JPH1070296A - Solar cell and fabrication thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To fabricate a solar cell in which contact resistance between a light receiving face electrode and an impurity diffusion layer is decreased and a high short circuit current can be obtained by providing an antireflection film suitable for modular solar cell. SOLUTION: Titanium oxides 2, 3 containing impurities are deposited on the light receiving face of a substrate 1. The titanium oxide 2 is doped heavier with impurities than the titanium oxide 3. A light receiving face electrode 8 is provided at a part of the light receiving face where the titanium oxide 2 is deposited. A rear electrode 6 is provided on the rear side of the light receiving face. More specifically, titanium oxides 2, 3 containing impurities are deposited on the light receiving face of the substrate 1 and impurity diffusion layers 4, 5 are formed in the vicinity of the light receiving face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a solar cell and a method for manufacturing the same.

[0002]

2. Description of the Related Art A conventional solar cell and a method for manufacturing the same are described, for example, in Japanese Patent Application Laid-Open No. H08-85874. This conventional solar cell and its manufacturing method will be described with reference to FIG. FIG. 5 is a cross-sectional view of a crystalline silicon substrate in this conventional solar cell manufacturing process.

[0005] As shown in FIG. 5 (a), on the surface of a p-type crystalline silicon substrate 31, in order to reduce the surface reflection of the light receiving surface and improve the short-circuit current, a minute difference of several μm to several tens μm is provided. And irregular grooves or grooves (hereinafter simply referred to as “irregularities”).

[0004] The method of forming the irregularities includes, for example, the substrate 3
There is a method of texture etching in which 1 is etched using a mixed solution of an aqueous solution of sodium hydroxide and alcohol of several percent. Further, there are a method of forming a large number of grooves using a dicing apparatus or a laser, and a method such as dry etching.

After forming the irregularities, as shown in FIG. 5 (b), C is formed using titanium alkoxide and phosphoric acid ester.
A uniform titanium oxide film 32 containing phosphorus as an impurity is formed by VD (Chemical Vapor Deposition).
With the uniform thickness, the function of preventing reflection by interference of the titanium oxide film 32 is improved.

Next, as shown in FIG. 5 (c), the substrate 31 on which the titanium oxide
Heat treatment at ℃. As a result, the impurities contained in the titanium oxide film 32 are diffused, and an impurity diffusion layer 33 is formed near the uneven surface of the substrate 31. And
A pn junction is formed.

Next, as shown in FIG. 5 (d), an aluminum paste 34 is printed on the back surface of the
By baking at a temperature of ° C., a back electrode and ap ⁺ layer 35 are formed. Thereafter, as shown in FIG. 5E, a silver paste is printed on the light receiving surface and baked to form a light receiving surface electrode 36. The light receiving surface electrode 36 comes into contact with the impurity diffusion layer 33 due to the effect of glass frit contained in the silver paste.

When a crystalline silicon solar cell is used as a module, a superstrate type is generally used in many cases. This module is composed of glass, a filler, a solar cell, a back surface material, a peripheral sealing material, and a frame material, and ethylene vinyl acetate (hereinafter, referred to as “EVA”) is frequently used as the filler.

[0009]

By the way, in order to reduce the surface reflectance of the solar cell and increase the short-circuit current, it is necessary to select a material having the best refractive index for the antireflection film. In general, assuming that the refractive index of the antireflection film is n, the refractive index of silicon is n _s , and the refractive index of the transmissive substance on the antireflection film is n _O , the optimum refractive index n of the antireflection film is n
It is given by n ² = n _O · n _s becomes conditions.

The refractive index n _{s of} silicon has a high sensitivity of 600
In the wavelength region of １1100 nm, it is about 3.5-4,
If only the solar cell, n _O is the optimum refractive index n of the antireflection film since it is 1 in the refractive index of air becomes 1.8 to 2.0.
When the solar cell is the above-described module, the refractive index n _O of glass or EVA is 1.4 to 1.5, and the optimum refractive index n is 2.2 to 2.5. As described above, in the case of the above-described module, an antireflection film having a different refractive index n from that of a solar cell alone is required.

In the manufacturing process of a low-cost crystalline silicon solar cell, the light-receiving surface electrode is provided using a silver paste. Contact resistance occurs at the contact portion between the silver paste and the impurity diffusion layer. This contact resistance increases as the impurity concentration of the impurity diffusion layer decreases.

Therefore, when a light receiving surface electrode is formed using silver paste, a sheet resistance of about several tens Ω / □ is used so that the series resistance loss is not increased due to an increase in contact resistance and the fill factor is not reduced. An impurity diffusion layer having a value is formed. In the conventional solar cell described in Japanese Patent Application Laid-Open No. H08-85874, when an impurity diffusion layer having a sheet resistance of about several tens Ω / □ is obtained, the refractive index of the antireflection film is about 1.7 to 1.8. Become.
With this refractive index, there is no problem as an antireflection film in the case of a solar cell alone, but there is a drawback that the refractive index is rather low in the case of a module.

According to the present invention, there is provided a solar cell and a method of manufacturing the same, wherein a contact resistance between a light receiving surface electrode and an impurity diffusion layer is small, and a large short circuit is provided by providing an antireflection film suitable for modularization. An object is to provide a solar cell capable of obtaining a current and a method for manufacturing the same.

[0014]

According to a first aspect of the present invention, a titanium oxide film containing impurities is provided on a light receiving surface of a silicon substrate to diffuse impurities near the light receiving surface. In a solar cell in which a layer is formed, first and second titanium oxide films containing impurities are provided on the light receiving surface, and the concentration of the impurities contained in the first titanium oxide film is the second titanium oxide film. A light-receiving surface electrode is provided on a portion of the light-receiving surface where the first titanium oxide film is provided, and a back surface electrode is provided on a back surface of the light-receiving surface. .

In such a configuration, by increasing the impurity concentration of the first titanium oxide film, an impurity diffusion layer having a high impurity concentration is formed on the substrate in the vicinity of where the first titanium oxide film is provided. . Since the light receiving surface electrode is provided on the substrate provided with the first titanium oxide film, the contact resistance between the light receiving surface electrode and the impurity diffusion layer does not increase, and the fill factor does not decrease. Further, a second titanium oxide film is provided on a portion to be a light receiving surface. This titanium oxide film has an effect as an antireflection film.

Since the concentration of the impurities contained in the second titanium oxide film is low, when the solar cell is modularized, it has a suitable refractive index as an antireflection film. In addition, the impurity concentration of the impurity diffusion layer is reduced in portions other than the portion where the light receiving surface electrode is formed on the light receiving surface. Therefore, recombination of carriers in the impurity diffusion layer is reduced, and the short wavelength sensitivity is improved by reducing the junction depth.

Further, in the second configuration of the present invention, the first
In the above structure, the light receiving surface is provided with irregularities or grooves having a predetermined height difference, and the light receiving surface provided with the irregularities or grooves is provided with the first and second titanium oxide films.

In such a configuration, irregularities or grooves having a predetermined height difference are provided on the surface of the substrate. The first and second titanium oxide films are formed on the surface provided with the irregularities or grooves. Thereby, the unevenness or the groove is provided on the light receiving surface of the solar cell, whereby the surface reflection is reduced and the short-circuit current is improved.

In the third structure of the present invention, the first titanium oxide film containing phosphorus is provided on a part of the surface of the substrate of p-type crystalline silicon, and the second titanium oxide film containing phosphorus on the surface is further provided. Is provided, and the substrate is heat-treated at a predetermined temperature to diffuse phosphorus contained in the first and second titanium oxide films to form a pn junction, and the first titanium oxide film is provided. The light receiving surface electrode is provided in the portion where the light is received.

In such a configuration, a first titanium oxide film containing phosphorus is provided on a part of the surface of the p-type crystalline silicon substrate using a metal mask or the like. Further, a second titanium oxide film is provided on the surface of the substrate. By subjecting the substrate provided with the first and second titanium oxide films to heat treatment at a predetermined temperature, phosphorus contained in the titanium oxide film is diffused,
An impurity diffusion layer is formed.

Thus, a pn junction is formed. First
The concentration of phosphorus contained in the titanium oxide film is high, and the impurity concentration of the impurity diffusion layer generated by the heat treatment is high. Further, the concentration of phosphorus contained in the second titanium oxide film is low, and the impurity concentration of the impurity diffusion layer generated by the heat treatment is low. And the first
By providing a light receiving surface electrode at the portion where the titanium oxide film is provided, a solar cell is manufactured.

In the fourth configuration of the present invention, the third configuration
In the configuration, the carrier gas is sent to a container containing the titanium compound and a container containing the phosphorus compound so that the carrier gas contains the titanium compound and the phosphorus compound, and the carrier containing the titanium compound and the phosphorus compound By supplying a gas to the surface of the substrate heated to a first predetermined temperature, the first titanium oxide film containing phosphorus is provided on the substrate, and a carrier gas containing the titanium compound and the phosphorus compound is further provided. By supplying to the surface of the substrate heated to a second predetermined temperature,
The second titanium oxide film containing phosphorus is provided.

In such a configuration, the carrier gas is sent to the container storing the titanium compound and the container storing the phosphorus compound. As a result, the carrier gas contains the titanium compound and the phosphorus compound. Then, the first titanium oxide film containing phosphorus is provided on the substrate by supplying the carrier gas to the substrate heated to a first predetermined temperature. Further, by heating the substrate to a second predetermined temperature and supplying a carrier gas containing a titanium compound and a phosphorus compound to the substrate, a second titanium oxide film is provided.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention is shown in FIGS.
This will be described with reference to FIG. FIG. 2 is a process diagram of manufacturing the solar cell of the present embodiment, and FIG. 1 is a cross-sectional view of a crystalline silicon substrate in the process.

In FIG. 2, first, in step S10, as described above, in order to reduce the surface reflection on the light receiving surface and improve the short-circuit current, the surface of the p-type crystalline silicon substrate 1 is
Asperities having a height difference of several μm to several tens μm are formed. As a result, as shown in FIG. 1A, minute irregularities are formed on the surface of the silicon substrate 1 which will be the light receiving surface.

In this embodiment, sodium hydroxide (Na
OH) Texture etching is performed at a solution temperature of about 90 ° C. using a mixed solution of an aqueous solution of OH and isopropyl alcohol to form irregularities. As described above, as the method of forming the irregularities, there are a method of forming a large number of grooves using a dicing apparatus or a laser, and a method of dry etching.

After forming the irregularities, a first titanium oxide film containing phosphorus is formed in step S11. FIG. 3 shows an atmospheric pressure CVD apparatus used for forming a titanium oxide film containing phosphorus. The silicon substrate 1 is transported by the belt 11 to the film forming unit 18 while being heated to a film forming temperature (described later) by the heater 13. In the film forming unit 18, the titanium compound and the phosphorus compound are supplied from the injector 14 to the surface of the substrate 1, and the supplied titanium compound and the phosphorus compound are thermally decomposed on the surface of the substrate 1 to form a titanium oxide film containing phosphorus. You.

The titanium compound and the phosphorus compound as raw materials are stored in bubbler containers 16 and 17, respectively. In this embodiment, tetra-i-propoxytitanium is stored in the bubbler container 16 as a titanium compound. on the other hand,
Triethoxylin is stored in the bubbler container 17 as a phosphorus compound. Fill the bubbler containers 16 and 17 with nitrogen (N
By introducing a carrier gas such as ₂ ), each raw material is included in the carrier gas up to a partial pressure corresponding to the vapor pressure and supplied to the substrate 1 together with the diluent gas.

In the film forming section 18, an exhaust gas 1 comprising a carrier gas, a diluent gas, undecomposed raw materials and decomposition products
5 is exhausted from both sides of the injector 14. In addition, 1
Reference numerals 9 and 20 denote rollers for moving the belt 11.
In addition, not only nitrogen (N ₂ ) but also an inert gas such as helium (He) and argon (Ar) can be used as the carrier gas and the diluent gas.

In FIG. 2, the first titanium oxide film is formed in step S11 by covering a portion of the substrate 1 other than the light-receiving surface electrode forming portion (described later) with a metal mask.
As shown in FIG. 1B, a titanium oxide film 2 containing a high concentration of phosphorus is formed on the light receiving surface electrode forming portion using a VD device. At this time, the temperature of the substrate 1 is set to 450 ° C. by heating the heater 13. The temperature of the bubbler container 16 containing the tetra-i-propoxy titanium is 85 ° C., and the flow rate of the carrier gas nitrogen (N ₂ ) to the bubbler container 16 is 1 liter / min.

The temperature of the bubbler container 17 containing triethoxyline is 45 ° C., and the flow rate of the carrier gas is 0.5 liter / min. The flow rate of nitrogen (N ₂ ) as a diluent gas is 5 liter / min, and the flow rate of oxygen (O ₂ ) is 7 liter / min. Under these conditions, a titanium oxide film 2 is formed as shown in FIG. 1B so as to have a thickness of about 50 nm.

After the formation of the first titanium oxide film, step S1
In step 2, a second titanium oxide film is formed. The metal mask was removed from the substrate 1, and the temperature of the substrate 1 was again increased to 350 ° C. using the atmospheric pressure CVD apparatus shown in FIG. It forms so that it may become. At this time, as shown in FIG. 1C, a titanium oxide film 3 containing a low concentration of phosphorus is formed on the entire light receiving surface. The relationship between the heating temperature of the substrate 1 and the concentration of phosphorus contained in the titanium oxide films 2 and 3 will be described later.

Next, in step S13, the substrate 1 on which the titanium oxide films 2 and 3 are formed is transferred to a quartz tube furnace (not shown), and heat-treated at about 900 ° C. for 30 minutes in a nitrogen atmosphere. Due to the diffusion of phosphorus contained in the titanium oxide films 2 and 3, a high concentration impurity diffusion layer 5 is formed near the titanium oxide film 2 as shown in FIG. 1D, and almost the entire light receiving surface is covered. A low-concentration impurity diffusion layer 4 is formed in a portion in contact with the titanium oxide film 3.

Using a normal pressure CVD apparatus shown in FIG. 3, the temperature of the substrate 1 is changed to 300 to 450 ° C. to form a phosphorus-containing titanium oxide film, and thereafter, an impurity formed by performing a heat treatment. FIG. 4 shows the relationship between the refractive index of the diffusion layer and the sheet resistance. However, the refractive index is measured by an ellipsometer, and the sheet resistance value of the impurity diffusion layer is measured by a four-probe method after dissolving a titanium oxide film containing phosphorus with hot concentrated sulfuric acid.

As shown in FIG. 4, as the temperature of the substrate 1 increases during film formation, the sheet resistance value of the impurity diffusion layer after the heat treatment decreases, and the refractive index of the phosphorus-containing titanium oxide film also decreases. That is, in FIG.
When a titanium oxide film is formed at a high temperature, it means that the sheet resistance becomes small and contains a high concentration of phosphorus. Next, when a titanium oxide film is formed on the substrate 1 at a low temperature in step S12, the sheet resistance becomes low. Means larger and contains lower levels of phosphorus.

Using the dummy substrate 1, the first titanium oxide film containing high-concentration phosphorus is formed on the entire surface of the substrate 1, and then heat-treated to measure the sheet resistance of the impurity diffusion layer. It was about 50 to 70 Ω / □. Similarly, a sheet resistance value of the impurity diffusion layer after forming a second titanium oxide film containing a low concentration of phosphorus and performing heat treatment is about 100 to 15
At 0Ω / □, the refractive index of the film was about 2.0. In the conventional solar cell, as described above, the refractive index of the film is 1.7 to 1.
Since it is 8, the second titanium oxide film has a suitable refractive index when the solar cell is modularized.

In FIG. 2, after heat treatment is performed in step S13, an aluminum paste is printed on the back surface of the substrate 1 by screen printing in step S14, and baked at about 700 ° C. to form a back electrode. As a result, FIG.
As shown in FIG. 7, the back electrode 6 and the p ⁺ layer 7 are formed.

Next, after positioning the mask on the light receiving surface electrode forming portion in step S15, a silver paste is printed by a screen printing method and baked at about 700 ° C. to form a light receiving surface electrode. Thereby, the light receiving surface electrode 8 is provided as shown in FIG. At this time, by the action of glass frit or the like in the silver paste, it penetrates the phosphorus-containing titanium oxide films 2 and 3 and contacts the impurity diffusion layer 5. In addition, in the present embodiment, the position of the mask for screen printing can be easily determined by visual observation because the substrate 1 has the titanium oxide film 2 containing a high concentration of phosphorus, so that the film thickness is large and the color is different. Can be done.

Finally, in FIG. 2, in step S16,
Apply solder coating and connect solar cells to lead wires (not shown)
String with Then, this solar cell is sandwiched between protective resins EVA, and thermocompression-bonded from above and below with glass and a back surface protection sheet to produce a module.

Table 1 shows a comparison of the photoelectric conversion efficiency between the modularized solar cell of this embodiment and the conventional solar cell. A conventional method for manufacturing a solar cell uses a normal pressure CVD apparatus shown in FIG. 3 to form a first titanium oxide film containing high-concentration phosphorus on the entire light-receiving surface, and then heat-treat it. An impurity diffusion layer is formed uniformly over the entire surface. The sheet resistance of the diffusion layer is about 50 to
70Ω / □, and the refractive index is about 1.8.

[0041]

[Table 1]

As shown in Table 1, the solar cell module according to the present embodiment has a
The contact resistance does not increase and the fill factor does not decrease. Since the impurity concentration on the light receiving surface is low, the refractive index increases, the surface reflectance is reduced, and the short wavelength sensitivity is improved. As a result, the conversion efficiency is improved, and the short-circuit current density is also improved. As described above, an antireflection film formed with a uniform film thickness can be made closer to an optimum refractive index. In addition, since the color of the antireflection film looks different at the position where the electrode is formed, the position of the electrode and the high concentration impurity diffusion layer can be easily adjusted.

When heat treatment is performed in step S13 (see FIG. 2), diffusion of impurities and formation of an antireflection film are performed simultaneously. Further, by using the atmospheric pressure CVD apparatus shown in FIG. 3, a titanium oxide film is uniformly provided on the surface of the substrate 1 provided with the unevenness. The impurity concentration of the impurity diffusion layer is reduced in portions other than the portion where the light receiving surface electrode is formed on the light receiving surface. Therefore, recombination of carriers in the impurity diffusion layer is reduced, and the short wavelength sensitivity is improved by reducing the junction depth.

[0044]

【The invention's effect】

<Effect of Claim 1> Since the impurity concentration contained in the first titanium oxide film is high, the contact resistance between the light receiving surface electrode and the impurity diffusion layer in contact therewith is small.
Therefore, the fill factor does not decrease. On the other hand, the second titanium oxide film becomes an antireflection film on the light receiving surface. Since the concentration of the impurity is low, the optimum refractive index of the titanium oxide film is obtained when the solar cell is modularized. In addition, the impurity concentration of the impurity diffusion layer is reduced in portions other than the portion where the light receiving surface electrode is formed on the light receiving surface. Therefore, recombination of carriers in the impurity diffusion layer is reduced, and the short wavelength sensitivity is improved by reducing the junction depth.

<Effect of Claim 2> By providing irregularities or grooves having a predetermined height difference on the light receiving surface, surface reflection on the light receiving surface is reduced, and short-circuit current is improved.

<Effect of Claim 3> By the method for manufacturing a solar cell according to the present invention, a solar cell provided with the first and second titanium oxide films containing phosphorus is manufactured.

<Effect of Claim 4> By supplying a carrier gas containing a titanium compound and a phosphorus compound to the surface of a substrate heated to a first predetermined temperature, a first titanium oxide film containing phosphorus is provided. . Further, by heating the substrate to a second predetermined temperature and supplying the carrier gas,
A second titanium oxide film containing phosphorus is provided. As described above, the concentration of phosphorus contained in the titanium oxide film varies depending on the heating temperature of the substrate.

[Brief description of the drawings]

FIG. 1 is a sectional view of a substrate in a manufacturing process of a solar cell according to an embodiment of the present invention.

FIG. 2 is a process diagram thereof.

FIG. 3 is a sectional view of the normal pressure CVD apparatus.

FIG. 4 is a diagram showing the relationship between the refractive index of the solar cell and the sheet resistance.

FIG. 5 is a cross-sectional view of a substrate in a conventional solar cell manufacturing process.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 p-type crystalline silicon substrate 2 titanium oxide film containing high-concentration phosphorus 3 titanium oxide film containing low-concentration phosphorus 4 low-concentration impurity diffusion layer 5 high-concentration impurity diffusion layer 6 back electrode 7 p ⁺ layer 8 Light receiving surface electrode 11 Belt 13 Heater 14 Injector 15 Exhaust gas 16 Bubbler container 17 Bubbler container

Claims (4)

[Claims] 1. A solar cell in which an impurity diffusion layer is formed near a light receiving surface by providing a titanium oxide film containing an impurity on a light receiving surface of a silicon substrate. 2 titanium oxide film is provided, the concentration of impurities contained in the first titanium oxide film is higher than the concentration of impurities contained in the second titanium oxide film, and A solar cell, wherein a light receiving surface electrode is provided in a portion where the first titanium oxide film is provided, and a back surface electrode is provided on a back surface of the light receiving surface. 2. The light-receiving surface is provided with irregularities or grooves having a predetermined height difference, and the light-receiving surface provided with the irregularities or grooves is provided with the first and second titanium oxide films. The solar cell according to claim 1. 3. The substrate according to claim 1, wherein the first titanium oxide film containing phosphorus is provided on a part of the surface of the substrate of p-type crystalline silicon, and the second titanium oxide film containing phosphorus is further provided on the surface. Is heat-treated at a predetermined temperature to diffuse phosphorus contained in the first and second titanium oxide films to form a pn junction,
A method for manufacturing a solar cell, wherein a light receiving surface electrode is provided in a portion where the first titanium oxide film is provided. 4. A carrier gas is sent to a container containing a titanium compound and a container containing a phosphorus compound so that the carrier gas contains the titanium compound and the phosphorus compound, and the carrier gas contains the titanium compound and the phosphorus compound. The first titanium oxide film containing phosphorus is provided on the substrate by supplying a carrier gas to the surface of the substrate heated to a first predetermined temperature, and the carrier gas further contains the titanium compound and the phosphorus compound. The method according to claim 3, wherein the second titanium oxide film containing phosphorus is provided by supplying the second titanium oxide film to the surface of the substrate heated to a second predetermined temperature.