Hybrid-Functional Calculations on the Incorporation of Na and K Impurities into the CuInSe₂ and CuIn₅Se₈ Solar-Cell Materials

Abstract

We have studied the energetics, atomic, and electronic structure of Na and K point defects, as well as the (Na–Na), (K–K), and (Na–K) dumbbells in CuInSe2 and CuIn5Se8 solar cell materials by hybrid functional calculations. We found that although Na and K behaves somewhat similar; there is a qualitative difference between the inclusion of Na and K impurities. Namely, Na will be mostly incorporated into CuInSe2 and CuIn5Se8 either as an interstitial defect coordinated by cations, or two Na impurities will form (Na–Na) dumbbells in the Cu sublattice. In contrast to Na, K impurities are less likely to form interstitial defects. Instead, it is more preferable to accommodate K either as KCu substitutional defect, or to form (K–K) dumbbells on Cu substitution positions. Our data show that all (Na–Na), (Na–K), and (K–K) dumbbells can form in both CuInSe2 and CuIn5Se8. In the Cu-poor CuIn5Se8 material the pristine Cu vacancies act as the most stable sites where Na and K can be inserted. The formation energy of Na-related defects is generally lower than the corresponding K-related defects, which would mean that if a defect site is already occupied by Na, then it is less likely that K is able to substitute Na during the postdeposition treatment. Regarding the electronic structure of the materials, Na and K point defects located in the Cu sublattice do not create deep defect levels in the gap, so they are not detrimental for the solar cell. In contrast, Se-related substitutional defects introduce defect levels in the gap, which act as charge traps, leading to severe degradation of the device efficiency. However, the formation energy of these Se-related defects are high so that they should have a low concentration in the material.