DIELECTRIC INTERFERENCE LAYERS FOR SOLAR. THERMAL COLLECTORS

J. Boudaden1and P. Oelhafen1,
11nstitut fur Physik, University of Basel, Klingelbergstrasse 82, CH-4056 Basel,

Switzerland

Tel: 41-61-267-37-15, Fax: 41-61-267-37-82, jamila. boudaden@unibas. ch
A. Schuler2, C. Roecker2 and J. — L. Scartezzini2
2 Laboratoire d’Energie Solaire et de Physique du Batiment LESO-PB, Ecole
Polytechnique Federale de Lausanne (EPFL), Batiment LE, CH-1015 Lausanne,

Switzerland

Abstract

Our aim is to study the possibility of integrating dielectric multilayer films deposited on glass substrates as a colored glazed cover for thermal solar collectors and building faces. The cover glass should ideally reflect only a narrow band of visible light while transmitting the rest of the sunlight spectrum to minimize energy loss. A compromise between the visible reflectance and the solar transmission has to be found. In our multilayer interference filters, we used two materials having respectively a high and a low refractive index. We studied two cases: Al2O3/SiO2 and TiO2/SiO2. The thin films were deposited by reactive magnetron sputtering. In-situ XPS characterizations were carried out for each film by transferring the sample from the deposition chamber to an ultra-high vacuum analysis chamber without breaking the vacuum. The growth rate of TiO2, SiO2 and A^O3 single layers on Si substrates were monitored by in-situ laser reflectometry. Spectroscopic ellipsometry was used to determine the optical constants and the thicknesses of every individual dielectric layer. Reflectivity measurements of the experimentally realized dielectric multilayers deposited on glass substrates confirmed their transparency and their good accordance with the simulation. The multilayers were also characterized by their solar transmission, visible reflectance and a factor of merit.

Keywords: multilayer, dielectric oxides, thermal solar collectors

1. INTRODUCTION

Transparent oxide films are widely employed as antireflection or high reflection coatings [1,2], band-pass filters [3] and narrow-band-filters [4] in various optical and electronic devices. The performances of these devices are based on interference effects obtained by alternating layers of high and low refractive indices.

Nowadays different deposition methods exist to produce dielectric oxide films. Thin film evaporation underwent rapid development and became a standard method for optical coatings [5]. Afterwards, alternative methods such as chemical vapor deposition [б], dip coating [7], sol gel method [8] and reactive sputtering [9] have been extensively studied.

The latter allows large area coatings and thicknesses uniformity combined with high growth rate deposition [10].

In our case, reflecting multilayers are used as a cover for solar collectors. A large fraction of power from the solar radiation must be transmitted through the coatings. The transparency of the film permits avoiding absorption energy losses within the coating. At the same time, the multilayer films should present a narrow reflection band in the visible range. This selective reflection fixes the color of the reflected light. A combination of different refractive indexes and thicknesses makes it possible to realize a wide range of reflected colors with an acceptable solar transmission [11].

In this work, we report an experimental study for the preparation of optical coatings based on Ti02/Si02 and Al203/Si02 multilayer dielectric films realized by reactive magnetron sputtering by depositing alternating layers of two materials. During the experimental realization, some important requirements must be fulfilled. The deposition technique must allow good control and reproducibility of the optical properties of any individual thin film combined with a high deposition rate. The interface between two layers should be as smooth as possible. To meet the above conditions Ti02, Si02 and Al203 are considered a suitable materials to coversolarthermal collectors.

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