Transport Properties of Organic Semiconductors

Abstract

Electrical and thermal transport coefficients of a large class of organic crystals are calculated from a Boltzmann equation treatment of narrow-band semiconduction. The latter, in turn, is based on the known tight-binding band structure of these crystals, and on an assumed relaxation time $\tau$. Specifically, assuming bandwidths $\lesssim k_{0}T$, explicit expressions are obtained for the electrical conductivity, Hall constant, magneto-conductivity, thermoelectric power, and thermal conductivity. These are expressed in terms of the relevant intermolecular transfer integrals and $\tau$, for various crystallographic directions. In most cases, the results are carried out to the first order in ($\frac{\mathrm{bandwidth}}{k_{0}T}$). The calculated anisotropy of the electrical conductivity agrees satisfactory with experiment. The Hall constant is found to be anomalous both in magnitude and sign. Finally, it is pointed out that the anisotropy of the thermoelectric power and magnetoconductivity can, in principle, provide information about the band structure.