Roles of sodium induced defects in CuInSe2 by first principles calculation

In order to investigate the roles of sodium in CuInSe2 absorber layer, the formation energy of sodium induced point defects and bound defects in CuInSe2 are calculated using first principles calculations within supercell approximation approach. Neutral defect NaCu and acceptor defect NaIn are found to form easily, which explains the enhanced p-type conductivity of CuInSe2 by sodium incorporation. More than that, sodium induced defects are energetically favorable to form bound defects with intrinsic defects (i.e. VCu, Cui, InCu, and VSe), which is responsible for the increased range of composition. The contour plot of total valence charge density suggests the chemical bonding between NaIn and Se atoms is stronger than that of Nai and NaCu.