JPS6159549B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrode material for semiconductor devices, and in particular provides an electrode material suitable for manufacturing semiconductor elements having relatively large electrodes such as solar cells in large quantities at low cost. It is. That is, the present invention relates to a slurry material for electrodes (hereinafter referred to as conductive paste) that is used when forming electrodes of semiconductor devices such as solar cells into an ohmic shape by applying a method such as printing and baking the electrodes. The present invention provides a conductive paste for producing highly efficient solar cells in large quantities at low cost and with good reproducibility.

In recent years, printing methods have been considered as a method for forming electrodes in solar cells, replacing the conventional vacuum evaporation methods and plating methods. The printing method is a method in which a viscous muddy substance prepared by mixing metal powder, glass powder, etc. with an organic solution, so-called conductive paste, is printed using a stencil screen or the like, and then fired. If this method is used, the work is significantly simplified compared to conventional vacuum evaporation methods, plating methods, etc., and continuous automation can be easily achieved. One of the biggest problems facing solar cells at present is the reduction of manufacturing costs, which is a decisive factor in the widespread use of solar cells.
For example, the electrode formation process accounts for a large proportion of the manufacturing cost of a silicon solar cell. Therefore, the printing method greatly contributes to reducing the manufacturing cost of solar cells, but when using this printing method to form the electrodes of solar cells, the most important thing is that the semiconductor exhibiting photovoltaic properties, that is, the electrode material It is possible to make ohmic contact with the substrate surface and to make it firmly adhere.

Conventionally, Ag-based, Ag/Pd-based, or Au-based conductive pastes have been mainly used to form electrodes by printing on sintered thick-film resistors and capacitors for hybrid integrated circuits. . These conductive pastes are Au, Ag or Ag and Pd.
This powder is made by adding lead-based glass powder, an organic binder such as ethyl cellulose, and an organic solvent such as Cellsolve to adjust the viscosity during printing.

However, according to the studies of the present inventors, simply printing and baking such a conductive paste on a semiconductor substrate for solar cells creates a barrier between the substrate and the electrode, resulting in a good electrode exhibiting low contact resistance. It was difficult to form. In order to form practical electrodes for silicon solar cells using these conventional conductive pastes, both the electrode formation surface, that is, the diffusion surface of silicon having a p/n junction, and the substrate surface must be either p ⁺ or n ⁺ . In order to achieve this, it was necessary to increase the surface impurity concentration to 10 ¹⁹ cm ^-3 or higher. Another disadvantage was that the firing temperature of the conductive paste was relatively high, close to 800°C. In order to provide low contact resistance to a silicon substrate, when using Au, Ag, or Aq/PD conductive paste electrodes that require high temperatures, the electrode material penetrates and diffuses into the silicon, resulting in photovoltaic properties. There was a risk that it would penetrate through the relatively shallow p/n junction layer exhibiting a 100% diameter and destroy the junction.

When using conventional conductive paste in this way,
It has been extremely difficult to form a solar cell electrode that has no penetration into the diffusion layer and exhibits low contact resistance. In addition, these conductive pastes include
An appropriate amount of glass powder is usually added in order to firmly and stably form the electrode on the substrate to be bonded after firing. In general, the components of these glassy powders vary depending on the type of metal powder and firing temperature, but they are usually made to have a low melting point. It also contains small amounts of glass powders such as boron, silicon, zinc, bismuth, and cadmium. Furthermore, in the Ag-based paste fired at a high temperature of 800° C. or higher, lead borosilicate glass powder is the main component of the vitreous powder. As described above, the glassy powder components in conventional conductive pastes contain a considerable amount of lead-based glass, even if the composition and firing temperature are different, and according to the inventors' study, it has already been found that As stated in Japanese Patent Application No. 52-91929, this lead-based glass is the reason why good electrodes cannot be formed directly from semiconductor substrates such as silicon.

As mentioned above, conductive pastes that have been commonly used as electrodes for thick film resistors and thick film capacitors have many drawbacks that make them impractical as electrodes for semiconductors, especially solar cells such as silicon. have.

The present invention eliminates the drawbacks of the conventional conductive pastes described above and provides a conductive paste that is extremely useful for electrodes on semiconductor substrates such as silicon.
This effect is particularly effective when the electrode is taken out from the diffusion layer side, such as a solar cell substrate, in which a shallow diffusion layer having a conductivity type different from that of the substrate is formed on one side, and from the substrate side.

That is, in the present invention, the solid components of the conductive paste include fine metal powders of silver and gold, and more preferably other vitreous powders that do not contain lead-based glass powders, such as zinc oxide, boron oxide, and silica. Glass powder in a specific ratio, and in order to make it easier to apply to the electrode formation substrate by printing or other methods, an appropriate amount of an organic binder such as ethyl cellulose or cellosolve and an organic solvent are added, and a sufficient amount is added. It is a viscous conductive paste that has been mixed and stirred.

According to the invention, as a solid metal component of the paste
Regarding newly adding Au powder to Ag powder,
More preferably, by removing lead-based glass from the glass component, it has become possible to form an excellent electrode for a silicon solar cell at a firing temperature approximately 200° C. lower than in the past.

To describe the features of the present invention, the eutectic temperature of Au with respect to Si is 370°C, which is lower than the eutectic temperature of Ag with respect to Si (830°C), so if Au powder is present in Ag powder, the firing temperature will be relatively low. However, Au preferentially causes an alloying reaction with silicon, making it possible to obtain a low contact resistance contact with silicon without raising the temperature to a high temperature of 800°C or higher, and to form a low resistance film of silver powder in the paste. Together, a preferred electrode is formed.

Furthermore, in the present invention, if the conductive paste applied to the silicon substrate does not contain lead-based glass, the silicon substrate will hardly be oxidized even if heated at 650° C. or lower in an oxygen-containing atmosphere. Therefore, even better ohmic contact can be achieved.

Furthermore, by appropriately changing the blending ratio of Ag and Au, the present invention enables diffusion in a p/n junction in which a p-type impurity is diffused into a diffusion layer having a different conductivity type from the substrate, such as an n-substrate, such as in a solar cell. Even if the layer is shallow, if appropriate conditions are selected, the electrode will not penetrate through the diffusion layer.

Examples of the present invention will be described below.

Example 1 35g of fine silver powder with a particle size of 0.5μm or less
3.5g of ultrafine gold powder with a particle size of 0.1μm or less
ZnO−B ₂ O ₃ whose main component is ZnO of 0.5 μm or less ―
Add a small amount of ethyl cellulose and carbitol to 2.5 g of SiO ₂ -based glass frit using an agate mixer and stir thoroughly.
Prepare a viscous muddy substance of about 300 cps, that is, a conductive paste. As a substrate for a solar cell, as shown in Fig. 1, a p ⁺ layer 2 with a diffusion depth of approximately 1.5 μm and an n ⁺ layer 3 are formed on the opposite surface on an n-type silicon substrate 1 with a specific resistance of 0.5 Ω-cm. The above conductive pastes 4 and 5 are printed on the surface using a 250 mesh stencil screen. The organic solvent is then removed using a hot air dryer at 120°C, followed by baking at 600°C for 20 minutes in an atmosphere of flowing nitrogen gas containing a trace amount of oxygen. When we measured the contact resistance between the electrode formed in this way and the p ⁺ and n ⁺ surfaces of the silicon substrate, we found that it was as low as 10 ^-2 to ^{10 -3} Ω−cm ² for both the p ⁺ and n ⁺ surfaces. It showed contact resistance, indicating that almost perfect ohmic contact was formed. In addition, when we looked at the diode characteristics with a carp tracer to see if there was any penetration to the p ⁺ diffusion surface, a good curve was observed. In addition, we applied −1.0V to the p ⁺ side electrode to reverse bias the pn junction and measured the leakage current, which was −1×10 ^-6 A/cm ² , which is the same as that when the electrode was formed by conventional vapor deposition. It was confirmed that there is almost no inferiority to current. In this way, the conductive paste of the present invention is more effective than conventional conductive pastes.
Despite being fired at a temperature as low as 200°C, the contact resistance is sufficiently low, and there is no visible penetration from the p ⁺ to the n substrate, which is important for the performance of solar cells, making it an excellent electrode for solar cells. It was a conductive paste.

Example 2 Next, the contact resistance, electrode surface resistance, and penetration level for the p ⁺ layer of conductive pastes prepared by changing the composition of the solid components of the conductive paste of the present invention, that is, silver, gold powder, and glassy powder, were investigated. We will explain the results of examining electrical characteristics such as leakage current.

Figure 2 shows the changes in the composition and electrical characteristics when the composition of silver and gold was changed.
Curve 20 shows the change in contact resistance of the p ⁺ surface, curve 21 shows the sheet resistance of the electrode, and curve 2
2 shows the leakage current indicating the degree of penetration. In addition, preparation of conductive paste, printing,
The firing is basically the same as the method shown in Example 1, but when mixing silver and gold in the conductive paste, a gold-free conductive paste is prepared in advance, and this conductive paste is An appropriate amount of Au powder was added to the mixture, and the mixture was thoroughly mixed and stirred while adjusting the viscosity to about 300 cps with carbitol.

As can be seen from Figure 2, a conductive paste that uses only silver powder as a metal component without adding any Au has a contact resistance of 10Ω− to the p ⁺ diffusion layer.
^cm2 , which was too high for practical use. In order to use a conductive paste as an electrode for a solar cell, it is preferable that the contact resistance and surface resistance be sufficiently low, as well as that the leakage current should be small.The composition of Au that almost satisfies these three conditions is Solid component 5
It was between ~25wt%. At 10 to 15 wt% Au, which is considered to be the optimum composition, the contact resistance is 10 ^-2 to ^{10 -3} Ω−.
cm ² , surface resistance 10 ^-3 to ^{10 -4} Ω/Sq, leakage current 10 ^-5
~10 ^-6 A/cm ² , showing electrical properties comparable to those of electrodes formed using conventional vapor deposition methods.

Furthermore, when Au decreases to less than 5wt%, the contact resistance becomes 1Ω- ^cm2 , and when it exceeds 20wt%, the leakage current tends to increase, which is due to the penetration of the Au powder into the diffusion layer in the conductive paste.
This is thought to be because the occurrence becomes more likely in proportion to the amount of Au powder blended. Further, according to the studies of the present inventors, the composition of the glass powder is 3 to 3% of the solid components.
10wt% is appropriate; below 3wt%, the adhesion to the substrate will be weak and problems will arise in terms of long-term reliability, while above 10wt%, the contact resistance and surface resistance will increase, and the series resistance will increase. It has become clear that this decreases the performance of solar cells.
In this way, Au is 5-20wt% and glass is 3-20wt%.
The appropriate proportion is 10 wt%, and at this time, the appropriate Ag proportion is 70 to 92 wt%.

Example 3 Next, the glass component in the conductive paste of the present invention will be explained. The vitreous powder in the conductive paste used in the above example is ZnO--B ₂ O ₃ which is primarily composed of ZnO excluding lead-based glass powder.
-Used SiO ₂ based glass powder. In this example, in order to replace most of the ZnO powder, which is the main component of the glass powder, when preparing a conductive paste in the same manner as in Example 1, the glass powder was
ZnO―B ₂ O ₃ ―SiO ₂ glass and PbO containing lead components
-B ₂ O ₃ - SiO As a result of forming electrodes using glass powder mixed with ₂ -based glass at a ratio of 1:3,
The contact resistance was as large as 1 Ω-cm ² on both the p ⁺ and n ⁺ surfaces, which was more than two orders of magnitude higher than the electrode formed in Example 1, which did not contain PbO. Furthermore, when a PbO--B ₂ O ₃ ---SiO ₂ -based glass powder in which all ZnO was replaced with PbO was used, the electrode was as large as 3 Ω-cm ² and could not withstand practical use as a solar cell.

For the above reasons, it is also important that the glass component in the conductive paste of the present invention does not contain any Pb-based glass powder, and we decided to replace this lead glass with ZnO-based glass, which has a relatively low melting point. This is achieved in this way. Furthermore, with the paste of the present invention, almost the same satisfactory results were obtained when an n-type diffusion layer was formed on a p-type silicon substrate.

As described above, the conductive paste of the present invention has a solid component composed of a mixed metal fine powder of silver and gold and a vitreous material containing zinc-based glass powder as a main component instead of preferably containing no lead component. This is an innovative conductive paste for solar cell electrodes that could not be achieved with conventional Ag-based, Ag/Pd-based, or Au-based pastes. If the conductive paste of the present invention is used,
Unlike conventional Ag-based pastes, which do not require high temperatures of 800°C or higher, problems such as penetration into the diffusion layer are solved at once. Therefore, if the conductive paste of the present invention is used, the advantages of printing and baking methods, which are expected to streamline the manufacturing process, can be fully utilized in place of the vacuum evaporation method and plating method that have been conventionally used for forming electrodes of solar cells. It is industrially very useful because highly efficient solar cells can be obtained at low cost and with good reproducibility.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of the structure of a silicon solar cell using a conductive paste according to an embodiment of the present invention, and FIG.
The figure is a characteristic diagram showing changes in contact resistance to a silicon substrate, electrode surface resistance, and leakage current when the blending ratio of gold powder is changed in the conductive paste according to the present invention. 1...n-type silicon substrate, 2...p ⁺ layer, 3...
...n ⁺ layer, 4, 5...conductive paste, 20...relationship between the content of Au in the conductive paste and contact resistance, 21...relationship between the content of Au and the surface resistance of the electrode, 22...Au content Relationship between amount and leakage current.

Claims (1)

[Claims] 1 Consists of a slurry material in which solid components consisting of silver, gold, and non-lead oxide low melting point glass are dispersed in an organic solvent containing an organic binder, and gold is included as the solid component of the mud material. 5-20% by weight, 70% silver
~92% by weight, non-lead oxide low melting point glass 3~10
% by weight.