Nuclear Instruments and Methods in Physics Research B 307 (2013) 362–365
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Nuclear Instruments and Methods in Physics Research B
journal homepage: www.elsevier.com/locate/nimb
Displacement damage dose approach to predict performance
degradation of on-orbit GaInP/GaAs/Ge solar cells
Ming Lu, Rong Wang ⇑, Yunhong Liu, Zhao Feng, Zhaolei Han, Chunyu Hou
Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology,
Beijing Normal University, Beijing 100875, PR China
a r t i c l e
i n f o
Article history:
Received 23 August 2012
Received in revised form 8 November 2012
Accepted 13 November 2012
Available online 25 January 2013
Keywords:
GaInP/GaAs/Ge solar cells
Displacement damage dose
Non-ionizing energy loss
Proton irradiation
Electron irradiation
a b s t r a c t
The displacement damage dose approach for analyzing and modeling the performance degradation of
triple-junction GaInP/GaAs/Ge solar cells in a space radiation environment is presented. The irradiation
effects of protons and electrons on GaInP/GaAs/Ge solar cells are analysed and then correlated with
the displacement damage dose. On this basis, on-orbit expected mission lifetime of GaInP/GaAs/Ge solar
cells shielded with silica coverglass at various thicknesses in circular orbits of 5000 km with 60° inclination and 20,000 km with 0° inclination is predicted, respectively.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments and results
The space radiation environment is a dynamic mixture of protons and electrons that varies with orbital altitude and inclination.
Exposure to these energetic charged particles typically degrades
the electrical performance of solar cells. Therefore, it is necessary
to investigate solar cells radiation effects and to predict the on-orbit expected mission lifetime of solar cells.
The displacement damage dose (Dd) approach [1], developed at
the U.S. Naval Research Laboratory (NRL), provides a means for
predicting on-orbit cell performance from a minimum of groundtest data. Dd equals the product (Dd = UNIEL) of particle fluence
U and the respective non-ionizing energy loss (NIEL) which here
refers to the rate of energy loss caused by atomic displacements.
Thus, the displacement damage effects on solar cells for protons
and electrons with different energies and fluences can be correlated with Dd.
Our works had shown the radiation effects on homemade
GaInP/GaAs/Ge solar cells with protons and electrons ground-radiation tests [2–5]. The aim of this study is to predict on-orbit homemade GaInP/GaAs/Ge expected mission lifetime in an actual space
radiation environment from the ground-test data using the Dd
approach.
GaInP/GaAs/Ge space solar cells were fabricated by metalorganic chemical vapor deposition (MOCVD). Solar cells mainly consist
of three sub-cells: GaInP top cell, GaAs middle cell, and Ge bottom
cell. Their dimensions are respectively about 1.2, 2.9, and 176 lm
in thickness. The detailed structure of the solar cells is shown in
Ref. [2].
The GaInP/GaAs/Ge 3 J solar cells were irradiated with 0.32,
1.00, and 3.00 MeV protons and 1.0, 1.8, and 11.5 MeV electrons,
respectively. The fluence ranged from 3 109 to 1 1012 cm2
for protons and 1 1012 to 3 1015 cm2 for electrons. I-V characteristics of the solar cells before and after irradiations were
measured at 25 °C under AM0 using a solar simulator with an illumination of 136.7 mWcm2.
The measured results for maximum power (Pmax) degradation
of GaInP/GaAs/Ge 3 J solar cells are shown in Fig. 1. The set of solid
curves on the right of Fig. 1 are the original protons and electrons
datum plotted against fluence using the abscissa along the top of
the figure. The superposed dot curve on the left of Fig. 1 is plotted
versus Dd using the below abscissa. The essence of the Dd method is
the calculation of the nonionizing energy loss (NIEL) as a function
of either protons or electrons energy for a cell material. Because
GaInP/GaAs/Ge solar cells degradation is primarily controlled by
the radiation response of the GaAs sub-cell [2,3], thus the NIEL(E)
of GaAs material was used to calculate Dd. For the proton calculation, the adjusted NIEL(E) has been adopted [4], because protons
⇑ Corresponding author.
E-mail address: wangr@bnu.edu.cn (R. Wang).
0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.nimb.2012.11.060
363
M. Lu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 362–365
0.8
curves, an electron to proton damage ratio (Rep) is applied, Rep is
defined as the ratio (=0.406) of Dxp/Dxe. The electron curve by multiplying the parameter Rep can be made to coincide with the proton
curve [5]. Thus, the superposed dot curve on the left of Fig. 1 shows
that all the measured electrons and protons datum can be represented by a single characteristic degradation curve using the Dd
approach.
0.7
3. Slowed-down proton and electron differential spectra
0.6
The slowed-down spectra shown in Fig. 2 and Fig. 3 were calculated by applying the continuous slowing down approximation
[8,9] and the proton and electron differential fluence spectra in
different orbit. The differential fluence spectra from the NASA
space model AP8MAX [10] and AE8MAX [10] were employed, corresponding to a circular orbit at 5000 km with 60° inclination in
Van Allen inner radiation belt and 20000 km with 0° inclination
in Van Allen outer radiation belt after five years. For omnidirectional proton radiation, the expression for the slowed-down differential energy spectrum f(e)p with taking into account all angle of an
omnidirectional flux, is as follows:
-2
Particle Fluence ( cm )
1E8
1.0
1E9
1E10
1E11
1E12
1E13
1E14
1E15
1E16
1E17
Normalized Maximum Power
0.9
0.5
Protons
0.32 MeV
1.00 MeV
3.00 MeV
0.4
0.3
1E8
1E9
1E10
1E11
1E12
1E13
Electrons
1.0 MeV
1.8 MeV
11.5 MeV
1E14
1E15
1E16
1E17
Displacement Damage Dose (MeV/g)
Fig. 1. The set of solid curves on the right are the original data against fluence,
while the dot curves on the left is against Dd.
f ðeÞp ¼
Table 1
Fitting parameters for proton and electron Dd curves of Pmax degradation.
Protons irradiation
k
0.27
Electrons irradiation
Dx (MeV/g)
2.46 109
k
0.27
Dx (MeV/g)
6.06 109
with relatively low energy can produce a non-uniform vacancies
production rate distribution at the active region of the GaAs subcell. For the electron calculation, the actual dose is redefined in
term of an equivalent dose, because electrons with sufficient
energy usually produce point defects and do not produce recoils
to generate the cascade defects caused by protons [6]. Thus, each
data set about electrons and protons is described by a single characteristic degradation curve given by the following expression [7]:
P max
Dd
¼ 1 k log 1 þ
Pmax0
Dx
ð1Þ
where Pmax0 and Pmax are the maximum power of solar cells before
and after irradiation, and k and Dx are fitting parameters whose
values are shown in Table 1. To collapse the electron and proton
1E17
-1
(
-2
Z p=2
b1
AaEa1 þ BbE
dh
Aaea1 þ Bbeb1
sin h cos h
0
ð2Þ
where g(E)p is the incident proton differential energy spectrum, E is
the incident energy, e is the energy emerging through the shield
coverglass, h is the angle between incident direction and normal
to the surface at the point of incidence, there are constants for
A = 7.019 a = 0.8542, B = 7.925, b = 1.824 from a fit of equation
R(E)=AEa + BEb to experimental measurements of the energy dependence of the range of protons in silica [8]. For omnidirectional
electron radiation, the f(e)e is as follows:
f ðeÞe ¼
gðEÞe
2
Z p=2
sin h cos h
0
aebþc ln e ðc lne e þ bþceln eÞ
aEbþc ln E ðc lnE E þ bþcEln EÞ
dh
Uncovered
25µm
Uncovered
1E15
500µm
1E14
1500µm
25µm
1E13
)
1E12
1E11
500µm
1E10
1500µm
1E9
1E7
1E-4
Electron spectra
Proton spectra
1E8
1E-3
0.01
0.1
1
10
Proton Energy (MeV)
100
ð3Þ
where g(E)e are the incident electron differential energy spectrum, E
is the incident energy, e is the energy emerging through the coverglass, h is the angle between incident direction and the normal to
the surface, there are constants for a = 2145.14, b = 1.22617,
c = 0.06936 from a fit of equation R(E)=aEb+clnE to experimental
measurements of the energy dependence of the range of electrons
in silica as compiled by Seltzer et al., and tabulated in the computer
code ESTAR [11].
The slowed-down proton and electron differential spectra with
different silica coverglass thicknesses were calculated by
5 year mission, 5000km,circular orbit, 60o Inclination
1E16
Differential Fluence cm MeV
gðEÞp
2
1E-4
1E-3
0.01
0.1
1
10
Electron Energy (MeV)
Fig. 2. Incident and slowed-down proton and electron differential spectra through silica coverglasses at 5000 km with 60° inclination after 5 years.
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M. Lu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 362–365
1E17
1E16
5 year mission
Uncovered
Uncovered
25µm
20000km,circular orbit
-2
Differential Fluence cm MeV
-1
(
1E15
500µm
0o Inclination
1E14
25µm
1500µm
)
1E13
1E12
75µm
1E11
1E10
1E9
150µm
1E8
Electron spectra
Proton spectra
1E7
1E-4
1E-3
0.01
0.1
1
1E-4
1E-3
Proton Energy (MeV)
0.01
0.1
1
10
Electron Energy (MeV)
Fig. 3. Incident and slowed-down proton and electron differential spectra through silica coverglasses at 20,000 km with 0° inclination after 5 years.
1.0
1E13
o
o
Electrons
Electrons
Protons
Protons
Total
Total
0.9
1E12
Normalized Maximum Power
Displacement Damage Dose (MeV/g)
5 years mission 20000km, 0 Inclination 5000km, 60 Inclination
1E11
1E10
1E9
0.8
0.7
0.6
0.5
0.4
5 years mission
0.3
5000km,60°Inclination
20000km,0°Inclination
0.2
1E8
0.1
0.0
1E7
0
200
400
600
800
1000
1200
1400
1600
0
200
numerically integrating expression (2) and (3), respectively. Figs. 2
and 3 show the results corresponding to a circular orbit at 5000 km
with 60° inclination and 20,000 km with 0° inclination after five
years, respectively.
4. Total displacement damage dose deposited
Z
duðEp Þ
duðEe Þ
NIELðEp ÞdEp þ Rep
dEp
dEe
n1
NIELðEe Þ
NIELðEe Þ
dEe
NIELð1MeVÞ
800
1000
1200
1400
1600
Fig. 5. Pmax as a function of the silica coverglass thickness after 5 years.
of Dd deposited in a solar cell. One can see from Fig. 4 that the displacement damage dose induced with protons is dominated, and
the effects of electrons irradiation are neglected in 5000 km orbit.
On the contrary, the effects of electrons are far greater than that of
protons in 20,000 km orbit, when the coverglass thickness exceed
approximately 100 lm.
5. Assessment mission lifetime of on-orbit solar cells
Total displacement damage dose deposited in GaAs sub-cell in
the 5000 km and 20,000 km orbit was calculated as a function of
silica shield coverglass thickness by expression (4) [12],
Dd ¼
600
SiO2 Thickness ( µm )
SiO2 Thickness (µm)
Fig. 4. Total displacement damage dose in solar cells as a function of silica
coverglass thickness after 5 years in a 5000 and 20,000 km circular orbits.
400
Z
ð4Þ
The end of life (EOL) values of Pmax were predicted by correlation the degradation characteristic curve in Fig. 1 with the total
displacement damage dose in Fig. 4. The results are shown in
Fig. 5, which shows the Pmax increases with the increase of coverglass thickness. However, the increase of the coverglass thickness
will result in the increase of the weight of a solar array. Thus, a
proper coverglass thickness should be chosen by a tradeoff
between the higher Pmax and the lighter weight.
duðE Þ
where dEpp and dudEðEee Þ are the slowed down proton and electron differential spectra, NIEL(Ep) and NIEL(Ee) are the NIEL for proton and
electron of energy E, respectively and NIEL(1 MeV) the NIEL for
1 MeV electron. Rep equals 0.406, and the exponent value of n
equals 1.29.
The calculated results are shown in Fig. 4. As expected, the
thicker shields have the effect of significantly reducing the value
6. Conclusions
The radiation response of homemade GaInP/GaAs/Ge solar cells
has been analysed in detail, and the degradation at different proton
and electron energies and fluences has been correlated in terms of
Dd. The assessment mission lifetime of on orbit GaInP/GaAs/Ge
M. Lu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 362–365
solar cells has been achieved from the ground-test data using the
Dd approach.
Acknowledgments
This work was supported by The National Natural Science Foundation of China under Grant Nos.10675023, 11075018 and by The
Fundamental Research Funds for the Central Universities.
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