Method of forming electrical connection on surface of substrate

Abstract

An apparatus includes a substrate and at least one metal contact within a contact area. The at least one metal contact may include a structure having a first region of a metal material separated by a second region in which the metal material is not located. The first region may be separated in two dimensions by a second region. The apparatus may also include at least one conductive wire coupled to the metallic material using a coupling material. At least some of the coupling material may also be located over the substrate at a second region within the at least one metal contact region within the contact region. The invention also discloses a method for forming the device.

Description

Technical Field

[0001] The present invention generally relates to a method for forming electrical connections on a surface of a substrate, and particularly, but not exclusively, to a method for forming electrical connections on a solar cell to electrically interconnect adjacent solar cells within a solar cell string. Background Art

[0002] Most solar photovoltaic (PV) modules consist of one or more strings of solar cells connected in series, encapsulated in a polymer with a glass front sheet and a glass back sheet or a polymer back sheet. To interconnect the individual solar cells within a string, each solar cell typically has one or more busbars to which solder-coated wires can be attached. The busbars collect photogenerated current from a plurality of thin metal wires, commonly referred to as "fingers," and cause the collected current to flow from one polarity of the solar cell to the other polarity on an adjacent solar cell via the wires. The arrangement of the fingers and vertically oriented busbars on the surface of the solar cell is commonly referred to as a "contact grid."

[0003] For the most commonly manufactured bifacial contact solar cells, the semiconductor region with a first polarity is on the front surface of the solar cell, and the semiconductor region with a second polarity is on the back surface of the solar cell. Therefore, the interconnecting wires must be "bent" from the back surface of one solar cell to the front surface of the adjacent cell.

[0004] However, with back-contact solar cells, each solar cell has both p-type and n-type contact areas on the back surface. This means that interconnecting wires do not need to run from the back to the front of adjacent solar cells. Back-contact solar cells eliminate shading of the solar cell's front surface because all metal contact areas are on the back of the solar cell. However, this benefit comes at the expense of greater manufacturing costs.

[0005] For both double-sided contact solar cells and back-contact solar cells, there are automatic wire threading machines that extend the interconnect wire across the busbars of the solar cell and form a bond between the interconnect wire and the busbars on the solar cell. This process requires alignment between the solar cell and the interconnect wire, which is usually wound on a spool and gradually unwound and transported to the solar cell for soldering. The bond is achieved by applying heat or infrared (IR) radiation, both of which soften the solder coating on the interconnect wire, causing it to flow over the busbar area and solidify into a bond connection when cooled.

[0006] In order to reduce the amount of metal required to interconnect each solar cell to adjacent solar cells, it is known to replace the fully metallized busbars with a series of contact pads connected to each other by thinner metal busbars.Figure 1 A corner of a solar cell 100 is shown having a plurality of thin metal grid lines 105 and a vertical busbar structure 110. The busbar structure included in this example comprises a plurality of parallel linear segments with contact pads 115 spaced apart along the length of the busbar structure.

[0007] Interconnecting solar cells with this grid pattern requires thermal or infrared (IR) soldering of the interconnect wires to the contact pads 115. Upon cooling, the contact pads of the solar cell bond to the interconnect wires through the solder joint. This type of contact grid pattern can reduce the amount of metal required on the solar cell while still providing an adequate bond between the interconnect wires and the solar cell and a low-resistance current path for photogenerated current to flow from one cell to an adjacent solar cell in the solar cell string.

[0008] The reduction in metal content achieved through this contact grid is particularly advantageous because most solar cells use silver as their metal contact areas, and silver is the most expensive material in solar cells after silicon. These contact pads are typically on the order of 1 mm 2 Up to 2mm 2 Slightly larger contact pads are typically used around the edges of the cells. While it's desirable to keep these pads as small as possible from a metal cost perspective, their width still needs to be large enough to accommodate the alignment tolerances of the automated solar cell stringing machines typically used in PV module manufacturing. The contact pads also need to have sufficient contact area to allow the molten solder to flow over the solar cell pad area, ensuring adequate bonding adhesion between the interconnect wire and the solar cell.

[0009] Reducing metal mass directly reduces the cost of photovoltaic modules. Costs could be further reduced if silver could be replaced with a metal such as copper, which is more conductive than the silver paste used to form the silver grid but is significantly cheaper. While attempts have been made to use copper electroplating as a silver alternative, the poor adhesion of the metal gridlines and busbar structures to the solar cell surface makes interconnecting solar cells in this manner difficult.

[0010] Therefore, improvements in technology are needed. Summary of the Invention

[0011] In a first aspect of the present invention, there is provided a method of forming an electrical connection on a surface of a substrate, the method comprising:

[0012] providing a substrate and a material for forming at least one metal contact on a surface of the substrate;

[0013] forming at least one metal contact on a surface of a substrate, comprising selecting at least one contact region on the surface of the substrate and forming a structure of metal material within the at least one contact region, the structure having a first region of metal material separated by a second region exposing the substrate, wherein the first region is separated in two dimensions by the second region;

[0014] providing at least one conductive line and a coupling material; and

[0015] The at least one conductive line is coupled to the formed at least one metal contact using a coupling material such that at least some of the coupling material is also located over the substrate at a second region within the at least one metal contact region within the contact region.

[0016] Throughout this specification, the term "wire" is used to refer to an elongated electrical conductor having any cross-sectional shape, such as, but not limited to, circular, rounded, rectangular, or triangular. Furthermore, a wire may have a cross-sectional shape that varies along a portion of the length of the wire, such as, for example, from a generally rectangular cross-sectional shape to a circular cross-sectional shape.

[0017] The material may be a polycrystalline material. The material may have a coefficient of thermal expansion greater than or equal to three times the coefficient of thermal expansion of the substrate. The material may have an elastic modulus greater than 50 GPa. The material may have a yield stress greater than 25 MPa. The material may be formed from a substantially single chemical element. The material may be a single material.

[0018] The coupling material may be in direct contact with the substrate in at least some of the second regions. Some of the coupling material may be located above and spaced apart from the substrate in at least some of the second regions and not in direct contact with the substrate in each second region.

[0019] The coupling material may be a solder material, and coupling the at least one conductive wire to the at least one formed metal contact may include soldering the at least one conductive wire to the at least one formed metal contact. Soldering the at least one conductive wire to the at least one formed metal material may, for example, include applying heat or applying infrared (IR) radiation.

[0020] Alternatively, the coupling material may be a conductive adhesive material, and the step of coupling the at least one conductive wire to the at least one formed metal contact may include adhering the at least one conductive wire to the at least one formed metal contact.

[0021] In one embodiment, the first regions of the metal material are interconnected, such as being integrally formed. For example, the first regions of the metal material may include one or more lines, which may or may not intersect with other lines. The first regions of the metal material may form a pattern or a grid-like structure, which may be a periodic structure. The first regions of the metal material may form an array within at least one contact region.

[0022] The at least one metal contact may further include at least some first regions of metal material, at least some of the first regions being separated from one another and not in direct contact with other first regions of metal material. The first regions of metal material may include island patterns such as dots, which may form a periodic structure and may form an array.

[0023] At least one metal contact may have a shortest extension in the plane of the metal contact of less than 2 mm, less than 1 mm, less than 0.5 mm, or even less than 0.2 mm. The first region of the metallic material may have a shortest extension in the plane of the metal contact of less than 100 μm, less than 50 μm, less than 20 μm, or even less than 15 μm. The second region may have a shortest extension in the plane of the metal contact of less than 500 μm, less than 100 μm, less than 50 μm, or even less than 25 μm.

[0024] The at least one electrically conductive wire may be coated with a solder material.Alternatively, the method may comprise the steps of providing the solder material separately from the at least one electrically conductive wire and soldering the at least one electrically conductive wire to the formed metal contact using the solder material.

[0025] The metal material may be any suitable material (e.g., Cu, Ni, Sn, or Ag), but in a specific embodiment, the metal material includes Cu or a combination of another metal such as Ni and Cu. In the latter case, Ni and Cu may be formed in separate layers.

[0026] The at least one conductive wire may be of any suitable form, and in one embodiment has a generally circular or rectangular cross-sectional shape.

[0027] The substrate may comprise a semiconductor material such as silicon. In one embodiment, the substrate forms part of a solar cell and the metal contacts are electrical contacts on the surface of the solar cell.

[0028] The inventors have observed that if the metal contact comprises a structure of a first region of metal material separated in two dimensions by a second region, and a coupling material or soldering material couples or joins the conductive line to the metal material and also flows over at least the second region (and may or may not be in direct contact with the substrate material in the second region), the adhesion of the metal contact to the substrate can be improved. Therefore, embodiments of the present invention provide the advantage of improving the adhesion of metal contacts, which may, for example, be part of a main grid line on the surface of a solar cell. The first region of metal material can also serve as a "hook" for a coupling material (such as solidified solder), which further increases the adhesion. In addition, because the first region of metal material is separated by the second region, the metal contact requires a reduced amount of metal material, such as copper.

[0029] The at least one metal contact may be one of a plurality of metal contacts, and the method may include forming the plurality of metal contacts on a surface of the substrate.The metal contact may form part of a busbar on a surface of a device such as a solar cell.

[0030] The substrate may be one of a plurality of substrates comprising a solar cell.The solar cell may be within or form part of a solar cell module.

[0031] At least one conductive wire can be used to interconnect adjacent solar cells by connecting one polarity of one solar cell to the opposite polarity of an adjacent solar cell. The solar cells can be arranged for double-sided contact or only for back-side contact. Adjacent solar cells can be interconnected to form a solar cell module.

[0032] In a second aspect of the present invention, there is provided a device having electrical connections formed by the method according to the first aspect of the present invention.

[0033] In a third aspect of the present invention, there is provided an apparatus comprising:

[0034] substrate;

[0035] at least one metal contact within the contact region, the at least one metal contact comprising a structure having a first region of metallic material separated by a second region in which no metallic material is located, wherein the first region is separated in two dimensions by the second region; and

[0036] at least one conductive line, the at least one conductive line being coupled to the metal material using a coupling material;

[0037] Wherein, at least some of the coupling material is also located above the substrate at a second region within the at least one metal contact region.

[0038] The material may be a polycrystalline material. The material may have a coefficient of thermal expansion greater than or equal to three times the coefficient of thermal expansion of the substrate. The material may have an elastic modulus greater than 50 GPa. The material may have a yield stress greater than 25 MPa. The material may be a single material. The material may be formed from a substantially single chemical element.

[0039] Some of the coupling material may be in direct contact with the substrate in at least some of the second regions. The coupling material may be above and spaced apart from the substrate in at least some of the second regions, with no coupling material in direct contact with the substrate in every second region.

[0040] The at least one metal contact may have a shortest extension in the plane of the metal contact of less than 2 mm, less than 1 mm, less than 0.5 mm or even less than 0.2 mm.

[0041] Furthermore, the first region of metallic material may have a shortest extension in the plane of the at least one metallic contact of less than 100 μm, less than 50 μm, less than 20 μm or even less than 15 μm.

[0042] The second region may have a shortest extension in the plane of the at least one metal contact of less than 500 μm, less than 100 μm, less than 50 μm or even less than 25 μm.

[0043] The device may be or may include a solar cell.The coupling material may be a solder material or a conductive adhesive.

[0044] The present invention will be more fully understood through the following description of specific embodiments of the present invention, which will be illustrated in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a schematic diagram of a corner of a solar cell having a metal contact grid including an array of fine metal grid lines and a busbar structure having a plurality of contact pads (prior art);

[0046] Figure 2A and Figure 2B Fully metallized contact pads are shown (prior art);

[0047] Figure 2C An example of a partially metallized contact pad including a pattern of metallic material according to an embodiment of the present invention is shown;

[0048] Figure 3A 、 Figure 3B 、 Figure 3C and Figure 3D is a flow chart illustrating a method according to an embodiment of the present invention;

[0049] Figure 4 is a scanning electron microscope image showing a cross-section of a copper-plated segment according to an embodiment of the present invention;

[0050] Figure 5 is a schematic diagram showing a pattern consisting of interconnected and spaced metal regions forming a pattern according to an embodiment of the present invention;

[0051] Figure 6A is a schematic cross-sectional view of a fully metallized contact pad;

[0052] Figure 6B is a schematic cross-sectional view illustrating a series of metal regions of a patterned contact pad according to an embodiment of the present invention;

[0053] Figure 7 is a table comparing the pull force and contact area of the busbar measured for solar cell interconnection wire structures having solid area contact pads and patterned contact pads according to an embodiment of the present invention;

[0054] Figure 8 This is a digital photo showing that during the tensile test, silicon fragments have broken off and Figure 5 The contact pads formed by the pattern shown are pulled out on the solar cell; and

[0055] 9a to 9h illustrate pattern arrangements of patterned contact pads according to an embodiment of the present invention.

[0056] 10 a to 10 h illustrate contact pads with various busbar structures in FIG. 9 a to 9 h . DETAILED DESCRIPTION

[0057] Embodiments of the present invention relate to forming metal contacts on a substrate, such as forming metal contacts on one or more solar cells that are electrically interconnected to form a string of cells that can be laminated into a solar photovoltaic (PV) module or panel. One or more solar cells can be connected to form a solar module.

[0058] Typically, the busbars are perpendicular to the fine lines within the contact grid, but this is not strictly required and other arrangements of fine lines and current collecting busbars may be used (eg, curved metal structures for non-planar surfaces).

[0059] Throughout the description, the term "contact grid" is used to refer to a plurality of thin metal grid lines and a plurality of busbar structures that make electrical contact with a solar cell.

[0060] The current flows from the solar cell into the fine metal grid lines and then into the connected busbar structure. The interconnecting wires are bonded to the busbar structure and transfer the current from the busbar structure to the adjacent solar cells in the interconnected solar cell string. The interconnecting wires can have a cross-sectional shape, for example, that is circular, triangular or rectangular. When the interconnecting wire has a rectangular cross-sectional shape, it is often referred to as a "ribbon" due to its flat shape. The shape of the interconnecting wire can vary along the solar cell string. For example, a circular wire can be used on the front surface of the solar cell and a wire with a rectangular cross-section can be used on the back surface. This arrangement has advantages because it maximizes the optical performance of the photovoltaic module due to the light redirecting properties of the circular wire, while reducing the required encapsulation dose due to the lower height of the wire with a rectangular cross-sectional shape.

[0061] Historically, busbar structures were fully metallized linear structures that ran along the length of the solar cell in a so-called H-shaped strip pattern. This arrangement ensured a large contact area between the solar cell and the interconnecting wires to form a solder joint connection during interconnection. Today, it is more common for busbar structures to consist of a series of connected contact pads, with bonding to the interconnecting wires occurring primarily at the contact pads rather than along the entire length of the busbar. This arrangement is often used to reduce the amount of metal required for the busbar structure on a solar cell.

[0062] Embodiments of the present invention provide a method for forming a metal contact on a solar cell, the metal contact comprising a first region made of a metal material within a contact region, the first region being separated by a second region and forming a two-dimensional structure, such as a pattern or grid within the contact region (which may have a minimum extension length of less than 2 mm, less than 1 mm, less than 0.5 mm, or less than 0.2 mm or other suitable extension length within the plane of the metal contact). The inventors have observed that such contact pads can enhance the adhesion between the busbar structure and the interconnecting wires compared to the adhesion conventionally achieved with solid metallized contact pads of similar size in the prior art. The method also reduces the metal mass required to form the contact grid on the solar cell.

[0063] One embodiment of the present invention utilizes a low-cost metal such as copper, but other conductive materials may be used in alternative embodiments. Since copper is less expensive than the silver typically used to form contact grids for solar cells, using copper rather than silver to form the contact grids can reduce the manufacturing cost of photovoltaic modules. Copper can be electroplated directly onto the surface of solar cells to form a metal structure with a conductivity close to that of pure copper, which is significantly higher than that of screen-printed silver paste, especially when it requires low-temperature curing. This means that the quality of copper required to form a given contact grid pattern can be lower than that required for screen-printed silver contact grids.

[0064] Although the present invention is described with reference to silicon-based solar cells and their interconnections, it should be clear to those skilled in the art that the method can also be applied to other solar cells, including thin film solar cells, such as solar cells containing cadmium telluride (CdTe), copper indium gallium selenide (CIGS), perovskite structures, and various series and multi-junction devices.

[0065] Figure 1 A corner of a common contact grid of a silicon-based solar cell 100 according to the prior art is schematically shown. The contact grid includes an array of fine metal grid lines 105 and a plurality of busbar structures 110 spaced apart on the solar cell 100. Solid metal contact pads 115 are regularly spaced apart along the busbar structures 110. When the solar cells 100 are interconnected, photocurrent is collected in the fine grid lines and flows along the fine grid lines to the nearest busbar structure 110. The current will then flow along the metal wires of the busbar structure 110 until it reaches the nearest contact pad 115, from which the current can flow into the interconnecting wire and flow to the adjacent solar cell.

[0066] When screen-printed silver is used as the contact grid of a solar cell, it is common to limit the distance that the current must flow along the metal fine grid lines 105 before being collected in the busbar structures, because this has the advantage of reducing the mass of silver paste that needs to be printed on the solar cell. Since the maximum current flowing along the fine grid lines is less than the current when the busbars are spaced farther apart, the height of the fine grid lines can be reduced, or more accurately, the cross-sectional area of the fine grid lines can be reduced. This means that for a solar cell manufactured on a 166mm (M6) silicon wafer, the total number of busbar structures 110 can be as many as 9 or 12, or even more in some cases. For larger solar cells, up to 20 busbar structures can be used to reduce the mass of silver required, thereby reducing the cost of cell metallization.

[0067] When using electroplated copper instead of silver, reducing the height of the copper busbars is not crucial. Therefore, to reduce costs, a larger number of busbar structures are not required. However, spacing the busbar structures closer together can reduce the magnitude of any stress that may be induced in the electroplated copper busbars. This can also reduce the likelihood of the busbars peeling or losing adhesion after electroplating.

[0068] During copper plating at room temperature, impurities and defects are incorporated into the grains of the deposited structure, generating compressive stresses. Faster plating speeds result in the incorporation of more impurities and crystal defects, which in turn increases the compressive stresses. The energy associated with these incorporated impurities / defects is typically dissipated during the self-annealing process, which causes grain growth and often changes in the primary crystal structure. This grain growth process can increase tensile stresses in the deposited structure. Ideally, the self-annealing process can counteract or balance the compressive stresses induced during deposition.

[0069] Thermal treatment of electroplated copper structures on solar cells can also introduce stresses in the solar cell due to the difference in the coefficient of thermal expansion (CTE) between copper and the solar cell absorber (e.g. silicon). The coefficient of thermal expansion (CTE) of polycrystalline copper is 17×10 -6 K -1 , about a rigid silicon wafer (2.6-3.3×10 -6 K -1 ). Furthermore, the elastic modulus of polycrystalline copper can reach as high as 100 GPa, and its yield / tensile stress is typically greater than 60 MPa. This means that the rapid heating and cooling of solar cells during the cell interconnection process generates stress in the copper electrodes and silicon wafers due to the large expansion and contraction of copper. This stress is not easily dissipated through plastic copper flow, and therefore also generates significant stress in the solar cell's silicon wafer.

[0070] In comparison, the thermal expansion coefficient of most industrially used silver pastes is typically around 10×10 -6 K -1 , (see, for example, C. Kohn et al., "Analysis of Warpage Effects Induced by Passivation Layers and Electrode Coatings in Silicon Solar Cells," 22nd European Conference on Photovoltaic Solar Energy, 2007), which is approximately half that of copper. Furthermore, the stresses induced by the different expansion and contraction rates of the silver solder pad and the silicon wafer are largely mitigated by the low elastic modulus of the cured silver paste (e.g., 7 GPa to 10 GPa reported by X. Gesheng et al., "Microindentation Study of Mechanical Properties of Cured Isotropic Conductive Adhesives (ICAs) Under Hygrothermal Aging," International Journal of Solids and Structures, 122-123, 81-90, 2017) and low yield / tensile stress (typically less than 20 MPa). Consequently, most silver pastes are able to adapt to heating and cooling changes through elastic and plastic deformation without inducing excessive stress in the metal electrodes or the solar cell itself.

[0071] Depending on the post-plating process implemented, the electroplated copper busbars are subjected to either compressive or tensile stress when interconnecting the solar cells. Both stress conditions can lead to adhesive failure between the copper and the solar cell surface, which can cause the busbars to detach from the cell surface. A denser busbar structure limits the thermal expansion and contraction of the busbar segments, reducing the likelihood of the busbars lifting or peeling from the surface.

[0072] It is generally known to use Figure 2A and Figure 2B The example shows a fully metallized contact pad 115. The contact pad 115 typically has a size of 0.8 mm to 1.2 mm and may intersect one or more metal fine grid lines depending on the fine grid line spacing used. Figure 2A and Figure 2BIn the embodiment, the spacing of the fine grid lines on the solar cell is 0.8mm, but different fine grid line spacings can be used depending on the type and design of the solar cell. Typically, different fine grid line spacings are used on the front and back surfaces of the bifacial solar cell. The contact pads 115 formed on the solar cell are typically square, rectangular or oval, and are typically wider than the interconnecting wires to allow for tolerance when the interconnecting wires are automatically aligned with the contact pads 115 during the cell stringing process. The diameter of the circular interconnecting wire can range from 200μm to 350μm. If an interconnecting flat ribbon is used, the width of the contact pad can be larger, with the flat ribbon width varying between about 0.6mm and 1.5mm. The thickness of the flat ribbon commonly used in manufacturing is 200μm to 250μm.

[0073] Embodiments of the present invention utilize patterning techniques, such as high-resolution patterning techniques, to replace the fully solid contact pad 115 with a patterned contact pad structure comprising metal regions separated by regions that expose the solar cell surface. Figure 2C One of many possible examples of such patterned contact pads is shown. The metal pattern of contact pad 215 and the area of the contact pad can vary depending on the properties of the metallization process (e.g., minimum width). In addition, the metal areas can be spaced apart from the openings in the pattern at the same or different periods in the two-dimensional pattern. Further examples of patterned metal contacts are further described below with reference to Figures 9 and 10.

[0074] Use as Figure 2C The patterned contact pads shown offer several advantages. First, when a solder-coated interconnect wire coated with a bonding material, such as solder, is brought into contact with the patterned contact pad 215 and heated, the coupling material can flow into the gaps or voids in the pattern and solidify upon cooling. This creates an interlocking, solid structure and more effectively distributes the shear stress generated when the bonding material cools after molten flow. Consequently, a stronger adhesive bond is formed between the interconnect wire and the contact pad.

[0075] like Figure 2C The patterned contact pads shown reduce lateral flow of bonding material, which is more pronounced in fully metallized contact pads. Upon heating, bonding material flows into the gaps between the metal segments, thereby reducing lateral diffusion within the plane of the solar cell. This results in a thicker bonding material layer over a smaller contact pad area, further reducing shear stress in the solder near the patterned contact pad interface. Unlike other approaches designed to reduce lateral flow of bonding material on substantially flat surfaces, embodiments of the present invention do not require modification of the physical properties of the bonding material, the solar cell surface, or the metal.

[0076] In addition, the patterned contact pad structure can be designed to regulate the stress generated in the electroplated copper used to form the contact grid. By allowing the metal structure (fine lines and contact pads) to expand and contract relative to the underlying silicon wafer to regulate this stress, the adhesion of the contact pad area to the solar cell surface can be enhanced, because the stress generated in the metal can destroy the adhesion of the metal to the solar cell surface. In one embodiment, this stress reduction is achieved by using a single metal (e.g., electroplated copper). As used herein, the term "single metal" or "single material" refers to a segment or portion that is partially or entirely composed of a metal or material that is not formed together with other metals or materials. In other words, a "single material" can be substantially formed from a single chemical element. For example, when a metal contact is made of copper, such as by electrodepositing copper on a surface, the metal contact is substantially free of other metals besides copper. However, it should be understood that a "single metal" or "single material" or "substantially a single chemical element" may contain trace impurities. A single material can also include materials such as screen-printed silver, and in this embodiment, silver is used as the "single material."

[0077] Finally, the amount of copper required to plate the contact pads can be reduced. As mentioned above, this reduces the amount of metal applied to the cell surface, thereby reducing costs. For certain types of solar cells, if the contact pad metal is in direct or indirect contact with the solar cell absorber material, the reduced metal area can also reduce potential charge carrier recombination at the metal-solar interface, thereby improving the solar cell's energy conversion efficiency.

[0078] The advantages of patterned contact pads can be achieved if the conductive lines are bonded by thermal or infrared radiation (e.g., infrared welding), or if bonding occurs during the lamination process. Bonding during lamination offers advantages for high-temperature-sensitive solar cells, such as silicon heterojunction (SHJ) cells.

[0079] The bonding material can be an alloy, solder or conductive adhesive. In one embodiment, all surfaces of the conductive wire are coated with a metal alloy or solder material. The solder material will form a solder joint between the conductive wire material and the patterned metal contact pads of the solar cell. The solder material can include, for example, SnPb solder, or a low-melting-point solder substitute such as SnBi, SnBiAg, SnBiIn or SnBiPb, or can be other suitable types of solder. Low-melting-point solder materials are advantageous for solar cells that are sensitive to high temperatures. Another advantage of these solder materials is that they can eliminate the use of lead in the final photovoltaic module.

[0080] Alternatively, the bonding material can be a conductive adhesive that includes a viscous polymer material with conductive particles. The viscous polymer can be an epoxy or acrylate material, and the conductive particles are typically silver particles or silver-plated copper particles, but other conductive materials can also be used. As explained with solder as the bonding material, when the adhesive material is heated, it can flow into the gaps between the metal segments of the pattern to form an interlocking contact structure. Examples of conductive adhesives are provided by companies such as Henkel and Loctite. Typically, the conductive adhesive is applied only to the surface of the interconnect wires that are in contact with the solar cell.

[0081] A specific embodiment of the present invention will now be described in more detail with reference to an n-type silicon heterojunction (SHJ) solar cell (a type of silicon semiconductor solar cell) in which a doped amorphous (alternatively, nanocrystalline or microcrystalline) silicon layer is used to form the electron collector and hole collector of the solar cell. For a double-sided contacted SHJ solar cell, typically the electron contact (also called the n-type contact) is formed on the front surface, while the hole contact (also called the p-type contact) is formed on the back surface of the solar cell. Both surfaces of the solar cell are coated with a transparent conductive oxide (TCO), which acts as an anti-reflective coating for the solar cell and promotes the flow of lateral current to the fine metal wires of the contact grid. In alternative cell designs, the hole contact can also be formed on the front surface of the solar cell.

[0082] The TCO may comprise indium tin oxide (ITO) with a ratio of In2O3:SnO2 ranging from 90:10 to 97:3 or 90:10 to 99:1. Typically, to achieve higher light capture, the front surface of the solar cell will use a higher ratio of In2O3:SnO2 to reduce parasitic absorption. Alternatively, the TCO may comprise a range of alternative materials including, but not limited to, transition metal doped SnO2, InWO, InCeO, InCsO, InTiO, InTaO and other indium-free TCOs such as aluminum doped zinc oxide (AZO). Typically, the thickness of the TCO is in the range of 60nm to 150nm, for example, between 80nm and 100nm. When the SHJ cell is illuminated, electrons are collected in the n-type doped surface silicon layer (typically at the front surface) and then flow into the TCO where they are conducted laterally to reach the nearest metal grid lines contacting the grid. Similarly, photogenerated holes are collected on the rear p-type silicon layer and flow through the TCO layer into the p-type (rear) contact grid.

[0083] The sheet resistance of the TCO is typically between 30 Ohm / sq and 110 Ohm / sq, for example between 40 Ohm / sq and 80 Ohm / sq. The fine grid line spacing is optimized to minimize electrical losses due to metal shadowing (which reduces current generation) and lateral resistance to current flow in the TCO layer.

[0084] Embodiments of the present invention are not limited to the design of the solar cell (e.g., the thickness of the surface contact layer on the silicon wafer, the optimization of the fine grid line spacing), and guidance on how to minimize the electrical performance loss of the solar cell contact grid is described in many textbooks, including "Solar Cells: Principles and Technology (Red Book)" by M. Green (ISBN: 0858235803).

[0085] Embodiments of the present invention are also applicable to other types of silicon-based solar cells, which can be fabricated on n-type or p-type silicon wafers. In these solar cells, the doped silicon regions can form the electron and hole collectors, and the metal contact grid can directly contact the doped silicon regions rather than the TCO. However, the optimization of the contact grid is essentially the same as described herein for SHJ cells.

[0086] Copper can be used to form the contact grid on both major surfaces of SHJ solar cells in a variety of ways. Copper pastes can be screen-printed directly onto the TCO surface; however, this approach has several drawbacks. First, the copper particles in these pastes must be silver-plated, limiting the cost advantage of using copper. Second, the resistivity of copper pastes is higher than that of pure copper, necessitating a higher copper loading to achieve the desired conductivity. Third, screen printing is limited to a resolution of approximately 30 μm if fine, continuous grid lines are to be reliably formed.

[0087] Alternatively, a metal layer (e.g., copper) can be sputtered or evaporated over the entire solar cell surface and then patterned into a contact grid with the desired geometry. When using this method, care must be taken to ensure that the removal of unwanted metal from the TCO surface does not affect the electrical performance of the solar cell. To reduce costs, the metal layer (usually copper) can be very thin and act as a seed layer. A mask layer can be formed on the metal layer, and a contact grid pattern is formed in the mask. The openings in the mask can then be exposed to a copper electrolyte, while the seed layer makes electrical contact. This allows copper to be electroplated onto the copper seed layer exposed in the mask openings, forming a thicker contact grid. Once the contact grid has been thickened by copper plating, the mask material and the seed layer outside the contact grid are removed, forming a copper contact grid on the surface of the solar cell. This method is called a "seed layer etch back" metallization method.

[0088] In a specific embodiment, the contact grid includes fine lines 105 and a busbar structure 110 with contact pads 115, which is electrochemically deposited directly on the TCO surface through a pre-patterned mask. Unlike the above-mentioned prior art methods, this method does not require a seed metal layer.

[0089] Now refer to Figure 3A An example of a method for forming a contact grid is described. Using this method, the width of the copper fine grid line can be as narrow as 10 μm (e.g., Figure 4 These fine feature sizes provide greater control over the bond formation mechanism between the solder and the contact pads on the solar cell.

[0090] In step 305 of method 300, a mask layer is formed on the TCO and patterned into openings for contacting the grid. The mask layer can comprise an organic resist polymer material, such as a novolac resin, and patterning can be achieved using photolithography (a technique well known in the art) or other patterning methods such as inkjet patterning (e.g., see: Z. Li et al., Patterning masks using polymers: insights and developments in silicon photovoltaics, International Materials Review, 61:6, 416-435, 2016). Another common masking method is direct printing of hot melt wax masks. In this method, wax is melted in the print head and solidifies in the mask pattern when it contacts the substrate (see: A. Descoeudres et al., Low temperature passivation and metallization processes for high-efficiency crystalline silicon solar cells, Solar Energy, 175, 54, 2018). Alternatively, a thin inorganic mask can be used and the inorganic material in the desired metal grid pattern can be removed by laser ablation or inkjet to achieve patterning (see T. Hatt et al., "Advances in Resist-Free Copper Plating for Silicon Heterojunction Solar Cell Metallization," AIP Conference Proceedings 2156, 020010, 2019). In fact, as long as these masking / patterning methods can be as Figure 2C The patterned contact pads shown achieve a segmented mask pattern of sufficient resolution that any of these masking / patterning methods can be used.

[0091] After forming the contact grid pattern in the mask layer, the TCO areas exposed by the mask can be pretreated in preparation for electroplating. Pretreatment ensures that the electroplated metal (to be deposited in step 315 of method 300) adheres securely to the TCO surface. This process can be, for example, an electrochemical wet chemical treatment, a chemical process, or a process utilizing hydrogen plasma. The type of pretreatment process employed depends on the type of TCO used, and embodiments of the present invention do not require the use of a specific pretreatment process.

[0092] In step 315 of method 300, a contact grid is plated onto the TCO through openings in the mask. If the equipment can directly contact the TCO layer, electrochemical copper deposition can be achieved on both the n-type and p-type surfaces. Current can then flow through the TCO layer to the openings in the mask that contact the plating electrolyte. Alternatively, the contact grid can be electrochemically deposited on the n-type and p-type surfaces using light-induced plating and forward-biased plating, respectively.

[0093] Photoinduced electroplating uses the light-induced current (and voltage) of a solar cell to drive the electrochemical deposition (electroplating) of metal onto the negatively polarized n-type surface of the solar cell. This process has been described in several publications, including A. Lennon et al., "The Evolution of Metal Electroplating for Silicon Solar Cell Metallization," Progress in Photovoltaics, Vol. 21, No. 7, pp. 1454-1468, 2012.

[0094] Forward-bias electroplating achieves electrochemical deposition on the p-type surface of a solar cell by applying a negative potential to the n-type surface. This negative potential forward-biases the solar cell's pn junction, making the p-type TCO surface the cathode. Forward-bias electroplating for silicon heterojunction (SHJ) cells is described by R. Boehme et al. in "Method for Fabricating Electrical Contacts for Silicon Solar Cell Structures," PCT International Publication No. WO2011117797.

[0095] After the contact grid is formed, the mask can be removed in step 320. The method used to remove the mask depends on the type of mask material. For example, if an organic resin is used, the mask can be removed by soaking or spraying with a dilute alkaline solution.

[0096] Figure 3B and Figure 3C An example of a method of forming a contact grid as involved in step 315 of method 300 is shown. Figure 3B , method 301 is shown, in which copper is first plated onto the TCO exposed in the mask opening in step 330 to form the main conductor. The amount of copper plated (i.e., the height of the fine grid lines) depends on the spacing between the fine grid lines and the main grid line structure, and can be optimized using methods familiar to those skilled in the art. After the plating is completed, the copper contact grid is covered with a thin layer of silver or tin to prevent oxidation of the copper when the individual solar cells are interconnected in the module. The covering layer can be formed using light-induced electroplating or forward bias electroplating (i.e., the same as the electroplating of the underlying copper), or it can be formed using an immersion or displacement process. In the latter case, the mask can also be removed before forming the covering layer (step 320 of method 300).

[0097] Figure 3CA further variation is shown and described in method 302 in which a nickel barrier layer is first plated on the TCO surface exposed by the mask opening in step 325 before the copper contact grid is plated. In cases where the TCO layer does not provide sufficient barrier, the nickel layer can provide additional barrier to copper diffusion into the active layer of the solar cell. The nickel layer can be plated using light-induced plating or forward-biased plating as previously described (step 315 of method 300).

[0098] In the illustrated embodiment, method 300 is performed on each of the n-type and p-type surfaces of a solar cell, thereby forming bifacial cells that can be interconnected into solar cell strings and then packaged into photovoltaic modules that receive light from both surfaces. Those skilled in the art will appreciate that the order in which the contact grids are formed can vary and depends on factors such as the choice of TCO material and the contact grid pattern.

[0099] Figure 3D A process flow or method 303 for a comparative "seed layer etch back" metallization process is shown. In step 360, a seed metal layer, such as copper, is deposited on each TCO surface of the solar cell, and then in step 365, a patterned mask is formed on both solar cell surfaces. The method of forming the pattern in the mask can be as described in methods 300, 301, and 302. Then, in step 370, a metal (e.g., copper) grid can be formed through the openings in the mask layer by directly contacting the external electrode to the metal seed layer on each solar cell surface and electroplating copper directly onto the seed layer metal (as described on page 17, lines 2 to 13). Typically, no pre-treatment step is required because the electroplated copper adheres strongly to the exposed seed layer. The mask is then removed in step 375 and the seed metal layer is removed / etched in step 380, leaving the formed contact grid on the surfaces of both TCO layers.

[0100] Regardless of how the n-type and p-type contact grids are formed, it is crucial that the contact grids adhere securely to the TCO surface. If the interfacial adhesion is not strong enough, it will be difficult to form a bond with the busbar structure through the bonding material to interconnect individual solar cells into a solar cell string. In the case of solder bonding, the heat applied to soften the solder and allow it to reflow can damage the fragile TCO metal bond and prevent cell interconnection.

[0101] When using automated equipment for cell interconnection, a strong bond between the contact grid and the TCO of the solar cell is particularly important because the interconnected cell strings are often lifted and moved for module layout. Although the use of a series of contact pads (such as Figure 12) instead of fully metallized busbars has the advantage, but this results in a reduction in the metal cell's bonding area. Therefore, the adhesion at the contact pads must be stronger than when using fully metallized busbars.

[0102] Figure 5 A schematic diagram of a region of a patterned contact pad 215 is shown. The contact pad 215 has first regions 217 of metal material separated by second regions 219 that expose the substrate. The first regions 217 are separated by the second regions 219 in two dimensions. In one embodiment, the first regions 217 of metal material are connected together, for example, by integral molding. In one embodiment, the first regions 217 form a periodic structure. For example, the first regions of metal material form a periodic structure. The contact pad has dimensions of 0.88 mm x 0.58 mm, with cross-hatched segments spaced 0.1 mm apart along the y-axis (height) and 0.15 mm apart along the x-axis. The width of the contact pad 215 is typically determined by the alignment tolerance of the automated stringing tool. This contact pattern is used for interconnecting SHJ solar cells fabricated on M6 wafers, where each cell has 12 busbar structures. The pattern of the contact pad 215 can be customized based on the cell area (i.e., wafer size), the number of busbar structures, and the alignment tolerance of the stringing process. Typically, as the wafer size increases, more main grid structures are required to keep the metal usage on the solar cell low. Serial devices with higher interconnect wire alignment accuracy can allow for narrower contact pads. If it is necessary to better bond the interconnect wire to the solar cell, the height of the patterned contact pad 215 (i.e., the size in the direction of the main grid structure) can be increased. However, in order to reduce costs, it is advantageous to keep the metal usage of each solar cell as low as possible.

[0103] Figure 6B is included Figure 5 , and illustrates a cross-sectional view of a linear segment 620 among a plurality of linear segments (eg, the first region 217 of metallic material) of the contact pad 215 . Figure 5 The arrow 510 in the diagram indicates the observation Figure 6B, and the plane of the cross section is perpendicular to arrow 510. The spacing (gap) between the linear segments of the cross-hatched pattern may be between 20 μm and 100 μm, for example between 30 μm and 60 μm. The width of the linear segments may be between 5 μm and 20 μm, for example, more specifically between 5 μm and 15 μm. In one embodiment, the first region 217 of the metallic material has a shortest extension length in the plane of the metallic contact of less than 100 μm, less than 50 μm, less than 20 μm or even less than 15 μm. In one embodiment, the second region 219 has a shortest extension length in the plane of the metallic contact of less than 500 μm, less than 100 μm, less than 50 μm or even less than 25 μm. In a specific arrangement, the patterned contact pads 215 are respectively formed using reference Figure 3A 、 Figure 3B 、 Figure 3C or Figure 3D The described method 300, 301, 302 or 303 is formed.

[0104] Having high aspect ratios for the individual segments in the pattern is also beneficial. However, the stronger adhesion provided by a more tightly spaced crosshatch or mesh pattern must be weighed against the increased metal content required to contact the grid. Ideally, an optimal balance between strong adhesion and low metal content is desired.

[0105] Figure 6B An electrical contact 650 is shown in which the solder bonding material 610 is in direct contact with the surface layer 625 of the silicon wafer 630. Those skilled in the art will appreciate that in an arrangement where the linear segments 620 are of sufficient height and are relatively closely spaced (high aspect ratio of the linear segments 620), the solder material 610 does not flow into the entire gap between the linear segments 620, such that at least some of the solder material is not in direct contact with the surface layer 625. In this case, the solder material 610 is located above and spaced apart from the surface layer 625.

[0106] Cross-section diagram Figure 4 The plated metal structure shown is particularly advantageous. Not only does it have an aspect ratio of approximately 1:1, but the capping regions 410 of the plated sections also act as "hooks" to help hold the solidified solder in place, thereby enhancing the solder bond. Figure 6B Adhesion between wire 605 and solar cell (silicon wafer 630 and surface layer 625) is shown. Although screen printing can also be used to form patterned contact pads, it usually results in metal structures with low aspect ratios without any "interlocking" features such as those described above. Figure 4 The "hook" shown.

[0107] During the cell interconnection process, a process often referred to as "string soldering" when the bonding material is solder, a solar cell having multiple busbar structures is bonded to a wire 605 by melting and subsequently solidifying a solder material 610, each busbar structure comprising multiple patterned contact pads 215. When the solar cell and wire are heated, the solder material 610 flows between the segments of the cross-hatched pattern of the patterned contact pads 215 and forms a cross-hatched pattern as it cools. Figure 6B The interlocking structure shown.

[0108] As a comparison, Figure 6A A cross section of an electrical contact 600 is shown with a fully metallized contact pad 615 (prior art). Like parts are numbered likewise. In this structure, a single planar solder-metal interface is formed upon cooling, and there are no interlocking structures to enhance adhesion.

[0109] In one embodiment, the substrate (e.g., silicon wafer 630) is one of a plurality of substrates that include, for example, solar cells within a solar cell assembly, and at least one conductive wire is used to interconnect adjacent solar cells within the solar cell assembly by connecting one polarity of one solar cell to the opposite polarity of an adjacent solar cell.

[0110] Strong adhesion of the wire 605 to the solar cell requires that the metal of the contact pad be firmly adhered to the solar cell and the wire 605. In the specific embodiment described with reference to the SHJ solar cell, the metal of the contact pad must be firmly adhered to the TCO on the surface of the solar cell to be contacted. For other types of silicon solar cells, such as passivated emitter back contact (PERC) cells or thermal oxide passivated contact (TOPCon) cells, strong adhesion between the contact pad metal and the silicon surface is required. For other solar cells, the metal can be in direct contact with other interface materials of the solar cell.

[0111] When one material is joined to another and heated, shear stress is generated during the assembly process due to the different rates of expansion and contraction (upon cooling) of the two materials. High levels of induced shear stress can lead to material fracture and / or adhesion failure at the interface between the two materials. Because the copper structure expands and contracts more than the silicon wafer, stress is induced in both the copper and silicon wafers. The solder in the middle undergoes plastic deformation, which reduces the magnitude of the generated stress. However, the use of patterned contact pads 215 can further reduce the generated stress because the shorter metal segments in the pattern reduce the physical extent of copper expansion / contraction. Orienting the metal segments of patterned contact pads 215 in non-parallel directions also distributes stress over a larger unmetallized area, reducing the magnitude of any concentrated stress. In solid metal contact pads, stress in the silicon after cooling can be concentrated at the edges of the contact pads. In one embodiment, the coefficient of thermal expansion of the first region of the metal material (e.g., linear segment 620) is greater than or equal to three times the coefficient of thermal expansion of the substrate (e.g., silicon wafer 630). The first region of metallic material (e.g., linear section 620) may have an elastic modulus greater than 50 GPa and / or a yield stress greater than 25 MPa. In one embodiment, linear section 620 is formed from a polycrystalline material. For example, linear section 620 may be formed from copper. In one embodiment, the material (e.g., linear section 620) is formed from a single material.

[0112] The use of patterned contact pads 215 can also advantageously reduce the diffusion or flow of solder material 610 during the heating phase. This is conducive to forming a thicker and thus more secure solder joint on a smaller pad area. The reduction in solder diffusion can further reduce the shear stress in the solder layer near the metal segment. The reduction in solder flow makes it possible to use smaller contact pads, provided that the wire 605 can be accurately aligned with the typical linear arrangement of the contact pads 215 in the main grid structure 110, which can further reduce the metal mass required for each battery. Using smaller contact pads can also reduce front surface shielding.

[0113] Another key advantage of the patterned contact pad 215 is that it can more easily withstand the stress generated in the metal of the contact pad and its adjacent fine grid lines when the solder cools and solidifies. The CTE of copper is much greater than the CTE of TCO and silicon. Therefore, when the contact pad is a large solid copper structure (i.e., a fully metallized contact pad), the shear stress generated in the copper contact pad during cooling can destroy the adhesion between the contact pad and the TCO, resulting in delamination between the contact pad and the TCO. According to an embodiment of the present invention, the use of patterned contact pads 215 can reduce the magnitude of the shear stress generated and distribute the vector direction of the stress over the contact pad area, thereby reducing the possibility of interface failure and the subsequent delamination of the patterned contact pad and the TCO of the solar cell.

[0114] For solar cells, the adhesion of the busbar structure to the solar cell is typically measured using busbar pull force. In this measurement, the interconnecting wire is pulled out of the solar cell while the solar cell remains fixed on a flat platform. The interconnecting wire is oriented at a fixed angle, typically 90°, to the plane of the solar cell. Most commonly, the interconnecting wire is bonded to the busbar structure using a strong solder. Alternatively, a conductive adhesive material can be used to bond the interconnecting wire to the solar cell.

[0115] For both bonding materials, pulling on the interconnect wire typically disrupts the interface between the solar cell and the busbar structure, and the amount of force measured provides a measure of how strongly the metal of the busbar structure adheres, or bonds, to the solar cell surface.

[0116] If the bond between the interconnecting wire and the metal contact pad on the solar cell is weak (for example, improper adhesive application or insufficient solder melting), pulling the interconnecting wire will destroy the bond between the bonding material and the contact pad metal. In this case, the measured force can hardly provide information about the strength of the adhesion between the main grid structure and the solar cell. If the bond between the interconnecting wire and the contact pad is stronger than the bond between the solar cell and the metal contact pad, the metal of the contact pad will typically be pulled out from the solar cell surface. In this case, the measured main grid pulling force is considered to be a measure of the adhesion strength of the metal (used to form the contact grid) to the solar cell surface.

[0117] Another consequence of this adhesion measurement can occur if there is a very strong bond between (i) the solar cell and the metal of the contact pads (of one or more busbar structures), and (ii) a very strong bond between the contact pads and the interconnect wires. In this case, the solar cell material (e.g., silicon wafer) itself can break, causing fragments of the wafer to break off from the solar cell when the interconnect wires are pulled.

[0118] When a fully metallized busbar structure is used, the measured pull force is typically averaged over the length of the busbar structure and normalized by the busbar width, the latter normalization typically being used to account for the area over which the metal of the busbar structure adheres to the solar cell. Therefore, the force is measured in N / mm. When the busbar structure consists of a series of discrete and independent contact pads, it is more common to measure the peak pull force for each contact pad and then average this peak pull force over all the contact pads on the solar cell. As described for the fully metallized busbar structure, this average peak pull force can also be normalized by the width of the contact pad to obtain a measured value (i.e., N / mm).

[0119] For the specific embodiment described in the present invention, the metal area of the contact pad is reduced compared to the fully metallized contact pad. In addition, depending on the pattern used for the patterned contact pad, the effective metal contact width in the "pull" direction may be different. Therefore, the adhesion of the contact pad is more appropriately expressed as the peak measured force (for the contact pad) divided by the average effective width of the metal pattern of the patterned contact pad. The unit of this indicator is still N / mm, which can be directly compared with the measurement value of the fully metallized contact pad. For example, Figure 5 The average effective metal width of the contact pads shown is only 0.17 mm (assuming a metal structure width of 12 μm). For a fully metallized contact pad, this effective width is approximately 20% of the contact pad width of 0.88 mm.

[0120] Figure 7 The table shows all metallized contact pads and the metallized contact pads formed on the SHJ solar cell using the method according to the specific embodiment of the present invention. Figure 5 The average "measured value" and the standardized peak busbar tension of the contact pad 215 of the pattern shown. The numerical values in the table represent the average value and standard deviation of the peak force measured by five consecutive contact pads in the copper-plated busbar structure. The average standardized peak force of the patterned contact pad 215 is about five times that of the contact pads of all metallizations. The standard deviation of the force measured for the patterned contact pad is also significantly reduced, which shows that using the patterned contact pad can achieve more uniform adhesion of the interconnect wire to the solar cell.

[0121] Furthermore, for many patterned contact pads 215, the interfacial adhesion at the contact pad / TCO interface was so strong that tensile force measurements resulted in cracking of the silicon wafer and / or silicon fragments falling off the wafer, demonstrating that extremely strong interfacial adhesion can be achieved through patterning of the contact pads. Figure 8 An example of a contact pad area on a solar cell is shown, with the contact pad having Figure 5 The pattern shown schematically in Figure 1, where busbar tension causes silicon fragments to break off across the entire thickness of the silicon wafer, illustrates the adhesion strength of the contacts to the silicon surface. For solar cells comprised of different photovoltaic materials, other failure mechanisms, such as delamination of the solar cell from the substrate, can occur due to strong interfacial adhesion between the contact pad metal and the solar cell.

[0122] The pattern used for patterned contact pads 215 can be tailored to specific adhesion characteristics. While high-resolution battery metallization patterns and high-aspect ratio conductive metal structures maximize adhesion, as described above, other patterns, contact pad shapes, and other metallization methods can also offer benefits.

[0123] Figure 9 shows Figure 5 There are several variations of patterned contact pads shown in . Figures 9A to 9C 、 Figure 9E and Figure 9F Contact pad patterns 215a, 215b, 215c, 215e, and 215f are shown, respectively, where connected linear metal segments are aligned along two major axes and have different peripheral shapes. When heated, the bonding material flows into the areas between the metal segments. Figure 9D 、 Figure 9G and Figure 9H Representative examples of contact pad patterns 215d, 215g, and 215h are shown, respectively, where the metal segments are not connected. In these variations, bonding material flows to electrically connect the metal segments and enhance the redundancy of current collection. The advantage of these non-connected metal variations is that they allow stress in the underlying silicon to be reduced around the metal circles (215g, Figure 9D )、Cross(215g, Figure 9G ) and square (215h, Figure 9H ) is distributed throughout the surrounding area.

[0124] Figures 10A to 10H Shown 9A to 9H , and the associated various busbar structures 110 are arranged and two exemplary fine grid lines 105 oriented perpendicular to the busbars. As previously mentioned, the busbars between the contact pads may include a series of thinner wires with varying spacing. While a single metal wire can be used to collect current from the fine metal grid lines, it is preferred to use multiple metal wires because this provides greater redundancy in current collection.

[0125] Those skilled in the art will appreciate that various possible contact pad designs can be employed in embodiments of the present invention. Not all metal segments of a contact pad pattern must be connected within the metal pattern, as once a bond is formed with the interconnecting wire, current can flow from the solar cell into the isolated metal segments of the pattern and then be transferred through the solder to the interconnecting wire. However, contact pad patterns with isolated metal segments can present difficulties when measuring solar cell efficiency before cell interconnection.

[0126] Throughout this specification, the term "comprise," or variations such as "comprising" or "including," will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0127] Any discussion of documents, acts, materials, devices, articles or the like included in this specification is not to be taken as an admission that any or all of the foregoing form part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.

Claims

1. A method for forming an electrical connection on a surface of a substrate, the method comprising: providing the substrate and a material for forming at least one metal contact on the surface of the substrate; forming the at least one metal contact on the surface of the substrate, comprising selecting at least one contact region on the surface of the substrate and forming a structure of metal material within the at least one contact region, the structure having a first region of metal material separated by a second region exposing the substrate, wherein the first region is separated in two dimensions by the second region; providing at least one conductive line and a coupling material; and The at least one conductive line is coupled to the formed at least one metal contact using the coupling material such that at least some of the coupling material is also located over the substrate at the second region within the at least one contact area.

2. The method according to claim 1, wherein The material is a polycrystalline material.

3. The method according to claim 1 or 2, wherein: The thermal expansion coefficient of the material is greater than or equal to three times the thermal expansion coefficient of the substrate.

4. The method according to any one of claims 1 to 3, wherein The material has an elastic modulus greater than 50 GPa.

5. The method according to any one of claims 1 to 4, wherein The material has a yield stress greater than 25 MPa.

6. The method according to any one of claims 1 to 5, wherein The material is a single material.

7. The method according to any one of claims 1 to 6, wherein Some of the coupling material is in direct contact with the substrate in at least some of the second regions.

8. The method according to any one of claims 1 to 6, wherein Some of the coupling material is located above and spaced apart from the substrate at at least some of the second regions and is not in direct contact with the substrate at each second region.

9. The method according to any one of claims 1 to 8, wherein The coupling material is a solder material, and coupling the at least one conductive wire to the at least one formed metal contact includes soldering the at least one conductive wire to the at least one formed metal contact.

10. The method according to any one of claims 1 to 8, wherein The coupling material is a conductive adhesive material, and coupling the at least one conductive wire to the at least one formed metal contact includes adhering the at least one conductive wire to the at least one formed metal contact.

11. The method according to any one of claims 1 to 10, wherein At least some of the first regions of metallic material are interconnected.

12. The method according to any one of claims 1 to 11, wherein The first region of the metal material is integrally formed.

13. The method according to any one of claims 1 to 12, wherein The first regions of the metal material form a periodic structure.

14. The method according to any one of claims 1 to 10, wherein The metal contacts include at least some first regions of the metal material, the at least some first regions being separated from each other and not in direct contact with other first regions of the metal material.

15. The method according to claim 14, wherein The first region forms a periodic structure.

16. The method according to any one of claims 1 to 15, wherein The at least one metal contact has a shortest extension in the plane of the metal contact of less than 2 mm, less than 1 mm, less than 0.5 mm or even less than 0.2 mm.

17. The method according to any one of claims 1 to 16, wherein The first region of metallic material has a shortest extension in the plane of the metallic contact of less than 100 μm, less than 50 μm, less than 20 μm or even less than 15 μm.

18. The method according to any one of claims 1 to 17, wherein The second region has a shortest extension in the plane of the metal contact of less than 500 μm, less than 100 μm, less than 50 μm or even less than 25 μm.

19. The method according to any one of claims 1 to 18, wherein The at least one conductive wire is coated with a solder material.

20. The method according to any one of claims 1 to 18, comprising: A soldering material is provided separate from the at least one conductive wire, and the at least one conductive wire is soldered to the formed metal contact using the soldering material.

21. The method according to any one of claims 1 to 20, wherein The metal material includes at least one of Cu, Ni, Sn and Ag or a combination thereof.

22. The method according to any one of claims 1 to 21, wherein The substrate forms part of a solar cell, and the at least one metal contact is an electrical contact on a surface of the solar cell.

23. The method according to any one of claims 1 to 22, wherein The at least one metal contact is one of a plurality of metal contacts, and the method includes forming a plurality of the metal contacts on the surface of the substrate.

24. The method according to any one of claims 1 to 23, wherein The substrate is one of a plurality of substrates including solar cells, and wherein the at least one conductive wire is used to interconnect adjacent solar cells by connecting one polarity of one of the solar cells to an opposite polarity of an adjacent solar cell.

25. A device having electrical connections formed by the method of any one of claims 1 to 24.

26. An apparatus comprising: substrate; at least one metal contact within the contact region, the at least one metal contact comprising a structure having a first region of metallic material separated by a second region in which the metallic material is not located, wherein the first region is separated in two dimensions by the second region; and at least one conductive line coupled to the metal material using a coupling material; Wherein at least some of the coupling material is also located above the substrate at the second region in at least one of the metal contact regions.

27. The device according to claim 26, wherein The material is a polycrystalline material.

28. The device according to claim 26 or 27, wherein The thermal expansion coefficient of the material is greater than or equal to three times the thermal expansion coefficient of the substrate.

29. The device according to any one of claims 26 to 28, wherein The material has an elastic modulus greater than 50 GPa.

30. The device according to any one of claims 26 to 29, wherein The material has a yield stress greater than 25 MPa.

31. The device according to any one of claims 26 to 30, wherein The material is a single material.

32. The device according to any one of claims 26 to 31, wherein Some of the coupling material is in direct contact with the substrate in at least some of the second regions.

33. The device according to any one of claims 26 to 31, wherein Some of the coupling material is located above and spaced apart from the substrate at at least some of the second regions, while no coupling material is in direct contact with the substrate at every second region.

34. The device according to any one of claims 26 to 33, wherein The at least one metal contact has a shortest extension in the plane of the metal contact of less than 2 mm, less than 1 mm, less than 0.5 mm or even less than 0.2 mm.

35. The device according to any one of claims 26 to 34, wherein The first region of metallic material has a shortest extension in the plane of the at least one metallic contact of less than 100 μm, less than 50 μm, less than 20 μm or even less than 15 μm.

36. The device according to any one of claims 26 to 35, wherein The second region has a shortest extension in the plane of the at least one metal contact of less than 500 μm, less than 100 μm, less than 50 μm or even less than 25 μm.

37. The device according to any one of claims 26 to 36, wherein The device comprises a solar cell, and wherein the coupling material is a solder material.

38. The apparatus according to any one of claims 26 to 36, wherein The device includes a solar cell, and wherein the coupling material is a conductive adhesive.

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