HK1058573A - Transparent polythiophene layers of high conductivity - Google Patents

Description

Transparent polythiophene layers of high conductivity

Technical Field

The invention relates to a method for producing a transparent conductive layer having a conductivity of at least 500S/cm, to a corresponding conductive layer and to the use thereof.

Background

Polythiophene layers are known, as are the electrochemical or chemical oxidation of suitable thiophenes for the production of polythiophene layers and their use for antistatic finishing of non-conductive or poorly conductive substrates.

EP 206133 a1 describes a method of applying a layer of a conductive polymeric heterocyclic compound, which is prepared using an oxidizing agent, to a non-conductive or poorly conductive substrate. The conductivity of the polymers which can be prepared according to EP 206133A 1 has a maximum of 100S/cm.

EP 253594A 2 describes, inter alia, thiophenes which are optionally substituted in the 3-and/or 4-position by alkyl and/or alkoxy groups, and also electrically conductive polymers which are obtained by chemical or electrochemical oxidation. The conductivity of the polymers and copolymers prepared by chemical oxidation described in EP 253594A 2 amounts to only up to 0.05S/cm. If the polymers and copolymers are prepared electrochemically by oxidation, the conductivity can be as high as 1050S/cm. However, a disadvantage of electrochemical oxidation is that the required equipment makes the electrochemical oxidation process significantly more complex than the chemical oxidation process. In addition, the polymers obtained by electrochemical oxidation are substantially insoluble, which greatly limits the potential applications of such polymers. Electrochemical oxidation generally results in uniform polymer films only having a layer thickness of up to about 200nm, beyond which the mechanical strength is low and quite rough.

U.S. Pat. No. 3, 4,521,589 describes A process for preparing poly-3-alkylthiophenes by reacting 3-alkyl-2, 5-dihalothiophenes with magnesium in the presence of nickel compounds in an inert organic solvent. The polythiophene obtained in this way has a conductivity of 9X 10^-14S/cm. The conductivity can be increased to about 0.5S/cm by reaction with iodine.

EP 203438 a1 and EP 257573 a1 describe a process for preparing organic solvent-soluble, substituted, electrically conductive polythiophenes and the use of such soluble polythiophene solutions for antistatic finishing of substrates which are not electrically conductive or have poor electrical conductivity. The conductivity of the polythiophenes described in EP 203438A 1 is preferably greater than 10^-2S/cm, but it is only possible to achieve the above-mentioned conductivity by doping the polymer with an electron donor, such as iodine. Even after doping, the maximum conductivity reaches only 15S/cm. Polythiophenes having a conductivity of up to 100S/cm can be prepared according to EP 257573A 1 by electrochemical polymerization.

EP 339340A 2 describes 3, 4-substituted polythiophenes which are prepared by oxidative polymerization of the corresponding thiophenes. The oxidizing agents used include iron (III) salts of organic acids and iron (III) salts of inorganic acids containing organic groups. For example, mention may be made of C₁-C₂₀Iron (III) salts of alkylsulfonic acids and iron (III) salts of aromatic sulfonic acids. The layer produced with this polythiophene has a conductivity of only 0.01 to 10S/cm. Also, if not chemically oxidized but electrochemically oxidized, the conductivity may rise to 200S/cm. But the disadvantages have been described above.

EP 820076 a2 describes a capacitor containing a solid electrolyte, the electrolyte of which comprises a conductive polymer. The solid electrolyte is composed of a polymer of pyrrole, thiophene, furan, aniline or derivatives thereof, and is doped with polysulfonic acid, organic sulfonic acid with hydroxyl or carboxyl, alicyclic sulfonic acid or benzoquinone sulfonic acid. EP 820076 a2 describes a method for impregnating a tantalum foil which has a large surface area and scatters light. Thus, the solid electrolyte described in EP 820076 a2 is not transparent, but opaque. The specific conductivity of the solid electrolyte is not described in EP 820076 a 2. According to EP 820076A 2, for the use of the polymer layer as a solid electrolyte, an electrical conductivity of the polymer layer of 10 to 100S/cm is sufficient. The method of preparing the higher conductivity polymer layer is not specifically described.

However, many applications require particularly high electrical conductivity, and it is therefore an object of the present invention to provide electrically conductive layers having a very high electrical conductivity, which are furthermore characterized by a high transparency.

Surprisingly, transparent, electrically conductive layers having particularly high conductivity can be produced using polymers prepared from suitable thiophenes by chemical oxidation, the oxidant used being an iron (III) salt of an alicyclic sulfonic acid.

Disclosure of Invention

The invention therefore relates to a process for the preparation of a transparent, electrically conductive layer having an electrical conductivity of more than 500S/cm, by polymerization of one or more thiophenes of the general formula (I)Wherein

R¹And R²Independently of one another, are optionally substituted, straight-chain or branched alkyl, aryl, alkylaryl or heterocyclic radicals having from 1 to 10 carbon atoms,

or R¹And R²Taken together as a straight or branched, substituted or unsubstituted alkylene group of 1 to 18 carbon atoms,

wherein the polymerization is carried out by chemical oxidation and the oxidizing agent used is an iron (III) salt of a cycloaliphatic sulfonic acid.

R¹And R²Independently of one another, are preferably straight-chain or branched alkyl radicals having 1 to 6 carbon atoms, C₆-C₁₀-aryl or C₁-C₆-alkyl-C₆-C₁₀-aryl, or R¹And R²Taken together as a straight chain, optionally substituted alkylene group containing from 1 to 10 carbon atoms.

The invention also relates to a conductive layer obtained by this method and to the use thereof.

The process of the invention is preferably carried out using compounds of the general formula (II)Wherein

R³Is- (CH)₂)_m-CR⁴R⁵-(CH₂)_n-, wherein

R⁴And R⁵Identical or different, are hydrogen, straight-chain or branched alkyl having 1 to 18 carbon atoms, OH, O-CH₂-CH₂-CH₂-SO₃H or O-alkyl having 1 to 18 carbon atoms, and

n and m are independent of each other and are each an integer of 0 to 9, and n + m is not more than 9.

R⁴And R⁵Independently of one another, preferably hydrogen, straight-chain alkyl having 1 to 6 carbon atoms, OH, O-CH₂-CH₂-CH₂-SO₃H or O-alkyl having 1 to 6 carbon atoms. R⁴And R⁵Hydrogen is particularly preferred.

Examples of compounds which can be used in the process of the invention are dimethoxythiophene, diethoxythiophene, dipropoxythiophene, dibutoxythiophene, methylenedioxythiophene, ethylenedioxythiophene, propylenedioxythiophene, butylenedioxythiophene, thiophenes substituted by hydroxy or alkoxy groups as described in U.S. Pat. No. 4,5,111,327, thiophenes bearing CH groups₂-O-(CH₂)_n-SO₃Thiophenes of the H group, in which n is an integer from 2 to 10, and substituted by alkyl, preferably by C₁-C₁₀-alkyl substitutedEthylenedioxythiophene.

Iron (III) salts which can be used in the process of the invention are iron (III) salts of cycloaliphatic sulphonic acids. The sulfonic acid moiety of the iron (III) salt is a sulfonic acid containing an alicyclic ring of 4 to 20 carbon atoms and one or more sulfonic acid groups.

Examples of cycloaliphatic sulfonic acids which can be used in the process according to the invention are cyclohexane sulfonic acid, methylcyclohexane sulfonic acid, cycloheptane sulfonic acid, camphorsulfonic acid and sulfonic acids which can be prepared, for example, by hydrogenation of aromatic sulfonic acids.

The process of the invention preferably uses iron (III) salts of camphorsulfonic acid, and racemates of (+) -iron (III) camphorsulfonate, (-) -iron (III) camphorsulfonate, (+) -iron (III) camphorsulfonate and (-) -iron (III) camphorsulfonate or any desired mixture of (+) -iron (III) camphorsulfonate and (-) -iron (III) camphorsulfonate may be used.

No further oxidizers and/or dopants have to be added and the use of these is preferably avoided.

The method according to the invention results in electrically conductive transparent layers which contain polythiophenes having positive charges in the polymer chain and counterions to these charges. The polymers present in the layers prepared by the process of the invention can be illustrated in a simplified and schematic manner by the formula (III).Wherein

R¹And R²The definition of (A) is as above,

X^-is the sulfonate ion corresponding to the iron (III) salt of the cycloaliphatic sulfonic acid used in the process of the invention,

n has an average value of 1 to 20, and m has an average value of 2 to 10,000.

The oxidative polymerization of the thiophenes of the formulae I and II by chemical oxidation can be carried out at temperatures of from-10 to +250 ℃ in general, preferably at temperatures of from 0 to 200 ℃ depending on the desired reaction time.

An inert organic solvent is continuously added to the thiophene to be used under reaction conditions to give a coating solution which can be applied to a substrate. Examples of inert organic solutions which may be mentioned are, in particular: aliphatic alcohols such as methanol, ethanol and propanol; aliphatic ketones such as acetone and butanone; aliphatic carboxylic acid esters such as ethyl acetate and butyl acetate; aromatic hydrocarbons such as toluene and xylene; aliphatic hydrocarbons such as hexane, heptane and cyclohexane; chlorinated hydrocarbons such as dichloromethane and dichloroethane; aliphatic nitriles such as acetonitrile; aliphatic sulfoxides and sulfones, such as dimethyl sulfoxide and sulfolane; aliphatic amides such as dimethylacetamide, dimethylformamide and N-methylpyrrolidone; aliphatic and araliphatic ethers, such as diethyl ether and anisole. The solvent used may also be water or a mixture of water and the abovementioned organic solvents, provided that the latter are miscible with water.

Thiophenes of the formulae I and II are oxidatively polymerized, theoretically requiring 2.25 equivalents of oxidizing agent per mole of thiophene (see, for example, J.Polymer. Sc.part A, Polymer Chemistry Vol.26, p.1287 (1988)). In practice, however, the oxidizing agent is used in a certain excess, for example in an excess of 0.1 to 2 equivalents per mole of thiophene.

The transparent conductive layer may be prepared by using thiophene and an oxidizing agent in combination or separately. For separate use, the substrate to be coated is first treated with an oxidant solution and then with a thiophene solution. If a combination of thiophene and an oxidizing agent is used, the substrate to be coated is generally coated only with a solution containing thiophene and an oxidizing agent. Since a part of thiophene is evaporated when used in combination, the amount of the oxidizing agent added to the solution can be reduced corresponding to the expected amount of thiophene lost.

Before the production of the coating, binders and/or crosslinkers, for example polyurethanes, polyacrylates, polyolefins, epoxy silanes, such as 3-glycidoxypropyltrialkoxysilane, can be added to the coating solution. In addition, in order to improve the scratch resistance of the coating, silanes or hydrolysis products of silanes, for example tetraethoxysilane-based silanes or hydrolysis products of silanes, may be added.

The coating solution applied to the substrate to be coated preferably contains from 1 to 30% by weight of the thiophene corresponding to the formula I and/or II and from 0 to 30% by weight of the binder, the percentages by weight being based on the total weight of the solution. The coating solution can be applied to the substrate using conventional methods, such as spraying, knife coating, spin coating, brushing, or printing.

After application of the coating solution, the solvent can be removed by evaporation at room temperature. In order to achieve a relatively high processing speed, it is advantageous, however, to increase the temperature at which the solvent is removed, for example at a temperature of from 20 to 250 ℃ and preferably at a temperature of from 40 to 200 ℃.

Removal of the solvent at elevated temperatures is beneficial because it has been found that the electrical conductivity of the layer of the invention can be increased by heat treating the coating at 50-250 c, preferably 100-200 c. This heat treatment can be carried out immediately after the removal of the solvent or at a time interval after the preparation of the coating.

After removal of the solvent (drying), the excess oxidizing agent in the coating is preferably washed away. For this purpose water may be used, which may optionally be mixed with organic sulfonic acids or lower alcohols, such as methanol and ethanol.

Substrates which can be coated by the process according to the invention are, in particular, inorganic transparent substrates made of glass, silica and ceramic materials, sheet-like transparent substrates made of organic plastics, for example transparent films made of polycarbonate, polyamide, polyolefin or polyester. If desired, the substrate can be coated with adhesion promoters, for example silanes, before the actual coating, for example in order to produce better adhesion.

The thickness of the applied coating after drying can generally be from 0.01 to 100 μm, depending on the desired conductivity and transparency of the coating.

The transparent conductive layer that can be prepared with the process of the present invention has a specific conductivity of at least 500S/cm, preferably at least 1000S/cm. Layer thickness and surface resistance as determined by profilometer (2X 2cm strip, resistance determined by commercially available resistivity meter), or specific conductivity using a commercially available four-point meter.

The film layer has high transmittance. The light transmittance is preferably at least 50%, particularly preferably at least 75%. The transmittance here is determined by transmission in the visible region (300-800nm) using a commercially available UV-VIS spectrometer and results in an arithmetic mean of at least three independent values.

The invention also relates to a transparent conductive layer obtained by the method of the invention.

The conductive transparent layer of the present invention is suitable for, for example, finishing of plastic films for packaging electronic components and plastic films for clean room packaging, antistatic processing of cathode ray tubes, antistatic processing of photographic films, transparent heaters, transparent electrodes, circuit boards, or electrically tintable window panes.

Examples

Specific conductivities were determined by measuring layer thickness (profilometer) and surface resistance (2X 2cm strip, resistance measured with a commercially available resistance meter), or using a commercially available four-point meter.

Example 1

0.25g of 3, 4-ethylenedioxy-thiophene (1.76mmol) and 5.0g (13.3mmol) of a butanol solution of 54% by weight of iron (III) camphorsulfonate salt are dissolved in one another and applied to the glass plates with the aid of a commercially available spin coater at different spin speeds. The coated glass plates were dried at room temperature and then conditioned at 80 ℃ for 1 hour. After cooling, the glass plate was rinsed with water and dried. Coating thickness and specific conductivity are listed in table 1.

TABLE 1

Rotational speed	Specific conductivity (S/cm)	Coating thickness (nm)
			1500	1035	420
2000	1276	340

Comparative example 1

0.25g of ethylenedioxythiophene (1.76mmol) and 5.0g (3.80mmol) of iron (III) p-toluenesulfonate (40% by weight) in butanol are dissolved in one another and applied to the glass plates with the aid of a commercially available spin coater at different spin speeds. The coated glass plates were dried at room temperature and then conditioned at 80 ℃ for 1 hour. After cooling, the glass plate was rinsed with water and dried. Coating thickness and specific conductivity are listed in table 2.

TABLE 2

Rotational speed	Specific conductivity (S/cm)	Coating thickness (nm)
			500	119	380
1000	122	240

Comparative example 2

0.25g of ethylenedioxythiophene (1.76mmol) and 5.0g (3.80mmol) of iron (III) phenol-4-sulfonate 43.6% by weight in butanol are dissolved with one another and applied to the glass plates with the aid of a commercially available spin coater at different spin speeds. The coated glass plates were dried at room temperature and then conditioned at 80 ℃ for 1 hour. After cooling, the glass plate was rinsed with water and dried. Coating thickness and specific conductivity are listed in table 3.

TABLE 3

Rotational speed	Specific conductivity (S/cm)	Coating thickness (nm)
			500	2.5	1300

Comparative example 3

0.25g of ethylenedioxythiophene (1.76mmol) and 5.0g (3.80mmol) of 25.9% by weight iron (III) methanesulfonate in butanol were dissolved in one another and applied to glass plates at different spin speeds with the aid of a commercially available spin coater. The coated glass plates were dried at room temperature and then conditioned at 80 ℃ for 1 hour. After cooling, the glass plate was rinsed with water and dried. Coating thicknesses and specific conductivities are listed in table 4.

TABLE 4

Rotational speed	Specific conductivity (S/cm)	Coating thickness (nm)
			500	102	1300

Claims (8)

1. A process for preparing a transparent, electrically conductive layer having a specific conductivity of at least 500S/cm by polymerizing a thiophene of the formula (I) or a mixture of thiophenes of the formula (I)Wherein

R¹And R²Independently of one another, is an optionally substituted, linear or branched alkyl, aryl, alkylaryl or heterocyclyl radical having from 1 to 10 carbon atoms,

or R¹And R²Combined together to be straight-chain or branchedSubstituted or unsubstituted alkylene of 1 to 18 carbon atoms,

characterized in that the polymerization is carried out by a chemical oxidation process, wherein the oxidant used is an iron (III) salt of a cycloaliphatic sulphonic acid.

2. A process for preparing transparent conductive layers according to claim 1, characterized in that the thiophene used is a compound of the formula (II)Wherein

R³Is- (CH)₂)_m-CR⁴R⁵-(CH₂)_n-, wherein

R⁴And R⁵Independently of one another, hydrogen, straight-chain or branched alkyl having 1 to 18 carbon atoms, OH, O-CH₂-CH₂-CH₂-SO₃H or O-alkyl having 1 to 18 carbon atoms, and

n and m are independent of each other and are each an integer of 0 to 9, and n + m is not more than 9.

3. A method for producing a transparent conductive layer according to claims 1 and 2, characterized in that the thiophene used is 3, 4-ethylenedioxythiophene.

4. A method for producing a transparent conductive layer according to claims 1 to 3, characterized in that the iron (III) salt used is an iron (III) camphorsulfonate salt.

5. The method for producing a transparent conductive layer according to claims 1 to 4, characterized in that the conductive layer has a light transmittance of at least 50%.

6. The method for producing a transparent conductive layer according to claims 1 to 5, characterized in that the specific conductivity of the transparent conductive layer is at least 1000S/cm.

7. Conductive layer prepared by at least one of the processes of claims 1 to 6.

8. Use of the conductive layer according to claim 7 in the finishing of plastic films for packaging electronic components and plastic films for clean room packaging, antistatic processing of cathode ray tubes, antistatic processing of photographic films, transparent heaters, transparent electrodes, circuit boards or electrically tintable pane.