Doped poly acetylene

Most conducting polymers, such as doped poly(acetylene), poly(p-pheny-lene), and poly(/ -phenylene sulfide), are not stable in air. Their electrical conductivity degrades rapidly, apparently due to reaction with oxygen and/or water. Poly(pyrrole) by contrast appears to be stable in the doped conductive state. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

FIGURE 6.3 Conductivities of doped poly acetylenes conductivities of insulators, semiconductors and metals are given for comparison. 

Recently, intense interest has been paid on doped poly(acetylene) because its film 68) showed markedly high conductivity on doping69 70 71 and the n- and p-type conductivities are depending on the dopants. The confirmation of p-n junction formation with the p- and n-type -fCH T, has roused great expectations to produce a polymer film solar cell.72a)... 

The doped poly(acetylene) forms various junctions such as a) a p-n junction from p- and n- -f CH, b) a hetero-Schottky junction from the inorganic semiconductor and metalic -f CH-, and c) a heterojunction from the inorganic semiconductor and semiconducting The bandgaps of -(-CH (trans- 0.6 eV, cis- 0.9 eV)... 

Others Semiconductors or insulators Doped poly acetylene, TTF-TCNQ... 

Figure 4.8-12 IR absorption spectra of neutral (a) and AsFj-doped poly(acetylene) (b), according to Rabolt et al., 1979.

Figure 1,17. Electronic energy levels associated with various types of defect structures found in doped poly acetylene.

When the 7r-systems of two or more double bonds overlap, as in conjugated dienes and polyenes, the 7r-electrons will be delocalized. This has chemical consequences, which implies that the range of possible chemical reactions is vastly extended over that of the alkenes. Examples are various per icy die reactions or charge transport in doped poly-acetylenes. A detailed understanding of the electronic structure of polyenes is therefore of utmost importance for development within this field. We will first discuss the structure of dienes and polyenes based on theoretical studies. Thereafter the results from experimental studies are presented and discussed. 

In the past three decades, several types of r-electron systems have shown very interesting features in electrical transport properties [1-4]. Charge-transfer complexes, intercalated graphite, conjugated polymers, carbon-60, carbon nanotubes, etc., are some of the well-known r-electron systems. Polymeric materials were considered as insulators before the discovery of metallic poly(sulfur nitride), [SN],, and the enhancement of conductivity in doped poly acetylene, (CH),, by several orders of magnitude [4, 5]. 

Fig. 19.8 Conductivity vs. air exposure time for I -doped poly acetylene (1) in the absence of and (2) in the presence of a antioxidizing agent (2-terbutyl,6-methyl phenol) (Data from Aldiss 1989)...

Since doped poly(acetylene)s were shown to possess metallic conductivity, this class of organic polymers has been studied intensively. Regardless of the catalyst system used, it is an inherent disadvantage of the washing process which is required to remove the catalyst or residues from the desired polymer preparation. Therefore a method was developed that avoids this pitfall and provides polyene films that form a solid layer on glass surfaces, ceramic plates, tubes, etc. 

The situation described by equ. 11a, b was used by S. Kivelson as the basis of a theory which describes the conductivity and the electronic behaviour of "lightly doped" poly(acetylene), i.e. the material which was treated with little oxidant (or reductand) as to bring the conductivity from the insulating state to the level of a good semiconductor. 

Fig. 11-45 ENDOR data on pure and doped poly (acetylene), bottom to top, increased doping. After Reference [468], reproduced with permission.

Winokur, M., Moon, Y. B., Heeger, A. J., Barker, J., Bott, D. C., and Shirakawa, H. (1987) "X-Ray Scattering from Sodium-Doped Poly acetylene Incommensurate-Commensurate and Order-Disorder Transformations" Phys. Rev. Lett. 58, 2329-2332. Murthy, N. S., Shacklette, L. W., and Baughman, R. H. (1989) "Structure of Lithium-Doped Polyacetylene" Phys. Rev. B 40, 12550-12553. 

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