Characterizing Morphology in Bulk Heterojunction Organic

PERSPECTIVE pubs.acs.org/JPCL

Characterizing Morphology in Bulk Heterojunction Organic Photovoltaic Systems Rajiv Giridharagopal and David S. Ginger* Department of Chemistry, University of Washington, Seattle, Washington 98195

ABSTRACT Organic semiconductors are an alternative to inorganic materials in solar cell applications. While the efficiencies of organic photovoltaics (OPVs) have been improving rapidly, they are currently below that required for widespread power generation. OPV performance is sensitive to the nanoscale texture, or film morphology, in the photovoltaic active layer, particularly in bulk heterojunction (BHJ) devices, and characterizing morphology across many length scales is currently a major experimental challenge. Here, we discuss several different experimental approaches for characterizing morphology in BHJ systems. These include techniques ranging from X-ray diffraction and spectroscopy to electron microscopy and scanning probe microscopy. These methods provide complementary information to guide future materials design and device optimization efforts. rganic photovoltaic (OPV) systems offer several potential advantages over inorganic devices. In particular, the promise of low-cost, highly scalable solution processing has made OPVs a source of intense research. Efficiencies of champion lab cells have increased rapidly in recent years. At the time of this writing, the NREL-certified (small cell) efficiency record for OPVs was 7.9%,1 and the most recent published efficiency record is 7.4% from the same group.2 However, improvements are still needed; integrated module efficiencies in the 8-10% range with lifetimes of roughly seven years have been cited as a target needed for organics to compete for commercial power generation.3 Efforts to develop new materials for use in lab champion cells as well as efforts to effectively translate these lab-scale efficiencies to large-scale module production must confront the challenge of characterizing and optimizing organic film morphology.

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electron-donor and -acceptor are most commonly a conjugated polymer and a fullerene derivative, respectively. The beststudied polymer donor materials are probably poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV), though new derivatives and polymer families are constantly being explored with more promising results.2,4-6 The most common fullerene derivatives include (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and (6,6)-phenyl-C71-butyric acid, the latter becoming prevalent due to its larger absorption cross section.4,7 When coated onto a substrate, the polymer and fullerene form a blended layer with a large internal interfacial area that is critical to OPV performance. The importance of this internal interface arises from the mechanism of OPV operation, which has been reviewed elsewhere.8,9 The energy diagram in Figure 1b summarizes the general operating principle. The absorption of a photon by an organic semiconductor results in the formation of an exciton (a neutral quasiparticle consisting of a bound electron-hole pair). The exciton diffusion length (the distance an exciton can diffuse before relaxing) is ∼10 nm in most semiconducting polymers.10 Thus, it is critical that a photogenerated exciton reach the donor/acceptor interface, where it can be dissociated into charges, during its brief lifetime. On the other hand, the active layer in an OPV device needs to be ∼100-200 nm thick, much thicker than the exciton diffusion length, in order to absorb most of the incident light. It is this length scale mismatch that has given rise to the study of bulk-heterojunction blends.3,11 If such a blend is to work as a solar cell, it must contain efficient pathways for charge transport to the electrodes in addition to its large internal surface area. The balance between maximizing the interfacial area and transport in this nanoscale

Efforts to develop new materials for use in lab champion cells as well as efforts to effectively translate these lab-scale efficiencies to large-scale module production must confront the challenge of characterizing and optimizing organic film morphology. At present, the OPV devices with the highest performance are based on the bulk heterojunction (BHJ) structure shown in Figure 1a, in which two different organic semiconductors, an electron-donor material, and an electron-acceptor material are blended to form an active photovoltaic layer. The

r 2010 American Chemical Society

Received Date: January 26, 2010 Accepted Date: February 23, 2010 Published on Web Date: March 23, 2010

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DOI: 10.1021/jz100100p |J. Phys. Chem. Lett. 2010, 1, 1160–1169


these effects with device performance requires detailed analysis of how the underlying film morphology is altered by these different processing variables and, in turn, how these changes in morphology affect the electronic properties of the device. In this Perspective, we highlight a selection of the different experimental techniques that are currently being used to investigate BHJ film morphology in model systems and point out some of the lessons that have been learned. These methods range from scanning probe microscopy approaches that combine topographic detail with local optoelectronic information to X-ray diffraction studies that provide details of crystallographic alignment within individual domains. X-ray Techniques: More than Just Scattering. X-ray-based probes are being used in BHJ studies to probe everything from local crystallographic properties using grazing incidence X-ray diffraction (GIXRD)19 to average domain sizes via small/wideangle X-ray scattering (SAXS/WAXS),20,21 maps of lateral composition with scanning transmission X-ray spectromicroscopy (STXM),22 and surface composition with both X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS).23 Techniques like GIXRD can determine crystallinity properties as well as packing distances near the BHJ surface. GIXRD uses X-rays at incidence angles below the critical angle; thus, only the evanescent component (which decays rapidly and thus is sensitive to the topmost 10-100 nm, depending on experimental parameters such as angle and intensity) is observed. The model polymer P3HT can order with either the (010) or (100) axis normal to the substrate. These two orientations are known as the face-on and edge-on conformations because the π-π stacking direction is perpendicular and parallel to the substrate, respectively.24 This packing is shown in Figure 1c. The P3HT stacking orientation can vary with the solvent,25,26 regioregularity,24 or the substrate surface energy.27,28 While edge-on stacking is desirable for a transistor geometry,24 the face-on orientation is likely preferable in an OPV architecture where charge transport normal to the electrodes is needed. Indeed, such face-on orientations have been observed using newer thiophene polymers in recent high-efficiency OPV devices.2 Regardless of the P3HT orientation, we know from studies of P3HT transistors that efficient hole transport benefits from increased crystallinity24,29 and from a limited presence of grain boundaries.30,31 In P3HT/PCBM films, GIXRD data have been used to underscore the complex influence that PCBM plays in P3HT crystallization and thus in device optimization. In unannealed as-cast films, PCBM molecules might act as defect sites that inhibit P3HT crystallization during the initial film formation, thus limiting performance.32,33 For example, annealing in P3HT/PCBM films is known to more than triple their power conversion efficiency from ∼1.1 to 3.6%, primarily due to enhanced crystallinity.34 GIXRD data have shown that the edge-on crystallization improves with annealing33,35 and with relatively high boiling point solvents such as dichlorobenzene.36 Grazing incidence SAXS on BHJ films showed similar results, with the radius of gyration in PCBM crystallites increasing from 15 to 23 nm along with the P3HT edge-on stacked crystallites increasing from 9.6 to 18 nm after annealing,

Figure 1. Schematics of (a) a typical BHJ structure, (b) the energy diagram illustrating the general operating principle, and (c) the edge-on versus face-on orientation of P3HT.

network creates an inherent design tension when optimizing blend structure, especially since many basic electronic properties such as carrier mobility and recombination rates are also sensitive to the film morphology.12

Organic photovoltaic performance is sensitive to the nanoscale texture, or film morphology, in the photovoltaic active layer, particularly in bulk heterojunction devices. Numerous factors can affect the morphology of an OPV film, including polymer regioregularity,13 choice of solvent,14 thermal annealing,15 solvent-vapor annealing,16 and the inclusion of additives during processing.17,18 Correlating


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linity from 21 to 12% by using P3HT of slightly lower regioregularity reduces the number of detrimental grain boundaries (Figure 2d). GIXRD data thus show that optimizing BHJ morphology can require a balance between competing factors. They were able to show similar efficiencies of ∼3.8-3.9% in both low and high regioregularity polymer devices despite the difference in P3HT crystallinity. The efficiency of the high regioregularity-based system decreased to nearly 2.0% with continued annealing, unlike the lower regioregularity case, implying that the defect propagation increases with annealing time and therefore showing that the understanding of morphological factors could be used to improve device stability as well as performance. GIXRD is typically used to probe micrometer-size or larger areas depending on the beam dimensions and does not generally provide high-resolution spatial information or chemical specificity. A surface-sensitive technique that can fill these gaps and provide chemical information is NEXAFS. NEXAFS involves the detection of emitted electrons following excitation of the sample with variable-energy polarized X-rays, typically near the carbon K-edge for studies on organic semiconductors. NEXAFS measurements are sensitive to secondary events such as Auger electrons and inelastically scattered photoelectrons. Also, because NEXAFS is polarization-sensitive, it can more readily distinguish variations in molecular orientation and local structure than XPS; however, NEXAFS does require high brightness beams afforded by synchrotron sources. NEXAFS studies on P3HT films have shown that the crystal orientation is not vertically uniform26 and that the orientation at the substrate interface can vary with spin coating rate38 and solvent.26 In P3HT/PCBM films, recent NEXAFS work has indicated that substrate surface energy plays a strong role in determining the chemical composition of the buried interface,23 with the relative interfacial PCBM concentration being higher on high-surfaceenergy substrates (e.g., bare SiO2) relative to that on lowsurface-energy substrates that have been coated with octadecyltrichlorosilane (OTS). The different NEXAFS spectra for these cases are shown in Figure 3a,b. However, in that study, annealing the BHJ film deposited on bare SiO2 substantially increased the interfacial P3HT concentration, with PCBM aggregating into micrometer-size crystals. Because it is known that P3HT alignment is sensitive to surface energy,28,29,39,40 the PCBM aggregation may also be sensitive to crystallization effects explored in GIXRD data. Regardless, the NEXAFS data indicate that substrate engineering to change the surface energy is a viable means to control the interfacial as well as lateral41,42 composition. Further studies designed to probe the molecular orientations at the buried interface in working devices would be insightful. Another technique made possible by high brightness, tightly focused synchrotron X-ray beams is STXM, which integrates raster scanning while monitoring X-ray absorption at constant beam energy, thus yielding spatially resolved Xray information. Varying the beam energy allows for chemically specific maps of the surface and surface-specific NEXAFS spectra. STXM has been used in several reports to analyze different polymer/fullerene22,43 and all-polymer44 organic photovoltaic systems. Recently, Watts et al. used STXM to

Figure 2. GIXRD data of a P3HT/PCBM film (a) before and (b) after thermal annealing, showing enhanced crystallization in both edge-on and face-on directions. (c) GIXRD data and schematic of how high P3HT crystallinity can result in an increased grain boundary density by varying regioregularity from (c) 86 to (d) 96%. Relevant diffraction peaks are labeled. Images (a) and (b) are taken from ref 13, adapted with permission from Macmillan Publishers Ltd., copyright 2006. Images (c) and (d) are taken from ref 19, adapted with permission from American Chemical Society, copyright 2008.

depending on the temperature.20 During the thermal annealing process, P3HT crystallizes, and the PCBM diffuses to form larger aggregates. After annealing, the P3HT phase is largely pure and contains virtually no PCBM, in contrast to MDMO-PPV where PCBM molecules can intercalate between the alkyl side chains to form a more intimately mixed system.37 PCBM intercalation in thiophene-based polymers has been shown via GIXRD studies by measurement of the lamellar spacing peaks in edge-on crystal regions,37 and it was also shown that intercalation can significantly affect the efficiency and absorption in a polymer/fullerene system.6 From these studies, it has been shown that a 1:1 polymer/fullerene ratio can produce drastically different efficiencies, depending on whether the system exhibits intercalation or not, and therefore, such effects are important to consider in optimizing devices. Furthermore, annealing in P3HT blends produces a broadened distribution of edge-on orientations,13 as shown in Figure 2a and b for unannealed and annealed films. While P3HT crystallization is important in BHJ films,35 Woo et al. recently postulated that overcrystallization of P3HT can result in detrimental grain boundary formation in P3HT/PCBM films.19 In P3HT/PCBM films, the higher crystallinity and tighter packing in highly regioregular P3HT forces greater PCBM phase segregation. The resulting PCBM crystallites, indicated by the strong GIXRD diffraction peaks in Figure 2c, are believed to propagate defects to increase grain boundary density throughout the P3HT film.19 Woo et al. further suggest that decreasing the degree of P3HT crystal-


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troscopic evidence of PCBM crystallization around existing PCBM grains and evidence of lower PCBM concentration in areas immediately outside of the crystallites due to diffusion. In fact, as shown Figure 3c-k, the PCBM crystals seem to be bordered by regions almost entirely depleted of PCBM, as has been previously proposed by others.45 This interpretation is consistent with optical microscopy data by Campoy-Quiles et al. showing that P3HT crystallization occurs, followed by PCBM diffusion through the lower-density areas between crystals both laterally and vertically. In their films, P3HT crystallization completed before PCBM diffusion, and the initial performance improvement at short annealing times from 1.1 to 3.6% efficiency was attributed to both P3HT crystallization and PCBM diffusion, while they attributed the improvements of subsequent annealing from 3.6 to 3.9% primarily to PCBM diffusion.34 Presumably, voids due to PCBM diffusion are filled in by P3HT, thus implying a certain level of mobility in the P3HTas well. The STXM results provide a visual explanation for why annealing increases polymer crystallinity in GIXRD data, as well as how overannealing can eventually decrease device performance due to significant PCBM aggregation. Efforts to control PCBM diffusion, for example, by blend composition or crystallinity, are critical to further improve BHJ efficiencies. Electron Tomography. Electron microscopy techniques provide valuable mapping data for analyzing OPV systems. Scanning electron microscopy,11 transmission electron microscopy (TEM),46,47 and, more recently, electron holography48 have provided useful insight into the structure of OPV films. Particularly with TEM-based approaches, it is possible to obtain spatially resolved information on how processing steps such as thermal annealing affect film morphology.49,50 Consistent with the GIXRD and STXM reports, basic TEM imaging reveals increased crystallization and segregation upon annealing (Figure 4a,b).51 Electron tomography has been used in recent work to provide three-dimensional images of the OPV system.50,52-55 In this context, “tomography” means a computationally derived three-dimensional reconstruction using two-dimensional TEM projections taken across a range of angles. The work of van Bavel et al. used electron tomography to determine how annealing methods affect the three-dimensional composition in a P3HT/PCBM film.52 Slices from the threedimensional reconstructed BHJ film are shown in Figure 4c. Their data reveal that annealing results in enhanced P3HT crystallization, as expected from GIXRD data; device efficiencies improved from 1.9 to 3.5%, which may, in part, be due to the P3HT crystallinity increasing from 30 to 40% upon annealing. The tomography data show that this crystallization extends vertically through the film, resulting in three-dimensional P3HT crystalline structures. Additionally, tomography data can be used to show that the P3HT composition varies considerably from the substrate interface to the top surface, with high concentration of crystalline P3HT at the bottom substrate. In contrast, when analyzing thicker, 200 nm films, the composition is fairly even throughout, shown in the slices in Figure 4c and the plot in Figure 4d. From these data, they concluded that the kinetics of film formation can play a significant role in vertical composition (in addition to any

Figure 3. NEXAFS spectra on (a) pristine P3HT and PCBM films and on (b) the buried P3HT/PCBM interface on SiO2 and OTS substrates, with evidence of a surface energy-dependent change in composition ratio. Peaks corresponding to spectral features in P3HTor PCBM are identified in (a). (c-k) STXM images of a P3HT/ PCBM film at three different annealing times, (c-e) 10 s, (f-h) 4 min, and (i-k) 40 min, in terms of (left) P3HT thickness in nm, (middle) PCBM thickness in nm, and (right) PCBM composition by volume percentage. The green areas are thick PCBM crystals whose composition could not be accurately calculated. During annealing, PCBM diffuses into the crystal from the surrounding area, while the P3HT composition remains largely unchanged. Images (a) and (b) are taken from ref 23, adapted with permission from the American Physical Society, copyright 2009. Images (c)-(k) are taken from ref 22, adapted with permission from the American Chemical Society, copyright 2009.

analyze PCBM diffusion in a P3HT/PCBM blend during the thermal annealing process.22 The STXM data provided spec-


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molecular weight and regioregularity. Many groups are using surface modifiers to guide phase separation41,42 and improve the performance and stability of various BHJ devices,56,57 and more experiments are needed before we will be certain of the microscopic origins of the electronic and morphological effects resulting from such modifications. Scanning Probe Microscopy. As powerful as they are, both X-ray techniques and electron microscopy methods lack the ability to correlate local structure with local performance. Scanning probe microscopy (SPM) can fill this gap.12,58 Atomic force microscopy (AFM) is the most commonly used SPM technique for analyzing these films and provides topographic images with nanometer-scale resolution. Aside from topography, AFM can be extended to measure electrical and optical data with high resolution. Electrically sensitive AFM methods that have been used to investigate OPV materials, including conducting AFM (cAFM),59,60 electrostatic force microscopy (EFM),61 and scanning Kelvin probe microscopy (SKPM).62,63

The techniques outlined here provide valuable, complementary information about the effects of processing on the BHJ film morphology, with each painting a different section of the overall picture. An optical variant of AFM, near-field scanning optical microscopy (NSOM), has been used to study BHJ blends for some time,64,65 with recent work focusing on local Raman and photoluminescence measurements via NSOM.66 Klimov et al. used NSOM on P3HT/PCBM to provide optical evidence of PCBM depletion around crystallites in the film, which has since been confirmed through other techniques such as the STXM data above.45 Additionally, McNeill et al. used NSOM to probe the local photocurrent generation of an allpolyfluorene copolymer photovoltaic blend and, surprisingly, concluded that the bulk of the photocurrent did not originate from the interfaces of the visible domains64 ; a result that is also consistent with time-resolved electrostatic force microscopy (trEFM) results67 and can be explained in terms of the local composition gradients measured by STXM.44 Although this result is likely the exception rather than the rule in OPVs, given that the photocurrent seems to arise from the polymer/fullerene interface in fullerene blends,68 it underscores the important role that imaging techniques can play in understanding device operation. Further adapting AFM to record optoelectronic information in BHJ films will be critical in order to correlate electrical data with device performance. For example, cAFM-based tunneling luminescence has been used to study the change in exciton relaxation in BHJ films upon thermal annealing.69 In this approach, the luminescence intensity is directly related

Figure 4. TEM images of a P3HT/PCBM (a) before and (b) after thermal annealing, with increasing crystallization causing heightened contrast in the images. (c) Reconstructed 3D tomography image and slices from the tomography image, showing the crystal structure near the bottom interface and near the the vacuum interface of the P3HT/PCBM film, indicating three-dimensional crystallization of the P3HT. (d) Crystalline P3HT composition percentage as a function of distance from the substrate interface (thickness), with the curves corresponding to different thickness films. Images are taken from ref 52, adapted with permission of the American Chemical Society, copyright 2009.

effects due to surface energies). The difference in vertical composition profiles may partially explain why they found higher efficiency in the 100 nm film than in the 200 nm film (3.5 versus 1.9%, respectively). They also found the same efficiency difference between unannealed and annealed films, 1.9 and 3.5%, which may be due to the increase in overall P3HT crystallinity from 19 to 55%.52 It should be noted, however, that TEM tomography is limited to differentiating between PCBM and crystalline P3HT, rather than simply PCBM and all P3HT, due to contrast limitations in noncrystalline P3HT. Given the strong effects of surface chemistry observed in the NEXAFS data, it appears that the control of vertical morphology may require balancing of kinetic and thermodynamic effects, and one might expect these factors to depend on annealing conditions as well as


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Figure 5. (a-c) SKPM images of an MDMO-PPV/PCBM film on ITO/PEDOT/PSS taken (a) before illumination, (b) under white light illumination (photovoltage measurement). and (c) after illumination. (d-f) pcAFM images of a P3HT/PCBM film at annealing times of (d) 0 s, (e) 1 min, and (f) 10 min. Photocurrent histograms are shown beneath each image. Annealing-induced crystallization allows for a higher average and maximum photocurrent with annealing, but increased heterogeneity also occurs, as indicated by the greater standard deviation in photocurrent. Images (a)-(c) are taken from ref 63, adapted with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2009. Images (d)-(f) are taken from ref 73, adapted with permission from the American Chemical Society, copyright 2009.

to poor exciton dissociation at the donor/acceptor interface. Additional data can be acquired through techniques based on SKPM or EFM. One SKPM-based technique is to measure the surface photovoltage by monitoring the change in the contact potential under illumination in all-polymer70 or polymera et al. measured the surface fullerene systems.71 Maturov photovoltage on MDMO-PPV/PCBM blends (Figure 5a-c) and found evidence of electron accumulation in both the polymerrich and fullerene phases. Combining their data with a drift diffusion device model, they concluded that lateral electron transport can be a limiting process in some OPV systems. Furthermore, under this assumption, they are able to account for the shape of device current/voltage curves without invoking a field-dependent geminate recombination rate,63,72 a result which will certainly be tested given the dominant role many researchers believe geminate recombination plays in OPVs. Our group has developed a time-resolved EFM (trEFM) technique for studying the charging rate behavior in OPV blends.67 This technique provides access to time-dependent data on 100 μs scales not measurable by steady-state EFM or SKPM experiments and can thus be used to study timedependent properties such as charge generation, trapping, and detrapping in OPV materials. Such results have also helped provide an electrical context for STXM-derived composition maps analyzed by McNeill et al. in recent work.44 Extending the frequency bandwidth of trEFM would allow the application of this method to a wider range of BHJ blends, as well as to studying the kinetics of fast local transport, trapping, and detrapping processes near specific morphological features. Several groups are now using conductive AFM (cAFM) in the dark or under illumination (photoconductive AFM,


pcAFM). It is possible to use pcAFM to record photocurrents under zero bias and thus to locally resolve short-circuit photocurrent generation/collection in a BHJ film. In pcAFM, our group uses a conducting AFM tip as the top contact in a solar cell and measures the generated photocurrent at each pixel due to laser illumination that has been focused onto a diffraction-limited spot on the sample and coaligned with the tip. Initially, we correlated photocurrent generation and film morphology in a blended MDMO-PPV/PCBM film using pcAFM to gain insight into charge generation in relation to possible subsurface variations and found that photocurrent is predominant at the polymer/fullerene boundaries in the film and can vary significantly from one fullerene crystallite to another.68 More recently, we reported photocurrent evolution and local variation in blended P3HT/PCBM films due to thermal annealing.73 Figure 5d-f shows a few representative pcAFM images as well as the associated photocurrent histograms taken on films annealed for different lengths of time. By analyzing the spatial distribution in short-circuit photocurrent maps, we observed that the maximum, average, and standard deviation of the photocurrent all increase with thermal annealing time. Additionally, we made the surprising observation that the dark electron, dark hole, and short-circuit photocurrent are not spatially coincident. The implication is that the underlying three-dimensional morphology plays a critical role in carrier transport and recombination. As such, we believe recent efforts to include 3D morphology into device models will continue to gain in importance.74,75 Understanding the relationship between local film morphology, processing parameters, and device performance is critical if efficiencies are to improve to the point where organic

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photovoltaics can become cost-effective. The techniques outlined here provide valuable, complementary information about the effects of processing on the BHJ film morphology, with each painting a different section of the overall picture. X-ray techniques like GIXRD studies show that thermal annealing increases P3HT crystallinity, with higher crystallinity possibly reducing device performance and thermal stability; STXM data show that the increased P3HT crystallinity can be attributed to PCBM diffusing away from the P3HT to form local crystallites upon annealing. Electron tomography studies show that the resulting P3HT domains extend three-dimensionally through the film but not uniformly as there is a vertical gradient in P3HT concentration. Though NEXAFS data indicate that this vertical gradient is surface-energy-dependent, the tomography data imply that the kinetics underlying film formation can also affect the P3HT ratio at the substrate interface depending on film thickness; in both cases, controlling the P3HT crystallinity at the interfaces may play a role in improving device efficiency by improving charge extraction. Electrical scanning probe studies such as pcAFM show that annealing increases the maximum and average short-circuit photocurrent in blended films while also showing increasing heterogeneity in the photocurrent, explaining why performance can eventually saturate during annealing. Together, these results provide valuable insight into how morphology-related factors, such as polymer crystallinity and orientation, PCBM diffusion, and vertical composition profiles, can be addressed on the way to attaining 8-10% efficiencies in manufactured bulk heterojunction organic photovoltaic solar cells with new materials. Donor/acceptor blends with better coverage of the solar spectrum and energy level offsets tuned to yield higher opencircuit voltages are needed to improve OPV performance, but new materials meeting these electronic requirements will not reach their inherent potential unless their morphology can be optimized. Fortunately, the lessons learned from the studies of P3HT/PCBM blend morphology can be applied to new materials currently under development. New donor and acceptor materials, as well as alternative transparent conducting substrates, may result in different BHJ morphologies. Even in P3HT/PCBM, we need a better understanding of the delicate balance between kinetic and thermodynamic factors leading to optimum lateral and vertical morphologies, especially to prepare large-area films with a minimum of postprocessing that do not exhibit excessive recrystallization or dewetting over time. Nevertheless, given the rapid progress that has been made to date, we are optimistic that these challenges can be met. As a field, we have shown that the crystalline structure and composition of BHJ films can be tailored through both intrinsic properties such as molecular weight and regioregularity of the organic materials and extrinsic factors such as substrate surface energy, film thickness, and processing solvents and additives. The application of a range of structural probes to characterize new BHJ blends should allow us to link performance with both local structure and electronic properties across different families of materials, allowing the field to predict the performance of devices prepared with different materials under different conditions. Such an experimental synthesis would


allow the field to move beyond trial and error and hopefully make organic OPVs economically viable.

The application of a range of structural probes to characterize new BHJ blends should allow us to link performance with both local structure and electronic properties across different families of materials. AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: ginger@ chem.washington.edu.

Biographies Rajiv Giridharagopal received a B.S. with high honors from the University of Texas at Austin in 2004 and M.S. and Ph.D. degrees from Rice University in 2007 and 2010, all in electrical engineering. His graduate work was supported by a NSF graduate research fellowship. Currently, he is a postdoctoral research associate in David S. Ginger's lab. David S. Ginger earned dual B.S. degrees in chemistry and physics from Indiana University in 1997 and a Ph.D. in physics from the University of Cambridge (U.K.) in 2001. After a postdoctoral fellowship at Northwestern University, he joined the University of Washington in 2003, where he is currently an Associate Professor of Chemistry. Further information can be found online at http:// depts.washington.edu/gingerlb/.

ACKNOWLEDGMENT The authors thank Dr. Yeechi Chen for help with illustrations and Andreas Tillack and Obadiah Reid for providing useful feedback. This review was made possible by a broad ongoing program to investigate different aspects of OPV performance supported by the NSF (DMR-0120967 and DMR0449422) (scanning probe microscopy development), AFOSR (equipment), DOE DE-FG02-07ER46467 (hybrid solar cells and imaging of P3HT/PCBM), and DE-SC0001084 (interfacial science), and ONR (morphology and interfacial effects on light trapping). D.S.G. also thanks the Camille Dreyfus Teacher-Scholar Awards program for support.

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