EP3472870A1

EP3472870A1 - Method for interconnecting photovoltaic cells, which have aluminum foil as back contact

Info

Publication number: EP3472870A1
Application number: EP17735407.3A
Authority: EP
Inventors: Henning Nagel; Jonas Bartsch; Mathias Kamp; Markus Glatthaar; Martin Graf; Jan Nekarda
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2016-06-19
Filing date: 2017-06-16
Publication date: 2019-04-24
Also published as: DE102016210910A1; CN109417102A; WO2017220445A1

Abstract

The present invention relates to a method for interconnecting photovoltaic cells, wherein (a) one side of an aluminum foil is treated in such a way with an alkaline, aqueous medium that contains Zn²⁺ that metal zinc is deposited on the treated side of the aluminum foil, thereby forming an aluminum foil with a single-side Zn coating, (b) the of the aluminum foil with the single-side Zn coating is applied to the back of a semiconductor component of a photovoltaic cell and is locally heated with laser radiation such that the aluminum foil connects to the back of the semiconductor component and a Zn-coated aluminum back contact is obtained, and (c) the Zn-coated aluminum back contact is connected to a metal contact of a second photovoltaic cell by means of a metal connector, said metal connector being fastened to the Zn-coated aluminum back contact by means of soldering or gluing.

Description

Process for interconnecting solar cells, the aluminum foil as

Have back contact

Solar cells typically include a semiconductor device comprising a first semiconductor material, a second semiconductor material, and a transition region (e.g., also referred to as a pn junction) between these two semiconductor materials. One of the semiconductor materials or also each of the

Semiconductor materials may be doped. Via a first metal contact, which is electrically connected to the first semiconductor material, and a second metal contact, which is electrically connected to the second semiconductor material, the voltage generated in a solar cell can be tapped.

One of the metal contacts may be mounted on the front or front side of the solar cell (often referred to as a front contact), while the other metal contact is located on the back of the unit cell (often referred to as the back contact). Alternatively, solar cells are also known in which the metal contacts are present exclusively on the back of the solar cell, e.g. in the form of a comb-like interdigital structure. In such exclusively

Contacting back solar cells can minimize shading effects.

The electrical contact on the back of solar cells such as silicon solar cells (back contact) is often a large area by screen printing of Applied aluminum paste. For the interconnection (eg in the form of an electrical series connection) of several solar cells solder pads made of silver paste are additionally printed on the back, are soldered to the connector. The overhanging other end of the connector is soldered to the front of adjacent solar cells, resulting in a series connection of solar cells to so-called solar cell strings.

A less expensive back contact can be made by positioning a metal foil over the back of the solar cell and then by means of

Laser radiation is locally melted, wherein the metal foil with the

Silicon solar cell in these areas connects. In an embodiment described in DE 10 2012 214 253, a cavity filled with a filling medium forms in the adjacent areas between metal foil and solar cell, in which the

Metal foil was not melted, which brings advantageous optical properties for the solar cell. As a metal foil is advantageous manner

Aluminum foil used, because when melting the film by means of laser, a local p ⁺ -Hdopierung is formed in the silicon, the recombination of

Reduced excess charge carriers at these points (so-called local back-surface field).

The electrical series connection of the solar cells with aluminum foil back contact to solar cell strings, as it is necessary for the production of solar modules, but is a challenge because the connectors are not conventionally soldered to the aluminum because of a very fast on the aluminum forming Al ₂ O ₃ layer can.

M. Kamp et. al., "Zincate processes for Silicon solar cell metallization", Solar Energy Materials & Solar Cells, 120, p. 332, 2014 describe a process in which aluminum is first applied via a physical vapor deposition on the back of a solar cell and this aluminum back contact subsequently coated with a metallic zinc by a zincate process. This metallic zinc layer in turn serves as the basis for the electrochemical deposition of a Ni / Cu metal layer stack on which a copper strip is soldered.

An object of the present invention is the interconnection of solar cells via a method with which solar cell connectors can be fastened on the metal contacts of the solar cells as simply and efficiently as possible. Another object is to provide interconnected solar cells having high adhesion between metal contact of the solar cell and solar cell connectors. The

Improvement of the adhesive power should preferably not be at the expense of other properties such as the efficiency of the solar cell.

The problem is solved by a method for interconnecting solar cells, wherein

(A) an aluminum foil is unilaterally treated with an alkaline, aqueous medium containing Zn ²⁺ , so that on the treated side of the

Aluminum foil deposits metallic zinc to form a one-sided Zn-coated aluminum foil,

(b) the unilaterally Zn-coated aluminum foil is applied over its untreated side on the back side of a semiconductor component of a first solar cell and locally heated with laser radiation, so that the aluminum foil connects to the back side of the semiconductor component and a Zn-coated aluminum - back contact is obtained

(c) the Zn-coated aluminum back contact is metallic

Connector is connected to a metal contact of a second solar cell, wherein the metallic connector is attached to the Zn-coated aluminum back contact by soldering or gluing. In the context of the present invention, it has been recognized that the zinc-coated aluminum obtained via a wet-chemical process is a very effective substrate for the attachment of the metallic connector (such as a copper ribbon). It can be realized high adhesion of the connector on the metallic contact of the solar cell. The application of further metallic layers on the galvanized aluminum (eg by electroplating) before soldering the connector is eliminated. Rather, the Zn-coated aluminum already represents a suitable substrate for soldering or Au & life of the connector. The application of the metallic connector by means of conventional

Soft soldering is very possible, with high adhesive forces and low electrical contact resistance between connector and aluminum foil are obtained. Sticking the connector (e.g., using electrically conductive adhesive) also provides high adhesive forces and low electrical contact resistance.

The presence of a metallic zinc layer does not adversely affect the semiconductor materials and the efficiency of the solar cell. However, aluminum foils coated with metals other than zinc are used

Production of the aluminum back contact used, it was found that the efficiency of the solar cell compared to the uncoated aluminum foil is reduced, probably due to incorporation of these metals in the locally highly doped areas. Surprisingly, it was found that these

Deterioration of the semiconductor material properties (e.g., the properties of silicon) does not occur with the process of the present invention. A possible reason for this could be that the deposited zinc evaporates in the areas exposed to laser radiation before the heated areas of the

Connect aluminum foil to the back of the solar cell. In this way, zinc is not or only in a negligible concentration in the

Semiconductor component of the solar cell installed. As already mentioned above, a solar cell includes a semiconductor device comprising a first semiconductor material, a second semiconductor material, and a transition region (eg, also referred to as pn junction) between these two semiconductor materials. One of the semiconductor materials or even each of the semiconductor materials may be doped. Via a first metal contact, which is electrically connected to the first semiconductor material, and a second metal contact, which is electrically connected to the second semiconductor material, the voltage generated in a solar cell can be tapped. In the context of the present invention, the Zn-coated aluminum back contact constitutes one of these

Metal contacts.

Depending on the nature of the solar cell (e.g., crystalline or amorphous silicon solar cell), one of ordinary skill in the art will know how to design the semiconductor device (i.e., type of semiconductor materials to be used, doping, etc.).

The solar cell is preferably a silicon solar cell, for example a monocrystalline silicon solar cell, a polycrystalline silicon solar cell or an amorphous silicon solar cell. However, the method according to the invention is also suitable for the interconnection of other solar cells, e.g. III-V semiconductor solar cells, II-VI semiconductor solar cells, I-III-VI semiconductor solar cells or organic solar cells.

As is well known to those skilled in the art, in the interconnection of solar cells, these are contacted with each other via a metallic connector. The metallic connector is in each case attached to one of the metal contacts of the adjacent solar cells. The interconnection can be a series connection or a parallel connection. A combination of series and parallel connection of the solar cells is possible. As stated above, in step (a) of the process according to the invention, an aluminum foil is treated on one side with an alkaline, aqueous medium containing Zn ²⁺ , so that metallic zinc forms on the treated side of the aluminum foil, forming a one-sided Zn-coated aluminum foil deposits.

The thickness of the aluminum foil is for example in the range of 0.5 μιη to 50 μιη, more preferably in the range of 2 μιη to 20 μιη. Such aluminum foils are commercially available.

The aluminum foil preferably contains aluminum in an amount of at least 80% by weight, more preferably at least 90% by weight. It may be pure metallic aluminum or an aluminum alloy. The purity of the metallic aluminum may vary over a wide range, provided that the electrical conductivity and / or mechanical properties are not adversely affected. For example, the aluminum contains further metallic elements in a total proportion of less than 1% by weight, more preferably less than 0.1% by weight or less than 0.01% by weight. Is the aluminum foil made of one

Made aluminum alloy, it preferably has a proportion of aluminum of at least 80% by weight, more preferably at least 90% by weight. Suitable metallic elements that can be alloyed with the aluminum are known to those skilled in the art.

Preferably, the aqueous medium to which the aluminum back contact is treated has a relatively high concentration of Zn ²⁺ . In a preferred embodiment, the Zn ²⁺ concentration in the alkaline aqueous medium is at least 1.5% by weight, more preferably at least 2.0% by weight, even more preferably at least 3.0% or even at least 4.0% by weight. Zn ²⁺ is in dissolved form, for example, by dissolving a Zn ²⁺ compound in relatively alkaline conditions (ie, relatively high pH) in the aqueous medium. For example, Zn ²⁺ may be present under alkaline conditions as a zincate (eg, [Zn (II) (OH) 4] ^{2 ~} or similar Zn ²⁺ -containing species) in the aqueous medium. This is known in principle to the person skilled in the art.

A suitable pH of the alkaline, aqueous medium is, for example,> 10, more preferably> 13. Optionally, the alkaline aqueous medium may contain further transition metal cations, preferably iron cations, nickel cations or copper cations or a combination of at least two of these Cations, included. In a preferred embodiment, the alkaline aqueous medium still contains Fe cations in a concentration of at least 0.0003% by weight, more preferably at least 0.001% by weight, e.g. in the range of 0.0003-30% by weight. If the alkaline, aqueous medium contains nickel cations, these may be present, for example, in a concentration of 0.1-5% by weight, more preferably 0.5-3% by weight. If the alkaline, aqueous medium contains copper cations, these can

for example, in a concentration of 0.01-1 wt%, more preferably 0.05-0.5 wt%.

The unilateral deposition of the metallic zinc from the Zn ²⁺ -containing aqueous medium on the aluminum foil preferably takes place without current. Electroless metal deposition is a coating process that operates without the use of an external power source.

Electroless deposition of metallic zinc onto an aluminum substrate using an alkaline Zn ²⁺ solution is well known to those skilled in the art. In this process, an Al 2 O 3 layer present on the aluminum is first dissolved. Exposed aluminum is oxidized and enters as aluminate Solution. Zn ²⁺ (eg in the form of zincate) is reduced to metallic Zn, which is deposited on the remaining aluminum.

Too thin a zinc layer can result in poor adhesion of soldered or glued connectors. In addition, for too thin zinc layers there may be a high contact resistance between connector and aluminum back contact. On the other hand, too thick zinc layers could not adhere enough to the aluminum foil. Preferably, the deposited on the aluminum foil Zn layer has a thickness in the range of 0.1 μιη to 5 μιη, more preferably 0.3 μιη to 2.5 μιη on.

Usually, the thickness of the aluminum foil during the Zn deposition step (a) is reduced by a value approximately equal to the thickness of the deposited Zn layer.

The duration of the treatment of the aluminum foil with the alkaline, aqueous Zn ²⁺ -containing medium in step (a) is, for example, 15 s to 250 s.

The zinc deposition step (a) is preferably carried out at a temperature in the range of 5-60 ° C, more preferably 5-45 ° C.

In step (a), the aluminum foil is treated on one side with the Zn ²⁺ -containing aqueous medium. This means that only one side of the aluminum foil is brought into contact with the Zn ²⁺ -containing medium and provided with a metallic Zn layer, whereas on the untreated side of the foil after step (a) no metallic Zn layer is present , As explained in more detail below, the film in step (b) via this untreated, ie no Zn layer having side attached to the back of the semiconductor device of the solar cell. Aluminum foils can have a rough and a smooth side due to their production. An improved light output of the solar cells has resulted when the rough side of the aluminum foil is coated with zinc and the smooth side rests on the back of the semiconductor device. In a preferred embodiment, therefore, the side of the aluminum foil is treated with the aqueous Zn ²⁺ -containing medium whose average surface roughness is greater compared to the opposite side.

In a preferred embodiment, the aluminum foil is held in a substantially horizontal position during the zinc deposition step (a) with the side to be coated with metallic zinc facing down and being contacted with the Zn ^{2+ -containing} aqueous medium. Substantially horizontal means that the film deviates at most 20%, more preferably at most 10% from an ideal horizontal position. The alkaline, aqueous Zn ^{2+ -containing} medium is located below the side of the aluminum foil to be coated and can be brought into contact with this side of the aluminum foil by conventional methods, for example by spraying or rinsing.

For example, the Zn ²⁺ -containing medium is in an open-topped container and the aluminum foil is moved over this container, wherein the Zn ²⁺ -containing medium comes into contact with the aluminum foil only on one side from below.

This horizontal positioning of the aluminum foil in step (a) and the resulting coating of the downwardly facing side of the aluminum foil has a positive influence on the microstructure of the deposited metallic zinc layer and leads to a further improvement of the adhesion between the aluminum back contact and the solar cell connector mounted thereon.

Alternatively, it is also possible that the aluminum foil in step (a) in

Essentially positioned vertically. In principle, however, every other is Positioning (eg in oblique orientation) of the semiconductor device in step (a) possible.

In a preferred embodiment, the aluminum foil is moved relative to the Zn ²⁺ -containing medium during the zinc deposition step (a). The relative speed between the aluminum foil and the aqueous Zn ²⁺ -containing medium is preferably at least 0.1 m / min, more preferably at least 0.2 m / min. As already mentioned above, this relative movement can be realized by moving the underside of the aluminum foil over a quiescent Zn ²⁺ -containing medium or by flowing a flowing Zn ²⁺ medium over a resting underside of the aluminum foil or through a Combination of these two variants. For example, the relative movement of the aluminum foil to the aqueous Zn ²⁺ -containing medium can be realized by a roll-to-roll process. The relative movement between the aluminum foil and the aqueous Zn ²⁺ -containing medium during the Zn deposition in step (a) has a positive influence on the microstructure of the deposited metallic zinc layer and leads to a further improvement of the adhesion between the aluminum back contact and the solar cell connector mounted thereon. As will be discussed in more detail below, it has been found to be particularly advantageous for the adhesion of a metallic connector secured by soldering or bonding to the Zn-coated aluminum back contact when the metallic zinc layer deposited in step (a) is present as a closed layer also on the surface of the closed zinc layer zinc crystallites with a diameter of more than 2.0 μιη in a number density of> 5000 per mm ² , more preferably> 10000 per mm ² , more preferably 5000-60000 per mm ² or 10000-50000 per mm ² are present; and / or wherein at least 1.5%, more preferably at least 3.0%, even more preferably 1.5-18.0%) or 3.0-15.0%) of the surface of the closed zinc layer by zinc crystallites having a diameter occupied by more than 2.0 μιη. In addition to the relatively large Zn crystallites (ie> 2.0 μηι), the metallic zinc layer preferably contains significantly smaller zinc crystallites with a

Diameter of less than 1.0 μm, preferably a relatively large portion of the surface (e.g., greater than 40% or greater than 50% or even greater than 60%) of the closed zinc layer through these smaller zinc crystallites

Diameter of less than 1, 0 μιη is occupied. The zinc crystallites with a diameter of more than 2.0 μm and the zinc crystallites with a diameter of less than 1.0 μm together preferably occupy at least 90%, more preferably at least 95%, of the surface of the closed zinc layer. The particle size distribution of the zinc crystallites on the surface of the closed zinc layer may be, for example, bimodal.

Such a closed metallic zinc layer with a relatively high

Number density of larger crystallites is shown in FIG. The regions in which relatively large Zn crystallites are present on the surface of the Zn layer are each surrounded by regions which are formed by significantly smaller Zn particles.

On the basis of the method parameters of step (a) described above, such a metallic zinc layer can be produced selectively.

Provided the attachment of the metallic connector to the zinc layer is by soldering, this specific structure with a relatively high density of larger crystallites remains at least in the non-soldered areas. Unless the

Fixing by gluing, the specific structure of the metallic zinc layer can also be retained in the glued areas.

In a preferred embodiment, the Zn coating obtained in step (a) is rinsed at least once with a rinsing liquid before step (b). At least for a first flush and optionally also for further flushes of the Zn-coated aluminum back contact before step (b), an aqueous rinse liquid of pH> 8.5, more preferably pH> 13, is preferably used.

The unilaterally Zn-coated aluminum foil is preferably subjected to drying before step (b), for example by suitable thermal treatment.

Optionally, step (a) may be repeated at least once before step (b) is performed. However, considering the process efficiency, it is preferable to perform step (a) only once.

As stated above, in step (b) of the method according to the invention, the one-sided Zn-coated aluminum foil is applied over its untreated side on the back of a semiconductor component of a first solar cell and locally heated with laser radiation, so that the aluminum foil with the Rear side of the semiconductor device connects and a Zn-coated aluminum back contact is obtained.

In accordance with the conventional understanding of those skilled in the art, the back side of the semiconductor device is the side which in use opposes the solar cell to the irradiated side (i.e., the front side), which is the side facing away from the light. The present on the back of a solar cell metal contact is commonly referred to as back contact.

Suitable process conditions for fixing an aluminum foil on the back of a semiconductor device of a solar cell using

Laser radiation are known in the art. In this context, reference may be made to the method described in DE 10 2012 214 253 A1.

The local heating preferably takes place in such a way that as much as possible of the deposited metallic zinc evaporates in this area and the aluminum foil at least briefly melts. As a result of the melting, an effective connection between the aluminum foil and the rear side of the semiconductor component occurs in this local area. Since zinc evaporates, disturbing contamination of the semiconductor device is minimized or even completely avoided in this local connection region. In the adjacent areas of the aluminum foil which have not been subjected to local heating by means of laser radiation, the metallic zinc coating is still present and can be used in step (c) for the attachment of the metallic connector by soldering or gluing. The local heating preferably takes place by means of laser radiation with several

Laser pulses per laser point. This promotes vaporization of the metallic zinc layer in the locally heated area and minimizes the risk of unwanted contamination of the semiconductor device. An advantageous embodiment of the method is that the aluminum foil is locally heated in step (b) by means of laser radiation circumferentially on the edge of the semiconductor device such that the aluminum foil is severed. This improves the adhesion of the film to the semiconductor device. Furthermore, it is advantageous if the adhesion of the aluminum foil is high, especially under the applied connectors. This is achieved, for example, by the fact that in process step (b) the area fraction of the area irradiated with laser radiation is

Aluminum foil in the area below and in the vicinity of the applied in step (c) connector is greater than in the remaining portion of the aluminum foil.

After attaching the one-sided Zn-coated aluminum foil on the back side of the semiconductor device, it can function as a Zn-coated aluminum back contact of the solar cell. In the context of the present invention, it is possible that the solar cell in addition to the aluminum back contact nor a metallic front contact on the

Front side of the solar cell has. This front contact can be configured in a known manner. For example, the front contact may have a grid structure. The front contact can be made of silver or a

Be made silver alloy. The front contact can already be on the

Semiconductor device are present when in step (b) the aluminum back contact is attached. Alternatively, the front contact may be applied to the semiconductor device simultaneously with the aluminum back contact or after attaching the aluminum back contact.

In order to minimize shading effects, it is alternatively also possible for the solar cell to be exclusively back contacted, ie only on the rear side of the solar cell

Semiconductor device metallic contacts for tapping the voltage present.

As will be described in more detail below, in step (c) the attachment of the metallic connector on the Zn-coated aluminum back contact takes place, in particular by soldering. Therefore, in an optional

Embodiment already before step (c) a solder material are applied to the metallic Zn layer of the aluminum back contact. This is preferred

Lot material applied at least in the areas on the Zn layer in which the metallic connector is to be attached. Suitable solder materials are known in the art and will be described in more detail below. As stated above, in step (c) of the method of the invention, the Zn-coated aluminum back contact is connected by a metallic connector to a metal contact of a second solar cell, wherein the metallic connector on the Zn-coated aluminum back contact of the first solar cell by soldering or sticking is attached. Metallic connectors for interconnecting solar cells are well known to those skilled in the art. Suitable metallic connectors are commercially available or can be prepared by conventional methods. The metallic connector is preferably ribbon or wire, with others

Shapes are in principle also possible. Preferably, the metallic connector is band-shaped.

If the metallic connector is fixed to the Zn-coated aluminum back contact by soldering, the metal connector may be coated with a solder material such as tin or a tin alloy. This eliminates the separate feeding of the solder material. As solder material suitable tin alloys are well known. These contain as alloying elements, for example, lead, silver and / or bismuth.

The solder material preferably has a melting temperature in the range of 180 ° C to 245 ° C.

In a preferred embodiment, the metallic connector is a copper tape, more preferably a tin or tin alloy coated copper tape.

Such "tin-plated" copper strips are commercially available.

The soldering is preferably carried out at a temperature of less than 450 ° C. This is commonly referred to as soft soldering. More preferably, the soldering temperature is in the range of 175 ° C to 400 ° C or 175 ° C to 300 ° C.

The soldering process uses conventional, preferably non-corrosive ("no cleari ^1" ) fluxes. The flux may be applied to the solder-coated metallic connectors (eg, the tinned copper tapes) and / or to the deposited zinc layer. If the attachment of the metallic connector on the Zn-coated aluminum back contact by gluing, so an electrically conductive adhesive is preferably used. Such adhesives are known to those skilled in the art and commercially available.

Preferably, the second solar cell, which is connected to the first solar cell, is also a solar cell on which according to the above

a Zn-coated aluminum back contact was attached. With regard to the metal contacts of this second solar cell (for example back contact and front contact or alternatively only back contacts), reference may therefore be made to the above statements.

Furthermore, the present invention relates to a solar cell string comprising at least two solar cells interconnected by a metallic connector, wherein at least one solar cell has a metallic zinc coated aluminum back contact and the metallic connector is directly soldered or bonded to this Zn coated aluminum back contact. Preferably, at least two, more preferably each of the solar cells has an aluminum back contact coated with metallic zinc, and each of these Zn-coated aluminum back contacts has a respective metal connector directly soldered or bonded thereto. Preferably, the solar cell string is obtainable by the method described above.

Thus, at least one of the solar cells interconnected in the solar cell string preferably has a Zn-coated aluminum back contact, which was produced by the method described above. Preferably, all in the Solar cell string connected solar cells to a Zn-coated aluminum back contact prepared in this way.

Preferably, the Zn-coated aluminum back contact of the solar cell has one or more areas in which a closed layer of metallic zinc is present, wherein also on the surface of the closed zinc layer zinc crystallites with a diameter of more than 2.0 μιη in a Number density of> 5000 per mm ² , more preferably> 10000 per mm ² , more preferably 5000-60000 per mm ² or 8000-55000 per mm ² or 10000-50000 per mm ² ; and / or wherein at least 1.5%, more preferably at least 3.0%>, even more preferably 1.5-30.0%) or 1.5-18.0%) or 3.0-15.0% of the surface the closed zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μm.

In addition to the relatively large Zn crystallites (i.e.,> 2.0 μιη), the metallic zinc layer preferably contains significantly smaller zinc crystallites with a

Diameter of less than 1.0 μm, preferably a relatively large portion of the surface area (e.g., greater than 40% or greater than 50% or even greater than 60%) of the closed zinc layer through these smaller zinc crystallites

Diameter of less than 1.0 μιη is occupied. Preferably, the zinc crystallites with a diameter of more than 2.0 μιη and the zinc crystallites with a diameter of less than 1.0 μιη jointly occupy at least 90%>, more preferably at least 95%> of the surface of the closed zinc layer. The particle size distribution of the zinc crystallites on the surface of the closed zinc layer may be, for example, bimodal.

Crystallite diameter, number density of Zn crystallites with a diameter of more than 2.0 μιη or less than 1.0 μιη on the surface of the Zn layer and the respective relative surface occupancy by these Zn crystallites are via scanning electron micrographs (SEM images ) of the Zn layer (in plan view) as well as the evaluation of the images by suitable Image evaluation software determines. The diameter of a crystallite is the diameter of a circle that corresponds in its area to the projection surface of the crystallite in the SEM image. If, for example, about 5% of the surface of the closed zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μm, this means that about 5% of the surface of the Zn layer shown in the SEM image in plan view is due to this Zinc crystallites with a diameter of more than 2.0 μιη is occupied.

For example, at least 90%, more preferably at least 97% of the

Surface of the Zn-coated aluminum back contact having such a structure, ie a closed layer of metallic zinc, wherein on the surface of the closed zinc layer zinc crystallites having a diameter of more than 2.0 μιη in a number density of> 5000 per mm ² , more preferably>

10,000 per mm ² , more preferably 5000-60000 per mm ² or 10000-50000 per mm ² ; and / or wherein at least 1.5%, more preferably at least 3.0%, even more preferably 1.5-18.0%) or 3.0-15.0%) of the surface of the closed

Zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μιη.

If the metallic connector has been soldered to the Zn-coated aluminum back contact, these high density, large area zinc crystallite areas may be where no metal connector has been attached.

For example, when an Sn alloy is used as a solder, in a region where a metallic connector has been soldered to the Zn-coated aluminum back contact, between the metal of the connector (e.g., Cu) and the metal

Aluminum or the aluminum alloy before a layer having an Sn matrix with dispersed therein Zn particles. Insofar as the metallic connector adheres to the Zn-coated

Aluminum back contact was attached, these areas with high number density large zinc crystallites can also be present where a metallic connector was attached.

As explained above, the zinc-coated aluminum obtained by a wet-chemical process is a very effective substrate for the attachment of another metal by soldering or gluing. In the above-described

Embodiments, the substrate was the aluminum back contact of the solar cell to which, when connected to other solar cells, metallic connector is to be attached by soldering or gluing.

It is known in principle that aluminum strips can be used as metallic connectors in the interconnection. Again, the problem of forming an Al ₂ O ₃ layer on the metallic aluminum, whereby a conventional soldering of the aluminum strip on the metal contact of a solar cell difficult or even prevented.

Therefore, the present invention also relates to a method for interconnecting solar cells, wherein

(i) treating a wire or tape-shaped aluminum substrate with an alkaline, aqueous medium containing Zn ²⁺ such that metallic zinc is deposited on the aluminum substrate to form a Zn-coated aluminum substrate,

(ii) bonding a metal contact of a semiconductor device of a first solar cell to a metal contact of a semiconductor device of a second solar cell through the Zn-coated aluminum substrate, wherein the Zn-coated aluminum substrate is fixed to the metal contacts by soldering or bonding, respectively. Wire or tape-shaped aluminum materials (eg, aluminum tapes) for interconnecting semiconductor devices are well known and commercially available.

The aluminum substrate may be made of metallic aluminum or an aluminum alloy. Regarding the properties of aluminum

(In particular, purity) and the aluminum alloy (in particular aluminum content) can be made to the above statements. The purity of the aluminum can vary over a wide range, provided that the electrical conductivity and / or mechanical properties are not adversely affected. For example, the aluminum contains further metallic elements in a total proportion of less than 1% by weight, more preferably less than 0.1% by weight or less than 0.01% by weight. Is the aluminum substrate made of one

Made aluminum alloy, it preferably has a proportion of aluminum of at least 80% by weight, more preferably at least 90% by weight. Suitable metallic elements that can be alloyed with the aluminum are known to those skilled in the art. With regard to the preferred conditions for the zinc deposition step (i), reference may be made to the above statements (see

Zinc deposition step (a)). It is thus preferred that the aqueous medium with which the aluminum substrate is treated has a relatively high concentration of Zn ²⁺ . In a preferred embodiment, the Zn ²⁺ concentration in the alkaline aqueous medium is at least 1.5% by weight, more preferably at least 2.0% by weight, even more preferably at least 3.0% or even at least 4.0% by weight. In a preferred embodiment, the aqueous medium contains Zn ²⁺ in a concentration of 1.5% by weight to 12.0% by weight, more preferably 2.0% by weight to 10.0% by weight, more preferably 3.0% by weight to 8, 0 wt% or 4.0 wt% to 8.0 wt%. A suitable pH of the alkaline, aqueous medium is, for example,> 10, more preferably> 13. Optionally, the alkaline aqueous medium may contain further transition metal cations, preferably iron cations, nickel cations or copper cations or a combination of at least two of these Cations, included. In a preferred embodiment, the alkaline, aqueous medium still contains Fe cations in a concentration of at least 0.0003% by weight, more preferably at least 0.001% by weight, for example in the range of 0.0003-30% by weight or 0.0003-0, 1% by weight. If the alkaline, aqueous medium contains nickel cations, these may be present, for example, in a concentration of 0.1-5% by weight, more preferably 0.5-3% by weight. If the alkaline, aqueous medium contains copper cations, these may be present, for example, in a concentration of 0.01-1% by weight, more preferably 0.05-0.5% by weight.

The deposition of the metallic zinc from the Zn ²⁺ -containing aqueous medium onto the aluminum substrate preferably takes place without current. Preferably, the deposited on the aluminum substrate Zn layer has a thickness in the range of 0.1 μιη to 5 μιη, more preferably 0.3 μιη to 2.5 μιη on.

The duration of the treatment of the aluminum substrate with the alkaline, aqueous Zn ²⁺ -containing medium in step (i) is, for example, 15 seconds to 250 seconds. The zinc deposition step (i) is preferably carried out at a temperature in the range of 5-60 ° C, more preferably 5-45 ° C.

The treatment of the aluminum substrate, for example, by immersion in the Zn ²⁺ -containing medium or by rinsing or spraying with the Zn ²⁺ -containing medium. The aluminum substrate (in particular an aluminum strip) may, for example, be positioned substantially horizontally in step (i). Alternatively, it is also possible that the aluminum substrate is positioned substantially vertically during step (i). In principle, however, any other positioning (eg in an oblique orientation) in step (i) is possible.

In a preferred embodiment, the aluminum substrate is moved relative to the Zn ²⁺ -containing medium during the zinc deposition step (i). The relative speed between the aluminum back contact and the aqueous Zn ²⁺ -containing medium is preferably at least 0.1 m / min, more preferably at least 0.2 m / min. This relative movement can be realized, for example, by moving the aluminum substrate over a quiescent Zn ²⁺ -containing medium or by flowing a flowing Zn ²⁺ medium over a resting aluminum substrate or by a combination of these two variants. The flow rate of the Zn ²⁺ -containing medium (and thus the relative speed with respect to the (moving or stationary) aluminum substrate) can be adjusted for example via the pump power. By the relative movement between the aluminum substrate and the aqueous Zn ²⁺ -containing medium during the Zn deposition in step (i), a further improvement of the adhesive force between the metal contact of a

Solar cell and the aluminum substrate (e.g.

Aluminum ribbon) can be achieved.

Preferably, the Zn-coated aluminum substrate is rinsed at least once with a rinsing liquid. With regard to suitable rinsing liquids and

Flush conditions can be referenced to the above statements.

Preferably, the Zn-coated aluminum substrate is dried before step (ii). Preferably, a solder material is applied to the Zn-coated aluminum substrate prior to step (ii). Suitable solder materials will be described below. As stated above, in step (ii), a metal contact of a

Semiconductor device of a first solar cell with a metal contact of a

Semiconductor device of a second solar cell through the Zn-coated

Aluminum substrate connected, wherein the Zn-coated aluminum substrate is attached by soldering or gluing on the metal contacts.

For soldering, common solder materials known to those skilled in the art can be used, e.g. Tin alloys. These contain as alloying elements

for example, lead, silver and / or bismuth. The solder material preferably has a melting temperature in the range of 180 ° C to 245 ° C. The soldering is preferably carried out at a temperature of less than 450 ° C. This is commonly referred to as soft soldering. More preferably, the soldering temperature is in the range of 175 ° C to 300 ° C.

The soldering process uses conventional, preferably non-corrosive, fluxes.

If the attachment by gluing, it is preferred to use an electrically conductive adhesive. Such adhesives are known to those skilled in the art and commercially available.

The metal contacts to which the metal connector is attached may be those commonly used for solar cells.

For example, the metal contact is a silver contact (e.g., a screen-printed silver contact), a Ni / Cu / Ag contact (e.g., via galvanic

Deposition produced), a Ni / Cu contact ((eg via galvanic deposition made) or an AI contact (eg produced by screen printing). The metal contact may be a back contact or a front contact.

Furthermore, the present invention relates to a Zn-coated wire or strip-shaped aluminum substrate, which preferably has one or more areas in which a closed layer of metallic zinc on the

Aluminum substrate is present, wherein also on the surface of the closed zinc layer zinc crystallites with a diameter of more than 2.0 μιη in a number density of> 5000 per mm ² , more preferably> 10000 per mm ² , more preferably 5000-60000 per mm ² or 8000-55000 per mm ² or 10000-50000 per mm ² ; and / or wherein at least 1.5%, more preferably at least 3.0%, even more preferably 1.5-30.0% or 1.5-18.0% or 3.0-15.0% of the surface of the closed zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μιη.

Crystallite diameter, number density of Zn crystallites with a diameter of more than 2.0 μιη or less than 1.0 μιη on the surface of the Zn layer and the respective relative Oberfiächen occupancy by these Zn crystallites are about Scanning electron micrographs (SEM images) of the Zn layer (in plan view) and the evaluation of the images by suitable

Image evaluation software determines. The diameter of a crystallite is the diameter of a circle that corresponds in its area to the projection surface of the crystallite in the SEM image.

If, for example, about 5% of the surface of the zinc layer is covered by zinc crystallites with a diameter of more than 2.0 μm, this means that about 5% of the surface of the Zn layer shown in the SEM photograph in plan view is caused by these zinc -Kristallite is occupied with a diameter of more than 2.0 μιη.

For example, at least 70%, more preferably at least 90%> of the

Surface of the Zn-coated aluminum substrate have such a structure, i. a closed layer of metallic zinc, wherein also on the surface of the closed zinc layer zinc crystallites with a

Diameter of more than 2.0 μιη in a number density of> 5000 per mm ² , more preferably> 10000 per mm ² , more preferably 5000-60000 per mm ² or 8000- 55000 per mm ² or 10000-50000 per mm ² are present; and / or wherein at least 1.5%), more preferably at least 3.0%>, even more preferably 1.5-30.0%) or 1.5-18.0%) or 3.0-15.0% of Surface of the closed zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μιη.

Furthermore, the present invention relates to the use of the above

described ribbon or wire-shaped Zn-coated aluminum substrate for interconnecting solar cells in the production of solar cell strings.

The invention will be described in more detail by the following example. example

A 9 μηι thick aluminum foil with a purity of 99% is passed from roll to roll at a speed of 1 m / min over 3 pools. The first basin is filled with an aqueous solution containing Zn ²⁺ containing 4% by weight of zinc ions, 15% by weight of NaOH and 0.001% by weight of iron ions. The second basin contains 1% caustic soda and the third basin demineralised water. The first tank serves to deposit the metallic zinc on one side of the aluminum foil, while with the tanks 2 and 3 the Zn-coated aluminum foil is rinsed. All 3 pools are filled to the upper edge of the pool. Due to the surface tension, the aluminum foil is pulled down by the liquids so that they wet the aluminum foil well, only on the lower side, ie on one side. In this way, the aluminum foil is only coated on one side with zinc. Because the aluminum foil is wider than the basins perpendicular to the transport direction, the basins are completely covered by the aluminum foil and due to the

Surface tension applied force from the aluminum foil well sealed, so that when transporting the aluminum foil over the edge of the pool, the wet chemical solutions stripped on the edge of the pool and thus be little delayed. The levels in the individual basins are kept constant with an accuracy of 3 mm or better by automatic refilling. Out

Transport speed and individual width of the basin results in the treatment times 90 s for tank 1, which is filled with the aqueous medium containing Zn ²⁺ , 20 s for tank 2, which is filled with 1% sodium hydroxide solution, and 20 s for tank 3 filled with demineralized water. After the wet-chemical treatment, the aluminum foil is heated by means of infrared radiation and warm

Air flow dried and finally rolled up. In roll form, the one-sided Zn-coated aluminum foil is protected against oxidation of the zinc layer and can be stored for long periods without a negative change in the properties. FIG. 1 shows a SEM image of the surface of the Zn-coated aluminum back contact. The photograph shows a closed metallic zinc layer, which has a relatively high proportion of large Zn crystallites with a diameter of at least 2 μm. Zinc crystallites with a diameter of more than 2 μm are present in a number density of about 30,000 per mm ² . 9% of the surface of the metallic zinc layer are coated with Zn crystallites with a diameter of at least 2 μιη. Subsequently, the roller with the zinc-coated aluminum foil is positioned in a film unwinder of the laser system. The film is unwound, placed with the non-zinc-coated side on the back of a semiconductor device (silicon substrate) of a solar cell (silicon solar cell) and sucked there with Unterduck. The zinc-coated side is locally illuminated with laser radiation. As a result, at least the Zn-coated aluminum foil is locally heated, so that melting of the aluminum foil occurs for a short time in these regions, whereby it bonds to the rear side of the semiconductor component.

Circumferentially at the edge of the semiconductor device, the Zn-coated

Aluminum foil lasered by means of other laser parameters such that the

Aluminum foil is severed and the protruding over the edge of the semiconductor device film can be removed well.

Due to its production, aluminum foil has a rough and a smooth side. An improved light output of the solar cells has resulted when the rough side of the aluminum foil is coated with zinc and the smooth side rests on the back of the semiconductor device.

It is important that the aluminum foil on the side, with the

Semiconductor component of the solar cell is attached, is not coated with zinc and there are no dried residues of the zinc-containing wet chemical solution Otherwise, the adhesion of the aluminum foil after the laser process is poor and the local highly doped regions in the silicon have an increased Shockley read-hall recombination of excess charge carriers. The Zn-coated aluminum foil fastened by lasers to the semiconductor device functions as a Zn-coated aluminum back contact. On this Zn-coated aluminum back contact, a soldered copper ribbon (tinned copper ribbon) is soldered by means of infrared heating at 275 ° C. On the solder joint previously a flux (Kester 952 s) was sprayed.

Figure 2 shows in cross section a SEM photograph of the area in which the metallic connector (the copper ribbon) was soldered onto the Zn coated aluminum back contact. Three layers can be seen. The uppermost layer is the aluminum layer of the back contact and the lowest layer is the copper of the metallic connector. In between lies the adhesion-promoting solder layer. In this is the solder material and the zinc of the zinc coating.

The copper ribbon shows good adhesion on the back contact of the solar cell.

The protruding end of the connector is soldered in a subsequent soldering in a known manner to the front of another solar cell. This gives a solar cell string in which the solar cells are connected in series. The solar cell strings are laminated with glass, ethylene vinyl acetate and polymer backsheet into a module.

Claims

claims

Method for interconnecting solar cells, wherein

(a) treating an aluminum foil unilaterally with an alkaline, aqueous medium containing Zn ²⁺ such that on the treated side of the aluminum foil, metallic zinc is deposited to form a one-sided Zn-coated aluminum foil;

(B) the one-sided Zn-coated aluminum foil is applied over its untreated side on the back of a semiconductor device of a first solar cell and locally heated with laser radiation, so that the aluminum foil with the back of the

Semiconductor device connects and a Zn-coated

Aluminum back contact is obtained

(c) the Zn-coated aluminum back contact through a

metallic connector is connected to a metal contact of a second solar cell, wherein the metallic connector is attached to the Zn-coated aluminum back contact by soldering or gluing.

The method of claim 1, wherein the alkaline aqueous medium contains Zn ²⁺ in a concentration of at least 1.5% by weight; and / or wherein the alkaline aqueous medium additionally contains iron cations, nickel cations, copper cations or a combination of at least two of these cations.

A method according to claim 1 or 2, wherein the deposition of the metallic zinc on the aluminum foil is carried out without current; and / or being on the Aluminum foil deposited zinc layer has a thickness in the range of 0.1 μιη to 5 μιη.

Method according to one of the preceding claims, wherein the side of the aluminum foil treated with the aqueous Zn ²⁺ -containing medium whose average surface roughness is greater compared to the opposite side.

A method according to any one of the preceding claims, wherein the aluminum foil is held in a substantially horizontal position during step (a), thereby contacting the downwardly facing side of the aluminum foil with the aqueous Zn ²⁺ -containing medium and thereby metallic zinc is coated.

Method according to one of the preceding claims, wherein the aluminum foil is moved relative to the aqueous Zn ²⁺ -containing medium, preferably with a relative speed of at least 0.1 m / min.

The method of any one of the preceding claims, wherein the unilaterally Zn-coated aluminum foil is rinsed at least once with an alkaline rinsing liquid prior to step (b); and / or wherein the unilaterally Zn-coated aluminum foil is dried before step (b).

Method according to one of the preceding claims, wherein the

Process step (a) and optionally the rinsing with a rinsing liquid and / or the drying of the one-sided Zn-coated aluminum foil is carried out in a roll-to-roll process.

Method according to one of the preceding claims, wherein in step Zn-coated aluminum foil on one side by the laser radiation is heated locally around the edge of the semiconductor device such that the aluminum foil is severed; and / or wherein in step (b) the area of the aluminum foil irradiated with the laser radiation in the region in which the metallic connector is applied in step (c) is greater than the surface area of the aluminum foil irradiated outside with the laser radiation of the metallic connector.

A method according to any one of the preceding claims, wherein the metallic connector and / or the Zn-coated aluminum back contact is coated with a solder material; and / or wherein the soldering is at a temperature of less than 450 ° C.

Method according to one of claims 1-9, wherein the bonding is carried out with an electrically conductive adhesive.

A solar cell string comprising at least two solar cells interconnected via a metallic connector, wherein at least one solar cell has a metallic zinc coated aluminum back contact, and the metallic connector is directly soldered or bonded to said Zn coated aluminum back contact.

A solar cell string according to claim 12, wherein the Zn-coated aluminum back contact of the solar cell has one or more regions in which a closed metallic zinc layer is present, and zinc crystallites more than 2 in diameter, on the surface of the closed zinc layer, 0 μιη present in a number density of> 5000 per mm ² ; and / or wherein at least 1.5% of the surface of the

closed zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μιη. Solar cell string according to claim 12 or 13, wherein on the surface of the closed zinc layer also present zinc crystallites with a diameter of less than 1.0 μιη, and the zinc crystallites with a

Diameter of more than 2.0 μιη and the zinc crystallites with a

Diameter of less than 1.0 μιη together occupy at least 90% of the surface of the closed zinc layer.

Method for interconnecting solar cells, wherein

(i) a wire or tape-shaped aluminum substrate having a

alkaline aqueous medium containing Zn ^{2+ is} treated such that metallic zinc is deposited on the aluminum substrate to form a Zn-coated aluminum substrate,

(ii) bonding a metal contact of a semiconductor device of a first solar cell to a metal contact of a semiconductor device of a second solar cell through the Zn-coated aluminum substrate, wherein the Zn-coated aluminum substrate is fixed to the metal contacts by soldering or bonding, respectively.

The method of claim 15, wherein the deposition of the metallic zinc in step (i) is according to the conditions of any one of claims 2, 3, 5, 6 or 7; and / or wherein attaching the Zn-coated aluminum substrate to the metal contact according to the conditions of one of

Claims 10 or 11 takes place.

Use of a tape or wire-shaped Zn-coated aluminum substrate for interconnecting solar cells.

18. Use according to claim 17, wherein the tape or wire-shaped Zn-coated aluminum substrate has one or more areas, in which is a closed layer of metallic zinc, wherein also on the surface of the closed zinc layer zinc crystallites with a diameter of more than 2.0 μιη in a number density of> 5000 per mm ² are present; and / or wherein at least 1.5% of the surface of the closed zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μιη.

A strip or wire-shaped Zn-coated aluminum substrate having one or more areas in which a closed layer of metallic zinc is present, wherein also on the surface of the closed zinc layer zinc crystallites with a diameter of more than 2.0 μιη in a number density of> 5000 per mm ² ; and / or wherein at least 1.5% of the surface of the closed zinc layer is occupied by zinc crystallites with a diameter of more than 2.0 μιη.