title: Potassium Postdeposition Treatment-Induced Band Gap Widening at Cu(In,Ga)Se₂ Surfaces--Reason for Performance Leap?
abstract: Direct and inverse photoemission were used to study the impact of alkali fluoride postdeposition treatments on the chemical and electronic surface structure of Cu(In,Ga)Se2 (CIGSe) thin films used for high-efficiency flexible solar cells. We find a large surface band gap (E(g)(Surf), up to 2.52 eV) for a NaF/KF-postdeposition treated (PDT) absorber significantly increases compared to the CIGSe bulk band gap and to the Eg(Surf) of 1.61 eV found for an absorber treated with NaF only. Both the valence band maximum (VBM) and the conduction band minimum shift away from the Fermi level. Depth-dependent photoemission measurements reveal that the VBM decreases with increasing surface sensitivity for both samples; this effect is more pronounced for the NaF/KF-PDT CIGSe sample. The observed electronic structure changes can be linked to the recent breakthroughs in CIGSe device efficiencies.
authors.affiliation: Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Evelyn
authors.initials: E
authors.lastname: Handick
authors.affiliation: Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology , Ueberlandstrasse 129, CH-8600 Duebendorf, Zurich, Switzerland.
authors.forename: Patrick
authors.initials: P
authors.lastname: Reinhard
authors.affiliation: Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Jan-Hendrik
authors.initials: JH
authors.lastname: Alsmeier
authors.affiliation: Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Leonard
authors.initials: L
authors.lastname: Köhler
authors.affiliation: Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology , Ueberlandstrasse 129, CH-8600 Duebendorf, Zurich, Switzerland.
authors.forename: Fabian
authors.initials: F
authors.lastname: Pianezzi
authors.affiliation: Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , Hahn-Meitner-Platz 1, 14109 Berlin, Germany.|Institut für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin , Brook-Taylor-Straße 6, 12489 Berlin, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Krause
authors.forename: Mihaela
authors.initials: M
authors.lastname: Gorgoi
authors.affiliation: SPring-8/JASRI, 1-1-1 Koto, Sayo-cho, Hyogo 679-5198, Japan.
authors.forename: Eiji
authors.initials: E
authors.lastname: Ikenaga
authors.affiliation: Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , Hahn-Meitner-Platz 1, 14109 Berlin, Germany.|Institut für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin , Brook-Taylor-Straße 6, 12489 Berlin, Germany.
authors.forename: Norbert
authors.initials: N
authors.lastname: Koch
authors.affiliation: Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Regan G
authors.initials: RG
authors.lastname: Wilks
authors.affiliation: Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology , Ueberlandstrasse 129, CH-8600 Duebendorf, Zurich, Switzerland.
authors.forename: Stephan
authors.initials: S
authors.lastname: Buecheler
authors.affiliation: Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology , Ueberlandstrasse 129, CH-8600 Duebendorf, Zurich, Switzerland.
authors.forename: Ayodhya N
authors.initials: AN
authors.lastname: Tiwari
authors.affiliation: Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH , Hahn-Meitner-Platz 1, 14109 Berlin, Germany.|Institut für Physik und Chemie, Brandenburgische Technische Universität Cottbus-Senftenberg , Platz der deutschen Einheit 1, 03046 Cottbus, Brandenburg, Germany.
authors.forename: Marcus
authors.initials: M
authors.lastname: Bär
keywords: alkali fluoride postdeposition treatments; chalcopyrites; direct and inverse photoemission; surface band gap; thin-film solar cells