Impact of Air-Annealing and Na₂S Rinsing on the Chemical and Electronic Structure of RbF Post-Deposition Treated Cu(In,Ga)Se₂ Surfaces

Alkali fluoride post-deposition treatments (PDTs) have significantly enhanced the power conversion efficiency (PCE) of Cu(In,Ga)Se₂ (CIGSe)-based thin-film solar cells (to above 23% on the laboratory scale). The PDT process aims to passivate the CIGSe surface and grain boundaries, and is typically followed by a rinsing step to remove alkali fluoride excess and unwanted surface phases (e.g., oxides). In a recent study on several thousand CIGSe samples, an additional conditioning of the CIGSe surface with air-annealing and/or rinsing with a diluted Na₂S solution was applied prior to the CdS buffer layer deposition, leading to PCEs up to 19% [1]. To exploit the full potential of these conditioning steps, their detailed mechanisms and impact on the chemical and electronic structure of the absorber surface need to be fully understood.
To enable a further insight-driven optimization, we have employed laboratory- and synchrotron-based x-ray photoelectron spectroscopy using the X-SPEC beamline [2] at the KIT Light Source to gain a depth-resolved understanding of the chemical and electronic structure of the absorber surface. Specifically, we focus on the RbF-PDT CIGSe surface with and without air-annealing and/or Na₂S rinse and correlate the findings with inductively coupled plasma - mass spectrometry of the rinsing solutions as well as the PCEs of corresponding solar cells.
We find no evidence of a Rb-In-Se environment after PDT (i.e., even without the rinsing step). The Na₂S rinse cleans the surface, reduces the Ga-content, and removes elemental Se from the surface. For the sample after annealing and Na₂S rinse, we find that S is incorporated into the outermost CIGSe surface, and that the valence band maximum is shifted away from the Fermi energy. Our results paint a detailed, depth-resolved picture of the chemical and electronic structure of the CIGSe surface after air-annealing and Na₂S rinsing and will be discussed in view of the performance of corresponding devices.

References
[1] R. Gutzler, W. Witte, A. Kanevce, D. Hariskos, and S. Paetel, “V_OC-losses across the band gap: Insights from a high-throughput inline process for CIGS solar cells,” Prog. Photovolt. Res. Appl., vol. 31, no. 10, pp. 1023–1031, 2023, doi: 10.1002/pip.3707.
[2] L. Weinhardt, R. Steininger, D. Kreikemeyer-Lorenzo, S. Mangold, D. Hauschild, D. Batchelor, T. Spangenberg, and C. Heske, “X-SPEC: a 70 eV to 15 keV undulator beamline for X-ray and electron spectroscopies,” J. Synchrotron Radiat., vol. 28, no. 2, pp. 609–617, 2021, doi: 10.1107/S1600577520016318.