KR101317228B1 - Aluminum paste compositon of the low bowing and high-efficiency silicon solar cells - Google Patents

Abstract

The present invention is to obtain a high efficiency characteristic by improving the bowing of the solar cell (cell) when the aluminum paste composition for forming a back field on the back of the silicon solar cell, the glass composition and the ceramic additive to the aluminum powder. .
The aluminum paste composition according to the present invention is composed of 60 to 90% by weight of aluminum powder, 1 to 10% by weight of binder resin, 0.1 to 10% by weight of lead-free glass additive, and 0.1 to 10% by weight of ceramic additive. 60 to 90% by weight of a conductive material having two or more different sizes.
According to the present invention, it is possible to improve the bowing characteristics of the solar cell can reduce the decrease in yield due to the breakage of the solar cell (cell) during the handling of the solar cell production, and also the solar cell by improving the bowing (bowing) Reducing the internal stress in the cell can ultimately improve solar cell efficiency.

Description

Aluminum paste composition for low warpage and high performance of silicon solar cell {ALUMINUM PASTE COMPOSITON OF THE LOW BOWING AND HIGH-EFFICIENCY SILICON SOLAR CELLS}

The present invention relates to a paste composition for realizing a low bending high property, which includes a lead-free glass composition and an oxide additive in an aluminum paste composition for forming a backside field on a back surface of a silicon solar cell.

A silicon solar cell is a semiconductor device that converts solar energy into electrical energy. It has a junction between a P-type semiconductor and an N-type semiconductor, and its basic structure is the same as a diode forming a pn junction. . In order to reduce the reflection loss, SiNx is deposited on the surface of the silicon (Si) wafer by PECVD (Plasma Enchanced Chemical Vapor Deposition) to form an antireflection film (ARC) layer. The front side of the wafer is screen printed with silver paste in a grid pattern, the back side of the wafer is printed with aluminum (Al) paste, and the silver or silver-aluminum can be used to connect individual cells to the solar cell module. Printed and fired to make a module by connecting to the ribbon (ribbon) material.

In the case of the rear part to which the aluminum paste is applied, an aluminum back field is formed. Aluminum metal acts as a p-type impurity when introduced into silicon as a group 3 element on the periodic table.

Since an internal field is formed due to a region of high additive concentration near the solar cell wafer rear electrode, electrons generated near the rear surface are prevented from recombining and disappearing to the rear aluminum electrode. When a high temperature belt type heat treatment is performed, a p ++ region is formed at the depth where aluminum metal and silicon contact and aluminum penetrates, thereby creating a barrier due to the potential difference formed therein, and the photoelectrons generated on the p-type side move toward aluminum. It prevents recombination loss and improves open voltage (Voc) and FF.

In the battery manufacturing process, about 50% of the wafer cost is currently being developed to reduce the thickness from 180 to 200 microns to 150 microns. In thin wafer applications, bowing increases due to the difference in the coefficient of thermal expansion (CTE) between aluminum and silicon, and the composition of the aluminum paste needs to be improved in order to reduce the wafer thickness.

That is, it is necessary to improve low bowing characteristics and electrical performance in the aluminum paste forming the aluminum backside field. In general, when the battery efficiency is improved by 0.1%, it is reported in the industry that there is a cost reduction effect of 1.5 billion won per year based on the output of 100 megawatts.

PbO-based glass has been used a lot in the paste composition for solar cell electrodes. Recently, Pb free and Cd free paste have been replaced in consideration of environmental aspects. Bi ₂ O ₃ -based glass is a material that can replace many of the advantages of PbO applications. The interaction with the silicon substrate is known to have an effect of improving the contact resistance.

The present invention was devised to solve the above problems, and its object is to minimize manufacturing defects due to cell breakdown due to bowing phenomenon that occurs during solar cell production handling. In order to improve the electrical performance of the solar cell manufactured thereby.

That is, the present invention is to provide a replacement material of the back electrode for producing a low bowing, high characteristics solar cell.

In order to solve the above technical problem, the present invention provides a paste mixture as a. Aluminum content is about 60-80% by weight of the mixture, b. Provided is an aluminum paste composition for a solar cell back electrode, wherein a lead-free glass additive comprises 0.1 to 2% by weight, 0.1 to 2% by weight of c oxide additive, d and the balance comprising an organic vehicle composed of a resin and a solvent.

In the present invention, the glass component of b. Is 50 to 70% by weight of Bi ₂ O ₃ , 10 to 20% by weight of B ₂ O ₃ , 5 to 20% by weight of ZnO, Al ₂ O ₃ based on the weight of the total glass composition 1 to 15 wt%, Na ₂ CO ₃ 1~5 provides a paste composition for aluminum rear solar cell electrode containing a weight%.

The present invention also provides an aluminum paste composition for a solar cell back electrode, characterized in that the oxide additive comprises a metal selected from Al, Si, Zn, Zr, Mg, and Ti or an oxide of these metals.

In addition, the present invention is an aluminum paste for solar cell back electrode, the oxide additive comprises at least one of Al ₂ O ₃ , SiO ₂ , ZnO, ZrO ₂ , MgO and TiO ₂ and the content is 0.1 to 2% by weight. To provide a composition.

In order to solve the warpage phenomenon after sintering due to the large difference in coefficient of thermal expansion (CTE) between aluminum (232 × 10 ^-7 / ° C) electrode and silicon (26 × 10 ^-7 / ° C) wafer in a solar cell, The addition of the glass composition and the addition of the ceramic additive to adjust the sintering coefficient of thermal expansion of the to realize a low warpage characteristics and at the same time can provide a paste composition made to have a high characteristic light conversion efficiency.

In general, the paste finally forms a coating film to impart electrical properties, affect the mechanical properties of the final coating film (functional phase) and the binder to impart the adhesion between the coating film and the applied substrate, and the operation of the paste It can be divided into vehicles that give sex. An inorganic binder is used for a baking paste. The raw materials used in the paste are largely divided into metal powders to impart conductivity, vehicles to impart workability and coating properties, and additives to improve desired properties. Vehicles can be further classified into polymer resins and organic solvents. have. The paste of the present invention comprises aluminum, glass compositions, oxide additives and organic vehicles. Each component is explained in full detail below.

A. Conductive Aluminum Powder.

Although the granular shape of the conductive material is spherical, amorphous, or plate-shaped or mixed type thereof, two or three kinds of spherical particles are preferably mixed and used for securing the uniformity of the composition. The mixing ratio of the aluminum powder, which is a conductive material, is about 10 to 20% by weight of D ₅₀ 1 to 3 microns based on a laser particle size analyzer, and about ₅₀ to 60% by weight of D _{50 5 to} 9 microns is preferable. When the D ₅₀ 5-9 micron is 80% by weight or more, the substrate bowing phenomenon after solar cell firing tends to increase.

B. Paste Glass Compositions.

The paste of the present invention contains 0.1 to 10% by weight of the glass component, preferably 0.1 to 2% by weight. Each glass component includes a Bi ₂ O ₃ -ZnO-B ₂ O ₃ based oxide frit comprising Bi ₂ O ₃ , ZnO, B ₂ O ₃ , Al ₂ O _3, and Na ₂ CO ₃ . Glass compositions generally use from about 0.5 to 10 microns but have a suitable particle size of 1 to 3 microns.

As the glass composition for a solar cell electrode, a V ₂ O ₅ -B ₂ O ₃ system, a P ₂ O ₅ system, a B ₂ O ₃ -ZnO-BaO system, and a Bi ₂ O ₃ system glass composition may be used. The electrode properties of the glass powder prepared by using the glass composition system, in the case of P ₂ O ₅ system has a problem in the mixing and melting of raw materials when manufacturing the glass powder due to the high hygroscopicity, and the initial powder due to moisture absorption during storage There is a disadvantage in that the characteristics cannot be maintained, and due to the low specific gravity, the electrode coating film density is low at the time of forming the electrode, and thus the conductive property of the electrode tends to be deteriorated. In addition, the B ₂ O ₃ -ZnO-BaO-based also has a low specific gravity, it is difficult to have an amorphous composition in the production of glass powder having a low glass transition temperature (Tg), which is not suitable as a glass powder for solar cell electrodes. For this reason, the glass powder composition according to the present invention has a low hygroscopicity, a high specific gravity, and a Bi ₂ O ₃ system capable of producing a low temperature glass transition temperature as the glass powder for solar cell electrodes. The Tg temperature of the Bi ₂ O ₃ -based glass powder may be variously prepared in a distribution of 350-500 ° C. according to an appropriate mixing ratio of the glass composition, and the content thereof may be 50 to 70 wt%. If the amount of B ₂ O ₃ is too large, the softening point of the glass becomes high, and if the amount is small, it is preferable that the breakthrough of the glass occurs during melting. Therefore, the amount of B ₂ O ₃ is preferably 10 to 20% by weight. It is used to improve chemical resistance, and when it is 20% by weight or more, crystallization occurs. Al ₂ O ₃ is suitable as a network former of the glass (15% by weight or less), and if it is higher than that the adhesion strength is reduced due to the high softening point. When Na ₂ CO ₃ is added at least 5% by weight, it is preferable that the water resistance and acid resistance of the glass is weakened to be 5% by weight or less. Bi ₂ O ₃ is a stable oxide that can cope with Pb system, and lowers the softening point of glass when the raw material is added, and increases the coefficient of thermal expansion.

C. Oxide Additives.

Oxide additives of the present invention comprise (a) a metal oxide selected from Al, Si, Zn, Zr, Mg, and Ti, (b) a metal selected from Al, Si, Zn, Zr, Mg, and Ti; any compound capable of producing the metal oxide of b) and (d) mixtures thereof.

The average particle size of the additional metal / metal oxide additive is preferably 20 microns or less, and preferably 10 microns or less. Preference is given to 0.1 to 2% by weight of the metal / metal oxide additive in the total composition.

D. Resin.

Depending on the application, it should be carefully selected to achieve the desired characteristics. In the case of the calcined paste like the solar cell paste, the polymer resin is burned at the time of final firing, and thus, a resin having good printability, dispersibility, and drying property of the coating is mainly selected. In general, a cellulose series is used. (Ethyl Cellulose) Nitro Cellulose, Hydroxy Propyl Cellulose, Ethyl Hydroxyethyl Cellulose, Hydroxyethylhydroxypropyl Cellulose Cellulose Acetate Butyrate ), Cellulose acetate propionate, polyvinyl alcohol polyvinyl butiral, polyester acrylic and phenol based materials 1 or 2 different species 0.5-10 weight% of comprised resin is preferable. If less than 0.5% by weight of the total content, there is a problem of poor adhesion and printability, and when used in excess of 10% by weight, there is a problem that the sintered density decreases due to the presence of trace ash (Ash).

E. Solvents.

As the solvent for a solar cell electrode paste, one or more of those having a boiling point of 150 ° C. or higher can be selected and used. Organic solvents used in pastes have many demanding properties and must be selected for each vehicle type. Basically, solvent should be good for the resin to be used. Especially in the case of screen printing, there may be volatilization of the solvent due to heat generated by friction between the squeeze and the screen mesh. In some cases, a high boiling point solvent having a low volatility of the solvent is basically used.

However, if the drying is too slow, the characteristics of the coating film may be impaired. Therefore, the volatility and the boiling point should be carefully considered. In addition, it is desirable to select a non-toxic and odorless solvent in consideration of the safety and environment of workers. Generally used organic solvents are aromatic hydrocarbons, ethers, ketones, lactones, ether alcohols, esters and diesters. A solvent may be used independently or may be used in combination of 2 or more type.

Description of Glass Composition Preparation

A glass composition having a Bi ₂ O ₃ -based glass powder as a basic composition is placed in a platinum crucible, melted at 1200-1500 ° C. for 1 hour, and then rapidly cooled to prepare a glass specimen. The prepared glass specimen was dry pulverized at 700 rpm or more using a disk mill equipment to prepare a glass powder having a final average particle size of 200 microns, and then 600 grams of 2 millimeter zirconia balls, 200 grams of pure water, and 100 grams of glass powder. After mixing, wet milling at 300 rpm for 30 minutes with a mono mill equipment to make a glass powder slurry, and dried at 100 for 12 hours to prepare a glass powder having a size of 10 microns or less.

The glass powder having a size of 10 microns or less was mixed again with 600 grams of 0.5 mm diameter zirconia ball and 160 grams of pure water, and wet pulverized at 300 rpm for 30 minutes using a mono mill equipment, and dried at 200 or less for 12 hours. Glass powders with an average particle size in the range of 1-2 microns were prepared.

Table 1 shows the composition, glass ionization temperature (Tg), and softening point of the glass powder prepared by the method described above.

Description of Paste Preparation

In the manufacturing process of the conductive paste composition according to the present invention, first, an organic binder and a solvent are put together in a mixer to dissolve well through stirring to prepare a vehicle. Subsequently, the conductive metal powder, the glass composition, the ceramic additive, and the vehicle are added to a planetary mixer to mix and stir.

The mixed paste is mechanically mixed and dispersed using a 3-roll mill. Subsequently, impurities such as particles having a large particle size, dust, and the like may be removed by filtering, and degassed with a degassing apparatus to remove bubbles in the paste, thereby manufacturing an aluminum paste composition for a solar cell.

Description of solar cell manufacturing

The printed sheet was printed on a 180 micron thick antireflective silicon wafer having an area of 243 square centimeters.

The rear aluminum paste was 36 microns in wire diameter and 5 microns in emulsion, and a 200 mesh stainless steel screen was used. The printing speed was adjusted to 200-250 millimeters per second, and the printing amount was adjusted to 1.6 ± 0.05 grams.

The front silver paste used a screen with a finger line line width of 100 microns and a grid line line width of 2 millimeters. Silver paste printed the front contact paste SOL-9235H available from Heraeus. A four-zone infrared (IR) belt furnace with a belt speed of 4 meters per minute was used to co-fire with the condition that the actual wafer maximum temperature was maintained at 750 ° C. or higher for 3 seconds.

Test Procedure-Evaluation of Manufacturing Glass Properties

The glass transition temperature (Tg) and softening temperature (Ts) of the prepared glass composition were measured at a heating rate of 10 ° C./min using differential scanning calorimetry (DSC: STA449C, Netzsch, Germany). The coefficient of thermal expansion (CTE) was measured at a heating rate of 10 ° C./min using a dilatometer (DIL402, Netzsch, Germany) and the coefficient of thermal expansion was measured from the slope in the range of 40 to 300 ° C.

Average particle size was measured using a particle size analyzer (Particle size analyzer: S3000, Microtrac, US).

Test procedure-bending

After firing, the solar cell was measured by moving the laser displacement meter on the flat surface, and the difference between the maximum displacement value and the minimum displacement value (wafer thickness) compared to the flat surface showed the amount of warpage of each sample. The measuring method is the same as the method of measuring the amount of warpage disclosed in Korea Laid-open Patent No. 10-2010-0088131.

Equation (1) Deflection (mm) = maximum displacement value (X2)-minimum displacement value (X1)

Test procedure-efficiency

The solar cell manufactured according to the above method was measured under a solar condition of an AM 1.5 by a solar simulator system (Oriel Instrument Co., model 94063A). In Tables 2, 3 and 4, FF is the filling factor obtained from the I-V curve, and Eff represents the light conversion efficiency. Voc is an open voltage, Isc is a short circuit current, Jsc is a current density, Pmax is a maximum output, Rsh is a parallel resistance, and Rs is a series resistance.

The weight percents of Examples I, II and III and Comparative Examples I, II and III described in Table 2 are shown in weight percent relative to the total paste composition.

In Table 2, the glass composition content was tested at 0.1, 1, 2, and 4 wt%, and when 1 wt% of Example II was added, the efficiency was 2.0% higher than that of Comparative Example II. It was confirmed that the bending characteristics and the efficiency characteristics were the best among the II, III and III. In addition, it was confirmed that the warpage characteristics were increased as the glass content was increased, and when 4 wt% of Example I and Comparative Example II were added, the warpage was large, resulting in breakage of the cell when measuring the efficiency, thereby measuring the light conversion efficiency. Could not proceed.

Since the effect of the oxide additive mixture on the aluminum paste of Table 3, the glass II was fixed to 1% by weight, and then tested. The oxide was characterized by the addition of Al ₂ O ₃ , SiO ₂ , ZnO, ZrO ₂ , MgO and TiO ₂ in 2% by weight, respectively.

Oxide additives and Example II weight percents shown in Table 3 are expressed in weight percent relative to the total paste composition.

As shown in Table 3, the addition of Al ₂ O ₃ , SiO ₂ , ZnO, ZrO ₂ , MgO and TiO ₂ showed a significant improvement in warpage compared to the unadded composition. In terms of efficiency, Al ₂ O ₃ and MgO addition were the best in terms of efficiency.

Al ₂ O ₃ and MgO wt% shown in Table 4 are expressed in wt% relative to the total paste composition.

When the addition amount of Example II was fixed at 1% by weight and the addition amount of Al ₂ O ₃ and MgO was adjusted to 0.1 to 8% by weight, the warpage characteristics were 0.8 mm and 16.2% when the Al ₂ O _{3 was} added in 1% by weight. Efficiency was measured, and the warpage characteristics of 1.0mm and 16.0% were measured when 1% by weight of MgO was added, and the Al ₂ O ₃ and MgO oxide additives were added to the lead-free glass additive to improve warpage characteristics and light conversion efficiency. Could increase.

Industrial availability

The present invention can minimize the manufacturing defects due to cell breakdown due to the bowing phenomenon that occurs during solar cell production handling, thereby improving the electrical performance of the manufactured solar cell.

In addition, the present invention can provide a replacement material of the back electrode for manufacturing low bowing, high characteristics solar cell.

Claims (4)

As a paste mixture
a. Aluminum content of about 60-80% by weight of the mixture
b 0.1 to 2 wt% lead-free glass additive
c Oxide additive 0.1-2 wt%
d and the rest of the organic vehicle consisting of a resin and a solvent
Made from a mixture comprising
The oxide additive includes a metal selected from Si, Zn, Zr, Mg, and Ti, or at least one of SiO _2, ZrO ₂ , MgO, and TiO ₂ . The glass component of claim 1, wherein the glass component of (b) is based on the weight of the entire glass composition.
Bi ₂ O ₃ 50-70 wt%
B ₂ O ₃ 10-20 wt%
ZnO 5-20 wt%
Al ₂ O ₃ 1-15 wt%
Na ₂ CO ₃ 1-5 wt%
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