raising the efficiency ceiling in multijunction solar...

Raising the Efficiency Ceiling in Multijunction Solar Cells

Richard R. King

Spectrolab, Inc.A Boeing Company

Stanford Photonics Research Center SymposiumSep. 14-16, 2009

Stanford, CA

• Martha Symko-Davies, Fannie Posey-Eddy, Larry Kazmerski, Manuel Romero, Carl Osterwald, Keith Emery, John Geisz, Sarah Kurtz – NREL

• Angus Rockett – University of Illinois• Rosina Bierbaum – University of Michigan, Ann Arbor• Pierre Verlinden, John Lasich – Solar Systems, Australia• Kent Barbour, Andreea Boca, Dhananjay Bhusari, Ken Edmondson, Chris Fetzer,

William Hong, Jim Ermer, Russ Jones, Nasser Karam, Geoff Kinsey,Dimitri Krut, Diane Larrabee, Daniel Law, Phil Liu, Shoghig Mesropian, Mark Takahashi, and the entire multijunction solar cell team at Spectrolab

This work was supported in part by the U.S. Dept. of Energy through the NREL High-Performance Photovoltaics (HiPerf PV) program (ZAT-4-33624-12), the DOE Technology Pathways Partnership (TPP), and by Spectrolab.

AcknowledgmentsAcknowledgments

R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009 2

• Global climate change and the solar resource

• Solar cell theoretical efficiency limits– Opportunities to change ground rules for higher terrestrial efficiency– Cell architectures capable of >70% in theory, >50% in practice

• Metamorphic semiconductor materials– Control of band gap to tune to solar spectrum– Dislocations in metamorphic III-Vs imaged by CL and EBIC

OutlineOutline

Tunnel Juncti

on

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF

n-GaInP emittern-AlInP windown+-GaInAs

contact

AR

p-Ge baseand substratecontact

p-GaInAsstep-graded buffer

Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJ

n++-TJTunnel J

unction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF


contact

AR



Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJ

n++-TJ

• High-efficiency terrestrial concentrator cells– Metamorphic (MM) and lattice-matched (LM) 3-junction

solar cells with >40% efficiency– 4-junction MM and LM concentrator cells– Inverted metamorphic structure, semiconductor bonded technology (SBT) for MJ terrestrial concentrator cells

• Concentrator photovoltaic (CPV) systems and economics

0.75-eV GaInAs cell 5

1.1-eV GaInPAs cell 4

semi-conductor

bondedinterface


1.7-eV AlGaInAs cell 2

2.0-eV AlGaInP cell 1

metal gridline


Global Climate Change


Vostok Ice Core Data

Years Before Present

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5000

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4000

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Tem

pera

ture

(°C

)

-10

-8

-6

-4

-2

0

2

4

(J.R. Petit, J. Jouzel, Nature 399:429-436)

• Antarctic ice core data allows for mapping of temperature and CO2 profiles

Climate and COClimate and CO22 Over the Over the Last 400,000 YearsLast 400,000 Years


Climate and COClimate and CO22 Over the Over the Last 400,000 YearsLast 400,000 Years



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Tem

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ture

(°C

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-10

-8

-6

-4

-2

0

2

4

CO

2 Con

cent

ratio

n (p

pmv)

150

200

250

300

350

(J.R. Petit, J. Jouzel, Nature 399:429-436)

• Clear correlation between temperature and CO2 levels




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pera

ture

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-2

0

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4

CO

2 Con

cent

ratio

n (p

pmv)

150

200

250

300

350315 ppm (NOAA, 1958)384 ppm (NOAA, 2004)

• CO2 has reached levels never before seen in measured history• If we do nothing, we allow this rising trend to continue at our own peril

Climate and COClimate and CO22 ––Recent HistoryRecent History


Rosina Bierbaum, Univ. of Michigan, IPCC

IPCC (2001) scenarios to 2100 IPCC (2001) scenarios to 2100

1000 years of Earth temperature history…and 100 years of projection

Temperature Anomaly by Temperature Anomaly by YearYear


The Solar Resource


5

6

5

6

• Entire US electricity demand can be provided by concentrator PV arrays using 37%-efficient cells on:

or ten 50 km x 50 km areasor similar division across US

Ref.: http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/

150 km x 150 km area of land

The Solar ResourceThe Solar Resource


Map source: http://www.nrel.gov/gis/images/map_csp_us_annual_may2004.jpg

CPV cost superiority40% cell efficiency


Map source: http://www.nrel.gov/gis/images/map_csp_us_annual_may2004.jpg



Concentrator Photovoltaic Concentrator Photovoltaic (CPV) Electricity Generation(CPV) Electricity Generation

Higher multijunction cell efficiency has a huge impact on the economics of CPV, and on the way we will generate electricity.


Solar Cell Theoretical

Efficiency


Ec

Ev

hν

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy

(W/m

2 .

eV)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on U

tiliz

atio

n Ef

ficie

ncy

AM1.5D, ASTM G173-03, 1000 W/m2

hν < Eg

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy

(W/m

2 .

eV)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on U

tiliz

atio

n Ef

ficie

ncy

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy to bandgap Eg = 1.424 eV

hν > Eg

insufficient energy to reach Ec

thermalization of carriers

Ec

Ev

e-

h+

hν

Energy Transitions in Energy Transitions in SemiconductorsSemiconductors


Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-Ga(In)As emitter

n+-Ge emitter

p-AlGaInP BSF

n-GaInP emittern-AlInP windown+-Ga(In)As

contact

AR


n-Ga(In)As buffer

Bottom Cell

p++-TJn++-TJ

p-Ga(In)As base

nucleation

Wide-bandgap tunnel junction

GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJn++-TJ

Ga(In)As middle cell

Tunnel junction

Buffer regionTunnel

Juncti

on

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF


contact

AR



Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJn++-TJ

Lattice-Matched (LM) Lattice-Mismatchedor Metamorphic (MM)

LM and MM 3LM and MM 3--Junction Junction Cell CrossCell Cross--SectionSection


0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell

Photon Utilization EfficiencyPhoton Utilization Efficiency33--Junction Solar CellsJunction Solar Cells


Ec

Ev

hνEg

qφn

qVqφp

V = voltage of solar cell

= quasi-Fermi level splitting

= ⏐φp - φn⏐

• Not all of bandgap energy is available to be collected at terminals, even though electron in conduction band has energy Eg

• Only qV = q⏐φp - φn⏐ is available at solar cell terminals

• Due to difference in entropy S of carriers at low concentration in conduction band, and at high concentration in contact layers: G = H - TS



Ec

Ev

hνEg

qφn

qVqφp

V = voltage of solar cell

= quasi-Fermi level splitting

= ⏐φp - φn⏐

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy

(W/m

2 .

eV)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy to bandgap Eg = 1.424 eV to Voc at 1000 suns to Voc at 1 sun



Detailed Balance Limit of Solar Cell Efficiency

• 30% efficient single-gap solar cell at one sun, for 1 e-/photon

• 44% ultimate efficiency for device with single cutoff energy

Shockley and Queisser (1961)Shockley and Queisser (1961)


• Assumptions for theoretical efficiency in Shockley and Quiesser (1961)

• Viewed from a different angle, these assumptions represent new opportunities, for devices that overcome these barriers

Assumption limiting solar cell efficiency Device principle overcoming this limitation

Single band gap energy Multijunction solar cells Quantum well, quantum dot solar cells

One e--h+ pair per photon Down conversion Multiple exciton generation Avalanche multiplication

Non-use of sub-band-gap photons Up conversion

Single population of each charge carrier type Hot carrier solar cells Intermediate-band solar cells Quantum well, quantum dot solar cells

One-sun incident intensity Concentrator solar cells

Assumptions Assumptions →→OpportunitiesOpportunities


35%

40%

45%

50%

55%

60%

1 10 100 1000 10000Incident Intensity (suns) (1 sun = 0.100 W/cm2)

Effic

ienc

y (%

)Detailed balance limit efficiencyRadiative recombination onlySeries res. and shadowing, optimized grid spacingNormalized to experimental efficiency

5003J & 4J MM solar cells

3J

3J

3J

4J

4J

4J

Theoretical Multijunction Theoretical Multijunction Cell EfficiencyCell Efficiency


Theoretical

95% Carnot eff. = 1 – T/Tsun T = 300 K, Tsun ≈ 5800 K93% Max. eff. of solar energy conversion

= 1 – TS/E = 1 – (4/3)T/Tsun (Henry)

72% Ideal 36-gap solar cell at 1000 suns (Henry)

56% Ideal 3-gap solar cell at 1000 suns (Henry)50% Ideal 2-gap solar cell at 1000 suns (Henry)

44% Ultimate eff. of device with cutoff Eg: (Shockley, Queisser)43% 1-gap cell at 1 sun with carrier multiplication

(>1 e-h pair per photon) (Werner, Kolodinski, Queisser)

37% Ideal 1-gap solar cell at 1000 suns (Henry)

31% Ideal 1-gap solar cell at 1 sun (Henry)30% Detailed balance limit of 1 gap solar cell at 1 sun

(Shockley, Queisser)

Measured

3-gap GaInP/GaInAs/Ge LM cell, 364 suns (Spectrolab) 41.6%3-gap GaInP/GaInAs/Ge MM cell, 240 suns (Spectrolab) 40.7%

3-gap GaInP/GaAs/GaInAs cell at 1 sun (NREL) 33.8%

1-gap solar cell (silicon, 1.12 eV) at 92 suns (Amonix) 27.6%1-gap solar cell (GaAs, 1.424 eV) at 1 sun (Kopin) 25.1%

1-gap solar cell (silicon, 1.12 eV) at 1 sun (UNSW) 24.7%

ReferencesC. H. Henry, “Limiting efficiencies of ideal single and multiple energy gap terrestrial

solar cells,” J. Appl. Phys., 51, 4494 (1980). W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction

Solar Cells,” J. Appl. Phys., 32, 510 (1961). J. H. Werner, S. Kolodinski, and H. J. Queisser, “Novel Optimization Principles and

Efficiency Limits for Semiconductor Solar Cells,” Phys. Rev. Lett., 72, 3851 (1994).

R. R. King et al., "Band-Gap-Engineered Architectures for High-Efficiency Multijunction Concentrator Solar Cells," 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009.

R. R. King et al., "40% efficient metamorphic GaInP / GaInAs / Ge multijunction solar cells," Appl. Phys. Lett., 90, 183516 (4 May 2007).

M. Green, K. Emery, D. L. King, Y. Hishikawa, W. Warta, "Solar Cell Efficiency Tables (Version 27)", Progress in Photovoltaics, 14, 45 (2006).

A. Slade, V. Garboushian, "27.6%-Efficient Silicon Concentrator Cell for Mass Production," Proc. 15th Int'l. Photovoltaic Science and Engineering Conf., Beijing, China, Oct. 2005.

R. P. Gale et al., "High-Efficiency GaAs/CuInSe2 and AlGaAs/CuInSe2 Thin-Film Tandem Solar Cells," Proc. 21st IEEE Photovoltaic Specialists Conf., Kissimmee, Florida, May 1990.

J. Zhao, A. Wang, M. A. Green, F. Ferrazza, "Novel 19.8%-efficient 'honeycomb' textured multicrystalline and 24.4% monocrystalline silicon solar cells," Appl. Phys. Lett., 73, 1991 (1998).

Maximum Solar Cell Maximum Solar Cell EfficienciesEfficiencies


Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface

GrowthDirection

cap

1.9 eV (Al)GaInP subcell 1

1.4 eV GaInAs subcell 2

graded MM buffer layers


Ge or GaAs substrate


cap

Metamorphic (MM) 3Metamorphic (MM) 3--Junction Cells Junction Cells –––– Inverted 1.0Inverted 1.0--eV GaInAs SubcelleV GaInAs Subcell


0

10

20

30

40

50

60

70

80

90

100

300 500 700 900 1100 1300 1500 1700 1900Wavelength (nm)

Cur

rent

Den

sity

per

Uni

t W

avel

engt

h (m

A/(c

m2 μ

m))

0

10

20

30

40

50

60

70

80

90

100

Exte

rnal

Qua

ntum

Effi

cien

cy (

%)

AM1.5D, low-AOD

AM1.5G, ASTM G173-03

AM0, ASTM E490-00a

1.89 1.41 eV0.67

0.95

eV

Solar Spectrum Partition for Solar Spectrum Partition for 33--Junction CellJunction Cell


cap

contactAR

(Al)GaInP Cell 1 2.0 eVwide-Eg tunnel junction

AlGa(In)As Cell 21.7 eV

tunnel junction

Ga(In)As Cell 31.41 eV

tunnel junction

AR

Ga(In)As buffer

Ge Cell 5and substrate

0.67 eV

nucleation

back contact

wide-Eg tunnel junction

GaInNAs Cell 41.1 eV

cap

contactAR


GaInP Cell 2 (low Eg)1.8 eV



tunnel junction


tunnel junction

AR

Ga(In)As buffer


0.67 eV

nucleation

back contact



• Divides available current

density above GaAs Eg

among 3-4 subcells

• Allows low-currentGaInNAs cell to be matched toother subcells

• Lower series resistance

Ref.: U.S. Pat. No. 6,316,715, Spectrolab, Inc., filed 3/15/00, issued 11/13/01.

55-- and 6and 6--Junction CellsJunction Cells


0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell



0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell 6-junction cell



0

10

20

30

40

50

60

70

80

90

100R

emai

ning

Fra

ctio

n of

Ava

ilabl

e Po

wer

(%)

0

10

20

30

40

50

60

70

80

90

100

photonenergiesin solar

spectrum

after carrierthermalizationto band edges

photonswith energy

above lowestband gap

6-junctionterrestrial

concentratorcell

morebalanced

1.9/1.4/1.0 eVbandgap

combination

after carrierextractionat quasi-

Fermi levels-- 1-sun


Fermi levels-- 500 suns

subcell 1 (top)

39%

subcell 3

31%

subcell 2

26%

no photogeneration

31%

20%

23%

11%

17%

25%

8%

17%

25%

5%

15%

23%

4%7%

8%

10%

14%

12%0

10

20

30

40

50

60

70

80

90

100

Rem

aini

ng F

ract

ion

of A

vaila

ble

Pow

er (%

)

0

10

20

30

40

50

60

70

80

90

100

photonenergiesin solar

spectrum

after carrierthermalizationto band edges

photonswith energy

above lowestband gap

6-junctionterrestrial

concentratorcell

morebalanced

1.9/1.4/1.0 eVbandgap

combination


Fermi levels-- 1-sun


Fermi levels-- 500 suns

subcell 1 (top)

39%

subcell 3

31%

subcell 2

26%

no photogeneration

31%

20%

23%

11%

17%

25%

8%

17%

25%

5%

15%

23%

4%7%

8%

10%

14%

12%

Ec

Ev

hνEc

Ev

hν

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy

(W/m

2 . eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on U

tiliz

atio

n Ef

ficie

ncy

AM1.5D, ASTM G173-03, 1000 W/m2

hν < Eg

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy

(W/m

2 . eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on U

tiliz

atio

n Ef

ficie

ncy

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy to bandgap Eg = 1.424 eV

hν > Eg

thermalization of carriers

Ec

Ev

e-

h+

hνEc

Ev

e-

h+

hν Ec

Ev

hνEg

qφn

qVqφp

Ec

Ev

hνEg

qφn

qVqφp

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy

(W/m

2 .

eV)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy to bandgap Eg = 1.424 eV to Voc at 1000 suns to Voc at 1 sun

GrowthDirection

cap





GrowthDirection

cap





cap





Ge or GaAs substrateGe or GaAs substrate


cap


capcap

0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.


33--Junction Cell Efficiency Junction Cell Efficiency Losses from 100% Losses from 100%


Metamorphic

Semiconductor

Materials


• Metamorphic = "changed form"

• Thick, relaxed epitaxial layers grown with different lattice constant than growth substrate

• Allows access to subcell band gaps desired for more efficient division of the solar spectrum in multijunction solar cells

• Also called lattice-mismatched

• Misfit dislocations are allowed to form in metamorphic buffer, which typically has graded composition and lattice constant

• Threading dislocations which can propagate up into active device layers grown on buffer are minimized as much as possible

Metamorphic (MM) Metamorphic (MM) Semiconductor MaterialsSemiconductor Materials


Bandgap vs. Bandgap vs. Lattice ConstantLattice Constant


Courtesy J. Geisz – NREL

0.6

0.8

1

1.2

1.4

1.6

1.8

2

5.6 5.65 5.7 5.75 5.8

Lattice Constant (angstrom)

Dire

ct B

andg

ap E

g (e

V)

Ge(indirect)

GaAs

disordered GaInP

ordered GaInP

GaInAs8%-In

GaInAs 12%-In

23%-In GaInAs

1%-In

35%-In

Bandgap vs. Bandgap vs. Lattice ConstantLattice Constant


0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200 1300 1400Wavelength (nm)

Inte

rnal

Qua

ntum

Effi

cien

cy (%

)

0.96

eV1.40

1.08

1.26

1.38

1.30

GaInAs single-junction solar cells

2.4% lattice mismatch

1.6% lattice mismatch

Internal QE of Metamorphic Internal QE of Metamorphic GaInAs Cells on GeGaInAs Cells on Ge


• Low dislocation density in active cell layers in top portion of epilayer stack:

~ 2 x 105 cm-2 from EBIC and CL meas.

• Dislocations confined to graded buffer layers in bottom portion of epilayer stack

GaInAs cap

GaInAs MC

GaInP TC

0.2 μm

Tunnel junction

Pre-grade buffer

Misfit dislocations

GaInAs gradedbuffer to 8%-In

0.2 μm

Ge substrate

Cross sectional TEMCross sectional TEMGaGa0.440.44InIn0.560.56P/ GaP/ Ga0.920.92InIn0.080.08As/ Ge As/ Ge

CellCell


Line of 0%relaxation


Qy

(Stra

in) Å

-1

Qx (Tilt) Å-1

Ge

Ga0.92In0.08As MC

GaInP TC

GradedBuffer

(115) glancing exit XRD



Qy

(Stra

in) Å

-1

Qx (Tilt) Å-1

Ge

Ga0.92In0.08As MC

GaInP TC

GradedBuffer

(115) glancing exit XRD

• GaInP/ 8%-In GaInAs/ Ge metamorphic (MM) cell structure

• Nearly 100% relaxed step-graded buffer →removes driving force for dislocations to propagate into active cell layers

• 56%-In GaInP top cell pseudomorphic with respect to GaInAs middle cell

HighHigh--Resolution XRD Resolution XRD Reciprocal Space Map (RSM)Reciprocal Space Map (RSM)




nucleation

buffer layerbuffer layer

emitter

1.39-eV GaInAsinverted LM subcell

base

contact

metal

hν

Inverted LatticeInverted Lattice--Matched (LM)Matched (LM)1.391.39--eV GaInAs SubcelleV GaInAs Subcell


Ge substrate


nucleation and pre-grade buffer

transparent MM transparent MM graded buffer layersgraded buffer layers

emitter

1.10-eV GaInAsinverted MM subcell

base

contact

metal

hν



Ge substrate




emitteremitter

0.970.97--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell

basebase

contactcontact

metal

hν



Ge substrate




emitteremitter


basebase

contactcontact

metal

hν




nucleationnucleation

buffer layerbuffer layerbuffer layerbuffer layer

emitter


base

emitter


base

contactcontact

metalmetal

50 μm8e-9766-1

1.39-eV ILM subcellGaInAs comp. 2% InLatt. mismatch 0.1%Disloc. density 2.5 x 105 cm-2

50 μm8e-9756-1

50 μm8e-9760-1

50 μm8e-9783-11

EBIC images and dislocation density of inverted metamorphic cell test structures

1.391.39--eV eV GaInAsGaInAs

1.10-eV IMM subcell23% In1.6%

3.9 x 106 cm-2


5.0 x 106 cm-2


6.3 x 106 cm-2

Ge substrateGe substrate

nucleation and pre-grade buffernucleation and pre-grade buffer



emitter


base

emitter


base

contactcontact

metalmetal






emitteremitter


basebase

emitteremitter


basebase

contactcontactcontactcontact

metalmetal






emitteremitter


basebase

emitteremitter


basebase

contactcontactcontactcontact

metalmetal


Dislocations in Inverted Dislocations in Inverted Metamorphic Cells Metamorphic Cells –– EBICEBIC


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 10 20 30 40 50In Composition for GaxIn1-xAs (%)

Ban

d G

ap E

g, O

pen-

Circ

uit V

olta

ge V

oc,

and

Ban

d G

ap-V

olta

ge O

ffset

(V

)

0

1

2

3

4

5

6

7

8

Dis

loca

tion

Den

sity

from

EB

IC (

106 c

m-2

)

Band gap Eg meas. by ext. QEMeas. Voc at ~1 sunWoc = (Eg/q) - Voc = band gap-voltage offsetDislocation density from EBIC

-0.07 0.64 1.36 2.07 2.79 3.51Lattice Mismatch Relative to Ge (%)

Inverted metamorphic (MM)GaxIn1-xAs solar cells

Dislocations in Inverted Dislocations in Inverted Metamorphic CellsMetamorphic Cells


0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50In Composition for GaxIn1-xAs (%)

Dis

loca

tion

Den

sity

from

EB

IC (

106 c

m-2

)an

d P

hoto

n In

tens

ity fr

om C

L (1

0 3 c

ps)

0

5

10

15

20

25

30

35

40

45

50

Car

rier L

oss

(%)

Dislocation density from EBICOverall % photon intensity from CL% carrier loss at each dislocation from CL

-0.07 0.64 1.36 2.07 2.79 3.51Lattice Mismatch Relative to Ge (%)

Inverted metamorphic (MM)GaxIn1-xAs solar cells

Dislocations in Inverted Dislocations in Inverted Metamorphic CellsMetamorphic Cells


Voltage depends on non-equilibrium concentrations of electrons and holes

kTqVi enpn /2=

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛=−≡⎟⎟

⎠

⎞⎜⎜⎝

⎛=

=== −−−−

pnNN

qkTVqEW

npn

qkTV

eNNeNNpn eNNn

VCg

i

kTqWVC

kTqVEVC

kTEVCi

gg

lnln 2

//)(/2

• Bandgap-voltage offset W ≡ (Eg/q) – V is a useful parameter for gauging solar cell quality, especially when dealing with semiconductors of many different bandgaps

• Basically a measure of how close electron and hole quasi-Fermi levels are to conduction and valence band edges

Solar Cell VoltageSolar Cell Voltage


0.0

0.5

1.0

1.5

2.0

0.6 1 1.4 1.8 2.2Bandgap Eg (eV)

Eg/q

, Voc

, and

(Eg/

q) -

Voc

(V)

VocEg from EQE(Eg/q) - Vocradiative limit

d-A

lGaI

nP

GaA

s1.

4 - e

V G

aInA

s

o-G

aInP

AlG

aInA

s

d-A

lGaI

nPd-

GaI

nP

d-A

lGaI

nP

0.97

-eV

GaI

nAs

GaI

nNA

s

1.10

-eV

GaI

nAs

1.24

-eV

GaI

nAs

1.30

-eV

GaI

nAs

Ge

(ind

irect

gap

)

AlG

aInA

s

Voc of solar cells with wide range of bandgaps and comparison to radiative limit

Band gap Band gap -- Voltage Offset (EVoltage Offset (Egg/q) /q) -- VVococfor Singlefor Single--Junction Solar CellsJunction Solar Cells


0.0

0.5

1.0

1.5

2.0

0.6 1 1.4 1.8 2.2 2.6 3Bandgap Eg (eV)

Eg/q

, Voc

, and

(Eg/

q) -

Voc

(V)

0

100

200

300

400

500

600

700

800

Inte

nsity

per

Uni

t Pho

ton

Ener

gy (

W/(m

2 . eV

))

VocEg from EQE(Eg/q) - Vocradiative limitAM1.5D, low-AOD

Voc of solar cells with wide range of bandgaps and comparison to radiative limit

d-A

lGaI

nP

GaA

s1.

4 - e

V G

aInA

s

o-G

aInP

AlG

aInA

s

d-A

lGaI

nPd-

GaI

nP

d-A

lGaI

nP

0.97

-eV

GaI

nAs

GaI

nNA

s1.

10-e

V G

aInA

s

1.24

-eV

GaI

nAs

1.30

-eV

GaI

nAs

Ge

(ind

irect

gap

)

AlG

aInA

s

Band gap Band gap -- Voltage Offset (EVoltage Offset (Egg/q) /q) -- VVococfor Singlefor Single--Junction Solar CellsJunction Solar Cells


High-Efficiency

Multijunction Cells


Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-Ga(In)As emitter

n+-Ge emitter

p-AlGaInP BSF

n-GaInP emittern-AlInP windown+-Ga(In)As

contact

AR


n-Ga(In)As buffer

Bottom Cell

p++-TJn++-TJ

p-Ga(In)As base

nucleation

Wide-bandgap tunnel junction

GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJn++-TJ

Ga(In)As middle cell

Tunnel junction

Buffer regionTunnel

Juncti

on

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF


contact

AR



Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJn++-TJ

Lattice-Matched (LM) Lattice-Mismatchedor Metamorphic (MM)

LM and MM 3LM and MM 3--Junction Junction Cell CrossCell Cross--SectionSection


Metamorphic (MM) Metamorphic (MM) 33--Junction Solar CellJunction Solar Cell

Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF


contact

AR



Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJn++-TJ

Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF


contact

AR



Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJn++-TJ

Lattice-Mismatchedor Metamorphic (MM)

0

0.05

0.1

0.15

0.2

0.25

0.3

0 0.5 1 1.5 2 2.5 3 3.5Voltage (V)

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)

MJ cell

subcell 1

subcell 2

subcell 3

• Metamorphic growth of upper two subcells, GaInAs and GaInP


0

10

20

30

40

50

60

70

80

90

100

300 500 700 900 1100 1300 1500 1700 1900

Wavelength (nm)

Cur

rent

Den

sity

per

Uni

t W

avel

engt

h (m

A/(c

m2 μ

m))

0

10

20

30

40

50

60

70

80

90

100

Exte

rnal

Qua

ntum

Effi

cien

cy (

%)

AM1.5D, low-AODAM1.5G, ASTM G173-03

AM0, ASTM E490-00aEQE, lattice-matched

EQE, metamorphic

External QE of LM and MM External QE of LM and MM 33--Junction CellsJunction Cells


1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

1.0 1.1 1.2 1.3 1.4 1.5 1.6

Eg2 = Subcell 2 Bandgap (eV)

E g1 =

Sub

cell

1 (T

op) B

andg

ap (

eV)

.

Disordered GaInP top subcell Ordered GaInP top subcell

38%

54%

42%

46%

50%

52%

3-junction Eg1/ Eg2/ 0.67 eV cell efficiency240 suns (24.0 W/cm2), AM1.5D (ASTM G173-03), 25oCIdeal efficiency -- radiative recombination limit

40.7% 40.1%

48%

44%

40%

MMLM

Metamorphic (MM) Metamorphic (MM) 33--Junction Solar CellJunction Solar Cell

• Metamorphic GaInAs and GaInP subcells bring band gap combination closer to theoretical optimum


Concentrator cell light I-V and efficiency independently verified by J. Kiehl, T. Moriarty, K. Emery – NREL

• First solar cell of any type to reach over 40% efficiency

SpectrolabMetamorphic

GaInP/ GaInAs/ Ge CellVoc = 2.911 V Jsc = 3.832 A/cm2

FF = 87.50%Vmp = 2.589 V

Efficiency = 40.7% ± 2.4% 240 suns (24.0 W/cm2) intensity0.2669 cm2 designated area25 ± 1°C, AM1.5D, low-AOD spectrum Ref.: R. R. King et al., "40% efficient

metamorphic GaInP / GaInAs / Ge multijunction solar cells," Appl. Phys. Lett., 90, 183516, 4 May 2007.

Record Record 40.7%40.7%--Efficient Efficient Concentrator Solar CellConcentrator Solar Cell


New World RecordNew World Record41.6% Multijunction Solar Cell41.6% Multijunction Solar Cell

• 41.6% efficiency demonstrated for 3J lattice-matched Spectrolab cell, a new world record• Highest efficiency for any type of solar cell measured to date• Independently verified by National Renewable Energy Laboratory (NREL)• Standard measurement conditions (25°C, AM1.5D, ASTM G173 spectrum) at 364 suns (36.4 W/cm2)• Lattice-matched cell structure similar to C3MJ cell, with reduced grid shadowing as planned for C4MJ cell• Incorporating high-efficiency 3J metamorphic cell structure + further improvements in grid design → strong potential to reach 42-43%champion cell efficiency

Ref.: R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009.

Concentrator cell light I-V and efficiency independently verified by C. Osterwald, K. Emery – NREL


24

26

28

30

32

34

36

38

40

42

44

0.1 1.0 10.0 100.0 1000.0

Incident Intensity (suns) (1 sun = 0.100 W/cm2)

Effic

ienc

y (%

) and

Voc

x 1

0 (V

)

0.78

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

Fill

Fact

or (

unitl

ess)

Efficiency

Voc x 10

Voc fit, 100 to 1000 suns

FF

• At peak 41.6% efficiency → 364 suns, Voc = 3.192 V, FF = 0.887• Efficiency still >40% at 820 suns, at 940 suns efficiency is 39.8%• Diode ideality factor of 1.0 for all 3 junctions fits Voc well from 100 to 1000 suns

41.6% Solar Cell41.6% Solar CellEff., Voc vs. ConcentrationEff., Voc vs. Concentration

41.6%


• At peak 41.6% efficiency → 364 suns, Voc = 3.192 V, FF = 0.887• Series resistance causes drop in Vmp above 400 suns, Voc continues to increase• Efficiency still >40% at 820 suns, at 940 suns efficiency is 39.8%

41.6% Solar Cell41.6% Solar CellLIV Curves vs. ConcentrationLIV Curves vs. Concentration

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0 0.5 1 1.5 2 2.5 3 3.5Voltage (V)

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)

2.6

6.6

17.6

59.8

127.3

364.2

604.8

940.9

Inc. Intensity (suns)1 sun = 0.100 W/cm2

41.6%


Chart courtesy of Larry Kazmerski, NREL

Best Research Cell Best Research Cell EfficienciesEfficiencies


Growth on Ge or GaAs substrate, followed by substrate removal from

sunward surface

GrowthDirection

cap





cap







cap


capcap

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0 0.5 1 1.5 2 2.5 3 3.5 4Voltage (V)

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)

MJ cell

subcell 1

subcell 2

subcell 3

Inverted Metamorphic (IMM) Inverted Metamorphic (IMM) 33--Junction CellJunction Cell

• Bottom ~1-eV GaInAs subcell is inverted and metamorphic (IMM)

• Upper two GaInAs and GaInP subcells are inverted and lattice matched (ILM)


1

1.1

1.2

1.3

1.4

1.5

1.6

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3Eg3 = Subcell 3 Bandgap (eV)

E g2 =

Sub

cell

2 B

andg

ap (

eV)

.

48%

53%

44%

46%

50%

52%

3-junction 1.9 eV/ Eg2/ Eg3 cell efficiency500 suns (50 W/cm2), AM1.5D (ASTM G173-03), 25oCIdeal efficiency -- radiative recombination limitX

51%

Inverted Metamorphic (IMM) Inverted Metamorphic (IMM) 33--Junction CellJunction Cell

• Raising band gap of bottom cell from 0.67 for Ge to ~1.0 eV for IMM GaInAs raises theoretical 3J cell efficiency


55--Junction Inverted Metamorphic Junction Inverted Metamorphic (IMM) Cells(IMM) Cells

transparent buffer


1.1-eV GaInAs cell 4transparent buffer




metal gridline

Ge or GaAs

growth substrate


cap

contactAR




tunnel junction

AR

Ga(In)As buffer


0.67 eV

nucleation

back contact


cap

contactAR




tunnel junction

AR

Ga(In)As buffer


0.67 eV


back contact


0

0.05

0.1

0.15

0.2

0.25

0 1 2 3 4 5Voltage (V)

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)

MJ cell

subcell 1

subcell 2

subcell 3

subcell 4

44--Junction Junction LatticeLattice--Matched CellMatched Cell

• Current density in spectrum above Ge cell 4 is divided 3 ways among GaInAs, AlGa(In)As, GaInP cells

•Lower current and I2R resistive power loss


44--Junction Upright Metamorphic Junction Upright Metamorphic (MM) Terrestrial Concentrator Cell(MM) Terrestrial Concentrator Cell

0.67-eV Ge cell 4 and substrate

transparent buffer



1.8-eV (Al)GaInP cell 1

metal gridline


1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5Eg3 = Subcell 3 Bandgap (eV)

E g2 =

Sub

cell

2 B

andg

ap (

eV)

.

38%46%

54%

56%

50%

42%

4-junction 1.9 eV/ Eg2/ Eg3/ 0.67 eV cell efficiency500 suns (50 W/cm2), AM1.5D (ASTM G173-03), 25oCIdeal efficiency -- radiative recombination limitX

58%

34%

44--Junction Cell Junction Cell Optimum Band Gap CombinationsOptimum Band Gap Combinations

• Lowering band gap of subcells 2 and 3, e.g., with MM materials, gives higher theoretical 4J cell efficiency


0

10

20

30

40

50

60

70

80

90

100

300 500 700 900 1100 1300 1500 1700 1900Wavelength (nm)

Exte

rnal

Qua

ntum

Eff

icie

ncy

(%)

0

200

400

600

800

1000

1200

1400

1600

Inte

nsity

Per

Uni

t Wav

elen

gth

(W

/(m2 μ

m))

AlGaInP subcell 1 1.95 eV

AlGaInAs subcell 2 1.66 eV

GaInAs subcell 3 1.39 eV

Ge subcell 4 0.72 eV

All subcells AM1.5DASTM G173-03

Measured 4Measured 4--Junction Cell Junction Cell Quantum EfficiencyQuantum Efficiency


Light ILight I--V CurvesV CurvesRecord Efficiency CellsRecord Efficiency Cells

• Light I-V curves for 3-junction upright MM (40.7%), 3J lattice-matched (41.6%), 3J lattice-matched at 822 suns (39.1%), and 4J lattice-matched cell (36.9%)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5Voltage (V)

Cur

rent

Den

sity

/ In

c. In

tens

ity (

A/W

) .

Metamorphic Lattice-matched LM, 822 suns 4J Cell

3J Conc. 3J Conc. 3J Conc. 4J Conc. Cell Cell Cell Cell

Voc 2.911 3.192 V 3.251 4.398 V Jsc/inten. 0.1596 0.1467 A/W 0.1467 0.0980 A/W Vmp 2.589 2.851 V 2.781 3.950 V FF 0.875 0.887 0.841 0.856 conc. 240 364 suns 822 500 suns area 0.267 0.317 cm2 0.317 0.208 cm2

Eff. 40.7% 41.6% 40.1% 36.9%

AM1.5D, AM1.5D, AM1.5D, AM1.5D, low -AOD spectrum ASTM G173-03 ASTM G173-03 ASTM G173-03 Prelim. meas. Independently confirmed meas. 25°C 25°C


SemiconductorSemiconductor--Bonded Technology Bonded Technology (SBT) Terrestrial Concentrator Cell(SBT) Terrestrial Concentrator Cell

InP growth substrateGaAs or Ge growth substrate



2.0-eV AlGaInP cell 10.75-eV GaInAs cell 5


GaAs or Ge growth substrate












semi-conductor

bondedinterface

metal gridline






semi-conductor

bondedinterface

metal gridline

– Low band gap cells for MJ cells using high-quality, lattice-matched materials– Epitaxial exfoliation and substrate removal– Formation of lattice-engineered substrate for later MJ cell growth– Bonding of high-band-gap and low-band-gap cells after growth– Electrical conductance of semiconductor-bonded interface– Surface effects for semiconductor-to-semiconductor bonding

• Wafer bonding for multijunction solar cells


cap

contactAR





tunnel junction


tunnel junction

AR

Ga(In)As buffer


0.67 eV

nucleation

back contact



cap

contactAR





tunnel junction


tunnel junction

AR

Ga(In)As buffer


0.67 eV


back contact



0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 1 2 3 4 5 6 7Voltage (V)

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)MJ cellsubcell 1subcell 2subcell 3subcell 4subcell 5subcell 6

66--Junction Solar CellsJunction Solar Cells


0

100

200

300

400

500

600

700

0 0.5 1 1.5 2 2.5 3 3.5 4Photon Energy (eV)

Inte

nsity

per

Uni

t Pho

ton

Ener

gy(W

/m 2

. eV

)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Phot

on u

tiliz

atio

n ef

ficie

ncy

.




Concentrator Photovoltaic (CPV)

Systems and

Economics


Combination of high efficiency & 500X concentration boosts output per semiconductor area by a factor of 1000.

• 1 football field of ~ 17% solar cells at 1-sun produces ~ 500 kW.

• By using MJ cells (> 35%) at concentration of 500 suns, same power is produced from smaller semiconductor area (or the football field produces 500 MW).

MJ cells are replaced by less expensive optics and common materials.

Leads to reduced cost of energy despite paying extra for tracking & cooling.

Concentrator PV Systems Concentrator PV Systems with Multijunction Cellswith Multijunction Cells


Courtesy of Solar Systems Pty. Ltd., Australia

Equipped with III-V MJ

cell receivers

• III-V MJ cells give 56% measured improvement in module efficiency relative to Si concentrator cells

Solar Systems, AustraliaSolar Systems, AustraliaHermannsburg Power StationHermannsburg Power Station


Courtesy of Solar Systems Pty. Ltd., Australia

TrackingTracking

StructureStructure

OpticsOptics

CoolingCooling

Operation Operation and and MaintenanceMaintenance

Balance of System CostsBalance of System Costs


Economics for Economics for Device PhysicistsDevice Physicists

Continuity equation:

...in $$ rather than charge carriers:

change in value of kWhr operating funds paid out to bankvalue of = generated – expenses – for interest and principalPV system by PV system for PV system on loan to buy PV system(profit or loss)

→( ) ⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛

=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

period payback

year5

yearm kWh

produced,energy ratio

conc.

m / cost

cell

W/moutput,

power peak

W/tcos

power,BOS

m/tcos

area,BOS

m / cost

tracking

m / cost

module

period payback year5in generated kWh per

cost systemPV

2

2222 2

J Rq Gq t

⋅∇−−=∂∂ ρ

J Rq Gq t

⋅∇−−=∂∂ ρ$$ $$gen $$exp F$$


Cell cost ranges

0.0

0.1

0.2

0.3

0.001 0.01 0.1 1 10Cell Cost ($/cm2)

PV S

yste

m C

ost /

kW

h G

ener

ated

in

5 Y

ear

Peri

od (

$/kW

h)

0.0

1.1

2.2

3.3

PV S

yste

m C

ost /

Pow

er O

utpu

t ($

/W)

5-Year Payback Threshold, at $0.15/kWh

500X Point-Focus Conc.

20%

30%

40%50% cell eff.

10%

20%15%

25% cell eff.

Fixed Flat-Plate

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛

⋅

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛

=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

period payback

year5

yearm kWh

produced,energy ratio

conc.

m / cost

cell

W/moutput,

power peak

W/tcos

power,BOS

m/tcos

area,BOS

m / cost

tracking

m / cost

module

period payback year5in generated kWh per

cost systemPV

2

2222 2

R. R. King et al., 3rd Int'l. Conf. on Solar Concentrators (ICSC-3), Scottsdale, AZ, May 2005

Increasing cell efficiency main priority for concentrators

Decreasing cell cost main priority for flat-plate

Terrestrial PV System Cost Terrestrial PV System Cost vs. Cell Costvs. Cell Cost


Larry Kazmerski, NREL

• Urgent global need to address carbon emission, climate change, and energy security concerns → renewable electric power can help• Theoretical solar conversion efficiency

– Examining built-in assumptions points out opportunities for higher PV efficiency– Multijunction architectures, up/down conversion, quantum structures, intermediate bands, hot-carrier effects, solar concentration → higher η– Theo. solar cell η > 70%, practical η > 50% achievable

• Metamorphic multijunction cells have begun to realize their promise– Metamorphic semiconductors offer vastly expanded of band gaps– 40.7% metamorphic GaInP/ GaInAs/ Ge 3J cells demonstrated– First solar cells of any type to reach over 40% efficiency

• New world record efficiency of 41.6% demonstrated– Highest efficiency yet measured for any type of solar cell– 41.6% efficiency independently verified at NREL (364 suns, 25°C, AM1.5D)

• Solar cells with efficiencies in this range can transform the way we generate most of our electricity, and make the PV market explode

SummarySummary


raising the efficiency ceiling in multijunction solar...

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