title: Band-Gap Widening at the Cu(In,Ga)(S,Se)2 Surface: A Novel Determination Approach Using Reflection Electron Energy Loss Spectroscopy.
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.
authors.affiliation: Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.
authors.forename: Dirk
authors.initials: D
authors.lastname: Hauschild
authors.affiliation: Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.
authors.forename: Evelyn
authors.initials: E
authors.lastname: Handick
authors.affiliation: Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.
authors.forename: Sina
authors.initials: S
authors.lastname: Göhl-Gusenleitner
authors.affiliation: Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Meyer
authors.affiliation: Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Schwab
authors.affiliation: Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.|Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT) , Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Benkert
authors.affiliation: AVANCIS GmbH , Otto-Hahn-Ring 6, 81739 Munich, Germany.
authors.forename: Stephan
authors.initials: S
authors.lastname: Pohlner
authors.affiliation: AVANCIS GmbH , Otto-Hahn-Ring 6, 81739 Munich, Germany.
authors.forename: Jörg
authors.initials: J
authors.lastname: Palm
authors.affiliation: Department of Physics, Chemistry and Pharmacy, University of Southern Denmark , DK-5230 Odense M, Denmark.
authors.forename: Sven
authors.initials: S
authors.lastname: Tougaard
authors.affiliation: Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT) , Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.|Department of Chemistry and Biochemistry, University of Nevada , Las Vegas (UNLV), 4505 Maryland Parkway, Las Vegas, Nevada 89154-4003, United States.|Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT) , Engesserstrasse 18/20, 76128 Karlsruhe, Germany.
authors.forename: Clemens
authors.initials: C
authors.lastname: Heske
authors.affiliation: Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT) , Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.|Department of Chemistry and Biochemistry, University of Nevada , Las Vegas (UNLV), 4505 Maryland Parkway, Las Vegas, Nevada 89154-4003, United States.|Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT) , Engesserstrasse 18/20, 76128 Karlsruhe, Germany.
authors.forename: Lothar
authors.initials: L
authors.lastname: Weinhardt
authors.affiliation: Experimental Physics VII, University of Würzburg , Am Hubland, 97074 Würzburg, Germany.
authors.forename: Friedrich
authors.initials: F
authors.lastname: Reinert
keywords: Cu(In,Ga)(S,Se)2; EELS; band gap; optical spectroscopy; thin-film solar cell