Thermoelectric power of doped polyaniline near the metal-insulator transition

The thermoelectric power (S) of various doped polyaniline (PANI) near the metal-insulator (M-I) transition was investigated as a function of temperature (T), and compared with the characteristic temperature dependence of conductivity (σ). For the PANI protonated by camphor sulfonic acid (CSA), S(300K) = +8 ∼ +12 μ V/K and linearly dependent upon temperature, indicating that microscopic transport is via the diffusion of charge carriers in the extended electronic states with relatively narrow bandwidth. In the homogeneous limit, the thermoelectric power is insensitive to the presence of microscopic disorder associated with the critical behavior of σ(T) near the M-I transition. For the samples doped by conventional protonic acids (HCl, H2SO4), the large scale inhomogeneity induces the strong temperature dependence of resistivity, ϱ(T) ∞ exp [(T0T)12], and additional negative contribution to the temperature dependence of thermoelectric power.