Dielectric function and double absorption onset of monoclinic Cu2SnS3: Origin of experimental features explained by first-principles calculations

Andrea Crovetto, Rongzhen Chen, Rebecca Bolt Ettlinger, Andrea Carlo Cazzaniga, Jørgen Schou, Clas Persson, Ole Hansen

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Abstract

In this work, we determine experimentally the dielectric function of monoclinic Cu2SnS3 (CTS) by spectroscopic ellipsometry from 0.7 to 5.9 eV. An experimental approach is proposed to overcome the challenges of extracting the dielectric function of Cu2SnS3 when grown on a glass/Mo substrate, as relevant for photovoltaic applications. The ellipsometry measurement reveals a double absorption onset at 0.91 eV and 0.99 eV. Importantly, we demonstrate that calculation within the density functional theory (DFT) confirms this double onset only when a very dense k-mesh is used to reveal fine details in the electronic structure, and this can explain why it has not been reported in earlier calculated spectra. We can now show that the double onset originates from optical transitions at the Γ-point from three energetically close-lying valence bands to a single conduction band. Thus, structural imperfection, like secondary phases, is not needed to explain such an absorption spectrum. Finally, we show that the absorption coefficient of CTS is particularly large in the near-band gap spectral region when compared to similar photovoltaic materials.
Original languageEnglish
JournalSolar Energy Materials & Solar Cells
Volume154
Pages (from-to)121-129
ISSN0927-0248
DOIs
Publication statusPublished - 2016

Keywords

  • CTS
  • Cu2SnS3
  • Optical properties
  • Band gap
  • Ellipsometry

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