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.

Defining a Technical Basis for Confidence in 

PV Investments – A Pathway to Service Life 

Prediction

Sarah Kurtz, John 

Wohlgemuth, Mike Kempe, 

Nick Bosco, Peter Hacke, 

Dirk Jordan, David Miller, 

and others from the 

community

IEEE Reliability Symposium

Phoenix, Arizona

September 17, 2013

NREL/PR-5200-60554
 NATIONAL RENEWABLE ENERGY LABORATORY

Outline

• Motivation: Importance of Reliability to PV Today

• Challenge: Squeezing 25 years into 3 months!!! 

• Qualification vs Qualification “plus” vs Comparative vs 

Quantitative

• What can we learn from history?

• What’s been demonstrated

• Room for improvement

• Proposal for Qualification “plus” testing:

• Additional tests

• More thorough testing

• Required quality program

• Comparative Testing (Climate/Application Specific)

• Quantitative Service Life Predictions

2
PV Industry has Grown to be Huge!

3

Area of Si passes 

microelectronics 

Tons of Si passes 

microelectronics 
Reliability Optimization is an 

Important Element of Growth

PV is now approaching ~$100 Billion/y

Reliability is key to continued natural growth
Growth of PV Industry Requires Reliability

Continued growth 

will require high 

reliability
Accelerating 25 y into 3 months is 

like hatching a chick in 6 hours!

Some processes cannot be accelerated quantitatively > 10X
Timeline Challenge

3 mo 6 mo 1 y 2 y 3 y

Common product development cycle today

Qualification test can be completed

Preferred quality management testing – the shorter the better

25-year service life test with X10 acceleration

Material testing for Qualification “plus” 

Module testing for Qualification “plus”
Timeline Challenge

3 mo 6 mo 1 y 2 y 3 y

Common product development cycle today

Qualification test can be completed

Preferred quality management testing

X10 acceleration to simulate 25 y

X100 acceleration to simulate 25 y

Some processes can be accelerated; 

For others, too much acceleration answers the wrong question!
Some Levels of Accelerated Testing

Qualification Qualification

“Plus” Comparative Service Life

Purpose

Minimum 

design 

qualification

Enhanced

design 

qualification

Comparison of 

products

Substantiation 

of warranty

Quantification Pass/fail Pass/fail Relative Absolute

Climate or 

application

(mounting)

Not 

differentiated

Not 

differentiated Differentiated Differentiated

Specificity Silicon, thinfilm, CPV

For today, 

discuss Si only

Package 

specific?

Product 

specific

Chamber test 

times

Modules: ~ 6 

weeks

Modules: ~ 3 

months

Materials: ~ 6 

months

TBD 3 years ?
 NATIONAL RENEWABLE ENERGY LABORATORY

Standard tests for qualifying modules – a starting point!

10

• Qualification tests were initially developed in 

1970s-1980s

• Today we have:

• IEC 61215 (silicon) or 

• IEC 61646 (thin film) or

• IEC 62108 (CPV).

• When carefully implemented, these have had 

great success for avoiding infant mortality in 

most PV systems!
 NATIONAL RENEWABLE ENERGY LABORATORY

Why we need to test “beyond” the standard

11

• Despite PV’s excellent success, some failures are 

seen in the field.

• New system designs cause new failures.

• Market is expanding into broader applications 

(climates) 

• Low prices motivate manufacturers to cut corners –

might the corner be cut too short?
 NATIONAL RENEWABLE ENERGY LABORATORY 12

Look first at what we’re seeing in the field:

A data set summarizing > 1 GW

We correlate field experience and results from 

accelerated testing to choose new tests
 NATIONAL RENEWABLE ENERGY LABORATORY

Return rates:

• 0.44% (for 3,400,000 

front-contact silicon 

modules with average 

age of 4.9 y)

• 0.005% (for > 8,000,000 

back-contact silicon 

modules with average 

age of 2.1 years)

Represents data for > 1 GW fleet

Most common failures – be sure to address these

Presented at 39th PVSC
 NATIONAL RENEWABLE ENERGY LABORATORY

Failures of internal electrical circuit

14

Mfg A: 

0.3% 

failure rate

Mfg B: 

1.5% 

failure 

rate

Mfg C: 

2.9% 

failure 

rate

Front

Back

Mfg E: 

0.1% 

failure 

rate

Front

Back

Degraaff

NREL Reliability 

Workshop, 2011

Note: 

• Manufacturers 

differ by > 10X

• Current IEC tests 

not designed to 

identify 1% failure 

rates 
 NATIONAL RENEWABLE ENERGY LABORATORY

Summary of Selected Field Studies

• Field failures differ, but some themes include:

– Thermal-fatigue-induced broken ribbons and solder bonds

– Encapsulant discoloration and delamination

– Diode or junction box issues

Kurtz, et al.

39th PVSC 2013
Wear-out mechanisms vary

Multi-Si Thin-film

-1

0

1

2

3

4

Degradation Rate (%/year)

Pmax Isc Voc FF

Label

42

IV Parameter

Pmax Isc Voc FF

36 31 33

Degradation Rate (%/year)

Pmax Isc Voc FF

-1

0

1

2

3

4

Data

Pmax Rd+ Isc Rd+ Voc Rd+ FF Rd+

Label

476

IV Parameter

Pmax Isc Voc FF

475 473 397

Degradation Rate (%/year)

0.0

0.5

1.0

1.5

2.0

2.5

0 10 20

Current (A)

Voltage (V)

10/18/2000 - 12 pm

10/15/2010 - 12 pm

Pmax correlated to Isc loss for Si Pmax correlated to FF loss for thin film

Jordan, 27th Eu. 

PVSEC, 2012
Some Levels of Accelerated Testing

Qualification Qualification

Plus Comparative Service Life

Purpose

Minimum 

design 

qualification

Enhanced

design 

qualification

Comparison of 

products

Substantiation 

of warranty

Quantification Pass/fail Pass/fail Relative Absolute

Climate or 

application

(mounting)

Not 

differentiated

Not 

differentiated Differentiated Differentiated

Specificity Silicon, thinfilm, CPV

For today, 

discuss Si only

Package 

specific?

Product 

specific

Chamber test 

times

Modules: ~ 6 

weeks

Modules: ~ 3 

months

Materials: ~ 6 

months

TBD 3 years ?
Testing beyond current IEC tests

• Ed. 3 of IEC 61215 will have multiple 

revisions/additions

– These will become available to the public over the 

next ~2 years

– Can we adopt improvements sooner?

• The California Energy Commission (CEC) 

has given us the opportunity to define an 

optional “plus” version of their Eligibility 

Requirements

• Next slides describe the “plus” requirements
 NATIONAL RENEWABLE ENERGY LABORATORY

Proposal to test “beyond” the standard

19

• Despite PV’s excellent success, some failures are seen 

in the field.

• Propose 7 new tests

• Low prices motivate manufacturers to cut corners –

might the corner be cut too short?

• Propose more frequent/extensive testing

• Propose oversight of quality management system

• Tests will continue to evolve
 NATIONAL RENEWABLE ENERGY LABORATORY

Proposal to test “beyond” the standard

20

• Despite PV’s excellent success, some failures are seen 

in the field.

• Propose 7 new tests

• Low prices motivate manufacturers to cut corners –

might the corner be cut too short?

• Propose more frequent/extensive testing

• Propose oversight of quality management system
 NATIONAL RENEWABLE ENERGY LABORATORY

#1 Proposed testing “beyond” IEC 61215

21

Failure type: Failure of connections within electrical circuit

Proposed change: Increase thermal cycling from 200 

cycles (~ 6 weeks) to 500 cycles (~ 15 weeks). 

Benefit: This type of failure can lead to fires and/or 

catastrophic module failure. 

Comment: There is evidence that 200 cycles gives 

confidence for ~ 10 y in the field; 500 cycles 

may give confidence closer to 25 y.

Front

Back

Degraaff

NREL Reliability 

Workshop, 2011

Wohlgemuth, 23rd EU 

PVSEC, p. 2663, 2008 

Wear-out mechanism
 NATIONAL RENEWABLE ENERGY LABORATORY

#2 Proposed testing “beyond” IEC 61215

22

Failure type: Cracked cells, etc. leading to disconnections

Proposed change: Apply Dynamic Mechanical Load 

before 50 thermal cycles and 10 humidity freeze cycles

Benefit: This type of failure can lead to fires and/or 

catastrophic module failure. 

Comment: Today’s thinner cells sometimes crack during 

shipping or snow/wind loading. At first, there may be no 

power loss, but after further stress, the metal interconnects 

may begin to break.

Note: this may be a faster way to achieve 

the stress of 500 thermal cycles –

uncovers many types of failures

Wohlgemuth, 32nd PVSC, paper 420, 2008

Pingel, 24th European PVSEC, p. 3459, 2009

Koch, 25th European PVSEC, p. 3998, 2010
 NATIONAL RENEWABLE ENERGY LABORATORY

#3 Proposed testing “beyond” IEC 61215

23

Failure type: System-voltage (potential-induced) 

degradation (module failure is caused by system)

Proposed change: Apply system voltage during damp 

heat or similar condition.

Benefit: For systems that float (active circuit ungrounded), 

the degradation can be dramatic (tens of % in first year). 

New inverter designs have enabled this style of system 

design. 

Comment: Test has been proposed to IEC.

Modules at negative end of 

string show degradation 

(indicated by black) http://www.nrel.gov/docs/fy12osti/54581.pdf
 NATIONAL RENEWABLE ENERGY LABORATORY

#4 Proposed testing “beyond” IEC 61215

24

Failure type: Degradation of encapsulant (yellowing, 

delamination)

Proposed change: Expose materials in “module-like” 

configuration to UV and heat for ~ 6 months (use test that 

was defined by STR ~ 15 y ago)

Benefit: Avoids degradation. Yellowing causes slow power 

decrease. 

Comment: May be done by 

encapsulant manufacturer.

Note: Historically, encapsulant manufacturers 

used this sort of test, so field failures were small. 

Now, newer manufacturers may or may not use 

adequate testing; it’s not required in IEC 61215.

Wohlgemuth, 39th PVSC, 2013 

Reid, SPIE, vol. 8825, 2013

Wear-out mechanism
 NATIONAL RENEWABLE ENERGY LABORATORY

#5 Proposed testing “beyond” IEC 61215

25

Failure type: Degradation of 

backsheet (cracking, delamination) 

leading to losses in safety or 

performance

Proposed change: Expose inner and 

outer layers of backsheet to UV 

consistent with outdoor exposure

Benefit: Avoids safety and 

performance issues (e.g. if backsheet

no longer provides adequate 

protection of the electrical circuit 

leading to shock hazard and/or to 

corrosion). 

Comment: May be completed by 

backsheet manufacturer. Gambogi, et al, EUPVSEC 2012, 

Gambogi, et al, NREL PVMRW 2013, 

Gambogi, et al, 39 Wear-out mechanism th PVSC 2013
 NATIONAL RENEWABLE ENERGY LABORATORY

#6 Proposed testing “beyond” IEC 61215

26

Failure type: Hot-spots from local reverse bias during 

partial shading

Proposed change: Use Edition 3 revised hot-spot test and 

retest random sample each month

Benefit: This type of failure can cause fires. 

Comment: Is planned for adoption into IEC 61215. 

ASTM E2481-06

TamizhMani & Sharma, SPIE 7048 (2008) 
 NATIONAL RENEWABLE ENERGY LABORATORY

#7 Proposed testing “beyond” IEC 61215

27

Failure type: Bypass diode

Proposed change: Increase bypass diode thermal test 

from 1 h to 96 h.

Benefit: Some manufacturers used cheaper diodes that 

passed the 1-h test, but failed in the field. 

Comment: Other bypass diode tests are in development. 

Failure of bypass diodes can lead to fires

Testing for Electrostatic Discharge (ESD) should be done in 

the factory

Zhang, et al, PVMRW 2013
 NATIONAL RENEWABLE ENERGY LABORATORY

Proposal to test “beyond” the standard

28

• Despite PV’s excellent success, some failures are seen 

in the field.

• Propose 7 new tests

• Low prices motivate manufacturers to cut corners –

might the corner be cut too short?

• Propose more frequent/extensive testing

• Propose oversight of quality management system
 NATIONAL RENEWABLE ENERGY LABORATORY

Test (sampling) methods changes

29

1. Choose all test samples randomly from production (no 

engineering samples and no cherry picking)

2. Increase number of test samples per leg from 2 to 5

3. Randomly select samples (at least 2 per leg) from 

production for retest at least once per 3 months (smallvolume manufacturers may have reduced requirement); 

address any failures.
 NATIONAL RENEWABLE ENERGY LABORATORY

Why we need to test “beyond” the standard

30

• Despite PV’s excellent success, some failures are seen 

in the field.

• Propose 7 new tests

• Low prices motivate manufacturers to cut corners –

might the corner be cut too short?

• Propose more frequent/extensive testing

• Propose oversight of quality management system
 NATIONAL RENEWABLE ENERGY LABORATORY 31

Quality Management: PV-specific ISO-9001

Organization controls the PV 

module’s design to align the 

expected lifetime with the 

warranty 

*Before launching their recent incentive 

program, Japan created: “JIS Q8901-

2012 Terrestrial photovoltaic (PV) 

modules-Requirement for reliability 

assurance system (design, production, 

and product warranty)” 

• Builds on Japanese standard*

• Report at 

http://www.nrel.gov/docs/fy13osti

/58940.pdf
Levels of Accelerated Testing

Qualification Qualification

Plus Comparative Service Life

Purpose

Minimum 

design 

qualification

Enhanced

design 

qualification

Comparison of 

products

Substantiation 

of warranty

Quantification Pass/fail Pass/fail Relative Absolute

Climate or 

application

(mounting)

Not 

differentiated

Not 

differentiated Differentiated Differentiated

Specificity Silicon, thinfilm, CPV

For today, 

discuss Si only

Package 

specific?

Product 

specific

Chamber test 

times

Modules: ~ 6 

weeks

Modules: ~ 3 

months

Materials: ~ 6 

months

TBD 3 years ?
33

International PV Module 

Quality Assurance Forum 

San Francisco, July, 2011 Goals: 

1. Create a QA Rating System to differentiate the relative durability of 

module designs

1) Compare module designs

2) Provide a basis for manufacturers’ warranties 

3) Provide investors with confidence in their investments

4) Provide data for setting insurance rates

2. Create a guideline for factory inspections of the QA system used 

during manufacturing.

Hosted by

NREL

AIST

PVTEC

Supported by

METI

JRC

US DOE

SEMI PV Group

A little history
34

The PV QA Task Force was formed at the conclusion of the Forum and consisted of 

five Task Groups:

Task Group 1: PV QA Guideline for Manufacturing Consistency 

(leaders Ivan Sinicco, Alex Mikonowicz, Yoshihito Eguchi, Wei 

Zhou, G. Breggemann)

Task Group 2: PV QA Testing for Thermal and mechanical fatigue including

 vibration (leader Chris Flueckiger, Tadanori Tanahashi)

Task Group 3: PV QA Testing for Humidity, temperature, and voltage

 (leaders John Wohlgemuth, Neelkanth Dhere, Takuya Doi)

Task Group 4: PV QA Testing for Diodes, shading and reverse bias

 (leaders Vivek Gade, Paul Robusto, Yasunori Uchida)

Task Group 5: PV QA Testing for UV, temperature and humidity 

 (leader Michael Köhl, Kusato Hirota, Jasbir Bath)

A little history

These groups began meeting by teleconference in summer of 2011.

Since then, five other task groups have been added.
35

Additional Task Groups:

Task Group 6: Communication of PV QA Ratings to the Community

 (leaders David Williams, Sarah Kurtz)

Task Group 7: PV QA Testing for Wind and Snow Loads

 (currently inactive)

Task Group 8: Thin Film Testing

(leaders: Neelkanth Dhere, Veronica Bermudez, Tobias Roschek, Shuuji Tokuda)

Task Group 9: CPV Testing

 (leaders: Itai Suez, Nick Bosco)

Task Group 10: Connectors

 (leader: Juris Kalejs)
 NATIONAL RENEWABLE ENERGY LABORATORY

Proposed comparative rating system

Qualification Qualification 

“plus” Comparative Service Life

IEC 61215

“Plus” list as

optional 

version of 

CEC Eligible 

Hardware list

IEC 60721-2-1 

Climate 

Designation

New Tests Require 

Additional Stress

Talk about next

Moderate Thermal cycling, UV, 

diodes

Warm Damp, 

Equable 

(Tropical)

Tests for delamination 

& moisture ingress in 

humid climates

Extremely Warm 

Dry (Desert)

Tests for higher 

temperatures

Classes

Rack mount Close-roof mount

Mounting can be as important as climate!
Levels of Accelerated Testing

Qualification Qualification

Plus Comparative Service Life

Purpose

Minimum 

design 

qualification

Enhanced

design 

qualification

Comparison of 

products

Substantiation 

of warranty

Quantification Pass/fail Pass/fail Relative Absolute

Climate or 

application

(mounting)

Not 

differentiated

Not 

differentiated Differentiated Differentiated

Specificity Silicon, thinfilm, CPV

For today, 

discuss Si only

Package 

specific?

Product 

specific

Chamber test 

times

Modules: ~ 6 

weeks

Modules: ~ 3 

months

Materials: ~ 6 

months

TBD 3 years ?
Service Life Prediction Standard?

• How would you write a standard for 

making a Service Life Prediction?

• The failure mechanisms that limit the life 

of a product vary
Steps to Service Life Prediction

1. Identify failure/degradation mechanisms that 

determine end of life

2. Quantify rates (model)

3. For given use environment, apply model to 

estimate expected lifetime

4. Verify model by comparing with field data

This step-by-step procedure is clear, 

but the actual tests are not;

Actual tests will build on comparative tests

This procedure is similar to quality management
 NATIONAL RENEWABLE ENERGY LABORATORY 40

Propose: Implement Service Life Prediction within 

Quality Management System

• The best quality management 

systems will include an 

assessment of whether the 

product can meet the warranty –

this is the best place to quantify 

Service Life

• A complete Service Life 

Prediction takes years to create 

and verify – most aspects of the 

product design must be “frozen” 

first

• Propose to differentiate with 

ratings:

• Platinum

• Gold

• Silver
 NATIONAL RENEWABLE ENERGY LABORATORY

Summary

41

• The PV Industry has grown

• Current IEC standards have provided excellent reliability

• There’s opportunity to take these to the next level

• Qualification “plus” tests will provide incremental 

improvement as optional requirement for CEC list 

• Seven tests that go “beyond” standard tests (**new**)

• Random product sampling and more frequent testing (**new**)

• Robust quality management system (PV-specific version of ISO 

9001) (**new**)

• International PV Module QA Task Force is developing 

comparative climate-specific tests

• Quality Management System is the pathway to defining a 

standard method for ascribing Service Life
 NATIONAL RENEWABLE ENERGY LABORATORY

Proposed approaches to PV Reliability 

Qualification Qualification 

“plus” Comparative Service Life

IEC 61215

What we 

have today

“Plus” list as

optional 

version of 

CEC Eligible 

Hardware list

What we’ll 

have next 

year

IEC 60721-2-1 

Climate 

Designation (for 

rack and closeroof mounting)

New Tests Require 

Additional Stress Implement 

through 

Quality 

Management

System:

• Platinum

• Gold

• Silver

Moderate Thermal cycling, UV, 

diodes

Warm Damp, 

Equable 

(Tropical)

Tests for delamination 

& moisture ingress in 

humid climates

Extremely Warm 

Dry (Desert)

Tests for higher 

temperatures

In development, using community’s collective wisdom