JP2007095972A - Solar cell silicon substrate and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-quality solar-cell silicon substrate with suppressed etch pit density in relation with an Fe concentration. <P>SOLUTION: The solar cell silicon substrate is composed so that the Fe concentration (a) and etch pit density (b) of the substrate satisfy an expression of log(b)<0.5 log(a)-2.5. A dash liquid expressed by HF:HNO<SB>3</SB>:CH<SB>3</SB>COOH=1:3:15 is used as a selective etchant. The manufacturing method includes a process for forming the solar cell silicon substrate by cutting a silicon ingot formed by unidirectionally solidifying a silicon melt in a mold. A liquid-surface temperature of the silicon melt at the central part is different as ≥20°C from that at the end in the unidirectional solidification process. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a silicon substrate for a solar cell and a method for manufacturing the same.

  Solar cells are expected to be put to practical use in a wide range of fields, from small households to large-scale power generation systems, as a clean petroleum alternative energy source. These are classified into crystalline, amorphous, and compound types depending on the type of raw material used, and most of those currently on the market are crystalline silicon solar cells. This crystalline silicon solar cell is further classified into a single crystal type and a polycrystalline type. Single crystal silicon solar cells have the advantage that conversion efficiency can be easily increased because the quality of the substrate is good, but the disadvantage is that the manufacturing cost of the substrate is high. On the other hand, polycrystalline silicon solar cells have been distributed in the market for a long time, but in recent years, the demand for environmental problems has been increasing, and higher conversion efficiency is required at lower cost.

  A silicon substrate used for a polycrystalline silicon solar cell is generally manufactured by a method called a casting method. This casting method is a method of forming a silicon ingot by cooling and solidifying a silicon melt in a mold made of quartz or the like coated with a release material. The ends of the silicon ingot are removed, cut into a desired size, and the cut out ingot is sliced to a desired thickness to obtain a silicon substrate for forming a solar cell.

  Description will be made using a general casting apparatus for casting silicon or the like disclosed in Patent Document 1. A melting crucible for melting the silicon raw material is disposed in the upper part of the casting apparatus while being held by the holding crucible, and a pouring port for pouring the silicon melt by inclining the melting crucible is provided at the upper edge of the melting crucible. It is done. A heating means is disposed around the melting crucible and holding crucible, and a mold into which a silicon melt is poured is disposed below the melting crucible and holding crucible. The melting crucible is made of, for example, high-purity quartz in consideration of heat resistance and the fact that impurities do not diffuse into the silicon melt. The holding crucible is used to hold a melting crucible made of quartz or the like because the melting crucible is softened at a high temperature in the vicinity of the silicon melt and cannot keep its shape, and the material is graphite or the like. As the heating means, for example, a resistance heating type heater or an induction heating type coil is used.

  The mold disposed at the lower part of the melting crucible and holding crucible is made of quartz, graphite or the like, and is used by applying a release material mainly composed of silicon nitride, silicon oxide or the like to the inside thereof. Further, a mold heat insulating material is installed around the mold to suppress heat removal. As the mold heat insulating material, a carbon-based material is generally used in consideration of heat resistance, heat insulating properties, and the like. In some cases, a cooling plate for cooling and solidifying the poured silicon melt is installed below the mold. These are all arranged in a vacuum vessel.

  In such a casting apparatus, the silicon raw material is charged into the melting crucible, the silicon raw material in the melting crucible is melted by heating means, and then all the silicon raw material in the melting crucible is melted, and then the crucible is tilted and melted. The silicon melt is poured from a pouring port at the upper edge of the crucible into a mold installed at the bottom.

  After pouring, the silicon in the mold is cooled from the bottom and solidified in one direction, and then slowly cooled while controlling the temperature to a temperature at which it can be taken out of the furnace. Finally, the silicon is taken out of the furnace and casting is completed.

Then, the obtained silicon ingot is cut into a desired size using a known cutting device such as a band saw cutting machine, and then sliced using a wire saw device or the like to form a silicon substrate for a solar cell. be able to.
Japanese Patent Laid-Open No. 11-180711

  However, the silicon substrate obtained by the above method has a region where etch pits exist, and the substrate characteristics such as diffusion length, which is considered to affect the solar cell characteristics, tend to decrease as the density of the etch pits increases. It is known to be. The etch pit refers to a defect portion that has become a hole as the substrate is etched with a selective etchant, and is due to the fact that the defect portion is etched earlier with respect to other portions.

  In addition, it is known that metallic impurities and foreign matters mixed in the manufacturing process of the silicon substrate, especially Fe, form deep levels in the silicon crystal and adversely affect device characteristics. There is a problem of being affected.

  Therefore, as a result of repeated experiments and examinations as to whether or not there is any causal relationship between the Fe concentration and the etch pit density of the silicon substrate, the present inventor found that the presence of Fe mixed in during the manufacturing process of the silicon substrate. We found that it was involved in the increase of etch pits. This relationship can be understood as follows.

  That is, in the manufacturing process of the silicon substrate, for example, as shown in FIG. 5, when crystal grains 21 having different plane orientations are growing in parallel, the crystal grain boundary portion 22 formed between the crystal grains 21. As a result of the growth being delayed due to the reduction of the compositional supercooling due to the concentration of Fe, the concentration of Fe further proceeds, whereby the Fe concentration region 23 in which a large amount of Fe is taken into the grain boundary portion 22 is formed. . In the subsequent solidification / cooling process, it is considered that Fe spreads in the crystal grains 21 by thermal diffusion. For this reason, it is considered that etch pits are generated in the vicinity of the crystal grain boundary part 22 due to the influence of the incorporation of a large amount of Fe and the growth of crystal grains in which dislocations are likely to occur due to disorder of crystal growth.

  From the above, in order to improve the characteristics of the solar cell, it is important to suppress the etch pit density by reducing the amount of Fe in the silicon melt, but in the casting method as described above, Fe is a graphite. Alternatively, since the silicon melt is mixed from a mold material such as silicon dioxide and a mold release material such as silicon nitride applied to the inner surface of the mold material, it is extremely difficult to form a silicon substrate in which the Fe concentration is suppressed.

  The present invention has been made in view of these problems, and an object thereof is to provide a high-quality silicon substrate for a solar cell in which the etch pit density is suppressed in relation to the Fe concentration.

The silicon substrate for solar cells of the present invention has an Fe concentration a and an etch pit density (EPD) b satisfying log (b) <0.5 log (a) −2.5. Here, the Fe concentration a (atoms / cm ³ ) indicates the number of Fe contained in the silicon substrate, and the etch pit density b (pieces / cm ² ) indicates the imaging of the silicon substrate in a microscope view. For an image, the average value of values obtained by performing image processing and measuring the number of etch pits as a binarized image by a black and white image and dividing it back by a unit area. As the selective etching solution, a dash solution represented by HF: HNO ₃ : CH ₃ COOH = 1: 3: 15 is used.

  The method for producing a silicon substrate for a solar cell according to the present invention is obtained by cutting a silicon ingot formed by unidirectionally solidifying a silicon melt inside a mold, and in the process of unidirectionally solidifying, The liquid surface temperature of the silicon melt is different by 20 ° C. or more between the central portion and the end portion.

  In the silicon substrate for solar cell of the present invention, the Fe concentration a and the etch pit density b satisfy log (b) <0.5 log (a) −2.5. Since it is relatively small, the characteristics of the solar cell can be improved.

  The method for producing a silicon substrate for a solar cell according to the present invention is obtained by cutting a silicon ingot formed by unidirectionally solidifying a silicon melt inside a mold, and in the process of unidirectionally solidifying, Since the liquid surface temperature of the silicon melt differs by 20 ° C. or more at the center and at the end, Marangoni convection from the silicon melt liquid surface (along the mold side surface) to the bottom occurs, and the stirring action by the convection causes It is possible to reduce the etch pit density b by suppressing non-uniform concentration of Fe at the grain boundary.

  Hereinafter, the present invention will be described in detail with reference to the drawings.

≪Silicon substrate manufacturing method≫
FIG. 1 is a cross-sectional view showing a casting apparatus used in the method for producing a silicon substrate for solar cells of the present invention, wherein (a) shows a crucible and (b) shows a mold. 1 is a crucible (1a is a melting crucible, 1b is a holding crucible), 2 is a pouring port, 3 is a heating means, 4 is a silicon melt, 5 is a mold, 6 is a mold release material, 7 is a mold heat insulating material, and 8 is a cooling plate , 9 indicates a mold upper part heating means. These are arranged in a casting furnace (not shown).

  First, in the crucible 1, the silicon raw material is melted by heating using a heating means 3 such as a resistance heating type heater or an induction heating type coil to obtain a silicon melt.

  Here, for the melting crucible 1a, high-purity quartz or the like is used in consideration of heat resistance and the fact that impurities do not diffuse into the silicon melt. The holding crucible 1b is intended to hold the shape of the melting crucible 1a during heating, and graphite or the like is used.

  Next, the crucible 1 is tilted, and the silicon melt is supplied from the pouring port 2 of the crucible 1 to the lower mold 5.

Here, the mold 5 is made of, for example, quartz or graphite, and a mold release material 6 made of general silicon dioxide or silicon nitride containing about 1e + 17 to 1e + 18 atoms / cm ³ of Fe is applied to the inner wall thereof.

  Next, by cooling the lower part of the mold 5 while keeping the upper part of the mold 5 at a high temperature, the supplied silicon melt is solidified in one direction from below to form a silicon ingot. That is, the ingot is formed in order to cool and solidify the silicon melt provided below the mold 5 and the mold heat insulating material 7 made of a carbon-based material for suppressing heat removal provided around the mold 5. The cooling plate 8 is controlled so as to be solidified in one direction from the lower side to the upper side of the mold 5.

  Here, as shown in FIG. 2, the liquid surface temperature of the silicon melt is maintained so that the central portion 11 is 20 ° C. higher than the end portion 12 (near the mold side surface). Thus, Marangoni convection 10 is continuously generated from the liquid surface of the silicon melt toward the bottom along the mold side surface, and effective convection is generated in front of the solid-liquid interface, and a stirring action occurs in the vicinity of the solid-liquid interface. As a result, the etch pit density b can be reduced by effectively suppressing the non-uniform concentration of Fe at the crystal grain boundary 22.

  The Marangoni convection 10 adjusts the heating output of the mold upper heating means 9 to control the temperature of the central portion 11 of the silicon melt 4 liquid surface and adjusts the heat removal effect of the cooling plate 8 to adjust the end 12 (the side surface of the mold). By controlling the temperature of the member 5 b and the upper end portion of the release material 6, the liquid surface temperature of the silicon melt is generated by making the central portion 11 higher than the end portion 12. The liquid surface temperature of the silicon melt is measured by installing a radiation thermometer in a viewing window (not shown) installed for the purpose of observing the liquid surface of the silicon melt from outside the casting furnace.

  Thereafter, the obtained silicon ingot is cut out as a silicon block having a size of 10 cm × 10 cm or 15 cm × 15 cm after the side surface is cut and removed by a band saw device or the like. Then, a plurality of silicon substrates 31 are formed by slicing to a predetermined thickness, for example, 300 μm or less, using a wire saw device or the like. In order to clean the mechanically damaged layer or the contaminated layer on the surface that has been cut or sliced, it is desirable that the surface is etched by a very small amount with NaOH, KOH, hydrofluoric acid, or hydrofluoric acid.

  In the silicon substrate 31 formed as described above, the Fe concentration a and the etch pit density b satisfy log (b) <0.5 log (a) −2.5. Thus, since the silicon substrate 31 has a relatively small etch pit density b with respect to the Fe concentration a, a solar cell having excellent characteristics can be formed.

≪Solar cell manufacturing method≫
Hereinafter, the manufacturing method of a solar cell is demonstrated using FIG.

  FIG. 3 is a view showing a solar cell formed using the silicon substrate for solar cell of the present invention, where (a) is a schematic sectional view, (b) is a plan view seen from above, and (c) is below. It is the bottom view seen from. In the figure, 31 is a silicon substrate, 32 is a reverse conductivity type diffusion region, 33 is an antireflection film, 34 is a surface electrode, 34a is a bus bar electrode on the front side, 34b is a finger electrode on the front side, 35 is a back electrode, 35a is An extraction electrode on the back surface side, 35b indicates a collecting electrode on the back surface side, and 36 indicates a back surface electric field region.

  As for the silicon substrate for solar cells of the present invention obtained as described above, first, it is desirable to form minute protrusions on the surface of the silicon substrate 31 by using a dry etching method or a wet etching method.

Thereafter, a reverse conductivity type diffusion region 32 exhibiting n-type conductivity is formed on one main surface side of the silicon substrate 31. As a method of forming the reverse conductivity type diffusion region 32, for example, the silicon substrate 31 is placed in a container, and a gas serving as a diffusion source is introduced while heating. For example, by flowing POCl ₃ , a phosphor glass serving as an impurity diffusion source containing P, which is an n-type dopant, is formed on the surface of the silicon substrate 31, and at the same time, thermal diffusion is performed on the surface of the silicon substrate 1. A diffusion method is common. Thereafter, for example, the phosphorus glass formed on the surface of the silicon substrate 31 is removed by immersing in a chemical such as a diluted hydrofluoric acid solution.

  Then, the remaining portion of the silicon substrate 31 on the light-receiving surface side is removed, and other portions are removed, followed by washing with pure water. As this removal method, for example, a film having resistance to hydrofluoric acid is applied to the surface side of the silicon substrate 31, and a reverse conductivity type diffusion other than the light receiving surface side of the silicon substrate 31 is performed using a mixed solution of hydrofluoric acid and nitric acid. After removing the region by etching, a film having resistance to hydrofluoric acid may be removed.

Next, an antireflection film 33 is formed on the light receiving surface side of the silicon substrate 31. The antireflection film 33 is made of, for example, a silicon nitride film, a silicon oxide film, or the like. For example, the silicon nitride film is a plasma that is deposited by making a mixed gas of silane (SiH ₄ ) and ammonia (NH ₃ ) into plasma by glow discharge decomposition. It is formed by a CVD method or the like. This antireflection film 33 is formed so as to have a refractive index of about 1.8 to 2.3 in consideration of a refractive index difference with the silicon substrate 31 and is formed to a thickness of about 500 to 1200 mm. . When the silicon nitride film is formed in the presence of hydrogen plasma in this way, it has a passivation effect at the same time, so that it has an effect of improving the electric characteristics of the solar cell in addition to the antireflection function.

  Next, the front electrode 34 and the back electrode 35 are formed. The current collecting electrode 35b on the back surface side constituting the back electrode 35 is mainly composed of a metal made of aluminum powder or the like, for example, and the organic vehicle and the glass frit are 10 to 30 parts by weight and 0.1 parts by weight with respect to 100 parts by weight of aluminum. An electrode material containing as a main component a first metal added to 5 parts by weight in paste form is used. As a specific shape, for example, as shown in FIG. 3C, an opening except for a portion where the extraction electrode 35a on the back surface side is formed is provided so as to be almost the entire back surface. As a coating method, a known method such as a screen printing method can be used. After the coating, the solvent is evaporated at a predetermined temperature and dried.

  The rear-side extraction electrode 35a constituting the back-side electrode 35 and the front-side bus bar electrode 34a and the finger electrode 34b constituting the front-side electrode 34 are mainly composed of a metal material having better solder wettability than the first metal, such as silver powder. An electrode material mainly composed of a second metal prepared by adding 10 to 30 parts by weight and 0.1 to 5 parts by weight of an organic vehicle and glass frit to 100 parts by weight of silver, respectively, is used.

  As for the surface electrode 34, as shown in FIG. 3 (b), a bus bar electrode 34a for taking out output from the surface as a general solar cell, and a current collecting finger provided so as to be orthogonal thereto What is necessary is just to form in a grid | lattice form by the electrode 34b.

  As a coating method, a known method such as a screen printing method can be used, and after coating, the solvent is evaporated at a predetermined temperature and dried.

  The surface electrode 34 and the back electrode 35 applied and dried as described above are baked at a maximum temperature of 600 to 800 ° C. for about 1 to 30 minutes, and are then baked on the silicon substrate 31. Can be formed. Further, at the same time as forming the current collecting electrode 35b on the back surface, a back surface electric field region 36 is formed which prevents aluminum from diffusing into the silicon substrate 31 and recombining carriers generated on the back surface. In addition, a portion corresponding to the surface electrode 34 of the antireflection film 33 is etched in advance, and an electrode material (silver paste or the like) mainly composed of the second metal is applied to the portion and baked to form the reverse conductivity type diffusion region 32. Conduction may be taken, or an electrode material (silver paste or the like) mainly composed of the second metal is directly applied on the antireflection film 33 and baked, and the antireflection film 33 is formed by a so-called fire-through method. May be made to be electrically connected to the reverse conductivity type diffusion region 32.

The solar cell obtained by the above method is a silicon substrate having a relatively small etch pit density b with respect to the Fe concentration a, and can have more excellent characteristics. Further, in the silicon substrate of the present invention, even when the Fe concentration a is higher than 1e + 14 atoms / cm ³ , the etch pit density b can be reduced as compared with the conventional method, and as a result, diffusion caused by forming phosphorous glass Due to the Fe gettering process, the passivation effect by the antireflection film 33 containing hydrogen, the back surface field region 36 formed by forming the aluminum electrode, etc., the conventional solar cell has a low conversion efficiency of 10% or less. However, it becomes possible to form a solar cell with higher characteristics.

  In addition, this invention is not limited to the above-mentioned embodiment, A various change, improvement, etc. are possible in the range which does not deviate from the summary of this invention, For example, the above-mentioned process does not use the crucible 1. The ingot may be formed in the mold 5 as it is after the silicon raw material is dissolved in the mold 5.

<Production of silicon substrate>
Examples of the present invention will be described below.

  Unless otherwise specified, the above-described casting apparatus shown in FIG. 1 was used as described above.

  As the mold 5, a melting crucible 1 a made of quartz was held by a holding crucible 1 b made of graphite, and a silicon raw material was put into the melting crucible 1 a.

  The heating means 3 was provided around the melting crucible 1a, the silicon raw material in the melting crucible 1a was dissolved by the heating means, and the silicon melt 4 was poured into the mold 1.

  Here, in order to accurately grasp the influence of Fe on the silicon substrate, the Fe powder was intentionally introduced into the silicon melt, and the amount thereof was variously changed.

  Thereafter, the silicon melt 4 is applied while applying a predetermined temperature gradient from below to above the mold 5 by the cooling plate 8 disposed below the mold 5 and the mold heating means 9 for heating the mold 5 from above. Was unidirectionally solidified.

  In Examples D, E, and F of the present invention, the temperature difference between the end portion 12 (region near the mold) and the central portion 11 on the surface of the silicon melt is set to 20 ° C., and the Marangoni convection 10 is generated while the silicon ingot is formed. Produced. The generation of Marangoni convection was confirmed by periodically dropping and floating a powdery silicon raw material of about φ5 μm on the surface of the silicon melt and observing the flow. Examples G, H, and I were set as other examples in which the amount of Fe input was larger than those of Examples D, E, and F.

  Moreover, in Comparative Examples A, B, and C of the conventional method, silicon ingots were produced without the above temperature difference. Note that A and D, B and E, and C and F have the same amount of Fe input to the silicon melt.

  By cutting the silicon ingot obtained by solidification in this manner, a silicon substrate having a thickness of 250 μm and an outer shape of 15 cm × 15 cm was obtained.

  As described above, for the silicon substrates manufactured by changing the amount of Fe in the silicon melt, the Fe concentration a and the etch pit density b of each substrate were measured. Here, the Fe concentration a is measured by ICP (inductively coupled plasma mass spectrometry / manufactured by Shimadzu Corporation, model number: ICPS-8100 type), and the etch pit density b is obtained by revealing etch pits using a selective etchant. The entire area of the silicon substrate is photographed with a 200 × microscope field of view, and the number of etch pits is measured as a binarized image by a black and white image by performing image processing on the photographed image. Measurements were performed over the entire area of the silicon substrate, and the measured values were averaged.

  The result is shown in FIG.

  As shown in FIG. 4, the silicon substrate according to the example satisfies log (b) <0.5 log (a) −2.5, and the etch pit density b is suppressed in relation to the Fe concentration a. It was found that the silicon substrate according to the comparative example does not satisfy the relational expression. In addition, although Examples G, H, and I had the same amount of Fe as Comparative Examples A, B, and C, it was found that the etch pit density was extremely small.

<Production of solar cell>
The silicon substrate obtained as described above is cleaned by etching the damaged layer on its surface with NaOH, and the silicon substrate is placed in a diffusion furnace and heated in phosphorus oxychloride (POCl ₃ ). Thus, phosphorus atoms were diffused on the surface of the silicon substrate 1 to a concentration of 1e + 20 atoms / cm ³ to form an n-type reverse conductivity diffusion region.

  A silicon nitride film having a thickness of 850 mm serving as an antireflection film was formed thereon by plasma CVD. On the back side of the silicon substrate 1, an electrode material in which aluminum powder, an organic vehicle, and glass frit were pasted was applied to the entire back side by screen printing and dried. Then, in order to form an extraction electrode for taking out the output on the back side and a grid-like surface electrode on the front side, an electrode material made of silver powder, an organic vehicle and glass frit in a paste form is applied by screen printing. And dried. Thereafter, baking was performed at a maximum temperature of 800 ° C. for 15 minutes in a baking furnace, and a front electrode and a back electrode were simultaneously formed to form a solar cell.

  The solar cell conversion efficiency was measured as a characteristic of the solar cell formed as described above.

The results are shown in Table 1.

  As shown in Table 1, using the silicon substrate of the present invention, the conversion efficiency of Comparative Example A having the same Fe input amount is 15.21%, while the conversion efficiency of Example D is 16.28%. It was found that all the solar cells formed in this way have better characteristics than solar cells formed using conventional methods. In Examples G, H, and I, the Fe concentration a and the etch pit density b of the substrate satisfy log (b) <0.5 log (a) −2.5, and the amount of Fe is respectively It was found that the conversion efficiency was higher than those of Comparative Examples A, B, and C that were the same.

It is sectional drawing which shows the casting apparatus used for the manufacturing method of the silicon substrate for solar cells of this invention, (a) is a crucible, (b) is a figure which shows a casting_mold | template. It is a cross-sectional schematic diagram for demonstrating the manufacturing method of the silicon substrate for solar cells of this invention, especially the Marangoni convection in a casting_mold | template. It is a figure which shows the solar cell formed using the silicon substrate for solar cells of this invention, (a) is schematic sectional drawing, (b) is the top view seen from upper direction, (c) is the bottom view seen from the downward direction FIG. It is a figure which shows the relationship between the Fe density | concentration in a casting ingot, and an etch pit density about the Example regarding the silicon substrate for solar cells of this invention, and a comparative example. It is an expansion schematic diagram of the silicon crystal grain and crystal grain boundary vicinity in the solidification process of the conventional silicon melt.

Explanation of symbols

1: crucible 1a: melting crucible 1b: holding crucible 2: pouring port 3: heating means 4: silicon melt 5: mold 6: mold release material 7: mold heat insulating material 8: cooling plate 9: mold upper heating means 10: silicon melting Liquid Marangoni Convection 11: Central part 12: End part 21: Crystal grain 22: Grain boundary part 23: Fe concentrated region 31: Semiconductor substrate (silicon substrate)
32: Reverse conductivity type diffusion region 33: Antireflection film 34: Front surface electrode 34a: Front side bus bar electrode 34b: Front side finger electrode 35: Back side electrode 35a: Back side extraction electrode 35b: Back side current collecting electrode 36 : Back surface electric field area

Claims (2)

  A silicon substrate for a solar cell in which the Fe concentration a and the etch pit density b of the substrate satisfy log (b) <0.5 log (a) −2.5. A method for producing a silicon substrate for a solar cell, comprising cutting a silicon ingot formed by unidirectionally solidifying a silicon melt inside a mold,
The method for producing a silicon substrate for a solar cell, wherein a liquid surface temperature of the silicon melt differs by 20 ° C. or more between a central portion and an end portion in the unidirectional solidification process.

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