JPH0892506A - Electrically conductive paste, method of electrode formation and solar cell - Google Patents

Abstract

PURPOSE: To provide an electrically conductive paste which is suitable for mass production because it can be readily formed into electrodes at a low cost and can form electrodes with good soldering properties and sufficient terminal tensile strength. CONSTITUTION: This electrically conductive paste comprises an epoxy resin of more than 900 molecular weight, silver powder in a weight ratio of 4-10 to the epoxy resin, and an imidazole curing agent in an amount twice or more of the minimum amount needed for curing the epoxy resin. This paste is used as an electrode material for solar cells. Further, it is heat-cured at 150 deg.C-200 deg.C to give the electrodes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive paste and an electrode forming method.

[0002]

2. Description of the Related Art Conventionally, electrodes whose characteristics deteriorate when exposed to high temperatures during electrode formation, for example, collector electrodes, terminal electrodes, and ohmic electrodes of solar battery cells, are made of Ag, Al, etc. by sputtering or vapor deposition. The electrode was formed by depositing a film of about 3 to 5 μm. And for this electrode,
Soldering with a solder containing Ag or ultrasonic waves has been performed.

As another method, an electrode is formed from a heat-curable conductive paint using a thermoplastic resin such as acrylic resin, and soldering is performed by dipping the solder.

[0004]

As described above, Ag,
When Al or the like is formed by sputtering or vapor deposition, a vacuum system is used at the time of film formation, which is disadvantageous to the mass production method by continuous operation.

Further, in order to prevent solder erosion, it is necessary to secure a sufficient film thickness, which requires a long film deposition time.

[0006] Further, as a measure for preventing this solder erosion, Ni electrolytic plating treatment and the like have been carried out, but the production process is increased and the cost is increased.

Further, in the case of a heat-curable conductive coating material using a thermoplastic resin such as an acrylic resin, the solder paste is remarkably corroded by the reflow temperature of the solder paste, and sufficient tensile strength of the terminal cannot be secured after soldering. It was

Therefore, an object of the present invention is to provide a conductive paste, an electrode forming method, and a solar cell, which are suitable for mass production of electrodes and which can form electrodes at low cost.

It is another object of the present invention to provide a conductive paste capable of forming an electrode having good solderability and having sufficient terminal tensile strength, an electrode forming method, and a solar cell.

[0010]

The present invention has been made to solve the above problems, and the conductive paste of the present invention comprises an epoxy resin having a molecular weight of 900 or more, and the epoxy resin. A silver powder having a weight ratio in the range of 4 to 10 and an imidazole-based curing agent in an amount twice or more the minimum amount required to cure the epoxy resin. Is.

Further, the electrode forming method of the present invention comprises a step of mixing an epoxy resin having a molecular weight of 900 or more with a silver powder having a weight ratio of 4 to 10 with respect to the epoxy resin, and the epoxy resin. A step of adding an imidazole-based curing agent in an amount twice or more the minimum amount required for curing the epoxy resin, a step of applying the conductive paste by screen printing, and a step of applying the conductive paste at 150 ° C. To 200 ° C. in the range of heating and curing.

The solar cell of the present invention has a molecular weight of 90.
0 or more epoxy resin, silver powder having a weight ratio with respect to the epoxy resin in the range of 4 to 10, and an imidazole-based compound having an addition amount that is twice or more the minimum addition amount required to cure the epoxy resin. A conductive paste containing a curing agent is applied by screen printing, and the temperature is 150 ° C to 200 ° C.
It is characterized in that it is provided with an electrode which is cured by heating within the range of ° C.

[0013]

With the conductive paste, electrode forming method, and solar cell of the present invention, it is possible to form fine silver powder particles,
Moreover, since the latent curing agent, which is an imidazole-based curing agent, is used, an increase in the viscosity of the conductive paste can be suppressed, which is suitable for screen printing. Therefore, the electrodes can be easily formed by using screen printing.

Further, since the molecular weight of the epoxy resin is 900 or more, unlike the epoxy resin having a molecular weight of less than 900, which can be heated and cured quickly, it can be allowed to proceed slowly.

Further, since the curing agent is an imidazole curing agent, the pot life, that is, the pot life until it reaches twice the initial viscosity can be lengthened.

When the weight ratio of the silver powder to the epoxy resin (silver powder / epoxy resin) is in the range of 4 to 10, the resin ratio is too high and the resin coats the conductive paste coating film so that the solder is used. It is possible to prevent poor attachment properties and to prevent the terminal tensile strength from being obtained due to solder erosion due to too high a silver powder ratio.

Further, since the amount of the imidazole-based curing agent added to the epoxy resin is a proper amount, that is, at least twice the minimum amount required to cure the resin, the tensile strength of the terminal is sufficient. Can be obtained.

Further, in the solar cell of the present invention, the conductivity of the electrode material is good.

[0019]

EXAMPLES Examples of the conductive paste of the present invention will be described below with reference to the case where it is used as an electrode material for solar cells. 1 and 2 are a cross-sectional view and a plan view of a crystalline Si solar cell 1. As shown in FIG. 1, the transparent electrode 5 is provided on the upper surface of the Si substrate 3 composed of the p-type semiconductor layer 3 a and the n-type semiconductor layer 3 b, and the transparent electrode 5 is provided on the lower surface of the Si substrate 3.
A mold layer electrode 7 is provided. Further, as shown in FIG. 2, a grid electrode 9 and a bus bar electrode 11 are provided on the upper surface of the transparent electrode 5.

This grid electrode 9 and bus bar electrode 1
1 is a conductive paste for forming an electrode according to the present invention,
That is, an epoxy resin as a conductive component, a silver powder,
It is an electrode formed of a conductive paste containing an imidazole-based curing agent.

Next, the conductive paste as the electrode material for the crystalline Si solar cell will be described in detail. The weight ratio of the silver powder to the epoxy resin in the conductive component (silver powder / epoxy resin) and the addition amount (phr) of the imidazole-based curing agent to 100 g of the epoxy resin were set as shown in Table 1, and solderability for each was set. Of Table 1, the tensile strength (N) of the terminal, and the specific resistance (μΩ-cm) are shown in Table 1.

[0022]

[Table 1]

In Table 1, the sample No. Those marked with * are samples outside the scope of the present invention, and others are examples of the present invention.

Sample No. 1 in Table 1 1 to sample No. Each of the conductive pastes indicated by 15 contains an epoxy resin as a conductive component, a silver powder, and an imidazole curing agent, and an electrode obtained by heating and curing these conductive pastes at 150 ° C. for 60 minutes. Is formed.

Here, the sample No. 1 to sample No. 1
3 shows the case where a solid epoxy resin Epicoat # 1004 (molecular weight of about 1600, epoxy equivalent of about 1000 g / egiv.) Manufactured by Mitsubishi Petrochemical Co., Ltd. was used as an epoxy resin, and a mixed solvent of methyl ethyl ketone, ethanol, and xylene was used. Used as an epoxy varnish dissolved in.

The sample No. Sample No. 14 shows the case where Epicoat # 1001 (molecular weight of about 900, epoxy equivalent of about 500 g / egiv.) Manufactured by Mitsubishi Petrochemical Co., Ltd. was used as the epoxy resin. 15 is an epoxy resin, Epicoat # 1007 (molecular weight: about 2900, epoxy equivalent: about 1800 g / egiv.) Manufactured by Mitsubishi Petrochemical Co., Ltd.
Shows the case of using.

By the way, the epoxy resin used in this embodiment has a molecular weight of about 900, about 1600, and about 2900, which has a molecular weight of 90.
This is because an epoxy resin having a value of less than 0 has poor solderability and cannot be soldered because heat curing proceeds rapidly.

The silver powder has an average particle size of 5 to 10
A flaky silver powder of μm is used.

Further, as the imidazole type curing agent,
2-Phenyl-4-methyl-5-hydroxymethylimidazole (Shikoku Kasei Co., Ltd., trade name: 2P4MH
Z) and 2-phenyl-4,5-dihydroxymethylimidazole (manufactured by Shikoku Kasei Co., Ltd., trade name: 2PHZ) are used. This phenyl hydroxymethyl imidazole has a secondary amine (-
Since it has an (NH-group), it is considered that the excess-NH-group acts as an auxiliary function of the flux activator when added in excess of the proper addition amount.

The appropriate addition amount of the imidazole-based curing agent is determined by the technical data of Shikoku Kasei Co., Ltd., and is the minimum addition amount required to cure the resin alone.

Next, solderability, tensile strength, and specific resistance in Table 1 will be described. As for solderability, a crystalline Si solar cell 1 obtained by heating and curing a conductive paste was coated with an inactive rosin-based flux, and
The solderability was observed by immersing in a eutectic solder bath containing 2% silver at 20 ° C.

A circle indicates 90 with respect to the solder coating area.
% Is solderable, Δ is 70% or more and less than 90% solderable area, and X is less than 70% soldered area. It is evaluated as a case where it was not possible. In addition, the solder erosion is described as being solder erosion during the solderability evaluation.

Sample No. 1 to sample No. 4, sample N
o. 7 to sample No. 9, and sample No. 12 to sample No. Each of the conductive pastes indicated by 15 had a solderability evaluation of ◯ and was 90% of the coated area.
% Or more can be soldered.

On the other hand, sample No. The conductive paste shown by 5 had a solderability evaluation of x, and less than 70% of the applied area could be soldered. In addition, the sample No. The conductive paste shown by 6 had solder erosion.

Sample No. No. 5 had a weight ratio of (silver powder / epoxy resin) of 3, and the epoxy resin ratio was too high to cover the conductive paste coating film with the epoxy resin, resulting in poor solderability, and thus sample No. ． 6
The reason is that the weight ratio of (silver powder / epoxy resin) is 11, and the ratio of silver powder is too high, which causes solder erosion. Incidentally, solder erosion means that silver powder is diffused from inside the conductive paste into the solder,
It is a state where it comes off.

Sample No. 10 and sample No. 1
In No. 1, the solderability evaluation is Δ, and 70% or more and less than 90% of the applied area can be soldered. Sample No. 10 and sample No. No. 11 is considered to have an insufficient flux effect because of poor solderability.

Therefore, in the evaluation of solderability, Sample N
o. 1 to sample No. 4, sample No. 7 to sample N
o. 9, and sample No. 12 to sample No. The conductive paste indicated by 15 is considered to have good solderability,
90% or more of the coated area was a solderable sample. In addition, the sample No. 5 and sample No. 6, and sample No. 10 and sample No. No. 11 had poor solderability.

Regarding the tensile strength (N) of the terminal, a commercially available 2% silver-containing eutectic solder paste was applied to the bus spar portion of the applied solar cell by screen printing to obtain 2 mm.
After the ribbon-shaped lead terminal having a width was soldered in a reflow furnace, the tensile strength (N) of the terminal was measured.

Sample No. 1 to sample No. 4, sample N
o. 7 to sample No. 9, and sample No. 12 to sample No. Each of the conductive pastes indicated by 15 had a good tensile strength of the terminal, and the tensile strength of 10 N or more required for mounting was obtained.

On the other hand, sample No. 5 has a solderability of 7
Less than 0%, sample No. No. 6 could not be measured due to solder erosion.

Further, the sample No. 10 and sample No. 1
1 has a terminal tensile strength of 7 (N) and 4 respectively.
(N), the tensile strength of 10 N or more required for mounting was not obtained.

Sample No. 10 and sample No.
No. 11 is because the addition amount of the imidazole curing agent of the trade name: 2P4MHZ (curing agent 1) and the trade name: 2PHZ (curing agent 2) was as small as 1 phr, so the solder bonding area was small and the tensile strength was low. .

Therefore, in the tensile strength of the terminal, the sample N
o. 1 to sample No. 4, sample No. 7 to sample N
o. 9, and sample No. 12 to sample No. Each of the conductive pastes indicated by 15 had a good tensile strength of the terminal, and the tensile strength of 10 N or more required for mounting was obtained. In addition, the sample No. 5 and sample N
o. 6, and sample No. 10 and sample No. In No. 11, the tensile strength of 10 N or more required for mounting was not obtained.

By the way, the proper addition amount (phr) of curing agent 1 and curing agent 2 to Epicoat # 1004 (molecular weight of about 1600, epoxy equivalent of about 1000 g / egiv.) Is 1 phr, that is, Epicoat # 1004 is cured alone. The minimum required addition amount to achieve this is 1 phr. For example, the proper addition amount of the curing agent 1 and the curing agent 2 to Epicoat # 828 (epoxy equivalent of about 200 g / egiv.) Is 5 phr. There is a correlation between the amount and the epoxy equivalent.

Therefore, in order to obtain the above-mentioned tensile strength, the addition amount of the curing agent 1 and the curing agent 2 should be at least twice the proper addition amount of 1 phr, that is, at least 2 phr to enhance the curing action. is necessary.

Regarding the specific resistance (μΩ-cm), a conductive paste is placed on a glass substrate with a width of 0.5 mm and a length of 250 mm.
Line is screen-printed, after heating and curing at 150 ° C. for 60 minutes, the resistance value between both ends of the line and the line film thickness are measured,
The specific resistance (μΩ-cm) was calculated.

Sample No. 1 to sample No. 4 and sample No. 6 to sample No. 6 15 has a specific resistance of 200 (μΩ-cm) or less, and the grid electrode 9 and the bus bar electrode 11 shown in FIGS.
The specific resistance is 200 μm in order to suppress the current loss due to the contact resistance with the transparent electrode 5 and the wiring resistance with the conductive paste.
-Cm or less is desirable, which is good.

On the other hand, sample No. 5 has a specific resistance of 920
(ΜΩ-cm), and the specific resistance is poor.

Therefore, regarding the specific resistance, the sample No. 1 to sample No. 4 and sample No. 6 to sample No. 6 1
Sample No. 5 is good. 5 is bad.

From the above, the sample No. 1 to sample N
o. Of the 15 samples, the sample N is within the scope of the present invention.
o. 1 to sample No. 4, sample No. 7 to sample N
o. 9, and sample No. 12 to sample No. It is 15.

Next, an electrode forming method using the conductive paste of the present invention will be described. (1) First, solid epoxy resin Epicoat # 1004 (molecular weight: about 1600, epoxy equivalent: about 1) manufactured by Mitsubishi Petrochemical Co., Ltd.
000 g / egiv. 1) is mixed with a mixed solvent of methyl ethyl ketone, ethanol and xylene, and a flake silver powder having an average particle size of 5 to 10 μm is mixed.

(2) Cool air was blown in while stirring to gradually diffuse the mixed solvent, and a solvent mixture was added by appropriately adding a mixed solvent of terpineol, diacetone alcohol, and butyl lactate for viscosity adjustment, which is suitable for printability. Adjust to viscosity.

(3) To this, an imidazole type curing agent having a trade name: 2P4MHZ or a trade name: 2PHZ is added and sufficiently kneaded and dispersed with a three-roll to produce a conductive paste. Here, considering the paste life of the imidazole-based curing agent for use as a one-component conductive paste, it is preferable that the curing agent for epoxy resin has a pot life of 30 days or more at room temperature.

(4) This conductive paste is used in FIGS.
After screen-printing as the grid electrode 9 and busper electrode 11 shown in (1), it is heated and cured at 150 ° C. for 60 minutes. The heat curing temperature may be set within the range of 150 ° C. to 200 ° C. so that soldering can be performed. Since it is formed by heating and curing at 200 ° C. or lower, it does not damage the film composition sensitive to thermal history. However, it is necessary to obtain sufficient tensile strength with the surface to be coated by heat curing, and it is preferable that the heat curing time is long in order to secure a solderability by making it in a semi-cured state when soldering. The heat curing time is usually about 20 to 90 minutes. If it is heated for 90 minutes or more, it will be completely cured and no solder will be attached, and if it is heated for 20 minutes or less, the solder will be eroded due to insufficient curing.

(5) Solder to a lead terminal or the like in a heated and semi-cured state. In this case, since the thermosetting resin is used in a semi-cured state, it has stronger adhesive strength than the thermoplastic resin and is less likely to be eaten by solder.

Here, the sample having the heat curing temperature set to 140 ° C. (heat curing time of 60 minutes) was sample No. 16 and sample No. 16 Sample No. 17 having a heat curing temperature of 210 ° C. (heat curing time of 30 minutes) was used. 18
And sample No. 19 is shown in Table 2.

[0057]

[Table 2]

Sample No. shown in Table 2 16 and sample N
o. Sample No. 17 suffered solder erosion. 1
8 and sample No. No. 19 had poor solderability.

Therefore, the heat curing temperature is 150 ° C. to 200 ° C.
It is necessary to set the temperature within the range of 0 ° C., and within this temperature range, it is possible to prevent the electrode from being eroded by solder or to have poor solderability, and it is possible to improve the reliability of quality.

By the way, in the above embodiment, as shown in FIGS. 1 and 2, the crystalline Si solar battery cell using the conductive paste for forming the electrode according to the above embodiment has been described. Needless to say, it is not limited to the electronic components of. For example, it can be used for a four-element series type amorphous Si solar cell as shown in FIGS. 3 and 4 and a II-VI group compound semiconductor (CdS-CdTe system) solar cell.

Further, in FIG. 3, 4 used in a calculator or the like is shown.
FIG. 3 is a cross-sectional view of an element series type amorphous Si solar cell 13, in which a transparent electrode 19 is formed on the surface of a glass substrate 15.
The amorphous Si semiconductor 17 having a pin structure composed of the p-type semiconductor layer 17c, the i-type semiconductor layer 17b, and the n-type semiconductor layer 17a, and the Ag-based electrode 21 are provided in layers. Among these, the Ag-based electrode 21 is an electrode formed of the conductive paste of the present invention.

Further, FIG. 4 is a cross-sectional view of a II-VI group compound semiconductor (CdS-CdTe system) solar cell 23, in which the n-type semiconductor CdS layer 27 and the p-type semiconductor CdTe are formed on the surface of the glass substrate 25. A pn-type semiconductor composed of the layer 29, a carbon-based electrode 31, and an Ag-based electrode 33 are provided. Among these, the Ag-based electrode 33 is an electrode formed of the conductive paste of the present invention.

Further, the present invention is not limited to the above embodiments, but various applications and modifications are possible within the scope of the gist of the present invention. For example, a conductive paste to which In or Mo is added within the range that does not impair the solderability is possible, and in this case, the ohmic contact property can be improved.

[0064]

The conductive paste of the present invention, the electrode forming method,
Also, in the solar cell, since the electrodes can be easily formed by using screen printing, it is suitable for mass production and can be manufactured at low cost. Further, since it has good conductivity, soldering by a solder dipping method or a reflow method using a solder paste is also possible.

Further, since the epoxy resin has a molecular weight of 900 or more, unlike the epoxy resin having a molecular weight of less than 900, which can be cured slowly by heating, the electrode can be easily formed because the epoxy resin can proceed slowly. .

Further, since the curing agent is an imidazole type curing agent, the pot life, that is, the pot life until the viscosity becomes twice the initial viscosity can be lengthened, so that a heating semi-cured state can be created for a long time. Therefore, the electrodes can be formed more easily.

Further, since the weight ratio of silver powder to epoxy resin (silver powder / epoxy resin) is in the range of 4 to 10, the resin ratio is too high and the resin covers the conductive paste coating film, resulting in solderability. It is possible to prevent the terminal strength from being deteriorated or the terminal tensile strength not being obtained due to solder erosion due to too high a silver powder ratio, so that good solderability and terminal tensile strength can be obtained. It is possible.

Further, the amount of the imidazole-based curing agent added to the epoxy resin is an appropriate amount, that is, at least twice as much as the minimum amount required to cure the resin. It is possible to obtain sufficient tensile strength of the terminal.

In the solar cell of the present invention, since the electrode material has good conductivity, an ohmic contact is formed,
It is possible to prevent a decrease in photoelectric conversion efficiency.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention.

FIG. 2 is a sectional view showing an embodiment of the present invention.

FIG. 3 is a sectional view showing an embodiment of the present invention.

FIG. 4 is a sectional view showing an embodiment of the present invention.

[Explanation of symbols]

1 Crystalline Si Solar Cell 3 Si Substrate 3a, 17c p-type Semiconductor Layer 3b, 17a n-type Semiconductor Layer 5,19 Transparent Electrode 7 n-type Layer Electrode 9 Grid Electrode 11 Bus Bar Electrode 13 Amorphous Si Solar Cell 15, 25 Glass Substrate 17b i-type semiconductor layer 21,33 Ag-based electrode 23 II-VI group compound semiconductor (CdS-)
CdTe system) 27 n-type semiconductor CdS layer 29 p-type semiconductor CdTe layer 31 carbon-based electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H01L 31/04

Claims (3)

[Claims] 1. An epoxy resin having a molecular weight of 900 or more,
A silver powder having a weight ratio with respect to the epoxy resin within a range of 4 to 10; and an imidazole-based curing agent added in an amount twice or more the minimum amount required to cure the epoxy resin. Characteristic conductive paste. 2. An epoxy resin having a molecular weight of 900 or more,
A step of mixing silver powder having a weight ratio with respect to the epoxy resin within a range of 4 to 10, and the epoxy resin,
A step of adding an imidazole-based curing agent in an amount twice or more the minimum amount required for curing the epoxy resin, a step of applying the conductive paste by screen printing, and a step of applying the conductive paste at 150 ° C. ~ 200 ° C
The method of forming an electrode, comprising: 3. An epoxy resin having a molecular weight of 900 or more,
A conductive paste containing a silver powder having a weight ratio with respect to the epoxy resin in the range of 4 to 10 and an imidazole-based curing agent in an amount twice or more the minimum amount required to cure the epoxy resin. Is applied by screen printing, and is provided with an electrode which is heat-cured within a range of 150 ° C to 200 ° C.