Joel T. Galofaro

Glenn Research Center, Cleveland, Ohio

Boris V. Vayner

Ohio Aerospace Institute, Brook Park, Ohio

Experimental Results of Thin-Film Photovoltaic

Cells in a Low Density LEO Plasma Environment:

Ground Tests

NASA/TM—2006-214430

November 2006
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Experimental Results of Thin-Film Photovoltaic

Cells in a Low Density LEO Plasma Environment:

Ground Tests

NASA/TM—2006-214430

November 2006

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Fourth World Conference on Photovoltaic Energy Conversion

sponsored by the Institute of Electrical and Electronics Engineers

Waikoloa, Hawaii, May 7–12, 2006

Joel T. Galofaro

Glenn Research Center, Cleveland, Ohio

Boris V. Vayner

Ohio Aerospace Institute, Brook Park, Ohio
Acknowledgments

The authors would like to express their gratitude to the Air Force Research Laboratory (AFRL) in bringing this manuscript to

publication. A special thanks to Pawel Tlomak AFRL and Michael Piszczor, Jr., NASA Glenn Research Center, for their helpful

dialogue and timely communications regarding the entire publication process.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

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Springfield, VA 22161

Available electronically at http://gltrs.grc.nasa.gov

Trade names and trademarks are used in this report for identification

only. Their usage does not constitute an official endorsement,

either expressed or implied, by the National Aeronautics and

Space Administration.

Level of Review: This material has been technically reviewed by technical management.
NASA/TM—2006-214430 1

Experimental Results of Thin-Film Photovoltaic Cells in a Low 

Density LEO Plasma Environment: Ground Tests 

Joel T. Galofaro 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Boris V. Vayner 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Abstract 

Plasma ground testing results, conducted at the Glenn Research Center (GRC) National Plasma 

Interaction (N-PI) Facility, are presented for a number of thin-film photovoltaic cells. The cells represent 

a mix of promising new technologies identified by the Air Force Research Laboratory (AFRL) under the 

CYGNUS Space Science Technology Experiment (SSTE-4) Program. The current ground tests are aimed 

at characterizing the performance and survivability of thin film technologies in the harsh low earth orbital 

space environment where they will be flown. Measurements of parasitic current loss, charging/dielectric 

breakdown of cover-slide coatings and arcing threshold tests are performed for each individual cell. These 

measurements are followed by a series of experiments designed to test for catastrophic arc failure 

mechanisms. A special type of power supply, called a solar array simulator (SAS) with adjustable voltage 

and current limits on the supply’s output, is employed to bias two adjacent cells at a predetermined 

voltage and current. The bias voltage is incrementally ramped up until a sustained arc results. Sustained 

arcs are precursors to catastrophic arc failure were the arc current rises to a maximum value for long 

timescales (often ranging between 30 to 100 μsec times). Normal arcs by comparison, are short lived 

events with a timescale between 10 to 30 μs. Sustained arcs lead to pyrolization with extreme cell damage 

and have been shown to cause the loss of entire array strings in solar arrays. The collected data will be 

used to evaluate the suitability of thin-film photovoltaic technologies for future space operations. 

Introduction 

The basic circuit element in all spacecraft photovoltaic (PV) power generation systems is configured 

from a number of solar cells connected in series forming a single string element. The entire array is then 

composed from a number of such identical string elements. The number of strings connected in series 

provides the full operational potential for the array; the number of strings connected in parallel provides 

the maximum operating current for the array. Several arrays are often connected to form a single wing 

and a typical spacecraft may have one or more wings making up its power generation system. Batteries 

are often employed to store power during times when the array is in full darkness (eclipse). The PV 

arrays, the reserve capacity of the batteries, the power distribution network and power management 

software all combine to define the overall robustness of the power system. 

The negative end of the solar array and batteries are typically grounded to the spacecraft structure, 

thereby providing a single point ground connection. It is the negative grounding scheme, coupled with the 

spacecraft structure and plasma environment, that is responsible for all related array/spacecraft 

interactions with the orbital plasma environment (refs. 1 to 6). While cell composition and physical layout 

do play important roles, they are not the most important factors in determining how the array and 

spacecraft structure interact the plasma environment. The single most important factor in determining the 

magnitude of the interactions with the space plasma environment lies with the designed operating 

potential or the design voltage of the spacecraft power system. Due to low mass and volume stowage 

limitations imposed by current solar cell technologies, the maximum on-orbit power supplied by the 
NASA/TM—2006-214430 2

current generation photovoltaic arrays is limited to only about 30 kW of power (ref. 7). A new generation 

of TFPV solar cell technologies seeks to overcome the mass and stowage limitations imposed by current 

day cell technologies. To that end a number of promising TFPV cell technologies are tested to see how 

they perform in a low density space plasma environment. 

A test plan was hammered out at the GRC/AFRL meeting in an early February of 2005. Originally 

the Air Force Research Laboratory (AFRL) had planned on conducting a plasma test on sixty sample 

coupons. Due to time and cost constraints the number of samples to be tested was reduced down to a more 

manageable number at the February meeting. The first order discussion entailed the need to test the cells 

as close as possible to their actual flight configuration. It was decided that the backside of the cells needed 

to be exposed to the plasma, just as they would be deployed in space. A matter of some conjecture was 

the need for a standardized way of testing solar cell samples. First the solar cells needed a rigid support to 

protect them from damage during evacuation and venting operations in the chamber. Furthermore, in 

order to avoid large deviations in the measured cell efficiency, both the contact and wire resistance are 

standardized using the same fixed length cable for all cells mounted on the test fixture. The outputs leads 

from each cell were connected using coaxial RG58U cables terminated with standard male type BNC 

connectors. The cable and cell contact points were then insulated to prevent them from arcing in the space 

plasma. AFRL contracted a major aerospace company to build the actual test fixture hardware, to mount 

the cells on the test fixture and to make all coaxial cable output connections to the cells. Two large test 

fixtures, with an area of approximately one square meter, arrived at the facility for testing in early 

September. The first test fixture was populated with a mix of nine sample coupons provided by three 

different solar cell manufactures (fig. 1(a) to (d)). The nine sample coupons comprised state of the art 

TFPV technologies of both amorphous silicon (a-Si) and Copper Indium Gallium Diselenide (CIGS) 

cells. A second test fixture populated with six a-Si cells from the same manufacturer was also supplied for 

testing (fig. 2(a) to (d)). The two test fixtures represent the most promising light weight TFPV 

technologies identified to date by the AFRL. 

Experimental Setup 

Plasma tests were performed in the 1.8 m long by 1.8 m diameter test section inside the Teney 

vacuum chamber geosynchronous (GEO) charging simulator (fig. 3(a)). The Teney chamber is capable of 

both Low Earth Orbit (LEO) and GEO operations. The vacuum system is backed by a holding pump and 

a 0.9 m (36 in.) diameter cryo-tub. The first test fixture (containing nine sample test coupons) was 

suspended at the far end the chamber (fig. 3(b)). The cells were electrically floated with respect to tank 

ground. Separate connections were made for each cells output leads using high voltage electrical 

feedthrough’s mounted on a large port located on the tank wall. A Penning type discharge source 

(mounted on the vacuum side of the tank door) ionizes a controlled flow of xenon gas neutrals flowing 

past a hot filament in magnetic field. The plasma source is electrically floated in the chamber with a 

positive bias being applied between the positive case anode and negative filament cathode. All measured 

tank pressures were obtained using an ionization gage. The arcing monitor detection system consists of a 

simple R-C circuit (fig. 3), a voltage probe, current probes, current probe amplifiers and a 4 channel 

400 Mhz oscilloscope (fig. 3(c)). The R-C circuit is used to bias the cell relative to the plasma just as it 

would be in space. The circuit is used to simulate the arcing conditions for cells by dumping an equivalent 

amount of charge from the capacitor that represents the total spacecraft capacitance. Typical spacecraft 

capacitances range from 1 to 4 μF. A capacitance value, C = 1 μF was used for all arcing tests. A 

Software driven oscilloscope control program arms the scope, puts it in a wait condition awaiting a 

change in state in the trigger signal. Once the oscilloscope is triggered, the control program instructs the 

scope to dump all waveform data from memory to non volatile disk storage. The control program 

proceeds to rearm the scope and awaits the next trigger event. Other equipment includes a color camera 

for imaging arcs, a DVD video recorder, color TV monitor, two programmable power supplies (source 

measure units), a solar array simulator power supply with adjustable voltage and current limits, and a 

quadrupole mass spectrometer for identifying gas species inside the chamber. 
NASA/TM—2006-214430 3
NASA/TM—2006-214430 4
NASA/TM—2006-214430 5
NASA/TM—2006-214430 6
NASA/TM—2006-214430 7

Experimental Results 

Prior to any plasma testing in the Teney vacuum chamber both test fixtures were delivered to the PV 

solar array simulator lab for characterization. As a control, a first round of baseline measurements were 

made in air under full illumination (1 sun) using a high power UV arc lamp. Measurements of short 

circuit current (Isc), open circuit voltage (Voc), maximum current (Imax), maximum voltage (Vmax), 

maximum power (Pmax) and cell efficiency (Eff %) were obtained for each cell on both test fixtures. 

The first test fixture was returned back to N-PIF in the beginning of September and installed inside 

the Teney vacuum chamber. Solar cell samples were allowed to outgas for 48 hr at the base pressure, 

P = 2.2×10–7 torr, prior to plasma testing. A pressure reading P = 1.1×10–5 torr was maintained during 

tests with the xenon plasma source operating. A floating spherical Langmuir probe was sweep through a 

range of potentials to obtain the volt-ampere characteristic curve. Applying standard reduction techniques, 

to the experimentally obtained curve, yielded the following xenon plasma parameters: Floating potential, 

Vf = 0.5 V, Plasma potential, Vp = 7.4 V, electron temperature, Te = 0.38 eV and electron number density, 

Ne = 5.4×105

 electrons per cubic centimeter. 

In order to simulate the cell interactions with the LEO plasma environment the cells are electrically 

floated in the chamber and immersed in the xenon plasma. The output leads of each cell are shorted 

together and negatively biased with a programmable power supply located outside the chamber. Parasitic 

current collection of a cell is a measure quality of insulator material of the cell. Large parasitic currents in 

the space plasma tend reduce the overall rated output power generated by the cell. In order to get a better 

understanding of the effect of parasitic current loss a current collection curve (voltampere curve) is 

obtained in-situ for each cell. (fig. 4(a) to (d)). Note that coupon D shown in figure 1(a) and (c) is actually 

composed of six separate a-Si cells labeled Da, Db, Dc, Dd, De, Df. Due to electrical breakdown and 
NASA/TM—2006-214430 8

subsequent and surface flashing concerns the voltage has been limited to bias voltages between –100 and 

100 V. The scaled parasitic current values shown in table A have been recalculated based on all cells 

having the same area. 

TABLE A.—SCALED PARASITIC CURRENT RESULTS 

Coupon Area 

[cm2

] 

Scale 

factor 

Current 

[A] 

Cell A 816 1 9.2709e–004 

Cell B 816 1 8.2099e–004 

Cell C 168 4.9 1.5766e–002 

Cell Da 14 58.3 2.1472e–001 

Cell Db 14 58.3 2.4060e–001 

Cell Dc 14 58.3 2.3597e–001 

Cell Dd 14 58.3 2.3451e–001 

Cell De 14 58.3 2.3574e–001 

Cell Df 14 58.3 5.4742e–002 

Cell E 180 4.5 1.9705e–002 

Cell F 180 4.5 1.9449e–002 

Cell G 17.5 46.6 9.0707e–002 

Cell H 63 12.9 4.5805e–002 

Cell I 68 12 3.7220e–002 

The next set of plasma experiments involved obtaining the arcing threshold of each cell. Arcing 

threshold is determined by negatively biasing each cell (through an R-C network, fig. 3(c)) and counting 

the number of arc events occurring in a predetermined time interval. If no arcs are observed at a given 

potential at the end of the time interval the bias level is increased and the test is repeated for the next time 

interval. The point at which arcs are first recorded determines the arcing threshold of the cell. To get a 

better handle on the maximum cell voltage encountered in orbit, each cell needs to be tested at the most 

negative potential developed by the cell in space. The most negative cell potential is actually specified by 

the designed operating voltage of the power system, which for the current tests is 280 V. Assuming a 

designed operating voltage, Vop = 280 V, and using the values of Vf = 0.5 V and Vp = 7.4 V specified 

earlier, the power system should float at a potential, VF = –273.1 V. (eq. (1)). In order to provide a large 

safety margin against catastrophic cell failure (encountered during times of severe geomagnetic substorm 

activity) it was decided to bias individual cells up to a maximum potential of –400 V (with the exception 

VF = (Vp −V f )−Vop (1) 

to cell D which tested up to –450 V). The results obtained from the arcing tests are summarized in 

table B. There appears to be two different kinds of arcs: one that is triggered normally and the one that 

appears as a surface flash with no trigger. Non-triggered arcs appear to be caused by micro discharges in 

the dielectric insulating layer. Because micro discharges are small by definition, they often occur well 

below the trigger level set on the current probe. 

To gain a better understanding of the mechanism behind such discharges, two different current probes 

(high current and low current) were employed, with the scope being triggered at a level set by the low 

current probe. Experience has shown that arcs (and maybe micro discharges) are often caused by 

absorbed water vapor at the triple junction sites on solar arrays (refs. 8 to 11). Micro discharges occurring 

in the surface layer are not large enough to cause the full charge stored in the capacitor to be discharged 

as in the case of an arc. A surface flash appears to generate micro discharges on the order of 70 mA of 

current (fig. 5(a) and (b) for details). Micro discharges occurring in the insulating layer of the film may 

however be considered as precursors to full fledged arcs. A question arose concerning the accumulated 

damage effects micro discharges (surface flashes) have on the long term survivability of dielectric 

insulator coatings in space. To help answer the coating survivability question a 1 hr long test was run at a 

sustained bias level of –170 V (with a 1 μF capacitor in parallel) on the large a-Si sample on cell A. The 
NASA/TM—2006-214430 9

arc rate appeared to slow down considerably after a large initial number of surface flashing events 

(99 flashes) with no apparent visual degradation to the coating of the cell (see time lapse in fig. 6). 

TABLE B.—ARCING RATES RECORDED AT INDICATED BIAS LEVEL 

(SSF = Single Surface Flash, MCF = Multiple Surface Flashes) 

Coupon Bias level 

[V] 

Arc rate 

[no./15 min] 

Comments 

Cell A –100 1 

 –150 0 

 –170 MSF scope not triggered 

 –200 MSF scope not triggered 

Cell B –125 1 

 –150 4 

 –150 5 

Cell C –100 5 

 –125 4 

Cell Da –150 14 

 –200 5 

 –250 4 

Cell Db –450 SSF no trigger 

Cell Dc –450 1 

Cell Dd –450 0 

Cell De –450 0 

Cell Df –450 0 

Cell E –100 2 

 –125 11 per minute 

 –150 8 

Cell F –100 2 

 –125 4, MSF flashes not triggered 

Cell G –350 1, SSF 1 trigger arc, 1 SSF 

 –400 4 

Cell H –300 2 

 –350 6 6 arcs in 5 min 

Cell I –350 5 
NASA/TM—2006-214430 10
NASA/TM—2006-214430 11

Another way of characterizing the cell is to determine how much voltage the cell is able to standoff 

without undergoing electrical breakdown. Electric field (E-field) strength is a direct measure of the how 

much voltage a given cell is able to withstand without arcing. An upper limit on the E-field strength can 

be estimated if the thickness z of the dielectric coating and the bias voltage φ at which the cell undergoes 

electrical breakdown is known. The magnitude of E-field strength can be estimated from the formulations 

given in equations 2(a) to (d). The E-field strength has dimensions of V/m and can be calculated directly 

from equation 2(d). Note that the E-field strength is measured in the z direction which is taken to be 

normal to the face of the cell. 

k k

z

j

x

i

x

E k grad k ˆ ˆ x ˆ y ˆ z ˆ ˆ ⋅ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

∂

∂φ +

∂

∂φ

+

∂

∂φ ⋅ = − φ⋅ = − G (2a) 

are unit normalvectorsand ˆ ,ˆ ,ˆ Where: E istheelectricfield vector andi j k G

z

k k

z

j k

y

i k

x

E k x y z z

∂

∂φ = ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ ⋅ ∂

∂φ ⋅ +

∂

∂φ ⋅ +

∂

∂φ ⋅ = − ˆ ˆ ˆ ˆ ˆ ˆ ˆ G (2b) 

o

z o

z z z

E k −

φ − φ = ∂∂φ ⋅ = ˆ G (2c) 

Letting:φ = 0 and = yields oz

z

E k φ ⋅ = ˆ G (2d) 

Thus, critical field strength can be estimated as 100 V/25 μm = 4 MV/m for cell A and as 25 V/10 μm = 

12.5 MV/m for the cell B. 

Sustained arcing tests were then run in a LEO environment on samples E and F (fig. 1(a) to (c)). The 

solar array simulator (SAS) was then placed between the shorted outputs of cells E and F (see fig. 3(d) for 

details). Cell E was attached to the negative end of the SAS and the positive end of the SAS was 

connected to cell F. Sample E was initially biased at –200 V and a SAS voltage of 120 V (with both 1 and 

2 A current limit settings) was used during the tests. Several triggered arc events were recorded between 

cells along with multiple surface flashes that were not triggered. No sustained arcs were observed during 

any of the tests. See figure 7(a) for further details. After SAS testing a current collection curve was run 

for samples E and F. The current collection results are shown in figure 7(b). The entire test fixture was 

then removed from the chamber and delivered to the PV solar array flash simulator lab for retesting in air. 

The second test fixture was installed in the chamber and both strings were allowed to outgas in the 

chamber for 48 hr under hard vacuum. The plasma source was then placed in an operational mode and 

adjusted until the same plasma conditions for the previous tests were met. A current collection curve was 

obtained on Strings 1 and 2. See figure 8(a) for details. Parasitic current results indicated that both 

Strings 1 and 2 collected more current than the virgin samples. 

Next sustained arcing tests were performed on Strings 1 and 2 using the setup shown in figure 3(d). 

Note that String 1 consists of three a-Si cells (J, K, and L) wired in series and String 2 is made up of three 

similar a-Si cells (M, N, and O) also wired in series (see figure 2(a) and (b) for details). Several 

measurements were initially performed with String 1 biased at –170 V with the negative end of the SAS 

being connected to String 1 and the positive end of the SAS connected to String 2. A SAS output voltage 
NASA/TM—2006-214430 12

of 120 V with a 2 A current limit was set for the sustained arc tests. Several trigger arc events were 

recorded, along with multiple surface flashing events that were not triggered. The SAS output connections 

to Strings 1 and 2 were then reversed. String 1 was connected to positive end of the SAS and the negative 

SAS end was connected to String 2. An output SAS voltage of 120 V with a 2 A current limit was used 

and several triggered and non-triggered arc events arc events were recorded. Figure 8(b) shows typical 

triggered response recorded during this part of the tests. No sustained arcs were observed for any of the 

tests run between Strings 1 and 2. A final round of cell performance measurements were then obtained in 

air on the second test fixture with mixed results. See table C for further details. 
NASA/TM—2006-214430 13

TABLE C.—REPORTED CHANGE IN EFFICIENCY AFTER PLASMA TESTING. PV SIMULATOR PERFORMANCE 

TEST IN AIR WITH FULLE (1 SUN) SOLAR UV IRRADIANCE SPECTRUM 

(NC = No Change, D = Degraded, I = Improved, L = Lost) 

Sample Cell type Area 

[cm2

] 

ΔEff (%) Remarks 

Cell A a-Si on stainless 816 9.14 I 

Cell B a-Si on stainless 816 10.01 I 

Cell C a-Si on Kapton (DuPont) 168 –37.08 D 

Cell Da a-Si 14 –94.76 D 

Cell Db a-Si 14 –99.18 L 

Cell Dc a-Si 14 –97.49 L 

Cell Dd a-Si 14 –95.98 L 

Cell De a-Si 14 –78.86 L 

Cell Df a-Si 14 –89.02 L 

Cell E CIGS with coating 180 26.57 I 

Cell F CIGS with coating 180 1.27 NC 

Cell G CIGS 17.5 97.88 I++

Cell H CIGS 63 –75.84 L 

Cell I a-Si 68 –68.81 D– –

Conclusions 

The scaled parasitic current results reported in table A indicate that the large 618 cm2 area a-Si cells 

A and B consistently returned the lowest and subsequently best leakage current figures, losing an average 

of two orders of magnitude less current than cells C, E, F, G, H, and I. The six individual cells (Da, Db, 

Dc, Dd, De, and Df) consistently returned the highest parasitic current results, losing on average three 

orders of magnitude more current, than either cells A or B. The parasitic current figures appear to be 

skewed and may be due to large scale edge effects acting on the smaller area cells. 

At first glance the arcing threshold results in table B may look encouraging. Unfortunately, 

combining the arc threshold results with the findings from the PV solar array flash simulator laboratory in 

table C, paints an entirely different picture all together. For example, three of the six amorphous silicon 

cells (d, e, and f) individual cells in sample D were able to standoff up –400 V without arcing. In fact cells 

d, e, and f of sample D had the highest recorded arc threshold values for any of the cells tested in table 

(B). However table C shows that five of the six cells in sample D were lost after arc testing and one cell 

(Da) from the same group of cells survived but was badly degraded. Similarly, cell H was lost after arc 

testing and cells C and I were badly degraded. Additionally cells A, B, and G improved in efficiency after 

arc test in plasma whereas cell F showed no appreciable change. 

So what does the above the compiled data mean? Why do some cells improve, while others degrade 

and why still other cells are lost entirely? First all CIGS and a-Si cells are not created equally. Particularly 

in the case of a-Si cells, the coatings are not completely uniform in thickness. The uniformity of the 

coating (thickness) may vary from point to point across the entire face of the insulating layer. 

Furthermore the coatings may also contain: voids, wrinkles, trapped air as well as both large and small 

(pinholes) scale defects, which are artifacts left over from the manufacturing process. With the previous 

two statements in mind an attempt will be made to describe why certain cells are lost. Obviously, 

something caused the cells to malfunction, resulting in a catastrophic loss of the cell as a direct 

consequence of plasma arc testing. Particularly for thin coatings, applied over an area of small 

dimensions, the cell will be even more perceptible to large potentials (and therefore large scale E-fields) 

than is the case for much larger cells having a greater surface area. For the same size arc, damage will 

scale according to the surface area, so smaller cells will incur more damage than larger cells. This appears 

to be the case of cells (a to f) in sample D. Now to the final question of why some cells degrade while 

others improve in efficiency after plasma arc testing. Generally speaking cell efficiency will depend on: 
NASA/TM—2006-214430 14

• The quality (uniformity of the insulating material) and who manufactures it. For example cells A and 

B, from the same manufacturer, improved while cells C and I supplied from a different manufacturer 

degraded. 

• Large area a-Si cells appear to be less susceptible to arc damage caused by large scale bias potentials 

then is the case for smaller area a-Si cells. For example the two large area (816 cm2

) a-Si cells: A and 

B had improved efficiency after arcing in LEO, while the intermediate sized a-Si cells: C (168 cm2

) 

and I (63 cm2

) degraded after arcing. 

• Arc damage in amorphous silicon cells appears to scale according to the surface area, so smaller cells 

will incur more damage than is the case for larger area cells. This appears to be the case of cells (a to 

f) in sample D. 

• CIGS cell performance appears to depend less on large scale potentials and surface area effects then 

is the case for a-Si cells. For example CIGS cell G which is small in area (17.5 cm2

) had the largest 

gain in efficiency (97.88 percent) for any of the cells tested. The Intermediate sized CIGS cells E and 

F either improved in efficiency (Cell E) or stayed the same (Cell F) after arc testing. 

In accordance with the designed array operating voltage (set by AFRL for arrays to be configured in 

arrays for space applications using the current a-Si and CIGS technologies) the expected magnitude of the 

plasma interactions between the cells and the LEO plasma environment calculated from equation (1) is 

–273 V. Inspection of table B shows that the only cells (b to f) in sample D, cells G, H, and I arced well 

above the specified –273 V limit. Unfortunately cells (b to f) in sample D and cell H were lost (cell I was 

degraded badly) after plasma arc testing. Only one cell (cell G) passed the –273 V limit. In retrospect 

other cells might have passed if the maximum limit of –273 V had been rigidly maintained throughout the 

experiment. 

Finally the sustained arc tests with the SAS placed between cells E and F show that the pulse width 

on triggered arcs between adjacent cells is only about 30 μS in length. Because the arc pulse width is 

small no sustained arcs were detected. This is not surprising since the gap thickness between cells E and F 

is large being about 0.5 cm in length (fig. 6(a)). The sustained arc tests with the SAS placed between 

adjacent Strings 1 and 2 indicate that although some arc pulse widths are extended in length, they are not 

sustained because the current falls back to zero at the end of the current pulse (fig. 8(b)). Extended arcs 

may be considered precursors to sustained arcs which would have resulted if the gap thickness between 

cells had been smaller in dimension. 

References 

1. Vaughn, J.A., Carruth Jr., M.R., Katz, I., Mandell, M.J., and Jongeward, G.A., “Electrical Breakdown 

Currents on Large Spacecraft in Low Earth Orbit,” Journal of Spacecraft and Rockets, vol. 31, no. 1, 

1994. 

2. Katz, I., Lilley, J.R., Greb, Macoy, J.E., Galofaro, J., and Ferguson, D.C., “Plasma Turbulence 

Enhanced Current Collection: Results from the Plasma Motor Generator Electrodynamic Tether 

Flight,” Journal of Geophysical Research, vol. 100, no. A2, 1995. 

3. Vayner, B.V., Ferguson, D.C., and Galofaro, J.T., Galofaro, “The Spacecraft Surfaces Degradation 

and Contamination Caused by Arcing in Low Density Plasmas,” XVIII ISDEIV Conference, 

Eindhoven, Netherlands, Aug. 17–21, 1998. 

4. Galofaro, J., Vayner, B., and Ferguson, D., “Electrical Breakdown of Anodized Coatings in Low 

Density Plasmas,” Journal of Spacecraft and Rockets, vol. 36, no. 4, 1999. 

5. Vayner, B., Galofaro, J., and Ferguson D., “Interactions of High-Voltage Solar Arrays with Their 

Plasma Environment: Ground Tests,” Journal of Spacecraft and Rockets, vol. 41, no. 6, 2004. 

6. Vayner, B., Galofaro, J., and Ferguson, D., “Interactions of High-Voltage Solar Arrays with Their 

Plasma Environment: Physical Processes,” Journal of Spacecraft and Rockets, vol. 41, no. 6, 2004. 
NASA/TM—2006-214430 15

7. Granata, J.E., Sahistrom, T.D., Hausgen, P., Messenger, S.R., Walters, R.J., Lorentzen, J.R., “ThinFilm Photovoltaic Radiation Testing and Modeling for a MEO Orbit,” Conference Record of the 

IEEE Photovoltaic Specialists Conference, Jan. 3–7, 2005, Lake Buena Vista, FL, pp. 607–610. 

8. Galofaro, J., Vayner, B., Degroot, W., and Ferguson, D., “The Role of Water Vapor and Dissociative 

Recombination Process in Solar Array Arc Initiation,” AIAA paper 2002–0938, 40th Aerospace 

Sciences Meeting and Exhibit, Jan. 14–17, 2002, Reno, Nevada. 

9. Galofaro, J., Vayner, B., Ferguson, D., and Degroot, W., “A desorbed Gas Molecular Ionization 

Mechanism for Arcing Onset in Solar Arrays Immersed in a Low Density Plasma,” AIAA Paper 

2002–2262, 33rd Plasmadynamics and Lasers Conference, May 20–23, 2002, Maui, Hawaii. 

10. Galofaro, J., Vayner, B., and Ferguson, D., “Daytime Solar Heating of Photovoltaic Arrays in a Low 

Density Plasmas,” AIAA Paper 2003–4177, 34th Plasmadynamics and Lasers Conference, June 23–

26, 2003, Orlando, Florida. 

11. Galofaro, J., Vayner, B., and Ferguson, D., “On Orbit Daytime Solar Heating Effects: A Comparison 

of Ground Chamber Arc Results,” NASA/TM—2004-212890, 8th Spacecraft Charging Conference, 

Huntsville, AL, October 20–24, 2003. 
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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21

Experimental Results of Thin-Film Photovoltaic Cells in a Low Density LEO

Plasma Environment: Ground Tests

Joel T. Galofaro and Boris V. Vayner

Arcing; Solar array arcing; Sustained arcs; CIGS thin film; Amorphous silicon thin films

Unclassified -Unlimited

Subject Category: 18

Prepared for the Fourth World Conference on Photovoltaic Energy Conversion sponsored by the Institute of Electrical and

Electronics Engineers, Waikoloa, Hawaii, May 7–12, 2006. Joel T. Galofaro, Glenn Research Center; and Boris V. Vayner,

Ohio Aerospace Institute, 22800 Cedar Point Road, Brook Park, Ohio 44142. Responsible person, Joel T. Galofaro,

organization code, RPV, 216–433–2294.

Plasma ground testing results, conducted at the Glenn Research Center (GRC) National Plasma Interaction (N-PI) Facility,

are presented for a number of thin-film photovoltaic cells. The cells represent a mix of promising new technologies

identified by the Air Force Research Laboratory (AFRL) under the CYGNUS Space Science Technology Experiment

(SSTE-4) Program. The current ground tests are aimed at characterizing the performance and survivability of thin film

technologies in the harsh low earth orbital space environment where they will be flown. Measurements of parasitic current

loss, charging/dielectric breakdown of cover-slide coatings and arcing threshold tests are performed for each individual

cell. These measurements are followed by a series of experiments designed to test for catastrophic arc failure mechanisms.

A special type of power supply, called a solar array simulator (SAS) with adjustable voltage and current limits on the

supply’s output, is employed to bias two adjacent cells at a predetermined voltage and current. The bias voltage is incrementally ramped up until a sustained arc results. Sustained arcs are precursors to catastrophic arc failure where the arc

current rises to a maximum value for long timescales often ranging between 30 to 100 μsec times. Normal arcs by

comparison, are short lived events with a timescale between 10 to 30 μsec. Sustained arcs lead to pyrolization with

extreme cell damage and have been shown to cause the loss of entire array strings in solar arrays. The collected data will

be used to evaluate the suitability of thin-film photovoltaic technologies for future space operations.