Polythiophenes crystallinity

Mo, Z. et al., X-ray-scattering from polythiophene Crystallinity and crystallographic structure, Macromolecules 18, 1972-1977, 1985. 

Polythiophene is a highly crystalline polymer with the chain analog to cis-polyacetylene. The sulfur atoms stabilize the structure and interacts poorly with... 

While the polymers seem to be amorphous in the solid state, there are signs of a transition in solution, possibly from a disordered coil state to a helix, which involves a temperature-dependent colour change 266). This change is also associated with precipitation of a partly-crystalline form. The crystals and solution apparently co-exist over a rather wide temperature range, which is unusual because polymers generally show a rather sharp transition from solubility to insolubility. This may reflect a broad distribution of molecular weights in the polythiophenes. 

Against this background of infusible conducting polymers, the development of the soluble polythiophenes is most interesting. Glass transition temperatures have been reported as 48 °C for poly(3-butylthiophene) and 145 °C for poly(3-methyl-thiophene) 261). These polymers also show crystallinity in films and can be crystallized from solution. Solution studies indicate that there are two chain conformations 262) and the availability of a non-conjugated conformation may be a key to the low transition temperatures and solubility, when compared to the stiff-chain conjugated polymers. 

As with crystalline organic semiconductors, there are only few systematic and quantitative studies of bias stress in polymeric TFTs. For example, it is still unclear whether in all polymers a positive VG leads to positive Vj shifts [7, 18]. Here we will concentrate on negative AVT after application of a negative Vg, which is nearly universally observed in p-type devices with a number of semiconductor-dielectric combinations. In the next section of this chapter we will focus on studies of poly-fluorene and polythiophene TFTs, because these are the polymer devices for which bias stress has been most thoroughly characterized in recent years. 

Wu Y, Liu P, Ong BS, Srikumar T, Zhao N, Botton G, Zhu S. (2005) Controlled orientation of liquid-crystalline polythiophene semiconductors for high-performance organic thin-film transistors. Appl Phys Lett 86 142102. 

Electrochemically prepared alkylated polythiophenes have been investigated by Gamier and coworkers.90 91 When comparing polythiophene and monosubstituted polyalkylthiophenes, these workers found an increase in crystallinity of the substituted thiophenes in comparison to the unsubstituted poly thiophene. The degree of crystallinity was low (5%), but the crystal structure was assigned to a hexagonal cell... 

A great deal of work has been done to characterize the structure of films cast from chemically prepared polythiophenes.96 Most of this work has been conducted on undoped PTh s because of their importance in electronic devices, such as thin-film transistors and photovoltaics. Depending on composition and processing conditions, the crystallinity can be as high as 30% in polythiophenes.97 A layered structure is usually formed in PTh crystals between extended chains held together by ir-slacking, with additional structural order introduced by alkyl side-chain interactions in functionalized PTh s.98 Chemically synthesized PTh powders exhibit a... 

Semiconducting polythiophene derivatives have been designed to assemble into large crystalline domains on crystallization and to possess an extended, planar... 

Conducting polymers such as polyacetylene, polypyrrole, polyaniline, polythiophene, etc. have been actively studied for use in various fields due to their interesting properties batteries,46 electrochromic displays,47 materials for supercapacitors,48 corrosion protection,49 protecting layers for static electricity,50 materials for organic electroluminescence displays,51 sensing materials,52 etc. Polypyrrole is reported to be extremely rigid, with a semi-crystalline structure. 

Devices made using these two techniques were essentially amorphous. Elec-tropolymerized pol3dhiophene has no strong organizing force for crystallinity, and the large groups used to solubilize polythiophene sterically interfere with chain-to-chain packing. 

Fig. 3.8. (a) a-hexithiophene, an oligomeric form of polythiophene. This material is typically vacuum deposited, (b) Adds two alkane groups at the ends, which improves the material crystallinity [29]. Despite the methyi groups the solubihty in common solvents is low. (c) An oligofluorene with strong seif-organizing properties when deposited on a heated substrate [28]... 

Unsubstituted polythiophene (PT) is an intractable material because it is neither soluble nor fusible, although films electrochemically prepared under certain conditions may be stretch-deformed. Generally speaking PT prepared by purely chemical routes turns out to be more crystalline than their electrochemical counterparts, as will be discussed in the following. 

Polythiophene, as obtained from Grignard-coupling polymerization from dibromothiophene, was vacuum deposited on various substrates and studied by electron diffraction by Yamamoto el al. [51]. They report diffraction patterns obtained from a single crystalline region, showing 44 distinct reflections of hkO-type. The indexing is in accordance with the a and 6-axes values obtained by Mo el al. Curiously the crystal is oriented with its c-axis perpendicular to the substrate, and the c-axis parameter could not be determined. The crystal is reported to deteriorate under the electron beam. 

Yamamoto et a. [60-62] and Kurata et al [63] prepared polythiophene layers by vacuum evaporation. To ensure the deposition of poly- and not oligothiophenes, they removed low molecular weight material ( < 15-17) by Soxhlet extraction with CHCI3, The evaporated material is assumed to consist of poly-thiophene with a molecular weight of about 1500-2000 and 20-25 thiophene units. The X-ray and electron diffraction data of Yamamoto et al. point to a (partially) crystalline film with the polymer axes oriented perpendicular to a carbon or gold substrate plane if sub = 423 K. At lower substrate temperatures, the orientation and crystallinity are worse. This behaviour is not only true for the first deposited layer, but at least up to 10 layers, i.e, 100 nm. On polyimide substrates, however, the molecules lie on the substrate plane and orient along the rubbing direction of the polyimide [61]. 

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