The Mechanism Behind the High zT of SnSe₂ Added SnSe at High Temperatures

Abstract

SnSe is a promising thermoelectric material due to its low toxicity, low thermal conductivity, and multiple valence band structures, which are ideal for high electronic transport properties. The multiple valence band structure has attracted many attempts to engineer the carrier concentration of the SnSe via doping, to place its fermi level at a position where the maximum number of valence bands can participate in the electronic transport. Up until now, ~5 × 10¹⁹ cm^-3 was the highest carrier concentration achieved in SnSe via doping. Recently, introducing SnSe₂ into SnSe was found to effectively increase the carrier concentration as high as ~6.5 × 10¹⁹ cm^-3 (at 300 K) due to the generated Sn vacancies. This high carrier concentration at 300 K, combined with the reduction in lattice thermal conductivity due to SnSe₂ micro-domains formed within the SnSe lattice, improved the thermoelectric performance (<i>zT</i>) of SnSe – <i>x</i>SnSe₂ as high as ~2.2 at 773 K. Here, we analyzed the changes in the electronic band parameters of SnSe as a function of temperature with varying SnSe₂ content using the Single Parabolic Band (SPB) model. According to the SPB model, the calculated density-of-states effective mass and the fermi level are changed with temperature in such a way that the Hall carrier concentration (<i>nH</i>) of the SnSe – xSnSe2 samples at 773 K coincides with the optimum <i>nH</i> where the theoretically maximum <i>zT</i> is predicted. To optimize the <i>nH</i> at high temperatures for the highest <i>zT</i>, it is essential to tune the 300 K <i>nH</i> and the rate of <i>nH</i> change with increasing temperature via doping.