Maximum power output for the GaP cells of this study was found to be on the order of 1 microW. This resulted from exposure to 200 and 40 KeV electrons at a flux of 2 x 10(exp 9) electrons/sq cm/s, equivalent to a 54 mCurie source. The efficiencies of the cells ranged from 5 to 9 percent for 200 and 40 KeV electrons respectively. The lower efficiency at higher energy is due to a substantial fraction of energy deposition in the substrate, further than a diffusion length from the depletion region of the cell. Radiation damage was clearly observed in GaP after exposure to 200 KeV electrons at a fluence of 2 x 10(exp 12) electrons/sq cm. No discernable damage was observed after exposure to 40 KeV electrons at the same fluence. Analysis indicates that a GaP betavoltaic system would not be practical if limited to low energy beta sources. The power available would be too low even in the ideal case. By utilizing high activity beta sources, such as Sr-90/Y-90, it may be possible to achieve performance that could be suitable for some space power applications. However, to utilize such a source the problem of radiation damage in the beta cell material must be overcome

Anspaugh, B.

Pool, F. S.

Stella, Paul M.

English

NASA Technical Reports Server

N91-19211
GaP Betavoltaic Cells as a Power Source*
F.S. Pool, P. Stella, B. Anspaugh
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California
Introduction
Since the launch of the very first satellite, photovoltaics has been the most widely
used spacecraft power source. Used not only for earth orbiters, but also for a wide
range of interplanetary applications such as a Mercury flyby, Venus orbiters, lunar
surface power, and Mars orbiters, photovoltaic systems have demonstrated great ver-
satility. However, photovoltaics are limited by the availability of solar illumination.
For missions that operate far from the sun and for those with lengthy non illuminated
periods, significant system compromises are required. In some cases the use of non PV
power sources, such as the RTG (radio-isotope thermoelectric generator), has been
implemented. Extending the range of applications of PV to these more difficult situ-
ations has been one of the challenges for PV technology development. Much of that
effort has addressed methods for reducing photovoltaic system mass and increasing
system conversion efficiency.
For many years the workhorse of space photovoltaics, the silicon solar cell, has
been reliably used to power heart pacers in the form of betavoltaics [ref. 1]. The beta
cell functions analogously to a solar cell, with the radiant energy of the sun replaced
by the emitted electron flux. The emitted high energy electrons (beta particles) from
the radioactive source traverse the cell material losing energy and thereby creating
electron-hole pairs [ref. 2]. Those carriers within a diffusion length of the junction
will be swept across contributing a current. An equivalent circuit for the device is
identical to that for a solar cell. Such converters are free from the limitations of a
solar light source and can operate where conventional photovoltaics are least capable.
During the past decade the use of microelectronic fabrication methods has al-
lowed the fabrication of a diverse variety of advanced solar cell structures. In view
of this progress, an investigation was undertaken in order to ascertain if a significant
improvement in betavoltaic conversion efficiency could be realized. In particular the
use of a GaP device was evaluated experimentally; GaP is an indirect wide-gap semi-
conductor [ref. 3]. The purpose of this study was to define the power limitations of
current GaP beta cells and examine the efficiency and extent of radiation damage in
GaP at the energies of interest. '
* This work was performed by the Jet Propulsion Laboratory, California Institute of
Technology, under contract to the National Aeronautics and Space Administration.
359
https://ntrs.nasa.gov/search.jsp?R=19910009898 2020-03-19T18:46:53+00:00Z
Experimental
The GaP cells used in this study, shown schematically in Figure 1, were fabricated
by Astropower Corporation. GaP was chosen as a cell material because of its wide
band gap. It has been shown that device efficiency is a function of band gap, with
wide-gap materials possessing higher efficiencies [refs. 1-2]. The GaP cells are a
lcm x lcm p/n structure grown by liquid phase epitaxy. As a comparison to the
performance of the GaP beta cells, Si back surface field 2 cm x 2cm n/p solar cells
fabricated by ASEC were exposed to identical conditions.
To approximately evaluate cell performance the average beta energy E_ was cho-
sen for two emitters, Ni 6a (E_ = 20 KeV, half-life 92 years) and Sr9°/Y 9° (E._ = 200
NeV/E 3 = 900 KeV, half-life 28 years and 62 hours respectively). The latter radioac-
tive source Sr 9° decays to a daughter nucleus y90 which also undergoes beta decay,
hence the notation Srg°/Y 9°. The Jet Propulsion Laboratory Dynamitron electron
accelerator was used as the source of monoenergetic incident electrons at 200 KeV
and 40 KeV. A flux • = 2 x l09 electron/cm2s, equivalent to a 54 mCurie source ( 1
Curie = 3.7 x 10 l° disintegrations/s), was directed on the cells.
Betavoltaic I-V data was collected with a Tektronix 4052 computer interfaced
to the experiment during exposure. Cell efficiencies were calculated as the ratio of
maximum power output of the cell to incident power of the electrons
r/ = Pmax/Pe
where Pe is the product of the electron flux, energy and cell area. All other parameters
such as fill factor, open circuit voltage and short circuit current were determined in
the usual way.
Results
In Figures 2 and 3 betavoltaic I-V curves are given for GaP beta cells and Si solar
cells. The solid lines refer to the initial exposure and the dotted lines represent cell
response after an exposure to a total fluence of 2 x 1012 electrons/era 2. A summary
of the cell characteristics is given in Tables 1-4.
The efficiency of the GaP cells was found to be 9 % at 40 KeV and 5_, with
exposure to 200 I(eV electrons. The reason for the lower device efficiency at 200 KeV
is due to the range of the electrons in GaP. At 40 KeV the electrons are stopped
within 25 pro, depositing all energy in the active region of the cell. At. 200 I(e\: the
incident electrons penetrate approximately 160 pm, depositing a substantial fi'action
of energy in the substrate, further than a diffusion length ( approximately 20 pro)
from the depletion region of the cell. A colnparison of the GaP cell efficiency to that of
Si during exposure demonstrates the effectiveness of GaP over Si in this application.
Typical Si heart pacer betavoltaic systems have etIiciencies on the order of only I<X>.
360 C-
Power output of the GaP cells was found to be 3.2 #W and 1.2 ttW under exposure
to 200 KeV and 40 KeV electrons respectively. The limitation in the power output of
the cells is primarily a function of the low incident power on the cells. The electron
power at a flux of 2 x 109/cm2s is only 65 yW at 200 KeV and 13 tlW at 40 KeV.
The flux was chosen to represent that from a ideal thin-film source of Ni 6a, 1 cm x 1
cm in area and 1 micron thick. A thin-film of the radioactive source is necessary to
minimize weight and avoid self-absorption of the emitted beta particles by the source
material.
To examine degradation of the cells, betavoltaic I-V data was taken after a pro-
longed exposure of 1000 seconds at the flux of 2 x 109/cm2s and compared to the
initial data. In addition, standard solar I-V and spectral response data were taken
as a diagnostic tool to evaluate the cells for radiation damage. GaP suffers a signifi-
cant degradation in performance after exposure to 200 KeV electrons at a fluence of
2 x 1012/cm 2. The beta I-V curve shown in Figure 3 shows a 32% decrease in Isc.
Cell degradation is also shown in Figures 4 and 5. A decrease in spectral response is
observed at wavelengths greater than 0.4a t,m, while the solar I-V shows a reduced
Isc of approximately 25%, consistent with the reduced Isc of the beta I-V shown in
Figure 3.
By contrast the exposure at 40 KeV produced no observable damage in GaP.
The dotted line of Figure ab is within the error of the experiment. The solar I-V
and spectral response data also show no discernable reduction in device performance.
The threshold for electron radiation damage in GaP may then be set in the range 40
KeV < E < 200 KeV.
Given the above data it is useful to discuss a model for a power source based on
betavoltaics. A system capable of producing power on the order of a few watts is
most practical using a high energy beta emitter such as Sr9°/Y 9°. The fundamental
limitation is in the power available from the radioactive source and radiation damage
to the cell. Due to the density of nickel and low beta energy the maximum source
power ideally achieved with Ni 6a would be on the order of 10 t_W. However, Sr 9°
is much less dense (2.6 gm /cm a) and represents an order of magnitude increase in
the beta particle energy spectrum. It has been shown that if the source thickness
for Sr9°/Y 9° is approximately 1/8 the range of the average energy beta particle (200
KeV) self-absorption may be neglected [ref. 4]. Since Sr 9° is readily available with
activities of 75 Curies/gm, a 1 cm x lcm source 35 ttm thick would have an activity
of about 1 Curie.
For a system based on Sr9°/Y 9°, an estimate of the power available may be made
from the average energy of the two emitted Sr9°/Y 9° betas i.e., 550 KeV. A 1 Curie
source will produce roughly 3 mW of power. Assuming an efficiency of 9% a cell
output power of 0.3 mW would result. To produce 5 watts of power about 16,000
cells would be required. Excluding packaging this system would weigh about 3 kg
and occupy a volume of 700 cm a. Although not an advantageous system in terms of
361
specific power it represents a battery capable of delivering several watts of power over
a period of 25 to 50 years without recharging.
Even with higher cell efficiencies, which are certainly possible, betavoltaic system
specific power will still remain low, especially when compared to typical space PV
systems. However, it is not intended that betavoltaics compete directly with pho-
tovoltaics. Instead the betavoltaic option is better suited to compete with R3'Gs or
batteries. For the latter case, betavoltaics would best match requirements for a long
lived, low power, continuous power source.
A possible solution to the problem of radiation damage is to operate the cell at
annealing temperatures. A GaP cell could operate at an elevated temperature due to
dissipation of radioactive energy not accessed by the device. If annealing properties
are rapid in GaP the cells could operate for extended periods under exposure to the
high energy electrons discussed above, tIowever, at this time annealing properties of
GaP are not known. The possibility of other radiation hard wide-gap materials has
not been investigated and could lead to the use of novel materials for this application.
Conclusions
Maximum power output for the GaP cells of this study was found to be on the
order of 1 IL\¥. This resulted from exposure to 200 KeV and 40 KeV electrons at a
flux of 2 x 109 electrons/cm2s, equivalent to a 54 mCurie source. The efficiencies of
the cells ranged from 5% to 9% for 200 KeV and 40 KeV electrons respectively. The
lower efficiency at higher energy is due to a substantial fraction of energy deposition
in the substrate, further than a diffusion length from the depletion region of the cell.
Radiation damage was clearly observed in GaP after exposure to 200 KeV electrons
at a fluence of 2 x 1012 electrons/cm 2. No discernable damage was observed after
exposure to 40 KeV electrons at the same fluence.
Analysis indicates that a GaP betavoltaic system would not be practical if limited
to low energy beta sources (E0 < 100 KeV). The power available would be too low
even in tile ideal case. By utilizing high activity bet a sources, such as Srg°/Y 9°, it
may be possible to achieve performance that could be suitable for some space power
applications. However, to utilize such a source the problem of radiation damage in
the beta cell material must l:,e overcome.
362
References
[ 1.] L. C. Olson, Energy Conversion13, 117, 1972.
[ 2.] L. C. Olson, 9th Intersociety Energy Conversion Engineering CoT_fer¢nce,
1974.
[ 3.] W. G. Pfann and W. van Roosbroeck, J. Appl. Phys., 25, 1422,195.i.
[ 4.] M. B. Panish and H. C. Casey, J. Appl. Phys., 40, 163, 1969.
363
Table I. - GaP Beta I-V Characteristics
Fluence = 2 x 109 electrons/cm2s
cell
i0
Ee(KeV)
200
4O
Jsc (_A/cm2 ) Voc(V ) FF Eff(%)
1.2 0.65 5
1.0 0.51 9
Table 2. - Beta I-V Characteristics GaP
Fluence = 2 x 1012 electrons/cm2s
cell
I0
Ee(KeV)
200
4O
Jsc (_A/cm2 ) Voc (v)
1.2
1.0
FF
0.64
0.52
Eff(%)
3
9
364
Table 3. - Si Beta I-V Characteristics
Fluence = 2 x 109 electrons/cm2s
cell
119
141
Ee(KeV)
200
40
Jsc (_A/cm2 )
29
Voc(V)
128
64
FF
0.38
0.25
Eff(%)
2
1
Table 4. - Si Beta I-V Characteristics
Fluence = 2 x 1012 electrons/cm2s
cell Ee(KeV ) Jsc(_A/cm 2) Voc(V ) FF Eff(%)
28119
141
200
40
84 0.33 1
60 0.26 1
365
GRID LINES-_
p- GaP
/.---.---- ANTI-RECOMBINATION
LAYER
I,./////////itit/lllliliti/l/i//_91--_ BACK CONTACT
SILVER PAD
Figure I. A schematic of the GaP beta cell structure.
366
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BETAVOLTAIC I-V Si
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¢_= 2xl 09/cm2s
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(D= 2xl 09/cm2s
t=Osec
..... t = 1000 sec
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----.---. t= Osec
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0 0.02 0.04 0.06
I I I
0.08 0.10 0.12 0.14
Voltage (V)
Figure 2. Betavoltaic I-V of si solar cells for (a) 200 KeY and
(b) 40 KeY electron exposures at a flux of 2 x 109
electrons/cm2s.
367
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0.004
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0.002
0.001
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0.003
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BETAVOLTAIC I-V GaP
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Figure 3. Betavoltaic I-V for GaP beta cells for (a) 200 KeV and
(b) 40 KeV electron exposures at a flux of 2 x 109
electons/cm2s.
368
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Spectral response of GaP beta cells before and after
exposure to (a) 200 KeY and (b) 40 KeV electrons to a
total fluence of 2 x 1012 electrons/cm2s.
369
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370


GaP betavoltaic cells as a power source

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