Thermopower doping dependence

Potential fluctuations due to charged defects and dopants provide the most plausible explanation of the energy difference between the thermopower and the conductivity in doped material. In undoped a-Si H either potential fluctuations or a broadened mobility edge which gives a continuous energy dependence of o( ) could be the explanation. 

Further information on the transport processes in a-Si H and on the influence of doping can be obtained, e.g., from measurements of the drift mobility (Allan et al., 1977 Moore, 1977), of the photoconductivity (Rehm et al., 1977 Anderson and Spear, 1977), as well as of the magnetic field dependence of the photo- and dark conductivity (Weller et al., 1981). In this chapter, however, we shall confine ourselves mainly to results of conductivity and thermopower measurements. Some results from Hall effect and photoconductivity studies are also discussed. 

A metallic state has been observed in high-quality samples of doped (CH) c, PPV, PANI, PPy, P3MeT, and PEDOT. The experimental evidence includes the following finite conductivity at millikelvin temperatures, linear temperature dependence of thermopower, linear term in specific heat, temperature-independent Pauli susceptibility, quantum corrections (weak localization and e-e interaction) to MC, metallic reflectance, and free-carrier absorption in the infrared. 

Although the thermopower, 5(7), of doped conducting polymers has been studied for many years, the evolution of 5(7) as a function of the extent of disorder is not yet fully understood. Usually, thermopower is not as sensitive to disorder as electrical conductivity, since the latter is strongly dependent on the scattering and hopping processes involved in charge transport in the disorder-induced localized regime. Kaiser [19] analyzed the... 

The thermopower data for various types of doped PANl are shown in Figs. 2.51a and 2.51b [172,193]. The room temperature value is approximately 10 /itV/K with small variations ( 2 p.V/K) depending on the details of the process for casting the film. The magnitude and positive sign of 5(7) are similar to those obtained from a number of partially doped p-type conducting polymers [89-92]. The positive sign of the thermopower is consistent with the calculated band structure of the metallic emeraldine salt, a three-quarter-filled tt band with one hole per... 

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 Langmuir-Blodgett (LB) films of alkylammonium-Au(dmit)2 and BEDO-TTF-alkylTCNQ show metallic temperature dependence at certain temperature ranges near RT. The ESR and thermopower measurements of the latter LB films suggest that the domain of the CT complex is metallic down to low temperatures and the conduction is activated by the domain boundaries. Superconducting thin films prepared by doping K into the Cgg LB film is reported with the onset 7). of 8.1 K. ... 

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