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Adhesion and Thin-Film 

Module Reliability 

Preprint 

T.J. McMahon and G.J. Jorgensen 

Presented at the 2006 IEEE 4th World Conference on 

Photovoltaic Energy Conversion (WCPEC-4) 

Waikoloa, Hawaii 

May 7–12, 2006 

Conference Paper 

NREL/CP-520-39732 

May 2006 
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ADHESION and THIN-FILM MODULE RELIABILITY* 

T.J. McMahon and G.J. Jorgensen

National Renewable Energy Laboratory, Golden, CO 80401 

ABSTRACT 

Among the infrequently measured but essential 

properties for thin-film (T-F) module reliability are the 

interlayer adhesion and cohesion within a layer. These 

can be cell contact layers to glass, contact layers to the 

semiconductor, encapsulant to cell, glass, or backsheet, 

etc. We use an Instron mechanical testing unit to measure 

peel strengths at 90° or 180° and, in some cases, a 

scratch and tape pull test to evaluate inter-cell layer 

adhesion strengths. 

We present peel strength data for test specimens 

laminated from the three T-F technologies, before and 

after damp heat, and in one instance at elevated 

temperatures. On laminated T-F cell samples, failure can 

occur uniformly at any one of the many interfaces, or nonuniformly across the peel area at more than one interface. 

Some peel strengths are << 1 N/mm. This is far below the 

normal ethylene vinyl acetate/glass interface values of >10 

N/mm. We measure a wide range of adhesion strengths 

and suggest that adhesion measured under higher 

temperature and relative humidity conditions is more 

relevant for module reliability. 

INTRODUCTION

Interlayer adhesion and cohesion within a layer 

can be a problem for thin-film (T-F) module reliability. To 

avoid delamination we must understand the many 

materials and interfaces that comprise a T-F module and 

how they respond to environmental stresses. A few years 

ago SnO2-to-glass delamination caused significant 

problems; A recurring problem is the “bubble” type of 

delamination found in some double-glass-laminated T-F 

modules, both shown in Fig. 1 [1]. The SnO2 delamination 

failures are caused by multiple stress factors that involve 

water vapor and ionic currents flowing through the sodalime glass or on its surface. A solution was found, and 

screening tests were developed for SnO2-coated glass 

[2,3]. The “bubble” type of delamination is caused by a 

reduction of the ethylene vinyl acetate’s (EVA) viscosity 

and adhesion at higher field temperatures along with 

tensile stress due to glass warpage. Encapsulant to 

backsheet or cell layers, backsheets, scribeline vias, 

layers within the cell, etc. are all subject to delamination 

failure. 

Sometimes solutions rely on reducing the internal 

stress in the T-F layer, for example, Mo on glass used for 

CIGS module construction [4]. As another example, we 

found that for an oxide barrier coating on polyethylene 

terephthalate (PET) to survive lamination without cracking, 

the internal stress in the coating must be below 5 x 109

Ncm-2 [5]. 

 

Fig. 1. "Bar-graph" delamination of SnO2/a-Si cells, left; 

“bubble” delamination, right[1] 

More recently, we have measured and tried to 

optimize the interfacial adhesion between soda-lime glass 

substrates and the most commonly used encapsulant material, EVA. Although initial adhesion values were 

important, we were looking for maximum adhesion after 

damp heat at 85°C/85% relative humidity (RH) exposure 

based on earlier work at the Jet Propulsion Laboratory [6]. 

At NREL, a similar effort to optimize the wet resistant 

strength of the interface between EVA and a smooth glass 

interface has recently been completed [7]. Specialized 

Technology Resources (STR) has reported on a recently 

developed EVA-based encapsulant to bond to Si cells that 

doesn’t degrade with damp heat [8]. Adhesion to glass 

surfaces that have been mechanically abraded, for the 

purpose of electrical isolation, is extremely important to 

achieve high module reliability. In one study we found 

loss in adhesion after damp heat to be greater for 

mechanically abraded surfaces than for smooth glass [9]. 

 One of the strategies for obtaining good adhesion 

to polymer surfaces as well as to back metal contacts is to 

deposit oxide barrier coatings before lamination. These 

polymers can be metal-impregnated back metal contacts, 

screen-printed grid-line contacts, back sheet materials 

such as PET, or possibly transparent front sheets of 

Tefzel. Barrier layers on PET can greatly reduce water 

vapor transmission and increase adhesion to EVA; peel 

strength values that were < 1 N/mm before coating 

increased to >10 N/mm after coating [4]. We use this 

oxide coating to facilitate our testing of one T-F cell 

interface. Oxide coatings can also enable us to produce a 

more durable module package. 

Peel strengths depend on the length of the 

ambient dry-out period after removal from the 85° C/85% 

*This work has been authored by an employee or employees of the Midwest Research Institute under Contract No. DE-AC36-99GO10337 with the U.S. 

Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United 

States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow 

others to do so, for United States Government purposes. 

1
RH exposure. Because hydrolysis weakens interfacal Si-O 

bonds [6], peel strength values measured immediately 

after removal from damp heat can be half as great as 

values after a 3-h dry out. Values measured after several 

days can show complete healing of a coating/PET 

interface to the pre-damp-heat condition. Increasing the 

temperature during peel significantly reduces adhesion. 

We suggest that adhesion measured under higher 

temperature and RH conditions is more relevant for 

module reliability. 

 PEEL TESTS

Table 1. Peel strengths for many sample types and 

interfaces.

Weathering 

Device: 

Source 

Failure 

Interface Time Type 

Peel 

Strength 

(N/mm) 

a-Si: EVA / a-Si 0 none 3.8 

A EVA / a-Si 92 h 85/85 3.1 

SiOxNy / EVA / SiOxNy 0 none 5.6 

 Ni paste/ 

Graphite/ 

CdTe: B 

SiOxNy / Ni 

paste 256 h 85/85 4.5 

EVA / SiOxNy 0 none 7.0 

SiOxNy / CdTe 0 none 2.0 

SiOxNy / 

CdTe: B 

SiOxNy / CdTe 256 h 85/85 0.3 

SiOxNy / EVA / SiOxNy 0 none 6.4 

CdTe: C 

EVA / SiOxNy 256 85/85 4.9 

CdTe: C 

EVA / MetalCdTe 0 none 1.1 

CdTe: C 

EVA / MetalCdTe 256 85/85 0.6 

EVA / MetalCdTe 0 none 1.0 

EVA / MetalCdTe 0 none 0.7 

CdTe: C 

(left) 

 (middle) 

 (right) Metal / CdTe 0 none 0.06 

CIGS:D1 CIGS / Mo 0 none 0.05 

CIGS:D2 

60°C 

80°C 

EVA / CIGS 

EVA / CIGS 

EVA cohesive 

0 

0 

0 

none 

none 

none 

7.0 

1.1 

0.05 

CIGS: E EVA / CIGS 0 none 0.9 

CIGS CIGS / Mo 0 none 0.3 

CIGS EVA / CIGS 258 h 85/85 0.6 

CIGS Tefzel / EVA 258 h 85/85 0.02 

CIGS 

Stainless / 

EVA 258 h 85/85 0.5 

CIGS EVA / CIGS 7 mo 

Cocoa, 

FL 0.8 

CIGS EVA / CIGS 16 mo 

Golden, 

CO 0.9 

CIGS 

Stainless / 

EVA 16 mo 

Golden, 

CO 0.8 

TPE/EVA/ EVA / Glass 0 none 5.5 

Glass: F 

control EVA / Glass 16 h 85/85 3.8 

EVA / Glass 

7 yr 

equal UV lamp 2.0 

TPE/EVA/ 

Glass: F 

exposed EVA / Glass + 16 h 85/85 1.5 

Tedlar/EVA EVA / Glass 0 none 3.2 

/ Glass: G 

control EVA / Glass 17 h 85/85 1.8 

EVA / Glass 

7 yr 

equal UV lamp 0.4 

Tedlar/EVA

/ Glass: G 

exposed EVA / Glass + 17 h 85/85 0.4 

Scotch 

Tape/Glass Tape / Glass 0 none 0.08 

ASTM 

Tape/Glass Tape / Glass 0 none 0.4 

 Table 1 shows peel strength adhesion values 

measured for a number of T-F module types from different 

sources (A thru E) and, for reference purposes, on other 

types of Si modules/samples (F, G, and 2-tape pulls), with 

and without 85° C/85% RH damp heat weathering. An 

example of such a T-F peel sample (CdTe:B) is shown in 

Fig. 2. The corresponding peel data from the right hand 

peel tab are shown in Fig. 3. The pull rate was 10 mm/min 

with data taken before and after 256 h of damp heat. 

 Tedlar/PET/EVA back sheet material from 

Madico is laminated to the module or mini-module with 

one sheet of STR’s 15295 P-UF EVA. Typically, 25 mmwide peel tab strips are defined with a razor blade and, in 

this case, mounted in the Instron for a 90° pull. Samples 

from B had low adhesion of the EVA to the back contact 

Ni-paste ( < 1 N/mm), so we deposited a SiOxNy film to 

increase the EVA/SiOxNy/Ni-paste interface strength. 

Because this is still the weakest interface, now failing at 

5.6 N/mm, we can say that all of the other cell adhesive 

and cohesive strengths are greater than 5.6 N/mm. 

 The Ni-paste contact area is shown on the right 

hand side of Fig. 2. After damp heat, the failure interface 

moves from the EVA/SiOxNy to the SiOxNy/Ni-paste with 

only small bits of the cell cracking from the SnO2. On the 

left side of Fig. 2, the cell always remains in contact with 

the SnO2: the dark areas are where the barrier coating 

remains on the CdTe, and the grey areas are where the 

coating comes off with the EVA. 

 

Fig. 2. Oxide-coated CdTe:B superstrate cell after 90°-

peel testing. Left, without a back contact; right, with a 

graphite/Ni paste back contact.

 

0

1

2

3

4

5

6

7

8

0 5 10 15 20 25 30 35 40

Peel Extension (mm)

After 256 h

Damp Heat

Before

Damp Heat

Fig. 3. Instron peel strength data in N/mm for the right 

hand side of the SiOxNy-coated CdTe:B pictured in Fig. 2. 

2
 

Fig. 4. Instron peels for the edge seal regions of the two 

Si-modules listed from sources F and G. The Al-channel 

frame is removed and never blocked much of this edgeseal area from UV exposure. 

 Similarly, for other T-F peel strengths where 

failure is not within the cell stack, e.g. a-Si:A, the lower 

limit of the cell adhesive and cohesive values is the value 

cited in the right hand column of Table 1, e.g. > 3.8 N/mm. 

The remaining T-F sample from source E was a flexible 

module package. Peel strengths for most of the interfaces 

in front of and behind the CIGS cell are shown in Table 1 

before and after damp heat and field exposures as noted. 

This package was never intended for the rigorous 

requirements of the power market; the peel strength 

values are somewhat lower than found for other cells. 

 Fig. 4 shows the pull tabs on two of the four Simodules measured for comparison purposes. Two sets of 

peel-strength values are entered in the table for each of 

the “F” and “G” module brands. Values measured for the 

two brands of aged (7-year UV equivalent) Si-modules at 

the EVA/glass interface near the edge are 2.0 and 0.4 

N/mm. These are down more than 60% from 5.5 and 3 

N/mm measured on the unexposed control modules; these 

are to be compared to typical EVA/glass values of > 10 

N/mm. Results after additional damp heat exposure for 

each of the four modules are listed as well. 

 A final but important point to make is the effect 

that the measurement temperature has on adhesion. The 

values measured in the edge seal region of modules “F” 

and “G” after UV (2.0 and 0.4 N/mm) will decrease even 

further with increasing temperature. Anyone who has deencapsulated a module with a heat-gun knows this. 

Because neither “F” or “G” would fit in the oven that is 

used with the Instron, we had to confine our temperaturedependent studies to the smaller CIGS:D2 sample. It had 

the room temperature value of 7.0 N/mm. As the table 

indicates, at T = 60°C the pull strength falls to 1.0 N/mm 

and at 80°C the EVA fails cohesively at 0.05 N/mm. This 

loss of adhesion and viscosity results in the bubble-type of 

failure shown in Fig. 1. Applying this train of thought to the 

modules shown in Fig. 4, any remaining strength left in the 

edge-seal region of “F” or especially “G” must be very 

small. We discuss this point later.

 

 

Fig. 5. ASTM scratch test images (1.4 mm wide) for four 

samples: top right, a-Si:A; top left, CdTe:B; bottom-left, 

CIGS:D2(>7 N/mm); bottom-right CIGS:D1(0.05 N/mm). 

 SCRATCH ADHESION TESTS 

 To measure cell layer adhesion properties only, 

without the complications of attaching pull-tabs and 

sometimes having to ascribe lower limit values as above, 

another, easier-to-do, adhesion scratch test can be 

performed. Initially developed for the paint industry, the 

ASTM D 3359-02 “Measuring Adhesion by Tape Test” was 

applied to all the cells tested. The test requires that a 

lattice pattern be scratched into the coated surface with six 

or seven lines in each direction. Any loose fragments are 

brushed away, the ASTM qualified tape is pressed firmly 

to that area, and, within 30 to 90 s, the tape is pulled back 

at a 180° angle. The last entry in Table 1 shows the 

ASTM-designated tape would exert a force of 0.4 N/mm 

on any cell material disturbed or loosened by the scratch 

tool. Some close-up after-test images of single, 

representative crossings, are shown in Fig. 5. We now 

correlate these images with results in Table 1. 

 The top-left image is of the a-Si:A that had a peel 

strength of 3.8 N/mm at the EVA/cell interface. It was the 

least affected by this test and, although we could only 

establish a lower limit value of 3.8 N/mm, it very likely has 

the highest overall adhesive and cohesive strength. The 

top-right image is of the CdTe:B that had a peel strength 

of 5.6 N/mm at the EVA/cell interface. These scratchfollowed-by-tape-pull results show virtually no removal of 

cell material in the area adjacent to the actual scratch 

mark. 

 The bottom-left image, D2, is from a commercial 

product with a peel strength > 7 N/mm, and the bottomright image, D1, is from a “reject” low adhesion test 

sample with only 0.05 N/mm. These side-by-side images 

reflect this difference, albeit in a non-quantifiable way. At 

the left, a fairly uniform but limited amount of cell material 

is fractured by the scratch mark. At the right, a large and 

non-uniform amount of cell material is fractured from the 

surface adjacent to the scratch marks. Referring to Table 

1, the failure interface is between the CIGS/Mo-interface. 

3
Adhesion at this interface was the focus of an earlier study 

that showed a sensitivty to the Se-processing [10]. 

 Within a given technology, it could be useful to 

apply the ASTM D 3359-02 test procedure to screen for 

cell-layer adhesion strength. It is testing adhesion in a 

very different way from the pull-testing that we usually do 

in our laboratory. Depending on the types of stress in the 

field, one or the other may be more valid. 

DISCUSSION 

Many field failures can be attributed to packaging 

deficiencies as shown in Fig. 1 and discussed in [1]. 

When damage does occur, it usually happens at elevated 

stress levels. In this paper, we have shown how interfacial 

adhesion values can vary within the layers of a T-F 

module. Large differences are found that depend on the 

source, technology, amount of exposure to stress, and 

measurement conditions (temperature and RH). 

Therefore, when we screen encapsulant materials, we 

should measure relevant properties under those higher 

stress conditions to avoid packaging-related failure. 

Specifying a minimum strength, which would 

insure a durable module, is problematic. It is unlikely that 

one value will be adequate for all technologies and for 

each interface therein. For example, modules like “F” and 

“G” have framed edges that mechanically hold the 

encapsulated module together at the edges so that when 

high temperature and RH occur in the field, the edge 

remains intact. The situation for a frameless or doubleglass laminate will require more strength. 

A minimal strength limit near the center of a 

module may not be adequate for the edge seal border 

region. Corrosion protection and reduction of interfacial 

water ingress may be more important considerations for 

requiring the highest possible adhesion strength[11]. 

Finally, when EVA is used as an encapsulant, as 

the softening temperature at 85°C is approached and 

exceeded, cohesive failure and viscous flow can occur if a 

tensile or shear stress is present. Mechanical stress in a 

double-glass laminated module can cause bubble-type 

delaminations or a compromised edge seal. 

CONCLUSIONS 

We presented peel strength values for the 

weakest interface of various T-F module technologies and 

measured how they are affected by environmental stress; 

some were quite low. 

 We showed that measurement at higher 

temperature and RH, and after extended UV and RH 

exposure reduce EVA’s adhesion strength compared with 

measurement before exposure and under normal 

laboratory conditions. If minimal guidelines were to be set 

for adhesion, they ought to include the effects of higher 

temperature and RH. An added problem associated with 

EVA is a softening near 85 °C which can also lead to 

failure. 

 Our data show that measured inter-layer 

adhesion within the T-F cell can be quite small. Within a 

technology the ASTM D 3359-02 “Measuring Adhesion by 

Tape Test” is useful as a screening test for cell adhesion. 

 While no minimum adhesion strength could be 

determined from mechanical considerations, others have 

shown that strong adhesion reduces both corrosion and 

interfacial water ingress, thus contributing to a durable 

module package in these additional ways. 

ACKNOWLEDGEMENTS 

 We thank K. Barth for the CdTe:B samples, 

allowing us to discuss and identify his prototype cell, and 

suggesting the ASTM D 3359-02 testing procedure. We 

are purposely vague about the other sources, but our 

thanks go out to them as well. The SiOxNy coating was 

deposited by S. Glick and J. Pankow of NREL. This work 

was funded by the U.S. Department of Energy under 

Contract Number DE-AC-36-99G010337. 

REFERENCES 

[1] T.J. McMahon, “Accelerated testing and failure of 

thin-film modules,” Prog. Photovolt: Res. Appl. 12(2004), 

p.234-248. And references therein. 

[2] K.W. Jansen and A.E. Delahoy, "A laboratory 

technique for the evaluation of electrochemical transparent 

conductive oxide delamination from glass substrates", 

Thin Solid Films 423 (2003) 153. 

[3] D.E. Carlson, R. Romero, Willing F, Meakin D, 

Gonzalez L, Murphy R, Moutinho HR, Al-Jassim M. 

Corrosion effects in thin-film photovoltaic modules. 

Progress in Photovoltaics 2003; 11: 377–386. 

[4] R. Wieting, R. Gay, H. Nguyen, J. Palm, C. 

Rischmiller, A. Seapan, D. Tarrant, and D. Willett, “CIS 

Thin-Film Manufacturing at Shell Solar: Practical 

Techniques in Volume Manufacturing,” Proc of the 31st 

IEEE PVSC Jan 2005, Orlando, FL, p. 177. 

[5] G. Barber, G. Jorgensen, K. Terwilliger, J. Pern, 

S. Glick, and T.J. McMahon, "New barrier coating 

materials for PV module backsheets", Proc. of the 29th 

IEEE PV Spec. Conf., New Orleans, LA, p. 1541 (2002). 

[6] E.P. Pleuddmann, Silane Coupling Agents, Chap. 

5, Plenum, New York, 1991. 

[7] F.J. Pern and G.J. Jorgensen, “Development of 

Damp-Heat Resistant Self-Primed EVA and Non-EVA 

Encapsulant Formulations at NREL,” Solar Rev Mtg., Nov 

2005, Denver, CO. 

 [8] R. Tucker, Solar Rev Mtg., Nov 2005, Denver, 

CO. 

 [9] T.J. McMahon and G.J. Jorgensen, “Electrical 

currents and adhesion of edge deleted regions of EVA-toglass module packaging,” Proc. of the 2001 NCPV 

Program Review Mtg., Lakewood, CO, Oct 2001; pp 137-

138. 

[10] Tarrant, D. E.; Gay, R. R. (1995). Research on 

High-Efficiency, Large-Area CuInSe2-Based Thin-Film 

Modules: Final Subcontract Report, 16 August 1993 - 30 

June 1995. 99 pp.; NREL Report No. TP-413-8121. 

(83014.PDF) 

[11] J. Tonge, DOW Chemical Corporation. 

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