Comparative study of sol–gel derived tin-doped indium- and aluminum-doped zinc-oxide coatings for electrical conducting and low-emitting surfaces

Cost-efficient transparent conductive oxide (TCO) compounds based on doped zinc oxide (e.g. aluminum zinc oxide – AZO) have been investigated in addition to the well-established functional coatings of doped indium oxide (e.g. indium tin oxide – ITO). The electrical, morphological and optical properties of both types of coatings are compared. For the deposition of transparent conductive oxides, such as tin-doped indium oxide and aluminum-doped zinc oxide on soda-lime glass, enhanced sol–gel processes have been used. Colloidal solutions consisting of inorganic precursors were applied via dip-coating technique, crystallized on the substrate and subsequently annealed in a reducing atmosphere in order to enrich the number of free charge carriers and the electrical conductivity, respectively. In addition to the conventional layer properties like high transparency in the visible spectral region, this work mainly focuses on the comparison of the infrared-optical characteristics in the mid and far infrared spectral range. The directional-hemispherical reflectance and transmittance of functional ITO and AZO multilayer structures with thicknesses of 50–500 nm were measured in the wavelength range between 0.25 and 35 μm and the thermal emission at ambient temperature was calculated and correlated with the electrical layer properties and the doping level of the coatings. To gain further insights, important characteristics of the coatings, such as layer thickness, surface smoothness, electrical conductivity, doping level, band gap energy as well as the position of the plasma wavelength were analyzed using UV–VIS- and IR-spectroscopy, SEM-, EDX-, XRD- and 4-point conductivity measurements. Both types of the functional coatings are highly transparent in the visible spectral region with over 80% and their specific resistivity reaches values up to 3 × 10⁻⁴ Ω cm. By applying multilayered coatings on soda-lime glass, the surface emittance was reduced from 89% to less than 20% in the infrared spectral range.