Structural Properties of Chalcopyrite-related 1:3:5 Copper-poor Compounds and their Influence on Thin-film Devices
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1165-M03-09
Structural Properties of Chalcopyrite-related 1:3:5 Copper-poor Compounds and their Influence on Thin-film Devices S. Lehmann*1, D. Fuertes Marrón2, J.M. Merino3, M. León3, E.J. Friedrich3, M. Tovar1, Y. Tomm1, C. Wolf1, S. Schorr4, Th. Schedel-Niedrig1, and M. Ch. Lux-Steiner1 1
Helmholtz-Zentrum Berlin für Materialien und Energie, Institut für heterogene Materialsysteme, Lise Meitner Campus, Glienicker Strasse 100, 14109 Berlin, Germany 2 Instituto de Energía Solar-ETSIT, Universidad Politécnica de Madrid, Spain 3 Universidad Autónoma de Madrid, Departamento de Física Aplicada, C-XII, 28049 Madrid, Spain 4 Free University Berlin, Dep. Geosciences, Malteserstr. 74-100, 12249 Berlin, Germany * Corresponding author: [email protected], Phone: +49 30 8062 2299, Fax: +49 30 80623199 ABSTRACT We present a new structural model for the ternary system Cu(InxGa1-x)3Se5 for the complete CuIn3Se5 - CuGa3Se5 solid solution. Due to similar X-ray scattering factors of the isoelectronic species Cu+ and Ga3+ involved, X-ray diffraction is not a suitable tool for a detailed structural analysis of occupancy factors in Ga-containing samples. Instead, neutron powder diffraction was applied on various samples covering the entire compositional range, complemented with X-ray analysis when appropriate. Based on Rietveld refinements of the neutron diffraction data, we have arrived at a new, ) for CuIII3Se5 (III=In,Ga) modified stannite-type structural model (space group compounds, characterized by 3 different cationic sites (Wyckoff positions 2a, 2b and 4d) which are occupied depending on the composition of the alloy. A significant difference in the distribution of the constituents within the cation sublattice has been found between In-free and In-containing compounds. Structural properties, such as lattice parameters, tetragonal distortion and occupancy factors of the cationic sublattice sites are also given. INTRODUCTION Chalcopyrite-based photovoltaic devices show highest conversion efficiencies among present thin-film architectures with values of 20% at laboratory scale [1]. This outstanding performance has been achieved for quaternary Cu(Inx,Ga1-x)Se2 (x~0.7) alloys. However, a strong correlation between the performance of the solar cell and the gallium content in the absorber layer has been recognized. One of the critical issues in this discussion is the formation of a copper-depleted, near-surface region in the CIGS layer with stoichiometry deviations from the bulk composition, abbreviated as 1:3:5 and 1:5:8 stoichiometries. In earlier reports, surfaces with such compositions have been found on CuInSe2, CuGaSe2 and Cu(Inx,Ga1-x)Se2 thin films [2-5]. These near-surface phases may have a positive influence on the performance of cells based on low band gap Cu(Inx,Ga1-x)Se2 material, due to type inversion of the p-type absorber close to the metallurgical heterojunction with the buffer layer and a measureable near-surface band gap widening of the absorber reducing the net recombination of minority carrier
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