JP3005633B2 - Method for producing polycrystalline silicon ingot for solar cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a polycrystalline silicon ingot used for a solar cell.

[0002]

2. Description of the Related Art As a material for a high-performance solar cell, a high-quality polycrystalline silicon ingot is indispensable. High quality polycrystalline silicon ingots have been conventionally produced by cooling molten silicon in one direction and growing crystals in the cooling direction. This unidirectional solidification is usually performed in a batch system using a mold.Recently, however, a continuous method of solidifying electromagnetically melted silicon in a bottomless crucible and pulling it downward has been developed. ing.

[0003]

In such a method for producing a polycrystalline silicon ingot by unidirectional solidification, the solidification rate and temperature gradient, which are thermal conditions in the process of solidifying silicon, determine the quality of the ingot. Considered as an important factor. These factors are related to each other, and in the past, complicated cooling control was performed while the influence of these factors on ingot quality was unclear. For this reason, sufficient control accuracy has not been obtained and the quality of the ingot has been reduced. Even if sufficient control accuracy is obtained, excellent ingot quality cannot be expected under the current situation where the influence of various factors on ingot quality has not been clarified.

An object of the present invention is to provide a method for producing a silicon ingot for a solar cell, which can realize high quality with simple control.

[0005]

Means for Solving the Problems The present inventors have proposed a polycrystalline silicon.
Unidirectional solidified ingot of Recon, especially cylindrical water-cooled bottomless crucible
Research on high quality polycrystalline silicon ingots produced by electromagnetic melting type continuous casting using uranium has been continued. The data collected so far are summarized in terms of the relationship between the thermal conditions of silicon in the process of solidification and the photoelectric conversion efficiency of solar cells collected from the manufactured silicon ingot. It was revealed.

[0006] The solidification rate, which has hitherto been considered to greatly affect the quality of the ingot, is almost independent of the photoelectric conversion efficiency that determines the performance of the solar cell. The factor that affects the photoelectric conversion efficiency is the temperature gradient of silicon in the solidification process, particularly the temperature gradient in a relatively narrow temperature range from the melting point of silicon, 1420 ° C. to 1200 ° C. The effect on the photoelectric conversion efficiency is Large, while the temperature gradient in other temperature ranges is almost independent of the photoelectric conversion efficiency. Therefore, if the temperature gradient in the temperature range from 1420 ° C. to 1200 ° C. is controlled, the photoelectric conversion efficiency is greatly improved by a relatively simple control operation.

The method for producing a polycrystalline silicon ingot for a solar cell according to the present invention has been developed based on such a new fact.
Electromagnetic melting continuous casting of unidirectionally solidified polycrystalline silicon ingots for solar cells using a cylindrical water-cooled bottomless crucible
When manufacturing the concrete, the silicon 1420 ° C. 12
The temperature gradient when passing through the temperature range up to
By controlling the temperature within the range of 25 ° C./cm, the quality of a polycrystalline silicon ingot as a solar cell is improved with a simple control.

[0008]

Many defects that deteriorate the photoelectric conversion efficiency of a solar cell occur when silicon passes through a temperature range from 1420 ° C. to 1200 ° C. The temperature gradient at this time is 25
By limiting the temperature to ° C./cm or less, the thermal stress generated inside the crystal is relaxed, and the generation of various defects that deteriorate the photoelectric conversion efficiency of the solar cell can be suppressed. The effect of suppressing defects is more remarkable as the temperature gradient is smaller. However, 15
A temperature gradient of less than ° C./cm is not realistic because it makes it difficult to maintain and control the temperature of the silicon melt and increases the manufacturing time and the manufacturing cost. Therefore, 1
The temperature gradient in the temperature range from 420 ° C. to 1200 ° C. was 15 to 25 ° C./cm. In the temperature range below 1200 ° C., the ingot quality has already been determined, and the temperature gradient in this temperature range hardly affects the ingot quality. 20-4 from the viewpoint of connection with
0 ° C./cm is desirable.

[0009]

Embodiments of the present invention will be described below.

The method for producing a polycrystalline silicon ingot for a solar cell according to the present invention is carried out using, for example, an electromagnetic melting type continuous casting apparatus shown in FIGS. This continuous casting apparatus includes a cylindrical bottomless crucible 2 housed in an airtight container 1. The bottomless crucible 2 is made of a conductive metal such as copper, and is forcibly cooled by cooling water flowing inside. The portion of the bottomless crucible 2 excluding the upper end is divided in the circumferential direction, and the induction coil 3 is arranged outside the divided portion. A heat insulating furnace 4 is provided below the bottomless crucible 2, and an electric heater 5 is provided therein.

In order to carry out the production method of the present invention, first,
After evacuating the airtight container 1 from the suction port 1a, an inert gas is injected into the airtight container 1 from the gas port 1b. At the same time, the bottom of the bottomless crucible 2 is closed with a seed ingot, and the raw material silicon
0 is charged. Next, in order to melt the raw material silicon 10 in the bottomless crucible 2, an alternating current of a predetermined frequency is passed through the induction coil 3. The silicon melt 11 in the bottomless crucible 2 is kept in a non-contact state with the inner surface of the bottomless crucible 2.
Then, the silicon melt 11 in the bottomless crucible 2 is gradually pulled downward while the silicon raw material 10 is supplied into the bottomless crucible 2 so that the unidirectional solidification of the polycrystalline silicon in which the crystal grows in the axial direction. The ingot 12 is manufactured continuously.

At this time, silicon is heated from 1420 ° C. to 12
The temperature gradient when passing through the temperature range up to 00 ° C is 15 ~
The heat radiation of the unidirectionally solidified ingot 12 is suppressed by the heat retaining furnace 4 so that the temperature becomes 25 ° C./cm. This control is performed by adjusting the output of the electric heater 5 in the heat retaining furnace 4 irrespective of the casting speed of the unidirectionally solidified ingot 12 (solidification speed of silicon).

In the continuous production of such a polycrystalline silicon ingot, as described above, the control of the temperature gradient of silicon in the process of solidification and the control temperature range greatly affect the quality of the ingot. The results of the survey are shown below. In addition,
The ingot quality was evaluated based on the photoelectric conversion efficiency of a solar cell collected from the manufactured ingot.

FIG. 3 shows the relationship between the temperature gradient and the quality of the ingot when the temperature gradient is controlled in the temperature range of 1240 to 1200 ° C. At 1420 to 1200 ° C., the smaller the temperature gradient, the better the ingot quality as a solar cell.
FIG. 4 shows a control result in a temperature range of 1200 to 1000 ° C., and FIG. 5 shows a control result in a temperature range of 1000 to 800 ° C., respectively. In a temperature range lower than 1200 ° C., there is no correlation between the temperature gradient and the slab quality. FIG. 6 shows the relationship between the solidification rate and the quality of the ingot, but there is no particular correlation between the two.

[0015]

[0016]

As is apparent from the above description, the method for producing a polycrystalline silicon ingot for a solar cell according to the present invention controls a narrow temperature range of silicon in a solidification process, so that control is easy and simple. Can be implemented. In addition, the quality improvement effect when implemented is large. Therefore, high quality of the solar cell can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic view of a casting apparatus suitable for carrying out the method of the present invention.

FIG. 2 is a perspective view showing a main part of the casting apparatus in a cutaway manner.

FIG. 3 is a table showing a relationship between a temperature gradient in a temperature range of 1240 to 1200 ° C. and ingot quality.

FIG. 4 is a chart showing a relationship between a temperature gradient in a temperature range of 1200 to 1000 ° C. and ingot quality.

FIG. 5 is a table showing a relationship between a temperature gradient in a temperature range of 1000 to 800 ° C. and ingot quality.

FIG. 6 is a table showing a relationship between a solidification rate and ingot quality.

[Explanation of symbols]

 2 Crucible without bottom 3 Induction coil 4 Insulation furnace 10 Raw material silicon 11 Silicon melt 12 Unidirectional solidified ingot

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-62-56395 (JP, A) JP-A-56-5399 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C30B 1/00-35/00

Claims (1)

(57) [Claims] 1. An electromagnetic casting method using a unidirectionally solidified polycrystalline silicon ingot provided to a solar cell using a cylindrical water-cooled bottomless crucible.
When manufacturing by melt-type continuous casting , 14
A method for producing a polycrystalline silicon ingot for a solar cell, wherein a temperature gradient when passing through a temperature range from 20 ° C to 1200 ° C is controlled within a range of 15 to 25 ° C / cm.