XPS studies of sodium compound formation and surface segregation in CIGS thin films [solar cells]

The authors report X-ray photoelectron spectroscopy (XPS) studies of device-quality CIGS thin films grown on soda-lime glass substrates. The copper atomic concentration profiles within the near-surface region indicate copper depletion, a previously reported observation attributed to the formation of the ternary defect compound Cu(In,Ga)₃Se ₅. Preferential surface segregation of sodium is also observed. Analyses of their data indicate that the group [I]/[III] atomic concentration ratio {[Cu]+[Na]}/{[In]+[Ga]} is constant to within the accuracy of the measurement. This result is strong evidence that the composition of CIGS films grown by multisource physical vapor deposition in the presence of sodium is thermochemically constrained to lie on the pseudobinary Na(In,Ga)Se₂-Cu(In,Ga)Se₂ tie-line in the ternary Na₂Se-Cu₂Se-(In,Ga)₂Se₃ composition diagram