Thermoelectric properties of semiconducting materials with parabolic and-mold band structures

By a combination of semi-analytical Boltzmann transport and first-principles calculations, we systematically investigate thermoelectric properties of semiconducting (gapped) materials by varying the degrees of polynomials in their energy dispersion relations, in which either the valence or conduction energy dispersion depends on the wave vector raised to the power of two, four, and six. Within the relaxation time approximation, we consider various effects such as band gaps, dimensionalities, and dispersion powers to understand the conditions that can give the optimal thermoelectric efficiency or figure of merit (ZT). Our calculations show that the socalled pudding-mold band structure produces larger electrical and thermal conductivities than the parabolic band, but no significant difference is found in the Seebeck coefficients of the pudding-mold and parabolic bands. Tuning the band gap of the material to an optimum value simultaneously with breaking the band symmetry, the largest ZT is found in a combination of two-contrasting polynomial powers in the dispersion relations of valence and conduction bands. This band asymmetry also shifts the charge neutrality away from the undoped level and allows optimal ZT to be located at a smaller chemical potential. We expect this work to trigger high-throughput calculations for screening of potential thermoelectric materials combining various polynomial powers in the energy dispersion relations of semiconductors. We give preliminary screening results for bulk PtS2 and FeAs2 compared with Si, where we indicate that the former two have better thermoelectric performance than the latter.