Recyclable silicon wafer and recycling method

Abstract

The invention relates to a method for recycling coated silicon wafers (1) having a heterojunction layer structure (2a, 2b) and to a method for producing recyclable coated silicon wafers having a heterojunction layer structure.

Description

[0001] RECYCLABLE SILICON WAFER AND RECYCLING PROCESS

[0002] INTRODUCTION

[0003] The invention relates to a method for recycling coated silicon wafers and a method for producing recyclable coated silicon wafers. The method according to the invention opens up the possibility of selectively removing coatings from silicon wafers without damaging the wafer and thereby reclaiming the coating agent.

[0004] BACKGROUND & STATE OF THE ART

[0005] There is no approach to circular recycling for silicon solar cells commonly used today. This is because these solar cells are manufactured using material combinations and methods that make subsequent separation extremely complex.

[0006] There are essentially two major types of solar cells: diffused junction solar cells and heterojunction solar cells.

[0007] In solar cells with a diffused semiconductor junction, another material is diffused into a silicon wafer in traces (PERC structure, or "passivated emitter and rear cell" structure). These materials must be considered contaminants if the silicon wafer is to be recycled and used for another purpose. The difficulty, however, is that the diffusion process is irreversible. The removal of the diffused materials requires the use of time-consuming processes that inevitably lead to material loss and damage to both the silicon wafer and the diffused material (SW Glunz, R. Preu, D. Biro in "Comprehensive Renewable Energy", Vol. 1 ISBN: 978-0-08-087873-7, 2012 by Elsevier doi:10.1016 / B978-0-08-087872-0.00117-7).

[0008] In silicon solar cells with a heterojunction structure, or solar cells manufactured using the layered stacking model (heterojunction with intrinsic thin layer "HIT," tunnel oxide passivated contact, "TOPCON"), no impurities diffuse into the silicon wafer. However, other materials are deposited in layers on the wafer surface in such a way that removing these materials without damaging the silicon wafer also requires complex, time-consuming, and costly processes. For this reason, such processes are not used in practice.

[0009] State-of-the-art silicon solar cells cannot currently be recycled at an economically viable cost without damaging the silicon wafer to the point where it cannot be reused in a new solar cell. Therefore, there is no way to economically recycle silicon solar cells. Currently, various approaches to recycling solar cells and solar modules are still in the development phase. In a currently widely used process, the discarded solar modules are shredded and their polymer components are subsequently incinerated. However, contaminants still remain on the silicon fragments. Therefore, recycled silicon from solar cells can currently only be used for metallurgical processes.

[0010] US 2014 / 0202517 A describes an organic solar cell with an alternative structure that completely dispenses with silicon or glass. This organic system can be recycled under relatively gentle conditions.

[0011] Publication CN 111599922 B describes a process for coating materials with a PEDOT:PSS thin film. Solvents and solvent mixtures, as well as temperature ranges and application methods that are advantageous for PEDOT:PSS coatings, are listed there.

[0012] Document CN 109226066 A describes a method for removing an ethylene vinyl acetate (EVA) coating. The solar cell unit, still coated with EVA, is subjected to ultrasonic treatment in an organic solvent at temperatures between 20 °C and 80 °C.

[0013] JP 2004-042 033 A describes a process for recycling silicon wafers and / or tempered glass from solar cells. In this process, a solar cell to be recycled is immersed in nitric acid heated to at least 50 °C. This method can be used to remove ethylene vinyl acetate (EVA) coatings from the glass / silicon body. The process is not suitable for removing active layers from a solar cell constructed in a layer stack. The process has the disadvantage that it requires very harsh etching conditions, which can also remove the wafer material.

[0014] TASK

[0015] The object to be achieved with the present invention was to develop a process which makes it possible to recycle coated silicon wafers which are present in a heterojunction layer structure, as is common in solar cells, without damaging the wafers.

[0016] The inventors of the present invention have surprisingly succeeded in developing a process for producing coated silicon wafers with a heterojunction layer structure, the coatings of which can be removed again using a simple recycling process without compromising the functionality and reuse of the recovered silicon wafers. By using suitable coating and functionalization materials, such coated silicon wafers with a heterojunction layer structure can be produced in which the individual coatings can be separated quickly and selectively by successively removing the coatings with suitable solvents.

[0017] DESCRIPTION OF THE INVENTION

[0018] [1] A method for the non-destructive recycling of a silicon wafer (1) which is coated on both sides of the surfaces (1a, 1b) with one or more identical or different surface coatings (2a, 2b) in the form of a heterojunction layer structure which can be selectively removed by a solvent, wherein the surface coatings (2a, 2b) are selectively removed with a suitable solvent or solvent mixture without damaging the silicon wafer (1).

[0019] [2] Method according to [1], wherein the solvent or solvent mixture for removing the surface coatings (2a, 2b) is also suitable for applying the removable surface coatings (2a, 2b) from a solution of the coating material in the solvent or solvent mixture to the surfaces (1a, 1b) of a silicon wafer (1).

[0020] [3] Process according to [1] to [2], wherein the solvent or solvent mixture is selected according to the solubility properties of the surface coatings (2a, 2b).

[0021] [4] Process according to [1] to [3], wherein the solvent or the solvent mixture is selected from one or more linear, branched, cyclic, heterocyclic, aromatic, heteroaromatic, saturated or unsaturated hydrocarbons selected from: unsubstituted hydrocarbons, halogenated hydrocarbons, alcohols, ethers, aldehydes, ketones, carboxylic acids, carboxylic acid esters, carboxylic acid anhydrides, carboxylic acid halides, thiols, thioethers, thioaldehydes, thioketones, thiocarboxylic acids, thioesters, amines, amides, nitriles, imines, oximes, hydrazines, hydrazones, sulfoxides, sulfinic acids, sulfinic acid esters, sulfonic acids, sulfonic acid esters, sulfonic acid halides and / or sulfonamides and wherein solvent mixtures may also contain water.

[0022] [5] Process according to [1] to [4], wherein the solvent or solvent mixture contains one or more additional molecular substituents in the linear, branched, cyclic, heterocyclic, aromatic, heteroaromatic, saturated or unsaturated molecular skeleton.

[0023] [6] Process according to [1] to [5], wherein the solvent or the solvent mixture is selected from water and / or one or more linear, branched, cyclic, heterocyclic, aromatic, heteroaromatic, saturated or unsaturated hydrocarbons, which are selected from: water and aqueous solutions, unsubstituted hydrocarbons such as in particular aromatics and heteroaromatics, halogenated hydrocarbons such as in particular chlorinated aromatics and heteroaromatics, alcohols such as in particular ethanol, ethers, aldehydes, ketones such as in particular GBL, amides such as in particular DMF, sulfoxides such as in particular DMSO, carboxylic acids, carboxylic acid esters and carboxylic acid anhydrides.

[0024] [7] Process according to [1] to [6], wherein one or more additives are added to the solvent or solvent mixture.

[0025] [8] Process according to [7], wherein one or more further additives are added to the solvent or solvent mixture, which increase the solubility of the surface coatings (2a, 2b) in the solvent or solvent mixture and / or chelate metals in the surface coating and / or enable the phase transition between solvent mixtures with different phases.

[0026] [9] Process according to [7] or [8], wherein one or more additives are added to the solvent or solvent mixture to increase the solubility of the surface coatings (2a, 2b), wherein the additives can preferably adjust a pH in the range between about 4 and about 10.

[0027]

[0010] Method according to [1] to [9], wherein the coated silicon wafer (1) is treated with a first solvent or solvent mixture (A) so that the surface coating (2a) is removed and is treated with a second, identical or different, solvent or solvent mixture (B) so that the surface coating (2b) is removed.

[0028]

[0011] Method according to [1] to

[0010] , wherein the treatment with the solvent or solvent mixture (A, B) comprises the targeted application of the solvent or solvent mixture to the surface coating (2a, 2b) or the single or multiple immersion of the coated silicon wafer (1) in one or more different solvents or solvent mixtures.

[0029]

[0012] Method according to [1] to

[0011] , wherein the immersion of the coated silicon wafer (1) is carried out automatically by guiding one or more coated silicon wafers by means of an ascending conveyor along a predetermined path through a basin filled with a suitable solvent or solvent mixture (A, B).

[0030]

[0013] Method according to [1] to

[0012] , wherein the targeted application of the solvent or solvent mixture comprises spraying and dripping.

[0014] Method according to [1] to

[0013] , wherein the solvent or solvent mixture is heated during the immersion of the coated silicon wafer (1).

[0031]

[0015] Method according to [1] to

[0014] , wherein the solvent or the solvent mixture is sonicated during the immersion of the coated silicon wafer (1) in an ultrasonic bath.

[0032]

[0016] Process according to [1] to

[0015] , wherein the surface coatings (2a, 2b) detached in the solvent or solvent mixture are recovered from the solution.

[0033]

[0017] Process according to [1] to

[0016] , wherein the surface coatings (2a, 2b) detached in the solvent or solvent mixture are recovered by phase extraction processes, distillation processes, crystallization processes or by mixed recovery steps.

[0034]

[0018] Method according to [1] to

[0017] , comprising the steps: i. providing a coated silicon wafer (1) with a heterojunction layer structure with surface coatings (2a, 2b); and ii. providing one or more solvents and / or solvent mixtures (A, B) suitable for selectively removing the surface coatings (2a, 2b); iii. optionally adding one or more additives to the solvents and / or solvent mixtures (A, B); and iv. selectively applying solvent or solvent mixture (A) to remove the surface coating (2a) onto the surface coating (2a); or v. immersing the silicon wafer (1) in the solvent or solvent mixture (A) to remove the surface coating (2a) so that at least the surface coating (2a) is completely covered by the solvent or solvent mixture (A); vi. optionallyHeating and / or sonicating the solvent or solvent mixture; and vii. Removing the solvent or solvent mixture in which the detached surface coating (2a) is dissolved; and viii. Repeating steps iv. to vii. with a solvent or solvent mixture (B) to remove the surface coating (2b); ix. If necessary, repeating steps iv. to vii. with further solvents or solvent mixtures to remove any additional coatings that may be present; and x. Recovering the detached materials of the surface coatings from the solvents or solvent mixtures by one or more recovery steps and recovering the uncoated silicon wafer (1).

[0035]

[0019] Method according to

[0018] , wherein the immersion of the silicon wafer (1) with a

[0036] Heterojunction layer structure with surface coatings (2a, 2b) into the solvent or solvent mixture (A, B) for detaching the

[0037] Surface coating (2a, 2b) with complete coverage of the

[0038] Surface coating (2a, 2b) by the solvent or solvent mixture (A, B) is carried out automatically along an ascending conveyor.

[0039]

[0020] A method for producing a recyclable silicon wafer (1) having a heterojunction layer structure with surface coatings (2a, 2b), wherein a silicon wafer is coated on both sides of the surfaces (1a, 1b) with at least one surface coating (2a, 2b), wherein the surface coating (2a, 2b) is characterized in that it is selectively removable by a suitable solvent without damaging the silicon wafer (1).

[0040]

[0021] Method according to

[0020] , wherein the surface coatings (2a, 2b)

[0041] Coating materials from the group comprising organic, inorganic and / or hybrid semiconductors, organic molecules which can form a self-organizing monolayer, metal oxide compounds, organometallic compounds, salt compounds, inorganic acids, silane compounds and / or mixtures thereof.

[0042]

[0022] Method according to

[0020] to

[0021] , wherein the surface coatings (2a, 2b) are doped.

[0043]

[0023] Method according to

[0022] , wherein the dopants are n- and / or p-dopants.

[0044]

[0024] Process according to

[0020] to

[0023] , wherein the surface coatings (2a, 2b) comprise mixed phases such as alloys, copolymers, salts, cocrystals and / or other mixed phases of the coating materials contained therein.

[0045]

[0025] Process according to

[0021] to

[0024] , wherein organic semiconductors are selected from PM6, PTQ10, D18, PEDOT, P3HT, spiro-MeOTAD, Y12, L8BO, C60, PCBM, COFs and mixtures thereof, preferably selected from PEDOT, PCBM, Y12, COFs and L8BO.

[0026] Process according to

[0021] to

[0025] , wherein inorganic semiconductors are selected from SiO x AIO X , NiO x , Cul, CuSCN, MoO x , SnO x , ZnO, TiCh and mixtures thereof.

[0046]

[0027] Process according to

[0021] to

[0026] , wherein the hybrid semiconductors are selected from metal halide perovskites and mixtures thereof.

[0047]

[0028] Process according to

[0021] to

[0027] , wherein organic molecules capable of forming a self-assembling monolayer are selected from ionomers and mixtures thereof, preferably from Me-4PACz.

[0048]

[0029] Process according to

[0021] to

[0028] , wherein organometallic compounds are selected from Grignard reagents, metal-organic frameworks (MOFS) and mixtures thereof, preferably from Zn-based MOFS.

[0049]

[0030] Process according to

[0021] to

[0029] , wherein salt compounds are selected from halide salts, ammonium salts, sulfate salts, phosphate salts, metal salts, semimetal salts and mixtures thereof.

[0050]

[0031] Process according to

[0021] to

[0030] , wherein inorganic acids are selected from phosphorus-containing Lewis and Bronsted acids, sulfur-containing Lewis and Bronsted acids, nitrogen-containing Lewis and Bronsted acids, halogen-containing Lewis and Bronsted acids, metal-containing Lewis and Bronsted acids and semi-metal-containing Lewis and Bronsted acids.

[0051]

[0032] Process according to

[0021] to

[0031] , wherein silane compounds are selected from halosilanes and mixtures thereof.

[0052]

[0033] Process according to

[0025] , wherein derivatives of the organic semiconductors are selected which are provided with additional molecular substituents which increase their solubility in a suitable solvent, preferably organic semiconductor derivatives end-capped with chlorosilanes.

[0053]

[0034] Process according to

[0020] to

[0033] , wherein one or more surface coatings are carried out by simultaneously applying one or more coating materials from a liquid phase or from the gas phase.

[0054]

[0035] Method according to

[0020] to

[0034] , wherein several surface coatings are applied on each side of the silicon wafer in different coating steps from a liquid and / or gas phase.

[0055]

[0036] Process according to

[0020] to

[0035] , wherein at least one coating step is carried out from a liquid phase by means of a solution, suspension, dispersion, or emulsion of one or more coating materials.

[0037] Process according to

[0036] , wherein at least one coating step is carried out from a liquid phase by means of inkjet printing, spray coating, dip coating, solution-based atomic layer deposition (ALD), sol / gel processes, or by the roll-to-roll printing process.

[0056]

[0038] Method according to

[0020] to

[0035] , wherein at least one coating step is carried out by a vapor deposition by means of atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), and organic hybrid vapor phase infiltration.

[0057]

[0039] Method according to

[0020] to

[0038] , wherein the surface coating (2a) and the surface coating (2b) are different.

[0058]

[0040] Method according to

[0039] , wherein one of the surface coatings (2a, 2b) is n-doped and the other surface coating is p-doped.

[0059]

[0041] Method according to

[0020] to

[0040] , wherein the surfaces (1a, 1b) of the silicon wafer (1) are each passivated before the application of the surface coatings (2a, 2b).

[0060]

[0042] Method according to

[0041] , wherein the passivation is carried out by applying a passivation layer (3a, 3b), wherein a passivation layer (3a, 3b) is preferably formed from an ionomer.

[0061]

[0043] Method according to

[0041] , wherein the passivation is integrated into the surfaces (1a, 1b) of the silicon wafer (1).

[0062]

[0044] Method according to

[0041] , wherein the passivation layer (3a, 3b) is formed as a passivating functional layer (4a, 4b) which combines the properties of the passivation layer (3a, 3b) and the surface coating (2a, 2b).

[0063]

[0045] Recyclable coated silicon wafer with heterojunction layer structure obtainable by the process according to

[0020] to

[0044] ,

[0064]

[0046] A solar cell equipped with one or more coated silicon wafers with heterojunction layer structure produced by the method according to

[0020] to

[0044] ,

[0065]

[0047] Use of the coated silicon wafer with a heterojunction layer structure produced by the process according to

[0020] to

[0044] for producing a recyclable solar cell.

[0048] Use of the process according to [1] to

[0019] for recycling a coated silicon wafer with a heterojunction layer structure obtainable by the process according to

[0020] to

[0044] .

[0066] The invention is described in detail below.

[0067] DETAILED DESCRIPTION OF THE INVENTION

[0068] I. Process for recycling a surface-coated silicon wafer

[0069] The invention described herein relates to a method for the non-destructive recycling of a silicon wafer (1) coated on both sides, in which both the silicon wafer itself and the coating materials forming the surface coatings can be recovered in such a way that the wafer is not damaged.

[0070] For the purposes of the present invention, the term "non-destructive" refers to the fact that the removal of the layered coatings takes place exclusively under conditions that do not damage the silicon wafer. This means, in particular, that mild pH conditions are chosen and no species are used that could attack a silicon surface.

[0071] The method according to the invention is therefore directed to coated silicon wafers (1) which are coated on both sides on the surfaces (1a, 1b) with one or more identical or different surface coatings (2a, 2b) in the form of a heterojunction layer structure, which can be primarily characterized in that they can be selectively removed with a suitable solvent without damaging the silicon body of the previously coated silicon wafer (1).

[0072] In relation to solar cells, a so-called heterojunction layer structure describes systems in which the surface coatings (2a, 2b) applied to the surfaces (1a, 1b) form a coating that is materially separate from the silicon wafer. In contrast to the solar cells frequently implemented in the prior art, the coating material does not diffuse into the silicon body, but rather deposits exclusively on the surface. Such a heterojunction layer structure is crucial for the recycling process according to the invention because it enables the removal (and application) of coating materials without the need for special precautions that would allow the removal of coating materials from a silicon matrix without damaging the silicon matrix.

[0073] For the heterojunction layer structure, it is further necessary that the layers (2a, 2b) applied to the solar cells or, where appropriate, passivating functional layers (4a, 4b) fulfill an electron or hole extraction function, i.e., are actively important for the functioning of the solar cell. In known solar cells, as frequently mentioned in the prior art, coatings with an ethylene-vinyl acetate copolymer (EVA) are described. However, such coatings merely serve to protect the silicon wafer from environmental influences and are not functional layers (2a, 2b) or (4a, 4b) within the meaning of the present invention.

[0074] In the sense of the method according to the invention, the material used to remove the surface coatings (2a, 2b) or, if appropriate, the passivating

[0075] Solvents suitable for functional layers (4a, 4b) are particularly also suitable for applying the coating materials to the surfaces (1a, 1b) of a silicon wafer. This means that, for example, a coating material AB can be dissolved in a solvent AL and the solution can be used to provide a silicon wafer (1), for example on the surface (1a), with a surface coating (2a). It is then possible to expose the coated silicon wafer (1) with the surface coating (2a) to the solvent AL, wherein the coating material AB is dissolved in the solvent AL and the uncoated, undamaged silicon wafer (1) is obtained.

[0076] In the sense of the method according to the invention, the solvent used for selectively removing the surface coatings (2a, 2b) is selected on the basis of its properties which enable the existing surface coatings to be selectively removed from the silicon body of the silicon wafer (1) without damaging the silicon.

[0077] The appropriate solvent is selected based on its solubility properties compared to the solids forming the surface coating. Numerous materials are suitable for forming a surface coating for a solar cell and thus influencing its electronic properties. Factors that play a role include the polarity of the solvent and the solid to be dissolved in it, and the often related ability of the solvent molecules to enter into weak, intermolecular interactions with those of the dissolved solid. If such a state of interaction between a dissolved solid molecule and the solvent molecules in the randomly distributed, disordered environment of a solution is energetically more favorable for the solid than remaining in the ordered (crystalline) or disordered (amorphous) solid phase, the solid passes into the solution.Therefore, another factor in selecting a suitable solvent for dissolving a solid forming the surface coating or the coating, in addition to the molecular identity of the solid to be dissolved, is its solid phase modification, such as whether it is crystalline or amorphous or in which of its possibly numerous crystalline modifications it exists.

[0078] For the purposes of the invention, depending on the surface coating, solvents or solvent mixtures can be used which can remove the most diverse coating materials forming the surface coatings of a solar cell. Examples include solvents from the groups of: linear, branched, cyclic, heterocyclic, aromatic, heteroaromatic, saturated or unsaturated hydrocarbons, for example those selected from: unsubstituted hydrocarbons, halogenated hydrocarbons, alcohols, ethers, aldehydes, ketones, carboxylic acids, carboxylic acid esters, carboxylic acid anhydrides, carboxylic acid halides, thiols, thioethers, thioaldehydes, thioketones, thiocarboxylic acids, thioesters, amines, amides, nitriles, imines, oximes, hydrazines, hydrazones, sulfoxides, sulfinic acids, sulfinic acid esters, sulfonic acids, sulfonic acid esters, sulfonic acid halides and / or sulfonamides, and mixtures thereof.In the case of solvent mixtures, water is also a preferred component.

[0079] For the purposes of the invention, it is particularly preferred if the selected solvents or solvent mixtures are selected from common, frequently used solvents that are familiar to the person skilled in the art. Such solvents are usually easily obtainable even in larger quantities, which can be considered advantageous for continuously carried out recycling processes. Therefore, preferred solvents or solvent mixtures according to the invention are selected from water and aqueous solutions as well as the group of linear, branched, cyclic, heterocyclic, aromatic, heteroaromatic, saturated or unsaturated hydrocarbons, which are more preferably selected from: unsubstituted hydrocarbons, halogenated hydrocarbons, alcohols, ethers, aldehydes, ketones, carboxylic acids, carboxylic acid esters and carboxylic acid anhydrides. In the case of solvent mixtures, solvent mixtures comprising water are also particularly preferred.

[0080] For the purposes of the present invention, aqueous solutions, as distinguished from water, refer to solutions in which additives, such as acids or bases, chelating agents, or solubilizers, are dissolved in water. Solvent mixtures with water refer to solvents from the group described above that are mixed with water.

[0081] One way to influence the solubility properties of the selected solvents or solvent mixtures so that the surface coatings can be removed from the silicon substrate surfaces without leaving residue is to modify the solvents with molecular substituents. Such molecular substituents can be selected from the group comprising halogens, a hydroxy group, an ether group, a ketone group, an aldehyde group, a carboxylic acid group, a carboxylic acid ester group, a carboxylic acid anhydride group, a carboxylic acid halide group, a thiol group, a thioether group, a thioaldehyde group, a thioketone group, a thiocarboxylic acid group, a thioester group, an amine group, an amide group, a nitrile group, an imine group, an oxime group, a hydrazine group, a hydrazone group, a sulfoxide group, a sulfinic acid group, a sulfinic acid ester group, a sulfinic acid halide group, and a sulfonamide group.The selection of the solvent for the process according to the invention depends primarily on the identity of the coating materials for the surface coatings (2a, 2b). For the purposes of the process according to the invention, water and aqueous solutions, unsubstituted aromatics and heteroaromatics, aromatics and heteroaromatics substituted with halogens, especially chlorine, alcohols such as ethanol, ketones such as γ-butyrolactate (GBL), amides such as DMF, and sulfoxides such as DMSO have proven to be particularly preferred solvents for the non-destructive recycling according to the invention. These solvents have in common that they are excellently suited to removing coating agents suitable for heterojunction layer structures. At the same time, they exhibit no damaging effects on silicon wafers.

[0082] A further possibility according to the invention to influence the solubility properties of the selected solvents or solvent mixtures in such a way that the surface coatings can be removed from the silicon carrier surfaces without leaving residues is to add one or more further additives to the solvent or solvent mixture.

[0083] For the purposes of the invention, one or more additives can be solid or liquid substances that are added to the solvent or solvent mixture. Even in the liquid phase, the neutral or ionized molecules or atoms that make up the sole or majority of a solution are in constant weak interaction with one another. The introduction of an additive can influence the nature of these weak interactions, which can increase the solubility of a solid to be dissolved in the solvent or solvent mixture to which one or more additives have been added. A typical example of such an effect is the variation of the pH value by adding an acidic or basic species to a solvent. The resulting protonation or deprotonation of solvent molecules or of molecules of a solid to be dissolved can, for example,increase the strength of possible attractive intermolecular interactions between profaned or deprotonated functional groups of solid molecules and any deprotonated or profanated functional groups of solvent molecules. The favored solid / solvent interactions may thus potentially improve the solubility of the solid in the solvent. Acidic and / or basic additives must not have a detrimental effect on the molecular integrity of the solid for the surface coatings and the silicon support surfaces (1a, 1b). Additives that are too strongly basic or too strongly acidic, which could attack the dissolved molecules, are unsuitable according to the invention. Strongly acidic / basic additives, which attack the surface of the silicon support, are also unsuitable. Of course, other types of additives that have a positive effect on the solubility properties can also be used.

[0084] For the purposes of the invention, it is preferred to add acidic or basic additives to the solvent or solvent mixture for use in the process according to the invention which enable the pH of the solution to be adjusted in the range between approximately 4 and approximately 10. For the purposes of the present invention, this means that either a mildly acidic or a mildly basic pH is adjusted, for example a pH of approximately 4 to approximately 6 or a pH of approximately 8 to approximately 10. The term "approximately" indicates that the values ​​are not to be understood as fixed limits, but rather indicate a range which, for the person skilled in the art, falls under the meaning of "mildly acidic" or "mildly basic". pH values ​​below or above these values, which would still be easily identified by the person skilled in the art as "mildly acidic" or "mildly basic", are also encompassed by the term.

[0085] It is further preferred according to the invention not to use any acidic or basic additives which have a damaging effect on the silicon wafer, such as strong hydroxy bases such as NaOH or KOH.

[0086] For the purposes of the invention, in connection with solvent mixtures, particular preference is given to additives that enable a phase transition between the different phases that may be present, so-called solubilizers. Such systems can exert attractive interactions via intermolecular interactions with molecules of a solid to be dissolved that forms the surface coating. At the same time, these solubilizers are highly soluble in the solvent or solvent mixture. This is often achieved by equipping larger organic framework molecules with functional groups. Due to the larger molecular framework, these molecules are rather non-polar but extremely polar at the functional groups. In this way, the solid to be dissolved is absorbed by the solubilizer and transported and released from the phase in which the solid is less soluble to the phase in which the solid is more soluble.Due to its external polarity, the additive is highly soluble in the phase in which the solid is less soluble. Due to the transport of the dissolved solid from the low-solubility phase to the more soluble phase, saturation does not occur in the low-solubility phase, allowing more solid to be dissolved overall. Such a system can be advantageous if the solid forming the surface coating is itself a mixture of substances, particularly if the species used in the mixture are soluble in different solvents. Surface coatings on silicon wafers can often be composed of such composite materials to positively influence the electronic properties of the silicon wafer surface.

[0087] For the purposes of the present invention, additives that are particularly well suited to chelating metals are also particularly preferred. For example, if a surface coating (2a, 2b) contains organometallic compounds, a chelating agent can remove the metal center from the bond, thereby allowing the coating to be re-dissolved.

[0088] The method according to the invention is carried out in that one or more, optionally identical or different solvents or solvent mixtures which can remove the surface coatings of a silicon wafer (1) are brought into contact with the surface coatings, so that the surface coatings are then selectively removed. For example, a solvent or solvent mixture (A) is brought into contact with a surface coating (2a), whereby this surface coating is removed. In a further step, a solvent or solvent mixture (B) is brought into contact with a further surface coating (2b), whereby this surface coating is also removed. In principle, however, it is also possible for the solvent or solvent mixture to be the same for several surface coatings (e.g. 2a, 2b).Particularly in the case of different surface coatings, it is preferred if the solvent or solvent mixture (A) dissolves exclusively one surface coating (2a), while another different solvent or solvent mixture (B) dissolves another different surface coating (2b). Thus, the various dissolved coating materials can be recovered from the different solvents or solvent mixtures without contamination.

[0089] Contacting the surface coatings with the solvent or solvent mixture can be achieved in various ways. For example, the solvent or solvent mixture can be applied specifically to the surface coatings, or the entire coated silicon wafer (1) can be immersed in the respective solvent or solvent mixture.

[0090] The method according to the invention can also be carried out automatically. One or more coated silicon wafers are conveyed through a tank filled with the solvent or solvent mixture by means of an appropriately sized ascending conveyor, such as an industrial conveyor belt. The running speed of the ascending conveyor and the length of the tank are selected so that the surface coatings are removed without residue during immersion. Of course, the silicon wafers can also be conveyed through separate tanks in two or more steps to remove the various surface coatings sequentially using different solvents or solvent mixtures.

[0091] However, the method according to the invention can also be carried out by spraying the surface coatings of the silicon wafer to be removed, preferably selectively, with the solvents or solvent mixtures, or by applying the solvents or solvent mixtures dropwise. In principle, any suitable method for applying the solvents or solvent mixtures is possible.

[0092] If the surface coatings of a coated silicon wafer are removed by immersion in a suitable solvent or solvent mixture (optionally selectively and / or automated by an ascending conveyor), further conventional steps can be performed during immersion to increase the solubility of solids in solvents or solvent mixtures. For example, the solvent or solvent mixture can be heated during immersion or prior to application. In principle, the solvent or solvent mixture can be heated to just below its boiling point. The saturation solubility of a solid in a solvent typically increases with increasing temperature. Thus, at higher temperatures, more coating material dissolves overall, and the dissolution process also proceeds more quickly.

[0093] Another way to increase the solubility of surface coatings in solvents or solvent mixtures is to sonicate the solvents or solvent mixtures while immersing the coated silicon wafers. Sonification, e.g., using ultrasound, is often used to increase the solubility of a poorly soluble solid in a solvent or solvent mixture by agitating the solid. Methods to mechanically assist the removal process, such as scraping, wiping, or scraping off the surface coating to be removed, are also conceivable and applicable in principle.

[0094] In the sense of the invention, the heating and sonication of the basin containing the solvent or solvent mixture can also be carried out simultaneously.

[0095] Since the process according to the invention is a recycling process, it should also be possible to recover not only the coating-free, undamaged silicon carrier, but also the coating materials that form the surface coatings to be removed. For this purpose, the solvent or solvent mixture in which the surface coatings were dissolved is collected. Different solvents or solvent mixtures that have removed different coating materials are preferably collected separately. For example, a solvent or solvent mixture (A) in which a surface coating (2a) was dissolved and a solvent or solvent mixture (B) in which a surface coating (2b) was dissolved are collected separately so that the respective removed coating materials of the different surface coatings can be recovered without contamination.The surface coating materials dissolved in the collected solvents or solvent mixtures can be separated by various basically known methods.

[0096] If the solvents or solvent mixtures used to remove the surface coatings have been heated and, if necessary, sonicated, it may be sufficient to simply stop the heating and sonication and allow the solution to cool to room temperature, or even further if necessary, to separate the coating materials from the solution in the form of solids. They can then be filtered, for example. Another option is to reduce the solubility of the dissolved surface coating in the solvent or solvent mixture, possibly by adding another substance, thus enabling rapid precipitation.

[0097] It is also conceivable to separate the solvent or solvent mixture from the surface coating materials dissolved therein by distillation processes so that the solid surface coating materials are recovered.

[0098] If the surface coatings were not or could not be removed separately in different solvents or solvent mixtures, a phase extraction process can be used to remove the different dissolved coating materials from the solvent or solvent mixture. For example, the pH of the solution can be changed, thereby influencing the molecular identity of a dissolved solid, which may then exhibit increased solubility in another solvent that is not miscible with the existing solvent or solvent mixture. The dissolved substance then migrates into the other solvent and can be recovered from there in an uncontaminated form using other methods.

[0099] According to the invention, the method for recycling a silicon wafer (1) coated on both sides of the surfaces (1a, 1b) with one or more identical or different surface coatings (2a, 2b) that can be selectively removed by a solvent, and wherein the surface coatings (2a, 2b) are selectively removed with one or more suitable solvents or solvent mixtures without damaging the silicon wafer (1), comprises the following steps: i. providing a coated silicon wafer (1) with a heterojunction layer structure with surface coatings (2a, 2b); and ii. providing one or more solvents and / or solvent mixtures (A, B) that are suitable for selectively removing the surface coatings (2a, 2b); iii. optionally adding one or more additives to the solvents and / or solvent mixtures (A, B); and iv.targeted application of solvent or solvent mixture (A) to remove the surface coating (2a) onto the surface coating (2a); or v. immersing the silicon wafer (1) in the solvent or solvent mixture (A) to remove the surface coating (2a) so that at least the surface coating (2a) is completely covered by the solvent or solvent mixture (A); vi. if necessary, heating and / or sonicating the solvent or solvent mixture; and vii. removing the solvent or solvent mixture in which the removed surface coating (2a) is dissolved; and viii. repeating steps iv. to vii. with a solvent or solvent mixture (B) to remove the surface coating (2b); ix. if necessary, repeating steps iv. to vii. with further solvents or solvent mixtures to remove any additional coatings that may be present; and x.Recovery of the detached materials of the surface coatings from the solvents or solvent mixtures by one or more recovery steps and recovery of the uncoated silicon wafer (1).

[0100] In the method according to the invention as described above, the immersion of the silicon wafer (1) in the solvent or solvent mixture for removing the surface coating(s) can be carried out automatically along an ascending conveyor while completely covering the surface coating(s) by the solvent or solvent mixture

[0101] The invention thus relates to a method for recycling coated silicon wafers whose one or more identical and / or different surface coatings can be removed from the surfaces of the silicon wafer by means of a simple contact process with suitable solvents or solvent mixtures. This process does not damage the silicon body or the surfaces of the silicon wafer. Furthermore, it is preferred if the surface coating materials of the surface coatings can be separated from the solvents or solvent mixtures again using simple extraction methods, in particular in such a way that the solids forming the surface coatings, separated from the solvents or solvent mixtures, contain as little impurities as possible.Impurities include, for example, traces of one of the other surface coating materials which remain during the recovery of a surface coating material after it has been separated from the solvent or solvent mixture.

[0102] II. Process for producing a recyclable surface-coated silicon wafer with a heterojunction layer structure

[0103] The described recycling process is suitable for recyclable silicon wafers with a heterojunction layer structure that are coated on both sides with at least one surface coating that can be selectively removed by a solvent or solvent mixture without damaging the silicon wafer. The surface coatings of the recyclable silicon wafers according to the invention are also the coatings that fulfill the energy-extracting function of commercially available solar cells.The present invention further relates to a method for producing a recyclable silicon wafer, wherein a silicon wafer (1) in the form of a heterojunction layer structure is coated on both sides of its surfaces (1a, 1b) with at least one surface coating (2a, 2b). The surface coatings (2a, 2b) may be identical or different and are characterized in that they can be selectively removed by a suitable solvent (or solvent mixture) without damaging the silicon wafer (1). Such surface coatings (2a, 2b) comprise coating materials from the group: organic, inorganic and / or hybrid semiconductors, organic molecules capable of forming a self-assembling monolayer, metal oxide compounds, organometallic compounds, salt compounds, inorganic acids, silane compounds and / or mixtures thereof.

[0104] The selectively removable coating materials that form the surface coating(s) can serve one or more different purposes. Their primary purpose is to influence the electronic properties of the surface coatings, i.e., to serve the purpose of energy extraction. Additional surface coatings can be provided, for example, those that positively influence solubility in a suitable solvent or solvent mixture, for example by acting as a passivation layer (3a, 3b) between the silicon wafer surface (1a, 1b) and the respective surface coatings (2a, 2b). Furthermore, they can improve the functionality of the coated silicon wafer in the form of a solar cell by forming thin tunnel layers that enable the flow of tunnel currents.In principle, it is preferred according to the invention if preferred solubility properties and preferred electronic properties are combined in one layer; however, separate passivation layers can also be applied to promote individual ones of these properties. In this case, care must be taken to ensure that the passivation layer can be removed just as selectively using a suitable solvent as any other layer. In this way, surface coatings can be made much more easily removable from the previously passivated silicon wafer surfaces (1a, 1b) than if they were applied directly to the pure silicon wafer surfaces. In order to enable the most complete recycling possible of all components, it is preferred that the components forming the passivating layer themselves can also be selectively removed from the silicon surface (1a, 1b) in a simple manner.

[0105] Another possibility is to apply surface coatings (2a, 2b) by depositing various selected coating materials simultaneously onto the surfaces (1a, 1b) of the silicon wafer (1), which form the surface coating on the wafer surface by chemical reaction between the deposited materials.

[0106] Surface coatings can also be formed on the wafer surface in the form of so-called mixed phases. Such mixed phases can contain coating materials with positive properties regarding the solubility of the surface coating, as well as coating materials that influence the electronic properties. In such mixed phases, the electronic properties of one coating material are retained, while the solubility properties in a solvent or solvent mixture are influenced by the other coating material in such a way that the surface coating can be selectively removed using the recycling process described above. The components forming the mixed phase (coating materials) can then be separated from each other from the solution in the manner described above.

[0107] According to the invention, at least two surface coatings (2a, 2b) applied to both sides of the surfaces (1a, 1b) of the silicon wafer (1) are doped with suitable materials, in particular n- and / or p-doped. The term "doping" refers to the introduction of an electron-rich species (electron donors, n-doping) into the crystal lattice of a semiconducting material, or the introduction of an electron-poor species (electron acceptors, p-doping) into the crystal lattice of a semiconducting material. These species are introduced into the crystal lattice of the semiconductor only to a very small extent, but they decisively influence the electronic properties of the coating.

[0108] When the surface coatings are in the form of mixed phases, the mixed phases include alloys, copolymers, salts, cocrystals and / or other mixed phases.

[0109] For the purposes of the invention, the term "alloys" refers to its generally accepted definition, i.e., in particular, a solid containing at least one metal and at least one other metallic, non-metallic, or semi-metallic species present in a stoichiometric ratio. In an alloy, metallic bonding predominates as the primary bonding type between the alloy components. The invention also encompasses alloys that are not based on a stoichiometric ratio of the components, i.e., are non-crystalline. For the purposes of the invention, surface coatings in the form of alloys are therefore always formed with a metallic species.

[0110] The term copolymer in the sense of the invention refers to any polymer species that comprises more than one monomer type. Copolymers (and polymers) are characterized by the covalent bonding of their monomer units. In contrast to alloys, they are mostly non-crystalline, but the invention also includes crystalline copolymers. Surface coatings in the form of copolymers in the sense of the invention correspond in particular to copolymerized organic semiconductor materials or organic molecules. However, it is also not excluded that other materials are present in the copolymers, for example when a porous copolymer structure is formed in whose pores other coating materials are embedded. The term salt in the sense of the invention refers to any type of crystalline coating that can be formed with charged, i.e. ionized, species.The ionic bond between a positively charged cation and a negatively charged anion is the primary bond type. Salts can, in principle, be formed with any of the coating materials according to the invention. Although this is not always the case, the inherent ionization of the salt-forming agents can increase solubility, especially in polar media, which is a preferred property for surface coatings within the meaning of the invention.

[0111] The term "cocrystal" in the context of the invention refers to crystalline mixed phases formed exclusively between neutral or between neutral and ionic species. Weaker intermolecular interactions such as hydrogen bonds, halogen bonds, T-T interactions, weak ionic interactions, etc. are the primary bond types. Cocrystals can, in principle, be formed with any of the coating materials according to the invention.

[0112] For the purposes of the invention, the term “mixed phase(s)” also includes mixed phases whose definitions lie between those mentioned above, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), clathraths, etc., which can, however, be obtained from the coating materials according to the invention and have desirable electronic and / or solubility properties.

[0113] With regard to the recyclability of the solar cell produced according to the invention by means of the recycling process according to the invention, a (mixed) phase of the coating material can be modified such that an increased solubility of the surface coating in a solvent or solvent mixture can be achieved and at the same time an excellent energy extraction performance is achieved.

[0114] The type and strength of interactions and the molecular charge of the coating materials in a surface coating are influenced, for example, by the solid-phase modification and also play a role in the dissolution process.

[0115] In the context of the invention, numerous materials are suitable as coating materials for the surface coatings of a recyclable solar cell.

[0116] For example, organic semiconductors can be selected from PM6, PTQ10, D18, PEDOT, P3HT, spiro-MeOTAD, Y12, L8BO, C60, PCBM, COFs, and mixtures thereof. The invention also encompasses derivatives of these compounds that increase their solubility in suitable solvents; in particular, chlorosilane-endcapped derivatives of the aforementioned compounds are preferred.

[0117] Furthermore, inorganic semiconductors can be selected from SiO x AlOx, NiO x , Cul, CuSCN, MoOx, SnO x, ZnO, TiO2, and mixtures thereof. The hybrid semiconductor materials are selected from metal halide perovskites and their mixtures, in which the halides are fluoride, chloride, bromide, and iodide.

[0118] For the purposes of the invention, organic molecules capable of forming a self-assembling monolayer can also be used as coating materials. Self-assembling monolayers (SAMs) are organic species that, in contact with a surface and / or phase boundary with which they can interact via intermolecular interactions, form a self-assembling, molecule-thick layer along the surface and / or phase boundary. The functional group of the organic molecules that can interact with the surface and / or phase boundary faces the surface and / or phase boundary. Organic molecules for SAMs typically have a "tail," often chain-like, portion in the molecular framework, which aligns itself, i.e., self-assembles, as forced by the interaction of the functional group with a surface and / or phase boundary.The advantage of such SAMs for applications according to the invention in surface coatings lies in the fact that only a weak bond is formed between the organic molecule and the surface and / or phase boundary, which can be easily broken by exposing the surface coating containing the SAMs to a suitable solvent, thus enabling easy removal of the surface coating. For the purposes of the invention, SAMs made of ionomeric, i.e., polymeric, structures with terminal ionizable functional groups are particularly preferred. Phosphonic acid groups, for example, are preferred.

[0119] In a preferred embodiment of the method according to the invention, surface coatings (2a, 2b) are selected from the outset so that they both adhere to the silicon surface (1a, 1b) and are also easily removable using a suitable solvent. Particularly suitable for this purpose are coating agents that contain a SAM, such as a MOF with a metal center and a SAM linker or a COF formed from SAMs. The term SAM linker refers to the framework-forming organic molecule of a MOF or COF, which is also capable of forming a SAM coating on a surface.

[0120] SAMs or COF or MOF compounds formed with SAMs can also serve as a passivating intermediate layer, i.e., as a removable layer that enables the application of an electron or hole extraction layer that is easier to remove from the passivating intermediate layer than from the direct silicon wafer surface. Thus, even complex surface coatings can be selectively applied and removed using the method according to the invention. MOFs or COFs, especially SAM-based MOFs or COFs, enable structural adaptability through the selection of the SAM linker, and thus precise adjustment of solubility. In combination with SAMs, this creates a highly tunable system in which each layer can be removed independently without damaging the silicon.

[0121] According to the invention, Grignard reagents and mixtures of Grignard reagents as well as metal-organic frameworks (MOFs) are selected for coating the recyclable silicon wafer (1) with organometallic compounds.

[0122] Salt compounds as coating materials for the surface coatings of the recyclable silicon wafer (1) are selected from halide salts, ammonium salts, sulfate salts, phosphate salts, metal salts, semimetal salts and mixtures thereof.

[0123] Inorganic acids as coating materials within the meaning of the invention are selected from phosphorus-containing Lewis and Bronsted acids, sulfur-containing Lewis and Bronsted acids, nitrogen-containing Lewis and Bronsted acids, halogen-containing Lewis and Bronsted acids, metal-containing Lewis and Bronsted acids and semi-metal-containing Lewis and Bronsted acids.

[0124] Finally, silane compounds as coating materials for the surface coatings according to the invention are selected from the halosilanes of fluorine, chlorine, bromine and iodine.

[0125] It is particularly preferred if organic semiconductors are selected from the group comprising PEDOT:PSS, PCDM, Y12, COFs, and L8BO. Among the organometallic compounds, MOFs are particularly preferred, with particular preference given to MOFs comprising a SAM linker, and further preference given to Zn-based MOFs comprising a SAM linker. A preferred SAM linker comprises, for example, a phosphonic acid group.

[0126] A crucial step in the production of surface coatings on recyclable silicon wafers is the choice of coating process. The coatings on the silicon wafer surfaces (1a, 1b) can be applied from the liquid or gas phase. The selected coating process can have a decisive influence on the phase modification of the surface coatings (2a, 2b). In general, coating from a liquid phase is better suited for obtaining coatings with organic compounds or mixed phases, especially mixed phases with organic compounds. In contrast, coating from a gas phase is particularly well-suited for obtaining high-purity surface coatings with atomic species such as metals.

[0127] According to the invention, the coating of the silicon wafer surfaces (1a, 1b) can be carried out in a single step, i.e., such that both surfaces are coated with the same material. This can be particularly advantageous if the selected coating is intended exclusively to exert a passivating effect, i.e., to ensure the removability of the electronically active coatings.

[0128] The coating of the silicon wafer surfaces (1a, 1b) with the

[0129] However, coating materials can also be applied in multiple steps, with individual surface coatings being applied selectively. This can also be done using vapor deposition or deposition of the coating materials from a liquid phase.

[0130] Possible liquid phases from which surface coatings according to the invention can be applied to the silicon wafer surfaces (1a, 1b) of the recyclable silicon wafer (1) include solutions, suspensions, dispersions or emulsions of one or more of the coating materials mentioned herein in a solvent or solvent mixture.

[0131] A coating step from the liquid phase can therefore be carried out, for example, by inkjet printing, spray coating, dip coating, solution-based atomic layer deposition (ALD), sol / gel processes and by a roll-to-roll printing process.

[0132] The inventive method for producing a coated silicon wafer is preferably carried out using a roll-to-roll printing process. A solution of the coating materials is stored in a tank such that a roller or roll comes into contact with the solution. The solution is evenly distributed over the roll by rotation. Additional rollers or rolls can be in contact with the first roller to ensure a more even and thinner distribution of the solution over the rolling surfaces. A silicon body can be moved between two rollers to coat it with the surface coatings on one or, if necessary, both sides.

[0133] If a coating step is to be carried out from the gas phase, a gas phase deposition can be carried out by means of atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), and organic hybrid gas phase infiltration within the meaning of the invention.

[0134] For the purposes of the invention, as well as in the context of solar cells in general, the surface coatings (2a) and (2b) applied to opposing surfaces of the silicon wafer (1a, 1b) differ. Even if both surface coatings (2a, 2b) consist largely of the same coating material, the surface coating (2a) and the surface coating (2b) are at least differently n- or p-doped according to the invention.

[0135] In order to ensure the recyclability of the silicon wafers (1) according to the invention, it can be advantageous within the meaning of the invention to first apply a passivation layer (3a, 3b) to the surfaces (1a, 1b) of the silicon wafer. The passivation layer has the property that it can be easily removed from the silicon wafer surface (1a, 1b) using a suitable solvent. The passivation layer can be applied in the same way as a surface coating (2a, 2b), i.e. via vapor deposition or from a liquid phase, preferably it is applied from a liquid phase. The actual surface coating (2a, 2b) can then be applied to the passivation layer (3a, 3b).The advantage of this is that the surface coatings (2a, 2b) can be removed more easily from the passivation layer (3a, 3b) using a solvent or solvent mixture (A, B) than when applied directly to the wafer surfaces (1a, 1b). At the same time, the passivation layer itself can be easily removed from the wafer surfaces (1a, 1b) using a suitable solvent or solvent mixture (A, B).

[0136] It is also possible within the meaning of the present invention for the passivation layer to be designed as a passivating functional layer (4a, 4b). This means that the passivation layer (3a, 3b) simultaneously assumes the functions of the surface coating (2a, 2b) and acts as a passivation layer and

[0137] Surface coating is identical. In accordance with a preferred embodiment of the invention, it is preferred if the surface coating is also a passivation layer.

[0138] For the purposes of the invention, SAM-forming organic compounds are particularly suitable for applying a passivation layer. This does not mean that these compounds are exclusively suitable for this purpose, but they do exhibit advantageous properties. Ionomers are suitable for this purpose. For example, the ionomer Nati® (sulfonated tetrafluoroethylene polymer (PTFE), distributed by DuPont Corp.) can be used for this purpose. Also suitable are compounds equipped with phosphonic acid groups, such as Me-4PACz.

[0139] However, the surfaces (1a, 1b) of the silicon wafer (1) can also be passivated in a different way according to the invention. For this purpose, other atomic species are integrated into the silicon lattice. If the inventive recycling process for coated silicon wafers is carried out with silicon wafers (1) that have integrated passivation on the surfaces (1a, 1b), the integrated passivation is not removed. The passivating species for integration into the silicon wafer surfaces are integrated into the silicon wafer surfaces (1a, 1b) using a vapor deposition process.

[0140] The invention therefore also relates to a manufacturing method for a coated silicon wafer, in which the coating materials are selected with regard to their removability and their electronic properties. By appropriately selecting the coating materials forming the surface coatings in combination with the process steps performed for the surface coating, surface coatings can be applied to a wafer surface that exhibits the most advantageous degree of both desired properties. The removability of the electronically active layers can also be controlled to the desired degree by passivation. Coated silicon wafers produced by the process according to the invention are recyclable and can be used as solar cells.

[0141] The invention also includes a recyclable coated silicon wafer with a heterojunction layer structure obtainable by the process described herein.

[0142] III. Solar cell

[0143] Finally, the invention also relates to a recyclable solar cell comprising the surface-coated silicon wafers with a heterojunction layer structure produced according to the invention. The solar cell is otherwise equipped with the usual components of a solar cell, including, for example, a housing, holding or fastening devices, one or more lenses, metal contact plates, and, if applicable, associated cabling for conducting electrical current.

[0144] According to the invention, the coated silicon wafer (1) with heterojunction layer structure can therefore be used to produce a recyclable solar cell which can be recycled according to the method according to the invention.

[0145] DESCRIPTION OF THE CHARACTERS

[0146] Fig. 1 Surface-coated silicon wafer, two coatings

[0147] Fig. 2 Surface-coated silicon wafer, more coatings, possibly passivated

[0148] Fig. 3 Surface-coated silicon wafer, two coatings with a passivating functional layer, in which the functions of layers (2a, 2b) and (3a, 3b) are combined

[0149] LIST OF REFERENCE SYMBOLS

[0150] (1) Coated silicon wafer

[0151] (la) Surface of the uncoated silicon wafer

[0152] (lb) Surface area of ​​the uncoated silicon wafer

[0153] (2a) Surface coating / possibly n- or p-doped

[0154] (2b) Surface coating / possibly n- or p-doped

[0155] (3a) Optional additional surface coatings / passivation layer

[0156] (3b) Optional additional surface coatings / passivation layer

[0157] (4) Silicon wafers

[0158] (4a) Optional passivating functional layer

[0159] (4b) Optional passivating functional layer EXAMPLES

[0160] I. Manufacturing example a. Double-sided coating with organic and inorganic semiconductor layer

[0161] A silicon wafer (1) can be coated with different coating materials on both sides of the surfaces (1a, 1b) using the roll-to-roll printing process.

[0162] By wetting the wafer surface (1a) via the roll-to-roll printing process with a coating solution of PCBM (([6, 6]-phenyl-Cei butyric acid methyl ester)) in chlorobenzene and subsequent evaporation of the solvent, a first PCBM surface coating (2a) is obtained.

[0163] By subsequently wetting the opposite wafer surface (1 b) via the roll-to-roll printing process with a coating solution of an acetic acid, aqueous solution of molybdenum oxide and subsequent evaporation of the solvent, a second MoO x Surface coating (2b) is obtained.

[0164] Since the solvents of the respective coating solutions function orthogonally, they do not dissolve or detach the coating material on the opposite wafer surface. b. Coating with a self-assembled monolayer

[0165] Optionally, the silicon wafer can be coated with one or more layers of SAM-forming organic compounds to apply a passivation layer before the surface coatings are applied.

[0166] For example, such a deposition of a self-assembling SAM layer can be achieved using the ionomer Nation® (sulfonated tetrafluoroethylene polymer (PTFE), distributed by DuPont Corp.).

[0167] By wetting the surface of the silicon wafer with a solution of the commercially available Nafion® in a mixture of aliphatic alcohols with water and subsequent drying in air until complete evaporation of the solvent, a silicon wafer coated with a Nafion® layer is obtained, which can then be coated with further surface coatings (2a, 2b) as described in Example 1a.

[0168] II. Recycling process

[0169] In order to recycle a coated silicon wafer according to the invention using the method according to the invention, the surface coatings are contacted with suitable solvents that selectively apply or dissolve the respective surface coatings. Using the example of a coated silicon wafer obtainable according to Example 1a, the surface coating can be removed by first contacting the coated silicon wafer with an acetic acid water solution, which is heated to 90°C, for example, and sonicated. This allows the MoO x Surface coating (2b) can be removed. By repeating this process using chlorobenzene as the solvent, the surface coating (2a) can also be removed.

[0170] This process can be performed by passing the coated silicon wafer at a speed of, for example, 1 m / s through a tank containing the solvents needed to remove the coatings. This allows the recycling process to be automated.

[0171] The surface coating materials dissolved in the solvents are then recovered, for example by distilling the solvents in a rotary evaporator.

[0172] III. Overview

[0173] The following table provides an overview of various suitable coating materials and suitable solvents for application (i.e., for preparing coating solutions) or removal (i.e., recycling) of the respective coating materials. The table presents combinations of surface coatings (2a, 2b) that can be selectively removed without the solvent affecting the other layer. Examples marked "No solvent" refer to cases where the material was applied to the surface in its pure form, rather than solvent-based.

[0174] Table 1. Examples of possible surface coatings (2a, 2b) in combination and suitable solvents.

[0175] DMSO: Dimethyl sulfoxide

[0176] DMF: dimethylformamide GBL: γ-butyrolactone

[0177] IV. Preferred embodiments

[0178] The following overview describes various suitable systems of coating materials and solvents for application (i.e. for preparing coating solutions) or for removal (i.e. for recycling) from the categories:

[0179] • organic coatings

[0180] • mixed coatings

[0181] • inorganic coatings I

[0182] • inorganic coatings II

[0183] • mixed coatings with “non-fullerene acceptors” (NFA) a. organic coatings

[0184] Materials:

[0185] Electron extraction (n-side): PCBM (C60-based fullerene derivative)

[0186] Hole extraction (p-side): PEDOT:PSS

[0187] Substrate: Silicon wafer Material description:

[0188] • PCBM: Excellent as an organic electron extractor layer, can be applied from non-polar solvents, especially chlorobenzene.

[0189] • PEDOT:PSS: A material that can be processed from aqueous solutions for a hole extraction layer, often used in organic photovoltaic systems.

[0190] Solvent removal and orthogonality:

[0191] • PCBM removal: Chlorobenzene was used to remove the PCBM from the silicon wafer. The silicon wafer was not damaged.

[0192] • PEDOT:PSS removal: The coated wafer was rinsed with an aqueous solution under mild acidic conditions until the PEDOT:PSS was removed. This process did not damage the silicon wafer or any other coatings present.

[0193] Result:

[0194] The combination of PCBM and PEDOT:PSS as coatings (2a, 2b) for the n- and p-sides demonstrates that fully organic coatings can be removable coatings in the sense of the present process. Both coating materials can be removed without damaging the silicon wafer using commonly used solvents. b. Mixed coatings

[0195] Materials:

[0196] • Electron extraction (n-side): SnO x

[0197] • Hole extraction (p-side): PEDOT:PSS

[0198] • Substrate: silicon wafer

[0199] Material description:

[0200] • SnO x : forms an electron extractor layer, can be deposited from a heated aqueous solution.

[0201] • PEDOT:PSS: A material for a hole extraction layer that can be processed from aqueous solutions.

[0202] Solvent removal and orthogonality:

[0203] • SnOx removal: SnO x can be removed from a silicon wafer without damage by exposure to mild alkaline aqueous solutions.

[0204] • PEDOT:PSS removal: The coated wafer was cleaned of PEDOT:PSS using mild acidic aqueous solutions as previously described. Result:

[0205] The inventive method can be used to produce solar cells with mixed coatings, where one coating side is inorganic and the other is organic. Since the layers can be removed under different conditions, the coating materials can be recycled. The silicon wafer is not damaged under the described conditions. c. Inorganic coatings I

[0206] Materials:

[0207] • Electron extraction (n-side): ZnO

[0208] • Hole extraction (p-side): NiO x

[0209] • Substrate: silicon wafer

[0210] Material description:

[0211] • NiOx and ZnO are robust and well-researched inorganic coatings for applications in phototechnology that are characterized by excellent stability against external influences.

[0212] Solvent removal and orthogonality:

[0213] • ZnO removal: ZnO can be removed from the silicon surface using mild alkaline, aqueous solutions.

[0214] • NiOx removal: NiO x can be removed from the silicon surface using mild acidic, aqueous solutions.

[0215] Result:

[0216] The method according to the invention is suitable for selectively removing the coatings from silicon wafers coated with different inorganic coatings. The silicon wafer is not damaged in the process. d. Inorganic coatings II

[0217] Materials:

[0218] Electron extraction (n-side): TiO2

[0219] Hole extraction (p-side): Cul or CuSCN

[0220] Substrate: Silicon wafer Material description:

[0221] • ÜO2: An electron extraction coating that can be removed under mild alkaline conditions.

[0222] • Cul / CuSCN: Copper-based hole extraction layers which are removable in polar solvents, allowing them to be easily removed selectively from the TiCh.

[0223] Solvent removal and orthogonality:

[0224] • ÜO2 removal: Can be removed from the silicon surface using mild alkaline aqueous solutions.

[0225] • Cul / CuSCN removal: Can be removed with alcohols, DMSO and GBL without affecting the TiO2 coating.

[0226] Result:

[0227] Another example of inorganic coatings. In this system, the hole extraction layer can be removed using various organic solvents, while the n-electron extraction layer is removed using a mild alkaline aqueous solution. e. Mixed coatings with "non-fullerene acceptors" (NFA)

[0228] Materials:

[0229] • Electron extraction (n-side): Y12 or L8BO

[0230] • Hole extraction (p-side): MoO x

[0231] • Substrate: silicon wafer

[0232] Material description:

[0233] • Y12 / L8BO: NFAs can form electron extraction coatings with high efficiency, which are currently of great research interest.

[0234] • MoO x : A material commonly used for hole extraction layers.

[0235] Solvent removal and orthogonality:

[0236] • Y12 / L8BO removal: the NFAs Y12 and L8BO can be excellently removed by halogenated organic solvents such as chlorobenzene.

[0237] • MoOx removal: MoO x can be removed from PEDOT:PSS using mild acidic aqueous solutions. Result:

[0238] The method according to the invention is suitable for removing novel materials, which are the subject of current research, from silicon surfaces as well as already known and established systems.

Claims

PATENT CLAIMS 1. A method for the non-destructive recycling of a silicon wafer (1) which is coated on both sides of the surfaces (1a, 1b) with one or more identical or different surface coatings (2a, 2b) in the form of a heterojunction layer structure which can be selectively removed by a solvent, wherein the surface coatings (2a, 2b) are selectively removed with a suitable solvent or solvent mixture without damaging the silicon wafer (1), 2. The method according to claim 1, wherein the solvent or solvent mixture for removing the surface coatings (2a, 2b) is also suitable for applying the removable surface coatings (2a, 2b) from a solution of the coating material in the solvent or solvent mixture to the surfaces (1a, 1b) of a silicon wafer (1).

3. The method according to claim 1 to 2, wherein the solvent or solvent mixture is selected according to the solubility properties of the surface coatings (2a, 2b) and is selected from one or more linear, branched, cyclic, heterocyclic, aromatic, heteroaromatic, saturated or unsaturated hydrocarbons selected from: water and aqueous solutions, unsubstituted hydrocarbons, halogenated hydrocarbons, alcohols, ethers, aldehydes, ketones, carboxylic acids, carboxylic acid esters, carboxylic acid anhydrides, carboxylic acid halides, thiols, thioethers, thioaldehydes, thioketones, thiocarboxylic acids, thioesters, amines, amides, nitriles, imines, oximes, hydrazines, hydrazones, sulfoxides, sulfinic acids, sulfinic acid esters, sulfonic acids, sulfonic acid esters, sulfonic acid halides and / or sulfonamides and in which solvent mixtures may also contain water.

4. A process according to claim 1 to 3, wherein the solvent or solvent mixture is selected from water and aqueous solutions, unsubstituted aromatic and heteroaromatic hydrocarbons, halogenated aromatic and heteroaromatic hydrocarbons, alcohols, ketones, amides and sulfoxides.

5. A process according to claim 1 to 4, wherein the solvent or one or more additives are added to the solvent mixture which increase the solubility of the surface coatings (2a, 2b) in the solvent or solvent mixture and / or the phase transition between solvent mixtures with different phases.

6. A process according to claim 1 to 5, wherein the solvent or Solvent mixture additives selected from acids, bases and chelating agents are added.

7. The method according to claim 1 to 6, wherein the coated silicon wafer (1) is treated with a first solvent or solvent mixture (A) so that the surface coating (2a) is removed and is treated with a second, identical or different, solvent or solvent mixture (B) so that the surface coating (2b) is removed, and wherein the surface coatings (2a, 2b) removed in the solvent or solvent mixture are recovered from the solution.

8. The method according to claim 1 to 7, comprising the steps: i. providing a coated silicon wafer (1) with a heterojunction layer structure with surface coatings (2a, 2b); and ii. providing one or more solvents and / or solvent mixtures (A, B) suitable for selectively removing the surface coatings (2a, 2b); iii. optionally adding one or more additives to the solvents and / or solvent mixtures (A, B); and iv. selectively applying solvent or solvent mixture (A) to remove the surface coating (2a) onto the surface coating (2a); or v. immersing the silicon wafer (1) in the solvent or solvent mixture (A) to remove the surface coating (2a) so that at least the surface coating (2a) is completely covered by the solvent or solvent mixture (A); vi. optionallyHeating and / or sonicating the solvent or solvent mixture; and vii. Removing the solvent or solvent mixture in which the detached surface coating (2a) is dissolved; and viii. Repeating steps iv. to vii. with a solvent or solvent mixture (B) to remove the surface coating (2b); ix. If necessary, repeating steps iv. to vii. with further solvents or solvent mixtures to remove any additional coatings that may be present; and x. Recovering the detached materials of the surface coatings from the solvents or solvent mixtures by one or more recovery steps and recovering the uncoated silicon wafer (1).

9. A method for producing a recyclable silicon wafer (1) having a heterojunction layer structure with surface coatings (2a, 2b), wherein a Silicon wafer is coated on both sides on the surfaces (1a, 1b) with at least one surface coating (2a, 2b), wherein the Surface coatings (2a, 2b) are n- and / or p-doped, and wherein the surface coating (2a, 2b) is characterized in that it can be selectively removed by a suitable solvent without damaging the silicon wafer (1).

10. The method according to claim 9, wherein the surface coatings (2a, 2b) contain coating materials from the group comprising organic, inorganic and / or hybrid semiconductors, organic molecules capable of forming a self-assembling monolayer, metal oxide compounds, organometallic compounds, salt compounds, inorganic acids, silane compounds and / or mixtures thereof.

11. The method according to claim 9 to 10, wherein the surface coatings (2a, 2b) are made of coating materials from the group of organic semiconductors comprising PEDOT, PCBM, Y12, covalent organic frameworks (COFs), L8BO and mixtures thereof and / or the group of inorganic semiconductors comprising SiO x AlOx, NiOx, CuSCN, MoO x , SnO x , ZnO, TiO2 and mixtures thereof and / or the group of organometallic compounds, in particular organometallic frameworks (MOFs) and / or the group of self-assembling monolayers (SAM).

12. The method according to claim 9 to 11, wherein one or more surface coatings are applied by simultaneously applying one or more coating materials from a liquid phase or from the gas phase; or wherein several surface coatings are applied on each side of the silicon wafer in different coating steps from a liquid and / or the gas phase.

13. The method according to claim 9 to 12, wherein the surfaces (1a, 1b) of the silicon wafer (1) are each passivated by applying a passivation layer (3a, 3b) before the application of the surface coatings (2a, 2b), or wherein the passivation is integrated into the surfaces (1a, 1b) of the silicon wafer (1).

14. The method according to claim 9 to 12, wherein the passivation layer (3a, 3b) is formed as a passivating functional layer (4a, 4b) which combines the properties of the passivation layer (3a, 3b) and the surface coating (2a, 2b).

15. The method according to claim 14, wherein the passivating functional layers (4a, 4b) are self-assembled monolayers (SAM), metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), in particular MOFs and COFs with SAM-based linkers and SAMs.

16. A solar cell recyclable according to a method according to claims 1 to 8, equipped with one or more coated silicon wafers (1) with a heterojunction layer structure obtainable according to the method according to claims 9 to 15.

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