First-principles study on the effect of Sn doping in Cu2S - Acanthite phase as a substitute to low chalcocite for modeling complex doping

Abstract

The major challenge in the application of Cu2S, as a solar absorber for photovoltaic, is its excessive Cu-vacancy formation tendency within the crystal structure that makes it a degenerate semiconductor. A recent study on Ag alloyed Cu2S acanthite phase has shown a reduced Cu-vacancy formation and a high Cu diffusion barrier. Though low chalcocite is the experimentally known ground state phase at room temperature, its low crystallinity makes it computationally expensive for complex doping modeling. In this work, we first establish the structural correspondence between a newly predicted acanthite and experimentally known low chalcocite phases of Cu2S. This study shows that the acanthite crystal structure of Cu2S can be used to model complex doping in the low chalcocite. The simulated pair distribution functions and diffusions of Cu at room temperature in acanthite and low chalcocite phases of Cu2S show that they have similar structural behavior. Pristine band structures and density of state plots of these two phases also show similarity. Next, we report density functional theory based first-principles investigations of Cu2-xSnxS systems for x = 0 to 0.31 in the acanthite phase for thermodynamic, electronic, and optical properties. We have found that Sn doping coupled with Cu vacancies creates charge-neutral defects and results in higher photoabsorption in the visible light spectra. It was found from these detailed studies that a low concentration of Sn doping is preferable. Then, Sn doping and Cu-vacancy related defects in low chalcocite Cu2S are also studied. Ab initio molecular dynamics simulations show that these compound defects in Cu2S do not negatively affect Cu diffusion inside the crystal. We propose a possible route to synthesize Sn doped acanthite like Cu2S.