Directing Hybrid Structures by Combining Self-Assembly of Functional

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Directing hybrid structures by combining self-assembly of functional block copolymers and ALD: a demonstration on hybrid photovoltaics Moshe Moshonov, and Gitti Frey Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03282 • Publication Date (Web): 02 Nov 2015 Downloaded from http://pubs.acs.org on November 5, 2015

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Directing hybrid structures by combining selfassembly of functional block copolymers and ALD: a demonstration on hybrid photovoltaics Moshe Moshonov and Gitti L. Frey Department of Materials Science and Engineering, Technion, Israel Institute of Technology, Haifa, 32000 Israel. KEYWORDS block copolymer, rod-coil, template, atomic layer deposition, self-assembly, photovoltaics, hybrid materials

ABSTRACT

The simplicity and versatility of block copolymer self-assembly offers their use as templates for nano- and meso-structured materials. However, in most cases, the material processing requires multi steps and the block copolymer is a sacrificial building block. Here, we combine a selfassembled block copolymer template and ALD of a metal oxide to generate functional hybrid films in a simple process with no etching or burning steps. This approach is demonstrated by using the

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crystallization-induced self-assembly of a rod-coil block copolymer, P3HT-b-PEO, and the ALD of ZnO. The block copolymer self assembles into fibrils, ~ 20 nm in diameter and microns long, with crystalline P3HT cores and amorphous PEO corona. The affinity of the ALD precursors to the PEO corona directs the exclusive deposition of crystalline ZnO within the PEO domains. The obtained hybrid structure possesses the properties desired for photovoltaic films: donor-acceptor continuous nano-scale interpenetrated networks. Therefore, we integrated the films into singlelayer hybrid photovoltaics devices, thus demonstrating that combining self-assembly of functional block copolymers and ALD is a simple approach to direct desired complex hybrid morphologies.

1. INTRODUCTION Hybrid systems are composed of functional organic and inorganic components and are useful in a variety of applications including coatings, energy harvesting and storage, sensing and catalysis, opto-electronic devices, and others. 1 In such structures, the synergistic combination of the two components allows properties and functionalities that are unattainable in the single phases. Frequently, the new features are induced by the hybrid organic/inorganic interface. Therefore, high interfacial area is desirable in most hybrid systems. 2 However, the distinct chemistries of the organic and inorganic components limit the concurrent processing of both components. Consequently, many approaches have been developed to direct hybrid systems with organic/inorganic phase separation on the nano-to-meso scale and at the same time control the chemical composition and interactions at the hybrid interface. One such hybrid system, which requires in addition to nano-scale phase separation also continuous pathways of both phases, is hybrid photovoltaics. In such devices the active layer is


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composed of phase-separated blends of an organic electron donor and an inorganic electron acceptor organized in a bulk heterojunction (BHJ) morphology. The key processes in such devices are the light absorption by the organic component, charge generation at the hybrid interface, charge transport through the active layer, and charge extraction at the electrodes. Efficient photovoltaic performance of hybrid devices, therefore, requires control over the active layer composition, non-homogeneous interfacial interactions and the morphology including phase separation, domain size and orientation. 3 Several techniques have been used to direct hybrid films for photovoltaic applications. The simplest approach is to combine the organic component with pre-formed inorganic nanostructures. For example, blending conjugated polymers with metal oxide nanoparticles, or depositing the organic phase on pre-formed metal oxide nanostructures. However, polymer infiltration into these nanostructures is partial and the polymer is mostly retained on the top surface. 4 To increase the interfacial area, a metal oxide sol gel precursor and conjugated polymer species were co-assembled from solution. Under such conditions, the organic and inorganic domains are mixed on the nano-scale, but the domains are often non-continuous hindering the charge transport and contributing to charge recombination. 5 Recently, we and others demonstrated that Atomic Layer Deposition (ALD), a common coating technique, can be used for the fabrication of hybrid BHJs. 6, 7 In a typical ALD process, two gaseous reactants are alternatingly introduced into a chamber, resulting in a self-limiting, conformal, layer-by-layer coating of surfaces. Attempts to coat polymer films by ALD showed that often the precursors diffuse into the polymer and deposit the inorganic phase inside the polymer film. 8, 9, 10, 11, 12, 13, 14, 15 The extent of the precursor diffusion into the polymer film depends on the presence and rate of chemical interaction between the precursor and functional groups on the polymer chains. For polymers


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with highly reactive groups, the reaction rate between the precursors and the polymer surface is fast so that the inorganic phase rapidly nucleates on the surface of the polymer film. Consequently, a conformal inorganic coating is formed on the surface of the polymer. However, if the polymer functional groups are non-reactive, the precursors can diffuse into the polymer and react within the film resulting in an inorganic phase embedded in the organic bulk. 7, 8, 14, 16 Importantly, the diffusion of the precursors into the film is also affected by the morphology of the film. Generally, polymeric crystalline regions are densely packed suppressing the precursor’s infiltration. The amorphous regions, on the other hand, are less dense and the precursors can diffuse through the free-volume voids between polymer chains. The diffusion of a ZnO precursor, diethyl zinc (DEZ), and water into the amorphous domains of P3HT was recently used to prepare hybrid photovoltaic films. 6 In this direct approach, the P3HT film was exposed to an alternating ALD sequence of DEZ and water forming crystalline ZnO particles, 5-10 nm, embedded in the amorphous regions of the polymer film. The hybrid film was integrated into PV devices and showed reasonable performances. However, although this fabrication process is simple, it attains only partial control of the morphology and does not ensure the formation of a continuous ZnO network. These limitations are overcome in a more complicated process involving the self-assembly of a sacrificial block copolymer (BCP) and etching processes. 17, 18 Namely, ZnO was deposited by ALD onto nanoscale patterns prepared using phase separated block copolymer PS-b-PI thin films. 19 After selectively etching the PI block, ZnO was deposited by ALD in the voids between the PS structures. Then, the PS block was burnt off and P3HT infiltrated into the now available voids. The final structure was a hybrid P3HT:ZnO BHJ with nano-scale separation directed by the length of the blocks and continuous pathways directed by the BCP self-assembly.


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In this study we take the combination of BCP self-assembly and ALD one step further to direct the hybrid BHJ in a simple process with no etching or burning steps. This is done by judiciously synthesizing P3HT-b-PEO, a BCP that is known to phase separate into continuous domains of crystalline P3HT and amorous PEO. Importantly, each block is selected for a specific role: the P3HT block will act as the donor in the hybrid BHJ and the π-π interactions in the phase separated crystalline P3HT domains will support the hole transport. The PEO, on the other hand, has high affinity to the ALD precursors and hence we speculate that exposure to the ALD process will result in selected growth of ZnO in the PEO domains. In this design, the BCP is the donor and a template for the acceptor of the BHJ. Gel Permeation Chromatography, (GPC) Nuclear Magnetic Resonance, (NMR) and Differential Scanning Calorimetry, (DSC) confirm the BCP synthesis, while optical measurements, High-Resolution Scanning Electron Microscopy, (HRSEM) High-Resolution Transmission Electron Microscopy, (HRTEM) and Grazing Incidence X-ray Diffraction (GIXRD) confirm the BCP self-assembly and selective deposition of ZnO in the PEO block. The processed films are then integrated into photovoltaic devices. This versatile process can be extended to numerous copolymers and metal oxide precursors, thereby providing the advantage of tuning the assembly size, shape, interfacial area and chemical interaction, which can later be used for diverse applications including catalysis and hybrid electronics. 2. EXPERIMENTAL DETAILS 2.1 Materials Toluene anhydrous 99.8%, THF inhibitor free, anhydrous 99.9%, Methanol anhydrous 99.8%, and hexane anhydrous 95% (Sigma-Aldrich), Methanol, Acetone, Chloroform and Isopropanol


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(AR, BioLab, Israel) were used as received. Poly(ethylene glycol) methyl ether azide, PEG average Mn 1000 gr/mole was purchased from Sigma-Aldrich and used as received. Poly (3,4ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS) was acquired from Haraeus (Clevios PVP AL 4083) and was filtered through a 0.45 μm poly(terafluoroethylene) (PTFE) filter before use. 2,5-dibromo-3-hexylthiophene (DBHT) 97% was purchased from SigmaAldrich (CAS Number 116971-11-0) and used as received. tert-Butylmagnesium chloride solution 2.0 M in diethyl ether (CAS Number 677-22-5) was purchased from Sigma-Aldrich and used as received. 1,3-Bis(diphenylphosphino)propane]dichloronickel(II) [Ni(dppp)Cl2] (CAS Number 15629-92-2) was purchased from Sigma-Aldrich and used as received. Ethynylmagnesium bromide solution 0.5 M in THF (CAS Number 4301-14-8) was purchased from Sigma-Aldrich and used as received. Copper(I) Iodide anhydrous, 99.995% trace metals basis, was purchased from Sigma-Aldrich and purified by the dissolution–precipitation process providing colorless samples. 2.2 Materials and synthesis Synthesis procedures and characterization of the polymers and BCPs are presented in the supporting information and synthesized according to the literature. 20 Namely, the ethynylterminated regioregular poly(3-hexylthiophene), P3HT GRIM polymerization procedure, and the poly(3-hexylthiophene)23-b-poly(ethylene oxide)23 (P3HT23-b-PEO23) click reaction procedure. Films were prepared by dissolving P3HT or P3HT-b-PEO (20 mg/ml) in dry toluene. The solutions were stirred for ~10 h at 60°C under N2 atmosphere. Films were spun onto PEDOT:PSS/ITO/glass, silicon or quarts substrates at 700 rpm for 20 sec, under N2 atmosphere (glove-box conditions). The spun films were then left in a covered glass petri dish for 1 hour.


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ALD of ZnO into the organic films was performed using an MVD100E Applied MST system with an integrated oxygen plasma module. Deposition temperature was set to 60°C and alternating 1 torr pulses of DEZ and water were applied. The precursor reaction time was limited to 1 sec for both DEZ and water in every cycle. The reaction chamber was purged with nitrogen between different precursor pulse injections. Four types of ALD processes were performed with 20, 40, 60, or 80 cycles. 2.3 Film Characterization The absorption spectra of the films were measured on quartz substrates using a Varian Cary 100 Scan UV-vis spectrophotometer in the 250-800 nm range. High-resolution scanning electron microscopy (HRSEM) cross section images of films on silicon substrates were obtained using the Zeiss Ultra plus high resolution scanning electron microscope (HRSEM), equipped with a Schottky field emission source, operating at 2 KeV. The images were acquired using both secondary electrons (in-lens detector) and backscattered electrons (Inlens energy selected detector), at a relatively low accelerating voltage of 1.5-2 kV at working distances of ~2mm. Energy dispersive spectrometer (EDS: JEOL JED-2300) was used for analyzing the composition of the sample. EDS measurements include Kα signals of sulfur, carbon and oxygen as well as the Lα signal of zinc. The Si (substrate) signal was observed in all EDS measurements to ensure that the interaction volume was larger than the film thickness. High-resolution transmission electron microscopy (HRTEM) measurements were conducted on a FEI Tecnai G2 T20 S-Twin microscope operating at 200 keV. Energy-filtered (EF) TEM


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measurements were performed on a FEI Titan 80-300 KeV S/TEM operating at 200 keV with filters set to sulfur L2,3 edge (165 eV) or to zirconium L2,3 edge (87 eV). Samples for EF-TEM examination were thinned by use of a focused ion beam (FIB) integrated in the Strata 400 STEM DualBeam system of a field-emission scanning electron microscope (FE-SEM). 2.4 Device fabrication and characterization Glass plates (12 x 12 mm2) covered with ~80 nm of patterned ITO strips were cleaned by sonication in acetone, methanol, and 2-propanol, followed by 15 min of UV-ozone treatment. PEDOT:PSS was filtered through a 0.45 mm PVDF filter and spun at 5000 rpm onto the ITO coated glass, followed by drying at 120°C for 15 min in ambient conditions. The active layers were then deposited as detailed above, followed by thermal deposition of the top metal layer, Al, through a shadow mask at a system pressure of ~10-6. Device characterization was performed in inert atmosphere in the dark and under100 mW/cm2 AM1.5G class solar simulator illumination (Newport Inc. 67005 lamp power-150W) by use of a Keithley 2400 source meter. 3. RESULTS AND DISCUSSION In this report, we use the self-assembly of a judiciously synthesized BCP, P3HT-b-PEO, in combination with ALD of ZnO to direct the complex morphology of hybrid photovoltaic BHJ films. The P3HT block is a good light absorber, electron donor and hole transporter. The PEO block, on the other hand, is selected to host the ALD deposition of the ZnO acceptor. Under such conditions the self-assembly of the BCP into distinct domains will be used as a temple for the ALD. BCP self-assembly is generally directed by the interplay between micro-phase separation, polymer crystallization and interfacial effects. In this case, the π-conjugation of P3HT implies a rod-coil BCP structure which imposes a more complicated self-assembly process with respect to


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common coil-coil BCPs. Recent studies reported the self-assembly of P3HT-b-PEO BCPs into continuous febrile morphologies. The π-π interactions of the P3HT block drive the crystallization of the febrile cores surrounded by the PEO amorphous corona. 21, 22, 23 The P3HT core width depends on the degree of polymerization of the P3HT block, and for the BCP used in this study the core width is ~8 nm. The average length of the fibers ranged from tens of nanometers to microns depending on the deposition technique and conditions. 24, 25, 26, 27 To study the use of self-assembled P3HT-b-PEO films as templates for ZnO ALD, we spun P3HT homopolymer and BCP films from toluene solutions and introduced them to the ALD system (see experimental). The deposition of ZnO into the block copolymer film, its location and effect on the different blocks is studied by comparing the optical absorption of pristine P3HT and P3HT-b-PEO BCP films before and after the ALD process. The absorption spectrum of pristine P3HT, Figure 1a, is composed of the two P3HT intra-molecular vibronic transitions at 518 and 552 nm; and the shoulder at 605 nm, indicative of ordered semi-crystalline domains 6, 7, 24, 25, 28 The effect of the ALD process on the absorption spectrum of P3HT was recently reported showing three contributions: a new absorption peak at ~300 nm, associated with ZnO; a broadening of the P3HT spectra to higher energies; and a slight reduction in the intensity of the 605 nm shoulder. These results indicated that the ZnO deposition occurs selectively in the amorphous regions of the semi-crystalline P3HT. 6 The absorption spectrum of the neat P3HT-b-PEO BCP, Figure 1b, shows the same transitions observed for pristine P3HT, confirming the high order of the P3HT block arraignment. Exposing the P3HT-b-PEO BCP to the ALD process also shows the uptake of ZnO, evident by the new peak at 300 nm, which increases with the number of cycles. However, in contrast to pristine P3HT, the ALD process does not induce a blue shift nor shoulder reduction in the P3HT-b-PEO BCP spectrum. We


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speculate that in the case of the P3HT-b-PEO BCP, the ALD precursors favor infiltration exclusively into the PEO segments. Consequently, ZnO is formed in the PEO domains and the P3HT block is maintained intact in a highly ordered crystalline form.

Figure 1: The absorption spectra of films before and after ALD cycles of DEZ and water: (a) P3HT homopolymer films, and (b) P3HT-b-PEO BCP. To confirm the presence of crystalline domains in P3HT and in the P3HT-b-PEO BCP after the ALD process we performed grazing incidence X-ray diffraction, GIXRD, measurements. The GIXRD patterns of P3HT and the P3HT-b-PEO block copolymer, before and after the ALD process, are presented in Figure 2. Quantitative analysis of the GIXRD patterns of both polymers prior to the ALD process (black lines in Figure 2) shows diffraction peaks at 2θ = 5.3°, 10.8° and 16.2° corresponding to the (100), (200), and (300) reflections of crystalline P3HT domains with the typical 1.6 nm lamellar distance. 29, 30 A weak peak is also noticed in both patterns at 2θ ~ 23 degrees, which corresponds to the ~ 0.38 nm π-π packing distance of P3HT, and suggests that the P3HT crystalline planes are mainly oriented normal to the films. Importantly, no additional peaks are observed in the pattern of the P3HT-b-PEO block


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copolymer indicating that only the P3HT block crystallizes, while the PEO block remains amorphous.

Figure 2: GIXRD patterns of (a) P3HT and (b) P3HT-b-PEO films before (black) and after 80 DEZ/water ALD cycles (blue). Patterns are staked on the Y axis for clarity. The GIXRD patterns of the polymers after 80 ALD ZnO cycles (blue lines in Figure 2) show that both films still include crystalline P3HT domains. However, obvious new peaks are observed at 2θ ~ 32°, 34°, 36° that directly correlate to the ZnO (001), (002) and (101) reflections, confirming that the ZnO grows in its wurtzite structure. 6 Importantly, all ZnO related peaks are of similar intensities indicating no preferred orientation. The ZnO GIXRD peaks are broad suggesting that the particles are in the nano-range size. Indeed, in our previous report we showed that ALD of ZnO into pristine P3HT results in the formation of ZnO nanoparticles, 5-10 nm, with no preferred orientation, embedded in the amorphous regions of P3HT. 6 The optical absorption spectra and the GIXRD patterns confirm that ZnO nanoparticles are deposited by ALD within P3HT-b-PEO block copolymer films. To determine the location, size and distribution of the nanoparticles in the P3HT-b-PEO block copolymer film we performed cross section HRSEM measurements. Figure 3 displays the


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cross section HRSEM images of the P3HT homopolymer and the P3HT-b-PEO block copolymer after 20, 40, 60 and 80 ALD cycles. The dark regions in the images represent the organic phase, while the bright domains are the ZnO. All images show that the ALD process results in the deposition of ZnO inside the film organic films and not on their surfaces. Furthermore, the films swell as a result of the ZnO intake. However, the size, amount and morphology of the incorporated ZnO depend on the number of ALD cycles. The images of the P3HT homopolymer (left panel of Figure 3) show that there is a strong correlation between the number of deposition cycles and the amount of ZnO in the film. After 20 deposition cycles only few small clusters of ZnO are uniformly distributed in the film. As the number of cycles increases, the size and abundance of the ZnO particles increase. Finally, after 80 cycles, the particles merge into an interconnected ZnO array, in good agreement with our previous report. 6 The SEM images of the P3HT-b-PEO BCP (right panel of Figure 3) also show a direct correlation between the number of cycles and amount of ZnO incorporated. However, in contrast to the homopolymer, after 20 cycles the BCP already hosts a high concentration of ZnO. After 40 cycles, the particles are arranged in large clusters. These clusters are significantly larger than those obtained in the homopolymer after the same number of cycles. After 80 ALD cycles the film is twice its original thickness due to the accumulation of ZnO clusters and the uptake of water molecules in the hygroscopic PEO block. 31 Therefore, the cross section images clearly show that the permeability and diffusivity of both precursors are significantly higher in the BCP compared to the homopolymer. It is therefore clear that the PEO block plays a vital role in the diffusion of the ALD precursors and growth of the ZnO particles.


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Figure 3: Cross section BSE HRSEM micrographs of films of P3HT homopolymer – left panel, and P3HT-b-PEO BCP – right panel; after 20, 40, 60 and 80 ALD cycles of DEZ and water. To understand the effect of the PEO block on the ALD process, HRSEM images of the surfaces of the homopolymer and BCP films after 80 ALD cycles are presented in Figure 4a and 4b, respectively. The images clearly shows that in contrast to the homopolymer surface where the ZnO particles are randomly scattered on the surface, the ZnO domains are organized on the BCP surface in continuous and extended fiber-like morphologies. These ZnO “fibers” have a uniform width of ~10 nm and extend over microns. Importantly, the “fibers” appear parallel to each other and the distance between adjacent “fibers” is also uniform and ~ 10 nm. The fiber-like morphology of the ZnO is in good agreement with the reported febrile morphology of self-


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assembled P3HT-b-PEO. As mentioned above, the rod-coil P3HT-b-PEO self-assembles into continuous nano-fibrils due to the crystallization of P3HT through π-π interactions. The fibril width and length depend on the deposition conditions and the molecular weight of both blocks. 24, 25, 26, 27

The agreement between the morphology of the deposited ZnO and the known

morphology of the self-assembled BCP, in combination with the affinity of PEO to the ZnO precursors, suggests that the self-assembly of the BCP acts as a template for the ALD of ZnO. Namely, that the affinity of the PEO block to the ZnO precursor induces the selective deposition of the ZnO only in the PEO blocks and hence the fibril morphology is translated to the ZnO during the ALD.

Figure 4: Surface BSE HRSEM micrographs of films that were exposed to 80 cycles of DEZ and water: a) P3HT homopolymer, and b) P3HT-b-PEO BCP. To confirm that the deposition of ZnO in the BCP is selectively in the PEO domains, and that this selectivity is not limited to the surface of the film only, we applied high-resolution cross-sectional energy-filtered transmission electron microscopy (EFTEM). In EFTEM, elementally distinct signals are measured and spatially resolved to sub-eV-resolution by electron energy loss spectroscopy (EELS), providing a nanoscale distribution map of the elements within


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a material. Sulfur atoms have a strong EELS signature centered at 165 eV, corresponding to the L2,3 loss edge, and therefore are used to map the distribution of the sulfur-bearing conjugated block, P3HT, in a cross-section image of the film. In parallel, zinc atoms have a strong EELS signature centered at 87 eV, corresponding to the L2,3 loss edge, and are therefore used to map the distribution of the zinc-bearing inorganic material, ZnO, in the cross-section. By comparing the sulfur and zinc distribution maps we can determine whether the ZnO is associated with the P3HT domain, the PEO-domain or both.


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Figure 5: High-resolution cross-sectional TEM images of a P3HT-b-PEO BCP film after 80 ALD cycles. (a) using the sulfur L2,3 edge transition (165 eV) and marking the sulfur rich area are in red; (b) using the Zinc L2,3 edge transition (87 eV) and marking the Zn-rich areas in blue; (c) an overlay of (a) and (b) , and d) a Bright-field TEM micrograph of the same region. The EFTEM and bright field cross sectional images of the same area and same magnification of a BCP film after 80 ALD cycles are shown in Figure 5. Figure 5a was acquired using the sulfur EELS signal and maps the distribution of sulfur in the cross section. The image


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clearly shows that sulfur is not evenly and randomly distributed in the film, but rather the film is composed of uniform-size sulfur-rich and sulfur-poor domains organized in a semi-pattern. This structure is nicely correlated with the expected self-assembly of the BCP into phase separated sulfur-rich P3HT domains and sulfur-poor PEO domains. Based on the surface images we speculate that during the evaporation of the solvent, the P3HT crystallizes and the BCP phase separates into self-organized febrile-like structures. The long-axis of the fibers is mainly in the film plane, as shown in the surface image (Figure 4), and the cross-section images (Figures 3 and 5) show cross-sections of the fibers. The EFTEM confirms that the ZnO is strictly correlated with the P3HT-poor domains. This result is evident from overlaying the sulfur map (Figure 5a) and the Zn map (Figure 5b). The overlay image, confirms unambiguously that the ZnO is preferentially located outside of the P3HT-domains of the BCP, and hence are present exclusively within the ~10 nm sized PEOdomains. The significant differences in the morphology and amount of ZnO particles incorporated in the BCP compared to homopolymer P3HT are due to the affinity of the PEO block to ZnO. In the homopolymer, the ZnO precursor molecules diffuse through the amorphous domains and some are randomly retained in the film. These molecules could then serve as nucleation centers for the particles. In the following cycles, the incoming precursors could either lead to the growth of the present nucleation centers, or form new centers. This nucleation-vsgrowth process occurs randomly in the film and hence the ZnO clusters are uniform in size and evenly distributed in the film. However, in case of the BCP, the affinity of PEO to ZnO leads to the selective growth of ZnO in the PEO domains. Therefore, in contrast to the homopolymer, the location and size of ZnO particles is dictated by the size and location of the PEO domains, i.e., by the self-assembly of the BCP. In the system studied here, the rod-coil BCP self-organizes into


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bundles of fiber-like structures composed of crystalline P3HT domains and amorphous PEO domains. After the ALD cycling, the PEO domains are saturated by ZnO particles. Interestingly and in contrast to the homopolymer P3HT, the optical properties of the P3HT-block of the BCP are not affected by the 80 ALD cycles and substantial uptake of ZnO (Figure 2). This result further confirms that the ZnO is hosted selectively by the PEO domains. The SEM and TEM images indicate that P3HT-b-PEO BCP can be used as a template for ALD of ZnO, and that the process results in the formation of a hybrid film with phase separated and continuous P3HT and ZnO fiber-like domains about ~10 nm in diameter. Furthermore, the absorption spectra and X-ray patterns reveal that both domains, P3HT and ZnO, are ordered/crystalline. Hence, the obtained film possesses the properties desired for hybrid BHJ photovoltaic films: donor-acceptor continuous nano-scale interpenetrated networks. Therefore, we integrated the films into single-layer hybrid photovoltaics devices, thus demonstrating that combining self-assembly of functional block copolymers and ALD is a simple approach to direct desired complex hybrid nano-scale morphologies.


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Figure 6: Typical photocurrent density-voltage characteristics (JV) under 1.5AM illumination (continuous lines), or in the dark (dash lines) of photovoltaic devices based on BCP films that were exposed to 60 (purple) or 80 (blue) ALD cycles. 25 devices of each type were measured and the standard deviations of the parameters are Voc ± 0.04 [V] and Jsc ± 5E10-3 mA/cm2. The general solar cell structure was: glass/ITO/PEDOT:PSS/BCP:ZnO/Al. The films obtained after 20 and 40 ALD cycles showed a photovoltaic effect but very poor performances, probably due to an incomplete ZnO network. The current density/voltage (JV) curves of the devices based on the films that were exposed to 60 and 80 ALD cycles are presented in Figure 6. The short circuit current density (Jsc) of films that were exposed to 80 cycles, 0.095 mA/cm2, is ~3 times higher than that obtained for films exposed to 60 cycles only, 0.035 mA/cm2. Since the amount and morphology of P3HT is identical in both films, as confirmed by the absorption spectra shown in Figure 2b, the increase in photocurrent is associated directly with the ZnO


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phase. Increasing the number of cycles increases the amount of ZnO in the film. This increase has two contributions: more hybrid interfacial area leading to more charge generation; and formation of a continuous and crystalline ZnO network that supports electron transport. Importantly, the SEM and TEM images clearly showed that, in contrast to the homopolymer where the ALD leads to randomly scattered spherical ZnO domains, the selective deposition in the PEO block of the BCP directs highly ordered and extended tube-like ZnO domains. The extended and continuous domain morphology is necessary for electronic devices that require charge transport. The open circuit voltage (Voc) of both devices is similar, ~0.3V, which is in good agreement with our previous report on photovoltaic devices prepared by non-controlled ALD of ZnO into P3HT films. 6 However, the Voc seems to slightly decrease with the number of ALD cycles. This decrease is in correlation with the slight increase in leakage currents (Figure 6 dashed lines) with number of cycles and could be associated with the ZnO network forming a contact between both electrodes. 4. CONCLUSION In summary, we demonstrated the utilization of a judiciously designed and synthesized amphiphilic conjugated BCP, P3HT-b-PEO, as a template for ALD of ZnO. The crystallizationdriven self-assembly and phase separation of blocks leads to the formation of fibrils with crystalline P3HT cores and PEO amorphous corona. The affinity of PEO towards the ALD precursor directs the growth of ZnO selectively within the PEO domains. Namely, the selfassembly of the BCP is used as a template for the in-situ growth of ZnO by ALD in the film. Therefore, the size, distribution and orientation of the inorganic phase are directed by the BCP through its chemical structure and self-assembly. Since BCP synthesis and self-assembly are highly versatile, the combination of BCP and ALD offers a new approach to direct functional


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hybrid material. Here, we demonstrated the approach for the fabrication of hybrid BHJ photovoltaic films. Acknowledgements We thank Dr. Yaron Kauffman for his assistance in acquiring the EF-TEM data presented in this work. We acknowledge Dr Guy Ankonina from the PV lab for assistance and guidance in the ALD process (all from the Technion, Israel Institute of Technology). Supporting Information Data supporting this study are provided as supplementary information accompanying this paper. Instrumentation used for the polymers characterization, (i.e. NMR, DSC and GPC) synthesis procedures and characterization of BCP by DSC and GPC are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Moshe Moshonov. Email: [email protected] Gitti L. Frey. Email: [email protected]

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Table of Contents, (TOC) Graphic Only


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Directing Hybrid Structures by Combining Self-Assembly of Functional

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