DE102012003866B4 - Method for contacting a semiconductor substrate, in particular for contacting solar cells, and solar cells - Google Patents

Abstract

The invention relates to a 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 at first a metallic seed structure (20) on the surface to be contacted by means of a LIFT process is generated and then a screen printing layer (22) on the seed structure (20) is printed by means of a screen printing process.

Description

The invention relates to a method for contacting a semiconductor substrate, in particular for contacting solar cells, in particular for producing front contacts on solar cells, in which first a metallic seed structure on the surface to be contacted by means of LIFT process (laser-induced-forward-transfer process) is produced and then at least one screen printing layer is printed on the seed structure by means of a Siebdurckverfahrens.

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 the 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 to the wafer by means of a laser. This is done by the so-called LIFT method (Laser Induced Forward Transfer). This can be followed by additional Siebdruckschritte, wherein the N-type and the P-type regions are contacted separately. Furthermore, an amplification of the structures produced by laser can be carried out galvanically or de-energized.

From Röder, T. C .; Hoffmann, E .; Kohler, J. R .; Werner, JH: "30 μm WIDE CONTACTS ON SILICONE CELLS BY LASER TRANSFER", in: 35th IEEE Photovoltaic Specialist Conference (PVSC), 2010, pages 3597-3599, DOI: 10.1109 / PVSC.2010.5614378 it is known, by means of the LIFT Method to produce seed structure front contacts directly through the anti-reflection coating on the front of solar cells. The fingers have a width of less than 30 microns.

A screen printing process is considered unsuitable for reinforcing the contact fingers made by the LIFT process. Rather, the contact fingers are reinforced by an electroplating process.

While this is a fundamental way to make thin front contacts on solar cells with good conductivity, the additional electroplating process is relatively expensive.

From the DE 10 2009 020 774 A1 For example, 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 produced in this way are known.

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

On a laboratory scale, it is possible to 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 far as possible. Problem with the use of copper To reinforce 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 ensure a very accurate placement of the reinforcement lines only over the metallic seed structure when using a galvanic process on an industrial scale. 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 over 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 the DE 43 30 961 C1 Furthermore, a method for the production of structured metallizations on surfaces is known, in which also first a laser nucleation of the substrate surface, such as the ITO surface of an LCD cell is performed by means of a LIFT method and then carried out a galvanic reinforcement, for example in a nickel bath becomes.

Here, too, the disadvantages described above arise.

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

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

From the US 2007/0169806 A1 Furthermore, a method for contacting solar cells is known, in which first a partial ablation of the passivation layer takes place by means of a laser process on the front side of the solar cell, and then the thus exposed surface areas are printed by means of an inkjet process.

This method also has the disadvantages described above.

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

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 on solar cells, in which first a metallic seed structure is produced on the surface to be contacted by means of a LIFT process and then at least one Screen printing layer is printed on the seed structure by means of a screen printing method, wherein the seed structure having first surface areas and the screen printing layer is generated with second surface areas by means of the screen printing process such that the second surface areas coincide with or extend within an outer contour of the first surface areas.

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

Metallization using the combination of the LIFT process and the subsequent screen printing process brings a new combination of two processes. The metallic seed structure generated by the LIFT method creates a low contact resistance to the solar cell. The screen printing layer brings in combination with a low line resistance. By creating the screen printing layer in such a way that its surface areas coincide with or run within the outer contour of the first surface areas produced by the LIFT method, it is ensured that the seed structures can perform their function as a diffusion barrier layer.

Furthermore, 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 with solar cells contacted on the back side, the same advantages generally result as with the solar cells contacted on the front side.

As the screen printing process is a tried-and-tested industrial process that enables extremely fine structures to be precisely produced in a cost-effective manner, a particularly cost-effective and precise production of high-quality contacts to solar cells results in combination with the LIFT process.

In a preferred development of the invention, the seed structure is made of nickel or a nickel alloy and the screen printing layer is produced from a copper- or nickel-containing screen printing paste.

The seed structure is preferably produced as a diffusion barrier layer.

This measure enables a very cost-effective production of high-quality strip conductors on the front side of solar cells. 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 produced 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 another LIFT process before the screen printing layer is applied.

Alternatively, the screen printing layer can basically also be made of nickel or a nickel alloy or another metal.

Although the use of nickel or a nickel alloy avoids the problems with the required diffusion barrier, nickel has 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 covering layer, preferably by a passivation layer, preferably on a front side of a solar cell.

In this way, the "shooting-through" of a silver-containing screen printing paste required in the prior art 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 form is preferably baked at a temperature of less than 400 ° C, preferably at most 330 ° C.

Since the furnace required for this purpose can be operated at significantly lower temperatures than in the high temperature step required in the prior art, in addition a considerable energy saving results, whereby the production costs for the solar cells thus produced are correspondingly reduced.

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 having a front contact or a rear contact having a seed structure of nickel or a nickel alloy and thereon a screen printing layer of copper, a copper alloy, nickel or a nickel alloy, wherein the seed structure as a diffusion barrier layer against the screen printing layer is formed, and wherein the seed structure first surface areas and the screen printing layer has second surface areas that match or extend within an outer contour of the first surface areas.

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

According to the present invention, by combining a seed structure formed as a diffusion barrier layer with a screen printing layer of copper or a copper alloy, a low contact resistance is combined with a low line resistance. Because of the design of the screen printing layer of copper or a copper alloy results in a low line resistance and cost manufacturability. It also ensures that the seed structure can safely perform its function as a diffusion barrier layer.

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

The metallic seed structure generated by the LIFT method causes a low contact resistance to the solar cell. The screen printing layer brings in combination with a low line resistance.

In this case, the seed structure preferably has a layer thickness of 0.1 to 100 nanometers, preferably 0.5 to 50 nanometers, more preferably 1 to 10 nanometers.

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

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

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

2a) to 2c) different phases of a LIFT process to produce the seed structure; and

3 a schematic representation of a solar cell with an attached screen printing form.

In 1 is a cross section through a silicon solar cell shown with a total of 10 is designated. The solar cell 10 indicates at its front an emitter 12 (n type) followed by a base layer 14 (p-type), to which a backside contact 16 consisting of aluminum is applied flat. Optionally, the backside contact 16 with a passivation layer 23 coated in amorphous silicon.

On the emitter 12 At the front is a passivation layer, which may consist of silicon nitride. At selected points is through the passivation layer 18 through a LIFT process, as will be explained below, a seed structure 20 generated, which is formed as a thin middle layer and the emitter 12 contacted directly. On top of this is a reinforcing layer in the form of a screen printing layer 22 made of a copper alloy. By the combination of seed structure 20 and screen printing layer 22 are so on the front of the solar cell 10 the front contacts 19 educated. The result is an H-shaped grid with narrow tracks 19 , the fingers that direct the collected power on busbars, such as busbars (not shown).

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

The LIFT process will now pass through the passivation layer 18 through a metallic seed structure 20 generated, which consists for example of nickel. This is done according to 2a) in the immediate vicinity in front of the substrate layer, ie before the passivation layer 18 , a carrier material 32 arranged in the form of a thin layer of glass or a thin film, on its the solar cell 10 facing side with a thin metal layer 34 is provided. This may be, for example, a nickel layer.

In 2 B) is now shown as from the thin metal layer 34 a part is replaced and according to 2c) through the passivation layer 18 through directly on the surface of the emitter 12 is shot. For this purpose, a pulsed laser 24 used by a lens 28 and a gap 30 through a laser beam 26 through the transparent carrier layer 32 on the metal layer 34 directed. The high energy of the pulsed laser beam makes the metal layer 34 locally detached and evaporated by the passivation layer 18 through to get on the surface of the emitter 12 , as in 2c) shown as a seed structure 20 quell. 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 representation in the 1 to 3 is purely schematic and does not reflect the actual proportions.

Preferably, a pulsed laser is used for the LIFT process. It may, for example, be a Nd: YAG laser with a wavelength of 532 or 1,064 nanometers.

The according to the 2a) to c) seed structure is according to 3 then reinforced.

For this purpose, a screen printing method is used. When screen printing is in accordance with 3 first a screen printing form 36 with a sieve 40 positioned on the surface of the solar cell so that the recesses in the sieve 40 exactly with the lines of the seed structure 20 correspond. Subsequently, a screen printing paste is applied with a doctor blade (not shown), then the screen printing form 36 removed and the solar cell 10 then 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.

Due to a sufficiently precise screen printing form 36 and accurate positioning before applying the screen printing paste will ensure that the screen printing layer produced 22 only within the surface areas of the seed structure 20 maximum, with the outline of the seed structure 20 corresponds.

This ensures that there is no direct contact between the screen printing layer 22 and the passivation layer 18 or the surface of the emitter 12 arises.

Because the seed structure 20 is prepared with sufficient strength, preferably with a thickness of about 1 to 10 nanometers, whereby a continuous layer is ensured, in this way a secure diffusion barrier layer between the screen printing layer 22 made of a copper alloy and the passivation layer 18 or the emitter 12 guaranteed.

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 screen printing layer 22 arise front contacts 19 with a very low contact resistance and a low line resistance.

The manufacturing process, which consists of a LIFT process combined with a subsequent screen printing step, can be carried out reliably on a very industrial scale in a very cost-effective manner with low energy consumption.

As the burning of the screen printing layer 22 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 (11)

Method for contacting a semiconductor substrate, in particular for contacting solar cells ( 10 ), in particular for the production of front contacts ( 19 ) to solar cells ( 10 ), in which initially a metallic seed structure ( 20 ) is produced on the surface to be contacted by means of a LIFT process and then at least one screen printing layer ( 22 ) on the seed structure ( 20 ) is printed by means of a screen printing process, wherein the seed structure ( 20 ) has first surface areas and the screen printing layer ( 22 ) is produced with second surface areas by means of the screen printing method such that the second surface areas coincide with or extend within an outer contour of the first area areas. Process according to claim 1, wherein the seed structure ( 20 ) is made of nickel or a nickel alloy and the screen printing layer ( 22 ) is made of a copper or nickel-containing screen printing paste. Process according to Claim 2, in which the seed structure ( 20 ) is prepared as a diffusion barrier layer. Process according to claim 1, wherein the seed structure ( 20 ) and the screen printing layer ( 22 ) are made of nickel or a nickel alloy. Method according to one of the preceding claims, in which 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. Method according to one of the preceding claims, in which the seed structure ( 20 ) is reinforced by means of a further LIFT process before the screen printing layer ( 22 ) is applied. Method according to one of the preceding claims, in which the seed structure ( 20 ) by a cover layer, preferably by a passivation layer ( 18 ), preferably on a front side of a solar cell ( 10 ), is produced. Method according to one of the preceding claims, in which a screen-printing paste is replaced by a screen-printing form ( 36 ) on the seed structure ( 20 ), which is baked at a temperature of less than 400 ° C, preferably at most 330 ° C. Method for contacting a solar cell ( 10 ), in which a backside passivation layer ( 23 ), preferably of amorphous silicon. Solar cell with a front contact ( 19 ) or with a back contact having a seed structure ( 20 ) of nickel or a nickel alloy and thereon a screen printing layer ( 22 ) of copper, a copper alloy, nickel or a nickel alloy, the seed structure ( 20 ) as a diffusion barrier layer opposite the screen printing layer (US Pat. 22 ), and wherein the seed structure ( 20 ) has first surface areas and the screen printing layer ( 22 ) has second surface areas which coincide with or extend within an outer contour of the first surface areas. Solar cell according to Claim 10, in which the seed structure ( 20 ) has a layer thickness of 0.1 to 100 nanometers, preferably 0.5 to 50 nanometers, more preferably 1 to 10 nanometers.

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