First Principles Modelling of Thermoelectric Materials

Thermoelectric semiconductor materials possess the unique ability to convert temperature differential into electricity or vice versa. This presents excellent opportunities for harvesting waste heat or cooling. Thermoelectric applications are found in many different areas ranging from medicine to space programs. The quality of the materials is determined by their thermoelectric properties like the Seebeck coefficient, electrical and thermal conductivity. In this thesis we model the thermoelectric properties from first principles. The materials of interest include Fe2VAl, NbFeSb, TaFeSb and Bi2Te3. The thermoelectric properties are analysed by considering the effects of doping, point defects, grain boundaries and size reduction. A number of key results are found. We show that the experimentally observed behaviour of the Seebeck coefficient in Fe2VAl can be theoretically modelled by enhancing the localisation of V electrons with the Hubbard model. We establish TaFeSb as a new thermoelectric material which exhibits 50 % better p-type thermoelectric properties than NbFeSb due to an increased scattering strength between Ta and potential dopants. We also note that mixing NbFeSb and TaFeSb does not have a negative impact on the electronic properties and could potentially lead to further improvements in the thermoelectric performance. We investigate the electronic thermoelectric properties of Bi2Te3 thin films. We find that the Seebeck coefficient increases dramatically when the film thickness is reduced to 1-2 nm. This leads to an overall increase in the power factor of the material and enhanced p-type thermoelectric performance. The successful calculation of the properties for a wide range of materials also shows that the developed in this project computational framework can be reliably used for further research on thermoelectrics.