NREL is operated by Midwest Research Institute • Battelle

A nati

O

Innovation for Our Energy Future

National Renewable Energy Laboratory

GaInAs 4th Junction fo

Next-Generation Lattice-Mism

Multijunction Solar Ce

D.J. Friedman, J.F. Geisz, A.G. Norman, M. Wanlas

Kurtz

National Renewable Energy Laboratory

NREL/PR-520-39909Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Convers

held May 7-12, 2006 in Waikoloa, Hawaii.
The Fine Print

Disclaimer and Government License

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Evolution of Multijunction D

• Need to make better use of spectrum, 

eV range

• How to do this...?

GaAs1.4 eVGaInP1.8 eV

future 

examp

in production

4 5 6 7 89

1

2 34

Energy (eV)

Ge

0.7 eV

GaInP1.8 eV

1.25 e

0.7 e
Outline

• Approaches to next-generation hi

efficiencies: survey of the field

• The inverted mismatched 3-juncti• Adding a 4th junction• Fabrication and testing of 4th junc• How low a bandgap do we really n• Outlook
Our Palette: the III-V All

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 

Lattice Constant (Å)

AlP

AlAs

GaP

GaAs

InP

In

Ge

Si
Lattice-Matched to Ge (and 

• The “standard” 3-junction device structu

– Lattice-matched: easy to grow good ma– Restricts available bandgap range

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 Lattice Constant (Å)

AlP

AlAs

GaP

GaAs InP

Ge

Junction #1

Ga0.5In0.5

P

Junction #2

(In)GaAs

Junction #3

Ge
Lattice-Matched to In

• Another example - lattice matching to In

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 Lattice Constant (Å)

AlP

AlAs

GaP

GaAs InP

Ge

Junction #InP

Junction #2

GaInAs
Expanding our Range of Ba

Options

• New materials lattice-matched to 

GaInNAs

– Need good PV materials

• Junctions grown separately, then 

– Mechanical stacking– Wafer bonding

• Lattice-mismatched epitaxy
New Semiconductor: GaIn

• GaInNAs: lattice-matched, desired bandga• But: short diffusion lengths >> poor device • MBE may help growth1,2• 5- or 6-junction structures may work around 

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 Lattice Constant (Å)

AlP

AlAs

GaP

GaAs InP

Ge

Junction #1

Ga0.5In0.5

P

Junction #2

GaAs

Junction #4

Ge

Junction #3Ga0.92In0.08

N0.03As0.97
Mechanically Stacked Jun

• Extremely wide range of materials/bandgaps accessible

high efficiencies; e.g. 32.6% for GaAs/GaSb back in 19

• Not a single chip; multiple growths required

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 

Lattice Constant (Å)

AlP

AlAs

GaP

GaAs

InP

InA

Ge

Junction #1

Ga0.5In0.5

P

Junction #2

GaAs

Junction 

GaSb
Wafer-bonded Stacked Jun

• Wide range of materials/bandgaps accessible• single chip / III-V integration with Si• Multiple growths required; requires transparent 

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 Lattice Constant (Å)

AlP

AlAs

GaP

GaAs InP

Ge

Junction #1

Ga0.5In0.5

P

Junction #2

GaAs

Junction #4

GaInAs

Junction #3

GaInAsP
Lattice-mismatched (“metamo

• Promising approach, competitive with lattice-ma• Challenge is to maintain materials quality of junc

after grade

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 Lattice Constant (Å)

AlP

AlAs

GaP

GaAs InP

Ge

Junction #1

Ga0.3In0.7

P

Junction #2

Ga0.8In0.2As

Ga

xIn1–xAs grade

Junction #3Ge
Inverted Lattice-Mismatched S

• Only the bottom junction is grown mismatched

– Similar philosophy to Varian GaInP/GaAs/substrate/GaInA

• Potential for very high efficiencies (38% achieved w.o. optim• Some complexities but also opportunities in the processing

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 Lattice Constant (Å)

AlP

AlAs

GaP

GaAs InP

Ge

Junction #1

Ga0.5In0.5P

Junction #2

GaAs

GaxIn1–xP grade

Junction #3

Ga0.75In0.25As
Processing of Inverted Str

Substrate

lattice-mismatched

1: 

Growt

h

epi growth

2: 

Bonding

foreign substrate

lattice-matched layers lots of freedom in choice

lightweight, flexible, etc. of foreign substrate:

3: 

S

low-Eg junction(s)

lattice-matched

high-Eg junctions

(GaAs or Ge)

grown first

Substrate

lattice-mismatched

low-Eg junction(s)

lattice-matched

high-Eg junctions

(GaAs or Ge)

f

la

l

h

F
Inverted LMM: Adding a 4th J

• Extend advantages of inverted 3-junction structure to • But: how far can we grade? How far do we need to 

2.42.01.61.20.80.4

Bandgap (eV)

5.5 5.6 5.7 5.8 5.9 6.0 Lattice Constant (Å)

AlP

AlAs

GaP

GaAs InP

Ge

Junction #1

Ga0.5In0.5

P

Junction #2

GaAs

first grade Ga

xIn1–x

P

Junction #3

Ga0.75In0.25As

second gradeGa

xIn1–xP,(InAs

x

Junction #4

Ga0.5In0.5As
4th Junction: Test Struc

• Accounts for eff

mismatched gro

• Bypasses comp

and especially othe other three j

• Tried bandgaps 

GaInAs Junction (0.7 eV)

GaInP Grade

GaInAs Junction (1 eV)

GaInP Grade

GaAs Junction (1.4 eV)

GaInP Junction (1.8 eV)

GaAs or Ge Substrate

(removed after growth)

Inverted 4 JunctionDesign:

GaInAs Ju

(0.74-0.88 

GaInP GGaInP G

GaAs Sub

4th Junc

Test Stru

GaInAs Isotyp

Simulated Junc

not

to

scale
X-ray Characterization of Strain

• XRD analysis critical to getting the com

correct

0.700.690.68

Qz (-1)

-0.495 -0.490 -0.485 -0.480

Q

x (-1

)

Ga0.76In0.24As (1 eV)

0.12% tensile strain

Ga0.49In0.51As (0

0.23% tensile st

MG155
TEM Characterization: 4th J

-220DF

Growth direction

• Dislocations largely confined to grade

Ga

xIn1-xAs (0.77 eV)

Ga

xIn1-xP grade

2 μm

-220DF

InP

MG298

0.77 eV 4th junction

(in

Ga

Simu

4

Te
Quantum Efficiency

• QE degrades with increasing mismatch (decreas

(note -- still room for improvement in these device

• QE of 0.88 eV device approaches performance r

(note -- QE achieved without significant depletion 

1.00.80.60.40.20.0

Internal quantum efficiency

0.7 0.8 0.9 1.0 1.1

Energy (eV)

 M M M

0.77 eV 0.88 eV

0.74 eV

band

gap

(eV):
Current-Voltage Character

• Eg=0.74eV junction leaky, others g

2010

0

-10

Current (mA/cm2

)

-0.1 0.0 0.1 0.2 0.3 

Voltage (V)

light level set as if under filter at~ E

g+0.35 eV

0.74 eV

0.77 e

0.88 

bandgap:
Dependence of VOC on Ban

• Lowest-Eg junction degraded• Higher-Eg junctions better 

(and should get closer to GaAs-like with concen

1.41.21.00.80.60.40.20.0

VOC (V)

0.8 1.0 1.2 1.4 1.6 

E

g (eV)

VOC

 measu extrap

GaInAs(for 4th junc)

GaInAs

GaAs

GaInAs(for 4th junc)
Improvement of Voc with Conc

• Voc does get closer to GaAs-like with c

as expected 

0.50.4

VOC (V)

0.01 0.1

JSC (mA/cm

2

)

expected for n=1GaAs-like device

Eg=0.88eV de(MF943n4)

n=1.5 fit
4th-Junction Eg - is 0.7 eV Ne

• Good devices increasingly challen

band gap decreases (i.e.mismatc

• Can we get away with a higher 4t

band gap?
Allowing Band Gap to V

• Can ra

Eg froand on

• Still 4% 

GaInP

• Real-w

the 0.9to be ~

dNph/dλ

0.7 0.8 0.9 

photon ener

low-AOD Spec

1.61.51.41.31.21.11.0

junction band gaps (eV)

0.7 0.8 0.9 1.0

4th junction band gap (eV)

55545352515049

efficiency (%)

2nd junction

3rd junction

idealized 4-junction efficiency500 suns, low-AOD spectrum

2nd and 3rd junctionband gaps
Device with 4th Junction Eg

Why consider this over the 0.7-eV option:

• High-quality 0.9-eV junction easier to m• Efficiencies:

– Only lose 1.6% efficiency compared to 0– Still 4% above the 1.85/1.41/1.0eV 3-jun

• Grade layers can be thinner:

– Less time to grow– Less source material used– Less strain/wafer bowing

Concerns:

• 1.6 eV junction needed: can it be as go• Tunnel junctions need to be demonstra
Outlook

• A cornucopia of promising approa

next-generation high efficiencies

– may be places for more than one

cost-performance niches

• Inverted multijunction approach 

– Extending to 4 junctions likely to b

efficiencies by several %, to ~45%

– 4-junction structure likely to use a

bottom junction
A Golden Age for Multijunct

Efficiency (%)

University

of Maine

Boeing

Boeing

Boeing

ARCO Boeing

NREL

Boeing

Euro-CIS

1975 1980 1985 1990 1995 

NREL

NREL

Japan

Energy

Spire

No. Carolina

State University

Multijunction ConcentratorsThree-junction (2-terminal, monolithic)Two-junction (2-terminal, monolithic)Crystalline Si CellsSingle crystalMulticrystallineThick Si FilmThin Film TechnologiesCu(In,Ga)Se2CdTeAmorphous Si:H (stabilized)Nano-, micro-, poly- Si

Multijunction polycrystalline

Emerging PVDye cells Organic cells(various technologies)

Varian

RCA

Solarex

UNSW

UNSW

ARCO

UNSW

UNSW

UNSW

Spire Stanford

Westinghouse

Georgia Tech UN

Georgia Tech Sharp

AstroP(smal

Matsushita

Monosolar Kodak

Kodak

AMETEK Photon Energy

University

So. Florida 

United 

United Solar

RCA

RCA

RCA

RCA RCA

RCA

Solarex

12

8

4

0

16

20

24

283236

EPFL

EPFL

40

Kaneka

(2μm on glass

Best Research-Cell Efficienci