JPH07183556A - Manufacture of photoelectric conversion element - Google Patents

Abstract

PURPOSE:To reduce surface reflection and to enhance a short-circuit current by a method wherein a plurality of uneven parts are formed on the surface on the light-receiving face of a semiconductor substrate by an ultrasonic machining method. CONSTITUTION:A plurality of grooves 12 are formed on the surface of a P-type polycrystal silicon substrate 11 by an ultrasonic machining machine. Then, an N-type diffused layer 13 is formed on the surface of the P-type polycrystal silicon substrate 11 on which the grooves 12 have been formed, and a titanium oxide layer as an antireflection film 15 is formed on it. Then, an inessential diffused layer on the rear is removed by an etching operation, and a BSF layer 16 and a rear electrode 17 are then formed by printing and firing an Al paste. Then, a silver paste is printed on a light-receiving face by a screen printing method, it is dried and fired, and a light-receiving-face electrode 18 is formed. At this time, the silver paste is printed in a flat part 19 on which the grooves 12 have not been formed. By the above process, a solar cell element is completed.

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 photoelectric conversion element having a surface with irregularities.

[0002]

2. Description of the Related Art A silicon solar cell will be described as an example.

In a silicon solar cell, reduction of surface reflection due to formation of irregularities on the surface is important as one of the elemental techniques for increasing the photoelectric conversion efficiency.

Conventionally, in a solar cell using a single crystal silicon substrate, a substrate having a (100) plane is etched in an alkaline solution to form pyramid-shaped irregularities on the surface. Further, by applying a resist on the surface, patterning the resist in a grid pattern, and then performing etching in an alkaline solution, it is possible to form inverted pyramid-shaped irregularities.

On the other hand, when a polycrystalline silicon substrate effective for cost reduction is used, the plane orientations in the substrate are various, so that uniform etching is performed in etching in an alkaline solution depending on the crystal plane orientation. Cannot be formed, and therefore reflection is not reduced. Therefore, in recent years, in polycrystalline silicon, a method of securing groove-shaped irregularities having a V-shaped or U-shaped cross section on the surface has been developed as a method of reducing surface reflection without depending on the crystal plane orientation. ing. As a method of processing the groove, a chemical processing of applying a resist on the surface, patterning and then etching, or a mechanical processing using a laser beam, a dicing machine or the like has been proposed.

A solar cell is completed by forming a junction by diffusing impurities on a substrate on which irregularities such as pyramids and grooves are formed, and then forming electrodes on the light-receiving surface and the back surface, respectively. As a method of forming the light-receiving surface electrode, there are a vapor deposition method, a screen printing method of a metal paste, and the like, but a screen printing method of a metal paste is generally used in a solar cell that requires cost reduction.

[0007]

As described above, various methods have been developed as a method of processing unevenness that does not depend on the plane orientation. However, it is difficult to mass-produce a low-cost solar cell because the above method has some problems with respect to the effect of reducing reflectance, cost, mass productivity, occupancy of the light-receiving surface electrode, and the like. This will be described in detail below.

First, in the chemical processing by etching, uniform grooves cannot be formed by anisotropic etching using an alkaline solution, and in isotropic etching using an acid solution, etching is performed in the width direction. Also progresses,
Low aspect ratio and little effect of reducing reflectance. Further, there is a problem that a costly process such as a resist coating process and a patterning process is required.

Next, in the method of processing with a laser beam, grooves are formed one by one, which is inferior in mass productivity, and simultaneous irradiation with a large number of laser beams requires a plurality of expensive laser processing machines. However, the problem is that it costs money.

Next, the processing by the dicing machine is relatively inexpensive in terms of cost, but since only one groove, or at most ten or more grooves, is processed on the substrate surface, it is not sufficient in terms of mass productivity. I can't say.

Further, the method of processing with a dicing machine has a problem that the metal paste is likely to spread along the concave portion of the groove and the metal paste is easily broken at the convex portion of the groove. This is because, in the method of processing with the dicing machine, the groove is formed on the entire surface of the substrate and the groove is formed immediately below the electrode. If there is unevenness on the surface directly below the electrode, when forming the electrode by screen printing,
The metal paste spreads along the concave portion of the groove, so that the minimum printable print line width becomes thicker than that when the surface is flat. As a result, the electrode occupancy increases as compared with the case where the surface is flat, and the short circuit current decreases. Further, as the electrode area increases, the contact area of the electrode with the diffusion layer also increases, and recombination in the vicinity of the electrode increases, so that the open circuit voltage decreases.
On the other hand, disconnection of the metal paste is likely to occur in the convex portion of the groove, and in this case, the fill factor is greatly reduced.

As described above, the formation of the groove-shaped irregularities on the surface has a great effect on reducing the surface reflection, and various methods have been developed as the processing method, but in reality,
There are problems such as increased cost and poor mass productivity.
Further, when the recess is formed just below the electrode, there is a problem that the occupation ratio increases due to the spread of the light-receiving surface electrode paste into the recess. Therefore, there is no example in which the processing of the uneven portion on the surface is introduced into the mass production process.

An object of the present invention is to solve the above problems and reduce the surface reflection due to the formation of groove-shaped irregularities, which does not depend on the plane orientation, and has a high productivity for a solar cell having an improved short-circuit current. It is to provide a method of manufacturing by the method.

[0014]

A method of manufacturing a photoelectric conversion element according to the present invention comprises forming a plurality of irregularities on the surface of a semiconductor substrate on the light receiving surface side by ultrasonic processing, and further, in this surface processing, the reflectance is increased. The formation of the concavo-convex portion for reduction is limited to a portion other than the electrode formation portion, and the electrode formation portion is left as a flat surface without forming a groove, or a concave portion having a flat bottom surface is simultaneously formed.

[0015]

In the ultrasonic machining method, the workpiece can be machined into a planar shape at once. Therefore, it is a low cost and highly productive method in that a large number of concavo-convex portions can be processed at the same time. In addition, a groove is formed in the light receiving portion of the light receiving surface side surface, and at the same time, a portion left as a flat surface in the electrode forming portion
Alternatively, a groove for forming a light receiving electrode having a flat bottom surface can be formed. Therefore, problems such as increase in occupancy due to the spread of the light-receiving surface electrode paste in the recesses of the grooves do not occur.

[0016]

EXAMPLES An example of the present invention will be described below.
In addition, a conventional example is also shown for comparison.

FIG. 1 is a perspective view of a polycrystalline silicon solar cell manufactured according to the present invention. A plurality of grooves 12 are formed on the surface of the P-type polycrystalline silicon substrate 11 by an ultrasonic processing machine as shown in FIG. 2 described later.

FIG. 2 is an explanatory view of the configuration of the ultrasonic processing machine. The ultrasonic processing machine is an ultrasonic oscillator 21 and an ultrasonic transducer 2.
2, a cone 23, a horn 24, a tool 25, an abrasive grain supply device 27 for supplying a slurry (a machining liquid containing abrasive grains) 26, a machining table 28, and the like, and a P-type polycrystalline silicon substrate 29 on the machining table 28. Fixed. The principle of ultrasonic processing is to apply the high frequency generated by the ultrasonic oscillator 21 to the ultrasonic oscillator 22 to generate ultrasonic vibration with a frequency of 20 to 40 KHz and an amplitude of several μm. The cone 23 and the horn 24 that resonate at a half wavelength are used to expand the amplitude so that the amplitude becomes 5 to 100 μm at the tip of the tool 25. When a slurry is poured between the P-type polycrystalline silicon substrate 29 and the tip of the tool 25 and a processing pressure is applied to the tool 25 to press it against the P-type polycrystalline silicon substrate 29, the abrasive grains vibrate and the silicon substrate The surface is crushed and the surface is processed into the shape of the tool 25.

FIG. 3 is a schematic perspective view of the machined surface of the tip of the tool 25, and FIGS. 4 to 6 are enlarged views of various shapes of the part marked with a circle in FIG. The tool 25 is, for example, 100 mm
The size of the corner polycrystalline silicon substrate was used.

In FIG. 4, a large number of minute U-shaped projections 41 having a width of 40 μm and a pitch of 100 μm were processed at the tip of the tool 25 so that the groove has a U-shaped cross section. Further, the notch 42 having a width of 150 μm is formed at a pitch of 2.5 mm in accordance with the dimension of the electrode so that the electrode formation portion remains without being processed.
Although not shown in the figure, the main electrode forming portion is 1.
A notch was made in the tool with a width of 5 mm.

As processing conditions, a frequency of 25 to 30K
H _Z , oscillator output 300 to 500 W, amplitude at tool tip 5 to 15 μm, processing pressure 0 to 50 gf, abrasive grain is boron carbide or diamond and grain size is # 500 to #
1000 was used, and pure water was used as the working liquid.

When the processing was carried out under the above conditions, a groove having a desired shape having a depth of 50 to 100 μm could be processed on the substrate in a processing time of 15 seconds to 2 minutes per sheet. Further, the upper surface of the electrode formation portion could be left flat. The size of the groove after processing is 5 to 5 compared to the size of the tool.
It was formed to be large by about 20 μm. This is due to the size of the abrasive grains.

In the conventional example, the groove was formed by a dicing machine. At this time, in order to correspond to the above-mentioned embodiment, the blade thickness of the dicing machine is set to 50 μm.
m, the groove depth was 70 μm, and the groove pitch was 100 μm. Since the pitch of the grooves is 100 μm, 1000 grooves were formed on the substrate having a 100 mm corner. Although the cutting speed was set to a relatively high speed of about 100 mm / s, the processing time per substrate was about 20 minutes because only one blade could be mounted on the dicing machine.

After cleaning the substrate having the U-shaped groove formed by the above steps, the substrate was etched with a mixed solution of hydrofluoric acid and nitric acid in order to remove a damaged layer during processing.

Next, as shown in FIG. 1, an N-type diffusion layer 13 is formed on the surface of the P-type polycrystalline silicon substrate 11 thus formed with a groove by gas diffusion using POCl _3. Then, a thin thermal oxide film was formed as a passivation layer 14 on the surface by a thermal oxidation method. A titanium oxide film was formed thereon as the antireflection film 15 by atmospheric pressure CVD. Then, after removing the unnecessary diffusion layer on the back surface by etching, the BSF layer 16 and the back surface electrode 17 were formed by printing and firing an Al paste. Next, silver paste was printed on the light-receiving surface by a screen printing method, and this was dried and baked to form the light-receiving surface electrode 18. At this time, the silver paste was printed on the flat portion 19 where the groove was not formed. Through the above steps, the solar cell element according to the present invention is completed.

The photoelectric conversion characteristics of the solar cell manufactured by this example are shown in the following Table 1 together with the characteristics of the solar cell manufactured by the conventional example.

[0027]

[Table 1]

The solar cell manufactured according to the present example has improved short-circuit current, open-circuit voltage and fill factor as compared with the solar cell manufactured according to the conventional example, which means that the occupancy rate of electrodes and the contact area are reduced. And that there is no disconnection of the paste.

In FIG. 5, the shape, width and pitch of the U-shaped projection 51 at the tip of the tool are the same as those shown in FIG.
In order to form a groove having a flat bottom surface on the electrode formation portion of the substrate, a convex portion 52 of 150 μm width is formed on the tool at a pitch of 2.5 mm according to the size of the electrode, and a main electrode (not shown) is formed. The formed portion was formed by processing a convex portion with a width of 1.5 mm.

When processing was carried out under the same conditions as in the above-mentioned embodiment, a groove having a desired shape with a depth of 50 to 100 μm was formed on the substrate in a processing time of 15 seconds to 2 minutes per sheet. In addition, a groove having a flat bottom surface could be processed in the electrode formation portion. After that, a solar cell was manufactured by the same steps as those in the above-described example. However, when forming the light-receiving surface side electrode, the silver paste was printed on a portion where a groove having a flat bottom was formed. Also in this embodiment, similar to the above-mentioned embodiments, the characteristics in which the short circuit current, the open circuit voltage, and the fill factor are higher than those of the conventional ones are obtained.

In FIG. 6, a large number of minute V-shaped projections 61 are formed at the tip of the tool at a tip angle of 50 ° and a pitch of 100 μm so that the groove has a V-shaped cross section. According to the dimensions of the electrode, so that the electrode formation part remains unprocessed,
Notches 62 having a width of 150 μm were formed at a pitch of 2.5 mm, and not-shown main electrode forming portions were notched in the tool with a width of 1.5 mm. When the processing was performed under the same conditions as in the above-mentioned embodiment, the processing time per sheet was 30 seconds, and a groove having a desired shape with a depth of 50 to 100 μm could be processed in the substrate. Could be left with a flat top surface. After this, a solar cell was produced by the same steps as those in the above-mentioned examples. In this case as well, similar characteristics to the conventional example were obtained in terms of short circuit current, open circuit voltage, and fill factor.

Although the above embodiments have been described with reference to a polycrystalline silicon substrate, they can be applied to a single crystal silicon substrate or another semiconductor substrate.

[0033]

According to the present invention, surface irregularities for reducing reflection can be formed on a semiconductor substrate at a high speed by using an inexpensive device which does not depend on the plane orientation of the substrate. As a result, it is possible to obtain a solar cell having a higher efficiency than ever before in a mass production process. Here, from the viewpoint of practicality, the surface unevenness has been described by taking a groove having a U-shaped or V-shaped cross section as an example, but a groove having another cross-sectional shape or a pyramid-shaped unevenness is formed by machining a tool. Can be processed at high speed,
It is obvious that it is effective in reducing the surface reflectance.

[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 configuration diagram of an ultrasonic processing machine.

FIG. 3 is a perspective view of a tip of a tool of an ultrasonic processing machine.

FIG. 4 is an enlarged view of a part of a tool processing surface.

FIG. 5 is an enlarged view of a part of a tool processing surface.

FIG. 6 is an enlarged view of a part of a tool processing surface.

[Explanation of symbols]

 11 P-type polycrystalline silicon substrate 12 Groove 13 N-type diffusion layer 14 Passivation layer 15 Antireflection film 16 BSF layer 17 Backside electrode 18 Light-receiving surface electrode 19 Flat part

Claims (3)

[Claims] 1. A method for manufacturing a photoelectric conversion element, which comprises forming a plurality of irregularities on the light-receiving surface side of a semiconductor substrate by ultrasonic processing. 2. The method for manufacturing a photoelectric conversion element according to claim 1, wherein the concave portion is formed in a portion of the light receiving surface side surface which is not the electrode forming portion, and the electrode forming portion is left as a flat surface without forming the concave portion. . 3. The recess according to claim 1, wherein a recess having an uneven bottom surface is formed in a portion of the surface on the light-receiving surface side which is not an electrode forming portion, and the electrode forming portion simultaneously forms a recess having a flat bottom surface. Method for manufacturing photoelectric conversion element.