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

Module Encapsulation 

Materials, Processing 

and Testing 

John Pern, Ph.D. 

National Center for Photovoltaics (NCPV)

National Renewable Energy Laboratory (NREL)

Golden, Colorado, USA

APP International PV Reliability Workshop

Dec. 4-5, 2008, SJTU, Shanghai, China

NREL/PR-520-44666
Outline

• Encapsulation Components (Commercial Products)

– Superstrates: Glass or Tefzel/Tedlar

– Encapsulants (EVA and Non-EVA - TPU, PVB, ... etc.)

– Substrates: Back Foils (or Backsheets) or Glass

– Edge Seals

– Materials Properties

• Processing

– Typical module constructions

– Module Lamination – Curing Process

• Materials-Level Testing

– Optical, Electrical, Mechanical

– Photothermal and damp heat tests

• Field-Degraded Modules (Photos)

– Effects of materials and encapsulation quality

• Conclusions
PV Module Production Line
Encapsulation:

Manufacturing Bottleneck in (c-Si) PV Module 

Fabrication Process

Importance of PV Module Packaging --

• High module reliability for 20-30 year service life

•“Packaging is the predominant cause of failure in 

modules” – remark of a DOE SETP PV Program reviewer, 2006
Typical PV Module Encapsulation Configurations Typical PV Module Encapsulation Configurations

I. Crystalline Si-based Module

Superstrate (Glass or Polymer Film)

C-Si Cell

Substrate (Polymer Film or Glass)

C-Si Cell

Common feature: 

Glass/EVA/c-Si Cells/EVA/backfoil

Backfoil selection:

TPT: Tedlar/PET/Tedlar 

TPE: Tedlar/PET/EVA 

PET: Polyester

EVA

EVA

Tab 

ribbon

II. Superstrate-Deposited Thin Film Module

Superstrate (TCO Glass)

Thin Film Solar Cell Array Connector 

Ribbon

EVA

Substrate (Polymer Film or Glass)

III. Substrate-Deposited Thin Film Module

Polymeric Support Base if SS Foil

Thin Film Solar Cell Array

Connector 

Ribbon

EVA

Substrate (TCO Glass or SS Foil)

Superstrate (Glass or Polymer Film)
Elements for Making Good 

Encapsulation of (c-Si) PV Modules

• Good (right) glass super-/substrate 

• High-performance encapsulant

• Good (right) backfoil or substrate 

• Good (correct) lamination process 

with proper handling of stack and 

temperature-pressure-time (T-P-t) 

profile

• Good edge seal if the design needs
• Polymeric Encapsulants (Pottants):

Most critical element in module encapsulation and reliability

– EVA (most commonly used and cheapest; the only field- proven over 20 years)

– Non-EVA 

• TPU

• PVB

• Silicones

• Silicone/PU hybrid 

• Ionomer

• Other new polymers

– UV-Curable Resin

• Edge Sealants (for Al-framed c-Si or thin-film modules)

– Polybutyl

– Silicones

– Desiccant-type

– PIB-type

Module Encapsulation Materials - 1
Module Encapsulation Materials - 2

Superstrate:

• Glass

– Low-Iron

– Tempered

– Plain or Textured

– UV filtering (Ce- glass)

– SiO2 AR Coatings

• Fluoropolymer

– Tefzel

– Tedlar

– THV220 

(to replace EVA/Tefzel)

Substrate:

• Polymer Multi-laminates

(Backsheet; Backfoil)

– Tedlar-based:

• TPT: Tedlar/PET/Tedlar

• TPE: Tedlar/PET/EVA

• TAT: Tedlar/Al foil/EVA

• TPAT: Tedlar/PET/Al foil/Tedlar

• TPOT: Tedlar/PET/Oxide/Tedlar

• PAP: PEN/Al foil/PET

– PET or PEN-based (to replace expensive 

Tedlar) 

• Protekt

• Teijin Teonex

• BaSO4-filled PET

• Glass
Primary Functions and Requirements 

of Encapsulant

Functions:

• Optical coupling –

refractive index (n) matching

• Electrical Insulation –

dielectric strength and volume 

resistivity

• Mechanical support – fixation of cells and adhesion 

strength

• Physical insulation –

separate cells & cell strings

• Physical protection –

from weathering-induced and 

environmental damages

• Thermal conduction

Requirements:

• High T% 

• Matching R.I. (n)

• High dielectric breakdown

• High volume resistivity

• High adhesion strength

• Mechanically strong, 

resistant to break or tear

• Low moisture absorption

• High resistance to UV- induced yellow-browning

• High resistance to UV and 

moisture-induced 

delamination
Select Encapsulant

• Based on Module Design and Construction

• Cost Consideration

• Processing Equipment, Method, Conditions

• Materials: EVA, PVB, TPU, Silicone, Ionomer, UV-curable resin,.. 

• Tests:

– Film transmission before and after processing or testing

• UV & Heat - induced yellowing (photothermal stability)

• damp heat and thermal cycle – induced yellowing

– Proper processing conditions (T-P-t profile) with your 

laminator

• Curing degree & gel% (EVA)

– Adhesion strength (e.g., 90o or 180o peel, or lap-shear test) 

• Initial (e.g., EVA to glass, Tedlar, or PET)

• thermal cycle

• humidity freeze

• damp heat 

– Electrical insulation (e.g., volume resistivity, breakdown V)

– Mechanical strength (tensile)
Select Glass 

Superstrate

• Low-Iron 

• Tempered (c-Si PV)

• Type:

– Plain or Textured

– UV-transmitting

– CeOx UV-Filtering

– SiO2 AR Coating

• Test T% and degree of

solarization

• Affect photo-(UV) stability 

of encapsulant 

• Correctly use the non- float (non-tinned) side

• Cleaning - affects 

adhesion (delamination)

0

10

20

30

40

50

60

70

80

90

100

250 300 350 400 450

Wavelength (nm)

Transmittance (%)

Borosilicate-t0

AFGKK-t0

Optiwhite-t0

Starphire-t0

Solarphire-t0

Solarphire-t1 (FSSS 49.5h)

Transmittance Spectra of Some 1/8"-Thick Glass Superstrates

50 %T UV Filtering

0

10

20

30

40

50

60

70

80

90

100

250 350 450 550 650 750 850 950 1050 1150 1250 1350 1450

Wavelength (nm)

Transmittance (%)

Solarphire-t0

Solarphire-t1 (FSSS 49.5h)

Solarization of CeOx - containing Solarphire Glass upon FSSS Exposure

0 h

49.5 h

UV-filtering CeOxglasses show 2.1~3.6% 

loss in T% (300-1500nm) 

due to solarization
Select Backfoil (Backsheet)

• Type (more commonly used) –

– TPT-primed: Tedlar/PET/Tedlar

– TPE: Tedlar/PET/EVA (low VA%)

– PET-based (polyester, primed or corona-treated)

– PEN-based

• Cost consideration

• Tests –

– adhesion strength with encapsulant 

– electrical insulation

– mechanical strength

– moisture-blocking (WVTR) 

– weathering durability

Madico’s TPT
Select Edge Sealant

• Primary function: to block moisture/water ingress

• Use depends on module design/construction

• Al-framed c-Si modules:

– Polybutyl (“hot butyl”)

– Silicones (needs to know moisture-blocking property)

– PIB-type (maybe the best)

– Others (“U”-shaped rubber tapes)

• Thin-Film CdTe and CIGS (glass/glass):

– Desiccant-type tapes (mechanical and adhesion strength 

may be weak)

• Conduct immersion/hot-pot leaking tests
An Example showing Various Tests Conducted to 

Determine Materials Properties and Processing

Material T% Gel Adhesion Strength

(400-1000 nm) (%) (N/mm) (90o

 Peel) T (o

C) RH (%) (g/m2

-day)

Tefzel (1.5 mil) ~94 20 85 1.80

Tefzel(1.5 mil)/EVA cured 38 82 6.55

Tefzel (5 mil) 87-92 7.60E+16

Cured Film Fast-Cured Uncured Cured To Plain KK glass

China EVA-1 91 ± 1 95 (deep texture) 2.3~5.8E+14

China EVA-2 91 ± 1 84 2.2E+14 1.1E+14

China EVA-3 91 ± 1 >90 8.8E+13

Europe EVA 91 ± 1 86

Japan EVA-1 91 ± 1 94 1.1E+14 1.2E+14 9~12

Japan EVA-2 91 ± 1 86 (deep texture) 3.4E+14 10~14

NREL EVAs 91 ± 1 88 0.6~5.5E+14 0.2~1.4E+16 9~12

US EVA-1 91 ± 1 88 0.8~1.1E+14 0.7~7.0E+14 9~10.5 20 88 7.02

38 82 28.45

US PVB 91 ± 1 0 4.4E+12 Glass/Glass only 39 100 33.36

Japan PVB 91 ± 1 0 1.7E+12 Glass/Glass only 39 100 40.05

Europe PVB 91 ± 1 0 8.4E+12 Glass/Glass only

US TPU 90 ± 2 0 6.4E+13

US TPU 90 ± 2 0 7.3E+14

Europe TPU 90 ± 2 0 1.1E+12

TAT (Tedlar/Al/Tedlar) 4.31E+14 20 87 0.10

85 100 0.83

TPT-primed 2.7~3.5E+15 ~ 4 - 9 (to EVA) 20 84 0.89

(EVA formulation dependent) 83 100 142.77

TPE Type 1.1~3.3E+16 > 12 (break up) 20 83 0.63

85 100 94.39

Teonex Q65F (PEN) 5.5E+16 28 100 1.04

85 100 35.24

Protekt HD 1.0E+17 40 100 3.20

85 100 60.02

Volume Resistivity Water Vapor Transmission Rate

(ohm-cm)
Encapsulation Process --

Double-Bag Vacuum Lamination (+ Oven) 

Superstrate (Glass or Polymer Film)

C-Si Cell

Substrate (Polymer Film or Glass)

C-Si Cell

Process with EVA:

One-Step, One (Two)-Temp: Lamination and 

Curing in the laminator (Fast-cure EVA)

Two-Step, One (Two)-Temp: Lamination in 

laminator, Cure in Oven (Slow-cure EVA)

=> No Industry-wide Standard!

High-performance vacuum 

laminator

• Use high-speed pump-down to ensure 

good vacuum before EVA is melted

• Ensure proper pressing of the module 

stack

• Optimize temperature-pressure-time 

(T-P-t) profile that is materials, 

formulation, and system-dependent

Typical Conditions for EVA: 

• Lamination:110o – 120oC for 4~10 min 

• Curing at 140o – 150oC for ~6 to 30 

min (depending on EVA formulation 

and process)

• Gel ≥ 80%

PVB and TPU: Roll-press possible and reworkable
Testing vs. Performance Reliability

Diurnal and Seasonal Temperature Variation; 

Rain/Snow/Hail; Air pollutants

Potential Degradation: Optical, 

Electrical, and/or Mechanical 

(+ Delamination) 

EVA Degradation: (Photo-discoloration vs. 

Photo-bleaching: O2 diffusion limited; 

acetic acid generation);

Moisture Ingress: Delamination, corrosion, 

current leakage, T% loss
Equipment for Accelerated Exposure Tests: 

Solar simulators, Weatherometers, Damp Heat Chamber

Xe arc lamps for better simulation of solar spectrum
EVA Yellowing Rate is affected by 

Formulation, Processing, UV Filtering, and 

Air Permeability of Superstrate

0

2

4

6

8

10

12

0 200 400 600 800 1000 1200

Exposure Time (h)

Net Yellowness Index Change (YI)

Discoloration of EVA Laminated with Various Superstrates

Starphire (UV)/ 

A9918 (slow cure)

Starphire (UV)/ 

15295 (fast cure)

Boro/ 

A9918

Ce-glass/ 

EVA

Tefzel/EVA

Laminates: 

Glass/EVA/Glass

Tefzel/EVA/Glass

Exposure: 

Full Spectrum Solar Simulator

1. Slow-Cure > Fast 

Cure

2. UV-transmitting 

glass >> UVfiltering (Ce glass)

3. No yellowing with 

air-permeable 

Tefzel 

(photobleaching)

4. Competition 

between photodiscoloration 

(yellowing) and 

Photo-bleaching
Accelerated Life 

Test (ALT) for 

Photothermal 

Stability of EVA

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200 1400

FS-SS Exposure Time (h)

Net Yellowness Index Change (YI)

(A new lamp was in use after 532 h)

Full Sectrum Solar Simulator Exposure

Laminates: (1/8") Borosilicate/EVA/Boro;

Exposure: Full-spectrum solar simulator 

at average ~6 UV suns and BPT~80-85o

C

EVA - 2

EVA - 1

EVA - 3

0

2

4

6

8

10

12

0 1000 2000 3000 4000 5000 6000

WOM Exposure Time (h)

Net Yellowness Index Change (YI)

EVA-2

EVA - 1

EVA - 3

Laminates: (1/8") Borosilicate/EVA/Boro;

Atlas Ci4000 Weatherometer (WOM) at 2.5 

UV suns (300-400 nm), 60o

C Chamber, 60% 

RH, 92-100o

C Black Panel Temp (BPT)

Atlas Ci4000 Weatherometer Exposure

EVA Yellowing Rate is:

1. dependant on product 

formulation

2. higher under solar 

simulator (~7 UV suns) 

than in weatherometer 

(2.5 UV suns)

3. different in two test 

methods, but the trend 

remains unchanged 

=> Shorter test time with 

greater light intensity
Photothermal 

Stability ALT of 

PVB and TPU

1. Both PVB and TPU 

showed much less UVinduced yellowing than 

EVA

2. PVB can be used only on 

glass/glass laminates

3. Delamination of TPU from 

UV-transmitter glass can 

be serious depending on 

product quality

4. Adhesion of TPU to glass 

can be largely degraded 

by damp heat exposure

0

1

2

3

4

5

6

7

8

9

0 200 400 600 800 1000 1200

Exposure Time (h)

Yellowness Index Change (YI)

Photothermal Stability of PVB and EVA Laminates

US EVA

Japan EVA

US

Japan

Japan 

PVB

Laminates: Starphire/PVB (or EVA)/Starphire

Exposure: FS-SS

PVB

Starphire: UVtransmitting glass

(delaminating)

0

2

4

6

8

10

12

14

16

0 1000 2000 3000 4000 5000 6000

Exposure Time (h)

Yellowness Index (YI)

Discoloration of Glass/EVA or TPU/Glass Laminates 

Exposure: Ci4000 Weatherometer

I(300-400 nm): 2.5 UV suns 

Chamber: 60o

C, RH: 60%

BPT: 92-95o

C

Glass:

Starphire: UV transmitting

KK Ce-glass: UV filtering

Boro/US 

EVA /Boro

KK/EVA/KK

Starphire/Europe 

TPU/Starphire

KK/US TPU-2/KK

Boro/ US 

TPUs
Mechanical 

Tests

PC-controlled 

Instron 

Mechanical Tester

Tensile Test 90o Peel Test T-Peel Test
EVA Adhesion 

Strength affected 

by Processing 

Condition

5

6

7

8

9

10

125 130 135 140 145 150 155 160

Cure Temp (oC)/8 min

Time-Weighted PS (N/mm)

50

55

60

65

70

75

80

85

90

95

100

Gel Content (%)

US EVA, DH ~520h

Effect of Gel% on Peel Strength upon DH Exposure

and by Damp Heat 

Exposure Time 5

6

7

8

9

10

11

12

13

0 100 200 300 400 500 600

Damp Heat Exposure Time (h)

90-degree Peel Strength (N/mm)

Peel Strength of Different EVA upon DH Exposure

NREL EVA

US EVA

Laminates: AFG KK Glass/EVA/TPE
Some Photos of Field Modules

• Degraded by 

– Yellow-browning

– Backsheet blistering & delamination

– Corrosion by moisture ingress

– Glass cracking by stress (glass/glass)

– Other factors
EVA Browning in Field PV Modules

1990 EVA Browning Crisis: Severe EVA browning on mirror-enhanced PV arrays at Carrisa PV 

Power Plant, CA. Annual Power Output degraded by >45% from 1986-1990 (original: ~6 MW)
Large T% Loss and 

so Power Loss due 

to EVA Browning
EPMA Composition Analysis for the New and Exposed Tab Ribbons

Auger Depth Profile Analysis

Sample Source

PV Module No.

New Tab

Carrizo 1B

Carrizo 309259

Carrizo 309262

Carrizo 309262

Solar Cell

Location

L9, R2

L9, R2

L9, R2

L7, R3

Side of

Tab Ribbon

EVA

EVA

EVA

EVA

EVA

Color

Light yellow

Brown

Dark brown

Dark brown

Pb-M

39.52

10.96

41.13

15.91

46.56

Sn-L

60.48

68.57

 5.80

30.60

 4.62

Cu-L

 5.35

13.27

20.41

 8.08

O-K

15.12

39.80

33.08

40.74

0

20

40

60

Depth (nm)

80

100

0 50 100 150 200 250 300 350 400

(a) New Tab Ribbon 

(Pb-Sn coated Cu foil)

Sn

Pb

C

Cu O

0

20

40

60

Depth (nm)

80

100

0 200 400 600 800 1000

Sn

Pb

C

Cu

Concentration (at. %) Concentration (at. %)

P110-A048804

(b) Carrizo No. 309262

O

Corroded Pb-Sn Alloyed Cu Tab Ribbons by 

Acetic Acid from Browned EVA
Irregular PVB Browning at ASU
Blistering & Delamination 

of Backfoils
Water Ingress and Corrosion
More Degraded 

Modules
Thin Film 

Modules at 

NREL OTF 
Field-Degraded Thin 

Film Modules

(photos: PowerLight)
Conclusions

• Proper selection and initial tests of encapsulation 

materials are important.

• Different encapsulant formulations (e.g., EVA) give 

different quality and performance.

• Encapsulation method and processing conditions 

can affect the laminate quality and reliability of PV 

modules.

• Adequate accelerated exposure tests can be useful 

to assess the performance expectation of materials 

and quality of processed components.

• Overall module reliability is determined by all 

component materials and processing factors.