Single crystalline
SnSe has been reported to exhibit the high thermoelectric zT value of 2.6 at 923 K along the b-axis
direction, due to its low thermal conductivity [Zhao, L. D.; et al. Nature
2014, 508, 373]. However,
the strongly anisotropic properties of the orthorhombic structure
degrade the thermoelectric performance of polycrystalline SnSe, resulting
in a low zT of 0.6 and 0.8 for Ag- and Na-doped SnSe,
respectively. Here, we prepared Ag0.01Sn0.99Se1–x
S
x
(x = 0, 0.10, 0.15, 0.20, and 0.35) compounds by
melting and hot press sintering. The compounds showed extremely low
thermal conductivity (0.11 W m–1 K–1 at 825 K for x = 0.15). Using transmission electron
microscopy images, we found that SnS alloying induced numerous nanoscale
point defects. A Debye–Callaway model analysis supported the
conclusion that the extremely low lattice thermal conductivity could
be attributed to the point defect scattering of phonons. This resulted
in a high zT of 1.67 at 823 K for the x = 0.15 sample, which is the state-of-the-art zT value for polycrystalline SnSe. Because the compounds are based
on the environmentally friendly and cheap materials Sn, Se, and S,
they make promising candidates for thermoelectric applications.
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