Variations in the Chemical and Electronic Impact of Post-Deposition Treatments on Cu(In,Ga)(S,Se)2 Absorbers

We present a comparative study that focuses on the variability of post-deposition treatments (NaF-PDT and KF-PDT) and their impact on the chemical and electronic structure of chalcopyrite thin film solar cell absorbers. For this purpose, two “extreme” chalcopyrite absorber systems are studied: Cu(In,Ga)(S,Se)2 with industrial relevance (STION), and Cu(In,Ga)Se2 with “research grade” properties (NREL). Samples were subjected to NaF-PDT and KF-PDT, and investigated using x-ray and ultra-violet photoelectron spectroscopy, Auger electron spectroscopy, as well as synchrotron-based soft x-ray emission spectroscopy. Considerably different alkali-induced effects are found for the two systems. In particular, we only detect a PDT-related Cu depletion on the NREL absorber surfaces (and only on those leading to high-efficiency devices). We also observe a reduction in the surface S/Se ratio for all alkali-treated STION absorbers, in addition to the presence of sulfates after the KF-PDT. After processing the PDT absorb...