DE102011016453A1 - Process for producing a screen printing form and solar cell produced therewith - Google Patents

Abstract

The invention relates to a method for producing a screen-printing form (10), in particular for front-side printing on solar cells (20), in which a first template structure (34) which defines a structure to be printed is first produced on the surface of a screen (14), and subsequently forming on the first template structure (34) at least one further template structure (42) which is aligned with the previous template structure (34) and together forms the screen printing form.

Description

The invention relates to a method for producing a screen printing form, which is particularly suitable for producing particularly fine structures in front side printing on solar cells. Furthermore, the invention relates to a screen printing form produced in this way and a solar cell printed in particular on the front side.

In the manufacture of crystalline silicon solar cells, metallization on the front side in industrial manufacturing is predominantly by means of a screen printing process, which is a low-cost, high-throughput technology.

The structure of a solar cell on the front corresponds to an H-shaped grid with narrow fingers which direct the current collected on the cell to wider busbars. Since the tracks on the front of the solar cell contribute to the shading and thus deteriorate the efficiency of the solar cell, the tracks should be as narrow as possible on the front, on the other hand have a good electrical conductivity.

A disadvantage of screen printing is the increasing influence of the screen mesh, the smaller the structures become. With structures as small as 100 microns or less, so-called "mesh marking" through the mesh becomes more and more prominent. This results in a wave-shaped image both in the width, as well as in the height of the structures. As a result, there are high finger and contact resistances and unnecessary consumption of the silver conductive paste for the front-side contacting.

Another application problem with screen printing on the front side of solar cells is the printing of contact fingers with reflective paint. If the contact grid on the front of the solar cell, no light can penetrate into the solar cell at this point. However, the light incident on the grating may be redirected to the cell by multiple reflection from the grating onto the modulus glass, which requires the printing of a reflective color, such as a white color. However, the viscosity of such colors is usually very low. During screen printing, the ink runs from the existing fingers onto the free surface of the solar cell and thus leads to shading losses, which reduce the efficiency of the module.

With previous screen printing technology for solar cells, fingers can be made in the width of about 80 to 100 microns and a height of about 20 to 25 microns, so that there is an aspect ratio of height to width of at most about 0.31. However, narrower fingers with nevertheless low electrical resistance were advantageous, which requires a higher aspect ratio.

In order to realize a fine line printing, a multiple printing of the electrically conductive paste could be carried out, which has been used industrially but little so far. For this purpose, a greater amount of equipment would be required, namely another screen printing device and another dryer.

The invention is therefore based on the object to provide a method for producing a screen printing form, which in particular fine structures can be printed, which are particularly suitable for the front side printing of solar cells. In this case, as uniform as possible cross-sections with a high aspect ratio, especially in fine line printing should be able to be generated.

This object is achieved by a method for producing a screen printing form, in particular for the front side printing on solar cells, in which on the surface of a screen, first a first template structure is generated, which defines a structure to be printed, and then at least one on the first template structure another stencil structure is created, which is aligned with the previous template structure and forms together with it the screen printing form.

The object of the invention is completely solved in this way.

According to the invention, a larger EOM (Emersion Over Mesh) is made possible by the creation of a two-stage or multi-level stencil structure. By this is meant that part of the stencil layer which builds up on the stencil carrier, ie on the screen, and whose thickness is the difference between the screen printing plate thickness and the screen thickness. With a larger EOM, a more consistent print can be achieved and mesh marking can be reduced, especially when printing fine textures.

In a preferred embodiment of the invention, in a subsequent template structure, a recess for a line structure of the printed image to be generated has a greater width than in a previous template structure.

In this case, the width may be increased by at least about 5%, in particular at least 10%, preferably at most 30%, more preferably at most 20%.

With such a form of a template structure, continuous lines can also be produced during fine-line printing while avoiding the mesh markings. Likewise, the aspect ratio can be improved. Overall, a stencil structure thus produced leads to a significant improvement in the electrical conductivity of the finger pressure on the front of solar cells or to avoid the mesh markings. At the same time, the pressure of narrower fingers is made possible, whereby a higher aspect ratio can be achieved. A line structure in a preceding and a subsequent template structure can thus be widened in a pyramid-like manner.

With such a configuration of a screen printing form, the problem of the so-called "bleeding", that is, the bleeding of the printed structure at the edges, can be reduced at the same time.

• (a) Auftragen einer lichtempfindlichen Kopierschicht auf die Oberfläche eines Trägers, das heißt eines Siebes;
• (b) Trocknen der Kopierschicht;
• (c) Belichten der Kopierschicht mit einer zu druckenden Struktur;
• (d) Entwickeln der Kopierschicht.

The first template structure can be created using the following steps:

• (a) applying a photosensitive copy layer to the surface of a support, ie, a screen;
• (b) drying the copy layer;
• (c) exposing the copy layer to a structure to be printed;
• (d) developing the copy layer.

In the subsequent production of a further stencil structure, the stencil structures to be produced on top of each other must be precisely aligned before the exposure. Then, steps (a) to (d) may be repeated with the previous template structure serving as a support for the subsequent template structure.

In this case, step (c) preferably comprises covering the copy layer with a film original and the exposure.

Instead of the variant of the production of the screen printing plate in a direct process in which an emulsion is applied directly to the screen, a so-called capillary film can be applied, followed by the usual steps of drying, exposure and development followed to produce a subsequent stencil structure.

A capillary film is a carrier film to which a prefabricated emulsion layer with a constant thickness is applied. The capillary film can be applied to the previous template structure, whereby sufficient adhesion can be produced by moistening or by applying a small amount of emulsion. The carrier film is removed after the application.

• (a) Applizieren einer lichtempfindlichen Kopierschicht auf die Oberfläche eines Siebes;
• (b) Trocknen der Kopierschicht;
• (c) Abdecken der Kopierschicht mit einer Filmvorlage, die eine zu druckende Struktur definiert;
• (d) Abdecken der Kopierschicht mit einer zweiten Filmvorlage seitlich versetzt zur ersten Filmvorlage;
• (e) Belichten der Kopierschicht in einem ersten Belichtungsschritt mit einem in Bezug auf die Kopierschicht gegenüber einem senkrechten Einfallswinkel um einen ersten Neigungswinkel zu einer ersten Seite hin geneigten Einfallswinkel;
• (f) Verschieben der zweiten Filmvorlage zur anderen Seite hin;
• (g) Belichten der Kopierschicht in einem zweiten Belichtungsschritt mit einem gegenüber einem senkrechten Einfallswinkel um einen zweiten Neigungswinkel zu einer der ersten Seite gegenüber liegenden zweiten Seite hin geneigten Einfallswinkel;
• (h) Entwickeln der Kopierschicht.

Alternatively, the invention is achieved by a method for producing a screen printing form, in particular for the front side printing on solar cells, with the following steps

• (a) applying a photosensitive copy layer to the surface of a screen;
• (b) drying the copy layer;
• (c) covering the copy layer with a film original defining a structure to be printed;
• (D) covering the copy layer with a second film original laterally offset from the first film original;
• (e) exposing the copying layer in a first exposure step with an angle of incidence inclined with respect to the copying layer from a normal angle of incidence by a first angle of inclination to a first side;
• (f) moving the second film original to the other side;
• (g) exposing the copying layer in a second exposure step with an angle of incidence inclined to a vertical angle of incidence by a second angle of inclination to a second side opposite the first side;
• (h) developing the copy layer.

Also in this way the object of the invention is completely solved.

Namely, by the exposure of the copy layer in two exposure step with oblique angles of incidence, which are inclined with respect to the perpendicular to different sides, oblique edges are produced in the copying layer. After development and subsequent washing left as inclined side edges.

In the subsequent printing using such a screen printing form, the thixotropy of the printing paste can be utilized, i. H. the property of liquefying down pressure action. It can thus reach more paste through the opening on the substrate surface to be printed through the funnel-shaped extension on the doctor side locally.

This makes it possible, in particular in fine line printing, to produce very fine structures while avoiding mesh marking.

In this case, the first inclination angle in the first exposure step is preferably + α and the second inclination angle in the second exposure step is -α.

This results in a symmetrical widening of the copy layer from the pressure side to the squeegee side on both sides.

The angle of inclination α is preferably at least 5 °, in particular at least 10 °, preferably at least 20 °, more preferably at least 30 °.

More preferably, the inclination angle α is at most 70 °, in particular at most 60 °, particularly preferably at most 50 °.

With such an inclination angle relative to the vertical, particularly favorable conditions result in the later screen printing.

The screen printing form produced according to the invention is preferably used for producing contacts, in particular fingers, on the front side of solar cells.

As already explained above, particularly fine fingers with a width of less than 80 micrometers, preferably in the range of 50 to 60 micrometers, or even lower, can be produced with high electrical conductivity and a high aspect ratio.

At the same time the bleeding, d. H. the fraying of the fingers at the edges are largely reduced and the paste consumption is minimized.

A screen printing plate produced according to the invention has a screen on which a template is provided, in which a recess in the template widens or tapers on a characteristic to be printed, starting from the screen to the printing side. In a first variant, in this case the recess of the template can widen in a pyramid-like manner on a feature to be printed to the pressure side. This embodiment results when the screen printing stencil is made in two layers or in multiple layers

According to a further embodiment of the invention, a recess in the template tapers at a feature to be printed obliquely to the print side.

This embodiment results when the screen printing plate is made by utilizing double exposure in different directions as described above.

A solar cell produced according to the invention has fingers with a finger width of at most 70 micrometers, which have an aspect ratio of height to width of at least 1: 3.

With the method according to the invention, furthermore, a solar cell can be produced, in which the fingers are printed with a color, in particular with a reflective color.

In this case, it is ensured by the screen printing form according to the invention that the ink is essentially applied only to the surface of the fingers and that bleeding is largely avoided.

It is understood that the features of the invention to be explained below and those to be explained below can be used not only in the respectively specified combination but also in other combinations or in isolation of the invention, without departing from the scope of the invention.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the drawings. Show it:

1 a view of a screen printing form according to the invention;

2 a partial perspective view of a silicon solar cell;

3a) to c)
a section through a screen printing form according to the invention in different stages of production;

4a) to d)
a section through a screen printing plate produced according to the invention in different stages of production, in a slightly modified version opposite 3 ;

4e)
an associated finger on a wafer for solar cell production printed by the screen printing form;

5 a partial view of the contacts on the front of a silicon solar cell according to 2 , wherein in the lower part a cross-section through a finger is shown in enlarged form, from which the aspect ratio is apparent;

6a , b)
a section through an alternative embodiment of a screen printing form with a copy of a film original with an indication of two different exposure steps according to 6a) and 6b) ;

7 the screen printing form according to 6 after developing;

8th a finger on an associated wafer for solar cell production, the with the screen printing form according to 7 can be produced and

9 a view of a conventional screen printing form with sieve and a slot-shaped opening for generating a finger, wherein in the right half of a hereby produced finger is shown, the mesh marking is visible.

In 1 a screen printing form according to the invention is shown schematically and in total with numeral 10 Denotes the screen printing form 10 has a screen frame 12 on, which serves to attach a screen printing stencil carrier. During the printing process, high mechanical stress on the screen and the frame is caused by the doctoring. Therefore, the frame is chosen depending on the screen used, the screen tension and the print motif. The size of the frame 12 depends directly on the size of the print motif. With a certain distance of usually at least 150 millimeters to the frame is the doctor blade surface 16 , Within the doctor blade area 16 is the template after all 18 recorded, which reproduces the print motif. The distance between the doctor blade surface 16 and the frame 12 is also called color rest.

The sieve 14 may consist of a fabric, such as a PET fabric or steel wire (Edelstau). In technical screen printing usually steel fabrics are used, since this finer structures can be achieved because they work with a lower thread size and can be used with a higher fabric tension.

In 2 is a partial perspective view of a silicon solar cell shown in total with 20 is called, the solar cell 20 has an emitter on its front 22 (n type) followed by a base layer 24 (p-type), to which a backside contact 26 consisting of aluminum is applied flat. On the emitter 22 At the front are front side contacts 28 arranged, which form an H-shaped structure, with two mutually parallel bus bars 30 with a larger cross section, from which starting on both sides fingers 32 extend with a significantly smaller cross-section. The front side contacts 28 consist of a silver alloy. They should be as small as possible to avoid shading losses, but have sufficient electrical conductivity.

With conventional screen printing forms so far fingers with a width of about 80 to 100 microns and a height of 20 to 25 microns can be produced.

If attempts are made to print narrower fingers with conventional screen printing forms, then mesh marking occurs, as in 9 is clear, which can lead to finger interruptions and high electrical resistance.

With a screen printing form according to the invention 10 can now significantly narrower fingers 32 having a width of 60 microns or less and an aspect ratio of 0.3 or more. At the same time, the mesh marking can be avoided.

3 shows the various phases in the production of a screen printing form according to the invention.

According to 3a) first becomes a first template structure 34 on a sieve 14 produced in a conventional manner. For this purpose, a UV-sensitive emulsion on the sieve 14 applied and then dried The thus produced copying layer 36 is then irradiated with the interposition of a film original, which reproduces the print pattern, with UV radiation in the wavelength range of 350 to 450 nanometers. In this case, the monomers combine to form long-chain polymers and the copy layer 36 loses its solubility in the sense of solvent (water). The copy layer 36 Thus, in the impingement of the radiation is insoluble in water, while the places covered by the film original, ie the image plane locations, remain water-soluble.

After exposure, these areas are washed out with a powerful jet of water so that the screen printing form remains. This step is also called development. A screen printing plate thus produced with a first conventionally prepared screen printing layer 34 is in 3a) shown in cross section.

On this first template structure 34 Subsequently, a second template structure is applied. In a manual production can this example, on both sides of a spacer 38 placed on the edge to specify the desired application thickness when applying an emulsion. In the present case the order thickness is 60 Micrometers.

In the following step according to 3b) is done with a squeegee 40 again applied UV-sensitive emulsion, so that a second copy layer 43 results

The screen printing form is then dried with the print side up in a drying oven.

After drying, the second copy layer has 43 a thickness of about 25 microns, as in 3c) shown. Subsequently, a Exposure in turn with high-energy UV radiation with the interposition of a film original 44 that is precise in terms of previously in the first template structure 34 generated structure must be aligned.

With a screen printing plate thus produced, a larger EOM thickness can be produced, allowing finer printing and more uniform structures.

While previously based on 3 the basic steps of producing a screen printing form with two-layer structure have been explained below, with reference to 4 the preferred structure for producing fingers on silicon solar cells 20 according to 2 explained in more detail.

The basic production corresponds to the previously based on 3 explained steps.

4a) shows a cross section through a screen printing form, in which on the screen 14 a first template structure 34 was generated in the conventional manner described above.

In the following step according to 4b) is then a second template structure 42 applied, which is a second copy layer 43 forms. Subsequently, according to 4c) a second movie template 44 precisely aligned and then exposed to UV radiation. Here is the second movie template 44 preferably designed so that these at a linear recess 52 has a slightly greater width than the associated recess 50 in the first template structure 34 , For example, the first recess could be 50 have a width of 50 microns while the second recess 52 has a width of about 60 microns, as dashed in the 4c) and 4d) is indicated.

4e) shows one with such a screen printing form 10 ' manufactured finger structure 48 on a substrate 46 The result is a sharply defined structure at the edges, avoiding bleeding with a good aspect ratio.

In 5 is a section of the front side contact shown schematically in the upper part, with a bus bar 30 and outgoing finger 32 , The fingers have a width w. In the lower part of 5 is a cross section through one of the fingers 32 shown enlarged. The width of the finger is w, while the height is h.

The aspect ratio a is defined as the ratio of height h to width w: a = h / w.

With a screen printing form according to the invention 10 ' according to 4 For example, an aspect ratio of about 0.4 can be achieved for an actual finger width of 50 microns. This represents a significant improvement over screen printing with conventional screen printing forms.

Conventional screen printing forms can be produced in single-stage screen printing as a result of the mesh marking (cf. 9 ) realize no finger width below 80 microns. The aspect ratio is significantly lower than 0.4.

In addition, the paste consumption is reduced compared to the conventional single-stage screen printing by the thinner finger widths and the larger aspect ratio, resulting in a significant cost savings.

At the same time the efficiency is significantly improved by the reduced shading on the front of a solar cell.

At the hand here by 3 and 4 described method is a manual method using a doctor blade. In industrial implementation, the process is usually carried out fully automatically. It is understood that this can achieve even higher precision and further improvement.

Based on 6 to 8th In the following, an alternative manufacturing method for producing a screen printing form will be described 10 '' explained.

The screen printing form 10 '' is produced in one stage, in contrast to the screen printing form described above

On a sieve 14 consisting of steel wire is first applied in the usual manner, a UV-sensitive emulsion to a copy layer 36 '' to create. Subsequently, the usual drying step in the drying oven.

Below is a first movie template 44 '' on the surface of the copy layer 36 '' hung up. However, in contrast to conventional methods in the prior art, the exposure is not subsequently effected with a single exposure step with vertically incident light, but in two successive exposure steps with obliquely incident UV radiation.

In a first exposure step, the UV radiation obliquely from the right incident on the first film original 44 '' directed, so that there is an inclination angle α of about 45 ° to the right with respect to the vertical. On the first film template is offset a second, wider film template 44 ''' hung up ( 6a) ). The second movie template 44 ''' Prevents UV radiation in the area to be developed later on one side. The incident radiation is indicated by solid lines.

Subsequently, the second film template 44 ''' shifted to the other side to prevent radiation on the other side. In a subsequent second exposure step, the incident UV radiation at the angle -α. inclined to the vertical, ie by 45 ° in the opposite direction ( 6b) ). Behind the movie template 44 '' thus results behind a recess 54 in the movie template 44 '' a V-shaped area of the master copy 36 '' which was not irradiated.

After development, a screen printing form results 10 '' according to 7 having a V-shaped recess which increases from the pressure side to the doctor side.

Will mm such a screen printing form 10 '' used to produce a fine structure, the thixotropic property is advantageously utilized, in particular when printing silver pastes on solar cells, in order to be able to produce particularly fine structures, since liquefaction of the screen printing paste results in the area of the funnel-shaped widened opening, resulting in a higher density Material discharge in this area and thus allows printing of fine structures with high aspect ratio.

Claims (19)

Method for producing a screen printing form ( 10 ), in particular for the front side printing on solar cells ( 20 ), in which on the surface of a screen ( 12 ) first a first template structure ( 34 ), which defines a structure to be printed, and in which subsequently on the first template structure ( 34 ) at least one further template structure ( 42 ) generated with the previous template structure ( 34 ) and together with it the screen printing form ( 10 ). Method according to Claim 1, in which the first template structure ( 34 ) is produced by the following steps: (a) applying a photosensitive copying layer ( 36 ) on the surface of a carrier ( 12 ); (b) drying the copying layer ( 36 ); (c) exposing the copying layer ( 36 ) with a structure to be printed; (d) developing the copy layer ( 36 ). Method according to Claim 1 or 2, in which, in order to produce a further template structure ( 42 ) steps (a) to (d) are repeated, the previous template structure ( 34 ) as support for the following template structure ( 42 ) serves. Method according to Claim 2 or 3, in which the step (c) comprises covering the copy layer ( 43 ) with a movie template ( 44 ) and exposing. Method according to one of the preceding claims, in which a previously generated template structure ( 34 ) is applied to a capillary film followed by drying, exposure and development to form a subsequent stencil structure ( 42 ) to create. Method according to one of the preceding claims, in which in a subsequent template structure ( 42 ) a recess ( 52 ) for the line structure to be generated has a greater width ( 50 ) than in a previous template structure ( 34 ) having. Process according to Claim 6, in which the width is increased by at least 5%, in particular by at least 10%, preferably by at most 30%, more preferably by at most 20%. Method according to one of the preceding claims, in which a line structure in a preceding ( 34 ) and a subsequent ( 42 ) Template structure is widened pyramidal. Method for producing a screen printing form ( 10 ), in particular for the front side printing on solar cells ( 20 ), with the following steps; (a) applying a photosensitive copying layer ( 36 '' ) on the surface of a sieve ( 14 ); (b) drying the copying layer ( 36 '' ); (c) covering the copy layer ( 36 '' ) with a first film template ( 44 '' ) defining a structure to be printed; (d) covering the copy layer ( 36 '' ) with a second film template ( 44 ''' ) laterally offset from the first film original; (e) exposing the copying layer ( 36 '' in a first exposure step, with an angle of incidence inclined with respect to the copy layer from a normal angle of incidence by a first inclination angle to a first side; (f) moving the second movie template ( 44 ''' ) to the other side; (g) exposing the copying layer ( 36 '' ) in a second exposure step with an angle of incidence inclined to a vertical angle of incidence by a second inclination angle to a second side opposite the first side; (h) developing the copy layer ( 36 '' ). The method of claim 9, wherein the first tilt angle in the first exposure step is + α and the second tilt angle in the second exposure step is -α. Method according to Claim 10, in which the angle of inclination (α) is at least 5 °, in particular at least 10 °, preferably at least 20 °, more preferably at least 30 °. The method of claim 10 or 11, wherein the inclination angle (α) is at most 70 °, in particular at most 60 °, preferably at most 50 ° Method according to one of the preceding claims, in which the screen printing form ( 10 . 10 '' ) for making contacts, in particular fingers ( 32 ), on the front of solar cells ( 20 ) is used. Method according to one of the preceding claims, in which the screen printing form ( 10 . 10 '' ) for printing on fingers ( 32 ) on the front of solar cells ( 20 ) is used with color. Screen printing form with a sieve ( 14 ), on which a stencil ( 18 ) is provided, in which a recess in the template ( 18 ) on a feature to be printed, starting from the screen ( 14 ) is widened or tapered towards the pressure side. Screen-printing form according to claim 15, in which the recess in the template ( 18 ) widened in a pyramid-like manner on a feature to be printed on the pressure side. Screen-printing form according to claim 15, in which the recess in the template ( 18 ) tapers obliquely on a feature to be printed on the pressure side. Solar cell, preferably produced according to one of the preceding claims, with finger contacts ( 32 ) having a finger width of at most 60 micrometers, having a height-to-width aspect ratio of at least 1: 3. Solar cell according to Claim 18, in which the finger contacts ( 32 ) are printed with a color, in particular with a reflective color.

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