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

Michelle Mezher, Lorelle M. Mansfield, Kimberly Horsley, Wanli Yang, Monika Blum, Lothar Weinhardt, Kannan Ramanathan, Clemens Heske

Research output: Contribution to journalArticlepeer-review

3 Scopus Citations


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 ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, and 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 absorbers to fully operating cells, we find that the PDT temperature has a significant impact on the resulting device efficiencies - both the NREL and STION absorbers can result in high-efficiency and low-efficiency devices, depending on KF-PDT processing parameters. The absorbers of low-efficiency KF-PDT devices show the largest Cu surface content after PDT, causing the valence band maximum to be closer to the Fermi energy, thus possibly leading to less efficient charge-carrier separation and/or enhanced recombination at the interface. Finally, we find varying degrees of Na, K, and/or F residuals on the different absorber surfaces after PDT, indicating a potential "hidden" parameter in employing PDTs for improved solar cell performance.

Original languageAmerican English
Pages (from-to)8641-8648
Number of pages8
JournalACS Applied Energy Materials
Issue number12
StatePublished - 23 Dec 2019

Bibliographical note

Publisher Copyright:
© 2019 American Chemical Society.

NREL Publication Number

  • NREL/JA-5K00-75198


  • alkali post-deposition treatment
  • chalcopyrite thin-film solar cell
  • photoelectron spectroscopy
  • potassium fluoride
  • sodium fluoride
  • X-ray emission spectroscopy


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