Thick semiconductor films for photothermal solar energy conversion

Abstract

Efficient and economical photothermal conversion of solar energy requires the use of spectrally selective surfaces for collection and retention of incident solar flux. This spectral selectivity can be obtained from an absorber–reflector tandem by overcoating an opaque metal film having high infrared reflectance with a thick film (1–5μm) of semiconductor having an appropriate band gap. The practical implementation of this design using films of silver and silicon on a variety of substrate materials is described. Chemical vapor deposition (CVD) is used for depositing the absorber and the antireflection layers. The spectral reflectance of coated samples has been measured at temperatures up to 500 °C and gives α/ε values in the range of 12–15, with a solar absorptance of 75%. The solar absorptance increases with temperature, while the thermal emittance typically increases by 2%–4% at 500 °C. Both the film adhesion and the spectral reflectance have remained stable after over 2000 cycles to 450 °C.