JPH04342496A - Production of polycrystal silicon cast mass for solar cell - Google Patents

Abstract

PURPOSE:To produce a polycrystal silicon cast mass for solar cell having excellent efficiency of photoelectric conversion. CONSTITUTION:In producing a polycrystal silicon cast mass by one-direciton solidification, temperature gradient when silicon is passed through a temperature area from 1420 deg.C to 1200 deg.C is controlled within the range of 15 to 25 deg.C/cm. Thereby controlled temperature area is narrowed and the control is facilitated. When the control is carried out, a larger effect on improving the efficiency of photoelectric conversion is achieved.

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 polycrystalline silicon ingots used in solar cells.

0002

BACKGROUND OF THE INVENTION High-quality polycrystalline silicon ingots are essential as materials for high-performance solar cells. High-quality polycrystalline silicon ingots have conventionally been produced by cooling molten silicon in one direction and growing crystals in the cooling direction. This unidirectional solidification is usually carried out in a batch process using a mold, but recently a continuous method has been developed in which electromagnetically melted silicon is solidified in a bottomless crucible and then pulled downward. ing.

0003

[Problems to be Solved by the Invention] In this method of producing polycrystalline silicon ingots by unidirectional solidification, the solidification rate and temperature gradient, which are thermal conditions during the solidification process of silicon, affect the quality of the ingot. It is considered as an important factor. These factors are interrelated, and in the past, complicated cooling control was performed without clarifying the degree of influence of these factors on ingot quality. Therefore, sufficient control accuracy could not be obtained, leading to a decline in the quality of the ingot. Furthermore, even if sufficient control accuracy is obtained, excellent ingot quality cannot be expected because the effects of various factors on ingot quality have not yet been clarified.

[0004] An object of the present invention is to provide a method for manufacturing silicon ingots for solar cells that can achieve high quality with simple control.

[0005]

[Means for Solving the Problems] The present inventors have been conducting research to improve the quality of polycrystalline silicon ingots. When we organized the data accumulated so far on the relationship between the thermal conditions of silicon during the solidification process and the photoelectric conversion efficiency of solar cells collected from manufactured silicon ingots, we found the following interesting facts. It was revealed.

[0006] The solidification rate, which was conventionally thought to have a great influence on the quality of the ingot, has almost no relation to the photoelectric conversion efficiency, which determines the performance of solar cells. The factor that affects photoelectric conversion efficiency is the temperature gradient of silicon during the solidification process, especially the temperature gradient in a relatively narrow temperature range from 1420°C to 1200°C, which is the melting point of silicon, and its influence on photoelectric conversion efficiency is On the other hand, temperature gradients in other temperature ranges have almost no relation to photoelectric conversion efficiency. Therefore, by controlling the temperature gradient in the temperature range from 1420° C. to 1200° C., the photoelectric conversion efficiency can be significantly improved with a relatively simple control operation.

The method for producing polycrystalline silicon ingots for solar cells of the present invention was developed based on this new fact.
When manufacturing polycrystalline silicon ingots for use in solar cells by unidirectional solidification, silicon is heated from 1420°C to 1200°C.
The temperature gradient when passing through the temperature range up to 15-25℃
By controlling the temperature within the range of °C/cm, the quality of the polycrystalline silicon ingot as a solar cell can be improved with simple control.

[0008]

[Operation] Many defects that deteriorate the photoelectric conversion efficiency of solar cells occur when silicon passes through a temperature range of 1420°C to 1200°C. The temperature gradient at this time is 25
By limiting the temperature to below .degree. C./cm, 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 is suppressed. This defect suppression effect becomes more pronounced as the temperature gradient becomes smaller. However, 15
A temperature gradient of less than 0.degree. C./cm is not practical because it makes it difficult to maintain and control the temperature of the silicon melt, and it also lengthens production time and increases production costs. 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 quality of the ingot has already been determined, and the temperature gradient in this temperature range has little effect on the quality of the ingot.In actual operation, temperature gradient control in the temperature range above 1200°C is necessary. 20 to 4 from the viewpoint of connection with
0°C/cm is desirable.

[0009]

[Examples] Examples of the present invention will be described below.

The method for manufacturing a polycrystalline silicon ingot for solar cells according to the present invention is carried out using, for example, an electromagnetic melting type continuous casting apparatus shown in FIGS. 1 and 2. This continuous casting apparatus includes a 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. A portion of the bottomless crucible 2 excluding the upper end portion is divided in the circumferential direction, and an induction coil 3 is disposed outside the divided portion. A heat-retaining furnace 4 is connected below the bottomless crucible 2, and an electric heater 5 is installed inside the furnace 4.

[0011] To carry out the manufacturing method of the present invention, first,
After the inside of the airtight container 1 is evacuated through the suction port 1a, an inert gas is injected into the airtight container 1 through the gas port 1b. In parallel with this, the bottom of the bottomless crucible 2 is closed with a seed ingot, and the granular raw material silicon 1 is transferred from the raw material charger 6 into the bottomless crucible 2.
Charge 0. 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 held in a non-contact state with respect to the inner surface of the bottomless crucible 2. Then, while replenishing the raw material silicon 10 into the bottomless crucible 2, the silicon melt 11 inside the bottomless crucible 2 is gradually drawn downward, thereby unidirectionally solidifying polycrystalline silicon in which crystals have grown in the axial direction. Ingots 12 are produced continuously.

[0012] At this time, silicon cooled from 1420°C to 12°C.
The temperature gradient when passing through the temperature range up to 00℃ is 15~
Heat radiation of the unidirectionally solidified ingot 12 is suppressed by the heat retention 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 retention furnace 4, regardless of the casting speed of the unidirectionally solidified ingot 12 (solidification speed of silicon).

In the continuous production of such polycrystalline silicon ingots, as described above, the control of the temperature gradient of the silicon during the solidification process and the controlled temperature range have a great influence on the quality of the ingot. The survey results are shown below. In addition,
The quality of the ingot was evaluated by the photoelectric conversion efficiency of the solar cells collected from the produced 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 1420 to 1200°C. At 1420 to 1200°C, the smaller the temperature gradient, the better the quality of the ingot as a solar cell. FIG. 4 shows the control results in the temperature range of 1200 to 1000°C, and FIG. 5 shows the control results in the temperature range of 1000 to 800°C. In the temperature range lower than 1200°C, no correlation is seen between temperature gradient and slab quality. Furthermore, although FIG. 6 shows the relationship between solidification rate and ingot quality, no particular correlation is observed between the two.

Although the above embodiment uses an electromagnetic melting type continuous casting apparatus, it is also possible to use a batch type casting apparatus using a mold.

[0016]

[Effects of the Invention] As is clear from the above explanation, the method for manufacturing polycrystalline silicon ingots for solar cells of the present invention controls the narrow temperature range of silicon in the solidification process, so it is easy to control. Can be implemented. In addition, the quality improvement effect when implemented is large. Therefore, high quality solar cells can be achieved.

[Brief explanation of drawings]

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

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

FIG. 3 is a chart showing the relationship between temperature gradient and ingot quality in the temperature range of 1420 to 1200°C.

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

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

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

[Explanation of symbols]

2 Bottomless crucible 3 Induction coil 4 Heat retention furnace 10 Raw material silicon 11 Silicon melt 12 One-way solidified ingot

Claims (1)

[Claims] Claim 1: When manufacturing polycrystalline silicon ingots to be used in solar cells by unidirectional solidification, silicon is
A method for manufacturing a polycrystalline silicon ingot for solar cells, characterized by controlling a temperature gradient within a range of 15 to 25°C/cm when passing through a temperature range from 0°C to 1200°C.