SCREEN-PRINTABLE BORON DOPING PASTE WITH SIMULTANEOUS INHIBITION OF PHOSPHORUS DIFFUSION IN CO-DIFFUSION PROCESSES

Abstract

The present invention relates to a novel printable boron doping paste in the form of a hybrid gel based on precursors of inorganic oxides, preferably of silicon dioxide, aluminium oxide and boron oxide, in the presence of organic polymer particles, where the pastes according to the invention can be used in a simplified process for the production of solar cells, where the hybrid gel according to the invention functions both as doping medium and as diffusion barrier.

Claims

1. A printable boron doping paste or gel based on a precursor of silicon dioxide, aluminium oxide or boron oxide, comprising at least one polymer as thickener selected from the group consisting of polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl acetate, polyvinylimidazole, polyvinylbutyral, methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, microcrystalline cellulose, sodium starch glycolate, xanthan gum, gellan gum, gelatine, agar, alginic acid, alginates, guar flour, pectin, carubin, polyacrylic acid, polyacrylate, associatively thickening polyurethane and a mixture thereof, which can be employed for local and/or full-area diffusion and doping on one side in solar cell production processes, and which paste or gel has been obtained from a precursor of silicon dioxide, aluminium oxide or boron oxide, or a mixture thereof, wherein the precursor of silicon dioxide is a symmetrically or asymmetrically mono- to tetrasubstituted carboxy-, alkoxy- or alkoxyalkylsilane, in which at least one hydrogen atom is bonded to the central silicon atom, or carboxy-, alkoxy- or alkoxyalkylsilane, which contain individual or different saturated, unsaturated branched, unbranched aliphatic, alicyclic or aromatic radicals, which are optionally functionalised at a position of the alkyl, alkoxide or carboxyl radical by one or more heteroatoms selected from the group consisting of O, N, S, Cl and Br, or tetraethyl orthosilicate, triethoxysilane, ethoxytrimethylsilane, dimethyldimethoxysilane, dimethyldiethoxysilane, triethoxyvinylsilane, bis[triethoxysilyl]ethane or bis[diethoxymethylsilyl]ethane, or a mixture thereof; the precursor of aluminum oxide is a symmetrically or asymmetrically substituted aluminium alcoholate (alkoxide), aluminium tris(-diketone), aluminium tris(-ketoester), an aluminium soap, an aluminium carboxylate, or aluminium triethanolate, aluminium triisopropylate, aluminium tri-sec-butylate, aluminium tributylate, aluminium triamylate, aluminium triisopentanolate, aluminium acetyl-acetonate or aluminium tris(1,3-cyclohexanedionate), aluminium monoacetylacetonate monoalcoholate, aluminium tris(hydroxyquinolate), mono- or dibasic aluminium stearate or aluminium triformate or aluminium trioctanoate, aluminium hydroxide, aluminium metahydroxide or aluminium trichloride, or a mixture thereof; the precursor of boron oxide is an alkyl borate, a boric acid ester of a functionalised 1,2-glycol, a boric acid ester of an alkanolamine, a mixed anhydride of boric acid or carboxylic acid, or boron oxide, diboron oxide, triethyl borate, triisopropyl borate, boric acid glycol ester, boric acid ethylene glycol ester, boric acid glycerol ester, boric acid ester of 2,3-dihydroxy-succinic acid, tetraacetoxy diborate and boric acid esters of the alkanolamines ethanolamine, diethanolamine, triethanolamine, propanolamine, dipropanolamine or tripropanolamine, or a mixture thereof.

2. The printable boron doping paste according to claim 1, comprising at least one polymer as thickener selected from the group consisting of polyvinylpyrrolidone, polyvinyl acetate, polyvinylbutyral, ethylcellulose and a mixture thereof.

3. The printable boron doping paste according to claim 1, comprising at least one polymer as thickener which interacts associatively and thus in a structure-forming manner with constituents of a hybrid sol and causes a significantly more pronounced structural viscosity than comparable pastes which comprise only polymeric thickening compounds.

4. The printable boron doping paste according to claim 1, comprising at least one polymer as thickener which interacts with constituents of a hybrid sol via coordinative and/or chelating mechanisms.

5. The printable boron doping paste according to claim 1, comprising additives selected from the group consisting of aluminium hydroxides, aluminium oxides, colloidally precipitated silicon dioxide, highly disperse silicon dioxide, tin dioxide, boron nitride, silicon carbide, silicon nitride, aluminium titanate, titanium dioxide, titanium carbide, titanium nitride, titanium carbonitride, and particulate formulation assistants which have a positive influence on the layer thickness of a dried paste.

6. The printable boron doping paste according to claim 1, which is based on a precursor of silicon dioxide, aluminium oxide or boron oxide.

7. The printable doping paste according to claim 1, which is based on a mixture of precursors of silicon dioxide, aluminium oxide and boron oxide.

8. The printable boron doping paste according to claim 1, which has been obtained on the basis of a precursor of silicon dioxide, which is a symmetrically or asymmetrically mono- to tetrasubstituted carboxy-, alkoxy- or alkoxyalkylsilane in which at least one hydrogen atom is bonded to the central silicon atom, carboxy-, alkoxy- or alkoxyalkylsilane which contain individual or different saturated, unsaturated branched, unbranched aliphatic, alicyclic or aromatic radicals, which are optionally functionalised at a position of the alkyl, alkoxide or carboxyl radical by one or more heteroatoms selected from the group consisting of O, N, S, Cl and Br, or a mixture thereof.

9. The printable boron doping paste according to claim 1, which has been obtained on the basis of a precursor of silicon dioxide, which is tetraethyl orthosilicate, triethoxysilane, ethoxytrimethylsilane, dimethyldimethoxysilane, dimethyldiethoxysilane, triethoxyvinylsilane, bis[triethoxysilyl]ethane or bis[diethoxymethylsilyl]ethane, or a mixture thereof.

10. The printable boron doping paste according to claim 1, which has been obtained on the basis of a precursor of aluminium oxide, which is a symmetrically or asymmetrically substituted aluminium alcoholate (alkoxide), aluminium tris(-diketone), aluminium tris(-ketoester), an aluminium soap, an aluminium carboxylate or a mixture thereof.

11. The printable boron doping paste according to claim 1, which has been obtained on the basis of a precursor of aluminium oxide, which is aluminium triethanolate, aluminium triisopropylate, aluminium tri-sec-butylate, aluminium tributylate, aluminium triamylate, aluminium triisopentanolate, aluminium acetylacetonate or aluminium tris(1,3-cyclohexanedionate), aluminium monoacetylacetonate monoalcoholate, aluminium tris(hydroxyquinolate), mono- or dibasic aluminium stearate or aluminium tristearate, aluminium acetate, aluminium triacetate, basic aluminium formate, aluminium triformate or aluminium trioctanoate, aluminium hydroxide, aluminium metahydroxide or aluminium trichloride, or a mixture thereof.

12. The printable boron doping paste according to claim 1, which has been obtained on the basis of a precursor of boron oxide, which is selected from the group consisting of alkyl borates, boric acid esters of functionalised 1,2-glycols, boric acid esters of alkanolamines, mixed anhydrides of boric acid and carboxylic acids, and mixtures thereof.

13. The printable boron doping paste according to claim 1, which has been obtained on the basis of a precursor of boron oxide, which is selected from the group consisting of boron oxide, diboron oxide, triethyl borate, triisopropyl borate, boric acid glycol ester, boric acid ethylene glycol ester, boric acid glycerol ester, boric acid ester of 2,3-dihydroxysuccinic acid, tetraacetoxy diborate and boric acid esters of the alkanolamines ethanolamine, diethanolamine, triethanolamine, propanolamine, dipropanolamine and tripropanolamine.

14. The printable boron doping paste according to claim 1, obtainable by bringing precursors to partial or complete intra- and/or interspecies condensation under water-containing or anhydrous conditions with the aid of the sol-gel technique, either simultaneously or sequentially, forming storage-stable, readily printable and printing-stable formulations.

15. The printable boron doping paste according to claim 14, obtainable by removal of volatile reaction assistants and by-products during the condensation reaction.

16. The printable boron doping paste according to claim 14, obtainable by adjustment of the precursor concentration, the water and catalyst content and the reaction temperature and time.

17. The printable boron doping paste according to claim 14, obtainable by addition of one or more condensation-controlling agents in the form of complexing agents and/or chelating agents, one or more solvents in predetermined amounts, based on the total volume, wherein the degree of gelling of the hybrid sols and gels formed is controlled.

18. A process for the production of solar cells, in which the printable boron doping paste according to claim 1 is printed onto silicon surfaces for the purposes of local and/or full-area diffusion and doping on one side by a printing process in the production of solar cells, optionally of highly efficient solar cells doped in a structured manner, and dried and subsequently brought to specific doping of the substrate by a suitable high-temperature process for release of the boron oxide precursor present in the dried paste to the substrate located beneath the boron paste.

19. A process for the production of solar cells, in which the printable boron doping paste according to claim 1 is processed and deposited by a printing process selected from spin or dip coating, drop casting, curtain or slot-die coating, screen or flexographic printing, gravure, ink-jet or aerosol-jet printing, offset printing, microcontact printing, electrohydrodynamic dispensing, roller or spray coating, ultrasound spray coating, pipe-jet printing, laser transfer printing, pad printing, flat-bed screen printing and rotational screen printing.

20. A process according to claim 24, wherein the silicon wafers are for photovoltaic, microelectronic, micromechanical or micro-optical applications.

21. A process according to claim 18, which is for the production of a product selected from the group consisting of PERC, PERL, PERT and IBC solar cells and comparable solar cells, where the solar cells have further architectural features, MWT, EWT, selective emitter, selective front surface field, selective back surface field and bifacial solar cells.

22. A process for boron doping of silicon, comprising achieving said doping with the printable boron doping paste according to claim 1, where the medium simultaneously acts as diffusion barrier or as diffusion-inhibiting layer against undesired diffusion of phosphorus through this medium and completely blocks or inhibits the latter to an adequate extent, so that the doping prevailing beneath these printed-on media is p type, i.e. boron-containing.

23. A process according to claim 22, wherein the doping of the printed substrate is carried out by temperature treatment, and doping of the unprinted silicon wafer surfaces with dopants of the opposite polarity is induced simultaneously and/or sequentially by gas-diffusion, where the printed-on boron doping paste act as diffusion barrier against the dopants of the opposite polarity.

24. A process for the doping of silicon wafers by boron doping pastes according to claim 1, comprising a) printing silicon wafers locally on one or both sides or over the entire surface on one side with said boron doping paste, the printed-on paste is dried, compacted, and the silicon wafers are subsequently subjected to subsequent gas-phase diffusion with, optionally, phosphoryl chloride, giving p-type dopings in the printed regions and n-type dopings in the regions subjected exclusively to gas-phase diffusion, or b) said boron doping paste printed over a large area onto the silicon wafer is compacted, and local doping of the underlying substrate material is initiated from the dried and/or compacted paste with the aid of laser irradiation, followed by high-temperature treatment, wherein diffusion and doping are induced for the production of two-stage p-type doping levels in the silicon, or c) the silicon wafer is printed locally on one side with said boron doping paste, where the structured deposition may optionally have alternating lines, the printed structures are dried and compacted, and the silicon wafer is subsequently coated over the entire surface on the same side of the wafer with the aid of PVD- and/or CVD-deposited phosphorus-doping dopant sources, where the printed structures of the boron doping paste are encapsulated, and the entire overlapping structure is brought to structured doping of the silicon wafer by high-temperature treatment, where the printed-on boron paste acts as diffusion barrier against the phosphorus-containing dopant source located on top and the dopant present therein, or d) the silicon wafer is printed locally on one side with said boron doping paste, where the structured deposition may optionally have alternating lines, the printed structures are dried and compacted, and the silicon wafers are subsequently coated over the entire surface on the same side of the wafer with the aid of phosphorus-doping doping inks or doping pastes, where the printed structures of the boron doping paste are encapsulated, and the entire overlapping structure is brought to structured doping of the silicon wafer by high-temperature treatment, and where the printed-on boron paste acts as diffusion barrier against the phosphorus-containing dopant source located on top and the dopant present therein, or e) the silicon wafer is printed locally on one side with said boron doping paste, where the structured deposition may optionally have alternating lines, the printed structures are dried and compacted, and the silicon wafer is subsequently printed on the same side of the wafer with a negative structure compared with the preceding print with the aid of a phosphorus paste, and the entire structure is brought to structured doping of the silicon wafer on one side and over the entire surface of the opposite side by high-temperature treatment in the presence of a conventional phosphorus-based gas-phase diffusion source, optionally, phosphoryl chloride, where the printed-on boron paste acts as diffusion barrier against the other phosphorus-containing diffusion sources present at the same time, or f) the silicon wafer is printed locally on one side with said boron doping paste, where the structured deposition may optionally have alternating lines, the printed structures are dried and compacted, and the silicon wafer is subsequently printed on the same side of the wafer with a negative structure compared with the preceding print with the aid of a phosphorus paste, the opposite side of the same wafer is subsequently printed with a further phosphorus doping paste, where the sequence of the printing steps of application of the phosphorus doping pastes need not necessarily take place in the said series, and the entire structure is brought to structured doping of the silicon wafer on one side and over the entire surface of the opposite side by high-temperature treatment, where the printed-on boron paste acts as diffusion barrier against the other phosphorus-containing diffusion sources present at the same time.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0084] Surprisingly, it has been found that boron-containing doping inks prepared on the basis of the sol-gel process can be formulated with the aid of classical thickeners in such a way that very readily printable formulations can be obtained therefrom. Suitable printing processes which may be considered are at least those mentioned below: spin or dip coating, drop casting, curtain or slot-die coating, screen or flexographic printing, gravure, ink-jet or aerosol-jet printing, offset printing, microcontact printing, electrohydrodynamic dispensing, roller or spray coating, ultrasound spray coating, pipe-jet printing, laser transfer printing, pad printing, flat-bed screen printing and rotational screen printing. The boron-containing doping inks formulated further into pastes are preferably, but not exclusively, printed onto silicon surfaces with the aid of the screen-printing process. The boron-containing doping inks are prepared here with the aid of the sol-gel process and consist at least of oxide precursors of the following oxides: aluminium oxide, silicon dioxide and boron oxide. The mixing ratios of the oxide precursors mentioned may be present in randomly selected proportions. Typical precursors of the oxides for the preparation of the boron-containing doping inks according to the invention, but not exclusively restricted to the said examples, which are also referred to as hybrid sols below, are presented below:

[0085] Aluminium oxide: symmetrically and asymmetrically substituted aluminium alcoholates (alkoxides), such as aluminium triethanolate, aluminium triisopropylate, aluminium tri-sec-butylate, aluminium tributylate, aluminium triamylate and aluminium triisopentanolate, aluminium tris(-diketones), such as aluminium acetylacetonate or aluminium tris(1,3-cyclohexanedionate), aluminium tris(-ketoesters), aluminium monoacetylacetonate monoalcoholate, aluminium tris(hydroxyquinolate), aluminium soaps, such as mono- and dibasic aluminium stearate and aluminium tristearate, aluminium carboxylates, such as basic aluminium acetate, aluminium triacetate, basic aluminium formate, aluminium triformate and aluminium trioctanoate, aluminium hydroxide, aluminium metahydroxide and aluminium trichloride and the like, and mixtures thereof.

[0086] Silicon dioxide: symmetrically and asymmetrically mono- to tetrasubstituted carboxy-, alkoxy- and alkoxyalkylsilanes, explicitly containing alkylalkoxysilanes, in which the central silicon atom can have a degree of substitution of [lacuna] by at least one hydrogen atom bonded directly to the silicon atom, such as, for example, triethoxysilane, and where furthermore a degree of substitution relates to the number of possible carboxyl and/or alkoxy groups present, which, both in the case of alkyl and/or alkoxy and/or carboxyl groups, contain individual or different saturated, unsaturated branched, unbranched aliphatic, alicyclic and aromatic radicals, which may in turn be functionalised at any desired position of the alkyl, alkoxide or carboxyl radical by heteroatoms selected from the group O, N, S, Cl and Br, and mixtures of the above-mentioned precursors; individual compounds which satisfy the above-mentioned demands are: tetraethyl orthosilicate and the like, triethoxysilane, ethoxytrimethylsilane, dimethyldimethoxysilane, dimethyldiethoxysilane, triethoxyvinylsilane, bis[triethoxysilyl]ethane and bis[diethoxymethylsilyl]ethane.

[0087] Boron oxide: diboron oxide, simple alkyl borates, such as triethyl borate, triisopropyl borate, boric acid esters of functionalised 1,2-glycols, such as, for example, ethylene glycol, functionalised 1,2,3-triols, such as, for example, glycerol, functionalised 1,3-glycols, such as, for example, 1,3-propanediol, boric acid esters with boric acid esters which contain the above-mentioned structural motifs as structural sub-units, such as, for example, 2,3-dihydroxy-succinic acid and enantiomers thereof, boric acid esters of ethanolamine, diethanolamine, triethanolamine, propanolamine, dipropanolamine and tripropanolamine, mixed anhydrides of boric acid and carboxylic acids, such as, for example, tetraacetoxy diborate, boric acid, metaboric acid, and mixtures of the above-mentioned precursors.

[0088] The possible combinations are furthermore not necessarily restricted to the above-mentioned possible compositions: further substances which are able to impart advantageous properties on the sols may be present as additional components in the hybrid sols. They may be: oxides, basic oxides, hydroxides, alkoxides, carboxylates, -diketonates, -ketoesters, silicates and the like of cerium, tin, zinc, titanium, zirconium, hafnium, zinc, germanium, gallium, niobium, yttrium, which can be used directly or in pre-condensed form in the sol-gel synthesis. The hybrid sols are sterically stabilised by the use of complexing and chelating substances, which may also control the condensation behaviour of the oxide precursors, in particular of aluminium, and also of other metal cations. Substances which are suitable in this respect are, for example, acetylacetone, 1,3-cyclohexanedione, isomeric compounds of dihydroxybenzoic acids, acetaldoxime, and in addition also those disclosed and present in the patent applications WO 2012/119686 A, WO2012119685 A1, WO2012119684 A, EP12703458.5 and EP12704232.3. The contents of these specifications are therefore incorporated into the disclosure content of the present application. The hybrid sols can be prepared with the aid of an anhydrous or water-containing sol-gel synthesis. In addition, further assistants can be used in the formulation of the hybrid sols according to the invention to form screen-printable pastes. Such assistants may be: [0089] surfactants, tensioactive compounds for influencing the wetting and drying behaviour, [0090] antifoams and deaerators for influencing the drying behaviour, [0091] strong carboxylic acids for initiation of the condensation reaction of oxide precursors, at least the following may serve as suitable carboxylic acids: formic acid, acetic acid, oxalic acid, trifluoroacetic acid, mono-, di- and trichloroacetic acid, glyoxalic acid, tartaric acid, maleic acid, malonic acid, pyruvic acid, malic acid, 2-oxoglutaric acid, [0092] high- and low-boiling non-polar and also polar protic and aprotic solvents for influencing the particle size distribution, the degree of pre-condensation, the condensation, wetting and drying behaviour and the printing behaviour, where these may be: glycols, glycol ethers, glycol ether carboxylates, polyols, terpineol, Texanol, butyl benzoate, benzyl benzoate, dibenzyl ether, butyl benzyl phthalate and others, and mixtures thereof, [0093] particulate additives for influencing the rheological properties, [0094] particulate additives (for example aluminium hydroxides and aluminium oxides, colloidally precipitated or highly disperse silicon dioxide, tin dioxide, boron nitride, silicon carbide, silicon nitride, aluminium titanate, titanium dioxide, titanium carbide, titanium nitride, titanium carbonitride) for influencing the dry-film thicknesses resulting after drying and the morphology thereof, [0095] particulate additives (for example aluminium hydroxides and aluminium oxides, colloidally precipitated or highly disperse silicon dioxide, tin dioxide, boron nitride, silicon carbide, silicon nitride, aluminium titanate, titanium dioxide, titanium carbide, titanium nitride, titanium carbonitride) for influencing the scratch resistance of the dried films, [0096] capping agents selected from the group acetoxytrialkylsilanes, alkoxytrialkylsilanes, halotrialkylsilanes and derivatives thereof for influencing the condensation rates and the storage stability, [0097] waxes and wax-like compounds, such as beeswax, Syncrowax, lanolin, carnauba wax, jojoba, Japan wax and the like, fatty acids and fatty alcohols, fatty glycols, esters of fatty acids and fatty alcohols, triglycerides, fatty aldehydes, fatty ketones and fatty -diketones and mixtures thereof, where the above-mentioned classes of substance should each contain branched and unbranched carbon chains having chain lengths greater than or equal to twelve carbon atoms, [0098] polymeric thickening, rheology-modifying additives, such as, for example, polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl acetate, polyvinylimidazole, polyvinylbutyral, methylcelluloses, ethylcelluloses, hydroxyethylcelluloses, hydroxypropylcelluloses, microcrystalline celluloses, sodium starch glycolates, xanthan and gellan gum, gelatine, agar, alginic acid and alginates, guar flour, pectin, carubin, polyacrylic acids, polyacrylates, associatively thickening polyurethanes, and mixtures thereof.

[0099] One synthetic method is based on the dissolution of oxide precursors of aluminium oxide in a solvent or solvent mixture, preferably selected from the group high-boiling glycol ethers or preferably high-boiling glycol ethers and alcohols, to which a suitable acid, preferably a carboxylic acid, and here particularly preferably formic acid or acetic acid, is subsequently added, and which is completed by the addition of suitable complexing agents and chelating agents, such as, for example, suitable -diketones, such as acetylacetone or, for example, 1,3-cyclohexanedione, - and -ketocarboxylic acids and esters thereof, such as, for example, pyruvic acid and esters thereof, acetoacetic acid and ethyl acetoacetate, isomeric dihydroxybenzoic acids, such as, for example, 3,5-dihydroxybenzoic acid, and/or oximes, such as, for example, acetaldoxime, and further cited compounds of this type, and also any desired mixtures of the above-mentioned complexing agents, chelating agents and agents which control the degree of condensation. A mixture consisting of the above-mentioned solvent or solvent mixture and water is then added dropwise to the solution of the aluminium oxide precursor at room temperature, and the mixture is subsequently warmed under reflux at 80 C. for up to 24 h. Gelling of the aluminium oxide precursor can be controlled specifically via the molar ratio of the aluminium oxide precursor to water, to the acid used and also the molar amounts and type of the complexing agents employed. The synthesis durations necessary in each case are likewise dependent on the above-mentioned molar ratios. The readily volatile and desired parasitic by-products occurring in the reaction are subsequently removed from the finished reaction mixture, which is optionally already furthermore diluted, by means of vacuum distillation. The vacuum distillation is achieved by stepwise reduction of the final pressure to 30 mbar at a constant temperature of 70 C. The hybrid gels are adjusted with respect to their desired properties, either after or even before the distillative treatment, by specific addition of suitable solvents which favour the rheology and printability of the paste, such as, for example, high-boiling glycols, glycol ethers, glycol ether carboxylates and furthermore solvents such as terpineol, Texanol, butyl benzoate, benzyl benzoate, dibenzyl ether, butyl benzyl phthalate, and solvent mixtures, and optionally diluted. In parallel to the dilution and adjustment of the paste properties, a mixture consisting of condensed oxide precursors of silicon dioxide and boron oxide is added. For this purpose, precursors of boron oxide are initially introduced in a solvent, such as, for example, dibenzyl ether, butyl benzyl phthalate, benzyl benzoate, butyl benzoate, THF or a comparable solvent, a suitable carboxylic anhydride, such as, for example, acetic anhydride, formyl acetate or propionic anhydride or a comparable anhydride, is added, and dissolved or brought to reaction under reflux until a clear solution is present. Suitable precursors of silicon dioxide, optionally pre-dissolved in the reaction solvent used, are added dropwise to this solution. The reaction mixture is subsequently warmed or refluxed for up to 24 h. After the mixing of all components, the paste rheology can furthermore be adjusted and rounded off in accordance with specific requirements corresponding to the assistants and additives likewise already described in detail above, where the use according to the invention of the said polymeric thickeners has a particular role. The thickeners are stirred into the mixture with vigorous stirring, where the stirring duration is dependent on the respective thickener used. The stirring-in of the thickener can optionally be completed with a vacuum treatment step, during which air bubbles stirred into the highly viscous mass are removed. Depending on the thickeners used, the resultant paste may have to be left to swell for a period of up to three days.

[0100] An alternative synthetic method is based on the preparation of a condensed sol of oxide precursors of silicon dioxide and boron oxide. For this purpose, precursors of boron oxide are initially introduced in a solvent, such as, for example, dibenzyl ether, butyl benzyl phthalate, benzyl benzoate, butyl benzoate, THF or a comparable solvent, a suitable carboxylic anhydride, such as, for example, acetic anhydride, formyl acetate or propionic anhydride or a comparable anhydride, is added and dissolved or brought to reaction under reflux until a clear solution is present. Suitable precursors of silicon dioxide, optionally pre-dissolved in the reaction solvent used, are added dropwise to this solution. The reaction mixture is subsequently warmed or refluxed for up to 24 h. Suitable solvents, such as, for example, glycols, glycol ethers, glycol ether carboxylates and furthermore solvents such as terpineol, Texanol, butyl benzoate, benzyl benzoate, dibenzyl ether, butyl benzyl phthalate, or solvent mixtures thereof, in which suitable complexing agents and chelating agents, such as, for example, suitable -diketones, such as acetylacetone or, for example, 1,3-cyclohexanedione, - and -ketocarboxylic acids and esters thereof, such as, for example, pyruvic acid and esters thereof, acetoacetic acid and ethyl acetoacetate, isomeric dihydroxybenzoic acids, such as, for example, 3,5-dihydroxybenzoic acid, and/or oximes, such as, for example, acetaldoxime, and further cited compounds of this type, and also any desired mixtures of the above-mentioned complexing agents, chelating agents and agents which control the degree of condensation, which are already predissolved in the presence of water, are subsequently added to the sol, and the mixture is stirred, where the temperature of the reaction mixture may increase at the same time. The duration of mixing of the two solutions can be between 0.5 minute and five hours. The entire mixture is heated with the aid of an oil bath, whose temperature is generally set to 155 C. After a duration of mixing of the entire solution completed from the two part-solutions which is known as suitable, a suitable aluminium oxide precursor, which has itself been pre-dissolved in one of the above-mentioned solvents or solvent mixtures, is subsequently added dropwise or allowed to run into the reaction mixture in such a way that the addition is completed in a time window of five minutes since the beginning of the addition. The reaction mixture now completed in this way is then warmed under reflux for one to four hours. The warm gelled mixture can then be modified further with respect to its Theological properties using further assistants already mentioned above, in particular and particularly preferably, however, through the use of the polymeric thickeners to be used in accordance with the invention. The thickeners are stirred into the mixture here with vigorous stirring, where the stirring duration is dependent on the respective thickener used. The stirring-in of the thickener can optionally be completed with a vacuum treatment step, during which air bubbles stirred into the highly viscous mass are removed. Depending on the thickeners used, the resultant paste may have to be left to swell for a period of up to three days.

[0101] Surprisingly, it has been found here that the polymers used during paste formulation can advantageously interact associatively with the constituents present in the hybrid sol. This interaction is based on coordination or chelate complex formation between the polymers stirred in for formulation and also the constituents present in the hybrid sol, in this case preferably those of aluminium.

[0102] In the following examples, the preferred embodiments of the present invention are reproduced.

[0103] As stated above, the present description enables the person skilled in the art to use the invention comprehensively. Even without further comments, it will therefore be assumed that a person skilled in the art will be able to utilise the above description in the broadest scope.

[0104] Should anything be unclear, it goes without saying that the cited publications and patent literature should be consulted. Accordingly, these documents are regarded as part of the disclosure content of the present description.

[0105] For better understanding and in order to illustrate the invention, examples are given below which are within the scope of protection of the present invention. These examples also serve to illustrate possible variants. Owing to the general validity of the inventive principle described, however, the examples are not suitable for reducing the scope of protection of the present invention to these alone.

[0106] Furthermore, it goes without saying to the person skilled in the art that, both in the examples given and also in the remainder of the description, the component amounts present in the compositions always only add up to 100% by weight, mol-% or vol.-%, based on the entire composition, and cannot exceed this, even if higher values could arise from the percent ranges indicated. Unless indicated otherwise, % data are therefore regarded as % by weight, mol-% or vol.-%.

[0107] The temperatures given in the examples and description and in the claims are always in C.

EXAMPLES

Example 1

[0108] 8 g of boron oxide were initially introduced in a glass flask and suspended in 80 g of acetic anhydride and 160 g of tetrahydrofuran. The mixture was brought to reflux, and 24.2 g of ethylene glycol monobutyl ether (EGB) were added. 24.2 g of diethoxydimethylsilane and 31 g of dimethyldimethoxysilane were subsequently added to the refluxing mixture, and this was warmed with boiling for 30 minutes. A solution consisting of 480 g of EGB and 250 g of Texanol, in which 2.5 g of water, 2 g of 1,3-cyclohexanedione and 4.2 g of acetaldoxime were dissolved, was added to the siloxane-containing solution and allowed to mix for 20 minutes. Over the same period, the reaction temperature was increased from 80 C. to 120 C. After the mixing, 50 g of aluminium tri-sec-butylate, dissolved in 400 g of dibenzyl ether, were allowed to run into the reaction mixture over the course of five minutes, and the completed mixture was left to react for a further 55 minutes. The reaction mixture was then freed from readily volatile solvents and reaction products by vacuum distillation at 70 C. until a final pressure of 30 mbar had been reached. Various pasty mixtures were prepared from the boron-containing doping ink by stirring in ethylcellulose.

TABLE-US-00001 TABLE 1 Mixtures of boron-containing doping inks subsequently thickened using ethylcellulose. Mixtures from a mass proportion between 2.9% and 3.4% were readily screen- printable. Paste mixtures having a mass proportion >5% of ethylcellulose were no longer printable. Mass of Mass of boron ink ethylcellulose Mass proportion of [g] [g] ethylcellulose [%] 200 2 0.99 200 3 1.48 200 4 1.96 200 5 2.44 200 6 2.91 200 7 3.38 200 8 3.85 200 9 4.31 200 10 4.76

Example 2

[0109] A paste in accordance with Example 1, characterised by a mass proportion of 4.3% of ethylcellulose, was printed onto a silicon wafer surface using a 350 mesh screen having a wire diameter of 16 m, an emulsion thickness of 8 m to 12 m, and furthermore using a squeegee speed of 200 mm/s and a squeegee pressure of 1 bar, and subsequently subjected to drying in a through-flow oven using the following heating zone temperatures: 350/350/375/375/375/400/400 C.

[0110] Paste mixtures having a mass proportion of greater than 5% and also those having a mass proportion of less than 2.5% cannot be processed by means of the screen printing process.

[0111] FIG. 2: shows a silicon wafer printed with the aid of a boron-containing doping paste according to the invention and in accordance with the composition and preparation of Example 1, after drying in a through-flow oven. The different colours (.fwdarw. interference colours) correspond to differences in locally present glass film thicknesses. Optimisation of the printing process results in a more homogeneous colour appearance of the printed wafer.

Example 3

[0112] 4 g of boron oxide were initially introduced in a glass flask and suspended in 40 g of acetic anhydride and 80 g of tetrahydrofuran. The mixture was brought to reflux, and 11.25 g of ethylene glycol monobutyl ether (EGB) were added. 12.1 g of diethoxydimethylsilane and 15.1 g of dimethyldimethoxysilane were subsequently added to the refluxing mixture, and this was warmed with boiling for 30 minutes. 32.5 g of the siloxane-containing solution were mixed with 69.8 g of a solution consisting of 240 g of EGB and 125 g of Texanol, and the heating temperature was increased from 80 C. to 120 C. over the course of 20 minutes with stirring of the reaction mixture. 1.75 g of 1,3-cyclohexanedione, 0.75 g of acetaldoxime and 0.5 g of water were dissolved in the reaction mixture. 10 g of aluminium tri-sec-butylate dissolved in 40 g of dibenzyl ether were subsequently added dropwise to the reaction mixture over the course of five minutes. After the addition, the mixture was left to react for a further 55 minutes. The reaction mixture was then subjected to vacuum distillation at 70 C. until a final pressure of 30 mbar had been reached in order to free the mixture from readily volatile solvents and reaction products. A mass loss of 31.74 g was determined here. Various pasty mixtures were prepared from the boron-containing doping ink by stirring in ethylcellulose: to this end, 5.1 g of ethylcellulose were stirred into 106.1 g of the doping ink. The paste was left to rest overnight after the stirring.

Example 4

[0113] The paste according to the invention in accordance with Example 3 was printed onto an alkaline-etched n-type CZ wafer with the aid of a 400 mesh screen having a wire diameter of 18 m. The other printing parameters corresponded to those which have already been described in Example 2(likewise the layout used). The printed wafer was coated by spray coating with the aid of a phosphorus-containing doping ink, and the wafer was subsequently subjected to a co-diffusion process at 935 C. for 30 minutes, followed by oxidation for five minutes in dry synthetic air, furthermore followed by a further drive-in step of 15 minutes. The boron-doped region was investigated by means of secondary ion mass spectrometry (SIMS). The principal doping of the wafer corresponded to p-doping with boron.

[0114] FIG. 3: shows SIMS doping profiles of an alkaline-etched n-type CZ wafer, printed with a doping paste according to the invention in accordance with Example 3. The doped structure has exclusively intense boron doping. The phosphorus doping corresponds to the background doping of the n-type wafer.

Example 5

[0115] Pastes according to the invention in accordance with Example 1 were investigated with respect to their dynamic viscosity with the aid of a cone-and-plate rheometer. The pastes had non-Newtonian flow properties.

TABLE-US-00002 TABLE 2 Dynamic viscosity of pastes according to the invention in accordance with Example 1. Dynamic Mass viscosity Mass of Mass of proportion of (forwards/ boron ink ethylcellulose ethylcellulose backwards).sup.1 [g] [g] [%] [Pa * s] 200 9 4.31 26.1/23.5 200 10 4.76 24.4/29.2 .sup.1Forwards and backwards curve.

[0116] In a separate batch, 150 g of ethylene glycol monobutyl ether, 75.9 g of Texanol and 121.9 g of dibenzyl ether were mixed. The viscosity of the solvent mixture was 3.47 mPa*s. In one case 3.5 g of Ethocel and in the second case 4.5 g of Ethocel were stirred into 100 g of the solvent in each case. Furthermore, the dynamic viscosity of the boron-containing doping ink was determined in accordance with Example 1. All media investigated exhibited Newtonian flow properties.

TABLE-US-00003 TABLE 3 Dynamic viscosity of pastes according to the invention in accordance with Example 1. Mass of Mass proportion Dynamic ethylcellulose of ethylcellulose viscosity [g] [%] [mPa * s] Solvent mixture 0.0 0.00 3.47 Solvent mixture 3.5 3.38 204 Solvent mixture 4.5 4.31 591 Boron ink 0.0 0.00 15.65

[0117] It becomes clear from a comparison of Tables 2 and 3 that the addition of the thickener to the solvent mixture in which the hybrid sols are dissolved allows the viscosity of the mixture to increase. Without an interaction with the active components of the hybrid sol, an increase in the viscosity to 600 mPas would be expected. By contrast, a corresponding paste mixture having the same mass proportion of ethylcellulose exhibits a dynamic viscosity of 26.1 Pa*s, i.e. approximately 45 times the expected value. For this reason, it can be assumed that the thickeners used in these examples undergo an associative interaction with parts of the hybrid sol, causing the structure formation taking place in the solution to be significantly increased compared with the structure formation taking place in the pure solvent mixture. This structure formation can be explained by means of complex and chelate complex formation of the polymer with the aluminium cores present in the hybrid sol.