Variational polaron transformation approach toward the calculation of thermopower in organic crystals

Based on the combination of the variational polaron transformation and the Green-Kubo formalism, a method toward the thermopower calculations in organic crystals is proposed, which covers a broad regime of electron-phonon interaction strengths and successfully recovers the small-polaron theory in the narrow-band limit and the modified Mott formula in the coherent limit, respectively. The application to a molecular chain reveals a crossover of thermopower with respect to the chemical potential and exhibits abnormal regions where the sign of thermopower is opposite to the sign of the carrier charge. This abnormal property may lead to a sign-inversion phenomenon of thermopower in terms of the temperature. It is also found that the incomplete dressing of the electron by the phonon cloud destroys the particle-hole symmetry and results in nonzero thermopower in a half-filled band.