Revisiting the defect physics in CuInSe2 and CuGaSe2

Using first-principles self-consistent electronic structure theory, we have calculated defect formation energies and defect energy levels in CuInSe₂. Contrary to previously accepted assumptions in the analysis of defects in CuInSe₂ we find that (i) it is much easier to form Cu vacancy in CuInSe₂ than to form cation vacancies in II-VI´s. (ii) Defect formation energies vary considerably both with the Fermi energy and the chemical potential of the atomic species and (iii) defect pairs such as (2V_Cu+In _Cu) have a remarkably low formation enthalpy. This explains the massive nonstoichiometry of CuInSe₂ and the appearance of ordered defect compounds CuIn₅Se₈, CuIn₃Se₅, Cu₂In₄Se₇ and Cu₃In₅Se₉. The fact that CuInSe₂ has good electrical properties despite this off-stoichiometry reflects the mutual passivation of In_Cu by V_Cu. Similar results are found for CuGaSe₂, except that (iv) it is more difficult to form (2V_Cu^-+Ga _Cu²⁺) in CuGaSe₂ than to from (2V_Cu^-+In_Cu²⁺) in CuInSe₂, and (v) the Ga_Cu donor levels are much deeper than the In_Cu donor levels. Thus, it is more difficult to dope CuGaSe₂ n-type