Optical properties of In2O3-coated cover glass for solar collectors

Abstract

Visible and infrared optical properties of tin-doped In2O3 films were examined with regard to their electrical properties so as to be able to meet the requirements made on a solar collector operated at 100 °C. Reflectances and transmittances were calculated on the basis of the Drude theory as functions of the density and mobility of carrier and the film thickness. A good agreement between measured and theoretical reflectances indicates that this theory can describe the optical properties of the films. The calculations predicted a carrier density of about 3 · 10^20/cm^3 for the cut-off wavelength at 2 μm, the most effective minimum thickness of 400 nm, and the highest possible carrier mobility of e.g. 40 cm^2/(V s) to raise the selectivity for solar energy. Selective transparent films could be prepared by controlling the mobility and density of carrier with the introduction of about 1 wt% SnO2 or with annealing the films with 5 wt% SnO2 in air at 400 °C. From these results, it is concluded that the evaluation of the optical selectivity of In2O3 films can be achieved by determining their electrical properties.