Realization of a cost-effective thermochromic solar absorber with a high emittance change based on VO2 and an infrared transparent intermediate layer

In this study, we present a thermochromic solar absorber coating that reaches a high thermal emittance change by using a thin, optically switching VOx film located on an infrared transparent interlayer (spacer) of Si or Ge with an optical thickness of λ/4 (for λ = 7 μm). Using this so-called “lambda/4-concept,” temperature-dependent reflection measurements in the spectral range between 2500 and 50 000 nm from an absorber with a 450 nm Si spacer and a VOx film oxidized from a 30-nm-thick V display an overall increase in emittance from ε(25 °C) = 12.2% to ε(150 °C) = 55.1%, resulting in a change of Δε = 42.9%. In addition, using an absorber with a 400 nm Ge spacer in combination with a VOx film oxidized from 17.5-nm-thick V, an increase in emittance from ε(25 °C) = 8.2% to ε(150 °C) = 49.2% with a change of Δε = 41.0% was achieved. In addition, the optical properties of Ge and Si thin films over a wide spectral range of 250–38 000 nm were determined using spectroscopic ellipsometry. Using this optical data and a simple optical model of the VOx film, reflectance simulations could be performed by using the ellipsometry analysis software WVASE©. It was shown by x-ray diffraction measurements that the optically switching VOx films oxidized from V in a belt furnace consist of the VO2 and the V2O5 phases. Using scanning electron microscopy images, the surface morphology of optically switching VOx films was compared with over-oxidized VOx films. A correlation between the surface morphology and the crystalline phases was revealed and applied to search for the optimal furnace parameters. This method could significantly reduce the time cost to achieve optically switching VOx coatings using an oxidation process.