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Contract No. DE-AC36-08GO28308

Degradation of Silicone 

Encapsulants in CPV Optics

Preprint

David C. Miller and Ian A. Tappan

National Renewable Energy Laboratory

Can Cai and Reinhold H. Dauskardt

Stanford University

Presented at the 2016 IEEE Photovoltaic Specialist Conference 

(PVSC)

Portland, Oregon

June 5–10, 2016

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NREL/CP-5J00-65895

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Degradation of Silicone Encapsulants in CPV Optics

Can Cai1, David C. Miller2, Ian A. Tappan2, Reinhold H. Dauskardt1

1

Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 USA

2

National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, CO 80401 USA

ABSTRACT — Concentrator photovoltaic (CPV) 

modules operate in extreme conditions, which poses a unique 

materials challenge for guaranteeing operational lifetimes of at 

least 25 years. The encapsulants used in the optical elements are 

critically susceptible to environmental degradation during 

operation. We studied the effects of weatherings on adhesion 

using an outdoor concentrator operating in excess of 1100 times 

the AM1.5 direct irradiance and in environmental chambers with 

broadband ultraviolet (UV) irradiation combined with controlled 

temperature and humidity. We observed a sharp initial increase 

in adhesion energy followed by a gradual decrease in adhesion as 

a result of both outdoor concentrator exposure and indoor UV 

weathering. We characterized changes in mechanical properties 

and chemical structures using XPS, FTIR, and DMA to 

understand the fundamental connection between mechanical 

strength and the degradation of the silicone encapsulant. 

Index Terms — adhesion, CPV, reliability, silicone, UV 

degradation

I. INTRODUCTION

The reliability of optical elements in CPV modules poses a 

unique materials challenge due to increased ultraviolet (UV) 

irradiance, elevated operating temperature and frequent

thermal cycling. We conducted a survey of the CPV industry 

to determine the critical interfaces and materials for study. The 

most critical interface for CPV reliability was determined to 

be the junction between the secondary optical element (SOE) 

and the anti-reflective (AR) coating on the CPV cell. The 

silicone adhesive used at the joint can be subject to greater 

than 1000 times the terrestrial solar flux, which can cause 

degradation and adhesive failure. Optical attenuation, 

including reflection loss at delamination at interfaces or cracks 

within the encapsulant, as well as loss of thermal 

management, even resulting in combustion of the encapsulant, 

have been observed in field studies [1].

Accordingly, the present study investigated the effects of 

photo-degradation on the silicone encapsulant and its adjacent 

interfaces. Specimens were aged in an outdoor concentrator to 

simulate operating conditions or in an environmental chamber 

with UV radiation to isolate environmental variables. 

Mechanical tests were carried out to determine the adhesion 

energy of the sample interface. Fourier transform infrared 

spectroscopy (FTIR) studies were conducted to examine the 

change in chemistry of bulk silicone as a result of weathering. 

We observed a significant change in adhesion energy along 

with change in failure mode as result of UV and outdoor 

weathering.

Fig. 1. Weathering of silicone specimens was conducted (a) in an 

environmental chamber under a UV lamp, (b) and in an outdoor 

concentrator. The samples are exposed through the transparent 

sapphire or glass superstrate (c), which was adhered to an aluminum 

plate (d) for mechanical testing of adhesion energy.

II. EXPERIMENTAL

Specimens were fabricated to model the adhesion of the 

silicone to the AR coatings of the multijunction CPV cells. 

Common AR materials, such as Al2O3 and silicon oxide, were 

modeled with sapphire wafers or B270 borosilicate glass, 

respectively, to allow for UV weathering and outdoor 

concentrator solar exposure. Two thermally cured silicone 

elastomers (Dow-Corning Sylgard 184 and NuSil Technology 

LS2-6140) were examined. To examine the effects of an 

adhesion promoting primer, a Ti-based primer (Dow-Corning 

92-023) was applied to both the aluminum beam substrate and 

the sapphire or borosilicate glass superstrate before 

application of the Sylgard silicone (Fig. 1c). The primer was 

not applied to specimens with the LS2-6140 silicone as its 

formulation contained an adhesion promoting additive. The 

silicone were cured following the curing procedure 

recommended by the manufacturers.

Silicone specimens were aged in an indoor environmental 

chamber under UV illumination for accelerated aging (Fig. 

1a). Outdoor exposure studies were conducted on Sylgard 184 

specimens using a solar concentrator (Fig. 1b). The 

concentrator optics allowed for 1100 times solar concentration 

onto a 1 cm2 area on the specimen, representing operating 

conditions in a contemporary CPV module design

a b

sapphire or borosilicate 

glass

silicone adhesive

Al beam

c d

P

a
2

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Similar to the examination of EVA in flat-panel PV [2-4], 

mechanical tests were carried out using a DTS 8.2 mechanical 

load frame (Fig 1d). The adhesion energy, Gc, was calculated 

using the equation:

GGcc = 6PP2aa2

BB2EE′h3 (1)

where P is the load at the start of crack propagation, a is the 

length of the crack, B is the width of the sample, E’ is the 

plain strain modulus, and h is the thickness of the aluminum 

beam [5]. Gc is the energy required to delaminate a unit 

surface area of silicone from the superstrate. The critical 

adhesion energy, Gc, characterizes the adhesive strength of the 

system, which can be used to quantify the degree of 

degradation.

III. RESULTS

The Sylgard 184 specimens aged in the outdoor 

concentrator (Fig. 2a) underwent an initial increase in 

adhesion energy up to ~250 J/m2 at ~700 hours of exposure 

followed by a subsequent decrease in adhesion energy to ~5

J/m2 at ~7000 hours of outdoor concentrator exposure. The 

failure mode for the field aged specimens changed from 

adhesive failure to cohesive failure with increased radiant 

exposure.

UV weathering at 110 o

C caused a significant change on the 

adhesion energy of the Sylgard 184 and LS2-6140 silicone 

adhesives (Fig. 2b). All specimens exhibited an increase in 

adhesion energy from 6.3 J/m2 to ~100 J/m2 after cumulative 

UV dosage, H, ~30 MJ/m2

. With further UV exposure, the 

adhesion energy decreased to ~20 J/m2 at H~200 MJ/m2

. The 

failure mode changed from adhesive failure at the interface of 

the silicone and the sapphire superstrate to cohesive failure 

within the silicone encapsulant with prolonged weathering.

The presence of the primer or adhesion promoter did not 

appear to affect the degradation of the silicone.

To determine the effect of temperature during UV exposure, 

the mechanical characterization was conducted for Sylgard 

184 with applied primer specimens aged at 50 o

C, 80 oC, and 

110 o

C (Fig. 4a and 4b). In the 80 oC series, the adhesion 

increased to a maximum of ~175 J/m2 at a cumulative UV 

exposure of ~50 MJ/m2

. In the 50 oC series, the adhesion 

increased to a maximum of ~150 J/m2 at a cumulative UV 

exposure of ~150 MJ/m2

. Specimens aged at 80 oC and 50 oC 

both exhibited change from adhesive to cohesive failure with 

increased cumulative radiant UV exposure.

Transmission FT-IR was conducted to determine the bulk 

chemistry of the silicone as a function of UV exposure (Fig. 

3a and 3b). The silicone specimen exhibited an increase in the 

Si-O peak at 1088 cm-1 as a result of UV exposure. The 

silicone exhibited a 6% increase in Si-O bonds after ~200 

MJ/m2 UV exposure (Fig. 3c). The measured percentage 

increase in Si-O bonds is normalized to the Si-O bonds in the 

silicone backbone.

Fig. 2. Specimens aged in outdoor concentrator exhibit increase 

followed by decrease in adhesion energy (a). Specimens with 

different silicone formulation and surface treatments (b) exhibited 

similar trends during UV aging at 110o

C.

IV. DISCUSSION

FTIR studies showed that the silicone was oxidized during 

exposure to UV at 110 o

C (Fig. 3a and 3b). Literature

identifies that UV-C radiation coupled with ozone treatment 

of silicone can cause oxidation of the silicone [6-7] but the 

effects of UV-A and UV-B radiation have not been widely 

studied. Traditionally, thermally cured silicone such as 

Sylgard 184 was thought to be unreactive to UV in the 300 nm 

to 400 nm range [8]. One possible mechanism for the 

oxidation of the silicone under these conditions is the cleavage 

of the Si-C bonds on the methyl side groups to form Si-OH 

groups [9], which can then react to from Si-O-Si bonds 

through a condensation reaction [6] (Fig. 3d).

Based on the calculated increase in Si-O bonds from FT-IR 

measurements ( 3c), the relationship between the increase in 

crosslink density, ΔN and H can be modeled by:

ΔNN = kkUUUUee

− EEaa

kkbbTTHH (2)

0 20 40 60 80 100 120 140 160 180 200

0

20

40

60

80

100

120

140

160

 Sylgard 184 With Primer

 Sylgard 184 No Primer

 LS2-6140 

Adhesion Energy, Gc (J/m2

)

Cumulative UV, Wtot (MJ/m2) b

110 oC Indoor UV Series

0 2500 5000 7500

0

50

100

150

200

250

300

Adhesion Energy, Gc (J/m2

)

Outdoor Exposure Time (hrs) a

Sylgard 184 Outdoor Series
3

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Fig 3. Transmission FTIR characterization of silicone after UV 

exposure (a) & (b) showed an increase in intensity for the Si-O 

stretching mode peak with increased cumulative radiant UV 

exposure, H. The percentage increase in Si-O bonds (c) can be 

calculated using the relative peak intensity at each H. A possible 

reaction scheme (d) for the oxidation of the silicone involves the 

formation of crosslinks following from UV mediated scission methyl 

side groups.

where kuv is a proportionality constant, similar to a frequency 

factor. The temperature effects are accounted for with an 

Arrhenius term where Ea is the activation energy, kb is the 

Boltzmann constant, and T is the temperature at which the 

specimens were aged.

The initial increase in Gc can be attributed to the increase in 

the number of bonds between the silicone and the underlying 

substrate and can be modeled as linearly proportional to the 

crosslink density between the silicone and the superstrate [10]: 

GGcc = kkss ∗ ∆NN + GG0 (3)

where ks is a proportionality constant and G0 is the initial 

adhesion energy with no UV exposure. Using Eq. (2) and Eq. 

(3), we obtain: 

GGcc = kk1HHee

− EEaa

kkbbTT + GG0 (4)

where k1 is the consolidated constant taking into account both 

ks and kUV. 

Data in the initial UV aging regime at 50 o

C, 80 oC, and 

110 o

C were used to determine the activation energy. The 

activation energy was calculated to be 32 kJ/mol, which is in 

the range of 20 kJ/mol to 40 kJ/mol reported for oxidation of 

silicone under UV-ozone treatments [11]. The activation 

energy for the degradation seen here is significantly less than 

the 90 kJ/mol to 125 kJ/mol activation energy reported for 

thermal oxidation of silicone [12]. The measured adhesion 

energy with no UV exposure, G0, was 6.3 J/m2

. Using 

the values for G0 and Ea, k1 was fitted to obtain 2.3*105 1/s.

Fig. 4. The linear fit (a) from Eq. 4 characterizes the initial 

increase in adhesion. The power function (b) from Eq. 6 characterizes 

the decrease in cohesion with the dotted line representing the ideal 

Mc

1/2 scaling for a planar crack and the solid line representing the 

fitted Mc

1.66 scaling for a meandering crack.

The dependence on temperature and cumulative radiant UV 

exposure in the model fit the observed trends in Gc (Fig. 6a) 

with the calculated value of Ea and the fitted value of k1.

With prolonged UV aging, the most stable crack path 

instead propagates through the bulk silicone [13]. The 

decrease in Gc can be attributed to embrittlement as a result of 

increased crosslinking in the bulk silicone material. Literature 

reports that the cohesion energy, Gc, in elastomeric polymers 

scales as

GGcc ~ nn ∗ MMcc

3

2 (5)

where n is the number of chains that bridge a crack front and 

Mc is the average molecular weight between crosslinks [14]. 

The term Mc

3/2 describes the average energy required to 

rupture one chain. n is directly proportional to the crosslink 

density, ΔN, as each additional crosslink creates two 

0 50 100 150 200 250 0

1

2

3

4

5

6

7

Increase in Si-O-Si Bonds (%)

Cumulative UV, H (MJ/m2

)

3500 3000 2500 2000 1500 1000 500

Absorbance (a.u)

Wavenumber (cm-1)

0 MJ/m2

42.3 MJ/m2

85.1 MJ/m2

151.3 MJ/m2

221.5 MJ/m2

a b

c

1120 1100 1080 1060 1040 1020 1000

Absorbance (a.u)

Wavenumber (cm-1)

0 MJ/m2

42.3 MJ/m2

85.1 MJ/m2

151.3 MJ/m2

221.5 MJ/m2

d

Si CH3+ O O Si OH+ H2C O

Si OH + Si OH Si O Si

0 50 100 150 200 250 300

0

50

100

150

200

250

50 o

C

80 o

C

110 oC

Adhesion Energy, Gc (J/m2

)

Cumulative UV, Wtot (MJ/m2

)

0 50 100 150 200 250 300

0

50

100

150

200

250

 50 oC

 80 o

C

 110 o

C

Adhesion Energy, Gc (J/m2

)

Cumulative UV, H (MJ/m2

)

a

b
4

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additional chains and as Mc scales inversely with ΔN, n would 

scale as Mc

-1

. Therefore Gc scales as ΔN-1/2 accounting for the 

competing effects of increasing volume density of chains and 

decreasing energy required to rupture the chains [15].

Using Eq. (2), the relationship between Gc and H can be 

written as

GGcc = kk2

�kk3 + HHee

− EEaa

kkbbTT�

1

2 (6)

where k2 is a proportionality constant that relates crosslink 

density to cohesion energy and k3 is a constant that reflects the 

initial degree of crosslinking. The constants k2 and k3 were 

fitted and the predicted behavior is plotted against the 

experimental values (Fig. 6b). The model, which scales as 

Mc

1/2, is indicated with a dashed line for all three aging 

temperatures.

The model predictions deviate from the experimentally 

measured Gc values at higher temperature and cumulative 

radiant UV exposure because it assumes a planar crack path. 

In materials with higher network connectivity, the crack will

meander through the material and follow a path of lower 

resistance compared to a planar crack [16].

A meandering crack path would affect the scaling of Gc

with ΔN since the crack would seek paths with shorter chains 

and corresponding lower rupture energy. In effect, the shortest 

chains determine the fracture energy of the bulk material in 

this regime. The fracture energy is accordingly more sensitive 

to changes in crosslink density and therefore would scale as 

ΔN<-1/2. A fitting of experimental data to determine the scaling 

reveal that Gc scales as ΔN-1.66, plotted as a solid line (Fig. 4b). 

The greater scaling exponent gives a better fit for the 

dependence of Gc on H. Experimental data in the literature 

shows similar trends of higher scaling power for materials 

with higher crosslink density. [24]

V. CONCLUSION

We studied the effects of terrestrial UV on silicone 

adhesives for CPV applications. Specimens aged in the 

outdoor concentrator exhibited an increase in adhesion energy 

to ~200 J/m2 with initial exposure and subsequent decrease in 

adhesion to ~2 J/m2 with prolonged exposure. Similar 

behavior was observed for UV weathering. The initial increase 

in adhesion was attributed to an increase in the population of 

bonds between the silicone and the underlying superstrate. 

The subsequent decrease in adhesion was attributed to the

embrittlement of the silicone as a result of UV-photochemical 

crosslinking. A fracture mechanics model was developed to

model the adhesion energy and can be used to predict the 

behavior of encapsulant materials with accelerated aging 

testing. We find that UV crosslinking could limit the life of 

silicone encapsulation used in CPV modules through reduced 

adhesion or embrittlement and subsequent cracking of the bulk 

material. The durability of materials under high UV flux must 

be considered to ensure the reliability of CPV modules.

ACKNOWLEDGEMENT

This work was supported by the U.S. Department of Energy 

under Contract No. DE-EE0006343. The work was executed

by the U.S. Department of Energy under Contract No. DEAC36-08-GO28308 with the National Renewable Energy 

Laboratory.

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