Thickness-dependent thermoelectric transporting properties of few-layered SnSe

Recently, SnSe of orthorhombic phase has attracted much attention due to its excellent performance in thermoelectricity, yet the thickness-dependent thermoelectric properties were poorly studied. We have investigated the thermoelectric transporting properties of few-layered (from 1 L to 4 L) and bulk SnSe by using density functional theory plus Boltzmann transport theory. We employed the electron-phonon Wannier method to obtain the accurate transporting parameters, i.e., carrier mobility as well as relaxation time, in contrast to those derived from deformation potential theory. Our results reveal that the band gap decreases from 0.91 eV (1 L) to 0.59 eV (bulk) as the thickness increases, and meanwhile the effective charge mass increases slightly. As bulk SnSe is thinned to the monolayer, the high frequency modes are stiffened while the low frequency modes are softened, and meanwhile the acoustic velocities become larger. At 300 K, the thermal conductivity of bulk SnSe takes 1.52, 0.80 and 0.41 W m−1 K−1 for κa, κb and κc respectively, in good agreement with the experimental values. Thermal conductivity of few-layered SnSe decreases slightly from 2.45 to 1.61 W m−1 K−1 for κa, and from 1.81 to 1.02 W m−1 K−1 for κb as the layer number increases from 1 L to 4 L. The EPW calculations reveal that the electron mobility of n-type films increases rapidly from 24.4 cm2/V s (1 L) to 179.6 cm2/V s (4 L) and the hole mobility of p-type films increases from 47.2 cm2/V s (1 L) to 1211.1 cm2/V s (4 L) at 300 K as the thickness increases. Compared with the monolayer, thicker film of SnSe has larger carrier mobility as well as lower thermal conductivity, which leads to better thermoelectric performance. At 700 K, the figure of merit is enhanced from 0.22 (1 L) to 0.58 (4 L) for n-type films in electron density of −0.55 × 1013/cm2. Our research casts some light on the future thermoelectric applications based on few-layered SnSe.