System and method for electrorefining of silicon

The present disclosure provides methods and systems for electrorefining high-purity materials, for example, silicon. An exemplary system includes at least one cathode, an anode, and a reference electrode. At least one controller, for example a potentiostat, is used to control the potential difference between a reference electrode and a cathode or anode. The system can be operated in a single phase or multiple phase operation to produce high-purity materials, such as solar-grade silicon.

TECHNICAL FIELD

The present disclosure generally relates to systems and methods for refining silicon. More particularly, the present disclosure relates to systems and methods of electrorefining high-purity silicon, for example using a multi-electrode electrorefining apparatus.

BACKGROUND

Conventional refining of high-purity silicon is highly energy-intensive. For example, producing silicon of sufficiently high-purity for use in solar cells, semiconductor devices, and computer chips requires a very high amount of electrical energy. Such high energy demand creates significant costs, and in many instances, makes it prohibitively expensive to refine sufficiently high-purity silicon.

For example, the Siemens method of producing high-purity silicon comprises reducing trichlorosilane to polycrystalline silicon, which occurs by decomposing trichlorosilane on high-purity silicon rods or plates. This process is accomplished at greater than 1000° C., and utilizes water cooling to reduce the temperature of the reactor wall. As such, it requires a very high amount of electrical energy, and consequently, significantly high energy costs.

Because of the high cost associated with producing sufficiently high-quality silicon, there exists a need for systems and methods which use less electrical energy or otherwise offer additional advantages over prior approaches. Accordingly, systems and methods to refine silicon in a more energy efficient way, such as reducing the amount of electrical energy required to refine silicon, are desired.

SUMMARY

The present disclosure provides improved systems and methods for the refining of high-purity materials, such as solar-grade silicon. In an embodiment, a system for electrorefining of silicon comprises an anode, a first cathode, and a reference electrode. Each of the anode, the first cathode, and the reference electrode are coupleable to an electrolyte. The system further comprises a controller configured to control the electrical potential between a reference electrode and at least one of the anode or the first cathode.

In another embodiment, a method for electrorefining of silicon comprises providing a system comprising an anode, a cathode, a reference electrode, an electrolyte, and a controller; applying an electrical potential between the reference electrode and the anode to cause silicon to dissolve from the anode into the electrolyte; and applying an electrical current between the anode and the cathode to cause silicon to deposit from the electrolyte onto the cathode.

In another embodiment, a method for electrorefining of silicon comprises applying a first electrical potential between a silicon-containing anode and a reference electrode to cause silicon to dissolve from the anode into an electrolyte. The electrolyte couples the anode and the reference electrode. The method further comprises applying a first electrical current between the anode and a first cathode to cause silicon to deposit from the electrolyte onto the first cathode; decoupling the anode from the electrolyte; coupling a second cathode to the electrolyte; applying a second electrical potential between the second cathode and the reference electrode to cause silicon to dissolve from the first cathode into the electrolyte; and applying a second electrical current between the first cathode and the second cathode to cause silicon to deposit from the electrolyte onto the second cathode.

DETAILED DESCRIPTION

The detailed description of various embodiments herein makes reference to the accompanying drawing figures, which show various embodiments and implementations thereof by way of illustration and best mode, and not of limitation. While these embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, it should be understood that other embodiments may be realized and that mechanical and other changes may be made without departing from the spirit and scope of the present disclosure.

Also, any reference to attached, fixed, connected or the like may include permanent, removable, temporary, partial, full and/or any other possible attachment option. Additionally, though the various embodiments discussed herein may be carried out in the context of electrorefining, for example electrorefining of silicon, it should be understood that systems and methods disclosed herein may be incorporated into other systems to refine high-purity materials, for example silicon, in accordance with principles of the present disclosure.

The various embodiments of an exemplary system, including at least one anode, at least one cathode, at least one reference electrode, and at least one system control comprise exemplary features hereinafter described. The following description and the annexed drawing figures set forth in detail and demonstrate certain illustrative embodiments of the disclosure. However, these embodiments are indicative of but a few of the various ways in which the principles disclosed herein may be employed. Other objects, advantages and novel features will become apparent from the following detailed description when considered in conjunction with the figures.

Systems and methods in accordance with principles of the present disclosure provide the ability to refine certain high-purity materials, for example solar grade silicon. To assist in understanding the context of the present disclosure, an exemplary high-purity silicon electrorefining system100in accordance with the present disclosure is illustrated inFIGS. 1A and 1B. It will be appreciated that, while various principles of the present disclosure are discussed herein with respect to electrorefining of silicon, principles of the present disclosure may suitably be applied to electrorefining of various other materials, such as, for example, copper or aluminum.

In various embodiments, electrorefining system100comprises a vessel102. Vessel102can be configured to contain a number of electrodes, each of the electrodes in contact with an electrolyte108. Vessel102can comprise, for example, a metallic and/or non-metallic material configured to facilitate an electrorefining operation. In various embodiments, vessel102comprises a glassy carbon crucible; however, any suitable material for vessel102may be utilized.

System100can further comprise an electrolyte108. Electrolyte108can comprise, for example, an electrically-conductive molten salt. In various embodiments, electrolyte108comprises a cation with a more negative standard reduction potential than silicon, for example potassium, magnesium, calcium, sodium, barium, and/or lithium. However, any cation that is suitable for use as a component of an electrolyte in the electrorefining of silicon is within the scope of the present disclosure.

In various exemplary embodiments, electrolyte108can further comprise an anion with an acceptably low cost and capable of forming a stable ionic bond with a cation. For example, chlorine, fluorine, and oxygen are widely used in industrial electrorefining processes and can be provided at a relatively low cost. In certain embodiments, chlorine and fluorine are preferable to oxygen as oxygen may form silicon dioxide on the surface of electrodes, which may lead to reduced performance of the electrode. However, any anion that is suitable for use as a component of an electrolyte in the electrorefining of silicon is within the scope of the present disclosure.

Because higher operating temperatures of an electrorefining system require higher electrical energy, and electrorefining systems operate more effectively at lower operating temperatures, electrolyte108can comprise, in various embodiments, a molten salt with a suitably low melting point. For example, a number of potential molten salt electrolytes108comprise salts which melt at temperatures less than 900° C., such as CaCl2(melting point 817° C.) and LiF (melting point 848° C.). In various embodiments, electrolyte108comprises LiCl, which has a melting point of approximately 610° C., a cation (Li) with a more negative standard reduction potential than silicon, and a relatively low cost, non-oxygen anion (Cl). However, any molten salt with a suitably low melting point that facilitates electrorefining of silicon is within the scope of the present disclosure.

In various embodiments, electrorefining system100further comprises at least one cathode110. Cathode110can comprise, for example, a high-purity silicon sheet or rod. In such configurations, silicon deposits on the surface of cathode110responsive to electrical energy being applied to electrorefining system100. In other embodiments, cathode110can comprise a non-silicon metal, such as tungsten. However, at higher operating temperatures (for example, at or greater than 600° C.), silicon can form silicides with metals such as tungsten. Therefore, in electrorefining systems operating at temperatures greater than 600° C., it is preferable for cathode110to comprise high-purity silicon. In various exemplary embodiments, systems and/or methods in accordance with principles of the present disclosure are operable over temperatures ranging from about 600° C. to about 1500° C.

Electrorefining system100can further comprise an anode120. In various embodiments, anode120can comprise a silicon-containing compound which dissolves as electrical energy is applied to electrorefining system100. In such configurations, electrical energy applied to electrorefining system100causes silicon from the anode to dissolve into electrolyte108. The dissolved silicon is then free to travel throughout the electrolyte.

Anode120can comprise, for example, a rod or sheet of metallurgical-grade silicon. In other embodiments, anode120comprises an alloy, such as silicon and copper. However, any silicon-containing compound which is capable of dissolving when electrical energy is applied is within the scope of the present disclosure.

In various embodiments, electrorefining system100can further comprise a reference electrode106. For example, reference electrode106can comprise a relatively inert and/or stable material with a known standard reduction potential. In certain embodiments, reference electrode106is comprised of at least one of glassy carbon or platinum. Reference electrode106can be configured to measure the potential difference between reference electrode106and either the cathode110or the anode120.

Electrorefining system100can further comprise a control system104. Control system104can be configured, for example, to adjust the electrical potential and/or current provided to electrorefining system100(or a portion thereof) by a power source. In various embodiments, control system104comprises a potentiostat. In such embodiments, control system104can maintain the potential difference between reference electrode106and cathode110or anode120, and can adjust the current provided between cathode110and anode120to maintain a desired electrical potential. It will be appreciated that in various exemplary embodiments, control system104is designed to control the voltage between a reference electrode and an anode (or cathode). In various exemplary embodiments, minimal and/or no electrical current flows through the reference electrode; rather, current flows between anode and cathode. Accordingly, in various exemplary embodiments, as the voltage applied to the anode is controlled, the voltage on the cathode may be adjusted by control system104(e.g., a potentiostat) to allow the amount of current required by the voltage on the anode (and vice versa).

In an exemplary embodiment, electrorefining system100can be utilized in a two-phase process for the electrorefining of high-purity silicon, for example solar-grade (i.e., 99.9999% pure or above) silicon. During operation of electrorefining system100, electrical potential is applied to anode120through control system104. As electrical potential is applied, silicon is dissolved from anode120into electrolyte108, along with one or more impurities. Responsive to electrical current flowing between anode120and cathode110, silicon and other species dissolved from anode120travel through electrolyte108and deposit on cathode110. Electrorefining system100can be configured to maintain a desired rate of dissolution from anode120and rate of deposition on cathode110, for example by varying the potential difference applied between anode120and reference electrode106. Alternatively, electrorefining system100may be configured to vary and/or modify the rate of dissolution from anode120and/or rate of deposition on cathode110.

As illustrated inFIG. 1A, in certain exemplary embodiments, during the first phase of operation, control system104is used to maintain a desired potential difference between anode120and reference electrode106. For example, it may be desirable to maintain the potential difference between anode120and reference electrode106at or below the standard oxidation potential of silicon at the operating temperature of electrorefining system100. By maintaining a relatively low potential difference (for example, a potential difference below the oxidation potential of silicon) between anode120and reference electrode106, a relatively high rate of dissolution of silicon from the anode is achieved, but the concentration of one or more impurities in electrolyte108is increased. After a sufficient amount of silicon has dissolved from anode120and/or a sufficient amount of time has passed, electrorefining system100can be operated in a second phase of operation.

Turning now toFIG. 1B, in various exemplary embodiments, in connection with the second phase of operation of electrorefining system100, anode120is removed and second cathode112is inserted into vessel102. During the second phase of operation, first cathode110is operated as an anode, and second cathode112is operated as a cathode. As electrical energy is applied to electrorefining system100and/or portions thereof, control system104is used to maintain a desired potential difference between second cathode112and reference electrode106. For example, it may be desirable to maintain the potential difference between second cathode112and reference electrode106between a minimum and maximum level. In various embodiments, the minimum potential difference is the standard reduction potential of silicon at the process temperature. In various embodiments, the maximum potential difference is between about 0.0 volts and 1.0 volts more negative than the reduction potential of silicon at the process temperature. By maintaining a relatively low potential difference, the amount of impurities deposited on second cathode112by electrolyte108is maintained at a relatively low level.

In various embodiments, the maximum potential difference between second cathode112and reference electrode106is limited by the target concentration of one or more impurities. As the potential difference increases, one or more impurities in electrolyte108are deposited more rapidly on second cathode112, which can increase the concentration of the one or more impurities. Therefore, it may be desirable to maintain a potential difference between second cathode112and reference electrode106between a minimum and maximum level, as described above. After a sufficient amount of high-purity silicon has deposited on the surface of second cathode112and/or a sufficient amount of time has passed, the second phase of operation of electrorefining system100can be terminated, and second cathode112removed for further processing.

In other exemplary embodiments, and with reference now toFIGS. 2A and 2B, an electrorefining system200can be operated in a two-phase process to electrorefine high-purity material, for example, solar-grade silicon.FIG. 2Aillustrates a first phase of operation of electrorefining system200. During the first phase of operation, the potential difference between a cathode210and a reference electrode206is controlled by control system204. As previously discussed in relation to the second phase of operation of electrorefining system100, the potential difference between cathode210and reference electrode206can be maintained at a desired level to achieve a suitably high rate of deposition of silicon and a suitably low concentration of impurities on the surface of cathode210.

FIG. 2Billustrates a second phase of operation of electrorefining system200in various exemplary embodiments. In connection with the second phase of operation, anode220is removed and second cathode212is inserted into vessel202. During the second phase of operation, first cathode210is operated as an anode and second cathode212is operated as a cathode. The potential difference between first cathode210(operating as an anode) and reference electrode206is controlled by control system204. As previously discussed in relation to the first phase of operation of electrorefining system100, the potential difference between first cathode210(operating as an anode) and reference electrode206can be maintained at a desired level to achieve a suitably high rate of dissolution of silicon and a suitably low rate of dissolution of impurities from first cathode210(operating as an anode).

Turning now toFIGS. 3A through 3C, in various embodiments, an exemplary electrorefining system300can be operated in a two-phase process to electrorefine high-purity material, for example solar-grade silicon. As illustrated inFIG. 3A, during a first phase of operation, electrorefining system300comprises a first cathode310and an anode320. During the first phase of operation, the electrical potential between anode320and a reference electrode306is controlled by a control system304. As discussed in relation to the first phase of operation of exemplary electrorefining system100, the potential difference between anode320and reference electrode306can be maintained at a desired level to achieve a suitably high rate of dissolution of silicon and a suitably low rate of dissolution of impurities from anode320.

In various exemplary embodiments, after a sufficient time of operation of electrorefining system300in the first phase configuration (for example, after a time of operation of between about three hours and about three days), as illustrated inFIG. 3B, anode320is removed and second cathode312is inserted into vessel302. As illustrated inFIG. 3C, during a second phase of operation, first cathode310is operated as an anode, and second cathode312is operated as a cathode. During the second phase, the potential difference between second cathode312and reference electrode306is controlled by control system304. As previously discussed in relation to the second phase of operation of electrorefining system100, the potential difference between second cathode312and reference electrode306can be maintained at a desired level to achieve a suitably high rate of deposition of silicon and a suitably low concentration of impurities on the surface of second cathode312. Such a two-phase operation allows for the use of a single electrorefining system300to subject the silicon of anode320to two stages of dissolution and deposition, which can improve the quality of silicon ultimately deposited on the surface of second cathode312by, for example, minimizing the concentration of one or more impurities.

In other exemplary embodiments, and with reference now toFIGS. 4A through 4C, an exemplary electrorefining system400can be operated in a two-phase process to electrorefine high-purity material, for example solar-grade silicon. As illustrated inFIG. 4A, during a first phase of operation, electrorefining system400comprises a first cathode410and an anode420. During the first phase of operation, the potential difference between first cathode410and a reference electrode406is controlled by a control system404. As discussed in relation to the second phase of operation of exemplary electrorefining system100, the potential difference between first cathode410and reference electrode406can be maintained at a desired level to achieve a suitably high rate of deposition of silicon and a suitably low rate of deposition of impurities on the surface of first cathode410.

In various exemplary embodiments, after a sufficient time of operation of electrorefining system400in the first phase configuration, as illustrated inFIG. 4B, anode420is removed and second cathode412is inserted into vessel402. As illustrated inFIG. 4C, during a second phase of operation, first cathode410is operated as an anode and the potential difference between first cathode410(operating as an anode) and reference electrode406is controlled by control system404. As previously discussed in relation to the first phase of operation of electrorefining system100, the potential difference between first cathode410(operating as an anode) and reference electrode406can be maintained at a desired level to achieve a suitably high rate of dissolution of silicon and a suitably low dissolution rate of impurities from first cathode410(operating as an anode). Such a two-phase operation allows for the use of a single electrorefining system400to subject the silicon of anode420to two stages of dissolution and deposition, which can improve the quality of silicon ultimately deposited on the surface of second cathode412by, for example, minimizing the concentration of one or more impurities.

In various exemplary embodiments and with reference now toFIG. 5, an electrorefining system500comprising a first control system504and a second control system505can be operated in a single-phase process for electrorefining of high-purity silicon. In such configurations, first control system504can comprise a potentiostat configured to maintain a potential difference between a first reference electrode506and a cathode510.

Second control system505can comprise, for example, a potentiostat configured to maintain a potential difference between a second reference electrode507and anode520. In an exemplary embodiment, second control system505maintains the electrical potential between second reference electrode507and the electrode not controlled by first control system504.

In various embodiments, electrorefining system500further comprises a partition530. For example, partition530can comprise a non-conductive material which separates an electrolyte508into a cathode segment534and an anode segment536. In such embodiments, partition530maintains physical isolation between cathode segment534and anode segment536.

Electrorefining system500can further comprise a counter electrode532and a molten alloy509. In such embodiments, a portion of counter electrode532is in contact with molten alloy509. A surface of molten alloy509can be in contact with a surface of electrolyte508of cathode segment534and anode segment536. In such configurations, molten alloy509operates as a cathode for anode segment536and as an anode for cathode segment534.

In various exemplary electrorefining processes, electrorefining system500is operated such that a first electrical current flows between anode520and counter electrode532, and a second electrical current flows between cathode510and counter electrode532. In an exemplary embodiment, control system504is configured to maintain a desired potential difference between cathode510and reference electrode506. Such a desired potential difference can comprise a potential difference which achieves a suitably high rate of deposition of silicon and a suitably low rate of deposition of impurities on the surface of cathode510. In such embodiments, control system505is configured to maintain a desired potential difference between anode520and reference electrode507. Such a desired potential difference can comprise a potential difference which achieves a suitably high rate of dissolution of silicon and a suitably low rate of dissolution of impurities from anode520. Separately controlling these two potential differences can be beneficial by, for example, achieving low impurity concentrations in the production of high-purity silicon.

Thus, the various systems and methods of electrorefining of the present disclosure provide means to produce sufficiently high-purity materials, for example silicon. The improved control of operating conditions, such as operating temperatures and various potential differences, beneficially increases the purity of electrorefined materials, for example solar-grade silicon.

The present disclosure has been described above with reference to a number of exemplary embodiments. It should be appreciated that the particular embodiments shown and described herein are illustrative of inventive principles and its best mode and are not intended to limit in any way the scope of the present disclosure. Those skilled in the art having read this disclosure will recognize that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present disclosure. Various aspects and embodiments of the present disclosure may be applied to fields of use other than electrorefining of silicon. Although certain aspects of the present disclosure are described herein in terms of exemplary embodiments, such aspects may be achieved through any number of suitable means now known or hereafter devised. Accordingly, these and other changes or modifications are intended to be included within the scope of the present disclosure.

While steps outlined herein represent exemplary embodiments of principles of the present disclosure, practitioners will appreciate that there are other steps that may be applied to create similar results. The steps are presented for the sake of explanation only and are not intended to limit the scope of the present disclosure in any way. Benefits, other advantages, and solutions to problems have been described herein with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as critical, required, or essential features or elements of any or all of the claims.

In the detailed description herein, references to “various embodiments”, “one embodiment”, “an embodiment”, “an example embodiment”, “an exemplary embodiment” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to utilize such feature, structure, or characteristic in connection with other embodiments if suitable, whether or not explicitly described. After reading the description, it will be apparent to one skilled in the relevant art(s) how to implement principles of the disclosure in alternative embodiments.

It should be understood that the detailed description and specific examples, indicating exemplary embodiments, are given for purposes of illustration only and not as limitations. Many changes and modifications may be made without departing from the spirit thereof, and principles of the present disclosure include all such modifications. Corresponding structures, materials, acts, and equivalents of all elements are intended to include any structure, material, or acts for performing the functions in combination with other elements. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” Moreover, when a phrase similar to “at least one of A, B, or C” or “at least one of A, B, and C” is used in the claims or the specification, the phrase is intended to mean any of the following: (1) at least one of A; (2) at least one of B; (3) at least one of C; (4) at least one of A and at least one of B; (5) at least one of B and at least one of C; (6) at least one of A and at least one of C; or (7) at least one of A, at least one of B, and at least one of C.