Pressure-induced enhancement of thermoelectric power factor in pristine and hole-doped SnSe crystals.

We evaluate the influence of pressure on the thermoelectric power factors PF ≡ S 2 σ of pristine and Na-doped SnSe crystals by measuring their electrical conductivity σ ( T ) and Seebeck coefficient S ( T ) up to ∼22 kbar with a self-clamped piston-cylinder cell. For both cases, σ ( T ) is enhanced while S ( T ) reduced with increasing pressure as expected, but their imbalanced variations lead to a monotonic enhancement of PF under pressure. For pristine SnSe, σ (290 K) increases by ∼4 times from ∼10.1 to 38 S cm -1 , while S (290 K) decreases by only ∼12% from 474 to 415 μV K -1 , leading to about three-fold enhancement of PF from 2.24 to 6.61 μW cm -1 K -2 , which is very close to the optimal value of SnSe above the structural transition at ∼800 K at ambient pressure. In comparison, the PF of Na-doped SnSe at 290 K is enhanced moderately by ∼30% up to 20 kbar. In contrast, the PF of isostructural black phosphorus with a simple band structure was found to decrease under pressure. The comparison with black phosphorus indicates that the multi-valley valence band structure of SnSe is beneficial for the enhancement of PF by retaining a large Seebeck coefficient under pressure. Our results also provide experimental confirmation on the previous theoretical prediction that high pressure can be used to optimize the thermoelectric efficiency of SnSe.