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Transparent conductive oxides on top of a black body absorber as alternative concept for high-temperature-stable solar-selective coatings

Krause, M.; Lungwitz, F.; Mendez, A.; Hoppe, M.; Sonnenberg, J.; Garcia-Valenzuela, A.; Munnik, F.; Grenzer, J.; Hübner, R.; Escobar Galindo, R.

After a short overview about the activities of our research group the concept of selective transmitter coatings on top of black body absorbers for the use in high-temperature solar thermal applications will be introduced.[1,2] Solar selective transmitters, which are also called heat mirrors, are characterized by a high solar transmittance and a high thermal reflectance. They can consist either of dielectric/metal/dielectric multilayers or of transparent conductive oxides (TCOs),[3] but only the latter one’s are suitable for high-temperature applications. The design of a TCO on top of a black body has a series of advantages compared to multilayer- or cermet-based solar-selective coatings (SSCs). Bare absorbers can be transformed into selective ones, the functionality is almost independent on film thicknesses, the fabrication is relatively easy and the concept is adaptable to specific requirements with respect to the operation temperature of the solar-thermal application.
The conceptual introduction will be followed by a review of recent developments in the field, which include the excellent high-temperature in-air stability of such type of solar coating when based on Sn-doped In₂O₃ (ITO).[4] In the main part of the talk, the development, optical modelling, properties and thermal stability of another TCO, Ta-doped SnO₂, are reported.[5] Its cutoff, i.e. the wavelength where it changes from transmitting to reflecting, is tunable from 1.7 µm to 2.4 µm. The optical properties of SnO₂:Ta are almost independent on the film thickness. The TCO is stable up to 800 °C in high vacuum and in air for 12 hours (at least) as shown by ion beam analysis, X-ray diffraction, ellipsometry and reflectometry. When the SnO₂:Ta is deposited on silicon and glassy carbon transforms these bare absorbers into selective ones. Finally, as part of the whole SSC concept, the formation, structure, and optical properties of dense, PVD-grown CuCr₂O₄ thin films is reported. This potential high-temperature absorber is obtained in high purity from as-deposited samples by a simple in-air annealing step at 800 °C and absorbs light in the whole solar range from 300 nm to 2500 nm.[6]

[1] Kennedy, C. E. Review of Mid- to High-Temperature Solar Selective Absorber Materials. Report No. NREL/TP-520-31267, (NREL - National Renewable Energy Laboratory, Golden, Colorado, USA, 2002).
[2] Granqvist, C. G. Transparent conductors as solar energy materials: A panoramic review. Solar Energy Materials and Solar Cells 91, 1529-1598, doi:10.1016/j.solmat.2007.04.031 (2007).
[3] Fan, J. C. C. & Bachner, F. J. TRANSPARENT HEAT MIRRORS FOR SOLAR-ENERGY APPLICATIONS. Applied Optics 15, 1012-1017, doi:10.1364/ao.15.001012 (1976).
[4] Wang, H., Haechler, I., Kaur, S., Freedman, J. & Prasher, R. Spectrally selective solar absorber stable up to 900 degrees C for 120 h under ambient conditions. Solar Energy 174, 305-311, doi:10.1016/j.solener.2018.09.009 (2018).
[5] Lungwitz, F. et al. Transparent conductive tantalum doped tin oxide as selectively solar-transmitting coating for high temperature solar thermal applications. Solar Energy Materials and Solar Cells 196, 84-93, doi:10.1016/j.solmat.2019.03.012 (2019).
[6] Krause, M. et al. Formation, structure, and optical properties of copper chromite thin films for high-temperature solar absorbers. Mater. 18, doi:10.1016/j.mtla.2021.101156 (2021).

  • Invited lecture (Conferences) (Online presentation)
    2021 Iberian Vacuum Meeting - RIVA Online, 04.-06.10.2021, Online, Spanien/ Portugal

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Publ.-Id: 33202