Solution-Cast Polythiophenes

Letizia, J.A. et al.. High electron mobility in solution-cast and vapor-deposited phen-acyl-quaterthiophene-based field-effect transistors Toward -type polythiophenes, J. Am. Chem. Soc. 127, 13476-13477, 2005. 

It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained firom substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regioregular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and polyisocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. 

Figure 8.26. Absorption spectra of the solution-cast films (approximately 0.1 /xm thickness) of various soluble polythiophenes ...

A. M. Rais, Doped polythiophene nanocomposite casting composition, useful in e.g. inkjet printing, screen printing and solution casting, comprises conductive polythiophene, dopant, organic hinder, crosslinker, carhon nano-tuhes, and chemical diluents. World Patent, WO2013133688-A1, 2013. 

As with polyanilines, polythiophenes can either be prepared directly by electropolymerization, or by casting from solutions (for alkyl-substituted thiophenes). Most interest has focused on the latter because of their improved mechanical properties compared with those of electrochemically prepared films. The factors influencing the mechanical properties of PTh s are reviewed in this section. 

Polythiophenes containing electron withdrawing alkyl carboxylates (35k-351) have also been synthesized.[212] OTFTs fabricated with the regioregular polythiophene (35k) gave mobilities of up to 0.07 cm V s compared to 0.0063 cm V s for the regiorandom polythiophene (351).[212] Another electron-with-drawing moiety, tetrafluorobenzene, has been substituted on the polyalkylthiophene backbone (35m). Spin-cast films of 35m from 0.5 wt % chloroform solution showed a mobility of up to 2 x 10 cm V s. [213]... 

Various processing routes of the soluble polythiophenes (synthesized either electrochemically or chemically) in combination with doping are summarized in Figure 8.13 [51]. For the purposes of spectroscopic measurements, the polymer films can be cast on a suitable substrate (quartz, KBr, ITO-coated glasses, etc.) in an appropriate thickness. Free-standing films obtained from thick solutions of the polymers are also available for these purposes and for conductivity measurements. Individual specific treatments can be seen in the subsequent sections (see, for example. Sections 4 and 6). 

Thin-film preparation is important, as in the case of the polythiophenes. The thin films can be obtained on suitable substrates such as glass slides either by casting from solution or by vacuum evaporation. In particular, the relatively low molecular weight of the materials enables a moderate rate of evaporation, ensuring the preparation of exceedingly uniform thin film [78]. Vacuum evaporation is usually carried out e.g. at a pressure of 10 Torr and at an appropriately elevated temperature of 180°C [79]. The thickness of the deposit can be controlled by adjusting the sample concentration in the solution or by changing the sample amount fed into a cmcible. 

The solution studies indicate that the ordered supramolecular disk structures with globally ordered extended conformations of polythiophene are desirable precursors to self-assembled PAT films, whereas the needle-like structures do not pack well and cannot give a globally ordered structure. This discovery implicitly has a profound effect on film casting and the resultant solid-state order (discussed below). The light-scattering data also suggest that solution spectra will vary with temperature, concentration, solvent composition, and the thermal history of the sample. 

Certain polymers, such as polyaniline, in their neutral form are soluble in A/-methyl pyrrolidone (NMP) [2], A/,A/-dimethylpropylene urea (DMPU) [3], and dimethyl sulfoxide (DMSO). Films of neutral polymer can be cast from these solutions. Chemical modification of monomers has proved to be effective in enhancing the modification solubility, especially with polythiophene [4,5]. In other instances, such as poly(p-phenylenevinylene), processibility has been achieved by synthesizing solu-tion-processible precursors, which could be subsequently converted to the conjugated polymer [6]. Nonetheless these techniques have failed to provide a method... 

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