Stability of the bandgap in Cu-poor CuInSe₂.

Recent photoluminescence studies report that the bandgap energy E(g) ≈ 1.0 eV of CuInSe(2) 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 CuInSe(2) 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 CuInSe(2) 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.