NASA Technical Reports Server (NTRS) 19910009863: Radiation resistance of thin-film solar cells for space photovoltaic power

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by  NASA Technical Reports Server (NTRS)

Radiation Resistance of Thin- ..........
Film Solar Cells for Space
_ (NASA--TM-IO_715) RADIATION RFSTSTANCE OF N91-1917o
THIN-FILM S_LAR CELLS FOR SPACE PHOTo]VOLTAIC
P_WER (NASA) 35 p CSCL 10B
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James R. Woodyard
Wayne State University
Detroit, Michigan
and ...........................
Geoffrey A. Landis
Le_sR_esea_c_h Center ..........................................................
Cleveland, Ohio
Janiiary--1991

Radiation Resistance of Thin-Film Solar Ceils
for Space Photovoltaic Power
James R. Woodyard
Wayne State University,
Electrical and Computer Engineering Department,
Detroit, MI 48202.
Geoffrey A. Landis*
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, OH 44135.
SUMMARY
Copper indium diselenide, cadmium telluride and amorphous silicon
alloy solar cells have achieved noteworthy performance and are currently
being investigated for space power applications. Cadmium sulfide cells
had been the subject of considerable effort but is no longer considered
for space applications. This article presents a review of what is known
about the radiation degradation of thin-film solar cells in space.
Experimental investigations of the electron and proton degradation
resistance of cadmium sulfide, copper indium diselenide, cadmium
telluride and amorphous silicon alloy cells are reviewed. Damage
mechanisms and radiation-induced defect generation and passivation in
the amorphous silicon alloy cell are discussed in detail due to the greater
amount of experimental data available.
1. INTRODUCTION
There is no universally accepted definition of a thin-film solar cell.
In this paper we will refer to a thin-film solar cell as one made from thin
amorphous and/or polycrystalline layers deposited on an inert substrate
or superstrate. Thin-film solar cells are made from thin polycrystalline
or amorphous semiconductor layers ranging from a fraction of a micron
to a few microns in thickness. Various deposition technologies are used
to fabricate cell structures on inert substrate or superstrate materials.
Typically, the materials used in the active layers of a thin-film solar
cell are high optical absorption direct-bandgap semiconductors. Optical
absorption coefficients greater than 105 cm -I result in a high level of
light absorption. This allows the active layers in thin-film solar cells to
be extremely thin, of the order of a few microns or less.
*NASA/.ewis/NRC Resident Research Associate at NASA Lewis.
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Thin-film solar cells are currently the focus of research for low-cost
terrestrial electricity production. Low materials usage, high throughput
and automated deposition result in the potential for low production costs.
Since large-area monolithicly-interconnected structures can be made,
highly automated manufacturing system may be employed. Initial
production efforts have focussed on amorphous silicon alloys; recently
copper indium diselenide and cadmium telluride have exhibited desirable
properties for low-cost large-area solar panels. Thin-film solar cells can
also be made from a wide variety of other ternary and quaternary
materials; few of these kinds of materials have been studied extensiveIy.
One of the advantages of thin-film solar cells for space power
applications is they have the potential for high specific power and
stowability [1]. The specific power is the total power produced by an
array divided by its mass; the mass includes the solar cell, substrate or
superstrate, optical coatings, protective windows, encapsulants, grids,
array supporting structures and connections to the power bus.
Stowabflity, the power per unit volume, is a measure of the array storage
volume needed to generate a required power level. The stowability will
be larger for materials which are flexible and self supporting.
Thin-film solar cells can potentially have very low weight if the cells
are deposited on low mass substrates or superstrates. However, most of
the current technology development programs are directed at terrestrial
use. The preferred substrate for terrestrial applications is typically about
1/4 inch thick glass, which is inexpensive and rugged but not
lightweight. Some consideration has been given to the use of light
substrates for the fabrication of high specific power a-Si arrays for space
power applications. Hanak and his collaborators [2,3] showed that a-Si
alloys can be made into flexible ultralight modules having thicknesses
from 7.5 to 50 microns with thin metal or polyimide sheet substrates.
Kishi and co-workers [4] fabricated amorphous silicon arrays on 20
micron thick plastic for a solar powered airplane. Others have also
reported on the use of thin substrates with a-Si alloy solar ceils; Wallace,
Sabinsky, Stafford and Luft reported on the use of thin polyimide
substrates [5]; Natakatani and co-workers of Teijin Ltd. studied cells on
thin polyethylene terepthalate substrates [6]. However, for other cell
materials, there is little or no research on alternative, lightweight
substrates and superstrates. Additionally, little work has been done in
the area of array design and encapsulation, and testing for space power
applications.
Thin active layers result in higher radiation resistance; the reason
suggested is that the number of electrically-active radiation-induced
defects is related to the active layer thickness. This topic will be
explored in more detail in the discussion on the radiation resistance of
a-Si alloy solar cells.
Conversion efficiencies of thin-film solar cells are often quoted for
terrestrial conditions, where the solar spectrum is filtered by passage
through the terrestrial atmosphere. Efficiencies are usually reported for
air mass I (AM1) and air mass 1.5 (AM1.5) spectra. Far fewer
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measurements have been made on cells under the extraterrestrial solar
spectrum, referred to as AM0. Efficiency measured under an AM0
spectrum is lower than under the AM1 or AM1.5 spectra because most of
the additional light energy in the AM0 spectrum is in the infrared and
ultraviolet regions of the spectrum, where the spectral responce is very
low.
A correction factor may be used to estimate an AM0 efficiency from a
measured AM1.5 efficiency. The exact correction factor will depend on
the spectral response of the cell. Typically, a reduction in the AM1.5
efficiency of 15 to 20 percent is used for cells with bandgaps in the range
of 1 to 1.5 eV. A multiplicative factor of 0.8 has been reported to correct
an a-Si alloy cell from an AM 1.5 to an AM0 efficiency [3].
Thin-film photovoltaic materials developed for terrestrial use which
may be adapted to space power arrays include CulnSe 2 and a-Si alloys [1].
These applications include solar electric propulsion, a manned Mars
mission, and lunar exploration and manufacturing. While the efficiencies
are low compared to space cells based upon crystalline materials, the
projected specific power levels are still extremely good. Additionally, the
development of multi-bandgap thln-film tandem structures has shown
that efficiencies can be improved significantly.
While thin-film technologies have not yet developed a record of
performance in space, there is a large manufacturing base which has been
developed for terrestrial applications: tens of megawatts per year for a-Si
alloys, a rapidly increasing capability for CulnSe2, and a moderately small
capability of perhaps tens of kilowatts per year for CdTe. By contrast,
U.S. space solar cell requirements have been nearly constant at 100-120
kW per year for nearly a decade.
Overall, for space power applications, thin-film solar cells have
potential advantages of:
-high specific power
-large area solar cells with integral series interconnections
-flexible blankets
-large body of array manufacturing experience
-potential low cost
-potential high radiation tolerance
with the corresponding potential disadvantages of:
-lower efficiency
-lack of spacecraft experience
-lack of current production with space qualified encapsulants and
lightweight superstrates and/or substrates.
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2. TYPES OF THIN-FILM SOIAR CEILS
2.1 CdS/ Cu2 S Thin-Film Solar Cells.
The first thin-film solar cell developed was the heteroJunction
cadmium sulfide/copper sulfide cell. In earlier references the cell was
often referred to as the "CdS" cell; the role played by the Cu2S layer was
obscure. The best efficiency was about 8% AM0 [7], and the cells had
excellent radiation resistance [8,9,10]. There was some difficulty in
achieving good stability against humidity. These cells were made obsolete
by the development of more stable and higher efficiency thin-film
materials.
2.2 CulnSe 2 Thin-Film Solar Cells
A thin-film photovoltaic material of great interest and currently
under investigation is copper indium diselenide (CuInSe 2) and copper
indium diselenide/copper gallium diselenide alloys [11]. Several
fabrication technologies have produced AM0 efficiencies of over 10%
[12]; the technologies for deposition of the cell layers onto substrates and
superstrates include vacuum evaporation, reactive sputtering, and
electroplating. In general, cell processing involves deposition
temperatures and/or annealing temperatures of about 450 ° C. The high
processing temperature is a problem for space applications where it
would be desirable to deposit the cell onto a thin polylmide material such
as Kapton. Deposition onto a thin substrate or superstrate has not yet
been demonstrated.
The bandgap of CuInSe2 is about 1.0 eV and the optical absorption
coefficient is greater than 10 5 cm-I above the band edge. Existing cells
consist of a layer of the active copper indium diselenide, typically about 3
microns in thickness; a front contact and heterojunction window of
either cadmium/zinc sulfide or zinc oxide plus cadmium sulfide, with a
thickness of about one micron; and a back contact of molybdenum. A
CuInSe 2 cell efficiency of 13% AM1.5 has been reported by Mitchell,
Eberspacher, Ermer and Pier [13]; the corresponding AM0 efficiency is
10.4% using a factor of 0.8 for conversion from AM1.5. Efflciencies of
12% AM0 are expected to be achievable in the near term.
Other I-III-VI 2 semiconductors have also been studied for solar cell
use. An AM1 efficiency of 10.2% has been reported for a CUInl_xGaxSe 2
cell [14]; while the efficiency is lower than CuInSe 2, it offers future
promise. Other materials of interest include CuGaSe2 and CuInS 2. No
radiation testing has been done on these materials to date.
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2.3 CdTe Thin-F_Im Solar Cells
Another high-absorption coefficient material suitable for thin-film
deposition is cadmium telluride (CdTe). The bandgap of CdTe is 1.44 eV,
very close to the optimum value for the AM0 spectrum. Thin-film CdTe
solar cells have been produced in thin-film form by a wide variety of
deposition methods, including vacuum evaporation, close-space
subiimation, eiectrodeposition, and screen printing [15]. Processing
temperatures of about 450 ° C are typical. Recently thin-film CdTe cells
have been made which resulted in corrected AM0 efficiencies as high as
9.8% [16]. Like CulnSe 2, it is currently not produced on thin substrates
or superstrates. Unlike CuInSe 2, most CdTe deposition methods produce
a cell on a superstrate: the cell is deposited inverted upon a glass sheet,
which then serves as a front cover.
2.4 a--Si Alloy Cells
The most intensively researched thin-film solar cell material, and
the material with the largest body of array manufacturing experience, is
amorphous silicon (a-Si) alloy. The material commonly referred to as
a-Si is an alloy containing silicon and other elements. Examples of a-Si
alloys are: silicon and hydrogen, a-Si:H; silicon, hydrogen and fluorine,
a-Si:H,F; silicon, germanium and hydrogen, a-Si,Ge:H; silicon,
germanium, hydrogen and fluorine, a-Si,Ge:H,F; and silicon, carbon and
hydrogen, a-Si,C:H. The hydrogen incorporation is necessary for good
electronic properties and can range from a few percent to as much as
15%. An amorphous Si alloy thin film differs from the other solar cell
materials in that its structure is amorphous rather than crystalline.
While several deposition methods have been used in the fabrication
of a-Si alloy devices, plasma enhanced chemical vapor deposition
(PECVD) is commonly used in large scale fabrication. The method
consists of flowing gases, such as silane at a few Torr pressure, through a
chamber in which there a plasma discharge is sustained by
radio-frequency power. Silicon complexes and hydrogen deposit on a
heated substrate maintained at about 250°C. Doped n and p layers are
produced by adding gases such as phosphine and diborane to the gas-flow
stream.
The optical bandgap of device grade a-Si:H ranges between 1.7 and
1.8 eV, which is a reasonable match to the solar spectrum. The bandgap
can be tailored further by addition of carbon to raise the bandgap or of
germanium to reduce it. Most testing is done at simulated AM1.5; two
published simulated AM0 measurements on single junction a-Si alloy
based cells report efflciencies of 4.85% [17] and 8.57% [18]. Cell
efficiencies have been improved with multi-bandgap multi-junction
structures, as well as bandgap profiling within a Junction. a-Si,Ge:H,F
alloys, along with bandgap profiling, have been used by Guha, Yang,
Pawlikiewicz, Glatfelter, Ross and Ovshinsky to fabricate a triple tandem
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structure with a 0.25 cm 2 area to achieve a 13.7% AM1.5 efficiency [19].
A difficulty experienced with amorphous silicon is light-induced
degradation, known as the Staebler-Wronski effect [20]. First generation
a-Si modules experienced between 20 and 40% degradation upon
exposure to light [21]. Yang, Ross, Mohr and Fourier found that the
performance of a triple cell with an initial efficiency of 11.2% degraded
about 15% [22]. Sabinsky and Stone reported on recent tests of cells
from three manufacturers; they concluded that "...multi-Junction cells
show reductions in efficiency of only around 10% for cells with initial
efficiencies as high as 11%" [21]. It is believed that future improvements
in materials and cell design, as well as a better understanding of the
physics of light-induced degradation, will reduce the degradation still
further.
Amorphous Si alloy solar cells for terrestrial use are the subject of
very large and active research programs, currently funded at several
million dollars per year. Much of this research will likely be applicable to
space.
3. RADIATION RESISTANCE OF THIN-FILM SOLAR CELLS
The data on radiation damage to thin-film cells is a large but
unorganized data set covering a wide range of cell types, fluences,
energies and test conditions including irradiations with both protons and
electrons. It must be kept in mind that results obtained by researchers
at one institution are not necessarily directly comparable to data on ceils
of nominally the same material fabricated at another institution. In
general, the details of cell structure and efficiency are different for
different investigations; they also change with time as improved cell
materials and designs supplant older ones in the quest for higher cell
efficiencies and improved stabilities. It should also be cautioned that the
older radiation resistance results may not apply to future ceils, since the
improved structures and materials may have a major influence on
radiation resistance. For example, recent a-Si solar cells are designed to
reduce degradation due to Staebler-Wronski effect; however, many of the
design and materials changes developed to minimize this effect are also
likely to reduce radiation damage effects.
For this reason, radiation damage values on cells alone are of only
partial value. It is also important to understand the mechanisms of
radiation damage, in order to be able to understand and predict the effect
of cell design changes on radiation tolerance.
This section will discuss the radiation resistance of completed solar
cells. Since considerable work has been done on the mechanisms of
radiation damage to amorphous silicon films, a separate section, section
4, will discuss these studies. The reason for discussing damage
mechanisms in a-Si alloy based materials at length is that a-Si is the only
material which has been the subject of fundamental studies of the
radiation effects on the opto-electronic properties.
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3.1 Electron Irradiation Resistance of CdS/Cu2S Cells.
While CdS/Cu2S cells are no longer considered a potential candidate
for photovoltaic power generation, considerable work was done between
1963 and 1968 to characterize the effects of radiation. The results are
still of some interest because they are qualitatively representative of the
effects of radiation on other types of thin-film cells. In general, thin-film
CdS/Cu2S solar cells have shown excellent radiation tolerance, with no
degradation in power for 1 MeV electron fluences up to 1016 cm -2. This
led to the hope that thin-film cells in general would have high radiation
resistances, an expectation which has for the most part been met.
The first electron irradiation studies of CdS/Cu2S cells were
reported by Schaefer in 1963 [23]. The initial efficiencies were about 2%
and the cells had Mylar covers which were about 25 microns thick. A
degradation of about 10% in the initial efficiencies was observed when
the cells were irradiated with 1 MeV electron fluences of about 1015
cm -2. The degradation was later interpreted as being caused by the
darkening of the Mylar covers [24].
Brandhorst and Hart reported electron irradiation results in 1964 on
CdS/Cu2S ceils [25]. The cells were encapsulated in various plastic films
of unspecified thicknesses. Ten cells were irradi_Ited in air using 0.60,
1.00 and 2.50 MeV electrons with fluences ranging between 1015 and
10 17 cm -2. The cells were mounted on a water-cooled plate;
temperatures during irradiation did not exceed 60°C. The initial cell
efficiencies were about 2% and the areas 2 cm 2. The short-circuit
current, open-circuit voltage, power, spectral response, series resistance
and shunt resistance were investigated. The cells ,encapsulated in Mylar
and "H film" [Kapton] showed no degradation at any of the energies and
fluences up to 1016 cm -2. At 4-10 r6 cm -2 and higher fluences, all 21
cells showed efficiency degradations of 50 to 90% when they were tested
immediately following irradiation. The degradation was primarily in the
short-circuit current. The spectral response measurements showed the
greatest decrease in carrier collection occurred in the blue region of the
spectrum. The initial efficiencies were restored when the cells were
placed in open-circuit conditions and exposed to various light levels;
light levels of 40 mW/cm 2 for 24 hours and "ambient room light for
several days" restored the efficiencies. No annealing was observed under
short-circuit or maximum-power conditions.
Bernard, Berry, Buisson, and Paillous investigated electron
irradiation of CdS/Cu2S cells and Acklar covers in vacuum and argon [26].
The radiation resistance of the covers was investigated both with and
without bonding adhesive. The electron beam energy was 600 keV prior
to entering the sample chamber. In order for the beam to enter the
chamber, it traversed two aluminum windows of 50 micron thicknesses.
The authors estimated the irradiating electron beam energy at the
surface of the cell was about 450 keV. An electron fluence of 1.7.1016
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cm-2 produced degradation in the cell spectral response primarily in the
blue region of the spectrum. The electron irradiation did not degrade
the Aklar covers but the bonding adhesive apparently degraded and
absorbed light primarily in the blue region of the spectrum; the adhesive
degradation accounted for most ff not all the cell degradation.
3.2 Proton Irradiation Resistance of CdS/Cu2S Cells.
Schaefer and Staffer tested the effect of 1.8 MeV and 3 MeV protons
on CdS/Cu2S ceils. The cells had efficiencies of about 2% and 25 micron
thick Mylar covers [24]. At a maximum fluence of 4.10 12 cm -2, the
efflciencies degraded about 2 and 4% for 1.8 and 3.0 MeV protons,
respectively; the degradations were due to decreases in the short-circuit
current. Some of this effect may be due to cover darkening.
Brandhorst and Hart irradiated CdS/Cu2S ceils with 2.0, 5.0, 7.0 and
10 MeV protons with fluences ranging between 1012 and 4.1014 cm -2.
The investigations were carried out under the same conditions as
discussed above for the electron irradiation studies [25]. The
post-irradiation maximum powers were 80% to 90% of the initial powers
with fluences of 4.1014 cm-2 ; the degradations were not strongly
dependent on the proton irradiation energies. They attributed the
degradation to loss of short circuit current and to increased series
resistance.
Degradation studies of unencapsulated CdS/Cu2 S cells were carried
out by Hui using low energy protons [27]. The effect of a protective Mylar
window was also investigated. Cells with 11 cm 2 areas and roughly 5%
efficiencies were irradiated with proton energies of 50, 100, and 400
keV and fluences ranging between 1012 and 5.1014 cm -2. Degradations of
about 20% were reported in both the open-circuit voltage and
short-circuit current at fluences of 5-10 14 cm-2. There was a slight
dependence on the irradiating proton energy; degradations in the
open-circuit voltage and short-circuit current were slightly larger at 100
keV. No improvement was observed in the radiation resistance of cells
constructed with 25 micron thick Mylar covers. However, a cover of
silicone with an unspecified thickness resulted in no degradation of ceils
exposed to 400 keV proton fluences up to about 1014 cm -2.
Curtin investigated proton irradiation of CdS/Cu2S ceils with 25
micron Kapton covers [28]. Cells were 2 cm 2 in area and had efficiencies
of 3.6%; they were irradiated in four cell groups with proton energies of
0.60, 0.80, 1.00, 1.20, 1.40, 2.00, 4.00 and 5.00 MeV. For proton
energies less than 1.40 MeV, no open-circuit voltage degradation was
observed suggesting the proton energy was too low to penetrate the
Kapton cover; degradation in the short-circuit was observed and shown
to be due to the degradation in the optical transmission of the Kapton
cover. The change in the optical properties of Kapton was verified by
measurements of the optical transmission as a function of proton
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