DE102010044132A1 - Method for applying structure in surface of silicon wafer, involves applying mask base material to surface of solar cell, coating selected pattern with plunger, and contacting liquid etching medium with etching mask - Google Patents

Abstract

The method involves applying a mask base material to a surface of a solar cell e.g. silicon wafer. A pattern selected according to a desired structure is impregnated (3) on the base material using a plunger. The base material is cured so that an etching mask is obtained. A liquid etching medium i.e. aqueous solution, is brought into contact with the etching mask to obtain the solar cell with a structured surface, where the mask does not have holes in a region of the surface to be structured. The base material is comprised of a network former i.e. sol-gel precursor. An independent claim is also included for a solar cell comprising a structured surface.

Description

This invention relates to an etching process for the targeted introduction of structures in surfaces of solar cells. A preferred embodiment relates to the surface structuring of silicon substrates.

Surface structuring finds application on a wide variety of substrates in numerous fields. Particularly in the field of photovoltaics, it is necessary that the surface structuring, for example of solar cell surfaces, meet high requirements.

Particular attention is paid to the surface structuring of solar cells based on silicon wafers, since the introduction of light-capturing structures into the surfaces of these silicon substrates can significantly increase the efficiency of such solar cells based on silicon wafers and thus of the corresponding modules. In solar modules, solar cells with mono-, multi- and polycrystalline silicon can be used. While it is comparatively easily possible with monocrystalline silicon to etch a uniform light-trapping pyramid structure into the surface by means of a wet-chemical etching process, which can also consist of several steps, this is not possible with multicrystalline or polycrystalline silicon, referred to below as multi-crystalline just like that. This is because, unlike monocrystalline material, multicrystalline silicon consists of several crystalline grains, which typically have different crystal orientations. These different crystal orientations and thus the corresponding crystal planes have different etch rates; For example, the etching rate of the (100) planes is significantly higher than that of the (111) planes. Consequently, in polycrystalline silicon without further action during the etching process, no homogeneous structures are produced, but rather irregular ones. With a uniform and homogeneous structuring of the silicon surface of multicrystalline material, an optimal light trapping structure can be generated for the later solar cell, and thus the solar module. This is reflected in an increased short-circuit current of the solar cell (and thus of the solar module), which is accompanied by an increase in efficiency.

In contrast to monocrystalline material, in the case of multicrystalline silicon further precautions must be taken in order to allow targeted structuring in the etching process. Otherwise, a statistical structuring of the surface is carried out, which with respect to the increase in efficiency has a lower potential than a targeted Lichteinfangstruktur.

In DE 3102712 A1 describes an etching process for introducing surface structures in solar cells. However, the surface to be structured is not provided with an etching mask. This results in the above-explained disadvantage of irregular structures in multicrystalline silicon due to the different crystal orientations. In addition, the use of photoresist is described here. However, this has no masking function, but serves to protect the non-structured substrate side. In this case, the photoresist is applied over its entire area on the side of the solar cell which faces away from the light and protects this side from the etching medium.

US 7,196,018 82 describes an etching process for multicrystalline silicon, in which a so-called direct etching paste is applied to the surface to be structured. The paste is applied to the surface by a printing process. An etching mask is not used here. Although this method can be carried out inexpensively, the etching paste can not be applied in such a finely structured manner by means of a printing method described in this document that sufficiently fine surface structures can be achieved.

It is therefore the object of the present invention to provide a process which can be carried out cost-effectively, which makes it possible to introduce precisely the finest structures in solar cells and, in particular, in silicon substrates.

The object is solved by the subject matters of the claims.

• • ein Maskengrundstoff auf die Oberfläche aufgebracht wird,
• • mit einem Stempel ein gemäß der gewünschten Struktur ausgewähltes Muster in den Maskengrundstoff geprägt wird,
• • der Maskengrundstoff in wenigstens einem Härtungsschritt gehärtet wird, so dass eine Ätzmaske erhalten wird,
• • und ein flüssiges Ätzmedium mit der Ätzmaske in Kontakt gebracht wird, um die gewünschte Struktur in der Oberfläche der Solarzelle zu erzielen, dadurch gekennzeichnet, dass die Ätzmaske keine Löcher im Bereich der zu strukturierenden Oberfläche aufweist.

The object is achieved in particular by a method for introducing a structure into a surface of a solar cell, wherein

• A mask base is applied to the surface,
• Stamping a pattern selected according to the desired structure into the mask base with a stamp,
• The mask base material is cured in at least one curing step, so that an etching mask is obtained,
• • and a liquid etching medium is brought into contact with the etching mask in order to achieve the desired structure in the surface of the solar cell, characterized in that the etching mask has no holes in the region of the surface to be structured.

The process steps can be carried out in any order which is obviously meaningful to the person skilled in the art.

That the etching mask has no holes in the area of the surface to be structured, means that the mask covers the entire surface to be structured and in particular has no macroscopic holes; In other words, the etch mask has no visible recesses. In particular, it has no recesses which are so large that the liquid etching medium would be in contact with the surface immediately after its application. Accordingly, the etching mask may well have pores; but these are not continuous and / or so small that they do not allow the direct contact of the etching medium with the solar cell surface. Instead, the etching medium must either diffuse through the mask (diffusion etching mask) or remove enough material to reach the solar cell (material removal mask). This is an essential aspect of the invention, which makes it possible, even with an anisotropic etching, to obtain a targeted structuring of the surface. The etching masks in the prior art always have holes through which the etching medium immediately comes into contact with the solar cell, so that it is etched irregularly and with the corresponding disadvantages. The etching process which takes place according to the invention is thus preferably an anisotropic etching, wherein the etching masks according to the invention naturally also function in the case of isotropic etching.

The stamp imprints the desired pattern by direct physical contact with the mask base.

The order of the mask base material on the surface is preferably over the entire surface.

The fact that the etching medium is liquid has advantages. Liquid phase structuring is significantly cheaper than gas-based etching processes or processes that require lasers, since the structure is defined by the stamp, a great flexibility is possible here (10 nm-500 μm structures).

According to the invention, a structure is understood to mean any targeted arrangement of changes in the surface properties of the solar cell which can be achieved with an etching medium according to this invention. In particular, "structure" is understood to mean a pattern of depressions which are produced using the etching medium in the surface of the solar cell.

The solar cell may preferably be a silicon element, such as for a solar cell, but also another semiconductive material, such as gallium arsenide. The solar cell can be monocrystalline or multicrystalline. Preferred solar cells consist of multicrystalline silicon elements.

The properties of the etching masks can be adjusted via a suitable choice of the mask base material. The mask base comprises at least one structurable material, which preferably comprises nanoparticles that are inorganic and / or organic.

The mask base preferably comprises UV-curable additives which allow the mask base to be cured by UV radiation. This has the advantage that the mask base material can be cured during the embossing step. Furthermore, the mask base material may be so pronounced that thixotropic embossing is possible.

The mask base further preferably comprises a sol-gel precursor. This sol-gel precursor preferably comprises a hybrid polymer material.

The mask base preferably comprises a base material capable of crosslinking to a network, and more preferably nanoparticles. The mask base preferably comprises 40 to 100% by weight of base material, more preferably 50 to 100% by weight, particularly preferably 60 to 90% by weight. Preferably, the mask base contains 0 to 60% by weight of nanoparticles, more preferably 0 to 50 Wt .-% and particularly preferably 10 to 40 wt .-%. In particularly preferred embodiments, the mask base consists of base material and nanoparticles in the proportions by weight described above.

The base material includes network builders, i. H. Molecules that can network with each other to form a network. The network can be linear or branched and have an oligomeric or polymeric structure; the network may be of organic or inorganic nature or a mixed form; the molecules themselves may be oligomers or polymers bearing organic and / or inorganic functional groups. Such network-forming constituents can consist of molecular, oligomeric and / or polymeric precursors which contain organically crosslinking groups and of organometallic and / or oxidic molecular or polymeric constituents, which may preferably undergo inorganic hydrolysis, crosslinking reactions and / or condensation reactions, and of polysiloxanes , Preferably, the base material consists of one or more of the aforementioned substances.

Preferably, the mask base may contain organic and / or inorganic fillers. In addition, the mask base may contain organic auxiliaries, such as leveling agents, crosslinkers and hardeners.

The nanoparticles are organic or inorganic; they may be in oxidic and / or non-oxidic form.

The choice of nanoparticles can functionalize the mask base material in such a way that, for example, organic or oxidic inorganic nanoparticles increase the resistance of the etching masks to acidic media.

The mask base material is a composition-preferably liquid, in particular molecular or colloid-disperse-composition which, after carrying out the process according to the invention, is cured by means of a reaction typical of sol-gel materials in order to form the etching mask. Optionally, the cure may be via a thermally-induced or UV-light-induced crosslinking reaction.

For the purposes of the invention, a hybrid-polymeric material is preferably also understood to mean a substance whose organic constituents have at least partially decomposed due to a thermal curing process.

In one embodiment, the etch mask comprises a condensate of one or more hydrolyzable and condensable silanes and / or metal alkoxides, preferably of Si, Ti, Zr, Al, Nb, Hf, B, and / or Ge. The corresponding condensable constituents in the base material of the mask base may preferably be selected from the group consisting of the acrylsilanes, epoxysilanes, acrylalkoxysilanes, acrylepoxysilanes, epoxyalkoxysilanes, allysilanes, vinylsilanes, fluoroalkylsilanes, aminosilanes, alkoxysilanes, metal alcoholates, metal oxide acrylates, metal oxide methacrylates, metal oxide acetylacetonates or mixtures thereof.

Preferred network former in the base material are: methacryloxypropylsilane, Glycidylpropylsilan, Zirkonsecundärbutylatacrylat, Titanethylatacrylat, Titanpropylatacrylat, Zirkonsecundärbuthylatmethacrylat, Titanethylatmethacrylat, Titanpropylatmethacrylat, tetraethoxysilane, tetramethoxysilane, methyltriethoxysilane, methyltrimethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, mercaptopropyltrimethoxysilane, aminopropylsilane, vinyltriethoxysilane, allyltriethoxysilane, phenyltriethoxysilane, triethoxysilylpropylsuccinic anhydride and / or Fluoroctylsilan,

The mask base preferably comprises base material having an inorganic degree of condensation of the hydrolyzate in the etching mask of greater than or equal to 50%, preferably greater than 70%, after carrying out the process of this invention.

The groups which crosslink in the sol-gel process, ie via inorganic hydrolysis or condensation (as constituent of the base material), may be the following functional groups:
TiR ₃ (X), TiR ₂ (X) ₂ , ZrR ₂ (X) ₂ , ZrR ₃ (X), SiR ₃ (X), SiR ₂ (X) ₂ , TiR (X) ₃ , ZrR (X) ₃ , AlR ₂ (X), AlR ₁ (X) ₂ SiR (X) ₃ and / or Si ₂ (X) ₆ , TiX ₄ , ZrX ₄ , SiX ₄ , AlX ₃ , TiR ₃ (OR), TiR ₂ (OR ) ₂ , ZrR ₂ (OR) ₂ , ZrR ₃ (OR), SiR ₃ (OR), SiR ₂ (OR) ₂ , TiR (OR) ₃ , ZrR (OR) ₃ , AlR ₂ (OR), AlR ₁ ( OR) ₂ Ti (OR) ₄ , Zr (OR) ₄ , Al (OR) ₃ , Si (OR) ₄ , SiR (OR) ₃ and / or Si ₂ (OR) ₆ , preferably OR = alkoxy, preferably methoxy , Ethoxy, n-propoxy, i-propoxy, butoxy, isopropoxyethoxy, methoxypropoxy, phenoxy, acetoxy, propionyloxy, ethanolamine, diethanolamine, triethanolamine, methacryloxypropyloxy glycidylpropyloxy, acrylate, methacrylate, acetylactone, ethyl acetate, ethoxyacetate, methoxyacetate, methoxyethoxyacetate or methoxyethoxyethoxyacetate or mixtures thereof , In particular embodiments, R is preferably methyl, ethyl, n-propyl, butyl, allyl, vinyl, aminopropyl and / or fluorotctyl. X is preferably Cl, Br, F or a mixture of these.

The mask base material is preferably applied at temperatures of 5 to 45 ° C, in particular 15 to 30 ° C. In preferred embodiments, the mask base is applied at room temperature.

The nanoparticles are preferably particles having particle sizes in the nanometer to micrometer range, which are suitable for controlling the resistance of the etching mask to the etching medium used and their permeability to the etching medium. These particles preferably have sizes of 0.5 nm to 10 μm, more preferably 2 nm to 150 nm, most preferably 4-40 nm.

Particle sizes in the range of <4 nm are determined by means of small angle X-ray analyzes, particle sizes between 4 nm and 20 μm by means of dynamic light scattering and particles> 20 μm by means of scanning electron microscopy. The particle size is defined as the average size of at least 20 particles.

The nanoparticles preferably consist essentially of carbides, nitrides, oxides, fluorides, oxynitrides and / or hydroxyfluorides. These anions are particularly preferably paired with cations of titanium, silicon, aluminum, calcium, yttrium, zirconium, magnesium, zinc, lanthanum, cerium, gadolinium, tin, boron, sodium, potassium and / or lithium. Specifically, these are titanium nitride (TiN), titanium oxynitride (TiON), silicon nitride (SiN), silicon carbide (SiC), aluminum nitride (AlN), titanium carbide (TiC), silicon oxynitride (SiON), titanium aluminum nitride (TiAlN), silicon dioxide (SiO ₂ ), magnesium fluoride (MgF ₂ ), magnesium hydroxyfluoride (MgOHF), calcium hydroxyfluoride (CaOHF), titanium dioxide (TiO ₂ ), boron oxide (B ₂ O ₃ ), aluminum oxide (Al ₂ O ₃ ), zirconium dioxide (ZrO ₂ ), Yttrium oxide (Y ₂ O ₃ ) and / or calcium fluoride (CaF ₂ ), cerium oxide (CeO ₂ ) and / or yttrium-stabilized zirconium oxide.

The nanoparticles preferably consist of silicon dioxide (SiO ₂ ), magnesium fluoride (MgF ₂ ), titanium dioxide (TiO ₂ ), aluminum oxide (Al ₂ O ₃ ), zirconium dioxide (ZrO ₂ ), yttrium oxide (Y ₂ O ₃ ), calcium fluoride (CaF ₂ ) or Mixtures thereof.

Preferably, the nanoparticles are reactively connected with silanol groups and / or other hydroxyl groups of metal oxides and / or their organometallic and / or hybrid polymer compounds, for example via condensation reaction.

In a preferred embodiment of the invention, these nanoparticles are reactively embedded in the network of the layer. This means that a chemical reaction of the preferred oxide nanoparticle surface and its hydroxyl groups with the displaceable functionalities of the base material has taken place.

These nanoparticles are preferably prepared by flame pyrolysis and / or precipitation reactions and / or syntheses under elevated pressure from the gas phase or liquid phases.

Particular preference is given to using the nanoparticles dispersed in nonaqueous, preferably alcoholic or nonpolar, solvents. For this purpose, the nanoparticles are preferably stabilized by means of surface-active reagents. For example, these may be: tetramethylammonium hydroxide, polyethylene, polylactic acid, polyamino acid, poly-caprolactone, paratoluene sulfonic acid, polyalkyl cyanoacrylate and / or polyethylene oxide block polyglutamic acid.

In a preferred development of the invention, a photoinitiator is admixed to the mask base material and hardening is carried out by means of electromagnetic radiation, in particular by means of UV light. Radical photoinitiators, such as, for example, 1-hydroxycydohexyl phenyl ketone and / or benzophenone, are preferably used as UV initiators for acrylate- or methacrylate-based mask base materials. For glycidyl-based sol-gel precursors, preference is given to cationic photoinitiators, for example from the group of iodonium salts, sulfonium salts and / or nonionic photoinitiators, for example diphenyliodonium nitrate, diphenyliodonium triflate, diphenyliodonium p-toluenesulfonate, N-hydroxynaphthalimide triflates, N-hydroxyphthalimide triflates, thiobis (cf. triphenyl sulfonium hexafluorophosphate) and / or (4-methylphenyl) [4- (2-methylpropyl) phenyl] (- 1) hexafluorophosphate.

Thus, the mask base can be stabilized in a very simple manner and, as provided in a preferred embodiment of the invention, can be thermally cured in subsequent steps.

In a particular embodiment of the invention, constituents of the base material of the mask may be polysiloxanes. For example, these may be methylpolymers and / or phenylpolysiloxanes which are, for example, hydroxyl, glycidyl and / or polyether-terminated.

Characteristic of a particular embodiment is that the mask base organic additives such as dipentaerythritol pentaacrylate, Hexandioldiarylat, trimethylolpropane triacrylate and / or succinic anhydride, are added as a curing agent.

For the preparation of mask base materials according to the invention, a thickener, for example polydispersed silica, cellulose and / or xanthan gum, can be used in the sol-gel precursor.

In a particular embodiment according to the invention, flow control auxiliaries, which can originate, for example, from the class of polyether-modified dimethylsiloxanes, are added to the sol-gel material.

The mask base is preferably applied to the surface by screen printing, pad printing, dip coating, roller coating, flooding, spraying or similar methods.

The stamp is preferably made of a polymeric material, such as silicone. The polymeric active ingredient is preferred because it is flexible and thus particularly suitable for solar cells of thinner 250 μm, such as silicon wafers, as well as translucent in a wavelength range of 200-400 nm, ie. H. in the UV range.

In special embodiments, however, the stamp can also consist of other materials which are not UV-transparent, such as, for example, metal foils or other materials. Then, the UV curing step is omitted and the embossing process is characterized in that it is either thixotropic or the structure is thermally cured. In the latter case, either a temperature-stable silicone is used as a stamp material or the stamp material chosen so that it burns in the thermal process with, so can not be used again.

The stamp is preferably produced starting from a master. The master already contains the structural information that will be found in the surface of the finished structured solar cell. The structure is preferably a regular, in particular periodic, structure in a preferred scale of 100 nm to 100 microns with an aspect ratio of 0.1 to 4, more preferably from 0.5 to 2. The structures preferably have a pattern of repeating structural elements on.

In particular embodiments, the structures may also be mutually rotated by 30 to 70 °, more preferably by 50 to 65 °, such as in a honeycomb structure.

Preferred structural elements are pyramids, inverted pyramids, cross lattices, moth eyes, honeycomb structures, because these have established themselves as light-capture structures. But there are also other structures conceivable.

However, in a particular embodiment, the structure may also not be periodically repeating or stochastic.

The master is molded with a commercially available, liquid or rigid polymer to produce a stamp. The impression can optionally take place under vacuum or ambient conditions. In addition, the stamp production at elevated temperature can take place in the range of 40 to 100 ° C, preferably 50 to 90 ° C, particularly preferably 60 to 80 ° C. It has been shown that for small structures in the range <400 nm, an elevated temperature in the production of stamps is advantageous, i. H. little to no impression defects are found in the stamp. The stamp can continuously in the embossing step, d. H. preferably with a rolling motion, or static, d. H. be applied flat, in order to achieve the best possible embossing result and to minimize air pockets.

In a particular embodiment, the stamp is made of a material which has a surface energy different from that of the layer to be embossed in order to minimize the adhesion between the stamp and the layer to be embossed.

In a further embodiment, the stamp may also be provided with a layer which has a surface energy different from that of the layer to be embossed. Such materials or coatings preferably contain fluorosilanes, but may also be other materials known to those skilled in the art.

Optionally, the stamp is also previously provided with a sol-gel layer to further increase the accuracy of the embossed image. This can be done for example by flooding, printing or similar methods.

Optionally, the embossing step takes place in a vacuum to prevent air bubbles in the etching mask. After removal of the stamp, an etching mask is preferably present in the form of a structured thin layer with an average layer thickness in the range from 0.05 to 10 μm. In particular embodiments, the etch mask has only a thickness of about 1 nm at certain locations. Naturally, these sites are sites with a short diffusion path, and the etching medium first reaches the solar cell at these locations.

Depending on the composition of the mask base, various types of etch masks can be obtained. Under the influence of the stamp, the layer of the masking base applied to the surface has recesses in which the etching mask has a smaller thickness than in the remaining areas. The etching mask obtained therefrom can be a material removal mask or a diffusion mask, depending on the type and amount of the nanoparticles and on the type and amount of the base material.

In the material removal mask ( ) occurs during the etching step, a uniform material removal, the mask is thinner. In the area of the depressions, no etching mask is present after a certain time, while the etching mask in the remaining areas further protects the solar cell against the attack of the etching medium. Consequently, a material removal is achieved on the solar cell only in the region of the wells. Such material removal masks are preferably to be used in isotropic etching media, wherein it must be ensured that the mask material etches more slowly than or as fast as the solar cell, ie the etching rate of the solar cell to be etched is higher than or equal to that of the mask material. In this case, the structures correspond to the stamp or have an elevation. In a particular embodiment, the mask material may also be designed in such a way that the etching rate is higher than that of the solar cell to be etched, so that partially highly flattened homogeneous structures can be realized.

The etching process can basically be carried out on both sides of the element to be etched or on one side. In a particular embodiment, the other side of the solar cell, which is not to be provided with a structure, is protected by a protective layer; such protective layers may consist of dense SiO ₂ . If the solar cell is a silicon element, this type of mask can be used with acidic etchants as they etch the silicon isotropically. This means that the (100) crystal plane has the same etch rate as the (111) crystal plane. In isotropic etching media, these masks produce hemispherical structures in the solar cell ("honeycomb structure"). According to one embodiment of this invention, the etching mask produced in the process is a material removal mask, which preferably consists of an organic polymer network and is preferably dense, ie has no microcracks. The etch mask preferably comprises pores to the extent of <20% by volume, more preferably <10% by volume, as determined by Baklanov ellipsometric porosimetry ( MR Baklanov, KP Mogilnikov, J. Vac. Sci. Technol. B 18, 3 (2000) 1385 ).

In the diffusion etching mask ( ) are also obtained under the influence of the punch depressions in the etching mask. Here, however, the etching mask is not attacked by the etching medium; the etching medium diffuses through the etching mask to the surface of the solar cell. For such an etching mask to be effective for multicrystalline material with an anisotropic etching solution, the diffusion rate of the etching medium through the mask must be smaller than the etching rate of the solar cell. Otherwise, irregular structures would be obtained in the surface. According to one embodiment of this invention, the etching mask produced in the process is a diffusion etching mask, which preferably consists of purely inorganic constituents and has pores or mesopores> 20% by volume, preferably> 50% by volume.

The etching medium serves to introduce the structure into the surface of the solar cell during the etching step. The etching medium is preferably liquid at room temperature, in particular an aqueous solution. Depending on the configuration of the solar cell and the method, a basic or acidic etching medium is selected.

Preferred basic etching media have a pK _B value of less than 3.25 and contain as bases sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide (TMAH) and / or ethylenediamine pyrocatechol (EDP). In this case, the concentration of the bases in the etching medium is preferably 5% to 100% (mass%), more preferably 10% to 50% (mass%).

Preferred acidic etching have a _pK a value of less than 3.25 and in a preferred embodiment as acids hydrofluoric acid (HF), sulfuric acid (H ₂ SO _4), nitric acid (HNO _3), hydrochloric acid (HCl), phosphoric acid (H ₃ PO ₄ ) and / or ammonium fluoride (NH ₄ F).

In this case, the concentration of the acids in the etching medium is preferably 30% by weight to 100% by weight.

The acidic etching media may preferably comprise additions of organic acids. These are preferably formic acid (HCOOH) and / or acetic acid (H ₃ CCOOH). The organic acids are preferably used in concentrations of 10 to 40%.

In another embodiment, buffered solutions may also be used.

The etching medium is brought into contact with the etching mask in an etching bath (etching step). The etching bath is preferably at 20 to 100 ° C, more preferably 50 to 90 ° C, tempered. The etching medium is preferably in contact with the etching mask for a period of 0.1 to 10 minutes, more preferably 1 to 5 minutes.

The method according to the invention preferably comprises a cleaning step after the etching step. The solar cell is washed with a cleaning medium. The cleaning medium is preferably an aqueous solution, in particular hydrofluoric acid. The cleaning step preferably takes about 10 seconds to 10 minutes.

In at least one curing step, the mask base is cured after it has been applied to the surface. During curing, the sol-gel conversion takes place. The sol-gel precursors crosslink to polymeric compounds, so that a resistant etching mask is obtained. This hardening step may be done before, during or after embossing by the stamp. Preferably, however, the curing step occurs during embossing. During curing, the mask base is cured thermally or photochemically, the photochemical curing is preferred. In photochemical curing, a stamp is preferably used which is permeable to the wavelength at which the curing process is initiated. It is preferred to cure with light in the UV wavelength range. In cases where photochemical curing is intended, the mask base preferably comprises a photoinitiator. In preferred embodiments, the mask base contains UV or thermally crosslinking organic monomers from the classes of acrylates, methacrylates and / or epoxides as the base material. The mass fraction of the organically crosslinkable polymers and / or hybrid polymer monomers is 10 to 100%, preferably 40 to 90%, very particularly preferably 50 to 70%, based on the starting monomers of the base material. Preference is given to a combination of several curing steps, in particular first a photochemical curing step and, after removal of the stamp, a subsequent thermal curing step.

In cases in which no photochemical curing is provided, the stamp can be removed either directly (so-called thixotropic embossing) after stamping or after a thermal curing step with stamp and the structure is transferred into the layer to be embossed.

According to the invention, the embossing punch is preferably pressed at room temperature and preferably atmospheric pressure.

If etching masks for alkaline media are to be obtained, the mask base preferably contains a high proportion of inorganic constituents, in particular an inorganic material content of between 10 and 50% (mass percent). Materials of ZrO ₂ and TiO ₂ are preferably used here as nanoparticles. The mass fraction of SiO ₂ of the inorganic material is preferably from 1 to 100%. In a preferred embodiment, the mask base contains from 0 to 30% (by weight) of non-organically crosslinkable organic components such as methyl groups and phenyl groups.

Mask stocks for acidic etchants preferably comprise 50 to 100% (mass percent) of organic components. Therefore, these masking bases are preferably cured photochemically or thermally to 300 ° C, especially at temperatures below 200 ° C. Preferably, these mask base materials contain as organic materials acrylates, methacrylates, vinyl derivatives and / or epoxides. In this case, via the ratio of the organic constituents to the inorganic, in particular hybrid polymer constituents, the etch barrier property of the etching masks, for example relative to hydrofluoric acid, can be set in a targeted manner.

In preferred embodiments, there is already a step of precuring the mask base on the surface prior to the embossing step. This precuring takes place thermally or photochemically, the photochemical precuring being particularly preferred.

Depending on the embodiment, the stamp is lifted off the masked layer before or after the curing step. In any case, the stamp is lifted before the etching step.

The method according to the invention preferably comprises a further step of the post-curing of the etching mask. The post-curing is preferably carried out at temperatures of 100 to 800 ° C, more preferably 300 to 740 ° C, in particular after the stamp has been lifted from the masked layer.

This invention further relates to a solar cell, in particular a silicon solar cell, which has been provided with a surface structure by a method according to this invention and an etching mask prepared according to this method. Moreover, this invention relates to the use of sol-gel precursors (base material) for producing an etching mask, in particular an etching mask as obtained in the process of the present invention.

Examples

Preparation of an Etching Mask, Example 1

In a vessel Methacryloxypropyltriethoxysilane (MPTES), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS) were submitted (base material). In this embodiment, about 0.6 moles of MPTES, about 0.2 moles of TEOS, and about 0.2 moles of MTEOS were used.

To this mixture was then added slowly with cooling and stirring 2.6 mol of distilled water added with 0.02 mol of para-toluenesulfonic acid. After stirring for 5 minutes, 500 g of a dispersion of 20% by mass of anatase nanoparticles having a crystallite size of 10 to 15 nm in n-butanol were added.

This solution was combined with a solution of titanium propylate and methacrylic acid. Here, 0.75 mol of MPTES, 0.2 mol of TEOS and 0.2 mol of MTEOS and a solution of 0.3 mol of titanium propylate and 0.3 mol of methacrylic acid were used.

After completion of the hydrolysis, which took about a period of about 24 hours, the volatile solvent was removed on a rotary evaporator at 120 mbar and 40 ° C. Subsequently, the solution was diluted to 20 mass% in terms of inorganic solid content by means of ethylene glycol monoethyl ether, and a photoinitiator was added to the composition. As photoinitiator, 2% (mass percentage) were based on the viscous hybrid polymer without solvent, of the photoinitiator 1-hydroxycyclohexyl phenyl ketone, which is available under the trade name Irgacure 184 ^® was added.

After making a film on polycrystalline Si wafer by screen printing using a 180 mesh and drying the solvent, a polymeric, silicone-type die was pressed into the low viscosity plastic gel film. The die is made of a material which is permeable in a wavelength range of> 230 nm, namely silicone, so-called Sil gels or rubber. As a structure of the stamper, a sinusoidal cross lattice having a period of 3 μm and a pattern depth of 4 μm was used. While the embossing stamp was in contact with the layer material, a first curing of the layer was carried out by means of a UV lamp which emits in the wavelength range of about 250 nm.

After removal of the embossing stamp, a further UV-based layer hardening and a thermal layer hardening took place at 450 ° C.

The mean layer thickness of the nanoparticle-functionalized layer was between 1 and 4 μm. The layer material had a refractive index of about 1.9.

Preparation of an Etch Mask, Example 2

In a vessel Glycidylpropyltriethoxysilane (GPTES), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS) were submitted (base material). About 0.6 moles of GPTES, 0.2 moles of TEOS and 0.2 moles of MTEOS were used. This solution was combined with a solution of aluminum secondary butylate and ethyl acetate, 0.1 mol each. An acidic dispersion of an aqueous nanoparticulate TiO ₂ dispersion, mixed with methanol and paratoluene sulfonic acid, was then added slowly to this solution while cooling and stirring. About 28 g of a TiO ₂ dispersion with 18 mass% anatase and a crystallite size of 7 to 12 nm, with about 60 g of methanol and 3.44 g of paratoluene sulfonic acid were added. After stirring for 5 minutes, 660 g of a dispersion of 20% by mass of anatase nanoparticles having a crystallite size of 10 to 15 nm in n-butanol were added.

After completion of the hydrolysis, which took a period of about 24 hours, the volatile solvent was removed at 120 mbar and 40 ° C bath temperature (ethanol) on a rotary evaporator. A photoinitiator is added to the resulting hybrid polymer sol having reactively embedded finely dispersed nanoparticles. As photoinitiator, 2% (mass percentage) were based on the viscous hybrid polymer of the cationic photoinitiator (4-methylphenyl) [4- (2-methylpropyl) phenyl] (- 1) hexafluorophosphate (Irgagure 250 ^®) were added. A one-sided coating was then performed by spin coating. After the solvent had dried, a polymeric, silicone-type embossing punch was pressed into the low-viscosity plastic gel film. The stamp is made of a material which is permeable in a wavelength range> 230 nm, namely silicone, so-called Sil gels or rubber. As the structure of the embossing die, a lattice structure consisting of a cross lattice having a period of 2 μm and a pattern depth of 3 μm was provided. While the stamper was in contact with the mask base material, a first curing of the layer was performed by means of a UV lamp emitting in the wavelength range of about 250 nm.

The alkaline etching was carried out with 10 molar NaOH at a bath temperature of 65 ° C. The etching time was 3 minutes. After the alkaline etching, residual constituents of the etching mask were removed by means of 1 molar HF.

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The drawings show exemplary, preferred embodiments of the present invention, they do not limit the scope of the invention.

shows a flow chart of the method according to the invention, wherein an embossing stamp is produced on the basis of a master structure ( 1 ) and a solar cell is provided with a layer of the mask base material ( 2 ). The mask base is then embossed with the die ( 3 ) by the stamp is pressed onto the mask base material and initially there remains. Optionally, the mask base material is photochemically activated before the embossing step ( 9 ). Also optional is the embossing step ( 3 ) a photochemical or thermal precuring step ( 10 ). It follows the removal of the embossing stamp from the layer of mask base ( 4 ), to which the curing step ( 5 ). This is followed by a wet-chemical etching step that is acidic ( 8th ) or alkaline ( 6 ) can be configured. Subsequently, the surface of the solar cell is cleaned ( 7 ).

illustrates an exemplary process flow, wherein
( 21 ) a mask base ( 12 ) on a solar cell ( 11 ) is applied,
( 22 ) under the influence of pressure ( 17 ) an embossing stamp ( 13 ) shapes the mask base,
( 23 ) which then under light or heat ( 16 ),
( 24 ) whereupon the embossing die is lifted off and a further hardening takes place under the effect of heat, so that
( 25 ) an etching mask ( 15 ), which under the action of an etching medium ( 18 )
( 26 ) makes the solar cell with structured surface available.

shows the principle of a Materialabtragsmaske invention. It is a solar cell ( 11 ) with an etching mask ( 15 ). Under the influence of an etching medium ( 18 ) the etching mask is removed. Where the etch mask is thinnest, the etch media first reaches the solar cell and selectively etches it at that location.

shows the principle of a diffusion etching mask according to the invention. It is a solar cell ( 11 ) and an etching mask ( 15 ), wherein the etching mask under the action of the etching medium ( 18 ) is not removed. The etching mask serves insofar as a barrier for the etching medium as this still through the etch mask must diffuse through before the solar cell is reached. At the thinnest points of the etching mask, the etching medium first reaches the solar cell and etches it at these locations.

LIST OF REFERENCE NUMBERS

1

Production of the embossing stamp of master structure

2

Coating of the solar cell with mask base material

3

Embossing of the mask base material by means of an embossing stamp

4

Remove the embossing stamp from the layer

5

curing

6

wet-chemical etching step, alkaline

7

Cleaning the surface, sour

8th

wet-chemical etching step, acidic

9

photochemical activation of the mask base material

10

photochemical or thermal precure step

11

solar cell

12

Mask base

13

dies

14

Solar cell with structured surface

15

etching mask

16

Light or heat

17

the effect of pressure

18

Action of the etching medium

21

Apply the mask base to the surface

22

Embossing a pattern in the mask base

23

Hardening of the mask base material

24

Lifting the embossing stamp

25

Post curing of the etching mask

26

Etching the solar cell

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 3102712 A1 [0005]
• US 719601882 [0006]

Cited non-patent literature

• MR Baklanov, KP Mogilnikov, J. Vac. Sci. Technol. B 18, 3 (2000) 1385 [0060]

Claims (12)

Method for introducing a structure into a surface of a solar cell ( 11 ), where • a mask base material ( 12 ) is applied to the surface, • with a stamp ( 13 ) a pattern selected according to the desired structure in the mask base material ( 12 ), • the mask base material ( 12 ) is hardened in at least one curing step, so that an etching mask ( 15 ) and a liquid etching medium with the etching mask ( 15 ) is brought into contact with a solar cell with a structured surface ( 14 ), characterized in that the etching mask has no holes in the region of the surface to be structured. Method according to claim 1, characterized in that the mask base material ( 12 ) Comprises nanoparticles. Method according to claim 1 or 2, characterized in that the mask base material ( 12 ) contains a base material comprising one or more network formers. A method according to claim 2 or 3, characterized in that the nanoparticles have average diameters of 0.5 nm to 10 microns, preferably 2 nm to 150 nm. Method according to one or more of claims 2 to 4, characterized in that the nanoparticles are selected from carbides, nitrides, oxides, fluorides, oxynitrides and hydroxy fluorides. Method according to one or more of the preceding claims, wherein the mask base material ( 12 ) Comprises nanoparticles in a proportion of 0 to 60% by weight. Method according to one or more of the preceding claims 3 to 6, wherein a network former is a sol-gel precursor. Method according to one or more of claims 3 to 7, wherein the mask base material ( 12 ) comprises the base material in a proportion of 40 to 100% by weight. Method according to one or more of the preceding claims, characterized in that the etching medium is an aqueous solution. Method according to one or more of the preceding claims, characterized in that the solar cell is a silicon wafer. Method according to one or more of claims 3 to 10, wherein a network former is selected from acryl silanes, epoxysilanes, Acrylalkoxysilanen, Acrylepoxysilanen, Epoxyalkoxysilanen, allylsilanes, vinylsilanes, fluoroalkylsilanes, aminosilanes, alkoxysilanes, metal alkoxides, Metalloxidacrylaten, Metalloxidmethacrylaten and / or Metalloxidacetylacetonaten. Solar cell with structured surface ( 14 ) obtainable by a process according to one or more of the preceding claims.

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