[DESCRIPTION] [invention Title]

ENVIRONMENT-FRIENDLY PASTE FOR ELECTRODE OF SOLAR CELL

AND SOLAR CELL USING THE SAME

[Technical Field]

The present invention relates to a paste for a solar cell electrode and a solar cell using the same, and more particularly, to a paste for an environmentally friendly solar cell electrode, which makes it possible to minimize the content of hazardous lead (Pb) thereby allowing electrodes of an environmentally friendly solar cell to be manufactured and to have a low series resistance (R _s ) and a high shunt resistance (R _sll ), and a solar cell using the paste.

[Background Art]

A semiconductor device using light, e.g., a solar cell, is in the limelight as clean energy resources by converting sunlight into useful electric energy, and commercialization of the solar cell is in progress at the present time. In general, electrodes are formed in the semiconductor device using light such as a solar cell and the like. At this time, the electrodes should have a small occupation area on a light receiving surface thereof such that light is prevented from being intercepted as far as possible. Therefore, electrodes of a solar cell are formed, e.g., in a grid pattern in the shape of a lattice.

Fig. 1 shows a sectional view of a general silicon wafer type solar cell.

Referring to Fig. 1, a solar cell generally comprises a semiconductor substrate 10 on which sunlight is incident; a front electrode 20 formed on a top portion of the semiconductor substrate 10; and a back electrode 30 formed on a bottom portion of the semiconductor substrate 10. The solar cell further comprises an anti-reflection coating 12 applied to a surface of the semiconductor substrate 10 to prevent reflection loss of the sunlight incident on the semiconductor substrate 10.

The semiconductor substrate 10 is made of a silicon wafer having an n-type silicon layer 10a and a p-type silicon layer 10b. Further, the back electrode 30 is mainly

made of an aluminum (Al) film. Specifically, a paste comprising aluminum powder as a principal material is applied to the p-type silicon layer 1 Ob and then Al ions are diffused into the p-type silicon layer 10b in a sintering process, so that the back electrode 30 comes into ohmic contact with the p-type silicon layer 10b. Further, the anti-reflection coating 12 comprises silicon nitride (Si ₃ N ₄ ) as an effective component for mainly preventing reflection loss of sunlight. The front electrode 20 is mainly formed of silver (Ag). Specifically, the front electrode 20 is formed by sintering the paste after printing a paste comprising silver powder as a principal material in a grid pattern on the anti-reflection coating 12. At this time, the front electrode 20 is penetrated into the anti-reflection coating 12 in the sintering process through heat treatment to come into ohmic contact with the n-type silicon layer 10a, thereby lowering series resistance of the solar cell.

Electrodes of a solar cell, specifically, the front and back electrodes 20 and 30 of the solar cell are formed by sintering a paste comprising the aforementioned conductive metals such as Ag and Al as principal materials. Further, the paste for the electrodes 20 and 30 comprises a glass frit as an inorganic sintering agent for allowing conductive metal components to adhere to the semiconductor substrate 10 through a sintering process, and an organic material such as an organic solvent as a dispersion medium for dispersing these solid components.

A paste comprising aluminum powder, a glass frit, an organic material, and the like is proposed in Japanese Laid-open Patent Publication No. 2001-202822 (Patent Document 1) and Korean Laid-open Patent Publication No. 2004-0025609 (Patent Document 2), as preceding documents related with a paste for the back electrode 30. Further, a paste comprising silver powder, a glass frit, an organic material (resin binder), and the like is proposed in Korean Laid-open Patent Publication No. 2007-0066938 (Patent Document 3), Korean Laid-open Patent Publication No. 2007-0067636 (Patent Document 4), and Korean Laid-open Patent Publication No. 2007-0084100 (Patent Document 5) as preceding documents related with a paste for the front electrode 20. As proposed in the preceding patent documents, the glass frit is used as an inorganic sintering agent containing a lead component with a low melting point.

The lead component lowers the melting point of the glass frit to improve adhesive

force of conductive metals such as Ag and Al to the semiconductor substrate 10, thereby being capable of enhancing wiring strength of the electrodes 20 and 30. Particularly, in case of a paste for the front electrode 20, a lead component contained in the glass frit is penetrated into a silicon nitride (Si ₃ N ₄ ) thin film layer that is the anti-reflection coating 12 to impart an ohmic contact with the silicon nitride (Si ₃ N ₄ ) thin film layer. Specifically, the lead component reacts with silicon nitride (Si ₃ N ₄ ) in the sintering process to corrode the anti-reflection coating 12, so that the Ag electrode is penetrated into the anti-reflection coating 12 to thereby come into ohmic contact with the n-type silicon layer 10a. Therefore, a glass frit containing a lead component is used as the glass frit in the paste for the electrodes 20 and 30. Particularly, it is essential that a paste for the front electrode 20 contain the lead component since the lead component functions as a penetration agent for forming an ohmic contact with the N type silicon layer 10a.

However, the paste for the electrodes 20 and 30 according to the prior art including the foregoing preceding patent documents causes environmentally severe problems since considerable quantities of a hazardous lead component are contained in the paste. Specifically, although the lead component is essentially contained in the front electrode 20, the front electrode could conventionally exhibit its role as a penetration agent according as the lead component is contained only when the lead component is contained in the glass frit in a considerable amount. For instance, Patent Document 5 proposes a paste comprising 1 to 15 weight percent (wt%) of a glass frit with respect to the total solid component weight, and 15 to 75 mole percent (mole%) of Pb and 5 to 50 mole percent (mole%) of SiO ₂ contained in the glass frit. When converting the lead component (Pb) into the weight basis, the amount of approximately up to 14.6 weight percent (wt%) of the lead component (Pb) is contained in 100 weight parts of the solid component. Therefore, the paste for an electrode according to the prior art has had flaws that severe environmental problems were generated due to high content of hazardous lead.

[Patent Document 1] Japanese Laid-open Patent Publication No. 2001-202822 [Patent Document 2] Korean Laid-open Patent Publication No. 2004-0025609 [Patent Document 3] Korean Laid-open Patent Publication No. 2007-0066938 [Patent Document 4] Korean Laid-open Patent Publication No. 2007-0067636

[Patent Document 5] Korean Laid-open Patent Publication No. 2007-0084100

[Disclosure]

[Technical Problem]

The present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a paste for a solar cell electrode, which makes it possible to minimize the content of hazardous Pb thereby allowing electrodes of an environmentally friendly solar cell to be manufactured and to have a low series resistance (Rs) and a high shunt resistance (Rsh), and a solar cell using the paste for a solar cell electrode. Specifically, an object of the present invention is to provide a paste for an environmentally friendly solar cell electrode, wherein the paste contains the hazardous lead component in a low content and penetrates an anti-reflection coating (Si ₃ N ₄ thin film) during heat treatment to allow electrodes to have excellent bonding strength with a semiconductor substrate (an N type silicon layer), thereby having a low series resistance (Rs) and a high shunt resistance (Rsh), and a solar cell using the paste for the environmentally friendly solar cell electrodes as a front electrode.

[Technical Solution]

The present invention for achieving the objects provides a paste for a solar cell electrode, comprising a solid component and a dispersion medium, wherein the solid component comprises at least one conductive powder selected from metal and a metal containing compound; a glass frit which does not contain a lead component; and lead component particles having a size of 1 to 1,000 nanometers (nm). It is preferable that the lead component particles have a size of 1 to 200 nanometers (nm). For instance, at least one selected from Pb, a lead alloy, and a lead oxide (the following formula 1) may be used as the lead component particles.

Furthermore, the present invention provides a paste for a solar cell electrode, which comprises a solid component and a dispersion medium, wherein the solid component comprises at least one conductive powder selected from metal and a metal containing compound; a glass frit which does not contain a lead component; and at least

one lead component compound selected from a lead alkoxide (the following formula 2), a lead polycondensation polymer (the following formula 3), and a complex of Pb with beta- diketones (the following formula 4)

In addition, the solid component further comprises a Bi component. Here, the Bi component is at least one selected from Bi metal and a Bi containing compound. For example, the Bi component is at least one selected from Bi, a bismuth alloy, a bismuth oxide (the following formula 5), a bismuth alkoxide (the following formula 6), a bismuth polycondensation polymer (the following formula 7), and a complex of Bi with beta- diketones (the following formula 8). If the Bi component, such as Bi single metal, a bismuth alloy, and a bismuth oxide (the following formula 5), is in the form of particles, the particles preferably have a size of 1 to 1,000 nanometers (nm), and more preferably 1 to 200 nanometers (nm).

According to a preferred embodiment of the present invention, it is preferable that the solid component comprise 40.0 to 99.0 weight parts of the conductive powder; 0.1 to 57.0 weight parts of the glass frit which does not contain Pb; and 0.1 to 3.0 weight parts of the lead or lead compounds. The solid component may further comprise 0.1 to 3.0 weight parts of the Bi or Bi compounds.

Furthermore, the present invention provides a solar cell, which comprises a semiconductor substrate; an anti-reflection coating applied to a top portion of the semiconductor substrate; a front electrode in contact with the semiconductor substrate by sintering after being formed on a top portion of the anti-reflection coating; and a back electrode formed on a bottom portion of the semiconductor substrate, wherein the front electrode is a sintered body of the paste according to the present invention.

[Advantageous Effects]

According to the present invention, the content of a hazardous lead component can be minimized. Specifically, according to the present invention, a paste for a solar cell electrode does not comprise a lead component in a state of being contained in a glass frit, but comprises a lead component of a nanometer (nm) size (preferably 200 nm or less) as a separate component or comprises a lead component in the form of compounds represented

by the following formulas 2 to 4, so that the surface area of the lead component is increased and activities such as reactivity thereof are improved, thereby obtaining an excellent penetration effect of an anti-reflection coating during heat treatment and improving bonding strength with a semiconductor substrate although the lead component is used in a low content. Therefore, according to the present invention, there are advantages in that it is possible to manufacture environmentally friendly electrodes since the consumption of the lead component can be reduced to not more than a half of the conventional consumption thereof and to improve electric properties of a solar cell by obtaining a low series resistance (R _s ) and a high shunt resistance (R _Sh ) due to an enhanced effect of penetrating the anti-reflection coating and the improved bonding strength with the semiconductor substrate. Further, according to the present invention, it is possible to exhibit excellent conductivity by forming electrodes to have a compact structure.

[Description of Drawings]

Fig. 1 is a sectional view of a general silicon wafer type solar cell.

Fig. 2 is a photograph showing Ag nano powder with an average particle size of 200 nm used in an embodiment of the present invention.

Fig. 3 is a photograph showing a dispersion solution containing nano-sized Pb(OH) ₂ powder used in an embodiment of the present invention.

Fig. 4 is a photograph showing a dispersion solution containing nano-sized Bi(OH) ₃ powder used in an embodiment of the present invention.

Fig. 5 is a photograph showing a dispersion solution containing nano-sized Bi ₂ O ₃ powder used in an embodiment of the present invention.

Fig. 6 is a photograph showing a cross section of an Ag electrode with a compact structure manufactured according to an embodiment of the present invention.

Fig. 7 is a photograph showing a cross section of a porous Ag electrode manufactured using conventional silver micro-particles.

[Best Mode]

Hereinafter, the present invention will be described in detail.

A paste for a solar cell electrode according to the present invention comprises a solid component, and a dispersion medium for dispersing the solid component. The solid component comprises a conductive powder, a lead-free glass frit as an inorganic sintering agent for bonding the conductive powder to a semiconductor substrate, and a lead component as a penetration agent for imparting an ohmic contact to an anti-reflection coating by penetrating it in a sintering process.

The conductive powder is at least one selected from metal and a metal containing compound, wherein the metal comprises a single metal and an alloy, and the metal containing compound comprises a metal oxide and a metal salt. For instance, the conductive powder is silver (Ag), copper (Cu), gold (Au), platinum (Pt), aluminum (Al), or mixtures or alloys of at least two thereof. Further, the conductive powder is a metal containing compound and may be selected, for example, from oxides of Ag or Al, or salts (including organic and inorganic salts) of Ag or Al. Such a conductive powder may be contained in a paste in the form of spherical or flake type particles or in the form of a colloid in which the spherical or flake type particles are dispersed. In addition, particles, which are selected from the foregoing metal or metal containing compound and coated with an organic material, may be used as the conductive powder.

Preferably, Ag, silver alloys, silver compounds (silver oxides or silver salts), and the like may be used as the conductive powder for a front electrode of a solar cell. The silver compound includes, for example, silver oxides such as a silver oxide (Ag ₂ O), and silver salts such as a silver chloride (AgCl), a silver nitrate (AgNO ₃ ), and a silver acetate (AgOOCCH ₃ ). Most preferably, Ag particles are used as the conductive powder for a front electrode of a solar cell. Further, Al, aluminum alloys, and an aluminum compounds (aluminum oxides or aluminum salts) may be preferably used as a conductive powder for a back electrode of a solar cell.

The conductive powder has a particle size of 0.01 to 30.0 micrometers (μm), and it is preferable that the conductive powder have a particle size of 100 to 500 nanometers (nm) since the conductive powder is excellent in sintering property when the particle size of the conductive powder is small.

Further, 40.0 to 99.0 weight parts of the conductive powder are preferably

contained with respect to the total solid component weight, i.e., 100 weight parts of the solid component. It is not preferable to contain the conductive powder in a content of less than 40 weight parts since the conductivity thereof drops, and it is not preferable to contain the conductive powder in a content of more than 99.0 weight parts since contents of the glass frit and the lead component are relatively decreased and thus the effect caused by containing them is insignificant. More preferably, the conductive powder is contained in a content of 70.0 to 99.0 weight parts with respect to the total solid component weight.

A lead-free glass frit in which no lead component is contained is used according to the present invention. In the present invention, it is possible to use any glass frit that can be sintered by heat treatment, contains no lead component, and contain at least a silicon (Si) component. The glass frits include, for example, an SiO ₂ based glass frit, an SiO ₂ -ZnO based glass frit (an Si-Zn-O based glass frit), an SiO ₂ -B2O3 based glass frit (an Si-B-O based glass frit), and an SiO ₂ - Bi ₂ O ₃ based glass frit (an Si-Bi-O based glass frit), wherein the SiO ₂ based glass frit means a glass frit comprising SiO ₂ as a principal component, and the SiO ₂ -ZnO based glass frit means a glass frit comprising SiO ₂ as a principal component and ZnO as a subsidiary component. Further, the glass frit can comprise oxides as other components in addition to the principal component and the subsidiary component. These oxides are one or more selected from, for example, Al ₂ O ₃ , Ta ₂ O ₅ , Sb ₂ O ₅ , ZrO ₂ , HfO ₂ , In ₂ O ₃ , Ga ₂ O ₃ , Y ₂ O ₃ , and Yb ₂ O ₃ .

It is preferable that the glass frit have a low melting point. Specifically, it is preferable that the glass frit have a softening point of 450 to 550 ⁰ C if Ag is used as the conductive powder. When Ag is used as the conductive powder, heat treatment (sintering) can be performed in a temperature range of about 600 to 800 ⁰ C. Here, it is hard to sinter Ag since the glass frit is promptly melted when the glass frit has a softening point of lower than 450 ⁰ C while good bonding strength of Ag with a semiconductor substrate may not be exhibited since good melting and flowing of the glass frit are not generated when the glass frit has a softening point of higher than 450 ⁰ C.

Further, it is preferable to contain the glass frit in a content of 0.1 to 57.0 weight parts with respect to the total solid component weight, i.e., relative to 100 weight parts of the solid component. It is not preferable to contain the glass frit in a content of less than

0.1 weight part since bonding strength drops to make it difficult to seek good bonding strength of the glass frit with the semiconductor substrate, and it is not preferable to contain the glass frit in a content of more than 57.0 weight parts since an effect of the glass frit due to an excessive content is not so great and contents of the conductive powder and the lead component decrease relatively and thus an effect caused by containing them is insignificant. More preferably, the glass frit is contained in a content of 1.0 to 20.0 weight parts with respect to the total solid component weight.

According to the present invention, the lead component is not contained in the paste in the form of being contained in the glass frit, but is contained as a separate component. Such a lead component functions as a penetration agent that penetrates an anti-reflection coating (Si ₃ N ₄ thin film) to impart an ohmic contact with a semiconductor substrate (an N type silicon layer).

The lead component is selected from particles with a size of 1 to 1,000 nanometers (nm) according to a first form of the present invention. The lead component particles, as particles with a size of 1 to 1,000 nm, are at least one selected from Pb and a Pb containing compound. The lead component particles preferably have a size of 1 to 200 nm. Here, Pb comprises a single Pb metal and a lead alloy, and the Pb containing compound comprises a lead oxide and a lead salt. Here, a lead oxide represented by the following formula 1 may be usefully used as the Pb containing compound.

[Formula 1]

Pb _p O _q H _r where p and q are integers or prime numbers greater than 0, and r is 0 or an integer or prime number greater than 0.

Further, at least one lead component compound selected from a lead alkoxide represented by the following formula 2, a lead polycondensation polymer represented by the following formula 3, and a complex of Pb with beta-diketones represented by the following formula 4 according to a second form of the present invention may be used as the lead component.

[Formula 2]

Pb _a (OR)b

where R is hydrogen or hydrocarbon, and a and b are integers or prime numbers greater than 0,

[Formula 3]

Pb _x O _y (OR) _z where R is hydrogen or hydrocarbon, and x, y, and z are integers or prime numbers greater than 0, and

[Formula 4]

0 0

I! Il

R-CCH ₂ C-R' where R and R' are hydrogen or hydrocarbons.

In Formulas 2 to 4, when R and R' are hydrocarbons, they may be hydrocarbons having various functional groups such as an alkyl group, an aryl group, and the like in chains. However, they are not limited thereto.

Further, in the present invention, the lead component is at least one selected from the foregoing lead component particles, at least one selected from the lead component compounds (i.e., the compounds represented by Formulas 2 and 3, and the complex of lead with the compound represented by Formula 4), or mixtures of the lead component particles and the lead component compounds (i.e., the compounds represented by Formulas 2 and 3, and the complex of lead with the compound represented by Formula 4). Specifically, the lead component is Pb, a lead alloy, the lead oxide represented by Formula 1, the lead alkoxide represented by Formula 2, the lead polycondensation polymer represented by Formula 3, the complex of Pb with beta-diketones represented by Formula 4, or mixtures of at least two thereof. For instance, the lead component is one selected from the lead oxide represented by Formula 1, the lead alkoxide represented by Formula 2, the lead polycondensation polymer represented by Formula 3, the complex of Pb with the compound represented by Formula 4, a mixture of the lead oxide represented by Formula 1 and the lead polycondensation polymer represented by Formula 3, and a mixture of the lead oxide represented by Formula 1 and the complex of Pb with the compound represented by Formula 4.

The lead component functions as an important requirement for accomplishing

objects of the present invention. Specifically, the lead component has improved surface area and reactivity and excellent sintering property due to excellent activities by allowing the paste to comprise the lead component as a component separate from the glass frit is contained in the paste in the form of particles with a size of not more than 1,000 nm, preferably not more than 200 nm, or in the form of the compounds represented by Formulas (i.e., the compounds represented by Formulas 2 and 3, and the complex of lead with the compound represented by Formula 4). More specifically, the paste comprises the lead component in the form of particles or compounds represented by Formulas as a component separate from the glass frit, so that excellent penetration of the lead component against an anti-reflection coating (Si ₃ N ₄ thin film) is obtained by improving surface area and reactivity of the paste even when the lead component is used in a small quantity, and excellent bonding strength with a semiconductor substrate is obtained by improving sintering property. Additionally, electric performance of a cell is improved due to a low series resistance (R _s ) and a high shunt resistance (R _Sh ) caused by the excellent penetration and bonding strength. Therefore, it is possible to manufacture environmentally friendly electrodes and improve electric performance of the cell since the content of a hazardous lead component can be minimized. Specifically, the consumption of the lead component can be reduced to not more than a half of that of the prior art.

Although it is desirable to comprise the lead component (lead component particles and lead component compounds) in a content as low as possible, it is difficult to obtain excellent penetration against the anti-reflection coating (Si ₃ N ₄ thin film) and improved bonding strength with the semiconductor substrate if the content of the lead component is too low. It is preferable to contain the total lead in the paste in a content of 0.1 to 3.0 weight parts with respect to the total solid component weight, i.e., 100 weight parts of the solid component, more preferable 0.1 to 1.5 weight part. It is not preferable to contain the lead component in a content of less than 0.1 weight part since the penetration and bonding strength may not be good, and it is not preferable to contain the lead component in a content of more than 3.0 weight parts since an effect of the lead component due to an excessive content is not so great and the excessive content is not desirable in the environmental aspect.

Further, according to another embodiment of the present invention, the solid component may further comprise a bismuth (Bi) component. The Bi component functions as a penetration agent of an anti-reflection coating (Si ₃ N ₄ thin film) like the lead component, thereby supports the lead component. According to the present invention, a Bi component plays the same role as the lead component, i.e., a penetration agent of the anti-reflection coating, so that the content of the lead component can be further minimized by containing the Bi component.

The Bi component is selected from Bi and a Bi containing compound. The Bi comprises single Bi metal and a bismuth alloy, and the Bi containing compound comprises a bismuth oxide and a bismuth salt. Additionally, the Bi containing compound comprises, for example, a bismuth oxide, a bismuth alkoxide, a bismuth polycondensation polymer, and a complex of Bi with beta-diketones, which are represented by the following formulas.

Specifically, a bismuth oxide represented by the following formula 5 may be usefully used as the Bi component.

[Formula 5]

Bi _p O _q H _r where p and q are integers or prime numbers greater than 0, and r is 0 or an integer or prime number greater than 0.

Further, a bismuth alkoxide represented by the following formula 6 may be usefully used as the Bi component.

[Formula 6]

Bi _a (OR) _b where R is hydrogen or hydrocarbon, and a and b are integers or prime numbers greater than 0.

Further, a bismuth polycondensation polymer represented by the following formula 7 may be usefully used as the Bi component.

[Formula 7]

Bi _x O _y (OR) _z where R is hydrogen or hydrocarbon, and x, y, and z are integers or prime numbers greater than 0.

Further, a complex of Bi with beta-diketones of the following formula 8 may be usefully used as the Bi component. [Formula 8]

0 0

Il Il

R-CCHX-R' where R and R' are hydrogen or hydrocarbons.

In Formulas 6 to 8, when R and R' are hydrocarbons, they may be hydrocarbons having various functional groups such as an alkyl group, an aryl group, and the like in chains. However, they are not limited thereto.

In the present invention, Bi, a bismuth alloy, the bismuth oxide represented by Formula 5, the bismuth alkoxide represented by Formula 6, the bismuth polycondensation polymer represented by Formula 7, the complex of Bi with beta-diketones represented by Formula 8, or mixtures of at least two thereof may be usefully used as the Bi component. Specifically, Bi, a bismuth alloy, the bismuth oxide represented by Formula 5, the bismuth alkoxide represented by Formula 6, the bismuth polycondensation polymer represented by Formula 7, the complex of Bi with the compound represented by Formula 8, a mixtures of Bi and the bismuth oxide represented by Formula 5, or a mixture of the bismuth oxide represented by Formula 5 and the bismuth polycondensation polymer represented by Formula 7 may be used as the Bi component.

When the Bi component is in the form of particles, it is desirable that the Bi component be nano-sized particles by reason of increased activities and improved sintering property due to the particles like the above-mentioned lead component. For instance, when the Bi component is in the form of particles, such as the single Bi metal, the bismuth alloy, and the bismuth oxide of the formula 5, it is preferable that the Bi component be particles with a size of 1 to 1,000 nanometers (run), more preferably 1 to 200 nanometers (nm). Further, the Bi component obtains excellent penetration against the anti-reflection coating and improves bonding strength with the semiconductor substrate since the activities is increased even when the Bi component is compounds, such as the bismuth alkoxide of Formula 6, the bismuth polycondensation polymer of Formula 7, and the complex of Bi with the compound of Formula 8.

It is preferable to contain the total Bi in the paste in a content of 0.1 to 3.0 weight parts with respect to the total solid component weight, i.e., relative to 100 weight parts of the solid component. It is not preferable to contain the Bi in a content of less than 0.1 weight part since it is difficult to serve to assist the lead component, i.e., to seek the improvement of penetration and bonding strength, and it is not preferable to contain the Bi in a content of more than 3.0 weight parts since an effect of the Bi component due to an excessive content is not so great. It is more preferable to contain the Bi in a content of 0.1 to 1.5 weight part with respect to the total solid component weight.

As described above, the paste for an electrode according to the present invention comprises the solid component and a dispersion medium for dispersing the solid component, wherein any dispersion medium capable of dispersing a solid component can be used. Although the dispersion medium is not particularly limited, the dispersion medium may be contained in a weight ratio of the solid component to the dispersion medium of 1 :0.05 to 60.0. The dispersion medium at least comprises, for example, water, organic solvents such as alcohols, glycols, and the like. A dispersion medium for improving dispersibility of the solid component may be further contained. For example, the dispersion medium includes alkyl amine, carboxylic acid amide, amino carboxylic acid salt, citrate salt, and the like. However, the dispersion medium is not limited thereto.

Furthermore, the paste for an electrode according to the present invention may further comprise a resin binder, additives, and the like, which are generally used, in addition to the solid component and the dispersion medium. The resin binder may be selected from organic materials. Although not limited specifically, the resin binder includes, for instance, polymethacrylate, ethyl cellulose, ethyl hydroxyethyl cellulose, rosin, ethylene glycol monobutyl ether monoacetate, and the like. Additionally, the additives include a stabilizer, a viscosity adjusting agent, and the like, and the specific types of the additives may be selected from those generally used in the art.

The paste for an electrode according to the present invention described above may have a viscosity of 10 to 500 Pa-s and be used as front and back electrodes of a solar cell. In particular, the paste may be usefully used as the front electrode of the solar cell. The paste for an electrode according to the present invention may be patterned on a

semiconductor substrate, more specifically on an anti-reflection coating, of a solar cell through a printing technique. The printing technique includes screen printing, roll-to-roll printing, gravure printing, offset printing, inkjet printing, and the like. The front electrode is preferably formed in a grid pattern in the shape of a lattice. After the patterning, it is desirable that the patterned paste be subjected to heat treatment at a temperature of 600 to 800 ⁰ C thereby to be sintered.

In the meantime, a front electrode of a solar cell according to the present invention comprises a sintered body of the paste according to the present invention described above.

Specifically, the solar cell according to the present invention comprises a semiconductor substrate; an anti-reflection coating applied to a top portion of the semiconductor substrate; a front electrode in contact with the semiconductor substrate through sintering after being formed on a top portion of the anti-reflection coating; and a back electrode formed on a bottom portion of the semiconductor substrate (silicon wafer), wherein the front electrode is a sintered body of the paste of the present invention described above.

The semiconductor substrate and the anti-reflection coating may be formed in an ordinary manner. Also, the back electrode may be formed by sintering an ordinary Al paste or the paste of the present invention described above.

Hereinafter, the present invention is described further in detail through examples and comparative examples. However, the following examples are provided merely for helping understand the present invention, and the present invention is not limited by the following examples.

[Preparation Example 1] - Silver Powder

Spherical silver powder (conductive metal powder) with an average particle size of 200 nm, which was purchased from Advanced Nano Products Co., Ltd, located at Buyong industrial complex, Kumho-ri, Buyong-myeon, Chungwon-kun, Chungcheongbuk-do, Korea, and of which a surface was coated with an organic material, was prepared. A photograph of the silver powder is shown in Fig. 2.

[Preparation Example 2] - Pb(OH) ₂ Dispersion Solution

A Pb(OH) ₂ dispersion solution with an average dispersed particle size of 50 nm was prepared by performing a ball-milling process of Pb(OH) ₂ nanopowder, which was purchased from Advanced Nano Products Co., Ltd located at Buyong industrial complex, Kumho-ri, Buyong-myeon, Chungwon-kun, Chungcheongbuk-do, Korea, to a concentration of 30 wt% using toluene as a solvent. There was a merit in that the Pb(OH) ₂ nanopowder could be distributed in a paste uniformly in the paste preparation process when preparing the dispersion solution from the Pb(OH) ₂ nanopowder by performing the ball-milling process. The toluene used as the solvent was not present in the paste since most of the toluene was volatilized in the paste preparation process using a three-roll mill. A photograph of the Pb(OH) ₂ dispersion solution containing nano-sized dispersed particles is shown in Fig. 3.

[Preparation Example 3] - Lead Complex (Lead (II) Acetylacetonate) Lead (II) acetylacetonate powder purchased from Sigma Aldrich Corporation in USA was used. The purchased powder was washed several times and then dried at room temperature. A content of lead contained in the powder was confirmed to be 51.107%.

[Preparation Example 4] - Bi(OH) ₃ Dispersion Solution

A Bi(OH) ₃ dispersion solution with an average dispersed particle size of 80 nm was prepared by performing a ball-milling process of Bi(OH) ₃ nanopowder with a primary average particle size of 50 nm, which was purchased from ANP (Advanced Nano Products Co., Ltd) located at Buyong industrial complex, Kumho-ri, Buyong-myeon, Chungwon- kun, Chungcheongbuk-do, Korea, to a concentration of 25 wt% using toluene as a solvent. There was a merit in that the Bi(OH) ₃ nanopowder could be distributed in a paste uniformly in the paste preparation process when preparing the dispersion solution from the Bi(OH) ₃ nanopowder by performing the ball-milling process. The toluene used as the solvent was not present in the paste since most of the toluene was volatilized in the paste preparation process using a three-roll mill. A photograph of the Bi(OH) ₃ dispersion

solution containing nano-sized dispersed particles is shown in Fig. 4.

[Preparation Example 5] - Bi ₂ O ₃ Dispersion Solution

Bi ₂ O ₃ powder with beige color was prepared by performing a sintering process of Bi(OH) ₃ nanopowder at 600 ⁰ C, which was purchased from ANP (Advanced Nano Products Co., Ltd) located at Buyong industrial complex, Kumho-ri, Buyong-myeon, Chungwon-kun, Chungcheongbuk-do, Korea. A Bi ₂ O ₃ dispersion solution with an average dispersed particle size of 50 run was prepared by performing a ball-milling process of the crushed Bi ₂ O ₃ powder to a concentration of 30 wt% using toluene as a solvent after performing a crushing process of the Bi ₂ O ₃ powder prepared through the sintering process. A photograph of the Bi ₂ O ₃ dispersion solution with nano-sized particles dispersed is shown in Fig. 5.

[Example 1]

A mixture was prepared by adding a lead-free Si-B-O based glass frit with an average particle size of 0.7 to 3.0 μm to the silver powder with an average particle size of 200 nm prepared in Preparation Example 1 in an amount of 3.0 wt% of the total paste weight. After dissolving ethyl cellulose functioning as a binder to a concentration of 30 wt% using Terpineol, the dissolved solution was added to the prepared mixture to be uniformly distributed therein. After additionally adding the Pb(OH) ₂ dispersion solution with an average dispersed particle size of 50 nm of Preparation Example 2 in an amount of 0.5 wt% of the total paste weight in consideration of the solid content and then preliminarily mixing the Pb(OH) ₂ dispersion solution with the mixture using a mixer, the paste was formed by repeatedly dispersing the mixture using a three-roll mill. Since the paste had a high viscosity in the three-roll milling process, Terpineol is added to the paste to adjust the viscosity of the paste, thereby preparing the paste for a solar cell electrode suitable for screen printing. When measuring a viscosity of the prepared paste using a Brookfield LVDV-II+Pro CPE-51 spindle, the viscosity was 210,000 cps at a rotation speed of 0.4 revolution per minute. Constituents' contents and viscosity of the paste according to this example are represented in the following table 1. In Table 1, the

contents are weight percents with respect to the total paste, and balances are residual amounts for a binder, a solvent for adjusting the viscosity, and the like.

[Example 2]

A paste was prepared by adding Terpineol using a three-roll mill and adjusting the viscosity in the same method as in Example 1 except that the Pb(OH) ₂ dispersion solution with the dispersed particle size of 50 run of Preparation Example 2 was added in an amount of 1.0 wt% of the total paste weight in consideration of the solid content. Constituents' contents and viscosity of the paste according to this example are represented in Table 1.

[Example 3]

A paste was prepared by adding Terpineol using a three-roll mill and adjusting the viscosity in the same method as in Example 1 except that the lead (II) acetylacetonate of Preparation Example 3 instead of the Pb(OH) ₂ dispersion solution was added in an amount of 0.5 wt% of the total paste weight. Constituents' contents and viscosity of the paste according to this example are represented in Table 1.

[Example 4]

A paste was prepared by adding Terpineol using a three-roll mill and adjusting the viscosity in the same method as in Example 3 except that the lead (II) acetylacetonate of Preparation Example 3 was added in an amount of 0.8 wt% of the total paste weight. Constituents' contents and viscosity of the paste according to this example are represented in Table 1.

[Example 5]

A paste was prepared by adding Terpineol using a three-roll mill and adjusting the viscosity in the same method as in Example 1 except that the Bi(OH) ₃ dispersion solution with an average dispersed particle size of 80 nm of Preparation Example 4 in addition to 0.5 wt% of the Pb(OH) ₂ dispersion solution was added in an amount of 0.5 wt% of the total paste weight in consideration of the solid content. Constituents' contents and

viscosity of the paste according to this example are represented in Table 1.

[Example 6]

A paste was prepared by adding Terpineol using a three-roll mill and adjusting the viscosity in the same method as in Example 3 except that the Bi ₂ O ₃ dispersion solution with an average dispersed particle size of 50 nm of Preparation Example 5 in addition to 0.5 wt% of the lead (II) acetylacetonate of Preparation Example 3 was added in an amount of 0.5 wt% of the total paste weight in consideration of the solid content. Constituents' contents and viscosity of the paste according to this example are represented in Table 1.

[Comparative Example 1]

A mixture was prepared by adding an Si-Pb-O based glass frit with an average particle size of 0.5 to 3.5 μm comprising 80 wt% of a lead compound to the silver powder with a particle size of 200 nm prepared in Preparation Example 1 in an amount of 3.0 wt% of the total paste weight. After dissolving ethyl cellulose functioning as a binder to a concentration of 30 wt% using Terpineol, the dissolved solution was added to the prepared mixture to be uniformly distributed therein. After preliminary mixing using a mixer, the paste was formed by performing repeatedly dispersing the mixture using a three-roll mill. Since the paste had a high viscosity in the three-roll milling process, Terpineol is added to the paste to adjust the viscosity of the paste, thereby preparing the paste for a solar cell electrode suitable for screen printing. Constituents' contents and viscosity of the paste according to this comparative example are represented in Table 1.

[Comparative Example 2]

A paste was prepared by adding Terpineol using a three-roll mill and adjusting the viscosity in the same method as in Comparative Example 1 except that the Si-Pb-O based glass frit with an average particle size of 0.5 to 3.5 μm comprising 80 wt% of a lead compound was adjusted in an amount of 0.5 wt% of the total paste weight and the lead-free Si-B-O based glass frit with an average particle size of 0.7 to 3.0 μm was added and adjusted in an amount of 3.0 wt% of the total paste weight. Constituents' contents and

viscosity of the paste according to this example are represented in Table 1.

[Table 1] Constituents' contents and viscosities of pastes

<Manufacture of solar cells and evaluation of electric properties thereof> After screen-printing the pastes according to Examples and Comparative Examples on 6-inch single crystalline silicon wafers of which backsides were coated with aluminum, the pastes were dried at 150 ⁰ C. Metallic 325 mesh masks were used in the screen printing, and a pattern for evaluation had finger lines with a width of 120 μm and bus lines with a width of 2 mm. Solar cell substrates (cells) were manufactured by firing the applied pastes in an infrared firing furnace at about 780 ⁰ C for about 4 minutes. Electric properties of the manufactured substrates (cells) were measured using a PASAN CT801 cell tester. The measured electric properties including conversion efficiency (Eff %), filling factor (FF %), open circuit voltage (Voc), short circuit current (Isc), maximum voltage (Vmp) and maximum current (Imp) of the solar cells to which the pastes according to Examples and Comparative Examples had been applied were represented in the following table 2.

[Table 2] Electric properties of solar cells according to paste compositions

Remarks Voc (V) Isc (A) Vmp (V) Imp (A) FF (%) Eff (%)

Example 1 0.612 8.002 0.501 7.100 72.635 15.315

As represented in Table 2, it can be seen that the pastes of Examples 1 to 6 of the present invention prepared by substantially reducing amounts of a lead compound used implement excellent electric properties through efficient contact of electrodes. Therefore, it can be seen that the paste for a solar cell electrode according to the present invention implements excellent electric properties and is environmentally friendly due to the reduction of the consumption of the lead compound. Furthermore, it can also be seen that electric properties, particularly FF % and Eff %, are very low when an Si-Pb-O based glass frit is used in a small amount of 0.5 wt% as in the paste of Comparative Example 2 in order to reduce consumption of the lead compound.

On the other hand, Fig. 6 is a photograph showing a cross section of an electrode manufactured using conductive Ag nanoparticles and lead oxide (Pb(OH) ₂ ) nanoparticles as an electrode using the paste according to Example 1, and Fig. 7 is a photograph showing a cross section of an electrode manufactured using conventional Ag particles with a micrometer (μm) particle size and a lead-containing glass frit with a micrometer (μm) particle size. It can be seen that the electrode of Fig 6 is formed in a compact structure and exhibits excellent conductivity due to its compact structure accordingly as compared with the electrode of Fig. 7.