Model oligothiophenes

Linear monodisperse tt-conjugated oligopyrroles and oligothiophenes as model compounds for polymers 99AG(E)1350. 

G. Zerbi, B. Chierichetti, and O. Inganas, Vibrational spectra of oligothiophenes as model of polythiophenes, J. Chem. Phys., 94 4637-4645, 1991. 

Toward an understanding of the conduction properties of polythiophenes (26) and polypyrroles (25) a large number of soluble oligomers has been prepared. Oligothiophenes, indeed, represent the most common model compounds for electrically conducting polymers [149]. Thereby, lower oligomers... 

The polaron model was also used by Waragai and Hotta [121] to analyze temperature-dependent data on methyl-substituted oligothiophenes. While their data did not cover a sufficient temperature range to discern a clear temperature dependence of the mobility, they noticed that [j. represented a drain voltage dependence, which they attributed to the dependence given by Eq. (14.69). Accordingly, the temperature-dependent mobility could be fitted to a thermally activated law of the form where Etot = fc/2 - ys/F, y being... 

Still, unsubstituted a-nT is a model system used in recent literature. One disadvantage towards application of the unsubstituted oligothiophenes however is the poor solubility of the rigid rods [34]. 

T.J., Building blocks for n-type molecular and polymeric electronics. Perfluoroalkyl versus alkyl-functionalized oligothiophenes (riT n = 2-6). Systematics of thin film microstructure, semiconductor performance, and modeling of majority charge injection in field-effect transistors, J. Am. Chem. Soc., 126, 13859-13874, 2004. 

It will be shown in this article that oligothiophenes are not only model substances for polythiophenes but often have superior properties. The results will show that a substantial knowledge could be collected over the last years but that there are also many unresolved problems to date. In particular, real applications are still rare and are limited to prototype studies. The major drawback is the poor long-term stability of the devices which is an inherent problem of nearly all organic-based systems. 

In the case of pure electrical measurements for substrates and contact materials Au, Ag, or Pt are preferred due to the p-type behaviour of as-prepared oligothiophenes. If simple band models are assumed for otnT and the contacts, materials like the noble metals with a workfunction of 5.3 eV (Au) or 5.6 eV (Pt) should lead to ohmic contacts whereas materials with low workfunction such as Al (4.28 eV) or Mg (3.66 eV) should form Schottky barriers. (For n-type behaviour, i.e. after n-doping or annealing in air, compare Section 4.2.2, the opposite is true.) Both types of contacts are necessary for electro-optical measurements. Here also one electrode has to be optically transparent. The most common material for the latter purpose is indium-oxide doped tin-oxide (ITO). This material is highly transparent and highly conductive but has the problem that the substrate always exhibits several spikes standing out of the surface. The other type of semi-transparent electrodes are ultra-thin metal films evaporated onto the organic film. 

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