JPH07193263A - Solar cell manufacturing method - Google Patents

Abstract

(57) [Summary] [Objective] The formation of the optical confinement structure of the silicon substrate and the formation of the BSF layer at low temperature are performed in one step. [Structure] An insulating film is formed on the back surface of a silicon substrate 6 having a PN junction formed on the light-receiving surface side, a plurality of openings are formed in this insulating film, and an Al paste is printed on the entire back surface including the openings. Then, by firing, the fine uneven structure and the BSF layer 11 along the uneven wall surface are simultaneously formed on the back surface of the silicon substrate 6 at a low temperature.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for manufacturing a solar cell,
In particular, it relates to the light confinement structure and the method for forming the high concentration layer on the back surface of the semiconductor substrate.

[0002]

2. Description of the Related Art Formation of a light confining structure for a substrate and formation of a back surface field layer (BSF layer) are mentioned as important techniques for improving the efficiency of a solar cell.

First, the necessity of a light confining structure and a manufacturing method will be described. For example, in a crystalline silicon solar cell, it is important to effectively use the light incident on the cell surface when considering high efficiency. For this reason, an antireflection film is formed on the surface of the solar cell, a fine pyramid or groove is formed to reduce reflection, and a fine pyramid or groove is also formed on the back surface so that the light reaching the back surface can be obliquely transmitted to the substrate. So-called light confinement is performed such as internally reflecting the light, or using a high-reflectance metal for the back electrode to increase the light reflected in the substrate.

In consideration of light confinement, as shown in FIG. 2, the structure of the front surface and the back surface is, for example, a groove 2 on the back surface orthogonal to a large number of grooves 21 on the surface of the silicon substrate 20.
It is theoretically said that the structure forming a large number of 2 is the best.

As a method of obtaining such a light confining structure, an anisotropic etching processing method using an alkaline aqueous solution using a SiO ₂ film as a mask for a single crystal substrate, or a mechanical processing method using a dicer is a polycrystalline method. For the substrate, a mechanical processing method using a dicer or the like has been proposed.

Next, the BSF layer will be described. BSF
The layer pushes back the carriers generated in the vicinity of the back surface of the substrate to the bonding layer side of the light receiving surface by the high electric field, and prevents the loss due to the recombination of carriers on the back surface.

Usually, in the case of a P-type semiconductor substrate, the BSF layer is
It has the same conductivity type as the substrate by using a method of thermally diffusing on the back surface at a high temperature of about 1000 ° C. using boron as an impurity, or a method of printing an Al paste on the back surface and alloying by heat treatment at about 740 ° C., In addition, a P ⁺ layer containing impurities having a higher concentration than the substrate is formed.

[0008]

A solar cell aiming at high efficiency has introduced the above-mentioned technique.

However, the steps of forming the light confining structure of the substrate and the step of forming the BSF layer are essentially different from each other, and in the current method of manufacturing a solar cell, they must be formed in independent steps. .

Also, after forming fine grooves on the back surface, BS
When forming the F layer, in the boron diffusion method, it is necessary to form a SiO ₂ film having a sufficient thickness as a diffusion mask for preventing the diffusion of boron on the light receiving surface side, and the diffusion temperature is 1000. There is a problem that a high temperature of about ℃ results in deterioration of substrate characteristics such as lifetime and deterioration of cell characteristics. In the Al alloy method, the BSF layer can be formed at a low temperature of about 740 ° C., but when the paste is printed on the groove and baked, the uneven shape of the groove is deformed by alloying and cannot be applied.

An object of the present invention is to provide a method of forming a light confining structure of a semiconductor substrate and forming a BSF layer at a low temperature in one step.

[0012]

According to the method of manufacturing a solar cell of the present invention, a PN junction is formed on the light receiving surface side of a semiconductor substrate, and an insulating film such as a SiN insulating film or a TiO ₂ insulating film is formed on the back surface of the PN junction. By forming a plurality of openings in the insulating film, printing an Al paste, for example, on the entire back surface including the openings and baking the same to form a fine concavo-convex structure on the back surface of the semiconductor substrate and the BSF along the concavo-convex wall surface. The layers are simultaneously formed at low temperature.

[0013]

The principle of operation of the present invention will be described.

In FIG. 3, after printing the Al paste on the back surface of the silicon substrate and baking it at 740 ° C., the substrate was obliquely polished to A
It is a graph of the result of having measured the spreading | diffusion state of 1 by the spreading resistance method. From this, it can be seen that an Al / Si alloy layer is formed up to a depth of about 7 μm from the substrate surface, and there is a diffusion layer below which Al is distributed with a concentration gradient. This is a reaction that occurs during the formation of the BSF layer of a normal solar cell. Next, in order to investigate the function of the Al / Si alloy layer, the spectral sensitivity characteristics before and after the removal of the alloy layer were compared using the same cell. Al on the surface after the alloy layer is removed
Was vapor-deposited. As a result, as shown in FIG. 4, the characteristics of both are almost the same, and it is understood that the BSF effect is due to the Al diffusion layer and the alloy layer functions as an electrode.

5 (a), 5 (b) and 5 (c) show the case where the back surface of the silicon substrate 6 is covered with the SiN film 1 and the openings 2, 2, ... It is sectional drawing of a process. (A) shows a state in which openings 2 having a width of 20 μm are provided in the SiN film 1 at intervals of 120 μm, and then an Al paste 3 is printed, (b) shows a state of firing at 740 ° C., (c) shows an Al / Si alloy layer. The cross section after removing is shown.
As a result, the alloying from the opening progresses isotropically, and the recess 4 having a width of about 120 μm and a depth of about 60 μm is formed, and the BSF layer 5 is formed along the wall surface of the recess 4. I understand that it will be done.

The fine recesses on the back surface shown in FIG.
By making the opening pattern on the iN film linear, a fine groove structure as shown in FIG. 2 can be obtained.

FIGS. 6A and 6B show how incident light is reflected by the back surface in the case where the back surface of the silicon substrate 6 is flat and the structure in which the groove is formed, respectively. FIG. The reflected light travels a longer distance in the substrate in the case of the groove structure than in the case of being flat, and thus contributes more to the photoelectric conversion. That is, the function of confining light is improved.

According to the method of the present invention, since the Al paste is fired at 740 ° C., it can be seen that the formation of the light confining structure of the substrate and the formation of the BSF layer are simultaneously performed at a low temperature.

[0019]

The present invention can be applied to either a polycrystalline silicon substrate or a single crystal silicon substrate.

First, the first embodiment will be described. Figure 1
FIG. 3 is a perspective view of an example of a solar cell formed by the method of the present invention using a polycrystalline silicon substrate. The silicon substrate 6 is a P-type cast polycrystalline silicon substrate having a 100 mm square and a thickness of 250 μm. First, grooves on the light-receiving surface side were processed. With a dicer, a blade with a thickness of 25 μm and a depth of 70
The grooves 14, 14 ... Of μm were formed at a pitch of 70 μm.
After that, etching with a mixed solution of hydrofluoric acid (HF) and nitric acid (HNO ₃ ) and etching with an aqueous solution containing sodium hydroxide (NaOH) and isopropyl alcohol (IPA) were performed to remove the damaged layer on the substrate surface. As a result, the light-receiving surface has a low-reflection surface structure having many fine grooves.

Then, thermal diffusion (850 ° C.) using phosphorus as an impurity with POCl ₃ is performed to form an N ⁺ layer 7 on the light-receiving surface, and then a thin (thickness) layer (thickness) is formed by a thermal oxidation method (800 ° C.). A SiO ₂ film 8 having a thickness of about 15 nm was formed. Then, normal pressure C using isopropyl titanate as a raw material
The TiO ₂ film 9 (having a thickness of about 5
5 mm) was formed.

Next, the N ⁺ layer remaining on the back surface of the substrate is removed by etching with a mixed solution of hydrofluoric acid and nitric acid, and a SiN film is formed on the back surface by plasma CVD using SiH ₄ , NH ₃ and N ₂ as source gases. After forming (thickness: 200 to 250 nm), this film was subjected to opening treatment in a line shape having a width of 20 μm and a pitch of 130 μm. The direction of this line was a direction orthogonal to the groove 14 formed on the light receiving surface side. The following conditions were used for forming the SiN film. Si as a source gas
The flow rates of H ₄ , NH ₃ and N ₂ are 10 _sccm and 15 respectively.
As _sccm, 50 _sccm, the substrate temperature 350 ° C., to form a pressure 0.75 Torr, the RF power as 100W.

Next, an Al paste was screen-printed on the entire back surface and baked at 740 ° C. in an N ₂ / O ₂ atmosphere. As a result, the groove 10 on the back surface, the BSF layer 11, and the back surface electrode 12 are simultaneously formed.

Next, an Ag paste to be the light-receiving surface electrode 13 is screen-printed on the surface, and then 58 in an N ₂ / O ₂ atmosphere.
After firing at 0 ° C., solder coating is performed to complete the solar cell.

In the above process, a high temperature of 1000 ° C., which is different from the diffusion of boron, is not used, and the lifetime is not reduced.

FIG. 7A is a graph showing the reflectance when light is incident from the front surface in the light confining structure in which fine grooves are formed on the back surface according to the present embodiment and the structure in which it is not formed. The light confining structure in which fine grooves are formed on the back surface according to the present embodiment has a lower reflectance on the long wavelength side than the structure not forming the groove, and has the effect of confining the long wavelength light reaching the back surface inside the substrate. I understand. As a result, as shown in FIG. 7B, the spectral sensitivity in the long wavelength region of the cell is improved and the short circuit current is increased.

Next, a second embodiment will be described. A P-type single crystal silicon substrate (100 mm square, 250 μm thick) is used to obtain a light-receiving surface having a large number of fine grooves and a low-reflection surface structure by the same method as in the first embodiment.

Next, by the same method as in the first embodiment, N
⁺ Layer and SiO ₂ film are formed, followed by plasma CVD
SiN film (thickness: about 85 nm) to be an antireflection film by the method
Was formed.

Next, the N remaining on the back surface of the substrate⁺Layer first
And remove it on the back
By atmospheric pressure CVD method using peel as a raw material, T at 580 ° C
iO ₂After forming a film (thickness about 55 nm), this film is
In the same manner as in Example 1, opening treatment was performed in a line shape. This rye
Direction is the direction orthogonal to the groove formed on the light-receiving surface side.
Is.

Next, an Al paste is screen-printed on the entire back surface and baked in the same manner as in the first embodiment, so that the groove structure on the back surface, the BSF layer, and the back surface electrode are simultaneously formed.

Next, the light-receiving surface electrode is formed in the same manner as in the first embodiment. Next, the Al / Si alloy layer on the back surface was removed with hydrochloric acid, and a copper thin film was formed on the uneven surface formed thereafter by a plating method to form a back surface electrode to complete a solar cell.

FIG. 8 shows the spectral sensitivities of a cell according to the second embodiment in which a fine groove is formed on the back surface and the alloy layer is removed to form a copper thin film, and a cell in which the alloy layer is used as an electrode as it is. It is a graph. The cell formed with the copper thin film according to the present embodiment has a higher spectral sensitivity in the long wavelength region than the cell in which the alloy layer is used as an electrode as it is, and by using a highly reflective metal such as copper as the electrode, It can be seen that the light confinement effect is further improved.

Although copper is used as the back electrode in this embodiment, gold, silver, etc. can also be used as the highly reflective metal, and the thin film forming method includes a vapor deposition method other than the plating method. . In addition to hydrochloric acid, aqua regia, hydrofluoric acid or the like can be used to remove the alloy layer.

[0034]

According to the present invention, since the back groove structure having a high light confinement effect is formed at a low temperature at the same time as the BSF layer, the cell characteristics are improved and the cell manufacturing process is simplified. Therefore, it is also effective in reducing the manufacturing cost.

[Brief description of drawings]

FIG. 1 is a perspective view of an example of a solar cell manufactured according to the present invention.

FIG. 2 is a perspective view showing a light confining structure.

FIG. 3 is a graph showing a carrier concentration distribution when an Al paste is printed and fired.

FIG. 4 is a graph showing the spectral sensitivities of a cell in which an alloy layer remains and a cell in which Al is evaporated after the alloy layer is removed.

5A to 5C are cross-sectional views of respective steps for explaining the principle of the present invention.

6 (a) and 6 (b) are diagrams showing incident light reflection states when the back surface is flat and when a groove is formed.

FIG. 7A is a graph showing the difference in reflectance depending on the presence or absence of fine grooves on the back surface, and FIG. 7B is a graph showing the difference in spectral sensitivity depending on the presence or absence of fine grooves on the back surface.

FIG. 8 is a graph showing spectral sensitivities of a cell in which an alloy layer is left and a cell in which a copper thin film is formed after removing the alloy layer.

[Explanation of symbols]

1 SiN Film 2 Opening 3 Al Paste 4 Dimple 5 BSF Layer 6 Silicon Substrate 7 N ⁺ Layer 8 SiO ₂ Film 9 TiO ₂ Film 10 Groove 11 BSF Layer 12 Backside Electrode 13 Light-Receiving Surface Electrode 14 Groove

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Nishida 22-22 Nagaike-cho, Abeno-ku, Osaka City, Osaka Prefecture

Claims (1)

[Claims] 1. A step of forming a PN junction on a semiconductor substrate, a step of forming an insulating film on the back surface of the substrate, a step of forming an opening in the insulating film, and a step of forming an insulating film on the insulating film including the opening. A step of printing a conductive paste containing a metal having a property of forming a high-concentration layer of the same conductivity type as that of the semiconductor substrate when diffused in the semiconductor substrate; and a semiconductor substrate made of metal diffused around the opening by heat-treating the paste. And a step of forming a high-concentration layer having a fine uneven wall surface of the same conductivity type as described above.