Defect properties of CuInSe2 and CuGaSe2 Original Research Article

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. InCu) for this ternary compound. Consequently, defect pairs such as (2VCu+InCu) 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 (2VCu+InCu) 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 View the MathML sourceInCu2+ by View the MathML source2VCu−. 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 GaCu donor level in CuGaSe2 is deeper than InCu donor level in CuInSe2, therefore, GaCu behaves as an electron trap in CuGaSe2, even when it is passivated by VCu. We have also calculated the band alignment between the CPCs and CuInSe2, showing that it could have significant effect on the solar cell performance.