SOLAR CELLS WITH THICK ALUMINUM CONTACTS

Abstract: An improved photovoltaic cell and method of making same is disclosed. The invention involves formation of a thick aluminum rear contact (12). In the preferred embodiment the cell is made by a single fire process that involves forming a glass layer (18) over the aluminum contact (12) so as to protect it against oxidation and corrosion. Also in the preferred embodiment of the invention, the rear side of the substrate preferably includes silver soldering pads (16) that extend through openings (28) in the thick aluminum rear contact (12), and the glass layer (18) has windows (28) exposing portions of the soldering pads (16) for solder attachment of connecting wire ribbons.

Description: SOLAR CELLS WITH THICK ALUMINUM C ' '■TACTS BACKGROUND 1. Field of Invention The present invention generally relates to photovoltaic cells. More particularly, the invention relates to the formation and protection of met " 1 ization layers on such photovoltaic cells. Summary of the Prior Art One method of making photovoltaic solar cells involves provision of semiconductor substrates in the form of flat sheets or wafers having a shallow p-n junction adjacent one surface thereof (commonly called the "front surface") . Such substrates, which may include an insulating anti-reflection ("AR") coating on their front surfaces, are commonly referred to as "solar cell blanks". The anti-reflection coating is transparent to solar radiation. In the case of silicon solar cells, the AR coating is often made of silicon nitride or an oxide of silicon or titanium. A typical solar cell blank may take the form of a thin rectangular EFG-grown silicon substrate of p-type conductivity having a thickness in the range of 0.012 to 0.016 inches, with a p-n junction located about 0.5 microns from its front surface, and also having a silicon nitride coating about 800 Angstroms thick covering its front surface. Equivalent solar cell blanks also are well known, e.g. those comprising single crystal silicon substrates and cast polycrystalline silicon substrates. The cells require electrical contacts (sometimes referred to as "electrodes") on both the front and rear sides of the semiconductor substrate in order to be able to recover an electrical current from the cells when they are exposed to solar radiation. For example, a common arrangement with solar cells having a silicon substrate is to make the rear contact of aluminum and the front contact of silver or nickel, preferably silver. These contacts are bonded to copper connecting ribbons or strips that are used to interconnect a plurality of cells in a series and/or parallel electrical circuit matrix according to the prior art and customer requirements. The contact on the front surface of the cell is generally made in the form of a grid, comprising an array of narrow fingers and at least one elongate bus that intersects the fingers. Further to improve the conversion efficiency of the cell, an AR coating as described above is applied at least to those areas of the first side of the cell that are not covered by the front contact. Aluminum is preferred for the rear contact for cost and other reasons. The rear contact may cover the entire rear surface of the solar cell blank, but more commonly it is formed so as to terminate close to but short of the edges of the blank. However, the exposed surface of an aluminum contact tends to oxidize in air, making soldering difficult. Therefore, to facilitate soldering, it has been found useful additionally to provide apertures in the aluminum coating, with silver soldering pads being formed in those apertures so as to slightly overlap the adjacent aluminum layer. These silver pads form ohmic bonds with the underlying substrate and also low resistance electrical connections with the aluminum contact, and are used as sites for soldering the tin-coated copper connecting ribbons or strips to the rear contact. Such a contact arrangement is disclosed in PCT International Publication No. WO 92/02952, based on U.S. Patent Application Serial No. 07/561,101, filed September 1, 1990 by Frank Bottari et al for "Method Of Applying Metallized Contacts To A Solar Cell". The front and rear contacts may be formed in various ways, but preferably they are formed by a paste printing/firing technique which involves printing a selected metal-containing paste or ink onto each surface of the solar cell blank and then firing that paste or ink in a suitable predetermined atmosphere so as to cause the metal constituent of the paste or ink to bond to the blank and form an ohmic contact therewith. The paste or ink comprises an organic vehicle in which particles of the selected metal are dispersed, and the firing is conducted so that the vehicle's components are removed by evaporation and/or pyrolysis. The printing may be conducted in various ways, e.g., by silk screen printing, pad printing or direct write printing techniques. One suitable pad printing technique is disclosed in PCT International Publication WO 92/02952, supra. U.S. Patent Application No. 666,334, filed 7 March 1991 by Jack I. Hanoka and Scott E. Danielson for "Method And Apparatus For Forming Contacts", discloses an improved method for direct writing a thick ink film onto the front surface of a solar cell blank to form a grid-like front contact. The teachings of those patent applications are incorporated herein by the foregoing reference thereto. For purposes of clarification and definiteness, as used herein the terms "ink" and "paste" are to be construed as essentially synonymous terms for describing fluid printing materials since they are used interchangeably by persons skilled in the art, although the term "ink" suggests a lower viscosity than the term "paste". The viscosity of the fluid printing material is adjusted according to the requirements of the method by which it is applied, e.g., silk screen printing, pad printing or direct write printing. Also, the term "metal paste" ("metal ink") is to be construed as denoting a metal-rich fluid comprising a selected metal in the form of discrete particles dispersed in an organic vehicle that is removable or destroyable on heating, e.g., via evaporation and/or pyrolysis. The vehicle typically comprises an organic binder and a solvent of suitable properties, e.g., ethyl or methyl cellulose as binder and Carbitol or terpineol as solvent. Thus, the term "aluminum metal paste" is to be construed as denoting a fluid aluminum-rich composition comprising aluminum particles dispersed in an organic vehicle. Further the term "glass frit paste" denotes fluid compositions comprising a selected glass frit dispersed in an organic vehicle of the type previously described, and the term "metal/glass frit paste", e.g., a silver metal/glass frit paste, is to be construed as a metal paste that essentially comprises a selected glass frit in a predetermined amount on a weight per cent basis. The grid-shaped contact on the front surface has been formed in various ways. For example, in some cases the procedure involves first forming the grid contact by a paste printing/firing method, and then covering at least those portions of the front surface of the substrate not covered by the grid contact with an AR coating. Another approach comprises first coating the semiconductor substrate with an AR coating, and thereafter forming the grid contact. This latter approach has been practiced in two different ways. One way involves chemically etching away portions of the anti-reflection coating so as to expose areas of the front surface of the semiconductor substrate in the desired grid electrode pattern, and then forming the grid contact on the front surface in the region where the anti-reflection coating has been etched away. The second way of forming the front contact utilizes the so-called "fired-through" method. That method utilizes a solar cell blank having an AR coating on its front surface and comprises the following steps: (1) applying a coating of a metal/glass frit paste to the surface of the AR coating in a predetermined pattern corresponding to the configuration of the desired grid electrode, and (2) heating the coated solar cell blank to a temperature and for a time sufficient to cause the metal/glass frit composition to dissolve and migrate through the anti-reflection coating and then form an ohmic contact with the underlying front surface of the substrate. The "fired through" method of forming silver contacts is illustrated by PCT Patent Application Publication WO 89/12312, published 14 December 1989, based on U.S. Patent Application, Serial No. 205,304, filed 10 June 1988 by Jack Hanoka for an "Improved Method of Fabricating Contacts for Solar Cells". The concept of firing metal contacts through an anti- reflection dielectric coating also is disclosed in U.S. Patent No. 4,737,197, issued to Y. Nagahara et al. for "Solar Cell with Paste Contact". In one prior art method of manufacturing solar cells having aluminum back contacts a so-called "double-fire" process is utilized. In that process, an aluminum metal paste is deposited on the rear surface of a solar cell blank in the desired pattern of the rear contact, the solar cell blank having an AR coating (preferably silicon nitride) on its front surface. The aluminum paste generally comprises 50-70 wt. % aluminum and is applied so as to provide 0.8 - 2.5 mg. of aluminum per cm 2 of coated substrate surface. Then, the solar cell blank is fired in a nitrogen atmosphere at a temperature and for a time adequate to produce an aluminum contact alloyed to the underlying silicon substrate in the manner described above. The alloying process involves melting the aluminum particles and the adjoining region of the substrate, and then cooling the solar cell blank to effect re-crystallization of the melted region of the substrate. The re-crystallized region comprises silicon highly doped with aluminum. The firing and cooling produces an aluminum contact on the rear surface of the substrate that is mechanically and electrically bonded to the re-crystallized region of the silicon substrate. Thereafter, a silver metal/glass frit paste is coated onto the AR layer so as to define a suitable grid contact pattern, as discussed above, following which a second firing operation is conducted in air to form a silver grid contact bonded to the front surface of the solar cell blank. When the double fire process involves forming silver soldering pads in openings in the aluminum back contact, the blocks or segments of silver metal paste used to form the pads are fired at the same time as the silver metal paste used to form the front grid electrode. Typically the silver metal paste used to form the soldering pcds contains a glass frit, as does the silver metal paste used to form the front contact that is fired through the AR coating. See Inter¬ national Application No. PCT/US91/06445, filed 6 September 1991 for "Electrical Contacts And Method Of Manufacturing Same", based on U.S. Application Serial No. 586,894, filed 24 September 1990, by David A. St. Angelo et al, which is incorporated herein by reference thereto. That patent application discloses that a small amount of nickel is preferably incorporated in the silver metal/glass frit paste used to form the soldering pads. The so-called "double-fire" process is costly because of the steps and equipment involved. Hence, efforts also have been made to develop a successful so-called "single-fire" process in which the aluminum back contact and a grid-like silver front contact are fired simultaneously. A primary concern of solar cell manufacturers is the need to increase the efficiency, reliability and useful life of solar cells and solar cell modules and panels, which necessarily involves efforts to decrease contact corrosion, particularly of the aluminum back contact. Although solar cell modules, i.e., modules comprising a plurality of solar cells connected in a suitable series and/or parallel circuit matrix are made so that the solar cells are sealed between substantially rigid front and back support sheets, with at least the front sheet being transparent, the solar cells are subject to deterioration because of some leakage of outside atmosphere through the protective module encapsulation. Such leakage tends to result in cell deterioration, in part by oxidation and corrosion of the aluminum contacts. Oxidation of the aluminum back contact reduces cell efficiency and also shortens the useful life expectancy of the cells and modules. Moreover, problems have been encountered in simultaneously firing the pastes used to form the silver front contacts and aluminum rear contacts in the same atmosphere. The silver paste must be fired in air. Unfortunately aluminum oxidation is accelerated when the aluminum-containing coating is fired in an oxygen-containing atmosphere, resulting in a porous aluminum contact on the rear side of the substrate. This porous aluminum metallization tends to degrade rapidly during conventional accelerated testing. Furthermore, there is a strong tendency for the aluminum to form what have been variously referred to as "balls" or "bumps" when fired in air. These anomalies in the rear contact tend to result in an increase in cell breakage in the course of interconnecting and encapsulating a plurality of cells together in a module. Copending U.S. Application Serial No. , filed on even date herewith by James A. Amick et al for "Method Of Encapsulating Metallization For Solar Cells" (Attorney's Docket No. MTA-83) discloses a method of coating the aluminum rear contact with a protective glass overcoating so as to minimize contact corrosion. The prior art also tends to suggest that increasing the thickness of the aluminum contact on the rear side of the solar cell may result in an improvement in overall cell efficiency. The reason for this is not fully understood, but it is believed that since a thicker aluminum contact can be achieved by increasing the amount of aluminum paste applied to the substrate, firing of the thicker paste layer will result in formation of a thicker aluminum metal contact, and also in formation of a thicker aluminum-doped (P + ) region or zone between the aluminum metal contact and the underlying substrate. The latter result is believed to provide an improved back surface field, resulting in improved cell efficiency. In this context, what is meant by a "thick" aluminum contact is one with a metal layer thickness on the outer surface of the solar cell blank in the order of 25 microns, in contrast to prior "thin" aluminum contacts which are characterized by a thickness not exceeding about 8 microns. By way of example, a "thick" aluminum contact may be obtained by coating an aluminum paste so as to provide 4.6 -.8.0 mg of aluminum per cm 2 of coated substrate surface, while a "thin" aluminum contact is produced if the paste is applied in a thickness providing aluminum in an amount equal to between 0.8 - 2.3 mg/cm 2 of coated substrate surface. A cell with a thin aluminum contact formed by a paste printing/firing technique has a P + region adjacent the aluminum contact measuring approximately 1-2 microns thick, whereas a cell with a correspondingly formed thick aluminum contact has a P + region adjacent to that contact measuring about 5-8 microns thick. This invention is based on a two-fold discovery: (1) providing thick aluminum contacts on thin (e.g., 0.012-0.016 inch thick) solar cell blanks, e.g., blanks comprising substrates made by the EFG crystal growth process, is not feasible when using the double fire process, because such thick aluminum contacts have been found to so warp the underlying semi-conductor substrates as a consequence of the nitrogen firing as to cause breakage of the cell; and (2) thick aluminum contacts are feasible with a single fire process as provided by this invention. SUMMARY OF THE INVENTION The present invention is directed to providing improved photovoltaic solar alls by achieving one or more of the following objects: (1) providing an improved solar cell and novel single fire method of manufacturing same wherein the solar cell is characterized by a thick aluminum back contact that improves the efficiency of the cell; (2) providing a single-fire sola" cell manufacturing method wherein the aluminum metallization that forms the rear contact is significantly thicker than heretofore but contains a minimum of "bumps" or "balls" resulting from oxidation during firing; (3) providing a method of making solar cells according to a single fire process that is characterized by the formation of a protective glass layer covering a thick aluminum back contact; and (4) providing an improved method of making solar cells wherein the rear surface of each solar cell is characterized by a thick aluminum contact and silver soldering pads extending through openings in the contact, with the aluminum contact overlapping the edges of the silver soldering pads. According to the preferred mode of practicing the invention, the foregoing and other objects of the invention are accomplished by providing a novel single fire manufacturing method characterized in that it commences with provision of a solar cell bl ink that comprises a silicon semiconductor substrate having a shallow p-n ;, motion adjacent its front surface, and a electrically insulating silicon nitride AR coating on its front surface. The blank is processed according to the novel single fire method so as to provide it with a thick aluminum rear electrical contact. According to the preferred form of the invention, silver is deposited onto two or more spaced areas of the rear surface of the solar cell blank so as to form two or more silver soldering pads, and an aluminum contact having apertures in registration with the silver deposits is alloyed to the rear surface of the substrate so as to make an ohmic contact therewith, with the apertures being sized so that the aluminum slightly overlaps the silver pads and the aluminum contact being a "thick" contact. Preferably, cells made according to this invention are characterized by a glass overcoating that covers the aluminum contact and extends beyond the outer edges of that contact so as to totally seal the aluminum contact from the external atmosphere. With the foregoing in mind, it will be understood that in a broad sense, and subject to changes or additions as herein suggested, the method of this invention generally includes the following steps: (1) providing a solar cell blank comprising a semiconductor substrate having a shallow p-n junction adjacent its front surface (about 0.5 microns deep) and a layer of an AR material such as silicon nitride coating that front surface; (2) selectively coating the rear surface of the substrate with an aluminum metal paste in a thickness that will provide a "thick" aluminum contact after firing; (3) drying that paste; (4) applying a layer of glass frit paste over the aluminum metal paste; (5) drying the glass frit paste; (6) selectively coating the front surface of the AR layer with a metal/glass frit paste so that the coating of said metal/glass frit paste forms a predetermined desired pattern of a front contact; and (7) heating the substrate to a temperature and for a time sufficient to (a) rapidly and efficiently cause the metal/glass frit paste to penetrate the AR layer but not the p-n junction and have the metal content thereof form an ohmic contact on the front surface of the substrate, (b) cause the aluminum metal of the aluminum metal paste to alloy with the rear surface of the substrate so as to form a rear contact, and (c) cause the glass frit in the glass frit paste to soften and fuse so as to form a mechanically adherent coating that seals off the rear contact. The preferred method of this invention is to precede step (2) with the step of applying a metal/glass frit paste to selected areas of the rear surface of the substrate so as to provide metal-containing paste blocks that are converted to soldering pads when the blank is fired according to step (7) . BRIEF DESCRIPTION OF THE DRAWINGS Throughout the drawings, like reference numerals identify like elements. Also the drawings are intended to be illustrative only. Therefore the thicknesses and depths of the various layers, coatings and regions are neither shown to scale nor shown exactly in accordance with their relative proportions, for convenience and clarity of illustration. Similarly, cross-sectional views are shown without cross-hatching for clarity. Also, for convenience and clarity of description, the various layers of metal and glass pastes that are applied to the solar cell blank are identified by the same reference numerals as the elements formed by firing those pastes. Fig. 1 is a top elevational plan view of an improved solar cell made in accordance with this invention; Fig. 2 is a bottom elevational plan view of the solar cell depicted in Fig. 1; Figs. 3-7 are diagramatic cross-sectional views in side elevation illustrating steps of a single fire method constituting a preferred embodiment of the invention; and Fig. 8 is a graphical representation of the temperature of a substrate as it undergoes firing in accordance with the invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Figs. 1-3 illustrate a preferred mode of practicing the invention. A cell 2 made according to the invention comprises a flat silicon EFG-grown substrate having on its front side a silver front contact 4 preferably in the form of a grid consisting of an array of narrow, elongate, parallel fingers 6 and at least one but preferably two bus bars 8 that interconnect fingers 6. Additionally, a thin AR coating 10 (see Figs. 3 and 7) in the form of an adherent layer of silicon nitride covers those portions of the front surface of the substrate that are not occupied by grid electrode 4. The rear side of cell 2 comprises an aluminum rear contact 12 (Fig. 2) that terminates short of the outer edges of the rectangular cell so as to have an uncoated margin portion 14 that extends along each side of the cell substrate, and also a plurality of silver soldering pads 16 that extend through openings in the rear contact and are fused to the underlying solar cell substrate. Although Fig. 2 shows eight soldering pads arranged in two parallel rows, it is to be understood that the number of soldering pads and the number of rows of soldering pads may be varied. In a general sense, to the extent described in the foregoing paragraph, the solar cell structure shown in Figs. 1, 2 and 7 is old, as disclosed by PCT International Publication No. WO 92/02952, supra. In accordance with the preferred mode of practicing this invention, an insulating overcoating 18 (Fig. 7) covers the rear contact 12 , edge portions of soldering tabs 16, and at least a portion of the margin area 14 of the rear surface that is not covered by the rear contact. The silver soldering pads 16 are formed before the aluminum rear contact 12, with the latter overlapping edge portions of the former. This is in contrast to the double fire process disclosed by said PCT International Application No. WO 92/02952 where the silver pads overlap the rear aluminum contact. The reasons for this difference are two-fold. First in the double fire process the rear aluminum contact is fired first and separately from the silver soldering pads in a nitrogen atmosphere. Second, it has been discovered that if the single fire process is practiced by applying first the aluminum paste and then the silver paste to form the rear contact and the soldering pads respectively, with the silver paste slightly overlapping the aluminum paste, the portions of the fired soldering pads that overlap the rear contact will tend to flake off and thus lower cell efficiency and reliability. It is believed that this flaking occurs because the organics in the paste used to form the rear contact are not fully driven off during firing in those areas where the silver metal soldering paste for forming the soldering pads overlies the aluminum metal paste. Referring now to Figs. 3-7, the preferred embodiment of this invention comprises a single fire process which commences with provision of a fiat solar cell blank 2 comprising a flat silicon substrate 20. Preferably substrate 20 is rectangular and constitutes a p-type polycrystalline sheet grown by the EFG method which has been processed so as to have a shallow p-n junction 22 located adjacent its front surface 24, and a silicon nitride AR coating 10 covering front surface 24. Preferably, the p-n junction is created by a suitable diffusion doping process according to well-known techniques, and the silicon nitride AR coating is formed using a suitable plasma deposition process as, for example, the one disclosed in International Patent Publication No. WO 89/0034, published 12 January 1989, describing an invention of Chaudhuri et al. The teachings of that publication are incorporated herein by reference thereto. Substrate 20 is relatively thin, typically having a thickness in the range of 0.012 to 0.016 inch and a resistivity of about 1-4 ohm-cm. The p-n junction 22 is located about 0.5 microns below the front surface of the substrate and the silicon nitride coating preferably has a thickness in the range of 600 to 1000 Angstroms, preferably about 800 Angstroms. The size of the solar cell blanks may vary, but the following description of how to practice the invention involves solar cell blanks measuring approximately 4 inches x 4 inches. Referring now to Fig. 3, a silver metal/glass frit paste is printed onto the rear side of the substrate 20 of a solar cell blank in the form of a plurality of rectangular soldering pad paste blocks 16. These blocks are then dried in air at a temperature and for a time sufficient to stabilize them so that they will not readily smear, e.g., by heating at 150 degrees C for 2-4 minutes. The size of blocks 16 may vary. Preferably, for a solar cell blank measuring approximately 4" x 4", the blocks 16 measure about 0.250" x .250". Next, as shown in Fig. 4, an aluminum paste 12 is applied to the rear surface of the solar cell b-ank so as to slightly overlap the dried silver paste soldering pad blocks 16 and leave an uncoated margin portion 14 (Fig. 2 ) . Preferably the aluminum paste is applied so as to leave rectangular windows 26 that frame and slightly overlap the silver paste blocks 16. Preferably, but not necessarily, windows 26 measure about 0.180 inch on each side, whereby the aluminum paste overlaps each side edge of the silver paste blocks by about 0.035 inch. Preferably, but not necessarily, the aluminum paste is applied so that the uncoated margin portion 14 of the rear side of the substrate has a uniform width of about 0.040". This aluminum layer is then dried in air, preferably at about 150 degrees C for 2-4 minutes. Then, as shown in Fig. 5, a coating of a glass frit paste 18 is applied over the dried aluminum paste so as to leave apertures or windows 28 that are slightly smaller than the windows 26 formed by the dried aluminum paste, with the result that the glass frit paste overlaps marginal portions of the dried silver paste blocks 16. Windows 28 are smaller than windows 26, preferably measuring about 0.150" x 0.150", so that at each window 28 the glass frit paste extends beyond the aluminum layer and overlaps the silver paste blocks 16 by about 0.015" at each side. Although not shown, it is to be understood that glass paste 18 extends beyond the outer boundary of aluminum metal paste layer 12, preferably but not necessarily to the periphery of the rear surface of the solar cell blank. The glass frit paste is applied in an amount such that after firing the glass overcoating 18 that remains has a thickness in the order of 4 microns. The glass paste 18 is then dried, preferably in air at about 150 degrees C for 1-4 minutes. Then a silver metal/glass frit paste 4 (Fig. 6) is applied to the silicon nitride layer in a suitable grid-defining pattern, e.g., the pattern shown in Fig. 1. This may be done in various ways, e.g., screen or pad printing or direct writing using a Micropen direct writing machine. Paste 4 is applied as a coating that is relatively thick in relation to the AR coating 10. Preferably, it is applied so as to have a thickness in the range of 20 to 50 microns after firing, with the ratio of silver content of the paste to coated substrate surface area being in the order of 10 mg./cm 2 . This silver/glass frit paste may then be dried in air to remove volatile solvents, preferably by heating at about 150 degrees C for 1-4 minutes. Thereafter, the coated solar blank is fired in an oxygen containing atmosphere, preferably in a radiant heated belt-type furnace having a maximum temperature in the range of 800-900 degrees C. The firing is conducted at a temperature and for a time sufficient to (1) remove the organic constituents of the several pastes, (2) fire their metal constituents (e.g., alloy the aluminum content of the aluminum paste with the substrate and form silver soldering pads 16 and the front grid contact 4) , and (3) optimize the properties of the solar cell, i.e., cell efficiency, fill factor and expected useful life. To accomplish all of the foregoing purposes, the substrate is heated so as to reach a peak temperature in the range of 780 to 810 degrees C, preferably a peak temperature of 790-800 degrees C. Preferably the actual time the substrate is held at a peak temperature of 780-810 degrees C is between 1 and 6 seconds, with the substrate having a temperature of at least 700 degrees C for between about 5 and 20 seconds. However, the total time that the cell remains in the furnace may vary, e.g., from about 2 to 10 minutes depending upon the "ramp-up" and "ramp-down" times, i.e., the time required to (a) heat the substrate up to the firing temperature and (b) cool the substrate down from the firing temperature to a temperature where it can be handled for subsequent cooling to room temperature and storage or other subsequent manufacturing operations. The silver metal/glass frit paste used to print the blocks 16 includes between 50 and 80 wt. % silver particles and between 4-15 wt. % glass frit in a vehicle comprising an organic binder such as ethyl cellulose or methyl cellulose and a solvent such as terpineol or Carbitol blended to provide the paste with a suitable viscosity, e.g. , a paste viscosity in the range of 50 to 1000 poise at 25 degrees C and a shear rate of 10 -1 second. Various commercially available silver pastes may be used to print blocks 16. Preferably, the silver metal paste for blocks 16 is based on DuPont's 4942 silver metal/glass frit paste modified with ESL Ni 2554 paste manufactured by Electro Science Labs of Pennsylvania. The ESL paste is believed to contain about 40-70% nickel and it is mixed with the DuPont paste so that the resulting DuPont/ESL paste mixture is believed to be made up of approximately 70 wt. % silver, 1 wt. % nickel, and 10 wt. % glass frit, with the vehicle making up the remainder of the paste mixture. The vehicle is estimated to comprise about 5 wt. % binder and about 24 wt. % solvent. In the case where blocks 16 are formed by pad printing, this DuPont/ESL paste mixture is diluted by adding 10-25 wt. % of Carbitol. The resulting paste is pad printed in a thickness so as to provide each block 16 with a silver content of 10 mg per cm 2 of coated substrate surface. As mentioned hereinabove, this invention is based on the discovery that a "thick" aluminum contact is feasible with the single fire process but not the double fire process. Therefore, this invention essentially comprises applying paste 12 in amounts adequate to form a "thick" aluminum contact, which is advantageous in that it improver cell efficiency. Preferably the paste is applied so as to provide an aluminum content in the range of 4.5 to 8 mg/cm 2 , with the result that firing will result in an aluminum metal contact having a thickness of about 20 to 30 microns, and a P + region in said substrate having a depth in the range of 5 to 8 microns. More specifically, with a silicon substrate having a thickness of 12 mils, it is desired to have solar cells with aluminum contacts that are 20 microns thick and a P + region about 5 microns deep, and with substrates of 16 mils thick it is desired to have aluminum contacts that ..re 30 microns thick and a P + region about 8 microns deep. Aluminum contacts with a thickness in excess of 30 microns are possible with the present invention, but warping and somewhat lower thicknesses tend to reduce yields of acceptable cells. The aluminum paste preferably comprises between about 50 and 70 wt. % aluminum particles, with the remainder of the paste consisting of a vehicle comprising an organic binder such as ethyl cellulose or methyl cellulose and a solvent such as Carbitol or terpineol blended to provide the paste with a viscosity suitable for printing the paste onto the solar cell blank. A suitable aluminum paste may be provided using commercially available pastes. By way of example, a suitable paste for pad printing is achieved by diluting Ferro FX53-015 aluminum paste, made by Ferro Company of Santa Barbara, California, with between 10-25 wt. % of a solvent such as Carbitol or terpineol. The glass frit paste may be a commercially available product diluted to provide the desired flow characteristics. By way of example, the glass frit used in the glass frit paste may be a zinc or lead borosilicate, but the zinc-type glass is preferred for the glass frit paste because of government restrictions on use and disposal of lead-containing materials. A suitable and preferred glass frit paste can be made by mixing the zinc borosilicate glass frit product #7574 of Corning Glass, of Corning, N.Y. with 3.5 wt. % ethyl or methyl cellulose as a binder and 42.5 wt. % of terpineol or Carbitol as a volatile solvent. A suitable lead borosilicate glass frit paste is the one sold by Ferro Company of Santa Barbara, California under the product code #1149. Although the specific composition of the Ferro #1149 product is proprietary, it is believed that that product contains about 60-70% lead borosilicate particles, 10% organic binder and 20-30% solvent. This paste product can be modified for pad printing by dilution with 10-25 wt. % Ferro solvent #800. Depositing a glass frit paste by pad printing rather than screen-printing subjects the semi-conductor substrate to reduced stresses and thereby reduces breakage problems. With respect to the silver metal/glass frit pastes used to form the soldering pads and the front grid contact, it is preferred to use pastes that embody lead or zinc borosilicate glass frits. Zinc and lead borosilicate glass frits are preferred in the present context for numerous reasons. First the glass frits are readily available and may be mixed into pastes which can be applied by conventional thick film methods such as silk screening, pad printing and direct writing technique. Second, metal pastes incorporating lead borosilicate frits are readily available. The softening points of the above-mentioned borosilicate glasses are compatible with the times and temperatures required for firing aluminum metal and silver metal pastes in the formation of solar ce?.l contacts. These glasses also bond well with aluminum surfaces, are stable chemically in the presence of moisture, and do not chemically react with aluminum, silver or silicon. Also, they are corrosion resistant. Various silver/glass frit pastes may be used to form the grid electrode. Suitable pastes consist of between 50 and 80 wt. % metal particles, 4 to 30 wt. % glass frit, and 10-25 wt. % organic compounds (i.e., the binders and solvents constituting the organic vehicle) . Commercially available silver/glass frit pastes and inks may be used. By way of example, the paste for forming the grid electrode may consist of Ferro 3349 paste. The Ferro paste as purchased is believed to comprise about 50-80 wt. % silver and about 10 wt. % glass frit. This paste may be diluted with Carbitol or terpineol to provide the flow characteristics required for the particular method used to print the grid electrode pattern. The invention will be better understood by reference to the following example. EXAMPLE Referring to Figs. 3-7, a 4" x 4" p-type, EFG grown polycrystalline silicon substrate 20 is provided having a conductivity of about 1-4 ohm-cm, a thickness of about 0.016 inch, a shallow p-n junction 22 formed about 0.5 microns from the front surface of the substrate by diffusion or some other suitable junction-forming process, and a silicon nitride AR coating 10 about 800 angstroms thick covering the front surface of the substrate 4, with the AR coating being formed substantially according to the method described in U.S. Patent No. 4,751,191, supra. Eight (8) mutually spaced small areas (each approximately 0.250 inches x 0.250 inches) of the rear surface of the substrate then are covered uniformly with a layer of a silver metal/glass frit paste made by diluting DuPont 4942 silver metal paste with ESL #2554 nickel paste and about 10-25 wt. % Carbitol so as to form printed silver paste blocks 16. The paste as applied comprises about 1 wt. % nickel. The application of the paste is accomplished using a pad printing technique, with the paste being applied in a thickness such that for each printed layer 16 the weight of silver per unit area of the coated surface of the substrate is approximately 10 mg/cm 2 . Paste blocks 16 are subsequently dried at 150° C in air for 2-4 minutes (see Fig. 3) . Thereafter, an area of the rear side of the substrate is coated with a layer of an aluminum paste consisting of Ferro FX53-015 aluminum metal paste diluted by adding between about 10-25 wt. % of Carbitol, with the aluminum metal paste being applied centrally on the rear surface of the substrate 4 by a pad printing technique so as to leave an uncoated band or margin area 14 measuring about 0.040 inches wide. The aluminum metal paste is printed so as to provide windows 26 as previously described that are sized so that the aluminum paste overlaps each side edge of the silver metal paste blocks 16 by about 0.035". The aluminum paste is applied so as to provide an aluminum content in metal paste layer 12 of about 8 mg per cm 2 of coated surface. For a 12 mil thick blank, the aluminum paste is applied so as to provide an aluminum content of about 5 mg/cm 2 of coated surface. The aluminum layer 12 is then dried at 150 degrees C for 2-4 minutes in air (see Fig. 4) . Thereafter, a layer 18 of a ,: ..ic-containing borosilicate glass frit paε*te cc;: rising Corning 7574 glass frit is pad printed over the dried aluminum paste 12 and the margin area 14 with a thickness such as to provide a glass layer about 4 microns thick after firing. The weight of the glass frit in layer 18 totals about 1.5 mg/cm 2 . The glass paste layer 18 extends to the edges of the substrate and ncludes eight rectangular apertures 28 aligned with silver metal paste blocks 16 that are sized so as to permit the glass frit paste to overlap each edge of each of the windows 26 in the aluminum paste layer 12 by approximately 0.015 inches (i.e., each window 28 measures approximately 0.150 x 0.150 inches) . The glass frit paste layer 18 is then dried at 150 degrees C for 1-4 minutes. Then a silver metal/glass frit paste is printed onto the silicon nitride coating 10 in a grid contact pattern as shown in Fig. 1. The paste comprises Ferro silver/glass frit paste #3349. The metal/glass frit paste is applied so as to provide (1) a silver content of about 10 mg/cm 2 on the coated area of the substrate, and (2) a grid contact having a thickness (height) in the order of 30 microns after the cell has been fired. Thereafter, the silicon blank is fired in an oxygen-containing atmosphere in a radiant-heated belt furnace for a period such that the substrate reaches a peak temperature of about 790 degrees C and is held at that peak temperature for between about 1 and 6 seconds, after which it is cooled rapidly. The exit zone of the furnace has a temperature of about 100-125 degrees C, and the conveyor belt is moved at a speed whereby each substrate is in the furnace for about 4 minutes, long enough to ramp the substrate up to its peak temperature and ramp it back down to about 100-125 degrees C as it leaves the furnace. The resulting cell has an aluminum metal contact with a thickness of about 30 microns and a P + region in the substrate adjacent the aluminum metal contact having a depth (thickness) of about 8 microns. These values are reduced if an aluminum paste layer with an aluminum content of 5 mg/cm 2 is applied to the substrate. Fig. 8 graphically illustrates the change in temperature of the substrate as it is processed according to the foregoing example. As can be seen from Fig. 8, the substrate undergoes a rapid increase in temperature up to a maximum temperature of about 790 degrees C, and then its temperature is decreased rapidly but substantially uniformly as it passes through a cooling zone in the furnace. Further cooling after the substrate leaves the furnace is done by radiant heat loss to a room temperature air atmosphere. Although not shown, it is preferred that the substrate be maintained at a temperature of 700 degrees C or higher for a period of 5-20 seconds. For best results, the substrate is held at a temperature of 700 degrees C or higher for about 12 seconds as shown in Fig. 8. The benefits of this invention are several. First and most important, it has been determined that trying to form thick aluminum contacts on a relatively thin substrate, e.g., a thickness of about 0.016 inch or less using a double fire process is not feasible due to a drastic reduction in solar cell yield because of warping or fracture of the solar cell substrates, but using a single fire process it is possible to produce cells with thick aluminum contacts on thin silicon substrates so as to reduce the tendency of the substrates to warp or fracture during or as a result of high temperature firing. Second, it provides a single fire method of making solar cells of the type described that results in cells with improved efficiency. A "single-fire" process incorporating the present invention requires fewer process steps and has been shown to result in higher V oc and J sc values than the double-fire process. Furthermore, notwithstanding the relatively thick aluminum metal layer that is deposited to form the rear contact, the single fire process of this invention avoids the creation of "bumps" in the rear aluminum contact during firing. Third, it accommodates application of a glass sealing layer that effectively reduces corrosion of the aluminum rear contact during use, thereby reducing the criticality of the need to encapsulate a module comprising a plurality of solar cells, and also increasing the life expectancy of the cells. Hence, a "single fire" process utilizing the present invention offers the advantage of a cost reduction coupled with an improvement in solar cell performance and enhanced reliability. Solar cells made with thick dimension contacts according to the foregoing single fire method using EFG grown silicon substrates typically exhibit fill factors in the range of 0.72 to 0.79 and efficiencies in the range of 12.5 to 14%, and also offer the advantage that they resist degradation under high temperature, high humidity conditions for times 50% or more greater than cells made without the glass overcoating, and also the breakage of substrates during the time that the pastes are being converted to contacts and soldering pads is substantially reduced. It is to be understood that the invention is subject to certain modifications, changes, variations and the like that are or will be obvious to those skilled in the art in view of the foregoing detailed description. Thus, for example, the rear side of the solar cell may not include separate soldering pads as shown at 16, but instead the aluminum contact may be uninterrupted over its length and breadth. Accordingly, the foregoing specification is intended to be illustrative but not limiting of the invention in its broadest aspects, and it is tc be appreciated and understood that the invention is limited only by the terms of the appended claims.

Claims: WHAT IS CLAIMED IS: 1. An improved photovoltaic cell comprising: a silicon substrate having a p-type conductivity, a thickness between about 12-16 mils, a front surface, a rear surface and a shallow p-n junction substantially adjacent said front surface; a front contact in mechanically adherent and electrical contact with said front surface; an anti-reflection coating adherently covering the portion of said front surface of said substrate not covered by said front contact; and a rear contact comprising a substantially uniform layer of aluminum metal alloyed to said rear surface so as to substantially cover same, said rear contact being characterized by between 4.5 and 8 mg of aluminum per cm 2 of substrate coated with said layer of aluminum metal, and the aluminum metal being alloyed with the silicon substrate by heating the substrate in air at a temperature above the silicon/aluminum eutectic temperature. 2. The cell of claim 1 having a P + region in said substrate adjacent said aluminum layer, with said P + region having a depth in the range of 5 to 8 microns. 3. The cell of claim 2 wherein said substrate is approximately 12 mils thick, said aluminum metal layer is approximately 20 microns thick, and said substrate has a P + region approximately 5 microns thick in contact with said aluminum layer. 4. The photovoltaic cell of claim 2 wherein said substrate is approximately 16 mils thick, said aluminum layer is approximately 30 microns thick, and said substrate has a P+ region approximately 8 microns thick in contact with said aluminum layer. 5. The photovoltaic cell of claim 3 wherein said aluminum layer preferably is characterized by about 8 mg aluminum per cm 2 of said rear surface. 6. A method of making a photovoltaic cell having improved efficiency comprising: (a) providing a silicon solar cell blan comprising a silicon substrate having p-type conductivity, a front surface, a rear surface, a shallow p-n junction adjacent said front surface, and an AR coating covering said front surface; (b) coating a substantial portion of said rear surface with an aluminum metal paste; (c) coating said AR coating with a layer of a silver/glass frit paste so as to define a selected front contact pattern on said AR coating; and (d) firing said substrate so that said substrate is maintained at a peak temperature of about 780 to about 810 degrees C for a time that is sufficient so that (1) the aluminum metal of said aluminum metal paste is caused to form an aluminum rear contact on said rear surface and (2) the silver metal and glass frit content of said silver/glass frit paste migrates through said AR coating so as to form a silver contact on said front surface of said substrate; characterized in that said aluminum metal paste is applied so as to provide an aluminum content in the range of 4.5 to 8 mg per cm 2 of coated rear surface area, whereby a thick aluminum contact is formed as a consequence of step (d) . 7. The method of claim 6 wherein step (d) involves heating said blank to a peak firing temperature between about 790 degrees C and 810 degrees C, and holding said blank at said temperature for 1-6 seconds. 8. The method of claim 6 wherein a P + region having a depth of about 5 to 8 microns is formed at the interface of said substrate and said rear aluminum contact. 9. The method of claim 6 wherein said peak temperature is about 790 degrees C. 10. The method of claim 6 wherein said firing is accomplished in a radiant belt-type furnace, and further wherein said substrate is moved through said furnace so that sequentially said substrate is brought up to said firing temperature, maintained at said firing temperature for a selected time, and thereafter cooled rapidly from said firing temperature. 11. The method of claim 6 wherein prior to step (b) a layer of silver metal/glass frit paste is applied tc at least one selected area of said rear surface, and further wherein said aluminum metal paste is applied to said rear surface so as define at least one opening in alignment with said at least one select d area, said aluminum paste overlapping but not comi ..etely covering said layer of silver paste at said at least one opening. 12. The method of claim 11 wherein the silver/metal glass frit paste applied to said rear surface is fired with the other pastes during step (d) . 13. The method of claim 6 wherein each of said pastes is dried in air at approximately 150° C for between about 1 and 4 minutes before another paste is applied. 14. A method for making a photovoltaic cell having an improved useful lifetime, said method comprising the steps of: (a) providing a solar cell blank comprising a semiconductor substrate having a front surface, a rear surface, a shallow p-n junction adjacent said front surface, and an AR coating covering said front surface; (b) applying a layer of a first metal paste to said rear surface of said substrate so as to cover selected first area portions of said rear surface, said first metal paste comprising a glass frit; (c) applying a layer of a second metal paste to said rear surface of said substrate so as to cover second area portions thereof that are not covered by said first metal paste, with said second metal paste layer overlapping portions of said first metal paste layer; (d) applying a metal/glass frit paste onto said AR coating in a predetermined electrode pattern; and (e) firing said blank in an oxygen-containing atmosphere at a temperature and for a time such that (1) said metal/glass frit paste penetrates through said AR coating sufficiently for the metal content of said metal/glass frit paste to form an electrical contact bonded to said front surface, (2) the metal content of the first metal paste applied in step (b) forms a mechanically adherent metal layer that forms an ohmic bond with said rear surface at each of said first area portions thereof, and (3) the metal content of the second metal paste applied in step (c) is alloyed to said substrate so as to form an ohmic rear contact with said second area portions of said rear surface and also form a low resistance contact with the metal content of said first metal paste; said second metal paste comprising aluminum metal and being applied to form a layer with a thickness such that the aluminum content of said layer is in the range of 4.6 - 8.0 mg per cm 2 of said second area portions. 15. The method according to claim 14 wherein said substrate has a thickness of about 0.016" and said aluminum paste is applied in a layer having a thickness so that the total aluminum content of said layer is about 8 mg per cm 2 of said rear surface. 16. The method of claim 14 wherein said substrate has a thickness of about 0.012" and said aluminum metal paste is applied in a layer having a thickness so that the total aluminum content of said layer is about 4.5 mg per cm 2 of said rear surface. 17. The method of claim 14 further including the step of applying a layer of a glass frit paste over said second metal paste prior to step (e) , whereby when step (e) is executed the glass frit glass paste will be converted to a glass overcoating that seals off said rear contact. 18. The method of claim 17 wherein said glass frit paste is applied over said second metal paste before said metal/glass frit paste is applied. 19. The method of claim 18 wherein said firing is accomplished by heating said blank to a peak firing temperature between about 780° C and 810° C. 20. The method of claim 19 wherein said blank is held at a peak firing temperature of 790 degrees C for 1-6 seconds.

Inventors: BOTTARI FRANK J, BOTTARI, FRANK, J.
Applicants: MOBIL SOLAR ENERGY CORP, MOBIL SOLAR ENERGY CORPORATION
Application year: 1993
Main classification: H01L31
IPC codes: H01L31