KR20110077731A

KR20110077731A - Solar cell

Info

Publication number: KR20110077731A
Application number: KR1020090134378A
Authority: KR
Inventors: 최종형
Original assignee: 엘지전자 주식회사
Priority date: 2009-12-30
Filing date: 2009-12-30
Publication date: 2011-07-07

Abstract

The present invention relates to a solar cell. The solar cell according to the present invention includes a silicon semiconductor substrate, an emitter layer formed on the substrate, an antireflection film formed on the emitter layer, and a front electrode connected to the emitter layer through the antireflection film, wherein the front electrode includes a silver powder and Nano powder, and the nano powder includes at least one of dry silica, a metal, and a metal oxide. As a result, the resistance of the front electrode can be reduced to improve the efficiency of the solar cell.

Description

Solar cell {Solar cell}

The present invention relates to a solar cell, and more particularly, to a solar cell having a front electrode formed of a nano-powder.

Recently, with the anticipation of depletion of existing energy sources such as oil and coal, there is increasing interest in alternative energy to replace them. Among them, solar cells are in the spotlight as next generation batteries using semiconductor devices that directly convert solar energy into electrical energy.

Solar cells are roughly classified into silicon solar cells, compound semiconductor solar cells, and tandem solar cells. Among them, silicon solar cells are the mainstream.

Meanwhile, the front electrode of a silicon solar cell is formed using a paste containing silver powder, an organic binder, glass frit, etc., and the paste has a low viscosity when a large amount of additive is added to secure the thixotropy of the paste. There may be difficulties in securing pattern characteristics.

An object of the present invention to provide a solar cell comprising a front electrode having a high aspect ratio by preventing the flow of the front electrode paste.

The solar cell according to the present invention for achieving the above object comprises a silicon semiconductor substrate, an emitter layer formed on the substrate, an antireflection film formed on the emitter layer and a front electrode connected to the emitter layer through the antireflection film; The front electrode includes silver powder and nano powder, and the nano powder includes at least one of dry silica, a metal, and a metal oxide.

In addition, the average particle size of nano powder is 7-20 nm.

In addition, the aspect ratio of the front electrode is 0.41 to 0.59.

In addition, the front electrode paste according to the present invention for achieving the above object is 60 to 95 parts by weight of silver (Ag) powder, 0.1 to 8 parts by weight of glass frit, 0.5 to 1.5 parts by weight of nano powder, binder 1 To 20 parts by weight and 1 to 20 parts by weight of the solvent.

According to the present invention, the paste for the front electrode includes a nano powder including at least one of dry silica, a metal, and a metal oxide, thereby improving the aspect ratio of the front electrode and lowering the resistance of the front electrode, thereby increasing the efficiency of the solar cell. Can improve.

Hereinafter, with reference to the drawings will be described the present invention in more detail.

1 is a view showing the structure of a solar cell according to an embodiment of the present invention.

Referring to FIG. 1, the solar cell according to the present invention includes a silicon semiconductor substrate 101, an emitter layer 103 formed on the substrate 101, an antireflection film 105 formed on the emitter layer 103, and The front electrode 109 penetrates the antireflection film 105 and is connected to the emitter layer 103, and may further include a rear electrode 107 connected to the rear surface of the substrate 101.

The substrate 101 may be doped with B, Ga, In, or the like, as a P-type impurity, with impurities, and the emitter layer 103 may be doped with P, As, Sb, or the like, with Group 5 elements, with N-type impurities. Can be doped.

As described above, when impurities of the opposite conductivity type are doped to the substrate 101 and the emitter layer 103, a P-N junction is formed at the interface between the substrate 101 and the emitter layer 103.

The antireflection film 105 immobilizes defects present in the surface or bulk of the emitter layer 103 and reduces the reflectance of the sunlight incident on the front surface of the substrate 101.

If the defect present in the emitter layer 103 is immobilized, the recombination site of the minority carriers is removed, thereby increasing the open voltage Voc of the solar cell. If the solar reflectance is reduced, the amount of light reaching the P-N junction is increased to increase the short circuit current (Isc) of the solar cell.

As such, when the open circuit voltage and the short circuit current of the solar cell are increased by the anti-reflection film 105, the conversion efficiency of the solar cell is improved by that amount.

The anti-reflection film 105 is, for example, a silicon nitride film, a silicon nitride film including hydrogen, a silicon oxide film, a silicon oxide nitride film, a single film selected from the group consisting of MgF ₂ , ZnS, TiO ₂ and CeO ₂ or two or more films. It may have a combined multilayer structure, but is not limited thereto.

The antireflection film 105 may be formed by vacuum deposition, chemical vapor deposition, spin coating, screen printing, or spray coating.

The front electrode 109 may be made of silver, and the rear electrode 107 may be made of aluminum, but is not limited thereto.

First, the back electrode 107 may be formed by printing a back electrode paste containing aluminum, quartz silica, a binder, or the like on the back surface of the substrate 101 and then performing heat treatment.

During the heat treatment of the back electrode 107, aluminum, which is an electrode constituent material, is diffused through the back surface of the substrate 101, so that a back surface field layer may be formed on the interface between the back electrode 107 and the substrate 101. .

When the rear electric field layer is formed, the carrier may be prevented from moving to the rear surface of the substrate 101 to be recombined. When the carrier is prevented from recombining, the open voltage is increased to improve the efficiency of the solar cell.

In addition, the front electrode 109 may be formed by screen printing the front electrode paste on the front electrode 109 forming point and then performing heat treatment. At this time, the silver contained in the metal paste becomes liquid at high temperature and recrystallizes into a solid phase through a predetermined process of the electrode, and emitter layer is formed by a punch through phenomenon penetrating the anti-reflection film 105 through the glass frit. (103).

On the other hand, as can be seen in Figure 1, the front electrode 109 is located at the top of the solar cell to cover the sunlight.

Therefore, it is important to minimize the area without degrading the function of the front electrode 109. Accordingly, the paste for the front electrode 109 according to the present invention includes at least one of metal powder, metal oxide, and dry silica. By including the nano-sized powder is formed, it is possible to increase the viscosity of the paste, prevent the flowability to easily maintain the shape of the front electrode 109.

As a result, the aspect ratio of the front electrode 109 is improved, so that the resistance of the front electrode 109 is reduced, and the area capable of absorbing light is widened, thereby improving efficiency of the solar cell.

Table 1 below shows the aspect ratio of the front electrode 109 and the resistance and standard test conditions of the front electrode 109 (Air Mass: 1.5, 1 sun: 1000 W / cm ² ^, battery temperature: 25 ° C.). Measured It is a table which shows the efficiency of a solar cell.

Aspect ratio Resistance (Ω) efficiency(%) 0.15 0.014 13.68 0.31 0.0098 14.6 0.39 0.0082 15.2 0.41 0.0075 17.1 0.43 0.0071 17.4 0.46 0.0069 17.5 0.53 0.0054 17.9 0.55 0.0070 17.5 0.57 0.0075 17.3 0.59 0.0077 17.0 0.61 0.0083 15.4 0.63 0.0089 14.9

As can be seen from Table 1, when the aspect ratio of the front electrode 109 is less than 0.41, it can be seen that the resistance of the front electrode 109 increases and the efficiency of the solar cell decreases rapidly. On the other hand, when the aspect ratio of the front electrode 109 is greater than 0.59, the height of the front electrode 109 becomes too high as the width of the front electrode 109 becomes too long, and thus the efficiency of the solar cell is greatly reduced.

Therefore, the aspect ratio of the front electrode 109 is preferably formed to be 0.41 to 0.59, thereby reducing the resistance of the front electrode 109, it is possible to improve the photoelectric conversion efficiency of the solar cell.

The paste for forming the front electrode 109 according to the present invention for this purpose is 60 to 95 parts by weight of silver (Ag) powder, 0.1 to 8 parts by weight of glass frit, 0.5 to 1.5 parts by weight of nano powder, binder 1 To 20 parts by weight and 1 to 20 parts by weight of the solvent.

First, the silver powder imparts conductivity to the paste, and the shape is not particularly limited as spherical or flake, but is preferably spherical in consideration of dispersibility.

On the other hand, when the amount of silver powder contained is less than 60 parts by weight, sufficient conductivity is not obtained in the front electrode 109 pattern obtained from the paste, and when it is contained in excess of 95 parts by weight, the viscosity may be too high and printing may be difficult. Not desirable

Further, it is preferable that the average particle size (D ₅₀₎ of the powder is 1.7 to 3.21㎛. If the average particle size (D ₅₀ ) of the silver powder is less than 1.7 μm, the fluidity of the paste may be deteriorated, and the workability may be degraded. On the other hand, if the average particle size (D ₅₀ ) exceeds 3.21 μm, the voids may be formed in the electrode after firing. As a result, electrical resistance of the front electrode 109 may be increased.

Glass frit may be included in 0.1 to 8 parts by weight. When the glass frit is included in less than 0.1 part by weight, the adhesive strength of the electrode pattern to be formed may not be sufficient, whereas when the glass frit is added in excess of 8 parts by weight, the sinterability of the electrode pattern is lowered and the resistance of the resulting electrode is reduced. Can increase.

In addition, the glass frit is preferably a fine powder in the range of 0.5 to 2 μm of the average particle size (D ₅₀ ) in order to effectively make a firing pattern without pinholes.

Examples of such glass frits may include lead oxides and / or bismuth oxides. Specifically, SiO ₂ _{-PbO-based, SiO 2 -PbO-B 2 O} 3 type, and _{Bi 2 O 3 -B 2 O 3} -SiO ₂ based powder may be a one or as mixtures of two or more thereof selected from the group consisting of, It is not limited to this.

Nano powder may be included in 0.5 to 1.5 parts by weight, it means a nano-sized powder formed by at least one of metal powder, metal oxide and dry silica.

The average particle size (D ₅₀ ) of the nanopowder is 7 to 20 nm, which is easy to adsorb to solvents and binders due to the high specific surface area, so that the viscosity of the paste can be easily increased even with a small amount, and the aspect ratio of the front electrode 109 described above. To prevent spreading of the paste and to maintain a high viscosity ratio.

Therefore, when the content of the nano-powder is contained less than 0.5 parts by weight, it is difficult to fully exhibit the above-described effect, while if it is contained in excess of 1.5 parts by weight, the viscosity of the paste is too high, so that the printability of the electrode pattern may be lowered, It is preferably included in 0.5 to 1.5 parts by weight.

The binder functions as a binder of each component before firing of the electrode pattern, and is preferably prepared by suspension polymerization for uniformity.

Such a binder may include a resin containing a carboxyl group, specifically, a carboxyl group-containing photosensitive resin having an ethylenically unsaturated double bond and a carboxyl group-containing resin not having an ethylenically unsaturated double bond.

For example, i) carboxyl group-containing resin obtained by copolymerizing an unsaturated carboxylic acid and a compound having an unsaturated double bond, and ii) a carboxyl group obtained by adding an ethylenically unsaturated group as a pendant to a copolymer of an unsaturated carboxylic acid and a compound having an unsaturated double bond. Containing photosensitive resin, iii) a carboxyl group-containing photosensitive resin obtained by reacting a copolymer of an acid anhydride having an unsaturated double bond with a compound having an unsaturated double bond, with a hydroxyl group and a compound having an unsaturated double bond, and the like. It is not.

Preferably, the binder is included in an amount of 1 to 20 parts by weight. If the amount of the binder is less than 1 part by weight, the distribution of the binder in the electrode pattern to be formed may become uneven, and patterning by selective exposure and development may be difficult. If it exceeds 20 parts by weight, it is easy to cause pattern collapse during firing of the electrode, and the resistance of the electrode may increase due to organic ash carbon after firing.

The solvent may dissolve the binder and may be mixed well with other additives. These include a-terpinol, buty cabitol acetate, texanol, buty cabitol, di-propylene glycol monomethyl ether And the like, including an aldehyde group, but are not limited thereto.

Such a solvent is preferably included in 1 to 20 parts by weight. When the content of the solvent is less than 1 part by weight, the paste may be difficult to apply uniformly. On the other hand, when it contains more than 20 parts by weight, sufficient conductivity of the electrode pattern may not be obtained, and adhesion to the substrate may be inferior. Because it is not desirable.

In addition, the paste for forming the front electrode 109 according to the present invention may further include additives such as a dispersant, a thickener, a thixotropic agent, and a leveling agent, and the amount of the additive may be included in an amount of 1 to 20 parts by weight.

On the other hand, the photopolymerizable monomer among the additives is used to promote the photocurability of the conductive electrode paste and to improve the developability.

As the photopolymerizable monomer, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, polyethylene glycol diacrylate, polyurethane diacrylate, trimethylol propane Triacrylate, pentaerythrite triacrylate, pentaerythrite tetraacrylate, trimethylolpropane ethylene oxide modified triacrylate, trimethylolpropanepropylene oxide modified triacrylate, dipentaery little pentaacrylate, dipentaeryse Little hexaacrylate and each methacrylate corresponding to the said acrylate; Mono-, di-, tri- or more polyesters of polybasic acids such as phthalic acid, adipic acid, maleic acid, ataconic acid, succinic acid, trimellitic acid and terephthalic acid with hydroxyalkyl (meth) acrylates. However, it is not limited to a specific thing, These can be used individually or in combination of 2 or more types.

2A to 2C schematically illustrate a process of forming a front electrode using the front electrode paste described above, and FIG. 3 is a view illustrating a change in viscosity of the front electrode paste according to rotation of a spindle (RPM). .

2A to 2C are roads schematically illustrating a process of forming a front electrode by screen printing. Referring to FIGS. 2A to 2C, a process of forming a front electrode will be described. First, a silicon semiconductor substrate 201 and an emitter layer will be described. 203 and the antireflection film 205 are sequentially formed.

Subsequently, after the screen mask 206 having an opening is selectively positioned on the anti-reflection film 205 to correspond to the position where the front electrode is to be formed, the squeeze rubber 210 is moved to move the opening of the front electrode paste 212. Patterned on the antireflection film 205 through.

At this time, as will be described later with reference to Figure 3, as the squeeze rubber 210 moves, the viscosity of the paste 212 is lowered and easily passes through the opening, and after passing through it, the shape of the pattern is increased by the viscosity increase again. It can be maintained.

Next, after the screen mask 206 is removed, the front electrode paste 212 is dried and baked to form the front electrode 209.

In the drying step, the solvent included in the paste 212 is evaporated, and the front electrode 209 is connected to the emitter layer 203 by a punch through phenomenon as shown in FIG. 2C.

On the other hand, Figure 3 is a view showing the viscosity change of the paste for the front electrode according to the rotation (RPM) of the spindle.

3 shows a change in viscosity of the paste for forming the front electrode according to the related art, and B shows a change in the viscosity of the paste for forming the front electrode according to the present invention. It can be seen that the change in viscosity is large.

Meanwhile, the viscosity change of the front electrode paste according to the rotation of the spindle represents the viscosity change of the paste due to the pressure applied to the paste according to the movement of the squeeze rubber 210 shown in FIGS. 2A and 2B.

Accordingly, FIG. 3 means a change in viscosity of the paste according to the pressure applied to the front electrode paste by the squeeze rubber 210.

That is, the paste 212 according to the present invention, which passes through the opening of the screen mask 206, has a viscosity that is lowered by the pressure applied by the squeeze rubber 210 due to its pseudoplastic property, so that the amount of passing through the opening increases. do.

In addition, after forming the shape of the electrode, since the viscosity is quickly restored by Thixotropy property, the spreading of the paste is reduced, and can be formed with a relatively high aspect ratio.

This phenomenon is because the front electrode paste of the present invention contains nano powder as described above, and the inorganic additive included has a high specific surface area and thus is easily adsorbed to solvents and binders, so that the viscosity of the paste can be increased even with a small amount. This is because it is easy to maintain the shape after printing by preventing the flow of the paste.

4A and 4B illustrate a conventional front electrode pattern and a front electrode pattern according to the present invention.

4A and 4B illustrate a case in which the front electrode is formed through the processes of FIGS. 2A to 2C described above with the pastes having the A and B compositions of FIG. 3, respectively.

Here, the composition of the A paste is 80 parts by weight of silver powder having an average particle size of 2 µm, 3 parts by weight of glass frit Bi2O3-B2O3-SiO2, 3.5 parts by weight of a binder prepared by suspension polymerization of MMA and MAA, and 10.5 parts by weight of a solvent butylcarbitol. Part and other additives include photopolymerizable monomer Trimethylopropane Triacrylate, DAROCUR TRO as photoinitiator, BYK-352 as leveling agent.

The composition of the B paste is 80 parts by weight of silver powder having an average particle size of 2 µm, 3 parts by weight of glass frit Bi2O3-B2O3-SiO2, 2.5 parts by weight of a binder prepared by suspension polymerization of MMA and MAA, and butyl carbitol as a solvent. 10.5 parts by weight, 1 part by weight of dry silica nanopowder having an average particle size (D ₅₀ ) of 15 nm, and other additives include the photopolymerizable monomer Trimethylopropane Triacrylate, DAROCUR TRO as a photoinitiator, and BYK-352 as a leveling agent.

Here, as can be seen in Figures 4a and 4b, it can be seen that the aspect ratio of the front electrode shown in Figure 4b including a small amount of nano-powder is significantly improved compared to Figure 4a.

Therefore, according to the present invention, the aspect ratio of the front electrode is improved, and thus the resistance of the front electrode is reduced, and the area capable of absorbing light is widened, thereby improving efficiency of the solar cell.

While the above has been shown and described with respect to preferred embodiments of the present invention, the present invention is not limited to the specific embodiments described above, it is usually in the technical field to which the invention belongs without departing from the spirit of the invention claimed in the claims. Various modifications can be made by those skilled in the art, and these modifications should not be individually understood from the technical spirit or the prospect of the present invention.

2A to 2C are schematic views illustrating a process of forming a front electrode.

3 is a view showing a viscosity change of the front electrode paste according to the rotation of the spindle (RPM).

Claims

Silicon semiconductor substrates;

An emitter layer formed on the substrate;

An anti-reflection film formed on the emitter layer; And

A front electrode penetrating the antireflection film and connected to the emitter layer;

The front electrode includes a silver powder and nano powder, the nano powder comprises at least one of dry silica, metal and metal oxide.

The method of claim 1,

The average particle size of the nano-powder is 7 to 20nm solar cell.

The method of claim 1,

A solar cell comprising a back electrode connected to the back of the substrate.

The method of claim 1,

The anti-reflection film is a silicon nitride film, silicon oxide film, silicon oxynitride film, MgF ₂ , ZnS, TiO ₂ and CeO ₂ any one single film selected from the group consisting of or a multi-layer film structure of a combination of two or more films.

The method of claim 1,

The average particle size of the silver powder is 1.7 to 3.21㎛ solar cell.

The method of claim 1,

Aspect ratio of the front electrode is 0.41 to 0.59 solar cell.

60 to 95 parts by weight of silver powder, 0.1 to 8 parts by weight of glass frit, 0.5 to 1.5 parts by weight of nano powder, 1 to 20 parts by weight of binder and 1 to 20 parts by weight of a solvent Paste for electrodes.

The method of claim 7, wherein

The nano powder is a paste for a front electrode of a solar cell comprising at least one of dry silica, metal and metal oxide.

The method of claim 7, wherein

The average particle size of the nano-powder is 7 to 20nm paste for the front electrode of the solar cell.

The method of claim 7, wherein

An average particle size of the silver powder is 1.7 to 3.21㎛ for the front electrode paste of the solar cell.