High Performance n-type Tin-Deficient SnSe Thermoelectric Polycrystal

Recently, the SnSe semiconductor was identified, in single-crystal form, as a mid- temperature thermoelectric material with record-high figure of merit [1], high power factor and surprisingly low thermal conductivity, with the figure of merit ZT exceeding 2.5. It can also be doped appropriately to obtain both p- and n-type behavior. Polycrystalline SnSe has similar excellent p-type properties when all contact with oxygen is avoided, and it has been demonstrated that the Sn and Se vacancies influence the transport properties. However, doping SnSe with donor or acceptor elements is more challenging than in other IV-VI chalcogenide semiconductors, like PbTe or GeTe. Thus, obtaining an n-type SnSe polycrystal is not an easy task.

Here we show that a straightforward, fast, and inexpensive arc-melting procedure followed by simple cold-pressing produces excellent n-type thermoelectric pellets at high temperature. A comparison of the calculated electronic density of states for both stoichiometric and tin-deficient SnSe (Sn_0.94Se) shows a broad defect-band right above the valence band for the tin-deficient SnSe [2], possibly contributing to the behavior of the Seebeck-coefficient. This is relevant in light of the Rietveld-refined occupancy factor obtained from Neutron powder diffraction (NPD) for our tin deficient SnSe with a Sn/Se ratio of 0.97. Although at lower temperatures this material has poor thermoelectric performance, it shows an exponential increase of the electrical conductivity with temperature, which along with the reasonably high Seebeck coefficient and the low thermal conductivity leads to the high ZT~1.8 above 800 K.

References

[1] L.-D. Zhao et al., Nature. 508 373–377 (2014).

[2] J. Gainza at al., J. Appl. Phys. 126 045105 (2019).