Role of sodium in the control of defect structures in CIS [solar cells]

The authors have grown CIS epitaxial films on single-crystal GaAs substrates under conditions that enhance the influence of surface effects on the resulting structures and their properties. There is a pronounced morphological dichotomy between indium-rich and copper-rich films. In addition, epilayers with nominally identical compositions and morphologies can exhibit fundamentally different ordering of the lattice in either the equilibrium chalcopyrite (CH) or metastable CuAu (CA) structure. The addition of sodium to In-rich CulnSe₂ epilayers during the initial stages of epitaxy both suppresses the formation of metastable Cu-CuInSe₂ and dramatically changes the film morphology. They discuss these results in the context of recent theories of island nucleation and their hypothesis that sodium acts as a surfactant during the growth of CuInSe₂ by destabilizing the (ln_Cu+2V_Cu) defect associate-complex in the near-surface transition layer, rejecting excess indium from the bulk of the growing film