A. H.; Gordiichuk, P. I.; Mannsfeld, S. C. B.; Herrmann, A.; Toney, M. F.; Barbero, D. R. Ultrahigh Mobility in an Organic Semiconductor by Vertical Chain Alignment. Adv. Mater. 2016, 28, 2359−2366. (42) Jimison, L. H.; Toney, M. F.; McCulloch, I.; Heeney, M.; Salleo, A. Charge-Transport Anisotropy Due to Grain Boundaries in Directionally Crystallized Thin Films of Regioregular Poly(3Hexylthiophene). Adv. Mater. 2009, 21, 1568−1572. (43) Koscher, E.; Fulchiron, R. Influence of Shear on Polypropylene Crystallization: Morphology Development and Kinetics. Polymer 2002, 43, 6931−6942. (44) Wie, J. J.; Nguyen, N. A.; Cwalina, C. D.; Liu, J.; Martin, D. C.; Mackay, M. E. Shear-Induced Solution Crystallization of Poly(3Hexylthiophene) (P3HT). Macromolecules 2014, 47, 3343−3349. (45) Kimata, S.; Sakurai, T.; Nozue, Y.; Kasahara, T.; Yamaguchi, N.; Karino, T.; Shibayama, M.; Kornfield, J. A. Molecular Basis of the Shish-Kebab Morphology in Polymer Crystallization. Science 2007, 316, 1014−1017. (46) Bielecka, U.; Lutsyk, P.; Janus, K.; Sworakowski, J.; Bartkowiak, W. Effect of Solution Aging on Morphology and Electrical Characteristics of Regioregular P3HT FETs Fabricated by Spin Coating and Spray Coating. Org. Electron. 2011, 12, 1768−1776. (47) Newbloom, G. M.; Kim, F. S.; Jenekhe, S. A.; Pozzo, D. C. Mesoscale Morphology and Charge Transport in Colloidal Networks of Poly(3-Hexylthiophene). Macromolecules 2011, 44, 3801−3809. (48) Chen, S. H.; Su, A. C.; Chang, C. S.; Chen, H. L.; Ho, D. L.; Tsao, C. S.; Peng, K. Y.; Chen, S. A. Aging of poly(2-Methoxy-5-(2′ethylhexyloxy)-1,4-Phenylenevinylene)/toluene Solutions and Subsequent Effects on Luminescence Behavior of Cast Films. Langmuir 2004, 20, 8909−8915. (49) Kleinhenz, N.; Rosu, C.; Chatterjee, S.; Chang, M.; Nayani, K.; Xue, Z.; Kim, E.; Middlebrooks, J.; Russo, P. S.; Park, J. O.; Srinivasarao, M.; Reichmanis, E. Liquid Crystalline Poly(3-Hexylthiophene) Solutions Revisited: Role of Time-Dependent Self-Assembly. Chem. Mater. 2015, 27, 2687−2694. (50) Kleinhenz, N.; Persson, N.; Xue, Z.; Chu, P. H.; Wang, G.; Yuan, Z.; McBride, M. A.; Choi, D.; Grover, M. A.; Reichmanis, E. Ordering of Poly(3-Hexylthiophene) in Solutions and Films: Effects of Fiber Length and Grain Boundaries on Anisotropy and Mobility. Chem. Mater. 2016, 28, 3905−3913. (51) Zhao, K.; Khan, H. U.; Li, R.; Su, Y.; Amassian, A. Entanglement of Conjugated Polymer Chains Influences Molecular Self-Assembly and Carrier Transport. Adv. Funct. Mater. 2013, 23, 6024−6035. (52) 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. (53) Wang, G.; Chu, P.-H.; Fu, B.; He, Z.; Kleinhenz, N.; Yuan, Z.; Mao, Y.; Wang, H.; Reichmanis, E. Conjugated Polymer Alignment: Synergisms Derived from Microfluidic Shear Design and UV Irradiation. ACS Appl. Mater. Interfaces 2016, 8, 24761−24772. (54) Qu, G.; Kwok, J. J.; Diao, Y. Flow-Directed Crystallization for Printed Electronics. Acc. Chem. Res. 2016, 49, 2756−2764.

(55) Chang, M.; Choi, D.; Egap, E. Macroscopic Alignment of OneDimensional Conjugated Polymer Nanocrystallites for High-Mobility Organic Field-Effect Transistors. ACS Appl. Mater. Interfaces 2016, 8, 13484−13491. (56) 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. (57) Chu, P.-H.; Kleinhenz, N.; Persson, N.; McBride, M.; Hernandez, J. L.; Fu, B.; Zhang, G.; Reichmanis, E. Toward Precision Control of Nanofiber Orientation in Conjugated Polymer Thin Films: Impact on Charge Transport. Chem. Mater. 2016, 28, 9099−9109. (58) Katz, E.; Yarin, A. L.; Salalha, W.; Zussman, E. Alignment and Self-Assembly of Elongated Micronsize Rods in Several Flow Fields. J. Appl. Phys. 2006, 100, 034313. (59) Kwiatkowski, J. J.; Jimison, L. H.; Salleo, A.; Spakowitz, A. J. A Boltzmann-Weighted Hopping Model of Charge Transport in Organic Semicrystalline Films. J. Appl. Phys. 2011, 109, 113720. (60) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney, M. F. Dependence of Regioregular Poly(3Hexylthiophene) Film Morphology and Field-Effect Mobility on Molecular Weight. Macromolecules 2005, 38, 3312−3319. (61) Himmelberger, S.; Vandewal, K.; Fei, Z.; Heeney, M.; Salleo, A. Role of Molecular Weight Distribution on Charge Transport in Semiconducting Polymers. Macromolecules 2014, 47, 7151−7157. (62) 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. (63) Koch, F. P. V.; Rivnay, J.; Foster, S.; Müller, C.; Downing, J. M.; Buchaca-Domingo, E.; Westacott, P.; Yu, L.; Yuan, M.; Baklar, M.; Fei, Z.; Luscombe, C.; McLachlan, M. A.; Heeney, M.; Rumbles, G.; Silva, C.; Salleo, A.; Nelson, J.; Smith, P.; Stingelin, N. The Impact of Molecular Weight on Microstructure and Charge Transport in Semicrystalline Polymer semiconductors−poly(3-Hexylthiophene), a Model Study. Prog. Polym. Sci. 2013, 38, 1978−1989. (64) Bronstein, H.; Hurhangee, M.; Fregoso, E. C.; Beatrup, D.; Soon, Y. W.; Huang, Z.; Hadipour, A.; Tuladhar, P. S.; Rossbauer, S.; Sohn, E.-H.; Shoaee, S.; Dimitrov, S. D.; Frost, J. M.; Ashraf, R. S.; Kirchartz, T.; Watkins, S. E.; Song, K.; Anthopoulos, T.; Nelson, J.; Rand, B. P.; Durrant, J. R.; McCulloch, I. Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-Hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells. Chem. Mater. 2013, 25, 4239−4249. (65) Wade, J.; Wood, S.; Beatrup, D.; Hurhangee, M.; Bronstein, H.; McCulloch, I.; Durrant, J. R.; Kim, J. Operational Electrochemical Stability of Thiophene-Thiazole Copolymers Probed by Resonant Raman Spectroscopy. J. Chem. Phys. 2015, 142, 244904. (66) Heeney, M.; Zhang, W.; Crouch, D. J.; Chabinyc, M. L.; Gordeyev, S.; Hamilton, R.; Higgins, S. J.; McCulloch, I.; Skabara, P. J.; Sparrowe, D.; Tierney, S. Regioregular poly(3-Hexyl)selenophene: A Low Band Gap Organic Hole Transporting Polymer. Chem. Commun. 2007, 5061−5063. (67) Shahid, M.; McCarthy-Ward, T.; Labram, J.; Rossbauer, S.; Domingo, E. B.; Watkins, S. E.; Stingelin, N.; Anthopoulos, T. D.; Heeney, M. Low Band Gap Selenophene−diketopyrrolopyrrolepolymers Exhibiting High and Balanced Ambipolar Performance in Bottom-Gate Transistors. Chem. Sci. 2012, 3, 181−185. (68) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Influence of the Ethylhexyl Side-Chain Content on the Open-Circuit Voltage in RrPoly(3-Hexylthiophene- Co −3-(2-Ethylhexyl)thiophene) Copolymers. Macromolecules 2012, 45, 3740−3748. (69) Howard, J. B.; Noh, S.; Beier, A. E.; Thompson, B. C. Fine Tuning Surface Energy of Poly(3-Hexylthiophene) by Heteroatom Modification of the Alkyl Side Chains. ACS Macro Lett. 2015, 4, 725− 730. (70) Chen, Z.; Lemke, H.; Albert-Seifried, S.; Caironi, M.; Nielsen, M. M.; Heeney, M.; Zhang, W.; McCulloch, I.; Sirringhaus, H. High Mobility Ambipolar Charge Transport in Polyselenophene Conjugated Polymers. Adv. Mater. 2010, 22, 2371−2375. 941


Article

Accounts of Chemical Research (71) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. Structurally Ordered Polythiophene Nanoparticles for High-Performance Organic ThinFilm Transistors. Adv. Mater. 2005, 17, 1141−1144. (72) Rivnay, J. Organic Electronics: Efficiency through Dilution. Nat. Mater. 2016, 15, 594−595. (73) Chang, M.; Su, Z.; Egap, E. Alignment and Charge Transport of One-Dimensional Conjugated Polymer Nanowires in Insulating Polymer Blends. Macromolecules 2016, 49, 9449−9456. (74) Chang, M.; Choi, D.; Wang, G.; Kleinhenz, N.; Persson, N.; Park, B.; Reichmanis, E. Photoinduced Anisotropic Assembly of Conjugated Polymers in Insulating Polymer Blends. ACS Appl. Mater. Interfaces 2015, 7, 14095−14103. (75) Choi, D.; Kim, H.; Persson, N.; Chu, P.-H.; Chang, M.; Kang, J.H.; Graham, S.; Reichmanis, E. Elastomer-Polymer Semiconductor Blends for High Performance Stretchable Charge Transport Network. Chem. Mater. 2016, 28, 1196. (76) Choi, D.; Chu, P.-H.; McBride, M.; Reichmanis, E. Best Practices for Reporting Organic Field Effect Transistor Device Performance. Chem. Mater. 2015, 27, 4167−4168. (77) Jain, A.; Ong, S. P.; Hautier, G.; Chen, W.; Richards, W. D.; Dacek, S.; Cholia, S.; Gunter, D.; Skinner, D.; Ceder, G.; Persson, K. A. Commentary: The Materials Project: A Materials Genome Approach to Accelerating Materials Innovation. APL Mater. 2013, 1, 011002. (78) Xiao, J.; Shi, J.; Liu, H.; Xu, Y.; Lv, S.; Luo, Y.; Li, D.; Meng, Q.; Li, Y. Efficient CH 3 NH 3 PbI 3 Perovskite Solar Cells Based on Graphdiyne (GD)-Modified P3HT Hole-Transporting Material. Adv. Energy Mater. 2015, 5, 1401943. (79) Jin, G.; Prabhakaran, M. P.; Ramakrishna, S. Photosensitive and Biomimetic Core-Shell Nanofibrous Scaffolds as Wound Dressing. Photochem. Photobiol. 2014, 90, 673−681. (80) Martino, N.; Feyen, P.; Porro, M.; Bossio, C.; Zucchetti, E.; Ghezzi, D.; Benfenati, F.; Lanzani, G.; Antognazza, M. R. Photothermal Cellular Stimulation in Functional Bio-Polymer Interfaces. Sci. Rep. 2015, 5, 8911.

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Nucleation, Growth, and Alignment of Poly(3-hexylthiophene

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