EP2786420A2

EP2786420A2 - Solar cell and process for producing a solar cell

Info

Publication number: EP2786420A2
Application number: EP12812364.3A
Authority: EP
Inventors: Wilhelm Stein
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-12-01
Filing date: 2012-11-14
Publication date: 2014-10-08
Also published as: TW201340361A; WO2013080072A3; US20150027527A1; WO2013080072A2; DE102011055912A1; CN104185904A

Abstract

The present invention relates to a solar cell and to a process for producing such a solar cell which involves producing a solar cell core in which a charge separation and a charge transfer to an emitter and to a base located on a side of the solar cell that is opposite from the emitter are provided when there is incident light in a front side of the solar cell; which involves producing an electrically conductive emitter-contact structure in the form of contact fingers that are in direct electrical contact with the emitter; and which involves producing a solderable metallic emitter-terminal structure in the form of conductor bars that are in direct electrical contact with the emitter-contact structure and transversely connect the contact fingers of the emitter-contact structure; wherein the emitter and the base are contacted on opposite sides of the solar cell. The object of the present invention is to propose a simply constructed and reliably operating solar cell of the generic type mentioned and a corresponding solar cell production process with which solar cell production costs can be reduced. This object is achieved by a solar cell and a solar cell production process of the generic type mentioned, wherein the solderable metallic emitter-terminal structure is produced at least from nickel, a nickel alloy, tin and/or a tin alloy, and a solder or an electrically conductive strip arrangement covered with a solder is applied to this emitter-terminal structure.

Description

Solar cell and method for producing a solar cell

The present invention relates to a method for producing a solar cell, comprising the steps of: producing a solar cell core, in which, upon incidence of light in a front side of the solar cell, a charge separation and a charge transport to an emitter and to a base located on a side of the solar cell opposite the emitter is provided; Producing an electrically conductive emitter contact structure in the form of contact fingers which are in direct electrical contact with the emitter; and manufacturing a solderable metallic emitter terminal structure in the form of bus bars which are in direct electrical contact with the emitter contact structure and which transversely connect the contact fingers of the emitter contact structure; wherein emitter and base are contacted on opposite sides of the solar cell. The invention further relates to a solar cell with a solar cell core, in which, upon incidence of light in a front side of the solar cell, charge separation and charge transport to an emitter and to a base located on a side of the solar cell opposite the emitter is provided; an electrically conductive emitter contact structure that is in direct electrical contact with the emitter; and a solderable emitter metal terminal structure that is in direct electrical contact with the emitter contact structure; wherein emitter and base are contacted on opposite sides of the solar cell.

Methods of the type mentioned are established in the industrial production of solar cells. For example, according to this method, standard solar cells are produced in which the solar cell core has a base of boron-doped crystalline silicon and a phosphorus-doped emitter located on the front side of the solar cell. The emitter of this solar cell is typically contacted by contact fingers made of silver. The contact fingers are interconnected by busbars, often referred to as busbars. On these busbars are soldered to the external interconnection of the solar cell connecting bands.

In a standard solar cell, the contact fingers and the busbars according to the prior art are produced in a common process step by screen printing from a silver paste. In this case, the base, which is typically opposite the emitter on the back of the solar cell, for example, by a Contacted aluminum layer, wherein the aluminum layer forms the base contact structure. Since aluminum is not solderable, a solderable silver connection structure is applied to this base contact structure.

The problem with the production of standard solar cells according to previously known technologies are their high production costs. The silver pastes used in the production of standard solar cells represent a significant proportion of the production costs. Therefore, there are approaches in the prior art to reduce the production costs of solar cells, in particular by limiting the consumption of silver.

From the document DE 10 2010 014 554 A1, a solar cell is known in which a copper wire is used as a busbar instead of a silver line, which is angalvanisiert to the contact fingers. A disadvantage of this solar cell and the associated production process are the need for special, suitable for the process plating. Furthermore, there is the danger when using copper that this differs into the silicon of the solar cell. To prevent this, very good barriers are needed between silicon and copper. Also problematic is the connection of serving as a busbar copper wires with the solar cell only at the small overlap areas with the contact fingers. Due to the small contact surfaces, the reliability of such solar cells is questionable.

Furthermore, from the document DE 10 2009 016 268 A1 rear-side contact solar cells, for example so-called interdigitated backside contact cells (IBC), known in which both the base and emitter are contacted on the solar cell back side, instead of the known Thick-film silver layers Multi-layer thin-film metallizations are used. A disadvantage of these solar cells is that their production is very complex and associated with correspondingly high costs. The high process costs arise, for example, from the relatively filigree backside structures, in which a separation of n- and p-areas must be ensured and therefore can only be realized with greater effort than in the production of standard solar cells.

It is therefore the object of the present invention to propose a simply constructed and reliably functioning solar cell of the above-mentioned type, which is more cost-effective in comparison with solar cells from the prior art. Furthermore It is an object of the invention to propose a corresponding manufacturing method for producing such a solar cell.

This object is achieved by a method of the type mentioned in the introduction, in which the solderable metallic emitter connection structure is produced at least from nickel, a nickel alloy, tin and / or a tin alloy, and to this emitter connection structure a solder or a coated with a solder, electrically conductive band assembly is applied.

The method according to the invention differs from conventional standard solar cell production methods, in which the emitter contact structure and the emitter connection structure on the front side of the solar cell are produced simultaneously in one process step, in that the emitter contact structure and the emitter connection structure are subsequently produced in different process steps be prepared and the emitter terminal structure is not formed from the proven in standard solar cells, very good conductive silver. Instead, according to the invention, a known, relatively poorly conductive material, such as nickel or tin or an alloy of nickel or tin, is used to form the emitter connection structure. This material mainly consists of either nickel or tin, but may also be a nickel-tin alloy. In principle, other alloys of nickel and / or tin, for example with vanadium, can be used. However, the emitter terminal structure need not be formed solely of a nickel, a nickel alloy, tin, and / or a tin alloy-containing layer. Thus, according to the invention, additional layers, such as, for example, an adhesion-promoting layer or a cover layer, can be applied to the solar cell below or above the nickel- and / or tin-containing layer.

Thus, the present invention is a completely unconventional way for solar cell contacting. In this case, expensive materials such as silver are explicitly dispensed with for producing the emitter connection structure, and are oriented to significantly poorer electrically conductive materials such as nickel, tin or an alloy of at least one of these materials. However, the electrical conductivity of this material is readily sufficient to suitably electrically contact the emitter contact structure. In addition, materials such as nickel, tin and / or an alloy of these materials, very good solderability, so that on an emitter terminal structure made of these materials very well a Lot or one with a Lot covered, electrically conductive tape assembly for contacting the solar cell can be applied. In the end, with the method according to the invention, a solar cell can be produced at a significantly lower cost than in the prior art, since according to the invention the use of high-priced materials, such as silver, for producing the emitter connection structure is dispensed with. By the procedure according to the invention, in which, on the one hand, a very good electrical contacting by means of the emitter contact structure and, on the other hand, a very good solderability by means of the emitter connection structure is provided, solar cells with high quality can also be produced.

A further advantage of the method according to the invention over solar cell standard technologies in which silver busbars are used as emitter connection structures is that the emitter connection structure materials proposed according to the invention do not negatively influence the silicon surface. Instead, conventional solar cells have the effect that the silver bus bars provided for soldering penetrate through the silicon nitride provided on the solar cell to the silicon surface. This reduces the efficiency of the solar cell. This is attempted to avoid this in the prior art by printing the contact fingers on the solar cells, subsequently performing a firing step, then applying the silver busbars and then merely drying the silver busbars so that the silver penetrates the silicon core Solar cell can be prevented. Such an effect is not to be feared, in particular if the emitter terminal structure materials proposed according to the invention are sputtered, for example.

As already mentioned above, the solderable metallic emitter terminal structure is as solderable as a conventional emitter terminal structure made of silver, for example. As a result, a solar cell produced by the method of the present invention can be further processed in a standard production process without major production upgrades. The external connection of the solar cell with other solar cells or external terminal poles of solar modules can be done without changes in the process flow. In the method according to the invention, for example, a tin-containing solder and an electrically conductive band arrangement can be applied to the emitter connection structure. The electrically conductive band arrangement can consist, for example, of two copper braided bands, which are soldered over the whole area on two current busbars. In this example assumes the band assembly of two copper bands the function of power collection and current dissipation of the solar cell surface.

In other possible embodiments of the method according to the invention, the current transport in the region of the emitter connection structure can not take place via a conductive band arrangement, but directly via a solder having a sufficiently large cross section, which can dissipate the current generated in the solar cell. The emitter connection structure itself has to transport the current only perpendicularly from the emitter contact structure to the solder or the solder-covered, electrically conductive band arrangement. However, lateral current transport over greater distances is not provided in the emitter connection structure.

The term solar cell core used according to the invention can be seen to different extents depending on the type of solar cell present. In a standard solar cell, the solar cell core typically includes the n-type crystalline core of the solar cell forming the base, the highly n-type emitter layer formed on the front surface of the solar cell by diffusion of phosphorus into the solar cell core, and a highly p-doped solar cell Reaction with back layer made of aluminum. The silicon nitride antireflection coating on the front side serves to minimize optical reflection and is no longer part of the solar cell core.

In another embodiment, the solar cell produced by the method according to the invention may be a so-called heterojunction solar cell, which is produced using a p-type Ausgangsswafers. The solar cell core of the heterojunction solar cell is formed for example by deposition of an undoped, ie intrinsic semiconductor layer, an n-type semiconductor layer and a transparent and conductive surface layer. The transparent conductive surface layer consists, for example, of indium tin oxide. On the one hand it assumes the function of antireflection coating and on the other hand also has electrical functions, such as the formation of the electrical Au ßenkontaktes the solar cell core.

On the front side of the solar cell, the largest possible area for the incidence of light is desired. Accordingly, the emitter connection structure should cover and shade the smallest possible area of the solar cell front side. Therefore, it is favorable on the solar cell front, the emitter terminal structure as narrow as possible train. For good electrical conductivity, therefore, the entire emitter connection structure with the solder or the solder-covered, electrically conductive band arrangement is to be reinforced on the solar cell front side. For solar cells where the emitter is on the back, however, reducing the pad area is not always necessary or desirable. On the rear side of the solar cell, the emitter connection structure can be regularly applied over a large area, in which case only the covering of a partial area of the emitter connection structure with solder or a solder-covered, electrically conductive band arrangement can be provided.

In one embodiment of the method according to the invention, the method further comprises the following method steps: producing a base contact structure which is in direct electrical contact with the base; Producing a solderable metallic base terminal structure that is in direct electrical contact with the base contact structure, at least one of nickel, a nickel alloy, tin and / or a tin alloy; and applying a solder or a solder-covered, electrically conductive tape assembly to the base terminal structure.

In this embodiment of the method according to the invention, the base connection structure is produced by a method which is similar to the method for producing the emitter connection structure according to the invention. Emitter and base are located on opposite sides of the solar cell. The requirements for the emitter connection structure and the base connection structure are largely due to the solderability, the material used for the external connection and the requirements of the solar cell as a whole.

On the front side of a solar cell, the respectively provided connection structure should shade the smallest possible area, irrespective of whether it is a solar cell in which the emitter connection structure or the base connection structure is located on the solar cell front side. As a result, it is also useful here, when the base terminal structure is on the solar cell front side, their structure sizes as low as possible and to realize a good electrical conductivity on the entire base terminal structure, a solder or a solder-covered, electrically conductive To provide band arrangement. On the solar cell rear side, such full-area coverage of the base connection structure is not absolutely necessary. Depending on the sequence of layers in the vicinity of the emitter connection structure or on the base connection structure, specific requirements may exist for the respective contact structure. For example, a barrier effect against copper, the diffusion of which must be regularly prevented in the solar cell core, required. If the neighboring layers of the respective connection structure allow a diffusion of copper into the solar cell core, then the corresponding contact structure itself must be a diffusion barrier for copper. On the other hand, if the respective contact structure is located on other layers, such as on a silicon nitride antireflection layer, which acts as a diffusion barrier for copper, then the corresponding connection structure itself does not have to have a diffusion barrier effect with respect to copper.

In a preferred embodiment of the method according to the invention, the emitter connection structure and / or the base connection structure is produced with a layer thickness of less than 5 μm. In a standard solar cell often emitter or base connection structures made of silver with a layer thickness of about 20 μιη be used. Such a large silver layer thickness is required because solder and silver are well dissolvable and a complete dissolution of the existing silver contact structure should be prevented in order not to damage the solar cell core. In contrast, in the mentioned embodiment of the method according to the invention, the emitter and / or base connection structure is preferably made considerably thinner and with a layer thickness of less than 5 μm. This is possible because nickel has a low solubility in the solder applied to the emitter and / or base terminal structure, and only a few nanometers of a nickel-made terminal structure is dissolved in soldering in the solder. This results in a much lower material consumption than conventional technologies. On the other hand, for the application of such thin layers, methods are also contemplated which, due to the required thick layer thickness, have hitherto not been suitable for reasons of efficiency, such as, for example, sputtering methods.

For reasons of further cost minimization, it is favorable to choose the layer thicknesses as small as possible. Therefore, according to a development of the method according to the invention, the emitter connection structure and / or the base connection structure is preferably produced with a layer thickness between 50 and 500 nm. Depending on the type of solar cell sufficient in the manufacturing process for the emitter terminal structure and / or the base connection structure for the functions of Terminal structure Layer thicknesses between 50 nm and 500 nm. In layer production processes that produce dense and low-defect layers, thinner layer thicknesses are generally sufficient to produce the respective connection structure. However, layer fabrication processes that produce lower quality layers require slightly higher layer thicknesses.

Particularly preferably, according to a further option of the method according to the invention, the emitter connection structure and / or the base connection structure are produced with a layer thickness between 100 nm and 150 nm. These layer thicknesses proved to be optimal in the practical implementation of the method according to the invention, since they are particularly efficient to produce and thus lead to low production costs of the solar cell, moreover allow a very good electrical contacting of the emitter and / or base contact structure and also very good are solderable.

In an advantageous variant of the method according to the invention, the emitter contact structure and / or the base contact structure is produced from a metal paste and / or from an electrolytic bath. From metal pastes cost-effective and fast thick metal layers can be produced. In addition, metal pastes can contain corrosive constituents, so that, for example, during the heat treatment of the metal paste to produce a metal, local etching and opening of an antireflection layer is simultaneously possible. Another inexpensive method for producing thick metal layers or for increasing the thickness of thin seed layers is the production from an electrolytic bath. In the electrolytic bath are metal ions, which are moved by electrical or electrochemical potentials in the direction of the contact structure and deposited there. Electrodeposition may be used to anneal or reinforce otherwise fabricated contact structures.

Preferably, the emitter contact structure and / or the base contact structure is formed with a layer thickness between 5 μιη and 50 μιη. These large layer thicknesses have proved to be favorable for the fulfillment of the procedural and constructive tasks of the emitter and / or base contact structures. Depending on the specific electrical resistance, the emitter and / or base contact structures need a minimum line cross section in order to achieve the required low line resistance of the contact structures. Furthermore, the required layer thickness of the contact structures also depends on that for producing the emitter and / or base layer. Contact structures used from. For example, the production of metal layers from metal pastes is a thick-film process that works flawlessly only with larger layer thicknesses. If the contact structures are made of a material having a lower resistivity than contact structures made of metal pastes, or if the contact structures on the back of the solar cell may occupy a larger area, then layer thicknesses of 5 μm or even less are sufficiently large.

In a particularly suitable variant of the method according to the invention, the emitter contact structure is produced in the form of contact fingers from a silver paste. The production of emitter contact structures from silver pastes on the front side of solar cells is established in the industrial production of solar cells. In the industrially widespread methods, however, not only the contact fingers but also the busbars electrically connected to the contact fingers are made from the silver paste. To produce the relatively wide busbars, much silver is needed in the prior art, which has a significant share of the total cost of the solar cell. With the method according to the invention, the amount of silver used in the production can be considerably reduced by the fact that in particular these busbars, that is to say the emitter terminal structure, are not made of silver, whereby the method according to the invention exhibits its economically advantageous effects. Silver contact fingers are typical for the front of the solar cell, but solar cells with silver contact fingers on the back can also be made.

In a further embodiment of the method according to the invention, the emitter connection structure and / or the base connection structure is produced by a locally occurring layer deposition. For the formation of connection structures on a partial surface of the solar cell, there is in principle the possibility of a full-surface deposition of a layer which is subsequently structured, or the possibility of a locally occurring layer deposition. Preferably, the local layer deposition is used, since this is easier and less expensive. Furthermore, locally occurring layer depositions for the relatively simple emitter and / or base connection structures are also sufficiently accurate. There are several possibilities for localized layer deposition, such as local inkjet printing with metal inks. As a particularly suitable embodiment variant of the method according to the invention, the deposition of the emitter connection structure and / or the base connection structure has proven using a shadow mask with a physical Vakuumabscheideverfahren. Physical vacuum deposition processes, such as sputtering, produce high-quality metal layers characterized by high density and good conductivity. Furthermore, in physical vacuum deposition typically a directional coating occurs, so that the desired structures can be made sufficiently sharp via shadow masks. In principle, however, other deposition methods, for example chemical vapor deposition, laser-assisted gas phase deposition or screen printing of metal pastes, are also possible for the production of emitter and / or base connection structure.

In an advantageous embodiment of the method according to the invention, the emitter connection structure and the base connection structure are produced simultaneously in one method step. The simultaneous implementation of these two process steps, a fast and cost-effective production is achieved. Practically, the simultaneous production takes place for example by the simultaneous operation of two sputtering sources on the front and the back of the solar cell.

According to a favorable configuration of the method according to the invention, the emitter contact structure and / or the base contact structure and in a later method step the emitter connection structure and / or the base connection structure are produced in at least one method step first by screen printing a metal paste. This process variant is particularly suitable for the comparison of the prior art modified production of standard solar cells. In the process, an aluminum paste is first printed over a large area on the back of the solar cell. Subsequently, silver contact fingers are produced by screen printing on the solar cell front side. The busbars which form the emitter connection structure or the base connection structure, however, are not screen printed on the front side of the solar cell. Subsequently, the solar cell is fired to convert the screen-printed pastes into metal layers. On the back side of the solar cell, the aluminum reacts with the silicon, forming a highly doped backside layer in good electrical contact with the aluminum backing layer. On the front side of the solar cell, the fingers are burned into the antireflection layer after the firing step and contact the emitter on the solar cell. Front. Thereafter, in this process variant according to the invention, the connection structures are produced on the front side and on the rear side of the solar cell, for example by sputtering of nickel. Sputtering preferably takes place via a sputtering mask on the front side of the solar cell. The back of the solar cell, however, is preferably coated over a large area, as this eliminates the expense of manufacturing and cleaning a sputtering mask on the back. After the firing step, a final measurement of the solar cell produced is typically carried out. In the method according to the invention, it may be necessary to modify the final measurement since pointed measuring tips could damage the thin nickel layers produced by sputtering.

In another option of the method according to the invention, this is carried out such that in at least one method step firstly the emitter connection structure and / or the base connection structure is produced and in a later method step the emitter contact structure and / or the base contact structure is produced. In this variant of the method, first the emitter connection structure and / or the base connection structure are applied to the solar cell core or an antireflection layer provided thereon. The emitter contact structure and / or the base contact structure is produced later, so that the contact structure does not come into direct contact with the solar cell core in the region of the connection structure. In the area of the busbars on the front side of the solar cells, no contact between the contact structure and the solar cell core is required. Instead, the contact between the contact structure and the solar cell core is rather disadvantageous than desirable, especially with flat doping profiles. The method variant described can be used in various technologies. In this way, for example, in the case of a standard solar cell, the silver contact fingers are prevented from being burned by the antireflection coating when the silver paste is fired in the area of the busbars.

In another embodiment, the process variant set forth above may be used in the production of a heterojunction solar cell. The heterojunction solar cell has temperature-sensitive, thin layers that define upper temperature limits for the manufacturing process. Accordingly, the production process of heterojunction solar cells differs significantly from the production process for standard solar cells. One such difference is the deposition of a transparent, electrically conductive oxide as an antireflection layer instead of a dielectric antireflection coating. The transparent electrically conductive oxide is used as an electrical conductor, so that the firing step for the silver paste in which the silver paste is baked in the dielectric antireflection layer is dispensable. In the production process of heterojunction solar cells, it has also proven useful to produce the emitter connection structure and / or the base connection structure in front of the emitter contact structure and / or the base contact structure.

In a typical embodiment of the method according to the invention, the emitter contact structure and / or the base contact structure are screen printed with a metal-polymer paste, and then a temperature treatment at temperatures below 300 ° C for the conversion of the metal-polymer paste into at least one Metal layer performed. In the production of heterojunction solar cells, the high temperatures of, for example, 850 ° C., which are set when firing silver paste during the production of standard solar cells, are inadmissible. The permitted temperatures are much lower here, for example below 350 ° C. A tried and tested variant for the production of metal structures at the allowed low temperatures is the screen printing of structures using metal polymer pastes, which can be converted by thermal treatment of below 300 ° C into conductive metal structures. However, these metal-polymer pastes, such as silver-polymer pastes, are high-priced products, so there is great interest in minimizing the need for metal-polymer pastes. With the mentioned embodiment of the method according to the invention, the emitter connection structure and / or the base connection structure are produced without the use of metal-polymer pastes, so that only a fraction of the amount of metal-polymer paste compared to a method in which the Terminal structures are made of a metal-polymer paste, is used.

In a preferred variant of the method according to the invention, in at least one method step, the solder is applied to the emitter connection structure and / or the base connection structure with a layer thickness of at least 20 μm or a copper band arrangement covered with solder to the emitter connection structure and / or the base Soldered connection structure. In this way, line cross sections are produced on the connection structures, which are sufficiently large for the conduction of the current generated in the solar cell. In this case, a copper strip arrangement is particularly preferably soldered on, that is, each bus bar is reinforced by a copper strip. The copper band may be a solid copper band. It can However, also be a copper braid, which is mechanically softer and leads to lower mechanical stress on the solar cell. However, the connection structures can also be amplified otherwise, for example, a solder with a large layer thickness can be applied to the connection structures, or the connection structure can be galvanically reinforced.

The object of the invention is further achieved by a generic solar cell in which the solderable metallic emitter terminal structure is formed at least of nickel, a nickel alloy, tin and / or a tin alloy, wherein on the solderable metallic emitter terminal structure a solder or a solder-covered, electrically conductive band assembly is provided.

In the solar cell according to the invention, the front side terminal structure of the solar cell is not made of silver, as is common in the prior art, but instead of nickel, a nickel alloy, tin and / or a tin alloy. These materials are known to have significantly lower electrical conductivity over silver. Because of this, such materials as nickel or tin or their alloys have not been considered at all for the formation of terminal structures on solar cells. However, materials such as nickel, tin and their alloys are characterized by the fact that they are very good solderability. Therefore, the solder applied to the emitter terminal structure according to the invention or the ribbon arrangement covered with the solder can be soldered very well to the emitter terminal structure formed of nickel, tin or a nickel and / or tin alloy. In addition, the electrical conductivity of the emitter terminal structure formed of nickel, tin or their alloys is quite sufficient to produce a suitable electrical contact between the emitter contact structure and the emitter terminal structure. Thus, according to the invention, it is possible to replace the busbars, which are usually made of silver and therefore costly more than 90% in the prior art, by substantially less expensive materials and thus save enormous costs in the production of solar cells.

In the case of a standard solar cell, the emitter is located on the solar cell front side, which is intended for irradiation with sunlight, so that in this type of solar cell the emitter connection structure is to be understood as a front-side connection structure. For other types of solar cells, the emitter terminal structure be provided on the back of the solar cell, wherein the base terminal in solar cells according to the invention is then on the opposite front. In the case where the emitter terminal structure is provided on the rear side of the solar cell, in one embodiment it may be arranged opposite the front side terminal structures in order to compensate mechanical forces from the front side. In another embodiment, however, the emitter connection structure may also be formed over a large area or almost over the entire area, for example in order to achieve a simplified production. In any case, a solder or a solder-covered, electrically conductive band arrangement is provided on the solderable metallic emitter connection structure. The solder or electrically conductive tape assembly forms on the emitter terminal structure a substantial portion of the wire cross-section required to transport the generated photocurrent.

In an advantageous development of the solar cell according to the invention, a base contact structure is provided in direct electrical contact with the base, and in direct electrical contact with the base contact structure, a solderable metallic base connection structure is provided which comprises at least nickel, a nickel alloy, Tin and / or a tin alloy is formed, wherein on the solderable metallic base connection structure, a solder or a solder-covered, electrically conductive band assembly is provided. In this development, not only the emitter contact structure on one side of the solar cell, but also the base contact structure on the other side of the solar cell made of nickel, a nickel alloy, tin and / or a tin alloy is formed. In this way, silver is not needed for any of the two connection structures, and a particularly large saving effect is realized over silver-made connection structures.

In a preferred embodiment of the solar cell according to the invention, the emitter connection structure and / or the base connection structure has a layer thickness of less than 5 μm. The emitter connection structure and / or the base connection structure does not primarily have the task of a conductor, but serves essentially to form a suitable basis for the application of the solder or the solder-covered, electrically conductive band arrangement. To fulfill this task, even small layer thicknesses of 5 μιη or considerably less than 5 μιη suffice. The required layer thickness depends inter alia on the production method used to form the respective connection structure. In manufacturing process, which produce less high-quality layers, greater layer thicknesses are required, and in other manufacturing processes which produce dense layers, substantially thinner layers are already sufficient to form suitable emitter and / or base connection structures. Furthermore, the required layer thickness is determined by the solubility of the material of the connection structure in the solder. The termination structure must be regularly thick enough to avoid complete dissolution of the termination structure in the solder or in the solder-covered, electrically conductive ribbon assembly.

In advantageous embodiments of the solar cell according to the invention, the emitter connection structure and / or the base connection structure have a layer thickness between 50 nm and 500 nm. Particularly preferably, the emitter connection structure and / or the base connection structure have a layer thickness between 100 nm and 150 nm. Fully functional solar cells are produced with these layer thicknesses, whereby the production costs for the connection structures are also low.

In a further embodiment of the solar cell according to the invention, the emitter connection structure and / or the base connection structure is formed from a metal paste and / or an electrodeposited material. The manufacture of a silver paste emitter terminal structure is an established and simple manufacturing process in the industry. The problem here, however, is the high price of silver, which also increases steadily. As metal pastes, however, not only silver pastes can be used, with p-doped emitters and the use of aluminum pastes is possible. Furthermore, in different types of solar cells, other metal pastes, such as copper pastes, are used. However, the emitter terminal structure does not necessarily have to be formed of a metal paste, and other manufacturing methods such as electrolytic deposition are also practicable.

In a preferred embodiment of the solar cell according to the invention, the emitter connection structure and / or the base connection structure has a layer thickness between 5 μm and 20 μm. The required layer thickness for the respective connection structure results from the required line cross section and the permissible line width. On the front of the solar cell narrow lines are required to shade as little solar cell surface. For narrow lines, a correspondingly greater layer thickness is used to realize the required cross-section. On the back of the solar cell, it is regularly not necessary that the lines are formed as narrow as on the solar cell front side. For larger line widths, therefore, lower layer thicknesses are sufficient to produce sufficiently small line resistances.

It is particularly favorable if, in one embodiment of the solar cell according to the invention, the emitter connection structure and / or the base connection structure has contact fingers formed from a silver paste. Terminal structures in the form of contact fingers are easy to control and therefore the preferred structures in the industry. By producing the contact fingers in a conventional manner from silver paste, the solar cell according to the invention can be produced in only slightly modified production lines. As a result, there are only slight resistance in the conversion of production lines for the production of the solar cell according to the invention.

According to a possible variant of the solar cell according to the invention, the emitter contact structure between solar cell core and emitter connection structure and / or the base contact structure between solar cell core and base connection structure is provided. In this embodiment, a direct electrical contact between the solar cell core and the connection structure is produced via the contact structure. This structure corresponds to the usual construction in standard solar cells.

In a preferred alternative embodiment of the solar cell according to the invention, the emitter connection structure between solar cell core and emitter contact structure and / or the base connection structure between solar cell core and base contact structure is provided. In this embodiment, the area under the connection structures is not or only partially used for contacting the solar cell core through the contact structure. This area is also not required for contacting the solar cell core, instead a smaller damage to the solar cell core is effected in the region of the connection structures and better diffusion barriers, for example against the diffusion of copper, are produced.

Preferred embodiments of the present invention, their structure, function and advantages will be explained in more detail below with reference to figures, wherein Figure 1 shows schematically an embodiment of a solar cell according to the invention in cross section;

FIG. 2 schematically shows a view of a front side of a device according to the invention

Showing solar cell in a top view;

FIG. 3 schematically shows a view of a rear side of a device according to the invention

Showing solar cell in a top view;

FIG. 4 schematically shows an alternative embodiment of a device according to the invention

Solar cell in cross-section shows;

Figure 5 schematically shows an alternative embodiment of an inventive

Solar cell in a plan view of the front shows; and

Figure 6 schematically an alternative embodiment of an inventive

Solar cell in a plan view on the back side.

Figure 1 shows schematically an embodiment of a solar cell 1 according to the invention in cross section. The solar cell 1 has a solar cell core 2, which has an emitter 3 and a base 4 on opposite sides of the solar cell core 2. When light enters the solar cell 1, a charge separation and a charge transport to the emitter 3 and to the on the emitter 3 opposite side of the solar cell 1 located base 4. The solar cell core 2, in addition to the layers shown in Figure 1 further layers, such as For example, an undoped layer between the emitter 3 and the base 4 or conductive layers on one of the two surfaces of the solar cell core 2, have.

The emitter 3 is in direct electrical contact with an electrically conductive emitter contact structure 5. In addition, an emitter connection structure 6 is provided on the emitter contact structure 5, which connects the individual lands of the emitter contact structure 5 transversely. As a result of the direct electrical contact, a current can flow in the direction of the solar cell surface out of the solar cell core 2 into the emitter contact structure 5 as far as into the emitter connection structure 6. The term direct electrical contact also includes cases in which the solar cell core 2 has other layers, not shown here, such as, for example, transparent conductive oxides. In the exemplary embodiment illustrated in FIG. 1, the emitter contact structure 5 is located on the front side 9 of the solar cell 1, which is provided for irradiation with light. In the illustrated embodiment, the emitter contact structure 5 is a thin line or finger of silver made of a silver paste. In the illustrated embodiment of the solar cell 1 according to the invention, only the emitter contact structure 5 made of silver is formed, the emitter terminal structure 6, however, is formed of a thin nickel layer. In other, not shown embodiments of the present invention, the emitter terminal structure 6 may also be formed of tin, a nickel alloy or a tin alloy.

In the exemplary embodiment illustrated in FIG. 1, the emitter connection structure 6 consists of three busbars, which are often referred to as busbars. However, the number of busbars shown has no further significance. In practice, any number of busbars for forming the emitter terminal structure 6 is conceivable. In the solar cell 1 according to the invention, a solder or an electrically conductive band arrangement 1 1 covered with a solder is provided on the emitter connection structure 6 made of nickel in this example. A large portion of the electrical conductor cross-section is taken over by the solder or the conductive band arrangement during operation of the solar cell 1, so that the emitter connection structure 6 itself does not have to be a low-resistance electrical conductor.

The emitter terminal structure 6 is a 150 nm thin nickel layer in the illustrated embodiment. However, the emitter terminal structure 6 is not limited to such a layer thickness. In other, not shown embodiments of the present invention, the layer thickness of the emitter terminal structure 6 between 100 nm and 150 nm or between 50 nm and 500 nm or generally below 5 μιη lie. In principle, however, layer thicknesses above 5 μm are also usable according to the invention for the emitter connection structure 6. On the nickel layer used in FIG. 1, it is possible to solder copper strips in the same soldering systems, so-called stringers, in which also strip arrangements can be soldered to standard solar cells of the prior art which have busbars made of silver.

On the back of the solar cell 1 is the base contact structure 7, which is a large-area aluminum layer in the illustrated embodiment. On the Outside the base contact structure 7, a solderable metallic base connection structure 8 is applied. In the exemplary embodiment shown, the base connection structure 8 is designed in the form of three busbars made of nickel. Again, the illustrated three busbars are merely demonstration, with any number of busbars for forming the base connection structure 8 is generally possible. In other embodiments of the present invention, not shown, tin, a nickel alloy or a tin alloy can also be used for the production of the base connection structure 8. In principle, other materials for the production of the base connection structure 8 can be used.

In the embodiment shown in FIG. 1, the emitter terminal structure 6 and the base terminal structure 8 are formed opposite each other by sputtering nickel via sputter masks. In another embodiment, not shown, the base connection structure 8 may also be deposited over the whole area, for example as a tin alloy layer.

FIG. 2 schematically shows the embodiment of the solar cell 1 according to the invention from FIG. 1 in a plan view of the front side 9 of the solar cell 1. In this case, the same elements with the same reference numerals as in Figure 1 are designated. In this view, it is clear that the emitter terminal structure 6 occupies a large area compared to the emitter contact structure 5, which would be associated with a correspondingly high cost in the formation of the emitter contact structure 5 made of silver. In the solar cell 1 according to the invention, however, the emitter terminal structure 6 is formed of inexpensive metals, so that the solar cell 1 according to the invention cheaper than a solar cell, in which both the emitter contact structure and the emitter terminal structure of a high-priced material, such as silver are made , can be produced.

FIG. 3 schematically shows the exemplary embodiment of the solar cell 1 according to the invention from FIGS. 1 and 2 in a plan view of the rear side 10 of the solar cell 1. The base contact structure 7 is applied to the back side of the solar cell 1 over almost the entire area. By contrast, the base connection structure 8 is formed locally in three strips, which are located opposite the strips of the emitter connection structure 6. In the illustrated embodiment, the base terminal structure 8 is made by sputtering over a sputtering mask. In other variants of the method, the strips can also by special linear plasma sources, by punctiform deposition sources, the relative be moved to the solar cell, electrochemically or otherwise deposited.

FIGS. 4, 5 and 6 schematically show an embodiment of an alternative solar cell 1 'according to the invention. 4 shows a cross section through the solar cell 1 ', Figure 5 shows a schematic view of the front side 9 of the solar cell 1' and Figure 6 shows a plan view of the back 10 of the solar cell 1 '. In the embodiment shown, the solar cell core 2 is a heterojunction solar cell in which the base 4 and the emitter 3 are made of different materials. In the illustrated embodiment, the base 4 is a crystalline silicon wafer on which the emitter 3 is deposited as a gradient layer. In this case, the solar cell core 2 also includes an electrically conductive, transparent oxide layer located on the emitter 3 but not shown here, which serves as an antireflection and connection layer. On the solar cell core 2, in this embodiment, first, the emitter terminal structure 6 is formed on the front side 9 by physical vapor deposition. Subsequently, the emitter contact structure 5 is produced by screen printing with a silver-polymer paste and subsequent temperature treatment at 300 ° C. On the rear side 10 of the solar cell 1 'is a substantially full-surface base contact structure 7 made of an aluminum layer and then the solderable metallic base terminal structure 8 as also formed over the entire surface layer.

The illustrated embodiments are merely illustrative and explanatory of the invention. By no means limit the invention to these exemplary embodiments. A person skilled in the art, on the other hand, knows on the basis of his specialist knowledge that the invention can also be realized modified by making use of the present description.

From the exemplary embodiments illustrated in FIGS. 1 to 6, the method used according to the invention for producing the solar cells 1, 1 'can be easily derived. In this method, a solar cell core 2 is first prepared or made available, in which when light enters the front side 9 of the solar cell 1, 1 ', a charge separation and a charge transport to the emitter 3 and to the emitter 3 opposite side 10 of Solar cell 1, 1 'located base 4 is provided. Subsequently, the electrically conductive emitter contact structure 5, which is in direct electrical contact with the emitter 3, is produced. In parallel, the base contact structure 7 can also be produced.

Either after or even before the production of the emitter contact structure 5, the solderable metallic emitter terminal structure 6 is produced, which is in direct electrical contact with the emitter contact structure 5. At the same time or also in a separate step, the base connection structure 8 can be produced on the opposite side of the solar cell 1, 1 '.

Nickel, a nickel alloy, tin and / or a tin alloy are used to produce the emitter connection structure 6. It is also favorable if nickel, a nickel alloy, tin and / or a tin alloy is used for the production of the base connection structure 8.

The emitter connection structure 6 is preferably formed with a relatively small layer thickness. Thus, in one embodiment of the method according to the invention, for example, only 100 to 150 nm thick nickel or nickel vanadium layers can be used as replacement for the silver busbars used in the prior art as emitter connection structure 6.

According to the invention, the emitter connection structure 6 or the busbars on the silicon solar cells produced in the further processing of the cells to modules for cell interconnection, that is, in the so-called stringing, needed in the module process. It is worth mentioning that, despite the changes made according to the invention in the manufacturing process of the solar cell 1, 1 'according to the invention does not necessarily change the further process flow to the finished module, as in other known in the art technologies, which also lower costs Cause silver pastes, that is the case. It can continue to work with standard cell connectors, standard solders and a standard module equipment, especially in the stringer used.

The thin nickel layer which can be used, for example, to produce the emitter connection structure 6, can be deposited, for example, by means of a vacuum deposition, such as a sputtering deposition. The solubility rate of nickel in the standard solder material is much lower than that of silver, which gives the possibility to work with the above-mentioned low layer thicknesses in the manufacture of the emitter terminal structure 6.

In order to achieve a corresponding structuring of the emitter terminal structure 6 already during the deposition, it is advisable to use a shadow mask in the deposition of the layer used in each case for the emitter terminal structure 6, wherein those areas in which the busbars (emitter terminal structure 6 ) are to be produced, exposed and coated accordingly.

As already mentioned above, the rear side 10 of the solar cell 1, 1 'can either be coated over the whole area to produce the base connection structure 8 or can also be formed only at specific positions of the busbars (base connection structure 8) to be produced using a shadow mask.

The respective sputter coating used can take place in a process step by parallel coating from above and below. Preferably, the placement of the respectively used sputter coating system with solar cell wafers is carried out using substrate carriers or carriers, in which the solar cells and their shadow masks are inserted. It is important to make sure that the shadow mask and the solar cell are as flush as possible so that there is a sharp edge and, if possible, no backsputtering in the active area of the solar cell.

In a subsequent method step, the nickel layer strips forming the emitter connection structure 6 are provided, for example, by means of the solder-covered, electrically conductive band arrangement 11 and soldered thereto. The solderability of the thin nickel layers in the stringing process is typically good. Similar pull-off forces are achieved as with pure silver busbars, that is> 2N.

The process flow used according to the invention is designed using back-passivated standard solar cells ("passivated emitter and rear contacts" (PERC) structure) as follows:

Screen printing of the emitter and base contact structures 5, 7 in the form of contact fingers; alternatively, formation of contact finger structures interrupted at the busbar positions

- Firing the contact finger structures - Deposition of the emitter and the base terminal structures 6, 8 via shadow masks on the front and on the back 9, 10 of the solar cell in a vacuum process

- End measurement on the flasher, wherein at the measuring table of the flasher preferably the measuring pins, which are usually placed on the busbars, must be replaced by a measuring strip.

In the production of heterojunction solar cells, the following process flow results according to the invention:

- Coating of the solar cell core 2 with a transparent, electrically conductive oxide (TCO) in the sputtering process

- Discharge

- Setting up a shadow mask

NiV coating on the front and back 9, 10 of the solar cell to form the emitter terminal structure 6 and the base terminal structure 8 (the NiV coating can be done in the same equipment as the TCO coating, but it can also a specially designed PVD system for a double-sided coating using a shadow mask are used)

- Printing the emitter contact structure 5 to form silver contact fingers on the solar cell front 9 (possibly with recess over the nickel busbars)

- drying the silver paste

- Final measurement on the flasher.

Although in the exemplary method variants described above, it has essentially been assumed that the nickel layers used, for example, for producing the emitter and base terminal structures 6, 8 are produced by a PVD method, it is also conceivable to use these layers by means of a Screen printing process. For this, however, appropriate pastes are required. Although printed layers are generally thicker than sputtered layers, this is acceptable in terms of manufacturing costs for the solar cell since the materials proposed by the invention, such as nickel, tin, nickel alloys or tin alloys, are significantly less expensive than the silver used in the prior art , By virtue of the solar cell 1, 1 'proposed according to the invention and the correspondingly proposed method according to the invention, 50 to 80% of the otherwise used silver paste can be saved in the solar cell production process of standard (PERC) solar cells. A particularly significant saving results when the process according to the invention is used for the production of heterojunction solar cells. In this case, more than 50% of the very expensive polymer silver paste can be saved.

The process of the present invention further leads to a marked improvement in the front-side soldering of heterojunction solar cells with an efficient and inexpensive standard cell-bonding process, which has not hitherto been possible in such a form in the prior art. The method of the invention is also characterized by being fully compatible with standard module manufacturing techniques. For example, the soldering can be done on the conventional stringer. A modification of the equipment in the module area is not necessary when using the method according to the invention, the standard equipment can continue to be used unchanged.

Despite the above-mentioned drastic savings possibilities when using the method according to the invention, the quality of the cell connection is comparable to standard soldering techniques. This results in withdrawal forces of> 2 N. In addition, the solar cells 1, 1 'according to the invention have no losses on the back surface field or the passivated back side by the backside busbars, which in the case of the solar cells 1, 1' produced according to the invention results in a comparison with the prior art The technique can result in higher efficiency of about 0.1 to 0.2%.

Claims

claims

1 . Method for producing a solar cell (1, 1 '), comprising the method steps:

- Producing a solar cell core (2), in which upon incidence of light in a front side (9) of the solar cell (1, 1 '), a charge separation and a charge transport to an emitter (3) and to a side opposite to the emitter of the solar cell ( 1, 1 ') located base (4) is provided;

- Producing an electrically conductive emitter contact structure (5) in the form of contact fingers, which are in direct electrical contact with the emitter (3); and

- Making a solderable metallic emitter terminal structure (6) in the form of bus bars, which are in direct electrical contact with the emitter contact structure (5) and connect the contact fingers of the emitter contact structure (5) transversely;

wherein emitter (3) and base (4) are contacted on opposite sides of the solar cell (1, 1 '),

characterized,

in that the solderable metallic emitter connection structure (6) is produced from at least one of nickel, a nickel alloy, tin and / or a tin alloy and to this emitter connection structure (6) a solder or a solder-covered, electrically conductive band arrangement (1 1) is applied.

2. The method according to claim 1, characterized in that the method further comprises the method steps:

- making a base contact structure (7) in direct electrical contact with the base (4);

- making a solderable metallic base terminal structure (8) which is in direct electrical contact with the base contact structure (7), at least of nickel, a nickel alloy, tin and / or a tin alloy; and

- Applying a solder or a solder-covered, electrically conductive tape assembly (12) on the base connection structure (8). A method according to claim 1 or 2, characterized in that the emitter connection structure (6) and / or the base connection structure (8) is produced with a layer thickness below 5 μιη.

Method according to at least one of the preceding claims, characterized in that the emitter connection structure (6) and / or the base connection structure (8) is produced with a layer thickness between 50 nm and 500 nm.

Method according to at least one of the preceding claims, characterized in that the emitter connection structure (6) and / or the base connection structure (8) is produced with a layer thickness between 100 nm and 150 nm.

Method according to at least one of the preceding claims, characterized in that the emitter contact structure (5) and / or the base contact structure (7) is produced from a metal paste and / or from an electrolytic bath.

Method according to at least one of the preceding claims, characterized in that the emitter contact structure (5) and / or the base contact structure (7) is formed with a layer thickness between 5 μιη and 50 μιη.

Method according to at least one of the preceding claims, characterized in that the emitter contact structure (5) is produced from a silver paste.

Method according to at least one of the preceding claims, characterized in that the emitter connection structure (6) and / or the base connection structure (8) is produced by a locally occurring layer deposition.

10. The method according to claim 9, characterized in that the emitter connection structure (6) and / or the base connection structure (8) using a Shadow mask is deposited by a physical vacuum deposition process.

1 1. Method according to at least one of the preceding claims, characterized in that the emitter connection structure (6) and the base connection structure (8) are produced simultaneously in one method step.

12. Method according to claim 1, characterized in that the emitter contact structure (5) and / or the base contact structure (7) are initially produced by screen printing a metal paste in at least one method step and the emitter connection structure (7) in a later method step. 6) and / or the base connection structure (8) is produced.

13. The method according to at least one of claims 1 to 1 1, characterized in that in at least one method step, first the emitter terminal structure (6) and / or the base terminal structure (8) is prepared and in a later method step, the emitter contact structure (5) and / or the base contact structure (7) is produced.

14. The method according to claim 13, characterized in that the emitter contact structure (5) and / or the base contact structure (7) are screen-printed with a metal-polymer paste and then a temperature treatment at temperatures below 300 ° C for the conversion of Metal-polymer paste is performed in at least one metal layer.

15. The method according to any one of the preceding claims, characterized in that applied in at least one method step, the solder on the emitter terminal structure (6) and / or the base connection structure (8) with a layer thickness of at least 20 μιη or a copper band assembly on the Emitter terminal structure (6) and / or the base terminal structure (8) is soldered.

16. Solar cell (1, 1 '), with

a solar cell core (2), in which, when light enters a front side (9) of the solar cell (1, 1 '), a charge separation and a charge transport to a Emitter (3) and to one on a the emitter (3) opposite side of the solar cell (1) located base (4) is provided;

- An electrically conductive emitter contact structure (5) in the form of contact fingers, which are in direct electrical contact with the emitter (3); and

- a solderable metallic emitter terminal structure (6) in the form of bus bars which are in direct electrical contact with the emitter contact structure (5) and connect the contact fingers of the emitter contact structure (5) transversely;

characterized in that the solderable metallic emitter terminal structure (6) is formed at least of nickel, a nickel alloy, tin and / or a tin alloy, wherein on the solderable metallic emitter terminal structure (6) a solder or one with a Lot covered, electrically conductive band assembly (1 1) is provided.

17. Solar cell according to claim 16, characterized in that in direct electrical contact with the base (4) a base contact structure (7) is provided and in direct electrical contact with the base contact structure (7) a solderable metallic base connection structure ( 8) is provided, which is formed at least from nickel, a nickel alloy, tin and / or a tin alloy, wherein on the solderable metallic base connection structure (8) a solder or a solder-covered, electrically conductive band assembly ( 12) is provided.

18. Solar cell according to claim 16 or 17, characterized in that the emitter connection structure (6) and / or the base connection structure (8) has a layer thickness of less than 5 μm.

19. Solar cell according to at least one of claims 16 to 18, characterized in that the emitter connection structure (6) and / or the base connection structure (8) has a layer thickness between 50 nm and 500 nm.

20. Solar cell according to claim 16, characterized in that the emitter connection structure (6) and / or the base connection structure (8) has a layer thickness between 100 nm and 150 nm.

21. Solar cell according to at least one of claims 16 to 20, characterized in that the emitter connection structure (6) and / or the base connection structure (8) is formed from a metal paste and / or an electrodeposited material.

22. Solar cell according to at least one of claims 16 to 21, characterized in that the emitter connection structure (6) and / or the base connection structure (8) has a layer thickness between 5 μιη and 20 μιη.

23. Solar cell according to at least one of claims 16 to 22, characterized in that the emitter connection structure (6) and / or the base connection structure (8) comprises contact pins formed from a silver paste.

24. Solar cell according to claim 16, wherein the emitter contact structure between the solar cell core and emitter connection structure and / or the base contact structure between the solar cell core and base connection structure (7) is provided.

25. The solar cell according to claim 16, wherein the emitter connection structure between the solar cell core and the emitter contact structure and / or the base connection structure between the solar cell core. 2. and base contact structure (7) is provided.