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Shanju Zhang, Candice I. Pelligra, Gayatri Keskar, Jie Jiang, Pawel W. Majewski, André D. Taylor, Sohrab Ismail-Beigi, Lisa D. Pfefferle, and Chinedum O. Osuji*

Directed Self-Assembly of Hybrid Oxide/Polymer Core/Shell Nanowires with Transport Optimized Morphology for Photovoltaics

Hybrid organic-inorganic solar cells are promising for the development of next generation low-cost, high efficiency photo-voltaics (PVs). They combine the facile solution processability, large optical extinction coefficients, and good hole mobility of conjugated polymers with the high electron affinity and elec-tron mobility of inorganic nanoparticles.[1] The most common configuration for effectively generating photocurrent with these materials is the bulk heterojunction (BHJ) device where intimate mixing of the polymer with inorganic nanoparticles generates a random bicontinuous morphology with nanometer-scale dimensions. The morphology and ultimately the perform-ance of this photoactive layer depend on a complex interplay of materials parameters and processing conditions such as tem-perature, polymer-solvent and particle-solvent interactions, sol-vent evaporation rate, solution composition (polymer volume fraction), and post-deposition treatments.[2] In particular, the role of polymer–particle miscibility has been well highlighted by recent reports.[3] Although there has been steady progress in improving device efficiency, the inherently disordered and kinetically-dictated structure of BHJ devices is suboptimal, particularly in terms of charge transport, but also in exciton utilization.[1a,4] An ideal hybrid device is one in which donor and acceptor materials are arranged in a densely packed vertical array. Nanometer-scale periodicity would minimize radiative decay of excitons, and the vertical alignment of the materials ensures a non-tortuous path for charge transport.[1a,4a,5] Such an ordered BHJ motif (OBHJ) has thus been the focus of recent interest,[6] but its realization, particularly in a manner compat-ible with low-cost fabrication of devices, remains elusive. A further refinement of the OBHJ entails atomic and molecular-scale control of the donor and acceptor materials such that their

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S. Zhang,[+] C. I. Pelligra, G. Keskar, P. W. Majewski, A. D. Taylor, L. D. Pfefferle, C. O. OsujiDepartment of Chemical and Environmental EngineeringYale University 9 Hillhouse Ave., New Haven, CT 06511, USA E-mail: chinedum.osuji@yale.eduJ. Jiang, S. Ismail-BeigiDepartment of Applied Physics Yale University New Haven, CT 06511, USA[+] Present address: Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, CA 93407

DOI: 10.1002/adma.201103708

fast-conduction axes are coincident with the non-tortuous vertical paths in the device. Taken together, these desired characteristics present significant challenges that require novel bottom-up approaches to provide the required order on multiple length scales.

Here, we propose a scheme to enable such multiscale control of structure in hybrid photovoltaic devices and we demonstrate its utility using ZnO and polythiophene derivatives as model materials. The concept is illustrated in Scheme 1. It is based on the synthesis of inorganic oxide/conjugated polymer core/shell nanowires (NWs). These nanoscale photovoltaic objects represent basic elements, which can then be arrayed vertically to arrive at the desired OBHJ motif. Three features are essen-tial. At the atomic level, the oxide nanowires are solvothermally produced as single crystals and thus offer the potential for elec-tron transport without loss of mobility due to grain boundary scattering. At the molecular level, the semiconducting polymer shell is ordered by side-on attachment of the polymer back-bone to the nanowire surface. For small diameter nanowires, this induces a coaxial arrangement of the polymer backbone with the nanowire and thus provides the promise of high hole mobility in the polymer parallel to the nanowire. Finally, at the mesoscale, the high-aspect ratio of the nanowires drives spon-taneous formation of a liquid crystalline mesophase at suffi-ciently high nanowire concentrations. This provides a handle for large-area alignment of the nanowires using magnetic fields or other external forces for low-cost fabrication.

ZnO was prepared using solvothermal synthesis.[7] The resulting NWs are single crystals with a [0001] growth direction (Supporting Information Figure 1). As-synthesized ZnO NWs are polydisperse with diameters of 29 ± 9.5 nm and lengths of 514 ± 423 nm (Supporting Information Figure 2). The NWs were functionalized by direct attachment of a bifunctional ligand, N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TAC) bearing ammonium and silyl groups where the silyl end-groups are reactive towards the ZnO surface (Scheme 1). Poly-thiophene derivatives bearing functional side chains, poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate)] (PTEBS) and poly[3-(potassium-6-hexanoate)thiophene-2,5-diyl] (P3KHT) were grafted onto the functionalized NWs using electrostatic inter-actions to form hybrid core-shell nanostructures (Scheme 1). The composites were isolated by centrifugation followed by removal of the supernatant containing excess polymer. Recovered ZnO-PTEBS and ZnO-P3KHT nanocomposites are red-orange and violet in color, respectively. UV-vis absorption spectra of dried films of the composite NWs show characteristic absorptions

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Scheme 1. Side-on chemical grafting of functionalized poly(alkyl thiophenes) onto ZnO NWs.

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of both ZnO (≈360 nm) and the conjugated polymers (≈450–550 nm) (Supporting Information Figure 3). Charge transfer between the polymer and the ZnO can be deduced from photo-luminescence studies. Measurements on nanocomposites in solution show up to ten-fold quenching of polymer emission relative to the equivalent quantity of pure polymer in solution (Supporting Information Figure 4) suggesting that there is inti-mate contact and effective charge transfer between the polymer shell and the ZnO core.

Chemical grafting of side-functionalized polythiophenes onto ZnO NWs was verified using Fourier transform infrared (FTIR) spectroscopy (Supporting Information Figure 5). New vibration bands are observed in the ranges of 800–1300 cm−1

Figure 1. a–c) HRTEM images of core/shell nanostructures of ZnO-PTEBS and d) possible polymer chain packing across interface.

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(sulfonate group) and 1200–1800 cm−1 (car-boxylate group) for ZnO-PTEBS and ZnO-P3KHT, respectively, suggesting successful coupling of the polymer to the nanowires. Additionally, the methylene group vibration peaks in the range of 2800–3000 cm−1 for both nanocomposites show either new peaks or blue-shifts, indicating ordered methylene structures in the hybrid nanocomposites.[8] The grafting density of the polymer was determined by thermogravimetric analysis (Supporting Information Figure 6). Typical composites exhibit 3.0 wt% and 7.5 wt% polymer loading for ZnO-PTEBS and ZnO-P3KHT, respectively.

High-resolution transmission electron microscopy (TEM) was employed to directly visualize individual core/shell nanowires (Figure 1). The bright-field images clearly show a uniform polymer shell (light con-trast) on the ZnO NW core (dark contrast) with shell thicknesses ranging from 2 to 8 nm (Figure 1a–c). A layered morphology can be readily discerned in the polymer shell. This suggests that the polymer coating the nanowires is well-ordered, with chain back-bones stacked parallel to the NW long axis

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(Figure 1d). This is remarkable considering the fact that both PTEBS and P3KHT do not exhibit observable crystallinity in the bulk, but it is consistent with observations of ordering of crystallizable macromolecules physisorbed on nanomaterial surfaces.[9] The interchain packing distance extracted from high-resolution TEM (HRTEM) is 0.8–0.9 nm. This suggests that backbone stacking in the radial direction is moderated by side chains, as schematically depicted in Figure 1, and not π–π interactions, which would lead to a much smaller interchain distance.[10] The equilibrium thickness in aqueous adsorption of polyelectrolytes at oppositely charged surfaces is a sensitive function of many parameters including polymer and surface charge densities, pH, ionic strength, polymer hydrophobicity,

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Figure 2. a) DSC heating curves of PTEBS and ZnO-PTEBS and b) WAXS data for PTEBS and ZnO-PTEBS

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and polymer concentration. Adsorption beyond a single chain monolayer, in our case the observed lamellar polymer packing at the nanowire surface, may be encountered, in particular where the polymer exhibits partial hydrophobic character, as observed both experimentally[11] and in simulations.[12] While the dependence of layer thickness on adsorption conditions has not been explored systematically here, the partical hydrophobic nature of the PTEBS and P3KHT chains is likely relevant in dictating the equilibrium shell thickness beyond a single mono-layer. It can be expected that conjugated polymer crystallinity at the ZnO interface will provide easy charge transport along the backbone, enabling charge mobility well in excess of that achieved in amorphous material.[13]

The idea that control may be exerted over backbone orien-tation by side-attachment of a polymer on a nanowire surface is intuitively compelling. In the general sense, a surface with a large radius of curvature relative to the polymer persistence length appears to the backbone as a 2D surface against which the polymer can adopt a configuration dictated by random-walk statistics. By contrast, a small radius of curvature relative to the persistence length appears to the backbone as a 1D object, a line, which then constrains the orientation of the backbone to be parallel to the nanowire axis.[14] Recent results from coarse-grained molecular dynamics simulations support this reasoning with phase diagrams constructed as a function of poly mer binding strength and nanowire thickness.[15] We used ab initio density functional theory (DFT) to explore this idea in the context of a side-chain functionalized poly(3-hexylthiophene) (P3HT) binding with the (101̄0) ZnO surface (see Supporting Information for computational details). The mismatch between the 0.78 nm side-chain periodicity and the surface-direction-dependent Zn atom periodicity leads to a competition between backbone strain energy and binding energy to the surface. The main diameter-dependent curvature effects are the bending of the backbone and the change of relative periodicities and the resulting strain energy. We find that the ground-state for large diameters corresponds to helical wrapping around the nanowires whereas for small diameters a coaxial arrangement is optimal. Similar results in terms of a cross-over from helical to on-axis arrangements for diameters below a critical value have also been recovered for poly(ethylene) chains adsorbed on carbon nanotubes.[16] Here, we predict a crossover diameter of roughly 25 nm. Despite the quantitative uncertainty introduced by the particular details of our modeling, these results are qual-itatively consistent with both reported coarse-grained data as well as basic concepts in polymer physics.

We contrast these results and the concept overall with intriguing data recently reported for conjugated polymers that are end-attached to inorganic nanomaterials[17] and where ordered structures are also reported involving hairpin folding of the polymer backbone.[18] We posit that the use of side-attachment as well as non-covalent reversible interactions offer a potentially more robust route to secure coaxial polymer arrangements at the nanowire surface and to provide the con-comitant high hole mobility along the nanowire axial direc-tion. The impact of the side chains on the band alignment in the system is unknown at this point however, and additional studies are needed to explore the charge transport characteris-tics of these materials in detail.

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The presence of ordered polymer structures in the hybrid nanocomposites was further confirmed using differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) (Figure 2). DSC heating plots show that bulk PTEBS exhibits only a glass transition at Tg = 125 °C while ZnO-PTEBS exhibits a strong endothermic peak at a melting temperature Tm = 210 °C (Figure 2a). WAXS shows an amorphous scattering halo at scattered wave vector q = 1.55 Å−1 for bulk PTEBS but strong Bragg reflections at q = 1.28 and 1.51 Å−1 for ZnO-PTEBS (Figure 2b). The corresponding d-spacings for the crystalline peaks are 4.9 and 4.2 Å, respectively. Similar results are also obtained for ZnO-P3KHT (Supporting Information Figure 7).

The core/shell NWs were made dispersible in aqueous media with the aid of tetramethylamonnium hydroxide (TMAH), which is a strong base. The dispersion undergoes phase transi-tions with increasing concentration until a fully homo geneous nematic liquid crystal (LC) phase is attained (Figure 3). The critical concentrations for lyotropic phase transitions were established by examining optical birefringence on increasing nanowire concentration, with the first appearance of birefringent nematic domains indicating the start of the biphasic regime. The size and volume fraction of these domains increased with nanowire concentration until a single phase with nematic order developed beyond a second critical concentration. Schlieren textures with two-fold symmetric black brushes are observed in the LC phase. Representative optical images in each phase for ZnO-PTEBS and ZnO-P3KHT are also shown in Supporting Information Figure 8,9, respectively. Using polarized optical

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Figure 3. Phase diagram of hybrid core/shell NWs in aqueous suspen-sions as a function of concentration. Typical optical images in isotropic, biphasic, and nematic liquid crystalline phases are shown at the top. The letters I, B, and LC represent isotropic, biphasic, and liquid crystalline phases, respectively. ϕ1

* and ϕ2* represent the critical concentrations of

biphasic and single liquid crystalline phase transitions, respectively. Figure 4. Bulk alignment of hybrid core-shell NWs. a) Optical image of ZnO-PTEBS aqueous suspension at 11 vol% concentration under mechanical shear, b) SEM image of the dried film from (a). c,d) SEM images of thin films of Co-doped ZnO-PTEBS aqueous suspension dried under a 6 T magnetic field from initial solutions of different concentra-tions: c) 5.5 vol% and d) 1.1 vol%.

microscopy, we estimated the biphasic range to be between 0.37 vol% and 3.1 vol% for ZnO-PTEBS and between 0.66 vol% and 3.5 vol% for ZnO-P3KHT (Figure 3). Correspondingly, the width of the coexistence regime is around 2.7 vol% and 2.8 vol% for ZnO-PTEBS and ZnO-P3KHT, respectively. These values are larger than the theoretical value of 1.2 vol% for mono disperse hard rod particles.[19] The widening of the biphasic region in both systems is attributed to the polydisper-sity of the NW dimensions and the likely segregation of longer NWs to the growing nematic domains.[7] The local organization of the hybrid NWs in the nematic director field were directly visualized by scanning electron microscopy (SEM) after drying (Supporting Information Figure 10). The remaining cooperative organization of individual hybrid NWs in the solid film demon-strates director patterns around the disclinations of strengths s = ±½ (Supporting Information Figure 10), consistent with the previous optical observations of two-fold symmetric black brushes.

It has long been recognized that lyotropic liquid crystals can be aligned in the bulk under external fields to produce highly ordered thin films. Likewise, mechanical forces can be used to align nanomaterials in polymer nanocomposites.[20] Here, we find that mechanical shear of lyotropic suspensions is suf-ficient to produce large-area, aligned domains of hybrid NWs (Figure 4a,b). Optical contrast of the aligned film under crossed polarizers is maximized when the shear direction lies at 45° to the polarizer (Figure 4a). SEM images of the solid film demon-strate that individual hybrid NWs are densely packed and highly aligned along the shear direction (Figure 4b). Bulk alignment of hybrid NWs under a static 6 T magnetic field was also investi-gated, but no significant effect was observed, likely due to low anisotropy of magnetic susceptibility of pristine ZnO. In order to achieve magnetically driven alignment, the magnetic suscepti-bility and anisotropy of susceptibility of the nanocomposites was increased by using ZnO nanowires synthesized with 5 at% Co-substitution.[21] At 5 at%, the Co does not significantly perturb

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the band structure of ZnO. When exposed to the 6 T static field, large-area monodomains in the modified Co-ZnO system were observed with the long axes of the core/shell nanowires clearly oriented perpendicular to the field (Figure 4c,d), which is consistent with the magnetocrystalline anisotropy previously reported for Co-doped ZnO.[21,22] This result is very promising for low-cost device fabrication given the ease with which magnetic fields may be applied over centimeter to meter length scales in concert with thermal and environmental control.

It is worth noting that the starting suspension concentra-tion was observed to affect magnetic alignment. Evaporating a concentrated suspension (5.5 vol%) under the field leads to near-uniform in-plane alignment (Figure 4c), while much more out-of-plane alignment developed upon evaporation of an ini-tially dilute (1.1 vol%) suspension (Figure 4d). The in-plane and out-of-plane arrangements observed are isoenergetic due to the degeneracy implied by the alignment of a rod-like ele-ment perpendicular to a uniaxial field—all rotational states of the element confined in the plane perpendicular to the field are equally probable. Generation of the ideal OBHJ format requires a uniform out-of plane alignment of the nanocomposites. We have previously shown that degenerate alignments of liquid crystalline mesophases can be conveniently broken by use of sample or field rotation.[23] In this case, production of vertically aligned core/shell nanowires can be effected by simple rota-tion of the field in the plane of the substrate. This is a focus of ongoing work.

In summary, we have demonstrated an entirely bottom-up approach for controlling structure across multiple length scales in organic–inorganic nanocomposites for photovoltaics. We have described the preparation of polythiophene-grafted ZnO

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core/shell nanocomposites, the formation of lyotropic mes-

ophases of these nanocomposites in suspension and their bulk alignment in thin films using mechanical shear and magnetic fields. The attachment of side-functionalized polythiophenes to the single-crystal ZnO NWs produced hybrid core/shell nano-structures in which the otherwise amorphous polymer formed a uniform crystalline layer with the chain backbones extended along the NW long axis. DFT calculations support the idea that the ability to produce coaxial nanowire–polymer backbone arrangements is dictated by the curvature of the nanowire. We expect that this motif of side-attached conjugated polymers offers a convenient means to effectively control the fast con-duction direction in hybrid photovoltaic devices. The ability to magnetically align the core/shell constructs bodes well for large-area processing of these materials. Additional efforts to characterize the anisotropic charge transport properties and to optimize process conditions for large-area alignment are clearly warranted.

Experimental SectionSynthesis of Core/Shell Hybrid Nanocomposites: In a typical procedure

for the preparation of ZnO nanowires, a zinc solution (6.67 × 10−3 m) was obtained by dissolving zinc nitrate salt (0.3 g) in ethanol at room temperature. Sodium hydroxide pellets (1.2 g) were added and the solution was stirred vigorously for 1 h under atmospheric conditions until all base was dissolved. The turbid solution was then sonicated for 30 min directly after mixing using a Branson Sonifier 450 tip sonicator with a power input of 30 W. Ethylenediamine (10 mL) was directly added as a growth-directing capping agent and the solution was sonicated for an additional 20 min in a Branson 2510 ultrasonic cleaner. Finally, each solution was transferred to a Teflon liner and sealed in a stainless steel autoclave to react at 120 °C for 2–3 days. The obtained reaction products were collected by centrifugation and washed with water and ethanol several times, then dried. Collected ZnO nanowires were bright white in color. In a standard functionalization protocol, ZnO nanowires (100 mg) were dispersed in n,n,n-trimethyl-3-(trimethoxysilyl)propan-1-aminium chloride (TAC) (6 mL, 1 × 10−3 m) in methanol solution and left to shake at 1000–1200 rpm for 1 day on a standard laboratory vortex mixer. Functionalized nanowires were thoroughly washed in methanol and dried. Finally, the functionalized nanowires were dispersed in of P3KHT (4.5 × 10−6 m) from Rieke Metals, Inc. or PTEBS from American Dye Source, Inc. in water solution. The dispersions were then set on the laboratory shaker for 2 days, after which the composites were washed thoroughly in water to remove excess polymer. Materials were stored wet to slow down the polymer degradation process and dried as needed. Core/shell nanocomposites suitable for magnetic alignment were obtained by following the above protocol for the synthesis of ZnO nanowires, substituting cobalt nitrate for zinc nitrate to achieve 5 at% Co in Zn lattice sites and reacting at 130 °C for 2 days.

Characterization Methods: UV-vis absorption spectra of dried films were taken on a Cary 3E spectrophotometer at 25 °C using an integrating sphere and Labsphere Spectralon SRM-99 diffuse reflectance standard. Photoluminescence emission spectra were obtained in solution on a Shimadzu RF-5301PC fluorometer with an excitation wavelength of 400 nm. Polymer concentrations in the composite and homopolymer control samples were normalized to probe fluorescence quenching using thermogravimetric analysis to determine polymer loading in the composites. Control polymers and composites were dissolved in 50 vol% 0.01 m NaOH/50 vol% ethanol solutions to 1.89 × 10−5 m in polymer. This basic solution system was chosen due to the fact that pure composite samples were not soluble under neutral or acidic conditions and fluorescence was enhanced in ethanol versus aqueous solutions. The thermal behavior of the system was studied using DSC Q200 (TA

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Instruments) with heating and cooling rates of 40 °C min−1. Polymer content and thermal degradation of the composites were studied using thermogravimetric analysis (TGA) on a TA Q500 with heating and cooling rates of 10 °C min−1. Optical textures of the suspensions were studied under cross-polarizers on a Zeiss Axiovert 200M optical microscope. After complete evaporation of the solvent, the dried samples were coated by a thin gold layer and examined in a Hitachi SU-70 scanning electron microscope with a 10 kV accelerating voltage. Quantitative analysis of nanowire dimensions was performed by imaging a large number of representative nanowires with resolution in either diameter, length, or both dimensions. Images were then analyzed using Image-J software. A FEI Tecnai-20 high-resolution transmission electron microscope was employed to image individual nanowires. FTIR spectra were recorded on a Nicolet Nexus 870-FTIR at a resolution of 4 cm−1 (1024 scans) by dispersing the polymer-coated nanowires in a KBr pellet. WAXS experiments were performed on a pin-hole collimated Rigaku instrument (SMAX3000) using 1.5405 Å Cu Kα radiation produced by a microfocus source. Scattering was recorded on an image plate with an accessible q-range of 0.8–3.2 Å−1.

Magnetic Alignment: Magnetic alignment experiments were performed with a superconducting electromagnet designed by American Magnetics Inc. capable of producing a 6 T static field. Samples drop-cast onto silicon wafers were mounted on a sample holder within the vertical bore of the magnet such that the plane of the sample substrate surface was parallel to the direction of the vertical field. The temperature of the sample holder was maintained at 35 °C by enlisting a heating coil and temperature controller, and the field was maintained at 6 T until all solvent was completely evaporated from the samples.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsS.Z. and C.P. contributed equally to this work. This work was primarily supported by the SOLAR program of the National Science Foundation under DMR-0934520. C.P. acknowledges financial support from the NSF Graduate Research Fellowship Program (GRFP). C.O. and P.M. acknowledge NSF support under DMR-0847534.

Received: September 27, 2011Published online: November 23, 2011

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