High-Efficiency Concentrator Photovoltaics –Partitioning the Sun's Spectrum with

Novel Materials in III-V Multijunction Cells

R. R. King, R. A. Sherif, C. M. Fetzer, D. C. Law, S. Kurtz1, P. C. Colter, K. M. Edmondson, T. D. Isshiki, H. L. Cotal, H. Yoon, G. S. Kinsey, J. H. Ermer, T. Moriarty1, J. Kiehl1,

K. Emery1, W. Metzger1, R. K. Ahrenkiel1, and N. H. Karam

Spectrolab, Inc., Sylmar, CA1National Renewable Energy Laboratory, Golden, CO

Global Climate & Energy ProjectSolar Energy Workshop

Stanford University – Oct. 18-19, 2004

Acknowledgments

• Donna Senft, Henry Yoo – Air Force Research Laboratory

• Martha Symko-Davies, Brian Keyes, Dan Friedman – NREL

• Dimitri Krut, Dave Joslin, Mark Takahashi, Greg Glenn, and the entire multijunction solar cell team at Spectrolab

This work was supported in part by the Air Force Research Laboratory (AFRL/VS) under DUS&T contract # F29601-98-2-0207, by the Dept. of Energy through the NREL High-Performance PV program (NAT-1-30620-01), and by Spectrolab.

Introduction

• Why hasn't photovoltaic electricity generation become widespread to date?

→ High costs not only of active semiconductor materials, but also more mundane materials, e.g., glass, metal, plastic→ High conversion efficiency offers the best hope to reduce these critical costs of cell environmental and mechanical protection, and balance-of-system (BOS) costs→ Use concentration to drastically reduce the area of high-efficiency cells, and therefore their cost

Outline

• Motivation: Concentrator and fixed flat-plate photovoltaic system economics

• Examples of concentrator primary optics used in PV systems

• Multijunction III-V solar cell approach to reaching very high efficiencies

• Research on novel semiconductor materials to improve range of subcell bandgaps that can be integrated into MJ cells

Outline

• Solar spectrum and theoretical efficiency

• Metamorphic (lattice-mismatched) photovoltaic materials

• Group-III sublattice ordering

• 3-junction GaInP/ GaInAs/ Ge solar cells –expt. results

• GaInNAsGaInNAs ~1-eV subcells lattice-matched to Ge

• 5- and 6-junction cellsFirst 6-junction cells built and tested

Terrestrial PV

Economics

Terrestrial PV

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PV S

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ost /

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5-Year Payback Threshold, at $0.15/kWh

6%10%

15%20%

25% cell eff.

Fixed Flat-Plate

Module and BOS cost assumptions from Swanson, Prog. Photovolt. Res. Appl. 8, 93-111 (2000).

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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

Terrestrial PV

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5-Year Payback Threshold, at $0.15/kWh20%

30%40%50% cell eff.

Fixed Flat-Plate

500X Point-Focus Conc.

Module and BOS cost assumptions from Swanson, Prog. Photovolt. Res. Appl. 8, 93-111 (2000).

Terrestrial PV

Cell cost ranges given in: Swanson, Prog. Photovolt. Res. Appl. 8, 93-111 (2000)

Module and BOS cost assumptions from Swanson, Prog. Photovolt. Res. Appl. 8, 93-111 (2000).

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yste

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ost /

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ear P

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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

Examples of PV

Concentrator Systems

Terrestrial Concentrators

Terrestrial III-V PV:• Highest efficiency PV technology

• Current record cell at 37.3%

• High concentration (~500-1000X) reduces importance of cell cost ⇒

Best to use highest efficiency cell

• High efficiency then reduces the• Primary optics cost• Cell cost for given power output• Balance-of-system costs

(supports, wiring, land prep. etc.)• Overall cost per watt

Terrestrial concentrator system on Mt. Haleakala – Courtesy ENTECH

See: "Terrestrial triple-junction array generates kilowatt solar power,"Compound Semiconductor, August 2004, p.29.

Solar Systems Dish Concentrators

Courtesy of Solar Systems, Australia

www.solarsystems.com.au

Dense Array of MJ Cells On-Sun in Dish Concentrator System

Arizona Public Service (APS) concentrator dish with Spectrolab dense array of 3-junction cells

A 1-kW Prototype

• First 1-kW prototype, grid-connected concentrator module using triple-junction cells installed at Arizona Public Service, June 2004.

Concentrator Solar Cell Receiver Assembly

Metallized Substrate

Solar CellBypass Diode

Solar Spectrum and

Theoretical Efficiency

Standard Solar Spectra

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Solar Spectrum Partitionfor 1-Junction cell

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Solar Spectrum Partitionfor 2-Junction cell

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AM1.5D 2-Junction Cell Efficiency Contours

AM1.5D 2-Junction Cell Efficiency Contours

Ga0.515In0.495P /Ga0.99In0.01As 2J

Ga0.44In0.56P /Ga0.92In0.08As 2J

Ga0.29In0.71P /Ga0.77In0.23As 2J

Bandgap vs. Lattice Constant

LM and MM 3-Junction Cell Cross-Section

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-Mismatchedor Metamorphic (MM)

Lattice-Matched (LM)

Solar Spectrum Partitionfor 3-Junction cell

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AM1.5 Direct, low-AOD

1.41 eV 0.67 eV1.85 eV

Voltage and Current DensityTradeoff vs. Eg

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adia

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omb.

AM0AM1.5 Direct, low-AODVoc X 10, AM1.5D low-AOD, 1 sunVoc X 10, AM1.5D low-AOD, 500 suns

1.305 1.414 eV

3-Junction Cell Efficiency vs. Eg

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omb.

AM0AM1.5 Direct, low-AODIdeal Eff., AM1.5D low-AOD, 1 sunIdeal Eff., AM1.5D low-AOD, 500 sunsVoc X 10, AM1.5D low-AOD, 1 sunVoc X 10, AM1.5D low-AOD, 500 suns

1.305 1.414 eV

Metamorphic

Photovoltaic Materials

Metamorphic GaInP/GaInAs/Ge 3-Junction Cell Cross-Section

Wide-bandgaptunnel junction

GaInP top cell

Ge bottom cell

Ga(In)As middle cell

Tunnel junctionTunnel

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

• Bandgap combination of lattice-mismatched GaInAs and GaInP provides better match to solar spectrum

• Top and middle subcells have same new lattice constant

• 0.5% lattice-mismatch

• Optimized buffer structure → reduced threading dislocation propagation intoactive cell regions

• 2.0 - 2.6 x 105 cm-2

threading dislocationdensity measuredby EBIC and CL

height

Lattice Const.

Ge

X%

InGaA

s

MMLM

Buffer region

EQE and PL of Subcells Matched to 1%-In and 8%-In GaInAs

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olum

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. uni

ts)EQE, lattice-matched

EQE, metamorphic

PL, lattice-matched

PL, metamorphic

1.890 1.813 eV 1.414 1.305 eV

Room Temperature PL DataAcross 100-mm Wafer

Top Cell –average of 678 nm = 1.827 ± 0.010 eV

(Lattice-matched GaInPtop cell is at 1.89 eV)

Middle Cell –average of 950 nm = 1.305 ± 0.005 eV

External QE of LM and MM 3-Junction Cells

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External QE of LM and MM 3-Junction Cells

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External QE of LM and MM 3-Junction Cells

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(mA

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EQE, metamorphicAM0AM1.5GAM1.5 Direct, low-AOD

Cross sectional TEMGa0.44In0.56P/ Ga0.92In0.08As/ Ge Cell

• Low dislocation density in active cell layers in top portion of epilayer stack:

~ 2 x 105 cm2 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 µm

Tunnel junction

Pre-grade buffer

Misfit dislocations

GaInAs gradedbuffer to 8%-In

0.2 µm

Ge substrateRef.: King et al., 28th IEEE PVSC, 2000, p. 982

EBIC and CL measurements courtesy of M. Romero – NREL

100%relaxed100%

strained

Ge Substrate

GaAs LayerGe Substrate

GaAs Layer

GaAs (0%-In) 1%-In GaInAs GaInP/8%-In GaInAs

12%-In GaInAs

Ge

• (115) glancing exit XRD

• Ga0.92In0.08As and Ga0.88In0.12Aslayers almost fully relaxed (90-100%)

• GaInP top cell fully strained w.r.t. Ga0.92In0.08As middle cell

GradedBuffer

Ga0.92In0.08As MCGaInP TC

Line of 100%relaxation

100%relaxed

Line of 0%relaxation

(100% strained)

XRD Reciprocal Space Maps

Dislocation Imaging in MM and LM GaInAs

8%-In MM GaInAs LM Control

EBIC images from S1 (left) and C1 (right). Eb = 3 keV, Ib = 150 pA.

50 µm 50 µm

50 µm 50 µm

EBIC images from S1 (left) and C1 (right). Eb = 3 keV, Ib = 150 pA.

50 µm 50 µm

50 µm 50 µm

Electron-Beam-Induced Current

(EBIC)

Cathodo-luminescence

(CL)

2.0 x 105 cm-2

Threading dislocation density0.9 x 105 cm-2

2.6 x 105 cm-2 1.8 x 105 cm-2

Pairing of defects indicative of

dislocation loops

Lifetime and Photovoltaic Parameters

Jo = Jsc e-qVoc/kT ni2 = NCNV e-Eg/kT

Example: 1%-In GaInAs 8%-In GaInAs

Meas. Voc: 1.010 V 0.856 VEg from PL: 1.406 eV 1.305 eV

τSRH → 430 ns 180 ns

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

radSRHA

io N

qwnJ

ττ112

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

−B

eNqwNJ

NkTEg

VC

oA

SRH/

1τ

Double Heterostructure Cross-Section

Tunnel Ju

nction

Top-Cell

-

Like D

H

Wide-Eg Tunnel

Middle-Cell

-

Like D

H

p-GaInP barrier

p-GaInP base

n+-Ge layer

p-AlGaInP barrier

p-AlGaInP barrierp+-GaInAs cap

n-Ge substrate

p-GaInAsstep-graded buffer

Substrate

p++-TJn++-TJ

p-GaInAs base

nucleation

p-GaInP barrier

p++-TJn++-TJ

Tunnel Ju

nction

Top-Cell

-

Like D

H

Wide-Eg Tunnel

Middle-Cell

-

Like D

H

p-GaInP barrier

p-GaInP base

p-GaInP barrier

n+-Ge layer

p-AlGaInP barrier

p-AlGaInP barrierp+-Ga(In)As cap

n-Ge substrate

n-Ga(In)As buffer

Substrate

p++-TJn++-TJ

p-Ga(In)As base

nucleation

Wide-bandgap tunnel junction

GaInP top-cell-like DH

Ge substrate

p++-TJn++-TJ

Ga(In)As middle-cell-like DH

Tunnel junction

Buffer region

Lattice-Mismatchedor Metamorphic (MM)

Lattice-Matched (LM)

Time-Resolved Photoluminescence: GaInP- and GaInAs-base DHs

1

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Phot

olum

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cenc

e In

tens

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uni

ts)

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AlGaInP/ GaInP/ AlGaInP double heterostructure

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GaInP/ GaInAs/ GaInP double heterostructure

• Double heterostructures grown with AlGaInP/GaInP and GaInP/GaInAs interfaces, in stack similar to MJ cells

• TRPL measurements at NREL

• Minority-carrier lifetime up to 2450 ns in 1%-In GaInAs on Ge substrate

Time-Resolved PL of LM & MM Double Heterostructures

1

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τ eff

Mea

sure

d by

TR

PL (

ns)

Base Material nid-GaInP (ordered) nid-GaInP (disordered) nid-GaInAs nid-GaInAs, recent data

1.422 eV (GaAs)

1.896 eV

1.818 eV

Eg = 1.407 eV

1.813 eV

1.887 eV

1.311 eV

1.736 eV

1.807 eV

• MM 8%-In GaInAs double heterostructure measured to have 600 ns τeff

→ lower limit of bulk lifetime

• ~60X longer than earlier measurements of lifetime in Ga0.92In0.08As DHs

Time-resolved PL meas. courtesy of W. Metzger, B. Keyes, and R. Ahrenkiel – NREL

Dislocation Imaging in 23%-In GaInAs

1 µm20 µm

23%-In GaInAs double heterostructure on Ge

Plan-View Transmission Electron Microscopy

(TEM)

disloc. density = 3.1 x 106 cm-2

Cathodoluminescence (CL)

disloc. density = 4.4 x 106 cm-2

Time-Resolved PL of LM & MM Double Heterostructures

0.1

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10

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0 5 10 15 20 25 30 35Indium Mole Fraction of GaInAs Lattice-Matched to Base (%)

τ eff

Mea

sure

d by

TR

PL (

ns)

Base MaterialRecent data nid-GaInAs, recent data p-GaInP (disordered)Previous data nid-GaInAs nid-GaInP (ordered) nid-GaInP (disordered)

Eg = 1.407 eV

1.813 eV

1.887 eV 1.311 eV

1.736 eV

1.807 eV

1.114 eV

1.619 eV

0.994 eV

1.529 eV

Time-resolved PL meas. courtesy of W. Metzger, B. Keyes, and R. Ahrenkiel – NREL

Open-Circuit Voltage of Metamorphic GaInAs Cells

0

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GaInAs Indium Composition (%)

Ope

n-C

ircui

t Vol

tage

Voc

(V)

and

G

aInA

s B

andg

ap E

g (e

V)

Voc

Eg

(Eg/q) - Voc

Open-Circuit Voltage of Metamorphic GaInAs Cells

• Eg/q - Vocexpected to be approx. constantin radiative limit

• Eg/q - Voc offsetis 430-490 mV for 8%- to 35%-InMM GaInAs

• Best LM cells show offset of ~370 mV

→ near radiative limit

• 23%-In GaInAscells with 1.1-eV Eghave nearly sameEg/q - Voc offsetas record Si cells

0

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Ope

n-C

ircui

t Vol

tage

Voc

(V)

and

G

aInA

s B

andg

ap E

g M

eas.

by

EQE

(eV) Voc

Eg(Eg/q) - VocRadiative limit

Nominal GaInAs Bandgap (eV)

1.4 0.91.01.11.21.3

GaA

s

Sili

con

InP

GaI

nNA

s

• High crystal quality maintained even for severe lattice mismatch

Group-III Sublattice

Ordering

Group-III Sublattice Ordering

[001]

Ga0.5In0.5Pfully ordered

(order parameter η = 1)

[100]

(111) planes

CuPtB ordering on [111] planes (shown)

or [111] planes

In practice: η = 0.4-0.5 , Eg ≈ 1.8 eVfor GaInP lattice matched to GaAs: Eg(η) = Eg(0) - (471 meV)·η2

[010]

GaP In

Group-III Sublattice Disorder

[001]

[100]

(111) planesGa0.5In0.5P

fully disordered (order parameter η = 0)

In practice: η = 0.0-0.1 , Eg ≈ 1.9 eVfor GaInP lattice matched to GaAs:

[010]

GaP In

Surface Reconstructionof GaInP

[110] cross-section of disordered GaInP epilayershowing [110]-orientedP dimers of the β(2 x 4) reconstruction.

The stresses caused insubsurface layers by the P dimers provide thethermodynamic driving force for ordering.

Gallium Phosphorus

[001]

[110]

[110]

[001][001]

[110]

[110]

[110]

[110][110]

(110)

2.33 Å

Indium

P dimer

Direct Meas. of GaInP Ordering from ½(115) XRD Peak

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14Relative Omega, referenced to 1%-In GaInAs Peak (degrees)

1/2(

115)

XR

D In

tens

ity d

ue to

Gro

up-II

I O

rder

ing

in G

aInP

(co

unts

/s)

GaInP Ordering State andLattice Match to GaInAsordered, GaInP LM to 1%-In GaInAspartially disordered, "disordered, "ordered, GaInP LM to 8%-In GaInAsdisordered, "

-2 0 2

½(1

15) X

RD

Inte

nsity

Due

to G

roup

-III

Subl

attic

e O

rder

ing

in G

aInP

(co

unts

/s)

1.813 eV

1.867 eV 1.887 eV1.736 eV

1.807

-2 0 2 -2 0 2

Wavelength-Resolved Photoluminescence: Eg Dependence on Ordering, Indium Content

0

500

1000

1500

2000

2500

3000

3500

1.65 1.70 1.75 1.80 1.85 1.90 1.95 2.00 2.05Photon Energy (eV)

Wav

elen

gth-

Res

olve

d PL

Inte

nsity

(arb

. uni

ts)

GaInP Ordering State andLattice Match to GaInAsordered, GaInP LM to 1%-In GaInAspartially disordered, "disordered, "ordered, GaInP LM to 8%-In GaInAsdisordered, "

Increasing group-III sublattice disorder

Lattice-Matched and Metamorphic

3-Junction GaInP/GaInAs/Ge Cells

– Expt. Results

Bandgap vs. Lattice Constant

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

3J Cell Eff. and Voc ––Top and Middle Cell Eg Combinations

1%-In

8%-In

23%

-In

1.91.8

1.7

1.6

1.5

10

12

14

16

18

20

22

24

26

28

30

32

3J C

ell A

pert

ure-

Are

a Ef

f. (%

) an

d V o

c X 1

0 (V

)

% In in GaInAs

Nominal Bandgap

(eV)

3J Eff. -- disord. GaInP 3J Voc -- disord. GaInP 3J Eff. -- ordered GaInP 3J Voc -- ordered GaInP

Metamorphic & Latt.-MatchedConcentrator & 1-sun Cells

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 3Voltage (V)

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Cur

rent

Den

sity

/ In

cide

nt In

tens

ity (

A/W

)

Latt.-matched Metamorphic Latt.-matched MetamorphicVoc 3.039 2.892 V 2.622 2.392 V Jsc/inten. 0.1390 0.1492 A/W 0.1437 0.1599 A/W Vmp 2.761 2.591 V 2.301 2.055 V FF 0.883 0.855 0.850 0.819conc. 175 309 suns 1.0 1.0 sunsarea 0.264 0.266 cm2 4.0 4.0 cm2

Eff. 37.3% 36.9% 32.0% 31.3% Aperture-area efficiency at 25C Total-area efficiency at 25C AM1.5D, low-AOD spectrum AM1.5G 1 sun = 0.100 W/cm2

Concentrator Cells 1-sun Cells

Record Efficiency Terrestrial Conc. Cell

Spectrolab GaInP/GaInAs/Ge Cell

Voc = 3.039 V Jsc = 2.435 A/cm2 FF = 88.3 % Vmp = 2.761 V

Efficiency = 37.3 ± 1.9%

175 suns (17.5 W/cm2) intensity 25 ± 1ºC, AM1.5D Low-AOD spectrum

• AM1.5 Direct, Low-AOD standard spectrum

• 0.264 cm2

aperture area

• 37.3% record efficiency,175 suns, 25°C

Concentrator cell light I-V meas. independently measured by T. Moriarty, J. Kiehl, K. Emery – NREL

37.3%-Efficient Conc. CellEfficiency, Voc, vs. Inc. Intensity

21

23

25

27

29

31

33

35

37

39

0.1 1 10 100 1000Concentration (suns)

Effic

ienc

y (%

)

2.1

2.3

2.5

2.7

2.9

3.1

3.3

3.5

3.7

3.9

Ope

n-C

ircui

t Vol

tage

Voc

(V)

Eff. near one sun

Eff. at concentration

Voc

37.3%

Record Cell EfficienciesEf

ficien

cy (%

)

Universityof Maine

Boeing

Boeing

Boeing

BoeingARCO

NREL

Boeing

Euro-CIS

200019951990198519801975

NREL/Spectrolab

NRELNREL

JapanEnergy

Spire

No. CarolinaState University

Multijunction ConcentratorsThree-junction (2-terminal, monolithic)Two-junction (2-terminal, monolithic)

Crystalline Si CellsSingle crystalMulticrystallineThin Si

Thin Film TechnologiesCu(In,Ga)Se2Amorphous Si:H (stabilized)CdTe

Emerging PVOrganic cells Varian

RCA

Solarex

UNSW

UNSW

ARCO

UNSWUNSW

UNSWSpire Stanford

Westing-house

UNSWGeorgia TechGeorgia Tech Sharp

AstroPower

NREL

AstroPower

Spectrolab

NREL

MasushitaMonosolar Kodak

KodakAMETEK

PhotonEnergy

UniversitySo. Florida

NRELNREL

Princeton UniversityKonstanz NREL

NRELCu(In,Ga)Se2

14x concentration

NREL

UnitedSolar

United Solar

RCA

RCARCA

RCA RCARCA

Spectrolab

University CaliforniaBerkeley

Solarex12

8

4

0

16

20

24

28

32

36

40

2005

40%

GaInNAs ~1-eV Solar Cells for

5- and 6-Junction Cells

Solar Spectrum Partitionfor 4-Junction cell

0

10

20

30

40

50

60

70

80

90

100

350 550 750 950 1150 1350 1550 1750 1950

Wavelength (nm)

0

10

20

30

40

50

60

70

80

90

100

Cur

rent

Den

sity

per

Uni

t Wav

elen

gth

(mA

/cm

2 µm)

AM0

AM1.5G

AM1.5 Direct, low-AOD

1.41 eV 0.67 eV1.85 eV 1.0 eV

5- and 6-Junction Cells

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 currentdensity above GaAs Egamong 3-4 subcells

• Allows low-currentGaInNAs cell to be matched toother subcells

• Lower series resistance

• Thinner bases havepotential for higherradiation resistance

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

Solar Spectrum Partitionfor 6-Junction cell

0

10

20

30

40

50

60

70

80

90

100

350 550 750 950 1150 1350 1550 1750 1950

Wavelength (nm)

0

10

20

30

40

50

60

70

80

90

100

Cur

rent

Den

sity

per

Uni

t Wav

elen

gth

(mA

/cm

2 µm)

AM0

AM1.5G

AM1.5 Direct, low-AOD

1.41 eV 0.67 eV1.8 1.1 eV2.0 1.6

Prototype 6-Junction Cell Growth

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

• Growth of (Al)GaInP/ GaInP/ AlGa(In)As/ Ga(In)Assubcells 1 through 4 and formation of Ge subcell 6 at Spectrolab

• Growth of GaInNAs subcell 5GaInNAs subcell 5 at NREL

• 6J cell fabrication and measurementat Spectrolab

6-Junction Prototype Cells

• 6-junction cell with active GaInNAs subcell

0.2584 cm2 area

Meas. Quantum Efficiency of Top 4 Junctions of 6J Cell

Full Process (AR Coated) Top 4 Junctions of 6J Cell

0

10

20

30

40

50

60

70

80

90

100

350 450 550 650 750 850 950Wavelength (nm)

Exte

rnal

Qun

atum

Effi

cien

cy (%

)

AM0 Jsc (mA/cm2)Cell 1 = 9.05 mA/cm2

Cell 2 = 8.97 mA/cm2

Cell 3 = 8.27 mA/cm2

Cell 4 = 8.36 mA/cm2

Measured External QE of GaInNAsSubcell in 5- and 6-Junction Cells

0

10

20

30

40

50

60

70

80

90

100

800 900 1000 1100 1200Wavelength (nm)

Qua

ntum

Effi

cien

cy (

%)

Low DMH GaInNAs in 6J cell Jsc = 1.93 mA/cm2 Eg = 1.17 eVMedium DMH GaInNAs in 6J cell Jsc = 2.53 mA/cm2 Eg = 1.11 eVHigh DMH GaInNAs in 6J cell Jsc = 3.14 mA/cm2 Eg = 1.09 eVGaInNAs in 5J structure from 2001 Jsc = 5.75 mA/cm2GaInNAs single-junction cell, int. QE Jsc = 11.2 mA/cm2

External QEno AR coat

Internal QESingle-junction

5- and 6-Junction Prototype Cells

0

1

2

3

4

5

6

0 1 2 3 4 5 6Voltage (V)

Cur

rent

Den

sity

(m

A/c

m2 )

6-junction GaInP/ GaInP/ AlGaInAs/ GaInAs/ GaInNAs/ Ge cell

5-junction GaInP/ GaInP/ AlGaInAs/ GaInAs/ GaInNAs cell on Ge

6-Junction 5-Junction

Voc 5.11 V 4.81 VJsc 4.80 mA/cm2 4.89 mA/cm2

FF 74.3 % 75.5 %Vmp 4.46 V 4.16 VEff. 13.47 % 13.13%Area 0.2584 cm2 0.2516 cm2

Preliminary light I-V meas., no AR coating

Summary• Concentrator and flat-plate PV economics

High η essential to mitigate costs of glass, plastic, metal, in module and balance-of-system

Optical conc. of sunlight reduces high cell cost dramatically → ~500X less cell area needed

Research on higher efficiency cells, in 40-50% range, has potential to make this market explode

• Practical concentration systemsReflective and refractive optics and tracking

systems are fielded, showing reliable performance

Summary• Record 3-junction cell efficiencies reached:

One-sun: 30.1% AM0 and 32.0% AM1.5GConc. cell, terrestrial, 175 suns: 37.3% eff.→ Independently verified at NREL under

AM1.5 Direct, low-AOD spectrum

• Metamorphic III-V double heterostructuresLong τ meas. out to 35%-In GaInAs and

82%-In GaInP → 2.4% lattice mismatch600 ns τ in 8%-In GaInAs: 60X increaseEg ↓ due to group-III sublattice orderingobserved to persist in metamorphic GaInP

Summary

• Metamorphic III-V single-junction cellsHigh Voc demonstrated on 1.1-eV & 0.95-eV cells

with 23% and 35%-In GaInAs→ comparable to 1.1-eV record Si cells

Low Eg/q - Voc = 430-490 mV → very high crystal quality

• Metamorphic cells represent new opportunity for bandgap engineering MJ cells

• 5- and 6-junction cellsUse active ~1-eV GaInNAs cell LM to GeFirst 6-junction cells built→ 5.1 V measured Voc