Method for electrically contacting and connecting a solar cell, and solar cell having electrical contacting and connection

Abstract

In a method for electrically contacting and connecting solar cells, a solar cell (1) is provided (20). At least one busbar (11) is formed on a first side of the solar cell (1) such that the at least one busbar (11) receives charge carriers generated in the solar cell, for example by contact with contacting fingers or from an electrically conductive surface coating of the solar cell. To do this, the at least one busbar (11), composed of an electrically conductive, thermoplastic material, is applied to the first side of the solar cell (1). The at least one applied busbar (11) is heated (23) to a temperature above its thermoplastic melting and / or softening temperature, and a conductive wire (13) is placed onto the at least partially melted, applied busbar (11) in this heated state. The at least one busbar (11) is then cooled (24) such that the conductive wire (13) is mechanically fastened to the solar cell (1) and electrically contacts the solar cell (1) by means of the electrically conductive, thermoplastic material of the busbar (11).

Description

[0001] Applicant: Karlsruhe Institute of Technology, a public corporation, MB&P Reference: K07802WO - hy / mu

[0002] Methods for electrically contacting and interconnecting a solar cell and solar cell with electrical contacting and interconnection

[0003] The invention relates to a method for electrically contacting and interconnecting a solar cell and a solar cell with electrical contacting and interconnection.

[0004] The invention lies in the field of materials research for application in the electrical contacting and interconnection of solar cells. Materials newly developed according to the invention are suitable for use both in currently market-dominating solar cell concepts, which can be exposed to high temperatures during the production process, such as PERC or TopCon, and in high-efficiency solar cell concepts that will gain market share in the future and which can only tolerate low temperatures of <200°C during production, such as heterojunction, perovskite, or tandem solar cells made of perovskite and silicon, for example.

[0005] Traditionally, solar cells are contacted and interconnected using various materials and processes. For contacting solar cells, which refers to electrical contacting, narrow contact lines, also called contact fingers, are applied to the surface of the solar cells. These contact fingers collect the charges generated within the solar cell. They terminate in current collectors and / or busbars, which transport the charges to module connecting wires. The busbars typically contain silver as a conductive component, while the connecting wires used to connect the cells into modules are usually made of copper.

[0006] Figure 1 shows the interconnection of a conventional solar cell 100, here using the example of a heterojunction half-cell with nine busbars and five solder pads per busbar. Thin contact fingers 110 are applied to the front side of the solar cell 100 shown in Figure 1. Furthermore, wider busbars 111 are applied to the front side of the solar cell 100 as current collectors, which conduct the electrical current from the contact fingers 110 to interconnection wires. The contact fingers 110 and the busbars 111 are conventionally applied separately from each other in screen printing processes using highly filled silver pastes.

[0007] Small, e.g. square, widenings called solder pads 112 are attached to the busbars 111, which provide a better mechanical connection to a copper wire (Cu wire, not shown in Fig. 1, called interconnection wire above) that is used to connect the solar cells to modules.

[0008] The interconnection of the solar cells using copper wire is achieved through a process called stringing. In this process, copper wires are soldered or glued to the busbars and solder pads using various solder materials or crosslinking, electrically conductive adhesives (ECAs), thus connecting the solar cells into modules. These ECAs are typically reactive polymers filled with silver particles and are usually applied and cured at temperatures below 200 °C during the stringing process. Copper wires coated with lead-tin alloy solder are primarily used and are soldered at process temperatures above 220 °C to create a mechanical and electrical connection between the cell (and, if applicable, fingers and busbars) and the interconnection wire.

[0009] For modern cell variants such as the heterojunction solar cell, alloys based on bismuth or indium are used, as these have a significantly lower melting temperature in the range of 150 °C.

[0010] An alternative, relatively new approach to interconnecting solar cells is described by the so-called shingling technique. Here, solar cells are cut into elongated strips, which are then stacked on top of each other like roof tiles and soldered or bonded using an ECA (Electronic Contact Application). Here, too, a busbar is first printed onto the cell along a cell overlap area.

[0011] Given the currently planned high investments in renewable energy sources in many countries, a significant expansion of photovoltaics is expected. This expansion requires large quantities of silver or other conductive metals, both for the metallization pastes used to print contact fingers, busbars, and solder pads, and for the conductive adhesives. This leads to the problem of a high strain on global resources. The solar industry's silver consumption already accounts for approximately 10% of global annual silver production. Furthermore, some solder materials used for cell interconnection also require scarce or environmentally problematic resources, such as indium and bismuth.

[0012] The invention is therefore based on the objective of enabling solar cells with reduced material consumption, in particular with reduced consumption of rare resources. A further objective of the invention may be to simplify the process of electrically contacting and interconnecting solar cells.

[0013] The problem is solved by the subject matter of the independent claims. The dependent claims relate to preferred embodiments.

[0014] One aspect concerns a method for the electrical contacting and interconnection of a solar cell. A solar cell is provided. Contact fingers can be formed on the first side of the solar cell such that they accept charge carriers generated within the cell. At least one busbar is formed on the first side of the solar cell such that it accepts charge carriers generated within the cell. This can be achieved, for example, by the busbar electrically contacting at least one of the contact fingers and conducting current from these contact fingers to interconnection wires of the solar cell. The busbar, at least one of which is made of an electrically conductive thermoplastic material, is applied to the first side of the solar cell.The at least one applied busbar is heated to a temperature above its thermoplastic melting and / or softening temperature, and in this heated state, a conductive wire is applied to the at least softened, applied busbar as a connecting wire. The at least one busbar is then cooled such that the conductive wire is mechanically attached to the solar cell by means of the electrically conductive, thermoplastic material of the busbar, and the solar cell is electrically contacted.

[0015] The solar cell can initially be provided unwired or partially metallized. It is wired and / or fully metallized, i.e., electrically contacted, by the method according to the invention. The solar cell can have been partially metallized either in high-temperature processes, such as for PERC or TopCON cells, or in low-temperature processes, such as for heterojunction or tandem solar cells. The solar cell can be a shingle strip, a half-cell, or a full cell.

[0016] The first side of the solar cell can be, for example, the front side of the solar cell, which is located on the side facing away from the sun during operation. The interconnection and wiring typically take place on two opposite sides of the solar cell, i.e., both on the side facing the sun and on the side facing away from the sun. The first side of the solar cell is not one of the narrow edges, but rather one of the two large surfaces of the (e.g., flat) solar cell.

[0017] On the first side, the contact fingers can be formed in a conventional manner, e.g., using a screen printing process with suitable screen printing pastes containing electrically conductive components such as silver and / or copper. The electrically conductive contact fingers can be applied to the first side approximately parallel to each other and / or at regular intervals, e.g., aligned along a preferred direction. The contact fingers can be configured to locally accommodate the charge carriers generated and / or produced within the solar cell. The contact fingers can have a maximum width of approximately 40 µm. Depending on the cell type, the contact fingers can be dried and / or cured, e.g., in HJT and / or tandem cells, or they can be sintered, particularly in PERC and / or TopCon silicon cells.

[0018] As an alternative to contact fingers, the solar cell can also be designed without contact fingers, i.e., it can have no such contact fingers. In this case, the solar cell can, for example, have a transparent, electrically conductive coating on its first side, which can be configured to locally accommodate the charge carriers that are "generated" and / or accumulate in the solar cell. The busbar can then be applied directly to the transparent, electrically conductive coating.

[0019] At least one busbar is formed on at least the first side of the solar cell. Depending on the cell type, busbars can also be formed on both large-area sides of the solar cell. In particular, the busbar can be designed, for example, such that it electrically contacts at least one or more of the contact fingers (if present) and conducts current from these contact fingers to interconnect wires of the solar cell. For this purpose, the busbar is also electrically conductive. Preferably, several such busbars are formed on the first side, which is why reference is often made below to multiple busbars, although this can equally apply to embodiments with only one busbar. The busbars can, for example, be formed at an angle to the contact fingers on the first side, in particular at a right angle.The busbars can be formed along a second preferred direction on the first side of the solar cell, in particular approximately equidistant and / or parallel to each other. The second preferred direction can, for example, be approximately perpendicular to the first preferred direction along which the contact fingers are formed.

[0020] In general, the busbar can be configured and / or designed so that it can directly receive charge carriers generated and / or accumulated in the solar cell, even from the first side of the solar cell, i.e., also at points on the solar cell where no contact fingers are formed. This applies particularly to areas on the solar cell where a transparent, electrically conductive coating is present. The solar cell can be electrically contacted at the interconnection wires. In particular, electrical interconnection with other solar cells can be established here.

[0021] According to the invention, at least one busbar is applied to the first side of the solar cell from an electrically conductive, thermoplastic material. The material can initially be in the form of a paste, for example, and applied to the solar cell using a predetermined application method. One of several different application methods can be predefined, which will be discussed in more detail below. The busbar material has at least two properties: First, it is thermoplastic, i.e., it contains thermoplastic polymers, and second, it is electrically conductive. For example, the material can consist of a thermoplastic polymer filled with conductive particles, which makes it conductive.

[0022] The main components of the material can be, for example, a suitable thermoplastic polymer, which preferably has a melting and / or softening temperature significantly above room temperature and / or the operating temperature of the solar cell, e.g., a melting temperature of at least approximately 80°C, and in particular at least approximately 100°C. Furthermore, the melting and / or softening temperature can preferably still be within a manageable range, e.g., up to a maximum of approximately 250°C. Another main component can be a suitable electrically conductive filler, such as silver, copper, silver-coated copper, graphite, carbon black, carbon, and / or another electrically conductive filler, such as silver-coated glass particles.

[0023] After application, the busbar is firmly bonded to the first side of the solar cell. For application, the electrically conductive, thermoplastic material can either be applied dissolved in a solvent, which is then dried, or it can be applied to the first side in a heated, or at least softened, state. The busbar is then positioned and attached to the first side of the solar cell as an electrically conductive, thermoplastic busbar.

[0024] The busbar can be unbranched, i.e., a single, unbranched line on the first side of the solar cell. In particular, the busbar can be approximately straight, which can, for example, simplify the precise application of the wire to the busbar.

[0025] After application, at least one applied busbar is heated to a temperature above its thermoplastic melting and / or softening point. This can be done either by heating only the busbar or by heating the entire solar cell, including all busbars, simultaneously. In this heated state, the busbar material becomes soft and malleable, allowing the conductive wire to be applied to the softened, applied busbar.

[0026] This can be done as part of a stringing process, whereby the applied busbar is brought into an adhesive state by increasing the temperature, so that the wire can be pressed into and / or onto the busbar.

[0027] The wire can be made of copper. Copper wires have proven suitable for the metallization of solar cells due to their good conductivity and other material properties.

[0028] The wire can be pressed into and / or onto the heated busbar, whereby the material of the busbar can deform so that the wire is bonded to the thermoplastic polymer on the first side.

[0029] The wire can be bonded to the first side of the solar cell using the electrically conductive, thermoplastic material.

[0030] Each busbar can be fitted with exactly one dedicated wire, the wire following or mimicking the direction of the busbar along the first side of the solar cell. The wire is preferably designed to extend beyond the busbar, allowing the solar cell to be reliably connected via the protruding wire. It can be attached to the solar cell by means of the busbar, at least substantially, along its entire length, i.e., from a first end of the busbar to a second end of the busbar.

[0031] Alternatively or additionally to heating the busbar, the conductive wire can also be heated so that when pressed onto the busbar, it softens it and thus adheres to it. This eliminates the need to heat the entire solar cell.

[0032] Finally, at least one busbar and / or the wire and / or the entire solar cell is cooled down again, e.g., to room temperature, so that the conductive wire is attached to the solar cell by means of the electrically conductive, thermoplastic material of the busbar, in particular by a metallurgical bond. In this attached state, the wire makes electrical contact with the solar cell, in particular with those contact fingers that are also contacted and / or crossed by the busbar on which the wire is applied. This contact of the solar cell by the wire can be enabled, supported, and / or enhanced by the electrical conductivity of the electrically conductive, thermoplastic material. The wire thus acts as the interconnection wire of the solar cell.

[0033] After cooling, this enables an electrically conductive, mechanically stable cell-wire contact, especially without the use of soldering materials and / or additional ECAs.

[0034] The process enables savings in raw materials and costs, while simultaneously simplifying the module manufacturing process. This applies to both conventional interconnection using copper wires and modern approaches such as shingling. The electrically conductive, thermoplastic material allows for the integration of multiple functions into a single material.

[0035] This eliminates the need for both a highly filled busbar (i.e., one filled with a high proportion of electrically conductive materials such as silver particles) and solder pads. Metallization and / or interconnection can be performed without solder pads. Furthermore, the use of additional solder and / or ECAs for securing the copper wires to the busbars is unnecessary. Instead, the busbar is applied as an electrically conductive, thermoplastic busbar.

[0036] Furthermore, this simplifies the process, as at least one process step can be eliminated, such as soldering and / or applying ECAs and / or forming solder pads. This also saves resources in the form of unnecessary high-silver busbars, solder pads, ECAs, and / or solder materials. Conventional metallization pastes for printing standard busbars are either sintered or based on polymers that cross-link to form a thermoset. In both cases, the metallization cannot be reversed to an adhesive state and is irreversible. This reversibility also allows for string repair by melting and replacing defective individual cells.

[0037] In one embodiment, the electrically conductive thermoplastic material comprises a thermoplastic polymer containing electrically conductive particles. For example, a polyamide can be used as the thermoplastic polymer, as it has proven to be sufficiently adhesive for the process. Furthermore, polyamide can be readily filled with conductive particles, resulting in a well-adhering paste from which the conductive busbars can be applied. Polyamide is also well-suited as a thermoplastic polymer because it is highly compatible with other fillers, such as the conductive particles and / or additives for adjusting flowability. Moreover, as a polymer, it can be applied with sufficient precision.

[0038] In a further development, the electrically conductive particles contain silver particles, and in particular, they can be formed as silver particles. The silver particles constitute a volume fraction of at most 30 vol%, and in particular at most 20 vol%, of the applied electrically conductive thermoplastic material. In at least one embodiment, the silver particles constitute a volume fraction of approximately 15 vol%. The vol% fraction refers here to the applied and (if a solvent was present) dried state of the electrically conductive thermoplastic material. The silver particles can be formed as pure silver particles. Alternatively or additionally, silver particles that consist only partially of silver, e.g., silver-coated glass particles, can also be used.In experiments, the inventors achieved reliable electrical contact using busbars made of an electrically conductive thermoplastic material containing only 15% silver particles as the sole conductive filler. This is significantly less than conventional metallization pastes used for busbar formation, which typically contain 60 to 80% silver. Therefore, this method allows for a significant reduction in the amount of silver required.

[0039] Alternatively or additionally to the embodiment with the silver particles, copper particles can be used as electrically conductive particles in an analogous manner, i.e. particles that contain at least copper and / or particles that are designed as pure copper particles.

[0040] In a further development, the electrically conductive particles contain carbon particles, and in particular, they can be formed as carbon particles. The carbon particles constitute a volume fraction of at most 35 vol%, and in particular at most 25 vol%, of the applied and optionally dried electrically conductive thermoplastic material. In at least one embodiment, the carbon particles constitute a volume fraction of approximately 22.5 vol%. In experiments, the inventors were able to achieve reliable contact using busbars made of an electrically conductive thermoplastic material, the sole conductor carrier of which was filled with only 22.5 vol% carbon particles. Carbon is a significantly cheaper and considerably less rare resource than, for example, silver.

[0041] In a further development, the thermoplastic polymer constitutes a volume fraction of at least 65 vol%, and in particular at least 80%, of the applied and optionally dried electrically conductive thermoplastic material. In some embodiments, the thermoplastic polymer can, for example, constitute a volume fraction of approximately 85 vol%, and in other embodiments, approximately 77.5 vol%. This means that only the remaining few vol% are sufficient for the electrically conductive components, such as silver particles. This allows for a fully functional and reliable electrical connection while simultaneously reducing the consumption of electrically conductive materials compared to conventional electrical contact pastes.

[0042] In one embodiment, the at least one busbar is applied to the solar cell as a thermoplastic screen printing paste and / or stencil printing paste using screen printing and / or stencil printing. The screen printing paste and / or stencil printing paste comprises a thermoplastic polymer dissolved in a solvent and filled with electrically conductive particles. The screen printing paste and / or stencil printing paste is dried to remove the solvent immediately after printing and / or before the application of the conductive wire. Currently, approximately 98% of all cell metallizations are applied using screen printing. Therefore, this method is well-established in solar cell manufacturing and requires relatively few adjustments when switching to the method according to the invention. Similarly, stencil printing can also be used for application.Applying busbars made of electrically conductive thermoplastic material requires no process modification, as printing this material can be directly integrated into the cell manufacturing process. First, the contact fingers are printed. In the subsequent printing process, the busbar is applied using the electrically conductive thermoplastic material. This can be done with thermoplastic screen printing paste, which consists of a thermoplastic polymer dissolved in a suitable solvent and filled with electrically conductive particles. One or more different additives can also be incorporated into the screen printing paste. These can, for example, improve and / or ensure the storage stability, homogeneity of the filler distribution, and / or printability of the screen printing paste.In particular, the screen printing paste can contain rheology additives and / or thixotropic and / or dispersing agents and / or thickeners and / or conductivity enhancers. After the screen printing paste is applied, it can be dried at temperatures of, for example, between 100 and 250 °C to remove the solvent. What remains is only the applied, electrically conductive, and thermoplastic busbar, which, upon increasing the temperature, returns to an adhesive state for contacting with conductive wire. This allows solar cells to be connected to form modules.

[0043] In one embodiment, at least one busbar is applied to the solar cell as a thermoplastic dispensing paste. The dispensing paste comprises a thermoplastic polymer dissolved in a solvent and filled with electrically conductive particles. Before the conductive wire is applied, the dispensing paste is dried to remove the solvent. The busbars made of this electrically conductive thermoplastic material can then be applied via dispensing. Similar processes are used for applying contact fingers or in the production of other electronic components, making this a well-established manufacturing method. The rheological properties of the dispensing paste can be pre-adjusted to the dispensing process to ensure good and / or optimized flow behavior during application.The dispensing paste, particularly the solvent, may contain additives, especially rheology additives and / or thixotropic and / or dispersing agents and / or thickeners and / or conductivity enhancers. After application, the dispensing paste is dried, just like the paste used for screen printing, before the thermoplastic busbars are used for contacting with conductive wire.

[0044] In one embodiment, the at least one busbar is applied to the solar cell as a solvent-free composite material using filament printing. Here, the electrically conductive, thermoplastic material is formulated as a solvent-free formulation. It can be applied, for example, as an electrically conductive thermoplastic filament using a process similar to material extrusion-based 3D printing and / or with a hot glue gun. In particular, the material can be applied using such a 3D printing process and / or with a hot glue gun. Application as a solvent-free formulation can enable high storage stability. This eliminates the need for solvent drying, which can accelerate the manufacturing process. This can reduce equipment costs, response time, and / or maintenance and cleaning times, thus improving the manufacturing process.

[0045] In one embodiment, the at least one busbar is applied to the solar cell as a thermoplastic transfer paste using pattern transfer printing. The transfer paste comprises a thermoplastic polymer, is dissolved in a solvent, and is filled with electrically conductive particles. Before the conductive wire is applied, the transfer paste is dried to remove the solvent. The transfer paste can be released from grooves in a plastic film positioned over the solar cell by local heating, e.g., using a laser beam, and thus deposited on the first side. The transfer paste used here can also contain additives, in particular solvents. After application, the transfer paste is dried before the thermoplastic busbars are used to attach the conductive wire.

[0046] In one embodiment, at least one busbar is applied to the first side of the solar cell with a layer thickness of at most approximately 50 pm, in particular from about 10 pm to about 30 pm. The layer thickness corresponds to the thickness and / or height of the busbar on the first side of the solar cell, i.e., approximately in the direction of a normal to the first side of the solar cell. This small layer thickness saves material and is sufficient to ensure reliable wire fixation and adequate electrical contact of the solar cell.

[0047] In one embodiment, at least one busbar with a layer width of at most approximately 200 pm, in particular from approximately 50 pm to approximately 100 pm, is applied to the first side of the solar cell. This layer width corresponds to an extent of the busbar transverse to the direction of the busbar's conduction, i.e., approximately perpendicular to the second preferred direction, and simultaneously approximately parallel to the surface plane of the first side of the solar cell. This small layer width saves material for the busbars and is sufficient to ensure reliable attachment of the wires when contacting the solar cell.

[0048] In one embodiment, at least one busbar made of electrically conductive thermoplastic material is applied directly to the first side of the solar cell, i.e., without first forming a busbar made of another material at that location. Thus, for example, no metallized base for the busbar, such as a high-metallic paste, is applied beforehand; instead, the busbar is applied directly to the first side of the solar cell, possibly onto the already formed contact fingers, which may be arranged, for example, perpendicular to the busbar on the first side. This saves material that would otherwise be required for conventional, highly metallized busbars.

[0049] In one embodiment, the electrically conductive wire is attached to the first side of the solar cell without soldering, solely by means of thermoplastic polymers of the electrically conductive thermoplastic material. The wire is attached exclusively by the tacky, softened polymers of the applied busbar, specifically without any additional solder or ECAs. This allows for material savings, as the electrically conductive thermoplastic material can be used for multiple purposes: as the material of the busbars, for attaching the wires, and for creating the electrical contacts.

[0050] In one embodiment, several busbars are formed approximately parallel to each other, particularly at equidistant intervals, on the first side of the solar cell from the electrically conductive thermoplastic material. Due to the parallel arrangement, the busbars do not contact each other directly, but rather each collects the charge of the contact fingers that cross them. Large solar cells, in particular, can have a large number of busbars.

[0051] In one embodiment, the contact fingers are formed parallel to each other along a first preferred direction on the first side of the solar cell, and the at least one busbar is formed along a second preferred direction on the first side of the solar cell, the second preferred direction being arranged approximately perpendicular to the first preferred direction. This arrangement enables efficient, planar dissipation of the charge carriers from the solar cell.

[0052] In one embodiment, a plurality of solar cells are provided, on each of which at least one busbar made of the electrically conductive thermoplastic material is applied. The same conductive wire is mechanically attached to the solar cells by means of the electrically conductive thermoplastic materials of the busbars, so that it electrically contacts all of the solar cells. This allows for effective electrical interconnection.

[0053] One aspect concerns a solar cell with electrical contacts and interconnection. Contact fingers can be formed on the first side of the solar cell, receiving charge carriers generated within the cell. At least one busbar is configured on the first side of the solar cell to receive charge carriers generated within the cell, for example, by electrically contacting several of the contact fingers and conducting current from the contact fingers to interconnection wires of the solar cell. This busbar, at least one of which is made of an electrically conductive thermoplastic material, is applied to the first side of the solar cell. A conductive wire is mechanically attached to the solar cell within the busbar, which is initially softened and then cooled, thus providing electrical contact to the solar cell.

[0054] In this case, the wire can be attached to the solar cell by means of the thermoplastic polymer contained in the electrically conductive thermoplastic material of the busbar, such that it makes electrical contact with it.

[0055] The solar cell can be electrically contacted using the method described above. Therefore, all descriptions of the method also apply to the solar cell and vice versa. Within the scope of this invention, the terms "essentially" and / or "approximately" may be used to include a relative deviation of up to 5% from a numerical value following the term, a deviation of up to 5° from a direction following the term, and / or from an angle following the term.

[0056] Preferred embodiments of the invention are described below by way of example. Here, identical or similar reference numerals can denote identical or similar features of the embodiments. Individual elements of the described embodiments are not limited to the respective embodiment. Rather, individual elements of the embodiments can be combined with one another to create new embodiments. This shows:

[0057] Figure 1 shows a conventional, metallized solar cell in a schematic representation;

[0058] Figure 2A shows a schematic representation of a section of a solar cell according to an exemplary embodiment;

[0059] Figure 2B shows an enlarged section of the solar cell shown in Fig. 2A in a perspective sectional view;

[0060] Figure 3 shows a section of a solar cell according to an exemplary embodiment in a photograph; and

[0061] Figure 4 shows a schematic flowchart of a method for contacting a solar cell.

[0062] Figure 1 shows a schematic representation of a conventional solar cell 100 as a heterojunction half-cell, in which several contact fingers 110 are formed parallel to each other on a first (top) side of the solar cell 100. Approximately perpendicular to these, several busbars 111 are formed on the top side, which electrically contact the contact fingers 110 and collect charge carriers from them. Solder pads 112 are arranged at regular intervals along the busbars 111.

[0063] The solar cell shown in Figure 1 consumes a relatively large amount of silver as a rare resource, since the contacting fingers 110, the busbars 111 and the solder pads 112 are made of highly filled silver / duromer composites or sintered pastes applied as pastes.

[0064] Figure 2A shows a schematic representation of a section of the first side of a solar cell 1 according to an exemplary embodiment. Contact fingers, which are not shown in Figure 2A, can be formed on the first side. These contact fingers can be arranged on the first side of the solar cell 1 like the contact fingers 110 shown in Figure 1, i.e., as thin, equidistant, and parallel strips, e.g., aligned in a first preferred direction along the first side of the solar cell 1. The contact fingers 110 can, for example, have a maximum layer width (definition as above for busbars) of approximately 40 pm.

[0065] In some embodiments, the solar cell 1 does not have contact fingers. Instead, a suitable coating can be applied, at least on the first side.

[0066] The first side of solar cell 1 can be, for example, the side of solar cell 1 facing away from the sun in its operating position, or in some cases, the side facing the sun in its operating position. Solar cell 1 is regularly electrically contacted and connected on both its sun-facing and sun-facing sides using the method described here.

[0067] Busbars 11 are arranged on the first side of solar cell 1. The busbars 11 are made of an electrically conductive, thermoplastic material and applied to the first side of solar cell 1.

[0068] The electrically conductive thermoplastic material may consist of a thermoplastic polymer which is infused and / or filled with conductive particles.

[0069] In a first embodiment, the electrically conductive thermoplastic material is composed as follows:

[0070] - approximately 0.85 volume parts polyamide;

[0071] - approximately 0.15 parts by volume of silver; and

[0072] - less than approximately 0.01 volume parts of ionic liquid.

[0073] In a second embodiment, the electrically conductive thermoplastic material is composed as follows:

[0074] - approximately 0.775 volume parts polyamide; and

[0075] - approximately 0.225 volume parts carbon.

[0076] In both embodiments, polyamide is used as a thermoplastic polymer, which gives the material its thermoplastic properties. In the first embodiment, silver particles are used as electrically conductive particles, while in the second, carbon particles are used. Both types of particles contribute to the electrical conductivity of the material. In the first embodiment, an ionic liquid is also used as a solvent, which can be used to apply the busbars 11. In the second embodiment, no solvent is used. Generally, the material can be used without a solvent when silver particles are used, just as it can be used with a solvent when carbon particles are used.

[0077] The thermoplastic busbar 11 is formulated to contain less silver than conventional metallization pastes with a silver content of 60 to 80 vol%. The lower fill level of the busbar 11 saves resources (and thus costs) and ensures sufficient adhesion between the wire 13 and the busbar 11, as well as between the busbar 11 and the solar cell 1. The electrically conductive thermoplastic busbars 11 can be applied in various ways, with different application methods requiring correspondingly adapted formulations, e.g., with or without solvents. For example, the busbars 11 can be applied by a dispensing process, a screen printing process, and / or a material extrusion-based filament printing process. In the embodiment shown in Figure 2A, the busbars 11 are designed as continuous busbars 11.Each busbar 11 extends on the first side of the solar cell 1 from a first edge of the solar cell 1 continuously to a second (e.g. opposite) edge of the solar cell 1.

[0078] Figure 2B shows a perspective sectional view of an enlarged section of the solar cell shown in Figure 2A. It shows in particular that an electrically conductive wire 13, e.g., a copper wire, is applied to each busbar 11. The wire 13 is bonded to the electrically conductive, thermoplastic material of the busbar 11. For this purpose, the busbar 11 can be heated above its thermoplastic melting and / or softening temperature, making it adhesive, in order to press the wire 13 into and / or onto the adhesive busbar 11 and bond it there. The busbar 11 can then be cooled again, during which time it hardens.

[0079] The busbar 11 is conductive enough to collect charge carriers from the contact fingers and / or the solar cell 1 and also to electrically connect the wire 13 to the solar cell 1. Current measurements have shown that this provides effective metallization of the solar cell 1. Furthermore, due to its thermoplastic properties, the busbar 11 is adhesive enough to mechanically connect the wire 13 to the solar cell 1, i.e., to stably and / or reliably attach the wire 13 to the solar cell 1.

[0080] In an alternative embodiment, so many thin busbars 11 with thin wires 13 are applied to the first side that the contacting fingers can be dispensed with.

[0081] Figure 3 shows in a photograph a section of an applied busbar 13, which is applied to the first side of a solar cell 1 according to an embodiment.

[0082] Since the solar cell 1 shown in Figure 1 is a prototype from a feasibility study, the busbars, which appear quite wide overall, do not yet have their intended width. In fact, they are significantly too wide to actually save resources. Much narrower busbars 11 would be sufficient.

[0083] Such a narrower busbar (core) is marked with reference numeral 11 in Fig. 3. The thin, essentially straight busbar 11 is sufficient for attaching and connecting the wire 13. Excess material 14 is shown on both sides of the busbar 11 in Fig. 3; this excess material was originally applied to the prototype and is not needed. In the actual embodiment, this excess material is not applied to the solar cell 1 at all, as a more precise application method is then used.

[0084] The thermoplastic electrically conductive busbar 13 is applied in thin layers, especially with layer thicknesses of about 10pm to about 30pm, and in small widths, especially in layer widths of about 50pm to about 100pm, which saves resources such as silver particles.

[0085] In comparison, the previously known TECC-Wire process requires at least three times the amount of polymer and conductive filler. With TECC-Wire, wires are completely coated with conductive hot melt adhesive and then bonded in place; approximately three-quarters of the wire coating is not required for mechanical fastening and electrical contact. For a mechanically stable and electrically conductive connection, it is sufficient to apply the adhesive only to the contact area between the wire and the solar cell. The thermoplastic busbars 11 according to the invention offer this advantage because they are applied to the solar cell 1, e.g., printed, instead of being applied to the wire 13. This ensures that the adhesive is applied only to the technically necessary areas. In this way, significant amounts of material and thus costs can be saved.In addition, large quantities of organic solvents, which are needed for coating the wires with the electrically conductive thermoplastics and subsequently have to be removed with high energy expenditure, can be saved.

[0086] Due to the relatively low particle density of the material used and the thermoplastic polymer, the busbar 11 can be reversibly bonded to a state that allows it to function not only as a current collector but also as a mechanical fastener for the wires 13 on the solar cell 1. Because of these two effects, the invention eliminates the need for conventional silver busbars (i.e., busbars with a high silver content of 40 vol.% or more), solder pads, solder material, and / or ECAs.

[0087] An advantage of the invention is that no solder or additional adhesive is required when connecting solar cells, since the busbar 11 applied during metallization has thermoplastic adhesive properties. This advantage can be utilized both when connecting solar cells with conductive wires and when shingling solar cells.

[0088] The invention enables both process simplification, since - depending on the conventional comparison method - process steps can be saved, and the saving of valuable resources.

[0089] For a solar cell 100, as shown in Figure 1, the printing costs for the busbars 111 and the solder pads 112 amount to approximately 1.05 to 1.3 cents / watt, depending on the current market price for silver. The busbars 111, which are highly silver-filled, are used for the solder joint with sufficient mechanical strength. In thermoplastic bonding of solar cells to wire, the high silver content is not necessary, which is why these costs can be reduced by about half in the inventive method based on the thermoplastic busbars 11.

[0090] In one embodiment, instead of continuous busbars 11, only slightly wider pads are formed on the linear contact fingers. This means that in this embodiment, the busbars 11 are not formed as continuous busbars 11 on the first side 11 of the solar cell 1, as in the embodiment shown in Fig. 2, but as segmentally interrupted busbars 11.

[0091] This can enable further material and / or cost savings. Additionally, costs are reduced by eliminating soldering materials or ECAs.

[0092] In general, the process speed can be increased because no further dosing and drying process of adhesive and / or solder material is required during cell interconnection, which can lead to further cost reductions.

[0093] Due to the relatively low particle fill level used and the thermoplastic polymer of the electrically conductive thermoplastic material, the busbar 11 can be reversibly bonded to a sticky state, enabling it to function not only as a current collector but also as a mechanical fastener for wires on the solar cell. Because of these two effects, silver busbars, solder pads, solder material, and / or ECAs can be completely omitted, at least in some embodiments.

[0094] In a feasibility study, the inventors developed a first electrically conductive, thermoplastic busbar paste as a material and applied it to heterojunction silicon solar cells on a laboratory scale using a dispensing process. This is shown in more detail in Fig. 3. After applying the busbars 11 by dispensing, the busbar paste was dried at 200 °C for 5 minutes to remove the solvent. After subsequent cooling, the solar cell was placed on a hot plate, heated above the melting and / or softening temperature of the polymer, and thus the thermoplastic busbars 11 were returned to an adhesive state.

[0095] In this state, a copper wire, designated as wire 13, could be pressed into the busbar 11. After a short cooling phase of the entire system, a mechanically stable connection was established between the busbar 11 and the solar cell 1. Adhesion between wire 13 and the solar cell 1 was achieved by means of the thermoplastic busbars 11, which withstood the usual tensile loads on the wire 13.

[0096] As shown in Figure 3, the thermoplastic busbars 11 arranged on the solar cell 1 were significantly wider than ultimately intended due to their very low viscosity. This is because the viscosity was not explicitly adjusted for the printing process. Furthermore, the application during the feasibility study was still done manually and with a relatively large nozzle opening.

[0097] By using an improved dispenser, for example one with a smaller nozzle opening, the busbars 11 can be applied more precisely and / or with a significantly narrower width. In particular, production can be automated for this purpose. For example, a robot can be used for application, which automatically and / or controllably applies the busbars 11 to a predetermined target thickness and / or width.

[0098] The feasibility study further demonstrated that the applied busbars 11 exhibit a high electrical conductivity of 832 ± 28 S / cm and / or low resistance. This was verified using a conductivity measuring device along the busbars 11. Furthermore, the feasibility study achieved an adhesion of at least 1.3 ± 0.3 N / mm (line load).

[0099] Fig. 4 shows in a flowchart individual steps of an embodiment of the method.

[0100] In this process, a solar cell is first provided in a provisioning step 20, e.g. a solar cell 1 as shown in Fig. 2 .

[0101] In an optional first development step 21, the contact fingers are formed on the first side of the solar cell, e.g., similarly to the contact fingers 110 shown in Fig. 1. This can be done, for example, using a conventional screen printing process. In some embodiments, no contact fingers are formed on the solar cell, but instead, for example, a suitable coating is applied. The process is fundamentally applicable to any type of solar cell on which at least one electrically conductive line, such as a busbar, is to be applied for electrical contact. In a second development step 22, the busbars are formed from the electrically conductive thermoplastic material on the first side of the solar cell, e.g., similar to the busbars 11 shown in Fig. 2A or 3. This can be done, for example, by dispensing, a screen printing process, and / or by material extrusion-based filament printing.

[0102] In a heating step 23, the applied busbars are heated above the thermoplastic melting and / or softening temperature of the polymer contained in the material, causing them to become sticky.

[0103] In application step 24, the conductive wires are applied to the heated, applied busbars, for example, similar to the wire 13 shown in Fig. 2. In particular, a dedicated wire can be applied to each busbar. This establishes both mechanical and electrical contact between the wire and the solar cell via the busbars.

[0104] In a cooling step 25, the busbars and / or the entire metallized solar cell are cooled below the melting and / or softening temperature of the thermoplastic polymer, in particular to room temperature and / or operating temperature.

[0105] This provides a connected solar cell from which electrical energy can be drawn via the wires, especially at the interconnection wires of the solar cell.

[0106] Reference symbol list

[0107] I Solar cell

[0108] II Busbar

[0109] 13 wires

[0110] 14 Excess material

[0111] 20. Setting up a solar cell: Forming contact fingers, forming busbars, heating busbars, applying a wire, cooling. Solar cell, contact fingers, busbar, solder pad

Claims

Applicant: Karlsruhe Institute of Technology Public corporation MB&P code: K07802WO - hy / mu Patent claims 1. Method for electrically contacting and interconnecting a solar cell (1 ) comprising the steps: Providing (20) the solar cell (1); and Forming (22) at least one busbar (11) on a first side of the solar cell (1) such that the at least one busbar (11) generated in the solar cell accepts charge carriers; wherein: - which at least one busbar (11 ) made of an electrically conductive thermoplastic material is applied to the first side of the solar cell (1 ); - at least one applied busbar (11) is heated to a temperature above its thermoplastic melting and / or softening temperature (23), and in this heated state a conductive wire (13) is applied to the at least softened, applied busbar (11); and - in which at least one busbar (11) is subsequently cooled (24) such that the conductive wire (13) is mechanically attached to the solar cell (1) by means of the electrically conductive thermoplastic material of the busbar (11) and the solar cell (1) is electrically contacted.

2. The method of claim 1, wherein the electrically conductive thermoplastic material comprises a thermoplastic polymer which is provided with electrically conductive particles.

3. Method according to claim 2, wherein the electrically conductive particles comprise silver particles and the silver particles constitute a volume fraction of at most 30 vol.%, in particular at most 20 vol.%, of the applied electrically conductive thermoplastic material.

4. Method according to claim 2 or 3, wherein the electrically conductive particles comprise carbon particles and the carbon particles constitute a volume fraction of at most 35 vol.%, in particular at most 25 vol.%, of the applied electrically conductive thermoplastic material.

5. Method according to any one of claims 2 to 4, wherein the thermoplastic polymer constitutes a volume fraction of at least 65 vol.%, in particular at least 80 vol.%, of the applied electrically conductive thermoplastic material.

6. A method according to any of the preceding claims, wherein the at least one busbar (11) is applied to the solar cell (1) as a thermoplastic screen printing paste and / or stencil printing paste by means of screen printing and / or stencil printing, wherein the screen printing paste and / or the stencil printing paste comprises a thermoplastic polymer, is dissolved in a solvent and is filled with electrically conductive particles; and wherein the screen printing paste and / or the stencil printing paste is dried prior to the application of the conductive wire (13) to remove the solvent.

7. A method according to any one of claims 1 to 5, wherein the at least one busbar (11) is applied to the solar cell (1) as a thermoplastic dispensing paste by means of dispensing, wherein the dispensing paste comprises a thermoplastic polymer, is dissolved in a solvent and is filled with electrically conductive particles; and wherein the dispensing paste is dried prior to the application of the conductive wire (13) to remove the solvent.

8. A method according to any one of claims 1 to 5, wherein the at least one busbar (11) is applied to the solar cell (1) as a thermoplastic transfer paste by means of pattern transfer printing, wherein the transfer paste comprises a thermoplastic polymer, is dissolved in a solvent and is filled with electrically conductive particles; and wherein the transfer paste is dried prior to the application of the conductive wire (13) to remove the solvent.

9. Method according to any one of claims 1 to 5, wherein the at least one busbar (11 ) is applied to the solar cell (1 ) as a solvent-free composite material by means of filament printing.

10. Method according to one of the preceding claims, wherein the at least one busbar (11 ) is applied to the first side of the solar cell (1 ) with a layer thickness of at most about 50pm, in particular from about 10pm to about 30pm.

11. Method according to one of the preceding claims, wherein the at least one busbar (11 ) is applied to the first side of the solar cell (1 ) with a layer width of at most about 200pm, in particular from about 50pm to about 100pm.

12. Method according to one of the preceding claims, wherein the at least one busbar (11 ) made of the electrically conductive thermoplastic material is applied directly, i.e. without first forming a busbar (11 ) made of another material at this location of the solar cell (1 ), to the first side of the solar cell (1 ).

13. Method according to one of the preceding claims, wherein the electrically conductive wire (11) is attached to the first side of the solar cell (1) without soldering, solely by means of thermoplastic polymers of the electrically conductive thermoplastic material.

14. Method according to one of the preceding claims, wherein several busbars (11 ) are formed approximately parallel to each other on the first side of the solar cell (1 ) from the electrically conductive thermoplastic material.

15. Method according to one of the preceding claims, wherein a plurality of solar cells (1 ) are provided, on each of which at least one busbar (11 ) made of the electrically conductive thermoplastic material is applied, and the same conductive wire (13) is mechanically attached to the solar cells (1 ) by means of the electrically conductive thermoplastic materials of the busbars (11 ) so that it electrically contacts all of the solar cells (1 ).

16. Solar cell (1 ) with electrical contacting and interconnection, comprising: at least one busbar (11 ) on a first side of the solar cell (1 ), which accepts charge carriers generated in the solar cell; wherein: - which at least one busbar (11 ) made of an electrically conductive thermoplastic material is applied to the first side of the solar cell (1 ); - in the electrically conductive thermoplastic material of the busbar (11) which is initially softened and then cooled down, a conductive wire (13) is mechanically attached to the solar cell and electrically contacts the solar cell.

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