Low-temperature acanthite-like phase of Cu2S : Electronic and transport properties

The mobility and disorder in the lattice of Cu atoms as liquidlike behavior is an important characteristic affecting the thermoelectric properties of ${\mathrm{Cu}}₂\mathrm{S}$. In this study, using a theoretical model called an acanthite-like structure for ${\mathrm{Cu}}₂\mathrm{S}$ at a low-temperature range, we systematically investigate the electronic structure, intrinsic defect formation, and transport properties by first-principles calculations. Therefore, previous experimental reports on the indirect band-gap nature of ${\mathrm{Cu}}₂\mathrm{S}$ were confirmed in this work with an energy gap of about 0.9--0.95 eV. As a result, the optical absorption coefficient estimated from this model also gives a potential value of $\ensuremath{\alpha}>{10}⁴\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ in the visible spectrum range. According to the bonding analysis and formation energy aspect, Cu vacancy is the most preferred defect to form in ${\mathrm{Cu}}₂\mathrm{S}$, which primarily affects the conductive behavior as a $p$ type, as experimentally observed. Finally, the transport properties of ${\mathrm{Cu}}₂\mathrm{S}$ system were successfully reproduced using an electron-phonon scattering method, highlighting the important role of relaxation time prediction in conductivity estimation instead of regarding it as a constant.