Gay, C.

Scott-Monck, J.

English

NASA Technical Reports Server

Development of High Efficiency, Radiation Tolerant, Thin Silicon
 
Solar Cell
 
CI4 
JPL Contract 954600
 
Third Quarterly Report N
 
0 
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for Period Covering
 
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1 March 1977 through 31 May 1977 HH 
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prepared by #I 44 
Charles Gay and John Scott-Monck 4 4J 0H W4 w 
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of 
P4 2 P r"Spectrolab, Inc. 

12500 Gladstone Avenue H
 
ZZ -; Sylmar, California 91342 t M
 
S -June 15, 1977 
-Zrll 
 "This work was performed for the Jet Propulsion Laboratory, 
; __ California Institute of Technology, sponsored by the National 
"<n Aeronautics and Space Administration under Contract NAS7-100." 
Spectrolab, Inc • 12500 Gladstone Ave., Sylmar, Calif 91342 / (213) 365-4611 / TWX. 910-496-1488 / a subsidiary of Hughes Aircraft Co. 
https://ntrs.nasa.gov/search.jsp?R=19770078920 2020-03-22T06:33:16+00:00Z
This report contains information prepared by Spectrolab, Inc. under
 
JPL subcontract. Its content is not necessarily endorsed by the
 
Jet Propulsion Laboratory, California Institute of Technology, or
 
the National Aeronautics and Space Administration.
 
i 
ABSTRACT
 
Back surface field cells ranging in thickness from 0.10 to 0.25mm in 0.05mm
 
steps were fabricated. Two ohm-cm cells, 0.15mm (.006") thick produced an
 
average power output in excess of 80 mW for 2 X 2cm configurations. Power
 
output was independent of cell thickness over the range of 0.15 to 0.25mm
 
for all silicon base resistivities used.
 
Textured cells 0.05mm (.002") thick were produced in 2 X 2cm size which
 
delivered power outputs as high as 62mW for non-field types and in excess
 
of 67mW when back surface field processing was employed. Reinforced
 
perimeter configurations as thin as 0.020mm were produced with polished and
 
textured surfaces.
 
ii
 
Table of Contents
 
Section Title Page
 
1.0 Summary 1
 
2.0 Introduction 1
 
3.0 Technical Discussion 2
 
4.0 Conclusions 9
 
5.0 Recommendations 9
 
6.0 New Technology 9
 
7.0 Three Month Projection 9
 
iii
 
1.6 Summary 
A twelve element matrix composed of 2 X 2cm textured back surface field cells
 
of varying resistivity (2-100 ohm-cm) and thickness (0.10-0.25mm) were
 
fabricated and delivered to JPL for evaluation. Outputs ranging up to 81mW
 
were observed and power was independent of silicon thickness from 0.15 to
 
0.25mm for all resistivities employed.
 
One hundred two ohm-cm textured 2 X 2cm cells 0.05mm thick were fabricated and
 
delivered to JPL during this period. Cell weight, including contacts, was
 
below 60mg, and this lot had an average output power of 57mW with the best
 
cell delivering 62.4mW. Some 0.05mm devices were made using back surface
 
field processing and they produced ten to fifteen percent more power than the
 
cells prepared for delivery. Some of these very thin cells showed evidence
 
of severe warping due to the mismatch in expansion coefficients between the
 
contact material and the silicon.
 
Ten samples of our reinforced perimeter structure were also made and delivered
 
to JPL. These parts consisted of a narrow (0.75mm) border approximately 0.20mm
 
thick bounding a 0.02mm silicon center section. This structure can be easily
 
handled and shows great promise as a method for large scale production of
 
very thin silicon cells.
 
2.0 Introduction
 
The purpose of this program is to develop the necessary technology to produce
 
very thin (0.10mm maximum), high output (17.5 mW/cm2), silicon solar cells
 
which will display improved radiation performance compared to present state­
of-the-art devices. The major emphasis shall be in the areas of back
 
surface field and selective etch technology. Higher resistivity (50 ohm-cm)
 
silicon than is normally used for space flight cells will be investigated as
 
a method for achieving the radiation objectives which are targeted as
 
15.5 mW/cm2 after 3 X 1014 equivalent 1 MeV electrons /cm
2 and 14.0 mW/cm2
 
.
after 1 X 1015 electrons/cm2
 
3.0 Technical Discussion
 
3.1 Back Surface Field (BSF) Matrix
 
A sixty cell matrix consisting of four thicknesses and three resistivities was
 
produced. The field was incorporated into these cells using aluminum paste
 
which was applied by screen printing techniques. This method has been
 
described in some detail in the Second Quarterly Report of this contract.
 
All cells were diffused using phosphine gas as the junction forming dopant
 
source. Sheet resistances of 120 ohms/square, corresponding to a junction
 
depth of "0.12/am were obtained for these samples. The cells were contacted
 
using tantalum-palladium-silver front contacts and chromium-palladium-silver
 
for the back surface metallization. The contact configuration, consisting
 
of a 1.0mm wide collector bar and 12 grids/cm, was deposited by electron
 
beam evaporation through bimetallic masks. Tantalum pentoxide (Ta205 ) was
 
used as the antireflection coating. These cells did not have ultraviolet
 
rejection filters. All cells were electrically tested under AMO conditions
 
(135.3mW/cm2 ) at 25 C. Table 1 is a summary of the electrical properties
 
of these cells.
 
It can be seen that there is no statistically significant difference in cell
 
output for cell thicknesses between 0.15 and 0,25mm. In fact the high end
 
range of the thinnest cells (0.10mm) overlaps the range for the thickest cells
 
in some cases. The variation in thickness within any sample group was tightly
 
controlled to ± 0.012mm. This was verified by weighing the individual samples
 
and calculating the silicon thickness knowing the weight of the contact metal.
 
One sample from each element of the matrix was randomly selected for spectral
 
response measurements. Table 2 shows the data obtained from these twelve
 
cells.
 
2
 
Thickness 

(mm) 

0.10 

0.15 

0.20 

0.25 

0.10 

0.15 

0.20 

0.25 

0.10 

0.15 

0.20 

0.25 

Table 1
 
Electrical Properties of BSF Matrix
 
Resistivity

(ohm-cm) 

2 

2 

2 

2 

10 

10 

10 

10 

100 

100 

100 

100 

I 

sc
(mA) 

159-162.5 

163.5-165 

165-167.5 

160-164.5 

159.5-160.5 

161.5-165 

163.5-166 

163.5-165 

161.5-165.5 

163-165.5 

165-167.5 

166-166.5 

V P 
oc(mV) max(mW) 
600-608 76.5-78.3 
609-612 81.1-81.6 
607-612 78.3-80.3 
608-613 78.5-80.3 
600-606 76.5-77.3 
600-605 76.8-77.8 
602-605 78.1-79.4 
602-608 77.3-78.3 
588-605 72.5-77.3 
599-604 7Q.0-77.5' 
594-597 75.5-77.0 
601-603 76.0-78.0 
3
 
TABLE 2 
BSF CELL MATRIX SPECTRAL RESPONSE (SINGLE SAMPLE) ,kA//W 
Filter 
Cell1.0/ 
TV .57 .5% .75 .8//8 .9 J .%.1.05.9 
0.10mm 
2s.-cm .14 .31 .40 .44 .47 .51 .55 .60 .64 .71 .74 .74 .30 
0.15mm 
2.s_--cm .14 .31 .40 .44 .47 .51 .55 .60 .64 .72 .75 .81 .37 
0.20m 
•.20m'.14 .31 .40 .44 .47 .51 .56 .61 .65 .73 .77 .86 .44 
0.25m 
2_.-cm .14 .31 .39 .43 .46 .51 .55 .60 .65 .72 .76 .88 .48 
0.10~mmlos-cm .14 .3i .39 .43 .46 .51 .55 .60 .65 .73 .75 .73 .28 
0.15rm 
10 a-cm .14 .31 .39 .43 .46 .51 .55 .60 .65 .73 .77 .82 .35 
0.20mm 
10-2-cm .14 .31 .39 .44 .47 .51 .55 .60 .65 .73 .78 .90 .46 
0.25mm 
10-a-cm .13 .29 .39 .43 .46 .50 .55 .60 .64 .72 .77 .90.51 
0.10mm 
100-G-cm0.m-m .14 .30 .39 .43 .46 .50 .54 .59 .63 .70 .72 .70 .30 
o.15ram100-52-cm .14 .30 .39 .43 .46 .50 .54 .59 .63 .70 .72 .757"3 34 
0.20mm 
l0.a.-cm .13 .30 .39 .43 .46 .50 .54 .59 .63 .70 .72 .81 .40 
0.25mm 
100-l-cm .14 .30 .39 .43 .46 .50 .54 .59 .63 .70 .7j I .84 .45 
4
 
3.2 
The influence of cell thickness was not evident for wavelengths equal to or less
 
than 800 nm, and did not become pronounced until 950 nm. The 0.10mm cells
 
showed very little difference in long wavelength response as a function of
 
base resistivity. The fact that the high resistivity 0.25mm cell had a
 
lower response than either the ten or two ohm-cm cell of the same thickness
 
is probably an anomoly caused by the sampling method used.
 
Thin (0.05mm) Cell Fabrication
 
A one hundred piece lot of texturized, 0.05 mm thick cells was completed during
 
this month. This lot was composed of cells from a variety of fabrication
 
schemes to test the effects of silicon blank preparation, back contact
 
metallization variations, contact application tooling and back surface
 
reflectors. The average cell output was approximately 57mW at 250and the
 
highest output was 62.4 mW as can be seen in Figure 1. The average cell
 
.mass was 57 mg of which approximately 15 mg was contact metallization.
 
3,2.1 Effect of Silicon Blank Preparation
 
The silicon slices used for production of 0.05 mm cells were obtained by
 
either etching an oversize 2 X 2 cm part or dicing a 2 x 2 cm blank from a
 
thinned round wafer. The latter technique is necessary if the aluminum
 
paste technology for back surface fields is employed. In either of the
 
above two cases it was found that lapping to remove blade marks from sawing
 
was not necessary. Blade marks are rarely more than 0.006 mm in amplitude
 
and they do not significantly affect the handling properties of thin cells.
 
If an oversize 2 X 2 blank is to be thinned, a one-step etch in sodium
 
hydroxide is used to assure squared edges. This leaves a surface which
 
might be best described as waffled or "orange-peel" in appearance. If the
 
wafer is cut from an oversize round, two possible surface finishes are
 
possible. The first is the "orange-peel" surface described above and the
 
other is the polished surface obtained by using solutions of nitric­
hydrofluoric and acetic acids either alone or followed by the hydroxide
 
etch. Comparisons of the mechanical properties of these two surface finishes
 
both before and after texturizing failed to show any significant advantage
 
in handling,
 
5
 
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Because of its size and flexibility, the 2cm X 2 cm blank of 0.05 mm thickness
 
is nearly as resistant to fracture as thicker parts of the same area. In the
 
case of larger area parts of the same thickness there was a higher incidence
 
of breakage which occurred most frequently during tweezer handling. The
 
forceps would induce a torsion at the edge causing a fracture which readily
 
propagated across the wafer. To overcome this problem edge reinforcement
 
was examined. Several techniques were found to be successful. The actual
 
wafer edge thickness could be increased by fusing a silicon border to the
 
wafer. - This was achieved by using an aluminum foil preform between the wafer
 
and reinforcing silicon and heating in an inert atmosphere to a temperature
 
in excess of 5770C.
 
A more direct approach involved thinning the center section. This could be
 
done using chemical etching or mechanical abrasion. The use of acids proved
 
to be ineffective for thin slices since the acid etched most rapidly at the
 
interface between the wafer and resist. When the center section was thinned
 
to 0.05 mm, it would frequently fall away from the border. Both the hydroxide
 
etching and abrasive techniques avoided this problem. Base etching was
 
accomplished with sodium hydroxide in 30% concentration at a temperature of
 
1050C and abrasive etching utilized a highly controllable silicon wafer
 
abrasion machine, which is commercially available. Subsequent chemical
 
etching to thin the wafer was found to be adequate for removal of the lattice
 
damage caused by abrasive etching even at the highest rates of removal
 
(25zm/minute). This latter technique was successfully employed in the
 
production of 0.05 mm cells with a back surface field.­
3.2.2 Back Contact Metallization on 0.05 mm Cells
 
Cells of 0.05 mm thickness which were contacted using the conventional
 
titanium-palladium-silver or chromium-palladium-silver back metallizations
 
warped severely after sintering. This was due to the quantity of silver
 
(3-41cm) deposited which is not matched in expansion coefficient with
 
silicon. It was possible to avoid this problem by applying a thinner
 
layer of silver (tslym), sintering, and then building up the silver thickness
 
in a second deposition step which did not require any further heat treatment.
 
7
 
Several successful alternatives were also devised. An aluminum-silver
 
contact was used which did'not require higher temperatures to achieve an
 
adherent, ohmic contact. A molybdenum-palladium-silver system was
 
successfully used in which 2 to 3pm of molybdenum provided the electrical
 
conductivity and good expansion match to silicon and the silver thickness
 
could correspondingly be reduced. Finally, a grid configuration back contact
 
was tested which helped eliminate warping 'and reddiced total cell weight.
 
3.2.3 Contact Application Tooling
 
Two approaches to contacting 0.05 mm cells were examined. One involved the
 
use of conventional bimetallic masking and cell holders, and the-other used
 
bimetallic masks on round wafers from which a 2 X 2 cm part would later be
 
diced. The latter technique was abandoned when it was found that substantially
 
fewer wafers could be contacted in a single pump-down and the dicing operation
 
caused a slight power loss. The one hundred, 0.05 mm cell complement sent
 
to JPL was contacted in the standard frames but a number of cells had poor
 
curve shape due to contact metal being deposited over the cell edge. Attempts
 
to clean the metal from the edges resulted in cell fracture due to the
 
fragile nature of these thin devices. New tooling has been ordered with
 
optimized grid spacing and locators which should eliminate this problem.
 
3.2.4 Back Surface Reflector
 
To investigate the possibility of utilizing longer wavelength radiation with
 
thin cells, a matrix of 0.10 mm thick, 1.5 ohm-cm cells was-prepared. The
 
back of the cell was either polished or texturized and was contacted with
 
one of several different metals. A slight advantage accrued to the smooth
 
surface in terms of short circuit current gain with an apparent further
 
increase observed for an aluminum metallization.
 
Perimeter Reinforced Thin Silicon Slices
 
The edge prdtected, hydroxide etching technique discussed in 3,2.1 was used
 
to prepare silicon slices in the form of 2 X 2 cm squares with a center
 
section 0.025 mm thick and a border 0.20 mm thick and 0.75 mm wide. The
 
average wafer mass was 47.7 mg of which 28.3 mg was attributable to the 
border thickness. This implies an average center section thickness of 
- 0.022 mm. Slices as thin as 0.006 mm were readily prepared using this 
technique. This-configuration could be easily handled without fracturing
 
the silicon.
 
8
 
3.3 
The reinforced perimeter configuration prevents severe cell warping caused
 
by contact metallization, allows standard handling and interconnect procedures
 
to be used, and can be used for fabrication of thin wrap-around contact tells.
 
Such a cell necessarily requires thick edges to allow the use of dielectric
 
pastes, which typically have poor expansion matches with silicon. At present
 
a number of such wafers have been prepared and have aluminum back contacts
 
which remain from the field incorporation step. It was found best to
 
leave the aluminum in place for good dielectric adherence, Several thick
 
film dielectric materials are now being tested to ascertain the optimum
 
combination of firing times, thicknesses and number of individual layers
 
required for a pinhole-free layer of glass.
 
3.4 Production of 0.05 mm BSF Cells
 
A 0.05 mm, 10 ohm-cm, back surface field 2 X 2 cm cell yeilded 67.6 mW at
 
250C as shown in Figure 2. This cell was prepared from a round wafer with
 
a thick border. This type of cell will be used with an optimized grid
 
configuration and a weld pad collector bar for the next delivery of 0.05 mm
 
cells to JPL.
 
4.0 Conclusions
 
With new tooling and further process optimization, it may be possible to
 
mass produce 0.05 mm cells with AMO efficiencies approaching thirteen percent.
 
5.0 Recommendations
 
Spectrolab recommends the deletion of two hundred solar cells of 0.01cm
 
thickness of highest available resistivity from the delivery schedule and
 
substitution of 0.15 mm, 2 ohm-cm BSF cells and reinforced perimeter cells
 
with center section thicknesses of 0.025 mm.
 
6.0 New Technology
 
Spectrolab has no new technology to report at this time.
 
7.0 Projected Work - Next Three Months 
9
 
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VOLTAGE (VOLTS X___ ) 
The next three months will see the completion of this contract and the
 
delivery of the following devices:
 
1. 	Eighty process integration matrix cells presently
 
awaiting tooling delivery.
 
2. 	Two hundred 0.05 mm cells also awaiting tooling
 
delivery.
 
3. 	Twenty wrap around cells.
 
4. 	Two hundred evaluation cells from pilot line
 
production.
 
ii
 


Development of high efficiency, radiation tolerant, thin silicon solar cell

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