LT6081B - New solar cell emtter contact grid forming method using the full chemical metallization coating method - Google Patents

Abstract

The invention relates to the field of semiconductor devices, particularly in relation to the solar cells. This invention is a method to produce solar cell emitter grid, with low resistance and good adhesion at which can be easily connected to the connecting cables. To realize this aim, instead of silver creates two layers of nickel and copper chemically coated coating which covered Ni performs ohmic contact function of silicon and copper layer reduces the resistance of the grid. This allows reduce emitter alloying level and increase solar cell efficiency. This solar cell has the following characteristics: a lower cost; easier availability of used items; significantly lower grid tracks that not contouring thin solar cell plates and practically does not shadow, volume; opportunity to thin solar cells.

Description

TECHNICAL FIELD

The present invention relates to semiconductor devices, in particular to solar cells. The present invention also relates to methods of manufacturing solar cells, in particular to contact formation and chemical metallization (chemical coating) in the preparation of characteristic structural elements for use in the construction and production of solar cells. The present invention also relates in particular to methods of forming an emitter contact wire.

TECHNICAL LEVEL

Semiconductor devices are devices that use semiconductor elements and are based on semiconductor properties. Semiconductors are materials whose electrical conductivity is between metals and dielectrics. By heating and / or illuminating a semiconductor with electromagnetic radiation, free charge carriers (electrons and holes) are formed in it and are involved in charge transfer processes in the semiconductor. Depending on the specifics of the application and the application conditions, the aforementioned new structural (composite) elements, which have new and different (different) properties, are constantly being developed in the world. On the one hand, as the world moves towards miniaturization of cells in the field of semiconductor devices, increasingly sophisticated methods of manufacturing such cells are emerging. On the other hand, more and more semiconductor elements are integrated with other elements or coated with different properties (layers). This is how the structural (composite) elements or so-called "sandwiches" are formed.

U.S. Pat. US2010186808, published Oct. 2010 July 29 This patent describes a step of forming an electrical contact to form a metal silicide for forming an ohmic contact between a metal and an alloy layer. For forming an ohmic contact, an annealing / tanning procedure is usually applied after plating on the alloy layer. This patent proposes not to use an annealing procedure, but to use dielectrics with strong quantum tunneling properties in the coating metals. However, this patent does not address the problem of coating metals for forming a contact wire.

Also known is Chinese patent no. CN101414646, Published 2009

April 22 This patent describes a method which allows the production of solar cells with a textured surface, i.e. the surface of the solar cell is not flat but has a certain geometry / architecture, such as the geometry of "repetitive" home roofs. This allows solar cells to be formed in the same working area of the sunlight, which, due to its textured surface, would allow more sunlight to be collected. However, this patent deals with the geometry of solar cells and does not deal with contact grids.

Also known is American Application No. WO2010120702, published Oct. 2010 October 21 The present application contemplates an improved embodiment of a conventional electrochemical coating method that utilizes solutions that improve the coating (deposit) rate. However, the electrochemical plating technique has one major drawback: the need to electrically connect each element.

Known Chinese patent no. CN101635319, Published 2010 January 27 This patent relates to a method of manufacturing solar cells which includes evaporation operations of contact metals. Although evaporation is an advanced and advantageous process, metal vapor is not only applied at the point where the contact is to be formed, but also at the point where it is not evaporated. Stencils or photolithography are used to avoid such side effects, but they are neither convenient nor fully effective. It also requires a very sophisticated and well-matched vacuum evaporator, which also has repeatability / reliability problems.

The closest to the prior art is Japanese patent no. JP9116177, published July 1997. May 2 This patent describes a process for the production of solar cells in which a copper (Cu) and an indium (In) substrate, formed by chemical metallization, is additionally deposited with CulnSe ₂ having good structural and thermal properties. Such material has a smooth surface, no holes and grooves, which allows the application of such layers in the production of diffractive gratings. However, this patent does not address the potential and advantages of a combination of nickel (Ni) and copper (Cu) on an alloyed silicon (Si) layer to reduce the cost of lattice forming technology and improve solar cell performance.

When increasing the efficiency of solar cells, the factors limiting its efficiency must be removed. Historically, silver (Ag) is commonly used to form the emitter contact wire. True, there have been attempts to use aluminum (Al) in earlier times, but it was abandoned for several reasons: aluminum (like most other metals) exhibits very strong recombination at the boundary with a semiconductor; silicon (Si) diffuses into aluminum (Al) and forms holes that shun the pn junction; soldering to aluminum (Al) is quite difficult and similar. However, Ag is also a rather problematic material with at least a few negative aspects, both technically and economically. One of the major technical drawbacks is that Ag material is not ideal for solar cell efficiency because it requires a sufficiently high degree of emitter doping (emitter surface concentration should be approximately 10 ² ° at / cm ³ ). Compared to Al, it is much better, but not ideal, and the degree of doping of the alloy layer is directly related to the efficiency of the solar cell: the lower the doping degree of the emitter, the higher the solar cell performance. Another technical problem is that with the Ag material, the size (width x height) of said contact wire strips is large enough (up to 80pm x 100 pm), which is also a problem since the width of the tracks competes with the surface of the dielectric layer. and altitude creates a "shadow problem": both factors directly reduce the effective area of the solar cell's incident rays. In addition to technical drawbacks, there are also several drawbacks in economic and resource terms. Ag is expensive enough material. Also, the material resources of Ag on Earth are not large: it is estimated that, if the utilization of Ag in the solar cell industry continues to develop at a similar rate, Ag will last a maximum of 30-40 years. All these shortcomings encourage the search for other materials: cheaper, more affordable and more efficient. It also encourages the search for new ways to produce solar cells: simpler, more convenient, faster and more efficient.

THE SUBSTANCE OF THE INVENTION

The object of the present invention is to provide a more efficient, faster, cheaper, simpler and more accessible method of forming an emitter contact wire that provides reliable operation of solar cells.

Currently, the most common emitter contact wire forming method based on silver (Ag) is used. Its (silver) coating on the dielectric is usually by silk screening and annealing. It is an object of the present invention to replace silver (Ag) with another element or compound / combination thereof in order to reduce the recombination process due to the emitter doping because, as mentioned above, a doped dopant means loss of solar cell efficiency.

SUMMARY OF THE INVENTION The present invention provides a method for producing a solar cell emitter screen having low impedance and good adhesion to which connecting cables can be easily connected. To accomplish this, a two-layer chemically coated coating of nickel and copper (Ni + Cu) is created in place of silver (Ag), in which the coated and sintered Ni performs an ohmic contact with silicon (Si) and the copper (Cu) layer reduces the mesh impedance. Since the contact resistance of such a compound with Si is low enough over a wider range of surface concentrations, the emitter may be less doped, which means that the entire solar cell can work more efficiently.

A further distinguishing feature of the present invention is that the said layers (Ni + Cu) are completely chemically coated, which prevents the inconvenient, expensive, slow and inefficient application of the applied coating methods (electrochemical, evaporation, metallic paste silk screening, etc.). currently applicable in the solar cell manufacturing industry.

The solar cell produced by the present invention has the following positive features (the essential features of the present invention are as follows):

low cost because the solutions used are inexpensive and easy to access;

the use of rare elements (such as Ag) whose natural resources may be depleted relatively quickly (approximately 30-40 years);

obtains lower volume (width x height) tracks (approx. 40 pm (width) x 15 pm (height)) that do not bend thin solar cell plates due to their weight and practically do not create shadows around these tracks (for comparison: technologically derived Ag tracks are approximately dimensions: 80 pm (width) x 100 pm (height));

full chemical application of the coating (Ni + Cu) coating method of the present invention provides the opportunity to further thin the solar cells, i.e., to save the parent material, Si;

this technique allows for a much faster and more convenient formation of the emitter contact wire structures (in terms of velocity versus electrochemical methods, almost the entire sequence), and also controls the coating rate of said elements.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 illustrates a solar cell composite plate (also known in the art as a cross-section of a solar cell plate) prior to processing by the present invention, consisting of an alloy semiconductor and a dielectric having an open / formed local opening.

FIG. Figure 2 shows a composite plate of a solar cell after pretreatment with activator solution and nickel solution when a nickel layer is formed.

FIG. 3 shows a composite plate of a solar cell after secondary treatment in an activator solution and a copper solution as the copper layer forms.

PREFERRED OPTIONS FOR IMPLEMENTATION

On the one hand, it is an object of the present invention to provide a new, more efficient, faster, accessible method of forming an emitter contact grid that enhances the efficiency of a solar cell and is suitable for a solar cell emitter with a lower surface impurity concentration. On the other hand, there is a need to make the new method less expensive, easier to use, easier to use and reliable. Currently, the emitter contact grid formation method based on Ag (as mentioned above) is the most widely used method. Ag coating is carried out by known methods of screen printing and tanning. In order to keep their contact resistance with the emitter within acceptable limits, the impurity atoms of the emitter must have a surface concentration of at least 10 ² ° at / cm ³ , which means that a highly doped emitter is required. Since a high-alloyed emitter means a loss of solar cell efficiency, a new and better solution has been sought, since reducing the surface concentration of the emitter impurity atoms directly improves the efficiency of the solar cell (SE). The aim of this work is to form a metal contact wire for a 6 emitter coated with a dielectric layer, which would have a low contact resistance at the average (1-5 x 10 ¹⁹ at / cm ³ ) concentration of the emitter impurity atoms. To minimize current losses, the metal mesh that is in ohmic contact with the semiconductor material (Si) must have a sufficiently low impedance. Also, said mesh must have good adhesion so that the connecting wires can be easily soldered to it. For this purpose, a two-layer chemically coated nickel-copper (Ni + Cu) coating is developed. Ni forms an ohmic contact with Si after the tanning procedure and the Cu layer reduces the mesh impedance. Both layers are chemically coated and thus self-adhesive to the opening up to Si in the dielectric. Ni contact resistance with Si is sufficiently low over a broad range of surface concentrations of phosphorus (P) and boron (B).

FIG. 1 illustrates a composite plate (E) of a solar cell prior to treatment, consisting of an alloy semiconductor (1, 3) and a dielectric (2) having an open / formed local opening (4). The solar cell plate (E) has dielectric layers (2) at the top and bottom. The doped semiconductor (1, 3) consists of two parts: a boron (B) doped p-type semiconductor (1) and a phosphorus (P) doped n-type semiconductor (3). We call the doped n-type semiconductor simply the importance of the emitter (3), namely the surface concentration of its impurity atoms, and we mentioned above. The said local aperture (4) of the required geometry corresponding to the contact grid topology is formed by any one of the currently known methods: photolithography, electron lithography, laser ablation and the like. In addition to P and B, the semiconductor may be doped with other suitable elements. Said emitter (3) may also be of p-type (respectively, the remainder of the semiconductor is then n-type). The plate (Fig. 1, E) (blank) thus prepared is immersed in an activator solution for cleaning the surface of the emitter (3) of said plate (E) from residues of the dielectric (2). The activator solution must necessarily contain fluorine ions (F ') and may also contain metal ions such as gold (Au) or palladium (Pd). After holding the activator solution for a period of time (10 s to 20 min), the plate (E) is transferred to the solution containing Ni ions (Ni solution) and kept there for the required time interval. Said Ni solution has a pH of about 8-10 and a temperature of about 50-95 ° C. Depending on the temperature and time of the solution, the Ni deposited in the orifice covers the entire open surface of the Si (emitter) (Fig. 2). FIG. 2 shows a composite plate (E) of a solar cell after pretreatment with an activator solution and a nickel (Ni) solution, whereby a layer of Ni (5) is formed which is in ohmic contact with the Si emitter (3). Depending on the type of reducing agent used in the solution, the chemically coated Ni layer contains up to 6% P or B (where P and B come from the components of the Ni solution, the proportions of which may vary as needed). The plate (E) with Ni deposited is annealed in an inert atmosphere to form a Ni ohmic contact with Si. Ni diffuses partially into Si, forming Ni silicide. Said annealing in an inert atmosphere is carried out at a temperature in the range of about 200-600 ° C, and the time (t) depends on the desired thickness of the silicide layer. The next step is to reduce the resistance of the contact wire. For this purpose, plate (E) is immersed in an activator solution which is usually acidic. Activator temperature 20-60 ° C. Plate (E) is maintained in the activator for approximately 30-90 s and then immersed in a solution containing copper (Cu) ions. The temperature of the copper solution is approximately 20-50 ° C and the pH range is approximately 12.0-13.3. Cu precipitates in the areas covered by the Ni layer (Fig. 3). FIG. 3 shows a composite plate (E) of a solar cell after secondary treatment in an activator solution and a copper (Cu) solution to form a copper layer. Retain the slide (E) in solution until the required layer of Cu is deposited. The plate (E) is then removed from the solution, washed and dried.

Here are some examples of implementation.

A first example of an embodiment wherein the pre-treatment preparation is by photolithography.

This plate with the emitter and the thermally grown S1O2 layer is covered with a photoresist layer and exposed via an emitter photo grid. It is then developed, duplicated, and buffered with SiO ₂ in areas not covered by a photoresist. The plate thus prepared is immersed in HF aqueous solution (4-8%) containing 0.05-0.08 g / l gold (Au) and kept for approximately 20-60 s. The plate is then removed from the activator solution and immersed in a 50-95 ° C nickel solution consisting of:

NiCl ₂ 6H ₂ O 20-40 g; NaH ₂ PO ₂ H ₂ O 5-15 g; (NH ₄ ) ₂ H C ₆ H ₅ O ₇ 50-75 g; NH 4 Cl - 30-50g.

In this Ni solution, the plate is held for 60 to 240 s, removed from it, rinsed with deionized water and dried. The plate is then immersed in hot dimethylformamide or other material used to remove the photoresist. It is then washed again with deionized water and, after drying, placed in a Ni tanning chamber. Ni combustion takes place in an inert or reducing atmosphere (N 2, Ar, and may also be H ₂ ) at 200-600 ° C, and the time may vary from about 5 s to about 60 min. Plates (E) are transferred from the Ni tanning process / chamber to the chemical variation chamber. The plate cartridge is immersed in HCl aqueous solution at 3060 ° C for 30-300 s, rinsed with deionized water and immersed in a ready-to-use propellant solution consisting of:

CuSO ₄ - 0.06 mol / L;

Trillon B 0.08 mol / L;

Glycine 0.06 mol / L;

Attachment A - 0-3 mg / l;

Attachment B - 0-0.156 mg / L;

NaOH -10 mol / l;

Formaldehyde 0.15 mol / L;

Other parameters are: temperature range: 20-50 ° C, charge size (active surface area) <1 dm ² / l, pH 12.3-13.3. Under these conditions, a Cu coating rate of 3-10 pm / h is achieved.

The second embodiment (laser pre-treatment preparation is laser ablation).

The plate with emitter plasmochemistry (PECVD) covers the Si _x N _y layer and laser-etches the emitter contact wire pattern as shown in Figs. 1. In the opening (4), Si, i.e. the emitter (3), exits the surface. Plate (E) thus prepared is immersed in the activator solution for a set time (20 seconds to 8 minutes and more). The retention time depends on the activator solution concentration, the ablation mode, and the initial Si _x N _y layer thickness. The activator is an aqueous solution of 4-40 percent HF or its salts. The plate (E) is then removed from the activator (activation solution) and immersed at 50-95 ° C in a nickel solution of the composition described in the first embodiment. In the nickel plating solution, the plate is held for about 60-240 s, removed from it, rinsed with deionized water and dried in a Ni ignition chamber. Ni combustion takes place in an inert or reducing atmosphere (N 2, Ar, may be present with H 2) at 200-600 ° C and the time may vary from about 5 s to about 60 min. Plates (E) are transferred from the Ni combustion process / chamber to the chemical propelling chamber. The plate cassette is immersed in HCl aqueous solution at 3060 ° C for 30-300 s, rinsed with deionized water and immersed in ready to use solution. The composition of the variation solution is also given in the first embodiment. The dwell time depends on the required thickness of Cu to be coated. Cu grows only on sites that are coated with Ni.

Thus, the new method of forming a solar cell emitter contact wire comprises the following steps:

1) preparing a solar cell composite wafer (E) consisting of a prepared geometry doped semiconductor (1), a dielectric (2), an emitter (3) and a preformed aperture (4) by chemical coating;

2) placing said solar cell composite plate (E) in the activator solution and maintaining it therein for a time interval to remove residual dielectric (2) from the emitter (3) surface;

3) placing said plate (E) in a solution of Ni and maintaining it thereon for a time interval, successively depositing Ni (5) on the surface of the emitter (3) of the plate (E);

(4) Depending on the preparation prior to treatment, if photolithography has been used, rinsing said plate (E) with deionized water and drying (if ablation has been applied, this step is omitted);

5) depending on the preparation and treatment, if photolithography has been used, immerse said plate (E) in dimethylformamide or another solution used to remove the photoresist (this step is omitted if ablation has been used);

6) placing said plate in a Ni combustion chamber and Ni combustion in an inert or reducing atmosphere;

7) placing said plate (E) in the activator solution and maintaining it therein for a time interval to remove nickel oxide derivatives from the Ni (5) surface;

8) placing said plate (E) in a Cu solution and maintaining it therein for a time interval, with the aim of depositing the Cu (6) on the nickel (5) by means of a (already other) reduction reaction;

9) rinsing said plate (E) with deionized water and drying.

At the end of this process, we obtain a composite plate (E) for the solar cell, which has the following distinctive (positive) properties:

low cost because the solutions used are inexpensive and easy to access;

unused rare elements whose natural resources may be exhausted relatively quickly;

smaller volume tracks (approximately 40 pm (width) x 15 pm (height)) that do not bend thin panels of solar cells due to their mass or create shadows around these tracks (for comparison: Ag tracks are approximately 80 pm (width) x 100 pm (height));

By applying the Ni + Cu coating technique of the present invention to a fully chemical process, it is possible to further thin the solar cells, i.e. further increase their efficiency;

this technique allows for a much faster formation of the emitter contact wire structures (compared to electrochemical methods in almost the whole sequence), and also the control of the coating rate.

In order to illustrate and describe the present invention, descriptions of preferred embodiments are set forth above. It is not a complete or limiting invention that seeks to determine the exact form or embodiment. | the description above should be viewed as an illustration rather than a limitation. Obviously, many modifications and variations may be apparent to those skilled in the art.

Embodiments are selected and described for the best understanding of the principles of the present invention and their best practical application for different embodiments with different modifications suitable for a particular application or embodiment, since the quantitative adaptations of this embodiment may vary from case to case. The scope of the invention is intended to be defined by the appended definition and its equivalents, in which all the foregoing terms have the widest possible meaning, unless otherwise stated. It will be appreciated that the embodiments described by those skilled in the art may make changes within the scope of the present invention as defined in the following definition.

Claims (4)

DEFINITION OF INVENTION 1. A composite element for the manufacture of semiconductor devices, in particular solar cells, consisting of dielectric layers, a (doped) semiconductor substrate and a molded opening to open the surface of the semiconductor for processing, in particular selective plating, characterized in that: the nickel (Ni) being chemically coated on the surface of the doped semiconductor substrate (emitter (3)) of the composite element (E) which forms an ohmic contact with said emitter (3); on the surface of said emitter (3), coated on the composite element (E) is chemically coated copper (Cu), which reduces the resistance of the solar cell grid and allows weaker alloying of the emitter (3), i.e. directly increases the performance of the solar cell. Process for the preparation of a composite element according to claim 1, characterized in that it comprises the following steps: preparation of solar cell composite plate (E) consisting of dielectric layers (2), (doped) semiconductor substrate / emitter (3) and molded orifice (4): placing said solar cell composite plate (E) in an activator solution and maintaining it therein for a time interval to remove residual dielectric (2) from the emitter (3) surface; placing said plate (E) in a nickel (Ni) solution and maintaining it thereon for a period of time to deposit Ni (5) on the surface of the plate (E) emitter (3); depending on the preparation of the plate (E) prior to treatment, if photolithography has been applied, washing of said plate (E) with deionized water and drying (if ablation is applied, this step is omitted); immersing said plate (E) in dimethylformamide or another solution for removal of the photoresist to remove photoresist residues (if ablation has been applied), depending on the preparation of the plate (E) prior to treatment if photolithography has been applied; placing said plate in a Ni combustion chamber and burning Ni in an inert or inert or reducing atmosphere; placing said plate (E) in an activator solution and maintaining it therein for a time interval to remove nickel oxide derivatives from the Ni (5) surface; placing said plate (E) in a solution of copper (Cu) and maintaining it therein for a time interval to coat the Cu (6) on Ni (5) by means of a (further) reduction reaction; washing and drying said plate (E) with deionized water; which allows: avoid the use of silver (Ag) in forming the emitter (3) mesh structure; faster formation of the emitter (3) mesh structure (compared to current electrochemical coating techniques); from a technological point of view it is more convenient and cheaper to form the structure of the emitter (3) grid; less alloying emitter (3), that is, reducing the emitter surface concentration of impurity atoms from about 10 ² at / cm ³ to about 1-5 x 10 ¹⁹ at / cm ³ ); easier control of the coating process, electrodes and their connections, evaporation equipment, etc.); forming thin mesh structures (volume approximately 40 pm x 15 pm instead of 80 pm x 100 pm); get rid of the negative shadow effect caused by the relief / height of the grid structures compared to currently used techniques); creating and shaping even thinner solar cells, i.e. reduce their production costs. 3. The method of manufacture of a composite element according to claim 2, characterized in that the above-mentioned nickel (Ni) solution used has the following composition at best: NiCl ₂ 6H ₂ O -20-40 g; NaH ₂ PO ₂ H ₂ O - 5-15 g; (NH ₄ ) ₂ HC ₆ H ₅ O 7 - 50-75 g; NH ₄ Cl 30-50 g; while the retention time in it is 60-240 s. Process for the manufacture of a composite element according to claim 2 and / or 3, characterized in that the composition of the above-mentioned copper (Cu) solution used is preferably as follows: CuSO ₄ - 0.06 mol / l; Triton B - 0.08 mol / l; Glycine - 0.06 mol / l; Appendix A 0-3 mg / L; Appendix B -0-0.156 mg / L; NaOH -10 mol / l; Formaldehyde 0.15 mol / L; where the retention time depends on the desired thickness of the Cu coated.