Phase Separation and Molecular Intermixing in Polymer–Fullerene

Letter pubs.acs.org/JPCL

Phase Separation and Molecular Intermixing in Polymer−Fullerene Bulk Heterojunction Thin Films Matthias A. Ruderer,† Robert Meier,† Lionel Porcar,‡ Robert Cubitt,‡ and Peter Müller-Buschbaum*,† †

Physik-Department, Lehrstuhl für Funktionelle Materialien, Technische Universität München, James-Franck-Str. 1, 85748 Garching, Germany ‡ Institut Laue Langevin (ILL), 6 Jules Horowitz, 38042 Grenoble, France S Supporting Information *

ABSTRACT: The phase separation and molecular intermixing in poly(3hexylthiophene) (P3HT)/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunction thin films are investigated as a function of the overall PCBM content. The structural length scales, phase sizes, and molecular miscibility ratio in bulk heterojunction films are probed with grazing incidence small-angle neutron scattering (GISANS). The PCBM content is varied between 9 and 67 wt %. For the symmetric P3HT/PCBM ratio, which is typically highly efficient in photovoltaic devices, a structure size of 20 nm, the largest PCBM phases, and 18 vol % dispersed PCBM in the amorphous P3HT phase are found. The molecularly dispersed PCBM content is found to increase with the overall PCBM content. Absorption measurements complement the GISANS investigation. SECTION: Macromolecules, Soft Matter

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studies focusing on the diffusion of [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) in poly(3-hexylthiophene) (P3HT) revealed miscibility of both components. Treat et al.13 investigated a bilayer system of P3HT as the top layer and PCBM as the bottom layer with dynamic secondary ion mass spectroscopy (DSIMS), electron microscopy, and grazing incidence wide-angle X-ray scattering (GIWAXS). They found a very fast diffusion of PCBM molecules in the amorphous part of the P3HT layer due to thermal annealing. Annealing at 150 °C below 1 min resulted already in a totally interpenetrated P3HT layer. Chen et al.14 investigated the same bilayer system with the same annealing procedure and similar methods to determine the PCBM diffusion. In addition, they found that the structural length scales of the annealed bilayer system are comparable to the blended system. Although both investigations agree in the fast diffusion process in the amorphous part of P3HT, the details of the PCBM diffusion process differ. Collins et al.15 used near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, SIMS, and GIWAXS to investigate the PCBM content in P3HT for the blended as well as the bilayer systems for different annealing temperatures. They found a mixing of PCBM in the amorphous regions of P3HT. Thus, in summary a fullerene/polymer system is not composed of two pure phases but also contains an intermixed phase of amorphous polymer and fullerene molecules.13−15 While the investigations mentioned above addressed mainly the diffusion and the miscibility of PCBM in the P3HT phase, we focus on the detailed structure, that is, phase sizes, structural

he performance of solution-processable organic photovoltaic devices improved continuously during the last year1−3 leading to a power conversion efficiency of 10.0%.4 Major improvements were achieved with the bulk heterojunction (BHJ) concept5,6 due to the increased inner acceptor− donor interface and with the development of new small band gap polymers.7 However, despite the increase in efficiencies, deep fundamental understanding is still limited. It is commonly accepted that the internal structure in the active layer of an organic solar cell is crucial for the performance of a photovoltaic device. For an efficient organic solar cell, a twocomponent system is required to separate the excitons and to generate free charge carriers, and structural length scales are needed in the range of the mean exciton diffusion length. Due to the self-assembly nature of the BHJ concept, the structure can only be controlled indirectly by external parameters such as the chosen material system, solvent, annealing conditions, and so forth.8 As the desired structure is typically a nonequilibrium state, the preparation history defines the resulting structure. The most successful organic solar cells contain a fullerene derivative as the electron acceptor and a conjugated polymer as the electron donor.1−3,7 When describing the device physics of such organic photovoltaic devices, models mainly rely on the assumption that the components exist as two separated pure phases.9 Furthermore, structural investigations of the active layers with, for example, scattering techniques so far assume pure phases only.10,11 First investigations of all-polymer solar cells with scanning transmission X-ray microscopy (STXM) revealed mixed phases.12 However, the resolution of STXM is limited, and molecular intermixing cannot be separated from phases in which small objects, that is, with a size below the resolution limit, are mixed with the second material. Recent © 2012 American Chemical Society

Received: January 10, 2012 Accepted: February 21, 2012 Published: February 21, 2012 683

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estimated. We determine a crystallinity between 60 and 40%, which decreases with increasing PCBM content. The crystallinity at around 50% is in good agreement with literature.21 However, the uncertainty for the determination of the crystallinity is about 15%. The high uncertainty results from the fitting of the vibronic transitions. Consequently, the amount of PCBM molecules does not influence the conjugation length of the P3HT chains but slightly hinders the P3HT crystallization. This finding is in contradiction to the studies on PCBM diffusion described above, which concluded that the P3HT crystals are not influenced by the PCBM.13−15 However, the systems in our study are BHJ systems, that is, P3HT and PCBM are already mixed in solution. Therefore, during spincoating PCBM can already influence the initial P3HT ordering. On the contrary, for the investigations using bilayer systems,13−15 P3HT orders undisturbed before annealing. The internal structure of the P3HT/PCBM BHJ films is determined with GISANS. Neutrons have already been used before to investigate the vertical material compositions with reflectivity22,23 or the lateral structure in the transmission geometry of P3HT/PCBM systems.14 For such systems, the scattering in grazing incidence geometry was reported only for X-rays so far.10,24 The main advantage of using neutron scattering is that standard substrates are typically transparent for neutrons. Therefore, neutrons are more sensitive to the material composition of the investigated sample. Compared to transmission and reflectivity experiments, the grazing incidence geometry accesses both lateral and vertical structures. In addition, neutrons interact with the atomic nucleus, and therefore, degeneration does not occur during the scattering experiment as it can occur for X-ray scattering. In the case of P3HT/PCBM systems for neutrons, the scattering length density (SLD) difference between P3HT (0.83 × 10−6 Å−2) and PCBM (4.3 × 10−6 Å−2) is sufficiently high, and no additional deuteration of one component is necessary. From the two-dimensional (2d) GISANS data (Figure 2 left column), lateral structures (horizontally scattered intensity) as well as material compositions (vertically scattered intensity) can be extracted. For a detailed quantitative analysis, we use the program IsGISAXS18 to simulate the scattering pattern of a given structural model in the framework of the DWBA. IsGISAXS was already used successfully to analyze GISAXS data, for example, of nanoparticles on surfaces,18,25 the evolution of aluminum electrodes on conducting polymers,26 as well as the structure of the blend of MEH-PPV with PVK for different film thicknesses.24 Most other models used for modeling GISAS data are adapted from transmission scattering experiments and therefore do not account for reflection−refraction effects.27 Consequently, exact modeling of a highly complicated system, for example, a mixture of small molecules (PCBM) with a semicrystalline polymer (P3HT), is not possible with these models. To fit the 2d GISANS data, PCBM objects with a certain size and distance in a P3HT matrix as well as bigger PCBM domains are chosen. The used model (see Figure 3) and its limitations are described in detail in the Supporting Information. The simulation and the data (see Figure 2) are in good agreement with each other. With increasing PCBM content, scattering in the horizontal direction (qy direction) is due to the formation of objects (PCBM domains) at a certain distance from each other. As a result of a larger distribution of the object sizes, the horizontal scattering gets more pronounced and

length scales, and molecular miscibility of the components, in a P3HT/PCBM BHJ. Grazing incidence small-angle neutron scattering (GISANS)16 combined with a detailed analysis is used to extract structural information as well as molecular mixing in P3HT/PCBM blend films. The analysis is based on the distorted wave Born approximation (DWBA)17 by applying the program IsGISAXS.18 Thin films are prepared via spincoating with subsequent thermal annealing at 140 °C. A broad mixing regime from 9 to 67 wt % PCBM content is investigated, whereas previous investigations on the influence of blending ratios of P3HT/PCBM with scattering techniques exploited only a very narrow blending regime and used very basic analyzing methods.11 In Figure 1, the absorbance of annealed P3HT/PCBM BHJ films for different PCBM contents is shown. The peak at

Figure 1. Wavelength-dependent absorbance spectra of P3HT/PCBM films with different PCBM content of 9 (cyan), 25 (green), 33 (red), 50 (black), and 67 wt % (magenta).

around 335 nm is attributed to PCBM, whereas the features above 400 nm result from P3HT. While the PCBM peak position does not change, the P3HT contribution shifts to a smaller wavelength with increasing PCBM content. A shift to smaller wavelengths in the absorbance spectra is due to an increase of the energy band gap, which is directly connected to the extension of the π-electron system on the P3HT molecules. Consequently, the conjugation length or the crystallinity of the P3HT molecules decreases for higher PCBM contents. Gao et al. attributed the blue shift to an increase of amorphous P3HT chains due to PCBM molecules disrupting π−π-stacking.19 In addition, all curves show vibrational excitations at 550 and 605 nm, which are attributed to crystalline P3HT phases. Further analysis of the absorption data using a weakly coupled H-aggregate model reveals information on the P3HT aggregation. The 0−0 and 0−1 vibronic transitions, that is, the absorption features at 605 and 550 nm, are fitted using this model. The details in fitting can be found elsewhere.19,20 From the 0−0/0−1 intensity ratios, the exciton bandwidth can be calculated, which is indicative of the conjugation length. A small exciton bandwidth is attributed to a large conjugation length. For the systems investigated in this study, the exciton bandwidth was found to be constant at about 270 meV, and therefore, also the conjugation length is constant for different PCBM content. Consequently, the blue shift of the main absorption peak has to be a consequence of less order in P3HT with increasing PCBM content. Gao et al. also compared the integrated areas from the contribution of aggregated and nonaggregated P3HT chains to the absorption spectra.19 From this ratio, the relative amount of crystalline and amorphous P3HT, that is, crystallinity, can be 684

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Figure 3. Schematic presentation of the model used for the GISANS simulations. The model is based on PCBM objects (gray) with a distance between adjacent domains (denoted structure sizes) in a P3HT matrix (red chains) with molecularly disperse PCBM. For clarity, the large domains are not shown.

shown in Figure 4. The complex interplay of the parameters in the model (solid line) fit the GISANS data (symbols) very well.

Figure 2. 2d GISANS data (left column) and corresponding 2d IsGISAXS simulations (right column) of P3HT/PCBM BHJ films with different PCBM content. The PCBM content increases from the bottom to top, as indicated. For all patterns, the same intensity color coding is used.

Figure 4. Horizontal (a) and vertical (b) line cuts of the GISANS data (symbols) and corresponding fits (solid lines) of P3HT/PCBM films. The PCBM content increases from the bottom to top. The Yoneda peak positions are highlighted by arrows. The curves are shifted along the y-axis for clarity of presentation.

narrows with higher PCBM content. The scattering in the vertical direction is dominated by two peaks, namely, the specular reflection (highest intensity, not included in the simulation) and the Yoneda peak (peak below the specular peak) (see also Figure S1, Supporting Information). The latter is a material-sensitive peak (a mechanistic description is given in the Supporting Information) and displays the material composition in the investigated sample. The position of the Yoneda peak shifts to higher qz values with increasing PCBM content and represents therefore the increase of the PCBM content. All of these described features are well represented by the simulations (Figure 2 right column). The exact Yoneda peak position is given by the actual molecular PCBM distribution in the P3HT matrix applied for the model. This includes the object sizes and the structural lengths as well as the SLDs of the phases. To draw an evident conclusion, the resulting simulation has to fit the horizontal as well as the vertical scattering simultaneously. Therefore, horizontal and vertical line cuts of the GISANS data are

The large PCBM domains contribute to the scattering in the horizontal line cuts (Figure 4a) at qy < 0.03 nm−1, and the small PCBM domains contribute to the intensity at higher qy values. The distance between these small PCBM domains is represented by the peaks at qy = 0.3 nm−1. The extracted parameters are presented in Figure 5a. The given error bars represent the distribution of the corresponding length scales put into the model. The structure size, resembling the distance between PCBM domains, is found to be broadly distributed around 80 nm for systems with a small PCBM content of 9 wt %. For the systems with higher PCBM content, the distance was revealed to be constant at about 20 nm, which is in the range of the exciton diffusion length and therefore beneficial for charge generation. While most analyzing methods are just able to extract structural length scales, the use of 685

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dispersed PCBM content is higher. From the UV/vis data analysis, we found an estimated crystallinity between 40 and 60%, and thus, a molecularly dispersed PCBM content (in the amorphous P3HT phase) between 10 and 35 vol %, respectively. The upper limit is in the range of previous reports.13,15 This value is in the region that was found to be within the miscibility regime (>42 vol % P3HT) of P3HT/ PCBM.28 However, Yin et al. reported a miscibility limit of 20 vol % of PCBM in P3HT, which is challenged by our investigation. 29 Independent of the actual amount of molecularly dispersed PCBM in the P3HT phase, single dispersed PCBM molecules will on the one hand enhance exciton separation but on the other hand act as traps and increase geminate recombination. Further investigations are needed to understand the influence of mixed phases on the performance on photovoltaic devices. In conclusion, we use GISANS measurements analyzed in the framework of the DWBA to reveal the detailed morphology of P3HT/PCBM BHJ films with different PCBM content, including structural length scales and domain and object sizes. For the 1:1 blending ratio, a distance of PCBM domains in the range of the exciton diffusion length and the largest PCBM phases are found. In addition, fitting of the scattering pattern exhibited molecularly dispersed PCBM in the P3HT phase. A molecularly dispersed PCBM content of up to 35% in the amorphous P3HT phase is revealed using the estimated crystallinity extracted from the weakly coupled H-aggregate model analysis of the absorption data. In addition, an increasing molecularly dispersed PCBM content as a function of the overall PCBM content is observed. Consequently, the idea of only pure phases in polymer/fullerene systems can be ruled out for PCBM concentrations at least for systems with a blending range from 9 to 67 wt % PCBM.

Figure 5. (a) Distances (structure size) and small (object radius) and large PCBM domains (domain radius) used for the simulations of the GISANS data depending on the PCBM content. The error bars represent the distribution of these lengths. (b) Fraction of the molecularly dissolved PCBM in the P3HT phase depending on the overall PCBM content. The solid lines are guides to the eyes.

advanced modeling reveals also object information.18 To fit the scattering data of such a complex system, up to three object types that all resemble PCBM domains have to be put into the model. For all blend ratios, large PCBM domains (Figure 5a, triangles) with a radius of 100−200 nm are detected. However, theses domains occur only rarely, which is expressed in a probability smaller than 0.05%. Highly probable (>90%) are smaller PCBM domains (Figure 5a, squares), dominating the morphology. Their radius ranges from 3 to 10 nm with the maximum object size at a PCBM content of 50 wt %, which is in the range of the blending ratio where typically the most efficient P3HT/PCBM solar cells are found. For the systems with 25 and 33 wt % PCBM content, an additional PCBM domain with a radius of 18 and 16 nm and with a low probability of 1 and 8%, respectively, is needed to describe the scattering data adequately. Such an additional PCBM domain size might resemble the nonequilibrium morphology. However, to fit the data and, in particular, the materialsensitive Yoneda peak position, conclusively, the SLD of the P3HT matrix has to be increased. An increase of the SLD of the matrix can only be explained by molecularly dispersed PCBM in the P3HT phase. In Figure 5b, the volume fraction of the molecularly dispersed PCBM γPCBM is plotted as a function of the overall PCBM content. For the 9 wt % PCBM content films, a very low amount of molecularly dispersed PCBM that lies in the resolution limit (about 1.5 vol %) is found. However, P3HT/PCBM films with a higher overall PCBM content show molecularly dispersed PCBM between 5 and 15 vol %. As we cannot distinguish with GISANS between the amorphous and the crystalline phases of P3HT, and PCBM is most probably embedded in the amorphous phase, the actual molecularly

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EXPERIMENTAL SECTION Sample Preparation. Poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C61 butyric acid methyl ester (PCBM) were purchased from Rieke Metals Inc. and Nano-C Inc., respectively. Both components were dissolved in chlorobenzene (CB, SigmaAldrich) and mixed with different ratios (9, 25, 33, 50, and 67 wt %) of PCBM. By adapting the concentration, thin films with a constant thickness of about 300 nm were prepared on acidic precleaned silicon substrates30 via spin-coating. The films were annealed at 140 °C for 10 min in an inert gas atmosphere. Absorption. Absorption data were taken with a UV/vis spectrometer (Lambda35, PerkinElmer) in a wavelength range from 260 to 1100 nm. The measurements were performed in transmission geometry. Structural Characterization. The structural characterization was carried out with grazing incidence small-angle neutron scattering (GISANS) at the small-angle diffractometer D22 at the Institut Laue-Langevin (ILL), Grenoble.16 The collimation length and the sample−detector distance were set to 17.6 and 11.2 m, respectively. The incident angle was chosen to be 0.6° and was therefore above the critical angle of the investigated materials. Consequently, the full film thickness was probed. A wavelength of λ = 0.6 nm with Δλ/λ = 10% was used. The resolvable structure size (structure factor) ranged from 15 nm to almost 1 μm. However, object sizes (form factor) of 2−3 nm and bigger are accessible at these settings, and consequently, PCBM aggregations of more than three PCBM molecules (the size of a PCBM molecule is about 1 nm) are detected as objects. The scattered intensity was recorded with a large-area 686

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multidetector (3He) consisting of 128 linearly sensitive Reuter−Stokes detector tubes. For data analysis, the program IsGISAXS was used and adapted to neutron scattering.18 IsGISAXS is based on the distorted wave Born approximation (DWBA),17 taking the form and structure factor of particles embedded in a film as well as reflection−refraction effects into account. IsGISAXS simulates the GISANS pattern that results from a given model. Line cuts as well as a 2d pattern can be fitted. The fits of the vertical line cuts are a combination of the IsGISAXS fits and additional contributions of the specularly reflected (Voigt) and the transmitted (Gaussian) signal, which are not included in the IsGISAXS program. Details of the used model are discussed in the main section and in the Supporting Information.

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Heterojunction Materials: Implications for Charge Transport. Adv. Mater. 2007, 19, 3656−3659. (10) Ruderer, M. A.; Guo, S.; Meier, R.; Chiang, H.-Y.; Körstgens, V.; Wiedersich, J.; Perlich, J.; Roth, S. V.; Müller-Buschbaum, P. SolventInduced Morphology in Polymer-Based Systems for Organic Photovoltaics. Adv. Funct. Mater. 2011, 21, 3382−3391. (11) Chiu, M.-Y.; Jeng, U.-S.; Su, M. S.; Wei, K.-H. Morphologies of Self-Organizing Regioregular Conjugated Polymer/Fullerene Aggregates in Thin Film Solar Cells. Macromolecules 2010, 43, 428−432. (12) McNeill, C. R.; Watts, B.; Thomsen, L.; Ade, H.; Greenham, N. C.; Dastoor, P. C. X-ray Microscopy of Photovoltaic Polyfluorene Blends: Relating Nanomorphology to Device Performance. Macromolecules 2007, 40, 3263−3270. (13) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Adv. Energy Mater. 2011, 1, 82−89. (14) Chen, D.; Liu, F.; Wang, C.; Nakahara, A.; Russell, T. P. Bulk Heterojunction Photovoltaic Active Layers via Bilayer Interdiffusion. Nano Lett. 2011, 11, 2071−2078. (15) Collins, B. A.; Gann, E.; Guignard, L.; He, X.; McNeill, C. R.; Ade, H. Molecular Miscibility of Polymer−Fullerene Blends. J. Phys. Chem. Lett. 2010, 1, 3160−3166. (16) Metwalli, E.; Moulin, J.-F.; Gebhardt, R.; Cubitt, R.; Tolkach, A.; Kulozik, U.; Müller-Buschbaum, P. Hydration Behavior of Casein Micelles in Thin Film Geometry: A GISANS Study. Langmuir 2009, 25, 4124−4134. (17) Rauscher, M.; Salditt, T.; Spohn, H. Small-Angle X-ray Scattering under Grazing Incidence: The Cross Section in the Distorted-Wave Born Approximation. Phys. Rev. B 1995, 52, 16855− 16863. (18) Lazzari, R. IsGISAXS: a Program for Grazing-Incidence SmallAngle X-ray Scattering Analysis of Supported Islands. J. Appl. Crystallogr. 2002, 35, 406−421. (19) Gao, Y.; Martin, T. P.; Niles, E. T.; Wise, A. J.; Thomas, A. K.; Grey, J. K. Understanding Morphology-Dependent Aggregation Properties and Photocurrent Generation in Polythiophene/Fullerene Solar Cells of Variable Compositions. J. Phys. Chem. C 2010, 114, 15121−15128. (20) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C. Role of Intermolecular Coupling in the Photophysics of Disordered Organic Semiconductors: Aggregate Emission in Regioregular Polythiophene. Phys. Rev. Lett. 2007, 98, 206406. (21) Pascui, O. F.; Lohwasser, R.; Sommer, M.; Thelakkat, M.; Thurn-Albrecht, T.; Saalwächter, K. High Crystallinity and Nature of Crystal−Crystal Phase Transformations in Regioregular Poly(3hexylthiophene). Macromolecules 2010, 43, 9401−9410. (22) Parnell, A. J.; Dunbar, A. D. F.; Pearson, A. J.; Staniec, P. A.; Dennison, A. J. C.; Hamamatsu, H.; Skoda, M. W. A.; Lidzey, D. G.; Jones, R. A. L. Depletion of PCBM at the Cathode Interface in P3HT/ PCBM Thin Films as Quantified via Neutron Reflectivity Measurements. Adv. Mater. 2010, 22, 2444−2447. (23) Kiel, J. W.; Kirby, B. J.; Majkrzak, C. F.; Maranville, B. B.; Mackay, M. E. Nanoparticle Concentration Profile in Polymer-Based Solar Cells. Soft Matter 2010, 6, 641−646. (24) Meier, R.; Ruderer, M. A.; Kaune, G.; Diethert, A.; Körstgens, V.; Roth, S. V.; Müller-Buschbaum, P. Influence of Film Thickness on the Phase Separation Mechanism in Ultrathin Conducting Polymer Blend Films. J. Phys. Chem. B 2011, 115, 2899−2909. (25) Laoufi, I.; Saint-Lager, M.-C.; Lazzari, R.; Jupille, J.; Robach, O.; Garaudee, S.; Cabailh, G.; P. Dolle, P.; Cruguel, H.; Bailly, A. Size and Catalytic Activity of Supported Gold Nanoparticles: An in Operando Study during CO Oxidation. J. Phys. Chem. C 2011, 115, 4673−4679. (26) Kaune, G.; Metwalli, E.; Meier, R.; Körstgens, V.; Schlage, K.; Couet, S.; Röhlsberger, R.; Roth, S. V.; Müller-Buschbaum, P. Growth and Morphology of Sputtered Aluminum Thin Films on P3HT Surfaces. ACS Appl. Mater. Interfaces 2011, 3, 1055−1062. (27) Collins, B. A.; Tumbleston, J. R.; Ade, H. Miscibility, Crystallinity, and Phase Development in P3HT/PCBM Solar Cells:

ASSOCIATED CONTENT

S Supporting Information *

Detailed description of the IsGISAXS model used. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 89 289 12451. Fax: +49 89 28912473. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.A.R. thanks the Bavarian State Ministry of Sciences, Research and Arts for funding this research work through the International Graduate School “Materials Science of Complex Interfaces” (CompInt).

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