First-principles modeling of alkali metal post deposition treatment effects in CIGS solar cells

The efficiencies of Cu(In, Ga)Se-2(CIGS) solar cells have increased very fast, thanks the alkali post deposition treatment (PDT). In the present work, we have considered the role of alkali metal atoms in the efficiency enhancement. First, we have investigated the effect of alkali metal atoms in the bulk CuInSe2(CIS) absorber and at the grain boundaries in terms of formation and migration energies and charge transition levels. We found that the copper sublattice is the most preferable for all the alkali metal atoms. A detailed comparison between different alkali metal atoms, with respect to the behavior at grain boundaries and in grain interiors has been done. Moreover, we have studied how alkali metal atoms contribute in the formation of the secondary phases. The secondary phase formation during the PDT process has been suggested on the basis of calculated reaction enthalpies.