EP2100337A2

EP2100337A2 - Solar cell and method for producing a solar cell

Info

Publication number: EP2100337A2
Application number: EP07847986A
Authority: EP
Inventors: Jörg Müller; Robert Wade; Markus Hlusiak
Original assignee: Q Cells SE
Current assignee: Q Cells SE
Priority date: 2006-12-08
Filing date: 2007-12-07
Publication date: 2009-09-16
Also published as: DE102006058267A1; WO2008068336A3; EP2107615A2; US20120266947A1; EP2107615A3; WO2008068336A2

Abstract

The invention relates to a solar cell (1), comprising a flat semiconductor substrate (13) having a front (11) and a back (12), a plurality of holes (14) connecting the front (11) and the back (12), and electric contacts (31, 32) collecting energy arranged exclusively on the back (12). The front (11) comprises highly doped regions (21, 25) and weakly doped regions (22) of a first type such that the holes (14) are located in a highly doped region (21) or are adjacent thereto. According to a first aspect of the invention, the highly doped regions (21) are arranged locally around the holes (14). According to a second aspect of the invention, the front (11) comprises at least one region (15) without holes and the highly doped regions comprise one or more regions (25), extending into the at least one region without holes (15). The invention further relates to methods for producing said solar cells.

Description

Solar cell and process for producing a solar cell

description

The invention relates to a solar cell and method for producing a solar cell.

BACKGROUND OF THE INVENTION

An emitter wrap-through (EWT) solar cell has no metallization on the front. The emitter is passed through a large number of small holes (d <100 microns) on the back of the cell and contacted there. The light-generated current is conducted via the emitter and the holes on the back of the cell to contacts arranged there and tapped there.

The strength of emitter doping plays an important role. On the one hand, a higher doping results in a smaller sheet resistance and thus contributes to the reduction of ohmic losses. In addition, the contact resistance between emitter and metallization at a high emitter doping is significantly lower. On the other hand, high doping reduces the ability of the cell, in particular, to convert short-wave light into current (so-called blue sensitivity). Accordingly, a compromise between good conductivity and blue sensitivity must be chosen for the doping.

In order to reduce the effective sheet resistance of an EWT solar cell, it is known, for example, from US 2005/0176164 A1, in the inner wall of the holes

Cell to make a higher doping than on the front of the cell. Such

Concept is also referred to as selective emitter or selective doping. From the

US 2005/0176164 A1 is also known (FIG. 3D of this publication) to form a heavily doped strip on the front side of an EWT solar cell, which comprises a plurality of the holes.

US Pat. No. 7,144,751 B1 describes a solar cell in which a higher doping is present in the inner wall of the holes of the solar cell and along a grid on the front side of the solar cell. The present invention is based on the object to provide a solar cell with a low effective sheet resistance at the same time high blue sensitivity and methods for producing such a solar cell.

SUMMARY OF THE INVENTION

In one aspect of the invention, the present invention provides a solar cell having a plurality of holes in which the front side has highly doped regions and lightly doped regions of a first doping type such that the holes of the solar cell each lie in a highly doped region or adjoin one another. The highly doped regions are arranged locally around the holes of the solar cell. The individual local highly doped regions are spatially separated from one another and thus do not form a coherent structure on the front side of the solar cell. It is thus provided that the front-side emitter has no homogeneous doping, but rather is highly doped in the immediate vicinity of the holes. Due to this special doping, the majority of the front side, which is lightly doped, has an emitter with high blue sensitivity. The heavily doped regions simultaneously reduce series resistance and contact resistance.

Among other things, the strength of the doping of the areas of the front side in the immediate vicinity of the holes is decisive for the series resistance of a solar cell. The reason for this is that the current in these areas flows approximately radially to the hole and thus in these areas the highest current density occurs. If the radius of the holes is thus heavily doped, the disadvantageous electrical resistance decreases. The circumference of the holes, which experiences a high doping, for example, has a radius of between a few 100 microns.

The solar cell according to the invention is, for example, an EWT solar cell or a metallization wrap-through (MWT) solar cell.

In one embodiment of the invention, the front side forms highly doped regions which are substantially circular. In this case, it can be provided that the highly doped regions form a circular ring with an inner radius and an outer radius, wherein the inner radius corresponds to the radius of the hole which the respective circular ring surrounds. gives. The difference in radius between outer radius and inner radius is, for example, between 50 μm and 300 μm, in particular between 100 μm and 200 μm.

In another embodiment, the front side forms highly doped regions which radiate star-shaped or fan-shaped from the respective holes. In this case, the highly doped regions, which radiate star-shaped or fan-shaped from the respective holes, in one embodiment each comprise finger-shaped regions.

In a further aspect of the invention, the semiconductor substrate and thus the front side of the solar cell have areas without holes. There may be various reasons for this. For example, there are certain areas of the solar cell in which holes are undesirable. These are in particular areas in which no holes are possible on the back, for example, because there solder joints or bus bars (so-called bus bars) are realized with the base polarity of the semiconductor substrate or because it is edge regions of the cell. Furthermore, it is desirable in principle for technological reasons to keep the number of holes as small as possible. Such technological reasons include, for example, the necessity of drilling the holes or the number of contact fingers and bus bars on the back. On the other hand, it is advantageous for a low series resistance to provide the largest possible number of holes. Thereby, at e.g. EWT solar cells always make a compromise regarding the number of holes. If, as by the present invention, a low series resistance can be realized even with a comparatively small number of holes, the holes may have a relatively large distance from one another, so that larger areas without holes lie between the holes.

In such hole-free areas, the distance to the next hole and thus the current path on the front side is much larger than desired. This leads to a large parasitic electrical series resistance in these areas. It is now provided in accordance with this further aspect of the invention that the heavily doped regions of the front comprise regions which extend into the hole-free regions. By way of example, these are finger-shaped regions which extend in a fan-like manner into the hole-free regions. These finger-shaped, heavily doped areas are well-conducting current paths that transport the current collected in the hole-free areas to a hole. This reduces the finger-shaped, highly doped rich the series resistance. At the same time, their transparency ensures good light utilization.

The highly doped regions as a whole, and in particular the finger-shaped regions which extend into hole-free regions, may in one embodiment be formed in trenches in the semiconductor substrate.

A method according to the invention for producing an emitter-wrap-through (EWT) solar cell comprises the following steps: providing a planar semiconductor substrate having a front side and a rear side,

- Full-surface application of a diffusion mask at least on the front,

Producing a plurality of holes in the semiconductor substrate which connect the front side and the rear side, selectively removing the diffusion mask such that the diffusion mask is removed at least in regions in which the holes are located or at which the holes adjoin,

Making a strong diffusion with a dopant of a first type, wherein the areas of the front side, in which the diffusion mask has been selectively removed, are high-doped,

Complete removal of the diffusion mask at least from the front,

- Making a slight diffusion with a dopant of the first type.

These steps do not necessarily have to be performed completely in the order given. For example, it can be provided that the holes are formed only after the selective removal of the diffusion mask, or even before the application of the diffusion mask.

Another method according to the invention for producing an emitter-wrap-through (EWT) solar cell comprises the following steps:

Providing a planar semiconductor substrate having a front side and a rear side,

Forming a plurality of holes in the semiconductor substrate connecting the front side and the back side, Making a strong diffusion with a dopant of a first type at least on the front side, the entire front side being heavily doped,

- Full-surface application of a diffusion mask at least on the front,

selectively removing the diffusion mask such that the diffusion mask is present only in defined areas in which the holes are located or adjacent to the holes, thereafter

Removing the heavily doped areas in the areas of the front, which lie outside the diffusion mask, and

Making a light diffusion with a dopant of the first type at least on the front side,

Complete removal of the diffusion mask at least from the front.

In this variant of the method, therefore, the entire front side is first heavily doped. The diffusion mask is patterned complementary to the diffusion mask of the method of claim 24. Outside the structured diffusion mask, the high doping is removed again. There is then a low doping in these areas. Finally, a complete removal of the diffusion mask takes place at least from the front side.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention will be explained in more detail with reference to the figures of the drawing with reference to its embodiments. It is understood that these figures represent only typical embodiments of the invention and are therefore not to be regarded as limiting their scope. Show it:

Figure 1 shows the top of an EWT solar cell with locally formed highly doped areas.

FIG. 2 shows a section through a partial region of the EWT solar cell of FIG. 1; FIG.

3 schematically shows the current flow on the front side of an EWT solar cell;

4 shows a plan view of a further embodiment of an EWT solar cell with locally highly doped regions; 5 shows a diffusion mask applied to a semiconductor substrate for masking a strong diffusion, wherein the diffusion mask has circular, local recesses in the area of holes of the semiconductor substrate;

FIG. 6 shows a diffusion mask applied to a semiconductor substrate for masking a strong diffusion, the diffusion mask having linear recesses, each containing a row of holes; FIG. and

Fig. 7 is a known in the prior art structure of current-collecting electrical contacts on the back of an EWT solar cell.

Figures 1 and 2 show an EWT solar cell with selectively formed on the top highly doped areas. The solar cell 10 includes a semiconductor substrate 13, for example, a silicon wafer, having a top 1 1 and a bottom 12. In the semiconductor substrate, a plurality of through holes 14 are formed connecting the top 11 to the bottom 12. The holes 14 are arranged like a grid, wherein the distance between two holes in one direction is between 0.2 and 0.8 mm and in the direction perpendicular thereto, for example 2 mm. The hole diameters are typically between 30 and 100 μm.

The holes 14 are made, for example, by laser drilling. However, other manufacturing methods such. B. etching or mechanical drilling conceivable.

The front side 1 1 of the semiconductor substrate 13 has a doping of a first type, for example an n-type doping. The semiconductor substrate 13 itself also has a doping of a second, opposite type, for example a p-doping. In the following, for the sake of simplicity, n-doping and p-doping will be mentioned, even if the doping can obviously be formed in reverse.

The n-doping is formed on the front side 1 1 and extends through the inner wall of the holes 14 to the bottom 12 of the semiconductor substrate 13. The bottom side 12 has, in addition to the n-doped regions 121 further, second regions 122, which have a p-type doping. This is the p-doping of the semiconductor substrate 13, which can optionally be locally reinforced by additional doping.

The n-doped regions 121 of the underside 12 are connected to first electrical contacts 31 in the form of a finger contact. The p-doped regions 122 of the underside 12 are connected to second electrical contacts 32, also in the form of a finger contact. The electrical contacts 31, 32 are electrically isolated from each other, for example, by means of a diffusion barrier (not shown).

In the illustrated EWT cell concept, the contacts for both poles are on the back of the cell. The n-type emitter region is passed through many of the tiny holes in the cell from the front to the back and only then contacted there.

It should be noted that Figures 1 and 2 do not show all the elements of a complete EWT solar cell. Only those elements are shown which are necessary for the understanding of the present invention. For example, the skilled person is aware that an EWT solar cell in addition to the elements shown on the upper side 1 1 for reflection reduction texturing and one or more passivation layers, such as a SiN _x layer may have. Furthermore, passivation layers and / or diffusion barriers for the electrical separation of the first and second electrical contacts 31, 32 can also be provided on the rear side 12. The provided electrical contacts 31, 32 may, for example, aluminum and silver or consist exclusively of silver. These are only examples of further embodiments that the EWT solar cells can have.

It is now provided that the front side 1 1, which forms the emitter adjacent to the holes 14 local highly doped regions 21 which thus have, for example, an n ++ - doping. These highly doped regions 21 have, for example, the shape of a circular ring, as shown in FIG. However, they can also assume other shapes, for example star-shaped or spiral-shaped. In this case, the heavily doped regions 21 are local in the sense that they do not touch and do not overlap one another.

Outside these areas in the other surface area 22, however, the surface is lightly doped (n + doping). In the highly doped regions 21 is the Sheet resistance is preferably less than 30 ohms / sq, preferably less than 15 ohms / sq, and in a preferred embodiment, about 5 ohms / sq. In the area of the remaining surface 22 there is a weak doping with a sheet resistance of, for example, more than 80 ohms / sq.

From Figure 3 it can be seen that the current density due to the fact that the current flows approximately radially to a hole 14 increases with decreasing distance from the hole 14, so that in the vicinity of a hole 14, the highest current density occurs. The high doping of this circumference of the holes 14 reduces the layer resistance of the EWT solar cell. At the same time, the predominant part 22 of the front side 1 1 has an emitter with a weak doping and, accordingly, a high blue sensitivity (i.e., a good ability to convert shortwave light into current).

FIG. 4 shows an alternative embodiment of an EWT solar cell in which certain local areas are heavily doped on the upper side 11 of the solar cell. For this purpose, in the exemplary embodiment of FIG. 4, highly doped regions 23 formed in rows, highly doped regions 24 formed in rows and elongate, finger-shaped narrow regions 25 are provided. At the crossing points of the heavily doped rows and columns 23, 24, the holes 14 are arranged, so that the front side in the immediate vicinity of the holes 14 as well as in the embodiment of Figures 1, 2 is highly doped.

The fingers 25 extend, starting from a hole 14, fan-shaped into a hole-free region 15. Such a hole-free region arises, for example, in that a current busbar (busbar), a soldering point or the like (see also FIG and therefore no holes can be formed in this area. However, this results in that the hole-free region 15 has a large distance to the next hole 14. This large distance can disadvantageously lead to a large parasitic electrical series resistance. The formation of highly doped fingers 25 ensures that the conductor carriers generated in the hole-free region 15 can be conducted via well-conducting current paths to the holes 14 and from these to the cell rear side 11 to the corresponding contacts 31, 32. Corresponding fingers can also be formed, for example, at edge regions of the solar cell 10 or in each case between two holes 14 of a grid, if a small number of holes are present in the solar cell and the grid is correspondingly large. It will also be pointed out that corresponding fingers which extend into hole-free regions can be realized in combination with the embodiment of FIG.

For example, the fingers 25 have a width less than or equal to 50 μm. In one embodiment, the fingers 25 have a variable width, whereby they preferably taper towards their end facing away from the associated hole.

It should be noted that the embodiment of Figures 1, 2 and 4 are to be understood as exemplary only. The highly doped regions may also have a different geometry, for example, be arranged quadrangular or oval around the individual holes around. For example, the finger-like regions 25 may each comprise only one finger, which is straight or curved (also helical).

To produce the highly doped regions on the top side 1 1 of the solar cells, the application of a diffusion mask to the semiconductor substrate 13 and a structuring of this diffusion mask is provided. FIG. 5 shows a diffusion mask 40 applied to the semiconductor substrate 13, which has circular local recesses 41 in the region of the holes 14. FIG. 6 shows a diffusion mask 40 applied to the semiconductor substrate 13, which has line-shaped or strip-shaped recesses 42, each of which comprises a row of holes. The mask 40 serves in each case to mask a strong diffusion, for example with phosphorus. The mask of Figure 6 is compared to the mask of Figure 5 easier to manufacture, but leads to an EWT solar cell with a lower efficiency due to a lower sensitivity to blue.

A mask according to FIGS. 5 or 6 is produced, for example, by initially applying a diffusion mask 40 to the semiconductor substrate 13 over the whole area. For this purpose, the semiconductor substrate 13 is oxidized, for example, so that an SiO ₂ layer is formed. However, a diffusion barrier can also be produced in another way. The existing example of silicon dioxide diffusion mask 40 has a thickness of, for example, 200 nm. This layer is not penetrated by the dopants within the usual diffusion conditions: since oxide impedes diffusion, the natural surface oxide also has a disturbing effect and prevents a uniform penetration of the dopant into the silicon crystal.

The diffusion mask 40 is applied at least on the front side of the solar cell, but preferably both on the front and on the back.

Before or after production of the holes (preferably after production of the holes), a selective removal of the diffusion mask in the subregions 41 and 42 of FIGS. 5 and 6 takes place. The selective removal of the diffusion mask in these regions can take place in various ways.

A first embodiment for this provides that an etching paste is applied in the corresponding areas on the front side. In the embodiment of FIG. 5, etching paste is applied in round areas 41, which each contain a hole 14. In the embodiment of FIG. 6, the etching paste is applied in strips, with each strip 42 containing a row of holes. The etching paste removes the diffusion mask in the applied areas.

In a second exemplary embodiment, the removal of the mask material in the regions 41, 42 in question takes place by laser ablation. A line or punctiform laser spot is used.

In a third embodiment, the diffusion mask is etched through a patterned etch layer. The structured etching layer is applied, for example, by screen printing, by an inkjet method or by dispensing.

A fourth embodiment uses the capillary effect. In this case, the underside 12 of the solar cell 10 is immersed in an etching solution. Due to the capillary effect, the etching solution is pulled through the holes 14 to the front 1 1. Here, a local area around the holes 14 is etched, with some etching solution flowing out of the holes and / or etching through the vapors of the solution. After the selective removal of the diffusion mask 40 in the subregions 41, 42 of the front side, a strong diffusion of a dopant now takes place. For example, there is a strong diffusion with phosphorus. For this purpose, it can be provided that the diffusion takes place in the throughflow method, wherein a carrier gas (Ar, N ₂ ) is enriched to a desired extent with dopant from a source and passed into a quartz tube, in which the semiconductor substrate is located. As a dopant source, for example, PH _{3 is} used. Alternatively, a liquid dopant source, such as POCl _{3, is} used. The respective liquid is then in a tempered bubbler vessel, which is purged by the carrier gas. With the carrier gas, the dopant passes into the quartz tube for diffusion.

Likewise, it is possible, for example, to carry out a high degree of diffusion via a printed diffusion paste, as described, for example, in US 2005/07 61 64 A1.

After performing the strong diffusion, the diffusion mask 40 is completely removed from the cell front side 1 1. Subsequently, a slight diffusion takes place to provide lightly doped regions on the front side of the solar cell. The light diffusion takes place for example also by means of phosphorus.

It is followed by other processes such as passivation processes, top surface texturing processes and processes for making positive and negative contacts on the back side of the solar cell in respective areas. These further steps are known to any person skilled in the art, so that will not be discussed further.

FIG. 7 shows a typical structure of a current-collecting back contact of an EWT solar cell. The rear side has first finger contacts 31 of a positive polarity, second finger contacts 32 of a negative polarity, and a total of four current bus bars (busbars) 33, 34, of which two are each of the same polarity. The currents collected via the finger contacts 31, 32 are picked off from the solar cell via the busbars 33, 34.

Claims

claims

A solar cell (10) comprising: - a planar semiconductor substrate (13) having a front side (1 1) and a rear side (12), a plurality of holes (14) forming the front side (1 1) and the rear side (12) connect, and only on the back (12) arranged current-collecting electrical see contacts (31, 32), wherein the front side (1 1) has a doping of a first type, the inner wall of the holes (14) a doping of the first type or a Metallization, the rear side (12) has first regions (121) which comprise the holes (14) and which have a doping of the first type, and second regions

(122) having a doping of a second type, the current collecting electrical contacts (31, 32), first contacts (31) contacting the first regions (121) of the back (12), and second contacts (32) connecting the contacting second areas (122) of the rear side (12), and the front side (1 1) has heavily doped areas (21) and lightly doped areas (22) of the first type, such that the holes (14) are each in one highly doped region (21) or adjacent to such,

characterized in that

the highly doped regions (21) are arranged locally around the holes (14), the individual local heavily doped regions (21) being spatially separated from one another.

2. Solar cell according to claim 1, characterized in that the local highly doped regions (21) are circular.

3. Solar cell according to claim 2, characterized in that the highly doped regions (21) form a circular ring with an inner radius and an outer radius, wherein the inner radius of the radius of the hole (14) corresponds to the respective ge circular ring surrounds, and wherein the difference in radius between outer radius and inner radius between 50 .mu.m and 300 .mu.m, in particular between 100 .mu.m and 200 .mu.m.

4. Solar cell according to claim 1, characterized in that the front side (1 1) forms highly doped regions which emit star-shaped or fan-shaped from the respective holes (2).

5. Solar cell according to claim 3, characterized in that the highly doped regions which emit star-shaped or fan-shaped from the respective holes (2), each comprise fingerfömige areas (25).

6. Solar cell according to one of claims 2 to 5, characterized in that the highly doped regions (21) have a diameter between 50 .mu.m and 1000 .mu.m, in particular between 100 .mu.m and 500 .mu.m.

7. A solar cell (10) comprising: a planar semiconductor substrate (13) having a front side (1 1) and a rear side (12), - a plurality of holes (14), the front side (1 1) and the rear side

(12) connect, and arranged exclusively on the back (12) current collecting electrical contacts (31, 32), wherein the front side (11) has a doping of a first type, - the inner wall of the holes (14) a doping of the first type or a metallization, the back side (12) having first regions (121) comprising the holes (14) and having a doping of the first type, and second regions (122) having a doping of a second type, - the current collecting elements electrical contacts (31, 32) comprise first contacts (31) which contact the first regions (121) of the rear side (12) and second contacts (32) which contact the second regions (122) of the rear side (12), the front side (11) has heavily doped regions (21, 23, 24, 25) and weakly doped regions (22) of the first type such that the holes (14) each have because they lie in or are adjacent to a highly doped region (21, 23, 24, 25),

characterized,

in that the semiconductor substrate (13) and thus the front side (11) has at least one area (15) without holes and the highly doped areas of the front area comprise one or more areas (25) which extend into the at least one hole-free area (15) ,

A solar cell according to claim 7, characterized in that the heavily doped regions of the front side comprise one or more finger-shaped regions (25) which extend into the at least one hole-free region (15).

9. Solar cell according to claim 8, characterized in that the finger-shaped regions (25) are each formed in a straight line.

10. Solar cell according to claim 8 or 9, characterized in that at least some of the finger-shaped regions (25) have a variable width.

1 1. A solar cell according to claim 10, characterized in that the finger-shaped portions (25) taper towards its end.

12. Solar cell according to one of claims 8 to 1 1, characterized in that a plurality of finger-shaped regions (25) extending from a hole (14) fan-shaped in a hole-free region (15).

13. Solar cell according to one of claims 8 to 12, characterized in that the finger-shaped regions (25) have a width of <50 microns.

14. Solar cell according to one of claims 8 to 13, characterized in that the finger-shaped regions (25) in trenches of the semiconductor substrate (13) are formed.

15. Solar cell according to one of the preceding claims, characterized in that the lightly doped regions (22) of the front side have a sheet resistance of> 80 ohms / sq.

16. Solar cell according to one of the preceding claims, characterized in that the highly doped regions (21, 23, 24, 25) have a sheet resistance kleinergleich 30 ohms / sq, in particular smaller equivalent 15 ohms / sq, in particular of about 5 ohms / sq.

17. Solar cell according to one of the preceding claims, characterized in that the highly doped regions (21, 23, 24, 25) are formed in on the semiconductor substrate (13) formed trenches.

18. Solar cell according to one of the preceding claims, characterized in that also the inner wall (141) of the holes (14) is highly doped.

19. Solar cell according to one of the preceding claims, characterized in that also arranged on the back (12) areas (121) of the doping of the first type are highly doped.

20. Solar cell according to one of the preceding claims, characterized in that the doping of the first type is an n-type doping and the doping of the second type is a p-type doping, or vice versa.

21. Solar cell according to one of the preceding claims, characterized in that the semiconductor substrate (13) is a p-doped or n-doped monocrystalline or polycrystalline silicon substrate.

22. Solar cell according to one of the preceding claims, characterized in that the first electrical contacts (31) and the second electrical contacts

(32) form two interlocking combs on the back (12) of the solar cell.

23. Solar cell according to one of the preceding claims, characterized in that the holes (14) are arranged in a grid and the distance between two holes (14) in a direction between 0.2 and 0.8 mm and in the direction perpendicular thereto between 1 and 2.5 mm.

24. A method for producing a solar cell comprising the steps of: providing a planar semiconductor substrate having a front side and a rear side;

- Whole-area application of a diffusion mask (40) at least on the front side (1 1),

- Producing a plurality of holes (14) in the semiconductor substrate (23) connecting the front side (1 1) and the back (12),

selectively removing the diffusion mask (40) such that the diffusion mask (40) is removed at least in regions (41, 42) in which the holes (14) lie or against which the holes (14) adjoin,

- making a strong diffusion with a dopant of a first type, wherein the areas (41, 42) of the front side (1 1), in which the diffusion mask was selectively removed, are highly doped,

Complete removal of the diffusion mask (40) at least from the front

(11 X

- Making a slight diffusion with a dopant of the first type.

25. A process for producing a solar cell comprising the steps of:

Providing a planar semiconductor substrate (13) having a front side (1 1) and a rear side (12),

- Making a strong diffusion with a dopant of a first type at least on the front side (1 1), wherein the entire front side (1 1) is highly doped,

- Whole-area application of a diffusion mask at least on the front side (1 1),

- selectively removing the diffusion mask (40) such that the diffusion mask is present only in defined areas in which the holes (14) or to which the holes (14) are adjacent, thereafter

Removing the heavily doped areas in the areas of the front side (1 1), which are outside the diffusion mask (40), - Making a slight diffusion with a dopant of the first type at least on the front side (1 1), and

- Complete removal of the diffusion mask (40) at least from the front (1 1).

26. The method of claim 25, wherein the step of removing the heavily doped regions in the regions outside the diffusion mask comprises ablation and / or etching.

27. The method according to claim 24 or 25, characterized in that the strong diffusion comprises a phosphorus diffusion and the light diffusion also comprises a phosphorus diffusion.

28. The method according to claim 24, characterized in that the diffusion mask (40) in the selective removal of the diffusion mask (40) in local areas

(41) around the respective holes (14) around is removed.

A method according to claim 25, characterized in that the diffusion mask (40) is maintained upon selective removal of the diffusion mask in local areas around the respective holes (14).

30. Method according to claim 24, characterized in that the diffusion mask (40) is removed during the selective removal of the diffusion mask (40) in strip-shaped areas (42) comprising the rows of grid-shaped holes (14).

31. The method according to claim 25, characterized in that the diffusion mask (40) is retained in the selective removal of the diffusion mask in strip-shaped areas which comprise rows of the grid-shaped arranged holes (14).

32. The method according to any one of claims 24 to 31, characterized in that the step of selectively removing the diffusion mask (40) comprises the application of an etching paste on the front side (1 1).

33. The method according to claim 32, characterized in that the etching paste is applied in strips, each containing entire rows of holes, or in local areas, each containing a hole.

34. The method according to any one of claims 24 to 31, characterized in that the selective removal of the diffusion mask (40) comprises a laser ablation of the mask material.

35. The method according to claim 34, characterized in that the laser ablation comprises the use of linear or punctiform laser spots.

36. The method according to any one of claims 24 to 31, characterized in that the selective removal of the diffusion mask (40) comprises etching of the diffusion mask by a patterned etching layer.

37. The method according to claim 36, characterized in that the structured etching layer is applied by screen printing, inkjet method or dispensing.

38. Method according to claim 24, characterized in that the selective removal of the diffusion mask (40) comprises wetting the rear side (12) of the cell (10) with an etching solution, wherein the etching solution penetrates through the holes ( 14) is pulled through from the back (12) of the cell to the front (1 1) and forms areas etched around the holes (14).

39. The method according to any one of claims 24 to 38, characterized in that the diffusion mask (40) by oxidation of the semiconductor substrate (13) over its entire surface is applied to this.