Microstructural Evolution of the Thin Films of a DonorâAcceptor

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Microstructural Evolution of the Thin Films of a Donor−Acceptor Semiconducting Polymer Deposited by Meniscus-Guided Coating Leo Shaw,† Hongping Yan,‡ Xiaodan Gu,†,‡ Pascal Hayoz,§ R. Thomas Weitz,∥ Daniel Kaelblein,*,∥ Michael F. Toney,*,‡ and Zhenan Bao*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § BASF Schweiz AG, R-1059, Mattenstrasse, 4058 Basel, Switzerland ∥ BASF SE, J542, 67056 Ludwigshafen, Germany ‡

S Supporting Information *

ABSTRACT: Crucial to the development and refinement of organic electronics is a fundamental understanding of how deposition processes affect the active material’s resulting microstructure in the thin film. Meniscus-guided coating (MGC) methods are attractive because of their amenability to high-throughput, industrially relevant continuous processes like roll-to-roll deposition, but the mechanism of solid film formation has not been elucidated and is valuable for the precise control of thin-film morphology and thus ultimate device performance. In this work, we investigate the microstructural evolution of thin films of a diketopyrrolopyrrole−terthiophene donor−acceptor polymer semiconductor using both in situ and ex situ X-ray diffraction methods. On the basis of a comparison of disorder between the film bulk and the top surface and a depth profiling of the out-ofplane orientation of crystallites, we find that faster coating speeds introduce more disorder into the resulting films because the stochastic nucleation of disordered crystallites at the meniscus air−liquid interface becomes more dominant than substratemediated nucleation. Our results suggest that there exist three separate deposition regimesnamely the shear-dominate, disorder-dominate, and Landau−Levich−Derjaguin regimesrevealed by observing both polymer alignment via dry film thickness and optical dichroism, a property sensitive to the flow and shear fields. At low coating speeds, the shear strain imparted upon the solution directly induces polymer alignment, causing an increase in dichroism as a function of coating speed. When solvent evaporation becomes too rapid as coating speeds increase, a decrease in the dichroic ratio is observed before the classical Landau−Levich−Derjaguin regime occurs at the highest coating speeds, resulting in isotropic films. The preservation of out-ofplane crystalline texture throughout the thickness of the films is seen only for lower coating speeds, and a study of different deposition temperatures similarly indicates that the lower overall solvent evaporation is conducive to this process. Increased paracrystalline disorder (i.e., peak broadening) is observed by grazing-incidence wide-angle X-ray diffraction at the top interface of the dry films and at higher coating speeds. Together, these results indicate that more rapid solvent evaporation at higher coating speeds causes increased disorder, which can cause the nucleation of misaligned crystallites, affect the dichroic ratio, and may frustrate the alignment of polymer molecules in the amorphous regions of the film. Because the polymer studied and the deposition technique used are representative models, these results are likely general for aggregating, semicrystalline donor− acceptor polymers deposited with MGC.

1. INTRODUCTION The continued development of organic semiconductors (OSCs) and devices incorporating them has opened up new application spaces beyond the reach of conventional, silicon© XXXX American Chemical Society

Received: February 14, 2018 Revised: April 30, 2018

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DOI: 10.1021/acs.macromol.8b00350 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules based technologies. Research in the field has realized electronics that are flexible, implantable, biodegradable, and transparent, allowing for their use in medicine, wearable and printable electronics, energy generation and storage, wireless sensing, and electronic skin.1−7 In fact, recent trends toward the study of polymeric OSCs over small molecules reflect the desire for the conferral and precise chemical tuning of desirable properties, such as intrinsic flexibility and solution processability. However, while the chemical properties and design of polymer OSCs inherently influence their charge transport behavior, how these materials are deposited can sometimes completely dominate their ability to effectively conduct charge. Laboratory-scale processing methods often suffice in proof-of-concept demonstrations, but the commercialization of OSC-based devices requires industrially and commercially feasible continuous deposition processes like meniscus-guided coating (MGC).8,9 In order to realize the new applications promised by polymer OSCs, a firm knowledge of the fundamental processes underpinning solid film formation is essential, and since these materials are solution-processed, an examination into the polymer crystallization mechanism from the solvent is needed. With an understanding of how the resulting thin films are formed, one can predict and control their microstructure, morphology, and crystallographic texture, which are crucial for the realization of high-performance devices. In this work, we use solution shearing, a technique similar to blade coating, as a representative The MGC technique shed light on the deposition mechanism of a diketopyrrolopyrrole−terthiophene donor− acceptor (D−A) polymerPDPP3T (Figure 1, top right)in order to gain insights into the complex interplay among solvent evaporation, shear strain within the solution, and polymer nucleation and growth. Given that applications like all-polymer, bulk-heterojunction solar cells involve complicated binary, ternary, and sometimes quaternary component systems, the mechanistic understanding we develop here for a singlecomponent system can inform the development of more complex models and deposition techniques,10−13 which are needed to achieve optimal device performance. Although others have previously examined the deposition of thin films from organic small-molecule solutions,14 colloidal suspensions,15,16 and solutions of conventional commodity polymers17 with MGC methods, in-depth studies on polymer OSCs, where strong interactions (both alkyl and π-stacking) can cause significant solution-phase aggregation and affect the crystallization process, have only emerged in recent years. Of the published research, poly(3-hexylthiophene-2,5-diyl)18 (P3HT) is one of the most studied model polymers in the field and is the subject of a large corpus of experimental and theoretical work. However, the community has since shifted attention from polythiophenes to alternating copolymers consisting of electron-rich (donor) and electron-deficient (acceptor) mersthat have since outperformed the former in terms of achievable field-effect mobilities. Because donor− acceptor polymers are characterized by very strong intra- and intermolecular interactions and increased backbone rigidity, many of the principles and models developed for the relatively flexible polythiophenes do not apply, and a new understanding of the behavior of popular moieties and chemical designs is needed. For this study, PDPP3T is chosen because of its high inplane alignment, which we previously reported and achieved without any pre- or postdeposition processing.19 It is also

Figure 1. (top) Deposition regimes demarcated by the trend in optical dichroic ratios as a function of coating speed. Figure adapted from ref 19. (top right) PDPP3T, the polymer studied. (bottom) A schematic of meniscus-guided coating (MGC) highlighting, in the out-of-plane direction, the relative influence of polymer nucleation either at the air−liquid interface or at the substrate surface, and their corresponding regime (bottom right). Red ovals indicate relatively ordered crystallites nucleated at the solid−liquid interface, while the blue ovals represent disordered ones nucleated from the air−liquid interface.

among the well-known class of diketopyrrolopyrrole-based conjugated polymers that have been found to give high charge carrier mobilities and can serve as a model semirigid conjugated polymer. Because the lowest-order transition dipole moment for D−A polymers (the HOMO−LUMO optical transition) is a vector generally aligned with the polymer backbone, the optical dichroic ratio obtained from polarized UV−vis spectroscopy gives a quantitative measure of the degree to which the polymer is aligned. PDPP3T films aligned along the coating direction exhibit a maximum dichroic ratio of about 7, and the dichroism is tunable by varying the coating speed (Figure 1, top). Surprisingly, however, the maximum occurs at a “critical” coating speed of only 0.2 mm s−1, even though the shear strain experienced by the solution monotonically increases as a function of coating speed. In other words, more uniaxial shear stress does not continue to enhance alignment beyond a certain coating speed. This hints at the influence of another underlying process that frustrates alignment, and thus PDPP3T’s ability to strongly align gives us an opportunity to probe what is occurring during film deposition. The polymer’s inclination toward aggregation in solution and its ability to form moderately to highly crystalline thin films allow us to track the evolution of solid film formation using Xray-based techniques sensitive only to crystalline ordering. Although substantial differences in aggregation and deposition behavior exist among semiconducting polymers,20,21 our goal here is to establish principles that may be applicable specifically to strongly aggregating, semicrystalline D−A polymers during B


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for χ between 2.0° and 90° were tracked as a function of time by recording exposures at intervals of 0.1 or 0.5 s. A Gaussian curve with a linear background was fit to the data. The intensity of a broad solvent scattering halo was tracked by analyzing cake slices centered at 1.275 Å−1 with a width of 1.2 Å−1 for χ between 70° and 90°. Data analysis was performed using WxDiff, IgorPro, and MATLAB. 2.4. UV−Vis Spectroscopy. Very dilute (∼0.000 002 mol %) solutions of PDPP3T in tetralin were measured at room temperature and at 140 °C in quartz cuvettes using a Cary 6000i UV−vis−NIR spectrometer (Agilent) without any polarization. Spectra of the neat solvent at both temperatures were used as backgrounds. The absorption in the region between 1200 and 1300 nm was averaged and subtracted from the data to zero the spectra. Discontinuities caused by a switch in the grating and the detector midscan were removed by manually shifting values. Data processing was performed with MATLAB. 2.5. Diffusion-Ordered Spectroscopy by Proton Nuclear Magnetic Resonance. An 11 mg mL−1 solution of PDPP3T in deuterated o-xylene was measured at 65 °C in a Varian Inova 600 MHz NMR spectrometer. Magnetic field gradient strengths (g) ranging from 2 to 57 G cm−1 were pulsed for 2.5 ms (δ) with a diffusion time of 0.132 s (Δ). The integrated intensities of the peaks in the range of between 0.875 and 1.9 ppm were fit as an exponential function of g2 to extract the diffusion coefficient D. Nonlinear fitting was performed with MATLAB. 2.6. Crystallinity Quantification. For the degree of crystallinity analysis, 2D-GIWAXS was performed at the Stanford Synchrotron Radiation Lightsource on beamline 7-2 with an X-ray energy of 15 keV using a Pilatus 300K-W detector oriented horizontally. The films were measured on a sample rotation stage with incidence angle α = 0.14° and rotated at 0.5° increments to resolve the (100) reflection as a function of the in-plane angle φ. A local specular scan35 was measured at α ≈ 1.5° to construct pole figures, which were then integrated in φ to yield the complete pole figure. Data analysis was performed using WxDiff, IgorPro, and MATLAB.

their deposition by methods that impart high shear strain rates in the solution. Previous characterization work along these lines, especially with the conducting ionomer blend PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)), does not typically focus on film solidification due to MGC or attempts to correlate deposition-related factors like postdeposition solvent treatments, solvent mixtures, or polymer blending to microstructural evolution.22−27 However, while such studies are useful for specific systems of interest, a basic model of polymer film solidification by industrially relevant coating methods would be a useful first step both in estimating the deposition conditions appropriate for a general semiconducting polymer and in developing more complex and detailed models for the deposition of multicomponent polymer solutions. Because our work here is among the few detailed studies that use a variety of X-ray characterization techniques to fully characterize both the film evolution process and the final microstructure of films deposited by MGC, we believe the characterization methods outlined and the insights gained here are a crucial step in the scale-up of laboratory coating processes toward industrially relevant fabrication schemes.

2. EXPERIMENTAL METHODS 2.1. Materials. Anhydrous 1,2,3,4-tetrahydronaphthalene (tetralin) and anhydrous trichloroethylene (TCE) were purchased from SigmaAldrich (St. Louis, MO). n-Octadecyltrimethoxysilane (OTMS) was purchased from Gelest (Morrisville, PA). Degenerately doped n-type, ⟨100⟩ Si wafers with native oxide (R < 0.005 Ω·cm) were used for Xray diffraction experiments. PDPP3T (Mw = 22 850 g mol−1, PDI = 1.77) and PDPP3T-2 (Mw = 36 082 g mol−1, PDI = 1.82) were synthesized according to previous reports.28 2.2. Solution Shearing. PDPP3T was solution sheared onto each sample from an 11 mg mL−1 tetralin solution at 140 °C with a gap height of ∼40 μm and a blade tilt angle of 8°. The substrates were cleaned with toluene, acetone, and isopropanol prior to deposition. The blade used was a silicon wafer treated with OTMS to form a dewetting surface. Each sample was allowed to sit on the heating stage for ∼3 min to drive off excess solvent after shearing. 2.3. Grazing-Incidence X-ray Diffraction. Grazing-incidence Xray diffraction/scattering (2D-GIWAXS) was performed at the Stanford Synchrotron Radiation Lightsource on beamline 11-3 using a MARCCD as the 2-D area detector. The X-ray energy was 12.735 keV, and for the out-of-plane texture study, the incidence angle α was chosen to be 0.06° and 0.12° to probe the top few nanometers of the films and the full thickness of the films, respectively. For the quantitative analysis, α was 0.04° and 0.20°, with the latter chosen to be above the critical angles of the film (∼0.105° based on an electron density of 0.4 Å−3 for conjugated polymers29 and this photon energy)30 and the Si substrate to avoid multiple reflection.31,32 All 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. Therefore, for these measurements, both the scattering vector Q and direction probed by the X-rays are perpendicular and parallel to the shearing direction respectively. The π-stacking anisotropy with respect to the direction parallel and perpendicular to the direction of shearing was determined by using a cake slice centered at 1.71 Å−1 with a width of 0.125 Å−1. A linear background resolved in polar angle χ was subtracted before calculating the approximate (010) intensity with the appropriate sin χ correction.33,34 For the in situ 2D-GIWAXS, a solution shearing setup developed for this purpose26 was used with an X-ray energy of 14 keV, 0.20° incidence angle, and 50 μm blade-to-substrate gap height. The horizontal beam width was 100 μm so that the illuminated volume was spatially resolved without reducing photon flux too much. Cake slices of the (100) reflection centered at 0.318 Å−1 with a width of 0.07 Å−1

3. RESULTS AND DISCUSSION 3.1. Deposition Regimes. Landau, Levich, and Derjaguin recognized that the thickness of a wet film deposited on a vertically translated substrate is a function of the capillary number Ca = ην/γ, where η is the liquid viscosity, ν is the substrate velocity (or coating speed), and γ is the surface tension.36 In this Landau−Levich−Derjaguin (LLD) regime, a wet film is first dragged out by viscous forces, and its thickness increases as a function of ν2/3 for large ν. At sufficiently low coating speeds, however, the liquid meniscus that forms between the liquid reservoir and the contact line on the substrate acts as a solvent evaporation front and is the locus of a variety of phenomena. As the contact line recedes, the liquid dries and, if it contains a solute or suspended materials, deposits them on the substrate under the direct influence of the flow fields and shear strain within the meniscus. Thus, in contrast to the LLD regime where solvent drying is separate from film deposition, this “evaporative” regime (a.k.a. convective assembly) is characterized by an intimate connection between solvent evaporation, fluid flow, and solid film formation, and film thickness in this regime decreases with increasing speed with an ideal power law exponent of −1 or −2.15,37 In these cases, two distinct regimes of deposition characterized by two different fluid mechanical mechanisms are revealed when considering dry film thickness as the key dependent variable. In our previous work, we found that the optical anisotropy of PDPP3T thin films deposited using solution shearing is tunable by simply controlling the coating speed and can reach a peak dichroic ratio of around 7 (Figure 1, top).19 Using lubrication theory,38 an estimation of the average shear strain across the C


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Figure 2. Parameters extracted from in situ grazing-incidence X-ray diffraction (2D-GIWAXS). (a) The normalized intensity of the (100) peak as a function of time, which has been shifted such that zero time is defined as the time at which the solvent scattering peak disappears. The data are normalized to unity at long times greater than 4 s (not shown). (b) The normalized (100) intensity as a function of the extent of solvent evaporation. As coating speed is increased, more of the (100) intensity appears later on during film evolution. (c) The position of the (100) peak over time. The position of the reflections appears very close to their equilibrium peak position. The oscillations at longer times for some speeds are so small in amplitude that they are likely the result of fitting error. (d) The evolution of the full width at half-maximum (fwhm) of the (100) reflection. The equilibrium values increase as a function of coating speed, indicating increased disorder.

fluid cross section when exiting the shearing blade indicates a linear dependence on shear speed. In this way, we intuitively expect alignment should either increase or plateau with higher unidirectional shear strain, so the films’ optical dichroism would monotonically increase as a function of shear speed before plateauing at its maximal value. However, the dichroic ratio curve exhibits a peak dichroic ratio that occurs at a relatively low coating speed (the critical shear speed), and the dichroism subsequently diminishes for coating speeds beyond this, contrary to our expectation. Just as thickness is the key variable demarcating a fluid dynamical transition between film deposition with (the evaporative regime) and without (the LLD regime) the direct influence of the meniscus, we postulate that optical dichroisma highly sensitive property related to the orientation of the entire ensemble of polymer molecules reveals another otherwise hidden transition point related to the nucleation and crystallization dynamics of the solute. We hypothesize that there exist three separate regimes during the thin-film deposition process separated by two transition points: (1) the fluid mechanical transition between the evaporative and LLD regimes and (2) a transition related to the crystallization rate where the solvent evaporation rate

relative to the movement of the contact line begins to enhance nucleation at the air−liquid interface (Figure 1, bottom). For the materials studied here, the former occurs around 5 mm s−1 and the latter at the critical shear speed of 0.2 mm s−1. Regime I, which we call the shear-dominate regime, occurs at low shear speeds and is dominated primarily by the shear stress imparted upon the solution, which in our system corresponds to shear speeds below 0.2 mm s−1 and average shear strains below about 5 s−1. At these low speeds, the amount of shear strain imparted directly induces polymer alignment, and films exhibit increasing dichroism as the coating speed is increased. At the other extreme, regime III (the LLD regime) involves a decoupling of deposition from blade motion, and the optical isotropy observed attests to the fact that even though the highest shear forces imparted by the moving blade occur in this regime, high nucleation densities and the relaxation of shear stress within polymer solution may prevent any alignment from being fixed in the resulting film. Again, this contrasts with the evaporative regime (consisting of regimes I and II) where the characteristic time scale for solid film formation vis-à-vis solvent evaporation exceeds or is comparable to that of the movement of the shearing blade. D


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in crystallinity across PDPP3T films and discuss those results in section 3.5. Figure 2 summarizes the results for films deposited at different shear speeds from an 11 mg mL−1 solution in 1,2,3,4tetrahydronaphthalene (a.k.a. tetralin, bp ∼207 °C) and with the substrate and polymer ink maintained at 140 °C. The normalized (100) intensity grows as a sigmoidal function of time (Figure 2a), with lower shear speeds achieving the maximum dry film intensity over a longer period as compared to the fast speeds. It takes about 9 s while shearing at 0.1 mm s−1 (regime I) to reach maximum intensity, as compared to 1 s at 0.7 mm s−1 (regime II) and 1.5 s at 50 mm s−1 (regime III). Given that in the evaporative regime the films deposited at lower speeds are thicker, our result is consistent with the fact that more solution is deposited per unit length traversed by the shearing blade as coating speed is decreased. In other words, the additional quantity of solution that dries to yield the greater dry film thickness naturally possesses more solvent to evaporate, thus requiring more time overall to fully evaporate if we assume the evaporation rates across all shear speeds are the same. This assumption is true given that the film thickness t is a power law function of coating speed v, whose exponent is −1.19 Although enhanced evaporation at the contact line is expected,40 the thickness scaling is consistent with an overall mass balance around the meniscus,15 and spatial variation in the solvent evaporation rate, to first order, is not important. We calculate that the total evaporation time Tevap to deposit a length l of polymer film at different coating speeds in fact depends on v−1 (see the Supporting Information for the analysis), indicating that at lower speeds more time is needed to completely evaporate the solvent from the greater quantity of solution necessary to deposit the thicker film. Because solvent remains in the film for a longer period of time at these lower speeds (regime I), polymer nucleation and growth processes occur at more modest supersaturation conditions, allowing more time for better ordering and (see section 3.4) selftemplated growth arising from the substrate−film interface. In contrast, at 0.7 mm s−1 in regime II, evolution of the film’s crystalline component occurs over a time scale 89% shorter than in the case at 0.1 mm s−1, which reduces the time available for film ordering and leads to kinetically trapped microstructural states characterized by increased disorder. Furthermore, because very thin solid films (90%). In regime III, almost all of the intensity is accounted for during the final phases of film solidification. These results again suggest that coating speeds in regimes II and III do not allow for enough time for good ordering of the films’ crystalline domains since the films dry quickly with little free solvent. Furthermore, when we observed the time-resolved evolution of the pole figures for each of the films (Figure S3), film reorganization is quite apparent for the lower coating speeds. In these plots, (100) intensity as a function of the polar angle χ is shown over time, with the zero time t = 0 pole figure denoted as a bold line. Intensity differences after this demarcating line can reveal changes over time in the polymer crystallite population. For the regime III film, the pole figures indicate that after all the solvent evaporates from the film lamella of all out-of-plane orientations formthere is an increase of intensity for all χ. In contrast, for regime I and II films there is a shift in intensity from close to the meridian (χ < 7°, Figure S3 insets) to between χ = 20° and 30° after time t = 0 when the majority of the solvent has evaporated. In other words, lamella whose unit cells are oriented almost completely out-of-plane at t = 0 rearrange and assume more tilted orientations in the almost-dry solid state. This is consistent with the ability for the films deposited at slower coating speeds to continue to evolve after solvent scattering intensity has vanished, but residual solvent remains and allows for further film reorganization. In contrast, the fact that intensity for all polar angles increases in the regime III film and that no shifts in intensity are seen indicate solvent evaporation is too rapid. We note that it is possible that the shift in χ of (100) intensity may also be related to a change in the unit cell geometry instead of reflecting a change in the relative orientation of crystallites to the substrate plane. In either case, this reorganization is nonetheless possible only in regimes I and II. The lamella peak positions do not systematically change during the course of deposition nor do they exhibit much variation in position at the beginning stages of drying (Figure 2c). With further solvent evaporation, the peak centers converge to an equilibrium value around 0.315−0.325 Å−1, corresponding to a d-spacing of about 19.6 Å. Moreover, because the differences in the long-time q-values are on the order of 0.005 Å−1, which are within the range of experimental error, we believe there is no trend among the equilibrium qvalues as a function of coating speed, as corroborated by the ex situ data. This contrasts with poly(2,5-bis(thiophene-2-yl)-(3,7diheptadecanyltetrathienoacene) (P2TDC17FT4), which exhibited a metastable lamella packing structure with higher dspacing at slow shear speeds, and poly(2,5-bis(3-hexadecyl-

thiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT-C16), which had increasing lamella spacing as a function of shear speed attributed to the incorporation of more gauche defects in its alkyl side chains.20 Also of note is the fact that the (100) reflection initially emerges very close to its corresponding equilibrium q value and rapidly converges to it. This significantly differs with reports on other polymers where peak positions for a polymer OSC could increase between 0.02 and 0.07 Å−1,26,27 indicating in those cases that lamella formed earlier on (in solvent) are more loosely associated via their alkyl side chains before adopting a more closely packed structure in the final film. Since those results focused on binary blends of polymers, whose film deposition process is complicated by the thermodynamics of phase separation, it is not a priori clear if such a phenomenon would be expected for a single-polymer system like PDPP3T. While the P3HT films have been shown to exhibit a time-dependent evolution in peak position,26 we see no evidence of that process here. In the case of PDPP3T, its low molecular weight is likely the source of the relatively high crystallinity of its films. A result of this is its strong aggregation, observable via solution-phase UV−vis spectroscopy even in highly diluted solution both at room temperature19 and at the deposition temperature (Figure S4) to form π-associated assemblies of individual polymer molecules. It is possible that the lack of any discernible timedependent variation of the films’ (100) q values and no trend in equilibrium q values as a function of coating speed are the direct result of these stable aggregates acting as the nucleation centers for crystallite growth in the solidifying film, similar to the case of spin-coated P3HT films.42 Furthermore, the fwhm profiles for each of the films (Figure 2d) lack a clear timedependent trend; the peak widths appear close to their respective equilibrium values, consistent with a distribution of crystallites that formed from the same population distribution of pre-existing nuclei, i.e., the aggregates in solution. 3.3. Quantitative Analysis of Crystalline Disorder. Unlike the lack of systematic variation in peak position, the fwhm from the in situ 2D-GIWAXS shows increasing equilibrium values as a function of coating speed (Figure 2d) and warrants further discussion. While qualitative analyses of peak positions and widths are sufficient for basic materials characterization, a more quantitative analysis of observed reflections can reveal deeper insights into the nature of crystalline disorder, in particular crystallographic directions as a function of shear speed. PDPP3T appears insensitive to the “disorder” that manifests as an increase in its lamella d-spacing seen in other polymers (as mentioned above), but other classically studied forms of disorder can be observed with X-ray techniques. The progressive peak broadening as a function of diffraction order is the result of cumulative disordera.k.a. disorder of the second kindwithin the probed crystallites and is composed of two primary contributions: the root-meansquare variance in the lattice parameter crystallite-to-crystallite eRMS and the paracrystalline disorder g.43,44 In the former, crystallites may have slightly different d-spacings (i.e., microstrain), and eRMS captures the variance in the lattice parameter over the entire population probed. The latter can be thought of as the effect of packing crystallographic planes imperfectly: a small deviation from the ideal lattice position (disorder) of one plane causes subsequent planes to also deviate. Because it measures this cumulative deviation of crystalline planes from their expected positions in a perfect lattice, the paracrystallinity value g is a metric of how nonideal the crystalline packing is for F


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Macromolecules a given crystallographic direction and thus allow us to quantify disorder as a function of shear speed during deposition. In this case, because the diffraction peaks are strong for the lamella (up to fourth order) and π-stacking (first order) directions, disorder can be quantified for the lamella and πstacking directions using methods based on the work of Warren and Averbach.45 We used a line cut 1° in width centered at χ = 6.9° for the (100) direction (since the lamella intensity is mostly out-of-plane) to adjust for the kinematically inaccessible zone caused by the grazing-incidence geometry and to minimize χ-dependent broadening. A line cut at χ = ∼90° (the horizon) is similarly used for the π-stacking direction since this reflection is primarily in-plane (the polymer is mostly edgeon). We also chose an incidence angle α below the critical angle of the polymer film to measure only the top few (∼5) nanometers of the polymer film because total external reflection occurs and an evanescent wave is established, allowing the technique to be surface-sensitive. We also measure with an incidence angle above the critical angles of both the film and the silicon substrate to probe the entire thickness of the polymer film while avoiding multiple reflection.31,32 A full Warren−Averbach analysis46,47 is not possible because the obtainable resolution in q would fail to capture the lowamplitude, high-frequency harmonics needed for the requisite fast Fourier transform, but a single-peak estimate of g provided by Rivnay et al.39 suffices for our purposes. The single-peak approximation outlined in their work, which assumes that the influence of column length (crystallite size) is negligible and that the influence of paracrystallinity is greater than that of lattice parameter variation (eRMS), has previously been validated for poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) thin films.48 Because PDPP3T does not exhibit ordering as strong as PBTTT, it follows from their analysis that both assumptions are valid in this case as well. Calculated g values using the (100) reflection are shown in Figure 3. For comparison, we calculated the paracrystallinity of thick PDPP3T films dropcast onto Si substrates and subsequently annealed either thermally at 200 °C for 30 min or with tetralin vapor for 18 h. At all coating speeds, the films exhibit values of g greater than that of the thermally annealed film and the solvent-vapor-annealed film; the former has the lowest value of about 8.2 ± 0.1% followed by about 9.0 ± 0.3% for the latter. Two trends are also apparent: (1) when we compare the top interface of the films (α = 0.04°) with the film bulk (α = 0.20°), we see slightly but consistently greater g values, indicating increased disorder at the film surface, and (2) within each set of data, we also observe that the paracrystallinity is a monotonically but modestly increasing function of coating speed, with the most disordered films being those deposited in regimes II and III. While the increase in bulk g values is rather slightwith an increase from 10.1 ± 0.4% for regime I films to greater than 10.7 ± 0.1% in regime IIIthe top surface of the films shows greater differences, beginning at 10.5 ± 0.1% and increasing to 12.0 ± 0.2%. These trends are a direct consequence of the increased peak breadths as seen in the fwhm values, whose error bars are beyond detector limitations (Figure S5), and the values themselves. The (200) reflection also reproduces this trend, although the calculated estimates have larger error bars (Figure S6). The slight difference is likely due to crystallite size also contributing to the peak widths. The results suggest that increasing the coating speed (and thus decreasing the time scale for polymer film deposition) has an effect on the paracrystalline disorder of the lamella stacking

Figure 3. Paracrystallinity g calculated from the (100) reflection as a function of coating speed for the bulk film (blue, α = 0.20°) and the top surface (red, α = 0.04°) with the confidence intervals for dropcast polymer films that are thermally annealed (200 °C, 30 min; dark gray) and annealed with tetralin solvent vapor (room temperature, 18 h). The values for the curves are averaged from the data obtained with the incident X-ray beam aligned both parallel and perpendicular to the shearing direction.

in the films. This supports the idea that faster total solvent evaporation and polymer deposition (as discussed previously) is producing crystallites with lamella that are packed slightly less perfectly, especially at the top surface of the films. Specifically, less time is available for polymer molecules within an aggregate to associate in the lamella direction, kinetically trapping the alkyl side chains in more disordered, nonequilibrium packing motifs and thus manifesting as an increase in g. Previous reports have noticed this increased disorder specifically for the πstacking peak,49 but in this case, we observe no systematic trend in g for the (010) direction (Figure S7) because π-stacking in such D−A polymers is typically quite disordered already.20,39,48 Consequently, the presence of such substantial disorder makes it difficult to resolve variations as a function of coating speed, especially with the larger error bars. Solution-phase UV−vis spectroscopy of very dilute tetralin solutions of PDPP3T heated to 140 °C (Figure S4) reveal vibronic features associated with aggregated polymer. This indicates that the polymer still aggregates at our deposition temperature via π interactions but, more importantly, can partly explain why variations in g are not seen in the (010) direction. In contrast, lamella stacking occurs because of solvent evaporation during deposition and is sensitive to the rate of supersaturation. Disorder in the (100) direction can thus reveal a difference in the rate of polymer nucleation or crystallite growth. We note that the g values for the (100) direction in the bulk are between 10 and 10.5%, while those for the (010) direction are between 10 and 14%. While these values are higher than those previously reported for polythiophene-based conjugated polymers (∼4% and 7%, respectively), our results are consistent with the few analyses that have been conducted for donor−acceptor polymers.48 It is not surprising that this class of materials have previously puzzled researchers as to the origin of their higher device performance despite exhibiting much higher degrees of disorder. G


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Figure 4. (left) 2D-GIWAXS images (α = 0.12°) for films deposited at four coating speeds onto bare Si substrates and with two orientations of the incident X-ray beam with respect to the coating direction. (right) Pole figures of the (010) peak for all conditions and with the two incidence angles for depth profiling. Asymmetry in scattering is sometimes observed and is caused by the films’ topology with respect to the stick−slip phenomena that occurs.10 The pole figures are drawn from the right half of the diffraction images. The images are denoted by the orientation of the incident beam with respect to the coating direction; the direction of the scattering vector Q is the opposite direction.

beam parallel) and a broad peak around χ = 50° (incident beam perpendicular) in the bulk of the films (Figure 4, right). The same trends are both seen at the top surface of the films, indicating some preferential orientation is maintained throughout the thickness of the film. We can see in Figure 4 that the texture of the film coated at 50 mm s−1 in LLD regime (III) bears a resemblance with the shear-dominate film (regime I), but with a major difference: the very slight preferential orientation through the bulk of the film is not reflected at the surface. For the (010) peak in both incidence directions, the low α (top surface) data indicates no preferential orientation even though the high α (bulk) curves show a small decrease in intensity at low χ. For the LLD regime, the polymer solution relaxes after experiencing very high shear strain, and the resulting liquid film drying process is effectively commensurate to that of dropcasting, which is why the parallel and perpendicular diffraction images are effectively the same (Figure 4, bottom). While preferential orientation of nuclei at the liquid−substrate interface is expected, the overall population of crystallites demonstrates no preferential orientation, suggesting that random nucleation at the air− liquid interface is the dominant process, resulting in disordered films with no optical dichroism. When we examine the film coated at 0.5 mm s−1 in regime II, we see the hallmarks of both regimes I and III. The film bulk has clear preferential orientation with respect to the (010) reflection, but the top interface exhibits nonethe strong intensity at χ = 35° (parallel) and χ = 15° (perpendicular) is absent from the low α data. In this way, it appears that templating from the substrate does extend into the film to some extent, but the top interface is dominated by randomly oriented crystallites. This suggests that within regime II nucleation occurs with comparable frequency at both interfaces, supporting the hypothesis that the drop in optical dichroism in regime II is in part due to the enhanced formation of

The fact that top surface of the films (α = 0.04°) exhibit higher paracrystallinity indicates more disordered crystallites are growing at the upper film surface during deposition, likely due to faster crystallite growth rates at the air−liquid interface, increased nucleation density, or both. Moreover, the trend of increasing disorder at the top interface as a function of shear speed further supports the hypothesis that the locus of polymer nucleation is different in regimes II and III as compared to those in regime I. 3.4. Out-of-Plane Crystallographic Texture and Microstructure. To further investigate this hypothesis, we examined the crystalline texture of the films to determine whether or not a higher nucleation density is implicated. Figure 4 shows the 2D-GIWAXS images for selected coating speeds with the incident X-ray beam oriented either parallel or perpendicular with the direction of shearing; i.e., the scattering vector Q is perpendicular or parallel to the shearing direction, respectively. The measured films are sheared onto untreated bare Si substrates and reveal substantial stick-and-slip motion during deposition, leading to an asymmetric scattering pattern10 quite noticeable for the film coated at 0.5 mm s−1. We note that this particular phenomenon has also been observed previously in polymer films subjected to postdeposition mechanical abrasion50 or stretching.51 The backgroundsubtracted intensity of (010) π-stacking reflection is plotted as a function of χ for α above and below the critical angle. The comparison of the pole figures of the two α allows us to perform a depth profiling of the crystallite ensemble’s orientation relative to the substrate normal.34 The pole figures are incomplete because the construction of a full pole figure requires higher incidence angles.35 Nonetheless, we can examine the crystalline texture of these films without the data near the meridian (χ = 0°). For a coating speed of 0.1 mm s−1 in regime I, the (010) reflection exhibits a plateau of intensity for χ > 45° (incident H


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examined the influence of the substrate on semiconducting polymer nucleation and found that while perfectly oriented crystallites form at the buried interface, the templating effect is highly local53 and extends only about one coherence length (∼10 nm) upward through the film thickness.33 We emphasize that a mitigating circumstance in previous reports is that their films are deposited with spin-coating, where much faster total solvent evaporation likely influences aggregation and crystallization behavior at the interfaces,49,54 and those deposition conditions do not apply here. Moreover, in cases where higher boiling point solvents are used for room-temperature spincoating, the slower solvent evaporation is also attributed to better ordering at the air−liquid interface,55−57 much like in the case of regime I films. While obtaining the dichroic ratio of these films as a function of thickness would provide additional corroboration, it is not possible because the UV−vis measurement is taken in a transmission geometry, which averages over the entire thickness. Furthermore, the alignment of this polymer cannot be detected using scanning probe methods of the top or bottom interfaces, making such a measurement quite difficult. We note that this model of the vertical evolution of crystalline microstructure contrasts with the mechanisms reported in other work.58 A few reports have suggested that the air−liquid interface can play a crucial role in overall film evolution and can induce organization ordered enough to facilitate the fabrication of transistors.30,59,60 While the final resulting microstructure at the upper film surface may have sufficient ordering for effective charge transport in some cases,55,56,61 reports of preferential out-of-plane or in-plane orientation caused by nucleation off of a free interface are relatively rare without the dramatic alteration of the surface free energy, for example by the chemical incorporation of fluoroalkyl groups in the polymer side chains.62−64 Even then, preferential orientation engenders the extreme cases of molecular orientation (edge-on, face-on, end-on);65 it is unlikely that nucleation at the air−liquid interface can produce the specific intermediate χ distributions observed here. Furthermore, in complete pole figures of the (100) peak (to be discussed below), no resolution-limited peak occurs at χ = 0° for all coating speeds, indicating that the substrate’s effect on nucleation in this case does not strongly orient crystallites to adopt the perfectly edge-on crystallites observed elsewhere but rather induces a distribution of orientations that are close to “edge-on”. We note that implicit in our discussion of the pole figures here is the simplifying approximation that the unit cell for the polymer is close to orthorhombic such that the lamellastacking (100), π-stacking (010), and backbone (001) directions are effectively orthogonal to one another. In principle, the polymer molecule may be tilted within the unit cell so that an intermediate χ mode for, say, the (100) reflection off of the meridian may correspond with true “edge-on” packing. In this way, it is possible that both interfaces may contribute to the films’ overall out-of-plane orientation. Despite this, however, the paracrystallinity and depth profiling data still support the hypothesis that the drop of optical dichroism is in fact caused by an increase in disorder and demarcates a transition into a regime of coating speeds characterized by rapid solvent evaporation and less-ordered crystallization. In the few reports where strong preferential orientation is induced by the deposition and not with specific chemical modification, an analysis of the fluid dynamics may explain the origin of ordering at the air−liquid interface. Recent work by

crystallites that are either misaligned or incapable of being aligned despite the higher shear strain at those coating speeds compared with regime I. For the film sheared at the critical shear speed of 0.2 mm s−1, the preservation of preferential orientation is beginning to weaken. The highest intensity for the (010) reflection appears around χ = 40° in the perpendicular direction for the bulk, but the surface’s crystallite population has begun to drift to higher χ from this mode and become more uniform. We note these distributions of orientations with a mode at intermediate angles not corresponding to face-on or edge-on packing are unexpected and warrant further study. In addition to the incomplete pole figure of the (010) intensity, we also analyze the anisotropy of the diffracted intensity, which we define as the integrated peak area when the coating directions of the films are oriented parallel versus perpendicular to the incident X-ray beam (Figure S8). The film bulk generally exhibits anisotropy following a trend similar to that of the dichroic ratio, with a peak at the critical shear speed and subsequent drop. We note that these values are substantially lower than the maximum achievable optical dichroic ratios (discussed further below). The surface of the films also exhibits a peak at the critical shear speed but is effectively isotropic in regime IIin line with the increased disorder expected in this regime. Both a full integration of the incomplete pole figure and an integration of just the intensity near the horizon (since the polymer is mostly edge-on) follow these trends and indicate the decrease in alignment that occurs beyond the critical shear speed and within regime II. 3.4.1. Growth Modes and Mechanism. On the basis of this experimental data and analysis of crystallographic texture, we propose that the primary locus of nucleation and subsequent crystallite growth changes as increased coating speed moves deposition from regime I to regimes II and III. We have summarized this picture of film evolution in the out-of-plane direction in Figure 1 (bottom left), which depicts the influence of the air−liquid interface relative to the substrate interface for the three regimes. The peak and subsequent drop-off in dichroic ratio are intimately related to the effect of solvent evaporation on the nucleation and growth processes. As long as the thicknesses of these films are not small enough to expect any substantial suppression of crystallinity caused by vertical confinement effects, we expect an effectively constant quantity of substrate-nucleated crystallites33 in all three regimes that form at the substrate interface during deposition.42 As increasing the coating speed causes a transition from regime I to II, the relative importance of the buried interface and the air−liquid interface for nuclei formation shifts, with random nucleation of disordered crystallites at the top dominating as shear speed is further increased. The texture of these films with respect to the out-of-plane direction corroborates this model because crystallographic texture appears preserved through the thickness of the film only for lower coating speeds. The results suggest that film evolution in regime I occurs in a bottom-up manner, and we propose that in addition to producing more ordered crystallites (see section 3.3), only nucleation off of the liquid−solid interface rather than the free interface can induce significant ensemble-level population modes of out-of-plane orientation in this case. Specifically, nucleation at the liquid−substrate interface is dominate in regime I and, as the film solidifies, tends to template further crystallite growth42,52 and orientation upward toward the air− liquid interface. This is in contrast with other reports that have I


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polymer film growth in the regime of coating speeds where film growth is relatively ordered (regime I versus regimes II and III). 3.5. In-Plane Microstructure and Degree of Crystallinity. With this picture of the out-of-plane microstructural evolution, we turn our attention to an analysis of the degree to which these films are crystalline and discuss the trends that arise in order to probe the films’ in-plane microstructure. Because these films exhibit optical dichroism in-plane, the typical 2D-GIWAXS experiments performed previously with two incidence directions are insufficient for a complete quantitative treatment of the data. For an accurate comparison between samples, the diffraction images must be integrated across all in-plane, azimuthal angles φall angles between those parallel and perpendicular to the coating direction. Two sets of diffraction images at two different incidence angles α were recorded while rotating the sample at 0.5° intervals from φ = −90° to φ = +90°, where φ = 0° is parallel with the coating direction. One set corresponds to a grazing-incidence geometry (α = 0.14°) that probes the entire film thickness, while the other is a local specular scan necessary to collect data in the kinematically inaccessible zone and to then construct the complete pole figure for the (100) reflection.35 A few representative φ-resolved pole figures can be found in the Supporting Information (Figure S11). Each set is integrated over all φ, and then the two sets are stitched together at a χ ≈ 10° to form the full φ-integrated (100) pole figure (Figure S12), which is then used to obtain the relative degree of crystallinity (rDoC) of a particular film by integrating in all χ and applying a geometric correction factor that varies as sin χ (no resolution-limited peaks are observed).33,34,70 Each data set is then normalized by the illuminated volume (substrate area and film thickness) and exposure time for that particular sample. The resulting values were then scaled relative to each other; an absolute degree of crystallinity is not possible because the molecular packing has not been modeled or solved for the polymer. We note that in the case of effectively amorphous or low-crystallinity polymers the use of relative values can be subject to relatively high error caused by sample-to-sample variation since the raw intensity values would already be small for films with a low volume fraction of crystalline domains. However, this is not the case for PDPP3T used herethe films are quite crystalline and typically require relatively short X-ray exposure times to observe clear diffraction peaks. The rDoC values for PDPP3T and for a higher molecular weight version (PDPP3T-2) are plotted as a function of shear speed in Figure 5 alongside the dichroic ratio curve (gray) for comparison; as previous reported, both molecular weights have identical dichroic ratio curves.19 The films’ crystallinity trends with their thickness, becoming more crystalline with decreasing thickness and increasing coating speed. This contrasts with results obtained for films of spin-coated regioregular P3HT33 and PBTTT,71 where the rDoC did not vary above a certain threshold thickness. In the evaporative regime, the rDoC monotonically increases until the transition into the LLD regime, where it drops as the films become thicker with increasing shear speed. When we compare this rDoC curve with that of the dichroic ratio, it is readily clear that critical shear speed for dichroism does not coincide with that for crystallinity. In other words, the film with the highest dichroism is not one of the most crystalline films. In fact, the film sheared at 0.2 mm s−1 is less than half as crystalline as the 0.8 mm s−1 film but has a dichroic ratio 2 times larger. The higher molecular weight polymer films (PDPP3T-2) have even higher

Qu et al. has investigated the alignment of a DPP− thienothiophene-based polymer capable of forming welldefined fibrils measurable using methods like atomic force microscopy (AFM).66 When that material is deposited using MGC, polymer alignment is enhanced only at the top surface of the dry films, which they attribute to extensional flow at the air−liquid interface of the meniscus. They argue that the Péclet number Pea dimensionless quantity that compares the rate of solvent evaporation to that of solute diffusionis high, suggesting the formation of a thin, skin-like layer of concentrated polymer at the air interface during coating that further inhibits crystallization through the bulk of the film. In their study, they estimate the diffusion constant of the polymer fibrils to be on the order of 10−6 cm2 s−1 based on the dynamics of a rod-like particle. In this case, the diffusion coefficient for PDPP3T at 65 °C is calculated to be 2 × 10−4 cm2 s−1 in a 11 mg mL−1 o-xylene solution based on diffusionordered spectroscopy (DOSY) using proton nuclear magnetic resonance (1H NMR) (Figure S9). Since the diffusion coefficient is expected to increase with increasing temperature, the value at 65 °C provides an upper bound on the Péclet number at the deposition temperature of 140 °C. We estimate this bound to be 10−2 at all measured coating speeds, indicating that skin-layer formation over the majority of the meniscus is unlikely.67 This low upper bound on Pe also indicates that as the volume fraction of solvent ϕs decreases while the film is drying, polymer is able to redistribute through solution, reducing the variation of ϕs as a function of the wet film thickness.68 Consequently, we expect that the majority of polymer nucleation to occur when the ϕs of the almost dry film is close to the value corresponding to the critical concentration for polymer nucleation, with ϕs varying weakly (regime I) or strongly69 (regimes II and III) through the film thickness. It is for this reason that we posit differences in the total time for solvent evaporation Tevap are important and can alter out-ofplane microstructure by inducing crystallization that is either rapid and disordered or slow and well-ordered. We provide a discussion of both ϕs and Tevap in the Supporting Information. In another point of contrast, Qu et al. find that bulk alignment as indicated by the anisotropy of the (010) πstacking peak from 2D-GIWAXS is quite weak and sometimes shows mild alignment in the direction transverse to coating. In contrast, in the PDPP3T films where we find the highest optical dichroism, we always observe (010) anisotropy in the film bulk (Figure S10). While extensional flow may in fact be involved with interfacial alignment in this case as well, film formation in the out-of-plane direction proceeds differently, as evidenced by our depth profiling of film texture. It is not surprising that the processes underlying the alignment of semiconducting polymers that form elongated fibrillar aggregates or nanostructures are different than those in this case. While in-plane or out-of-plane alignment can be induced at the air−liquid interfaces of thin films deposited by MGC,30,59,60 confounding variables like the existence of other fluid dynamical phenomena can explain the preferential orientation induced at the free surface during deposition. Modeling and an analysis of the influence of diffusion constants, temperature gradients, dynamic viscosity, and non-Newtonian behavior may be necessary to fully explain the differences observed in these systems. Nonetheless, our work here addresses the case where at least one possible mechanismskin-layer formationis suppressed and suggests that in the absence of these other factors a templating effect is likely at play during conjugated J


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the proportion of aligned crystallites or by frustrating the alignment of the polymer molecules in the amorphous regions during the drying process. Nonetheless, it becomes apparent that MGC methodssolution shearing in particularare able to align the amorphous regions of the film, a result previously postulated19 and now confirmed. We note that field-effect mobilities typically reported in studies of semiconducting polymers are a complex function of a variety of nonprocessing factorsmolecular weight, contact resistance, channel length, voltage sweep rate, etc.making it difficult to directly attribute the effect of enhanced crystallinity or alignment on overall device performance. Some materials are relatively insensitive to film crystallinity beyond a certain threshold, while others may be dramatically influenced by these changes. In this work, we avoid further device-based characterization because of these confounding influences and instead focus on relatively objective measures of crystallinity via X-ray diffraction to understand the effect of deposition on microstructure. 3.6. Effect of Deposition Temperature. Lastly, to further examine the effect of rapid solvent evaporation, the rDoC for films deposited at different temperatures and shear speeds was analyzed for the films sheared at the critical shear speed at each temperature.19 In Figure 6, we can see that the 140 °C sample, whose dichroic ratio is about 7, is 70% as crystalline as the film sheared at 80 °C, whose dichroic ratio is about 4.5. The downward trend in crystallinity is also apparent with the two samples sheared at 185 °C, which had dichroic ratios of about 2.5. Given that the films had comparable thicknesses of about 150−200 nm, increasing the temperature, which increases the solvent vapor pressure and the overall evaporation rate, drops film crystallinity likely because of the shorter time for effective crystallite growth and chain rearrangement in the drying thin film. Moreover, the trend again shows a difference between the attained dichroism and film crystallinity. When we look at the out-of-plane orientation of these films (each sheared at the critical shear speed), we observe that the two films deposited at lower temperatures follow our previous discussion regarding the preservation of crystalline texture through the thickness of these films. However, for the two highest temperature films, they resemble the regime III film there is effectively no preferential orientation or texturingand are consistent with the very fast solid film formation process expected. It appears that the effective window of coating speeds corresponding to regime II is sensitive to deposition temperature and overall solvent evaporation rate and can adopt the hallmarks of regime III filmslack of the preservation of crystalline texture, etc. In this way, there must be a balance between the chosen deposition temperature and the desired ordering of the resulting films. Lower temperatures for a given solvent allow for more well-behaved film growth but shift the deposition window of possible coating speeds to lower velocities since the critical shear speed and LLD transition point both move toward lower speeds. Higher temperatures can move the deposition window to faster speeds, but overall ordering suffers. It is possible to choose solvents based off their boiling points and pick coating speeds feasible for a chosen deposition temperature to confer good in-plane alignment, for example. However, we note that the type of solvent and its specific associative interactions with the polymer of interest can affect the latter’s aggregation behaviorour study on these effects is forth-

Figure 5. Relative degree of crystallinity of films as a function of coating speed for two molecular weights of PDPP3T. The range of coating speeds between about 1 and 10 mm s−1 corresponds to the transition between the evaporative and LLD regimes, and films deposited at these speeds are too thin and discontinuous for analysis. We highlight the existence of these separate regimes by not linking the data points across this gap of speeds. The dichroic ratio data and curve (light gray) are shown for reference and are adapted from ref 19. The latter is meant to guide the eye and not to impose a functional form on the coating speed dependence.

crystallinity than those of PDPP3T but identical dichroism curves. To understand this, we must consider what is probed by Xray diffraction and by polarized UV−vis spectroscopy. The former probes only ordered portions of the films corresponding to crystallinity; the latter depends on the alignment of the transition dipole moments of the material relative to the polarization of the incident light and without regard to packing. The difference between the rDoC and dichroic ratio curves reflects the fact that the most crystalline films are not the most aligned ones. As mentioned earlier, the degree of optical dichroism for most all films also is not reproduced in the estimated anisotropy of (010) diffraction intensity using the two sample orientations (parallel and perpendicular to the coating direction) (Figure S10). While the discrepancy between the dichroic ratios and the (010) anisotropy could be explained by having less polymer alignment in the film bulk as compared to their interface, the dichroic ratio is an ensemble-averaged value that is greater than unity for all of the films coated between 0.1 and 0.8 mm s−1. In the case where there are interfacial differences in the alignment of polymer fibrils, the dichroic ratios are close to one66 likely because the films are isotropic overall when bulk-averaged. For PDPP3T, however, such strong alignment likely persists to some degree through the film. Together, (1) the mismatch between coating speed corresponding to the most crystalline films and the most aligned films and (2) the difference in magnitude of the optical dichroism and the (010) peak anisotropy both indicate that the crystalline component of the films’ microstructure are not the only elements contributing to the films’ dichroisma significant portion arises from the amorphous regions of the films. From the view of the alignment process presented here, it is unclear whether the formation of randomly oriented crystallites influences polymer alignment either by reducing K


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Figure 6. Relative degree of crystallinity (top) and depth profiling of the partial (010) pole figures (bottom) for films coated at the critical shear speeds at three different temperatures.

comingand make solvent selection a more complex process than simple screening by boiling point.

nucleation regime (II) does indeed cause more crystallite formation and growth to happen when less solvent is available in the drying film to plasticize the polymer and facilitate reorganization. The data also reveal that aggregates in solution are likely to be nuclei for crystallites in the film because the Q(100) and fwhm values do not change over time. A singlepeak estimate of paracrystallinity demonstrated that the top interface is always more disordered than the bulk, and the

4. CONCLUSION In summary, in situ 2D-GIWAXSspecifically, the evolution of (100) peak intensity and χ-resolved out-of-plane intensity indicates that faster total solvent evaporation in the surface L


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nucleation and crystallization do not occur. Lastly, we believe that for the aggregates to be aligned, intuitively they must be anisotropically shapedpolymers with spherical aggregates in solution with a particular solvent cannot be aligned. In our case, high dichroism, if any at all, is achievable only for specific solvents, and we detail our analysis of these effects in a subsequent report. Although these results shed light on the complicated array of processes that occur during MGC, much work needs to be done in examining the various other phenomenaupstream flow and shear fields, viscosification near the contact line, interfacial (Marangoni) flows, concentration gradients, evaporation of solvent blends, temperature gradients, etc.affecting the final thin-film morphology. The MGC of semicrystalline conjugated polymers is a complex interplay of these factors, and certain chemical and macromolecular properties may alter the relative importance of some processes over others.66 Nonetheless, the general principles uncovered here provide a better understanding of the phenomenon underlying deposition using MGC techniques overall. Given that the crystallization process and the resulting microstructure of the solid film are crucial considerations for the development of organic electronic devices, we believe these insights are a crucial step for the continued refinement of solution-phase deposition in highthroughput, industrially relevant fabrication processes.

degree of disorder increases monotonically as coating speed increases. The films’ out-of-plane crystalline texture is not preserved at higher coating speeds, which is consistent with a bottom-up ordering of these films during solidification in the shear-dominate regime (I) that is disrupted by the rapid solvent evaporation of regimes II and III. By varying the deposition temperature, we observe that this texturing is further affected by overall solvent evaporation, with higher temperature films exhibiting no preferential orientationsimilar to regime III (LLD) filmseven at the respective critical shear speeds. By quantifying the relative degree of crystallinity of these films, we established that the amorphous regions of the film are aligned by this type of coating, a phenomenon not previously reported for materials lacking intrinsic liquid-crystalline behavior. These extensive X-ray analyses indicate that, in addition to the commonly known transition between the evaporative and the LLD regimes, there exists another transition related to solute crystallization that is intimately involved with solvent evaporation. The transition is caused by of the competition between the relative importance of substrate-nucleated crystallites and upward crystallite growth versus stochastic nucleation of disordered crystallites at the air−liquid interface, which disrupts well-ordered film growth and in-plane alignment. PDPP3T’s ability to be aligned by solution shearing proves to be useful in demarcating a regime of slow, wellordered film deposition and one of greater disorder and rapid solvent evaporation. In the Supporting Information, we have elaborated on these observations by suggesting a model of the deposition mechanism that relates the solvent volume fraction ϕs of the drying thin film as a function of film height (Figure S13). While our work has focused on a strongly aggregating and crystalline donor−acceptor polymer, we believe that this study has uncovered principles general to polymer semiconductors deposited by MGC methods. For example, we believe shear strain imparted by the coating blade acts primarily on polymer aggregates as opposed to free molecules. The relative paucity of reports demonstrating the alignment of nonliquid-crystalline semiconducting polymers suggests that specific conditions are crucial for high in-plane alignment, and it is mostly in cases of clear solution-phase polymer aggregation where there is evidence of alignment in deposited films. Second, the alignment of polymer tie-chains in the amorphous film fraction is likely intimately related to the alignment of aggregates, especially when considering the polymer molecules that are only partially incorporated in a crystallite that are otherwise dangling out of these crystallites. We speculate that fully amorphous semiconducting polymers cannot be aligned in single-step processes because of (1) the need for crystallites to act as “anchors” for tie-chain alignment during unidirectional coating and (2) the relatively low molecular weights of these semiconductors (104− 105 Da) as compared to conventional commodity polymers where factors like entanglement density become more significant. Moreover, amorphous polymers clearly would not have two separate regimes within the evaporative regime since nucleation and crystallite growth are not important processes during drying. Third, our three-regime model is a straightforward explanation of the peak-like (as opposed to plateauing) behavior of polymer alignment that accounts for the fact that (1) the solute will naturally undergo a nucleation and crystal growth process and (2) faster total solvent evaporation reduces the time for good crystalline ordering. This expands the tworegime model previously developed for colloidal systems where

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00350. Plots of 2D-GIWAXS intensity data for the (100) reflection and solvent scattering halo, partial pole figures of the (100) peak, solution-phase UV−vis spectra for PDPP3T solutions, fwhm and paracrystallinity values for the (100), (200), and (010) peaks, anisotropy comparison between the 2D-GIWAXS and optical dichroism data, DOSY-NMR data for a PDPP3T solution, φresolved and fully integrated (100) pole figures, model for solution concentration profile during solvent evaporation, analysis of total solvent evaporation time (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(D.K.) E-mail [email protected]. *(M.F.T.) E-mail [email protected]. *(Z.B.) E-mail [email protected]. ORCID

Leo Shaw: 0000-0003-1182-5537 R. Thomas Weitz: 0000-0001-5404-7355 Michael F. Toney: 0000-0002-7513-1166 Zhenan Bao: 0000-0002-0972-1715 Present Addresses

X.G.: School of Polymers and High Performance Materials, University of Southern Mississippi, Hattiesburg, MS 39406. R.T.W.: Physics of Nanosystems, Physics Department, NanoSystems Initiative Munich (NIM) and Center for NanoScience (CeNS), LMU Munich, Amalienstrasse 54, 80799 Munich, Germany. M


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.S. gratefully acknowledges support from the Kodak Graduate Fellowship. The authors acknowledge support from the US Department of Energy, Office Basic Energy Sciences, Division of Material Science and Engineering, Program on Physical Behaviors of Materials (DE-SC0016523) and BASF SE. 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 DE-AC02-76SF00515. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under Award ECCS-1542152. We also thank Stephen R. Lynch for his help with performing the NMR experiments.

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ABBREVIATIONS D−A, donor−acceptor; UV−vis, ultraviolet−visible; LLD, Landau−Levich−Derjaguin; 2D-GIWAXS, two-dimensional grazing-incidence wide-angle X-ray scattering; fwhm, full width at half-maximum; PDI, polydispersity

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