Tuning the Morphology of Solution-Sheared P3HT:PCBM Films - ACS

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Tuning the Morphology of Solution-Sheared P3HT:PCBM Films Julia A. Reinspach,†,‡ Ying Diao,†,‡,⊥ Gaurav Giri,§ Torsten Sachse,∥ Kemar England,† Yan Zhou,† Christopher Tassone,‡ Brian J. Worfolk,†,‡ Martin Presselt,∥ Michael F. Toney,*,‡ Stefan Mannsfeld,*,‡,# and Zhenan Bao*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § Department of Chemical Engineering, MIT, Cambridge, Massachusetts 02139, United States ∥ Institute of Physical Chemistry, Friedrich-Schiller-University Jena, D-07743 Jena, Germany ‡

S Supporting Information *

ABSTRACT: Organic bulk heterojunction (BHJ) solar cells are a promising alternative for future clean-energy applications. However, to become attractive for consumer applications, such as wearable, flexible, or semitransparent power-generating electronics, they need to be manufactured by high-throughput, low-cost, large-area-capable printing techniques. However, most research reported on BHJ solar cells is conducted using spin coating, a single batch fabrication method, thus limiting the reported results to the research lab. In this work, we investigate the morphology of solution-sheared films for BHJ solar cell applications, using the widely studied model blend P3HT:PCBM. Solution shearing is a coating technique that is upscalable to industrial manufacturing processes and has demonstrated to yield record performance organic field-effect transistors. Using grazing incident small-angle X-ray scattering, grazing incident wideangle X-ray scattering, and UV−vis spectroscopy, we investigate the influence of solvent, film drying time, and substrate temperature on P3HT aggregation, conjugation length, crystallite orientation, and PCBM domain size. One important finding of this study is that, in contrast to spin-coated films, the P3HT molecular orientation can be controlled by the substrate chemistry, with PEDOT:PSS substrates yielding face-on orientation at the substrate−film interface, an orientation highly favorable for organic solar cells. KEYWORDS: organic electronics, BHJ, solution-shearing, X-ray diffraction, UV−vis absorption spectroscopy

1. INTRODUCTION The field of organic electronics has been growing tremendously within the past decade.1−7 For organic photovoltaic applications, organic bulk heterojunction (BHJ) solar cells have attracted particular interest due to their promising potential for future clean-energy applications. Because of their versatile tunable chemical structures, they can be developed to enable flexible, stretchable, or semitransparent devices. Moreover, because they can be deposited from solution (in contrast to, e.g., vacuum-based vapor deposition techniques), they can be fabricated using large-scale, low-cost, roll-to-roll compatible coating techniques, such as inkjet printing, slot-die coating, or doctor blading.8 Organic BHJs typically consist of a blend of an electrondonating polymer and an electron-accepting fullerene that form an interpenetrating network of phase-separated domains.9−11 Recently, different material systems are also gaining increasing interest, for example, small-molecule donors12−14 and allpolymer solar cells.15−18 The performance of a BHJ solar cell is closely related to the morphology of this interpenetrated network, in particular, the phase separation between the donor © 2016 American Chemical Society

and acceptor domains, crystallinity, crystallite orientation, and vertical distribution of the respective domains.9,19,20 As a consequence, tremendous effort has been put into understanding the influence of processing conditions on the resulting morphology, such as influence of donor−acceptor blend ratio,21 solvent,22,23 thermal or solvent-vapor annealing,23−26 and use of solvent additives.27,28 However, the majority of these reported studies rely on noncontinuous batch fabrication methods such as spin-coating and dropcasting. This poses a challenge to transfer the results to large-scale, roll-to-roll compatible fabrication methods, because of the difference in film formation kinetics, which plays a crucial role in the close relationship between processing conditions and resulting morphology. In addition, although some reports have been published on organic solar cells fabricated by doctor blading,21,28−36 inkjet printing,37,38 and all-roll-to-roll printing,39 the relationship Received: October 2, 2015 Accepted: December 28, 2015 Published: January 15, 2016 1742

DOI: 10.1021/acsami.5b09349 ACS Appl. Mater. Interfaces 2016, 8, 1742−1751

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ACS Applied Materials & Interfaces

Figure 1. Normalized UV−vis absorption spectra of P3HT and P3HT:PCBM sheared from chloroform at 25 °C (a,c) and 1,2-dichlorobenzene at 130 °C (b,d). With increasing shearing speed, the spectrum shifts to larger wavelengths, and the A00/A01 peak intensity increases, indicators for increased aggregation and order of the polymer chains.

between processing conditions and morphology is still to be fully understood. We have previously reported on a roll-to-roll compatible fabrication technique called solution shearing.40 Here, a film is deposited by sandwiching a solution in between a moving blade and a stationary, temperature-controlled substrate, thus exposing a wet film as the blade moves along the substrate. This technique is different from other blade-coating techniques (such as doctor blading, slot-die coating, etc.), since the solution is covered by the blade, and therefore, solvent evaporation only occurs at the exposed solution front.41 Using this technique, it was demonstrated that the crystal structure of the small-molecule organic semiconductor TIPSpentacene can be controlled and tuned by the shearing parameters, resulting in record charge carrier mobilities.42−44 Thus, because of its compatibility with large-scale, roll-to-roll compatible fabrication techniques and its potential to tune film morphology, solution shearing is highly interesting for fabrication of organic BHJ solar cells. In this paper, we investigate how the morphology of blends made from the polymer poly(3-hexylthiophene) (P3HT) and the fullerene phenyl-C61-butyric-acid-methyl-ester (PCBM) can be controllably tuned by solution shearing. P3HT:PCBM is the most widely studied donor−acceptor system in the literature, and as such, it is a well-suited model system for understanding the effect of solution shearing on BHJ morphology. We find that, for pristine P3HT films, morphological characteristics such as P3HT aggregation and conjugation length are highly dependent on the shearing speed but very little on the choice of solvents when the solvent evaporation rate, and thus film formation times, is kept constant. For P3HT:PCBM films, however, we find that the solvent plays a more important role, most likely due to the difference in the P3HT and PCBM solubility, which can lead to different phase

separation and crystallization dynamics. In addition, we demonstrate that the substrate interface plays a crucial role in whether P3HT stacks face-on or edge-on on the substrate surface. This finding opens the opportunity to control polymer orientation at the substrate−film interface for films fabricated by solution shearing.

2. RESULTS In this section, we discuss the influence of processing parameters, in particular, shearing speed and choice of solvent, on the morphology of solution-sheared P3HT and P3HT:PCBM films. Films were prepared as described in the Experimental Section and characterized using grazing incident small-angle X-ray scattering (GISAXS), grazing incident wideangle X-ray scattering (GIWAXS), and UV−vis absorption spectroscopy. X-ray scattering, on one hand, has proven to be a powerful tool to study organic polymer and BHJ thin films,24,25,27,36,45 yielding important morphological information on the sample’s crystallinity, that is, long-range order of πstacked conjugated polymers.46,47 UV−vis absorption spectroscopy, on the other hand, probes the local interaction of πstacked conjugated segments (also called “aggregates”), even though they do not necessarily need to exhibit long-range order.46 Thus, GIWAXS and GISAXS provide information about molecular orientation (i.e., face-on vs edge-on stacking),48 crystallinity, crystallite alignment, and phase-separation size, while UV−vis absorption spectroscopy yields information about local aggregation and interaction of the polymers. 2.1. UV−vis Absorption Spectroscopy. UV−vis spectra were taken of P3HT and P3HT:PCBM films solution-sheared at different speeds from the solvents chloroform and 1,2dichlorobenzene. PEDOT:PSS coated glass slides were used as substrates. The films from chloroform were prepared at a substrate temperature of 25 °C, while the films from 1,21743


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Figure 2. (a) Fraction of aggregated P3HT in films sheared from chloroform (CF) at 25 °C and 1,2-dichlorobenzene (ODCB) at 130 °C. (b) Excitonic bandwidth W as calculated using the ratios of the A00 and A01 peak intensities. Lines were added to guide the eye.

dichlorobenzene were obtained at a temperature of 130 °C. Those two temperatures were chosen to provide a similar rate of solvent evaporation, despite the very different boiling points (TB, chloroform = 62.2 °C; TB, 1,2dichlorobenzene = 180.5 °C). Thus, the film drying time was comparable for both solvents. The spectra were then fitted using a theoretical model first described by Spano et al.49,50 (cf. Figures S1 and S2). In this model, P3HT is treated in terms of weakly interacting H-aggregates, that is, assuming weak interchain molecular coupling between the polymer chains within the aggregate. Following the experimental procedure described by, for example, Turner and Clark,51,52 we extracted the fraction of amorphous versus aggregated P3HT, as well as the excitonic bandwidth (W). More details about the fitting procedure can be found in the Supporting Information. Figure 1 shows normalized UV−vis absorption spectroscopy profiles. Qualitatively, it can be seen that an increase in solution-shearing speed results in an absorption redshift and more pronounced vibronic shoulders. This is usually ascribed to a higher fraction of aggregated P3HT within the film.51−53 In addition, the A00/A01 peak ratio (i.e., the ratio of the peaks at ∼600 and ∼560 nm) increases with shearing speed, which is due to more coplanar chain-segments within the polymer, and thus, a larger conjugation length.54 When comparing the spectra of the two different solvents, that is, chloroform and 1,2-dichlorobenzene, one observes that the spectra for pure P3HT films are almost identical for both solvents (cf. Figure 1a,b). However, for the P3HT:PCBM blends, the situation is different (cf. Figure 1c,d). Films sheared from 1,2-dichlorobenzene (Figure 1d) exhibit a lower absorption in the wavelength region between 400 and 500 nm, which is the spectral region where PCBM absorption is significant. Thus, there is higher PCBM absorption in films sheared from chloroform solutions compared to those sheared from 1,2dichlorobenzene. An explanation for this trend could possibly be found in the solubility of the respective components: for P3HT, the solubility is very similar for both solvents (14.7 mg/ mL for 1,2-dichlorobenzene and 14.1 mg/mL for chloroform55). PCBM, however, has a higher solubility in 1,2dichlorobenzene (42.1 mg/mL for 1,2-dichlorobenzene and 28.8 mg/mL for chloroform55)as a consequence, there might be an earlier onset of PCBM aggregation during film formation in chloroform, which might inhibit P3HT crystallization,56 thus leading to a higher fraction of aggregated PCBM versus P3HT in the final film.

Figure 2a shows the fraction of aggregated P3HT within the films as extracted by the Spano analysis. As already qualitatively observed from the spectra, it can be seen that an increase in shearing speed results in an increase in the fraction of aggregates. Pure P3HT films have a higher percentage of aggregation compared to blend films. As expected from the similarity of the spectra, P3HT films from both chloroform and 1,2-dichlorobenzene exhibit a very similar percentage of aggregation for a given shearing speed. In contrast, BHJ blend films from chloroform have a lower fraction of P3HT aggregation compared to films from 1,2-dichlorobenzene. As discussed above, one explanation for this might be the earlier onset of PCBM aggregation in chloroform compared to 1,2dichlorobenzene, which in turn might inhibit P3HT crystallization in P3HT:PCBM films. However, another contributing effect must be the higher substrate temperature for 1,2dichlorobenzene films (25 °C for chloroform, vs 130 °C for 1,2-dichorobenzene). Higher temperate results in an effective thermal annealing and, thus, would lead to a higher P3HT aggregation content. Since we do not observe this effect for pure P3HT films, the difference in PCBM absorption and P3HT aggregation observed for P3HT:PCBM films is probably due to an interplay of earlier PCBM aggregation (and thus, slower P3HT aggregation) and P3HT thermal annealing. Figure 2b shows the excitonic bandwidth W, which, for phenylene- and thiophene-based conjugated polymers, is inversely related to the conjugation length.54 Thus, a lower value of W corresponds to a higher number of coplanar polymer segments along the polymer backbone. From the graph, it can be seen that an increase in shearing speed leads to a lower value of the excitonic coupling W and, thus, to a larger conjugation length. P3HT:PCBM films from 1,2-dichlorobenzene have the lowest values of W, while P3HT:PCBM films from chloroform have the highest W. Interestingly, W does not follow the same trend as the percentage of aggregation in terms of the effects of solvent and blend ratio. Thus, the percentage of aggregation does not seem to be correlated to the intramolecular coupling, that is, the conjugation length, of the aggregates. 2.2. Grazing Incident Wide-Angle X-ray Scattering. Films were prepared under the same experimental conditions as the UV−vis absorption spectroscopy, that is, the films from chloroform were sheared at a substrate temperature of 25 °C, while the films from 1,2-dichlorobenzene were sheared at a temperature of 130 °C. The prepared films were analyzed by grazing-incidence wide-angle scattering at the Stanford 1744


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Figure 3. GIWAXS images of films sheared from chloroform (CF) and 1,2-dichlorobenzene (ODCB) at speeds from 0.25 to 2.5 mm/s.

Qxy ≈ 1.6 Å−1. When the shearing speed increases, the stacking orientation changes to a mixed orientation (face-on and edgeon), as evidenced by the emergence of an out-of-plane peak at Q z ≈ 1.6 Å−1 (see also Figure S6). Face-on orientation has been deemed beneficial for solar cell performance since it can facilitate charge transport in the vertical direction between the electrodes. A dependence of stacking direction on the processing conditions has also been reported for spin-coated films. For example, Verploegen et al. obtained predominantly face-on orientation for P3HT:PCBM blends spincast from chloroform at room temperature, while blends spincast from chlorobenzene exhibited mainly edge-on behavior.25 The authors attributed this to a prolonged solvent evaporation when using chlorobenzene, which allows the chains to reach a more favorable energetic state (i.e., edge on).25 DeLongchamp et al. reported P3HT films with predominantly face-on orientation for high spin-coating speeds, while films spin-coated at slow speeds showed predominantly edge-on behavior.61 Thus, they also associated edge-on orientation with a prolonged solvent evaporation time. Sirringhaus et al. observed edge-on orientation for rapidly grown spin-coated films and face-on orientation for films fabricated by slow casting.62 Duong et al.

Synchrotron Radiation Lightsource (SSRL). More details can be found in the Experimental Section. Figure 3 shows GIWAXS images of P3HT and P3HT:PCBM films solution-sheared from 1,2-dichlorobenzene and chloroform at shearing speeds ranging from 0.25 to 2.5 mm/s on PEDOT:PSS coated Si substrates. All films exhibit typical P3HT (l00) lamellar spacing distances (cf. Figure S7 in Supporting Information). Films from 1,2-dichlorobenzene generally exhibit a slightly larger spacing compared to chloroform films (∼16.4 Å for ODCB vs ∼16.0 Å for CF), and pure P3HT films have larger spacings than blend films. However, no clear trend with shearing speed can be deduced. Also, we did not observe any shearing-induced alignment effect (in the substrate plane), as has been reported for, for example, strain-aligned57 and blade-coated P3HT,58 or for dip-coated PBTTT59 (cf. Figures S3−S5 in Supporting Information). Our findings are in agreement with results we have reported previously for P3HT films sheared from Decalin, which also exhibited isotropic behavior.60 The most striking observation from Figure 3 is the change in orientation of the P3HT lamellae and π−π stacks: for slow speeds, the lamellar stacking direction is predominantly edgeon, as evident from the strong in-plane (010) π-stacking peak at 1745


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Figure 4. GIWAXS images of films sheared at 1 mm/s from 1,2-dichlorobenzene onto different substrates: PEDOT:PSS, PTS, PSS and ZnO. For comparison, the background GIWXS pattern of the blank substrate was recorded as well.

found that for spin-coated P3HT films, a highly ordered edgeon layer crystallizes from the substrate−polymer interface, while fast crystallization from the bulk results in more disordered, face-on crystallites.46 Thus, it appears that long drying times produce films with higher fractions of crystalline P3HT in edge-on configuration, while fast drying leads to kinetic trapping and, thus, to more disordered films with predominant face-on orientation. So how can this shearing-speed dependence of the molecular orientation in solution-sheared P3HT:PCBM blend films be explained? As observed for spin-coated films, kinetic trapping could be one explanation: for higher shearing speeds, the liquid film thickness deposited by the blade is thinner.63 This results in an increased surface-to-volume ratio and, thus, to a higher evaporation rate of the solvent per volume, which could lead to kinetic trapping of the face-on configuration for thinner films. Another explanation might be spatial confinement, as has been reported for, for example, TIPS−pentacene43 or thin films of PBTTT.45 In that case, spatial confinement from both the film−air and the film−substrate interface could lead to the formation of a metastable arrangement of P3HT (i.e., face-on). Another factor that can influence molecular orientation is the substrate surface. For example, Kline et al. and Jimison et al. showed that surface treatment with octadecyltrichlorosilane (OTS) results in well-oriented P3HT crystallites.64,65 However, those surfaces induced an increase in edge-on texture, while face-on surface-induced texture for P3HT has, to our knowledge, so far only been reported for graphene.66 To investigate the influence of substrate interaction on the morphology of solution-sheared films, we also prepared P3HT films on PTS-treated substrates as well as on substrates

spin-coated with ZnO, PSS, and PEDOT:PSS. All of those surfaces exhibit complete wetting behavior for both chloroform and 1,2-dichlorobenzene. GIWAXS images of the films are shown in Figure 4. Interestingly, all films exhibit a predominantly edge-on behavior of the conjugated plane except the films sheared on PEDOT:PSS. All films were sheared at the same experimental conditions (1 mm/s shearing speed), and thus, the films have similar thicknesses. The observed difference in molecular orientation between PEDOT:PSS and the other substrates can therefore not only be due to kinetic trapping induced by a decrease in drying time, or by confinement effect, as hypothesized above. Instead, it must also be related to the interaction between the PEDOT:PSS substrate and P3HT. To support the hypothesis that the face-on texture is indeed induced by the PEDOT:PSS surface, we performed GIWAXS depth-profiling on P3HT and P3HT:PCBM films sheared at 1 mm/s. The results are shown in Figures S8 and S9. As expected, the distribution of crystallites is rather random at shallow X-ray incidence angles where only an ∼5 nm thick region at the top surface of the film (i.e., the air−film interface) is probed. In contrast, at higher incident angles (i.e., when probing the full film thickness), the films on the PEDOT:PSS surface exhibit a predominantly face-on texture, while the films on PTS, ZnO, and PSS exhibit a predominantly edge-on texture. Thus, for P3HT films on PEDOT:PSS, the P3HT orientates largely face-on at the substrate−film interface, and it transitions to a more randomly distributed film toward the film−air interface. The cause of this orientation is not certain but is likely some type of interaction of the P3HT and the PEDOT:PSS surface. 1746


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trend has been observed for P3HT:PCBM films slot-die-coated from chlorobenzene:69 here, a higher temperature resulted in larger P3HT crystallites but smaller PCBM domains. The authors attribute this to a crystallization-driven phase separation, leading to increased P3HT crystal growth at higher temperatures and, therefore, to a reduced PCBM diffusion and domain formation. However, for the temperature range investigated in their study, that is, room temperature and 40 °C, annealing effects most likely did not play a major role. A small domain size is essential for efficient exciton diffusion in BHJ solar cells; however, at the same time, efficient charge transport requires high conductivity of the donor material, and thus, for the case of crystalline donor polymers such as P3HT, high crystallinity and aggregation are needed. Here, even though films for chloroform have slightly smaller domain size, 1−2 dichlorobenzene films should outperform films from chloroform because of their higher order in the P3HT material.

To verify that the change from edge-on to face-on orientation is not related to other processing parameters, we tested the sensitivity of our results to shearing parameters that might affect the meniscus shape, possible solution degradation, and sample condition. In particular, we varied the blade− substrate distance between 100 and 300 μm, used blade tilt angles of 0 and 1° (tilt angles larger than 1° result in a large solution−air interface for the back meniscus, and thus, the solution concentration is not well-controlled), and tested the effect of surface contact angle by using newly treated (contact angle ca. 85−90°) and old blades (contact angle reduced by ca. 20−40°). The effect of solution age was investigated by using freshly-prepared and four-month old solutions. Also, we remeasured films after storing them in air for one month. However, within the parameter range used, we did not find any correlation. We also investigated how these different surfaces influence morphology of spin-coated and also spin-coated and then annealed films. Interestingly, we did not find a clear correlation here (cf. Figure S10). An explanation for this could be the difference in film formation for spin-coated and sheared films: in contrast to solution-shearing, spin-coating is dominated by kinetic effects, that is, large centrifugal forces and nonequilibrium film drying, which might make the P3HT less sensitive to difference in substrate chemistry. 2.3. Grazing Incident Small-Angle X-ray Scattering. The films prepared for GIWAXS in Section 2 were also measured at the GISAXS beamline at the Advanced Light Source in Berkeley and were analyzed using the unified fitting approach.67 Figure 5 shows the domain size Rg as a function of

3. CONCLUSIONS In summary, we have investigated the morphology of solutionsheared films of P3HT and P3HT:PCBM on organic photovoltaic-relevant substrates as a function of shearing speed, solvent, and substrate surface, using GIWAXS, GISAXS and UV−vis spectroscopy. When using the solvents chloroform and 1,2-dichlorobenzene and matching their evaporation rates by adjusting the substrate temperature, we find that pure P3HT films exhibit similar fractions of aggregates and similar conjugation lengths. However, for P3HT:PCBM films, we observe different morphologies: (1) films from chloroform exhibit higher PCBM absorption. (2) Films from 1,2dichlorobenzene show higher fractions of aggregation and higher conjugation lengths. (3) PCBM domain size, as extracted by GISAXS, is higher for films from 1,2dichlorobenzene. We attribute those differences in morphology to two effects: first, since PCBM solubility in chloroform is lower than in 1,2-dichlorobenzene, PCBM aggregation in chloroform might start earlier, thus leading to a higher PCBM absorption in chloroform films. Since we do not observe a difference in aggregation for pure P3HT films, and since PCBM was reported to slow the P3HT crystallization in blend films,56 the lower fraction of P3HT aggregation in chloroform films can be explained by that. However, a second effect that plays a role is annealing: since the films from 1,2-dichlorobenzene are sheared at a higher temperature, thermal annealing should lead to P3HT crystallization and, thus, might lead to a higher fraction of aggregates in the film. In addition, thermal annealing enlarges PCBM domains, which is consistent with a larger domain size for 1,2-dichlorobenzene films as observed by GIWAXS. The finding that for solution-sheared films, P3HT aggregation and molecular orientation for pure P3HT films can be made similar for films from both 1,2-dichlorobenzene and chloroform by adjusting the solvent evaporation speed opens new possibilities in low-temperature solar cell manufacturing, which are not possible for spin-coated films. Other room temperature, low-boiling point solvents with better PCBM solubility than chloroform might yield the same aggregation, domain size, and crystallinity as current spincoated and annealed films, however, at the advantage of much easier processing. We further find that the molecular orientation of P3HT and P3HT:PCBM films is highly dependent on the substrate surface: for PEDOT:PSS films sheared at faster speeds, the

Figure 5. Domain size RG calculated from GISAXS using the unified fitting approach for solution-sheared P3HT:PCBM films.

shearing speed for P3HT:PCBM blend (full profiles and fits can be found in Figures S12 and S13). Since the signal-to-noise ratio of the data is low and the model is only approximate, the fits do not yield exact numbers for the actual domain size. Regardless, the trend in the raw data (cf. Figure S12) is reflected in the fitting results (Figure 5): first, there does not seem to be a significant dependence of domain size on shearing speed, and second, it can be seen that the domain size differs for the two different solvents. For chloroform films, the domains are smaller compared to films from 1,2-dichlorobenzene. We attribute the larger domain sizes in 1,2-dichlorobenzene films (8.5−9 vs 7 nm, respectively) to the higher substrate temperature (25 °C for chloroform, vs 130 °C for 1,2dichorobenzene), which leads to annealing of the films and, thus, to larger domains.35,68 This is consistent with the higher fraction of aggregated P3HT in P3HT:PCBM films from 1,2dichlorobenzene as observed by UV−vis spectroscopy. However, we also would like to point out that an opposite 1747


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Solution Shearing of P3HT:PCBM Films. To obtain similar surface and interface conditions as in a solar cell, all morphology studies were performed on PEDOT:PSS unless otherwise noted. Solution shearing was performed in ambient conditions (T = 21 °C, humidity 30−45%), using a substrate-blade distance of 100 μm and a substrate-blade angle of 0° (i.e., the substrate and the blade were parallel). Films of P3HT and P3HT:PCBM 1:1 blends were solutionsheared from two different solvents, namely, 1,2-dichlorobenzene and chloroform, and using shearing speeds of 0.1, 0.25, 0.5, 1.0, and 2.5 mm/s. For each sample, a solution volume of 15−20 μL was used. Films from 1,2-dichlorobenzene were solution-sheared at a substrate temperature of 130 °C, while films from chloroform were deposited at a temperature of 25 °C. Thus, despite their different boiling points (T = 60 °C for chloroform and T = 180 °C for 1,2 dichlorobenzene), the speed of solvent evaporation is similar for both solvents, facilitating a more direct comparison. The solvent evaporation rate was determined by evaporating 100 μL of the respective solvent on the shearing stage and adjusting the temperature until the evaporation times matched. In particular, 100 μL of 1,2-dichlorobenzene at 130 °C evaporate within 153 s, while 100 μL of chloroform at 25 °C evaporate within 150 s. As a consequence, within experimental variability, same shearing speeds resulted in similar film thicknesses for both solvents (cf. Figure S11). After shearing, the samples were left on the substrate stage for ca. 60 s, to allow residual solvent to evaporate, and were then removed from the heated sample stage. Spin-Coating of P3HT:PCBM Films. Spin-coating was performed in ambient conditions at 1500 rpm for 60 s. For annealing, films were transferred into a nitrogen glovebox and baked at a hot plate for 20 min at 150 °C. UV−vis Absorption Measurements. Optical characterization of the films was performed using an Agilent Cary 6000i UV−vis−NIR spectrometer equipped with an InGaAs detector. Spectra were obtained within a wavelength range from 800 to 250 nm. GIWAXS. GIWAXS data were acquired at SSRL beamline 11−3. For our experiments, a photon energy of 12.73 keV and a sample− detector distance of 400 mm were used. To mitigate background air scattering and radiation damage, samples were imaged in a helium chamber. All images were collected at an exposure time of 30 to 300 s and a grazing incidence angle of 0.12°, unless otherwise noted. For image analysis, the software WxDiff developed by S. C. B. Mannsfeld was used. GISAXS. GISAXS data were obtained at ALS beamline 7.3.3 using a photon energy of 10 keV and a grazing incidence angle of 0.16°. The sample-to-detector distance was 3800 mm. All experiments were performed in air and using an exposure time of 30−60 s. For GISAXS data reduction and analysis, the Nika and Irena packages of Igor Pro (Wavemetrics) were used.70

conjugated plane orientates face-on to the substrate−film interface, while for PSS, PTS, and OTS surfaces, we observe edge-on behavior. We hypothesize that the edge-on behavior is due to strong substrate−polymer interaction in the case of PEDOT:PSS, caused by P3HT-stabilized PEDOT hole polarons at the PEDOT:PSS-P3HT interface. Face-on orientation at the interface facilitates charge transport through the plane, which is beneficial for BHJ solar cell performance, due to increased charge carrier mobility and increased short circuit current. Furthermore, the ability to tune molecular orientation at the interface opens the possibility for better charge transport control in other organic electronic devices, such as transistors or sensors. Understanding the morphology−processing relationship for processing methods that are compatible with large-scale, industrial processing is crucial for the progress of organic solar cells and other organic, polymer-based electronics. One generally important finding from this study on the model system P3HT:PCBM is that morphological evolution for solution-sheared films is quite different from that of spincoated films. Besides the more expected dependence of the morphology on the processing conditions (shearing speed, solvent), we were able to demonstrate that substrate effects play a crucial role in the resulting morphology for solution-sheared films. Thus, substrate design can be used to obtain desired molecular packing in a variety of conjugated polymer systems.

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EXPERIMENTAL SECTION

Materials. Regioregular P3HT (Sepiolid P100, by BASF in cooperation with Rieke) with a molecular weight of MN = 12 480 g/ mol and a polydispersity index of 1.7 (as measured by gas-phase chromatography in tetrahydrofuran) was used. C60PCBM was bought from Nano-C. Both materials were used as received. PEDOT:PSS (CLEVIOS P VP AI 4083 or CLEVIOS PH 1000) was purchased from Heraeus and filtered twice prior to spin-coating (PES membrane, 0.45 μm). PSS (Aldrich, 30 wt % in H2O, Mw of ∼70 000) was diluted in a ratio of 1:4 with deionized H2O and filtered twice (PES membrane, 0.45 μm). ZnO solutions were prepared in ethanol prepared at a concentration of 100 mg/mL and filtered once (0.2 μm, PTFE). Solutions. Solutions were prepared from chloroform (SigmaAldrich, Reagent Plus Grad, 99%) and 1,2-dichlorobenzene (SigmaAldrich, Reagent Plus Grad, 99%) at a concentration of 30 mg/mL for the solution-sheared films. For the spin-coated films, concentrations of 30 mg/mL for the 1,2-dichlorobenzene solution and 15 mg/mL for the chloroform solutions were used. The concentrations were adjusted to yield comparable film thicknesses. Solutions were prepared using the following procedure: first, P3HT and PCBM were dissolved separately and stirred on a hot plate for at least 2 h at 25 °C (chloroform) and 50 °C (1,2-dichlorobenzene) in ambient conditions. The solutions were then blended in a 1:1 ratio and stirred on a hot plate for ca. 30 min before using. Substrate Preparation. Samples were prepared on silicon wafers (3 in. diameter, 500 μm thickness, native oxide) for the GIWAXS/ GISAXS experiments and on glass substrates (VWR micro cover glass, 200 μm, 25 × 25 mm) for UV−vis absorption spectroscopy measurements. The substrates were first cleaned in UV−ozone for 20 min. Then, they were spin-coated with either PEDOT:PSS, ZnO, PSS, or PTS-treated. PEDOT:PSS samples were spin-coated for 60 s at 4000 rpm and then baked at 150 °C for 20 min in air. PSS samples were spin-coated for 60 s at 3000 rpm and subsequently baked in air at 150 °C for 20 min. ZnO samples were spin-coated for 15 s at 5000 rpm and baked for 15 min at 200 °C. The resulting film thicknesses are ca. 30 nm for PEDOT:PSS, ca. 140 nm for PSS, and ca. 45 nm for ZnO. PTS-treated substrates were obtained by immersing the samples for 15 h in a 3 wt % PTS toluene solution at 90 °C. After surface treatment or spin-coating, the Si wafers were cut into pieces of 1 cm × 2.5 cm and used for solution shearing with P3HT or P3HT:PCBM.

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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/acsami.5b09349. UV−vis absorption spectroscopy modeling theory; UV− vis fits; GIWAXS of films probed parallel and perpendicular to the X-ray beam; (100) integrated intensity profiles; (010) integrated intensity profiles; plot of azimuthal FWHM; plot of Qz; depth-profile GIWAXS of films on different substrates; (010) integrated intensity profiles of depth-profile GIWAXS on different substrates; GIWAXS profiles of spin-coated, and spin-coated and annealed films on different substrates; table of film thicknesses; GISAXS integrated intensity profiles; GISAXS fits. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Z.B.) 1748


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ACS Applied Materials & Interfaces *E-mail: [email protected]. (M.F.T.) *E-mail: [email protected]. (S.M.)

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Present Addresses ⊥

Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States. # Center for Advancing Electronics Dresden, Dresden University of Technology, D-01062 Dresden, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge support by the Department of Energy, Bridging Research Interactions through collaborative Development Grants in Energy (BRIDGE) program under Contract No. DE-FOA-0000654-1588 and by the Department of Energy, Laboratory Directed Research and Development funding, under Contract No. DE-AC02-76SF00515. We thank the team at beamline 7.3.3 at the Advanced Light Source (ALS) in Berkeley for great support and valuable input. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We thank the beamline engineers and scientists at Stanford Synchrotron Radiation Laboratory for their valuable input and support. J.R. acknowledges postdoctoral support from the Swedish Knut and Alice Wallenberg Foundation. M.P. gratefully acknowledges financial support by the Carl-Zeiss-Foundation and by the German Bundesministerium für Bildung und Forschung (FKZ: 03EK3507). T.S. acknowledges financial support from the German Federal Environmental Foundation (DBU).

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Tuning the Morphology of Solution-Sheared P3HT:PCBM Films - ACS

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