Description

The invention relates to a conductive paste comprising from 30 to 97 % by weight of electrically conductive particles, from 3 to 70 % by weight of an organic medium and from 0 to 20 % by weight of a glass frit. Conductive pastes or inks can be used to form electrodes, such as conductive grid lines, for example silver grid lines, and bus bars on the surface of semiconductor substrates or substrates from an insulating material. A particularly preferred use is screen printing of electrodes on semiconductor substrates for the production of solar cells or photovoltaic cells which convert solar energy to electrical energy when photons from sunlight excite electrons on semiconductors from the valance band to the conduction band. The electrons which flow to the conduction band are collected by the metal electrodes that contact the semiconductors.

Besides of printing electrodes on semiconductor substrates for producing solar cells or photovoltaic cells, conductive pastes or inks can be used for printing grid lines on insulating sub- strates for producing printed electronic circuit boards or hybrid circuits on ceramic substrates.

To print fine lines onto either a semiconductor substrate or an insulating substrate, generally screen printing processes are used for cost efficient mass production. Nevertheless, to generate uniform narrow lines without line interruptions is challenging for screen printing, particularly for high speed screen printing. The industrial printing speed depends on the application requirement. It ranges from 80 mm/s to 800 mm/s, preferably not slower than 150 mm/s, for example in solar cell printing, the printing speed is 150 mm/s to 300 mm/s.

When the conductive paste is used for printing on semiconductor substrates, the paste usually comprises electrically conductive particles, which typically are metal powders, an organic medium and optionally a glass frit. The organic medium typically comprises at least one organic liquid, such as organic solvents or organic salts or other organic compounds having liquid form at room temperature. The organic medium comprises optionally a polymeric component. To form metal contacts, the conductive paste is printed onto a substrate. Depending on the type of ma- terials, the substrate is then heated at a temperature in the range from about 150 °C to about 950 °C, where the organic medium is decomposed and the inorganic species form the conductive tracks and electrical contact to the substrate.

Such conductive pastes are disclosed for example in CN-A 102831951 , CN-A 102592710, CN- A 104078097, WO-A 2006/1 16376, CN-B 102831951 or US 6,787,342.

However, to be used in screen printing processes, a paste has to be released in a quantitative way from the surfaces of the materials the screen is made of and completely transferred to the surface of the substrate. On the other hand, the paste must show sufficient adhesion on the surface of the substrate the paste is printed on to achieve fine lines which have a sufficient thickness and do not show any interruptions. Therefore it has been an object of the invention to provide a conductive paste which can be printed by screen printing without adhering in a too strong way on the material of the screen and which has a sufficient adhesion to form fine grid lines without seepage on the substrate on which the paste is printed. This object is achieved by a conductive paste comprising from 30 to 97 % by weight of electrically conductive particles, from 3 to 70 % by weight of an organic medium and from 0 to 20 % by weight of a glass frit, wherein the organic medium comprises 0.1 to 50 % by weight of the organic medium of a polar solvent. This polar solvent shows partial phase separation in a mixture of 0.1 to 90 % by weight of said polar solvent, 0 to 89.9 % by weight butyl carbitol acetate and 10 % by weight of tetradecane.

Surprisingly it has been found that a paste which comprises an organic medium with a polar solvent which shows partial phase separation in a mixture of 0.1 to 90 % by weight of the polar solvent, 0 to 89.9 % by weight butyl carbitol acetate and 10 % by weight tetradecane releases from the material of the screen in a sufficient manner and adheres on the substrate for forming fine grid lines without seepage and interruptions.

In the context of the present invention the partial phase separation in a mixture of 0.1 to 90 % by weight of the polar solvent, 0 to 89.9 % by weight butyl carbitol acetate and 10 % by weight tetradecane only defines the polar solvent. It is not mandatory that either butyl carbitol acetate or tetradecane are present in the conductive ink.

In the present application phase separation describes the partial spatial separation of two liquids in a mixture of these two liquids at 25 °C and ambient pressure due to partial immiscibility of these two liquids.

Partial phase separation is macroscopically visible via the formation of an interface between two spatially separated phases which can be characterized by an abrupt change of physical properties comprising density, refractive index, color, viscosity or turbidity. Phase separation can exist without a macroscopically visible interface via formation of an emulsion. Such emulsions can be stable or change with time into partially phase separated systems due to differences in density of the continuous phase and the emulsion droplets. In phase separated mixtures of immiscible liquids the phase with components of lower density is located above the phase with components of higher density.

Miscibility of two liquids is enabled by formation of a negative Gibb's enthalpy of mixing ΔΘ in kJ/mol. Following the theory of thermodynamics ΔΘ is defined by ΔΘ = ΔΗ - T AS, whereas ΔΗ is the enthalpy of mixing in kJ/mol, T is the temperature in Kelvin and AS is the entropy of mixing in kJ/(K mol). Mixtures of two liquids which show negative ΔΗ and positive AS are therefore completely miscible resulting in no phase separation.

Immiscibility or phase separation is never complete due to the presence of a chemical equilibrium. One skilled in the art would describe a chemical equilibrium with its equilibrium constant K. Following the theory of thermodynamics Gibb's free enthalpy of mixing ΔΘ is related to the chemical equilibrium constant via

ΔΘ = -RT ln(K), whereas R is the universal gas constant, T the temperature and ln(K) the natural logarithm of the equilibrium constant. This equation describes the impossibility of complete phase separation of two immiscible liquids.

In practice a mixture comprising 10 g of tetradecane, 0 to 90 g of butyl carbitol acetate and 0 to 90 g of a polar solvent comprising propylene carbonate, butylene carbonate, DBE4, DBE5, tri- acetin or tripropionin can result in formation of an interface with one separated upper phase containing less than 10 g of tetradecane whereas the remaining tetradecane is still dissolved in the second lower separated phase comprising tetradecane, butyl carbitol acetate and said polar solvent. The mass of said remaining tetradecane which is not present in the upper phase depends on the equilibrium constant K and the respective Gibb's free enthalpy of mixing of the respective mixture of solvents. The upper phase contains mainly tetradecane due to the higher density of the other solvents.

Screens for screen printing are manufactured by covering selected areas in meshes of stainless steel wires with a polymer layer which is called screen emulsion. During the screen printing pro- cess the paste can only flow through the areas of the screen which is not covered by the screen emulsion. For this reason the screen printing paste has to show minimum adhesion on the surface of the stainless steel wires and on the surface of the screen emulsion.

One of skill in the art will recognize that adhesion between the paste surface and the screen materials is dominated by attractive dispersive forces between the paste surface and the surfaces of the screen materials. These attractive dispersive forces between the paste surface and the screen materials can be reduced in the following way: the paste contains unpolar solvents like aliphatic solvents or fluorinated hydrocarbons and polar solvents. Polar solvents and unpolar solvents show limited miscibility. Due to the limited miscibility there is an enrichment of liquid solvents on the paste surface. The phase separated liquids form a lubrication layer on the paste surface and reduce the adhesion between the paste surface and the surface of other materials. By this lubrication layer adhesion of the paste on the screen will be reduced. In a preferred embodiment of the present invention the organic medium has a boiling point of at least 230 °C. The higher the boiling point of a solvent the lower the vapour pressure of this solvent at room temperature. Solvents with a boiling point below 230 °C show too high vapour pressure at room temperature and cause therefore too fast paste drying during the screen print- ing process. Paste drying during the screen printing process increases the paste viscosity resulting in line interruptions during fine line screen printing due to limited paste flow.

In one embodiment of the invention, the organic medium additionally comprises at least one second organic compound which is not miscible with the polar solvent. The second organic compound preferably is selected from the group consisting of hydrocarbon-based lubricating oils, fluorinated components, fatty acid esters, esters of caproic acid, caprylic acid, capric acid, undecanoic acid and mixtures thereof.

Hydrocarbon-based lubricating oils useful in this invention include all common mineral oil base stocks. This would include oils that are naphthenic, paraffinic or aromatic in chemical structure. Naphthenic oils are made up of methylene groups arranged in ring formation with paraffinic side chains attached to the rings. The pour point is generally lower than the pour point for paraffinic oils. Paraffinic oils comprise saturated, straight chain or branched hydrocarbons. The straight chain paraffins of high molecular weight raise the pour point of oils and are often removed by dewaxing. Aromatic oils are hydrocarbons of closed carbon rings of a semi- unsaturated character and may have attached side chains. This oil is more easily degraded than paraffinic and naphthenic oils leading to corrosive by-products. In reality a base stock will normally contain a chemical composition which contains some proportion of all three types (paraffinic, naphthenic and aromatic). For a discussion of types of base stocks, see for example "Motor Oils and Engine Lubricating by A. Schilling, Scientific Publications, 1968, section 2.2 to 2.5". Preferably, the second organic compound has a boiling point of at least 230°C, too.

According to the invention the composition contains 0.1 to 50 % by weight of the polar solvent which shows partial phase separation in a mixture of 0.1 to 90 % by weight of the polar solvent, 0 to 89.9 % by weight butyl carbitol acetate and 10 % by weight tetradecane. Preferably the amount of the polar solvent is in the range from 0.5 to 30 % by weight and particularly preferred is an amount of 1 to 20 % by weight.

In one embodiment of the invention, the polar solvent is selected from the group consisting of carbonate esters, glycerol esters, alcohols and mixtures thereof.

If a carbonate ester is used as polar solvent, the carbonate ester preferably is selected from propylene carbonate, butylene carbonate or mixtures thereof. If on the other had a glycerol es- ter is used as polar solvent, the glycerol ester is selected from triacetin, tripropionin or mixtures thereof.

If an alcohol is used as polar solvent, the alcohol is preferably selected from 1 ,3-propanediol, 1 ,4-butanediol, 1 ,5-pentanediol or mixtures thereof.

The electrically conductive particles present in the electrically conductive paste may be particles of any geometry composed of any electrically conductive material. Preferably, the electrically conductive particles comprise carbon, silver, gold, aluminum, platinum, palladium, tin, nickel, cadmium, gallium, indium, copper, zinc, iron, bismuth, cobalt, manganese, molybdenum, chromium, vanadium, titanium, tungsten, or mixtures or alloys thereof or are in the form of core-shell structures thereof. Preferred as material for the electrically conductive particles are silver or aluminum, particularly silver due to good conductivity and good oxidation resistance. The mean particle size of the electrically conductive particles preferably is in the range from

10 nm to 100 μηη. More preferably, the mean particle size is in the range from 100 nm to 50 μηη and particularly preferred, the mean particle size is in the range from 500 nm to 10 μηη. The electrically conductive particles may have any desired form known to those skilled in the art. For example, the particles may be in the form of flakes, rods, wires, nodules, spheres or any mix- tures thereof. Spherical particles in context of the present invention also comprise particles with a real form which deviates from the ideal spherical form. For example, spherical particles, as a result of the production, may also have a droplet shape or be truncated. Suitable particles which can be used to produce the conductive paste are known to those skilled in the art and are commercially available. Particularly preferably, spherical silver particles are used. The advan- tage of the spherical particles is their improved rheological behavior compared to irregular shaped particles.

The proportion of electrically conductive particles in the composition is in the range from 30 to 97 % by weight. The proportion is preferably in the range from 70 to 95 % by weight and par- ticularly preferred in the range from 85 to 92 % by weight. This weight percentage of solid particles is often referred as solids content.

The particle shapes and sizes do not change the nature of this invention. Particles can be used as mixtures of different shapes and sizes. It is known to those skilled in the art that the particles with mixtures of different shapes or sizes can result in higher or lower viscosity when they are dispersed in the same organic medium. In such case, it is known to those skilled in the art that the organic medium needs to be adjusted accordingly. The adjustment can be but is not limited to variations of solids content, solvent content, polymer content, thixotrope content and/or surfactant content. As an example, typically when nano-sized particles are used to replace micron- sized particles, the solids content has to be reduced to avoid an increase of the viscosity of the paste, which results in higher contents of organic components. The electrically conductive particles, especially when made of a metal, generally are coated with organic additives in the course of production. In the course of preparation of the composition for printing conductor tracks, the organic additives on the surface are typically not removed, such that they are then also present in the conductive paste. The proportion of additives for stabiliza- tion is generally not more than 10 % by weight, based on the mass of particles. The additives used to coat the electrically conductive particles may, for example, be fatty amines or fatty amides, for example dodecylamine. Further additives suitable for stabilizing the particles are, for example, octylamine, decylamine, and polyethyleneimines. Another embodiment may be fatty acids, fatty acid esters, with or without epoxylation, for example, lauric acid, palmitic acid, oleic acid, stearic acid, or a salt thereof. The coating on the particles does not change the nature of this invention.

In an embodiment, the conductive paste additionally comprises a glass frit. If a glass frit is present in the paste, any glass frit known to a skilled person can be used, either based on lead- containing compositions or on lead-free compositions. The glass frit is not bound to any particular shape or form. The mean particle size of the particles of the glass frit used is in the range from 10 nm to 100 μηη. The mean particle size of the glass frit particles is more preferably in the range from 100 nm to 50 μηη and particularly preferred in the range from 500 nm to 10 μηη. The particles used may have any desired form known to those skilled in the art. For example, the particles may be in the form of flakes, rods, wires, nodules, spheres or any mixture thereof.

Spherical particles in this context mean that the real form of the particles deviates from the ideal spherical form, for example, spherical particles, as a result of the production, may also have a droplet shape or be truncated. Suitable particles which can be used as glass frit are known to those skilled in the art and are commercially available. Especially preferred, spherical particles are used. The advantage of spherical particles is their improved rheological behavior compared to irregular shaped particles. According to the invention, the glass frit content is in the range from 0 to 20 % by weight, preferably from 0 to 10 % by weight and most preferably from 1 to 5 % by weight, based on the total mass of the conductive paste. The organic medium in the conductive paste additionally may comprise at least one solvent. In one embodiment of the invention, the solvent comprises one or more solvents selected from liquid organic components having at least one oxygen atom. The liquid organic component having at least one oxygen atom is selected from alcohol, ester alcohol, glycol, glycol ether, ketone, fatty acid ester or terpene derivatives, excluding dibasic esters. The liquid organic component for example may be benzyl alcohol, texanol, ethyl lactate, diethylene glycol monoethyl acetate, diethylene glycol monobutylether, diethylene glycol dibutylether, diethylene glycol monobutyl- ether acetate, butyl cellosolve, butyl cellosolve acetate, propylene glycol monometylether, propylene glycol monomethylether acetate, dipropylene glycol monomethylether, propylene glycol monomethylpropionate, ethylether propionate, dimethylamino formaldehyde, methylethylketone, gamma-butyrolactone, ethyl linoleate, ethyl linolenate, ethyl myristate, ethyl oleate, methyl myristate, methyl linoleate, methyl linolenate, methyl oleate, dibutyl phthalate, dioctyl phthalate and terpineol. The solvent being a liquid organic component having at least one oxygen atom can be used in the conductive paste either as single solvent or as a solvent mix. In case a solvent mix is used, the solvent can additionally comprise 5 to 50 wt% of at least one dibasic ester based on the total mass of the solvent mixture. The dibasic ester preferably is selected from dimethyl esters of adipic acid, glutaric acid, succinic acid or mixtures thereof.

When either a single solvent or a solvent mixture is used, it is necessary that organic binders can be dissolved greater than 2 wt% in the selected single or mixed solvents, such that the organic medium comprises at least 2 wt% dissolved binder based on the total mass of the organic medium.

In an embodiment of the invention, the paste additionally comprises from 0.1 to 20 wt% of at least one additive selected from surfactants, thixotropic agents, plasticizers, solubilizers, defoamers, desiccants, crosslinkers, inhibitors, complexing agents and/or conductive polymer particles. The additives may be used individually or as a mixture of two or more of them.

When a surfactant is used as an additive, it is possible to use only one surfactant or more than one surfactant. In principle, all surfactants which are known to those skilled in the art or are described in the prior art, can be suitable. Preferred surfactants are singular or plural compounds, for example anionic, cationic, amphoteric or nonionic surfactants. However, it is also possible to use polymers with pigment-affinitive anchor groups, which are known to a skilled person as surfactants.

In case the electrically conductive particles are pre-coated with a surfactant, the conductive paste may not comprise an additional surfactant as additive.

Besides the solvent and the further organic additives, the conductive paste also may comprise organic binders in a range from 0.1 to 20 % by weight. The organic binder can be selected from natural or synthetic resins and polymers. As known to those skilled in the art, selections are based on but not limited to solvent compatibility and chemical stability. For example, the common binders as disclosed in the prior art comprise cellulose derivatives, acrylic resin, phenolic resin, urea-formaldehyde resin, alkyd resin, aliphatic petroleum resin, melamine formaldehyde resin, rosin, polyethylene, polypropylene, polystyrene, polyether, polyurethane, polyvinyl acetate and copolymers thereof.

The inventive paste particularly is used in screen printing processes for producing electrically conductive pattern on a substrate. Particularly preferred, the paste is used to print electrodes on semiconductors for producing solar cells. The invention further is related to process for producing electrodes on a semiconductor substrate comprising the steps of: (a) screen printing a conductive paste according to any of claims 1 to 1 1 on a semiconductor substrate in a predetermined pattern to form a printed semiconductor substrate,

(b) drying the printed semiconductor substrate at a temperature in the range from 100 to 300 °C,

(c) heating the dried printed semiconductor substrate with the printed composition to a sintering temperature in the range from 650 to 900 °C to sinter the electrically conductive particles.

Suitable semiconductor substrates are for example such to produce photovoltaic cells comprising an n-type region, a p-type region, a p-n junction and conductive gridlines. Photovoltaic cells comprise optionally an anti-reflection layer on the surface of substrate. The semiconductor substrate can be mono-crystalline silicon, multi-crystalline silicon, amorphous silicon coated solid substrate, or a substrate whose surface is coated with polycrystalline or amorphous transparent conductive oxides (TCO), such as indium tin oxide (ITO), ZnO based transparent conductive oxides, such as indium gallium zinc oxide (IGZO), indium zinc tin oxide (IZTO), indium zinc oxide (IZO), indium tungsten oxide (IWO), gallium zinc oxide (GZO). The conductive gridlines are formed by the screen printing process from a conductive paste.

The screen printing in step (a) is carried out in any known manner known to a skilled person. The printing process particularly can be an industrial high speed screen printing process. In such a process the printing speed depends on the application requirement. It ranges from 80 mm/s to 800 mm/s, preferably not slower than 150 mm/s, for example in solar cell printing, the printing speed is 150 mm/s to 300 mm/s.

After printing the conductive paste on the semiconductor substrate, the printed semiconductor substrate is dried at a temperature in the range from 200 to 300 °C. The drying step preferably is carried out for a duration in the range from 10 to 50 sec, particularly preferably in the range from 15 to 30 sec.

After drying, the dried printed semiconductor substrate is heated to a sintering temperature in the range from 700 to 900 °C to sinter the electrically conductive particles. In the heating step the printed semiconductor substrate is heated from the drying temperature to the sintering tem- perature within 5 to 50 sec, preferably within 5 to 35 sec, the sintering temperature is kept for less than 5 sec, and afterwards the printed semiconductor substrate is cooled down to room temperature within 3 to 60 sec, preferably within 3 to 30 sec. The whole heating step is carried out for a duration in the range from 10 to 80 sec, preferably from 15 to 50 sec. Examples

Conductive pastes according to the compositions in table 1 have been screen printed with the screen parameters as shown in table 3. Table 1 : Paste composition

The content of triacetin has been increased by reduction of solvent blend containing butyl carbitol, butyl carbitol acetate, methyl oleate and aliphatic solvent. MMA stands for methylmethacry- late. SMA stands for stearylmethacrylate.

The slip of the pastes has been measured with a commercial rheometer by using plate-plate geometry.

The rotation speed of the upper plate is indicated in rounds per minute (rpm). Shear stress is indicated in Pascal. During application of constant shear stress the rotation speed of the upper plate will depend on the viscosity of the paste. Higher paste viscosity will cause slower rotation speed at constant shear stress. Wall slip effects can dominate the rotation speed at constant shear stress in case of weak adhesion at the interface between the paste and the stainless steel plate. Slip effects can be caused by formation of a thin layer of liquid solvents on the paste surface. More paste slip will cause faster rotation speed at constant shear stress.

Table 2: Slip measurement

Screen prints have been conducted with the following screen parameters: Table 3: screen parameters mesh count [wires/inch] 360

wire thickness [μηη] 16

emulsion thickness [μηη] 17

fabric thickness [μηη] 22

line opening [μηη] 35

Number of lines 102

Screen tension [N] 26 pm 2

Busbar type 2 continuous

Angle between finger opening and mesh [°] 30

Results of the printing process are shown in Figures 1 to 6. Figures 1 to 3 show microscope pictures of printed lines of examples 1 to 3 after drying at 250 °C.

Figures 4 to 6 show microscope pictures of printed lines of examples 1 to 3 after sintering at 800 °C

The amount of the transferred paste is shown in table 4. In this table the paste transfer per cell describes the difference of the wafer weight before printing and the wafer weight after printing.

Table 4: paste transfer per cell

Table 5: characteristics of the printed lines

Triacetin is the most polar solvent in compositions E1 , E2 and E3. The higher the content of triacetin the stronger are the phase separation effects which result in formation of a thin liquid layer on the paste surface. This thin liquid layer is causing paste slip during viscosity measurement in plate-plate-geometry. Increased paste slip causes also more complete paste release from the surface of the screen materials. Therefore the measured weight of paste which is transferred from the screen to the substrate is increasing with increasing content of triacetin. As further can be seen in the figures, an increasing amount of triacetin results in less seepage. Figures 1 and 4 shows the lines of example E1 , figures 2 and 5 the lines printed with the ductive paste E2 and figures 3 and 6 lines printed with the conductive paste E3.