Electronic properties of energy harvesting Cu-chalcogenides: p-d hybridization and d-electron localization

Abstract Copper-based chalcogenides with the familiar diamond-like structure are superior thin-film light absorbers. Remarkably, several closely related chalcogen-deficient structures have recently been recognized as promising thermoelectric converters with ultra-low thermal conductivities. The combination of various constituent elements provides an ample space for material design. However, first-principles investigations on their electronic properties are complicated by the involvement of the shallow Cu-3d semi-core states. Although Cu-3d electrons are intrinsically rather localized, they also strongly hybridize with delocalized chalcogen-p states. The seemly incompatible tendencies require delicate theoretical treatments, and the conventional local density approximation (LDA) within density functional theory (DFT) is inadequate. In this work, we carefully analyze the characteristics of the Cu-3d states, which form the basis for an in-depth understanding of the electronic properties of Cu-chalcogenides to provide insightful guidance for their energy harvesting applications. We show that the band-edge electronic properties can be reliably predicted by the DFT+U+G₀W₀ or the mBJ+U methods. Moreover, the simple mBJ+U method works equally well for moderate-gap systems and theoretically challenging narrow-gap systems. Cu-chalcogenides with narrow-gaps usually contain heavy elements, in which relativistic effects are expected to play an important role. We find that the orbital contraction effect tends to reduce the energy of the s conduction states in diamond-like structures, and might lead to a structural transformation to chalcogen-deficient structures. Although the spin-orbit coupling effect can be quite strong, its influence on the band structure can also be suppressed by strong hybridizations.