Method of preparing solar cell front contacts

Details Method of preparing solar cell front contacts Method of preparing solar cell front contacts

2000-03-10

2002-01-15

Diamond, Alan (Department: 1753)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S098000, C438S256000, C136S256000

Reissue Patent

active

RE037512

ABSTRACT:

OBJECT OF THE INVENTION
The present invention is related to a method of preparing contacts on the surface of semiconductor substrates. The present invention is also related to products obtained by this method and more particularly to a solar cell.
STATE OF THE ART
Conventional screen printing is currently used in a mass scale production of solar cells. Typically, the top contact pattern of a solar cell consists of a set of parallel narrow finger lines and wide collector lines deposited essentially at a right angle to the finger lines on the semiconductor substrate or wafer.
Such front contact formation of crystalline solar cells is performed with standard screen printing techniques. It has advantages in terms of production simplicity, automation, and low production cost.
Low series resistance and low metal coverage (low front surface shadowing) are basic requirements for the front surface metallization.
According to the document Hybrid Circuit No. 30, January 1993, “Thick-film Fine-line Fabrication Technique—Application to Front Metallization of Solar Cells,” by A. Dziedzic, J. Nijs, and J. Szlufcik, minimum metallization widths of 100-150 &mgr;m are obtained using conventional screen printing. This causes a relatively high shading of the front solar cell surface. In order to decrease the shading a large distance between the contact lines, i.e., 2 to 3 mm is required. On the other hand, this implies the use of a highly doped, conductive emitter layer. However, the heavy emitter doping induces a poor response of the solar cell to short wavelength light. Narrower conductive lines can be printed using ultra-thin stainless steel wire screens with a high mesh density of 325 or 400. A thin masking emulsion with a thickness of 5-15 &mgr;m is required to produce a line definition on the screen of at least 50 &ngr;m.
Although a line width of 50 &mgr;m can be achieved, the line thickness decreases below 10 &mgr;m measured after the firing process. This gives rise to increased line resistance causing high power dissipation, particularly in the main collector lines.
The fact that the fingers are ultra-thin can result in the interruption of such fingers.
Another main disadvantage of the ultra-thin screens is their higher cost and lower durability and/or reliability.
An alternative technique to the standard screen printing is the application of an etched or electroformed metal mask. The manufacturing process of such mask involves etching of a cavity pattern on the one side of the metal foil and a mesh pattern on the reverse side. Photoresist masking and precise mask positioning are necessary for double-sided etching of the metal foil. This implies a complicated design and a very high screen cost.
In the case of conventional wire mesh screens as well as in the case of the metal etched screens, the open area (mesh openings) is usually not higher than 50% of the pattern. The open area defines the maximum amount of paste transferred to the substrates and at the same time the wet line thickness. Another important point is that a small mesh aperture requires utilization of special inks formulated for fine line printing. This is in conflict with most of the commercially available silver pastes for solar cell front contact metallization. Silver powder has a tendency to create agglomerates of particles in the paste. In addition, a flake-shaped silver powder, usually used in the paste formulation for a solar metallization, increases the tendencies to create agglomerates of particles in the paste.
The modern solar cell processing includes growing of thin thermal oxide (50-250 Å) on the top emitter surface using methods well known in microelectronics. Such an oxide layer passivates defects and recombination centers always present on the semiconductor surface. This process leads to an improvement of cell response to solar short wavelength radiation that in effect gives rise to a higher cell efficiency. Although commercially available screen printed pastes produce good contact to non-oxidized silicon surfaces, the firing through thermal oxide gives difficulties in obtaining high quality contacts with low resistance.
It should also be noted that the solar cell manufacturing process includes in most cases a step of applying an antireflection (AR) coating which can be deposited before or after the contact formation. If the AR layer is deposited before contact printing, it often gives rise to the problem of high contact resistance between silicon and printed contacts. This problem occurs particularly when silicon nitride is used as an antireflection coating.
If an AR layer is deposited after the contact formation, another problem is raised which is the soldering of the collector lines during the module fabrication.
The solution to this problem brings the “firing through” method described in PCT Document WO 89/12312, wherein the authors apply the commercially available silver paste “Ferro #3349” to “fire through” a silicon nitride ARC. A “fired through” TiO
2
AR layer is described in the paper by Nunoi, et al. “High performance BSF silicon solar cell with fired through contacts printed on AR coating”, 14th IEEE PV Specialists Conference—1980, San Diego, USA, pp. 805-810.
PCT Document WO 92/22928 describes a solar cell and a method to make it wherein an antireflective coating is deposited on a semiconductor substrate before a first set of narrow elongated parallel electrodes are printed thereon and wherein finally a second set of elongated electrodes are affixed to each of the first electrodes.
It should be noted that the paste or the ink used in order to form the array of narrow elongated parallel electrodes is such that it penetrates said antireflective material and forms mechanically adherent and low electrical resistance contact with the front surface of the semiconductor substrate. This means that not all the conventional pastes can be used. Furthermore, in order to have such good contact between the semiconductor substrate and the narrow elongated parallel electrodes, a step of “firing through” is necessary.
The firing at the same time through the thermally grown silicon dioxide and antireflection coating (particularly silicon nitride) layers, although described in the technical literature, usually gives problems of high contact resistance and is difficult to achieve with commercial pastes.
E.P.O. Document EP-A-0002550 describes a method of forming a contact configuration for soldering a metal connection on a region of the surface of a semiconductor body comprising the provision by serigraphy, on at least a part of said region, of a conductive paste which comprises at least a principal metal, said paste then being vitrified thermally such that the dopant migrates into at least a surface part on the region of a surface of the semiconductor body.
OBJECTS OF THE INVENTION
The present invention has an object to provide improved semiconductor devices such as solar cells which do not have the drawbacks of the prior art.
More particularly, the present invention aims to form semiconductor devices such as solar cells wherein the electrical contacts exhibit a low series of resistance and a low metal coverage which also provides a low front surface shadowing.
Many other advantages will be mentioned hereunder in the description of the main characteristics of the present invention.
SUMMARY OF THE INVENTION
The present invention provides a method of forming the top contact pattern of a solar cell, which consists of a set of parallel narrow finger lines and wide collector lines deposited essentially at the right angles to the finger lines on a semiconductor substrate, characterized in that it comprises the following steps:
(a) screen printing and drying the set of narrow finger lines;
(b) printing and drying the wide collector lines on top of the set of finger lines in a subsequent step;
(c) firing both finger lines and collector lines in a single final step in order to form an ohmic contact between the finger lines and the semiconductor substrate and between the finger lines and the wide collector lines.
A

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