Note: Descriptions are shown in the official language in which they were submitted.
~3~
1 5 7 0 : RD S
1 THIN SOLAR CELLS
Background
I linted glass plates in architectural
Sll structures has become a significant feature of contexPo-
rary design. These tinted glass plates are used 2x-
tensively in the construction of ofrice buildings, schools,
hospitals, factories and other structures to reduce glare
and provide heat absorption and lower the operatins costs
10¦ for air conditioning. In many instances passive plate
I glass surfaces encompass entire bui~dinss and consequen~ly
could be a significant source of electrical energy, if
provided with a photovoltaic capability. Preliminary
calculations indicate that even with low efficiency
photovoltaic responses, su~ficient curren~ could be
generated 'o offset a portion if not all of electric
power requirements for the enclosed structure.
Present commercial solar cells are significantly
limited by voltage and ef'iciency ar.d in exPer men.ing
with thin films on glass substr2tes, an additional
goal was establishea to increas2 the voltage and
efficiency of a photovoltaic cell by combining the
appropriate properties of e:~istins materials into a
laminated or compound structure.
Regardless of the method of construction, in
the end each completed cell must be rearranged in
groups or "zrrays", and this requiremen' ~ictates the
final objective of this study which was to devise a
~Z~33 ~2
means by which these complete arrays consisting of sheets of
multiples of identical cells, can be printed or generated
simultaneously together with the neeessary circuitry, the
complete sheet fully equipped and ready for installation.
Brief Summary of the Invention
In accordance with the present invention there is
provided a semi-transparent solar cell comprising:
a ~assivated a~ subst,r~t~;
a first electrically conductive transparent
film on the glass substrate;
a photovoltaic semiconductor layer over the
conductive film having a P-N junction parallel to the
glass substrate and suffieiently thin to be substantially
transparent;
; a second electrically conductive transparent film
over the semiconduetor layer and eleetrieally isolated
from the first eonductive film;
an eleetrieally insulating transparent layer over
the second conductive film;
a first electrieally conduetive bus bar on the
glass substrate along one edge of the semieonductor layer
and connected to the first eonduetive film; and
a second electrically conductive bus bar on the
glass substrate along the opposite edge of the
semieonduetor layer and connected to the second
conductive film.
-- 2
~:3l33~
Solar cells and arrays of solar cells are made as thin
films or insulating substrates. In an exemplary embodiment a
thin conductlve film is deposited on a passivated glass
substrate and a photoactive semiconductor film is deposited over
the conductive film. The semiconductor film has a P-N junction
parallel to the substrate and can extend beyond at least some of
the edge of the conductive film for insulation. ~nother
conductive film is deposited over the semi-conductor and is
insulated from the first conductive film. Contacts made to
edges of the conductive films form solar or photovoltaic cells.
In an array of such cells the edge of the second conductive film
of one cell can overlap the edge of the first conductive film of
an adjacent film for connecting the cells in series. A
plurality of semitransparent film may be stacked to absorb
selective portions of the spectrum. A technique of stacking
thin films may be employed using diagonal displacement of a mask
providing successive layers with exposed and covered edges for
suitable electrical connections.
- 2a
~ 3;3~
1 Brief Description of the Drawings
. , . . . __
FIG. 1 is an exploded perspective v.iew of many of the
layers in a solar cell;
FIG. 2 is a plan view of the solar cell of FIG. l;
FIG. 3 is a plan view of a second embodiment of solar
cell;
FIG. 4 is an exploded transverse cross section of
the second embodiment of solar cell;
FIG. 5 is an exploded perspective view of a thi.rd
embodiment of solar cell;
FIG. 6 is a plan view of the solar cell of FIG. 5;
FIG. 7 is an exploded transverse cross section of a
multiple cell array of solar cells;
FIG. 8 is a plan view of the multiple cell array
lS of solar cells;
FIG. 9 is a fragmentary enlargement of a part of
the array of FIG. 8;
FIG. 10 is a plan view of another array of solar
cells;
FIG, 11 is an exploded transverse cross section of
third array of solar cells;
FIG. 12 is a plan view of the array of FIG. 11; and
FIG. 13 is a schematic view of a float glass facility
with means for mass producing solar cells.
FIG. 14 is an exploded view of a laminated solar cell
array.
1 Descr iptiOIl , .~ ,-
~ 33 ~2
Since the principal objective of this research
program was to develop photovoltaic pl2te glass, ~he
S efforts to main~ain the transDarency of -the end
product ~Jas a prima~ con~ideration. In using the
application of thin rilms to ~Gssivat2dglass sub~
strates, a significant effort was ~.ade to reduce o~
restrict '~he total thic!ness of the applied films to
0 5000 Angstro~s or less, which in effect would restric
the transparency of the end product to abou~ fifty
percent. The transparenc~ of a series Gr films
became additionally significant later in the projest
when a series of semi-transparent film combinations
or layers were stacked, one upon the other, so tha'
each photovoltaic layer would absorb a particular
portion of the remaining spectrum to which it was
exposed, extracting in total all of the available
light incident to the cell.
In so restrictinq this P-N- iunction semioon~ll~tor f; l.m An~ th.
electrically conducti~e layers, a remarkable discovery
was made- In effect, it has now been proven that the
photovolt~ic activity or propzgation of elect-ical/
current across the depletion zone and within the¦
P-N-homojunction ac.ual1y ~ccurs at a thickness ~f
less than a micron and depending on the conceniratio~.
cf dopents in all probability the thickne~s of this
zone can be less than 2000 Angstroms. In a~dition
_ 4
~33~
1 to the e~perimen's ~7ith ul~ra thin semi-transpa~-en~
phO.ovoltaic material5,a sisnificant effort was made
t~ select a highlv conductive but normally '~ansparent
ma~erlal and enhanced indillm tin ox~s proved t~ be
51 satisfactorv.
The expanded dra~ing in Figure 1 illustrates the
structure of one of the first embodiments of the family
of prototypes of large surface semi transparent photo-
voltaic devices fabricated to test the principles and
~O experimental production techniques for large surface
cells. A commercial glass plate, passivated principally
with aluminum oxide, forms the base or substrate of a
structure consisting of a series of planes or layers
comprised of one or more vacuum deposited thin films.
Figure 2 is a plan view of the same embodiment demon-
strating the outline~ and geometric patterns of the
various layers of thin films applied to a ylass substrate.
These planes are deposited upon the glass substrate
in such a manner as to conform to prescribed geometric
patterns. The geometric pattern for each of the planes
is dictated by the function of the film or films within
the plane, and the changes in patterns serve to isolate
or insulate the various deposited active layers wnich,
in total, comprise a photovoltaic cell.
~5
The geometric patterns are laid out in masks and
placed over the glass substrate so that each of the plar.es
or layers is ~eposited through its respective mask. As
each successive plane of films is applied, the masking
is changed to ouiline the next required pattern.
~L33 ~2
1 In the first plane a
pair of heavy aluminum bus bars 2 and 3 ar~ deposited
upon the 8-inch by 10-inch passivated glass substrate 1.
The bus bars 2 and 3, serve as the electrical
terminals or the completed cell ~r.d the thickne-.s
is determined by tr.e estimated amperage to be generated
at the pe~k load of the cell. FOL exæmple, in the case
of the cell depicted in F1gUreS 1 ar.d 2, approximately
~ive microns of al~min~ was used.
T~e geometric outlir.e and location of the bus ba-s
2 and 3 in Figure 1 can also be seen at 10 and 11
respectively in the pl~n view of Fi,ure 2.
Having esta~lished the aluminum bus bars 2 and 3
two conductor films 4 and 5 are deposited so they contact
bus bar 2 at 6. This conductor la~er i5 comprised of a
first aluminum film 4 of a thickness of appl-oximately
50 angstroms, which serves as a bond between the passi-
vated glass substrate 1 and the gold conductive fil~ 5.
This gold film is approximately one to two hundred
An~stroms, with a resistivity of about five oh~5 per
square centimeter and sufficient residual transparency
to pass the remaining light.~
At this point in the fabrication of the cell, with
the bus bars deposited and the conductive layer super-
imposed, the entire plate is removed and a new mask
affixed for the next series of new 1ayers.
~:L;33 ~2
1 Referring again to Fisure ~, the bus bars are seen
at ll and 12 and the alumin~Lm-cold layer ~o~posed o' Films
4 and 5 is outlined with t~e dash-dot bo-der at 13.
S The next series of fi~s in Figure 1 2re comprised
of pre-doped amorphorous-silicon wherein Film 7 is
phosphorous doped a-silicGn sublayer of appro~imatelv
3,000 angstroms, and film ~ is a boron doped a-silicon
overlay of approximately 5~0 Angstroms.
The outline of this layer can be seen in the plan
view of Figure 2 as the dash outline at 14, which is
deposited withln the confin2s of the subordinate
aluminum-gold conductive '2yer at 13.
The ~argin 15 was provided in an ef~ort to avoid
exposing the aluminum-gold la~er to po~sible direct
contact and shorting from the superimposed or outer
conductor.
As a further protection against shorting along
the edses of the subordin~te alu~.inum-gold layers
and a-silicon, an insulator strip of aluminum oxide i5
deposited at 9 in Figure 1 and outlined with short
dashes at 16 in Figure 2. This insulator strip 9 is
about 5 microns in thicknes~ which W25 considered heavy
enough to prevent contact between the lower and up~er
conductive layers.
~33 ~2
1 Having insulated the righthand margin of Layers
4, 5, 7 and 8 in Figure 1, a outer or top conductor layer
(not shown in Figure 1) of a single gold film of
approximateiy 100 Angstroms was deposited within the
margins of all of the previous layers, as shown at 17
in Figure 2, and fully extending to contact the cover
of the second bus bar 12 at 18 as shown in Figure 2.
Again the plate was remo~ed from the chamber and
0 r~masked wlth the appropri2te geometric patte~n prior
to the application of each new layei
At this point, the entire stack of layers comprising
the cell is complete. Because of the extreme sensitivity
of these layers to atmospheric water vapor and dust, a
~-otective layer of aluminum oxide 10 (Fiallre 1) is placed
over the entire cell, with the exception of the bus bars
which must remain exposed for contact. As a suggested
further protection, a layer of polyvinyl butyral, together
with a second glass protective sheet, can be bonded over
the aluminum oxide coating.
~ 33 ~2
1 Materials and Processes
B~cause of its excellent surface and other
properties, glass offers the best medium for thin
film application. It is an insulator, is corrosion
and weather resistent, and its limited coefficient
of expansion reduces the risk of fracturing the
materi~ls which are bonded to the surface, and when
heated, the mel~ing point of glass closely matches
the meltins points of the other active materials which
comprise a solar cell.
In the a?plications which are discussed in this
paper, it is essential to precondition the glass with
an aluminum oxide which passivates the glass and
prevents sodium ions from ~nisrating and contaminatiny
the adjacent photovoltaiç layers.
It should also be noted, however, that in addition
to glass substrates film can be deposited on other
materials such as polished metal or fiberglass. There
are also several types of cells produced from caomium
sulfide which are applied to copper. In the particular
case of sputtering/ success has been reportcd in the
deposit of thin films on polished steel. Although the
concepts which are discussed in this paper are specifi-
cally directed to use on glass, these other substrates
can also be considered as a useful specific for certain
forms of cells.
~IL33~2
1 In the e:~periments which have been performed to
date, several conductcrs and combinations of conductors
suitable for thin film deposition have been tried which
are, for the most part, common to the semi-con2uctor
S and solar cell industry in an effort to select a
suitable material which would have transparent characteri-
stics and retain hignly conductive properties. The
materials which were used in these experiments were
basically aluminum, gold, and indium-tin oxide (IT0).
Aluminum is highly conductiv2 and lends itself readily
to vacu~m applicatlon. It has, however, the dis~dvantase
of acting as a dopant for the silicon~ Further, aluminum
creates an undesirable discoloration when applied to
glass in thicknesses greate~ than a few Angstroms. The
advantase of aluminum is the use as a bonding agent
between the glass substrate and other conductors such
as gold. Aluminum also acts as an excellent reflector
and was suggested as an reflec~or for residual liqht
within the Composite Cell which is discussed elseYh2re
in this paper.
Gold is the best high grade conductorr In thickness
of 50 to ~00 Angstroms, gold is acceptably transparen~
which satisfied the primary criteria for this family
of cells~ However, in addition to its high cost, gold
has an other si~nificant disadvantage~ Unfortunately,
it can also be absor~ed by the silicon which creates an
alloy co~monly known as thel~urple pla~ue". This promotes
"recombination", a condition which cannot be tolerated in
solar cell construction~ Studies with the electron
I -10-
~ 3~'~
1 microscope have shown that in co~ination with cther
materials such ~s ~Iuminum, gold will spread to an e~treme-
ly thin, homogeneous, hiqhly conducti~e f:;lm and therefore
it h~s utility as a primary conduc~or in these pro-totypes.
Ho~ever, when applied to glass substrates without a bonding
agent, such as aluminum, gold tends to bead, producing
'islands" on the substrate as opposed to a closed net~-or1;~
ITO has excellent Ee~tures for transinitting light and,
for purposes of these test cells, has proven to be e~
tremely valuable. Its internal resistance, however, is far
greater than the metals, but its transparent properties and
electrical pro~erties~ when enhanced ~Jith a metallic
interlayer (gold), are suitable for these experiments.
Apparently ITO ~il] not mate with the silicon.
In examining the properties of the various photo-
voltaic materials which are readily available today, and
further, from exploring literature on the subject, it is
clear that silicon, because of the suitability-of most of
its properties, was the best candidate as a photovoltaic
material for experimental use. Efficiencies of 12 percent
have been reported with crystalline silicon which is
currently recognized as satisfactory. Other materials,
such as gallium arsenide, czdmium sulide, the tellurides,
and a range of o~her glass-like amorphous materials,
chalropyrites, are ~ery promising but for the moment
present some serious, technical and economic proble~s
when considered for large scale use. The arsenides, and
to some e~t~nt, the cadmium salts are poisonous.
30 I
1~ ~1~ ;3 3 ~ ;~
1 Amorphous silicon created by decomposition or
silane gas in an ion ch~.~er is the most promising
rlla~erial^
Althou~h vacuum equipment e~;ists today by which
S large sheets of window glass can be tinted for use in
architectural construction, a process ~hich is pri.marily
electron gun vacu~m deposition may not be suita~le
for the production of silicon vacuum ~eposited cells
since the silicon when deposited tends to become
microcrystalline when applied to a heated substrate.
It is known that the presence of hydrogen can influence
this result b~ forming amorphous a-silicon.l~he
presence of that hydrogen would bond the silicon and
thus convert it into an amorphous state at the pro~er
tempera~ure.
Sputtering svstems, which are also described here
impose cell size limitations, since the ~argets and
magnets involved are relatively small that only small-
20 sized individual cells can be produced. But thisprocess does have a utility as a means of ma~ing excellellt
laboratorv samples and in particular the compound cells
to which a portion of this work is directed.
30 I .
~3~4~
1 Here a~ain, the cell is cGmurised of planes or
layers containing one or more films of selected materia~,s.
Referring to Figures 3 ana 4 these planes are seen as the base
conductive layer 21,the photoemissive layer 2 and a
second outer conductive layer 23, all on a glass substrate
24.
This embodiment was prepared in an io~-plasrna
vacuum cha~ber by the process which is co~monly called
sputtering.
The confines of the sputtering chan~er restricted
the size of ~he cell to a 4-inch square gl~ss plate.
This was exposed through a single 3-1~2 inch by 3-1/2 ir.ch
aperture in a mask which was repeatedly used to outline
each of the successive layers bv simpl~ shifting the glass
plate diagonally to tllree equally spaced index positions engraved
in the ~ask adjacent to the aperture. This shifting can
be accomplished externally by simple mec}lanical me~ns,
and there~ore the substrate does have to be repeatedly
removed from the chamber.
As before, the glass substrate 24 was commercially
passivated, primarily with alu~inum oxide, to eliminate
degradation of the a-silicon layers by the migration of
the sodium atoms from within the glass.
~ 33~2
1 It is then placed within the sputtering chamher and
raised to a temperature of 500 K. To the passivated
substrate first layer21 of discreet films of ITO and gold
is appiied, within the outline o~ the first mas~ing position
as shown in Figure 3~ The gold w2s used in min~te q~lanti'ies
to enhance the conductive properties of the ITO. TG prevent
"re-combination" the a-silicon was isolated from the gold
by the ITO f ilm .
In this case a combination of ITO and gold was selected
to reduce the resistivity of the conductive layers sub-
stantially below the internal resistance of the a-silicon
layers.
The substrate is then shifted to a second position
to apply through the same aperture in the mask a plurality
of doped a-silicon layers at 22, constituting a P-N-junction.
Extreme care is taken during the shifting not to scratch
or disturb the surface of the cell.
Borane and phosphorous doped layers of a-silicon
can now be applied by either the use of pre-doped a-
silicon collars, which are referred to in the trade as
"targets", or by the deposition of pure silicon target
25 which can be doped when ionized by borane and phosphane,
~ 3 ~2
1 respectively. The phosphane and horane are introduced
as gases -to the argon supplemented vacuum atmosphere and
are infused during ion deposition. The co~bined layers
of doped a~silicon should be controlled to a total thickness
5 of less than Z,500 Angstroms. The gold is appro~imately
50 ~ngstroms thick and the ITO as much as 2 microns.
An alternative means of glow discharge is accomplishecl
throu~h the use of silanP g2s in combination with borane and
phosphane. ~ere the entire deposition process is carried
out by decomposing these gases. The experimental results
are technically excellent but at this point are not considered
financially reasible for production.
Withol~t opening the vacuum chamber, the substrate
is shifted to the third index position, the chamber turret
rotated, and a top conductive layer 3 of first ITO and second
gold i~ deposited~ As in the previous cells r the top and
bottom conductive layers of ITO and gold are separated and
partially insulated by properly positioning the geometry
of the photoemlssi~e layer of doped a-silicon~
The resulting cell bears the characteristics which are
necessary to be adaptable to limited manufacture for test
purposes~ This cell is far superior to its predecessorc
~ld is readily adaptable to limited laboratory manufactl~re.
1~133 ~2
1 In preparing the previous prototypes which are
comprised of three or more layers of phoioemissive and
conductive material, a unique embodiment evolved by which
higher efficiencies and higher voltages can be achieved.
This cell is best produced for test purposes by means of
sputtering, but it is believed, and gain an objective,
to also produce the cell by applying the required coats
to a fused glass substrate at the time the glass is being
drawn.
The following description, therefore,is pre~ented
with both construction methods in mind, but will be
specifically directed toward producin~ a prototype by use
of sputtering.
~5
To a 4-inch by 4-inch passivated glass substrate 31,
as shown in Figure 5, the base conductive layer 2 is
deposited. This layer 32,is comprised of a three to
five micron film of aluminum, over which is applied a
500 Angstrom film of IT0, the combination of which is to
act a~ a conductor and reflector. The IT0 is added as a
protection for the doped a-silicon layers which follow,
s-nce, under the influence of heat, aluminum will migrate
and dope the a-silicon. The purpose of the reflector, in
~ 3~2
1 tnis configuration, is to utili~e all of the available
light energy from the visible spectruml by reflecting
any residual photons bac~ into a stack of layers of
photoemiss ve and conductive materials to bc deposited
above. Therefore, as shown in Flgure 5, at a temperature
of approximately 300 K, to the pre~iousl~ applied
aluminum-ITO layer 32, first pai.r of alternately doped
P-N a-silicon films which comprise ~ayer 332re deposited
through the aperture by which layer31 was outlined but
after the substrate is shifted to achieve partial in-
sulation along ed~e 39 as was the procedure in pre-J ous
descriptions. This P-N junstion 33is appro~imatPly
2,000 Angstroms in total thicknessr and, therefore, absorbs
that portion of the ~isible spectrum which has penetrated
to this layer~ it being understood that layer 33 is the
bottom P-N junction in a state of three congruent P-N
junctions.
Accordingly, dixectly above and adjac~nt, is a second
ITO and gold conducti~e interlayer 34totalling appro~imately
200 Angstroms, to which a second 2,000 An~strom P-N doped
a-silicon la~er35 is deposited, as before. At 36a second
200 Angstrom ITO and yold conductive interlayer is deposited,
followed by a third 2,000 Angstrom P-N doped a-silicon layer at 37
2S
~Z~3~3 ~
This procedure could be continued to the point at which all of the
spectrum was absorbed. Three superimposed photoconductive layers
in series is considered sufficient.
To th~ last P-N juncture, a final layer of three microns
of ITO with a fraction of gold is deposited at 38, as shown in
Figure 5. In all cases the gold was insulated from the a-silicon
by ITO.
It must be noted here that these layers can be applied to
the substrate in reverse order in such a manner as to create a
back contact mirror with the reflected surface as the top or
outside coat of the sandwich. This has certain advantages if a
nonreflective glass surface is employed to help reduce the
reflection and consequent loss of the sunlight. Furthermore, the
prototype as depicted in Figure 4 utilizes a variant of the
masking technique, which has been discussed throughout this paper,
but also requires finish etching to delineate the margins at 40.
Again referring to Figure 5, the principle of stacking
can provide greater efficiencies in individual solar cells. By
utilizing the etching techniques which are, incidentally, common
in the semi-conductor industry, together with appropriate masking,
a variant of the series of clean-cut photovoltaic stacks such as
shown in Figures 7 to 9 can be produced.
. .
~ 33~
1 Multiple Cell Array
In the previous sections in which the "Preferred
E~bodimen~" and the "Multil.ayered Composit Cell" were
l discussed, it was found that photovoltaic and conductive
51 substances can be applied to a num~er of different substrate
materials .hrough a screen or ~ask ~rnich, when moved,
~ould permit the depositon of a series of linked cells.
To understand this concept, please refer to Figure .8,
which depicts a sheet of materi21, again, pre erably slass,
on which a matrix o~ individual cells in the form of small,
identical rectangles is shown. Accordingly, a passlvz~ed
glass substrate 41is covered by a single mask wh.ich
contains plurality of unifoxm rectangular openings. These
-rectangular openings permit the passage and subsequent de-
position of materials which comprise a solar cell~ The first
layer of these materials as in previous cells, would be en-
hanced ITO shown at 42. ~aving de?osited the ITO, and ~ask
or screen is moved to a second position, and successive
layers of P a~d N doped silicon.~3 are vaporized and
deposited. Now the mask is moved to a third inde~ po~i.t_on
equal to thc seccnd under position and the outer layer of
ITO 44is applied. This outer layer ITO contacts thc inner
layer ITO at45, and a chain or array of cells 'in series has
been produced on a single sheet. By linking the contacts
~hich lie along the margin at 46 to the contacts which lie
~ 33 ~2
1 along the margin at 47, voltages due to the serial
connections of cells in each row and currents due to the
parallel connections of several rows can be obtained.
Alternatively, the rows can be connected at alternate
ends to successive rows to make a long sinuous path of
cells with higher voltage.
A significant fea-ture of the array of linked solar
cells lies in the fact that it can be produced by the
shifting of a single mask or its substrate without
necessitating the removal of the work in progress from
the vacuum chamber and therefore eliminates the risk
of contamination.
This technique is also proposed for use with a
"silk screen" process wherein each layer is applied as
a slurry and dried prior to the application of successive
layers.
An example of a sinuous array of solar cells is
illustrated in ~igure 10 which has twelve cells connected
in series. The cells are deposited on a glass substrate
51 suitably coated to prevent contamination of overlying
layers. Each cell has a first rectangular conductive
film 52 deposited in a rectangular area on the substrate.
Next, a rectangular semiconductor film 53 with a P-N
junction parallel to the substrate is deposited over the
first conductive film. One edge 39 of the conductive
film extends beyond the semiconductor for making electrical
contact. The other edges of the semiconductor film extend
3~
-20-
~ 3 ~
1 beyond the edges of the first conductive film, thereby
providing electrical isolation of the edges of the
conductive film~ Next, a s~cond rectangular conductive
film 54 is deposited over the semiconductor. This
second film is displaced laterally from the first
conductive film and extends beyond an edge of the semi-
conductor film so as to overlap the first conductive
film of an adjacent solar cell as at 55. At least a
portion of the edges of the semiconductor film extend
beyond the edges of the second conductive film to assure
electrical isolationO At the ends of the rows of cells,
interrow connections are made with the rectangular
conductive areas 57 turned 90 to overlap the cell at
the end of the adjacent row as at 58.
Another variant of a serially connected multiple cell
array is illustrated in Figures 11 and 12. In this array,
the substrate 61 is a passivated sheet of glass. A
rectangular conductive layer 62 is deposited on the
substrate along one edge. Overlying a portion of this
layer 62 there is a semiconductor film 63 which extends
beyond the edge of the first conductive layer 6~. A
second layex of conductive material 64 is deposited next,
overlapping part of the semiconductor layer and extending
beyond it onto the substrate. The portion of the second
layer of conductive material on the substrate is analogous
to the first layer,and another layer 65 of semiconductor
is deposited thereon. Such layers are overlapped
30 I
~,33 ~
1 successively across the substrate like shingles to
form an array of solar cells connected in series.
Electrical connection to the array is made via the
exposed edge of the first conductive layer 62 and an
edge of the last conductiv~ layer 66 at the opposite
edge of ~he suhstrate. ~ similar array can be made
with overlapping of layers, like illustrated in Figure 7.
The application of the various photovoltaic compounds
and conductive materials in a ~olten state or ionized by
10 means of vacuum process which rely on the evaporation of
targets of parent materials, and the ~nowledge that the most
efficie~t silicon solar cells are made from molten silic~n,
clearly demonstrates that the u~ilization of heat in th~
manufact~ring process of the so}ar celis is beneficial.
Consequently, a means by which the materials can be applied
to the slass substrate at the time the glass is first dr2wn
from a m~lten liquid would represent a signi~icant advance.
Figure 6 is representative of a system by which each
of the materials can be applied in a molten state to a
plate glass substrate during the drawing.
The schematic drawing of Figure13 represents a typical
Pilkington Bros. Ltd. float glass racility. A furnace at 71
is linked to a ~olten tin bath at72, the glass is continuously
withdrawn in a single ribbon through a heat trea~ing le'n~ at 73
and ~o a cutting line at 74. Assuming that the molten slass
is drawn at approximately 1,600 C, a temperature matching
can be made with tne ingredients for a continuousiy withdra~n
-~2-
~,33~Z
1 solar cell array. The melting points or optimum
deposition temperatures of aluminum o~lde, indium tin
oxide and silicon, for example, closely match that of
the cooling molten glass, and a mean temperature should
be achleved to accommodate, without the risk of
evaporation, each of the mating materials. At this
point, it is important to recognize that alternative
methods such as the use of sprays, powders, or decompo-
sition of gases to deposit active coatings onto the
molten glass, can be considered. However, the process
of this discussion is limited to the application of the
active materials by successive thin molten films.
Figure 13 depicts a variant of the Pilrington
process in which these materials seen are to be applied
sequentially as the glass passes over the molten tin bath.
The molten plate glass substrate 75 is totally coated with
a continuous sheet of aluminum oxide 76, which is withdrawn
from container 77. When cooled, this will passivate the
glass substrate. To the passivated base, ribbon stripes
78 of enhanced ITO are delivered through ports or slots
from a separate container 79, and these stripes extend
along the entire length of the substrate and are separated
from each other by a margin which is sufficient to prevent
contact of a particular stripe with any adjacent stripe.
Over these ITO stripes 78 a continuous sheet of P-doped
silicon 80 is applied from an individual container Bl. The
outer margins of sheet 81 are held within the border of
the subs~rate 75. A second corresponding sheet of N-doped
~33 ~
1 silicon is applied frorn another container 83. These two
sheets, when fused together, are to form a P-N junction,
but it would be noted that an alternative means of gas
doping one of the sheets could be substituted. The step
is followed by a second or top layer o ITO stripes 84
which are applied from a container 85 and are identical
in dimension and position to the preceding ITO stripes 78
By this process a single array of multiple stripes with
contacts above and below an inner layer of P~N doped
silicon homojunction emerges from the heat treating lehr
73 and can be cut into single arrays at 74. For purposes
of this discussion, 4-feet by 8-feet sheets are cut.
Following cutting, a margin of approximately one inch
is etched along one of the eight-feet sides of the finished
glass plate array. This etching is cut to a depth which
is sufficient to remove the two top layers and the P-N doped
silicon exposing the ends of the inner layer of ITO stripes
as contacts.
By connecting the top ITO stripes and the exposed
contacts of the inner ITO layers, combinations of curxents
and voltages can be selected. This embodiment, as well as
any of the others hereinabove described, can be protected
by covering the solar cells with a second layer of glass,
or of plastic, by means of a conventional polyvinyl butyral
interlayer laminated onto the glass substrate by techniques
like those presently used to make safety glass.
~3~ ~2
1 Manufacturing Low Cost Arrays
._
A low cost solar array can be manufactured through
the application of a plasma spray process which is becoming
widely used in industry. The plasma process equipment is
produced in this country by a number of companies,
principally Union Car~ide and Tafa, for the application of
numerous metal coatings and ceramics to various industrial
and aerospace products. The coats can be applied either by
melting through induction, electric arc, or by RF generators.
0 For use in this process, the RF application of conductive
and photovoltaie materials in the presence of an oxygen-free
atmosphere is the appropriate procedure.
Since plasma sprays ean be effectively applied to a
number of materials, the process should not be eonstrued as
limited solely to glass. There are many applieations for
solar eells in which metal substrates can be used as well
as plastics and plastics in combination with metal, as
"printed circuit boards," to which photovoltaie compounds
could b~ ~pplied. The preferred substrate is glass.
In th~ particular ease of the use of plasma spray, selec-
tion of the mas~ is significantly important, since the mask
may have to be disposable at the end of eaeh operation~ Two
kinds of masks are ~herefore considered~
1. A semipermanent metal, carbon, or plastic mask.
2. A completely disposable imp-egnated paper or plastic
mask.
-25-
~33 ~2
1 The su~gestion for the disposa~le nature of the mask lies in
the fact that as these coats are applied a buildup occurs along
the aperture or periphery of the opening of each of the indi-
vidual holes which, throush repeated use, will distort or
destroy the tolerance of the dimensions of each cell component.
The so-called semipermanent mask is al50 subject to a substan-
tial buildup of material along the edges of the aperture
through which material is being applied, and would therefore
have to be cleaned or discarded after successive applications.
The second form of mask, he disposable variety, has the
advantage of being cle~n for each application. The only con-
ceivable disadvantage would be the necessity for an additional
labor step. But in either case, the mask has to be arfixed to
the substrate for each operation. This attachment procedure
could be automatic in both cases, but for the moment it is
contemplated as a hand operation, even in limited production.
The semipermanent masks can be made of materials such as
metal or plastic impregnated with carbon or made of sheet
graphite to which at least one of the components, sillcon, will
not adhere.
The disposable masks should be paper, in whic:l thc
apertures are cut or punched out of a continuously flowing
roll of paper, and individual sheets cut to the size of the
substrate. If these paper masks are partially coated with
pressure sensitive adhesive, they can be applied to the
substrate at the top and rolled on with just sufficient
spots or areas of adhesive to "tack" the mask down to the
~33-~
1 substrate. The difficult part of applying the disposable
masks lies in handling ~he substrate, and ex~reme care
will have to be taken with thi~ operation to see that
contamination or scratching does not occur during the
time the mask is applied or removed.
Disposable masks have the advantage of offering a time to
inspect the product between coats, which is a beneficial
feature in any manufacturing process, since ~or ea~h set of
coats a new mask ~ill be required, making three mas~s neces-
sary for the construction of one array.
It must be noted here, du_ing a description of the mas~-
ing process, that the application of heat may be necessary as
an integral step or after the masking and coat application
sequence is completed. Heat treatiny is anticipated as a neces-
sary component for the constru_tion of the completed array.
Therefore, the selection of t~is material for the ~ask is an
impGrtant consideration, since the mask can beco.~e charred or
distorted with excess heat.
A series o glass plates are prepared in advance of the
construction procedure. These plates are cut to size, edges
tri~med, and passivated principally with aluminum oxide prior
to the application of the first of the set of three masks.
After cutting, cleaning, and passivating the glass s~b-
2~ stra~e, a properly cleaned or new mask is fitted to the glass.
~33 ~Z
1 This first mask is arranged to permit depositions of the
first conductive material to the glass substrate. It
should be noted a~ this point that this first conductive
material can either represent the top of the cell to be
constructed or the base coat. In the particular case of
this configuration~ the first conductive material will
constitute the top electrical component of the solar array
and will lie directly adjacent and in contact with the
aluminum oxide passivated surface of the glass plate. The
conductive m~terial, in this instance, is principally
enhanced indium tin oxide (ITO). Under certain clrcum-
stances, this enhanced ITO will not be considered fully
conductive, in which instance a conductive grid can be
applied to the glass substrate prior to this step,and the
ITO coating makes electrical contact with the grid.
Having affixed the mask to the substrate, the assembly
is placed in a plasma chamber in which the inert atmosphere
is continuously replaced to remain free of oxygen. Either
nitrogen or argon i5 suitable as an inert atmosphere.
The ITO is fed as a powder into the "gun," which heats
the material substantially above its melting point and
deposits the ITO as melted droplets on the substrate. It
is assumed at this juncture that the substrate can remain cool,
but it may be ne~cssary to heat the subs~rat~ to appro~imately
300C in order to get a significant bond between the compo-
nents of the struoture. Again, it must be ~oted that in the
event disposablc masks are used, they will have to be con-
s~ructed of a material which will withstand heat. For pur- :
poses of this eY.planation, it is assu~.ed that the first coat
of ITO is to be app~ied a~ ambient t~mperature.
I -28-
~ 2
1 ~ollo~ing the deposition o~ the first conductive coat,
the assembly is removed from the chamber and the masX care-
fully re~oved and the treated substrate irspected. This entire
procedure must be done in a clean room a.mosphere, since, as
in the case of vacuum deposi~ion, dust will, to some deg-ee,
destroy the suali~y of the end product.
Having removed the first mask, a second mzs~ is affixed
in preparatio~ for the application of the photovoltaic coats.
A~ain, the assembly is introduced to the deposition
chamber, and an RP plasma spray or pre-doped boron-silicon is
applied. Here the silicon must be introduced to the RF "gun"
in rod form. Normally, metals and ceramics deposited through
plasma guns are fed as a powder into the melting zone, but
there is a significant risk of contamination in atte~.pting to
pulverize silicon even in a non-oxidizing or nitrogen a.mos-
phere. Fifty parts per million will oxidize silicon and
render it useless. It is anticipated that the total thick-
ness of the coat of silicon which will constitute a P-N
structure will be on the order of five microns, and conse-
quently the timing of the spraying is critical. During the
spraying procedure, the pre-doped silicon is f~sed in an argon
gas stream to which two additional gases, phosphane and h~dro-
gen, are to be introduced. Approximately t~o percen. of
hydro~en is considered to be necessary, in order to provide a
reducing atmosphere, and further to enter into thé crystalline
structure o~ the cell to help satisfy any existing loose bonds.
~33~
1 In addition to supplying hydrogen, it is conte~plated that
at least one of the dopants be introduced to t~.e pl2sma as a
gas. Since the silicon rod was pre-doped with boron, phos-
phane ~ill be injected during the increment of time requiredor the deposition of the first micron of silicon material.
~he balance of approximately four microns of boron doped 5il-
icon will be applied without the presence of the phosphane.
This spraying o~eration is continuous and the struc~ure
will be a sradient homojunction which in effec~ will create a
N+, N, P, P~ junction.
One significant feature of placma deposition lies in the
fact that these melted, extremely hot, coa~s can be applied to
cold substrates. Following the deposition of a P-N junction,
as described above, and the removal of the masks, an inter-
mediate step of sintering can be included. This step consists
of applying the direct heat from the silicon-free gas plasma
to the entire substrate and deposited coats to just raise the
temperature of the silicon structure to its melting point, and
permit the formation of a multi-crystalline structure. Again,
alternative procedure would be to elevate the temperature of
the glass substrate to a point at which fracturing would not
occur, and then apply the silicon layers. The ultimate
25 ¦ objective of this step of applying the photovoltaic silicon
~ coat is to uniformly crystallize the silicon homojunction.
-30-
~ 3~
1 Following the deposition of photovoltaic layers, the
heat treating and removal of the second mask, the third mask
is attached, the apertures of which will constitute the
geometry of the final conductive layer. This layer can be
made principally of aluminum, and simply applied by the RF
plasma "gun" directly to the pre-deposited semiconductor
layers.
The encapsulation should begin by applying a general coat
of aluminum oxide over the entire completed array. Although
this coat serves as an additional handling protection, it may
not in the end be necessary, but has proved a useful precaution
in the preparation of prototypes which led to the development
of this configuration.
The final encapsulation follows the procedure for the
manufacture of "safety glass."
The temperature of formation of the crystalline structure
of silicon is critical, paxticularly for this hybrid means of
creating crystalline wafers. The objective is to produce a
single wafer of a minimum number of independent crystals.
Wafers consisting of imperfections in the form of independent
crystals are acceptable for solar cell construction. For
¦ example, the Tyco Ribbon Process produces a wafer which is
¦ not suitable for the semiconductor industry, but is a satis-
¦ factory quality solar cell.
25 ¦ The silicon can be introduced to the plasma chambe~ as
¦ a powder, and extreme care must be taken to prevent this powder
¦ from oxidizing during the process steps. The silicon should
¦ be ground to the largest grain size compatible with the plasma
~33~
1 spray system and then washed in a dilute solution of hydrogen
fluoride. The solution of hydrogen fluoride will be gradually
exchanged with distilled water and acetone to a point at which
acetone has replaced the hydrogen fluoride solution. The
silicon can then be dried in a chamber containing hydxogen
and an i~ert gas. The prepared powder is then introduced to
the ion plasma chamber through the appropriate hopper, thus
assuring that the least possible exposure to latent oxygen
has been assured.
In mass production it is also expected that both dopants
can be introduced as gases, since both borane and phosphane
do not react with argon and hydrogen in the plasma.
Figure 14 is an exploded view o~ an array of solar cells
91 on a glass substrate 92 protected with an overlying pro-
tective layer. The solar eells are initially protected witha layer of aluminum oxide, not shown. A conventional inter-
layer 93 of polyvinyl butyral, polyurethane, silicone or the
like is laid over the array of cells and a layer 94 of glass,
polycarbonate, aerylic or the like, is laid over the inter-
layer. The sandwich is then bonded with heat and pressure ina conventional manner. The sandwich also ineludes busbar
leads 95 along eaeh edge making electrical eontaet with solar
eells in each row. The leads 95 extend beyond the edge of the
sandwich for conneetion to external cireuits. These busbars
can, for example, be strips of metal foil held in place in
¦ contact with a conductive layer of the cells by pressure of
¦ the interlayer. Conduetive adhesive ean be ineluded to enhance
strength and contact if desired.
I -32-
r ~ ~ IL 3 3 ~
1 It can be desirable to avoid edge effects in the
photoemissive semiconductor layer where the N and P layers
are indistinct. This can be provided by depositing one of
the layers, for example, an N-doped layer of silicon. A
narrcw band of aluminum oxide or other electrical insulator
is then deposited along an edge of the semiconductor over-
lapping the edge a couple millimeters or less. The other
semiconductor layer, for example, P-doped silicon, i5 then
deposited over the first with its edge overlapping the
insulating layer. This keeps feathered edges of the silicon
apart and minimizes edge effects.
It is often desirable to provide an array of a plurality
of electrically connected solar cells instead of a few large
area cells. The resistance of thin films is such that
effective power generation can be minimal at substantial
distances from low resistance electrical connections. For
example, in a 4-foot by 8-foot window having a transparent
(or semi-transparent) solar cell over the entire area, the
center may be ineffective in generating useful power. Smaller
windows or subdivi~.ion of the larger pane into a plurality of
~olar cells can increase the power.
~ IL3;~4;Z
1 In the event that aluminum is used as one of the conduc
tive electrodes and deposited first upon the glass in the
,e~ers2 sequence of the process just described, the temperature
of the glass can only be elevated to 577C, the critical point
at which a eutectic would form. Again, e:~periments will h2ve
to be conducted to see at what substrate temperaturQ and rate
of formation this process can hest be carried out;
For production, the silicon will have t~ be introduc~d to
the plasma chæmber as a powder, and extreme care rnust be taken
to prevent this powder from oxidizins dur~ng the p~ocess steps.
The silicon should be gro~nd to ~he largest grain size com-
patible ~th the plasma sp a~- system and then washed in a
dilute solution of hydrosen fluoride. The solution of hydro-
gen fluoride will be g,adually exchansed with distilled wa~e~and acetone to a point at which acetone has replaced the hydro-
gen fluoride solution. The silicon can then be dried in a
chamber containing hydrogen ancl an iner~ gas. The prepared
powder is then introduced to the ion plasma cha~ber through
the appropriate hopper, thus assuring that the least possible
exp~sure to latent oxygen has been assured.
Ir. mass producti~n it is also e~pected that boih dopants
can b~ introduced as gases, since both borane and phospnan~ do
not reac~ with arson and hydrogen in the plasma.
