Solar Energy Materials & Solar Cells 107 (2012) 316–321

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Solar Energy Materials & Solar Cells

0927-02

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journal homepage: www.elsevier.com/locate/solmat

Preparation of novel SiO2 protected Ag thin films with high reflectivityby magnetron sputtering for solar front reflectors

Y.J. Xu a,n, Q.W. Cai b, X.X. Yang a, Y.Z. Zuo a, H. Song b, Z.M. Liu a, Y.P. Hang b

a Key Laboratory of Distributed Energy of Guangdong Province, Dongguan University of Technology, Dongguan 523808, PR Chinab School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China

a r t i c l e i n f o

Article history:

Received 21 January 2012

Received in revised form

4 July 2012

Accepted 7 July 2012Available online 25 July 2012

Keywords:

Magnetron sputtering

Solar front reflectors

SiO2 protected Ag films

High reflectivity

Aging test

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.solmat.2012.07.002

esponding author. Fax: þ86 76922862605.

ail address: hnllxyj@126.com (Y.J. Xu).

a b s t r a c t

The Essential Macleod Program (EMP) has been used to successfully assist in the design of a SiO2

protected Ag thin film. The film is applied through magnetron sputtering onto a glass substrate for use

in a solar front reflector. In the following experiments, Ag films were first deposited on glass substrates

using direct current (DC) magnetron sputtering, and then SiO2 films were deposited as a protective

layer onto the surface of the Ag films using radio frequency (RF) magnetron sputtering. The reflectivity

of the obtained samples was calculated and tested under light wavelengths ranging from 250 to

2500 nm with films of Ag and SiO2 of different thicknesses. The solar reflectivity (SR) and the light

reflectivity (LR) of the 130 nm Ag film and the 320 nm SiO2 film were found to be up to 96.66% and

98.84%, respectively, which is almost identical to the calculated results based on the designed model.

Additionally, the deposited films exhibited high anti-corrosion properties in harsh abrasion and aging

resistance tests. More importantly, the films were proven capable of operating in high-temperature

systems by testing under different annealing temperatures. The high performance of the films was

attributed primarily to the SiO2 layer, which served as a good means of protection without experiencing

serious degradation of reflectivity, demonstrating their potential in applications for solar front

reflectors.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Solar thermal electric power systems are one of the majortechnologies for converting sunlight to electricity [1–5]. Owing tothe widespread application of solar thermal electric power genera-tion, the development of advanced solar reflector materials thatmaintain high performance for decades in outdoor service and arecapable of being manufactured in large volumes at a competitive costis urgently demanded [5–8]. For reflector materials in solar thermalapplications, a high reflectivity in the entire wavelength range of thesolar spectrum (300 to 2500 nm) is crucial as part of the opticalrequirements. Moreover, the performance of the reflector materialsstrongly depends on the films formed on the substrate [9].

In particular, metallic (or metallic compound) thin films formedon various substrates have received increased attention in the solarenergy field because of their excellent properties, such as highconductivity, conspicuous reflectivity within long electromagneticwavelengths and superior transmission [10–13]. Due to their highreflectivity (97% of the sunlight) and applicability as well as theirflexibility that allows them to be molded into a variety of curvedsurfaces [14], metallic film reflective materials have recently attractedincreasing scientific interest for their role in solar energy applications,

ll rights reserved.

such as solar reflectors. Based on their superior performance overtraditional reflective materials such as glass, metallic thin filmsexhibit significant potential for future commercialization [15]. It hasbeen reported that only free electron-like metals, which obey theDrude model, are suitable to be used as reflectors for solar thermalapplications. Among the Drude metals, silver and aluminum [14],which are two important reflective metals with a hemisphericalreflectivity of 97% and 92%, respectively, are recognized as the bestsolar reflectors. Nevertheless, the free electron-like metals, whichexhibit limited corrosion resistance, are often used in back surfacemirrors. These metals are always evaporated on the back of a glass orpolymer substrate to protect the metal from oxidation. Among thestate-of-the-art solar reflector materials, back-surface-silvered low-iron glass or polymethylmethacrylate (PMMA) [16,17] are the twoestablished materials. However, glass mirrors tend to be brittle, heavyand fragile. Although front surface mirrors so far have been developedto be bendable and lightweight, their optical performance severelydegrades in only a couple of months if the surface is not protected[18,19].

It is well known that approximately 80–85% of optical reflec-tivity is determined by the quality of the films that cover the solarreflectors. Therefore, it is desirable to use various alloys [20] orprotective layers [17,21,22] to improve film adhesion, opticalreflectivity and anti-corrosion behavior compared to pure metal-lic films [23]. Kennedy et al. [21] provides a basic knowledge ofsolar front reflectors with a structure of A12O3 (4 mm)/Ag

Y.J. Xu et al. / Solar Energy Materials & Solar Cells 107 (2012) 316–321 317

(70 nm)/Cu (40 nm) acting as the layers of protection, reflectionand substrate, respectively. The results revealed outstandingoptical performance with a reflectivity of 95% in an aging testover 3000 h. Their new structure highlights the development ofnew films for solar reflectors. However, the thickness of the A12O3

protective layer is 4 mm, which markedly increases the cost. Thus,although many films have been reported to be suitable for solarreflector materials, few of them consist of novel structures thatcan maintain high reflectivity, continuously endure harsh condi-tions in outdoor environments and can be fabricated at a low cost.Such films for solar reflector materials are expected to have bothacademic and industrial significance.

The EMP was employed to conduct the simulation process toguide the design of the films for solar front reflectors. Because theprogram was previously applied primarily to the simulation of theoptical parameters of thin films under single-wavelengths orcenter wavelengths, the simulation of the optical parameters forthin films under a wide range of wavelengths has not yet beenreported. In a solar concentration system, it is very important thatthe simulation and calculation for reflectivity and transmission ofthe thin films focus on a wide range of wavelengths, such as thevisible region (300–780 nm) and the sunlight region (300–2500 nm), and not just on the single wavelength.

In this paper, a novel structure composed of Ag film protectedby SiO2 was designed by EMP and fabricated using RF magnetronsputtering applied to the solar front reflector. Our films can beapplied onto the surface of any substrates. Here, we choose glassas the substrate because it was free and easily accessible. Thereflectivity of the obtained samples was tested and calculatedunder the light wavelengths ranging from 250 to 2500 nm withAg and SiO2 films of different thicknesses. To test the potentialapplication in a high-temperature environment, harsh abrasionand aging resistance tests were conducted to verify the anti-corrosion behavior of the obtained films. The effect of differentannealing temperatures on the reflectivity was also investigatedto test the durability of the films. The aim of this paper is todetermine how the annealing temperature and the thickness ofthe film together affect its optical and anti-corrosion behaviors.

2. Materials and methods

The experiments performed in this study are divided intosequent phases:

(1) preparing SiO2/Ag/glass samples; (2) determining the optimumthickness for the protective and reflective layers; (3) simulating theSR and LR with the EMP; (4) testing the anti-abrasion and anti-agingperformances; and (5) analyzing the influence of annealing tempera-ture on the thin film reflective materials.

2.1. Preparation of SiO2 protected Ag thin films

Ag and SiO2 thin films are deposited with high vacuum multi-functional magnetron sputtering equipment (JPGP-450, Sky, China)using Ag (^¼60 mm�5 mm, 99.99% purity) and SiO2 targets(^¼60 mm�5 mm, 99.99% purity) on glass (32 mm�25.4 mm�1.2 mm) substrates at room temperature. Prior to deposition, thesubstrates were ultrasonically cleaned with anhydrous ethyl alcoholand then washed with deionized water several times. The substrateswere then soaked in a cleaning solution (a mixture of ammonia,hydrogen peroxide and deionized water with a mass ratio of 1:2:5)and then heated for 15 min. Finally, the substrates were cleaned withdeionized water and placed in a vacuum oven for experimental useimmediately after being dried with an air dryer. The high-purityworking gas (argon, with 99.999% purity) was introduced into thesputtering chamber after it was evacuated to a pressure of

6.1�10�4 Pa. Before the films were deposited, all targets were pre-sputtered with argon ions for 5 min to identify samples with poorsurface adsorption. During sputtering, the argon gas flow rate waskept at 22 sccm and the chamber pressure was maintained at 0.7 Pa.The Ag sputtering was performed in DC mode (40 W) [24], whereasthe SiO2 sputtering was performed in RF mode at a higher power of280 W [25] due to the low sputtering yield of the SiO2 ceramic target.The distance between substrate and target was kept at 75 mm for alldepositions. The deposition rates of Ag and SiO2 were 64.4 nm/minand 9.1 nm/min, respectively. All the prepared samples were thenplaced in petri dishes and then stored in a vacuum oven to avoidcontamination.

2.2. Design and simulation of SiO2 protected Ag thin films

For the optical characterization, the EMP was employed andthe calculated results were compared with measured opticalproperties. The EMP is a comprehensive software package forthe design, analysis, manufacture and troubleshooting of thin filmoptical coatings [26,27]. The simulation method for the EMP wasperformed by the adhering to the following steps. First, construc-tion parameters such as the refractive index and the extinctioncoefficients of Ag and SiO2, which were calculated using ellipso-metry measurements, were input. Second, simulations with vari-able parameters such as the wavelength ranges (250 to 2500 nm)and the number of layers (glass/Ag/SiO2/air), were designed.Finally, an analysis of each layer with a variable thickness of Agand SiO2 thin films under wavelengths ranging from 250 to2500 nm was performed to determine the optical properties ofeach layer, and to determine whether each layer is appropriate forthe optimal simulation with system modification.

2.3. Characterization

The thickness of the films was measured using an AS500 levelmeter, and a normal reflectivity measurement of the films under lightwavelengths ranging from 250 to 2500 nm was conducted on a UV–vis–NIR spectrophotometer (PerkinElmer, Lambda 950), with a lightinjection angle set at 81. The reflectivity of the prepared samples wascalculated according to ISO 9050-2003. SR re and LR rv,o of films shallbe calculated using the following two formulas (according to ISO9050-2003: Glass in building–Determination of light transmittance,solar direct transmittance, total solar energy transmittance, ultraviolettransmittance and related glazing factors):

re ¼Xl ¼ 2500 nm

l ¼ 300 nm

r0ðlÞSlDl=Xl ¼ 2500 nm

l ¼ 300 nm

SlDl ð1Þ

rv,o ¼Xl ¼ 780 nm

l ¼ 380 nm

r0ðlÞDlVðlÞDl=Xl ¼ 780 nm

l ¼ 380 nm

DlVðlÞDl ð2Þ

The chemical compositions of the surface of the films wereanalyzed through Axis Ultra DLD X-ray photoelectron spectro-scopy (XPS) (X-ray with the Mono Al Ka energy of 1486.6 eV,10 mA�15 kV were used and calibrated internally by carbondeposit C 1s (285.0 eV). The full spectrum of the CAE scanningmode was performed at 160 eV).

The bilayer films comprised of Ag and SiO2 were etched by afocused ion beam (FIB, 30 kV and 100 pA) through a hole with adiameter of 1 mm which was bombarded vertically. Then, themorphology and the grain size of the films were observed using ascanning electron microscope (Nova Nano SEM 430, 10 kV).

The films deposited on the substrates were scrubbed 100 timeswith a brown brush and then the films were subjected to the XPSto investigate the abrasion resistance [28]. For durability testing, a10 cm�10 cm sample was aged for 486 h in a Q-sun/Xe-3-HS

0.95

1.00

tivity

Y.J. Xu et al. / Solar Energy Materials & Solar Cells 107 (2012) 316–321318

climatic test chamber from Q-LAB (according to ASTMG155). Theedges of the sample were sealed with glass cement. A 3 h testcycle was repeated for 162 times, a 1 kW xenon arc lamp wasutilized to radiate the sample under the light wavelength rangingfrom 280–3000 nm and the water was sprayed on them for 9 minonce every 1.5 h. The temperature of the back panel ranged from45 to 85 1C with a relative humidity of 6575% [29].

0 50 100 150 200 250 3000.80

0.85

0.90 LR-Calculated LR-ExperimentalLi

ght r

efle

c

Ag film thickness (nm)

Fig. 2. The influence of Ag film thickness on LR with a fixed SiO2 film thickness of

228 nm.

0 100 200 300 400 500 6000.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

SR-Simulation SR-Experimental

Sol

ar re

flect

ivity

SiO2 film thickness (nm)

Fig. 3. The influence of SiO2 film thickness on SR with a fixed Ag film thickness of

130 nm.

3. Results and discussion

3.1. The influence of Ag film thickness on the reflectivity

In this experiment, the influence of the thickness of the Ag film onits reflectivity was first investigated. As shown in Figs. 1 and 2, thecalculated results of both the LR and the SR match well with theexperimental results. The SR and LR changed with the increase of theAg film thickness under a 228 nm-thick SiO2 protection layer. The SRand LR values are less than 85% when the Ag film thickness is lowerthan 32 nm. However, the reflectivity sharply increases until the filmthickness reaches 64 nm; at this thickness, the reflectivity becomesstable with almost no change until the film thickness reaches 300 nm.The experimental maximum values for SR of 95.95% appeared whenthe Ag film was approximately 130 nm-thick, which is a little lessthan the calculated results.

The observed phenomenon in reflectivity can be explained asfollows: (1) the increase in film thickness can surely improve thereflectivity until a maximum reflectivity is reached, which isapproximately 130 nm in experiments undertaken for this study;and (2) the continued increase in film thickness after 130 nmcannot overcome the increase in the roughness of the grainsurface, which leads to more serious light scattering issues thatnegatively affect the reflectivity [30].

Therefore, the Ag film thickness is fixed at 130 nm based onstudies on the influence of the thickness of the SiO2 protectionlayer on the reflectivity in the subsequent experiments.

3.2. Thickness determination of SiO2 film

3.2.1. The influence of SiO2 film thickness on reflectivity

The influence of the thickness of the SiO2 protection layer onreflectivity is illustrated in Figs. 3 and 4. The thickness of theAg film was fixed at 130 nm which was optimized above. Theexperimental reflectivity for the SR and the LR can reach up to

0 50 100 150 200 250 3000.80

0.85

0.90

0.95

1.00

SR-Calculated SR-ExperimentalS

olar

refle

ctiv

ity

Ag film thickness (nm)

Fig. 1. The influence of Ag film thickness on SR with a fixed SiO2 film thickness of

228 nm.

0 100 200 300 400 500 6000.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

LR-Calculated LR-Experimental

Ligh

t ref

lect

ivity

SiO2 film thickness (nm)

Fig. 4. The influence of SiO2 film thickness on LR with a fixed Ag film of thickness

130 nm.

100

Y.J. Xu et al. / Solar Energy Materials & Solar Cells 107 (2012) 316–321 319

96.66% and 98.84%, respectively, when the thickness of the SiO2

film is approximately 320 nm. It has been shown that for thisbilayer structure, the increase in the SiO2 film thickness from 46to 320 nm resulted in an increase in the reflectivity and adecrease in the reflectivity when the thickness continues increas-ing from 320 to 600 nm. However, consistent with the report byWang et al. [31], compared with the experimental values, thecalculated values are higher for the SR but lower for the LRbecause the 320 nm-thick SiO2 films effectively protect the Agfilms from being oxidized and/or sulfurized. Ag films exposed tothe air will be unavoidably affected by the oxygen in the atmo-sphere. This will result in a decreased reflectivity when the SiO2

film is less than 320 nm-thick. At SiO2 film thicknesses greaterthan 320 nm, the strength of the absorption and reflection ofsunlight by the SiO2 increases with film thickness, and less lightcan penetrate through the film. As a result, the reflectivitydeclines when the SiO2 film is thicker than 320 nm even thoughthe oxidization of Ag has been successfully avoided.

500 1000 1500 2000 25000

20

40

60

80

Before abrasion test After abrasion testS

olar

refle

ctiv

ity (%

)

Wavelength (nm)

Fig. 6. Reflectivity of the film structure (substrate/Ag (130 nm)þSiO2 (320 nm)/air)

before and after the abrasion test.

3.2.2. Abrasion resistance

So far there are no standardized test methods to test theabrasion resistance of the solar reflective films. However, due tothe regular cleaning process of the solar reflective films, thereflectivity of the materials will decline, which is caused by themechanical friction. We speculated that the abrasion resistance isof great importance to the films. Therefore, the films deposited onthe substrates were scrubbed 100 times with a brown brush totest the abrasion resistance. No Ag remains on the film surfaceafter abrasion resistance tests when the SiO2 protection layer isthinner than 220 nm (as shown in Fig. 5). Therefore, we speculatethat SiO2 films with a thickness greater than 220 nm could serveas effective protection for the Ag film (see Table 1).

Fig. 6 shows the results of the reflectivity of the film structure(substrate/Ag (130 nm)þSiO2 (320 nm)/air) before and after theabrasion test. After the abrasion testing, a decrease of 96.62% to

1000 800 600 400 200 0

Inte

nsity

(a.u

.)

Binding Energy (eV)

Si2pSi2sC1s

O1s

Fig. 5. XPS spectrum of the film surface after brushed (SiO2: 220 nm, Ag: 130 nm).

Table 1XPS analysis of the sample surfaces after abrasion resistance testing.

Thickness (SiO2) (nm) 170 180 190

Ag existence Yes Yes Yes

Tests are based on Ag film thickness that is fixed at 130 nm.

95.85% was observed in SR, and a decrease of 98.58% to 98.14%was observed in LR. Only a slight decrease (less than 1%) in thereflectivity was observed in the experiments, demonstrating highanti-corrosion properties.

Therefore, the optimum structure of the reflective material issubstrate/Ag (130 nm)þSiO2 (320 nm)/air where the SR is 96.62%and LR is 98.58% (see Fig. 6).

3.2.3. Durability test

Durability resistance is a very important indicator of thecharacteristics of solar front reflective materials. The SR decreasedfrom 96.54% to 95.62% while the LR decreased from 97.47% to96.93% (glass/Ag (130 nm)/SiO2 (310 nm)) after the aging tests ofapproximately 486 h (see Fig. 7). The specular reflectivity loss ofSR and LR are 0.95% and 0.55% respectively, which is less than 2%.

200 210 220 230 240

Yes Yes No No No

500 1000 1500 2000 2500

0

20

40

60

80

100

Before aging test After aging test

Sol

ar re

flect

ivity

(%)

Wavelength (nm)

Fig. 7. Reflectivity of the film structure (substrate/Ag (130 nm)þSiO2 (320 nm)/

air) before and after the aging test.

Y.J. Xu et al. / Solar Energy Materials & Solar Cells 107 (2012) 316–321320

The results reveal that this film structure has a good agingresistance.

3.3. The influence of annealing temperature on the reflectivity

of Ag–SiO2 films

Because the solar front reflective materials are predominantlyutilized in middle-high temperature solar energy systems, it isvery important to study the characteristics of thin films underhigh temperatures. The Ag films were deposited on the glass

0 100 200 300 400 5000.90

0.92

0.94

0.96

0.98

1.00

LR-Reflectivity SR-Reflectivity

Sol

ar re

flect

ivity

Annealing temperature (°C)

Fig. 8. The influence of different annealing temperatures on the reflectivity of the

film with a structure of glass/Ag (130 nm)/SiO2 (320 nm)/air.

SiO2

Ag

Ag

glass

Fig. 9. SEM images of Ag–SiO2 films deposited on glass after etching: (a)

substrates under vacuum conditions and then annealed at differ-ent temperatures for 2 h at intervals of 50 1C. The reflectivity ofthe samples with a structure of glass/Ag (130 nm)/SiO2 (320 nm)/air were tested at different annealing temperatures, as shown inFig. 8. The results indicate that the reflectivity of the obtainedfilms is highest when the samples were annealed at approxi-mately 350 1C (SR: 96.62%, LR: 98.58%).

The reasons why the reflectivity can be improved during theannealing process have been investigated and analyzed. First, themorphology of the Ag film annealed at 300 1C. was studied indetail. If no SiO2 layer was deposited, the Ag film would beinevitably oxidized before being treated with the vacuum anneal-ing. Therefore, the obtained film surface was first etched by FIB.Fig. 9 shows the SEM images of the obtained films after etching,which were deposited on the glass substrate and annealed for 2 hat 300 1C. It is observed that the average grain size of the Agannealed at 300 1C increased from 60 to 160 nm and simulta-neously, the structure became more compressed compared to thesample that did not undergo an annealing treatment. This isbecause the Ag particles have sufficient free energy to enhancesurface migration, grain growth and grain aggregates during theannealing treatment, which causes a reconstruction of the crys-tals and positive changes in the optical properties of the films. Asa Ag grain grows, the crystal surface energy is released and thedefects in the film are reduced. Finally, the grain surface areabecomes smaller, which reduces the stress in the Ag film. There-fore, the annealing treatment has the ability to increase the filmgrain size and reduce its stress to improve the reflectivity [32].Furthermore, the structure of the SiO2 film becomes morecompact after being annealed at 300 1C, which may furtherimprove the abrasion resistance of the tested samples.

Ag

SiO2

Ag

glass

and (c) annealed at 300 1C; (b) and (d) without annealing treatment.

Y.J. Xu et al. / Solar Energy Materials & Solar Cells 107 (2012) 316–321 321

The annealing also enhances the adhesion between the films andthe substrate, which can further extend its useable life.

4. Conclusion

In conclusion, novel SiO2 protected Ag thin films deposited onglass substrates used for solar front surface reflectors weresuccessfully designed theoretically and prepared experimentally.The Ag films were deposited on the glass substrates using DCmagnetron sputtering whereas the SiO2 films were deposited onthe Ag surface as the protection layer using RF magnetronsputtering. The reflective materials with an optimum structureof glass/Ag (130 nm)/SiO2 (320 nm) have the highest SR of 96.66%and LR of 98.84%. Furthermore, the experimental results matchwell with the calculated results. This structure also exhibits goodanti-aging and anti-abrasion properties. When the structure ofglass/Ag (130 nm)/SiO2 (320 nm) was exposed to an annealingtreatment of 350 1C, the SR and LR increased to 96.62% and98.58%, respectively. These novel films have demonstrated thepotential application for use in the solar energy industry.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (no. 51076032).

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