KR20150114451A - Lead-free electrically conductive composition for solar cell electrode - Google Patents

Abstract

The present invention relates to a glass frit composition including one or more of Na2O, Al2O3 and PbO as an additive but not containing CaO, wherein a main composition is Bi-Zn-B-Si-O. Furthermore, the invention discloses the glass frit composition including one or more of Ba2O3, ZrO, K2O2 and LiCl additionally. The glass frit composition has a low interfacial resistance value, and in particular, has outstanding etching characteristics to an anti-reflective layer when the anti-reflective layer is formed as a front electrode on a Si substrate coated with the anti-reflective layer regarding a solar cell and indicates great interfacial resistance characteristics by forming an outstanding ohmic contact with the Si substrate.

Description

TECHNICAL FIELD [0001] The present invention relates to a lead-free conductive composition for a solar cell electrode,

The present invention relates to a conductive composition for a solar cell electrode, and more particularly, to a lead-free conductive composition for a solar cell electrode having an excellent interface resistance property despite being lead-free.

In recent years, interest in renewable energy has been rapidly increasing, and research on Si solar cells that obtain energy using solar light has been accelerated. Such Si solar cells have been widely commercialized and widely used, but since efficiency is not so high yet, active research is under way to increase such efficiency.

Such a Si solar cell generally has a schematic structure as shown in Fig.

1, an ordinary Si solar cell includes an n-type Si layer 4, a light absorbing layer 3 and a p-type Si layer 2 sequentially laminated on the upper and lower surfaces of the stacked body, And the rear electrode 5 are formed. An antireflection film 6 is formed on the top surface of the front electrode 1.

In this structure, when sunlight reaches the light absorbing layer 3 through the antireflection film 6 and the p-type Si layer 2, an electron-hole pair is generated in the Si layer. The holes are accumulated in the p-type Si layer 2 and electrons are accumulated in the n-type Si layer 4 by the internal electric field formed by the p-type Si layer 2 and the n-type Si layer 4, respectively. When an external electric circuit is connected to the front electrode 1 and the rear electrode 5 respectively connected to the p-type Si layer 2 and the n-type Si layer 4, the electrons and holes flow into the circuit, Electric current flows through the electric circuit.

On the other hand, as the material of the front electrode 1, Ag paste is mainly used. This Ag paste requires a high electrical conductivity to minimize the resistance of the electrode after firing and a sufficient adhesion to the Si substrate is required to secure long-term reliability of the electrode. In order to maximize the light absorption area, it is necessary to be able to form a fine print pattern in a printing process and require a relatively large printing thickness. In order to satisfy this requirement, the Ag paste generally uses glass frit as a main material for securing adhesion between the Ag powder having excellent conductivity and the Si substrate, and further contains solvents and various organic additives to ensure good printing properties. do.

Particularly, the glass frit in the composition of the front electrode 1 is formed by etching the antireflection film 6 (generally SiN _x ) coated on the surface of the Si layer 2 to form the p-type Si layer 2 and the front electrode 1 ) Forms an ohmic contact with each other, thereby forming a circuit of the solar cell, increasing the efficiency and increasing the adhesion, thereby securing the reliability.

Such glass frit is mainly made of a Pb-based composition having a low melting point composition in consideration of the low possible heat treatment temperature in the solar cell manufacturing process. An example of this is disclosed in detail in Korean Patent Laid-Open No. 10-2011-52212 (published on Aug. 18, 2011). However, such a Pb-based glass frit composition contains a PbO composition and causes environmental pollution and is harmful to the human body, so its use tends to be restricted. Recently, a lead-free Bi-based composition has been developed to replace this.

The glass frit included in the composition of the front electrode (1) greatly influences the interface resistance with the Si layer and subsequently the power generation efficiency of the solar cell, although it is used in a very small amount. However, studies on the compositional properties of the glass frit to improve the interfacial resistance have not yet been sufficiently conducted, so that the optimum composition has not yet been developed through the verification.

Accordingly, the present invention is to provide a lead-free conductive composition for a solar cell electrode which is free from lead and has excellent interfacial resistance characteristics.

The lead-free conductive compositions of the present invention for achieving the objects, comprising a conductive powder and glass frit, the glass frit is Na ₂ O or of the main component consisting of _{Bi 2 O 3, ZnO, B} 2 O 3 and SiO ₂ comprising the first additive is an additive composition which is a PbO, when the first additive is a Na ₂ O the glass frit may further include a second additive selected from the group consisting of Ba ₂ O _3, K ₂ O ₂ and LiCl and , The glass frit does not contain CaO.

At this time, the main component of the glass frit is Bi ₂ O ₃ 50 to 90 wt%, ZnO 35 wt% or less, B ₂ O ₃ 5 to 20 wt%, SiO ₂ 10 wt% or less. The first additive may be added in an amount of 1 to 5 wt% based on the total amount of the glass frit.

The first additive may be Na ₂ O, and the glass frit may further include a second additive selected from the group consisting of Ba ₂ O ₃ , ZrO ₂ , K ₂ O _2, and LiCl. At this time, the second additive may be added in an amount of 1 to 3 wt% based on the total amount of the main components.

The glass frit may be contained in an amount of 10 wt% or less based on the conductive powder. The transition temperature (T _g ) of the glass frit may range from 350 to 550 ° C, and the melting temperature may range from 800 to 1200 ° C. The glass frit may have an average particle size ranging from 0.5 to 10 mu m.

In the present invention, the main composition is Bi-Zn-B-Si-O and Na ₂ O, Al ₂ O ₃ And PbO, wherein the glass frit composition does not contain CaO, and additionally at least one of Ba ₂ O ₃ , ZrO ₂ , K ₂ O ₂ and LiCl is added. These glass frit compositions have a low interfacial resistance value and exhibit excellent etching properties for the antireflection film and excellent ohmic contact with the Si substrate when formed as a front electrode on an Si substrate coated with an antireflection film, Thereby exhibiting excellent interfacial resistance characteristics.

1 is a schematic structural view of a general solar cell.
2 is a view for explaining a method of measuring an interface resistance between a front electrode and a Si substrate formed of an Ag paste according to the present invention.
3A to 3 D are electron micrographs and EDS analysis results of each front electrode formed of an Ag paste according to an embodiment of the present invention.
3A and 3B are front electrodes of the sample composition (G6) of Table 1, which is the main constituent without additives,
Figs. 3C to 3D show the front electrode by the sample composition (G6-Ca3) in which 3% Ca was added.

In the present invention, new compositions in which various additives are added to a Bi-based glass composition as a low melting point glass composition for obtaining an optimal glass frit composition for a front electrode of a solar cell are presented.

Accordingly, the glass frit according to the present invention is a glass frit having a main composition of Bi-Zn-B-Si-O and Na ₂ O, Al ₂ O ₃ And PbO, but may be a composition that does not contain CaO. The additive is preferably added in an amount of 1 to 5 wt% based on the total amount of the glass frit.

1, the glass frit in the front electrode 1, that is, the front electrode in the form of an Ag paste, is formed by etching the antireflection film 6 (SiN _x ) coated on the surface of the Si substrate 2, And functions to form an ohmic contact between the substrate 2 and the front electrode 1. However, as will be described in detail below, according to the present invention, when containing the CaO in the glass frit composition, the glass frit due to the influence of Ca instead of moving towards the Si substrate 2, but rather, by remaining in the Ag electrode (1), SiN _x And the result is that the ohmic contact can not be achieved. Thus, the interface resistance between the front electrode 1 and the Si substrate 2 increases sharply due to the CaO content.

Further, according to the present invention, when Ba is added to the main composition Bi-Zn-B-Si-O together with Na, Ba helps the etching effect of the antireflection film to further lower the interface resistance between the front electrode and the Si substrate .

The novel glass frit composition of the present invention as described above has an average particle size ranging from 0.5 to 10 mu m. In addition, such a glass frit composition is mixed with a conductive powder such as Ag to prepare an electrode paste. Preferably, such a glass frit composition may be contained in an amount of 10 wt% or less based on the conductive powder. The conductive powder may be any known conductive material such as Cu, Al, Pd including Ag, and the present invention is not limited to the Ag powder. The front electrode of the solar cell can be formed by printing this conductive paste on the surface of the Si substrate coated with the anti-reflection film (SiN _x ).

In addition, such an electrode paste composition may include a conventional organic vehicle solution to prevent drying of the paste during the printing process and to control fluidity, and the organic vehicle solution may include a solvent, a dispersant, and a binder. These solvents, dispersants and binders can be used as all known materials in the art, for example, α-terpineol and diethylene glycol monobutyl ether acetate as solvents and BYK-111 (BYK-111 , Germany), and ethyl cellulose may be used as the binder.

Casting

As described above, the present invention assumes that the main composition is Bi-Zn-B-Si-O. Particularly, in the present invention, B and Si are elements that form a network in the glass, but a transition temperature (T _g ) of the glass frit is preferably in the range of 350 to 550 ° C And the melting point is in the range of 800 to 1200 DEG C, it is preferable to reduce the amount of these used so that dense network structure can not be formed in the glass. Further, it is desirable to increase the amount of Bi, which can serve as a modifier for releasing the bond relatively, or the amount of ZnO whose role between them is unclear. For this purpose, as an embodiment of the present invention, glass frit has Bi ₂ O ₃ 50 to 90 wt%, ZnO 0 to 35 wt%, B ₂ O ₃ 5 to 20 wt%, and SiO ₂ And preferably 0 to 10 wt%.

Table 1 shows examples of the main composition of the present invention. The ratio of Bi and Zn to the sum of the Si content and the B content is set. Particularly, in the case where the Si content and the B content are fixed, The measured interfacial resistances for various samples varied.

These sample compositions can be prepared by any known production method including an oxide mixing method and the present invention is not limited thereto. If fixed, the content of Si and B as above, in the content ratio changes in a variety of Bi and Zn did not increase the glass transition temperature (T _g) may largely satisfy the basic requirements that must be a low melting point. The above composition samples having the compositions of Bi ₂ O ₃ , ZnO, B ₂ O ₃ and SiO ₂ under these conditions were prepared by melting the raw materials at a temperature of 900 ° C. or higher, rapidly quenching and pulverizing them, mixing them with Ag powder or the like, . Then, the Ag paste was printed on the Si substrate coated with the SiN _x layer as the front electrode, and the interfacial resistance between the front electrode and the Si substrate was measured for each composition. One detailed embodiment of this manufacturing and measuring process is as follows i) to iii):

i) First, Bi ₂ O ₃ , ZnO, B ₂ O ₃ and SiO ₂ (99.9%, Daejung chemical and metals Co. Ltd., Korea) were used as glass raw materials, Lt; / RTI > for 2 hours. Then, in order to obtain a glass having no crystallinity, 900 deg. C melting glass was quenched by using a cooling roller and dry-milled for 2 hours using? 10 zirconia balls. The glass frit thus obtained was filtered through # 63 sieve and wet milled for 24 hours to obtain amorphous glass frit, respectively.

ii) Using this glass frit, α-terpineol (99.5%, Daejung chemical and metals Co. Ltd., Korea) and diethylene glycol monobutyl ether acetate (98.0%, manufactured by Junsei chemical Co. Ltd., Japan) as a solvent. , 20 vol% of dispersant BYK-111 (BYK-chemie, Germany) and 24 vol% of ethyl cellulose (Acros Organics, USA) as a binder were weighed and dissolved in an oven at 100 ° C., The frit was added in an amount of 7.5 vol% based on the Ag powder, primary mixing was performed using a conditioning vacuum chamber (ARV-200, Thinky, Japan) and paste mixing was performed using a 3-roll mill to prepare a final Ag paste.

iii) The Ag paste thus prepared was screen-printed as a front electrode on a Si substrate coated with a SiN _x layer having a thickness of 80 nm as shown in Fig. 2, and then heat-treated at 750 to 800 ° C. After the heat treatment, the interfacial resistance between the front electrode and the Si substrate was measured using a digital multimeter.

The interfacial resistance values measured as above are shown in Table 1. As the addition amount of Bi ₂ O ₃ increases, the value of interfacial resistance tends to be lowered at 750 ° C. sintering. In the case of sintering at 800 ° C., Although it does not show a similar tendency, it can be seen that the interface resistance of G6 sample with 78wt% of Bi ₂ O ₃ is the lowest at 100 Ω in both sintering temperatures and is suitable as the glass frit composition for the front electrode of solar cell .

Casting + Additive

Table 2 shows the measured interfacial resistance values of various composition samples prepared by adding additives such as Na, Pb and Al to the above-mentioned main composition in order to improve the interfacial resistance characteristic.

In Table 2, the additives added to each composition are Na, which is the most commonly used modifier for general glass, Pb, which is widely used for low-temperature glass composition, and Ca and Al, which are commonly used for glass as di- and trivalent ions. Materials known in the field including NaHCO ₃ , PbO, CaCO ₃ , and Al ₂ O ₃ can be used as the raw materials, and the same manufacturing process and measurement process as those in Table 1 can be used. As a result, Value.

Referring to Table 2, it is observed that the Na-added sample composition (G6-Na) has the lowest measured interfacial resistance. Particularly, the sample composition (G6-Ca1 to G6-Ca3) to which Ca was added showed a sharp increase in interfacial resistance in the case of 3% Ca addition sample composition (G6-Ca3) It is observed that the interfacial resistance value is increased by about 10 times or more as compared with the case (G6).

In this regard, 3% Ca added to the sample composition (G6-Ca3) and each producing an Ag paste by the sample composition (G6) shown in Table 1 without addition of Ca and the anti-reflection film in such a Ag paste (SiN _x) with the coated Si It is necessary to examine how the addition of the glass frit composition of Ca affects the increase of the interface resistance in these two types of front electrodes when the front electrodes are printed and sintered on the substrate. 3A and 3B show electron microscope photographs and EDS analysis results of the front electrodes formed as described above. FIGS. 3A to 3B show the results of EDS analysis of the front electrodes formed by the sample composition (G6) , And Figs. 3C to 3 D are for the front electrode by the sample composition (G6-Ca3) in which 3% of Ca is added.

FIG. 3A shows that the sintering of the front electrode is performed relatively well. In addition, in FIG. 3B, when the line scan was performed from the front electrode surface toward the Si substrate, a large amount of Si was detected at the contact portion and the distribution of Ag was constant, indicating that Si was diffused from the Si substrate toward the front electrode .

On the other hand, in FIG. 3C, the sintering of the front electrode does not proceed sufficiently. The results of the EDS analysis in FIG. 3D show that when the line scan is performed from the surface of the front electrode toward the Si substrate, the amount of Ag is drastically reduced and a large amount of Bi and Si are detected. Since Bi is the main constituent of the glass, its detection means that the glass frit is present in a lump shape. In other words, although the same amount of glass frit composition is contained in the Ag paste, the glass frit does not move toward the Si substrate due to the effect of Ca, but rather remains in the Ag electrode, so that the SiN _x layer is not etched, resulting in an ohmic contact . That is, when Ca is added to the glass frit composition, the interface resistance rapidly increases.

Table 3 below also shows the interfacial resistance values of the glass frit compositions to which an additional additive is added to the Na addition composition (G6-Na) of Table 2 having the lowest interface resistance.

The materials of the additional additive composition in Table 3 can be materials known in the art including, for example, Ba ₂ O ₃ , ZrO ₂ , K ₂ O ₂ and LiCl, and Table 3 shows the same manufacturing process as in Tables 1 and 2 The measurement was carried out to obtain the interfacial resistance values for each composition. The addition amount of the additive in Table 3 is preferably 1 to 3 wt% based on the total amount of the main components.

Referring to Table 3, it is observed that when Ba is further added, the interface resistance is lower than the interface resistance obtained by adding Na. It is considered that Ba has obtained a low interfacial resistance by widening the adhesion area between Ag paste and Si substrate by etching the antireflection film coated on Si substrate like Na.

On the other hand, in the case of the Ag paste to which Zr, K, and Li are further added, the interface resistance slightly increases, but all of them can be used as a glass frit composition.

In the above-described embodiments and examples of the present invention, the powder characteristics such as the average particle size, distribution and specific surface area of the composition powder, and the purity of the raw material, the amount of the impurity added, and the heat treatment conditions, It is quite natural for a person of ordinary skill in the field to have such a possibility.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the present invention and the advantages thereof, , Changes, additions, and the like are to be regarded as falling within the scope of the claims.

Claims (12)

A lead-free conductive composition comprising a conductive powder and a glass frit,
Wherein the glass frit comprises a composition comprising a main component consisting of Bi ₂ O ₃ , ZnO, B ₂ O ₃ and SiO ₂ to which a first additive consisting of Na ₂ O or PbO is added, wherein the first additive is Na ₂ O the glass frit further comprises a second additive selected from the group consisting of _{Ba 2 O 3, K 2 O} 2 , and LiCl,
Wherein the glass frit does not contain CaO. The method according to claim 1,
Wherein the main component of the glass frit has the following content.
Bi ₂ O ₃ 50 to 90 wt%;
ZnO 35% wt% or less;
B ₂ O ₃ 5 to 20 wt%; And
10 wt% or less of SiO ₂ . 3. The method according to claim 1 or 2,
Wherein the first additive is added in an amount of 1 to 5 wt% based on the total amount of the glass frit. The method according to claim 1,
Wherein the second additive is added in an amount of 1 to 3 wt% based on the total amount of the main component. The method according to claim 1,
Wherein the glass frit is contained in an amount of 10 wt% or less based on the conductive powder. The method according to claim 1,
Wherein the lead-free conductive composition further comprises an organic vehicle solution. The method according to claim 1,
Transition temperature of the glass frit (T _g) is a lead-free conductive compositions, characterized in that in the range 350 ~ 550 ℃. The method according to claim 1,
Wherein the melting temperature of the glass frit is in the range of 800 to 1200 占 폚. The method according to claim 1,
Wherein the glass frit has an average particle size ranging from 0.5 to 10 mu m. The method according to claim 1,
Wherein the conductive powder is at least one of Ag, Cu, Al and Pd. The method according to claim 6,
Wherein the organic vehicle solution comprises a solvent and a dispersant. A method according to claim 1 or 2, wherein the lead-free conductive composition is used as a composition of a front electrode of a solar cell.

Images