Effect of intrinsic vacancy defects on the electronic properties of monoclinic Ag2S

The formation energy and electronic properties of intrinsic defects in Ag2S are studied using first-principle calculations. Three possible intrinsic defect configurations are considered, including V-Ag_I, V-Ag_II and V-S. The calculation results show that V-Ag defects are more energetic favorable than V-S defects. It is found that electrical neutral V-Ag_I is energetically favorable over the negative charged V-Ag_I below E-F = 0.335eV. We find electronic conductivity could be enhanced by introducing intrinsic V-Ag defects into crystalline Ag2S. Our electronic band structures show that the curvatures of conduction band minimum of Ag2S are much flatter than the curvatures of valence band maximum, suggesting that the effect masses of holes are much lighter than those of electrons. Compared to pristine Ag2S, the effective masses of holes get larger by introducing vacancy into crystal. Fermi level is passing through the valence band, suggesting Ag2S with V-Ag defect would be a better conductivity. Non-vanishing N(E-F) as well as light effect masses of holes make Ag2S suitable for construction of highly conductive p-type materials.