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Contract No. DE-AC36-99-GO10337 

G.J. Jorgensen, K.M. Terwilliger, M.D. Kempe, 

and T.J. McMahon 

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3–7, 2005 

February 2005 • NREL/CP-520-37366 

Testing of Packaging Materials 

for Improved PV Module 

Reliability 
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TESTING OF PACKAGING MATERIALS 

FOR IMPROVED PV MODULE RELIABILITY 

G. J. Jorgensen, K. M. Terwilliger, M. D. Kempe, and T. J. McMahon 

National Renewable Energy Laboratory (NREL), Golden, CO 80401 

ABSTRACT 

A number of candidate alternative encapsulant and 

soft backsheet materials have been evaluated in terms of 

their suitability for photovoltaic (PV) module packaging 

applications. Relevant properties, including interfacial adhesion and moisture transport, have been measured as a 

function of damp-heat (85°C / 85% relative humidity) exposure. Based on these tests, promising new encapsulants with improved properties have been identified. Backsheets prepared by industry and at NREL have been 

found to provide varying levels of moisture ingress protection. To achieve significantly improved products, further 

development of these candidates is ongoing. The relative 

effectiveness of various packaging strategies to protect PV 

devices has also been investigated. 

INTRODUCTION 

To survive in harsh operating environments, PV modules generally rely on packaging materials to provide requisite durability. These include glass/glass and 

glass/breathable backsheet constructions laminated with 

various encapsulant and/or edge-seal materials. Thin-film 

PV manufacturers are interested in replacing glass backsheets (and possibly frontsheets) with soft cover layers. 

Such constructions can eliminate glass breakage due to 

edge pinching and provide a more durable mechanical 

package. In addition, lighter weight can lead to lower cost. 

Alternative encapsulants that provide better moisturebarrier properties and improved adhesion with weathering 

are also being contemplated. For some configurations in 

which a thin-film device is directly deposited onto a clear 

superstrate material, solar transparency of the encapsulant is unnecessary. This expands the number of candidates that can be considered, allowing the use of less expensive materials and lamination processes. To achieve 

these benefits, the reliability of the resulting PV module 

must be demonstrated. In particular, the damaging effects 

of moisture ingress must be averted. Water can weaken 

interfacial adhesive bonds, resulting in delamination and 

increased ingress paths, consequent loss of passivation, 

electrochemical corrosion, and ultimately, device failure. 

The ability of combined packaging elements to protect 

thin-film aluminum coatings deposited onto glass substrates was assessed as a function of damp-heat exposure. Glass/glass laminate constructions were often found 

to trap harmful compounds that catalyzed moisture-driven 

corrosion of the aluminum. Constructions with breathable 

backsheets allow higher rates of moisture ingress, but also 

allow egress of corrosive substances. 

EXPERIMENTAL 

A number of new encapsulants and backsheets have 

been evaluated as improved packaging materials. The 

primary properties of interest were adhesion (using an 

Instron 5500R mechanical testing unit) as a function of 

damp-heat exposure, and moisture transport (measured 

with a Mocon Permatran-W® 3/31 instrument). Peel 

strength measurements (both 90° and 180°) have been 

made to allow screening of alternate encapsulant formulations and to test the durability of interfacial adhesion. Lap 

shear measurements were also made to allow more rigorous intercomparisons between different encapsulants, 

subjected to damp-heat exposure. By measuring the timedependent permeability of backsheets and encapsulants 

the diffusivity and solubility of these materials can be derived and used to compute moisture-ingress time scales 

[1]. Finally, the ability of combinations of packaging components to protect PV devices from corrosion was assessed. 

Encapsulants 

NREL has investigated the adhesion properties of a 

number of candidate encapsulants. STR’s standard fastcure ethylene vinyl acetate (EVA) product (which contains 

a self-priming additive and is designated 15295P) was 

considered the control material. We have evaluated 

whether additional primers (used to prime glass substrates) or alternative substrate-cleaning procedures can 

improve the adhesion between glass and STR’s EVA [2]. 

Other samples were also tested including silicones and 

primers from GE and Dow Corning, an experimental material from BRP Manufacturing (BRP-C), and an experimental fluorocarbon from Saint-Gobain (THV). 

A number of silane adhesion promoters designed to 

improve adhesion of EVA to glass were screened by priming the glass substrates and preparing samples at NREL 

having the construction: TPE / EVA / Primed glass, where 

TPE is a commercial Tedlar-PET-EVA backsheet material 

(PET is polyethylene terephthalate) and EVA is STR’s 

1
15295P product. The 90° peel strength was measured 

between EVA and glass as a function of damp-heat exposure. Some samples demonstrated an improvement in 

adhesion compared to the STR control laminated to an 

unprimed glass substrate; some samples were so adherent that they did not initiate peel, even after 775 h exposure to damp-heat [2]. The most promising primers are 

being compounded into EVA for further testing. 

NREL-prepared alternate encapsulant formulations 

were also tested [2]. These included EVA, and an ethylene 

copolymer of methacrylate with glycidyl functional groups 

having various silane coupling agents incorporated into the 

base resin. Some materials were found to be similar or 

inferior in performance to STR’s 15295P EVA. However, 

one material was quite promising and experienced no peel 

initiation after 743 h of damp-heat exposure. 

The lap shear strength of several encapsulants was 

measured as a function of damp-heat exposure (see Fig. 

1). Significant loss in adhesion occurred for the EVA control material (EVA 85/85); a ~50% decrease in lap shear 

strength was found after 1000 h of exposure. Even at ambient temperature and humidity conditions (EVA Ambient), 

a noticeable loss (~20%) in lap shear strength results after 

1000 h. The detrimental effect of moisture is evident by

comparison with samples exposed to a room-temperature 

dry-air purge (EVA Dry 23°C), which shows no loss in 

adhesion. An experimental material from BRP Manufacturing demonstrated outstanding retention (even a slight increase) of lap shear strength with exposure (BRP-C 

85/85). The THV material (THV 85/85) exhibited a precipitous drop in lap shear strength in a fairly short timeframe. 

The effect of two glass-cleaning procedures on the 

adhesion of EVA to glass was compared. One procedure 

used various acid treatments; the other used a commercial 

product (Billco #013-701) commonly used by the PV industry during their manufacturing process. The acid treatment resulted in a water-droplet contact angle of ~5°, 

whereas the Billco cleaning resulted in a contact angle of 

~52°. Although the contact-angle measurements suggest 

significant differences in the cleaned glass surface energy, 

90° peel strength measurements made as a function of 

damp-heat exposure were indistinguishable between the 

two cleaning methods (see Fig. 2). The sample construction was: TPE / EVA / Cleaned Glass. 

0

2

4

6

8

10

12

0 100 200 300 400 500

Time of Exposure (h)

Peel Strength (N/m

m)

Glass cleaned with Billco

Glass cleaned with acids

Fig. 2. 90° peel strength at EVA/glass interface as a 

function of accelerated exposure. 

Backsheets 

Polymer films coated with dense inorganic layers can 

provide improved moisture-barrier properties. We have 

previously investigated sputter-deposited inorganic coatings on PET as candidate backsheets for PV modules [3]. 

A number of other backsheet constructions have been 

characterized recently. These include an NREL plasmaenhanced chemical vapor deposition (PECVD)-coated 

PET, three commercial products from Isovolta, an experimental laminate from DuPont Teijin Films, ten PECVDcoated PET samples from AKT, two multilayer-coated PET

samples from PNNL, and an uncoated/unlaminated liquid 

crystal polymer (LCP). 

0

2

4

6

8

10

12

14

1 10 100 1000 10000

Time of Exposure (h)

Lap Shear Strength (MPa)

THV 85/85 BRP-C 85/85

EVA Ambient EVA Dry 23°C

EVA 85/85

EVA

Ambient

EVA THV 85/85 85/85

EVA Dry

23°C

BRP-C 

85/85

The various samples are at different stages of evaluation; preliminary adhesion and moisture transport results 

are summarized in Table 1. A series of hierarchical tests 

are typically performed. For polymer films with inorganic 

coatings, the quickest screening test is a Scotch tape peel 

test in which the tape is applied to the coating and peeled 

off. If none of the coating is removed, then it passes this 

test. Samples passing the tape peel test are then subjected to laminated peel strength and water vapor permeability characterization. Water vapor permeability was 

measured as a function of temperature (see Fig. 3). LCPs 

have very low permeability values, typically 100 times 

lower than uncoated PET. LCPs also provide a better 

moisture-barrier than experimental PET films having inorganic coatings. However, the LCP material by itself is very

fragile and may be better suited for use in a laminate construction; it is also a relatively expensive material. Polyethylene naphthalate (PEN) has been identified as a highperformance material for the food packaging industry. Although it does have better moisture-barrier properties than 

PET, uncoated PEN is inadequate for PV applications. 

The water vapor transmittance rate (WVTR) of DuPont 

Teijin’s experimental PEN / Al / PET laminate is below the 

detection limit of our Mocon unit, even at 85°C (see open 

Fig. 1. Lap shear strength as a function of accelerated 

exposure. 

2
circle in Fig. 3). If adhesion to EVA can be maintained 

during damp-heat exposure, this would be a very useful 

backsheet. The Isovolta Tedlar-SiOx-PET backsheet exhibits barrier properties intermediate between uncoated 

PET and some experimental oxide/nitride-coated PET

films. PNNL’s multilayer-coated PET exhibits very low

WVTR. However, it does not adhere to EVA during lamination. 

Table 1. Preliminary test results for various backsheet 

materials.

Combined Packaging

Several experiments were performed to quantify the 

relative effectiveness of various combined packaging 

strategies and components (i.e., backsheets, encapsulants, edge sealants) in preventing moisture-induced degradation of thin-film devices. The performance of small 

(laboratory-scale) PV devices is difficult to characterize 

without compromising the integrity of the protective package. However, exposure to damp-heat aggressively corrodes aluminum, which is often used in PV modules (e.g., 

as interconnects and back contacts). Consequently, thinfilm aluminum coatings were vacuum deposited onto glass 

substrate test articles to simulate a PV device and provide 

a rapid visual indication of damage. The extent of degradation was documented with digital imagery as a function 

of time of exposure to damp-heat. Without any packaging, 

the unprotected aluminum corrodes vary rapidly (see Fig. 

4). A number of backsheet / encapsulant combinations 

were laminated to the aluminized glass and were exposed 

to damp-heat for 700 h. 

Sample Results

LCP Excellent WVTR

NREL PECVD-coated 

PET

Passes initial tape peel 

test; fails tape peel test 

within 500 h. Poor WVTR

properties (i.e., not much 

better than uncoated 

PET); excellent initial peel 

strength; total delamination of backsheet from 

EVA after 25 h exposure 

to damp-heat 

Isovolta commercial 

products 

Laminates with Al foil or 

an SiOx coating have 

maintained intermediate 

peel strength up to 2000 h 

damp-heat exposure; the 

Tedlar-PET-Tedlar construction degrades with 

exposure; Laminate with 

SiOx coating exhibits 4-6 X 

improvement in WVTR

over uncoated PET 

Experimental laminate 

from DuPont Teijin 

Films 

Excellent WVTR results; 

backsheet progressively

delaminates from EVA 

with damp-heat exposure 

(total delamination after 

500 h) 

PECVD-coated PET

from AKT

2 of 10 samples continue 

to pass scotch tape peel 

test after 1850 h dampheat exposure 

PNNL multilayercoated PETPasses scotch tape peel 

test after 634 h damp-heat 

exposure; fails after 967 h; 

multilayer coating does not 

adhere to EVA; excellent 

WVTR

-6

-5

-4

-3

-2

-1

0

1

2

3

0.0027 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.0035

1/T (K-1)

ln(P)

PET; 0.58 PEN; 0.59 PVF-SiOx-PET; 0.63 Coated PET; 0.44 LCP; 0.72 PEN/Al/PET

Material; Activation Energy (eV)

0.01

0.1

1

10

85 60 37.8 20

P (mm-g/

m2/d)

Temperature (°C)

Fig. 3. Arrhenius plot of permeability for backsheets at 

85% relative humidity (RH). 

Samples with a breathable PET backsheet provided 

very good protection of the Al layer (see Fig. 5). Although 

moisture readily passes through the breathable PET backsheet, only slight corrosion is evident after damp-heat exposure. One possible explanation for this result is that the 

aluminum interface is passivated against corrosion by the 

EVA encapsulant. If this is true, then other constructions of 

EVA / Al-glass should not corrode. 

(a) (b) Corroded Al

Uncorroded Al 

Fig. 4. Unprotected aluminized glass substrates (a) as 

deposited and (b) after 19 h exposure at 85°C/85% 

RH.

3
However, substantial bulk corrosion of Al occurred for 

Glass / EVA / Al-glass samples. Moisture ingress occurred 

from the edges on a time scale of about 250 h [1]. A 

“doughnut” configuration was used whereby the glass 

backsheet was laminated to the aluminized glass substrate using EVA along the edges and in the center such 

that there was a ring-shaped area in which there was no 

EVA in contact with the aluminum (see Fig. 6). After 700 h 

of damp-heat exposure, the aluminum in contact with EVA 

corroded, whereas the area of aluminum that was not in 

contact with EVA did not corrode. This suggests that EVA 

does not passivate aluminum, and that corrosion of the 

aluminum may be catalyzed by by-products (possibly acetic acid) within the encapsulant that cannot readily egress 

through the glass backsheet. 

Results for other package combinations include: 

• Samples with the BRP-C encapsulant exhibited 

excellent protection of the aluminum layer, independent of the type of backsheet. 

• Samples with the experimental DuPont Teijin 

backsheet did not exhibit aluminum corrosion, 

even though its adhesion to EVA was severely

compromised by damp-heat exposure. 

• Properly wetted edge seal materials demonstrated an outstanding ability to prevent moisture 

ingress in glass / glass constructions, even when 

a bead-blasted edge-delete existed. 

CONCLUSIONS (a) (b)

Improved packaging materials are required to increase reliability of thin-film PV modules. We have evaluated a large number of backsheet and encapsulant materials in terms of their moisture-barrier properties and their 

ability to maintain good adhesion during damp-heat exposure. Several promising packaging candidates have been 

identified. We have also investigated the effectiveness of 

several combined packaging strategies and constructions 

to provide increased protection of PV modules. Glass / 

glass laminate constructions were often found to trap 

harmful compounds that catalyzed moisture-driven corrosion of aluminum. Constructions with breathable backsheets allow higher rates of moisture ingress, but also 

allow egress of deleterious substances, thereby reducing 

corrosion. 

Fig. 5. PET / EVA / Aluminized glass (a) initial and (b) 

after 700 h exposure at 85°C/85% RH. 

ACKNOWLEDGEMENTS 

Samples of EVA laminated to primed glass substrates 

were prepared at NREL by John Pern [2]. NREL PECVDcoated PET backsheets were prepared by Steve Glick and 

Joe DelCueto. Aluminized glass substrates were prepared 

at NREL by Cheryl Kennedy. This work was performed 

under U.S. Department of Energy Contract No. DE-AC36-

99-G010337. 

(a) (b) REFERENCES 

[1] M. D. Kempe, “Control of Moisture Ingress into Photovoltaic Modules”, Thirty-First IEEE PVSC, 2005, this proceedings. 

[2] J. F. Pern, and G. J. Jorgensen, “Enhanced Adhesion 

of EVA Laminates to Primed Glass Substrates Subjected 

to Damp Heat Exposure”, Thirty-First IEEE PVSC, 2005, 

this proceedings. 

[3] G. D. Barber et al., “New Barrier Coating Materials for 

PV Module Backsheets”, Twenty-Ninth IEEE PVSC, 2002, 

pp. 1541-1544. 

EVA Uncorroded 

Al Fig. 6. Glass / EVA (partial) / Aluminized glass (a) initial 

and (b) after 700 h exposure at 85°C/85% RH. 

4
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13. SUPPLEMENTARY NOTES

A number of candidate alternative encapsulant and soft backsheet materials have been evaluated in terms of their 

suitability for photovoltaic (PV) module packaging applications. Relevant properties, including interfacial adhesion 

and moisture transport, have been measured as a function of damp-heat (85°C / 85% relative humidity) exposure. 

Based on these tests, promising new encapsulants with improved properties have been identified. Backsheets 

prepared by industry and at NREL have been found to provide varying levels of moisture ingress protection. To 

achieve significantly improved products, further development of these candidates is ongoing. The relative 

effectiveness of various packaging strategies to protect PV devices has also been investigated. 

15. SUBJECT TERMS

PV; encapsulant; module reliability; soft backsheet materials; packaging applications; moisture ingress protection; 

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