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Degradation in PV Encapsulant 

Strength of Attachment: An 

Interlaboratory Study Towards a 

Climate-Specific Test

Preprint

David C. Miller, Eleonora Annigoni, Amal Ballion, 

Jayesh G. Bokria, Laura S. Bruckman, David M. Burns, 

Xinxin Chen, Jiangtao Feng, Roger H. French, Sean 

Fowler, Christian C. Honeker, Michael D. Kempe, 

Hussam Khonkar, Michael Köhl, Laure-Emmanuelle 

Perret-Aebi, Nancy H. Phillips, Kurt P. Scott, Fanny 

Sculati-Meillaud, and John H. Wohlgemuth

Presented at the 43rd IEEE Photovoltaic Specialists Conference

Portland, Oregon

June 5–10, 2016

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Degradation in PV Encapsulant Strength of Attachment:

An Interlaboratory Study Towards a Climate-Specific Test

David C. Miller1, Eleonora Annigoni2, Amal Ballion3, Jayesh G. Bokria4, Laura S. Bruckman5, David M. Burns6, 

Xinxin Chen7

, Jiangtao Feng7, Roger H. French5, Sean Fowler8, Christian C. Honeker9, 

Michael D. Kempe1

, Hussam Khonkar10, Michael Köhl3, Laure-Emmanuelle Perret-Aebi11, Nancy H. Phillips6, 

Kurt P. Scott12, Fanny Sculati-Meillaud2

, and John H. Wohlgemuth1

1

National Renewable Energy Laboratory (NREL), 15013 Denver West Parkway, Golden, CO 80401, USA 2

École Polytechnique Fédérale de Lausanne (EPFL), Rue de la Maladière 71B, Ch-2002 Neuchâtel, Switzerland 3

Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstrasse 2, 79110 Freiburg, Germany 4

Specialized Technology Resources, Inc. (STR), 10 Water Street, Enfield, CT 06082, USA 5

Case Western Reserve University (CWRU), White 538, 10900 Euclid Avenue, Cleveland, OH 44106, USA 6

The 3M Company, 3M Center, Building 235-67-15, St. Paul, MN 55144, USA 7

China National Electric Apparatus Research Institute Co., Ltd. (CEI), Guangzhou, 510663, P. R. China 8

Q-Lab Corporation, 800 Canterbury Road, Cleveland, OH, 44145, USA 9

Fraunhofer Center for Sustainable Energy Systems (CSE), 5 Channel Center, Boston, MA 02210, USA 10King Abdulaziz City for Science and Technology (KACST), 17 King Abdullah Road, Riyadh, 11442, Saudi Arabia

11

Centre Suisse d'Electronique et Microtechnique SA. (CSEM), Rue Jaquet-Droz 1, Ch-2002 Neuchâtel, Switzerland 12Atlas Material Testing Technology L.L.C., 1500 Bishop Court, Mount Prospect, IL 60056, USA 

Abstract — Reduced strength of attachment of the encapsulant 

resulting from the outdoor environment, including ultraviolet 

(UV) radiation, may decrease photovoltaic (PV) module lifetime 

by enabling widespread corrosion of internal components. To 

date, few studies exist showing how the adhesion of PV 

components varies with environmental stress. We have conducted 

an interlaboratory experiment to provide an understanding that 

will be used to develop climatic specific module tests. Factors 

examined in the study included the UV light source (lamp type), 

temperature, and humidity to be proposed for use in accelerated 

aging tests. A poly (ethylene-co-vinyl acetate) (EVA) formulation 

often used in veteran PV installations was studied using a 

compressive shear test - to quantify the strength of attachment at 

the EVA/glass interface. Replicate laminated glass/polymer/glass 

coupon specimens were weathered at 12 institutions using a 

variety of indoor chambers or field aging. Shear strength, shear 

strain, and toughness were measured using a mechanical loadframe for the compressive shear test, with subsequent optical

imaging and electron microscopy of the separated surfaces. 

Index Terms — durability, reliability, Xenon

I. INTRODUCTION

The adhesion of encapsulation was identified as a critical issue 

in the early development of photovoltaic (PV) module 

technology [1],[2]. That work sought to develop encapsulation 

formulations whose cohesive strength exceeded the adhesive 

strength at interfaces. To achieve this, a bifunctional coupling 

agent (with chemical groups on one end being reactive with 

glass and chemical groups on the opposite end being reactive 

with the encapsulant) was developed that could be included in 

the formulation to act as a primer. It was theorized at the time 

that the coupling agent would form an interpenetrating 

network, i.e., an intermediate layer covalently bonded to the 

glass and encapsulant [1]. Since then, delamination has often 

been observed in veteran PV installations in a wide variety of 

locations and climates [3]. Systematic quantitative studies of 

encapsulant adhesion are, however, presently lacking in the PV 

literature. In their absence, qualitative examinations provide 

anecdotal support for insights including that the 

encapsulation/cell interface is typically weaker than the 

encapsulation/glass interface [3],[4],[5], with observations of 

delamination being reported at both interfaces in veteran 

installations [3]; that the use of primer, enabled with an 

adequate degree of cure, facilitates good adhesion [1],[2],[6]; 

and that delamination often precedes corrosion in fielded PV 

modules [1],[7]. The widespread observation of delamination 

[3] suggests that it may be aided by the complex history of 

conditions present during the life of a module. It remains to be 

established if encapsulant adhesion correlates with 

discoloration.

Existing protocols examining the general robustness of PV 

modules include the IEC 61730 (safety) [8] and IEC 61215 

(type qualification) [9] standards. Specifically, IEC 61730-2

edition 2 only directly examines adhesion of the backsheet 

(using a Peel test) or cemented joint (using an Overlap Shear 

test) whereas IEC 61215-2 only directly examines adhesion of 

cables and the junction-box (Robustness of Terminations

tests), indirectly examining adhesion of the encapsulant

following the ultraviolet (UV) Preconditioning, Humidity 

Freeze, and Thermal Cycling tests. IEC 61215-2, however, 

does specify an ultraviolet radiation (UV) preconditioning of 

54 MJ⋅m-2 (15 kWh⋅m-2

), which would be encountered after 40 

days of field exposure to the AM1.5G UV spectrum. IEC 

61730-2 now prescribes up to 4x this UV exposure - also much 

less than a 25 year service life. The recent development of 

standards related to the materials and components used in PV 

modules, e.g., the IEC 62788 series, has highlighted the need 

for improved understanding of adhesion in PV technology that 

might also apply to encapsulation.

We have conducted an interlaboratory study to provide the 

understanding that will be used to develop climate-specific 

sequence tests, e.g., the IEC 62892 series. The experiments
2

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and results for the examination of the degradation of strength 

of attachment will be described here.

The goals of the encapsulant attachment study include:

•Quantify the significance of the factors of UV, 

temperature (T), relative humidity (RH), and time. Which 

factors most reduce the attachment during weathering? What 

range of values should be applied for aging?

•Determine if there is significant coupling between UV, T,

and RH. What factors should be applied in a weathering test 

for attachment?

•Investigate the spectral requirements for light sources, 

e.g., compare Xe-arc or UVA-340 fluorescent lamps. Does 

visible light affect aging?

II. EXPERIMENTAL

Glass/encapsulant/glass coupons were artificially aged using

a variety of different steady state conditions. The glass 

consisted of textured Solite glass (AGC Solar/Asahi Glass 

Co.), representative of glass used in the PV industry. Unlike 

the silica glass used in Ref. [10], Solite does absorb some 

terrestrial UV. Furthermore, Solite is subject to solarization—

its UV cut-off wavelength (where it becomes 10% 

transmitting) was verified here to change from 295 nm to 297 

nm following UV exposure. A single encapsulation material, 

“EVA-B” as in Ref. [10]—similar to the STR product 

PHOTOCAP 15295P/UF—was studied in the attachment test.

EVA-B represents the material found in many recently

examined veteran installations [3].

Weathering specimens were made by first laminating a 36.8-

cm x 36.8-cm glass/encapsulant/glass stack using a periphery 

inset lamination frame composed of a matched-thickness

removable layer to ensure good (0.45 ± 0.03 mm) thickness 

uniformity. Lamination was carried out for 8 minutes at 145°C. 

The resulting samples were then diced into 76-mm x 76-mm 

coupons using an abrasive water jet saw (Precision Waterjet 

Co.) Water jet cutting was found to provide a surface 

roughness comparable to carbide or diamond resin dicing 

blades (average and root-mean-square roughness on the order 

of 1-2 μm), but importantly with less chipping of the exposed 

glass surfaces at the specimen edges. Diced coupons were then 

preconditioned in temperature/humidity chambers for at least 1 

month to achieve a baseline moisture concentration in the 

polymer similar to that applied in the different artificial aging

tests.

The description here is primarily focused on materials 

exposed using Ci5000 chambers (ATLAS Material Testing 

Technology LLC), which are equipped with a Xe lamp and 

“Right Light” filter. The default exposure settings for the 

Ci5000 used an irradiance of 1.0 W⋅m-2

⋅nm-1 at 340 nm; an

indoor chamber temperature set point of 60°C; an uncontrolled 

black panel temperature; and a chamber atmosphere controlled 

to 30% RH, resulting in a measured sample temperature of 

~63°C. Exposure times are reported for the cumulative radiant 

exposure, H, provided for the wavelength range 295≤λ≤360 

nm so that the results may be more readily compared between 

Xe and UVA-340 sources [11].

The attachment coupons were aged for 0, 15, 30, 45, 90, 

135, or 180 cumulative days. Strength measurements were 

obtained after the coupons were diced into 25-mm x 25-mm 

specimens using the water jet cutter and then conditioned for at 

least 2 weeks to equilibrate their internal moisture content 

similar to that during aging. The coupons were marked before 

dicing to be able to distinguish the specimens’ interior and 

periphery. Two coupons were aged to each cumulative UV 

dose. At least 5 replicates are used for the periphery 

measurements, where the orientation of the periphery 

specimens was alternated (i.e., the outside edge was in-line or 

perpendicular to the applied displacement).

One additional experiment was performed to quantify the 

effect of specimen conditioning. Here, unaged specimens were 

either desiccated or maintained at 45°C and 85% RH for 1 

month before testing in addition to specimens subject to 

H=333 GJ⋅m-2 at 60°C/50% RH that were dessicated for 1 

month after aging.

As shown in Fig. 1, attachment was measured using the 

compressive shear test (CST) method with custom grips, 

similar to Ref. [6]. The displacement controlled CST was 

performed at a constant strain rate of 0.05 s-1 (for −1

= ⋅ • •

i xy γ δ h , 

where •

xy γ represents the strain rate, s-1

; •

δ the displacement 

rate, m⋅s

-1

; and hi the initial encapsulation specimen thickness

of 0.45-mm) and data acquisition rate of 100 Hz using a 

mechanical load-frame (ReNew 1125/75/82, Instru-Met 

Corp.), load-cell (GR A30-33, Instron), and the custom grips. 

Two universal joints (Grey and Prior Machine Co.) were used 

to compensate the alignment of the grips during the test, shown 

in Fig. 1 (inset). Data were excluded for samples with substrate 

cracking prior to failure. Substrate cracking, typically visually

or auditorily evident during testing, often corresponded to 

excursions in the data profiles. Good dicing and specimen 

handling practices to maintain good surface quality and 

prevent chips or cracks at the edges of the glass were critical to 

preventing substrate cracking.

Fig. 1. The equipment for the compressive shear test includes a 

load-frame, load-cell, and custom grips. As indicated in the inset, 

specimens are sheared apart when the movable top grip is displaced 

relative to the fixed lower grip.

δ

Load-cell

Load-frame

Specimen
3

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As indicated in Equation 1, the strain (γxy) was calculated 

from the crosshead displacement (δ, m) based on the initial hi

and known test angle, θ, of 45°. Because the strain was 

inferred from the crosshead displacement, it does not account

for the compliance of the instrument and grips. As in Equation 

2, shear stress was calculated from the measured load (F, N)

based on the initial specimen geometry (the area, A, m2

) and θ. 

Toughness was automatically calculated from the measured 

area of stress/strain data profile.

(1)

(2)

III. RESULTS

Fig. 2 shows the results for Xe aged specimens including the 

maximum shear strength (max[τxy], Pa) and toughness (UT, 

J⋅m-3

). The max[τxy] was taken from the peak of the 

stress/strain data profile; UT was determined from the start of 

the test, γxy = 0, up to failure, the maximum γxy. Results are 

shown for specimens cut from the interior (-i) and periphery (-

p) of the coupons. The equivalent IEC 61215-2 UV dose 

(295≤λ≤360 nm) is indicated in the bottom left of the figure 

for Xe (left hash mark) and UVA-340 sources (right hash 

mark). Fig. 2 shows a significant loss of strength, exceeding 

50% of the original strength. A similar decrease in attachment 

strength exceeding 66% of the original strength was observed 

for specimens aged at Fraunhofer ISE using fluorescent UV 

lamps, not shown. In that case, asymptotic loss similar to that 

depicted in Fig. 2 was also observed for specimens aged in that 

chamber (operated at 2.2 W⋅m-2

⋅nm-1 at 340 nm, with the 

chamber temperature set point of 60°C, and its atmosphere 

controlled to 50% RH).

Fig. 2. Change in max[τxy] and UT with H (295≤λ≤360nm) for 

coupons aged in the NREL Ci5000 chamber (Xe lamps, with the 

chamber controlled at 60°C and 50% RH).

Fig. 3 shows representative (median) τxy/γxy profiles from 

the data in Fig. 2. The elastic region (initial τ and γ) of the 

specimens is not overtly affected with age. The hyperelastic 

behavior (e.g., observed at the greatest τ and γ for the unaged 

specimen, which may strain nearly 500% - five times the 

encapsulant thickness) no longer occurs with prolonged aging

(e.g., where strain is limited to 300% for H≥>0.17 MJ⋅m-2

).

The self-similar shape of the data profiles in Fig. 3 is 

consistent with the general correlation between τxy and UT in 

Fig. 2. Some variability in sharpness of the τxy peak was 

observed in specimens at all ages.

Fig. 3. Overlay of τxy/γxy profiles for the median of the data 

profiles (from periphery specimens) in Fig. 1. H is noted along with 

the cumulative test duration (days).

Table provides the previous max[τxy] data in Fig. 2 for H

= 0 MJ⋅m-2 with additional results from the sets of pre- or postconditioned specimens (any aged specimens periphery 

located). Additional specimens in Table include: (a) unaged 

specimens desiccated at 25°C; (b) unaged specimens 

maintained at 45°C and 85% RH; and (c) specimens aged for H

= 333 MJ⋅m-2 and then desiccated at 25°C for 1 month.

TABLE I

COMPARISON OF CONDITIONED SPECIMENS RELATIVE TO FIG. 2.

H, RADIANT 

EXPOSURE 

{GJ·m-2

}

PRE-CONDITION max[txy]

{MPa}

UT, 

TOUGHNESS

{mJ}

0.000 Fig. 2, desiccated 13.64±2.04 21.540±3.388

0.000 (a), desiccated 14.45±1.31 23.720±2.995

0.000 (b), 45°C/85% RH 9.71±1.08 14.640±1.988

0.333 Fig. 2, 45°C/85% RH 4.84±0.75 7.206±1.321

0.333 (c), desiccated 7.95±0.98 12.100±1.889

Fig. 4 shows the previous max[τxy] data in Fig. 2, overlaid with 

additional results from specimens weathered at 3M at 30% RH. 

All specimens in Fig. 4 were aged at the same irradiance (1.0 

W⋅m-2

⋅nm-1 at 340 nm) and chamber temperature (60°C, 

resulting in a measured specimen temperature of ~63°C), but at 

different moisture concentrations (30% RH or 50% RH). As in 

Fig. 2, and Fig. 4, the periphery-located specimens are more 

rapidly affected than those obtained from the interior of the 

aged coupons. This may be quickly verified in each figure as 

the lighter of the like-colored data (the periphery-located 

specimens) are more affected (greater reduction in max[τxy]) 

[ ] i

i xy hh

2

2

cos 1 δγ = δ θ ⋅ = −

[ ] A

F

xy F A

2

2

cos 1 = ⋅ = − τ θ
4

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than the accompanying darker like-colored data. As in the 

other results, a fit has been added to the discrete data points

strictly to guide the eye. For example, max[τxy] is decreased 

through the experiment at 50% RH, resembling asymptotic 

behavior, represented using an exponential decay fit. max[τxy] 

appears to be enhanced for H < 166 MJ⋅m-2 at 30% RH;

therefore, a more complex sigmoidal fit has been applied to 

group the data.

Fig. 4. Overlay of data for samples weathered in separate Xe 

chambers at the same irradiance (1.0 W⋅m-2

⋅nm-1 at 340 nm) and 

chamber temperature (60°C) but RH of: (a) 30%, colored green; and 

(b) 50%, colored blue.

Change in τxy with age is examined in Fig. 5 for

specimens weathered using separate Xe chambers at 3M 

maintained at different temperature, but all using 1.0 W⋅m2

⋅nm-1 at 340 nm and a 30% chamber RH. Limited (if any) 

change is observed for the specimens aged at 45°C; a possible 

increase in attachment strength is initially observed for 60°C; 

and an asymptotic decrease in τxy is observed for 80°C. As in 

Fig. 5, a lesser decrease in τxy was seen for specimens aged at 

Fraunhofer ISE using a fluorescent UV chamber maintained at 

40°C (not shown). The asymptotic decrease in τxy for the 

moisture equilibrated specimens aged at 80°C/30% RH in Fig. 

5 is less (∆max[τxy] of 41% from the dry reference) than that 

observed in Fig. 2 for 60°C/50% RH (∆max[τxy] of 54%). As 

in Fig. 5, a substantial asymptotic decrease in τxy (12.3 MPa) 

was seen for a specimen aged at Fraunhofer ISE using a 

fluorescent UV chamber maintained at 80°C (not shown).

The typical morphology of failed specimens is shown in Fig. 

6. For many of the specimens, the majority of the encapsulant 

typically remained on one side of the detached glass. This was

confirmed using scanning electron microscopy (not shown) 

and may be observed at a site along the bottom edge of the 

detached specimen substrate, identified with an arrow in the 

inset of Fig. 6. To clarify, a thin layer of encapsulant typically 

remained adjacent to the textured surface of the superstrate. 

Most often, the majority of the encapsulant remained on the 

substrate, suggesting UV damage near the incident 

polymer/glass interface. This was corroborated during 

specimen preparation for failure analysis, when the irradiated 

surface was much easier to separate when dicing transmittance 

specimens [10] into smaller pieces for analysis. Furthermore, 

the detached surface was used to identify specimens facing 

differently during weathering at one laboratory (with the serial 

number surface scribed towards the lamp), when the opposite 

specimen orientation was used at the other laboratories.

Localized delamination was sometimes observed at 

specimen edges. An example appears in Fig. 6, where a 

crescent shaped portion of encapsulant is detached at the left 

edge of specimen substrate. Delamination at the specimen 

edges always occurred along the loading direction, regardless 

of specimen orientation.

Fig. 5. Comparison of change in τxy with applied temperature. The 

same default irradiance and RH conditions were used in separate Xe 

chambers, maintained at 45°C, 60°C, or 80°C, respectively.

Fig. 6. Optical photograph showing the typical morphology of 

failed specimens. An arrow marks an area on the inset where the 

remaining encapsulation may be discerned from the glass substrate.

IV. DISCUSSION

Regarding the results in Fig. 2, the asymptotic behavior 

suggests an overriding degradation mode such as UV- or 

hydrothermal- degradation. The data further suggest an 

asymptotic limit for the degradation of the max[τxy] at the 

EVA/glass interface. From Ref. [12], the final H is equivalent 

to 4.5 years in Phoenix, AZ (based solely on the UV dose 

between 295 nm and 385 nm). The decrease in strength in Fig. 

2 exceeds 25%, the threshold typically used in the relative 

thermal index (RTI, i.e., UL 746C) test and 50%, the threshold 

in the relative thermal endurance (RTE i.e., IEC 60216) test, 

which is also applied for cemented joints in IEC 61730-2. The 

loss of strength therefore raises the question: should the 

traditional test protocol (e.g., 50% loss of strength) or the 

device application (such as a minimum threshold) motivate 

2 mm 1 cm
5

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decision making about the attachment of weathered 

encapsulant interfaces?

The loss of the hyperelastic in Fig. 3 suggests the results 

in Fig. 2 are dominated by the mechanical properties of the 

EVA. An increase in crosslink density in the EVA might 

explain the embrittlement in Fig. 3; such behavior has been 

observed in the gel content measurements of artificially aged 

EVA [13]. Regarding the rounding of the τxy peaks in Fig. 3, 

this likely relates to the suddenness of failure, where rounding 

often suggests yielding or slip at the interface.

The additional desiccated unaged specimens, marked as 

(a) in Table, resemble the previous unaged specimens. This 

overlap demonstrates that the test is repeatable. The data 

importantly clarify the meaning of the unaged specimens in 

Fig. 2, Table, Fig. 4, and Fig. 5, which were obtained for 

desiccated specimens to standardize the presentation of results 

between different experiments. The remaining data in Fig. 2, 

Fig. 4, and Fig. 5 (not colored gray), however, were obtained 

from aged specimens controlled at the internal water 

concentration present during UV weathering.

The wet pre-conditioned unaged specimens, marked (b) in 

Table, show a 3.93 MPa reduction in max[τxy] relative to the 

desiccated reference specimens in Fig. 2. The effect of added 

moisture is comparable to the effect of continued chamber 

weathering, i.e., an additional 4.87 MPa reduction in max[τxy].

This is confirmed in the reverse characterization, where the 

max[τxy] for aged specimens, (c) in Table, increased by 3.11 

MPa when water was removed. Table importantly confirms 

that controlling the moisture content of specimens at the time 

of each measurement is essential to measurement and 

interpretation of weathering studies related to attachment.

The significant difference in results at 30% RH and 50% 

RH confirms that moisture plays a significant role in change in

attachment between EVA and glass. This suggests the mode of 

hydrothermal degradation in Fig. 4. The more rapid and 

significant reduction in max[τxy] at the coupon periphery may 

result from oxygen facilitated degradation. For example, 

oxygen bleaching was observed to affect the degradation and 

subsequent properties of transmittance specimens in Ref. [13]. 

To clarify, the location-specific results should not follow from 

the equilibration of moisture, as the aged specimens were preconditioned before weathering as well as after aging and also 

after being diced. An initial increase in strength of attachment 

with aging, as suggested for the specimens aged at 30% RH, 

can occur, e.g., based on increased substrate bond population.

An acute dependence on the temperature present during 

weathering is evident from the very different data profiles in 

Fig. 5. The disparate profiles may result from: (1) a threshold 

of UV, T, or RH required to cause substantial damage; (2) the 

consequences of absorbed moisture, i.e., water facilitated 

polymer plasticization or hydrothermal degradation instead of

UV degradation; or (3) phase transition, where EVA melts at 

~60°C. Additional experiments in progress, including 

examination at 40°C, 60°C, and 80°C but with no light present 

as well as characterization at multiple temperatures above the 

melting transition for EVA should clarify the meaning of Fig. 

5. The results in Fig. 5 may fortuitously suggest that in many 

areas of the world, where extreme T and RH are not both 

present, there may be little or no loss of attachment with UV 

exposure. In comparison, Weathering at 85°C and 20% RH is 

presently advocated in the proposed IEC 62788-7-2 weathering 

of PV polymeric materials standard.

The failure morphology in Fig. 6 may identify cohesive 

failure near the irradiated interface rather than delamination.

This observation is consistent with the loss of hyperelastic 

behavior with age in Fig. 3. If so, the max[τxy] and UT in Fig. 

2 are limited by EVA degradation, not the interface with the 

glass. Failure may instead occur at the interface between an

interpenetrating network between the EVA and primer, where 

the network has a more significant thickness than immediately 

evident in Ref. [1]. It remains to be validated if either failure 

mode is observed in PV modules.

Regarding the specimen appearance in Fig. 6, the EVA was 

usually not overtly discolored except at higher temperatures 

(80° C). The reduced UV transmittance (i.e., UV cut-off 

wavelength of 295 nm) of the Solite glass superstrate used in 

this study likely reduces UV damage, thereby contributing to 

the observed neutral appearance.

The localized delamination at the edges in Fig. 6 may result

from increased stress intensity, which was observed in finite 

element analysis of the test method [6]. The rounding of the 

τxy peaks in Fig. 3 may result from such inelastic deformation 

at the edges of the encapsulant.

Several considerations might provide further understanding 

regarding the degradation observed in this study. First, the 

textured glass may affect the results by creating a 

heterogeneous stress field near the EVA/glass interface, 

moving the failure zone away from the interface. Textured 

glass is commonly used in PV modules. Second, the 

mechanical state invoked during the CST (which here created a 

stress by applying a large displacement) can vary significantly 

from that in a fielded PV module (which can include stresses 

concentrated about components including cells and

interconnect ribbons as well as about voids/flaws in the 

encapsulant itself). The study here would also benefit from a 

fracture mechanics test method, which would directly examine 

adhesion rather than strength of attachment. Fracture 

mechanics can be directly related to the underlying physics of 

adhesion in order to also study its degradation. Third, it is 

unclear if the best suited aging method is applied in this study. 

While some weakness is identified here based on failure 

proximate to the irradiated interface, the coupling agents 

developed for EVA were theorized to be robust to UV [1],[2]. 

Those early studies instead aged using immersion in heated or 

boiling water. T and/or RH cycling might instead more closely 

emulate the degradation seen in PV modules. A more recent 

study [14] instead examines if UV weathering could be applied

prior to the Cyclic (“Dynamic”) Mechanical Load test, 

followed by the Humidity Freeze test followed by the Damp 

Heat test to invoke delamination and subsequent corrosion 

damage in PV modules. The steady state weathering results 

presented here will be compared to those obtained from 

identical coupons, naturally aged at locations including Miami 
6

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

and Phoenix. Fourth, metal corrosion (e.g., of grid lines or 

interconnects, possibly facilitated by acetic acid) or glass 

corrosion (e.g., ion migration following with potential induced 

degradation) or gas formation (from residual reactions) may 

not occur in the simple coupon material-only specimens 

examined here. Fifth, manufacturing and process control (glass 

cleaning and contamination control) may significantly 

contribute to loss of adhesion in veteran PV modules. In this 

case, components in addition to those used in the specimens in 

this study would also be required for examination. 

Manufacturing-related adhesion problems might be better 

addressed in the IEC 62941 standard for a quality management 

system in PV manufacturing rather than IEC 61730 or IEC 

62125. Lastly, it should be pointed out that only permanent 

UV damage has been examined here whereas UV present 

during an applied mechanical stress may have different effects 

and consequences. Remember also that the glass/encapsulant 

interface may not be weakest interface in many PV modules.

V. CONCLUSIONS

The UV photodegradation of the strength of attachment of 

PV encapsulation materials is under examination. Key interim 

results of the interlaboratory study include the following:

Evidence of permanent damage from UV weathering was 

identified and attributed to the degradation of the EVA 

encapsulant. The strongest evidence comes from the many 

specimens showing failure proximate to their UV irradiated 

surface. Asymptotic degradation occurred during a UV dose of 

<700 MJ⋅m-2

, suggesting a 6-month artificial aging test may be 

appropriate for UV weathering. It is cautioned that combined 

UV and mechanical stress, not examined here, may have 

different effects and consequences.

The results of weathering have been found to be very 

condition-sensitive, i.e., likely very factor coupled. For 

example, the loss of strength for weathering at 80°C was found 

to be much greater than that obtained at 60°C. Regarding 

moisture, plasticization and hydrothermal degradation may 

contribute to the loss of strength. So, in areas of the world

where extreme T and RH are not both present, there may be 

little or no loss of attachment with UV exposure. The results 

here suggest careful consideration of temperature and humidity 

in weathering or qualification related standards.

Cohesive failure of the EVA or delamination between EVA 

and a thick interpenetrating network with the coupling agent 

(primer) is implied from the failure morphology observed in 

this study. This result may follow from: the use of textured 

glass (and corresponding heterogeneous stress field near the 

interface); the mechanical state invoked during the CST (where 

stress results from a large applied displacement); the steady 

state nature of the weathering applied (as opposed to a more 

complex or cyclic aging); and the underlying cause of 

delamination in PV modules (which may include corrosion, 

contamination, gas formation, or process control issues). 

Specific examination of -adhesion- would likely benefit from a 

fracture mechanics test method rather than the strength of 

attachment approach used here.

Initial results suggest the effects of water, which may 

include plasticization and hydrothermal degradation, are at 

least comparable to that of UV damage. Weathering studies 

related to attachment must therefore treat the effect of water 

before and during aging as well as making sure to control the 

moisture level before measurement. Pre-conditioning 

specimens to achieve an internal water concentration beforeweathering or -measurement will extend the overall test 

duration, but is essential to obtain valid results.

REFERENCES

[1] D.R. Coulter, E.F. Cuddihy, E.P. Pluedmann, “Chemical 

Bonding Technology for Terrestrial Photovoltaic Modules”, 

DOE-JPL-1012-91, 1983, pp. 1-65.

[2] J.L. Koenig, F.J. Boerio, E.P. Plueddemann, J. Miller, P.B. 

Willis, E.F. Cuddihy., Chemical Bonding Technology: Direct 

Investigation of Interfacial Bonds”, DOE/JPL-1012-120, 1986, 

pp. 1-58.

[3] J.H. Wohlgemuth, T. Silverman, D.C. Miller, P. McNutt, M.D. 

Kempe and M. Deceglie, “Evaluation of PV Module Field 

Performance”, Proc IEEE PVSC, 2015.

[4] N.G. Dhere, M.B. Pandit, “Study of Delamination in 

Acceleration Tested PV Modules”, Proc. Euro PVSEC, pp. 572-

575, 2001.

[5] G.J. Jorgensen, T.J. McMahon, “Accelerated and Outdoor Aging 

Effects on Photovoltaic Module Interfacial Adhesion Properties”, 

PIP, 16, pp. 519-527, 2008.

[6] V. Chapuis, S. Pelisset, M. Raeis-Barneoud, H.-Y. Li, C. Ballif, 

and L.-E. Perret-Aebi, “Compressive-Shear Adhesion 

Characterization of PVB and EVA at Different Curing Times 

Before and After Exposure to Damp-Heat Conditions”, PIP, 22 

(4), pp. 405–414, 2014.

[7] T.J. McMahon, G.J. Jorgensen, “Adhesion and Thin-Film 

Module Reliability”, Proc. IEEE World Conf. Energy Conv., pp. 

2062-2065, 2006. 

[8] “IEC 61730-2 Photovoltaic (PV) module safety qualification -

Part 2: Requirements for testing,” Edition 2, International 

Electrotechnical Commission: Geneva, 1–57 (2016).

[9] “IEC 61215-2 Terrestrial photovoltaic (PV) modules – design 

qualification and type approval,” Edition 1, International 

Electrotechnical Commission: Geneva, 1–41(2016).

[10] D.C. Miller, et. al., “Degradation in PV Encapsulation 

Transmittance: An Interlaboratory Study Towards a ClimateSpecific Test”, Proc IEEE PVSC, 2015, 972.

[11] S. Fowler, “Developing steady state exposure conditions in an 

ASTM G154 fluorescent UV test chamber for backsheet 

materials,” Proc. NREL PVMRW, 2014.

[12] http://atlas-mts.com/online-tools/weather-summary-reports/

[13] A. W. Czanderna and F. J. Pern. “Encapsulation of PV Modules 

Using Ethylene Vinyl Acetate Copolymer as a Pottant: A Critical 

Review,” SOLMAT, 43, 101-181, 1996.

[14] J.H. Wohlgemuth, P. Hacke, N.S. Bosco, D.C. Miller, M. D. 

Kempe, S.R. Kurtz, “Assessing the Causes of Encapsulant 

Delamination in PV Modules”, Proc. IEEE PVSC, 2016.

ACKNOWLEDGMENT

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

under Contract No. DE-AC36-08-GO28308 with the National 

Renewable Energy Laboratory. Instruments and materials are 

identified in this paper to describe the experiments. In no case 

does such identification imply recommendation or 

endorsement by NIST or NREL.