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Module Technology: 

Current Practice and Issues

MRS Workshop on 

Photovoltaic 

Materials and 

Manufacturing Issues

John Wohlgemuth

5 October 2010, 

Denver, CO

NREL/PR-5200-49293
NATIONAL RENEWABLE ENERGY LABORATORY

PURPOSE OF MODULE PACKAGE

•Mechanical support – hold the cells in place pointing 

toward the sun.

•Dielectric protection – keep high voltage away from 

people and keep current from flowing out of the array 

circuit (to ground or in a loop).

•Protect the cells, diodes and interconnects from the 

weather (UV, rain, humidity, hail etc.)

•To couple as much light energy as possible into the 

solar cells (at all angles at the wavelengths that the cell 

can utilize).

•To minimize the temperature increase of the cells.

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Module Package Should

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•Be qualified to IEC 61215 or 61646.

•Be safety certified to UL 1703 if being sold in US.

•Be safety certified to IEC 61730 parts 1 and 2.

•Have at least a Class C fire rating if it is going to be 

used on a building.

•Carry an extended warranty. (25 years is typical for 

crystalline silicon – would expect at least 20 years for a 

thin film product)
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TYPICAL CONFIGURATIONS

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Glass Superstrate

o Glass/encapsulant/cells/encapsulant/backsheet

o Glass/thin film cells/encapsulant/backsheet

o Glass/encapsulant/thin film cells/glass or other 

substrate

Flexible 

o Transparent frontsheet/encapsulant/thin film 

cells/flexible substrate
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Glass/encapsulant/cells/encapsulant/backsheet

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GLASS

EVA CELLS

BACKSHEET
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Glass/thin film cells/encapsulant/backsheet

Glass

Backsheet

Thin Film

PV

Encapsulant

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Glass/encapsulant/thin film cells/substrate

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Glass

Substrate

Encapsulant

Thin Film PV
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Transparent frontsheet/encapsulant/thin film 

cells/flexible substrate

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Transparent 

Frontsheet

Encapsulant

Flexible Substrate

Thin Film PV
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Typical component selection for Cr-Si

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•Superstrate – Glass

•Substrate or Backsheet – Tedlar and /or PET 

•Encapsulant – EVA 

• (also PVB, TPU and Ionomers)

•Edge Seal – usually none
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Typical components for Thin Films

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•Superstrate – Glass

•Substrate - Glass

•Encapsulant – EVA 

• (also PVB, TPU and Ionomers) 

•Edge Seal – PIB (polyisobutylene)with 

additives
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Status of Packaging Technologies – Cr Si

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• More than 20 years of field experience with the 

present package.

• Modules easily pass the IEC 61215 Qualification Test

• EVA discoloration from 1990’s appears to have been 

solved, but must be revisited every time a new EVA 

formulation or supplier appears.

• Adhesion of encapsulant to glass and backsheet 

appears adequate for 25 year life as long as processing 

is under control.

• Annual module degradation rates of < 0.5% per year 

are being reported.
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Status of Packaging Technologies – Thin Films

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• Edge seals appear to do a good job of keeping 

moisture away from sensitive thin film layers.

• Most modules pass the IEC 61646 Qualification Test

• Recent thin film modules appear to suffer much less 

degradation than earlier types.

• Many thin film modules are now sold with a 25 year 

power warranty, matching today’s crystalline silicon 

modules.
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Issues

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• Cost – We would like to reduce module cost 

so lower encapsulant material cost and 

process costs would be an advantage.

• Minimize degradation rate – Understand why 

today’s modules are degrading and reduce or 

eliminate the degradation.

• Lifetime – Why stop at 25 years? Can 

modules be built to survive 40 or even 50 

years?
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Evaluating Cr-Si for Improving Performance

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• With Cr-Si most known failure mechanisms are evaluated using the 

accelerated tests within the qualification sequence.

• To assess long term performance the test durations must be increased 

considerably.

• When that is done, good quality crystalline Si modules can survive10 

times as many thermal cycles as the qualification test calls for with 

minimum degradation.

•UV testing using the methodology developed by STR to study EVA 

discoloration shows that today’s formulations do not suffer discoloration or 

optical loss over levels that STR estimated was equivalent to 25 years of 

outdoor exposure.

• On the other hand after 2 to 3 times as much damp heat exposure, EVA 

encapsulated Cr-Si modules with breathable backsheet start to suffer 

severe degradation of fill factor and output power.

• So it appears that moisture induced degradation is likely to be one of the 

mechanism that leads to module failure and may be a major culprit in the 

power degradation observed in today’s modules.
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Long Term Thermal Cycle Testing

200 Cycles = 10 Years in Golden, CO

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Thermal 

Cycles

Module A

Pmax (W)

Module A

% Change 

in Pmax

Module B

Pmax (W)

Module B

% Change 

in Pmax

Module C

Pmax (W)

Module C

% Change 

in Pmax

Start 165.3 159.5 161.6

500 161.3 -2.49% 158.1 -0.9% 160.6 -0.6%

1000 163.1 -1.3% 159.1 -0.3% 160.7 -0.5%

1500 158.9 -3.9% 157.3 -1.4% 159.3 -1.4%
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Damp Heat Induced Degradation 

Based on 20 Years = 1000 hours at 85 C/85% RH 

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-80

-70

-60

-50

-40

-30

-20

-10

0

0 20 40 60 80 100 120

Projected Years of Operation

Power Loss (%)
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Extending Test Duration for Cr-Si Modules Qualified to 

IEC 61215 

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Manufacturer DMLC/TC/HF 1250 Hr DH 500 TC

A Pass Fail Fail

B Pass Fail Pass

C Pass Fail Fail

D Pass Fail Pass

E Pass Fail Pass

F Pass Pass Pass

G Pass Fail Pass

H Pass Pass Pass

J Pass Fail Pass

K Pass Pass Pass
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Cr-SI Performance – Replacing EVA

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• We can replace EVA with alternate encapsulant like silicone, 

urethane, or Ionomer. This has its own issues:

• Cost –most cost more than EVA

• Long term reliability – do we trade one problem for another?

• Degradation rate – Will the change improve the overall degradation 

rate?

• Some preliminary data indicates that with use of silicone 

encapsulants Cr-Si modules do not suffer long term 

degradation in damp heat.

• The main issue with using a new encapsulant is the lack of long 

term field data and its correlation with the accelerated tests.

• Without the field data we can never be 100% sure that the new 

material will work better (or even match) the performance of the 

EVA.
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Cr-SI Performance – Improving EVA

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• An alternate route to improving lifetime and reducing 

degradation rates is to figure out why/how the cells 

degrade in EVA after long term damp heat and then to 

alleviate the problem.

• Historically degradation of cells in EVA has been 

attributed to generation of acetic acid and its attack upon 

the cell metallization system. But there are 2 problems with 

this explanation:

1. There is no visual evidence of corrosion of the front grid lines after 

long term damp heat testing even though fill factors are reduced.

2. Experiments from 1980’s indicated that neither acetic acid nor 

acetic acid vapors attacked the grid lines on solar cells.
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Cr-SI Performance Improvements

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We have 2 possible routes to improve the package

– Replace EVA with an encapsulant that allows the module to 

survive long term damp heat and then see if these modules 

actually perform better in the field.

– Determine the damp heat degradation mechanism in EVA 

modules and develop improvements to the system to stop the 

degradation.

Today we should be trying both because it is not clear 

which will lead to longer module lifetime and low 

degradation rates.
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Issues for Newer Technologies

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• What about those PV technologies that do not 

have 20 or 25 years of field experience?

• Does passing the qualification test prove that 

the modules will survive for 20 or 25 years?

• No, because we do not know what failure 

mechanisms are likely to occur in these 

products over this time period. 

• Without knowing the failure mechanisms it is 

not possible to identify the stress tests 

necessary to evaluate the product.
NATIONAL RENEWABLE ENERGY LABORATORY

Polyisobutylene Edge Seals - Slow Ingress of Moisture

Thanks to Mike Kempee - NREL

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PIB #1

PIB #2

Ca Detector Exposed to 85 C/85% RH

0 h 163 h 652 h 1230 h

0 h 1490 h 2780 h 4664 h

Delaminations

Reactions
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Evaluating Thin Films for Improving Performance

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So the major issues with today’s thin film products are:

– What is the expected lifetime?

– What degradation rates will the modules experience?

– Will the edge seals survive for 25 years and continue to provide 

the necessary protection to the moisture sensitive components

TF needs: 

1. Data on what failure mechanisms are occurring. 

This is exactly the type of information that is not being supplied by 

the large commercial thin film module manufacturers!

2. Accelerated test procedures that adequately stress edge 

seals.

Do we have to develop these tests or can we borrow them from a 

related industry (for example from IGUs)?
NATIONAL RENEWABLE ENERGY LABORATORY

SUMMARY

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• As the volume of the PV market grows the issues of 

module lifetime, failure rates and degradation rates become 

more important.

• These issues represent financial risk to PV investors.

• Quantifying the risk levels and implementing solutions to 

improve the performance are key to the overall commercial 

success of PV.