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An improved photovoltaic cell and a method for making such cells is provided. In a generic sense, the preferred method comprises the step of (1) applying a coating of an aluminum metal paste onto the rear surface of the substrate in a suitable rear contact pattern, (2) applying a layer of a glass fr

An improved photovoltaic cell and a method for making such cells is provided. In a generic sense, the preferred method comprises the step of (1) applying a coating of an aluminum metal paste onto the rear surface of the substrate in a suitable rear contact pattern, (2) applying a layer of a glass frit paste so as to envelop the rear contact, and (3) firing those pastes so as to form a rear contact protected by a glass overcoating. A front contact also is formed by applying a silver metal/glass frit paste onto the anti-reflection coating in a related front contact pattern. The front and rear contacts may be fired simultaneously or in two different steps. However, a single firing method is preferred because thicker aluminum contacts are possible. In a preferred embodiment, the cell provided by the invention includes (1) a silicon substrate having a shallow p-n junction adjacent its front surface, (2) a plurality of silver soldering pads bonded to the rear surface of the substrate, (3) an aluminum contact on the rear surface of the substrate having openings that expose at least portions of said silver pads, (4) a silver contact surrounded by an anti-reflection coating on the front surface of the substrate, and (5) a glass layer overlying the aluminum contact so as to protect it against oxidation and corrosion. The glass layer has windows exposing portions of the soldering pads for solder attachment of connecting wire ribbons.

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1. A photovoltaic cell comprising: a silicon semiconductor substrate having a front surface, a rear surface, and a shallow p-n junction adjacent said front surface; a first conductive metal layer in mechanically adherent and electrical contact with said rear surface, said first metal layer essen

1. A photovoltaic cell comprising: a silicon semiconductor substrate having a front surface, a rear surface, and a shallow p-n junction adjacent said front surface; a first conductive metal layer in mechanically adherent and electrical contact with said rear surface, said first metal layer essentially comprising aluminum; a second conductive metal layer in adherent and electrical contact with said front surface, said second conductive metal layer defining a predetermined grid electrode pattern; a radiation-transparent AR coating covering at least that portion of said front surface of said substrate not covered by said second conductive metal layer; and a protective coating covering and sealing said first conductive metal layer, said protective coating being a glass selected from the group consisting of a lead-containing borosilicate glass, a zinc-containing borosilicate glass, and mixtures of said zinc-containing borosilicate glass and said lead-containing borosilicate glass. 2. The photovoltaic cell of claim 1 wherein said AR coating comprises silicon nitride. 3. The photovoltaic cell of claim 2 wherein said AR coating is approximately 800 Angstroms thick. 4. The photovoltaic cell of claim 1 wherein said first metal layer comprises aluminum metal in a concentration corresponding to an aluminum weight of between about 400-700 mg per a 3.781×3.781 inch area. 5. The photovoltaic cell of claim 4 wherein said first metal layer is between about 20 and 30 microns thick. 6. The photovoltaic cell of claim 1 wherein said second metal layer comprises silver metal and an inorganic glass frit. 7. The photovoltaic cell of claim 1 wherein said second metal layer has a thickness greater than the thickness of said anti-reflection coating. 8. The photovoltaic cell of claim 1 wherein said glass is a lead borosilicate glass. 9. The photovoltaic cell of claim 1 wherein said glass is a zinc borosilicate glass. 10. The photovoltaic cell of claim 1 wherein said glass has a thickness of approximately 4 microns. 11. The photovoltaic cell of claim 1 wherein said glass will soften and flow at a temperature of between about 760° C.-800° C. 12. 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 metal paste to said rear surface of said substrate so as to substantially cover a substantial portion of same; (c) applying a layer of an inorganic glass frit paste over the metal paste applied to said rear surface in step (b); (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 and the metal content of said metal/glass frit paste forms a mechanically adherent and electrical contact with said front surface of said substrate, (2) the metal content of the metal paste applied in step (b) forms a mechanically adherent and electrical metal contact with said rear surface of said substrate, and (3) the glass frit in said glass frit paste fuses so as to form an adherent protective glass layer encasing the metal contact formed on said rear surface of said substrate. 13. The method of claim 12 wherein said semiconductor substrate is made of silicon, the metal paste used in step (b) is an aluminum metal paste, and said metal/glass frit paste is a silver/glass frit paste. 14. The method of claim 13 wherein said AR layer comprises silicon nitride and is approximately 800 Angstroms thick. 15. The method of claim 13 wherein the layer of glass frit paste is approximately 10 microns thick prior to step (e). 16. The method of claim 13 wherein the glass frit in said metal/glass frit paste is selected from the group consisting of a lead-containing borosilicate glass, a zinc-containing borosilicate glass, and mixtures of said lead-containing borosilicate glass and said zinc-containing borosilicate glass. 17. The method of claim 13 wherein the metal paste in step (b) is applied by a pad printing technique. 18. The method of claim 13 wherein said firing step (e) is performed in air. 19. The method of claim 18 wherein said substrate is fired by heating it to a peak temperature between 780 degrees C. and 810 degrees C., with a temperature above 700 degrees C. for a time not exceeding 20 seconds. 20. The method of claim 12 wherein the glass frit content of said glass frit paste comprises a lead-containing borosilicate glass, a zinc-containing borosilicate glass, or a mixture thereof. 21. The method of claim 12 wherein said substrate is fired by heating it to a peak temperature of 780-810 degrees C. for only 1-6 seconds. 22. The method of claim 12 wherein the paste applied in step (b) is dried before step (c). 23. The method of claim 12 wherein the paste applied in step (c) is dried before step (d). 24. The method of claim 12 wherein the paste applied in step (b) is dried before step (c), and the paste applied in step (c) is dried before step (d). 25. The method of claim 24 wherein said drying steps are performed in air at approximately 150 degrees C. 26. The method of claim 12 wherein in step (b) said metal paste is applied so as to form a layer having at least two or more apertures each exposing a selected portion of said rear surface of said substrate, and further wherein after step (b) but before step (c) a second metal paste is applied so as to cover those selected exposed portions of said rear surface, and further wherein said glass frit paste is applied so as to form openings aligned with said apertures but sized so that said glass frit paste overlaps said first-mentioned metal paste at said apertures. 27. The method of claim 12 wherein said substrate is between about 12 and 16 mils thick. 28. The method of claim 12 wherein prior to step (b) another metal paste comprising a solderable metal is applied to said rear surface such that a plurality of discrete areas of said rear surface are formed with blocks of said another metal paste, and said first-mentioned metal paste is applied so as to form a metal paste layer having at least two or more apertures each exposing a selected portion of one of said blocks, but with said metal paste layer overlapping edge portions of said blocks, and further wherein said glass frit paste is applied so as to form openings aligned with said apertures but sized so that said glass frit paste overlaps said first-mentioned metal paste at said apertures. 29. The method of claim 28 wherein said another metal paste is a silver metal paste and said first-mentioned paste is an aluminum metal paste. 30. The method of claim 12 wherein said substrate is fired in step (d) so that it undergoes a change in temperature substantially as illustrated in FIG. 12. 31. 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; (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 layer of an inorganic glass frit paste over the layer of said second metal paste applied to said rear surface in step (c); (e) applying a metal/inorganic glass frit paste onto said AR coating in a predetermined electrode pattern; and (f) 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 a low electrical resistance contact bonded to said front surface, (2) the metal content of the 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, (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 (4) the glass frit in said glass frit paste fuses so as to form an adherent continuous glass layer encasing said rear contact. 32. The method of claim 31 wherein said glass frit paste is approximately 10 microns thick prior to firing. 33. The method of claim 31 wherein the glass frit in said metal/glass frit paste is a lead-containing borosilicate glass. 34. The method of claim 31 wherein the glass frit in said glass frit paste is selected from the group consisting of lead containing borosilicate glass, zinc containing borosilicate glass, and mixtures of lead containing borosilicate glass and zinc containing borosilicate glass. 35. The method of claim 31 wherein said first and second metal pastes are applied to said solar cell blank by pad printing, and said metal/glass frit paste is applied to said solar cell blank by a direct writing technique. 36. The method of claim 31 wherein said substrate is silicon, and said firing step is performed in an oxygen containing atmosphere. 37. A photovoltaic cell comprising: a silicon semiconductor substrate having a front surface, a rear surface, and a shallow p-n junction adjacent said front surface; a first electrically conductive metal layer in mechanically adherent and electrical contact with said rear surface, said layer consisting essentially of aluminum and including two or more openings therethrough at selected locations on said rear surface of said substrate; an electrically conductive metal soldering pad disposed in each of said openings, each of said soldering pads having a peripheral portion in electrical contact with said first conductive layer and a surface in mechanically adherent and electrical contact with said rear surface; a second electrically conductive metal layer in adherent and electrical contact with said front surface, said second metal layer defining a predetermined grid electrode pattern; a radiation-transparent AR coating covering at least that portion of said front surface of said substrate not covered by said second metal layer; and a protective coating of an electrically-insulative, corrosion-resistant inorganic material covering and sealing said first metal layer, said protective coating defining two or more apertures therethrough in registration with said two or more openings, said apertures being smaller than said openings but large enough to leave at least the central regions of said soldering pads exposed for connection to outside circuit elements. 38. The photovoltaic cell of claim 37 wherein said soldering tabs fully fill said openings in said first metal layer. 39. The photovoltaic cell of claim 38 wherein said apertures are sized so that said protective coating overlaps edge portions of said soldering tabs. 40. The photovoltaic cell of claim 37 wherein said soldering tabs essentially comprise silver metal. 41. The photovoltaic cell of claim 37 wherein said second metal layer essentially comprises silver metal and said AR coating is silicon nitride. 42. The photovoltaic cell of claim 41 wherein said soldering tabs essentially comprise silver metal. 43. The photovoltaic cell of claim 37 wherein said protective coating consists of a glass from the group consisting of lead containing borosilicate glass, zinc containing borosilicate glass, and mixtures of lead containing borosilicate glass and zinc containing borosilicate glass.