First-principles study of defect physics in a photovoltaic semiconductor Cu₂ZnGeSe₄

Cu₂ZnGeSe₄ is of interest for the development of next-generation thin-film photovoltaic technologies. The additional number of elements in Cu₂ZnGeSe₄ increases the flexibility of material properties relative to binary and ternary semiconductors. However, a variety of intrinsic lattice defects are formed that have a significant impact on its photovoltaic performance. Here we study the intrinsic point defects in Cu₂ZnGeSe₄ using the density functional theory calculations with both the generalized gradient approximation and the hybrid functional. Examination of the thermodynamic stability of Cu₂ZnGeSe₄ shows that the stable chemical potential region for the formation of stoichiometric compound is small. Under Zn-poor condition, the shallow acceptor, Cu_Zn antisite is the dominant defect with the lowest formation energy, accounting for the observed p-type conductivity, while in a Ge-rich condition, the Fermi level is close to the middle of band edge because of the lower formation energy of Zn_Cu. We will discuss the effects of these defects on the photoelectric properties of Cu₂ZnGeSe₄.