JP5843588B2 - Method for manufacturing crystalline solar cell - Google Patents

Description

The present invention relates to the production how crystalline solar cells.

  In recent years, solar cells have been increasingly and widely used from the viewpoint of effective use of natural energy. As a material constituting a solar cell, crystalline silicon has been mainly used so far.

A solar cell using crystalline silicon includes a substrate made of p-type crystalline silicon that exhibits a photoelectric conversion function between a light-receiving surface that receives light and a back surface that faces the light-receiving surface. A high-concentration p-type silicon layer (p ⁺ layer) is formed on one main surface side of the substrate, and an n-type silicon layer (n layer) is formed on the other main surface side. A p-type silicon layer (p layer) having a lower concentration than the p ⁺ layer is formed between the p ⁺ layer and the n layer. By irradiating the substrate with light, of the electrons and holes excited in the p layer, the electrons are captured by the electrode connected to the n layer and the holes are captured by the electrode connected to p ⁺ , respectively. Current generated by trapped electrons and holes can be generated.

  That is, the crystalline silicon solar cell has a function of converting the energy of irradiated light into electric energy that can be output to the outside. In particular, crystalline silicon solar cells have a high rate of conversion to electrical energy (photoelectric conversion efficiency) in the energy of irradiated light, and occupy an important position in the solar cell market. Technological development is being promoted (Non-Patent Document 1).

In the manufacture of a crystalline silicon solar cell using a polycrystalline or single crystal silicon substrate, a film made of POCl ₃ is generally formed on the substrate and then annealed (non-patent document). 2). This annealing process is performed to reduce the surface resistance of the substrate by diffusing phosphorus (P) in the depth direction from the surface of the silicon substrate.

  However, phosphorus deposited on the end face of the substrate is annealed and diffuses into the substrate, causing a decrease in resistance. That is, a low-resistance layer is formed on the end surface of the substrate and is electrically connected to the back surface. As a result, the intention to form a PIN junction solar cell on both sides is short-circuited at the end face.

  Conventionally, this has been dealt with by separating and discarding the outer peripheral portion of the substrate. Specifically, a method called edge cutting using a laser, that is, the outer peripheral portion of the substrate is irradiated with a laser, separated and discarded. However, in this method, the region irradiated with the laser and separated does not contribute to the photoelectric conversion and becomes a useless region. Furthermore, laser irradiation is necessary for edge cutting, which is disadvantageous in terms of cost and process.

Plasma Fusion Res. Vol. 85, N0.12 (2009) 820-824 “COMPARISON OF DIFFERENT TECHNIQUES FOR EDGE ISOLATION” A. Hauser, G. Hahn

The present invention has been devised in view of such a conventional situation, and does not require laser irradiation, and can prevent formation of a conductive portion (low resistance region) on the substrate end face. and purpose thereof is to provide a method of manufacturing.

According to a first aspect of the present invention, there is provided a method for manufacturing a crystalline solar cell, comprising: a crystalline solar cell including a crystalline substrate that exhibits a photoelectric conversion function between a light receiving surface that receives light and a back surface that faces the light receiving surface; In the manufacturing method, a flat substrate made of single crystal or polycrystalline silicon is used as the crystal substrate, and the outer peripheral end surface of the substrate is a region where the entire region forms an ion implantation side and a region which forms a non-ion implantation side and the entire outer peripheral end surface of the substrate together with the one main surface of the substrate with respect to one main surface side including the entire outer peripheral end surface of the substrate. In the exposed state, phosphorus (P) ions are implanted, and the substrate is heated, and phosphorus (P) adhered to the outer peripheral end surface portion of the substrate during the first step. A second step of evaporating and removing the coating comprising And a third step of performing an annealing process on the substrate at least in order.
In the method for producing a crystalline solar cell according to claim 2 of the present invention, in claim 1, the temperature of the second step is higher than the temperature of the first step and lower than the temperature of the third step. It is characterized by.
The method for producing a crystalline solar cell according to claim 3 of the present invention is characterized in that, in claim 1 or claim 2, in the third step, the oxygen partial pressure is 1% or less .

In the method for producing a crystalline solar cell of the present invention, a flat substrate made of single crystal or polycrystalline silicon is used, and the outer peripheral end surface of the substrate is a region where the entire region forms an ion implantation side and a region which forms a non-ion implantation side. has a shape which is divided into a beta, of the base body, for the one main surface side of including the entire area of the outer peripheral end face of the base body, the entire region of the outer peripheral end surface of the base body with one main surface of said substrate In the exposed state, phosphorus (P) ions are implanted, and the substrate is heated , and phosphorus (P) adhered to the outer peripheral end surface portion of the substrate during the first step. And a third step of performing an annealing process on the substrate at least in order. Thereby, in this invention, since the coating film which consists of phosphorus (P) adhering to the outer peripheral end surface part of the said base | substrate in the 1st process is removed by evaporating by heat-processing in a 2nd process, it is a 3rd process. Even if the annealing treatment is performed, a conduction portion (low resistance region) is not formed on the outer peripheral end face of the substrate . As a result, there is no decrease in resistance at the outer peripheral end surface of the substrate , and electrical continuity with the back surface can be avoided. As a result, in the method for manufacturing a crystalline solar cell according to the present invention, it is possible to prevent the formation of a conductive portion (low resistance region) on the outer peripheral end face of the substrate without requiring laser irradiation. Further, there is no loss of the photoelectric conversion region due to laser irradiation.

Sectional drawing of the crystalline solar cell in this invention. Sectional drawing of the ion implantation apparatus used for manufacture of a crystalline solar cell. Sectional drawing which shows typically a mode that the phosphorus film is formed in the end surface part of a board | substrate. Sectional drawing which shows typically how to put a needle | hook in the measurement of resistance value by a 4-probe method. The graph which shows the relationship between the oxygen partial pressure at the time of annealing using a furnace, and sheet resistance. The graph which shows the relationship between the oxygen partial pressure at the time of annealing using RTA, and sheet resistance.

  Hereinafter, an embodiment of a method for producing a crystalline solar cell according to the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the present invention.
FIG. 1 is a diagram illustrating a configuration of a crystalline solar cell 100 according to the present embodiment. The crystalline solar cell 100 is a solar cell including a crystalline substrate 101 that exhibits a photoelectric conversion function between a light receiving surface that receives light and a back surface that faces the light receiving surface. The crystalline substrate 101 is a flat substrate made of p-type single crystal or polycrystalline silicon.

The crystalline substrate (base body) 101 includes a p-type semiconductor layer (p ⁺ layer) 102 and an n-type semiconductor layer on one main surface (back surface) 101a side and the other main surface (light-receiving surface) 101b side, respectively. (N layer) 103 is provided. The n layer 103 is covered with a silicon nitride film (SiN _x film) 104 formed on the other main surface 101b side of the crystal substrate. Further, the n layer 103 is electrically connected to the light receiving surface electrode 105 formed so as to penetrate the SiN _x film. The p ⁺ layer 102 is covered with a back electrode 106 formed on the one main surface 101b side of the crystal substrate.

  The crystalline solar cell of the present invention is manufactured by a method as described later, and an end surface region connected to a surface opposite to the surface into which the ions are implanted is a non-conductive portion. This eliminates the resistance drop at the substrate end face and avoids electrical continuity with the back face.

  In FIG. 1, the two main surfaces 101a and 101b of the substrate are shown to be flat, but both have a microscopic textured shape.

[Method of manufacturing crystalline solar cell]
A method for manufacturing the crystalline solar cell 100 according to this embodiment shown in FIG. 1 will be described.
In the method for producing a crystalline solar cell according to the present invention, a first step of implanting phosphorus (P) ions into one main surface side of the substrate, a heat treatment is performed on the substrate, and the first step And a third step of evaporating and removing the film made of phosphorus (P) attached to the end surface portion of the substrate and a third step of performing an annealing process on the substrate. And

  Conventional crystal solar cells include texture formation, p-type ion implantation, first annealing treatment, first hydrofluoric acid (HF) treatment, n-type ion implantation, second annealing treatment, second hydrofluoric acid (HF) treatment, insulating film It is manufactured by sequentially performing the steps of formation and electrode formation.

  On the other hand, the crystalline solar cell 100 according to this embodiment includes texture formation, p-type ion implantation, first annealing treatment, hydrofluoric acid (HF) treatment, n-type ion implantation (first step), and heat treatment (second step). Step), second annealing treatment (third step), insulating film formation, and electrode formation steps are sequentially performed. That is, the manufacturing method of the crystalline solar cell 100 according to the present embodiment is different from the conventional manufacturing method of the crystalline solar cell in that the heat treatment is performed after the n-type ions are implanted and before the second annealing treatment.

  In the present invention, the film made of phosphorus (P) radicals attached to the end face portion of the substrate in the n-type ion implantation step is removed by evaporation by heat treatment. In addition, no conductive portion (low resistance region) is formed on the end face of the substrate. As a result, in the method for manufacturing a crystalline solar cell according to the present invention, it is possible to prevent the formation of a conductive portion (low resistance region) in the substrate end face region without requiring laser irradiation. Further, there is no loss of the photoelectric conversion region due to laser irradiation.

  Each process for manufacturing the crystalline solar cell 100 according to the present embodiment will be described in detail. First, in the texture forming step, a wet etching process using, for example, potassium hydroxide (KOH) or sodium hydroxide (NaOH) as an etchant is performed on the substrate 101. Then, organic substances and metal contaminants remaining on the substrate 101 after the treatment are removed using hydrofluoric acid, so that the two main surfaces 101a and 101b are processed into a textured shape.

  Next, p-type ions such as boron (B) ions are implanted into one main surface 101a of the base 101 processed into a textured shape. The implantation of p-type ions is performed using an ion implantation apparatus 200 shown in FIG.

  FIG. 2 is a cross-sectional view of an ion implantation apparatus 200 used in a p-type ion implantation process and an n-type ion implantation process described later. The ion implantation apparatus 200 includes a vacuum chamber 201, a plasma generating means by ICP discharge using a permanent magnet 205, an RF introducing coil 206, an RF introducing window (quartz) 212, and a vacuum exhaust means (not shown). The inside of the vacuum chamber 201 is separated into a plasma generation chamber and a plasma processing chamber by mesh electrodes 208 and 209. A substrate support table 204 that supports a substrate (substrate) 203 to be processed is disposed in the plasma processing chamber. Note that the mesh electrode 208 has a floating potential and has a function of stabilizing the potential of the plasma 207. The mesh electrode 209 has a function of extracting a positive ion from the plasma 207 when a negative potential is applied thereto.

  The inside of the vacuum layer 201 is decompressed, and a gas containing impurity atoms to be injected into the base 203 is introduced into the plasma generation chamber. Then, the plasma 207 is excited using the plasma generating means to ionize the impurity atoms, and p-type or n-type ions extracted via the mesh electrodes 208 and 209 can be implanted into the substrate. it can.

Here, the implantation amount of the p-type ions is based on the relationship between the sheet resistance of the p-type semiconductor layer 102 after the first annealing process, which will be described later, and the photoelectric conversion efficiency of the crystalline solar cell. It is determined as the optimum value. However, it is assumed that the concentration of p-type ions in the p-type semiconductor layer 102 is at least higher than the concentration of p-type ions in the substrate 101. By providing a p ⁺ layer having a high concentration of p-type ions between the back electrode 106 and the substrate 101, the electric field is weakened between the back electrode 106 and the substrate 101, and the electrical resistance is reduced.

  Next, a first annealing process is performed in a nitrogen atmosphere on the substrate 101 into which p-type ions have been implanted. The conditions for the first annealing treatment are determined as optimum conditions corresponding to the diffusion coefficient of P-type ions inside the substrate. That is, the temperature of the first annealing treatment is desirably about 900 ° C. Further, the time for the first annealing treatment is preferably about 30 to 60 minutes.

  Note that an oxide film is formed on the surface of the substrate 101 during the first annealing treatment. Oxygen ions contained in the oxide film diffuse into the substrate 101 and function as recombination centers, thereby reducing the photoelectric conversion efficiency. Therefore, it is desirable to remove the oxide film formed on the surface of the substrate 101 by performing hydrofluoric acid treatment (HF treatment) between the first annealing treatment and the implantation of P ions.

  Next, n-type ions such as phosphorus (P) are implanted into the other main surface 101b of the base 101 subjected to the first annealing treatment. The n-type ion implantation is performed using an ion implantation apparatus 200 shown in FIG.

  Here, the amount of n-type ions implanted is determined when the solar cell 100 is manufactured from the relationship between the sheet resistance of the n-type semiconductor layer 103 after the second annealing process described later and the photoelectric conversion efficiency of the crystalline solar cell. It is determined as the optimum value.

  Next, the substrate 101 is subjected to heat treatment, and the coating made of phosphorus (P) attached to the end surface 101c portion of the substrate 101 during the n-type ion implantation step is removed by evaporation.

  In the above-described n-type ion implantation step, when phosphorus (P) is implanted by a plasma doping method, as shown in FIG. 3, film formation with radicals occurs in addition to ion implantation. On the outer peripheral end surface 101c of the substrate, radicals wrap around not only on the ion implantation side (region α) but also on the non-ion implantation side (region β). Therefore, the radical film formation region is the entire area of the outer peripheral end surface 101 of the substrate. Also, in ion implantation, due to multiple scattering due to beam diffusion, part of ions is also implanted into the ion implantation side (region α) of the outer peripheral end surface 101c of the substrate. If the phosphorus on the outer peripheral end face is annealed in a later step, it diffuses into the substrate and causes a decrease in resistance. That is, a low resistance region (conduction portion) is also formed on the outer peripheral end surface of the base body, and is electrically connected to the back surface. As a result, the intention is that a solar cell with a PIN junction is formed on the front and back sides, but there is a problem that a short circuit occurs at the outer peripheral end face.

  Therefore, in the present invention, a heat treatment is performed before the annealing process, and the film made of phosphorus (P) adhering to the end face 101c portion of the substrate 101 during the ion implantation is evaporated and removed. As a result, even if annealing is performed, no conductive portion (low resistance region) is formed in the substrate end face region. This eliminates the resistance drop at the end surface of the substrate and avoids electrical continuity with the back surface. Further, since laser irradiation is not required as in the prior art, an increase in cost and process can be avoided. Furthermore, there is no loss of the photoelectric conversion region due to edge cut.

In the heat treatment of this step, the heating temperature is higher than the temperature of the n-type ion implantation step and lower than the temperature of the second annealing step.
Phosphorus takes various forms, but phosphorous formed from radicals is usually white phosphorus. White phosphorus (P4) is composed of tetrahedral molecules, and is a white waxy solid at normal temperature and pressure, having a specific gravity of 1.82, a melting point of 44.1 ° C., and a boiling point of 280 ° C. By heating at a temperature equal to or higher than the boiling point of phosphorus, it is possible to evaporate and remove the film formed on the surface of the substrate before annealing, or the one that has been injected into Si but emerges on the surface by diffusion. However, if the temperature is too high, it will be annealed and phosphorus will diffuse into the substrate.

  Since the temperature of the n-type ion implantation process is about 100 to 200 ° C., and the temperature of the second annealing treatment is about 850 ° C. as described later, the temperature of the heating process is specifically, for example, 200 to 800 ° C. It is preferable to set it at about ° C. Thereby, phosphorus can be evaporated without being annealed, and the phosphorus coating adhering to the end face of the substrate can be removed.

  In the heating step, the oxygen partial pressure is preferably set to 1% or less, for example. When the phosphorous film adhering to the end face portion of the substrate is oxidized, the portion becomes a conducting portion (low resistance region), and therefore it is important to prevent the phosphorous film from being oxidized. Further, by preventing oxidation of the phosphorus film, the phosphorus film can be removed even at a lower temperature.

  Next, a second annealing process is performed in an oxygen atmosphere on the substrate 101 into which n-type ions have been implanted. The conditions for the second annealing treatment are determined as optimum conditions according to the diffusion coefficient of n-type ions inside the substrate. That is, the temperature of the second annealing treatment is desirably about 850 ° C., and is at least lower than the temperature of the first annealing treatment. Further, the time for the second annealing treatment is preferably about 30 to 60 minutes.

  Furthermore, in this embodiment, it is preferable that the oxygen partial pressure is 1% or less in the second annealing treatment. When the phosphorous film adhering to the end face portion of the substrate is oxidized, the portion becomes a conducting portion (low resistance region), and therefore it is important to prevent the phosphorous film from being oxidized.

If the oxygen partial pressure during the second annealing treatment is greater than 1%, the sheet resistance of the obtained substrate is small, but if the oxygen partial pressure is 1% or less, the sheet resistance rapidly increases to, for example, 1000 Ωcm or more. This is presumably due to the fact that there is a large amount of phosphorus ion-implanted in the substrate surface, but phosphorus adhering as a radical film is dominant rather than implantation at the substrate end surface.
As an actual solar cell, there is no problem as a league current if the substrate end face resistance is 1000 Ωcm or more.

  Furthermore, it is preferable to suppress the water concentration to 1000 ppm or less during the second annealing treatment. It is important to exclude water, because the more water there is, the more phosphorous oxidation will be promoted.

  Next, an SiNx film is formed on one main surface 101a of the base so as to cover the n layer as an insulating film forming step. The SiNx film is a film that functions as an antireflection film, and can be formed using a plasma CVD method. In the present embodiment, the thickness of the SiNx film is 80 [nm].

  Next, as the electrode forming step, the light receiving surface electrode 105 is formed so as to penetrate the SiNx film and be electrically connected to the n layer. As a material constituting the light-receiving surface electrode 105, for example, silver (Ag) can be used, and it is formed by using a fire-through (fired through) method.

As an electrode formation step, a back electrode 106 is further formed on the other main surface 101b of the base so as to cover the p ⁺ layer. As a material constituting the back electrode 106, for example, aluminum (A1) can be used, and it is formed by a screen printing method.

  As described above, according to the method for manufacturing a crystalline solar cell according to the present embodiment, the film made of phosphorus (P) attached to the end face portion of the base body during n-type ion implantation is subjected to heat treatment. Since it is removed by evaporation, a conducting portion (low resistance region) is not formed on the end face of the substrate even after annealing. This eliminates the resistance drop at the end surface of the substrate and avoids electrical continuity with the back surface. As a result, in the method for manufacturing a crystalline solar cell according to the present invention, it is possible to prevent the formation of a conductive portion (low resistance region) on the end face of the substrate without requiring laser irradiation. Further, there is no loss of the photoelectric conversion region due to laser irradiation.

[Experimental example]
The optimum conditions for the heat treatment based on the experimental example using the crystalline solar cell 100 manufactured by the above-described manufacturing method will be described.
Using an ion implantation apparatus as shown in FIG. 2, plasma was generated by ICP (Inductive Coupled Plasma) discharge using a magnet, and ions were extracted and implanted using a mesh electrode.

A p-type silicon substrate with semiconductor specifications was used as the base. The substrate was set below the electrode in parallel with the electrode, and a predetermined beam amount (5 × 10 ¹⁵ Dose / cm ² ) was injected. The implantation energy was 3 kev and the substrate temperature was room temperature. In the ion source, PH ₃ gas was introduced and 13.56 MHz was applied to generate PH _x ⁺ ions and irradiate the substrate. After ion implantation, heat treatment was performed at 600 ° C. for 30 minutes.
After the heat treatment, annealing was performed at 850 ° C. for 30 minutes using a furnace. At this time, a mixed atmosphere of nitrogen gas and oxygen gas (N ₂ + O ₂ = 5000 sccm) was used as the atmosphere, and the flow rate ratio of N ₂ and O ₂ was changed.
Finally, the surface oxide film was removed using 1% HF.

The resistance values of the surface and end face of the substrate obtained as described above were measured with a four-probe. FIG. 4 shows an image of how to place the needle when measuring the resistance value.
The relationship between oxygen partial pressure and sheet resistance during annealing is shown in Table 1 and FIG.

As can be seen from Table 1 and FIG. 5, in the substrate plane, the sheet resistance is as small as 46 Ωcm ² when the oxygen partial pressure is 20%, but 62 Ωcm ² , 0.1 when the oxygen partial pressure is 1%. % Is gradually increasing to 70 Ωcm ² . On the other hand, at the end face of the substrate, it was found that the oxygen partial pressure was as small as 120 Ωcm ² when the oxygen partial pressure was 20%, but at 1%, it became 1000 Ωcm ² or more and increased rapidly. This is because there is a large amount of phosphorus ion-implanted in the substrate surface, but the phosphorus adhering as a film rather than implantation is dominant on the substrate end surface.
As an actual solar cell, there is no problem as a league current if the end face resistance is 1000 Ωcm or more. This shows that the oxygen partial pressure during the annealing process may be 1% or less.

In the annealing treatment, annealing was similarly performed using an RTA (Rapid Thermal Annealing) apparatus instead of the furnace. At this time, the temperature of the RTA is increased at 100 ° C./second.
The relationship between the oxygen partial pressure (unit:%) and the sheet resistance (unit: Ω / □) during annealing is shown in Table 2 and FIG.

  As can be seen by comparing FIG. 5 and FIG. 6, the resistance is slightly smaller than that in the case of annealing in the furnace. This is because the evaporation of phosphorus and phosphoric acid starts at 300 to 400 ° C, whereas diffusion does not occur unless the temperature is 800 ° C or higher. While it takes time to raise the temperature in the furnace (20 ° C./min), RTA is due to the very high temperature rise. That is, because the time from the evaporation temperature of phosphorus and phosphoric acid to the diffusion temperature is short, evaporation is less than in the case of the furnace.

  As mentioned above, although the manufacturing method and crystalline solar cell of the crystalline solar cell of this invention were demonstrated, this invention is not limited to the example mentioned above, In the range which does not deviate from the meaning of invention, it can change suitably.

  The present invention can be widely applied to the case where the photoelectric conversion efficiency of a solar cell using crystalline silicon is increased.

  100 crystal solar cell, 101 substrate, 101a, 101b main surface, 102 p-type semiconductor layer, 103 n-type semiconductor layer, 104 nitride film.

Claims (3)

A method for manufacturing a crystalline solar cell comprising a crystal substrate that exhibits a photoelectric conversion function between a light-receiving surface that receives light and a back surface that faces the light-receiving surface, wherein the crystal substrate is a single crystal or polycrystalline silicon The outer peripheral end surface of the base body is provided with a shape in which the entire region is divided into a region α forming an ion implantation side and a region β forming a non-ion implantation side,
Phosphorus (P) ions in a state where the entire outer peripheral end surface of the substrate is exposed together with the one main surface of the substrate to one main surface side including the entire outer peripheral end surface of the substrate. A first step of injecting;
Performing a heat treatment on the substrate, evaporating and removing the coating made of phosphorus (P) attached to the outer peripheral end surface portion of the substrate in the first step;
And a third step of performing an annealing process on the substrate at least in order.   2. The method for producing a crystalline solar cell according to claim 1, wherein the temperature in the second step is higher than the temperature in the first step and lower than the temperature in the third step.   The method for manufacturing a crystalline solar cell according to claim 1 or 2, wherein in the third step, the oxygen partial pressure is set to 1% or less.

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