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

10.26.2009

Solar Power 

International 2009

Anaheim, CA

NREL/PR-520-46924

CPV 101: Intro to CPV Technology, 

Opportunities and Challenges
Outline

• Overview of PV – Opportunity for CPV

• Fundamentals of concentrating PV

• Why CPV?

• Design considerations

• Bird’s eye view

• Sorting it out

• Worm’s eye view

• Status of industry

• Standards

• Many companies

• Is it a turning point?

National Renewable Energy Laboratory 2 Innovation for Our Energy Future
Outline

• Overview of PV – Opportunity for CPV

• Fundamentals of concentrating PV

• Advantages

• Primary approaches (High & low concentration)

• Designing from the system perspective

• Design considerations

• Thermodynamic limit of concentration

• Refractive vs reflective optics

• Concentration ratio, f number, etc.

• Thermal considerations

• Keeping it clean and dry

• Cells

National Renewable Energy Laboratory 3 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
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 (flat plate)

Lifetime is critical

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

Examples of price estimates ($/W)

PV is already competitive for 

peak power in some locations
National Renewable Energy Laboratory Innovation for Our Energy Future

Growth of photovoltaic (PV) industry

6

Area of Si passes 

microelectronics

2001 

Tons of Si passes 

microelectronics

2006 
Growth of PV industry - opportunity

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

(These milestones do not consider low capacity factor nor growth of electricity demand)
Opportunity – what’s needed?

1. Low cost

• Reduce use of semiconductor material

• Higher efficiency can reduce area costs 

(installation, land, & BOS costs)

• Long lifetime reduces cost of electricity

2. Scalability

• CapEx costs

• Time to ramp production

National Renewable Energy Laboratory 8 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

Reduce cost by reducing 

use of semiconductor
Concentrating Photovoltaic Systems: CPV

Dish: requires active cooling

Microdishes can be 

passively cooled

Fresnel lenses focus light on small 

cells: Passive cooling

Lens

Cell

Heat sink

Receiver or 

cell assembly

Many designs
Concentrating PV (CPV) vs Concentrating solar power (CSP)

National Renewable Energy Laboratory 11 Innovation for Our Energy Future

CPV

• Appropriate for > 50 kW

• Usually requires no water

• Low maintenance

• Good match to load 

profile (better than fixed PV; 

not as good as CSP)

CSP

• Appropriate for > 100 MW

• Heat generates steam to run 

conventional power plant

• Possibility of storage – run 

into the evening

• Supplement fuel for 

conventional plant
Scope of this presentation – high & low X

National Renewable Energy Laboratory 12 Innovation for Our Energy Future

High concentration ~ 500X Low concentration: 1.5 - 200X

Multijunction cells ~ 40% Silicon cells 15-25%

(cells are ~ $4/cm2) (may use 1-sun silicon)

Both approaches are aggressively pursued today

Both will be discussed throughout presentation

Silicon 

cell

III-V 

cell
Reduce semiconductor material

National Renewable Energy Laboratory 13 Innovation for Our Energy Future

500X – GaInP/GaAs/Ge

0.007 g/W

500X – GaInP/GaAs/GaInAs 

(reuse wafer)

0.001 g/W

Assumptions:

150 μm Ge X 1 cm2 X 5.3 g/cc = 0.08 g

25 mW X 500 X 0.85 /cm2 = 10.6 W

0.08 g/10.6 W = 0.007 g/W

10 μm of epi X 1 cm2 X 5.3 g/cc = 0.0053 g

Silicon 

cell

III-V 

cell

1-sun Si – as low as 5 g/W

2-sun Si – 2.5 g/W

20-sun Si – 0.25 g/W

For comparison:

1 μm CdTe @ 12% 

0.05 g/W

1 μm X 1 cm2 X 5.9 g/cc = 0.00059 g

12 mW/cm2 implies 0.00059 g/ 0.012 W = 0.05 g/W

Less semiconductor can mean lower cost; better scalability
Potential for low cost

National Renewable Energy Laboratory 14 Innovation for Our Energy Future

• CPV is estimated to 

have similar or lower 

costs than other 

technologies

• Uncertainty is larger 

than the difference 

between the 

technologies

• Must be at largevolume production 

before costs become 

apparent

• World benefits from 

exploring multiple 

options 

Swanson, "The Promise of Concentrators," Prog. PV. 8, 93 (2000) 
Smallness enables use of highest efficiency cells

15
One “winner” or many technologies?

National Renewable Energy Laboratory Innovation for Our Energy Future

Nickel 

cadmium

Lithium ion

Lead acid

Lithium

Nickel 

metal 

hydride

Alkaline

Different technologies for different applications

16

CPV markets

• Sunny locations

• Large systems

• Area constrained

CPV advantages

• Scaleable

• Better match to 

demand

• High efficiency

• Low T coefficient 

(good kWh/kW)
Scalability

1. Is expected

• Easier to scale up production volume for 

mirrors or lenses

• Semiconductor use is small

2. Demonstration is yet to come

• First Solar has demonstrated for thin film

• Still needs to be demonstrated for CPV

3. Most companies are developing or 

demonstrating reliable product – are we 

close to a company being ready to ramp?

National Renewable Energy Laboratory 17 Innovation for Our Energy Future
Be careful not to be confused

1. High efficiency can translate to higher electricity 

production but not always

• CPV uses direct beam; diffuse light may not be focused 

– so less sunlight is available

• Tracking usually increases available sunlight

2. High efficiency can translate to reduced land use, 

but not necessarily

• Trackers may shade each other

• Loss with shading can be very dependent on design and 

geometry

National Renewable Energy Laboratory 18 Innovation for Our Energy Future
CPV progress/status

Multijunction cells > 41% in lab; 37-39% in production; 

systems as high as 25% AC

About a dozen multijunction cell companies (30-40%)

About three dozen companies high-X CPV

About two dozen companies low-X CPV

Some companies working on 1 MW installations

Production capability now > 100 MW/y

Why has it taken so long???

National Renewable Energy Laboratory 19 Innovation for Our Energy Future
Why so hard? – need infrastructure

In 1990s, PV community decided that buildingintegrated, customer-owned, customer-sited would 

be the future: little interest in CPV, so little funding 

Investment in CPV came later than for other 

technologies, so CPV infrastructure development 

lags

Some of today’s investors are secretive, preventing 

companies from working together to create 

infrastructure

National Renewable Energy Laboratory 20 Innovation for Our Energy Future

Infrastructure = standards, knowledge of how to test for reliability, 

development of supply chain, etc.
Why so hard? – many tradeoffs

National Renewable Energy Laboratory 21 Innovation for Our Energy Future

Performance

Cost Reliability
Use two views

National Renewable Energy Laboratory 22 Innovation for Our Energy Future

Design

Bird’s eye view

Diagnose

Worm’s eye view
Outline

• Overview of PV – Opportunity for CPV

• Fundamentals of concentrating PV

• Advantages

• Primary approaches (High & low concentration)

• Designing from the system perspective

• Design considerations

• Thermodynamic limit of concentration

• Refractive vs reflective optics

• Concentration ratio, f number, etc.

• Thermal considerations

• Keeping it clean and dry

• Cells

National Renewable Energy Laboratory 23 Innovation for Our Energy Future
Fundamentals - concentrating optics

National Renewable Energy Laboratory 24 Innovation for Our Energy Future

Finite size of sun limits concentration – acceptance 

angle must be at least as big as sun’s disk

Sun emits light in all directions: Sun

Earth Sun tan θ = (sun radius)/(sun-earth distance)

tan θ = (7 X 108 m)/(1.5 X 1011 m)

θ = 0.27 °

θ
Fundamentals – non-imaging optics

National Renewable Energy Laboratory 25 Innovation for Our Energy Future

Nonimaging Optics – Roland Winston, Juan Minano, 

Pablo Benitez, Academic Press, 2004.

Concentration limit (Cmax):

linear focus Cmax = n/(sin θ) ~ 200 X (if n=1) 

point focus Cmax = n2/(sin2 θ) ~ 40,000 X (if n=1)

In practice, a larger acceptance angle is desired to 

allow alignment and tracker error; for C = 500 X, the 

(half) acceptance angle may approach 2.5°, or higher 

if n>1

An optical design may be judged by its acceptance 

angle relative to the concentration ratio

Earth Sun tan θ = (sun radius)/(sun-earth distance)

tan θ = (7 X 108 m)/(1.5 X 1011 m)

θ = 0.27 °
Fundamentals – acceptance angle

National Renewable Energy Laboratory 26 Innovation for Our Energy Future

If acceptance angle of optics is ± 0.27°, then there is 

zero tolerance for alignment and imperfections: 

measured acceptance angle will be ~ 0.03°

Acceptance angle measured for module or system may 

reflect alignment more than optics

Reported acceptance angle may be quoted for 90%, 

80%, or 50% point

Earth Sun tan θ = (sun radius)/(sun-earth distance)

tan θ = (7 X 108 m)/(1.5 X 1011 m)

θ = 0.27 °
Fundamentals – non-imaging optics

National Renewable Energy Laboratory 27 Innovation for Our Energy Future

For low-X approaches, tracking may not be essential

Can’t collect diffuse light outside of this angle for given concentration

For point focus, concentration is squared

1

2

4

6

8

10

2

4

6

Maximum acceptance angle (°)

2 3 4 5 6 7 8 9

10

2

Concentration in one axis

n=1

n=1.5
Reflective vs refractive: statistics are shifting 

National Renewable Energy Laboratory 28 Innovation for Our Energy Future

Find Boes quote

Wisdom or misdirection??

Number of companies developing CPV
Refractive vs reflective – alignment tolerance

National Renewable Energy Laboratory 29 Innovation for Our Energy Future

Refractive elements are more 

tolerant to misalignment

1° alignment error causes 

~0.5° change in refracted light

Reflective elements must 

be accurately aligned

1° alignment error causes 

2° change in reflected light

1° misalign

2° rotation

Mirror
Refractive vs reflective – chromatic aberration

National Renewable Energy Laboratory 30 Innovation for Our Energy Future

Refractive 

chromatic aberration

Blue light has shorter 

focal length

Reflective

NO chromatic aberration

Mirror
Refractive vs reflective – chromatic aberration

National Renewable Energy Laboratory 31 Innovation for Our Energy Future

Refractive 

chromatic aberration

Miller, et al SPIE 2009

Focal length may 

change by ~2%
Refractive – changes with T & RH are small

National Renewable Energy Laboratory 32 Innovation for Our Energy Future

Refractive 

aberration

Ralf Leutz

Focal length 

change < 1%
Effect of chromatic aberration – from R. Winston

National Renewable Energy Laboratory 33 Innovation for Our Energy Future

Difference in optical efficiency is mostly due to the absorption 

spectrum of the materials.

Difference in acceptance angle is due to “chromatic aberration”. 

Configuration: Primary + Secondary

•Primary: Fresnel lens, material: PMMA

•Secondary: non-imaging optics, material: glass

Geometric Concentration: 711X

Maximum Incident angle on cell: 65 Degree

System is designed for 550 nm

Source Spectrum Optical 

Efficiency

Acceptance Angle

(Degrees)

550 nm 84.8% 1.68

AM1.5 clipped 

between 

300nm and 1900nm

83.2% 1.62
Refractive vs reflective – many details

National Renewable Energy Laboratory 34 Innovation for Our Energy Future

Refractive 

Fresnel lens –

imperfections

Soiling; abrasion

Glass vs PMMA vs 

Glass/silicone

Reflective

Front vs back-surface 

mirror

Soiling; abrasion

Off axis, or shade cell

Mirror

1. Cost

2. Weight

3. Optical

performance
Concentration ratio – for multijunction

National Renewable Energy Laboratory 35 Innovation for Our Energy Future

Cell cost drives design to 

> 1000X

Can cell cost decrease?

If you’re designing for over 

500X, ask yourself why 

you’ll be successful

Tracker drives design to < 

~500X

Smart tracker isn’t enough

- thermal expansion

- wind

- cost of rigid structure 
Concentration ratio – for silicon

National Renewable Energy Laboratory 36 Innovation for Our Energy Future

Higher concentration:

- Lower cell cost

- Cell 

packaging/cooling is 

smaller area

Lower concentration:

- Can use Si modules

- Tracking accuracy is 

easier

- Easiest product 

development
Cell size design trade offs

National Renewable Energy Laboratory 37 Innovation for Our Energy Future

Large cells and optics

Reduced part count

Helps make structure rigid

Extreme is dish with 

replaceable receiver

Can use active cooling

Modularity can be advantage

Small cells and optics

Reduced materials cost

Aesthetic appeal

Heat is distributed

Smaller currents
Design trade offs – f number

National Renewable Energy Laboratory 38 Innovation for Our Energy Future

Higher f number

Alignment tolerance is wider 

(bigger depth of fields)

If you use low f number, analyze the effects of imperfect optics 

and alignment

Lower f number

Reduces thickness

Innovative designs may 

have aesthetic appeal
Design trade offs – thermal management

National Renewable Energy Laboratory Innovation for Our Energy Future

Cell

Heat sink

Optic

Electrical 

contact

Small ∆T

Electrical isolation

No voids

- Pure, single-crystal materials usually have good thermal 

conductivity

- Impurities and structural defects (dislocations or grain 

boundaries) can affect thermal conductivity

39
Thermal resistivity varies with composition

National Renewable Energy Laboratory 40 Innovation for Our Energy Future

Adachi, J. Appl. Phys. 54(4) p.1844 (1983)

Thermal transport through a pure, single crystal is much higher than for imperfect crystal
Crude thermal analysis - ∆T within cell

National Renewable Energy Laboratory 41 Innovation for Our Energy Future

Composition Thickness (μm) Thermal 

conductivity 

(W/cmK)

∆T for heat 

flux of 23 

W/cm2 (°C)

GaInP 1 0.05 0.05

GaAs 3 0.46 0.02

Ge 175 0.6 0.7

GaInP(50%In) 2.7 0.05 0.1

AlGaInP(grade) 1 0.05 0.05

GaInAs(4%In) 2.7 0.2 0.03

GaInP(grade) 3 0.05 0.1

GaInAs(37%In) 3 0.05 0.1

500 suns @ 850 W/m2; 85% optical efficiency; 35% cell efficiency: the 

waste heat is 23 W/cm2. (Ignore radiative transfer)

3J on Ge

Inverted 3J 

metamorphic

T drop within cell is not a serious problem under most circumstances
Crude thermal analysis – ∆T to heat sink

National Renewable Energy Laboratory 42 Innovation for Our Energy Future

Composition Thickness 

(μm)

Thermal 

conductivity 

(W/cmK)

∆T for heat flux 

of 23 W/cm2

(°C)

Solder (epoxy) 30 0.4 (0.02) 0.2 (4)

Cu 250 3.9 0.1

AlN (Al2O3) 600 1.7 (0.25) 0.8 (6)

Cu 250 3.9 0.1

Solder (epoxy) 30 0.4 (0.02) 0.2 (4)

Direct 

bonded 

copper

Credle, Dehmel, 

Schulz-Harder

ICSC5 (2008)
Crude thermal analysis – ∆T to ambient

National Renewable Energy Laboratory 43 Innovation for Our Energy Future

Technology Mounting ∆T for ~1000 W/m2

(°C)

Flat plate Open rack ~30

Flat plate Insulated on back ~60

Conclude:

1.∆T within cell is small

2.∆T between cell and heat sink is larger

3.∆T between heat sink and ambient is largest

T drop from heat sink to ambient may be similar 

to flat-plate’s module-to-ambient ∆T

Do the optics act as insulation?
Design trade offs – how to keep the dirt out?

National Renewable Energy Laboratory 44 Innovation for Our Energy Future

Issue: need to keep dirt and water out

• Condensation of water can obscure lenses

• Condensation of water can fry cells

• If air tight, then pressure changes deform system

• Cells may run hot

Mundane issue can be huge engineering challenge
Design trade offs – keep the water out

National Renewable Energy Laboratory 45 Innovation for Our Energy Future

Water condensation on lenses

Mundane issue can be huge engineering challenge

Araki, ICSC5, 2008
Bird’s eye view – factory vs installation

National Renewable Energy Laboratory 46 Innovation for Our Energy Future

Build at the factory

Reduces installation costs

Build in the field

Reduces transport costs
Bird’s eye view – wind effects

National Renewable Energy Laboratory 47 Innovation for Our Energy Future

The losses associated with wind 

stow depend on the local weather 

and the control parameters:

-Wind speed for stow

-Time stay in stow

Araki, 33rd PVSC 2008

High winds can cause loss of 

efficiency if acceptance angle is small
Bird’s eye view – system shading

National Renewable Energy Laboratory 48 Innovation for Our Energy Future

Pedestal

Carousel

Pivot

Shading can affect system performance;

Shading of every cell is different from 

shading one cell in a string

New solutions are now available
Bird’s eye view – system configuration

National Renewable Energy Laboratory 49 Innovation for Our Energy Future

Pedestal is less disruptive

• Dual land use

• Bureau of Land Management –

don’t disrupt habitat

• Minimal site preparation

• Fast installation

Carousel

• Can maximize land use

• Avoid being in the wind, so 

don’t need as much strength

• Can use on roof top

• Need to be able to adjust

• Soiling; plant growth?

Amonix photo General Energy photo
Bird’s eye view – 1987 Barstow installation

National Renewable Energy Laboratory 50 Innovation for Our Energy Future

Amonix photo

General Energy photo

More than 1 MW installed in 1980s
Markets by location; CPV sunshine is better in west

3-4 kWh/m2/day

Two-axis tracked

Direct (DNI)

Fixed, latitude tilt

Global

http://rredc.nrel.gov/solar/old_data/nsrdb/1961-1990/redbook/atlas/

6-7 kWh/m2/day

5-6 kWh/m2/day

4-5 kWh/m2/day
Markets by location; but sunshine isn’t everything

http://votesolar.org/images/Germany_US_2.jpg

kWh per installed kW per 

yr

Germany has one of biggest markets despite poor solar resource

Germany
Property Tax Incentives for Renewables

State exemption or special assessment + local government option

www.dsireusa.org / October 2009

Puerto Rico

Local governments authorized to offer exemption (no state exemption or assessment)

State exemption or special assessment only

DC

See this website for related information
Reliability - bond to heat sink

National Renewable Energy Laboratory Innovation for Our Energy Future

Cell

Heat sink

Optic

Electrical 

contact

Small ∆T

Electrical isolation

No voids

T cycle OK

• Borrowing experience from power electronics and DBC (direct 

bonded copper) makes this a smaller issue

• Currently, there is a debate about the best way to test this bond –

see standards section

54
Reliability – UV exposure 

National Renewable Energy Laboratory 55 Innovation for Our Energy Future

Analysis of transmitted optical spectrum enabling accelerated testing of 

CPV designs

SPIE 2009 David Miller, et al
Reliability – UV exposure 

National Renewable Energy Laboratory 56 Innovation for Our Energy Future

SPIE 2009 David Miller, et al

PMMA absorbs UV strongly, protecting cell, so UV stress is not so high
Outline

• Overview of PV – Opportunity for CPV

• Fundamentals of concentrating PV

• Advantages

• Primary approaches (High & low concentration)

• Designing from the system perspective

• Design considerations

• Thermodynamic limit of concentration

• Refractive vs reflective optics

• Concentration ratio, f number, etc.

• Thermal considerations

• Keeping the dirt and water out

• Cells (multijunction & silicon)

National Renewable Energy Laboratory 57 Innovation for Our Energy Future
Smallness enables use of highest efficiency cells

58
Why multijunction?

Power = Current X Voltage

5x1017

4

3

2

1

0

Solar spectrum

0 1 2 3 4

Photon energy (eV)

Band gap

of 0.75 eV

5x1017

4

3

2

1

0

Solar spectrum

0 1 2 3 4

Photon energy (eV)

Band gap

of 2.5 eV

High current, 

but low voltage

High voltage, 

but low current

Highest efficiency: Absorb each color of light with a material 

that has a band gap equal to the photon energy
in

4 5 6 7 8 9

1

2 3 4

Energy (eV)

Multijunction cells use multiple materials to 

match the solar spectrum
Expected efficiency depends on band gap

20

1.0 1.2 1.4 1.6 1.8 2.0

band gap (eV)

1.0

Voc (V)

40

20

Jsc

2

) 0.90

0.80

FF

Efficiency

Voc

Jsc

FF

Could have 

higher efficiency 

for 

monochromatic 

light

or, we should 

look for a set of 

materials to 

match portions of 

the spectrum
Efficiency increases with concentration

I ∝ flux

V ∝ log(flux)

Power = I*V ∝ flux*log(flux) 

Efficiency ∝ log(flux)

Graphs from http://www.emcore.com/assets/photovoltaics/CTJ_B_Web.pdf

Efficiency increases with 

concentration until the series 

resistance becomes a problem

At 7 A and 3 V, 4 mΩ causes 1% loss 
Response of three junctions

National Renewable Energy Laboratory 63 Innovation for Our Energy Future

Data for commercial cells

http://www.spectrolab.com/DataSheets/TerCel/C1MJ_CDO-100.pdf

http://www.emcore.com/assets/photovoltaics/CTJ_B_Web.pdf

GaInP InGaAs Ge

Three junctions are measured 

using light bias
Temperature coefficients: ~-0.2%/°C

National Renewable Energy Laboratory 64 Innovation for Our Energy Future

Data for commercial cells

http://www.spectrolab.com/DataSheets/TerCel/C1MJ_CDO-100.pdf

http://www.emcore.com/assets/photovoltaics/CTJ_B_Web.pdf

GaInP InGaAs Ge

Temperature Dependence at 800 Suns 

∆Voc = -4 mV/°C

∆Jsc = 7.2 mA/°C

∆Efficiency = -0.06% (absolute)/°C

Temperature coefficients

• smaller than for c-Si

• smaller at higher conc.

• can depend on spectrum

Friedman “Modeling of tandem cell 

temperature coefficients” 25th PVSC, 

p. 89 (1996).
Is there room to grow?

Theoretical & experimental efficiencies

Marti & Araujo, Solar Energy Mat. 

& Solar Cells 43 p. 203 (1996) 

Kurtz, et al Prog. In PV, 2008.

80

60

40

20

0

1 2 3 4 5 6 7

Number of junctions

Theoretical

(detailed balance)

Amorphous 

Single-crystal

Polycrystalline

One sun

Infinite

junctions

80

60

40

20

0

1 2 3 4 5 6 7

Number of junctions

Theoretical

(detailed balance)

Single-crystal

Poly-crystalline

Concentrated sunlight

Infinite

junctions

Higher efficiencies by: 1. more junctions, 2. use concentration, 

3. improve material quality
Approaches to multijunction

+

-

Monolithic

+

-

-

+

4 (or more)-terminal

Mechanical stack

1

2

Wafer bonded

Many other 

configurations

41.1%

champion 

Fraunhofer

42.8%

champion

DARPA

+

-
Multijunction cells can be 

assembled in many ways
in

4 5 6 7 8 9

1

2 3 4

Energy (eV)

Choose materials with band gaps that 

span the solar spectrum

For series 

connection 

(monolithic 

approach): equal 

photocurrents
Lattice-matched 3 junction

2.8

2.4

2.0

1.6

1.2

0.8

0.4

Bandgap (eV)

5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1

Lattice Constant (Å)

AlP

AlAs

GaP

GaAs

GaSb

InP

InAs

Ge

Si

Lattice matched materials give high crystal quality

41.6%

Spectrolab

1.9 eV

1.4 eV

0.7 eV

Current record: http://boeing.mediaroom.com/index.php?s=43&item=810
Lattice-mismatched 3 junction

2.8

2.4

2.0

1.6

1.2

0.8

0.4

Bandgap (eV)

5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1

Lattice Constant (Å)

AlP

AlAs

GaP

GaAs

GaSb

InP

InAs

Ge

Si

41.1%

Dimroth

2009

1.8 eV

1.3 eV

0.7 eV

http://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/worldrecord-41.1-efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-ise
41.1% efficiency cell (Fraunhofer ISE) 

3 junctions: top two are mismatched 

Ge bottom cell 

and substrate

Grade

Ga0.83In0.17As middle cell

Ga0.35In0.65P top cell

http://www.ise.fraunhofer.de/press-and-media/press-releases/press-releases-2009/world-record-41.1-

efficiency-reached-for-multi-junction-solar-cells-at-fraunhofer-ise

1.75 eV

1.3 eV

0.7 eV

Not to scale

Tunnel junctions 

not shown

41.1% record by Fraunhofer ISE

n-on-p junctions 

connected by 

tunnel junctions
Inverted lattice-mismatched (IMM)

2.8

2.4

2.0

1.6

1.2

0.8

0.4

Bandgap (eV)

5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1

Lattice Constant (Å)

AlP

AlAs

GaP

GaAs

GaSb

InP

InAs

Ge

Si

Lattice matched materials are grown first

40.8%

Geisz

APL

2008

1.8 eV

1.3 eV

0.9 eV
GaInP/Ga(In)As/GaInAs Ultra-Thin Tandem Cell

1.8 eV GaInP

1.3 eV GaInAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

GaAs Substrate

Advantages:

• Path to higher efficiency – 40.8% 

so far

• Reuse of substrate or use of 

impure substrate can reduce cost 

(and use of semiconductor material)

Inverted metamorphic approach

Invented by Mark Wanlass

40.8%: John Geisz, APL, 2008

R&D 100 Award.
Handle

GaInP/Ga(In)As/GaInAs Ultra-Thin Tandem Cell

1.8 eV GaInP

1.3 eV GaInAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

GaAs Substrate

1.8 eV GaInP

1.3 eV GaInAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

GaAs Substrate

Advantages:

• Path to higher efficiency – 40.8% 

so far

• Reuse of substrate or use of 

impure substrate can reduce cost 

(and use of semiconductor material)

Inverted metamorphic approach

Invented by Mark Wanlass

40.8%: John Geisz, APL, 2008

R&D 100 Award.
Handle

GaInP/Ga(In)As/GaInAs Ultra-Thin Tandem Cell

1.8 eV GaInP

1.3 eV GaInAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

Advantages:

• Path to higher efficiency – 40.8% 

so far

• Reuse of substrate or use of 

impure substrate can reduce cost 

(and use of semiconductor material)

Inverted metamorphic approach

Invented by Mark Wanlass

40.8%: John Geisz, APL, 2008

R&D 100 Award.
Lattice mismatched growth (IMM)

Step grade of composition can 

confine defects to graded 

layers Geisz, et al Appl. Phys. Lett. 93, p. 123505 (2008)
Next generation inverted latticemismatched

2.8

2.4

2.0

1.6

1.2

0.8

0.4

Bandgap (eV)

5.4 5.5 5.6 5.7 5.8 5.9 6.0 6.1

Lattice Constant (Å)

AlP

AlAs

GaP

GaAs

GaSb

InP

InAs

Ge

Si

The inverted structure opens the parameter space

1.9 eV

1.5 eV

1.1 eV

0.7 eV
Ways to add In to GaAs to make GaInAs

Ordered Random Quantum

wells

Quantum

dots
Combine multiple materials

Modular approach is limited only by creativity

- band gap combinations matched to solar spectrum

- material quality should be excellent
Useful numbers (& challenge) 

• 1 MW requires ~ 1000 4 inch (100 mm) wafers @ 500X

• Estimates may be as optimistic as 350 4” wafers/1 MW @ 1000X

• Actual numbers depend on yield, active area/wafer, optical 

losses, etc.

• Cost target for largest (up to 4 TW/yr) market is $1/W for module 

($2/W installed), with cell being small part of that (~$100/4 inch 

wafer for 4 TW/yr market) 

• $1000/wafer can enter market now, but will limit size of market in 

future

500 X

0.1 W/cm2 40,000 cm2 4 kW input

4 m2

50 W/cm2 78 cm2 1 kW output

Irradiance

At lens

At cell

Area Power

100 mm wafer
Effect of changing spectrum 

National Renewable Energy Laboratory 81 Innovation for Our Energy Future

Derived from 

McMahon, 29th PVSC

The instantaneous power is somewhat sensitive to cell 

design, but the energy is much less sensitive. The loss is a 

few per cent, but the average performance is fairly consistent.

Complicates troubleshooting.

1.02

1.00

0.98

0.96

0.94

0.92

Relative power or energy

1.0 1.2 1.4 1.6 1.8 2.0 2.2

Relative top-cell thickness

Power at noon on 

Hot Sunny Day

Energy for day

 Cold Sunny Day

 Nice Day

 Hot Sunny Day
Effect of changing spectrum

National Renewable Energy Laboratory 82 Innovation for Our Energy Future

Araki “Which is the Best Number of Junctions for Solar 

Cells under Ever-changing Terrestrial Spectrum?” 3rd

WCPEC (2003)
Outline

• Overview of PV – Opportunity for CPV

• Fundamentals of concentrating PV

• Advantages

• Primary approaches (High & low concentration)

• Designing from the system perspective

• Design considerations

• Thermodynamic limit of concentration

• Refractive vs reflective optics

• Concentration ratio, f number, etc.

• Thermal considerations

• Open vs closed

• Cells (multijunction & silicon)

National Renewable Energy Laboratory 83 Innovation for Our Energy Future
Smallness enables use of highest efficiency cells

84
Silicon concentrator cells (<250X)

• For low concentration, may be able to use one-sun 

cells or modules

• For higher concentration, need to lower series 

resistance: 6 A for a 150 mm cell @ 1 sun.

• Auger recombination limits efficiency above ~100X

• SunPower was first to offer ‘off-the-shelf’ silicon 

concentrator cells (for ~250X)

• Today, SunPower makes the highest efficiency one-sun 

cells

• Many companies are capable of making these cells, but 

availability of silicon concentrator cells has been a 

problem for 20 years

National Renewable Energy Laboratory 85 Innovation for Our Energy Future
Silicon cells – back point contact

• No grids on front

• Carefully passivated front and minimal contact area on 

back can lead to high efficiency

• Possible to handle large currents because contacts are 

on back

• 22% efficiency at one sun; concentration can increase 

to ~28%

• T coeff -0.38%/°C (at one sun) 

National Renewable Energy Laboratory 86 Innovation for Our Energy Future

n p n p n p n p n p n
Silicon cells – buried laser groove

• Front grids are put in a groove formed by a laser

• Reduces shadowing losses for given grid conductance

• Goes to high current

National Renewable Energy Laboratory 87 Innovation for Our Energy Future

http://pvcdrom.pveducation.org/MANUFACT/BCSC.HTM
Bird’s eye view – many tradeoffs

There are dozens of design tradeoffs/choices with no 

clear winners and optimum may change

New ideas/technologies will affect optimal design

Optimal design is very dependent on application

Each company reaches a different conclusion

National Renewable Energy Laboratory 88 Innovation for Our Energy Future

What will CPV systems look 

like 100 years from now?
Commodity market

National Renewable Energy Laboratory 89 Innovation for Our Energy Future

Luxury

100 years ago

Costs pennies 

Today

Maturing industry

Think of the light bulb

How much has it changed in the last 100 years?
Valley of death – too many options

National Renewable Energy Laboratory 90 Innovation for Our Energy Future

The engineer wants to tweak, then gets lost in the valley of death.

Often, the business manager rather than the engineer should 

decide when to move into manufacturing. 

Valley of death

The grass is greener.....
Outline

• Overview of PV – Opportunity for CPV

• Fundamentals of concentrating PV

• Why CPV?

• Design considerations

• Bird’s eye view

• Sorting it out

• Worm’s eye view

• Status of industry

• Standards

• Many companies

• Improving performance

• Ramping up

National Renewable Energy Laboratory 91 Innovation for Our Energy Future
Use two views

National Renewable Energy Laboratory 92 Innovation for Our Energy Future

Design

Bird’s eye view

Diagnose

Worm’s eye view
Worm’s eye view – what happened?

National Renewable Energy Laboratory 93 Innovation for Our Energy Future

“Fast” optics 

Reduces part count for 

thickness

Innovative designs may have 

aesthetic appeal

Many losses, what happened?

Ideal performance for solar resource - 100%

Electricity out – 64%

Output for 

prototype is less 

than expected
Worm’s eye view – what happened?

National Renewable Energy Laboratory 94 Innovation for Our Energy Future

“Fast” optics 

Reduces part count for 

thickness

Innovative designs may have 

aesthetic appeal

So many potential losses, how do we identify solutions?

Ideal performance for solar resource - 100%

First reflection loss - 96%

Imperfect optics - 93%

Second reflection loss - 89%

Secondary optics loss – 84%

Cell nonuniform illumination - 82%

Cell temperature - 75%

Cell spectrum - 73%

Cell stringing - 70%

Resistance of wiring - 69%

Tracker misalignment - 65%

Electricity out - 64%
Worm’s eye view – Start with components

National Renewable Energy Laboratory 95 Innovation for Our Energy Future

Characterize components first

Cell

Lens

Then move to lens-cell combination

Move to module only after understand single cell

Module may show different effects because of variable alignment, etc.
Worm’s eye view – Spectral issues

National Renewable Energy Laboratory 96 Innovation for Our Energy Future

Demand matched reference cells from your cell supplier

Why use matched reference cells?

- Quantify optical efficiency for each junction

- Depth of field and acceptance angle may be different for each 

junction (Use special mount that allows you to move each cell)

- Evaluate current matching of multijunction cell for optical design 

(may vary as a function of alignment)

- Reference cells quantify variation in spectrum

- Thorough characterization before start stringing cells

MJ top middle bottom MJ top mid bottom

Collimating tubes
Worm’s eye view – Use all parameters

National Renewable Energy Laboratory 97 Innovation for Our Energy Future

Short-circuit current - optical efficiency

Open-circuit voltage – cooling (adjust for concentration 

using transient)

Fill factor for reference cells - electrical resistance or 

shorts; non-uniform illumination

McMahon – PIP 2008

Fill factor for multijunction cell –

spectral effects for cells, but 

what about for modules?
Worm’s eye view – Sorting out a module

National Renewable Energy Laboratory 98 Innovation for Our Energy Future

Be creative; cover the optics; use thin-film filters with partial 

transmission

Characterize module at maximum power point – short-circuit will 

miss many problems

Module should have same acceptance angle as single cell/optic

If not, measure cell temperature or use filter to see which cell is 

limiting the current; bypass diodes should not be hot; fill factor of 

module should be similar to ff of single cell without showing 

evidence of bypass diodes turning on

Forward bias emission should be consistent
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

• However, modularity of CPV may be an advantage

National Renewable Energy Laboratory 99 Innovation for Our Energy Future
Outline

• Overview of PV – Opportunity for CPV

• Fundamentals of concentrating PV

• Why CPV?

• Design considerations

• Bird’s eye view

• Sorting it out

• Worm’s eye view

• Status of industry

• Standards

• Many companies

• Is it a turning point?

National Renewable Energy Laboratory 100 Innovation for Our Energy Future
Standards for CPV

• Standards provide a foundation for the industry

• Challenging because CPV comes in so many flavors

• CPV standards were not developed early on

• CPV standards are now progressing quickly

National Renewable Energy Laboratory 101 Innovation for Our Energy Future
Power rating – 850 or 1000 W/m2?

Flat plate

• 1000 W/m2

National Renewable Energy Laboratory 102 Innovation for Our Energy Future

CPV

• 850 W/m2

1000

950

900

850

800

750

700

DNI (Direct normal irradiance) W/sq m

900 950 1000 1050 1100 1150

GNI (Global normal irradiance) W/sq m

890

920

860 Data taken in 

Golden, CO 

(no filtering)

Tags give ratio 

of DNI/GNI 

normalized to 

1000 W/m2
Power rating – ambient, cell, or heat sink T? 

Flat plate module rating 

• 1000 W/m2 

• 25°C module T 

• Si: T coef ~ -0.38 or -0.5%/°C 

National Renewable Energy Laboratory Innovation for Our Energy F

103 

CPV module rating 

• 850 W/m2 

• 20°C ambient T 

• T coef ~ -0.24%/°C 

Graph shows 

expected power 

assuming irradiance 

is same as irradiance 

used for rating 

Is this fair? 

Implies higher 

capacity factor 

Which gives better 

indication of 

performance? 

1000

950

900

850

800

750

Power for 1000 W module (W)

700

0 10 20 30 40

Ambient Temperature (°C)

CPV

Tcoef = -0.24%/°C

Silicon open rack

Tcoef = -0.38%/°C

ΔT @ 1000 W/sq m = 29°C

Silicon close-roof mount

Tcoef = -0.5%/°C

ΔT @ 1000 W/sq m = 49°C
Standards – Power rating

National Renewable Energy Laboratory 104 Innovation for Our Energy Future

• Currently being developed by IEC

• PVUSA used 850 W/sq m DNI, 20°C ambient T, 

and 1 m/s wind

• ASTM E2527 uses 850 W/sq m DNI, 20°C 

ambient T, and 4 m/s wind

• ASTM G173 spectrum for direct beam, 

integrates to 900 W/sq m

• Debate is ongoing about 

• 1000 vs 850 W/sq m irradiance

• 20°C ambient vs 25°C “module” T

• 1 m/s vs 4 m/s wind speed if use ambient
Standards – Qualification test: IEC 62108

National Renewable Energy Laboratory 105 Innovation for Our Energy Future

• Requires 7 modules and 3 receivers

• Tests include:

• Outdoor exposure – Cumulative DNI 1000 kWh/m2

• Thermal cycling – 500 cycles from -40 to 110°C*

• Bypass diode

• Humidity freeze – 20 cycles from -40 to 85°C* (85% 

RH)

• Damp heat – 85°C, 85% relative humidity for 1000 h

• Mechanical load

• Terminations

• Hail impact

• Hot spot 

*Other options are available if 110°C is too hot for lenses
Standards – Qualification test: IEC 62108

National Renewable Energy Laboratory 106 Innovation for Our Energy Future

• Current debate has to do with application of 

forward bias current during thermal cycling

Cell

Heat sink

Inject heat into cell to give real T profile

• Forward bias current causes 

cell failure if:

- Thermal control is lost 

(what we want)

- Cell is defective (not what 

we want)

Forward bias current may be best way to detect failure. 

How much is optimal?
Concentrator system companies using low-X (mostly Si)

North America

– Covalent Solar

– ENTECH (> 100 kW in 

1990s)

– Greenfield Solar

– JX Crystals (>100 kW in ‘07)

– MegaWatt Solar (50 kW in 

‘08)

– Netcrystal

– Opel International

– Optony

– Pacific Solar Tech

– Prism Solar Technologies

– QD Soleil

– Skyline Solar

– Solaria

– Solbeam

– Stellaris

– SV Solar

– Thales Research

Europe/Israel

– Abengoa Solar

– Archimedes

– Cpower

– Maxxun

– Pythagoras Solar

– Silicon CPV

– Whitfield Solar

– WS Energia (263 kW in 

‘08)

– Zytech Solar

Australia

– Sunengy

Asia

Everphoton
Concentrator system companies using III-V cells

North America

– Abengoa Solar

– American CPV

– Amonix

– Boeing

– Concentrating Technologies

– Cool Earth Solar

– Emcore

– Energy Innovations

– EnFocus Engineering

– ENTECH

– GreenVolts

– IBM

– Menova Energy

– Morgan Solar

– Opel International

– Pyron Solar 

– Scaled Solar

– SolarTech

– SolFocus

– Soliant Energy

– SUNRGI

– Xtreme Energetics

Europe

– Concentracion Solar La Mancha

– Concentrix Solar

– ENEA

– Guascor Foton

– Isofoton

– Sol3g

– SolarTec

– Zytech Solar

Australia

– Solar Systems

– Green & Gold

Asia

– Arima Ecoenergy

– Daido Steel

– Delta Electronics (ending 12/09)

– ESSYSTEM

– EverPhoton

– Sharp
Amonix

•Founded in 1989

•Original design used 

Silicon; now use III-V

•~410 kW in Arizona

•~200 kW in Nevada

•~1 MW in Spain

•~8 MW jointly with 

Guascor in Spain

•Report 25% AC 

efficiency with III-V

National Renewable Energy Laboratory 109 Innovation for Our Energy Future
Concentrix

•100 kW Casaquemada, 

Spain 23% AC efficiency 

(6 kW 25% and 27% for 

module)

•25 MW/y production 

capacity

•Spun off from 

Fraunhofer ISE in 2005

National Renewable Energy Laboratory 110 Innovation for Our Energy Future
SolFocus

•200 kW in Puertollano, Spain

•300 kW in Almoguera, Spain

•10 MW field started in Greece

•$150M in funding; founded in 2005

•Design has relatively large acceptance angle

National Renewable Energy Laboratory 111 Innovation for Our Energy Future

Glass
Boeing

•Using reflective optics 

(off-axis)

•Wide acceptance 

angle

National Renewable Energy Laboratory 112 Innovation for Our Energy Future
Semprius

•Is example of companies bringing in new 

approaches

•Printing technique allows parallel assembly

•Large part count is acceptable when use 

parallel assembly

•Reduce amount of material to reduce cost

National Renewable Energy Laboratory 113 Innovation for Our Energy Future
Many creative designs

Enfocus

National Renewable Energy Laboratory 114 Innovation for Our Energy Future

Cool Earth Solar
Abengoa

•1.2 MW low concentration in Sevilla, Spain in 

2008 (largest low-X CPV); 2 GWh/yr

•1.5 X (Iso-Photon) and 2.2 X (Artesa & 

SolarTech )

•2-axis tracked

National Renewable Energy Laboratory 115 Innovation for Our Energy Future

http://www.abengoasolar.com/sites/solar/en/our_projects/solucar/sevilla_pv/index.html

http://www.abengoasolar.com/sites/solar/resources/pdf/en/Sevilla_PV.pdf

Mirror

Module
III-V cell companies with datasheets 

Spectrolab (cells and cells with welded leads)

– www.spectrolab.com/prd/terres/cell-main.htm

– Minimum average efficiency: 36% (38.5% announced) @ 50 

W/cm2

– Plan to ship 35 MW of cells in 2009 and 100 MW in 2010 

(lattice-matched, 3-junction 500X cells)

EMCORE (cells and receivers)

– www.emcore.com/solar_photovoltaics

– Typical efficiency: 39% @ 500 suns

CESI (cells) – www.cesi.it/pagina_2.asp?livello=2&cp=03040000&c2=03040800&c3=&cc=&lang=EN

– Efficiency > 30% @ > 100 suns
Other companies with multijunction 

cell capability (data sheets on request)

North America

– Cyrium

– JDSU 

– Microlink

– RFMD

– Solar Junction

– Spire

Europe

– Azur Solar (RWE)

– IQE

– QuantaSol

Asia

– Arima

– Epistar

– Sharp

– VPEC

Research laboratories, 

universities, and companies 

in R&D or stealth phases not 

included in this list
Microlink – remove cost of substrate?

National Renewable Energy Laboratory 118 Innovation for Our Energy Future
Supply & Demand - of multijunction cells

Current supply

– Emcore/Spectrolab/Azur 

epi capacity: hundreds of 

MWs (500X)

– Capacity depends on 

space cell demand

Future supply

– New companies could 

dramatically increase 

supply

– Emcore/Spectrolab/Azur 

will expand under 

contract (6 months - 3 

years lead)

Current demand

– Actual installation rates 

MW/yr

– Tens of MWs purchases 

for planned expansion

Future demand

– Projections vary 

dramatically 

– Potential for GWs

– Expansion limited by 

automation

– Expansion limited by risk 

of unproven product

– Expansion limited by 

banking crisis
Receivers - system integration

(some designs require mounting of cells directly to optics)

Needs:

– System determines receiver design, so every CPV 

system may need new design

– Need automated cell mounting

– Receiver designs must be carefully tested

Current status:

– Off-the-shelf receivers typically use 1 cm X 1 cm 

cells

– Manufacturing of receivers remains a challenge 

for many companies

The need for custom-designed receivers (and their integration 

with the optics) is still challenged (should this be the job of the 

cell supplier, the system integrator, or a 3rd company?)...
Other business needs

Substrates (Ge; could be GaAs in future)

Optics

Structural materials

Heat sinks

Electrical isolation

Alignment tools (automated assembly)

Trackers

Power conditioning
What has changed?

Companies entering field now tend to be bigger 

and more experienced at large-scale 

production (e.g. RFMD, JDSU)

System efficiencies are commonly ~25%

Several companies approaching 1 MW in field

Several companies are setting up 

manufacturing lines with 10s of MW capability

Is the industry nearing a turning point?
Turning point for industry?

National Renewable Energy Laboratory 123 Innovation for Our Energy Future

Last year, 

PHOTON International 

predicted 50 MW in 2009

60

50

40

30

20

10

0

CPV installations (MW)

2008 2009

Year

Predicted

Is the CPV industry 

ready to ramp 

production?

PHOTON International
National Renewable Energy Laboratory Innovation for Our Energy Future

What will CPV look like 

100 years from now?

Olson: “Many options are a 

curse and a blessing”

124

Thank you to the many 

who contributed to this 

and to the growing CPV 

industry