Life Prediction of the GaInP/GaAs/Ge Triple-junction
Solar Cells in Space
Jieru Meng, Jing Feng*, Quan Sun, Zhengqiang Pan, Tianyu Liu
College of Information System and Management
National University of Defense Technology
Changsha, Hunan, P.R. China, 410073.
fengjing@nudt.edu.cn

Abstract—GaAs solar cells are being used in the space
industry due to their good properties. It is thus essential to
investigate their life prediction for better use. We focused on the
research of establishing an effective model to calculate the
damping of the lifetime of the GaInP/GaAs/Ge triple-junction
solar cells in the space. First of all, the interrelated telemetry
parameters of the GaInP/GaAs/Ge 3J solar cells are obtained,
then the output current is selected as the crucial performance
parameter to analyze the degradation of cells, a mathematical
model are established by analyzing the variation tendency of the
telemetric current. Finally, the predicted lifetime of 3J solar
cells was obtained. It provided a theoretical way to analyze 3J
solar cells used on satellite, which is a necessary and basic work
for the life prediction of other modern cells, single, double or
triple junctions made of GaInP, GaAs and Ge materials in the
space.
Keywords: GaInP/GaAs/Ge 3J solar cells, current, life
prediction.
I. INTRODUCTION
GaAs solar cells are being used widely in the space because
of their good characteristic such as high quantum efficiency
and good irradiation tolerance. As we all know, it can be
considered the most abundant renewable source of energy
available to guarantee space satellites’ better mission,
especially the GaInP/GaAs/Ge triple-junction solar cells have
become the mainstream at present, having very high
conversion efficiency and also high radiation hardness.
Therefore, appropriate lifetime prediction of the 3J solar cells
is useful to guarantee that they can provide more stable and
abundant source for satellites[1].
However, for the reason that the 3J solar cells are directly
exposed to the harsh space environment, they have to
withstand the alternate effects of the mutable temperatures,
the corrosion from atomic oxygen, large amounts of
radiations etc. As a result, their electrical performance
degrades seriously. This causes a serious problem for the
power supplies of satellites operating in space[2]-[[8]. So, it is
important to predict the lifetime of the in space from some
experimental results as accurately as possible. Then we can
make decisions that how long the 3J solar cells can offer to the
satellites and how to maintain the satellites.
Presently, there are many published papers which have
reported on different fields.
Mostly, many scholars noted that the 3J solar cells for
space use are required to have radiation hardness more often.
Focusing on degradation behavior due to proton or electron
irradiation, there are two representative lifetime prediction
methods for space solar cells, the equivalent fluence method
and the displacement damage dose method[9]. The first
method was to determine the equivalent of 1 MeV electron
fluence, which would cause the same damage to the cell as the
specified space environment. The second method was to
calculate the displacement damage dose for a given mission
has been determined, on the basis of the energy dependence of
the damage coefficients.
Moreover, Summer et al.[10] had reviewed the degradation
effect of low-energy protons in space orbits. In the meantime,
Hu Jianmin[1] found that low-energy proton or electron
radiation would result in more serious degradation of the solar
cell than that of higher-energy proton (>200 keV) radiation. A
fact that the degradation behavior of solar cells from proton
irradiation is more serious than that from electrons was
carried out by Wang R[11].
Furthermore, some temperature accelerated life tests have
been conducted to evaluate the reliability of GaAs solar cells.
For instance, Núñez N[12] extrapolated the evolution of the
relative power at any temperature of solar cell operation by
the value that was calculated for III–V terrestrial concentrator
solar cells.
Nevertheless, there is no report about a systematic model to
describe the damping of the lifetime of the 3J solar cells as far
as we investigated, so it is no use to do remaining life
prediction of the 3J solar cells. On the one hand, the
degradation happens in an extremely complicated
environment, a lots of complex factors interacted with each
other. On the other hand, which parameter choose to describe
the tendency of the damping can not be determined solidly.
This paper provides a theoretical way combined with the
real situation the 3J solar cells suffered in the space. We
obtained several telemetric parameters of the 3J solar cells in
the space in section 2. Then we chose current, which can best
illustrate the performance degradation of cells, as the key
parameter to set up a useful model in section 3. Establishing a
degradation model to calculate the lifetime when the 3J solar
cells works for different years in section 4. Finally, the
conclusion is given in the last section
II. ACQUISITION AND PROCESSING OF EXPERIMENTAL DATA
A. Experiment and original data acquisition
Firstly, the 3J solar cells are installed on the satellite, and
then we monitor the satellite’s on-orbit behaviors in order to
get the relevant data of the solar cells for three years. we have
already acquired the original experimental data for some
different types, that are the telemetric parameters, such as the
original output current and solar illumination angle cosine
values. We obtain the three aspects data every day during
three years, which is shown in Fig.1(a)-(b).
0.205
0.2
The original current (mA)
0.195
0.19
0.185
P represents the solar cell output power,
V is the output voltage, V  is the bus voltage, V D 1 is the
By definition,
0.18
0.175
0.17
average potential drop of the electric cable and connector,
0.165
VD 2
is the average potential drop of isolation diode, and I is
the output current.
If the satellite’s bus voltage is unchanged, the attenuation
of the solar cell output power is mainly reflected by the
attenuation of the solar cell output current according to
equation (2).That is, the larger is the solar cell output power,
the higher is the photoelectric conversion efficiency. In a
word, the output current can directly indicate the main
performance of the 3J solar cells, it is thus determined as the
degradation performance of the 3J solar cells in this paper.
0.16
0.155
0
200
400
600
Time (d)
800
1000
1200
Fig. 1(a) The original current
The solar illumination angle cosine value
1
0.95
0.9
0.85
The output power of solar cell is the most important
property itself. Moreover, the output current is an essential
indicator to measure the solar cell power. This is because the
formula of the solar cell output power is[13][14]:
P  I V
(2)
V  V  VD1  VD 2
0
50
100
150
200
Time（ d）
250
300
350
IV. DEGRADATION MODEL
400
Fig. 1(b) The solar illumination angle cosine values
B. Data pre-processing and the result
When the satellites make missions in the space, it is
evident that the 3J solar cells can convert the solar energy
into the useful power to make sure the normal tasks.
However, during the process, the relationship between the
current and the solar incidence angle can be implied by the
following formula:
(1)
I max  I / cos 
I max is the current of vertical incidence angle, I is the
original output current, and cos  is the solar illumination
angle cosine value.
Given that, the everyday current of vertical incidence
angle of the 3J solar cells is calculated throughout a time
period of three years. Then the original current data after
above pre-processing is shown in Fig. 2.
A. Modeling Procedure
It is confirmed that solar radiation energy is most important
factors of which influence the lifetime of solar cells[11]. It is
also reported that the solar radiation intensity varies annually
which is subordinated to a cosine function or so, as seen in
Fig. 3[15][16]. So a hypothesis should be made that solar
radiation intensity can be expressed as follows:
2
(3)
S  S 0  c  cos( 
 t)
T
In which the aspection of the solar radiation intensity is
consistent with cosine function. Here, the solar radiation
constant( S 0 ), namely Galileo data, is1353 W/m2
35
30
25
20
The everyday current of vertical incidence angle (mA)
0.2
15
0.195
10
5
0.19
0
0
200
400
600
800
1000
1200
0.185
Fig.3 Variations of the solar radiation intensity
0.18
0.175
0
200
400
600
Time （ d）
800
1000
1200
Fig. 2. The everyday current of vertical incidence angle of the 3J solar cells
during three years.
III. DEGRADATION FEATURE DETERMINATION
A very important procedure of degradation modeling is
to determine the degradation feature. Here, the reasons why
we choose the output current as degradation feature will be
discussed below.
In conclusion, we have demonstrated that the degradation
of the 3J solar cells can be accounted by the attenuation of the
current of vertical incidence angle, which is mostly dependent
on the solar radiation intensity.
Liu Zhen[17] calculated the total output current of solar
array through numerical statistic and mathematical analysis ,
which looks approximately as a linear relationship. Based on
this, the model we proceed to compute the degradation model
of the current of vertical incidence angle is therefore as
follows:
D(t )  I max  (a  b  t )  (
In this expression, (a 
rate of the current,
S
S0
S
) 
S0
(4)
b  t ) represents the attenuation
means the influence coefficient of the
solar radiation intensity, and I max is the current of vertical
incidence angle, i.e.,   N (0, )
Taking the formula (3) into (4), the parameters in this
expression are as follows: t is the time in orbit, a,b ,c are
2
model parameters, a  0 means the original current of
vertical incidence angle when t  0 ; b  0 means the
attenuation rate of the current one day; c, all stand for the
periodic diversification of the solar radiation intensity.
B. Parameter Estimation
In order to enhance the precision evaluation result, we
should estimate the parameters of the model according to the
measure data, i.e., the current value of vertical incidence
angle calculated. By definition:
RSS 
n
 (D(ti )  (a  b  t ) 
i 1
S 2
) 
S0
n


i
Point estimation
0.1937
-2.91e-6
32.3
The current
0
0.1983
1
0.1972
2
0.1961
3
0.1950
4
0.1940
5
0.1929
8
0.1896
The calculating data in Table 2 indicate that the decline rate
of the current of vertical incidence angle is similar every year,
which is 0.5% for one year.
If the current threshold is given, a judgment and
computation can be done to predict the lifetime of the 3J solar
cells. However, for the sake of the regular mission of satellite,
there must be abundant energy provided by the solar cells, so
in a general situation, the output current is obliged to a secure
extent. Once beyond, the 3J solar cells are definite to be
renewed to maintain the normal work. So the degradation
model can be used to predict the cells' remaining useful
lifetime.
2
V. CONCLUSION
1
(5)
After minimizing RSS, we can obtain the evaluated
parameters. Accordingly, the parameter estimates for each of
the models appear in Table 1, and the experimental data used
for the validation of the models are described in Fig. 4. The
picture shows the original points (blue solid line, the proposed
curve to fit the original points (red solid line ) and the
predicted current data for the next three years(black solid line
).
TABLE I: THE RESULT OF PARAMETER ESTIMATION
Parameter
a
c
b
Time on-orbit

-0.06966
0.2
0.198
0.196
The proposed method of describing the damping of the 3J
solar cells’ output current plays a key role for researching on
its life prediction.
The key performance parameter which we choose to build
the model is based on effective theories. Considering the
complex space environment comprehensively and three years
of the telemetry data from the real satellite make the results
more accurate. Therefore we can monitor and maintain the
solar cells in order to make sure that they provide abundant
power for satellites in the space.
Moreover, it served as a guide for us to do research of other
GeAs solar cells, whose performance degradation can be
depicted by the decay of the output current. In spite of these,
future research should analyze more factors influencing the
solar cells respectively, only by this can we create a model
which is much more comprehensive and can applied to all
kinds of solar cells.
0.194
Current
(mA)
0.192
ACKNOWLEDGMENT
0.19
0.188
This work is supported by the Chinese National Science
Foundation grant 61273041 and 71271212.
0.186
0.184
0.182
0
500
1000
1500
2000
2500
Time(d)
Fig. 4 The fit by the power function
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Based on experimental parameters, we calculated the current
[2]
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[3]
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[4]
given in Table 2.
TABLE II: THE PREDICTED RESULTS OF THE CURRENT VALUE OF
VERTICAL INCIDENCE ANGLE
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