EP2304814A2

EP2304814A2 - Method for producing electrodes for solar cells

Info

Publication number: EP2304814A2
Application number: EP09765772A
Authority: EP
Inventors: Rene Lochtman; Norbert Wagner; Juergen Kaczun; Juergen Pfister
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2008-06-18
Filing date: 2009-06-09
Publication date: 2011-04-06
Also published as: CA2728055C; US8247320B2; MY155485A; CN102067323A; IL209780A0; RU2011101492A; WO2009153192A3; IL209780A; US20110151614A1; JP2011524646A; RU2505889C2; KR20110030539A; CA2728055A1; WO2009153192A2; TW201005960A; TWI580059B

Abstract

The invention relates to a method for producing electrodes for solar cells, the electrode being designed as an electrically conducting layer on a substrate (1) for solar cells. In a first step, a dispersion containing electrically conducting particles is transferred from a support (7) onto the substrate (1) by irradiation of the dispersion with a laser (9), and in a second step, the dispersion transferred onto the substrate (1) is dried and/or cured to form the electrically conducting layer.

Description

Process for producing electrodes for solar cells

description

The invention relates to a method for producing electrodes for solar cells, wherein the electrode is formed as an electrically conductive layer on a substrate for solar cells.

Solar cells generally comprise a semiconductor substrate having a number of p-type and n-type doped regions which together produce a potential difference and a voltage when exposed to sunlight. In order to be able to tap the voltage, electrodes are applied to the surfaces of the semiconductor substrate.

Currently, the application of the electrodes is generally carried out by screen printing.

The preparation of the electrodes by screen printing is e.g. described in EP-A 1 911 584, US-A 2007/0187652 or US 4,375,007.

As an alternative, e.g. in WO 2008/021782 discloses first applying a metal layer to the semiconductor material, applying a resist by an ink-jet printing method, which covers the areas which are to form the structure of the electrode and then the uncovered areas of the metal layer by an etching process to remove. Finally, the Abdecklack is removed again.

The introduction of contact openings in a passivation layer on a semiconductor substrate, e.g. by laser-based systems is described in EP-A 1 833 099. After the introduction of the contact openings, a metal is introduced into the contact openings by a direct-writing metallization process. As a direct writing metallization method, e.g. Called ink jet method or extrusion method. Finally, a highly conductive material is applied to the previously deposited contact material and between the contacting openings.

From DE-A 10 2006 033 887 it is known to apply an electrically conductive layer to a substrate by transferring an electrically conductive polymer having a transfer layer from a transfer film to the substrate.

A disadvantage of the printing and embossing methods known from the prior art is, on the one hand, that the printing resolution is limited, in particular in screen printing, and printed conductors with a width of less than 120 μm can not be printed. Efficient power generation in solar cells, however, requires the greatest possible use of solar cells. Therefore, it is desirable to print even printed circuit structures with smaller dimensions.

Another disadvantage of the printing and embossing processes is that they are not contactless and due to the applied pressure through the contact, e.g. with screen and squeegee screen printing, the substrate can break. In non-contact methods, no pressure is exerted on the substrate, so that the risk of breakage of the substrate is significantly reduced. The non-contact methods known from the prior art are generally etching methods which have the disadvantage that acids and alkalis must be used for etching and subsequent removal of the covering lacquer. In addition, several complex process steps are required.

The object of the invention is to provide a method for the production of electrodes for solar cells, wherein the electrode is formed as an electrically conductive layer, which makes it possible to image the electrically conductive layer even in very fine structures and in a simple manner without the use large quantities of environmentally hazardous substances can be carried out.

The object is achieved by a method for producing electrodes for solar cells, wherein the electrode is formed as an electrically conductive layer on a substrate for solar cells, comprising the following steps:

a) transferring an electrically conductive particle-containing dispersion from a support to the substrate by irradiation of the dispersion with a laser,

b) drying and / or curing of the dispersion transferred to the substrate to form the electrically conductive layer.

As a substrate for the solar cell to which the electrically conductive layer is applied, for example, all rigid or flexible substrates are suitable, which are suitable for the production of solar cells. Suitable substrates are, for example, monocrystalline, multicrystalline or amorphous silicon, III-V semiconductors, for example GaAs, GaSb, GaInP, GaInP / GaAs, GaAs / Ge or II-VI semiconductors, for example CdTe or I-III-VI semiconductors For example, CulnS ₂ , CuGaSe ₂ or those of the general formula ABC ₂ , wherein A is copper, silver, gold, B aluminum, gallium or indium and C is sulfur, selenium or tellurium. Furthermore, all rigid or flexible substrates are suitable, which are coated with the aforementioned semiconductor materials. Such rigid and flexible substrates are for example glass or plastic films.

In a first step, a dispersion containing electrically conductive particles is transferred from a carrier to the substrate. The transfer takes place by irradiation of the dispersion on the support with a laser.

The electrically conductive layer which is transferred to the substrate may be full-surface or structured. By transferring the dispersion with the laser, it is also possible to produce very fine structures, for example with dimensions of less than 120 μm, preferably of less than 100 μm, in particular of less than 80 μm. These dimensions mainly affect the width of individual webs.

Before the dispersion is transferred with the electrically conductive particles contained therein, this is preferably applied over the entire surface of the carrier. Alternatively, it is of course also possible that the dispersion is applied in a structured manner on the carrier. However, a full-surface application of the dispersion is preferred.

Suitable carriers are all transparent to the respective laser radiation materials, such as plastic or glass. For example, when using IR lasers, polyolefin films, PET films, polyimide films, polyamide films, PEN films, polystyrene films or glass can be used.

The carrier can be both rigid and flexible. Furthermore, the carrier can be in the form of a tube or endless foil, sleeve or flat carrier.

Suitable laser beam sources for generating the laser beam are commercially available. In principle, all laser beam sources can be used. Such laser beam sources are, for example, pulsed or continuous gas, fiber, solid-state, diode or excimer lasers. These can each be used if the respective carrier is transparent to the laser radiation, and the dispersion which contains the electrically conductive particles and is applied to the carrier absorbs the laser radiation sufficiently to form a cavitation bubble by converting light energy into heat energy in the electrically conductive layer.

Preferred laser sources are pulsed or continuous (cw) IR lasers, for

Example Nd: YAG laser, Yb: YAG laser, fiber or diode laser used. These are inexpensive and available with high performance. Particular preference is given to continuous (cw) IR lasers. Depending on the absorption behavior of the dispersion, the contains electrically conductive particles, but also lasers with wavelengths in the visible or in the UV frequency range can be used. For example, Ar lasers, HeNe lasers, frequency-multiplied IR solid-state lasers or excimer lasers such as ArF lasers, KrF lasers, XeCI lasers or XeF lasers are suitable for this purpose. Depending on the laser beam source, the laser power and the optics and modulators used, the focal diameter of the laser beam is in the range between 1 μm and 100 μm.

The wavelength of the laser beam which the laser generates is preferably in the range from 150 to 10,600 nm, in particular in the range from 600 to 10,600 nm.

In order to produce the structure of the electrically conductive layer, it is also possible to arrange a mask in the beam path of the laser or to apply a mapping method known to the person skilled in the art.

In a preferred embodiment, the desired parts of the dispersion applied to the support and containing the electrically conductive particles are transferred to the substrate by means of a laser focused on the dispersion.

To carry out the method according to the invention, the laser beam and / or the carrier and / or the substrate can be moved. The laser beam can be moved, for example, by optics with rotating mirrors known to those skilled in the art. The support may, for example, be in the form of a rotating endless film which is continuously coated with the dispersions containing the electrically conductive particles. The substrate can be moved, for example, by means of an XY table or as an endless film with unwinding and winding device.

The dispersion transferred from the support to the substrate generally contains electrically conductive particles in a matrix material. The electrically conductive particles may be particles of any geometry made of any electrically conductive material, of mixtures of different electrically conductive materials or of mixtures of electrically conductive and non-conductive materials. Suitable electrically conductive materials are, for example, carbon, such as carbon black, graphite, graphene or carbon nanotubes, electrically conductive metal complexes or metals. Preference is given to nickel, copper, silver, gold, aluminum, titanium, palladium, platinum and alloys thereof or metal mixtures which contain at least one of these metals. Particularly preferred are aluminum, copper, nickel, silver, titanium, carbon and mixtures thereof.

The electrically conductive particles preferably have an average particle diameter of 0.001 to 100 μm, preferably 0.002 to 50 μm and in particular preferably from 0.005 to 15 microns. The average particle diameter can be determined by means of laser diffraction measurement, for example on a Microtrac X100 device. The distribution of the particle diameter depends on their production method. Typically, the diameter distribution has only one maximum, but several maxima are also possible. In order to achieve a particularly dense packing of the particles, different particle diameters are preferably used. For example, particles having an average particle diameter of more than 1 μm may be mixed with nanoparticles having an average particle diameter of less than 100 nm.

The surface of the electrically conductive particles may at least partially be provided with a coating ("coating") Suitable coatings may be inorganic or organic Inorganic coatings are, for example, SiO _2. Of course, the electrically conductive particles may also be coated with a metal or metal oxide. The metal may also be present in partially oxidized form.

If two or more different metals are to form the electrically conductive particles, this can be done by mixing these metals. In particular, it is preferred if the metals are selected from the group consisting of aluminum, silver, copper, nickel, titanium, platinum and palladium.

However, the electrically conductive particles may also include a first metal and a second metal, wherein the second metal is in the form of an alloy with the first metal or one or more other metals, or the electrically conductive particles contain two different alloys.

In addition to the selection of electrically conductive particles, the shape of the particles has an influence on the properties of the dispersion after a coating. With regard to the shape, numerous variants known to the person skilled in the art are possible. The shape of the electrically conductive particles may be, for example, acicular, cylindrical, platelet-shaped or spherical. These particle shapes represent idealized shapes, wherein the actual shape, for example due to production, may vary more or less strongly therefrom. For example, drop-shaped particles in the context of the present invention are a real deviation of the idealized spherical shape.

Electrically conductive particles having various particle shapes are commercially available. If mixtures of electrically conductive particles are used, the individual mixing partners can also have different particle shapes and / or particle sizes. It is also possible to use mixtures of only one type of electrically conductive particles having different particle sizes and / or particle shapes. In the case of different particle shapes and / or particle sizes, the metals aluminum, silver, copper, nickel, titanium, platinum and palladium and carbon are also preferred.

When mixtures of particle shapes are used, mixtures of spherical particles with platelet-shaped particles are preferred. In one embodiment, for example, spherical silver particles are used with platelet-shaped silver particles and / or carbon particles of other geometries. In an alternative embodiment, spherical silver particles are combined with platelet-shaped aluminum particles.

As already stated above, the electrically conductive particles in the form of their powders can be added to the dispersion. Such powders, for example metal powders, are common commercial goods and can easily be produced by known processes, for example by electrolytic deposition or chemical reduction from solutions of metal salts or by reduction of an oxidic powder, for example by means of hydrogen, by spraying or atomizing a molten metal. especially in cooling media, such as gases or water. Preference is given to the gas and water atomization and the reduction of metal oxides. Metal powders of the preferred grain size can also be made by grinding coarser metal powders. For this purpose, for example, a ball mill is suitable.

Platelet-shaped electrically conductive particles can be controlled by optimized conditions in the production process or subsequently obtained by mechanical treatment, for example by treatment in a stirred ball mill.

Based on the total weight of the dried coating, the proportion of electrically conductive particles in the range of 20 to 98 wt .-%. A preferred range of the proportion of the electrically conductive particles is from 30 to 95% by weight, based on the total weight of the dried coating.

Suitable matrix materials are, for example, binders having a pigment-affine anchoring group, natural and synthetic polymers and their derivatives, natural resins and synthetic resins and their derivatives, natural rubber, synthetic rubber, proteins, cellulose derivatives, drying and non-drying oils and the like. These can - but need not - be chemically or physically curing, for example, air-curing, radiation-curing or temperature-curing.

The matrix material is preferably a polymer or polymer mixture.

Preferred polymers as the matrix material are ABS (acrylonitrile-butadiene-styrene); ASA (acrylonitrile-styrene-acrylate); acrylated acrylates; alkyd resins; Alkylvinylacetate; Alkylenvi- nylacetat copolymers, in particular methylene vinyl acetate, ethylene vinyl acetate, butylene vinyl acetate; Alkylenvinylchlorid copolymers; amino resins; Aldehyde and ketone resins; Cellulose and cellulose derivatives, in particular hydroxyalkylcellulose, cellulose esters, such as acetates, propionates, butyrates, carboxyalkylcelluloses, cellulose nitrate; Ethyl cellulose, methyl cellulose, epoxy acrylates; epoxy resins; modified epoxy resins, for example bifunctional or polyfunctional bisphenol A or bisphenol F resins, epoxy novolac resins, brominated epoxy resins, cycloaliphatic epoxy resins; aliphatic epoxy resins, glycidyl ethers, vinyl ethers, ethylene-acrylic acid copolymers; Hydrocarbon resins; MABS (containing transparent ABS with acrylate units); Melamine resins, maleic anhydride copolymers; methacrylates; Natural rubber; synthetic rubber; Chlorinated rubber; Natural resins; rosins; Shellac; Phenol resins; Polyester; Polyester resins, such as phenylester resins; polysulfones; polyether; polyamides; polyimides; Polybutylene terephthalate (PBT); Polycarbonate (for example, Makrolon ^® of Bayer AG); polyester; polyether; polyethylene; Polyethylene thiophene; Polymethyl methacrylate (PMMA); Polyphenylene oxide (PPO); Polystyrenes (PS), polyvinyl compounds, in particular polyvinyl chloride (PVC), PVC copolymers, PVdC, polyvinyl acetate and copolymers thereof, optionally partially hydrolyzed polyvinyl alcohol, polyvinyl acetals, polyvinyl acetates, polyvinyl pyrrolidone, polyvinyl ethers, polyvinyl acrylates and methacrylates in solution and as a dispersion, and their copolymers, polyacrylic esters and polystyrene copolymers; Polystyrene (impact or not impact modified); Polyurethanes, uncrosslinked or crosslinked with isocyanates; Polyurethane acrylates; Styrene-acrylic copolymers; Styrene-butadiene block copolymers (for example, Styroflex ^® or Styrolux ^® from BASF AG, K-Resin ™ CPC); Proteins, such as casein; SIS. Furthermore, mixtures of two or more polymers can form the matrix material.

Particularly preferred polymers as matrix material are acrylates, acrylate resins, cellulose derivatives, such as cellulose ethers, for example methylcelluloses, ethylcelluloses or cellulose esters, methacrylates, methacrylate resins, melamine and amino resins, polyalkylenes, polyimides, epoxy resins, modified epoxy resins, polyvinyl ethers, phenolic resins, polyurethanes, polyesters, polyvinyl acetals, Polyvinyl acetates, polyvinyl alcohols, polystyrenes, Polystyrene copolymers, polystyrene acrylates, styrene-butadiene block copolymers, alkylene-vinyl acetates and vinyl chloride copolymers, polyamides and copolymers thereof.

Based on the total weight of the dry coating, the proportion of the organic binder component is from 0.01 to 60% by weight. Preferably, the proportion is 0.1 to 45 wt .-%, more preferably 0.5 to 35 wt .-%.

The dispersion containing the electrically conductive particles may additionally contain a glass frit. The proportion of the glass frit, based on the dry coating, is preferably in the range of 0.1 to 15 wt .-%, preferably in the range of 0.5 to 10 wt .-% and particularly preferably in the range of 1 to 5 wt. -%. The glass used for the glass frit has a softening point generally ranging from 450 to 550 ^° C.

The glass frit added to the dispersion can be alkali metal oxides, for example Na ₂ O, K ₂ O, Li ₂ O, alkaline earth metal oxides, for example MgO, CaO, SrO or BaO, or further metal oxides, for example B ₂ O ₃ , Bi ₂ O ₃ , Al ₂ O ₃ , SiO ₂ , ZnO, TiO ₂ , ZrO ₂ , PbO, AgO or WO ₃ . The oxides may each be contained individually or as a mixture of two or more oxides in the glass frit. If two or more oxides are contained as a mixture in the glass frit, then any mixing ratio of the individual oxides is possible.

In order to be able to apply the dispersion containing the electrically conductive particles and the matrix material to the support, the dispersion may furthermore be admixed with a solvent or a solvent mixture in order to adjust the viscosity of the dispersion which is suitable for the respective application method. Suitable solvents are, for example, aliphatic and aromatic hydrocarbons (for example n-octane, cyclohexane, toluene, xylene), alcohols (for example methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, amyl alcohol), polyhydric alcohols such as glycerol, ethylene glycol, propylene glycol, neopentyl glycol, alkyl esters (for example methyl acetate, ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), alkoxy alcohols (for example methyl xypropanol, methoxybutanol, ethoxypropanol), alkylbenzenes (for example, ethylbenzene, isopropylbenzene), butylglycol, butyldiglycol, alkylglycolacetates (for example, butylglycolacetate, butyldiglycolacetate, propyleneglycolmethyletheracetate), diacetonealcohol, diglycoldialkylether, diglycolmonoalkylether, dipropyleneglycoldialkylether, dipropyleneglycolmonoalkylether, diglycolalkyletheracetate, dipropyleneglycolalkyletheracetate , Dioxane, dipropylene glycol and ethers, Diethy glycol and ether, DBE (dibasic esters), ethers (for example diethyl ether, tetrahydrofuran), ethylene chloride, ethylene glycol, ethylene glycol acetate, ethylene glycol dimethyl ester, cresol, lactones (for example butyrolactone). clay), ketones (for example acetone, 2-butanone, cyclohexanone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK)), methyl diglycol, methylene chloride, methylene glycol, methyl glycol acetate, methylphenol (ortho-, meta-, para-cresol), pyrrolidones (For example, N-methyl-2-pyrrolidone), propylene glycol, propylene carbonate, carbon tetrachloride, toluene, trimethylolpropane (TMP), aromatic hydrocarbons and mixtures, aliphatic hydrocarbons and mixtures, alcoholic monoterpenes (such as terpineol), water and mixtures of two or more of these solvents.

Preferred solvents are alcohols (for example ethanol, 1-propanol, 2-propanol, butanol), alkoxyalcohols (for example methoxypropanol, ethoxypropanol, butylglycol, butyldiglycol), butyrolactone, diglycol dialkyl ethers, diglycol monoalkyl ethers, dipropylene glycol dialkyl ethers, dipropylene glycol monoalkyl ethers, esters (for example ethyl acetate , Butyl acetate, butylglycol acetate, butyl diglycol acetate, diglycol alkyl ether acetates, dipropylene glycol alkyl ether acetates, DBE, propylene glycol methyl ether acetate, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate), ethers (for example tetrahydrofuran, dioxane), polyhydric alcohols, such as glycerol, ethylene glycol, Propylene glycol, neopentyl glycol, ketones (for example, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone), hydrocarbons (for example, cyclohexane, ethylbenzene, toluene, xylene), N-methyl-2-pyrrolidone, water, and mixtures thereof.

In liquid matrix materials, the respective viscosity can alternatively be adjusted via the temperature during the application, or via a combination of solvent and temperature.

The dispersion may further contain a dispersant component. This consists of one or more dispersants.

In principle, all dispersants known to the person skilled in the art for use in dispersions and described in the prior art are suitable. Preferred dispersants are surfactants or surfactant mixtures, for example anionic, cationic, amphoteric or nonionic surfactants. Cationic and anionic surfactants are described, for example, in "Encyclopedia of Polymer Science and Technology", J. Wiley & Sons (1966), Vol. 5, pp. 816-818, and in "Emulsion Polymerization and Emulsion Polymers", editors P. Lovell and M. El-Asser, published by Wiley & Sons (1997), pages 224 to 226. However, it is also possible to use polymers known to those skilled in the art with pigment-affine anchor groups as dispersants.

The dispersant may be used in the range of 0.01 to 50% by weight based on the total weight of the dispersion. Preferably, the proportion is 0.1 to 25 wt .-%, particularly preferably 0.2 to 10 wt .-%. Furthermore, further additives such as thixotropic agents, for example silica, silicates such as aerosils or bentonites or organic thixotropic agents and thickeners such as polyacrylic acid, polyurethanes, hydrogenated castor oil, dyes, fatty acids, fatty acid amides, plasticizers, wetting agents, defoamers, lubricants , Driers, crosslinkers, photoinitiators, complexing agents, waxes, pigments, conductive polymer particles, can be used.

The proportion of the filler and additive component based on the total weight of the dry coating is preferably 0.01 to 50 wt .-%. Further preferred are 0.1 to 30 wt .-%, particularly preferably 0.3 to 20 wt .-%.

If the electroconductive particles in the dispersion on the support itself do not sufficiently absorb the energy of the energy source, for example the laser, absorbers can be added to the dispersion. Depending on the laser beam source used, it may be necessary to select different absorbents or mixtures of absorbents which effectively absorb the laser radiation. In this case, the absorbent is either added to the dispersion or an additional separate absorption layer is applied between the support and the dispersion, which contains the absorbent. In the latter case, the energy is absorbed locally in the absorption layer and transferred to the dispersion by conduction.

Suitable absorbents for laser radiation have a high absorption in the laser wavelength range. In particular, absorbers are suitable which have a high absorption in the near-infrared as well as in the longer-wave VIS range of the electromagnetic spectrum. Such absorbents are particularly suitable for absorbing the radiation of high-performance solid-state lasers, for example Nd-Y AG lasers and IR diode lasers. Examples of suitable absorbers for the laser radiation are dyes which absorb strongly in the infrared spectral range, for example phthalocyanines, naphthalocyanines, cyanines, quinones, metal complex dyes, such as dithiolenes or photochromic dyes.

Furthermore, suitable absorbents are inorganic pigments, in particular intensively colored inorganic pigments such as chromium oxides, iron oxides, iron oxide hydrates or carbon in the form of, for example, carbon black, graphite, graphene or carbon nanotubes.

Particularly suitable as absorbers for laser radiation are finely divided carbon types and finely divided lanthanum hexaboride (LaB ₆ ). In general, from 0.005 to 20% by weight of absorbent based on the weight of the electrically conductive particles in the dispersion is used. Preference is given to using 0.01 to 15% by weight of absorbent and particularly preferably 0.1 to 10% by weight of absorbent in each case based on the weight of the electrically conductive particles in the dispersion.

The amount of absorbent added is chosen by the person skilled in the art, depending on the respectively desired properties of the dispersion layer. In this context, one skilled in the art will further appreciate that the added absorbents not only affect the rate and efficiency of dispersion of the dispersion by the laser, but also other properties such as adhesion of the dispersion to the support, curing or electroless and / or galvanic coatability of the electrically conductive layer.

In the case of a separate absorption layer, this consists in the best case of the absorbent and a thermally stable, optionally crosslinked material, so that it is not decomposed itself under the action of the laser light. In order to effect an effective conversion of light into heat energy and to achieve poor heat conduction into the electrically conductive layer, the absorption layer should be applied as thinly as possible and the absorption medium should be present in the highest possible concentration, without the layer properties, for example adhesion to the support negative to influence. Suitable concentrations of the absorbent in the absorption layer are 25 to 95 wt .-%, preferably 50 to 85 wt .-%.

The energy needed to transfer the portion of the dispersions containing the electroconductive particles may be either on the dispersion-coated side or on the dispersion, depending on the laser used and / or the material from which the support is made be applied opposite side. If necessary, a combination of both process variants can be used.

The transfer of the components of the dispersion from the support to the substrate can be carried out both on one side and on both sides. The two sides can be coated simultaneously with the dispersion in succession or, for example, by using two laser sources and two carriers coated with the dispersion, also from both sides.

To increase productivity, it is possible to use more than one laser source. In a preferred embodiment of the method according to the invention, the dispersion is applied to the support before transferring the dispersion from the support to the substrate. The application is carried out, for example, by a coating method known to the person skilled in the art. Such coating methods include, for example, casting such as curtain coating, roll coating, brushing, knife coating, brushing, spraying, dipping or the like. Alternatively, the dispersion containing the electrically conductive particles is printed onto the carrier by any printing method. The printing process by which the dispersion is printed is, for example, a roll or sheet printing process, for example screen printing, gravure printing, flexographic printing, letterpress printing, pad printing, inkjet printing, offset printing or magnetographic printing processes. However, it is also possible to use any further printing method known to the person skilled in the art.

In a preferred embodiment, the dispersion is not completely dried on the support and / or cured, but transferred to the substrate in a wet state. This makes it possible, for example, to use a continuously operated printing unit in which the dispersion on the support can be constantly renewed. With this process control, a very high productivity can be achieved. Printing units that are continuously colored, the expert, for example, from DE-A 37 02 643 known. In order to avoid sedimentation of particles from the dispersion, it is preferred if the dispersion is stirred and / or recirculated in a receiver tank prior to application to the support. Furthermore, it is preferred for adjusting the viscosity of the dispersion if the feed tank, in which the dispersion is contained, can be tempered.

In a preferred embodiment, the carrier is designed as an endless belt transparent to the respective laser radiation, which is moved, for example, by means of internal transport rollers. Alternatively, it is also possible to carry out the carrier as a cylinder, wherein the cylinder can be moved via inner transport rollers or is driven directly. The coating of the carrier with the dispersion containing the electrically conductive particles is then carried out, for example, by a method known to the person skilled in the art, for example with a roller or a roller system from a storage container in which the dispersion is located. By rotation of the roller or the roller system, the dispersion is absorbed and applied to the carrier. By moving the carrier past the coating roller, a full-surface dispersion layer is applied to the carrier. In order to transfer the dispersion to the substrate, the laser beam source is arranged inside the endless belt or the cylinder. For transferring the dispersion, the laser beam is focused on the dispersion layer and strikes by the one for this transparent support to the dispersion and, at the point where it meets the dispersion, transfers the dispersion to the substrate. Such a commissioned work is described for example in DE-A 37 02 643. The dispersion is transferred, for example, by virtue of the fact that the dispersion is at least partially evaporated by the energy of the laser beam and the dispersion is transferred by the resulting gas bubble. The dispersion not transferred from the support to the substrate can be reused in a next coating step.

The layer thickness of the electrically conductive layer which is transferred to the substrate by means of the transfer by the laser preferably varies in the range between 0.01 and 50 μm, more preferably between 0.05 and 30 μm and particularly preferably between 0.1 and 20 microns. The electrically conductive layer can be applied both over the entire surface as well as structured.

A structured application of the dispersion to the carrier is advantageous if certain structures are to be produced in large quantities and the structured application reduces the amount of dispersion which has to be applied to the carrier. This makes it possible to achieve a more cost-effective production.

In order to obtain a mechanically stable, structured or full-surface electrically conductive layer on the substrate, it is preferred that the dispersion with which the structured or full-surface electrically conductive layer is applied to the substrate is physically dried or cured after application. Depending on the matrix material, the drying or curing takes place, for example, by the action of heat, light (UVA / is) and / or radiation, for example infrared radiation, electron radiation, gamma radiation, X-radiation, microwaves. To trigger the curing reaction, if necessary, a suitable activator must be added. Curing can also be achieved by combining various methods, for example by combining UV radiation and heat. The combination of the curing processes can be carried out simultaneously or sequentially. For example, by UV or IR radiation, the layer may initially only be cured or dried, so that the structures formed no longer flow apart. Thereafter, the layer can be further cured or dried by the action of heat.

When the substrate is heat-resistant, particularly when the substrate does not include a plastic film, it is preferable to fire the substrate having the electrically conductive layer coated thereon after drying and / or curing the dispersion transferred to the substrate to form the electrically conductive layer a consistently to produce electrically conductive surface on the substrate and to make contact with the active semiconductor layer of the substrate.

For firing, the substrate with the electrically conductive layer applied thereto is brought to a temperature in the range from 600 to 900 ^° C. for a period of generally 30 seconds to 20 minutes with a temperature profile adapted to the respective formulation and the substrate in a gradient oven. As a result, part of the metal of the electrically conductive layer begins to diffuse into the semiconductor material. The penetration depth of the metal into the substrate is adjusted by the temperature and the duration. The diffusion of the metal into the substrate results in a firm connection of substrate and electrically conductive layer.

For firing usually an infrared furnace is used. However, any other suitable furnace can be used with which the necessary temperatures for firing can be set. It is also possible to use both continuously operated furnaces, for example as a continuous furnace, or intermittently operated furnaces.

In one embodiment of the invention, at least one metal layer is deposited on the structured or full-surface electrically conductive layer by electroless and / or galvanic coating.

When the substrate is fired with the electrically conductive layer deposited thereon, the electroless and / or galvanic deposition of the metal layer can be done both before firing and after firing.

The coating can be carried out by any method known to those skilled in the art. In this case, the composition of the electrolyte solution used for the coating depends on which metal the electrically conductive layer is to be coated on the substrate. Typical metals which are deposited on the electrically conductive layer by electroless and / or galvanic coating are, for example, silver, gold, nickel, palladium, platinum or copper. The layer thicknesses of the one or more deposited layers are in conventional, known in the art areas.

Suitable electrolyte solutions which can be used for coating electrically conductive structures are known to the person skilled in the art.

If the electrically conductive particles consist of materials which oxidize easily, it may additionally be necessary to at least partially protect the oxide layer beforehand. to remove it. Depending on the implementation of the method, for example, when using acidic electrolyte solutions, the removal of the oxide layer can already take place simultaneously with the onset of metallization, without an additional process step is required.

If the electrically conductive particles contain a material which can easily oxidize, in a preferred variant of the method, before forming the metal layer on the structured or full-surface electrically conductive layer, the oxide layer is at least partially removed. The removal of the oxide layer can take place, for example, with acids, such as concentrated or dilute sulfuric acid or concentrated or dilute hydrochloric acid, nitric acid, citric acid, phosphoric acid, amidosulfonic acid, formic acid or acetic acid.

After the galvanic coating, the substrate can be processed further in accordance with all steps known to the person skilled in the art. For example, existing electrolyte residues can be removed from the substrate by rinsing and / or the substrate can be dried.

In an alternative embodiment, at least one metal layer is first deposited on the dried and / or hardened electrically conductive layer by electroless and / or galvanic coating, and subsequently the composite, comprising the substrate with an electrically conductive layer formed thereon, onto which a further metal layer is fired, fired.

The process according to the invention for producing electrically conductive layers on a substrate can be operated in a continuous, partially continuous or batchwise manner. It is also possible that only individual steps of the process are carried out continuously while other steps are carried out discontinuously.

In addition to the production of an electrically conductive layer, it is also possible with the method according to the invention to successively transfer a plurality of layers to the substrate. For example, after performing the method for producing the first conductive layer, at least one further structured or full-surface electrically conductive layer can be applied by a printing method as described above. The at least one further electrically conductive layer may, for example, have a different composition of electrically conductive particles. In this case, it is possible, for example, for the proportion of electrically conductive particles in the dispersion to be greater, for electrically conductive particles made of a different material or electrically conductive particles of the same materials to be used in a different manner. their mixing ratio or with other particle geometry are used for the further electrically conductive layer.

In addition to the one-sided generation of an electrically conductive layer on the substrate, it is also possible to apply the dispersion both on the top and on the underside of the substrate to form the electrically conductive layer. In this case, electrically conductive layers are produced both for the front-side contact and for the back-side contact of solar cells.

An embodiment of the invention is shown in the single drawing and will be explained in more detail in the following description.

The single figure shows schematically a device for carrying out the method according to the invention.

For the production of electrodes for solar cells, a substrate 1 with a transport device 3 shown only schematically here is supplied to a coating device 5. As transport device 3, any, known in the art transport device is suitable. For example, the transport device 3 may comprise a belt on which the substrate 1 is positioned and which is guided around rollers to move the substrate 1. Alternatively, it is also possible, for example, to use feeders with which the substrate 1 is positioned in the coating device 5. Any other suitable transport device known to a person skilled in the art can also be used.

The coating device 5 comprises a carrier 7, which is coated with a dispersion. In order to coat the substrate 1, the carrier 7 coated with the dispersion is irradiated with a laser 9. As a result, the dispersion dissolves from the carrier 7 and is transferred to the substrate 1. This is done, for example, by evaporating a small amount of the solvent contained in the dispersion and generating a shockwave in the dispersion, which subsequently produces a droplet which separates from the carrier.

The dispersion transferred to the substrate 1 contains electrically conductive particles. In this way, an electrically conductive layer is produced on the substrate 1. In addition to the electrically conductive particles, binders also contained in the dispersion can be transferred to the substrate 1. It forms a layer on the substrate 1, which contains both particles and binder. The transfer of the dispersion from the carrier 7 to the substrate 1 takes place, for example, in the form of droplets 11. A structured coating can be produced on the substrate 1 by using, for example, a mask. Preferably, it is also possible to achieve the structuring by shifting the laser, wherein the laser is switched on and off simultaneously depending on the structure. This can be done, for example, by an acousto-optical modulator or pulsing the laser. Depending on the diameter of the laser beam incident on the carrier 7, it is also possible to produce very fine structures with dimensions of less than 120 μm. The layer thickness is preferably in the range between 0.01 and 50 microns.

In the embodiment shown in Figure 1, the carrier 7 is guided over inner rollers 13. The movement of the carrier 7 is shown by an arrow 15.

Since, after the application of the coating to the substrate 1, the dispersion is no longer completely applied to the carrier 7, it is necessary to re-coat the carrier 7 after application of the coating to the substrate 1 with the dispersion. For this purpose, a feed tank 17 is provided which contains the dispersion. In the embodiment shown here, a roller 19 is immersed in the reservoir 17. With an applicator roll 21, the dispersion is applied to the carrier 7. In order to remove the unconsumed dispersion on the carrier during the new coating, it is necessary for the application roller 21 to move in opposite directions to the carrier 7. The application roller 21 may, for example, have a structuring, so that the dispersion is applied in a structured manner to the carrier 7. In this case, there is likewise a structured application to the substrate 1. In general, however, the dispersion is applied over the entire surface to the carrier 7.

As an alternative to the embodiment shown here, in which the dispersion is applied to the carrier 7 by means of a roller application method, it is also possible to use any other application method, for example screen printing, gravure printing, inkjet printing or flexographic printing.

After transferring the dispersion onto the substrate 1 with the aid of the laser 9, the coating thus produced is dried or cured. After curing, it is possible to electrolessly or galvanically coat the coating on the substrate 1. The further process steps are carried out in devices suitable for this purpose. For this purpose, the substrate 1 is moved, for example, with the transport device 3 in a further treatment unit. This is shown by an arrow 23. LIST OF REFERENCE NUMBERS

1 substrate

3 transport device

5 coating device

7 carriers

9 lasers

1 1 droplets

13 roll

15 movement of the carrier 17 storage container

19 roller

21 applicator roll

23 Transport of the substrate

Claims

1Patentansprüche

1. A process for the production of electrodes for solar cells, wherein the electrode is formed as an electrically conductive layer on a substrate (1) for solar cells, comprising the following steps:

a) transferring an electrically conductive particle-containing dispersion from a support (7) onto the substrate (1) by irradiation of the dispersion with a laser (9),

b) drying and / or curing of the dispersion transferred to the substrate (1) to form the electrically conductive layer.

2. The method according to claim 1, characterized in that before the transfer in step a), the dispersion is applied to the carrier (7).

3. The method according to claim 2, characterized in that the application of the dispersion to the carrier (7) by a coating method, in particular by a printing, casting, rolling or spraying, takes place.

4. The method according to any one of claims 1 to 3, characterized in that the dispersion is stirred in a storage container (17) prior to application to the carrier and / or recirculated and / or tempered.

5. The method according to any one of claims 1 to 4, characterized in that the laser (9) is a solid-state laser, a fiber laser, a diode laser, a gas laser o- an excimer laser.

6. The method according to any one of claims 1 to 5, characterized in that the laser (9) comprises a laser beam having a wavelength in the range of 150 to

10600 nm generated.

7. The method according to any one of claims 1 to 6, characterized in that the electrically conductive particles contain at least one metal and / or carbon.

8. The method according to claim 7, characterized in that the electrically conductive particles contain a metal which is selected from the group consisting of aluminum, silver, copper, nickel, titanium, platinum and palladium. 2

9. The method according to any one of claims 1 to 8, characterized in that the electrically conductive particles have different particle geometries.

10. The method according to any one of claims 1 to 9, characterized in that the dispersion contains an absorbent.

1 1. A method according to claim 10, characterized in that the absorbent is carbon or Lanthanhexaborid.

12. The method according to any one of claims 1 to 1 1, characterized in that the dispersion contains a glass frit.

13. The method according to any one of claims 1 to 12, characterized in that the electrically conductive layer is electroless and / or electroplated after drying and / or curing.

14. The method according to any one of claims 1 to 13, characterized in that the electrically conductive layer is fired.

15. The method according to claim 13 or 14, characterized in that before the electroless and / or galvanic coating of the electrically conductive layer, an optionally present oxide layer is removed from the electrically conductive particles.

16. The method according to any one of claims 1 to 15, characterized in that the dispersion is applied to the top and bottom of the substrate to form the electrically conductive layer.

17. The method according to any one of claims 1 to 16, characterized in that the carrier is a permeable for the laser radiation used rigid or flexible plastic or glass.