IE52714B1 - Aluminum-magnesium alloys in low resistance contacts to silicon coated with si3n4 - Google Patents

Description

FIELD OF THE INVENTION The present invention relates to the art of converting solar energy into electrical energy by means of a silicon solar cell, and more particularly, to a thick-film paste composition for making low resistance contacts to such a cell.

BACKGROUND OF THE INVENTION It is well known that radiation of an appropriate wavelength falling on a P-N junction of a semiconductor body serves as a source of external energy to generate ho’le-electron pairs in that body. Because of the potential difference which exists at a P-N junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer which has been metallized, i.e., provided with metal contacts which are electrically conductive.

To provide a low cost method of generating an electrical current from the P-N junction region of the silicon wafer, it is common practice to metallize the wafer by a screen printing and firing sequence. Commercially available metallizing inks which are employed for depositing contacts on the surface of the wafer generally contain a metal powder, a finely divided glass frit, and an organic vehicle. Typical metal powders are those of silver, aluminum, nickel, gold, or copper, or alloys of these with precious metals, including platinum and palladium.

There is extensive use of SijN^ in solar cell technology as an anti-reflection coating, which also serves as a masking protective layer. It has good adhesion and stability when deposited on silicon. In a specific embodiment, silicon solar cells are coated with Si3N4 on the front N-type side as an anti-reflection coating, and, in the process, the back P-type side also becomes coated with Si,N.. In order to make electrical contact to 3 4 the underlying silicon substrate, an etchant step is often employed. It is state of the art technology that the Si^N^ is removed where contact is made, the front side etched in pattern form for application of the front side contact, and the back side similarly etched for application generally of large area backside contact. There would be a cost saving if this etching step could be eliminated.

The article Thick film, plating cut cost of laying solar cell contacts and conductors in Electronics International, Vol.53, No.20, Sept. 11, 1980 (New York) pages 40-41, discloses a process of metallizing a silicon solar cell having an anti-reflective coating of Si^N^ without the need for etching the Si^N^ coating, using an air-firable nickelbased thick-film ink.

It has now been found that metallizing paste formulations containing aluminum-magnesium alloys can also perform this function.

British Patent Specification No. 1 558 764 is directed to a method for making photo-voltaic cells in which the semi-conductive body of the cell is coated with a passivating layer of TiC>2 or SiO^ on which is printed a conductive composition which comprises a mixture of highly conductive metal such as Ag or Au, glass frits and a material which will react with an underlying passivating layer such as titanium hydride, dispersed in an organic medium. It is a disadvantage of this method that it requires the passivating layer to obtain adequate adhesion.

SUMMARY OF THE INVENTION In terminating Si^N^-coated solar cells with base metal contacts such as of Ni-Sb alloys or ; aluminum, improvement in electrical characteristics are obtained, and the firing window, i.e., the temperature range for satisfactory firing, is widened, when 50 Al:50 Mg alloy powder is incorporated in the metallizing paste. In a specific embodiment, the invention resides in a thick-film metallizing paste for use in providing low resistance electrically conductive contacts (terminations) to a silicon solar cell coated with Si-^N^, having a P-type and an N-type region, and a P/N junction, said paste comprising an organic vehicle containing, in particle form, a mixture of a major amount of a metal powder, e.g., aluminum or a Ni-Sb alloy, - 4 53714 a minor amount of finely divided glass frits, and a small amount of a 50 Al:5O Mg alloy. The invention further resides in the process of metallizing the cell, and in the resulting product. The metallization process typically comprises screen-printing one surface of the cell with the metallizing paste of this invention and firing at a temperature of at least 500°C.

DETAILED DESCRIPTION This application is related to U.S. Patent Specification Nos. 4,361,718 and 4,342,795. The present invention is demonstrated by the Examples which follow.

EXAMPLE 1 A front surface textured silicon solar cell, constructed by applying N-type impurities 0.4 to 0.5 microns in depth into a P-type silicon wafer that had been etched to form pyramidal texture on the diffusion side, and having a Si-^N^ anti-reflection coating, was metallized to provide metal contacts or terminations. The metallizing paste was screen-printed on the N surface of the wafer and was composed of an organic vehicle (ethyl cellulose/dibutyl phthalate' in terpineol), NiSb alloy, glass frits, and a minor amount of ..a* 50 Al:50 Mg alloy. Composition of the glass frits in 1 by wt, was PbO 83, PbFj 4.9, an^ Three samples of paste were prepared, and the terminations were nitrogen-fired, the metal components were'varied, The proportions as indicated in 50 AlsSOMg following tabulations: Glass Paste # NiSb 1 88 12 0 2 83 12 4.7 3 79 12 9.3 - 5 - The soldered electrical characteristics of the nitrogen-fired solar cells are listed in Table I.

TABLE I Termi- nation Firing Temp °C VOc (mv) Fill FactorR Series (ohm)Rshunt (ohm) 1 500 - - Large (Infinity) 1 550 580 0.57 1.7 II 1 575 520 0.31 1.4 II 2 500 560 - 8.6 It 2 550 595 0.50 1.2 It 2 575 595 0.54 0.73 ' If 3 500 590 - 6.0 n 3 550 600 0.50 2.0 II Table I demonstrates that the termination not containing the 50 A1j50 Mg alloy has a window with lower firing temperature near 550eC, while this window is extended to at least 500°C when the alloy is present. Table I also demonstrates that the electrical characteristics of the solar cell, i.e., series conductivity and Vac (voltage generated across the cell when illuminated by one sun with no current 25 flowing through cell) are enhanced when use is made of this alloy. This demonstrates that the 50 Al:50 Mg alloy was effective in penetrating the sigN^ coating and making contact with the N-type region.

EXAMPLE XI -The 50 Al:50 Mg alloy was incorporated into of 3 samples of a thick-film aluminum base metallizing paste, and these were applied by screen-printing to the back P-type surface of a silicon solar cell coated with Si,N.. The 35 3 4 resulting terminations were air-fired. Table IT shows the back contact resistance in ohms of the solar cells so terminated (making use of a two-probe measurement between parallel conductor lines on the P surface of the silicon cell). - 7 52714 φ Μ JJ <0 Ο ιη ιο S rH I Ν Μ ο ο ο Η ιη u Φ 0« ε φ ο fro ο CDH C <0 Η Μ •Η &4 S ο ο «Η ί ο ιη CM m w w U «Η 0 < Ο ιη ιη 1 ! I Ο ο Ο ο ιη 1 I I Μ co ιη ιη ο (Q • « • pH CM CM in Ο ι—1 r-i CM ·· η σ> < S 00 CO C ο ο • m Ο co 0 •H m m 00 ιη m fH Η © O £o pH • • • < «Η ρ* 10 pH p*I co co 10 Φ 4J CO ι—( CM cn 11 11 s « (0 CU rH CM It is evident from Table II that there is a significant reduction in contact resistance of formulations to which 50 Al:50 Mg had been added, compared to that where the 50 Al;50 Mg was absent.

The foregoing Examples are illustrative only. It is to be understood that'other vehicles, other metal powders, other glass frit compositions, and other Al:Mg alloys may be employed, to the extent that they function to form a thick-film metallization paste for use in providing low resistance electrically conductive contacts to a silicon solar cell-coated with si^t^. While screen printing is disclosed above, other methods of application to the substrate such as brushing, spraying, stamping, etc. could be used. The organic vehicle employed in the printing paste is generally employed in an amount such that the printing paste will contain 70-90% solids and 10-30% vehicle. A number of inert liquid vehicles commonly used in the art are described in greater detail in O.S. Patent Specification. No. 4,172.919,colunn 4, lines 3-28, which lines are incorporated by reference herein.

Claims (12)

1. CLAIMS:1. A thick-film metallizing paste for use in providing low resistance electrically conductive contacts to a silicon solar cell coated with 5 Si^N^, said paste comprising an organic vehicle containing, in particle form, a metal powder, finely divided glass frits, and a 50 Al:50 Mg alloy.

2. The metallizing paste of claim 1 wherein the metal powder is Al powder. 10

3. The metallizing paste of claim 1 wherein the metal powder is a powder of a NiSb alloy.

4. The metallizing paste of claim 1 wherein the organic vehicle contains, in particle form, a mixture of a major amount of a NiSb alloy, a minor 15 amount of finely divided glass frits, and a small amount of a 50 Al:50 Mg alloy.

5. The metallizing paste of claim 1 wherein the organic vehicle contains, in particle form, a mixture of a major amount of aluminum, a minor amount 20 of finely divided glass frits, and a small amount of a 50 Al:50 Mg alloy.

6. A process of metallizing a silicon solar cell having a P-type and an N-type region, a P-N junction and a Si 3 N^ coating which comprises screen-printing the coated 25 N-type surface of said cell with the metallizing paste of 10 52714 claims 3 or 4, and firing the printed surface at a temperature of at least 500°C to form electrically conductive contacts thereon.

7. A solar cell prepared by the process of 5 claim 6.

8. A process of metallizing a solar cell having an Si 3 N 4 coating, a P-type and an N-type region, a P-N junction and a Si^N^ cpating which comprises screen-printing the coated Prtype surface of said cell with 10 the metallizing paste of claims 2 or 5, and firing the printed surface at a temperature of at least 600°C to form electrically conductive contacts thereon.

9. A solar cell prepared by the process of claim 8. 15

10. A metallizing paste containing an aluminum alloy, and substantially as'herein described with reference to the Examples.

11. A method of making a solar cell, in which electrically conductive contacts are formed by using a 20 metallizing paste according to claim 1, substantially as herein described.

12. A solar cell made by a method according to claim 11.