Thin film multilayer design types for colored glazed thermal solar collectors

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Solar Energy Materials & Solar Cells 89 (2005) 219–231

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Thin film multilayer design types for coloredglazed thermal solar collectors

A. Schulera,�, J. Boudadenb, P. Oelhafenb, E. De Chambriera,C. Roeckera, J.-L. Scartezzinia

aLaboratoire d’Energie Solaire et de Physique du Batiment LESO-PB, Ecole Polytechnique Federale de

Lausanne EPFL, Batiment LE, 1015 Lausanne, SwitzerlandbInstitut fur Physik der Universitat Basel, Klingelbergstr. 82, 4056 Basel, Switzerland

Received 30 September 2004; accepted 17 November 2004

Available online 10 May 2005

Abstract

Multilayered interference filters of dielectric thin films have been designed for the

application as energy-efficient coloration of collector cover glasses. The optical behavior of

the designed multilayers is analyzed by computer simulations yielding the CIE color

coordinates, the relative luminosity, the degree of solar transmission, and a figure of merit

which is a measure for the energy effectiveness of the coloration. A high performance should

be achieved with a number of individual layers reasonable for large-scale deposition.

Constraints on the refractive index of the dielectric films are given by the availability of

suitable thin film materials. The challenge lies in finding the best combination of material

choice and layer thicknesses. We describe several types of multilayer designs for which the

computer simulations yield promising results.

r 2005 Elsevier B.V. All rights reserved.

PACS: 42.79.Ek; 42.79.Wc

Keywords: Solar thermal energy; Solar collector glazing; Colored solar collectors; Thin film interference

coatings; Multilayered optical filters

see front matter r 2005 Elsevier B.V. All rights reserved.

.solmat.2004.11.015

nding author. Tel.: +4121 693 4544.

dress: [email protected] (A. Schuler).

www.elsevier.com/locate/solmat

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A. Schuler et al. / Solar Energy Materials & Solar Cells 89 (2005) 219–231220

1. Introduction

The issue of color becomes more and more important for thermal solar collectors,and has attracted interest recently [1–3]. This might be related to a generally growingattention towards architectural integration of solar energy systems into buildings[4–7]. A recent opinion poll [1] showed, that 85% of architects would prefer differentcolors besides black, even if a lower efficiency would have to be accepted. Thermalsolar collectors, typically equipped with black, optical selective absorber sheets,exhibit in general good energy conversion efficiencies. However, the black color, andsometimes the visibility of tubes and corrugations of the metal sheets, limits thearchitectural integration into buildings.One solution to this problem is to color the absorber sheets. Optical selective

absorber coatings are usually deposited by processes such as magnetron sputtering[8–10], vacuum evaporation [11], electrochemical processes [12], sol–gel technology[13], or as selective paint (thickness-sensitive or thickness-insensitive) [3,14].Niklasson and Granqvist [15] described the pioneering work in this area within acomprehensive overview. Modifying the parameters of the deposition process canresult in a colored appearance. Following this approach, the absorbersurface combines the functions of optical selectivity (high solar absorption/lowthermal emission) and colored reflection. Tripanagnostopoulos [16] reports adifferent solution: his group used non-selective colorful paints as absorbercoatings for glazed and unglazed collectors, and compensated the energy losses byadditional booster reflectors. Alternatively, we propose to establish a coloredreflection not from the absorber but from the cover glass. This approach has theadvantage that the black, sometimes ugly absorber sheet is then hidden by thecolored reflection. In addition to that, the functions of optical selectivity and coloredreflection are separated, giving more freedom to layer optimization. No energyshould be lost by absorption in the coating: all energy, which is not reflected, shouldbe transmitted. Therefore, multilayer interference stacks of transparent materialsshould be ideally suited for this purpose. A recent feasibility study showedencouraging results [17]. By employing optical methods such as real-time laserreflectometry, spectroscopic ellipsometry and spectrophotometry, the deposition ofmultilayered interference stacks can be monitored very precisely [18]. In this articlewe describe a variety of multilayer designs, which can be employed to achieve thedesired characteristics.

2. Theory

The propagation of electromagnetic waves in stratified media has beendiscussed by Born and Wolf [19]. The field of optics of thin films has been reviewedby various authors, e.g. Heavens [20], Holland [21], Anders [22], Knittl [23], andMacleod [24]. Due to the multiple reflections between the different interfaces, theproblem of the optical behavior of a multilayered thin film stack is non-trivial. It canbe treated, though, by the method of characteristic matrices, which defines one

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A. Schuler et al. / Solar Energy Materials & Solar Cells 89 (2005) 219–231 221

matrix Mr per individual layer. From this matrix product, transmission andreflectance spectra can be computed. Extended calculations are usually carried outby a computer.The visible reflectance RVIS is a measure for the brightness of a surface as it

appears to the human eye under certain illumination conditions. Its determination isbased on the photopic luminous efficiency function V ðlÞ and depends on the choiceof the illuminant I ILLðlÞ:

RVIS ¼

RRðlÞ � I ILLðlÞ � V ðlÞdlR

I ILLðlÞ � V ðlÞdl, (1)

where RðlÞ is the simulated or measured reflectance of the sample.For the assessment of colored solar collectors it is useful to introduce a figure of

merit. In a previous publication [17] we defined the ratio of the visible reflectanceRVIS under daylight illumination D65 and the solar reflectance Rsol (based on thesolar spectrum AM1.5 global [25,26]) as figure of merit M:

M ¼ ðRVIS under daylight illumination D65Þ=ðRsol for AM1:5 globalÞ. (2)

Being large in the case of high visible reflectance or low solar energy losses Rsol, thisnumber describes the energy efficiency of the visual perception (brightness per energycost).An alternative approach is to use for both the evaluation of the visual impression

and the energetic reflection losses the same illuminant Isol.

M 0 ¼ ðRVIS under solar illumination AM1:5 globalÞ=ðRsol for AM1:5 globalÞ.

(3)

In practice, the resulting values for M and M0 are always rather close to each other.It can be shown that the principal upper limit for M and M0 amounts to the value ofapprox. six (in the ideal case). Of course the energy-effectiveness of the coloredreflection should not be the only criterion for the evaluation of a colored collectorglazing. A sufficient solar transmission is certainly one of the most importantrequirements.

3. Simulations

Searching for designs of multilayer interference stacks suitable for the consideredapplication, several promising design types could have been identified. Those will bedescribed in the following section:

3.1. Two-layered systems

From the field of anti-reflection coatings, two-layered coating designs such as theV- and the W-design are known [24]. These systems owe their name to the shape ofthe reflection minimum. The effect of anti-reflection extends over a certainwavelength range, but aside from this region considerable reflectance peaks occur,

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which can be used to produce a colored reflection. The region of anti-reflectionenhances the solar transmission. Creating a region of higher reflectance at shortwavelengths, blue and green colors of reflection can be easily achieved incombination with a good solar transmission. As an example we show the W-design.Both sides of the substrate are coated identically, as it occurs often in sol–gel dipcoating. Our calculation takes into account the multiple reflections between the twosides. The optical model has the structure

air==L2H==glass=2HL==air;

where the letters ‘‘L’’ and ‘‘H’’ mean quarterwave layers of low and high refractiveindex, respectively. The corresponding layer thicknesses t(L) and t(H) have thus beenchosen to n � tðHÞ ¼ l0=2 and n � tðLÞ ¼ l0=4, where the so-called ‘‘designwavelength l0’’ indicates the center of the region of anti-reflection (here:l0 ¼ 800 nm). A refractive index of 1.52 has been assumed for the glass substrate.The resulting reflectance spectra are displayed in Fig. 1. For the shown examples, theresulting colors are in the region of bluish green. Within a rather large region thereflectance is lower than the one of an uncoated substrate (approx. 8%). Due to thepartial anti-reflection, the achieved color saturation and as well the solartransmission are remarkable. A survey of the characteristic figures, the colorcoordinates x and y, the visible reflectance RVIS, the solar transmission Tsol and thefigure of merit M ¼ RVIS/Rsol, is given in Table 1.

3.2. Three-layered system

Starting from a classical V-design, we have studied the influence of adding a thirdlayer. We consider a glass substrate with a refractive index of 1.52 being coated onone side by a stack of three layers, the first layer being 30 nm thick with a refractiveindex of 2.2, the second layer 140 nm thick with n ¼ 1.46, and a third layer ofvariable thickness with n ¼ 2.2. By variation of the thickness t3 of the topmost layerfrom 0 to 50 nm, we obtain the reflectance spectra displayed in Fig. 2. The thin blackline illustrates the reflectance for the two-layered V-design (thickness t3 ¼ 0 nm).Adding the additional layer results in an increase of the colored reflection. Thespectrum corresponding to the case of a 30 nm thick top layer exhibits a strongenhancement of the reflection peak and still a region of anti-reflection at awavelength of 1000 nm. For the top layer being 50 nm thick, the region of anti-reflection is already less pronounced. Also from the point of view of the solartransmission Tsol(%), the region between 10 and 40 nm appears most interesting.Table 2 shows the numerical results for the color coordinates x and y, the visiblereflectance RVIS, the solar reflectance Rsol, the solar transmission Tsol and the figureof merit M ¼ RVIS/Rsol, in dependence on the thickness t3 of the third layer.

3.3. Maxima of higher order

Towards shorter wavelengths, the reflectance spectra for dielectric thin film stacksexhibit rapid oscillations between the maxima and minima of higher order. By using

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Table 1

CIE color coordinates x and y, the visible reflectance RVIS, the solar transmission Tsol and the figure of

merit M ¼ RVIS/Rsol, as computed for the curves displayed in Fig. 1

n(H) n(L) t(H) (nm) t(L) (nm) x y RVIS (%) Tsol (%) M ¼ RVIS/Rsol

1.8 1.47 222 136 0.18 0.27 9.1 93 1.21

2.2 1.47 182 136 0.21 0.32 21 86 1.53

1.8 1.38 222 145 0.19 0.29 9.4 93 1.42

2.2 1.38 182 145 0.23 0.34 23 86 1.63

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

refl

ecta

nce

2500200015001000500

wavelength (nm)

refractive indices 1.8 and 1.47 2.2 and 1.47 1.8 and 1.38 2.2 and 1.38

Fig. 1. Simulated reflectance spectra for W-designs. The glass substrate has been assumed to be coated at

both sides (optical model: air//L2H//glass//2HL//air). The design wavelength l0 has been chosen to

800 nm.


high enough layer thicknesses, the maxima can be placed in the visible spectralregion. Fig. 3 illustrates an example showing the oscillations in the high-wavelengthregion which are used for the coloration. For e.g. a color of pink two peaks,corresponding to blue and red contributions, are necessary. The calculation is basedon literature data for the refractive indices of SiO2 and TiO2 [27–29]. For simplicity,coatings have been assumed to be transparent within the solar spectral region. Layerthicknesses of 213 and 258 nm are assumed for the TiO2 and the SiO2 layers,respectively. Color coordinates of x ¼ 0.39 and y ¼ 0.24 have been found,accompanied by a visible reflectance of 9.8%, while the solar transmission amountsto 84.6%. Some more examples are listed in Table 3. The selection comprises two-,three- and four-layered systems. Layer 1 is next to the glass. A refractive index of1.52 has been assumed for the glass, the coating is supposed to be only on one side ofthe substrate.

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Table 2

CIE color coordinates x and y, the visible reflectance RVIS, the solar reflectance Rsol, the solar transmission

Tsol and the figure of merit M ¼ RVIS/Rsol, as computed for the curves displayed in Fig. 2

t3 (nm) x y RVIS (%) Rsol (%) Tsol (%) M ¼ RVIS/Rsol

0 0.22 0.22 8.1 10 90 0.81

10 0.24 0.27 17 12 88 1.42

20 0.27 0.31 28 16 84 1.75

30 0.29 0.34 37 20 80 1.85

40 0.31 0.36 43 24 76 1.79

50 0.34 0.38 46 27 73 1.70

0.5

0.4

0.3

0.2

0.1

refl

ecti

vity

2500200015001000500

wavelength (nm)

thickness of topmost layer (nm):

50 40 30 20 10 0

Fig. 2. Simulated reflectance spectra for the three-layered system. The glass substrate has been assumed to

be coated at only one side. Adding the additional layer to the V-design results in a strong enhancement of

the colored reflection.


3.4. Quarterwave stacks

In quarterwave stacks, all individual layers are of the optical film thicknessn � t ¼ l0=4, where l0 is called the design wavelength. Usually layers of a high indexmaterial (H) alternate with layers of a low refractive index material (L), resulting in astack of the form HLHLHLy . Often, these filters are employed as high-reflectivitymirrors, exhibiting a nearly perfect reflectance over a large-frequency band. Thelarger the difference in the refractive indices, the larger is the spectral region of highreflection. We are interested in the opposite, a narrow reflection peak, which can inprinciple be created by employing a large number of layers (e.g. 40 layers for a

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0.4

0.3

0.2

0.1

0.0

ref

lect

ance

2500200015001000500

wavelength (nm)

design 'A': purplish pink solar spectrum AM1.5

Fig. 3. Simulated reflectance spectra for the design ‘A’ (layer 1: 213 nm TiO2, layer 2: 258 nm SiO2). The

glass substrate has been assumed to be coated at only one side. For a color of pink two peaks,

corresponding to blue and red contributions, are necessary.

Table 3

Color coordinates x and y, the visible reflectance RVIS, the solar reflectance Rsol, the solar transmission Tsol

and the figure of merit M ¼ RVIS/Rsol, as computed for a variety of two-, three- and four-layered systems

Design Layer Color

coordinates

Approx.

color

RVIS

(%)

Tsol

(%)

M ¼

RVIS/Rsol

1 2 3 4

TiO2

(nm)

SiO2

(nm)

TiO2

(nm)

SiO2

(nm)

x y

A 213 258 0.39 0.24 Pink 9.8 85 0.64

B 126 304 0.22 0.14 Blue 6.7 84 0.41

C 15 410 0.35 0.42 Yellow 14 90 1.45

D 88 224 0.48 0.38 Orange 16 83 0.93

E 12 378 12 0.35 0.41 Yellow 17 89 1.47

F 21 45 259 109 0.22 0.15 Blue 6.3 88 0.54

G 12 378 12 378 0.29 0.39 Green 14 90 1.36

H 104 37 103 231 0.52 0.35 Orange 16 80 0.79


linewidth in the order of 20 nm [30]). Here we consider a system, where both sides ofthe glass are coated each by a stack of five individual layers. Consequently, thestructure of the optical model is air//HLHLH//glass//HLHLH//air. The refractiveindices n(H) ¼ 1.65 and n(L) ¼ 1.47 have been chosen (in combination with arefractive index of the glass substrate of 1.52). Our calculation has been performed

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0.35

0.30

0.25

0.20

0.15

0.10

refl

ecta

nce

2500200015001000500

wavelength (nm)

0˚ 20˚ 40˚ 60˚

Fig. 4. Simulated reflectance spectra for a design based on a five-layered quarterwave stack on each side of

the glass substrate (optical model: air//HLHLH//glass//HLHLH//air). The design wavelength l0 has beenchosen to 550nm. Angles of reflection from 01 to 601 have been assumed.

Table 4

Position of the reflectance maximum, color coordinates x and y, the visible reflectance RVIS, the solar

transmission Tsol, the transmission of an uncoated glass Tglass, the difference Tglass�Tsol between the latter

and the solar transmission, and the figure of merit M ¼ RVIS/Rsol, as computed for the curves displayed in

Fig. 4. The values for the angle of incidence of 801 are added

Angle Pos. max. x y RVIS (%) Tsol (%) Tglass (%) Tglass�Tsol (%) M ¼ RVIS/Rsol

01 548 0.35 0.44 34 84 92 8 2.1

201 534 0.32 0.42 33 84 92 7 2.1

401 499 0.27 0.35 29 84 91 7 1.8

601 456 0.25 0.29 24 80 84 5 1.2

801 — 0.30 0.33 56 45 46 1 1.0


for various angles of incidence, rising from normal incidence in steps of 201. Agraphical representation is given in Fig. 4, the figures are summarized in Table 4. Fornormal incidence, the FWHM amounts to 167 nm, the visible reflectance to 34%.The solar transmission (84%) is acceptable; the energy loss compared to an uncoatedglass amounts only to 8%. The angular dependence of the reflectance is illustrated inFig. 4. For an angle of incidence of 201, the curve does not change significantly; theposition of the maximum shifts slightly from 550 to 534 nm. For 401, the shiftcontinues to 499 nm. For 601, a blueshift in the order of magnitude of the peak widthis accompanied by a rise of the background level. For 801, the background level risesfurther, the peak is barely distinguishable, and the properties are quite close to those

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of uncoated glass (representation in Fig. 4 omitted for clarity). With increasing angleof incidence, the difference between the transmission of uncoated glass Tglass and thetransmission Tsol of the coated system decreases, while the figure of merit M

converges to unity. In Table 4 a survey of the characteristic numbers is given.By adapting film thicknesses according to the relation n � t ¼ l0=4, the peak

position l0 can be shifted easily. For design wavelengths l0 of 450 and 650 nm, colorsof blue (x ¼ 0.20, y ¼ 0.24) and orange (x ¼ 0.44, y ¼ 0.36 ) are obtained. The solartransmission increases slightly or changes barely (86% and 84%, respectively), whilethe visible reflectance decreases (17% and 29%, respectively).For a high solar transmission it would be preferable to lower the level of

background oscillation shown in Fig. 4. In order to achieve anti-reflection, the use oflow-index materials is often advantageous. A common choice is magnesium fluorideMgF2, with a refractive index around 1.38 [24,31]. One can take this compound aslow-index material and silicon oxide as high-index material. Thus, we assumen(L) ¼ 1.38 and n(H) ¼ 1.47. The refractive index of the glass is now larger than thetwo; in agreement with the phase jump relations at the interfaces we choose an evennumber of layers, six layers on each side of the glass: air//(LH)^3//glass//(HL)^3//air. Layer thicknesses have been adjusted with respect to the product of the refractiveindex and the individual thickness of each layer being equal to a quarter of thechosen design wavelength (550 nm). Again, we take into account the multiplereflections between the two sides of the glass. The resulting spectra are shown inFig. 5. For normal incidence, the level of background oscillation could be loweredfrom 9% to 6%, thus gaining some percent in solar transmission. A well-definedpeak is formed, shifting for rising angles from 550 nm towards lower wavelengths.The transmission losses relative to the uncoated glass are really small or evennegative (gains). The relative luminosity for normal incidence amounts 21%, whilethe solar transmission is only 1% worse than that of uncoated glass. The appearingcolor is a yellowish green of good saturation (x ¼ 0.36, y ¼ 0.48), shifting for higherangles of incidence towards bluish green (401) and blue (601). Due to the lowersensitivity of the human eye in the spectral region of the color blue, the relativeluminosity decreases from 21% (at 01) to 13% (at 601), before rising to 53% (at 801).The latter is related to the general increase of the background level at high angles,yielding finally a white reflection (x ¼ 0.31, y ¼ 0.32 at 801). A survey of thecharacteristic numbers is given in Table 5.

4. Discussion

Several design types have been proposed to achieve an energy-efficient colorationof solar collector glazing. Known anti-reflection coatings such as the two-layered V-and W-design can be modified to create a colorful reflection in the visible spectralregion and a region of anti-reflection, e.g. in the infrared. This approach is especiallysuitable to create blue and green colors in combination with a high solartransmission. Addition of a third layer to a V-design leads to a strong enhancementof the reflectance peak. Already with such a three-layered coating design of moderate

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Table 5

Position of the reflectance maximum, color coordinates x and y, the visible reflectance RVIS, the solar

transmission Tsol, the transmission of an uncoated glass Tglass, the difference Tglass�Tsol between the latter

and the solar transmission, and the figure of merit M ¼ RVIS/Rsol, as computed for the curves displayed in

Fig. 5. The values for the angle of incidence of 801 are added

Angle Pos. max. x y RVIS (%) Tsol (%) Tglass (%) Tglass�Tsol (%) M ¼ RVIS/Rsol

01 549 0.36 0.48 21 91 92 1 2.2

201 534 0.32 0.47 20 91 92 1 2.2

401 490 0.22 0.33 15 90 91 1 1.5

601 436 0.25 0.23 13 85 84 0 0.9

801 397 0.31 0.32 53 47 46 �1 1.0

0.25

0.20

0.15

0.10

0.05

refl

ecta

nce

2500200015001000500

wavelength (nm)

0˚ 20˚ 40˚ 60˚

Fig. 5. Angle-dependent reflectance spectra, computed for a quarter wave stack with low refractive

indices, n(L) ¼ 1.38 and n(H) ¼ 1.47. According to phase jump relations, the optical model has the

structure air//(LH)^3//glass//(HL)^3//air. The level of background oscillation is lower that in Fig. 8, thus

gaining some percent in solar transmission.


total coating thickness, considerable peak heights and thus a strong visiblereflectance can be achieved.Therefore this design is attractive for large-area coating processes where

production costs scale with the number of individual layers and the total coatingthickness. Another way to create a colored reflection is to make use of the maxima ofhigher order appearing towards short wavelengths. In this regime the shapes of thereflectance spectra become rather complex, but can be used to produce colors wheremore than one reflection peak is needed. A systematic approach to the problem ofachieving a single, isolated reflectance peak is the one of quarterwave stacks. Already

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with stacks of five layers and a suitable choice of refractive indices, sufficientlynarrow reflectance peaks can be produced. Interference colors are in general angle-dependent. This could both increase (nice effect, high-tech image) or decreaseacceptance of the proposed collector covers, which is a subject of discussion witharchitects, manufacturers and end-users. However, for the shown example of a five-layered quarterwave stack, the peak shift at 601 angle of reflection is in the sameorder of magnitude as the half peak width. Additionally, if the multilayered coatingis only applied at the inner side of the collector, the angular dependence can bereduced by diffusing elements, such as rough surfaces/interfaces or a diffusinginterlayer. Common thin film deposition processes are magnetron sputtering,plasma-enhanced chemical vapor deposition, vacuum evaporation, or sol–gel dipcoating. Transparent oxides such as silicon dioxide (nE1.47), aluminum oxide(nE1.65), or titanium dioxide (nE2.2) can routinely be deposited [24,32,33].Intermediate refractive indices would be accessible by the synthesis of mixed oxides.Nanocomposite mixed oxides can be modeled in the framework of effective mediumtheories, such as, e.g. the Bruggeman or the Ping Sheng theory [34,35], bothproviding analytical expressions for the resulting optical properties. By choosing alow-index material such as magnesium fluoride, the level of background oscillationin the reflectance spectra can be lowered, thus gaining color saturation and somepercent in solar transmission. However, a lower film stability (hardness anddurability) might be disadvantageous [24]. Multilayer design examples have beenshown with both sides of the substrate coated, as it occurs naturally in the sol–geldip-coating process, and also for the case of only one side of the substrate coated, asit is easily achieved by, e.g. magnetron sputtering (or in sol–gel dip-coating bycovering one side of the substrate with a plastic foil). With coatings on both sides,higher color-saturation and better partial anti-reflection can be achieved, but forreasons of protection (dirt, abrasive effects) it might also be interesting to have onlythe inner side of the glazing coated.With all coating processes, care has to be taken for a superior film homogeneity,

which is essential for interference filters. Vacuum processes yield in general high-quality films, but a considerable investment into the vacuum coating machines isnecessary already in the start-up phase. The scale-up of a vacuum process, which hasbeen developed in the laboratory, is possible, but non-trivial [36]. This is much easierfor sol–gel dip-coating. Once the right solutions and withdrawal speeds are found,the size of the glass pane does not alter the basic process parameters. Here, one mainproblem is to avoid dust, which creates defects and harms the coating quality. Costsrise with the repeated baking of multilayered coatings on large glass panes. Onesolution to the problem can be special precursors, which enable a film hardening byultraviolet light [37].

5. Conclusions

Multilayered interference filters of dielectric thin films have been designed for theapplication as energy-efficient coloration of collector cover glasses. The optical

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behavior of the designed multilayers is analyzed by computer simulations yieldingthe CIE color coordinates, the relative luminosity, the degree of solar transmission,and a figure of merit which is a measure for the energy effectiveness of thecoloration. For several types of multilayer design the computer simulations yieldpromising results. Hereby, constraints such as a realistic choice of refractive indices,a limited number of layers in the stack, and a not excessive total stack thickness havebeen respected. The way for the experimental realisation of energy-efficient coloredglazed solar collectors has thus been opened up.

Acknowledgments

Financial support of this work has been provided by the Swiss Federal Office ofEnergy SFOE. Authors are grateful to Dr. I. Hagemann, and Dr. P. W. Oliveira forinspiring discussions.

References

[1] W. Weiss, I. Stadler, Facade Integration—a new and promising opportunity for thermal solar

collectors, Proceedings of the Industry Workshop of the IEA Solar Heating and Cooling Programme,

Task 26 in Delft, The Netherlands, April 2, 2001 (http://www.fys.uio.no/kjerne/task26/pdf/

industry_workshop_delft.pdf).

[2] I. Stadler, Facade integrated solar thermal collectors, Proceedings of the Congress Solar Space

Heating, Graz, Austria, 2001.

[3] B. Orel, A. Surca Vuk, A. Vilcnik, B. Jelen, M. Kohl, M. Heck, Thickness insensitive spectrally

selective (TISS) paint coatings for glazed and unglazed solar building facades, Proceedings of the

ISES World Congress 2003, O2 45, Gotheborg, Sweden.

[4] A.G. Hestnes, Sol. Energy 67 (4–6) (1999) 181.

[5] C. Roecker, P. Affolter, J. Bonvin, J.-B. Gay, A.N. Muller, Sol. Energy Mater. Sol. Cells 36 (1995)

381.

[6] I.B. Hagemann, Architektonische Integration der Photovoltaik in die Gebaudehulle, Ph.D. Thesis

accepted at the Technical University of Aachen, ISBN 3-481-01776-6, Muller, Koln, 2002.

[7] J.-L. Scartezzini, G. Courret, Sol. Energy 73 (2) (2002) 123.

[8] W. Graf, F. Brucker, M. Kohl, T. Troscher, V. Wittwer, L. Herlitze, J. Non-cryst. Solids 218 (1997)

380.

[9] A. Schuler, J. Geng, P. Oelhafen, S. Brunold, P. Gantenbein, U. Frei, Sol. Energy Mater. Sol. Cells 60

(2000) 295.

[10] A. Schuler, I.R. Videnovic, P. Oelhafen, S. Brunold, Sol. Energy Mater. Sol. Cells 69 (2001) 271.

[11] M. Lazarov, P. Raths, H. Metzger, W. Spirkl, J. Appl. Phys. 77 (1995) 2133.

[12] H. Tabor, U.S. Patent 2,917,817 (1955).

[13] L. Kaluza, B. Orel, G. Drazic, M. Kohl, Sol. Energy Mater. Sol. Cells 70 (2001) 187.

[14] B. Orel, I. Radoczy, Z. Crnjak Orel, Solar Wind Technol. 3 (1986) 45.

[15] G.A. Niklasson, C.G. Granqvist, J. Mater. Sci. 18 (1983) 3475.

[16] Y. Tripanagnostopoulos, M. Souliotis, Th. Nousia, Sol. Energy 68 (2000) 343.

[17] A. Schuler, C. Roecker, J.-L. Scartezzini, J. Boudaden, I.R. Videnovic, R.S.-C. Ho, P. Oelhafen, On

the feasibility of colored glazed thermal solar collectors based on thin film interference filters, Sol.

Energy Mater. Sol. Cells 84 (2004) 241.

[18] J. Boudaden, R.S.-C. Ho, P. Oelhafen, A. Schuler, C. Roecker, J.-L. Scartezzini, Towards colored

glazed solar collectors, Sol. Energy Mater. Sol. Cells 84 (2004) 225.

http://www.fys.uio.no/kjerne/task26/pdf/industryworkshop_delft.pdf

http://www.fys.uio.no/kjerne/task26/pdf/industryworkshop_delft.pdf

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[19] M. Born, E. Wolf, Principles of Optics, seventh ed., Cambridge University Press, Cambridge, MA,

1999.

[20] O.S. Heavens, Optical Properties of Thin Solid Films, Butterworths, London, 1955.

[21] L. Holland, Vacuum Deposition of Thin Films, Chapman & Hall, London, 1956.

[22] H. Anders, Dunne Schichten fur die Optik, Wissenschaftliche Verlagsgesellschaft, Stuttgart, 1965

(English translation Thin films in Optics, Focal Press, 1967).

[23] Z. Knittl, Optics of Thin Films, Wiley, London, 1976.

[24] H.A. Macleod, Thin-Film Optical Filters, Institute of Physics Publishing, Bristol and Philadelphia,

1986, 2001.

[25] R. Hulstrom, R. Bird, C. Riordan, Solar Cells 15 (1985) 365.

[26] The American Society for Testing and Materials ASTM, standard spectra ASTM E-891, E892,

G-159, G-173, downloadable in electronic form from http://rredc.nrel.gov/solar/standards/

[27] E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, New York, 1985.

[28] E.D. Palik, Handbook of Optical Constants of Solids II, Academic Press, New York, 1991.

[29] L. Ward, The Optical Constants of Bulk Materials and Films, Adam Hilger, Bristol, 1995.

[30] A. Schuler, C. Roecker, J. Boudaden, P. Oelhafen, J.-L. Scartezzini, Coatings for colored glazed

thermal solar collectors and solar active glass facades, Proceedings of the CISBAT 2003 Conference,

8 October 2003, Lausanne, Switzerland.

[31] J.T. Cox, G. Hass, J.B. Ramsay, J. Phys. 25 (1964) 250.

[32] O.S. Heavens, S.D. Smith, J. Opt. Soc. Am. 47 (1957) 469.

[33] G. Hass, Vacuum 2 (1952) 331.

[34] D.A.G. Bruggeman, Ann. Phys. 5. Folge. 24 (1935) 636.

[35] P. Sheng, Phys. Rev. Lett. 45 (1980) 60.

[36] F. Milde, M. Dimer, Ch. Hecht, D. Schulze, P. Gantenbein, Vacuum 59 (2–3) (2000) 825.

[37] M. Mennig, P.W. Oliveira, H. Schmidt, Thin Solid Films 351 (1–2) (1999) 99.

http://rredc.nrel.gov/solar/standards/

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Thin film multilayer design types for colored glazed thermal solar collectors