Electronic Structure and Optical Properties of Cu2ZnGeSe4: First-Principles Calculations and Vacuum-Ultraviolet Spectroscopic Ellipsometric Studies

S. G. Choi, J. S. Park, A. L. Donohue, S. T. Christensen, B. To, C. Beall, S. H. Wei, I. L. Repins

Research output: Contribution to journalArticlepeer-review

19 Scopus Citations

Abstract

Cu2ZnGeSe4 is of interest for the development of next-generation thin-film photovoltaic technologies. To understand its electronic structure and related fundamental optical properties, we perform first-principles calculations for three structural variations: kesterite, stannite, and primitive-mixed CuAu phases. The calculated data are compared with the room-temperature dielectric function ρ=ρ1+iρ2 spectrum of polycrystalline Cu2ZnGeSe4 determined by vacuum-ultraviolet spectroscopic ellipsometry in the photon-energy range of 0.7 to 9.0 eV. Ellipsometric data are modeled with the sum of eight Tauc-Lorentz oscillators, and the best-fit model yields the band-gap and Tauc-gap energies of 1.25 and 1.19 eV, respectively. A comparison of overall peak shapes and relative intensities between experimental spectra and the calculated ρ data for three structural variations suggests that the sample may not have a pure (ordered) kesterite phase. The complex refractive index N=n+ik, normal-incidence reflectivity R, and absorption coefficients α are calculated from the modeled ρ spectrum, which are also compared with those of Cu2ZnSnSe4. The spectral features for Cu2ZnGeSe4 appear to be weaker and broader than those for Cu2ZnSnSe4, which is possibly due to more structural imperfections presented in Cu2ZnGeSe4 than Cu2ZnSnSe4.

Original languageAmerican English
Article number054006
Number of pages10
JournalPhysical Review Applied
Volume4
Issue number5
DOIs
StatePublished - 19 Nov 2015

Bibliographical note

Publisher Copyright:
© 2015 American Physical Society.

NREL Publication Number

  • NREL/JA-5J00-64401

Keywords

  • Cu2ZnGeSe4
  • electronic structure
  • ellipsometry
  • optical properties

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