Substrate for use in preparing solar cells

Conductive material is combined with other substances to form a composite material for use as a conductive back face substrate for a thin silicon wafer solar cell. In at least one embodiment, a conductive composite substrate material is fabricated by filling granular conductive material with a mineral or ceramic or other small particulate with a low CTE; the composite is cast and fired so that it has an electrically conductive continuous phase and a discontinuous phase that will control and match the CTE of the substrate to be equal to or close to that of silicon, thereby diminishing the effects of bowing from CTE-mismatch.

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to methods and processes for fabricating photoelectric devices, and more particularly to methods and processes for fabricating the back face substrate for silicon wafer solar cells.

2. Description of the Related Art

In most current embodiments of solar cell technology, a solar cell comprises a silicon wafer built on a substrate (the “back face substrate”). In this respect, a silicon wafer solar cell closely resembles other silicon wafer semiconductor devices. Accordingly, certain methods and technologies used to fabricate silicon wafers for use in conventional semiconductor devices are also used to fabricate silicon wafers for solar cells. However, the choice of materials for the back face substrate of a solar cell hinges upon different criteria than the choice of substrate material for the back face substrate of a conventional semiconductor device. For example, when fabricating a silicon wafer with a back face substrate for a conventional semiconductor device, as for example for use in an integrated circuit, it is usually desirable for the substrate material to be an electrical insulator. By contrast, with a solar cell, it is desirable for the back face substrate to be an electrical conductor. Thus, the back face substrate of a solar cell is often referred to as a “back face conductor”.

In order to improve silicon utilization and reduce the material costs of producing silicon wafer solar cells, the trend in the industry has been to reduce the thickness of silicon wafers as much as is practical. Wafer thickness of between 180 microns and 200 microns is typical of the present state of the technology. Customary silicon wafer specifications at present call for a square-faced wafer that is slightly over 12 millimeters on each side of the square; thus, the length of the wafer, on any side or diagonal, is considerably greater than the thickness of the wafer.

Silicon wafer solar cells are fabricated using processes that require the silicon and the substrate material or materials to attain high temperatures—usually several hundred degrees Celsius, with the exact temperatures varying depending on the type of fabrication process used and the nature of the substrate material, among other considerations. When a silicon wafer solar cell cools after fabrication, the solar cell often will experience bowing due to a difference in the coefficient of thermal expansion (CTE) of the substrate material versus the CTE of the silicon. As the substrate material cools and contracts to a greater degree than the silicon, the contracting substrate material pulls the silicon into a curved or bowed shape. The greater the difference between the CTE of the substrate material and the CTE of silicon, the greater the bowing. Additionally, bowing generally will be greater with thinner wafers, as thinner wafers generally flex more easily than thicker wafers.

Significant bowing can damage a silicon wafer, for example by leading to separation between the silicon layer of the solar cell and the back face substrate, or by causing the silicon wafer to crack. Therefore, it is desirable to limit bowing as much as possible while still producing a thin silicon wafer with a conductive back face substrate. In addition to bowing during the fabrication process, a silicon wafer used as a solar cell may also experience bowing due to changes in temperature during use. Therefore, a silicon wafer solar cell should be designed to minimize bowing due to extremes of summer and winter weather. In particular, a silicon wafer solar cell should be designed to withstand exceptionally low winter temperatures, which may be infrequent but can nevertheless cause failures such as those discussed above when they do occur.

One approach to limiting the degree of bowing in a finished silicon wafer solar cell is to select materials for fabrication of the solar cell which decrease as much as is feasible the mismatch between the CTE of the silicon layer and the CTE of the back face substrate. With conventional semiconductor devices, practitioners have used for the back face substrate various ceramic materials that have a CTE close to the CTE of silicon. However, this class of substrate materials is generally ill-suited for use in a solar cell because the ceramic materials with a low CTE close to the CTE of silicon are generally insulators and are poor electrical conductors. As discussed above, with a solar cell, it is desirable for the back face substrate to be an electrical conductor.

As electrical conductors, the metals shown in the Table 1 as follows make attractive candidates for use as conductive back face substrates:

However, essentially all of the candidate metals have a high mismatch of CTE as compared to silicon. Gold and platinum have high melting points and would present high material costs for use in silicon wafers. Copper likewise has a high melting point. Silver has a slightly lower melting point, but silver's lack of elongation and relatively high cost also make it impractical to use as a substrate material for a conducting back face substrate. Aluminum is attractive as a material for a conductive back face, as it has a low melting point and is a good electrical conductor. However, Aluminum by itself as a substrate presents the possibility of significant bowing because of the great difference in CTE between silicon (which is approximately 3 parts per million per degree Celsius) and aluminum (which is approximately 23 parts per million per degree Celsius).

In light of the above, it is desirable for the back face conductor of a solar cell to be highly conductive electrically. It is also desirable for the back face conductor to be highly reflective in the ultraviolet (UV) to infrared (IR) range of the electromagnetic spectrum. A back face conductor is desired which is able to survive and operate in a wide range of thermal conditions, including cold winter conditions and summer heat. Additionally, for solar cells to be competitive with technological alternatives, it is desirable to produce a solar cell using materials for the back face conductor which are not prohibitively expensive.

BRIEF SUMMARY OF THE INVENTION

Disclosed herein are methods and processes for fabricating photoelectric devices and more particularly to methods and processes for fabricating a conductive back face substrate for silicon wafer solar cells. In many embodiments of the present invention, a conductive material, such as aluminum, is combined with other substances having lower CTE to form a composite material for use as a back face substrate. This composite material takes advantage of the high conductivity of aluminum, while the other substances reduce the overall CTE of the composite material and thereby diminish the adverse effects of bowing from CTE mismatch. In at least one embodiment of the present invention, a conductive composite substrate material is fabricated by mixing particulate aluminum with a particulate additive comprising a mineral, ceramic, or other small particulate with a low CTE, and casting and firing the mixture so that the aluminum forms a continuous phase that will be electrically conductive, and the additive forms a discontinuous phase that will control and match the CTE of the mixture to be equal to or close to that of silicon. In several embodiments, the additive is a material with a high temperature melting point, a small particle size, and a low CTE. In several embodiments, the additive is co-milled with or mixed into a fine aluminum powder in order to make a castable or shapeable precursor or body, which can be shaped into a back face substrate, wherein the low CTE particles become the discontinuous phase in the back face substrate. This body is then fired to the melting point of aluminum, at which point, upon solidification, the additive particles, which are very strong in compression, are put into compression as the aluminum seeks to shrink, causing the aluminum to build up tensile stresses which are relieved by elongation. In several embodiments, by tailoring the distribution of particle sizes and amount of volume filled with the low CTE particles, a conductive aluminum-based back face substrate member is made having a tailored CTE essentially close to that of silicon.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed herein are methods and processes for fabricating photoelectric devices, and more particularly, methods and processes for fabricating a back face substrate for a silicon wafer solar cell. In several embodiments, the methods and processes of the present general inventive concept provide for the manufacture of a back face substrate for a silicon wafer solar cell having a relatively high conductivity, and also having a relatively low CTE. According to several features of the present general inventive concept, a back face substrate is provided which includes a finely divided particulate additive bonded in a continuous phase of conductive material, the particulate additive imparting a low CTE to the back face substrate while the continuous phase of conductive material maintains conductivity of the back face substrate. In many embodiments of the present invention, a continuous phase of the back face substrate is provided which comprises aluminum or other such conductive material, and is combined with a discontinuous phase comprising at least one of other substances having a lower CTE than the continuous phase, such that a composite substrate material is formed for use as a back face substrate. This composite substrate material takes advantage of the high conductivity of the aluminum continuous phase, while the discontinuous phase comprising other substances reduces the overall CTE of the composite material, thereby diminishing the adverse effects of bowing from CTE-mismatch.

According to one embodiment of a method of manufacture of the present general inventive concept, a conductive composite substrate material is fabricated by first mixing a measure of first particulate material comprising a conductive material with a measure of second particulate material comprising a mineral, ceramic or other small particulate having a low CTE. The mixture is then cast to a desired shape, and thereafter is heated such that a composite material is formed having a continuous phase that is electrically conductive, and a discontinuous phase imposing a low overall CTE to the composite material that is equal to, or close to, that of silicon.

FIG. 1illustrates a flow diagram of a method of manufacture in accordance to one embodiment of the present general inventive concept. As illustrated inFIG. 1, in one embodiment, a conductive material is provided10for use as the continuous phase of a conductive composite substrate material, and an additive material is provided12for use as the discontinuous phase of the conductive composite substrate material. The conductive material is preferably a metal having sufficiently high conductivity such that the resultant continuous phase of the conductive composite substrate material, as will be further discussed below, exhibits an overall conductivity sufficiently high to enable use of the conductive composite substrate material as a back face substrate of a solar cell. Generally, the additive is a material with a high temperature melting point, which is strong in compression, and which has a CTE approximately equal to, or close to, the CTE of silicon. In certain embodiments, the conductive material is selected from the group consisting of platinum, gold, copper, silver, and aluminum, and in a preferred embodiment, is aluminum. In one embodiment, the additive material is silica.

It is desired that both the conductive material and the additive be of a generally fine particle size, such that following provision10,12of the conductive material and the additive, the conductive material and additive may be combined16to form a mixture of fine-grained conductive material and additive. In certain embodiments, provision10,12of the conductive material and the additive includes provision of materials which are of a small average particle size. In other embodiments, following provision10,12of the conductive material and the additive, the conductive material and/or additive are milled to a desired average particle size. It will be understood that such milling of the conductive material and/or additive may occur in an operation separate from the operation of combining16the conductive material and the additive, or as part of the same operation. For example, as illustrated inFIG. 1, following provision10,12of the conductive material and the additive, the conductive material and additive are combined with an optional binder14which, in certain embodiments, is selected to promote adherence of the conductive material and the additive, and also to limit oxidation of the conductive material and/or the additive. Thereafter, the conductive material, additive, and binder are milled together16, such that the collective particle size distribution of the conductive material, additive, and binder is reduced, thereby forming a particulate mixture of the conductive material, additive, and binder.

In one embodiment, the method of the present general inventive concept begins with provision10of a conductive material defined by a measure of granular aluminum, such as commercially available aluminum powder or flake, having an average particle size of less than 10 microns, and provision12of an additive defined by a measure of fused silica powder having an average particle size of approximately 5 microns. The silica powder and the granular aluminum are co-milled16with a binder14which can later be removed from the mixture of silica and aluminum absent significant oxidation of the aluminum. In one embodiment, the fused silica is at least 99.5% pure, such as is commercially manufactured and sold by Minco, Inc., C-E Minerals, Inc., or Precision Electro Minerals Co. (PEMCO), Inc., and has a CTE of approximately 0.59 parts per million per degree Centigrade (PPM) and a melting point close to 1800 degrees Centigrade. In several embodiments, the ratio of silica to aluminum in the milled composite substrate material ranges from 30% to 84%.

Following the operation(s) of milling and mixing16the conductive material, the additive, and optionally the binder, the resultant composite mixture is shaped18into a desired shape, such as for example a flat, planar member shape desirable for use as a back face substrate in a solar cell. In several embodiments, such shaping18is accomplished by setting the composite substrate material into a boat or other such mold which maintains and retains the desired shape of the mixture. In several embodiments, the boat is fabricated from a material which maintains the desired shape of the mixture during subsequent heating20of the boat and mixture, as described below, which is not reactive with the components of the mixture, and which also allows release of the finished substrate following heating20. For example, in some embodiments, the boat is fabricated from boron nitride, and in other embodiments, from a mixture of boron nitride and titanium diboride, of the type commonly used as evaporation boats for chemical vapor deposition of aluminum. It will be recognized that such a boat may be capable of making many thousands to tens of thousands or more substrates and is not reactive with the aluminum of the mixture.

FIG. 2illustrates a cross-sectional side view of a composite mixture layer24set in a boat26for heating in accordance with one embodiment of the present general inventive concept. In some embodiments, a foil or coating of the conductive material28is applied to the surface of the boat26before heating is performed, thereby interposing a relatively pure conductive material interlayer28between the boat26and the substrate material layer24. Additionally, in some embodiments, a coating of boron30is provided along a top surface of the substrate material layer24so as to improve the interface and doping mechanics and metrics of the finished substrate material layer24. Referring toFIGS. 1 and 2, following shaping18of the composite mixture, such as by placement of a composite mixture layer24into the boat26, the composite mixture is heated20at least to the melting point of the conductive material. For example, in an embodiment in which the conductive material is aluminum, the composite mixture is heated20to at least the melting point of aluminum, which is approximately 660 degrees Centigrade at standard pressure.

FIGS. 3 and 4are schematic diagrams showing close-up views of the composite mixture24prior to (FIG. 3) and after (FIG. 4) the operation of heating the mixture20discussed above. It will be understood thatFIGS. 3 and 4are merely schematic diagrams, and that the relative shapes and diameters of the various conductive material particles and additive particles are not depicted to scale. As shown inFIG. 3, the above-discussed milling/mixing16of the conductive material with the additive results in a distribution of additive particles34throughout the conductive material particles32. As shown inFIG. 4, once the mixture24is heated to at least the melting point of the conductive material32, the conductive material32forms a continuous liquid matrix36having additive particles34suspended therein. In several embodiments, the ratio of additive particles34to conductive material particles32is such that, following heating20of the mixture24, the additive particles34maintain intimate contact with one another, such that at least a portion of the weight of each additive particle34may bear upon at least one adjacent additive particle34. However, it will be recognized that such intimate contact of the additive particles34is not necessary to accomplish the present general inventive concept. Furthermore, it will be recognized that the specific ratio of additive particles34to conductive material32necessary to accomplish such intimate contact may vary, at least in part, upon the specific particle size distribution and particle shapes defined by the various particles of conductive material32and additive34.

Referring now toFIGS. 1 and 4, following the heating process20, the composite substrate material24is cooled22. It will be understood that such cooling22of the composite substrate material24may be by active means, or by passively allowing the composite substrate material24to cool. It will be understood that, as the conductive material32cools22, the melting and resolidification of the conductive material32produces a continuous, electrically-conductive phase36surrounding the additive particles34. As the mixture cools to below the melting point of the conductive material32and begins to solidify, the conductive material32begins to thermally contract, putting the various additive particles34into compression and stretching the conductive material32around the additive particles34. However, the discontinuous phase of additive particles34interspersed among the continuous phase of conductive material32resists thermal contraction of the composite substrate material24. Accordingly, tensile stresses within the continuous phase of conductive material32are relieved by elongation. Thus, the overall thermal contraction of the composite substrate material24is controlled by the additive particles34, and the composite substrate material24exhibits an effective overall CTE of the composite substrate material24which is approximately equal to that of the additive material, in other words, approximately equal to, or close to, that of silicon. By tailoring the distribution of particle sizes and amount of volume filled with the low CTE particles, a conductive back face substrate member with a tailored CTE essentially close to that of silicon is produced.

In several embodiments of the present general inventive concept, such as the embodiment ofFIG. 2, the substrate24is manufactured initially, and may later be used as a component for the manufacture of a solar-grade silicon wafer in situ on the substrate24. In other embodiments, manufacture of the substrate24is performed simultaneously with manufacture of a solar cell. For example,FIG. 5illustrates a cross-sectional side view of a composite mixture layer24aset in a boat26afor heating in accordance with another embodiment of the present general inventive concept, in which a solar cell is manufactured simultaneously with the manufacture of the substrate layer24a. In the illustrated embodiment ofFIG. 5, once the above-discussed operations of forming a composite mixture24of conductive material particulates32and additive particulates34, a layer of composite mixture24ais deposited into a boat26awhich defines an internal cavity having the size and shape desired for shaping a finished solar cell. A conductive material interlayer28is provided along a top surface of the layer of composite mixture24a, and a layer of boron dopant30is provided above the conductive material interlayer28. A silicon layer38is provided above the dopant layer30and the underlying conductive material interlayer28and composite mixture layer24a. Thereafter, the contents of the boat26aare heated20as discussed above to form the substrate24having the above-discussed continuous phase of conductive material32and discontinuous phase of additive material34.

Once the contents of the boat26aare heated20to above the melting point of the conductive material, the conductive material interlayer28melts and, in certain embodiments, fuses with the adjacent conductive material forming the composite substrate24. Thus, a composite substrate24is formed which includes an external layer of pure conductive material in contact with the adjacent layers30,38of the solar cell. It will be recognized that, in embodiments in which the conductive material32is aluminum, during subsequent cooling22of the composite substrate material24as described above, the conductive material interlayer28, which in several embodiments is constructed to be very thin, such as for example having a thickness of only a few thousandths of an inch, tends to thermally contract at a rate of approximately 24 PPM, i.e., the CTE of aluminum itself. However, as discussed above, the composite substrate material24to which the conductive material interlayer28is fused tends to thermally contract at a collective rate much lower than that of the conductive material interlayer28, such as for example approximately 3 PPM in certain embodiments. In the embodiment ofFIG. 5, the conductive material interlayer28is further restrained along a surface of the conductive material interlayer28opposite the composite substrate material24a, by virtue of lamination with the dopant layer30and/or the silicon layer38. Thus, the interface of the composite substrate material24awith the silicon layer38and associated dopant layer30serves to limit the ability of the conductive material interlayer28to impart a bowing strain to the silicon layer38, thereby leaving the silicon layer38essentially strain and stress free, both at standard temperature and pressure and at significantly higher and lower temperatures.

Several additional benefits resulting from the above-described general inventive concept will be recognized by one of skill in the art. For example, in the event a substrate24is produced in which several fused silica particles34are present at the reflective interface of the substrate24and the silicon38, since the fused silica34is transparent in the spectrum of interest, those photons with marginal band gap energy may, in certain instances, be reflected back into the body of the silicon38by the fused silica34. In the case where those photons with marginal band gap energy impact and are transmitted through a particle of fused silica34, such photons may, in certain instances, be reflected at an angle created by the refractive interface of the substrate24aand the silicon38, thus creating a reflectance angle with a longer path through the silicon38, and thereby increasing the probability that the photon may be captured by the solar cell. Furthermore, the above-described general inventive concept allows the possibility for construction and utilization of a very thin solar-grade silicon wafer in a solar cell, such as for example 50 microns or less. Utilization of such a very thin solar-grade silicon wafer will lower costs of silicon per watt produced by the solar cell by decreasing silicon usage and increasing output. Also, it will be recognized that the above-discussed general inventive concept makes possible the construction of solar cells that are capable of withstanding extreme hot and cold environments, a limiting factor today and one which will make solar modules much more reliable in all environments.

It will be recognized that, in other embodiments, the composite substrate material24may comprise aluminum milled with other materials used to affect the CTE of the composite substrate material or to affect other properties of the final back face substrate layer. In some embodiments, these other materials that are included in the composite substrate material24include, for example, silver, copper, germanium, gallium, gallium arsenide, and a number of alloys and ceramic materials. In some embodiments, these other materials that are included in the composite substrate material include particulate or granular silver, particulate or granular copper, particulate or granular germanium, particulate or granular gallium, or particulate or granular gallium arsenide. Persons of skill in the art will recognize that various combinations of these materials are possible and are contemplated by the present general inventive concept.