QA TG5: UV, temperature and humidity

http://pvqataskforceqarating.pbworks.com/ ⇒ goto 5. UV, temperature, and humidity

2013 Thin Film PV Reliability Workshop

Golden, Colorado

Thursday, February 28, 9:40-10:00

Task-Force coordinated by:

Michael Köhl, Fraunhofer ISE, Germany

michael.koehl@ise.fraunhofer.de

Kusato Hirota, Toray Industries, Inc., Japan

Kusato_Hirota@nts.toray.co.jp

Jasbir Bath, SEMA, USA

jasbir_bath@yahoo.com

Presented by David Miller, NREL, David.Miller@nrel.gov

1

NREL/PR-5200-59336

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.
Needs and Approaches (Motivating Questions)

 Service life assessment needs to take UV-degradation seriously into 

account (up to 3000 kWh/m2 in the desert for 25 years)

 Different suitable artificial UV radiation sources are available for ALT 

with varying spectral distribution of the irradiation

 Different spectral sensitivities of the tested materials have to be

expected

 Are comparable tests in different labs possible ?

 Can we accelerate tests by increasing UV intensity?

 Can we accelerate tests by increasing the sample temperature?

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3

Goal and Activities for QA TG5 (UV, T, RH)

•IEC qualification tests (61215, 61646, 61730-2) presently prescribe up to 137 days 

equivalent (IEC 60904-3 AM 1.5) UV-B dose

•Goal develop UV & temperature facilitated test protocol(s) that may be used to assess 

materials, components, and modules relative to a 25 year field deployment.

Core Activities:

1: (weathering and climates... location dependent information)

e.g., known benchmark locations... Miami, FL; Phoenix, AZ

2: (standards from other fields of work) 

summary exists from Kurt Scott et. al.

3: (test conditions) 

4-1 (collect information about observed failure mechanism)

e.g., the literature, site inspections

4-2 (find appropriate models for ALT procedures)

5: (suitable UV sources) 

summary exists from David Burns et. al.

6: (proposal for accelerated service testing) 

7: (laboratory verification of acceleration of proposed test standard/failure mechanism)

Japan mini-module study, Sophia round-robin, Ea interlaboratory study 
4

Overview of the QA TG5-Japan Activities

Objectives:

(1) Develop the procedure for a suitable UV weathering test using mini modules. 

 Factors during the test: irradiation intensity, temperature, humidity 

 Experiment will help determine: test duration + characteristics to measure 

(2) A combination test or a sequential test series (if appropriate) .

 UV weathering + Dynamic Mechanical load test 

 UV weathering + DH Test 

Provisional schedule:

•4 cell mimi-module test 2000 cumulative hours: 2013 June 

• Examination of UV weather resistant test of 1 cell module: 2013 October 

• Examination of a compound or sequential test: 2013 October 

• International proposal for a new comparative UV weathering test system and 

certification including the test of a full-size module, a mini module, and materials:

2014 May. 
UV weathering test of 4-cells small size module QA Task-5 Japan

Irradiance ・・・ 90 W / m2 (UV 300-400nm)

 Nearly 2x UV (ASTM G173 Xenon Lamp)

Chamber temp. ・・・ ・ ・ 65 °C 

Chamber humidity. ・・・ No Control

 ( typical 1 –10%RH)

Test Modules ・・・ 4-cells, polycrystalline Si

Termination ・・・ Open circuit

Backsheet ・・・ Multilayer laminated PET

Encapusulant ・・・ EVA (fast cure) 

 EVA A ・・・ Within the shelf life 

 EVA B ・・・ Over the shelf life

Sample ID and Test sequence

120410-02

120710-01

120710-02 120410-04

120410-03 120710-03

* The front or back side is irradiated 

 Module layout in the UV chamber

X: Thermocouple gage

□:Junction BOX 5
6

0%

20%

40%

60%

80%

100%

120%

Initial UV330h UV660h UV990h UV1314h

Isc Change

120410-02(CH1)

120410-03(CH4)

120710-01(CH2)

120710-02(CH3)

0%

20%

40%

60%

80%

100%

120%

Initial UV330h UV660h UV990h UV1314h

Voc Change

120410-02(CH1)

120410-03(CH4)

120710-01(CH2)

120710-02(CH3)

0%

20%

40%

60%

80%

100%

120%

Initial UV330h UV660h UV990h UV1314h

Pmax Change

120410-02(CH1)

120410-03(CH4)

120710-01(CH2)

120710-02(CH3)

0%

20%

40%

60%

80%

100%

120%

Initial UV330h UV660h UV990h UV1314h

FF Change

120410-02(CH1)

120410-03(CH4)

120710-01(CH2)

120710-02(CH3)

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3

Voltage[V]

Current[A]

Initial

UV330h

UV660h

UV990h

UV1314h

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3

Voltage[V]

Current[A]

Initial

UV330h

UV660h

UV990h

UV1314h

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3

Voltage[V]

Current[A]

Initial

UV330h

UV660h

UV990h

UV1314h

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3

Voltage[V]

Current[A]

Initial

UV330h

UV660h

UV990h

UV1314h

Output power performance QA Task-5 Japan 

120410-02(CH1)

120410-03(CH4)

120710-01(CH2)

Irradiation on Front :990h + on Back :324h

120710-02(CH3)

Isc decreased

 1.5 to 2% approximately

Pmax decreased

 1.5 to 2% approximately

No major performance loss. 

Isc↓ with Pmax ↓ is consistent with encapsulation discoloration.
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Discoloration of the Backsheet QA Task-5 Japan

6

Measurement position

7 Glass

BS

7 6

Measurement position

(Cross sectional view)

Test sequence I :

Front side 990h → + Back side 324h

Slight yellowing of BS was observed.

Yellowing of BS differs on a cell vs. off 

of a cell.

When UV light irradiation was carried 

out on the front side, after irradiation on 

back side, yellowing of the backsheet 

increased significantly. 

Cell → Result: higher temperature on cell? 

Test sequence II :

Back side 330h → + Front side 984h

*7 measured at 

 990hrs,1314 hrs only

off

of cell +UV324h +UV330h +UV660h +UV984h

on cell
UV – Round Robin Light and Back-Sheets

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 Aim: 

Comparison of the effect of different UV- sources on 

glass/encapsulant/backsheet laminates with different materials

 Spectral distribution of different UV-light sources leads to different 

degradation on different materials

 Stronger UV testing needs better definition of the test conditions

Spectra of radiation sources used in PV testing 280 290 300 310 320 330 340 350 360 370 380 390 400

0

50

100

150

200

250

300

350

400

450

500

550

μW/cm2nm

Wavelength in nm

UV-spectrum measured 

on a German mountain (2600m altitude)
UV – Round Robin Samples

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 Samples: 

 manufacturers provide different back-sheet types

 ISE produces laminates (usual glass and EVA, 13x20 cm) 

and 300 sample holders (till end of February)

3 long-pass filters

2 neutral density filters (grids)

Unfiltered area
UV – Round Robin Procedure

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 Time frame: September 2013

 Samples: 

 manufacturers provide different backsheet types 

 ISE produces laminates (usual glass and EVA, 13x20 cm)

 direct radiation on the back side and on the front glazing

 Testing procedure:

 2 temperature levels: 60°C, 80°C (e.g.) (Assessment of 

sample temperatures)

 Irradiation: integral UV dose: min. 120 kWh/m2

 Light sources and (spectral distribution) characterised 

radiometrically (Fluorescence, Metal-halide, Xenon)

 3 longpass and 2 neutral density filters provided by ISE
 UV – Round Robin Procedure

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 Characterisation procedures after 0, 30, 60, 120 kWh (when available):

 Spectral hemispherical reflectance (UV-VIS-NIR)

Calculation of Yellowness Index or adequate degradation indicator

 Raman / Micro-Raman spectroscopy

 FTIR-ATR measurements for BS

Calculation of carbonyl-index

 Optical microscopy/AFM investigation for microcracks in BS

 Fluorescence for encapsulants

 And .....?
 UV – Round Robin Schedule

 Preparation and Testing

Purchasing of components (filters, etc) is finished

Back-sheet materials are collected

Production of Mini-modules and filter-holders in March 2013

Distribution of samples to test labs beginning of April 2013

Testing till August 2013 (at least 120 kW/m2)

intermediadte telecons or meetings at NRELMRW, TC82 WG2 meeting)

 Final characterisation of the samples and evaluation of data by

Fraunhofer ISE August - September 2013

 Final discussion of the results during PVSEC2013 or fall meeting of

TC82 WG2 

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Motivation for the Ea Interlaboratory Experiment

•As in Kempe, “Group 3: Understanding the Temperature and Humidity 

Environment Inside a PV Module ”, knowing Ea is critical to prescribing and 

interpreting a <UV and temperature> mediated test.

•Unfortunately, Ea is not known for the common UV PV degradation modes.

The modified Arrhenius equation





 



 −





 





=

RT

n Ea

e

T

T k A

Critical unknowns 0

(Goals for the interlaboratory experiment):

1. Quantify Ea, so that applied test conditions can be interpreted.

2. Provide a sense of the range of Ea that may be present by examining “known 

bad”, “known good”, and “intermediate” material formulations.

3. Determine if there is significant coupling between relevant aging factors, 

i.e., UV, temperature, and humidity.

What factors does TG5 need to consider?

4. Investigate the spectral requirements for light sources by 

comparing Ea for different sources, i.e., Xe-arc, UVA 340. 

Is visible light required in addition to UV light?
Details of the Ea Test Specimens 14

•(4) custom EVA formulations, (1) TPU product proposed for study.

• EVA to be extruded at NREL; specimens to be laminated at NREL. 

•50x50mm2 quartz/encapsulation/quartz geometry for transmittance.

•Details of adhesion experiment to be determined.

Ingredient Comment Mass {g} Mass {g} Mass {g} Mass {g} 

Elvax PV1400 Dupont EVA resin, 33 wt% VAc 100 100 100 100

Dow Corning Z6030 Silane primer, gama-methacroyloxy propyl trimethoxysilane 0.5 0.5 0.5 0.5

Tinuvin 770 Hindered amine light stabilizer (HALS) 0.13 0.13 0.13 N/A

Tinuvin 123 Non-basic aminoether-hindered amine light stabilizer (NOR-HALS) N/A N/A N/A 0.13

TBEC Curing agent, OO-Tertbutyl-O-(2-ethyl-hexyl)-peroxycarbonate, 

0.133kPa at 20C. N/A 1.5 1.5 1.5

Lupersol 101 Curing agent, 2,5-Bis(tert-butylperoxy)-2,5-dimethylhexane 1.5 N/A N/A N/A

Naugard P Phosphite anti-oxidant (AO) 0.25 0.25 N/A N/A

Tinuvin 328 Benotriazole UV absorber (UVA) N/A N/A N/A 0.3

Cyasorb 531 Benzophenone UV absorber 0.3 0.3 0.3 N/A

Comments "Known bad", 

"slow cure" 

"Intermediate", 

"fast cure"

"Intermediate", 

"fast cure" "Known good"

quartz/EVA/quartz specimen

Kempe et. al., Proc. PVSC 2009, 1826-1831.

Photo of aged PV module

Miller, from APS-STAR site

? ?

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The Ea Interlaboratory Experiment Enables a Wider Range of Study

•Discoloration & adhesion will be studied in detail at different institutions using 

the same make & model of instrument (i.e., Ci5000, QUV).

•This overcomes the difficulty of limitedly-available aging equipment.

•A standard condition (70°C in chamber) allows a broad variety of other 

instruments to also be compared.

•Rate of degradation will be compared against field data to allow site specific 

acceleration factors to be computed.

•Outdoor data should help verify validity of the test.

•Separate experiment at NIST (same EVA’s) will determine action spectrum

LIGHT SOURCE, FILTER No light

field deployment

(outdoors)

UV LIGHT INTENSITY 0 W•m-2

CHAMBER RELATIVE HUMIDITY {%}

match for

"very low" 

(~7%) 25 ambient

CHAMBER TEMEPRATURE {°C} 50 70 90 50 70 70 50 60 70 50 70 90 70 ambient

3M (Ci5000) 3M (Ci5000) 3M (Ci5000) ATLAS (Ci5000) Mitsui(SX120) NREL (Ci5000) CWRU (QUV)ATLAS (UVTEST) QLAB (QUV)

Fraunhofer

(custom)

Fraunhofer

(custom)

Fraunhofer

(custom) NREL ATLAS (EMMA in Phoenix)

QLAB (QSUN XE3) QLAB (QSUN XE3) NREL (XR260) NREL (UV suitcase) CWRU (5x in Cleveland)

ATLAS (SunTest XXL) Fraunhofer (custom) ATLAS (rack in Phoenix)

Suga (SX75) Suga (FDP) ATLAS (rack in Miami

NREL (rack in Golden)

PARTICIPANT 

(INSTRUMENT MODEL)

UVA 340 fluorescent (no filter)

NOMINAL (245.5 W•m-2 for 300≤λ≤400

50 ("high")

Xe Arc (right-light/cira filter)

NOMINAL (92 W•m-2 for 300≤λ≤400)

20 ("low") 50 ("high") ~7% ("very low")

UVA 340 fluorescent (no filter)

NOMINAL (0.92 W•m-2@ 340 nm)

Summary of participating laboratories and test 

conditions
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Degradation Mechanisms for Crystalline Si PV

Failure/degradation mechanisms from the literature†:

• Corrosion of AR coating on glass (Group3/Group 5)

• Corrosion of cells (Group 3/Group 5)

• Corrosion of electrical interconnects (Group 3/Group 5)

• Crazing of glass. Crazing/roughening of front surface (Group3/Group 5)

• Delamination of encapsulation (Group3/Group 5)

• Diode failure during “hot spots” (Group 4)

• Discoloration of encapsulation (Group 5)

• Embrittlement of back sheet (Group 5)

• Embrittlement of encapsulation (Group 5)

• Embrittlement of junction box material and wire insulation (Group 5)

• Fatigue of solder bonds (Group 2)

• Fatigue of interconnects [open circuits/arcing ] (Group 2)

• Fracture of cells (Group 2)

• Fracture of glass/superstrate (Group 2)

• Ground faults (Group3/Group 5)

• Junction box and module connection failures (Group 2)

• Soiling of glass/superstrate (TBD)

• Structural failures (TBD)

† based on Wohlgemuth, “PV Modules: Validating Reliability, Safety and Service Life”, Intersolar 2012 Conf.

*e.g., D. C. Jordan and S. R. Kurtz, “Photovoltaic Degradation Rates—an Analytical Review”, PIP, 21 (1), 2013, pp. 12-29.

Literature*, site inspections, and industry feedback suggest these are most common 
17

The Si PV cell (cross-section)

http://www.viridiansolar.co.uk/Assets/Images/Technical/laminate_small.jpg

http://www.staffordarea.saveyourenergy.org.uk/what/solar/monoPVarray.jpg

Wohlgemuth, “Reliability PV Cells, Modules and Systems”, Tutorial, IEEE PVSC, 2011.

front glass

(could be 

textured, 

with AR)

encapsulation

encapsulation

backsheet

PV cells

also: junction box & diodes; cables & connectors; frame & grounding

string interconnect

interconnect ribbon

(site soldered to 

busbars on cells)

front contact (“gridline”)

n-type Si (could have AR coating)

p-type Si

pn junction

front contact (“bus bar”)

back 

contact

•Si flat-panel PV modules are complex 

devices, containing many components.

•TF modules may not have as many 

layer/components. This simplicity may aid 

reliability. 

•The QA TG’s to date have really only 

considered Si flat-panel PV (consider this 

slide).

Si PV module (front view) The Si PV module laminate (cross-section)

The Components of a Monocrystalline Silicon Module 
Key Differences Between Monocrystalline Silicon & TF PV 

•Device layer may be deposited on superstrate (CdTe, a-Si) or substrate (CIGS).

-UV may be filtered by superstrate devices-

•An edge seal may be present.

•Interconnection accomplished scribing TCO or metal layer 

(vs. gridlines, ribbons, solder joints, etc. in crystalline Si).

•Alternatives to EVA encapsulation.

•Often no backsheet. Maybe glass instead. Maybe different form factor (e.g.,shingles).

Substrate/superstrate may consist of a thin flexible ceramic/polymeric layer 

(laminate or other).

•Different diode protection schemes, with smaller j-box.

•May use adhesive facilitated rails for mounting.

Some differences between Si, CdTe, and CIGS TF device implementations

http://www.advanced-energy.com/upload/Image/Newsletters/2008Q4PVSunTimes/TCO_Order_800x252.jpg 

polymeric materials, subject to UV degradation

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19

Additional Degradation Mechanisms for Thin Film PV

Failure/degradation mechanisms from the literature†:

•Delamination of edge-seal (Group 3/ Group 5/ Group 8)

 -Kempe et. al. have shown coupled (UV, T, & RH) effects-

•Electro-chemical corrosion of TCO – Group 5

•Inadequate Edge Deletion – Group 8

•Light Induced Degradation of a-Si (performance issue only?)

•Shunts at laser scribes – Group 8

•Shunts at impurities in films – Group 8

•Other?

† based on Wohlgemuth, “PV Modules: Validating Reliability, Safety and Service Life”, Intersolar 2012 Conf.

*e.g., D. C. Jordan and S. R. Kurtz, “Photovoltaic Degradation Rates—an Analytical Review”, PIP, 21 (1), 2013, pp. 12-29.

(Substrate not shown)

(Interconnects not shown)
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QA TG5: Discussion for Thin Film PV

TF in General:

•Are there other key components that were not identified?

•Are there other relevant features/considerations for TF? 

Your UV experience:

•What UV facilitated degradation modes have you observed?

•How significant is UV damage to you?

•What UV facilitated degradation modes are the most urgent?

Feedback for QA TG5:

•Are the objectives, activities, & experiments relevant to you?

•Can you help/contribute in the existing QA TG5 groups?

(all of three groups have regular meetings - refer also to 

Europe, Japan, and US points of contact) 

•Where and how significant is UV degradation for TF? 

(How does UV change TG8)?
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Summary of QA TG5 (UV, T, RH)

•Goal develop UV & temperature facilitated test protocol(s) that may be used to assess 

materials, components, and modules relative to a 25 year field deployment.

Round-robin (under Sophia project)

•Emphasis on backsheet materials

•Examination of source (spectral) dependence

Mini-module round-robin (QA Task-5 Japan)

•Examining backsheet and encapsulation

•Apply a combination or series of aging plus dynamic mechanical or DH tests? 

Ea interlaboratory study

•Examining discoloration and delamination of encapsulation

•Quantify coupled and (irradiation) source dependent effects

Application to TF PV:

•Significant differences in components/materials between c-Si and TF PV

•What are your experiences? How much is UV relevant?

•Can you contribute to QA TG5 (UV, T, and RH)?

•What is unique to QA TG8 (TF PV)?