Semiconductor-metal transition in doped (CH)x: Thermoelectric power

Thermoelectric power studies of polyacetylene have been carried out as a function of dopant concentration and temperature. The thermopower of pure trans-(CH)_x is large $\text{(S =}\text{+850 μ V}\text{°K}\text{)}$ and positive consistent with p-type material. With iodine doping, (CHI_y)_x, the thermopower remains positive over the full range of concentration 0 < y < 0.22. The semiconductor-metal transition is clearly observed at n_c ? 3 mole %; S falls dramatically from $\text{S =}\text{+850 μ V}\text{°K}$ at y = 0.003 to $\text{S =}\text{+30 μ V}\text{°K}$ at y = 0.03. At higher concentrations, S remains nearly constant saturating at $\text{+18 μ V}\text{°K}$ in the heavily doped metallic polymer. Temperature dependences are consistent with metallic behavior at the highest dopant concentrations and hopping transport in the undoped and lightly doped polymer.