GaAs/Ge solar cells with lot average efficiencies in excess of 18 percent were produced by MOCVD growth techniques. A description of the cell, its performance and the production facility are discussed. Production GaAs/Ge cells of this type were recently assembled into circuits and bonded to aluminum honeycomb panels to be used as the solar array for the British UOSAT-F program

Gillanders, M.

Lillington, D.

Rolph, R.

Smith, B.

Vijayakumar, P.

Yang, H.

English

NASA Technical Reports Server

N91-30212
PRODUCTION STATUS OF GaAs/Ge SOLAR CELLS AND PANELS
B. Smith, M. Gillanders, P. Vijayakumar, D. Lillington
Spectrolab, Inc.
Sylmar, CA
and
H. Yang, and R. Rolph
Hughes Aircraft Company
Torrance, CA
GaAs/Ge solar cells with lot average efficiencies in excess of 18.0% have been
produced by MOCVD growth techniques. A description of the cell, its performance
and the production facility are given. Production GaAs/Ge cells of this type
were recently assembled into circuits and bonded to aluminum honeycomb panels to
be used as the solar array for the British UOSAT-F program.
INTRODUCTION
GaAs/Ge solar cells are in demand for applications involving spacecraft solar
arrays which require increasingly higher power outputs per unit area but have no
capacity for growth due to limitations on the array envelope. GaAs is grown onto
a germanium wafer by metallo-organic chemical vapor deposition (MOCVD)
techniques. The use of germanium as a growth substrate instead of gallium
arsenide, enables solar cells to be manufactured which are robust enough to
survive the assembly processes and handling required to produce solar arrays.
To meet this demand for high efficiency, rugged GaAs/Ge solar cells, Spectrolab
has recently commissioned a new production facility which is able to fabricate
solar cells in excess of 18.0% efficiency in a range of sizes varying from 2cm x
2cm to 6cm x 6cm. To date this facility has processed over 2000 4.5cm x 4.5cm
wafers and produced material from which single junction GaAs/Ge, wrapthrough
GaAs/Ge and dual junction GaAs/Ge solar cells have been fabricated. These cell
types are presently being integrated into solar cell circuits for use on military
and scientific research programs.
Earlier this year single junction GaAs/Ge cells of dimension 2cm x 4cm were
produced and assembled into the UOSAT-F solar array. This effort is noteworthy
on many counts. The program was completed in an extremely short time frame - 3
months; the bare cell lot average efficiency was 18.3% with a maximum of 19.7%
the delivered solar array had an average circuit efficiency of 18.2% with a
maximum of 18.6%.
In the following sections, a description of the solar cell design is given,
including some unique features devised to increase cell stability at high
temperatures. The new Spectrolab facility is described with its projected
capacity for mass production of GaAs/Ge solar cells. Cell performance
distributions from different programs are shown and finally, a description of the
design and build of the UOSAT-F solar array is given including acceptance test
performance data of the completed array.
9-1
https://ntrs.nasa.gov/search.jsp?R=19910020898 2020-03-19T17:29:23+00:00Z
MOCVDMANUFACTURING FACILITY
Spectrolab has established a high volume MOCVD manufacturing facility, currently
occupying 7000 sq. ft. with room for expansion to 30,000 sq. ft. This area
presently houses 3 MOCVD barrel reactors, each of which has produced GaAs solar
cellsof over20%at AM0, 28°Cefficiency.
Currently, the problem with large scale barrel MOCVD reactors is that gas
depletion, which occurs as the source gases traverse the wafer, results in large
variations in layer thickness and doping uniformity which ultimately translate to
reduced yield and radiation hardness.
Based on this fact, and the need for improved uniformity to meet the more
stringent needs of advanced solar cells such as GalnP2/GaAs or AIGaAs/GaAs
tandem cells, our strategy has been to pursue the development of improved, MOCVD
systems to replace the conventional barrel design. Delivery of the first reactor
is scheduled for delivery in the second quarter of 1992 with commissioning
completed by the third quarter of 1992. Additional systems will be added to meet
the needs of the market as it matures.
Our MOCVD strategy is also driven by the need for substantial cost reduction and
the ability to provide an enabling technology for larger area GaAs/Ge solar cells
up to 8cm x 8cm in size. The reactor is therefore being designed with 4"
diameter wafer capability to provide a growth option for Air Force programs
currently utilizing large area silicon cells or as a possible growth option for
Space Station Freedom. Alternatively up to six 2cm x 4cm cells can be laser
scribed from a 4" wafer, substantially reducing the cost of smaller area cells
below current levels. Further cost reduction will also be possible through a
three-fold reduction in cycle time thus substantially increasing the reactor
throughput.
CELL DESCRIPTION
The cross section of the GaAs/Ge cell is shown in Figure 1.
The starting wafer upon which the active GaAs layers are grown is 7 mils thick,
4.5cm x 4.5cm n-type Ge, chem-mechanically polished on the _ont side and
chemically etched on the backside. The wafer is doped to 5E17 cm-" with Sb and
oriented several degrees off <100> toward the nearest <111> plane.
MOCVD growth of GaAs is performed at low pressure. Growth conditions have been
optimized to ensure electrical performance uniformity of +_. 3% between the top and
bottom rows.
The _n-type GaAs buffer is between 2 to 6 microns thick and is doped to 5E18
cm--' with Si. It is grown using a specific nucleation/growth procedure
designed to minimize the diffusion of Ga into the wafer, thus assuring the
elimination of an active Ge junction.
The base is typically 2 to 3 microns thick and is doped to nominally 2E17 cm -3
with Si.
The p-type emitter is doped to between 2 and 4E18 cm -3 with Zn and is typically
0.5 microns thick. The thickness is controlled using SPC to ensure radiation
hardness which is known to be a strong function of emitter thickness.
9-2
An AI0 85Ga015As window serves to passivate the surface of the emitter by
providlfig an" optically transparent heterof3ace at the cell surface. It is
typically 500 A thick and is heavily doped to 2E18 era- with Zn.
The dual antireflection (AR) coating used is Ta205/AI20 3. The window
surface is specially treated prior to AR coating to guarantee subsequent
environmental stability.
A noteworthy feature of the cell design is the front contact GaAs cap structure.
This provides a very effective barrier against metal diffusion into the sensitive
junction region. Cells manufactured at Spectrolab with this type of contact have
shown thermal stability up to 550°C for several minutes duration and for over
500 hours at 350°C. Th_ GaAs cap is typically 0.3 to 0.5 microns thick and is
heavily doped (>lEI9 cm -z) with Zn to allow good ohmic contact to be achieved
between the grid and the front of the cell. It also allows higher efficiencies to
be achieved, compared to other designs, since it reduces the recombination
velocity beneath the contact, allowing higher Voc to be achieved.
Both front and back silver contact layers are typically 5 microns thick and are
weldable using Ag or Ag/Kovar interconnects without degrading cell performance.
CELL PERFORMANCE
Spectrolab has processed approximately 2000 Ge wafers on its manufacturing line
into cell sizes from 2cm x 2cm, up to 6cm x 6cm. AM0 efficiencies exceeding 18%
have been achieved for each cell type.
In Figure 2 we show the efficiency distribution of approximately 640 weldable 2cm
x 4cm GaAs/Ge cells fabricated for the UOSAT-F flight program. The average
efficiency was 18.3%. Representative cells, flown on the NASA Lewis Research
Center Lear jet under near ideal AM0 conditions confirmed the inactivity of the Ge
wafer.**
In Table 1 we also show the efficiencies of thirty two 4cm x 4cm cells recently
fabricated. The average AM0, 28°C efficiency was 18.3% with a best efficiency
of 19.2%.
A limited number of 6cm x 6cm prototypes have also been made from 6.5cm x 6.5cm
wafers. The IV curve of the best cell with an AM0 efficiency of 18.2% is shown
in Figure 3. Further improvements in efficiency are expected by increasing the
grid height to over 12 microns.
** The authors would like to thank David Brinker of NASA LeRC for assistance in
performing the lear jet measurement.
9-3
UOSAT-F
The UOSAT series of micro-satellites are built by the University of Surrey.
Their primary goal is to provide a vehicle for research into satellite
engineering topics at low cost. UOSAT-F is the fifth in the series and will fly
an RAE Solar Cell Technology experiment, including indium phosphide and cleft
GaAs cells for NASA-Lewis. The payload also includes a CCD Earth imaging camera.
The solar arrays are body mounted and fairly small area. To maximize power,
therefore, GaAs is the material of choice for the array solar cells. The UOSAT-3
and -4 arrays were made by Mitsubishi and FIAR/CISE/EEV respectively. UOSAT-F
provided an opportunity to fly an American GaAs/Ge solar array for the first time
on an European satellite. Specifically it enabled Spectrolab to combine its
skills as cell manufacturer and panel assembler to design build and test the
array in an extremely short timeframe. UOSAT-F is scheduled for ARIANE launch in
early May 1991.
UOSAT-F SOLAR PANEL ASSEMBLY
Cells taken from the distribution shown in Figure 2 were interconnected by means
of welded silver-plated Kovar interconnects. Welding was chosen as the method of
interconnection since earlier panel assembly experience had shown that solder
wets GaAs and sometimes wicks under the interconnect and over the cell edge, thus
shorting out the GaAs cell junction just below the surface. Attrition through
the welding line was, as a result, less than 1%.
The cells were covered with 6 mil ceria doped microsheet and then interconnected
into circuits of 42 cells series, 2 parallel.
The array consisted of three panels, each panel having two circuits bonded to it
with CV 2568 silicone adhesive.
Each circuit was wired with 24 awg redundant wiring which were later connected
directly into the spacecraft power system since the panels were body mounted.
The six circuits were tested on a LAPSS and found to have efficiencies varying
from 17.9% to 18.6%.
The panel outputs at maximum power varied from 32.6 watts to 33.9 watts at 38
volts. The I-V curve for the latter is shown in Figure 4. The three completed
panels are shown in Figure 5.
CONCLUSION
Spectrolab's MOCVD GaAs/Ge solar cells are now available in lot average
efficiencies in excess of 18.0% in dimensions varying from 2cm x 2cm to 6.5cm x
6.5cm. Production capability is to be significantly increased with commissioning
of a new, improved design reactor which will allow production of 8cm x 8cm cells.
Spectrolab's capability for integrating these cells into solar panels has been
proven by the fabrication of the UOSAT array with circuit efficiencies as high as
18.6%.
9-4
Table 1 PERFORMANCE DISTRIBUTION OF 4cm x 4cm GaAs/Ge CELLS
Voc Isc Vmp Imp Pmp Cff Elf
(mY) (mA) (mY) (mA) (=W) (%) (%)
* AMO, 28 C
I019 488 870 455 396 79.6
1013 474 874 444 389 80.9
1022 484 893 450 402 81.4
1018 468 873 444 387 81.3
1020 466 883 439 388 81.7
1025 468 896 444 398 83.0
1029 485 890 458 407 81.5
1020 465 888 435 386 81.5
1031 483 910 456 415 83.3
1025 455 893 438 391 83.9
1032 481 900 456 410 82.7
1031 484 894 443 396 79.4
1028 482 888 448 398 80.3
1031 490 892 463 413 81.7
1022 469 872 435 379 79.1
1026 469 886 433 384 79.7
1025 485 878 438 385 77.3
1027 478 894 444 397 80.9
1025 469 886 430 381 79.2
1028 475 902 449 405 83.0
1029 480 892 449 401 81.1
1031 488 892 467 417 82.8
1027 476 890 440 392 80.1
1031 468 904 447 404 83.7
1018 459 886 427 378 80.9
1023 478 900 448 403 82.5
1021 472 900 442 398 82.5
1023 471 898 439 394 81.9
1026 474 898 449 403 82.9
1026 483 891 444 396 80.0
1028 477 908 449 408 83.1
1025 470 900 436 392 81.5
Average Effy =
18.3
18.0
18.6
17.9
17.9
18.4
18.8
17.8
19.2
18.1
19.0
18.3
18.4
19.1
17.5
17 .7
17 .8
18.3
17.6
18.7
18.5
19.2
18.1
18.7
17 5
18 6
18 4
18 2
18 6
18 3
18.8
18.1
18.3
GaAs GAP
WINDOW _
EMITTER
BASE --
SUFFER
............. x
Figure 1
i N-Ge 5E17 cm-3!
/ "\
DAR _,
WINDOW
P EMITTER
iq
N /
/
/
X\ /'
\, //
II I III IIII
GaAs/Ge CELL CROSS-SECTION
9-5
NUMBER OF CELLS
Ir iCELL SIZE 2CM X 4CM200 -_"
150 -_
50 _ : ,_
0 / _ i "
17 17,5 18 18.5 19 19.5
EFFICIENCY (%)
640 CELLS , 16.5% (18.30% AVO)
Figure 2 EFFICIENCY DISTRIBUTION FOR UOSAT CELLS
2O
1200
II00
1000
900
<f 800
£
v 700
600
(D
500
L-
400
CO
300
200
100
00,'0'
Figure 3
i i i i i i i I i i i I I t i i i i i I l I i I , i 1 I i i i I i i i I i i i I i i i I i i
Isc (mA)= 1090
Voc (V)= 1.022
Imp (mA)= 1040
Vm# [V)= 0.875
Prop (roW)= 911
cf+ (_,)= 81.8
Elf (s)= 18.2
t t I i i t 1 i i i t i i i I i t i I t i i I i i t t i t i I t i I i i
0.1 0,2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Voltage (V)
i i i I i i
,0 1,1
!
._-
._-
._-
.-
._-
._-
._-
_-
i
1.2
IV CURVE FOR 6.5cm x 6.5cm GaAs/Ge SOLAR CELL
9-6
ZUOSAT P/N 028554 S/N 00_ 3--26-9_
/
l I I I I I I I li I
25 50
V
Figure 4 UOSAT PANEL POWER OUTPUT
_ TOTAL.
Isc = .9967
Voc = 43.!6
Vtst = 35.80
Itst= .9245
Pmax = 33.85
ipmx = .89_8
Vpmx = 37.96
Tamb = 22.30
Tout = 25.00
FF = .7869
- ' .... ' ..... L.'_ ...............
Figure 5 SCHEMATIC OF UOSAT PANEL LAYOUT
9-7



Production status of GaAs/Ge solar cells and panels

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