title: Correlating the Local Defect-Level Density with the Macroscopic Composition and Energetics of Chalcopyrite Thin-Film Surfaces.
abstract: The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin-film solar cells. Especially, impurity-driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect-level density and unusual optoelectronic grain-boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provides evidence that Na diffusion from the glass substrate does not affect the surface defect passivation or grain-boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6 eV) due to Na. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat-induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n interface in standard chalcopyrite thin-film solar cells is dominated by band bending within the CIGSe absorber rather than the result of Na impurities or an n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as the origin for Cu depletion and band-gap widening at the surface of chalcopyrite thin films.
authors.affiliation: †Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany.|‡Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, 48149 Münster, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Bröker
authors.affiliation: †Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany.|‡Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, 48149 Münster, Germany.
authors.forename: Dennis
authors.initials: D
authors.lastname: Kück
authors.affiliation: †Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany.|‡Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, 48149 Münster, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Timmer
authors.affiliation: ⊥Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Iver
authors.initials: I
authors.lastname: Lauermann
authors.affiliation: ⊥Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Bünyamin
authors.initials: B
authors.lastname: Ümsür
authors.affiliation: ⊥Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Dieter
authors.initials: D
authors.lastname: Greiner
authors.affiliation: ⊥Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany.
authors.forename: Christian A
authors.initials: CA
authors.lastname: Kaufmann
authors.affiliation: †Physikalisches Institut, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany.|‡Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, 48149 Münster, Germany.
authors.forename: Harry
authors.initials: H
authors.lastname: Mönig
keywords: chalcopyrite thin films; photoelectron spectroscopy; scanning tunneling spectroscopy; solar cells; surface defects