sign in sign up
<<
Home , Band gap

Band-Gap-Engineered Architectures for High-Efficiency Multijunction Concentrator Solar Cells

Richard R. King, A. Boca, W. Hong, X.-Q. Liu, D. Bhusari, D. Larrabee, K. M. Edmondson, D. C. Law, C. M. Fetzer, S. Mesropian, and N. H. Karam

Spectrolab, Inc. A Boeing Company

24th European Photovoltaic Solar Energy Conference Sep. 21-25, 2009 Hamburg, Germany

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 1 AcknowledgmentsAcknowledgments

• Carl Osterwald, Keith Emery, Larry Kazmerski, Martha Symko-Davies, Fannie Posey-Eddy, Holly Thomas, Manuel Romero, John Geisz, Sarah Kurtz – NREL • Rosina Bierbaum – University of Michigan, Ann Arbor • Pierre Verlinden, John Lasich – Solar Systems, Australia • Kent Barbour, Russ Jones, Jim Ermer, Peichen Pien, Dimitri Krut, Hector Cotal, Mark Osowski, Joe Boisvert, Geoff Kinsey, Mark Takahashi, and the entire multijunction team at Spectrolab This work was supported in part by the U.S. Dept. of Energy through the NREL High-Performance (HiPerf PV) program (ZAT-4-33624-12), the DOE Technology Pathways Partnership (TPP), and by Spectrolab.

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 2 OutlineOutline

contact AR +-GaInAs n ll e n-AlInP window C n-GaInP emitter p • Solar cell theoretical efficiency limits To

p-GaInP base l e n n u T g -E e d p-AlGaInP++ BSF i – Opportunities to change ground rules for higher terrestrial efficiency p -TJ W n++-TJ ll n-GaInP window e C n-GaInAs emitter le idd M – Cell architectures capable of >70% in theory, >50% in practice p-GaInAs base

p-GaInP BSF

p-GaInAs n o step-graded ti c n buffer u J l e n ++ n p -TJ u T l • Metamorphic materials n++-TJ el nucleation C m n+ o -Ge emitter tt o B p-Ge base and substrate – Control of band gap to tune to solar spectrum contact

gridline semi- • High-efficiency terrestrial concentrator cells conductor bonded interface – Metamorphic (MM) and lattice-matched (LM) 3-junction 2.0-eV AlGaInP cell 1 solar cells with >40% efficiency 1.7-eV AlGaInAs cell 2 1.4-eV GaInAs cell 3

– 4-junction MM and LM concentrator cells 1.1-eV GaInPAs cell 4 – Inverted metamorphic structure, semiconductor 0.75-eV GaInAs cell 5 bonded technology (SBT) for MJ terrestrial concentrator cells

• The solar resource and concentrator photovoltaic (CPV) system economics

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 3 High-Efficiency Multijunction Cell Architectures

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 4 MaximumMaximum SolarSolar CellCell EfficienciesEfficiencies Measured Theoretical

References C. H. Henry, “Limiting efficiencies of ideal single and multiple terrestrial solar cells,” J. Appl. Phys., 51, 4494 (1980). 95% Carnot eff. = 1 – T/Tsun T = 300 K, Tsun ≈ 5800 K W. Shockley and H. J. Queisser, “Detailed Balance Limit of Efficiency of p-n Junction 93% Max. eff. of solar energy conversion Solar Cells,” J. Appl. Phys., 32, 510 (1961). J. H. Werner, S. Kolodinski, and H. J. Queisser, “Novel Optimization Principles and = 1 –TS/E = 1 –(4/3)T/Tsun (Henry) 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 72% Ideal 36-gap solar cell at 1000 suns (Henry) cells," Appl. Phys. Lett., 90, 183516 (4 May 2007). M. Green, K. Emery, D. L. King, Y. Hishikawa, W. Warta, " Tables (Version 27)", Progress in Photovoltaics, 14, 45 (2006). A. Slade, V. Garboushian, "27.6%-Efficient 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 56% Ideal 3-gap solar cell at 1000 suns (Henry) Tandem Solar Cells," Proc. 21st IEEE Photovoltaic Specialists Conf., Kissimmee, 50% Ideal 2-gap solar cell at 1000 suns (Henry) 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). 44% Ultimate eff. of device with cutoff Eg: (Shockley, Queisser) 43% 1-gap cell at 1 sun with carrier multiplication 3-gap GaInP/GaInAs/Ge LM cell, 364 suns (Spectrolab) 41.6% (>1 e-h pair per ) (Werner, Kolodinski, Queisser) 3-gap GaInP/GaInAs/Ge MM cell, 240 suns (Spectrolab) 40.7%

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

3-gap GaInP/GaAs/GaInAs cell at 1 sun (NREL) 33.8% 31% Ideal 1-gap solar cell at 1 sun (Henry) 30% Detailed balance limit of 1 gap solar cell at 1 sun 1-gap solar cell (silicon, 1.12 eV) at 92 suns (Amonix) 27.6% (Shockley, Queisser) 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 5 MetamorphicMetamorphic (MM)(MM) 33--JunctionJunction SolarSolar CellCell

contact AR +-GaInAs n ll MJ cell n-AlInP window e C 0.3 p n-GaInP emitter o T subcell 1

p-GaInP base l e n n subcell 2 u 0.25 T g -E e d p-AlGaInP++ BSF i subcell 3 p -TJ W n++-TJ ll n-GaInP window e 0.2 C le n-GaInAs emitter d id M p-GaInAs base 0.15

p-GaInP BSF

p-GaInAs 0.1 n o step-graded ti c n buffer u J l e n ++ n p -TJ u T l 0.05 n++-TJ el C nucleation CurrentDensity / Incident Intensity (A/W) m n+ o -Ge emitter tt o B p-Ge base 0 and substrate 00.511.522.533.5 contact Voltage (V) Lattice-Mismatched or Metamorphic (MM)

• Metamorphic growth of upper two subcells, GaInAs and GaInP

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 6 ExternalExternal QEQE ofof LMLM andand MMMM 33--JunctionJunction CellsCells

100 100 AM1.5D, low-AOD AM1.5G, ASTM G173-03 90 AM0, ASTM E490-00a 90 EQE, lattice-matched 80 EQE, metamorphic 80 m))

μ 70 70 2

60 60

50 50

40 40

30 30 Current Density per Unit Current Density

Wavelength (mA/(cm 20 20 External Quantum Efficiency (%) 10 10

0 0 300 500 700 900 1100 1300 1500 1700 1900 Wavelength (nm)

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 7 MetamorphicMetamorphic (MM)(MM) 33--JunctionJunction SolarSolar CellCell

. 3-junction E / E / 0.67 eV cell efficiency 2.1 g1 g2 240 suns (24.0 W/cm2), AM1.5D (ASTM G173-03), 25oC Ideal efficiency -- radiative recombination limit 2 LM 1.9 MM 40.7% 40.1% 1.8 54% 1.7 52% 1.6 50% 48% 1.5 46%

= Subcell 1 (Top) Bandgap (eV) (eV) Bandgap (Top) 1 Subcell = 44% 1.4 g1 42% E 40% 38% 1.3 1.0 1.1 1.2 1.3 1.4 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 8 RecordRecord 40.7%40.7%--EfficientEfficient ConcentratorConcentrator SolarSolar CellCell

Spectrolab • First solar cell of Metamorphic GaInP/ GaInAs/ Ge Cell any type to reach

Voc = 2.911 V 2 over 40% efficiency Jsc = 3.832 A/cm FF = 87.50% Vmp = 2.589 V 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 9 MetamorphicMetamorphic (MM)(MM) 33--JunctionJunction CellsCells –––– InvertedInverted 1.01.0--eVeV GaInAsGaInAs SubcellSubcell

Ge or GaAs substrate Growth Direction Ge or GaAs substrate

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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 10 InvertedInverted MetamorphicMetamorphic (IMM)(IMM) 33--JunctionJunction CellCell

1.6 3-junction 1.9 eV/ Eg2/ Eg3 cell efficiency 500 suns (50 W/cm2), AM1.5D (ASTM G173-03), 25oC X

. Ideal efficiency -- radiative recombination limit 1.5

1.4 53% 52%

51% 1.3 50%

48% 1.2

= Subcell 2 Bandgap (eV) 46%

g2 1.1 E

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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 11 44--JunctionJunction UprightUpright MetamorphicMetamorphic (MM)(MM) TerrestrialTerrestrial ConcentratorConcentrator CellCell

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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 12 44--JunctionJunction CellCell OptimumOptimum BandBand GapGap CombinationsCombinations

1.7 4-junction 1.9 eV/ Eg2/ Eg3/ 0.67 eV cell efficiency 500 suns (50 W/cm2), AM1.5D (ASTM G173-03), 25oC X . Ideal efficiency -- radiative recombination limit 1.6

1.5

58% 1.4

56% 1.3 54%

1.2 50% = Subcell 2 Bandgap (eV) = Subcell 2 Bandgap 38% g2 46% E 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 13 55--JunctionJunction InvertedInverted MetamorphicMetamorphic (IMM)(IMM) CellsCells

g Ge ro o wt r G h a su As bs tr at metal e 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 14 SemiconductorSemiconductor--BondedBonded TechnologyTechnology (SBT)(SBT) TerrestrialTerrestrial ConcentratorConcentrator CellCell

• Wafer bonding for multijunction solar cells – Low band gap cells for MJ cells using high-quality, GaAs ormetal Ge growth gridlinesubstrate lattice-matched materials semi- GaAs or Ge growth substrate – Epitaxial exfoliation and conductor substrate removal bonded 2.0-eV AlGaInP cell 1 interface 2.0-eV AlGaInP cell 1 – Formation of lattice- 1.7-eV AlGaInAs cell 2 engineered substrate for 1.7-eV AlGaInAs cell 2 later MJ cell growth 1.4-eV GaInAs cell 3 – Bonding of high-band-gap 1.4-eV GaInAs cell 3 and low-band-gap1.4-eV GaInAs cells cell after 3 growth 1.1-eV GaInPAs cell 4 – Electrical1.7-eV conductanceAlGaInAs cell of 2 semiconductor-bonded 0.75-eV GaInAs cell 5 2.0-eV AlGaInP cell 1 interface – Surface effects for GaAs or Ge InP growth substrate semiconductor-to-growth substrate semiconductor bonding

15 66--JunctionJunction SolarSolar CellsCells

0.14

0.12

contact 0.1 AR AR cap 0.08 MJ cell (Al)GaInP Cell 1 2.0 eV subcell 1 0.06 subcell 2 wide-Eg tunnel junction subcell 3 GaInP Cell 2 (low Eg) 0.04 subcell 4 1.78 eV subcell 5 0.02

wide-Eg tunnel junction (A/W) Intensity Incident / Density Current subcell 6

AlGa(In)As Cell 3 0 1.50 eV 01234567 wide-Eg tunnel junction Voltage (V) 700 AM1.5D, ASTM G173-03, 1000 W/m21.4 Ga(In)As Cell 4 Utilization efficiency of photon energy 1.22 eV 1-junction cell 600 3-junction cell 1.2 tunnel junction 6-junction cell

GaInNAs Cell 5 500 1 . 0.98 eV tunnel junction

eV) 400 0.8 Ga(In)As buffer 2 . 2 nucleation 300 0.6 Ge Cell 6 (W/m and substrate 200 0.4 0.67 eV back contact

Intensity per Unit Photon Energy 100 0.2 Photon utilization efficiency 0 0 00.511.522.533.54 Photon Energy (eV)

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 16 ModeledModeled TerrestrialTerrestrial ConcentratorConcentrator CellCell EfficiencyEfficiency

Detailed balance limit efficiency 60% Radiative recombination only Series res. and shadowing, optimized grid spacing 3J Normalized to experimental efficiency 4J 55%

50% 4J

3J 45% Efficiency (%) Efficiency 4J

40% 3J

3J & 4J MM solar cells 35% 500 1 10 100 1000 10000 Incident Intensity (suns) (1 sun = 0.100 W/cm2)

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 17 High-Efficiency Multijunction Cell Results

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 18 LMLM andand MMMM 33--JunctionJunction CellCell CrossCross--SectionSection

contact contact AR AR n+-Ga(In)As n+-GaInAs

n-AlInP window ll n-AlInP window ll e e n-GaInP emitter C n-GaInP emitter C p p o o T T p-GaInP base p-GaInP base GaInP top cell l l e e nn nn u u T T g g -E -E p-AlGaInP BSF e p-AlGaInP BSF e d d ++ i i p -TJ W p++-TJ W Wide-bandgap tunnel junction n++-TJ n++-TJ ll ll n-GaInP window e n-GaInP window e C C n-Ga(In)As emitter le n-GaInAs emitter le d d id id M M Ga(In)As middle cell p-Ga(In)As base p-GaInAs base n o ti c n u J l p-GaInP BSF e p-GaInP BSF ++ nn Tunnel junction p -TJ Tu n++-TJ p-GaInAs n o step-graded ti c n n-Ga(In)As buffer buffer u J l Buffer region e n p++-TJ n Tu ll l e n++-TJ el nucleation C C m nucleation m + o + o n -Ge emitter tt n -Ge emitter tt o o B B Ge bottom cell p-Ge base p-Ge base and substrate and substrate contact contact Lattice-Matched (LM) Lattice-Mismatched or Metamorphic (MM) R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 19 NewNew WorldWorld RecordRecord 41.6%41.6% MultijunctionMultijunction SolarSolar CellCell • 41.6% efficiency demonstrated for 3J lattice-matched Spectrolab cell, a new world record • Highest efficiency for any type of solar cell measured to date • Independently verified by National Renewable Energy Laboratory (NREL) • Standard measurement conditions (25°C, AM1.5D, ASTM G173 spectrum) at 364 suns (36.4 W/cm2) • Lattice-matched cell structure similar to C3MJ cell, with reduced grid shadowing as planned for C4MJ cell • Incorporating high-efficiency 3J metamorphic cell structure + further improvements in grid design Ref.: R. R. King et al., 24th European Photovoltaic Solar → strong potential to reach 42-43% Energy Conf., Hamburg, Germany, Sep. 21-25, 2009. champion cell efficiency

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

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 20 41.6%41.6% SolarSolar CellCell Eff.,Eff., VocVoc vs.vs. ConcentrationConcentration

44 Efficiency 0.98 Voc x 10 41.6% 42 0.96 Voc fit, 100 to 1000 suns 40 FF 0.94

38 0.92 x 10 (V) oc 36 0.90

34 0.88

32 0.86

30 0.84 Fill Factor (unitless)

28 0.82 Efficiency (%) and V 26 0.80

24 0.78 0.1 1.0 10.0 100.0 1000.0 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 21 41.6%41.6% SolarSolar CellCell LIVLIV CurvesCurves vs.vs. ConcentrationConcentration

0.16 41.6% 0.14

Inc. Intensity (suns) 0.12 1 sun = 0.100 W/cm2

2.6 0.1 6.6

0.08 17.6

59.8 0.06 127.3

0.04 364.2 604.8 0.02 940.9 Current Density / Incident Intensity (A/W)

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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 22 BestBest ResearchResearch CellCell EfficienciesEfficiencies

Chart courtesy of Larry Kazmerski, NREL

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 23 SpectrolabSpectrolab CellCell GenerationsGenerations inin DOEDOE TPPTPP ProgramProgram

43% C4MJ •Terrestrial 43 concentrator 42 40% cell efficiency 41 •Goals in 40 38.5% Technology 39 37.5% Pathways 37% 38 Partnership 37 (TPP) Production (%) Cell Efficiency

36 20072008200920102015 Year

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 24 SpectrolabSpectrolab C1MJ,C1MJ, C2MJ,C2MJ, andand C3MJC3MJ CellCell ProductsProducts

40% C1MJ ηAVG = 38.2% 35% C2MJ

30% C3MJ ηAVG = 37.5%

25% ηAVG = 36.9% 20%

15%

% of Population Population of of % % 10%

5%

0% 39.5%39.5% 38.5%38.5% 39.0%39.0% 37.5%37.5% 38.0%38.0% 35.5%35.5% 36.0%36.0% 36.5%36.5% 37.0%37.0% 34.0%34.0% 34.5%34.5% 35.0%35.0% Efficiency η at Max. Power

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 25 PrototypePrototype 3J3J MetamorphicMetamorphic CellCell BuildsBuilds

Corrected LIV Data for Prototype 3J Metamorphic (MM) 5%-In Cells 10 runs, 22 wafers, 205 cells

5% 3J-MM 35.7% 36.0% 36.3% 36.6% 36.9% 37.2% 37.5% 37.8% 38.1% 38.4% 38.7% 39.0% 39.3% 39.6% 39.9% 40.2% 40.5% 40.8% 3J MM Cell Efficiency Bin Isc (A) Voc (V) FF Eff Average 7.601 3.090 0.845 39.6% Std. dev. 0.135 0.022 0.009 0.8%

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 26 44--JunctionJunction LatticeLattice--MatchedMatched CellCell

contact AR AR MJ cell cap 0.25 subcell 1 (Al)GaInP Cell 1 1.9 eV wide-Eg tunnel junction subcell 2 0.2 AlGa(In)As Cell 2 subcell 3 1.6 eV subcell 4

wide-Eg tunnel junction 0.15

Ga(In)As Cell 3 1.4 eV 0.1

tunnel junction Ga(In)As buffer 0.05 nucleation Current Density / Incident Intensity (A/W) Ge Cell 4 and substrate 0 0.67 eV 012345 back contact 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 27 MeasuredMeasured 44--JunctionJunction CellCell QuantumQuantum EfficiencyEfficiency

100 1600

90 1400 80 1200 70 AlGaInP subcell 1 AlGaInAs subcell 2 1.95 eV 1.66 eV GaInAs subcell 3 Ge subcell 4 1000 60 1.39 eV 0.72 eV m))

All subcells AM1.5D μ 2 50 ASTM G173-03 800

40 600 (W/(m 30 400 20 Intensity Per Unit Wavelength

External Quantum Efficiency (%) Efficiency Quantum External 200 10

0 0 300 500 700 900 1100 1300 1500 1700 1900 Wavelength (nm)

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 28 LightLight II--VV CurvesCurves RecordRecord EfficiencyEfficiency CellsCells

0.16 . 0.14

0.12

0.10

3J Conc. 3J Conc. 3J Conc. 4J Conc. 0.08 Cell Cell Cell Cell Metamor phic Lattice-matched LM, 822 suns 4J Cell

Voc 2.911 3.192 V 3.251 4.398 V J /inten. 0.1596 0.1467 A/W 0.1467 0.0980 A/W 0.06 sc 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 0.04 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 Current Density / Inc. Intensity (A/W) Density / Inc. Intensity (A/W) Current 0.02 Prelim. meas. Independently confirmed meas. 25°C 25°C 0.00 0 0.5 1 1.5 2 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 29 The Solar Resource and CPV Economics

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 30 TheThe SolarSolar ResourceResource

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

R. R. King et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 31 ConcentratorConcentrator PhotovoltaicPhotovoltaic (CPV)(CPV) ElectricityElectricity GenerationGeneration

CPV cost superiority CPV cost superiority 40% cell efficiency 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 32 SummarySummary

• Urgent global need to address 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 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 et al., 24th European Photovoltaic Solar Energy Conf., Hamburg, Germany, Sep. 21-25, 2009 33