Enhanced IR-optical and electrical properties of sol-gel derived tco thin films for energy-efficient window applications

M. Rydzek1, N. Wolf1, M. Arduini-Schuster1 and J. Manara1

1Bavarian Center for Applied Energy Research (ZAE Bayern), Germany

Keywords: infrared radiation, sol-gel technique
property: electrically conducting, low-emitting
material: transparent conductive oxides

The capability of creating electrically conductive sheets on glass by depositing a thin current-carrying layer that maintains a high visible transmittance has opened up a wide field of applications. There is a particular interest in highly conductive, hyaline thin films in the field of modern architecture, due to the necessity of spectrally selective glazing and low-emitting surfaces. Lowering the thermal emittance of a window by applying a highly infrared reflecting metal-oxide thin film leads to an improvement in the thermal properties. Therefore, the thermal insulating effect of a building equipped with such a glazing rises as well as the thermal comfort, which both result in a reduction in energy consumption. At ambient temperature, heat transition from the surroundings onto a surface due to thermal radiation lies in the same order of magnitude as the heat transition caused by convection. The key benefit of sol-gel deposited transparent conductive oxide (TCO) coatings lies in their mechanical and chemical durability. While sputtered TCO coatings and thin transparent metal layers, for example, need specific protection against various environmental influences, sol-gel deposited coatings have a significantly higher resistance and therefore excellent weatherability. Other noticeable advantages of the sol-gel based deposition technique lie in the good adherence between coating and surface, the possibility of functionalizing complexly shaped substrates, as well as the low equipment costs and an easy controlling of doping levels due to the usage of inorganic metal-oxide precursors. The aim of this work is to study sol-gel derived tin-doped indium oxide (ITO) and aluminum-doped zinc oxide (AZO) thin films for highly transparent electrically conductive and infrared reflective surfaces to improve the thermal comfort and to reduce the thermal emittance of a glazing. For thin solid films based on transparent conductive oxides, the infrared-optical properties of the coatings are closely correlated with the electrical properties. Therefore, the complex refractive index, the complex dielectric function, as well as the reflectance of the coatings are correlated and associated with the electrical characteristics. As a result, the spectral emittance, based on the directional-hemispherical reflectance and transmittance values of the enhanced coatings is presented from the ultraviolet over the visible up to the far infrared wavelength region. Moreover, the important structural characteristics of the coatings are presented and discussed.

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