On November 26, 2015, Science Magazine published the outcome of a study in which the research group headed by Prof. He Jiaqing at the Department of Physics was one of the main participants: Ultra-high power factor and thermoelectric performance in hole-doped single-crystal SnSe. The study was jointly undertaken by Beijing University of Aeronautics and Astronautics, Northwestern University, South University of Science and Technology of China, the University of Michigan and the California Institute of Technology.
As society progresses, the issues related to energy and the environment have become the most serious challenge facing humanity in the new century. Thermoelectric materials are new energy materials used for the direct conversion of heat into electricity, or vice versa, have a wide range of applications in areas such as thermoelectric cooling and cogeneration and play an important role in improving the utilization of existing energy sources and alleviating the energy crisis. However, the large-scale commercial application of thermoelectric materials is facing the low-efficiency, high-cost bottleneck; therefore, the development and utilization of cheap, low-toxicity thermoelectric materials made from abundantly available elements is gaining ever-increasing attention from researchers.
Compared with traditional thermoelectric material PbTe, SnSe is made from relatively inexpensive and abundantly available elements, and is less toxic and more eco-friendly. However, for a long time, it has failed to attract much attention due to the fact that its polycrystalline thermoelectric figure of merit ZT is very low. In 2014, a research group at Northwestern University reported in Nature a peak ZT of 2.62 at 923K, realized in SnSe single crystals which exhibited ultra-low thermal conductivity along the b axis, setting a new record for bulk thermoelectric materials. The finding caused a great deal of excitement in the field of thermoelectricity. However, since the intrinsic carrier concentration is too low, the thermoelectric performance of the material at low and moderate temperatures is unsatisfactory. According to the article published in Science, in order to solve this issue, we need to regulate the electrical conductivity and thermal electromotive force of SnSe by adjusting its energy band structure, thereby greatly improving its thermoelectric performance at low and moderate temperatures. The internal mechanism is rooted in the very complex valence band structure of SnSe. The energy gaps between valence bands with different effective masses and carrier mobility are very small. When the Fermi level enters and approaches multiple valence bands, multiple valence bands can participate in electrical transport at the same time. The principle can be figuratively described as follows: a highway (corresponding to a single valence band) is crowded with vehicles, so the traffic is very slow; but if the same number of vehicles are assigned to multiple parallel roads, not only will vehicles travel fast, but the number of vehicles passing through a given location per unit time will also increase. Through this method of regulation of the Fermi level, the thermal electromotive force of SnSe is greatly improved, significantly increasing the thermoelectric figure of merit ZT of SnSe at low and moderate temperatures. In theory, a thermoelectric conversion efficiency of about 16.7% can be achieved. The research group headed by Professor He Jiaqing is also doing more in-depth research on the SnSe thermoelectric material. More relevant results will be published.
The complex valence band structure of SnSe and the mechanism for “multi-channel” coordinated transport of its charge carriers
The work received strong support from SUSTC Start-up Fund, Shenzhen Key Laboratory of Thermoelectric Materials and Guangdong Natural Science Foundation Major Basic Research Cultivation Project when carried out at SUSTC.
Read more:
http://www.sciencemag.org/content/early/2015/11/24/science.aad3749.abstract
