Prof. JIANG Jun’s team at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS) reported excellent thermoelectric performance along the out-of-plane direction of n-type tin selenide (SnSe) single crystals by alloying with PbBr₂. This study, published in Energy & Environmental Science (Energy Environ. Sci.), revealed that the thermoelectric performance of n-type SnSe along the out-of-plane direction is very sensitive to the lattice parameter a, owing to the Fermi surface dynamics.

Thermoelectric materials offer a unique solution for the issues of energy and the environment, thanks to their ability in the direct conversion from waste heat into electricity. Recently, the layered chalcogenide SnSe has attracted great interest owing to its remarkable thermoelectric performance as well as the traits of being resource-rich and environment-friendly.  

The researchers fabricated PbBr₂-doped SnSe single crystals by a temperature gradient method. In contrast to the p-type SnSe, the n-type SnSe was found to prefer the out-of-plane direction, achieving the maximum ZT of 2.1 at 770 K.

Interestingly, they observed a sharp drop of the carrier mobility at a doping level of x = 3%. Combined with theoretical calculations, the X-ray diffraction (XRD) measurements indicated that the Fermi surface dynamics driven by the lattice variation is the underlying reason. By introducing Ge doping, the lattice parameter a in x = 3% was decreased, resulting in the greatly enhanced carrier mobility and the maximum ZT of 1.7.  

The study revealed that the thermoelectric performance of n-type SnSe is driven by the subtle structures of the Fermi surface, thus the Fermi surface dynamics plays a crucial role in the thermoelectric performance optimization. It has provided a new perspective for the investigation of thermoelectric materials with layered structures.

Figure: Fermi-surface dynamics and improved thermoelectric performance in PbBr₂-doped SnSe_0.95 (Image by NIMTE)