JP5131860B2 - Silicon sheet and solar cell - Google Patents

Description

  The present invention relates to a silicon sheet used for solar cells and the like.

  Silicon substrates (silicon sheets) used for manufacturing solar cells are disclosed in, for example, Japanese Patent Application Laid-Open No. 11-21120 (Patent Document 1) and Japanese Patent Application Laid-Open No. 11-92284 (Patent Document 2). Generally manufactured by a casting method. In the casting method, silicon melted in the crucible is gradually cooled from the bottom of the crucible to solidify the silicon melt. It is a manufacturing method.

  However, the casting method takes a long time of several tens of hours to manufacture one silicon ingot in order to grow without causing cracks in the ingot and from the viewpoint of ensuring semiconductor quality. In addition, a slicing process for cutting a silicon substrate from an ingot takes a long time, and even a slicing technique using a multi-wire saw requires several tens of hours. Therefore, it is difficult to significantly reduce the cost in manufacturing a silicon substrate using a casting method.

  On the other hand, as another silicon substrate manufacturing method, a silicon ribbon growth method using a web method or an EFG (edge-defined film-fed growth) method which does not require slicing has been studied. In recent years, the RGS (ribbon growth on substrate) method for producing a thin silicon ribbon directly from a silicon melt has been attracting attention with the aim of faster growth (Non-patent Document 1).

  The RGS method performs high-speed growth of a silicon ribbon by high-speed heat transfer (heat removal) from a surface close to a solidification growth surface. Specifically, the lower support plate supporting the open lower surface is moved relative to the side support frame supporting the periphery of the molten silicon in the lateral direction while cooling, so that the silicon is placed on the lower support plate. This is a method for growing a ribbon at a high speed. Hereinafter, a silicon ribbon creation method such as a web method, an EFG method, or an RGS method is referred to as a ribbon creation method.

  As another method for producing a silicon substrate, a method (sheet forming method) in which a base is brought into contact with a silicon melt and a sheet-like silicon substrate is directly obtained by solidification from a liquid phase is disclosed, for example, in JP-A-2001-2001. No. 223172 (Patent Document 3).

  The silicon substrate obtained by the ribbon forming method and the sheet forming method described above is a polycrystal, but in the polycrystal, (1) the diffusion rate of the substance is high at the grain boundary, and (2) the substance is solidified. Two phenomena are generally known that impurities are likely to segregate at the grain boundaries during (solidification).

  From this, when the impurity concentration is high, impurity segregation increases in the grain boundary portion. Therefore, it is considered that boron, which is an impurity, segregates at the grain boundary in the polycrystalline silicon substrate, and the resistance value of the grain boundary part is lower than that of the surrounding area.

  As a result, assuming a state where a part of the wafer (silicon sheet) in the solar cell module is shaded, in the reverse withstand voltage test, which is a test in which a current flows in the reverse direction, the pn junction layer on the light receiving surface side is Since it works so as not to conduct electricity, a current (short-circuit current) flows through a low resistance portion in the wafer surface. For this reason, the current flows concentrated on the grain boundary portion having the low resistance.

  In addition, according to Non-Patent Document 2, the specific resistance value of the current polycrystalline silicon sheet for solar cells is generally about 1 to 3 Ω · cm, which indicates that the solar cell characteristics are empirically improved by experiments. Theoretically, it is suggested that even if the specific resistance value exceeds 3 Ω · cm, the lifetime increases as the specific resistance increases to about 10 Ω · cm, and the solar cell characteristics are not greatly deteriorated.

  However, if the specific resistance exceeds 10 Ω · cm, the increase in lifetime will be small, and the loss of generated current will increase and the efficiency will deteriorate, so a wafer with a high specific resistance exceeding 10 Ω · cm is desirable. Absent.

Japanese Patent Laid-Open No. 11-21120 JP-A-11-92284 JP 2001-223172 A

26th PVSC, 1997, pp. 91-93 L. J. Geerlings et al., “Base Doping and Recombination Activity of Impurities in Crystalline Silicon Solar Cells”, PROGRESS IN PHOTOVOLTAICS: RESERCH AND APPLICATIONS Prog. Photovolt: Res.Appl., 2004, 12, pp309-316

  An object of the present invention is to impart good semiconductor characteristics to a silicon sheet used for solar cells and the like, and to further increase the efficiency of a solar cell using the silicon sheet.

  The present invention is a silicon sheet formed by bringing a contacted body into contact with a silicon melt and containing 0.04 to 0.2 ppmw of boron. Moreover, it is preferable to have an unevenness | corrugation in both or one main surface. Moreover, it is preferable that the uneven | corrugated period of both the said main surfaces is the same.

  Moreover, it is preferable that the thickness of the thinnest part is 100 micrometers or more. Moreover, it is preferable that the size of the unevenness is 200 μm or less. Moreover, it is preferable to have an average thickness in the range of 100 μm to 1 mm.

  The present invention further relates to a solar cell provided with the above silicon sheet.

  The silicon sheet of the present invention imparts good semiconductor characteristics in a silicon sheet used for solar cells and the like, and can further increase the efficiency of a solar cell using the silicon sheet.

It is typical sectional drawing parallel to the thickness direction of the silicon sheet by this invention. It is typical sectional drawing for demonstrating the basic procedure of an example of a sheet | seat formation method, (a) shows the growth stage of a silicon sheet, (b) shows the state from which a silicon sheet peels from a base | substrate. Yes. The current value in the reverse withstand voltage test of the silicon sheet of this invention and a prior art example is made into a graph. It is a graph showing the relationship between the boron concentration and specific resistance of the silicon sheet of the present invention and a conventional example. It is a flowchart which shows an example of the process of producing a solar cell using the silicon sheet by this invention. It is sectional drawing which shows typically an example of the solar cell produced using the silicon sheet by this invention. It is a perspective view which shows an example in case the silicon sheet of this invention has an unevenness | corrugation on both surfaces. It is typical sectional drawing for demonstrating the magnitude | size of the unevenness | corrugation of the silicon sheet of this invention, (a) is a fragmentary sectional view of the silicon sheet shown to (b), (b) is a silicon sheet which has an unevenness | corrugation on both surfaces Sectional drawing (c) is a sectional view of a silicon sheet having irregularities only on one surface. It is a typical perspective view which shows the surface shape of the base | substrate which can be used in order to produce the silicon sheet by this invention, (a) shows the base | substrate with which the periodic groove | channel was formed in the surface, (b) is periodic on the surface. The base | substrate in which the typical pyramid-like unevenness | corrugation was formed is shown.

(First embodiment)
Embodiments according to the present invention will be described with reference to FIGS. 1 to 7 and Tables 1 and 2. Here, in this embodiment, a silicon sheet created by a method (sheet forming method) in which a substrate is brought into contact with a silicon melt and solidified from a liquid phase to obtain a sheet-like silicon substrate will be described. Silicon having the characteristics described below, even in other manufacturing methods such as a web method, an EFG (edge-defined film-fed growth) method, and a ribbon production method such as an RGS (ribbon growth on substrate) method The method is not particularly limited as long as the sheet is obtained. In the present embodiment, the “substrate” is used as the “contacted body”. However, the “contacted body” used in the present invention is a “rotating body” in the Web method, and “underside support” in the RGS method. “Flat plate”, “substrate” in the case of a sheet forming method, and the like, which are all formed by dipping and contacting a silicon melt to form a silicon sheet on the surface.

  FIG. 1 is a cross-sectional view of a polycrystalline silicon sheet obtained by a ribbon forming method or a sheet forming method. The polycrystalline silicon sheet 1 has a grain boundary 11 from a light receiving surface side surface 12 to a back surface side surface 13. . As described above, impurities are likely to segregate at the grain boundaries 11, and current flows in a concentrated manner at the grain boundary portions having low resistance in the reverse breakdown voltage test.

(Production method)
Next, a method for producing the silicon sheet of the present invention by the sheet forming method will be described with reference to FIG. As shown in the schematic cross-sectional view of FIG. 2A, the surface of the heat-resistant substrate 2 temperature-controlled by the temperature control means 5 capable of heating and cooling to a temperature lower than the melting point of silicon, 1415 ° C. Is brought into contact with (or immersed in) the silicon melt 3 in the crucible 4 to grow the silicon sheet 1 on the surface of the substrate 2. After the silicon sheet 1 having a necessary thickness is grown, the substrate 2 to which the sheet is attached is taken out from the crucible 4. When the silicon sheet 1 attached to the substrate 2 is cooled from a high temperature, the substrate 2 and the silicon sheet are caused by the difference in thermal expansion coefficient between the substrate 2 and the silicon sheet 1 as shown in FIG. 1 is separated naturally or by applying a small impact to the substrate 2 to obtain a silicon sheet 1 formed directly by solidification from the liquid phase.

  That is, since the substrate 2 is at a temperature lower than the temperature of the silicon melt 3, silicon crystal nuclei are generated everywhere on the surface of the substrate. The crystal nuclei grow in one direction toward the direction in contact with the silicon melt 3 to form a polycrystalline silicon sheet.

  In the polycrystalline silicon sheet, the larger the average crystal grain size, the lower the grain boundary density, which causes a decrease in semiconductor characteristics, and the carrier diffusion length is increased, thereby improving the semiconductor characteristics of the silicon sheet. By this improvement effect, the silicon sheet directly formed by solidification from the liquid phase can be used for devices such as solar cells.

  Moreover, as a material of the base | substrate 2 used for the said sheet | seat formation method, the base | substrate which formed graphite and the silicon carbide on the surface by the thermal CVD method can be used, for example. Ceramics, heat resistant metals that can withstand high temperatures, carbons that are partially or wholly coated with ceramics, ceramics, or heat resistant metals can also be used.

  Further, as the temperature control means 5 of the base 2, for example, a gas refrigerant that provides a space near the surface of the base 2 opposite to the surface in contact with the silicon melt and introduces nitrogen, argon, air, or the like under pressure. In addition, various means such as a liquid refrigerant system including a temperature control means 5 for controlling the temperature by embedding a metal pipe such as stainless steel or copper in the base 2 can be adopted. Thus, the polycrystalline silicon sheet 1 can be formed on the surface of the substrate 2 at high speed and stably, but the temperature control means 5 may not be provided in the sheet forming method.

  Further, the temperature of the silicon melt at the time of manufacturing the silicon sheet is usually within a range from a supercooling temperature of 1380 ° C. or higher to a higher temperature of 1600 ° C. (for example, 1450 ° C.) depending on the balance with the growth conditions of the sheet. ). After the silicon melt surface reaches a specified height, the temperature of the substrate 2 is controlled by the temperature control means 5, and the surface temperature of the substrate 2 is lower than the melting point of silicon by 1000 ° C. to 120 ° C. (for example, 1200). The surface may be immersed in the silicon melt in a state stabilized to (° C.).

  As described above, in the silicon sheet according to the present invention, the initial temperature of the substrate 2 is controlled in a temperature range lower by about 120 ° C. to 1000 ° C. than the silicon melting point (1415 ° C.), or by using a graphite material having an appropriate thickness. The heat capacity of the substrate 2 is appropriately set, or a temperature control means 5 for heating and cooling the substrate 2 is used with a refrigerant, or the immersion time of the substrate 2 in the silicon melt 3 is controlled so as to obtain a silicon sheet having an optimum thickness. By setting the basic conditions such as promoting the solidification of the silicon solution, a polycrystalline silicon sheet can be formed on the surface of the substrate 2 at high speed and stably.

(Example 1 and Comparative Example 1)
Next, the characteristics of the silicon sheet according to the present invention will be described with reference to Tables 1 and 2 and FIGS. Table 1 shows a silicon sheet (Example 1) prepared by a sheet forming method with a thickness of 324 μm having a boron concentration of 0.05 ppmw and a specific resistance of 10 Ω · cm, and a boron concentration of 0.15 ppmw and a specific resistance of 2 Ω · cm. 3 is a table comparing current values in a reverse withstand voltage test of a silicon sheet (Comparative Example 1) prepared by a sheet forming method having a thickness of 336 μm, and FIG. 3 is a graph showing the results.

(Reverse pressure test)
In Table 1, the current value (reverse current value) when a reverse voltage of 1 to 15 V is applied to the silicon sheets of Example 1 and Comparative Example 1 in the thickness direction of the sheet is shown. In addition, the reverse direction is a direction opposite to the direction (forward direction) in which light is generated by receiving light when a silicon sheet is used as a solar cell, and the current value was measured by an ammeter.

  From the results shown in Table 1 and FIG. 3, in the silicon sheet of Example 1 (specific resistance 10Ω), the current flows as compared with the silicon sheet of Comparative Example 1 (specific resistance 2Ω) even when the reverse voltage is increased. It can be seen that a silicon sheet having a small amount and a high specific resistance is less likely to cause a short-circuit current. Here, the reverse withstand voltage test is evaluated by the heat generation amount. However, if the resistance value of the wafer is doubled, the current amount is ½, and the effect of suppressing the heat generation is doubled. When a solar cell is produced using this silicon sheet, a solar cell having good characteristics can be obtained.

(Comparison of specific resistance)
Next, the relationship between boron concentration and specific resistance will be described with reference to Table 2 and FIG. Table 2 compares the relationship between the boron concentration and the specific resistance of a silicon sheet obtained by a casting method as an example of a conventional silicon sheet and a silicon sheet obtained by the sheet forming method. FIG. 4 is a graph of the results in Table 2. From this, it can be seen that the specific resistance of the silicon sheets prepared by the casting method and the sheet forming method tends to increase as the boron concentration decreases.

  However, the relationship between the boron concentration and the specific resistance shows different characteristics between the silicon sheet obtained by the casting method and the silicon sheet obtained by the ribbon making method and the sheet forming method, and silicon obtained by a general casting method. Compared with the specific resistance value of the sheet, it can be seen that the silicon sheet produced by the sheet forming method containing the same concentration of boron has a much higher specific resistance.

  The reason why the silicon sheet manufactured by the sheet forming method as described above exhibits a high specific resistance due to factors other than the boron concentration is not clear, but the sheet forming method has a temperature change in the manufacturing process compared to the casting method or the like. It is presumed that this has an influence on the crystal grain size and the like, and that some difference in crystal structure occurs.

  In addition, it can be seen from FIG. 4 that the increase in the specific resistance of the silicon sheet prepared by the sheet forming method increases rapidly from the boron concentration around 0.2 ppmw. As described above, the reverse withstand voltage test is evaluated by the calorific value, and if the specific resistance value increases, the current value decreases proportionally. Therefore, if the specific resistance increases rapidly, the current value decreases rapidly, causing a short circuit. A sheet that is less likely to generate current is obtained.

  In addition, since it is difficult to produce a solar cell having good characteristics with a silicon sheet having a specific resistance of 10 Ω · cm or more from Non-Patent Document 2, a silicon sheet produced by the sheet forming method contains a boron concentration of 0.04 ppm or more. It is necessary to have been.

  This is because if the boron concentration is less than 0.04 ppm, the specific resistance of the entire silicon sheet increases, the lifetime increases, the loss of the generated current increases, and the efficiency decreases.

  As described above, the silicon sheet of the present invention is a polycrystalline silicon sheet containing 0.04 to 0.2 ppmw of boron, but the boron concentration in the silicon sheet may be 0.04 to 0.1 ppmw. It is preferable because the specific resistance value is large, and it is more preferable that the boron concentration is 0.04 to 0.07 ppmw because the specific resistance value increases more rapidly.

(Solar cell)
As an example of a method for producing a solar cell using the silicon sheet of the present invention, a method according to the procedure shown in the flowchart of FIG. 5 can be given. In this embodiment, the silicon sheet is a p-type semiconductor, but it may be an n-type semiconductor. When forming a p-type or n-type silicon sheet, it is desirable to mix a dopant such as boron (B) or phosphorus (P) when the raw silicon is melted.

  In the flow chart of FIG. 5, first, in steps Sl and S2, the silicon sheet was cleaned and the surface was etched using a mixed solution of nitric acid and hydrofluoric acid. In subsequent step S3, texture etching was performed on the light incident side main surface of the sheet using sodium hydroxide. As this etching, a dry etching method using plasma discharge or the like can be used, but by using a wet etching method, a surface texture can be formed at a lower cost.

  Next, in step S4, an n-type diffusion layer was formed by PSG diffusion (diffusion method using a phosphorus silicate glass film). In step S5, the PSG film formed on the surface was removed with hydrofluoric acid, and then a silicon nitride film was formed as an antireflection film on the light receiving surface side main surface.

  Next, in step S6, the diffusion layer formed on the back side was removed using a mixed solution of nitric acid and hydrofluoric acid. In step S7, an alloy layer and a back electrode were simultaneously formed on the back side using Al paste. Finally, in step S8, the electrode on the light receiving surface side was formed by screen printing of a silver paste material.

  In this way, a solar battery cell as shown in the schematic cross-sectional view of FIG. 6 is produced. The solar battery cell of FIG. 6 is formed including a silicon sheet 6, a diffusion layer 61, a photoelectric conversion layer 62, an alloy layer 63, a front electrode 64, and a back electrode 65.

(Second Embodiment)
The shape of the silicon sheet of the present embodiment is characterized in that both or one of the main surfaces has irregularities, which will be described with reference to FIGS.

  In FIG. 7, grooves along the rotation direction of the base body 8 having the base top portion 81 and the base bottom portion 82, or fine uneven surfaces arranged regularly or irregularly are formed on the surface of the base body 8. Such grooves and fine irregular surfaces formed on the surface of the substrate 8 have a function of speeding up the growth of the silicon sheet 7.

  When the substrate 8 having such a shape is brought into contact with or immersed in the silicon melt, the grooves and irregularities of the substrate 8 are likely to be the starting point of silicon crystal growth. Has an uneven shape. Here, by determining the groove and uneven distribution of the substrate 8, the uneven distribution and groove interval of the silicon sheet 7 can be determined, and the silicon sheet 7 can form a shape with a certain degree of regularity. It is. In FIG. 7, a base having periodic grooves formed on the surface as shown in FIG. 9A is used. However, periodic pyramidal irregularities are formed on the surface as shown in FIG. 9B. Even if the base is used, a silicon sheet having irregularities can be obtained.

  Here, having both the or one main surface has irregularities means having a height difference (undulation) in the thickness direction on both or one of the two main surfaces excluding the edge surface of the silicon sheet. For example, a sheet having irregular periodicities such as a groove shape and a pyramid shape can be mentioned, but the periodicity does not mean only a sheet having periodicity uniformly over the entire sheet, but a part of the sheet. It only needs to have periodicity, and does not require strict periodicity that is artificially designed. Further, irregularities with irregular cycles may be used. Moreover, the shape which has only a recessed part or a convex part may be sufficient.

  Next, the uneven shape of the silicon sheet 7 will be described in detail with reference to FIG. The silicon sheet of the present invention is particularly effective when both main surfaces have unevenness, and furthermore, when the unevenness periods of both the main surfaces are the same as shown in FIGS. 8 (a) and 8 (b). It is even more effective. Here, the same period of concaves and convexes on both main surfaces means a state in which the period in which concave and convex portions appear alternately on the main surface is substantially the same on both sides.

  When both main surfaces have unevenness, a portion with a thin sheet thickness is likely to be formed locally. Particularly, when the unevenness period of both main surfaces is the same, a portion with a locally thin sheet thickness (both This is because there is a high possibility that a short-circuit current in the thickness direction at that portion will occur because there are a large number of portions where the main surface becomes concave portions.

  Further, the size of the unevenness is a height from the reference surface to the top of the convex portion on one main surface in FIG. 8 (a) which is a partial sectional view of a silicon sheet having unevenness on both sides as shown in FIG. 8 (b). The height difference (c) or (c ′) between the height (a) or (a ′) and the height (b) from the reference surface to the bottom of the concave portion adjacent to the convex portion. The reference surface may be an average value of the heights of both main surfaces, or may be an average value of the heights of the bottoms of the recesses of both main surfaces, and the reference surface is relative to the thickness direction of the silicon sheet. Any plane can be used as long as it is a vertical plane.

  8 (a) and 8 (b) show a sheet having irregularities on both main surfaces. However, a silicon sheet having irregularities only on the upper surface and a lower surface as shown in FIG. The size of the unevenness on the upper surface is defined in the same manner as described above. Here, the plane is not limited to a plane in a strict sense, and includes a plane having irregularities with an error of less than 10 μm.

  Further, the silicon sheet of the present invention is the sum of the distances from the reference surface of the first main surface side bottom portion 72 and the second main surface side bottom portion 74 of the same period in FIG. It is preferable that the minimum value of e (the thickness of the thinnest portion of the silicon sheet) which is between the main surface side bottom portion 72 and the second main surface side bottom portion 74 is 100 μm or more. This is because in such a case, a locally thin portion is hardly formed and a short-circuit current is hardly generated.

  If the size of the unevenness is too large, inconvenience is caused in application to a solar cell or the like. Therefore, the size of the surface unevenness of the silicon sheet 7 is preferably 200 μm or less. In this case, a process such as slicing or polishing is performed. The sheet can be used for a solar cell or the like without passing, and the surface etching time can be shortened or the surface etching can be omitted. In addition, since the diffusion length of the carrier in the silicon sheet is 30 μm or more, a solar cell with relatively good conversion efficiency can be obtained.

  The silicon purity of the silicon sheet of the present invention is preferably 5 or more, and in this case, a good device characteristic value can be obtained even when used for a solar cell or the like. From the viewpoint of the characteristics of the solar cell, it is more preferable that the purity of the silicon sheet is 7 or more.

  The average thickness of the silicon sheet is preferably set in the range of 100 μm to 1 mm. By setting the average thickness of the silicon sheet to 100 μm or more, high handling properties can be obtained in the solar cell manufacturing process using the sheet. Further, by making the sheet thickness 1 mm or less, the sheet manufacturing time can be shortened, and a low-cost silicon substrate can be provided.

  From the viewpoint of ease of sheet production, it is more preferable that the average thickness of the sheet is in the range of 200 to 600 μm. The average thickness of the silicon sheet is the average value of the sheet thickness at each point in the plane of the silicon sheet, and is the average value of the sheet thickness including the thickness of the convex portion and the thickness of the concave portion. In the manufacturing method of the present invention, by directly manufacturing a silicon sheet having such a thickness from a silicon melt, a silicon ingot slicing step or the like as in the case of the casting method becomes unnecessary, and good semiconductor characteristics are obtained. be able to.

  According to the present invention, a silicon sheet can be efficiently produced without slicing the silicon ingot, and the specific resistance can be set to an appropriate value for use in solar cells and the like. As a result, it is possible to prevent occurrence of short-circuit current in the thickness direction in the portion where the sheet is easily short-circuited without deteriorating the characteristics required for applications such as solar cells. In this case, it is possible to provide a highly efficient product such as a solar cell that avoids power loss in some cases.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  1,6,7 silicon sheet, 11 grain boundary, 12 light-receiving surface side surface, 13 back surface side, 2,8 substrate, 3 silicon melt, 4 crucible, 5 temperature control means, 61 diffusion layer, 62 photoelectric conversion layer, 63 Alloy layer, 64 Front electrode, 65 Back electrode, 71 First main surface side top of silicon sheet, 72 First main surface side bottom of silicon sheet, 73 Second main surface side top of silicon sheet, 74 Silicon The second main surface side bottom of the sheet, 81 base top, 82 base bottom.

Claims (7)

  A silicon sheet formed by bringing a contacted body into contact with a silicon melt and containing 0.04 to 0.2 ppmw of boron.   The silicon sheet according to claim 1, wherein both or one main surface has irregularities.   The silicon sheet according to claim 2, wherein the concave and convex periods of both main surfaces are the same.   The silicon sheet in any one of Claims 2-3 whose thickness of the thinnest location is 100 micrometers or more.   The silicon sheet in any one of Claims 2-4 whose magnitude | size of the said unevenness | corrugation is 200 micrometers or less.   The silicon sheet according to claim 2, which has an average thickness within a range of 100 μm to 1 mm.   The solar cell provided with the silicon sheet in any one of Claims 1-6.

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