MULTILAYER FILMS FOR COLOURED GLAZED

SOLAR COLLECTORS

INAUGURALDISSERTATION

zur Erlangung der Würde eines Doktors der Philosophie

vorgelegt der Philosophisch–Naturwissenschaftlichen Fakultät

der Universität Basel

von

Jamila Boudaden aus Agadir, Marokko

Basel, 2009

Genehmigt von der Philosophisch-Naturwissenschaftlichen Fakultät auf Antrag von

Prof. Dr. P. Oelhafen Prof. Dr. E. Meyer

Basel, den 22. April 2008 Prof. Dr. Eberhard Parlow, Dekan

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ABSTRACT

In this work a solution to the problem of black colour appearance which dominates

the external aspect of buildings covered by solar thermal collectors is proposed.

Multilayered thin films on the glass surface, consisting of oxides materials such as

SiO2, Al2O3, TiO2 or a mixture of these oxides were deposited by reactive magnetron

sputtering on glass and their optical properties were examined. As the interface

between the sputtered layers on glass emerged as important, the interfaces formed

between TiO2 and SiO2 and between Al2O3 and SiO2 were studied by X-ray

photoelectron spectroscopy. The reflectivity of the film on glass system was shown to

be a narrow band in the visible region while the rest of the sunlight is transmitted

through the glass due to the use of a near zero absorption materials. In addition, the

desired colour of the reflected light in the visible range was obtained by adapting the

oxide film thicknesses. Such optical properties besides the film’s stability as

demonstrated in accelerated ageing tests make the coloured glazing aesthetically

pleasing and suitable as a cover glass for thermal solar collectors.

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Table of contents General introduction 1 CHAPTER I: Experimental techniques and thin film characterisation 1 Thin film deposition method 7

1.1 Magnetron sputtering 7 1.2 Sputter system 9

2 Photoelectron spectroscopy 10

2.1 Introduction 10 2.2 Principle of photoemission and photoelectron spectroscopy 11 2.3 Electron escape depth 13 2.4 Three-step model versus one-step model 15 2.5 Photoelectron spectroscopy applied to insulating materials 16

2.5.1 Analysis of very thin films 17 2.5.2 Calibration by an internal reference 17 2.5.3 Calibration by an external reference (gold layer) 17 2.5.4 Surface charge neutralisation by an electron beam 17 2.5.5 Analysis of the Auger parameter 18 2.5.6 Parameters insensitive to charging effects 18

2.6 Experimental set-up 19 2.7 Data analysis 21

3 Laser reflectometry 22 4 Spectroscopic ellipsometry 23

4.1 Introduction 23 4.2 Principles of ellipsometry 24 4.3 Ellipsometer 26 4.4 Data analysis 27 4.5 Effective medium approximation 28

5 Total reflectivity and transmission 30 6 Optical properties of multilayer films 31

6.1 Solar reflectivity, solar transmission, visible reflectance 31 6.2 Merit factor 33 6.3 Colour coordinates in CIE Lab system 35

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CHAPTER II: TiO2/SiO2 - SiO2/TiO2 interfaces and TiO2 – SiO2 multilayers 1 TiO2/SiO2 and SiO2/TiO2 interfaces 41

1.1 Introduction 41 1.2 Experimental details 42 1.3 XPS results 43

1.3.1 TiO2 on SiO2 43 1.3.2 SiO2 on TiO2 46

1.4 Discussion 47 1.4.1 TiO2 on SiO2 48 1.4.2 SiO2 on TiO2 48

1.5 Conclusion 53 2 TiO2 – SiO2 multilayers for coloured glazed solar collectors 54

2.1 Introduction 54 2.2 Thin film deposition 55 2.3 Laser Reflectometry 55 2.4 Ellipsometry 56

2.4.1 Single layers on silicon substrate 57 2.4.2 Multilayers on silicon substrate 59

2.5 Transmission Electron Microscopy 61 2.6 Simulation 62

2.6.1 Solar transmission and visible reflectance 62 2.6.2 Peak position of the reflectivity curves 64

2.7 Experimental realisations and ageing tests 65 2.8 Conclusion 67

CHAPTER III: SiO2/Al2O3 - Al2O3/SiO2 interfaces and Al2O3 – SiO2 multilayers 1 SiO2/Al2O3 and Al2O3/SiO2 interfaces 71

1.1 Introduction 71 1.2 Experimental details 72 1.3 XPS results 73

1.3.1 SiO2 on Al2O3 73 1.3.2 Al2O3 on SiO2 74

1.4 Discussion 75 1.5 Conclusion 78

2 Al2O3 – SiO2 multilayers for coloured glazed solar collectors 78

2.1 Introduction 78 2.2 Experimental details 79 2.3 Optical characterisation 80

2.3.1 Laser Reflectometry 80 2.3.2 Ellipsometry 81

2.4 Multilayered films 83 2.4.1 Multilayers on silicon substrate 83 2.4.2 Simulation of the solar transmission and visible reflectance 85

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2.4.3 Experimental realisation on glass 87 2.5 Ageing test 89 2.6 Conclusion 91

CHAPTER IV: TiO2 – SiO2 and TiO2 – Al2O3 mixed oxides 1 TiO2 – SiO2 composite films for coloured glazed solar collectors 95

1.1 Introduction 95 1.2 Experimental details 96 1.3 Results and discussion 98

1.3.1 XPS 98 1.3.2 Laser Reflectometry 102 1.3.3 Ellipsometry 104

1.4 Experimental realization of multilayered films 107 1.4.1 Optical properties 107 1.4.2 Accelerated ageing test 111

1.5 Conclusion 112 2 TiO2 – Al2O3 composite films for coloured glazed solar collectors 113

2.1 Introduction 113 2.2 Experimental details 115 2.3 Results and discussion 116

2.3.1 XPS 116 2.3.2 Laser reflectometry 119 2.3.3 Spectroscopic ellipsometry 120

2.4 Multilayered films with mixed oxide 124 2.4.1 Optical properties of multilayer films on silicon 124 2.4.2 Optical properties of multilayer films on glass 126 2.4.3 Ageing test 131

2.5 Conclusion 132 General conclusion 135

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General introduction

1

1 General introduction

The low price of fossil fuels is the most important reason for limiting the heavy use of

solar thermal energy. However, oil prices have increased by 20% the last ten years.

For this principal reason a fast transition to an energy structure based on renewable

energy is of utmost importance to limit the high dependency on imported fuels. Solar

thermal energy is considered as an adequate alternative energy resource for heating

and cooling to replace fossil fuels. In 2005, approximately 10 GWth of solar thermal

capacity were in operation in Europe. It could be increased to reach 200 GWth by

2030, when solar thermal energy will be used in the majority of buildings [1]. To meet

this realizable objective, it is expected that the solar thermal collectors will cover,

together with photovoltaic modules, the entire south-oriented roof area of buildings.

In addition to the roof areas, south facing facades also have to be used as active

solar absorption surfaces. Therefore, the solar collectors have to be completely

integrated into the building envelope components. Building integration is considered

to be a huge barrier for their development. It concerns the overall image of the solar

system in the building. From the point of view of the architects, the aesthetic aspect

is the main reason for talking about building integration.

One motivation in our work is finding a solution to the problem of black colour

appearance due to the black body which dominates the external aspect of buildings

covered by solar thermal collectors. Until today, no satisfying economically

interesting solution to increasing the architectural attractiveness of solar collectors

has been found. A study showed that more than 80% of architects and engineers

rated as important the possibility to choose a custom colour [2, 3]. For two-thirds of

them this is even an essential requirement. On the choice of the actual colours, the

majority of architects preferred the colour grey, independent of their geographical

origin. Another study conducted by AEE INTEC showed that 85% of architects prefer

any colour besides black [4].

One recent idea is the use of coloured glazing of cover glass for thermal solar

collectors and building faces by depositing a multilayer thin film on the glass surface.

The ideal reflectivity of the glass-film system should be a narrow band of the visible

General introduction

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light while transmitting the rest of the sunlight towards the black body to minimize

energy losses, see on Figure 1.

In this way, one part of the solar energy in the visible spectrum is invested to make it

more aesthetically pleasing and the other part of energy, most of the energy, will

pass through the cover, be absorbed and converted to heat in the black surface of

the absorber sheet of the solar collectors [5]. However, a compromise has to be

found bet