JPH0936403A - Production of basic body and solar cell employing it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a production method for basic body from which a step for slicing an ingot can be eliminated and an inexpensive high quality solar cell having high mass productivity. SOLUTION: The method for producing a basic body supporting a semiconductive layer comprises a first step for injecting a material of Si into the planar groove of a mold 101 extending vertically to the carrying direction, a second step for sustaining the mold at a temperature higher than the melting point of Si to melt 106 the Si material and filling the planar groove with molten Si, a third step for passing the mold through a furnace provided with a negative temperature gradient in the carrying direction of mold, and a fourth step for solidifying 107 the molten Si during passage of the mold through the furnace. A semiconductor layer for fabricating a photoelectric conversion element is formed on the basic body.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a substrate manufacturing method and a solar cell. More specifically, the present invention relates to a method for manufacturing a substrate which is inexpensive and has a surface with few impurities. Also,
By using the substrate according to the above method for producing a substrate,
The present invention relates to a solar cell having improved photoelectric conversion characteristics.

[0002]

2. Description of the Related Art Solar cells have been widely studied as a driving energy source for various devices and a power source for systematic connection with commercial power.

[0003] In such a solar cell, it is desired that a photoelectric conversion element can be formed on a substrate such as a metal that can be obtained at a low price because of cost requirements.

On the other hand, a semiconductor made of Si is generally used as a photoelectric conversion element constituting a solar cell. In terms of efficiency of converting light energy into electric energy,
It is preferable to use single crystal Si as the semiconductor. However, amorphous Si is advantageous in terms of increasing the area and cost of the solar cell. In recent years,
The use of polycrystalline Si is being studied for the purpose of simultaneously obtaining a low cost comparable to that of amorphous Si and a high photoelectric conversion efficiency comparable to that of single crystal Si.

However, in a conventional solar cell made of single crystal Si or polycrystalline Si, a lump crystal is sliced,
Since it is used after being processed into a plate-like body, it is difficult to reduce the thickness to 0.3 mm or less. Therefore, since the thickness is more than necessary and sufficient to absorb the amount of light, the effective use of the material was insufficient. That is, in order to reduce the cost, it was necessary to further reduce the thickness.

[0006] Recently, as a method for solving the above-mentioned thinning, a method of forming a silicon sheet by a spin method of pouring molten Si droplets into a mold has been proposed. However, even with this method, the minimum thickness is 0.1 m.
The thickness is about m to 0.2 mm, and the thinning is still insufficient as compared with the film thickness (20 to 50 μm) necessary for absorbing light as crystalline Si. Further, such thinning makes it difficult for the silicon sheet itself to maintain its strength as a substrate. As a result, there is a need for another inexpensive substrate that necessarily supports the silicon sheet.

As such another inexpensive substrate, for example, metallic grade Si (T. Warabisako, T. Saitoh, E. Kuroda, H.
Itoh. N. Nakamura and T. Tokuyama, “Efficient Solar
Cells from Metallurgical-GradeSilicon ", Proceedin
gs of the 11th Conferenceon Solid State Devices, T
okyo, 1979; Japanese Journal of Applied Physics,
19 (1980) Supplement 19-1, p.539). T.
Warabisako et al. Reported an attempt to form a substrate by using the above-mentioned metal-grade Si, and form a Si layer having a film thickness necessary and sufficient for light absorption on the substrate to obtain a solar cell.

However, in the method of T. Warabisco, et al., A plate-like substrate is prepared by slicing metal-like Si into a lump crystal by a pulling method. Therefore, the process is the same as the conventional process using single crystal Si as a substrate, and there is a problem that the merit of metal-grade Si, which is an inexpensive material, is not utilized. Also, at present,
In order to obtain sufficient characteristics, it is necessary to perform the pulling up twice, and there is also a problem that it takes a lot of time to manufacture.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a substrate having a surface which is inexpensive and has a small amount of impurities.

Another object of the present invention is to provide a solar cell having improved photoelectric conversion characteristics by using the substrate produced by the above method for producing a substrate.

[0011]

A method of manufacturing a substrate according to the present invention is a method of manufacturing a substrate for supporting a semiconductor layer, wherein the mold having a plate-shaped groove in a direction perpendicular to a conveying direction is the plate. A first step of injecting a raw material made of Si into the inside of the groove having a groove shape; and a second step of melting the raw material made of Si by filling the template groove at a temperature higher than the melting point of Si to fill the plate-shaped groove. And a step of moving the mold in a furnace provided with a negative temperature gradient in a direction in which the mold is conveyed.
And a fourth step of solidifying the melted Si while the mold moves in the furnace.

It is preferable that the material of the mold is selected from carbon graphite, silicon carbide, and silicon nitride, and the raw material of Si is metallic grade Si.

Further, it is desirable that the inner surface of the plate-shaped groove is coated with a release agent containing at least Si ₃ N ₄ . The base is suitably used for a photoelectric conversion element.

The solar cell of the present invention is characterized in that the semiconductor layer forming a photoelectric conversion element is formed on the substrate according to any one of claims 1 to 5.

[0015]

In the first aspect of the present invention, the first to fourth steps are provided.
Since the steps are provided, it is possible to obtain a method for manufacturing a base body in which the following points are improved.

First, since the mold having the plate-shaped groove in the direction perpendicular to the carrying direction is provided with the first step of injecting the raw material made of Si into the plate-shaped groove, the plate-shaped groove is formed. The raw material can be uniformly injected into the groove.

Further, since the second step is provided in which the template is kept at a temperature higher than the melting point of Si and the raw material made of Si is melted to fill the plate-like groove, the injected raw material is sufficiently melted. Becomes

Further, the third mold is moved in a furnace provided with a negative temperature gradient in the direction in which the mold is conveyed.
Since a step is provided, when the molten metal-grade Si is solidified, the negative temperature gradient causes the solidification of Si to start from the surface of the mold facing the conveying direction, and the surface opposite to this solidifies last. To do. As a result, the impurities contained in the metallurgical Si are segregated in the vicinity of the surface on the side opposite to the traveling direction in which it is finally solidified. Therefore, the surface of the mold facing the carrying direction is obtained as the surface of the substrate containing relatively few impurities.

Furthermore, since the fourth step of solidifying the melted Si is provided while the mold moves in the furnace, a plate-like substrate made of metal-grade Si can be obtained without post-processing. . Therefore, the step for slicing the substrate shape, which is required in the conventional pulling method, can be omitted, and the time and cost can be improved.

According to the second aspect of the present invention, since the material of the mold is selected from carbon graphite, silicon carbide or silicon nitride, it is possible to form a mold that can be used repeatedly. As a result, cost reduction can be achieved and an inexpensive method for manufacturing a substrate can be obtained.

In the third aspect of the present invention, since the raw material made of Si is metallic grade Si, the material cost can be kept very low when compared with high purity Si or the like. as a result,
It is possible to obtain an inexpensive substrate manufacturing method having a strength equivalent to that of a normal wafer.

According to a fourth aspect of the present invention, since the inner surface of the plate-like groove is coated with a release agent containing at least Si ₃ N ₄ , the plate-like substrate made of metal-grade Si after solidification. Can be easily removed from the mold. As a result, a large number of plate-shaped substrates can be processed at one time.

According to a fifth aspect of the present invention, the substrate is
Since it is for a photoelectric conversion element, the material can be effectively used as compared with the conventional single crystal Si substrate or polycrystalline Si substrate. As a result, it is possible to obtain a method for manufacturing a base body that can supply a solar cell with low manufacturing cost.

According to a sixth aspect of the present invention, since the semiconductor layer forming the photoelectric conversion element is formed on the substrate according to any one of the first to fifth aspects, a semiconductor layer made of Si or the like is grown. The migration of impurities from the base to the semiconductor layer can be reduced. As a result, the gettering process that was conventionally performed as a countermeasure can be omitted.
Cost reduction can be achieved.

[0025]

[Example embodiment]

(Substrate) The metal-grade Si used for the substrate according to the present invention has a low purity, specifically, 1 ppm to 2 of an impurity element.
% Content is cheap and easy to use. It is also possible to reduce the amount of impurities by performing a treatment with an acid such as hydrochloric acid in advance, if necessary, after the powder is formed and before being melted.

(Mold) The mold according to the present invention has a plate-like groove provided in the longitudinal direction, and the number of grooves may be one or plural per one mold. It may have a structure in which a plurality of molds having grooves are connected.
As the material of the mold, carbon graphite is used in terms of easiness of processing and price. However, a material for releasing molten / solidified Si can be applied, and the melting point is S
Any material higher than that of i can be used, and silicon carbide, silicon nitride or the like can be used.

(Release Agent) As the release agent applied in the mold according to the present invention, a release agent having a large contact angle without causing a reaction with molten Si is selected. Specifically, Si ₃ N ₄
Is used as the main component, and if necessary, SiO ₂
Etc. are added. As a method of coating the mold release agent in the mold, an organic solution or silanol solution in which powdery Si ₃ N ₄ is dispersed is sprayed into the mold, and heat treatment is performed at 400 ° C. or higher to form the film.

(Furnace) As the furnace according to the present invention, an electric furnace is preferable from the viewpoint of controllability. Further, in order to melt Si in the same furnace, a portion kept at a temperature equal to or higher than the melting point of Si and a portion provided with a negative temperature gradient from a temperature equal to or higher than the melting point of Si to a temperature equal to or lower than the melting point of Si. It is desirable that the structure has a structure such that the template can move between these sites.

The temperature gradient provided in the furnace and the speed at which the mold moves in the region having the temperature gradient are appropriately determined depending on the solidification conditions of the melted Si. However, from the viewpoint of the crystallinity of the solidified sheet and the segregation effect of impurities, the temperature decreasing rate (= temperature gradient × moving rate) received when the mold moves is set to be about −30 ° C./min or less. Is preferred.

[0030]

EXAMPLES The method for producing a substrate and the solar cell according to the present invention will be described below in more detail with reference to Examples, but the present invention is not limited to these Examples.

Example 1 In this example, a method of manufacturing a sheet substrate by melting / solidifying metal grade Si will be described based on the substrate manufacturing process shown in FIG. Mold 10 of FIG.
For No. 1, a carbon product having plate-shaped grooves in the longitudinal direction was used. Further, for the purpose of easily taking out the solidified Si on the surface of the groove, a Si ₃ N ₄ film was applied to the inner surface thereof.

In the following, the manufacturing method will be described along with steps. (1) The powdery metal grade Si 103 is fed to the feeder 102.
It was charged into the groove in the mold through the through hole, the opening of the groove was closed with a carbon lid 104, and it was placed in an electric furnace as shown in FIG. (2) The powdery metallurgical grade Si was made into the melt 106 by holding for a certain period of time in an electric furnace set at a constant temperature higher than the melting point of Si.

(3) The mold 101 was slowly moved at a constant speed in a portion having a temperature gradient in the horizontal direction provided in the same electric furnace. At this time, as shown in FIG. 1, the temperature gradient was set to be negative with respect to the moving direction of the mold. (4) After the mold passed through the part of the furnace near the melting point, the temperature was dropped to room temperature at a sufficiently distant place. (5) The solidified plate-shaped sheet substrate was taken out from the mold.

The results of the elemental analysis near the surface of the sheet substrate obtained by the above steps (1) to (5) will be described below.

Table 1 shows the results of the impurity analysis. In Table 1, “Metal grade Si” is the result of analysis of the raw material, and “advancing direction side” and “opposite side” indicate the case of the formed sheet substrate. Here, the "traveling direction side" means the surface on the side facing the traveling direction of the mold. On the other hand, the “opposite side” is the opposite side of the “traveling direction side”.

From Table 1, it was found that there was a considerable migration of impurities in the process of melting / solidification. In particular, it was confirmed that the amount of impurities on the “traveling direction side” was much smaller than that on the “opposite side”.

[0037]

[Table 1]

From the results shown in Table 1, when the mold 101 passes through the part of the furnace corresponding to the vicinity of the melting point, the melt 106 starts to solidify from the surface facing the advancing direction, and the opposite surface finally solidifies. Therefore, it was found that the impurities contained in the metal-grade Si segregated near the surface on the side opposite to the traveling direction. See Se
When the crystal grain boundaries were revealed by cco etching,
The crystal grain size of the obtained sheet was expanded to several mm to several cm, which was equivalent to that of the Si ingot obtained by the ordinary casting method.

(Example 2) In this example, a case will be described in which a Si layer is crystal-grown on the sheet substrate obtained in Example 1 by the CVD method. (1) HF / H on the surface of the sheet substrate on the side where the impurities are segregated
Etched back by several tens of μm with a NO ₃ based etchant. (2) SiH ₂ Cl ₂ is used as the source gas and the growth temperature is set to 1
The temperature was set to 050 ° C. and the growth rate was about 0.8 μm / min. (3) After the growth, the surface of the Si layer was observed with an optical microscope and a scanning electron microscope.

As a result, the surface of the obtained Si layer was equivalent to the surface of the sheet substrate, and a relatively flat Si layer was formed. Further, the crystal grain size also inherits the size of the base sheet. Furthermore, the etch pit density on the surface of the obtained Si layer was about 1 × 10 ⁴ pieces / cm ² .

(Example 3) In this example, the case where the thin film solar cell is formed on the Si layer produced in Example 2 as an active layer will be described.

The process of forming a thin film solar cell will be described below. (1) P was implanted into the surface of the Si layer by an ion implantation method under the conditions of 80 keV and 1 × 10 ¹⁵ / cm ² .
After that, anneal (temperature 800 ℃, time 30 minutes),
An n ⁺ layer was formed. (2) On the n ⁺ layer, a collector electrode (Cr (0.02μ
m) / Ag (1 μm) / Cr (0.004 μm)) / transparent electrode (ITO (0.085 μm)) was formed by vacuum evaporation. (3) A1 was vapor-deposited on the back surface of the sheet substrate to form a back surface electrode.

AM1.5 (100 mW / 100 mW / for the thin film crystal solar cell produced by the above steps (1) to (3)
cm ² ) I-V characteristics under light irradiation were measured. As a result, when the cell area was 2 cm ² , the open circuit voltage was 0.57 V, the short circuit photocurrent was 29 mA / cm ² , and the fill factor was 0.74.
A conversion efficiency of 12.2% was obtained.

From the results of this example, it was found that a thin film crystal solar cell having good characteristics can be formed by using a sheet substrate obtained by melting / solidifying metallurgical Si and laminating a Si layer thereon.

(Embodiment 4) This embodiment differs from Embodiment 1 in the following two points in the method of manufacturing a sheet substrate by melting / solidifying metal grade Si. (A) As the powdery metal-grade Si, one obtained by leaching impurities through a powdery metal-grade Si into a hydrochloric acid / peroxide water mixed solution heated at 120 ° C., followed by washing / drying was used.

(B) The powdery metallurgical grade Si produced in (a) above was filled in the grooves in the carbon graphite mold using a feeder. However, on the inner surface of the groove of the mold, a silanol solution in which Si ₃ N ₄ powder was previously dispersed was applied to the inside of the mold and heat-treated at 400 ° C. to form a mold release film. Other points were the same as in Example 1.

In the following, the manufacturing method will be described along with steps. (1) The mold is put into an electric furnace having the structure shown in FIG.
It was kept at a constant temperature (1500 ° C) above the melting point of. (2) When a certain time (30 minutes to 1 hour) has passed,
Negative temperature gradient set at another site in the same furnace (1 ℃ /
mm) at a speed of 10 mm / min. (3) Even after Si was completely solidified, it continued to move for a while, and when the mold came to a temperature region sufficiently lower than the melting point, the heater of the electric furnace was turned off and the temperature of the mold was lowered to room temperature.

The results of elemental analysis of the vicinity of the surface of the sheet substrate obtained by the above steps (1) to (3) will be described below.

Table 2 shows the results of the impurity analysis. In Table 2, "the advancing direction side" means the surface on the side facing the advancing direction of the mold. On the other hand, the “opposite side” is the opposite side of the “traveling direction side”.

[0050]

[Table 2] From Table 2, it was found that the amount of impurities on the surface facing the advancing direction of the mold was much smaller than that on the opposite surface.

When the crystal grain boundaries were revealed by Secco etching, the crystal grain size of the obtained sheet substrate was expanded to several mm to several cm. In the case of the Si ingot obtained by the ordinary casting method, Was equivalent to.

Example 5 This example differs from Example 1 in the following two points in the method of manufacturing a sheet substrate by melting / solidifying metal grade Si. (A) A mold made of SiC as shown in FIG. 1 was prepared, and a silanol solution in which Si ₃ N ₄ powder was dispersed was applied to the inner surface of the groove in the mold, and heat treatment was performed at 600 ° C. for mold release. A film was formed.

(B) Impurities were leached through a powdery metallic grade Si into a hydrochloric acid / peroxide water mixed solution heated at 120 ° C., washed with water / dried, and then filled in a groove in a mold using a feeder. . Other points were the same as in Example 1.

The above manufacturing method will be described below step by step. (1) The mold is put into the electric furnace having the structure shown in FIG.
The temperature was kept constant at 80 ° C. (2) When about 40 minutes have passed, a temperature gradient of −0.5 ° C./mm set at another site in the same furnace was changed to 15
It was moved at a speed of mm / min. (3) Even after Si was completely solidified, it continued to move for a while, and when the mold came to a temperature region sufficiently lower than the melting point, the heater of the electric furnace was turned off and the temperature of the mold was lowered to room temperature.

(4) The solidified plate-shaped sheet was taken out of the mold, and the surface of the sheet substrate on which impurities were segregated was HF /
Etched back by several tens of μm by HNO ₃ type etchant. (5) The etched back surface was roughened by sandblasting, and then the sheet substrate was subjected to gettering treatment at 1100 ° C. for 3 hours.

Table 3 shows the results of elemental analysis on the surface of the sheet substrate obtained by the above steps (1) to (5) facing the advancing direction of the mold.

[0057]

[Table 3] From Table 3, it was found that the amount of impurities on the surface on the side facing the advancing direction of the mold was smaller after the getter treatment than before the treatment.

(Embodiment 6) This embodiment differs from Embodiment 1 in the following two points in the method of manufacturing a sheet substrate by melting / solidifying metal grade Si. Then, a Si layer was deposited on the obtained sheet substrate by a CVD method to form a solar cell.

(B) A mold made of Si ₃ N ₄ as shown in FIG. 1 was prepared and used. (B) Impurities were leached through a powdery metal-grade Si into a hydrochloric acid / peroxide water mixed solution heated to 120 ° C., washed with water / dried, and then filled in a groove in a mold using a feeder. Other points were the same as in Example 1.

The method of manufacturing the above-mentioned sheet substrate and the formation of the solar cell will be described below along with the steps. (1) The mold is put into the electric furnace having the structure shown in FIG.
The temperature was kept constant at 80 ° C. (2) After 1 hour, a temperature gradient of −0.3 ° C./mm set at another site in the same furnace was changed to 30 m.
It was moved at a speed of m / min. (3) Even after Si was completely solidified, it continued to move for a while, and when the mold came to a temperature region sufficiently lower than the melting point, the heater of the electric furnace was turned off and the temperature of the mold was lowered to room temperature.

(4) The solidified plate-shaped sheet was taken out of the mold, and the surface of the sheet substrate on which impurities were segregated was HF.
/ HNO ₃ type etchant was used to etch back several tens of μm. (5) The etched back surface was roughened by sandblasting, and then the sheet substrate was subjected to gettering treatment at 1100 ° C. for 3 hours. (6) SiH ₂ Cl ₂ is used on the surface on the side where the amount of impurities is reduced (the surface facing the advancing direction of the mold), and the growth temperature 1
A Si layer of 40 μm was formed at 050 ° C. and a growth rate of about 0.8 μm / min.

(7) On the surface of the Si layer, P was thermally diffused at a temperature of 900 ° C. using POCl ₃ as a diffusion source to form an n ⁺ layer. The junction depth was about 0.5 μm. (8) The dead layer on the surface of the formed n ⁺ layer was removed by etching. As a result, a junction depth having an appropriate surface concentration of about 0.2 μm was formed. (9) A transparent conductive film (about 0.1 μm) made of ITO was formed on the n ⁺ layer by an electron beam evaporation method.

(10) A collector electrode (C
r (0.02 μm) / Ag (1 μm) / Cr (0.00
4 μm)) was formed by vacuum evaporation. (11) On the back surface of the sheet substrate, Al was deposited to form a back surface electrode.

AM1.5 (100 mW) was applied to the thin film crystal solar cell manufactured by the above steps (1) to (11).
/ Cm ² ) I-V characteristics under light irradiation were measured. As a result, when the cell area is 2 cm ² , the open circuit voltage is 0.56 V, the short-circuit photocurrent is 31 mA / cm ² , and the fill factor is 0.75.
A conversion efficiency of 13.0% was obtained.

From the results of this example, it was found that a thin film crystal solar cell having good characteristics can be formed by using a sheet substrate obtained by melting / solidifying metallurgical Si and laminating a Si layer thereon.

[0066]

As described above, according to the present invention,
A method for manufacturing a substrate can be obtained in which the step of slicing from an ingot can be omitted.

Further, by using the substrate formed by the above-mentioned substrate manufacturing method, it is possible to obtain a solar cell of high mass productivity, low cost, and good quality.

[Brief description of drawings]

FIG. 1 is a schematic view illustrating a manufacturing process of a substrate according to the present invention.

[Explanation of symbols]

 101 mold, 102 feeder, 103 metal grade Si, 104 lid, 105 heater, 106 melt Si, 107 solidified Si, 108 impurity segregation region, 109 furnace tube.

Claims (6)

[Claims] 1. A method of manufacturing a substrate for supporting a semiconductor layer, wherein a mold having a plate-shaped groove in a direction perpendicular to a conveying direction is filled with a raw material made of Si into the plate-shaped groove. A first step of maintaining the mold at a temperature higher than the melting point of Si, melting the raw material of Si to fill the plate-like groove, and a negative direction in which the mold is conveyed. In a furnace provided with a temperature gradient, a third step of moving the mold, and while the mold moves in the furnace,
A fourth step of solidifying the melted Si, the method for producing a substrate. 2. The method of manufacturing a substrate according to claim 1, wherein the material of the mold is selected from carbon graphite, silicon carbide, and silicon nitride. 3. The method for producing a substrate according to claim 1, wherein the raw material made of Si is metallurgical grade Si. 4. The inner surface of the plate-shaped groove has at least S.
The method for producing a substrate according to any one of claims 1 to 3, wherein a release agent containing i ₃ N ₄ is coated. 5. The method for producing a substrate according to claim 1, wherein the substrate is for a photoelectric conversion element. 6. A solar cell, wherein the semiconductor layer forming a photoelectric conversion element is formed on the substrate according to any one of claims 1 to 5.