JP2004140087A - Polycrystalline silicon substrate for solar cell and method for manufacturing the same, and method for manufacturing solar cell using the substrate - Google Patents

Polycrystalline silicon substrate for solar cell and method for manufacturing the same, and method for manufacturing solar cell using the substrate Download PDF

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Publication number
JP2004140087A
JP2004140087A JP2002301918A JP2002301918A JP2004140087A JP 2004140087 A JP2004140087 A JP 2004140087A JP 2002301918 A JP2002301918 A JP 2002301918A JP 2002301918 A JP2002301918 A JP 2002301918A JP 2004140087 A JP2004140087 A JP 2004140087A
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Prior art keywords
base
surface
polycrystalline silicon
solar cell
layer
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JP2002301918A
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Japanese (ja)
Inventor
Shunichi Ishihara
Masaaki Iwane
Yukiko Iwasaki
Masaki Mizutani
Katsumi Nakagawa
Akiyuki Nishida
Hiroshi Sato
Noritaka Ukiyo
Toshihito Yoshino
中川 克己
佐藤 宏
吉野 豪人
岩崎 由希子
岩根 正晃
水谷 匡希
浮世 典孝
石原 俊一
西田 彰志
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Canon Inc
キヤノン株式会社
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    • YGENERAL 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
    • Y02TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00Energy generation through renewable energy sources
    • Y02E10/50Photovoltaic [PV] energy
    • Y02E10/54Material technologies
    • Y02E10/546Polycrystalline silicon PV cells

Abstract

An object of the present invention is to use a low-purity Si as a main raw material and achieve a conversion efficiency equivalent to that of a conventional polycrystalline Si substrate when a solar cell is fabricated, while being able to reduce the cost compared to a conventional polycrystalline Si substrate. A substrate for a solar cell, and a method for manufacturing a solar cell with high conversion efficiency using the substrate.
A polycrystalline Si substrate for a solar cell formed by growing a high-purity polycrystalline Si layer on a surface of a base sliced from a polycrystalline Si ingot obtained by melting and solidifying low-purity Si by a liquid phase growth method, The polycrystalline Si layer (102) substantially covers the front surface and the end face of the base (101), and an opening (104) is formed on the back surface at least in a part of which the base is exposed. The same conductivity type as the base, a specific resistance of 0.1 to 10 Ω · cm, and at least a part of the surface of the polycrystalline Si layer is a facet surface (103) formed as a result of the growth.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a low-cost polycrystalline silicon substrate whose production is not easily restricted by silicon resources, and a method of manufacturing a solar cell using this substrate with high efficiency.
[0002]
[Prior art]
With increasing environmental awareness, solar cells have become widely used. For the production of general-purpose solar cells, a monocrystalline or polycrystalline silicon substrate is mainly used. Single crystal silicon for solar cells is basically pulled up by the same Czochralski method used in the production of silicon for semiconductors. Polycrystalline silicon, on the other hand, can be produced by melting and solidifying silicon in a crucible, and has a higher production throughput than single crystals. They are often used, have a limited supply, and cannot be so inexpensive.
[0003]
Therefore, it has been attempted to purify polycrystalline silicon produced using inexpensive unpurified silicon (metal-grade silicon) that has only been directly reduced from silica stone without using a silicon purification process for semiconductors such as the Siemens method. . For example, K. Hanazawa, M .; Abe, H .; Baba, N .; Nakamura, N .; Yuge, Y .; Sakaguchi, Y .; Kato, S.M. Hiwasa, M .; Obashi proposes a technique of removing a large amount of phosphorus and boron contained in metal-grade silicon using an EB gun or a plasma torch to obtain a silicon raw material for a solar cell (12). th PVSEC June 11-15 2001 processing p265-268, Non-Patent Document 1). However, even with this method, boron is particularly difficult to remove and requires two-step treatment, so that it is expected that there is a limit to cost reduction.
[0004]
Attempts have also been made to directly grow polycrystalline silicon on a base made of a material other than silicon. However, the growth must usually be performed at a high temperature of 1000-1500 ° C. From the viewpoint of expansion coefficient matching, it is difficult to use metal or glass as a base, and therefore, attempts have been made to use glassy carbon or ceramics. However, polycrystalline silicon films grown on this type of base tend to have small crystal grains and poor surface flatness, and have not been put to practical use. Moreover, glassy carbon and ceramics are far from inexpensive materials for use in solar cells.
[0005]
Therefore, a method has been proposed in which a base is made of inexpensive metal-grade silicon, and a solar cell is formed using a substrate on which a high-purity silicon layer having a predetermined thickness is grown. For example, Haruo ITO, Tadashi SAITOH, Noboru NAKAMURA, Sunao Matsubara, Terunori WARABI SISAKO, Takashi TOKUYAMA are SiH on a metal-grade silicon base. 2 Cl 2 A silicon solar cell is grown by a CVD method using the method described above to produce a solar cell as a prototype (J. Crys. Growth 45 (1978) 446-453, Non-Patent Document 2). Noguchi, Sano and Iwata also developed semiconductor-grade high-purity polycrystalline silicon on the base of metal-grade silicon described in claim 1-3 of Japanese Patent Application Laid-Open No. Hei 5-033661, and We are proposing to use batteries.
[0006]
According to these methods, the base is made of silicon while having a low purity, and there is no problem of inconsistency in heat resistance and coefficient of thermal expansion. In addition, since the grown polycrystalline silicon film inherits the crystallinity of the base, a higher quality polycrystal based on glassy carbon or ceramics can be grown.
[0007]
However, a method of growing silicon from a vapor phase such as CVD has a problem in that the number of sheets that can be fed per batch is limited, and that the film is peeled off from the inner wall of the apparatus during growth. Further, when growing on a base made of low-purity silicon such as metal-grade silicon, impurities such as metals and boron and phosphorus contained in the base are once released into the gas phase and then re-entered into the high-purity silicon layer. Even if the purity of the raw material silicon gas to be used is increased, the grown silicon layer tends to be contaminated with a metal or to have a low resistance unsuitable for the manufacture of a solar cell.
[0008]
T. H. Wang, T .; F. Cisek, C.I. R. Schwartfeger, H .; M. Moutinho, R .; Matson has proposed a method using a liquid phase growth method for growing a high-purity silicon layer on metal-grade silicon (Solar Cell Materials and Solcr Cells 41/42 (1996) 19-30, Non-Patent Document 3). Nishida has also proposed in Japanese Patent Application Laid-Open No. 10-098205 (Patent Document 2) that a high-purity silicon layer is grown on a base made of metal-grade silicon by a liquid phase method and used for a solar cell. Various new methods are also disclosed for the method of forming the solar cell, and this is an effective means for reducing the cost of the solar cell.
[0009]
In the liquid phase growth method, a thick silicon layer can be easily grown, the ratio of waste silicon material used is small, and if the supersaturation degree of the melt is controlled, the influence of base impurities on the high-purity silicon layer is not significant. Since it is less than when growing from a phase, a high-quality polycrystalline silicon layer is relatively easy to obtain, and the compatibility with a metal-grade silicon-based substrate is high. However, regardless of the type of the method of growing the high-purity silicon layer, the manufacturing process of the solar cell tends to be gradually contaminated by impurities contained in the base, and the characteristics of the manufactured solar cell tend to deteriorate. Therefore, it has been difficult to use a substrate made of low-purity silicon such as metal-grade silicon for production even though it is inexpensive.
[0010]
Also, growing a thick silicon layer requires a lot of time and silicon raw material, which increases costs. Although the liquid phase growth method provides a higher growth rate than other methods, it is still desirable to use a thin polycrystalline silicon layer in order to increase production throughput. In order for a thin polycrystalline silicon layer to absorb incident light sufficiently, it is desirable to form a fine uneven structure called texture on the surface of the crystal. A part of the grown silicon layer is lost, which is not desirable from the viewpoint of light absorption.
[0011]
As described above, many problems still remain in the production of polycrystalline silicon substrates for solar cells based on low-purity silicon such as metal-grade silicon, and in the manufacture of solar cells using such substrates. Was.
[0012]
[Patent Document 1]
Japanese Patent Application Laid-Open No. H05-036611 (Claim 1-3)
[Patent Document 2]
JP-A-10-098205
[Non-patent document 1]
K. Hanazawa, M .; Abe, H .; Baba, N .; Nakamura, N .; Yuge, Y .; Sakaguchi, Y .; Kato, S.M. Hiwasa, M .; Obashi "12 th PVSEC June 11-15 2001 processing ", p. 265-268
[Non-patent document 2]
Haruo ITO, Tadashi SAITOH, Noboru NAKAMURA, Sunao Matsubara, Terunori WARABISAKO, Takashi Tokuyama, J. Crys. Growth, 1978, 45, p. 446-453
[Non-Patent Document 3]
T. H. Wang, T .; F. Cisek, C.I. R. Schwartfeger, H .; M. Moutinho, R .; Matson, Solar Cell Materials and Solcr Cells, 1996, 41/42, p. 19-30
[0013]
[Problems to be solved by the invention]
The present invention uses low-purity silicon as a main raw material and achieves a conversion efficiency equivalent to that of a conventional polycrystalline silicon substrate when a solar cell is built, while achieving a much lower cost than a conventional polycrystalline silicon substrate. It is an object of the present invention to provide a solar cell substrate capable of performing the following. Another object of the present invention is to provide a method for manufacturing a solar cell having high conversion efficiency using this substrate.
[0014]
[Means for Solving the Problems]
The present invention has been made in view of such a situation, and a high-purity polycrystalline silicon layer is formed on a base formed by slicing an ingot made using low-purity silicon represented by metal-grade silicon. The present invention relates to a novel grown polycrystalline silicon substrate for a solar cell and a method for manufacturing a high-efficiency solar cell using the substrate.
[0015]
[Solution 1]
On a polycrystalline silicon substrate for a solar cell formed by growing a high-purity polycrystalline silicon layer on a surface of a base sliced from a polycrystalline silicon ingot obtained by melting and solidifying low-purity silicon by a liquid phase growth method,
The polycrystalline silicon layer substantially covers the front surface and the end surface of the base, and at least a part of the back surface has an opening in which the base is exposed, and the polycrystalline silicon layer has the same conductivity type as the base and has a specific resistance. A polycrystalline silicon substrate for a solar cell, characterized in that the polycrystalline silicon substrate has a surface area of 0.1 Ω · cm or more and 10 Ω · cm or less and at least a part of the surface of the polycrystalline silicon layer is a facet surface formed as a result of growth.
[0016]
[Solution 2]
The low-purity silicon is melted and solidified to form a polycrystalline silicon ingot, and a flat base is cut out of the ingot. At least the surface and the end face of the base have the same conductivity type as the base and a specific resistance of 0.1 Ω · cm. A method for producing a polycrystalline silicon substrate for a solar cell, wherein a polycrystalline silicon layer having a facet face of at least 10 Ω · cm or less and at least a part of the surface is liquid-phase grown.
[0017]
[Solution 3]
An emitter layer of a conductivity type different from the polycrystalline silicon layer is formed on a surface of the polycrystalline silicon substrate for a solar cell according to the first aspect, an antireflection layer is formed on a surface of the emitter layer, and a back surface is formed on a back surface of the base. A method for manufacturing a solar cell, comprising: forming an electrode pattern; forming a grid electrode pattern on the surface of an antireflection layer; and firing the back electrode pattern and the grid electrode pattern.
[0018]
[Solution 4]
4. A method for manufacturing a solar cell according to claim 3, wherein in the step of forming the emitter layer, the back surfaces of the substrates are bonded to each other for processing.
[0019]
[Solution 5]
In the method of manufacturing a solar cell according to the third or fourth aspect, after forming the emitter layer, the polycrystalline silicon layer is removed to a depth that does not reach the base in the peripheral portion of the substrate, and the emitter layer is isolated. Solar cell manufacturing method.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
(Silicone as raw material)
The cheapest and most abundant silicon raw material is metal-grade silicon obtained by directly reducing silica. It is not produced in Japan but imported from Norway, Brazil, China and other countries. In general, the purity is designated as 98 to 99.5%, but the type and concentration of impurities actually contained vary depending on the raw material silica. A typical example is shown in Table 1.
[0021]
[Table 1]
[0022]
The main impurities include heavy metals such as Fe, Cr, and Cu. These impurities form a deep level in silicon and become recombination centers, so that the solar cell characteristics are significantly impaired. In addition, since heavy metals are easily diffused, if a heavy metal is contained in the base material at a high concentration, contamination tends to spread over a wide range in a process of growing a high-purity silicon layer and a process of manufacturing a solar cell. Further, the metal impurities aggregate to form fine particles, which may cause shunting of the solar cell.
[0023]
Further, impurities such as boron and phosphorus which serve as dopants are also contained at a high concentration. Generally, as shown in Table 1, when an ingot is formed with a relatively high boron concentration, an ingot often shows a p-type (specific resistance of about 0.1 Ω · cm), but may become an n-type depending on a used raw material.
[0024]
In addition, even if it is originally a semiconductor-grade or solar-cell-grade silicon raw material, if the concentration of a dopant such as boron or phosphorus is high and the specific resistance is out of the standard (generally, 0.1 Ω · cm or less as described later), the solar cell However, the obtained solar cell has low efficiency and is not practical. Since such a raw material can be obtained at a considerably lower price than ordinary high-purity silicon, it can be effectively used as a "low-purity silicon" as a raw material of the present invention.
[0025]
(Explanation of manufacturing process of polycrystalline silicon substrate for solar cell)
(Ingot formation and slicing)
The base of polycrystalline silicon is formed by slicing a polycrystalline silicon ingot obtained by melting and solidifying raw silicon filled in a crucible into a predetermined thickness with a wire saw. FIG. 4 shows an example of an ingot solidifying apparatus suitable for carrying out the present invention. The solidification of the raw silicon melted in the crucible 201 progresses gradually from the bottom surface of the crucible to the surface (along the direction 207) while the interface between the solidified portion 204 and the molten portion 206 remains flat. It is desirable. To this end, cooling may be performed entirely while maintaining the temperature of the heater 202 provided at the lower part of the crucible 201 slightly higher than the temperature of the heater 203 provided at the upper part of the crucible 202. Then, the grain boundaries 205 grow from the bottom surface of the crucible 201 toward the surface. Such a solidification method is called unidirectional solidification. At this time, the concentration of heavy metal impurities can be reduced to some extent by the segregation effect, but the concentration of boron and phosphorus cannot be reduced because the segregation effect is extremely weak.
[0026]
The above method of Haru ITO and the like is intended to remove boron and phosphorus which cannot be removed by directional solidification as easily as possible, but it requires two steps more steps than directional solidification, and considerably. Cost increase factor.
[0027]
In the case of the present invention, heavy metals are removed to the extent possible by unidirectional solidification, but no further purification is performed. Therefore, since phosphorus and boron cannot be removed, the specific resistance is often too low, and the formed polycrystalline silicon is not practical as a solar cell as it is.
[0028]
The formed ingot is sliced into a flat plate having a thickness of 200 to 350 μm using an inner peripheral blade type cutting machine or a wire saw. For use in solar cells, the use of highly productive wire saws is preferred. Since the wire saw crease remains on the sliced base surface and dirt adheres, etching is performed after cleaning. The surface of a solar cell substrate is often roughened with an alkaline etchant to form a textured structure, but in the case of a base, the surface shape of the silicon layer grown on it is different from the surface of the original base In many cases, the shape differs from the shape, which is meaningless and causes abnormal growth. Rather, the surface of the base is preferably smoothed after the solvent cleaning by, for example, planar etching with a mixed solution of nitric acid, acetic acid, and hydrofluoric acid for several minutes.
[0029]
(Liquid phase growth)
In the liquid phase growth of silicon, a metal having a low melting point such as tin, indium, gallium, aluminum, or copper is dissolved, and silicon is dissolved therein to form a melt. Among them, indium has a moderately low melting point, is easy to handle, and is suitable for growing high-quality silicon which is hardly dissolved in silicon. Copper has high solubility of silicon and is suitable for growing silicon at high speed.
[0030]
5 and 6 are sectional views of a liquid phase growth apparatus suitable for carrying out the present invention. First, the crucible is heated by a cylindrical heater 304 surrounding the crucible 301 and melted at a temperature of about 600 ° C. to about 1200 ° C. depending on the type of melt until silicon is saturated to form a melt 302. As a silicon raw material to be dissolved, metal-grade silicon having many impurities is inappropriate, but semiconductor-grade (purity of about 10N to 11N) silicon is not required, and solar cell-grade (purity of about 6N to 7N) silicon may be used. Subsequently, the base 305 of polycrystalline silicon is immersed in the melt. Although the number of bases is three in FIGS. 5 and 6, it is possible to grow tens or hundreds of bases according to the size of the crucible. Before starting the liquid phase growth, usually, the temperature of the melt 302 is once increased from the saturation temperature of silicon to be unsaturated, and then the base 305 is immersed, and a part of the base is dissolved in the melt to adapt the surface. It is not preferable to use a graded silicon base because impurities in the base dissolve into the melt. If the base surface is appropriately etched and a flow of a reducing gas such as hydrogen is formed inside the container accommodating the base and the crucible, the temperature of the melt is about several degrees to several tens degrees Celsius higher than the saturation temperature of silicon. Even if the base is immersed after being lowered, the surface of the base is adapted to the melt, and there is no fear that impurities are dissolved in the melt.
[0031]
After dipping the base 305 in the melt 302, the melt is cooled. When the melt is cooled, the insoluble silicon precipitates on the base 305. Since the base is polycrystalline silicon, the deposited silicon layer becomes polycrystalline following the base. Cooling is often performed gradually at a constant rate. Such a method is called a slow cooling method. In the liquid phase growth method, in addition to this, the solute solid such as silicon and the base are both immersed in the melt, and the solute is maintained at a relatively high temperature and the base is maintained at a relatively low temperature. There is a technique called a temperature difference method in which a solute is eluted / diffused and a solute is grown on a base. The temperature difference method can keep the temperature of each part constant throughout. Therefore, the temperature difference method is preferably used in the growth of a compound semiconductor in which uniformity in the thickness direction of a grown film is particularly required, but is also suitable for the growth of silicon. Applied to The conductivity and specific resistance of the polycrystalline silicon layer are affected by the melt. Indium, gallium, aluminum and the like are themselves p-type dopants. When such a metal is used for a melt, the dopant often forms a solid solution with silicon to become p-type. Among them, indium has a low solid solution in silicon and its conductivity is easily controlled. Although tin is slightly dissolved in silicon, it is electrically inactive and easy to control the conductivity because of the group IV element. When these melts are used, p-type and n-type can be freely controlled by dissolving dopants such as boron, aluminum, gallium, phosphorus and antimony in the melt together with silicon and performing liquid phase growth.
[0032]
When used as an active layer of a solar cell, the specific resistance of the polycrystalline silicon layer is preferably about 0.1 to 10 Ω · cm. If the specific resistance is higher than this, n + / P junction (or p + / N junction) is not sufficiently formed, and the open-circuit voltage is particularly reduced. Conversely, if the specific resistance is lower than this, the depletion layer does not sufficiently expand, and furthermore, the recombination of carriers increases, and in particular, the short-circuit photocurrent decreases. In addition, the base and the polycrystalline silicon layer need to be of the same conductivity type so as not to form a reverse junction with the junction formed by the emitter layer. In addition, bases formed from metal-grade silicon tend to have low resistance, but low-resistance bases increase the long-wavelength sensitivity of solar cells due to the back surface field effect, and make it easier to make electrical contact with the back electrode. There are benefits.
[0033]
When the active layer of the solar cell is used, the thickness of the polycrystalline silicon layer is preferably at least about 100 μm because the absorption of incident light increases as the thickness is increased. This increases the cost. Therefore, as generally employed in crystalline silicon solar cells, a method of forming a texture structure on the surface by etching with an alkaline solution or the like, extending the optical path length of incident light, and increasing absorption can be considered. However, this method is less preferred because it loses the polycrystalline silicon layer that has grown.
[0034]
When liquid phase growth is performed on a base made of crystalline silicon, a plane (facet plane) having a specific plane orientation, particularly a (111) plane, tends to appear preferentially on the surface of the grown crystalline silicon. This is thought to be because liquid phase growth occurs in a state close to thermal equilibrium. FIGS. 1 and 2 show a state where the surface orientation of the surface of the base 101 is other than (111). Since the facet surface 103 has an inclination with respect to the surface of the base 101, fine irregularities having a pitch of several μm to several tens μm are formed on the surface of the polycrystalline silicon layer 102. Further, in the base of polycrystalline silicon, the orientation of the facet plane 103 is uniform within a crystal grain, but the orientation is different in different crystal grains, and the orientation is random as a whole. Even with the polycrystalline silicon layer 102 having a thickness of about 20 to 50 μm, light absorption equivalent to that of a flat polycrystalline silicon layer having a thickness of 100 μm can be obtained by the action of the fine unevenness formed by the facet surface 103. This method is advantageous in terms of cost because all the grown silicon can be used and an etching step is not required as compared with the method by etching.
[0035]
In the present invention, the base contains a high concentration of the dopant element. In particular, when metal-grade silicon is used as a raw material, heavy metal impurities that cannot be completely removed are included. When such a base is used, a dopant element or a heavy metal impurity may diffuse into a processing apparatus from a surface of the base exposed in a solar cell manufacturing process, and may adversely affect characteristics of the completed solar cell. Above all, the emitter layer on the surface using a high temperature (when the polycrystalline silicon layer is p-type, n + In the thermal diffusion process for forming the mold layer), the influence is likely to appear. Therefore, from the viewpoint of preventing impurity diffusion, it is desirable to cover the entire surface of the base with a high-purity polycrystalline silicon layer when performing liquid phase growth. On the other hand, when the back surface of the base is covered with a relatively high-resistance polycrystalline silicon layer, it is difficult to make electrical contact on the back surface. Therefore, as shown in FIGS. 1 and 2, liquid phase growth is performed on a predetermined region on the back surface of the base 101 so that the base surface is exposed, while a high-purity polycrystalline silicon layer is formed on the surface and the end surface 105 of the base. It is good to completely cover with 102. 1 shows a case where the exposed portion 104 is formed on the entire back surface, and FIG. 2 shows a case where the exposed portion 104 is formed only on a predetermined portion of the back surface. When the substrate thus manufactured is passed through a solar cell manufacturing process, the diffusion of impurities can be suppressed by applying a cover to the exposed portion 104 or stacking two substrates back to back. Further, since the exposed portion 104 has a low resistance, it is possible to easily make an electrical contact with the base.
[0036]
5 and 6 incorporate a mechanism for forming the exposed portion 104 only on the back surface of the base during liquid phase growth. In the apparatus shown in FIG. 5, the base 305 is supported between the support plate 306 and the fall prevention claw 307. In this cross-sectional view, only two locations of the fall prevention claw 307 are shown, but in actuality, at least three locations are provided to stably support the base 305. Here, when the base 305 is immersed in the melt 302, as shown in FIGS. 5 and 6, the base 305 having a lower specific gravity than the melt 302 comes into close contact with the support plate 306 by buoyancy, and the support plate 306 is Since it is made slightly larger, growth occurs on the front and end faces of the base, but no growth occurs on the back face. Further, in the apparatus shown in FIG. 6, since the support plate 306 is made slightly smaller than the base 305, growth occurs on the periphery of the back surface in addition to the front surface and the end surface of the base. However, in the portion in close contact with the support plate 307, no growth occurs, and an exposed portion as shown in FIG. 2 is formed.
[0037]
(Explanation of solar cell manufacturing process)
FIG. 3 shows a cross-sectional structure of an example of a solar cell manufactured by the method of the present invention.
[0038]
(Formation of emitter layer)
As a method of forming the emitter layer 106, a method of growing a thin silicon layer heavily doped with a conductivity type opposite to that of the polycrystalline silicon layer on the surface of the polycrystalline silicon layer 102 grown by liquid phase, There is a method of changing the conductivity type of several thousand angstroms on the outermost surface by performing thermal diffusion or ion implantation of a dopant on the surface of the crystalline silicon layer. As an n-type diffusion source, a coating solution containing phosphorus is coated, or POCl 3 Formed on the surface of polycrystalline silicon by oxidation while flowing an inert gas containing 2 O 5 Layers can be used. BBr as a p-type diffusion source 3 Formed on the surface of polycrystalline silicon by oxidation while flowing an inert gas containing 2 O 3 Layers can be used. The depth of the junction of the emitter layer is about 1000 to 5000 Å, and the surface sheet resistance is about 10 to 100 Ω / □. In order to obtain such an emitter layer by thermal diffusion, a treatment at a temperature of about 700 to 900 ° C. for several minutes to several tens of minutes is necessary. As described above, boron, phosphorus, heavy metal, etc. Impurities may diffuse. In the solid phase, the diffusion length of boron and phosphorus in the solid phase is short, and the concentration of heavy metals is reduced by unidirectional solidification. However, when a CVD furnace is used or a dopant is thermally diffused in a diffusion furnace in forming the emitter layer, impurities may diffuse from the gas phase. On the other hand, when using the base proposed in the present invention, in which at least the surface and the end face are covered with a high-purity polycrystalline silicon layer, two substrates can be placed back to back and put into a CVD furnace or a diffusion furnace to obtain a gas phase. The risk of impurity diffusion in the semiconductor device can be minimized.
[0039]
(Formation of anti-reflection layer and grid electrode)
Since silicon has a high refractive index of about 3.4 and a high reflectance with respect to air, it is necessary to form an appropriate anti-reflection layer 107 on the surface. As the antireflection layer, a transparent film having a refractive index of about 1.8 to 2.3 and a high transparency and made of silicon nitride, titanium oxide, zinc oxide, zinc sulfide or the like and having a thickness of about 600 to 900 angstroms is used. As a method for depositing the antireflection layer 107, a sputtering method, a thermal CVD method, a plasma CVD method, or the like is generally used. In the case of titanium oxide, it can be formed by applying and baking a coating solution. In some cases, the antireflection film has a function of preventing recombination of carriers on the surface in addition to the optical function. From this viewpoint, silicon nitride has been widely used because it is particularly effective.
[0040]
A grid electrode 108 is formed on the surface of the emitter layer to extract a photocurrent. Since the grid electrodes 108 are shaded by the incident light, it is desirable that the width is as small as possible and the number of the grid electrodes 108 is small. Further, the grid electrode 108 needs to form a good electrical contact with the emitter layer 106. From this viewpoint, a silver paste pattern containing glass frit is generally formed by printing and firing. Since the antireflection film generally has a high resistance, the grid electrode 108 needs to directly contact the emitter layer 106. However, forming the anti-reflection layer from above the grid electrode interferes with the solder coat 109 applied to the printed grid electrode to reduce the resistance of the grid electrode. A method of forming a grid electrode after exposing a region to be etched to expose an emitter layer is adopted. Alternatively, there is a method in which a pattern of the grid electrode 108 is printed from above the antireflection layer 107, and the antireflection layer is penetrated by baking to make contact with the emitter layer 106 (fire-through method). This method does not require the etching of the anti-reflection layer and the alignment of the grid electrode pattern, and has a high productivity.
[0041]
(Formation of back electrode and isolation of emitter layer)
In a general crystalline silicon solar cell, in order to make electrical contact with the back surface, an aluminum paste is often printed and fired to form the back surface electrode 110, particularly when the polycrystalline silicon layer is p-type. The aluminum paste often shrinks when fired, causing the substrate to bend. In particular, when an electrode is formed on the entire back surface, the bending becomes remarkable. If bending becomes a problem, the back electrode 110 may be formed in a divided pattern instead of over the entire surface as shown in FIG.
[0042]
As described above, the emitter layer 106 is formed on the surface of the polycrystalline silicon layer. However, if the emitter layer contacts the surface of the back electrode 110 or the base, a photocurrent leaks and the solar cell characteristics are significantly impaired. In the present invention, since at least the surface of the base and the end face 105 are substantially covered with the polycrystalline silicon layer, there is little possibility of such a leak. Also, in the CVD process or the thermal diffusion process for forming the emitter layer, if the processing is performed with the back surfaces of the substrates back-to-back, the emitter layer is particularly unlikely to move to the back surface, and the risk of leakage is further reduced. However, when it is desired to particularly suppress the leak between the emitter layer 106 and the back electrode 110 or the base 101, the diffusion source of the dopant may be formed by printing in a pattern avoiding the peripheral portion of the substrate when forming the emitter layer, or The emitter layer may be removed by etching or by scribing the peripheral surface. When performing etching or scribing on the emitter layer in the peripheral portion of the substrate, it is desirable to substantially remove the emitter layer in a predetermined region. Conversely, if the emitter layer is removed until the surface of the base is exposed, it becomes easier to leak. Need to control the depth of removal. When a substantially insulating antireflection film such as silicon nitride is used, if the isolation is performed before the formation of the antireflection film, the effect of preventing leakage is further enhanced.
[0043]
【Example】
Ingots were made from nuggets of chemical grade metal grade silicon from Norway. After 60 kg of the nugget was washed with an acid, it was put into the apparatus shown in FIG. The crucible 201 has a bottom surface of 30 cm square and a depth of 40 cm. After lowering the lower heater 202 and the upper heater 203 and melting and degassing all the silicon for 10 hours, the output of the lower heater 202 is gradually reduced, and silicon is removed from the bottom of the crucible 201 as shown in FIG. Coagulated. Coagulation was completed over 10 hours, and the outputs of both heaters were gradually lowered to cool for 10 hours. The grain boundaries 205 extended vertically in the solidified ingot. A sample was sliced from this ingot, the surface was etched, and the hole resistance was measured. As a result, the specific resistance was 0.02 Ω · cm for the p-type. Cut off the part within 5 cm from the surface of the ingot and within 2.5 cm from the bottom and inner walls of the crucible with a band saw, and furthermore, divide four blocks of 125 mm square and 250 mm long so that the longitudinal direction is perpendicular to the crystal growth direction 207. The base was cut out from the block, and 2,000 pieces of 125 mm □ × 300 μm thick base were cut out from this block with a multi-wire saw, washed with a solvent, and planar-etched with a mixed solution of nitric acid / acetic acid / hydrofluoric acid for 2 minutes to remove the wire saw remaining on the base. Removal gave a glossy surface.
[0044]
On the surface of the base thus obtained, a polycrystalline silicon layer was grown using a liquid phase growth apparatus equivalent to that shown in FIG. However, in the apparatus used in the present embodiment, four groups of 51 support plates 306 supporting the base are arranged in parallel in a vertical direction, so that 204 substrates can be grown in one batch. First, indium was charged into the crucible 301, heated to 950 ° C., and melted while maintaining this temperature. Next, a p-type solar cell-grade polycrystalline silicon plate having a thickness of 3 mm was set instead of the base, immersed in dissolved indium, and silicon was dissolved in indium and saturated to prepare a melt 302. This polycrystalline silicon plate was once pulled up, and instead 200 bases prepared in advance were attached. However, four bases made of n-type polycrystalline silicon were also attached for measuring the specific resistance. The base is held outside the melt 302 by four fall prevention claws 307. After replacing the atmosphere around the crucible with hydrogen, the melt 302 was started to be cooled at a rate of 1 ° C. per minute. When the temperature of the melt reached 945 ° C., the base was immersed in the melt, the growth was continued for 1 hour, and then the base was pulled out of the melt. Since some indium adhered to the contact portion between the fall prevention claw 307 and the base 305, the whole was directly applied to hydrochloric acid for 1 hour to remove indium just in case. Thereafter, when the base 302 was removed, a polycrystalline silicon layer 102 of about 30 μm was grown on the base 101.
[0045]
Hereinafter, the configurations of the substrate and the solar cell will be described with reference to FIGS. When the surface was observed with a metallographic microscope, fine irregularities with a pitch of 5 to 10 μm were observed. When the cross section was further cut out and observed, it was determined that the irregularities were formed of terraces oriented in a fixed direction for each crystal grain, and were facet faces 103 accompanying the crystal growth. When the specific resistance of the polycrystalline silicon layer grown on the n-type base was measured by four-probe measurement, the specific resistance was 0.8 to 1.2 Ω · cm. Here, the reason why the n-type base is used is that a depletion layer is formed between the n-type base and the p-type polycrystalline silicon layer 102, the grown polycrystalline silicon layer is electrically separated from the base, and the specific resistance is accurately measured. This is for better measurement. Further, the polycrystalline silicon layer completely covered not only the surface of the base but also the end face 106 as shown in FIG. 1, but no growth was observed on the rear face. Thus, a polycrystalline silicon substrate for a solar cell was completed. Similarly, the growth was performed 10 times, and the growth was performed on all of the 2000 bases. The cross-sectional structure and specific resistance of the polycrystalline silicon layer were confirmed at each growth, but the reproduction was good.
[0046]
Subsequently, a solar cell was prototyped using this polycrystalline silicon substrate. First, in order to form the emitter layer 106, a coating solution containing phosphorus was applied by a spinner. After the coating solution was dried, 100 substrates were placed in a horizontal heat treatment furnace, two by two, back to back, and phosphorus was thermally diffused at 900 ° C. in a nitrogen atmosphere, and then the film of the coating solution was etched. Removed. This process was repeated 20 times, and thermal diffusion was performed on 2000 substrates.
[0047]
Next, in order to form a silicon nitride film as the antireflection film 107, the substrate was put into a load-lock type plasma CVD apparatus. The substrate was spread on a 550 mm square susceptor at a temperature of 300 ° C. Sixteen substrates were arranged at an interval of 10 mm. While a silane gas, an ammonia gas, and a nitrogen gas were mixed and flowed, an RF voltage was applied to the cathode facing the substrate, and discharge was continued for 5 minutes to deposit a silicon nitride film on the surface. In addition to this, it took 15 minutes per batch including the time for temperature rise and transportation. The deposited silicon nitride film 107 was deposited so as to cover the end face 106 as well. The reflection spectrum of the surface was measured with a spectral reflectometer equipped with an integrating sphere, and found to have a minimum at a wavelength of 580 nm and a reflectance of 10% or less in a wavelength range of 450 nm to 1000 nm. When a silicon nitride film is deposited on a silicon wafer whose surface is polished under the same conditions, the minimum is 650 nm and the reflectance is 10% or less in the range of 550 nm to 800 nm. The effect was clearly recognized.
[0048]
Next, using a screen printer, first, an aluminum paste was printed as the back electrode 110 and dried, and then a silver paste pattern was printed as the grid electrode 108 on the front surface and dried. This was put into an infrared belt firing furnace. The baking furnace was provided with a zone of 450 ° C. and a zone of 800 ° C., and two substrates were arranged side by side. The belt was driven at a speed of 100 mm / min. The silver particles penetrated the antireflection film 107 and reached the emitter layer 106, and good electrical contact was made with the emitter layer. On the other hand, the aluminum paste melted the aluminum and made good electrical contact with the back surface of the base.
[0049]
Finally, in order to form the solder coat layer 109, two substrates were accommodated in a cassette at a time, immersed in a flux tank and dried in hot air, immersed in a solder flow tank for a predetermined time, the cassette was pulled up, and the flux was washed and dried. . The solder was only coated on the grid of silver paste.
[0050]
By the above process, 2000 solar cells were manufactured. One sheet was taken out of every 100 sheets in the order of production, and the characteristics were evaluated using a solar simulator having an irradiation light spectrum of AM1.5. The conversion efficiency of the 20 solar cells fell between 13.5% and 14.0%, and there was no tendency to systematically change in the order of production.
[0051]
Thus, it has been found that stable production can be continued even when the conventional production process of the crystalline solar cell is used as it is. Further, in the present invention, a high-purity silicon layer having a thickness of 30 μm has a conversion efficiency equivalent to that of a conventionally used polycrystalline silicon substrate having a thickness of about 300 μm (consuming a thickness of 500 μm by adding a slice allowance). It was confirmed that resources can be used effectively and that solar cells can be further reduced in price.
[0052]
[Comparative example]
To see the effect of the present invention, a solar cell was prototyped using the liquid phase growth apparatus shown in FIG.
[0053]
7, the base 305 is fixed to the support plate 306 with the peripheral cover 308. The peripheral portion cover 308 covers not only the back surface of the base but also the peripheral portion of the end surface 105 and the front surface of the base, so that the polycrystalline silicon layer does not grow on these portions. Therefore, in the method of forming the emitter layer 106 of the embodiment, a leak occurs between the emitter layer and the base. In order to avoid this, the emitter layer was removed in a line shape by etching around the formed emitter layer and then isolated. As a result, the effective opening area of the solar cell was lost by 5%.
[0054]
Except for the above, 2,000 solar cells were manufactured in the same process as in the example, and one out of every 100 cells was sampled and the extraction characteristics were evaluated as in the example. When 1800 sheets were manufactured and the characteristics of 18 solar cells were measured, a clear tendency was seen in which the conversion efficiency was reduced in the order of manufacturing from 13.1% of the first sheet to 9.3%. SIMS analysis of the polycrystalline silicon layer of the solar cell with reduced efficiency showed that iron and boron other than phosphorus were detected near the surface. Further, the detection amount was small in a portion deep from the surface, and it was presumed that the metal was diffused from the surface of the polycrystalline silicon layer. On the other hand, no iron or boron was detected from the first solar cell of the example or the solar cell of this comparative example.
[0055]
Then, the quartz glass member of the heat treatment furnace for forming the emitter layer was removed and replaced with a new one. Thereafter, the efficiency of 12.8% and 12.6% of two sheets taken out of the 200 sheets successively manufactured did not show a clear decrease in characteristics. This is because the front surface and the end face 105 of the base were completely covered with the polycrystalline silicon layer 102 in the substrate of the example, and the back face of the base was not exposed by combining two substrates, whereas the end face 105 of the base was exposed in the comparative example. It is predicted that the heat treatment furnace was gradually contaminated because the surface and the periphery of the surface were exposed. Even in the substrate of the comparative example, the surface of the base is covered with a phosphorus glass film formed from a phosphorus coating solution during the heat treatment, but does not seem to be effective in preventing diffusion of impurities.
[0056]
【The invention's effect】
According to the present invention, a polycrystalline silicon substrate for a solar cell equivalent to the conventional one can be obtained with the use amount of the high-purity silicon raw material of 1/10 or less of the conventional one. Therefore, the cost of the solar cell can be reduced as compared with the case where the conventional polycrystalline silicon substrate is used, and the production amount is hardly restricted. In addition, the substrate of the present invention has the same shape as the conventional polycrystalline silicon substrate and can be flowed to the conventional solar cell production line as it is, so that new investment in the solar cell production line is not required.
[Brief description of the drawings]
FIG. 1 is a diagram showing a cross section of a polycrystalline silicon substrate according to the present invention.
FIG. 2 shows a cross section of another polycrystalline silicon substrate according to the present invention.
FIG. 3 is a diagram showing a cross section of a polycrystalline silicon solar cell manufactured by the method of the present invention.
FIG. 4 is a diagram showing a configuration of an apparatus for manufacturing a polycrystalline silicon ingot suitable for carrying out the present invention.
FIG. 5 is a diagram showing a configuration of a liquid phase growth apparatus suitable for carrying out the present invention.
FIG. 6 is a diagram showing a configuration of another liquid phase growth apparatus suitable for carrying out the present invention.
FIG. 7 is a diagram showing a configuration of a liquid phase growth apparatus used for implementing a comparative example.
[Explanation of symbols]
101 base
102 Polycrystalline silicon layer
103 facet face
104 Base back exposed part
105 Base end face
106 Emitter layer
107 Anti-reflection layer
108 grid electrode
109 Solder coat layer
110 Back electrode
201 crucible
202 Lower heater
203 Upper heater
204 Solidified polycrystalline silicon
205 grain boundary
206 molten silicon
207 Growth Direction
301 crucible
302 Melt
303 shaft
304 heater
305 base
306 Support plate
307 Fall prevention claw
308 Peripheral cover

Claims (3)

  1. On a polycrystalline silicon substrate for a solar cell formed by growing a high-purity polycrystalline silicon layer on a surface of a base sliced from a polycrystalline silicon ingot obtained by melting and solidifying low-purity silicon by a liquid phase growth method,
    The polycrystalline silicon layer substantially covers the front surface and the end surface of the base, and at least a part of the back surface has an opening in which the base is exposed, and the polycrystalline silicon layer has the same conductivity type as the base and has a specific resistance. A polycrystalline silicon substrate for a solar cell, characterized in that the polycrystalline silicon substrate has a surface area of 0.1 Ω · cm or more and 10 Ω · cm or less and at least a part of the surface of the polycrystalline silicon layer is a facet surface formed as a result of growth.
  2. The low-purity silicon is melted and solidified to form a polycrystalline silicon ingot, and a flat base is cut out from the ingot. At least the surface and the end face of the base have the same conductivity type as the base and a specific resistance of 0.1 Ω · cm. A method for producing a polycrystalline silicon substrate for a solar cell, wherein a polycrystalline silicon layer having a facet face of at least 10 Ω · cm or less and at least a part of the surface is liquid-phase grown.
  3. An emitter layer of a conductivity type different from the polycrystalline silicon layer is formed on a surface of the polycrystalline silicon substrate for a solar cell according to claim 1, an antireflection layer is formed on a surface of the emitter layer, and a back surface is formed on a back surface of the base. A method of manufacturing a solar cell, comprising: forming an electrode pattern; forming a grid electrode pattern on the surface of an antireflection layer; and firing the back electrode pattern and the grid electrode pattern.
JP2002301918A 2002-10-16 2002-10-16 Polycrystalline silicon substrate for solar cell and method for manufacturing the same, and method for manufacturing solar cell using the substrate Withdrawn JP2004140087A (en)

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008525297A (en) * 2004-12-27 2008-07-17 エルケム ソウラー アクシエセルスカプ Method for producing directional solidified silicon ingot
JP2011527112A (en) * 2008-07-01 2011-10-20 サンパワー コーポレイション Front contact solar cell having conductive layers formed on the front and rear surfaces
JP2013149896A (en) * 2012-01-23 2013-08-01 Innovation & Infinity Global Corp Polycrystalline silicon substrate for composite solar cell, and solar cell using the same
WO2013140597A1 (en) * 2012-03-23 2013-09-26 三洋電機株式会社 Solar cell, solar cell module, and solar cell manufacturing method
JPWO2013140597A1 (en) * 2012-03-23 2015-08-03 パナソニックIpマネジメント株式会社 Solar cell, solar cell module, and solar cell manufacturing method

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008525297A (en) * 2004-12-27 2008-07-17 エルケム ソウラー アクシエセルスカプ Method for producing directional solidified silicon ingot
JP2011527112A (en) * 2008-07-01 2011-10-20 サンパワー コーポレイション Front contact solar cell having conductive layers formed on the front and rear surfaces
JP2013149896A (en) * 2012-01-23 2013-08-01 Innovation & Infinity Global Corp Polycrystalline silicon substrate for composite solar cell, and solar cell using the same
WO2013140597A1 (en) * 2012-03-23 2013-09-26 三洋電機株式会社 Solar cell, solar cell module, and solar cell manufacturing method
CN104205352A (en) * 2012-03-23 2014-12-10 三洋电机株式会社 Solar cell, solar cell module, and solar cell manufacturing method
JPWO2013140597A1 (en) * 2012-03-23 2015-08-03 パナソニックIpマネジメント株式会社 Solar cell, solar cell module, and solar cell manufacturing method

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