WO2017099470A1 - Paste composition for forming solar cell electrode - Google Patents

Abstract

Disclosed is a conductive paste composition, which has an excellent continuous printability and is used for forming a front electrode of a solar cell by screen printing. The conductive paste composition comprises a conductive metal powder, a glass frit, a binder, a thixotropic agent, and an organic solvent, and has a viscosity expressed by mathematical formula 1 below and a loss angle expressed by mathematical formulas 2 and 3 below. [Mathematical formula 1] 15 Pa·s ≤ η₁₀₀ ≤ 35 Pa·s; [Mathematical formula 2] 5° ≤ Loss angle ≤ 20° when strain is 0.01%; and [Mathematical formula 3] 70° ≤ Loss angle ≤ 85° when strain is 300%, wherein η₁₀₀ represents the viscosity value at a shear rate of 100 s^-1, and the loss angle is measured by the oscillatory strain step test (1 Hz) in which the strain is continuously repeated between 0.01% and 300%.

Description

Paste composition for solar cell electrode formation

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a paste composition for forming a solar cell electrode, and more particularly, to a conductive paste composition having excellent continuous printability and for forming a solar cell front electrode by screen printing.

A solar cell is a device that obtains power by using a photovoltaic effect in which electricity is generated when light is incident on a semiconductor substrate. A solar cell is typically a front surface of a semiconductor substrate made of p-type silicon or the like. And a cathode electrode is formed on the surface to which sunlight is irradiated, and an anode electrode is formed on the back surface. The solar cell electrode is formed by screen printing and baking an electrode forming paste composition on a substrate, and the electrode forming paste composition comprises an electrically conductive powder, a glass frit, an organic solvent, and a cellulose resin binder. It consists of a conductive organic medium containing.

In order to improve the efficiency of a solar cell, the electrically conductive paste composition which can form the electrode of a fine line width, and is excellent in continuous printability is calculated | required. In the early 2000s, a front electrode having a line width of about 100 μm was used, but recently, using a fine line width mask having a pattern hole of 40 μm or less, for example, 36 to 40 μm, A front electrode pattern (wiring pattern) having a line width is formed. However, when the electrode pattern is printed using a conventional conductive paste composition and a fine line width mask, printability problems such as disconnection in the electrode (circuit) pattern, clogging of the mask mesh, or the like are formed. The aspect ratio (height / width ratio) of the pattern is low, so that there is a disadvantage in that the performance of the solar cell is lowered or the production yield is lowered.

Accordingly, an object of the present invention is to provide a paste composition for forming a solar cell electrode having excellent continuous printability when screen printing a conductive paste on a semiconductor substrate using a mask having a fine line width pattern.

Another object of the present invention is to provide a paste composition for forming a solar cell electrode capable of suppressing electrode disconnection and mask mesh clogging and forming a highly accurate printed wiring.

In order to achieve the above object, the present invention comprises a conductive metal powder, a glass frit, a binder, a thixotropic agent and an organic solvent, the viscosity represented by the following equation (1), the loss angle represented by the following equations (2) and (3) It provides a paste composition for forming a solar cell electrode having an angle).

[Equation 1] 15 Pa.s ≤ η ₁₀₀ ≤ 35 Pa.s

Equation 2 When the strain (Strain) 0.01%, 5 ° ≤ Loss angle ≤ 20 °

Equation 3 When the strain (Strain) 300%, 70 ° ≤ Loss angle ≤ 85 °

Here, η ₁₀₀ represents a viscosity value at a shear rate of 100 s ⁻¹ , and the loss angle is measured by an Oscillatory Strain Step test (1 Hz) in which a strain rate of 0.01% and 300% is continuously repeated.

Preferably, the paste composition for forming a solar cell electrode of the present invention has a S.M.C (Strain at Modulus Cross-point) value represented by Equation 4 below.

Equation 4 25% ≤ S.M.C ≤ 75%

Here, the SMC value represents the strain value at the point where the storage modulus and the loss modulus intersect when the storage modulus and the loss modulus are plotted against the strain. .

Moreover, this invention is (1) 80-94 weight% of silver (Ag) powder as electroconductive metal powder; (2) 1 to 5 weight percent of glass frit; (3) 0.1 to 2.0 wt% of an ester or ether binder; (4) 0.6-1.0 wt% amide wax thixotropic agent; (5) 0.1 to 3 weight percent of thixotropic aid; And (6) 3 to 8% by weight of a solvent selected from a carbitol solvent (glycol ether), an ester solvent, and a mixture thereof. The present invention also provides a solar cell electrode formed by screen printing and firing a paste composition for forming a solar cell electrode on a semiconductor substrate.

The paste composition for forming a solar cell electrode according to the present invention is excellent in continuous printability when screen printing a conductive paste on a semiconductor substrate using a mask having a fine line width pattern, and suppresses electrode disconnection and mask mesh clogging, High precision printed wirings can be formed.

1 is a view showing a solar cell and an electrode structure that can be prepared using a solar cell electrode paste composition according to the present invention.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. As used herein, the terms "comprise" or "having" are intended to indicate that there are features, numbers, steps, components, or combinations thereof, and that one or more other features, numbers, steps, configurations, etc. It does not exclude the possibility of the presence or the addition of elements, or combinations thereof. As the invention allows for various changes and numerous modifications, particular embodiments will be illustrated and described in detail below. However, this is not intended to limit the present invention to the specific disclosed form, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

According to the present invention, in order to prepare a paste composition for forming a solar cell electrode having excellent continuous printability, the rheological index of the paste composition may include viscosity (Viscosity, η), modulus of elasticity, loss angle, and the like. Rheological properties should be adjusted. In the present invention, the rheological properties (viscosity, modulus of elasticity, loss angle, etc.) of the paste composition were measured at 25 ° C. using a Rheometer (manufactured by TA instrument, product name: Discovery Hybrid Rheometer-2), and slip occurred during measurement. Plate SST 25 mm sandblast spindle (accessory from TA instrument) was used to reduce the pressure, and 1000-grit sandpaper was used for the stage to raise the paste to be measured. In measuring the viscosity, the spindle gap was set to 100 μm. In addition, when measuring rheology such as modulus and loss angle, the spindle gap was set to 200 μm.

The paste composition for forming a solar cell electrode according to the present invention includes a conductive metal powder, a glass frit, a binder, a thixotropic agent, and an organic solvent, and has a viscosity represented by the following Equation 1.

[Equation 1] 15 Pa.s ≤ η ₁₀₀ ≤ 35 Pa.s

Here, η ₁₀₀ represents the viscosity value at the shear rate 100 s ⁻¹ .

The paste composition according to the present invention has a viscosity (η ₁₀₀ ) of 15 to 35 Pa · s, specifically 20 to 35 Pa · s, at a shear rate of 100 s ⁻¹ so as to be suitable for pattern formation by screen printing. The viscosity η ₁₀₀ is generally measured at a shear rate of 100 s ⁻¹ by simulating the shear rate when the paste is transferred through a screen mask when printing the paste composition. Under these conditions, if the viscosity is too low, the aspect ratio of the printed pattern may be low, or the pattern may be unevenly collapsed. If the viscosity is too high, defects such as mask mesh clogging and pattern disconnection may occur. Can be.

In addition, the paste composition for forming a solar cell electrode according to the present invention has a loss angle represented by the following equations (2) and (3).

Equation 2 When the strain (Strain) 0.01%, 5 ° ≤ Loss angle ≤ 20 °

Equation 3 When the strain (Strain) 300%, 70 ° ≤ Loss angle ≤ 85 °

The loss angle is measured by an Oscillatory Strain Step test (1 Hz) which continuously repeats a strain of 0.01% and 300%. The loss angle at 0.01% strain of the paste composition according to the present invention is specifically 7 to 18 °, and the loss angle at strain 300% is specifically 72 to 83 °. Loss tangent can be expressed as the ratio of Loss Modulus (G ″) / Storage Modulus (G ′) and is zero if it is completely elastic and infinite if it is completely viscous. Δ corresponding to the angle of Loss tangent, tanδ) means a loss angle (ie, tan δ = G ″ / G ′, 0 ° ≦ δ ≦ 90 °). Therefore, the smaller the loss angle, the more solid-like. The greater the loss angle, the more liquid-like. The paste composition of the present invention has a loss angle represented by Equations (2) and (3). When both of the conditions are satisfied, the paste composition is viscous when the strain is high during printing and is easily discharged, and when plate separation occurs immediately after printing. The elastic recovery speed can be excellent to form a uniform pattern shape. In the Oscillatory Strain Step test (1Hz), if the loss angle exceeds 20 °, if the loss angle exceeds 20 °, the elastic recovery speed is low immediately after printing, making it difficult to realize fine patterns, and the loss angle of less than 5 ° is theoretically excellent. Although recovery characteristics can be exhibited, it can be said that the actual paste is difficult to achieve. At a strain rate of 300%, if the loss angle exceeds 85 °, it can theoretically show excellent discharge characteristics during printing, but it is difficult to achieve with actual pastes. If it is less than 70 °, it is a paste with poor discharge characteristics during printing. This can cause various printing defects such as poor illumination and disconnection.

In addition, the paste composition for forming a solar cell electrode according to the present invention preferably has a S.M.C (Strain at Modulus Cross-point) value represented by Equation 4.

Equation 4 25% ≤ S.M.C ≤ 75%

SMC values are plotted at the Modulus Cross-point where the storage modulus and loss modulus cross (reverse) when plotting the storage modulus and loss modulus to strain. Strain values are shown. The S.M.C value of the paste composition according to the present invention is more specifically 30 to 60%. S.M.C means the point at which the internal structure of the paste is deformed and the storage modulus and loss modulus are reversed when some shear shear deformation occurs during printing. An ideal paste would have the properties of a solid without deformation (before SMC, high storage modulus) and the properties of liquid when deformed (after SMC, high loss modulus). When a certain deformation is received) The value of SMC greatly affects the printability. If the S.M.C value is too large (greater than 75%), the fluidity decreases during the printing process, and the frequency of occurrence of electrode disconnection increases, causing problems in continuous printability. On the other hand, if the S.M.C value is too small (less than 30%), despite the low strain of the paste, the fluidity of the paste composition is increased, so that the line width of the printing pattern may increase.

The paste composition for forming a solar cell electrode according to the present invention includes a conductive metal powder, a glass frit, a binder, a thixotropic agent and an organic solvent, and adjusts the organic solvent, the type and molecular weight of the binder, the type and content of the thixotropic agent, and the like. , Viscosity (η), SMC value and the loss angle can be adjusted as in Equations 1 to 4. Specifically, the paste composition for forming a solar cell electrode according to an embodiment of the present invention, (1) 80 to 94% by weight of silver (Ag, Silver) powder as the conductive metal powder, (2) 1 to 5% by weight of glass frit (3) 0.1 to 2.0 wt% of ester or ether binder, (4) 0.6 to 1.0 wt% of amide wax thixotropic agent, (5) 0.5 to 3 wt% of thixotropic aid, (6) carbitol solvent (glycol ether) And 3 to 8% by weight of a solvent selected from ester solvents and mixtures thereof. Hereinafter, each component which can be used for the paste composition of this invention is demonstrated concretely.

The silver (Ag) powder used in the paste composition of the present invention is a component that imparts conductivity to the electrode formed by the paste composition, and any silver powder or particles commonly used for forming a solar cell electrode can be used without limitation. The content of the silver powder is 80 to 94% by weight, specifically 85 to 92% by weight relative to the total paste composition. If the content of the silver powder is too small, it is difficult to secure the conductivity as the front electrode, if too large, the uniformity or paste viscosity of the paste composition may be lowered and printability may be lowered. The silver powder may be used irrespective of its appearance, but for example, spherical particles, plate particles, or a mixture thereof may be used, and more specifically, spherical particles may be used. In the case of using spherical particles, excellent dispersibility is advantageous for realizing fine line width in printing. The number average particle diameter of the silver powder particles may be about 0.1 to 5 μm, specifically about 1 to 2.5 μm. If the particle size of the silver powder is too small, the size of the voids between the particles may be small, which may prevent the formation of ohmic contacts of the glass frit during sintering, and if the particle size is too large, the dispersibility in the paste may be reduced. As a result, the printability of the paste may be degraded, and thus, fine line width may be difficult to achieve.

The glass frit serves to form ohmic contacts between the semiconductor substrate and the electrode at the time of sintering after application of the electrode forming paste. The content of the glass frit is 1 to 5% by weight, specifically 1.5 to 4% by weight, based on the total paste composition. If the content of the glass frit is too small, ohmic contact formation between the electrode and the semiconductor substrate may be insufficient, the contact resistance may increase, and if too large, the line resistance in the electrode may be excessively increased.

The ester binder or the ether binder used in the paste composition of the present invention imparts viscoelasticity to the paste composition and maintains the shape of the pattern when the pattern is formed. As the ester-based binder or ether-based binder, ester-based or ether-based binders generally used in solar cell electrode compositions may be used without particular limitation, and for example, polyester-based binders, polyester-polyol-based binders, poly Ether binders, polyether polyol binders, mixtures thereof and the like can be used. The weight average molecular weight (Mw) of the binder may be 50,000 to 70,000. Since the weight average molecular weight of the binder has a great influence on the viscosity of the composition, it is possible to mix and use binders having different molecular weights in order to more accurately match the target viscosity. If the weight average molecular weight of the binder is too small or too large, there is a fear that a viscosity suitable for screen printing (15 to 35 Pa · s at a shear rate of 100 s ⁻¹ ) may not be obtained. With respect to the total paste composition, the content of the binder is 0.1 to 2% by weight, specifically 0.2 to 1% by weight. If the content of the binder is too small, there is a fear that the minimum strength and rheology due to the use of the binder may not be secured. If the content of the binder is too large, the conductivity of the electrode formed of the paste composition may be lowered.

Amide wax thixotropic agent is an additive for forming a network of a network structure inside the paste composition to impart thixotropy (the rheological property), and the commercially available amide wax thixotropic agent is particularly limited. Can be used without Since the content of the thixotropic agent greatly affects the rheological properties of the paste, accurate adjustment is required according to the printing method and the characteristics of the printing press. The content of the amide wax thixotropic agent is 0.6 to 1.0% by weight relative to the total paste composition. If the content of the thixotropic agent is too small, there is a fear that the aspect ratio of the pattern during paste printing becomes small, and if the content of the thixotropic agent is too large, printing defects such as mask mesh clogging and pattern disconnection may occur.

The thixotropic aid, together with the thixotropic agent, serves to improve the rheological properties of the paste composition, thereby increasing the aspect ratio of the electrode pattern formed of the paste composition. Specific examples of the thixotropic aid include siloxane-based compounds such as rosin, rosin ester, polysiloxane, cyclosiloxane, silica powder, aliphatic amine, and carbohydrate. Carboxylic acid amides, combinations thereof, and the like. The content of the thixotropic aid is 0.1 to 3% by weight, specifically 0.5 to 2% by weight, based on the total paste composition. If the content of the thixotropic aid is too small, the aspect ratio of the electrode pattern may be reduced, or the durability of the electrode pattern may be reduced. If the content of the thixotropic agent is too large, the role of the binder and the thixotropic agent may be inhibited, and the thixotropic property may be lowered.

The solvent used in the composition of the present invention dissolves the binder and thixotropic agent of the paste composition to impart fluidity and improves printability. A solvent having a boiling point of 170 ° C. or higher for application to a printing process (Glycol ether), at least one solvent selected from ester solvents and mixtures thereof. A glycol ether solvent is generally referred to as a carbitol solvent, and examples of the carbitol solvent include carbitol, carbitol acetate, butyl carbitol, butyl carbitol acetate, diethylene glycol dibutyl ether, and dibutyl ether. Ethyl carbitol, dimethyl carbitol, diethyl glycol methyl ethyl ether and the like can be exemplified. Examples of the ester solvent include EEP (ethyl ethoxy propionate), 2-ethylhexyl acetate, DBE (dibasic ester), Texanol, Texanol isobutylate, terpinol and the like. Depending on the properties required by the paste, two or more solvents can be mixed and used. In order to compensate for the solubility of binder and thixotropic agent having different solubility in a single solvent, a mixture of solvent having excellent solubility in binder dissolution and solvent having excellent solubility in thixotropic agent dissolution is used for It can compensate for the poor properties of the solvent and can also help to make deformation of the paste's internal structure easy. The content of the solvent is 3 to 8% by weight, specifically 5 to 7% by weight, based on the total paste composition. In addition, the content of dibutyl glycol dibutyl ether solvent is preferably 0.1 to 2% by weight based on the total paste composition. If the content of the solvent is too small, the viscosity of the composition may be high, the printability may be reduced due to the speed of drying the solvent, and if the content of the solvent is too high, the viscosity of the paste composition may be excessively low. There exists a possibility that printability may fall.

If necessary, the paste composition for forming a solar cell electrode according to the present invention may further include conventional additives such as a dispersant and a stabilizer. The dispersant makes the dispersion state of the paste composition uniform, maintains thixotropic properties, maintains the viscosity relatively high during storage, and improves the storage stability of the paste composition. As the dispersant, a dispersant generally used in a solar cell electrode paste composition may be used without particular limitation, and specifically, an ionic or nonionic surfactant, an amide compound, or the like may be used. For example, an aliphatic ammonium salt ( Aliphatic ammonium salts, Aliphatic carboxylic acid salts, and mixtures thereof may be used. In the case of using a salt-type flocculent dispersant, ions having functional groups can form a network with nano-sized inorganic particles in a solvent, expand and form a gel structure to prevent sedimentation, thereby securing rheological stability. To improve storage stability. Specific examples of the dispersant include polyhydroxy carboxylic acid esters, polyhydroxy carboxylic acid amides, polyamine amides, and unsaturated polyamine amides. salts, Aliphatic carboxylic acid hydroxylalkylamine salts, Long chain fatty acid derivative amine salts, Long chain fatty acid alkyl amine salts, Modified polyester amine salt, High molecular weight carboxylic acid amide amine salt, Modified phosphate ester amine salt, Alkyl carboxylic acid Carboxylic acid alkylol alkylammonium salt, Alkylamm of polycarboxylic acid onium salt of a polycarboxylic acid, Polycarboxylic acid salt of polyamine amides, pentanoic acid-5- (dimethylamino) -2-methyl-5-oxo-methyl ester and lithium chloride (Pentanoic acid 5- (dimethylamino) -2-methyl-5-oxo-methyl ester and Lithium chloride), N- (tallowalkyl) -1,3-propanediamineoleate (N- (Tallow alkyl) -1,3- propanediamine oleates), polysiloxane unsaturated carboxylic acid salts, and the like. The use amount of the said dispersing agent is 0.01 to 2 weight% normally with respect to the whole paste composition. If the amount of the dispersant is too small, the dispersibility or storage stability of the paste composition may be lowered. If the amount of the dispersant is too small, the dispersant may be deteriorated from the rheological properties (viscosity, modulus of elasticity, loss angle, etc.) suitable for printing.

The stabilizer in the additive serves to maintain the viscosity of the paste by adjusting the pH of the paste composition, thereby lowering the reactivity of the composition components. For example, if the paste composition excluding the stabilizer is basic, an acid compound having excellent compatibility and miscibility with the composition may be added as a stabilizer in order to lower the pH of the composition. As the acid compound stabilizer, an aliphatic carboxylic acid, aliphatic amine, a mixture thereof, and the like may be used. Specific examples thereof include oleic acid and linoleic acid. , Malonic acid, glycolic acid, unsaturated polycarboxylic acid, unsaturated polycarboxylic acid, polysiloxane copolymer, excidic polyester polyamide, excidic polyether (Acidic polyether) etc. can be illustrated. On the other hand, if the paste composition except the stabilizer is acidic, a base compound having excellent compatibility and miscibility with the composition may be added as a stabilizer in order to increase the pH of the composition. Such base compound stabilizers include aminopropyldiethanolamine, 2-[(1-methylpropyl) amino] ethanol (2-[(1-methyl propyl) amino] ethanol), Sec-butylamine, diethylamine (Diethylamine), Diethylaminopropylamine, Diiso propylamine, Dimethylaminopropyl aminopropylamine, Ethyldiisopropylamine, Ethylmethylamine, 3 3-Isopropoxy propylamine, Monoethylamine, Monoisopropyl amine, 3-Methoxypropylamine, Triethylamine, Tributyl Tributylamine, Trioctylamine, Tetramethylpropylenediamine, 2-Amino-2-ethyl-1,3-propanediol (2-Amino- 2-Ethyl-1,3-Propane-Diol ), 2-amino-2- Butyl and the like can be given 1-propanol (2-Amino-2-Methyl-1- Propanol). The amount of the stabilizer used is usually 0 to 2% by weight, for example, 0.001 to 2% by weight based on the total paste composition. Depending on process conditions and other compositions, stabilizers may not be included. However, if the content of the stabilizer is excessively high, there is a fear that it will deviate from the rheological properties (viscosity, modulus of elasticity, loss angle, etc.) suitable for printing.

Examples of such antioxidants include phenol compounds, aromatic amine compounds and the like, which are usually classified as primary antioxidants and UV stabilizers. Antioxidants that decompose the produced peroxide groups into stable radicals are classified as secondary antioxidants. Hindered Phenolics are one of the most commonly used primary antioxidants, which are simple phenolics, bisphenolics, polyphenolics, and thiobisphenolic compounds. It can be classified as a compound (thiobisphenolics). The most representative of the hindered phenolic compounds is dibutyl hydroxy toluene (BHT), which is an antioxidant of polyolefins, styrenes, vinyls, and elastomers. BHT has very weak chain terminators and high volatility weaknesses. Substitution of the methyl group in the para position of the BHT with a long aliphatic group reduces the volatility but decreases the reactivity of the OH group for the same weight. Bisphenol-based compounds and polyphenol-based compounds have low volatility and relatively low equivalent weight due to high molecular weight compared to the former. Phenolic compounds have nonstaining and nondiscoloring in general cases, but when oxidized, some compounds are converted to highly chromophoric structures. High Molecular Weight (HWM) phenolic compounds are used in combination with BHT to achieve optimal processability. Well-known polyphenolic antioxidants include 8-Hydroxyquinoline, 8-Hydroxyquinoline sulfate, 8-Hydroxyquinoline sulfate, 8-Hydroxyquinoline-5- sulfonic acid), tetrakis (methylene-3,5-di-t-butyl-4-hydroxyhydrocinnamate) methane (methane-3.5-di-t-butyl-4-hydroxyl hydrocinnamate) methane and the like. Thiobisphenols are not so effective as terminators of peroxy radicals, but they also act as peroxide decomposers when processing temperatures exceed 100 ° C. Secondary amines, like phenolic compounds, act as hydrogen donors, and they also act as peroxide decomposers at high temperatures, so amine groups belonging to primary antioxidants Because of the role of chain terminators and peroxide decomposers, they are more effective than phenolic compounds, but because of their discoloring characteristics, their use is limited in areas where color is important or in the appearance of products. It is most used for polyolefins containing these.

Secondary antioxidants include a variety of trivalent phosphorous and divalent sulfur-containing compounds, and many are known are organophosphites, thioesters, Thioamide, thioamine, and the like. Secondary antioxidants are related to protective stabilizers, and therefore they prevent the diffusion of alkoxy and hydroxy radicals by the decomposition of hydroperoxides. Phosphites convert hydroperoxides into alcohols and convert them into phosphates. An important drawback of phosphite is its sensitivity to hydrolysis. Some hydrolysis can reduce this sensitivity when combined with various additives. Hydrolysis of Phosphite ultimately results in the formation of Phosphate acid, which leads to corrosion of the processing equipment. Phosphite Stabilizer can be used synergistically with Hindered Phenolics. In some cases, it also increases stability against Ultraviolet Exposure. Aliphatic esters of β-thiodipropionic acid are very effective peroxide decomposers for long-term heat exposure applications. Thioesters have an excellent synergistic effect when used with primary antioxidants. The usage-amount of the said antioxidant is 0-2 weight% normally with respect to the whole paste composition, for example, 0.001-2 weight%, and can be used selectively. Depending on the process conditions and other compositions, antioxidants may not be included. However, if the amount of the antioxidant is excessively high, there is a fear that it will deviate from the rheological properties (viscosity, modulus of elasticity, loss angle, etc.) suitable for printing.

Using the solar cell electrode paste composition according to the present invention, it is possible to produce a solar cell and a solar cell front electrode by conventional methods used in the art. 1 is a view showing a solar cell and an electrode structure that can be prepared using a solar cell electrode paste composition according to the present invention. As shown in FIG. 1, a solar cell to which the composition of the present invention may be applied is formed on a p-type silicon substrate 10 including an n-type semiconductor portion 12 on a front surface thereof, and the n-type semiconductor portion 12. And a rear electrode 30 formed on the front electrode 20 and the p-type silicon substrate 10. An anti-reflection film 14 may be formed on the top surface of the n-type semiconductor unit 12 except for the front electrode 20. When the solar cell electrode paste composition according to the present invention is screen printed on the n-type semiconductor portion 12 and fired, an electrode of the solar cell, specifically, a solar cell front electrode 20 can be formed. .

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. The following examples are intended to illustrate the invention, but the scope of the invention is not limited by these examples.

EXAMPLES 1-3, COMPARATIVE EXAMPLES 1-4 Preparation of the paste composition for solar cell electrode formation

To the components and contents (% by weight) as shown in Table 1, an ester binder is mixed with a carbitol solvent A, B and an ester alcohol solvent to form an organic vehicle, and the amide wax thixotropic agent and thixotropic agent are formed in the formed organic vehicle. Auxiliaries were added and stirred with a high speed mixer. The dispersant and the glass frit were added to the organic vehicle after the stirring was completed, stirred with a mixer, silver powder was added and stirred again, and then dispersed using a 3 roll mill to prepare a paste composition.

In Table 1 below, the ester binder is cellulose acetate butyrate having a weight average molecular weight of 50,000 to 70,000, the carbitol solvent A is butyl carbitol acetate, and the carbitol solvent B is diol. Butyl carbitol (Diethylene glycol dibutyl ether), the ester alcohol solvent is TEXANOL (TEXANOL, 2,2,4-trimethyl-1,3-pentanediol isobutyrate). In addition, a thiamide wax was used as a thixotropic agent, a rosin ester compound as a thixotropic agent, and an alkyl diamine dioleate as a dispersant.

	Comparative Example 1	Comparative Example 2	Example 1	Example 2	Example 3	Comparative Example 3	Comparative Example 4
Silver powder	88	87.9	87.8	87.6	87.5	87.3	87.2
Glass frit	3	3	3	3	3	3	3
Diamide Wax	0.4	0.5	0.6	0.8	0.9	1.1	1.2
Thixotropic supplements	1.2	1.2	1.2	1.2	1.2	1.2	1.2
Ester binder	0.7	0.7	0.7	0.7	0.7	0.7	0.7
Dispersant	0.5	0.5	0.5	0.5	0.5	0.5	0.5
Carbitol Solvent A	4.8	4.8	4.8	4.8	4.8	4.8	4.8
Carbitol Solvent B	0.5	0.5	0.5	0.5	0.5	0.5	0.5
Ester alcohol	0.9	0.9	0.9	0.9	0.9	0.9	0.9

Experimental Example 1-1 Rheological Properties (S.M.C Value) Measurement

For the compositions of Examples 1 to 3 and Comparative Examples 1 to 4, while changing the strain (Strain) to 0.01 to 1000% (Strain sweep), the storage modulus (G ') and the loss modulus (Loss Modulus) of the composition , G "), and typical storage modulus (G '), loss modulus (G") and SMC (Strain at Modulus Cross-point) values are shown in Table 2 below.

	Strain (%)	Comparative Example 1	Comparative Example 2	Example 1	Example 2	Example 3	Comparative Example 3	Comparative Example 4
Storage modulus (G ')	0.01	34200	101000	142000	203000	295000	428000	531000
	0.1	21900	55000	82200	130000	183000	251000	351000
	One	7100	14800	23100	38200	52000	81500	131000
	10	910	2200	3650	5250	8250	12600	19300
	100	151	290	310	570	730	990	1470
	1000	15	27	50	100	150	200	280
Loss modulus (G ")	0.01	6090	25100	48000	75000	123000	130000	158000
	0.1	5510	18000	33000	54900	73000	94400	110000
	One	3220	8500	12700	23000	25100	35200	54200
	10	801	2000	3020	4120	5510	6100	7500
	100	380	410	750	870	1110	1481	1600
	1000	120	220	250	320	405	520	741
S.M.C (%)		17	22	35	47	59	77	92

From Table 2, the composition of the embodiment can be seen that S.M.C (Strain at Modulus Cross-point,%) is 25 to 75%.

Experimental Example 1-2 Rheological Properties (Loss Angle and Viscosity) Measurement

For the compositions of Examples 1-3 and Comparative Examples 1-4, Loss Angle and viscosity (η, viscosity at shear rate 100 s ⁻¹ ) at 0.01% and 300% strain were measured. It is shown in Table 3 below.

	% Strain	Comparative Example 1	Comparative Example 2	Example 1	Example 2	Example 3	Comparative Example 3	Comparative Example 4
Loss angle (°)	0.01	10	11	12	13	14	18	23
	300	75	74	73	72	70	68	64
Viscosity (η, Pa.s)		12.724	15.891	20.602	27.05	28.871	32.473	36.742

From the above Table 3, the compositions of the examples have a loss angle of 10 to 20 ° at a strain rate of 70 to 75 ° at a strain rate of 300% and a viscosity η at a shear rate of 100 s ⁻¹ of 20 to 29 Pa. It can be seen that s.

Experimental Example 1-3 Evaluation of Printing Properties of Paste Composition

The solar cell front electrode was printed using the paste composition of Examples 1-3 and Comparative Examples 1-4. As a silicon wafer for printing electrodes, a sheet resistance cell (Cell) having a sheet resistance of 90 Ω / □ was used. The backside silver electrode paste was printed and dried on the back side of the silicon substrate to form a backside silver electrode, and the backside aluminum electrode paste was dried after screen printing to overlap a portion of the backside Ag electrode. The drying temperature of each paste was 170 ° C. At this time, the mask for printing was a mask having a total thickness of 47 μm of 360 mesh, and the front electrode was formed by using a finger line having a width of 40 μm and a bus bar pattern having a width of 1.5 mm. Formed. The line width and thickness of the formed solar cell front electrode were measured using KEYENCE's VK Analyzer, and the clogging defect and scrap characteristic defect (defect of printing) of the mask mesh used for screen printing were measured using Loupe. The evaluation was performed to classify the defects into 0 to 10 levels. 0 was classified as no blockage of the mesh and 10 when the blockage of the mesh occurred more than 10 points. In the case of the scrap characteristics, when the paste is applied on the mask with a scraper when printing in the same process, it is uniformly applied when there is no abnormality. Cases should be divided into 10 levels as shown in Table 4 below.

Experimental Example 1-4 Fabrication and Performance Evaluation of Solar Cells

The printed matter of Experimental Examples 1-3 was dried at 170 ° C., and then calcined at 960 ° C. to evaluate the performance after fabricating the solar cell. The electrical characteristics (I-V curve) of the solar cell were measured using a solar simulator, and are shown in Table 4 together. In Table 4, Isc [A] is a short-circuit current flowing by shorting the solar cell electrode terminal with short-circuit current, and Voc [V] is an open-circuit voltage measured by opening the solar cell electrode terminal with open-circuit voltage. Rs [mΩ] is a resistance acting in series between the solar cell upper and lower electrodes. FF [%] is the fill factor, which is the ratio of the product of the maximum output voltage and the maximum output current to the product of the open-circuit voltage and the short-circuit current. Efficiency [%] is the ratio of the incident light energy and the output of the solar cell per unit area. Is defined.

	Comparative Example 1	Comparative Example 2	Example 1	Example 2	Example 3	Comparative Example 3	Comparative Example 4
Line width after firing	68.71	66.62	59.31	58.46	57.95	57.20	55.88
Thickness after firing	15.28	15.37	15.88	15.93	16.12	16.38	17.34
Mesh clogging	One	One	0	0	One	2	3
Scrap attribute	0	0	0	0	0	5	10
Continuous printing characteristics	One	One	One	2	2	7	9
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Isc [A]	8.426	8.458	8.517	8.526	8.551	8.577	8.584
Voc [V]	0.635	0.635	0.636	0.636	0.635	0.635	0.635
Rs [mΩ]	4.426	4.397	4.498	4.563	4.650	4.800	4.913
FF [%]	78.574	78.459	78.720	78.866	78.807	78.521	77.874
Efficiency [%]	17.276	17.316	17.522	17.585	17.597	17.572	17.446

As can be seen from Table 4, the continuous printing characteristics were poor when the S.M.C value (%) was large or the loss angle at the strain rate of 300% was small (Comparative Examples 3 and 4). In this case, since the elasticity of the paste composition is larger than that of the viscosity, it is difficult to reverse the elasticity of the viscous material due to its strong solid like property when subjected to a certain deformation during printing, so that the discharge of the paste becomes difficult and the line width is thin and thick. Is high, but poor printing due to disconnection and the like increases, and continuous printability is very poor. On the contrary, when the SMC value (%) is small from 0 to 30% (Comparative Examples 1 and 2), the paste composition is generally low in elasticity and strong in viscosity, and the viscosity is low because the absolute value of viscoelasticity when the deformation is small is small. . This shows that the internal structure of the paste is easily deformed in spite of small deformation, and thus the viscosity is easily reversed. It is advantageous for continuous printing, but it is not suitable because of its large line width and low thickness. Also, Isc was influenced by the line width, Rs and FF were affected by the printing defect, and relatively high efficiency value was obtained when the content of thixotropic agent amide wax was 0.6 ~ 1.0%. Could.

[Examples 4 to 6 and Comparative Examples 5 to 7] Preparation of a Paste Composition for Forming a Solar Cell Electrode

Depending on the properties required by the paste, two or more solvents can be mixed and used. It is possible to improve the miscibility of the whole composition by mixing a solvent having excellent solubility in binder dissolution and a thixotropic agent dissolution, and may also serve to facilitate deformation of the internal structure of the paste. As shown in Table 5 below, butyl carbitol acetate (carbitol-based solvent A, Diethylene glycol monobutyl ether acetate), which is a carbitol-based solvent having excellent solubility in binder dissolution, has excellent solubility in thixotropic agents. While changing the composition of TEXANOL (ester-based alcohol, 2,2,4-trimethyl-1,3-pentanediol isobutyrate) and dibutyl carbitol (carbitol-based solvent B, Diethylene glycol dibutyl ether) having the same composition as in Example 1 By the method, a paste composition was prepared. Evaluation was performed by changing the compositions of butyl carbitol acetate (carbitol solvent A) and dibutyl carbitol (carbitol solvent B) in the order of Comparative Examples 4 to 5, Examples 4 to 6, and Comparative Example 7.

	Comparative Example 5	Comparative Example 6	Example 4	Example 5	Example 6	Comparative Example 7
Silver powder	87.6	87.6	87.6	87.6	87.6	87.6
Glass frit	3	3	3	3	3	3
Diamide Wax	0.8	0.8	0.8	0.8	0.8	0.8
Thixotropic supplements	1.2	1.2	1.2	1.2	1.2	1.2
Ester binder	0.7	0.7	0.7	0.7	0.7	0.7
Dispersant	0.5	0.5	0.5	0.5	0.5	0.5
Carbitol Solvent A	5.3	5	4.8	4.3	3.3	3.1
Carbitol Solvent B	0	0.3	0.5	One	2	2.2
Ester alcohol	0.9	0.9	0.9	0.9	0.9	0.9

Experimental Example 2-1 Rheological Properties (S.M.C Value) Measurement

For the compositions of Examples 4-6 and Comparative Examples 5-7, the strain sweep was varied from 0.01 to 1000%, the storage modulus (G ') and the loss modulus (Loss Modulus) of the composition. , G ″) is shown in FIG. 1, and typical storage modulus (G ′), loss modulus (G ″), and SMC values are shown in Table 6 below.

	Strain (%)	Comparative Example 5	Comparative Example 6	Example 4	Example 5	Example 6	Comparative Example 7
Storage modulus (G ')	0.01	431000	428000	172000	141000	122000	115600
	0.1	288225	275625	131000	112000	100000	93600
	One	81900	78435	36200	37400	36700	35400
	10	8610	8130	5480	5170	5480	5670
	100	820	800	510	389	421	560
	1000	210	198	91	69	72	95
Loss modulus (G ")	0.01	153725	145850	75000	33600	32160	31160
	0.1	114975	111195	51500	33000	31320	31220
	One	39532.5	38902.5	21000	22500	20500	20800
	10	5260	4820	4320	4150	3870	3540
	100	857	860	835	821	740	652
	1000	445	420	280	219	255	210
S.M.C (%)		93	90	46	38	53	78

From Table 6, the composition of the embodiment can be seen that S.M.C (Strain at Modulus Cross-point,%) is 25 to 75%.

Experimental Example 2-2 Rheological Properties (Loss Angle and Viscosity) Measurement

For the compositions of Examples 4-6 and Comparative Examples 5-7, Loss Angle and Viscosity (η, Viscosity at Shear Rate 100 s ⁻¹ ) at 0.01% and 300% Strain It is shown in Table 7 below.

	% Strain	Comparative Example 5	Comparative Example 6	Example 4	Example 5	Example 6	Comparative Example 7
Loss angle (°)	0.01	21	18	13	12	14	23
	300	67	68	72	73	75	64
Viscosity (η, Pa.s)		42.15	38.81	27.05	24.53	23.74	27.59

From the above Table 7, the composition of the examples, the loss angle is 10 to 15 ° at 0.01% strain, 70 to 75 ° at 300% strain, viscosity (η) at shear rate 100 s ^-1 is 23 to 28 Pa. It can be seen that s.

Experimental Example 2-3 Evaluation of Printing Properties of Paste Composition

Using the paste compositions of Examples 4-6 and Comparative Examples 5-7, the solar cell front electrode was formed in the same manner as Experimental Example 1-3, and the line width and thickness of the solar cell front electrode were used for screen printing. Clogging defects, squeegee characteristics, and scrap characteristics defects of the mask mesh were classified into levels of 0 to 10 using a loupe. 0 was classified as no blockage of the mesh and 10 when the blockage of the mesh occurred more than 10 points. In the case of squeegee property and scrap property, it is 0 when there is no abnormality when printing in the same process, and there is no abnormality. It is classified as shown in Table 8 below.

Experimental Example 2-4 Fabrication and Performance Evaluation of Solar Cells

Using the paste compositions of Examples 4-6 and Comparative Examples 5-7, a solar cell was fabricated in the same manner as Experimental Example 1-4, and the electrical characteristics (IV curve) of the solar cell were measured using a solar simulator. As shown in Table 8 below.

	Comparative Example 5	Comparative Example 6	Example 4	Example 5	Example 6	Comparative Example 7
Line width after firing	54.80	55.48	57.21	58.12	58.40	56.34
Thickness after firing	16.83	16.65	15.91	16.24	16.41	16.10
Mesh clogging	2	2	0	0	0	One
Scrap attribute	3	2	0	0	0	0
Squeegee Characteristics	3	2	0	0	0	0
Continuous printing characteristics	5	4	2	One	2	5
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Isc [A]	8.581	8.578	8.526	8.521	8.517	8.534
Voc [V]	0.628	0.629	0.628	0.628	0.629	0.628
Rs [mΩ]	4.581	4.512	4.329	4.351	4.317	4.364
FF [%]	78.368	78.275	78.866	78.812	78.756	78.768
Efficiency [%]	17.354	17.355	17.352	17.330	17.337	17.347

As shown in Table 8, when the carbitol-based solvent B content is 0.5 to 2% by weight (Examples 4 to 6), the SMC is rapidly lowered and the loss angle is increased. It is considered that the elastic value is maintained without deterioration when low deformation is applied, so that continuous excellent print quality can be maintained during continuous printing. It is believed that the relatively nonpolar carbitol-based solvent B acted as a printing property improvement aid to soften the network of networks in the paste. In this case, the thixotropic aid may be added to further increase elasticity and elastic recovery rate.

When the content of the carbitol-based solvent B was 2% by weight or more (Comparative Example 7), the characteristics dropped drastically. This is due to the repulsive force with the ester-based cellulose binder having a polar characteristic, it is determined that the aggregation phenomenon occurs in the paste internal structure. When high deformation was applied in a composition having a carbitol-based solvent B of 2% by weight or more, the loss modulus (Loss Modulus) was decreased and the storage modulus was increased, thereby showing a deterioration in printing characteristics. In general, when the strain modulus crosspoint (SMC) decreases, the viscosity decreases as the solvent content increases or the elasticity value decreases when the strain simulating a stationary state decreases, so that the line width of the printed electrode is generally large. The thickness is lowered. However, in the present invention, without increasing the solvent content, by mixing two or more solvents to change the composition to maintain the initial elastic value, and to reduce the S.M.C to increase the loss angle improved continuous printability.

In Comparative Examples 4 and 5, a high SMC value and a low loss angle result in high print defects.In Comparative Example 7, despite the low viscosity, compared to Comparative Example 4, SMC is high and the loss angle is low, so it is continuous printability. This showed a sharp drop. On the other hand, in Examples 4 to 6, the loss angle value was large, so that the fluidity was relatively high, thereby improving continuous printing and other printability.

Claims (10)

1. A conductive paste composition comprising a conductive metal powder, a glass frit, a binder, a thixotropic agent, and an organic solvent, and having a viscosity represented by the following formula (1), and a loss angle represented by the following formulas (2) and (3).

   [Equation 1] 15 Pa.s ≤ η ₁₀₀ ≤ 35 Pa.s

   Equation 2 When the strain (Strain) 0.01%, 5 ° ≤ Loss angle ≤ 20 °

   Equation 3 When the strain (Strain) 300%, 70 ° ≤ Loss angle ≤ 85 °

   Here, η ₁₀₀ represents a viscosity value at a shear rate of 100 s ⁻¹ , and the loss angle is measured by an Oscillatory Strain Step test (1 Hz) in which a strain rate of 0.01% and 300% is continuously repeated.
2. The conductive paste composition of claim 1, wherein the conductive paste composition has a S.M.C (Strain at Modulus Cross-point) value represented by Equation 4 below.

   Equation 4 25% ≤ S.M.C ≤ 75%

   The S.M.C value represents the strain value at the intersection of the storage modulus and the loss modulus when the storage modulus and the loss modulus are plotted against the strain.
3. The viscosity (η ₁₀₀ ) at a shear rate of 100 s ⁻¹ is 20 to 35 Pa · s, the SMC value is 30 to 60%, and the loss angle at a strain of 0.01% is 7 to 18%. And a loss angle at a strain of 300% is 72 to 83%.
4. The paste composition of claim 1, wherein the binder is an ester binder or an ether binder having a weight average molecular weight (Mw) of 50,000 to 70,000.
5. The paste composition of claim 1, wherein the organic solvent comprises at least one solvent selected from a carbitol solvent, an ester solvent, and a mixture thereof.
6. The method of claim 5, wherein the organic solvent. A paste composition for forming a solar cell electrode, comprising 0.1 to 2% by weight of a dibutyl carbitol solvent, based on the total paste composition.
7. (1) 80 to 94% by weight of silver (Ag) powder as the conductive metal powder;

   (2) 1 to 5 weight percent of glass frit;

   (3) 0.1 to 2.0 wt% of an ester or ether binder;

   (4) 0.6-1.0 wt% amide wax thixotropic agent;

   (5) 0.1 to 3 weight percent of thixotropic aid; And

   (6) A paste composition for forming a solar cell electrode comprising 3 to 8% by weight of a solvent selected from a carbitol solvent, an ester solvent, and a mixture thereof.
8. The method of claim 7, wherein the thixotropic aid comprises at least one component selected from the group consisting of rosin, rosin ester, polysiloxane, cyclosiloxane, silica powder, aliphatic amine, carboxylic acid amide, and combinations thereof. The paste composition for forming a solar cell electrode.
9. The method of claim 7, further comprising 0.01 to 2% by weight dispersant selected from aliphatic ammonium salts, aliphatic carboxylates and mixtures thereof and 0.001 to 2% by weight stabilizer selected from aliphatic carboxylic acids, aliphatic amines and mixtures thereof. A paste composition for forming a solar cell electrode.
10. The solar cell electrode formed by screen-printing and baking the paste composition for solar cell electrode formation of any one of Claims 1-9 on a semiconductor base material.

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