Patent Application
20090280050
Embodiments of the present invention generally relate to methods and associated apparatuses for the preparation of multi-crystalline silicon ingots. More specifically, embodiments of the present invention relate to apparatuses and methods for the directional solidification of multi-crystalline silicon ingots having fewer crystal defects than conventional methods.
Alternative power sources have been studied with greater interest as a result of the sharp rise in oil and gasoline prices. Solar power is one of the promising technologies for generating clean, renewable electricity. Solar cells, also called photovoltaic cells, are devices which convert solar energy into electricity. These cells have evolved significantly over the past two decades, with experimental efficiencies increasing from less than about 5% in 1980 to almost 40% in 2008.
In the early development of solar cells, single-crystal or semi-conductor grade silicon was employed. However, crystalline silicon ingots of this type are expensive due to the cost associated with creating the crystalline structure. One of the traditional methods of creating a single crystal of silicon is by the Czochralski process. In this process, polysilicon is melted in a cylindrical crucible. The melt can be doped to create n-type or p-type silicon. A seed crystal is introduced to the melt causing crystal growth. The crystal is pulled from the melt, creating a cylindrical single crystal ingot. Single-crystal silicon wafers, less than about 300 μm thick, are then cut from this cylindrical ingot.
However, it is now known that single-crystal silicon is not required for producing efficient solar cells. Multi-crystalline, also referred to as multi-crystalline, silicon ingots can be created using the directional solidification process, sometimes called the oriented solidification process. Directional solidification employs a rectangular-shaped crucible, often heated from the sides and bottom. Generally, the crucible is filled with polysilicon and melted in an inert atmosphere. Once melted, the crucible is allowed to cool in a controlled manner from the bottom up. Heat loss during cooling occurs at the sides of the crucible by vertically moving graphite heat shields, to allow radiative heat loss from the crucible and the silicon.
Upon cooling, nucleation occurs, resulting in crystal growth upward from the bottom of the crucible. The ingot produced is rectangular in shape, as opposed to the cylindrically shaped ingot from the Czochralski process. The directional solidification process causes any impurities in the silicon to be pushed to the top of the crucible where they concentrate in the top layer of the ingot. This layer is subsequently cut from the ingot, leaving substantially pure multi-crystalline silicon.
Today, the majority of solar cells are manufactured using crystalline silicon wafers. Over 50% of crystalline silicon solar cells are manufactured using multicrystalline silicon wafers which are manufactured by directional solidification. However, there are several undesirable aspects to the current process: excessive power consumption and the interface is always concave resulting in higher defect density in the resulting ingot. To remove these defects, the sides of the rectangular ingot must be ground off, resulting in loss of about 1 cm of the outer silicon surface. Therefore, there is a continuing need in the art for methods and apparatus to create multi-crystalline silicon ingots with lower defect density.
Aspects of this invention involve the use of horizontally moving heat shields at the bottom of the crucible in directional solidification process and apparatus. This allows for controlled heat loss from the bottom of the crucible, resulting in controlled growth of crystals and a convex interface between the solid and liquid silicon during solidification as compared to a concave interface in the prior art. This design also results in a flatter crystallization interface, lower defect density, less stress and fewer defects in the ingot center as compared to current state of the art. This invention also enables faster crystallization rates while maintaining a desirable interface shape (no bending at edges and an overall convex shape). Furthermore the total heat loss through the movable bottom heat shields will be lower than the heat loss during crystallization as compared to prior art.
One or more embodiments of the invention are directed to an apparatus for producing multi-crystalline silicon ingots by directional solidification. The apparatus comprises a crucible having four sides and a bottom. The top of the crucible can be open or closed depending upon the specific application. The crucible is placed within a crucible holder. A plurality of heaters surrounds at least a portion of the crucible holder. The heaters are capable of causing silicon within the crucible to melt. At least two moveable heat shields below the crucible holder are adapted to move in the same plane as the crucible bottom. The moveable heat shields can be made of any suitable material, such as graphite, graphite felt or other graphite insulation, but is not limited to graphitic materials and may also be made of suitable metals, such as molybdenum, acting as heat reflector.
The heaters of some embodiments are located adjacent the four sides of the crucible holder. In other embodiments a heater is located above the crucible. In still further embodiments a heater is located below the crucible. A cooler located below the crucible may also be present. A water cooled jacket surrounding the apparatus may also be employed.
The moveable heat shields according to one or more embodiments comprises four members adapted to move so that an opening having a similar shape to the crucible bottom is formed. The members of some embodiments can move independently of each other. A specific embodiment has two movable, partially overlapping shields. Another embodiment involves two rotating overlapping shields.
In other embodiments, one or more temperature probes are disposed within the apparatus. A control mechanism for monitoring the temperature probes may be present. The control mechanism may also be able to adjust the location of, and the extent of movement of, individual moveable heat shields to controllably extract heat from the molten silicon in the crucible.
Additional embodiments of the invention are directed to methods of producing multi-crystalline silicon ingots by directional solidification. The methods comprise the transferring of silicon into a crucible located within a furnace. The crucible may have a bottom and four sides. A top for the crucible may also be present. The crucible is held within a crucible holder. The crucible is heated with heating elements located adjacent the crucible holder sides. The crucible is surrounded at least on the bottom with a moveable heat shield. The silicon within the crucible melted and then cooled in a controlled manner to achieve controlled solidification of the silicon by moving the heat shields. A multicrystalline silicon ingot is produced where the grain size in the center of the ingot is substantially uniform with respect to the grain size at the edge of the ingot.
In other embodiments, the heat shield comprises four members adapted to move so that an opening having a similar shape to the crucible bottom can be formed.
Further embodiments of the invention are directed to multi-crystalline silicon ingots. The ingots comprise four sides and a solid-liquid silicon interface during solidification. The solid-liquid interface is controlled by moving the heat shields. The multi-crystalline silicon ingot of some embodiments has an interface which curls downward at the intersection of the solid-liquid interface with the walls of the crucible. The multi-crystalline silicon ingot of other embodiments has an interface which is perpendicular to the ingot side. The multi-crystalline silicon ingot of further embodiments shows uniform grain size from the center of the ingot to the edges of the ingot.
Before describing several exemplary embodiments of the invention, it is to be understood that the invention is not limited to the details of construction or process steps set forth in the following description. The invention is capable of other embodiments and of being practiced or being carried out in various ways.
As used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly indicates otherwise. Thus, for example, reference to “an ingot” includes a combination of two or more ingots, and the like.
Referring to
In one or more embodiments, the heat shields 18 are opened by a control mechanism (not shown). The control mechanism can be a simple control mechanism such as a handle and/or track that allows the heat shields 18 to be opened and closed manually. In certain embodiments, the control mechanism may include an automated mechanism such as a motor or other suitable device to control the extent of opening of the heat shields 18. In a specific embodiment, the control mechanism is in communication with a sensor that can measure the temperature within the chamber 10 using strategically placed temperature probes 28 throughout the chamber 10. Suitable temperature probes may include thermocouples or pyrometers. By evaluating the temperature profile at various locations within the chamber 10, the control mechanism can open any one or all of the heat shields 18. This could be achieved by utilizing a microprocessor or computer using a feedback control system that would adjust the extent of opening of the heat shields based upon the temperature readings from the temperature probes 28. This can allow for a uniform temperature to be maintained across the bottom of the crucible 12, resulting in a uniform ingot.
The shape of the interface 26 is correlated to the quality of the ingot produced.
The interface 26 achieved according to embodiments of the present invention can be seen in
Crucibles for use with embodiments of the invention are generally rectangular in shape, having four sides and a bottom. The crucible can be made of any material suitable to contain liquid silicon without causing contamination (e.g., quartz with a silicon nitride coating). High purity quartz is presently a preferred material for the production of silicon ingots. The crucible holder can be made of any material suitable for holding the crucible (e.g., graphite).
Accordingly, one or more embodiments of the invention are directed to an apparatus for producing multi-crystalline silicon ingots by directional solidification. The apparatus comprises a crucible having four sides and a bottom. The top of the crucible can be open or closed depending upon the specific application. The crucible is placed within a crucible holder. A plurality of heaters surround at least a portion of the crucible holder. The heaters are capable of causing silicon within the crucible to melt. At least two moveable heat shields below the crucible holder are adapted to move in the same plane as the crucible bottom. The moveable heat shields can be made of any suitable material, such as graphite, graphite felt or other graphite insulation, but is not limited to graphitic materials.
The heaters of some embodiments are located adjacent the four sides of the crucible holder. In other embodiments, a heater is located above the crucible. In still further embodiments a heater is located below the crucible. A cooler located below the crucible may be present. A water cooled jacket surrounding the apparatus may also be employed.
The moveable heat shields of one or more embodiments comprise up to four members adapted to move so that an opening having a similar shape to the crucible bottom is formed. The members of some embodiments can move independently of each other. Other embodiments include two linear movable shields which overlap when closed (
In other embodiments, one or more temperature probes are disposed within the apparatus. A control mechanism for monitoring the temperature probes may be present. The control mechanism may also be able to adjust the location of individual moveable heat shields to controllably extract heat from the molten silicon in the crucible.
Additional embodiments of the invention are directed to methods of producing multi-crystalline silicon ingots by directional solidification. The methods comprise the transferring of silicon into a crucible located within a furnace. The crucible may have a bottom and four sides. A top for the crucible may also be present. The crucible is held within a crucible holder. The crucible is heated with heating elements located adjacent the crucible holder sides. The crucible is surrounded at least on the bottom with a moveable heat shield. The silicon within the crucible melted and then cooled in a controlled manner to achieve controlled solidification of the silicon by moving the heat shields. A multicrystalline silicon ingot is produced where the grain size in the center of the ingot is substantially uniform with respect to the grain size at the edge of the ingot.
In other embodiments, the heat shield comprises four members adapted to move so that an opening having a similar shape to the crucible bottom can be formed. It will be understood that the configuration of four shields shown in
Further embodiments of the invention are directed to multi-crystalline silicon ingots. The ingots comprise four sides and a solid-liquid silicon interface during solidification. The solid-liquid interface is controlled by moving the heat shields. The multi-crystalline silicon ingot of some embodiments has an interface which curls downward at the intersection of the solid-liquid interface with the walls of the crucible. The multi-crystalline silicon ingot of other embodiments has an interface which is perpendicular to the ingot side. The multi-crystalline silicon ingot of further embodiments shows uniform grain size from the center of the ingot to the edges of the ingot.
Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.
This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Patent Application No. 61/047,939, filed Apr. 25, 2008, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61047939 | Apr 2008 | US |
