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Manipulating the Morphology of P3HT−PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing Eric Verploegen,†,§ Chad E. Miller,† Kristin Schmidt,† Zhenan Bao,§ and Michael F. Toney*,† †

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States

§

S Supporting Information *

ABSTRACT: Using grazing incidence X-ray scattering, we observe the effects of solvent vapors upon the morphology of poly(3-hexylthiophene)−phenyl-C61-butyric acid methyl ester (P3HT−PCBM) bulk heterojunction thin film blends in real time; allowing us to observe morphological rearrangements that occur during this process as a function of solvent. We detail the swelling of the P3HT crystallites upon the introduction of solvent and the resulting changes in the P3HT crystallite morphology. We also demonstrate the ability for tetrahydrofuran vapor to induce crystallinity in PCBM domains. Additionally, we measure the nanoscale phase segregated domain size as a function of solvent vapor annealing and correlate this to the changes observed in the crystallite morphology of each component. Finally, we discuss the implications of the morphological changes induced by solvent vapor annealing on the device properties of BHJ solar cells. KEYWORDS: solar cells, nanostructures, thin films, conducting polymers, characterization tools more equilibrium morphology to be achieved.11−14 Additionally, alkane thiol additivesdue to their high boiling point and selective solubility for the fullerene componenthave been used to control the nanoscale phase segregation of polymer− fullerene BHJ blends by promoting the formation of fullerene clusters.15−17 Solvent vapor annealing (SVA) can be effectively used as a postprocessing technique to manipulate the morphology of polymer−fullerene BHJs.8,11,18−24 SVA differs from slower evaporation rates during spin-casting, as SVA provides molecular mobility for local rearrangements25 but does not fully redissolve the components. As a consequence, SVA does not typically induce large scale phase segregation. In contrast, during slow evaporation of a high boiling point solvent, the components are essentially in a concentrated solution, and phase segregation on a larger length scale, 10−100 nm, can occur. Several reports have shown that chloroform and chlorobenzene vapor annealing results in increases in the short circuit current, fill factor, and power conversion efficiency as a result of increased ordering of the polymer component and local demixing of the fullerene component. This demonstrates the utility of SVA for postprocessing of BHJ solar cells.19,20,22,24 Thermal annealing is widely used as a postprocessing technique for improving the efficiency of BHJ solar cells.11,26−31 Increases in the crystallinity of both the polymer and fullerene domains and subsequent increases in charge

1. INTRODUCTION Polymer−fullerene bulk heterojunctions (BHJs) have attracted significant interest as a potentially inexpensive active layer for solar cell devices used to convert solar energy to electricity.1−4 The BHJ often consists of polymer (electron donor) and fullerene (electron acceptor) components blended together and phase segregated on the nanometer length scale. In a BHJ solar cell, light is absorbed in the active layer, generating a bound electron−hole pair (i.e., exciton). The exciton must then diffuse to an interface between the electron donor and acceptor phases, where the charges can be separated and transported to their respective electrodes, resulting in a photocurrent. Controlling the molecular packing within each domain, the orientation of the crystallites within the domains, and the nanoscale phase segregation between domains are all critical for optimizing the performance of the devices. Several strategies have been used to manipulate and control the morphology of polymer−fullerene BHJs, including variations in the thin film deposition conditions and postprocessing techniques, such as thermal and solvent vapor annealing.5 Bulk heterojunction thin films are effectively prepared by spin-casting a solution of the polymer and fullerene components dissolved in an organic solvent. It has been shown that evaporation time of the solvent has a significant impact on the resulting BHJ thin film morphology. Higher spincasting speeds and longer the spin-casting times lead to faster evaporation of the solvent from the film, providing less time for molecular rearrangements and resulting in a more disordered morphology.6−10 Solvents with high boiling points evaporate slowly from the polymer−fullerene thin films, allowing for a © 2012 American Chemical Society

Received: July 22, 2012 Revised: September 15, 2012 Published: September 20, 2012 3923

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out-of-plane, indicating the π−π stacking direction is predominantly perpendicular to the substrate. Schematics of thin films with random, perpendicular, and parallel P3HT lamellae orientations and the relative orientation of the P3HT crystallographic axes are shown in Figure S2 (Supporting Information). Additionally, the thin films spin-cast from chlorobenzene show a smaller full width at half-maximum (fwhm) of the nominally out-of-plane (200) peak, showing that the coherence length of the crystallites is larger in the thin films spin-cast from chlorobenzene than those spin-cast from chloroform (see Table S1, Supporting Information). These trends are also observed for the 3:1 and 1:1 P3HT−PCBM blend thin films. The choice of spin-casting solvent does not appear to have any significant effect upon the PCBM morphology; in all cases, broad rings indicate that the PCBM is amorphous or consists of very small crystallites (∼2 nm). These results indicate that spin-casting from chlorobenzene allows the P3HT component to arrange in a lower free-energy state compared to thin films spin-cast from chloroform due to the higher boiling point and slower evaporation rate of chlorobenzene. Other groups have shown that P3HT−PCBM blends spincast from chlorobenzene have longer time to self-organize, exhibiting improved P3HT crystallinity, which leads to an enhancement of the absorption, an increase in the charge carrier mobility, and increased power conversion efficiency.32,33 Yu showed that for P3HT−PCBM blends spin-cast from a series of solvents (chloroform, chlorobenzene, o-dichlorobenzene, and 1,2,4-trichlorobenzene) improved P3HT crystallinity is observed as the solvent boiling point is increased.33 AlIbrahim et al. observed that P3HT−PCBM blend thin films spin-cast from chlorobenzene display greater P3HT ordering and a higher power conversion efficiency than those spin-cast from chloroform.32 However, in this study, thermal annealing of the P3HT−PCBM thin films spin-cast from chlorobenzene did not result in significant morphological rearrangements or increases in device properties, while thermal annealing of the thin films spin-cast from chloroform resulted in greater P3HT ordering and power conversion efficiency than the thin films spin-cast from chlorobenzene. This indicates that the thin films spin-cast from chloroform, with their less stable morphology, are more susceptible to morphological manipulation and device performance improvements than their lower free-energy counterparts spin-cast form chlorobenzene. We will, thus, focus our investigation on thin films spin-cast from chloroform, as the orientation of the crystallites is more favorable for charge transport in a solar cell device34,35 and these films present a less ordered state, so that the effects of SVA can be more clearly observed. 2.2. In-Situ Solvent Vapor Annealing. Using GIWAXS, we investigated, in situ, the effects of exposing P3HT, PCBM, and BHJ blend thin films to vapor from several organic solvents, including acetone, methanol, chlorobenzene, dichlorobenzene, toluene, chloroform, hexane, and tetrahydrofuran. Exposure to acetone and methanol vapor produced less significant changes in the BHJ morphology than the other solvents. Additionally, chlorobenzene, dichlorobenzene, and toluene vapor annealing yielded effects similar, although not completely identical, to chloroform vapor annealing. Thus, for the sake of brevity and to highlight the different effects of the solvents, we will focus the discussion on SVA with chloroform, hexane, and tetrahydrofuran (THF).

carrier mobilities are often achieved with thermal annealing.1,28,29 However, thermal annealing can lead to degradation of conjugated polymers and flexible substrates that are not stable at high temperatures, and it can result in coarsening of the nanoscale phase segregation, which is unfavorable for exciton diffusion to the BHJ interface, leading to exciton recombination and lower power conversion efficiency.1,21,28 To achieve higher power conversion efficiencies for polymer−fullerene solar cells, more precise control over the morphology of the active layer is needed. As described above, SVA provides potential advantages over thermal annealing, namely selective annealing of individual components and more controlled nanoscale phase segregation. However, there is still a lack of fundamental understanding regarding the details of how various solvent vapors induce morphological changes in the polymer−fullerene BHJ active layer; a better understanding of SVA will lead to improved processing methods of BHJ thin films. To this end, we use grazing incidence X-ray scattering (GIXS) to investigate the morphology of poly(3-hexylthiophene) (P3HT)−phenyl-C 61 -butyric acid methyl ester (PCBM) BHJ thin films before, during, and after exposure to solvent vapor. The goal of this work is to better understand the morphological rearrangements taking place by capturing how the solvent affects the morphology during processing of the film; this understanding can be translated into the design of processing parameters that include SVA to achieve a more optimal morphology and device performance. Specifically, we discuss swelling of P3HT, changes in P3HT crystallite size and orientation, PCBM crystallization with THF vapor annealing, and changes in the nanoscale phase segregated domain size. Finally, we will discuss the implications that these morphological changes have on the performance of polymer−fullerene solar cell devices.

2. RESULTS The following section will describe the results of SVA on four aspects of the BHJ morphology: the P3HT layer spacing, the P3HT π−π stacking, the crystallinity of PCBM, and the nanoscale phase segregation of the bulk heterojunction. We describe the effects of SVA with chloroform, hexane, and THF upon neat P3HT, neat PCBM thin films, and BHJ blends of the two components with several blend ratios. However, first we will discuss the differences observed between spin-casting the films from chloroform and chlorobenzene, before any postprocessing, as it is important to consider the initial state of the thin films. 2.1. Spin Casing Solvent. Grazing incidence wide-angle Xray scattering (GIWAXS) images of neat P3HT, 3:1 P3HT− PCBM blend, 1:1 P3HT−PCBM blend, 1:3 P3HT−PCBM blend, and neat PCBM thin films, spin-cast from 2 mg/mL in chloroform and 2 mg/mL in chlorobenzene are shown in Figure S1 of the Supporting Information. Neat P3HT thin films spin-cast from chlorobenzene have stronger preferential orientation relative to the substrate compared to those spincast from chloroform. The neat P3HT thin film spin-cast from chlorobenzene shows an orientation similar to thermally annealed films (P3HT lamella direction is perpendicular and the π−π stacking direction is predominantly parallel to the substrate), as evidenced by the lack of out-of-plane π−π scattering and the presence of strong in-plane π−π scattering.26 In contrast, for the neat P3HT thin films spin-cast from chloroform, strong scattering from the (020) peak is observed 3924

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For the SVA experiments, we bubbled helium through the organic solvent of choice, and this helium and solvent vapor mixture is flowed through the sample chamber. This allows us to characterize the thin film morphology before, during, and after exposure to solvent vapor with GIWAXS. We also investigated thin films that were solvent annealed in a slightly different solvent annealing procedure, where the thin film was placed under a glass cover along with a dish of the desired solvent for 30 min. In all cases, the resulting morphologies were similar to the thin films solvent vapor annealed with the in situ GIWAXS chamber. Below, we will discuss the results of SVA on the P3HT layer spacing, the P3HT π−π stacking, the crystallinity of PCBM, and the nanoscale phase segregation of the bulk heterojunction. 2.2.1. Solvent Vapor Annealing Effect on the P3HT Layer Spacing and fwhm. For analysis of the P3HT layer spacing, we considered the in-plane (100) peak at q ∼ 0.38 Å−1. The region selected for subsequent quantitative analysis is shown in Figure S3 of the Supporting Information. For the neat P3HT thin film spin-cast from 2 mg/mL in chloroform, a P3HT layer spacing increase of 3.0%, 3.8%, and 2.6% was observed upon the introduction of chloroform, hexane, and THF, respectively (see Figure 1a). However, after the removal of solvent, the layer spacing decreases but was still larger than the original value (1.6%, 1.5%, and 0.4% for chloroform, hexane, and THF, respectively). Table S2 (Supporting Information) shows the P3HT layer spacing before, during, and after exposure to solvent vapors for neat P3HT, 3:1 P3HT−PCBM blend, and 1:1 P3HT−PCBM blend thin films spin-cast from 2 mg/mL in chloroform. Vapor annealing with each solvent resulted in roughly similar P3HT layer swelling behavior for the 3:1 and 1:1 P3HT−PCBM blend thin films (see Figure S4a in the Supporting Information and Figure 2a, respectively). A significant decrease of radial fwhm (along q) of the inplane (100) peak was observed as a result of vapor annealing with chloroform or THF for the neat P3HT, 3:1 blend, and 1:1 blend thin films (see Figures 1b, S4b (Supporting Information), and 2b, respectively). This decrease in the radial fwhm is due to an increase in the coherence length, which could result from either an increase in the crystallite size through consumption of amorphous regions or an increase in paracrystalline order.36,37 In contrast, while hexane vapor annealing did result in a significant swelling of the P3HT layers, the decrease in the radial fwhm of the in-plane (100) peak was much less significant than seen with chloroform or THF. For the thin films annealed with hexane vapor, the radial fwhm of the inplane (100) peak is greater than 0.054 Å−1 for the neat P3HT, 3:1 blend, and 1:1 blend thin films, compared to vapor annealing with chloroform or THF, which resulted in a fwhm less than 0.050 Å−1 for all three thin films. Similar results were obtained from the same experiments for neat P3HT, 3:1 P3HT−PCBM blend, and 1:1 P3HT−PCBM blend thin films spin-cast from 10 mg/mL in chloroform and annealed with chloroform or THF vapor (see Table S4 and Table S5 in the Supporting Information). 2.2.2. Solvent Vapor Annealing Effect on the P3HT π−π Stacking Distance. The nominally out-of-plane (020) peak observed at q ∼ 1.65 Å−1 corresponds to the π−π stacking of the thiophene backbone.38 The region selected for subsequent quantitative analysis is shown in Figure S3 of the Supporting Information. For neat P3HT and 3:1 P3HT−PCBM blend thin films, vapor annealing with chloroform or THF resulted in a decrease

Figure 1. (a) Layer spacing, (b) layer fwhm, (c) π−π stacking distance, (d) π−π fwhm for P3HT thin films spin-cast from 2 mg/mL in chloroform. The thin films exposed to chloroform vapor (■), hexane vapor (○), and THF vapor (△) are shown. The layer spacing values were determined by integration of the in-plane (100) peak, and the π−π stacking distance was determined by integration of the nominally out-of-plane (020) peak. The region selected for quantitative analysis is shown in Figure S3 of the Supporting Information.

in the out-of-plane π−π stacking distance upon the introduction of solvent vapor, and a further decrease was 3925

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Figure 2. (a) Layer spacing, (b) layer fwhm for 1:1 blend thin films spin-cast from 2 mg/mL in chloroform. The thin films exposed to chloroform vapor (solid squares), hexane vapor (open circles), and THF vapor (open triangles) are shown. The layer spacing values were determined by integration of the in-plane (100) peak. Figure 3. 2-D GIWAXS images of 1:1 blend thin films spin-cast from 10 mg/mL in chloroform (a) as-cast, (b) after expose to chloroform vapor for 30 min, and (c) after expose to THF vapor for 30 min.

observed upon the removal of the solvent vapor (see Figure 1c and Figure S4c (Supporting Information)). A decrease in the out-of-plane π−π stacking distance of at least 1.5% was observed for both the neat P3HT and 3:1 P3HT−PCBM blend thin films after vapor annealing with chloroform or THF. In contrast, no significant change in the out-of-plane π−π stacking distance (less than a 0.25%) was observed for hexane vapor annealing of neat P3HT and 3:1 P3HT−PCBM blend thin films. We were not able to accurately measure changes in the π−π stacking distance for 1:1 P3HT−PCBM blend thin films due to the weak scattering intensity of this feature and the large background from PCBM. Additionally, vapor annealing with chloroform or THF resulted in a greater than 6% decrease in the radial fwhm (along q) of the nominally out-of-plane (020) scattering peak; in contrast, hexane vapor annealing resulted in less than a 1.5% decrease in the fwhm of the same (020) peak. This decrease in the peak width indicates an increase in the paracrystalline order for the P3HT π−π stacking.36,39 After SVA, the nominally out-of-plane (020) scattering peak remains significantly stronger than the in-plane (020) peak, see Figure 3. In contrast, for thermal annealing above the P3HT melting temperature, the nominally out-of-plane (020) peak completely disappears and the in-plane (020) peak significantly increases in intensity for all P3HT containing films.26 This has implications for the application of these materials in solar cells, as the orientation of the π−π stacking influences the charge carrier mobility in a given direction. Similar results were obtained from the same experiments and analysis of neat P3HT, and 3:1 P3HT−PCBM blend thin films spin-cast from 10 mg/mL in chloroform and annealed with chloroform or THF vapor (see Table S4 and Table S5 in the

Supporting Information). These results show that chloroform and THF vapor annealing allow for rearrangements of the π−π stacking between polymer backbones, where hexane vapor does not. 2.2.3. Solvent Vapor Annealing Effect on the PCBM Component. GIWAXS of neat PCBM thin films spin-cast from 10 mg/mL in chloroform show broad isotropic peaks at q ∼ 0.69 Å−1 and 1.37 Å−1 from an amorphous PCBM phase or very small PCBM crystallites (the region selected for subsequent quantitative analysis is shown in Figure S5 of the Supporting Information). Annealing with chloroform vapor did not result in a significant change in the peak position or shape. In contrast, when the neat PCBM thin film was exposed to THF vapor, the appearance of several distinct crystalline peaks at q ∼ 0.63 Å−1, 0.73 Å−1 and 1.42 Å−1 was observed (see Figure 4a). There is, however, no conclusive evidence of preferential orientation relative to the substrate for the neat PCBM thin films annealed with either chloroform or THF vapor. These in situ vapor annealing GIWAXS measurements reveal that the crystallization takes place upon the introduction of THF vapor and remains after the THF vapor is removed from the chamber. Due to the high parasitic scattering from the THF vapor, we cannot accurately determine the rate of crystallite growth; however, no significant increase in crystallinity is observed after 10 min of exposure to THF vapor. It should also be noted that after THF vapor annealing the film is semicrystalline, with a significant portion of 3926

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annealed with THF vapor display isotropic rings as a result of scattering form the randomly oriented PCBM crystallites, the thermally annealed thin films exhibit crystallites with strong preferential orientation relative to the substrate (see Figure S6, Supporting Information). GIWAXS of the 1:3 and 1:1 P3HT−PCBM blend thin films annealed with THF vapor also show similar evidence of PCBM crystallization (peaks at q ∼ 0.63 Å−1 and 0.73 Å−1), and annealing with chloroform vapor shows no signs of inducing PCBM crystallization (Figure 4b and c, respectively). Additionally, the (100) peak of the P3HT component can be seen at q ∼ 0.37 Å−1 in the 1:3 and 1:1 P3HT−PCBM blend thin films after annealing with chloroform or THF vapor. The small shoulder visible at q ∼ 0.75 Å−1 in the 1:1 P3HT−PCBM blend thin film vapor annealed with chloroform originates from the (200) peak of the P3HT component (Figure 4c). This peak can be more clearly seen in the neat P3HT and 3:1 P3HT−PCBM blend thin films, annealed with chloroform or THF vapor (Figure S7). The same trends for the thin films spin-cast from 2 mg/mL in chloroform are observed, but the features are not as distinct due to the reduced thickness of the films. 2.2.4. Nanoscale Phase Segregation. We used grazing incidence small-angle X-ray scattering (GISAXS) in order to investigate the nanoscale phase segregation of the bulk heterojunction. These length scales are roughly 1 order of magnitude larger than those from the P3HT and PCBM crystallites discussed previously and require a different experimental configuration, which can measure scattering at smaller angles. We investigated 1:1 P3HT−PCBM blend thin films spin-cast from 10 mg/mL in chloroform without postprocessing (as-cast) and exposed to chloroform, hexane, or THF vapor. Additionally, we thermally annealed as-cast thin films for 30 min at 140 and 175 °C for comparison. Figure 5

Figure 4. GIWAXS of (a) neat PCBM, (b) 1:3 P3HT−PCBM blend, and (c) 1:1 blend P3HT−PCBM thin films spin-cast from 10 mg/mL in chloroform. The as-cast thin films (solid line), thin films exposed to chloroform vapor (dashed line), and thin films exposed to THF vapor (dotted line) are shown. The data are presented with arbitrary units and have been shifted for clarity. The region selected for integration is from q = 0.3 to 1.6 Å−1 and β = 20 to 70° (shown in Figure S5 of the Supporting Information). The peaks observed at q ∼ 0.63 Å−1, 0.73 Å−1, and 1.42 Å−1 originate from crystalline PCBM. The peaks observed at q ∼ 0.37 Å−1 and ∼0.75 Å−1 are the (100) and (200) peaks of the P3HT component, respectively.

Figure 5. GISAXS of 1:1 P3HT−PCBM blend thin films spin-cast from 10 mg/mL in chloroform after different annealing conditions. The in-plane region selected for integration is shown in Figure S8 in the Supporting Information. The lines indicate the unified fit function used to extract information about the domain sizes. The data was shifted with respect to each other for better visibility.

amorphous PCBM in the thin film, as evidenced by the presence of broad isotropic features. The appearance of these scattering features after exposure to THF vapor is similar to the peaks observed when the neat PCBM thin films were annealed above the PCBM cold crystallization temperature (150 °C).26 However, the integrated radial profiles of the thermally annealed PCBM containing thin films cannot be directly compared to those from the thin films annealed with THF vapor due to the differences in the orientation of the crystallites. While the neat PCBM thin films

shows the in-plane GISAXS data with the corresponding fits (the region selected for in-plane integration is shown in Figure S8 of the Supporting Information). We fit these GISAXS scattering curves using the unified equation, which combines Guinier’s and Porod’s law to extract information about the domain sizes and the domain interface roughness, respectively. Details about the model used can be found in ref 40. The ascast thin film exhibits a power-law dependence of 2 and does 3927

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measure the peak width of the (020) peak in the as-cast 1:1 P3HT−PCBM blend thin films due to the weak scattering intensity of this feature, it is apparent that an increase in the ordering of the π−π stacking occurs as a result of chloroform vapor annealing, as evidenced by the increased scattering intensity of the out-of-plane (020) peak in Figure 3. Additionally, the crystallite orientation is an important factor for device applications, as charge transport is required perpendicular to the substrate in order for the charge to be collected at the electrodes. Charge transport in P3HT is significantly higher along the (001) direction (parallel to the polymer backbone) and the (010) direction (parallel to the π−π stacking), compared to the (100) direction (perpendicular to the layers).43 In nearly all cases, spin-casting of conjugated polymers yields polymer chains orientated parallel to the substrate; therefore, it is favorable to have the π−π stacking direction perpendicular to the substrate. Spin-casting from chlorobenzene or dichlorobenzene results in a higher degree of P3HT crystalline order than spin-casting from chloroform; however, spin-casting from chlorobenzene or dichlorobenzene results in P3HT π−π stacking predominately oriented parallel to the substrate, where spin-casting from chloroform results in the more favorable π−π stacking predominately oriented perpendicular to the substrate. Thermal annealing of P3HT containing thin films above the P3HT melting temperature results in a reorientation such that the π−π stacking is predominately oriented parallel to the substrate. However, chloroform vapor annealing can be used to increase the crystalline order without inducing large scale reorientation, thus preserving the favorable initial orientation, with the π−π stacking predominately perpendicular to the substrate. Chloroform vapor annealing leads to phase segregation of P3HT and PCBM with domain sizes smaller (Figure 5) than that obtained from thermal annealing. GISAXS data confirms that chloroform vapor annealing promotes phase segregation on a ∼52 Å length scale, which is smaller than the domain size of ∼86 Å that we observed for thermal annealing at 140 °C. Pronounced phase segregation was not observed in the as-cast films likely because the amorphous P3HT mixes too well with PCBM. As the crystallinity of P3HT increases due to chloroform vapor annealing, the phase segregation of crystalline P3HT and PCBM takes place. It has been reported in the literature that chloroform vapor annealing results in increases in the short circuit current, fill factor, and power conversion efficiency,19,20 with these increases in device performance being attributed to nanoscale phase segregation and increased crystallinity of the P3HT component. The results presented in this work illustrate that chloroform vapor annealing swells the P3HT layers, providing enough molecular mobility for rearrangements of the P3HT alkyl side chains and the P3HT backbone, leading to increased P3HT crystalline order and inducing nanoscale phase segregation, without allowing for large scale reorientation of the P3HT crystallites. 3.2. Hexane Vapor Annealing. To compare the effects of chloroform vapor annealing with a solvent that has lower P3HT and PCBM miscibility, we performed hexane vapor annealing experiments on the same P3HT, PCBM, and BHJ blend thin films. Hexane vapor annealing of neat P3HT thin film resulted in P3HT layer swelling similar to chloroform and THF vapor annealing, indicating that P3HT is soluble in hexane. However, only a 3% decrease in the radial fwhm of the in-plane (100)

not show any Guinier regime, both indicating that the domains are poorly phase segregated (i.e., highly intermixed) with no well-defined domain sizes within the length scales probed (∼35−1000 Å). After exposure to hexane vapor, no changes are observed, indicating similar nanoscale morphology as the ascast thin films. Exposure to chloroform and THF vapor, however, results in the appearance of weak shoulders at about 0.035 Å−1 and 0.029 Å−1, respectively, and the power-law exponent increases to about 3. These features indicate that for the 1:1 P3HT−PCBM blend thin films phase-segregation took place during solvent annealing with chloroform or THF vapor, resulting in domain sizes of ∼52 Å and ∼62 Å, respectively (see Table S6, Supporting Information). In comparison, thermal annealing at 140 and 175 °C results in the appearance of shoulders near 0.022 Å−1 and 0.014 Å−1, respectively, indicting phase segregation with dominant domain sizes of ∼86 Å and ∼121 Å, respectively.

3. DISCUSSION In the following section, we will compare the effects of SVA with chloroform, hexane, and THF upon the crystallinity of the P3HT and PCBM phases, and the nanoscale phase segregation of these two components. We will discuss how each solvent interacts with the P3HT and PCBM components to result in the observed morphological changes of the thin film. 3.1. Chloroform Vapor Annealing. Chloroform vapor annealing results in molecular rearrangements and growth of P3HT crystallites, no changes in the PCBM crystallinity, and the nanoscale phase segregation of P3HT and PCBM domains. The neat P3HT, 3:1 P3HT−PCBM blend, and 1:1 P3HT− PCBM blend thin films all showed a layer spacing swelling of greater than 3% during exposure to chloroform vapor. After the removal of the chloroform vapor the layer spacing of all the thin films remained at least 1.5% larger than the as-cast value. This indicates that a rearrangement of the alkyl side chains occurs, similar to what is known to occur with thermal annealing,26 resulting in the larger P3HT layer spacing after chloroform vapor annealing. Figure 3 shows increased scattering intensity for the P3HT layers and the π−π stacking peaks for 1:1 blend thin films after expose to chloroform vapor; however, there are no indications of large scale crystallite reorientation, as seen with thermal annealing above the P3HT melting temperature.26 The decrease in the radial fwhm (along q) of the in-plane (100) peak observed with chloroform vapor annealing indicates that local rearrangements (increased crystallite size through consumption of neighboring amorphous regions and/or increased paracrystalline order) have taken place. The π−π stacking distance and the paracrystalline order both have a significant effect on the charge transport in conjugated polymer systems;39−41 thus, we are keen to explore the effects of SVA upon the π−π stacking in polymer−fullerene BHJ blends. We observe a decrease in the π−π stacking distance with chloroform vapor annealing for the neat P3HT and the 3:1 P3HT−PCBM blend thin films. Shorter π−π stacking distances have been shown to increase the charge transfer integral, which describes the electronic wave function overlap between adjacent molecules, and increase the charge carrier mobility.42 These films also exhibit a decrease in the radial fwhm of the nominally out-of-plane (020) peak, indicating increased paracrystalline order in the π−π stacking after chloroform vapor annealing. It has been shown that decreased paracrystalline order is related to decreased charge transport, due to the movement of holetraps deeper into the band gap.39,41 While we could not 3928

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of the nucleation process for PCBM crystallite growth resulting from exposure to THF vapor, the lack of preferential orientation indicates that it is likely that THF acts as nucleation sites for PCBM crystallization within the bulk of the thin film and/or coordinates the growth of existing small crystallites. Tang et al. have also used THF vapor annealing to induce the formation of crystalline PCBM aggregates in P3HT−PCBM BHJ thin films.21 In this work, a two step process was used where THF was first used to induce PCBM crystallite formation, followed by carbon disulfide vapor annealing, which further increased the P3HT crystallinity, resulting in improved solar cell efficiency compared to thermal annealing. By varying the THF vapor pressure and exposure time, the PCBM cluster size can be controlled and the fill factor of the solar cell device can be optimized. For 1:1 P3HT−PCBM blend thin films, our GISAXS measurements reveal that THF vapor annealing results in phase segregation on the length scale of ∼62 Å, slightly larger than observed with chloroform vapor annealing. The crystallization of PCBM induced by THF vapor annealing allows for increased molecular mobility and greater domain coarsening compared to chloroform vapor annealing. However, both chloroform and THF vapor annealing result in phase segregation on a smaller length scale than thermal annealing at temperatures above 140 °C. It has been shown that crystallization of PCBM leads to improvements in the electron mobility;46,47 however, it is important to control the crystallization, as over coarsening of the domains leads to exciton recombination and lower power conversion efficiencies. Thus, THF vapor annealing could be a powerful tool for the controlled crystallization of PCBM in a number of polymer−fullerene BHJ blends.

peak were observed, compared to the greater than 16% decreases observed for chloroform and THF vapor annealing. This suggests that the swelling only takes place in the alkyl chain region of the P3HT layer and not the conjugated backbone, preventing significant increase crystalline order and crystallite growth due to consumption of neighboring amorphous regions. Similarly, for the 1:1 P3HT−PCBM and 3:1 P3HT−PCBM blend thin films vapor annealed with hexane, swelling of the P3HT layers occurred but the decrease in the radial fwhm of the in-plane (100) peak with was significantly less than with chloroform or THF vapor annealing. Additionally, hexane vapor annealing does not induce significant changes in the π−π stacking distance or the radial fwhm (along q) of the (020) peak, corroborating the conclusion that the hexane vapor does not induce rearrangements of the backbone. Due to the hydrophobicity of hexane, while it incorporates into the alkyl chain portion of the P3HT layers, resulting in an increase in the layer spacing, it does not provide mobility for rearrangements of the backbone. Thus, despite the swelling of the P3HT layers observed with hexane vapor annealing, there is not sufficient molecular mobility for P3HT crystallite growth or changes in the π−π stacking, as we observed with chloroform and THF vapor annealing. No change in the scattering from the PCBM component was observed with exposure to hexane vapor. GISAXS of the 1:1 P3HT−PCBM blend thin film annealed with hexane vapor showed no change compared to the as-cast thin film. Neither of these thin films show significant scattering features that could be attributed to distinct nanoscale phase segregation. Hexane vapor annealing does not induce defined nanoscale phase segregation of the bulk heterojunction blend, as this does not increase the crystallinity of P3HT because there is not enough molecular mobility for large scale rearrangement. Since hexane vapor annealing does not produce morphological rearrangements that are favorable for improving BHJ solar cell performance, it is not surprising that there are no reports in the literature regarding the effects of hexane vapor annealing on the BHJ device properties. 3.3. THF Vapor Annealing. THF vapor annealing had similar effects upon the P3HT component as chloroform, namely, swelling of the P3HT layers, decrease in the fwhm of the (100) and (020) peaks (an increase in the crystalline order of the P3HT layers and the π−π stacking, respectively), and a reduction in the π−π stacking distance, phase segregation of P3HT and PCBM domains, all without significant reorientation of the P3HT crystallites. Additionally, an enhancement of PCBM crystallinity was observed upon THF vapor annealing. It is interesting to note that PCBM has a lower solubility in THF (2 mg/mL)44 than in chloroform (25 mg/mL)45 and exposure to THF vapor results in the appearance distinct scattering from PCBM crystallites, where chloroform does not. This indicates that solvent miscibility is not the most important driver for inducing crystallinity in a given component within a BHJ blend. While THF provides the molecular mobility necessary for PCBM crystallization, there is an additional interaction between THF and PCBM that induces PCBM crystallization that is not present for chloroform vapor annealing. For the neat PCBM thin films annealed above the PCBM cold crystallization temperature (150 °C), heterogeneous nucleation takes place at the substrate interface, leading to crystallites that have a preferential orientation relative to the substrate.26 Although we cannot precisely determine the nature

4. CONCLUSION We have used GIXS to investigate the morphology of P3HT− PCBM BHJ thin films before, during, and after exposure to solvent vapor with the goal of better understanding how the solvent affects the morphology. We have shown that chloroform and THF vapor annealing both result in a swelling of the P3HT layers, a decrease in the π−π stacking distance, an increase in the crystalline order within the P3HT component, and phase segregation of P3HT and PCBM domains. Additionally, we showed that THF vapor annealing induces crystallinity in the PCBM component. In contrast, hexane results in a swelling of the P3HT layers but does not allow for rearrangement of the polymer backbone or crystallization of PCBM. GISAXS showed that vapor annealing with chloroform and THF results in smaller phase segregated domains (∼52 Å and 62 Å, respectively) than thermal annealing at temperatures above 140 °C (>130 Å), which is more favorable for exciton dissociation. Additionally, SVA does not result in a significant reorientation of the P3HT layers; leaving the π−π stacking direction predominately perpendicular to the substrate, which is more favorable for charge transport in a solar cell device than the predominately parallel π−π stacking direction seen with thermal annealing. The morphological changes we observe with SVA have significant implications on the performance of polymer− fullerene solar cell devices. It has been shown in the literature that reduction of the π−π stacking distance and an increase in the paracrystalline order of the π−π stacking can lead to improvements in the charge transport properties, through increased electronic wave function overlap and a reduction of 3929

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hole-traps deep in the band gap, respectively.39−41 Similarly, increased PCBM crystallinity is associated with improved electron transport and improved power conversion efficiencies.21,46,47 With SVA, these improvements are possible without inducing excessive nanoscale phase segregation, exposing the conjugated polymers to potentially damaging high temperatures or leading to large scale reorientation of the P3HT domains all of which would lead to a decrease in solar cell device performance. Thus, it has been concludedthrough this work and the work of others19−21,23,24that the milder annealing condition produced through SVA can lead to a more favorable polymer−fullerene BHJ morphology for solar cell devices, compared to thermal annealing.

at room temperature; thus, the only source of temperature difference between the solvent and the film would be from evaporative cooling. No condensation of solvent is observed on the film or the chamber. Collection times range from 120 s to 180 s depending on the scattering strength of the films. A similar custom built chamber was used for the GISAXS experiments at ALS beamline 7.3.3; however, in situ measurements were not possible because parasitic scattering from the solvent vapor dominates the scattering at the q range of interest. We did not carefully monitor the temperature of the solvent vapor, because the experimental runs were short (ca. 30 min) resulting in only small changes in temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables as described in the text. This material is available free of charge via the Internet at http:// pubs.acs.org.

5. EXPERIMENTAL SECTION

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Poly(3-hexylthiophene) (P3HT) with a molecular weight of ∼50 000 g/mol and ∼95% regioregular, was purchased from Rieke Metals Inc. Phenyl-C61-butyric acid methyl (PCBM) ester was purchased from Nano-C. Chloroform, chlorobenzene, hexane, and tetrahydrofuran were purchased from Sigma Aldrich. Five solutions were prepared: 2.0 mg/mL of P3HT, 1.5 mg P3HT + 0.5 mg/mL PCBM, 1.0 mg/mL P3HT + 1.0 mg/mL PCBM, 0.5 mg/mL P3HT + 1.5 mg/mL PCBM, and 2.0 mg/mL PCBM, each in chloroform. Additional films were prepared with a total concentration of 10 mg/mL. Silicon wafers were cut (∼ 20 mm × 30 mm) and rinsed with deionized water, toluene, acetone, and isopropyl alcohol, followed by a 15 min oxygen plasma etch. The solutions were spin-cast onto bare silicon wafers at 400 rpm for 10 s followed by 1000 rpm for 60 s and let dry overnight in a nitrogen glovebox. The film thicknesses were measured using a Veeco Dektak Stylus Profilometer with a tip radius of 12.5 μm. Solutions spin-cast from 2.0 mg/mL resulted in films with thicknesses of 25 to 20 nm. Solutions spin-cast from 10.0 mg/mL resulted in films with thicknesses of 130 to 70 nm (the film thickness decreased with increasing PCBM content). To ensure reproducibility between measurements the remaining film was cut into several pieces each ∼15 mm in length; this allows us to investigate the effects of different processing conditions on films from the same wafer. There was no measurable change in the film thickness after solvent vapor annealing. GIWAXS measurements were performed at the Stanford Synchrotron Radiation Lightsource (SSRL) using beamline 11−3 with a photon wavelength of 0.0976 nm. The scattering intensity was detected on a 2-D image plate (MAR-345) with a pixel size of 150 μm (2300 × 2300 pixels), and the detector was placed 400 mm from the sample center. Data analysis was performed using the software package WxDiff, provided by Dr. Stefan Mannsfeld ([email protected]). The beam size was 50 μm × 150 μm, which resulted in a beam footprint on sample 150 μm wide over the entire length of the 15 mm long sample. The overall resolution in the GIWAXS experiments, dominated by the sample size, is ∼0.08 Å−1. GISAXS measurements were carried out at beamline 7.3.3 at the Advanced Light Source with a photon wavelength of 0.1240 nm.48 The scattering intensity was detected with a CCD detector (Pilatus 1M) with a pixel size of 172 μm (981 × 1043 pixels), and the detector was placed 4075 mm from the sample center. Data reduction and analysis was performed using the NIKA49 and IRENA50 software package written by Jan Ilavsky at Argonne National Laboratory for use with the commercially available software Igor Pro. The beam size was 250 μm × 1000 μm, which resulted in a beam footprint on sample 1000 μm wide over the entire length of the 15 mm long sample. For both the GIWAXS and GISAXS the incidence angle was 0.12° to tune the scattering to be from the bulk of the film. A custom built chamber and sample stage was used for in situ GIWAXS experiments.26 The sample stage was enclosed in an aluminum chamber with a ∼200 cm3/min flow of helium into the chamber; solvent vapors were introduced into the chamber by bubbling the helium through the given solvent (see Figure S9 of the Supporting Information). The solvent was subsequently removed from the chamber by purging with helium. All experiments were performed

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This publication was partially based on work supported by the Center for Advanced Molecular Photovoltaics, Award No. KUS-C1-015-21, made by King Abdullah University of Science and Technology (KAUST). E.V. would like to thank the Eastman Kodak Corporation and the Kodak Fellows Program for support. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The authors would like to thank Alex Hexemer, Eric Schaible, Cheng Wang, and Steven Alvarez for their help with the GISAXS experiments at the Advanced Light Source. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract DE-AC02-05CH11231.

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ABBREVIATIONS GIXS = grazing incidence X-ray scattering GIWAXS = grazing incidence wide-angle X-ray scattering GISAXS = grazing incidence small-angle X-ray scattering BHJ = bulk heterojunction P3HT = poly(3-hexylthiophene) PCBM = phenyl-C61-butyric acid methyl ester THF = tetrahydrofuran SVA = solvent vapor annealing fwhm = full width half-maximum REFERENCES

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