Research Article www.acsami.org
Potassium Postdeposition Treatment-Induced Band Gap Widening at Cu(In,Ga)Se2 Surfaces − Reason for Performance Leap? Evelyn Handick,*,† Patrick Reinhard,‡ Jan-Hendrik Alsmeier,† Leonard Köhler,† Fabian Pianezzi,‡ Stefan Krause,†,§ Mihaela Gorgoi,∥ Eiji Ikenaga,⊥ Norbert Koch,†,§ Regan G. Wilks,†,∇ Stephan Buecheler,‡ Ayodhya N. Tiwari,‡ and Marcus Bar̈ *,†,∇,# †
Renewable Energy, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany Laboratory of Thin Films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, CH-8600 Duebendorf, Zurich, Switzerland § Institut für Physik and IRIS Adlershof, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 6, 12489 Berlin, Germany ∥ Methods and Instrumentation for Synchrotron Radiation Research and ∇Energy Materials In-Situ Laboratory (EMIL), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Albert-Einstein-Strasse 15, 12489 Berlin, Germany ⊥ SPring-8/JASRI, 1-1-1 Koto, Sayo-cho, Hyogo 679-5198, Japan # Institut für Physik und Chemie, Brandenburgische Technische Universität Cottbus-Senftenberg, Platz der deutschen Einheit 1, 03046 Cottbus, Brandenburg, Germany ‡
S Supporting Information *
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 (EgSurf, 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 EgSurf 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. KEYWORDS: chalcopyrites, thin-film solar cells, alkali fluoride postdeposition treatments, surface band gap, direct and inverse photoemission
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INTRODUCTION
A common way to introduce alkali elements into the CIGSe absorber grown on alkali-free substrates is the so-called postdeposition treatment (PDT),2,4 where alkali-based compounds (commonly NaF, KF) are evaporated on top of the heated CIGSe layer. The addition of alkali elements in a PDT allows more control over the amount incorporated into the CIGSe layer, rather than relying on alkali diffusion at elevated temperature from the substrate (usually soda-lime glass) through the Mo contact layer. It has long been known that Na diffusion into the absorber is needed to produce highperforming solar cells. Hence, for alternate substrates like flexible materials (i.e., nonsoda-lime glass) it is required to add
Recently, chalcopyrite-based thin-film solar cells have experienced a significant leap in performance, kickstarting research and commercialization efforts by showing that this technology has the potential to supersede polycrystalline Si wafer based technologies in terms of performance, cost, and application flexibility. The improved performance of Cu(In,Ga)Se 2 (CIGSe) based devices, with reproducible conversion efficiencies above 21%,1 is tied to the controlled introduction of alkali elements (especially Na and/or K) after growth of the absorber.1,2 The underlying beneficial impact of the addition of alkali elements in general, and potassium in particular, on the chemical, electronic, or even topographic (a KF-PDT induced “surface nanopatterning” was reported in ref 3) properties is not yet clearly and unambiguously understood. © XXXX American Chemical Society
Received: September 30, 2015 Accepted: November 19, 2015
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DOI: 10.1021/acsami.5b09231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces sodium separately. So recent explorations have shown for highly performing cells on both rigid1 and flexible substrates,2 the best results are obtained by applying a KF-PDT. In this Article, we compare the effects of NaF and both NaF and KF (hereafter, “NaF/KF-PDT”) on the properties of lowtemperature (
