Predictability of thermoelectric figure of merit for the single crystal from first principles

The first-principles calculations have been widely used to predict the thermoelectric figure of merit (ZT) for the single crystal, but such precise calculations are confined to phonon-limited intrinsic system, and the defects present in practical samples cannot be easily modeled. So, a question arises as to whether the phonon-limited ZT can represent the thermoelectric potential of the material. In this work, we demonstrate that the exclusion of defect scattering could provide ZT close to the actual values in single crystal by analyses of the ZT involved parameters. This is also illustrated in the case of SnSe by performing first-principles calculations of phononlimited ZT compared with available experimental data. The calculated ZT can reproduce the measured values, indicating that the calculations possess predictive capability. On the basis, strong and anisotropic thermoelectric enhancement of SnSe induced by pressure is found. The pressure significantly increases the thermoelectric power factor of n-type SnSe along its a crystallographic axis and that of p-type SnSe along the b axis, while the increase of lattice thermal conductivity is relatively small. As a result, a large enhancement of ZT is obtained. By applying pressure up to 3.2 GPa, at room temperature, the a-axis ZT of n-type SnSe is as high as 1.2, while the b-axis ZT of p-type SnSe reaches up to 0.8, which are more than twice larger than the values at ambient pressure. This study provides strong confidence and support for the application of first-principles calculations to search high-performance thermoelectric materials and thus would be an important reference for future researches.