The flight performance of NASA Lewis Research Center's indium phosphide homojunction solar cell module on the LIPS 3 satellite is presented. A module of four n+p cells was fabricated and has been on-orbit on the LIPS 3 spacecraft since 1987. The experimental objective is the measurement of InP cell performance in the natural radiation environment of the 1100 kilometer altitude, 60 deg inclination, circular orbit. Flight data from the first year is near expected values, with no degradation in short-circuit current. The temperature dependence of current-voltage parameters is included along with the laboratory radiation tolerance studies necessary for normalization and analysis of the data. Details of the cell structure and flight module design are also discussed

Brinker, David J.

English

NASA Technical Reports Server

NASA Technical Memorandum 102 1 3 1 
, On-Orbit Results of the LIPS III/InP 
Homojunction Solar Cell Experiment 
( N A S A - T U - 1 0 2 1 3 1 )  O N - O R B I T  RESULTS O F  T H E  H89-26292 
LIPS 3 / h P  B O f l O 3 U N C T I C N  SOLAR CELL 
EXPEEIHENT (NASA., Leu* Research C e n t e r )  - J P  CSCL l 0 A  U n c l a s  
G3/44 3219605 
David J.  Brinker 
Lewis Research Center 
Cleveland, Ohio 
Prepared for the 
European Space Power Conference 
sponsored by the European Space Agency 
Madrid, Spain, October 2-6, 1989 
https://ntrs.nasa.gov/search.jsp?R=19890016921 2020-03-20T01:28:56+00:00Z
ON-ORBIT RESULTS OF THE LIPS III/InP HOMOJUNCTION 
SOLAR CELL EXPERIMENT 
David J. Brinker 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio USA 
ABSTRACT 
The flight performance of NASA-LeRC's indium 
phosphide homojunction solar cell module on the 
LIPS I11 satellite is presented. A module of 
four n+p cells was fabricated and has been on- 
orbit on the LIPS I11 spacecraft since 1987. 
The experimental objective is the measurement of 
InP cell performance in the natural radiation 
environment of the 1100 kilometer altitude, 60" 
inclination, circular orbit. Flight data from 
the first year is near expected values, with no 
degradation in short-circuit current. Included 
in the paper is the temperature dependence of 
current-voltage parameters and the laboratory 
radiation tolerance studies necessary for 
normalization and analysis of the data. Details 
of the cell structure and flight module design 
are also discussed. 
Keywords: Indium Phosphide, Solar Cells, Flight 
Experiment, LIPS I11 Spacecraft 
1. INTRODUCTION 
Upon recognizing its potential for space power 
generation, the Lewis Research Center embarked, 
in 1984, on a program to develop indium phos- 
phide solar cells. The program has the goal of 
maximizing end-of-life power through the optim- 
ization of conversion efficiency and radiation 
tolerance. Cells of both the ntp and p+n con- 
figurations have been developed with an effi- 
ciency of 18.8% Air Mass Zero ( A M O )  achieved. 
An extensive laboratory investigation of radi- 
ation tolerance has also been undertaken, with 
radiation tolerance superior to both silicon and 
gallium arsenide clearly demonstrated (Refs. 1, 
2). However, limited opportunities for space 
flight had prevented actual testing of InP in a 
natural radiation environment. The decision by 
the Naval Research Laboratory to launch the 
third Living Plume Shield (LIPS 111) satellite 
provided the first opportunity to obtain flight 
data on InP solar cells. 
present the on-orbit performance of n+p InP 
homojunction solar cells on the LIPS I11 flight 
experiment, and compare it with expected results 
This paper will 
based on laboratory testing. Details of solar 
cell and module design and fabrication will be 
discussed as well. 
2. EXPERIMENT DESIGN 
The flight module developed for the LIPS I11 
satellite contains four n+p homojunction indium 
phosphide solar cells along with a temperature 
sensor. The module is pictured in Figure 1. 
The cells were made at Rensselaer Polytechnic 
Institute by the open tube diffusion of sulfur 
into single crystal p-InP substrates (Ref. 3 ) .  
The substrates used were Czochralski grown, 
<loo> oriented wafers do ed with zinc to a con- 
for emitter formation was a vacuum evaporated 
thin film of Ga2S3 which in turn was encapsu- 
lated with silicon dioxide. Open tube diffusion 
in a nitrogen atmosphere at 670 "C for 25 min- 
utes created the junction. Au and Au-5XZn were 
used for front and rear ohmic contacts respec- 
tively with a single layer antireflection 
coating of silicon monoxide. A total cell area 
of 0.313 cm2 was defined by mesa etching, with 
front contact obscuration of 15%. Details of 
the cell structure are seen in Figure 2. 
Fifteen cells, with AM0 efficiencies ranging 
from 11.4 to 14.3 percent were obtained for 
fabrication into two modules of four cells each, 
a prime flight module and one reserve module. 
The two highest efficiency cells of the lot were 
retained to serve as secondary standards. The 
total area, AM0 (25 "C) current-voltage param- 
eters of the four cells used in the prime flight 
module are listed in Table 1. The I-V curves, 
taken before incorporation into the module, were 
measured using a zenon arc lamp solar simulator 
in our laboratory. 
The flight modules, prime and reserve, were 
fabricated by Spectrolab, Inc. of Sylmar, Cali- 
fornia, USA under contract to the Lewis Research 
Center. The module substrate is a 1.6 mm thick 
aluminum sheet (alloy 6061-T6), 5 cm square. 
Electrical insulation from the substrate and 
consequently the spacecraft was provided by a 75 
micrometer thick sheet of mica ply bonded to the 
aluminum substrate with DC93-500 silicone adhe- 
sive. After rear contact was made by soldering 
centration of 5x1Ol6 cm- 5 . The source of sulfur 
1 
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 insulated substrate 
adjacent to each cell, two per cell for front 
and rear contact. The rear contact circuit was 
completed by soldering the silver-plated Kovar 
interconnects to this termination tab. The 
front contact circuit was completed by ultra- 
sonically ball bonding 25 micrometer diameter 
gold wire to the cell busbar and the adjacent 
termination pad. For redundancy, six wires were 
used for each cell. Because cell area was de- 
lineated by mesa etching, it was necessary to 
insulate the wires from the p-InP substrate. 
This was accomplished with 25 micrometer Kapton 
sheet bonded with DC93-500 to the InP substrate 
adjacent to the mesa. 
A 300 micrometer fused silica coverglass 
was bonded with DC93-500 adhesive to each cell. 
The coverglass adhesive also served to encap- 
sulate 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. Neg- 
ative 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 c e l l ,  providing a 
four-wire, independent connection for each 
cell. Kapton insulated, multi-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 provide 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 cells (Figure 
1). 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 125 micrometer thick, silver-coated, 
Teflon second surface mirror. 
The current-voltage characteristics of the 
four cells on the completed prime flight module 
were again measured in our solar simulator imme- 
diately prior to delivery to the Naval Research 
Laboratory for integration on the spacecraft. 
This data (Table 2 )  provided a reference for 
analysis of flight data as well as measure of 
cell degradation due to module assembly 
processes and cell glassing. It should be noted 
that there is no correlation between Tables 1 
and 2 with regards to the ordering of the cells. 
3 .  FLIGHT PERFORMANCE 
The Living Plume Shield I11 satellite was 
launched in the Spring of 1987 into a nearly 
circular orbit of 1100 kilometers altitude with 
an inclination of 60". To date, data from the 
first thirteen months of flight has been 
received, corresponding to approximately 5200 
revolutions of the earth. Each of the four 
cells in the experiment are providing meaningful 
data. (Details of the satellite's design, the 
data acquisition system and the experiments on- 
board have been published in References 4 and 5.) 
The current-voltage characteristics of cell 
number 4 for three orbits throughout the first 
year is shown in Figure 3 .  Also plotted is the 
pre-flight data measured in our solar simulator 
prior to delivery to the Naval Research Labor- 
atory. This curve is designated Day 0. Since 
module temperature ranged from between 1 and 34 
"C ,  this data has been normalized to a standard 
temperature of 25 "C. Since the module design 
was new, knowledge of the temperature dependence 
of open tube diffused InP cells was necessary 
for normalization. As part of the Lewis 
Research Center's InP cell development program 
the temperature dependence of maximum power, 
open-circuit voltage, short-circuit current and 
fill factor had been previously determined for 
this type of cells (Ref. 6, 7 ) .  The values used 
in normalizing the flight data to a standard of 
25 "C is summarized in Table 3. All data has 
also been normalized with respect to solar 
intensity. 
Short-circuit current as a function of time 
on-orbit is plotted in Figure 4 for cell numbers 
2 and 4 .  Again, all data has been normalized 
with respect to temperature and solar inten- 
sity. The data shown for these cells is repre- 
sentative of all four cells on the module. The 
variation in I, is f 2 %  with the average value 
remaining constant. 
4 .  DISCUSSION 
The currents at voltages below the maximum power 
point (Figure 3) are uniformly low with respect 
to the pre-flight solar simulator data, a de- 
crease of about 4 % .  
dust contamination problems which occurred on 
the launch pad (Ref. 4 ) .  Significant variation 
in the data at the "knee" or maximum power point 
occurs for all cells. Whether this variation is 
inherent to the cells or is caused by the LIPS 
data system has not been determined as yet and 
will require the acquisition of flight data from 
subsequent orbits. Regrettably, the open- 
circuit voltage is never reached in any of the 
data, and analysis has been unable to reliably 
determine it. 
In much of the data received from the first year 
on-orbit, an anomalous drop in current has been 
observed for all cells. This decrease, at most 
10% in magnitude occurs between 0.3 and 0 .7  
volts. In all cases, the current returned to 
expected values as the voltage returned to the 
maximum power point. The decrease also occurred 
in the pre-flight data taken through the LIPS 
data acquisition system. 
sionally disappeared during the first year, 
including the two most recent months. This 
anomalous behavior seems to be a condition of 
the those experiments with small short-circuit 
currents, with no explanation of the effect 
apparent at this time. 
This may likely be due to 
The effect has occa- 
2 
The principal objective of the experiment is to 
measure the degree of performance degradation of 
InP homojunction cells in the natural radiation 
environment of the LIPS I11 spacecraft. A 
secondary motivation is the comparison of actual 
flight data with laboratory radiation tolerance 
studies. This is of importance because damage 
equivalences do not exist for InP because of its 
recency as a space cell material and the as yet 
lack of standardization of cell type and 
configuration. Extensive radiation tolerance 
studies of InP n+p open tube diffused cells have 
been conducted as a consequence of early 
research which showed indium phosphide superior 
to both silicon and gallium arsenide in a 
laboratory radiation environment (Ref. 8). Open 
tube diffused n+p cells identical to the flight 
cells were irradiated with 1 MeV electrons to a 
fluence of 3x1015 e-/cm2 (Ref. 7). The 
dependency of efficiency, open-circuit voltage 
and short-circuit current on electron fluence is 
shown in Figures 5, 6 gnd 7. 
Silicon equivalent fluences were used for flight 
data analysis in lieu of InP equivalences; 
6.49x1011/yr equivalent 1 MeV e-/cm2 for elec- 
trons and 2.28x10I3/yr equivalent 1 MeV e-/cm2 
for protons (Ref. 9). From Figure 7 and the 
silicon equivalent fluences, little decrease in 
short-circuit current is expected during the 
first year. This is confirmed by Figure 4, the 
plot of short-circuit current as a function of 
days on-orbit. 
A second effect of possible influence on the 
radiation performance of InP solar cells is the 
discovery that light illumination of InP cells 
during electron irradiation significantly 
reduces degradation (Ref. 10). Although data 
received thus far is insufficient to determine 
if this effect is active, it may take on 
increased significance as time on-orbit 
increases. 
5. CONCLUSION 
Four n+p indium phosphide homojunction solar 
cells are currently on-orbit on the LIPS I11 
spacecraft and returning meaningful data. 
Analysis of flight data from the first year 
reveals that all cells continue to operate at 
expected performance levels. Short-circuit 
current has not decreased, in agreement with 
laboratory radiation tolerance studies of 
similar open tube diffused junction cells and 
Cel l  voc * I s ,  1 FF * 
number v percent 
TABLE 1 - Am, 25 O C  I -V  PARAMETERS o f  InP 
CELLS PRIOR TO F A B R I C A T I O N  OF MODULE 
E F F ,  
percent 
KK-125E 
-126C 
-127A 
-128C 
0.810 7.77 80 .9  11.87 
.811 7.78 81.2 11.94 
.813 7.85 81 .7  12.15 
.813 7.95 81 .5  12.27 
the known radiation flux of the orbit. 
Additional flight data is necessary to confirm 
the superior radiation tolerance of indium 
phosphide in the space radiation environment. 
6. REFERENCES 
Average 0.812 7.84 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
9. 
10. 
81.3  I 12.06 I 
Weinberg I, Swartz C K & Hart R E 1989, InP 
Solar Cell Research in the United States- 
Comparison with Non Photovoltaic Power 
Sources, this conference. 
Weinberg I & Brinker D J 1988, Progress in 
InP Solar Cell Research, Proc. 23rd Inter- 
society Energy Conversion Enaineering Conf, 
Denver, 1-5 August 1988, vol 3, 121-126. 
Parat K K, Bothra S, Borrego J M & Ghandhi 
S 1987, Solar Cells in Bulk InP, Made by an 
Open Tube Diffusion Process, Solid State 
Electronics vol 30, 283-289. 
Severns J G et a1 1988, The Experiments of 
Average 0.802 
LIPS 111. Proc. Space Photovoltaic Research 
& Technology Conf., Cleveland, 19-21 April 
1988, NASA, 331-348. 
7.70 78.8 11.32 
Severns, J G et a1 1988, LIPS I11 - A Solar 
Cell Test Bed in Space, Proc. 20th Photo- 
voltaic Specialists Conf., Las Vegas, 26-30 
September 1988, IEEE 88CH2527-0, 801-807. 
Weinberg I et a1 1987, Radiation and Tem- 
perature Effects in GaAs, InP and Si Solar 
Cells, Proc. 19th Photovoltaic Specialists 
Conf., New Orleans, 4-8 May 1987, IEEE 
87CH2400-0, 548-567. 
Weinberg I et a1 1987, Comparative Radi- 
ation Resistance, Temperature Dependence 
and Performance of Diffused Junction InP 
Solar Cells, Solar Cells, vol 22, 113-124. 
Yamaguchi M et a1 1984, Electron Irradi- 
ation Damage in Radiation-Resistant InP 
Solar Cells, Jap. J. Appl. Phvs.. vol 23, 
302-307. 
Tada H Y, Carter J R, Anspaugh B E & 
Handbook, JPL Publication 82-69. 
Ando K & Yamaguchi M 1985, Radiation Re- 
sistance on InP Solar Cells Under Light 
Illumination, Appl. Phys. Lett., vol 47, 
Downing R G 1982, Solar Cell Radiation 
846-848. 
0 800 7 86 78.2 11.45 I :  I :e02 I 7:47 I 78 .3  1 10.91 
.805 7.63 80 .2  11.47 I .800 I 7.83 1 78.4 I 11.44 1 
3 
TABLE 3 - TEMPERATURE DEPENDENCY TERMS - n*p I n P  
Open-circuit  v o l  t a a e ,  m V / K  . . . . . . . . . .  -2 .37  
Shor t - c i r cu i t  c u r r e n t ,  n A / c m 2 - K  . . . . .  +2.23x10-2 
F i l l  f a c t o r ,  percent/K . . . . . . . . .  - 5 . 6 6 ~ 1 0 - 2  
t 
i 
p ( Z n  : 4.6x1016/cm3) 
1 
SUBSTRATE 
3 
A 
Figure  1. InP Homojunction c e l l  f l i g h t  
module 
CELL NUMBER 4 
+ Figure  2 .  N P InP c e l l  s t r u c t u r e  
8 
DAY 
0 
0 39 
0 221 
U \ 
0 . 2  . 4  .6 .8 1.0 
VOLTAGE. V 
Figure  3.  F l i g h t  I-V cu rves  of c e l l  number 4 (Day 0 
is  p r e - f l i g h t  s imula to r  d a t a )  
ORIGINAL PAGE 
BLACK AND WHITE P!-”3TXRAPH 
4 
10 
8: 
7 -  
V 
+ Cell 4 . Cell 2 
'1  
0 1 OD 200 300 400 
Days From Launch 
Figure 4 .  I, vs. days from launch 
.8 I I I 1 1 1 1 1 1  I I I I I I I I I  I I I I I I I , ]  
1013 1014 1015 1016 
1 MEV ELECTRON FLUENCE, CM-2 
Figure 5. Normalized efficiency vs. 1 MeV 
electron fluence 
gz 
L U  
$ 8  I I I I I Ill1 I I I I I Ill1 I I I I ll1,l 
1013 1014 1015 10'6 
1 HEV ELECTRON FLUENCE, C M - ~  
Figure 6. Normalized open-circuit voltage vs. 
1 MeV electron fluence 
1 HEV ELECTRON FLUENCE. CH-L 
Figure 7. Normalized short-circuit current vs. 
1 MeV electron fluence 
5 
National Aeronautics and 
1. Report No. 
NASA TM-102131 
Report Documentation PaQe 
2. Government Accession No. 
7. Key Words (Suggested by Author@)) 
Indium phosphide 
Solar cells 
Flight experiment 
LIPS I11 spacecraft 
7. Author@) 
David J. Brinker 
18. Distribution Statement 
Unclassified - Unlimited 
Subject Category 44  
9. Performing Organization Name and Address 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 44135-3191 
3. Security Classif. (of this report) 
Unclassified 
2. Sponsoring Agency Name and Address 
National Aeronautics and Space Administration 
Washington, D.C. 20546-0001 
21. No of pages 22. Price' 20. Security Classif. (of this page) 
Unclassified 6 A02 
3. Recipient's Catalog No. 
5. Report Date 
6. Performing Organization Code 
8. Performing Organization Report No. 
E-492 1 
10. Work Unit No. 
506-41-1 1 
11.  Contract or Grant No. 
13. Type of Report and Period Covered 
Technical Memorandum 
14. Sponsoring Agency Code 
5. Supplementary Notes 
Prepared for the European Space Power Conference sponsored by the European Space Agency, Madrid, Spain, 
October 2-6, 1989. 
6. Abstract 
The flight performance of NASA Lewis Research Center's indium phosphide homojunction solar cell module on the 
LIPS 111 satellite is presented. A module of four n+p cells was fabricated and has been on-orbit on the LIPS I11 
spacecraft since 1987. The experimental objective is the measurement of InP cell performance in the natural 
radiation environment of the 1100 kilometer altitude, 60" inclination, circular orbit. Flight data from the first 
year is near expected values, with no degradation in short-circuit current. Included in the paper is the temperature 
dependence of current-voltage parameters and the laboratory radiation tolerance studies necessary for normalization 
and analysis of the data. Details of the cell structure and flight module design are also discussed. 
*For sale by the National Technical Information Service, Springfield, Virginia 221 61 NASA FORM 1626 OCT 86 


On-orbit results of the LIPS 3/InP homojunction solar cell experiment

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