Mechanisms of Enhanced Thermoelectricity in Chalcogenides

Abstract

Thermoelectric materials can provide solutions to power generation and refrigeration challenges. Layered chalchogenides are of particular interest, with bismuth telluride and lead telluride being the most common compounds. Bismuth telluride is often used for room temperature applications, while its solid solutions with antimony or selenium as well as lead tellurides show better thermoelectric properties at elevated temperatures. Regrettably, the efficiency of the known thermoelectric materials is still low. Evidently, bringing thermoelectric energy harvesting to commercial viability is a materials challenge: How can we obtain materials with figure of merit above 3? This question drives the research community since the successes of nanoengineering in the 1990s. Nowadays, high-pressure technology is a promising frontier for making further advances in thermoelectric material performance.

The main goal of this thesis is to understand the electronic and thermoelectric properties of selected materials using density functional theory and semi-classical Boltzmann transport theory. Bulk and monolayer CuSbS2 and CuSbSe2 are studied to clarify the role of the interlayer coupling for the thermoelectric properties. The calculated band gaps of the bulk compounds turn out to be in agreement with experiments and significantly higher than those of the monolayers, which thus show lower Seebeck coefficients. Since also the electrical conductivity is lower, the monolayers are characterised by lower power factors. Therefore, the interlayer coupling, even though it is weak, is found to be essential for the thermoelectric response. We study Cu (Sb,Bi)(S,Se)2 under hydrostatic pressure up to 8 GPa, considering the van der Waals interaction, as these compounds have layered structures. We find an indirect band gap that decreases monotonically with increasing hydrostatic pressure. Only CuBiS2 shows an indirect-indirect band gap transition around 3 GPa, leading to conduction band convergence with a concomitant 20% increase in the Seebeck co-efficient. This enhancement results from a complex interplay between multivalley and multiband effects as well as changes of the band effective masses. The variation of the electronic band structure of AB2Te4 (A = Pb, Sn and B = Bi, Sb) under hydrostatic pressure up to 8 GPa is analyzed in detail and its consequences for the material properties are explained.