TOWARDS COLOURED GLAZED THERMAL SOLAR COLLECTORS J. Boudaden , R. S-C. Ho and P. Oelhafen Institut für 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. Schüler, C. Roecker and J. - L. Scartezzini Laboratoire d’Energie Solaire et de Physique du Bâtiment LESO-PB, Ecole Polytechnique Fédérale de Lausanne (EPFL), Bâtiment LE, CH-1015 Lausanne, Switzerland Abstract – Multilayers of TiO 2 and SiO 2 dielectric coatings deposited by reactive sputtering have been characterized by X-ray photoelectron spectroscopy (XPS), in-stitu real-time laser reflectometry, spectroscopic ellipsometry and spectrophotometry. Measurements have been performed on individual dielectric layers of TiO 2 , SiO 2 and multilayers. The experiments show a good agreement between the different techniques. Optical properties can be modelled properly using a Cauchy dispersion model. The optical constants of an individual layer are confirmed by the optical measurements on more complex multilayer structures. Reflectivity of the three-layer films fulfils the requirements for a first step towards coloured glazed thermal solar collectors. 1. INTRODUCTION Dielectric thin films are widely employed in various optical devices and components like high-reflection mirrors (Bhattacharyya et al 2001), waveguides (Garapon et al, 1996), narrow-band filters (Ouellette et al, 1991), dyes (Gomeza M.M. et al, 2003) and antireflection or high reflection coatings (Selhofer et al, 1999). The performances of these devices are based on interference effects by alternating layers of high and low refractive indexes. Different optical properties are obtained by depositing multilayers of two or more different materials. Two important requirements must be fulfilled. On one hand, the optical properties of each layer should be uniform. On the other hand, the interface between two layers should be as smooth as possible for a good modelling. Evaporation coating technique underwent rapid development and became a standard method for optical coating (Zhong DishengXu Guangzhong and Liu Wi, 1991). Afterwards, alternative methods such as chemical vapour deposition (C. Martinet et al, 1997), dip coating (H. Köstlin et al, 1 997) , sol gel method (Que et al, 2000) and reactive sputtering (Hill, 1997) have been extensively studied. The later allows large area coatings and thickness uniformity combined with high rate deposition (Vergöhl et al, 1999). TiO 2 is one of the most interesting dielectric materials since it is transparent to visible light, has a high refractive index (at λ = 550 nm n = 2.54 for anatase and 2.75 for rutile), a low absorption (Mardare et al, 2002), a good hardness and forms a stable device with SiO 2 in discrete and mixed coatings (Gluck, et al, 1999). SiO 2 is a low- index material and transparent from the UV to the NIR (Tabata et al, 1996). TiO 2 /SiO 2 systems have been extensively used to realize a wide variety of optical devices. (Ouellette et al, 1991) reported on a TiO2–SiO2 notch filter prepared by ion beam reactive sputtering. (Wong et al, 1998) have reported the possibility of preparation of optical reflection filter using Helicon plasma sputtering. They have prepared multilayers with a wide range of refractive indexes from (n = 1.47 to 2.2) by mixing TiO 2 and SiO 2 . In modern architecture, large glass planes are used as facades in commercial buildings and glazing in a residential home for day lighting. In our case, reflecting multilayers will be used as a cover for solar collectors. For this purpose the reflecting multilayers consisting of oxides materials have to fulfil some requirements. Firstly, large amount of power from solar radiation must be transmitted through the coatings. Moreover, there is a need for zero or near zero absorption materials to avoid energy loss within the coating. Lastly, another critical factor is a narrow reflection in the visible range fixing the colour of the reflected light. In this work, we report an experimental study for the preparation of optical multilayer coatings based on TiO 2 /SiO 2 dielectric films for solar application. A combination of different refractive index and thickness could help to realize a wide range of reflected colours with an acceptable solar transmission. In the following we will concentrate on three types of samples: • single layered samples on substrate (TiO 2 // Si-substrate), (SiO 2 // Si-substrate) • double layered samples on substrate (SiO 2 / TiO 2 // substrate) • triple layered samples (TiO 2 / SiO 2 / TiO 2 // substrate).