Outstanding thermoelectric performances for both p- and n-type SnSe from first-principles study

The highest thermoelectric figure of merit (ZT) value of 2.6 has been experimentally determined in p-type single crystalline SnSe. In this work, we propose possible method to further enhance the thermoelectric performance of SnSe by comparing different thermoelectric performances of p- and n-type SnSe using the first-principles method and the semiclassical Boltzmann theory. There are relatively stronger intra-layer interactions between Sn and Se atoms for the layered single crystalline SnSe. The Sn p and s electrons near the Fermi level are pushed away along the a-direction because of the anti-bonding interaction between Sn and Se atoms. The pushed away electrons would hinder holes transport in the a-direction. This leads to the lowest electrical conductivity and ZT value along the a-direction for the p-type single crystalline SnSe. For n-type SnSe, the Sn p and Se p electrons are primarily distributed along the a-direction near the Fermi level. This would lead to high electrical conductivity and large Seebeck coefficient simultaneously in the a-direction. Combined with its ultralow thermal conductivity in the a-direction, the ZT value (∼3.1 at 770 K by a rough estimation) of n-type SnSe along the a-direction is probably much higher than that of p-type one. The peak values of S2στ of the p-type SnSe correspond to different carrier concentrations along the a- and b-directions. This is one reason for the low experimental peak ZT value of 0.6 for the p-type polycrystalline SnSe at 750 K. For the n-type SnSe, the S2στ can attain their peak values almost at the same carrier concentration along the a-, b-, and c-directions. This suggests that the power factor value of n-type polycrystalline SnSe would be much higher than that of p-type one at 770 K.