WO2012111478A1 - Conductive paste and solar cell - Google Patents

Abstract

A conductive paste which contains an Ag powder, a first glass frit, a second glass frit that has a softening point higher than that of the first glass frit by 20˚C or more, and an organic vehicle. The first and second glass frits are free from lead and contain at least B and Si, with the molar ratio of B₂O₃ relative to SiO₂ being 0.4 or less. The first glass frit and the second glass frit are contained in the conductive paste in a weight ratio of from 1/4 to 4/1. A light-receiving surface electrode (3) is formed using the conductive paste. Consequently there can be achieved: a conductive paste for forming an electrode for solar cells, which is free from lead and still capable of stably achieving high conversion efficiency even if fired over a wide temperature range; and a solar cell which is manufactured using the conductive paste.

Description

Conductive paste and solar cell

The present invention relates to a conductive paste and a solar cell, and more particularly to a conductive paste suitable for forming an electrode of a solar cell, and a solar cell manufactured using this conductive paste.

A solar cell usually has a light-receiving surface electrode of a predetermined pattern formed on one main surface of a semiconductor substrate. Further, an antireflection film is formed on the semiconductor substrate excluding the light receiving surface electrode, and the reflection loss of incident sunlight is suppressed by the antireflection film, thereby converting the conversion efficiency of sunlight into electric energy. Has improved.

The light receiving surface electrode is usually formed as follows using a conductive paste. That is, the conductive paste contains conductive powder, glass frit, and organic vehicle, and the conductive paste is applied to the surface of the antireflection film formed on the semiconductor substrate to form a conductive film having a predetermined pattern. To do. Then, the glass frit is melted in the firing process, and the antireflection film under the conductive film is decomposed and removed, whereby the conductive film is sintered to form the light receiving surface electrode, and the light receiving surface electrode and the semiconductor substrate are bonded together. They are bonded to make them both conductive.

This method of disassembling and removing the antireflection film in the firing process and bonding the semiconductor substrate and the light-receiving surface electrode is called fire-through, and the conversion efficiency of the solar cell is greatly increased in fire-through performance. Dependent. That is, it is known that if the fire-through property is insufficient, the conversion efficiency is lowered and the basic performance as a solar cell is inferior.

Further, in this type of solar cell, it is preferable to use a glass frit having a low softening point in order to increase the adhesive strength between the light receiving surface electrode and the semiconductor substrate.

Conventionally, lead-based glass frit has been used as a glass frit with a low softening point. However, since lead (Pb) has a large environmental load, the appearance of a new material to replace lead-based glass frit is required. ing.

From this viewpoint, Patent Document 1 discloses that the glass frit has a softening point of 570 to 760 ° C., and the glass frit has a molar ratio of B ₂ O ₃ / SiO ₂ of 0.3 or less. A conductive paste containing B ₂ O ₃ and SiO ₂ and containing less than 20 mol% Bi ₂ O ₃ has been proposed.

In Patent Document 1, although it is a lead-free conductive paste that does not contain lead, the adhesive strength between the light-receiving surface electrode and the semiconductor substrate is large even when baked at a relatively low temperature, and between the light-receiving surface electrode and the semiconductor substrate. It is possible to obtain a solar cell having a low contact resistance.

International Publication No. 2007/102287 (Claim 2, Paragraph Number [0016])

In a conventional solar cell such as Patent Document 1, as described above, a conductive paste is applied to an antireflection film on a semiconductor substrate, and this is fired as an object to be fired.

However, since the object to be fired passes through the firing furnace at a high speed, fire-through is not sufficiently performed in the firing process, and thus there is a possibility that an antireflection film remains between the light receiving surface electrode and the semiconductor substrate. . As a result, there is a portion where the light receiving surface electrode is not bonded to the semiconductor substrate, which may cause a decrease in conversion efficiency.

In addition, in the mass production process of solar cells, a large amount of the material to be baked passes through the baking furnace at a high speed. As a result, the firing temperature tends to fluctuate, and products with insufficient fire-through may be discharged from the firing furnace.

Therefore, in order to stably produce solar cells using the conductive paste, it is necessary to strictly control the firing conditions. In particular, when a glass frit having a high softening point of 570 to 760 ° C. is used as in Patent Document 1, high conversion efficiency can be obtained under optimum firing conditions, but it is easily affected by fluctuations in the firing temperature, It is difficult to obtain high conversion efficiency stably.

Moreover, in patent document 1, since only one kind of glass frit having a high softening point is used, after the binder resin or the like contained in the conductive paste disappears by thermal decomposition or combustion, the glass component softens. In the meantime, the adhesion between the conductive film to be the light-receiving surface electrode and the semiconductor substrate remains extremely weak. If the sintering of the conductive powder proceeds excessively in such a state, the number of contact points between the conductive powder and the semiconductor substrate that existed before firing decreased, and therefore, the desired high conversion It becomes difficult to obtain efficiency.

The present invention has been made in view of such circumstances, and it is a lead-free electrode for forming an electrode for a solar cell that can stably obtain high conversion efficiency even when fired in a wide temperature range. It aims at providing the electrically conductive paste and the solar cell manufactured using this electrically conductive paste.

The present inventor conducted intensive studies to achieve the above object, and as a result, the molar ratio of B ₂ O ₃ to SiO ₂ was adjusted to 0.4 or less, and the two types of non-softening points differing by 20 ° C. or more. By using a lead-based glass frit, if the blending ratio of both glass frit is in the range of 1/4 to 4/1, good fire-through property can be secured even if the temperature fluctuates during firing. It was possible to obtain a solar cell having a desired high conversion efficiency in a wide range of firing temperatures.

This invention is made | formed based on such knowledge, Comprising: The electrically conductive paste which concerns on this invention is an electrically conductive paste for forming the electrode of a solar cell, Comprising: Conductive powder and 1st glass Containing a frit, a second glass frit having a softening point higher by 20 ° C. or more than the first glass frit, and an organic vehicle, wherein the first and second glass frits do not contain Pb, At least B and Si are contained, and the molar ratio of B to Si is 0.4 or less in terms of SiO ₂ and B ₂ O ₃ , respectively, and the first glass frit and the second glass frit The content ratio of is characterized by being 1/4 to 4/1 in weight ratio.

In the conductive paste, adhesion is secured by the first glass frit having a relatively low softening point, while the light receiving surface electrode and the semiconductor are secured by the second glass frit having a softening point of 20 ° C. or more higher than that of the first glass frit. Excessive glass flow to and from the substrate can be suppressed, thereby ensuring a good fire-through property even when fired in a wide temperature range, and stably obtaining a solar cell having high conversion efficiency Is possible.

In the conductive paste of the present invention, the softening point of the first glass frit is 510 to 570 ° C., and the softening point of the second glass frit is preferably 530 to 680 ° C.

Furthermore, in the conductive paste of the present invention, the first glass frit is such that Bi ₂ O ₃ is 20 to 40 mol%, BaO is 5 to 20 mol%, and Al ₂ O ₃ is 5 mol% or less. Each is preferably contained.

In the conductive paste of the present invention, the second glass frit has a Bi ₂ O ₃ content of 5 to 30 mol%, a BaO content of 5 to 25 mol%, and an Al ₂ O ₃ content of 5 mol% or less. It is preferably contained.

This makes it possible to obtain a conductive paste with good fire-through and chemical durability.

In the conductive paste of the present invention, it is preferable that the first and second glass frit contain at least one of Bi and Ba.

Furthermore, the conductive paste of the present invention preferably contains ZnO.

Thereby, the fire-through property can be further promoted, and a solar cell having a low contact resistance between the electrode and the semiconductor substrate can be realized.

In the conductive paste of the present invention, the conductive powder is preferably Ag powder.

Further, in the solar cell according to the present invention, an antireflection film and a light receiving surface electrode penetrating the antireflection film are formed on one main surface of the semiconductor substrate, and the electrode is the conductive paste according to any one of the above Is characterized by being sintered.

This makes it possible to obtain a solar cell having a stable and high conversion efficiency even when fired in a wide temperature range.

According to the conductive paste of the present invention, the conductive powder such as Ag powder, the first glass frit having a softening point of, for example, 510 to 570 ° C., and the first glass frit having a softening point of, for example, 530 to 680 ° C. And a second glass frit having a softening point higher by 20 ° C. or more than the organic vehicle, and the first and second glass frits do not contain Pb and contain at least B and Si. , The molar ratio of B to Si is 0.4 or less in terms of SiO ₂ and B ₂ O ₃ , respectively, and the content ratio of the first glass frit and the second glass frit is expressed as a weight ratio. Since it is 1/4 to 4/1, the first glass frit having a relatively low softening point ensures adhesion, while the second glass frit having a softening point higher than the first glass frit by 20 ° C. or more. Can suppress the flow of excess glass between the light-receiving surface electrode and the semiconductor substrate, thereby ensuring good fire-through performance and stable high conversion efficiency even when fired in a wide temperature range. It is possible to obtain a solar cell having the same.

Moreover, according to the solar cell of the present invention, an antireflection film and an electrode penetrating the antireflection film are formed on one main surface of the semiconductor substrate, and the electrode is formed of the conductive paste according to any one of the above. Since it is sintered, a solar cell having high conversion efficiency can be obtained stably even when fired in a wide temperature range.

It is principal part sectional drawing which shows one Embodiment of the solar cell manufactured using the electrically conductive paste which concerns on this invention. It is the enlarged plan view which showed the light-receiving surface electrode side typically. It is the enlarged bottom view which showed the back electrode side typically.

Next, an embodiment of the present invention will be described in detail.

FIG. 1 is a cross-sectional view of an essential part showing an embodiment of a solar cell manufactured using a conductive paste according to the present invention.

In this solar cell, an antireflection film 2 and a light receiving surface electrode 3 are formed on one main surface of a semiconductor substrate 1 containing Si as a main component, and a back electrode 4 is formed on the other main surface of the semiconductor substrate 1. Has been.

The semiconductor substrate 1 has a p-type semiconductor layer 1b and an n-type semiconductor layer 1a, and an n-type semiconductor layer 1a is formed on the upper surface of the p-type semiconductor layer 1b. The semiconductor substrate 1 can be obtained, for example, by diffusing impurities on one main surface of a single-crystal or polycrystalline p-type semiconductor layer 1b to form a thin n-type semiconductor layer 1a. As long as the n-type semiconductor layer 1a is formed on the upper surface of the layer 1b, its structure and manufacturing method are not particularly limited. The semiconductor substrate 1 has a structure in which a thin p-type semiconductor layer 1b is formed on one main surface of the n-type semiconductor layer 1a, or a p-type semiconductor layer 1b on a part of one main surface of the semiconductor substrate 1. A structure in which both the n-type semiconductor layer 1a and the n-type semiconductor layer 1a are formed may be used. In any case, the conductive paste according to the present invention can be used effectively as long as it is the main surface of the semiconductor substrate 1 on which the antireflection film 2 is formed.

In FIG. 1, the surface of the semiconductor substrate 1 is shown in a flat shape, but the surface is formed to have a fine concavo-convex structure in order to effectively confine sunlight to the semiconductor substrate 1.

The antireflection film 2 is formed of an insulating material such as silicon nitride (SiN _x ), suppresses reflection of light to the light receiving surface of sunlight indicated by an arrow A, and allows sunlight to be quickly and efficiently applied to the semiconductor substrate 1. Lead. The material constituting the antireflection film 2 is not limited to the above silicon nitride, and other insulating materials such as silicon oxide and titanium oxide may be used, and two or more kinds of insulating materials may be used. May be used in combination. In addition, as long as it is crystalline Si, either single crystal Si or polycrystalline Si may be used.

The light receiving surface electrode 3 is formed on the semiconductor substrate 1 through the antireflection film 2. The light-receiving surface electrode 3 is formed by applying a conductive paste of the present invention, which will be described later, onto the semiconductor substrate 1 by using screen printing or the like to produce a conductive film and baking it. That is, in the baking process for forming the light receiving surface electrode 3, the antireflection film 2 under the conductive film is decomposed and removed and fired through, whereby the light receiving surface electrode is formed on the semiconductor substrate 1 so as to penetrate the antireflection film 2. 3 is formed.

Specifically, as shown in FIG. 2, the light-receiving surface electrode 3 has a large number of finger electrodes 5a, 5b,... 5n arranged in a comb-like shape and intersects with the finger electrodes 5a, 5b,. Bus bar electrode 6 is provided, and finger electrodes 5a, 5b,... 5n and bus bar electrode 6 are electrically connected. Then, the antireflection film 2 is formed in the remaining region excluding the portion where the light receiving surface electrode 3 is provided. In this way, the electric power generated in the semiconductor substrate 1 is collected by the finger electrodes 5n and taken out to the outside by the bus bar electrodes 6.

Specifically, as shown in FIG. 3, the back electrode 4 is formed on the back surface of the current collecting electrode 7 and the current collecting electrode 7 made of Al or the like formed on the back surface of the p-type semiconductor layer 1b. It is comprised with the extraction electrode 8 which consists of Ag etc. which were electrically connected with the current collection electrode 7. FIG. Then, the electric power generated in the semiconductor substrate 1 is collected by the collecting electrode 7 and is taken out by the extracting electrode 8.

Next, the conductive paste of the present invention for forming the light receiving surface electrode 3 will be described in detail.

The conductive paste of the present invention contains conductive powder, two types of lead-free glass frit (first and second glass frit) having different softening points, and an organic vehicle.

The first and second glass frit both contain at least B and Si, and satisfy the following formulas (1) to (3).

Ts ₂ −Ts ₁ ≧ 20 (1)
α / β ≦ 0.4 (2)
1/4 ≦ x / y ≦ 4/1 (3)

Here, Ts ₁ is the softening point of the first glass frit, Ts ₂ is the softening point of the second glass frit, α is the molar content of B ₂ O ₃ in each glass frit, and β is in each glass frit. The molar content of SiO ₂ , x is the content weight of the first glass frit, and y is the content weight of the second glass frit.

That is, the conductive paste of the present invention contains at least two lead-free glass frit containing B and Si and having different softening points, and the softening point Ts ₂ of the _second glass frit is the same as that of the first glass frit. 20 ° C. or more higher than the softening point Ts _1, the molar ratio α / β of B ₂ O ₃ to SiO ₂ is 0.4 or less, and the weight ratio x / of the first glass frit to the second glass frit y is set to 1/4 to 4/1.

Thus, even when fired in a wide temperature range, good fire-through properties can be ensured, and a solar cell having high conversion efficiency can be obtained stably.

Hereinafter, the reason why the glass frit satisfies the above formulas (1) to (3) will be described.

(1) Softening points Ts ₁ and Ts _{2 of} the first and second glass frit
Si-B-Bi-Ba glass frit containing SiO ₂ , B ₂ O ₃ , Bi ₂ O ₃ , and BaO is a promising substitute for lead glass frit because it has good fire-through properties. is there.

Of the Si-B-Bi-Ba-based glass frit, the glass frit having a low softening point easily flows at the interface between the light-receiving surface electrode 3 and the semiconductor substrate 1 (n-type semiconductor layer 1a) during the baking process, and is reflected. Since the decomposition / removal of the prevention film is promoted, it can contribute to the improvement of the fire-through property. It is also possible to improve the adhesive strength between the light receiving surface electrode 3 and the semiconductor substrate 1.

However, the lead-free glass frit having a low softening point flows excessively at the interface between the conductive film to be the light-receiving surface electrode 3 and the semiconductor substrate 1 and diffuses toward the semiconductor substrate 1 to cause the semiconductor substrate 1 to flow. There is a risk of erosion. As a result, the pn junction formed between the n-type semiconductor layer 1a and the p-type semiconductor layer 1b may be destroyed, and a desired high conversion efficiency may not be obtained. In this case, if the conductive powder is excessively diffused to the semiconductor substrate 1 side through the glass frit, the parallel resistance is lowered, and as a result, the voltage when the output terminal is opened, that is, the open circuit voltage Voc is lowered. High conversion efficiency cannot be obtained.

Therefore, in the present embodiment, the first glass frit having a low softening point and the second glass frit having a softening point higher by 20 ° C. or more than the first glass frit are contained in the conductive paste, thereby Excessive flow of glass frit at the interface between the light receiving surface electrode 3 and the semiconductor substrate 1 is suppressed.

As described above, the glass component is appropriately flowed by the first glass frit, the adhesion at the interface is ensured, the decomposition and removal of the antireflection film is promoted, and the fire-through property is secured, while the second glass frit is secured. By suppressing the excessive flow of the glass component at the interface, it is possible to prevent the pn junction from being destroyed, and to suppress the decrease of the parallel resistance to avoid the decrease of the open circuit voltage Voc. The conversion efficiency is obtained.

Here, the softening point of the second glass frit Ts ₂ and the difference between the softening point Ts ₁ of the first glass frit be at least 20 ° C. above, when both the softening point difference ΔTs is reduced to below 20 ° C., This is because it is not much different from the case where one type of glass frit is contained in the conductive paste, and high conversion efficiency cannot be obtained stably.

The softening points Ts ₁ and Ts ₂ of the first and second glass frits are not particularly limited as long as the difference between the softening points ΔTs between the commonly used glass frits can be 20 ° C. or more. However, in order to obtain a desired high conversion efficiency in a wider firing temperature range, the first glass frit is preferably 510 to 570 ° C., and the second glass frit is preferably 530 to 680 ° C.

(2) B ₂ O ₃ molar ratio to SiO ₂ α / β
Glass is composed of a network oxide that becomes amorphous to form a network network structure, a modified oxide that modifies the network oxide to make it amorphous, and an intermediate oxide between the two. Composed. Of these, SiO ₂ and B ₂ O ₃ both act as network oxides and are important constituents.

In the conductive paste for electrode formation of a solar cell, the conductive powder is dissolved in the glass frit during firing of the conductive film, and the dissolved conductive powder is reduced on the semiconductor substrate 1 and deposited as metal particles. This facilitates the formation of electrical contact between the conductive powder and the semiconductor substrate 1.

However, if the molar ratio α / β of B ₂ O ₃ to SiO ₂ exceeds 0.4, the content molar amount of B ₂ O ₃ becomes excessive and the amount of conductive powder dissolved in the glass frit increases. It becomes difficult for the conductive powder dissolved in the glass frit to be deposited on the semiconductor substrate 1. As a result, a large amount of a glass component stays at the interface between the light-receiving surface electrode 3 and the semiconductor substrate 1 after firing, thereby inhibiting the formation of electrical contact.

Therefore, in the present embodiment, the molar ratio α / β of B ₂ O ₃ with respect to SiO ₂ is set to 0.4 or less.

(3) Weight ratio x / y between the first glass frit and the second glass frit
As described above, by including two types of lead-free glass frit (first and second glass frit) having different softening points of 20 ° C. or more in the conductive paste, the light-receiving surface electrode 3 and the semiconductor substrate 1 are separated from each other. The excessive flow of the glass frit at the interface can be suppressed, thereby preventing the pn junction from being broken, and the decrease in the open circuit voltage Voc can be avoided by suppressing the decrease in the parallel resistance. Conversion efficiency can be obtained.

However, when the weight ratio x / y between the first glass frit and the second glass frit is less than 1/4, the second glass frit is excessively contained, while the weight ratio x / y is 4/1. If it exceeds, the first glass frit is excessively contained.

Thus, when either one glass frit is contained in a larger amount than the other glass frit, even if there is a difference of 20 ° C. or more in the softening point of both glass frit, one kind of glass frit is eventually obtained. It is not much different from the case where glass frit is contained in the conductive paste, and stable and high conversion efficiency cannot be obtained.

Therefore, in this embodiment, the weight ratio x / y between the first glass frit and the second glass frit is set to 1/4 to 4/1.

The total content of the glass frit is not particularly limited, but is preferably 1 to 6 parts by weight with respect to 100 parts by weight of the conductive powder.

As described above, in the present embodiment, since the glass frit satisfying the above formulas (1) to (3) is contained in the conductive paste, the first glass frit having a relatively low softening point has the adhesiveness. On the other hand, excessive glass flow between the light-receiving surface electrode and the semiconductor substrate can be suppressed by the second glass frit having a softening point 20 ° C. higher than that of the first glass frit. While preventing destruction, it is possible to prevent the open circuit voltage Voc from decreasing by suppressing the decrease in parallel resistance. As a result, while being a lead-free conductive paste, a solar cell having high conversion efficiency stably even when fired in a wide temperature range without impairing fire-through properties, electrical contact properties, adhesive strength, etc. Can be obtained.

The components of the first and second glass frit are not particularly limited as long as they contain Si and B. From the viewpoint of obtaining good battery characteristics, the above-described Si—B—Bi is used. -Ba type is preferred. The composition of each glass component is such that the first glass frit contains 20 to 40 mol% Bi ₂ O ₃ , 5 to 20 mol% BaO, and 5 mol% or less Al ₂ O _3. The second glass frit preferably contains 5 to 30 mol% Bi ₂ O ₃ , 5 to 25 mol% BaO, and 5 mol% or less Al ₂ O _3. .

Bi ₂ O ₃ has an effect of adjusting the fluidity of glass as a modified oxide, and further promotes fire-through properties, and therefore plays an important role in a conductive paste for solar cells.

However, since Bi ₂ O ₃ has an effect of lowering the softening point, when the content molar amount of Bi ₂ O ₃ increases, the softening point of the glass is excessively lowered and crystallization becomes easy. Therefore, in the case of the first glass frit having a low softening point, the content molar amount is 20 to 40 mol%, and in the case of the second glass frit requiring a higher softening point than the first glass frit, The amount is preferably 5 to 30 mol%.

BaO, like Bi ₂ O ₃ , also has the effect of adjusting the fluidity of glass as a modified oxide, and contributes to the promotion of fire-through properties. Accordingly, the molar amount of BaO is determined in relation to the molar amount of Bi ₂ O ₃ having the same action. For example, in the case of the first glass frit, the content of the second glass frit is 5 to 20 mol%. In this case, 5 to 25 mol% is preferable.

Alkaline earth metal oxides other than BaO, such as MgO, SrO, and CaO, can be used because they have the effect of adjusting the fluidity of the glass as a modified oxide, as well as BaO, but are good. From the viewpoint of obtaining fire-through properties, it is preferable to use BaO.

In addition, Al ₂ O ₃ acts as an intermediate oxide, and by containing an appropriate amount, Al ₂ O ₃ can suppress crystallization of glass, obtain a stable amorphous glass, and improve the chemical durability of the glass frit. Can do.

However, if the content molar amount of Al ₂ O ₃ exceeds 5 mol%, it becomes easier to crystallize. Therefore, when Al ₂ O ₃ is included, it is 5 mol% or less, preferably 0.1%. ~ 5 mol%.

The conductive powder is not particularly limited as long as it is a metal powder having good conductivity, but it maintains good conductivity without being oxidized even when the baking treatment is performed in the air. Ag powder that can be used is preferred. The shape of the conductive powder is not particularly limited, and may be, for example, a spherical shape, a flat shape, an irregular shape, or a mixed powder thereof.

Further, the average particle diameter of the conductive powder is not particularly limited, but from the viewpoint of securing a desired contact point between the conductive powder and the semiconductor substrate 1, in terms of spherical powder, 1. 0 to 5.0 μm is preferable.

Moreover, it is also preferable to contain ZnO in the conductive paste. That is, ZnO promotes the decomposition and removal of the antireflection film 2 during the firing of the conductive paste to enable smooth fire-through, and lowers the contact resistance between the light receiving surface electrode 3 and the semiconductor substrate 1.

The organic vehicle is prepared such that the binder resin and the organic solvent are in a volume ratio of 1 to 3: 7 to 9, for example. The binder resin is not particularly limited, and for example, ethyl cellulose resin, nitrocellulose resin, acrylic resin, alkyd resin, or a combination thereof can be used. Also, the organic solvent is not particularly limited, and α-terpineol, xylene, toluene, diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether, diethylene glycol monoethyl ether acetate, etc. alone or in combination thereof Can be used.

In addition, it is also preferable to add one or a combination of plasticizers such as di-2-ethylhexyl phthalate and dibutyl phthalate to the conductive paste as necessary. It is also preferable to add a rheology modifier such as a fatty acid amide or a fatty acid, and a thixotropic agent, a thickener, a dispersant, etc. may be added.

The conductive paste is prepared by weighing and mixing the conductive powder, the first and second glass frit, the organic vehicle, and various additives as necessary at a predetermined mixing ratio, and so on. It can be easily manufactured by dispersing and kneading using the above.

As described above, in the present embodiment, the conductive powder such as Ag powder, the first and second glass frits, and the organic vehicle are contained, and the glass frit satisfies the above formulas (1) to (3). Therefore, the first glass frit having a relatively low softening point ensures adhesion, while the second glass frit having a softening point 20 ° C. higher than that of the first glass frit allows the light-receiving surface electrode and the semiconductor substrate to adhere to each other. In this way, it is possible to suppress excessive glass flow between them, and even when fired in a wide temperature range, it is possible to obtain a solar cell having stable and high conversion efficiency while ensuring good fireability. Become.

The first glass frit contains Bi ₂ O _{3 in an amount} of 20 to 40 mol%, BaO in an amount of 5 to 20 mol%, and Al ₂ O _{3 in an amount} of 5 mol% or less. Glass frit contains 5 to 30 mol% of Bi ₂ O ₃ , 5 to 25 mol% of BaO, and 5 mol% or less of Al ₂ O ₃ . Can be obtained.

Also, the inclusion of ZnO can promote the fire-through property, and a solar cell with low contact resistance between the electrode and the semiconductor substrate can be realized.

And the said solar cell will have high conversion efficiency stably even if it burns in a wide temperature range.

The present invention is not limited to the above embodiment. In the above embodiment, the case where both Bi ₂ O ₃ and BaO are included is exemplified as a preferred form of the glass frit. However, since both are modified oxides and promote fire-through properties, only one of them is included. It is good also as a component composition containing this.

Moreover, it is also preferable to contain various oxides in the glass frit as required. For example, TiO ₂ and ZrO ₂ can be drastically improved in chemical durability of glass only by being contained in a small amount in a glass frit. However, since it may act as a nucleating agent if contained in a large amount, when these TiO ₂ and ZrO ₂ are contained in the glass frit, the content in the glass frit is preferably 5 mol% or less.

_{Further, Li 2 O, Na 2 O} , an alkali metal oxide _{K 2} O, etc., similar to the _Bi 2 _{O 3,} because it has a function of adjusting the softening point of the glass, also preferably be contained as appropriate. However, if alkali metal oxide is contained in a large amount in the glass frit, the chemical durability of the glass frit may be lowered. Therefore, the content of the alkali metal oxide in the glass frit is 10 mol% or less. It is preferable to do this.

Next, specific examples of the present invention will be described.

[Sample preparation]
(Production of glass frit)
SiO ₂ , B ₂ O ₃ , Bi ₂ O ₃ , BaO, and Al ₂ O ₃ were blended so as to have a blending ratio as shown in Table 1 in mol% to prepare glass frits A to L. Then, thermal analysis was performed using TG-DTA (thermogravimetric-differential thermal analyzer), and the softening points of the glass frits A to L were measured. That is, 5 mg of a sample is accommodated in an alumina container, α alumina is used as a standard sample, and the measuring apparatus is heated at 20 ° C. per minute while supplying air into the measuring apparatus at a flow rate of 100 mL / min. It heated with the profile and the TG curve and the DTA curve were created from the weight change with respect to temperature. And the softening point in each sample was measured from such TG curve and DTA curve.

Table 1 chemical composition of the glass frit A ~ _L, B 2 molar quantity of _{O 3} alpha and the molar content of SiO ₂ beta molar ratio alpha / beta (hereinafter, referred to as _"B 2 _O 3 / SiO _2" )) And the softening point Ts.

As is apparent from Table 1, the glass frit A to J has a B ₂ O ₃ / SiO ₂ of 0.4 or less, indicating a glass frit composition within the scope of the present invention.

On the other hand, in the glass frit K and L, B ₂ O ₃ / SiO ₂ is 3.83 and 2.49, respectively, and greatly exceeds 0.4, indicating a glass frit composition outside the scope of the present invention. Yes.

In Example 1, a conductive paste was prepared using these glass frits A to L, and further, a solar battery cell was prepared using this conductive paste, and the characteristics were evaluated.

(Preparation of conductive paste)
As the conductive powder, spherical Ag powder having an average particle diameter of 1.6 μm and ZnO having a specific surface area of 10 m ² / g were prepared.

Next, an organic vehicle was produced. That is, the organic cellulose was prepared by mixing the ethyl cellulose resin and texanol so that the binder resin was 10% by weight of ethyl cellulose resin and the organic solvent was 90% by weight of texanol.

These are blended with rheology modifiers such as fatty acid amides and fatty acids and organic vehicles so that the Ag powder is 82.0% by weight, ZnO is 4.5% by weight, and the total glass frit is 2.0% by weight. After mixing with a planetary mixer, the mixture was kneaded with a three-roll mill, thereby producing conductive pastes of sample numbers 1 to 16.

The glass frit contained in the conductive paste was blended so that the total amount was 2% by weight, as shown in Table 2, by appropriately selecting and combining glass frits A to J.

(Production of solar cells)
An antireflection film having a thickness of 0.1 μm was formed by plasma enhanced chemical vapor deposition (PECVD) over the entire surface of a single crystal Si-based semiconductor substrate having a length of 50 mm, a width of 50 mm, and a thickness of 0.2 mm. In this Si-based semiconductor substrate, P is diffused into a part of the p-type Si-based semiconductor layer, whereby an n-type Si-based semiconductor layer is formed on the upper surface of the p-type Si-based semiconductor layer.

Next, an Al paste mainly composed of Al and an Ag paste mainly composed of Ag were prepared. Then, an Al paste and an Ag paste were appropriately applied to the back surface of the Si-based semiconductor substrate and dried to form a back electrode conductive film.

Next, screen printing is performed using the conductive paste, and the conductive paste is applied to the surface of the Si-based semiconductor substrate so that the film thickness after baking is 20 μm, and a conductive film for the light-receiving surface electrode is manufactured. did.

Next, each sample was placed in an oven set at a temperature of 150 ° C. to dry the conductive film.

Thereafter, a belt-type near infrared furnace (CDF7210, manufactured by Despatch) was used, and the conveyance speed was adjusted so that the sample passed between the inlet and the outlet in about 1 minute, and the maximum firing temperature of 760 to 800 in the air atmosphere. The solar cells of Sample Nos. 1 to 16 were prepared by firing at 0 ° C. and sintering the conductive paste to form the light-receiving surface electrode. The reason why the maximum firing temperature is set to 760 to 800 ° C. is that the optimum maximum firing temperature varies depending on the paste composition.

(Sample evaluation)
For each sample number 1 to 16, a solar simulator (SS-50XIL, manufactured by Eihiro Seiki Co., Ltd.) was used, and a current-voltage characteristic curve was measured under conditions of a temperature of 25 ° C. and AM (air mass) -1.5. From this current-voltage characteristic curve, a fill factor (FF) represented by Equation (4) was obtained.

FF = Pmax / (Voc × Isc) (4)

Here, Pmax is the maximum output of the sample, Voc is the open circuit voltage, and Isc is the short circuit current.

Also, the conversion efficiency η was obtained from the maximum output Pmax, the area A of the light receiving surface electrode, and the irradiance E based on the formula (5).

Η = Pmax / (A × E) (5)

Table 2 shows the paste composition, the glass frit softening point difference ΔTs, the fill factor FF, and the conversion efficiency η of each sample Nos. 1 to 16.

Sample No. 13 was found to contain only glass frit C having a softening point Ts of 542 ° C. in the conductive paste, and the conversion efficiency η was as low as 16.06%.

Sample No. 14 contains glass frits C and D in the conductive paste, but the softening point difference ΔTs is as small as 5 ° C. (glass frit C: 542 ° C., glass frit D: 547 ° C.). The conversion efficiency η was slightly improved as compared with Sample No. 12 containing only the kind, but the conversion efficiency η was still low at 16.15%, and a sufficient conversion efficiency η could not be obtained.

Sample No. 15 has a softening point difference ΔTs of 22 ° C. and a difference of 20 ° C. or more, but includes glass frit K outside the scope of the present invention in which B ₂ O ₃ / SiO ₂ is 3.83, The conversion efficiency was greatly reduced to 13.56%. This is because glass frit K containing a large amount of B ₂ O ₃ / SiO ₂ is contained, so that a large molten glass stays at the interface between the light-receiving surface electrode and the semiconductor substrate, resulting in a high contact resistance, resulting in conversion. The efficiency η seems to have decreased.

Sample No. 16 has a softening point difference ΔTs of 51 ° C. and a difference of 20 ° C. or more, but includes glass frit L outside the scope of the present invention in which B ₂ O ₃ / SiO ₂ is 2.49. For the same reason as Sample No. 15, the conversion efficiency η decreased to 13.80%.

On the other hand, Sample Nos. 1 to 12 use a combination of two types of glass frit having a softening point difference ΔTs of 20 ° C. or more, and B ₂ O ₃ / SiO ₂ is also 0.10 to 0.39. And the blending ratio of each glass frit is also 1/1, so that the conversion efficiency η is 16.36 to 16.75% and has a high conversion efficiency η of 16.35% or more. It was found that a battery was obtained.

Sample No. 12 has a softening point of the first glass frit of 597 ° C., which is a high temperature exceeding 570 ° C., but when the maximum firing temperature is optimally adjusted, a desired high conversion efficiency η is obtained. It was.

Using the glass frits E, F and H produced in Example 1, the conductive pastes of sample numbers 21 to 24 were prepared by the same method and procedure as in Example 1 so that the paste composition shown in Table 3 was obtained. Produced.

Next, solar cells of sample numbers 21 to 24 were produced in the same manner and procedure as in Example 1 except that the maximum firing temperature was set to 4 different temperatures between 780 and 820 ° C., respectively, and firing was performed.

For each sample, the fill factor FF and the conversion efficiency η were measured by the same method and procedure as in Example 1.

Table 3 shows the paste composition, firing temperature (baking maximum temperature), fill factor FF, conversion efficiency η, and determination result for each sample Nos. 21 to 24.

In addition, the determination result made the sample whose conversion efficiency (eta) 16.35% or more (circle) (pass) and the sample whose conversion efficiency is less than 16.35% made x (fail).

Sample No. 23 contains only glass frit E having a softening point Ts of 567 ° C. in the conductive paste, so the conversion efficiency η is low at 13.90 to 15.89%, and the conversion efficiency depends on the firing temperature. It was found that η varies.

In Sample No. 24, since the conductive paste contains only glass frit H having a softening point Ts of 659 ° C., a favorable conversion efficiency η of 16.66% is obtained when the firing temperature is 810 ° C. However, at other firing temperatures, the conversion efficiency η was as low as 13.28 to 15.96%. That is, it was found that although the conversion efficiency η can be obtained at the optimum firing temperature, the conversion efficiency η varies depending on the firing temperature.

Thus, it was found that when only one kind of glass frit is contained, the conversion efficiency η varies greatly depending on the firing temperature, and the desired high conversion efficiency η cannot be stably obtained in a wide firing temperature range.

On the other hand, Sample No. 21 uses glass frits E and H having a softening point difference ΔTs of 73 ° C., B ₂ O ₃ / SiO ₂ are both 0.4 or less, and the blending ratio of the two types of glass frits Therefore, it was found that a solar cell having a high conversion efficiency η of 16.47 to 16.66% over a wide temperature range of 780 to 810 ° C. was obtained.

Sample No. 22 uses glass frits F and H having a softening point difference ΔTs of 43 ° C., B ₂ O ₃ / SiO ₂ are both 0.4 or less, and the mixing ratio of the two types of glass frits is Although the softening point of the first glass frit was 597 ° C. and exceeded 570 ° C., the conversion efficiency η was reduced to 13.90% when the firing temperature was lowered to 780 ° C. That is, when the softening point of the first glass frit is excessively high, the firing temperature region for obtaining high conversion efficiency is wider than that of sample numbers 23 and 24, but is slightly narrower than that of sample number 21. I understood.

Using the glass frits A and J produced in Example 1, the conductive paste and solar cells of sample numbers 31 to 39 were prepared by the same method and procedure as in Example 1 so that the paste composition shown in Table 4 was obtained. A cell was produced.

For each sample, the fill factor FF and the conversion efficiency η were measured by the same method and procedure as in Example 1.

Table 4 shows the paste composition, the weight ratio x / y (hereinafter referred to as “A / J”) of the glass frit A and the glass frit J, the fill factor FF, and the conversion efficiency η in each of the samples 31 to 39. Is shown.

Sample No. 38 had a large A / J of 9.00 and an excessive content of glass frit A having a low softening point, so the conversion efficiency η was reduced to 15.85%. This is because glass frit A having a low softening point is excessively contained, so that an excessive flow of the glass component occurs at the interface between the conductive film to be the light-receiving surface electrode and the semiconductor substrate during firing, and the glass component becomes a semiconductor substrate. In addition, Ag is excessively diffused into the semiconductor substrate through the molten glass, leading to a reduction in parallel resistance and a reduction in the open-circuit voltage Voc. As a result, a high conversion efficiency η cannot be obtained. This is probably because of this.

On the other hand, in Sample No. 39, A / J was as small as 0.11, and the content of glass frit J having a high softening point was excessive, so the conversion efficiency η decreased to 16.15%. This is because glass frit J having a high softening point is excessively contained, so that an excessive flow of the glass component does not occur at the interface between the conductive film to be the light receiving surface electrode and the semiconductor substrate, but the glass component is the light receiving surface. It seems that it tends to float on the surface of the electrode, leading to a decrease in adhesive strength, resulting in a decrease in conversion efficiency.

In contrast, Sample Nos. 31 to 37 have an A / J of 0.25 to 4.0 (1/4 to 4/1), which is within the range of the present invention. % High solar cell was obtained.

Even when a lead-free conductive paste is used, a solar cell having a desired high conversion efficiency can be stably obtained in a wide firing temperature range.

DESCRIPTION OF SYMBOLS 1 Semiconductor substrate 2 Antireflection film 3 Light-receiving surface electrode (electrode)

Claims (8)

A conductive paste for forming a solar cell electrode,
Containing a conductive powder, a first glass frit, a second glass frit having a softening point higher by 20 ° C. or more than the first glass frit, and an organic vehicle;
The first and second glass frits do not contain Pb, contain at least B and Si, and the molar ratio of B to Si is 0.4 or less in terms of SiO ₂ and B ₂ O ₃ , respectively. And
A conductive paste characterized in that the content ratio of the first glass frit and the second glass frit is 1/4 to 4/1 by weight. The conductive paste according to claim 1, wherein the first glass frit has a softening point of 510 to 570 ° C, and the second glass frit has a softening point of 530 to 680 ° C. The first glass frit contains 20 to 40 mol% Bi ₂ O ₃ , 5 to 20 mol% BaO, and 5 mol% or less Al ₂ O ₃ , respectively. The conductive paste according to claim 1 or 2. The second glass frit contains 5 to 30 mol% of Bi ₂ O ₃ , 5 to 25 mol% of BaO, and 5 mol% or less of Al ₂ O ₃ , respectively. The electrically conductive paste in any one of Claim 1 thru | or 3. 3. The conductive paste according to claim 1, wherein the first and second glass frits contain at least one of Bi and Ba. The conductive paste according to any one of claims 1 to 5, wherein the conductive paste contains ZnO. The conductive paste according to claim 1, wherein the conductive powder is an Ag powder. An antireflection film and an electrode penetrating the antireflection film are formed on one main surface of the semiconductor substrate,
A solar cell, wherein the electrode is formed by sintering the conductive paste according to any one of claims 1 to 7.

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