Transport near the metal-insulator transition: Polypyrrole doped withPF6

Abstract

Heavily doped polypyrrole-hexafluorophosphate, PPy(PF6), undergoes a metal-insulator (M-I) transition at resistivity ratio p, = p(1.4 K)/p"(300 K) = The effect of the partially screened Coulomb interaction is substantial at low temperatures for samples on both sides of the M-I transition. In the insulating regime, the crossover from Mott variable-range hopping (VRH) to Efros-Shklovskii hopping is observed. In the metallic regime, the sign of the temperature coeScient of 0 the resistivity changes at p, =2. At T=1. 4 K, the interaction length L&=(AD/k&T)' =30 A. Since this is smaller than the inelastic-scattering length, L;"=300 A, the contribution to p(T) from the electron-electron interaction is dominant. Application of high pressure decreases p" induces the transition into the metallic regime, and enables fine tuning of the M-I transition. For samples close to the M-I transition, the thermoelectric power is proportional to the temperature in both the metallic and insulating regimes. The correlation length (L, ) increases as the disorder, characterized by p", approaches the M-I transition from either side. The expected divergence in L, at the M-I transition is qualitatively consistent with the values for L, inferred from the extrapolated cr(0) in the metallic regime and from analysis of the VRH magnetoresistance in the insulating regime. Thus, by using p" to characterize the magnitude of the disorder, a complete and fully consistent picture of the M-I transition in PPy(PF6) is developed. I. INTRODUCirONRecent studies of doped polypyrrole (PPy), polymerized electrochemically at relatively low temperatures ( -20'C to -30'C), have shown that the roomtemperature conductivity oar=200 -500 S/cm, increasing to approximately 1000 S/cm after tensile drawing. 'A positive temperature coefficient of the resistivity (TCR) was' reported for temperatures below T =10-20 K for PFs-doped PPy, PPy(PFs). ' To date, however, the physical aspects of these phenomena as related to the metallic nature of heavily doped conjugated polymers have not been clearly understood. Models suggested earlier, ' such as the electron-hopping (or tunneling) conduction, small-polaron tunneling, local superconductivity, etc. seem to be either inappropriate or incapable of describing the entire range for the data (i.e. , in both metallic and insulating samples.Many of the properties that characterize heavily doped conducting polymers, such as relatively high electrical conductivity, temperature independent magnetic susceptibility, linear temperature dependence of thermoelectric power, ' '" absorption throughout the infrared with no energy gap, ' etc. , suggest that the electronic structure is that of a metal. However, the disorder generated during synthesis and during the doping process plays a critical role; microscopic disorder and/or structurally amorphous regions can dominate the transport.%'e present the results of a systematic study of the transport properties of PPy(PFs) near the disorderinduced metal-to-insulator (M I) transition. The ex-tent of di...

Doping, density of states, and conductivity in polypyrrole and poly(p-phenylene vinylene)

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