Thermoelectric semiconductors can convert temperature differences into electricity or electricity into temperature differences. This offers great potential for the use of wasted heat or cooling. These materials can be used in a variety of fields, from healthcare to space exploration. The effectiveness of the materials is evaluated by their thermoelectric
properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. The aim of this PhD thesis is to investigate the electronic structure using first-principle methods for potential thermoelectric applications. Materials of interest include Copper and Tin based ternary /quaternary compounds, and monolayers of SnS2, SnSe2 and Janus SnSSe. Density functional theory, ab initio molecular dynamics and Boltzmann transport theory are used to study the electronic and phonon transport properties. In the first part of the thesis, electronic structure calculations were performed on both monoclinic and disordered cubic forms of Cu2SnS3(CTS). The impact of structural disorder on thermoelectric properties was examined through these simulations. The results, obtained through first-principle calculations, revealed the existence of band tails in the electronic density of states for the disordered structure,
and low-lying optical modes in the disordered cubic structure. This was found to be caused by a significant variation in Sn bonding, leading to strong anharmonicity as measured by the Grüneisen parameter. The findings from the first principle calculations were supported by Nuclear inelastic scattering experiments. Furthermore, the effect of grain size
on Cu2SnS3 was studied using first-principles calculations on various ordered and disordered surfaces. The density of states (DOS) revealed that the surface of CTS is conductive due to the presence of dangling bonds. Furthermore, calculations of the formation energy showed that the stoichiometric CTS, Cu-vacant and Cu-rich systems are
energetically more favourable, while the formation of Sn-vacant and Sn-rich systems is less likely. In the subsequent study, the impact of Ag substitution at the Sn site at various concentrations was investigated. The Fermi level for Ag-substituted systems was found to lie deep within the valence band, with the shift of the Fermi level inside the valence
band increasing with substitution increasing the carrier concentration. The incorporation of Ag into the system decreases the root mean squared displacement of the other cations and anions, which reduces the scattering of phonons and thereby increases the lattice thermal conductivity. A comparative study of various polymorphs of CTS,
Cu2ZnSnS4 and Cu2ZnSnSe4 was done. Ab-initio molecular dynamics was performed on CTS, CZTS and CZTSe. The root mean squared displacement value for the disordered polymorph was higher than for the ordered phase, indicating increased static disorder. This corresponds to the static (temperature-independent) distortion of the crystalline lattice due to the disorder of the cations and is associated with higher anharmonicity and bond inhomogeneity in the disordered phase, which is then directly responsible for the ultra-low thermal conductivity. In the final part of the thesis, thermoelectric properties of dichalcogenide monolayer of SnS2, SnSe2 and Janus SnSSe was performed. Density functional theoretical calculations points out the hexagonal Janus SnSSe monolayer as a potential high-performing
thermoelectric material. Results for the Janus SnSSe monolayer show an ultra-low thermal conductivity originating from the low group velocity of the low-lying optical modes, leading to superior zT values of 0.5 and 3 at 300 K and 700 K for the p-type doping, respectively. The successful calculation of properties for materials shows that the computational work done in this thesis can be used for further research into thermoelectricity.
| Identifer | oai:union.ndltd.org:unitn.it/oai:iris.unitn.it:11572/378687 |
| Date | 25 May 2023 |
| Creators | Nautiyal, Himanshu |
| Contributors | Nautiyal, Himanshu, Scardi, Paolo |
| Publisher | Università degli studi di Trento, place:Trento |
| Source Sets | Università di Trento |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | firstpage:1, lastpage:203, numberofpages:203 |
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