Achieving High Thermoelectric Figure of Merit in Polycrystalline SnSe via Introducing Sn Vacancies

Abstract

Thermoelectric power generation technology has emerged as a clean "heat engine" that can convert heat to electricity. Recently, the discovery of an ultrahigh thermoelectric figure of merit in SnSe crystals has drawn a great deal of attention. In view of their facile processing and scale-up applications, polycrystalline SnSe materials with ZT values comparable to those of the SnSe crystals are greatly desired. Here we achieve a record high ZT value ∼2.1 at 873 K in polycrystalline SnSe with Sn vacancies. We demonstrate that the carrier concentration increases by artificially introducing Sn vacancies, contributing significantly to the enhancements of electrical conductivity and thermoelectric power factor. The detailed analysis of the data in the light of first-principles calculations results indicates that the increased carrier concentration can be attributed to the Sn-vacancy-induced Fermi level downshift and the interplay between the vacancy states and valence bands. Furthermore, vacancies break translation symmetry and thus enhance phonon scattering, leading to extralow thermal conductivity. Such high ZT value ∼2.1 is achieved by synergistically optimizing both electrical- and thermal-transport properties of polycrystalline SnSe. The vast increase in ZT for polycrystalline SnSe may accelerate practical applications of this material in highly effective solid-state thermoelectric devices.

High‐Performance Thermoelectric SnSe: Aqueous Synthesis, Innovations, and Challenges

“…For Sn and/or Se vacancies, the calculated results indicate that Sn vacancy (V Sn ) possess much lower formation energy than Se vacancy (V Se ), explaining the p‐type electrical transport behavior in pure SnSe. Meanwhile, the results indicate that forming extra Sn vacancies in SnSe matrix is much easier than forming extra Se vacancies, which has been experimentally confirmed 3,17,25,270. Besides, the formation of interstitial Sn (Sn i ) and Se (Se i ) are much more difficult than that of V Sn and V Se , indicating that it is much harder to form interstitial atoms in pure SnSe.…”

“…As can be clearly seen, the induced Sn vacancies do not obviously affect the bandgap, but E F obviously moves into the valence band, indicating that the Sn vacancy changes SnSe into a degenerate p‐type semiconductor, and in turn resulting in significantly increased p 22,197. Considering that pristine SnSe possess a low p of only ≈2 × 10 17 cm −3 ,3 which is far away from its optimized value of ≈3 × 10 19 cm −3 to achieve a high S 2 σ ,3,22,197 creating more Sn vacancies in SnSe matrix is an effective strategy to improve the thermoelectric performance of pure SnSe 3,17,22,25,270,299. For Se vacancies, because their formation energy is much larger than that of Sn vacancies, it is difficult to induce a high concentration of Se vacancies in SnSe matrix to realize high performance n‐type SnSe 66…”

High Thermoelectric Performance in Polycrystalline SnSe Via Dual‐Doping with Ag/Na and Nanostructuring With Ag8SnSe6

“…It is mainly linked to the lower electrical conductivity [24] and higher thermal conductivity stemmed from the effects of Sn oxides of polycrystalline SnSe than that of single crystalline SnSe. [31] In particular, a high ZT value (≈1.3 at 773 K) has been reported for Ag doped SnSe samples. [31] In particular, a high ZT value (≈1.3 at 773 K) has been reported for Ag doped SnSe samples.…”

Thermally enhanced Fröhlich coupling in SnSe

“…SnSe combines large carrier conductivity σ and Seebeck coefficient S, with highly anharmonic lattice dynamics [2-4]. Anharmonic effects limit the lattice thermal conductivity κ via phonon-phonon scattering, thus contributing to a record-high figure of merit ZT = (S 2 σ/κ)T ∼ 2.6, which may be even further improved through doping and alloying [5][6][7][8].As the operational conditions of thermoelectric devices typically involve large temperatures, a quantummechanical description of the electronic and lattice properties of SnSe across the temperature domain of its thermodynamical stability is key to unravel the microscopic origin of this outstanding thermoelectric performance. Recent experimental investigations have unveiled a pervasive influence of temperature on the electronic and transport properties of SnSe.…”

“…SnSe combines large carrier conductivity σ and Seebeck coefficient S, with highly anharmonic lattice dynamics [2][3][4]. Anharmonic effects limit the lattice thermal conductivity κ via phonon-phonon scattering, thus contributing to a record-high figure of merit ZT = (S 2 σ/κ)T ∼ 2.6, which may be even further improved through doping and alloying [5][6][7][8].…”