CA1213342A - Thin solar cells - Google Patents
Thin solar cellsInfo
- Publication number
- CA1213342A CA1213342A CA000407715A CA407715A CA1213342A CA 1213342 A CA1213342 A CA 1213342A CA 000407715 A CA000407715 A CA 000407715A CA 407715 A CA407715 A CA 407715A CA 1213342 A CA1213342 A CA 1213342A
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- Prior art keywords
- solar cell
- conductive
- layer
- conductive film
- silicon
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E10/00—Energy generation through renewable energy sources
- Y02E10/50—Photovoltaic [PV] energy
- Y02E10/547—Monocrystalline silicon PV cells
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
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- Photovoltaic Devices (AREA)
Abstract
Abstract of the Disclosure Solar cells and arrays of solar cells are made as thin films or insulating substrates. In an exemplary embodiment a thin conductive film is deposited on glass and a semiconductor film is deposited over the metal.
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. Another conductive film is deposited over the semiconductor 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. In one aspect of the invention the films are thin enough to be trans-parent. In another aspect, a plurality of semitrans-parent films are stacked to absorb selective portions of the spectrum. In another aspect a technique of stacking thin films using diagonal displacement of a mask provides successive layers with exposed and covered edges for suitable electrical connections.
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. Another conductive film is deposited over the semiconductor 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. In one aspect of the invention the films are thin enough to be trans-parent. In another aspect, a plurality of semitrans-parent films are stacked to absorb selective portions of the spectrum. In another aspect a technique of stacking thin films using diagonal displacement of a mask provides successive layers with exposed and covered edges for suitable electrical connections.
Description
~3~
1 5 7 0 : RD S
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.
~: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
1 5 7 0 : RD S
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.
~: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
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
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 ~ 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.
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
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 ~ 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.
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 ~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~
~ 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 ~,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.
~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.
~ 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.
~ 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.
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.
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 ~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~
~ 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 ~,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.
~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.
~ 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.
~ 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.
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.
Claims (11)
1. A semi-transparent solar cell comprising:
a passivated glass substrate;
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 sufficiently thin to be substantially transparent;
a second electrically conductive transparent film over the semiconductor layer and electrically isolated from the first conductive film;
an electrically insulating transparent layer over the second conductive film;
a first electrically conductive bus bar on the glass substrate along one edge of the semiconductor layer and connected to the first conductive film; and a second electrically conductive bus bar on the glass substrate along the opposite edge of the semiconductor layer and connected to the second conductive film.
a passivated glass substrate;
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 sufficiently thin to be substantially transparent;
a second electrically conductive transparent film over the semiconductor layer and electrically isolated from the first conductive film;
an electrically insulating transparent layer over the second conductive film;
a first electrically conductive bus bar on the glass substrate along one edge of the semiconductor layer and connected to the first conductive film; and a second electrically conductive bus bar on the glass substrate along the opposite edge of the semiconductor layer and connected to the second conductive film.
2. A solar cell as recited in Claim 1 wherein the insulating layer comprises a nonconductive metal oxide.
3. A solar cell as recited in Claim 1 wherein the insulating layer comprises a layer of glass laminated to the glass substrate.
4. A solar cell as recited in claim 1 wherein the insulating layer comprises a layer of transparent plastic over the second conductive film.
5. A solar cell as recited in any one of claims 1 to 3 wherein the semiconductor layer comprises a layer of P-doped silicon and a layer of N-doped silicon.
6. A solar cell as recited in any one of claims 1 to 3 wherein the conductive films include indium-tin oxide.
7. A solar cell as recited in any one of claims 1 to 3 wherein the passivated glass substrate comprises a layer of aluminium oxide between the glass and the first electrically conductive transparent film.
8. A solar cell as recited in claim 1 wherein the photovoltaic semiconductor layer is formed by the steps of:
melting silicon and subjecting the molten silicon to a gas jet sufficiently energetic to eject droplets of molten silicon; and depositing such droplets on a conductive substrate,the gas jet and environment adjacent the substrate being non-oxidizing.
melting silicon and subjecting the molten silicon to a gas jet sufficiently energetic to eject droplets of molten silicon; and depositing such droplets on a conductive substrate,the gas jet and environment adjacent the substrate being non-oxidizing.
9. A solar cell as recited in claim 8 wherein the gas jet includes a doping agent for silicon.
10. A solar cell as recited in claim 9 wherein the doping agent is selected from the group consisting of boron and phosphorus.
11. A solar cell as recited in claim 9 wherein the gas includes a gas selected from the group consisting of borane and phosphane.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000407715A CA1213342A (en) | 1982-07-21 | 1982-07-21 | Thin solar cells |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000407715A CA1213342A (en) | 1982-07-21 | 1982-07-21 | Thin solar cells |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1213342A true CA1213342A (en) | 1986-10-28 |
Family
ID=4123256
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000407715A Expired CA1213342A (en) | 1982-07-21 | 1982-07-21 | Thin solar cells |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1213342A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115075443A (en) * | 2022-08-22 | 2022-09-20 | 天津耀皮工程玻璃有限公司 | Large-layout internal-spliced film BIPV building curtain wall power generation glass |
-
1982
- 1982-07-21 CA CA000407715A patent/CA1213342A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN115075443A (en) * | 2022-08-22 | 2022-09-20 | 天津耀皮工程玻璃有限公司 | Large-layout internal-spliced film BIPV building curtain wall power generation glass |
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