EP2735002B1 - Method for producing thin electrically conductive layers of silver, a silver layer, a silver complex, the solution of said silver complex, and the use of the silver complex in a solution - Google Patents

Description

The invention relates to a method for producing thin electrically conductive layers of silver on a surface of substrates, a silver complex and a solution of the silver complex and the use of a silver complex solution. It can preferably be used for the production of electrically conductive structural elements, such as electrodes or conductor tracks, which may also be optically transparent, as is the case, for example, with thin-film solar cells or light-emitting diodes.

For these applications, the use of different materials, such as electrically conductive oxides (TCOs), modifications of carbon (CNTs), electrically conductive polymers and metals is known. Except for the metals, however, the other substances have an increased electrical resistance, if the formed layers should also be optically transparent. In addition, their use is associated with increased costs.

Metal layers, which are preferably made of silver, can hitherto be produced in the required thickness only by means of a known vacuum coating technique, in the required homogeneity while at the same time maintaining sufficient optical transparency, which likewise increases the production costs.

In addition, various possibilities for the reduction of silver compounds to pure silver are known. But a high effort and / or an increased use of energy is required.

The JP 2000 144440 A discloses an electrolytic solution for the electroless deposition of silver. The WO 2006/076611 A2 discloses a process for the preparation of metal nanoparticles. The US 2009/0041404 A1 and the US 2005/0221116 A1 each reveal a deposition of a 100 nm thick silver layer on a substrate.

It is therefore an object of the invention to provide options for producing electrically conductive thin layers on substrate surfaces, which are inexpensive and flexible to produce and provide a simple way for a reduction of a silver compound to pure silver.

According to the invention, this object is achieved by a method having the features of claim 1. The claim 7 relates to a silver complex and the claim 8 is a solution of the silver complex. Advantageous embodiments and further developments of the invention can be achieved with features described in the subordinate claims.

In the method according to the invention for producing thin electrically conductive layers of silver on a surface of substrates, the procedure is that, in a first method step, silver nitrate (AgNO ₃ ) and 2-pyrrolidone in a solvent, preferably water or an ethanol / water mixture be solved. It is also possible, for example, to use water / acetone or water / THF as solvent.

Following dissolution, the solution obtained can be subjected directly to a second process step. However, it is favorable that the solution is kept at room temperature with exclusion of light for several days is, and there is an evaporation of liquid.

Instead of 2-pyrrolidone, one could also use 3-pyrrolidone. Due to the deviating oxygen composition, a different crystal structure and a somewhat different reduction behavior to silver could occur.

Following this process step, in a second process step, the solution is applied to a surface of the substrate to be coated and then subsequently in a third process layer a chemical reduction, which leads to the separation of silver from the other contained chemical components, by irradiation with electromagnetic radiation from the wavelength spectrum of the UV light over a period of at least 15 min, preferably 20 min performed. Thereafter, in a fourth process step, a heat treatment is carried out at a temperature of at most 500 ° C., preferably at 250 ° C., more preferably 220 ° C. over a period of at least 30 min, preferably 60 min, and one at least almost exclusively of silver formed layer on the surface of the substrate. Remains of other chemical elements and compounds with a content of <2% can remain in the layer.

The maximum temperature depends on the temperature permissible for a substrate to be coated and, if appropriate, the melting temperature of silver. Thus, the substrate material should not be adversely affected by the temperature used and at least neither deform nor chemically react. Diffusion processes with components contained in the substrate material should also be avoided.

In the first process step, silver nitrate and 2-pyrrolidone [Ag (pyl) 2] form NO3 - (C8H14Ag1N3O5) as a complex that can crystallize. The monoclinic space group C2 / c (No. 15) with four formula units forms a cell unit. The asymmetric unit contains the 2-pyrrolidone molecules, half a silver atom, half a nitrate anion, which occupy certain positions of a double axis, one of which passes through the N1-O4 bond, as shown in FIG FIG. 1 evident.

The silver atom has a disordered irregular AgO ₆ geometry consisting of two oxygen atoms of a pyrrolidone molecule and four oxygen atoms of the nitrate anion. The distances between the silver atom and the oxygen atoms are in the range 2.358 to 2.683 angstroms. The nitrate anions have a linker function and connect the structure in 1-D polymer chains in one direction. The silver atoms are arranged with linear polymer chains through μ ² -O oxygen-bridge atoms with significantly larger distances between Ag1 and O4 of 2.683 angstroms compared to the distance between Ag1 and O2 of 2.536 angstroms. In the crystal structure, the 1-D polymer chains in the 2D network are linked by intermolecular NH ... O bonds.

In order to avoid or at least reduce the crystallization of the [Ag (pyl) ₂ ] NO ₃ during application to the substrate surface, which may occur due to evaporation of the solvent, a pyrollidone derivative as crystallization inhibitor, preferably tert-butylpyrrolidone, may be used at least 2% by mass to a maximum of 20% by mass of the solution are added. This addition may take place before, during or after the first process step. By using a crystallization inhibitor, a homogeneous and uniform silver layer can be obtained.

In the first process step for the preparation of the solution, a ratio of water to ethanol of 1 to 4, but it can also be selected smaller amounts of water.

For silver and 2-pyrrolidone, a molar ratio of 1 to 2 should be maintained.

In addition, during the first process step, any influence of electromagnetic radiation should be avoided and the solution enclosed in a hermetically sealed optically nontransparent container.

Before performing the second process step, the surface of the substrate should be cleaned. For this purpose, a suitable liquid, which can be selected taking into account the substrate material, are used. For example, for a glass substrate, this may be "piranha solution", So be an aqueous solution of Peroxomonosschwefelsäure. It can be cleaned with ultrasound support.

The order of the solution after the first process step on the surface of the substrate, which can also consist of a preferably optically transparent polymer in addition to the aforementioned glass, can be done by a dip, spin coating or a printing process. It should only be ensured that a constant layer thickness can be achieved in order to achieve homogeneous electrical and / or optical properties of the layer formed over the coated surface.

For example, and preferably those which are to be counted as nanoimprint lithography or microcontact printing can be used as the printing method. It is thus also possible to produce geometrically differently structured layers on substrate surfaces. This is also possible with screen printing technology. In the simplest way, a substrate can also be immersed in the prepared solution and pulled out again (dipcoating) before the third and fourth process steps are carried out. The formation of the layers can also be done by spin coating.

Care should be taken that the layers formed by the invention should not exceed a maximum layer thickness of 200 nm. Layer thicknesses in the range 50 nm to 100 nm are to be preferred in order to be able to comply with a sufficient electrical conductivity.

The tert-butylpyrrolidone can be prepared from 20 ml of tetrahydrofuran (THF), triethylamine and tert-butylamine, which are purged with argon and mixed together at a temperature of 0 ° C. 4-Chlorobutyryl chloride is added to the mixture and vigorous stirring is carried out at this temperature. Subsequently, triethylamine hydrochloride is filtered out and a double washing with THF is carried out. The resulting filtrate is concentrated under reduced pressure and the resulting starting material is mixed with acetate and then washed once with HCL and twice with brine. The resulting organic phase can be dried on a substrate with MgSO4 and the solvent removed at reduced pressure.

The tert-buty-4-chlorobutanamide then obtained is dissolved in THF and added to a solution of potassium tert-butylate with THF. After prolonged stirring in an ice bath, this mixture from the container can be placed in another container and mixed therein with ethyl acetate and then washed twice with brine. The resulting organic phase can be dried again with MgSO ₄ and the solvent removed under reduced pressure. After distillation at reduced pressure and a temperature of 75 ° C, the tert-butylpyrrolidone can be obtained as a colorless liquid.

This production is for example of K. Takao et al. In "Molecular and Crystal Structures of Uranyl Nitrate Complexes with N-Alkylated 2-Pyrrolidone Derivatives: Design and Optimization of Promising Precipitant for Uranyl Ion"; Cristal Growth &Design;2008; 8 (7); Pp. 2364-2376 described.

With the invention, sufficiently conductive thin silver layers can be produced which have electrical resistivities <10 Ω / □ at a thickness of 100 nm, which is a sheet resistance, which is expressed by the "□".

Below, the invention will be explained in more detail by way of example.

It shows FIG. 1 in schematic form, the linkage function of nitrate anions connecting the 1-D polymer chains of [Ag (pyl) ₂ ] NO ₃ .

When preparing a thin layer of silver on a substrate, first 0.34 g (2 mmol) of silver nitrate was dissolved in 1 ml of deionized water. To this solution was added 0.34 g (4 mmol) of 2-pyrrolidone. Already in the preparation of this solution or even afterwards, the influence of electromagnetic radiation, in particular that of light, has been reduced by suitable measures, such as e.g. prevented by the use of a radiopaque vessel.

The formed reaction product [Ag (pyl) ₂ ] NO ₃ crystallizes out during storage at normal room temperature and the water as solvent evaporates slowly.

For the production of thin layers, the water, preferably after several days storage of the solution previously prepared in a first process step can be added with ethanol, so that a ratio of water to ethanol of 1 to 4 and maintained a 0.8 M solution.

The substrate was preliminarily cleaned with a solution of 30% H ₂ O ₂ and three parts of concentrated H ₂ SO ₄ over a period of 30 minutes, then thoroughly washed with deionized water and then dried.

On the surface of the substrate thus prepared was applied the previously prepared solution with the [Ag (pyl) ₂ ] NO ₃ by immersion and extraction, and then over a period of 20 minutes with electromagnetic radiation in a UV box with an F-emitter irradiated. The irradiation took place in the wavelength range between 280 nm and 400 nm, so that a partial region of the radiation used was above 315 nm and the remainder in the UV range.

In this irradiation, reduction of [Ag (pyl) ₂ ] NO _{3 occurs} and pure silver is obtained.

This was followed by a heat treatment in which heated at a heating rate of 5 K / min to a maximum temperature of 220 ° C and the temperature was kept for 60 min to remove the remaining constituents of the pyrrolidone.

In this case, an approximately 100 nm thick layer could be obtained on the substrate, which had an electrical resistance of <10 Ω / □.

When the solution was applied to the substrate surface by microcontact printing, a layer thickness of 60 nm of the silver layer could be achieved. Line structures with a width of 20 μm can be printed. However, it is possible to print smaller, filigree structures down to the "nanometer range".

The solution obtained by the first process step may be added to suppress the crystallization of the [Ag (pyl) ₂ ] NO ₃ complex 10% by mass of the pyrrolidone derivative tert-butylpyrrolidone. As a result, a disorder in the polymer chain can be achieved, which prevents the crystallization, which facilitates the application of the solution to the substrate surface.

Claims (9)

1. A method for producing thin electrically conductive layers of silver on a surface of substrates, in which
   in a first method step silver nitrate and 2-pyrrolidone are dissolved in a solvent, preferably water or an ethanol/water mixture, and in so doing the formation of [Ag(Pyl)₂]NO₃ takes place by complexing, and following this method step
   in a second method step the solution is applied to a surface of the substrate to be coated, and then
   in a third method step a chemical reduction, which results in the separation of silver from the other chemical components contained, is carried out by irradiation with electromagnetic radiation from the wavelength spectrum of UV-light over a period of at least 15 min., and following thereon
   in a fourth method step a heat treatment is carried out at a temperature of at most 500°C, preferably at 220°C over a period of at least 30 min, preferably 60 min, and in so doing a layer formed at least virtually exclusively of silver on the surface of the substrate is obtained.
2. A method according to Claim 1, characterised in that the solution obtained in the first method step is kept at room temperature with light excluded for several days, and in so doing evaporation of liquid takes place.
3. A method according to Claim 1 or 2, characterised in that in the fourth method step a heating rate in the range from 3K/min to 10 K/min, preferably of 5 K/min, is maintained.
4. A method according to one of the preceding claims, characterised in that a molar ratio of silver to 2-pyrrolidone of 1 to 2 is maintained.
5. A method according to one of the preceding claims, characterised in that in or directly after the first method step a pyrrolidone derivative, preferably tert. butylpyrrolidone with a proportion of at least 2 mass % to a maximum of 20 mass % is added to the solution.
6. A method according to one of the preceding claims, characterised in that in the second method step the coating is carried out with a printing or dipping process.
7. A crystalline silver complex having the formula [Ag(2-pyrrolidone)₂]NO₃, produceable by crystallising a complex of silver nitrate and 2-pyrrolidone, wherein a monoclinic space group C2/c with four formula units to a cell unit is formed.
8. A solution of the silver complex according to Claim 7 containing silver nitrate, 2-pyrrolidone, water and a further organic solvent, characterised in that the solution additionally contains tert. butylpyrrolidone.
9. A solution according to Claim 8, characterised in that the solution contains ethanol and tetrahydrofuran or acetone as further organic solvents.

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