Tuning the Vertical Phase Separation in Polyfluorene:Fullerene Blend

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Tuning the Vertical Phase Separation in Polyfluorene:Fullerene Blend Films by Polymer Functionalization Ana Sofia Anselmo,† Lars Lindgren,‡ Jakub Rysz,§ Andrzej Bernasik,^ Andrzej Budkowski,§ Mats R. Andersson,‡ Krister Svensson,† Jan van Stam,# and Ellen Moons†,* †

Department of Physics and Electrical Engineering, Karlstad University, SE-65188 Karlstad, Sweden, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 G€oteborg, Sweden, § Institute of Physics, Jagiellonian University, 30-059 Krakow, Poland, ^ Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, 30-059 Krakow, Poland, # Department of Chemistry and Biomedical Sciences, Karlstad University, SE-65188 Karlstad, Sweden. ‡

bS Supporting Information ABSTRACT: Achieving control over the nanomorphology of blend films of the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester, PCBM, with light-absorbing conjugated polymers is an important challenge in the development of efficient solutionprocessed photovoltaics. Here, three new polyfluorene copolymers are presented, tailored for enhanced miscibility with the fullerene through the introduction of polymer segments with modified side chains, which enhance the polymer’s polar character. The composition of the spincoated polymer:PCBM films is analyzed with dynamic secondary ion mass spectrometry (dSIMS). The dSIMS depth profiles demonstrate compositional variations perpendicular to the surface plane, as a result of vertical phase separation, directed by the substrate. These variations propagate to a higher degree through the film for the polymers with a larger fraction of modified side chains. The surface composition of the films is studied by Near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Quantitative analysis of the NEXAFS spectra through a linear combination fit with the spectra of the pure components yields the surface composition. The resulting blend ratios reveal polymer-enrichment of the film surface for all three blends, which also becomes stronger as the polar character of the polymer increases. Comparison of the NEXAFS spectra collected with two different sampling depths shows that the vertical composition gradient builds up already in the first nanometers underneath the surface of the films. The results obtained with this new series of polymers shed light on the onset of formation of lamellar structures in thin polymer:PCBM films prepared from highly volatile solvents. KEYWORDS: morphology, polymer
fullerene interaction, solar cell

’ INTRODUCTION Solution processability is one of the most attractive properties of polymer-based photovoltaic devices. The one-step method of film preparation leading to the bulk heterojunction (BHJ) architecture was first reported in 19951 and is, because of its simplicity, the prevalent method for preparing polymer-based electronics. In a BHJ, the interface between the electron-donating and the electron-accepting material is distributed throughout the bulk of the film. The internal nanostructure of the film influences the overall performance of polymer-based optoelectronic devices.2,3 In particular for photovoltaic devices,4
8 the film morphology can affect the absorption of light, the generation of excitons, their separation into charge carriers, and the transport of charges to their respective electrodes, which all contribute to the effect on the overall performance. To image the morphology of films of conjugated polymers blended with the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM), several techniques have been used, as summarized in a few recent papers.4,5,9
11 Compositional variations r 2011 American Chemical Society

in the vertical direction, perpendicular to the film surface, have received particular attention.5,12 Simultaneous to the development of structural models to describe vertical phase separation mechanisms during thin film formation, several high-resolution techniques were developed and applied to monitor these vertical composition gradients. Given the typical ∼100 nm thickness of the layer and the compositional dominance of carbon, most characterization techniques need to rely on indirect properties to be able to distinguish the components and obtain a depth profile of the film. Examples of such indirect properties are density in cross-sectional scanning electron microscopy13 and electron tomography,9,14,15 optical parameters in variable-angle spectroscopic ellipsometry,16 and scattering density differences in neutron reflectometry.17 Other methods can give direct chemical information. Near-edge X-ray absorption fine structure spectroscopy Received: July 31, 2010 Revised: March 12, 2011 Published: April 11, 2011 2295

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Chemistry of Materials (NEXAFS)18,19 probes X-ray induced transitions into the unoccupied states and is highly sensitive to the chemical environment of the element at study, e.g., carbon. Thereby NEXAFS provides direct fingerprints of the film composition. However, the information depth of NEXAFS is limited to the surface region. Another technique, complementary to NEXAFS, which is both element specific and provides a full depth profile, is dynamic secondary ion mass spectrometry (dSIMS). The vertical phaseseparation in a polymer:PCBM blend was imaged directly for the first time using this technique.20,21 Altering the final film morphology can be done, for instance, by choice of solvent,22
25 surface energy of the substrate,26 and blend components.25,27,28 However, because of the complexity of the interactions and dynamics involved in the drying process of a polymer blend solution, controlling the final film morphology remains challenging. Chemical design of the blend components is a useful tool for altering the final film morphology and for tailoring the optical and electronic properties of the polymers. A successful polymer synthesis strategy to improve overlap of the polymer absorption spectrum with the solar spectrum makes use of a donor
acceptor
donor (D
A
D) segment structure in the polymer backbone.29
32 By tuning the strength of the acceptor in the D
A
D segment, it is possible to narrow the energy gap of the material. Alternating polyfluorene copolymers (APFO) follow this strategy and have a fluorene unit alternating with the D
A
D segment to form the backbone of the polymer.33
36 By controlling the polar character of the side-chains, the miscibility between polymer and fullerene, and hence the BHJ morphology, can be tuned.37 From earlier work on PCBM blends of a series of polyfluorene copolymers, it is known that the interaction parameter (χ) between the polymer and the PCBM is a crucial factor, that determines the final morphology of the film. Lamellar phases (vertical phase separation) were observed in blends with small χ parameter, while lateral phases were formed, through breakup of the layers, for blends with large χ parameter.25 In particular, the formation of lamellar structures (or multilayer) was reported for blends of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(40 ,70 -di-2-thienyl20 ,10 ,30 -benzothiadiazole] (APFO3, also referred to as LBPF5, PFDTBT or F8TBT) with PCBM, when spincoated from chloroform solutions.20 This lamellar structure, with a polymer-enriched surface composition, was demonstrated by dSIMS depth profiles20,26 and correlated with the weak polymer-fullerene repulsion (low value of χ) for the APFO3:PCBM pair and with the kinetics of the drying film.25,26 The effect of kinetics and choice of solvent on the film morphology of this blend is discussed in earlier work.21 An ultraviolet photoemission spectroscopy study has also shown that the surface composition of such a APFO3: PCBM film with bulk weight ratio 1:4 was close to 1:1.38 In this work, we explore the onset of the region of lamellar phase separation, toward the transition to lateral phase separation, and use a set of new polyfluorene copolymers (APFO-Green11, APFO-Green12 and APFO-Green13) to tune the miscibility of the polymers in the electron acceptor material PCBM. This family of APFO-Green polymers is designed to gradually enhance the miscibility between the polymer and the PCBM. Indeed, by introducing segments with modified side chains and the ability to form H-bonding with PCBM, the polymers’ miscibility with PCBM is expected to increase. The distribution of the components in these films is studied by dSIMS depth profiles. In addition, NEXAFS is used to analyze the composition

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at the surface of the film. Fits of the NEXAFS spectra provide a way to evaluate the polymer:fullerene content quantitatively in different depth regimes. The blend films show stronger vertical phase separation, as well as stronger polymer-enrichment of the surface as the polar character of the polymer and its miscibility with PCBM increase.

’ EXPERIMENTAL SECTION Materials. The polyfluorene random copolymers APFO-Green11, APFO-Green12, and APFO-Green13 have a general chemical structure MmNn that is shown in Figure 1. These copolymers were synthesized with varying fractions of a phenolic monomer in which the side chains on the D
A
D segment have an
OH group in the pyrazine unit, resulting in segment N in Figure 1, instead of the hexyl-ether in segment M. The synthesis of the monomers and the polymerization were described earlier.37 APFO-Green11 was synthesized with no phenolic monomer; APFO-Green12 with a 5% feed; and APFO-Green13 with a 10% feed. The polymerization was done in anisole. Molecular weights and polydispersity indices (PDI) were determined by matrix-assisted laser desorption/ionization
time-of-flight (MALDI-TOF) mass spectrometry and size exclusion chromatography (SEC), using polystyrene standards and 1,2,4-trichlorobenzene (135 C) as the solvent. The values of the number average molecular weight (Mn) obtained from SEC were 23000, 26000, and 17000 for APFO-Green11, 12, and 13, respectively, while the ones from MALDI-TOF were in the range of 4000
5500 for all three polymers. This discrepancy between the MALDI-TOF and the SEC values is likely due to the rather high PDI values and the fact that MALDI tends to overestimate the lowmolecular-weight fraction. The substituted fullerene, PCBM, and its pentadeuterated form, d5-PCBM, were both purchased from Solenne BV (The Netherlands) and used as received. For sample preparation, silicon (Si) substrates with a (001) orientation, n-type As doping and a resistivity of 0.001
0.003 Ω cm were used. The substrates were cleaned according to the standard RCA method, without the final HF-etching step, so preserving a hydrophilic surface.39
41 Sample Preparation. The thin-films for atomic force microscopy (AFM) and dSIMS measurements were prepared by spin-coating a blend solution of polymer:d5-PCBM in a 1:4 w/w ratio (total concentration of solids of 12 mg/mL) from chloroform (CF) at 3000 rpm for 60 s onto the silicon substrate. The blend ratio 1:4 was chosen because this is the optimized blend ratio for APFO3:PCBM solar cells, and the present polymers are closely related to APFO3. Films of pure polymer were prepared in a similar way from polymer solutions (12 mg/mL in CF). For dSIMS measurements, the samples were covered with a 50
100 nm thick layer of polystyrene (PS) by the lift-off/float-on method. This PS layer serves as a sacrificial layer to ensure stable sputtering conditions before the active layer is reached. For the NEXAFS measurements, the blend and pristine films were prepared in the same way, with the exception that nondeuterated PCBM was used. All samples described above were prepared and analyzed in air. Characterization. To examine the topography of the surface, the thin films were imaged by AFM using a Nanoscope IIIa Multimode instrument (Veeco, USA), in tapping mode and a Si tip (TESP). Roughness values, extracted from the height images, refer to the standard deviation from the average height, root-mean-square (rms) roughness. Thin-film thicknesses were determined by step-height analysis of images collected by AFM in contact mode across a scratch in the polymer film. Pure component films had a typical thickness of about 70
90 nm, whereas blend films were about 60 nm thick. Thickness values were also measured over a longer distance with a stylus profilometer DEKTAK 8 (Veeco, USA) confirming these values. dSIMS depth profiles were obtained using a VSW apparatus equipped with a liquid metal ion gun (FEI Company, USA) in the dynamic mode. The 2296

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Figure 1. Chemical structures of the polyfluorene copolymers APFO-Green11 to 13 and of the fullerene derivatives PCBM and d5-PCBM.

Figure 2. AFM images (1  1 μm) of the APFO-Green11:PCBM blend film: (a) height and (b) corresponding phase image. (Note the z-scale in the phase image, chosen to optimally cover the distribution of phase values.) samples were gradually sputtered with a Gaþ primary ion beam of 5 keV, scanning over a 100 μm  100 μm region. Secondary ions, with mass to charge ratios (m/q) of 14, 24, 26, 28, and 32, were collected from the central part of the sputtered region (50%) and analyzed with a quadrupole mass spectrometer (Balzers, Liechtenstein). These secondary ions were chosen because they serve as labels for the blend components and the substrate. The use of deuterated PCBM provides a label for this material (m/q = 14; corresponding to CD
, but also CH2
, clusters). The d5PCBM and the polymers also contribute to a mixed signal (m/q = 26; CN
, C2D
, C2H2). However, the m/q = 32 signal (S
) is exclusively assigned to the polymer and can be compared to the m/q = 26 signal to aid interpretation. The signal at m/q = 24 for C2
monitors the total carbon content in the film. The m/q = 28 signal is assigned to sputtered silicon ions and its increase indicates that the substrate has been

reached.42 The depth resolution of the resulting profiles was about 10 nm, similar to earlier work.43,44 Sputter rates were determined which, in combination with the measured film thicknesses, allowed sputtering time to be converted into depth values. The NEXAFS measurements were carried out at the beamline D1011 of the synchrotron storage ring MAX II at Max Lab, in Lund, Sweden. NEXAFS spectroscopy was used in both partial (PEY) and total (TEY) electron yield modes to probe the near-surface region of the polymer blend films at different depths. TEY- and PEY-NEXAFS spectra at the C K-edge were collected simultaneously, with a 40 incident angle measured from the surface normal (5 degrees off from the magic angle). PEY spectra were collected using an applied entrance grid voltage of
150 V on the multichannel plate detector. In this way the surface sensitivity is increased and only electrons escaping from the top surface layers are 2297

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detected. In TEY-NEXAFS all the electrons that escape the surface potential are detected (including inelastically scattered electrons). The raw TEY and PEY data were divided by the respective spectra of a reference sample of freshly sputtered film of gold evaporated on mica and normalized to the incident photon flux, to remove background effects.45
47

’ RESULTS AND DISCUSSION Figure 2 shows the height image and corresponding phase image (1  1 μm micrographs) of the APFO-Green11:d5-PCBM sample obtained by tapping mode AFM. The topography of the surfaces of the other two blends looks similar (images not shown). Larger scale images showed no significant structural change. Though the height images are smooth for all three polymer blends, a small variation in the rms roughness values is observed: 0.76 ( 0.05 nm, 0.67 ( 0.02 nm, and 0.58 ( 0.03 nm for APFO-Green11:d5PCBM, APFO-Green12:d5-PCBM, and APFO-Green13:d5PCBM, respectively. These roughness values are lower than the ones for the corresponding pure polymer films, which are approximately 1 nm for all three polymer films (images not shown). The roughness values of the blends are similar to those previously observed for blend films of APFO3 with PCBM, prepared under similar conditions.20,27,42 AFM-images were also recorded for the corresponding blends with nondeuterated PCBM and the morphologies, as well as the roughness values obtained, were not significantly different. Dynamic Secondary Ion Mass Spectrometry. The dSIMS depth profiles of the three blend films are shown in Figure 3. To improve clarity, these profiles are divided into three distinct regions, separated by dashed lines. From left to right, they correspond to the polystyrene (PS) sacrificial layer, the active layer (Act. L.) and the silicon substrate (Si). The flat profile for the carbon content signal (m/q = 24) indicates constant density throughout the sample. The depth profile of APFO-Green11:d5-PCBM, presented in Figure 3a, shows a fairly homogeneous composition as indicated by the polymer-exclusive signal (m/q = 32) and by the mixed signal (m/q = 26). This homogeneity is only disrupted by an S
signal increase, followed by a steep decrease, closer to the Si substrate. When comparing this with the mixed signal, the same increase is present; however the drop in this signal is not as steep as for the polymer-exclusive one. This indicates some stratification at the substrate interface, with a polymer-depleted (PCBM-rich) layer immediately at the silicon surface, preceded by a polymer-enriched layer. This is likely due to surface-directed phase separation mechanisms, as previously reported for APFO3:PCBM films prepared from a high vapor pressure solvent.20 During film formation, the higher surface energy component, the fullerene, migrates to the high surface energy silicon substrate, initiating substrate-induced phase stratification to yield a more energetically stable situation.20,26,42 Figure 3b shows the profile for the APFO-Green12:d5-PCBM sample. The same stratification at the substrate interface is noticeable in this sample, albeit with a less sharp decrease of the S
signal. At the free surface, i.e., PS-polymer film interface, a peak in the mixed-signal is observed. This is likely to indicate polymer enrichment of the surface, although the profile for the polymer-exclusive signal (m/q = 32) is hard to resolve in this region. Within the bulk, and after the initial peak, the steady increase of the S
signal indicates a gradient in the polymer content. In the dSIMS profile for the APFO-Green13:d5-PCBM sample (Figure 3c) this self-stratification process is intensified and two clear peaks are observed

Figure 3. SIMS profiles of (a) APFO-Green11:d5-PCBM, (b) APFOGreen12:d5-PCBM, and (c) APFO-Green13:d5-PCBM blend films, spin-coated from chloroform on a silicon substrate (PS, Act. L., and Si refer to the polystyrene sacrificial layer, the active layer and the silicon substrate, respectively); the dashed lines mark the limits of the active layer and are drawn at 50% increase of the sulfur signal (S
, m/q = 32) and 50% decrease of the carbon signal (C2
, m/q = 24).

both in the polymer and in the mixed profiles. The S
signal shows two areas of polymer-enrichment: at the free surface and within the film (depth ∼40 nm). This variation is confirmed in the mixed-signal. However, at the interface with the substrate, a contribution from the fullerene generates a shoulder feature in the mixed signal not present in the polymer profile. The fullerene label (m/q = 14) mirrors the two-peak profile, with the increase at the substrate explaining the shoulder feature of the mixed profile. Although quantitative composition data cannot be reliably deduced from the dSIMS profiles due to the environmental 2298

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Figure 4. Carbon K-edge NEXAFS spectra measured in total (TEY) and in partial (PEY) electron yield of (a) PCBM; (b) APFO-Green11; (c) APFO-Green12; and (d) APFO-Green13.

dependence of sputter rates and detection sensitivity, these results clearly show an increased self-stratification as the polar character of the polymer and the tendency to interact with PCBM increases, by insertion of the phenolic groups. Fullerene enrichment at the substrate, and its depletion in the bulk of the film, is visible for all three APFO-Green:PCBM blend films studied here. Additionally, there is polymer enrichment at the free surface of the blend film for APFO-Green13, in particular. Near-Edge X-ray Absorption Fine Structure Spectroscopy. The NEXAFS spectra of the pure polymers and of the PCBM are shown in Figure 4. Both the spectra collected in PEY and in TEY detection modes are presented, which differ in the sampling depth. In PEY the largest contribution was calculated for organic molecules and found to be from electrons that come from the first 2
3 nm, for the used electron grid bias.48 In TEY-NEXAFS, the sampling depth can be as deep as 10 nm. However, recent studies point toward a much smaller electron scattering length in polymers due to short secondary electron ranges
for P3HT films it was determined that 90% of the TEY signal comes from the first 5 nm and 63% from the first 2.5 nm below the surface.49 If this can be generalized to most organic semiconductors, then any difference between the PEY and TEY spectra can give an indication of compositional changes in the region just a few nanometers beneath the surface. The PEY- and the TEY-NEXAFS spectra of the polymers (Figure 4) are almost identical, indicating a homogeneous composition in the near-surface region of the films. The spectra of PCBM show a slight difference in the π*resonance peaks (∼284
286 eV) that could be due to degradation of the topmost surface layer. Figure 5 shows the spectra for

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Figure 5. Carbon K-edge NEXAFS spectra measured in total (TEY) and in partial (PEY) electron yield of (a) APFO-Green11:PCBM; (b) APFO-Green12:PCBM; and (c) APFO-Green13: PCBM.

Figure 6. Carbon K-edge NEXAFS spectra in total yield of APFOGreen11:PCBM blend (in 1:4 weight ratio) and the respective best linear combination fit, which was found to have the fitting coefficients 1:3.3 w/w. The residual plot is also shown (shifted, to improve clarity).

the blend films. The PEY- and the TEY-NEXAFS spectra are now very different, with a change in shape and an increase in intensity of the π*-resonance peaks in the TEY-NEXAFS spectra compared to the PEY-NEXAFS spectra. These differences occur in all three polymer-PCBM blends. Different composition at different depths indicates that vertical phase separation occurs in all blend films. Because NEXAFS spectra at the C-edge exhibit clearly distinguishable fingerprint patterns specific for a molecule, the spectra of pure components can be used to find the composition ratios in the surface layer of films consisting of mixtures of these 2299

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Table 1. Blend Ratios (by weight) and Corresponding Polymer Weight Fractions for the Best Linear Combination Fit of the Experimental NEXAFS Spectra of 1:4 w/w (20% polymer) Blend Films APFO-Green11:PCBM

APFO-Green12:PCBM

APFO-Green13:PCBM

PEY-NEXAFS

1:1.9 (34% pol.)

1:1.6 (39% pol.)

1:1.4 (42% pol.)

TEY-NEXAFS

1:3.3 (23% pol.)

1:2.8 (26% pol.)

1:1.9 (34% pol.)

components. This can be done when the pure components’ spectra show well separated peaks, as is the case. As an example, the TEY-NEXAFS spectrum of APFO-Green11:PCBM blend is given in Figure 6. Contributions from both of the components can be recognized in the blend film spectrum (dotted line). Assuming that the contribution of any orientational differences in the pure films and blend films on the spectra is minimal, and assuming no reacting components, and similar scattering lengths for different blend components, a fit of the blend film spectrum (full line) was made as a linear combination of the pure components spectra. In order to be able to compare the obtained ratios with the bulk blend ratio, the resulting fit coefficients were converted from volume ratios to weight ratios, taking into account their densities. The density of the polymers is estimated to be 1 g/cm3; the reported density of PCBM ranges from 1.3 to 1.5 g/cm3.17,50 Analogous fits were carried out for the other polymer blends (fitted spectra are provided in the Supporting Information). The resulting blend ratios and corresponding polymer weight fractions for all three blends with bulk weight ratios 1:4 are summarized in Table 1, using the PCBM density value of 1.5 g/cm3. The values in Table 1 deviate significantly from the bulk weight ratio 1:4 (or 20% polymer), demonstrating that all the blend films are polymer-enriched at the surface. Considering the lower published density value for the PCBM (FPCBM = 1.3 g/cm3) leads to approximately 3% higher weight percentage of polymer at the surface. This surface enrichment with polymer is more pronounced as the polar character of the polymers increases, reaching a polymer to fullerene ratio of 1 to 1.4 (or 42% polymer) for the surface of the APFO-Green13:PCBM film. A gradient in composition with depth is noticeable already within the first 5 nm from the surface of the films, shown by the higher PCBM content obtained from the fit of the TEY-NEXAFS spectra, compared to the PEY-NEXAFS ones. The fact that the ratio calculated from the TEY-NEXAFS of the APFO-Green11:PCBM film (1:3.3) is quite close to the overall ratio of 1:4 is consistent with the fact that no polymer-enrichment was detected with dSIMS. The calculated ratios for the other two blend films also correlate very well with the dSIMS results as they show increased surface segregation in the polymer with its increasing polar character. Apart from a more quantitative estimate of the composition, the NEXAFS technique also yields additional information on the composition of the surface region, with a depth accuracy that could not be reached by other techniques. Most remarkably, for samples with dSIMS depth profiles that appear flat, NEXAFS could clearly show polymer enrichment of the surface-most region, compared to the bulk composition. Figure 7 schematically illustrates the difference between the surface sensitivities of PEY and TEY measuring modes when compared to dSIMS. From these combined results, we draw the conclusion that the functionalization of an increasing fraction of the polymer segments by phenolic side groups enhances the polymer enrichment at the free surface of the polymer:PCBM blend film and extends the selfstratification to deeper regions in the film. This can be correlated to

Figure 7. Schematic illustration of a sample with vertical gradients in composition: polymer-enriched regions are colored blue and fullereneenriched regions are colored red. The zoomed-in area represents the depth range of NEXAFS measurements, compared with the profile obtain with dSIMS which probes the sample all the way through, from the free surface until the substrate.

the enhanced miscibility of the components, due to the increased polar character of the polymer and its enhanced tendency to form H-bonds with PCBM. The situation observed here resembles the one reported earlier for PCBM blends with polyfluorene copolymers, spin-cast under similar conditions (1:4 w/w polymer:PCBM ratio from chloroform).25,27 The blends with APFO3, known for weak repulsive interactions with PCBM, formed lamellar film structures with four alternating layers, rich in PCBM and polymer at the substrate and at the free surface, respectively. The four-layer film structure observed in this study for the PCBM blend with APFO-Green13 is similar to that with APFO3.20,26 Careful comparison of the dSIMS depth profiles of these two blends shows that APFO3:PCBM exhibits even more pronounced phase separated lamellar structure than APFO-Green13:PCBM. In turn, islands (lateral domains) have been observed in other polyfluorene:PCBM blends with stronger polymer
fullerene repulsion, due to the break-up of initially formed multilayers.25,27 Therefore the observations presented in this study correspond to the onset of the lamellarlateral transition, rather than the complete transition,51 visible in blends spin-cast from rapidly evaporating solvent,25 and induced by reduced fullerene
polymer miscibility or decreased polar character of the polymer. Caution should be taken to ascribe the full cause of the enhanced vertical phase separation to the polymer structure, because the differences in the polymers’ molecular weight distributions and polydispersities could also have some effect on the morphology. This effect is, however, believed to be minor in the present range of molecular weights.52 The implications of vertical phase separation in these film blends for photovoltaic performance are reported elsewhere.

’ CONCLUSIONS A novel set of polyfluorenes, APFO-Green11, 12, and 13, was synthesized, with tailored monomer, aiming at better chemical miscibility with the fullerene derivative PCBM. Morphology studies 2300

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Chemistry of Materials revealed a larger extent of vertical phase separation in blend films as the miscibility of the polymer with PCBM increases, preventing the onset of lamellar-lateral transition from occurring at the free surface. In-depth SIMS profiles show that such vertical segregation extends all the way from the substrate and is driven by surface energy minimization mechanisms. The quantitative blend composition close to the surface was calculated for two different NEXAFS sampling depths, demonstrating that surface enrichment in polymer occurs already in the first few nanometers of the film.

’ ASSOCIATED CONTENT

bS

Supporting Information. Linear combination fits and corresponding residual plots for the NEXAFS spectra of the three blend films (PDF). This material is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Dr. Andrzej Dzwilewski, Eindhoven University of Technology (The Netherlands), for his contribution to the NEXAFS measurements, Stefan Hellstr€om, Chalmers University of Technology, G€oteborg (Sweden), for the molecular weight analysis, and Dr. Christian M€uller, Link€oping University (Sweden), for fruitful discussions. We gratefully acknowledge the financial support from the Swedish Research Council and the Swedish Energy Agency. ’ REFERENCES (1) Yu, G.; Gao, J.; Hummelen, C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (2) Moons, E. J. Phys.: Condens. Matter 2002, 14, 12235. (3) Arias, A. C. J. Macromol. Sci. Polym. Rev. 2006, 46, 103. (4) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45–61. (5) Chen, L.-M.; Xu, Z.; Hong, Z.; Yang, Y. J. Mater. Chem. 2010, 20, 2575. (6) Peet, J.; Heeger, A. J.; Bazan, G. C. Acc. Chem. Rev. 2009, 42, 1700. (7) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21, 1323. (8) McNeill, C. R.; Greenham, N. C. Adv. Mater. 2009, 21, 3840. (9) Yang, X.; Loos, J. Macromolecules 2007, 40, 1353–1362. (10) DeLongchamps, D. M.; Kline, R. J.; Fischer, D. A.; Richter, L. J.; Toney, M. F. Adv. Mater. 2010, 23, 319. (11) Slota, J. E.; He, X.; Huck, W. T. S. Nano Today 2010, 5, 231. (12) Xu, Z.; Chen, L.-M.; Yang, G.; Huang, C.-H.; Hou, J.; Wu, Y.; Li, G.; Hsu, C.-S.; Yang, Y. Adv. Funct. Mater 2009, 19, 1227. (13) Hoppe, H.; Glatzel, T.; Niggemann, M.; Hinsch, A.; Lux-Steiner, M. C.; Sariciftci, N. S. Nano Lett. 2005, 5, 269. (14) Maturova, K.; van Bavel, S. S.; Wienk, M. M.; Janssen, R. A. J.; Kemerink, M. Nano Lett. 2009, 9, 3032. (15) Ingan€as, O.; Zhang, F.; Tvingstedt, K.; Andersson, L. M.; Hellstr€om, S.; Andersson, M. R. Adv. Mater. 2010, 22, E100. (16) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Nat. Mater. 2008, 7, 158. (17) Kiel, J. W.; Kirby, B. J.; Majkrzak, C. F.; Maranville, B. B.; Mackay, M. E. Soft Matter 2010, 6, 641.

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