A first-principles study of the electronic structures and transport properties of IV-VI thermoelectric compounds

Abstract

Binary IV-VI chalcogenides have drawn vast attention in the thermoelectric community due to their outstanding performance and great potential for large-scale thermoelectric applications. Recently, compounds based on PbTe, SnSe and GeTe have achieved ZTs >2.5, which has stimulated a large amount of work in optimizing the thermoelectric performance based on the IV-VI chalcogenides. At the same time, the first-principles calculations based on density functional theory (DFT) has become an efficient and effective tool in theoretical material design and optimization in recent decades. The key to enhancing ZT is to promote the electrical transport properties, the strategies of which mainly include optimizing the carrier concentration, band convergence and introducing a resonant level. Therefore, in this dissertation, theoretical efforts have been made based on these strategies. First, theoretical studies have been conducted to explore the thermoelectric properties of SnSe analogs (SnS and GeSe in the Pnma phase) based on DFT methods and Boltzmann transport theory. The doping effects of various elements on the electronic structure and power factor have been investigated. It has been found that the pnictogen group elements (Sb and Bi) can induce significant resonant states near the conduction band minimum (CBM) and a more delocalized electron density along the out-of-plane direction in n-type SnS, which contribute to an enhancement of the normalized power factor along the out-of-plane direction in the low temperature range. However, pnictogen group elements (Sb and Bi) are not as good as halogen group elements (Br and I) in n-type GeSe, as they will induce a decrease in DOS near the CBM, leading to a lower normalized power factor. Second, pressure engineering was employed in pristine GeTe (R3m phase) to achieve band convergence. Hydrostatic pressure is shown to increase valley degeneracy, decrease the band effective mass and enhance the electrical transport property. The power factor was computed based on both the constant relaxation time approximation and an explicit calculation of electron-phonon coupling. Finally, the effectiveness of n-type dopants in tuning the carrier concentration of GeTe was evaluated based on the calculations of defect formation energies. It is theoretically proven that pnictogen group elements (Sb and Bi) tend to substitute the Ge site and act as donors, which can effectively lower the carrier concentration to the optimal value under Ge-rich conditions.