Improvement of thermoelectric performance of SnTe-based solid solution by entropy engineering

Abstract

SnTe is a good alternative to PbTe in the thermoelectric (TE) applications, in that it is a compound with no toxic element Pb. Besides, the compound SnTe has a relatively narrow bandgap (0.3–0.4 eV) and high Sn vacancy concentration (Sn<sub>v</sub>) as well. Accordingly, it gives a high carrier concentration (10<sup>21</sup> cm<sup>–3</sup>) at room temperature (RT), which is not favorable for thermoelectrics, therefore the regulation of both the electronic and phonon scattering mechanisms is strongly required. Up to date, there have been many approaches to improving its TE performance. The typical examples are those involving the valence band convergence, nanostructuring, substitutional and interstitial defects, and lattice softening, which are all practical and effective to improve the TE performance of SnTe. However, in this work the entropy is taken as an indicator to design the SnTe-based TE material with multicomponents and then optimize its TE performance. The detailed scheme involves the chemical composition design step by step. At first, SnTe alloys with 5% GaTe to form a solid solution Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te, aiming to increase the solubility of the foreign species. The second step is to form another solid solution (Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub> via the alloying Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te with 5% Ag<sub>2</sub>Se. The purpose of this step is to reduce the p-type carrier concentration of the system, for the species Ag<sub>2</sub>Se is a typical n-type semiconductor. The last step is to form a series of solid solutions (Sn<sub>0.95–<i>x</i></sub>Ge<sub>0.05</sub>Bi<sub><i>x</i></sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub> by substituting different amounts of Bi on Sn in (Sn<sub>0.95</sub>Ge<sub>0.05</sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub>, to further enhance the configurational entropy (Δ<i>S</i>). Because of the above approaches, both the carrier concentration and thermal conductivity decrease while the highest TE figure of merit (<i>ZT</i>) increases from 0.22 for the pristine SnTe to ~0.8 for the alloy (Sn<sub>0.95–<i>x</i></sub>Ge<sub>0.05</sub>Bi<sub><i>x</i></sub>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub> (<i>x</i> = 0.075). This result proves that the entropy engineering is a practical way to improve the TE performance of SnTe, and at the same time it illustrates that it is very important to harmonize the entropy engineering with other electronic and phonon scattering mechanisms, in order to improve the TE performance of SnTe effectively.