Stability of the bandgap in Cu-poor CuInSe2

Recent photoluminescence studies report that the bandgap energy E-g approximate to 1.0 eV of CuInSe2 is stable for Cu-poor compounds [Cu]/[In] < 1, despite the fact that Cu vacancies and (In-Cu + 2V(Cu)) complexes increase the energy gap. In this work, the impact on E-g due to the presence of native defects is analyzed using a screened hybrid density functional approach. We demonstrate that the formation energy of neutral (Cu-In + In-Cu) anti-site dimers decreases for CuInSe2 compounds when [Cu]/[In] decreases. This is explained in terms of the octet rule for the Se atoms next to the (In-Cu + 2V(Cu)) defects. As a consequence, Cu-poor CuInSe2 involves the large [(In-Cu + 2V(Cu)) + (Cu-In + In-Cu)] complexes where the anti-site defects stabilize E-g, in agreement with experimental findings.