Silicon solar cells with efficiencies ranging from 17 to 17.7 percent are described. These cells were processed on low-resistivity FZ substrates using techniques recently developed for high efficiency terrestrial solar cells. Preliminary results indicate that the high efficiency cell is more susceptible to radiation damage in that it retains a smaller proportion of its original power output when compared to conventional space cells after exposure to 5 times 10 to the 14th power 1 MeV electrons. However, the cell does maintain a greater overall power output than the conventional cells to which it was compared. Furthermore, this cell does not demonstrate post-electron irradiation photon decay as has been described for cells processed on 1-10 ohm-cm float zone silicon

Crotty, G. T.

English

NASA Technical Reports Server

N8 7- 26 423
PROCESSING AND CHARACTERIZATION OF HIGH-EFFICIENCY
SILICON SOLAR CELLS FOR SPACE
G. T. Crotty
California Institute of Technology
Jet Propulsion Laboratory
Silicon solar cells with AM0 efficiencies ranging from 17 to 17.7% are described.
These cells were processed on low-resistivity FZ substrates using techniques recently
developed for high-efficiency terrestrial silicon solar cells. Not surprisingly, pre-
liminary results indicate that the JPL high efficiency cell is more susceptible to
radiation damage in that it retains a smaller proportion of its original power output
when compared to conventional space cells after exposure to 5 x 10 1 1MeV electrons.
However, the JPL cell does maintain a greater overall power output than the conven-
tional cells to which it was compared. Furthermore, this cell does not demonstrate
post-electron irradiation photon decay as has been described for cells processed on
I-I0 ohm-cm float zone silicon.
INTRODUCTION
To meet NASA goals for high performance, arrays of 300 W/kg will require thin
solar cells with BOL efficiencies >20%, or thin film devices with equivalent power-
to-weight ratios (ref. I). A number of technologies such as GaAs, thin film multi-
junction cells, indium phosphide and a-Si:H show promise of meeting or exceeding
these goals and are currently in various stages of development.
Until these technologies mature, however, silicon will continue to be the work-
horse of the space photovoltaics industry. This is most likely to be the case for
some time and silicon should be expected to maintain a place in spacecraft power
systems even as other technologies begin to prove themselves as viable alternatives.
The theoretical maximum efficiency for a silicon solar cell at AMO is between
21 to 24%. Actual achievable efficiency will be considerably lower, but it is prob-
ably not unrealistic to strive for 18 to 19%. Recently, considerable progress in
increasing terrestrial silicon solar cell efficiency has been made. JPL's DOE-
supported efforts have resulted in silicon solar cells with AMI.5, global efficiencies
as high as 20%. At AMO, these same cells measure 17.7%.
These cells, although initially designed primarily for terrestrial use, may
conceivably find application in space. This paper will describe the design and
processing of the JPL high-efficiency cell. Also discussed, will be preliminary
tests of these cells to moderate 1MeV electron radiation and comparison to conven-
tional space cells. Post-electron irradiation photon degradation will also be
discussed.
81
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DESIGNANDPROCESSING
The JPL high-efficiency silicon solar cell incorporates many of the recent
developments in solar cell processing technology designed to reduce minority carrier
recombination in the bulk, at the front surfaces and under the contacts. Figure I
is a diagrammatic view of the JPL high-efficiency cell. The structure of this cell
is similar to the microgrooved PESCcell reported by Green, et al. (ref. 2), in that
the contacts rest on mesasabove a textured surface. The mesasoffer two advantages.
Oneis that since the surface is not textured, it reduces the total surface area
contacted by the metal. The other advantage is that they aid in the alignment of
the masks during various photolithography steps.
Following texturing, the N-type emitter is formed using a phosphine source at
825°C. Total diffusion time is 15 minutes. After removing the diffusion glass, the
wafers are oxidized at 900°C in an atmosphere composedof 94.5% N2, 4.5% 02, and
i% HCI. This step results in an oxide that has been measuredon planar <100> surfaces
to be 100-120 A thick. Following oxidation, the wafers are annealed at 600°C for
one hour. The furnace is then cooled to 450°C. After cooling, the wafers are kept
at 450°C for one hour, and then pulled from the furnace. This annealing step was
chosen because it has been found to recover the minority carrier lifetimes in float
zone ingots after high temperature treatments (ref. 3).
Ohmicback contacts are formed by electron beamevaporation of AI, Ti, Pd and
Ag followed by a 20 minute sinter at 490°C in hydrogen. Front contact to the cells
is madeby opening 4-vm-square contact windows in the oxide and then evaporating the
Ti, Pd and Ag grid lines and bus bar over the windows. These windows were spaced at
50, 100, 200 and 400 _m centers for each of the four cells processed on an individual
wafer. This spacing results in actual contact areas of 0.575 to 0.075% of the total
cell surface. Table I lists the spacing percent contact area for each cell measured.
Shadowingfrom the front contact grid amounts to approximately 3.3% of the total cell
surface.
The primary purpose for varying the contact area was to determine the effect of
contact area on Voc and series resistance. From Table I, Voc does not show the
expected increase with decreasing contact area. This may be because recombination
in the base and emitter, along with band bending effects have begun to dominate so
that recombination under the contacts is no longer relevant. As can be seen from
the fill factors in Table I, series resistance does not increase with decreasing
area.
RADIATIONTESTS
To gain someunderstanding of the extent to which these cells might degrade in
the space environment, cell 2B from Table 1 was irradiated with 5 x 1014 1 MeV
electrons. Light I-V data for this cell is summarizedin Table II.
The power degradation of the JPL cell was compared to that of two types of
cells routinely used in space photovoltaic arrays (ref. 4). K6-3/4 is a 2-mil cell
with a dual layer AR coating. K7 is similar in design to the JPL cell in that it is
textured, has a dual layer AR coating and is 8 mils thick. Both K6-3/4 and K7 were
fabricated on i0 ohm-cmbase material and have back surface fields and reflectors.
82
Figure 2 exhibits the power loss of cells K6-3/4 and K7 through a range of 1MeV
electron fluences along with the JPL cell before and after 5 x 1014 1MeV electrons.
Cells K-6/34 and K7 retained 75 to 85%of their original power. The JPL cell exhibits
considerably more deterioration in that it maintains only 67%of its original power
output. Nevertheless, the power output of this cell is still 2.3% greater than for
cell K6-3/4 and 4.3% greater than cell K7.
Float zone silicon has not typically been used on space arrays in part because
of the additional photon degradation observed for devices made on I-I0 ohm-cmFZ
material (ref. 5). Even though devices processed on float zone material may initially
demonstrate higher performance than devices made on Cz material, after electron
irradiation, the performance of the float zone devices deteriorates to where they
are comparable to Cz devices after photon irradiation. Therefore, Cz is the material
of choice since it is less costly and easier to obtain.
Photon degradation was also expected for the JPL cell. Table II summarizesthe
light I-V data for cell 2B following a 24-hour photon irradiation period carried out
after 5 x 1014 i MeVelectron irradiation. Surprisingly, no further degradation
is observed for this cell processed on low-resistivity float zone material.
These radiation studies to date are encouraging, even though the results are
very preliminary. If the trend toward higher post-irradiation power output continues,
it is possible that this material and device structure may find future mission appli-
cations.
CONCLUSION
The significant increase in efficiency exhibited by the JPL high-efficiency
cell is derived primarily from optimization of relatively conventional design
approaches. These include very high-quality starting material, textured surfaces,
front surface passivation, reduced contact area and reduced contact shadowing.
The behavior of these cells in radiation is encouraging in that they maintain a
higher power output after 5 x 1014 1MeV electrons than for conventional space cells
and do not suffer from photon degradation.
83
REFERENCES
I. Scott-Monck, J. A.; and Stella, P. M.: Advanced Photovoltaic Solar Array
Program--A Preliminary Assessment. Proceedings of the 21st IECEC, San Diego,
California, August 1986, p. 1458.
2. Green, M. A.; Blakers, A. W.; Wenham, S. R.; Narayanan, S.; Wilson, M. R.;
Taouk, M.; and Szpitalak, T.: Improvements in Silicon Solar Cell Efficiency.
Proceedings of the 18th IEEE Photovoltaics Specialists Conference, Las Vegas,
Nevada, October 1985, p. 39.
3. Graff, K.; Peiper, H.; and Goldbach, G.: Carrier Lifetime Doping of P-Type
Silicon by Annealing Processes. Semiconductor Silicon 1973, Edited by Howard R.
Huff and Ronald Burgess, The Electrochemical Society, Princeton, New Jersey,
p. 170.
4. Anspaugh, B. E.; Downing, R. G.; Tada, H. Y.; and Carter, J. R., Jr.: Solar Cell
Radiation Handbook, Third Edition, JPL Publication 82-69, pp. 3-60 to 3-162.
5. Crabb, R. L.: Photon Induced Degradation of Electron Irradiated Solar Cells.
Proceedings of the 9th IEEE Photovoltaic Specialists Conference, May 1972, p. 329.
84
Table I - AM0I-V Data for 7 of the JPL High-Efficiency Silicon Solar Cells
Cell S* Contact Voc Isc Fill Effic.
ID (_m) Area (%) (mV) (mA) Factor (%)
1A 400 0.575 647.0 179.4 0.799 17.14
ID 50 0.075 649.2 179.2 0.804 17.27
2B 200 0.29 650.4 176.8 0.810 17.20
3B 200 0.29 652.8 178.1 0.802 17.23
3C 100 0.15 660.1 178.4 0.815 17.74
4A 400 0.575 649.2 178.9 0.801 17.20
4D 50 0.075 649.7 178.9 0.805 17.30
*S = contact spacing
Table II - I-V Data for Cell #2B
Voc
(mv)
650.4
587.2
588.7
Isc
(mA)
176.8
126.3
127.7
Fill
Factor
0.810
0.795
0.794
Effic.
(z)
17.2
11.50
11.51
Remarks
Prior to 5 x 1014 1MeV
electron irradiation
After irradiation
After 24-hr photon
irradiation
85
NG
IN OXIDE
4#m x 4_um
ZD
SURFACE
FRONT
CONTACT
(Ti-Pd-Ag)
OXIDE
250-300_m
Figure i.
N +
3_m
DE
(_ 100AI
P-TYPE
FZ
0. 18 OHM-CM
'BACK
METAL
(AI-Ti-Pd-Ag)
Schematic diagram of the JPL high-efficiency silicon solar cell
Figure 2.
24
22
2o
18
o
"' 16
14
10
I I | I I I I I] I I I I IIII I l I I I I I II
--_K7
-- 6
0 lO 161013 1014 1015
2
ELECTRON FLUENCE, e/cm
Maximum power vs. I MeV electron fluence for JPL high-efficiency cell and
2 conventional space cells. Maximum power after 5 x 1014 e/cm 2 is 15.6
mW/cm 2 for the JPL cell, 15.2 mW/cm 2 for K6-3/4 and 14.9 mW/cm 2 for K 7.
86


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