JPH0613632A - Solar cell and its manufacture - Google Patents

Abstract

PURPOSE:To form V-shaped trenches on the surface of a silicon substrate and, continuously, form P-N junctions at the bottom parts of the V-shaped trenches. CONSTITUTION:V-shaped trenches 2 are formed on the surface of a P-type silicon substrate 1 by changing the width of a laser beam successively. The laser beam is applied to the bottom parts of the V-shaped trenches 2 with the energy density changed in a suitable atmosphere to form N-type diffused layers 3. Thus local P-N junctions between the N-type diffused layers 3 and the P-type silicon substrate 1 are continuously formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solar cell having an uneven surface and a method for manufacturing the same.

[0002]

2. Description of the Related Art Generally, a basic method of manufacturing a solar cell using a silicon substrate and having irregularities on the surface is to first form irregularities for reducing surface reflection on the surface of the silicon substrate which is the light incident side. After that, a doped layer for forming a PN junction is formed, and finally electrodes on the front surface and the back surface are formed.

The texture treatment for forming irregularities on the surface of a single crystal solar cell is performed for a substrate having a (100) plane orientation.
In an alkaline aqueous solution such as NaOH or KOH,
Anisotropic etching in the (100) direction and the (111) direction is performed to form pyramid-shaped irregularities on the substrate.

Similarly, even in a polycrystalline solar cell, anisotropic etching with an alkaline aqueous solution is performed.
Although the reflectance is lower than that of a flat surface, it is not as effective as the texture treatment of a single crystal substrate. This is because the plane orientation of each crystal grain existing in the polycrystalline substrate is not uniform, and pyramids are not formed at plane indices other than (100).

Therefore, in a polycrystalline solar cell, surface reflection is reduced by forming a V-shaped groove by isotropic etching using photolithography or mechanical processing using a laser, a dicer, or the like. It is the current situation.

When grooves are formed on the surface of the substrate, surface reflection can be reduced. For example, when the PN junction is formed by the thermal diffusion method using POCl ₃ which is commonly used as an impurity, the It has been spread over the entire surface,
As compared with a flat surface, the unevenness increases the bonding area. This increase in the junction area causes an increase in leakage current and a decrease in open circuit voltage.

For this reason, various efforts have been made to reduce the area of the PN junction called a point junction, but the SiO ₂ film is patterned by photolithography which is often used in a so-called LSI process, and the diffusion barrier is used. The present situation is that only a layer having a doped layer is formed.

[0008]

In the photolithography described above, when the surface has irregularities, there are drawbacks that the patterning accuracy is poor and the reliability tends to be a problem.
Further, in order to form unevenness to reduce the surface reflectance and to form a partial PN junction, a plurality of steps including complicated steps such as photolithography are required, which makes it practical and low cost. There was a big problem in terms of conversion. An object of the present invention is to simplify the manufacturing process by forming a groove for reducing the surface reflectance and forming a partial PN junction by continuous laser processing.
To obtain a highly efficient and inexpensive solar cell.

[0009]

According to the present invention, a V-shaped groove and a V-shaped groove are formed on the surface of a silicon substrate by laser light.
The PN junction at the bottom of the V-shaped groove was continuously formed.

[0010]

By forming the PN junction only on the bottom of the V-shaped groove, the area of the junction is reduced, and as a result, the leakage current is reduced and the open circuit voltage is improved.

[0011]

EXAMPLE FIG. 1 (a) is a schematic sectional view of a polycrystalline solar cell according to the present invention, and FIG. 1 (b) is a perspective view thereof.

A large number of V-shaped grooves 2, 2, ... Are formed on the surface (light-receiving surface side) of the P-type silicon substrate 1, and N-type diffusion layers 3, 3 ,. Has been done. A SiO ₂ film 4 serving as a passivation film and a conductive antireflection film 8 are laminated on the surface. The conductive antireflection film 8 is provided with the light-receiving surface electrodes 5, 5, ... Through the holes of the SiO ₂ film 4.
Are connected to the N type diffusion layers 3, 3 ,. A BSF layer 6 and a back surface electrode 7 are formed on the back surface. A current extraction part 5-1 is provided at one end of the surface of the cell obtained as described above.

An example of a manufacturing method in which the polycrystalline P-type silicon substrate 1 is used and the laser light is the XeCl ₃ excimer laser will be described below with reference to the drawings.

First, the formation of the V-shaped groove will be described.
2A to 2D show a V-shaped groove 2 formed by laser light 2.
2, ... Indicates the principle of processing. When the surface of the P-type silicon substrate 1 is irradiated with laser light, silicon is melted and evaporated on the irradiation surface. For example, the P-type silicon substrate 1 is adsorbed on a stage whose moving speed is variable and which can raise the substrate temperature, and the energy density is 23.6 J /
cm ² and continuous irradiation with an oscillation frequency of 100 Hz,
It has been found that it penetrates a polycrystalline silicon substrate having a thickness of 400 μm at room temperature in about 30 seconds. Therefore, FIG.
As shown in (a) to (d), a V-shaped processed shape can be obtained by initially widening the irradiation range and decreasing the irradiation shape in accordance with the processing depth. The shaded portion is the melt evaporation layer 1-1. Although the slope of the V-shaped groove 2 is a straight line in FIG. 2D, it is actually composed of a number of steps.

From the results of the above processing experiment, 23.6 J /
It was found that with the energy density of cm ² , the substrate can be processed in the depth direction by 1 μm by irradiation of about 8 times.

FIG. 3 shows an example of processing the V-shaped groove 2 formed according to the present invention. The V-shaped groove 2 has a laser beam irradiation shape of 50 × 50 μm square, and the stage is moved in the X direction in steps of 6 μm for each irradiation so that the irradiation position moves to the end of the P-type silicon substrate 1. Then, the irradiation shape is reduced by the Y-axis slit, and the operation of moving the stage in the opposite X direction is repeated, so that there is a step of about 1 μm in the depth direction and a width of 50 μm.
V-shaped grooves 2, 2, ... Are formed on the entire surface of the substrate by moving the stage sequentially in the Y direction.

Next, the formation of a PN junction at the bottom of the V-shaped groove 2 will be described.

FIG. 4 shows a laser beam spot size of 2 ×.
2 μm, the energy density was changed in a PCl ₃ gas atmosphere, and the surface resistance of the diffusion layer was examined.
About 1000Ω / sq in ^{7J / cm 2, 0.8J / cm} 2
At 100 Ω / sq and 1 J / cm ² more than 20-30
It becomes the value of Ω / sq. Based on this result, the energy density was set to 0.8 J / cm ² , and only the bottom of the V-shaped groove 2 was irradiated with laser light to form the N-type diffusion layers 3, 3 ,. A PN junction was formed only on the bottom of the mold groove 2. It has been confirmed that the V-shaped groove shape hardly changes at such an energy density. Further, since the pitch of the joint portion is 50 μm, which is smaller than the diffusion length of the minority carriers of 150 to 300 μm in a normal crystal substrate, the minority carriers are not recombined and disappeared in the bulk.

Next, the processing on the light receiving surface side and the back surface will be described. As a passivation film, SiH ₄ and N
By heating the substrate to 400 to 500 ° C. in a mixed gas atmosphere of ₂ O, a 150 Å SiO ₂ film 4 was formed. Since the surface passivation effect of forming the SiO ₂ film 4 of about 200 Å on the surface is larger in the region without the doped layer than in the doped layer, recombination of photogenerated carriers occurring on the surface of the photoelectric conversion element is reduced. It is possible to increase the effect.

After that, Al (CH₃)₃In a gas atmosphere
And set the spot size of the laser light to 1 x 1 μm, and
Lugie density of 0.1-0.5 J / cm²As a PN connection
Only the bottom of the V-shaped groove 2 where the gap is formed is irradiated with laser light.
Thin SiO by irradiation ₂At the same time the membrane is removed
, Al (CH₃)₃Photodecomposes and deposits Al about 5 μm
Then, the light-receiving surface electrodes 5, 5, ... Are formed.

Next, as the conductive antireflection film 8, a normal pressure C
A conductive SnO ₂ film was formed by the VD method.

Further, the BSF layer 6 and the back surface electrode 7 are formed on the back surface of the substrate by printing and firing an Al paste to complete the solar cell.

Photoelectric conversion characteristics of the solar cell according to the present invention,
Table 1 below shows a comparison with the characteristics of a solar cell in which a junction was formed with POCl ₃ on a conventional textured surface.

[0024]

[Table 1] It is understood that the solar cell according to the present invention has improved open circuit voltage and short-circuit current as compared with the conventional solar cell, and the recombination of minority carriers is reduced. Further, by using a laser, the spot size can be set to an arbitrary size up to about 1 μm by the optical system, and the processing accuracy equivalent to that of conventional photoetching can be obtained by patterning by direct drawing, which is complicated as before. V-groove processing and diffusion can be performed by a series of processes without performing any process. As the laser light, other ArF or Kr of XeCl ₃ is used.
An excimer laser such as F can also be used. As for the wavelength of the laser light, if ultraviolet light of 350 nm or less is selected, the penetration depth into the silicon substrate is very shallow, and the damage to the substrate due to heat is small, so that the characteristics of the solar cell are deteriorated by the high temperature treatment. Nor.

The present invention can also be applied to a single crystal silicon substrate.

[0026]

According to the present invention, the V-shaped groove and the PN junction at the bottom of the V-shaped groove can be continuously formed on the surface of the silicon substrate by a series of laser processing processes with simple equipment. It is possible to obtain a solar cell that is more efficient and less expensive than a solar cell.

[Brief description of drawings]

1 (a) and 1 (b) show a schematic sectional view and a perspective view of a solar cell according to the present invention, respectively.

2A to 2D show a V-shaped groove processing step.

FIG. 3 is an explanatory diagram showing a relationship between laser light and V-shaped groove processing.

FIG. 4 is a graph showing a relationship between laser energy density and diffusion layer surface resistance.

[Explanation of Codes] 1 P-type silicon substrate 2 V-shaped groove 3 N-type diffusion layer 4 SiO ₂ film 5 Light-receiving surface electrode 6 BSF layer 7 Backside electrode 8 Conductive antireflection film

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koji Okamoto 22-22 Nagaike-cho, Abeno-ku, Osaka-shi, Osaka Within Sharp Corporation (72) Yuji Yokozawa 22-22 Nagaike-cho, Abeno-ku, Osaka, Osaka Within the corporation

Claims (2)

[Claims] 1. A solar cell in which a PN junction is formed only on the bottom of a V-shaped groove formed on the surface of a silicon substrate. 2. A method of manufacturing a solar cell, which comprises continuously forming a V-shaped groove on the surface of a silicon substrate and forming a PN junction at the bottom of the V-shaped groove by laser light.