WO2004055909A1 - Silicon wafer for solar cell and the same manufacturing method - Google Patents

Silicon wafer for solar cell and the same manufacturing method Download PDF

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Publication number
WO2004055909A1
WO2004055909A1 PCT/KR2003/002725 KR0302725W WO2004055909A1 WO 2004055909 A1 WO2004055909 A1 WO 2004055909A1 KR 0302725 W KR0302725 W KR 0302725W WO 2004055909 A1 WO2004055909 A1 WO 2004055909A1
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Prior art keywords
silicon
silicon wafer
solar cell
binder
sintered
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PCT/KR2003/002725
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French (fr)
Inventor
Hong-Jae Lee
Etsuo Otsuki
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Samwha Electronics Co., Ltd.
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Priority claimed from KR1020020079667A external-priority patent/KR20040051961A/en
Priority claimed from KR1020030090044A external-priority patent/KR20050057839A/en
Priority claimed from KR1020030090043A external-priority patent/KR20050057838A/en
Application filed by Samwha Electronics Co., Ltd. filed Critical Samwha Electronics Co., Ltd.
Priority to AU2003302960A priority Critical patent/AU2003302960A1/en
Publication of WO2004055909A1 publication Critical patent/WO2004055909A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • H01L31/182—Special manufacturing methods for polycrystalline Si, e.g. Si ribbon, poly Si ingots, thin films of polycrystalline Si
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/546—Polycrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a silicon wafer for a solar cell, and more particularly to a method for manufacturing a polycrystalline silicon wafer for a solar cell at a low cost within the range of a wide application of silicon raw material.
  • single crystal silicon wafer has high purity and excellent power generation efficiency
  • polycrystalline silicon wafer will be used more preferably in view of the cost aspect than single crystal silicon wafer.
  • polycrystalline silicon wafer for a solar cell is manufactured by cutting and grinding ingot.
  • the ingot is made by melting residual products of silicon wafer generated in the manufacturing process of semiconductor device.
  • the processes of cutting and grinding ingot cause high cost.
  • raw material of silicon is supplied from residual products of silicon wafer generated in the process of semiconductor device, its production ratio depends on how many semiconductor devices are manufactured. This causes problems in that demand of silicon wafer for a solar cell cannot be met if the production ratio of semiconductor device s small.
  • the present invention is directed to a silicon wafer for a solar cell and a method for manufacturing the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art.
  • An object of the present invention is to provide a method for manufacturing a high density silicon wafer for a solar cell that can solve problems such as high cost caused by a conventional method for manufacturing polycrystalline silicon wafer and instability in supply of polycrystalline silicon wafer.
  • Another object of the present invention is to provide a method for manufacturing a silicon wafer by a powder metallurgical method without melting silicon powders.
  • Other object of the present invention is to provide a method for manufacturing a silicon wafer considering various conditions such as adjusting particle size of silicon raw material powders, adding a proper binder, and sintering silicon powders.
  • a method for manufacturing a silicon wafer for a solar cell includes the steps of grinding silicon raw material powders to make silicon powders having a particle size of 0.5 ⁇ m ⁇ 7.2 ⁇ m, mixing an organic binder such as poly vinyl butyral (PVB) , camphor, poly vinyl alcohol (PVA), polyethylene glycol (PEG), and paraffin with the silicon powders, drying the mixture, pressing the dried mixture at a pressure of 1 ⁇ 3 ton/cm 2 to form a green body, and sintering the green body under the hydrogen gas atmosphere (non-oxidizing atmosphere) at a temperature of 1300°C ⁇ 1400°C for 1 - 10 hours.
  • PVB poly vinyl butyral
  • PVA poly vinyl alcohol
  • PEG polyethylene glycol
  • FIG. 1 illustrates a structure of a solar cell.
  • each compound suitable for silicon wafer for a solar cell can be obtained by the results of measuring the strength of a formed body made based on each compound, impurities contained in a sintered body, and photoelectric conversion efficiency of a solar cell based on the sintered body.
  • the silicon wafer for a solar cell according to the present invention can be used by surface cleaning based on mechanical grinding or chemical treatment without cutting process .
  • the surface of silicon wafer is partially dissolved to obtain polycrystalline and high purity characteristics, thereby improving the photoelectric conversion efficiency.
  • the raw material powder of silicon may be used as silicon powder or residual products generated from the process of semiconductor device. More preferably, the raw material powder of silicon can stably be obtained with high efficiency from byproducts generated by decomposing a silicon compound in the process of refining silicon.
  • the density of the sintered body is obtained by maintaining particle size of powder at 7.2 ⁇ m or below and sintering the powder at a temperature of 1300°C ⁇ 1400°C for 1 - 10 hours under the hydrogen gas atmosphere or the inert gas atmosphere containing hydrogen of 10 ⁇ 100vol%.
  • PVB has a degree of polymerization between 1000 and 2000, and paraffin has a composition of molecular weight to have a melting point of 45°C ⁇ 80°C.
  • a silicon powder having an average particle size of 8 ⁇ m was pulverized by a ball mill using ethanol as a dispersion media.
  • Each sample of formed bodies of 70mm x 70mm x 2mm was manufactured by adding a PVB having a degree of polymerization with 2000 to the silicon powder by 0.2 ⁇ 15wt% using ethanol as a solvent, mixing them with each other, drying them, and pressing them at a pressure lton/cm 2 .
  • each formed body was sintered under the inert gas atmosphere containing hydrogen of 10 ⁇ 100vol% at a temperature of 1360°C for 5 hours.
  • the formed body was shrunk by about 5 ⁇ mm x 56mm x 1.6mm by the sintering process.
  • Samples of the sintered body of the silicon wafer 56mmx 56mm x 1mm were made by grinding the surface of the sintered body
  • the density of the sintered body was measured by maintaining the theoretical density of silicon as 2.3g/cc, dividing the actually measured density of the sintered body by the theoretical density, and converting it to percentage .
  • Flexural strength of the sintered body was measured by cutting the sintered body in size of 30mm x 3mm x 1mm, processing the sintered body, placing the sintered body on a measuring instrument of which span length is set to 20mm, and pressing the center of the sintered body (three-point flexural strength test) .
  • the following table 1 shows PV characteristics such as carbon contents of the sintered body, its forming strength, and photoelectric conversion efficiency.
  • the sample of the green body can be manufactured in case that the content of the binder is 0.4wt%. However, in this case, it is difficult to control the green body.
  • the content of the binder is required by 0.5wt% or greater per powder.
  • the strength of the sintered body was measured at a sufficient level but the content of carbon contained m the sintered body increased with increase of the binder.
  • the binder of 0.5 ⁇ 10wt% is appropriate as a result of the measurement of the sample.
  • a silicon powder having an average particle size of 8 ⁇ m was pulverized by a ball mill using ethanol as a dispersion media.
  • PVB and paraffin were selected as organic binders while bentomte, sodium silicate, and sodium alginic acid were selected as inorganic binders to manufacture corresponding sintered bodies of silicon wafer, thereby measuring characteristics of the sintered body.
  • the respectively selected binders were added to the silicon powders by 3wt% to manufacture the sintered bodies of silicon wafer in the same manner as the first embodiment.
  • Table 2 shows the density and PV characteristics of the sintered bodies.
  • the inorganic binder containing a metallic element made PV characteristics a zero (0) so as not to function as silicon wafer. It is assumed that the metallic element remaining in the sintered body causes PV characteristics to be degraded.
  • organic compounds based on oxygen, hydrogen and carbon are used as binders of silicon wafer in the powder metallurgy method.
  • a silicon powder having an average particle size of 8 ⁇ m was pulverized by a ball mill using ethanol as a dispersion media.
  • Sintered bodies of silicon wafer were manufactured by selecting both organic compounds and reversible organic compounds as samples of binders. The density and handling strength of the sintered bodies were then measured and compared.
  • the organic compounds include PVB, PVA, camphor, and PEG while the thermoplastic organic compounds include polyethylene, polypropylene, and polystyrene .
  • the content of each binder was limited to 2.0wt%, and sintered bodies of silicon wafer were manufactured in the same manner as the first embodiment.
  • PVB, PEG, and camphor were added using alcohol as a solvent
  • PVA was added using aqua as a solvent
  • paraffin, polyethylene, polypropylene, and polystyrene were added by heating and melting processes.
  • the strength of the sintered bodies was determined by the handling strength when moving the green body formed in size of 70mm x 70mm x 2mm to a board for sintering. This is the reason why the green body is easily damaged when handling it in case that the forming strength is weak.
  • thermoplastic organic binders such as polyethylene, polypropylene, and polystyrene have poor handling strength because the strength of the green bodies is weak.
  • the sintered bodies of silicon wafer were manufactured by increasing contents of the thermoplastic organic binders such as polyethylene, polypropylene, and polystyrene to 5wt% so as to improve the handling strength.
  • the characteristics of the sintered bodies were then measured and compared. In this case, the handling strength was improved as shown in Table 4 but PV characteristics were degraded due to increase of carbon content in the green body. As a result, it was found that it is difficult for the sintered bodies to be practically used.
  • thermoplastic organic binders such as polyethylene, polypropylene, and polystyrene are not appropriate for the binders.
  • the handling strength depending upon the polymerization degree of PVB was measured.
  • Sintered bodies of silicon wafer were manufactured in the same manner as the first embodiment, and the handling strength and state of the green bodies were experimented by varying the polymerization degree of PVB used as a binder.
  • the polymerization cgiee is maintained as 1400 to 2000 when PVB is used as a binder .
  • the handling strength of the green body depending upon the hardening strength of PVA was measured.
  • Sintered bodies of silicon wafer were manufactured in the same manner as the first embodiment, and the handling strength and state of the green bodies were experimented by varying the hardening strength of PVA used as a binder.
  • the results of experiment indicate that the handling strength is poor because the green body is weak when the hardening strength of PVA is lower than 90. On the other hand, the green body has sufficient handling strength when the saponification degree is higher than 90.
  • the same processes as those of the second embodiment were performed, and handling strength of the formed body and change of carbon contents in the sintered body were experimented using a molecular weight of paraffin, i.e.', a melting point of paraffin, as a parameter when paraffin is used as a binder.
  • Paraffin is characterized in that a melting point is varied depending upon a molecular weight.
  • paraffin has preferably a molecular weight that can maintain a melting point at 50°C ⁇ 76°C.
  • a silicon powder having a mean particle size of 0.5 ⁇ m, 0.7 ⁇ m, 1.4 ⁇ m, 2. l ⁇ m, 3. O ⁇ m, 4.8 ⁇ m, 5.7 ⁇ m, and 7.2 ⁇ m was made by pulverizing silicon powder of 8 ⁇ m in a ball mill using ethanol as a dispersion media.
  • Each sample of green bodies of 70mm x 70mm x 2mm was manufactured by adding paraffin of 1.5wt% to the silicon powder, mixing them with each other by heating and melting processes, drying them, and pressing them at a pressure lton/cm 2 . Subsequently, each green body was sintered under the inert gas atmosphere containing hydrogen gas of 10 ⁇ 100vol% at a temperature of 1360°C for 5 hours.
  • the green body was shrunk by about 56mm x 56mm x 1.6mm by the sintering process.
  • Samples of the sintered body of the silicon wafer of 56mmx 56mm x 1mm were made by grinding the surface of the sintered body.
  • the density of the sintered body was measured by maintaining theoretical density of silicon as 2.3g/cc, dividing the actually measured density of the sintered body by the theoretical density, and converting it to percentage .
  • the flexural strength of the sintered body was measured by cutting the sintered body by 30mm x 3mm x 1mm, processing the sintered body, placing the sintered body on a measuring instrument of which span length is set to 20mm, and pressing the center of the sintered body (three-point flexural strength test) .
  • Table 8 The results of Table 8 indicate that the density of the sintered body is improved and a fine pore size and uniform strength are obtained when the silicon powder has an average particle size smaller than 5 ⁇ m.
  • the strength of the sintered body is improved when the silicon powder has an average particle size smaller than 2 ⁇ m.
  • Eighth Embodiment A silicon powder having an average particle size of l ⁇ m was made by filling silicon powder of 8 ⁇ m into a ball mill to undergo milling using ethanol as a dispersion media .
  • Each sample of sintered bodies of silicon wafer was manufactured in the same manner as the seventh embodiment by applying the conditions of Table 9 under the non- oxidation atmosphere. Oxygen contents of each sintered body were then measured.
  • the manufactured sintered body was ground at a thickness of 0.5mm and cleaned with oxygen to form n + and p + layers by a solid state reaction process.
  • a solar cell as shown in FIG. 1 was made to measure photoelectric conversion efficiency.
  • a reference numeral 1 denotes n + and p + layers
  • a reference numeral 2 denotes a collecting electrode
  • a reference numeral 3 denotes a transparent electrode film.
  • a sample of sintered bodies of silicon wafer was manufactured in such a manner that the concentration of oxygen under the treatment condition of powder and the processes of drying the green body, mixing it with the binder, and loading the furnace of the green body were strictly controlled to obtain oxygen contents of the sintered bodies as shown in Table 10.
  • the photoelectric conversion efficiency of the sample was then measured. Table 10
  • the results of experiment in the ninth embodiment indicate that the photoelectric conversion efficiency of silicon wafer is improved when the oxygen content of the sintered body is less than 10,000ppm.
  • a silicon powder having an average particle size of 7.2 ⁇ m, 5.7 ⁇ m, 3. O ⁇ m, 2. l ⁇ m, 1.4 ⁇ m, and 0.7 ⁇ m was made by pulverizing silicon powder by Aldrich in a ball mill using ethanol as a dispersion media.
  • the silicon powder was dried and then paraffin of 1.5vol% was added thereto. Subsequently, the powder was pressed at a pressure of l.Oton/cm 2 to form green bodies having the density of 56-58%.
  • the green bodies were sintered under the vacuum of 4 x 10 ⁇ 5 Torr, the high purity Ar gas atmosphere, and the high purity hydrogen gas atmosphere at a temperature of
  • Table 11 Relation among the density of sintered bodies obtained using silicon powder, sintering atmosphere, and mean particle size of powder. As will be apparent from Table 11, hydrogen sintering is indispensable for high density sintering, and the particle size of powder should be smaller than 5 ⁇ m to obtain the relative density of 90% or greater.
  • the green bodies of silicon powders manufactured in the tenth embodiment were sintered under the hydrogen- argon mixture gas atmosphere at a temperature of 1360°C for 5 hours.
  • the density of each sintered body was shown in Table 12.
  • a certain content of hydrogen should be mixed under the sintering atmosphere, Preferably, the mixture content of hydrogen is at least 10vol% or greater.
  • the green bodies of silicon powders manufactured in the tenth embodiment were sintered at various temperatures n the hydrogen gas atmosphere.
  • the density of each sintered body was then measured and shown in Table 13.
  • Table 13 Effect of sintering temperatures to the density of sintered bodies
  • the results of experiment indicate that the mean particle size of powders fine and higher sintering temperature is required to obtain a high density. It is preferable that the sintering temperature is higher than 1200°C and lower than a melting point of silicon .
  • PVB was dissolved in Acetone, and a certain content of silicon powders was mixed with the dissolved solution. A green sheet was then made by a doctor blade method at a thickness of about 500 ⁇ m. Since the content of the binder in the fourteenth embodiment is greater than that of the thirteenth embodiment, it takes time to remove the binder but the density of the sintered body was obtained equally in comparison with the thirteenth embodiment.
  • the surface of the sintered bodies manufactured in the eleventh embodiment was ground, and n + layer 1, a collecting electrode 2, and a transparent electrode film 3 were then formed to constitute a cell as shown in FIG. 1.
  • the photoelectric conversion efficiency of the cell was measured using a crystalline system solar cell output measuring method named JISC8913.
  • Pm has a maximum output of a solar cell in I-V (current x voltage) between short current and open voltage.
  • Table 15 efficiency of solar cell manufactured using sintered bodies having various types of density.
  • silicon powders obtained by thermally decomposing monosilane at a temperature of 760°C silicon powders by grinding silicon layers obtained in the process of semiconductor device and metal-grade silicon powders were used as raw materials, and a cell was constituted in the same manner as the fourteenth embodiment. The results were shown in Table 16.
  • Table 16 the density of sintered body manufactured using various materials of silicon and efficiency of solar cell
  • the method for manufacturing a silicon wafer for a solar cell according to the present invention has the following advantages.
  • the silicon wafer for a solar cell is manufactured by a powder metallurgy method using either one selected from poly vinyl butyral (PVB), camphor, poly vinyl alcohol (PVA), polyethylene glycol (PEG) , and paraffin or its mixture as a binder of a powder silicon material.
  • the sintered body has a sintering density of 90% or greater, contents of oxygen of 10,000 ⁇ pm or below, and an average pore size of 2 ⁇ m.
  • the method for manufacturing a silicon wafer for a solar cell according to the present invention enables stable supply of silicon material by a powder metallurgy method at a low cost within the range of a wide application of silicon raw material.

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Abstract

A method for manufacturing a silicon wafer for solar cell is disclosed in which the silicon wafer is pressed and sintered by a powder metallurgy method. The method enables stable supply of silicon material at a low cost within the range of a wide application of silicon raw material. The method includes the steps of mixing at least one organic material selected from PVB, camphor, PVA, PEG, and paraffin with silicon powder, the organic material being used as a binder, and pressing them to form a green body, and sintering the formed body by heating it under the non-oxidizing atmosphere containing hydrogen gas.

Description

SILICON WAFER FOR SOLAR CELL AND THE SAME MANUFACTURING METHOD
TECHNICAL FTE D
The present invention relates to a silicon wafer for a solar cell, and more particularly to a method for manufacturing a polycrystalline silicon wafer for a solar cell at a low cost within the range of a wide application of silicon raw material.
BACKGROUND ART
Recently, exploitation of various substitute energy sources for thermal-power generation and nuclear power generation has rapidly increased due to the problems such as global warming, environmental disruption, and the exhaustion of petroleum.
The use of solar cells has previously been known to be most suitable for the solutions of the problems. However, the use of solar cells has been limited because solar cells are a relatively expensive method of generating electricity. Especially, the manufacturing cost of silicon wafer is the basic reason why the high cost of power generation module is caused.
Although single crystal silicon wafer has high purity and excellent power generation efficiency, polycrystalline silicon wafer will be used more preferably in view of the cost aspect than single crystal silicon wafer.
It is general tendency that demand of solar-based power generation module using polycrystalline silicon wafer goes on growing. In this case, there exist some problems related to the cost of polycrystalline silicon wafer and stability of its supply. For this reason, the solar- based power generation module fails to substitute for the conventional power generation system.
There is a fixed idea that a high density sintered body cannot be obtained by a powder metallurgy method which is not subject to melting because it is difficult to sinter silicon of covalent bond. Therefore, polycrystalline silicon wafer for a solar cell is manufactured by cutting and grinding ingot. The ingot is made by melting residual products of silicon wafer generated in the manufacturing process of semiconductor device. However, the processes of cutting and grinding ingot cause high cost. In addition, since raw material of silicon is supplied from residual products of silicon wafer generated in the process of semiconductor device, its production ratio depends on how many semiconductor devices are manufactured. This causes problems in that demand of silicon wafer for a solar cell cannot be met if the production ratio of semiconductor device s small.
DISCLOSURE OF THE INVENTION Accordingly, the present invention is directed to a silicon wafer for a solar cell and a method for manufacturing the same that substantially obviate one or more of the problems due to limitations and disadvantages of the related art. An object of the present invention is to provide a method for manufacturing a high density silicon wafer for a solar cell that can solve problems such as high cost caused by a conventional method for manufacturing polycrystalline silicon wafer and instability in supply of polycrystalline silicon wafer.
Another object of the present invention is to provide a method for manufacturing a silicon wafer by a powder metallurgical method without melting silicon powders. Other object of the present invention is to provide a method for manufacturing a silicon wafer considering various conditions such as adjusting particle size of silicon raw material powders, adding a proper binder, and sintering silicon powders.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for manufacturing a silicon wafer for a solar cell includes the steps of grinding silicon raw material powders to make silicon powders having a particle size of 0.5μm ~ 7.2μm, mixing an organic binder such as poly vinyl butyral (PVB) , camphor, poly vinyl alcohol (PVA), polyethylene glycol (PEG), and paraffin with the silicon powders, drying the mixture, pressing the dried mixture at a pressure of 1~3 ton/cm2 to form a green body, and sintering the green body under the hydrogen gas atmosphere (non-oxidizing atmosphere) at a temperature of 1300°C ~ 1400°C for 1 - 10 hours.
In the present invention, degradation of semiconductor characteristics of the silicon wafer can be reduced by appropriately controlling the particle size of the silicon powders and the content of the organic binder such as PVB added to the silicon powders as impurities. Thus, a high-density silicon wafer efficient for a solar cell can be obtained. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawing, which is included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawing: FIG. 1 illustrates a structure of a solar cell.
BEST MODE FOR CARRYING OUT THE INVENTION
Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawing. In the present invention, among the conditions of a high density sintered body in the processes of powder metallurgy including grinding, mixing, forming and sintering, a binder suitable for characteristics of silicon wafer for a solar cell is noted. Upon reviewing factors that affect performance of the solar cell, it was found that an inorganic binder deteriorates semiconductor characteristics by degrading the purity of the sintered body.
On the other hand, it was found that specifying both a compound from an organic binder and its molecular weight could control the process of forming a high density silicon wafer and reduce degradation of semiconductor characteristics of silicon wafer. In the present invention, either one selected from poly vinyl butyral (PVB) , camphor, poly vinyl alcohol (PVA), polyethylene glycol (PEG), and paraffin or its mixture is used as a binder of a powder silicon material. The molecular weight of each compound suitable for silicon wafer for a solar cell can be obtained by the results of measuring the strength of a formed body made based on each compound, impurities contained in a sintered body, and photoelectric conversion efficiency of a solar cell based on the sintered body.
The silicon wafer for a solar cell according to the present invention can be used by surface cleaning based on mechanical grinding or chemical treatment without cutting process . The surface of silicon wafer is partially dissolved to obtain polycrystalline and high purity characteristics, thereby improving the photoelectric conversion efficiency.
The raw material powder of silicon may be used as silicon powder or residual products generated from the process of semiconductor device. More preferably, the raw material powder of silicon can stably be obtained with high efficiency from byproducts generated by decomposing a silicon compound in the process of refining silicon.
The higher the density of the sintered body is, the higher its characteristics and efficiency are. However, it is preferable that the relative density of the sintered body, i.e., ratio of theoretical density of the sintered body, is greater than 90% considering sintering conditions.
The density of the sintered body is obtained by maintaining particle size of powder at 7.2μm or below and sintering the powder at a temperature of 1300°C ~ 1400°C for 1 - 10 hours under the hydrogen gas atmosphere or the inert gas atmosphere containing hydrogen of 10~100vol%. PVB has a degree of polymerization between 1000 and 2000, and paraffin has a composition of molecular weight to have a melting point of 45°C~80°C.
Although the technology of silicon wafer for solar cell is directly introduced to the sintered body in embodiments of the present invention, for improving of efficiency, it may be possible to control grain size and impurities by partially dissolving the sintered body.
A method for manufacturing silicon wafer for a solar cell according to the present invention will be described with .reference to the preferred embodiments.
First Embodiment
A silicon powder having an average particle size of 8μm (measured by laser diffraction scattering type particle size distribution analyzer by Japan) was pulverized by a ball mill using ethanol as a dispersion media.
Each sample of formed bodies of 70mm x 70mm x 2mm was manufactured by adding a PVB having a degree of polymerization with 2000 to the silicon powder by 0.2~15wt% using ethanol as a solvent, mixing them with each other, drying them, and pressing them at a pressure lton/cm2.
Subseguently, each formed body was sintered under the inert gas atmosphere containing hydrogen of 10~100vol% at a temperature of 1360°C for 5 hours.
The formed body was shrunk by about 5βmm x 56mm x 1.6mm by the sintering process. Samples of the sintered body of the silicon wafer 56mmx 56mm x 1mm were made by grinding the surface of the sintered body
The density of the sintered body was measured by maintaining the theoretical density of silicon as 2.3g/cc, dividing the actually measured density of the sintered body by the theoretical density, and converting it to percentage .
Flexural strength of the sintered body was measured by cutting the sintered body in size of 30mm x 3mm x 1mm, processing the sintered body, placing the sintered body on a measuring instrument of which span length is set to 20mm, and pressing the center of the sintered body (three-point flexural strength test) .
The following table 1 shows PV characteristics such as carbon contents of the sintered body, its forming strength, and photoelectric conversion efficiency.
Table 1
Figure imgf000008_0001
In table 1, since the strength of the green body is weak in case that the content of the binder is 0.2wt%, a sample of the green body could not be manufactured.
Further, the sample of the green body can be manufactured in case that the content of the binder is 0.4wt%. However, in this case, it is difficult to control the green body.
In case that the content of the binder was greater than 0.7wt%, there was no problem in controlling the green body.
Therefore, it is noted that the content of the binder is required by 0.5wt% or greater per powder.
The strength of the sintered body was measured at a sufficient level but the content of carbon contained m the sintered body increased with increase of the binder.
It is also noted that the PV characteristics of the sintered body was remarkably deteriorated when the content of carbon was lOOppm or greater.
Consequently, it is noted that the binder of 0.5~10wt% is appropriate as a result of the measurement of the sample.
Second Embodiment
A silicon powder having an average particle size of 8μm was pulverized by a ball mill using ethanol as a dispersion media. PVB and paraffin were selected as organic binders while bentomte, sodium silicate, and sodium alginic acid were selected as inorganic binders to manufacture corresponding sintered bodies of silicon wafer, thereby measuring characteristics of the sintered body.
The respectively selected binders were added to the silicon powders by 3wt% to manufacture the sintered bodies of silicon wafer in the same manner as the first embodiment. Table 2 shows the density and PV characteristics of the sintered bodies.
When the respective binders were added to the silicon powders in the process of sintering the silicon wafer, PVB was added using alcohol as a solvent while bentonite, sodium silicate, and sodium alginic acid were added using aqua as a solvent. Paraffin was added by heating and melting processes. Table 2
Figure imgf000010_0001
As will be apparent from Table 2, the inorganic binder containing a metallic element made PV characteristics a zero (0) so as not to function as silicon wafer. It is assumed that the metallic element remaining in the sintered body causes PV characteristics to be degraded.
A result of experiments according to the second embodiment, it is preferable that organic compounds based on oxygen, hydrogen and carbon are used as binders of silicon wafer in the powder metallurgy method.
Third Embodiment
A silicon powder having an average particle size of 8μm (measured by laser diffraction scattering type particle size distribution analyzer by Japan) was pulverized by a ball mill using ethanol as a dispersion media.
Sintered bodies of silicon wafer were manufactured by selecting both organic compounds and reversible organic compounds as samples of binders. The density and handling strength of the sintered bodies were then measured and compared. In this case, the organic compounds include PVB, PVA, camphor, and PEG while the thermoplastic organic compounds include polyethylene, polypropylene, and polystyrene .
The content of each binder was limited to 2.0wt%, and sintered bodies of silicon wafer were manufactured in the same manner as the first embodiment.
In the process of sintering the silicon wafer, PVB, PEG, and camphor were added using alcohol as a solvent, PVA was added using aqua as a solvent, and paraffin, polyethylene, polypropylene, and polystyrene were added by heating and melting processes.
The sintered bodies manufactured as above were compared with one another so that their handling strength depended upon kinds of the binders. The results of the comparison were shown in Table 3.
Since it was difficult for the strength of the sintered bodies to be measured due to their small size, the strength of the sintered bodies was determined by the handling strength when moving the green body formed in size of 70mm x 70mm x 2mm to a board for sintering. This is the reason why the green body is easily damaged when handling it in case that the forming strength is weak.
Table 3
Figure imgf000011_0001
As will be apparent from Table 3, PVA, PVB, PEG, and paraffin have good handling strength while camphor has average handling strength.
However, thermoplastic organic binders such as polyethylene, polypropylene, and polystyrene have poor handling strength because the strength of the green bodies is weak.
The sintered bodies of silicon wafer were manufactured by increasing contents of the thermoplastic organic binders such as polyethylene, polypropylene, and polystyrene to 5wt% so as to improve the handling strength. The characteristics of the sintered bodies were then measured and compared. In this case, the handling strength was improved as shown in Table 4 but PV characteristics were degraded due to increase of carbon content in the green body. As a result, it was found that it is difficult for the sintered bodies to be practically used.
Table 4
Figure imgf000012_0001
Therefore, it was noted that the thermoplastic organic binders such as polyethylene, polypropylene, and polystyrene are not appropriate for the binders.
Fourth Embodiment
In the fourth embodiment, the handling strength depending upon the polymerization degree of PVB was measured. Sintered bodies of silicon wafer were manufactured in the same manner as the first embodiment, and the handling strength and state of the green bodies were experimented by varying the polymerization degree of PVB used as a binder.
The results of experiment indicate that the handling strength is poor because the green body is weak when the polymerization degree of PVB is low. On the other hand, since the green body is too strong when the polymerization degree is high as much as 2400, crack occurs in the formed body in the sintering process or the green body is likely to be damaged. For this reason, it was found that it is difficult for the sintered bodies to be practically used.
Table 5
Figure imgf000013_0001
Therefore, it is preferable that the polymerization cgiee is maintained as 1400 to 2000 when PVB is used as a binder .
Fifth Embodiment
In the fifth embodiment, the handling strength of the green body depending upon the hardening strength of PVA was measured. Sintered bodies of silicon wafer were manufactured in the same manner as the first embodiment, and the handling strength and state of the green bodies were experimented by varying the hardening strength of PVA used as a binder.
As shown in Table 6, the results of experiment indicate that the handling strength is poor because the green body is weak when the hardening strength of PVA is lower than 90. On the other hand, the green body has sufficient handling strength when the saponification degree is higher than 90.
Table 6
Figure imgf000014_0001
Sixth Embodiment
In the sixth embodiment, the same processes as those of the second embodiment were performed, and handling strength of the formed body and change of carbon contents in the sintered body were experimented using a molecular weight of paraffin, i.e.', a melting point of paraffin, as a parameter when paraffin is used as a binder. Paraffin is characterized in that a melting point is varied depending upon a molecular weight.
As shown in Table 7, the results of experiment indicate that paraffin has preferably a molecular weight that can maintain a melting point at 50°C~76°C.
Figure imgf000015_0001
Seventh Embodiment
A silicon powder having a mean particle size of 0.5μm, 0.7μm, 1.4μm, 2. lμm, 3. Oμm, 4.8μm, 5.7μm, and 7.2μm (measured by laser diffraction scattering type particle size distribution analyzer by Japan) was made by pulverizing silicon powder of 8μm in a ball mill using ethanol as a dispersion media. Each sample of green bodies of 70mm x 70mm x 2mm was manufactured by adding paraffin of 1.5wt% to the silicon powder, mixing them with each other by heating and melting processes, drying them, and pressing them at a pressure lton/cm2. Subsequently, each green body was sintered under the inert gas atmosphere containing hydrogen gas of 10~100vol% at a temperature of 1360°C for 5 hours.
The green body was shrunk by about 56mm x 56mm x 1.6mm by the sintering process. Samples of the sintered body of the silicon wafer of 56mmx 56mm x 1mm were made by grinding the surface of the sintered body.
The density, Mean diameter of pore, flexural strength, and damaged state in grinding of the respectively formed sintered bodies were measured as shown in Table 8.
The density of the sintered body was measured by maintaining theoretical density of silicon as 2.3g/cc, dividing the actually measured density of the sintered body by the theoretical density, and converting it to percentage .
The flexural strength of the sintered body was measured by cutting the sintered body by 30mm x 3mm x 1mm, processing the sintered body, placing the sintered body on a measuring instrument of which span length is set to 20mm, and pressing the center of the sintered body (three-point flexural strength test) .
Table 8
Figure imgf000016_0001
The results of Table 8 indicate that the density of the sintered body is improved and a fine pore size and uniform strength are obtained when the silicon powder has an average particle size smaller than 5μm.
Particularly, it is noted that the strength of the sintered body is improved when the silicon powder has an average particle size smaller than 2μm.
Eighth Embodiment A silicon powder having an average particle size of lμm was made by filling silicon powder of 8μm into a ball mill to undergo milling using ethanol as a dispersion media .
Each sample of sintered bodies of silicon wafer was manufactured in the same manner as the seventh embodiment by applying the conditions of Table 9 under the non- oxidation atmosphere. Oxygen contents of each sintered body were then measured.
The manufactured sintered body was ground at a thickness of 0.5mm and cleaned with oxygen to form n+ and p+ layers by a solid state reaction process. A solar cell as shown in FIG. 1 was made to measure photoelectric conversion efficiency. In FIG. 1, a reference numeral 1 denotes n+ and p+ layers, a reference numeral 2 denotes a collecting electrode, and a reference numeral 3 denotes a transparent electrode film.
Table 9
Figure imgf000017_0001
Figure imgf000018_0001
The results of experiment as above indicate that powder of silicon wafer can preferably be treated under the non-oxidation ambient condition.
Ninth Embodiment
A sample of sintered bodies of silicon wafer was manufactured in such a manner that the concentration of oxygen under the treatment condition of powder and the processes of drying the green body, mixing it with the binder, and loading the furnace of the green body were strictly controlled to obtain oxygen contents of the sintered bodies as shown in Table 10. The photoelectric conversion efficiency of the sample was then measured. Table 10
Figure imgf000018_0002
The results of experiment in the ninth embodiment indicate that the photoelectric conversion efficiency of silicon wafer is improved when the oxygen content of the sintered body is less than 10,000ppm.
Tenth Embodiment
A silicon powder having an average particle size of 7.2μm, 5.7μm, 3. Oμm, 2. lμm, 1.4μm, and 0.7μm was made by pulverizing silicon powder by Aldrich in a ball mill using ethanol as a dispersion media.
The silicon powder was dried and then paraffin of 1.5vol% was added thereto. Subsequently, the powder was pressed at a pressure of l.Oton/cm2 to form green bodies having the density of 56-58%.
The green bodies were sintered under the vacuum of 4 x 10~5Torr, the high purity Ar gas atmosphere, and the high purity hydrogen gas atmosphere at a temperature of
1360°C for 5 hours. The relative density of each sintered body was shown in Table 11.
Table 11 Relation among the density of sintered bodies obtained using silicon powder, sintering atmosphere, and mean particle size of powder.
Figure imgf000019_0001
As will be apparent from Table 11, hydrogen sintering is indispensable for high density sintering, and the particle size of powder should be smaller than 5μm to obtain the relative density of 90% or greater.
Eleventh Embodiment
The green bodies of silicon powders manufactured in the tenth embodiment were sintered under the hydrogen- argon mixture gas atmosphere at a temperature of 1360°C for 5 hours. The density of each sintered body was shown in Table 12.
Table 12 Effect of hydrogen content in the sintering atmosphere to the density of sintered bodies.
Figure imgf000020_0001
As will be apparent from Table 12, a certain content of hydrogen should be mixed under the sintering atmosphere, Preferably, the mixture content of hydrogen is at least 10vol% or greater.
Twelfth Embodiment
The green bodies of silicon powders manufactured in the tenth embodiment were sintered at various temperatures n the hydrogen gas atmosphere. The density of each sintered body was then measured and shown in Table 13. Table 13 Effect of sintering temperatures to the density of sintered bodies
As shown in Table 13, the results of experiment indicate that the mean particle size of powders fine and higher sintering temperature is required to obtain a high density. It is preferable that the sintering temperature is higher than 1200°C and lower than a melting point of silicon .
Thirteenth Embodiment PVB was used as a binder, and various contents were mixed with silicon powders.
If the content of the binder is less than 0.3%, breakage occurs when forming a green body, thereby failing to obtain yield of practical products. Also, even though PVB was replaced with PVA or paraffin, the same results were obtained.
The density of each sintered body sintered at a temperature of 1360°C for 5 hours was shown in Table 14.
Table 14 Effect of binder that affects forming silicon powders and the density of the sintered body
Figure imgf000021_0002
Figure imgf000022_0001
Fourteenth Embodiment
PVB was dissolved in Acetone, and a certain content of silicon powders was mixed with the dissolved solution. A green sheet was then made by a doctor blade method at a thickness of about 500μm. Since the content of the binder in the fourteenth embodiment is greater than that of the thirteenth embodiment, it takes time to remove the binder but the density of the sintered body was obtained equally in comparison with the thirteenth embodiment.
Fifteenth Embodiment
The surface of the sintered bodies manufactured in the eleventh embodiment was ground, and n+ layer 1, a collecting electrode 2, and a transparent electrode film 3 were then formed to constitute a cell as shown in FIG. 1.
The photoelectric conversion efficiency of the cell was measured using a crystalline system solar cell output measuring method named JISC8913. In this case, Pm has a maximum output of a solar cell in I-V (current x voltage) between short current and open voltage.
I-V characteristic curve was measured using a four- terminal method. η (%)=Pm/ (A x Er) x 100 The density of each sintered body and the photoelectric conversion efficiency was shown in Table 15.
Table 15; efficiency of solar cell manufactured using sintered bodies having various types of density.
Figure imgf000023_0001
Sixteenth Embodiment
Instead of the aforementioned sample, silicon powders obtained by thermally decomposing monosilane at a temperature of 760°C, silicon powders by grinding silicon layers obtained in the process of semiconductor device and metal-grade silicon powders were used as raw materials, and a cell was constituted in the same manner as the fourteenth embodiment. The results were shown in Table 16.
Table 16; the density of sintered body manufactured using various materials of silicon and efficiency of solar cell
Figure imgf000023_0002
As shown in Table 16, characteristics of the sintered body were improved in case that silicon compound decomposed powder was used. On the other hand, in case that layer silicon was used, the characteristics were degraded. However, since the process of manufacturing a sintered body and the process of constituting a cell are not optimized in the sixteenth embodiment, it is too early to conclude from the results of Table 16 that layer silicon or metal-grade silicon cannot be used.
INDUSTRIAL APPLICABILITY
As aforementioned, the method for manufacturing a silicon wafer for a solar cell according to the present invention has the following advantages.
In the present invention, the silicon wafer for a solar cell is manufactured by a powder metallurgy method using either one selected from poly vinyl butyral (PVB), camphor, poly vinyl alcohol (PVA), polyethylene glycol (PEG) , and paraffin or its mixture as a binder of a powder silicon material. Also, the sintered body has a sintering density of 90% or greater, contents of oxygen of 10,000ρpm or below, and an average pore size of 2μm.
The method for manufacturing a silicon wafer for a solar cell according to the present invention enables stable supply of silicon material by a powder metallurgy method at a low cost within the range of a wide application of silicon raw material.
While the present invention has been described and illustrated herein with reference to the preferred embodiments thereof, it will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention that come within the scope of the appended claims and their equivalents. "

Claims

What is claimed is
1. In a silicon wafer for a solar cell manufactured by adding a binder to silicon powder having a mean particle size of 0.5μtm - 7.2μιm, mixing them with each other, and undergoing sintering by a powder metallurgy method, wherein the sintered body has a sintered density of 90% or greater and contents of oxygen of 10,000ppm or below.
2. The silicon wafer for a solar cell according to claim 1, wherein the sintered body has a mean pore size of 2μιm or below.
3. The silicon wafer for a solar cell according to claim 1 or 2, wherein the binder is of any one selected from poly vinyl butyral (PVB), camphor, poly vinyl alcohol (PVA), polyethylene glycol (PEG), and paraffin.
4. The silicon wafer for a solar cell according to claim 1, wherein the silicon powder is made by grinding any one selected from silicon scrap, metal silicon, and silicon powder obtained by decomposing a chemical compound.
5. A method for manufacturing a silicon wafer for a solar cell comprising the steps of: mixing at least one organic material selected from PVB, camphor, PVA, PEG, and paraffin with silicon powder, the organic material being used as a binder, and pressing the mixture to form a formed body; and sintering the green body by heating it under the hydrogen gas atmosphere for predetermined hours.
6. The method according to claim 5, wherein the mixture is pressed at a pressure of 1-3 ton/cm2, and the green body is sintered at a temperature of 1300°C - 1400°C for 1 - 10 hours under the inert gas atmosphere containing hydrogen of 10~100vol%.
7. The method according to claim 5, wherein the binder is mixed with the silicon powder by 0.2~15wt%.
8. The method according to claim 5, wherein the PVB has polymerization degree between 1000 and 2000.
9. The method according to claim 5, wherein the PVA has saponification degree of 90 or greater.
10. The method according to claim 5, wherein the paraffin has a composition of molecular weight to have a melting point of 45°C-80°C.
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