WO2016137059A1

WO2016137059A1 - Silver paste composition, front electrode for solar cell formed using it, and solar cell employing it

Info

Publication number: WO2016137059A1
Application number: PCT/KR2015/005870
Authority: WO
Inventors: Jin Kwon Lee; Sung Koo Kang; Jin Hyun Kim; Ji Myung Shim; Ji Hyun Kim; Young Jun Song; Joon Geol Park; Hye Sung Lee
Original assignee: Dae Joo Electronic Materials Co Ltd
Current assignee: Dae Joo Electronic Materials Co Ltd
Priority date: 2015-02-27
Filing date: 2015-06-11
Publication date: 2016-09-01
Anticipated expiration: 2017-08-27
Also published as: CN107258002A; KR101693841B1; TWI556457B; CN107258002B; TW201631787A; KR20160104917A

Abstract

Provided are a silver paste composition for a front electrode for a solar cell, a front electrode for a solar cell formed using it, and a solar cell employing it. The silver paste composition of the present invention has excellent adhesion to a substrate and reduced contact resistance to an anti reflection coating, thereby manufacturing a highefficiency solar cell.

Description

SILVER PASTE COMPOSITION, FRONT ELECTRODE FOR SOLAR CELL FORMED USING IT, AND SOLAR CELL EMPLOYING IT

The present invention relates to a silver paste composition, a front electrode for a solar cell formed using it, and a solar cell employing it.

A solar cell which is a battery generating electric energy from solar energy has a long life because the solar energy which is an eco-friendly energy source is infinite. Meanwhile, the depletion of traditional energy sources like petroleum or coal is predicted, and as a result the solar cell has recently received most attention as an energy source for replacing these traditional energy sources.

The solar cell is largely divided a silicon solar cell, a compound semiconductor solar cell, a tandem solar cell depending on a source material. Among these, the silicon solar cell is a mainstream.

The silicon solar cell includes a silicon wafer configured of an anti reflection coating (ARC) which makes light absorb good while reducing light reflection, a silicon wafer configured of a p/n junction, an emitter, and a base, and a front electrode and a rear electrode inducing electricity generated by the light into external circuits. Both electrodes are formed by alternately carrying out printing and drying in an order of a bus bar, rear aluminum, and front silver (Ag) paste and then performing co-firing within a temperature ranging from 600 to 950 ℃.

The front electrode of the silicon solar cell is formed by an interfacial reaction of metal paste for forming the front electrode with the anti reflection coating. In this case, the silver included in the metal paste contacts an emitter layer by a punch through phenomenon penetrating through the anti reflection coating via glass frit while being in a liquid state at high temperature and then being again re-crystallized to a solid state.

The glass frit generates the interfacial reaction with the anti reflection coating to etch the anti reflection coating and some of elements of the glass frit are reduced by an oxidation-reduction reaction to be generated as products. The existing glass frit uses lead oxide (PbO) as a main component and therefore is subjected to the interfacial reaction and then the lead is reduced, which causes environmental problems.

To solve the above problems, lead-free glass frit using bismuth oxide (Bi₂O₃) instead of the lead oxide has been introduced. However, the bismuth oxide-based glass frit may have a lower contact resistance between the electrode and a substrate than the existing glass frit including the lead oxide.

Therefore, glass frit which may manufacture a solar cell which is more eco-friendly and has more excellent performance than the existing solar cell needs to be urgently developed.

[Related Art Document]

[Patent Document]

Korean Patent No. 10-1276671

An object of the present invention is to provide a silver paste composition for a front electrode for a solar cell capable of forming the front electrode for the solar cell.

Another object of the present invention is to provide a front electrode for a solar cell formed by using a silver paste composition for a front electrode for a solar cell.

Still another object of the present invention is to provide a solar cell employing a front electrode for a solar cell.

The present invention provides a silver paste composition for a front electrode for a solar cell having excellent etching ability to generate etching within a wide range to expand a region of the front electrode, have low contact resistance, and have high conversion efficiency.

In one general aspect, a silver paste composition includes: (a) silver powder; (b) lead-free glass frit including Bi₂O₃, TeO₂, and V₂O₅; and (c) an organic vehicle, wherein the glass frit does not include lead (Pb), in particular, PbO.

The glass frit may include at least one selected from the group consisting of SiO₂, ZnO, Li₂O, B₂O₃, Al₂O₃, CuO, Na₂O, ZrO₂, MgO, P₂O₅, CaO, BaO, SnO, SrO, K₂O, TiO₂, and MnO₂.

The glass frit may include 40 to 80 wt% of Bi₂O₃, 1 to 30 wt% of TeO₂, and 1 to 30 wt% of V₂O₅.

The glass frit may include 40 to 80 wt% of Bi₂O₃, 1 to 30 wt% of TeO₂, and 1 to 30 wt% of V₂O₅, 0.1 to 10 wt% of ZnO, 0.1 to 15 wt% of SiO₂, 0.1 to 10 wt% of Li₂O, and 0.1 to 10 wt% of B₂O₃.

The glass frit may be included in 0.1 to 15 wt% for the silver paste composition.

The silver powder may have a glass transition temperature which is equal to or less than 420 ℃ and a melting point which is equal to or less than 550 ℃.

The organic vehicle may be prepared by adding at least one selected from the group consisting of cellulose resins such as ethyl cellulose, methyl cellulose, nitrocellulose, and cellulose esters, acrylic resins such as rosin or alcoholic polymethacrylate, and acrylic ester, and polyvinylbased resins such as polyvinyl alcohol and polyvinyl butyral to at least one selected from the group consisting of trimethyl pentanyl diisobutylate (TXIB), dibasic ester, butyl carbitol (BC), butyl carbitol acetate, butyl carbitol, butyl cellosolve, butyl cellosolve acetate, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dimethyl adipate, dimethyl glutarate, propylene glycol monomethyl ether propionate, ethyl ether propionate, terpineol, propylene glycol monomethyl ether acetate, dimethyl amino formaldehyde, methyl ethyl ketone, gamma butyrolactone, ethyl lactate, and Texanol.

A front electrode for a solar cell is formed of the silver paste composition for the front electrode for the solar cell according to the exemplary embodiment of the present invention.

A solar cell employing the front electrode for the solar cell according to the exemplary embodiment of the present invention is provided.

According to the exemplary embodiments of the present invention, the silver paste composition has the low contact resistance with the anti reflection coating while having the excellent etching ability, and as a result the solar cell employing the front electrode for a solar cell formed using the silver paste composition may have the high efficiency.That is, the silver paste composition according to the exemplary embodiment of the present invention may improve the adhesion between the substrate and the front electrode, have the excellent etching ability to perform the etching within a wide range so as to expand the front electrode area which is formed by recrystallizing silver, and have the low contact resistance with the anti reflection coating and the high open circuit voltage to improve the efficiency of the solar cell.

The present invention provides a silver paste composition for a front electrode for a solar cell. The silver paste composition according to the exemplary embodiment of the present invention includes:

(a) silver powder;

(b) lead-free glass frit including Bi₂O₃, TeO₂, and V₂O₅; and

(c) an organic vehicle.

The silver paste composition according to the exemplary embodiment of the present invention has excellent etching ability and is ecofriendly without PbO, by including the Bi₂O₃ instead of including lead, in particular, PbO.

Further, the silver paste composition according to the exemplary embodiment of the present invention more improves the etching ability while increasing adhesion between a substrate and a front electrode and lowers a contact resistance with the anti reflection coating to increase an open circuit voltage, by including TeO₂ and V₂O₅ as well as the Bi₂O₃.

The glass frit according to the exemplary embodiment of the present invention may include 40 to 80 wt% of Bi₂O₃, 1 to 30 wt% of TeO₂, and 1 to 30 wt% of V₂O₅.

To lower the contact resistance with the anti reflection coating while having the higher etching ability, the glass frit according to the exemplary embodiment of the present invention preferably may further include at least one selected from the group consisting of SiO₂, ZnO, Li₂O, B₂O₃, Al₂O₃, CuO, Na₂O, ZrO₂, MgO, P₂O₅, CaO, BaO, SnO, SrO, K₂O, TiO₂, and MnO₂, and more preferably may further include ZnO, SiO₂, Li₂O, and B₂O₃, and the glass frit may include 40 to 80 wt% of Bi₂O₃, 1 to 30 wt% of TeO₂, and 1 to 30 wt% of V₂O₅, 1 to 10 wt% of ZnO, 0.1 to 10 wt% of SiO₂, 0.1 to 10 wt% of Li₂O, and 0.1 to 10 wt% of B₂O₃.

To increase conversion efficiency, the glass frit according to the exemplary embodiment of the present invention may preferably include Bi₂O₃, TeO₂, V₂O₅, ZnO, SiO₂, Li₂O, and B₂O₃. In more detail, the glass frit may include 40 to 70 wt% of Bi₂O₃, 5 to 20 wt% of TeO₂, 1 to 20 wt% of V₂O₅, 1 to 10 wt% of ZnO, 3 to 10 wt% of SiO₂, 0.5 to 5 wt% of Li₂O, and 1 to 8 wt% of B₂O₃.

The silver (Ag) powder according to the exemplary embodiment of the present invention is conductive metal which gives electrical characteristics to the silver paste composition for forming the front electrode for the solar cell and also includes silver oxide, silver alloy, silver compound, and a material which may educe the silver powder by other firing in addition to pure silver powder, in which the material may be used alone or a mixture of at least two thereof may be used.

A shape of the silver powder may be applied without being specifically limited as long as the silver powder has a shape known to the technical field of the present invention. For example, the silver powder may include, but not limited to, a spherical shape, a flake shape, or a combination thereof. A gain size of the silver powder may be controlled to a proper range in consideration of a desired sintering speed, an effect in a process of forming an electrode, etc. An average grain size d50 of the silver powder according to the exemplary embodiment of the present invention may have, but not limited to, a size ranging from about 0.5 to 4.0 ㎛.

The silver powder of which the purity is equal to or more than 80%, preferably, 95% or more may be used, but the silver powder is not specifically limited as long as it has purity for meeting conditions commonly required for the electrode.

The silver powder may be included at about 60 to 95 wt% for every 100 wt% of silver paste composition, preferably, about 70 to about 85 wt% at which the viscosity of paste is low or the phase separation is made, in consideration of an economical aspect without causing poor printability.

The silver powder may be included at about 80 to 99 wt% for every 100 wt% of solid content, among the solid contents from which an organic solvent is excluded in the silver paste composition.

The glass frit according to the exemplary embodiment of the present invention may be included at about 0.1 to 15 wt% for the silver paste composition.

The glass frit according to the exemplary embodiment of the present invention may preferably have a glass transition temperature which is equal to or less than 420 ℃ and a melting point which is equal to or less than 550 ℃.

That is, the silver paste composition according to the exemplary embodiment of the present invention includes 60 to 95 wt% of silver powder for every 100% of silver paste composition, 0.1 to 15 wt% of glass frit, and the balance of organic vehicle.

As the organic vehicle according to the exemplary embodiment of the present invention, the organic vehicle used in the electrode paste for the solar cell may commonly be used and may be, for example, a mixture of polymer and a solvent. Preferably, the organic vehicle is prepared by adding at least one selected from the group consisting of cellulose resins such as ethyl cellulose, methyl cellulose, nitrocellulose, and cellulose esters, acrylic resins such as rosin or alcoholic polymethacrylate, and acrylic ester, and polyvinylbased resins such as polyvinyl alcohol and polyvinyl butyral to at least one selected from the group consisting of trimethyl pentanyl diisobutylate (TXIB), dibasic ester, butyl carbitol (BC), butyl carbitol acetate, butyl carbitol, butyl cellosolve, butyl cellosolve acetate, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dimethyl adipate, dimethyl glutarate, propylene glycol monomethyl ether propionate, ethyl ether propionate, terpineol, propylene glycol monomethyl ether acetate, dimethyl amino formaldehyde, methyl ethyl ketone, gamma butyrolactone, ethyl lactate, and Texanol.

To easily disperse the silver powder and prevent the conversion efficiency of the solar cell from deteriorating due to the increase in resistance by the residual carbon after firing, the organic vehicle according to the exemplary embodiment of the present invention may be preferably included at a range between 4 wt% to 35 wt% for every 100 wt% of composition.

The silver paste composition according to the exemplary embodiment of the present invention may further include organic additives, in which the organic additives may be selected from a dispersant, a thickener, a thixotropic agent, a leveling agent, and the like. The dispersant may include, but not limited to, SOLSPERSE made by LUBRISOL Co., DISPERBYK180, 110, 996, and 997, etc. made by BYK Co., The thickener may include, but not limited to, BYK410, 411, 420, etc. made by BYK Co. The thixotropic agent may include, but not limited to, THIXATROL MAX made by ELEMENTIS Co., and ANTITERRA203, 204, 205, etc. made by BYK Co. The leveling agent may include, but not limited to, BYK3932 P, BYK378, BYK306, BYK3440, etc. made by BYK Co. The organic additives may be included at about 1 to 20 wt% for every 100 wt% of silver paste composition.

Further, the present invention provides the front electrode for the solar cell formed by using the silver paste composition.

The front electrode is formed by a process of printing, drying, and firing the silver paste composition on the wafer substrate. The printing method may use screen printing, etc., but is not specifically limited.

Further, the present invention is to provide the solar cell including the front electrode for the solar cell.

The solar cell according to the exemplary embodiment of the present invention includes a first conductivity type substrate, a second conductivity type emitter layer formed on the substrate, an anti reflection coating formed on the emitter layer, a front electrode penetrating through the anti reflection coating to be connected to the emitter layer and manufactured using the silver paste composition and a rear electrode formed on a rear surface of the substrate.

The first conductivity type substrate may be selected from a P type or an N type. The second conductivity type emitter layer is selected to have an opposite conductivity type to the substrate. To form a P+ layer, group 3 elements are doped with impurities and to form an N+ layer, group 5 elements are doped with impurities. For example, to form the P+ layer, B, Ga, and In may be doped and to form the N+ layer, P, As, and Sb may be doped. The PN junction is formed at the interface between the substrate and the emitter layer and is a part which receives sunlight to generate a current using a photovoltaic effect. Electrons and holes generated by the photovoltaic effect are each attracted to the P layer and the N layer and thus move to each of the electrodes bonded to the lower portion of the substrate and the upper portion of the emitter layer and electricity may be generated by applying a load to the electrodes.

The anti reflection coating reduces reflectance of sunlight incident on the front surface of the solar cell. When the reflectance of the sunlight is reduced, a light quantity reaching the PN junction is increased, and thus a shortcircuit current of the solar cell is increased and the conversion efficiency of the solar cell is improved.

The anti reflection coating may have, for example, a single layer of any one selected from a silicon nitride film, a silicon nitride film including hydrogen, a silicon oxide film, and a silicon oxynitride film or a multilayer of a combination of at least two thereof, but is not limited thereto.

The front electrode and the rear electrode may be manufactured by known various technologies, but are preferably formed by a screen printing method. The front electrode is formed by screenprinting the silver paste composition according to the present invention on the front electrode forming point and heattreating the silver paste composition. When the heat treatment is performed, the front electrode punches the anti reflection coating by a punch through phenomenon and thus contacts the emitter layer.

The rear electrode is formed by printing the paste composition including aluminum as the conductive metal on a rear surface of the substrate and then heattreating the paste composition. At the time of heattreating the rear electrode, the aluminum is diffused through the rear surface of the substrate, and thus a backside field layer may be formed at a boundary surface between the rear electrode and the substrate. When the backside field layer is formed, carriers may be prevented from being recombined by moving to the rear surface of the substrate, thereby improving the conversion efficiency of the solar cell.

Hereinafter, the detailed embodiment of the present invention will be described in detail. Terms and words used in the present specification and claims are not to be construed as a general or dictionary meaning but are to be construed as meaning and concepts meeting the technical ideas of the present invention based on a principle that the inventors can appropriately define the concepts of terms in order to describe their own inventions in best mode.

[Examples 1 to 11] Preparation of glass frit

Components described in the following Table 1 were mixed at an indicated ratio (wt%), melted at 1100 ℃ for 30 minutes, quenched at pure water (H₂O), and then rapidly cooled. The rapidly cooled glass melt was pulverized by a ballmill pulverizer to prepare the glass frit having an average grain size of 1 to 3 ㎛.

The components and contents of the glass frit corresponding to each embodiment were shown in the following Table 1.

Table 1

Glass Component	Example
Glass Component	1	2	3	4	5	6	7	8	9	10	11
TeO₂	2	1	1	20	20	10.5	8	8	8	8	8
SiO₂	5	7	7	7	7	7	6.3	3.7	7.5	6.8	7.5
ZnO		4	4	4	4	4	6.4	7.5	6.8	7.5	3.8
Bi₂O₃	69	59	78	59	40	59	65	65	65	65	65
Li₂O	4	4	4	4	4	4	2.3	3.8	0.7	0.7	3.7
V₂O₅	20	20	1	1	20	10.5	7	7	7	7	7
PbO
B₂O₃		5	5	5	5	5	5	5	5	5	5

[Comparative Examples 1 to 2] Preparation of glass frit

Comparative Examples 1 to 2 prepared the glass frit by the same method as Examples 1 to 11, except for the components and contents described in the following Table 2 in Examples 1 to 11 were used.

Table 2

Glass Component	Comparative Example 1	Comparative Example 2
TeO₂	31	49
SiO₂	7	0.5
ZnO	5
Bi₂O₃		6.5
Li₂O	3	1.5
V₂O₅
PbO	52	42
B₂O₃	2	0.5

[Examples 12 to 22 and Comparative Examples 3 and 4] Preparation of paste composition

The silver paste compositions were each prepared using the glass frit prepared according to Examples 1 to 11 and Comparative Examples 1 and 2.

Three kinds of silver powders were selected based on the average grain size and a combination of the silver powders having 0.3 ㎛ and 2 wt%, 1.6 ㎛ and 43.5 wt%, and 2.2 ㎛ and 43.5 wt% was used. The glass frit used 2 wt% of compositions of Examples 1 to 11 which are shown in the above Table 1 and Comparative Examples 1 and 2. As an organic binder, cellulose ester (CAB38220 made by EASTMAN Co.) and ethyl cellulose resin (ECN50 made by AQUALON Co.) were each used at 1 wt%, as an organic solvent, 2.0 wt% of trimethyl pentanyl diisobutylate (TXIB), 3.0 wt% of dibasic ester (mixture of dimethyl adipate /dimethyl glutarate/ dimethyl succinate made by TCI Co.), 1.0 wt% of butyl carbitol(BC) were used, and as an additive, 0.5 wt% of thixotropic adjuster (THIXATROL MAX made by ELEMENTIS Co.) and 0.5 wt% of dispersant (SOLSPERSE made by LUBRISOL Co.) were added, thereby preparing the silver paste composition.

[Examples 23 to 33 and Comparative Examples 5 and 6] Manufacturing of solar cell.

For the manufacturing of the solar cell, a single crystal silicon wafer of 156 ㎜ was used and the emitter layer having a sheet resistance of 100 Ω/sq was formed by doping phosphorous (P) by a diffusion process using POCl₃ at 810 ℃ in a tube furnace and the anti reflection coating was formed at a thickness of 70 ㎚ by depositing the silicon nitride film on the emitter layer by plasma enhanced chemical vapor deposition (PECVD) using precursors SiH₄ and NH₃.

The aluminum powder having a size of 3 ㎛ and the leadfree glass, ethyl cellulose (STD10 made by DOW Co.) as the organic binder, and terpineol as the solvent were used. 75 wt% of aluminum powder (JINHAO Co.) and 2 wt% of leadfree glass powder were mixed with 23 wt% of organic binder and then the mixture was dispersed at an agitation speed of 2000 rpm for 4 hours, thereby preparing the aluminum paste. The aluminum paste was coated on the rear surface of the substrate, which is formed with the anti reflection coating, at a thickness of 30 ㎛ by the screen printing method and then the substrate coated with the aluminum paste was dried for 60 seconds in a drying furnace of 250 ℃. The front electrode was manufactured by coating the silver paste composition prepared in Examples 12 to 22 of the present invention and Comparative Examples 3 and 4 at a thickness of 20 ㎛ by the screen printing method and then drying it for 60 seconds in the drying furnace of 200 ℃.

The substrate of which the front electrode is printed was fired for 1 minute in a belt firing furnace of 820 ℃ to manufacture the solar cell.

Electrical performance of the acquired solar cell was measured using a cell simulator to which a light source made by Oriel Co., is applied. The performance of the solar cell was measured under a 1.0 sun condition and the measured values were represented by the photo conversion efficiency (%) of the following Table 3.

Table 3

	Conversion Efficiency	Contact resistance
Example 1	1.000	0.992
Example 2	1.011	0.943
Example 3	1.016	0.894
Example 4	1.038	0.846
Example 5	1.027	0.854
Example 6	1.022	0.878
Example 7	1.032	0.821
Example 8	1.038	0.805
Example 9	1.016	0.927
Example 10	1.022	0.911
Example 11	1.016	0.886
Comparative Example 1	1.000	1.000
Comparative Example 2	1.005	0.984

Claims

A silver paste composition for a front electrode for a solar cell, comprising:

(a) silver powder;

(b) leadfree glass frit including Bi₂O₃, TeO₂, and V₂O₅; and

(c) an organic vehicle.
The silver paste composition of claim 1, wherein the glass frit includes at least one selected from the group consisting of SiO₂, ZnO, Li₂O, B₂O₃, Al₂O₃, CuO, Na₂O, ZrO₂, MgO, P₂O₅, CaO, BaO, SnO, SrO, K₂O, TiO₂, and MnO₂.
The silver paste composition of claim 1, wherein the glass frit includes 40 to 80 wt% of Bi₂O₃, 1 to 30 wt% of TeO₂, and 1 to 30 wt% of V₂O₅.
The silver paste composition of claim 1, wherein the glass frit includes 40 to 80 wt% of Bi₂O₃, 1 to 30 wt% of TeO₂, and 1 to 30 wt% of V₂O₅, 0.1 to 10 wt% of ZnO, 0.1 to 15 wt% of SiO₂, 0.1 to 10 wt% of Li₂O, and 0.1 to 10 wt% of B₂O₃.
The silver paste composition of claim 1, wherein the glass frit has a glass transition temperature which is equal to or less than 420 ℃ and a melting point which is equal to or less than 550 ℃.
The silver paste composition of claim 1, wherein the glass frit is included in 0.1 to 15 wt% for the silver paste composition.
The silver paste composition of claim 1, wherein the organic vehicle is prepared by adding at least one selected from the group consisting of cellulose resins, acrylic resins, and polyvinylbased resins to at least one selected from the group consisting of trimethyl pentanyl diisobutylate (TXIB), dibasic ester, butyl carbitol (BC), butyl carbitol acetate, butyl carbitol, butyl cellosolve, butyl cellosolve acetate, propylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dimethyl adipate, dimethyl glutarate, propylene glycol monomethyl ether propionate, ethyl ether propionate, terpineol, propylene glycol monomethyl ether acetate, dimethyl amino formaldehyde, methyl ethyl ketone, gamma butyrolactone, ethyl lactate, and Texanol.
A front electrode for a solar cell formed of the silver paste composition for the front electrode for the solar cell of any one of claims 1 to 7.
A solar cell employing the front electrode for the solar cell of claim 8.