JP2005129602A - Solar cell and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a solar cell wherein deterioration of life time does not exist, variation is little in characteristic, impairment with time is little even in more prolonged time using and conversion efficiency is more high when the solar cell is manufactured from silicon single crystal, and the solar cell. <P>SOLUTION: In the method for manufacturing the solar cell from silicon single crystal, the silicon single crystal is grown by using polycrystalline silicon wherein content of silicon isotope<SP>28</SP>Si is at least 92.3% as material. The silicon single crystal is sliced and processed into a silicon wafer. The solar cell is formed on the silicon wafer, thereby providing the solar cell. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a solar cell using a silicon wafer useful as a material for the solar cell, and a solar cell.

  When a silicon single crystal is used as a material for manufacturing a solar battery cell, reduction of manufacturing cost is a major issue as well as improvement of conversion efficiency. Hereinafter, the technical background using a silicon single crystal as a material for solar cells will be described.

  First, the characteristics of the solar cell will be described based on the substrate material constituting the solar cell. When solar cells are classified based on their substrate materials, they can be broadly classified into three types: “silicon crystal solar cells”, “amorphous (amorphous) silicon solar cells”, and “compound semiconductor solar cells”. Crystalline solar cells include “single crystal solar cells” and “polycrystalline solar cells”. Among them, a solar cell having a high conversion efficiency which is the most important characteristic as a solar cell is a “compound semiconductor solar cell”, and the conversion efficiency reaches nearly 25%. However, it is very difficult to make a compound semiconductor as a material for compound semiconductor solar cells, and there is a problem in general dissemination in terms of manufacturing cost of solar cell substrates, and its use is limited. Yes.

  Here, “conversion efficiency” is a value indicating “ratio of energy that can be extracted by converting electric energy into the solar cell with respect to the energy of light incident on the solar cell”. (Also referred to as photoelectric conversion efficiency).

  The solar cell with the next highest conversion efficiency after the compound semiconductor solar cell is a silicon single crystal solar cell. The conversion efficiency is about 20%, which is close to that of a compound semiconductor solar cell, and a solar cell substrate can be procured relatively easily. Furthermore, although the conversion efficiency is about 5 to 15%, which is not as high as that of the above-mentioned two solar cells, the production cost of the solar cell substrate material is low, so that a silicon polycrystalline solar cell or an amorphous silicon solar cell is used. Batteries and the like have also been put into practical use.

  Next, a general method for manufacturing a silicon single crystal solar cell will be briefly described. First, in order to obtain a silicon wafer as a substrate of a solar battery cell, the Czochralski method (hereinafter sometimes referred to as CZ method) or the floating zone melting method (hereinafter referred to as FZ method) may be used. ) To grow a cylindrical silicon single crystal rod. Further, the single crystal rod is sliced and processed into a thin wafer having a thickness of, for example, about 300 μm, and the wafer surface (substrate) to be a solar cell is obtained by removing the processing distortion on the surface by etching the wafer surface with a chemical solution. It is done. This wafer is subjected to impurity (dopant) diffusion treatment to form a pn junction surface on one side of the wafer, electrodes are attached to both sides, and finally light energy loss due to light reflection is reduced on the sunlight incident side surface. A solar battery cell is completed by attaching an antireflection film for the purpose.

  Recently, the demand for solar cells is expanding as one of clean energy against the background of environmental problems, but the high energy cost is an obstacle to its spread compared with general commercial power. In order to reduce the cost of the silicon crystal solar cell, it is necessary to further increase the conversion efficiency while reducing the manufacturing cost of the substrate. For this reason, the substrate of a single crystal solar cell is not suitable for electronics for producing a so-called semiconductor element, or a corn portion, tail portion, etc. that are not a product in a single crystal rod are used as raw materials. Lowering the cost of substrate materials has been done. However, the procurement of such raw materials is unstable and the amount is limited. Considering the future increase in demand for silicon single crystal solar cells, this method requires the required amount of solar cell substrate. Is difficult to produce stably. Therefore, it is increasingly important to increase the conversion efficiency.

  Moreover, in a solar cell, in order to obtain a larger current, it is important to manufacture a larger area solar cell. As a method for obtaining a large-diameter silicon wafer as a substrate for producing a large-area solar cell, a large-diameter silicon single crystal can be easily produced, and the strength of the produced single crystal is excellent. The CZ method is suitable. For this reason, the production of silicon crystals for solar cells is mainly performed by the CZ method.

  On the other hand, a silicon wafer as a substrate material for a single crystal solar cell cannot be used as a solar cell substrate unless the substrate lifetime value, which is one of its characteristics, is 10 μs or more. In order to obtain a solar cell with high conversion efficiency, it is required that the substrate lifetime is preferably 200 μs or more.

  However, when the silicon single crystal produced by the CZ method is processed into a solar cell, if the solar cell is irradiated with strong light, the lifetime of the solar cell substrate is reduced, and the conversion efficiency is sufficient to cause photodegradation. Thus, there is a demand for improvement in the performance of solar cells.

  It is known that the reason why the lifetime is reduced and the photodegradation is caused by the influence of boron and oxygen existing in the single crystal substrate. Currently, the p-type is the main conductivity type of wafers used as solar cells, and boron is usually added to the p-type wafer as a dopant. And the single crystal rod used as the material of this wafer is manufactured by CZ method (including magnetic field applied method; Magnetic field applied CZ method (hereinafter sometimes referred to as MCZ method)) or FZ method. However, in the FZ method, the manufacturing cost of the single crystal rod is higher than that of the CZ method. In addition, as described above, the CZ method is easier to manufacture a large-diameter silicon single crystal. Manufactured by the CZ method, which can produce a large-diameter single crystal at a relatively low cost.

  When a silicon single crystal is grown by the CZ method, the single crystal is pulled up from the silicon melt contained in the quartz crucible, so oxygen eluted from the inner surface of the crucible into the silicon melt is unavoidably pulled up as interstitial oxygen in the crystal. Is captured. The interstitial oxygen concentration contained in a crystal immediately after pulling (as-grown) or a silicon wafer produced from this crystal is called an initial interstitial oxygen concentration, and a CZ silicon single crystal wafer containing such interstitial oxygen In addition, it is known that when heat treatment is applied in a later process, oxygen precipitates are precipitated in the bulk.

  Thus, there is a problem that high concentration of oxygen exists in the crystal produced by the CZ method, and therefore the lifetime and photodegradation are caused by boron and oxygen in the p-type CZ method silicon single crystal. There is.

  In order to solve such problems, the present applicant has disclosed a method in which Ga (gallium) is used instead of B (boron) as a p-type dopant in the previous application. It has become possible to prevent a decrease in lifetime due to influence (see, for example, Patent Document 1).

  Further, regarding the relationship between the initial oxygen concentration of the silicon wafer manufactured by the CZ method and the oxygen precipitation amount depending on the heat treatment conditions, almost no oxygen precipitation occurs when the initial oxygen concentration is about 15 ppm (JEIDA: Japan Electronics Industry Promotion Association standard) or less. It is known that there is no such information (see, for example, FIG. 8 of Non-Patent Document 1).

  By the way, in recent years, when attention is focused on rapid high integration, high speed operation and high power in silicon integrated circuit elements, heat generation of the elements themselves has become a problem. Since heat generation in an integrated circuit causes a reduction in device speed due to a decrease in carrier mobility or causes malfunction or destruction, countermeasures against heat generation are an important issue, and a solution to that is required. The same applies to power devices using a silicon substrate.

Natural silicon (Si) existing in nature is composed of a stable isotope of ²⁸ Si, 92.23%, ²⁹ Si, 4.67%, and ³⁰ Si, 3.10%. to solve the problem, the content of isotope ^{28 Si,} the thermal conductivity of silicon is improved by the 92.23% higher than 98% is a content of ^{28 Si} in natural silicon, heat dissipation The technique of raising is disclosed (for example, refer patent document 2). Si having an increased content of one kind of isotope can be produced by a gas diffusion method, a centrifugal separation method, a laser separation method, or the like (see, for example, Patent Document 3).

International Publication No. 00/73542 US Pat. No. 5,144,409 JP 2001-199792 A Takao Abe, Advanced Electronics Series 1-5 “Silicon”, published by Baifukan, May 1994, p. 195

  In the case of manufacturing a solar battery cell from a silicon single crystal, the present invention has no decrease in lifetime, little variation in characteristics, and less deterioration over time even in long-term use. It aims at providing the method and solar cell which manufacture a high photovoltaic cell.

In order to achieve the above object, the present invention is a method for producing a solar battery cell from a silicon single crystal, wherein at least silicon with a silicon isotope ²⁸ Si content of 92.3% or more is used as a raw material. A method of manufacturing a solar cell is provided, wherein a single crystal is grown, the silicon single crystal is sliced and processed into a silicon wafer, and a solar cell is formed on the silicon wafer.

Thus, a silicon single crystal grown from a polycrystalline silicon raw material in which the content rate of ²⁸ Si exceeds the content rate of natural silicon has high thermal conductivity, carrier mobility, and low oxygen precipitation. By forming a solar battery cell on a silicon wafer obtained by slicing and processing a simple silicon single crystal, a solar battery cell having high conversion efficiency and little deterioration over time can be manufactured.
In order to increase the conversion efficiency and reduce the deterioration over time, the content of ²⁸ Si is more preferably 94% or more, and even more preferably 98% or more.

In this case, it is preferable that the initial interstitial oxygen concentration of the silicon wafer is 16 ppma or less.
As described above, when ²⁸ Si is a silicon wafer having a content exceeding the natural silicon content and the initial interstitial oxygen concentration is 16 ppma (JEIDA) or less, it is applied at the time of manufacturing the solar cell. Almost no oxygen precipitation occurs due to the heat treatment performed, and there is almost no Bulk Micro Defect (BMD) formed by oxygen precipitation, so it is possible to prevent a decrease in lifetime, less variation in characteristics, higher conversion efficiency and time A favorable solar battery cell having characteristics with less deterioration can be manufactured.

The resistivity of the silicon wafer is preferably 0.1 to 5 Ω · cm.
As described above, if the resistivity of the silicon wafer is 0.1 Ω · cm or more, the amount of dopant is small, so that it is possible to prevent the lifetime from decreasing due to the Auger recombination phenomenon in which minority carriers are captured by the dopant. If the resistivity is 5 Ω · cm or less, it is possible to suppress a decrease in conversion efficiency due to power consumption inside the cell accompanying an increase in the internal resistance of the solar battery cell.

The silicon wafer is preferably a p-type silicon wafer using Ga as a dopant.
Thus, by using a silicon wafer as a dopant instead of B and using p-type conductivity, it is possible to prevent the lifetime from being reduced and the light deterioration due to the presence of B and oxygen in the wafer.

In this case, the Ga concentration is preferably 3 × 10 ^{15 to} 5 × 10 ¹⁷ atoms / cm ³ (Claim 5).
Thus, when the Ga concentration is 3 × 10 ¹⁵ atoms / cm ³ or more, the resistivity is 5 Ω · cm or less, and the conversion efficiency is increased by the power consumption inside the cell accompanying the increase in the internal resistance of the solar battery cell. The decrease can be suppressed, and if it is 5 × 10 ¹⁷ atoms / cm ³ or less, the lifetime can be prevented from decreasing due to the Auger recombination phenomenon in which minority carriers are captured by Ga atoms.

Moreover, this invention provides the photovoltaic cell manufactured by said manufacturing method (Claim 6).
Thus, a silicon single crystal grown from a polycrystalline silicon raw material in which the content rate of ²⁸ Si exceeds the content rate of natural silicon has high thermal conductivity, carrier mobility, and low oxygen precipitation. A solar battery cell formed on a silicon wafer obtained by slicing and processing a silicon single crystal becomes a solar battery cell with high conversion efficiency and little deterioration over time.
In order to increase the conversion efficiency and reduce the deterioration over time, the content of ²⁸ Si is more preferably 94% or more, and still more preferably 98% or more.

Next, the present invention is a solar battery cell formed on a silicon single crystal wafer, wherein the silicon wafer has a silicon isotope ²⁸ Si content of 92.3% or more. A solar battery cell is provided (claim 7).

Thus, ^{28 Si} and content beyond the content of natural silicon, ²⁸ because of the high purity ^Si, thermal conductivity and carrier mobility higher oxygen precipitation also formed small silicon wafer solar cells The cell can be a solar cell having high conversion efficiency and little deterioration over time.
Also in this case, in order to increase the conversion efficiency and reduce the deterioration with time, the content of ²⁸ Si is more preferably 94% or more, and further preferably 98% or more.

In this case, the silicon wafer preferably has an initial interstitial oxygen concentration of 16 ppma or less.
Thus, if the solar cell is formed on a silicon wafer in which ²⁸ Si exceeds the content of natural silicon and the initial interstitial oxygen concentration is 16 ppma or less, the purity of the silicon isotope itself is high and Since the oxygen concentration is low, almost no precipitation of oxygen occurs due to the heat treatment performed when manufacturing the solar battery cell, and since there is almost no BMD, the lifetime can be prevented from being reduced, and there is little variation in characteristics. A solar cell with higher efficiency and less deterioration over time can be obtained.

The silicon wafer preferably has a resistivity of 0.1 to 5 Ω · cm.
As described above, if the resistivity of the silicon wafer is 0.1 Ω · cm or more, the amount of dopant is small, so that it is possible to prevent the lifetime from decreasing due to the Auger recombination phenomenon in which minority carriers are captured by the dopant. If the resistivity is 5 Ω · cm or less, it is possible to obtain a solar cell in which a decrease in conversion efficiency due to power consumption inside the cell accompanying an increase in the internal resistance of the solar cell is suppressed.

The BMD density in the silicon wafer is preferably 5 × 10 ⁸ / cm ³ or less.
Thus, if the BMD density in the silicon wafer is 5 × 10 ⁸ / cm ³ or less, it is possible to prevent the lifetime from rapidly decreasing due to the heat treatment and to maintain the conversion efficiency at a high level. Thus, a solar battery cell with little variation in characteristics and little deterioration with time can be obtained.

The silicon wafer is preferably a p-type silicon wafer having Ga as a dopant.
In this way, if the silicon wafer is a p-type wafer having Ga as a dopant instead of B, lifetime reduction and photodegradation due to the presence of B and oxygen in the wafer can be prevented.

In this case, the Ga concentration is preferably 3 × 10 ^{15 to} 5 × 10 ¹⁷ atoms / cm ³ (claim 12).
Thus, when the Ga concentration is 3 × 10 ¹⁵ atoms / cm ³ or more, the resistivity is 5 Ω · cm or less, and the conversion efficiency due to the power consumption inside the cell accompanying the increase in the internal resistance of the solar battery cell. The solar cell in which the decrease in lifetime is prevented by the Auger recombination phenomenon in which minority carriers are captured by Ga atoms as long as the decrease can be suppressed and the number of carriers is 5 × 10 ¹⁷ atoms / cm ³ or less. And can.

The silicon wafer preferably has a diameter of 100 mm or more.
As described above, if the silicon wafer has a diameter of 100 mm or more, a large-area solar battery cell can be manufactured at a low manufacturing cost, so that further improvement in the performance of the solar battery and an increase in demand can be expected. In particular, it is more preferable to use a large-diameter silicon wafer having a diameter of 200 mm or 300 mm, for which demand has been increasing in recent years.

The solar cell preferably has an area of 100 cm ² or more.
Thus, if the area of the solar battery cell is 100 cm ² or more, one solar battery cell has a large area, and a large current can be obtained from one solar battery cell, which is effective for electric power. Conventionally, it has been difficult to obtain a large-area and high-efficiency solar battery cell. However, according to the present invention, the efficiency of a large-area solar battery can be improved.

Moreover, it is preferable that the conversion efficiency of the said photovoltaic cell exceeds 21.1% (Claim 15).
In this way, if the solar cell is formed from a silicon wafer of 92.3% or more in which the content of the isotope ²⁸ Si exceeds the content of natural silicon, the conversion efficiency is higher and the conversion efficiency due to light degradation is higher. It can be set as a photovoltaic cell with little fall and deterioration with time, and the conversion efficiency can exceed 21.1%. In particular, conventional solar cells with an area of 100 cm ² or more and conversion efficiency exceeding 21.1% have not been practically used, but the solar cells of the present invention have a cell area of 100 cm ² or more. Even so, conversion efficiencies exceeding 21.1% can be achieved.

The solar battery cell is preferably for space use (claim 16).
The solar battery cell of the present invention is formed from a silicon wafer having a content of isotope ²⁸ Si of 92.3% or more, and since it has high purity, it is affected by various radiations such as electron beams in outer space. It is extremely small, and hardly deteriorates over time. Therefore, the solar battery cell of the present invention is suitable for space use.

Moreover, it is preferable that the said photovoltaic cell is a thing whose fall of the conversion efficiency by light degradation is 0.4% or less (Claim 17).
Thus, the solar cell of the present invention has high conversion efficiency, almost no decrease in conversion efficiency due to light degradation, and almost no deterioration over time. Become.
Here, the reduction in conversion efficiency due to light deterioration is the conversion efficiency before irradiation for 30 hours of stationary light such as a halogen lamp used in a solar simulator subtracted from the conversion efficiency after irradiation. Deterioration is the result of subtracting the conversion efficiency after irradiation from the conversion efficiency before irradiating stationary light such as a halogen lamp for 1000 hours and maintaining the temperature of the solar battery cell at 100 ° C.

Hereinafter, the present invention will be described in more detail.
The present inventors have determined that the content of silicon isotope ²⁸ Si contained in a silicon wafer forming a solar cell (natural silicon existing in the natural world is 92.23% for ²⁸ Si and 4.67% for ²⁹ Si. , ³⁰ Si has a composition of 3.10%), and crystal defects such as interstitial oxygen and oxygen precipitates in the wafer bulk, that is, regardless of whether or not BMD coexists with B, The inventors have conceived that the conversion efficiency may be affected, and as a result of diligent research, have arrived at the present invention.

  As described above, in a solar battery cell formed on a B-doped p-type silicon wafer sliced from a silicon single crystal grown by the CZ method, photodegradation of conversion efficiency occurs due to the coexistence of oxygen and B. On the other hand, the applicant disclosed a method of using Ga instead of B as a p-type dopant, thereby preventing a decrease in lifetime due to the influence of B and oxygen.

  However, although the influence of B and oxygen was excluded using Ga as a dopant, the lifetime was lowered depending on the produced solar battery cell, and the characteristics of the solar battery cell were sometimes varied. Further, not only such variations in characteristics but also a phenomenon that even when Ga doped, the characteristics deteriorate over time when irradiated for a longer period of time. Such a phenomenon has been a problem due to a decrease in yield when manufacturing solar cells, a decrease in conversion efficiency of the entire solar cell module, and instability in use for many years.

The present inventors speculated that this phenomenon may be caused by the content of the isotope ²⁸ Si contained in the silicon wafer. Regarding Ga, it is known that when the Ga concentration is increased, the frequency of minority carriers approaching Ga increases, and a so-called Auger recombination phenomenon occurs in which the lifetime is reduced by being trapped by Ga. On the other hand, such an effect is not known for oxygen atoms, but by increasing the content of isotope ²⁸ Si, the silicon crystal is highly purified, and other isotopes ²⁹ Si and ³⁰ Si It is conceivable that the strain field formed by the silicon lattice is reduced, and by reducing the strain field, the number of precipitation nuclei for oxygen atoms to become precipitates is reduced and the amount of precipitated oxygen is reduced.

However, conventionally, no consideration has been given to the content of the isotope ²⁸ Si in the silicon wafer in which such a solar battery cell is formed. Therefore, variations in characteristics of the solar battery cell and deterioration over a long period of use are not considered. It is thought that there was.

On the other hand, as described above, in recent years, heat generation of the element itself due to rapid high integration, high speed operation and high power in the silicon integrated circuit element has become a problem, and the heat generation of the integrated circuit is due to a decrease in carrier mobility. Measures against heat generation have become an important issue because it causes the device to slow down or cause malfunction or destruction, and a technique for increasing the content of ²⁸ Si has been disclosed as a solution. Such a problem of heat generation and its solution seemed to be unrelated to silicon solar cells, but I thought that it could be applied to solar cells with high conversion efficiency and little deterioration over time. The present invention has been completed as a result of studying various conditions based on such a basic idea.

In accordance with the present invention, a silicon single crystal is grown using polycrystalline silicon having a content of isotope ²⁸ Si of 92.3% or more as a raw material, and the silicon single crystal is sliced and processed into a silicon wafer. By forming solar cells on a suppressed silicon wafer, it is possible to manufacture more efficient solar cells with less variation in characteristics and less deterioration over time.

Hereinafter, embodiments of the present invention will be described, but the present invention is not limited thereto.
In order to grow the silicon single crystal of the present invention in which the content of isotope ²⁸ Si is 92.3% or more, the content of at least isotope ²⁸ Si produced by a conventional gas diffusion method or the like is 92.3. A conventional method commonly used in the CZ method may be applied, in which more than% raw material silicon polycrystal is contained in a quartz crucible and melted, the seed crystal is brought into contact with the silicon melt, and pulled up while rotating. If the inner surface of the seed crystal or the crucible is composed of an isotope ²⁸ Si content of 92.3% or more, the ²⁸ Si content can be increased stably and more preferably. It is.
Further, if the diameter of the silicon single crystal grown at this time is 100 mm or more, particularly 200 mm or 300 mm, a large-diameter silicon wafer can be obtained, and thus a large area solar cell is manufactured at a low manufacturing cost. Can be suitable for

  At this time, it is preferable that the initial interstitial oxygen concentration in the silicon single crystal to be grown is 16 ppma or less. For the control of the oxygen concentration, for example, the crucible rotational speed is decreased, the introduced gas flow rate is increased, and the atmospheric pressure is decreased. The oxygen concentration range can be easily achieved by conventional means such as temperature distribution of silicon melt and adjustment of convection. When the oxygen concentration is raised to about 10 ppma or lower, the oxygen concentration can be lowered to about 7 ppma according to the so-called MCZ method in which the magnetic field is applied to raise the oxygen concentration.

  Next, the silicon single crystal is sliced to a predetermined thickness with a cutting machine such as a wire saw, a band saw or an inner peripheral cutting machine, and processing such as etching removal of processing distortion on the surface is performed to form a solar cell. Obtain a silicon wafer.

  As a general solar cell manufacturing process using a silicon wafer, there are mainly a pn junction forming step, an electrode forming step, an antireflection film forming step, and the like, and the solar cell is thus formed. In the pn junction forming step, a pn junction is usually formed by introducing an n-type impurity into the surface of a p-type silicon wafer. For the impurity introduction at this time, a gas diffusion method, a solid phase diffusion method, an ion implantation method, or the like is used. The heat treatment is performed at a temperature of several hundred to 1000 ° C. or higher. The electrode forming step is a step of forming a metal to be an electrode by a vapor deposition method, a plating method, a printing method, or the like, and a heat treatment of about several hundred degrees C. is applied. Furthermore, in the antireflection film forming step, a deposited film is formed by a CVD (Chemical Vapor Deposition) method, a PVD (Physical Vapor Deposition) method, or the like, and a heat treatment of about several hundred to 800 ° C. is also applied at that time.

In the solar cell formed in this way, if the area of one cell is a large area of 100 cm ² or more, a large current can be obtained from one solar cell, so the number of cells necessary to obtain a desired current is small. This is preferable because it reduces the number, increases the overall conversion efficiency, and reduces the cost.

In the above silicon wafer, the density of BMD formed from oxygen precipitates is preferably 5 × 10 ⁸ / cm ³ or less.
Here, in order for oxygen precipitates to be formed by heat treatment of the silicon wafer, oxygen is supersaturated at the heat treatment temperature and nuclei (precipitation nuclei) in which precipitates are formed. The presence of minute oxygen precipitates and metal impurities is a condition.

That is, what heat treatment conditions are used depends on the manufacturing conditions unique to various solar cells, and the relationship between the initial oxygen concentration and the BMD density after the heat treatment varies depending on the heat treatment conditions. Therefore, by using a silicon wafer having an isotope ²⁸ Si content of 92.3% or more according to the present invention, oxygen precipitation itself is less likely to occur, but the heat treatment conditions in the production of each solar cell are specified in advance. If the relationship between the BMD density existing after the heat treatment and the initial oxygen concentration of the silicon wafer used for producing the solar battery cell is experimentally determined, a silicon wafer having a BMD density of 5 × 10 ⁸ / cm ³ or less Therefore, it is possible to reliably determine the initial oxygen concentration necessary for achieving the above.

  Precipitation nuclei are also present in an as-grown silicon wafer, but are also formed when supersaturated oxygen becomes minute oxygen precipitates by heat treatment at a temperature of about 650 to 900 ° C. In order for such precipitation nuclei to grow and become large oxygen precipitates, heat treatment at a temperature of 900 to 1100 ° C. is required. Heat treatment at 900 ° C. or lower requires a very long time for growth because oxygen diffusion is slow, and large oxygen precipitates are hard to be generated. Will not be formed.

  Therefore, the BMD density can be extremely reduced if all the heat treatments for forming solar cells using silicon wafers exceed 1100 ° C. However, in practice, as described above, low-temperature processes mainly under 1000 ° C. Used. In other words, at the stage where solar cells are formed by such a process, oxygen precipitates of a very large size are not formed, but at least the heat treatment at a temperature at which minute oxygen precipitates forming precipitation nuclei are formed. Therefore, the generation of such precipitation nuclei becomes a cause of lifetime reduction.

On the other hand, when a high temperature of about 900 to 1100 ° C. is employed as the heat treatment temperature for forming the solar battery cell, the size of the oxygen precipitate increases and the lifetime decreases, so that the oxygen precipitation as in the present invention. Is suppressed, and the effect of preventing a decrease in lifetime due to the solar battery cell having a BMD density of 5 × 10 ⁸ / cm ³ or less is further increased and more effective.

  The measurement of the BMD density after the solar battery cell is formed can be measured by an OPP (Optical Precipitate Profiler) method or by cleaving the wafer and selectively etching the cleaved surface. When the size of the BMD is small, for example, heat treatment may be performed at 1000 ° C. for 16 hours to grow oxygen precipitates to a detectable size and then measure.

  The OPP method applies a normal-sky type differential interference microscope. First, the laser beam emitted from the light source is separated into two linearly polarized beams with 90 ° phase orthogonal to each other by a polarizing prism. Incident from the mirror side of the wafer. At this time, when one beam crosses the defect, a phase shift occurs and a phase difference from the other beam occurs. A defect is detected by detecting this phase difference with a polarization analyzer after passing through the back surface of the wafer.

In order to increase the conversion efficiency, the resistivity of the silicon wafer is preferably 0.1 to 5 Ω · cm. When the resistivity is higher than 5 Ω · cm, the resistivity of the wafer is higher than necessary, so that power is consumed by the internal resistance of the solar battery cell, and conversion efficiency may be reduced. In addition, when the resistivity is lower than 0.1 Ω · cm, the resistivity of the wafer is extremely lowered, so that the lifetime of minority carriers may be reduced due to Auger recombination inside the wafer. . In order to further prevent photodegradation of the solar battery cell, it is preferable to dope Ga as a p-type dopant of the silicon single crystal wafer. The Ga concentration at this time is preferably 3 × 10 ^{15 to} 5 × 10 ¹⁷ atoms / cm ³ (resistivity is 5 to 0.1 Ω · cm) for the above-described reason.

In order to produce a silicon single crystal having an isotope ²⁸ Si content of 92.3% or more to which Ga is added, a poly-single crystal silicon raw material having a content of ²⁸ Si of 92.3% or more is put in the crucible. After filling and melting, Ga is added to the silicon melt as a dopant, a seed crystal is brought into contact with the silicon melt, and it may be pulled up while rotating. In this case, the addition of Ga to the melt in the crucible is carried out by growing a silicon crystal to which a high concentration of Ga has been added in advance and crushing the high concentration Ga doped silicon crystal to obtain an appropriate amount of a dopant by calculation. Only by adding it to the silicon melt, an accurate amount of Ga can be doped to achieve a desired resistivity. In addition, even in the inner surface of the crucible, the seed crystal, and the high-concentration Ga-doped silicon, a silicon single crystal can be manufactured with a more stable content rate by using a ²⁸ Si content rate of 92.3% or more.

And the solar cell produced in this way can be made into a solar cell with higher conversion efficiency, less decrease in conversion efficiency due to light deterioration, and less deterioration over time. It can be more than 1%. In particular, in the past, solar cells using silicon having an area of 100 cm ² or more and conversion efficiency exceeding 21.1% have not been practically used, but the solar cell of the present invention has an area of 100 cm. ^{Even if it is two} or more, the conversion efficiency exceeding 21.1% can be achieved, and further, the reduction in conversion efficiency due to light deterioration can be made 0.4% or less, so the life of the cell is also long. Therefore, it is extremely effective for applications such as electric power.
In addition, since the solar cell thus produced is formed from a silicon wafer having a content of isotope ²⁸ Si of 92.3% or more, the influence of various radiations in outer space is extremely small. There is almost no deterioration over time due to radiation.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited to these examples.
(Examples 1-5)
As an example, crushed polycrystalline silicon material content of 98% of the silicon isotope ^{28 Si,} the content of the previously high concentration isotopes ²⁸ with the addition of Ga of ^Si this by growing 98% silicon crystal The prepared dopant was prepared so that the resistivity at the shoulder of the silicon crystal rod to be grown was 1 Ω · cm. Then, using a quartz crucible having a diameter of 600 mm, a silicon single crystal rod having a crystal diameter of 200 mm and ²⁸ Si content added with Ga of crystal orientation <100> and having a content of 98% is set to a normal initial oxygen concentration. Three were pulled by the CZ method, and two were pulled by the MCZ method to obtain a low oxygen concentration.

(Comparative Examples 1-4)
As a comparative example, the same polysilicon material content of the silicon isotope ^{28 Si} is the native silicon, and the content of the previously high concentration isotopes ²⁸ with the addition of Ga of ^Si is grown to the same silicon crystal with the natural silicon The dope prepared by crushing this was prepared so that the resistivity at the shoulder of the silicon crystal rod to be grown was 1 Ω · cm, as in the example. Then, similarly crystal diameter 200mm and example, a predetermined initial oxygen concentration using the same silicon single crystal ingot content and natural silicon ²⁸ Si with the addition of Ga, a quartz crucible having a diameter of 600mm crystal orientation <100> In order to obtain a low oxygen concentration, three wires were pulled by the normal CZ method and one by the MCZ method.

These nine pulled silicon single crystal rods were cut into a block state, and sliced into a wafer having a thickness of 2 to 3 mm to obtain a silicon wafer. As pre-processing conditions, the slice wafer is treated with a mixed acid of HF: HNO ₃ = 5%: 95%, and the slice damage layer on both sides is removed by etching and then cleaned, and then AM (Air Mass) is applied to the wafer surface. After irradiating stationary light for 30 hours under the condition of 1.5, the natural oxide film on the surface was removed with HF. Subsequently, chemical passivation (CP) treatment using a mixed solution of iodine and ethanol was performed to reduce carrier recombination on the crystal surface. After measuring the lifetime using the microwave-PCD method (optical conductivity attenuation method), 800 ° C., 30 minutes as a heat treatment assuming the manufacturing process of the solar cell without actually forming the solar cell. (Pn junction forming process) After three-step heat treatment of + 600 ° C., 30 minutes (electrode forming process) + 700 ° C., 60 minutes (antireflection film forming process), the lifetime can be measured again, and further BMD can be detected In order to grow to size, heat treatment was performed at 1000 ° C. for 16 hours, and the BMD density was measured by the OPP method. The results are shown in Table 1.

As can be seen from Table 1, when the silicon isotope ²⁸ Si content in the silicon single crystal was 98% as in Examples 1 to 5, the silicon wafer was subjected to a heat treatment for manufacturing a solar cell. Nevertheless, it was confirmed that the lifetime was not decreased and stable characteristics were exhibited.

On the other hand, when the silicon isotope ²⁸ Si content in the silicon single crystal is the same as that of natural silicon as in Comparative Examples 1 to 4, oxygen precipitation is more likely to occur than in the examples of the present invention. It can be seen that the lifetime after the heat treatment for producing is reduced, and particularly in Comparative Examples 3 and 4 where the BMD density is 5 × 10 ⁸ / cm ³ or less, the lifetime is also reduced.

Next, a silicon wafer having the same resistivity is selected from the silicon wafers used above, and a large 10 cm × 10 cm square (cell area 100 cm ² ) solar cell is produced as a solar cell, and the conversion is performed. Efficiency was measured. The conversion efficiency of solar cells is measured by placing the solar cells on a measuring table whose temperature is adjusted to 25 ° C. and irradiating the cells with steady light under a condition of AM of 1.5 with a solar simulator using a halogen lamp as the light source. Then, the conversion efficiency of the solar cell was calculated by measuring the voltage and current that could be extracted from the cell. The conversion efficiency here is a value defined by the following equation.
[Conversion efficiency] = [Electric power extracted from per unit cell area] / [Light energy irradiated per unit cell area] × 100 (%)
The measurement results are also shown in Table 1.

As shown in Table 1, when the content of silicon isotope ²⁸ Si in the silicon wafer is 98% (Examples 1 to 5), the conversion efficiency is as high as 21.6 to 22.6%. It can be seen that light energy is efficiently converted into electric energy. In addition, the conversion efficiency after irradiating the solar cell with light at 25 ° C. for 30 hours (for light degradation investigation) and 100 ° C. for 1000 hours (for time degradation investigation) is almost the same as the initial value without almost changing. The results show that the conversion efficiency is stable, and it is confirmed that a stable and large-sized solar cell with high conversion efficiency can be obtained without causing light deterioration and deterioration over time. A solar cell without photodegradation produced from a silicon single crystal having a silicon isotope ²⁸ Si content higher than that of natural silicon is expected to be suitable as a space solar cell with little degradation due to electron beam irradiation. .

On the other hand, when the silicon isotope ²⁸ Si content in the silicon wafer is the same as that of natural silicon (Comparative Examples 1 to 4), the conversion efficiency of the solar cell is slightly lower than that of the example of the present invention before photodegradation. . Shows relatively high conversion efficiency of 1 to 20.9%, and the same conversion efficiency after light degradation due to 30 hours of steady light irradiation, but after a long time of steady light irradiation such as 100 hours, Due to the deterioration, it decreased to 16.5 to 19.2%, and conversion efficiency exceeding 21% could not be obtained stably. ^In a solar cell using a silicon wafer having the same Si content as ²⁸ Si, it is difficult to produce a stable solar cell while maintaining high conversion efficiency over time, even with Ga doping. It is done.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

Claims (17)

A method for producing a solar battery cell from a silicon single crystal, comprising growing at least a silicon single crystal from polycrystalline silicon having a silicon isotope ²⁸ Si content of 92.3% or more as a raw material. A method of manufacturing a solar cell, comprising slicing and processing into a silicon wafer, and forming a solar cell on the silicon wafer.   2. The method for manufacturing a solar cell according to claim 1, wherein an initial interstitial oxygen concentration of the silicon wafer is 16 ppma or less.   The method for manufacturing a solar cell according to claim 1 or 2, wherein the resistivity of the silicon wafer is 0.1 to 5 Ω · cm.   The method for manufacturing a solar cell according to any one of claims 1 to 3, wherein the silicon wafer is a p-type silicon wafer having Ga as a dopant. 5. The method for manufacturing a solar cell according to claim 4, wherein the Ga concentration is 3 × 10 ^{15 to} 5 × 10 ¹⁷ atoms / cm ³ .   A solar battery cell manufactured by the manufacturing method according to claim 1. A solar battery cell formed on a silicon single crystal wafer, wherein the silicon wafer has a silicon isotope ²⁸ Si content of 92.3% or more.   The solar cell according to claim 7, wherein the silicon wafer has an initial interstitial oxygen concentration of 16 ppma or less.   The solar cell according to claim 7 or 8, wherein the silicon wafer has a resistivity of 0.1 to 5 Ω · cm. 10. The solar battery cell according to claim 7, wherein a BMD density in the silicon wafer is 5 × 10 ⁸ / cm ³ or less. 11.   The solar cell according to any one of claims 7 to 10, wherein the silicon wafer is a p-type silicon wafer having Ga as a dopant. The solar cell according to claim 11, wherein the Ga concentration is 3 × 10 ^{15 to} 5 × 10 ¹⁷ atoms / cm ³ .   The solar cell according to any one of claims 7 to 12, wherein the silicon wafer has a diameter of 100 mm or more. The solar cell according to any one of claims 7 to 13, wherein the solar cell has an area of 100 cm ² or more.   The solar cell according to any one of claims 7 to 14, wherein the solar cell has a conversion efficiency exceeding 21.1%.   The solar cell according to any one of claims 7 to 15, wherein the solar cell is for space use.   The solar cell according to any one of claims 7 to 16, wherein the solar cell has a decrease in conversion efficiency due to light degradation of 0.4% or less.