TW201301528A - Conductive paste and solar cell - Google Patents

Abstract

This conductive paste contains an Ag powder, a binder resin and an organic solvent, while additionally containing an alkali metal compound having a melting point of 1,000 DEG C or less. It is particularly preferable to use Li as the alkali metal. As a compound form of the alkali metal compound, a carboxylic salt or an amide having a melting point of 400 DEG C or less is preferable, and a carbonate or a borate may also be used. A light-receiving surface electrode (3) is formed using this conductive paste. Consequently, there can be provided a conductive paste for solar cell electrodes, which enables good battery characteristics, while achieving good fire-through performance and low electrode resistivity.

Description

Conductive paste and solar cell

The present invention relates to a conductive paste and a solar cell. More specifically, the present invention relates to a conductive paste suitable for forming an electrode of a solar cell and a solar cell produced using the conductive paste.

A solar cell is generally formed with a specific pattern of light-receiving surface electrodes on one main surface of a semiconductor substrate. Further, an anti-reflection film is formed on the semiconductor substrate other than the light-receiving surface electrode, and the reflection loss of the incident sunlight is suppressed by the anti-reflection film, thereby improving the conversion efficiency of sunlight to electric energy.

The light-receiving surface electrode is usually formed by applying a conductive paste to the surface of the anti-reflection film to form a conductive film having a specific pattern, and firing the film. In other words, since the antireflection film of the lower layer of the conductive film is formed by an insulator such as tantalum nitride (SiN _x ), the antireflection film is decomposed and removed during the firing of the light-receiving surface electrode, and the conductive film is sintered to form a light-receiving surface electrode. And the light-receiving surface electrode and the semiconductor substrate are connected to each other.

The method of decomposing and removing the anti-reflection film in the calcination process to cause the semiconductor substrate and the light-receiving surface electrode to follow is referred to as "fire through", and the conversion efficiency of the solar cell largely depends on the fire-through property. In other words, when the burn-through property is insufficient, an anti-reflection film remains between the light-receiving surface electrode and the semiconductor substrate, so that the conductivity between the light-receiving surface electrode and the semiconductor substrate is lowered, and as a result, the conversion efficiency is lowered, and the solar cell is basically used. Poor performance.

Therefore, in order to improve the characteristics of solar cells, it is necessary to improve the fire-through property. Since the electrical conductivity of the conductive powder such as Ag is inferior, an inorganic oxide such as ZnO is added to the conductive paste to improve the fire-through property.

For example, Patent Document 1 proposes a thick film conductive composition in which an Ag powder, an additive containing Zn, and one or a plurality of lead-free glass frits are dispersed in an organic medium.

In Patent Document 1, a conductive paste (thick film conductive composition) containing 2 to 10% by weight of ZnO and 0.5 to 4% by weight of a glass frit is used, and a good bonding strength is obtained and converted. A solar cell that is efficient.

Further, in Patent Document 2, an Ag powder, a ZnO powder, a lead-free glass frit, and an organic solvent are proposed, and the lead-free glass frit is based on the total glass frit, Bi ₂ O ₃ >5 mol%, and B ₂ O ₃ <15 mol. %, BaO < 5 mol%, SrO < 5 mol%, Al ₂ O ₃ < 5 mol%, (content of ZnO / content of Ag powder) × 100 thick film conductive composition.

In Patent Document 2, by setting the content of ZnO and Ag powder to be (content of ZnO/content of silver powder) × 100>2.5, typically, ZnO in the composition is set to 0.5 to 15.0% by weight. And seek for the improvement of the electrical performance of solar cells.

Further, Patent Document 3 proposes a solar cell including a contact made of a mixture, and the mixture contains a solid portion and an organic portion before calcination, and the solid portion contains a conductive metal component such as Ag: about 85 to about 99% by weight and glass component: about 1 to about 15% by weight, and the glass component does not contain lead.

Further, Patent Document 3 discloses that the solid portion is a solar cell contact in which a specific oxide such as SnO or ZnO or a specific composite oxide such as 2Li ₂ O‧5V ₂ O _{5 is} added to the glass component.

Further, in Patent Document 3, by using the conductive paste having the above composition, even if the glass component does not contain lead having a large environmental load, the anti-reflection film can be removed during the firing of the light-receiving electrode. The contact resistance between the light-receiving surface electrode and the semiconductor substrate can be reduced.

Prior technical literature Patent literature

Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-332032 (refer to claim 1, paragraph number [0024], [0031], [0058], etc.)

Patent Document 2: Japanese Patent Laid-Open Publication No. 2010-524257 (refer to claim 1, paragraph number [0026] to [0028], etc.)

Patent Document 3: Japanese Patent Laid-Open Publication No. 2008-543080 (refer to Request No. 1, Paragraph No. [0017])

However, in the above-mentioned Patent Documents 1 to 3, the conductive paste contains an additive such as an inorganic oxide such as ZnO or a glass frit to improve the fire permeability, but the content of the additives is large, and the content of the conductive powder is high. Relatively less, there is a problem that the specific resistance of the electrode becomes high.

The present invention has been made in view of such circumstances, and an object thereof is to provide a conductive paste for a solar cell electrode which has good fire-through property, a low specific resistance of an electrode, and which can obtain good battery characteristics, and uses the conductivity. A solar cell made by paste.

The present inventors conducted intensive studies to achieve the above object, and as a result, it has been found that only an alkali metal compound having a melting point of 1000 ° C or less is contained in the conductive paste, and even if it does not contain an inorganic oxide, it is effective to cause permeation. The content ratio of the metal component derived from the conductive powder can be increased in the electrode, and the specific resistance of the electrode can be lowered.

The present invention has been completed based on the above-described findings, and the conductive paste of the present invention is characterized in that it is used to form a conductive paste of an electrode of a solar cell, and contains a conductive powder, a binder resin, a solvent, and a melting point. It is an alkali metal compound of 1000 ° C or less.

Thereby, fire-through can be effectively produced even without adding an inorganic oxide. Further, since the content ratio of the metal component derived from the conductive powder can be increased in the electrode, the specific resistance of the electrode can be lowered, whereby the conversion efficiency of the solar cell can be improved.

Further, the alkali metal compound having a lower melting point tends to flow at the substrate interface from the time of drying to the calcination. Therefore, even if a trace amount is added, the burnt property peculiar to the alkali metal can be sufficiently produced. Therefore, the lower the melting point of the alkali metal compound, the better, preferably 800 ° C or lower, more preferably 400 ° C or lower.

That is, the conductive paste of the present invention preferably has a melting point of the alkali metal compound of 800 ° C or less.

Further, in the conductive paste of the present invention, it is preferred that the alkali metal compound has a melting point of 400 ° C or lower.

Further, in the conductive paste of the present invention, it is preferable that the alkali metal compound contains at least one of a carboxylate and an amine.

In this case, a better burntability can be ensured.

Further, in the conductive paste of the present invention, it is preferable that the alkali metal compound contains at least one of a carbonate and a borate.

In this case, even if the melting point is relatively high or a small amount is added, a conductive paste for a solar cell electrode having desired burnt property and a low specific resistance of the electrode can be realized.

Further, in the conductive paste of the present invention, it is preferable that the alkali metal element contained in the alkali metal compound is lithium.

In this case, a conductive paste which is more excellent in fire-through property and can achieve low line resistance can be obtained.

Further, the conductive paste of the present invention preferably has a content of the alkali metal compound of 2% by weight or less (excluding 0% by weight).

Thereby, it is possible to realize a conductive paste in which the specific resistance of the electrode is not increased and the fire-through property is good and the low-line resistance can be obtained.

Further, in the conductive paste of the present invention, it is preferable that the conductive powder is Ag powder.

Further, the solar cell of the present invention is characterized in that an antireflection film and an electrode penetrating the antireflection film are formed on one main surface of the semiconductor substrate, and the electrode is sintered by the conductive paste described in any one of the above. .

The conductive paste of the present invention contains an electroconductive powder (preferably Ag powder), a binder resin and a solvent, and contains an alkali metal compound having a melting point of 1000 ° C or less, so that it can be effective even without adding an inorganic oxide. The ground is burnt through. Moreover, since the content ratio of the metal component derived from the conductive powder can be increased in the electrode, the specific resistance of the electrode can be lowered, thereby improving Conversion efficiency of solar cells.

Further, according to the solar cell of the present invention, the antireflection film and the electrode penetrating the antireflection film are formed on one main surface of the semiconductor substrate, and the electrode is sintered by the conductive paste described in any one of the above. Therefore, it is possible to obtain a solar cell having good conductivity between the semiconductor substrate and the electrode, low specific resistance of the electrode, and good conversion efficiency.

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

Fig. 1 is a cross-sectional view of an essential part of an embodiment of a solar cell produced by using the conductive paste of the present invention.

In the solar cell, the anti-reflection film 2 and the light-receiving surface electrode 3 are formed on one main surface of the semiconductor substrate 1 having Si as a main component, and the back surface electrode 4 is formed on the other main surface of the semiconductor substrate 1.

The semiconductor substrate 1 includes 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 it is on the upper surface of the p-type semiconductor layer 1b. When the n-type semiconductor layer 1a is formed, the structure and production method thereof are not particularly limited. Further, the semiconductor substrate 1 may be used in 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 is formed on a part of one main surface of the semiconductor substrate 1 and The structure of both of the n-type semiconductor layers 1a. The conductive paste of the present invention can be effectively used as long as the main surface of the semiconductor substrate 1 on which the anti-reflection film 2 is formed. Furthermore, in FIG. 1, the surface of the semiconductor substrate 1 is described as flat. The flat shape is such that the sunlight is effectively enclosed in the semiconductor substrate 1, and the surface thereof is formed to have a fine uneven structure.

The anti-reflection film 2 is formed of an insulating material such as tantalum nitride (SiN _x ), and suppresses the reflection of sunlight reflected by the arrow A on the light-receiving surface, thereby guiding the solar light to the semiconductor substrate 1 quickly and efficiently. The material constituting the anti-reflection film 2 is not limited to the above-described tantalum nitride, and other insulating materials such as ruthenium oxide (SiO ₂ ) or titanium oxide (TiO ₂ ) may be used, or two or more kinds of insulation may be used in combination. Sexual material. Further, in the case of a crystalline Si system, any of single crystal Si and polycrystalline Si can be used.

The light-receiving surface electrode 3 penetrates the anti-reflection film 2 and is formed on the semiconductor substrate 1. The light-receiving surface electrode 3 can be formed by applying a conductive paste of the present invention to the semiconductor substrate 1 by screen printing or the like to form a conductive film and firing it. In the firing process in which the light-receiving surface electrode 3 is formed, the anti-reflection film 2 of the lower layer of the conductive film is decomposed and removed, and is fired, whereby the light-receiving surface electrode 3 is formed on the semiconductor substrate 1 so as to penetrate the anti-reflection film 2.

Specifically, as shown in FIG. 2, the light-receiving surface electrode 3 is provided with a plurality of finger electrodes 5a, 5b, ... 5n in a comb-tooth shape, and is connected with the finger electrodes 5a, 5b, .. The bus electrodes 6 are provided in a manner of being crossed, and the finger electrodes 5a, 5b, ... 5n are electrically connected to the bus electrodes 6. Further, the anti-reflection film 2 is formed in a remaining region other than 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 electrodes 6.

Specifically, as shown in FIG. 3, the back electrode 4 is formed by a p-type semiconductor The collector electrode 7 including Al or the like on the back surface of the bulk layer 1b is configured to include an extraction electrode 8 including Ag or the like which is formed on the back surface of the collector electrode 7 and electrically connected to the collector electrode 7. Further, electric power generated in the semiconductor substrate 1 is concentrated in the collector electrode 7, and electric power is extracted by taking out the 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 a conductive powder, a binder resin, and an organic solvent, and contains an alkali metal compound having a melting point of 1000 ° C or less.

In this way, when the conductive paste of the present invention contains an alkali metal compound, even if the conductive paste does not contain an inorganic oxide such as ZnO or a glass frit, it can be burnt through during the firing of the solar cell.

That is, a considerable amount of an inorganic oxide such as ZnO is previously added to improve the fire permeability. Therefore, in the light-receiving surface electrode 3, the content ratio of the metal component derived from the conductive powder is lowered, and as a result, the specific resistance of the electrode is increased.

However, according to the findings of the present inventors, it has been confirmed that an alkali metal compound containing an alkali metal element and having a low melting point has a peculiar property of causing fire-through property.

Therefore, in the present embodiment, the conductive paste contains an alkali metal compound having a melting point of 1000 ° C or lower, whereby the fire-through property is generated. Further, it is only necessary to add a small amount of the burnt property by the alkali metal compound, and it is not necessary to add a considerable amount as in the case of the prior inorganic oxide.

In other words, in the present invention, since only a trace amount of the alkali metal compound is contained, the fire-through property can be ensured, so that the gold derived from the conductive powder in the light-receiving surface electrode 3 can be improved. The content ratio of the genus component. Therefore, the specific resistance of the electrode can also be lowered, and the conversion efficiency of the solar cell can be further improved.

The content of the alkali metal compound contained in the conductive paste is not particularly limited, and as described above, the desired fire permeability can be ensured in a small amount. However, when the content of the alkali metal compound is excessive, the specific resistance of the electrode tends to increase, so that it is preferably 2% by weight or less (excluding 0% by weight or less).

Further, the alkali metal compound is not particularly limited as long as it has a low melting point and contains an alkali metal element, and preferably has a melting point of 1000 ° C or less, preferably a melting point of 800 ° C or less, and more preferably a melting point of 400. An alkali metal compound below °C. That is, when the melting point is lowered, the alkali metal compound flows on the interface with the n-type semiconductor layer 1a of the semiconductor substrate 1 from the time of drying to the time of firing. Therefore, by containing a trace amount of the alkali metal compound, a sufficient amount of the alkali metal element can be present on the surface of the n-type semiconductor layer 1a, and the fire-through property at the interface with the n-type semiconductor layer 1a can be dramatically improved.

Further, as the alkali metal compound, a carboxylate such as a dodecanoate, an octadecanoate, an acetate or an oxalate containing an alkali metal element, a guanamine or the like can be preferably used.

Among them, even in the case where the melting point exceeds 400 ° C, the alkali metal-containing carbonate or borate may contain only a trace amount to ensure good fire-through property.

Further, the alkali metal element is not particularly limited, and Li, K, Na or the like can be used, and Li can be preferably used from the viewpoint of obtaining a smaller contact resistance Rc.

Further, as the conductive powder, any metal having good conductivity is used. The powder is not particularly limited, and an Ag powder which does not oxidize even when it is subjected to a calcination treatment in the air to maintain good conductivity can be preferably used. Further, the shape of the conductive powder is not particularly limited, and may be, for example, a spherical shape, a flat shape, an amorphous shape, or a mixed powder of the above.

In addition, the average particle diameter of the conductive powder is not particularly limited, and from the viewpoint of ensuring a desired contact point, the conductive powder and the semiconductor substrate 1 are preferably 1.0 to 5.0 μm in terms of spherical powder.

The binder resin contained in the conductive paste is not particularly limited, and for example, an ethyl cellulose resin, a nitrocellulose resin, an acrylic resin, an alkyd resin, or a combination thereof may be used.

Further, the organic solvent is not particularly limited, and α-rosinol, xylene, toluene, diethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether may be used singly or in combination. , diethylene glycol monoethyl ether acetate, and the like.

Further, the binder resin and the organic solvent are prepared, for example, in a volume ratio of 1 to 3:7 to 9, whereby an organic vehicle is produced.

Further, the conductive paste of the present invention can exert a desired effect even if it does not contain a glass frit, and can contain a glass frit in order to improve the adhesion between the light-receiving surface electrode 3 and the semiconductor substrate 1. In this case, the glass frit is not particularly limited, and in view of the environment, it is preferable to use a lead-free glass frit such as a lead-free Si-B-Bi-based glass frit.

Further, if the specific resistance of the electrode does not become a problem, an inorganic oxide may be added.

Further, the conductive paste can be weighed by using a conductive powder, an organic vehicle, and optionally various additives in a specific mixing ratio. The mixture is mixed and dispersed, and kneaded using a three-roll mill or the like to be easily produced.

In this manner, the present embodiment contains a conductive powder such as Ag powder, a binder resin, and a solvent, and contains an alkali metal compound having a melting point of 1000 ° C or less. Therefore, it can be burnt through without adding an inorganic oxide. Further, since the content ratio of the conductive powder in the light-receiving surface electrode 3 can be increased, the specific resistance of the electrode can be lowered, whereby the conversion efficiency of the solar cell can be improved.

In addition, when the melting point of the alkali metal compound is 800 ° C or lower, particularly 400 ° C or lower, since the melting point of the alkali metal compound becomes low, it is easy to form an interface with the n-type semiconductor layer 1a during drying to calcination. flow. Therefore, even if a trace amount is added, the burnt property peculiar to the alkali metal can be sufficiently produced.

Further, by allowing the alkali metal compound to contain any one of a carboxylate and an amine, more excellent fire permeability can be obtained.

Further, when the alkali metal compound contains any one of a carbonate and a borate, even if the melting point is relatively high, the conductivity of the solar cell electrode having a desired burnt property and a low line resistance can be obtained in a small amount. paste.

Moreover, when the alkali metal element is lithium, it is possible to achieve a conductive paste having a better fire-through property and a low specific resistance.

Further, a solar cell having good conductivity between the semiconductor substrate 1 and the light-receiving surface electrode 3 and having a low specific resistance and excellent conversion efficiency can be obtained.

Furthermore, the present invention is not limited to the above embodiments. Further, it is preferred to add a small amount of a plasticizer such as di-ethylhexyl phthalate or dibutyl phthalate to the conductive paste as needed or a combination thereof. Further, a rheology modifier such as a fatty acid guanamine or a fatty acid is preferably added, and a thixotropic agent, a thickener, a dispersant or the like may be added.

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

Example 1 [Production of conductive paste]

(Sample No. 1~10, 13, 15, 16)

A spherical Ag powder having an average particle diameter of 1.0 μm was prepared as a conductive powder, and an additive shown in Table 1 was prepared.

Then, an organic vehicle is produced. In other words, the ethyl cellulose resin was mixed with TEXANOL so that the ethyl cellulose resin as the binder resin was 10% by weight, and the TEXANOL (trade name) as the organic solvent was 90% by weight to prepare an organic vehicle.

Then, the Ag powder was blended in an amount of 80.0% by weight, the additive was 0.2% by weight, and the organic vehicle was 19.8% by weight. The mixture was mixed by a planetary mixer and kneaded by a three-roll mill to prepare a sample. Conductive pastes Nos. 1 to 10, 13, 15 and 16.

(sample number 11)

A Si-B-Bi-based lead-free glass frit was prepared in addition to a spherical Ag powder having an average particle diameter of 1.0 μm and lithium dodecanoate as an additive.

Then, an organic vehicle was prepared in the same manner as above, and then Ag powder was 80.0% by weight, additive was 0.2% by weight, Si-B-Bi-based lead-free glass frit was 2% by weight, and organic vehicle was 17.8% by weight. In the manner of mixing these, the mixture was mixed by a planetary mixer and kneaded by a three-roll mill to prepare a conductive paste of sample No. 11.

(sample number 12)

Conductive sample No. 12 was produced in the same manner as above except that the spherical Ag powder having an average particle diameter of 1.0 μm was 80% by weight, and the organic vehicle was added in an amount of 20% by weight, and the alkali metal compound was not added. Sex paste.

(sample number 14)

A sample was prepared in the same manner as described above except that the spherical Ag powder having an average particle diameter of 1.0 μm was 80% by weight, the zinc oxide as an additive was 4.6% by weight, and the organic vehicle was added to 15.4% by weight. No. 14 conductive paste.

(sample number 17)

The spherical Ag powder having an average particle diameter of 1.0 μm is 80% by weight, the Si—B—Bi—Li-based lead-free glass frit is 2.0% by weight, and the organic vehicle is added to 18.0% by weight, and the alkali metal compound is not added. A conductive paste of sample No. 17 was produced in the same manner as above except the above.

(sample number 18)

The spherical Ag powder having an average particle diameter of 1.0 μm was 80% by weight, the Si—B—Bi-based lead-free glass frit was 2.0% by weight, and the organic vehicle was added to 18.0% by weight, and the alkali metal compound was not added. Otherwise, the conductive paste of sample No. 18 was produced in the same manner as above.

[Measurement of Melting Point]

The additive used for the preparation of the sample was subjected to thermal analysis using TG-DTA (Thermo-Gravimetric/Differential Thermal Analyzer) and the melting point was measured. That is, the sample is stored in an alumina container 5 mg, the standard sample is made of α-alumina, and air is supplied to the measuring device at a flow rate of 100 mL/min, and the measuring device is heated by a calcination distribution of 20° C. in 1 minute, and is produced according to the weight change with respect to temperature. TG (Thermo-Gravimetric) curve and DTA (Differential Thermal) curve. Then, the melting point of each additive was measured based on the TG curve and the DTA curve.

[Evaluation of sample]

As shown in FIG. 4, a specific electrode pattern was formed on the antireflection film, and the contact resistance Rc was obtained by a TLM (Transmission Line Model) method.

That is, the whole of the polycrystalline Si-based semiconductor substrate 11 having a lateral X of 50 mm, a longitudinal Y of 50 mm, and a thickness T of 0.2 mm by plasma enhanced chemical vapor deposition (PECVD). An anti-reflection film 12 having a film thickness of 0.1 μm was formed on the surface.

Here, as the material of the anti-reflection film 12, TiO ₂ is used for sample No. 10, and SiN _x is used for others. Further, the Si-based semiconductor substrate 11 is formed on the surface of the p-type Si-based semiconductor layer to form an n-type Si-based semiconductor layer.

Then, each of the conductive pastes of sample Nos. 1 to 18 was used for screen printing to produce a conductive film having a specific pattern and having a film thickness of 20 μm. Then, each sample was placed in an oven set to a temperature of 150 ° C to dry the conductive film.

Thereafter, a belt type near-infrared furnace (CDF7210, manufactured by Despatch Co., Ltd.) was used to adjust the conveying speed by transferring the sample from the inlet to the outlet in about 1 minute, and calcining at a maximum calcination temperature of 750 ° C in an atmosphere. Samples of sample numbers 1 to 18 of the electrodes 13a to 13f were formed.

Here, the distances L1 to L5 of the respective electrodes 13a to 13f are measured, and as a result, the distance L1 between the electrode 13a and the electrode 13b is 200 μm, and the distance L2 between the electrode 13b and the electrode 13c is 400 μm, and the electrode 13c and the electrode 13d are The distance L3 is 600 μm, the distance L4 between the electrode 13d and the electrode 13e is 800 μm, and the distance L5 between the electrode 13e and the electrode 13f is 1000 μm. Also, the length Z of the electrodes is 30 mm.

Then, the contact resistance Rc was obtained for each of the sample numbers 1 to 18 by the TLM method.

This TLM method is widely known as a method for evaluating the contact resistance of a film sample, and the contact resistance Rc is calculated by using a transfer line theory and considering the electrode and the lower semiconductor substrate in consideration of a so-called transfer line circuit. That is, the electrode length Z 13a ~ 13f, the sheet resistance R _SH n-type semiconductor layer of Si-based, inter-electrode distance L, between the inter electrode resistance R, equation (1) is established.

R=(L/Z)×R _SH +2Rc......(1)

According to the mathematical formula (1), it is clear that the inter-electrode resistance R has a linear relationship with the distance L between the electrodes. Therefore, each of the resistors R under the inter-electrode distance Ln (n = 1 to 5) is measured, and L is estimated to be 0, thereby obtaining 2Rc, and the contact resistance Rc can be calculated from the 2Rc.

Therefore, in the present embodiment, the respective resistances R at the distance Ln between the electrodes were measured, and the contact resistance Rc of each sample of the sample numbers 1 to 18 was calculated. In addition, the sheet resistance R _{SH of the} n-type Si-based semiconductor layer can be calculated from the straight line derived from the above formula (1), based on the slope when the horizontal axis is L and the vertical axis is R. This is 30 Ω/cm.

In addition, the length of each sample of sample Nos. 1 to 18 was measured to be 30 mm in length and width. Line resistance of 200 μm electrode. Then, the line resistance was divided by the length and multiplied by the sectional area, thereby measuring the specific resistance of the electrode. Further, the line resistance was measured using a digital voltmeter, and the sectional area of the electrode was measured using a contact surface roughness meter.

Table 1 shows the specifications of the conductive paste of sample Nos. 1 to 18, the contact resistance Rc, and the specific resistance of the electrodes.

In the sample No. 12, the electrodes 13a to 13f were formed only of Ag, and the alkali metal compound was not contained. Therefore, the specific resistance of the electrode was low, but no burn-through occurred, and the contact resistance Rc was excessively high, and the contact resistance Rc could not be measured.

The sample resistance 13 was also unable to measure the contact resistance Rc. The reason for this is considered to be that although zinc oxide is contained as an additive, the content thereof is 0.2% by weight, so that no fire-through occurs.

Sample No. 14 has a lower contact resistance Rc of 2.5 Ω, but the specific resistance of the electrode is increased to 6.60 μΩ. Cm. It is considered that since the content of zinc oxide is increased to 4.6 wt%, the fire-through property is good, and the contact resistance Rc is lowered, but the content of Ag is relatively lowered, so that the specific resistance is high.

The specific resistance of the electrode of sample No. 15 was good, 5.28 μΩ. Cm, but the contact resistance Rc is extremely high at 4532 Ω. It is considered that although a carboxylate is contained as an additive, the carboxylate does not contain an alkali metal element, and as a result, the fire-through property is inferior.

The specific resistance of the electrode of sample No. 16 is good, 3.96 μΩ. In the same manner as in sample No. 15, the alkali metal element was not contained in the carboxylate, and therefore the contact resistance Rc became high at 770 Ω.

On the other hand, it is understood that Sample Nos. 1 to 11 each contain 0.2% by weight of an alkali metal compound having a melting point of 1000 ° C or less, so that the contact resistance Rc is low, the fire permeability is good, and the specific resistance of the electrode is lowered.

Further, it was found that the contact resistance Rc of the sample numbers 4 to 7, 10, and 11 containing lithium carboxylate was 7 Ω or less, and particularly excellent results were obtained.

Further, as shown in the sample No. 7, it was found that when lithium is contained, a good contact resistance Rc of 10 Ω or less can be obtained even if it is an amine.

Further, as shown in the sample Nos. 8 and 9, it is understood that when lithium is contained, a good contact resistance Rc of 30 Ω or less can be obtained even if it is a carbonate or a borate.

Further, it is clear from the comparison between sample No. 5 and sample No. 10 that by containing a trace amount of the additive of the present invention, it is possible to perform desired permeation without any kind of antireflection film, and to obtain a good contact resistance Rc. Thus, a good ohmic contact can be obtained between the semiconductor substrate-electrodes.

Moreover, it can be clearly confirmed from the comparison between the sample No. 5 and the sample No. 11, even if the glass frit is contained in the conductive paste to impart adhesion to the semiconductor substrate 11, the specific resistance to the contact resistance Rc or the electrode is hardly observed. The effect is to obtain the desired good ohmic contact.

In addition, sample Nos. 17 and 18 were investigated as the form of inclusion of Li as an alkali metal element. From the comparison between the sample No. 17 and the sample No. 18, it can be clearly confirmed that since the glass frit itself has a fire-through property, the contact resistance Rc of both of them becomes 100 Ω or less, but the Si-B-Bi-based lead-free glass frit The contact resistance Rc is lowered as compared with the Si-B-Bi-Li-based lead-free glass frit containing Li. In other words, even if Li is contained in the glass frit, the effect of lowering the contact resistance Rc of Li does not occur, and the addition of the additive to the conductive paste causes an effect of lowering the contact resistance Rc.

Fig. 5 is a graph showing the relationship between the melting point Tm of each sample and the contact resistance Rc for sample numbers 4 to 9, the horizontal axis being the melting point Tm (°C), and the vertical axis being the contact resistance Rc (Ω).

As is clear from Fig. 5, when the melting point Tm is lowered, the contact resistance Rc is also lowered, and in particular, the melting point Tm is preferably 800 ° C or lower, more preferably 400 ° C or lower.

Example 2

A conductive paste of sample Nos. 21 to 24 in which the content of lithium dodecanoate was different was prepared using lithium dodecanoate as an alkali metal compound. Further, the sample No. 22 is the same as the conductive paste of the sample No. 5 of the first embodiment.

Then, the contact resistance Rc and the specific resistance of the electrode were determined for each of the sample numbers 21 to 24 in the same manner as in the first embodiment.

Table 2 shows the melting points, contents, and measurement results of lithium dodecanoate contained in each of the conductive pastes of Sample Nos. 21 to 24.

From the results of Table 2, it is clear that when the content of lithium dodecanoate is increased, the contact resistance Rc hardly changes, but the specific resistance of the electrode tends to increase. Therefore, it is understood that the lithium dodecanoate content is preferably 2% by weight or less in consideration of the specific resistance of the electrode.

Industrial availability

A solar cell having high conversion efficiency is realized by using a conductive paste having good burnt property and a low specific resistance of the electrode.

1‧‧‧Semiconductor substrate

1a‧‧‧n type semiconductor layer

1b‧‧‧p-type semiconductor layer

2‧‧‧Anti-reflective film

3‧‧‧Photometric surface electrode (electrode)

4‧‧‧Back electrode

5a‧‧‧ finger electrode

5b‧‧‧ finger electrode

5n‧‧‧ finger electrode

6‧‧‧Concurrent electrode

7‧‧‧ Collecting electrode

8‧‧‧Removing the electrode

11‧‧‧Si-based semiconductor substrate

12‧‧‧Anti-reflective film

13a~13f‧‧‧electrode

A‧‧‧ arrow

L1~L5‧‧‧Distance

X~Z‧‧‧ length

Fig. 1 is a cross-sectional view showing an essential part of an embodiment of a solar cell produced by using the conductive paste of the present invention.

Fig. 2 is an enlarged plan view showing the side of the light-receiving surface electrode.

Fig. 3 is a schematic enlarged plan view showing the back electrode side.

Fig. 4 is a plan view schematically showing an electrode pattern produced by the embodiment.

Fig. 5 is a graph showing the relationship between the melting point Tm of the alkali metal compound used in the examples and the contact resistance Rc.

1‧‧‧Semiconductor substrate

1a‧‧‧n type semiconductor layer

1b‧‧‧p-type semiconductor layer

2‧‧‧Anti-reflective film

3‧‧‧Photon surface electrode

4‧‧‧Back electrode

Claims (9)

A conductive paste comprising an electroconductive powder, a binder resin, and a solvent, and an alkali metal compound having a melting point of 1000 ° C or less, which is used to form an electrode of a solar cell. The conductive paste of claim 1, wherein the alkali metal compound has a melting point of 800 ° C or less. The conductive paste of claim 2, wherein the alkali metal compound has a melting point of 400 ° C or less. The conductive paste according to any one of claims 1 to 3, wherein the alkali metal compound contains at least one of a carboxylate and an amine. The conductive paste of claim 1, wherein the alkali metal compound contains at least one of a carbonate and a borate. The conductive paste according to any one of claims 1 to 3, wherein the alkali metal element contained in the alkali metal compound is lithium. The conductive paste according to any one of claims 1 to 3, wherein the content of the alkali metal compound is 2% by weight or less (excluding 0% by weight). The conductive paste according to any one of claims 1 to 3, wherein the conductive powder is Ag powder. A solar cell characterized in that an antireflection film and an electrode penetrating the antireflection film are formed on one main surface of a semiconductor substrate, and the electrode is sintered by the conductive paste according to any one of claims 1 to 8. .