3.1. Spectrometric characterization of spectral-normal reflectance in UV to mid-IR regime
The specular reflectance of the fabricated solar absorber was measured by a Fourier Transform Infrared (FTIR) spectrometer (Thermo Fisher, iS50) along with a variable-angle reflectance accessory (Harrick Scientific, Seagull) at an incidence angle of 8° from 0.4 to 20 μm in wavelength with a spectral resolution of 4 cm−1 in wavenumber. Due to the excellent geometric symmetry in the x and y directions of the sample, the spectrometric measurement was performed with unpolarized waves as the radiative property has negligible polarization-dependence at the near-normal direction. The reflectance from 0.4 to 1.1 μm in wavelength was measured by an internal Si detector, while an internal DTGS detector was employed at longer wavelengths beyond 1.1 μm. An aluminum mirror was used as the reference and the measured reflectance is normalized based on the theoretical reflectance of aluminum. The measured reflectance was averaged over three measurements (each with 32 scans) by interchanging the sample and reference to reduce the occasional errors during the measurement. In order to check the uncertainty of FTIR measurements, the reflectance of a reference Si sample (Virginia Semiconductor, Boron doped with resistivity of 60 Ω cm) was measured and compared with its theoretical value, showing the measurement uncertainty within 2%.