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 α-Ag₂S crystal, a ductile inorganic semiconductor reported experimentally [Nat. Mater. 2018, 17, 421]. The semiconductor α-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 × 10^-3 V/K for n-type and 0.21 × 10^-3 V/K for p-type α-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 α-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 α-Ag₂S. Our work demonstrates the great advantage of the α-Ag₂S crystal as a ductile thermoelectric material and sparks new routes to improve its figure of merit.