Performance data for the NASA Lewis Research Center indium phosphide n+p homojunction solar cell module on the LIPS 3 Flight Experiment is presented. The objective of the experiment is to measure the performance of InP cells in the natural radiation environment of the 1100 km altitude, 60+ deg inclination orbit. Analysis of flight data indicates that the performance of the four cells throughout the first year is near expected values. No degradation in short-circuit current was seen, as was expected from radiation tolerance studies of similar cells. Details of the cell structure and flight module design are discussed. The results of the temperature dependency and radiation tolerance studies necessary for normalization and analysis of the data are included

Brinker, David J.

Hart, Russell E., Jr.

Smith, Brian S.

Weinberg, Irving

English

NASA Technical Reports Server

NASA Technical Memorandum 101390
InP Homojunction Solar Cell
Performance on the LIPS III
Flight Experiment
David J. Brinker, Russell E. Hart, Jr., and Irving Weinberg
Lewis Research Center
Cleveland, Ohio
and
Brian S. Smith
Spectrolab, Inc.
Sylmar, California
_i_E_ (_) II _ CSCL ICA
Nd_-1212J
Prepared for the
20th Photovoltaic Specialists Conference
sponsored by the Institute of Electrical and Electronics Engineers
Las Vegas, Nevada, September 26-30, 1988
https://ntrs.nasa.gov/search.jsp?R=19890002752 2020-03-20T05:53:54+00:00Z

InP HOMOJUNCTION SOLAR CELL PERFORMANCE
ON THE LIPS III FLIGHT EXPERIMENT
David J. Brinker, Russell E. Hart, Jr., and Irving Weinberg
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
Brian S. Smith
Spectrolab, Inc.
Sylmar, California 91342
LO
#
L_
SUMMARY
Performance data for NASA Lewis Research Center indium phosphide n+p homo-
junction solar cell module on the LIPS III Flight Experiment is presented.
The objective of the experiment is to measure the performance of the InP cells
in the natural radiation environment of the II00 km altitude, 60+ ° inclination
orbit. Analysis of flight data indicates that the performance of the four
cells throughout the first year is near expected values. No degradation in
short-circuit current was seen, as was expected from radiation tolerance stud-
ies of similar cells. Details of the cell structure and flight module design
are discussed. The results of the temperature dependency and radiation toler-
ance studies necessary for normalization and analysis of the data are included.
INTRODUCTION
Indium phosphide (InP) solar cells are excellent candidates for space
power generation because their potential for high efficiency is coupled with
an inherent radiation resistance and low temperature annealability. A develop-
ment program, underway since 1984, has the goal of maximizing the end-of-life
efficiency of InP space solar cells through the optimization of conversion
efficiency and radiation tolerance. Cells of both the n+p and p+n configura-
tions have been developed with an efficiency of 18.8 percent Air Mass Zero
(AMO) achieved. An extensive investigation of radiation tolerance and tempera-
ture effects has also been initiated and continues as part of this ongoing
development program. However, limited opportunities for space flight and its
recency as a space photovoltaic material had prevented actual space flight
testing of InP solar cells. The decision of the Naval Research Laboratory to
build and launch the third Living Plume Shield (LIPS Ill) satellite provided
the first opportunity to obtain invaluable flight data on InP solar cells. In
this paper we report on the module design and early on-orbit performance of
InP n+p homojunction solar cells on the LIPS Ill spacecraft.
InP MODULE DESIGN
InP Solar Cell Characterization
The indium phosphide solar cells used in this experiment were made at
Rensselaer Polytechnic Institute by the open tube diffusion of sulfur into
p-type InP substrates (ref. I). The substrates were LEC grown, <lO0> oriented
wafers doped with zinc to a concentration of 5xlO 16 cm-3. After vacuum
evaporation of a Ga2S3 film, which in turn was encapsulated with silicon
dioxide, the emitter was formed by open tube diffusion in a nitrogen ambient
at 670 °C for 25 min. Cell fabrication was completed with deposition of Au
and Au-5% Zn for emitter and base ohmic contacts and a single layer antireflec-
tion coating of SiO. A total cell area of O.313 cm2 was defined by mesa etch-
ing, with front contact obscuration of 15 percent. Details of the cell
structure are seen in figure I.
Fifteen cells, with AMO efficiencies ranging from 11.36 to 14.31 percent,
were received. The two highest efficiency cells were reserved for use as stan-
dards; the remainder were used for fabrication of two flight modules of four
cells each, a prime flight module and one back-up module. Table I lists the
total area, AMO (25 °C) performance as received from RPI of the four cells
selected for the prime flight module.
The operating temperature of a solar cell for a given orbit is, to a
large extent, a function of array design. Since no previous flight experience
exists for InP and the module design was new, it is necessary to know the tem-
perature dependence of open tube diffused InP cells to enable normalization of
flight data. The temperature dependence of maximum power, open-circuit volt-
age, short-circuit current and fill factor for these cells was previously
determined as part of the Lewis Research Center's InP cell development program
(refs. 2 and 3). Table II summarizes the values used in normalizing the
flight data to a standard temperature of 25 °C.
The prime objective of the InP flight experiment is to measure the degree
of performance degradation in the natural radiation environment of the LIPS
spacecraft orbit. As a consequence of early research which showed indium phos-
phide superior to both gallium arsenide and silicon in a laboratory radiation
environment, extensive radiation tolerance studies of open tube _iffused n+p
InP solar cells have been performed (ref. 3). The cells were irradiated
with ] MeV electrons in a Van de Graaf generator to a maximum fluence of
3xlO 15 e-/cm 2. The dependency of short-circuit current, open-circuit voltage
and efficiency on electron fluence is shown in figures 2, 3 and 4. No damage
equivalents exist for InP solar cells due to the immaturity of InP as a space
cell material and the as yet lack of standardization of cell type and configu-
ration. For this reason, Si equivalent fluences were used for analysis
purposes; 6.49xlOll/yr equivalent I MeV e-/cm 2 for electrons and 2.28xlO13/yr
equivalent 1MeV e-/cm 2 for protons (ref. 4). It has been demonstrated that
light illumination of InP cells while they are undergoing electron irradiation
significantly reduces performance degradation (ref. 5). This effect will also
be considered in the analysis of the InP module flight data.
Flight Module Fabrication
The InP flight module on LIPS III, shown in figure 5, was fabricated by
Spectrolab, Inc. under contract to Lewis Research Center. The module substrate
is a 1.6 mm thick aluminum sheet (alloy 6061-T6), 5 cm square. Electrical
insulation was provided by 3 mil mica ply bonded to the A1 substrate with
DC93-500 adhesive. After rear contact was made by soldering a silver-plated
Kovar interconnect to each cell, the cells were bonded to the substrate with
CV2568 adhesive. Gold plated Kovar termination tabs were bonded to the sub-
strate adjacent to each cell, two per cell for front and rear contact. The
rear contact circuit was completed by soldering the silver-plated Kovar inter-
connects to this termination tab. The front contact circuit was completed by
ultrasonically bail bonding one mil gold wire to the cell busbar and the adja-
cent termination pad. For redundancy, six wires were used for each cell.
Because cell area was delineated by mesa etching, it was necessary to insulate
the wires from the p-lnP substrate. This was accomplished with l mil Kapton
sheet bonded with DC93-500.
Each cell was individually covered with 12 mil CMX fused silica micro-
sheet, bonded with DC93-500. The coverglass adhesive also served to encapsu-
late the gold front contact wires and provide some added degree of mechanical
strength. Two additional gold-plated Kovar termination strips were bonded to
the substrate to serve as common negative current and voltage bus strips.
Negative leads were then routed from each cell to the bus strips and soldered.
Redundant leads from the bus strips were provided for the module pigtail.
Redundant positive leads for the module pigtail were connected directly to the
termination tabs for each cell, providing a four-wire, independent connection
for each cell. Kapton insulated, 26 AWG stranded wire was used for all
connections.
Module temperature measurement capability was provided using a ceramic-
based, platinum resistance thermal detector (RTD). The RTD was bonded with
DC93-500 adhesive to the rear of a fifth InP cell. A U-shaped piece of mica
ply was tailored to fit around the RTD and the gap filled with DC93-500 to pro-
vide a flat surface for mounting on the substrate. The RTD/cell assembly was
then bonded to the center of the module between the four active ceils (see
fig. 5). Kapton insulated wire was then spliced to the RTD leads and added to
the module pigtail. Module fabrication was completed with the covering of all
exposed areas of the substrate with a 5 mil, silver-coated, Teflon second sur-
face mirror.
After fabrication was completed, the current-voltage characteristics of
the flight module was measured in our X-25L solar simulator. This provided a
reference for analysis of flight data as well as measure of cell degradation
due to module assembly processes and cell glassing. Table Ill tabulates the
results of this measurement. It should be noted that there is no correlation
between tables I and Ill with regards to the ordering of ceils. Based on the
average value, the largest change was in fill factor, a decrease of
3.1 percent.
FLIGHT EXPERIMENT RESULTS
The LIPS III satellite is in a nearly circular orbit of II00 km altitude
with an inclination of 60+ °. Details of the LIPS Ill satellite design, the
data acquisition system and the experiments have been published by the satel-
life's builders at the Naval Research Laboratory (refs. 6 and 7). To date,
data through the first 383 days of flight has been received, which corresponds
to 5131 revolutions about the earth.
Each of the four cells on the InP module survived the launch environment
and continue to provide meaningful data. Module temperature has ranged
between I and 34 °C. The current-voltage characteristics of cell number 4 for
three orbits is shown in figure 6. Also plotted is the preflight data mea-
sured in NASA Lewis' solar simulator prior to delivery of the module to the
Naval Research Laboratory for satellite integration. This curve is designated
Day O. The flight data has been normalized with respect to solar intensity
and temperature (to a standard of 25 °C). The current at voltages below the
maximum power point is uniformly low with respect to the preflight simulator
data, a decrease of approximately 4 percent. Significant variation in the
data occurs near the maximum power point. Further analysis of the flight data
is necessary to determine if this variation is caused by the data acquisition
system or is inherent to the cells. Unfortunately, open-circuit voltage is
never reached in the flight data, the three curves shown in figure 6 being
representative of data from all four cells. More extensive analysis may yield
the open-circuit voltage.
An anomalous drop in current has been observed for all cells in much of
the data. This decrease, at most lO percent in magnitude, generally occurs
between 0.3 and 0.7 V. In all cases, the current returned to expected values
as the voltage increased to the maximum power point. The drop also occurred
in preflight data taken through the LIPS data acquisition system. The effect
has occasionally disappeared during the first year on-orbit, including the
most recent 2 months. The drop seems to be a condition of only the small
current experiments, and no explanation of the effect is apparent at this time
(ref. 8).
A plot of short-circuit current as a function of time on orbit is shown
in figure 7 for 2 of the 4 cells. All data has been normalized with respect
to temperature and solar intensity. The variation in Isc is ±2 percent with
the average value remaining constant during the first year on-orbit. This
result is as expected based on laboratory radiation studies (fig. 2) and the
equivalent electron and proton fluences in this orbit.
CONCLUSION
A module of four n+p, homojunction InP solar cells has been designed and
fabricated and was launched on the LIPS III Flight Experiment. Preliminary
analysis of flight data from the first year reveals that short-circuit current
has not decreased, in agreement with laboratory radiation tolerance studies.
Anomalous drops in current at voltages between 0.3 and 0.7 V were seen in much
of the data; however, the drop did not effect short-circuit current and maxi-
mum power determination. This drop also occurred in preflight data taken
through the LIPS data acquisition system. More extensive analysis of the
flight data continues, and only more time on-orbit will confirm the superior
radiation tolerance of InP solar cells to the natural radiation environment of
space.
ACKNOWLEDGMENTS
The authors wish to thank Mr. K.K. Parat and Professors S.K. Ghandhi and
J.M. Borrego of Rensselaer Polytechnic Institute for fabricating, on very short
notice, the InP cells utilized in this experiment.
REFERENCES
I. K.K. Parat, S. Bothra, J.M. Borrego, and S.K. Ghandhi, "Solar Cells in
Bulk InP, Made by an Open Tube Diffusion Process," Solid State Elec
iron., VoI. 30, pp. 283-287, 1987.
2. I. Neinberg, C.K. Swartz, R.E. Hart Jr., and R.L. Statler, "Radiation and
Temperature Effects in Gallium Arsenide, Indium Phosphide and Silicon Solar
Cells," in 19th IEEE Photovoltaic Specialists Conference, IEEE, Piscataway,
NJ, 1987, pp. 548-557.
3. I. Weinberg, C.K. Swartz, R.E. Hart Jr., S.K. Ghandhi, J.M. Borrego, K.K.
Parat, and M. Yamaguchi, "Comparative Radiation Resistance, Temperature
Dependence and Performance of Diffused Junction Indium Phosphide Solar
Cells," Solar Cells, Vol. 22, pp. 113-124, 1987.
4. H.Y. Tada, J.R. Carter, B.E. Anspaugh, and R.G. Downing, "Solar Cell
Radiation Handbook," JPL Publication 82-69, NASA CR-169662, 1982.
5. K. Ando, and M. Yamaguchi, "Radiation Resistance on InP Solar Cells Under
Light lllumination," Appl. Phys. Lett., Vol. 47, pp. 846-848, 1985.
6. J.C. Severns, R.M. Hobbs, N,P. Elliott, R.H. Towsley, and G.F. Virshup,
"The Experiments of LIPS III," in Space Photovoltaic Research and Techno-
logy Conference, NASA CP- , NASA, 1988, to be published.
7. J.G. Severns, "The LIPS III Flight Experiment," in 20th IEEE Photovoltaic
Specialists Conference, IEEE, Piscataway, NJ, 1988, to be published.
8. J.G. Severns, private communication.
TABLE I. - AMO, 25 °C I-V PARAMETERS of InP
CELLS PRIOR TO FABRICATION OF MODULE
Cell Voc, Isc, FF, EFF,
number V mA percent percent
KK-125E 0.810 "7.77 80.9 11.87
-126C .811 7.78 81.2 11.94
-127A .813 7.85 81.7 12.15
-128C .813 7,95 81.5 12.27
Average 0.812 7,84 81,3 12,06
TABLE II. - TEMPERATURE DEPENDENCY TERMS - n+p InP
Maximum power, mW/cm2-K ......... -4.82x10 -2
Open-circuit voltage, mV/K .......... -2.37
Short-circuit current, mA/cm2-K ..... +2.23xi0-2
Fill factor, percent/K ......... -5.66×10-2
TABLE Ill. - AMO, 25 °C l-V PARAMETERS OF
FLIGHT MODULE IN SOLAR SIMULATOR
Cell Voc, Isc,
number V mA
l 0.800 7.86
2 .802 7.47
3 .805 7.63
4 .800 7.83
Average 0,802 7.70
FF, EFF,
percent percent
78.2 11.45
78.3 10.91
80.2 II.47
78.4 11.44
78.8 11.32
/-- M COAT
/ SiO
n
,-Au
/
)__1-_ .068
(S >1018/cm 3 --_.040
p (Zn : 4.6xlo16/cm3)
pm
IJm
SUBSTRATE
FIGURE 1. - n+p InP CELL STRUCTURE.
1.0
,8 I , l,lJl,l I I I Il_lll I I l il,l,l
1013 1014 1015 1016
I REV ELECTRONFLUENCE, cH-2
FIGURE 2. - NORRALIZEDSHORT-CIRCUIT CURRENTVERSUS1 REV
ELECTRONFLUENCE.
_. ,,, 1.0
0 I---
N
--, I---
0 ,--,
Z u
.9--
.8 I I I ililll I i l,lll,l I I lll,l,l
1013 1014 1015 1016
11_EV ELECTRONFLUENCE, cM-2
FIGURE 3. - NORNAL[ZEDOPEN-CIRCUIT VOLTAGEVERSUS1 NEV
ELECTRONFLUENCE.
ORIGINALPAGEIS
OF POORQUALITY
r-_ >,-
N Z
Z _J
1,0 E
.9 m
.8
i013
I , I_I=ILI I t I_11111 I I l il=lll
1014 1015 1016
I MEV ELECTRONFLUENCE, CM-2
FIGURE 4. - NORMALIZEDEFFICIENCY VERSUSI MEV ELECTRON
FLUENCE.
FIGURE 5. - INP FLIGHT MODULE.
1 CM
8G
CELL NUMBER4
m
0 39
D 221
0 31o
I I I
.2 .4 .6 .8
VOLTAGE, V
FIGURE 6. - FLIGHT I-V CURVES OF INP CELL NUMBER 4 (DAY 0 IS PRE-FLIGHT
SOLAR SIMULATOR DATA).
I
1.0
8.oF-
7.5
CELL
7.0 -- NUMBER
0 4
D 2
G.5 I I I I I
100 200 300 400
DAYS FROM LAUNCH
FIGURE 7. - Isc VERSUS DAYS FROM LAUNCH (TEMPERATURE AND SOLAR INTENSITY
CORRECTED).
Report Documentation PageNational Aeronautics and
Space Administration
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
NASA TM-101390
5. Report Date4. Title and Subtitle
InP Homojunction Solar Cell Performance on the LIPS III Flight Experiment
7. Author(s)
David J. Brinker, Russell E. Hart, Jr., Irving Weinberg and Brain S. Smith
6. Performing Organization Code
8. Performing Organization Report No.
E-4454
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
10. Work Unit No.
506-41-11
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the 20th Photovoltaic Specialists Conference sponsored by the Institute of Electrical and Electronics
Engineers, Las Vegas, Nevada, September 26-30, 1988. David J. Brinker, Russell E. Hart, Jr., and
Irving Weinberg, NASA Lewis Research Center; Brian S. Smith, Spectrolab, Inc., Sylmar, California 91342.
16. Abstract
Performance data for NASA Lewis Research Center indium phosphide n+p homojunction solar cell module on
the LIPS III Flight Experiment is presented. The objective of the experiment is to measure the performance of
InP cells in the natural radiation environment of the 1100 km altitude, 60+ ° inclination orbit. Analysis of flight
data indicates that the performance of the four cells throughout the first year is near expected values. No degra-
tion in short-circuit current was seen, as was expected from radiation tolerance studies of similar cells. Details of
the cell structure and flight module design are discussed. The results of the temperature dependency and radiation
tolerance studies necessary for normalization and analysis of the data are included.
17. Key Words (Suggested by Author(s))
LIPS IlI
Solar cells
Indium phosphide
Space power
18. Distribution Statement
Unclassified - Unlimited
Subject Category 44
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages
Unclassified Unclassified 10
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InP homojunction solar cell performance on the LIPS 3 flight experiment

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