TW201500313A - Solar cell and solar cell forming paste composition for solar cell - Google Patents

Abstract

The present invention provides a paste composition for forming an aluminum electrode capable of achieving excellent electrical characteristics. The paste composition provided by the present invention contains an aluminum powder, a glass frit and an organic vehicle, and the glass frit has the following conditions: (1) In terms of the molar ratio converted to an oxide, substantially consists of: SiO2: 5 to 40 mol%; B2O3: 5 to 40 mol%; ZnO: 10 to 50 mol%; Bi2O3: 5 to 45 mol%; and alkaline earth Metal-like oxide: 0 to 25 mol%; (2) When the total amount of the glass frit is 100 mol%, the ratio of the molar content of MZn of ZnO to the molar content of Bi2O3 (MZn/MBi) is 0.8 or more and 2.5. the following.

Description

Solar cell and solar cell forming paste composition for solar cell Technical field

The present invention relates to a solar cell (cell) and a paste composition therefor. More specifically, it relates to a paste composition for forming an aluminum electrode on the back side of a light receiving surface of a solar cell.

The present invention claims the priority of Japanese Patent Application No. 2013-096933, filed on May 2, 2013, the entire content of which is hereby incorporated by reference.

Background technique

A typical example of a solar cell that converts solar light energy into electric power is a solar cell in which a crystalline germanium (single crystal or polycrystal) is used as a substrate, and a so-called crystalline germanium solar cell. As the above-described crystalline lanthanide solar cell, for example, a single-sided light-receiving solar cell 10 as shown in Fig. 1 is known.

The solar cell 10 includes an n-Si layer 16 on one surface (light-receiving surface side) of the p-type germanium substrate (Si wafer) 11, and an anti-reflection film 14 and a light-receiving surface electrode (surface) on the surface of the n-Si layer. Electrode) 12. Further, a p ⁺ layer 24 is provided on the other surface (back surface side) of the substrate 11 , and an aluminum electrode 20 and an external connection electrode 22 are provided on the surface of the p ⁺ layer.

Typically, the aluminum electrode 20 is formed by applying (coating) a paste containing aluminum powder (including an ink-like or slurry-like form, the same applies hereinafter) to the ruthenium substrate 11 and firing it. . At the time of the firing, an Al-Si alloy layer (not shown) is formed on the interface between the p-type germanium substrate 11 and the aluminum electrode 20, and the aluminum is diffused into the p-type germanium substrate 11 (p-Si layer 18). Forming a p ⁺ layer 24. By the p ⁺ layer 24, that is, the BSF (Back Surface Field) layer, recombination of the light generating carrier can be prevented, and electrical characteristics (for example, light conversion efficiency) (so-called BSF effect) can be improved.

As a conventional technique related to this, various kinds of paste compositions for forming an aluminum electrode are disclosed in Patent Documents 1 to 4. For example, Patent Document 1 discloses a paste composition using a glass of a Bi ₂ O ₃ main body.

Advanced technical literature Patent literature

[Patent Document 1] Japanese Patent Application Publication No. 2010-222238

[Patent Document 2] Japanese Patent Application Publication No. 2008-159917

[Patent Document 3] Japanese Patent Application Publication No. 2013-504199

[Patent Document 4] Japanese Patent Application Publication No. 2007-081059

Summary of invention

However, according to the review by the inventors of the present invention, in the case of using the paste composition of the prior art, it is difficult to advance during firing (when the BSF layer is formed). The reaction of the substrate with aluminum is performed, or the reaction is excessively generated locally so that the Al-Si alloy layer is thickened and agglomeration of bubbles or Al occurs. As a result, the BSF layer becomes an inhomogeneous layer and has insufficient electrical properties or durability (for example, water resistance).

The present invention has been made in view of the above circumstances, and an object thereof is to provide a paste composition for forming an aluminum electrode which can exhibit excellent electrical characteristics for a long period of time.

Further, another related object is to provide a solar battery including an aluminum electrode formed using the paste composition.

The paste composition provided by the present invention (i.e., a composition prepared into a paste) is an aluminum electrode for forming a solar cell. The paste composition contains aluminum powder, a glass frit, and an organic vehicle. Further, the glass frit has the following conditions (1) and (2).

(1) The molar ratio converted to an oxide is substantially composed of SiO ₂ : 5 to 40 mol %; B ₂ O ₃ : 5 to 40 mol%; ZnO: 10 to 50 mol%; ₂ O ₃ : 5 to 45 mol%; and at least one of MgO, CaO, SrO and BaO: 0 to 25 mol%.

(2) so that all of the glass frit 100mol%, the molar content ratio of ZnO and 2.5 M _{Zn Bi} with respect to the ratio of M (M _Zn / M _Bi) the Bi ₂ O ₃ mole ratio of 0.8 or more comprising the following.

By using the paste composition (aluminum paste) of the foregoing configuration, the interfacial reaction between aluminum and ruthenium can be uniformly and appropriately produced. Therefore, generation of bubbles or Al aggregation can be greatly suppressed, and a homogeneous BSF layer can be formed on the back side of the tantalum substrate. Therefore, the electrical characteristics of the solar cell can be greatly improved (for example, light conversion efficiency) Rate, open voltage or fill factor). In addition, since the aluminum electrode also has excellent adhesion strength and durability (for example, water resistance), even when moisture penetrates into the interior of the battery unit, the electrical characteristics can be maintained in a good range and can be stabilized for a long period of time. use. In this way, by using the paste composition of the above configuration, an aluminum electrode capable of achieving both electrical characteristics and durability (for example, water resistance) at a high level can be realized.

Further, regarding the glass frit constituting the paste composition, the term "substantially constituted" includes a term consisting of only the main oxide component; and a molar ratio converted into an oxide. The content of 7 mol% or less (preferably 5 mol% or less, and 4 mol% or less) of the entire glass is contained in the subcomponent other than the main oxide component.

In a preferred embodiment, the glass frit is substantially composed of the following composition in terms of a molar ratio converted to an oxide, namely: SiO ₂ : 10 to 35 mol %; B ₂ O ₃ : 10 to 30 mol %; ZnO: 20 to 40 mol%; Bi ₂ O ₃ : 10 to 40 mol%; and at least one of MgO, CaO, SrO, and BaO: 0.1 to 20 mol%.

By using the glass frit having such a composition, the characteristics (for example, the aforementioned electrical characteristics, the subsequent strength, and the durability) of the aluminum electrode formed of the paste composition can be further improved, and higher reliability can be achieved.

In a preferred aspect, the glass frit does not contain Pb and an alkali component. That is, at least the component is not intended to be added to the above-mentioned glass frit (a case where, for example, an unavoidable impurity is allowed to be introduced). By using a Pb-free glass frit, the agglomeration of bubbles or Al can be further reduced, and a more homogeneous aluminum electrode can be formed. Moreover, water resistance or resistance can be improved by using an alkali-free glass frit. Pharmaceutical properties (eg alkali resistance). In addition to this, the coefficient of thermal expansion can be suppressed to be lower. This is also desirable from the standpoint of integration with the thermal expansion coefficient of the tantalum substrate.

In a preferred embodiment, the softening point (starting softening point) of the glass frit is 420 ° C or higher and 620 ° C or lower. The interfacial reaction between aluminum and ruthenium can be promoted by using a glass frit having a softening point in such a temperature range. Therefore, a homogeneous BSF layer can be stably formed, and an aluminum electrode excellent in electrical characteristics such as light conversion efficiency, open voltage, or fill factor can be formed. In addition, the softening point of the glass frit can be determined by a general differential thermal-thermal weight measurement (TG-DTA; Thermogravimetry-Differential Thermal Analysis).

In a preferred embodiment, when the entire paste composition is 100% by mass, the content of the glass frit is 0.01% by mass to 1% by mass. In the range in which the BSF effect can be sufficiently enjoyed as described above, the content of the glass frit is reduced as compared with the prior art, whereby the occurrence of bubbles or the decrease in water resistance can be further suppressed. Further, since the glass frit exhibits insulation, the electrical conductivity of the aluminum electrode can be improved by reducing it.

In a preferred embodiment, the organic vehicle contains a resin (for example, an acrylic resin) having a decomposition temperature (decomposition end temperature) of 400 ° C or less as an organic binder. Since such a binder has good degreasing characteristics (that is, it is easily burned through during firing), it is possible to prevent excessive reaction of bismuth with aluminum under the influence of heat absorption during burn-through. Therefore, by using a paste composition containing the above glass frit and the binder, a more homogeneous BSF layer can be formed, and it can be adapted to achieve more excellent (for example, at least one of electric properties, durability, and strength). ) aluminum electrode. In addition, the decomposition temperature of the organic binder can be used in general. The differential thermal-thermal weight simultaneous measurement (TG-DTA) was used for the measurement.

As described above, an aluminum electrode excellent in electrical characteristics and durability can be formed by using any of the paste compositions (aluminum paste) disclosed herein. Therefore, according to the present invention, a solar cell having the aluminum electrode disclosed herein can be provided. The solar cell described above is excellent in, for example, a light conversion efficiency or a fill factor and is excellent in electrical characteristics, durability, or reliability.

10‧‧‧Solar battery

11‧‧‧p-type germanium substrate (Si wafer)

12‧‧‧Photometric surface electrode (surface electrode)

14‧‧‧Anti-reflection film

16‧‧‧n-Si layer (n ⁺ layer)

18‧‧‧p-Si layer

20‧‧‧Aluminum electrode (back electrode)

22‧‧‧External connection electrode

23‧‧‧ openings

24‧‧‧p ⁺ layer (BSF layer)

Fig. 1 is a cross-sectional view schematically showing a configuration example of a solar cell in an embodiment.

Form for implementing the invention

Hereinafter, preferred embodiments of the present invention will be described. In addition, what is necessary for the implementation of the present invention other than those specifically mentioned in the present specification (for example, a paste preparation method, a general manufacturing procedure of a solar cell (battery unit) not characterized in the present invention) can be used as It is grasped according to the design matters of those having ordinary knowledge in the technical field to which the invention pertains to the prior art in the art. The present invention can be implemented in accordance with the teachings of the present disclosure and the technical common knowledge in the field.

Paste composition

First, the paste composition for forming an aluminum electrode provided by the present invention will be described in detail. The paste composition for forming an aluminum electrode disclosed herein is an aluminum paste used for forming an aluminum electrode in a solar cell, and is an electrode forming material containing at least aluminum powder, a glass frit, and an organic vehicle solution and preparing a paste. . As far as the object of the invention can be achieved, There are no special restrictions.

Aluminum Powder

In the present specification, the term "aluminum powder" means an aggregate of particles containing aluminum (Al) as a main component, and is typically an aggregate of particles composed of an Al monomer. However, even a trace amount of Al is contained. The alloy of the impurity or the Al main body may be included in the "aluminum powder" as used herein as long as it is an aggregate of particles of the aluminum main body. Further, the aluminum powder itself can be produced by a conventionally known production method, and no special manufacturing means is required. The particles constituting the aluminum powder to be used are typically spherical, but are not limited to a so-called spherical shape, and may contain particles such as a sheet shape or an irregular shape.

The properties of the aluminum powder to be used are not particularly limited. However, from the viewpoint of forming a fine electrode pattern, the average particle diameter D ₅₀ is usually preferably 10 μm or less, and typically, an average particle diameter of 2 μm can be suitably used. ~8 μm (for example, 4 μm to 6 μm) of aluminum powder. In addition, in the present specification, the "particle diameter" means a value measured by a particle size distribution measuring apparatus according to a laser diffraction method, and the so-called D ₅₀ means a particle diameter at a cumulative volume of 50% in a particle size distribution (so-called Median particle size). Further, the particle size distribution of the aluminum powder is preferably narrow (the particle diameter is uniform). The foregoing index may use a ratio (D ₁₀ /D ₉₀ ) of the particle diameter (D ₁₀ ) at a cumulative volume of 10% in the particle size distribution according to the laser diffraction method to the particle diameter (D ₉₀ ) at a cumulative volume of 90%. When the particle diameters of the constituent powders are substantially equal, D ₁₀ /D ₉₀ =1 is formed, and conversely, the wider the particle size distribution, the closer D ₁₀ /D ₉₀ is to zero. Here, an aluminum powder having a relatively uniform particle diameter of about 0.2 of D ₁₀ /D ₉₀ can be suitably used.

The proportion of the aluminum powder in the entire paste composition is not particularly limited, and is usually about 55 to 85% by mass, and for example, it can be set to 60. Mass%~80% by mass. By setting it as the said range, the aluminum electrode can be provided with sufficient electroconductivity. Further, the reaction with ruthenium can be appropriately produced, and the BSF layer can be formed appropriately. Therefore, high electrical characteristics (especially light conversion efficiency) can be achieved.

"Glass"

The glass frit (glass powder) is an inorganic additive that enhances the bonding strength (peel strength) of the aluminum electrode. The glass frit (glass composition) contained in the paste composition (aluminum paste) disclosed herein contains SiO ₂ , B ₂ O ₃ , ZnO, and Bi ₂ O ₃ as essential constituent components. In other words, it is a glass frit in which an oxide of Si, B, Zn, and Bi is used as a basic constituent element (that is, a SiO ₂ -B ₂ O ₃ -ZnO-Bi ₂ O ₃ system is essential). The glass frit is substantially composed of a composition of SiO ₂ : 5 to 40 mol %; B ₂ O ₃ : 5 to 40 mol %; ZnO: 10 to 50 mol %; ₂ O ₃ : 5 to 45 mol%; and at least one of MgO, CaO, SrO and BaO: 0 to 25 mol%.

Further, the glass frit having a preferred composition is substantially composed of the following composition in terms of a molar ratio converted into an oxide, that is, SiO ₂ : 10 to 35 mol %; B ₂ O ₃ : 10 to 30 mol %; ZnO : 20 to 40 mol%; Bi ₂ O ₃ : 10 to 40 mol%; and at least one of MgO, CaO, SrO, and BaO: 0.1 to 20 mol%.

Among them, the glass frit is preferably composed of the following composition in terms of a molar ratio converted into an oxide, that is, SiO ₂ : 10 to 35 mol %; B ₂ O ₃ : 10 to 30 mol %; ZnO: 20 to 30 mol. %; Bi ₂ O ₃ : 10 to 20 mol%; BaO: 10 to 20 mol%; and at least one of MgO, CaO and SrO: 0.1 to 2 mol%.

Hereinafter, each constituent component will be described in order.

Cerium oxide (SiO ₂ ) is a component constituting the glass skeleton. The proportion of the SiO ₂ in the entire glass is preferably about 5 mol% or more, and is preferably 10 mol% or more in terms of the molar ratio of the oxide. Thereby, at least one of water resistance, chemical resistance, and thermal shock resistance can be improved. Further, the upper limit is preferably about 40 mol% or less, and is preferably 35 mol% or less. Thereby, the softening point of the frit can be prevented from becoming too high, and the aluminum electrode can be formed by a lower firing temperature.

Boron oxide (B ₂ O ₃ ) is a component having high effects of improving the thermal stability of the glass frit (adjusting the coefficient of thermal expansion) while lowering the softening point of the glass frit. In order to exhibit such an effect, the proportion of B ₂ O ₃ in the entire glass is preferably about 5 mol% or more, and is preferably 10 mol% or more, based on the molar ratio of the oxide. Thereby, the reaction of aluminum and ruthenium can be more uniformly produced, and a homogeneous BSF layer can be formed. Further, the upper limit is preferably about 40 mol% or less, and is preferably 30 mol% or less. Thereby, the thermal stability of the glass frit can be improved, and the durability (for example, mechanical strength or water resistance) of the aluminum electrode can be improved.

Zinc oxide (ZnO) is a component which has a higher coefficient of thermal expansion than other components and which has a high effect of improving the thermal stability (adjustment of thermal expansion coefficient) of the glass frit. Further, it is a component which has high thermal shock resistance and is difficult to be immersed in water and can achieve chemically stable properties and durability. In order to exhibit such an effect, the proportion of ZnO in the entire glass is preferably about 10 mol% or more, and is preferably 20 mol% or more, based on the molar ratio of the oxide. Further, the upper limit is preferably about 50 mol% or less, and is preferably 40 mol% or less.

Bismuth oxide (Bi ₂ O ₃ ) is a component having high effects of improving the thermal stability of the glass frit (adjusting the coefficient of thermal expansion) while lowering the softening point of the glass frit. In order to exhibit such an effect, the proportion of the total amount of Bi ₂ O ₃ in the entire glass is preferably about 5 mol% or more, and is preferably 10 mol% or more. Further, the upper limit is preferably about 45 mol% or less, and is preferably 40 mol% or less.

Further, in the glass batch to be included within the compositions disclosed herein of the paste, the molar content ratio of ZnO with respect to M _Zn Bi ₂ O ₃ ratio of M _Bi (M _Zn / M _Bi) containing a molar ratio of 0.8 Above and 2.5 or less (except that M _Bi and M _{Zn are} not deviated from the range of the aforementioned ratio). By including such a glass frit in the paste composition, a homogeneous BSF layer can be formed, and an aluminum electrode capable of achieving both electrical properties and durability (for example, water resistance) at a high level can be realized.

Further, in addition to the above-mentioned four essential constituent components, the glass frit may contain an optional component. In one aspect, the constituents of the glass frit further contain at least one alkaline earth metal oxide such as magnesium oxide (MgO), calcium oxide (CaO), strontium oxide (SrO), and barium oxide (BaO). Thereby, the coefficient of thermal expansion can be adjusted, and the stability of the glass can be improved by the diversification of the glass composition (constituting a plurality of types of metal elements). In view of the above, the proportion of the alkaline earth metal oxide in the entire glass is preferably about 0.01 mol% or more, and preferably 0.1 mol% or more, based on the molar ratio of the oxide. . Further, the upper limit is preferably about 25 mol% or less, and preferably 20 mol% or less. Further, MgO is also a component which can adjust the viscosity of the glass frit when it is melted, and CaO is also a component which can improve the hardness of the glass frit and improve the abrasion resistance. Therefore, it is preferable to contain a magnesium component and/or a calcium component, and it is preferable to contain both.

In another aspect that is suitable, the constituents of the glass frit further contain Al ₂ O ₃ . Alumina (Al ₂ O ₃ ) is a component that controls the fluidity at the time of melting of the glass frit and interferes with the adhesion stability at the time of formation of the aluminum electrode. The proportion of Al ₂ O ₃ in the entire glass frit may be about 10 mol% or less, and is preferably, for example, about 0.01 mol% to 5 mol%, based on the molar ratio of the oxide. If the Al ₂ O ₃ content is too high compared to the above range, the chemical resistance of the glass may be lowered.

In addition, TiO ₂ , ZrO ₂ , V ₂ O ₅ , Nb ₂ O ₅ , FeO, Fe ₂ O ₃ , Fe ₃ O ₄ may be contained for various purposes as long as the effects of the present invention are not significantly impaired. An oxide component such as CuO, Cu ₂ O, SnO, SnO ₂ , P ₂ O ₅ , La ₂ O ₃ or CeO ₂ . The ratio of the constituent components is preferably 3 mol% or less (typically 2 mol% or less, for example, 1 mol% or less) of the entire glass composition.

In one aspect, the paste composition disclosed herein does not contain an alkali metal oxide as an alkali component (for example, lithium oxide (Li ₂ O), sodium oxide (Na ₂ O), potassium oxide (K ₂ O). )). Specifically, the proportion of the alkali metal oxide in the entire glass frit may be about 1 mol% or less, and is preferably less than 1 mol%, preferably 0.5 mol% or less, more preferably 0.1 mol% or less. . Thereby, water resistance or chemical resistance can be improved. Further, it is possible to prevent the coefficient of thermal expansion from becoming too high.

Moreover, it is not preferable to contain PbO as a constituent component of a glass frit in view of the viewpoint of lead limitation, etc.. Specifically, the proportion of PbO in the entire glass frit is preferably about 0.1 mol% or less, and more preferably 0.01 mol% or less. Thereby, the aggregation of bubbles or Al can be further reduced, and a homogeneous BSF layer can be formed. From the point of view of environmental considerations or workability, it is also advisable to make no Pb.

The properties of the glass frit to be used are not particularly limited. However, in order to stably fix (burn) the aluminum electrode on the ruthenium substrate, the average particle diameter D ₅₀ is usually preferably 20 μm or less. Typically, the average value can be suitably used. A glass frit having a particle diameter of 0.01 μm to 10 μm (for example, 0.1 μm to 5 μm).

In an ideal aspect, the glass frit has a softening point of 420 ° C or higher (typically 430 ° C or higher) and 620 ° C or lower (typically 600 ° C or lower). The interfacial reaction between aluminum and ruthenium can be promoted by the use of a glass frit having a softening point in such a temperature range.

The proportion of the glass frit in the entire paste composition is not particularly limited, and is usually about 5% by mass or less, and typically 0.01% by mass to 5% by mass, for example, 0.01% by mass to 3% by mass. In particular, in a suitable aspect, the proportion of the glass frit in the entire paste composition is 0.01% by mass to 1% by mass. By setting it as the said range, the BSF layer can be formed suitably. Further, by reducing the content of the glass frit, the amount of warpage of the substrate can be reduced. Furthermore, the conductivity can be improved and the electrical characteristics of the aluminum electrode can be further improved.

<Organic media>

The organic vehicle is a liquid medium for dispersing the solid matter (i.e., aluminum powder or glass frit) as described above, and typically contains an organic solvent and an organic binder.

The organic solvent is not particularly limited as long as it can disperse aluminum powder or glass frit, and an organic solvent conventionally used in such a paste composition can be used. Specifically, a high-boiling organic solvent such as ethylene glycol and a diethylene glycol derivative (glycol ether solvent), tetracarbitol, diethylene glycol monobutyl ether acetate, terpineol or the like can be used. Kind or combination of multiple species.

The organic binder can impart the composition of the paste disclosed herein as long as it is The good viscosity and the film forming ability (adhesion to the substrate) may be used, and an organic binder conventionally used in such a paste composition may be used without particular limitation. For example, an acrylic resin, an epoxy resin, a phenol resin, an alkyd resin, a cellulose polymer, a polyvinyl alcohol, a rosin resin, or the like can be exemplified. In one aspect, a resin having a decomposition temperature of 400 ° C or less is contained. By using such a binder, aggregation of bubbles or Al can be prevented, and a more homogeneous BSF layer can be formed. From this point of view, in particular, an acrylic resin can be suitably used.

The ratio of the organic solvent to the organic binder as the organic vehicle can be arbitrarily set in consideration of the viscosity, coatability, and the like of the intended paste composition. When the total amount of the solid material (aluminum powder and the glass frit) in the paste composition is 100% by mass, the content is generally 1% by mass or more and 5% by mass or less (more preferably 2% by mass or more). And the ratio of 4 mass % or less) contains an organic binder.

Further, although it is not particularly limited, the proportion of the organic vehicle liquid in the entire paste composition is usually about 10% by mass to 30% by mass, and preferably 15% by mass to 25% by mass.

Like the conventional aluminum paste for solar cells, such a paste composition can be easily prepared by mixing the above materials (typically aluminum powder, glass frit (glass powder) and a suitable organic vehicle). For example, the paste composition can be prepared by mixing the aluminum powder and the glass frit with the organic vehicle at a predetermined blending ratio by using a general kneader. Stirring is carried out.

The paste composition disclosed herein can be treated in the same manner as the aluminum paste which has been conventionally used for forming an aluminum electrode (and further a p ⁺ layer, that is, a BSF layer) as a back surface electrode on a substrate, and is not particularly limited. A conventionally known method can be used. Specifically, the formation of an aluminum electrode can be provided as follows.

First, the paste composition is applied (coated) to a ruthenium substrate by a method such as a screen printing method, a dispenser coating method, or a dip coating method to form a desired film thickness or a coating film pattern. The thickness of the coating film of the aluminum electrode is preferably 100 μm or less, preferably 80 μm or less, more preferably 60 μm or less, and particularly preferably 30 μm ± 10 μm. By forming the aluminum electrode with such a film thickness, the BSF effect in the back surface electrode can be effectively exhibited.

Next, using any drying mechanism, the paste coating is dried by a suitable temperature (for example, 100 ° C to 300 ° C). After drying, the film is dried by heating in a suitable firing furnace (typically a high-speed firing furnace) under appropriate heating conditions (for example, 600 ° C to 900 ° C, and preferably 700 ° C to 800 ° C) for a predetermined time. Burnt. Thereby, the paste coating material is baked on the ruthenium substrate, and the aluminum electrode 20 and the p ⁺ layer (BSF layer) 24 as shown in FIG. 1 can be formed later. That is, the aluminum electrode 20 (constituting the back surface electrode) is formed on the p-type germanium substrate 11 by firing, and at the same time, aluminum atoms are diffused into the substrate to form a p ⁺ layer 24 containing impurities of aluminum.

The paste composition disclosed herein contains the SiO ₂ -B ₂ O ₃ -ZnO-Bi ₂ O ₃ system as a basic predetermined glass frit as described above. Thereby, a homogeneous BSF layer can be formed, and excellent electrical characteristics can be achieved. Moreover, the adhesion strength or durability (for example, water resistance) of the aluminum electrode can also be improved. Therefore, the content of the glass frit contained in the purpose of improving the durability of the aluminum electrode can be reduced as compared with the prior art. As a result, in the solar cell obtained by using the paste composition, deformation (warpage, etc.) of the substrate which may cause a difference in thermal expansion coefficient between the substrate and the glass component can be effectively suppressed and prevented. Further, in the aluminum electrode which is formed by using the paste composition, the content of the glass component which is insulating can be reduced, and good conduction can be exhibited.

Further, in addition to the use of the paste composition disclosed herein to form an aluminum electrode, materials or procedures for the manufacture of solar cells may be the same as conventionally known. For example, as in the prior art, after the n ⁺ layer or the antireflection film is formed on the substrate, the same silver paste as in the prior art is used for screen printing on the back side. Dry, and the silver paste is also printed on the side of the light receiving surface. Dry to a pattern. Then, the paste composition disclosed herein is printed. It is dried to partially overlap with one of the fields of silver paste formation on the back side, and is fired. In this way, a solar cell (typically a crystalline system solar cell) having a back electrode formed using the paste composition can be produced.

Solar Cell

A typical example of the structure of the above-described crystalline lanthanide solar cell can be exemplified as shown in FIG. In general, the solar cell 10 of the present embodiment is configured by a tantalum substrate (Si wafer) 11 and a light-receiving surface electrode 12 formed on one surface side (surface side) of the substrate 11 and an aluminum electrode 20, This is formed on the other surface side (back side) of the substrate 11.

The tantalum substrate 11 of the present embodiment is composed of a crucible such as a crystalline crucible or an amorphous crucible. The properties of the substrate 11 are not particularly limited. For example, the thickness may be set in consideration of the desired size of the solar cell or the strength (for example, breaking strength) of the substrate, and is usually set to 5 μm to 300 μm, for example, It is preferably about 5 μm to 200 μm.

In the structure of the embodiment, n-Si formed by pn junction formation The layer 16 is located on the light-receiving side of the p-Si layer (p-type crystalline germanium) 18 of the substrate 11. Further, the anti-reflection film 14 made of titanium oxide or tantalum nitride is formed on the surface of the n-Si layer 16 by a general chemical vapor deposition method (CVD) or the like. Further, typically, a light-receiving surface electrode (surface electrode) 12 made of Ag is formed by screen printing and firing a silver paste, and is provided on the surface of the anti-reflection film 14.

Moreover, the aluminum electrode 20 formed using the paste composition for forming an aluminum electrode disclosed herein is formed on the back side of the p-Si layer 18 together with the external connection electrode 22. In the present embodiment, the external connection electrode 22 is formed in a linear shape on the back side of the p-Si layer 18, and is substantially entirely integrated except for the field in which the external connection electrode 22 is formed, and an aluminum electrode having a back surface electric field effect is formed. 20. Further, in the structure of the present embodiment, the aluminum electrode 20 is formed to cover one portion of the external connection electrode 22 (specifically, both edge portions of the linear structure), and has an opening portion 23 through which the external connection electrode 22 is exposed. .

Hereinafter, several test examples of the present invention will be described. However, the present invention is not intended to be limited to the above test examples.

Here, the difference in the composition (composition component and ratio) of the glass frit which is a solid component of the paste composition for forming an aluminum electrode is different from each other.

In the test, six kinds of glass samples (samples 1 to 6) having a composition (mol%) and a softening point (°C) as shown in Table 1 below were used.

Each of the glass frits (samples 1 to 6) shown in Table 1 was used, and a total of six kinds of aluminum pastes (test examples 1 to 6) were produced. As described above, the aluminum paste of each test example differs only in the properties of the glass frit, and the other components (aluminum powder, organic vehicle liquid) or blending ratio are made the same.

The contents of the aluminum paste of each test example are as follows.

(1) Aluminum powder: Aluminum powder having an average particle diameter of 4.5 μm was blended and used in an amount of 75 mass% of the entire paste.

(2) Glass frit: A glass frit in which the properties of any of the above samples 1 to 6 are blended and used by any addition amount of 0.01 to 1.00% by mass of the entire paste.

(3) Organic vehicle liquid: In an amount of about 22% by mass of the entire paste, tetrabutyl carbitol acetate was blended and used as an organic solvent. In addition, an acrylic resin was blended and used as an organic binder in an amount of about 2% by mass of the entire paste.

Next, the aluminum paste of Test Examples 1 to 6 prepared as described above was used, and an aluminum electrode was formed on one surface (back surface) of the tantalum substrate.

Specifically, a p-type single crystal ruthenium substrate (plate thickness: 200 μm) for a commercially available 125 mm square solar cell was prepared, and the surface thereof was subjected to an alkali etching treatment with an aqueous sodium hydroxide solution. Then, the phosphor-containing solution is applied to the light-receiving surface of the ruthenium substrate on which the fine uneven structure (tissue structure) is formed by the etching treatment, and by heat treatment, n-thickness of about 0.5 μm is formed on the light-receiving surface of the ruthenium substrate. Si layer (n ⁺ layer).

Next, an antireflection film (tantalum nitride film) having a thickness of 50 nm or more and 100 nm or less is formed on the n-Si layer by a plasma CVD (PECVD) method. Further, after a silver paste for forming a predetermined surface electrode (Ag electrode) is used, and a coating film (having a thickness of 20 μm or more and 50 μm or less) constituting the surface electrode (Ag electrode) is formed on the antireflection film by a screen printing method, Similarly, the coating film constituting the back surface electrode (Ag electrode) was formed into a pattern (linearly formed by a width of about 5 mm) and dried.

The paste composition of any of the above Test Examples 1 to 6 was printed (coated) on the back side of the substrate by screen printing (using a stainless steel mesh SUS #250) to partially overlap the Ag electrode. And a coating film having a film thickness of about 30 μm was formed. The substrate was fired in an air atmosphere by a high-speed firing furnace at a firing temperature of about 700 ° C to 800 ° C for 1 minute to form an aluminum electrode. Thereby, the solar cells of the test examples 1 to 6 were obtained.

[confirmation of appearance]

The aluminum electrode after the firing was observed, and it was confirmed whether or not the appearance of bubbles or aluminum agglomeration occurred. The results are displayed in the "Appearance" column of Table 2. In addition, in this column, "○" indicates that the problem is 5 or less, "△" is 5 to 20, and "X" is 20 or more.

As shown in Table 2, Test Example 2 using a glass frit not containing ZnO and Test Example 3 using a glass frit using M _Zn /M _Bi ≦0.6 (that is, having a small content of ZnO with respect to Bi ₂ O ₃ ) Among them, it can be confirmed that many appearances are poor (typically bubbles). It is generally believed that the reason is that the aluminum and the strontium are partially overreacted when the binder is burned through. On the other hand, in Test Example 1 using a glass frit containing ZnO as a main component and Test Examples 3 to 5 using a glass frit using M _Zn /M _Bi ≧ 0.8 (M _Zn /M _Bi ≧ 1.0), poor appearance was observed. Less, and form a homogeneous BSF layer.

[Water resistance evaluation]

The battery cells (solar cells) for test evaluation prepared above were used, and the hot water resistance of the aluminum electrodes was evaluated. Specifically, hot water which has been maintained at a temperature of 80 ° C is prepared, and each test evaluation battery unit is immersed in hot water so that the aluminum electrode is sufficiently brought into contact with hot water. Further, the time until the generation of bubbles from the aluminum electrode (that is, until the aluminum reacts with moisture to generate hydrogen) is measured. In addition, the upper limit is set to 10 minutes. The results are shown in the "water resistance" column of Table 2 in terms of water resistance time.

As shown in Table 2, in Test Examples 2 and 3, which had many problems in the confirmation of the appearance, the aluminum electrode was deteriorated within 10 minutes, and the hydrothermal resistance was relatively low. relatively Here, in Test Examples 1 and 3 to 5, it was found that the water heat resistance was relatively high.

[Electrical characteristic evaluation]

The battery cells (solar cells) for test evaluation prepared above were used, and the electrical characteristics were evaluated in accordance with JIS C8913 (1998). Table 2 shows light conversion efficiency (Eff), open voltage (V _OC ), short circuit current (I _SC ), fill factor (FF), series resistance (R _S ), and parallel resistance (R _Sh ). In addition, in Table 2, the result of the test example 1 was 100 and it was represented by relative evaluation.

As shown in Table 2, the ZnO in the molar ratio of containing the test with respect to the M _Bi (M _Zn / M _{Bi) Bi 2} O ₃ mole ratio of satisfied containing 0.8 to 2.5 (particularly 1 to 2) the ratio M _Zn In Examples 4 to 6, a significant increase in the light conversion efficiency (Eff) using the increase in the open voltage (V _OC ) was observed, and it was found that excellent electrical characteristics were exhibited. Among them, in Test Examples 4 and 5, in addition to the aforementioned V _OC , an increase in Eff due to an increase in the fill factor (FF) was also observed, and it was found that more excellent electrical characteristics were exhibited.

[Peel strength evaluation]

The battery cell (solar cell) for test evaluation prepared above was used, and the peel strength (tape pull-out force) of the aluminum electrode was measured. Specifically, after the Sumitomo 3M Scotch tape was pressed against the aluminum electrode forming portion of each test evaluation battery unit by a finger, the tape was peeled off, and the tape attached to the peeled tape was observed by visual observation. The appearance of the electrode. The results are shown in the "Peeling" column of Table 2. In the column, "○" indicates that the electrode is not attached to the pressed tape. On the other hand, the case where the electrode is attached to the pressed tape is indicated by "x".

As shown in Table 2, in Test Example 1, the strength was low. General recognition The reason for this is that the interface reaction between aluminum and bismuth is suppressed at the time of firing. On the other hand, in Test Examples 2 to 6, it was found that the relatively high bonding strength was exhibited.

As is apparent from the above-described embodiments or test examples, the aluminum electrode formed using the paste composition disclosed herein can form a BSF layer homogeneously, and is excellent in electrical conductivity, adhesion strength, or water resistance. Therefore, in the solar cell including the electrode described above, electrical characteristics (for example, light conversion efficiency, open voltage, or fill factor) can be greatly improved, and the high performance can be stably exhibited for a long period of time.

The present invention has been described in detail above. The present invention may be embodied in other specific forms, and various modifications may be made without departing from the spirit and scope of the invention.

10‧‧‧Solar battery

11‧‧‧p-type germanium substrate (Si wafer)

12‧‧‧Photometric surface electrode (surface electrode)

14‧‧‧Anti-reflection film

16‧‧‧n-Si layer (n ⁺ layer)

18‧‧‧p-Si layer

20‧‧‧Aluminum electrode (back electrode)

22‧‧‧External connection electrode

23‧‧‧ openings

24‧‧‧p ⁺ layer (BSF layer)

Claims (8)

A paste composition for forming an aluminum electrode, which is a paste composition for forming an aluminum electrode of a solar cell, comprising aluminum powder, a glass frit, and an organic vehicle, wherein the glass frit has the following conditions: (1) converted into an oxide The molar ratio is substantially composed of the following composition: SiO ₂ : 5 to 40 mol %; B ₂ O ₃ : 5 to 40 mol %; ZnO: 10 to 50 mol %; Bi ₂ O ₃ : 5 to 45 mol % And at least one of MgO, CaO, SrO, and BaO: 0 to 25 mol%; (2) when the entire glass frit is 100 mol%, the molar content of the ZnO M _{Zn is} relative to the aforementioned Bi ₂ O ₃ The ratio (M _Zn /M _Bi ) of the molar content M _Bi is 0.8 or more and 2.5 or less. The paste composition of claim 1, wherein the glass frit is substantially composed of the following composition in terms of a molar ratio converted into an oxide, namely: SiO ₂ : 10 to 35 mol %; B ₂ O ₃ : 10~ 30 mol%; ZnO: 20-40 mol%; Bi ₂ O ₃ : 10-40 mol%; and at least one of MgO, CaO, SrO and BaO: 0.1-20 mol%. The paste composition of claim 1 or 2, wherein the glass frit is simultaneously measured according to the differential heat-heat weight, and has a softening point of 420 ° C or more and 620 ° C or less. In the paste composition according to any one of claims 1 to 3, wherein the content of the glass frit is 100% by mass, the content of the glass frit is 0.01 to 1% by mass. The paste composition according to any one of claims 1 to 4, wherein the glass frit does not contain Pb and an alkali component. The paste composition according to any one of claims 1 to 5, wherein the organic vehicle contains a resin having a decomposition temperature of 400 ° C or less which is simultaneously measured according to the differential heat-heat weight as an organic binder. The paste composition according to any one of claims 1 to 6, wherein the organic vehicle contains an acrylic resin as an organic binder. A solar cell comprising: a germanium substrate; a light receiving surface electrode formed on a light receiving surface side of one surface of the substrate; and an aluminum electrode formed on a back side of the other surface belonging to the substrate, and as claimed in claim 1 The paste composition of any one of 7 is fired.