Structural, electronic, and optical properties of bulk Cu2Se

•Structural, electronic, and optical properties of bulk Cu2Se were investigated.•The results are based on the state-of-the-art GW–Bethe–Salpeter equation approach.•van der Waals forces greatly modify the structure and energetics of layered Cu2Se.•The layered phase is semiconducting in contrast to the metallic fluorite one.

By using first-principles calculations within the density functional theory and the many-body perturbation theory, we investigate the structural, electronic, and optical properties of bulk Cu2Se with a recently discovered low-temperature layered configuration. We demonstrate that the effects of the van der Waals forces significantly modify the interlayer binding and distance in the layered Cu2Se, while the band gap is invariant. Our density functional theory and post-processing GW calculations reveal that for the layered structure, GW correction remedies the serious band-gap underestimation of the density functional theory from 0.12 eV to 0.99 eV. By solving the Bethe–Salpeter equation, we find that the optical gap of the layered Cu2Se is 0.86 eV, which is in close agreement with previous experimental observations. In addition, we show that the high-temperature fluorite structure has no band gap, even after GW correction, explaining that the band gap controversy among the theories stems from different structural models. This work may serve as an important guide in designing and evaluating photovoltaic devices using Cu2Se-based materials.