JP2000101111A - Solar cell manufacturing method - Google Patents

Abstract

(57) Abstract: Provided is a method for manufacturing a solar cell capable of improving energy conversion efficiency by preventing destruction at a pn junction of a semiconductor substrate. SOLUTION: Fine irregularities 2 are formed on the surface of a polycrystalline silicon substrate 1 by dry etching. Thereafter, the corners of the irregularities 2 are smoothed by wet etching. After the etching, impurity diffusion is performed on the polycrystalline silicon substrate 1 to form a pn junction. Thereafter, a conductive paste is baked on the front and back surfaces of the polycrystalline silicon substrate 1 to form a front surface electrode 7 and a back surface electrode 6, respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a solar cell, and more particularly, to a method for forming irregularities for reducing reflection on the surface of a semiconductor substrate.

[0002]

2. Description of the Related Art Generally, in a single crystal silicon solar cell,
Anisotropic etching using an alkaline solution is performed on a silicon substrate to form a fine pyramid or inverted pyramid structure on the surface of the silicon substrate, thereby reducing reflection on the substrate surface.

The above-described anisotropic etching utilizes the fact that the etching rate of silicon is different between the (100) crystal orientation plane and the (111) crystal orientation plane. Therefore, in the case of a polycrystalline silicon substrate including crystal grains having various plane orientations, the occupancy of the crystal grains having the (100) crystal orientation plane in the substrate is estimated to be about 20%, and the remaining (100) Crystal grains having a plane orientation other than the crystal orientation plane occupy a large part, and there is a disadvantage that a fine pyramid structure capable of reducing surface reflection is not formed.

As a method of solving this drawback, for example,
JP-A-9-148603 discloses a method of mechanically processing the surface of a silicon substrate, JP-A-3-89518 discloses a method of processing the surface of a silicon substrate using a laser, and JP-A-9-167850. For performing wet etching using an aqueous acid solution, see JP-A-9-1026.
No. 25 discloses a method of performing dry etching using a mask forming gas and an etching gas, respectively.

[0005] For example, "Tadashi Kadoma, Yoshinori Ishizaki, Yoji Saito, IEICE Technical Report, Vol. 96, No. 39
6, the p19-24,1996 ", in a single crystal silicon substrate, chlorine trifluoride gas (ClF ₃₎ and techniques have been proposed regarding isotropic etching using, according to this etching, the silicon substrate 10% surface reflectance
It is reported that it can be reduced to the extent. In the above etching, fine random irregularities are formed on the silicon substrate surface only by exposing the silicon substrate to a ClF ₃ gas atmosphere. Therefore, there is a possibility that a large amount of silicon substrates can be processed at one time, and there is an advantage that ion damage is not caused because of plasmaless etching.

[0006]

The inventors of the present invention made a trial production of a solar cell by applying etching using ClF ₃ gas to a polycrystalline silicon substrate.
It has been found that the following problems occur.

Generally, in a method for manufacturing a solar cell,
After irregularities are formed on the surface of the silicon substrate, impurities are diffused to form a pn junction, and a silver (Ag) paste printed by a screen printing method or the like is fired on the surface of the silicon substrate to form a light receiving surface electrode. Form.

In the polycrystalline silicon substrate etched by using the ClF ₃ gas, the irregularities formed on the substrate surface become fine, but the tips of the irregularities become sharp. P just below the light-receiving surface electrode
In some cases, the n-junction was electrically destroyed. Therefore, in the characteristics of the solar cell, the leakage current increases,
As a result, a decrease in fill factor and open circuit voltage was observed.

The present invention has been made in view of the above problems, and provides a method of manufacturing a solar cell capable of preventing a pn junction of a semiconductor substrate from being broken and improving energy conversion efficiency while reducing surface reflection. The purpose is to:

[0010]

The object of the present invention is to form fine irregularities on the surface of a semiconductor substrate by performing different kinds of etchings a plurality of times, further smooth the corners of the irregularities, and perform etching. Later, p
A solar cell is manufactured by forming an n-junction and then forming electrodes on the front and back surfaces of the semiconductor substrate.

More specifically, after fine irregularities are formed on the surface of the semiconductor substrate by dry etching, the corners of the irregularities are smoothed by wet etching, and impurities are diffused into the semiconductor substrate. A pn junction is formed, and then a conductive paste is fired on the front and back surfaces of the semiconductor substrate to form electrodes.

In the dry etching, the semiconductor substrate is exposed to a chlorine gas, for example, a chlorine trifluoride gas. In wet etching, the semiconductor substrate is immersed in an alkaline aqueous solution, for example, an aqueous solution of sodium hydroxide or potassium hydroxide. Alternatively, it may be immersed in an aqueous acid solution, for example, a mixed solution of nitric acid and hydrofluoric acid.

As described above, the semiconductor substrate is exposed to a chlorine gas such as chlorine trifluoride gas and is immersed in an alkaline aqueous solution such as an aqueous sodium hydroxide solution, and is etched a plurality of times to form the semiconductor substrate on the substrate surface. The corners of the fine irregularities become smooth. Therefore, for example, in the case of fine irregularities formed only by exposing the semiconductor substrate to chlorine trifluoride gas, the pn junction was destroyed when the electrode was formed, but the corners of the irregularities were smooth. Can prevent it.

[0014]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a diagram illustrating a method for manufacturing a solar cell according to one embodiment of the present invention. Hereinafter, the manufacturing procedure of the solar cell will be described with reference to FIG.

The feature of the present embodiment is that fine irregularities are formed on the surface of a silicon substrate by performing different types of etching a plurality of times, and the corners of the irregularities are further smoothed.
An object of the present invention is to improve the energy conversion efficiency of a solar cell. Specifically, the p-type polycrystalline silicon substrate 1 having a thickness of about 0.4 mm is set in a quartz boat, and is loaded into a quartz or stainless steel tube furnace that can be decompressed.
After reducing the pressure in the tube furnace, the substrate temperature is kept at about 30 ° C.

Here, dry etching is performed on the silicon substrate 1. In dry etching, a ClF ₃ gas as an etching gas and an Ar gas as a diluting gas are introduced into the tube at a flow rate of 0.2 l / min and 2 l / min, respectively. Approximately 10 kPa total pressure with pressure adjustment valve
Etching for about 15 minutes while maintaining
As shown in FIG. 1, unevenness 2 finer than 1 μm is formed on the surface of the silicon substrate 1. At this time, fine irregularities 2 are also formed on the back surface of the silicon substrate 1. The etching gas is chlorine (Cl) instead of ClF ₃ gas.
Other chlorine gas such as gas, oxygen (O ₂ ) gas, sulfur hexafluoride (SF ₆ ) gas, or the like may be used.

Next, wet etching is performed on the dry-etched silicon substrate 1. That is, sodium hydroxide (NaO) which is an alkaline aqueous solution
Etching is performed for about 5 minutes at room temperature using a 10% aqueous solution of H) to change the shape of the unevenness 2 of the silicon substrate 1 as shown in FIG. 2A and 2B are diagrams showing a state of change of the unevenness 2, wherein FIG. 2A shows a state after dry etching and FIG. 2B shows a state after wet etching. FIG. 3 shows the surface of the silicon substrate 1 after the dry etching, and FIG. 4 shows the surface after the wet etching. According to these figures, it can be understood that the corners of the unevenness 2 are smoothed and the shape of the unevenness 2 is increased by further etching with an alkaline aqueous solution after the dry etching. Specifically, it was observed that the size of the unevenness 2 was 4 μm to 8 μm. In the wet etching, NaO
Instead of the H aqueous solution, a potassium hydroxide (KOH) aqueous solution which is the same alkaline aqueous solution may be used. Further, a mixed solution of nitric acid (HNO) and hydrofluoric acid (HF), which is an aqueous acid solution, can also be used.

Next, impurities are diffused on the surface of the silicon substrate 1 using an impurity diffusion source such as phosphorus oxychloride (POCl ₃ ) to form an n ⁺ layer 3 as shown in FIG. A pn junction is formed. After forming the n ⁺ layer 3,
Unnecessary n ⁺ layer 3 on the back and side surfaces of silicon substrate 1 is removed. That is, other portions are removed except for the n ⁺ layer 3 on the surface side of the silicon substrate 1. Specifically, a resist film for etching is formed only on the surface side of the silicon substrate 1,
After immersion in a mixed solution of nitric acid and hydrofluoric acid to remove the n ⁺ layer 3 other than the surface side, the resist film is removed, and the silicon substrate 1 is washed with pure water.

[0019] Then, as shown in FIG. (D), n ⁺ layer 3
An anti-reflection film 4 is formed thereon. The anti-reflection film 4 is a film for efficiently absorbing light incident on the silicon substrate 1. For example, a silicon nitride film formed by a plasma CVD method using a mixed gas of silane and ammonia as a raw material, or a titanate alkoxide And a titanium oxide film formed by a normal pressure CVD method.

Next, as shown in FIG. 2E, an aluminum paste as a conductive paste is printed on the back surface of the silicon substrate 1 and fired at about 700 ° C. using a belt firing furnace.
A p ⁺ layer 5 and a back electrode 6 are formed.

Then, as shown in FIG. 1F, the surface (light receiving surface) of the silicon substrate 1 is coated with a conductive paste of Ag.
The surface electrode 7 is formed by printing and baking the paste. At this time, the antireflection film 4 where the surface electrode 7 is to be formed
Is removed. The front electrode 7 and the back electrode 6
May be formed using a plating method, a vacuum evaporation method, or the like.

As described above, a solar cell having a structure in which fine irregularities 2 with smooth corners are formed on the surface of a polycrystalline silicon substrate 1 by performing dry etching with a chlorine gas and further performing wet etching with an alkaline aqueous solution. Can be manufactured. Conventionally, on a polycrystalline silicon substrate, sharp irregularities are formed at the tip by performing only dry etching, and when a conductive paste is heated and fired to form a surface electrode, the pn junction may be broken. However, by performing etching as described above a plurality of times, the corners of the unevenness 2 become smooth, the tip of the unevenness 2 becomes less sharp, and the pn junction can be prevented from being broken. Also, in a polycrystalline silicon substrate, ClF ₃
Since etching using a gas can be applied, a large amount of silicon substrates can be processed, and advantages such as no ion damage due to plasmaless etching can be obtained.

<Example 1> Based on the above manufacturing method,
A sample that was wet-etched after dry etching was manufactured (hereinafter, referred to as “Example 1”). On the other hand, as Comparative Example 1, a sample prepared by performing only wet etching using a mixed solution of NaOH aqueous solution and isopropyl alcohol at about 90 ° C., which is generally called texture etching, was prepared. In Comparative Example 1, the processes after forming the n ⁺ layer are the same as those in Example 1.

FIGS. 5 and 6 show the surface of the silicon substrate of Comparative Example 1. In particular, FIG. 5 shows crystal grains in a low reflection portion.
Indicates crystal grains in a high reflection portion. Here, as compared with the surface of the silicon substrate of Example 1 (see FIG. 4), in Example 1, almost no difference in surface shape due to crystal grains was observed.

FIG. 7 is a diagram showing the spectral reflectances of the low reflection part and the high reflection part of Example 1 and Comparative Example 1.
According to the figure, the spectral reflectance of Example 1 is lower than that of the low reflection portion of Comparative Example 1, but is lower than that of the high reflection portion with a large difference except for some wavelength regions. In Example 1, it can be seen that the characteristic uneven structure of the surface has a remarkable effect on the suppression of reflection.

Table 1 shows the IV characteristics (short-circuit current density J _SC , open-circuit voltage V _OC , fill factor FF, conversion efficiency Effi., Leak current, etc.) of Example 1 and a comparative example. 1
₃ shows the IV characteristics of a sample subjected to texture etching and Comparative Example 2 a sample subjected to only ClF ₃ gas etching. In Comparative Example 2, n
^Subsequent processes for forming the ⁺ layer are manufactured in the same process as in the first embodiment.

[0027]

[Table 1] According to Table 1, in the solar cell of Example 1, the surface reflectance was reduced, so that the short-circuit current density was improved by about 6% as compared with Comparative Example 1, and the conversion efficiency was also about 6 to 7 % Has improved. In addition, since the junction breakdown due to the paste electrode can be prevented, the leak current is significantly reduced, the open-circuit voltage and the fill factor are improved as compared with Comparative Example 2, and the conversion efficiency is about 6 to 7
% Has improved.

As described above, by performing dry etching and further performing wet etching, the corners of the fine irregularities 2 formed on the surface of the silicon substrate 1 are smoothed,
In addition to preventing the destruction of the pn junction, the surface reflectance can be reduced as compared with the etching in which only dry etching or only wet etching is performed. As a result, characteristics such as short-circuit current density are improved, and conversion efficiency is improved. Can be increased.

Further, even if the silicon substrate 1 is polycrystalline, the irregularities 2 can be formed uniformly without depending on the plane orientation.
Light absorption increases, and conversion efficiency can be improved.

<Example 2> As Example 2, in wet etching, instead of an NaOH aqueous solution, the sample was immersed in an aqueous solution in which nitric acid and hydrofluoric acid were mixed at a ratio of 1:10 for about 2 minutes to change the shape of the unevenness. A solar cell was manufactured. Other manufacturing processes of the solar cell were performed in the same manner as in the above embodiment. Experiments also confirmed that the same effect was obtained in the conversion efficiency and the like in Example 2.

Further, the present invention is not limited to the above embodiment, and many modifications and changes can be made to the above embodiment within the scope of the present invention. For example, in the above embodiment, the etching is performed twice, that is, the dry etching and the wet etching. For example, the dry etching is performed after the dry etching, the wet etching, or the wet etching is further performed after the dry etching and the wet etching. Etching may be performed more than twice, for example. Further, in the above embodiment, the n ⁺ layer is formed on the p-type polycrystalline silicon substrate, but the p ⁺ layer may be formed on the n-type polycrystalline silicon substrate.

[0032]

As described above, according to the present invention, dry etching is performed on a semiconductor substrate using a chlorine gas, and then wet etching is performed on the surface of the semiconductor substrate using an alkaline aqueous solution or an acid aqueous solution. Since the corners of the irregularities can be smoothed, the pn junction can be prevented from being broken during the formation of the electrode. Furthermore, in the characteristics of the solar cell, an increase in leakage current can be prevented, and as a result, characteristics such as open-circuit voltage and fill factor can be improved, and a solar cell with increased conversion efficiency can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a method for manufacturing a solar cell according to an embodiment of the present invention, wherein (a) shows a case where dry etching is performed, (b) shows a case where wet etching is performed, and (c) shows a case where p is
When an n-junction is formed, (d) when an antireflection film is formed, (e) when a back electrode is formed, and (f) when a front electrode is formed.

FIG. 2 shows a cross-sectional view of a silicon substrate of a solar cell, and (a)
Is after dry etching, (b) is after dry and wet etching

FIG. 3 is a scanning electron micrograph showing the surface of a silicon substrate after dry etching.

FIG. 4 is a scanning electron micrograph showing the surface of a silicon substrate after dry and wet etching.

FIG. 5 is a scanning electron micrograph showing a surface of a low reflection portion of a silicon substrate in a comparative example.

FIG. 6 is a scanning electron micrograph showing the surface of a highly reflective portion of a silicon substrate in a comparative example.

FIG. 7 is a diagram showing spectral reflectances of solar cells of Examples and Comparative Examples.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 polycrystalline silicon substrate 2 unevenness 3 n ⁺ layer 4 antireflection film 5 p ⁺ layer 6 back electrode 7 front electrode

Claims (4)

[Claims] 1. A method of forming fine irregularities on a surface of a semiconductor substrate by performing different types of etchings a plurality of times,
A method of manufacturing a solar cell, further comprising smoothing corners of the irregularities, forming a pn junction on the semiconductor substrate after the etching, and thereafter forming electrodes on a front surface and a back surface of the semiconductor substrate. 2. After forming fine irregularities on the surface of the semiconductor substrate by performing dry etching, the corners of the irregularities are smoothed by performing wet etching, and impurity is diffused into the semiconductor substrate to perform pn. A method for manufacturing a solar cell, comprising: forming a bonding portion; and sintering a conductive paste on the front and back surfaces of the semiconductor substrate to form electrodes. 3. The solar cell according to claim 2, wherein the dry etching is performed by exposing the semiconductor substrate to a chlorine gas, and the wet etching is performed by immersing the semiconductor substrate in an alkaline aqueous solution or an acid aqueous solution. Manufacturing method. 4. The chlorinated gas is chlorine trifluoride gas, the alkaline aqueous solution is a sodium hydroxide aqueous solution or a potassium hydroxide aqueous solution, and the acid aqueous solution is a mixed solution of nitric acid and hydrofluoric acid. The method for manufacturing a solar cell according to claim 3.