The Power Extension Package (PEP) is the prime focus of a development program to produce low cost solar cells. The PEP is a 32 kilowatt flexible substrate, retrievable, solar array system for use on the Space Shuttle. Solar cell cost will be reduced by increasing cell area and simplifying cell and coverglass fabrication processes and specifications. The cost goal is to produce cells below $30 per watt. Two and ten ohm-cm silicon cells were investigated. This paper describes a unique radiation damage test and side-by-side comparison of candidate cell types with pre-and post-irradiation airplane calibration of outer space short-circuit current

Baraona, C. R.

Hart, R. E., Jr.

Swartz, C. K.

English

NASA Technical Reports Server

NASA Technical Memorandum 82656
(NASA-TH-82656) COHPABATIVE BADIATICN
TESTIHG OF SOLAB CELLS fOR THE SHUTTLE PQWEB
EXTENSION PACKAGE (NASA) 4 p HC A02/MF A01
CSCL 10A
N81-27605
Unclas
26863
Comparative Radiation Testing of
Solar Cells for the Shuttle Power
Extension Package
Cosmo R. Baraona, Clifford K. Swartz, and Russell E. Hart, Jr.
Lewis Research Center
Cleveland, Ohio
Prepared for the
Fifteenth Photovoltaic Specialists Conference
sponsored by the Institute of Electrical and Electronics Engineers
Kissimmee, Florida, May 12-15, 1981
NASA
https://ntrs.nasa.gov/search.jsp?R=19810019067 2020-03-21T12:57:25+00:00Z
COMPARATIVE RADIATION TESTING OF SOLAR CELLS FOR THE SHUTTLE POWER EXTENSION PACKAGE
Cosmo R. Baraona, Clifford K. Swartz and Russell E. Hart, Jr.
NASA Lewis Research Center
Cleveland, Onio 44135
Introduction - The Power Extension Package (PEP) is a reusable array under development (1,2,3) by
NASA Johnson Space Center. The PEP is designed to operate from tne Space Shuttle Orbiter cargo bay
(figure 1) and to supplement the Orbiter's fuel cell/cryogenic power source. The PEP offers a major
increase in Orbiter power level and mission duration. The PEP array w i l l operate in low earth orbit for
a cumulative time of about 3 years and w i l l be subjected to performance degradation by space radiation
equivalent to 1 or 2x10^ 1 MeV electrons/cm^. Laboratory tests to determine the amount of cell
degradation due to radiation are thus necessary for proper cell selection and system design. The purpose
of this paper is to report the results of radiation damage tests of candidate low-cost PEP cells
conducted at the NASA-Lewis Research Center.
Six different cell types with the best potential for meeting the system-requirements were supplied by
the two PEP cell prime contractors for evaluation. The cells had combinations of resistivity, back
surface reflector (BSR), back surface field (BSF), and antireflection (AR) coating. The code used to
describe the cells is shown in figure 2a.
The PEP system designer must select the solar cell technologies that will yield the best combination
of beginning and/or end of life performances, lowest cost and acceptable reliability and risk. This
multiplicity of technologies mandates side-by-side comparative testing.
Procedure - The test conditions are shown in figure 2b. Two groups of cells were irradiated. In
each group 3 cells of each type were randomly arranged in the test fixture. ATI the measured performance
data were taken at 28° C. The orbital operating temperature can then be calculated if the thermal
absorptivity (which varies depending on cell type) is known.
The cells to be used for the PEP array will be 5.9x5.9 cm in size to reduce cost However, the cells
used for this radiation damage test were 2x2 cm in size so that greater numbers of cells could be testeo,
thus increasing tne statistical reliability of the data. As long as the small area cells have the same
characteristics (material, processes, spectral response, etc.) as the large area cells and the cells are
irradiated uniformly over their area, cell size is not expected to affect the radiation test results.
The highest fluence attained in this test was 3x10^ e/cm2 which exceeds the maximum expected for the
'PEP mission life as described previously.
Two methods of fluence measurement were used: A Faraday cup current integrator and control cells
with known radiation degradation behavior. The maximum power of these control cells agreed well with
previous data confirming the accuracy of tne Faraday cup and the uniformity of irradiation.
After irradiation, tne cells were annealed for 17 hours at 60° C in air without illumination to
ensure that the measured performance would be stable. Because of the low fluences and the use of
Czochralski-grown silicon, photon degradation effects were not expected. Therefore the standard
procedure of annealing with post irradiation illumination was not used.
Tne current-voltage measurements of all the cells were made with an X-25 xenon arc solar simulator.
The light intensity of the simulator was adjusted until the measured snort-circuit current of an
aircraft-flown primary reference cell agreed with its aircraft projected air mass zero (AMD) value. The
reference cells used at the start of the tests were either actual candidate PEP cells flown and
calibrated specifically for this test or cells with spectral responses closely matched to the PEP cells
to be tested. A good spectral response match is vital to the overall accuracy of the test. The aircraft
calibration procedure is described elsewhere (4).
After irradiation to 3x10^ e/cm?. the PEP cells had degraded such that their spectral response no
longer closely matched the spectral response of the unirradiated reference cells Therefore, these
irradiated PEP cells themselves were aircraft-flown and their accurate AMD short-circuit currents (ISc)
measured. The maximum power and Isc data at all the fluences were then adjusted based on the change in
calibration due to the spectral response shifts during irradiation. At the highest fluence these
adjustments increased cell output over that measured with the non-irradiated standards from 1.2% to 4.4%
depending on cell type. The multilayer AR coated cells required the highest adjustments. In other
words, without adjustment the data could have been in error by as much as 4.4% at 3x10^ e/cm?. At
zero fluence, no adjustments of this type were needed. At intermediate fluences the adjustments ranged
between zero (at Ixl0'3 e/cm2) and 4.4% and were proportional to the logarithm of the fluence.
Results and Discussion - The average maximum power of each 6-cell group at each fluence for the 10
and ¥ ohm-cells is shown in figures 3 and 4 respectively. For each point, the data spread was less than
3%. These data show that the A10FRM, (i.e. the ten ohm-cm cell with a BSF, a BSR and a multilayer AR
coating) had the highest beginning of life (BOL) power (76.8 mW) under laboratory conditions. At
3xl014 e/cm2 fluence, the A10FRM and the A2RM cells both had the highest average power (56.4 mW).
The S2FRT, a unique 2 ohm-cm BSF cell, has about 10% higher BOL power (72.4 mW) than the S2RT (65.6 mW)
due to the BSF. However, the S2FRT degrades more quickly than the S2RT and at 3xl014 the power
increase due to the BSF effect is almost completely eliminated (53.2 vs 52.8 mW). Comparison of S2FRM
(73.1 mW) and S2FRT (72.4 mW) shows about a 3.7% boost in power due to the multilayer AR coating.
2In terms of normalized power (i.e. maximum power at zero fluence divided by maximum power after
3xlO'4 e/cm2) the 2 ohm-cm BSR cells degraded to 0.80 of original; the 10 ohm-cm BSR/BSF cells
degraded to 0.74 and the 2 ohm-cm BSR/BSF cells degraded to 0.73 of their original power. .
These radiation damage results are necessary, but not sufficient, information needed by the designer
to select the cell to be used on PEP. These tests show that the cell type with the highest power output
at room temperature was the 10FRM at BOL. The 10FRM and 2RM groups had equal power at EOL. The lower
thermal absorptivity (measured.elsewhere) of non-BSF cells results in lower orbital operating temperature
and high power output. Thus the 2 ohm-cm BSR cell w i l l have the best EOL performance in orbit.
Additional information needed for cell selection .which are still being generated include the influence of
array design on operating temperature and the cell cost and delivery schedule. The radiation test
results described here are a vital starting point in the cell selection process.
REFERENCES
1. C. J. Souza, "Large Area Flexible Solar Array Design for Space Shuttle Application," Energy to
the 21st Century; 15th Interaoclety Energy Conversion Engineering Conference. New York:
American Institute of Aeronautics and Astronautics, Inc., 1980, Vol. 1, pp. 410-414.
2. D. Michaels, R. Opjorden, "Large Area, Low-Cost Space Solar Cells," Presented at the Proceedings
of the 15th Photovoltaic Specialists Conference. Orlando, FL, May 12-15, 1981.
3. K. Matthel, et al., "Optimization of Large Area Solar Cells for Low-Cost Space Applications,"
Presented at the Proceedings of the 15th Photovoltaic Specialists Conference. Orlando, FL,
May 12-15, 1981.
4. H. W. Brandhorst, Jr., "Airplane Testing of Solar Cells," Proceedings of the 4th Photovoltaic
Specialists Conference. June 2-3, 1964, pp. C21-17; also NASA TN D-2508.
2a) Cell Description Code:
A or S-cell manufacturer.
10 or 2-base resistivity in ohm-ciii.
F or R-back surface field or reflector.
T or M-T3205 or multilayer AR coating.
P 180-2 ohm-cm control cell, (all cells
2x2x.02 cm, no coverglass, conventional
contacts).
2b) Test Conditions
o Two irradiation groups of 22 cells each;
4-P180: 3 each: A10FRM, A2RM, S2RM, S2FRT,
S2RT, SIOFRT. ., ,
o 1 MeV electron flux < 10'^e/cmVsec in air.
o Cell temperature < 40° C during irradiation,
60° C for 17 hours in air post irradiation
annealing.
o Measurements: AMO IV at 28° C, spectral re-
sponse and aircraft calibration of AMO ISc-
Shuttle Orbiter
Cargo Bay
Remote Manipulator
System
Solar Array
Figure 1. - PEP system with the space shuttle Figure 2. - a) Code, b) Conditions
AIOFRM
1013
Fluence (e/cm2)
10'
Figure 3 - Pmax vs fluence for 10 ohm-en
75 •
50
Figure 4
io13 io14
Fluence (e/cm?)
Pm=.v vs fluence for 2 ohm-cmin a A
1. Report No.
NASA TM-82656
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
COMPARATIVE RADIATION TESTING OF SOLAR CELLS FOR
THE SHUTTLE POWER EXTENSION PACKAGE
5. Report Date
6. Performing Organization Code
7. Author(s)
Cosmo R. Baraona, Clifford K. Swartz, and Russell E. Hart, Jr.
8. Performing Organization Report No.
506-55-42
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
10. Work Unit No.
E-922
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D. C. 20546
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the Fifteenth Photovoltaic Specialists Conference sponsored by the Institute of
Electrical and Electronics Engineers, Kissimmee, Florida, May 12-15, 1981.
16. Abstract
The Power Extension Package (PEP) is the prime focus of a development program to produce
low cost solar cells. The PEP is a 32 kilowatt flexible substrate, retrievable, solar array
system for use on the Space Shuttle. Solar cell costs will be reduced by increasing cell area
and simplifying cell and coverglass fabrication processes and specifications. The cost goal
is to produce cells below $30 per watt. Two and ten ohm-cm silicon cells were investigated.
This paper describes a unique radiation damage test and side-by-side comparison of candidate
cell types with pre- and post-irradiation airplane calibration of outer space short-circuit
current.
17. Key Words (Suggested by Author(s))
Solar cell
Radiation damage
Photovoltaic cells
18. Distribution Statement
Unclassified - unlimited
STAR Category 44
19. Security Qassif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
21. No. of Pages 22. Price*
' For sale by the National Technical Information Service, Springfield, Virginia 22161
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Comparative radiation testing of solar cells for the shuttle power extension package

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