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Testing Protocol for Module 

Encapsulant Creep

National Renewable Energy Laboratory – Photovoltaic Module 

Reliability Workshop

NREL-PVMRW

Michael D. Kempe, David C. Miller, John H. Wohlgemuth, Sarah R. Kurtz, 

John M. Moseley, Qurat (Annie) Shah, Govindasamy Tamizhmani, 

Keiichiro Sakurai, Masanao Inoue, Takuya Doi, and Atsushi Masuda

February 29, 2012

NREL/PR-5200-54583
2

Background, Concerns and Objectives

• Background:

o Creep is the permanent deformation of a solid material under 

the influence of mechanical stresses.

o PV manufacturers are using thermoplastic materials.

o Qualification tests only test to 850C whereas modules can reach 

1050C outdoors, though only for a short time.

• Concerns:

o Live components may be exposed.

o Cells, tabbing, busbars, and etc. may be stressed and broken.

o Internal short circuits may be created.

• Objectives:

o Evaluate the potential for creep in outdoor exposure.

o Provide guidance for the risks and for the design needs with 

thermoplastic materials.

o Provide a basis for modifying standards to account for materials 

with the potential to creep
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Outline

• Experimental Materials Used

• Outdoor Exposure Results

• Indoor Exposure

• Conclusions
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Eight Representative Encapsulants Studied
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Thin Film Mock Modules

(1) 3.18 mm TCO glass with edge delete

(2) Encapsulant

(3) 3.18 mm back glass with through hole for electrical contact to TCO

(4) Black Paint, thermocouples and rails on back

(5) 2.5 cm fiberglass matte insulation, 46.5 m2K/W2 (R 6.7)

(6) 2.5 cm polyisocyanurate sheating foam insulation board, 45.1 m2K/W2 (R 6.6)

(7) 1.3 cm plywood back
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Thin Film Mock Modules

(1) 3.18 mm TCO glass with edge delete

(2) Encapsulant

(3) 3.18 mm back glass with through hole for electrical contact to TCO

(4) Black Paint, thermocouples and rails on back

(5) 2.5 cm fiberglass matte insulation, 46.5 m2K/W2 (R 6.7)

(6) 2.5 cm polyisocyanurate sheating foam insulation board, 45.1 m2K/W2 (R 6.6)

(7) 1.3 cm plywood back
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Thin Film Mock Modules

(1) 3.18 mm TCO glass with edge delete

(2) Encapsulant

(3) 3.18 mm back glass with through hole for electrical contact to TCO

(4) Black Paint, thermocouples and rails on back

(5) 2.5 cm fiberglass matte insulation, 46.5 m2K/W2 (R 6.7)

(6) 2.5 cm polyisocyanurate sheating foam insulation board, 45.1 m2K/W2 (R 6.6)

(7) 1.3 cm plywood back
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Crystalline Silicon Module Setup

(1) 3.18 mm glass

(2) Encapsulant

(3) UMG polycrystalline Si Solar Cells

(4) Encapsulant

(5) PVF/PET/PVF backsheet

(6) 9 cm Fiberglass matte insulation, 104 m2K/W2 (R 15)

(7) 1.3 cm plywood
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Deployed in Arizona Summer 2011

• Modules mounted in Mesa Arizona from May to 

September, 2011.

• Array oriented at a 330 tilt and an azimuth of 

2550 south so that the array more directly faced 

the sun at the hottest part of the day.

• A single no-cure-EVA mock module was 

deployed in Golden Colorado.
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Only the No-Cure-EVA Module Crept Significantly

Creep in Arizona

Creep in Colorado
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Minor Creep in TPO-3 and TPO-1

PDMS

0.005

TPO-3

0.090

NC-EVA 

(Arizona)

3.0

PVB

0.016

TPU

0.005

EVA

-0.013

TPO-1

0.032

TPO-4

0.004

NC-EVA 

(Colorado)

0.49

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

Final Lab Measured Creep (mm)
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Outdoor Results Summary

• Crystalline Si Modules

o No Signs of Creep

o All Passed Wet High Pot

o Only TPO-4 showed performance loss attributable to 

cell breakage. Probably from the lamination process.

• Thin Fim Mock Modules

o The NC-EVA Module Crept 3 mm.

o The NC-EVA appears to be crosslinking as it ages.

o The TPO-1, and TPO-3 crept 32 and 90 microns, 

respectively.

o All Passed the Wet High Pot Test.
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Step Stress Test Parallels Outdoor Data

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

65 70 75 80 85 90 95 100 105 110 115 120 125 130 135

Creep (mm)

Last Temperature (0C)

Total Creep of Mock Modules, 200 h Step Stress 

NC-EVA TPU

TPO-3 TPO-4

TPO-1 EVA

PVB PDMS

The next temperature 

step resulted 

in > 1 cm of movement.
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Step Stress Test Parallels Outdoor Data

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

4

65 70 75 80 85 90 95 100 105 110 115 120 125 130 135

Creep (mm)

Last Temperature (0C)

Total Creep of Mock Modules, 200 h Step Stress 

NC-EVA TPU

TPO-3 TPO-4

TPO-1 EVA

PVB PDMS

The next temperature 

step resulted 

in > 1 cm of movement.

DMA Crossover (δ=450, and G’=G”) Temperatures

690C

790C

1050C

1150C 1200C
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TPU Thin Film Mock Module Formed Bubbles Upon Heating

TPU after 1000C

No Creep

Pass Wet Hi-Pot
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TPU Thin Film Mock Module Formed Bubbles Upon Heating

TPU after 1000C

No Creep

Pass Wet Hi-Pot

TPU After 1050C

No Creep

Pass Wet Hi-Pot
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TPU Thin Film Mock Module Formed Bubbles Upon Heating

TPU after 1000C

No Creep

Pass Wet Hi-Pot

TPU After 1050C

No Creep

Pass Wet Hi-Pot

TPU after 1100C

0.023 mm Creep

Pass Wet Hi-Pot
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TPU Thin Film Mock Module Formed Bubbles Upon Heating

TPU after 1000C

No Creep

Pass Wet Hi-Pot

TPU after 1100C

0.023 mm Creep

Pass Wet Hi-Pot

TPU after 1200C

0.482 mm Creep

Pass Wet Hi-Pot
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TPU Thin Film Mock Module Formed Bubbles Upon Heating

TPU after 1000C

No Creep

Pass Wet Hi-Pot

TPU after 1100C

0.023 mm Creep

Pass Wet Hi-Pot

TPU after 1300C

>1 cm Creep

Fail Wet Hi-Pot
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NC-EVA Did not Form Bubbles

NC-EVA Mock Module 

After 800C Exposure.

TPU Mock Module 

After 1300C exposure.

Glass

Displacement
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Some Creep in NC-EVA in Chamber at 850C

Before Exposure

After 850C Step Stress

NC-EVA Crystalline Si Module (Electroluminescence)

Gap

No Gap

However, there was no performance loss.
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Temperature Non-Uniformity Decreases Creep

Up to a ~150C temperature variation was seen.
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X-Si Modules Show Similar Thermal Gradients

Despite reaching very high temperatures around 1020C, the module tabbing and 

backsheet were able to prevent large cell movements.

Strings were connected vertically, if it had been mounted with horizontal strings, 

cells in the center may have been more likely to move.
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Conclusions

• Even without any peroxide for curing, NC-EVA, 

modules are not likely to creep significantly in 

most environments and mounting configurations.

• A Creep evaluation test should account for the 

possibility of polymer chain scission or crosslinking.

• Thermal non-uniformities dramatically reduce the 

propensity for creepage.

• The current proposal for IEC 61730 part 1, is to 

expose all modules for 200 h to a temperature 

between 100 and 110 0C.
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Acknowledgements

• Adam Stokes, Alain Blosse, Ann Norris, Bernd Koll, Bret 

Adams, Casimir Kotarba (Chad), Crystal Vanderpan, David 

Trudel, Dylan Nobles , Ed Gelek, Greg Perrin, Hirofumi 

Zenkoh, James Galica, Jayesh Bokria, John Pern, Jose 

Cano, Kartheek Koka, Keith Emergy, Kent Terwilliger, 

Kolakonu, Mowafak Aljasim, Nick Powell, Niki Nickel, 

Pedro Gonzales, Peter Hacke, Ryan Smith, Ryan Tucker, 

Sam Samuels, Steve Glick, Steve Rummel, Tsuyoshi 

Shioda, and Yamamichi Masaaki

• This work was supported by the U.S. Department of 

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

National Renewable Energy Laboratory.