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

Authors

Michelle Mezher, University of Nevada, Las VegasFollow
Lorelle M. Mansfield, National Renewable Engery Laboratory
Kimberly Horsley, University of Nevada, Las VegasFollow
Wanli Yang, Lawrence Berkeley National Laboratory
Monika Blum, University of Nevada, Las VegasFollow
Lothar Weinhardt, University of Nevada, Las VegasFollow
Kannan Ramanathan, STION
Clemens Heske, University of Nevada, Las VegasFollow

Document Type

Article

Publication Date

11-7-2019

Publication Title

ACS Applied Energy Materials

First page number:

1

Last page number:

8

Abstract

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.

Keywords

Alkali post-deposition treatment; Chalcopyrite thin-film solar cell

Disciplines

Chemical Engineering

Language

English

Repository Citation

Mezher, M., Mansfield, L. M., Horsley, K., Yang, W., Blum, M., Weinhardt, L., Ramanathan, K., Heske, C. (2019). Variations in the Chemical and Electronic Impact of Post-Deposition Treatments on Cu(In,Ga)(S,Se)2 Absorbers. ACS Applied Energy Materials 1-8.
http://dx.doi.org/10.1021/acsaem.9b01565