JOURNAL OF MATERIALS SCIENCE 27 (1992) 210 214 Evaluation of electrical conduction in iodine-doped polypyrrole HARI SINGH NALWA* Department of Material Systems Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan Electrical conductivity of polypyrrole has been measured after doping with different iodine concentrations. A thermally activated electrical conductivity was found which was pseudo- ohmic and increased with doping level. The results can also be fitted by log cr versus T -1/2 and tog o- versus T-1/4 dependences, instead of the Arrhenius log cr versus T -1 dependence. From these results it was concluded that within the experimental scatter no significant distinction can be made between these different temperature dependence laws. Hence these data can only enable one to speculate about the true underlying transport model, rather than to draw decisive conclusions. Electrical conductivity results predicting the role of iodine dopant concentration on the conduction process of semiconducting polypyrrole are discussed. 1. Introduction Conjugated organic polymers have attracted much attention in the scientific community over the past decade because of their unique electronic properties [1]. The electrical conductivity of conjugated organic polymers could be raised from the insulating to the semiconducting and even to the metallic regime by doping with either electron acceptors or electron don- ors [1, 2], An essential common feature which induces high electrical conductivity is a reaction of the conju- gated n-electron backbone with an appropriate oxidizing or reducing dopant. The delocalization of ~-electrons along the conjugated polymer chain is believed to participate in the conduction process. The highly conjugated polyacetylene has a degenerate ground state and the charge-carrying species formed on doping have been described as solitons [3-5]. Polypyrrole, polythiophene and poly(p-phenylene), on the other hand, have a nondegenerate ground state and bipolarons have been considered as the charge- carriers formed on doping [6, 7]. A neutral soliton has a spin 89 while the charge soliton is spinless. Likewise, a polaron has a spin 89 whereas a bipolaron is spinless [1]. These spinless mobile species formed on doping have been considered to be the basis of the conduction process in conjugated polymer solids. Several conduction mechanisms have been pro- posed to describe conducting polymers; problems arise from the doping mechanism and from the com- plexity of polymeric materials, therefore there are some controversies with the analysis and correct inter- pretation of the conduction process [4]. A theoretical model of Mott's variable range hopping (VRH) conduction can predict transport processes occurring in disordered semiconducting materials [8]. With this model the conductivity-temperature data are best plotted as log o versus T -x, where cr is the electrical conductivity, T is the temperature and x is a constant ranging from 1 to 88 The x = 89 behaviour evinces a pseudo-one-dimensional conduction [9, 10], whereas x = 89 indicates a two-dimensional conduction [11], and x = 88 predicts a three-dimensional conduction process. Thus an experimental linear log cr versus T-1/4 behaviour is taken as a proof of a variable- range hopping in three dimensions, most often sugges- ted for conducting polymers. Sickel et al. [12] proposed that the size and the shape of the dopant ions also influence the conduction process and a T -:/2 behaviour observed in doped polyacetylene indicates a fluctuation-induced tunnelling conduction. In this conduction process, tiny conducting domains, formed on doping, participate in the delocalization of charge carriers by tunnelling between conducting is- lands. Structurally, this process resembles a metallic composite (inhomogeneous) system in which metallic particles are embedded in an insulating matrix. It has been thought that the nonuniform doping causes het- erogeneity which leads to the formation of nonstoi- chiometric charge-transfer complexes between the polymer and the dopant. Audenaert et aI. [13] re- ported that the conduction process in polyacetylene is affected by the iodine concentrations. Epstein et al. [14] investigated the conductivity-temperature characteristics of iodine-doped trans-polyacetylene and suggested an intersoliton electron hopping mech- anism. The changes introduced in electrical conductivity are influenced not only by the structure and nature of a dopant but also by the doping concentrations and doping procedures [12]. Dopants also cause morpho- logical as well as structural changes in the polymer matrix [4]. All such factors contribute in determining *Present address: Hitachi Research Laboratory, Hitachi Ltd, 4026 Kuji-cho,Hitachi-shi, Ibaraki 319-12,Japan. 21 0 0022-2461/92 $03.00 + .12 9 1992 Chapman & Hall