Uniaxial Tensile Strain Induced the Enhancement of Thermoelectric Properties in n-Type BiCuOCh (Ch = Se, S): A First Principles Study

It is well known that the performance of thermoelectric measured by figure of merit ZT linearly depends on electrical conductivity, while it is quadratic related to the Seebeck coefficient, and the improvement of Seebeck coefficient may reduce electrical conductivity. As a promising thermoelectric material, BiCuOCh (Ch = Se, S) possesses intrinsically low thermal conductivity, and comparing with its p-type counterpart, n-type BiCuOCh has superior electrical conductivity. Thus, a strategy for increasing Seebeck coefficient while almost maintaining electrical conductivity for enhancing thermoelectric properties of n-type BiCuOCh is highly desired. In this work, the effects of uniaxial tensile strain on the electronic structures and thermoelectric properties of n-type BiCuOCh are examined by using first-principles calculations combined with semiclassical Boltzmann transport theory. The results indicate that the Seebeck coefficient can be enhanced under uniaxial tensile strain, and the reduction of electrical conductivity is negligible. The enhancement is attributed to the increase in the slope of total density of states and the effective mass of electron, accompanied with the conduction band near Fermi level flatter along the Γ to Z direction under strain. Comparing with the unstrained counterpart, the power factor can be improved by 54% for n-type BiCuOSe, and 74% for n-type BiCuOS under a strain of 6% at 800 K with electron concentration 3 × 1020 cm−3. Furthermore, the optimal carrier concentrations at different strains are determined. These insights point to an alternative strategy for superior thermoelectric properties.