On May 18, 2018, the team led by Prof. Zhao Lidong from the School of Materials Science and Engineering published their latest findings about thermoelectric materials online in Science. Entitled “3D charge and 2D phonon transports leading to high out-of-plane ZT in n-type SnSe crystals”, the research brought the charge transport through layered n-type SnSe (3D charge transport) into reality by making use of the lowest thermal conductivity in layered SnSe crystals in the out-of-plane direction (two-dimensional (2D) phonon transport) and doping SnSe with bromine to enhance the hybridization of delocalized electrons. Such “2D phonon /3D charge” transports significantly enhance the thermoelectric performance of n-type SnSe, offering new insights into new efficient thermoelectric materials—2D layered ones (Science360 (2018) 778-783).
Heat can be converted to electricity by semiconductor materials. This technology has the advantages of small-sized systems, high reliability, zero emission and a wide range of operating temperatures, attracting wide attention in the field of clean energy. Thermoelectric conversion technology is also irreplaceable in deep space exploration and space probes. The efficiency of thermoelectric conversion is a key indicator for the performance of thermoelectric materials, and it is determined by the materials figure of merit ZT. However, the complicated interplay among different parameters makes it a great challenge to achieve high ZT. For a long time, application of materials with low thermal conductivity and ways to lower the conductivity have been two directions in increasing ZT.
After discovering the strong nonresonant effect (Nature508 (2014) 373-377) and complicated multiple valence band structure (Science351 (2016) 141-144) of SnSe, Prof. Zhao’s team have been focusing on research into SnSe materials, especially the transports of phonons and electrons in 2D layered bulk materials (Adv.Mater.29 (2017) 1702676). They observed strong phonon scattering effect in the 2D interface of layered SnSe (Fig. 1 (A)), which results in low out-of-plane thermal conductivity (theoretical minimum of ~0.18 W/mK at 773K). Based on this, high thermoelectric performance will be realized if high electrical conductivity can be achieved in this direction.
In a simplified version of ZT proposed by the team, the increase of electrical conductivity requires the simultaneous optimization of carrier mobility and effective mass. At 800K, there is a phase transition from Pnma to Cmcm in SnSe, based on which light conduction bands and heavy conduction bands go through a process of degeneracy convergence (increasing effective mass and reducing carrier mobility) and degeneracy removal convergence (doing the opposite) by the adjustment of the concentration of doping electrons. The process optimizes the product of carrier mobility and effective mass (Fig. 1(B)) and helps SnSe maintain a high electrical conductivity within the range of operating temperatures.
According to the comparison between n-type SnSe and p-type SnSe in which electrons and holes are doped, delocalized Sn and Se p electrons near the conduction band minimum (CBM) contribute to more orbital overlap along the out-of-plane direction, and the charge density of n-type SnSe will be high enough to fill the interlayer gap, turning out-of-plane transports of electrons into reality (Fig. 1(C)).
Fig. 1 “2D phonon/3D charge” transports remarkably boost the thermoelectric performance of n-type SnSe.
To put it simply, the layered structure of the intrinsic SnSe is like a wall that stops the transport of both phonons and holes. Delocalized electrons of the conduction bands increase the charge density and customize a passage for electrons between different walls (Fig. 2) when heavy electrons are doped with SnSe. High charge density, as well as structural changes of energy bands and increased symmetry in the crystal structure caused by a continuous phase transition, contributes to the outstanding out-of-plane electrical conductivity of SnSe. At a temperature over 700K, the out-of-plane “3D change” transport properties in n-type SnSe are comparable to those along the in-plane direction. The “2D phonon/3D charge” transport properties improved the thermoelectric performance of n-type SnSe greatly (thanks to the third-party reports issued by Netzsch and Cryoall).
Fig. 2 “2D phonon /3D charge” transports: (a) The electrons of the conduction bands are delocalized, increasing the charge density and offering a passage for electrons, while phonons and holes are stopped by the interlayers. (b) The plane (phonon) unrestricted by the track is stopped by hills (interlayers), the train (electron) can go through the tunnel, and the car (hole) cannot go into the tunnel because it does not match the track.
Collaborators of the research include the teams of Prof. Huang Jiaqing (co-corresponding author of the paper), Prof. Huang Li and Prof. Wang Kedong of Southern University of Science and Technology, the team of Prof. Li Jingfeng of Tsinghua University, Zhu Fangyuan, an associate research fellow of Shanghai Institute of Applied Physics, Chinese Academy of Sciences, and the team of Prof. Chen Yue of the University of Hong Kong. The work was supported by the National Natural Science Foundation of China, Beijing Municipal Science and Technology Commission, Plan 111 of the Ministry of Education, the Ten Thousand Talents Plan, etc.
The Paper: http://science.sciencemag.org/content/360/6390/778
Introduction of Prof. Zhao: http://shi.buaa.edu.cn/zhaolidong/zh_CN/index.htm
Reported by Xiao Jie
Edited by Wang Qing and Li Mingzhu
Translated by Li Mingzhu