Electrical Contacts for a Solar Cell

The present invention relates in general to photovoltaic cells having electrical contacts on their front and/or back surfaces. More specifically, it relates to a method of forming one or both of those electrical contacts and to the cells so produced.

In the foregoing U.S. Pat. No. 4,392,010 and U.S. Pat. No. 4,297,391, I have described photovoltaic cells that are of a general type well-known in the art. In those cells a host material, e.g., a monocrystalline silicon wafer, has been doped with a suitable impurity so that the doped wafer has p or n characteristics. Thereafter, an impurity of a conductivity type opposite to that of the dopant is diffused or otherwise applied to the doped wafer to form a p-n photovoltaic junction at or closely adjacent to the front, light-receiving surface of the wafer. When light energy impinges on the front, light-receiving surface of the cell, electron-hole pairs are excited in the silicon. Because of the presence of the p-n junction, which term is meant to include both p-n and n-p junctions, electrons will be directed toward one or the other of the major surfaces of the cell, and holes toward the other major surface. In a typical n-p cell, electrons are directed to the front, light-receiving surface of the cell and holes toward the back surface. To make the cell operable it is typically necessary to apply electrical contacts to both the front and back surfaces of the cell so that electrons excited by impingement of light on the silicon or other host material will be conducted away from the cell and, after causing work to be done, back to the cell to complete the circuit.

Many problems associated with the application of an electrically conductive grid to the surface of a photovoltaic cell have been mitigated in accordance with the invention disclosed and claimed in the aforementioned U.S. Pat. No. 4,392,010. In that patent a method is described in which a contact is applied to the surface of a photovoltaic cell by forming particles of electrically conductive material, generally a metal, at a temperature in excess of the alloying temperature of the material and silicon, when silicon is the host material for the cell. The particles are then sprayed to a surface of the cell at a temperature at which they will become adhered to the silicon or other host material from which the cell is formed. Preferably, electrical contact between the silicon and the contact material is accomplished without deep alloying when the contact is being applied to the front surface of the cell. True alloying may take place. However, use of the term, alloying, is not limited to a strict, technical definition of the term but includes adherence at somewhat lower temperatures where the mode of adherence might better be termed, sintering. Such methods of spraying have become commonly known as flame spraying, arc spraying, or plasma spraying. Common to all three forms is the fact that fine particles, atoms, or ions of a metallic material, such as aluminum, are directed toward the work surface, in this case the surface of a solar cell. Consequently, for the purposes of the present invention these forms of spraying will be used synonymously and the disclosure of such spraying in my prior patent is desired to be incorporated by reference herein where requisite.

The aforementioned U.S. Pat. No. 4,392,010 described a problem which is present when aluminum, the material preferred for use for both front and back contacts of the cells therein described, is used. Aluminum is economically priced, and has good electrical characteristics that enable it to serve as a conductor of electricity either for the front, patterned grid of a solar cell or for the back, usually continuous contact layer on the rear, protected surface of the cell. While aluminum meets the basic criteria for a suitable conductor of electricity, it has been found that, as exacerbated when plasma spraying is used to apply aluminum to the surface of the cell, the difference in the coefficients of thermal expansion of aluminum and silicon has resulted in cells being bent or warped to some extent as both the silicon host material and the aluminum contact cool. When the back contact of the cell has been plasma sprayed with aluminum in a continuous layer, such warpage is emphasized. As a result, it has been found desirable to apply the aluminum to the back surface of the solar cell in the form of a pattern rather than a continuous layer, thereby at least to mitigate warpage and bending of the cells during cooling. While warpage will be less evident on the front surface of the cell, where the aluminum contact is applied in the form of a grid rather than a continuous surface, it may be present there as well.

Moreover, when aluminum alone is applied, e.g., to the back surface of the cell, the resulting surface has a grainy characteristic, rather than being smooth. The grainy, almost sandpaper-like surface is resistent to facile attachment of parts such as electrical wiring leads, to the back surface of the cell. The same problem is present, perhaps to a lesser extent, where the grainy texture is on the front grid of the cell, particularly where busbars are to be affixed to collecting fingers of the grid. As a consequence, while the so-formed cells are operable, these deficiencies in the use of aluminum contacts sprayed onto the front and/or back surfaces of the cell have been identified.

The aforementioned problems were overcome by reducing the quantity of aluminum used and incorporating with it a soft soldering metal such as zinc. It was taught that aluminum and zinc could be sprayed onto the surface of a cell in a mixture of constant composition, or that several discrete layers of varying composition could be applied, such that the metallic layer adjacent to the silicon of the cell would be largely or entirely aluminum while the external metallic surface would be largely or entirely zinc or other solderable soft metal.

The foregoing U.S. Pat. No. 4,392,010 indicated that, while zinc appeared to be the most suitable solderable soft metal for use at that time, it was contemplated that tin, lead, copper and silver, among other metals, could be substituted for or used in conjunction with zinc.

I have now ascertained that several other solderable metals and alloys can, in fact, be substituted for zinc, and that some of these other metals offer valuable benefits above and beyond those provided by zinc.

Other metals which I have used successfully in this manner include:

(1) Tin/zinc alloy containing from 20% to 40% zinc.

(2) Nickel.

(3) Copper.

(4) Copper/aluminum alloy containing about 90% copper.

(5) Copper/tin alloy containing about 93% copper

(6) Babbitt metal containing 90% tin, 7 antimony, 3% copper.

(7) Tin.

(8) Tin/silver alloy containing from 1% to 5% silver.

(9) Tin/lead alloys having tin/lead ratios in the range from 60/40 to 63/37.

(10) Tin/lead alloys having tin/lead ratios in the range from 60/40 to 65/35 and containing from 1% to 5% of a third metal selected from the group comprised of silver, antimony, cadmium, and bismuth.

Pure silver would also be applicable, and it would provide the best electrical conductivity, but silver is excessively expensive. Copper conductivity is nearly as good as that of silver, but caution must be exercised in spraying copper, because copper, improperly applied, could cause shunting of the active p-n junction of the solar cell.

Nickel and tin are especially good choices for corrosion resistance, while tin is especially easy to solder. Pure tin wire is very soft and consequently difficult to manipulate in this application, but a small amount of silver added to the tin hardens it sufficiently for easy handling.

The tin/lead alloys, with or without the addition of a third metal, are commercial solders, and they yield products possessing the desirable solderability properties of these solders.

Considering all of the requirements for solar cell metallization, the preferred metal for use with aluminum at this time appears to be a tin alloy containing a small amount of silver. A commercially available alloy which I find especially suitable contains about 96% tin and 3.6 to 4.4% silver.

While the present invention has been described with reference to best modes thereof, numerous alterations and modifications will be apparent to those of skill in the art without departing from the spirit of the invention. As to all such obvious alterations and modifications, it is desired that they be included within the preview of my invention, which is to be limited only by the scope, including equivalents, of the following appended claims.