Thermoelectric performance of tetragonal silicon allotrope tP36-Si from first-principles study

Diamond-like cubic silicon (d-Si) has become a mainstay material for new energy and modern electronics industries. Nevertheless, such material hosts a high lattice thermal conductivity, resulting in a small thermoelectric figure of merit (ZT), which greatly limits its applications in thermoelectric conversion field. tP36-Si is a newly predicted allotrope of silicon with direct band gap, and its total energy is close to d-Si, which indicates that it is likely to be experimentally prepared in years to come. In this article, the thermoelectric properties of this novel new silicon allotrope are researched by combining semi-classical Boltzmann transport theory with first-principles calculation. Electron transport of this new silicon allotrope possesses obvious anisotropy, while the anisotropy of phonon thermal conductivity is slight. Compared to d-Si and other silicon allotropes (Si-2(4), oP32-Si), lower lattice thermal conductivity (23.68 W/mK) and higher power factor (72.63 W/mK(2)) are revealed in tP36-Si. Further analysis shows that the lower phonon thermal conductivity principally comes from the inhibition of group velocity and relaxation time of phonon. The thermoelectric performance of tP36-Si is evaluated according to the electronic relaxation time obtained from the deformation potential (DP) theory, where the peak value of ZT along the ar lattice direction of n-type (p-type) under 700 K is close to 2.18 (0.64), which is much above that of Si24(0.69, 0.51) and d-Si(0.07). The finding illustrates the excellent thermoelectric property of tP36-Si and demonstrate that this new silicon allotrope is an appropriate and promising potential thermoelectric materials.