KR20180071855A - Conductive paste composition for front electrode of solar cell and front electrode of solar cell using thereof - Google Patents

Abstract

The present disclosure provides a paste for a foreside electrode of a solar cell, as a paste for a foreside electrode of a solar cell, which comprises a copper nitride compound, an antioxidant, and a binder. Accordingly, the first aspect of the present disclosure is to provide a copper nitride-based paste for the foreside electrode of the solar cell, which has a remarkably low specific resistance even at high-temperature firing while replacing expensive silver paste.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a paste composition for a front electrode of a solar cell and a front electrode for a solar cell using the same,

The present disclosure relates to a copper nitride-based paste composition for a front electrode of a solar cell and a solar cell using the same. More particularly, it is a paste composition comprising a copper nitride compound, an antioxidant, and a binder.

Conductive pastes used as electrode materials require high-temperature firing processes. Conductive pastes can be used as electrode materials for solar cells, digitizers, FPCB, LTCC, and MLCC.

Conductive pastes for electrode materials are easily oxidized during the high temperature baking process in the atmosphere and have a problem of conductivity. Although most of silver paste is used because silver is not easily oxidized when using silver as an electrode material, silver is expensive and there are many attempts to replace it. There is a problem of increasing the specific resistance value at the time of high-temperature firing, although research for replacing some or all of the materials with other low-cost materials is ongoing.

The solar cell basically consists of a p-n junction structure. It consists of an antireflection film for absorbing light into the solar cell and a front electrode and a rear electrode for pulling out the electron-hole pair formed inside the silicon.

The front electrode plays an important role in the efficiency of solar cells and modules connecting several solar cells. First, when light enters the solar cell, negative electrons and electrons escape through interaction with light and the material that constitutes the semiconductor of the solar cell, and positive holes are generated, This is called a photoelectric effect. The front electrode collects electrons generated therefrom without loss to make an electrical path.

When manufacturing an electrode for a solar cell, an electrode is formed on the side where the antireflection film is formed. As a method for producing the electrode, a paste containing a conductive powder such as silver powder, a glass frit, a resin binder and, if necessary, an additive is coated on the antireflection layer and fired.

In order to improve the power generation efficiency of the solar cell, the characteristics of the electrode are important. For example, lowering the series resistance of the electrode can reduce the current loss, thereby increasing the power generation efficiency. Various techniques have been proposed to achieve this goal.

Most of the front electrodes for conventional crystalline silicon solar cells use silver paste. However, in order to reduce the cost of the cell due to an increase in the silver price, studies have been continuously carried out to manufacture the paste by minimizing the silver content in the paste.

Techniques for forming electrodes with Ni / Cu / Ag structure using plating have been developed to replace silver, but processes requiring the removal of the antireflection layer are required and are manufactured in the pilot line due to environmental problems such as wastewater, It is not applied to production.

Hitachi has produced a paste for use as a front electrode for solar cells based on CuP, but does not disclose any specific resistance values at all.

US Pat. No. 2,110,131,217A1 discloses a technique for making a solar cell electrode using copper-based or core-shell type particles. However, since a high-temperature process is essentially involved in manufacturing a solar cell, oxidation problems And there is no disclosure of a method for solving the problem.

In the case of Napra, Japan, a resistivity of 3 × 10 ^-5 Ω · cm was achieved using a copper alloy and LMPA. However, this is not a high-temperature firing process of 800 ° C. or higher, but a process condition of 300 ° C. or lower. , It is not known whether the resistivity of the electrode can be attained to 2 x 10 ^-4 Ω · cm even at high temperature firing.

Japanese Patent Application Laid-Open No. 2005-243500 discloses a technique for manufacturing an electrode having conductivity. Specifically, in the conductive paste comprising the organic binder, the solvent, the glass frit, the conductive powder and at least one metal selected from Ti, Bi, Zn, Y, In and Mo or a metal compound thereof, A compound has an average particle diameter of 0.001 탆 or more and less than 0.1 탆 and discloses a conductive paste capable of providing high conductivity with a semiconductor and excellent adhesiveness. However, when the paste is baked, there is a problem that the contact resistance increases due to shrinkage of the coating film and microcracks occur.

These problems may adversely affect the characteristics of the solar cell. For example, there may be a problem that the in-plane uniformity of the solar cell is lowered and a conversion efficiency of the solar cell is lowered.

In addition, there is a conductive paste using a silver-coated copper composite (for example, a silver-copper core shell) as an approach for reducing the amount of expensive silver, but these core shell structures have problems in that the silver core is exposed ≪ / RTI > The exposed copper cores are not suitable for replacing the conventional silver paste in manufacturing the front electrode of the solar cell because the surface is oxidized to increase the resistivity and the conductivity is not ensured.

Therefore, the first aspect of the present disclosure is to provide a copper nitride-based paste for a solar cell front electrode, which has a remarkably low specific resistance even at high temperature firing while replacing expensive silver paste.

A second aspect of the present disclosure is to provide a solar cell front electrode including the paste of the first aspect.

The solar cell front electrode paste for achieving the first aspect of the disclosure includes a copper nitride compound, an antioxidant, and a binder.

According to one embodiment of the present disclosure, the copper nitride compound includes copper (I) (copper (I) nitride).

According to one embodiment of the present disclosure, the antioxidant is at least one selected from the group consisting of a fatty acid, a boric acid compound, a fluoride compound, and a reducing metal.

According to one embodiment of the present disclosure, the fatty acid comprises lauric acid, palmitic acid, stearic acid, or propionic acid, and the boric acid compound includes boron oxide, potassium borate, or potassium borate, Comprises acidic potassium fluoride, and the reducing metal comprises boron.

According to one embodiment of the present disclosure, the binder is selected from the group consisting of a cellulose resin, polyvinyl alcohol, acrylic resin, butyral resin, castor oil fatty acid modified alkyl resin, epoxy resin, phenol resin, rosin ester resin, polymethacrylate, Ethylene glycol monobutyl ether monoacetate, and ethylene glycol monobutyl ether monoacetate.

According to one embodiment of the present disclosure, the paste comprises 55-70 wt% copper nitride compound, 1-20 wt% antioxidant, and a binder as a balance.

According to one embodiment of the present disclosure, the solar cell front electrode paste further includes at least one additive selected from the group consisting of a dispersing agent, a thixotropic agent, a defoamer, a plasticizer, a viscosity stabilizer, a pigment, a UV stabilizer and a coupling agent do.

The solar cell front electrode for achieving the second aspect of the disclosure includes the solar cell front electrode paste according to the first aspect of the present disclosure.

The present disclosure can replace expensive silver by forming a front electrode based on a copper nitride compound using a copper nitride compound instead of silver in comparison with a front electrode composed of silver, It is possible to provide an electrode having a low specific resistance value in the process. In addition, the addition of an antioxidant enables firing in the atmosphere, which does not require a process under non-atmospheric conditions, such as under a nitrogen condition, so that the unit cost is lowered and is suitable for mass production.

Fig. 1 shows the electrode structures on the front and rear surfaces in which silver electrodes of a typical crystalline silicon solar cell are used.
2 shows the resistivity according to the boron content of the paste formed when boron is used as the antioxidant in the paste according to the first aspect of the present disclosure.

The solar cell front electrode paste for achieving the first aspect of the disclosure includes a copper nitride compound, an antioxidant, and a binder.

Copper nitride paste for solar cell front electrode manufacture

Organic binders, organic solvents, and the like can be used to make the metal powder viscous and capable of screen printing in paste production.

The copper nitride-based compound constituting the copper nitride-based paste of the present disclosure may include copper nitride (I), copper nitride (II), or a powder composed of a material capable of copper nitride precipitation by firing, But the present invention is not limited thereto. Preferably, the copper nitride compound may include copper (I) nitrate.

According to one embodiment of the present disclosure, the content of the copper nitride-based compound in the copper nitride-based paste is 55 to 63 wt%, 57 to 68 wt%, 60 to 68 wt%, and 55 to 70 wt% -70 wt%, and preferably 57-67 wt%, such as 57-63 wt% and 60-65 wt%. When the content of the copper nitride compound is less than 55 wt%, the content of the copper nitride in the paste is too small to lower the conductivity. On the contrary, when the content of the copper nitride compound exceeds 70 wt%, the oxidation of copper nitride occurs too much during the high-temperature firing process for producing electrodes, and the specific resistance increases.

The inventor of the present invention has found that when a copper nitride-based paste composed of only a copper nitride compound instead of silver is used at all, the resistivity is excessively high, but when a certain level of an antioxidant is added, the resistivity can be remarkably reduced.

Although it is not intended to be limited by theory, it is believed that the addition of an antioxidant inhibits the oxidation of copper nitride during high-temperature baking of the front electrode, thereby suppressing the increase in the specific resistance of the copper nitride-based paste.

According to one embodiment of the present disclosure, the antioxidant may be at least one selected from the group consisting of a fatty acid, a boric acid compound, a fluorinated compound, and a reducing metal. The fatty acid may preferably comprise lauric acid, palmitic acid, stearic acid, or propionic acid. The boric acid compound may preferably comprise boron oxide, potassium borate, or potassium borofluoride. The fluorinated compound may preferably include acidic potassium fluoride. The reducing metal may preferably comprise boron.

According to one embodiment of the present disclosure, the antioxidant content may be 1-20 wt%, preferably 9-18 wt%, more preferably 10-14 wt%, of the total paste. When the content of the antioxidant is less than 1 wt%, the oxidation inhibiting effect is insignificant and it is difficult to lower the resistivity value. When the content of the antioxidant is more than 20 wt%, the amount of the antioxidant increases.

According to one embodiment of the present disclosure, more preferably, the antioxidant may be boron. The boron may be in an oxide form, and the boron oxide is not particularly limited to the oxidation number of the boron, and may include all of the boron-oxidized form. For example, the boron oxide may be B ₂ O ₃ , B ₂ O, or B ₆ O. The boron oxides may be used alone or in admixture of two or more. Preferably the boron oxide is B ₂ O ₃ .

According to one embodiment of the present disclosure, the boron content may be 1-20 wt%, preferably 9-18 wt%, more preferably 10-14 wt%, of the total paste. If the boron content is less than 1 wt%, the oxidation inhibiting effect is insignificant and it is difficult to lower the resistivity value. When the boron content is more than 20 wt%, the boron content is high and the resistivity is increased due to the heat treatment. The graph of Fig. 2 shows the tendency of the resistivity value according to the boron content in the paste. On the other hand, depending on the content of boron, boron has a high specific surface area, which affects the paste dough and printability.

In the present disclosure, any resin binder may be used for the copper nitride-based paste for the front electrode of the solar cell, but it is preferable to use a resin binder such as a cellulose resin, polyvinyl alcohol, acrylic resin, butyral resin, castor oil fatty acid modified alkyl resin, Phenol resin, rosin ester resin, polymethacrylate, or ethylene glycol monobutyl ether monoacetate, and more preferably, a cellulose-based or acrylic-based resin can be used as a binder.

The content of the resin binder is preferably in the range of 30 wt% or less based on the total weight of the paste. When the content of the resin binder exceeds 30 wt%, the viscosity is lowered and there is a problem in printability. The binder content should be selected within the range of no problems with viscosity and printability, and no mixing problems with paste pastes, fillers and vehicles.

The copper nitride conductive paste of the present disclosure may further contain at least one additive selected from the group consisting of a thickener, a stabilizer, a dispersant, a thixotropic agent, a defoamer, a plasticizer, a viscosity modifier, a pigment, a UV stabilizer, . The amount of the additive can be determined depending on the properties of the finally obtained conductive paste. The amount of the additive can be suitably determined by those skilled in the art.

The copper nitride conductive paste of the present disclosure is preferably applied to a desired portion of the front surface of the solar cell by screen printing, but it is preferable that the copper nitride conductive paste has a predetermined range of viscosity when applied with such printing.

In the present disclosure, water or an organic solvent can be used as a viscosity modifier. Examples thereof include ester solvents of hexane, cyclohexane, cycloheter, amide, ketone, terpene, polyhydric alcohol ester, alcohol and alcohol, preferably dihydro-terpineacetate, Phenol, and butyl ketyl acetone.

As described above, the paste having the conductivity of the present disclosure is used to form an electrode mainly composed of copper nitride, which can be placed on the front side of the solar cell. That is, the paste of the present disclosure is printed on the front side of the solar cell and dried. In addition, a rear electrode composed of aluminum or silver may be formed on the rear side of the solar cell.

Manufacture of solar cell front electrode

In the case of the solar cell front, it consists of a finger electrode collecting electrons and a bus bar electrode capable of direct / parallel connection of the cell and the cell. In the case of the rear surface, it is composed of the entire aluminum electrode and is composed of the same bus bar electrode as the front surface.

The solar cell according to one aspect of the present disclosure may use a single crystal silicon or a polycrystalline silicon wafer, or may be a thin film silicon. In the case of a single crystal silicon wafer, it is formed by a pulling method or the like. In the case of a polycrystalline silicon wafer, it may be formed by a casting method or the like.

The silicon ingot formed by the impression method or the casting method is sliced to a predetermined thickness, and then its surface is etched with NaOH, KOH, hydrofluoric acid or the like to be cleaned.

When a p-type silicon wafer is used, the n-layer can be formed by diffusing pentavalent elements such as phosphorus, and the depth of the diffusion layer can be varied by controlling the diffusion temperature and time.

An antireflection film may be formed on the n-layer. The antireflection film reduces the reflectance of the surface of the solar cell with respect to the incident light, thereby increasing the amount of light absorption and thus increasing the generation of current.

The antireflection film may be a single layer film or a multilayer film of SiN _x , TiO ₂ , SiO ₂ , MgO, ITO, SnO ₂ , or ZnO. The anti-reflection film may be formed by a thin film deposition process such as sputtering or CVD (Chemical Vapor Deposition).

In the upper part of the antireflection film, a front electrode manufactured using a copper nitride-based paste for a front electrode according to one aspect of the present disclosure is formed.

The paste of the present disclosure is screen printed and printed in a predetermined pattern, and dried using an infrared ray drying furnace. During the firing, it penetrates the antireflection film and is connected to the n-layer. From this, a front electrode of a solar cell can be manufactured.

Solar cell manufacturing

The front electrode paste is used to manufacture a solar cell. After the paste is textured on the entire surface of the wafer, an n-layer is formed, an antireflection film is formed thereon, screen printing is performed on the entire surface in a predetermined pattern and dried using an infrared drying furnace. Thereafter, the wafer is printed on the back surface with an aluminum paste or the like, and then dried in the same manner. The cells formed by the above process are usually baked at 800 ° C or higher for 3 to 5 seconds to form crystallites using a firing furnace to prepare cells.

Example

Example 1-7: Conductive paste production

Example 1-6 and Example 1-6 having the contents of the following Table 1 were applied to copper (I) (Copper (I) nitride), boron powder (B95-1617, boron, purity 95-97% 6 instead of boron as an antioxidant.

Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Copper nitride (I) 71.7 wt% 69.4 wt% 67.2 wt% 64.9 wt% 62.6 wt% 59.4 wt% 65.0 wt% Antioxidant 3.8wt%
(Boron) 6.1wt%
(Boron) 8.3 wt%
(Boron) 10.6 wt%
(Boron) 12.9 wt%
(Boron) 16.1 wt%
(Boron) 10.5 wt%
(Palmitic acid) bookbinder 24.5 wt% 24.5 wt% 24.5 wt% 24.5 wt% 24.5 wt% 24.5 wt% 24.5 wt% Resistivity
(Ω · cm) 0.22 1.37x10 ^-2 5.58x10 ^-3 6.06x10 ^-5 3.10x10 ^-5 5.34x10 ^-4 6.85X10 ^-5

The binder was mixed with solvent butyl carbitol and 1-dodecanol mixed at a ratio of 53: 35 at a ratio of ethyl cellulose N300 and ethyl cellulose N22 of 3.76: 7.52, respectively, followed by stirring for 3 days. And it has aging time of day for bubble removal.

Copper (I) nitride and boron are mixed and dispersed in the prepared binder. The paste was prepared by adjusting the interval between the rolls using a 3 roll mill equipment and repeating 5 times. The paste was printed at the room temperature for one day or stored at 4 ° C for 60 days in a refrigerator.

The prepared paste was applied to a silicon wafer by a screen through a thick film process and the resistivity was measured after heat treatment in an atmospheric environment. The heat treatment temperature is the same as a sintering condition using a silver paste for a solar cell, and the firing condition is carried out at 800 ° C to 900 ° C. The higher the temperature, the higher the resistivity of the electrode due to oxidation or compound formation, and therefore it is required to achieve a specific resistance value below a certain level. In the case of Example 1-6, the heat treatment was carried out at 850 占 폚 for 1 minute in the atmosphere.

In Example 7, palmitoic acid was used instead of boron and the other conditions were the same.

Unlike Examples 1-7, in the case of an antioxidant-free paste, an oxide of copper (I) oxide is produced as nanowires and the copper (I) nitrate particles are melted together. When melting occurs, interfusion does not occur and electron transfer is thought to be limited. On the other hand, in the case of the paste to which the antioxidant was added as in Example 1-7, the antioxidant was first oxidized first in place of the copper (I) nitrate so that the copper (I) nitrate was hardly oxidized and the copper (I) To improve the electron transport.

Method of measuring resistivity

The surface resistivity was measured by measuring the sheet resistance using four probe points, measuring the thickness of the same electrode, and multiplying the measured sheet resistance. The sheet resistance was measured with a LORESTA-GP / MCP-T610 sheet resistance meter manufactured by Mitsubishi Chemical Co., Ltd., and a DIGIMICRO MF 501 thickness meter of Nikon Metrology was used for the electrode thickness measurement.

The results of the measurement of the resistivity values of the pastes 1-7 are shown in Table 1.

In the paste of the present disclosure, in the case of paste 4-7 containing 9.0-18.0 wt% of the antioxidant, the specific resistance value was 3.10 x 10 ^-5 to 5.34 x 10 ^-4 Ω · cm. In the paste for the front electrode of the solar cell, the specific resistance value of the electrode is preferably as low as possible, preferably 2.0 x 10 < ^-4 > In the present disclosure, an antioxidant is added to a copper nitride-based paste so that the antioxidant can improve the low non-resistance property by suppressing oxidation of the copper nitride.

Comparative Example 1-4: Paste without addition of boron

Resistivity values were measured by baking at 500 ° C and 850 ° C using a paste containing silver added to the CuP alloy base shown in Table 2. The CuP alloy was manufactured by Pometon.

Paste
base Composition (wt%) Firing condition Resistivity (Ω · cm) CuP Ag Binder Comparative Example 1 CuP + Ag 20wt% 500 ℃ 7.1 x 10 ^-5 67wt% 850 ℃ 4.5 x 10 ^-4 13wt% Comparative Example 2 CuP + Ag 40wt% 500 ℃ 4.1 x 10 ^-4 47wt% 850 ℃ 1 x 10 ^-3 13wt% Comparative Example 3 CuP + Ag  60wt% 500 ℃ 2.46 x 10 ^-3 27wt% 850 ℃ 1.83 x 10 ⁴ 13wt% Comparative Example 4 CuP 87wt% 500 ℃ 1.12 x 10 ³ 0wt% 850 ℃ - ^a 13wt%

- ^a means that the specific resistance can not be measured.

The results of Table 2 show that the resistivity of the CuP alloy based paste to which silver was added without addition of boron was improved as the silver content increased at 500 ° C and 850 ° C baking temperature. It is considered that the silver acts as the main electrode and the resistivity is lowered. On the other hand, it was confirmed that the resistivity of the paste having the same silver content was increased when the firing temperature was changed to 500 ° C and 850 ° C, and when the firing temperature was 850 ° C, when the paste was fired at 500 ° C. In the case of low content of silver, the resistivity value according to firing temperature showed a larger difference. For example, were a 7.4x10 ⁶ times the specific resistance increases when the content is 27 wt% if, at 500 ℃ the specific resistance compared inde 2.46x10 ^-3 Ω · cm, a specific resistance at 850 ℃ is increased as the firing temperature 1.83x10 ⁴ Ω · cm .

The resistivity values after baking of the CuP paste (Comparative Example 4) without addition of boron (Comparative Example 4) and the Ag-added CuP paste (Comparative Example 1-3) were not sufficiently low enough to be suitably used for solar cell electrodes, and the lowest resistivity Comparative Example 1 was recorded. Although the comparative example 1 has a resistivity value of 7.1 x 10 < ^-5 > OMEGA .cm, the silver content is 67 wt%, even though it has a resistivity improving effect of 2.0 x 10 < ^-4 > Alternatively, this disclosure was able to achieve a resistivity of less than 2.0 x 10 < ^-4 > [Omega] -cm by using copper (I) nitride and a small amount of boron without the use of expensive silver at all.

As a result, in the case of CuP based alloys, even when silver was added, no paste was found to have a specific resistance value of 2.0 x 10 < ^-4 > In Comparative Example 1 paste, the silver content was excessively 67 wt%, indicating that the economic effect of substituting silver was not significant.

Comparative Example 5-12: Paste with boron added

The paste was baked at 800-900 ° C using a paste containing boron added to the CuP alloy base described in Table 3, and the resistivity was measured. Pometon (Comparative Example 5-8) and Poongsan Co. (Comparative Example 9-12) were used as the CuP alloy.

Paste-based
Composition (wt%) Resistivity
(Ω · cm) CuP Boron Binder Comparative Example 5 CuP 67.82% 3.08x 10 ^-1
 5.35% 26.83% Comparative Example 6 CuP 62.30% 3.44x 10 ^-1
9.84% 27.87% Comparative Example 7 CuP 55.07% - ^a 13.04% 31.88% Comparative Example 8 CuP 50.67% - 16.00% 33.33% Comparative Example 9 CuP 67.82% 6.24x 10 ^-3
5.35% 26.83 Comparative Example 10 CuP 62.30% 1.14x 10 ^-1
9.84% 27.87% Comparative Example 11 CuP 55.07% - 13.04% 31.88% Comparative Example 12 CuP 50.67% - 16.00% 33.33%

- ^a means that the specific resistance can not be measured.

In the case of a paste based on a CuP alloy, there was an effect of improving the resistivity in the case of adding boron rather than in the case of not adding boron. However, there was no paste that achieved the resistivity value 2.0 x 10 < ^-4 > . The lowest resistivity was 6.24 x 10 < ^-3 > OMEGA .cm in Comparative Example 9 paste.

As a result, the addition of boron to the paste did not show a significant improvement in resistivity when added to CuP, unlike the case where copper was added to copper (I).

Claims (8)

As a paste for solar cell front electrode,
A paste for a solar cell front electrode comprising a copper nitride compound, an antioxidant, and a binder. The method according to claim 1,
Wherein the copper nitride compound includes copper (I) nitride (Copper (I) nitride). The method according to claim 1,
Wherein the antioxidant is at least one selected from the group consisting of a fatty acid, a boric acid compound, a fluoride compound, and a reducing metal. The method of claim 3,
Wherein the fatty acid comprises lauric acid, palmitic acid, stearic acid, or propionic acid,
The boric acid compound includes boron oxide, potassium borate, or potassium borofluoride,
Wherein the fluorinated compound comprises acidic potassium fluoride,
Wherein the reducing metal comprises boron. The method according to claim 1,
The binder may be selected from the group consisting of a cellulose resin, polyvinyl alcohol, acrylic resin, butyral resin, castor oil fatty acid modified alkyl resin, epoxy resin, phenol resin, rosin ester resin, polymethacrylate, and ethylene glycol monobutyl ether monoacetate Wherein the conductive paste is at least one selected from the group consisting of silver, silver, and silver. The method according to claim 1,
Wherein the paste comprises 55 to 70 wt% of a copper nitride compound, 1 to 20 wt% of an antioxidant, and a binder as a balance. The method according to claim 1,
The solar cell front electrode paste further comprises at least one additive selected from the group consisting of a dispersant, a thixotropic agent, a defoamer, a plasticizer, a viscosity stabilizer, a pigment, a UV stabilizer, and a coupling agent. Paste. A solar cell front electrode comprising the solar cell front electrode paste according to any one of claims 1 to 7.

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