Microsc. Microanal. 20, 1507–1513, 2014 doi:10.1017/S1431927614001615
© MICROSCOPY SOCIETY OF AMERICA 2014
Visualization of Hierarchical Nanodomains in Polymer/Fullerene Bulk Heterojunction Solar Cells Jianguo Wen,1,* Dean J. Miller,1,* Wei Chen,2,5 Tao Xu,3 Luping Yu,3 Seth B. Darling,4,5 and Nestor J. Zaluzec1 1 Argonne National Laboratory, Electron Microscopy Center, Nanoscience and Technology Division, 9700 South Cass Avenue, Argonne, IL 60439, USA 2 Argonne National Laboratory, Materials Science Division, 9700 South Cass Avenue, Argonne, IL 60439, USA 3 Department of Chemistry, The James Franck Institute, The University of Chicago, 929 E 57th Street, Chicago , IL 60637, USA 4 Argonne National Laboratory, Center for Nanoscale Materials, Nanoscience and Technology Division, 9700 South Cass Avenue, Argonne, IL 60439, USA 5 Institute for Molecular Engineering, The University of Chicago, 5747 South Ellis Avenue, Chicago, IL 60637, USA
Abstract: Traditional electron microscopy techniques such as bright-field imaging provide poor contrast for organic films and identification of structures in amorphous material can be problematic, particularly in high-performance organic solar cells. By combining energy-filtered corrected transmission electron microscopy, together with electron energy loss and X-ray energy-dispersive hyperspectral imaging, we have imaged PTB7/PC61BM blended polymer optical photovoltaic films, and were able to identify domains ranging in size from several hundred nanometers to several nanometers in extent. This work verifies that microstructural domains exist in bulk heterojunctions in PTB7/PC61BM polymeric solar cells at multiple length scales and expands our understanding of optimal device performance providing insight for the design of even higher performance cells. Key words: organic photovoltaics, energy-filtered transmission electron microscopy, chromatic aberration correction, electron energy-loss spectroscopy, X-ray energy-dispersive spectroscopy
I NTRODUCTION Polymer/fullerene bulk heterojunction (BHJ) solar cells represent one of the most promising photovoltaic technologies owing to their compatibility with high-throughput processing (Lee et al., 2008; Pingree et al., 2009; Brabec et al., 2010; Green et al., 2011; He & Yu, 2011; Li et al., 2012; Darling et al., 2013). BHJ solar cells that blend PTB7, a semiconducting copolymer (C41H53FO4S4)n, with PC61BM fullerenes (C72H14O2) are among the best performers with respect to power conversion efficiency (~8%) (Yu et al., 1995; Liang et al., 2010; Green et al., 2011; He et al., 2011; Chen et al., 2012). Understanding the morphology and microstructure of these polymer/fullerene blends and their correlation with photovoltaic mechanisms is of critical importance in improving the performance of these materials (Hoppe & Sariciftci, 2006; Giridharagopal & Ginger, 2010; Brabec et al., 2011; Brady et al., 2011; Pearson et al., 2011; Ruderer & Müller-Buschbaum, 2011; DeLongchamp et al., 2012; McNeill, 2012; Pfannmöller et al., 2012; Schindler et al., 2012). Using wide- and small-angle X-ray scattering, Chen et al. (2011) reported a hierarchical morphology in PTB7/fullerene BHJ solar cells, which they deduced mainly from reciprocal space characterization. Such hierarchical morphologies spanning multiple length scales are believed to Received August 22, 2013; accepted May 28, 2014 *Corresponding author. [email protected]; [email protected]
be a key factor determining the performance of these—and likely most—polymer/fullerene organic photovoltaic (OPV) devices, and challenge our assumption about what constitutes an “ideal morphology” for OPVs. Given that this complex multiphase system is sensitive to processing, chemistry, and local environment, structural assemblies at a broad range of length scales must be simultaneously controlled to impart optimal optoelectronic performance; to do so requires a simplified and qualitative morphological description as well as an appropriate methodology to readily observe and measure their parameters. In this work, we report on the imaging of hierarchical domains ranging in size from several hundred nanometers to several nanometers for PTB7/PC61BM BHJs using a combination of analytical methodologies using both conventional and chromatic aberration (Cc)-corrected energy-filtered transmission electron microscopes (EFTEM). Progress established in the course of our morphological characterization serves as the foundation for further improving the efficiency of polymer solar cells to both realize their largescale commercial use and establish a direct space (imaging) methodology to confirm the resultant morphologies.
MATERIALS
AND
METHODS
PTB7 was synthesized as described by Li et al. (2012). Blended film samples were spin-coated from a PTB7/
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PC61BM 1:1 stock solution with a concentration of 10 mg/mL for both the PTB7 copolymer and PC61BM on a poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/ PSS) modified Si substrate under the same conditions as those used for fabrication of functional OPV devices. The films were then transferred to Cu grids or holey carbon grids by floating on the surface of water and then dissolving the PEDOT/PSS layer. Holey carbon grids were used to provide mechanical strength when films could not form a continuous foil on a regular Cu grid. TEM, EFTEM, and electron energy-loss spectroscopy (EELS) measurements were performed on the Argonne National Laboratory prototype TEAM instrument having a Cs and Cc image corrector developed in partnership with FEI (Hillsboro, OR, USA) and CEOS GmbH (Heidelberg, Germany), hereafter referred to as ACAT (Rose & Wan, 2005; Kabius et al., 2009; Miller et al., 2011). EFTEM and EELS on ACAT were conducted using a Gatan Tridiem Image Filter system (Gatan, Pleasanton, CA, USA). Both 80 and 200 kV accelerating voltages were used to study these thin films, with no visible difference or indication of beam damage observed at either voltage when appropriately controlled electron doses were employed. X-ray energydispersive spectroscopy (XEDS) hyperspectral imaging and corresponding scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) and hyperspectral X-ray data were collected on a Tecnai F20 S/TEM (FEI, USA) operated at 200 kV and equipped with an EDAX Phoenix SiLi System (EDAX Inc., Mahwah, NJ, USA). Position and time stamped data acquizition facilitated post analysis elimination of spectral data recorded beyond damage thresholds. In both instruments, high dose irradiation produced visible damage to the BHJ films, as is typically observed for most polymeric compounds in the TEM environment. Data under these conditions were not used in the results presented herein.
RESULTS
AND
DISCUSSION
The BHJ films blending PTB7 with PC61BM studied in this research had a film thickness comparable to that used in working OPV devices, namely about 80 nm as measured using X-ray reflectivity. Bright-field TEM imaging at an infocus condition showed virtually no contrast across the BHJ film owing to the weak phase contrast. With large defocus values (−20 to − 50 μm), bright-field TEM images gave rise to contrast features having dimensions of ~100–300 nm, however, such large values of defocus resulted in delocalization in TEM images with contrast oscillations that obscured details of actual microstructure. Figure 1a presents a representative bright-field TEM image (at defocus of − 20 μm) of the PTB7/PC61BM-BHJ polymer while Figure 1b shows the same area imaged in-focus in an EFTEM zero-loss (elastic) image. Although these TEM images show coarse features that are similar in dimension, the detailed structures of the same area differ. This can be appreciated by using a fortuitous reference point (a dark impurity/contamination spot) as a marker as is present in Figure 1. Not surprisingly, one can see that the light and dark features in these two images are generally uncorrelated. While moderately defocused bright-field TEM imaging is occasionally a simple way to visualize discrete structures, the contrast produced by large defocussing for the materials studied in this work obscures any microstructural information. For the case of the in-focus elastic EFTEM image (Fig. 1b), a potential interpretation can be put forward that the observed intensity variation results from variations in local film thickness, density, or combinations thereof. An atomic force microscopy examination of BHJ films indicated that the root mean square roughness was 1.87 nm over a scan area of 2 × 2 μm. This thickness variation computes to be ∼2.5%, while by comparison, the percentage of intensity variation in Figure 1b is about 30%. On this basis, we conclude that the
Figure 1. a: Bright-field transmission electron microscopic (TEM) image (defocus of − 20 μm) of PTB7/PC61BM bulk heterojunction (BHJ) film, (b) energy-filtered zero-loss (elastic) TEM image (slit width = 10 eV, defocus ~ 0 μm) of the same area. Neither image provides definitive microstructural information about the BHJ film.
Hierarchical Nanodomains in Organic Solar Cells
Figure 2. Scanning transmission electron microscopy/highangle annular dark-field image of PC61BM:PTB7 = 1:1 bulk heterojunction film, revealing broad diffuse domains of variable size, shape, and direction. Note: outlier pixels owing to detector noise spikes have been normalized to neighboring pixels in this image.
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contrast in Figure 1b arises mainly from the mass density variations rather than thickness variations. The denser (dark) domains in this elastic EFTEM image can be attributed either to a higher proportion of PC61BM or to a higher concentration of PTB7 crystallites with respect to amorphous PTB7. In order to determine which of these two interpretations is correct it became necessary to invoke complementary measurements using both STEM and hyperspectral imaging. In contrast to the generally nebulous structures of the BHJ film shown in Figure 1, obtained using TEM imaging, STEM/HAADF images of the PTB7/PC61BM-BHJ film (Fig. 2) illustrate more directly the existence of broad, diffuse domains that vary in size, shape, and direction. In order to discern the details of these domains we applied multiple spectroscopy imaging methods to elucidate the nature of these domains. To begin, we note that previous EELS studies have shown that the individual components of generic OPV films, i.e., polymers and fullerenes, have different low-loss peaks (hereafter for simplicity referred to as plasmon peaks) (Egerton, 1996; Pines, 1999) and in an OPV formulated using a different polymer, poly
Figure 3. a: Low-loss electron energy-loss spectra of a series of bulk heterojunction (BHJ) films with different blend ratios of polymer to fullerene, illustrating the variation in low-loss (plasmon) energies with blend ratio. b–d: Energyfiltered transmission electron microscopic plasmon loss images of the BHJ of PTB7:PC61BM = 1:1 illustrating diffuse domain in three length scale regimes: (b) ~200–300 nm at energy loss of 20 eV, (c) ~50–100 nm at 25 eV, and (d) ~5–10 nm at 30 eV. All images recorded in ACAT with an imaging slit width of 2 eV at 200 keV, all images were recorded under conditions to minimize electron beam damage (Wen et al., 2012).
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(3-hexyl thiophene) (P3HT) has a plasmon peak lying approximately halfway between two loss maxima of the polymer and the fullerene material, respectively, as reported (Herzing et al., 2010; Pfannmöller et al., 2011). Figure 3a presents EELS data from the low-loss region for five compositions of thin BHJ films studied in this work: pure PTB7, PTB7:PC61BM (ratio of 4:1), PTB7:PC61BM (1:1), PTB7: PC61BM (1:4), and pure PC61BM (Wen et al., 2012). Pure PTB7 polymer and pure fullerene were found to have plasmon peak positions at 21.5 ± 0.1 and 25.0 ± 0.1 eV. The plasmon peak energy values determined for the PTB7: PC61BM-BHJ polymer used here differs from the earlier work on P3HT-BHJ polymer by 0.8 ± 0.1 eV, a readily measurable difference in ACAT. Similar to the results reported for other polymer blends, with increasing fullerene content, the plasmon peak maximum position for the blended PTB7:PC61BM compounds shifts to higher energies when transitioning from the pure polymer (PTB7) toward pure fullerene (PC61BM). The plasmon maximum energy for our BHJ with PTB7:PC61BM = 1:1, lies approximately halfway between at 23 ± 0.1 eV. EFTEM imaging in ACAT using these plasmon losses emerged as an effective method to investigate the distribution of PTB7 and PC61BM domains in this BHJ film, principally because PTB7 and PC61BM have sufficiently distinct plasmon peak positions. To realize such images, we
utilized the imaging of ACAT to first obtain an in-focus image (Gaussian focus) at zero loss (elastic scattering) and then changed incident electron energy without loss of image focus (Haider et al., 2009; Wen et al., 2013) to the desired loss value to collect the EFTEM plasmon loss images. Three energy losses (δE): 20, 25, and 30 eV were chosen for EFTEM imaging, the results for which are presented in Figures 3b–3d. With increasing energy loss from 20 to 30 eV, the EFTEM images show increasing detailed microstructure. The size of the broad diffuse domains revealed in EFTEM images were ~200–300 nm (δE = 20 eV), which transition to regions of ~50–100 nm (δE = 25 eV), and finally to a narrow distribution of ~5–10 nm (δE = 30 eV). In earlier studies of a different polymer, P3HT/BHJ, EFTEM plasmon loss images showed a contrast inversion between 20 and 28 eV. In that work, regions that appear bright at 20 eV became dark at 28 eV (Herzing et al., 2010; Pfannmöller et al., 2011), from which the authors concluded that their normalized bright domains correspond to P3HT in the 20 eV image and to PC61BM in the 28 eV image. Our observations for PTB7/ PC61BM-BHJ do not show this contrast reversal, and cannot be attributed to any contrast normalization processes used in previous studies. Instead they can be interpreted as indicating the existence of hierarchical morphologies as deduced by Chen et al. (2011), mainly from their reciprocal space/ diffraction work.
Figure 4. Hyperspectral imaging illustrating the variation in S and C distributions in PTB7:PC61BM: (a) reverse contrast zero-loss energy-filtered transmission electron microscopic (EFTEM) image [for comparison with the scanning transmission electron microscopy/high-angle annular dark field (HAADF) (d)] with slit width of 10 eV, (b) EFTEM elemental mapping of sulfur (L shell) and (c) carbon (K shell) in the same regions of interest (ROI). d: STEM/HAADF image and X-ray energydispersive spectroscopic (XEDS) elemental images of (e) sulfur (K shell), and (f) carbon (K shell) in the same ROI. Note: the EFTEM and XEDS imaging in this figure were obtained from the same specimen, but recorded using different instruments and for different specimen ROI’s. All elemental images are background corrected.
Hierarchical Nanodomains in Organic Solar Cells
Figure 5. The sulfur and carbon variation for bulk heterojunction film of PTB7:PC61BM obtained from (a) X-ray energy-dispersive spectroscopy (XEDS) hyperspectral line scan (spectra extracted) showing the correlation of C/S ratios with domain structure. b: XEDS summed spectra (normalized at S-K peak) from the “bright” regions, “dark” regions. c: Background subtracted electron energy-loss spectra from the “bright” and “dark” regions show different ratio between S and C. As noted in Figure 4, different regions of interest, instruments, and areas were employed.
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To further confirm the presence of domains, we used hyperspectral X-ray and electron-loss imaging methods to visualize the distribution of PTB7 and fullerene in the PTB7: PC61BM = 1:1 BHJ films. Based on the fact that only the PTB7 copolymer contains sulfur, we first employed energyfiltered elemental mapping at the core-loss edges of sulfur (L2,3 edge at 165 eV) and carbon (K edge at 284 eV) to investigate the elemental distributions in the BHJ film. Background-corrected EFTEM mapping of sulfur and carbon using the standard three-window method (Edgerton, 1996; Williams & Carter, 1996) is shown in Figures 4b and 4c, respectively, while Figure 4a shows the corresponding reversed contrast zero-loss EFTEM image (for comparison with STEM/HAADF images). At 80 nm thick it would not be surprising if no single region of the film was single phase, and ideally in the perfect OPV both the polymer and fullerene compounds would be distributed everywhere. The EFTEM maps quite clearly show the existence of regions that are speciated with concentrated zones of higher sulfur content. It is also apparent that the distribution of sulfur and carbon are positively correlated, i.e., the strongest signals for both sulfur and carbon occur in the same places, although sulfur regions are generally dimensionally smaller than the carbon regions. STEM/HAADF imaging and XEDS elemental mapping from hyperspectral data cubes were used to further investigate this speciated distribution. Figure 4d presents a STEM/ HAADF image for a different, but nominally similar, area of the same PTB7/PC61BM-BHJ film using an inner collection of 115 mrad. As in Figure 2, Figure 4d reveals larger diffuse domains several hundred nanometers in size, while Figures 4e and 4f are the corresponding background subtracted XEDS hyperspectral images for S and C, respectively, and mirror the EFTEM mapping results. Our inelastic scattering images, HAADF imaging, and sulfur- and carbon-mapping by EFTEM and XEDS are all positively correlated, i.e., the strongest scattering occurs in the same regions for each image. As only the polymer contains sulfur, it is reasonable to conclude that each of these approaches directly indicates the location of the sulfur-containing polymer phase. The distribution of the fullerene is, however, more challenging to directly measure, since the PTB7 polymer also contains carbon. Fortunately, the ratio between carbon and sulfur can be used to probe the regions richer in fullerene. Figure 5a shows the relative ratio between carbon- and sulfur-integrated XEDS peak intensity at different locations in the film and a clear variation in the C/S ratio is readily observed. To improve statistical sampling, individual spectra for a series of bright and dark regions (as indicated in the inset) were summed from the hyperspectral data cube and the resulting spectra normalized at the sulfur K X-ray peak. The composite XED spectra are presented in Figure 5b, again showing the correlation of higher C/S ratio in dark regions of the film. Finally, EELS core-loss spectral imaging was also carried out in these areas to further verify the relative ratio between carbon and sulfur. Figure 5c compares background subtracted EELS spectra from dark and light regions mirroring the XEDS results, i.e., both XEDS and
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Figure 6. Energy-filtered transmission electron microscopic images at 30 eV with a slit width of 2 eV for samples (a) pure polymer, (b) PTB7:PC61BM = 4:1, (c) PTB7:PC61BM = 2:1, (d) PTB7:PC61BM = 1:1, (e) PTB7:PC61BM = 1:4, and (f) pure PC61BM illustrating the variation in domain formation with fullerene content. Note: random debris from the specimen preparation process appears as dark ~10–20 nm rectilinear features in (a).
EELS indicate that the dark regions are relatively richer in carbon, hence indicative of a higher proportion of fullerene. The hierarchical morphology and domain size in our OPV/BHJ was also strongly influenced by the blending ratio of the polymer to fullerene. We verified this by preparing films of different polymer to fullerene ratios and then studying the formation domains as a function of the fullerene content using the EFTEM plasmon imaging discussed earlier. Figure 6 presents an overview of this by comparing EFTEM plasmon images taken at δE = 30 eV for specimens of (a) pure polymer, (b) PTB7:PC61BM = 4:1, (c) PTB7: PC61BM = 2:1, (d) PTB7:PC61BM = 1:1, (e) PTB7: PC61BM = 1:4, and the pure fullerene (f) PC61BM. In this figure one can see that as an increasing amount of fullerene is incorporated into the blended composite, spatially dispersed domains begin to form (Figs. 6b–6e), the size of which decreases with increasing fullerene content reaching the 5–10 nm dimension at PTB7:PC61BM = 1:4.
CONCLUSIONS We have obtained evidence for hierarchical morphologies in high-performance PTB7/PC61BM solar cells using a suite of analytical electron optical methods (EFTEM, STEM/HAADF, XEDS, and EELS). This hierarchical domain structure was
previously inferred from X-ray/synchrotron scattering results and consists of diffuse domains of broad length scales ranging from several hundred nanometers to several nanometers, each appearing to have dimensions related to their PTB7 and fullerene content. This correlates with the general understanding of improved performance of an OPV solar cell. These results differ from that obtained for a P3HT/BHJ solar cell, which is both compositionally different and also of poorer energy conversion efficiency, which is not surprising given the difference in efficiencies of the two different formulations. In addition, EFTEM plasmon loss imaging combined with XEDS hyperspectral imaging was used successfully to study the morphology of OPVs at multiple length scales, and may prove to be a useful method to observe key structural features critical to optimal performance of organic material blends, thus providing new insight for the design of even higher performance polymeric solar cells.
ACKNOWLEDGMENTS Use of the Electron Microscopy Center, the Advanced Photon Source (APS), and the Center for Nanoscale Materials at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. W. Chen gratefully
Hierarchical Nanodomains in Organic Solar Cells
acknowledges financial support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award No. KC020301. L. Yu and T. Xu acknowledge support from NSF, NSF-MRSEC, AFOSR, and DOE on the synthesis of polymers. This work was also supported by a University of Chicago-Argonne Strategic Collaborative Initiative Seed Grant, the University of Chicago and the Department of Energy under Department of Energy Contract No. DE-AC02-06CH11357 awarded to UChicago Argonne, LLC, the operator of Argonne National Laboratory.
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Visualization of Hierarchical Nanodomains in Polymer/Fullerene Bulk Heterojunction Solar Cells Jianguo Wen,1,* Dean J. Miller,1,* Wei Chen,2,5 Tao Xu,3 Luping Yu,3 Seth B. Darling,4,5 and Nestor J. Zaluzec1 1 Argonne National Laboratory, Electron Microscopy Center, Nanoscience and Technology Division, 9700 South Cass Avenue, Argonne, IL 60439, USA 2 Argonne National Laboratory, Materials Science Division, 9700 South Cass Avenue, Argonne, IL 60439, USA 3 Department of Chemistry, The James Franck Institute, The University of Chicago, 929 E 57th Street, Chicago , IL 60637, USA 4 Argonne National Laboratory, Center for Nanoscale Materials, Nanoscience and Technology Division, 9700 South Cass Avenue, Argonne, IL 60439, USA 5 Institute for Molecular Engineering, The University of Chicago, 5747 South Ellis Avenue, Chicago, IL 60637, USA
Abstract: Traditional electron microscopy techniques such as bright-field imaging provide poor contrast for organic films and identification of structures in amorphous material can be problematic, particularly in high-performance organic solar cells. By combining energy-filtered corrected transmission electron microscopy, together with electron energy loss and X-ray energy-dispersive hyperspectral imaging, we have imaged PTB7/PC61BM blended polymer optical photovoltaic films, and were able to identify domains ranging in size from several hundred nanometers to several nanometers in extent. This work verifies that microstructural domains exist in bulk heterojunctions in PTB7/PC61BM polymeric solar cells at multiple length scales and expands our understanding of optimal device performance providing insight for the design of even higher performance cells. Key words: organic photovoltaics, energy-filtered transmission electron microscopy, chromatic aberration correction, electron energy-loss spectroscopy, X-ray energy-dispersive spectroscopy
I NTRODUCTION Polymer/fullerene bulk heterojunction (BHJ) solar cells represent one of the most promising photovoltaic technologies owing to their compatibility with high-throughput processing (Lee et al., 2008; Pingree et al., 2009; Brabec et al., 2010; Green et al., 2011; He & Yu, 2011; Li et al., 2012; Darling et al., 2013). BHJ solar cells that blend PTB7, a semiconducting copolymer (C41H53FO4S4)n, with PC61BM fullerenes (C72H14O2) are among the best performers with respect to power conversion efficiency (~8%) (Yu et al., 1995; Liang et al., 2010; Green et al., 2011; He et al., 2011; Chen et al., 2012). Understanding the morphology and microstructure of these polymer/fullerene blends and their correlation with photovoltaic mechanisms is of critical importance in improving the performance of these materials (Hoppe & Sariciftci, 2006; Giridharagopal & Ginger, 2010; Brabec et al., 2011; Brady et al., 2011; Pearson et al., 2011; Ruderer & Müller-Buschbaum, 2011; DeLongchamp et al., 2012; McNeill, 2012; Pfannmöller et al., 2012; Schindler et al., 2012). Using wide- and small-angle X-ray scattering, Chen et al. (2011) reported a hierarchical morphology in PTB7/fullerene BHJ solar cells, which they deduced mainly from reciprocal space characterization. Such hierarchical morphologies spanning multiple length scales are believed to Received August 22, 2013; accepted May 28, 2014 *Corresponding author. [email protected]; [email protected]
be a key factor determining the performance of these—and likely most—polymer/fullerene organic photovoltaic (OPV) devices, and challenge our assumption about what constitutes an “ideal morphology” for OPVs. Given that this complex multiphase system is sensitive to processing, chemistry, and local environment, structural assemblies at a broad range of length scales must be simultaneously controlled to impart optimal optoelectronic performance; to do so requires a simplified and qualitative morphological description as well as an appropriate methodology to readily observe and measure their parameters. In this work, we report on the imaging of hierarchical domains ranging in size from several hundred nanometers to several nanometers for PTB7/PC61BM BHJs using a combination of analytical methodologies using both conventional and chromatic aberration (Cc)-corrected energy-filtered transmission electron microscopes (EFTEM). Progress established in the course of our morphological characterization serves as the foundation for further improving the efficiency of polymer solar cells to both realize their largescale commercial use and establish a direct space (imaging) methodology to confirm the resultant morphologies.
MATERIALS
AND
METHODS
PTB7 was synthesized as described by Li et al. (2012). Blended film samples were spin-coated from a PTB7/
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PC61BM 1:1 stock solution with a concentration of 10 mg/mL for both the PTB7 copolymer and PC61BM on a poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/ PSS) modified Si substrate under the same conditions as those used for fabrication of functional OPV devices. The films were then transferred to Cu grids or holey carbon grids by floating on the surface of water and then dissolving the PEDOT/PSS layer. Holey carbon grids were used to provide mechanical strength when films could not form a continuous foil on a regular Cu grid. TEM, EFTEM, and electron energy-loss spectroscopy (EELS) measurements were performed on the Argonne National Laboratory prototype TEAM instrument having a Cs and Cc image corrector developed in partnership with FEI (Hillsboro, OR, USA) and CEOS GmbH (Heidelberg, Germany), hereafter referred to as ACAT (Rose & Wan, 2005; Kabius et al., 2009; Miller et al., 2011). EFTEM and EELS on ACAT were conducted using a Gatan Tridiem Image Filter system (Gatan, Pleasanton, CA, USA). Both 80 and 200 kV accelerating voltages were used to study these thin films, with no visible difference or indication of beam damage observed at either voltage when appropriately controlled electron doses were employed. X-ray energydispersive spectroscopy (XEDS) hyperspectral imaging and corresponding scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) and hyperspectral X-ray data were collected on a Tecnai F20 S/TEM (FEI, USA) operated at 200 kV and equipped with an EDAX Phoenix SiLi System (EDAX Inc., Mahwah, NJ, USA). Position and time stamped data acquizition facilitated post analysis elimination of spectral data recorded beyond damage thresholds. In both instruments, high dose irradiation produced visible damage to the BHJ films, as is typically observed for most polymeric compounds in the TEM environment. Data under these conditions were not used in the results presented herein.
RESULTS
AND
DISCUSSION
The BHJ films blending PTB7 with PC61BM studied in this research had a film thickness comparable to that used in working OPV devices, namely about 80 nm as measured using X-ray reflectivity. Bright-field TEM imaging at an infocus condition showed virtually no contrast across the BHJ film owing to the weak phase contrast. With large defocus values (−20 to − 50 μm), bright-field TEM images gave rise to contrast features having dimensions of ~100–300 nm, however, such large values of defocus resulted in delocalization in TEM images with contrast oscillations that obscured details of actual microstructure. Figure 1a presents a representative bright-field TEM image (at defocus of − 20 μm) of the PTB7/PC61BM-BHJ polymer while Figure 1b shows the same area imaged in-focus in an EFTEM zero-loss (elastic) image. Although these TEM images show coarse features that are similar in dimension, the detailed structures of the same area differ. This can be appreciated by using a fortuitous reference point (a dark impurity/contamination spot) as a marker as is present in Figure 1. Not surprisingly, one can see that the light and dark features in these two images are generally uncorrelated. While moderately defocused bright-field TEM imaging is occasionally a simple way to visualize discrete structures, the contrast produced by large defocussing for the materials studied in this work obscures any microstructural information. For the case of the in-focus elastic EFTEM image (Fig. 1b), a potential interpretation can be put forward that the observed intensity variation results from variations in local film thickness, density, or combinations thereof. An atomic force microscopy examination of BHJ films indicated that the root mean square roughness was 1.87 nm over a scan area of 2 × 2 μm. This thickness variation computes to be ∼2.5%, while by comparison, the percentage of intensity variation in Figure 1b is about 30%. On this basis, we conclude that the
Figure 1. a: Bright-field transmission electron microscopic (TEM) image (defocus of − 20 μm) of PTB7/PC61BM bulk heterojunction (BHJ) film, (b) energy-filtered zero-loss (elastic) TEM image (slit width = 10 eV, defocus ~ 0 μm) of the same area. Neither image provides definitive microstructural information about the BHJ film.
Hierarchical Nanodomains in Organic Solar Cells
Figure 2. Scanning transmission electron microscopy/highangle annular dark-field image of PC61BM:PTB7 = 1:1 bulk heterojunction film, revealing broad diffuse domains of variable size, shape, and direction. Note: outlier pixels owing to detector noise spikes have been normalized to neighboring pixels in this image.
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contrast in Figure 1b arises mainly from the mass density variations rather than thickness variations. The denser (dark) domains in this elastic EFTEM image can be attributed either to a higher proportion of PC61BM or to a higher concentration of PTB7 crystallites with respect to amorphous PTB7. In order to determine which of these two interpretations is correct it became necessary to invoke complementary measurements using both STEM and hyperspectral imaging. In contrast to the generally nebulous structures of the BHJ film shown in Figure 1, obtained using TEM imaging, STEM/HAADF images of the PTB7/PC61BM-BHJ film (Fig. 2) illustrate more directly the existence of broad, diffuse domains that vary in size, shape, and direction. In order to discern the details of these domains we applied multiple spectroscopy imaging methods to elucidate the nature of these domains. To begin, we note that previous EELS studies have shown that the individual components of generic OPV films, i.e., polymers and fullerenes, have different low-loss peaks (hereafter for simplicity referred to as plasmon peaks) (Egerton, 1996; Pines, 1999) and in an OPV formulated using a different polymer, poly
Figure 3. a: Low-loss electron energy-loss spectra of a series of bulk heterojunction (BHJ) films with different blend ratios of polymer to fullerene, illustrating the variation in low-loss (plasmon) energies with blend ratio. b–d: Energyfiltered transmission electron microscopic plasmon loss images of the BHJ of PTB7:PC61BM = 1:1 illustrating diffuse domain in three length scale regimes: (b) ~200–300 nm at energy loss of 20 eV, (c) ~50–100 nm at 25 eV, and (d) ~5–10 nm at 30 eV. All images recorded in ACAT with an imaging slit width of 2 eV at 200 keV, all images were recorded under conditions to minimize electron beam damage (Wen et al., 2012).
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(3-hexyl thiophene) (P3HT) has a plasmon peak lying approximately halfway between two loss maxima of the polymer and the fullerene material, respectively, as reported (Herzing et al., 2010; Pfannmöller et al., 2011). Figure 3a presents EELS data from the low-loss region for five compositions of thin BHJ films studied in this work: pure PTB7, PTB7:PC61BM (ratio of 4:1), PTB7:PC61BM (1:1), PTB7: PC61BM (1:4), and pure PC61BM (Wen et al., 2012). Pure PTB7 polymer and pure fullerene were found to have plasmon peak positions at 21.5 ± 0.1 and 25.0 ± 0.1 eV. The plasmon peak energy values determined for the PTB7: PC61BM-BHJ polymer used here differs from the earlier work on P3HT-BHJ polymer by 0.8 ± 0.1 eV, a readily measurable difference in ACAT. Similar to the results reported for other polymer blends, with increasing fullerene content, the plasmon peak maximum position for the blended PTB7:PC61BM compounds shifts to higher energies when transitioning from the pure polymer (PTB7) toward pure fullerene (PC61BM). The plasmon maximum energy for our BHJ with PTB7:PC61BM = 1:1, lies approximately halfway between at 23 ± 0.1 eV. EFTEM imaging in ACAT using these plasmon losses emerged as an effective method to investigate the distribution of PTB7 and PC61BM domains in this BHJ film, principally because PTB7 and PC61BM have sufficiently distinct plasmon peak positions. To realize such images, we
utilized the imaging of ACAT to first obtain an in-focus image (Gaussian focus) at zero loss (elastic scattering) and then changed incident electron energy without loss of image focus (Haider et al., 2009; Wen et al., 2013) to the desired loss value to collect the EFTEM plasmon loss images. Three energy losses (δE): 20, 25, and 30 eV were chosen for EFTEM imaging, the results for which are presented in Figures 3b–3d. With increasing energy loss from 20 to 30 eV, the EFTEM images show increasing detailed microstructure. The size of the broad diffuse domains revealed in EFTEM images were ~200–300 nm (δE = 20 eV), which transition to regions of ~50–100 nm (δE = 25 eV), and finally to a narrow distribution of ~5–10 nm (δE = 30 eV). In earlier studies of a different polymer, P3HT/BHJ, EFTEM plasmon loss images showed a contrast inversion between 20 and 28 eV. In that work, regions that appear bright at 20 eV became dark at 28 eV (Herzing et al., 2010; Pfannmöller et al., 2011), from which the authors concluded that their normalized bright domains correspond to P3HT in the 20 eV image and to PC61BM in the 28 eV image. Our observations for PTB7/ PC61BM-BHJ do not show this contrast reversal, and cannot be attributed to any contrast normalization processes used in previous studies. Instead they can be interpreted as indicating the existence of hierarchical morphologies as deduced by Chen et al. (2011), mainly from their reciprocal space/ diffraction work.
Figure 4. Hyperspectral imaging illustrating the variation in S and C distributions in PTB7:PC61BM: (a) reverse contrast zero-loss energy-filtered transmission electron microscopic (EFTEM) image [for comparison with the scanning transmission electron microscopy/high-angle annular dark field (HAADF) (d)] with slit width of 10 eV, (b) EFTEM elemental mapping of sulfur (L shell) and (c) carbon (K shell) in the same regions of interest (ROI). d: STEM/HAADF image and X-ray energydispersive spectroscopic (XEDS) elemental images of (e) sulfur (K shell), and (f) carbon (K shell) in the same ROI. Note: the EFTEM and XEDS imaging in this figure were obtained from the same specimen, but recorded using different instruments and for different specimen ROI’s. All elemental images are background corrected.
Hierarchical Nanodomains in Organic Solar Cells
Figure 5. The sulfur and carbon variation for bulk heterojunction film of PTB7:PC61BM obtained from (a) X-ray energy-dispersive spectroscopy (XEDS) hyperspectral line scan (spectra extracted) showing the correlation of C/S ratios with domain structure. b: XEDS summed spectra (normalized at S-K peak) from the “bright” regions, “dark” regions. c: Background subtracted electron energy-loss spectra from the “bright” and “dark” regions show different ratio between S and C. As noted in Figure 4, different regions of interest, instruments, and areas were employed.
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To further confirm the presence of domains, we used hyperspectral X-ray and electron-loss imaging methods to visualize the distribution of PTB7 and fullerene in the PTB7: PC61BM = 1:1 BHJ films. Based on the fact that only the PTB7 copolymer contains sulfur, we first employed energyfiltered elemental mapping at the core-loss edges of sulfur (L2,3 edge at 165 eV) and carbon (K edge at 284 eV) to investigate the elemental distributions in the BHJ film. Background-corrected EFTEM mapping of sulfur and carbon using the standard three-window method (Edgerton, 1996; Williams & Carter, 1996) is shown in Figures 4b and 4c, respectively, while Figure 4a shows the corresponding reversed contrast zero-loss EFTEM image (for comparison with STEM/HAADF images). At 80 nm thick it would not be surprising if no single region of the film was single phase, and ideally in the perfect OPV both the polymer and fullerene compounds would be distributed everywhere. The EFTEM maps quite clearly show the existence of regions that are speciated with concentrated zones of higher sulfur content. It is also apparent that the distribution of sulfur and carbon are positively correlated, i.e., the strongest signals for both sulfur and carbon occur in the same places, although sulfur regions are generally dimensionally smaller than the carbon regions. STEM/HAADF imaging and XEDS elemental mapping from hyperspectral data cubes were used to further investigate this speciated distribution. Figure 4d presents a STEM/ HAADF image for a different, but nominally similar, area of the same PTB7/PC61BM-BHJ film using an inner collection of 115 mrad. As in Figure 2, Figure 4d reveals larger diffuse domains several hundred nanometers in size, while Figures 4e and 4f are the corresponding background subtracted XEDS hyperspectral images for S and C, respectively, and mirror the EFTEM mapping results. Our inelastic scattering images, HAADF imaging, and sulfur- and carbon-mapping by EFTEM and XEDS are all positively correlated, i.e., the strongest scattering occurs in the same regions for each image. As only the polymer contains sulfur, it is reasonable to conclude that each of these approaches directly indicates the location of the sulfur-containing polymer phase. The distribution of the fullerene is, however, more challenging to directly measure, since the PTB7 polymer also contains carbon. Fortunately, the ratio between carbon and sulfur can be used to probe the regions richer in fullerene. Figure 5a shows the relative ratio between carbon- and sulfur-integrated XEDS peak intensity at different locations in the film and a clear variation in the C/S ratio is readily observed. To improve statistical sampling, individual spectra for a series of bright and dark regions (as indicated in the inset) were summed from the hyperspectral data cube and the resulting spectra normalized at the sulfur K X-ray peak. The composite XED spectra are presented in Figure 5b, again showing the correlation of higher C/S ratio in dark regions of the film. Finally, EELS core-loss spectral imaging was also carried out in these areas to further verify the relative ratio between carbon and sulfur. Figure 5c compares background subtracted EELS spectra from dark and light regions mirroring the XEDS results, i.e., both XEDS and
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Figure 6. Energy-filtered transmission electron microscopic images at 30 eV with a slit width of 2 eV for samples (a) pure polymer, (b) PTB7:PC61BM = 4:1, (c) PTB7:PC61BM = 2:1, (d) PTB7:PC61BM = 1:1, (e) PTB7:PC61BM = 1:4, and (f) pure PC61BM illustrating the variation in domain formation with fullerene content. Note: random debris from the specimen preparation process appears as dark ~10–20 nm rectilinear features in (a).
EELS indicate that the dark regions are relatively richer in carbon, hence indicative of a higher proportion of fullerene. The hierarchical morphology and domain size in our OPV/BHJ was also strongly influenced by the blending ratio of the polymer to fullerene. We verified this by preparing films of different polymer to fullerene ratios and then studying the formation domains as a function of the fullerene content using the EFTEM plasmon imaging discussed earlier. Figure 6 presents an overview of this by comparing EFTEM plasmon images taken at δE = 30 eV for specimens of (a) pure polymer, (b) PTB7:PC61BM = 4:1, (c) PTB7: PC61BM = 2:1, (d) PTB7:PC61BM = 1:1, (e) PTB7: PC61BM = 1:4, and the pure fullerene (f) PC61BM. In this figure one can see that as an increasing amount of fullerene is incorporated into the blended composite, spatially dispersed domains begin to form (Figs. 6b–6e), the size of which decreases with increasing fullerene content reaching the 5–10 nm dimension at PTB7:PC61BM = 1:4.
CONCLUSIONS We have obtained evidence for hierarchical morphologies in high-performance PTB7/PC61BM solar cells using a suite of analytical electron optical methods (EFTEM, STEM/HAADF, XEDS, and EELS). This hierarchical domain structure was
previously inferred from X-ray/synchrotron scattering results and consists of diffuse domains of broad length scales ranging from several hundred nanometers to several nanometers, each appearing to have dimensions related to their PTB7 and fullerene content. This correlates with the general understanding of improved performance of an OPV solar cell. These results differ from that obtained for a P3HT/BHJ solar cell, which is both compositionally different and also of poorer energy conversion efficiency, which is not surprising given the difference in efficiencies of the two different formulations. In addition, EFTEM plasmon loss imaging combined with XEDS hyperspectral imaging was used successfully to study the morphology of OPVs at multiple length scales, and may prove to be a useful method to observe key structural features critical to optimal performance of organic material blends, thus providing new insight for the design of even higher performance polymeric solar cells.
ACKNOWLEDGMENTS Use of the Electron Microscopy Center, the Advanced Photon Source (APS), and the Center for Nanoscale Materials at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. W. Chen gratefully
Hierarchical Nanodomains in Organic Solar Cells
acknowledges financial support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award No. KC020301. L. Yu and T. Xu acknowledge support from NSF, NSF-MRSEC, AFOSR, and DOE on the synthesis of polymers. This work was also supported by a University of Chicago-Argonne Strategic Collaborative Initiative Seed Grant, the University of Chicago and the Department of Energy under Department of Energy Contract No. DE-AC02-06CH11357 awarded to UChicago Argonne, LLC, the operator of Argonne National Laboratory.
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