Density profiles in sputtered molybdenum thin films and their effects on sodium diffusion in Cu(InxGa1−x)Se2 photovoltaics

Molybdenum (Mo) thin films were sputtered onto soda lime glass (SLG) substrates. The main variable in the deposition parameters, the argon (Ar) pressure p_Ar, was varied in the range of 6-20 mTorr. Ex situ spectroscopic ellipsometry (SE) was performed to find out that the dielectric functions ε of the Mo films were strongly dependent on p_Ar, indicating a consistent and significant decrease in the Mo film density ρ_Mo with increasing p_Ar. This trend was confirmed by high-angle-annular-dark-field scanning transmission electron microscopy. ε of Mo was then found to be correlated with secondary ion mass spectroscopy profiles of Sodium (Na) in the Cu(In_xGa_1-x)Se₂ (CIGS) layer grown on top of Mo/SLG. Therefore, in situ optical diagnostics can be applied for process monitoring and optimization in the deposition of Mo for CIGS solar cells. Such capability is demonstrated with simulated optical transmission and reflectance of variously polarized incident light, using ε deduced from SE.