NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Predicting the Performance of 

Edge Seal Materials for PV

National Renewable Energy Laboratory – Photovoltaic 

Module Reliability Workshop

NREL-PVMRW

Michael Kempe 

Dhananjay Panchagade

Arrelaine Dameron

Matthew Reese

March 1, 2012 NREL/PR-5200-54582
2

Edge Seals - Introduction

•Many PV technologies are sensitive to moisture. 

Even with impermeable front- and back-sheets, 

moisture can penetrate from the sides. Edge 

seals are incorporated around the perimeter to 

prevent this ingress.

•Here we use a Ca-based method to evaluate the 

moisture ingress time for edge seal materials.

•Then we use this data to model the performance 

when deployed outdoors.

2

Seal Encapsulant

Glass

Glass

H2O

w
3

Outline

• Ca film method for moisture ingress 

determination.

• Finite element modeling of moisture 

ingress.

• Investigation of failure modes.

–Edge Pinch

–UV Light

–Heat and Humidity

3
4

Test Sample Designed to Mimic Module Edge

4

Seal Encapsulant

Glass

Glass

H2O

w

50 mm

Glass (3.18 mm)

Polymer Film (~0.5 mm)

Ca (100 nm)

Glass (3.18 mm)

H2O

Ca + 2 H20 → Ca(OH)2 + H2

Module Edge

Test Sample
5

Oxidation of Ca Indicates Moisture Ingress

5

 Ca + 2 H20 → Ca(OH)2 + H2

Mirror-Like → Transparent

→
6

Moisture Ingress Varies Greatly in Encapsulants

6

 

PDMS 

 

Ionomer

#1

Exposed to 

85°C and 

85% RH 

0 h 1.5 h 3 h 4.5 h

0 h 67 h 240 h 652 h

50 mm

50 mm
7

Polyisobutylene Edge Seals Slow Ingress

7

 

PIB #1 

 

PIB #2

Exposed to 

85°C and 

85% RH 

0 h 163 h 652 h 1230 h

0 h 1490 h 2780 h 4664 h

50 mm

50 mm

Delaminations

Reactions
8

Moisture Ingress Rate Governed by Diffusion

8
9

Moisture Ingress Rate Governed by Diffusion

9

( ) ( ) ( ) C (Calcium) X

D t C C C C

X

D t C P

m n

P

m n

P

m n

P

m n

P

m n

P

m n  −



 





∆

∆

+ + + + − ∆

∆

=

+ − + −

+

2 1, 1, , 1 , 1 2 ,

1

, 1 4

H2O

H2O
10

Permeation Measured at Low RH

10
11

Low RH Measurements Reduce Extrapolation Errors

0

10

20

30

40

50

60

70

80

90

100

0

102030405060708090

100

Relative Humidity (%)

Temperature (0C)

 

 

500-550

450-500

400-450

350-400

300-350

250-300

200-250

150-200

100-150

50-100

0-50

Hours/Year

Bangkok Thailand RH and Temperature for outside of a Glass/Glass Rack Mounted Module
12

Edge Seal Modeling

• The use of fillers, pigments, and desiccants makes the 

determination of modeling parameters much more difficult.

12

100%

RH% S S e kT

Ea

m o

s 



 





−

=





 



 −

=

kT

Ea

eff o

D

D D e

Mobile phase water absorption is split between the 

polymer matrix and the mineral components.

Assume linearity with relative humidity.

Mobile phase water diffusivity is an effective 

diffusivity. This accounts for a rapid equilibration 

between adsorbed and dissolved water.

A non-reversible reaction with water that 

immobilizes the water.

RH2O
13

Getting the Modeling Parameters

13

RH2O

Measured by weighing samples before humidity exposure, after 

humidity exposure, and after drying.

o EaS S ,

Measured by exposing to controlled humidity then drying in a 

TGA to determine moisture loss. 

Curvature of K vs %RH is determined by the ratio of S to RH2O

Do EaD ,

Estimate from other parameters and fit to Ca data. Specifically 

the difference between 45 and 850C curves.
14

0

0.02

0.04

0.06

0.08

0.1

0 1 2 3 4 5 6 7 8 9 10 11 12

Ingress Depth (cm)

Time (months)

 

Insulated Back, Glass/Polymer

Close Roof, Glass/Glass

Open Rack, glass/glass

Open Rack, Glass/Polymer

Ingress Estimated Using Finite Element Analysis

14

Denver Colorado

Used TMY3 Data and Temperature estimates similar to King et al, and Kurtz et al.

X = K t
15

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8 10

Ingress Depth (cm)

Time (Years)

 

Insulated Back, Glass/Polymer

Close Roof, Glass/Glass

Open Rack, glass/glass

Open Rack, Glass/Polymer

Square Root Relation Works to Longer Times

15

Denver Colorado

Used TMY3 Data and Temperature estimates similar to King et al, and Kurtz et al.

X = K t
16

Preliminary Results for Different Climates

16

A sensitivity analysis gave about ±15% on K and Width, and ±30% on 20 yr equivalent time.
17

Edge Seal Failure Modes and Stresses

• Heat.

• Humidity (85C/85% RH).

• Adhesion to edge delete region.

• UV Light.

• Edge Pinch

17
18

Laser Edge Delete Did Not Increase Ingress

18

0.1

1

10 100 1000 10000

Distance (cm)

Time (h)

 

Edge Deleted Sides

Non-Deleted Sides
19

Edge-Seals May Have Edge Pinch

19

Edge Pinch

(lamination pressure 

cause the glass to bend 

around the perimeter)

Large tensile stress in 

polymer

Idea Edge Profile

(no bend in glass at the 

module perimeter)

Very little stress in polymer

Glass

Encapsulant

/Edge Seal

Glass

Glass

Encapsulant

/Edge Seal

Glass

Schematic side views of module edge
20

Edge Seal Test Specimen

20

0.30 mm of 

edge pinch

0.2 mm thick polymer

Schematic side view of test sample

Glass (3.18 mm)

Ca film (100 nm)

Edge Seal

Glass (3.18 mm)

Photographic top view

0.5 mm thick polymer

100 mm

50 mm
21

Edge Seals With Pinch Resist 85°C and 85% RH

21

No Exposure 170 h 85°C/85% RH 674 h 85°C/85% RH

Only small signs of minor delamination on ends exposed to tensile stress. 

Edge pinch is 0.31±0.01 mm for all exposures.
22

UV Light Can Delaminate Edge Seals With Pinch 

22

No Exposure

0.32±0.01 mm pinch

621 h

60°C/60% RH/2.5 UV Suns

0.02±0.01 mm pinch

165 h 

60°C/60% RH/ 2.5 UV Suns

0.02±0.01 mm pinch
23

UV Light Can Delaminate Edge Seals With Pinch 

23

No Exposure

0.32±0.01 mm pinch

621 h

60°C/60% RH/2.5 UV Suns

0.02±0.01 mm pinch

165 h 

60°C/60% RH/ 2.5 UV Suns

0.02±0.01 mm pinch

Light exposure on non-Ca film backside.

Very significant delamination on ends exposed to tensile stress.

Polymer 

pulled 

away 

from edge

Delamination

on front side

Intact Ca Film
24

UV Light Alone is Much Less Damaging

24

50 mm 50 mm

Unexposed 1962 h 60°C/60%RH/2.5 UV suns

PIB #2

PIB #1
25

IEC TC82 WG2 Edge Seal Standards Development

• Under IEC TC82 WG2 a group has formed to 

work on developing standard test methods 

for testing PV packaging materials.

– Encapsulants

– Back Sheets and Front Sheets

– Adhesives

– Edge Seals/Pottants

• If you would like to help with the edge seal 

standards development, please contact me.

25
26

What edge seal parameters are important?

1. Adhesion is the most important parameter.

a) Must be maintained after environmental exposure.

b) Residual stress in glass will affect adhesion.

c) Material may expand as it absorbs water.

d) Good surface preparation is necessary.

2. Breakthrough time is the next most important.

a) The 12 mm edge delete perimeter should be wide enough 

to keep moisture out.

3. Module mounting configuration is not important.

a) Hotter installations tend to dry out the module partially 

countering the effects of increased diffusivity.

4. The steady state transmission is less important. 

a) The amount of permeate is very low.

b) Ideally one will not reach steady state.

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27

Conclusions

• An edge seal width of 1 cm can be 

capable of keeping moisture out for 20 

years in almost any climate.

• Delamination is the main concern for 

edge seal performance.

• Edge Seals should be assembled without 

edge pinch to ensure good adhesion.
28

Acknowledgements

Sarah Kurtz

John Wohlgemuth

David Miller

Joshua Martin

This work was supported by the U.S. 

Department of Energy under Contract No. 

DE-AC36-08-GO28308 with the National 

Renewable Energy Laboratory.