Band-Gap Widening at the Cu(In,Ga)(S,Se)2 Surface: A Novel Determination Approach Using Reflection Electron Energy Loss Spectroscopy

Dirk Hauschild; Evelyn Handick; Sina Göhl-Gusenleitner; Frank Meyer; Holger Schwab; Andreas Benkert; Stephan Pohlner; Jörg Palm; Sven Tougaard; Clemens Heske; Lothar Weinhardt; Friedrich Reinert

doi:10.1021/acsami.6b06358

Band-Gap Widening at the Cu(In,Ga)(S,Se)2 Surface: A Novel Determination Approach Using Reflection Electron Energy Loss Spectroscopy

ACS Appl Mater Interfaces. 2016 Aug 17;8(32):21101-5. doi: 10.1021/acsami.6b06358. Epub 2016 Aug 5.

Authors

Dirk Hauschild¹, Evelyn Handick¹, Sina Göhl-Gusenleitner¹, Frank Meyer¹, Holger Schwab¹, Andreas Benkert^{1

2}, Stephan Pohlner³, Jörg Palm³, Sven Tougaard⁴, Clemens Heske^{2

5

6}, Lothar Weinhardt^{2

5

6}, Friedrich Reinert¹

Affiliations

¹ Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.
² Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT) , Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
³ AVANCIS GmbH , Otto-Hahn-Ring 6, 81739 Munich, Germany.
⁴ Department of Physics, Chemistry and Pharmacy, University of Southern Denmark , DK-5230 Odense M, Denmark.
⁵ Department of Chemistry and Biochemistry, University of Nevada , Las Vegas (UNLV), 4505 Maryland Parkway, Las Vegas, Nevada 89154-4003, United States.
⁶ Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT) , Engesserstrasse 18/20, 76128 Karlsruhe, Germany.

PMID: 27463021
DOI: 10.1021/acsami.6b06358

Abstract

Using reflection electron energy loss spectroscopy (REELS), we have investigated the optical properties at the surface of a chalcopyrite-based Cu(In,Ga)(S,Se)2 (CIGSSe) thin-film solar cell absorber, as well as an indium sulfide (InxSy) buffer layer before and after annealing. By fitting the characteristic inelastic scattering cross-section λK(E) to cross sections evaluated by the QUEELS-ε(k,ω)-REELS software package, we determine the surface dielectric function and optical properties of these samples. A comparison of the optical values at the surface of the InxSy film with bulk ellipsometry measurements indicates a good agreement between bulk- and surface-related optical properties. In contrast, the properties of the CIGSSe surface differ significantly from the bulk. In particular, a larger (surface) band gap than for bulk-sensitive measurements is observed, providing a complementary and independent confirmation of earlier photoelectron spectroscopy results. Finally, we derive the inelastic mean free path λ for electrons in InxSy, annealed InxSy, and CIGSSe at a kinetic energy of 1000 eV.

Keywords: Cu(In,Ga)(S,Se)2; EELS; band gap; optical spectroscopy; thin-film solar cell.