Solar cell interconnection

A solar cell can include a conductive foil having a first portion with a first yield strength coupled to a semiconductor region of the solar cell. The solar cell can be interconnected with another solar cell via an interconnect structure that includes a second portion of the conductive foil, with the interconnect structure having a second yield strength greater than the first yield strength.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well known devices for direct conversion of solar radiation into electrical energy. Generally, solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation impinging on the surface of, and entering into, the substrate creates electron and hole pairs in the bulk of the substrate. The electron and hole pairs migrate to p-doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are connected to conductive regions on the solar cell to direct an electrical current from the cell to an external circuit.

Solar cells can be interconnected together in series to provide a string of solar cells, which in turn can be connected in series to form a module. In some instances, interconnecting solar cells can be challenging.

DETAILED DESCRIPTION

“Comprising.” This term is open-ended. As used in the appended claims, this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/components include structure that performs those task or tasks during operation. As such, the unit/component can be said to be configured to perform the task even when the specified unit/component is not currently operational (e.g., is not on/active). Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. §112, sixth paragraph, for that unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.). For example, reference to a “first” portion of a conductive foil does not necessarily imply that this portion is the first portion in a sequence; instead the term “first” is used to differentiate this portion from another portion (e.g., a “second” portion).

“Coupled”—The following description refers to elements or nodes or features being “coupled” together. As used herein, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically.

Although many of the examples described herein are back contact solar cells, the techniques and structures apply equally to other (e.g., front contact) solar cells as well. Moreover, although much of the disclosure is described in terms of solar cells for ease of understanding, the disclosed techniques and structures apply equally to other semiconductor structures (e.g., silicon wafers generally).

Solar cell interconnects and methods of forming solar cell interconnects are described herein. In the following description, numerous specific details are set forth, such as specific process flow operations, in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to one skilled in the art that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known fabrication techniques, such as lithography techniques, are not described in detail in order to not unnecessarily obscure embodiments of the present disclosure. Furthermore, it is to be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

This specification first describes example solar cells that can be interconnected with the disclosed interconnects, followed by a more detailed explanation of various embodiments of interconnect structures. The specification then includes description of example methods for forming the interconnect structures. Various examples are provided throughout.

In a first example solar cell, a conductive foil is used to fabricate contacts, such as back-side contacts, for a solar cell having emitter regions formed above a substrate of the solar cell. For example,FIG. 1Aillustrates a cross-sectional view of a portion of a solar cell having conductive contacts formed on emitter regions formed above a substrate, in accordance with an embodiment of the present disclosure. In various embodiments, the conductive foil is also used to form an interconnect structure that has a higher yield strength than the conductive foil of the conductive contacts, as described in more detail below.

Some challenges exist in coupling conductive foil to the solar cell and to interconnect conductive foil of adjacent cells. As one example, ratcheting can occur which can reduce reliability and lifetime of a solar cell and modules. Ratcheting is a form of plastic deformation of metal that is characterized by non-planar distortion of the foil, which can lead to reliability issues in the field. As another example, wafer bowing can occur due to thermal stress mismatch between materials (e.g., between silicon and metal) and can cause trouble with processing (e.g., alignment) and handling. The relationship between the yield stress of the metal and the impact on both ratcheting and bowing is opposite. For instance, high yield stress metal can be good for ratcheting but bad for bow. The inverse is true for low yield stress metal. The disclosed structures and techniques can inhibit wafer bowing and ratcheting and result in improved lifetime and performance of the resulting solar cells and modules.

Referring toFIG. 1A, a portion of solar cell100A includes patterned dielectric layer224disposed above a plurality of n-type doped polysilicon regions220, a plurality of p-type doped polysilicon regions222, and on portions of substrate200exposed by trenches216. Conductive contacts228are disposed in a plurality of contact openings disposed in dielectric layer224and are coupled to the plurality of n-type doped polysilicon regions220and to the plurality of p-type doped polysilicon regions222.

In one embodiment, the plurality of n-type doped polysilicon regions220and the plurality of p-type doped polysilicon regions222can provide emitter regions for solar cell100A. Thus, in an embodiment, conductive contacts228are disposed on the emitter regions. In an embodiment, conductive contacts228are back contacts for a back-contact solar cell and are situated on a surface of the solar cell opposing a light receiving surface (direction provided as201inFIG. 1A) of solar cell100A. Furthermore, in one embodiment, the emitter regions are formed on a thin or tunnel dielectric layer202.

In some embodiments, as shown inFIG. 1A, fabricating a back-contact solar cell can include forming thin dielectric layer202on the substrate. In one embodiment, a thin dielectric layer is composed of silicon dioxide and has a thickness approximately in the range of 5-50 Angstroms. In one embodiment, thin dielectric layer performs as a tunnel oxide layer. In an embodiment, the substrate is a bulk monocrystalline silicon substrate, such as an n-type doped monocrystalline silicon substrate. However, in another embodiment, the substrate includes a polycrystalline silicon layer disposed on a global solar cell substrate.

Trenches216can be formed between n-type doped polysilicon (or amorphous silicon) regions220and p-type doped polysilicon regions222. Portions of trenches216can be texturized to have textured features. Dielectric layer224can be formed above the plurality of n-type doped polysilicon regions220, the plurality of p-type doped polysilicon regions222, and the portions of substrate200exposed by trenches216. In one embodiment, a lower surface of dielectric layer224can be formed conformal with the plurality of n-type doped polysilicon regions220, the plurality of p-type doped polysilicon regions222, and the exposed portions of substrate200, while an upper surface of dielectric layer224is substantially flat. In a specific embodiment, the dielectric layer224is an anti-reflective coating (ARC) layer.

A plurality of contact openings can be formed in dielectric layer224. The plurality of contact openings can provide exposure to the plurality of n-type doped polysilicon regions220and to the plurality of p-type doped polysilicon regions222. In one embodiment, the plurality of contact openings is formed by laser ablation. In one embodiment, the contact openings to the n-type doped polysilicon regions220have substantially the same height as the contact openings to the p-type doped polysilicon regions222.

Forming contacts for the back-contact solar cell can include forming conductive contacts228in the plurality of contact openings226and coupled to the plurality of n-type doped polysilicon regions220and to the plurality of p-type doped polysilicon regions222. Thus, in an embodiment, conductive contacts228are formed on or above a surface of a bulk N-type silicon substrate200opposing a light receiving surface201of the bulk N-type silicon substrate200. In a specific embodiment, the conductive contacts are formed on regions (222/220) above the surface of the substrate200.

Still referring toFIG. 1A, conductive contacts228can include a conductive foil134. In various embodiments, conductive foil can be aluminum, copper, other conductive materials, and/or a combination thereof. In some embodiments, as shown inFIG. 1A, conductive contacts228can also include one or more conductive (metal or otherwise) regions, such as regions130and132inFIG. 1A, between conductive foil134and a respective semiconductor region. For example, a first conductive region130can include (e.g., aluminum, aluminum/silicon alloy, etc.), which can be printed, or blanket deposited (e.g., sputtered, evaporated, etc.).

In one embodiment, the conductive foil134and the one or more conductive regions130and132can be welded, thermally compressed, or otherwise coupled to the semiconductor region of the solar cell and therefore in electrical contact with the emitter regions of the solar cell100A. As described herein, in some embodiments, as shown inFIGS. 1A and 1B, one or more conductive regions (e.g., sputtered, evaporated, or printed aluminum, nickel, copper, etc.) may exist between the conductive foil and the emitter regions. Thermally compressed conductive foil is used herein to refer to the a conductive foil that has been heated at a temperature at which plastic deformation can occur and to which mechanically pressure has been applied with sufficient force such that the foil can more readily adhere to the emitter regions and/or conductive regions.

In some embodiments, the conductive foil134can be aluminum (Al) foil, whether as pure Al or as an alloy (e.g., Al/Silicon (Al/Si) alloy foil). In one embodiment, the conductive foil134can also include non-Al metal. Such non-Al metal can be used in combination with or instead of Al particles. Although much of the disclosure describes metal foil and metal conductive regions, note that in some embodiments, non-metal conductive foil (e.g., conductive carbon) and non-metal conductive regions can similarly be used in addition to or instead of metal foil and metal conductive regions. As described herein, metal foil can include Al, Al—Si alloy, tin, copper, and/or silver, among other examples. In some embodiments, conductive foil can be less than 5 microns thick (e.g., less than 1 micron), while in other embodiments, the foil can be other thicknesses (e.g., 15 microns, 25 microns, 37 microns, less than 50 microns, etc.) In some embodiments, the type of foil (e.g., aluminum, copper, tin, etc.) can influence the thickness of foil needed to achieve sufficient current transport across the solar cell. Moreover, in embodiments having one or more additional conductive regions130and132, the foil can be thinner than in embodiments not having those additional conductive regions.

Moreover, in various embodiments, the type and/or thickness of the conductive foil can affect the yield strength of the portion of the conductive foil coupled to the solar cell and the portion of the conductive foil that overhangs the edge of the solar cell and is part of the interconnect structure.

In various embodiments, conductive regions130and132can be formed from a metal paste (e.g., a paste that includes the metal particles as well as a binder such that the paste is printable), from a metal powder (e.g., metal particles without a binder, a powder of Al particles, a layer of Al particles and a layer of Cu particles), or from a combination of metal paste and metal powder. In one embodiment using metal paste, paste may be applied by printing (e.g., screen printing, ink-jet printing, etc.) paste on the substrate. The paste may include a solvent for ease of delivery of the paste and may also include other elements, such as binders or glass frit.

In various embodiments, the metal particles can be fired (before and/or after the conductive foil and conductive regions are coupled together), also referred to as sintering, to coalesce the metal particles together, which can enhance conductivity and reduce line resistance thereby improving the performance of the solar cell. But heating from firing or the bonding process can also reduce the yield strength of the conductive foil, which can reduce reliability and lifetime of the solar module from ratcheting. Accordingly, techniques and structures disclosed herein can provide for a sufficiently low yield strength for the conductive foil over the solar cell to inhibit bowing yet also provide for a sufficiently high yield strength of the foil of the interconnect structure so as to inhibit ratcheting.

Turning now toFIG. 1B, a cross-sectional view of a portion of an example solar cell having conductive contacts formed on emitter regions formed in a substrate is illustrated, according to one embodiment. For example, in this second exemplary cell and similar to the example ofFIG. 1A, conductive foil can be used to fabricate contacts, such as back-side contacts, for a solar cell having emitter regions formed in a substrate of the solar cell.

As shown inFIG. 1B, a portion of solar cell100B includes patterned dielectric layer124disposed above a plurality of n-type doped diffusion regions120, a plurality of p-type doped diffusion regions122, and on portions of substrate100, such as a bulk crystalline silicon substrate. Conductive contacts128are disposed in a plurality of contact openings disposed in dielectric layer124and are coupled to the plurality of n-type doped diffusion regions120and to the plurality of p-type doped diffusion regions122. In an embodiment, diffusion regions120and122are formed by doping regions of a silicon substrate with n-type dopants and p-type dopants, respectively. Furthermore, the plurality of n-type doped diffusion regions120and the plurality of p-type doped diffusion regions122can, in one embodiment, provide emitter regions for solar cell100B. Thus, in an embodiment, conductive contacts128are disposed on the emitter regions. In an embodiment, conductive contacts128are back contacts for a back-contact solar cell and are situated on a surface of the solar cell opposing a light receiving surface, such as opposing a texturized light receiving surface101, as depicted inFIG. 1B.

In one embodiment, referring again toFIG. 1Band similar to that ofFIG. 1A, conductive contacts128can include a conductive foil134and in some embodiments, one or more additional conductive regions, such as conductive regions130and132. The conductive foil134, and the one or more conductive regions can be coupled (e.g., welded, thermally compressed, or otherwise) to the semiconductor region of the solar cell and/or to one or more conductive regions between the foil and the semiconductor region and therefore in electrical contact with the emitter regions of the solar cell100A. The conductive contact description ofFIG. 1Aapplies equally to the conductive contact ofFIG. 1Bbut is not repeated for clarity of description.

Turning now toFIG. 2A, the illustrated solar cell includes the same features as the solar cell ofFIG. 1Aexcept that the example solar cell ofFIG. 2Adoes not include the one or more additional conductive regions (regions130and132ofFIG. 1A). Instead, conductive foil134is bonded directly to the semiconductor region of the solar cell.

Similarly, the illustrated solar cell ofFIG. 2Bincludes the same features as the solar cell ofFIG. 1Bexcept that the example solar cell ofFIG. 2Bdoes not include the one or more additional conductive regions (regions130and132ofFIG. 1B). Instead, conductive foil134is bonded directly to the semiconductor region of the solar cell.

Although certain materials are described herein, some materials may be readily substituted with others with other such embodiments remaining within the spirit and scope of embodiments of the present disclosure. For example, in an embodiment, a different material substrate, such as a group III-V material substrate, can be used instead of a silicon substrate.

Note that, in various embodiments, the formed contacts need not be formed directly on a bulk substrate, as was described inFIGS. 1B and 2B. For example, in one embodiment, conductive contacts such as those described above are formed on semiconducting regions formed above (e.g., on a back side of) as bulk substrate, as was described forFIGS. 1A and 2A.

In various embodiments, the conductive foil of solar cells ofFIGS. 1A-1B and 2A-2Bincludes an overhang region (e.g., tab) that extends beyond the edge of the cell and can be coupled to an overhang region of an adjacent cell to interconnect the two cells together. In some embodiments, the overhang portion of a particular solar cell can extend less than 2 mm over its edge.

Turning now toFIGS. 3-8, various examples of solar cell interconnect structures configured to inhibit ratcheting and wafer bowing are illustrated.

FIG. 3illustrates two solar cells, solar cells300aand300b, coupled together via an interconnect structure. In the illustrated example, a portion302aof a conductive foil is coupled to solar cell300aand a portion302bof another conductive foil is coupled to solar cell300b. The interconnect structure can include overhang portions304aand304bof the conductive foils. As shown, the overhang portions304aand304bcan be coupled via one or more joints306, which can be formed via laser or electrical welding, soldering, or some other technique. In various embodiments, the portions of the conductive foils disposed above and coupled to the solar cells have a lower yield stress than the yield stress of the interconnect structure. Accordingly, the portion of the conductive foil that includes lower yield stress metal, which can help inhibit wafer bowing, is the portion that is coupled to the wafer. And the portion of the foil that is used to form the interconnect structure can be higher yield stress metal, which can inhibit ratcheting. Accordingly, such a foil can inhibit both wafer bowing and ratcheting. One example technique to form dual tempered conductive foil is described atFIGS. 10-13.

FIG. 4illustrates another example interconnect structure. Similar toFIG. 3, a portion402aof a conductive foil is coupled to solar cell400aand a portion402bof another conductive foil is coupled to solar cell400b. In contrast toFIG. 3, the interconnect structure ofFIG. 4includes an additional material408coupled to and between overhang portions404aand404b. In one embodiment, additional material408can be a material such that the collective yield strength of the interconnect structure is high enough to inhibit ratcheting. In one embodiment, the conductive foils, including the portions disposed over and coupled to the solar cells and the overhang portions, have a lower yield strength to inhibit bowing. The additional material, however, can have sufficiently high yield strength such that, when added to the lower yield strength foil of the overhang portions, the collective interconnect structure has a high enough yield strength to inhibit ratcheting.

In various embodiments in which additional material408is positioned between the overhang tabs, as inFIG. 4, additional material408is a conductive material. It can be the same material as the overhang tabs (e.g., soft aluminum overhang tabs, and hard aluminum additional material) or different.

FIG. 5illustrates another example interconnect structure for use in interconnecting solar cells. The interconnect structure is similar to the interconnect structure ofFIG. 4except that the additional material508is not between overhang portions504aand504b. Instead, in the example ofFIG. 5, additional material508is disposed between solar cells500aand500b, which can provide and/or maintain a consistent spacing or gap between the solar cells. For a back contact solar cell, additional material508is located on the sunny side of the interconnect structure and can be visible from the sunny side of a solar module. Accordingly, in one embodiment, the additional material508can be colored or otherwise made such that the visible portion of the interconnect structure, as viewed from the front of the module, is a similar color to the solar cells and therefore blends in.

In some embodiments, additional material508can be a conductive material or in some instances, it can be a non-conductive material as long as the additional material508can be coupled (e.g., welded, soldered, wrapped around, tied around, etc.) to the overhang tabs and as long as the interconnect structure collectively has sufficient yield strength to inhibit ratcheting.

Various other examples also exist. For example, in one embodiment, instead of the additional material being on the front side of the overhang tabs or between the overhang tabs, the additional material can be on the back side of the overhang tabs. In another embodiment, the additional material can be wrapped around the overhang tabs and then coupled to the overhang tabs to form the interconnect structure.

In the examples ofFIGS. 4 and 5, the additional material is shown at a non-zero angle relative to the solar cells. In some embodiments, the additional material can be coupled such that the interconnect structure is slightly out of plane from the solar cell, which can result in strain relief for the interconnect structure, thereby further inhibiting ratcheting. Other examples of stress relief interconnect structures also exist. One such example is illustrated inFIG. 6. As shown inFIG. 6, additional material608includes a bend (e.g., c-shaped bend) such that additional material is coupled separately to each overhang tab. Such an interconnect structure can result in improved strain relief and further inhibit ratcheting. Although the illustrated additional material shows a two-axis bend, in some embodiments, the bend can be a three-axis bend.

The additional material of the interconnect structure can be a variety of shapes. The interconnect structure can be a simple ribbon, a channel shape (e.g., for additional stiffness), a bowtie shape (for aesthetics and connection in the diamond areas of the module). The additional material can have other materials or properties to modify the joint or reliability. Such materials or properties include coating for corrosion protection (e.g., metal, oxide, or nitride) or for use in coupling to foil (e.g., solder coating on the additional material), adhesive properties for adhesion to module materials (e.g., encapsulant) or to the overhang portions (e.g., solder material coating), or multiple layers for different expansion and contraction properties.

FIGS. 7 and 8illustrate example interconnected solar cells, according to various embodiments. For ease of explanation, the metal on solar cells700and710is not illustrated as patterned (e.g., finger pattern). As shown, solar cells700and710are interconnected via multiple interconnect structures720at the corners of the solar cells. The right-most dashed lines illustrate the edge of the overhang tab from the conductive foil of the left solar cell and the left-most dashed lines illustrate the edge of the overhang tab from the conductive foil of the right solar cell. In some embodiments, joint730can be a weld joint, solder joint, or some other coupling, and can be the location at which the overlapping overhang tabs are coupled together.

As shown in the example ofFIG. 8, instead of interconnecting the solar cells at their respective corners, the solar cells are connected at the overlapping overhang edges820of the solar cells with a plurality of interconnect joints830. In one embodiment, one or more of the interconnect joints830can correspond to a separate piece of additional material, such as a separate piece of hard foil. Or, in one embodiment, a continuous piece of additional material can be above, between, or under the overlapping edges and coupled to the overlapping overhang edges at the locations of interconnect joints830. In some embodiments, however, such as in the embodiment ofFIG. 3, no additional material is used. In such embodiments, joints730and830can simply be regions in which one overhang foil is coupled to another.

In one embodiment, one or more stress relief features can be added to the interconnect structure after it has been formed. For example, in one embodiment, one or more relief cutouts can be formed in the interconnect structure to further relieve stress.

Turning now toFIG. 9, a flow chart illustrating a method for forming a solar cell interconnect region is shown, according to some embodiments. In various embodiments, the method ofFIG. 9may include additional (or fewer) blocks than illustrated. For example, in some embodiments, coupling an interconnect material to the overhang tabs, as shown at block908, may not be performed.

As shown at902, a portion of a conductive foil can be coupled to a solar cell. For example, in one embodiment, a portion of the conductive foil disposed over the solar cell can be coupled to a semiconductor region of the solar cell. Coupling can be achieved by laser or thermal welding, soldering, thermocompression, among other techniques.

As illustrated at904, a portion of another conductive foil can be coupled to another solar cell. Similar to the description at block902, in one embodiment, a portion of the other conductive foil disposed over the other solar cell can be coupled to a semiconductor region of the other solar cell. As was the case with block902, coupling can be achieved by laser or thermal welding, soldering, thermocompression, among other techniques. In various embodiments, blocks902and904can be performed sequentially or can be processed at substantially the same time.

At906, other portions of the conductive foils can be coupled together to form an interconnect structure. In one embodiment, the other portions are overhang portions that extend past the edge of the solar cells. Various examples are illustrated inFIGS. 3-8. The overhang portions can at least partially overlap and the overlapping regions can be welded, soldered, or otherwise coupled together such that the cells are electrically and mechanically interconnected together.

In some embodiments, the overhang portion of the foils can have higher yield strength than the portion of the foil disposed over and coupled to the solar cell such that wafer bowing and ratcheting can be inhibited. In one embodiment, the foil can be fabricated or modified to be dual tempered, such that the overhang portion is a hard foil and the solar cell portion is a soft foil.FIGS. 10-13illustrate one example embodiment or modifying the foil to be dual tempered.

In some embodiments, however, an additional material can be coupled to the overhang foil portions to form the interconnect having the higher yield strength as illustrated at908. For example, in one embodiment, the additional material can be placed between the overhang foil portions or on the front or back side of the overhang foil portions and the additional material, and both overhang foil portions can be coupled together to collectively form the interconnect. As one simple example, the overhang portions may be the same lower yield strength, soft foil as the solar cell portions of the foil but the additional material may have high enough yield strength to make the overall interconnect structure have a sufficiently high yield strength to inhibit ratcheting.

In one embodiment, the coupling of the two overhang portions and the coupling of the additional material at block906and908can be performed substantially simultaneously, or block906can be performed first, or block908can be performed first. As one example, the additional material can be welded to one of the overhang portions first, and then the other overhang portion can be welded to the already welded overhang portion and additional material. Other variations also exist.

In some embodiments, the additional material is conductive whereas in other embodiments, the additional material may not be conductive or may not be as conductive as the foil. In such embodiments, the overhang portions may make direct contact to one another without the additional material being between the overhang portions. In such embodiments, the additional material can provide mechanical integrity and allow for sufficient yield strength to inhibit ratcheting but may not be relied upon to carry current from one cell to the other.

Turning now toFIG. 10, a flow chart illustrating a method for forming a dual tempered conductive foil is shown, according to some embodiments. In various embodiments, the method ofFIG. 10may include additional (or fewer) blocks than illustrated.

As shown at1002, a hot bonding technique can be performed to couple a portion of a conductive foil to a solar cell. Similar to blocks902and904ofFIG. 9, in one embodiment, a portion of the conductive foil disposed over the solar cell can be coupled to a semiconductor region of the solar cell. Hot bonding techniques include thermocompression bonding and heated welding. For thermocompression bonding, the conductive foil can be heated to temperatures above 200 degrees Celsius and mechanical force (e.g., via plate, roller, etc.) can be applied with pressure of at least 1 psi.

In one embodiment, the conductive foil used in the method ofFIG. 10is a hard foil with high yield strength before the hot bonding technique is applied. One example hard foil is 7020 Series Aluminum foil but other hardness Al foils or other non-Al foils can be used.

As illustrated at1004, a second portion of the conductive foil, which can be a portion that corresponds to overhang portions that extend past the edge of the solar cell, can be cooled during the hot bonding technique of block1002. The result of blocks1002and1004is that the original hard, higher yield strength conductive foil is softened and modified into a lower yield strength foil in the portion over the solar cell as it is coupled to the solar cell to inhibit wafer bowing yet substantially retains its hardness in the overhang portions of the foil to inhibit ratcheting once interconnected.

Clamping the overhang portions to cool them can be difficult, especially when the overhang portions may only extend 2 mm or less past the edge of the solar cell. In one embodiment, a larger overhang portion may exist during the dual tempering process and the overhang portion may then be trimmed after the foil has been tempered. For example, during the dual tempering process, the overhang portions may extend about 10 mm past the edges of the wafer such that the overhang portion is sufficiently enough for the clamp to hold the overhang portion. After the dual tempering process, the overhang portions can then be trimmed to a small length (e.g., 2 mm long, 1 mm, etc.).

FIGS. 11-13illustrate cross-sectional views of portions of an example dual tempering technique. As shown inFIG. 11, wafer1102can be located on a surface1100, such as a wafer chuck. In one embodiment, surface1100can be heated during the process to help the foil bond to the solar cell. Hard foil1104can be placed on wafer1102(e.g., on a backside of the wafer for a back contact solar cell) and clamped via clamps1106aand1106b. Not illustrated, the foil can be pressed, vacuumed, or otherwise be mechanically held in place and sufficiently taut.

FIG. 12illustrates a hot bond1108being applied to the portion of the foil over the solar cell. At the same time, clamps1106aand1106bcan be cooled (e.g., air chilled, water or coolant chilled, etc.) such that the heat from hot bond1108does not transfer enough to the overhang portions of the foil to lower the yield strength of the overhang portions.

Instead, as shown atFIG. 13, the resulting foil includes a lower yield strength portion1110disposed over and bonded to the solar cell1102and higher yield strength overhang portions1112aand1112b, each of which can be coupled to an overhang portion of a respective adjacent solar cell to electrically interconnect the cells but also be stiff enough to inhibit ratcheting. In one embodiment, the interconnect structure can simply be the coupled together higher yield strength overhang portions or it can also include the additional material as described herein.