Raising the Efficiency Ceiling in Multijunction Solar Cells€¦ · 1 Raising the Efficiency...

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

• 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

• 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

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

p-Ge baseand substratecontact

p-GaInAsstep-graded buffer

Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJ

n++-TJTunnel J

unction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF

contact

Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJ

n++-TJ

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

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

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

solar cells with 41.6% efficiency

• Concentrator photovoltaic (CPV) systems and economics

0.75-eV GaInAs cell 5

1.1-eV GaInPAs cell 4

semi-conductor

bondedinterface

1.7-eV AlGaInAs cell 2

2.0-eV AlGaInP cell 1

metal gridline

Global Climate Change

Vostok Ice Core Data

Years Before Present

(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

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

• Clear correlation between temperature and CO2 levels

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

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

Climate and COClimate and CO22 ––Recent HistoryRecent History

Rosina Bierbaum, Univ. of Michigan, IPCC

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

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

Temperature Anomaly by Temperature Anomaly by YearYear

The Solar Resource

• 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

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

Direct Normal Solar Radiation

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

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

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

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

Direct Normal Solar RadiationTwo-Axis Tracking Concentrator *

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

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

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

Direct Normal Solar RadiationTwo-Axis Tracking Concentrator *

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

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

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

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

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

Solar Cell Theoretical

Efficiency

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

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

h < Eg

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

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

Energy Transitions in Energy Transitions in SemiconductorsSemiconductors

Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-Ga(In)As emitter

n+-Ge emitter

p-AlGaInP BSF

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

contact

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

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF

contact

Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJn++-TJ

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

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

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

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

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

= p - n

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

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

Detailed Balance Limit of Solar Cell Efficiency

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

• 44% ultimate efficiency for device with single cutoff energy

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

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

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

Assumption limiting solar cell efficiency Device principle overcoming this limitation

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

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

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

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

One-sun incident intensity Concentrator solar cells

Assumptions Assumptions OpportunitiesOpportunities

Theoretical

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

= 1 – TS/E = 1 – (4/3)T/Tsun (Henry)

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

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

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

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

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

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

(Shockley, Queisser)

Measured

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Maximum Solar Cell Maximum Solar Cell EfficienciesEfficiencies

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

GrowthDirection

1.9 eV (Al)GaInP subcell 1

1.4 eV GaInAs subcell 2

graded MM buffer layers

Ge or GaAs substrate

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

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

AM1.5D, low-AOD

AM1.5G, ASTM G173-03

AM0, ASTM E490-00a

1.89 1.41 eV0.67

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

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

Ga(In)As buffer

Ge Cell 5and substrate

0.67 eV

nucleation

back contact

wide-Eg tunnel junction

GaInNAs Cell 41.1 eV

contactAR

GaInP Cell 2 (low Eg)1.8 eV

tunnel junction

Ga(In)As buffer

0.67 eV

nucleation

back contact

• Divides available current

density above GaAs Eg

among 3-4 subcells

• Allows low-currentGaInNAs cell to be matched toother subcells

• Lower series resistance

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

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

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

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

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

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

photonenergiesin solar

spectrum

after carrierthermalizationto band edges

photonswith energy

above lowestband gap

6-junctionterrestrial

concentratorcell

morebalanced

1.9/1.4/1.0 eVbandgap

combination

after carrierextractionat quasi-

Fermi levels-- 1-sun

Fermi levels-- 500 suns

subcell 1 (top)

subcell 3

subcell 2

no photogeneration

photonenergiesin solar

spectrum

after carrierthermalizationto band edges

photonswith energy

above lowestband gap

6-junctionterrestrial

concentratorcell

morebalanced

1.9/1.4/1.0 eVbandgap

combination

Fermi levels-- 1-sun

Fermi levels-- 500 suns

subcell 1 (top)

subcell 3

subcell 2

no photogeneration

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

2 . eV

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

h < Eg

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

2 . eV

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

h > Eg

thermalization of carriers

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

GrowthDirection

Ge or GaAs substrateGe or GaAs substrate

capcap

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

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

Metamorphic

Semiconductor

Materials

29Courtesy J. Geisz – NREL

Bandgap vs. Bandgap vs. Lattice ConstantLattice Constant

5.6 5.65 5.7 5.75 5.8

Lattice Constant (angstrom)

Ge(indirect)

disordered GaInP

ordered GaInP

GaInAs8%-In

GaInAs 12%-In

23%-In GaInAs

35%-In

Bandgap vs. Bandgap vs. Lattice ConstantLattice Constant

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

eV1.40

GaInAs single-junction solar cells

2.4% lattice mismatch

1.6% lattice mismatch

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

Metamorphic = "changed form"

• 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

Tunnel junction

Pre-grade buffer

Misfit dislocations

GaInAs gradedbuffer to 8%-In

Ge substrate

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

CellCell

Line of 0%relaxation

in) Å

Qx (Tilt) Å-1

Ga0.92In0.08As MC

GaInP TC

GradedBuffer

(115) glancing exit XRD

in) Å

Qx (Tilt) Å-1

Ga0.92In0.08As MC

GaInP TC

GradedBuffer

(115) glancing exit XRD

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

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

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

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

nucleation

buffer layerbuffer layer

emitter

1.39-eV GaInAsinverted LM subcell

contact

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

Ge substrate

nucleation and pre-grade buffer

transparent MM transparent MM graded buffer layersgraded buffer layers

emitter

1.10-eV GaInAsinverted MM subcell

contact

Ge substrate

emitteremitter

0.970.97--eV GaInAseV GaInAsinverted MM subcellinverted MM subcell

basebase

contactcontact

Ge substrate

emitteremitter

basebase

contactcontact

nucleationnucleation

buffer layerbuffer layerbuffer layerbuffer layer

emitter

contactcontact

metalmetal

50 m8e-9766-1

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

50 m8e-9756-1

50 m8e-9760-1

50 m8e-9783-11

EBIC images and dislocation density of inverted metamorphic cell test structures

1.391.39--eV eV GaInAsGaInAs

1.10-eV IMM subcell23% In1.6%

3.9 x 106 cm-2

5.0 x 106 cm-2

6.3 x 106 cm-2

Ge substrateGe substrate

nucleation and pre-grade buffernucleation and pre-grade buffer

emitter

contactcontact

metalmetal

emitteremitter

basebase

emitteremitter

basebase

contactcontactcontactcontact

metalmetal

emitteremitter

basebase

emitteremitter

basebase

contactcontactcontactcontact

metalmetal

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

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

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

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

Inverted metamorphic (MM)GaxIn1-xAs solar cells

Dislocations in Inverted Dislocations in Inverted Metamorphic CellsMetamorphic Cells

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

ity fr

rier L

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

Band Gap-Voltage

Offset

= p - n

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

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

0.6 1 1.4 1.8 2.2Band Gap Eg (eV)

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

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

of solar cells with wide range of band gaps

• 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

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

kTqWVC eNNpn /

VqEW g

ocgoc VqEW

qkTW VCln

phgoc J

W lnln

phoc J

kTV ln

JnJK /

2 kTqVpho

oceJJ /

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

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

gcco e

chkTqqQJ /

goc JkT

EchkTq

qkTW 11)(2ln

xchkTqdE

chqqQJ

EkTEco

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

0.6 1 1.4 1.8 2.2Band Gap Eg (eV)

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

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

of solar cells with wide range of band gaps

• 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

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

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

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

Energy Production in

MJ Solar Cells

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

250 500 750 1000 1250 1500 1750 2000 2250

Wavelength (nm)

/(m2 m

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

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

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

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

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

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

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

Time of Day (hours)

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

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

Time of Day (hours)

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

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

Time of Day (hours)

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

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

100000

200000

300000

400000

500000

600000

700000

800000

900000

Time of Day (hours)

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

Experimental

Multijunction Cell

Results

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

Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-Ga(In)As emitter

n+-Ge emitter

p-AlGaInP BSF

contact

n-Ga(In)As buffer

Bottom Cell

p++-TJn++-TJ

p-Ga(In)As base

nucleation

GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJn++-TJ

Tunnel junction

Buffer region

Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-Ga(In)As emitter

n+-Ge emitter

p-AlGaInP BSF

contact

n-Ga(In)As buffer

Bottom Cell

p++-TJn++-TJ

p-Ga(In)As base

nucleation

GaInP top cell

Ge bottom cell

n-GaInP window

p++-TJn++-TJ

Tunnel junction

Buffer regionTunnel

Juncti

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF

contact

Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJn++-TJ

Tunnel Ju

nction

Top Cell

Wide-Eg Tunnel

Middle Cell

p-GaInP BSF

p-GaInP base

n-GaInAs emitter

n+-Ge emitter

p-AlGaInP BSF

contact

Bottom Cell

p++-TJn++-TJ

p-GaInAs base

nucleation

n-GaInP window

p++-TJn++-TJ

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

300 500 700 900 1100 1300 1500 1700 1900

Wavelength (nm)

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

AM0, ASTM E490-00aEQE, lattice-matched

EQE, metamorphic

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

1.0 1.1 1.2 1.3 1.4 1.5 1.6

Eg2 = Subcell 2 Bandgap (eV)

E g1 =

Disordered GaInP top subcell Ordered GaInP top subcell

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%

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

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

Spectrolab CPV Cell Spectrolab CPV Cell GenerationsGenerations

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

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

0.1 1.0 10.0 100.0 1000.0

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

Efficiency

Voc x 10

Voc fit, 100 to 1000 suns

• 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

Chart courtesy of Larry Kazmerski, NREL

Best Research Cell Best Research Cell EfficienciesEfficiencies

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

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

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

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

Spectrolab 3J Metamorphic Spectrolab 3J Metamorphic Concentrator CellsConcentrator Cells

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

E g2 =

38%46%

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

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

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

0.67-eV Ge cell 4and substrate

transparent buffer

1.48-eV AlGaInAs cell 21.79-eV AlGaInP cell 1

metal gridline

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

MJ cell

subcell 1

subcell 2

subcell 3

subcell 4

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

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

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

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

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

0.67-eV Ge cell 4

metal gridline

0 1 2 3 4 5 6Voltage (V)

MJ cell

subcell 1

subcell 2

subcell 3

subcell 4

subcell 5

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

InP growth substrateGaAs or Ge growth substrate

2.0-eV AlGaInP cell 10.75-eV GaInAs cell 5

GaAs or Ge growth substrate

semi-conductor

bondedinterface

metal gridline

semi-conductor

bondedinterface

metal gridline

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

• Wafer bonding for multijunction solar cells

contactAR

tunnel junction

Ga(In)As buffer

0.67 eV

nucleation

back contact

contactAR

tunnel junction

Ga(In)As buffer

0.67 eV

nucleationnucleation

back contact

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

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

66--Junction Solar CellsJunction Solar Cells

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

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

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

Matched (LM)C3MJ+

3J Meta-morphic (MM)low mismatch

3J Meta-morphic (MM)high mismatch

3J InvertedMetamorphic

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

l Effi

MJ CellEfficiency

1.88-eV GaInP cell 1

metal gridline

transparent buffer

metal gridline

transparent buffer

metal gridline

transparent buffer

1.40-eV GaAs cell 2

metal gridline

Ge or GaAs

growth substrate

transparent buffer

metal gridline

transparent buffer

1.12-eV GaInAs cell 3transparent buffer

metal gridline

Ge or GaAs

growth substrate

1.73-eV AlGaInAs cell 21.42-eV GaAs cell 3

metal gridline

Ge or GaAs

growth substrate

semiconductorbonded

interface

transparent buffer

1.77-eV AlGaAs cell 21.465-eV AlGaAs cell 3

metal gridline

Ge or GaAs

growth substrate

1.12-eV GaInNAsSb cell 4

metal gridline

0.67-eV Ge cell 4

metal gridline

Concentrator Photovoltaic (CPV)

Systems and

Economics

Courtesy of Solar Systems Pty. Ltd., Australia

Equipped with III-V MJ

cell receivers

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

Solar Systems, AustraliaSolar Systems, AustraliaHermannsburg Power StationHermannsburg Power Station

Courtesy of Solar Systems Pty. Ltd., Australia

TrackingTracking

StructureStructure

OpticsOptics

CoolingCooling

Operation Operation and and MaintenanceMaintenance

Balance of System CostsBalance of System Costs

Economics for Economics for Device PhysicistsDevice Physicists

Continuity equation:

...in $$ rather than charge carriers:

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

period payback

yearm kWh

produced,energy ratio

m / cost

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

2222 2

J Rq Gq t

$$ $$gen $$exp F$$

Cell cost ranges

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

5-Year Payback Threshold, at $0.15/kWh

500X Point-Focus Conc.

40%50% cell eff.

20%15%

25% cell eff.

Fixed Flat-Plate

period payback

yearm kWh

produced,energy ratio

m / cost

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

2222 2

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

Increasing cell efficiency main priority for concentrators

Decreasing cell cost main priority for flat-plate

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

80Larry Kazmerski, NREL

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

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

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

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

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

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

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

SummarySummary

Raising the Efficiency Ceiling in Multijunction Solar Cells€¦ · 1 Raising the Efficiency...

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