PHYSICS OF DOPED TIN DIOXIDE FILMS FOR SPECTRAL-SELECTIVE SURFACES

The spectral selectivity of tin dioxide layers has been optimised in an experimental study using Drude's theory. This theory gives a free electron model which makes it possible to compute the spectral reflectance as a function of electrical properties of a conducting material. By measuring the reflectance, on the one hand, and by measuring the Hall coefficient and electrical conductivity, on the other, the validity of this model has been checked. For an optimum spectral selectivity it is shown that, besides a minimum layer thickness of 0.5 μm, a high electron mobility of at least 4.0 × 10-3 m2/Vs and an electron number density of about 3 × 1026 m-3 is favourable. We could obtain these conditions with a fluorine-doped tin oxide coating. Applying this coating on a black-enamelled steel gives a good spectral-selective surface for solar collectors with a solar absorptance of 0.92 and a thermal emittance of 0.15. © 1979.