Promising thermoelectric performance in van der Waals layered SnSe2

Abstract

a b s t r a c tSnSe as a lead-free IVeVI semiconductor, has attracted intensive attention for its potential thermoelectric applications, since it is less toxic and much cheaper than conventional PbTe and PbSe thermoelectrics. Here we focus on its sister layered compound SnSe 2 in n-type showing a thermoelectric performance to be similarly promising as SnSe in the polycrystalline form. This is enabled by its favorable electronic structure according to first principle calculations, its capability to be effectively doped by bromine on selenium site to optimize the carrier concentration, as well as its intrinsic lattice thermal conductivity as low as 0.4 W/m-K due to the weak van der Waals force between layers. The broad carrier concentration ranging from 0.5 to 6 Â 10 19 cm À3 realized in this work, further leads to a fundamental understanding on the material parameters determining the thermoelectric transport properties, based on a single parabolic band (SPB) model with acoustic scattering. The layered crystal structure leads to a texture in hot-pressed polycrystalline materials and therefore anisotropic transport properties, which can be well understood by the SPB model. This work not only demonstrates SnSe 2 as a promising thermoelectric material but also guides the further improvements particularly by band engineering and texturing approaches.

Cu Intercalation and Br Doping to Thermoelectric SnSe2 Lead to Ultrahigh Electron Mobility and Temperature‐Independent Power Factor

“…The difference in the magnitude of S (|S|) among the samples mainly arises from their different n H values given the single conduction band nature of SnSe 2 . [59] The S values at 300 and 773 K for SnCu x Se 2−y Br y (x = 0, 0.005 and y = 0, 0.02) in this work and SnSe 2−z Br z (z = 0.001-0.05) from the previous report [46] fit well on the lines, indicating that Cu and Br doping does not reduce the effective mass in the conduction band near the Fermi level. It is calculated based on a single Kane band model by assuming acoustic phonon interaction is the dominant scattering mechanism (see the Supporting Information for the details).…”

“…It also exhibits µ H values more than 1.5 times higher than any other SnSe 2 -based materials reported so far such as SnSe 2−z Br z (z = 0.001-0.05) [46] and SnSe 2−m Cl m (m = 0-0.07) (Figure 4c). As a result, SnCu 0.005 Se 1.98 Br 0.02 shows n H and µ H values higher than those of the control samples over the entire temperature range of the measurement.…”

“…For example, the polycrystalline SnSe 2 sample with carrier concentration of 4.3 × 10 18 cm −3 shows 80% lower electron mobility of 7.4 cm 2 V −1 s −1 [40] than the single crystal sample with the similar carrier concentration. The achieved PF is record high to date for both polycrystalline SnSe 2 [42,43,45,46] and SnSe. The examples are densifying the nanoparticles prepared by colloidal synthesis, [42] packing the nanoplatelets delicately, [43] and embedding dense nanoscale precipitates into SnSe 2 nanopowders.…”

“…For example, SnCu 0.005 Se 1.98 Br 0.02 shows ≈320 and ≈50 S cm −1 higher σ than SnSe 1.992 Br 0.008 [46] and single crystal SnSe 0.985 Br 0.015 at 300 K, [47] respectively, despite their similar S values. For example, SnCu 0.005 Se 1.98 Br 0.02 shows ≈320 and ≈50 S cm −1 higher σ than SnSe 1.992 Br 0.008 [46] and single crystal SnSe 0.985 Br 0.015 at 300 K, [47] respectively, despite their similar S values.…”

“…[44,46] The n H is nearly constant at ≈5.6 × 10 19 cm −3 up to ≈523 K, subsequently increasing rapidly to a maximum ≈1.4 × 10 20 cm −3 at 773 K. The electron doping efficiency, namely, the number of electron donation per Cu atom to the SnSe 2 lattice, amounts to ≈0.3 e − from 300 to 523 K and to ≈1 e − at 773 K. In contrast, the maximum doping efficiency at 300 K for bulk SnCu 0.005 Se 2 and nanocomposite SnSe 2 -1.9% Cu [42] is only ≈0.01 and 0.03 e − , respectively. Its trend with respect to temperature is a combination of those of SnSe 1.98 Br 0.02 and SnCu 0.005 Se 2 .…”