Enhanced out-of-plane thermoelectric performance of Cmcm SnSe phase by uniaxial strain

The electronic structure and thermoelectric property of SnSe are calculated by first-principles methods and Boltzmann transport theory. The deformation potential approximation and single parabolic band model are employed to compute the relaxation time. Owing to the accurate electronic structure calculated by Tran-Blaha modified Becke-Johnson (TB-mBJ) potential, the theoretical data (Seebeck coefficient, electrical conductivity and figure of merit ZT) are in good agreement with experimental data. Results show that the uniaxial strain not only affects the electrical structure, but also changes the thermal conductivity. As the uniaxial strain increases, the band gap, density of states of mass (mdos) and inertia mass (mi) also increases, indicating that the Seebeck coefficient S increases, while the electrical conductivity a decreases. At carrier concentration of 3 x 10(18) cm(-3), 4 x 10(18) cm(-3), 5 x 10(18) cm(-3), 6 x 10(18) cm(-3) and 7 x 10(18) cm(-3), the corresponding optimal value of ZT is 1.91, 2.78, 3.51, 4.17 and 4.73, respectively. These results show that the strain not only leads to a lower elastic property, which causes the phonon modes softening and phonon propagation slowing down, but also enhances the conduction band minimum (CBM) convergence, which improves the Seebeck coefficient. This work provides insightful data to understand the enhanced thermoelectric properties of SnSe induced by the strain. (C) 2019 Published by Elsevier B.V.