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

4.27.2009

IEEE IRPS

Montreal, Canada

NREL/PR-520-44970

Reliability Challenges for Solar Energy
Outline

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

• Key approaches to solar electricity

• Solar thermal

• Crystalline silicon

• Thin film – amorphous silicon, CdTe, CIGS

• Concentrator – low- and high-concentration approaches

• Importance of reliability to success of solar

• Reliability issues specific to each approach

• Silicon – strong performance; continuous improvement; 

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
National Renewable Energy Laboratory Innovation for Our Energy Future

Growth of photovoltaic (PV) industry

0.01%-0.1% of electricity now comes from PV - extrapolates to > 5% in 2020

competitive with conventional electricity for 0.1% - 1% of market; more in future

Rogol, PHOTON International August 2007, p 112.

3

40%/yr 

growth 

‘97-’07

Area of Si passes 

microelectronics 

Tons of Si passes 

microelectronics 
Solar energy is abundant

National Renewable Energy Laboratory Innovation for Our Energy Future

Convenient truth: small area can supply our energy needs

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

4

5-6 kWh/sq m/day

>10 kWh/sq m/day

At 10% 

efficiency, 

area needed 

for US 

electricity
Parabolic trough is the primary 

technology today

Resurgence of interest

~ 400 MW installed

Currently generates ~ 0.01% of US 

electricity

Can generate electricity into the 

evening & use fuel into the night

80

60

40

20

0

MW completed/yr in US

1985 1990 1995 2000 2005

Year

Solar thermal

Parabolic trough

Solar thermal electric

4 GW planned in US by 2014
1-MW Arizona Trough Plant –

near Tucson, AZ - 2006

64 MW Solargenix 

Parabolic Trough 

Plant in Nevada -

2007

National Renewable Energy Laboratory 6 Innovation for Our Energy Future
National Renewable Energy Laboratory 7 Innovation for Our Energy Future
National Renewable Energy Laboratory Innovation for Our Energy Future

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

8

Reduce cost by reducing 

use of semiconductor
National Renewable Energy Laboratory Innovation for Our Energy Future

Many technology choices

9
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

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Cost of electricity: two or three parts

Upfront costs for PV and coal plants are converging

Ongoing costs are less for PV

Operation only during daylight hours increases cost by ~X4

Key remaining question is life of PV plant (30 years?) 

National Renewable Energy Laboratory Innovation for Our Energy Future

6

5

4

3

2

1

0

Initial cost ($/W)

Old coal New coal Clean coal PV

1. Initial price (estimates*) 2. Operation and maintenance

- Fuel cost (Coal PV)

- Operation (Coal PV)

- Maintenance (Coal PV)

3. Total electricity generated

- Capacity factor (Coal PV)

(Coal ~100%; PV ~ 25%)

- Life of plant (Coal PV?) *Fortnightly’s SPARK, p. 10, May 2008

11

Examples of price estimates ($/W)

PV is already competitive for 

peak power in some locations
Importance of reliability & durability

National Renewable Energy Laboratory Innovation for Our Energy Future

Cost of solar electricity depends on degradation, lifetime, & ongoing costs 

Those paying for upfront investment want guarantee of system life

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1.0

0.8

0.6

0.4

0.2

0.0

Relative performance or cost

0 5 10 15 20 25

Year

Performance of good system

Operation and maintenance costs 

for systems with problems

Performance of systems with problems

O&M costs 

for good system
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 13 Innovation for Our Energy Future
Silicon modules

SiliconS

National Renewable Energy Laboratory Innovation for Our Energy Future

Glass

EVA

EVA

Backsheet

Silicon cell Tab

Common encapsulation materials

EVA - Ethylene vinyl acetate

PET - polyethylene terephthalate

PVF - poly vinyl fluoride (Tedlar)

Si module cross section

14
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

15
National Renewable Energy Laboratory Innovation for Our Energy Future

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)

Prog. PV 2008; 16:249

Currently, most reports imply that c-Si module failures are 

dominated by improper installation, lightning strikes, critters, etc.

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Today’s qualification standard

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

National Renewable Energy Laboratory Innovation for Our Energy Future

www.iec.ch

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Silicon modules – remaining challenges

New materials (reduce cost, improve performance) –

will these have same reliability?

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 degradation rates are 0.5%/yr (difficult to 

measure); field failure rates are often < 0.1%/yr

National Renewable Energy Laboratory 18 Innovation for Our Energy Future
Measurement of degradation rates takes years

National Renewable Energy Laboratory Innovation for Our Energy Future

Need precise measurement of irradiance, temperature, etc.

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Module degradation

Inverter replacement

Inverter 

performance
Thin-film approaches on the market

National Renewable Energy Laboratory Innovation for Our Energy Future

CuIn(Ga)Se CdTe Amorphous silicon

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Thin-film approach

National Renewable Energy Laboratory Innovation for Our Energy Future

• First Solar (Toledo, OH) - 5th biggest 

PV company in world in 2007

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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

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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

23
Hurdle for thin films: uniformity of deposition

National Renewable Energy Laboratory 24 Innovation for Our Energy Future
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

25
Module metastability

National Renewable Energy Laboratory Innovation for Our Energy Future

• Reversible vs irreversible changes

• Amorphous silicon degrades in light and 

recovers when annealed in the dark

• CIGS and CdTe can show transients

• Light soaking – IEC standard calls for 600-

1000 W/m2 @ 50°C (±10°C) for dose of >43 

kWh/m2 under resistive load until power is 

stable within 2%

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Thin-film qualification test

National Renewable Energy Laboratory Innovation for Our Energy Future

• IEC 61646 – Thin-film design qualification 

includes 19 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 to produce 90% of rated power 

– Light soaking - 600-1000 W/m2 @ 50°C 

(±10°C) for dose of >43 kWh/m2 under 

resistive load until Pmax is stable within 2%

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Damp heat can cause degradation

ZnO (and other transparent conductors) react with 

moisture, causing increase in series resistance

CuInGaSe may react with moisture

National Renewable Energy Laboratory Innovation for Our Energy Future

Transparent Conductive Oxides

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Many possible failure mechanisms 

• See tutorial by Dave Albin

• 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 29 Innovation for Our Energy Future
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

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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 31 Innovation for Our Energy Future
Bonds to heat sink and optics

National Renewable Energy Laboratory Innovation for Our Energy Future

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

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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

• Everyone wants to bring a product to market 

immediately

National Renewable Energy Laboratory 33 Innovation for Our Energy Future
Concentrator technology

Creative optical 

designs?

National Renewable Energy Laboratory 34 Innovation for Our Energy Future
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

• In general, PV industry can benefit from the reliability 

testing experience of the microelectronics industry 

National Renewable Energy Laboratory 35 Innovation for Our Energy Future

Let’s work together to help PV grow!
National Renewable Energy Laboratory Innovation for Our Energy Future

Planet powered by 

renewable energy

By year 2100 or before?

36

Thank you to :

Nick Bosco

Joe del Cueto

Chris Deline

Ed Gelak

Steve Glick

Mike Kempe

Tom Moricone

Bill Marion

David Miller

Matt Muller

Jose Rodriguez

Bill Sekulic

Kent Terwilliger

David Trudell

Many others

Thank you for your attention!