Using first-principles electronic structure theory, defect formation energies and defect transition levels in CuInSe2 and CuGaSe2 were calculated. It was shown that (i) it was easy to form Cu vacancies in CuInSe2, and (ii) it was also relatively easy to form cation antisite defects (e.g. InCu) for this ternary compound. Consequently, defect pairs such as (2VCu+InCu) had a remarkably low formation enthalpy. As a result, the formation of a series of Cu-poor compounds (CPCs) such as CuIn5Se8 and CuIn3Se5, was explained as a repeat of (2VCu+InCu) pairs in CuInSe2. The very efficient p-type self-doping ability of CuInSe2 was explained by the easy formation of the shallow Cu vacancies. The electrically benign character of the natural defect in CuInSe2 was explained in terms of an electronic passivation of the InCu2+ by 2VCu-. For CuGaSe2, it was found that (i) the native acceptor formation energies and transition energy levels were similar to that in CuInSe2, but the donor formation energy was larger in CuGaSe2. (ii) The GaCu donor level in CuGaSe2 was deeper than InCu donor level in CuInSe2, therefore, GaCu behaves as an electron trap in CuGaSe2, even when it was passivated by VCu. The band alignment between the CPCs and CuInSe2 was also calculated; showing that it could had a significant effect on the solar cell performance.

Defect Properties of CuInSe2 and CuGaSe2. S.H.Wei, S.B.Zhang: Journal of the Physics and Chemistry of Solids, 2005, 66[11], 1994-9