Several Hughes gallium arsenide (GaAs) concentrator cells were exposed to 1-MeV electrons at fluences up to 1x10 to the 15th power electrons/sq cm. Performance data were taken after several fluences, at two temperatures, and at concentration levels from 1 to approx. 150x AMO. Data at 1 sun and 25 deg C were taken with an X-25 xenon-lamp solar simulator. Data at concentration were taken using a pulsed solar simulator with the assumption of a linear relationship between short-circuit current and irradiance. The cells are 5 by 5 mm with a 4-mm diameter illuminated area

Curtis, H. B.

Swartz, C. K.

English

NASA Technical Reports Server

N86-17860
PERFORMANCE OF HUGHES GaAs CONCENTRATOR CELLS
UNDER 1-MeV ELECTRON IRRADIATION
Henry B. Curtis and Clifford K. Swartz
NASA Lewis Research Center
Cleveland, Ohio
Several Hughes gallium arsenlde (GaAs) concentrator cells were exposed to l-MeV
electrons at fluences up to IxlO 15 electrons/cm 2. Performance data were taken
after several fluences, at two temperatures (25 and 80 °C), and at concentration
levels from l to ~150x AMO. Data at 1 sun and 25 °C were taken with an X-25
xenon-lamp solar simulator. Data at concentration were taken using a pulsed solar
slmulator with the assumption of a linear relationship between short-clrcult current
and Irradlance. The cells are 5 by 5 mm with a 4-mm-dlameter illuminated area.
INTRODUCTION
The use of concentrating optics for space photovoltalc power generation has been
under consideration for some time. The potential advantages of concentrators
include higher cell efficiency, better radiation resistance, and lower cost. One
possible optical design out of several Is the miniature Cassegralnlan concept
developed by TRW (ref. l). This design involves small GaAs cells operating at a
concentration level of lO0 to 130x AMO. The cells are 5 by 5 mm wlth a 4-mm-
diameter illuminated area which leaves about half the cell area covered wlth outer
bus-bar.
One of the unanswered questions Involvlng concentrator cells Is their perform-
ance degradation at concentrated light levels after electron irradiation. As part
of an ongoing concentrator-cell program at NASA Lewis, several Hughes concentrator
GaAs cells were irradiated wlth l-MeV electrons. The data presented here are
intended to be a first look at the performance of concentrator cells after electron
bombardment.
EXPERIMENTAL DESCRIPTION
Flve Hughes GaAs small-area concentrator cells were individually mounted In
separate cell holders. The holders consist of a small bottom metal base and a
washerllke metal top with a beveled hole slightly larger than the illuminated area
of the cell. These two pieces supply both a permanent support for the cell and an
area for the four-wlre electrical attachment. The cells remained In their holders
throughout all electron irradiations and performance measurements. There were no
cover glasses attached to the cells nor was there any shielding by optical elements
during the irradiations.
Electron irradiations using l-MeV electrons were performed at the NASA Lewis
dynamltron and at the Naval Research Laboratory Van de Graff generator. (The elec-
tron irradiation facilities at NRL were made available through the cooperation of
Richard Statler and Robert Farr.) The cells were irradiated to a total fluence of
18g
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I_IO] 5 electrons/cm2, ' wlth performance measurements made at several intermediate
fluence levels. The performance measurements consisted of the following:
(1) I-V data at 25 °C and 1AMO using an X-25 xenon solar simulator and a
reference cell
(2) I-V data at 25 °C at several concentrations up to 100 times AMO and above
using a pulsed xenon solar simulator and the linear assumption between
Irradlance and short-circuit current
(3) Short-circuit current data at one fixed concentration at both 25 and 80 °C
tn order to set the current scale at the elevated temperature
(4) I-V data at 80 °C at several concentrations as tn step (2)
During I-V measurements the cells tn their holders are mounted to a temperature-
controlled block. The concentration level on the cell Is varied by a combination
of changing the distance from the light source and using and not using a fresnel
lens. Since the duration of the light pulse from the flash simulator Is Just
2 msec, there is no heating effect from the concentrated light. The elapsed time
at 80 °C was about 90 mln for each cell. Several repeat measurements were made at
1 sun and 25 °C after the elevated temperature measurements tn order to determine
If any annealing had taken place.
RESULTS AND DISCUSSIONS
The data presented In the tables and figures are the average of the flve cells
which were carried throughout the electron irradiations. Table I shows the initial
I-V parameters before irradiation. One cell was somewhat lower In current and
efficiency than the others and therefore lowered the averages. Two cells had an
efficiency of 19 percent at IOOx AMO and 80 °C.
Figure 1 Is a plot of cell efficiency versus concentration level for the unlr-
radiated case and two irradiated cases for data taken at 25 °C. One common factor
among the three curves Is that they get flatter wlth increasing electron fluence.
This indicates that there Is more power loss due to electron irradiation at concen-
trated levels than at l sun. Thls can be seen more clearly In figure 2, which shows
the ratio of maximum power Pmax after irradiation to the initial value as a
function of electron fluence for both AMO and IOOx at 25 °C.
Table II shows the ratios of short-clrcult current Isc, open-clrcult voltage
Voc, flll factor, and Pmax after irradiation to the unlrradlated values for
several fluence levels at 25 °C and at l and lOOx AMO. At l sun the power
degrades to 78.9 percent of the unlrradlated value while It drops to 74.1 percent
at lOOx AMO. Both the voltage and flll-factor changes contribute to the greater
power loss after irradiation at concentration. Note that the flll factor increases
by 3 percent at l sun after irradiation while It decreases by 1.6 percent at lOOx
AMO. The ratios for short-clrcult current are the same for both solar irradiation
levels due to the linear current-lrradlance assumption.
There Is somewhat more Voc degradation at lOOx AMO than at l sun (0.91
versus 0.925). Thls indicates that even though the unlrradlated Voc Is larger
at IOOx AMO than at l sun, It Is still taking a larger percentage drop after
190
irradiation. Thls can be seen more clearly In table Ill, which lists the differ-
ences in Voc between lOOx AMO and l sun measured after different fluence
levels The initial Voc difference of 180 mV drops to about 150 mV after
Irradlatlon to IxlO 14 electrons/cm 2 and then remains fairly constant for the
final two electron fluences.
The decrease in the Voc difference indicates that the cell Is becoming more
diffusion current dominated and less space-charge recombination current dominated,
as the electron fluence increases. A difference of about 120 mV (60 mV/decade for
2 decades) would be expected for a cell wlth an n value of I. At present, we hav(
no good explanation for the change In Voc differences.
Figure 3 shows the degradation In Pmax at lOOx AMO for both 25 and 80 °C.
The curves are nearly identical indicating that the effects on cell performance due
to electron irradiation are essentially the same at the two temperatures.
Performance data were also taken at 25 °C after the 60 to 90 mln spent at 80 °C
for measurement purposes In order to determine If there were any annealing effects.
In all cases, there was no annealing due to the tlme spent at 80 °C.
CONCLUDING REMARKS
Flve small Hughes GaAs concentrator cells were irradiated wlth l-MeV electrons
to a total fluence of lxlO 15 electrons/cm 2. After several different intermedi-
ate fluences, performance measurements were made at both 25 and 80 °C at different
Irradlance levels. The major conclusions are as follows:
I. The drop In Pmax after irradiation was larger at lOOx AMO than at
l sun.
2. There was no significant difference in the degradation of cell performance
when measured at 25 or 80 °C.
3. There was no annealing due to about 90 mln spent at 80 °C for measurements.
REFERENCE
I. Patterson, Robert E.: Preliminary Concept of a lO0-Kllowatt Miniaturized
Cassegrainian Concentrator Solar Array. Space Photovoltalc Research and Tech-
nology 1983, NASA CP-2314, Oct. 1983, pp. 157-162.
191
TABLEI. - I-V PARAMETERS
FORUNIRRADIATEDCELLS
Concentration
Temperature, °C
Short-clrcult
current,
Isc, mA
Open-clrcult
current,
Voc, V
Fill
Efficiency,
percent
l lO0
25 80
3.46 365.5
0.947 1.041
0.755 0.815
14.5 18.3
TABLEII. - RATIOSOF IRRADIATEDTO INITIAL
VALUESFORSEVERALFLUENCES
Irradlatlon, Short-clrcult Open-clrcult Fill Maximum
electrons/cm 2 current, current, power,
Isc Voc Pmax
1 sun
I xl 013
3xlO13
ixlO 14
3xlO14
ixlO 15
0.982
.965
.925
.893
.828
0.990
.980
.964
.947
.925
l.OlO
1.007
l.Ol9
1.024
1.030
0.982
.954
.908
.866
.789
lOOx AMO
ixlO 13
3xlO 13
ixlO 14
3xlO 14
ixlO 15
0.982
.965
.925
.893
.828
0.978
.962
.941
.932
.910
0.997
.994
.992
.988
.984
0.957
.924
.863
.822
.741
192
TABLEIII. - DIFFERENCESIN
CURRENTV BETWEEN
OC
lOOx AMO AND l SUN
AT 25 °C
Electron
Irradlatlon,
electrons/cm 2
Unl rradlated
IxlO 13
3xlO 13
lxlO 14
3xlO 14
IxlO 15
Voc
difference,
mV
180
165
157
149
152
150
20
18
16
14
12
10
Unirradiated
3.2 I0.0 31.6 I00.0
Sunlight concentration
Figure I. - Cell efficiency versus sunlkjht concentration for
different electron fiuences.
.2
1.0
.9
.8
.l
_ [] IOOXAMO __
I I I
--VA 1013 1014 1015
Electron fluence/cm 2
Figure 2. - Pmax ratio versus electron fluence for 1 sun and
IOOXANIOat 25 °C.
.2
2
1.0
.9
.8
.1
c
-- 080
f J __l
0 lO13 1014 !015
Electron fluence/cm2
Figure 3.- Pmax ratioversus electron fluence for lOOXAk;Oat
25 and 80 °C.
193


Performance of Hughes GaAs concentrator cells under 1-MeV electron irradiation

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