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 at the same time high Sn vacancy concentration (Sn<sub>v</sub>). Accordingly, it gives a high carrier concentration (10<sup>21</sup> cm<sup>-3</sup>) at room temperature (RT), which is not favorable in thermoelectrics, thereby the regulation of both the electronic and phonon scattering mechanisms is strongly required. Up to date, there are many approaches to improve its TE performance. The typical examples are those involving the valence band convergence, nanostructuring, substitutional and interstitial defects, and lattice softening etc., which are all practical and effective to improve the TE performance of SnTe. However, we in this work take the entropy 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, as 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<i><sub>x</sub></i>Te)<sub>0.95</sub>(Ag<sub>2</sub>Se)<sub>0.05</sub> by substituting different amount 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 reduce while the highest TE figure of merit (ZT) 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<i><sub>x</sub></i>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.

Electronic Structure- and Entropy-Driven Design of Thermoelectric Chalcogenide Cu5Sn2Se7 Leading to the Optimization of Carrier Concentration and Reduction in Thermal Conductivity

“…For example, the Δ S follows almost linearly with the composition (Figure ), while the physical parameters ( α , σ , κ L , and ZT) do not. Such inharmonious contributions were also observed in the SnTe system . Particularly, we might overlook some additional transport mechanisms that are hard to be unraveled at this moment, for instance, the thermal entropy ( normalΔ S ( T ) = prefixtrue∫ 300 T C normalp ( T ) T normald T ) and the temperature-dependent solubility-based dynamic doping effect, , in addition to the additional lattice disorder and crystal structure distortion.…”

“…Such inharmonious contributions were also observed in the SnTe system . Particularly, we might overlook some additional transport mechanisms that are hard to be unraveled at this moment, for instance, the thermal entropy ( normalΔ S ( T ) = prefixtrue∫ 300 T C normalp ( T ) T normald T ) and the temperature-dependent solubility-based dynamic doping effect, , in addition to the additional lattice disorder and crystal structure distortion. In addition, these additional transport mechanisms are all of significance in regulating the electronic and phonon transport at high temperatures; that is why we observed the abnormal changes in TE performance around ∼700 K (see Figure ).…”

“…Such inharmonious contributions were also observed in the SnTe system. 52 Particularly, we might overlook some additional transport mechanisms that are hard to be unraveled at this m o m e n t , f o r i n s t a n c e , t h e t h e r m a l e n t r o p y (…”

“…) 52 and the temperature-dependent solubility-based dynamic doping effect, 53,54 in addition to the additional lattice disorder and crystal structure distortion. In addition, these additional transport mechanisms are all of significance in regulating the electronic and phonon transport at high temperatures; that is why we observed the abnormal changes in TE performance around ∼700 K (see Figure 9).…”

Chemical Composition Modulation Realizing Remarkable Improvement of Thermoelectric Performance in CuInTe2-Based Alloy

“…In this work, however, we propose a chemical composition modulation strategy trying to realize electro-acoustic coordination: 25,26 that is, alloying GeTe in CIT at first, aiming to gain degenerated or converged valence bands through improving its structural symmetry. 27−31 After that, we introduce a proper Cu deficiency (V Cu ) 32,33 to adjust the carrier concentration (n H ) through unpinning the Fermi level (F r ) in the madgap and, at the same time, suppress the phonon velocity and/or strengthen the structural anharmonicity through increasing lattice disorder.…”