Electronic structure and thermoelectric properties of biaxial strained SnSe from first principles calculations

Abstract

Biaxial strain effects on the electronic structure and thermoelectric properties of the Pnma phase SnSe are investigated by first-principles calculations and Boltzmann transport theory. The biaxial strains ε ab, ε ac and ε bc were applied on the ab, ac and bc planes from −6% to 6%, respectively. The band gap decreases under the compressive strains, and increases under the tensile strains except for ε ab = 6%. The ε bc can tune the band gap in a large range from 0 eV to 0.88 eV. A semiconductor to metal transition is observed at ε bc < = −4%. The biaxial strains also influence the electronic band structure of SnSe with Pnma phase. The momentum alignment and energy convergence of the electronic bands induced by the biaxial strains are observed. At ε ac = −6%, the Pnma SnSe transits from an indirect bandgap to a direct bandgap material. The calculated Seebeck coefficient values for unstrained SnSe are in good agreement with the experimental results. The calculated results indicate that the biaxial strains ε ab, ε ac and ε bc can improve the Seebeck coefficient S, electrical conductivity σ/τ and power factor PF/τ of the Pnma SnSe. However the biaxial strains have different effects on the three components of these thermoelectric properties along the a, b and c axes. For unstrained SnSe, the PFa/τ of p-type SnSe is the smallest one while the PFa/τ of n-type SnSe is the largest one among the three components of PF/τ at 300 K and 750 K. The PF/τ of n- and p-type SnSe can be significantly enhanced compared with those of the unstrained one for 300 K and 750 K, respectively. These findings indicate that the thermoelectric performance of SnSe used at room temperature and high temperature can be improved by the suitable biaxial strains.