raising the efficiency ceiling in multijunction solar cells€¦ · 1 raising the efficiency...

1

Raising the Efficiency Ceiling in Multijunction Solar Cells

Richard R. King

Spectrolab, Inc.A Boeing Company

UCSB Center for Energy Efficient Materials SeminarSanta Barbara, CA

Feb. 16, 2011

2

• Carl Osterwald, Keith Emery, Larry Kazmerski, Martha Symko-Davies, Fannie Posey-Eddy, Holly Thomas – NREL and DOE

• Gerald Siefer – Fraunhofer, ISE

• Geoff Kinsey – Amonix, Inc.

• Andreea Boca – Emcore

• Dhananjay Bhusari, Xing-Quan Liu, William Hong, Chris Fetzer, Ken Edmondson, Hector Cotal, Robyn Woo, Shoghig Mesropian, Diane Larrabee, Dimitri Krut, Daniel Law, Joe Boisvert, Nasser Karam, Russ Jones, Jim Ermer, Peichen Pien, Pete Hebert, Kent Barbour, Mark Takahashi, Andrey Masalykin,

...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, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011

3

• 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

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

• Band gap-voltage offset– Theoretical basis, experimental data, and MJ cell design

• Energy production in cells– Next-generation 4, 5, and 6J cells under changing spectrum

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

solar cells with 41.6% efficiency

• 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


4

Global Climate Change


5

Vostok Ice Core Data

Years Before Present

0

5000

0

1000

00

1500

00

2000

00

2500

00

3000

00

3500

00

4000

00

4500

00

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


6

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



0

5000

0

1000

00

1500

00

2000

00

2500

00

3000

00

3500

00

4000

00

4500

00

Tem

pera

ture

(°C

)

-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


7



0

5000

0

1000

00

1500

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2000

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2500

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3000

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3500

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4000

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4500

00

Tem

pera

ture

(°C

)

-10

-8

-6

-4

-2

0

2

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


8

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


9

The Solar Resource


10

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


11

Solar Resource in EuropeSolar Resource in Europewith 40% CPV Cellswith 40% CPV Cells


Direct Normal Solar Radiation

Two-Axis Tracking Concentrator *Cost effective region (0.10 € /kWhr = 0.14 $ /kWhr) for:

40% eff. III-V multijunction CPV cell (500 suns)4.7 kWhr/(m2 day) = 1720 kWhr/(m2 year)

26% eff. silicon CPV cell (100 suns)6.7 kWhr/(m2 day) = 2450 kWhr/(m2 year)

* Estimated from mapped global irradiation on fixed, inclined flat-plate panels, and correlation with direct normal radiation in other regions.

Direct Normal Solar RadiationTwo-Axis Tracking Concentrator *

Cost effective region (0.10 € /kWhr = 0.14 $ /kWhr) for: 40% eff. III-V multijunction CPV cell (500 suns)

4.7 kWhr/(m2 day) = 1720 kWhr/(m2 year)26% eff. silicon CPV cell (100 suns)

6.7 kWhr/(m2 day) = 2450 kWhr/(m2 year)

Direct Normal Solar RadiationTwo-Axis Tracking Concentrator *

Cost effective region (0.10 € /kWhr = 0.14 $ /kWhr) for: 40% eff. III-V multijunction CPV cell (500 suns)

4.7 kWhr/(m2 day) = 1720 kWhr/(m2 year)26% eff. silicon CPV cell (100 suns)

6.7 kWhr/(m2 day) = 2450 kWhr/(m2 year)

* Estimated from mapped global irradiation on fixed, inclined flat-plate panels, and correlation with direct normal radiation in other regions.

12

Solar Resource in the USSolar Resource in the USwith 40% CPV Cellswith 40% CPV Cells


13

Solar Cell Theoretical

Efficiency


14

Ec

Ev

hEc

Ev

h

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

h < 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

h > Eg

insufficient energy to reach Ec

thermalization of carriers

h+

e-

h+

e-

Energy Transitions in Energy Transitions in SemiconductorsSemiconductors


15

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


16

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


17

Ec

Ev

hEg

qn

qVqp

V = voltage of solar cell

= quasi-Fermi level splitting

= p - n

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

• Only qV = qp - n 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



18

Ec

Ev

hEg

qn

qVqp



= p - n

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



19

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)


20

• 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


21

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


22

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


23

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

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


24

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


25

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



26

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



27

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

hEc

Ev

h

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

h < 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

h > Eg

thermalization of carriers

Ec

Ev

e-

h+

hEc

Ev

e-

h+

h Ec

Ev

hEg

qn

qVqp

Ec

Ev

hEg

qn

qVqp

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


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%


28

Metamorphic

Semiconductor

Materials


29Courtesy J. Geisz – NREL

Bandgap vs. Bandgap vs. Lattice ConstantLattice Constant

30

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


31

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


Metamorphic = "changed form"

32

• 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 m

Tunnel junction

Pre-grade buffer

Misfit dislocations

GaInAs gradedbuffer to 8%-In

0.2 m

Ge substrate

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

CellCell


33

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)


34



nucleation

buffer layerbuffer layer

emitter

1.39-eV GaInAsinverted LM subcell

base

contact

metal

h

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


35

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

h



36

Ge substrate




emitteremitter

0.970.97--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell

basebase

contactcontact

metal

h



37

Ge substrate




emitteremitter


basebase

contactcontact

metal

h



38


nucleationnucleation

buffer layerbuffer layerbuffer layerbuffer layer

emitter


base

emitter


base

contactcontact

metalmetal

50 m8e-9766-1

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

50 m8e-9756-1

50 m8e-9760-1

50 m8e-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


39

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


40

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


41

Band Gap-Voltage

Offset


42

Ec

Ev

hEg

qn

qVqp



= p - n

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




43

Experimental and Theoretical Experimental and Theoretical Band GapBand Gap--Voltage Offset Voltage Offset WWococ

0.0

0.5

1.0

1.5

2.0

0.6 1 1.4 1.8 2.2Band Gap Eg (eV)

E g/q

, Voc

, and

(Eg/q

) - V

oc (

V)

measured Vocmeas. Eg from EQEWoc = (Eg/q) - Vocradiative recomb. onlydetailed balance model

d-A

lGaI

nP

GaA

s1.

4 - e

V G

aInA

s

o-G

aInP

AlG

aInA

s

d-A

lGaI

nP d

-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 and band gap-voltage offset Woc = (Eg/q) - Voc

of solar cells with wide range of band gaps

Si

(indi

rect

gap

)

0.79

-eV

GaI

nAs

• Expt. band gap-voltage offset is very flat from 0.67-2.1 eV• Borne out by calc. from Eg-dependent B, and from detailed balance model


44

Semiconductor Eqns. Formulated in Semiconductor Eqns. Formulated in Terms of Band GapTerms of Band Gap--Voltage Offset Voltage Offset WW

• Woc formulation has more physical basis, related to NC , NV rather than ni2

• Far more invariant with respect to Eg , good for multiple subcells in MJ cells• Makes Woc a convenient indicator of solar cell quality – amount of SRH recombination vs. radiative recombination – for wide range of Eg

• Ko ≡ Jo / ni2 has nearly all band gap dependence taken out

kTqVi enpn /2

2ln

inpn

qkTV

kTqWVC eNNpn /

VqEW g

ocgoc VqEW

pnNN

qkTW VCln

ph

VCo

o

phgoc J

NNKq

kTJJ

qkT

qE

W lnln

o

phoc J

Jq

kTV ln

kTqW

VC

ph

i

oo

oceNN

JnJK /

2 kTqVpho

oceJJ /


45

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 Shockley and QueisserQueisser (1961) (1961) Detailed Balance ModelDetailed Balance Model


46

Band GapBand Gap--Voltage Offset Voltage Offset WWococCalc. from Detailed Balance ModelCalc. from Detailed Balance Model

• Woc decreases ~70 mV as band gap drops from 2.0 eV to 0.7 eV

• Relatively insensitive to band gap Eg

• Ideal photon recycling implicitly assumed in detailed balance model, leading to lower Woc

• Consistent with experiment and calc. for radiative recombination only, without photon recycling

cg kTE

c

gcco e

kTE

chkTqqQJ /

2

23

3

1)(2

ph

goc JkT

EchkTq

qkTW 11)(2ln

2

23

3

dxe

xchkTqdE

eE

chqqQJ

gg

cx

xc

EkTEco

1)(2

12 2

23

3

/

2

23


47

Experimental and Theoretical Experimental and Theoretical Band GapBand Gap--Voltage Offset Voltage Offset WWococ

0.0

0.5

1.0

1.5

2.0

0.6 1 1.4 1.8 2.2Band Gap Eg (eV)

E g/q

, Voc

, and

(Eg/q

) - V

oc (

V)

measured Vocmeas. Eg from EQEWoc = (Eg/q) - Vocradiative recomb. onlydetailed balance model

d-A

lGaI

nP

GaA

s1.

4 - e

V G

aInA

s

o-G

aInP

AlG

aInA

s

d-A

lGaI

nP d

-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 and band gap-voltage offset Woc = (Eg/q) - Voc

of solar cells with wide range of band gaps

Si

(indi

rect

gap

)

0.79

-eV

GaI

nAs

• Expt. band gap-voltage offset is very flat from 0.67-2.1 eV• Borne out by calc. from Eg-dependent B, and from detailed balance model


48

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


49

Energy Production in

MJ Solar Cells


50

1 1 –– Transmittance Transmittance Dependence on Air MassDependence on Air Mass

0

500

1000

1500

2000

2500

250 500 750 1000 1250 1500 1750 2000 2250

Wavelength (nm)

Inte

nsity

Per

Uni

t Wav

elen

gth

. (W

/(m2 m

))

0

0.25

0.5

0.75

1

1.25

(1 -

Tran

smitt

ance

)

(uni

tless

)

AM0, ASTM G173-03AM1.5D, ASTM G173-031 - transmittance for air mass 1.51 - transmittance for air mass 2.5

• Attenuation in atmosphere from absorption and scattering is greatest at short


51

Subcell Light ISubcell Light I--V Curves for V Curves for 66--Junction CellJunction Cell

0

0.05

0.1

0.15

0.2

0.25

0.3

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

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

• Atmospheric attenuation severely limits top subcell current at very high air mass, but little energy is available at those times

• Effect on energy production winds up being fairly small as a result

6:15 AM onAutumnal equinox

9:00 AM onAutumnal equinox

at design point


52

66--Junction Subcell Currents Junction Subcell Currents Over Typical DayOver Typical Day

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

6.50

7.50

8.50

9.50

10.5

0

11.5

0

12.5

0

13.5

0

14.5

0

15.5

0

16.5

0

17.5

0

Time of Day (hours)

Jsc

/ Int

ensi

ty (

A/W

)

subcell 1subcell 2subcell 3subcell 4subcell 5subcell 6

6J CellOptimized for:

9:00 AM, Autumnal Equinox

• Atmospheric attenuation severely limits top subcell current at very high air mass, but little energy is available at those times

• Effect on energy production winds up being fairly small as a result


53

66--Junction Cell Light IJunction Cell Light I--V Parameters V Parameters for Changing Air Massfor Changing Air Mass

0%

20%

40%

60%

80%

100%

120%

6.00

7.00

8.00

9.00

10.0

0

11.0

0

12.0

0

13.0

0

14.0

0

15.0

0

16.0

0

17.0

0

18.0

0

Time of Day (hours)

J-R

atio

for S

ubce

lls 1

and

2,

FF, a

nd E

ffic

ienc

y (%

)

0.00

0.04

0.08

0.12

0.16

0.20

0.24

Jsc

/ Int

ensi

ty (A

/W) a

ndA

ir M

ass

/ 100

(un

itles

s)

J-ratio: Jph,1 / Jph,2FFEff.Jsc / intensityAir mass / 100

• Inverse relationship between FF and J-ratio partially counteracts effect of top subcell current mismatch on overall MJ cell efficiency


54

33--, 4, 4--, 5, 5--, and 6, and 6--Junction Cell Junction Cell Efficiency over Typical DayEfficiency over Typical Day

0%

10%

20%

30%

40%

50%

60%

6.00

7.00

8.00

9.00

10.0

0

11.0

0

12.0

0

13.0

0

14.0

0

15.0

0

16.0

0

17.0

0

18.0

0

Time of Day (hours)

Mul

tijun

ctio

n C

ell E

ffic

ienc

y O

ver

Day

on

Aut

umna

l Equ

inox

(%

)

3-junction cell

4-junction cell

5-junction cell6-junction cell

Current balance optimized for 9:00 AM, Autumnal Equinox,

for all MJ cells

• At 6:15 AM: 6J cell has lower efficiency than 3J, 4J, or 5J cells• 6:15 AM – 7:15 AM: 4J cell has highest efficiency• By 7:30 AM: 6J cell is higher than 4J, and thereafter is the highest efficiency cell type, over the most significant energy-producing hours of the day


55

Cumulative Energy Produced Cumulative Energy Produced for 3for 3--, 4, 4-- , 5, 5--, 6, 6--Junction CellsJunction Cells

0

100000

200000

300000

400000

500000

600000

700000

800000

900000

6.00

7.00

8.00

9.00

10.0

0

11.0

0

12.0

0

13.0

0

14.0

0

15.0

0

16.0

0

17.0

0

18.0

0

Time of Day (hours)

Cum

ulat

ive

Ener

gy /

Are

a Pr

oduc

ed b

y M

J C

ell O

ver

Day

on

Aut

umna

l Equ

inox

(J/

cm2 )



Current balance optimized for 9:00 AM, Autumnal

Equinox, for all MJ cells

• The higher efficiency 4J and 5J cells produce more energy than conventional 3J cells

• 6J cells produce more than any of the other cell designs by the end of a typical day


56

Experimental

Multijunction Cell

Results


57

LatticeLattice--Matched and Matched and Metamorphic Cell StructureMetamorphic Cell Structure

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


contact

AR


n-Ga(In)As buffer

Bottom Cell

p++-TJn++-TJ

p-Ga(In)As base

nucleation


GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJn++-TJ


Tunnel junction

Buffer region

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


contact

AR


n-Ga(In)As buffer

Bottom Cell

p++-TJn++-TJ

p-Ga(In)As base

nucleation


GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJn++-TJ


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

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


58

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

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


59

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


60

Spectrolab CPV Cell Spectrolab CPV Cell GenerationsGenerations

0%

5%

10%

15%

20%

25%

30%

% o

f Cel

ls

34.0

%

34.5

%

35.0

%

35.5

%

36.0

%

36.5

%

37.0

%

37.5

%

38.0

%

38.5

%

39.0

%

39.5

%

40.0

%

40.5

%

41.0

%

41.5

%

Efficiency at 50 W/cm2 (%)

C1MJ Production

C2MJ Production

C3MJ Production

C4MJ Preliminary Data

avg = 36.9%

avg = 37.7%

avg = 38.5%

avg = 39.4%

The new C4MJ cell is a metamorphic (MM) GaInP/ GaInAs/ Ge

cell design


61

World Record 41.6% World Record 41.6% Multijunction Solar CellMultijunction Solar Cell

• 41.6% efficiency demonstrated for 3J lattice-matched Spectrolab cell, a 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 GaInP/ GaInAs/ Ge cell structureRef.: 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


62

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 Cellvs. Concentrationvs. Concentration

41.6%


63

Chart courtesy of Larry Kazmerski, NREL

Best Research Cell Best Research Cell EfficienciesEfficiencies

40%


64

Spectrolab 3J Metamorphic Spectrolab 3J Metamorphic Concentrator CellsConcentrator Cells

• Multiple Spectrolab 3J MM terrestrial concentrator cells were confirmed with efficiencies over 41% at independent test lab at Fraunhofer ISE

• Record efficiency upright MM 3J cell verified at 41.6% efficiency

AM1.5D, ASTM G173-03 spectrum 41.6%

SpectrolabSpectrolabMetamorphicGaInP/GaInAs/Ge3-Junction Cell

T = 25°C, Oct. 2010• Reaches parity with

41.6% lattice-matched 3J cell –earlier record efficiency for any type of solar cell

• Full-sized 1 cm2 area cell


65

SpectrolabSpectrolabMetamorphic41.6%-EfficientGaInP/GaInAs/Ge3-Junction Cell

• 3J MM cell reaches record 41.6% efficiency at 484 suns –very close to 500 sun design point

• Relatively high metal coverage supports large 1 cm2 cell area and high concentration


Spectrolab 3J Metamorphic Spectrolab 3J Metamorphic Concentrator CellsConcentrator Cells

66

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


67

44--Junction Upright Junction Upright Metamorphic Solar CellMetamorphic Solar Cell


0.67-eV Ge cell 4and substrate

transparent buffer


1.48-eV AlGaInAs cell 21.79-eV AlGaInP cell 1

metal gridline

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 0.5 1 1.5 2 2.5 3 3.5 4 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

68

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

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

• Subcell EQE for 4-junction terrestrial concentrator cell prototype

• AlGaInP/ AlGaInAs/ GaInAs/ Ge 4J cell structure


69

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

• 40.7% metamorphic 3J cell first solar cell to reach over 40%• 41.6% lattice-matched 3J cell current record efficiency cell• Same cell at 822 suns 40.1% eff.• 4J terrestrial conc. lattice-matched cell at 500 suns 36.9% eff.

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


70

55--Junction Upright LatticeJunction Upright Lattice--Matched Matched Cell with EpitaxiallyCell with Epitaxially--Grown Ge SubcellGrown Ge Subcell


0.67-eV Ge cell 4



metal gridline


0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0 1 2 3 4 5 6Voltage (V)

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)

MJ cell

subcell 1

subcell 2

subcell 3

subcell 4

subcell 5

71

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


72

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

nucleationnucleation

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


73

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

.




74

Terrestrial Conc. Cell Designs Terrestrial Conc. Cell Designs from 40% to 50%from 40% to 50%


(a) (b) (c) (d) (e) (f) (g) (h) (i) (j)3J Lattice-

Matched (LM)C3MJ+

3J Meta-morphic (MM)low mismatch

C4MJ

3J Meta-morphic (MM)high mismatch

3J InvertedMetamorphic

(IMM)

4J Meta-morphic (MM)high mismatch

4J Double-Grade InvertedMetamorphic

(MMX2)

5J Lattice-Matched (LM)

w. epitaxial Ge subcell

5J Lattice-Matched (LM)w. GaInNAsSb

subcell

5J Lattice-Matched (LM)

SemiconductorBonded (SBT)

6J Triple-GradeInverted

Metamorphic(MMX3)

MJ Cell 39.42% 40.00% 40.54% 43.26% 44.44% 47.87% 43.25% 47.43% 47.64% 50.91% EfficiencyChange 0.0% 1.5% 2.8% 9.7% 12.7% 21.4% 9.7% 20.3% 20.9% 29.2% in Power from C3MJ+ Efficiencies for AM1.5D, ASTM G173-03 spectrum, 50.0 W/cm2 (500 suns), 25°C

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Subc

ell B

and

Gap

s (e

V)

34%

36%

38%

40%

42%

44%

46%

48%

50%

52%

MJ

Cel

l Effi

cien

cy (

%)

C1 Eg

C2 Eg

C3 Eg

C4 Eg

C5 Eg

C6 Eg

MJ CellEfficiency



1.88-eV GaInP cell 1

metal gridline

transparent buffer



metal gridline


transparent buffer



metal gridline


transparent buffer


1.40-eV GaAs cell 2


metal gridline

Ge or GaAs

growth substrate


transparent buffer



metal gridline

transparent buffer


1.12-eV GaInAs cell 3transparent buffer


metal gridline

Ge or GaAs

growth substrate



1.73-eV AlGaInAs cell 21.42-eV GaAs cell 3


metal gridline

Ge or GaAs

growth substrate

semiconductorbonded

interface

transparent buffer




1.77-eV AlGaAs cell 21.465-eV AlGaAs cell 3


metal gridline

Ge or GaAs

growth substrate


1.12-eV GaInNAsSb cell 4



metal gridline

0.67-eV Ge cell 4



metal gridline


75

Concentrator Photovoltaic (CPV)

Systems and

Economics


76

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


77

Courtesy of Solar Systems Pty. Ltd., Australia

TrackingTracking

StructureStructure

OpticsOptics

CoolingCooling

Operation Operation and and MaintenanceMaintenance

Balance of System CostsBalance of System Costs


78

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

J Rq Gq t

$$ $$gen $$exp F$$


79

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


80Larry Kazmerski, NREL

83

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

• Solar cell theoretical efficiency limits– Examining built-in assumptions presents opportunities for higher PV efficiency– Theo. solar cell η > 70% , practical η > 50% achievable

• Metamorphic semiconductor materials– Long even with high mismatch vastly expanded of band gaps for solar cell design

• Band gap-voltage offset Woc = (Eg/q) - Voc– Expt. Woc largely indep. of Eg, from 2.1 to 0.67 eV, borne out theoretically Woc form of semiconductor eqns. useful for dealing with many band gaps, e.g., MJ cells

• Energy production modeling in multijunction solar cells– Changing spectrum over day and year impacts current balance in MJ cells, but...– 4, 5, 6-junction cells have much higher η over vast majority of energy-producing hours Greater energy production for next-generation 4J, 5J, and 6J cells than for 3J

• Experimental multijunction cell results– Lattice-matched and metamorphic 3-junction GaInP/ GaInAs/ Ge 41.6% solar cell efficiency Highest η for any type of solar cell with single growth run

• 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 cells€¦ · 1 raising the efficiency...

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