Nanomechanical Imaging of the Diffusion of Fullerene into Conjugated

Aug 16, 2017 - *E-mail: [email protected]., *E-mail: [email protected]., *E-mail: [email protected]. Cite this:ACS Nano 11, 9, 8660-...
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Nanomechanical Imaging of the Diffusion of Fullerene into Conjugated Polymer Dong Wang,*,† Ken Nakajima,‡ Feng Liu,*,§,⊥ Shaowei Shi,† and Thomas P. Russell*,†,§,∥ †

Beijing Advanced Innovation Center for Soft Matter Science and Engineering & State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China ‡ Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan § Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Department of Physics and Astronomy and Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiaotong University, Shanghai 200240, China ∥ Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: The large Young’s modulus difference between chemically modified fullerene (PCBM) and a conjugated polymer was used to nanomechanically map the diffusion of PCBM into PTB7, a high-efficiency low-bandgap conjugated polymer. The sharp tip in nanomechanical atomic force microscopy ensures a high-resolution nanomechanical characterization of the diffusion front, with the intrinsic benefits of revealing the mechanical properties of the mixtures. Localized structure changes induced by diffusion were investigated by grazing incidence X-ray diffraction methods. We found a most unusual diffusion behavior that shows Case II characteristics, where a front of PTB7 saturated with PCBM moves into the pure PTB7 with a linear time dependence. This diffusion is due mainly to a majority fraction of the disordered PTB7 that has continuous paths for PCBM diffusion without obvious energetic barriers, and as diffusion proceeds, the paths for diffusion gets larger, leading to a step in the concentration profile. The donor/ acceptor-dependent diffusion constants may also contribute to the observed Case-II-like diffusion front. KEYWORDS: atomic force microscopy, nanomechanical mapping, organic photovoltaics, diffusion, structure, bilayer

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where the order and state of aggregation of the components can be varied. A large amount of research has been devoted to understanding and optimizing the morphology of BHJ active layer thin films. Details of the morphological framework, composition, and size statistics have been reported and reviewed.4,5 Innovative experiments using high-powered characterization techniques have been used to observe the formation of the morphology in situ.6−11 While X-ray scattering and diffraction can be used to characterize the fibrillar network of the crystalline hole-conducting polymer,10 the formation of an interconnected network within the framework established by the fibrils requires the growth of the crystals in a matrix composed of a mixture of the hole-conducting polymer with an

anipulating the nano- and mesoscale structure of a material is important for tailoring the properties and function of a material.1−4 In most cases, the structure of a material is determined by a complex interplay of multiple kinetics processes, including crystallization, diffusion, and phase separation, which lead, ultimately, to kinetically trapped morphologies.4 This is particularly true for the active layer of organic photovoltaic (OPV) devices. In the active layer of the device, a bicontinuous morphology having domains that are tens of nanometers in size is essential for device performance. Photons are absorbed, driving the holeconducting polymer into an excited state, leading to the generation of excitons, bound electron−hole pairs, that diffuse to the interface with the electron acceptor. The electron and hole are separated at the interface, and the electron and hole diffuse to their respective electrodes to produce a current. Processing conditions can be used to optimize the morphology of the active layer into multilength scale, bicontinuous domains, tens of nanometers in size, of the hole and electron acceptors, © 2017 American Chemical Society

Received: December 17, 2016 Accepted: August 16, 2017 Published: August 16, 2017 8660

DOI: 10.1021/acsnano.6b08456 ACS Nano 2017, 11, 8660−8667

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Figure 1. (a) Scheme of the bilayer sample and QNM-AFM investigation; (b) typical force−distance curve of PCBM and PTB7 in QNM-AFM characterization; (c) Young’s modulus map across the interfacial regions for the 30 s (top) and 8 min (bottom) annealed thick bilayer samples.

show a surprisingly low degree of order in the polymer, far lower in the bulk heterojunction than in pure PTB7.18 The structural characteristics of PTB7 are fundamentally different from P3HT, and, as such, the mechanism of interdiffusion should be significantly different. Quantitative nanomechanical atomic force spectroscopy (QNM-AFM) was used on cross sections of thick films of each material to monitor the interdiffusion of the components. The thick films were used to avoid issues associated with the reflection of the diffusing species at the boundaries of the sample. Grazing incidence Xray diffraction (GIXD) was used to characterize the morphology as PCBM diffused into PTB7, where disruption of ordered regions was observed in the PTB7 matrix. The combined results of AFM and GIXD showed that the bulk diffusion of PCBM into PTB7 is initially rapid and can be described by a Fickian type of diffusion. However, there is an apparent saturation of PCBM in PTB7 of ∼50% such that a diffusion front of this concentration diffuses into the pure layer, where the position of the front varies linearly with time. There is, also, a Fickian precursor that precedes the propagating front, a process reminiscent of a Case II diffusion.

electron acceptor, such as chemically modified fullerene (PCBM). Parameters of critical importance are the interactions between the components, the growth rate of the crystals, and the mechanism and rate of diffusion of the noncrystalline electron acceptor away from the growth front of the crystals. These will define the growth habit of the crystals, the size scale of the crystalline domains, and the nature of the interfibrillar noncrystalline domains. Perhaps the simplest way to measure interdiffusion is to construct a bilayer composed of a thin film of the holeconducting polymer placed on top of a layer of the electron conductor, generally PCBM. By monitoring the concentration profile normal to the surface of the film after the bilayer has been thermally annealed for different periods of time, the concentration profiles normal to the diffusion front can be determined, from which the mechanism of diffusion and rate of diffusion can be determined.12,13 Dynamic secondary ion mass spectroscopy (DSIMS) and neutron reflectivity have been used to measure the interdiffusion of poly(3-hexylthiophene) (P3HT) and PCBM.12−16 The interdiffusion has been shown to be non-Fickian, where a sharp interface was found between the two layers, but the concentration of PCBM ahead of the diffusion front was found to be uniform, increasing with the time allowed for interdiffusion. These results suggest that the diffusion of PCBM into P3HT is very rapid, almost ballistic in nature, where, within very short time periods, the PCBM can diffuse completely through P3HT layers, several tens of micrometers in thickness.12,13 Here, we focus on the understanding the interactions and miscibility of a low-band-gap polymer, poly[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and PCBM. PTB7 is one of the most efficient OPV polymeric materials, which delivers a power conversion efficiency >10%.17 PTB7 shows a much lower crystallinity than P3HT, and the crystallites assume a face-on orientation at the electrode interface. 18 Solution cast thin films of PTB7:PCBM mixtures without the additive diiodooctane (DIO) show large phase-separated domains and an aggregation of PCBM, due to the limited solubility of PCBM in PTB7, while with DIO, they result in a more finely dispersed mixture of the PTB7-rich and PCBM-rich phases.18−20 The fractions of crystalline regions of PTB7 in the blends, whether with or without the additive DIO, however, are the same, but both

RESULTS AND DISCUSSION Atomic force microscopy is one of the most effective techniques to probe the surface morphology of materials with high resolution. In the current study, the peak force (quantitative nanomechanical) mode is used to study the interdiffusion of PTB7 and PCBM. A schematic of the bilayer diffusion experiment is shown in Figure 1a, where the probe scans a cross section of the bilayer film to record the mechanical strength of the film as a function of distance normal to the interface between the polymer and PCBM. When the tip of the AFM cantilever presses against the sample surface, interaction forces increase at a rate proportional to local stiffness, enabling quantitative imaging of the phase composition of soft material blends. Shown in Figure 1b are the force− separation curves of PTB7 and PCBM. The reduced Young’s modulus, E*, can be obtained by fitting the curve using the Derjaguin−Muller−Toropov (DMT) model,21 which takes into account the adhesive force between the tip and the surface. The DMT theory is expressed by the following equation: E* = 8661

3(Ftip − Fadh) 4 Rd3

(1) DOI: 10.1021/acsnano.6b08456 ACS Nano 2017, 11, 8660−8667

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ACS Nano where Ftip is the force on the AFM tip, Fadh is the adhesive force, R is the tip radius, and d is the deformation value. E* is the reduced elastic modulus and is related to the Young’s modulus, Es, by Es = (1 − νs 2)E*

understanding the physical procedure of PTB7 and PCBM mixing. Figure 3a−e shows a series of Young’s modulus profiles across the interfaces, from short to prolonged thermal annealing times. The profiles were obtained by a section that is normal to the interface to minimize the influence of the interface morphology. A low base value from PTB7 and a high value plateau from PCBM are evident. The modulus variance reflects intrinsic heterogeneities or fluctuations in both PTB7 and PCBM. Previous modeling and experimental studies have shown that there are mechanical heterogeneities, several nanometers in size, in polymer glasses arising from fluctuations in the viscoelastic properties or density fluctuations.26 Surface topography can also cause variations in the measured Young’s modulus.22 The interdiffusion is of more interest in the current study, where the change of the Young’s modulus provides sufficient information to assess the change in the concentration across the interface (the modulus profile can be converted to a composition profile based on a linear relationship between the Young’s modulus and material composition; see Figure S1 in the Supporting Information). As shown in Figure 3a−c and corresponding concentration profiles in Figure S2, the modulus/concentration profiles across the interfaces annealed at 150 °C for short periods of time show a smooth gradient that can be fit by a hyperbolic tangent function, indicating a typical Fickian-type diffusion. However, with further annealing a plateau in the concentration develops at a PCBM concentration of ∼50% (Figure 3d,e), clearly deviating from Fickian behavior. The Young’s modulus of this plateau was 6−7 GPa, indicating a PCBM concentration of 44−54%. While PCBM is miscible with PTB7, it is evident from these data that there is a saturation concentration corresponding to the concentration of this interfacial layer. The width of the transition region in Young’s modulus/concentration profiles is seen to increase with time, where the front edge of the plateau represents the diffusion of the PCBM into the PTB7 and the displacement of this front into the PCBM varies linearly with time. Assuming that the interdiffusion at the initial stage is Fickian, as shown in Figure 3a−c, the diffusion coefficient D can be determined from27

(2)

where νs is the Poisson ratio. From eqs 1 and 2 Es can be determined. A fit to Figure 1b gives a Young’s modulus of 11.9 GPa for PCBM and 1.20 GPa for PTB7. The Young’s modulus values of PCBM and PTB7 are consistent with their bulk values reported in our previous work.23 While there have been no other reports on the mechanical properties of PTB7, similar types of polymers, such as P3HT, have a Young’s modulus of 1.3 GPa from buckling measurements, of 1.4 GPa from nanoindentation, and of 2.2 GPa from QNM-AFM.24,25 P3HT is more ordered than PTB7, so a lower Young’s modulus would be expected for PTB7, as observed. The drastic difference in the mechanical properties of PTB7 and PCBM provides good contrast for mechanical imaging, making QNM-AFM a useful tool for studying the interdiffusion. Figure 1c shows the Young’s modulus maps of PTB7 (red) and PCBM (purple) and the interface between the two materials (color variation). After 30 s, the interface between PTB7 and PCBM was narrow and sharp, whereas after 8 min, the interface was diffusive. The Young’s modulus histograms of the above two samples are shown in Figure 2. Short annealed bilayer samples (30 s)

L = 2(Dt )1/2

(3)

where L is the diffusion length and t is the diffusion time. Consequently, D can be determined from the slope of L vs t1/2. From the weighted least-squares fits, constrained to pass through the origin, D = (1.72 ± 0.04) × 10−13 cm2/s is obtained for the PTB7/PCBM system at 150 °C (Figure 4) at early times (below 5 min). After the initial rapid diffusion of the PCBM into the PTB7, an interfacial layer forms that broadens in time, but the concentration of PCBM in this layer remains constant. If all of the data are normalized, so that the initial decrease in the PCBM concentration superposes, as shown in Figure 5, it is seen that even after 2 min of interdiffusion the system is driving toward the formation of this interfacial plateau, where there is a clear Fickian precursor at the leading edge of the plateau, where the shape of this leading edge remains essentially constant. The width of this plateau, essentially the position of the lead edge, is seen to vary linearly with time, as shown in Figure 6. The only exception to this is the data at 30 s, where the plateau has not been fully established. This type of concentration profile is far different than that observed for the diffusion of PCBM into P3HT, a more crystalline hole-conducting polymer. There, a very rapid,

Figure 2. Young’s modulus histogram of 30 s (top) and 8 min (bottom) annealed thick bilayer samples. New features showed up in 8 min annealed bilayer samples, indicating a stabilized mixing state.

showed distinctive mechanical features from PTB7 and PCBM. The statistical distribution of the Young’s modulus can be described by a Gaussian function with a mean value of 1.30 ± 0.12 GPa for PTB7 and 11.7 ± 1.60 GPa for PCBM. When bilayer samples were annealed for a longer time, a new Young’s modulus peak at 6.1 GPa was clearly observed, which is distinctly different from the Young’s modulus peaks of PTB7 and PCBM, indicating that there is a mixed layer at the interface between PTB7 and PCBM with a concentration of ∼50% PCBM. At the front and back of this mixed layer are concentration gradients into the two pure layers that can be described by a hyperbolic tangent function, which is characteristic of a Fickian diffusion. The formation of a mixed interfacial layer indicated a new phase was established during the interdiffusion process, which should be taken into account in 8662

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Figure 3. Young’s modulus profiles across the interface of 30 s (a), 2 min (b), 5 min (c), 8 min (d), and 15 min (e) annealed at 150 °C for thick bilayer samples.

Figure 6. Plot of the front of the plateau form as a function of time.

Figure 4. Diffusion length as a function of the square root of the diffusion time in the initial diffusion stage (below 5 min).

amorphous regions of the polymer. The diffusion of PCBM into the P3HT can be linked to the diffusion of water into a sponge, where the material is swollen but the framework, the P3HT crystals, remains essentially unchanged. From previous studies we know that the diffusion of PCBM into the PTB7 results in a reduction of ordering of the PTB7.19 Consequently, the picture that emerges for the diffusion of the PCBM into the PTB7 is one where there is an initial diffusion of the PCBM into the PTB7 that, over short distances, is Fickian in nature. However, unlike P3HT, the low degree of ordered regions of PTB7 are dispersed in the essentially continuous disordered majority fraction in PTB7 matrix, which has continuous paths for diffusion without obvious energetic barriers. As diffusion proceeds, the paths for diffusion get larger, leading to a step in the concentration profile. Diffusion in the disordered fraction proceeds much more readily and faster than the diffusion in the ordered region. These factors are considered to account for the observed diffusion behavior, in which a diffusion front of this concentration diffuses into the pure where the position of the front varies linearly with time, a process reminiscent of a CaseII-like diffusion. Recent work has shown that, however, vitrification and its donor/acceptor ratio dependence that can be inferred by the cold-crystallization transition in DSC measurements can also

Figure 5. Shifted diffusion of concentration profiles of 150 °C annealed samples.

near ballistic penetration of PCBM into the P3HT is observed, and the diffusion in that case can be described by a diffusion of the PCBM into a network formed by the P3HT crystals, where there is a very rapid penetration of the PCBM into the 8663

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ACS Nano impact the diffusion properties of the system.28,29 As shown in the first heating curves of PCBM:PTB7 in Figure S4, both the temperature and the transition of cold crystallization of PCBM increase as a function of PTB7 concentration, indicating the diffusion constants are related to the state of vitrification in the disordered region of PTB7. Therefore, observation of higher diffusion at higher PCBM concentration is consistent with the observed Case II diffusion front; that is, the PCBM diffusion can catch up with the PCBM precursor as the PCBM increases. It should be noted that the observed diffusion is not the same as Case II diffusion. But it definitely shows Case II characteristics, where a front of PTB7 saturated with PCBM moves into the pure PTB7 with a linear time dependence. Regarding the interface and diffusion in a rubbery−glassy interface, the diffusion and material transport can happen below the glass transition temperature (Tg) of the glassy material, which is very similar to the current case in a thorough study of the microstructure and chain diffusion at a rubbery/glassy polymer interface (PS/PPO) by Lin et al., and forms a frontier profile quite similar to Case II diffusion.30 At the specified diffusion temperature of 150 °C, where PTB7 is rubbery and PCBM is in the glassy state, the interdiffusion requires the plasticization of the glassy PCBM by the neighboring rubbery PTB7. Leman et al. have also shown polymer (poly[N-9′heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)], PCDTBT) Tg is critical to the interdiffusion and material transportation between the polymer and PCBM.31 Here, the diffusion constant was impacted by the fullerene concentration, which was consistent with the current observations. Interdiffusion at different temperatures of bilayer samples was also performed. Samples were annealed for 5 min at different temperatures (from 50 to 150 °C), so that only the initial stages of the interdiffusion were observed. Typical concentration profiles, shown in Figure 7, could be described

Figure 8, over the entire temperature range investigated, the data fall on a straight line with a slope yielding EA = 16.8 ± 1.7

Figure 8. Diffusion kinetics data D vs 1/T plot, to estimate the energy barrier for diffusion.

kcal/mol for the PTB7/PCBM system. This activation energy is very similar to the value of 15.6 kcal/mol, reported by Kramer and co-workers,16 for the diffusion of PCBM into P3HT. Our reported value is within the range of that expected for the diffusion of small molecules into a polymer, 11.9−19.1 kcal/mol,32 and indicates that initial interdiffusion of PCBM and PTB7 is essentially the diffusion of a disordered PCBM into amorphous or disordered PTB7. The structure of PTB7 in the bilayers was measured by GIXD as a function of annealing temperature (Figure 9a). Thin bilayer samples were fabricated to ensure full penetration of PCBM into PTB7. A face-on orientation is observed in the GIXD with lattice parameters similar to that reported previously. The (100) peak in the in-plane direction and (010) peak in the out-of-plane direction are summarized and analyzed (Figure 9b). In lattice spacing, the (100) peak, which is the lamellae spacing between PTB7 π−π stacked thin sheets, showed an increase in the spacing at ∼110 °C, indicating an increase in the local side-chain disorder as the temperature decreased that continued and then leveled off at a spacing of 2 nm as the temperature was decreased further. In total, an increase in the (100) distance of ∼3.7% was observed. This can result from a change in the conformation of the alkyl chains from a cis to gauche conformation with decreasing temperature. Concurrently, the persistence of the lattice size along the (100) direction, as determined from a Scherrer analysis of the (100) reflection, underwent an abrupt decrease from ∼6 nm to ∼4.5 nm, in total a 25% reduction, indicating that the local disordering of local packing of the chains gave rise to a much larger disorder in the longer range packing of the chains, which could be expected. The (010) reflection, on the other hand, went through a slight reduction in the spacing at ∼110 °C, from ∼0.4 nm to 0.39 nm, indicating a tighter stacking of the chains, while the persistence of the lattice along this direction was virtually unchanged. The peak area of both (100) and (010) peaks showed a continuous decrease with increasing temperature. These results show that during thermal annealing the introduction of PCBM into PTB7 leads to a preferential plasticization of the smaller or more disordered PTB7, i.e., those chains with a higher number of gauche conformations, giving rise to an apparent decrease in the (100) d-spacing with increasing temperature. This would be expected since the chains with a larger separation would be able to more easily accommodate and interact with the PTB7 chains, allowing those chains to be more readily dissolved during the

Figure 7. Five-minute diffusion of bilayers under different annealing temperatures.

with a hyperbolic tangent, from which a diffusion coefficient could be determined. Diffusion coefficients of 1.63 × 10−13, 1.48 × 10−13, 7.06 × 10−14, and 1.55 × 10−14 cm2/s at 150, 130, 90, and 50 °C, respectively, were found, with a clear reduction in the diffusion coefficient, as would be expected, as the temperature was decreased. For an activated (Arrhenius) diffusion process, D = D0 exp(−EA/RT), where EA is the activation energy, D0 is the pre-exponential factor, R is the universal gas constant, and T is the diffusion temperature. A value for the apparent activation energy for diffusion, EA, can be obtained by plotting D as a function of 1/T, and, as shown in 8664

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Figure 9. (a) In-plane (IP) and out-of-plane (OOP) GIXD line cut profiles of annealed thin film bilayers; the color scale indicates the annealing temperature. (b) Detailed structure analysis of PTB7 crystals under different processing procedures. PTB7 and PCBM films was measured to be 1.8 ± 0.19 nm and 1.5 ± 0.16 nm for 5 × 5 μm2, respectively. The PSS layer was ∼10 nm and, since it is water-soluble, served as a sacrificial layer to float the PTB7 films onto the water surface, where the film was retrieved with a PCBM-coated substrate forming a bilayer. The bilayer films were then placed under vacuum for 30 min at room temperature to remove trapped water. Subsequently, the bilayers were placed in a vacuum oven at a given temperature to allow the PTB7 and PCBM to interdiffuse. After specified times, the bilayer samples were rapidly removed from the vacuum oven and quenched in liquid nitrogen to arrest the interdiffusion and freeze-in the morphology. The bilayers were then embedded in an epoxy resin at room temperature and ultramicrotomed using a Leica EM FC6 (Leica Microsystems GmbH Wetzlar, Germany) at −80 °C to obtain a flat surface for AFM characterization. The cutting was done normal to the diffusion direction to minimize the introduction of artifacts arising from shearing across and interface between two mechanically different materials. Nanomechanical Mapping Measurement. Nanomechanical mapping was performed in PeakForce QNM (Quantitative NanoMechanics) mode on a Bruker MultiMode AFM at ambient conditions. The oscillation frequency of the Z-piezo was 1.0 kHz, and the peak force amplitude was set at 150 nm. The samples were scanned at peak tapping forces of approximately 50 nN using rectangular silicon cantilevers with a nominal spring constant of 30 N/ m (OMCL-AC160TS-R3, Olympus Micro Cantilevers). The actual spring constant was measured by a thermal tuning method. The tip geometry, before and after imaging, was checked by tapping mode imaging of a tip-check sample (Aurora Nanodevices, Canada) at scan rate of 1 Hz. The Young’s modulus, E, is obtained by fitting the unloading curve using the DMT model.21 A detailed description of the nanomechanical mapping measurement can be found in our previous work.22 Grazing Incidence X-ray Diffraction. GIXD was done at beamline 7.3.3 at Lawrence Berkeley National Lab (LBNL). The sample was put inside a helium chamber, and a Pilatus 1M detector (LBNL) was used to collect the signal. X-ray energy was 10 keV, and incidence angle was 0.18°. The beam center and sample to detector distance were calibrated by using a silver behinet (AgB) standard. Data

interdiffusion. This, in turn, would allow widening the diffusion paths, allowing further interdiffusion of the PCBM into the continuous disordered regions between the crystals and give rise to the observed diffusion behavior that shows Case II characteristics.

CONCLUSIONS In conclusion, we demonstrated a comprehensive analysis of the diffusion of PCBM into PTB7 using QNM-AFM mapping, in which the composition variation at the interface can be visualized with high spatial resolution (∼8 nm). The mechanical properties of both pure regions and the diffuse interlayers were directly measured. We found a most unusual diffusion process that shows Case II characteristics, where a front of PTB7 saturated with PCBM moves into the pure PTB7 with a linear time dependence. This diffusion is due mainly to a majority fraction of the disordered PTB7 that has continuous paths for PCBM diffusion without obvious energetic barriers, and as diffusion proceeds, the paths for diffusion get larger, leading to a step in the concentration profile. The donor/ acceptor-dependent diffusion constants may also contribute to the observed Case-II-like diffusion front. Evidence for the plasticization or disordering was afforded by grazing incidence X-ray diffraction. Experiments performed as a function of the temperature yielded an activation energy that was comparable to that seen for small-molecule diffusion into polymers. MATERIALS AND METHODS Materials and Sample Preparation. PTB7, with a molecular weight Mw of ∼200 kg mol−1 and molecular weight distribution of ∼4, was obtained from 1-Material. PC71BM was obtained from Nano-C Inc. Thick films of PTB7 and PC71BM were prepared by drop-casting chlorobenzene solutions (∼30 mg/mL) onto cleaned or PSS-modified silicon wafers, respectively. The films were then dried in air for 1 day and then under vacuum at room temperature for 7 days to ensure complete removal of solvent. Surface roughness, Rq (rms), of the 8665

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ACS Nano are processed by the Nika software package. The bilayer sample was prepared in a manner similar to those used in QNM-AFM measurements. However, to ensure full penetration of PCBM in PTB7, a thin PTB7 top layer (∼100 nm) was used and the bottom PCBM layer was ∼50 nm. In a typical procedure, the thin layers of PCBM and PTB7 were first spin-coated from chlorobenzene solutions onto a Si wafer, respectively. The films were then dried under vacuum at room temperature for 7 days to ensure complete removal of the solvent. Subsequently, the PTB7 layer was floated onto a deionized water surface and picked up onto the PCBM layer that was already on a Si wafer. The bilayer thin films were dried in a vacuum overnight, and then the same thermal treatment was carried out as used in the thick bilayer interdiffusion experiment.

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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/acsnano.6b08456. Additional information (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Dong Wang: 0000-0003-2326-0852 Ken Nakajima: 0000-0001-7495-0445 Shaowei Shi: 0000-0002-9869-4340 Thomas P. Russell: 0000-0001-6384-5826 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51673016). F.L. and T.P.R. were supported by the U.S. Office of Naval Research under contract N00014-15-1-2244. Portions of this research used resources (Beamlines 7.3.3 and 11.0.1.2) of the Advanced Light Source and Molecular Foundry, which are DOE Office of Science User Facilities. REFERENCES (1) Rivnay, J.; Mannsfeld, S. C. B.; Miller, C. E.; Salleo, A.; Toney, M. F. Quantitative Determination of Organic Semiconductor Microstructure from the Molecular to Device Scale. Chem. Rev. 2012, 112, 5488−5519. (2) DeLongchamp, D. M.; Kline, R. J.; Herzing, A. Nanoscale Structure Measurements for Polymer-Fullerene Photovoltaics. Energy Environ. Sci. 2012, 5, 5980−5993. (3) Chen, W.; Nikiforov, M. P.; Darling, S. B. Morphology Characterization in Organic and Hybrid Solar Cells. Energy Environ. Sci. 2012, 5, 8045−8074. (4) Liu, F.; Gu, Y.; Shen, X.; Ferdous, S.; Wang, H. W.; Russell, T. P. Characterization of the Morphology of Solution-Processed Bulk Heterojunction Organic Photovoltaics. Prog. Polym. Sci. 2013, 38, 1990−2052. (5) Collins, B. A.; Tumbleston, J. R.; Ade, H. Miscibility, Crystallinity, and Phase Development in P3HT/PCBM Solar Cells: Toward an Enlightened Understanding of Device Morphology and Stability. J. Phys. Chem. Lett. 2011, 2, 3135−3145. (6) Schmidt-Hansberg, B.; Sanyal, M.; Klein, M. F. G.; Pfaff, M.; Schnabel, N.; Jaiser, S.; Vorobiev, A.; Müller, E.; Colsmann, A.; Scharfer, P.; Gerthsen, D.; Lemmer, U.; Barrena, E.; Schabel, W. 8666

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