EP2643855A1 - Method for forming a fibrous layer - Google Patents

Method for forming a fibrous layer

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Publication number
EP2643855A1
EP2643855A1 EP11794862.0A EP11794862A EP2643855A1 EP 2643855 A1 EP2643855 A1 EP 2643855A1 EP 11794862 A EP11794862 A EP 11794862A EP 2643855 A1 EP2643855 A1 EP 2643855A1
Authority
EP
European Patent Office
Prior art keywords
mixture
silicon
silicon substrate
substrate
fibrous layer
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP11794862.0A
Other languages
German (de)
French (fr)
Inventor
Jean-Paul Garandet
Armand Bettinelli
Béatrice Drevet
Etienne Pihan
Philippe Thony
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Original Assignee
Commissariat a lEnergie Atomique CEA
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Commissariat a lEnergie Atomique CEA, Commissariat a lEnergie Atomique et aux Energies Alternatives CEA filed Critical Commissariat a lEnergie Atomique CEA
Publication of EP2643855A1 publication Critical patent/EP2643855A1/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0236—Special surface textures
    • H01L31/02363—Special surface textures of the semiconductor body itself, e.g. textured active layers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022408—Electrodes for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/022425—Electrodes for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/547—Monocrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a method for forming a surface of a surface of a silicon substrate, a fibrous layer having an average network pitch of less than or equal to 2 ⁇ m. This method is particularly advantageous in the context of the development of photovoltaic cells, to form in their rear face, a layer of fibrous structure capable of providing diffraction of infrared photons.
  • Photovoltaic cells are essentially made from mono- or poly-crystalline silicon.
  • these standard silicon-based industrial cells have a rear-facing electric field, also called BSF ("Back Surface Field") obtained by an aluminum-silicon (Al-Si) eutectic alloy formed by annealing of a aluminum layer deposited by screen printing on a silicon substrate. This annealing of the contacts on the rear face is carried out according to a standard technology in a passage oven.
  • BSF Back Surface Field
  • such annealing requires the assembly to a temperature of about 800 ° C for a few seconds, to form a liquid alloy between silicon and aluminum.
  • the first stages of solidification of this liquid alloy lead to the deposition of a single-phase layer of Al-saturated Si of a few microns, which forms the rear field (BSF) of the photovoltaic cells.
  • the eutectic temperature of the Al-Si system (577 ° C) once reached, the solidification becomes biphasic and leads to a structure formed of silicon lamellae in an aluminum matrix.
  • such a structure which generally has interlamellar spacings of the order of 10 to 20 microns unfortunately has a major topological disorder.
  • the structures present on the rear face of a photovoltaic cell produced from this standard method do not allow diffraction of the infrared photons not absorbed by the silicon of the cell, ie photons of wavelength less than 1 , 1 micron corresponding to the forbidden band of silicon, and which would therefore be likely to generate charge carriers.
  • the techniques of microelectronics can achieve by etching of organized reliefs, networks having a good regularity and an average pitch suitable for the diffraction of infrared photons.
  • the spacings of the resulting fibrous eutectic structure are greater than 2 microns, and are therefore not suitable for diffraction of infrared photons.
  • Fast quenching techniques can also provide structures with a reduced network pitch.
  • quenching techniques induce high levels of stress.
  • the structures obtained after quenching prove, moreover, fragile and not very manipulable, and therefore do not allow to continue the subsequent steps essential for the development of photovoltaic cells. Therefore, there remains the need to be able to achieve a significantly reduced average network pitch structure and in particular, advantageously less than or equal to 2 microns, capable of diffracting infrared photons not absorbed by silicon, by an otherwise compatible with the standard technology for developing photovoltaic cells, in particular compatible with annealing of the contacts in a passage oven.
  • the present invention aims precisely to provide a method that satisfies the aforementioned requirements.
  • the present invention relates, according to a first of its aspects, to a method of forming, on the surface of a face of a silicon substrate, a fibrous layer (22) having a mean network pitch of less than or equal to at 2 ⁇ m, comprising at least the steps of:
  • step (2) exposing at least the coated side of said substrate of step (1) to a heat treatment conducive to (a) forming a molten alloy comprising silicon, aluminum and said modifying elements, and (b) ) the subsequent solidification of said molten alloy under conditions conducive to the formation of at least one layer (22) having a two-phase eutectic structure made of silicon-based fibers in an aluminum-based matrix, with an average network pitch less than or equal to 2 ⁇ m, characterized in that said mixture of step (1) further comprises from 20 to 60% by weight, relative to its total weight, of one or more addition elements chosen from gallium, indium, tin, zinc and their mixtures.
  • a fibrous layer having an average network pitch of less than or equal to 2 ⁇ m by implementing a liquid alloy comprising, in addition to silicon, aluminum and one or more modifying elements, a significant amount of one or more metal elements selected from gallium (Ga), iridium (In), tin (Sn) and zinc (Zn).
  • a liquid alloy comprising, in addition to silicon, aluminum and one or more modifying elements, a significant amount of one or more metal elements selected from gallium (Ga), iridium (In), tin (Sn) and zinc (Zn).
  • a layer of fibrous structure having a mean network pitch of less than or equal to 2 ⁇ m, particularly suitable for the diffraction of infrared photons, especially of wavelength less than 1.1 ⁇ m. corresponding to the forbidden band of silicon.
  • a fibrous structure at the rear of a cell thus allows the "collection" of infrared photons by diffraction, and the improvement of the efficiency of the photovoltaic cell.
  • step (2) of the process according to the invention can be carried out with the industrial techniques usually employed for the production of photovoltaic cells, more precisely by the standard technology for cooking in a passage oven.
  • the process of the invention does not require significant modifications of the usual process for producing photovoltaic cells. More particularly, as developed subsequently, it is possible according to the method of the invention, to form in a single step, the rear surface field (BSF) and the diffractive fibrous layer.
  • BSF rear surface field
  • Figure 1 shows a schematic cross section of a modified silicon substrate (10) obtained at the end of step (2) of the method of the invention.
  • the present invention relates to a device, in particular a photovoltaic cell, comprising a modified silicon substrate obtained according to the method described above.
  • Groups IA and IIA mentioned above refer to the numbering retained (Roman numerals from I to VIII according to Newlands, and letters A and B according to Moseley) well known to those skilled in the art, to designate the elements in the periodic table of elements. , also called "Mendeleev's Table".
  • step (1) of the method of the invention consists in having a silicon substrate, one of whose faces is covered at least in part with the mixture considered according to the invention.
  • substrate refers to a basic structure on the face of which is applied the mixture considered according to the invemion.
  • the silicon base substrate used in step (1) of the process of the invention can be of various kinds. In particular, as developed in the following, it can be chosen with regard to the method of elaboration of the photovoltaic cell.
  • the silicon substrate used in the process of the invention must be crystalline and have a grain size of at least 1 mm, preferably 1 cm or more.
  • the silicon substrate used in the process according to the invention may be doped or undoped.
  • the silicon used in the process according to the invention may be doped, in particular by a p-type dopant such as, for example, boron, aluminum, indium and gallium or by an n-type dopant such as by phosphorus, antimony and arsenic.
  • the silicon substrate may, where appropriate, be juxtaposed on the opposite side to that coated with the mixture according to the invention, with other layers of materials.
  • the substrate may, if appropriate, undergo prior to its implementation in the method of the invention, one or more transformations dedicated, for example, to confer particular properties.
  • the silicon substrate used in step (1) of the method of the invention may be a p-type silicon plate, in particular comprising at least one pn junction on the face opposite to that coated with the mixture according to the invention, and having optionally been previously subjected to one or more anti-reflection treatment (s).
  • Such a silicon wafer can be made according to conventional techniques falling within the skills of a person skilled in the art.
  • Its thickness may, for example, vary from 100 to 300 ⁇ m, in particular from 150 to
  • the substrate modified at the end of step (2) of the method according to the invention can then form, integrally, as it is, the rear face of the photovoltaic cell already (partly) made .
  • the silicon substrate that is suitable for the treatment according to the invention may be a so-called "low cost” substrate, of the metallurgical silicon type, purified by segregation prior to its implementation in the process of the invention. .
  • Silicon substrate metallurgical silicon type means silicon substrates containing high concentrations of impurities, especially metal, of the order of 1 to 100 ppm by weight.
  • This silicon which may be monocrystalline silicon or multicrystalline silicon, that is to say silicon whose grains have a size of 1 mm 2 to several cm 3 and whose growth is columnar, generally contains impurities. such as Fe, Cr, Cu ... at much higher concentrations than electron-quality crystalline silicon.
  • impurities such as Fe, Cr, Cu ... at much higher concentrations than electron-quality crystalline silicon.
  • Such a silicon substrate may have a thickness ranging from 200 to 700 ⁇ m, in particular ranging from 300 to 500 ⁇ m,
  • the modified substrate at the end of step (2) of the process of the invention, can be used, as described later, by one or more subsequent steps, as an epitaxial substrate. adapted to the development of a cell by recrystallization of a thin layer of silicon.
  • the mixture considered in the process of the invention comprises at least:
  • one or more modifying elements chosen from the elements of columns IA and ⁇ of the periodic table, in particular strontium, sodium and their mixture;
  • the aluminum is present in the mixture of step (1) of the process of the invention in a content ranging from 40 to 80% by weight, preferably from 55 to 65% by weight. , based on the total weight of said mixture.
  • the modifying element (s) is (are) present in the mixture of step (1) in a content ranging from 0.01 to 0.1%, preferably from 0.02 to 0.06% by weight, relative to the total weight of said mixture.
  • the mixture considered according to the invention comprises from 20 to 60% by weight of said element (s) of addition.
  • the element (s) of addition is (are) present in said mixture of step (1) in a content ranging from 35 to 45% by weight, relative to the weight total of said mixture, preferably about 40%.
  • the additive element is zinc or tin.
  • the mixture of the different metallic elements may be in the form of a powder.
  • the powder mixture has a particle size D50 expressed in volume ranging from 2 to 10 microns.
  • the particle size can be measured for example by laser particle size according to a technique known to those skilled in the art.
  • the mixture in the form of a powder, considered according to the invention is formed by mixing the different metal elements, each in the form of a powder.
  • a master alloy comprising the various elements used in the composition of the mixture of the invention is produced and then consecutively reduced to powder.
  • the mixture of the invention can be made by mixing a powder obtained by grinding an aluminum parent alloy and 5% by weight of modifying element (s), with a powder obtained by mixing an aluminum powder and the addition element (s) in the form of powder (s).
  • the mixture considered according to the invention comprises, in addition to the mixture of the different powders, at least one binder.
  • a mixture is a screen printing paste, which can be easily spread on the silicon base substrate.
  • the binder makes it possible in particular to ensure the dispersion and cohesion of the powder mixture. It is generally a resin dissolved in a solvent, chosen from cellulose resins and acrylic resins. As examples, ethylcellulose dissolved in a solvent such as teipinole, n-butyl methacrylate dissolved in a glycol ether.
  • the silicon substrate coated on one of its faces of the mixture must be subjected to a drying step to evaporate the solvent and then to a debinding step, for purposes of to eliminate, prior to step (2), the binder (s).
  • the mixture may further comprise glass frits.
  • glass frits generally consist of a mixture of SiO 2 , B 2 O 3 , ZnO, PbO and Bi 2 O 3 . They advantageously make it possible to pierce the insulating layers, to facilitate the densification of the metal particles, to create an electrical contact and to create a snap on the substrate.
  • the coated surface of said silicon substrate of step (1) of the process according to the invention is exposed to a heat treatment that is conducive to:
  • the formation of the molten alloy (a) can be obtained by exposing the coated face of the substrate of step (1) to a temperature below silicon melting temperature, in particular ranging between 600 ° C and 850 ° C, preferably between 700 ° C and 750 ° C, for a period of the order of one minute.
  • the metal elements of the mixture considered according to the invention and the silicon melt to form a molten alloy by establishing the thermodynamic equilibrium.
  • the melted zone is exposed to conditions permitting the solidification of the molten alloy. These conditions require in particular a cooling of the melted zone below the melting temperature.
  • This cooling can be progressive, with several cooling rates during the same cycle, from 5 ° C / s to 50 ° C / s.
  • the fibrous layer (22) considered according to the invention having a two-phase eutectic structure consisting of silicon-based fibers in an aluminum-based matrix, and
  • step (2) of the process of the invention leads to the formation of an outer layer (23) of eutectic structure having at least three phases, said outer layer (23) comprising the majority of said element (s). ) addition.
  • step (2) of the process of the invention leads to the formation of an intermediate layer (21) between said fibrous layer (22) and said silicon substrate (20), of single-phase structure and comprising predominantly silicon.
  • FIG. 1 represents the different layers of the silicon substrate (10) obtained at the end of step (2) of the method of the invention.
  • steps (a) and (b) are carried out continuously.
  • the heat treatment may be carried out in a heating chamber into which the silicon substrate according to the invention is introduced.
  • This chamber is particularly suitable for ensuring the exposure of the face of the substrate coated with the mixture described above, to a heating under the aforementioned conditions.
  • the silicon substrate and said enclosure may be moved relative to one another so that any melted zone in step (a) is moved consecutively towards the enclosure zone, suitable for its solidification (b) by cooling.
  • the silicon substrate that is moved through the enclosure.
  • this heat treatment can be carried out according to the standard method of annealing the contacts, generally via tube furnaces, static or dynamic.
  • This heat treatment can be carried out under air or under a non-oxidizing atmosphere such as a stream of argon, helium, etc.
  • the cooling step it can be done by natural cooling after turning off the heating source or by forced cooling, for example by passing on the substrate, a flow of air.
  • step (2) is performed by introducing the silicon substrate of step (1) into a pass-through furnace, under standard operating conditions, conventionally used for the production of photovoltaic cells, and well known to those skilled in the art.
  • the fibrous layer (22) formed according to the method of the invention has a mean network pitch of less than or equal to 2 ⁇ m,
  • said fibrous layer (22) has a mean pitch ranging from 0.5 to 1.5 ⁇ m.
  • said fibrous layer (22) may have a thickness of between 1 and 20 ⁇ m, preferably between 5 and 10 ⁇ m.
  • silicon-based fibers the fact that said formed fibers mainly comprise silicon, in other words consist of more than 99.99% by weight of silicon.
  • the matrix "based on aluminum” mainly comprises aluminum, that is to say it consists of 98.5% by weight of aluminum. In fact, the maximum solubility of silicon in aluminum is about 1.5% by weight at the eutectic temperature.
  • the single-phase layer (21) contiguous with the base silicon substrate (20) can, in the case where it is p-type , play The role, within a photovoltaic cell, rear surface field, also called BSF (Back Surface Field), that is to say the role of electric field repelling minority carriers in the back of the cell .
  • BSF Back Surface Field
  • the method according to the invention can advantageously be implemented to form in a single step, both the rear surface field of a photovoltaic cell and the desired diffractive fibrous layer.
  • the upper layer (23) contiguous to the fibrous layer (22) of the invention is of three-phase structure for the case where a single addition element is used in the mixture in question according to the invention.
  • This layer (23) is of no interest for the diffraction of infrared photons, but may have the advantage of conducting electricity which is advantageous for contacting and assembly in modules.
  • the method of the invention is carried out, as mentioned above, from a p-type silicon plate, on which a pn junction has already been made, and possibly one or more treatments) anti-reflections.
  • the modified substrate obtained at the end of step (2) of the process according to the invention can then integrally form, as such, the rear face of the photovoltaic cell.
  • this photovoltaic cell will have on the rear face, the single-phase layer (21) constituting the BSF, and the fibrous layer (22) of the invention, allowing the diffraction of infrared photons not absorbed by silicon.
  • the present invention relates to a device, in particular a photovoltaic cell, formed wholly or partly of a modified silicon substrate, as obtained at the end of step (2). ) of the method described above.
  • said modified silicon substrate is obtained according to the method of the invention, from a p-type silicon wafer, comprising at least one pn junction on its other face and possibly having been previously subjected to anti-oxidation treatment. -reflets.
  • the method of the invention is implemented to form an epitaxial substrate adapted to the recrystallization of one or more thin layers of silicon.
  • the silicon substrate of step (1) may be, as specified above, a metallurgical silicon substrate, purified by segregation.
  • the method of the invention may furthermore comprise a step (3) comprising the elimination of the eutectic layer (s) (23) in at least three phases formed at the end of the process. step (2) and contiguous to the fibrous layer considered according to the invention, and the removal of the aluminum matrix from the fibrous layer.
  • This step (3) can be carried out according to techniques known to those skilled in the art, in particular by a chemical etching of the substrate obtained at the end of step (2) of the process of the invention, in particular at the using orthophosphoric acid.
  • Such pickling step (3) eliminates all the metal elements other than silicon.
  • the substrate is in the form of a fakir carpet consisting of silicon needles.
  • These needles may in particular have a height ranging from 2 .mu.m to 10 .mu.m, in particular around 5 .mu.m.
  • Such a substrate is suitable for deposition of amorphous or nanocrystalline silicon layers by a type of PVD (iii) technology without risking clogging of the spaces between the needles.
  • the fiber layer will also be, according to this embodiment, the rear face of the final cell.
  • the present invention relates to a device, formed in whole or part of a modified silicon substrate, as obtained at the end of step (3) of the described method.
  • the present invention relates to a device, in particular a photovoltaic cell, characterized in that an auxiliary silicon layer is superimposed on said modified silicon substrate, as obtained at the end of step (3) of the process of the invention.
  • An alloy containing 60% by weight of Al and 40% by weight of Zn is produced by mixing powders of micron size (D 50 of between 2 and 20 ⁇ m).
  • the Sr is added in the form of powders obtained by grinding an Al-5% by weight Sr alloy so that the Sr content in the Al-Zn-Sr alloy is 500 ppm by weight.
  • These powders are agglomerated with a binder of cellulosic type (ethylcellulose dissolved in terpineol), and possibly glass frits, to form a paste suitable for screen printing.
  • This paste is deposited on a p-type Si plate on which the p-n junction and the anti-reflection treatments have already been made.
  • the assembly is introduced into a passage furnace to reach a maximum temperature of 70 ° C., which leads to dissolve a portion of the Si of the substrate to ensure thermodynamic equilibrium.
  • the first structure deposited during the cooling is single-phase and grows epitaxially on the Si substrate, it acts as a back repellent field for the application.
  • a ternary eutectic structure is then formed with a medium composition rich in Zn.
  • An alloy containing 60% by weight of Al and 40% by weight of Sn is produced by mixing powders of micron size (Dso of between 2 and 10 ⁇ m). Sr is added in the form of powders obtained by grinding a parent alloy Al-5% by weight of Sr so that the Sr content in the Al-Sn-Sr alloy is 500 ppm by weight. These powders are agglomerated with a binder of acrylic type (n-butyl methacrylate dissolved in a glycol ether), and possibly glass frits, to form a paste suitable for screen printing.
  • a binder of acrylic type n-butyl methacrylate dissolved in a glycol ether
  • This paste is deposited on a low cost metallurgical Si substrate purified by segregation.
  • the assembly is introduced into a passage oven to reach a maximum temperature of 700 ° C, which leads to dissolve a portion of the Si substrate to ensure thermodynamic equilibrium.
  • the first structure deposited during the cooling is single-phase and grows by epitaxy on the Si of the substrate.
  • the resolidified assembly is subjected to chemical etching (for example orthophosphoric acid) to keep only the Si.
  • the substrate is in the form of a carpet of fakir consisting of Si needles from a height close to 5 microns with a spacing of the order of 1.2 microns.

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  • Electromagnetism (AREA)
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  • Life Sciences & Earth Sciences (AREA)
  • Sustainable Development (AREA)
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  • Photovoltaic Devices (AREA)
  • Crystals, And After-Treatments Of Crystals (AREA)
  • Inorganic Fibers (AREA)

Abstract

The present invention relates to a method for forming, on the surface of one of the sides of a silicon substrate, a fibrous layer having a mean lattice pitch of no more than 2 µm, without requiring soaking. The invention also relates to devices, in particular photovoltaic cells, comprising a silicon substrate produced by means of such a method.

Description

Procédé de formation d'une couche fibreuse  Method of forming a fibrous layer
La présente invention porte sur un procédé de formation en surface d'une des faces d'un substrat de silicium, d'une couche fibreuse présentant un pas moyen de réseau inférieur ou égal à 2 μm. Ce procédé est particulièrement avantageux dans le cadre de l 'élaboration de cellules photovoltaïques, pour former en leur face arrière, une couche de structure fibreuse, apte à assurer la diffraction des photons infrarouges. The present invention relates to a method for forming a surface of a surface of a silicon substrate, a fibrous layer having an average network pitch of less than or equal to 2 μm. This method is particularly advantageous in the context of the development of photovoltaic cells, to form in their rear face, a layer of fibrous structure capable of providing diffraction of infrared photons.
Les cellules photovoltaïques sont pour l'essentiel fabriquées à partir de silicium mono- ou poly-cristallin. D'une manière générale, ces cellules industrielles standards à base de silicium présentent un champ électrique face arrière, encore appelé BSF (« Back Surface Field ») obtenu par un alliage eutectique Aluminium-Silicium (Al-Si) formé par recuit d'une couche en aluminium déposée par sérigraphie sur un substrat de silicium. Ce recuit des contacts en face arrière est réalisé selon une technologie standard dans un four à passage.  Photovoltaic cells are essentially made from mono- or poly-crystalline silicon. In general, these standard silicon-based industrial cells have a rear-facing electric field, also called BSF ("Back Surface Field") obtained by an aluminum-silicon (Al-Si) eutectic alloy formed by annealing of a aluminum layer deposited by screen printing on a silicon substrate. This annealing of the contacts on the rear face is carried out according to a standard technology in a passage oven.
Plus précisément, un tel recuit requiert de porter l'ensemble à une température de l'ordre de 800 °C pendant quelques secondes, pour former un alliage liquide entre le silicium et l'aluminium. Au refroidissement, les premiers stades de solidification de cet alliage liquide conduisent au dépôt d'une couche monophasée de Si saturée en Al de quelques microns, qui forme le champ arrière (BSF) des cellules photovoltaïques. La température eutectique du système Al-Si (577 °C) une fois atteinte, la solidification devient biphasée et conduit à une structure formée de lamelles de silicium dans une matrice d'aluminium. Toutefois, une telle structure qui présente généralement des espacements inter lamellaires de l'ordre de 10 à 20 μm, possède malheureusement un désordre topologique important.  More specifically, such annealing requires the assembly to a temperature of about 800 ° C for a few seconds, to form a liquid alloy between silicon and aluminum. On cooling, the first stages of solidification of this liquid alloy lead to the deposition of a single-phase layer of Al-saturated Si of a few microns, which forms the rear field (BSF) of the photovoltaic cells. The eutectic temperature of the Al-Si system (577 ° C) once reached, the solidification becomes biphasic and leads to a structure formed of silicon lamellae in an aluminum matrix. However, such a structure which generally has interlamellar spacings of the order of 10 to 20 microns unfortunately has a major topological disorder.
En conséquence, les structures présentes en face arrière d'une cellule photovoltaïque élaborée à partir de ce procédé standard, ne permettent pas une diffraction des photons infrarouges non absorbés par le silicium de la cellule, i.e. des photons de longueur d'onde inférieure à 1,1 μm correspondant à la bande interdite de silicium, et qui seraient donc susceptibles de générer des porteurs de charges.  Consequently, the structures present on the rear face of a photovoltaic cell produced from this standard method do not allow diffraction of the infrared photons not absorbed by the silicon of the cell, ie photons of wavelength less than 1 , 1 micron corresponding to the forbidden band of silicon, and which would therefore be likely to generate charge carriers.
Afin d'améliorer le rendement des cellules photovoltaïques, il serait donc souhaitable de pouvoir réaliser une structure en face arrière des cellules, permettant la diffraction des photons infrarouge non absorbés par le silicium, et ainsi améliorer leur "collecte" au sein de la cellule. Le problème d'absorption des photons se pose avec une acuité particulière dans le cas des méthodes basées sur la recristallisation de couches minces déposées par des technologies sous vide en phase vapeur (par exemple, par les techniques CVD (ii) et PVD (iii)). S 'agissant de couches souvent très fines (généralement inférieures à 20 μm, souvent de l'ordre du μm), la mise en œuvre de moyens permettant d'allonger le chemin optique des photons en face avant et/ou arrière des cellules est nécessaire à l'obtention de rendements de conversion énergétique intéressants pour les applications industrielles. In order to improve the efficiency of the photovoltaic cells, it would therefore be desirable to be able to make a structure on the back of the cells, allowing the diffraction of the infrared photons not absorbed by the silicon, and thus improve their "collection" within the cell. The problem of photon absorption is particularly acute in the case of methods based on the recrystallization of thin layers deposited by vapor phase vacuum technologies (for example, by the CVD (ii) and PVD (iii) techniques). ). With regard to often very thin layers (generally less than 20 μm, often of the order of μm), the implementation of means for extending the optical path of the photons in front and / or back of the cells is necessary to obtain interesting energy conversion efficiencies for industrial applications.
Pour ce faire, les techniques de la microélectronique permettent de réaliser par gravure de reliefs organisés, des réseaux présentant une bonne régularité et un pas moyen adapté à la diffraction des photons infrarouges.  To do this, the techniques of microelectronics can achieve by etching of organized reliefs, networks having a good regularity and an average pitch suitable for the diffraction of infrared photons.
Toutefois, ces techniques présentent l'inconvénient d'être particulièrement coûteuses. En particulier, elles ne sont pas compatibles avec la technologie standard de recuit en four à passage, usuellement utilisée dans l'élaboration des cellules photovoltaïques, et nécessitent dès lors d'opérer des adaptations majeures du procédé de fabrication des cellules photovoltaïques.  However, these techniques have the disadvantage of being particularly expensive. In particular, they are not compatible with the standard annealing technology passing through, usually used in the development of photovoltaic cells, and therefore require major adaptations of the manufacturing process of photovoltaic cells.
Egalement, il est bien connu, notamment dans le domaine de la métallurgie, de pouvoir transformer la forme lamellaire de l'eutectique Al-Si en une forme fibreuse, par ajout, à l'alliage fondu, d'un modificateur comme le sodium (Na) ou le strontium (Sr). Différentes théories ont été développées pour tenter d'expliquer l'obtention d'une telle structure fibreuse (iv).  Also, it is well known, particularly in the field of metallurgy, to be able to transform the lamellar form of Al-Si eutectic into a fibrous form, by adding a modifier such as sodium to the molten alloy ( Na) or strontium (Sr). Different theories have been developed to try to explain the obtaining of such a fibrous structure (iv).
Toutefois, comme précisé ci-dessus, dans les conditions standards de recuit en four à passage, plus particulièrement, pour une solidification de l'alliage fondu à des vitesses de l'ordre de S à 25 um/s, correspondant à des vitesses de refroidissement de l'ordre de 10 à 50°C/s caractéristiques des fours à passage, les espacements de la structure eutectique fibreuse obtenue sont supérieures à 2 μm, et ne sont donc pas adaptés pour une diffraction des photons infrarouges.  However, as specified above, in the standard conditions of annealing furnace passage, more particularly, for a solidification of the molten alloy at speeds of the order of S to 25 um / s, corresponding to speeds of cooling of the order of 10 to 50 ° C / s characteristics of passing furnaces, the spacings of the resulting fibrous eutectic structure are greater than 2 microns, and are therefore not suitable for diffraction of infrared photons.
Les techniques de trempe rapide peuvent par ailleurs permettre d'obtenir des structures avec un pas de réseau réduit. Malheureusement, outre leur difficulté de mise en œuvre sur des échantillons massifs, les techniques de trempe induisent des niveaux de contrainte élevés. Les structures obtenues à l'issue de trempes, s'avèrent, en outre, fragiles et peu manipulables, et ne permettent pas, par conséquent, de poursuivre les étapes ultérieures indispensables pour l'élaboration des cellules photovoltaïques. Par conséquent, il demeure le besoin de pouvoir réaliser une structure de pas moyen de réseau significativement réduit et en particulier, avantageusement inférieur ou égal à 2 μm, apte à assurer la diffraction des photons infrarouge non absorbés par le silicium, par un procédé par ailleurs compatible avec la technologie standard d'élaboration des cellules photovoltaïques, en particulier compatible avec un recuit des contacts en four à passage. Fast quenching techniques can also provide structures with a reduced network pitch. Unfortunately, besides their difficulty of implementation on massive samples, quenching techniques induce high levels of stress. The structures obtained after quenching, prove, moreover, fragile and not very manipulable, and therefore do not allow to continue the subsequent steps essential for the development of photovoltaic cells. Therefore, there remains the need to be able to achieve a significantly reduced average network pitch structure and in particular, advantageously less than or equal to 2 microns, capable of diffracting infrared photons not absorbed by silicon, by an otherwise compatible with the standard technology for developing photovoltaic cells, in particular compatible with annealing of the contacts in a passage oven.
La présente invention vise précisément à proposer un procédé donnant satisfaction aux exigences précitées.  The present invention aims precisely to provide a method that satisfies the aforementioned requirements.
En particulier, la présente invention concerne, selon un premier de ses aspects, un procédé de formation, en surface d'une face d'un substrat de silicium, d'une couche fibreuse (22) présentant un pas moyen de réseau inférieur ou égal à 2 μm, comprenant au moins les étapes consistant à :  In particular, the present invention relates, according to a first of its aspects, to a method of forming, on the surface of a face of a silicon substrate, a fibrous layer (22) having a mean network pitch of less than or equal to at 2 μm, comprising at least the steps of:
(1) disposer d'un substrat de silicium dont l'une des faces est recouverte au moins en partie d'un mélange comprenant au moins de l'aluminium et au moins un élément modificateur choisi parmi les éléments des colonnes IA et HA du tableau périodique, et  (1) having a silicon substrate, one of the faces of which is covered at least in part with a mixture comprising at least aluminum and at least one modifying element chosen from the elements of columns IA and HA of the table periodic, and
(2) exposer au moins la face enduite dudit substrat de l'étape (1) à un traitement thermique propice à (a) la formation d'un alliage fondu comprenant le silicium, l'aluminium et lcsdits éléments modificateurs, et à (b) la solidification consécutive dudit alliage fondu dans des conditions propices à la formation d'au moins une couche (22) présentant une structure eutectique biphasée constituée de fibres à base de silicium dans une matrice à base d'aluminium, avec un pas moyen de réseau inférieur ou égal à 2 μm, caractérisé en ce que ledit mélange de l'étape (1) comprend en outre de 20 à 60 % en poids, par rapport à son poids total, d'un ou plusieurs éléments d'addition choisis parmi le gallium, l'indium, l'étain, le zinc et leurs mélanges.  (2) exposing at least the coated side of said substrate of step (1) to a heat treatment conducive to (a) forming a molten alloy comprising silicon, aluminum and said modifying elements, and (b) ) the subsequent solidification of said molten alloy under conditions conducive to the formation of at least one layer (22) having a two-phase eutectic structure made of silicon-based fibers in an aluminum-based matrix, with an average network pitch less than or equal to 2 μm, characterized in that said mixture of step (1) further comprises from 20 to 60% by weight, relative to its total weight, of one or more addition elements chosen from gallium, indium, tin, zinc and their mixtures.
Contre toute attente, les inventeurs ont découvert qu'il est possible ainsi d'accéder à une couche fibreuse présentant un pas moyen de réseau inférieur ou égal à 2 μm, en mettant en œuvre un alliage liquide comprenant, outre le silicium, l'aluminium et un ou plusieurs éléments modificateurs, une quantité importante d'un ou plusieurs éléments métalliques choisi(s) parmi le gallium (Ga), l'iridium (In), l'étain (Sn) et le zinc (Zn). Un tel procédé est d'autant plus surprenant que les procédés standards de réalisation des cellules photovoltaïques cherchent habituellement à éviter toute contamination préjudiciable du silicium par les éléments métalliques, connus pour jouer le rôle de centres recombinants pour les porteurs de charge minoritaires (v). Unexpectedly, the inventors have discovered that it is thus possible to access a fibrous layer having an average network pitch of less than or equal to 2 μm, by implementing a liquid alloy comprising, in addition to silicon, aluminum and one or more modifying elements, a significant amount of one or more metal elements selected from gallium (Ga), iridium (In), tin (Sn) and zinc (Zn). Such a method is all the more surprising since the standard methods for producing photovoltaic cells usually seek to avoid any detrimental contamination of the silicon by the metal elements known to act as recombining centers for the minority charge carriers (v).
Ainsi, le procédé selon l'invention est avantageux à plusieurs titres.Thus, the method according to the invention is advantageous for several reasons.
Tout d'abord, il permet d'accéder à une couche de structure fibreuse présentant un pas moyen de réseau inférieur ou égal à 2 μm, particulièrement adapté à la diffraction des photons infrarouges, notamment de longueur d'onde inférieure à 1,1 μm correspondant à la bande interdite du silicium. Une telle structure fibreuse à l'arrière d'une cellule permet ainsi la "collecte" des photons infrarouges par diffraction, et l'amélioration du rendement de la cellule photovoltaïque. Firstly, it provides access to a layer of fibrous structure having a mean network pitch of less than or equal to 2 μm, particularly suitable for the diffraction of infrared photons, especially of wavelength less than 1.1 μm. corresponding to the forbidden band of silicon. Such a fibrous structure at the rear of a cell thus allows the "collection" of infrared photons by diffraction, and the improvement of the efficiency of the photovoltaic cell.
En outre, l'étape (2) du procédé selon l'invention peut être réalisée avec les techniques industrielles usuellement employées pour l'élaboration des cellules photovoltaïques, plus précisément par la technologie standard de cuisson en four à passage. Ainsi, de manière avantageuse, le procédé de l'invention ne nécessite pas de modifications importantes du procédé usuel d'élaboration des cellules photovoltaïques. Plus particulièrement, comme développé par la suite, il est possible selon le procédé de l'invention, de former en une seule étape, le champ de surface arrière (BSF) et la couche fibreuse diffractante.  In addition, the step (2) of the process according to the invention can be carried out with the industrial techniques usually employed for the production of photovoltaic cells, more precisely by the standard technology for cooking in a passage oven. Thus, advantageously, the process of the invention does not require significant modifications of the usual process for producing photovoltaic cells. More particularly, as developed subsequently, it is possible according to the method of the invention, to form in a single step, the rear surface field (BSF) and the diffractive fibrous layer.
D'autres caractéristiques, avantages et modes d'application du procédé selon l'invention assortiront mieux à la lecture de la description qui va suivre, donnée à titre illustratif et non limitatif en référence à la figure 1 annexée.  Other characteristics, advantages and modes of application of the method according to the invention will be better suited to the reading of the description which follows, given by way of illustration and without limitation with reference to the appended FIG. 1.
Plus précisément, la figure 1 représente une coupe transversale schématique d'un substrat de silicium (10) modifié obtenu à l'issue de l'étape (2) du procédé de l'invention.  More specifically, Figure 1 shows a schematic cross section of a modified silicon substrate (10) obtained at the end of step (2) of the method of the invention.
11 convient de noter que, pour des raisons de clarté, les différentes couches visibles sur la figure 1 sont représentées en échelle libre, les dimensions réelles des différentes parties n'étant pas respectées.  It should be noted that, for the sake of clarity, the different layers visible in Figure 1 are shown in free scale, the actual dimensions of the different parts are not respected.
Selon un autre de ses aspects, la présente invention concerne un dispositif, notamment une cellule photovoltaïque, comportant un substrat de silicium modifié obtenu selon le procédé décrit précédemment. Les groupes IA et IIA précités font référence aux numérotations retenues (chiffres romains de I à VIII selon Newlands, et lettres A et B selon Moseley) bien connues de l'homme de l'art, pour désigner les éléments dans la classification périodique des éléments, encore appelée « table de Mendeleïev ». According to another of its aspects, the present invention relates to a device, in particular a photovoltaic cell, comprising a modified silicon substrate obtained according to the method described above. Groups IA and IIA mentioned above refer to the numbering retained (Roman numerals from I to VIII according to Newlands, and letters A and B according to Moseley) well known to those skilled in the art, to designate the elements in the periodic table of elements. , also called "Mendeleev's Table".
Dans la suite du texte, les expressions « compris entre ... et , , . », « allant de ... à ... » et « variant de ... à ... » sont équivalentes et entendent signifier que les bornes sont incluses, sauf mention contraire.  In the rest of the text, the expressions "between ... and,,. "," Ranging from ... to ... "and" varying from ... to ... "are equivalent and mean that the terminals are included, unless otherwise stated.
ETAPE (1) STEP 1)
Comme précisé précédemment, l'étape (1) du procédé de l'invention consiste à disposer d'un substrat de silicium dont Tune des faces est recouverte au moins en partie du mélange considéré selon l'invention.  As specified above, step (1) of the method of the invention consists in having a silicon substrate, one of whose faces is covered at least in part with the mixture considered according to the invention.
Substrat de silicium Silicon substrate
Dans le cadre de la présente invention, le terme « substrat » fait référence à une structure de base sur la face de laquelle est appliqué le mélange considéré selon l'invemion.  In the context of the present invention, the term "substrate" refers to a basic structure on the face of which is applied the mixture considered according to the invemion.
Le substrat de base en silicium mis en œuvre dans l'étape (1) du procédé de l'invention peut être de diverses natures. En particulier, comme développé dans la suite, il peut être choisi au regard du mode d'élaboration de la cellule photovoltaïque.  The silicon base substrate used in step (1) of the process of the invention can be of various kinds. In particular, as developed in the following, it can be chosen with regard to the method of elaboration of the photovoltaic cell.
Le substrat de silicium utilisé dans le procédé de l'invention doit être cristallin et présenter une strocture en grains de taille au moins égale à 1mm, préférentiellement à 1 cm ou plus.  The silicon substrate used in the process of the invention must be crystalline and have a grain size of at least 1 mm, preferably 1 cm or more.
Le substrat de silicium utilisé dans le procédé selon l'invention peut être dopé ou non dopé. Ainsi, le silicium utilisé dans le procédé selon l'invention peut être dopé, notamment par un dopant de type p tel que par exemple le bore, l'aluminium, l'indium et le gallium ou par un dopant de type n tel que par exemple le phosphore, l'antimoine et l'arsenic.  The silicon substrate used in the process according to the invention may be doped or undoped. Thus, the silicon used in the process according to the invention may be doped, in particular by a p-type dopant such as, for example, boron, aluminum, indium and gallium or by an n-type dopant such as by phosphorus, antimony and arsenic.
Le substrat de silicium peut, le cas échéant, être juxtaposé sur la face opposée à celle enduite du mélange selon l'invention, à d'autres couches de matériaux. Le substrat peut, le cas échéant, subir préalablement à sa mise en œuvre dans le procédé de l'invention, une ou plusieurs transformations dédiées, par exemple, à lui conférer des propriétés particulières. Selon une première variante de réalisation, le substrat de silicium mis en œuvre dans l'étape (1) du procédé de l'invention peut être une plaque de silicium de type p, comportant en particulier au moins une jonction p-n sur la face opposée à celle enduite du mélange selon l'invention, et ayant éventuellement été préalablement soumis à un ou plusieurs traitement(s) anti-reflets. The silicon substrate may, where appropriate, be juxtaposed on the opposite side to that coated with the mixture according to the invention, with other layers of materials. The substrate may, if appropriate, undergo prior to its implementation in the method of the invention, one or more transformations dedicated, for example, to confer particular properties. According to a first variant embodiment, the silicon substrate used in step (1) of the method of the invention may be a p-type silicon plate, in particular comprising at least one pn junction on the face opposite to that coated with the mixture according to the invention, and having optionally been previously subjected to one or more anti-reflection treatment (s).
Une telle plaque de silicium peut être réalisée selon des techniques classiques relevant des compétences dé l'homme du métier.  Such a silicon wafer can be made according to conventional techniques falling within the skills of a person skilled in the art.
Son épaisseur peut par exemple varier de 100 à 300 μm, notamment de 150 à Its thickness may, for example, vary from 100 to 300 μm, in particular from 150 to
200 μm. 200 μm.
Dans le cadre de cette première variante, le substrat modifié à l'issue de l'étape (2) du procédé selon l'invention peut alors former, intégralement, tel quel, la face arrière de la cellule photovoltaïque déjà (en partie) réalisée.  In the context of this first variant, the substrate modified at the end of step (2) of the method according to the invention can then form, integrally, as it is, the rear face of the photovoltaic cell already (partly) made .
Selon une seconde variante de réalisation, le substrat de silicium convenant au traitement selon l'invention, peut être un substrat dit "bas coût", de type silicium métallurgique, purifié par ségrégation préalablement à sa mise en œuvre dans le procédé de l'invention. According to a second variant embodiment, the silicon substrate that is suitable for the treatment according to the invention may be a so-called "low cost" substrate, of the metallurgical silicon type, purified by segregation prior to its implementation in the process of the invention. .
Par substrat en silicium de type silicium métallurgique, on entend désigner des substrats en silicium contenant des concentrations élevées en impuretés, notamment métalliques, de l'ordre de 1 à 100 ppm en poids. Ce silicium, qui peut être du silicium monocristallin ou du silicium multicristallin, c'est-à-dire du silicium dont les grains ont une taille de 1 mm2 à plusieurs cm3 et dont la croissance est colonnaire, contient de &çon générale des impuretés métalliques telles que Fe, Cr, Cu..., à des concentrations bien plus élevées que le silicium cristallin de qualité électronique. Au regard de la présence des impuretés, il est peu onéreux et particulièrement avantageux pour être transformé en substrat possédant une forte valeur ajoutée. Silicon substrate metallurgical silicon type means silicon substrates containing high concentrations of impurities, especially metal, of the order of 1 to 100 ppm by weight. This silicon, which may be monocrystalline silicon or multicrystalline silicon, that is to say silicon whose grains have a size of 1 mm 2 to several cm 3 and whose growth is columnar, generally contains impurities. such as Fe, Cr, Cu ... at much higher concentrations than electron-quality crystalline silicon. In view of the presence of impurities, it is inexpensive and particularly advantageous to be transformed into a substrate having a high added value.
Un tel substrat de silicium peut présenter une épaisseur allant de 200 à 700 μm, en particulier allant de 300 à 500 μm, Selon un mode de réalisation particulier, à l'issue de l'étape (2) du procédé de l'invention, le substrat modifié peut être utilisé, comme décrit par la suite, par une ou plusieurs étapes ultérieures, comme substrat d'épitaxie adapté à l'élaboration d'une cellule par recristallisation d'une couche mince de silicium. Such a silicon substrate may have a thickness ranging from 200 to 700 μm, in particular ranging from 300 to 500 μm, According to a particular embodiment, at the end of step (2) of the process of the invention, the modified substrate can be used, as described later, by one or more subsequent steps, as an epitaxial substrate. adapted to the development of a cell by recrystallization of a thin layer of silicon.
Le choix d'un substrat de silicium convenable tait partie des compétences de l'homme du métier, qui sélectionnera la nature du substrat de silicium de base à mettre en œuvre dans le procédé de l'invention, selon la technique d'élaboration de la cellule photovoltaïque correspondante. The choice of a suitable silicon substrate was part of the skill of the skilled person, who will select the nature of the base silicon substrate to be used in the method of the invention, according to the technique of developing the corresponding photovoltaic cell.
Mélange Mixed
Comme précisé précédemment, le mélange considéré dans le procédé de l'invention comprend au moins :  As stated above, the mixture considered in the process of the invention comprises at least:
- de l'aluminium ;  - aluminum;
- un ou plusieurs éléments) modificateurs) choisi(s) parmi les éléments des colonnes IA et ΠΑ du tableau périodique, en particulier le strontium, le sodium et leur mélange ; et  one or more modifying elements) chosen from the elements of columns IA and ΠΑ of the periodic table, in particular strontium, sodium and their mixture; and
- un ou plusieurs éléments) d'addition choisi(s) parmi le gallium, l'indium, l'étain, le zinc et leurs mélanges.  one or more elements of addition chosen from gallium, indium, tin, zinc and their mixtures.
Selon un autre mode de réalisation particulier, l'aluminium est présent dans le mélange de l'étape (1) du procédé de l'invention en une teneur allant de 40 à 80 % en poids, de préférence de 55 à 65 % en poids, par rapport au poids total dudit mélange. According to another particular embodiment, the aluminum is present in the mixture of step (1) of the process of the invention in a content ranging from 40 to 80% by weight, preferably from 55 to 65% by weight. , based on the total weight of said mixture.
Selon un mode de réalisation particulier, l'(les) éléments) modificateurs) est (sont) présent(s) dans le mélange de l'étape (1) en une teneur allant de 0,01 à 0,1 %, de préférence de 0,02 à 0,06 % en poids, par rapport au poids total dudit mélange. According to a particular embodiment, the modifying element (s) is (are) present in the mixture of step (1) in a content ranging from 0.01 to 0.1%, preferably from 0.02 to 0.06% by weight, relative to the total weight of said mixture.
Comme précisé précédemment, de tels éléments sont connus pour leur capacité à modifier la structure de l'eutectique Al-Si. Lors de sa solidification, le silicium de l'eutectique Al-Si croit normalement sous forme lamellaire, encore appelée « forme aciculaire ». S'il est modifié par ajout d'un élément modificateur, il croît alors sous une forme fibreuse. Selon une caractéristique essentielle de l'Invention, le mélange considéré selon l'invention comprend de 20 à 60 % en poids dudit/desdits élément(s) d'addition. As previously stated, such elements are known for their ability to modify the structure of Al-Si eutectic. During its solidification, the silicon of the eutectic Al-Si normally grows in lamellar form, also called "acicular form". If it is modified by adding a modifying element, it then grows in a fibrous form. According to an essential characteristic of the invention, the mixture considered according to the invention comprises from 20 to 60% by weight of said element (s) of addition.
D'une manière préférentielle, l'(les) élémerit(s) d'addition est (sont) présents) dans ledit mélange de l'étape (1) en une teneur allant de 35 à 45 % en poids, par rapport au poids total dudit mélange, de préférence d'environ 40 %.  Preferably, the element (s) of addition is (are) present in said mixture of step (1) in a content ranging from 35 to 45% by weight, relative to the weight total of said mixture, preferably about 40%.
. Selon un mode de réalisation particulier, l'élément d'addition est le zinc ou l'étain. Selon un mode de réalisation particulier de l'invention, le mélange des différents éléments métalliques peut se présenter sous forme de poudre.  . According to a particular embodiment, the additive element is zinc or tin. According to a particular embodiment of the invention, the mixture of the different metallic elements may be in the form of a powder.
Avantageusement, le mélange de poudre présente une granulométrie D50 exprimée en volume allant de 2 à 10 μm.  Advantageously, the powder mixture has a particle size D50 expressed in volume ranging from 2 to 10 microns.
La granulométrie peut être mesurée par exemple par granulométrie laser selon une technique connue de l'homme du métier.  The particle size can be measured for example by laser particle size according to a technique known to those skilled in the art.
Dans une variante de réalisation, le mélange sous forme d'une poudre, considéré selon l'invention est formé en mélangeant les différents éléments métalliques, se présentant chacun sous la forme d'une poudre.  In an alternative embodiment, the mixture in the form of a powder, considered according to the invention is formed by mixing the different metal elements, each in the form of a powder.
Dans une autre variante de réalisation, un alliage mère comprenant les différents éléments entrant dans la composition du mélange de l'invention est réalisé, puis consécutivement réduit en poudre.  In another variant embodiment, a master alloy comprising the various elements used in the composition of the mixture of the invention is produced and then consecutively reduced to powder.
A titre d'exemple, le mélange de l'invention peut être réalisé par mélange d'une poudre obtenue par broyage d'un alliage mère constitué d'aluminium et de 5 % en poids d'élément(s) modificateurs), avec une poudre obtenue par mélange d'une poudre d'aluminium et du(des) éléments) d'addition sous forme de poudre(s).  By way of example, the mixture of the invention can be made by mixing a powder obtained by grinding an aluminum parent alloy and 5% by weight of modifying element (s), with a powder obtained by mixing an aluminum powder and the addition element (s) in the form of powder (s).
De manière avantageuse, le mélange considéré selon l'invention comprend, outre le mélange des différentes poudres, au moins un liant. Un tel mélange constitue une pâte de sérigraphie, pouvant être aisément étalée sur le substrat de base en silicium. Advantageously, the mixture considered according to the invention comprises, in addition to the mixture of the different powders, at least one binder. Such a mixture is a screen printing paste, which can be easily spread on the silicon base substrate.
Le liant permet notamment d'assurer la dispersion et la cohésion du mélange des poudres, Il s'agit généralement d'une résine dissoute dans un solvant, choisie parmi les résines cellulosiques et les résines acryliques. On peut citer à titre d'exemples, l'éthylcellulose dissoute dans un solvant tel que le teipinéol, le méthacrylate de n-butyle dissout dans un éther de glycol. The binder makes it possible in particular to ensure the dispersion and cohesion of the powder mixture. It is generally a resin dissolved in a solvent, chosen from cellulose resins and acrylic resins. As examples, ethylcellulose dissolved in a solvent such as teipinole, n-butyl methacrylate dissolved in a glycol ether.
Lorsque le mélange met en œuvre un ou plusieurs liant(s), le substrat de silicium enduit sur l'une de ses faces du mélange doit être soumis à une étape de séchage pour évaporer le solvant puis à une étape de déliantage, à des fins d'éliminer, préalablement à l'étape (2), le ou les liantes).  When the mixture uses one or more binder (s), the silicon substrate coated on one of its faces of the mixture must be subjected to a drying step to evaporate the solvent and then to a debinding step, for purposes of to eliminate, prior to step (2), the binder (s).
L'homme du métier est à même de mettre en œuvre les techniques de déliantage connues, de préférence par décomposition thermique, en étuve par exemple. Selon encore un autre mode de réalisation, le mélange peut comprendre, en outre, des frittes de verre. Ces frittes de verre sont généralement constituées d'un mélange de SiO2, B2O3, ZnO, PbO et Bi2O3. Elles permettent avantageusement de percer les couches isolantes, de faciliter la densification des particules métalliques, de créer un contact électrique et de créer un accrochage sur le substrat. Those skilled in the art are able to implement known debinding techniques, preferably by thermal decomposition, in an oven for example. According to yet another embodiment, the mixture may further comprise glass frits. These glass frits generally consist of a mixture of SiO 2 , B 2 O 3 , ZnO, PbO and Bi 2 O 3 . They advantageously make it possible to pierce the insulating layers, to facilitate the densification of the metal particles, to create an electrical contact and to create a snap on the substrate.
La réalisation du mélange considéré selon l'invention sous la forme d'une pâte de sérigraphie convenable fait partie des compétences de l'homme du métier, qui étalera une telle pâte de sérigraphie sur l'une des faces du substrat de silicium, par des moyens adaptés. The production of the mixture according to the invention in the form of a suitable screen printing paste is within the skill of those skilled in the art, which will spread such a screen printing paste on one of the faces of the silicon substrate, by means of adapted means.
ETAPE (2) 2ND STEP)
Procédé de formation de la couche fibreuse  Method of forming the fibrous layer
Dans une seconde étape essentielle du procédé de l'invention, la face enduite dudit substrat de silicium de l'étape (1) du procédé selon l'invention est exposée à un traitement thermique propice à :  In a second essential step of the process of the invention, the coated surface of said silicon substrate of step (1) of the process according to the invention is exposed to a heat treatment that is conducive to:
(a) la formation d'un alliage fondu comprenant le silicium et lesdits éléments modificateurs, et  (a) forming a molten alloy comprising silicon and said modifying elements, and
(b) la solidification consécutive dudit alliage fondu dans des conditions propices à la formation de la couche fibreuse (22) selon l'invention.  (b) the subsequent solidification of said molten alloy under conditions conducive to the formation of the fibrous layer (22) according to the invention.
Plus particulièrement, la formation de l'alliage fondu (a) peut être obtenue en exposant la face enduite du substrat de l'étape (1) à une température inférieure à la température de fusion du silicium, en particulier variant entre 600 °C et 850 °C, de préférence entre 700 °C et 750 °C, pendant une durée de l'ordre de la minute. More particularly, the formation of the molten alloy (a) can be obtained by exposing the coated face of the substrate of step (1) to a temperature below silicon melting temperature, in particular ranging between 600 ° C and 850 ° C, preferably between 700 ° C and 750 ° C, for a period of the order of one minute.
A une telle température, les éléments métalliques du mélange considéré selon l'invention et le silicium fondent pour former un alliage fondu par établissement de l'équilibre thermodynamique.  At such a temperature, the metal elements of the mixture considered according to the invention and the silicon melt to form a molten alloy by establishing the thermodynamic equilibrium.
L'ajustement des conditions de température et de durée font partie des compétences de l'homme du métier.  The adjustment of the temperature and duration conditions are within the skill of the person skilled in the art.
Dans un stade consécutif (b), la zone fondue est exposée à des conditions permettant la solidification de l'alliage fondu. Ces conditions requièrent en particulier un refroidissement de la zone fondue en dessous de la température de fusion. In a subsequent stage (b), the melted zone is exposed to conditions permitting the solidification of the molten alloy. These conditions require in particular a cooling of the melted zone below the melting temperature.
Ce refroidissement peut être progressif, avec plusieurs vitesses de refroidissement au cours d'un même cycle, de 5 °C/s à 50 °C/s.  This cooling can be progressive, with several cooling rates during the same cycle, from 5 ° C / s to 50 ° C / s.
Au cours du refroidissement (b), apparaissent successivement, comme représenté sur la figure 1 :  During cooling (b), appear successively, as shown in FIG. 1:
• une couche monophasée (21) comprenant à base de silicium, qui croît par épitaxie sur la partie du substrat de silicium (20) restée solide ;  • a single-phase layer (21) comprising silicon-based, which grows epitaxially on the part of the silicon substrate (20) remained solid;
• la couche fibreuse (22) considérée selon l'invention, présentant une structure eutectique biphasée constituée de fibres à base de silicium dans une matrice à base d'aluminium, et  The fibrous layer (22) considered according to the invention, having a two-phase eutectic structure consisting of silicon-based fibers in an aluminum-based matrix, and
• une ou plusieurs couche(s) (23) de structure eutectique à au moins trois phases, dont la ou les compositions) moyennes) est(sont) voisine(s) de celle dudit/desdits éléments) d'addition. Ainsi, l'étape (2) du procédé de l'invention conduit à la formation d'une couche externe (23) de structure eutectique présentant au moins trois phases, ladite couche externe (23) comprenant la majorité du ou desdits élément(s) d'addition.  One or more layers (23) of eutectic structure with at least three phases, whose average composition (s) is (are) close to that of said addition element (s). Thus, step (2) of the process of the invention leads to the formation of an outer layer (23) of eutectic structure having at least three phases, said outer layer (23) comprising the majority of said element (s). ) addition.
Par ailleurs, l'étape (2) du procédé de l'invention conduit à la formation d'une couche intermédiaire (21) entre ladite couche fibreuse (22) et ledit substrat de silicium (20), de structure monophasée et comprenant majoritairement du silicium. La figure 1 représente les différentes couches du substrat de silicium (10) obtenu à l'issue de l'étape (2) du procédé de l'invention. Furthermore, step (2) of the process of the invention leads to the formation of an intermediate layer (21) between said fibrous layer (22) and said silicon substrate (20), of single-phase structure and comprising predominantly silicon. FIG. 1 represents the different layers of the silicon substrate (10) obtained at the end of step (2) of the method of the invention.
Selon un mode de réalisation particulièrement préféré, les étapes (a) et (b) sont réalisées en continu.  According to a particularly preferred embodiment, steps (a) and (b) are carried out continuously.
Le traitement thermique peut être réalisé dans une enceinte chauffante dans laquelle est introduit le substrat de silicium selon l'invention.  The heat treatment may be carried out in a heating chamber into which the silicon substrate according to the invention is introduced.
Cette enceinte est apte en particulier à assurer l'exposition de la face du substrat enduite du mélange décrit précédemment, à un chauffage dans les conditions précitées.  This chamber is particularly suitable for ensuring the exposure of the face of the substrate coated with the mixture described above, to a heating under the aforementioned conditions.
Le substrat de silicium et ladite enceinte peuvent être animés d'un mouvement l'un par rapport à l'autre de manière à ce que toute zone fondue en étape (a) soit déplacée consécutivement vers la zone de l'enceinte, propice à sa solidification (b) par refroidissement.  The silicon substrate and said enclosure may be moved relative to one another so that any melted zone in step (a) is moved consecutively towards the enclosure zone, suitable for its solidification (b) by cooling.
Plus particulièrement, c'est le substrat de silicium qui est déplacé au travers de l'enceinte.  More particularly, it is the silicon substrate that is moved through the enclosure.
De manière avantageuse, ce traitement thermique peut être réalisé selon le procédé standard de recuit des contacts, généralement via des fours à lampes, statiques ou dynamiques. Ce traitement thermique peut être réalisé sous air uu sous atmosphère non oxydante tel qu'un flux d'argon, d'hélium, etc.  Advantageously, this heat treatment can be carried out according to the standard method of annealing the contacts, generally via tube furnaces, static or dynamic. This heat treatment can be carried out under air or under a non-oxidizing atmosphere such as a stream of argon, helium, etc.
Quant à l'étape de refroidissement, elle peut s'effectuer par refroidissement naturel après avoir éteint la source de chauffage ou encore par un refroidissement forcé, par exemple par passage sur le substrat, d'un flux d'air. As for the cooling step, it can be done by natural cooling after turning off the heating source or by forced cooling, for example by passing on the substrate, a flow of air.
De manière avantageusement, l'étape (2) est réalisée via introduction du substrat de silicium de l'étape (1) dans un four à passage, dans des conditions de fonctionnement standards, classiquement mis en œuvre pour l'élaboration des cellules photovoltaïques, et bien connues de l'homme du métier.  Advantageously, step (2) is performed by introducing the silicon substrate of step (1) into a pass-through furnace, under standard operating conditions, conventionally used for the production of photovoltaic cells, and well known to those skilled in the art.
Caractéristiques de la couche fibreuse formée selon l'invention Characteristics of the fibrous layer formed according to the invention
Comme précisé précédemment, la couche fibreuse (22) formée selon le procédé de l'invention, présente un pas moyen de réseau inférieur ou égal à 2 μm, Avantageusement, ladite couche fibreuse (22) présente un pas moyen allant de 0,5 à 1,5 μm. As specified above, the fibrous layer (22) formed according to the method of the invention has a mean network pitch of less than or equal to 2 μm, Advantageously, said fibrous layer (22) has a mean pitch ranging from 0.5 to 1.5 μm.
En particulier, ladite couche fibreuse (22) peut présenter une épaisseur comprise entre 1 et 20 μm, de préférence entre 5 et 10 μm.  In particular, said fibrous layer (22) may have a thickness of between 1 and 20 μm, preferably between 5 and 10 μm.
Au sens de l'invention, on entend par fibres « à base de silicium », le fait que lesdites fibres formées comprennent majoritairement du silicium, autrement dit sont constituées à plus de 99,99 % en poids de silicium.  Within the meaning of the invention, the term "silicon-based" fibers, the fact that said formed fibers mainly comprise silicon, in other words consist of more than 99.99% by weight of silicon.
La matrice « à base d'aluminium » comprend majoritairement de l 'aluminium, autrement dit est constituée de 98,5 % en poids d'aluminium. De fait, la solubilité maximale de silicium dans l'aluminium est d'environ 1,5 % en poids à la température eutectique.  The matrix "based on aluminum" mainly comprises aluminum, that is to say it consists of 98.5% by weight of aluminum. In fact, the maximum solubility of silicon in aluminum is about 1.5% by weight at the eutectic temperature.
Quant aux autres couches formées à l'issue de l'étape (2) du procédé de l'invention, la couche monophasée (21) contigiie au substrat de silicium de base (20), peut dans le cas où il est de type p, jouer Le rôle, au sein d'une cellule photovoltaïque, de champ de surface arrière, encore appelé BSF (Back Surface Field), c'est-à-dire le rôle de champ électrique repoussant les porteurs minoritaires en face arrière de la cellule. As for the other layers formed at the end of step (2) of the process of the invention, the single-phase layer (21) contiguous with the base silicon substrate (20) can, in the case where it is p-type , play The role, within a photovoltaic cell, rear surface field, also called BSF (Back Surface Field), that is to say the role of electric field repelling minority carriers in the back of the cell .
Le procédé selon l'invention peut avantageusement être mis en œuvre pour former en une seule étape, à la fois le champ de surface arrière d'une cellule photovoltaïque et la couche fibreuse diffractante souhaitée.  The method according to the invention can advantageously be implemented to form in a single step, both the rear surface field of a photovoltaic cell and the desired diffractive fibrous layer.
La couche supérieure (23), contigiie à la couche fibreuse (22) de l'invention est de structure triphasée pour le cas où un seul élément d'addition est utilisé dans le mélange considéré selon l'invention. The upper layer (23) contiguous to the fibrous layer (22) of the invention is of three-phase structure for the case where a single addition element is used in the mixture in question according to the invention.
Elle est de structure à quatre phases, voire plus de quatre phases, lorsqu'au moins deux éléments d'addition sont introduits dans le mélange considéré selon l'invention.  It is of four-phase structure, or even more than four phases, when at least two additive elements are introduced into the mixture considered according to the invention.
Cette couche (23) est sans intérêt pour la diffraction des photons infrarouges, mais peut présenter l'intérêt de conduire l'électricité ce qui est avantageux pour la prise de contact et l'assemblage en modules. Selon une première variante d'élaboration d'une cellule photovoltaïque, le procédé de l'invention est réalisé, comme évoqué précédemment, à partir d'une plaque de silicium de type p, sur laquelle ont déjà été réalisés une jonction p-n, et éventuellement un ou plusieurs traitements) anti-reflets. This layer (23) is of no interest for the diffraction of infrared photons, but may have the advantage of conducting electricity which is advantageous for contacting and assembly in modules. According to a first alternative embodiment of a photovoltaic cell, the method of the invention is carried out, as mentioned above, from a p-type silicon plate, on which a pn junction has already been made, and possibly one or more treatments) anti-reflections.
Le substrat modifié obtenu à l'issu de l'étape (2) du procédé selon l'invention peut alors former, intégralement, tel quel la face arrière de la cellule photovoltaïque. En particulier, cette cellule photovoltaïque présentera en face arrière, la couche monophasée (21) constituant le BSF, et la couche fibreuse (22) de l'invention, permettant la diffraction des photons infrarouges non absorbés par le silicium.  The modified substrate obtained at the end of step (2) of the process according to the invention can then integrally form, as such, the rear face of the photovoltaic cell. In particular, this photovoltaic cell will have on the rear face, the single-phase layer (21) constituting the BSF, and the fibrous layer (22) of the invention, allowing the diffraction of infrared photons not absorbed by silicon.
Ainsi, selon un autre de ses aspects, la présente invention a pour objet un dispositif, notamment une cellule photovoltaïque, formée en tout ou partie d'un substrat de silicium modifié, tel qu'obtenu à l'issue de l'étape (2) du procédé décrit précédemment.  Thus, according to another of its aspects, the present invention relates to a device, in particular a photovoltaic cell, formed wholly or partly of a modified silicon substrate, as obtained at the end of step (2). ) of the method described above.
En particulier, ledit substrat de silicium modifié est obtenu selon le procédé de l'invention, à partir d'une plaque de silicium de type p, comportant au moins une jonction p-n sur son autre face et ayant éventuellement été préalablement soumis à un traitement anti-reflets.  In particular, said modified silicon substrate is obtained according to the method of the invention, from a p-type silicon wafer, comprising at least one pn junction on its other face and possibly having been previously subjected to anti-oxidation treatment. -reflets.
L'invention permet également avantageusement The invention also advantageously
- une réduction des contraintes thermomécaniques et la courbure des plaquettes suite à l'étape de recuit des contacts ; et  a reduction of the thermomechanical stresses and the curvature of the wafers following the step of annealing the contacts; and
- la possibilité d'ajouter à la pâte de sérigraphie une source de bore pour augmenter le niveau de dopage du BSF.  the possibility of adding to the screen printing paste a source of boron to increase the doping level of the BSF.
Dans une seconde variante d'élaboration d'une cellule photovoltaïque, le procédé de l'invention est mis en œuvre pour former un substrat d'épitaxie adapté à la recristallisation d'une ou plusieurs couches minces de silicium.  In a second alternative embodiment of a photovoltaic cell, the method of the invention is implemented to form an epitaxial substrate adapted to the recrystallization of one or more thin layers of silicon.
Selon cette variante, le substrat de silicium de l'étape (1) peut être, comme précisé précédemment, un substrat de type silicium métallurgique, purifié par ségrégation.  According to this variant, the silicon substrate of step (1) may be, as specified above, a metallurgical silicon substrate, purified by segregation.
Selon cette variante, le procédé de l'invention peut comprendre en outre une étape (3) comprenant l'élimination de la ou des couche(s) eutectique(s) (23) à au moins trois phases formées à l'issue de l'étape (2) et contigües à la couche fibreuse considérée selon l'invention, et l'élimination de la matrice d'aluminium de la couche fibreuse. Cette étape (3) peut être réalisée selon des techniques connues de l'homme du métier, en particulier par un décapage chimique du substrat obtenu à l'issu de l'étape (2) du procédé de l'invention, notamment à l'aide de l'acide ortho-phosphorique. According to this variant, the method of the invention may furthermore comprise a step (3) comprising the elimination of the eutectic layer (s) (23) in at least three phases formed at the end of the process. step (2) and contiguous to the fibrous layer considered according to the invention, and the removal of the aluminum matrix from the fibrous layer. This step (3) can be carried out according to techniques known to those skilled in the art, in particular by a chemical etching of the substrate obtained at the end of step (2) of the process of the invention, in particular at the using orthophosphoric acid.
Une telle étape de décapage (3) permet d'éliminer l'ensemble des éléments métalliques autres que le silicium.  Such pickling step (3) eliminates all the metal elements other than silicon.
A l'issue de l'étape de décapage (3), le substrat se présente sous la forme d'un tapis de fakir constitué d'aiguilles de silicium.  At the end of the etching step (3), the substrate is in the form of a fakir carpet consisting of silicon needles.
Ces aiguilles peuvent notamment présenter une hauteur allant de 2 μm à 10 μm, en particulier d'environ 5 μm.  These needles may in particular have a height ranging from 2 .mu.m to 10 .mu.m, in particular around 5 .mu.m.
Un tel substrat est adapté au dépôt de couches de silicium amorphe ou nanocristallin par une technologie de type PVD (iii) sans risquer le bouchage des espaces entre les aiguilles.  Such a substrate is suitable for deposition of amorphous or nanocrystalline silicon layers by a type of PVD (iii) technology without risking clogging of the spaces between the needles.
Puis un recuit en phase solide induit une recristallisation de cette couche de silicium amorphe ou nanocristallin, à partir des aiguilles pour constituer la couche active de la cellule photovoltaïque.  Then a solid phase annealing induces a recrystallization of this layer of amorphous or nanocrystalline silicon, from the needles to form the active layer of the photovoltaic cell.
La couche de fibres constituera également, selon ce mode de réalisation, la face arrière de la cellule finale.  The fiber layer will also be, according to this embodiment, the rear face of the final cell.
Ainsi, selon encore un autre de ses aspects, la présente invention a pour objet un dispositif, formé en tout ou partie d'un substrat de silicium modifié, tel qu'obtenu à l'issue de l'étape (3) du procédé décrit précédemment  Thus, according to yet another of its aspects, the present invention relates to a device, formed in whole or part of a modified silicon substrate, as obtained at the end of step (3) of the described method. previously
En particulier, la présente invention concerne un dispositif, notamment une cellule photovoltaïque, caractérisé en ce qu'une couche de silicium annexe est superposée audit substrat de silicium modifié, tel qu'obtenu à l'issue de l'étape (3) du procédé de l'invention.  In particular, the present invention relates to a device, in particular a photovoltaic cell, characterized in that an auxiliary silicon layer is superimposed on said modified silicon substrate, as obtained at the end of step (3) of the process of the invention.
L'invention va maintenant être décrite au moyen des deux exemples suivants, illustrant plus particulièrement les deux variantes de mise en œuvre du procédé de l'invention dans l'élaboration d'une cellule photovoltaïque. The invention will now be described by means of the two following examples, illustrating more particularly the two variants of implementation of the method of the invention in the development of a photovoltaic cell.
Ces exemples sont bien entendu donnés à titre illustratif et non limitatif de l'invention. EXEMPLES These examples are of course given by way of illustration and not limitation of the invention. EXAMPLES
Exemple 1  Example 1
Procédé selon l'invention mis en œuyre pour l'élaboration d'une cellule photovoltaïque par recuit de contact arrière  Process according to the invention put in use for the production of a photovoltaic cell by rear contact annealing
Un alliage contenant 60 % en poids de Al et 40 % en poids de Zn est réalisé en mélangeant des poudres de taille micronique (D50 compris entre 2 et 20 uxn). Le Sr est ajouté sous forme de poudres obtenues par broyage d'un alliage mère Al-5 % en poids de Sr de sorte que la teneur en Sr dans l'alliage Al-Zn-Sr est de 500 ppm en poids. Ces poudres sont agglomérées avec un liant de type cellulosique (éthylceliulose dissoute dans le terpinéol), et éventuellement des frittes de verre, pour former une pâte adaptée à la sérigraphie. An alloy containing 60% by weight of Al and 40% by weight of Zn is produced by mixing powders of micron size (D 50 of between 2 and 20 μm). The Sr is added in the form of powders obtained by grinding an Al-5% by weight Sr alloy so that the Sr content in the Al-Zn-Sr alloy is 500 ppm by weight. These powders are agglomerated with a binder of cellulosic type (ethylcellulose dissolved in terpineol), and possibly glass frits, to form a paste suitable for screen printing.
Cette pâte est déposée sur une plaque de Si de type p sur laquelle ont déjà été réalisés la jonction p-n et les traitements anti-reflet  This paste is deposited on a p-type Si plate on which the p-n junction and the anti-reflection treatments have already been made.
L'ensemble est introduit dans un four à passage pour atteindre une température maximale de 7S0 °C, ce qui conduit à dissoudre une partie du Si du substrat pour assurer l'équilibre thermodynamique.  The assembly is introduced into a passage furnace to reach a maximum temperature of 70 ° C., which leads to dissolve a portion of the Si of the substrate to ensure thermodynamic equilibrium.
La première structure déposée au cours du refroidissement est monophasée et croît par épitaxie sur le Si du substrat, elle joue le rôle de champ répulsif arrière pour l'application.  The first structure deposited during the cooling is single-phase and grows epitaxially on the Si substrate, it acts as a back repellent field for the application.
Puis la température de l'eutectique biphasé atteinte, une structure constituée de fibres à base de silicium, dans une matrice à base d'aluminium, et d'espacement moyen 1,4 μm est obtenue.  Then the temperature of the two-phase eutectic reached, a structure consisting of silicon-based fibers in an aluminum-based matrix, and a mean spacing of 1.4 μm is obtained.
Se forme ensuite une structure eutectique ternaire, avec une composition moyenne riche en Zn.  A ternary eutectic structure is then formed with a medium composition rich in Zn.
Exemple 2 Example 2
Procédé selon l'invention mis en oeuyre pour la formation d'un substrat d' épitaxie adapté à la recristallisation de couches minces pour l'élaboration d'une cellule photovoltaïque  Process according to the invention put in use for the formation of an epitaxial substrate adapted to the recrystallization of thin layers for the development of a photovoltaic cell
Un alliage contenant 60 % en poids de Al et 40 % en poids de Sn est réalisé en mélangeant des poudres de taille micronique (Dso compris entre 2 et 10 μm). Le Sr est ajouté sous forme de poudres obtenues par broyage d'un alliage mère Al-5 % en poids de Sr de sorte que la teneur en Sr dans l'alliage Al-Sn-Sr est de S00 ppm en poids. Ces poudres sont agglomérées avec un liant de type acrylique (méthacrylate de n-butyle dissout dans un éther de glycol), et éventuellement des frittes de verre, pour former une pâte adaptée à la sérigraphie. An alloy containing 60% by weight of Al and 40% by weight of Sn is produced by mixing powders of micron size (Dso of between 2 and 10 μm). Sr is added in the form of powders obtained by grinding a parent alloy Al-5% by weight of Sr so that the Sr content in the Al-Sn-Sr alloy is 500 ppm by weight. These powders are agglomerated with a binder of acrylic type (n-butyl methacrylate dissolved in a glycol ether), and possibly glass frits, to form a paste suitable for screen printing.
Cette pâte est déposée sur un substrat bas coût de type Si métallurgique purifié par ségrégation.  This paste is deposited on a low cost metallurgical Si substrate purified by segregation.
L'ensemble est introduit dans un four à passage pour atteindre une température maximale, de 700 °C, ce qui conduit à dissoudre une partie du Si du substrat pour assurer l'équilibre thermodynamique.  The assembly is introduced into a passage oven to reach a maximum temperature of 700 ° C, which leads to dissolve a portion of the Si substrate to ensure thermodynamic equilibrium.
La première structure déposée au cours du refroidissement est monophasée et croît par épitaxie sur le Si du substrat.  The first structure deposited during the cooling is single-phase and grows by epitaxy on the Si of the substrate.
Puis, la température de l'eutectique biphasé atteinte, une structure constituée de fibres comprenant majoritairement du silicium, dans une matrice comprenant majoritairement de l'aluminium est obtenue.  Then, the temperature of the biphasic eutectic reached, a structure consisting of fibers comprising predominantly silicon, in a matrix comprising predominantly aluminum is obtained.
Enfin, quand la température de l'eutectique ternaire invariant est atteinte, une structure eutectique ternaire de composition moyenne riche en Sn se forme.  Finally, when the temperature of the invariant ternary eutectic is reached, a ternary eutectic structure of medium Sn-rich composition is formed.
L'ensemble resolidifié est soumis à décapage chimique (par exemple à l'acide ortho-phosphorique) pour ne garder que le Si. Le substrat se présente sous la forme d'un tapis de fakir constitué d'aiguilles de Si d'une hauteur voisine de 5 μm avec un espacement de l'ordre de 1,2 μm.  The resolidified assembly is subjected to chemical etching (for example orthophosphoric acid) to keep only the Si. The substrate is in the form of a carpet of fakir consisting of Si needles from a height close to 5 microns with a spacing of the order of 1.2 microns.
Références References
(i) F. Huster, 20th Européen Photovoltaic Solar Energy Conférence and Exhibition, Barcelone, 6-10 June 2005, 2DV2.49 ;  (i) F. Huster, 20th European Photovoltaic Solar Energy Conference and Exhibition, Barcelona, 6-10 June 2005, 2DV2.49;
(ii) S. Reber, A. Hurrle, A. Eyer, G. Wilke, "Crystalline silicon thin film solar cells - récent results at Fraunhofer ISE", Solar Energy, 77 (2004) 865-875 ;  (ii) S. Reber, A. Hurrle, A. Eyer, G. Wilke, "Crystalline silicon thin film solar cells - recent results at Fraunhofer ISE", Solar Energy, 77 (2004) 865-875;
(iii) M. Aoucher, G. Farhi, T. Mohatnmed-Brahirn, J. Non-Crystalline Solids, 227-230 (1998) 958 ;  (iii) M. Aoucher, G. Farhi, T. Mohatnmed-Brahirn, J. Non-Crystalline Solids, 227-230 (1998) 958;
(iv) M.M. Makhtouf, H.V Guthy, Journal of Light Metals 1 (2001) 199-218 ; (iv) M. M. Makhtouf, H. V. Guthy, Journal of Light Metals 1 (2001) 199-218;
(v) J.R. Davis, Jr et al., « Impurities in silicon solar cells », IEEE transactions on Electron Devices 27 (1980) 677-687. (v) J. R. Davis, Jr et al., "Impurities in silicon solar cells," IEEE Transactions on Electron Devices 27 (1980) 677-687.

Claims

REVENDICATIONS
1. Procédé de formation, en surface d'une face d'un substrat de silicium, d'une couche fibreuse (22) présentant un pas moyen de réseau inférieur ou égal à 2 μm, comprenant au moins les étapes consistant à : A method of forming, on the surface of one side of a silicon substrate, a fibrous layer (22) having an average network pitch of less than or equal to 2 μm, comprising at least the steps of:
(1) disposer d'un substrat de silicium dont l'une des faces est recouverte au moins en partie d'un mélange comprenant au moins de l'aluminium et au moins un élément modificateur choisi parmi les éléments des colonnes IA et ΠΑ du tableau périodique, et  (1) having a silicon substrate, one of the faces of which is covered at least in part with a mixture comprising at least aluminum and at least one modifying element chosen from the elements of columns IA and ΠΑ of the table periodic, and
(2) exposer au moins la face enduite dudit substrat de l'étape (1) à un traitement thermique propice à (a) la formation d'un alliage fondu comprenant le silicium, l'aluminium et lesdits éléments modificateurs, et à (b) la solidification consécutive dudit alliage fondu dans des conditions propices à la formation d'au moins une couche (22) présentant une structure eutectique biphasée constituée de fibres à base de silicium dans une matrice à base d'aluminium, avec un pas moyen de réseau inférieur ou égal à 2 μm, caractérisé en ce que ledit mélange de l'étape (1) comprend en outre de 20 à 60 % en poids, par rapport à son poids total, d'un ou plusieurs éléments d'addition choisis parmi le gallium, l'indium, l'étain, le zinc et leurs mélanges.  (2) exposing at least the coated side of said substrate of step (1) to a heat treatment conducive to (a) forming a molten alloy comprising silicon, aluminum and said modifying elements, and (b) ) the subsequent solidification of said molten alloy under conditions conducive to the formation of at least one layer (22) having a two-phase eutectic structure made of silicon-based fibers in an aluminum-based matrix, with an average network pitch less than or equal to 2 μm, characterized in that said mixture of step (1) further comprises from 20 to 60% by weight, relative to its total weight, of one or more addition elements chosen from gallium, indium, tin, zinc and their mixtures.
2. Procédé selon la revendication 1, caractérisé en ce que ladite couche fibreuse (22) présente un pas moyen allant de 0,5 à 1 ,5 μm.  2. Method according to claim 1, characterized in that said fibrous layer (22) has a mean pitch ranging from 0.5 to 1, 5 microns.
3. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que ladite couche fibreuse (22) présente une épaisseur allant de 1 à 20 μm, de préférence de S à 10 μm.  3. Method according to any one of the preceding claims, characterized in that said fibrous layer (22) has a thickness ranging from 1 to 20 microns, preferably from 5 to 10 microns.
4. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'(les) éléments) d'addition est(sont) présents) dans ledit mélange de l'étape (1) en une teneur allant de 35 à 45 % en poids, par rapport au poids total dudit mélange.  4. Method according to any one of the preceding claims, characterized in that the (the) elements) of addition is (are) present in said mixture of step (1) in a content ranging from 35 to 45 % by weight, based on the total weight of said mixture.
5. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'élément modificateur est choisi parmi le strontium, le sodium et leur mélange.  5. Method according to any one of the preceding claims, characterized in that the modifying element is selected from strontium, sodium and their mixture.
6. Procédé selon l'une quelconque des revendications, caractérisé en ce que l'(les) éléments) modificateur(s) est(sont) présents) dans le mélange de l'étape (1) en une teneur allant de 0,01 à 0,1 % en poids, par rapport au poids total dudit mélange, en particulier de 0,02 à 0,06 % en poids. 6. Process according to any one of the claims, characterized in that the modifying element (s) is (are) present in the mixture of step (1) in a content ranging from 0.01. at 0.1% by weight, relative to the total weight of said mixture, in particular from 0.02 to 0.06% by weight.
7. Procédé selon l'une quelconque des revendications précédentes; caractérisé en ce que ledit mélange de l'étape (1) est sous forme d'une poudre, présentant une granulométrie DSO exprimée en volume allant de 2 à 10 μm. 7. Method according to any one of the preceding claims; characterized in that said mixture of step (1) is in the form of a powder, having a DSO particle size in volume ranging from 2 to 10 μm.
8. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que ledit mélange de l'étape (1) comprend en outre au moins un liant, en particulier choisi parmi les résines, et plus particulièrement choisi parmi les résines cellulosiques et les résines acryliques.  8. Method according to any one of the preceding claims, characterized in that said mixture of step (1) further comprises at least one binder, in particular chosen from resins, and more particularly chosen from cellulose resins and resins. acrylic resins.
9. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que le mélange de l'étape (1) comprend en outre des frittes de verre.  9. Method according to any one of the preceding claims, characterized in that the mixture of step (1) further comprises glass frits.
10. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'alliage fondu en étape (2) est formé par exposition dudit substrat de l'étape (1) à une température allant de 600 °C à 850 °C, de préférence de 700 °C à 750 °C.  The process according to any one of the preceding claims, characterized in that the molten alloy in step (2) is formed by exposing said substrate of step (1) to a temperature ranging from 600 ° C to 850 ° C. preferably 700 ° C to 750 ° C.
11. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'étape de solidification (b) de l'alliage fondu en étape (2) d'effectué à une vitesse de refroidissement allant de 5 à 50 °C/s.  11. Process according to any one of the preceding claims, characterized in that the step of solidification (b) of the molten alloy in step (2) is carried out at a cooling rate ranging from 5 to 50 ° C / s.
12. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'étape (2) conduit à la formation d'une couche intennédiaire (21) entre ladite couche fibreuse (22) et ledit substrat de silicium (20), de structure monophasée et comprenant majoritairement du silicium.  12. A method according to any one of the preceding claims, characterized in that step (2) leads to the formation of an intermediate layer (21) between said fibrous layer (22) and said silicon substrate (20). of single-phase structure and mainly comprising silicon.
13. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que le substrat de silicium est une plaque de silicium de type p, comportant au moins une jonction p-n sur son autre face, et ayant éventuellement été préalablement soumis à un traitement anti-reflets.  13. Method according to any one of the preceding claims, characterized in that the silicon substrate is a p-type silicon wafer, comprising at least one pn junction on its other face, and having optionally been previously subjected to anti-oxidation treatment. -reflets.
14. Procédé selon l'une quelconque des revendications 1 à 12, caractérisé en ce qu'il comprend en outre une étape (3) comprenant l'élimination de la ou des couche(s) eutectique(s) (23) à au moins trois phases formée(s) à l'issue de l'étape (2) et l'éliinination de la matrice d'aluminium de la couche fibreuse (22).  14. Method according to any one of claims 1 to 12, characterized in that it further comprises a step (3) comprising the elimination of the eutectic layer (s) (s) (23) to at least three phases formed at the end of step (2) and the elinination of the aluminum matrix of the fibrous layer (22).
15. Procédé selon la revendication précédente, caractérisé en ce que l'étape (3) consiste en un décapage chimique du produit obtenu à l'issu de l'étape (2), plus particulièrement à l'aide de l'acide ortho-phosphorique. 15. Method according to the preceding claim, characterized in that step (3) consists of a chemical etching of the product obtained at the end of step (2), more particularly with the aid of orthoacidic acid. phosphoric.
16. Procédé selon l'une quelconque des revendications 14 et 15, caractérisé en ce que le substrat de silicium est un substrat de type silicium métallurgique purifié par ségrégation. 16. Method according to any one of claims 14 and 15, characterized in that the silicon substrate is a metallurgical silicon substrate purified by segregation.
EP11794862.0A 2010-11-24 2011-11-21 Method for forming a fibrous layer Withdrawn EP2643855A1 (en)

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