Patent Publication Number: US-2010116339-A1

Title: Method for producing solid phase sheet and photovaltaic cell employing solid phase sheet

Description:
TECHNICAL FIELD 
     The present invention relates to a method for producing a solid phase sheet by deposition of a semiconductor melt on a surface of a base body, and a photovoltaic cell employing the solid phase sheet. 
     BACKGROUND ART 
     A typical method for producing a solid phase sheet such as a polycrystalline silicon wafer employed for a photovoltaic cell includes the steps of forming an ingot made of a semiconductor material and slicing the ingot thin. However, slicing requires a highly advanced technique such that the semiconductor material will not be lost more than the amount corresponding to the thickness of a cutting wire or a circular blade used in slicing. 
     Instead, a method for producing a solid phase sheet as described in Patent Document 1 (Japanese Patent Laying-Open No. 2002-237465) can be employed. The method for producing a solid phase sheet as described in Patent Document 1 is performed by the following steps. First, base bodies each having a surface sectioned by a circumferential groove into a peripheral section and an inner section surrounded by the circumferential groove are dipped consecutively at regular intervals in an inert atmosphere into a crucible in which a semiconductor material having been heated and melted is stored. The semiconductor material is a high-purity silicon material doped with a dopant such as phosphorus or boron. A solid phase sheet made of the semiconductor material is formed on the surface of the inner section of a base body. The formed solid phase sheet is then separated from the base body and cut into desired dimensions by a laser, dicer or the like according to applications to provide a product such as a wafer for photovoltaic cell. 
     The above-mentioned method for producing a solid phase sheet employing a base body allows efficient production of a flat solid phase sheet with high dimensional accuracy directly from a melt of semiconductor material while reducing material losses. However, this method causes a state with what is called “a residual melt” where a melt of semiconductor material condenses by surface tension along a trailing end in the moving direction on the solid phase sheet formed in the inner section when the base body is moved and taken out from the melt of semiconductor material in which the base body has been dipped. This residual melt is formed substantially linearly along the trailing end. 
     The residual melt has a larger heat capacity than and a different rate of solidification from an area of the solid phase sheet surrounding the residual melt. Accordingly, the melt of semiconductor material undergoes solidification expansion at the time of solidification into solid semiconductor, by which its volume is larger in the solid state than in the melted state. When the residual melt starts solidification, the area of the solid phase sheet surrounding the residual melt has already been solidified. Delayed solidification expansion of the residual melt will cause cracks in its surrounding area of the solid phase sheet. It has been pointed out that this disadvantageously causes defects in the solid phase sheet formed in the inner section qualified as a product, surrounded by the peripheral section, resulting in degraded yield and increased product cost. 
     Patent Document 1: Japanese Patent Laying-Open No. 2002-237465 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     In light of the foregoing problems, the present invention provides a method for producing a uniform and defect-free solid phase sheet while preventing cracks from occurring in the solid phase sheet portion that may be caused by the above-mentioned residual melt. The invention also provides a method for producing a solid phase sheet with improved yield. The invention further provides an inexpensive photovoltaic cell employing the solid phase sheet with improved yield. 
     Means for Solving the Problems  
     The present invention relates to a method for producing a solid phase sheet of a semiconductor material on a surface of a base body by bringing the base body into contact with a melt of semiconductor material. The surface of the base body is sectioned by a circumferential groove into a peripheral section and an inner section surrounded by the circumferential groove or has the peripheral section and the inner section partly connected to each other. A slit is provided in a surface of the inner section. 
     According to the present invention, the slit is preferably provided at a trailing side in a moving direction in which the surface of the base body is brought into contact with the melt of the semiconductor material. It is also preferable that a plurality of slits extend from the circumferential groove into the inner section in the moving direction in which the surface of the base body is brought into contact with the melt of the semiconductor material. 
     Further, according to the present invention, the slit preferably has a width of not less than 1 mm and not more than 5 mm. The slit preferably has a length of not less than 5 mm and not more than 20 mm. The slit preferably has a depth of not less than 1 mm and not more than the thickness of the base body. 
     Furthermore, according to the present invention, a portion divided by the slit preferably has a width of not less than 5 mm and not more than 50 mm. The slit preferably has an end with a curvature, the end being opposite to a side that crosses the circumferential groove. 
     A method for producing according to the present invention relates to a method for producing a solid phase sheet employing, as a product, a solid phase sheet from which a region provided with the slit has been cut away. 
     A photovoltaic cell according to the present invention relates to a photovoltaic cell employing, as a product, the solid phase sheet formed by the method for producing the solid phase sheet. 
     Effects of the Invention  
     The present invention can provide a method by which a flat and uniform solid phase sheet may be produced directly from a melt of semiconductor material with improved yield while reducing losses of the semiconductor material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an embodiment of a base body according to the present invention provided with slits in an inner section surrounded by a circumferential groove. 
         FIG. 2  is a schematic perspective view showing an embodiment in which the base body according to the present invention provided with slits in an inner section surrounded by a circumferential groove is dipped into a melt of semiconductor material for forming a solid phase sheet. 
         FIG. 3  is an enlarged cross sectional view showing an embodiment of a base body according to the present invention provided with slits in an inner section surrounded by a circumferential groove. 
         FIG. 4  is a plan view showing an embodiment of a base body according to the present invention provided with slits in an inner section surrounded by a circumferential groove. 
         FIG. 5  is a schematic cross sectional view showing an example in a method for producing a solid phase sheet according to the present invention. 
         FIG. 6  is a perspective view showing an embodiment of a base body according to the present invention provided with slits in an inner section surrounded by a circumferential groove. 
     
    
    
     DESCRIPTION OF THE REFERENCE SIGNS 
       101 ,  201 ,  301 ,  401 ,  505 ,  601  base body;  102 ,  403 ,  602  circumferential groove;  103 ,  207 ,  302 ,  402 ,  603  slit;  104 ,  206  residual melt;  202  dipping mechanism arm section;  203  melt;  204 ,  501  crucible;  105 ,  205 ,  303 ,  507  solid phase sheet;  404 ,  604  slit end;  502  chamber;  503  silicon-melting heater;  504  dipping mechanism;  506  silicon melt;  508  transportation conveyor;  509  device subsidiary chamber;  510  gate valve; S 11  surface; S 21  direction of arrow; S 61  inner section 
     BEST MODES FOR CARRYING OUT THE INVENTION 
     &lt;Basic Structure of Base Body&gt; 
     A method for producing a solid phase sheet according to the present invention employs an improved base body to be employed in a method for producing a solid phase sheet by bringing the base body into contact with a melt of semiconductor material to cause uniform deposition of the semiconductor material on the surface of the base body. Means used for bringing the base body into contact with the melt of semiconductor material may take the form of bringing the base body into contact with the melt of semiconductor material in a crucible, or moving the crucible storing the melt of semiconductor material with the base body fixed. The base body employed is one having a surface sectioned by a circumferential groove into a peripheral section and an inner section surrounded by the circumferential groove as mentioned above, or one having the peripheral section and the inner section partly connected to each other. In the case of producing a solid phase sheet in an embodiment shown in  FIG. 2  as will be described later, the solid phase sheet will be formed on the back side of the base body. In this case, it is preferable to employ the one having the peripheral section and the inner section partly connected to each other in order for preventing the solid phase sheet from falling down. A base body employed is provided with slits in the surface of the inner section. It is particularly preferable to provide a plurality of such slits extending linearly in a direction perpendicular to the circumferential groove. However, any slits that make an angle of 45 to 90° with respect to the circumferential groove may be employed without any problem. The slits may be corrugated, for example, instead of being linear. Preferably, slits are formed at one side corresponding to the trailing side in the moving direction in which the base body is brought into contact with a melt of semiconductor material. The moving speed when the base body is brought into contact with the melt of semiconductor material is preferably 300 to 1000 cm/min. Although the material for the base body itself is not particularly limited, one having a high heat resistance is preferable considering that the melt of semiconductor material is 1420 to 1500° C. A material of good thermal conductivity is also preferable. High-purity graphite is more preferable in terms of ease of processing. In terms of preventing adhesion and cohesion of a semiconductor material to and on the surface of the base body to reduce the residual melt, graphite is preferably employed as the material for the base body. In addition, the base body may also be made of, for example, silicon carbide, quartz, boron nitride, alumina, and zirconium oxide. 
     Specific description will be made below based on  FIG. 1  showing an embodiment of a base body provided with slits in an inner section surrounded by a circumferential groove. A base body  101  from which a solid phase sheet  105  is to be produced is characterized in that slits  103  are formed in the inner section of a surface  511  of base body  101  divided by a circumferential groove  102 . When base body  101  is brought into contact with a melt of semiconductor material, a state with a residual melt  104  occurs where the melt of semiconductor material condenses by surface tension at a trailing side in the moving direction of the solid phase sheet formed in the inner section. By providing slits  103  in the inner section of base body  101 , solid phase sheet  105  (hatched portion) formed in the inner section of base body  101  is divided by slits  103  in a region to be located at the trailing side in the moving direction of base body  101 , so that the residual melt is separated. 
     Herein, residual melt  104  has reduced volume as compared to a residual melt caused by a conventional production method. Residual melt  104  has a larger heat capacitance as compared to its surrounding area and a lower cooling speed as compared to an area free from residual melt  104 . A possible consideration is that cracks are more likely to occur in produced solid state sheet  105  due to this difference in cooling speed. However, slits  103  partly dividing solid state sheet  105  have not only the effect of separating residual melt  104  but also the effect of relieving stress towards the solid state sheet due to solidification expansion of residual melt  104 . This can prevent cracks from extending into the inner section, and allows a defect-free solid phase sheet of 200 to 400 μm thick to be provided as a product. 
     A melt of semiconductor material generally has a high viscosity, and hence a higher surface tension as compared to water or the like. It is therefore preferable for separating the residual melt to set conditions for the width, length and depth of each slit as well as the width of a portion divided by slits. The condition setting will be described below in detail. 
     &lt;Width of Slit&gt; 
     The above-mentioned slits for separating the residual melt on the solid phase sheet have an appropriate width. If the slits are excessively narrow in width, the solid phase sheet will be deposited in a bridge-like form over the slits due to surface tension of the melt of semiconductor material, which prevents the essential object of separating the residual melt from being achieved. If the slits are excessively large in width, the melt of semiconductor material will enter the slits to be filling in the respective slits upon solidification in the slits. This interferes with heat contraction of the solid phase sheet, which in turn causes cracks in the solid phase sheet portion formed in the inner section of the base body. 
     Description will be made below based on  FIG. 2  schematically showing an embodiment of forming a solid phase sheet with a melt of semiconductor material. A base body  201  is provided on a leading edge of a dipping mechanism arm section  202 , By dipping base body  201  in a direction of arrow S 21  into a crucible  204  in which a melt  203  of semiconductor material is stored, a solid phase sheet  205  (hatched portion) is formed on the surface of base body  201 . Herein, melt  203  of semiconductor material has viscosity. Accordingly, when a slit  207  has a width of less than 1 mm, solid phase sheet  205  may start to be deposited in a manner spanning over slits  207 , which prevents the object of dividing the trailing end of solid phase sheet  205  from being achieved. When slit  207  has a width of more than 5 mm, melt  203  of semiconductor material may start to enter slit  207  to fill in slit  207 , which prevents the object of dividing the trailing end of solid phase sheet  205  from being achieved. Thus, the result of experiments shows that slit  207  provided in the aforementioned inner section preferably has a width of not less than 1 mm and not more than 5 mm. To divide the trailing end of solid phase sheet  205  stably by slit  207 , it is particularly preferable to set the width of the slit in the vicinity of 3 mm which is a median value between 1 mm at which melt  203  of semiconductor material starts to span over slits  207  and 5 mm at which the melt of semiconductor material starts to enter slit  207 . 
     &lt;Length of Slit&gt; 
     In  FIG. 2 , melt  203  of semiconductor material left on the surface of the trailing part of base body  201  when base body  201  is pulled out from melt  203  of semiconductor material condenses by surface tension, so that a residual melt  206  is formed at the trailing part of divided solid phase sheet  205 . Slit  207  provided in the inner section of base body  201  sectioned by the circumferential groove needs to have a length longer than an average diameter of residual melt  206  on the solid phase sheet on the inner section of base body  201 . Herein, as to determination of the size of residual melt  206 , it is known that a circular residual melt has a diameter of at least 5 mm and about 20 mm at most, many ranging from approximately 7 to 8 mm, depending on the surface tension of the melt of semiconductor material. This shows that slit  207  provided in base body  201  needs to have a length of at least 5 mm or more. According to the result of experiments, the largest diameter of residual melt  206  never exceeds 20 mm. Herein, the region of the solid phase sheet provided with the slits will be removed to produce a product, as will be described later. Therefore, the longer the length of slit  207  than necessary, the smaller the area of solid phase sheet  205  that can be utilized. Thus, taking the foregoing into account, slit  207  provided in the inner section of base body  201  preferably has a length of not less than 5 mm and not more than 20 mm. In consideration of the effective area of solid phase sheet  205  qualified as a product and the dimensions of residual melt  206 , a length of about 10 mm is particularly preferable. 
     &lt;Depth of Slit&gt; 
     Specific description will now be made based on  FIG. 3  showing an enlarged cross sectional view of slits provided in the inner section of the base body. If a slit  302  provided in the inner section of a base body  301  has a depth of less than 1 mm, a melt of semiconductor material in slit  302  will contact the bottom of slit  302 , where a solid phase sheet  303  is deposited. The deposition of solid phase sheet  303  within slit  302  prevents the object of dividing solid phase sheet  303  on the inner section of base body  301  by slit  302  from being achieved. Thus, the stress caused by the residual melt formed between slits  302  in the inner section of base body  301  cannot be relieved, so that solid phase sheet  303  is more likely to be damaged, resulting in degraded yield. Therefore, slit  302  provided in the inner section of the base body sectioned by the circumferential groove preferably has a depth of not less than 1 mm such that the melt of semiconductor material will not be deposited within slit  302 . Further, slit  302  cutting through base body  301  presents no particular problem in production of a solid phase sheet. It is therefore preferable to set the depth of slit  302  at not less than 1 mm and not more than the thickness of base body  301 . Although slit  302  having a depth of not less than 1 mm can separate the melt of semiconductor material, a depth of not less than 3 mm is particularly preferable for slit  302  because the depth of not less than 3 mm enables reliable separation. 
     &lt;Width of Portion Divided by Slits&gt; 
     Description will be made below based on  FIG. 1 . The width of each portion divided by slits  103  located at the inner section of base body  101  is also relevant to the size of residual melt  104  that will remain thereat. Since residual melt  104  formed measures at least 5 mm and about 20 mm at most as previously mentioned, each portion divided by slits  103  needs to have a width of at least 5 mm or more. The result of experiments shows that, when the width of each portion divided by slits  103  exceeds 50 mm, cracks are more likely to occur in an area of solid phase sheet  105  surrounding the residual melt due to heat contraction of the residual melt. That is, the effect of division by slits  103  is reduced when the width of each portion divided by slits  103  exceeds 50 mm, so that the influence of stress due to the different rate of solidification of residual melt  104  spreads across solid phase sheet  105 , resulting in defects such as breakage and degraded product yield. Therefore, the portion divided by slits  103  preferably has a width of not less than 5 mm and not more than 50 mm. Considering that the size of residual melt  104  mostly ranges from about 7 to 8 mm as mentioned earlier, and that the influence of stress should be minimized, it is particularly preferable that the portion divided by slits  103  provided in the inner section of base body  101  have a width of about 10 mm. 
     &lt;Shape of Slit End&gt; 
     Description will now be made based on  FIG. 4  showing a top view of a base body. A residual melt will be formed at a portion divided by slits  402  provided in a base body  401 . The different rate of solidification when this residual melt solidifies causes stress to be concentrated on each slit end  404  opposite to the side that crosses a circumferential groove  403  of base body  401 , from which cracks are likely to extend into the solid phase sheet. At this stage, if slit end  404  has a corner, stress concentrates further on that part, making the solid phase sheet more likely to break down. Accordingly, providing slit end  404  with a curvature can avoid stress concentration on one point, leading to improved product yield. The smaller the curvature at this stage, the more smooth the transition from the linear segment to the curved segment. A curvature of ½ the width of slit  402  is particularly preferable since the stress is minimized when the curvature is a semicircle. That curvature is also particularly preferable in terms of processing. 
     &lt;Product (Solid Phase Sheet) and Photovoltaic Cell&gt; 
     The solid phase sheet produced by the above-described method of the present invention can be employed as a wafer for photovoltaic cell as it is uniform without defects such as cracks. The method for producing a photovoltaic cell in this case includes forming a doped layer, an anti-reflective coating, a photoreceiving electrode, a rear face electrode, and the like by a well-known method for producing a photovoltaic cell, as well as cutting a solid phase sheet into desired dimensions for use as a wafer for photovoltaic cell. The photovoltaic cell of the present invention can provide increased production yield as compared to a photovoltaic cell produced employing a solid phase sheet produced from a conventional base body. 
     Herein, in the case of producing a product, in particular a photovoltaic cell, from a solid phase sheet having a region provided with slits (hereinafter also referred to as a slit region) on the solid phase sheet, it is preferable to cut off and remove the slit region. The reason will be described below based on the case of employing the solid phase sheet for a wafer for photovoltaic cell as an example. 
     Bringing a base body into contact with a melt of high-purity silicon material doped with a dopant such as phosphorus or boron forms a residual melt in the slit region where the wafer for photovoltaic cell is divided by slits provided in the inner section of the base body. Since the residual melt is in the state where the melt of semiconductor material condenses by surface tension to project from its surrounding planar area, there exist residual stress and defective shape. This causes a problem such as damages in the process of producing a photovoltaic cell. Accordingly, in order to use the solid phase sheet including the slit region as a product, specialized processing needs to be performed according to residual melt, leading to increased cost. Further, considering the conversion efficiency in a module as an assembly of wafers for photovoltaic cells employing the slit region as a photoreceiving face, it is assumed that the conversion efficiency is reduced and that the number of wafers for photovoltaic cells necessary for obtaining rated outputs increases. In this case, a photovoltaic cell unnecessarily increases in weight and size, resulting in increased cost in total. Accordingly, cutting off the slit region to produce a product can avoid the above problems. Further, the slit region of the solid phase sheet that cannot be used in a product can be reutilized by remelting after being cut off and removed, which in turn provides improved yield. 
     In the following, the present invention will be described in more detail illustrating examples, however, the present invention is not limited to these examples. 
     EXAMPLE 
     Description will be made referring to  FIG. 5  showing a method for producing a solid phase sheet employed in an embodiment of the present invention. A silicon raw material with the boron concentration adjusted such that a solid phase sheet to be obtained had a specific resistance of 2 Ω·cm was fed into a crucible  501  made of high-purity graphite, which was placed in a chamber  502 . Chamber  502  was then evacuated. Thereafter, an argon gas was introduced, and was kept introduced from above chamber  502  always at 100 L/min while maintaining 800 hPa. 
     A thermoelectric couple for controlling a silicon-melting heater  503  located around crucible  501  was set at a set temperature of 1500° C., to bring silicon into a completely melted state to obtain a silicon melt  506 . Since silicon fed initially decreased in volume by melting, silicon was supplied additionally, to thereby bring the surface of silicon melt  506  at a predetermined level. The control temperature was then set at 1430° C. and was kept for 30 minutes to stabilize the temperature of silicon melt  506 . A base body  505  made of high-purity graphite on which a solid phase sheet was to be formed was then set at the leading edge of a dipping mechanism  504 . 
     Base body  505  was dipped into silicon melt  506  at a speed of 500 cm/min to cause deposition of a solid phase sheet  507  on the surface of base body  505 . Base body  505  with solid phase sheet  507  formed thereon was then transferred to a device subsidiary chamber  509  by a transportation conveyor  508 , and a gate valve  510  partitioning device subsidiary chamber  509  and chamber  502  body was closed. Then, device subsidiary chamber  509  was evacuated by a rotary pump. After the atmosphere was let in, base body  505  with solid phase sheet  507  formed thereon was taken out of device subsidiary chamber  509 . Solid phase sheet  507  formed on base body  505  could easily be detached. Solid phase sheet  507  obtained from the inner section surrounded by the circumferential groove was then cut into dimensions of 126 mm×126 mm by means of a laser cutting apparatus. 
     Herein, a base body of similar form to that shown in  FIG. 6  representing the base body employed in the present example was employed. The base body was a square measuring 150 mm per side, and base body  601  had a thickness of 20 mm. Slits  603  were provided at the one side that will be taken out lastly from silicon melt  506  in a region sectioned in an inner section S 61  surrounded by a circumferential groove  602 . At this time, slit  603  had a width of 3 mm, a length of 10 mm and a depth of 3 mm. A portion divided by slits  603  had a width of 10 mm. A slit end  604  of slit  603  opposite to the side that crosses circumferential groove  602  had a curvature of semicircle having a radius of 1.5 mm. 
     Under the above-described conditions, 1000 solid phase sheets were produced. A load of 50 N/mm 2  was applied by a bend tester to the laser-cut solid phase sheets to identify the yield. A favorable result of 85.1% was obtained. 
     Then, a photovoltaic cell was produced employing a solid phase sheet with dimensions of 126 mm×126 mm obtained by laser cutting, as a wafer for photovoltaic cell. A wafer for photovoltaic cell of the present example will be described below as a p-type substrate. First, the obtained p-type substrate was etched with a mixed solution of nitric acid and hydrofluoric acid, and then subjected to alkaline etching with sodium hydroxide. Thereafter, an n layer was formed on the p-type substrate by POCl 3  diffusion. After removing a PSG (Phospho Silicate Glass) film formed on the surface of the p-type substrate that will be the photoreceiving face side with hydrofluoric acid, a silicon nitride film was formed on the n layer at the photoreceiving face side of the photovoltaic cell by plasma CVD (chemical vapor deposition). An n layer formed on the surface of the photovoltaic cell at the rear face side was then removed by etching with a mixed solution of nitric acid and hydrofluoric acid to uncover the p-type substrate, on which aluminum paste was printed by a screen printing technique and burnt, so that the rear face electrode and a p+ layer were formed at the same time. A solder coating was then applied, to complete the photovoltaic cell. 
     Cell properties of 100 photovoltaic cells produced by the above-described method were measured under irradiation of AM 1.5 (100 mW/cm 2 ) to obtain favorable results of: an average open-circuit voltage of 580 mV; a short-circuit current of 30.8 mA/cm 2 ; an average fill factor of 0.736; and an average conversion efficiency of 13.1%. 
     It should be construed that embodiments and examples disclosed herein are by way of illustration in all respects, not by way of limitation. It is intended that the scope of the present invention is defined by claims, not by the description above, and includes all modifications equivalent in meaning and scope to the claims. 
     INDUSTRIAL APPLICABILITY 
     Providing a method for producing a uniform and defect-free solid phase sheet without causing cracks can provide improved yield, resulting in effective utilization of resources. Further, employing the solid phase sheet as a wafer for photovoltaic cell can provide a high-performance photovoltaic cell.