AVS 62nd International Symposium & Exhibition
    Thin Film Friday Sessions
       Session TF+MI-FrM

Paper TF+MI-FrM6
Application of High Refractive Index Layers to Perfect Absorbers for Solar and Thermal Radiations

Friday, October 23, 2015, 10:00 am, Room 111

Session: Thin Films for Light Trapping, Plasmonic, and Magnetic Applications
Presenter: Motofumi Suzuki, Kyoto University, Japan
Authors: M. Suzuki, Kyoto University, Japan
K. Nishiura, Kyoto University, Japan
S. Masunaka, Kyoto University, Japan
K. Namura, Kyoto University, Japan
Correspondent: Click to Email
In this presentation, we demonstrate that high refractive index materials such as β-FeSi2 are key to achieve anti-reflective interference coatings on an opaque substrate. Β-FeSi2 is known as an eco-friendly semiconductor and its bulk refractive (n) and extinction (k) indices are higher than 5 and zero, respectively, in infrared (IR) region (λ>1.55 μm). We have reported that the high refractive index of β-FeSi2 is quite useful to reduce the reflectance of metal substrates and that β-FeSi2 thin films/stainless steel substrate systems show nice spectrally selective absorption properties in IR region. For sputtered polycrystalline β-FeSi2 thin films, we recently found that k≈0.3 in IR region, which is different from the bulk value. Thus, we redesigned antireflective-layered structures on an opaque substrate and prepared them. As the results, a system of β-FeSi2/W shows perfect absorption properties, where absorptance reaches higher than 99% at desired wavelength regions, while that in other regions is lower than a few %. On the other hand, another interesting optical property of β-FeSi2 is that both n and k are considerably high in visible to NIR region (λ<1.55 μm). This enables us to design multilayered broadband absorbers for VIS to λ<2.0 μm. The designed multilayers consist of SiO2/β-FeSi2/SiO2/β-FeSi2/W, where the upper β-FeSi2 layer absorbs VIS and NIR (λ<1.0 μm) and the bottom β-FeSi2 layer/W absorbs IR (1.0 μm<λ<2.0 μm). The optimized multilayers absorb more than 95% of solar energy and the eminence at 450 °C is lower than 6%. No significant change in absorptive properties in both single and multilayered absorbers has been recognized after they are annealed in air at least up to 500 °C. The perfect absorbers with high refractive index layers are useful for applications to solar selective absorbers for solar thermal power generation and spectrally selective thermal emitters for thermophotovoltaic power generation, IR heaters, radiation cooling.