Defect properties of CuInSe2 and CuGaSe2

Using first-principles electronic structure theory, we have calculated defect formation energies and defect transition levels in CuInSe₂ and CuGaSe₂. We show that (i) it is easy to form Cu vacancies in CuInSe₂, and (ii) it is also relatively easy to form cation antisite defects (e.g. In_Cu) for this ternary compound. Consequently, defect pairs such as (2V_Cu+In_Cu) have a remarkably low formation enthalpy. As a result, the formation of a series of Cu-poor compounds (CPCs) such as CuIn₅Se₈ and CuIn₃Se₅, is explained as a repeat of (2V_Cu+In_Cu) pairs in CuInSe₂. The very efficient p-type self-doping ability of CuInSe₂ is explained by the easy formation of the shallow Cu vacancies. The electrically benign character of the natural defect in CuInSe₂ is explained in terms of an electronic passivation of the by . For CuGaSe₂, we find that (i) the native acceptor formation energies and transition energy levels are similar to that in CuInSe₂, but the donor formation energy is larger in CuGaSe₂. (ii) The Ga_Cu donor level in CuGaSe₂ is deeper than In_Cu donor level in CuInSe₂, therefore, Ga_Cu behaves as an electron trap in CuGaSe₂, even when it is passivated by V_Cu. We have also calculated the band alignment between the CPCs and CuInSe₂, showing that it could have significant effect on the solar cell performance.