Photovoltaic Reliability R&D Toward a 

Solar-Powered World

Sarah Kurtz and Jennifer Granata

with help from many others

National Renewable Energy Laboratory,

Sandia National Laboratories

SPIE

San Diego, CA

August 2-7, 2009

NREL/PR-520-46412

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.
Outline

• A vision of a solar-powered world

• Importance of reliability to success of solar

• Working together to establish reliability

• R&D issues related to:

• Product Development

• Quality Assurance during manufacturing

• Lifetime Predictions

• Current status

• Technology-specific R&D issues

– Selected highlights from SPIE

2
How fast can a world change?

3

?
Solar power is within reach

4

?

What do we need to do to create a 

solar-powered world?
Growth of PV industry

5

PV shipments have been doubling every two years
Growth of PV industry

1

10

100

GW of PV shipped

2000 2005 2010 2015 2020

Year

Annual new electricity capacity 1996-2006*

*www.eia.doe.gov/emeu/international/electricitycapacity.html (4012-2981 GW)/10 yr

Annual replacement of electricity capacity for 20 yr cycle

If we can maintain the current

growth rate, PV will reach major

milestones in less than 10 yrs

If we can maintain the 

current growth rate, 

PV will reach major 

milestones in < 10 yrs
Why is reliability important?

7

1

10

100

GW of PV shipped

2000 2005 2010 2015 2020

Year

Annual new electricity capacity 1996-2006*

Annual replacement of electricity capacity for 20 yr cycle

If we can maintain the current

growth rate, PV will reach major

milestones in less than 10 yrs

Improved reliability helps to reduce life-cycle cost:

- Longer lifetime

- Slower degradation

- Lower O&M costs

Improved reliability improves customer satisfaction

- Good performance builds customer confidence

- Better confidence inspires investors 

With reliability, this 

graph leads to a 

solar-powered world
Why is reliability important?

8

1

10

100

GW of PV shipped

2000 2005 2010 2015 2020

Year

Annual new electricity capacity 1996-2006*

Annual replacement of electricity capacity for 20 yr cycle

If we can maintain the current

growth rate, PV will reach major

milestones in less than 10 yrs

Improved reliability helps to reduce life-cycle cost:

- Longer lifetime

- Slower degradation

- Lower O&M costs

Improved reliability improves customer satisfaction

- Good performance builds customer confidence

- Better confidence inspires investors 

Without reliability: 

a Ponzi scheme? -

Bill Marion
Reliability should be considered 

by venture capitalists

9

Venture capitalists funding dot.com look for

- Novel (secret) idea

- Return on investment in a couple of years

Venture capitalists funding PV need to look for

- Good approach (not necessarily novel)

- Excellent implementation

- Plan for enough time to check reliability

Developing a PV product is difficult!!!

The investors must recognize that the 

potential return on investment is huge, 

but will take time.

Community should demand public 

demonstration of reliability before IPO.
Building a solarpowered world

Years of standards development/field experience
Building a solarpowered world

Years of standards development/field experience

Universities

-Educate work force

-Original research to extend 

useful PV-reliability knowledge

National labs

-Build foundation of PV-reliability 

knowledge through R&D

-Long-term or larger projects
Building a solarpowered world

Companies (& investors)

-Develop reliable products

-Manufacture quality products

-Customer satisfaction

Complementary roles

Years of standards development/field experience

Universities

-Educate work force

-Original research to extend 

useful PV-reliability knowledge

National labs

-Build foundation of PV-reliability 

knowledge through R&D

-Long-term or larger projects
Building a solarpowered world

Defining 

complementary 

roles allows more 

efficient use of 

resources

Solar-powered world

Companies (& investors)

-Development of reliable product

-Manufacturing of quality product

-Customer satisfaction

Years of standards development/field experience

Universities

-Educate work force

-Original research to extend 

useful PV-reliability knowledge

National labs

-Build foundation of PV-reliability 

knowledge through R&D

-Long-term or larger projects

Social acceptance and utility acceptance of PV
What do we mean by reliability?

14

For a solar-powered world, 

Reliability means: 

the lights go on when the 

switch is flipped

For today’s talk, 

Reliability means a PV system working as expected when 

the sun is shining (with low O&M costs and long life)
Developmental stages 

Product 

development

-Identify failure modes

-Understand failure 

mechanisms

-Test for failures

-Mitigate

Quality 

assurance

-Test raw and 

refined materials

-Control process

-Test final products

Predict 

reliability

-Identify useful tests

-Understand all 

components

-Make predictions

Achieving excellent reliability is a step-by-step process; 

you can’t skip the early steps and expect to be successful 

with the final steps
Developmental stages

Product 

development

-Identify failure modes

-Understand failure 

mechanisms

-Test for failures

-Mitigate

Quality 

assurance

-Test raw and 

refined materials

-Control process

-Test final products

Predict 

reliability

-Identify useful tests

-Understand all 

components

-Make predictions

Achieving excellent reliability is a step-by-step process; 

you can’t skip the early steps and expect to be successful 

with the final steps
Product development

Reliability should be considered from day 1 forward

Lots of tools:

-Advanced Product Quality Planning

-Design Failure Modes Effects and Analysis

-Fault Tree Analysis

-Design for Manufacturability

-Design Review Based on Failure Mode (Toyota) 

Product development

-Identify failure modes

-Understand failure 

mechanisms

-Test for failures

-Mitigate
Prototype testing

Identify serious failure/degradation

Diagnose failure

Mitigate failure Devise test for failure

Model adequacy 

of mitigation

This cycle has been effective at 

improving PV module reliability and in 

developing standard for qualification 

test; not done yet!

Product development

-Identify failure modes

-Understand failure 

mechanisms

-Test for failures

-Mitigate
History of Si module qual. test: JPL 

(Jet Propulsion Lab) Block buys

Test I II III IV V

Year 1975 1976 1977 1978 1981

Thermal 

Cycle (°C)

100 cycles

-40 to +90

50 cycles

-40 to +90

50 cycles

-40 to +90

50 cycles

-40 to +90

200 cycles

-40 to +90

Humidity 70 C, 90%RH, 

68 hr

5 cycles

40 C, 90%RH 

to 23 C

5 cycles

40 C, 90%RH 

to 23 C

5 cycles

54 C, 90%RH 

to 23 C

10 cycles

85 C, 85%RH 

to -40 C

Hot spots - - - - 3 cells, 100 

hrs

Mechanical 

load

- 100 cycles 

± 2400 Pa

100 cycles 

± 2400 Pa

10000 cyc. 

± 2400 P

10000 cyc. 

± 2400 Pa

Hail - - - 9 impacts 

3/4” - 45 mph

10 impacts 

1” - 52 mph

NOCT - - - Yes Yes

High pot - < 15 μA 

1500 V

< 50 μA 

1500 V

< 50 μA 

1500 V

< 50 μA 

2*Vs+1000

19
JPL Block buys led to 

dramatic improvements

• One study claimed (Whipple, 1993):

• Pre-Block V: 45% module failure rate

• Post-Block V: <0.1% module failure rate

20

Today’s qualification test retains similarities to JPL tests

• IEC 61215 - Crystalline silicon design qualification 

includes 18 test procedures 

– Thermal cycling - 200 cycles -40°C to +85°C

– Humidity freeze - 10 cycles +85°C, 85% RH to -40°C

– Damp heat - 1000 hrs at +85°C, 85% RH

– Wet leakage current - Wet insulation resistance X area > 40 

MΩm2 at 500 V or system voltage

– Requirement is typically to retain 95% of original power 

production

• IEC 61646 (thin film) and IEC62108 (CPV) are similar

www.iec.ch
Developmental stages 

Product 

development

-Identify failure modes

-Understand failure 

mechanisms

-Test for failures

-Mitigate

Quality 

assurance

-Test raw and 

refined materials

-Control process

-Test final products

Predict 

reliability

-Identify useful tests

-Understand all 

components

-Understand field 

failures

-Make predictions

Achieving excellent reliability is a step-by-step process; 

you can’t skip the early steps and expect to be successful 

with the final steps
Silicon modules –

R&D needs for QA

Quality assurance – R&D opportunities

• IEC standards do not address periodic retesting (when?)

• What QA tests/controls are needed? (e.g. Si purity, EVA 

cure)

• How can we keep the cost of the QA low, while keeping 

confidence high and learning as much as possible?

22

Quality assurance

-Test raw and refined materials

-Control process

-Test final products

SunTech raised question 

of purity of silicon in 2008

QA must be in place before confident predictions can be made
Developmental stages 

Product 

development

-Identify failure modes

-Understand failure 

mechanisms

-Test for failures

-Mitigate

Quality 

assurance

-Test raw and 

refined materials

-Control process

-Test final products

Predict 

reliability

-Identify useful tests

-Understand all 

components

-Make predictions

Achieving excellent reliability is a step-by-step process; 

you can’t skip the early steps and expect to be successful 

with the final steps
Predictive Model Development

Improved Field 

O&M methods

Degradation Rates 

Real-time Reliability Studies

Failure Statistics 

Accelerated Life Tests Field O&M and Failure Data 

Predictive Model

Process and 

Design 

Improvements 

New Failure 

Mode ID and 

Mitigation

Failure 

Mechanism ID 

and Mitigation 

Improved ALT 

and Qual Test 

Design

Reliability vs 

Time

Availability vs 

Time

Field Degradation vs 

Time

Community’s 

collective data
Predictive Model Development

Reliability and availability analysis of a fielded PV system

34th PVSC Elmer Collins
Systems Approach:

Status – System level

• Understand how each component fits into the system 

• The overall reliability of the system is dominated by the 

least reliable component(s)

Studies of c-Si systems typically 

show few module failures;

Inverters typically dominate O&M 

cost

(Prog. PV 2008; 16:249)

Unscheduled maintenance costs

69% 

10% 

21% 

Inverters

System

PV Reliable Si modules are 

demonstrated, but not 

guaranteed 
Systems model helps to 

evaluate status

Module performance can be good; Some inverters have short lifetimes

27

1200

1000

800

600

400

Power rating (W)

Time

1.0

0.9

0.8

0.7

0.6

0.5

Inverter efficiency

inverter

replaced

-0.14%/yr

inverter

efficiency

DC power

2003 2005 2007

 ASE Americas Si modules

 Xantrex Suntie 2500
Inverters are improving, 

but still need more

28

Inverters suffer from early failures in the field, temperature-related 

failures, & mismatch between PV voltage & inverter window. 

Qualification/p

erformance 

standards for 

inverters and 

BOS are not 

well defined

Limited 

warranties 

may be 

longer

14

12

10

8

6

4

2

0

Minimum guarantee (y)

1 10 100 1000

Power of inverter (kW)

Inverter Guarantees

Source: PHOTON International, April, 2009
Documented 

degradation rates

29

31st IEEE PVSC p.2085 (2006)

Vazquez, Prog. in PV (2008) 

About two-thirds of degradation rates are measured as < 1%/yr
Past success does not guarantee future success

30 About two-thirds of degradation rates are measured as < 1%/yr

Thin film

C-Si

33rd PVSC, TamizhMani, 2008

70%
Reliability R&D

31

With highlights of presentations at SPIE this week

Current/recent studies
Improved stress testing

32

Development and application of a UV light source for PV-module testing

Wed. 8:30 am Michael Kohl (7412-2)

Stress tests need to be 

- More complete (all stresses)

- More thorough

- More quantitative

- Faster

- Less expensive 

Accelerated stress testing of hydrocarbon-based encapsulants for 

medium-concentration CPV applications 34th PVSC Michael Kempe

Application of the NREL test-to-failure protocol for PV modules

Silicon Workshop (Aug 2009) Peter Hacke

Outdoor monitoring and high voltage bias testing of PV modules as 

necessary test for assuring long-term reliability

Thurs. 3:50 pm Neelkanth Dhere (7412-28)
Improved performance requires 

vigilance for reliability

33

Effects of cerium removal from glass on photovoltaic module performance and stability

Thurs. 3:05 pm Michael Kempe (7412-26)

60°C/60% humidity; 2.5 suns UV

Removing Ce from glass may improve performance and 

decrease cost, but will it affect reliability?
Separate reversible from 

irreversible changes

34

Striving for a standard protocol for preconditioning or stabilization of polycrystalline thinfilm PV modules

Wed. 9:15 am Joseph del Cueto (7412-3)

Light-soaking and power measurements of thin-film modules

Mon. 9:35 am Karl-Anders Weiss (7409-24)

9

8

7

6

5

4

Efficiency (%)

June 

2003

July

2008

Dec

2008

Dec

2008

July

2008

Jan 2009

0.2 kWh/m 2

Jan 2009

0.2 kWh/m 2

2.75 kWh/m 2

tested same day

3.15 kWh/m 2

tested next day

3.15 kWh/m 2

tested next day

0.25 kWh/m 2

start test

0.25 kWh/m 2

start test

50°C

overnight

50°C

overnight

0°C

overnight

CIGS modules

Rated efficiency

 CIGS no exposure

 CIGS 3-year exposure in Florida

After exposure, thin-film modules show reversible and irreversible changes

One 

module 

showed 

30% 

increase
Thin-film layers can change

35

Degradation and capacitance voltage hysteresis in CdTe devices

Thurs. 10:30 am David Albin (7412-18)

Understanding and mitigating effects of nonuniformities on reliability of thin-film PV

Thurs. 11:40 am Victor Karpov (7412-21)

The effect of metal foil tape degradation on the long-term reliability of PV modules

Thurs. 3:30 pm Rob Sorensen (7412-27)

Change in CdTe 

cells after annealing 

(Albin)
Moisture can degrade TCO

36

Stability of TCO window layers for thin-film CIGS solar cells upon damp 

heat exposures

Thurs. 10:55 am Rajalakshmi Sundaramoorthy (7412-19)

Thurs. 11:15 am John Pern (7412-20)

ITO and SnO2 are stable in 85% humidity, 85°C Pern, 2008

Flexible configuration is especially difficult: harden the cell or the package?
When is concentrated UV a 

problem for CPV?

37

Analysis of transmitted optical spectrum enabling accelerated testing of 

CPV designs

Wed. 11:35 am David Miller (7407-16)

Does concentrated sunlight at cell contain damaging UV dose?

Stress in large-area optics for solar concentrators

Wed. 10:25 am Ralf Leutz (7412-5)

Are optics durable?
Bond to heat sink is R&D 

topic for CPV

38

Reliability testing the die attach of CPV cell assemblies

34th PVSC Nick Bosco 

IR image of void in die attach
Summary

• Solar is growing rapidly; could become a 

significant source of electricity within 10 yrs

• Excellent performance of silicon modules has 

been demonstrated in the field; but new 

products may repeat old mistakes

• Inverters currently dominate system failures

• Many R&D needs are best met by community 

working together

• Need to ensure reliability to build foundation 

for a solar-powered world

39
Planet powered by 

renewable energy

By year 2100 or before?

40

Thank you to :

Dave Albin

Glenn Alers

Nick Bosco

Joe del Cueto

Chris Deline

Ed Gelak

Steve Glick

Peter Hacke

Dirk Jordan

Mike Kempe

Tom Moricone

Bill Marion

David Miller

Matt Muller

John Pern

Jose Rodriguez

Bill Sekulic

Ryan Smith

Kent Terwilliger

David Trudell

Thank you for 

your attention!

Thank you to :

Michael Quintana

Mike Mundt

Elmer Collins

Peggy Rhodes

Jeff Mahn

Rob Sorensen

Many others

Special thanks to Dan 

Ton and DOE/SETP