Deposition of In 2O 3SnO 2 layers on glass substrates using a spraying method

Abstract

Thin films of doped indium oxide (In 2O 3) or of tin oxide (SnO 2), as well as mixed compounds of the two (indium tin oxide (ITO)), can be prepared in a highly conductive form with the important property of being highly transparent in the visible and near-IR regions of the solar spectrum. Moreover, these layers are physically and chemically resistant and adhere well to many substrates. Potential applications are numerous: specifically as coatings for heat mirror systems as well as in solar cell technology. The “spraying” method of preparation uses a hydrolysis reaction at relatively low temperatures (400–600 °C) in an alcoholic solution of tin or indium chloride. The basic physical, electrical and optical properties of the films are reviewed in relation to the dopant concentrations and stoichiometric deviations. (Typical parameters for ITO are resistivity ϱ≈5×10 −4 ohms cm, transmission coefficient T≈80–90% in the wavelength range 0.3–1.5 microm and sheet resistance R □≈10 ohms.) In recent years the economic contingencies related to the energy crisis have led to an increase of interest in these layers, but the marked rise in the number of publications does not correspond to concomitant improvements in layer quality or to a better understanding of the properties. Some basic theoretical limitations can be pointed out: electron-hole pair creation limits transparency at short wavelengths and the steep increase of reflectivity due to free carrier absorption limits it at long wavelengths. The latter factor is directly related to the electrical conductivity. A fundamental conflict between transparency and conductivity is therefore built in. The properties of these layers are compared with those of chemically identical layers prepared by different methods, as well as with those of some other transparent and conducting films of quite different compositions.