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Technology Overview

Solar Power International 2010

Los Angeles, CA

Sarah Kurtz 

Principal Scientist; Reliability 

Group Manager

Andreas Bett, Fraunhofer

Nancy Hartsoch, SolFocus

October 11, 2010

NREL/PR-5200-49713
Outline

• CPV systems – optimize the system

• Two primary approaches: multijunction or Si

• Multijunction cells

- Multiple companies/technologies >40%

- Strong technology/Strong business

• Optics: creativity could lead to new things

• Tracking: many designs, but still biggest 

reliability concern

• Standards: 

- Power rating: creating order from chaos 

• Slow start, but bright future for CPV

National Renewable Energy Laboratory 2 Innovation for Our Energy Future
Three approaches to PV (and lower cost)

National Renewable Energy Laboratory 3 Innovation for Our Energy Future

Front

Solar cell

Back

2. Thin film

3. Concentrator (CPV)

1. Silicon

CPV: Reduce 

semiconductor material; 

use high-efficiency cells
The Challenge of Concentrator PV

Simultaneously 

consider the whole 

system!

Optics

Cells

Module

Tracking/B

OS

Optimize the whole concentrator system !!!

Optimize:

• Performance

• Cost 

• Reliability

• Manufacturability

• Ease of shipping, installation, alignment, maintenance
National Renewable Energy Laboratory Innovation for Our Energy Future

Two primary concentrator approaches

High concentration

• 35% - 40% III-V cells

• 400X – 1500 X

Low concentration

• 15% - 25% Silicon cells

• 2X – 100 X

Amonix Skyline Solar

5
Multijunction (MJ) solar cells

National Renewable Energy Laboratory 6 Innovation for Our Energy Future
Why MJ? 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 voltage, but low current

Subbandgap light is lost

High current, but low voltage

Excess energy lost to heat

White light can be converted most efficiently by 

multiple materials
in

4 5 6 7 8 9

1

2 3 4

Energy (eV)

Choose materials with band gaps 

that span the solar spectrum

Multiple junctions –

currently 3 junctions 

in champion cells
Success of GaInP/GaAs/Ge (or ?) cell

Mars Rover powered by 

GaInP/GaAs/Ge cells

This very successful space cell is currently 

being engineered into systems for terrestrial use

Not a laboratory curiosity: records are often set on 

production hardware

Currently, eight groups claim ≥ 40% cells

Four cell architectures have achieved > 40%

Cells have been well tested for space applications

Spire:

42.3%!
Multijunction (MJ) solar cells 

≥ 40% - multiple designs

National Renewable Energy Laboratory 10 Innovation for Our Energy Future
Lattice-matched 3 junction is commercially available

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 though 

they do not provide optimal band gap combination

41.6% @ 364 suns

King 2009; 

24th PVSEC

Spectrolab

1.9 eV

1.4 eV

0.7 eV

Mars Rover powered 

by multijunction cells

(cells are well tested)

GaInP

Ga(In)As
Lattice mismatched growth gives new opportunities

Step grade of composition 

can confine defects to graded 

layers

1 μm

GaAsP 

step grade

220DF

GaAs0.7P0.3

GaP

XTEM

Larger 

lattice constant

Smaller 

lattice constant
New World Record – triple junction grown on 2 sides

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

Growth on both sides of wafer gives flexibility

42.3% @ 406 

suns

Spire – just 

announced

1.9 eV

1.4 eV

1.0 eV

GaInP

GaAs

GaInAs

GaAs substrate

(estimated, 

since details 

of the cell 

have not 

been 

published; 

“bifacial”)
Lattice-mismatched triple junction on Ge

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 mismatched materials give close to optimal band gap 

combination, but are more difficult to grow with high yield

41.1% @ 454 suns

Guter 2009, APL

Fraunhofer

1.7 eV

1.2 eV

0.7 eV

GaInAs

GaInP
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 followed by mismatched –

provides pathway to four-junction and higher efficiencies

40.8%

Geisz

APL

2008

(NREL)

1.8 eV

1.3 eV

0.9 eV

GaInP

Ga(In)As

GaInAs
Handle

GaInP/GaAs/GaInAs Ultra-Thin Tandem Cell

1.8 eV GaInP

1.3 eV GaAs

Transparent GaInP grade

Metamorphic 0.9 eV InGaAs

GaAs Substrate

1.8 eV GaInP

1.3 eV GaAs

Transparent GaInP grade

Metamorphic 1.0 eV InGaAs

GaAs Substrate

1.3 eV GaInAs

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

Inverted metamorphic approach

Invented by Mark Wanlass; 40.8%: Geisz, APL, 2008.
Quantum dot triple junction cells ~40% by Cyrium 

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

1.9 eV

1.4 eV + QD

0.7 eV

GaInP

Ga(In)As
Dilute nitride unique to Solar 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

This makes six different multijunction structures that could 

be viable for moving past 40%

1.9 eV

1.4 eV

GaInP

GaInNAs?

1.0 eV

~40%

@ 1000 

suns

Dilute nitride bottom 

junction with Eg of 

0.8 -1.4 eV

Platform for future 

generations of 

higher efficiency 

cells built on a high 

reliability latticematched 

architecture.

3-junction 

lattice 

matched
Kurtz, Prog. In PV, 2008.

80

60

40

20

0

Maximum Efficiency (%)

1 2 3 4 5 6

Number of junctions

Theoretical

(detailed balance)

Amorphous 

Single-crystal

Polycrystalline

One sun

80

60

40

20

0

Maximum Efficiency (%)

1 2 3 4 5 6

Number of junctions

Theoretical

(max. conc.)

Single-crystal

Polycrystalline

Concentrated sunlight

1000 kW/m2

500 kW/m2

(232X)

(180-

1000X)

(360X)

(14X)

Efficiency limit for multijunction cells

45% may be practical; 50% may be achievable 
Companies making multijunction CPV cells

Company Name/Web Link Location Comment

Arima Taipei, Taiwan Reported achieving >40% cells.

Azur Space (RWE) Heilbronn, Germany Reported 36% efficiency; custom designs available.

CESI Milano, Italy Datasheet reports efficiency >30%.

Compound Solar Technology Hsinchu Science Park, Taiwan Website shows I-V curve with 33.4% efficiency

Cyrium Ottawa, Canada Datasheet describes typical > 39% cells

Emcore Albuquerque, NM, USA Datasheet describes typical 39% cells and receivers at ~500 X. 

Epistar Hsinchu, Taiwan Multijunction cells in development

IQE Cardiff, Wales, UK Has demonstrated state-of-the-art efficiencies

JDSU Milpitas, CA, USA Advertises multijunction concentrator cells on website

Microlink Devices Niles, IL, USA Multijunction cells removed from substrate in development

Quantasol Kingston upon Thames, Surrey, UK Multijunction cells with quantum wells 

RFMD Greensboro, NC, USA Multijunction cells in development

Sharp Japan Has demonstrated high efficiencies; has not indicated plans for 

external commercialization.

Solar Junction San Jose, CA, USA “Approaching 40%”

Spectrolab (Boeing) Sylmar, CA, USA

Datasheet describes minimum average 36% cells and cell 

assemblies at 50 W/cm2. Will ship 35 MW in 2009, and plan to 

ship 100 MW in 2010 (@500X).

Spire Boston, MA, USA Announced achievement of 42.3% efficiency.

VPEC Ping-jen city, Taiwan Multijunction cells in development

Addition of companies like JDSU and RFMD adds financial credibility
Silicon concentrator cells

• Some companies use one-sun silicon cells

• SunPower sold Si CPV cells off the shelf a decade ago, but 

made a business decision to stop

• NaREC is currently the primary company with this business 

model

• Supply of Silicon concentrator cells remains a problem for 

this segment of the community
Optics –

Creativity can take 

us to new worlds

National Renewable Energy Laboratory 22 Innovation for Our Energy Future
Choices for optics – blessing or curse?

Refractive vs reflective

Add secondary to increase acceptance angle?

Small vs large elements

Planar (Fresnel) vs shaped (domed) elements

Acrylic vs silicone-on-glass vs many other materials

Short vs long focal length (f number)

Point focus (MJ CPV?) vs line focus (Si CPV?)

Filled (solid) optics vs transmission through air

Use of wave guides

Use of luminescence for concentration

National Renewable Energy Laboratory 23 Innovation for Our Energy Future
Examples of Concentrating Elements
Trackers –

Choice may 

depend on 

application 

National Renewable Energy Laboratory 25 Innovation for Our Energy Future
Choices for trackers

Pedestal vs distributed support

One axis (for Si CPV?) vs two axis

Small (individually tracked) vs large elements

Height

Circular (carousel: rotate & roll) vs linear (tilt & roll)

Planar mounting vs staggered mounting

Open- vs closed-loop tracking

Hydraulic vs direct drive

Stow position (up or down?); stow condition

Dual use of land

National Renewable Energy Laboratory 26 Innovation for Our Energy Future
Land use – complicated trade off

High efficiency is often assumed to mean fewer acres/MW

Packing density is trade off between shading and energy production

Pedestal

Carousel

Use of pedestals often results in higher shading losses, but provides 

opportunity for dual use of land

Carousel or tilt & roll approaches may allow closer packing

Complicated: creativity may minimize shading losses and identify new approaches
Reliability – an important challenge

Reports of reliability issues include:

- Trackers

- Inverters, data acquisition, etc.

- Longevity may be limited by optics, thermal control of 

cells, dirt getting into the light path

- Only a handful of companies have > 10 y experience in 

the field

- Most companies are aggressively applying accelerated 

testing

Most companies are considering “design for reliability” from the start

Convincing banks of long-term reliability is key hurdle to growth
Amonix, CA, USA

• C ~ 500x

• Fresnel lens

• Up to 70 kW/pedestal

• $130M new funding 

this year

• Installed: 

~14 MW Si-based

~2 MW MJ

• > 40 MW in progress

• Production capacity 

30 MW/y (plans to 

expand)

Examples for III-V based Concentrator Systems

Amonix and Solar Systems in Australia 

have had product in the field for the longest
SolFocus, CA USA

• C ~ 650 X

• Multiple reflections 

within glass

• ~9 kW/pedestal

• Designed for low 

chromatic aberration 

and high acceptance 

angle

• Installed:

~ 2 MW on sun

> 10 MW in progress

• Production capacity 

50 MW/y

Examples for III-V based Concentrator Systems
Concentrix Solar, 

Germany

• C ~ 385x

• Fresnel lens

• Up to 70 kW/pedestal

• Glass/glass 

construction

• Installed:

~ 1 MW on sun

> 1 MW in progress

• Production capacity 

25 MW/y

Examples for III-V based Concentrator Systems
Opel, Canada

• C ~ 500x

• Fresnel lens

• ~ 4 kW/pedestal

• Staggered alignment

• Installed:

~ 500 kW

• Scaling up for 

production

Examples for III-V based Concentrator Systems
Daido Steel, Japan

• C ~ 500x

• Domed Fresnel lens

• ~ 15 kW/pedestal

• Staggered alignment

• Installed:

~ 200 kW

• Advanced prototype 

development

Examples for III-V based Concentrator Systems
Emcore, NM, USA

• C > 1000x

• Fresnel lens

• Tilt & Roll

• Suggest < 5 

acres/MW is possible

• Installed > 1 MW of 

design on pedestal

Examples for III-V based Concentrator Systems
Soliant Energy, 

CA, USA

• C ~ 1000x

• Fresnel lens

• Tilt & Roll

• Designed for 

rooftop installation

• Preparing to start 

manufacturing

Examples for III-V based Concentrator Systems
Energy Innovations, 

CA, USA

• C = 1200x

• Fresnel lens

• “Sunflower” uses 

single-module tracker

• Low-profile, lowweight design for 

carport, rooftop, or 

field

• Installed ~50 kW

Examples for III-V based Concentrator Systems
Semprius, NC, USA

• 0.36 mm2 microcells assembled with 

proprietary printing technique

• 31.5% InGaP/GaAs cell efficiency at 800x

• Plano convex silicone-on-glass primary

• Tiny glass ball lens secondary

• RD&D systems under test in NC and AZ

• Advanced prototype development using

3-J cells and 1,111x concentration

Examples for III-V based Concentrator Systems

Source Wafer

Transfer

Stamp

Receiver Substrate

Printed

Solar Cells

RD&D System

Transfer printing method provides parallel assembly
Morgan Solar, 

Canada

• Light-guide optic

• C ~ 1000X

• Light flows laterally; 

very thin optic

• Prototype 

development

Examples for III-V based Concentrator Systems
Cool Earth Solar 

• Balloon with back 

reflector

• Water-cooled cells

• Can use a range of 

concentrations; either 

silicon or 

multijunction cells

• Steel band is used to 

point at the sun

• Advanced prototype 

development

Examples of Concentrator Systems
Solaria, CA, USA

• Linear focus

• Thin, refractive optics

• C ~ 2x

• Si cells

• Marketed as a flat-plate 

module 

• Passed certification

• Shipped to a dozen leading 

companies

• enXco (EDF) has invested in 

Solaria and will procure 

several MW in next 6 months 

(100s of MW planned in 

future)

Examples for Low Concentration Systems
Skyline Solar, CA, USA

• Linear focus

• Reflective optics aimed 

at opposite side

• C ~ 10x

• Si cells

• Carefully designed 

heat sink: cells operate 

at ~ same temperature 

as flat plate 

• “Thin-film” mirrors

• ~ 150 kW on sun

• Mirrors shaped in 

automotive factory

Examples for Low Concentration Systems
Banyan Energy, CA, 

USA

• Linear focus

• Aggregated total 

internal reflection

• C < 10x

• Si cells

• Prototype development

Examples for Low Concentration Systems
International standards efforts

IEC TC82 WG7 current projects:

• Power Rating

• Safety 

• Energy Rating

• Tracker specification

• Acceptance test

• Others

National Renewable Energy Laboratory 43 Innovation for Our Energy Future

UL:

• Safety 
Power rating: order out of chaos

In the past, companies chose rating conditions:

Irradiance: 850, 900, or 1000 W/m2?

Temperature: 25°C cell or 20°C ambient?

(Affects $/W, performance ratio, and other metrics)

Chaos

National Renewable Energy Laboratory 44 Innovation for Our Energy Future

IEC WG7 committee has now tentatively chosen:

Irradiance: 900 W/m2

Test condition:

25°C cell

(same as flat plate)

Operating condition:

20°C ambient

(like California’s rating)

This is progress, but be careful (this is too new to be implemented)
Current status & what to expect next

National Renewable Energy Laboratory 45 Innovation for Our Energy Future
High-concentration CPV Status

Some companies are moving into production phase

> 50 companies are developing prototypes

~ 20 are testing > 2nd generation design

~ 30% efficiencies are being reported

Company Installed capacity In progress - other 

projects at planning stage

Manufacturing 

capacity

Amonix ~ 2 MW multijunction

~14 MW silicon 

(Guascor)

30 MW (Xcel, Alamosa, 

Colorado); 12 MW (Tucson 

Electric); 2 MW (Tucson 

Electric/Univ. of Arizona)

30 MW/yr 

(>100 MW/yr)

Solfocus ~ 2 MW 10 MW in a variety of 

projects

50 MW/yr

Concentrix ~ 1 MW 1 MW (Chevron, Questa, 

NM); 100 kW (Abu Dhabi)

25 MW/yr 

(100 MW/yr)

Solaria (lowX)Shipments to ~ dozen 

customers

10s of MWs in next 2Q 40 MW/yr
Market is emerging

National Renewable Energy Laboratory 47 Innovation for Our Energy Future

Utility Requests for Proposals Planned capacity

Arizona Public Service Small Gen RFP – 5-15 MW

RFP – 15-50 MW

70 MW

100 MW

Salt River Project PV-RFP – 12 kV interconnection 50 MW

Southern California 

Edison

Renewables Standard Offer RFP - ≤ 20 MW

RFO for solar PV – 200 kW-2 MW (≤ 20 MW)

500 MW

50 MW

Pacific Gas & Electric Utility-owned program/RFP - ≤ 20 MW

PV PPA Program/RFP - ≤ 20 MW

75 MW

50 MW

Total 895 MW

Thank you to Amonix for compilation of data
Convergence provides opportunity

National Renewable Energy Laboratory 48 Innovation for Our Energy Future

Companies entering 

production phase

Market emerging for 

utility-scale projects

Opportunity for 

growth for CPV

Some flat-plate 

companies 

have lead times

What to expect?

Expect growth of CPV!

Rate of growth will be limited by 

demonstration of long-term reliability 

(needed to be “bankable”)
Summary

• Many options for optics, trackers, and cells to 

consider in CPV system development

• High-efficiency multijunction cells enable highconcentration CPV; while low-X CPV reduces 

use of silicon

• Module efficiencies of 30% are enabling

• Convergence of product development & market 

emergence provides growth opportunity

For more information:

• Panel tomorrow: “Will 2010 be a Turning Point for CPV?”

• CPV-7 in April, Las Vegas

• http://www.nrel.gov/docs/fy10osti/43208.pdf

National Renewable Energy Laboratory 49 Innovation for Our Energy Future