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
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++-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
1.4-eV GaInAs cell 3
1.7-eV AlGaInAs cell 2
2.0-eV AlGaInP cell 1
metal gridline
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
4
Global Climate Change
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
5
Vostok Ice Core Data
Years Before Present
0
5000
0
1000
00
1500
00
2000
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2500
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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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
6
Climate and COClimate and CO22 Over the Over the Last 400,000 YearsLast 400,000 Years
Vostok Ice Core Data
Years Before Present
0
5000
0
1000
00
1500
00
2000
00
2500
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3000
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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
350
(J.R. Petit, J. Jouzel, Nature 399:429-436)
• Clear correlation between temperature and CO2 levels
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
7
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
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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The Solar Resource
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
11
Solar Resource in EuropeSolar Resource in Europewith 40% CPV Cellswith 40% CPV Cells
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
13
Solar Cell Theoretical
Efficiency
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
p-Ge baseand substratecontact
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
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++-TJn++-TJ
Lattice-Matched (LM) Lattice-Mismatchedor Metamorphic (MM)
LM and MM 3LM and MM 3--Junction Junction Cell CrossCell Cross--SectionSection
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
Energy Transitions in Energy Transitions in SemiconductorsSemiconductors
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
18
Ec
Ev
hEg
qn
qVqp
V = voltage of solar cell
= quasi-Fermi level splitting
= 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
Energy Transitions in Energy Transitions in SemiconductorsSemiconductors
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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)
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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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
1.0 eV GaInAs subcell 3
Ge or GaAs substrate
Ge or GaAs substrate
cap
Metamorphic (MM) 3Metamorphic (MM) 3--Junction Cells Junction Cells –––– Inverted 1.0Inverted 1.0--eV GaInAs SubcelleV GaInAs Subcell
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
(Al)GaInP Cell 1 2.0 eVwide-Eg tunnel junction
GaInP Cell 2 (low Eg)1.8 eV
wide-Eg tunnel junction
AlGa(In)As Cell 31.6 eV
tunnel junction
Ga(In)As Cell 41.41 eV
tunnel junction
AR
Ga(In)As buffer
Ge Cell 6and substrate
0.67 eV
nucleation
back contact
wide-Eg tunnel junction
GaInNAs Cell 51.1 eV
• 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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
Photon Utilization EfficiencyPhoton Utilization Efficiency33--Junction Solar CellsJunction Solar Cells
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
Photon Utilization EfficiencyPhoton Utilization Efficiency66--Junction Solar CellsJunction Solar Cells
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
after carrierextractionat quasi-
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
after carrierextractionat quasi-
Fermi levels-- 1-sun
after carrierextractionat quasi-
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
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
1.9 eV (Al)GaInP subcell 1
1.4 eV GaInAs subcell 2
graded MM buffer layers
1.0 eV GaInAs subcell 3
GrowthDirection
cap
1.9 eV (Al)GaInP subcell 1
1.4 eV GaInAs subcell 2
graded MM buffer layers
1.0 eV GaInAs subcell 3
cap
1.9 eV (Al)GaInP subcell 1
1.4 eV GaInAs subcell 2
graded MM buffer layers
1.0 eV GaInAs subcell 3
Ge or GaAs substrateGe or GaAs substrate
Ge or GaAs substrate
cap
Ge or GaAs substrateGe or GaAs substrate
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
.
AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell 6-junction cell
33--Junction Cell Efficiency Junction Cell Efficiency Losses from 100% Losses from 100%
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
28
Metamorphic
Semiconductor
Materials
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
33
Line of 0%relaxation
Line of 100%relaxation
Qy
(Stra
in) Å
-1
Qx (Tilt) Å-1
Ge
Ga0.92In0.08As MC
GaInP TC
GradedBuffer
(115) glancing exit XRD
Line of 0%relaxation
Line of 100%relaxation
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)
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
34
Ge or GaAs substrate
Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
35
Ge substrate
Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface
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
Metamorphic (MM) 3Metamorphic (MM) 3--Junction Cells Junction Cells –––– Inverted 1.10Inverted 1.10--eV GaInAs SubcelleV GaInAs Subcell
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
36
Ge substrate
Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface
nucleation and pre-grade buffer
transparent MM transparent MM graded buffer layersgraded buffer layers
emitteremitter
0.970.97--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell
basebase
contactcontact
metal
h
Metamorphic (MM) 3Metamorphic (MM) 3--Junction Cells Junction Cells –––– Inverted 0.97Inverted 0.97--eV GaInAs SubcelleV GaInAs Subcell
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
37
Ge substrate
Growth on Ge or GaAs substrate, followed by substrate removal from sunward surface
nucleation and pre-grade buffer
transparent MM transparent MM graded buffer layersgraded buffer layers
emitteremitter
0.840.84--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell
basebase
contactcontact
metal
h
Metamorphic (MM) 3Metamorphic (MM) 3--Junction Cells Junction Cells –––– Inverted 0.84Inverted 0.84--eV GaInAs SubcelleV GaInAs Subcell
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
38
Ge or GaAs substrateGe or GaAs substrate
nucleationnucleation
buffer layerbuffer layerbuffer layerbuffer layer
emitter
1.39-eV GaInAsinverted LM subcell
base
emitter
1.39-eV GaInAsinverted LM subcell
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
0.97-eV IMM subcell33% In2.3%
5.0 x 106 cm-2
0.84-eV IMM subcell44% In3.1%
6.3 x 106 cm-2
Ge substrateGe substrate
nucleation and pre-grade buffernucleation and pre-grade buffer
transparent MM transparent MM graded buffer layersgraded buffer layers
transparent MM transparent MM graded buffer layersgraded buffer layers
emitter
1.10-eV GaInAsinverted MM subcell
base
emitter
1.10-eV GaInAsinverted MM subcell
base
contactcontact
metalmetal
1.101.10--eV eV GaInAsGaInAs
Ge substrateGe substrate
nucleation and pre-grade buffernucleation and pre-grade buffer
transparent MM transparent MM graded buffer layersgraded buffer layers
transparent MM transparent MM graded buffer layersgraded buffer layers
emitteremitter
0.970.97--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell
basebase
emitteremitter
0.970.97--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell
basebase
contactcontactcontactcontact
metalmetal
0.970.97--eV eV GaInAsGaInAs
Ge substrateGe substrate
nucleation and pre-grade buffernucleation and pre-grade buffer
transparent MM transparent MM graded buffer layersgraded buffer layers
transparent MM transparent MM graded buffer layersgraded buffer layers
emitteremitter
0.840.84--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell
basebase
emitteremitter
0.840.84--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell
basebase
contactcontactcontactcontact
metalmetal
0.840.84--eV eV GaInAsGaInAs
Dislocations in Inverted Dislocations in Inverted Metamorphic Cells Metamorphic Cells –– EBICEBIC
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
41
Band Gap-Voltage
Offset
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
42
Ec
Ev
hEg
qn
qVqp
V = voltage of solar cell
= quasi-Fermi level splitting
= 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
Energy Transitions in Energy Transitions in SemiconductorsSemiconductors
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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 /
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
49
Energy Production in
MJ Solar Cells
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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 )
3-junction cell4-junction cell
5-junction cell6-junction cell
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
56
Experimental
Multijunction Cell
Results
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
n-GaInP emittern-AlInP windown+-Ga(In)As
contact
AR
p-Ge baseand substratecontact
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 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
n-GaInP emittern-AlInP windown+-Ga(In)As
contact
AR
p-Ge baseand substratecontact
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
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++-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
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++-TJn++-TJ
Lattice-Matched (LM) Lattice-Mismatchedor Metamorphic (MM)
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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%
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
63
Chart courtesy of Larry Kazmerski, NREL
Best Research Cell Best Research Cell EfficienciesEfficiencies
40%
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
67
44--Junction Upright Junction Upright Metamorphic Solar CellMetamorphic Solar Cell
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
0.67-eV Ge cell 4and substrate
transparent buffer
1.12-eV GaInAs cell 3
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
70
55--Junction Upright LatticeJunction Upright Lattice--Matched Matched Cell with EpitaxiallyCell with Epitaxially--Grown Ge SubcellGrown Ge Subcell
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
0.67-eV Ge cell 4
1.40-eV GaInAs cell 3
1.71-eV AlGaInAs cell 22.00-eV AlGaInP cell 1
metal gridline
0.67-eV Ge cell 5and substrate
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
1.4-eV GaInAs cell 3
1.7-eV AlGaInAs cell 2
2.0-eV AlGaInP cell 10.75-eV GaInAs cell 5
1.1-eV GaInPAs cell 4
GaAs or Ge growth substrate
1.4-eV GaInAs cell 3
1.7-eV AlGaInAs cell 2
2.0-eV AlGaInP cell 1
1.4-eV GaInAs cell 3
1.7-eV AlGaInAs cell 2
2.0-eV AlGaInP cell 1
GaAs or Ge growth substrate
1.4-eV GaInAs cell 3
1.7-eV AlGaInAs cell 2
2.0-eV AlGaInP cell 1
GaAs or Ge growth substrate
semi-conductor
bondedinterface
metal gridline
0.75-eV GaInAs cell 5
1.1-eV GaInPAs cell 4
1.4-eV GaInAs cell 3
1.7-eV AlGaInAs cell 2
2.0-eV AlGaInP cell 1
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
72
cap
contactAR
(Al)GaInP Cell 1 2.0 eVwide-Eg tunnel junction
GaInP Cell 2 (low Eg)1.78 eV
wide-Eg tunnel junction
AlGa(In)As Cell 31.50 eV
tunnel junction
Ga(In)As Cell 41.22 eV
tunnel junction
AR
Ga(In)As buffer
Ge Cell 6and substrate
0.67 eV
nucleation
back contact
wide-Eg tunnel junction
GaInNAs Cell 50.98 eV
cap
contactAR
(Al)GaInP Cell 1 2.0 eVwide-Eg tunnel junction
GaInP Cell 2 (low Eg)1.78 eV
wide-Eg tunnel junction
AlGa(In)As Cell 31.50 eV
tunnel junction
Ga(In)As Cell 41.22 eV
tunnel junction
AR
Ga(In)As buffer
Ge Cell 6and substrate
0.67 eV
nucleationnucleation
back contact
wide-Eg tunnel junction
GaInNAs Cell 50.98 eV
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
.
AM1.5D, ASTM G173-03, 1000 W/m2Utilization efficiency of photon energy 1-junction cell 3-junction cell 6-junction cell
Photon Utilization EfficiencyPhoton Utilization Efficiency66--Junction Solar CellsJunction Solar Cells
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
74
Terrestrial Conc. Cell Designs Terrestrial Conc. Cell Designs from 40% to 50%from 40% to 50%
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
(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
0.67-eV Ge cell 3and substrate
1.40-eV GaInAs cell 2
1.88-eV GaInP cell 1
metal gridline
transparent buffer
1.20-eV GaInAs cell 2
1.69-eV GaInP cell 1
metal gridline
0.67-eV Ge cell 3and substrate
transparent buffer
1.35-eV GaInAs cell 2
1.83-eV GaInP cell 1
metal gridline
0.67-eV Ge cell 3and substrate
transparent buffer
0.98-eV GaInAs cell 3
1.40-eV GaAs cell 2
1.88-eV GaInP cell 1
metal gridline
Ge or GaAs
growth substrate
0.67-eV Ge cell 4and substrate
transparent buffer
1.12-eV GaInAs cell 3
1.48-eV AlGaInAs cell 21.79-eV AlGaInP cell 1
metal gridline
transparent buffer
0.72-eV GaInAs cell 4
1.12-eV GaInAs cell 3transparent buffer
1.48-eV AlGaInAs cell 21.95-eV AlGaInP cell 1
metal gridline
Ge or GaAs
growth substrate
0.75-eV GaInAs cell 5
1.14-eV GaInPAs cell 4
1.73-eV AlGaInAs cell 21.42-eV GaAs cell 3
2.00-eV AlGaInP cell 1
metal gridline
Ge or GaAs
growth substrate
semiconductorbonded
interface
transparent buffer
0.70-eV GaInAs cell 6
0.97-eV GaInAs cell 5transparent buffer
1.20-eV GaInAs cell 4transparent buffer
1.77-eV AlGaAs cell 21.465-eV AlGaAs cell 3
2.00-eV AlGaInP cell 1
metal gridline
Ge or GaAs
growth substrate
0.67-eV Ge cell 5and substrate
1.12-eV GaInNAsSb cell 4
1.40-eV GaInAs cell 3
1.71-eV AlGaInAs cell 22.00-eV AlGaInP cell 1
metal gridline
0.67-eV Ge cell 4
1.40-eV GaInAs cell 3
1.71-eV AlGaInAs cell 22.00-eV AlGaInP cell 1
metal gridline
0.67-eV Ge cell 5and substrate
75
Concentrator Photovoltaic (CPV)
Systems and
Economics
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
77
Courtesy of Solar Systems Pty. Ltd., Australia
TrackingTracking
StructureStructure
OpticsOptics
CoolingCooling
Operation Operation and and MaintenanceMaintenance
Balance of System CostsBalance of System Costs
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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$$
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011
80Larry Kazmerski, NREL
81Larry Kazmerski, NREL
82Larry 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
R. R. King, UCSB Center for Energy Efficient Materials Seminar, Santa Barbara, CA, Feb. 16, 2011