ANNO Hiroaki

The effect of Ga−P co-doping on the electronic structure has been investigated for p-type clathrate thermoelectric semiconductor Ba8Cu6Ge40 by density functional theory (DFT) to study the possibility of carrier control. In Ba8Cu6Ge40, the Fermi energy EF lies in the valence band, and Ba8Cu6Ge40 is a p-type degenerate semiconductor. For Ga−P, Ga2−P, and Ga3−P co-doping, the EF lies in the conduction band, the energy gap, and the valence band, respectively, indicating that the p/n carrier type and carrier concentration change with the Ga/P ratio. In comparison with Ba8Cu6Ge40, the effect of Ga−P co-doping on the energy dispersion relation at the valence band edge is greater than that at the conduction band edge. Therefore, the effect on the effective mass is greater in the valence band than in the conduction band.

We report on the electronic structure and electronic transport properties of skutterudite CoSb3 based on density functional theory utilizing the nonequilibrium Green’s function method. CoSb3 has a non-parabolic (linear) dispersion relation near the top of the valence band, and the hole effective mass is much smaller than the electron effective mass. This is the reason for the characteristic property that hole mobility is higher than electron mobility. This is completely different from that of ordinary semiconductors. The four-membered ring of Sb, which is one of the features in the crystal structure, is important in relation to the electronic structure and electronic properties. The relation of these crystal structure features to the electron transport properties is discussed. Then, the optimization of thermoelectric properties is discussed based on the chemical potential dependence of thermoelectric properties.

Semiconducting clathrates are attracting a great deal of attention as potential candidates of thermoelectric material based on a design concept called Phonon Glass and Electron Crystal (PGEC). Since most of the thermoelectric clathrates are n-type, developing a p-type clathrate with high thermoelectric performance is an important issue. In this study, the electronic structure and thermoelectric properties of Ba8Cu6Ge40 clathrate are calculated based on density functional theory (DFT) to search for new p-type clathrates. An energy gap is formed in the electronic band structure of Ba8Cu6Ge40, and the Fermi energy lies in the valence band, indicating that Ba8Cu6Geぃis a p-type semiconductor. The effective mass of the valence band is larger than that of the conduction band. The Seebeck coefficient for p-type is larger than that for n-type, reflecting the difference in effective mass. According to the dependence of Seebeck coefficient and electrical conductivity on the chemical potential, adjusting the Fermi energy, corresponding to the carrier concentration, to the optimum value improves the power factor. Therefore, the results of DFT calculation show that Ba8Cu6Ge40 has excellent properties as a candidate for p-type thermoelectric materials.

Recently, nanostructured materials or nanocomposites, rather than thin films or superlattices, are of increasing interest in creating a new material with high thermoelectric figure of merit. The approach of nanoscale control of materials by introducing nanostructures, such as nanoinclusions, nanointerfaces, etc., may have a significant influence on the transport properties due to the energy filtering effect of the potential barrier at interfaces, or strong scattering effect of phonons and/or carriers at interfaces, whose density increases with decreasing size of structure. On the other hand, it is also of importance to elucidate the mechanism of enhancement in the Seebeck coefficient for nanostructured material systems. The density functional theory (DFT) using non-equilibrium Green’s function (NEGF) method may be a powerful tool to calculate the transport properties of nanoscale systems. There are, however, few studies of nanoscale system for thermoelectric clathrates by DFT using NEGF method. Thus, we adopt the DFT using NEGF method to calculate the transport properties of nanoscale system of Ba8Au6Si40/nanogap/Ba8Au6Si40, where nanogap acts as a potential barrier, as a model of nanoscale clathrate Ba8Au6Si40 system, to investigate the effect of nanointerface on the transport properties of thermoelectric clathrates. For Ba8Au6Si40/nanogap/Ba8Au6Si40 system, the Seebeck coefficient value at EF is greatly enhanced. The electrostatic potential difference was found to be large at the nanogap for Ba8Au6Si40/nanogap/Ba8Au6Si40 system. The calculation suggests that the potential barriers at nanointerface, such as grain boundary, have a significant influence on the Seebeck coefficient. We also discuss the effect of nanointerface on the electron and thermal conductance.