Phenyl-C61-butyric Acid Methyl Ester Thin Films - ACS Publications

Feb 21, 2012 - The nanostructure of thermally annealed thin films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCB...
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Interfacial Free Energy Driven Nanophase Separation in Poly(3hexylthiophene)/[6,6]-Phenyl-C61-butyric Acid Methyl Ester Thin Films Giovanni Li Destri,† Thomas F. Keller,‡ Marinella Catellani,§ Francesco Punzo,∥ Klaus D. Jandt,‡ and Giovanni Marletta*,† †

Laboratory for Molecular Surfaces and Nanotechnology (LAMSUN), Department of Chemistry, University of Catania and CSGI, V.le A Doria 6, 95125 Catania, Italy ‡ Institute of Materials Science and Technology (IMT), Chair in Materials Science, Friedrich Schiller University, Löbdergraben 32, D-07743 Jena, Germany § Istituto per lo Studio delle Macromolecole (ISMAC), CNR − Via Bassini 15, 20133 Milano, Italy ∥ Dipartimento di Scienze del Farmaco, Divisione Chimica, University of Catania, V.le A Doria 6, 95125 Catania, Italy S Supporting Information *

ABSTRACT: The nanostructure of thermally annealed thin films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61-butyric acid methyl ester (PCBM) blends on hydrophobic and hydrophilic substrates was studied to unravel the relationship between the substrate properties and the phase structure of polymer blends in confined geometry. Indeed, the nature of the employed substrates was found to affect the extent of phase separation, the PCBM aggregation state and the texture of the whole system. In particular, annealing below the melting temperature of the polymer yielded the formation of PCBM nanometric crystallites on the hydrophobic substrates, while mostly amorphous microscopic aggregates were formed on the hydrophilic ones. Moreover, while an enhanced in-plane orientation of P3HT lamellae was promoted on hydrophobic substrates, a markedly tilted geometry was produced on the hydrophilic ones. The observed effects were interpreted in terms of a simple model connecting the interface free energy for the blend films to the different polymer chain mobility and diffusion velocity of PCBM molecules on the different substrates.

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INTRODUCTION Polymer thin films have last attracted much interest because of both their peculiar structural and thermal behavior1 as well as their unique properties which make this new class of materials very promising in many fields such as coatings,2 biomaterials,3 and electronics.4,5 From a purely fundamental point of view, polymer thin films represent a model case for studying the variation of structural and thermal properties from bulk to nanoconfined systems. Indeed, a nanoconfinement effect on the glass transition temperature6 as well as peculiar phase separation7 and crystallization8−13 processes have recently been found. Among the environmental parameters which may play a role in determining the nanostructure of polymer films, polymer/ substrate interfacial interactions can have a striking effect since preferential wetting may drive the phase separation process.14 Moreover, substrate nature has been found to affect the chain mobility of polymers and, in turn, to enhance or lower the crystalline order.15 © 2012 American Chemical Society

A special case of polymer films, for which the effect of interfacial interactions is not clarified yet, is given by polymer/ small molecules blends: the very different molecular sizes between the two components cause an increased mixing entropy with respect to polymer blends in which both components have similar molecular weight. This effect may indeed lead to different phase separation processes in the two cases. A particularly interesting case, because of its technological relevance, is given by poly(3-hexylthiophene)/[6,6]phenyl-C61-butyric acid methyl ester (PCBM) blends which have been extensively studied as active materials in organic photovoltaic cells.16,17 Since the photovoltaic efficiency is strongly dependent upon the nanostructure of the blend layer, a comprehensive structural investigation both in terms of polymer crystallinity18 and phase separation19 has been carried out in last years. In particular, a nanoscopic phase separation is Received: January 16, 2012 Revised: February 17, 2012 Published: February 21, 2012 5257

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interface, while GIXRD is the most suitable technique to characterize the crystalline structure of polymeric thin films across their whole thickness.23,24 Moreover, the X-ray diffraction technique allows one to estimate not only the crystalline structure but also the texture of the film, if GIXRD data are combined with the analysis of rocking curves, thus providing an almost comprehensive view on the out of plane growth of polymeric crystallites.25 Thus, the comparison of AFM and GIXRD results are expected to allow the identification of surface-confined and “bulk” rearrangements in the investigated films. Our results clearly show that interfacial interactions are a key parameter in determining the extent of phase separation across the whole film, and demonstrate that PCBM nanophases can be obtained by controlling the relevant IFE values.

requested to enhance the charge carrier generation; moreover, high crystallinity has been found to increase the light absorption yield as well as the charge mobility. For this purpose, many experimental attempts have been performed in order to optimize the nanostructure of P3HT/PCBM thin films and thermal annealing close to 130 °C has been found to induce the best structural evolution in terms of both phase separation and crystallization.18 Despite that, there is no experimental evidence on the effect that interfacial interactions may have on the structuring process of P3HT/PCBM thin films during and after thermal annealing, also due to the limited number of studied systems. Indeed, the most investigated substrate is indium tin oxide (ITO) covered by poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT-PSS) which ensures the optimal work function alignment,17 thus allowing optimal charge injection from the photoactive layer to the electrode, while very little work has been performed on different substrates. However, since interfacial interactions have been shown to markedly affect phase separation, in terms of extent, segregation, and crystal orientation, the overall structural effect given by optimized interface may strikingly increase the photovoltaic efficiency. Only very recently Friend and co-workers demonstrated that substrate nature induces spinodal decomposition waves in P3HT/PCBM films annealed at 130 °C for 15 min, so that thin layers of PCBM and P3HT at the two interfaces respectively were formed.20 Moreover, Bulliard et al. showed how the surface free energy of substrates may drive the morphological reorganization of the active layer upon thermal annealing and affect the device efficiency.21 These approaches, therefore, appear basically aimed to determine the influence of surface free energy (SFE) on the overall film composition and vertical phase separation, in an optimized device, that is, at just one annealing temperature. Finally, a detailed study19 has evidenced that bulk P3HT:PCBM blends show an eutectic behavior with an eutectic temperature of 200 °C and composition of 35% in weight of PCBM. In the case of P3HT:PCBM thin films, it was shown that thermal annealing induces phase separation even below the eutectic temperature19 by the diffusion of PCBM molecules to form aggregates whose structure is not clarified yet. The present paper is demonstrated that rather the interfacial free energy (IFE) than the SFE affects the blend phase separation and the crystalline structure of the film components upon thermal annealing. Indeed, IFE is known to strongly affect the structural features of polymeric thin films by modifying the macromolecular chain mobility and, in turn, the related thermal22 and structural15 properties. Noteworthy, for the P3HT:PCBM blends of interest of the present paper, IFE values can be easily estimated from SFE values of the substrate and the blend components as obtained by contact angle measurements. Accordingly, the effect of hydrophilic and hydrophobic substrates on the reorganization of thermally annealed P3HT:PCBM thin films was investigated by comparing the results of structural and morphological techniques. Indeed, the structural evolution of polymeric thin films can be simply followed by means of atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD), two techniques that provide complementary structural data. In particular, AFM is a powerful tool to characterize the surface morphology and the crystal orientation at the film/atmosphere

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EXPERIMENTAL SECTION

Materials. Regioregular P3HT Mn = 15 × 103 g mol−1, Mw = 25 × 103 g mol−1 (polydispersity 1.76) was purchased from Merck and purified by repeated extractions in a Soxhlet apparatus with methanol, diethyl ether, hexane, and chloroform. The molecular weight distribution was obtained via a modular multidetector size exclusion chromatography system from Waters with UV−visible diode array detector, in THF (tetrahydrofuran) using a calibration curve obtained with polystyrene standards. The average molecular weight was Mw = 25.6 × 103 g·mol−1, Mn = 14.6 × 103 g·mol−1, and the polydispersity Mw/Mn = 1.75. PCBM, purity higher than 99.9%, was purchased from Solenne BV and used without further purifications. Substrates and Film Preparation. Hydrophilic substrates were prepared by treating a ⟨100⟩ silicon wafer (from Wacker Siltronic, Burghausen, Germany) with a basic piranha mixture (3 mL of NH4OH, 3 mL of H2O2, and 15 mL H2O at 60 °C for 10 min). The treatment removed the native oxide layer and produced a new oxidized layer homogeneously terminated by silanol groups.26 The treated substrates were dried using a dry N2 flux. Hydrophobic substrates were prepared by dipping the silicon wafer, previously treated according to the above-described procedure, in a 40 mM solution of octadecyltrichlorosilane (OTS) in 15 mL hexadecane and 5 mL CHCl3 at 40 °C for 1 h. The hydrophilic substrates had a static water contact angle θ = 3.8° ± 0.5°, while the hydrophobic substrate had a contact angle θ = 104.1° ± 1.1°. P3HT:PCBM 1:1 in weight solutions with concentration of 1.5 mg mL−1 were obtained by mixing the proper amount of pure P3HT and PCBM chloroform solutions. Chloroform was used as solvent in view of its low-boiling point, preventing the reorganization of polymer molecules during the spin coating deposition process27 and, therefore, allowing, upon annealing of the films, the maximization of the molecular reorganization, leading to a better crystallization and thus of any effect of substrate nature and thickness on it. Before the film deposition, the blend solution was kept under stirring for 12 h to ensure the complete dissolution of PCBM. Thin films of P3HT:PCBM blend were obtained by spin coating at 1200 rpm the solutions on the substrates immediately after the abovedescribed treatments both for hydrophilic and hydrophobic terminations. The resultant films had a thickness of 100 ±10 nm as determined by spectroscopic ellipsometry. Thermal annealing treatments were performed by heating the samples at temperatures ranging between 150 and 250 °C for 30 min in vacuum, at approximately 10−3 Torr, to prevent polymer degradation processes in atmosphere. Optical Microscopy. Optical microscopy (OM) images were obtained with a DM 4000B from Leica (Wetzlar, Germany) used in reflection mode. Atomic Force Microscopy. Atomic force microscopy (AFM) images were obtained with a Nanoscope IIIa apparatus from Digital Instruments (Santa Barbara, CA) used in tapping mode in air to record simultaneously topography, amplitude, and phase images of samples. By isotropically applying the fast Fourier transform filtering on the 5258

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Figure 1. Optical micrographs of unannealed P3HT:PCBM films on hydrophilic substrate (a), annealed at 150 °C (b), 200 °C (c), 250 °C (d) and unannealed P3HT:PCBM films on hydrophobic substrate (e), annealed at 150 °C (f), 200 °C (g), and 250 °C (h). Arrows show the PCBM aggregates formed upon annealing.

Table 1. Average Size and Average Number per 1000 μm2 of PCBM Aggregates of P3HT:PCBM Films Deposited on Both Hydrophobic (conventionally indicated as “Phob”) and Hydrophilic (conventionally indicated as “Phil”) Substrate and Annealed at Different Temperatures Phil unan size (μm) number of aggregates/1000 μm2

Phil 150 °C

Phil 200 °C

Phil 250 °C

1.5 ± 0.2 4.0 ± 1.1

5.8 ± 0.7 4.2 ± 0.8

7.8 ± 0.6 3.6 ± 1.2

amplitude images, monodimensional frequency plots were obtained, further allowing the calculation of the power spectral density (PSD). The PSD analysis was performed for at least six independent measurements. Transmission Electron Microscopy. Samples for transmission electron microscopy (TEM) were prepared by spreading few drops of a concentrated water solution of poly(acrylic acid) (PAA) on the P3HT/PCBM films. After the solution was dried, a film of PAA on the samples was formed. By gently peeling the PAA film, the P3HT/ PCBM films were removed from the substrate. Finally, the PAA was dissolved in water leaving the P3HT/PCBM films floating on the water surface. The samples were subsequently fished on a copper TEM grid. TEM analysis was performed on a JEOL 3010 instrument operated at 300 kV. X-ray Diffraction Measurements. X-ray diffraction measurements were performed with a Rigaku Ultima IV type III diffractometer (Rigaku, Tokyo, Japan) equipped with cross beam optics (CBO) by using a Kα wavelength emitted by a Cu anode. Careful alignment of source and detector with respect to the sample was reached by using a thin film attachment with three degrees of freedom. In order to avoid beam defocusing, the measurements were carried out in parallel beam mode. Divergence of the primary beam was reduced by a 5° Soller slit, while divergence of the diffracted beam was reduced by a 0.5° horizontal Soller slit.28 For GIXRD measurements, the incident angle was kept at 0.1° to avoid any significant scattering from the substrates. Rocking curve measurements were performed by keeping the scattering angle 2θ = 5.4°and by gradually varying the incident angle from 2° to 3.5°. Mercury29 was used to simulate the powder diffraction patterns of PCBM and to generate the relative hkl versus F2 files (see the Supporting Information), where F is the structure factor and is related to the integrated intesity IH at a given Bragg angle according to the following equation:

Phob unan

Phob 150 °C

Phob 200 °C

Phob 250 °C

0.7 ± 0.2 1.4 ± 0.3

0.9 ± 0.2 2.2 ± 0.3

6.3 ± 1.6 2.6 ± 0.3

described as a combination of attractive van der Waals forces (Lifshitz−van der Waals component) and Lewis acid−base polar interactions as described by eq 1 γ = γ LW + 2 γ+γ−

(1)

where γ is the Lifshitz−van der Waals component, γ is the acid component, and γ‑ is the basic one. Contact angle measurements of three different liquids on a solid surface make possible, providing surface free energy components of the liquids are known, the determination of the surface free energy of the solid as well as the Lifshitz−van der Waals together with the acid and the basic components by simply solving the three equations which describe the spreading of the three liquids on the solid surface +

LW

(

(1 + cos θ)γL = 2

γSLW γ LW L +

− γ+ S γL +

+ γ− S γL

)

(2)

The subscripts L and S refer to the liquid and solid surface tension components, respectively, and γL is the total surface tension of the liquid. In our case, the three employed liquids were water, glycero,l and tricresyl phosphate (TCP) whose surface tensions and the three related components are reported in the instrument database.

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RESULTS AND DISCUSSION Phase Separation and P3HT Crystallite Orientation. The degree of phase separation on different substrates can be clearly seen by means of microscopy techniques. Indeed, the diffusion of PCBM molecules leads in general to the formation of aggregates which can be revealed, depending on the dimensions, by optical microscopy, sensitive only to microscopic aggregates, or by transmission electron microscopy, revealing also nanometric aggregates. In Figure 1, the optical micrographs of P3HT:PCBM 1:1 films deposited on both hydrophilic and hydrophobic substrates and annealed at different temperatures are reported. In particular, for each given temperature, some microscopic PCBM aggregates appear, with their number being lower and their size being in average smaller for hydrophobic substrates (see Table 1). In particular, according to Figure 1 and Table 1, a striking difference can be observed for samples annealed at 200 °C. Indeed, while for hydrophilic substrates a significant number of

IH = k1k2I0 LPTE |F |2 where k1 and k2 are physical constants, I0 is the intensity of the incident X-ray beam, L is the Lorentz factor and depends on the diffraction technique, P is the polarization factor, T is the transmission factor, and E is the extinction coefficient. Static Contact Angle Measurements. Contact angle measurements were performed by using a OCA 20 apparatus (DataPhysics Instruments GmbH, Filderstadt, Germany). In order to calculate the surface free energy of P3HT/PCBM films and the two substrates, the three liquids technique was applied. Surface free energy (γ) may be 5259

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microscopic PCBM aggregates is found, none of them are detected for films on the hydrophobic ones, where microscopic PCBM aggregates are formed only at 250 °C. A high resolution study, performed by TEM for the 200 °C annealed films onto the hydrophobic substrates, was performed in order to reveal the fate of PCBM molecules in films annealed below the P3HT melting temperature onto these substrates, where microscopic aggregates are missing (see Figure 2).

AFM analysis gives more insight on the overall morphology of the films onto the two types of substrates, that is, roughness and crystallinity of the film surfaces. Figures 3 and 4 report the height images of P3HT:PCBM 1:1 films before and after annealing at different temperatures and

Figure 2. TEM image of unanneled P3HT:PCBM (1:1 in weight) films deposited on hydrophobic substrate (a) and annealed at 200 °C (b). The darker regions are formed by PCBM molecules immersed in the brighter polymeric matrix.

Figure 4. Mean roughness values for P3HT:PCBM thin films deposited on both hydrophilic and hydrophobic substrates unannealed and annealed at various temperature. The data were obtained by averaging four different images each of 1 μm2 size for each sample The lines are drawn to guide the eyes.

Indeed, it is known that TEM would powerfully visualize the phase separation between P3HT and PCBM, due to the different electron density of the two molecules which allows a good contrast between PCBM (darker) and polymeric (brighter) domains.18 In particular, TEM results show that before annealing PCBM is homogeneously dispersed inside the polymeric matrix (see Figure 2a), while after annealing a huge quantity of nanoscopic PCBM darker spots is formed inside the P3HT brighter matrix (Figure 2b). Accordingly, PCBM does not appear to be dissolved inside the polymer, but rather to phase separate into nanoscopic aggregates. Moreover, the density of the PCBM nanostructures, whose average size ranges between 30 and 70 nm, appears very high, corresponding to the formation of a very large interface between the nanoaggregates and the polymer phase.

the related measured roughness for the various samples. The film resistance to the tapping mode AFM measurements was verified by repeating twice the measurement after each scanning and comparing the results. No tip-induced changes were detected with the employed AFM settings. It is clearly shown that, by increasing the annealing temperature, the roughness of all the films increases, but the temperature dependence is strikingly different for the two substrates. In particular, for the hydrophilic substrate, a steady increase with the annealing temperature can be seen up to 250 °C, while for hydrophobic substrates the roughness remains almost constant up to 200 °C, that is, up to the eutectic temperature of the blend, to yield an abrupt rise at 250 °C, that is, above the melting temperature of the polymer.

Figure 3. AFM height images (Z = 10 nm) of P3HT:PCBM (1:1 in weight) films deposited on hydrophilic substrates unannealed (a) and annealed at 150 °C (b), 200 °C (c), and 250 °C (d) and of P3HT:PCBM (1:1 in weight) films deposited on hydrophobic substrates unannealed (e) and annealed at 150 °C (f), 200 °C (g), and 250 °C (h). 5260

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Figure 5. AFM amplitude images of P3HT:PCBM films deposited on hydrophilic substrates and annealed at 150 °C (a), 200 °C (b,) and 250 °C (c) and of P3HT:PCBM films deposited on hydrophobic substrates and annealed at 150 °C (d), 200 °C (e), and 250 °C (f).

Figure 6. PSD distribution of lamellar width obtained by the AFM images of P3HT:PCBM films deposited on hydrophilic substrates and annealed at 150 °C (a), 200 °C (b), and 250 °C (c) and of P3HT:PCBM films deposited on hydrophobic substrates and annealed at 150 °C (d), 200 °C (e), and 250 °C (f).

our case, the high roughness can be assumed as diagnostic of a strongly demixed system. The second relevant structural parameter of interest is the crystallinity of the overall system after annealing treatments. In order to gain a thorough understanding of this parameter for the whole films, both AFM and GIXRD analysis were performed. In particular, AFM analysis provides valuable information for P3HT crystallites at the film surface. Typical examples are reported in Figure 5, showing that, as previously observed for the case of pure P3HT films, the substrate nature in itself does

The AFM data, in connection with the optical microscopy results, suggest that the observed substrate-dependence of roughness with temperature are basically due to the occurrence of strong phase separation processes in films of immiscible molecules such as in the system here considered. Indeed, as it was reported that phase separation in the P3HT:PCBM system basically occurs by diffusion of PCBM molecules through the polymeric matrix toward the film/air interface,30−32 the roughness increase of the film can be reasonably connected to inhomogeneity induced by such diffusion process. Thus, in 5261

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not affect the crystalline structure of the polymer, which forms lamellae by folding its backbone perpendicularly to the substrate plane.15,25 The AFM data were analyzed in terms of power spectral density (PSD), as reported in Figure 6. The PSD analysis, indeed, isotropically identifies the correlation distances of the repeating features in the images, which, for lamellae-forming polymers like P3HT, can be interpreted in terms of lamellar widths.15 Moreover, these correlation distances, according to literature, are related to the folding period of the polymer chains. Therefore, a sharp PSD distribution is diagnostic of a high amount of lamellae. Noticeably, the observed PSD distribution becomes sharper and sharper for both substrates by increasing the annealing temperature, suggesting that the number of lamellae increases with temperature. Moreover, the PSD analysis can also provide an insight on the different in plane orientation of the polymeric crystals at the film surface. As far as the folding period of the P3HT here employed is constant at a fixed temperature,33 the changes in the P3HT lamellae apparent widths are actually due to changes in lamellae tilting angle.15,34 Interestingly, the maximum of this width distribution is shifted to lower values, ranging between 14 and 15 nm, as compared to the pure polymer films,15 and it is independent of the substrate nature. Both the results suggest that the optimal orientation of lamellae, corresponding to an apparent width of 16 nm for the pure polymer films,15 is in fact hindered by the presence of PCBM molecules. GIXRD Results. In order to fully characterize the crystalline structure of P3HT/PCBM thin films, GIXRD analysis was carried out, focusing attention on the samples annealed at 150 and 200 °C, whose comparison clearly shows the marked structural evolution depending on substrate nature and annealing temperature. Figure 7 reports the out-of-plane grazing incidence diffractograms of P3HT/PCBM thin films deposited on both substrates and annealed at different temperatures. Unannealed samples present, as expected, a low degree of crystallinity of the films, as, in fact, only the ⟨100⟩ peak of the P3HT lamellar stacking is clearly visible, while the two higher order reflections are much less intense. Upon annealing, a sharp increase of polymer crystallinity is observed on both substrates as it is revealed by the decrease of full width at half-maximum of the ⟨100⟩ peak, as well as by the appearance of very intense ⟨200⟩ and ⟨300⟩ lamellar stacking peaks. Moreover, sharp peaks also appear at 9.2°, 17.5°, and 19.6°, that are diagnostic of the formation of a PCBM crystalline phase. It is noted that this is the first time, to the best of our knowledge, that such sharp peaks in P3HT/PCBM thin films have been detected. The single crystal structure of PCBM was already solved and published,35 and two relative .cif files (CCDC 221976 and 211977) were deposited to the Cambridge Structural Database (CCDC). They correspond to two different structures which are the results of different crystallization attempts performed by means of two solvents: chlorobenzene and dichlorobenzene. Molecules of these solvents are actually present in the PCBM unit cells although their coordinates were modeled in the case of dichlorobenzene only. Even if there are strong similarities between the two unit cells, the use of different crystallization solvents resulted in a packing in two different space groups: monoclinic for the one containing one dichlorobenzene molecule and triclinic for the one bearing two chlorobenzene molecules. The two deposited .cif files were used to simulate PCBM powder diffraction patterns. The so inferred hkl versus

Figure 7. Grazing incidence diffractograms of P3HT:PCBM unanneled films deposited on both hydrophilic and hydrophobic substrates (a), annealed at 150 °C (b) and 200 °C (c) with the complete indexing of crystalline phases (see the text). The subscripts P, M, and T of the Miller indices refer, respectively, to P3HT, monoclinic PCBM, and triclinic PCBM crystalline structures.

intensity list (reported in the Supporting Information) was used in order to shed light over the possible attribution of the correct Miller indices to the most intense peaks shown in the GIXRD diffractograms in Figure 7. As a consequence, the peak at 9.2° can be assigned both to the ⟨111⟩ plane of triclinic and ⟨101⟩ plane of the monoclinic structure, the peak at 17.5° can be assigned to both the ⟨2,−1,1⟩ plane of the triclinic cell and the ⟨2,2,−1⟩ plane of the monoclinic one. Finally, the peak at 19.6° is assigned to the ⟨1,1,−3⟩ plane of the triclinic structure and the ⟨2,2,1⟩ plane of the monoclinic one. In spite of the superimposition of the above-mentioned characteristic peaks, there is some evidence that suggests the prevalence of the monoclinic structure over the triclinic one. On one hand, the progressive reduction of the amount of solvent molecules in the unit cell, passing from two chlorobenzene molecules to one dichlorobenzene molecule, gave rise to a space group change 5262

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from triclinic to monoclinic. This is also true while diminishing the overall amount of volume occupied by solvent molecules in the unit cell, which is even more probable when using chloroform instead of benzene derivatives. Thus, remembering that the experimental conditions we used for the film deposition are quite different from the ones used for those single crystal harvests, that is, we used chloroform instead of chlorine derivatives of benzene, we can suppose that there is a higher tendency of the PCBM alone to crystallize in the monoclinic space group instead of the triclinic. On the other hand, a more quantitative argument can be used to strengthen this hypothesis: as reported in the hkl versus intensities files (see the Supporting Information), all the indexed peaks are in agreement with high intensity peaks for the monoclinic form only and not for the triclinic one. As a result, although we cannot exclude that a small fraction of the present crystals could have crystallized in the triclinic space group, we can assume that the vast majority of the crystals are monoclinic. As shown in Figure 7, unlike P3HT peaks, the shape of the described PCBM reflections changes with both the substrate nature and the annealing temperature. In fact, for samples annealed at 150 °C, the films on the hydrophobic substrate show a higher degree of crystallinity of the PCBM phase, as indicated by both the higher relative intensity and narrower FWHM of the above-mentioned peaks. Thus, while on OTS the shoulder at 9.2° is visible, the same feature is absolutely lacking on hydrophilic substrates, as well as the two peaks at 17.5° and 19.6° which appear sharper and better defined on the hydrophobic substrate than on the hydrophilic one. On the other hand, the two peaks at 17.5° and 19.6° still visible at 150 °C for samples on hydrophilic substrates disappear at 200 °C and the background significantly increases, suggesting a predominantly amorphous growth of PCBM aggregates, in agreement with the reported loss of crystalline order in strongly phase separated thin films.36,37 Such a strong microscopic phase separation, indeed, was shown to occur for the P3HT/PCBM films on hydrophilic substrates at 200 °C, as above demonstrated by optical micrographs (see Figure 1). In turn, the very low crystallinity of PCBM induced by the phase separation can be explained in terms of a fast phase separation process that does not allow an efficient crystallization. On the contrary, no marked change in PCBM crystallinity, according to the GIXRD peak shape and background, occurs for films on the hydrophobic substrate by changing the annealing temperature. This is fully consistent with the lack of significant microscopic phase separation in the 150−200 °C range, already revealed by optical microscopy data. On the other hand, the high degree of PCBM phase crystallinity corresponds to the nanometric phase separation confirmed by the above-reported TEM results (see Figure 2) Figure 8 reports the rocking curves of the ⟨100⟩ P3HT peak. They give an insight on the in-plane orientation of the polymeric crystallites, as a function of both substrate and annealing temperature. In fact, the intensity of the rocking peak is a function of the number of planes which are in the right position to give rise to scattering. Thus, keeping the incident angle fixed, positioning the detector at a given angle, the one corresponding to the ⟨100⟩ peak in our case, and by gently rocking the detector around a small angle range, we can determine the amount of “contributing” scatterers present in our sample. More precisely, the mosaic spread of the sample can be easily quantified. For this purpose, both the peak width

Figure 8. Rocking curves of ⟨100⟩ peaks of P3HT:PCBM films deposited on hydrophilic substrates unannealed and annealed at 150 and 200 °C (a) and of P3HT:PCBM films deposited on hydrophobic substrates unannealed and annealed at 150 and 200 °C (b).

and the signal-to-noise ratio are diagnostic of the degree of “edge-on” orientation of the lamellar stacking. In particular, the signal-to-noise ratio is taken as the ratio between the number of counts at the maximum of the peak and the average count number of the background. Indeed, the peak corresponds to the perfectly aligned crystalline planes, while the background intensity is produced by randomly tilted planes. Accordingly, as shown in Figure 8 for unannealed films, poor orientation is found for both substrates, indicated by the low values of signal-to-noise ratios, respectively, about 4 for hydrophobic and 2 for hydrophilic substrates. The rocking curves of annealed samples on hydrophobic substrate show lower noise and a sharper peak with respect to the unannealed case, while on hydrophilic substrates, upon annealing, the rocking curves show a noise increase. Hence, the annealing treatment induces a strongly anisotropic, that is, perpendicular to the substrate plane, lamellar growth on the hydrophobic substrates. On the hydrophilic one, instead the increase of the amount of polymeric crystallites, caused by the above-mentioned thermal annealing, is due to the formation of randomly in-plane oriented lamellae. Interface Free Energy. As reported above, PSD analysis of AFM data clearly indicates that the in-plane order of lamellae at the film surface is roughly identical for all the investigated samples by changing both substrate and temperature. On the other hand, the XRD rocking curves clearly show that different degrees of in-plane ordering arise depending on the substrate. Accordingly, the comparison of the two sets of data suggests that different ordered phases are mostly localized at the buried film−substrate interface, with the differences being clearly due 5263

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higher chain mobility leading to more efficient crystallization and, in turn, to a better in-plane orientation.15 On the other hand, for PCBM it is known that not only its diffusion is responsible of phase separation32 but also that the light PCBM molecules tend to quickly migrate away from hydrophilic substrates upon thermal annealing toward the atmosphere/film interface.30 It is to mention that Friend and co-workers reported the formation of a PCBM “wetting” layer on top of a PEDOT:PSS hydrophilic film.20 The different results in the present paper may be ascribed to the very different chemical nature of the employed substrates, and particularly to the fact that the hydrophilic surfaces in our case are purely inorganic. Our results can be explained in terms of simple minimization arguments for the total IFE of the blend/hydrophilic substrate. This factor, indeed, produces a strong PCBM concentration increase at the film/air interface, promoting the formation of large microscopic aggregates. Straightforwardly, the reverse behavior is observed for hydrophobic substrates, where the lower IFE promotes the nanocrystalline phase separation of PCBM molecules at the blend/substrate interface, revealed by the TEM and GIXRD results above-described. It is worthwhile noting that the IFE at the blend/hydrophobic substrate interface is not sufficiently high to force the quick migration of the PCBM molecules toward the film/air interface. In other words, it does not cause the microscopic speciation observed for hydrophilic substrates. Thus, the above-reported experimental results clearly demonstrate the driving action of the interfacial free energy for hydrophilic and hydrophobic substrates, and its role in inducing a complex nanostructuring event cascade, involving the direct link among PCBM diffusion, phase separation and stratification of the PCBM:P3HT blends. A last, still controversial, point concerns the mechanism underlying the crystallization behavior depending on the substrates and the role of PCBM:P3HT phase separation on the crystalline features. With respect to this point, in agreement with literature,43,44 we argue that the speed of the migration processes of PCBM, depending on the IFE, affects the extent of phase separation and, in turn, the crystallization process of the two components of the blend, i.e., the number and width of the P3HT lamellae and the aggregation of PCBM molecules in nano- or microcrystallites. As already reported the crystallization of the components of a polymer blend is a process which is often subsequent to demixing20,45−47 and is strongly affected by this step in terms of kinetics and morphology. In this framework, the quick diffusion of PCBM toward the film/air interface, induced by the high IFE substrate, should be responsible of the lower crystallinity of the PCBM component, demonstrated for the hydrophilic substrates. On the other hand, the lower IFE between PCBM and hydrophobic substrate does not provide a sufficient driving force for the speciation of PCBM at the film/air interface. Thus, the higher PCBM

to the substrate chemical structure, that is, by the related interfacial interactions. Figure 9 shows a schematic picture of the structural evolution induced by thermal annealing on the two different substrates.

Figure 9. Schematic structural evolution of P3HT:PCBM thin films on the two different substrates. The hydrophobic substrates promote the formation of highly crystalline nanoscopic PCBM aggregates as well as high texture of P3HT lamellae. The hydrophilic substrate induces segregation of predominantly amorphous microscopic PCBM aggregates at the film/air interface and low in plane orientation of polymeric lamellae.

In order to quantitatively evaluate these interfacial interactions, the surface free energy (SFE) of the two substrates and thin films of each of the blend components were measured by employing the three liquids technique (see Experimental Section). From these values, the interface free energies (IFEs) between molecules and the two substrates were in turn calculated by using the Fowkes−Van Oss−Chaudurhy−Good (FOCG) model,38 which exploits the Good−Girifalco−Fowkes combining rule39 with suitable expressions for the Lewis acid− base interactions across the interface. 40 The resulting experimental values are reported in Table 2. It must be recalled that, as previously noted, the results are obtained under the assumption that the data, obtained experimentally at room temperature, undergo linear changes at the annealing temperatures.41,42 The IFE values are about 1 order of magnitude lower for both PCBM and P3HT on OTS than on SiOx substrates. This fact allows an explanation of the different degree of phase separation as well as the relative lamellar in-plane ordering on the two substrates. Indeed, as to P3HT, the lower IFE evaluated for the hydrophobic substrates prompts, upon annealing, a

Table 2. Calculated surface Free Energy Lifshitz−van der Waals (γLW), Acid (γ+), and Basic (γ−) Components of the Four Solid Surfaces and Calculated Interfacial Free Energies (IFEs) between the P3HT Film and the Two Substrates (γS‑P3HT) and between PCBM and the Two Substrates (γS‑PCBM) γLW (mN/m) SiOx OTS P3HT PCBM

40.28 22.59 27.23 39.97

± ± ± ±

0.16 1.04 0.04 0.41

γ+ (mN/m) 2.78 0.10 0.09 0.00

± ± ± ±

0.05 0.02 0.02 0.00

γ− (mN/m)

γS−P3HT (mN/m)

γS‑PCBM (mN/m)

± ± ± ±

12.64 ± 3.28 0.16 ± 0.09

14.36 ± 0.84 1.71 ± 1.07

47.11 1.87 6.99 6.52 5264

0.15 0.22 0.13 0.20

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material is available free of charge via the Internet at http:// pubs.acs.org.

crystallinity, demonstrated by the GIXRD diffractograms, would be determined by the slower PCBM reorganization across the whole film. In other words, the kinetics of the P3HT/PCBM phase separation should act as the controlling factor of the overall crystallinity of the system, including both the formation of PCBM nanocrystallites as well as the texture of the P3HT lamellae. It is important to stress that, as far as the data we are considering in the present paper essentially pertain to equilibrium thermodynamical states, they do not allow the derivation of kinetical arguments. Therefore, further studies have to be performed before a reliable mechanism relating the crystallinity features and the velocity of phase separation can be established.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support of PON 2011 “Photovoltaic Systems and conversion electronics with innovative materials” (MIUR − Rome). The Deutscher Akademischer Austauschdienst (DAAD), VIGONI-Programme, Grant Number D/07/ 15305, is acknowledged for supporting the researcher exchange travels.

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CONCLUSIONS The interfacial interactions between P3HT/PCBM thin films and, respectively, hydrophilic and hydrophobic substrates have been shown to induce different nanostructures of thermally annealed films. Indeed, below the melting temperature of the polymer, the extent of phase separation is lower on hydrophobic substrates than on hydrophilic ones, accompanied by the formation of homogeneously distributed highly crystalline nanometric PCBM aggregates and highly in-plane oriented P3HT lamellae. On the contrary, a stronger phase separation is found for the P3HT/PCBM thin films on hydrophilic substrates at the same temperatures, including the formation of microscopic mostly amorphous PCBM aggregates and predominantly tilted P3HT lamellae. These results therefore provide convincing experimental evidence on the effect that interfacial interactions have on the structuring process of P3HT/PCBM thin films during and after thermal annealing, supporting recent literature pointing to the substrate-induced spinodal decomposition waves in unannealed films. Furthermore, the experiments performed at several annealing temperatures in our case allow one to rationalize the P3HT/PCBM thin film reorganization upon thermal annealing in terms of the nanoscopic aggregation of PCBM small molecules in high mobility conditions, that is, on hydrophobic substrates. The described effects have been accounted for in terms of the relative IFE for the two blend/substrate systems. In fact, a lower IFE (for hydrophobic substrates) is seen to prompt higher mobility of P3HT chains, inducing a higher in-plane orientation, as well as a reduced repulsion between PCBM and the substrate, preventing the PCBM migration toward the film/ air interface and allowing the nucleation of highly dispersed PCBM nanocrystallites. Obviously, the reverse behavior is then expected for the high IFE system, involving the hydrophilic substrates. Finally, it is to stress that the reported results clearly demonstrate how IFE engineering can play a key role in modulating the nanostructure and nanocrystallinity of polymeric blend thin films and in turn in optimizing the 3D structure of polymeric blend-based devices and tailoring their functional properties, as, for instance, charge carrier generation, light absorption yield, as well as the charge mobility.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Simulated powder pattern and list (hkl vs F2) of Bragg peaks of monoclinic and triclinic crystalline form of PCBM. This 5265

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