Wafer-based solar cell, and method for manufacturing a wafer-based solar cell

Abstract

The invention relates to a wafer-based solar cell comprising a semiconductor wafer (1) that is made of semiconductor material and has a semiconductor wafer surface, at least one p-doped region (2), and at least one n-doped region (3), wherein the p-doped region (2) and / or the n-doped region (3) are each electrically contacted, via opening regions (8) and current contact regions (5) located therein, with a metal electrode structure (4) attached to the semiconductor wafer surface, and each metal electrode structure (4) covers an electrode coverage area on the semiconductor wafer surface. According to the invention, the opening regions (8) occupy less than 25%, preferably less than 15%, and particularly preferably less than 10% of the electrode coverage area, and the current contact regions (5) are formed with an area density of 1000 to 50,000, preferably 3000 to 30,000, and particularly preferably 5000 to 15,000 current contact regions (5) per square millimetre of electrode coverage area. The invention also relates to a method for manufacturing such a wafer-based solar cell.

Description

The present invention relates to a wafer solar cell and a method for producing such a wafer solar cell. Wafer solar cells within the meaning of the present invention are considered to be all solar cells which are produced by means of a semiconductor wafer, wherein the semiconductor wafer used forms the structural framework of the solar cell. This means that a substrate material in addition to the semiconductor wafer is not required. Such a wafer solar cell having a semiconductor wafer made of semiconductor material has a semiconductor wafer surface and at least one p-doped region and at least one n-doped region, wherein electrical contact is made with the p-doped region and / or the n-doped region in each case with a metal electrode structure attached to the semiconductor wafer surface via current contact regions, and each metal electrode structure covers an electrode covering face on the semiconductor wafer surface. The metal electrode structure is important for removing charge carriers in the form of an electrical current flow from the wafer solar cell. Typically, the semiconductor wafer surfaces of a wafer solar cell are passivated by dielectric passivation layers. In order to be able to remove the charge carriers from the semiconductor wafer, the metal electrode structures of the two polarities must penetrate the electrically non-conductive dielectric passivation layers at least in certain regions. To do this, the passivation layer is removed in certain regions such that opening regions are created. Electrical contact between the semiconductor surfaces and the metal electrode structures is made in these opening regions by means of so-called current contact regions. The current contact regions usually do not occupy the same area as the opening regions. In order to produce the opening regions and the current contact regions, metal pastes are printed onto the passivation layers, for example in a screen printing method, and the metal pastes etch or burn through the dielectric passivation layer at least in certain regions in a subsequent tempering step. Within the opening regions of the passivation layer that are created in this way, electrical metal semiconductor contacts are created in the form of the current contact regions mentioned. These current contact regions are decisive for the electrical contact resistance between the metal electrode structure and the doped semiconductor wafer surfaces of the wafer solar cell. This contact resistance should be as low as possible. Therefore, the compositions of the metal pastes concerning, for example, their glass frits etching the passivation layers and / or the subsequent tempering step are adapted such that comparatively large areas of the passivation layer are etched away in order to increase the chance of obtaining electrically highly conductive metal semiconductor contacts in the form of the current contact regions. However, the opening regions of the passivation layers represent recombination centres for the charge carriers separated in the semiconductor wafer, with the result that the efficiency of the wafer solar cell is reduced by the increased, unwanted recombination. Usually, a compromise must therefore be reached between the efficiency which decreases due to increased recombination and the areal opening portion of the passivation layers influencing the recombination centres and the contact resistance. This compromise often provides for a degree of opening of the passivation layers in the form of opening regions with a coverage of less than 50% of the electrode covering face below the metal electrode structure. An excessively high contact resistance and the degree of contact recombination activity are the main causes of poor efficiencies of the wafer solar cell after applying the metal electrode structure. A so-called LECO (Laser Enhanced Contact Optimization) process allows a highly controllable and localized improvement of the contact resistance of the wafer solar cell after the metal electrode structure has been applied and burnt in. Such a method is described, for example, in DE 10 2016 009 560 A1. In the method, electrical contact is made with the wafer solar cell on its front side and rear side at the respective metal electrode structure which is formed as a contact grid on the front side. A wafer solar cell is then illuminated locally under reverse voltage. The local illumination is generated, for example, by a laser, wherein the wafer solar cell is scanned by the laser for the purpose of processing the entire surface of the wafer solar cell. The current that is established flows from a laser-illuminated operating point to the contacts of the wafer solar cell. A portion of the voltage drops due to the solar cell contacts on account of resistance. A point close to the contact thus experiences different effective process parameters than a point far away from the contact. The applied reverse voltage, which has a significant influence on the quality of the process, is thus distributed inhomogeneously over the wafer solar cell. The contact resistance can be subsequently improved in this manner by generating more current contact regions without increasing the density of the recombination centres when considered in terms of area. There is also a need for a wafer solar cell with improved contact resistance and a method for producing it. It is an object of the invention to provide a wafer solar cell and a method for producing a wafer solar cell, wherein the wafer solar cell has an improved contact resistance for increasing the efficiency of the wafer solar cell. According to the invention, this object is achieved by a wafer solar cell having the features of Claim 1 and by a production method having the features of Claim 7. Advantageous developments and modifications are specified in the dependent claims. The wafer solar cell has an improved contact resistance and thus an improved filling factor. The method for producing this wafer solar cell results in a wafer solar cell with an improved contact resistance without the recombination rate at the electrode surface significantly increasing. The invention provides for the opening regions to occupy less than 25%, preferably less than 15% and particularly preferably less than 10% of the electrode covering face and for the current contact regions to be formed with a surface density of 1000 to 50 000, preferably 3000 to 30 000 and particularly preferably 5000 to 15 000 current contact regions per square millimetre of electrode covering face. It has been found that, at the low area coverage rate of the opening regions, the claimed, special microstructure of the current contact regions, namely in the form of the sum of a multiplicity of microscopic current contact regions, leads to an advantageous combination of a comparatively low contact resistance with a low recombination activity. This combination entails an increased efficiency of the wafer solar cell. The area coverage rate of the current contact regions is preferably less than 1%, even more preferably less than 0.2%, based on the electrode covering face. Preferably, the individual current contact regions have an area of less than 3 pm2, preferably less than 1.5 pm2 and particularly preferably less than 0.5 pm2 and, for a large part of the current contact regions, it holds true that the current contact regions are arranged such that they are spaced apart from one another. In the present case, a large part is understood as meaning a portion of more than 50%. The wafer solar cell is advantageously designed in such a way that the opening regions consist of many individual regions which are spaced apart from one another, wherein these are arranged predominantly within polygonally adjacent, pyramid-shaped semiconductor structures and the pyramid-shaped semiconductor structures have polygonally arranged edges with lengths in the range of more than one micrometre. Further preferably, the wafer solar cell is characterized in that only one opening region is respectively arranged within the majority of the pyramidshaped semiconductor structures. A majority should be understood as meaning a portion of more than 50%. In one advantageous development of the wafer solar cell, the opening regions within the pyramid-shaped semiconductor structures occupy smaller areas, - 5 -preferably areas of less than 50% and particularly preferably of less than 25%, in comparison with the areas covered by these pyramid-shaped semiconductor structures. The pyramid-shaped semiconductor structures form three-dimensional surfaces. Therefore, the wafer solar cell is advantageously designed such that the opening regions are predominantly arranged in the region of the tips of the pyramid-shaped semiconductor structures which, considered in terms of their three-dimensional topology, are spatially closest to the semiconductor material with which electrical contact is to be made. The semiconductor wafer material is preferably silicon. The p-doped region and the n-doped region may be arranged on the same side of the wafer solar cell or on opposite sides. The wafer solar cell has a front side representing a side facing the sun and a rear side representing a side facing away from the sun. The metal electrode structure on the front side is preferably formed as a plurality of electrode fingers with a busbar or without a busbar or as a contact grid. The metal electrode structure on the rear side can be formed as a plurality of electrode fingers with a busbar or without a busbar, as a contact grid or over the full surface. In one preferred embodiment, the wafer solar cell has a passivation layer between the metal electrode structure and the semiconductor material in regions around the opening regions. The passivation layer is usually constructed from the materials selected from the group consisting of: AlOx (aluminium oxide), SiNx (silicon nitride), SiOxNy (silicon oxynitride), SiOx (silicon oxide) and / or poly-Si (polycrystalline silicon). The passivation layer may consist of a single one of these materials, but is preferably constructed as a layer stack combination of a plurality of these materials. The passivation layer, whether as an individual layer or as a layer stack, preferably has a total layer thickness of 70 to 200 nm. The dielectric passivation layer reduces the recombination activity of the electrical charge carriers separated in the semiconductor material. Preferably, the wafer solar cell has an additional separating layer between the metal electrode structure and the semiconductor material in regions around the opening regions on the passivation layer. Preferably, the additional separating layer is selected from the group consisting of: transparent, electrically conductive oxides, such as indium-tin oxide, tungsten-doped indium oxide, aluminium-doped zinc oxide, zinc-doped indium oxide, and non-electrically conductive oxides selected from the group consisting of: nickel oxide, titanium oxide and magnesium fluoride. This additional separating layer has a layer thickness of less than 80 nm, preferably less than 40 nm and particularly preferably less than 20 nm. In the process of producing the wafer solar cell, the metal electrode structure is applied to this additional separating layer. The additional barrier of the separating layer additionally contributes to the fact that the opening regions only occupy a small portion of the electrode covering face and a wafer solar cell with a comparatively high specific contact resistance is first created. In one embodiment that is furthermore preferred, the current contact regions located in opening regions have metal crystallites and / or metal-silicon alloys which adjoin or protrude into the metal electrode structure. These enable an improved contact resistance. These advantageous structures are formed, for example, by a combination of the structural features described above and the method steps during production that are explained in more detail below. The wafer solar cell is preferably a PERC (Passivated Emitter Rear Contact), a TopCon (Tunnel Oxide Passivated Contact) or an IBC (Interdigitated Back Contact) solar cell. The invention also relates to a method for producing a wafer solar cell comprising the preceding structural features, wherein the following method steps are used: - providing a semiconductor wafer made of semiconductor material and comprising a semiconductor wafer surface and at least one p-doped region and at least one n-doped region with at least one p-n junction and a passivation layer deposited on the semiconductor surface, - applying a metal electrode structure to the passivation layer such that the metal electrode structure covers an electrode covering face on the semiconductor wafer surface, - heating the semiconductor wafer in a first thermal treatment step in a first temperature window of 500 to 850 degrees Celsius such that the metal electrode structure makes electrical contact with the semiconductor material via opening regions of the passivation layer, which occupy less than 25%, preferably less than 15% and particularly preferably less than 10% of the electrode covering face, and via current contact regions located in these opening regions, and such that the resulting wafer solar cell has an efficiency of less than 18% in the case of an average specific contact resistance (that is to say a contact resistance averaged over the surface of the wafer solar cell) of more than 10 mOhm cm2, preferably more than 20 mOhm cm2, particularly preferably more than 50 mOhm cm2, - performing a further treatment step for thermal and / or electrical contact resistance optimization such that the current contact regions are formed within the opening regions with a surface density of 1000 to 50 000 preferably 3000 to 8000 and particularly preferably 4000 to 6000 current contact regions per square millimetre of electrode covering face. As a result of the fact that the opening regions occupy less than 25%, preferably less than 20% and particularly preferably less than 15% of the electrode covering face, the passivating layers of the front and / or rear side are damaged only in comparatively few regions. As a result, the passivation properties of the passivation layer, which are beneficial to the efficiency of the wafer solar cell, are retained to a comparatively larger extent. The first and second treatment steps can be applied if only one or both of the front and rear sides of the provided semiconductor wafer material provided with the metal electrode structure has or have the features described above. The wafer solar cell produced is preferably bifacial. The metal electrode structure is preferably applied by spray printing or screen printing a metal paste. This has, for example, Ni, Cu, Ag, Al, Si and / or their alloys. Preferably, the passivation layer is produced in the form of a monolithic layer or a layer package on the semiconductor wafer provided in such a way that a comparatively high contact resistance is provided at a high passivation quality. This can be achieved by adding an additional separating layer and / or by making the passivation layer thicker than usual. In one preferred embodiment, the passivation layer is formed with a layer thickness of more than 50 nm, preferably in the range of 70 to 200 nm. With alternative or additional preference, an additional separating layer is provided on the passivation layer with a layer thickness of less than 40 nm, preferably less than 30 nm. This additional separating layer is located on that side of the passivation layer which faces away from the semiconductor wafer material. The further treatment step is preferably selected, as an individual step or as a combination of steps, from the group consisting of: - a LECO step (Laser Enhanced Contact Optimization), - a tempering step, - a laser heating step and - photonic sintering. The individual or combination step also ensures that a contact resistance that is sufficiently low for the wafer solar cell is created. Further current contact regions are generated within the opening regions without the area portion of the opening regions increasing significantly. A LECO step should be understood as meaning that electrical contact is made with the wafer solar cell on both sides - i.e. front and rear sides -, a reverse voltage is applied and at the same time a light source or point light source, such as a laser beam, scans a surface of the wafer solar cell and heats it locally. At least one of the electrical front and rear side contacts is not over the full surface or movable, e.g. alternately with another contact, and so illumination of this side with the point light source is possible. The electrical contacts are connected to a voltage source and a voltage of less than the breakdown voltage of the wafer solar cell is preferably applied in the reverse direction. Then the light source is guided over one or both of the front and rear sides of the wafer solar cell so that it / they is / are illuminated locally. In particular, bifacial wafer solar cells can be illuminated and processed from both sides. Preferably, the front side of the wafer solar cell, which represents the side facing the sun, has a lower degree of metallization than the rear side, which represents the side facing away from the sun. On the front side, optical and electrical losses are balanced with a lower degree of metallization, since the solar cell side facing the sun should be shaded as little as possible. On the rear side, however, the optical losses play a minor role, and so the optimum shifts to a higher degree of metallization to a larger contact surface, e.g. by means of more and / or wider contact fingers. An increased number of fingers also shortens the distances in the wafer solar cell for the generated charge carriers. If the wafer solar cell is illuminated from the rear side,the front side with the higher resistances lies on the full-surface contact. The resistance in the full-surface contact is very low. The rear side has a lower resistance as described, and so there are lower voltage losses I voltage drops from the contact point to the operating point, i.e. the illuminated point. The method therefore acts more homogeneously across the entire wafer solar cell. A tempering step should be understood as meaning heating to a temperature in a range of 500°C to 850°C in a furnace. A laser heating step should be understood as meaning heating to a temperature in a range of 500°C to 850°C using a laser. Photonic sintering should be understood as meaning sintering, i.e. connecting materials below the typical melting point to form a solid, using pulsed laser light with local heating. In one embodiment that is furthermore preferred, the further treatment step comprises a LECO step in which the wafer solar cell is irradiated with a laser from its rear side facing away from the sun or the front side facing the sun. It may be advantageous to couple in the laser radiation via that side of the wafer solar cell which scatters the light less. This is, for example, the surface with low roughness, which is e.g. chemically etched or mechanically polished. Typically, this is the rear side of the wafer solar cell. This means that penetrating light only acts locally and generates a current over a smaller area. The resulting local current flow then also acts on a smaller area and can be better controlled. Preferably, the further treatment step comprises a LECO step in which the wafer solar cell is irradiated with a power density of 200 to 1 500 000 W / cm2. In one preferred embodiment, the further treatment step comprises a LECO step in which an applied reverse voltage is 1 to 40 V in the opposite direction of the forward direction of the voltage applied to the solar cell and a local current of 0.5 to 20 A flows. This local current can be achieved, for example, by suitable light sources such as lasers with specific wavelengths. Light sources, preferably lasers, with the power density described above are advantageous. Further advantages and properties of the invention are explained using preferred exemplary embodiments that are described below. The figures are not drawn to scale, but rather are purely schematic and illustrative. In the figures: Fig. 1 shows a cross-sectional view of a wafer solar cell according to the prior art; Fig. 2 shows a cross-sectional view of a wafer solar cell according to the invention; Fig. 3 shows a cross-sectional view of a further wafer solar cell according to the invention; Fig. 4 shows a flowchart of a production method according to the invention; Fig. 5 shows a microscope image of a wafer solar cell subjected to multiple steps of the method shown in Fig. 4; and Fig. 6 shows a further microscope image of a wafer solar cell subjected to the method shown in Fig. 4. Fig. 1 shows a cross-sectional view of a wafer solar cell according to the prior art. The wafer solar cell has a semiconductor wafer 1 having a semiconductor wafer surface comprising a front side and a rear side. An n-doped region 3 is formed in the region of the front side, whereas a p-doped region 2 is formed in the region of the rear side. A passivation layer 6 is respectively applied to both the n-doped region 3 and the p-doped region 2. A metal electrode structure 4 in the form of finger electrodes is respectively arranged on each of the passivation layers 6. The metal electrode structure 4 covers an electrode covering face when considered in the plan view of the front side or rear side. The metal electrode structures 4 are each electrically connected to the n-doped or p-doped regions 3, 2 via current contact regions 5 which are located within opening regions 8 of the passivation layer 6. In this wafer solar cell, the opening regions 8 of the passivation layer 6 occupy significantly more than 50% of the electrode covering face because the metal electrode structures 4 have formed opening regions 8 almost completely through the passivation layers 6 during the production process. The spatial arrangement of the n-doped region 3 and p-doped region 2 may be the reverse of the arrangement shown in Figure 1. Fig. 2 shows a cross-sectional view of a wafer solar cell 1 according to the invention. The wafer solar cell 1 shown in Fig. 2 corresponds to the wafer solar cell shown in Fig. 1, with the difference that the opening regions 8 of the passivation layer 6 occupy less than 25% of the electrode covering face and, from a microscopic point of view, form a surface density of current contact regions 5 in the range from 1000 to 10 000, preferably from 3000 to 8000 and particularly preferably from 4000 to 6000 current contact regions 5 per square millimetre of electrode covering face. Fig. 3 shows a cross-sectional view of a further wafer solar cell according to the invention. The wafer solar cell 1 shown in Fig. 3 corresponds to the wafer solar cell shown in Fig. 1, with the difference that an additional separating layer 7 is respectively arranged between the passivation layer 6 and the metal electrode structure 4. In the production method, the additional separating layer 7 supports the desired, low surface coverage of the opening regions 8 of the passivation layer 6 in the previously described microscopic structure. Fig. 4 shows a flowchart of a method according to the invention. The production method comprises the following steps: First, a step 21 is carried out in the form of providing a semiconductor wafer made of semiconductor material and comprising a semiconductor wafer surface and at least one p-doped region and at least one n-doped region with at least one p-n junction and a passivation layer deposited on the semiconductor surface. Step 21 is followed by a step 22 in the form of applying a metal electrode structure to the passivation layer by means of a screen printing step. After application, the metal electrode structure covers an electrode covering face on the semiconductor wafer surface. Step 22 is followed by a step 23 in the form of heating the semiconductor wafer in a first thermal treatment step in a first temperature window of 500 to 850 degrees such that that the opening regions 8 occupy less than 25% or preferably less than 15% or even more preferably less than 10% of the electrode covering face and the metal electrode structure makes electrical contact with the semiconductor material via some current contact regions 5 located within opening regions 8, and such that the resulting wafer solar cell has an efficiency of less than 18% in the case of a specific contact resistance averaged over the surface of the wafer solar cell of more than 10 mOhmcm2, preferably more than 20 mOhmcm2 or particularly preferably more than 50 mOhmcm2. Step 23 is followed by a step 24 in the form of carrying out a further treatment step for thermal and / or electrical contact resistance optimization, wherein, in addition to the already existing current contact regions, further current contact regions 5 are formed with a surface density of 1000 to 50 000 or 3000 to 30 000 or 5000 to 15 000 current contact regions per square millimetre of electrode covering face. Fig. 5 shows a microscope image of a plan view of the passivation layer of a wafer solar cell below its metal electrode structure. This wafer solar cell was subjected to multiple steps of the method shown in Fig. 4. The wafer solar cell shown has gone through steps 21, 22, 23 of the method shown in Fig. 4, but not step 24. In the microscope image shown, the metal electrode structure of the wafer solar cell has been removed. The passivation layer 6 has a multiplicity of small opening regions 8 which are formed only at tips of pyramid-shaped semiconductor structures of the semiconductor wafer material covered by the passivation layer 6. The passivation layer 6 is thus missing at the pyramid tips of these pyramid-shaped semiconductor structures. In a few of the opening regions 8, electrical current contact regions 5 have formed, via which the current flow between the semiconductor material and metal electrode structure takes place. However, the density and quality of these electrical current contact regions is so low and of such poor quality that there is a specific contact resistance of more than 10 mOhmcm2, preferably more than 20 mOhmcm2, particularly preferably more than 50 mOhmcm2, averaged over the surface of the wafer solar cell. The passivation layer 6, on the other hand, is intact in the region of the side surfaces of the pyramid-shaped semiconductor structures, i.e. in the regions that are predominant in terms of area on the semiconductor wafer surface. Fig. 6 shows a microscope image of a plan view of the passivation layer of a wafer solar cell below its metal electrode structure. The wafer solar cell has gone through steps 21, 22, 23 and 24 of the method shown in Fig. 4. In the microscope image shown, the metal electrode structure of the wafer solar cell has again been removed. As can be seen from a comparison with the microscope image made on the same scale from Fig. 5, the surface density of 5 opening regions 8 in the passivation layer 6 in the pyramid-shaped semiconductor structures is substantially unchanged despite the further method step 24. However, further current contact regions 5 which are additionally created by the further method step 24 are present in the opening regions 8. This explains on a microscopic level that, with a consistently high 10 proportion of covering face through the passivation layer 6, the average specific contact resistance of the wafer solar cell decreases due to the increased number of current contact regions 5 in already existing opening regions 8. of reference signs: Semiconductor wafer p-doped region n-doped region Metal electrode structure Current contact regions Passivation layer Separating layer Opening regions First method step Second method step Third method step Fourth method step

Claims

1. Wafer solar cell having a semiconductor wafer (1) made of semiconductor material and comprising a semiconductor wafer surface and at least one p-doped region (2) and at least one n-doped region (3), wherein electrical contact is made with the p-doped region (2) and / or the n-doped region (3) in each case with a metal electrode structure (4) attached to the semiconductor wafer surface via opening regions (8) and current contact regions (5) situated in the latter, and each metal electrode structure (4) covers an electrode covering face on the semiconductor wafer surface, characterized in thatthe opening regions (8) occupy less than 25%, preferably less than 15% and particularly preferably less than 10% of the electrode covering face, and wherein the current contact regions (5) are formed with a surface density of 1000 to 50 000, preferably 3000 to 30 000 and particularly preferably 5000 to 15 000 current contact regions (5) per square millimetre of electrode covering face.

2. Wafer solar cell according to Claim 1, characterized in that the individual current contact regions (5) have an area of less than 3 pm2, preferably less than 1.5 pm2 and particularly preferably less than 0.5 pm2 and, for a large part of the current contact regions (5), it holds true that the current contact regions (5) are arranged such that they are spaced apart from one another.

3. Wafer solar cell according to Claim 1 or 2, characterized in that the opening regions (8) consist of many individual regions which are spaced apart from one another, wherein these are arranged predominantly within polygonally adjacent pyramid-shaped semiconductor structures and pyramid-shaped semiconductor structures have polygonally arranged edges with lengths in the range of more than one micrometre.

4. Wafer solar cell according to Claim 3, characterized in that only one opening region (8) is respectively arranged within the majority of the pyramid-shaped semiconductor structures.

5. Wafer solar cell according to Claim 4, characterized in that the opening regions (8) within the pyramid-shaped semiconductor structures occupy smaller areas, preferably areas of less than 50% and particularly preferably of less than 25%, in comparison with the areas covered by these pyramid-shaped semiconductor structures.

6. Wafer solar cell according to Claim 4 or 5, characterized in that the opening regions (8) are predominantly arranged in the region of the tips of the pyramidshaped semiconductor structures which, considered in terms of their threedimensional topology, are spatially closest to the semiconductor material with which electrical contact is to be made.

7. Wafer solar cell according to any one of the preceding claims, characterized in that the wafer solar cell has a passivation layer (6) between the metal electrode structure (4) and the semiconductor material in regions around the opening regions (8).

8. Wafer solar cell according to Claim 7, characterized in that the wafer solar cell has an additional separating layer (7) between the metal electrode structure (4) and the semiconductor material in regions around the opening regions (8) on the passivation layer (6).

9. Wafer solar cell according to Claim 8, characterized in that the additional separating layer (7) is selected from the group consisting of:transparent, electrically conductive oxides, such as indium-tin oxide, tungsten-doped indium oxide, aluminium-doped zinc oxide, zinc-doped indium oxide, and non-electrically conductive oxides selected from the group consisting of: nickel oxide, titanium oxide and magnesium fluoride.

10. Wafer solar cell according to Claim 8 or 9, characterized in that the additional separating layer (7) has a layer thickness of less than 80 nm, preferably less than 40 nm and particularly preferably less than 20 nm.

11. Wafer solar cell according to any one of the preceding claims, characterized in that the current contact regions (5) located in opening regions (8) have metal crystallites and / or metal-silicon alloys which adjoin or protrude into the metal electrode structure (4).

12. Method for producing a wafer solar cell, comprising the following steps - providing a semiconductor wafer (1) made of semiconductor material and comprising a semiconductor wafer surface and at least one p-doped region (2) and at least one n-doped region (3) with at least one p-n junction and a passivation layer (6) deposited on the semiconductor surface,- applying a metal electrode structure (4) to the passivation layer (6) such that the metal electrode structure (4) covers an electrode covering face on the semiconductor wafer surface,- heating the semiconductor wafer in a first thermal treatment step in a first temperature window of 500 to 850 degrees such that the metal electrode structure (4) makes electrical contact with the semiconductor material via opening regions (8) of the passivation layer, which occupy less than 25%, preferably less than 15% and particularly preferably less than 10% of the electrode covering face, and via current contact regions (5) located in these opening regions, and such that the resulting wafer solar cell has an efficiency of less than 18% in the case of an average specific contact resistance of more than 10 mOhmcm2, preferably more than 20 mOhmcm2, particularly preferably more than 50 mOhmcm2,- performing a further treatment step for thermal and / or electrical contact resistance optimization such that the current contact regions (5) are formed within the opening regions (8) with a surface density of 1000 to 50 000, preferably 3000 to 30 000 and particularly preferably 5000 to 15 000 current contact regions (5) per square millimetre of electrode covering face.

13. Method according to Claim 12, characterized in that the passivation layer (6) is formed with a layer thickness of more than 50 nm, and / or an additional separating layer (7) is provided on the passivation layer (6) with a layer thickness of less than 40 nm, preferably less than 30 nm.

14. Method according to either one of Claims 12 and 13, characterized in that the further treatment step is selected, as an individual step or as a combination of steps, from the group consisting of:- a LECO step - Laser Enhanced Contact Optimization,- a tempering step,- a laser heating step and- photonic sintering.

15. Method according to any one of Claims 12 to 14, characterized in that the further treatment step comprises a LECO step in which the wafer solar cell is irradiated with a power density of 200 to 1 500 000 W / cm2.

16. Method according to any one of Claims 12 to 15, characterized in that the further treatment step comprises a LECO step in which an applied reverse voltage is 1 to 40 V in the opposite direction to the forward direction and a local current of 0.5 to 20 A flows.

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