Temperature Dependence of the Electrical Conductivity of Poly(Benzo[1,2S-b:4,5S-bS'] DithiopheneS-4,8S-Diyl Vinylene) an
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relationship in all three samples at low temperatures. We also analyze the temperature dependence of the resistivity of PBDV by using the thermal fluctuation-induced tunneling model. INTRODUCTION
Conjugated polymers are known to exhibit remarkable electrical properties. There has been a substantial interest in understanding physical processes that are responsible for the mobility of the charge carriers and related temperature dependence of the electrical conductivity in conducting polymers.1- 9 Unlike saturated polymers, many conjugated polymers are known to exhibit conducting behavior on redox doping, with dc electrical conductivities as high as 105 f-1 cm- 1 measured for polyacetylene. A linear chain structure makes them additionally interesting from the point of view of reduced dimensionality which is expected to influence their electronic properties. The electrical conductivity of the conjugated polymers results from mobile charge
carriers introduced into their n electronic configuration. At sufficiently high doping levels, the transport of charges is believed to occur mainly along the conjugated chains and via inter-chain hopping processes. The temperature dependence of the electrical conductivity of such materials is usually described by using the Variable Range Hopping (VRH) model. Whereas this model qualitatively describes the temperature behavior of the conductivity, the resulting values of N(Ef) and a--1 have been found to be unphysical in several cases. 10 - 12 The intrinsic electron-phonon scattering, scattering of the charge carriers off defects, dimensionality and microstructure of the film are among important parameters that could significantly affect transport properties of these materials. 13,14
In an earlier paper, we had discussed a(T) and microscopic structure of poly(pyrrole tosylate) and poly(pyrrole fluoride). 10 These early data had provided useful information about what appeared to be a difficulty in applying the VRH model to a(T) in poly(pyrrole tosylate) and poly(pyrrole fluoride). Specifically, the temperature dependence of a in both of these conducting polymers could not be explained satisfactorily by the VRH model. For example, a fit of the VRH model to the temperature dependence of the conductivity had provided results for the bipolaron localization lengths of 0.02 A and 6 x 10-8 A for poly(pyrrole tosylate) and poly(pyrrole fluoride), respectively. The results for the density of states at the Fermi energy were 1 x 1027 and 3 x 1029
eV- 1 cm- 3 , respectively. Obviously, these values have no physical significance. Similar results 701
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
3 x 1029 eV- 1 cm- 3 , respectively. Obviously, these values have no physical significance. Similar results have also been reported by others. 11, 12 For example, in the cases of oxidized and substituted poly(pyrrole), o- 1 has been determined to fall within a range from 10-4 A to 3 x 10-9 A. The corresponding values for N(Ef) range from 7 x 1033 to 3 x 1046 eV-1 cm- 3 . Here we r
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