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

Abstract

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 × 10 20 cm −3 . Furthermore, the optimal carrier concentrations at different strains are determined. These insights point to an alternative strategy for superior thermoelectric properties.Newly discovered thermoelectric materials BiCuOSe and BiCuOS [BiCuOCh (Ch = Se, S)] have attracted attentions due to their intrinsically low thermal conductivity [8][9][10][11], whose conductive layers (Cu 2 Se 2 ) 2− or (Cu 2 S 2 ) 2− are alternately stacked with an insulating layer (Bi 2 O 2 ) 2+ , composing of the ZrSiCuAs structure type, and this layered structure may be an important factor affording its low thermal conductivity [12,13]. Because of the low lattice thermal conductivity, efforts to improve thermoelectric performance ZT of these compounds have mainly focused on enhancing their power factor S 2 σ. To obtain high power factor, several approaches to increase the electrical conductivity of BiCuOCh, such as doping [14,15], pressure [16], and strain [17], have been attempted. As the electrical conductivity, σ, and Seebeck coefficient, S, are coupled, improving electrical conductivity will reduce Seebeck coefficient [1]. Compared with p-type BiCuOSe, n-type BiCuOSe possesses higher electrical conductivity [16]. On the other hand, it is also noticed that the power factor, S 2 σ, depends linearly on electrical conductivity, σ, but quadratically on Seebeck coefficient S. Thus, it is an alternative pathway to achieve the enhancement of power factor for n-type BiCuOCh via enhancing Seebeck coefficient while keeping electrical conductivity with only slight reduction.Recently, band engineer...

Strain‐induced effects on thermoelectric performance of layered LaCuOSe

“…In the literature, Zou et al. found that application of 6% uniaxial tensile strain along the c direction can significantly improve the power factor by 54% for n‐type BiCuOSe and by 74% for n‐type BiCuOS at 800 K 65 . The enhancement can be attributed to the significantly enhanced Seebeck coefficient resulting from the increased electron effective mass 65 .…”

“…This is different from the cases of BiCuOCh (Ch = S, Se, Te). In the literature, Zou et al found that application of 6% uniaxial tensile strain along the c direction can significantly improve the power factor by 54% for n-type BiCuOSe and by 74% for n-type BiCuOS at 800 K. 65 The enhancement can be attributed to the significantly enhanced Seebeck coefficient resulting from the increased electron effective mass. 65 This differs from the case of n-type doped LaCuOSe, for which the power factor is reduced under 5% and 10% tensile strain.…”

“…In the literature, Zou et al found that application of 6% uniaxial tensile strain along the c direction can significantly improve the power factor by 54% for n-type BiCuOSe and by 74% for n-type BiCuOS at 800 K. 65 The enhancement can be attributed to the significantly enhanced Seebeck coefficient resulting from the increased electron effective mass. 65 This differs from the case of n-type doped LaCuOSe, for which the power factor is reduced under 5% and 10% tensile strain. The degradation of band degeneracy, caused by the separation of heavy and light conduction bands, ultimately leads to a reduction in the Seebeck coefficient and reduced power factor for n-type LaCuOSe.…”