JPH0337751B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to an electrode material for a semiconductor device, and particularly to an electrode material suitable for manufacturing a semiconductor element having a relatively coarse pattern of electrodes such as a solar cell. [Background of the Invention] The figure shows a typical configuration example of a solar cell as an example of a semiconductor element. Si with n ⁺ /P/P ⁺ junction formed
It has a structure in which a light-receiving surface electrode 4 and a back-surface electrode 5 are formed on the light-receiving surface and the back surface of the substrate. More generally,
An antireflection film or the like is also provided. An important issue for solar cells in recent years is to reduce manufacturing costs.
As for the forming method, low-cost plating methods and printing methods are being considered instead of the conventional vacuum deposition method. Among these methods, printing methods in particular are being widely studied because they are easy to automate and have high productivity. This printing method is a method in which a paste-like substance (hereinafter referred to as conductive paste) made by kneading metal powder, glass powder, etc. with an organic binder and an organic solvent is applied by screen printing, etc., and then fired. Silver powder is commonly used. Many such conductive pastes are commercially available for use in forming electrodes of solar cells, thick film circuit boards, and the like. On the other hand, in the formation of electrodes for solar cells, etc., the electrodes are required to have high adhesive strength, low contact resistance to silicon, and no penetration to the diffusion layer (low leakage current). However, the present inventors have discovered various commercially available Ag-based, Ag-
According to the results of studies on Pd-based conductive pastes, the following problems occurred when any of the conductive pastes was applied by printing onto the bond-forming silicon wafer shown in the figure, dried, and fired. In other words, with Ag-based or Ag-Pd-based conductive pastes for thick-film circuit boards, a barrier is formed between the silicon wafer and the electrode, resulting in high contact resistance, and when baked at relatively high temperatures, the bond breaks down and leaks occur. An increase in current was observed. Some Ag-based conductive pastes for solar cells are difficult to form a barrier between the silicon wafer and the electrode, but all have high contact resistance, and when examining the current-voltage characteristics when solar cells are irradiated with light, the fill factor However, it was not possible to create highly efficient solar cells due to their small size. Furthermore, when the firing temperature was set to a relatively high temperature, the contact resistance tended to decrease, but this caused the problem of increased leakage current. As described above, it has been extremely difficult to form electrodes with low contact resistance using the above-mentioned conventional conductive pastes without causing bond breakdown. [Object of the Invention] An object of the present invention is to provide a material that does not have the drawbacks of the conventional conductive pastes described above and is very useful as an electrode material for semiconductor devices such as solar cells. [Summary of the Invention] The electrode material of the present invention comprises silver powder, at least one metal selected from titanium and magnesium,
At least one phosphoric acid compound selected from silver phosphate, nickel phosphate, and magnesium phosphate,
It is characterized by consisting of an organic binder, an organic solvent, and glass powder added as necessary. The present invention differs from conventional conductive pastes in that it contains at least one metal selected from titanium and magnesium, and at least one phosphoric acid compound selected from silver phosphate, nickel phosphate, and magnesium phosphate. This means that if a conductive paste containing these metals and phosphorous compounds is printed on a substrate such as silicon and fired, it will not adhere to the substrate even at a relatively low temperature (<750°C) that will not cause bond failure. This is due to the discovery that it is possible to form electrodes with extremely low contact resistance. It is believed that the reason why the electrode material of the present invention is capable of forming a very good electrode as described above compared to conventional conductive pastes is as follows. That is, when a conventional conductive paste is printed on, for example, a silicon substrate and fired, an insulating silicon oxide film is formed on the silicon surface due to oxygen contained in the firing atmosphere. Furthermore, when the conductive paste uses lead oxide-based low melting point glass, this silicon oxide film is also generated due to the reaction between lead oxide and silicon. Since a silicon oxide film is thus formed on the silicon surface, it is expected that the contact resistance between the fired electrode and the silicon will become extremely high. On the other hand, with the electrode material according to the present invention, it is thought that a silicon oxide film is generated in the same way as described above, but the metals (titanium, magnesium) contained in the electrode material react with the silicon oxide film, causing reduction of the silicon oxide film,
Formation of silicide compounds of these metals occurs,
This lowers the contact resistance between the electrode and silicon, and furthermore, phosphoric acid compounds (silver phosphate,
It is expected that the addition of nickel phosphate, magnesium phosphate) will improve the contact between the silver powder in the paste and the silicide compound described above, resulting in a very low contact resistance. The components of the electrode material of the present invention will be explained in further detail below. Ag powder, organic binder,
The organic solvent can be the same as that used in conventional conductive pastes. As the silver powder, those having a particle size of 1 μm or less can be used, as the organic binder, cellulose compounds and polymethacrylate compounds, and as the organic solvent, polyhydric alcohols can be particularly preferably used. The metals titanium and magnesium are preferably used in powder form. However, since these metal powders have high activity, it is preferable to use ones that have been stabilized by a method such as forming a thin oxide film on the powder surface. Titanium and magnesium may be used alone or in combination. Furthermore, it is also possible to use these alloy powders or to coat the surface of the silver powder with these metals. In addition, even if one type of phosphoric acid compound such as silver phosphate, nickel phosphate, or magnesium phosphate is used,
Two or more types may be used in combination. Furthermore, the present invention does not necessarily require glass to be included. However, when glass is added, the adhesive strength of the formed electrode to the semiconductor element is improved. Furthermore, the resistance of the electrode to solder is also improved. For this reason, especially when used for forming electrodes of solar cells, it is rather preferable to incorporate glass. The type of glass used here is not particularly limited. Furthermore, by blending Pd powder into the electrode material of the present invention, the resistance to solder of the formed electrode is further improved, and by blending Pd powder, the welding strength of the electrode is improved. When the electrode material of the present invention is used particularly for forming electrodes of solar cells, the blending ratio of at least one metal selected from titanium and magnesium is 100% of silver powder.
The amount is preferably 0.5 to 40 parts by weight. If the blending ratio is less than 0.5 parts by weight, the contact resistance of the formed electrode to silicon will increase, and if the blending ratio exceeds 40 parts by weight, the conductive resistance of the formed electrode will increase, which tends to cause a decrease in the efficiency of the solar cell. For the same reason, the blending ratio of at least one phosphoric acid compound selected from silver phosphate, nickel phosphate, and magnesium phosphate is 100% silver powder.
5 to 40 parts by weight is suitable. [Examples of the Invention] The present invention will now be described in more detail with reference to Examples. Example 1 10 g of silver powder with a particle size of 1 μm or less and titanium powder (silver powder) with a particle size of 10 μm or less whose surface has been stabilized.
(0.5 to 40 parts by weight per 100 parts by weight), silver phosphate (5 to 40 parts by weight per 100 parts by weight of silver powder), and 1 g of lead borosilicate glass frit were weighed.
A viscous solution prepared by dissolving 10 parts by weight of ethylulose (10 cps) in 90 parts by weight of α-terpineol was thoroughly kneaded and the viscosity reached approximately 200 poise (shear rate
100/sec) paste-like electrode material was prepared. As shown in the figure, a P-type silicon substrate 1 (specific resistance 1 to 5 Ωcm,
A 0.2-0.4 μm deep N ⁺ layer 2 (surface sheet resistance
50~70Ω/port) and aluminum on the opposite side.
The P ⁺ layer 3 was formed by thermal diffusion to a depth of 2 μm. Next, the above paste-like electrode material was screen printed on the n ⁺ layer 2 of this P type silicon substrate 1 in a comb-shaped pattern and on the P ⁺ layer 3 in a solid pattern. After drying,
A light-receiving surface electrode 4 and a back surface electrode 5 were formed. Next, this substrate was heated for 600 min in a nitrogen gas atmosphere containing 50 ppm of oxygen.
℃ for 10 minutes. The electrode-voltage characteristics (IV characteristics) of the solar cell created in this way were investigated, and the electrode contact resistance (Rc), leakage current at reverse bias (1V),
The fill factor (FF), open circuit voltage (Voc), and short circuit current (Isc) were investigated. As shown in Table 1, solar cells using the electrode material of the present invention containing titanium powder and silver phosphate are:
Rc was significantly lower, FF and Isc were larger, and as a result, efficiency was significantly improved compared to the case where titanium powder and silver phosphate were not blended as shown as a comparative example. Also, the leakage current is
It is on the order of 10 ^-6 A/cm ² and the problem is

[Table] Not completely recognized. In this way, the electrode material of the present invention can be used at relatively low temperatures.
Even when fired at 600℃, Rc is sufficiently low and the thickness of the n ⁺ layer is
Despite being extremely thin at 0.2 to 0.4 μm, there was no increase in leakage current, and it was confirmed that the material was much superior to conventional conductive pastes as electrode materials. Example 2 An example of the present invention will be described in which titanium and magnesium are blended as metals, and silver phosphate, nickel phosphate, and magnesium phosphate are blended as phosphoric acid compounds. Titanium and magnesium metal powders (forming a thin oxide film on the surface), phosphoric acid compounds (silver phosphate, nickel phosphate, magnesium phosphate),
Various combinations of glass frits (lead borosilicate type, zinc borosilicate type), and ethyl cellulose are added to this.
Thoroughly knead while adding a viscous liquid prepared by dissolving 10 parts by weight in 90 parts by weight of α-terpineol until the viscosity is approximately 200.
We prepared various paste electrode materials with different compositions of voids (shear rate 100/sec). This electrode material was screen-printed on the surface of a bond-forming silicon substrate similar to that in Example 1, and after drying at 150°C for 10 minutes, it was placed in a nitrogen gas atmosphere containing 10 ppm of oxygen.
It was baked at 600°C for 10 minutes. The characteristics of the solar cell thus produced were investigated in the same manner as in Example 1, and the results are shown in Table 2 together with the inorganic components of the electrode material. The electrode material of the present invention, which is a mixture of titanium and magnesium metals and phosphoric acid compounds (silver phosphate, nickel phosphate, magnesium phosphate), has a lower Rc than the compositions of Comparative Examples 1 and 2. F.
F, Isc were large, and as a result, efficiency was also significantly improved. Further, the leakage current was on the order of 10 ^-6 A/cm ² in all cases, and no problem was observed. As described above, it was confirmed that the electrode material of the present invention shown in Example 2 also had a very superior effect compared to the conventional conductive paste.

〔Effect of the invention〕

As described above, the electrode material of the present invention enables the formation of electrodes with low contact resistance, even when fired at relatively low temperatures, without causing junction breakdown or an increase in leakage current, even for semiconductor elements with shallow junctions. This is an innovative material that can Therefore, when the electrode material of the present invention is used to form electrodes of a solar cell, a solar cell with much higher efficiency can be obtained than when a conventional conductive paste is used. Furthermore, the electrode material of the present invention can be applied by a printing method, and electrodes can be formed at low cost and with high productivity, making it very useful industrially. Furthermore, the electrode material of the present invention can also be used to form electrodes of light receiving elements other than solar cells and other semiconductor devices.

[Brief explanation of drawings]

The figure is a cross-sectional view showing a typical configuration of a solar cell. 1... P-type silicon substrate, 2... n ⁺ layer, 3...
...P ⁺ layer, 4... Light-receiving surface electrode, 5... Back electrode.

Claims (1)

[Claims] 1 Silver powder and 0.5 to 40 parts per 100 parts by weight of silver powder
parts by weight of at least one metal selected from titanium and magnesium; and 5 to 40 parts by weight of at least one phosphoric acid compound selected from silver phosphate, nickel phosphate, and magnesium phosphate based on 100 parts by weight of silver powder. An electrode material for a semiconductor device, comprising an organic binder, an organic solvent, and glass powder added as necessary.