Chatu Kumarasinghe (Engineering)
Modifying the bandstructure of half-Heuslers to improve their thermoelectric power factor
Half-Heuslers (HHs) have impressive thermoelectric power factors as a result band and orbital degeneracy combined with weak electron-phonon scattering. Investigations into their band structures, reveal that they have multiple bands that can be aligned through different band engineering approaches, giving opportunity to further improve their power factor. We explore the power factor optimisation of Co-based p-type HHs TiCoSb, NbCoSn and ZrCoSb using ab-initio Density Functional Theory (DFT) calculations and semi-classical Boltzmann transport.
First we develop simplified, computationally inexpensive models based on effective mass, non-parabolic band approximations, for NbCoSn and TiCoSb to study potential improvements in the powerfactor as a result of band alignment. We show that this model provides a good approximation for the ab-initio derived power factor, within 0.25eV of the valence band edge. Using this simple model, we identify that there is potential for improvement under different electron scattering mechanisms (constant rate of scattering, inter-band scattering only, inter and intra-band scattering).
Finally, using more computationally expensive DFT and semi-classical Boltzmann transport calculations, we investigate how strain can be used to modify effective masses and band convergence to improve the power factor in TiCoSb, NbCoSn and ZrCoSb under different methods of electron scattering.
Our results can provide a generic insight into the design and optimization of complex band structure thermoelectric materials.