Mixing Behavior in Small Molecule:Fullerene ... - ACS Publications

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Mixing Behavior in Small Molecule:Fullerene Organic Photovoltaics Stefan D. Oosterhout,† Victoria Savikhin,†,⊥ Junxiang Zhang,‡ Yadong Zhang,‡ Mark A. Burgers,§ Seth R. Marder,‡ Guillermo C. Bazan,§ and Michael F. Toney*,† †

SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Building 137, Menlo Park, California 94025, United States School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States § Department of Materials and Chemistry & Biochemistry, University of California, Santa Barbara, California 93106, United States ⊥ Electrical Engineering Department, Stanford University, 350 Serra Mall, Stanford, California 94305, United States ‡

S Supporting Information *

ABSTRACT: We report a novel method to determine the amount of pure, aggregated phase of donor and acceptor in organic photovoltaic (OPV) bulk heterojunctions. By determination of the diffraction intensity per unit volume for both donor and acceptor, the volume content of pure, aggregated donor and acceptor in the blend can be determined. We find that for the small molecule X2: [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) system, in contrast to most polymer systems, all the PCBM is aggregated, indicating there is negligible miscibility of PCBM with X2. This provides an explanation why the performance of OPV devices of X2:PCBM are high over a large range of PCBM concentrations. This is in contrast to many other OPV blends, where PCBM forms a mixed phase with the donor and does not provide sufficient transport for electrons when the PCBM concentration is low. This study demonstrates that a mixed phase is not necessarily a requirement for good OPV device performance.

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INTRODUCTION An organic photovoltaic (OPV) bulk heterojunction device typically comprises an active layer containing a blend of two organic materials: an electron donor (such as a polymer or small molecule) and an electron acceptor (oftentimes a fullerene derivative) sandwiched in between a hole and electron-selective contact. Light absorption in the donor leads to the formation of a Coulombically bound electron−hole pair (exciton), which has a diffusion length of 5−10 nm,1−4 before it will recombine to the ground state. The exciton can be dissociated at a donor:acceptor interface, forming a free hole and electron that can move through the donor and acceptor materials, respectively, to the electrodes.5 To ensure a high charge separation efficiency, the donor and acceptor are blended to form a bulk heterojunction (BHJ), by casting both materials from a common solution. In such a BHJ, there are typically three phases: pure donor, pure acceptor, and molecularly mixed donor:acceptor. The morphology of the resulting BHJ has a high impact on its device efficiency.6,7 The size of the pure domains should be around 10−20 nm, to ensure that the excitons can migrate to a donor:acceptor interface to charge separate.4 At the same time, the pure phases need to be interconnected to provide a path for free charges to reach the charge collecting electrodes.7 The impact of the mixed phase is currently not well understood, although there is some evidence that a mixed phase enhances charge generation.8 This can be rationalized by the fact that excitons created in a mixed phase are always in close vicinity of an interface, and © 2017 American Chemical Society

optical excitation thus always leads to charge separation. In addition, the optical gap of the donor is typically slightly broadened in a mixed, amorphous phase compared to the pure phase.9 This induces an energy cascade from the mixed phase to the pure phase that provides a driving force for free charges to move out of the mixed phase, into the pure phase.10,11 The interface between the donor and the acceptor phases is likely not sharp, but may be gradual; there is not a molecularly abrupt interface between donor and acceptor. The pure and mixed phases are important for the performance of the device and have been investigated for only a few polymer (or small molecule):fullerene systems.6,8,12−18 The mixed phase has been demonstrated to exist for polymers including P3HT,13,16 PTB7,19 and PBDTTPD.15 In the case of PBDTTPD,15 diffraction measurements were performed on polymer layers with different fullerene contents, and no fullerene diffraction was observed up to 20 wt % of fullerene; thus, it was concluded that ∼20 wt % fullerene mixes with the polymer. Small molecules behave differently from polymers and usually have a stronger tendency to aggregate, and it is presently unclear to what extent [6,6]-phenyl-C61butyric acid methyl ester (PCBM) forms mixed phases with most small molecules. Mukherjee et al.20 showed that PC71BM is able to intermix with the small molecule (p-DTS(FBTTh2)2). Received: January 6, 2017 Revised: February 21, 2017 Published: February 22, 2017 3062

DOI: 10.1021/acs.chemmater.7b00067 Chem. Mater. 2017, 29, 3062−3069

Article

Chemistry of Materials

Figure 1. Chemical structure of X2.

Figure 2. GIWAXS data of X2:PCBM blends. The percentage corresponds to the vol % of PCBM present in the layer.

In addition, Lan et al.21 reported a miscibility with PC71BM of 18%. Yet, the strong aggregation behavior of small molecules is expected to have a significant effect on the extent to which fullerene mixes with the molecules, as aggregation of the donor may have a tendency to expel fullerene from a mixed phase, leading to a smaller quantity of mixed phase in the BHJ. With the increasing interest in small molecule OPV systems, it is important to determine whether the mixed phase is as prevalent as it is in polymer:fullerene systems, and to determine how this affects the device efficiency. Here, we investigate the well-studied, small molecule X2:PCBM system (Figure 1).22,23 X2:PCBM has the unusual property that devices have the maximum efficiency (>6%) in a broad range of different PCBM contents, while most, but not all, other systems are sensitive to the exact amount of fullerene present in the BHJ.24−29 A key difference between most polymers and small molecules such as X2 is the ordered nature of X2: it is known that polymers form disordered, amorphous phases even in the absence of fullerene, while X2 forms an aggregated phase with less or possibly even no amorphous phase. In addition, X2 shows an ordered nature despite being processed from the low-boiling point solvent chloroform (without any additives), which reduces the time for a material to aggregate. Due to the ordered nature of X2, not only can the PCBM scattering be used to probe the quantity of pure PCBM phase, but also the X2 diffraction can be used to probe the quantity of pure X2 in the BHJ.

Here we report a method that allows for quantification of the aggregated PCBM and X2 content in a BHJ, as a function of PCBM content in the layer. Interestingly, we find that the amount of molecularly mixed X2:PCBM phase is negligible in X2:PCBM BHJs. Additionally, the device performance is high for a broad range of PCBM contents,22 and we find that the minimum content of both pure, aggregated X2 and pure PCBM is 30 vol % in all these blend ratios. This result shows that a mixed phase is not a requirement for high device performance, but rather, a minimum amount of pure material seems to be required to provide a pathway for sufficient charge transport to the electrodes for the X2:PCBM OPV system.

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RESULTS AND DISCUSSION GIWAXS. In order to quantify the amounts of pure phases and mixed phase, grazing incidence wide-angle X-ray scattering (GIWAXS) was performed on a series of bulk heterojunctions with varying PCBM content (Figure 2). By observing the PCBM diffraction as a function of PCBM added to the layer, the amount of aggregated (vs nonaggregated/molecularly dispersed) PCBM in the BHJ can be inferred, similar to the method used by Bartelt et al.15 The GIWAXS for pure X2, pure PCBM, and two selected X2:PCBM blends is shown in Figure 2, while the data for all other samples can be found in the Supporting Information (Figure S1). Figure 2a displays the GIWAXS pattern of the pure X2, which shows the (100) lamellar stacking peak in the

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Figure 3. (a) Radially integrated GIWAXS data for pure X2, pure PCBM, and a series of blends. The data are normalized at q = 1 Å−1. (b) Intensity of the normalized curves at q = 0.66 and 1.4 Å−1. The black lines are a guide to the eye.

Figure 4. Influence of X2 peak positions (a, c) and fwhm (b, d) on the PCBM content of the layer.

out-of-plane (OOP) direction at q = 0.39 Å−1, and the πstacking peak (010) at q = 1.79 Å−1 in the in-plane (IP) direction. X2 has therefore a preferred edge-on orientation with respect to the silicon substrate. Figure 2d shows the scattering pattern of pure PCBM; while PCBM is generally an amorphous material, there is sufficient order in PCBM domains to give rise to isotropic scattering peaks at q = 0.66 and 1.4 Å−1.30 Upon addition of PCBM to X2 (Figure 2b,c) there are two effects that can be observed. First, both the lamellar and the π-stacking diffraction of X2 becomes more angularly isotropic, which shows that the orientation of X2 with respect to the substrate becomes more isotropic. Additionally, the PCBM scattering at q = 1.4 Å−1 is obscured by diffraction that is also present in pure X2 (around q = 1.4 Å−1), which can make it difficult to judge from the 2D data whether pure PCBM contributes to the diffraction at low PCBM loading in Figure 2b. In order to determine whether PCBM diffraction is present in these blends, the images are integrated over the entire polar angle (χ) range (from OOP to IP) to quantify all PCBM and X2 aggregates in the film (refer to the Experimental Section and Supporting Information for details).31 The χ-integrated 1D data for the entire data setpure X2, pure PCBM, and blends with various PCBM contentswas normalized by dividing the entire curve by the intensity value at q = 1.0 Å−1, and plotted in Figure 3a. Normalization at q = 1.0 Å−1 was used because here there is only diffuse scattering from both X2 and PCBM, and this is proportional to the total film

thickness and sample area. Thus, normalizing at this q takes account of scattering volume and allows for a comparison of the curves. The percentages used correspond to the volume % PCBM added to the layer. Already from the normalized diffraction data, judging from the relative intensity at q = 0.66 and q = 1.4 Å−1, it is apparent that PCBM contributes to the scattering significantly, even at low PCBM content. The intensity at q = 0.66 and 1.4 Å−1 as a function of PCBM content is displayed in Figure 3b. The observed PCBM scattering even at very low PCBM contents is a strong indication that for this material system, not much, if any, PCBM exists in a mixed phase. Lamellar and π-Stacking Spacing. At this point, in addition to the increase in observed PCBM scattering, another apparent effect of blending PCBM with X2 is the shift in q and broadening of the lamellar stacking peak around q = 0.38 Å−1 (see Figure 3a). It is uncertain why the lamellar stacking peak shifts position upon blending with PCBM, but it has been observed before for X2.22 One possible explanation is that PCBM disrupts the order along the lamellar direction, which leads to a broadening of the peak and a shift to smaller q. This effect has been observed before for PBTTT.32 We have plotted the lamellar and π-stack peak position and full-width at halfmaximum (fwhm) as a function of PCBM content in Figure 4. Adding a small amount of PCBM to the X2 causes the lamellar stacking peak to shift to lower q, indicating that the average lamellar stacking distance is increased. Additionally, the π3064


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Figure 5. (a) Simulated scattering profiles for X2:PCBM blends (thin black lines), overlaid on measured data (thick colored lines), assuming full aggregation of both X2 and PCBM and (b) fits of the scattering profiles (thin black lines), overlaid on measured data (thick colored lines). The traces are offset for clarity. Additional traces can be found in Figures S2 and S3.

Since the normalized scattering profiles in Figure 3 suggest that (nearly) all PCBM forms a pure phase, and no intermixing with X2 takes place, we have first simulated (rather than fit) how the scattering profile would look in the case of no mixing, using eq 1 and the X2 and PCBM scattering intensity per unit volume, and overlaid this with the experimental data in Figure 5a for select X2:PCBM blends (all other overlays can be found in Supporting Information Figure S2). As can be seen from the figure, the simulated patterns describe the measured data fairly accurate up to 42 vol % of PCBM. When the PCBM contents are further increased, the X2 scattering is clearly overestimated by the simulation (peaks at q = 0.38 and 1.79 Å−1), while the PCBM scattering appears to remain accurate (peaks at q = 0.7 and 1.4 Å−1). Thus, due to the accurate representation of the PCBM by the fit, this confirms our conclusion from the normalized scattering data in Figure 3, that PCBM does not mix with X2 to any significant extent. Since the X2 scattering is overestimated in the simulation when the PCBM concentration is high, it follows that not all X2 in those layers is in the aggregated state. In order to quantify the amount of aggregated X2 in more detail, the experimental data has been fit to a combination of the scattering for X2 and PCBM, using eq 1. The fits yield an estimate of aggregated vol % of X2 and PCBM. Fits are plotted in Figure 5b on top of the measured data (additional data can be found in Figure S3). These fits are a more accurate representation of the data than the simulated patterns in Figure 5a, indicating that these fits give a more accurate measure of the actual quantity of aggregated X2 and PCBM. Here it is also observed that the change in width of the lamellar stacking peak at q = 0.38 Å−1 appears to induce an error on the fit at higher PCBM contents. However, the difference in peak area of the lamellar stacking for the fit and the experimental data (determined by fitting of the peak to a Lorentzian) is small and only reaches a larger difference (up to 17%) when the PCBM content is high (see Table S1). This means that the amount of aggregated X2 is slightly underestimated by our fit. However, this uncertainty of aggregated X2 content of up to 17% in the fit results does not affect the trend that is observed in the data: increasing the PCBM content decreases the relative amount of aggregated X2 in the film. A summary of the fits is presented in Figure 6. Solid diagonal lines are added to the graphs in Figure 6a,b, to indicate where the fitted data points are expected in the case where all X2 and PCBM would be fully aggregated, and contribute to the scattering. We find that, for PCBM (Figure 6b), these points are very close to the diagonal line. This is further evidence that nearly all PCBM is in an aggregated state in these X2:PCBM bulk heterojunctions, and there is no significant molecularly

stacking distance becomes smaller. Further, the breadth (or fwhm) of the lamellar peak becomes broader with increasing PCBM contents, while the breadth of the π-stacking peak remains unchanged. The peak broadening is indicative of more disorder in the lamellar stacking of the X2 phase when the PCBM contents are higher, as the “regularity” of the lamellar distance is decreasing. It is possible that the shift and broadening affects our analysis below to some degree, and this will be pointed out where appropriate. Mixed Phase Analysis Methodology. Here we report a method to quantif y the amount of pure X2, pure PCBM, and mixed phase in the layers. The method relies on careful measurement of layer thickness and sample size. The diffraction intensity per unit volume for X2 and PCBM is determined, after which the diffraction data of the various blends can be described using a linear combination of X2 and PCBM diffraction (eq 1), from which the volumetric amount of pure X2 and PCBM in the film can be calculated. The remaining (nonscattering) volume is then assumed to be amorphous as either a pure phase or a mixed X2:PCBM phase. The details of the fitting process are described in the Supporting Information. Icalc(q) = VX2 ×IX2(q) + VPCBM × IPCBM(q) + A ×ISi(q) (1)

where VX2 and VPCBM are the volumes of the respective phase in the blend layer (in μm3), IX2(q) and IPCBM(q) are the intensity per unit volume as determined by the scattering intensity of the pure materials, ISi(q) is the scattering intensity of a pure silicon substrate, and A is a constant. The A × ISi(q) term is added to correct for differences in background intensity, which arise from slight differences in sample size. The absolute value of A is smaller than 0.1 for all fits, so background intensity fluctuation is a small effect. At this point, it is necessary to address that any significant change in molecular packing of X2 would change the structure factor and, therefore, the scattering intensity. However, as demonstrated in the previous section, the peak positions do not change significantly in this regard, and therefore we argue that the molecular packing of X2 remains the same. Any changes to the structure factor of X2 are minor, and hence eq 1 is justified. It is important to point out that all percentages used in this paper are vol % of the entire layer, as opposed to the more commonly used mass % or mass blend ratio. Further, the vol % of PCBM added to the layer experimentally is different from the vol % of aggregated PCBM, which is determined using GIWAXS data and the fitting routines. Vol % is always a percentage of the volume of the entire layer, unless explicitly stated otherwise. 3065


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The absence of a mixed phase in X2:PCBM, while still achieving an EQE of 0.7,43 suggests that the presence of a mixed phase may not be a requirement for good device performance. In fact, the absence of a mixed phase results in a significant amount of pure fullerene phase at lower fullerene contents and allows electron transport to be sufficiently high at low fullerene contents, as is evident by high device performance at these PCBM contents.22 This explains why the efficiency of X2:PCBM is tolerant to blending ratio. This is illustrated in Figure 7, where the efficiency of X2:PCBM photovoltaic

Figure 6. Results from fitting scattering data: fraction of (a) aggregated X2, (b) aggregated PCBM, and (c) amorphous phase as a function of PCBM content in the layer. The diagonal lines in (a) and (b) show where the fit results are expected in the case of fully aggregated X2 and PCBM in the layer, respectively.

mixed phase present in these layers. We do note that the results at high PCBM content are slightly above the diagonal line, which is nonphysical; we attribute this to the experimental error. Looking at the fit results for X2 (Figure 6a), most of the X2 is in the aggregated state for a PCBM content of less than ∼40 vol %. When the PCBM content is higher, part of the X2 is no longer in the aggregated state, but disordered and nonscattering. This nonscattering volume of the layer is displayed in Figure 6c. The observation that part of the X2 is amorphous is consistent with the simulated scattering traces in Figure 4, which show an overestimation of aggregated X2 for these blends.

Figure 7. Aggregated X2, PCBM, and nonscattering material content of the active layer (top) compared to the device performance of these layers (bottom). The device efficiency is data from Y. Huang et al.22

devices (from Y. Huang et al.22) is compared to the pure X2 and PCBM contents in the active layer. In addition, hole mobility remains high (>1.8 × 10−4 cm2 V−1s−1) up to a PCBM content of 52 vol % (Table S2), which accounts for high device performance when the quantity of aggregated X2 is above the 30 vol % threshold. Efficiency that is tolerant to the blend ratio has been reported before for the PTB7−th:PC71BM OPV system by W. Huang et al., which has a high efficiency in the 50−70 wt % of PC71BM range.25 They found that, due to the mixing of the materials, there are no interconnected pathways for electrons and hence low device efficiency when PC71BM concentration is low, while the polymer is no longer able to aggregate and transport holes sufficiently when the PC71BM concentration is high. These observations are consistent with what we observe for the X2:PC61BM OPV system. Exciton Diffusion and Charge Transport. The mechanism of charge generation in the absence of a mixed phase is different from a blend where a mixed phase is present, such as PBDTTPD:fullerene, where a significant fraction of free charges are created in a mixed phase. The X2:PCBM system is in fact more comparable to the hybrid organic−inorganic P3HT:ZnO system, where due to the nature of the electron acceptor, ZnO, there is also no mixed phase.7 Consequently, exciton diffusion becomes an important process for charge generation, as all excitons originate in a pure X2 or pure PCBM domain, rather than (partially) in an intimately mixed phase. Because there is no energy cascade pushing excitons to domain boundaries, the size of the domains will have a greater impact on the device performance in blends without mixed phase.7 The domain size of X2:PCBM has been well documented and is around 23 nm in the optimized efficiency range.22 It is

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DISCUSSION Comparison to Other Systems. The observation that there is negligible mixed phase present in the small molecular X2:PCBM photovoltaic system stands in stark contrast to what has been observed for several polymer:fullerene OPV systems. For P3HT:PCBM, a miscibility of P3HT and PCBM of up to 20% was reported.16,33−36 The existence of the mixed phase has also been demonstrated for PTB7, 19 PTB7-th, 25 and PBDTTPD.15,37 The small molecule p-DTS(FBTTh2)2 has been reported to have a mixed phase at least for some deposition conditions,20 which means the existence of a mixed phase does not appear to be unique to polymers, but it is likely that the strong tendency of the donor to aggregate has an influence on the miscibility of donor and fullerene. It is clear that the existence of a mixed phase does not necessarily decrease device performance, since many systems that have a mixed phase reach an external quantum efficiency (EQE) of 0.7 or higher,15,38−41 and PBDTTPD reaches an internal quantum efficiency of near unity.42 However, it is at present not clear whether a mixed phase is actually a requirement for good device performance. On the other hand, it is clear that an interconnected, pure phase of both donor and acceptor is required, in order to transport free charges to their respective electrodes efficiently.7,15,29,37 3066


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PCBM was purchased from Nano-C and used as received. GIWAXS Sample Preparation. Silicon substrates were purchased from Silicon Quest Int’l (item 908-006) and cut to ∼1 × 1 cm. Organic layers were spin-cast onto the substrate in a nitrogen-filled glovebox from a 20 mg/mL total solids concentration in chloroform, at a spin rate of 2000 rpm. The substrates were annealed for 10 min at 100 °C inside the glovebox. The edges of the samples were removed to eliminate edge effects in the GIWAXS experiment. The vol % of PCBM in the layer was converted from the mass blend ratio using a density of 1.1 g/cm3 for X2 and 1.5 g/cm3 for PCBM. Thickness Measurements. The thickness of the layers was determined by analysis of the Kiessig fringes as measured in X-ray reflectivity measurements, measured on a PANalytical X’Pert diffractometer. The thicknesses obtained are summarized in Figure S3 in the Supporting Information. GIWAXS Measurements. GIWAXS was measured at the Stanford Synchrotron Radiation Lightsouce (SSRL) beamline 11-3 in a heliumfilled chamber with an X-ray wavelength of 0.9752 Å and sample to detector distance of 30 cm at an incident angle of 0.20°. The incident angle is well above the critical angle of the thin film as well as the silicon substrate, to reduce intensity fluctuation due to slight errors in the exact incidence angle around the critical angle (∼0.11°), as well as to eliminate reflection effects of the incident beam on the silicon substrate. The vertical slit size was 0.05 mm, resulting in a beam size that is larger than the sample, to ensure quantitative comparisons are valid. Spectra were recorded on a Rayonix MX225 X-ray detector and processed using the Nika45 software package for Wavemetrics Igor, in combination with WAXStools, our custom written Igor script. Further computational details and fitting methods are provided in the Supporting Information. Mobility Measurements. Hole mobilities for the various X2:PCBM blends were obtained by fabrication of single carrier diodes, adopting the architecture ITO/MoOx/active layer/Au. MoOx was deposited through a shadow mask by thermal evaporation at a rate of 0.1 Å s−1 under vacuum below 10−6 Torr, yielding a 9 nm thick film. Active layers were deposited as described above. Au top contacts (50 nm) were deposited through a shadow mask by thermal evaporation at a rate of 0.2 Å s−1 under vacuum below 10−6 Torr. Electrical characterization was carried out on a Keithley 2602 system source unit. The mobility was determined by fitting the current−voltage curve in the space-charge limited current regime using the Mott−Gurney law, while accounting for built-in voltage, series resistance, and field activation.46

possible that the exciton diffusion length is longer in highly ordered materials such as X2, allowing for larger domain size while still maintaining high exciton splitting, compared to more disordered materials. Besides exciton diffusion, charge transport is an important parameter for high device performance, which remains higher when more pure phase is present, as demonstrated by high device performance and the mobility, summarized in Table S2. The fact that there is no mixed phase thus has several possible consequences regarding exciton diffusion and charge transport, which should be considered in future detailed analyses of X2:PCBM, as well as other OPV systems that lack a mixed phase. Domain Purity. Because X2:PCBM films are solution cast, it is likely that there is a low concentration of PC61BM molecules present in the X2 domains. Bartelt et al.15 have reported an annealing study of PBDTTPD:PC61BM, where, upon annealing, PC61BM was demonstrated to migrate from a mixed phase to pure PC61BM domains, leaving isolated PC61BM molecules in the polymer-rich domains. This resulted in trapping of electrons on these isolated PC61BM molecules, leading to a loss in photocurrent. The high external quantum efficiency (EQE) of X2:PC61BM (0.7)43 demonstrates that the low concentration of isolated PC61BM molecules does not inhibit the performance to a significant extent. Donor:Acceptor Interface. The fact that the amount of mixing is negligible suggests that the interface between the donor and the acceptor must be fairly sharp compared to most other reported systems, although we cannot rule out slight mixing at the interface. Additionally, we speculate that the amorphous X2 is located at the interface with PCBM, while the aggregated X2 is more likely to be inside the pure X2 domains. If X2 has a slightly larger band gap in the amorphous phase, compared to the aggregated phase, this might pose a barrier for excitons to reach the interface with PCBM. It is currently unclear to what extent this effect influences the device performance, but due to the high reported EQE of X2:PCBM OPV devices, this barrier is not detrimental for device performance. It would be interesting to investigate this in future research.

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CONCLUSION To conclude, we have demonstrated that, in contrast to most polymer:fullerene systems, there is negligible mixing in the small molecule X2:PCBM system. Instead, all PCBM in the active layer is in an aggregated state, most of the X2 is in an aggregated state, and only a limited amount of X2 is in an amorphous, nonordered state. We speculate that this is more common to small-molecule OPV systems, since small molecules have a stronger tendency to aggregate than their polymeric counterparts. More work is required to assess the mixed phase in other small molecule blends. Further, we have found that, in efficient X2:PCBM devices, a minimum amount of aggregated material is present, namely, 30 vol % for both X2 and PCBM, which seems to be a requirement for decent hole and electron transport to their respective electrodes; a similar effect has been observed for different systems.25,34 Finally, this study demonstrates that a mixed phase is not necessarily a requirement for good device performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00067. Additional fitting details, GIWAXS analysis methods, mobility and layer thickness data, and the experimental details of the large-scale synthesis of X2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(M.F.T.) E-mail: [email protected]. ORCID

Stefan D. Oosterhout: 0000-0001-9648-5206 Seth R. Marder: 0000-0001-6921-2536 Guillermo C. Bazan: 0000-0002-2537-0310 Notes

The authors declare no competing financial interest.

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EXPERIMENTAL SECTION

ACKNOWLEDGMENTS Work was supported by the Department of the Navy, Office of Naval Research, Award No. N00014-14-1-0580 and Grant No.

Materials. X2 was synthesized according to a literature procedure,44 which has been modified to allow for large-scale synthesis. The details are provided in the Supporting Information. 3067


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N00014-14-1-0711. 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.

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