Process for producing multicrystalline silicon ingots by the induction method and apparatus for carrying out the same

Abstract

This invention is related to obtaining multicrystalline silicon using induction method. The method comprises melting and casting of a pool in the form of a melting space, crystallization of a multicrystalline silicon ingot, and its controlled cooling by means of a heating equipment set. After the pool melting and casting is terminated, crystallization of the remaining part of the multicrystalline silicon ingot is finished along with the controlled cooling of the whole ingot; the ingot is then removed together with a movable bottom and the heating equipment set; and its controlled cooling continues. At the same time, another heating equipment set is supplied to the vacated place with another movable bottom; then the new movable bottom is moved into the water-cooled crucible; and the process steps are repeated in order to produce the next ingot. The method is implemented using an apparatus that additionally includes a platform installed in the controlled cooling compartment and designed to revolve on its axis. This platform holds at least two heating equipment sets. The proposed invention ensures increased output of multicrystalline silicon suitable for production of solar cells.

Description

TECHNICAL FIELD[0001]The invention relates to producing polycrystalline silicon, particularly to producing multicrystalline silicon by the induction method, and can be used in manufacture of solar cells from multicrystalline silicon.[0002]Crystal silicon is used for producing solar cells to converse solar energy into electrical energy. Single-crystal silicon is usually used for this purpose.BACKGROUND ART[0003]Recently completed research has demonstrated that polycrystalline silicon formed by the large crystals, so called multicrystalline silicon, allowed to reach the efficiency of converting solar energy into electric energy close to that of single-crystal silicon. Production capacity of equipment for producing multicrystalline silicon is several times higher than for single-crystal silicon, and its technology is easier than the technology for obtaining single-crystal silicon. The use of multicrystalline silicon enables to reduce the cost of solar panels and to start their producti...

AI Technical Summary

Benefits of technology

[0018]An object of the invention is to improve the process for producing multicrystalline silicon ingots by the induction which would lead to increased output of multicrystalline silicon suitable for producing solar cells due to the proposed relocation of an ingot and heating equipment with a movable bottom during controlled cooling.

[0025]Besides, the proposed staging of the controlled cooling of a multicrystalline silicon ingot which can be implemented without interrupting the overall cooling process allows to be flexible in regulating the length of the produced ingot depending on the amount of impurities in the starting batch.

[0027]A reduced period of induction melting downtime, and lack of dependency of a part of the controlled cooling process on melting and casting processes permit to increase capacity of the system for producing multicrystalline silicon.

Problems solved by technology

A disadvantage of the above process for producing multicrystalline silicon ingots by induction method is the appearance of thermal stress in the ingot resulting in lower quality degradation of plates produced using such ingot.

Thermal stresses in the ingot and in the plates made of such ingot results in decreased efficiency of energy conversion by solar cells formed of these plates.

In addition, the output factor of good plates also decreases due to their rupture caused by thermal stresses.

When concentration of impurities in the pool exceeds its limit established for each specific impurity, multicrystalline silicon becomes unsuitable for producing solar cells.

In addition, each step of inserting the separation device into the furnace melting volume and resuming the process of melting and casting takes about 7.2 hours.

The step of inserting a separation device into a melting volume requires high precision, since even a small misalignment error during the installation of the separation device may result in its jamming and damage of the water-cooled crucible, consequently resulting in the compulsory termination of melting and casting.

Besides, inserting of a separation device made of foreign material—specifically silicon nitride, or graphite—into the melt-pool gives rise to contamination of the lower part of the ingot becomes impure, and as a result to reduce quality and output factor of a good silicon.

The need to resume melting on the top of already produced silicon ingot leads to the need of stopping its movement into the heating equipment and keeping it inside the water-cooled crucible for a long time.

This results in uncontrolled cooling of this part of the ingot, appearance of thermal stresses and microcracks in that area, and consequently, in the need of rejecting the upper part of the ingot.

A drawback of the current system is a low production capacity, especially when a batch with a high content of impurities is used.

A drawback of the prior art apparatus is degradation of ingot quality and decrease of production output when using silicon lump batch with a high content of impurities, for example, metallurgical-grade silicon characterized by an increased content of iron (Fe) and aluminum (Al) admixtures.

However, upon producing the ingots of the identified length time period required to remove the ingot from the water-cooled crucible and the controlled cooling compartment increases relative to the time of induction melting and casting, and the equipment capacity decreases.

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