Doped conjugated polymers electrical conductivity

Table 2 shows the present state-of-the-art for the electrical conductivity of doped conjugated polymers. The magnitude of the electrical conductivity in polymers is a complex property determined by many stmctural aspects of the system. These include main-chain stmcture and TT-ovedap, molecular... 

Fig. 9.34 Correlation of electrical conductivity and charge carrier mobility for a doped conjugated polymer thermally dedoped device and o separately doped devices. The fits, thick and thin lines, are described in the text. Reproduced with permission of the American Institute of Physics from Jamett et al. (1995).

As mentioned in the introduction, the electrical conductivity upon doping is one of the most important physical properties of conjugated polymers. The conductivity ranges from lOOOOOS/cm for iodine-doped polyacetylene [41], 1000 S/cm for doped and stretched polypyrrole [42], to 500 S/cm for doped PPP [43], 150 S/cm for hydrochloric acid doped and stretched polyaniline [44], and 100 S/cm for sulfuric acid doped PPV [45] to 50 S/cm for iodine-doped poly thiophene [46]. The above listed conductivities refer to the unsubstituted polymers other substitution patterns can lead to different film morphologies and thus to a different electrical conductivity for the same class of conjugated polymer in the doped state. 

Polyheterocycles. Heterocychc monomers constitute a third class of monomers that can he polymerized to form fully conjugated polymers the most common of these monomers, shown in equation 7, are p5urole (X = NH), thiophene (X = S), and furan (X = O). They can he doped to give electrical conductivity when pol5unerization occurs in the 2,5-positions. These monomers can be polymerized both by electrochemical and chemical methods. The polyheterocycles have received considerable attention for their electron-rich nature, which leads to materials that are easily oxidized and therefore more stable in the oxidized state. Additionally, the increased structural complexity of polyheterocycles relative to polyacetylenes makes structural modifications possible for improved processability. 

ELECTRICAL BEHAVIOR OF POLYMERS 785 Table 14.6 Structures and conductivity of doped conjugated polymers (59)... 

One active area of research is completely missing. These are the optical and electrical properties, with effects such as the high conductivity of doped conjugated polymers, electro-luminescence in polymeric light emitting diodes, or the ferro- and piezoelectricity of poly(vinylidene fluoride), to cite only a few examples. There is no good reason for this omission, only that I did not want to overload the book with another topic of different character which, besides, mostly employs concepts which are known from the physics of semi-conductors and low molar mass molecules. 

Metallic properties of doped conjugated polymers are observed in the temperature dependence of conductivity, magnetoresistance, thermopower, magnetic susceptibility, and infrared reflectivity. However, the materials of this class are not yet really metallic with long mean free paths they remain just on the metallic side of disorder-induced M-I transition. This implies that significantly higher electrical conductivities will be obtained with continued improvement of the materials. 

The incorporation of electrically conductive polymers (doped conjugated polymers) into non-conductive polymers to impart electrical conductivity has been demonstrated in a number of examples. Conjugated polymers shown below are generally non-conductive (semiconductive), but exhibit electrical conductivities often in excess of 1 S/cm with acid dopants, such as mineral acids (e.g., HCl, H2SO4), iodine, or organic acids such as dodecyl benzene sulfonic acid. 

Recently, polyimines include the synthesis of long alkoxy (Cg-Cig) side chain derivatives [188,189], which are presumably soluble to some extent in organic solvents and derivatives containing fluorene cardo unit [190]. Trifluoromethyl groups [191] in the polymer backbone provide solubility in organic solvents. Studies of the electrical conductivity of doped conjugated aromatic polyimines and alkoxy derivatives have been reported [188], and the values are in the range of 10 to 10 S/ cm. 

---