A-sexithienyl

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Electronic Structure and Energy Transfer in Solid a-Sexithienyl... 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion-radicals. These devices are based on the sandwich-type electrode structures containing organic layers as the electron/hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion-radicals lowers the barrier height for the electron or hole injection. Anion-radicals (e.g., anion-radicals from 4,7-diphenyl-l,10-phenanthroline—Kido and Matsumoto 1998 from tetra (arylethynyl) cyclooctatetraenes—Lu et al. 2000 from bis (1-octylamino) perylene-3,4 9,10-bis (dicarboximide)s— Ahrens et al. 2006) or cation-radicals (e.g., cation-radicals from a-sexithienyl—Kurata et al. 1998 l,l-diphenyl-2-[phenyl-4-A/,A- /i(4 -methylphenyl)] ethylene— Umeda et al. 1990, 2000), all of them are electron or hole carriers. 

Added to that, kinetics can also be simulated with time-dependent decay rate parameters. Dexheimer et al. and Lanzani et al have identified such dynamics in C60 (ps regime) and a-sexithienyl thin films (fs regime) from pump-probe experiments [6,7]. They implemented aForster rate equation with a 5— function pulse... 

Figure 14 shows the unpolarized absorption spectra of films of polythiophene [124] and its hexamer a-sexithienyl [125]. The hexamer solution spectrum is at almost the same energy. As in Fig. 12a, the oligomer shows structure on its rising edge, and the polymer spectrum is structureless. But in Fig. 14 the absorption thresholds differ by = 0.2 eV only one would then say that conjugation extends in PT over only about six monomers. One could show similar data for many other CPs. 

Figure 14 Optical absorption of polythiophene (dashed line) and of the corresponding hexamer a-sexithienyl (solid line).

There are a few such measurements on poly thiophene, where a fXpn value of 10 5 to 10 4 cm2/V s has been found [227]. Even accounting for trapping, this is a very small value. The most systematic measurements have been performed on several unsubstituted or substituted thiophene oligomers [226,228]. Values of xFE are between 10-7 and 10-1 cm2/V s, the largest values for the hexamers. For instance, a-sexithienyl has a xFE value of 2 x 10 3 to a few 10 2, depending on the insulator. The carriers are holes. 

It is interesting to compare these values with the result of another method, the analysis of current-voltage relation for space-charge-limited currents. Such a study on a-sexithienyl [229] has given an effective mobility p, 2 x 10 2 cm2/V s, limited by traps 0.28 eV above the top of the valence band. Thus the trap-free microscopic mobility is likely to be at least two orders of magnitude larger, a typical value for a molecular crystal. 

Work with a-sexithienyl TFTs yielded more promising values The field-effect mobility reaches 3 x 10"2 cm2/V s, with currents and voltages similar to those with CPs [227,254]. For comparison, amorphous silicon FETs [256] usually have x 0.1 to 0.3 cm2/V s, sometimes nearly reaching 1 cm2/V s [257], with /D 1 to 100 xA at voltages of about 10 V, and such devices are used in liquid-crystal display panels [234]. 

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