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

Journal Article · Thu Nov 19 00:00:00 EST 2015 · Physical Review Applied

DOI:https://doi.org/10.1103/PhysRevApplied.4.054006· OSTI ID:1233696

Choi, Sukgeun ^[1]; Park, Ji-Sang ^[1]; Donohue, Andrea ^[2]; Christensen, Steven T. ^[1]; To, Bobby ^[1]; Beall, Carolyn ^[1]; Wei, Su-Huai ^[1]; Repins, Ingid L. ^[1]

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1. National Renewable Energy Lab. (NREL), Golden, CO (United States)
2. J. A. Woollam Co. Inc., Lincoln, NE (United States)

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 functionϵ=ϵ1+iϵ2 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 ϵ data for three structural variations suggests that the sample may not have a pure (ordered) kesterite phase. We found that the complex refractive index N=n+ik, normal-incidence reflectivity R, and absorption coefficients α are calculated from the modeled ϵ 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 .

• Research Organization: National Renewable Energy Lab. (NREL), Golden, CO (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Solar Energy Technologies Office
• DOE Contract Number: AC36-08GO28308
• OSTI ID: 1233696
• Report Number(s): NREL/JA-5J00-64401
• Journal Information: Physical Review Applied, Vol. 4, Issue 5; Related Information: Physical Review Applied; ISSN 2331-7019
• Publisher: American Physical Society
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
36 MATERIALS SCIENCE
Cu2ZnGeSe4
Ellipsometry
Optical properties
Electronic structure