Orienting semi-conducting π-conjugated polymers.

Macromolecular Rapid Communications

Review

Orienting Semi-Conducting π-Conjugated Polymers Martin Brinkmann,* Lucia Hartmann, Laure Biniek, Kim Tremel, Navaphun Kayunkid The present review focuses on the recent progress made in thin film orientation of semiconducting polymers with particular emphasis on methods using epitaxy and shear forces. The main results reported in this review deal with regioregular poly(3-alkylthiophene)s and poly(dialkylfluorenes). Correlations existing between processing conditions, macromolecular parameters and the resulting structures formed in thin films are underlined. It is shown that epitaxial orientation of semi-conducting polymers can generate a large palette of semi-crystalline and nanostructured morphologies by a subtle choice of the orienting substrates and growth conditions.

1. Introduction Plastic electronics has emerged as a major and challenging new research field in the past few decades. This is largely due to the availability of new semi-conducting polymers (SCPs) that allow for low-cost solution processing, via, e.g., spin coating or inkjet printing and give thus access to new or improved opto-electronic properties in thin film devices.[1–7] Early on, the existence of correlations between orientation and electronic properties in thin films of SCPs has been recognized, pointing to the intrinsic anisotropic properties inherent to highly structured SCPs.[8,9] In most cases, orientation of a polymer film can be understood in two different ways namely: i) the existence of a preferential contact plane of the crystalline domains on a substrate, Dr. M. Brinkmann, Dr. L. Hartmann, Dr. L. Biniek, Dr. N. Kayunkid Institut Charles Sadron, CNRS-Université de Strasbourg, 23 rue du Loess, 67034 , Strasbourg Cedex, France E-mail: [email protected] K. Tremel, IPOC, Institut für Polymer Chemistry, Pfaffenwaldring 55, 70569, Stuttgart, Germany. Macromol. Rapid Commun. 2014, 35, 9−26 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ii) the presence of a long-range in-plane preferential orientation of the polymer chains. The existence of preferential contact planes of crystallites on a substrate is essential as it can influence charge transport properties and device performances to a great extent.[8] In the case of regioregular poly(3-hexylthiophene) (P3HT), the existence of so-called edge-on and face-on orientations of the polymer backbones on a substrate is well documented.[10–14] Whereas in-plane transport is favored in case of edge-on orientation (π-stacking direction and chain axes lie in-plane), face-on orientation (chain axes lie in-plane and π-stacking direction is normal to the substrate) is expected to be favorable in the case of photovoltaic devices when the active layer is sandwiched between two electrodes. Beside the possibility to control the contact plane of crystalline domains, it is also of importance to master the in-plane orientation of the chains to exploit the intrinsic anisotropic properties of SCPs, e.g., charge transport and light emission.[15–19] As for device manufacture, the design of dense arrays of organic field effect transistors (OFETs) implies to find strategies to reduce parasitic current paths between successive OFETs. This can be achieved by preferential alignment of the SCP to generate anisotropic charge transport.[16,19] One potential

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DOI: 10.1002/marc.201300712

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application of highly oriented SCP layers lies in the replacement of conventional backlights in LCD displays by polarized light emitting devices (POLEDs).[19] More recently, Yang and co-workers[20] demonstrated the possibility to prepare P3HT-based solar cells with an aligned active layer of P3HT prepared by mechanical rubbing. Such polarized solar cells would help recover part of the backlight in an LCD display that is lost when using a conventional polarizing filter. Orientation of SCP layers by epitaxy or mechanical rubbing can have various structural and morphological implications that will affect the multi-scale charge transport process.[21] On a molecular scale, intrachain and interchain transport within crystalline domains must be distinguished.[8] Intrachain transport is mainly affected by the rupture of conjugation (e.g., by chemical defects altering the regioregularity of the chain) whereas interchain transport depends crucially on the π-stacking between adjacent chain segments. However, because of the predominance of intrachain transport,[21] charge transport in conjugated polymers is by essence highly anisotropic, which justifies the need to develop simple and efficient processes for long-range chain orientation in thin films. Besides orientation, nanostructuring of conjugated polymers is another yet essential aspect that influences for instance photovoltaic activity in organic solar cells (OSC).[22,23] In short, charge transport is sensitive to the level of order of semicrystalline polymers at multiple length scales ranging form molecular to nano- and meso-scales. As shown here, epitaxy is an elegant method to achieve long-range orientation and/or nanopatterning in SCP thin films. The first section of this review reports various methods developed in the last decade to orient conjugated polymers without the use of an orienting substrate. In particular, methods that make use of shear forces (friction transfer, rubbing) are described. The second half of this review is devoted to orientation methods that take benefit of the orienting ability of substrates whether inorganic, organic (molecular crystals), or polymeric (oriented poly(imide), oriented poly(tetrafluoroethyelene) (PTFE)). 1.1. Orientation of SCP by Shear Forces This category of orientation methods does not make use of any specific alignment layer. This is advantageous from a practical point of view, as the oriented SCP layers can be more readily integrated in a device (especially when the active layer is sandwiched between two electrodes) without taking care of any alignment substrate. Two representative methods are i) mechanical rubbing of a SCP thin film and ii) friction transfer of a polymer on a substrate as illustrated in Figure 1. Both these methods exploit shear forces to align polymer chains in the direction of rubbing or friction. In some cases, change of chain conformation is induced from coiled to stretched chain as for instance in

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Martin Brinkmann obtained his Ph.D. in physics from the University of Strasbourg in 1997. He spent several years as a postdoctoral fellow at CNR in Bologna and the Massachussetts Institue of Technology in Cambridge (U.S.A) before moving to the Institut Charles Sadron (CNRS) in Strasbourg in 2000 as a senior scientist. He investigates fundamental aspects of organic thin film growth and structure, especially epitaxy of pi-conjugated materials using transmission electron microscopy. Lucia Hartmann received her master-diploma in physics from the University of Saarland (Germany) and the University of Nancy (France) in 2008. She then moved to the University of Strasbourg (France) where she completed her Ph.D. in 2012. She was working under the supervision of Dr. Martin Brinkmann at the Centre National de la Recherche Scientifique (CNRS) in Strasbourg and Dr. Frederic Chandezon at the Comissariat aux Energies Atomique et aux energies Alternatives (CEA) in Grenoble. Her research activity focuses on the field of organic semiconductors and hybrid organic–inorganic optoelectronic materials. Currently, she is a postdoctoral research assistant in the Laboratory of Macromolecular and Organic Materials at the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland under the guidance of Dr Holger Frauenrath.

Laure Biniek obtained her Ph.D. in Chemistry from the University of Strasbourg in 2010. Following a postdoc at Imperial College within Iain McCulloch's group in 2011, she is now working in the team of Martin Brinkmann at Institut Charles Sadron in France. Her research focuses on the design, synthesis, and characterization of semi-conducting polymers for OPV and OFET applications. She is currently working on organization and structural characterization of conjugated molecules.

Kim Tremel received her master in chemistry in 2010 from the University of Freiburg (Germany) working on micron-scale patterning of conjugated polymers. She is currently a Ph.D. student at the University of Stuttgart under the supervision of Prof. Sabine Ludwigs. Her research interests focus on molecular order and charge transport in thin films of semi-crystalline, conjugated polymers with the aim to better understand structure–property relations.

Dr. Navaphun Kayunkid received his Ph.D. in Physics and Chemical Physics in 2012 from the University of Strasbourg, France. After graduation, he became a post-doctoral fellow at the Institut Charles Sadron (ICS), Strasbourg, France. His research focused on the determination of the crystal structure of conjugated polymers, using transmission electron microcopy, and electron diffraction analysis. At present, he is a lecturer and researcher at College of Nanotechnology, King Mongkut’s Institute of Technology (Ladkrabang, Thailand).

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Figure 1. Schematic illustration of two orientation methods of SCPs: a) Orientation by mechanical rubbing (buffing). A rotating cylinder covered by a microfiber or velvet cloth is applied on a polymer-coated substrate that moves at a constant velocity. The sample holder can be heated during rubbing to ease alignment of SCPs. (Reproduced with permission.[35] Copyright 2013, American Chemical Society. b) Friction transfer deposition of an oriented SCP film. A polymer pellet is applied with a given pressure against a moving hot substrate whereby a thin oriented polymer layer is deposited.

the generation of shish-kebab structures of polyolefins in the presence of elongational flow.[24] 1.1.1. Orientation by Mechanical Rubbing Rubbing has been widely used to prepare polyimide alignment layers in the fabrication of flat-pannel liquid crystal displays.[25,26] Mechanical rubbing is usually achieved by a cloth on a cylinder rotating in contact with a polymeric substrate that is translated at a constant speed. Rubbing parameters are the applied pressure and the so-called rubbing length, i.e., the length of the rubbing tissue applied on a given point of the sample.[26] The rubbing length can be adjusted by controlling the number of rubbing cycles. Although widely used for polyimides, only a few reports have described the orientation of SCPs by this method. In a very early work, Kanetake et al.[27] used rubbing to align polydiacetylene films. Uniaxially oriented layers were prepared by rubbing of a photopolymerized film of the diacetylene precursor deposited by evaporation. Hamaguchi and Yoshino used rubbing to prepare oriented films of poly(2,5-dinonyloxy-1,4-phenylenevinylene) for polarized light emission.[28] Polarized light emitting diodes were also prepared by rubbing poly(paraphenylenevinylene) films prepared by using a chlorine polyelectrolyte precursor


and subsequent conversion in the presence of lithium hydroxide.[29] Heil et al.[30] investigated the influence of rubbing on the field-effect mobility of P3HT OFETs. Oriented P3HT films with dichroic ratio of 5.1 were prepared and the charge mobility was improved when rubbing in the direction perpendicular to the source and drain contacts.[30] The orientation of P3HT by rubbing was investigated from a structural point of view by Brinkmann et al.[31] with par¯w ticular emphasis on the influence of molecular weight M on the polymer alignment. Rubbing of the P3HT films was found to cause i) the orientation of the chains parallel to the rubbing direction and ii) a flip of the contact plane from initially edge-on to face-on. However, the maximum in-plane orientation is a function of molecular weight (see Figure 2). High in-plane orientation is obtained for P3HT ¯w ≥ ¯w ≤ 7.9 kDa but a very poor alignment for M with M 50 Da (see Figure 2). Re-orientation of π-stacked chains in ¯w films (M ¯w > 50 kDa) is hindered by the reduced high-M chain mobility caused by the presence of chain entanglements and tie chains interconnecting crystalline P3HT ¯w-dependence domains (see illustration in Figure 2). This M of P3HT chain alignment and re-orientation of crystalline domains is further manifested in the anisotropy of charge mobility μ///μ⊥. The anisotropy of charge mobility μ///μ⊥ measured in the directions parallel and perpendicular to the rubbing is clearly correlated with the level of in-plane ¯w P3HT films with alignment achieved by rubbing: low-M the highest in-plane alignment showed the highest charge transport anisotropy (μ///μ⊥ ≈ 20) . Rubbing was also used in the case of blends of P3HT and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Vohra et al.[32] demonstrated that rubbing of a P3HT/ PCBM mixture leads to an ideal vertical donnor–acceptor concentration gradient with an enrichment in P3HT at the lower interface with the substrate.[32] Devices based on this graded bilayer structure showed an improved power conversion efficiency (PCE) of 3.81%. Mechanical rubbing was further applied to new and highly promising SCPs,[33,34] e.g., poly(2,5-bis(3-dodecyl2-yl)thieno[3,2-b]thiophene) (C12-pBTTT) and poly{[N,N′bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6diyl]-alt-5,5´-(2,2´-bithiophene)} (p(NDI2OD-T2)), two representative hole- and electron-transporting materials respectively.[35,36] In all studies on alignment of polymer films via rubbing, the process was applied to thin films maintained at room temperature.[25,26] However, our recent studies have demonstrated that for certain SCPs, e.g., C12-pBTTT and p(NDI2OD-T2), a remarkable improvement of orientation is observed when the polymer films are maintained at a temperature in the range 50 °C–125 °C during rubbing.[35,36] The improved alignment is attributed to the increased plasticity of the polymer films as the layers of alkyl side chains are more and more disordered. As an example, Figure 3 and 4 illustrate the typical electron diffraction, high-resolution


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4b). In both cases, periodic fringed patterns were observed. These patterns are related to the Z-contrast between layers of π-stacked sulfur-rich conjugated backbones and layers of alkyl side chains. However, in the case of p(NDI2OD-T2), HR-TEM revealed in addition a periodic pattern along the chain axis direction. This periodic pattern is due to a segregated mode of stacking of napthalenediimide and bithiophene units forming separate columns.[35] For both polymers, the initial alignment achieved by rubbing was further amplified by thermal annealing with a substantial increase in dichroic ratio. Provided the right temperature is selected, annealing can lead to either new crystal polymorphs (form II for p(NDI2OD-T2)) or to a change of the preferential contact plane of crystalline domains. In the case of pBTTT, a change to edge-on–oriented domains is observed when the annealing temperature exceeds 200 °C.[35] Accordingly, it was possible to compare the hole transport anisotropy in oriented films of pBTTT showing pure face-on or pure edgeon crystalline domains. Very high hole mobility anisotropies μ///μ⊥ ≈ 70 were observed for the oriented face-on C12pBTTT films whereas much lower values of ≈7 were observed in edge-on films.[36] The possibility to align efficiently SCPs in thin films was further exploited to prepare highly oriented hybrid films Figure 2. a) UV–vis absorbance spectra of P3HT thin films ( Mw = 17.2 kDa) oriented by made of P3HT and CdSe nanorods. a) rubbing for the incident light polarized parallel (//) and perpendicular (⊥) to the rub- When mechanically rubbed, a hybrid bing direction. b) Evolution of the dichroic ratio measured at 550 nm for rubbed P3HT film of P3HT/CdSe nanorods (NRs) films corresponding to different molecular weights in the range 6.4 kDa–70 kDa. The shows highly aligned nanorods and inset represents the Mw-dependence of the dichroic ratio corresponding to the highest polymer chains parallel to the rubbing observed orientation. c) and d) characteristic HR-TEM image and electron diffraction pattern of a rubbed P3HT film with = 17.2 kDa (4 rubbing cycles) respectively. e) Sche- direction R as illustrated in Figure 5. The high level of alignment of both P3HT matic illustration of the orientation of a low- Mw P3HT sample upon increasing the number of rubbing cycles. The inset in the upper left corner of the HR-TEM images corand CdSe NRs is further evidenced in responds to the Fast Fourier Transforms. Reproduced with permission.[31] the electron diffraction pattern of the films (see Figure 5b). It consists of a set of equatorial h 0 0 reflections of P3HT TEM, and UV–vis spectra of highly oriented films of C12characteristic of face-on oriented domains and oriented pBTTT and p(NDI2OD-T2), respectively. Interestingly, for reflections of CdSe nanorods (in particular the strong C12-pBTTT, rubbing at high temperature induced a change meridional (0 0 2)CdSe reflection). of the nanocrystal’s orientation from initially edge-on in For a set of hybrid films with different NR concentrathe as-deposited films to face-on orientation in the rubbed tion, the analysis of ED patterns highlights the strong corlayers. Using low-dose high resolution TEM, it was posrelation between the in-plane orientation degree of P3HT sible to observe the layers of π-stacked polymer chains in chains and CdSe NRs. This correlation indicates an orienfor both C12-pBTTT and p(NDI2OD-T2) (see Figure 3c and tation mechanism of the NRs via the surrounding P3HT

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matrix. This is further in agreement with low-dose HR-TEM results (see Figure 5c) showing that the local orientation of individual NRs matches that of the surrounding π-stacked P3HT chains. For high NR concentrations (CdSe/P3HT ratio in weight above 3), bundling of NRs into larger aggregates prevents their efficient alignment by rubbing. These preliminary results on mechanical alignment of hybrid thin films open interesting perspectives for the development of new hybrid materials with highly anisotropic opto-electronic properties using a very simple and versatile alignment method. 1.1.2. Alignment by Friction-Transfer The principle of the friction-transfer method developed by Smith and Wittmann to prepare oriented poly (tetrafluoroethylene) (PTFE) layers[38,39] was used extensively by the group of Yase to prepare highly oriented SCPs films.[40–47] Various semi-conducting polymers, e.g., poly(2,5-dioctyloxy-1,4-phenylenevinylene), poly(dimethylsilylene), poly(9,9′dioctyl-fluorene) (PFO), or regioregular poly(3-alkylthiophene)s were successfully oriented by this method. The polymer powder is first compressed in the form of a cylindrical pellet under high pressure (125–235 MPa). The polymer pellet is then slid at a constant speed of 1–2 cm s−1 on a hot substrate as shown in Figure 1b. In the case of P3HT, the sliding is performed at 150 °C (100 °C for P3DDT). Nagamatsu and co-workers[43] fabricated highly oriented P3HT and poly(3-dodecylthiophene) (P3DDT) films with typical dichroic ratio 10–100 (see Figure 6). The very high level of in-plane alignment of the polymer chains parallel to the direction of friction transfer was confirmed by grazing incidence X-ray diffraction (GIXD) measurements.[44] GIXD indicated that the π

Figure 3. a) UV–vis polarized optical absorption of the oriented C12-pBTTT film with parallel and perpendicular incident light polarization. b) Evolution of the dichroic ratio in C12-pBTTT films maintained at various temperatures Tr during rubbing. c) ED pattern of a thin films of C12-pBTTT after rubbing at T = 125 °C. d) Low-dose HR-TEM images of some highly oriented areas in C12-pBTTT thin films oriented by rubbing at T = 125°C. e) Schematic illustration of the re-orientation of crystalline domains of C12-pBTTT upon mechanical rubbing. The initial films show edge-on oriented domains with a random in-plane orientation of the b and c axes. At T ≥ 50 °C, partial disordering of the dodecyl side chains results in an increased plasticity of the films, which is necessary to align and re-orient C12-pBTTT nanocrystals towards face-on orientation. Reproduced with permission.[36] Copyright 2013, American Chemical Society.



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Figure 4. a) UV–vis absorption of the oriented p(NDI2OD-T2) films oriented by rubbing at 80 °C and subsequently annealed at 220 °C and slowly cooled (0.5 °C min−1) for incident light polarized parallel (//) and perpendicular (⊥) to the incident light polarization. b) HR-TEM image showing the oriented stacks of π-stacked p(NDI2OD-T2) chains. c) and d) Electron Diffraction pattern and corresponding schematic indexation. The rubbing direction is indicated by an arrow. (Results obtained in collaboration with Prof. S. Ludwigs, to be published, see ref. [35]).

Figure 6. a) Polarized UV–vis absorption spectrum of P3HT films oriented by friction transfer. b) and c): GIXD line scans obtained for friction-transferred P3HT. The scattering vector q is oriented b) perpendicular and c) parallel to the sliding direction. Reproduced with permission.[40] Copyright 2003, American Chemical Society.

Figure 5. a) BF of an oriented P3HT/CdSe thin film after rubbing. b) Electron diffraction pattern. The main reflections from P3HT and CdSe nanocrystals are indexed. c) High-resolution low-dose TEM image of the rubbed P3HT/CdSe film. The fringed pattern reveals the layers of π-stacked P3HT chains with a 1.65 nm period running parallel to the CdSe NRs. d) Schematic illustration of the top view of an oriented P3HT/CdSe film. Reproduced with permission.[37] Copyright 2013, American Chemical Society.

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stacking direction (b axis) was perpendicular to the substrate plane, i.e., the conjugated backbone adopts a face-on orientation, as observed for the rubbed P3HT layers. Moreover, the in-plane orientation of the chains was remarkable: the 1 0 0 reflection showed an FWHM of 10°, similar to that observed for epitaxied P3HT (vide infra). For oriented poly(3dodecylthiophene) films, the carrier mobility was measured in the direction parallel and perpendicular to the friction transfer, leading to carrier mobility anisotropy μ///μ⊥ = 8.[46] The friction transfer method was also applied to insoluble polymers like poly(benzo(1,2-d:4,5-d′)bisthiazole-2,6-diyl) (3,4,4′′′,4′′-tetradodecyl(2,2′:5′,2′′:5′′,23-quaterthiophene)5,5′′-diyl).[47] The anisotropy of charge transport was measured in OFETs with an oriented active layer consisting of





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pure face-on or pure edge-on oriented domains by adjusting the temperature of the substrate during rubbing (100 °C for face-on and 240 °C for edge-on oriented films). In the case of PFO, the very high level of alignment obtained by friction transfer lead to single-crystal-like electron diffraction patterns of the films after proper annealing at 210 °C and slow cooling to room temperature.[43] The highly oriented PFO layers were further used to fabricate highly polarized light-emitting diodes.[44] Some of the best devices showed luminances of 1800 Cd m−2 and a polarization ratio of 45. In addition, postdeposition vapor treatments allowed for the fabrication of oriented PFO films of the β form. Highly polarized βform emission was observed in corresponding polymer light-emitting diodes[45] with an integrated polarization ratio of 51. The possibility to achieve such highly polarized light emission is of major interest in the field of LCD technology for the replacement of conventional and highly energy consuming backlight panels. The benefit of the friction transfer method lies in the ease and rapidity of the process but thickness control of the transferred layer is difficult. Moreover, the necessity to anneal the substrate to high temperatures may be detrimental for flexible polymeric substrates such as polyethylene terephthalate. 1.1.3. Strain-Alignment An alternative alignment method of SCP is to use stretching of an active layer deposited onto a given polymeric substrate, e.g., polyethylene. Dyreklev et al.[48] achieved polarized electroluminescence out of a thin film of poly(3-(4-octyphenyl)-2,2′-bithiophene) with a dichroic ratio of 2.4. More recently, the group of DeLongchamp and co-workers[49] obtained high in-plane orientation of P3HT thin films by using a similar method of strain alignment. This method makes use of a polydimethylsiloxane (PDMS) substrate on which the P3HT film is first transferred. Upon straining of the PDMS substrate, alignment of P3HT is enforced via a plastic deformation of the film. The strained P3HT film is then transferred to an oxidized substrate after lamination and the PDMS substrate is removed. The oriented P3HT films exhibit a dichroic ratio A///A⊥ of 4.8 for an applied strain of 170%. As seen in Figure 7, the level of alignment measured via the anisotropy of UV–vis absorption is proportional to the applied strain. The phi-scan of the 1 0 0 reflection hints at an FWHM of 23°. GIXD measurements indicate that strain-orientation implies i) preferential alignment of P3HT backbones along the strain direction and also ii) edge-on → face-on reorientation of crystalline domains. As a consequence of backbone alignment parallel to the direction of strain application, charge transport anisotropies μ///μ⊥ ≈ 9 were reported and attributed to the intrinsic anisotropy of transport


Figure 7. a) Evolution of the dichroic ratio A///A⊥ as a function of strain applied to the P3HT thin films. b) X-ray diffraction phi-scan of the 2 0 0 reflection for a P3HT thin film after straining to 112%. c) Anisotropy of mobility in strain aligned P3HT films for transistors with different channel lengths. The lower inset illustrates the relative orientation of the gold electrodes relative to the strain direction. Reproduced with permission.[49]

(μ///μ⊥ correspond to the hole mobilities measured in the direction parallel and perpendicular to the applied strain). Although very interesting, the method of strain-alignment has some limitations in terms of large-scale device manufacture because of the use of soft polymeric substrates (PDMS or PE in the previous examples). In order to integrate the oriented active layers in devices, e.g., OFETs or OLEDs, a transfer to a suitable substrate is necessary. 1.1.4. Orientation by Flow Coating DeLongchamp et al.[50] investigated a second orientation method based on “flow-coating“ following the initial work


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Figure 8. a) Orientation of a pBTTT film using flow-coating. b) Schematic illustration of the oriented ribbon phase of pBTTT. c) AFM image of a flow-coated pBTTT film after thermal annealing. Reproduced with permission.[50] Copyright 2009, ACS.

of Meredith et al.[51] and Beers et al.[52] In its principle, the method is very similar to the doctor-blade method used to prepare large surfaces of polymer films. However, as demonstrated by DeLongchamp and co-workers, orientation of the polymer chains is enforced by flow-coating when applied to pBTTT. As seen in Figure 8, a spreading blade and a translating stage on which the silicon substrate is fixed are used to prepare the films. A solution of pBTTT in chlorobenzene is deposited at the edge of the blade, which is subsequently moved rapidly over the substrate at 2 mm s−1. The as-prepared films exhibit limited orientation, but annealing at 250 °C generates highly oriented films as inferred from ellipsometry measurements and AFM showing a nanoribbon morphology. This is further assessed by the GIXD results showing a clear difference in the patterns obtained when the incident X-ray beam is parallel or perpendicular to the flow direction. From these results, it is shown that the pBTTT chains orient parallel to the flow direction and generate a so-called terraced ribbon morphology. The mechanism by which a limited number of oriented seeds in the as-prepared films leads to large-scale orientation of the entire film after annealing is not fully elucidated so far but it is suggested that i) pBTTT adopts a lyotropic nematic state in solution or ii) directional drying of the film causes the orientation. Charge transport along the chain direction and perpendicular to it was measured in OFETs, with an oriented film of pBTTT. A rather low anistropy of charge mobility (μ///μ⊥ in the range 3–5) was evidenced and explained in terms of crystalline domain dimensions along both parallel and perpendicular directions in the device.[53,54] 1.2. Orientation of Conjugated Polymers on Substrates

1.2.1. Epitaxy of Semi-Crystalline Polymers Epitaxy can be defined as a directed growth of a material on the surface of a crystalline substrate along one or several preferential crystallographic directions. In most cases, the mutual orientation of the overlayer and substrate

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can be described by a 2 × 2 matrix relating the 2D lattices of both overlayer and substrate although 1D epitaxial mechanisms are also often encountered in the case of polymers.[55] While epitaxy is largely used in the semiconductor industry in silicon-based device manufacture, the use of epitaxy in the field of polymers has been more marginal, especially in the case of SCPs. Epitaxy of semi-crystalline polymers on aromatic organic crystals is an original and elegant method to grow highly crystalline and oriented polymer thin films with a controlled and regular organization of crystalline domains on a surface. Epitaxy of semi-crystalline polymers can be achieved on a large variety of substrates including, inorganic substrates such as NaCl or KCl,[56,57] aromatic molecular crystals,[58,59] polymeric alignment layers,[38] and infusible aromatic salts.[60,61] Epitaxy of polyolefins on inorganic substrates was first investigated in 1950s by Willems and by Fischer.[61,62] A significant progress towards a better understanding of polymer epitaxy was accomplished by Lotz and Wittmann in the early eighties[58,59,64] using transmission electron microscopy to observe the characteristic structural and morphological features in epitaxied polymer films. Moreover, the latter authors investigated epitaxial orientation of polymers on polymeric and organic substrates, e.g., aromatic molecular crystals. One of the major motivations for the study of polymer epitaxy was the need for a precise understanding of the nucleating ability of certain organic compounds for the crystallization of polyolefins.[58,59] The studies by Lotz and Wittmann uncovered epitaxial relationships based on 2D lattice match of polyethylene and the substrate crystals of p-terphenyl and anthracene. In this perspective, epitaxy was clearly identified as a major driving force explaining the efficiency of certain, albeit not all, aromatic crystals used as nucleating agents for polyolefins.[9,58,64] Epitaxial growth of polymers on aromatic molecular crystals is manifested by the development of peculiar patterns formed by crystalline lamellae growing along specific in-plane directions on the substrate surface. For instance, para-terphenyl and anthracene lead to textured and oriented polymer films of polyethylene (PE) when the growth is performed on the surface of uniform single crystals of these aromatic molecules.[58,59] Parts a and b of Figure 9 depict the characteristic film morphology and the electron diffraction pattern for epitaxied PE films grown on anthracene substrate whereas part c) and d) show the schematic interpretation of the ED pattern and the orientation of PE chains on the anthracene crystal respectively. The overall orientation of crystalline lamellae of PE in thin films epitaxied on anthracene and p-terphenyl is shown in Figure 9e. While polyolefins and the studies of fundamental structural issues were at the forefront of academic research





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Figure 9. a) Electron diffraction pattern and b) Bright field showing a cross-hatched pattern of oriented crystalline lamellae of PE epitaxied on anthracene. c) Schematic interpretation of the experimental ED pattern and d) illustration of the orientation of PE chains on the surface of an anthracene crystal. e) Schematic illustration of the orientation of PE crystalline lamellae as observed on substrates of p-terphenyl and anthracene. Note the two different (1 1 0) and (1 0 0) contact planes of PE crystals on p-terphenyl and anthracene, respectively. Adapted with permission.[58]



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Figure 10. a) Scheme illustrating the three main steps of the oriented P3HT thin film preparation method, b) and c) POM images of an oriented area of P3HT grown epitaxially on K-BrBz at T = 180 °C. The K-BrBz crystals has been removed by rinsing with ethanol. The remaining epitaxied P3HT area has the initial lozenge shape of the K-BrBz crystal. The epitaxied P3HT area shows a variation of the absorbance vs orientation of the incident light polarization. d) TEM bright-field image showing an oriented and nanostructured P3HT film after removal of the K-BrBz substrate. Note that the shape of the oriented P3HT film matches the initial rhombic-shaped K-BrBz crystal. Adapted with permission.[65] Copyright 2009, American Chemical Society.

in the early eighties, new functional polymers, e.g., SCPs have raised much interest over the past few decades. In the following, we will review a certain number of cases of oriented growth of SCPs either by epitaxy on molecular crystals or on polymer alignment layers. 1.2.2. Epitaxy of SCPs on Aromatic Crystal Surfaces Aromatic molecules such as acenes can be easily grown in the form of large single crystals that are suitable substrates for epitaxy of polymers. Anthracene and p-terphenyl crystals have been extensively used to address the epitaxial growth of polyethylene. However, in contrast to the latter polymers, SCPs usually exhibit very high melting temperatures (typically 240 °C for P3HT). Accordingly, the choice of aromatic crystal substrates is more restricted. One possibility is to use aromatic salt substrates, e.g., potassium4-bromobenzoate (KBrBz) or potassium acid phthalate (KAP), which can withstand very high annealing temperatures.[56,57] Both these substrates are suited for the epitaxial orientation of rr-P3HT generating however different patterns and orientations. The preparation of the oriented layers implies first to grow 10–100 μm large single crystals of the substrate from saturated solutions (see Figure 10).

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Figure 11. TEM BF images showing the oriented nanostructured patterns of P3HT formed on substrates of KBrBz (a and b) and KAP (c and d). (b) Note the unidirectional pattern obtained in the case of KAP versus cross-hatched pattern of P3HT domains on KBrBz. For KBrBz, epitaxy of P3HT generates a twofold in-plane orientation of P3HT crystalline domains as indicated by the blue and orange arrows depicting the in-plane orientations of the b and c axes. Adapted with permission.[65] Copyright 2011, American Chemical Society.

Then, the crystals are brought in contact with the polymer film, either by depositing a solution of single crystals onto the polymer film or by casting a polymer film on the top of single crystals of KAP or KBrBz. Isothermal annealing for several hours is then applied to the whole so as to promote the epitaxial orientation of the polymer from the melt state. Finally, the substrate material is readily removed by rinsing the films with water, which leaves large areas of epitaxied CPs films as shown in Figure 10d. As illustrated in parts a and b of Figure 11, epitaxial growth of P3HT on the surface of an aromatic salt (potassium 4-bromobenzoate) leads to highly crystalline, oriented and nanotextured P3HT films which consist of a regular network of interconnected semi-crystalline domains oriented along two preferential in-plane directions (see Figure 12a). The overall crystallinity and the level of in-plane orientation of the P3HT films are controlled by the temperature of isothermal crystallization (Tiso). Well-defined electron diffraction patterns with sharp reflections obtained for Tiso = 180 °C (see Figure 11b) indicate that the crystalline domains grow with a unique (1 0 0)P3HT contact plane on the K-BrBz substrate (so-called “edge-on“ orientation). The P3HT chains are oriented along two preferred in-plane directions of the K-BrBz substrates namely the [0 ± 2 1] directions. During annealing of the polymer film, the surface of the aromatic salt undergoes a topographic reconstruction resulting in a regular nanostructured “hill and valley“ morphology that





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new textures and nanomorphologies of SCPs. A further example concerns the use of potassium acid phthalate (KAP) substrates in a very similar approach. This type of substrate was originally used for the alignment of a series of diactylene molecules by Thierry and co-workers.[61] As seen in Figure 11, orientation of P3HT is also achieved on substrates of KAP, leading to a (1 0 0) contact plane with a unique in-plane orientation of the P3HT chain direction. 1.2.3. Directional Epitaxial Crystallization

Figure 12. a) Scheme showing the organization of the nanostructured P3HT films grown on K-BrBz substrate. Crystalline zones are shown in red while amorphous interlamellar zones are in blue. b) Schematic representation of the preferential nucleation and orientation of P3HT crystalline domains at the steps of a reconstructed K-BrBz substrate. The P3HT chains are running parallel to the [0 2 1]K-BrBz or the [0 -2 1]K-BrBz directions. The height of the π-stacked P3HT chains matches closely the observed step height of the K-BrBz substrate. Reproduced with permission.[65] Copyright 2011, American Chemical Society.

templates and orients the growth of P3HT in the form of oriented nanoribbons. Preferred orientation of P3HT crystalline domains occurs at step edges of the substrate (see Figure 12b). This is because of the matching between the layer period of P3HT aP3HT and the terrace height of the K-BrBz substrate aK-BrBz at the annealing temperature of 180 °C, i.e., aP3HT = 1.75 nm versus aK-BrBz/2 = 1.745 nm.[65] Accordingly, as illustrated in Figure 12b, in the absence of a matching between in-plane unit cell parameters of the substrate and P3HT, it is a matching of unit cell parameters along the substrate normal that gives rise to this unique type of epitaxial growth of P3HT on KBrBz. This results opens new perspectives in terms of epitaxial crystallization of conjugated polymers as numerous organic (as opposed to the inorganic KBr) salts such as potassium acid phthalates can be used to generate


One of the first examples of efficient epitaxial orientation of P3HT was obtained by using the directional epitaxial crystallization method on 1,3,5-trichlorobenzene (TCB) as proposed by Brinkmann and Wittmann.[66–72] The originality of this approach lies in the use of a crystallizable aromatic solvent, in this case TCB, which can successively play the role of solvent for the polymer and, once crystallized, the role of substrate for epitaxy. After orientation, TCB is readily removed by evaporation in primary vacuum leaving large areas of highly oriented P3HT. This original method was further improved in order to prepare films of P3HT with a high uniformity in terms of thickness and inplane orientation.[37] Figure 13 describes the various steps used to prepare P3HT films by this DEC method. In brief, the method improves the level of long-range orientation by using an orienting substrate of PTFE to guide the crystallization of the TCB. In addition, a zone-melting technique is used to control precisely the rate of crystal growth of TCB and achieve large and uniform surfaces of TCB crystals suitable for the epitaxial growth of P3HT. The improvement in the overall in-plane orientation of the films leads to both an increase of the observed dichroic ratio in the UV–Vis absorption spectra (in excess of 13) and to a substantially narrower angular spread of the in-plane orientation distribution of the P3HT crystals (full width at half maximum of 9°) as evidenced by GIXD measurements (see Figure 13e).[69] The morphology of the P3HT films was investigated by transmission electron microscopy, revealing an alternation of crystalline lamellae and amorphous interlamellar zones in both bright- and dark-field imaging modes (see Figure 14). The semi-crystalline morphology of P3HT was further evidenced by low-dose HR-TEM revealing fringed patterns with a 1.65 nm period corresponding to layers of π-stacked P3HT chains separated by alkyl side chains. HR-TEM showed very clearly the alternation of crystalline lamellae and amorphous (featureless) interlamellar zones. The total lamella period (crystalline plus amorphous) was shown to vary as a function of the molecular ¯w of P3HT.[67] weight M Orientation of P3HT on TCB has been explained in terms of 1D-epitaxy as the stacking periodicity of TCB molecules matches almost perfectly the repeat period


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Figure 13. a) Different steps in the preparation of highly oriented P3HT films by an improved DEC method. The use of a local zone melting as shown in the step 3, allows to grow TCB crystals with a high uniformity of in-plane orientation (Reproduced with permission from ref.[37] © 2011, Wiley-VCH). b,c) POM images for the incident light polarization horizontal for a P3HT thin film grown by DEC. d) Polarized UV–vis absorption spectra. e) X-ray diffractograms of oriented thin films of P3HT film. Red curve: out-ofplane configuration. Blue curve: GIXD with q ⊥ cP3HT. Black curve: q// cP3HT. The inset in e) corresponds to the in-plane omega scan around the (100)P3HT reflection for the oriented P3HT film. Adapted with permission from ref. [69] Copyright 2012, Royal Society of Chemistry.

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of 3-hexylthiophene monomers in the P3HT chain.[66] Using the epitaxial orientation of P3HT, Salleo and coworkers[16] investigated the anisotropy of charge transport in oriented films. Higher mobility was observed in the direction of the polymer chains with typically μ// > 10−2 cm2/V s, whereas charge transport measured in the direction perpendicular to the chains yielded μ⊥ < 10−3 cm2/V s. The origin of this anisotropy was attributed to the existence of a high density of grain boundaries between fiber-like domains, limiting charge transport in the direction perpendicular to the chain axis. The high orientation and crystallinity ¯w P3HT films grown by DEC of low- M made a structural refinement of form I by electron diffraction possible. The ¯w P3HT thin structure of oriented low- M films was analyzed by the rotation-tilt method that provided characteristic ED patterns corresponding to specific projections in reciprocal space. Based on these data, molecular modeling using a trial-and-error method was used to refine a structural model.[70] Interestingly, this study showed that despite a 3.8 Å stacking period of the polythiophene backbones, short 3.4 Å contacts between successive backbones are observed in form I (see Figure 14e,f). This result illustrates the fact that epitaxy of SCPs can be a means to circumvent the difficult (sometimes impossible) task of single crystal growth to refine crystal structures of SCPs. The directional epitaxial crystallization method was also used to orient polyfluorenes, e.g., poly(9,9′-di-n-octyl-2,7fluorene) (PFO).[71,72] Dialkylfluorenes tend to form crystalline lamellae made of extended chains (not folded chains like P3HT) because of the higher persistence length of the polymer chains. The consequence is that no significant amorphous interlamellar zones are observed in oriented PFO films but rather narrow grain boundaries between successive crystalline lamellae. This is particularly apparent in the dark-field image shown in Figure 15. Interestingly, the quality of the ED patterns obtained in oriented





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of P3HT and P3HT:fullerene blends. Instead of using pure TCB as a crystallizable solvent, a mixture of TCB and chlorobenzene was used as a solvent for P3HT and P3HT:fullerene. Upon evaporation of chlorobenzene, TCB generates spherullitic crystalline structures of large dimensions (several 100 μm in diameter) that are subsequently decorated by epitaxied P3HT.[73] 1.2.4. Epitaxy: Towards Oriented and Nanostructured Fibers

Figure 14. a) Bright-field TEM morphology showing a periodic alternation of crystalline lamellae separated by amorphous interlamellar zones. b) High-resolution TEM image showing the packing of P3HT chains within lamellae. c) Corresponding ED pattern. d) Schematic representation of the semi-crystalline structure of P3HT (see text). Reproduced with permission.[72] Copyright 2009, L’actualité chimique.

PFO films after annealing at 210 °C was sufficient to allow for a precise structural refinement using the method of rotation-tilt.[71] In this way, it was possible to uncover the space group and unit cell symmetry. More recently, Müller et al.[73] adapted the directional epitaxial method to prepare spherullitic-like structures

Figure 15. a) Diffraction pattern of a highly oriented PFO film grown by DEC on TCB and subsequently annealed at 210 °C for 10 min and slowly cooled to ambient (0.4 °C min−1). b) Dark-field image obtained by selecting the 0 0 8 reflection and showing the alternation of crystalline PFO lamellae separated by narrow black grain boundaries. c) Calculated ED pattern using the structure shown in d). Reproduced with permission.[72] Copyright 2009, L’actualité chimique.


A further example of original polymer morphogenesis is illustrated in Figure 16 showing the so-called shish-kebab fibers of P3HT.[74] Highly-oriented fibers of regioregular poly(3-alkylthiophene)s (P3ATs) with a “shish-kebab” morphology have been prepared by oriented epitaxial crystallization in a mixture of 1,3,5-trichlorobenzene (TCB) and pyridine. The superstructure of the P3AT fibers consists of an oriented thread-like core several hundreds of micrometers long (the “shish”) onto which lateral crystalline fibrils made of folded polymer chains (the “kebabs”) are connected with a periodicity of 18 to 30 nm (see Figure 16). The P3AT chain axis is oriented parallel to the fiber axis whereas the π-stacking direction is perpendicular to it. The oriented character of the shish-kebab fibers results in polarized optical absorption and photoluminescence. The

Figure 16. a) Optical image taken with crossed polarizers, showing highly birefringent TCB needles decorated with oriented P3HT fibers (the orientation of the polarizer and analyzer are indicated by two arrows). b) TEM bright-field image of a bundle of P3HT fibers with a shish-kebab superstructure after removal of both pyridine and TCB. c) Enlarged bright-field image of a shish-kebab fiber of P3HT showing the tight packing of nanofibrils (kebabs) along the backbone of the fiber (shish). d) Schematic structure of a shish-kebab fiber. The directions of the unit cell axes are indicated by arrows. (Reproduced with permission from ref.[74] © 2009, Wiley-VCH).


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formation of oriented precursors by epitaxial orientation of polymer chains onto long needles of a molecular crystal, here TCB, appears to be an original alternative to the shishkebab crystallization usually performed under external flow conditions. 1.3. Epitaxy on Oriented Polymer Substrates 1.3.1. Polyimide Substrates The most widely used polymer alignment layers are based on polyimide films oriented by mechanical rubbing.[24] Polyimides (PI) have a high thermal stability compatible with the thermotropic alignment at temperatures exceeding 200 °C, which are necessary to align the liquid crystalline phase of hairy-rod-like SCPs. Highy oriented films of various polyfluorenes were obtained on PI substrates as illustrated for poly(9,9-di(2-ethylhexyl)fluorene2,7-diyl) (PF2/6) in Figure 17.[75–83] Although high in-plane orientation of PFs can be observed, the orientation is most of the time uniaxial with the chain axis oriented parallel to the rubbing direction of the polyimide, i.e., no unique contact plane of crystalline domains is obtained. As illustrated

in Figure 17b, several contact planes of crystalline PF2/6 domains coexist. Of importance is also the fact that the efficiency of the thermotropic alignment in the nematic glassy phase is a function of the polymer molecular weight (that controls the viscosity of the melt). This has been demonstrated in the case of poly(9,9′-dioctylfluorene-co¯w polymers are required benzothiadiazole) for which low-M to form large and highly oriented monodomains with improved opto-electronic properties.[77] For the class of poly(dialkylfluorenes), the level of in-plane alignment is a function of the alkyl side chain length: shorter side chains leading to higher orientation parameters.[80,81] Although rubbed polyimide films show a very high orienting capability, limitations inherent to rubbing, e.g., scratching and surface contamination can affect the quality of the subsequently deposited SCP films. Therefore, a different approach based on photoalignment of azobenzene side-chain polymers has been developed recently.[84–86] When irradiated with linearly polarized UV light, azobenzene side chain polymers align preferentially along the UV light polarization direction. Such oriented polymer films proved very efficient to align SCPs like polyfluorenes in the liquid crystalline phase.[84] To prepare alignment layers, Sakamoto et al.[84,85] used a polyamic acid containing azobenzene (Azo-PAA), which is transformed to a thermally, optically, and chemically stable polyimide film upon photoisomerization reaction. PFO films were oriented on such photoaligned polyimide substrates and showed a high polarization in emission with a DR of 30 at 432 nm. Using the same photoaligned substrates, OFETs with inverted gate architecture were prepared using oriented active layers of poly[(9,9′dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2). High charge-transport anisotropies were measured with μ// = 0.016 cm2/V s and μ⊥ = 0.002 cm2/V s [85] (μ// and μ⊥ are the hole mobilities measured parallel and perpendicular to the chain direction). 1.3.2. Semicrystalline Polymeric Substrates

Figure 17. a) GIXD 2D images of an oriented PF2/6 thin film oriented on a rubbed polyimide substrate as obtained for the incident X-ray beam oriented along the rubbing direction (top) and perpendicular to it (bottom). Blue and red indices refer to the domains with orientation I and II on the substrate. B) Schematic illustration of the three different populations of PF2/6 domains assuming chain alignment along the rubbing direction (z axis). (Reproduced with permission from ref.[78] © 2007, American Chemical Society).

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Besides oriented polyimide substrates, Yan and coworkers[86] also investigated the possibility to use highly oriented polyethylene substrates to align P3HT films. Free standing and highly oriented PE substrates were prepared by the hot-draw method proposed originally by Petermann and Gohil.[86–88] In hot-drawn PE films, the chains of PE are oriented along the drawing direction whereas periodically spaced crystalline lamellae are oriented perpendicular to that direction. As demonstrated by the ED pattern in Figure 18, P3HT spin-coated from a chloroform solution on PE tends to align with the chain axis parallel to that of PE. Moreover, the (100) lattice plane of P3HT is in contact with the PE substrate. The observed epitaxy was explained in terms of a 1D lattice matching between the interchain





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Figure 18. AFM height image of an epitaxially oriented P3HT film on an oriented PE substrate. b) Electron diffraction pattern of the oriented P3HT film on PE. The arrows point at the reflections of P3HT. Reproduced with permission.[86] Copyright 2011, ACS.

distances of PE in the (110) lattice plane and P3HT in the (100) lattice plane. 1.3.3. Orientation on Friction-Transferred Poly(tetrafluoroethylene) Substrates Beside polyimides, friction-transferred PTFE substrates are particularly well suited for the orientation of SCPs. The alignment capability of PTFE substrates was evidenced by Wittmann and Smith following the original work on oriented PTFE films by Pooley and Tabor.[38,89] Frictiontransfer of a PTFE rod on a hot substrate (T = 250–300 °C) leads to a sub-100 nm thick oriented PTFE layer with a biaxial orientation, i.e., the polymer chain direction cPTFE is parallel to the direction of friction transfer and the films have a unique (0 1 0) contact plane. By essence, PTFE forms different polymorphs and a solid–solid phase transition is observed around T = 19 °C between a low-temperature phase (phase II) consisting of 13/6 helices and an intermediate-temperature phase (phase IV) with 15/7 helices.[90–92] Numerous polymers and small molecules were successfully oriented on these substrates.[93–104] Regarding SCPs, the initial studies were devoted to PPV-based systems obtained using a precursor route. Films with rather limited orientation were obtained (dichroic ratio around 2) presumably because alignment was limited to the thin adjacent layer atop the PTFE substrate.[101] However, since the development of alkyl-substituted SCPs that can be processed from solution, very high orientations were reported for several SCPs, in particular poly(9,9-di(n-octyl) fluorene-2,7-diyl) (PFO), poly(9,9-di(2-ethylhexyl)fluorene2,7-diyl) (PF2/6), and poly(9,-hexyl-9-(2′-ethylhexyl)fluorene-2,7-diyl).[98,99] The orientation achieved in the as-cast PFs thin films on PTFE is relatively minor but annealing above or close to the melting temperature promotes the crystallization and the formation of lamellar crystals oriented perpendicular to the PTFE fiber direction. Figure 19 depicts the typical ED pattern, BF morphology, AFM topographic image, and dark-field image of a highly oriented


Figure 19. a) Electron diffraction pattern of a highly oriented PF2/6 thin film ( Mn = 43.7 kDa) grown on an oriented PTFE substrate after annealing at 180 °C (1 min) and slow cooling to room temperature (0.4 °C min−1). The layer lines have been indexed on the basis of a 21/4 helix as discussed in the text. Additional reflections on the meridian have been highlighted by asterisk as well as the position of the diffuse halo; b) Corresponding TEM bright-field image. The PTFE chain axis direction cPTFE is indicated by a white arrow; c) Topographic AFM image showing the periodic lamellar structure of the PF2/6 thin film. d) Dark-field image obtained by selecting the intense 0 0 21 reflection. Reproduced with permission.[103] Copyright 2009, ACS.

PF2/6 film. The dichroic ratio in absorption increases from ≈2 before annealing to 4–5 after annealing (6–9 for photoluminescence). The ED pattern indicates a perfect biaxial orientation, i.e., the crystalline lamellae of PF2/6 grow with a unique (0 1 0)PF2/6 contact plane on the PTFE substrate. This is at variance with films oriented on rubbed polyimides, which show a uniaxial orientation.[78] The ED pattern of PF2/6 films grown on PTFE is single crystal like with extremely sharp reflections, which allowed for a determination of the chain conformation. Contrary to the predicted 5/1 or 5/2 helical conformation, the ED results indicate a more complex 21/4 helical conformation. The interest of PTFE substrates lies mainly in the nucleating ability for systems that crystallize slowly and/or with difficulty.[95,96] This holds for alternating copolymers made of naphthalenebisimide and bithiophene units, e.g., poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′bithiophene)} p(NDI2OD-T2) that belong to a new and promising class of electron-transporting polymers.[105,106] After melt-annealing above 300 °C, spin-coated films of p(NDI2OD-T2) on SiO2 substrates are essentially


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Figure 20. a,b) Bright-field and electron diffraction pattern of an oriented p(NDI2OD-T2) thin film grown on oriented PTFE. The inset corresponds to the calculated fast Fourier transform (c) Corresponding phase mode AFM image in tapping mode. The arrows indicate the direction of the PTFE chains. The asterisk in (b) points at the (1 0 0) reflection of the PTFE substrate. Reproduced with permission.[108] Copyright 2012, American Chemical Society.

amorphous[107] whereas for the same preparation conditions, highly oriented films with a well-defined lamellar structure are observed on PTFE substrates (see Figure 20).[108] Interestingly, the use of such orienting methods allowed to identify two different forms of p(NDI2OD-T2) corresponding to segregated and mixed stacking modes of naphthalenebisimide and bithiophene units in the crystal.[108] On PTFE, only the structure characterized by a mixed stacking mode was observed in the form of lamellar crystalline domains with essentially edge-on orientation (π-stacking direction perpendicular to the chain axis of PTFE) and the chain axis parallel to cPTFE.

alignment methods will certainly be privileged. Beyond pure SCP films, hybrid materials comprising a SCP and inorganic nanoparticles with tailored opto-electronic properties are gaining increasing interest as new functional materials. In this perspective, epitaxy of SCPs seems an interesting approach to achieve growth control, alignment, and nanostructuring. Acknowledgements: This work has been supported by the French National Agency (ANR) in the frame of its program in Nanosciences and Nanotechnologies (MYOSOTIS project n°ANR-08-NANO-012–01) and the ANR project PICASSO (project ANR-11-BS08–0009.) L.H. acknowledges financial support from the Région Alsace. The IRTG project SOMAS is gratefully acknowledged for financial support.

2. Conclusion and Outlook This review has demonstrated that orientation of SCPs remains a timely issue in plastic electronics with both applicative and fundamental aspects. Applications like organic photovoltaics bear intrinsic interests related to nanostructuring and orientation-control of SCPs in thin films. In this perspective, epitaxy-driven nucleation and growth in donor–acceptor (D/A) blends can be envisioned as an elegant approach to fine tune the structure of the D/A interfaces. A first step in that direction has been recently demonstrated by Emrick and co-workers[109] who evidenced a shish-kebab-like morphology in a binary mixture of P3AT and perylenebisimides. Beside material design, epitaxy is a unique method to prepare SCPs films with high levels of crystallinity and orientation. Such films are ideally suited for fine structural investigations using electron diffraction, especially for the present polymers that seldom crystallize in the form of extended single crystals.[110] More challenging seems the up-scaling of epitaxy-based orientation methods for device manufacture, and this is an area where shear-based and non-contact

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Received: September 12, 2013; Revised: October 17, 2013; Published online: December 2, 2013; DOI: 10.1002/marc.201300712

Keywords: conjugated transport properties

polymers;

nanostructures;

charge-

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