Effects of heavy bromine doping on the thermoelectric performance and dynamic stability of SnSe2 polycrystals

Abstract

SnSe2 is a material with a layered crystal structure, which is abundant in the Earth's crust and is non-toxic. It exhibits anisotropic thermoelectric properties due to the anisotropy between its interlayer and intralayer electrical and thermal transport properties. However, its thermoelectric performance is intrinsically poor due to its lower electrical transport properties. In our study, we successfully synthesized n-type Bromine-doped SnSe2 polycrystals using a combined method of mechanical alloying (MA) and spark plasma sintering (SPS). Bromine (Br) acts as an electron donor, and its heavy doping has been achieved by substituting it with selenium (Se) atoms. This substitution results in a significant increase in carrier concentrations, thereby improving the thermoelectric performance of SnSe2. This Br doping has led to a notable improvement in the power factor, which increased to approximately 755 µWm−1K−2 at 573 K. The most significant anisotropy was observed in thermal conductivity, and Br doping led to an ultralow thermal conductivity of approximately 0.6 Wm−1K−1 at 748 K. Furthermore, density functional theory (DFT) was used to investigate the electronic structure and phonon dispersions. The results of our DFT calculations showed a shrinking bandgap and increased degeneracy, which supports the observed improvement in the thermoelectric performance of Bromine-doped SnSe2 polycrystals. Moreover, the absence of imaginary modes in the phonon dispersion confirmed the dynamical stability of the system after heavy Br doping. Finally, a maximum figure of merit (ZT) of 0.59 was obtained at 748 K for the SnSe1.95Br0.05 sample parallel to the SPS pressing direction, which is nearly sixfold higher than the value of pure SnSe2.