Raising the Efficiency Ceiling in Multijunction Solar Cells

Richard R. King Spectrolab, Inc. A Boeing Company

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

Acknowledgments • 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 HighPerformance Photovoltaics (HiPerf PV) program (ZAT-4-33624-12), the DOE Technology Pathways Partnership (TPP), and by Spectrolab.

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

2

Outline • Global climate change and the solar resource

contact AR n+-GaInAs n-AlInP window n-GaInP emitter

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

• 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

p-GaInP base

p-AlGaInP BSF p++-TJ n++-TJ

W

id

n-GaInP window n-GaInAs emitter p-GaInAs base

M

E e-

g

Tu

e dl id

ll

el nn

Ce

ll

p-GaInP BSF p-GaInAs step-graded buffer p++-TJ

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

Ce

p To

n++-TJ

Tu

el nn

nucleation

n+-Ge emitter

t Bo

Ju

m to

nc

t io

Ce

n

ll

p-Ge base and substrate contact

metal gridline

semiconductor bonded interface

2.0-eV AlGaInP cell 1 1.7-eV AlGaInAs cell 2 1.4-eV GaInAs cell 3 1.1-eV GaInPAs cell 4 0.75-eV GaInAs cell 5

• Concentrator photovoltaic (CPV) systems and economics

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

3

Global Climate Change

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

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Climate and CO2 Over the Last 400,000 Years Vostok Ice Core Data 4

Temperature (°C)

2 0 -2 -4 -6 -8

Years Before Present

0

50 00 0

45 00 00 40 00 00 35 00 00 30 00 00 25 00 00 20 00 00 15 00 00 10 00 00

-10

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

• Antarctic ice core data allows for mapping of temperature and CO2 profiles R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

5

Climate and CO2 Over the Last 400,000 Years Vostok Ice Core Data 350

Temperature (°C)

2 300

0 -2

250 -4 -6

200

-8

Years Before Present

0

50 00 0

150

45 00 00 40 00 00 35 00 00 30 00 00 25 00 00 20 00 00 15 00 00 10 00 00

-10

CO2 Concentration (ppmv)

4

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

• Clear correlation between temperature and CO2 levels R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

6

Climate and CO2 – Recent History Vostok Ice Core Data

Temperature (°C)

2 300

0 -2

250 -4 -6

200

-8

0

50 00 0

150

45 00 00 40 00 00 35 00 00 30 00 00 25 00 00 20 00 00 15 00 00 10 00 00

-10

CO2 Concentration (ppmv)

350 315 ppm (NOAA, 1958) 384 ppm (NOAA, 2004)

4

Years Before Present

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

7

Temperature Anomaly by Year 1000 years of Earth temperature history… and 100 years of projection

IPCC (2001) scenarios to 2100

Rosina Bierbaum, Univ. of Michigan, IPCC R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

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The Solar Resource

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

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The Solar Resource

5 6

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

• Entire US electricity demand can be provided by concentrator PV arrays using 37%-efficient cells on: 150 km x 150 km area of land

or or

ten 50 km x 50 km areas similar division across US

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

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Concentrator Photovoltaic (CPV) Electricity Generation

CPV cost superiority 40% cell efficiency

CPV cost superiority 50% cell efficiency

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

Higher multijunction cell efficiency has a huge impact on the economics of CPV, and on the way we will generate electricity. R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

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Solar Cell Theoretical Efficiency

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

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Energy Transitions in Semiconductors insufficient energy to reach Ec

hν

thermalization of carriers

Ec

hν

Ev

Ec

e-

Ev

600

1.2

500

1

hν < Eg

400

0.8

300

0.6

200

0.4

100

0.2 0

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

4

AM1.5D, ASTM G173-03, 1000 W/m2 1.4 Utilization efficiency of photon energy to bandgap Eg = 1.424 eV

700

1.2

600

1

500

hν > Eg

400

0.8

300

0.6

200

0.4

100

0.2

Photon Utilization Efficiency

AM1.5D, ASTM G173-03, 1000 W/m2 1.4

Photon Utilization Efficiency

700

Intensity per Unit Photon Energy (W/m 2 . eV)

Intensity per Unit Photon Energy (W/m 2 . eV)

h+

0

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

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

3

3.5

4

13

LM and MM 3-Junction Cell Cross-Section contact

contact

AR

AR n+-Ga(In)As n-AlInP window n-GaInP emitter T

GaInP top cell

p-GaInP base

p-AlGaInP BSF

Wide-bandgap tunnel junction

p++-TJ n++-TJ

W

E eid

n-GaInP window n-Ga(In)As emitter

Ga(In)As middle cell

p-Ga(In)As base

p-GaInP BSF

Tunnel junction Buffer region

p++-TJ

M

id

g

e C

T el nn u T

e dl

el C

p++-TJ n++-TJ

l

u lJ

M

id

d

el nn u T

g

le

el C

l

p-GaInP BSF p-GaInAs step-graded buffer

m tto o B

l

n tio nc

n-Ga(In)As buffer

n+-Ge emitter

W

E eid

n-GaInP window n-GaInAs emitter p-GaInAs base

e nn Tu

el C

op

p-GaInP base

p-AlGaInP BSF

n++-TJ

nucleation

Ge bottom cell

op

n+-GaInAs n-AlInP window n-GaInP emitter

ll

l Ce

l

p++-TJ

lJ

n++-TJ

nucleation

n+-Ge emitter

p-Ge base and substrate

p-Ge base and substrate

contact

contact

Lattice-Matched (LM)

e nn Tu

n io ct n u

m tto o B

Ce

ll

Lattice-Mismatched or Metamorphic (MM)

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

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Photon Utilization Efficiency 3-Junction Solar Cells

500

1

400

0.8

300

0.6

200

0.4

100

0.2

0

Photon utilization efficiency

Intensity per Unit Photon Energy (W/m 2 . eV)

600

.

AM1.5D, ASTM G173-03, 1000 W/m21.4 Utilization efficiency of photon energy 1-junction cell 3-junction cell 1.2

700

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

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

4 15

Energy Transitions in Semiconductors Ec

hν

V = voltage of solar cell

Eg

qφn

qV Ev

qφp

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

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Energy Transitions in Semiconductors V = voltage of solar cell

Eg

qφn

hν

qV qφp

Ev

=

⏐φp - φn⏐

AM1.5D, ASTM G173-03, 1000 W/m2 1.4 Utilization efficiency of photon energy to bandgap Eg = 1.424 eV 1.2 to Voc at 1000 suns to Voc at 1 sun

700

Intensity per Unit Photon Energy 2. (W/m eV)

= quasi-Fermi level splitting

600 500

1

400

0.8

300

0.6

200

0.4

100

0.2

Photon utilization efficiency

Ec

0

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

4

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

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Shockley and Queisser (1961)

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

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Assumptions → Opportunities • 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 Down conversion Multiple exciton generation Avalanche multiplication Up conversion

One e--h+ pair per photon

Non-use of sub-band-gap photons Single population of each charge carrier type

One-sun incident intensity

Hot carrier solar cells Intermediate-band solar cells Quantum well, quantum dot solar cells Concentrator solar cells

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

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Theoretical Multijunction Cell Efficiency Detailed balance limit efficiency Radiative recombination only Series res. and shadowing, optimized grid spacing Normalized to experimental efficiency

60%

3J 4J

Efficiency (%)

55% 50% 4J 3J

45%

4J

40%

3J

3J & 4J MM solar cells 35% 1

500

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

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

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Maximum Solar Cell Efficiencies Measured Theoretical References C. 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).

95% 93%

Carnot eff. = 1 – T/Tsun T = 300 K, Tsun ≈ 5800 K 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% 50%

Ideal 3-gap solar cell at 1000 suns Ideal 2-gap solar cell at 1000 suns

(Henry) (Henry)

44% 43%

Ultimate eff. of device with cutoff Eg: (Shockley, Queisser) 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

31% 30%

Ideal 1-gap solar cell at 1 sun (Henry) Detailed balance limit of 1 gap solar cell at 1 sun (Shockley, Queisser)

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

(Henry)

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%

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

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

Ge or GaAs substrate

Ge or GaAs substrate

Growth Direction

cap

1.9 eV (Al)GaInP subcell 1 1.4 eV GaInAs subcell 2 graded MM buffer layers

1.0 eV GaInAs subcell 3

Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

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0.6 7 eV

0.9 eV 5

100

1.4 1

1.8 9

Solar Spectrum Partition for 3-Junction Cell 100

AM1.5D, low-AOD AM1.5G, ASTM G173-03 AM0, ASTM E490-00a

80

90 80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0 300

500

700

900

1100

1300

1500

1700

External Quantum Efficiency (%)

Current Density per Unit Wavelength (mA/(cm 2μm))

90

0 1900

Wavelength (nm) R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

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5- and 6-Junction Cells contact AR

AR cap contact AR

AR cap

(Al)GaInP Cell 1

(Al)GaInP Cell 1

2.0 eV

wide-Eg tunnel junction

2.0 eV

wide-Eg tunnel junction

GaInP Cell 2 (low Eg) 1.8 eV wide-Eg tunnel junction

AlGa(In)As Cell 2 1.7 eV

AlGa(In)As Cell 3 1.6 eV

wide-Eg tunnel junction

wide-Eg tunnel junction

Ga(In)As Cell 3 1.41 eV

Ga(In)As Cell 4 1.41 eV

tunnel junction

tunnel junction

GaInNAs Cell 4 1.1 eV

GaInNAs Cell 5 1.1 eV

tunnel junction

tunnel junction

Ga(In)As buffer

Ga(In)As buffer

nucleation

nucleation

Ge Cell 5 and substrate 0.67 eV

Ge Cell 6 and substrate 0.67 eV

back contact

back contact

• Divides available current density above GaAs Eg among 3-4 subcells • Allows low-current GaInNAs cell to be matched to other subcells • Lower series resistance

Ref.: U.S. Pat. No. 6,316,715, Spectrolab, Inc., filed 3/15/00, issued 11/13/01. R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

24

Photon Utilization Efficiency 3-Junction Solar Cells

500

1

400

0.8

300

0.6

200

0.4

100

0.2

0

Photon utilization efficiency

Intensity per Unit Photon Energy (W/m 2 . eV)

600

.

AM1.5D, ASTM G173-03, 1000 W/m21.4 Utilization efficiency of photon energy 1-junction cell 3-junction cell 1.2

700

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

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

4 25

Photon Utilization Efficiency 6-Junction Solar Cells

500

1

400

0.8

300

0.6

200

0.4

100

0.2

0

Photon utilization efficiency

Intensity per Unit Photon Energy (W/m 2 . eV)

600

.

AM1.5D, ASTM G173-03, 1000 W/m2 1.4 Utilization efficiency of photon energy 1-junction cell 3-junction cell 1.2 6-junction cell

700

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

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

4 26

3-Junction Cell Efficiency Losses from 100%

300 200 100 0 0

0.5

1

80

70

39% 60

subcell 2 14% 26%

40

20

23%

1 0.8 0.6

10

26% photon energies in solar spectrum

after carrier thermalization to band edges

photons with energy above lowest band gap

23%

11%

8%

5% after carrier extraction at quasiFermi levels -- 1-sun

7% 4%

more 25% balanced

after carrier extraction at quasiFermi levels -- 500 suns

1.9/1.4/1.0 eV bandgap combination

23%

6-junction terrestrial concentrator cell

subcell 3

0.4

thermalization of carriers

hν

0.2

3.5

20% E

h+

photon energies in solar spectrum

photons with energy hν > Eg above lowest band gap

600 500 400 300 200

1.2 1 0.8 0.6 0.4

no photogeneration 100

0.2 0

0 0.5

Eg

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

4

Ge or GaAs substrate

after carrier thermalization to band edges 700 600 500 400 300 200

8%

qφp

5%

Ge or GaAs substrate

after carrier extraction at quasiFermi levels -- 1-sun

AM1.5D, ASTM G173-03, 1000 W/m2 1.4 Utilization efficiency of photon energy to bandgap Eg = 1.424 eV 1.2 to Voc at 1000 suns to Voc at 1 sun

1 0.8 0.6 0.4

100

0.2

11%

cap

AM1.5D, ASTM G173-03, 1000 W/m2 1.4 Utilization efficiency of photon energy 1-junction cell 3-junction cell 1.2 6-junction cell

12%

600

30

500 400

1 0.8

10%

300

0.6

20

200

0.4

8%

100

0

Growth Direction

after carrier more extraction 1.9 eV (Al)GaInP subcellbalanced 1 at quasi1.9/1.4/1.0 eV 1.4 eV GaInAs subcell 2 Fermi levels bandgap graded MM buffer layers -- 500 suns combination

40

700

0

qφn

qV Ev

AM1.5D, ASTM G173-03, 1000 W/m2 1.4 Utilization efficiency of photon energy to bandgap Eg = 1.424 eV

700

0

15% c

hν

Ev

4

17%

17%

e-

10

25% 0

50

14%

8%

15%

20%

10

20

17%

17%

31%

0

0

10%

subcell 2 20

Ec

60

30

subcell 3

31%

70

12%

23%

31%

50 40

1.2

hν < Eg

3

25%

25%

30

80

60

31%

50

0

90

70

no photogeneration

AM1.5D, ASTM G173-03, 1000 W/m2 1.4

1.5 2 2.5 Photon Energy (eV)

39%

7%1

0.5

0.2

10

1.5 2 2.5 Photon Energy (eV)

4%

6-junction terrestrial concentrator cell

0 3

3.5

4

0

1.0 eV GaInAs subcell 3

0

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

4

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

27

.

Remaining Fraction of Available Power (%)

30

80

subcell 1 (top)

100

Photon utilization efficiency

400

40

90

Intensity per Unit Photon Energy (W/m 2 . eV)

500

50

subcell 1 (top)

Photon utilization efficiency

600

60

90

Intensity per Unit Photon Energy 2. (W/m eV)

Intensity per Unit Photon Energy 2. (W/m eV)

700

70

100

Photon Utilization Efficiency

Ev

80

100

Photon Utilization Efficiency

Ec

90

Intensity per Unit Photon Energy (W/m 2 . eV)

hν

Remaining Fraction of Available Power (%)

100

Metamorphic Semiconductor Materials

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

28

Metamorphic (MM) 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 R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

29

Bandgap vs. Lattice Constant

Courtesy J. Geisz – NREL R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

30

Bandgap vs. Lattice Constant 2

Direct Bandgap Eg (eV)

1.8

disordered GaInP 1.6

ordered GaInP

1.4

GaAs 1%-In

1.2

GaInAs

8%-In 12%-In GaInAs

1

23%-In GaInAs

0.8

35%-In

Ge (indirect)

0.6 5.6

5.65

5.7

5.75

5.8

Lattice Constant (angstrom) R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

31

0.96 eV

1.08

1.30 1.26

1.40 1.38

Internal QE of Metamorphic GaInAs Cells on Ge

Internal Quantum Efficiency (%)

100 90 80 70 60

GaInAs single-junction solar cells

50 40

1.6% lattice mismatch 2.4% lattice mismatch

30 20 10 0 300

400

500

600

700 800 900 1000 1100 1200 1300 1400 Wavelength (nm)

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

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Cross sectional TEM Ga0.44In0.56P/ Ga0.92In0.08As/ Ge Cell

• Low dislocation density in active cell layers in top portion of epilayer stack: from ~2x EBIC and CL meas. 105

cm-2

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

GaInAs cap

GaInP TC

Tunnel junction GaInAs MC

0.2 μm

GaInAs graded buffer to 8%-In

Misfit dislocations Pre-grade buffer

Ge substrate

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

33

High-Resolution XRD Reciprocal Space Map (RSM) • GaInP/ 8%-In GaInAs/ Ge metamorphic (MM) cell structure

Ge

Qy (Strain) Å-1

Graded Buffer

Ga0.92In0.08As MC GaInP TC

(115) glancing exit XRD

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

Qx (Tilt) Å-1 Line of 0% relaxation

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

Line of 100% relaxation

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

34

Inverted Lattice-Matched (LM) 1.39-eV GaInAs Subcell hν

metal contact

1.39-eV GaInAs inverted LM subcell base emitter

buffer layer nucleation

Ge or GaAs substrate

Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

35

Metamorphic (MM) 3-Junction Cells –– Inverted 1.10-eV GaInAs Subcell hν

metal contact

1.10-eV GaInAs inverted MM subcell base emitter

transparent MM graded buffer layers nucleation and pre-grade buffer

Ge substrate

Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

36

Metamorphic (MM) 3-Junction Cells –– Inverted 0.97-eV GaInAs Subcell hν

metal contact

0.97-eV GaInAs inverted MM subcell base emitter

transparent MM graded buffer layers nucleation and pre-grade buffer

Ge substrate

Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

37

Metamorphic (MM) 3-Junction Cells –– Inverted 0.84-eV GaInAs Subcell

hν

metal contact

0.84-eV GaInAs inverted MM subcell base emitter

transparent MM graded buffer layers nucleation and pre-grade buffer

Ge substrate

Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

38

Dislocations in Inverted Metamorphic Cells – EBIC

8e-9766-1

50 μm

1.39-eV ILM subcell GaInAs comp. Latt. mismatch Disloc. density

2% In 0.1% 2.5 x 105 cm-2

8e-9756-1

50 μm

0.97-eV IMM subcell

0.84-eV IMM subcell

23% In 1.6% 3.9 x 106 cm-2

33% In 2.3% 5.0 x 106 cm-2

44% In 3.1% 6.3 x 106 cm-2

metal

metal contact

1.39-eV GaInAs

base

8e-9783-11

50 μm

1.10-eV IMM subcell

contact

1.39-eV GaInAs inverted LM subcell

8e-9760-1

50 μm

1.10-eV GaInAs

1.10-eV GaInAs inverted MM subcell base

metal

metal

contact

contact

0.97-eV GaInAs

buffer layer

transparent MM graded buffer layers

nucleation

nucleation and pre-grade buffer

Ge or GaAs substrate

Ge substrate

base

base

emitter

emitter

emitter

emitter

0.84-eV GaInAs

0.840.84-eV GaInAs inverted MM subcell

0.970.97-eV GaInAs inverted MM subcell

transparent MM graded buffer layers

transparent MM graded buffer layers

nucleation and pre-grade buffer

nucleation and pre-grade buffer

Ge substrate

Ge substrate

EBIC images and dislocation density of inverted metamorphic cell test structures R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

39

Dislocations in Inverted Metamorphic Cells Lattice Mismatch Relative to Ge (%) 0.64

1.36

2.07

2.79

3.51

Inverted metamorphic (MM) GaxIn1-xAs solar cells

1.2

8 7 6

1.0

5 0.8 4 0.6 3 0.4

2 Band gap Eg meas. by ext. QE Meas. Voc at ~1 sun Woc = (Eg/q) - Voc = band gap-voltage offset Dislocation density from EBIC

0.2

1

0.0

Dislocation Density from EBIC (10 6 cm -2)

Band Gap Eg, Open-Circuit Voltage Voc , and Band Gap-Voltage Offset (V)

1.4

-0.07

0 0

10

20 30 In Composition for Gax In1-x As (%)

40

50

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

40

Dislocations in Inverted Metamorphic Cells Lattice Mismatch Relative to Ge (%)

and Photon Intensity from CL (10 3 cps)

Dislocation Density from EBIC (10 6 cm -2)

9

0.64

1.36

2.07

2.79

3.51

Inverted metamorphic (MM) GaxIn1-xAs solar cells

8

45 40

7 Dislocation density from EBIC

35

Overall % photon intensity from CL

6

50

% carrier loss at each dislocation from CL

30

5

25 4

20

3

Carrier Loss (%)

-0.07

15

2

10

1

5

0

0 0

10

20 30 In Composition for Gax In1-x As (%)

40

50

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

41

Solar Cell Voltage Voltage depends on non-equilibrium concentrations of electrons and holes

pn = n e

2 qV / kT i

n = NC NV e 2 i

− E g / kT

kT ⎛ pn ⎞ V= ln⎜⎜ 2 ⎟⎟ q ⎝ ni ⎠

pn = NC NV e

− ( E g − qV ) / kT

= NC NV e − qW / kT

kT ⎛ NC NV W ≡ (Eg q ) − V = ln⎜⎜ q ⎝ pn

⎞ ⎟⎟ ⎠

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

42

Band gap - Voltage Offset (Eg/q) - Voc for Single-Junction Solar Cells Voc of solar cells with wide range of bandgaps and comparison to radiative limit

d-AlGaInP

d-AlGaInP

d-AlGaInP

d-GaInP

o-GaInP

AlGaInAs

AlGaInAs

1.4 - eV GaInAs GaAs

1.30-eV GaInAs

1.24-eV GaInAs

0.5

1.10-eV GaInAs

1.0

GaInNAs

1.5

0.97-eV GaInAs

Voc Eg from EQE (Eg/q) - Voc radiative limit Ge (indirect gap)

Eg/q, Voc, and (Eg/q) - Voc (V)

2.0

0.0 0.6

1

1.4 Bandgap Eg (eV)

1.8

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

2.2 43

Band gap - Voltage Offset (Eg/q) - Voc for Single-Junction Solar Cells Voc Eg from EQE (Eg/q) - Voc radiative limit AM1.5D, low-AOD

800 700

1.5

600

d-AlGaInP

d-AlGaInP

d-GaInP d-AlGaInP

o-GaInP

AlGaInAs

AlGaInAs

1.4 - eV GaInAs GaAs

400 1.24-eV GaInAs 1.30-eV GaInAs

0.5

GaInNAs 1.10-eV GaInAs

1.0

0.97-eV GaInAs

500

300 200 100

0.0

Intensity per Unit Photon Energy (W/(m2 .eV))

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

Ge (indirect gap)

Eg/q, Voc, and (Eg/q) - Voc (V)

2.0

0

0.6

1

1.4

1.8 2.2 Bandgap Eg (eV)

2.6

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

3 44

High-Efficiency Multijunction Cells

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

45

LM and MM 3-Junction Cell Cross-Section contact

contact

AR

AR n+-Ga(In)As n-AlInP window n-GaInP emitter

GaInP top cell

To

p-GaInP base

p-AlGaInP BSF

Wide-bandgap tunnel junction

p++-TJ n++-TJ

W

E eid

n-GaInP window n-Ga(In)As emitter

Ga(In)As middle cell

p-Ga(In)As base

p-GaInP BSF

Tunnel junction Buffer region

p++-TJ

M

id

g

dl

l

el nn u T

e

el C

p++-TJ n++-TJ

l

tio nc u lJ

W

n-GaInP window n-GaInAs emitter M

id

m tto o B

el nn u T

g

dl

l

e

el C

l

n

n-Ga(In)As buffer

n+-Ge emitter

p

E eid

p-GaInP BSF p-GaInAs step-graded buffer

Ce

el C

p-GaInP base

p-GaInAs base

e nn u T

To

p-AlGaInP BSF

n++-TJ

nucleation

Ge bottom cell

p

el C

n+-GaInAs n-AlInP window n-GaInP emitter

ll

p++-TJ

lJ

n++-TJ

nucleation

n+-Ge emitter

p-Ge base and substrate

p-Ge base and substrate

contact

contact

Lattice-Matched (LM)

e nn Tu

n tio c un

m tto o B

Ce

ll

Lattice-Mismatched or Metamorphic (MM)

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

46

Metamorphic (MM) 3-Junction Solar Cell

0.3

n+-GaInAs n-AlInP window n-GaInP emitter

Ce

p To

p-GaInP base

p-AlGaInP BSF p++-TJ n++-TJ

W

i

-E de

n-GaInP window n-GaInAs emitter p-GaInAs base

M

g

Tu

e dl id

ll

el nn

Ce

ll

p-GaInP BSF p-GaInAs step-graded buffer n Tu

p++-TJ n++-TJ

ne

nucleation

n+-Ge emitter

t Bo

l

n t io nc u J

m to

l Ce

l

p-Ge base and substrate contact

C urrent D ensity / Incident Intensity (A /W )

contact AR

MJ cell 0.25

subcell 1 subcell 2

0.2

subcell 3

0.15

0.1

0.05

0 0

0.5

1

1.5

2

2.5

3

3.5

Voltage (V)

Lattice-Mismatched or Metamorphic (MM)

• Metamorphic growth of upper two subcells, GaInAs and GaInP R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

47

External QE of LM and MM 3-Junction Cells 100

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

90

AM0, ASTM E490-00a

90

Current Density per Unit Wavelength (mA/(cm 2μm))

EQE, lattice-matched

80

EQE, metamorphic

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0 300

500

700

900

1100

1300

1500

1700

External Quantum Efficiency (%)

100

0 1900

Wavelength (nm) R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

48

Metamorphic (MM) 3-Junction Solar Cell Eg1 = Subcell 1 (Top) Bandgap (eV) .

2.1 3-junction Eg1/ Eg2/ 0.67 eV cell efficiency 240 suns (24.0 W/cm2), AM1.5D (ASTM G173-03), 25oC 2 Ideal efficiency -- radiative recombination limit

1.9

MM 40.7%

1.8

LM 40.1%

54%

1.7

52%

1.6

50% 48%

1.5

46% 44%

1.4

42%

40%

1.3 1.0

1.1

1.2

1.3

1.4

38%

1.5

1.6

Eg2 = Subcell 2 Bandgap (eV) Disordered GaInP top subcell

Ordered GaInP top subcell

• Metamorphic GaInAs and GaInP subcells bring band gap combination closer to theoretical optimum R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

49

Record 40.7%-Efficient Concentrator Solar Cell

Spectrolab Metamorphic GaInP/ GaInAs/ Ge Cell Voc Jsc FF Vmp

= = = =

2.911 V 3.832 A/cm2 87.50% 2.589 V

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

Efficiency = 40.7% ± 2.4% 240 suns (24.0 W/cm2) intensity 0.2669 cm2 designated area 25 ± 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.

Concentrator cell light I-V and efficiency independently verified by J. Kiehl, T. Moriarty, K. Emery – NREL R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

50

New World Record 41.6% 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

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

• Incorporating high-efficiency 3J metamorphic cell structure + further improvements in grid design → strong potential to reach 42-43% champion cell efficiency

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

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

51

41.6% Solar Cell Eff., Voc vs. Concentration Efficiency

Efficiency (%) and Voc x 10 (V)

41.6%

Voc x 10

42

0.98 0.96

Voc fit, 100 to 1000 suns

40

0.94

FF

38

0.92

36

0.90

34

0.88

32

0.86

30

0.84

28

0.82

26

0.80

24

0.78 1000.0

0.1

1.0

10.0

100.0

Fill Factor (unitless)

44

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

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

52

41.6% Solar Cell LIV Curves vs. Concentration Current Density / Incident Intensity (A/W)

0.16

41.6%

0.14 Inc. Intensity (suns) 2 1 sun = 0.100 W/cm

0.12

2.6

0.1

6.6

0.08

17.6 59.8

0.06

127.3 364.2

0.04

604.8

0.02

940.9

0 0

0.5

1

1.5

2

2.5

3

3.5

Voltage (V)

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

53

Best Research Cell Efficiencies

Chart courtesy of Larry Kazmerski, NREL R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

54

Inverted Metamorphic (IMM) 3-Junction Cell

Ge or GaAs substrate

Ge or GaAs substrate

Growth Direction

cap

1.9 eV (Al)GaInP subcell 1 1.4 eV GaInAs subcell 2 graded MM buffer layers

1.0 eV GaInAs subcell 3

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

Current Density / Incident Intensity (A/W )

0.16 0.14 0.12 0.1

MJ cell subcell 1

0.08

subcell 2 0.06

subcell 3

0.04 0.02 0 0

0.5

1

1.5

2

2.5

3

3.5

4

Voltage (V)

• Bottom ~1-eV GaInAs subcell is inverted and metamorphic (IMM) • Upper two GaInAs and GaInP subcells are inverted and lattice matched (ILM)

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

55

Inverted Metamorphic (IMM) 3-Junction Cell 1.6

3-junction 1.9 eV/ Eg2/ Eg3 cell efficiency 2

o

Eg2 = Subcell 2 Bandgap (eV) .

500 suns (50 W/cm ), AM1.5D (ASTM G173-03), 25 C X

Ideal efficiency -- radiative recombination limit

1.5

1.4

53% 52% 51%

1.3

50%

48%

1.2 46%

1.1 44%

1 0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

Eg3 = Subcell 3 Bandgap (eV)

• Raising band gap of bottom cell from 0.67 for Ge to ~1.0 eV for IMM GaInAs raises theoretical 3J cell efficiency R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

56

5-Junction Inverted Metamorphic (IMM) Cells

gro Ge o wt r G h s aA ub s str ate metal gridline

2.0-eV AlGaInP cell 1 1.7-eV AlGaInAs cell 2 1.4-eV GaInAs cell 3 transparent buffer

1.1-eV GaInAs cell 4 transparent buffer

0.75-eV GaInAs cell 5 R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

57

4-Junction Lattice-Matched Cell AR

AR cap

(Al)GaInP Cell 1

1.9 eV

wide-Eg tunnel junction

AlGa(In)As Cell 2 1.6 eV wide-Eg tunnel junction

Ga(In)As Cell 3 1.4 eV tunnel junction

Ga(In)As buffer nucleation

Ge Cell 4 and substrate 0.67 eV

Current Density / Incident Intensity (A/W )

0.25

contact

MJ cell subcell 1

0.2

subcell 2 subcell 3 0.15

subcell 4

0.1

0.05

0 0

1

back contact

2

3

4

5

Voltage (V)

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

58

4-Junction Upright Metamorphic (MM) Terrestrial Concentrator Cell

metal gridline

1.8-eV (Al)GaInP cell 1 1.55-eV AlGaInAs cell 2 1.2-eV GaInAs cell 3 transparent buffer

0.67-eV Ge cell 4 and substrate

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

59

4-Junction Cell Optimum Band Gap Combinations 1.7

4-junction 1.9 eV/ Eg2/ Eg3/ 0.67 eV cell efficiency

Eg2 = Subcell 2 Bandgap (eV) .

2

1.6

o

500 suns (50 W/cm ), AM1.5D (ASTM G173-03), 25 C X Ideal efficiency -- radiative recombination limit

1.5 58%

1.4 56%

1.3 54%

1.2

50% 38% 46%

1.1 34%

42%

1 0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Eg3 = Subcell 3 Bandgap (eV)

• Lowering band gap of subcells 2 and 3, e.g., with MM materials, gives higher theoretical 4J cell efficiency R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

60

100

1600

90

1400

80

1200

70

AlGaInP subcell 1 1.95 eV GaInAs subcell 3 1.39 eV All subcells

60 50

AlGaInAs subcell 2 1.66 eV Ge subcell 4 0.72 eV AM1.5D ASTM G173-03

1000 800

40

600

30

400

20

Intensity Per Unit Wavelength (W/(m2μ m))

External Quantum Efficiency (%)

Measured 4-Junction Cell Quantum Efficiency

200

10 0 300

0

500

700

900 1100 1300 Wavelength (nm)

1500

1700

1900

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

61

Light I-V Curves Record Efficiency Cells Current Density / Inc. Intensity (A/W) .

0.16 0.14 0.12 0.10 3J Conc. Cell

0.08

3J Conc. Cell

Metamorphic V oc Jsc /inten. V mp FF conc. area

0.06 0.04

Lattice-matched

2.911 0.1596 2.589 0.875 240 0.267

LM, 822 suns

3.192 V 0.1467 A/W 2.851 V 0.887 364 suns 0.317 cm2

Eff. 40.7%

41.6%

AM1.5D, low -AOD spectrum

0.02

3J Conc. Cell 3.251 0.1467 2.781 0.841 822 0.317

36.9%

AM1.5D, ASTM G173-03

Independently confirmed meas. 25°C

0.00 0

0.5

1

1.5

4J Cell 4.398 V 0.0980 A/W 3.950 V 0.856 500 suns 0.208 cm2

40.1%

AM1.5D, ASTM G173-03

4J Conc. Cell

2

AM1.5D, ASTM G173-03 Prelim. meas. 25°C

2.5

3

3.5

4

4.5

Voltage (V)

• 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%) R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

62

Semiconductor-Bonded Technology (SBT) Terrestrial Concentrator Cell • Wafer bonding for multijunction solar cells – Low band gap cells for MJ cells using high-quality, lattice-matched materials – Epitaxial exfoliation and substrate removal – Formation of latticeengineered substrate for later MJ cell growth – Bonding of high-band-gap and low-band-gap cells after 1.4-eV GaInAs cell 3 growth 1.7-eV conductance AlGaInAs cellof 2 – Electrical semiconductor-bonded 2.0-eV AlGaInP cell 1 interface – Surface effects forGe GaAs or semiconductor-togrowth substrate semiconductor bonding

semiconductor bonded interface

GaAs or Ge metal gridline growth substrate GaAs or Ge growth substrate 2.0-eV AlGaInP cell 1 2.0-eV AlGaInP cell 1 1.7-eV AlGaInAs cell 2 1.7-eV AlGaInAs cell 2 1.4-eV GaInAs cell 3 1.4-eV GaInAs cell 3 1.1-eV GaInPAs cell 4 0.75-eV GaInAs cell 5

InP growth substrate

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

63

6-Junction Solar Cells

0.14 AR

AR cap

(Al)GaInP Cell 1

2.0 eV

wide-Eg tunnel junction

GaInP Cell 2 (low Eg) 1.78 eV wide-Eg tunnel junction

AlGa(In)As Cell 3 1.50 eV wide-Eg tunnel junction

Ga(In)As Cell 4 1.22 eV tunnel junction

GaInNAs Cell 5 0.98 eV tunnel junction

Ga(In)As buffer nucleation

Ge Cell 6 and substrate 0.67 eV

Current Density / Incident Intensity (A/W )

contact

0.12

0.1

0.08 MJ cell 0.06

subcell 1 subcell 2 subcell 3

0.04

subcell 4 subcell 5

0.02

subcell 6

0 0

1

2

3

4

5

6

7

Voltage (V)

back contact

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

64

Photon Utilization Efficiency 6-Junction Solar Cells

500

1

400

0.8

300

0.6

200

0.4

100

0.2

0

Photon utilization efficiency

Intensity per Unit Photon Energy (W/m 2 . eV)

600

.

AM1.5D, ASTM G173-03, 1000 W/m2 1.4 Utilization efficiency of photon energy 1-junction cell 3-junction cell 1.2 6-junction cell

700

0 0

0.5

1

1.5 2 2.5 Photon Energy (eV)

3

3.5

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

4 65

Concentrator Photovoltaic (CPV) Systems and Economics

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

66

Concentrator PV Systems with Multijunction Cells • 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). Combination of high efficiency & 500X concentration boosts output per semiconductor area by a factor of 1000. MJ cells are replaced by less expensive optics and common materials. Leads to reduced cost of energy despite paying extra for tracking & cooling.

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

67

Solar Systems, Australia Hermannsburg Power Station

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

Courtesy of Solar Systems Pty. Ltd., Australia

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

68

Balance of System Costs Optics Cooling

Tracking

Structure Operation and Maintenance Courtesy of Solar Systems Pty. Ltd., Australia R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

69

Economics for Device Physicists Continuity equation:

∂ρ ∂t

= qG − qR − ∇ ⋅ J

...in $$ rather than charge carriers:

∂$ ρ$ = $$ qG qR J$$ gen − $$ exp − ∇ ⋅ F ∂t change in value of = PV system (profit or loss)

→

⎛ PV system cost ⎞ ⎜ ⎟ ⎜ per kWh generated in ⎟ = ⎜ 5 year payback period⎟ ⎝ ⎠

value of kWhr generated by PV system

–

operating expenses – for PV system

funds paid out to bank for interest and principal on loan to buy PV system

⎛ module ⎞ ⎛ tracking ⎞ ⎛ BOS, area⎞ ⎛ BOS, power ⎞⎛ peak power ⎞ ⎛ cell ⎞ ⎜ ⎟+⎜ ⎟+⎜ ⎟+⎜ ⎟⎜ ⎟+⎜ ⎟ ⎜ cost / m 2 ⎟ ⎜ cost / m 2 ⎟ ⎜ cos t / m 2 ⎟ ⎜ cos t / W ⎟⎜ output, W/m 2 ⎟ ⎜ cost / m 2 ⎟ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠ 5 year ⎛ energy produced, ⎞⎛ ⎞ ⎜ ⎟⎜ ⎟ ⎜ kWh m 2 ⋅ year ⎟⎜ payback period ⎟ ⎝ ⎠⎝ ⎠

(

⎛ conc. ⎞ ⎜ ⎟ ⎜ ratio ⎟ ⎝ ⎠

)

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

70

Terrestrial PV System Cost vs. Cell Cost Fixed Flat-Plate PV System Cost / kWh Generated in 5 Year Period ($/kWh)

3.3

500X Point-Focus Conc. Cell cost ranges

0.2

2.2

10% 5-Year Payback Threshold, at $0.15/kWh

0.1

20%

15% 20%

30%

25% cell eff.

40% 50% cell eff.

Decreasing cell cost main priority for flat-plate

Increasing cell efficiency main priority for concentrators

⎛ module ⎞ ⎛ tracking ⎞ ⎛ BOS, area⎞ ⎛ BOS, power ⎞⎛ peak power ⎞ ⎛ cell ⎞ ⎟ ⎟+⎜ ⎟⎜ ⎟+⎜ ⎟+⎜ ⎟+⎜ ⎛ PV system cost ⎞ ⎜⎜ ⎟⎜ output, W/m 2 ⎟ ⎜ cost / m 2 ⎟ ⎜ ⎟ ⎝ cost / m 2 ⎟⎠ ⎜⎝ cost / m 2 ⎟⎠ ⎜⎝ cos t / m 2 ⎟⎠ ⎜⎝ cos t / W ⎠ ⎠ ⎝ ⎠⎝ ⎜ per kWh generated in ⎟ = 5 year ⎞ ⎛ energy produced, ⎞⎛ ⎜ 5 year payback period⎟ ⎟ ⎜ ⎟ ⎜ ⎝ ⎠ ⎜ kWh m 2 ⋅ year ⎟⎜ payback period ⎟ ⎠ ⎠⎝ ⎝

(

0.0 0.001

0.01

1.1

⎛ conc. ⎞ ⎟ ⎜ ⎜ ratio ⎟ ⎠ ⎝

)

0.1

1

PV System Cost / Power Output ($/W)

0.3

0.0 10

2

Cell Cost ($/cm ) R. R. King et al., 3rd Int'l. Conf. on Solar Concentrators (ICSC-3), Scottsdale, AZ, May 2005 R. R. King, Stanford Photonics Research Center Symposium, Stanford, CA, Sep. 14-16, 2009

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Larry Kazmerski, NREL

Larry Kazmerski, NREL

Larry Kazmerski, NREL

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

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