KR20110016726A - Aluminium paste for a back electrode of solar cell - Google Patents

Abstract

PURPOSE: An aluminum paste for a back electrode of a solar cell is provided to minimize aluminum bubbles, bump, and yellowing of an aluminum back electrode or the warping of the solar cell in a plasticizing process, and to improve Isc and Voc values. CONSTITUTION: An aluminum paste for a back electrode of a solar cell includes, based on the weight of the composition, 65~75 weight% of aluminum powder having a particle size distribution of 0.01~5 micron and 25~35 weight% of organic vehicles, and does not contain glass frits. The organic vehicle includes organic solvents and polymer resins. The boiling point of the organic solvent is 150~300 °C.

Description

Aluminum paste for a back electrode of solar cell

The present invention relates to an aluminum paste for a back electrode of a solar cell.

In the silicon crystalline solar cell, a PN junction is formed by diffusing phosphorus (P) on the surface of a P-type silicon wafer, and an antireflection film is coated to reduce reflection loss of sunlight incident on the P-type silicon wafer. The Ag electrode is formed, and Ag and Al electrodes are formed on the back side P layer. The silver paste used as the front electrode is connected to the N-side of silicon to form an electrode, and the aluminum paste is connected to the backside of silicon, that is, the P layer, to form an electrode.

Silicon crystalline solar cells generally use P-type silicon substrates with a thickness of 180-220 μm. An n-type impurity layer having a thickness of 0.2-0.6 μm is formed on the front surface of the substrate, and a SiNx layer and a front electrode are formed thereon to prevent reflection. An aluminum electrode is formed on the rear side. The aluminum electrode is formed by applying an aluminum paste by screen printing and the like and drying it, followed by a two-stage baking process of low temperature (about 600 ° C.) and high temperature (800-950 ° C.). In this firing process, aluminum diffuses into the P-type silicon wafer to form an Al-Si alloy layer. The diffusion layer (P ⁺ layer) forms a back surface field (BSF) that prevents recombination of electrons generated in the solar cell and improves the collection efficiency of the generated carriers. The efficiency of the solar cell depends on the thickness and uniformity of the BSF layer formed as described above. As the thickness becomes thinner, the efficiency of the solar cell decreases and the efficiency increases when the thickness becomes thicker.

On the other hand, in recent years, in order to reduce the cost of solar cells, it is a trend to thin the substrate of the silicon wafer. When the thickness of the silicon wafer is thinned, the warpage of the wafer occurs due to the difference in the expansion coefficient between the silicon wafer and the aluminum, and thus the wafer Phenomena such as cracking also occur.

In order to improve this problem, it is necessary to form a thin thickness of the aluminum electrode, which is the rear electrode, which can be achieved by reducing the amount of the aluminum paste applied. However, if the coating amount of aluminum paste is reduced, the thickness of the BSF layer, which is the rear electric field layer, is reduced, which reduces the efficiency of the solar cell, and also causes the problem that the generation of aluminum bubbles or bumps (Bμmp) increases in the electrode layer during the firing process. do. The aluminum bubbles or bumps (Bμmp) generated as described above decrease the smoothness of the back surface of the wafer, and as stress is concentrated thereon, the battery breaks in the solar cell manufacturing process or the module manufacturing process.

Conventional techniques proposed to reduce the warpage of the solar cells and the generation of aluminum bubbles during the firing process are as follows. Korean Patent Publication No. 10-0825580 describes aluminum powder having a size of 0.5-10 μm and aluminum paste having metal alkoxide added to an organic vehicle, and Korean Patent Publication No. 10-2008-0068638 describes an aluminum powder having a size of 2-20 μm. And aluminum paste containing metal hydroxides in addition to glass frit and organic vehicle, and Korean Patent Publication No. 10-2008-0057230 includes a plasticizer in addition to aluminum powder, glass frit and organic vehicle having a size of 2-20 μm. Aluminum paste is described, and also the Republic of Korea Patent Publication No. 10-2008-0104179 includes 4-10μm aluminum particles and alkali glass frit, including boron ethoxide, titanium ethoxide (Titaniμm) Aluminum paste with Ethoxide, Fμmed silica, etc. is described. .

However, the aluminum pastes introduced in the above patent documents all contain organic or inorganic additives in addition to aluminum, glass frit, and organic vehicle. However, these additives remain as residues or voids in the firing process of the paste, which not only lowers the resistance of the paste, but also lowers the uniformity, thereby causing a problem that adversely affects the efficiency of the solar cell. In addition, since the aluminum pastes have a maximum particle size of 10-20 μm, it is difficult to form a uniform contact with the back surface of the textured solar cell, so that aluminum bumps (bμmp) may be generated by internal pores. Has a high disadvantage.

The present invention is to solve the above problems of the prior art,

Minimizes the generation of aluminum bubbles, bumps (Bμmp) and yellowing of the solar cell during the firing process and the aluminum back electrode layer, greatly improves the Isc and Voc values, and dramatically improves the efficiency of the solar cell. It is an object to provide an aluminum paste for a battery back electrode.

In addition, an object of the present invention is to provide an aluminum paste for solar cell back electrodes that does not include glass frit and provides cost reduction and simplification of manufacturing process as well as the above effects.

Provided is an aluminum paste for a solar cell back electrode, comprising 65 to 75% by weight of aluminum powder having a particle size distribution of 0.01 μm to 5 μm and 25 to 30% by weight of an organic vehicle solution, and containing no glass frit. .

In addition,

It provides a solar cell manufacturing method comprising the step of forming a back electrode using the aluminum paste.

According to the present invention, the aluminum paste composition improves the contact between the textured silicon wafer and the aluminum paste, thereby minimizing the warpage of the solar cell and the generation of aluminum bubbles, bumps (Bμmp) and yellowing on the aluminum back electrode layer during the firing process. And not only greatly improve the Voc value, but also dramatically improve the efficiency of the solar cell.

In addition, by not including the glass frit, in addition to the above effects, it provides a cost reduction and a simplification of the manufacturing process.

The present invention includes an aluminum paste for solar cell back electrode, including 70 to 75% by weight of aluminum powder and 25 to 30% by weight of an organic vehicle solution having a particle size distribution of 0.01 μm to 5 μm based on the total weight of the composition. It is about.

The aluminum powder contained in the paste composition of the present invention uses a particle size distribution of 0.01 μm-5 μm.

In general, in the case of silicon solar cells, the front and rear surfaces are textured to broaden the light receiving area of sunlight. In general, the form of single crystal texturing has a pyramidal structure, and the height of the pyramid structure is about 2 μm to 15 μm and the width is about 2 μm to 20 μm. In addition, polycrystalline silicon texturing has an irregular maze shape. The aluminum paste is printed on the silicon wafer having the above structure by screen printing, gravure printing, or offset printing, and dried, and then the aluminum electrode is formed through a firing process. In this process, if the size of the aluminum particles is too large, the contact between the aluminum paste and the silicon wafer is poor, and voids are formed between the paste and the texture structure of the silicon wafer after printing and drying. These pores are ejected through the aluminum paste layer to the surface of the aluminum electrode during the firing process, and bumps and bubbles of aluminum are generated along with this process. Therefore, the particle size distribution of the aluminum powder included in the paste is preferably in the range of 0.01 μm to 5 μm. If the particle size is less than 0.01 μm, aluminum bumps are generated in the post-printing firing process, the wafer warpage is increased, and if the particles are larger than 5 μm, the filling rate of the particles is lowered, which causes a problem of lowering the efficiency. do.

When the paste is manufactured using aluminum powder having a particle size distribution in the above range, not only the paste can penetrate deeply between the texturing structures of the wafer but also the porosity in the paste is reduced. Therefore, the back side BSF layer is formed uniformly, the resistance of the aluminum electrode is also lowered, and the warping of the wafer is also suppressed. Therefore, in the solar cell manufactured using the aluminum powder as described above, the maximum output current Isc is increased and the efficiency is also increased. In addition, it is also possible to obtain an effect of preventing the yellowing phenomenon occurring in the aluminum electrode after the firing process.

In the aluminum paste of the present invention, the aluminum powder is preferably included in 65 to 75% by weight, and if included in less than 65% by weight, the aluminum layer printed after firing becomes thinner, so that the rear BSF layer is not sufficiently formed so that the efficiency is high. The problem of falling occurs, and if it exceeds 75% by weight, the printing thickness becomes too thick, which may result in warping of the silicon wafer.

In general, the glass frit is included in the aluminum paste to increase adhesion to the substrate and induce a decrease in melting temperature. However, the aluminum paste of the present invention is characterized by maximizing solar cell efficiency without including any glass frit component.

The reason why it is possible to maximize the solar cell efficiency without including the glass frit seems to be that the present invention uses aluminum powder having a particle size distribution of 0.01 μm to 5 μm. That is, when the metal particles are used in nano size or sub micro size, the melting temperature lowering effect, which is inherent to the metal, is exhibited. In the aluminum paste of the present invention, the aluminum particles are inherent to the melting point of aluminum. It appears to be melting at temperature and eutectic with silicon wafer to form BSF layer smoothly without glass frit.

Paste of the present invention comprises an organic vehicle solution of 25 to 35% by weight based on the total weight of the composition, the organic vehicle solution is prepared by dissolving the polymer resin in an organic solvent, if necessary, thixotropic agents, wetting agents, additives, etc. It may include.

The organic vehicle solution used in the present invention comprises at least 75% by weight of solvent, 1-30% by weight of polymer resin, about 5% by weight of wetting agent and thixotropic agent, and 1-15% by weight of the total weight of the solution. can do.

The solvent is a solvent having a breaking point in the range of 150-300 ℃ to prevent the drying of the paste during the printing process and to control the fluidity. Widely used solvents include glycol ether-based tripropylene glycol methyl ether, dipropylene glycol n-propyl ether, dipropylene glycol n-butyl ether, tripropylene glycol n-butyl ether, propylene glycol phenyl ether, diethylene glycol ethyl ether. , Diethylene glycol n-butyl ether, diethylene glycol hexyl ether, ethylene glycol hexyl ether, triethylene glycol methyl ether, triethylene glycol ethyl ether, triethylene glycol n-butyl ether, ethylene glycol phenyl ether, terpinol, Texanol ^® Ethylene glycol etc. are mentioned.

Examples of the polymer resin include polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, ethyl cellulose, rosin, phenol resin, acrylic resin and the like. The content of the polymer resin is preferably 1-30% by weight, preferably 5-25% by weight based on the total weight of the organic vehicle solution. If the amount of the polymer resin is less than 1% by weight, the printability and dispersion stability of the paste may be lowered. If the amount of the polymer resin is more than 30% by weight, the paste may not be printed.

As the thixotropic agent and the humectant, any one generally used in the art may be used without limitation.

The additives include those generally used in this field such as dispersants. Commercially available surfactants can be used as the dispersant, and these can be used alone or in combination of two or more thereof. The surfactant may be, for example, an ether type such as alkyl polyoxyethylene ether, alkylaryl polyoxyethylene ether, polyoxyethylene polyoxypropylene copolymer as a nonionic surfactant; Ester ether types such as polyoxyethylene ether of glycerin ester, polyoxyethylene ether of sorbitan ester, and polyoxyethylene ether of sorbitol ester; Ester type such as polyethylene glycol fatty acid ester, glycerin ester, sorbitan ester, propylene glycol ester, sugar ester, alkyl polyglucoside; Nitrogen-containing types such as fatty acid alkanolamides, polyoxyethylene fatty acid amides, polyoxyethylene alkylamines, amine oxides; Polymeric surfactants include polyvinyl alcohol, polyvinylpyrrolidone, polyacrylic acid, polyacrylic acid-maleic acid copolymers, poly 12-hydroxystearic acid, and the like.

Commercially available products include Hypermer KD (manufactured by Uniqema), AKM 0531 (manufactured by Nippon Yuji Co., Ltd.), KP (manufactured by Shin-Etsu Chemical Co., Ltd.), and POLYFLOW (manufactured by Kyowa Co., Ltd.). ), EFTOP (manufactured by Tochem Products), Asahi guard, Suflon (above, manufactured by Asahi Glass), SOLSPERSE (manufactured by Geneva), EFKA (EFKA) Chemicals Co., Ltd.), PB 821 (made by Ajinomoto Co., Ltd.), BYK-184, BYK-185, BYK-2160, Anti-Terra U (by BYK Co., Ltd.), etc. are mentioned.

The dispersant is preferably 1 to 10% by weight, more preferably 1 to 5% by weight based on the total weight of the organic vehicle solution.

Paste preparation according to the present invention can be easily produced using a planetary mixer that performs both rotation and revolution at the same time. That is, the above-mentioned components may be prepared by mixing and dispersing the solid content and the organic vehicle solution appropriately by putting the above-mentioned components into the planetary mixer container according to the corresponding composition ratio and stirring. The viscosity of the paste thus prepared is from 20,000 to 200,000 cps at 5 rpm, preferably from 40,000 to 100,000 cps (measured with Brookfield HBDV-III Ultra Rheometer, spindle CPE-52 at 25 °). .

In addition,

It provides a method of manufacturing a solar cell comprising the step of forming a back electrode using the aluminum paste.

The solar cell manufactured by the above method minimizes warpage and generation of aluminum bubbles and bumps (Bμmp) in the electrode layer, thereby improving Isc and Voc values as well as dramatically improving the efficiency of the solar cell.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following examples are intended to illustrate the present invention more specifically, but the scope of the present invention is not limited by the following examples. The following examples can be appropriately modified and changed by those skilled in the art within the scope of the present invention.

Example 1 Preparation of Aluminum Paste

74% by weight of aluminum powder with a particle size distribution of 0.04 to 5μm and 26% by weight of an organic vehicle solution in which ethyl cellulose was dissolved and dissolved in glycol ether were added sequentially, followed by rotating and rotating at a speed of 3 at 1,000 rpm. The mixture was stirred for a minute to prepare an aluminum paste.

Comparative Example 1: Preparation of Aluminum Paste

An aluminum paste was prepared in the same manner as in Example 1, except that an aluminum powder having a particle size distribution of 0.04 to 5 μm was added at a content of 64 wt% and an organic vehicle solution was added at 36.0 wt%.

Comparative Example 2: Preparation of Aluminum Paste

An aluminum paste was prepared in the same manner as in Example 1, except that an aluminum powder having a particle size distribution of 0.04 to 5 μm was added in a content of 76 wt% and an organic vehicle solution was added at 24.0 wt%.

Comparative Example 3: Preparation of Aluminum Paste

An aluminum paste was prepared in the same manner as in Example 1, except that 0.5 wt% of PbO-Al ₂ O ₃ -SiO ₂ -B ₂ O ₃ -based glass frit was further added, and an organic vehicle solution was added at 25.5 wt%. It was.

Comparative Example 4: Preparation of Aluminum Paste

An aluminum paste was prepared in the same manner as in Example 1, except that 0.5 wt% of ZnO-Al ₂ O ₃ -SiO ₂ -B ₂ O ₃ -based glass frit was further added, and an organic vehicle solution was added at 25.5 wt%. It was.

Comparative Example 5: Preparation of Aluminum Paste

An aluminum paste was prepared in the same manner as in Example 1 except that the aluminum powder having a particle size distribution of 2 to 10 μm was used instead of the aluminum powder having a particle size of 0.04 to 5 μm.

Comparative Example 6: Preparation of Aluminum Paste

An aluminum paste was prepared in the same manner as in Example 1, except that an aluminum powder having a particle size distribution of 5 to 15 μm was used instead of an aluminum powder having a particle size of 0.04 to 5 μm.

Test Example: Manufacturing and Characteristic Test of Solar Cell

A surface texturing process was performed on a 156 × 156 mm, 200 μm thick single crystal wafer to form a pyramid height of about 4-6 μm, followed by coating SiNx on the N-side of the wafer. Subsequently, the Bus Bar was printed on the back surface of the wafer using silver paste, followed by drying. Then, the pastes exemplified in Examples and Comparative Examples were applied and dried using a 250-mesh screen printing plate. The coating amount was printed to be 1.5 ± 0.1 g before drying and dried, and the Finger Line was printed and dried using silver paste on the front SiNx side.

The silicon wafer passed through the above process was fired in an infrared continuous firing furnace such that the temperature of the firing region was 720-900 ° C to manufacture a solar cell.

The firing process may be performed by simultaneous firing of the front and rear surfaces while passing the silicon wafer into a belt furnace. At this time, the belt furnace includes a burn-out section of about 600 ° C. and a firing section around 800 ° C. to 950 ° C., and burns away the organic material in the paste, and then melts the silver and aluminum on the front and rear surfaces to form an electrode. do.

After matching the four corners of the solar cell manufactured above with the bottom, the degree of lifting of the center portion was measured to evaluate the degree of warpage of the solar cell. In addition, the occurrence of bumps (bμmp) and aluminum bubbles in the rear aluminum electrode region was visually observed to count the number, and yellowing was visually observed and evaluated. The evaluation results are shown in Table 1 below.

The efficiency of the solar cell was evaluated using SCM-1000, a solar cell performance evaluation device of FitTech, and the results are shown in Table 2 below.

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Al powder 0.04-5 μm 0.04-5
μm 0.04-5 μm 0.04-5 μm 0.04-5 μm 2-10
μm 5-15
μm Al content 74wt% 64wt% 76wt% 74wt% 74wt% 74wt% 74wt% Glass Frit Content (%) 0 0 0 0.5 0.5 0 0 Deflection (mm) 0.2 0.5 0.2 0.3 0.8 1.2 0.2 0.5 2.5-3 0.5 1.0 1.8 2.5 Bump Count 0 0-2 0 1-2 3-5 5-8 0 Al electrode
Yellowing X X X X O O OO

[Note] Al electrode yellowing: X = none, O = partial occurrence, OO = large quantity

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 3 Comparative Example 4 Comparative Example 5 Pmax (W) 4.065 3.9171 4.0152 4.0839 3.743 4.009 3.9389 efficiency (%) 17.02 16.393 16.80 17.09 15.66 16.78 16.48 FF (%) 77.9 75.316 77.91 77.21 72.55 76.79 77.18 Isc 8.392 8.4075 8.3322 8.4829 8.3515 8.3881 8.2789 Voc 0.6220 0.6186 0.6185 0.6235 0.6178 0.6225 0.6164 Rs 0.00714 0.0082 0.00749 0.00799 0.01096 0.00739 0.00771

[Note] Isc = short circuit current (A)

Voc = open voltage (V)

Rs = Series Resistance

FF = Fill Factor

Claims (4)

An aluminum paste for solar cell back electrode, comprising 65 to 75% by weight of aluminum powder and 25 to 35% by weight of an organic vehicle solution having a particle size distribution of 0.01 μm to 5 μm relative to the total weight of the composition. The aluminum paste of claim 1, wherein the organic vehicle solution comprises an organic solvent and a polymer resin, and the organic solvent has a boiling point of 150 to 300 ° C. The aluminum paste of claim 1, wherein the viscosity of the aluminum paste is 20,000 to 200,000 cps at 5 rpm. A method of manufacturing a solar cell, comprising the step of forming a back electrode using the aluminum paste of claim 1.