Optical Properties of Doped Conducting Polymers

In spite of the clear signatures of the metallic state, however, the heavily doped conjugated polymers do not exhibit traditional metallic behavior in transport [173] and optical properties [1155]. Instead, they show a negative temperature coefficient of the resistivity dp/dT 0) the dc conductivity is activated with temperature, so that the dc conductivity decreases by several orders of magnitude as the temperature is lowered. Furthermore, the optical spectra do not show the Drude-like behavior expected for a typical metal in the infrared. Rather, an electronic pseudo-gap of about 0.1-0.2 eV has been observed, below which the optical conductivity is suppressed [1155]. These nonmetallic behaviors arise from the disorder of the sample, originating from a combination of molecular-scale disorder and mesoscale inhomogeneity. 

Optical Spectra of Conducting Polymers in the Metallic Regime 

Above the 1.4-eV minimum, o(w) starts to increase again around 2.5 eV, corresponding to the onset of interband transition, and evolves a peak at 2.8 eV. The 2.8-eV peak is consistent with previous spectroscopic results obtained from the emeraldine salt [1182-1185]. For a metal, the onset of interband absorption is associated with transitions from the Fermi surface to the next higher 

Ejfective Number of Carriers. The integrated spectral weight, eff( ) provides information on the free carriers responsible for 

From the saturation value above 1.2 eV, the effective number of carriers per unit cell, involved in the intraband Drude-like con- 

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Before closing this topic, let s consider the band structure and optical properties of doped conductive polymers, once again considering the most widely studied structure, poly acetylene. If all C—C bond distances were equivalent, one would expect a partially-filled Ti-band and metallic conductivity (Figure 5.64a). However,... 

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