First-principles study of anisotropic thermoelectric transport properties of IV-VI semiconductor compounds SnSe and SnS

摘要

We conduct comprehensive investigations of both thermal and electricaltransport properties of SnSe and SnS using first-principles calculationscombined with the Boltzmann transport theory. Due to the distinct layeredlattice structure, SnSe and SnS exhibit similarly anisotropic thermal andelectrical behaviors. The cross-plane lattice thermal conductivity $\kappa_{L}$is 40-60% lower than the in-plane values. Extremely low $\kappa_{L}$ is foundfor both materials because of high anharmonicity. It is suggested thatnanostructuring would be difficult to further decrease $\kappa_{L}$ because ofthe short mean free paths of dominant phonon modes (1-30 nm at 300 K) whilealloying would be efficient in reducing $\kappa_{L}$ considering that therelative $\kappa_{L}$ contribution ($\sim$ 65%) of optical phonons isremarkably large. On the electrical side, the anisotropic electricalconductivities are mainly due to the different effective masses of holes andelectrons along the $a$, $b$ and $c$ axes. This leads to the highest optimal$ZT$ values along the $b$ axis and lowest ones along the $a$ axis in both$p$-type materials. However, the $n$-type ones exhibit the highest $ZT$s alongthe $a$ axis due to the enhancement of power factor when the chemical potentialgradually approaches the secondary band valley that causes significant increasein electron mobility and density of states. SnSe exhibits larger optimal $ZT$scompared with SnS in both $p$-type and $n$-type materials. For both materials,the peak $ZT$s of $n$-type materials are much higher than those of $p$-typeones along the same direction. The predicted highest $ZT$ values at 750 K are1.0 in SnSe and 0.6 in SnS along the $b$ axis for the $p$-type doping whilethose for the $n$-type doping reach 2.7 in SnSe and 1.5 in SnS along the $a$axis, rendering them among the best bulk thermoelectric materials forlarge-scale applications.