Thermoelectric material (TE) is one of the important functional materials to realize the conversion of electricity from waste heat. Since SnTe contains toxic-free elements and possesses the high-symmetry rock salt crystal structure, it has gained much attention in the thermoelectric field. However, the pristine SnTe compound suffers from the poor electrical properties due to the high intrinsic carrier concentration, the small band gap, and large energy difference between light (L) and heavy (Σ) bands. Therefore, hunting for the effective dopants and exploring the influence mechanism of dopants on the thermoelectric properties (electrical properties) of SnTe are of great significance for further improving its thermoelectric performance.
Recently, professor ZHANG Yongsheng from Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences found that the screened As/Sb elements can facilitate not only the band convergence of light (L) and heavy (Σ) bands (Delta E(L-Σ)) but also increasing the band gap (Eg) of SnTe, which improved its thermoelectric properties
The results were published in Chemistry of Materials.
After scrutinizing the electronic structure of SnTe and analyzing the dominant orbital contributions for VBM and CBM, the team used cation substitution to adjust the thermoelectric properties of SnTe.
It is time-consuming to use Density Functional Theory (DFT) supercell calculations to consider the effects of many different defects or dopants on the electronic structures of SnTe. Therefore, they constructed the simple tight binding model (TB) to quickly investigate the influence of various trivalent cations on the band structure, and screened the effective dopants of As and Sb, which not only reduced the Delta E(L-Σ), but also increased Eg of SnTe. With band structure modifications, the electrical properties of SnTe were significantly improved.
They further synthesized the predicted samples (Sn0.92Ge0.04Sb0.04Te and Sn0.92Ge0.04As0.04Te) to verify their thermoelectric properties. The result is encouraging. Ge-Sb or Ge-As co-doping in SnTe is favorable for the improvement of Seebeck coefficients, power factor, and the zT value. Besides, Ge-As co-doping in SnTe possesses the high weighted mobility and conversion efficiency.
Their work provided a methodology to efficiently screen the promising codopants in the SnTe-based materials and detect important candidates to improve the thermoelectric power generation and conversion efficiency.
The research was supported by the financial support from the National Natural Science Foundation of China and NSF DMREF. The calculations were performed in the Computational Science of HFIPS, the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Science and AM-HPC. Numerical computations were performed at the Hefei advanced computing center.
Band and PDOS structure of pristine SnTe compound. Contour maps of energy difference in light and heavy bands and band gap with varying relative on-site energies of the cation and anion s and p orbitals. (Image by ZHANG Xuemei)
Unfolded band structures of Ge/As co-doping in SnTe. (Image by ZHANG Xuemei)
Fig. 3 ZT values (a), calculated (b) weighted mobility and (c) conversion efficiency for the pristine SnTe, and Ge-Sb/As codoped SnTe compounds. (Image by ZHANG Xuemei)