Assembly of PCBM and tn-ZnPc Molecular Domains and Phase

Nov 7, 2017 - We present the results of investigations aimed at understanding the mechanisms by which small, mutually immiscible organic molecules sel...
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Article Cite This: Langmuir 2017, 33, 13657-13668

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Assembly of PCBM and tn-ZnPc Molecular Domains and Phase Separation of Molecular Mixtures from Liquid Solution on Si(111) Miriam Cezza, Colin S. Qualters, and Raymond J. Phaneuf* Department of Materials Science and Engineering, University of Maryland, College Park 20742, Maryland, Unites States ABSTRACT: We present the results of investigations aimed at understanding the mechanisms by which small, mutually immiscible organic molecules self-assemble into domains during phase separation from liquid solutions in the presence of a solid substrate. As an example system, we investigated molecular mixtures of tetranitro zinc-phthalocyanine (tnZnPc), an electron donor, and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), an electron acceptor, in chloroform solution, deposited onto native oxide-covered Si(111) substrates. We found qualitatively different behavior in the formation of domains of the two types of molecules, seemingly because of a large difference in their solubilities in the solvent. The morphology of PCBM molecule domains varies widely depending on the solvent evaporation rate, the presence/absence of tn-ZnPc molecules in the solution, solvent annealing conditions, and seemingly the presence/absence of trace impurities. By contrast, we find evidence that the shape of the tn-ZnPc domains is insensitive to evaporation; these form while in liquid suspension, and the assembly of domains into clusters varies according to the rate and mode of solvent evaporation.

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grow outward from it. The result would be a “vertical” phase separation, with a morphology approximating to that regarded as optimal for devices such as OSCs.1 If the system is not too far from equilibrium, once the phase separation begins, the dominant lateral domain spacing, as they grow upward from the substrate, should be controlled by capillarity, specifically by the ratio of the interface free energy per area between domains and the “supersaturation” or driving force per volume for the separation.5 The energetic barrier to nucleation of the phase separation, in which the formation of the individual molecular domains requires overcoming an energy of activation associated with the formation of the interfaces between these and the initial uniform solution,4 should be proportional to the interface free energy per area times the square of the lateral domain spacing;5 this barrier can be reduced for “heterogeneous” nucleation, as long as the individual solute domains wet the substrate.5 In principle, an appropriate templating of the substrate,6,7 with a lateral template period matching the fastest propagating lateral domain spacing, might be employed to direct the initial phase separation. This simple model, however, leaves out a number of effects that might lead to different modes of solute domain assembly and thus different domain morphologies. Among these is the competing mechanism of spontaneous (spinodal) decomposition,8,9 which should dominate if the nucleation barrier is small enough. This would lead to phase separation via local

INTRODUCTION There is a great deal of interest in the scientific and engineering communities in understanding the effects that control the phase separation of immiscible organic molecules from solution. On the engineering side, this interest is in large part because of applications to organic solar cells (OSCs), where a finely dispersed network of boundaries between the domains of electron donor and electron acceptor molecules can enhance the photoconversion efficiency of such devices.1 The optimum domain size in such systems is thought to be comparable to the diffusion length of an exciton before recombination, typically approximately 10 nm.1,2 A second important consideration in the OSC photoconversion efficiency is the degree of molecular order or crystallinity within domains, which is generally thought to increase the exciton diffusion length.3 On the science side, such systems are of interest for understanding how the kinetics and thermodynamics of phase separations combine to determine the dominant molecular domain morphology. A grand challenge for eventual OSC applications is to achieve phase separation on a practical time scale, into mesoscale domains, in which there is a good crystalline order. An appealing potential approach for meeting this challenge is suggested by a highly simplified model of the kinetics of phase separations in the regime of nucleation and growth,4 where interface effects can direct where the assembly starts and how it propagates. In this model, the solvent allows an initially uniform mixture of two solutes, which would be immiscible in a solid film. As the solvent evaporates, the individual solutes become supersaturated and precipitate into alternating domains, each of which would nucleate at the substrate and © 2017 American Chemical Society

Received: September 13, 2017 Revised: October 27, 2017 Published: November 7, 2017 13657

DOI: 10.1021/acs.langmuir.7b03233 Langmuir 2017, 33, 13657−13668

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Langmuir fluctuations in the composition from the initial uniform solution, which grows exponentially with time because of an instability, rather than metastability of that solution, making it difficult to direct by using a substrate. This mechanism may well govern the phase separation at the small lateral domain spacing desired, given the expected power law scaling of the nucleation barrier to this. A second set of complicating effects is associated with the possibility that each solute may interact differently with the substrate, the solvent, or both. This would lead to a difference in the wettability of the substrate by the individual solute domains in the former case and a difference in the liquid-phase solubility in the latter. Sequential, rather than parallel precipitation of solutes can result. A third effect is that associated with “solvent vapor annealing” (SVA), in which a solvent continues to interact with the precipitated solute domains after its evaporation.10−13 In relatively closed environments, the solvent vapor does not immediately escape, but instead can evaporate and recondense repeatedly, maintaining contact with the precipitated film, increasing the mobility of film molecules,14 and potentially favoring more highly ordered self-assembly of molecules within domains.12,15−17 Yet, a fourth potentially complicating effect can arise from the presence of “impurities” in solution, leading to instabilities in domain growth fronts.18,19 Finally, in deposition from liquid-phase solution, convective currents are generally present and may produce considerably different domain morphologies than that suggested by the simple picture considered above.20−23 While motivated by potential applications for OSCs, the study we present here is intended to probe the underlying science of phase separations in such a system, rather than the engineering of OSCs. In this article, we present the results of investigations of phase separation in a prototypical donor/ acceptor molecular mixture system: [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and tetranitro zinc-phthalocyanine (tn-ZnPc) molecules deposited from chloroform-based liquid solutions onto Si(111) substrates. We find that varying the deposition technique and the solvent evaporation rate results in a rich variety of domain morphologies, providing evidence for a number of these complicating effects during phase separation.

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donor molecules for this study because of their thermal and chemical stability.25 Additionally, phthalocyanines functionalized with nitro functional groups are more soluble in organic solvents.26 PCBM is a fullerene derivative molecule created by adding a phenyl group and an ester group to the buckminsterfullerene C60 (illustrated in the right panel of Figure 1). It shows enhanced solubility over C60 in many organic solvents.27 It also acts as an electron acceptor with a high electron mobility. The molecules are deposited from chloroform-based solutions onto native oxide-covered silicon substrates using a variety of techniques described below. We chose chloroform as the solvent based on preliminary solubility tests. The choice of Si(111) substrates was mainly due to the near-atomic surface flatness available. We prepared the substrates via RCA cleaning28 prior to molecular deposition; this process leaves a native oxide at the surface. “Individual solute molecular” solutions (i.e., PCBM in CHCl3 and tn-ZnPC in CHCl3) were prepared in chloroform at concentrations of 0.25 mM; to improve dissolution, these solutions were sonicated for 10 min. “Molecular mixture” solutions were prepared by subsequently mixing equal volumes of individual solute molecular solutions, resulting in a mixture whose ratio of concentrations is equal to 1:1. This procedure produced individual molecular concentrations, which were halved compared to that for individual solute molecular solutions, that is, 0.125 mM. Both individual solute molecules and molecular mixtures were deposited onto the substrate in a N2-atmosphere glovebox using one of three different methods: (1) spin-coating, (2) drop-casting, and (3) substrate immersion-in-solution. For this last method, we explored two variations: substrate immersion in “open” and “nearly closed” containers. These deposition methods are illustrated schematically in Figure 2. Spin-coating results in a solvent evaporation rate, which increases with the substrate rotation frequency.29,30 We investigated the results of this mode of deposition at a relatively high substrate rotation frequency of 2000 rpm for a duration well beyond the approximately 0.8 s that it takes for the solvent to disappear visually. Because of the rapid evaporation, we can only estimate the average rate based upon visual observations of the time it takes for a 50 μL drop to disappear. From a frame-by-frame analysis of video sequences recorded during spin-coating, we find an average solvent evaporation rate of 0.060 ± 0.007 mL/s. This by far is the fastest rate for those investigated in this study. In analogy to the result of quenching the temperature of a solution beneath a phase boundary associated with a miscibility gap, a sufficiently abrupt increase in the solute concentration (effectively a “quench”) might be expected to favor spinodal decomposition8,9 over nucleation of the phase separation.4 In addition, a rapid loss of the solvent might be expected to suppress the ordering of molecules within the clusters. The drop-casting method allows molecular deposition at lower rates of solvent evaporation than spin-coating; we measure rates very close to that for pure solvent under ambient conditions. In our experiments, we oriented the substrate surfaces precisely horizontally to avoid the solution running off the surface. This method is illustrated schematically in the second panel of Figure 2. During molecular deposition via immersion-in-solution, the substrate was placed into a cylindrical container with one open end (diameter ≈ 1.8 cm). We found that the rate of evaporation of the solvent was approximately 1 order of magnitude slower than that for the drop-casting method. The sample was supported on a pedestal above the container bottom, as illustrated schematically in Figure 2. In this geometry even once the solvent has evaporated to a level below the sample surface, it should act as the source of vapor, and continued interaction with deposited molecular domains10−13 might be expected to occur. In a variation on this technique, we decreased the evaporation rate further by covering the cylindrical well with a cap containing a small aperture (diameter ≈ 0.5 cm). We expect that this more closed configuration should result in a higher solvent partial pressure above the surface, once the liquid has evaporated to a level below the sample support, presumably extending the time during which the solvent vapor interacts with molecular domains.

EXPERIMENTAL SECTION

tn-ZnPcs are phthalocyanine (Pc) molecules with nitro functional groups (−NO2) attached to each of four isoindole groups (illustrated schematically in the left panel of Figure 1). Nitro functional groups withdraw electrons strongly from the metal core of a Pc molecule, making these molecules very good electron-donor organic semiconductors.24 Phthalocyanines were selected as interesting electron-

Figure 1. Structures of tn-ZnPc and PCBM molecules used in this study. 13658

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Figure 2. Schematic illustrations of the methods of deposition used in this work. The last panel on the right illustrates a variation on the deposition by substrate immersion to achieve even lower evaporation rates. The experimentally determined evaporation rates during spincoating, drop-casting, immersion in solvent in a nearly open-topped, and nearly close-topped containers are (3.6 ± 0.4), (0.030 ± 0.002), (0.00347 ± 0.00001), and (0.0017 ± 0.0005) mL/min, respectively; the first was determined by a frame-by-frame analysis of the video images recorded during spin-coating, the second was determined by placing the substrate on a balance and recording the mass versus time during drop-casting, and the last two were determined by placing the container on a balance and recording the mass versus time during solvent evaporation. This represents a range of more than 3 orders of magnitude.

ray intensity. (By contrast, as we show later in this report, there is indirect evidence for crystallinity of PCBM clusters resulting from much slower solvent evaporation rates.) The spherical cap height measured above the substrate is 1.5 ± 0.5 nm, whereas the apparent diameter at the base (neglecting tip convolution effects) of the spherical cap domains is 80 ± 6 nm, corresponding to an apparent contact angle of 4.3° ± 1.5°. We find that because of the smallness of cap heights, deconvolution from the atomic force microscope tip shape (nominally 8 nm terminal radius, with a 30° conical angle) has a negligible effect on this value; an approximate, rigid tip/rigid feature deconvolution31 increases the contact angle by only approximately 0.02°. The measured number of domains per area is 145 ± 1 caps/μm2, whereas the measured area fraction of the surface covered by the PCBM domains (calculated from AFM images, using the area projected by the PCBM domains onto the surface plane) is 59 ± 1 area %. We find that for spin-casting deposition from a (1:1) PCBM/ tn-ZnPc molecular mixture molecules, compared to the deposition from PCBM-alone in CHCl3 solution, the cap heights increase, to 3.9 ± 0.9 nm, and the apparent cap base diameters increase, to 100 ± 5 nm. This corresponds to an apparent contact angle of 8.9° ± 2.0°. A rigid tip/rigid cap deconvolution changes this very slightly to 9.0° ± 2.0°. The approximate doubling of the contact angle for the mixture compared to that for deposition of PCBM alone is consistent with a modification of the relevant PCBM/substrate interface tensions so as to reduce the wettability of the substrate by PCBM molecules. Below, we present evidence that this is due to the presence of a “wetting layer” of either tn-ZnPc or a related impurity on the Si substrate. As compared to deposition from a PCBM-alone in CHCl3 solution, the area-density of spherical cap domains and the fractional area coverage of spherical caps on the surface from the mixed solution both decrease to 69 ± 2 caps/μm2 and 41 ± 1 area %, respectively. Finally, we analyzed the arrangement of spherical cap domains for evidence of a dominant spacing. Fourier analysis of our AFM images shows that there is indeed a well-defined periodicity for spin-cast deposition both from PCBM-only solution and from a solution of a (1:1) mixture with tn-ZnPc. The values of the dominant lateral cap spacing are 53 ± 10 nm for the PCBM-individual molecule solution and 59 ± 11 nm for the molecular mixture solution, that is, the same to within the uncertainty in the determination. This might indicate that under conditions of rapid solvent evaporation, the kinetics are those of spontaneous (spinodal) decomposition, rather than nucleation.8,9 Indeed, one signature of spinodal decomposition is the dominance of a fastest growing wavelength in the evolving system.8,32−34 Our observation of a preferred spacing might, however, instead indicate that the propagation of the domain growth front is cellular due to the presence of a trace

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RESULTS AND DISCUSSION Fastest Evaporation: Spin-Casting of PCBM-Only, tnZnPc-Only, and (1:1) Mixture. Example results for PCBM, initially dissolved in chloroform and then deposited onto the substrate via spin-coating, are shown in the form of atomic force microscopy (AFM) topography maps in Figure 3A. We find that in this case, PCBM molecules assemble into spherical cap-shaped domains. The lack of anisotropy in the domain shape suggests a disordered molecular arrangement within them; indeed X-ray diffraction (XRD) measurements on these samples do not show detectable Bragg peaks in the scattered X-

Figure 3. (A,B) AFM topography maps showing the spherical cap molecular domains resulting from spin-coating deposition from (A) PCBM-alone in chloroform solution and (B) from a (1:1) PCBM/tnZnPc mixture in chloroform solution. (C,D) AFM topography maps showing rod-shaped molecular domains resulting from spin-coating deposition from tn-ZnPc-alone in chloroform solution (C) and from a (1:1) PCBM/tn-ZnPc mixture in chloroform solution (D). All depositions were performed at 2000 rpm. (B) shows the magnification of a region between rod-shaped domains in the image in (D). 13659

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Figure 4. Histogram of hydrodynamic radii (Rh) of particles in a tn-ZnPc in chloroform solution (left) and in a (1:1) mixture of tn-ZnPc/PCBM in chloroform solution (right) at different overall concentrations (individual molecular concentrations are one-half of the listed values in the right panel), as determined by DLS.

Figure 5. Force curves measured (A) between a spherical probe (1.9 μm radius) AFM tip and a native oxide-covered Si(111) surface (black open squares) and between the same spherical probe and “flat” regions between rod-shaped domains resulting from the deposition from tn-ZnPc-alone in chloroform solution (red open circles); (B) between a conical (8 nm radius, 30° conical angle) AFM probe tip and a native oxide-covered Si(111) surface (black open squares) and between the conical probe tip and flat regions between molecular domains deposited from (1:1) tn-ZnPc/PCBM mixture in chloroform solution (red open circles).

impurity in solution; more systematic, time-resolved studies would be required to distinguish between these possibilities. We find evidence for qualitatively different behavior in the assembly of domains of tn-ZnPc molecules during the spincoating deposition. As seen in the AFM image of Figure 3C, assembly from an individual tn-ZnPc molecular solution is into large rod-shaped clusters. The measured height of these clusters is 172 ± 29 nm, whereas the apparent width, that is, without atomic force microscope tip deconvolution, is 328 ± 35 nm. A simple rigid tip/rigid feature deconvolution results in smaller widths, 242 ± 26 nm. This indicates an anisotropy in the shape even after deconvolution, with the rod height/width ratio equal to 0.71 ± 0.14. It may reflect an anisotropy during molecular assembly along with a preference for an orientation that maximizes the attractive van der Waals interaction with the substrate, a deformation of cylindrically shaped rods because of that interaction, or a deformation of the rods during AFM imaging. We find that the measured rod lengths vary considerably, ranging from 0.29 to 2.3 μm. The area fraction of the surface covered by tn-ZnPc rods is 10.25 ± 0.05 area %. Given the low solubility of tn-ZnPc in chloroform (our experimentally estimated value is ∼0.28 mg/mL, i.e., 0.37 mM) and the high evaporation rate of the solvent, it seems possible that individual molecules might start to agglomerate into rods while still in liquid suspension, which then simply settle onto

the substrate. To address this possibility, we carried out dynamic light scattering (DLS) measurements from solutions of tn-ZnPc in CHCl3 over a wide range of concentrations. Our results, shown in Figure 4, indeed indicate the presence of agglomerates in suspension even at low concentrations, with a dominant hydrodynamic radius Rh ≈ 100 nm. A careful examination of the DLS spectrum in the left panel of Figure 4 shows a smaller peak in the size distribution for higher concentrations at a much larger value of the hydrodynamic radius, that is, in micrometers. This second peak, which we attribute to micron-size assemblies of rods, is much stronger in the measured DLS spectra from tn-ZnPc-alone in solution than in tn-ZnPc/PCBM molecular mixture solutions. Evidently, the presence of PCBM in solution reduces the tendency toward rod agglomeration. In both cases, these results support the hypothesis that the rods observed in the AFM topography maps of Figure 3C,D are formed while in liquid solution and subsequently settle out onto the substrate. Figure 3D shows the AFM image of the rod-shaped clusters, which assemble during the spin-coating deposition of tn-ZnPc from a (1:1) molecular mixture solution. We find that the dimensions of the domains are smaller compared with those resulting from a solution of tn-ZnPc-alone in chloroform; we do not see evidence for any very-large rods, as might be expected from the bimodal distribution seen in the DLS spectra 13660

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Figure 6. Microscopy images of the structures assembled via drop-casting of PCBM in chloroform solution onto a Si(111) surface: (A) light optical image acquired near the edge of the sample; (B) light optical image acquired in a region intermediate between the sample edge and center; (C) light optical image acquired near the center of the sample showing a continuous “blanket phase”; (D) AFM topography map of a circular depletion zone appearing in the blanket phase; and (E) corresponding AFM phase map across the structure visible at the center of the void.

finite % yield in the synthesis of a complicated molecule such as tn-ZnPc, along with the limit to which the procedures used in synthetic chemistry can purify the desired end product.37 We also find evidence for the presence of a wetting layer in the films deposited from a (1:1) PCBM/tn-ZnPc mixture in chloroform solution; in this case, we performed force curve measurements in the small areas between PCBM spherical capshaped domains formed on deposition. Because of the relatively small interdomain separation, we used a much smaller probe for these measurements: a conical AFM tip with a terminal radius of ∼8 nm, a conical angle of ∼30°, and a height of ∼15 μm. We compared the adhesion forces with those obtained between the tip and bare silicon and again found a larger adhesive force in areas between PCBM cap-shaped clusters (Figure 5B). This result is consistent with the hypothesis given above in the discussion of Figure 3, that a wetting layer is present in films resulting from deposition from a mixed solution, which lowers the substrate surface tension, reducing the wettability by the PCBM domains. We note in passing that the substantial difference in the values of the adhesion forces measured with a large spherical and a much smaller conical probe is expected qualitatively because of the difference in the contact area between the probe and the sample; the contact area is much smaller for the conical tip, producing smaller interaction forces.36 To summarize this section, we find that the self-assembly of PCBM molecules into spherical cap-shaped domains during the spin-coating onto Si substrates from chloroform-based solution is measurably affected by the presence/absence of tn-ZnPc in solution, with a wetting angle which differs by approximately a factor of 2. By contrast, tn-ZnPc molecule domain shapes are seemingly less sensitive to the presence/absence of PCBM. Our results suggest that PCBM molecules assemble into cap-shaped domains at the substrate, whereas the formation of rod-shaped tn-ZnPc domains instead apparently occurs in liquid solution. Finally, subsequent to spin-coating from solutions containing tn-ZnPc, we observe evidence for a wetting layer on the Si

in the right panel of Figure 4. A possible interpretation is that in a mixed solution, there is a preference for somewhat smaller tnZnPc rod-shaped domains, along with an increased tendency for individual domains to aggregate into rod-clusters while still in solution, especially at higher concentrations of tn-ZnPc. The rod heights measured above the substrate are 127 ± 29 nm, whereas the apparent widths are 196 ± 28 nm; tip shape deconvolution yields rod widths of 141 ± 21 nm with a height/ width ratio of 0.90 ± 0.24. The area coverage is smaller compared to that for deposition from a tn-ZnPc-alone solution. We find a value of 4.5 ± 0.5%, that is, roughly half of that from Figure 3C, which is in good agreement with the halving of concentration of tn-ZnPc molecules in the mixed solution. As a means of characterizing the chemical termination of the various structures resulting from deposition from the solution onto the silicon substrate, we measured adhesion forces (Fad) between them and an atomic force microscope tip. Of most interest for this discussion are measurements in the “flat” regions between the molecular domains, which we contrast with similar measurements from a “bare” (i.e., native oxidecovered) Si(111) substrate. For such measurements, subsequent to deposition from tn-ZnPc-alone in chloroform solution, we used a spherical, SiO2 AFM probe whose radius was approximately 1.9 μm. The measured adhesion forces between the probe and the surface differ by approximately a factor of 2; 352 ± 7 nN for bare silicon versus 656 ± 7 nN for the “flat” regions between rod-shaped domains of tn-ZnPc, as shown in Figure 5A. The much larger adhesive force in the second case is consistent with the presence of a “wetting” layer with either (1) a substantial electrostatic dipole moment,35 (2) a large polarizability,35 (3) a significantly lower modulus leading to a pronounced increased contact area in the latter case,35,36 or (4) some combination of these. In any case, the results argue strongly that the termination of the flat region is different from bare silicon, as would be the case if there is a wetting layer between the rods. Such a wetting layer could be made up of tnZnPc molecules, or possibly some related impurity. The presence of such an impurity would not be surprising given the 13661

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Figure 7. (A) Plan-view light optical image and (B,C) grazing incidence SEM images of domains resulting from immersive deposition of PCBM on native oxide-terminated Si(111) (solvent evaporation rate = 0.00347 mL/min). (D−F) Light optical images of structures resulting from immersive deposition at a solvent evaporation rate of 0.0017 mL/min: (D) pyramidal assembly of PCBM plate domains; (E) detail of central hexagonal domain from image (D); and (F) dendritic domain assembly.

shape transition is that the spherical cap domains represent a “first-formed” structure for which the energetic barrier for separation from the liquid solution is relatively small, and that the spherical caps, dendritic film, and blanket phase represent a “cascade” series of increasingly stable structures that form in analogy to what has been observed in precipitation from solid solution.38 In this interpretation, the initiation of the evaporation front at the edges gives the regions near the center a more prolonged contact between the solution and substrate, allowing sufficient time for phase changes. Analysis of our AFM topography maps indicates that the average PCBM coverage near the sample center is approximately a factor of 10 higher than near its edges: θ ≈ 8 × 1015 molecules/cm2 versus 8 × 1014 molecules/cm2. The increasing concentrations of PCBM molecules toward the center of the substrate subsequent to drop-cast deposition is qualitatively consistent with what would be expected if precipitation of the excess solute occurs over a solution/substrate contact area, which shrinks with time. We note, however, that such a simple picture neglects a number of complicating effects, including the convective currents associated with the heat of vaporization, the “Marangoni flow.” Previous investigators proposed that such an effect can also contribute to an enhanced deposition rate near the center of a sessile droplet, reversing the tendency for the formation of “coffee-ring” deposits at the edges.20 We return to this issue below. Slowest Evaporation: Immersion Deposition of PCBM from Chloroform Solution. In this section, we discuss the results of lowering the evaporation rate of the solvent yet further during PCBM deposition, by immersion of the substrate into the solution. We oriented the Si substrates horizontally in most of these investigations; we discuss the significance of this in more detail in a later section. In the first part of this investigation, the solution was held in a cylindrical container whose top surface was open to atmosphere. Measurements of the solution weight versus time showed that this approach produced a solvent evaporation rate of 0.00347 mL/min, that is, approximately 1 order of magnitude slower than that for drop-casting. In this approach, even after evaporation of the bulk solution to a level below that of the substrate, the

substrate made up of either tn-ZnPc molecules or possibly some associated impurity. Intermediate Evaporation Rate: Drop-Casting of PCBM from Chloroform Solution. Deposition via dropcasting produces slower solvent evaporation than spin-coating, allowing solute molecules to interact with each other more extensively during assembly into domains. Visually, we observe that solvent evaporation initiates at the edges of the substrate and that the evaporation front progresses inward toward its center. We find that the resulting PCBM domains show a variation in their morphology with the position across the substrate. As seen in the light optical microscopy image in Figure 6A, PCBM molecules drop-cast from a chloroform-based solution assemble into spherical cap-shaped domains near the sample edges; the diameters of these domains are ∼2−3 μm, that is, more than an order of magnitude larger than those produced by spin-casting. Figure 6B shows a light optical image recorded from the same sample, but in a region located between the edge and center of the sample. Instead of spherical cap domains, here the image shows a dendritic, nearly continuous film. XRD measurements do not show sharp reflections, suggesting that this phase is amorphous. Figure 6C shows a light optical image acquired near the sample center; the appearance is uniform, suggesting the presence of a continuous, “blanket” phase of PCBM covering the substrate. Visual observations in this region show a brownish cast. Significantly, AFM observations in this region reveal occasional, nearly circular voids in the blanket phase, within which regions of much taller, anisotropic structures are found. An example is visible in the AFM images shown in Figure 6D,E. A plausible interpretation is that the growth of these anisotropic structures depletes the surrounding blanket phase of PCBM molecules, resulting in the surrounding voids. We find additional evidence, presented below, that such domains may be the precursor to much larger, crystalline domains of PCBM that dominate the film structure at even slower solvent evaporation rates. We now consider what might cause the shape transition from spherical cap domains near the sample edge to the continuous blanket phase near its center. One plausible explanation for the 13662

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Figure 8. SEM images of tn-ZnPc rod-shaped domains deposited from single component solution via (A) drop-casting, (B) immersive deposition (substrate oriented vertically) from a tn-ZnPc solution at a solvent evaporation rate of 0.00347 mL/min (open-topped container), and (C) immersive deposition (substrate oriented vertically) from a tn-ZnPc solution at a solvent evaporation rate of 0.0017 mL/min (nearly close-topped container).

that dendritic growth in some systems can be an indication of the presence of small concentrations of impurities in the solution.5,18,34 Indeed, the overall morphology of assemblies like that shown in Figure 7F is reminiscent of that resulting from Mullins−Sekerka instabilities,39,40 which occur in the presence of a subcritical spatial gradient in the supersaturation of an impurity. To conclude this section, at the slow rates of solvent evaporation characteristic of immersive deposition, PCBM molecules assemble into large hexagonal domains on the substrate. The slowest solvent evaporation rates studied here promote dendritic assembly of these domains, suggestive of growth instability and consistent with the presence of an unidentified impurity in the solution. Intermediate and Slow Evaporation: Drop-Casting and Immersive Deposition of tn-ZnPc. In this section, we present the results of deposition from a tn-ZnPc-alone in chloroform solution onto native oxide-terminated Si(111) substrates via drop-casting and via substrate immersion. We find that drop-casting of tn-ZnPc from chloroform-based solution onto Si substrates results in a bright blue sample (visual appearance) in contrast to the brown cast obtained for PCBM films deposited in a similar manner. SEM images like those shown in Figure 8A reveal that this procedure results in the formation of rod-shaped domains, whose size and shape are essentially identical to those observed after spin-casting deposition. Significantly, the density of such domains is not uniform across the substrate. Rather, we observe concentric, high-density, “coffee-ring” assemblies of these rod-shaped domains. Further, while coffee-ring domain assemblies are visible across the entire substrate, larger concentrations occur at the edges of the sample than near the center. Previous observations of coffee-ring particle assemblies subsequent to sessile droplet evaporation have been interpreted as resulting from an outward flow of solution from the droplet bulk21 because of the pinning of the contact line between the droplet and substrate by particles. Shen et al.23 proposed that multiple colloidal particles are required to pin the droplet edge, and that a competition between the rate at which particles assemble and that at which the solvent evaporates determines whether or not coffee-ring deposits form. We suppose that in our case, the relative rates are such that a relatively small number of rods assemble locally, pinning the droplet edge only until the net surface tension force exceeds that resulting from the interaction between these rods and the droplet. The pinning is thus intermittent, and arrays of concentric coffee-rings result. The observation of higher concentration of coffee-ring rod assemblies near the edges is consistent with the progressive

chloroform solvent vapor is expected to continue to interact with the deposited film, allowing some degree of SVA. Indeed, unlike after either spin-coating or drop-casting deposition, we find evidence for Cl atoms on the PCBM domains subsequent to immersive deposition, from the energy dispersive X-ray spectrometry (EDS) analysis. Figure 7A,B shows light optical and scanning electron microscopy (SEM) images, respectively, of single hexagonal plate domains resulting from immersive deposition from a solution of PCBM in chloroform. These are very large: the observed lateral domain widths range from 40 to 100 μm, and the plate thicknesses range from 250 to 700 nm. The presence of facets in their shapes indicates anisotropy in the surface energy, in the growth velocity, or in both; it is strongly suggestive of a crystalline order. The domain shape change, as compared to those seen for spin-casting and drop-casting, is likely due to the far slower solvent evaporation rate, which gives individual solute molecules more time to interact with each other during molecular phase separation from liquid solution, presumably allowing enhanced molecular self-ordering. One interesting possibility is that the shape change results from passing through a spinodal: that the slow rate of solvent evaporation allows the system sufficient time for crystalline nucleation after passing through the phase boundary, whereas the spherical cap domains seen for faster evaporation rates result from fast quenching beneath the spinodal curve.8,9 A second possibility, mentioned in the previous section, is that the system passes through a cascade of phases of increasing stability, with the order in which the phases form dictated by the relative heights of the energetic barriers associated with the successive transitions.38 Interestingly, in addition to single domains like that seen in Figure 7A,B, in some locations on the substrate, we observe larger scale assemblies of multiple domains, for example, the vertical stack seen in Figure 7C. This may indicate that heterogeneous nucleation of a new domain is favorable over sites on existing domains. PCBM deposition at solvent evaporation rates approximately a factor of 2 lower (∼0.0017 mL/min) was achieved by covering the cylindrical container holding the solution with a cap containing a small aperture. An example of the resulting structures is seen in the light optical image of Figure 7D. Rather than simply forming larger hexagonal plates, this very slow evaporation produces pyramidal, dendritic assemblies of such domains; compare, for example, Figure 7A,E. The assembly is seemingly formed around a central hexagonal domain, visible at the apex (Figure 7E). These results suggest that the individual domain sizes effectively self-limit, that is, beyond a certain size, additional domains nucleate at those that already exist. We note 13663

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whereas the larger scale assembly into domain clusters on the substrate varies considerably. Drop-Casting and Immersive Deposition from (1:1) tn-ZnPc/PCBM Mixed Solutions. We find that drop-casting of (1:1) mixtures of PCBM and tn-ZnPc in chloroform solution onto Si substrates results in an apparent gray sample color, noticeably different from both the bright blue cast resulting from deposition from tn-ZnPc-alone in CH3Cl and the brown cast resulting from deposition from PCBM-alone in CH3Cl. As we discuss below, the AFM maps of the films resulting from drop-casting from mixed solution show a number of structures observed after deposition from chloroform solutions of PCBMalone and tn-ZnPc-alone. A comparison of height-contrast and phase-contrast AFM images (Figure 9A,B) shows evidence that a blanketlike phase

depletion of rods from the droplet as its contact line with the substrate moves inward. This morphology contrasts sharply with that observed for drop-casting of PCBM, where no such assemblies of domains are observed. We interpret this difference as due to two related effects. First, the solubility of tn-ZnPc in chloroform is very low compared to that of PCBM (determined experimentally to be ∼0.28 mg/mL for the former, compared to ∼25 mg/mL for the latter); this provides a much higher supersaturation of tn-ZnPc at a given concentration, seemingly allowing for homogeneous nucleation of rod-shaped domains within liquid solution in the former case, whereas the lower rate of supersaturation favors heterogeneous nucleation of PCBM onto the substrate in the latter. Second, the much larger rod-shaped tn-ZnPc domains move more slowly through solution via diffusion and/or capillary flow than the isolated PCBM molecules, making them more likely to be trapped at the receding edge of the droplet during solvent evaporation. We next explored the effect of decreasing the rate of solvent evaporation, using deposition via immersion of the Si substrate into a solution of tn-ZnPc-alone in chloroform. As for the PCBM-alone in chloroform solution we carried this out using both open-topped (Figure 8B) and nearly close-topped containers (Figure 8C). In each of these two cases, we observe rod-shaped domains whose sizes and shapes are similar to those resulting from either spin-coating or from drop-casting deposition. Small quantities of chlorine atoms were detected on the rod-shaped domains by EDS, indicating that the solvent molecules adsorb onto these and raising the possibility that some degree of SVA may occur during the assembly of preformed rod-shaped domains into rod-clusters on the substrate. An interesting and different aspect of deposition of tn-ZnPc via substrate immersion in solution is that the substrate orientation within the solution affects the larger scale assembly of domains within the film. Placing the substrate in a horizontal orientation at the bottom of the container leads to very nonuniform films: some areas show a large accumulation of rods, whereas others show no rods at all. By contrast, we found that the walls of the container were densely coated with horizontally oriented, densely packed “bands” of rod-shaped domains. We attribute the assembly of rods into these bands to a similar mechanism as that leading to coffee-ring assemblies during drop-casting: a preference for settling out of rods at the contact line between the solution surface and the wall of the container, which again moves in a stick−slip fashion during evaporation.23−25 We found that we can achieve deposition of similar dense bands on the substrate during immersive deposition of tn-ZnPc by orienting the substrate vertically in the container. Analysis of SEM images like those in Figure 8B,C shows that the area-density covered by rods in such a band is even higher than that within the coffee-rings, which form during drop-casting (Figure 8A). Qualitatively, this is consistent with the much larger amount of solution, and thus solute, used in immersive deposition. In this case, there is no observable difference between the morphology resulting from immersion in an open-topped container (at a solvent evaporation rate of 0.00347 mL/min) and in a nearly close-topped container (at an evaporation rate of 0.0017 mL/min, i.e., approximately a factor of 2 slower). To summarize this section, our results indicate that over a very wide range of solvent evaporation rates, the shape of individual tn-ZnPc domains formed during drop-casting and solvent immersion deposition remains essentially unchanged,

Figure 9. (A) Topography (height-contrast) and (B) phase-contrast AFM images of the structures that result from drop-casting (1:1) tnZnPc/PCBM mixed solutions in chloroform onto silicon.

enshrouds the cluster of tn-ZnPc rod-shaped domains. The edges of the rods, sharp in the phase map, are diffuse in the topography map. In the phase map, contrast is sensitive to the elastic modulus of the components, and the apparently stiffer rods dominate the map, whereas the blanket, well-visible in the height contrast map, is almost invisible. As no such evidence for a blanket-like phase was found subsequent to immersive deposition in tn-ZnPc-alone in chloroform, we believe it to be made up of PCBM molecules. Interestingly, the variation of the molecular domain shape and coverage with position across the sample is generally consistent with what was observed for drop-casting from individual solute solutions. Rod-shaped domains are concentrated in a series of concentric coffee-ring assemblies, whereas spherical cap-shaped domains, presumably of PCBM, form near the sample edges and are replaced with a more uniform-density blanket phase closer to the center. Our interpretation is that the larger rod-shaped domains of the more highly saturated tnZnPc molecules move slowly in suspension, leading to an intermittent pinning of the receding droplet edge, whereas the more highly soluble PCBM molecules nucleate heterogeneously into domains on the surface of the substrate, coalescing to envelop the rods of tn-ZnPc near the sample center. An interesting difference between the blanket phase observed for the mixed solution and that for drop-casting of PCBM alone is the absence of small, anisotropic domains like that seen in Figure 6D,E in the mixture. This may indicate that crystallization of PCBM domains is impeded by the presence of tn-ZnPc or may simply be a consequence of the lower individual molecular concentration in the mixed solution. 13664

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Figure 10. (A−C) SEM images of the structures resulting from immersive deposition of the Si substrate in a (1:1) tn-ZnPc/PCBM mixture in chloroform solution, with the substrate oriented vertically at a solvent evaporation rate of 0.00347 mL/min. (A) Irregular, compact, plate-shaped domains (presumably of PCBM) coexisting with rod-shaped (tn-ZnPC) clusters; (B) dendritic PCBM domains in high rod-density areas; and (C) stacking of PCBM plates in low rod-density areas. (D,E) Light optical images of the structures resulting from deposition from a (1:1) tn-ZnPc/ PCBM mixed solution via immersion with the substrate oriented vertically at a solvent evaporation rate of 0.0017 mL/min; (D) an area with a high density of tn-ZnPc rods and (E) an area with a low density of rods.

Given the observed effect of substrate orientation on the rod density during immersive deposition of tn-ZnPc alone, we investigated both vertical and horizontal substrate orientations in the deposition from immersion in (1:1) tn-ZnPc/PCBM mixtures in solution. Our results are illustrated in Figure 10. Immersive deposition with the substrate oriented vertically and at a solvent evaporation rate of 0.00347 mL/min (nearly open-topped container) results in the formation of compact, but irregularly shaped plate domains, presumably of PCBM molecules, as seen in Figure 10A. This is in contrast to the formation of hexagonal plate domains for immersive deposition from a solution of PCBM alone (Figure 7A). We also observe assemblies of rod-shaped domains, presumably of tn-ZnPc molecules, on these samples. The density of rods again is not uniform across the substrate; rather, they cluster in horizontal bands (of width ≈ 286 μm), separated by areas with a low density of rods. The presence of chlorine atoms is detectable from the EDS measurements on agglomerations of tn-ZnPc rod-shaped domains and on PCBM dendrites and plates, again raising the possibility that some degree of SVA may occur during the assembly of domains. Interestingly, we find a correlation between the morphology of the plate-shaped domains and the local density of tn-ZnPc rods. In areas where the concentration of rods is lower, we observe stacking of relatively compact irregular plates (Figure 10C). In areas with high densities of rods, compact domains are replaced by dendritic shapes, as seen in Figure 10B. The presence of tnZnPc seemingly impedes the transformation of the PCBM blanket phase into hexagonal plates and drives an instability in the shape of the domains. The simplest explanation for these effects is that either some fraction of the tn-ZnPc molecules or an associated impurity remains dispersed in solution or on the substrate, with the concentration of this dispersed phase varying with the local density of rod-shaped domains. We detect a small Zn signal in the EDS measurements performed at the positions of PCBM dendrites and plates, supplying some evidence for such an interpretation.

We repeated these experiments at a lower solvent evaporation rate of 0.0017 mL/min (immersion in solution in a nearly close-topped container). Once more we observed alternating horizontal bands of low and high density of tn-ZnPc rod domains. Again, we find a variation in the degree of dendritic character of the plate-shaped domains, which correlates with the local density of rod-shaped domains (Figure 10D,E). This correlation is qualitatively similar to that discussed above: areas with a high concentration of rods show more highly branched, dendritic assemblies of the domains, as seen in Figure 10D, whereas areas with a low concentration of rod-shaped domains show a less dendritic assembly of plates, that is, showing fewer secondary arms (Figure 10E). A comparison with the above results (Figure 10B,C) shows that the dendritic character is more evident for deposition at this slower solvent evaporation rate. We speculate that, in analogy with the Mullins−Sekerka instability, unstable growth of PCBM domains might be favored by a small spatial gradient in the supersaturation of some impurity, perhaps tnZnPc, at the growth front.18,39,40 As a control experiment, we carried out deposition from PCBM-alone in chloroform (i.e., without tn-ZnPc), immersing the substrate into the solution in a vertical orientation. We observe that hexagonal PCBM plates form (Figure 11A), independent of the orientation of the substrate. This supports the supposition that the absence of regular hexagonal domains of PCBM in the mixed film is due to the interaction with tnZnPc molecules. As seen on a coarser scale in Figure 11B, we again observe a dendritic character in the larger scale assembly of these domains, qualitatively consistent with what was seen in the case of deposition from the mixture. If the dendrites are taken as an indication of cellular growth, as speculated above, these results suggest that the impurity in this case is not tnZnPc. We note, however, that when PCBM is mixed with tnZnPc, the concentration of impurities might be different and depends on the local density of rods. In this way, the tn-ZnPc 13665

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Finally, we note that the dendrite arms observed here are much smaller than in the case of deposition from PCBM-alone in chloroform at the lowest evaporation rate (Figure 7F). This again suggests an impurity effect: either the presence of dispersed tn-ZnPc molecules or an associated impurity in solution. To conclude this section, mixing tn-ZnPc and PCBM in a (1:1) ratio in chloroform solution results in no evident change in the individual tn-ZnPc domain shapes from those in the absence of PCBM: rods are observed in each case. In sharp contrast, the PCBM domain shape is strongly affected by the presence of tn-ZnPc in solution. The formation of regular, seemingly crystalline, hexagonal PCBM domains is seemingly suppressed in the mixture. This is despite the low evaporation rates and the action of SVA. It seemingly indicates that some dispersed tn-ZnPc molecules or an associated impurity37 remain in solution, changing the mode of the PCBM domain assembly.18

Figure 11. Light optical images of the structures resulting from deposition via immersion of the Si substrate into PCBM-alone in chloroform solution, using a vertical substrate orientation, with a solvent evaporation rate of 0.0017 mL/min. (A) Relatively highmagnification image showing stacking of hexagonal domains; (B) lower magnification image showing the dendritic character of domain assemblies on a larger scale.

clusters may exert an indirect influence on the assembly of the PCBM domains into larger scale structures. Finally, we compare the morphologies of domains resulting from immersion of a horizontally oriented Si substrate into a (1:1) mixture of PCBM/tn-ZnPc in chloroform solution with those from PCBM-alone in chloroform. We performed the deposition at the lowest solvent evaporation rate studied here (0.0017 mL/min). We again find that most of the tn-ZnPc rodshaped domains deposit onto the walls of the container. Images of the substrate show areas with a very low density of rods alternating with irregular bands of high densities of rods. Light optical images of the resulting structures are shown in Figure 12A,B.

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SUMMARY AND CONCLUSIONS We find that the assembly of PCBM and tn-ZnPc molecules into domains during deposition from chloroform-based liquid solutions onto a native oxide-terminated Si substrate is considerably more complex than suggested by a simple, symmetrical, capillarity-driven vertical phase separation model. We observe a range of different domain morphologies, depending especially on the solvent evaporation rate as well as the presence of a second solute and even substrate orientation. We interpret this complexity as due to a number of effects, summarized below. PCBM, which has a much higher solubility in chloroform, precipitates onto the substrate with domain morphologies that vary widely over the range of solvent evaporation rates investigated. For the highest rates, the domains are isotropic and apparently amorphouswith a near-periodicity in their spacing. Slowing the evaporation rate causes an abrupt change in the domain size and shape to micron-scale faceted hexagonal domains. This shape change could be due to either a cascade of phase transformations, with the sequence dictated by the lowest energetic barrier between successive phases, or a change from spontaneous decomposition kinetics to nucleation and growth of crystalline domains. Reduction of the solvent evaporation rate yet further results in the dendritic assembly, suggesting an impurity-mediated instability. tn-ZnPc displays a much lower solubility in chloroform leading to an apparent aggregation in solution into rod-shaped domains, which settle out of suspension onto the substrate. Individual domain morphology is insensitive to the supersaturation rate over the entire range we have investigated, although the larger scale assembly of these domains varies considerably. As a result, the dominant mechanism for the phase separation between tn-ZnPc and PCBM is insensitive to the solvent evaporation rate over the entire range studied here: aggregation of the former while in liquid solution, with the latter subsequently precipitating onto the substrate as a nucleated process. Interactions between dissimilar molecules manifest themselves differently for the two solutes, with the presence of tnZnPc apparently lowering the wettability of the substrate by PCBM domains at fast supersaturation rates and suppressing the crystalline order within these domains at slower rates. In addition, we find evidence for the effect of an unknown, presumably dilute impurity on the stability of the PCBM

Figure 12. Light optical images of the structures resulting from immersive deposition of a (1:1) tn-ZnPc/PCBM mixed solution in chloroform with the substrate oriented horizontally at a solvent evaporation rate of 0.0017 mL/min. Areas with a low density of rodshaped domains of tn-ZnPc (A) show evidence for dendritic assembly of PCBM domains, whereas areas with high densities of rods (B) show simple stacking of PCBM domains.

In direct contrast to the correlation observed for immersive deposition in mixed solution for a vertically oriented substrate, for a horizontal substrate orientation, it is in regions with a low density of rods that dendritic PCBM domains are found (Figure 12A). Simple stacking of PCBM domain plates is found for this substrate orientation instead of regions with high densities of rods (Figure 12B). Although we do not have a detailed understanding of this reversed correlation, we believe that it is likely controlled by the convection currents that are generated inside the solution during evaporation.20,22 These currents likely move the dispersed tn-ZnPc molecules within the solution in specific directions, resulting in an effective interaction between PCBM and tn-ZnPc domains, which differs depending on the substrate orientation. 13666

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(12) Treossi, E.; Liscio, A.; Feng, X.; Palermo, V.; Müllen, K.; Samorì, P. Temperature-Enhanced Solvent Vapor Annealing of a C3 Symmetric Hexa-peri-Hexabenzocoronene: Controlling the SelfAssembly from Nano- to Macroscale. Small 2009, 5, 112−119. (13) Mascaro, D. J.; Thompson, M. E.; Smith, H. I.; Bulović, V. Forming oriented organic crystals from amorphous thin films on patterned substrates via solvent-vapor annealing. Org. Electron. 2005, 6, 211−220. (14) Lin, Y.-C.; Müller, M.; Binder, K. Stability of thin polymer films: influence of solvents. J. Chem. Phys. 2004, 121, 3816−3828. (15) Yang, Y.; Liu, C.; Gao, S.; Li, Y.; Wang, X.; Wang, Y.; Minari, T.; Xu, Y.; Wang, P.; Zhao, Y.; Tsukagoshi, K.; Shi, T. Large [6,6]-phenyl C61 butyric acid methyl (PCBM) hexagonal crystals grown by solventvapor annealing. Mater. Chem. Phys. 2014, 145, 327−333. (16) Li, G.; Yao, Y.; Yang, H.; Shrotriya, V.; Yang, G.; Yang, Y. “Solvent Annealing” Effect in Polymer Solar Cells Based on Poly(3hexylthiophene) and Methanofullerenes. Adv. Funct. Mater. 2007, 17, 1636−1644. (17) Colle, R.; Grosso, G.; Ronzani, A.; Gazzano, M.; Palermo, V. Anisotropic molecular packing of soluble C60 fullerenes in hexagonal nanocrystals obtained by solvent vapor annealing. Carbon 2012, 50, 1332−1337. (18) Langer, J. S. Instabilities and pattern formation in crystal growth. Rev. Mod. Phys. 1980, 52, 1−28. (19) Ginzburg, V. V.; Peng, G.; Qiu, F.; Jasnow, D.; Balazs, A. C. Kinetic model of phase separation in binary mixtures with hard mobile impurities. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1999, 60, 4352−4359. (20) Hu, H.; Larson, R. G. Analysis of the effects of Marangoni stresses on the microflow in an evaporating sessile droplet. Langmuir 2005, 21, 3972−3980. (21) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Contact line deposits in an evaporating drop. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 62, 756−765. (22) Xu, X.; Luo, J.; Guo, D. Radial-velocity profile along the surface of evaporating liquid droplets. Soft Matter 2012, 8, 5797−5803. (23) Shen, X.; Ho, C.-M.; Wong, T.-S. Minimal size of coffee ring structure. J. Phys. Chem. B 2010, 114, 5269−5274. (24) Ogunsipe, A.; Nyokong, T. Effects of substituents and solvents on the photochemical properties of zinc phthalocyanine complexes and their protonated derivatives. J. Mol. Struct. 2004, 689, 89−97. (25) Cid, J.-J.; García-Iglesias, M.; Yum, J.-H.; Forneli, A.; Albero, J.; Martínez-Ferrero, E.; Vázquez, P.; Grätzel, M.; Nazeeruddin, M. K.; Palomares, E.; Torres, T. Structure−Function Relationships in Unsymmetrical Zinc Phthalocyanines for Dye-Sensitized Solar Cells. Chem.Eur. J. 2009, 15, 5130−5137. (26) Nemykin, V. N.; Lukyanets, E. A. Synthesis of substituted phthalocyanines. ARKIVOC 2010, 2010, 136. (27) Thompson, B. C.; Fréchet, J. M. J. Polymer−fullerene composite solar cells. Angew. Chem., Int. Ed. 2008, 47, 58−77. (28) Kern, W. The evolution of silicon wafer cleaning technology. J. Electrochem. Soc. 1990, 137, 1887−1892. (29) Meyerhofer, D. Characteristics of resist films produced by spinning. J. Appl. Phys. 1978, 49, 3993−3997. (30) Bornside, D. E.; Macosko, C. W.; Scriven, L. E. Spin coating of a PMMA/chlorobenzene solution. J. Electrochem. Soc. 1991, 138, 317− 320. (31) Shen, J.; Zhang, D.; Zhang, F.-H.; Gan, Y. AFM tip-sample convolution effects for cylinder protrusions. Appl. Surf. Sci. 2017, 422, 482−491. (32) Higgins, A. M.; Jones, R. A. L. Anisotropic spinodal dewetting as a route to self-assembly of patterned surfaces. Nature 2000, 404, 476− 478. (33) Phaneuf, R. J.; Bartelt, N. C.; Williams, E. D.; Swiech, W.; Bauer, E. Crossover from metastable to unstable facet growth on Si(111). Phys. Rev. Lett. 1993, 71, 2284−2287. (34) Kandel, D.; Weeks, J. D. Step motion, patterns, and kinetic instabilities on crystal surfaces. Phys. Rev. Lett. 1994, 72, 1678−1681.

growth front. By sharp contrast, the individual tn-ZnPc domain morphology is insensitive to the presence of PCBM. We find evidence that convection currents and a “stick−slip” motion of the liquid/substrate/atmosphere contact line have major effects on the large-scale arrangements of tn-ZnPc aggregates on the substrate as well as on the correlation between the morphology of PCBM domains and their proximity to the rod-shaped tn-ZnPc aggregates. Finally, we note that a large part of the deviation of the domain geometries, which assemble in this system from a simple, ideal vertical phase separation, seemingly derives from the dramatically different solvent−solute interactions for the individual molecules. Mixed solutions in which the individual solubilities are close to each other might be expected to lead to a more ideal vertical phase separation, which is tunable via the supersaturation rate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Raymond J. Phaneuf: 0000-0002-1797-2856 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank C. Zelhoffer for assistance with measurements of solvent evaporation rates. This work was supported by the NSF-MRSEC at the University of Maryland under grant number DMR0520471.

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