J. Phys. Chem. 1983,
87, 2289-2292
2289
Effect of Dopant on the Physicochemical and Electrical Properties of Organic Conducting Polymers G. Tourillon and F.
Gamier*
Laboratoire de Photochlmle Solaire, ER 241, CNRS, 94320 Thiais, France (Received: March 25, 1983)
Electrochemically synthesized polythiophene and poly-substituted thiophene have been characterized by visible, IR, and XPS spectroscopy, and compared to polypyrrole. A high intrachain conductivity, increasing with the doping level, is demonstrated for the thiophene derivatives, which show a marked metallic behavior. On the other hand, the relatively low macroscopic conductivity, measured on pressed pellets, is related to poor interchain contacts and to morphological inhomogenities.
The doping process of organic polymers, like polyacetylene (CH)„ has been shown to give a very interesting property, allowing these compounds to evolve from an insulator state to a highly conducting state.1"1 A large number of works has already been devoted to the characterization of the dopant-polymer interaction, and, when considering its reversibility, a limit of the doping level of 5-15% has been observed for chemically synthesized polymers.4 5More recently6"7 electrochemically synthesized polymers have been reversibly doped to 25-30%, and high conductivities have been obtained. In order to evaluate the dopant effect of this new class of polymers, we have analyzed a series of polythiophene and poly-substituted thiophene by using XPS, visible and IR spectroscopies, and dc conductivity measurements. We show that, although a high metallic behavior exists in the polymeric chains, the relatively low macroscopic conductivity must be associated with poor interchain contacts. 23
I
xszxsz
Ss/
J
II
possibility of reaching a very high doping level, 50%, with the couple poly(3-methylthiophene)-CF3S03" and (ii) the existence of very small amount of dopant in all reduced polymers, explaining their semiconducting behavior ( 10'7 "1 cm"1). States I and II have been characterized spectroscopically. ~
Undoped State II The visible absorption spectrum of the polymer (Figure characterized by an intense band with a maximum at 430 nm for polypyrrole, as already determined (Figure la), and at 480 nm for polythiophene (Figure lb) and poly(3-methylthiophene) (Figure lc). These maxima correspond to the values expected for a long conjugated chain of these heterocycles.8 Using a band model, we calculated the bandgap to be about 2-2.5 eV. The absorption coefficients are very high, comprising between 10® and 4 X 10® cm"1, analogous to those observed for semiconductors with a direct gap (~106 cm"1).9 The poly 3,4-disubstituted thiophene (dimethyl and diethyl) absorption band maxima lie, however, at 330 and 280 nm, respectively. We have recently shown,10 that this deviation is associated with a twist between the monomeric units in the polymeric chain, due to substituent steric interactions, which cause a very high loss in electronic conjugation. The IR spectrum is characteristic of the considered polyheterocycle with absorption bands related to those of the parent monomer. Small frequency shifts and some new bands, associated with the polymer, are observed.11 An interesting feature is shown in the 3000-3100-cm"1 frequency range. Whereas for polythiophene no peak can be observed, poly(3-methylthiophene) exhibits a peak at 3050 cm"1 (Figure 2a) characteristic of C-H vibrations in the d-position relative to the S atom. This result is consistent with a very high regularity of the latter polymer, with aa! coupling of the monomer units. Polythiophene,
Experimental Section
1) is
These polymers have been electrochemically generated Pt, as previously described,7 in an electrolytic medium involving CH3CN, the monomer, and the supporting salt MX (M+ = Li+, N(Bu)4+; X" = C104", BF4", PF
