WO2013124254A1

WO2013124254A1 - Method for contacting a semiconductor substrate, more particularly for contacting solar cells, and solar cells contacted thereby

Info

Publication number: WO2013124254A1
Application number: PCT/EP2013/053238
Authority: WO
Inventors: Juergen H. Werner; Renate ZAPF-GOTTWICK
Original assignee: Universitaet Stuttgart
Priority date: 2012-02-23
Filing date: 2013-02-19
Publication date: 2013-08-29
Also published as: DE102012003866B4; CN104137270A

Abstract

A method is specified for contacting a semiconductor substrate, more particularly for contacting solar cells (10), more particularly for producing front contacts (19) on solar cells (10), in which a metal seed structure (20) is initially produced by means of a LIFT process on the surface to be contacted, and then a reinforcement layer (22) is printed onto the seed structure (20) by means of a printing method, more particularly a screen printing method or an inkjet method.

Description

METHOD FOR CONTACTING A SEMICONDUCTOR SUBSTRATE, ESPECIALLY CONTACTING SOLAR CELLS, AND CONTACTING THEREOF

SOLAR CELLS

The invention relates to a method for contacting a semiconductor substrate, in particular for contacting solar cells, in particular for the production of front contacts to solar cells, in which first a metallic seed structure on the surface to be contacted by LIFT process (laser induced-forward transfer Process) is generated.

The invention further relates to a solar cell with a front contact having a seed structure of nickel or a nickel alloy and thereon a reinforcing layer. Finally, the invention also relates to solar cells which are contacted on the back side. From DE 10 2009 020 774 A1 a method for contacting a semiconductor substrate by means of a LIFT process according to the aforementioned type and a solar cell with a front contact thus produced are known.

Thereafter, the metallic seed structure produced by the LIFT process is to be reinforced in a subsequent step, including in particular a galvanic process is preferred.

On a laboratory scale can be hereby produce solar cells with a low contact resistance, which also have a low line resistance, if a copper or silver layer is used for reinforcement.

However, the use of silver pastes is very expensive, which is why the use of silver as the contact material should be avoided as possible. The problem with the use of copper to enhance the seed structure is the adverse effect of copper by diffusion into the solar cell.

Although it is proposed according to the above-mentioned application to form the metallic seed structure produced by the LIFT process as a diffusion barrier layer, however, it is difficult to use a galvanic process on an industrial scale, a very accurate placement of the reinforcement lines only on the metallic seed structure guarantee. Because it must be prevented in any case that the reinforcing layer extends partially over areas of the solar cell, which are not completely separated from the metallic seed structure compared to the copper capping, otherwise the effect would no longer be guaranteed as a diffusion barrier.

On the other hand, the front contact in solar cells must be as narrow as possible in order to keep the optical losses as low as possible.

From DE 43 30 961 C1, a method for producing structured metallizations on surfaces is also known, in which also initially a Laser germination of the substrate surface, such as the ITO surface of an LCD cell is performed by means of an LIFT method and then a galvanic reinforcement is made, for example in a nickel bath.

Again, the disadvantages described above arise.

A method for producing electrically conductive surfaces on an electrically non-conductive substrate is also known from WO 2008/080893 A1, in which first a dispersion is transferred from a carrier by irradiation with a laser to the substrate, then the on the substrate-transferred dispersion is dried to form a base layer and finally an electroless or electroplating of the base layer takes place.

This method is particularly suitable for the production of electrically conductive surfaces on electrically non-conductive substrates.

A method for contacting solar cells is also known from US 2007/0169806 A1, in which first on the front side of the solar cell, a partial ablation of the passivation layer by means of a laser process is carried out and then the thus exposed surface areas using an Ink JET method be printed.

Also this method is subject to the disadvantages described above.

From DE 10 2009 053 776 A1 a method for contacting a solar cell according to the aforementioned type is known. Here, a dopant is introduced to form an emitter layer. In this case, a metal-coated film is brought into contact with the back side of the silicon wafer and the metal is transferred onto the wafer with the aid of a laser. This is done by the so-called LIFT method (Laser Induced Forward Transfer). Furthermore, an amplification of the structures produced by laser can be carried out galvanically or de-energized. From Röder, TC; Hoffmann, E .; Köhler, JR; Werner, JH: "30mm WIDE CONTACTS ON SILICONE CELLS BY LASER TRANSFER", in: 35th IEEE Photovoltaic Specialist Conference (PVSC), 2010, pp. 3597-3599, DOI: 10.1109 / PVSC.2010.5614378 it is known, by means of the LI FT- method seed structure - make front contacts directly through the anti-reflection coating on the front of solar cells. The fingers have a width of less than 30 mm. Here, the contact fingers are reinforced by an electroplating process.

Although this is a fundamental possibility to produce thin front contacts to solar cells with good conductivity, but the additional electroplating is relatively complex.

Against this background, the invention has for its object to provide a method for contacting a semiconductor substrate, in particular for contacting solar cells, specify with the most cost-effective manner, in particular the front contacts for solar cells can be produced, with the highest possible efficiency can be achieved should.

Furthermore, an improved solar cell is to be specified, which can be produced at high efficiency with the lowest possible cost on an industrial scale.

This object is achieved by a method for contacting a semiconductor substrate, in particular for contacting solar cells, in particular for the production of front contacts to solar cells, in which first a metallic seed structure is produced on the surface to be contacted by means of a LIFT process and Subsequently, at least one reinforcing layer is printed on the seed structure by means of a printing process, in particular by means of a screen printing process or an inkjet process.

The object of the invention is completely solved in this way. The metallization by means of the combination of the LIFT process and the subsequent printing process brings a new combination of two methods. The metallic seed structure generated by the LI FT method provides a low contact resistance to the solar cell. The reinforcing layer in combination with this results in a low line resistance.

In this way, the proportion of contact and line resistance at the series resistance of the solar cell is kept low. The series resistance influenced thereby is thus kept low, so that the fill factor of the solar cell is as large as possible.

Even when contacted at the back solar cells are basically the same advantages as the front contacted solar cells.

Since both the screen printing method and the ink-jet method are tried-and-tested industrial methods with which extremely fine structures can be precisely produced in a cost-effective manner, a particularly cost-effective and precise production results in combination with the LIFT method of high-quality contacts to solar cells.

In a preferred embodiment of the invention, the seed structure on first surface areas and the reinforcing layer second surface areas, wherein the seed structure and the reinforcing layer are coordinated such that the second surface areas coincide with an outer contour of the first surface areas or extend within it.

In this way, it is possible that the seed structure acts as a diffusion barrier layer.

In a further preferred embodiment of the invention, the seed structure and / or the reinforcing layer of nickel, titanium, aluminum, copper or alloys thereof are produced. Particular advantages are obtained when the seed structure is made of nickel or a nickel-containing material, since nickel is particularly well suited as a diffusion barrier.

In a preferred embodiment of the invention, the seed structure of nickel or a nickel alloy is prepared and the reinforcing layer made of a copper or nickel-containing screen printing paste.

By this measure, a very cost-effective production of high-quality interconnects on the front of solar cells is possible. The nickel-containing seed structure produced by the LIFT process is produced with sufficient thickness to act as a diffusion barrier layer. Thus, a copper-containing or nickel-containing screen printing paste can be applied to this, so as to produce an electrically very well conductive reinforcing layer. In particular, this makes it possible to dispense with the use of silver and instead to replace a nickel-containing seed structure in conjunction with a reinforcing layer, so that there is a high cost savings.

The seed structure is preferably prepared with a layer thickness of 0.1 to 100 nanometers, more preferably from 0.5 to 50 nanometers, more preferably from 1 to 10 nanometers.

In this way, the thickness of the seed structure is kept as low as possible, but sufficient to act as a diffusion barrier layer can. The aim is in any case a continuous layer, so that the diffusion barrier effect is achieved.

For this purpose, the seed structure can also be reinforced by means of a further LIFT process before the reinforcing layer is applied.

Alternatively, the reinforcing layer can basically also be made of nickel or a nickel alloy or any other metal. While the use of nickel or a nickel alloy avoids the problems with the required diffusion barrier, nickel has a much lower conductivity than copper, so the line resistance can not be set as low as when using copper.

According to a further advantageous embodiment of the invention, the seed structure is produced by a cover layer, preferably through a passivation layer, preferably on a front side of a solar cell.

In this way, the required in the prior art "shooting through" a silver-containing screen printing paste can be avoided. The required high-temperature step, which takes place in the prior art at about 700 to 800 ° C, completely eliminated.

According to the invention, the screen printing paste applied to the seed structure by a screen printing mold is preferably baked at a temperature of less than 400 ° C, preferably at most 330 ° C.

Since the ovens required for this purpose can be operated at significantly lower temperatures than in the high-temperature step required in the prior art, there is also a considerable energy saving, whereby the production costs for the solar cells thus produced are reduced accordingly.

In a further preferred embodiment of the method according to the invention, a backside passivation layer, preferably of amorphous silicon, is applied.

Because of the avoidance of the high-temperature step and the possible baking at temperatures of less than 400 ° C can be used in this way the advantageous passivation with an amorphous silicon layer, which would not withstand a high-temperature step, as in the prior art. The invention is further achieved by a solar cell with a front contact or with a back contact, having a seed structure of nickel or a nickel alloy and thereon a reinforcing layer of copper, a copper alloy, nickel or a nickel alloy, wherein the seed structure as a diffusion barrier layer is formed opposite to the reinforcing layer.

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

According to the invention, by combining a seed structure formed as a diffusion barrier layer with a copper or copper alloy reinforcing layer, a low contact resistance is combined with a low line resistance. Because of the design of the reinforcing layer of copper or a copper alloy results in a low line resistance and cost manufacturability.

Even with solar cells contacted on the back side, basically the same advantages result as with solar cells contacted on the front side.

The metal seed structure generated via the LI FT method causes a low contact resistance to the solar cell. The reinforcing layer in combination with this results in a low line resistance.

In a preferred development of this embodiment, the seed structure has first area regions and the reinforcement layer second surface regions which coincide with or extend within an outer contour of the first surface regions.

This arrangement ensures that the seed structure can safely perform its function as a diffusion barrier layer. Here, the seed structure preferably has a layer thickness of 0.1 to 100 nanometers, preferably from 0.5 to 50 nanometers, more preferably from 1 to 10 nanometers.

With such a layer thickness, the effect is securely ensured as a diffusion barrier layer.

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

1 shows a simplified cross section through a solar cell according to the invention;

Fig. 2a) to 2c)

different phases of a LIFT process for generating the

crop structure; and

Fig. 3 is a schematic representation of a solar cell with an attached

Screen printing form.

In Fig. 1 is a cross section through a silicon solar cell is shown, which is generally designated 10. The solar cell 10 has on its front side an emitter 12 (n-type), followed by a base layer 14 (p-type), to which a rear-side contact 16 made of aluminum is applied flatly. Optionally, backside contact 16 is coated with a passivation layer 23 of amorphous silicon.

On the emitter 12 is located on the front side of a passivation layer, which may consist of silicon nitride, for example. At selected points, a seed structure 20 is produced through the passivation layer 18 by means of an LIFT process, as will be explained below, which is formed as a thin middle layer and directly contacts the emitter 12. On top of this is a reinforcing layer in the form of a Reinforcing layer 22 applied from a copper alloy. The combination of seed structure 20 and reinforcing layer 22 on the front side of the solar cell 10, the front contacts 19 are formed. The result is an H-shaped grid with narrow tracks 19, the fingers that guide the collected power to current rails, such as busbars (not shown).

The principle of the LIFT process is explained in detail in DE 10 2009 020 774 A1, which is hereby incorporated by reference in its entirety.

By means of the LIFT process, a metallic seed structure 20, which consists for example of nickel, is now produced through the passivation layer 18. For this purpose, a substrate 32 in the form of a thin glass layer or a thin film is arranged in the immediate vicinity of the substrate layer, ie before the passivation layer 18, which is provided on its side facing the solar cell 10 with a thin metal layer 34. This may be, for example, a nickel layer.

FIG. 2 b) shows how a part is detached from the thin metal layer 34 and, according to FIG. 2 c), is shot through the passivation layer 18 directly onto the surface of the emitter 12. For this purpose, a pulsed laser 24 is used which directs a laser beam 26 through the transparent carrier layer 32 onto the metal layer 34 through a lens 28 and a gap 30. Due to the high energy of the pulsed laser beam, the metal layer 34 is locally peeled off and evaporated through the passivation layer 18 to deposit on the surface of the emitter 12, as shown in Fig. 2c), as a seed structure 20. This layer is referred to herein as a "seed structure" because it is enhanced by an additional process step, screen printing.

It is understood that the illustration in FIGS. 1 to 3 is purely schematic and does not reflect the actual size ratios. Preferably, a pulsed laser is used for the LIFT process. It may be, for example, a Nd: YAG laser with a wavelength of 532 or 1 .064 nanometers.

The seed structure produced according to FIGS. 2 a) to c) is subsequently reinforced according to FIG. 3.

For this purpose, a screen printing method is used. In the case of screen printing, a screen printing form 36 with a screen 40 on the surface is first positioned on the solar cell in accordance with FIG. 3 in such a way that the recesses in the screen 40 correspond exactly to the lines of the seed structure 20. Subsequently, a screen printing paste is applied with a doctor blade (not shown), then the screen printing plate 36 is removed and the solar cell 10 is subsequently treated in a suitable oven to burn in the screen printing paste. This is preferably done with a substantially copper-containing screen printing paste at a temperature of less than 400 ° C, preferably at most 330 ° C.

By means of a sufficiently precise screen printing plate 36 and accurate positioning prior to application of the screen printing paste, it is ensured that the generated reinforcing layer 22 is located only within the surface regions of the seed structure 20, corresponds at most to the outline of the seed structure 20.

This ensures that no direct contact between the reinforcing layer 22 and the passivation layer 18 or the surface of the emitter 12 is formed.

In this way, since the seed structure 20 is made of sufficient strength, preferably about 1 to 10 nanometers thick, to ensure a continuous layer, a secure diffusion barrier layer is formed between the copper alloy reinforcement layer 22 and the passivation layer 18, respectively Ensures the emitter 12. In this way, the adverse effects of copper diffusion, which occurs even at room temperature, safely avoided.

The combination of the nickel or nickel alloy seed structure 20 and the precisely applied reinforcing layer 22 results in front contacts 19 having a very low contact resistance and a low line resistance.

The manufacturing process, which consists of a LIFT process as combined with a subsequent printing step, can be carried out on an industrial scale in a reliable manner very cost-effective with low energy consumption by an order by screen printing or ink-jet method.

Since the baking of the print layer 22 in the case of a screen printing process at a temperature below 400 ° C, preferably at most 330 ° C, takes place, in addition, the back 16 with a passivation layer 23, preferably made of amorphous silicon, are occupied.

Claims

claims

Method for contacting a semiconductor substrate, in particular for contacting solar cells (10), in particular for producing front contacts (19) on solar cells (10), in which initially a metallic seed structure (20) on the surface to be contacted by means of an LIFT process (Laser Induced Forward Transfer process) is generated and then at least one reinforcing layer (22) is applied by means of a printing process, in particular by means of a screen printing process or an ink-jet process.

The method of claim 1, wherein the seed structure (20) has first surface areas and the reinforcement layer (22) has second surface areas, wherein the seed structure (20) and the reinforcement layer (22) are matched to one another such that the second surface areas have an outer contour first surface areas coincide or run within it.

The method of claim 1 or 2, wherein the seed structure (20) and / or the reinforcing layer are made of nickel, titanium, aluminum, copper or alloys thereof.

The method of claim 3, wherein the seed structure (20) and the reinforcing layer (22) are made of a copper or nickel containing material.

Method according to one of the preceding claims, in which the seed structure (20) is produced as a diffusion barrier layer.

A method according to any one of the preceding claims, wherein the seed structure (20) and the reinforcing layer (22) are made of nickel or a nickel alloy.

7. The method according to any one of the preceding claims, wherein the seed structure (20) is produced with a layer thickness of 0.1 to 100 nanometers, preferably from 0.5 to 50 nanometers, more preferably from 1 to 10 nanometers.

A method according to any one of the preceding claims, wherein the seed structure (20) is reinforced by a further LI FT process before the reinforcement layer (22) is applied.

9. The method according to any one of the preceding claims, wherein the seed structure (20) through a cover layer, preferably through a passivation layer (18) therethrough, preferably on a front side of a solar cell (10) is generated.

10. The method according to any one of the preceding claims in which a screen printing paste by a screen printing plate (36) on the seed structure (20) is applied, which is baked at a temperature of less than 400 ° C, preferably at most 330 ° C.

1 1. Method for contacting a solar cell (10), in which a backside passivation layer (23), preferably of amorphous silicon, is applied.

12. A solar cell with a front contact (19) or with a rear contact having a seed structure (20) of nickel or a nickel alloy and thereon a reinforcing layer (22) made of copper, a copper alloy, nickel or a nickel alloy, wherein the seed structure ( 20) is formed as a diffusion barrier layer opposite to the reinforcing layer (22).

13. A solar cell according to claim 12, wherein the seed structure (20) has first surface areas, and the reinforcing layer (22) has second area areas that coincide with or extend within an outer contour of the first area areas.

14. Solar cell according to claim 12 or 13, wherein the seed structure (20) has a layer thickness of 0.1 to 100 nanometers, preferably from 0.5 to 50 nanometers, more preferably from 1 to 10 nanometers.