National Renewable Energy Laboratory Innovation for Our Energy Future

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.

NREL

Sarah Kurtz

12.8.2009

Atlas Technical 

Conference on 

Accelerated Ageing and 

Evaluation

Phoenix, Arizona

NREL/PR-520-46874

Reliability Challenges for Solar Energy
Outline

• Solar – a huge success, but still a long way to go

• Importance of reliability to success of solar

• A little history

• Reliability issues specific to three key approaches

• Silicon – continuous improvement requires continued 

testing; quantitative predictions

• Thin film – uniform, large-area deposition for product 

development and sensitivity to moisture; metastabilities

• Concentrator – product development; simultaneous 

optimization of multiple components

National Renewable Energy Laboratory 2 Innovation for Our Energy Future
Solar energy is abundant

Convenient truth: small area can supply our energy needs

Sunlight reaching earth in 1 hour is enough to power the world for 1 year

5-6 kWh/sq m/day

>10 kWh/sq m/day

At 10% 

efficiency, 

area needed 

for US 

electricity
National Renewable Energy Laboratory Innovation for Our Energy Future

Growth of photovoltaic (PV) industry

4

Area of Si passes 

microelectronics 

Tons of Si passes 

microelectronics 
Growth of PV industry

If we can maintain the 

current growth rate, PV 

will reach major 

milestones in < 10 yrs

1

10

100

GW of PV shipped worldwide annually

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
1

10

100

GW of PV shipped worldwide annually

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

Growth of PV industry

If reliability is poor, what 

happens to this success story?
How to satisfy the investor?

Historically, degradation & failure mechanisms have 

been found in the field that were not found in 

accelerated testing

Predictive models need to be validated with field data

Big challenge: How can we give 30-year predictions for 

degradation & failure rates when the product has only 

been in the field for 1-2 years?

National Renewable Energy Laboratory 7 Innovation for Our Energy Future
National Renewable Energy Laboratory Innovation for Our Energy Future

History of Si module qualification 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

8
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

• Studies of c-Si modules show that module failures 

are small (inverters dominate when cost is low)

Currently, most reports imply 

that c-Si module failures are 

dominated by improper 

installation, lightning strikes, 

critters, etc.

(Prog. PV 2008; 16:249)

Unscheduled maintenance costs

69% 

10% 

21% 

Inverters

System

PV
Today’s qualification standards are similar

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

Review: Osterwald & McMahon, Progress in PV 17, p11 (2009)
Three key approaches to photovoltaic (PV) panels

Front Solar cell

Back

2. Thin film

3. Concentrator

1. Silicon

Two strategies to reduce 

semiconductor material Conventional approach

Reduce cost by reducing use 

of semiconductor
National Renewable Energy Laboratory Innovation for Our Energy Future

Many technology choices

12
One “winner” or many technologies?

National Renewable Energy Laboratory Innovation for Our Energy Future

Nickel cadmium

Lithium ion

Lead acid

Lithium

Alkaline Nickel metal hydride

Different technologies for different applications

13
Silicon modules

SiliconS Glass

EVA

EVA

Backsheet

Silicon cell Tab

Common encapsulation materials

EVA - Ethylene vinyl acetate

PET - polyethylene terephthalate

PVF - poly vinyl fluoride 

Si module cross section
Silicon modules – remaining challenges

New materials (reduce cost, improve performance) –

will these have same reliability; how to test for?

Continued quality assurance (e.g. impurities in Si give 

light-induced degradation)

Arcing, grounding, power conditioning, other systemrelated problems

Confident, long-term, quantitative predictions

Typical module degradation rates are 0.5±0.5%/yr 

(difficult to measure); field module failure rates are 

often < 0.1%/yr

National Renewable Energy Laboratory 15 Innovation for Our Energy Future
Past success does not guarantee future success

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

Thin film

C-Si

33rd PVSC, TamizhMani, 2008

70%
Measurement of degradation rates takes years

Need precise measurement of 

irradiance, temperature, etc. 17

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
Documented degradation rates

National Renewable Energy Laboratory 18 Innovation for Our Energy Future

31st IEEE PVSC p.2085 (2006)

Vazquez, Prog. in PV (2008) 
Thin-film approaches on the market

National Renewable Energy Laboratory Innovation for Our Energy Future

CuIn(Ga)Se CdTe Amorphous silicon

19

Large variety of approaches is challenge to reliability engineer
Typical thin-film structures

National Renewable Energy Laboratory Innovation for Our Energy Future

Glass

ITO or TCO

CdS

CdTe

Metal

Glass for strength

CdTe uses superstrate

Glass for protection

ZnO or TCO

CdS

CuInGaSe

Molybdenum

Glass

CuInGaSe uses substrate

Not to 

scale

20

EVA

EVA
Monolithic module integration

National Renewable Energy Laboratory Innovation for Our Energy Future

+

Contact

Cell

(Inactive)

Contact

Cell

(Inactive)

Active Cell

Glass

5 x 120 μm

Conductor

Device

Conductor

– w

21
Hurdle for thin films: uniformity of deposition

National Renewable Energy Laboratory 22 Innovation for Our Energy Future

Quality assurance remains a big problem for thin-film products
Hurdle for thin films: uniformity of interconnections

National Renewable Energy Laboratory Innovation for Our Energy Future

+

Contact

Cell

(Inactive)

Contact

Cell

(Inactive)

Active Cell

Glass

5 x 120 μm

Conductor

Device

Conductor

– w

23

Quality assurance remains a big problem for thin-film products
Metastability: Reversible vs irreversible changes

• Amorphous silicon degrades in light and recovers 

when annealed in the dark

• CIGS and CdTe can show transients

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

As high as 

30% 

metastability 

has been 

observed
Flexible CIGS requires reduced sensitivity to moisture

ZnO (and other transparent conductors) react with 

moisture, causing increase in series resistance

Two strategies:

- Harden the cell (e.g. Sundaramoorthy, et al 34th PVSC)

- Harden the packaging (barrier coatings)

National Renewable Energy Laboratory 25 Innovation for Our Energy Future
Many possible failure mechanisms 

• CdTe has shown instability of back contact 

(diffusion?)

• Edge seal may allow water into glass/glass module

• Partial shunts or conducting diodes may be seen at 

scribe lines or other defected areas

• Adhesion to glass can be problem

• Role of sodium is important in CuInGaSe modules, 

but sodium can move

• Currently, the biggest effort with CuInGaSe is to try to 

put it on a flexible substrate – requires excellent 

barrier coating unless cell can be hardened to 

moisture
National Renewable Energy Laboratory Innovation for Our Energy Future

Range of concentrator approaches

High concentration

• 35% - 40% cells

• 400X – 1500 X

Low concentration

• 15% - 25% cells

• 2X – 100 X

Amonix JX Crystals

27
CPV challenges – many interactions

CPV can fail in many ways, but it can be difficult to 

understand where the problem is and to fix the 

problem without creating a new problem

• Tracking – optics must be aligned with the sun

• Optics – durability can be problem, soiling; optics 

affect rest of system

• Cell – must be encapsulated, but not affected by UV; 

size of cell affects rest of system

• Heat sink – must be electrically isolated, but excellent 

thermal contact

• Modularity may be benefit!

National Renewable Energy Laboratory 28 Innovation for Our Energy Future
UV transmission depends on design

Analysis of transmitted optical spectrum enabling accelerated testing of 

CPV designs

SPIE 2009 David Miller, et al

Low UV High UV
Bonds to heat sink and optics

Cell

Heat sink

Optic Optical bond with 100% 

Transmission

Electrical 50 W/sq cm

contact

Small ∆T (<10°C)

Electrical isolation

No voids

T cycle OK

• Borrowing experience from power electronics and 

DBC (direct bonded copper) makes this a smaller 

issue

• Intense UV may be a substantial problem, but 

optics may not transmit UV

IR image of void in 

die attach

Bosco, et al 34th

PVSC
Concentrators – reliability challenges

• Wide variety of designs

• Qualification test is not well established

• Companies spend time developing their own 

accelerated tests to speed product development 

cycles

• Very few companies have heritage with field testing

• Response to different stress conditions not well 

understood (sand, etc.)

• Everyone wants to bring a product to market 

immediately

National Renewable Energy Laboratory 31 Innovation for Our Energy Future
Concentrator technology

Creative optical 

designs?
PV is growing up!

National Renewable Energy Laboratory 33 Innovation for Our Energy Future

Observe failure

Identify mechanism

Develop test

Mitigate problem

Identify next failure

Prototype

Failure analysis

Identify mechanism

Develop (ALT) test

Measure acceleration

Predict performance

Mature product

When do we move from focus on mitigation to focus on prediction?
Summary

• Solar is growing rapidly and could become a 

significant source of electricity within our lifetimes

• Silicon modules are performing well in the field; 

reliability testing of new designs is still important

• CdTe and CuInGaSe modules are sensitive to 

moisture, so must be carefully sealed; only 

amorphous Si modules are available in flexible form

• Concentrator PV is in product development stage, but 

is benefiting from expertise in other industries

• How do we predict 30 years of performance for 

product that has been in the field < 1-2 years?

National Renewable Energy Laboratory 34 Innovation for Our Energy Future
Planet powered by 

renewable energy

By year 2100 or before?

Thank you for your attention!

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