Wafer solar cell and method for producing a wafer solar cell

Abstract

The invention relates to a wafer solar cell with a semiconductor wafer (1) made of semiconductor material, having a semiconductor wafer surface and 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 with a metal electrode structure (4) applied to the semiconductor wafer surface via opening regions (8) and current contact regions (5) located therein, and each metal electrode structure (4) covers an electrode covering 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 covering area, and wherein the current contact regions (5) are arranged with an areal density of 1000 to 50.000, preferably 3000 to 30,000, and particularly preferably 5000 to 15,000 current contact regions (5) are formed per square millimeter of electrode covering area. Furthermore, the invention relates to a method for producing such a wafer 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 in the sense of the present invention are all solar cells which are produced with the aid of a semiconductor wafer, wherein the semiconductor wafer used forms the structural framework of the solar cell. That is, a substrate material in addition to the semiconductor wafer is not required. Such a wafer solar cell comprising 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 the p-doped region and / or the n-doped region is electrically contacted via current contact regions in each case with a metal electrode structure mounted on the semiconductor wafer surface, and each metal electrode structure covers an electrode covering surface on the semiconductor wafer surface.The metal electrode structure is important in order to discharge charge carriers from the wafer solar cell in the form of an electrical current flow. Usually, 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 regions. For this purpose, the passivation layer is removed in regions, so that opening regions are produced. In these opening regions, the electrical contact is made between the semiconductor surfaces and the metal electrode structures by means of so-called current contact regions. In this case, the current contact regions usually do not take up the same area as the opening regions.In order to produce the opening regions and the current contact regions, metal pastes are printed on the passivation layers, for example, in a screen printing method, and in a subsequent tempering step, the metal pastes etch or burn at least in regions through the dielectric passivation layer. Within the opening regions of the passivation layer formed in this way, electrical metal-semiconductor contacts in the form of the said current contact regions are formed. 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 relating to, for example, their glass frits etching the passivation layers and / or the subsequent annealing step are adapted in such a way 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 centers for the charge carriers separated in the semiconductor wafer, so that the efficiency of the wafer solar cell is lowered by the increasingly occurring, undesirable recombination. Usually, therefore, a compromise must be made between the efficiency which decreases due to increased recombination and the areal opening proportion of the passivation layers, which influences the recombination centers and the contact resistance. This compromise often provides that an opening degree of the passivation layers is in the form of opening regions with a coverage of less than 50% of the electrode covering area below the metal electrode structure.Too high a contact resistance and the degree of activity of the contact recombination are the main causes of poor efficiencies of the wafer solar cell after the metal electrode structure is deposited. A so-called LECO (Laser Enhanced Contact Optimization) process allows a well-controllable and localized improvement of the contact resistance of the wafer solar cell after the application and firing of the metal electrode structure. Such a method is described, for example, in DE 10 2016 009 560 A1. In the method, the wafer solar cell is electrically contacted on its front side and its rear side on the respective metal electrode structure, which is formed on the front side as a contact grid. A wafer solar cell is then locally illuminated under reverse voltage. The local illumination is generated, for example, by a laser, wherein the wafer solar cell is scanned by the laser in order to process the entire surface of the wafer solar cell. The current that arises flows from a laser-illuminated working point to the contactings of the wafer solar cell. Due to the solar cell contacts, a portion of the voltage drops due to resistance. A point close to the contact thus experiences other effective process parameters than a point far from the contact. The applied reverse voltage, which has a considerable 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 way by producing more current contact regions without the density of the recombination centers being increased when viewed in the area.There remains a need for a wafer solar cell with improved contact resistance and a method of making the same.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 associated therewith an improved fill factor. The method for manufacturing this wafer solar cell results in a wafer solar cell having an improved contact resistance without significantly increasing the recombination rate at the electrode surface.According to the invention, it is provided that the opening areas occupy less than 25%, preferably less than 15% and particularly preferably less than 10% of the electrode covering surface and that the current contact areas are formed with an area density of 1000 to 50,000, preferably 3000 to 30000 and particularly preferably 5000 to 15000 current contact areas per square millimeter of electrode covering surface.It has been found that, with the low surface 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 brings about an increased efficiency of the wafer solar cell. The area coverage rate of the current contact regions is preferably below 1%, more preferably below 0.2%, based on the electrode coverage area.The individual current contact regions preferably have an area of less than 3 μm 2, preferably less than 1.5 μm 2 and particularly preferably less than 0.5 μm 2 and it applies for a large part of the current contact regions that the current contact regions are arranged spaced apart from one another. A major part is understood here to mean a proportion of more than 50%.The wafer solar cell is advantageously designed such that the opening regions consist of many individual regions spaced apart from one another, wherein these are arranged predominantly within polygonally adjoining pyramidal semiconductor structures and the pyramidal semiconductor structures have polygonally arranged edges with lengths in the range of more than one micrometer.Furthermore, the wafer solar cell is preferably characterized in that only one opening region is arranged within the majority of the pyramidal semiconductor structures. A majority is understood to mean a proportion of more than 50%.In an advantageous development of the wafer solar cell, the opening regions within the pyramidal semiconductor structures occupy smaller areas, preferably areas of less than 50% and particularly preferably of less than 25%, compared to the areas covered by these pyramidal semiconductor structures.The pyramidal semiconductor structures form three-dimensional surfaces. Therefore, the wafer solar cell is advantageously designed such that the opening regions are for the most part arranged in the region of the tips of the pyramidal semiconductor structures which, viewed in their three-dimensional topology, are spatially closest to the semiconductor material to be electrically contacted.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 constituting a sun-facing side and a back side constituting a sun-averted side. The metal electrode structure on the front side is preferably designed as a plurality of electrode fingers with busbar or busbarless or as a contact grid. The metal electrode structure on the rear side can be designed as a plurality of electrode fingers with busbar or busbarless, as a contact grid or over the entire surface.In a 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 composed of the materials selected from the group consisting of: AlOx(aluminum oxide), SiNx(silicon nitride), SiOxNy(silicon oxynitride), SiOx(silicon oxide) and / or poly-Si (polycrystalline silicon). The passivation layer can 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 a single 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, aluminum-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 of less than 40 nm and particularly preferably of less than 20 nm. In the manufacturing process of 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 opening regions occupying only a small proportion of the electrode covering surface and firstly a wafer solar cell having a comparatively high specific contact resistance is produced.In a further preferred embodiment, the current contact regions lying in opening regions comprise metal crystallites and / or metal-silicon alloy which adjoin the metal electrode structure or project into the latter. These enable an improved contact resistance. These advantageous structures are formed, for example, by a combination of the above-described structural features and the method steps explained in more detail below during production.The wafer solar cell is preferably a PERC (Passivated Emitter Rear Contact), a TopCon (Tunnel Oxide Passivated Contact) solar cell or an IBC (Interdigitated Back Contact) solar cell.The invention also relates to a method for producing a wafer solar cell having the preceding structural features, wherein the following method steps are used:providing a semiconductor wafer made of semiconductor material having a semiconductor wafer surface and at least one p-doped region and at least one n-doped region having at least one p-n junction and a passivation layer deposited on the semiconductor surface,applying a metal electrode structure to the passivation layer so that the metal electrode structure covers an electrode covering surface 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 in such a way that the metal electrode structure is electrically contacted with the semiconductor material over 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 surface and over current contact regions located in these opening regions, and that the wafer solar cell produced has an efficiency of less than 18% at an average specific contact resistance (i.e. contact resistance averaged over the surface of the wafer solar cell) of more than 10 mOhm cm 2, preferably more than 20 mOhm cm 2, particularly preferably more than 50 mOhm cm 2.performing a further treatment step for thermal and / or electrical contact resistance optimization in such a way that the current contact regions are formed within the opening regions with an area density of 1000 to 50,000, preferably 3000 to 8000 and particularly preferably 4000 to 6000 current contact regions per square millimeter of electrode covering area.Because the opening areas occupy less than 25%, preferably less than 20% and particularly preferably less than 15% of the electrode covering surface, the passivating layers of the front and / or rear side are damaged only by comparatively few areas. As a result, the passivation properties of the passivation layer which are detrimental to the efficiency of the wafer solar cell are retained to a comparatively greater extent.The first and second treatment steps may be applied when only one or both of the front and back sides of the provided semiconductor wafer material provided with the metal electrode structure has or have the features described above. The produced wafer solar cell is preferably bifacial.The metal electrode structure is preferably applied by spray printing or screen printing a metal paste. This comprises, for example, Ni, Cu, Ag, Al, Si and / or alloys thereof.The passivation layer is preferably produced in the form of a monolithic layer or a layer package on the provided semiconductor wafer 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 making the passivation layer thicker than usual.In a preferred embodiment, the passivation layer is formed with a layer thickness of more than 50 nm, preferably in the range from 70 to 200 nm. Alternatively or additionally preferably, an additional separating layer having a layer thickness of less than 40 nm, preferably less than 30 nm, is provided on the passivation layer. This additional separating layer is located on the side of the passivation layer facing away from the semiconductor wafer material.The further treatment step is preferably selected as a single step or as a combination of steps selected from the group consisting of:LECO (Laser Enhanced Contact Optimization) step,annealing step,laser heating step; andphotonic sintering.The single or combination step furthermore ensures that a contact resistance sufficiently low for the wafer solar cell is produced. In this case, further current contact regions are generated within the opening regions without the proportion of the surface area of the opening regions significantly increasing.An LECO step is understood to mean that the wafer solar cell is electrically contacted on both sides-i.e. front side and rear side-a reverse voltage is applied and simultaneously 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-side and rear-side contacts is not displaceable over the full surface or, for example, in alternation with a further contact, so that illumination of this side with the point light source is possible. The electrical contacts are connected to a voltage source and a voltage is preferably applied in the reverse direction smaller than the breakdown voltage of the wafer solar cell. Then, the light source is passed over one or both of the front and back surfaces of the wafer solar cell so as to be locally illuminated.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 away from the sun, has a lower degree of metallization than the rear side, which represents the side facing away from the sun. Optical and electrical losses balance out on the front side with a lower degree of metallization, since as little shading as possible is to be effected on the solar cell side facing the sun. On the rear side, however, the optical losses play a downstream role, and therefore the optimum shifts to a higher degree of metallization to a larger contact area, for example by more and / or wider contact fingers. An increased number of fingers also shortens the paths 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-area contacting. The resistance in the full-area contacting is very low. As described, the rear side has a lower resistance, so that lower voltage losses / voltage drops are produced from the contact point to the working point, i.e. the illuminated point. The method thus acts more homogeneously over the entire wafer solar cell.A tempering step is understood to mean heating to a temperature in a range from 500° C. to 850° C. in a furnace.A laser heating step is understood to mean heating to a temperature in a range from 500° C. to 850° C. using a laserPhotonic sintering is understood to mean sintering, i.e. joining materials below the typical melting point to form a solid body, using pulsed laser light with local heating.In a further preferred embodiment, the further treatment step comprises a LECO step in which the wafer solar cell is irradiated with a laser from its sun-averted rear side or the sun-averted front side. It can be advantageous to carry out the coupling-in of the laser radiation via that side of the wafer solar cell which scatters the light less. This is, for example, the low roughness surface which is, for example, chemically etched or mechanically polished. As a rule, this is the rear side of the wafer solar cell. Penetrating light thereby 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.The further treatment step preferably has an LECO step in which the wafer solar cell is irradiated with a power density of 200 to 1,500,000 W / cm 2. In a preferred embodiment, the further treatment step comprises an LECO step in which a reverse applied voltage is 1 to 40 V against the voltage applied to the forward direction of 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 having specific wavelengths. Light sources, preferably lasers, having the above-described power density are advantageous.Further advantages and properties of the invention are explained with reference to preferred exemplary embodiments described below. The figures are not drawn to scale, but are embodied purely schematically and by way of example.The following are shown: 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 flow diagram of a production method according to the invention; FIG. 5 shows a micrograph of a wafer solar cell subjected to a plurality of steps of the method shown in FIG. 4 ; and FIG. 6 shows a further micrograph 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 with a semiconductor wafer surface comprising a front side and a back side. An n-doped region 3 is formed in the region of the front side, while a p-doped region 2 is formed in the region of the rear side. Both on the n-doped region 3 and on the p-doped region 2, a passivation layer 6 is applied in each case. A metal electrode structure 4 in the form of finger electrodes is arranged on each of the passivation layers 6. The metal electrode structure 4 covers an electrode covering surface when viewed in a plan view on the front side or rear side. The metal electrode structures 4 are each in electrical connection with the n- or p-doped regions 3, 2 via current contact regions 5, which are located within opening regions 8 of the passivation layer 6. The opening regions 8 of the passivation layer 6 in this wafer solar cell occupy significantly more than 50% of the electrode covering surface because the metal electrode structures 4 have formed opening regions 8 almost over the full surface through the passivation layers 6 in the course of the production process. The spatial arrangement of n-doped region 3 and p-doped region 2 can be the reverse of the arrangement shown in FIG. 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 area and, viewed microscopically, form an area 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 millimeter of electrode covering area.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 arranged in each case between the passivation layer 6 and the metal electrode structure 4. The additional separating layer 7 assists in the production method the desired, low surface area coverage of the opening regions 8 of the passivation layer 6 in the microscopic structure described above.FIG. 4 shows a flow diagram of a method according to the invention. The method for manufacturing comprises the following steps: First, a step 21 takes place in the form of providing a semiconductor wafer made of semiconductor material comprising a semiconductor wafer surface and at least one p-doped region and at least one n-doped region having 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 the deposition, the metal electrode structure covers an electrode covering surface 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 the opening regions 8 occupy less than 25%, or preferably less than 15%, or even more preferably less than 10%, of the electrode covering surface and the metal electrode structure is electrically contacted with the semiconductor material via some current contact regions 5 which are located within opening regions 8, and that the resulting wafer solar cell has an efficiency of less than 18% at a specific contact resistance averaged over the surface of the wafer solar cell of more than 10 mOhmcm 2, preferably more than 20 mOhmcm 2 or particularly preferably more than 50 mOhmcm 2. 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 further current contact regions 5, current contact regions are formed with an area density of 1000 to 50,000 or 3000 to 30000 or 5000 to 15000 current contact regions per square millimeter of electrode covering area.FIG. 5 shows a micrograph 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 several steps of the method shown in FIG. 4. The wafer solar cell shown has passed through steps 21, 22, 23 of the method shown in FIG. 4, but not through step 24. The passivation layer 6 has a plurality of small opening portions 8 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 absent from the pyramid tips of these pyramidal semiconductor structures. In a few of the opening regions 8, electrical current contact regions 5 have formed, via which the current flow between 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 a specific contact resistance of more than 10 mOhmcm 2, preferably more than 20 mOhmcm 2, particularly preferably more than 50 mOhmcm 2 averaged over the surface of the wafer solar cell is present. In the region of the side surfaces of the pyramidal semiconductor structures, i.e. in the regions on the semiconductor wafer surface which are predominant in terms of area, the passivation layer 6, on the other hand, is intact.FIG. 6 shows a micrograph of a plan view of the passivation layer of a wafer solar cell below its metal electrode structure. The wafer solar cell has passed through steps 21, 22, 23, and 24 of the method shown in FIG. 4. In the microscope photograph shown, the metal electrode structure of the wafer solar cell has been removed again. As can be seen from a comparison with the microscope photograph of FIG. 5 taken on the same scale, the surface density of opening regions 8 in the passivation layer 6 in the pyramidal semiconductor structures is substantially unchanged despite the further method step 24. However, additional current contact regions 5 additionally formed by the further method step 24 are present in the opening regions 8. This explains at a microscopic level that, with a constantly high proportion of covering surface area due to the passivation layer 6, the mean specific contact resistance of the wafer solar cell decreases due to the increased number of current contact regions 5 in already present opening regions 8.List of reference numbers:1 Semiconductor wafer 2 P-doped region 3 N-doped region 4 Metal electrode structure 5 Current contact regions 6 Passivation layer 7 Separating layer 8 Opening regions 21 First method step 22 Second method step 23 Third method step 24 Fourth method stepReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2016 009 560 A1

[0004]

Claims

Wafer solar cell having a semiconductor wafer (1) made of semiconductor material, having a semiconductor wafer surface and 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 covering surface on the semiconductor wafer surface, characterized in that the opening regions (8) occupy less than 25%, preferably less than 15% and particularly preferably less than 10% of the electrode covering surface, and wherein the current contact regions (5) have an area density of 1000 to 50,000, preferably 3000 to 30000 and particularly preferably 5000 to 15000 current contact regions (5) are formed per square millimeter electrode covering surface.Wafer solar cell according to claim 1, characterised in that the individual current contact regions (5) have an area smaller than 3 μm 2, preferably smaller than 1.5 μm 2 and particularly preferably smaller than 0.5 μm 2 and for a large part of the current contact regions (5) it applies that the current contact regions (5) are arranged spaced apart from one another.Wafer solar cell according to claim 1 or 2, characterised in that the opening regions (8) consist of many individual regions spaced apart from one another, wherein these are arranged predominantly within polygonally adjoining pyramidal semiconductor structures and pyramidal semiconductor structures have polygonally arranged edges with lengths in the range of more than one micrometer.Wafer solar cell according to claim 3, characterised in that within the majority of the pyramidal semiconductor structures only one opening region (8) is arranged in each case.Wafer solar cell according to claim 4, characterised in that the opening areas (8) within the pyramidal semiconductor structures occupy smaller areas, preferably areas of less than 50% and particularly preferably of less than 25%, compared to the areas covered by these pyramidal semiconductor structures.Wafer solar cell according to Claim 4 or 5, characterized in that the opening regions (8) are arranged for the most part in the region of the tips of the pyramidal semiconductor structures which, viewed in their three-dimensional topology, are spatially closest to the semiconductor material to be electrically contacted.Wafer solar cell according to one of the preceding claims 1, 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).Wafer solar cell according to claim 7, characterized in that the wafer solar cell comprises 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).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, aluminum-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.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.Wafer solar cell according to one of the preceding claims, characterized in that the current contact regions (5) lying in opening regions (8) have metal crystallites and / or metal-silicon alloys which adjoin the metal electrode structure (4) or project into the latter.Method for producing a wafer solar cell, comprising the following steps - providing a semiconductor wafer (1) made of semiconductor material having a semiconductor wafer surface and at least one p-doped region (2) and at least one n-doped region (3) having 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 surface 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) occupies over 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 surface, A further treatment step for thermal and / or electrical contact resistance optimization is carried out by means of current contact regions (5) located in these opening regions, and in that the resulting wafer solar cell has an efficiency of less than 18% at an average specific contact resistance of more than 10 mOhmcm 2, preferably more than 20 mOhmcm 2, particularly preferably more than 50 mOhmcm 2 in such a way that the current contact regions (5) are formed within the opening regions (8) with an area density of 1000 to 50,000, preferably 3000 to 30000 and particularly preferably 5000 to 15000 current contact regions (5) per square millimeter of electrode covering area.Method according to claim 12, characterised in that the passivation layer (6) is formed with a layer thickness of more than 50 nm and / or an additional separating layer (7) with a layer thickness of less than 40 nm, preferably less than 30 nm, is provided on the passivation layer (6).Method according to one of claims 12 or 13, characterised in that the further treatment step is selected as a single step or as a combination of steps from the group consisting of: - LECO step - laser enhanced contact optimisation, - tempering step, - laser heating step and - photonic sintering.Method according to one of Claims 12 to 14da, characterized in that the further treatment step has a LECO step in which the wafer solar cell is irradiated with a power density of 200 to 1,500,000 W / cm 2.Method according to any of claims 12 to 15da characterized in that the further treatment step comprises a LECO step in which a reverse applied voltage is 1 to 40 V opposite to the forward direction and a local current of 0.5 to 20 A flows.

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