Characterization of spectral selectivity of tin-doped In2O3 thin films using spectroscopic ellipsometry

• Optical constants of ITO, Al2O3, and SiO2 were obtained in the solar range by SE measurement.
• Optical constant of ITO film from 2500 nm to 50 μm was calculated by minds of Drude model extrapolation.
• The spectral selectivity of ITO film with different thickness was increased by minds of AR-coating.
• Calculated spectra of ITO AR stack in the range from 300nn to 50 μm are verified and the error is analyzed.

Transparent conductive oxide (TCO) films on cover glass are one of the key materials for solar thermal collector. Currently, the benefit of using TCO single layer is limited due to its insufficient optical selectivity. The objective of this paper is to increase the spectral selectivity of ITO film with different thickness by whole solar spectrum antireflection treatment. The sputter-deposited ITO thin films on glass substrate are characterized by variable-angle spectroscopic ellipsometry in the spectral range of 300 nm to 2500 nm. Based on suitable dielectric function models such as Drude, Gaussian, and Tauc–Lorentz, the theoretical spectra for the transmittance and reflectance of ITO film with different thickness in the spectral range from 300 nm to 50 μm are calculated. Also, the multi-layered AR coatings for ITO film with different thickness in the broad solar spectrum are designed using optical software. For ITO film with thickness from 40 to 120 nm, an increase of the solar transmittance more than 6% is achieved, without compromise in the infrared emissivity. Following such AR designs, ITO three-layer AR coating (glass/Al2O3/ITO (120 nm)/SiO2) was fabricated. The solar transmittance has been improved from 79.1% without any AR coatings to 84.7% with a three-layer AR coating, and the IR emissivity remains about 0.2 unchanged.