Thermoelectric transport properties with non-parabolicity, degeneracy and multiplicity of band edges: The case of anisotropic p-type SnSe

Efficient ab initio computational methods for the evaluation of transport
properties of thermoelectric (TE) materials, are highly important for the development of
energy harvesting technologies. The BOLTZTRAP code [1] has been widely used for
this aim. However, its current version relies only on the
constant relaxation time (RT) approximation, within the Boltzmann transport
equation. Here, we extend the implementation of the BOLTZTRAP code
by incorporating realistic k-dependent RT models
of the main scattering processes, namely, screened polar and
nonpolar scattering by optical phonons, scattering by acoustical phonons, and scattering
by screened ionized impurities. The RT models are based on a smooth Fourier
interpolation of the Kohn-Sham eigenvalues and its derivatives, thus, taking
into account non-parabolicity, degeneracy
and multiplicity of the band edges on equal footing, at a very low computational
cost. To test our approach, we determined the anisotropic TE
transport properties of the Pnma phase of p-type SnSe. Our results for the evolution
of TE coefficients with both temperature and chemical potential
are in agreement with the experimental data.

[1] G.K.H. Madsen, D.J. Singh, Comput. Phys. Commun. v.175, p.67-71, 2006.