WO2014181985A1 - Method of manufacturing electrode of solar cell and solar cell using the same - Google Patents

Abstract

The present invention relates to a method of manufacturing an electrode of a solar cell, and a solar cell using the same. Provided herein is a method of manufacturing an electrode of a solar cell, wherein a second conductive layer is formed on a first conductive layer (silicide layer) using a conductive past composition containing no glass frit or a minimum amount of glass frit. Therefore, the present invention provides a solar cell having low contact resistance and high photovoltaic power generation efficiency.

Description

[DESCRIPTION]

[Invention Title]

METHOD OF MANUFACTURING ELECTRODE OF SOLAR CELL AND SOLAR CELL USING THE SAME

[Technical Field]

The present invention relates to a method of manufacturing an electrode of a solar cell using a conductive paste composition having excellent dispersion stability and electrical conductivity, and to a solar cell using the same.

The present application claims the priority benefits from Korean Patent Application No.

10-2013-0050834 filed on May 6, 2013, the entire contents of which are fully incorporated herein by reference.

[Background Art]

Recently, with the prediction of exhaustion of the exiting energy resources such as petroleum, coal and the like, altemative energy sources thereto have attracted considerable attention. Among these altemative energy sources, a solar cell, directly converting soar energy into electric energy, has been in the limelight as a next-generation cell using a semiconductor device.

Solar cells are largely classified into silicon solar cells, compound-semiconductor solar cells and tandem solar cells. Among these solar cells, silicon solar cells are most frequently used.

Generally, such a silicon solar cell includes: a semiconductor substrate and a semiconductor emitter layer which are composed of P-type and N-type semiconductors having different conductive types from each other, an anti-reflective coating film formed on the semiconductor emitter layer; a conductive electrode layer formed on the anti-reflective coating film; a front electrode formed on the conductive electrode layer, and a rear electrode foimed on the other side of the semiconductor substrate. Therefore, a P-N junction is formed at the interface between the semiconductor substrate and the semiconductor emitter layer. In such a silicon solar cell, the transparent conductive electrode layer provided thereon with the front electrode is formed by the interfacial reaction between metal paste and the anti- reflective coating film. In this case, when silver included in the metal paste is liquefied at high temperature and then recrystallized into a solid phase, it makes contact with the semiconductor emitter layer by a punch through phenomenon in which the recrystallized silver penetrates the anti-reflective coating film through the intermediary of glass frit powder. The glass frit powder causes an interfacial reaction with the anti-reflective coating film so as to etch the anti-reflective coating film. This interfacial reaction is an oxidation-reduction reaction, and, through this reaction, some elements are reduced into byproducts. Therefore, in a conventional process of forming a front electrode using silver-containing paste by screen printing, a predetermined amount of glass frit is included in the silver-containing paste in order to penetrate the anti- reflective coating film at the time of forming a front electrode. However, when glass frit is included in the paste, there are problems in that it has a negative influence upon the electrical conductivity of the front electrode, and the dispersion stability of the paste also becomes poor.

Therefore, it is necessary to develop a novel method of manufacturing a front electrode of a solar cell, by which the adhesivity between an emitter layer and a transparent electrode layer can be increased, and the electrical conductivity of a solar cell and the dispersion stability of metal paste for forming a conductive layer can be improved.

[Disclosure]

[Technical Problem]

It is an object of the present invention is to provide a method of manufacturing a n electrode of a solar cell, wherein an electrode is formed on a metal silicide layer using a conductive paste composition including no glass frit or a minimum amount of glass frit, thus having excellent the dispersion stability of metal paste and improving the electrical c onductivity of a solar cell, and to provide a solar cell using the method.

[Technical Solution]

In order to accomplish the above object, an aspect of the present invention prov ides a method of manufacturing an electrode of a solar cell, including the steps of: formi ng an emitter layer on a substrate; forming an anti-reflective coating film on the emitter I ayer; removing a partial region of the anti-reflective coating film to form an opening; formi ng a first conductive layer directly connected with the emitter layer in the opening of the anti-reflective coating film; and forming a second conductive layer on the first conductive I ayer, wherein the second conductive layer is formed by printing a conductive paste com position on the first conductive layer, wherein the conductive paste composition includes 6 0 to 95 wt% of metal powder, 0 to 1 wt% of glass frit, 1 to 20 wt% of a binder and 1 to 20 wt% of a solvent.

In the method, the metal powder may include metal particles having an average particle diameter (D50) of 0.5 to 4 μηι, selected from the group consisting of Ag, Au, Al, Ni, Pt, Cu, and alloys thereof. The metal powder may include sphere-shaped or flake-s haped Ag powder having an average particle diameter (D₅₀) of 0.5 to 4 μιη.

Further, the glass frit may include 5 to 30 wt% of Si0₂, 50 to 90 wt% of PbO, 0.1 to 10 wt% of AI2O3 and 0.1 to 10 wt% of Zr0₂. Selectively, the glass frit may furth er include ZnO and Li₂0, and the composition thereof may include 5 to 30 wt% of SiO₂, 50 to 90 wt% of PbO, 0.1 to 10 wt% of AI₂O₃ and 0.1 to 10 wt% of ZrO₂, 0.1 to 10 wt% of ZnO and 0.1 to 10 wt% of Li₂O.

Further, the binder may include: a carboxyl-containing photosensitive resin which is a copolymer of an unsaturated carboxylic compound and a compound having an unsa turated double bond; a carboxyl-containing photosensitive resin which obtained by adding an ethylenically unsaturated group, as a pendant group, to a copolymer of an unsaturat ed carboxylic compound and a compound having an unsaturated double bond; and a ca rboxyl-containing photosensitive resin which obtained by a reaction of a copolymer of an unsaturated carboxylic compound and a compound having an unsaturated double bond with a compound having a hydroxyl group and an unsaturated double bond.

The solvent may include at least one selected from the group consisting of a-ter pineol, butyl carbitol acetate, texanol, butyl carbitol, and dipropylene glycol monomethyl et her. The second conductive layer may be formed by electrohydrodynamic printing, sc reen printing, inkjet printing, dispensing printing or plating.

The first conductive layer may be a metal silicide layer including metal particles selected from the group consisting of Ni, Ti, Co, Pt, Pd, Mo, Cr, Cu, W, and alloys ther eof.

The method may further include the step of performing a firing process at a tern perature of 400 ^°C to 980 ^CC for 0.1 to 20 min after the formation of the first conductive la yer and the second conductive layer.

The opening may be formed by photolithography, laser etching or an etching pa ste method.

Another aspect of the present invention provides a solar cell, including: a substra te; an emitter layer formed on one side of the substrate; an anti-reflective coating film for med on the emitter layer, a front electrode penetrating the anti-reflective coating film to b e connected to the emitter layer; and a rear electrode formed on the other side of the s ubstrate, wherein, according to the method, the front electrode includes: a first conductive layer formed in the opening of the anti-reflective coating film and directly connected with the emitter layer; and a second conductive layer formed on the first conductive layer.

[Advantageous Effects]

According to the present invention, when a front electrode is manufactured by forming a second conductive layer on a first conductive layer (metal silicide layer) using a conductive past composition containing no glass frit or a minimum amount of glass frit, the dispersion stability of metal paste is improved compared to that of conventional metal paste, thus improving the electrical conductivity of a solar cell. Therefore, the present invention provides a solar cell having high adhesivity between an emitter layer and a transparent conductive layer and having low contact resistance and high photovoltaic power generation efficiency.

[Description of Drawings]

FIG. 1 is a schematic sectional view showing a structure of a solar cell accordin g to an embodiment of the present invention.

[Reference Numerals]

1: silicon semiconductor substrate

2: emitter layer

3: anti-reflective coating film

4: first conductive layer of front electrode

5: second conductive layer of front electrode

6: rear electrode

[Best Mode]

Hereinafter, a method of manufacturing an electrode of a solar cell according to an embodiment of the present and a solar cell using the same will be described in detai I.

The method of manufacturing an electrode of a solar cell according to an embo diment of the present invention includes the steps of: forming an emitter layer on a subs trate; forming an anti-reflective coating film on the emitter layer; removing a partial region of the anti-reflective coating film to form an opening; forming a first conductive layer dire ctly connected with the emitter layer in the opening of the anti-reflective coating film; and forming a second conductive layer on the first conductive layer, wherein the second con ductive layer is formed by printing a conductive paste composition on the first conductive layer, the conductive paste composition including 60 to 95 wt% of metal powder, 0 to 1 wt% of glass frit, 1 to 20 wt% of a binder and 1 to 20 wt% of a solvent.

The method of manufacturing an electrode of a solar cell according the present invention is characterized in that a specific conductive paste composition including no gla ss frit or including a very small amount of glass frit is used in forming the second condu ctive layer on the first conductive layer (metal silicide layer).

When a conductive electrode layer is formed on the metal silicide layer in this w ay, the content of glass frit in the conductive paste composition is small, so the dispersio n stability of the conductive paste composition is excellent, and the content of glass frit, having non-conducting properties, decreases and the content of silver (Ag) increases, thu s improving the electrical characteristics thereof. Further, when the conductive paste com position of the present invention is used, the effect of reducing silver (Ag) by primary low er printing and secondary upper printing at the time of forming the conductive electrode I ayer can be expected. In addition, if the composition of the present invention is used to form the conductive layer, the effect of reducing silver (Ag) can be expected by the primary lower printing and the second upper printing process.

The conductive paste composition used in the present invention essentially includ es metal powder, a binder and a solvent, and may further include a minimum amount of glass frit.

Preferably, the conductive paste composition used in forming the second conduct ive layer may include 60 to 95 wt% of metal powder, 0 to 1 wt% of glass frit, 1 to 20 wt% of a binder, 1 to 20 wt% of a solvent and 0.01 to 5 wt% of an additive. Hereinafter, the ingredients included in the conductive paste composition of the p resent invention will be described in more detail, respectively.

Metal powder

The conductive paste composition of the present invention includes metal powde r as conductive powder for conferring electrical characteristics.

The metal powder may include metal particles having an average particle diamet er (D₅o) of 0.5 to 4 μιτι, selected from the group consisting of Ag, Au, Al, Ni, Pt, Cu, an d alloys thereof. Preferably, the metal powder may include sphere-shaped or flake-shap ed Ag powder having an average particle diameter (D₅o) of 0.5 to 4 pm.

Specifically, in the present invention, the silver (Ag) powder may include silver ox ide, silver alloy, and other materials capable of precipitating silver powder by firing as wel I as pure silver (Ag) powder. These materials may be used independently or in a mixtu re thereof.

The silver powder may be commercially-available sphere-shaped or flake-shaped silver powder. The flake-shaped silver powder may be formed by a commonly-used m ethod.

According to an embodiment of the present invention, the silver powder may ha ve an average particle diameter (D50) of 0.5 to 4 pm.

Further, the purity of the silver powder is not particularly limited as long as it sati sfies general requirements necessary as an electrode. The purity of the silver powder u sed in the present invention may be 90% or more, preferably, 95% or more.

The amount of the metal powder in the conductive paste composition of the pre sent invention may be 60 to 95 wt%, preferably, 80 to 95 wt%, based on the total weig ht of the conductive paste composition. When the amount of the metal powder is less t han 60 wt%, the conductive paste composition is phase-separated, or the viscosity there of is lowered, thus causing a problem of printability. Further, when the amount thereof i s more than 95 wt%, the viscosity of the conductive paste composition becomes high, a nd thus it is difficult to print this composition.

Glass frit

In the conductive paste composition used in forming the second conductive layer, the amount of glass frit may be 0 to 1 wt%, based on the total weight of the conductiv e paste composition. More preferably, the amount thereof may be 0 to 0.5 wt%.

The conductive paste composition of the present invention may not include glass frit at all or may include a very small amount of glass frit, thus making the contact bet ween an electrode layer and a silicon layer easy due to the formation of the first conduc tive layer (metal silicide layer). Further, thanks to the presence of glass frit, the dispersio n stability of the conductive paste composition is improved, thus greatly improving the pri ntability thereof.

Glass frit is used to effectively make an electrode into a fired pattern having no pinhole, and may include SiO₂, PbO and at least one metal oxide selected from the gro up consisting of AI₂O₃, Ζ1Ό2, ZnO and Li₂0.

More specifically, according to an embodiment of the present invention, the glass frit may be a glass composition including 5 to 30 wt% of SiO₂, 50 to 90 wt% of PbO, 0.1 to 10 wt% of AI2O3 and 0.1 to 10 wt% of ZrO₂.

According to another embodiment of the present invention, the glass frit may be a glass composition including 5 to 30 wt% of SiO₂, 50 to 90 wt% of PbO, 0.1 to 10 w t% of AI2O3 and 0.1 to 10 wt% of ZrO₂, 0.1 to 10 wt% of ZnO and 0.1 to 10 wt% of L i₂O.

The AI2O3, ZrO₂, ZnO and U2O included in the glass frit serves to maintain low viscosity as well as form a stable glass phase during an interfacial reaction. When a gl ass ingredient has low viscosity during the interfacial reaction, the probability of contact b etween PbO and an anti-reflective coating film becomes high, and thus etching may take place in a larger area. As such, when etching takes place relatively frequently, the are a of a front electrode formed by the recrystallization of metal powder (preferably, Ag pow der) becomes larger, so the contact resistance between a substrate and a front electrode becomes lower, thereby improving the performance of a solar cell and enhancing the c ontact strength between the substrate and the front electrode.

Further, the glass frit may further include ingredients generally used in the glass composition in addition to the above ingredients. Such ingredients may be used regardl ess of kind, and all the ingredients well known in the related field may be used. For ex ample, the glass frit may further include, but is not limited to, Na₂O.

Further, according to an embodiment of the present invention, the glass frit may have an average particle diameter (D₅₀) of 0.5 to 10 μιτι, preferably, 0.8 to 5 pm. Wh en the average particle diameter of the glass frit is present within the above range, an el ectrode can be effectively made into a fired pattern having no pinhole. Therefore, it is p referred that the average particle diameter thereof be present within the above range.

Meanwhile, the glass frit is used in the form of solid powder after its ingredients are melted and then solidified and powdered through a series of procedures below.

According to an embodiment of the present invention, when the ingredients of th e glass frit are melted together under the atmospheric pressure, the intermolecular bonds of the ingredients are cut, so these ingredients lose the properties of metal oxide, are u niformly mixed in a molten state, and have vitreous properties after the following cooling process. In the melting process of the ingredients, the melting temperature thereof is not particularly limited as long as each of the ingredients can be sufficiently melted at this melting temperature. For example, the melting temperature may be 900 to 1,500^°C, but is not limited thereto. Further, the melting time thereof is not particularly limited as long as all the ingredients can be sufficiently melted for the melting time while maintaining th e above melting temperature. The melting time may be suitably selected depending on the kinds of the ingredients and the melting temperature thereof. For example, the melti ng time may be 10 min to 1 hr, but is not limited thereto.

Next, the molten mixture is cooled to obtain solid glass frit. That is, the molten ingredients are hardened through a cooling process to form sold glass frit. The cooling r ate may be suitably selected depending on the kinds of the ingredients of the glass frit, but, preferably, the ingredients may be rapidly cooled. Specific cooling conditions may b e determined with reference to the state diagram of each of the ingredients. When the cooling rate is excessively low, the crystallization of the molten ingredients occurs, and th us a glass phase may not be formed. For example, the molten mixture may be cooled to 25 ~ 50 ^°C for 1 to 5 min under the atmospheric pressure, but is not limited thereto. In order to obtain the above high cooling rate, methods generally used in the related fie Id may be used without limitations. For example, a method of extruding the molten mixt ure into a sheet to increase surface area and a method of precipitating the molten mixtu re with water may be employed, but is not limited thereto.

Next, the solid glass frit is pulverized into glass frit powder. Since the above-me ntioned glass frit is too large in volume to mix with metal paste, it is preferred that this g lass frit be pulverized into glass frit powder. After the pulverization of the solid glass frit, the average particle diameter of the obtained glass frit powder may be 1 to 10 μιη, but is not limited thereto. When the average particle diameter thereof is present within the above range, the glass frit powder is uniformly dispersed in the metal paste, thus very ef ficiently causing an interfacial reaction. As the method of pulverizing the solid glass frit o btained by cooling into glass frit powder, pulverization method generally used in the relat ed field may be used without limitations. For efficient pulverization, the pulverization proc ess may be performed in two stages. In this case, the primary and secondary pulveriza tions may be repeatedly performed in the same manner; and the primary pulverization m ay be performed by coarse pulverization, and the secondary pulverization may be perfor med by fine pulverization. Here, the coarse pulverization means that the above-mentione d solid glass frit is pulverized to a proper in order to make the fine pulverization thereof easy, not that the average particle diameter of the glass frit powder obtained thereby is li mited to a predetermined average particle diameter. The fine pulverization is a process of pulverizing the coarsely pulverized glass frit into glass frit powder having a desired av erage particle diameter. Further, if necessary, each of the primary and secondary pulveri zation processes may be selectively performed by either dry pulverization or wet pulveriz ation. As generally known in the related field, in wet pulverization, water, ethanol or the like may be added, but is not limited thereto. Binder

The conductive paste composition of the present invention includes a binder. The binder functions to bind the ingredients before the firing of an electrode patt em, and, for uniformity, may be prepared by suspension polymerization.

The binder may include carboxyl-containing resins, for example, a carboxyl-contai ning photosensitive resin having an ethylenically unsaturated double bond and a carboxyl- containing photosensitive resin having no ethylenically unsaturated double bond. More sp ecifically, the binder may include, but is not limited to: a carboxyl-containing photosensitiv e resin which is a copolymer of an unsaturated carboxylic compound and a compound having an unsaturated double bond; a carboxyl-containing photosensitive resin which obta ined by adding an ethylenically unsaturated group, as a pendant group, to a copolymer of an unsaturated carboxylic compound and a compound having an unsaturated double bond; and a carboxyl-containing photosensitive resin which obtained by a reaction of a c opo!ymer of an unsaturated carboxylic compound and a compound having an unsaturate d double bond with a compound having a hydroxyl group and an unsaturated double bo nd.

The binder may be included in an amount of 1 to 20 wt%, based on the total weight of the conductive paste composition. The amount of the binder is less than 1 w t%, the distribution of the binder in the formed electrode pattem may become ununiform, and thus the patterning cannot be easily performed by selective exposure and develop ment. In contrast, when the amount thereof is more than 20 wt%, a pattem may be ea sily destroyed at the time of firing an electrode, and the resistance of the electrode may be increased by carbon ash after firing the electrode.

Solvent

The conductive paste composition includes a solvent.

The solvent can dissolve the binder, and can be easily mixed with other additive s. Examples of the solvent may include, but are not limited to, a-terpineol, butyl carbitol acetate, texanol, butyl carbitol, and dipropylene glycol monomethyl ether.

The solvent may be included in an amount of 1 to 20 wt%, based on the total weight of the conductive paste composition. When the amount of the solvent is less tha n 1 wt%, it is difficult to uniformly apply the conductive paste composition. In contrast, when the amount thereof is more than 20 wt%, an electrode pattem having sufficient co nductivity cannot be obtained, and the adhesivity between the conductive paste compositi on and the substrate may be deteriorated.

Additives

The conductive paste composition of the present invention may further include a n additive, such as a dispersant, a thickener, a thixotropic agent, a leveling agent or the like, in addition to the above-mentioned ingredients. The additive, if necessary, may be included in an amount of 0 to 5 wt%, based on the total weight of the conductive past e composition.

Examples of the dispersant may include, but are not limited to, DISPERBYK-180, 110, 996, and 997, manufactured by BYK Corporation. Examples of the thickener may include, but are not limited to, BYK-410, 411 an d 420, manufactured by BYK Corporation.

Examples of the thixotropic agent may include, but are not limited to, ANTI-TER RA-203, 204 and 205, manufactured by BYK Corporation.

Examples of the leveling agent may include, but are not limited to, BYK-3932 P,

BYK-378, BYK-306 and BYK-3440, manufactured by BYK Corporation.

Hereinafter, the method of manufacturing an electrode of a solar cell according t o the present invention will be described in detail.

The solar cell includes a substrate, an emitter layer, an anti-reflective coating film and a conductive layer. Here, the conductive layer may be a front electrode. The con ductive layer may have a multi-layered structure including a first conductive layer and a s econd conductive layer.

Further, in the present invention, when the conductive layer has a multi-layered structure, the first conductive layer making contact with the emitter layer does not include glass frit, and the second conductive layer formed on the first conductive layer does not include glass frit or include a very small amount of glass frit. According to the present invention, the effect of reducing silver (Ag) can be expected by primary lower printing an d secondary upper printing at the time of forming a conductive electrode layer.

According to an embodiment of the present invention, the conductive layer may i nclude a first conductive layer, as a silicide layer, and a second conductive layer formed of the above mentioned conductive paste composition of the present invention.

For this purpose, first, a substrate is provided, and then an emitter layer is form ed on the substrate.

The emitter layer may be formed by a method well known in the related field.

Therefore, the method of forming the emitter layer is not limited.

Subsequently, an anti-reflective coating (ARC) film is formed on the emitter layer.

Specifically, the process of forming the anti-reflective coating film on the emitter I ayer may be performed using a general electrode preparation process. Here, the substr ate may be doped with a P-type impurity, and the emitter layer may be doped with an N-type impurity.

The anti-reflective coating film may be a single-layer film or multi-layer film made of any one selected from the group consisting of silicon nitride, hydrogen-containing silic on nitride, silicon oxide, MgF₂, ZnS, T1O2, CeO2, and mixtures thereof.

Subsequently, a partial region of the anti-reflective coating film is removed to for m an opening therein.

In this method, the opening may be formed by photolithography, laser etching or an etching paste method. Preferably, the opening may be formed by electrohydrodyna mic printing using etching paste or screen printing using etching paste. More preferably, the opening may be formed by electrohydrodynamic printing using etching paste. The process of forming the opening using photolithography or laser printing may be performe d by a generally known method, and is not particularly limited.

When the opening is formed using the etching paste, specifically, the etching pa ste is printed on the anti-reflective coating film, and then the anti-reflective coating film pri nted with the etching paste is immersed into a 0.1% KOH solution and partially removed using ultrasonic waves.

The size and shape of the opening of the anti-reflective coating film must corres pond to the size and shape of the conductive layer. The reason for this is that the con ductive layer is formed in the opening of the anti-reflective coating (ARC) film to directly make contact with the emitter layer to form a silicide compound. For example, the shap e of the opening corresponds to the shape of an electrode, and the anti-reflective coatin g film may be removed along the line width of the electrode, for example, a line width o f 60 Mm.

When a composition for forming a conductive layer is printed in the opening of t he anti-reflective coating film by electrohydrodynamic printing to form a conductive layer, t he conductive layer directly makes contact with the emitter layer to form a silicide compo und, thereby reducing the contact resistance of an electrode.

Next, the first conductive layer, which is a silicide layer directly making contact w ith the emitter layer, is formed in the opening of the anti-reflective coating film.

The first conductive layer may be formed on the opening of the anti-reflective la yer by electrohydrodynamic printing such that it directly makes contact with the emitting I ayer without an additional intermediate layer.

The conductive layer may be formed by non-contact direct printing, preferably, el ectrohydrodyanmic printing. The eletrohydrodynamic printing is a technology of injecting droplets using an electric field. In the electrohydrodynamic printing, when a predetermine d voltage is applied between an injection nozzle and a substrate, an electric field is gene rated, and simultaneously electric charges are concentrated on the surface of fluid aroun d the injection nozzle. In this case, when the pressure of fluid injected from the nozzle i s higher than the surface tension of fluid due to the electric charges generated from the surface of fluid and the electric field, the spherical surface of fluid is converted into the T aylor cone-shaped surface thereof, and thus jet or spray droplets still smaller than the siz e of the nozzle are generated. Due to the formation of Taylor cone by the electric field, jet or spray droplets having a size of several hundreds of nanometers (nm) to several t ens of micrometers (pm) can be generated from the nozzle having a relative large size of several tens of micrometers (pm) to several hundreds of micrometers (pm), and thus t wo advantages of the improvement of line width resolution and the reduction of clogging frequency can be realized, and the linearity of the injected droplets can be improved by optimizing the electric field.

The electrohydrodynamic printing is advantageous in that the physical damage of a silicon substrate can be minimized because it is not necessary to form an electrode by pressuring the silicon substrate, similarly to screen printing, in that the consumption of raw material can be reduced, and in that an electrode can be formed using electrode materials having a wide viscosity range because the electrohydrodynamic printing process is not greatly influenced by the viscosity of the electrode material. For this reason, in t he case of a nickel layer on which a thin electrode layer is to be applied, the thin electr ode layer can be formed by printing low-viscosity ink, and, in the case of an Ag electrod e that must have a high aspect ratio, this Ag electrode can be formed using high-viscosi ty ink. Therefore, electrohydrodynamic printing can overcome the limitations of ink-jet pri nting and screen printing.

Further, the metal particles applicable to the composition for forming the first con ductive layer may be used without limitations as long as they are conductive metal partic les capable of forming the emitting layer and the silicide compound. For example, when the conductive layer includes the first conductive layer making contact with the emitter I ayer and the second conductive layer formed on the first conductive layer, the first cond uctive layer may include metal particles selected from the group consisting of Ni, Ti, Co, R, Pd, Mo, Cr, Cu, W, and alloys thereof. Preferably, the first conductive layer may incl ude nickel particles.

Further, the composition for forming the first conductive layer according to the pr esent invention may be an ink composition, and may further include a solvent, a binder and a dispersant in addition to the metal particles. The generally used ingredients and c ontents are applicable even to the present invention.

For example, the binder functions to bind the ingredients before the firing of an electrode pattern, and, for uniformity, may be prepared by suspension polymerization. Th e binder may be used without limitations as long as it is a polymer that can be dissolve d in a main solvent. Specifically, the binder may include carboxyl-containing resins, for e xample, a carboxyl-containing photosensitive resin having an ethylenically unsaturated dou ble bond and a carboxyl-containing photosensitive resin having no ethylenically unsaturate d double bond. Examples of the binder may include, but are not limited to, i) a carboxy l-containing photosensitive resin which is obtained by the copolymerization of an unsaturat ed carboxylic compound and a compound having an unsaturated double bond; ii) a carb oxyl-containing photosensitive resin which obtained by adding an ethylenically unsaturated group, as a pendant group, to a copolymer of an unsaturated carboxylic compound and a compound having an unsaturated double bond; and iii) a carboxyl-containing photosen sitive resin which obtained by a reaction of a copolymer of an unsaturated carboxylic co mpound and a compound having an unsaturated double bond with a compound having a hydroxyl group and an unsaturated double bond. These binders may be used indepe ndently or in a combination of two or more. The preferred examples of the binder may include an alkylphenol-formaldehyde copolymer (for example, Tackirol, manufactured by Taoka Chemical Corporation), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), ethylcell ulose, and the like.

The binder may be included in an amount of 0.1 to 5 wt%, based on the total weight of the composition for forming the first conductive layer. The amount of the bind er is less than 0.1 wt%, the distribution of the binder in the formed electrode pattem ma y become non-uniform, and thus the patterning cannot be easily performed by selective exposure and development. In contrast, when the amount thereof is more than 5 wt%, a pattem may be easily destroyed at the time of firing an electrode, and the resistance of the electrode may be increased by carbon ash after firing the electrode.

The dispersant may be included in an amount of 0.5 to 5 wt%, based on the t otal weight of the composition for forming the first conductive layer. The dispersant may be a copolymer including an acid group having an acid value of 50 mg KOH/g or mor e and an amine value of 100 mg KOH/g or less. Examples of commercially available p hosphate copolymers satisfying the above acid value and amine value may include BYK 102 (acid value: 101 mg KOH/g, amine value: Omg KOH/g), BYK110 (acid value: 53mg KOH/g, amine value: Omg KOH/g), BYK 45 (acid value: 76mg KOH/g, amine value: 71 mg KOH/g), BYK180 (acid value: 94mg KOH/g, amine value: 94mg KOH/g), BYK995 (a cid value: 53mg KOH/g, amine value: Omg KOH/g), and BYK996(acid value: 71 mg KOH/ g, amine value: Omg KOH/g), which are manufactured by BYK Chemie Corporation.

As the solvent that can be used in the composition for forming the first conducti ve layer, all solvents generally used in forming a conductive electrode layer may be use d as long as they can dissolve the binder and can be easily mixed with other additives.

Preferably, the solvent may be a mixture of a low-boiling solvent, as a main solvent, a nd a high-boiling solvent.

The low-boiling solvent, as a main solvent, may be low-boiling glycol ethers eac h including a hydroxyl group at the end thereof and having a boiling point of 130^°C to 2 10^°C, preferably, 170 to 210^°C. When the boiling point of the low-boiling solvent is lower than 130°C, the jetting characteristics of the composition for forming the first conductive I ayer become poor, and, when the boiling point thereof is higher than 210^°C, the drying r ate of the composition after jetting becomes low, and thus the spreading phenomenon th ereof becomes severe.

The low-boiling solvent may be included in an amount of 30 to 70 wt%, based on the total weight of the composition for forming the first conductive layer. When the a mount of the low-boiling solvent is less than 30 wt%, it is difficult to uniformly apply ink. In contrast, when the amount thereof is more than 70 wt%, an electrode pattern having sufficient conductivity cannot be obtained, and the adhesivity between the composition a nd the substrate may be deteriorated.

Preferred examples of the low-boiling solvent may include: diethylene glycol mon oalkyi ethers, such as diethylene glycol monomethyl ether (methyl carbitol), diethylene gly col monoethyl ether (ethyl carbitol), diethylene glycol monobutyl ether (butyl carbitol) and t he like; and ethylene glycol monoalkyl ethers, such as ethylene glycol ethyl ether (ethyl c ellosolve), ethylene glycol propyl ether (propyl cellosolve), ethylene glycol n-butyl ether (n- butyl cellosolve)) and the like, wherein the alkyl is a straight-chained or branched alkyl gr oup.

The high-boiling solvent may be a solvent including a hydroxyl group at the end thereof and having a boiling point of 240 °C to 300 ^°C, preferably, 240 to 270 °C. Exampl es of the high-boiling solvent may include, but are not limited to, diethylene glycol, glycer ol, and tripropylene glycol methyl ether.

The high-boiling solvent may be included in an amount of 3 to 10 wt%, based on the total weight of the composition for forming the first conductive layer. When the a mount of the high-boiling solvent is less than 3 wt%, the jetting characteristics of the co mposition for forming the first conductive layer become poor, and, when the amount ther eof is more than 10 wt%, the drying rate of the composition after jetting becomes low, a nd thus the spreading phenomenon thereof becomes severe.

Further, if necessary, the composition for forming the first conductive layer may f urther include at least one additive selected from among a thickener, a thixotropic agent and a leveling agent. The additive may be included in an amount of 1 to 20 wt%, bas ed on the total weight of the composition for forming the first conductive layer.

In the method of preparing the composition for forming the first conductive layer according to the present invention, stirring and pulverization processes may be performed by ball milling, and a milling process may be performed in order to prepare glass frit na noparticles. Additionally, a filtering process may be performed in order to remove coagul ated particles or particles exceeding a predetermined diameter range. More preferably, t his method may be performed by flame spray pyrolysis, plasma treatment or the like, bu t is not limited thereto.

The composition for forming the first conductive layer may also be used in formi ng a front electrode of a silicon solar cell. Metal powder, such as nickel powder, is bon ded to silicon to form metal silicide, thus reducing the contact resistance between a silico n substrate and an electrode.

When the above procedures are completed, a second conductive layer is forme d on the first conductive layer.

The second conductive layer may be formed by printing the above-mentioned co nductive paste composition.

The printing method for forming the second conductive layer may be performed by screen printing, inkjet printing, dispensing printing, silver light induced plating (LIP), plati ng, such as electroless plate, or electrohydrodynamic printing. Preferably, the printing me thod for forming the second conductive layer may be performed by electrohydrodynamic printing.

The second conductive layer may include metal particles selected from the grou p consisting of Ag, Au, Al, Ni, Pt, Cu, and alloys thereof. Preferably, the second conduc tive layer may include silver particles.

In this case, the metal particles of the second conductive layer may be included in the first conductive layer in an amount of 2 to 10 parts by weight, based on 100 par ts by weight of the metal particles of the first conductive layer, thus increasing the adhes ivity between the first conductive layer and the second conductive layer. The method according to the present invention may further include the step of p erforming a firing process at a temperature of 400 ^°C to 980 ^°C for 0.1 to 20 min after the formation of the first conductive layer and the second conductive layer. When the cond uctive layer includes the first conductive layer and the second conductive layer, the firing processes may be respectively performed after the formation of each of the first and se cond conductive layers, or may be simultaneously formed after the formation of both the first conductive layer and the second conductive layer.

In the method, the thickness of the first conductive layer may be 50 nm to 1 μ m. When the thickness of the first conductive layer is less than 50 nm, which is excess ively thin, it is difficult to form a silicide layer, and thus the first conductive layer does not make contact with the emitting layer. Further, when the thickness thereof is more than 1 pm, which is excessively thick, the conductivity of the first conductive layer is not goo d compared to that of silver (Ag), and thus the resistance thereof increases, thereby bloc king the flow of electric current.

Further, it is preferred that the upper part of the second conductive layer be exp osed over the anti-reflective coating film through the opening formed in the anti-reflective coating film. The thickness of the second conductive layer may be 5 to 25 pm. When the thickness of the second conductive layer is less than 5 μητι, the linear resistance th ereof increases, thus decreasing electric current. Further, when the thickness thereof is more than 25 pm, the second conductive layer causes a shading phenomenon for blocki ng light, thus increasing the unit cost thereof.

Meanwhile, according to another embodiment of the present invention, there is p rovided a solar cell, including: a substrate; an emitter layer formed on one side of the su bstrate; an anti-reflective coating film formed on the emitter layer; a front electrode penetr ating the anti-reflective coating film to be connected to the emitter layer; and a rear elect rode formed on the other side of the substrate, wherein, according to the method, the fr ont electrode includes: a first conductive layer formed in the opening of the anti-reflective coating film and directly connected with the emitter layer; and a second conductive layer formed on the first conductive layer.

In the solar cell, as described above, a substrate (for example, silicon wafer) pro vided with an anti-reflective coating (ARC) film is etched by photolithography, laser etchin g or screen printing to form an opening, and then a silicide forming material is printed in the opening to form a front electrode.

Subsequently, a first conductive layer of the front electrode is formed using the s ilicide forming material, and then a second conductive layer of the front electrode is form ed.

Thereafter, a rear electrode is formed by subsequent processes, and then co-firi ng is performed to manufacture a solar cell. Further, a silicide layer having low contact resistance at the interface of an electrode and an emitter layer is formed by the co-firin 9-

According to the method of the present invention, a solar cell having high efficie ncy can be manufactured by using an Ag electrode layer including a small amount of gl ass frit.

FIG. 1 is a schematic sectional view showing a structure of a solar cell accordin g to an embodiment of the present invention.

Referring to FIG. 1 , The solar cell according to an embodiment of the present in vention includes a first conductive type silicon semiconductor substrate 1 , a second cond uctive type emitter layer 2 formed on one side of the substrate 1 , an anti-reflective coati ng film 3 formed on the emitter layer 2, a first conductive layer 4 penetrating the anti-refl ective coating film 3 to be directly connected to the emitter layer 2, a second conductive layer 5 formed on the first conductive layer 4 using a conductive paste composition, an d a rear electrode 6 formed on the other side of the substrate 1.

In the structure of the solar cell of the present invention, a part of the first cond uctive layer 4 directly makes contact with the emitter layer 2 through at least one openin g formed in the anti-reflective coating film 3 to be electrically connected with the emitter I ayer 2. Further, the upper part of the second conductive layer 5 may be exposed over t he anti-reflective coating film 3 through at least one opening formed in the anti-reflective coating film 3, and the lower part thereof may make contact with the first conductive lay er 4.

In this case, the substrate 1 may be doped with a P-type impurity, such as Gro up III elements including B, Ga, In and the like, and the emitter layer 2 may be doped with an N-type impurity such as group V elements including P, As, Sb and the like. Lik e this, when the substrate 1 and the emitter layer 2 are respectively doped with conducti ve impurities opposite to each other, a P-N junction is formed at the interface between t he substrate 1 and the emitter layer 2.

The anti-reflective coating film 3 serves to immobilize the defects existing on the surface of the emitter layer 2 or in the bulk thereof and decrease the reflectance of sol ar light incident on the front side of the substrate 1. When the defects existing in the e mitter layer 2 is immobilized, the recombination sites of a minority carrier are removed, a nd thus the open voltage (Voc) of a solar cell increases. Further, when the reflectance of solar light decreases, the amount of light reaching the P-N junction increases, and thu s the short-circuit current (Isc) of a solar cell increases. As such, when the open voltag e and short-circuit current of a solar cell are increased by the anti-reflective coating film 3, the conversion efficiency thereof is increased in proportion thereto.

Meanwhile, as shown in FIG. 1, the front electrode is located at the uppermost portion of a solar cell to block solar light. Thus, it is important to minimize the area of t he front electrode without deteriorating the function thereof.

Further, the rear electrode 6 may include, but is not limited to, aluminum. For e xample, the rear electrode 6 may be formed by printing the other side of the substrate 1 with an ink composition including aluminum, quartz, silica, a binder and the like and the n heat-treating the ink composition. During the heat treatment of the rear electrode, alu minum, which is an electrode forming material, is diffused through the back side of the s ubstrate 1 , and thus a back surface field layer may be formed at the interface of the re ar electrode 6 and the substrate 1. When the back surface field layer is formed, it is p ossible to prevent carriers from being recombined by the movement thereof toward the b ack side of the substrate 1, and, when the recombination of the carriers is prevented, th e open voltage of a solar cell increases, thus improving the efficiency of a solar cell. [Mode for Invention]

Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, these Examples are set forth only to illustrate the present invention, and the scope of the present invention is not limited thereto. Preparation Example 1>

SiO₂ 17.0 wt%, AI₂O₃ 8.7 wt%, PbO 66.0 wt%, ZnO 6.0 wt%, Li₂O 1.7 wt%, a nd Zr0₂ 0.6 wt% were mixed using a ball mill, and then dried at 80 ^°C. This mixture w as melted at 1000^°C, and then quenched at room temperature. This resulting mixture w as coarsely pulverized using a disk mill, and then finely pulverized using a planetary mill to prepare glass frit having an average particle size of 5 pm or less.

Preparation Example 2>

Si0₂ 7.0 wt%, AI2O3 10.75 wt%, PbO 64.55 wt%, ZnO 6.0 wt%, and B₂O₃ 1.7 wt% were mixed using a ball mill, and then dried at 80 ^°C. This mixture was melted at 1000^°C, and then quenched at room temperature. This resulting mixture was coarsely pulverized using a disk mill, and then finely pulverized using a planetary mill to prepare glass frit having an average particle size of 5 pm or less.

<Examples 1 to 3 and Comparative Examples 1 to 3>

Silver paste compositions were prepared according to the ingredients and conten ts (unit: wt%) given in Table 1 below.

In this case, as silver (Ag) powder, Ag particles (brand name: 4-8F, manufacture d by Dowa Corporation) having an average particle diameter (D₅o) of 2.0 pm and Ag pa rticles (brand name: 2-1 C, manufactured by Dowa Corporation) having an average particl e diameter (D₅₀) of 0.8 prn were used.

Further, ethylcellulose (Std 10, manufactured by Dow Chemical Corporation) was used as a binder, BCA (butyl carbitol acetate) was used as a solvent, and a dispersant (KD-4, manufactured by Croda Corporation) was used as an additive.

[Table 1]

<Example 5>

A polycrystalline silicon wafer having a thickness of 156 mm was doped with ph osphorus (P) at 900 ^°C in a tube furnace by a diffusion process using POCI₃ to form an emitter layer having 100 Ω/sq sheet resistance.

A silicon nitride film was deposited on the emitter layer by plasma enhanced ch emical vapor deposition (PECVD) to form an anti-reflective coating film having a thicknes s of 80 pm.

The anti-reflective coating film was printed to a line width of 60 pm using etchin g paste (SolarEtch BES TypelO, manufactured by Merck Corporation) by electrohydrodyn amic (EHD) printing, and then dried at 330 ^°C by a belt dryer. The dried wafer was im mersed into a 0.1 % KOH solution charged in a bath, and was then treated by ultra-soni cation to remove and then dry the anti-reflective coating (ARC) film printed with the etchi ng paste, thereby forming an opening. Subsequently, a nickel layer (that is, a first cond uctive layer of a front electrode) was formed in the opening using a nickel-containing con ductive paste composition by electrohydrodynamic (EHD) printing. Here, the nickel-contai ning conductive paste composition was prepared by mixing 40 wt% of nickel particles ha ving a particle diameter of about 50 nm, 50 wt% of ethyl cellosolve as a main solvent, 7 wt% of tripropyleneglycolmethylether as a high-boiling solvent, 1 wt% of polyvinylpyrrolid one K15 as a binder and 2 wt% of BYK996 as a dispersant for 2 hours using a ball m ill.

A second conductive layer of a front electrode was formed on the formed nickel layer (that is, the first conductive layer of a front electrode) using the silver paste of Ex ample 1 including no glass frit. Further, the back side of the wafer was screen-printed with Al paste (ALSOLAR, manufactured by Toyo Aluminium K. K Corporation). Thereaft er, drying was performed at a temperature of 300 ^°C for 60 seconds in a belt firing furna ce, and then sintering was performed at a peak temperature of 850 ^°Cf or 20 seconds in a belt firing furnace to form a first electrode (first conductive layer), a second electrode (second conductive layer) and a rear electrode. In this case, the finger width of the for med front electrode was 70 μιη, and the thickness thereof was about 10 μηι.

<Example 6>

A solar cell was manufactured in the same manner as in Example 5, except th at the silver paste of Example 2 was used at the time of forming the second conductive layer of a front electrode.

<Example 7>

A solar cell was manufactured in the same manner as in Example 5, except th at the silver paste of Example 3 was used at the time of forming the second conductive layer of a front electrode.

<Example 8>

A solar cell was manufactured in the same manner as in Example 5, except th at the silver paste of Example 4 was used at the time of forming the second conductive layer of a front electrode.

<Comparative Example 4>

A solar cell was manufactured in the same manner as in Example 5, except th at the silver paste of Comparative Example 1 was used at the time of forming the seco nd conductive layer of a front electrode.

<Comparative Example 5>

A solar cell was manufactured in the same manner as in Example 5, except th at the silver paste of Comparative Example 2 was used at the time of forming the seco nd conductive layer of a front electrode.

Comparative Example 6>

A solar cell was manufactured in the same manner as in Example 5, except th at silver paste of Comparative Example 3 was used at the time of forming the second c onductive layer of a front electrode.

Comparative Example 7>

A solar cell was manufactured using Sol9411 (manufactured by Heraeus Corpor ation) without performing an ARC grooving process and a nickel printing process in the same manner as in Example 5. This method belongs to a general solar cell manufactu ring method.

<Comparative Example 8>

A solar cell was manufactured using the silver paste of Comparative Example 2 without performing an ARC grooving process and a nickel printing process in the same manner as in Example 5. This method belongs to a general solar cell manufacturing method. [Experimental Example]

Evaluation of electrical performance of solar cells

The electrical performance of the solar cells manufactured in Examples 3 to 4 a nd Comparative Examples 5 to 10 was measured using a solar tester (Model: NCT-M-1 80A, manufactured by NPC Incorporated in NJ, Dumont in U.S.A. under a solar conditio n of AM 1.5 according to the ASTM G-173-03. The results thereof are given in Table 2 below. Here, Jsc is a short-circuit current density measured at a zero output voltage, and Voc is an open-circuit voltage measured at a zero output current. Further, F.F.[%] i s a fill factor, and Eff [%] is efficiency.

[Table 2]

As seen from the results of Table 2 above, when a front electrode is formed us ing the silver paste (Example 7) including 0.5 wt% of glass frit at the time of manufacturi ng a solar cell, the optimum solar cell characteristics are exhibited. Further, due to the f ormation of a nickel (Ni) electrode layer, the contact between a silicon wafer and a silver (Ag) electrode become easy, thus exhibiting low contact resistance. For this reason, th e amount of glass frit is minimized to allow a solar cell to have high Jsc and F.F. value s. In a shallow emitter, the properties and content of glass frit are important. When the properties and content of glass frit are excessive, an emitter layer is punched through t o be shunted, thus having a great influence on F.F. Further, since this basic process is a process of removing anti-reflective coating (ARC) film and introducing a silicide layer, t he content of glass frit, which is a non conductor, can be reduced, and thus it is possibl e to manufacture a solar cell having excellent cell characteristics.

Further, in the case of the silver paste of Example 5 including no glass frit, it ca n be ascertained that the efficiency of a solar cell is exhibited to some degree even whe n this silver paste does not include glass frit. Further, in the case of the silver paste of Example 5 including 0.2 wt% of glass frit, it can be ascertained that its effect is very ex cellent. The results of the silver paste of Example 8 are somewhat inferior to those of t he silver pastes of Comparative Examples 7 and 8, but it can be ascertained that its gla ss frit content is relatively low and is minimized to such a degree that the efficiency of a solar cell can be exhibited.

Claims

[CLAIMS]

[Claim 1 ]

A method of manufacturing an electrode of a solar cell, comprising the steps of: forming an emitter layer on a substrate;

forming an anti-reflective coating film on the emitter layer;

removing a partial region of the anti-reflective coating film to form an opening; forming a first conductive layer directly connected with the emitter layer in the op ening of the anti-reflective coating film; and

forming a second conductive layer on the first conductive layer,

wherein the second conductive layer is formed by printing a conductive paste co mposition on the first conductive layer, wherein the conductive paste composition includes 60 to 95 wt% of metal powder, 0 to 1 wt% of glass frit, 1 to 20 wt% of a binder and 1 to 20 wt% of a solvent.

[Claim 2]

The method of claim 1 , wherein the metal powder includes metal particles havin g an average particle diameter (D₅₀) of 0.5 to 4 pm, selected from the group consisting of Ag, Au, Al, Ni, Pt, Cu, and alloys thereof.

[Claim 3]

The method of claim 2, wherein the metal powder includes sphere-shaped or fla ke-shaped Ag powder having an average particle diameter (D50) of 0.5 to 4 pm.

[Claim 4]

The method of claim 1 , wherein the glass frit includes 5 to 30 wt% of Si0₂, 50 to 90 wt% of PbO, 0.1 to 10 wt% of Al₂0₃ and 0.1 to 10 wt% of Zr0₂.

[Claim 5]

The method of claim 1 , wherein the glass frit includes 5 to 30 wt% of S1O2, 50 to 90 wt% of PbO, 0.1 to 10 wt% of Al₂0₃ and 0.1 to 10 wt% of Zr0₂, 0.1 to 10 wt% of ZnO and 0.1 to 10 wt% of Li₂O.

[Claim 6]

The method of claim 1 , wherein the binder includes:

a carboxyl-containing photosensitive resin which is a copolymer of an unsaturate d carboxylic compound and a compound having an unsaturated double bond;

a carboxyl-containing photosensitive resin which obtained by adding an ethylenica lly unsaturated group, as a pendant group, to a copolymer of an unsaturated carboxylic compound and a compound having an unsaturated double bond; and

a carboxyl-containing photosensitive resin which obtained by a reaction of a cop olymer of an unsaturated carboxylic compound and a compound having an unsaturated double bond with a compound having a hydnoxyl group and an unsaturated double bond.

[Claim 7]

The method of claim 1 , wherein the solvent includes at least one selected from the group consisting of a-terpineol, butyl carbitol acetate, texanol, butyl carbitol, and dipro pylene glycol monomethyl ether.

[Claim 8]

The method of claim 1, wherein the second conductive layer is formed by electr ohydrodynamic printing, screen printing, inkjet printing, dispensing printing or plating.

[Claim 9]

The method of claim 1, wherein the first conductive layer is a metal silicide layer including metal particles selected from the group consisting of Ni, Ti, Co, Pt, Pd, Mo, C r, Cu, W, and alloys thereof.

[Claim 10] The method of claim 1 , further comprising the step of performing a firing proces s at a temperature of 400^°C to 980^°C for 0.1 to 20 min after the formation of the first c onductive layer and the second conductive layer.

[Claim 11 ]

The method of claim 1 , wherein the opening is formed by photolithography, lase r etching or an etching paste method.

[Claim 12]

A solar cell, comprising:

a substrate;

an emitter layer formed on one side of the substrate;

an anti-reflective coating film formed on the emitter layer;

a front electrode penetrating the anti-reflective coating film to be connected to th e emitter layer and

a rear electrode formed on the other side of the substrate,

wherein, according to the method of any one of claims 1 to 11 , the front electro de includes: a first conductive layer formed in the opening of the anti-reflective coating fil m and directly connected with the emitter layer; and a second conductive layer formed o n the first conductive layer.