There has been a renaissance of interest in exploring highly efficient thermoelectric materials as a possible route to address the worldwide energy generation, utilization, and management. This review describes the recent advances in designing high-performance bulk thermoelectric materials. We begin with the fundamental stratagem of achieving the greatest thermoelectric figure of merit ZT of a given material by carrier concentration engineering, including Fermi level regulation and optimum carrier density stabilization. We proceed to discuss ways of maximizing ZT at a constant doping level, such as increase of band degeneracy (crystal structure symmetry, band convergence), enhancement of band effective mass (resonant levels, band flattening), improvement of carrier mobility (modulation doping, texturing), and decrease of lattice thermal conductivity (synergistic alloying, second-phase nanostructuring, mesostructuring, and all-length-scale hierarchical architectures). We then highlight the decoupling of th...

Rationally Designing High-Performance Bulk Thermoelectric Materials

The deceptively simple material SnSe has surprised the scientific community by showing an unexpectedly low thermal conductivity and high power factor and it has become a very promising thermoelectric material. Both the electrical and thermal transport properties of SnSe are outstanding. It is remarkable that a binary compound exhibits strong anharmonic and anisotropic bonding, and after hole doping it shows an exceptionally high power factor because of a high electrical conductivity and a strongly enhanced Seebeck coefficient. The latter is enabled by the contribution of multiple electronic valence bands. In this perspective, we discuss the natural features of SnSe, including crystal structures, electronic band structures, and physical and chemical properties. We also compare the electrical transport properties of single crystals and polycrystalline SnSe. The thermal conductivities of polycrystalline samples show wide variation from laboratory to laboratory, with some values being higher than those of single crystals and some lower, which has caused confusion and controversy. To address the issues regarding the thermal transport properties of SnSe, we systematically summarize the reports for SnSe variants, discuss them along with some of our own new results, and offer possible explanations. Finally, some possible strategies are proposed toward future enhancements of the thermoelectric figure of merit of SnSe.

SnSe: a remarkable new thermoelectric material

https://espace.library.uq.edu.au/view/UQ:e38288a/UQe38288a_OA.pdf

High-performance SnSe thermoelectric materials: Progress and future challenge

Thermoelectric generators, capable of directly converting heat into electricity, hold great promise for tackling the ever-increasing energy sustainability issue. The thermoelectric energy conversio...

Advances in thermoelectrics

Recently SnSe, a layered chalcogenide material, has attracted a great deal of attention for its excellent p-type thermoelectric property showing a remarkable ZT value of 2.6 at 923 K. For thermoelectric device applications, it is necessary to have n-type materials with comparable ZT value. Here, we report that n-type SnSe single crystals were successfully synthesized by substituting Bi at Sn sites. In addition, it was found that the carrier concentration increases with Bi content, which has a great influence on the thermoelectric properties of n-type SnSe single crystals. Indeed, we achieved the maximum ZT value of 2.2 along b axis at 733 K in the most highly doped n-type SnSe with a carrier density of −2.1 × 1019 cm−3 at 773 K. The good thermoelectric figures of merit of p-type tin selenide single crystals are actively studied. Here, the authors show that n-type SnSe can also reach a figure of merit of around 2, at high temperatures, when doped with bismuth.

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Achieving ZT=2.2 with Bi-doped n-type SnSe single crystals

Tin telluride (SnTe) has recently attracted lots of interest due to its potential thermoelectric application as a lead-free rock-salt analogue of PbTe. However, pristine SnTe samples have high hole concentration due to the presence of intrinsic Sn vacancies, and shows a low Seebeck coefficient and high electrical thermal conductivity, resulting in poor thermoelectric performance. In this report, we show that zone-melted SnTe systems with additional Mn (1–7 mol%) can control the hole concentration by reducing the Sn vacancies, and modulate the electronic band structure by increasing the band gap and decreasing the energy separation between the light and heavy hole valence bands. Therefore, alloying with additional Mn enhances the contribution of the heavy hole valence band and significantly improves the Seebeck coefficient in SnMnxTe with the highest value of ∼270 μV K−1. A record power factor of 31.9 μW cm−1 K−2 has been obtained at 820 K. The maximum thermoelectric figure of merit ZT of ∼1.25 is found at 920 K for the high quality crystalline ingot of p-type SnMn0.07Te.

Valence band engineering and thermoelectric performance optimization in SnTe by Mn-alloying via a zone-melting method

Tin selenide (SnSe) compound has attracted much attention due to its unprecedented high ZT (∼2.6) in single crystals. The polycrystalline SnSe materials were then prepared to improve the mechanical performance for large-scaled application. However, the ZT values of 0.3–0.8 were much lower due to their poor electrical properties. In the present study, the zone melting method is employed to prepare the polycrystalline SnSe samples, which show highly textured structures and strong anisotropic thermoelectric performance. A maximum power factor (S2σ) of 9.8 μW cm−1 K−2 was obtained in the polycrystalline samples, which is comparable with that of SnSe single crystals, resulting in a peak ZT of 0.92 at 873 K. The zone-melted ingot was then pulverized into powders and the bulk material was prepared by the spark plasma sintering (SPS) technique. As a result, the ZT value was enhanced to be over 1.0, owing to the slight reduction of lattice thermal conductivity and maintenance of electrical performance. The present investigation indicates that the TE performance of the SnSe compound can be significantly improved by the texture modulation.

Enhanced thermoelectric performance in p-type polycrystalline SnSe benefiting from texture modulation

N-type SnSe compound has been synthesized through melting with spark plasma sintering. By doping BiCl3, the carrier concentration of SnSe is significantly increased, leading to a large enhancement of electrical conductivity. Meanwhile, the SnSe0.95-BiCl3 samples also exhibit higher Seebeck coefficient and lower lattice thermal conductivity, compared with polycrystalline SnSe. Consequently, a high power factor of similar to 5 mu W cm(-1) K-2 and a ZT of 0.7 have been achieved at 793 K. The synergistic roles of BiCl3 doping in SnSe provide many opportunities in the optimization of n-type SnSe materials. (C) 2016 AIP Publishing LLC.

Optimization of thermoelectric properties in n-type SnSe doped with BiCl3

SnCdxTe materials were synthesized by the zone-melting method for this thermoelectric performance study. The X-ray diffraction results show that the lattice parameter decreases with increasing x, following Vegard's law of rock-salt structure SnTe and CdTe. Besides, the room temperature Seebeck coefficients of the SnCdxTe system are enhanced to >60 μV K−1, larger than those of Cd-doped SnTe synthesized by spark plasma sintering. A large power factor of ∼25 μW cm−1 K−1 is achieved in SnCd0.12Te at 820 K, which rivals those of high performance PbTe-based materials. As a result, the highest ZT of ∼1.03 at 820 K was achieved for SnCd0.12Te.

Liu Zhu

Papers

Valence band engineering and thermoelectric performance optimization in SnTe by Mn-alloying via a zone-melting method

Enhanced thermoelectric performance in p-type polycrystalline SnSe benefiting from texture modulation

Optimization of thermoelectric properties in n-type SnSe doped with BiCl3

Enhanced thermopower in rock-salt SnTe–CdTe from band convergence

Synthesis of SnTe/AgSbSe 2 nanocomposite as a promising lead-free thermoelectric material