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 non-stoichiometry 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)(-)+Ca-Cu(2+)) in CuGaSe2 than to from (2V(Cu)(-)+In-Cu(2+)) 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.