EP2896070A2

EP2896070A2 - Method for improving the adhesion of plated metal layers to silicon

Info

Publication number: EP2896070A2
Application number: EP13753849.2A
Authority: EP
Inventors: Richard Russell; Loic TOUS
Original assignee: Katholieke Universiteit Leuven; Interuniversitair Microelektronica Centrum vzw IMEC
Current assignee: Katholieke Universiteit Leuven; Interuniversitair Microelektronica Centrum vzw IMEC
Priority date: 2012-09-17
Filing date: 2013-08-22
Publication date: 2015-07-22
Also published as: WO2014040834A2; CN104603956A; WO2014040834A3; JP2015528645A

Abstract

A method is disclosed for fabricating photovoltaic cells, the method comprising providing plated metal contacts with a contact pattern comprising a plurality of fingers and at least one first pinning element at the free ends of each of the plurality of fingers; and associated photovoltaic cells.

Description

Method for improving the adhesion of plated metal layers to silicon

Technical field

The disclosed technology relates to methods for improving the adhesion of plated metallayers, such as plated Cu contacts, to silicon. The method can advantageously be used for improving the adhesion of plated metal contacts of silicon photovoltaic cells. Background art

New concepts and fabrication processes are being developed for silicon photovoltaic cells, in view of implementing these concepts and processes in a production environment. An example of such a development towards industrial production is the use of plating processes for the formation of the metal contacts, as an alternative to screen printing processes.

A typical process flow using plating, e.g. for the front side metallization of silicon photovoltaic cells, comprises providing a dielectric layer, e.g. an antireflection coating, over the entire front surface, and then locally removing the antireflection coating, thereby exposing the underlying silicon surface at locations where metal contacts need to be provided. Locally removing the antireflection coating can for example be done by means of laser ablation. After local removal of the antireflection coating, front side metal contacts are provided in the exposed silicon regions by metal plating to form the front side metal contacts.

Due to their excellent contact properties, Ni layers or nickel silicide layers can for example be used for forming good contacts.

Nickel silicide layers can be formed by providing a thin nickel layer (e.g. by electroless plating) on the silicon surface in the openings created in the antireflection coating, followed by an annealing or sintering step to induce silicidation, resulting in the formation of a nickel-silicon alloy (nickel silicide). In processes for fabricating silicon photovoltaic cells, on top of the nickel silicide layer at least one additional metal layer (such as for example a Cu layer) is electroplated using the nickel silicide layer as a seed layer to form a low resistance front side metal pattern.

In alternative processes for fabricating silicon photovoltaic cells, a Ni layer is first provided, e.g. by light-induced plating, in the openings created in the antireflection coating and next a Cu layer is provided on the Ni layer, e.g. by electroplating. Afterwards a sintering step is performed to convert the Ni layer at least partially into a nickel silicide layer.

In such processes with plated metal contacts, it is a challengeto achieve good adhesion between the different metal layers and between the metal layers and the underlying silicon substrate. Poor adhesion between the metal layers or between the metal layers and the silicon substrate can result in peeling off of a metal layer, resulting in an unacceptable lowering of the cell efficiency. The risk of a metal layer peeling off is typically higher for narrower metal features. For example, for a typical front side metal grid of photovoltaic cells comprising a plurality of narrow lines (fingers) connected to at least one broader line (busbar), the problem of metal peeling off is typically higher for the narrow fingers than for the broader busbars.

Individual photovoltaic cells are typically electrically connected in a module by means of Cu ribbons that are soldered to the busbars of the cells. If the adhesion between the metal stack (including the busbars and the soldered ribbons) and the silicon substrate is poor, the electrical contact can degrade and module efficiency can drop to failure levels.

Summary of the disclosure

Certain inventive aspects relate to a method for fabricating photovoltaic cells with plated metal contacts wherein the risk of peeling off of the plated metal contacts is substantially reduced and/or wherein the adhesion of metal ribbons soldered to a busbar is improved as compared to existing methods. In one inventive aspect the method comprises providing plated metal contacts with a contact pattern comprising a plurality of fingers and at least one first pinning element or anchoring element at the free ends of each of the plurality of fingers. It is an advantage of providing a first pinning element at a free and of a finger that the risk of finger peeling off is substantially reduced.

In one inventive aspect the method comprises providing plated metal contacts with a contact pattern comprising at least one busbar, a plurality of fingers connected to at least one busbar and at least one second pinning element or anchoring element at finger ends that are in physical contact with the at least one busbar, preferably at each finger end that is in physical contact with the at least one busbar. It is an advantage of providing such second pinning elements that it improves the adhesion of ribbons soldered to the at least one busbar.

In one inventive aspect the method comprises providing plated metal contacts with a contact pattern comprising at least one busbar, a plurality of fingers connected to at least one busbar, at least one first pinning element at the free ends of each of the plurality of fingers, and at least one second pinning element at finger ends that are in physical contact with the at least one busbar.

A method according to an inventive aspect can advantageously be used for providing a front side metallisation pattern of photovoltaic cells. A fabrication method can for example comprise: providing a dielectric layer, e.g. an antireflection coating, over the entire front surface of a silicon substrate, locally removing the antireflection coating, thereby exposing the underlying silicon surface at locations where metal contacts need to be provided in accordance with a contact pattern of the present disclosure; and providing front side metal contacts in the exposed silicon regions by metal plating to form the front side metal contacts.

Certain inventive aspects relate to photovoltaic cells with plated metal contacts wherein the risk of peeling off of the plated metal contacts is substantially reduced and/or wherein the adhesion of metal ribbons soldered to a busbar is improved as compared to existing photovoltaic cells. In one inventive aspect the photovoltaic cells haveplated metal contacts with a contact pattern comprising a plurality of fingers and at least one first pinning element or anchoring element at the free ends of each of the plurality of fingers.

In one inventive aspect the photovoltaic cells have plated metal contacts with a contact pattern comprising at least one busbar, a plurality of fingers connected to at least one busbar and at least one second pinning element or anchoring element at finger ends that are in physical contact with the at least one busbar.

In one inventive aspect the photovoltaic cells have plated metal contacts with a contact pattern comprising at least one busbar, a plurality of fingers connected to at least one busbar, at least one first pinning element at the free ends of each of the plurality of fingers, and at least one second pinning element at finger ends that are in physical contact with the at least one busbar.

A first pinning element can be any element or feature that leads to a local increase of the contact area between a finger and the underlying silicon at a free end of the finger. A first pinning element can be provided by locally increasing the finger width at a free finger end or it can be an element having a linear, square, rectangular, polygonal, circular or oval shape or any other suitable shape, the element being provided at a free finger end.

A second pinning element can be any element or feature that leads to a local increase of the contact area between a finger and the underlying silicon at an end of the finger in contact with a busbar. A second pinning element can be provided by locally increasing the finger width at a finger end in contact with a busbar or it can be an element having a linear, square, rectangular, polygonal, circular or oval shape or any other suitable shape, the element being provided at a finger end in contact with a busbar.

Certain objects and advantages of various inventive aspects have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the disclosure. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. Further, it is understood that this summary is merely an example and is not intended to limit the scope of the disclosure. The disclosure, both as to organization and method of operation, together with features and advantages thereof, may best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings.

Brief description of the drawings

Figure 1 shows an example of a typical front side metallization pattern of photovoltaic cells.

Figure 2 shows an example of a front side metallization pattern according to one inventive aspect.

Figure 3 shows examples of first pinning elements according to inventive aspects of the present disclosure.

Figure 4 shows an example of a front side metallization pattern according to one inventive aspect.

Figure 5 shows examples of second pinning elements according to inventive aspects of the present disclosure.

Figure 6 shows a detail of Figure 2, zooming in on a particular finger line.

In the different drawings, the same reference signs refer to the same or analogous elements.

Detailed description of preferred embodiments

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure and how it may be practiced in particular embodiments. However, it will be understood that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures and techniques have not been described in detail, so as not to obscure the present disclosure. While the present disclosure will be described with respect to particular embodiments and with reference to certain drawings, the disclosure is not limited hereto. The drawings included and described herein are schematic and are not limiting the scope of the disclosure. It is also noted that in the drawings, the size of some elements may be exaggerated and, therefore, not drawn to scale for illustrative purposes.

Furthermore, the terms first, second, third and the like in the description, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the disclosure described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising" should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B.

In the context of the present disclosure, the front surface or front side of a photovoltaic cell is the surface or side adapted for being oriented towards a light source and thus for receiving illumination. In case of bifacial photovoltaic cells, both surfaces are adapted to receive impinging light. In such case, the front surface or front side is the surface or side adapted for receiving the largest fraction of the light or illumination. The back surface, back side, rear surface or rear side of a photovoltaic cell is the surface or side opposite to the front surface. The front side of a substrate is the side of the substrate corresponding to the front side of the photovoltaic cell, while the rear side or back side of the substrate corresponds to the back side of the photovoltaic cell.

In the context of the present disclosure, fingers are relatively narrow conductive lines, for example having a width in the range between about 10 micrometer and 150 micrometer, that are typically distributed over the cell area and that collect(photogenerated) current from the underlying semiconductor. The fingers can transport the photogenerated current to at least one busbar. In preferred embodiments the finger lines are defined by lateral longitudinal sidewalls which can be parallel. In certain embodiments the lateral longitudinal sidewalls can form a tapered structure. The tapered structure can have a relatively narrow and a relatively wide end. The relatively narrow end can be directed away from the bus bar, i.e. it can correspond to the free end of the finger line.

A busbar is a metallized area that is substantially wider (e.g. having a width in the range between about 1 mm and 3 mm) than the fingers and that is used for collecting (photo generated) current from the fingers and for soldering a cell to cell interconnect material (e.g. interconnect ribbon) to. In the context of the present disclosure, a free end of a finger refers to an end of a finger (line) that is not in direct physical contact with a busbar. In another view the free end can be seen as an end pointing away from a busbar, or as the end being remote from a respective busbar.

It can be noted that in the context of the present disclosure, the finger lines and busbars are typically produced at the same time, by means of for instance a single metallization (plating) process. They are thus typically formed in a single metalized layer and are referred to as different and non-overlapping parts of such a metalized layer. The process of providing an interconnect ribbon on top of the bus bar is known to the skilled person. The ribbon typically comprises a copper strip comprising a tin lead solder coated on both sides thereof. The process can for instance comprise:

-applying a flux to the bus bar to clean its surface;

-placing the ribbon on top of the busbar;

-soldering the ribbon to the underlying metal pattern, by for instance using a soldering iron, for instance at a temperature of 325°C. For instance, the soldering can be performed by providing a set of (e.g. 5) soldered spots, each for instance approximately 1 cm long, along the bus bar, preferably with equal spacing between the spots.

The present disclosure is further illustrated for examples wherein the front side contacts of silicon photovoltaic cells are provided by means of metal plating. However, the present disclosure is not limited thereto. A method according to an inventive aspect of the present disclosure can for example also be used for providing rear side contacts of silicon photovoltaic cells, such as for bifacial cells, or for example for providing rear side contacts of back-contact cells such as interdigitated back contact cells.

It is known that in a silicon photovoltaic cell fabrication process wherein the front contacts are provided by metal plating, such as e.g. Cu plating, it is a challengeto achieve good adhesion between the patterned plated metal layer and the silicon substrate. Poor adhesion between the metal pattern and the silicon substrate can result in peeling off of the metal pattern, resulting in an unacceptable lowering of the cell efficiency. A typical front contact pattern of a photovoltaic cell contains a plurality of relatively narrow metal lines (fingers), each of the fingers being electrically connected to at least one wider metal line (busbar), as for example illustrated in Figure 1 . Alternatively, the front contact pattern may be busbar-free and the narrow metal fingers may be electrically connected to an interconnect ribbon via a conductive adhesive. The risk of the metal pattern peeling off is typically higher for narrower metal features. This may be related to the relatively lower metal-to-silicon contact area for narrower metal features. The risk of peeling off increases for narrower lines, for thicker plated layers, with increasing stress in the metal, e.g. Cu (related to chemistry and/or deposition rate) and for longer metal lines. Therefore, for a photovoltaic cell, the problem of metal peeling off may be higher for the narrow fingers than for the broader busbars.

In one inventive aspect the present disclosure provides a method for fabricating photovoltaic cells with plated metal contacts wherein the risk of peeling off of patterned plated metal contacts is reduced as compared to existing methods and/or wherein the adhesion of metal ribbons soldered to a busbar is improved as compared to existing methods.

The present disclosure is further described for embodiments wherein the plated metal layers are part of a front side metallization pattern of silicon photovoltaic cells. However, the present disclosure is not limited thereto and the method can be used for other applications, such as for example for plated metal layers that are part of the rear side metallisation of bifacial photovoltaic cells or of back-contact photovoltaic cells.

Figure 1 shows an example of a typical front side metallization pattern of a photovoltaic cell. In the example shown, the metallization pattern comprises two busbars 100 and a plurality of fingers 101 , 102, 103. However, the present disclosure is not limited thereto and any other suitable metallization pattern known by a person skilled in the art can be used. In the example shown in Figure 1 , each of the fingers 101 , 102, 103 is connected to at least one busbar 100 at a finger end 12. Fingers 101 , 103 have a free end 1 1 and an opposite end 12 connected to a busbar 100. Fingers 102 have both ends 12 connected to a busbar 100.

The method according to an embodiment may comprise providing first pinning elements or anchoring elements at the free ends 1 1 of the fingers of a metallization pattern of a photovoltaic cell. It was surprisingly found that providing such first pinning elements at the free ends of the fingers substantially reduces the risk of peeling off. The method may comprise providing second pinning elements or anchoring elements at finger ends 12 that are in physical contact with a busbar 100. It was surprisingly found that providing such second pinning elements improves the adhesion of ribbons soldered to the busbar.

In one inventive aspect the present disclosure provides a photovoltaic cell having a plated metal contact pattern containing a plurality of metal fingers, wherein the metal pattern further contains first pinning elements or anchoring elements, e.g. pinning lines or anchoring lines, at the free ends of the plurality of metal fingers.

In one inventive aspect the present disclosure provides a photovoltaic cell having a plated metal contact pattern containing a plurality of fingers and at least one busbar, wherein the metal contact pattern further containssecond pinning elements or anchoring elements at finger ends that are in physical contact with a busbar.

In a method for fabricating silicon photovoltaic cells with plated front contacts, typically an antireflection coating is first provided on the front surface, and then locally removed by means of laser ablation at the location where metal contacts need to be provided. The width of the laser openings is typically in the range between about 5 micrometer and 50 micrometer, e.g. in the order of about 10 micrometer, at the location where contact fingers are to be provided. The metal contacts are then formed in the openings by means of metal plating, with a typical thickness of the plated metal layer in the range between about 5 micrometer and 35 micrometer, e.g. in the order of about 10 micrometer. For example, when using a laser opening of 10 micrometer and a metal thickness of 10 micrometer, this results in a typical plated line (finger) being about 30 micrometer wide (due to lateral overplating) with only about 1 0 micrometer being in contact with the silicon. The remaining part is plated over the antireflection coating where adhesion is very poor, i.e. much poorer than the adhesion of the finger line to the underlying silicon substrate.

One approach that can be used for improving finger adhesion is creating wider laser lines, but this leads to increased shading losses. Alternatively the plating thickness can be reduced, having a disadvantage of reduced finger conductivity.

In a method according to the present disclosure, finger adhesion is improved with only a limited increase in shading loss and without reducing the finger conductivity.

In a method of the present disclosure, first pinning elements or anchoring elements are provided at the free ends of each of the plurality of fingers. It was surprisingly found that this is sufficient to substantially improve finger adhesion and to substantially reduce peeling off.

In one aspect, a first pinning element can be provided by locally increasing the width of a finger 101 , 103 at a finger end 1 1 . For example, locally increasing the width of the finger FW can comprise increasing the finger width by a factor of about 2 to 5, such as for example by a factor of about 3, the increased width referred to as PW. However, the present disclosure is not limited thereto. The width of the finger can be increased over a length PL that is in the range between about 5% and 50% of the finger length FL, measured from the edge of the finger end, for example between about 1 0% and 30% of the finger length, the present disclosure not being limited thereto. An example of a front side metallization pattern with first pinning elements 21 formed by locally increasing the finger width at the free ends 1 1 is schematically illustrated in Figures 2 and 6, the latter providing a detail of Figure 2.

Scotch tape peel tests were performed to illustrate the improved results when using fingers with first pinning elements according to aspects of the present invention, as compared to typical state of the art fingers with constant width and no pinning elements. These scotch tape peel tests have been performed on cells comprising 18 finger lines on both sides of a central bus bar. A scotch tape was applied in the area in between the respective cell edge and the busbar. The scotch tape was applied such that it extended from the respective cell edge to the edge of the busbar (without covering any portion of the bus bar itself), and such that it was parallel to the fingers, thereby covering the respective 18 fingers. The scotch tape was then removed (peeled off). Then the number of finger lines were counted that were released from the substrate by the process. The process was repeated on the left and right side of the bus bar. This was repeated for two photovoltaic cells comprising state of the art finger lines, and two photovoltaic cells comprising finger lines according to embodiments of the present invention, wherein pinning elements were provided only at the free end of the finger lines. The four photovoltaic cells were further identical. The results are provided in Table 1 . It is clear to the skilled person that there generally is a clear improvement of the adhesion of the plated metal finger lines to the underlying silicon substrate. This is also quantitatively supported by the large difference in average number of released finger lines between the state of the art and the embodiments of the present invention.

Table 1 : comparative results Figure 6 further illustrates that the finger lines are preferably defined by parallel longitudinal sidewalls. In a first portion of the finger 101 , referred to as portion I, the remote portion, the finger width PW is larger and defined by a set of parallel sidewalls 21 1 . In a second portion, directly adjacent and in physical contact with a busbar or ribbon, referred to as portion I I, the finger with FW is relatively smaller and delimited by a set of parallel sidewalls 101 1 . The transition can be of the step-type, as illustrated. Alternatively the transition can be more gradual. The transition can then for instance comprise a corresponding transition structure in a transition area, wherein the width of the fingers evolves gradually from a width FW to a width PL.

Moreover, one or both of the first portion I and second portion I I of the finger lines can have a tapered structure. For instance, the second portion II can be tapered, and thenarrowerside of a respective tapered structure can be pointing away from or can be remote from the busbar.

In another aspect, a first pinning element can contain an element having alinear, square, rectangular, polygonal, circular or oval shape or any other suitable shape, the element being provided at a free finger end 1 1 . Examples are schematically illustrated in Figure 3, showing finger ends 1 1 with different first pinning elements21 . The first pinning element 21 can be a pinning line, wherein the longitudinal direction of the pinning line can be parallel to the longitudinal direction of a finger (corresponding to a local increase of the finger width as illustrated in Figure 2) or wherein the longitudinal direction of the pinning line can be substantially orthogonal to the longitudinal direction of a finger or at any suitable angle with respect to the longitudinal direction of a finger. It is an advantage that providing first pinning elements in accordance with the present disclosure substantially improves finger adhesion, while not significantly increasing shadowing losses or decreasing finger conductivity.

More in general, any element or feature that leads to a local increase of the contact area between a finger at a free end 1 1 of the finger and the underlying silicon can be used as a first pinning element 21 .

In a method according to the present disclosure, secondpinning elements or anchoring elements can be provided at the finger endsthat are in physical contact with the at least one busbar. It was surprisingly found that such second pinning elements at the bus bar side improve the adhesion of ribbons soldered to the busbar. This improvement may be related to the ribbon soldering process, wherein solder flowing to the pinning elements may result in an increased solder contact area and thus a better adhesion.

In one aspect, a second pinning element can be provided by locally increasing the width of a finger 101 , 102, 103 at a finger end 12 in physical contact with a busbar. For example, locally increasing the width of the finger can comprise increasing the finger width by a factor of about 2 to 5, such as for example by a factor of about 3. However, the present disclosure is not limited thereto. The width of the finger can be increased over a length that is in the range between about 5% and 50% of the finger length, for example between about 10% and 30% of the finger length, the present disclosure not being limited thereto. An example of a front side metallization pattern with second pinning elements 22 formed by locally increasing the finger width at the finger ends 12 is schematically illustrated in Figure 4.

In another aspect, a second pinning element can contain an element having a linear, square, rectangular, polygonal, circular or oval shape or any other suitable shape, the element being provided at a finger end 12 next to a busbar 100. Examples are schematically illustrated in Figure 5, showing finger ends 12 connected to a busbar 100, with different second pinning elements 22. The second pinning element 22 can be a pinning line, wherein the longitudinal direction of the pinning line can be parallel to the longitudinal direction of a finger (corresponding to a local increase of the finger width as illustrated in Figure 4) or wherein the longitudinal direction of the pinning line can be substantially orthogonal to the longitudinal direction of a finger or at any suitable angle with respect to the longitudinal direction of a finger. It is an advantage that providing second pinning elements in accordance with the present disclosure substantially improves adhesion of metal ribbons soldered to a busbar 100, while not significantly increasing shadowing losses or decreasing finger conductivity.

More in general, any element or feature that leads to a local increase of the contact area between a finger and the underlying silicon at an end 12 of the finger in physical contact with a busbar 100 can be used as a second pinning element 22. A variation to the wider pinning line only close to the busbar is to extend the wider line from the busbar to about half the total finger length. This has the advantages that it increases the overall contact area to the silicon which permits metallization schemes to be used with higher specific contact resistances (e.g. for lower surface doping levels of the silicon at the contact) whilst maintaining the same cell efficiency; and it moves the optimum copper thickness corresponding to minimumresistive power losses to lower values, thus lowering the plating time required.

The method according to an embodiment of the present disclosure may comprise providing a metallization pattern containing first pinning elements at the free finger ends and containing second pinning elements at the ends of the fingers in contact with a busbar.

The foregoing description details certain embodiments of the disclosure. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the disclosure may be practiced in many ways. It should be noted that the use of particular terminology when describing certain features or aspects of the disclosure should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the disclosure with which that terminology is associated.

While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the technology without departing from the spirit of the invention.

Claims

1 . A method for fabricating photovoltaic cells, the method comprising providing plated metal contacts with a contact pattern comprising a plurality of fingers and at least one first pinning element at the free ends of each of the plurality of fingers.

2. A method according to claim 1 , comprising providing plated metal contacts with a contact pattern comprising at least one busbar, a plurality of fingers connected to said at least one busbar and at least one second pinning element at finger ends that are in physical contact with the at least one busbar.

3. A method according to claim 2, comprising providing a second pinning element at each finger end that is in physical contact with the at least one busbar.

4. A method according to any of claims 1 to 3, further comprising soldering a ribbon to the at least one contact pattern.

5. A method according to any of the previous claims, further comprising:

- providing a dielectric layer over the entire front surface of a silicon substrate;

- locally removing the dielectric layer, thereby exposing the underlying silicon surface at locations where metal contacts need to be provided in accordance with a contact pattern according to any of the previous claims; and

- providing the metal contacts in the exposed silicon regions by metal plating to form front side metal contacts.

6. A photovoltaic cell comprising plated metal contacts with a contact pattern comprising a plurality of fingers and at least one first pinning element or anchoring element at the free ends of each of the plurality of fingers.

7. A photovoltaic cell according to claim 6, comprising plated metal contacts with a contact pattern comprising at least one busbar, a plurality of fingers connected to at least one busbar and at least one second pinning element or anchoring element at finger ends that are in physical contact with the at least one busbar.

8. A photovoltaic cell according to claim 7, comprising a second pinning element at each finger end that is in physical contact with the at least one busbar.

9. A photovoltaic cell according to any of claims 6 to 8, further comprising a substrate underlying said finger, wherein said first pinning element is a feature that leads to a local increase of the contact area between a respective finger and said substrate underlying said free end of said finger.

10. A photovoltaic cell according to claim 9, wherein said first pinning element is embodied by or comprises a local increase of the finger width at a free finger end.

1 1 . A photovoltaic cell according to any of claims 6 to 10, further comprising a substrate underlying said finger, wherein said second pinning element is a feature that leads to a local increase of the contact area between a respective finger and said substrate at an end of the finger in contact with a busbar.

12. A photovoltaic cell according to any of claims 6 to 1 1 , wherein said second pinning element is embodied by or comprises a local increase of the finger width at an end of the finger in contact with a busbar.

13. A photovoltaic cell according to any of the claims 6 to 12, wherein the width of the finger at a free finger end or at an end of the finger in contact with said busbar is increased over a length that is in the range of 5% to 50% of the finger length.

14. A photovoltaic cell according any of the previous claims 6 to 13, wherein said first and/or second pinning element has a linear, square, rectangular, polygonal, circular or oval shape.