High efficiency Al sub x Ga sub 1-x As/GaAs heteroface solar concentrator cells have been developed for space applications. The cells, which were grown using metalorganic chemical vapor deposition (MOCVD), have been fabricated in both the p-n and n-p configurations. Magnesium and zinc are used as the p-type dopants, and Se is used as the n-type dopant. The space cells, which are designed for use in a Cassegrainian concentrator operating at 100 suns, AMO, have a circular illuminated area 4 mm in diameter on a 5 mm by 5 mm cell. These cells have exhibited flash-tested efficiencies as high as 23.6 percent at 28 C and 21.6 percent at 80 C

Ford, C. W.

Hamaker, H. C.

Macmillan, H. F.

Virshup, G. F.

Werthen, J. G.

English

NASA Technical Reports Server

N8 7- 26 417
HIGH-EFFICIENCY GaAs CONCENTRATOR SPACE CELLS
J.G. Werthen, G.F. Virshup, H.F. MacMillan, C.W. Ford, and H.C. Hamaker
Varian Research Center
Palo Alto, California
High-efficiency AlxGal_xAs/GaAs heteroface solar concentrator cells have been
developed for space applications. The cells, which were grown using metalorganic
chemical vapor deposition (MOCVD), have been fabricated in both the p-n and n-p con-
figurations. Magnesium and zinc are used as p-type dopants, and Se is used as the
n-type dopant. The space cells, which are designed for use in a Cassegrainian con-
centrator operating at I00 suns, AMO, have a circular illuminated area 4 mm in dia-
meter on a 5 mm x 5 mm cell. These cells have exhibited flash-tested efficiencies
as high as 23.6% at 28°C and 21.6% at 80°C.
INTRODUCTION
Solar energy systems using GaAs cells in a concentrator array offer the potential
for very high conversion efficiency along with low array costs. Several concepts for
light-weight, radiation-resistant space concentrators have been proposed such as the
point-focus minLature Cassegrainian array (Ref. l) and the parabolic trough design
(Ref. 2). GaAs devices have greater potential than Si cells in such applications
due to their more optimal bandgap and more favorable temperature behavior. The
latter factor enables the GaAs cells to operate efficiently at extreme concentrations
~i000 suns or at the higher operating temperatures which are inherent to concentrator
systems. In this paper, we report AlxGal_xAs/GaAs heteroface solar concentrator
cells grown by metalorganic chemical vapor deposition (MOCVD) for use in Cassegrain-
ian arrays.
CELL DESIGN AND FABRICATION
The concentrator cells were designed with the aid of a detailed computer model.
The model simulates the performance of GaAs solar cells and can be used to optimize
the relevant cell parameters, e.g., layer thicknesses, doping levels of the emitter
and base, grid pattern, and antireflection (AR) coating. Details of the model may
be found elsewhere (Ref. 4). The design conditions for the Cassegrainian cell were
defined to be i00 suns, AMO at an operating temperature T = 80°C, with a circular
illuminated area of 0.126 cm 2 (4-mm diameter) on a 5 mm x 5 mm die. Under such
conditions, it is crucial that the emitter and the grid pattern each have very low
series resistance. At the same time, the spectral response should remain high and
the obscuration must be minimized. The structures of the cells that resulted from
these computer optimizations are very similar; this structure is shown in Fig. i.
In addition to the emitter and base, the devices include a highly doped buffer layer
to minimize surface recombination effects at the back of the base and to provide a
smooth surface upon which the overlying structure is grown. The AI0.9Gao. IAs window
layer passivates the front surface of the emitter and, in combination with the
single-layer AR coating, minimizes the reflectance. Finally, the GaAs cap layer
enables excellent ohmic contact by the top grid pattern while simultaneously minimi-
zing the possibility of the grid metallization diffusing into the semiconducting
25
https://ntrs.nasa.gov/search.jsp?R=19870016984 2020-03-20T09:51:08+00:00Z
materials during processing. The cap layer is selectively etched away between the
grid lines prior to the application of the3AR coating. Doping levels for the cells
are approximately 2 x 1018 and 7 x lOl7cm- in the emitter and base, respectively.
For the n-p cells, the emitter thickness was reduced to about 0.15 _m.
The multilayer structure was grown in a horizontal rf-heated MOCVDreactor at
730°C, as described in Ref. 5. Growth rates were 0.06 Dm/min. In the p-n cells, Mg
and Se were used as the dopants, whereas Se and Zn were used as the dopants in the
n-p cells. Conventional photolithographic techniques were used to lay down the top
grid pattern. Metallizations, which were deposited by evaporation, were Au/Ge/Ni/Au
and Pd/Au for n- and p-type GaAsohmic contacts, respectively. The front grid pat-
tern was subsequently plated with Ag and/or Au to a total thickness of 2 _m. The
ARcoating was deposited by plasma deposition at 350°C.
TESTING
One-sun efficiencies were determined using Spectrolab XT-10 simulators both at
Varian and at Sandia National Laboratories. For AMOmeasurements, the simulator
intensity was set using a balloon-flight-calibrated GaAsp-n solar cell which had a
similar spectral response to the concentrator cells. A xenon-lamp flash tester was
used to determine device performance under concentration. The incident power upon
the cell in the flash tester was determined by assuming that the short-circuit
current I was linearly dependent upon the solar concentration. The exposed scrib-scing streets at the edges of the cells were maskedoff from any light to assure the
accuracy of the measurements. Efficiencies were based on the total illuminated
area, and no correction was madefor obscuration.
RESULTSOFCASSEGRAINIANCELLS
Figure 2 showsthe current-voltage (I-V) characteristics of one of the best p-n
Cassegrainian cells at one-sun, AMO,and T = 28°C. The efficiency N = 20.8% is
quite high, with a particularly high value of the fill factor FF = 0.86. Such a
value for FF is attainable due to the low series resistance which results from the
optimization of the grid pattern for high concentration. The open-circuit voltage
Voc is 1.02 V, and Jsc = 32.2 mA/cm2. As shown in Fig. 3, the samecell at a con-
centration of 91 suns has N = 23.3%, while maintaining the good fill factor. Curve
fitting of these data yield an estimate of ~5 m_ for the series resistance in
this cell. The conversion efficiency versus concentration is plotted in Fig. 4. The
data show that _ has its maximumvalue near i00 suns, which is very near the intended
operating concentration. However, comparable performance is attainable over a com-
paratively wide range between 80 and 200 suns, and even at 400 suns, N exceeds 22%.
The temperature dependenceof the cell at 94 suns, AMOis shownin Fig. 5. As
expected, N drops linearly with increasing temperature, and a least-squares fit to
the data yields a temperature coefficient of -0.036%/°C. At the intended operating
conditions of ~i00 suns and T = 80°C, N = 21.6%. Radiation damageexperiments have
been performed on similar Varian Cassegrainian p-n cells at NASA-Lewis(Ref. 6).
After a fluence of 1 x 1015cm-2 I-MeV electrons on the bare cells, the 100-sun
efficiency was reduced to 79%of its initial value. Using this result and those
presented above, end-of-life (EOL) efficiencies could potentially exceed 17%under
operating conditions. Since in the Cassegrainian module the cells are shielded from
muchof the radiation, as shownin Ref. I, the actual EOLefficiency could be signi-
ficantly higher.
26
Although the data presented in Figs. 2-5 indicate the performance of one of the
best cells, the results are reproducible. Figure 6 shows the distribution of one-
sun efficiencies for all cells, including the one discussed above, fabricated from
the samewafer. Of the for_-eight devices which were obtained, twenty-seven exhi-
bit N > 20%, and eighty-five percent have N >17%. Lower cell efficiencies are
almost invariably due to lower values of FF and, to a lesser extent, Voc. The
observed values of I are relatively unchangedfrom cell to cell. Whenthe valuesc
of N under concentra_zon are used, the yield is even more impressive, as shownin
Fig. 7. Over half of the tested devices have N > 23%,and ninety-three percent
exceed N = 22%. Several other wafers have shownsimilar efficiency distributions,
thus indicating the feasibility of producing these high-efficiciency cells in large
quantities using MOCVD.
Although most of the Cassegrainian cells which have been fabricated to date
have had the p-n configuration, several n-p cells have also been made. Figure 8
shows the one-sun I-V characteristics of one of these cells. Comparisonwith the
data in Fig. 2 shows that the performance of this cell is very similar to the best
p-n cells. The n versus concentration data for another n-p device, which is plotted
in Fig. 9, shows an even higher peak efficiency of 23.6%. Furthermore, the decrease
in N at greater concentrations is less pronounced, presumably due to the superior
majority-carrier mobility in the n-type emitter. The computer modeling results
indicate that the n-p structures maypotentially have higher efficiencies than p-n,
primarily due to better quantumefficiencies.
CONCLUSIONS
High-efficiency GaAsconcentrator cells have been developed for both space and
terrestrial applications. Their optimal structures were determined with the aid of
a computer model which realistically simulates the performance of AlxGal_xAs/GaAs
heteroface cells. Cassegrainian cells have shown100-sun, AMOefficiencies as high
as 23.6% at T = 28°C and 21.6% at T = 80°C. The space cells have shownexcellent
radiation resistance to I-MeV electrons, so in combination with the protective
design of the magnifying element, the Cassegrainian arrays should offer excellent
end-of-life performance. Although most of the development efforts have focused on
the p-n configuration, several high-efficiency n-p cells were also fabricated. The
computer modeling indicates that even higher efficiencies maybe obtained in the
latter case.
This research was supported in part by NASA/Lewisunder contract No. NAS3-23876.
.
.
.
.
5.
REFERENCES
Patterson, R. E.; Rauschenbach, H. S.; and Cannady, M. D.: Conference Record
of the 16th IEEE Photovoltaic Specialists Conference, IEEE, New York, 1982, p.
39.
Stern, T. G; and Hayes, E. W: Conference Record of the 17th IEEE Photovoltaic
Specialists Conference, IEEE, New York, 1984, p. 326.
Arvizu, D. E.; and Edenburn, M. W.: Proc. 105th ASME Winter Annual Meeting,
1984, 84-WA/SOL-13.
Hamaker, H. C.: J. Appl. Phys. 58, 2344, 1985.
Lewis, C. R.; Dietze, W. T.; and Ludowise, M. J.: J. Electron. Mater. 12, 507,
27
o1983.
Curtis, H. B.; and Swartz, C. K.: Conference Record of 18th IEEE Photovoltaic
Specialists Conference, IEEE, New York, 1985, p. 356.
FRONT CONTACT
f 4.0
AH COAl IN(3
0.6 pm /
p+ GaAs
300 A p A,_0.9Ga0.1As WINDOW
0.6 .um p GaAs EMITTER
3.2 .um n GaAs BASE
0.6 #m n+ GaAs BUFFER
_, 400 #m n GaAs SUBSTRATE
1"
BACK CONTACT
Figure i: Optimized p-n solar cell
structure obtained from
the computer model.
A
,_ 3.0
E
I-
z 2.0
iii
o 1.0
I I I I I
0
= 20.8%
F F = 0.86
Voc = 1.02 V
Jsc = 32.2 mA/cm 2
lX, AM0
I
I
I
I
I
I
I
I
I
II I I I
0.2 0.4 0.6 0.8 1.0 1.2
VOLTAGE (V)
Figure 2: Current vs voltage for a p-n
Cassegrainian solar cell at
T = 28% and 1-sun, AMO. The
dashed lines indicate voltage
and current at maximum power
point.
0.4
A 0.3
v
k-
Z
,,, 0.2
n-
O
0.1
0
Figure 3:
0
= 23.4%
m
F F = 0.870
Voc = 1.13 V
- Isc = 0.377 A
91X, AM0
0.2 0.4 0.6 0.8
VOLTAGE (V)
t
1.0 1.2
Current vs voltage for cell
discussed in Fig. 2 at 91
suns. Dashed lines indicate
voltage and current at maxi-
mum power point.
28
24
23
z 22
E
w21
20
l , i I wwww| ! g w wiDwi_ w ! i w lw,i
i I I iliil ! S Ji • |mmn| I m m a =it*
10 100 1000
CONCENTRATION
Figure 4: AMO efficiency versus solar concentration
for cell discussed in Fig. 2 at T = 28°C.
23.5
23.0 -
>" 22.5-
r,.)
Z
U.I
__ 22.0-
U.
W
21.5 -
21.0
20
, , i i , i
90X, AM0
mO
O O
I I I I I I
30 40 50 60 70 80 90
TEMPERATURE (°C)
Figure 5: AMO efficiency versus temperature at 94
suns for cell discussed in Fig. 2. The
line represents a least-squares fit to
the data and has a slope of -0.036%/°C.
29
25
20
o 15
P,
lO
25
0
0-17 17-18 18-19 19-20 20-21 21-22
EFFICIENCY RANGE (%)
Figure 6: One-sun, AMO efficiency distribution
of all Cassegrainian cells processed
from a single wafer, including cell
discussed in Fig. 2.
30
25
20
I.I.,
O 15
ILl
,'_10
z 5
90X, AM0
°
-
E
0-20 20-22 22-22.5 22.5-23 23-23.5
EFFICIENCY RANGE I%)
Figure 7 : Efficiency distribution of all Casse-
grainian cells processed from wafer
of Fig. 6 at 90 suns, AMO and
T = 28°C.
3O
4.0
A
<_ 3.0
E
I--
z 2.0
uJ
n-
n-
O 1.0
--'lr_lr
" FF = 0.86
Voc = 1.02 V
- Jsc = 32.3 mA/cm 2
lX, AM0
0 0.2 0.4 0.6 0.8 1.0 1.2
VOLTAGE (V)
Figure 8: Current versus voltage for n-p Cassegrainian
solar cell at T = 28°C and 1-sun, AMO. The
dashed lines indicate voltage and current
at the maximum power point.
24
23
_ 22
21
20
w
19
18
1 10 100 1000
CONCENTRATION
Figure 9 : AMO efficiency versus solar concentration
for a n-p Cassegrainian solar cell at
T = 28°C.
31


High-efficiency GaAs concentrator space cells

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