JP2018195649A - Method for manufacturing crystal-type solar battery - Google Patents

Abstract

To provide a method for manufacturing a solar battery, which can prevent the outward diffusion in which ions implanted into a substrate by use of a non-mass separation type ion implantation system pass through the substrate to outside in annealing for activation.SOLUTION: A method for manufacturing a crystal type solar battery has a crystal substrate of a first conductivity type which develops a photoelectric converting function, and comprises at least, in turn, a first step S2 of using, as the crystal substrate, a flat plate-like base body of monocrystalline or polycrystalline silicon, and implanting ions of a conductivity type different from the first conductivity type into one principal face side of the base body, a second step S3 of implanting ions having the same conductivity type as the first conductivity type into the other principal face side of the base body, a third step S4 of forming a cap film so as to cover both the principal faces of the base body subjected to ion implantation in the first and second steps, and a fourth step S5 of performing an annealing treatment on the base body having gone through the third step.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for manufacturing a crystalline solar cell, in which ions implanted into a substrate using a non-mass separation type ion implantation apparatus can suppress outward diffusion that escapes out of the substrate during activation annealing.

  Solar power generation using solar energy is a power generation system that is expected as an alternative technology for fossil fuels. Various structures and configurations have been developed. Among these, crystalline silicon (Si) solar cells are most commonly used due to their performance such as photoelectric conversion efficiency and superior manufacturing costs.

  The structure of a photoelectric conversion element in a crystalline silicon solar cell is roughly classified into a homojunction type in which an emitter layer is formed by a diffusion method and a heterojunction type using amorphous silicon. In the homojunction type, PERRC (passivated emitter and rear contact) structure with electrodes on both sides, PERL (passivated emitter rear locally diffused) structure, IBC (Interdigitated Back Contact) structure without electrodes on the light receiving surface, etc. With this structure, conversion efficiency can be improved (for example, Patent Document 1).

For example, as shown in FIG. 16, a crystalline silicon solar cell 100 is a substrate made of n-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. (Hereinafter also referred to as a silicon substrate) 50. A high-concentration p-type region 51 is formed on one main surface side of the substrate forming the light receiving surface, and an n-type region 52 is formed on the other main surface side forming the back surface. By irradiating the substrate with light, holes which are minority carriers excited in the substrate are connected to the electrode 57 connected to the p + region 51, and electrons which are majority carriers are connected to the n ⁺ type region. 58 can be collected, and a current can be generated by the collected electrons and holes. In FIG. 16, reference numerals 54A and 54B denote a passivation film, reference numeral 55A denotes an anti-reflection film (ARC), and reference numeral 55B denotes a back light confinement film, which is provided as necessary.

Conventionally, patterning of such p ⁺ -type region and n ⁺ -type region is performed by forming a p ⁺ layer or n ⁺ layer by a solid phase diffusion method or a vapor phase diffusion method, and then patterning by laser or etching. A complicated process was required. For this reason, the manufacturing cost is increased and the manufacturing process becomes complicated. Therefore, the manufacturing process is simplified and the cost reduction is required.

An ion implantation technique using a mask has attracted attention as a technique for shortening the manufacturing process and reducing the cost in place of the patterning technique by laser or etching (Patent Document 2). Among them, the non-mass separation type ion apparatus is expected from the low cost.
In the non-mass separation type ion implantation apparatus, all ions in the separated plasma in the ion source are implanted. For example, when BF ₃ gas is used, all ions from BF ₃ ^{+ to} B ⁺ are implanted. Boron ions are implanted into the surface layer of the silicon substrate with a concentration profile different from that of normal single ion implantation.

  Also, in order to prevent damage, ions implanted to lower the implantation energy have an implantation profile in which many ions exist in a depth region of several nm to several tens of nm from the substrate surface. For this reason, in the case of boron at the time of activation annealing, an out-diffusion that causes boron to escape from the inside of the silicon substrate to the outside is caused and the acceptor concentration is reduced, while on the other hand, a dopant that does not become an acceptor However, there has been a problem that a large number remain in the silicon substrate. This phenomenon of outward diffusion is similar to that of phosphorus.

  When the above-described outward diffusion occurs, the resistance value of the ion implantation region changes or a distribution of resistance value occurs in the ion implantation region (for example, in the substrate depth direction), which affects the power generation characteristics of the solar cell. There was a risk of effect. Therefore, in order to stabilize the power generation characteristics of the solar cell, the solar cell having a structure in which the implanted ions (boron, phosphorus) do not cause outward diffusion at the time of activation annealing performed after ion implantation. Development was expected.

JP 2012-060080 A Japanese Patent Laid-Open No. 2017-017219

  The present invention has been made in consideration of the above points, and is a method for manufacturing a crystalline solar cell having a structure that does not cause outward diffusion during activation annealing after ion implantation into a substrate. Provide a method.

A manufacturing method of a crystalline solar cell according to claim 1 of the present invention is a manufacturing method of a crystalline solar cell including a first conductive type crystal substrate that expresses a photoelectric conversion function, A first step of implanting ions of a conductivity type different from the first conductivity type into one main surface side of the substrate using a flat substrate made of crystal or polycrystalline silicon; A second step of implanting ions of the same conductivity type as the first conductivity type into the other main surface side; and one main surface of the substrate ion-implanted by the first step and the second step A third step of forming a cap film so as to cover the other main surface and a fourth step of performing an annealing process on the substrate that has undergone the third step are provided at least in order.
That is, in the method for manufacturing a crystalline solar cell according to claim 1 of the present invention, when the first conductivity type of the crystal substrate is n-type, the conductivity type (p ⁺ ) different from the first conductivity type in the first step. Ions [for example, boron (B) ions] are implanted, and in the second step, ions of the same conductivity type (n ⁺ ) as the first conductivity type [for example, phosphorus (P) ions] are implanted. Conversely, when the first conductivity type of the crystal substrate is p-type, ions of a conductivity type (n ⁺ ) different from the first conductivity type are implanted in the first step, and the first conductivity type is different in the second step. Ions of the same conductivity type (p ⁺ ) are implanted.

A method for producing a crystalline solar cell according to a second aspect of the present invention is the method according to the first aspect, wherein the cap film is formed on one main surface and the other main surface of the base by a vapor deposition method (for example, It is a film deposited by a CVD method or a PVD method).
The method for producing a crystalline solar cell according to claim 3 of the present invention is the method according to claim 1 or 2, wherein the cap film is a single layer film selected from a silicon oxide film, an aluminum oxide film, and a silicon nitride film, or The method for producing a crystalline solar cell according to claim 1, wherein the method is a laminated film of these.
The method for producing a crystalline solar cell according to claim 4 of the present invention is characterized in that, in any one of claims 1 to 3, the cap film has a thickness of 30 nm to 300 nm.
The method for producing a crystalline solar cell according to claim 5 of the present invention is characterized in that, in any one of claims 1 to 3, the cap film has a thickness of 50 nm to 300 nm.

According to the method for manufacturing a crystalline solar cell according to the present invention, after ion implantation (first step, second step) into the substrate, a cap film is formed so as to cover the ion-implanted region (third step). ) And then an annealing process (fourth step) is performed. That is, annealing is performed in a state where the ion implantation region is sealed with the cap film. Therefore, the present invention provides a method for manufacturing a solar cell having a structure in which implanted ions (boron, phosphorus) do not cause outward diffusion during activation annealing performed after ion implantation.
The cap film is preferably a film deposited on one main surface and the other main surface of the substrate by a vapor deposition method (CVD method, PVD method).
The cap film is preferably a single layer film selected from a silicon oxide film, an aluminum oxide film, and a silicon nitride film, or a laminated film thereof.
The thickness of the cap film is preferably 30 nm to 300 nm, more preferably 50 nm to 300 nm.

The flowchart which shows the manufacturing method of the crystalline solar cell which concerns on this invention. The typical sectional view showing an example of the crystalline solar cell produced by the manufacturing method of FIG. Sectional drawing of the ion implantation apparatus used for manufacture of a crystalline solar cell. Sectional drawing of the cap film forming apparatus used for manufacture of a crystalline solar cell. Sectional drawing of the annealing treatment apparatus used for manufacture of a crystalline solar cell. The graph which shows the result (depth range: 0-> 1000nm) of SIMS. The graph which shows the shallow area | region (depth range: 0-> 100 nm) of FIG. Graph showing sheet resistance (cap film = SiO _x : presence / absence of cap film, ion implantation condition dependency). Graph showing sheet resistance (cap film = SiN _y : presence / absence of cap film, film formation method dependence). A graph showing sheet resistance (cap film = SiO _x : film thickness dependence). The graph which shows the case where the horizontal axis of FIG. 10 is a wide range (0-> 300 nm). The graph (cap film: silicon oxide) which shows the characteristic [J (current density) -V (voltage) curve] of a solar cell. The typical sectional view showing the example which applied the cap to the crystal system solar cell of PERC structure. The typical sectional view showing the example which applied the cap to the crystal system solar cell of PERL structure. The flowchart which shows the manufacturing method of the conventional crystalline solar cell. FIG. 16 is a schematic cross-sectional view illustrating an example of a crystalline solar cell manufactured by the manufacturing method of FIG. 15.

  Hereinafter, based on a preferred embodiment, the present invention will be described with reference to the drawings.

<Crystal solar cell>
FIG. 1 is a flowchart showing a method for manufacturing a crystalline solar cell according to the present invention. FIG. 2 is a schematic cross-sectional view showing an example of a crystalline solar cell manufactured by the manufacturing method of FIG.
As shown in FIG. 2, a crystalline solar cell 1A (1) manufactured by the manufacturing method according to the present invention has a light receiving surface (upper surface in FIG. 2) that receives light and a back surface (in FIG. 2) that faces the light receiving surface. It is a solar cell provided with the crystal substrate 10 which expresses a photoelectric conversion function between the lower surface).

  The crystal substrate 10 is a flat substrate made of a first conductivity type (n-type) single crystal or polycrystalline silicon. The crystal substrate (base body) 10 is preferably provided with an antireflection structure (also referred to as a surface micro uneven structure) on one main surface (light receiving surface) 10a. The antireflection structure brings about a reflectance reduction effect on the light receiving surface and a confinement effect of incident light. Further, as the antireflection structure, an uneven structure having a large inverted pyramid may be provided (FIGS. 13 and 14).

The crystal substrate (base body) 10 has a region 11 in which ions [boron (B) ions] having a conductivity type (p ⁺ ) different from the first conductivity type are implanted into one main surface (light receiving surface) 10a side. And the other main surface (rear surface) 10b side each having a region 12 in which ions [phosphorus (P) ions] of the same conductivity type (n ⁺ ) as the first conductivity type are implanted.

  The region 11 is covered with a stacked structure 16A in which a passivation film 14A and an ARC film 15A are sequentially stacked. The region 12 is covered with a stacked structure 16B in which a passivation film 14B and an ARC film 15B are sequentially stacked.

  Further, the p-type region 11 is electrically connected to a first electrode (also referred to as a light receiving surface electrode) 17 formed so as to penetrate the laminated structure 16A. The n-type region 12 is electrically connected to a second electrode (also referred to as a back electrode) 18 formed so as to penetrate the laminated structure 16B.

<Method for producing crystalline solar cell>
The method for producing a crystalline solar cell according to the present invention comprises at least the first to fourth steps described below in order.
As described above, the flat substrate 10 made of single crystal or polycrystalline silicon is used as the crystal substrate. The crystal substrate in the present embodiment is a silicon substrate of the first conductivity type (n) that exhibits a photoelectric conversion function, and a substrate in which a reflection confinement structure is formed on the light receiving surface side that receives light is preferably used (see FIG. 1 S1).

First, ions [boron (B) ions] having a conductivity type (p ⁺ ) different from the first conductivity type are implanted into one main surface side (surface side serving as a light receiving surface) of the substrate (hereinafter referred to as “boron (B) ions”). Also referred to as the first step: S2 in FIG. In FIG. 2, reference numeral 11 denotes a p-type region into which boron is implanted.
Next, ions of the same conductivity type (n ⁺ ) as the first conductivity type [phosphorus (P) ions] are implanted into the other main surface side (the back surface side facing the light receiving surface) of the substrate. (Hereinafter also referred to as the second step: S3 in FIG. 1). In FIG. 2, reference numeral 12 denotes an n-type region implanted with phosphorus.

Next, cap films 13A and 13B are formed so as to cover one main surface and the other main surface of the base body ion-implanted in the first step S2 and the second step S3 (hereinafter also referred to as a third step: FIG. 1 S4).
Thereafter, annealing is performed on the substrate that has undergone the third step S4 (hereinafter also referred to as a fourth step: S5 in FIG. 1). As a result, a p-type region is formed on the front surface side of the substrate, and an n-type region is formed on the back surface side of the substrate.

If necessary, the cap layers 13A and 13B are removed from the substrate that has undergone the fourth step S5 (S6 in FIG. 1) to form passivation films 14A and 14B (S7 in FIG. 1), the ARC film 15A, 15B is formed (S8 in FIG. 1).
Finally, on the surface side of the substrate, a first electrode 17 is formed which is electrically connected to the p-type region of the substrate. On the back side of the substrate, a second electrode 18 is formed which is electrically connected to the n-type region of the substrate (S9 in FIG. 1).

The ion implantation in the first process and the second process described above is performed using, for example, an ion implantation apparatus 1200 shown in FIG.
FIG. 3 is a cross-sectional view of an ion implantation apparatus 1200 used for the p-type ion implantation step (first step) and the n-type ion implantation step (second step) in the present invention. The ion implantation apparatus 1200 includes a vacuum chamber 1201, a permanent magnet 1205, an RF introduction coil 1206, plasma generation means by ICP discharge using an RF introduction window (quartz) 1212, and vacuum exhaust means (not shown). The inside of the vacuum chamber 1201 is separated into a plasma generation chamber and a plasma processing chamber by mesh electrodes 1208 and 1209.

  In the plasma processing chamber, a substrate support 1204 that supports a substrate (corresponding to the substrate 10 after the antireflection structure is formed) 1203 as an object to be processed is arranged. Note that the mesh electrode 1208 has a floating potential and has a function of stabilizing the potential of the plasma 1207. Further, the mesh electrode 1209 has a function of extracting a positive ion from the plasma 1207 when a negative potential is applied thereto.

The inside of the vacuum layer 1201 is depressurized, and a gas containing p-type impurity atoms injected into the surface of the substrate 1203 [one main surface (light receiving surface) 10a side of the crystal substrate 10] is introduced into the plasma generation chamber. Then, the plasma 1207 is excited using the plasma generating means to ionize the impurity atoms, and p-type ions extracted through the mesh electrodes 1208 and 1209 can be implanted into the surface of the substrate 1203. . As a result, the crystal substrate 10 was implanted with ions of a conductivity type (p ⁺ ) different from the first conductivity type [for example, boron (B) ions] on one main surface (light receiving surface) 10a side. Region 11 is obtained [first step].

  Next, the upper and lower surfaces of the substrate 1203 are reversed by the moving means (not shown) of the substrate 1203 and placed on the substrate support 1204. Thus, the back surface of the substrate 1203 [the other main surface side of the crystal substrate 10 (the back surface side facing the light receiving surface)] is disposed at a position facing the mesh electrodes 1208 and 1209.

A gas containing n-type impurity atoms to be injected into the back surface of the substrate 1203 is introduced into the plasma generation chamber. Then, the plasma 1207 is excited using plasma generating means to ionize impurity atoms, and n-type ions extracted through the mesh electrodes 1208 and 1209 can be implanted into the back surface of the substrate 1203. . Thereby, the crystal substrate 10 has ions of the same conductivity type (n ⁺ ) as the first conductivity type (for example, phosphorus (P) ions) with respect to the other main surface (the back surface side facing the light receiving surface) 10b. To obtain a region 12 in which is injected [second step].

Next, a cap film is formed as a third step so as to cover the regions 11 and 12.
The cap film is formed using, for example, an inter-back type CVD apparatus 1400 shown in FIG.

In the manufacturing apparatus 1400 of FIG. 4, the substrate 1458 (corresponding to the substrate 10 on which the region 11 and the region 12 are formed) is transferred to a preparation take-out chamber (L / UL) 1451 and a heating chamber (H) by a transfer means (not shown). 1452, the first film formation chamber (S1) 1453, and the second film formation chamber (S2) 1454 are movable.
Exhaust means 1451P, 1452P, 1453P, and 1454P are arranged in each of the chambers 1451, 1452, 1453, and 1454 individually so that the internal space can be decompressed.

  First, the substrate 1458 is introduced from the manufacturing apparatus into the preparation / extraction chamber 1451 that is set to atmospheric pressure from the outside (atmosphere). Thereafter, the charging / unloading chamber 1451 is made into a reduced-pressure atmosphere using the exhaust means 1451P. Next, the substrate 1458 is transferred from the charging / unloading chamber 1451 in a reduced pressure atmosphere to the heating chamber 1452 and subjected to a desired heat treatment by the heating means 1459.

Next, the substrate 1458 after the heat treatment is transferred from the heating chamber 1452 to the first film formation chamber 1453 and passed in front of the target 1462 made of silicon, thereby forming a cap film on the surface (region 11) of the substrate 1458. A silicon oxide film is formed. Thereby, the cap film 13 </ b> A is formed so as to cover the region 11.
At that time, the substrate 1458 is brought to a desired temperature by the temperature adjusting means 1461. The target 1462 is placed on a backing plate 1463. During sputtering, a desired process gas is introduced from a gas supply source 1465, and desired power is supplied to the backing plate 1463 from a power source 1464.

Next, the substrate 1458 on which the cap film 13A is formed is transferred from the first film formation chamber 1453 to the second film formation chamber 1454 and passes in front of the target 1472 made of silicon, whereby the back surface (region) of the substrate 1458 is obtained. 12) A silicon oxide film is formed thereon as a cap film. Thereby, the cap film 13B is formed so as to cover the region 12.
At that time, the substrate 1458 is brought to a desired temperature by the temperature adjusting means 1471. The target 1472 is placed on the backing plate 1473. During sputtering, a desired process gas is introduced from the gas supply source 1475, and desired power is supplied to the backing plate 1473 from the power source 1474.
Table 1 shows typical manufacturing conditions of the cap film (silicon oxide film) described above.

  Then, the substrate 1458 on which the cap film 13A and the cap film 13B are formed is transferred from the second film formation chamber 1454 to the preparation / extraction chamber 1451 and taken out from the manufacturing apparatus to the outside (atmosphere).

The film thickness of the cap films 13A and 13B is preferably 30 nm to 300 nm, and more preferably 50 nm to 300 nm.
As will be described later, when the thickness of the cap film is 30 nm or more, the sheet resistance value can be suppressed by about 10% lower than when the cap film is not provided (when no cap film is provided). When the film thickness of the cap film is 50 nm or more, the sheet resistance value can be further suppressed, and can be reduced by about 40% compared to the case without the cap film.

  Note that the cap film 13A and the cap film 13B are not limited to silicon oxide films. Instead of the silicon oxide film, an aluminum oxide film or a silicon nitride film may be used. It is not limited to these single layer films, and may be a laminated film of two or more layers including a silicon oxide film, an aluminum oxide film, and a silicon nitride film.

Next, an annealing process is performed as a fourth step so as to cover the cap films 13A and 13B. That is, annealing is performed in a state where the ion implantation regions (region 11 and region 12) are sealed with the cap film.
The annealing process in the fourth step is performed using, for example, an annealing apparatus 1500 shown in FIG. An annealing apparatus 1500 of FIG. 5 includes a first buffer chamber 1502, a first heating chamber 1503, a second heating chamber 1504, a third heating chamber 1505, and a second buffer chamber 1506.

The annealing process by the annealing apparatus 1500 is performed through the following first to fifth steps. That is, as a first step, the substrate to be processed [the substrate in which the ion implantation regions (regions 11 and 12) are sealed with the cap film until the first heating chamber 1503 reaches a desired temperature rising state. Corresponds to the first buffer chamber 1502 supplied with nitrogen (N ₂ ) gas. Next, as a second step, the substrate 1501 is heated at 600 ° C. using a lamp in the first heating chamber 1503 to which oxygen gas (O ₂ ) is supplied. Next, as a third step, in the second heating chamber 1504 to which oxygen gas (O ₂ ) is supplied, the substrate 1501 is heated at a desired annealing temperature using a lamp. Next, as a fourth step, the substrate 1501 is cooled in the third heating chamber 1505 so as to reach room temperature. Then, as a fifth step, the substrate 1501 is kept in the second buffer chamber 1506 supplied with nitrogen (N ₂ ) gas until preparation for unloading from the annealing apparatus 1500 is completed.

  The conditions for the annealing treatment are determined as optimum conditions corresponding to the activation and recrystallization of n-type ions and p-type ions inside the substrate. For example, the annealing temperature is desirably 900 ° C. or higher. Moreover, it is desirable that the time required for the annealing treatment is about 30 to 60 minutes.

Next, after removing the cap films 13A and 13B from the annealed substrate, passivation films 14A and 14B and antireflection films 15A and 15B are provided as necessary. These are produced by a desired sputtering apparatus. In the case where the cap films 13A and 13B have the functions of the passivation films 14A and 14B, the cap films 13A and 13B can be used for the passivation films 14A and 14B.
Examples of the passivation films 14A and 14B include aluminum oxide, silicon nitride, and silicon oxide.
As the antireflection films 15A and 15B, for example, a structure in which thin films having different refractive indexes and film thicknesses are stacked in multiple layers, a film having a refractive index change in the depth direction of the film, and the surface has an uneven shape. Membrane, etc.

  Finally, the electrodes 17 and 18 are formed by a known manufacturing method. Known manufacturing methods include, for example, a perforation method and a through-through method. In the following description, the point contact means a portion (part) that is electrically connected to the ion implantation region (region 11 and region 12).

  The perforated method is to form a laminate composed of the above-described cap film, passivation film and antireflection film on both the front and back surfaces of the substrate, and then to form the laminate using only a point contact portion or a laser or etching paste. Remove to form an opening. In this opening, the substrate is brought into contact with aluminum or silver serving as an electrode.

  In the fire through method, a laminate composed of the above-described cap film, passivation film, and antireflection film is formed on both the front and back surfaces of the substrate, and then a silver paste for Fire through is applied to the point contact portion. This is a technique for making contact by causing a silver paste and a laminate to react and fire through during cell firing.

By passing through the process mentioned above, the solar cell of the structure shown in FIG. 2 can be manufactured.
In the following, the influence of the annealing treatment of the implanted ions was evaluated. Here, the case where the implanted ions are boron (B) was evaluated using SIMS (Secondary Ion Mass Spectrometry). At that time, the annealing treatment was performed under certain treatment conditions (atmosphere, 1000 ° C., 60 min).

Four types of samples 1 to 4 shown in Table 2 were prepared and evaluated using SIMS.
Here, the cap film was silicon oxide (SiO _x ), and the film thickness was 50 nm. Samples 1 and 2 having different injection energies under two conditions (10 keV, 5 keV) were prepared. For comparison, Samples 3 and 4 without a cap film were also prepared. All samples 1 to 4 are not provided with a passivation film, an antireflection film, or an electrode.

6 and 7 are graphs showing the SIMS results, where the horizontal axis represents the evaluated depth (position) and the vertical axis represents the boron concentration observed at the depth (position).
6 and 7, the horizontal axis represents the depth in the implantation region 1. 0 on the horizontal axis represents the surface of the implantation region 1, that is, the portion where the electrode contacts. The greater the number on the horizontal axis, the greater the depth in the implantation region 1.
FIG. 6 is a graph showing the depth range of 0 to 1000 nm, and FIG. 7 is a graph showing the shallow region (depth range: 0 to 100 nm) of FIG. 6 in an enlarged manner.

The following points became clear from FIGS.
(A1) The tendency for the boron concentration to decrease with increasing depth from the surface was the same for all samples 1-4. While it gradually decreases in the range of 0 to 400 nm, it tends to decrease when it exceeds 400 nm.
(A2) Depending on the presence or absence of the cap film, a difference was confirmed in the boron concentration in the depth range of 0 to 400 nm. Samples 1 and 2 provided with a cap film had a boron concentration about 2 to 3 times higher than Samples 3 and 4 provided with no cap film.
(A3) In particular, in the range where the depth is 0 to 20 nm, the tendency of (A2) is remarkably strong. Samples 1 and 2 provided with a cap film had a boron concentration about 10 times that of Samples 3 and 4 provided with no cap film.
(A4) In Samples 1 and 2 provided with a cap film, the tendency of the above (A2) and (A3) did not depend on the implantation energy, and the boron concentration was a file in the same depth direction.
(A5) In the samples 3 and 4 where no cap film is provided, the tendency of the above (A2) and (A3) depends on the implantation energy. Sample 3 with a large implantation energy had a boron concentration about twice as high as that of sample 4 with a small implantation energy.

From the above results, it was confirmed that the cap film has an effect of preventing outward diffusion of implanted ions (boron) during activation annealing performed after ion implantation.
Although not described in detail here, the same results as (A1) to (A5) described above were obtained when phosphorus was implanted instead of boron.
From this, it was found that the structure provided with the cap film does not depend on the type of implanted ions (boron, phosphorus) and contributes to suppressing the outward diffusion that occurs during the activation annealing performed after the ion implantation. .

FIG. 8 is a graph showing the results of measuring the sheet resistance for the samples 1, 3, and 4 described above. That is, Sample 1 is a case where a cap film made of silicon oxide (film thickness 50 nm) is provided, and Samples 3 and 4 are cases where a cap film is not provided. Sample 1 and sample 3 have the same injection energy of 10 keV. Sample 4 has an implantation energy of 5 keV.
8 is a sheet resistance value in a solar cell manufactured by a solid phase diffusion method using an APCVD method (atmospheric pressure CVD method). The target of the ion implantation method was the numerical value of this comparative example.

The following points became clear from FIG.
(B1) In Sample 3 and Sample 4 where no cap film is provided, the sheet resistance is about 110Ω / □, and the level of the comparative example (60Ω / □: indicated as “Ref” in FIG. 8) has not been reached. there were.
(B2) In Sample 1 provided with a cap film, the sheet resistance was found to be halved (110 → 55Ω / □).
As described above, the provision of the cap film prevents the outward diffusion of the implanted ions (boron) during the activation annealing performed after the ion implantation. As a result, the target APCVD method is equivalent to the solid phase diffusion method. It was confirmed that the sheet resistance value can be realized by the ion implantation method.

FIG. 9 is a graph showing the sheet resistance values of four samples (samples 6, 7, 8, and 9) in which the film forming method and the ion implantation amount (Dose) are changed. Regarding the film forming method, Samples 6 and 8 are APCVD methods (atmospheric pressure CVD methods), and Samples 7 and 9 are PECVD methods (plasma CVD methods). The ion implantation amount (Dose [ions / cm ² ]) is 8E + 15 for samples 6 and 7 and 4E + 15 for samples 8 and 9. The film thickness of the cap film was constant (300 nm).
Details of each sample are shown in Table 3. The sheet resistance shown in Table 3 is a numerical value after annealing (1000 ° C., 60 min).

From FIG. 9 and Table 3, the following points became clear.
(C1) In the APCVD method and the PECVD method, substantially the same sheet resistance value was confirmed.
(C2) Even if the ion implantation amount differs twice, there is no difference in the tendency of the sheet resistance to increase as the implantation amount in (C1) decreases.
From the above, it was confirmed that the effect of providing the silicon oxide film as the cap film was the same between the APCVD method and the PECVD method.

FIG. 10 is a graph showing the sheet resistance values of samples 1a, 1b, and 1c prepared by changing the thickness of the cap film (silicon oxide film) in the extremely thin range in the sample 1 described above. .
The thicknesses [nm] of the cap films of Sample 1a, Sample 1b, and Sample 1c are 0, 2.7, and 4.7 in order. That is, the sample 1a corresponds to the case where the cap film is not provided (sample 3 described above).
FIG. 11 is a graph showing the sheet resistance value of each sample prepared by changing the film thickness of the cap film (SiO _x film) in a wide range (0 to 300 nm) in the sample 1 described above.

The following points were clarified from FIGS.
(D1) It was found that the ultra-thin cap film (2.7 nm, 4.7 nm) had no influence on the sheet resistance value (FIG. 9).
(D2) By setting the film thickness to 30 nm or more, the sheet resistance value is less than 100Ω / □, indicating a decreasing tendency.
(D3) By setting the film thickness to 50 nm or more, a sheet resistance value of about 60Ω / □ can be obtained. Equivalent sheet resistance values were confirmed in the film thickness range of 50 nm to 300 nm.
From the above, when a silicon oxide film is provided as a cap film, a film thickness of 30 nm or more is preferable in order to reduce the sheet resistance value. By providing a cap film having a thickness in the range of 50 nm or more and 300 nm or less, a sheet resistance value of about 60Ω / □ is stably obtained, which is more preferable.

FIG. 12 is a graph showing the characteristic [J (current density) -V (voltage) curve] of the solar cell. In FIG. 12, in the case of a solar cell (α) manufactured using the APCVD method (atmospheric pressure CVD method) as a diffusion source, the symbol APCVD uses the present invention (ion implantation method) and the cap film (oxidation method). In the case of a solar cell (β) provided with a silicon film (50 nm), respectively. For comparison, the latter was also examined for a solar cell (γ) without a cap film.
Table 4 shows the evaluation results of the three types of solar cells. The evaluation parameters shown in Table 3 are the short circuit current density (J _SC ), the open circuit voltage (V _OC ), the fill factor (FF), the conversion efficiency (E _ff ), the series resistance (R _S ), and the shunt resistance (R _Sh ). It is.

From FIG. 12 and Table 4, the following points became clear.
(D1) By providing a cap film (silicon oxide film, 50 nm), numerical values are improved in all evaluation parameters (comparison between solar cells β and γ). As a result, the conversion efficiency (E _ff ) was improved by 3.3%.
(D2) The solar cell of the present invention (ion implantation method) provided with a cap film (silicon oxide film, 50 nm) has the same characteristics as the solar cell (α) manufactured by solid phase diffusion in the APCVD method. (Comparison of solar cells β and α).
From the above, it was confirmed that according to the manufacturing method (ion implantation method) of the present invention in which a silicon oxide film is provided as a cap film, a solar cell of the same level as that of the APCVD method can be manufactured.

  As a result of changing the cap film from the silicon oxide film to the silicon nitride film and the aluminum oxide film and performing similar verification, the same result as that of the silicon oxide film was obtained. That is, even when the cap film is a silicon nitride film and an aluminum oxide film, the film thickness is preferably 30 nm or more, and more preferably 50 nm or more and 300 nm or less.

FIG. 13 is a schematic cross-sectional view showing an example in which the above-described cap film is applied to a crystalline solar cell having a PERC (passivated emitter and rear contact) structure.
Reference numerals in FIG. 13 are the same as those in FIG. That is, reference numeral 10 denotes a crystal substrate 10 which is a flat substrate made of a first conductivity type (n-type) single crystal or polycrystalline silicon. However, in the case of FIG. 13, the crystal substrate (base body) 10 is different from FIG. 2 in that one main surface (light receiving surface) 10a has an inverted pyramid structure. The other points are the same as in FIG.

By providing the cap films 13A and 13B on both the front surface forming the light receiving surface and the back surface located on the opposite side of the light receiving surface, the outward diffusion of the doped ions is reduced. Further, after removing the cap films 13A and 13B on both sides, the p ⁺ layer and the n ⁺ layer are respectively provided with the passivation films 14A and 14B, the ARC film 15A, and the back light confinement film 15B to be a laminated structure. Even if such a laminated structure is provided, the surface side forming the light receiving surface reflects the ARC films 15A and 15B where the inverted pyramid structure is located on the outermost surface, and therefore depends on the thickness of the laminated structure. Therefore, the effect of suppressing surface reflection can be obtained stably.

FIG. 14 is a schematic cross-sectional view showing an example in which the above-described cap film is applied to a crystalline solar cell having a PERL (passivated emitter rear locally diffused) structure.
Each symbol in FIG. 14 is the same as that in FIG. That is, reference numeral 10 denotes a crystal substrate 10 which is a flat substrate made of a first conductivity type (n-type) single crystal or polycrystalline silicon. 15 also differs from FIG. 2 in that the crystal substrate (base body) 10 has an inverted pyramid structure on one main surface (light receiving surface) 10a as in FIG. doing. The other points are the same as in FIG.
Similar to the PERC structure of FIG. 13, the above laminated structure can be applied to the PERL structure of FIG. 14. Therefore, even in the crystal solar cell having the PERL structure, the operation and effect of applying the cap film of the present invention can be obtained.

  According to the present invention, after ion implantation (first step and second step) into the substrate, a cap film is formed so as to cover the ion-implanted region (third step), and then an annealing process (first step). By performing the four steps, annealing is performed in a state where the ion implantation region is sealed with the cap film. Thus, the present invention contributes to providing a method for manufacturing a crystalline solar cell having a structure in which implanted ions (boron and phosphorus) do not cause outward diffusion during activation annealing performed after ion implantation. .

  The present invention can be widely applied to a method for manufacturing a crystalline solar cell. This manufacturing method is suitably used for manufacturing a solar cell having a structure that requires high power generation efficiency per unit area.

  S1 texture formation, S2 first step (boron implantation), S3 second step (phosphorus implantation), S4 third step (cap formation), S5 fourth step (annealing), S6 cap removal, S7 passivation formation, S8 ARC formation , S9 Electrode formation.

Claims (5)

A method for producing a crystalline solar cell comprising a first conductivity type crystal substrate that exhibits a photoelectric conversion function,
As the crystal substrate, a flat substrate made of single crystal or polycrystalline silicon is used,
A first step of implanting ions of a conductivity type different from the first conductivity type into one main surface side of the substrate;
A second step of implanting ions of the same conductivity type as the first conductivity type on the other main surface side of the substrate;
A third step of forming a cap film so as to cover one main surface and the other main surface of the base body ion-implanted by the first step and the second step;
A fourth step of performing an annealing process on the substrate that has undergone the third step;
Are provided in order at least in order. The manufacturing method of the crystalline solar cell characterized by the above-mentioned.   2. The method for manufacturing a crystalline solar cell according to claim 1, wherein the cap film is a film deposited by vapor phase film formation on one main surface and the other main surface of the substrate. .   3. The crystalline solar cell according to claim 1, wherein the cap film is a single layer film selected from a silicon oxide film, an aluminum oxide film, and a silicon nitride film, or a laminated film thereof. Production method.   4. The method for producing a crystalline solar cell according to claim 1, wherein a thickness of the cap film is 30 nm or more and 300 nm or less. 5.   4. The method for manufacturing a crystalline solar cell according to claim 1, wherein the cap film has a thickness of 50 nm to 300 nm. 5.

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