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Wide-band-gap InAlAs solar cell for an alternative multijunction approach
Marina S. Leite, Robyn L. Woo, William D. Hong, Daniel C. Law, and Harry A. Atwater
Citation: Appl. Phys. Lett. 98, 093502 (2011); doi: 10.1063/1.3531756
View online: http://dx.doi.org/10.1063/1.3531756
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Published by the American Institute of Physics.
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Wide-band-gap InAlAs solar cell for an alternative multijunction approach
Marina S. Leite,
1,
a

Robyn L. Woo,
2
William D. Hong,
2
Daniel C. Law,
2
and
Harry A. Atwater
1
1
California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, USA
2
Boeing-Spectrolab Inc., 12500 Gladstone Avenue, Sylmar, California 91342, USA

Received 23 September 2010; accepted 5 December 2010; published online 28 February 2011

We have fabricated an In
0.52
Al
0.48
As solar cell lattice-matched to InP with efficiency higher than
14% and maximum external quantum efficiency equal to 81%. High quality, dislocation-free
In
x
Al
1−
x
As alloyed layers were used to fabricate the single junction solar cell. Photoluminescence of
In
x
Al
1−
x
As showed good material quality and lifetime of over 200 ps. A high band gap In
0.35
Al
0.65
As
window was used to increase light absorption within the
p
-
n
absorber layer and improve cell
efficiency, despite strain. The InAlAs top cell reported here is a key building block for an InP-based
three junction high efficiency solar cell consisting of InAlAs/InGaAsP/InGaAs lattice-matched to
the substrate. ©
2011 American Institute of Physics
.

doi:
10.1063/1.3531756

The need for high efficiency photovoltaics has recently
attracted considerable interest in multijunction solar cells
based on III-V semiconductors.
1
Such devices can achieve
efficiencies over 40% under AM 1.5 global illumination
2
and
can potentially be used both in terrestrial and spacial appli-
cations. Usually, multijunction solar cell designs are based
on materials grown in Ge or GaAs
3
due to the well under-
stood optical and electronic properties of the lattice-matched
alloys. The most common configuration consists of a top
InGaP cell, a middle GaAs cell, and a Ge bottom cell, with
modeled efficiency up to 40% under 1 sun illumination.
4
,
5
However, the large bandgap difference between Ge and
GaAs leads to poor current matching between these subcells
in a multijunction solar cell design. Direct wafer bonding
combined with layer transfer has been proposed to integrate
subcells with distinct lattice spacing.
6
Also, inverted meta-
morphic growth allows for the fabrication of GaInP/GaAs/
InGaAs triple junction cells with efficiencies higher than
30% under 1-sun illumination.
7
Additionally, by adding two
independent metamorphic junctions, an efficiency over 40%
was achieved.
8
In this approach, current matching is opti-
mized for a lattice-mismatched In
x
Ga
1−
x
As bottom junction.
Therefore, a graded InGaP buffer layer is required prior to
bottom cell growth, which can result in dislocations depend-
ing on the material growth conditions. Additionally, growth
of a thick grade layer adds to the total growth time and cost
in a commercial setting.
In order to enable excellent current matching between
the middle and bottom subcells and also to work with lattice-
matched epitaxial layers, we propose an alternative InP-
based approach for a triple junction solar cell formed by a
combination of InAlAs

1.47 eV

/InGaAsP

1.06 eV

/
InGaAs

0.74 eV

alloys. Detailed balance calculations indi-
cate that this multijunction solar cell can achieve over 46%
efficiency at 100 suns illumination with subcells connected
in series.
9
Here, we present an InAlAs solar cell lattice-
matched to InP which is a promising option for a top junction
in an InP-based multijunction configuration. We discuss in
detail the material crystal quality and optical properties. So-
lar cells were fabricated with an efficiency of 14.2% at AM
1.5 1 sun illumination and maximum external quantum effi-
ciency of 81.0%. This performance is attributed to the high
bandgap top window which reduces surface recombination
and increases light absorption within the
p
-
n
absorber layer
despite an increase in strain. The overall performance of the
cell supports the possibility of a high efficiency InAlAs/
InGaAsP/InGaAs triple junction cell.
In
x
Al
1−
x
As alloys have been used as window layers in
low bandgap InGaAs solar cells
10
and showed very good
efficiencies. It was also suggested
11
that a wide-band-gap
In
x
Al
1−
x
As window layer could boost the efficiency of InP
solar cells by more than 20% due to the almost negligible
window layer light absorption. Figure
1
shows a schematic
and a photograph of the fabricated InAlAs solar cells. The
single junction InAlAs solar cells were grown on 50 mm,
p
-type InP

001

on-axis substrates using Veeco E400 metal-
organic vapor phase epitaxy reactor operated at low pressure.
The main precursors used in the layers are trimethylindium,
trimethylaluminum, and arsine. Growth temperatures typi-
cally ranged from 600 to 750 °C depending on the layers.
The
p
-
n
absorber layer consists of an In
0.52
Al
0.48
As
n
-doped
layer 200 nm thick with a carrier concentration of 1.0
a

Electronic mail: mleite@caltech.edu.
15nm n
+
In
0.53
Ga
0.47
As
InP
p
-
typesubstrate
500nm p-type bufferlayer
20nm n
+
In
0.35
Al
0.65
As
200nm n-type In
0.52
Al
0.48
As
1.5μm p-type In
0.52
Al
0.48
As
20nm p
+
In
0.35
Al
0.65
As
InP
p
typesubstrate
1cm
FIG. 1.

Color

Top: Schematic of InAlAs/InP solar cell showing layer
thickness and composition. Blue: In
0.35
Al
0.65
As window layers
E
g
=1.98 eV. Light gray: In
0.52
Al
0.47
As
p
-
n
absorber layer
E
g
=1.47 eV.
White: In
0.53
Ga
0.47
As cap layer
E
g
=0.77 eV. Yellow: metallic contacts.
Bottom: Photograph of fabricated InAlAs solar cells with 1

1cm
2
each.
APPLIED PHYSICS LETTERS
98
, 093502

2011

0003-6951/2011/98

9

/093502/3/$30.00
© 2011 American Institute of Physics
98
, 093502-1
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10
18
cm
−3
and a 1.5

m thick layer with the same com-
position,
p
-doped with 1.0

10
17
cm
−3
. The junction is
sandwiched by a top In
0.35
Al
0.65
As window layer 20 nm thick
and a back surface field with the same thickness and compo-
sition. The top layer is highly doped to minimize surface
recombination.
The In
0.35
Al
0.65
As alloy used for the window layer has a
lattice spacing of 5.8001 Å, being 1.17% tensile with respect
to the InP substrate and the
p
-
n
absorber layer. Figure
2
shows an x-ray diffraction

-2

measurement at

004

re-
flection for the InAlAs solar cell. The
p
-
n
absorber layer
peak is superposed to the substrate sharp peak, confirming
that the In
0.52
Al
0.48
As layer is lattice-matched to InP. The
window layer diffraction condition

broad peak and fringes

demonstrates that this layer is coherently-strained with re-
spect to InP and corresponds to a 20 nm thick In
0.35
Al
0.65
As
alloy, matching the nominal composition. As shown in Fig.
2
the measurement is in very good agreement with x-ray simu-
lation for a similar unrelaxed structure. Therefore, although
the In
0.35
Al
0.65
As window is highly strained with respect to
the InP substrate


1.17%

it is below the critical thickness
for the kinetic growth conditions used and is therefore
dislocation-free. A pseudomorphic and defect-free wide-
band-gap window is ideally suited to reduce surface recom-
bination velocity. The dependence of In
x
Al
1−
x
As bandgap en-
ergy with material lattice spacing indicates that a small
increase in strain can abruptly increase the window bandgap
and, as a consequence, reduce parasitic window light absorp-
tion by this layer.
The In
0.52
Al
0.48
As alloy optical properties were investi-
gated by room temperature photoluminescence. Very little
optical information is available for In
x
Al
1−
x
As alloys. Figure
3

a

shows the photoluminescence spectrum for a
p
-type
In
0.52
Al
0.48
As
/
InP double heterostructure. The peak at 1.34
eV is due to the InP substrate; 1.47 eV corresponds to the
In
0.52
Al
0.48
As layer. The interface between InAlAs and InP
results in a thin InAsP layer which gives rise to a lower
energy luminescence band at 1.23 eV. This peak is known to
be due to a mixed type I-II heterostructure recombination.
12
The well defined peaks demonstrate the optical quality of the
grown In
0.52
Al
0.48
As. Time-resolved room temperature pho-
toluminescence measurements for the same alloy showed an
exponential decay with a lifetime of 206 ps, as in Fig.
3

b

.
In order to determine the minority carrier diffusion length of
In
0.52
Al
0.48
As alloy, Hall measurements were taken in a 930
nm thick In
0.52
Al
0.48
As
n
-doped layer. Hall mobility was
measured to be 667

8cm
2
/
V s for
N
d
=

2.68

0.02


10
18
cm
−3
carrier density. Therefore we estimate a lower
bond on the minority carrier diffusion length in the
In
0.52
Al
0.48
As alloy grown of approximately 1

m.
One dimensional device modeling was performed
13
,
14
in
order to assess the performance of In
x
Al
1−
x
As wide-band-gap
window layers. The results are shown in Table
I
for
In
0.52
Al
0.48
As solar cells lattice-matched to InP with a struc-
ture similar to the one presented in Fig.
1
. The effect of a 20
nm thick In
0.35
Al
0.65
As window layer was investigated,
which can be synthesized dislocation-free, depending on spe-
cific growth conditions. According to the device modeling,
the overall cell performance is strongly affected by the pres-
ence of the Al-rich top window layer

see Fig.
4

a

for light
I-V experimental curves

. In the absence of a top window,
the short circuit current

J
sc

is significantly reduced due to
surface recombination in the
n
-type layer of the cell, and cell
efficiency

decreases

10.7%

. According to the device
modeling, an In
0.35
Al
0.65
As window can boost cell efficiency
to 19.6%.
15
In order to improve the top contact of the cell a thin
InGaAs contact layer was used.
10
Although the low bandgap

0.74 eV

of this layer causes parasitic absorption to the solar
cell, overall cell performance was significantly improved
compared to a cell without the cap layer. Electrical measure-
ments were performed using a solar simulator with active
illumination area under AM 1.5 global solar spectrum with 1
sun total intensity

100 mW
/
cm
2

. Figure
4

a

shows a light
I-V curve for a representative cell with 1.0 cm
2
. InAlAs so-
lar cell photovoltaic characteristics are maximum power P
m
=14.2 mW
/
cm
2
with V
mp
=809.0 mV and J
mp
=17.5 mA
/
cm
2
. An open circuit voltage

V
oc

of 990 mV
and a short circuit current density

J
sc

of 19.3 mA
/
cm
2
y
(
counts
)
window
measurement
p-n
absorberlayer
andInPsubstrate
30 31 32 33
ω
(degrees)
I
ntens
i
t
simulation
FIG. 2.

Color online

X-ray diffraction measurement and simulation of the
cell structure showing a pseudomorphic window layer. Window broad peak
and fringes correspond to a 20 nm unrelaxed In
0.35
Al
0.65
As layer. Measure-
ment settings: lambda=1.540 56 Å,

004

reflection, receiving slit: 1/2°.
200
300
4
00
InAsP
In
0.52
Al
0.48
As
(
counts
)
InPsubstrate
(a)
1.2 1.4 1.6 1.8
0
100
200
s
interface
E(V)
Intensity
10
-1
10
0
y
(a.u.)
I(t)=A+Be
-t/
τ
A=0.0051+
0.0003
B=0.74+
0.01
τ
=206+
1.6psec
E
nergy
(
e
V)
(b)
0 1000 2000 3000
10
-2
Intensit
y
Time(psec)
Time(psec)
FIG. 3.

Color online

a

Room temperature photoluminescence measure-
ment of the In
0.52
Al
0.48
As 300 nm layer as a function of wavelength showing
material bandgap and alloy/InP interface peak.

b

Time-resolved photolu-
minescence measurement on the same alloy showing a lifetime of 206 ps.
The dashed line corresponds to the exponential decay behavior.
093502-2 Leite
etal.
Appl. Phys. Lett.
98
, 093502

2011

Downloaded 26 Nov 2012 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
were measured. The resulting cell efficiency is 14.2% with
fill factor

FF

of 74.4%, promising for a wide-band-gap top
junction cell. The fabricated InAlAs cells were found to be
very stable under ambient conditions. An external quantum
efficiency

EQE

of 81.0% was achieved for the same cell

see Fig.
4

b


. The EQE drops rapidly for wavelength
828 nm as a consequence of In
0.52
Al
0.48
As bandgap en-
ergy.
The overall cell efficiency is higher than for cells using
lower bandgap window layers. An In
0.47
Al
0.53
As window
with
E
g
=1.76 eV and a lattice mismatch of

0.37% with
respect to InP resulted in a relative efficiency that was 18%
lower

see Fig.
4

a


. The high bandgap of the In
0.35
Al
0.65
As
window layer allows for more light absorption in the
p
-
n
absorber layer. Additionally, the EQE improved by 5% in the
blue region of the spectrum

not shown

.
Summarizing, by controlling defects such as dislocations
in In
x
Al
1−
x
As layers with different compositions we fabri-
cated a wide-band-gap InAlAs solar cell. The InAlAs
p
-
n
absorber layer was lattice-matched to the InP substrate and a
top In
0.35
Al
0.65
As window layer was used to prevent surface
recombination and provide more light absorption into the
absorber layer. High quality material was achieved from both
structural and optical properties’ standpoints. The InAlAs
fabricated solar cells showed an efficiency of 14.2% and an
EQE of up to 81.0%. The demonstration of an InAlAs wide-
band-gap solar cell presented here opens up the possibility
for an innovative multijunction solar cell design based on
InP lattice-matched alloys.
The authors acknowledge D. M. Callahan, J. S. Fakonas,
G. M. Kimball, J. N. Munday, and D. M. O’Carroll and
financial support from the Department of Energy—Solar En-
ergy Technologies Program under Grant No. DE-FG36-
08GO18071.
1
R. R. King,
Nat. Photonics
2
,284

2008

.
2
M. J. Griggs, D. C. Law, R. R. King, A. C. Ackerman, J. M. Zahler, and
H. A. Atwater,
Proceedings of the 4th World Conference on Photovoltaic
Energy Conversion

IEEE, New York, 2006

, p. 857.
3
J. Olson, T. Gessert, and M. Al-Jassim,
Proceedings of the 18th IEEE
Photovoltaic Specialists Conference

IEEE, New York, 1985

,p.552.
4
F. Dimroth, U. Schubert, and A. W. Bett,
IEEE Electron Device Lett.
21
,
209

2000

.
5
Handbook of Photovoltaic Science and Engineering
, edited by A. Luque
and S. Hegedus

Wiley, West Sussex, England, 2003

.
6
D. C. Law, R. R. King, H. Yoon, M. J. Archer, A. Boca, C. M. Fetzer, S.
Mesropian, T. Isshiki, M. Haddad, K. M. Edmondson, D. Bhusari, J. Yen,
R. A. Sherif, H. A. Atwater, and N. H. Karam,
Sol. Energy Mater. Sol.
Cells
94
,1314

2010

.
7
J. F. Geisz, S. Kurtz, M. W. Wanlass, J. S. Ward, A. Duda, D. J. Friedman,
J. M. Olson, W. E. McMahon, T. E. Moriarty, and J. T. Kiehl,
Appl. Phys.
Lett.
91
, 023502

2007

.
8
J. F. Geisz, D. J. Friedman, J. S. Ward, A. Duda, W. J. Olavarria, T. E.
Moriarty, J. T. Kiehl, M. J. Romero, A. G. Norman, and K. M. Jones,
Appl. Phys. Lett.
93
, 123505

2008

.
9
M. J. Archer

private communication

.
10
Y. Takeda, M. Wakai, T. Ikeoku, and A. Sasaki,
Sol. Energy Mater. Sol.
Cells
26
,99

1992

.
11
R. K. Jain, G. A. Landis, D. M. Wilt, and D. J. Flood,
Appl. Phys. Lett.
64
, 1708

1994

.
12
D. Vignaud, X. Wallart, F. Mollot, and B. Sermage,
J. Appl. Phys.
84
,
2138

1998

.
13
Modeling was performed using automat for simulation of heterostructures

AFORS-HET

free software.
14
R. Stangl, M. Kriegel, and M. Schmidt,
Proceedings of the 4th World
Conference on Photovoltaic Energy Conversion

IEEE, New York, 2006

,
p. 1350.
15
The purpose of the modeling presented here is to determine quantitatively
how a top window can affect the overall cell performance; therefore no
antireflection coating was included on the simulations.
TABLE I. InAlAs solar cell figures of merit obtained by one device modeling
14
of the structure shown in Fig.
1
without and with a 20 nm thick InAlAs top window layer with different compositions and therefore band gap
energies. The modeling takes multiple internal and external reflections into account and normal incidence of
light. The last row shows the parameter measured for the InAlAs solar cell with In
0.35
Al
0.65
As windows,
In
0.53
Ga
0.47
As cap layer, and an antireflection coating. In all cases EQE refers to the maximum value of the
external quantum efficiency achieved at 765 nm.
V
oc

mV

J
sc

mA
/
cm
2

FF

%



%

EQE

%

Without top window
984
12.6
87
10.7
53.4
With 1.7 eV top window
1017
19.8
88
17.7
68.0
With 2.0 eV top window
1064
20.8
89
19.6
82.0
Experiment
990
19.3
74.4
14.2
81.0
()
25
m
2
)
20eVwindow+
(
a
)
10
15
20
2.0eVwindow
17V id
t
density(mA/c
m
2
.
0eVwindow+
InGaAscapandARcoating
(
b
)
1.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0
5
1
.
7
e
V
w
i
n
d
ow
Voltage(V)
Curren
t
()
0.4
0.6
0.8
EQE
300 400 500 600 700 800 900
0.0
0.2
Wavelen
g
th
(
nm
)
FIG. 4.

Color online

a

Light I-V curve under AM 1.5 global illumination
for the fabricated InAlAs solar cells using a 1.7 eV window layer

squares

,
a 2.0 eV window layer

diamonds

, and a 2.0 eV window plus an InGaAs
cap layer and antireflection

AR

coating

circles

. Best cell characteristics
are shown in Table
I
.

b

External quantum efficiency

EQE

of the InAlAs
cell with cap layer and AR coating as a function of light wavelength.
093502-3 Leite
etal.
Appl. Phys. Lett.
98
, 093502

2011

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