AlGaAs-GaAs heteroface space concentrator solar cells have been fabricated by metalorganic chemical vapor deposition. AMO efficiencies as high as 21.1% have been observed both for p-n and np structures under concentration (90 to 100X) at 25 C. Both cell structures are characterized by high quantum efficiencies and their performances are close to those predicted by a realistic computer model. In agreement with the computer model, the n-p cell exhibits a higher short-circuit current density

Ford, C. W.

Hamaker, H. C.

Lewis, C. R.

Virshup, G. F.

Werthen, J. G.

English

NASA Technical Reports Server

N86- i 7845 !
HIGH-EFFICIENCY AIGaAs-GaAs CASSEGRAINIAN CONCENTRATOR CELLS
J. G. Werthen, H. C. Hamaker, G. F. Virshup, C. R. Lewis, and C. W. Ford
Varian Research Center
Palo Alto, California
AIGaAs-GaAs heteroface space concentrator solar cells have been fabricated by
metalorganic chemical vapor deposition. AMO efficiencies as high as 21.1% have been
observed both for p-n and np structures under concentration (90-100X) at 25°C. Both
cell structures are characterized by high quantum efficiencies and their performances
are close to those predicted by a realistic computer model. In agreement with the
computer model, the n-p cell exhibits a higher short-circuit current density.
INTRODUCTION
GaAs solar cells are becoming increasingly important for space applications. An
attractive approach is offered by the concentrator space cell (Ref. i), which has the
potential for reaching efficiencies higher than those predicted for one-sun space
solar cells (Ref. 2).
A concentrator cell will operate at a temperature of approximately 80°C; however,
the predicted efficiency is still higher than for one-sun cells operating at 25°C.
In this work, we are studying heteroface AIGaAs-GaAs p-n and n-p small-area concen-
trator solar cells grown by metalorganic chemical vapor deposition (MOCVD). The
cells are intended for use in a miniaturized Cassegrainian concentrator assembly
operating at approximately i00 suns, AMO. The cell has a circular configuration with
4-mm diameter active area, and the total dimensions are 5 mm x 5 mm. Cell efficien-
cies as high as 21.1% have been obtained under simulated 92 suns, AMO.
CELL DEVELOPMENT
The concentrator cells are designed by a realistic computer model which solves
the transport equations and determines the current-voltage (I-V) characteristics.
The model uses measured values of relevant parameters such as mobility, minority-
carrier diffusion lengths, absorption coefficients, etc. Cells are optimized for
operating conditions such as temperature and concentration. Under concentrated
light, it is crucial that the emitter and grid pattern on top of the solar cell each
have very low series resistance. At the same time, the internal spectral response
must remain high and the obscuration must be minimized. A one-dimensional distri-
buted resistance model incorporating dark current mechanisms such as injection and
recombination is used to determine the I-V curve. Obscurations caused by the grid
pattern and the AIGaAs window layer are also included in the model.
The cell structure used in the model is shown in Fig. I. Starting from the
substrate, a highly-doped buffer layer (0.5 _m) is grown to provide a smooth surface
for overlying growths and also reduce recombination at the back of the cell. Next
Work supported by NASA-Lewis Research Center under Contract NAS3-23876.
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• $ _ 4
lie the two active layers, the base and the emitter, followed by a thin, highly-doped
AIGaAs (90% AI) window layer. This layer reduces the surface recombination velocity
in the emitter, and its thickness is optimized to couple with the antireflection (AR)
coating to minimize the reflectance. Finally, the cell structure is terminated with
a highly-doped GaAs cap layer serving as a contact layer for good ohmic contacts.
This layer, which also protects the cell during processing, is selectively etched
away immediately before deposition of a single-layer AR coating.
Calculations of the quantum efficiency are limited to the emitter, base, and
depletion regions. Two optimized responses are shown in Fig. 2 for p-n and n-p
structures, respectively, with the individual contributions from the various regions
being indicated. In the p-n structure, most photons are absorbed in the p-type
emitter, whereas in the n-p structure, all regions make significant contribution to
the photogenerated current. Inherently lower resistivity of n-type versus p-type
GaAs allows the use of a much thinner emitter in the n-p configuration. Consequently,
a better radiation tolerance for n-p cells may be expected.
In addition to the quantum efficiency, the computer model finds the optimum
performance. Figure 3 shows the I-V characteristics of an optimized p-n structure
under 100X, AMO at 80°C. Efficiencies of 20.7% are predicted for both p-n and n-p
cell structures. With the addition of gradients both in doping and in composition,
even higher efficiencies are predicted, as indicated in Table I. These gradients can
be added both in the base and emitter regions where they give rise to built-in fields
that aid in the collection of photogenerated carriers. Over a 2% increase in effi-
ciency is observed with the additional gradients.
For each optimized cell structure, the computer model provides doping levels and
layer thicknesses. Thus far, fabrication and testing efforts have been concentrated
primarily on cells without intentional gradients. The structures are grown in a
horizontal rf-heated MOCVD reactor at 730°C, as described in Ref. 3. Selenium and
zinc or magnesium (Ref. 4) are used as n-type and p-type dopants, respectively. An
optimized grid pattern (shown in Fig. 4) is defined using conventional photolitho-
graphic techniques. Metallizations are typically deposited by evaporation to a
thickness of 0.2 _m, and normally consist of Au/Ge/Ni/Au for n-type GaAs and Pd/Au
for p-type GaAs. The front contact grid pattern is plated to 3-pm thickness.
Cell performances were measured both at Varian and at Sandia National Labora-
tories. All measurements are based on the total circular area with a 4-mm diameter.
Figure 5 shows the I-V characteristics for a p-n cell under simulated concentrated
light. The measurement was obtained at 28°C. The cell efficiency versus solar
concentration is shown in Fig. 6, together with the theoretically-predicted behavior.
The poor performance of the experimental cell at one sun may be attributed to shunt
currents which become less significant at higher concentrations. However, the meas-
ured values are still below the theoretical values, which indicates that further
improvements are necessary.
Cells having the n-p configuration have also been fabricated. Compared to p-n
cells, these cells show larger values of short-circuit current, in agreement with
the theoretical prediction. Similar behavior has been observed for large-area, 1-sun
GaAs cells (Ref. 5). However, lower values of open-circuit voltage and fill factor
are predicted and also experimentally observed. Therefore, the efficiencies of p-n
and n-p cells are comparable. The n-p structure has a very thin emitter, which may
be of importance for increased radiation hardness. Experiments will have to be con-
ducted to assess whether the n-p cell exhibits better radiation tolerance. The
62
addition of doping and compositional gradients, which provide built-in fields, may
also improve radiation hardness by retaining good carrier collection even in damaged
material.
High-efficiency cells intended for muchhigher concentrations have also recently
been fabricated for terrestrial use. In this case, the doping levels were raised
further to accommodatemuchhigher current levels. These cells have the p-n configu-
ration and are dopedwith Mg in the emitter. Efficiencies in excess of 26%have been
observed at 753X (AMI.5, i00 mW/cm2),as shownin Fig. 7.
CONCLUSIONS
Heteroface AIGaAs-GaAscells for space concentrator applications have been
demonstrated with efficiencies as high as 21.1%at 92X, AMO. Both p-n and n-p cell
structures have been fabricated using MOCVD.The cells still need to be tested at
temperatures up to 80°C. Similarly, the effects of intentional gradients in doping
and composition must be evaluated. Radiation hardness maybe increased by including
gradients and using the n-p configuration.
REFERENCES
io Patterson, R. E.; and Rauschenback, H. S.: Miniaturized Cassegrainian Concen-
trator Concept Demonstration. TRW Space & Technology Group, Space Photovoltaic
Research and Technology, 1982, NASA Conf. Pub. 2256, April 1982.
. Knechtli, R. C.; Loo, R. Y.; and Kamath, G. S.: High-Efficiency GaAs Solar
Cells. IEEE Trans. Electron Devices, Vol. ED-31, 1984, p. 577.
a Lewis, C. R.; Dietze, W. T.; and Ludowise, M. J.: The Growth of Magnesium-Doped
GaAs by the OM-VPE Process. J. Electron. Mater., Vol. 12, 1983, p. 507.
. Lewis, C. R.; Ford, C. W.; and Werthen, J. G.: Magnesium Doping of Efficient
GaAs and Ga^ 751n^ 25As Solar Cells Grown by Metalorganic Chemical Vapor Depo-u u.
sition. Appl. Phys. Lett., Vol. 45, 1984, p. 895.
. Werthen, J. G.; Hamaker, H. C.; Virshup, G. F.; and Ford, C. W.: 18.7% Effi-
cient (1-Sun, AMO) Large-Area GaAs Solar Cells. Appl. Phys. Lett., Vol. 46,
1985, p. 776.
63
TABLEI. RESULTSOFCELLOPTOPTIMIZATIONRUNSFORCELLS
OPERATINGUNDER100X, AMOAT 80°C.
E = exponential profile
L = linear profile
U = constant level of dopant or aluminumfraction
Type Doping Profile Aluminum Profile Efficiency (%)
p-n U U 20.71
p-n E U 22.50
p-n E L 22.70
p-n E E 23.15
p-n L U 22.23
p-n L L 22.54
p-n L E 23.15
n-p U U 20.79
n-p E U 22.73
n-p E L 22.85
n-p E E 23.08
n-p L U 22.46
n-p L L 22.50
n-p L E 23.13
64
0.5 pm
0.03 pm
0.2-0.5 pm
3.0 pm
350 pm
FRONT CONTACT
f
GaAs p(n) f COATING
A_GaAs p(n) WINDOW
GaAs p(n) EMITTER
GaA$ n(p) BASE
GaA$ n(p) SUBSTRATE
"_-,.,BACK
CONTACT
I,IJ
¢n 1.0 ..... , , , ,
o.e ," _.
0.6 ' I /,/p-emitter
o., . ,,..
Z_ 0.2 . I ",_ , depletion -I
-- 1.0 1.4 1.8 2.2 2.6 3.0
PHOTON ENERGY (eV)
v_ 1.0
i0.8
0.6
_00.4
Z< 0.2
Z
mm
, w v I I .... |= I
F'"".. p-base .. ¢"'.
J % n-eml_er
/ '"-.....
_ [ ".... ,/ _
L I depletion s" '_'-*P'_ J
F I .>LS--" ".. \ i
0
1.0 1.4 1.8 2.2 2.6 3.0
PHOTON ENERGY leVI
Fig. i Schematic solar cell
structures in the computer
model development.
Fig. 2 Internal spectral response
versus photon energy deter-
mined by the computer model-
ing program for optimized
(a) p-n and (b) n-p
structures.
65
0.45 I I l I i I
o..;o
C 0.35
U
R
R
E 0.30 --
N
T
0.25 --
(
A
m 0.20 --
P
e
r
e 0.15 --
8
)
0.10 --
O. 05 --
0.00
0.0
= 20.7%
FF = O. 831
V = 1.08V
oc
J = 3.08 A/cm 2
sc
AMO, 100X
80°C
I i I i i
0.2 0.4 0.6 0.8 I .0
VOLTAGE (volt, s)
J
7
J
1.2
Fig. 3 Current-voltage characteristics for an
optimized p-n cell as predicted by computer
model. The cell operating conditions are
indicated.
I_ 4 mm _l
F 7
Fig. 4 Grid pattern for space concentrator cell.
66
o.
E
c-
t.
3
_2
38M1829-5
Dmli zmvod
.45 __-- FEB 2B 1905 8:1_ Fttl MSTT-----'----
= 21.1%
FF = 0.836
V = 1.13V
OC
J = 2.76 A/cm 2
SC
AMO, 91.6X
28°C
.4
.35
.3
.25
.2
.15
.I
•05
8 -.G -.4 -.2 0 .2 .4 .S .8 I .2
Voltage(volts)
Fig. 5
23
22
21
2O
Current-voltage characteristics for
a p-n cell under simulated concentrated
light.
I _ L . L _ L - l - ,I I I I
/"
lg --
18
,,
16
15
0 10 20
I I I I I I I
30 40 50 80 ?0 80 go
SOLAR CONCZNTRATIOH
100
Fig. 6 CeLl efficiency versus solar concentration
at 28°C, AMO: computer model (R),
experimental (o).
67
0.5
0
0
| ! ! I
ii = 26.1%
F F = 0.866
VOC -- 1.17 V
ISC = 3.94 A
AM1.5, 753 X
28"C
I I
0.2 0.4 0.6 0.8 1.0 1.2
VOLTAGE (VOLTS)
Fig. Current-voltage characteristics for
an experimental p-n solar cell under
simulated terrestrial concentrated
light.
58


High-efficiency AlGaAs-GaAs Cassegrainian concentrator cells

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