Methods and Apparatuses for Manufacturing Cast Silicon From Seed Crystals

Abstract

Methods and apparatuses are provided for casting silicon for photovoltaic cells and other applications. With these methods, an ingot can be grown that is low in carbon and whose crystal growth is controlled to increase the cross-sectional area of seeded material during casting.

Description

[0001]This application claims the benefit of U.S. Provisional Application No. 60 / 951,155, filed Jul. 20, 2007. The entire disclosure of U.S. Provisional Application No. 60 / 951,155 is hereby incorporated by reference into this specification.[0002]This application was made with U.S. Government support under Subcontract No.: ZAZ-6-33628-11 under prime contract with the National Renewable Energy Laboratory awarded by the Department of Energy. The Government has certain rights in this invention.DESCRIPTION[0003]1. Technical Field[0004]The present invention generally relates to the field of photovoltaics and to methods and apparatuses for manufacturing cast silicon for photovoltaic applications. The invention further relates to new forms of cast silicon that can be used to manufacture devices, such as photovoltaic cells and other semiconductor devices. The new silicon can have a monocrystalline, near-monocrystalline, bi-crystal, or geometric multicrystalline structure and can be manufactu...

AI Technical Summary

Benefits of technology

[0013]During the melting steps, heat can be conducted through the seed crystal to ensure melting of the feedstock while maintaining at least a portion of the seed crystal as a solid to initiate crystal growth orientation during the solidification steps. During the solidification steps, it is desirable to reduce and / or prevent heat loss through walls of a crucible, such as to minimize an amount of multicrystalline material produced. Since the bottom of the crucible is in thermal communication with a heat sink and the material for the crucible typically includes a higher thermal conductivity than molten silicon, the sides of the crucible cool as well. Surprisingly and unexpectedly, a configuration of a thermally conducting material and a thermally insulating material placed under the crucible allows control of heat flow to improve solidification by increasing an amount of monocrystalline silicon produced from the seed crystals.

[0019]In accordance with the present invention, there is also provided a crucible for the casting of silicon having a bottom surface and a plurality of side walls, wherein at least one of the plurality of side walls tapers inwards toward a center of the crucible at an angle from about 1° to about 25° with respect to a plane perpendicular to a bottom surface of the crucible and viewed in a direction extending upwards from the bottom surface. The tapered side wall or walls may reduce the vessel cross-sectional area taken in the direction away from the bottom surface.

[0020]In accordance with the present invention, there is also provided a crucible for the casting of silicon having a bottom surface and a plurality of side walls, wherein at least one of the plurality of side walls tapers outwards from a center of the crucible at an angle greater than about 2°, with respect to a plane perpendicular to a bottom surface of the crucible and viewed in a direction extending upwards from the bottom surface. The tapered side wall or walls may increase the vessel cross-sectional area taken in the direction away from the bottom surface.

[0024]In accordance with the present invention, there is also provided a method of manufacturing cast silicon, comprising: placing at least one monocrystalline seed crystal having at least about 10 cm by about 10 cm area on a bottom surface of a crucible that rests on a partially insulating base plate; introducing solid or liquid silicon feedstock and partially melting the seed crystal, extracting heat through the seed crystal in such a way that a convex solid boundary increases the cross-sectional area of monocrystalline growth; bringing the silicon to a first temperature and cooling it, preferably uniformly, down to a second temperature; cutting a slab from a side of the cast silicon opposite the seed crystal; cleaning the slab using a chemical process; and using the large slab as a new seed layer for a subsequent casting process.

Problems solved by technology

Imperfections can include individual impurities, agglomerates of impurities, intrinsic lattice defects and structural defects in the silicon lattice, such as dislocations and stacking faults.

Many of these imperfections can cause a fast recombination of electrical charge carriers in a functioning photovoltaic cell made from crystalline silicon.

This can cause a decrease in the efficiency of the cell.

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