α-Ag2S: A Ductile Thermoelectric Material with High ZT

Using first-principles calculation and Boltzmann electron/phonon transport theory, we present an accurate theoretical prediction of thermoelectric properties of the alpha-Ag2S crystal, a ductile inorganic semiconductor reported experimentally [Nat. Mater. 2018, 17, 4211. The semiconductor alpha-Ag2S has ultralow thermal conductivity associated with high anisotropy, which can be attributed to the complex crystalline structure and weak bonding. The optimal values of the Seebeck coefficient are 0.27 x 10(-3) V/K for n-type and 0.21 X 10(-3) V/K for p-type alpha-Ag2S, respectively, which are comparable to those of many promising thermoelectric materials. As a consequence, a maximum ZT value of 0.97/1.12 can be realized for p-type/n-type alpha-Ag2S at room temperature. More interestingly, the value of ZT can be further enhanced to 1.65 at room temperature by applying 5% compressive strain. Moreover, we find that the electronic thermal conductivity is a major factor limiting the ZT, which is several times the lattice thermal conductivity for n-type alpha-Ag2S. Our work demonstrates the great advantage of the alpha-Ag2S crystal as a ductile thermoelectric material and sparks new routes to improve its figure of merit.