WO2012136512A2

WO2012136512A2 - Method for producing a screen printing mold and solar cell produced therewith

Info

Publication number: WO2012136512A2
Application number: PCT/EP2012/055381
Authority: WO
Inventors: Matthias SAUERESSIG; Renate ZAPF-GOTTWICK
Original assignee: Universität Stuttgart
Priority date: 2011-04-08
Filing date: 2012-03-27
Publication date: 2012-10-11
Also published as: WO2012136512A3; DE102011016453A1

Abstract

The invention relates to a method for producing a screen printing mold (10), in particular for the face printing of solar cells (20), in which first a first stencil structure (34) is generated on the surface of a screen (14), said stencil structure defining a structure to be printed, and in which subsequently at least one further stencil structure (42) is generated on the first stencil structure (34), said further stencil structure being aligned with the previous stencil structure (34) and forming the screen printing mold together therewith.

Description

Process for producing a screen printing form and solar cell produced therewith

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 production of crystalline silicon solar cells, the metallization is carried out at the front in industrial manufacturing mainly by means of a screen printing process, which is a low-cost technology with high throughput. 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 bus bars. 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 ever-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 of solar cells is the printing of contact fingers with reflective color. 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 area of the solar cell and thus leads to shading losses, which reduce the efficiency of the module.

With the 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 an aspect ratio of height to width of at most about 0.31 results. However, narrower fingers would be advantageous yet low electrical resistance, 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 in which subsequently on the first Template structure is generated at least one further template structure, which is aligned with the previous template structure and together forms 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 generation of a two- 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 by at most 30%, more preferably by at most 20%.

With such a shape of a stencil structure can be produced during fine line printing continuous lines 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 can be reduced at the same time the problem of so-called "bleeding", that is, the bleeding of the print structure at the edges.

The first template structure can be generated approximately with 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 one above the other must be exactly 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.

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

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

In a capillary film is a carrier film to which a prefabricated emulsion layer is applied with a constant thickness. The capillary film can be applied to the previous stencil 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.

Alternatively, the invention is solved 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 perpendicular 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.

According to the invention namely by the exposure of the copy layer in two exposure steps with oblique angles of incidence, which are inclined relative to the vertical to different sides, generated in the copy layer oblique edges. 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. E. the property of liquefying under pressure. 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 + ot and the second inclination angle in the second exposure step is -ot. This results in a symmetrical widening of the copy layer from the pressure side to the squeegee side on both sides.

Preferably, the inclination angle α is 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 a tilt angle relative to the vertical, particularly favorable conditions result in the later screen printing.

Preferably, the screen printing forme produced according to the invention for the production of contacts, in particular of fingers, used on the front 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, and with high electrical conductivity and a high aspect ratio can be produced hereby.

At the same time, bleeding, i. 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 widened or tapers on a characteristic to be printed, starting from the screen to the printing side. In a first variant, the recess of the template may be pyramid-shaped on a feature to be printed on the pressure side. to broaden like that. 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 pressure side.

This embodiment results when the screen printing form 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, a solar cell can further 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 above-mentioned and the features of the invention to be explained below not only in the combination, but also in other combinations or in isolation of the invention can be used 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: Figure 1 is a view of a screen printing form according to the invention; Figure 2 is a partial perspective view of a silicon solar cell; FIG. 3 a) to c)

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

FIGS. 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 with respect to Figure 3;

FIG. 4e)

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

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

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

FIG. 7 shows the screen printing form according to FIG. 6 after development; 8 shows a finger on an associated wafer for solar cell production, which can be produced with the screen printing form according to FIG. 7, and FIG

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

In Fig. 1, a screen printing form according to the invention is shown schematically and generally designated by numeral 10. The screen printing form 10 has a screen printing frame 12, 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 surface 16 finally the template 18 is recorded, which reproduces the print motif. The distance between the doctor blade surface 16 and the frame 12 is also called color rest.

The screen 14 may be made of a fabric, such as a PET fabric or steel wire (stainless steel). 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 Fig. 2 is a partial perspective view of a silicon solar cell is shown, which is generally designated 20. The solar cell 20 has on its front side an emitter 22 (n-type), followed by a base layer 24 (p-type) to which a rear-side contact 26 made of aluminum is applied flatly. On the emitter 22 front side contacts 28 are arranged on the front side, which have a H- form a shaped structure, with two bus bars 30 parallel to each other with a larger cross-section, from which extend starting on both sides fingers 32 with a much smaller cross-section. The front side contacts 28 are made of a silver alloy. They should be as small as possible to avoid shading losses, but have sufficient electrical conductivity.

So far, fingers with a width of about 80 to 100 micrometers and a height of 20 to 25 micrometers can be produced with conventional screen printing forms.

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

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

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

According to Fig. 3a), first a first template structure 34 is produced on a wire 14 in a conventional manner. For this purpose, a UV-sensitive emulsion is applied to the wire 14 and then dried. The copying layer 36 thus produced is then irradiated with UV radiation in the wavelength range from 350 to 450 nanometers with the interposition of a film original which reproduces the printed pattern. Here, the monomers combine to form long-chain polymers and the copy layer 36 loses its solubility in the sense of solvent (water). The copying layer 36 thus becomes water-insoluble in the impingement region of the radiation, while the places covered by the film template, 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 produced in this way with a first conventionally produced screen printing layer 34 is shown in cross section in FIG. 3a).

A second template structure is subsequently applied to this first template structure 34. In the case of a manual production, for example, a spacer 38 can be applied on both sides on both sides to predetermine the desired application thickness when applying an emulsion. In the present case, the application thickness is 60 microns.

In the subsequent step according to FIG. 3b), UV-sensitive emulsion is again applied with a doctor blade 40, so that a second copying 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 43 has a thickness of about 25 microns, as shown in Fig. 3c). Subsequently, an exposure takes place again with high-energy UV radiation with the interposition of a film original 44, which must be precisely aligned with respect to the structure previously generated in the first template structure 34.

With a screen printing plate thus produced, a larger EOM thickness can be produced, which allows a finer pressure and more uniform structures. While the basic steps of producing a screen printing plate having a two-layer structure were explained above with reference to FIG. 3, the preferred structure for producing fingers on silicon solar cells 20 according to FIG. 2 will be explained in more detail below with reference to FIG.

The basic production corresponds to the previously explained with reference to FIG. 3 steps.

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

In the subsequent step according to FIG. 4b), a second stencil structure 42 is applied thereto, which forms a second copying layer 43. Subsequently, according to FIG. 4c), a second film original 44 is precisely aligned and subsequently exposed to UV radiation. Here, the second film original 44 is preferably designed so that it has a slightly larger width at a line-shaped recess 52 than the associated recess 50 in the first template structure 34. For example, the first recess 50 could have a width of 50 microns, while the second Recess 52 has a width of about 60 microns, as dashed lines in Fig. 4c) and 4d) is indicated.

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

5, a section of the front side contacting is shown schematically in the upper part, with a bus bar 30 and fingers 32 extending therefrom. The fingers have a width w. In the lower part of Fig. 5 is a cross section through one of the fingers 32 is 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 an inventive screen printing form 10 'according to FIG. 4, an aspect ratio of about 0.4 can be achieved for an actual finger width of 50 micrometers. This represents a significant improvement over screen printing with conventional screen printing forms.

With conventional screen printing forms can be in the single-stage screen printing as a result of the mesh markings (see Fig. 9) realize no finger width below 80 microns. The aspect ratio is significantly lower than 0.4.

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

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

The method described herein with reference to Figs. 3 and 4 is a manual method using a squeegee. In industrial implementation, the process is usually carried out fully automatically. It is understood that this can achieve even higher precision and further improvement.

An alternative manufacturing method for producing a screen printing form 10 "is explained below with reference to FIGS. 6 to 8.

The screen printing form 10 "is produced in one stage in contrast to the previously described screen printing form. On a wire 14 consisting of steel wire, a UV-sensitive emulsion is first applied in the usual way in order to produce a copying layer 36 ", followed by the usual drying step in the drying oven.

Subsequently, a first film original 44 "is placed on the surface of the copy layer 36". However, in contrast to conventional methods in the prior art, the exposure is subsequently not 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 is directed obliquely from the right onto the first film original 44 ", so that an angle of inclination α of approximately 45 ° results to the right with respect to the vertical , wider film original 44 "'launched (Fig. 6a)). The second film original 44 "'prevents the UV radiation in the area to be developed later on the one side .The incident radiation is indicated by solid lines.

Subsequently, the second film original 44 "'is shifted to the other side to prevent the incidence of radiation on the other side." In a subsequent second exposure step, the incident UV radiation is tilted at the angle -a from the vertical, ie 45 ° in the opposite direction (Figure 6b).) Behind the film master 44 ", a V-shaped region of the master copy 36", which was not irradiated, thus results behind a recess 54 in the original film 44 ".

After development, a screen printing form 10 "results as shown in FIG. 7, which has a V-shaped recess, which increases from the pressure side to the doctor side. If such a screen printing form 10 "is used in order 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 openings are formed in the area of the funnel-shaped widened opening liquefaction of the screen printing paste results, which leads to a higher material output in this area and thus enables printing of fine structures with a high aspect ratio.

Claims

claims

1. A method for producing a screen printing form (10), in particular for the front side printing on solar cells (20), wherein on the surface of a screen (12) first a first template structure (34) is defined which defines a structure to be printed, and in at least one further stencil structure (42) is subsequently produced on the first stencil structure (34), which is aligned with the previous stencil structure (34) and together forms the screen printing form (10).

2. The method of claim 1, wherein the first template structure (34) is generated with the following steps:

(a) applying a photosensitive copy layer (36) to the surface of a support (12);

(b) drying the copy layer (36);

(c) exposing the copy layer (36) to a structure to be printed;

(d) developing the copy layer (36).

3. The method of claim 1 or 2, wherein the steps (a) to (d) are repeated to produce a further template structure (42), wherein the previous template structure (34) serves as a support for the subsequent template structure (42).

The method of claim 2 or 3, wherein step (c) comprises covering the copy layer (43) with a film master (44) and exposing.

A method according to any one of the preceding claims, wherein a capillary film is applied to a previously created stencil structure (34) followed by drying, exposure and development to produce a subsequent stencil structure (42).

6. The method according to any one of the preceding claims, wherein in a subsequent template structure (42) has a recess (52) for the line structure to be generated has a greater width (50) than in a previous template structure (34).

7. The method of claim 6, wherein the width is increased at least by 5%, in particular by at least 10%, preferably by at most 30%, more preferably by at most 20%.

8. The method according to any one of the preceding claims, wherein a line structure in a preceding (34) and a subsequent (42) stencil structure is widened in a pyramid.

9. A method for producing a screen printing form (10), in particular for the front side printing on solar cells (20), comprising the following steps:

(a) applying a photosensitive copy layer (36 ") to the surface of a screen (14);

(b) drying the copy layer (36 ");

(c) covering the copy layer (36 ") with a first film original (44") defining a structure to be printed;

(D) covering the copy layer (36 ") with a second film original (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 copying layer from a normal angle of incidence by a first angle of inclination to a first side;

(f) moving the second film original (44 "') to the other side;

(g) exposing the copy layer (36 ") in a second exposure step with an angle of incidence inclined to a perpendicular angle of incidence by a second angle of inclination 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 + a and the second tilt angle in the second exposure step is -a.

11. The method according to claim 10, wherein the angle of inclination (a) at least 5 °, in particular at least 10 °, preferably at least 20 °, more preferably at least 30 °.

12. The method according to claim 10 or 11, wherein the angle of inclination (a) is at most 70 °, in particular at most 60 °, preferably at most 50 °.

13. The method according to any one of the preceding claims, wherein the screen printing form (10, 10 ") for the production of contacts, in particular of fingers (32), on the front of solar cells (20) is used.

14. The method according to any one of the preceding claims, wherein the screen printing form (10, 10 ") for printing fingers (32) on the front of solar cells (20) is used with color.

15. Screen printing form with a sieve (14) on which a template (18) is provided, wherein a recess in the template (18) on a characteristic to be printed, starting from the screen (14) widened or tapered towards the pressure side.

16. Screen printing form according to claim 15, wherein the recess in the template (18) widens in a pyramid-like manner on a feature to be printed on the pressure side.

17. 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.

18. A solar cell, preferably produced according to one of the preceding claims, with finger contacts (32) having a finger width of at most 60 micrometers, which have an aspect ratio of height to width of at least 1: 3.

19. A solar cell according to claim 18, wherein the finger contacts (32) are printed with a color, in particular with a reflective color.