On September 27, Professor Zhao Lidong’s group in the School of Materials Science and Engineering, Beihang University, published an article entitled “High thermoelectric performance in low-cost SnS0.91Se0.09 crystals” in Science. The research reports the temperature-dependent interplay of several separate electronic bands in hole-doped tin sulfide (SnS) crystals and boosts the synergistic optimization between effective mass (m*) and carrier mobility (μ) by introducing selenium (Se), which enhances the thermoelectric performance of SnS crystals featuring low-cost, Earth-abundan, and environmentally friendly (Science 365 (2019) 1418-1424).
The first author: He Wenke (a PhD student enrolled in 2018)
Supervisor and corresponding author: Zhao Lidong
The first Institute: The School of Materials Science and Engineering, Beihang University
Thermoelectric technology takes advantage of Seebeck Effect (an electromotive force by a temperature gradient) and Peltier Effect (the presence of heating or cooling at an electrified junction of two different conductors) to boost the conversion between thermal energy and electricity. It is a competitive technology, for its system are small in size, free from mechanical wear with no moving parts, noiseless and non-polluting. It provides an environmentally friendly route for power generation through harvesting waste heat, as well as for refrigeration by solid-state coolingand is geared specifically for deep-space exploration.
As an important parameter to measure the properties of thermoelectric materials, the thermoelectric conversion efficiency is determined by the dimensionless figure of merit (ZT) for a given thermoelectric material, expressed by ZT= S2σT/(κlat+κele), where S is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature (in kelvin), and κlat and κele are the phonon and carrier contributions to the thermal conductivity, respectively.
Thus, a high-performance thermoelectric material should have high thermoelectromotive force (to keep high thermoelectric voltage), high electrical conductivity (to reduce Joule losses) and low thermal conductivity (to maintain a steep temperature gradient).
However, these thermoelectric parameters are intertwined, making manipulation of any single parameter targeted to improving the overall thermoelectric performance a challenge. Several strategies have emerged for improving ZTs in recent years. For example, reducing thermal conductivity by introducing nanostructures or all-scale hierarchical architectures, optimizing power factors (PF = S2σ) through band convergence, band flattening, or density of states (DOS) distortion, decoupling thermoelectric parametersthrough embedding magnetic nanoparticles, developing new materials with intrinsically low thermal conductivity or seeking high-performance thermoelectrics through reliable high-throughput material screening.
In 2014, SnSe was discovered to be a type of thermoelectrics with strong anharmonicity (Nature 508 (2014) 373-377). And then, features like multiple valence bands (Science 351 (2016) 141-144) and three-dimensional (3D) charge and 2D phonon transport (Science 360 (2018) 778-783) were found in SnSe.
In the meanwhile, Prof. Zhao’s group has worked towards developing low-cost, environmental friendly and Earth-abundant thermoelectrics. Compared with other IV–VI thermoelectric materials (such as PbTe, PbSe, PbS, SnTe, SnSe, with the abundance of Te in the Earth being 0.001 parts per million (ppm), Se 0.05ppm and S 420ppm), SnS is far superior when considering toxicity and elemental abundance. However, the enhancement of the electrical conductivity and thermoelectric force of SnS remains to be a challenging task.
Featuring high electronegativity and wide band gap, sulfides are not commonly regarded as good conductors of electricity. After two years’ research, the group finds a novel approach to synthesize the polycrystalline SnS (J. Mater. Chem. A, 2 (2014) 17302-17306) whose carrier mobility is 10-15 times higher than normal polycrystalline by regulating anisotropic crystal growth (J. Mater. Chem. A, 6 (2018) 10048-10056), which successfully improves the electrical conductivity of SnS.
Thermoelectric materials demand not only good electrical conductivity, but also asteep temperature gradient. However, it is controversial as the two factors are conditioned by carrier concentration in an anti-correlated way. Therefore, a main task of the research is to optimize a dimensionless quality factor (β) for a given thermoelectric material, characterized by β ∝ (m*)3/2μ, where m* and μ are the effective mass and carrier mobility (they are also anti-correlated), respectively.
In their experiment, first, atomic positions under different temperatures are derived from variable-temperature synchrotron radiation x-ray diffraction data. On the basis of density function theory (DFT) calculated band structure, it is investigated that there is a temperature-dependent evolution and interplay of multiple valence bands in SnS, involving two-band convergence, two-band crossing and two-band divergence. The study also found that the interplay of multiple valence bands can be promoted by introducing Se in SnS (See Fig. 1).
Fig. 1 Through the manipulation of the electronic band structure (Se alloying) , valence bands are sharpened and electrical transport properties are improved with more valence bands involved in electrical transport
Besides, it is found that the introduction of Se can also sharpen the multiple valence bands (which reduces effective mass and increases carrier mobility) and activate more bands (the fourth valence band) to be involved in the electrical transport (which maintains a large effective mass).
The Se alloying, which enhances carrier mobility and contributes to a large effective mass and therefore results in an optimized quality factor β, allows SnS crystal to exhibit high electrical conductivity over the entire working temperature range, even higher than that of the SnSe crystal with multiple valence bands in it (Science 351 (2016) 141-144). Moreover, the maximum figure of merit ZT has increased from ~1.0 to ~1.6 and the average ZT of the entire working temperature has reached ~1.25 after introducing Se.
Compare with other IV-VI compound, SnS is more superior as it is environmental-friendly, highly-efficient, cost effective and can be commercially applied inthermoelectrics in the future.
The work is a joint effort of 27 collaborators from 11 institutes including the research group led by Prof. Li Jingfeng from Tsinghua University, Prof. He Jiaqing’s group from Southern University of Science and Technology, Prof. Stephen J. Pennycook’s group from National University of Singapore, Prof. Michihiro Ohta’s group from the National Institute of Advanced Industrial Science and Technology (Sangyō Gijutsu Sōgō Kenkyū-sho), Hao Lijie and Niu Changlei from China Institute of Atomic Energy, Song Jianming from China Academy Of Engineering Physics, Xu Wei from the Institute of High Energy Physics of the Chinese Academy of Sciences and Prof. Wang Guangtao from Henan Normal University.
Various advanced tests and measuring methods are involved in the research, such as synchrotron radiation x-ray diffraction (SR-XRD), density function theory (DFT) calculations, angle-resolved photoemission spectroscopy (ARPES), x-ray absorption fine structure spectroscopy (XAFS), inelastic neutron scattering (INS) experiment, scanning transmission electron microscopy (STEM), stability test on thermoelectric properties after neutron radiation and thermoelectric conversion efficiency test.
The project is supported by the National Natural Science Foundation of China (51788104), National Key R&D Program of China (2018YFA0702100, 2018YFB0703600), National Natural Science Foundation of China (51772012, 51632005, 51571007), Beijing Natural Science Foundation (JQ18004) and Program of Introducing Talents of Discipline to Universities, or 111 Plan (B17002), etc.
The article is available at:
https://science.sciencemag.org/content/365/6460/1418
The link to the website of Prof Zhao’s research group:
http://shi.buaa.edu.cn/zhaolidong/zh_CN/index.htm
Edited by Jia Aiping and Xiong Ting
Reviewed by Tan Hualin
Translated by Xiong Ting
