EP1075708A2

EP1075708A2 - Method for producing contact structures in solar cells

Info

Publication number: EP1075708A2
Application number: EP99929034A
Authority: EP
Inventors: Ralf LÜDEMANN; Sebastian Schaefer
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 1998-04-29
Filing date: 1999-04-27
Publication date: 2001-02-14
Also published as: EP1091420A3; US6423567B1; AU739818B2; DE19819200B4; WO1999056324A3; DE19819200A1; AU4600599A; EP1091420A2; JP2002513212A; WO1999056324A2

Abstract

The invention relates to a method for producing contact structures in semiconductor components, especially solar cells, and semiconductor components having these contact structures. According to the invention, recesses which extend through the first layer to or into a second layer located under said first layer are etched after positioning an etching mask (3) over a first layer or a series of layers (2). The etching is carried out in such a way that the etching mask is etched underneath and/or at least one area of the first layer (2) is provided with negative flanks. Afterwards, an electrically conductive material (9) is inserted in the recesses, whereby the etching mask (3) or the first layer (2) forms a shadow mask for inserting the material. The conductive material is placed in the recesses only up to a height at which there is still no contact between the conductive material (9) and the first layer (2). The structures permit the emitter to contact the base without additional masking. As a result, the invention makes it possible to produce metal contacts on solar cells in an easier and economical manner.

Description

Process for the production of contact structures in

Solar cells

The invention relates to a method for producing contact structures in solar cells, and to solar cells with these contact structures. The invention relates in particular to structures of trenches or holes and methods for their implementation, which enable novel contacts to be made with solar cells.

Solar cells are components that convert light into electrical energy. They usually consist of a semiconductor material which contains regions or layers of different mobility for positive and negative charge carriers, n- or p-conducting regions. The areas are called emitters or bases. Positive and negative charge carriers generated by incident light are separated and can be removed by means of metallic contacts provided on the respective areas. Accordingly, only those charge carriers that reach the contacts and do not previously recombine with another charge carrier contribute to the usable electrical power of the solar cells.

The metallic contacts are generally produced by evaporating metal, which is then galvanically thickened, by electroless deposition of nickel or by printing a metal paste on the surface of the areas to be contacted. If these areas do not extend over the entire solar cell surface or for other reasons (eg shadowing of the light, increased recombination at the contact, etc.) The metal may only be applied in places, as described below, this is ensured by using masks during manufacture.

When metal is vapor deposited, a photolithographically produced resist mask (3) is usually applied directly to the surface of the layer (1) to be contacted, as shown in FIG. A metal layer (4) is then deposited from the metal vapor (5) over the entire surface and the paint mask is removed.

Another structuring option is to put on or clip on a shadow mask (3), as shown in FIG. 1b. The further process steps take place as described in connection with FIG. Printing processes can also be used to apply the metallization. Here, too, masks are used, for example screen printing masks (3) or preformed clichés (stamp or pad printing).

Figure lc shows the screen printing process, in which metal in the form of a metal paste (6) is pressed into the spaces between the mask by means of a doctor blade (7).

The metal contacts produced with the methods just described are usually realized in a comb-like form, i.e. they form so-called grids.

In its simplest form, solar cells consist of full-surface base and emitter areas, with the emitter mostly on the side facing the light

(Front) lies (see Figure 2). This allows the base to be contacted by applying metal all over the back. The emitter comes with contacted a grid with the aim of losing as little light as possible through reflection on the metal contact for the solar cell. One speaks of the shadowing of the solar cell through the contact. The less the shading, ie the more light can get into the solar cell, the greater the current efficiency of the cell per area, and thus the higher the efficiency. In order to guarantee electricity transport with low resistance, the distance between the grid fingers must not be too large, and the number and cross-section must not be too small. A certain amount of shadowing has to be accepted.

Figure 2 shows a solar cell with the base layer (1), the emitter layer (2) and the emitter grid (8) for contacting the emitter. The front, from which the light (10) is incident, is formed by the emitter layer (2). The base contact (9) is applied over the entire surface of the back of the base layer (1).

WO 88/03709 AI describes such a solar cell. With this solar cell the

Emitter layer contacted via trenches, which in turn are in contact with a grid applied to the surface of the solar cell. The trenches are produced in this solar cell using an etching process. Due to the arrangement and shape of the grids, undesirable shading of the surface of the solar cell must also be accepted.

One way to reduce shadowing from the metal contacts is to use the

To design grid fingers high and narrow, as described in SW Glunz et al. , Optimized High-Efficiency Silicon Solar Cells with J _sc = 42 mA / cm ² and η = 23.3%, 14 ^th EU PVSEC Barcelona, Spain (1997). This reduces the solar cell area covered by the contacts without reducing the cross-section and thus the conductivity of the grid. However, the process requires a special paint mask, which ensures that the grid only grows upwards during the galvanic thickening of the evaporated metal. Such fine contacts cannot be printed on according to the current state of the art.

Another concept, the so-called "buried contact cell", is presented in US Pat. No. 4,726,850. This is based on trenches produced by lasers (see also US 4626613), which are filled with metal. The shading of the solar cell is determined by the width of the trenches and can be minimized, while the cross-section of the contacts is retained by the depth of the trenches. Another advantage of such buried contacts is the larger contact area between the metal and the solar cell, which reduces the contact resistance.

A disadvantage of this method, however, is that the trenching using a laser is a very complex process. Here, the laser beam must be guided over the entire surface of the solar cell via a precise deflection or displacement device. The action of the laser beam also damages the solar cell, so that the trenches must then be etched in order to remove the damaged layer. This also applies to mechanically generated, for example sawn, trenches. In addition to increasing efficiency, simplifying production and developing cheaper starting materials represent the most important challenges for solar cell technology. In terms of contacting, the elimination of masks during metallization means a significant simplification. Two recently described methods for the mask-free production of grids are based on the texturing of the surface by sawn trenches similar to a sawtooth pattern.

For example, in M. Verbeek et al. Are Mechanically Grooved High Efficiency Silicon Solar Cells with Self-Aligned metallization, 25 ^th IEEE PVSC Washington, USA (1996), a first method proposed, metallized by oblique vapor deposition due to the mutual shading only the tips of the "saw teeth" in which . This is shown schematically in Figure 3a. The figure shows the sawn surface of a layer (1) to be contacted, at the tips of which metallization (4) is produced only on one side by oblique vapor deposition (5).

A second method is described by C. Gerhards et al. , Mechanically V-Textured Low Cost Multi Crystalline Silicon Solar Cells with a Novel Printing Metalliza- tion, described 26 ^th IEEE PVSC Anaheim, United States (1997). By producing "saw teeth" of different heights, the method enables selective metallization by means of "roller printing", in which a metal paste (6) is applied by means of a doctor blade (7), as shown in FIG. 3b.

In both methods, however, an approximately 3-5 μm thick damaged layer must be removed by etching after sawing. They are also suitable due to the Mechanical tension occurring during sawing is not suitable for sensitive materials such as some strip-drawn materials or thin, separated layers. They are also unsuitable for thin-film solar cells, since the minimum saw depths that can be achieved are usually greater than the thickness of the active solar cell layers (3-50 μm).

In the course of developing cheaper starting materials, the concept of thin-film solar cells on an inexpensive substrate is of particular importance. The solar cell consists only of a thin semiconductor layer (3-50 μm), which is applied to a carrier material. However, many of these substrates are not conductive. Therefore, the base cannot be contacted from the back through the substrate. Instead, a so-called single-sided grid must be used, which consists of two interlocking grids, each for contacting the base and the emitter.

FIG. 4a shows an example of the structure of a thin-film solar cell with one-sided contact. The base layer (1) is applied here on an insulating substrate (11). The selective emitter regions or layers (2) are embedded in the base layer. Both the emitter regions and the base are contacted by intermeshing emitter (8) or base grids (9), as shown in the figure. The concept of single-sided contacting can simultaneously be used to connect several solar cells to one another on a substrate, as in German patent application P 197 15 138.8 is described.

The same statements also apply to the rear contact cell, a concept for highly efficient solar cells, as shown in FIG. 4b. Here, both contacts and the selective emitter regions (2) are made on the back of the base layer (1) in order to completely eliminate the shadowing of light (10) on the front. If the contacts are implemented as narrow grids (8, 9), light that reaches the solar cell from the rear can also contribute to electricity generation (so-called "bifacial cell").

To date, however, this one-sided contacting has only been possible using very complex methods. A so-called selective emitter is generated over several mask steps, i.e. the emitter does not consist of a homogeneous layer, but of sub-areas that correspond to the shape of the emitter grid. The manufacturing process is shown in Figure 5. The surface of the base layer (1) is first masked (FIG. 5a). The material for producing the emitter regions (2) is then diffused into the surface of the base layer (1) at the locations specified by the mask (3) (FIG

5b). In a subsequent “lift-off” process, the mask (3) is removed (FIG. 5c), so that the structure shown in FIG. 5d with the base (1) and selective emitter regions (2) results. In this way, areas belonging to the base (1) are retained on the surface and can be contacted on the surface. However, the precise application of the respective metal contacts to the corresponding emitter or base area represents a critical adjustment problem and requires additional masks.

It is therefore an object of the present invention to provide a method for producing contact structures in solar cells and solar cells with contact structures that can be implemented in a simple and inexpensive manner.

Furthermore, the method should enable the simple implementation of single-sided contacting of thin-film and rear-side contact solar cells.

The object is achieved with the method according to claims 1 and 2 or with the solar cell according to claim 11. Advantageous refinements are the subject of the dependent claims.

According to one aspect of the present invention

After the etching mask has been positioned over the first layer or layer sequence, the depressions are etched through to the first layer or layer sequence up to or into the underlying second layer. According to the invention, the etching takes place in such a way that the etching mask is underetched. Subsequently, electrically conductive material is introduced into the depressions, the etching mask forming a shadow mask for the introduction of the material due to the undercut and the resulting protruding edges. The conductive material is introduced into the depressions only to an extent or up to a height such that there is no contact between the conductive material and the first shift or shift sequence. Finally, the etching mask is removed.

According to a second aspect of the present invention (claim 2), the first steps are carried out as in the method according to claim 1. According to the second aspect of the invention, however, the etching of the depressions is carried out in such a way that at least a region of the first layer or layer sequence receives or undercuts negative flanks. After the etching step, the etching mask can already be removed. Subsequently, electrically conductive material is introduced into the depressions, in which case the region of the first layer or layer sequence with negative flanks forms a shadow mask for the introduction of the material due to the resulting protruding edges. Here too, the conductive material is only introduced into the depressions to an extent or up to a height such that there is no contact between the conductive material and the first layer or layer sequence.

Due to the particularly advantageous aspects of the method according to the invention according to claims 1 and 2, contacting of the second layer through the first layer can be realized without additional masking or isolation. This enables a base layer to be contacted through the emitter layer of a solar cell. With the method, any pattern, such as trenches or holes and combinations of trenches and holes, can be realized in one step. When using the methods, there is no significant damage to the solar cell, and the material to be treated is not subjected to mechanical stress.

In the process according to the invention of claims 1 and 2, for example, photolithographically structured lacquer layers can be used as etching masks. It is also possible to use oxide, nitride or metal layers, for example, as etching masks. A particular advantage of the method is the possibility of using shadow masks that are simply placed on or clamped on, so that no additional masking step is required. The electrically conductive material can be introduced by known methods, such as, for example, the methods described in the introduction to the description in connection with FIG. 1.

A solar cell according to the invention, which has been provided with a contact structure in accordance with the method according to claim 2, has depressions or openings in the first layer or layer sequence which extend to or into the second layer or layer sequence. The depressions have inclined flanks, at least in a region of the first layer or layer sequence, the mutual spacings of which increase with the depth (negative flanks). Electrically conductive material is introduced into the depressions only to an extent or up to a height such that there is no contact between the conductive material and the first layer or layer sequence. In this case, inclined flanks are to be understood to mean both straight and curved flanks, in other words any flank shape that deviates from the normal (to the surface of the solar cell or the first layer or etching mask). Of course, this also includes steps in the flank, which have horizontal areas which lead to the enlargement of the cross section of the depression with the depth, as oblique or negative flanks in the present sense.

The method according to the invention is an improvement of known contact structures or contacting methods for solar cells. The core of the method according to claims 1 and 2 are specially shaped depressions. The specified etching processes are preferably implemented using a plasma etching process (cf. claim 8).

The depressions in the form of trenches or holes are distinguished by the fact that they cause partial self-masking due to specific undercut or flank shape (cf. FIG. 6, claim 2). This shape of the depressions can be realized by specifically controlling the etching or else by different etching behavior of the materials in different surface layers. Due to the (self-) masking achieved in this way, parts of the structure are not metallized during vapor deposition, so that short circuits between the emitter and base are prevented during the metallization.

It is therefore possible for the first time to have a contact structure in which the base of a solar cell is homogeneously fusion of the emitter can be contacted from the emitter side.

In addition to the targeted shaping of the depressions, this self-masking effect can also be achieved by etching processes which do not attack the mask necessary for producing the structure, but undercut so that the mask partially masks the flanks of the depressions during the metallization (see FIG. 7, Claim 1). Since the same mask is used for the structure generation and for the metallization, these are so-called self-aligning contact structures.

In order to minimize shadowing of the solar cell and at the same time to ensure a high current conductivity of the contacts and a low contact resistance, the trenches or holes should be narrow and deep, i.e. have a high aspect ratio (ratio of depth to width).

The trench or hole structures according to the invention cannot be produced by means of lasers or by mechanical shaping. The structures can be realized to a limited extent, for example, by wet chemical etching. On the one hand, the property of some etching solutions can be exploited to etch faster in certain crystal directions than in others. This can lead to the desired structures on appropriately cut single-crystal material. However, their size and geometry are determined by the crystal structure of the material.

On the other hand, etching solutions can be used which etch equally quickly in all crystal directions, and undercut the etching mask, but do not attack. However, this purely isotropic etching erosion only produces hemispherical structures, ie a poor aspect ratio.

Plasma etching methods are therefore preferably used, with which the slope or undercut can advantageously be checked. A high aspect ratio can also be achieved with plasma etching processes.

In plasma etching, reactive and / or inert gases are ignited into plasmas by means of high-frequency and / or microwave radiation. The resulting radicals can react with the sample surface (isotropic chemical etching) and / or ions are accelerated thereon by an electric field. In the case of reactive ion etching (RIE, “Reactive Ion Etching”), mainly reactive ions are used, which not only knock out surface atoms or support chemical reactions through their energy, but can also react with surface atoms. The directed ion current causes anisotropic removal. The ratio of isotropic and anisotropic etching and thus the undercutting or

Flank shape of the structures can be influenced in a targeted manner. This is independent of the material to be etched and its crystal orientation and can therefore also be used for inexpensive multicrystalline material.

The possible damage to the treated material in all plasma processes is extremely critical for solar cells. Unlike most other semi- In solar cells, the power loss should be kept as low as possible. This means that the recombination of charge carriers must be largely suppressed so that as many charge carriers as possible reach the electrical contacts. To ensure this, in this case extremely low-damage plasma-assisted etching processes are used that have no negative effects on the solar cell. This is achieved through low ion energies and a combination of etching gases adapted to the respective process.

The fact that plasma etching processes are dry-chemical processes extends the field of application beyond the wet-chemical processes. For example, when using a plasma etching process to produce the contact structures according to the invention, it is advantageously possible to treat thin-film solar cells on porous substrate materials. Since plasma etching processes are dry processes, the material to be treated does not come into contact with liquids. Shadow masks can also be used as an etching mask, which is much cheaper than the photo-lithographic masks required for wet chemistry.

The invention is explained in more detail below on the basis of exemplary embodiments in conjunction with the figures. Show here

1 shows examples of metallization processes in solar cells according to the prior art; Figure 2 is a schematic of a simple solar cell according to the

State of the art; FIG. 3 examples of a maskless metallization of sawtooth structures according to the prior art;

FIG. 4 examples of single-sided contacting of a thin-film solar cell (a) and a rear-side contact cell (b) according to the prior art; FIG. 5 shows an example of the steps for producing a selective emitter according to the prior art; FIG. 6 examples of the design of self-masking contact structures according to the invention; Figure 7 Examples of the self-adjusting

Metallization due to self-masking by undercutting the etching mask according to one aspect of the invention; FIG. 8 shows an example of the steps in the self-adjusting production of buried contacts according to the invention; FIG. 9 shows an example of the contacting of the base of a solar cell according to the invention through a homogeneous emitter layer; FIG. 10 shows an example of the result of the metallization of a structure without using the method according to the invention; FIG. 11 shows an example of the metallization of the emitter and base according to the invention in the case of rear-side contact; and

12 shows an example of the inventive equal ^¬ term metallization of Einseitenkontakt- grids. The examples of methods for metallization, contacting and configuration of solar cells in the prior art shown in FIGS. 1 to 5 have already been explained in the introduction to the description.

FIG. 6 shows examples of contact structures which were produced by undercutting or by etching negative flanks in a region of the emitter layer (2) located on the base layer (1) of a solar cell. All structures shown have a self-masking effect in the case of metal vapor deposition, so that after the metal has been deposited on the surface of the emitter layer, the emitter contact (8) and in the depressions the base contact (9) is formed in the form shown schematically. The depressions are only filled to the extent that the metal is deposited only for as long as there is no contact between the base contact (9) and the emitter layer (2). The flanks of the depressions shown in the figure show (in the figures from left to right) negative straight flanks, vertical flanks in the base layer with a negative area in the emitter layer, any flank shape with a negative area in the emitter layer, and a flank shape Isotropic etching with a negative area in the emitter layer. The flank shapes shown can arise, for example, from different etching behavior in the emitter and base.

FIG. 7 shows examples of self-masking by under-etching the etching mask (3) according to one aspect presented the invention. The meaning of the reference numerals corresponds to that of FIG. 6. The etching mask (3) itself is not attacked during this under-etching. The exact shape of the depressions no longer plays a role, ie no negative flanks in the emitter layer (2) as in FIG. 6 are required.

The right illustration shows an SEM image of a structure realized with the method. This structure can be generated, for example, in an ECR reactor in the downstream mode. When using an etching gas such as sulfur hexafluoride SF ₆ at a gas flow of 30 sccm, a pressure of 3 Pa, a microwave power of 400W, a sample temperature of 10 ° C, a distance between sample and plasma of 200 mm and an etching time of 15 min Trench depth of 15 μm can be achieved. If additional high frequency (RIE) power of 20 W is used, the trenches can be made narrower and deeper in the same time.

One application example (example 1) of the method according to the invention is the self-aligning metallization of buried contacts in a solar cell. The method represents an improvement in the trenches produced by laser or by sawing with regard to aspect ratio and damage to a buried contact cell.

If the surface is structured before emitter formation, an emitter contact can be implemented. If the structuring takes place on the base area of the solar cell, a base contact is realized.

A mask (3) is used to create the trenches (or holes). This can either be a photo - IS

lithographically generated paint mask or a shadow mask. Correspondingly opened, existing layers (oxides, nitrides, metals, etc.) on the solar cell (1) can also be used as a mask. A very narrow, deep structure can be produced by plasma etching (e.g. RIE or microwave-assisted RIE methods such as ECR-RIE) without damaging the mask, as is shown in FIG. 8a. An undercut of the mask (3) can be seen in FIG. 8a. Accordingly, the mask also becomes

Metallization used and only then removed in a so-called "lift-off" process. After removing the mask, a metallized trench (4) is obtained (FIG. 8b). This eliminates the need to adjust the metallization mask to the structure. The metallization can be done either by vapor deposition or by filling the structures with metal paste. This can be done by doctoring or printing over the entire surface.

Another exemplary embodiment according to the invention (example 2) allows for the first time the contacting of the base of a solar cell through a homogeneous emitter, ie through an emitter layer covering the entire surface. This is shown in Figure 9. This corresponds to a self-adjusting contact on the base with one-sided contact. A single mask is sufficient for the production of the trenches or holes according to the invention and the subsequent metallization. Both the elaborate manufacture of a selective emitter and the adjustment of the metallization mask to the structure are eliminated. This provides a simple solar cell contact from one side, which applies to both thin-film cells insulating substrate as well as for back contact cells.

The trenches or holes are produced as in the previous example (example 1) by using a mask (3) on the emitter layer (2) (FIG. 9a). The emitter layer (2) is located on the base layer (1). In the method, either the mask (3) is under-etched (Figures 9b and 9d) or the flank shape of the recesses is etched (Figure 9c) so that masking of the emitter (2) is ensured during the metallization. The metallization is carried out by vapor deposition and possibly subsequent galvanic thickening of the metal. Since the vapor deposition does not take place exactly vertically, the emitter area in the structure or the depressions must also be masked, comparable to casting a shadow. This is ensured by the structure according to the invention. For the implementation of the base contact (9), the deposition or application of the metal may only take place up to a height within the depressions at which the metallization (9) is not yet in contact with the emitter layer (2). This can be seen in the lower figures of FIGS. 9b to 9d. The cross-sectional shape of the metallization (9) shown results from the masking. In principle, the trenches or holes can also be produced wet-chemically in this application. However, since the etching rate is then isotropic, ie horizontally and vertically the same, only very broad structures can be realized, which result in a correspondingly large shading of the solar cell or a high contact resistance and low current conductivity of the contacts. FIG. 9d shows a form of the vertical fung or trench, as can be generated by wet-chemical etching.

Suitable etching solutions or materials or crystal directions or the appropriate choice of parameters in the plasma etching process for producing the structures according to the invention can be found in the specialist literature at any time.

If no structure according to the invention with a self-masking effect is used for the contacting, and if the contacting does not take place according to the method according to the invention of undercutting a mask (3) with subsequent metallization through the same mask, then there are short circuits (12) between emitter (2) and base contact 9), as shown in Figure 10. The solar cell is therefore unusable.

FIG. 11 shows an exemplary embodiment (example 3) for the simultaneous self-adjusting metallization of emitter and base in the case of rear-side contact cells. Since the entire rear side can be metallized in the case of rear-side contact cells, the trenches or holes according to the invention enable the self-adjusting contacting of the emitter and base at the same time. As in the previous example (example 2), structures are etched into the base (1) by the emitter (2). The self-masking shape of the structural flanks is crucial. FIG. 11 shows the underetching of regions of the emitter layer (2) near the surface, as a result of which the depression in the region of the emitter layer receives oblique flanks, the spacing of which increases with depth. The mask is then removed and it is a full-surface metallization follows. The metallization on the emitter layer (2) forms the emitter contact (8), the metallization in the recess forms the base contact (9). As explained in Example 1, different types of masks for generating the structures are possible. Due to the shape of the trenches or holes with protruding edges of the regions of the emitter layer near the surface, the metal is deposited as shown in FIG. 11, so that a short circuit between emitter (8) and base contact (9) is excluded. With the example shown, the emitter (2) and the base (1) are advantageously contacted simultaneously when a single mask is used.

Contrary to the previous example (example 3), the emitter contact (8) cannot be implemented over the entire surface, for example for realizing an emitter grid. For this, as shown in FIG. 12, an additional metallization mask (13) is required. The metallization of emitter (2) and base (1) can still be carried out in one step. After creating the depressions in accordance with the previous example (example 3), the additional mask (13) for defining the emitter contact is applied. During the subsequent full-surface metallization, the emitter and base are contacted simultaneously. The additional mask is then removed. This form of simultaneous metallization of the emitter and base can also be used for one-sided contacting from the front. It has the same advantages as in the previous example.

Claims

claims

1. Method for producing contact structures in solar cells, which have at least a first layer or layer sequence (2) over a second layer (1), with the following steps:

Positioning an etching mask (3) over the first layer or layer sequence (2), the etching mask determining the position of the contact structures;

Etching depressions at the locations predetermined by the etching mask (3) through the first layer or layer sequence (2) up to or into the underlying second layer (1) in such a way that the etching mask (3) is underetched;

Introducing electrically conductive material (9) into the depressions, the etching mask (3) being a

Forms a shadow mask for the introduction of the material, and the conductive material is only introduced in such a way that there is no contact between the conductive material (9) and the first layer or layer sequence (2); and then removing the etching mask (3).

2. Method for producing contact structures in solar cells which have at least a first layer or layer sequence (2) over a second layer (1), with the following steps:

Position an etching mask (3) over the first

Layer or layer sequence (2), the etching mask

Determines the position of the contact structures; - Etching recesses on the through the etching mask (3) predetermined positions through the first layer or layer sequence (2) up to or into the underlying second layer (1) in such a way that at least a region of the first layer or layer sequence (2) is undercut or receives negative flanks;

Removing the etching mask (3); and

Introducing electrically conductive material (9) into the recesses, the undercut area or the negative flanks of the first layer or layer sequence (2) forming a shadow mask for the introduction of the material, and the conductive material is only introduced in such a way that there is no contact between the conductive material (9) and the first layer or layer sequence (2).

3. The method according to claim 2, characterized in that the underetched area or the area with negative flanks is a near-surface area of the first layer or layer sequence (2).

4. The method according to any one of claims 1 to 3, characterized in that the under-etching of the etching mask (3) or the etching of negative flanks in a region of the first layer or layer sequence (2) by targeted control of parameters of the etching process and / or by different Etching behavior of the materials used for the etching mask and / or first layer or layer sequence is realized.

5. The method according to any one of claims 2 to 4, characterized in that the conductive material (9) simultaneously with the Introduction into the depressions is applied over the entire area to the first layer or layer sequence (2).

6. The method according to any one of claims 2 to 4, characterized in that before the introduction of the conductive material (9) a new mask (13) for defining a contact structure for the first layer or layer sequence (2) is applied to the first layer or layer sequence is then introduced into the contact structure for the first layer or layer sequence at the same time as it is introduced into the depressions, and the new mask (13) is subsequently removed.

7. The method according to any one of claims 1 to 6, characterized in that the first layer (2) is an emitter layer and the second layer (1) is a base layer of the solar cell.

8. The method according to any one of claims 1 to 7, characterized in that a plasma etching process is used for the etching.

9. The method according to any one of claims 1 to 8, characterized in that the positioning of the etching mask (3) is carried out by application to the first layer or layer sequence (2).

10. The method according to any one of claims 1 to 8, characterized in that the positioning of the etching mask (3) by attachment as a shadow mask over the first layer or layer sequence (2).

11. Solar cell having at least a first layer or layer sequence (2) over a second layer (1)

Wells or openings in the first layer or layer sequence, which extend up to or into the second

Extend layer, and the inclined at least in a region of the first layer or layer sequence

Have flanks, the mutual distances between them

Increase depth; and electrically conductive material (9) in the

Wells or openings which are introduced in such a way that there is no contact between the conductive material (9) and the first layer or layer sequence (2).

12. Solar cell according to claim 11, characterized in that the first layer is the emitter layer and the second layer is the base layer of the solar cell, so that the base contact can be realized through the emitter layer.

13. Solar cell according to claim 11 or 12, characterized in that the depressions or openings are trenches and / or holes.

14. Solar cell according to one of claims 11 to 13, characterized in that the first layer or layer sequence has the oblique flanks in a region close to the surface.

15. Solar cell according to one of claims 11 to 14, characterized in that the depressions have a high aspect ratio.