The achievement of high ZT in n-type SnSe single crystal

SnSe is a two dimensional (2D) layered semiconductor with strong Sn-Se bonding along $b-c$ plane and weaker bonding along $a$ axis direction, resulting in a strong anisotropic transport properties. Recently, Zhao \textit{et al}. reported that high thermoelectric power factor and low thermal conductivity at high temperature make SnSe as a very good p-type thermoelectric material; ZT values along $b$ and $c $axes are up to 2.6 and 2.3 at 923 K, respectively. They attributed the remarkably high ZT value along the $b$ axis to the intrinsically low lattice thermal conductivity in SnSe. More recently, two first-principles calculations predicted good thermoelectric performances in both n- and p-type SnSe's and better n-type thermoelectric properties than p-type SnSe and J. Yang \textit{et al.} predicted ZT\textasciitilde 3.1 in n- type SnSe. Here, we report that n-type SnSe single crystals were successfully synthesized by substituting Bi at Sn sites. In addition, it was found that the carrier concentration increases with Bi content, which has a great influence on the thermoelectric properties of n-type SnSe single crystals. Indeed, we achieved the maximum \textit{ZT} value of 2.2 along b axis at 733 K in the most highly doped n-type SnSe with a carrier density of -2.1$\times$ 10$^{19}$ cm$^{-3\, }$at 773 K.