JP2015050277A - Solar cell and process of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solar cell and such including an electrode containing no special material.SOLUTION: A rear face passivation film (25) has a multilayer film structure including an aluminum oxide film and a silicon nitride film. The refractive index of the silicon nitride film is equal to or higher than 2.4 and equal to or lower than 3.1. A back electrode (28) penetrates the rear face passivation film (25) by a fire-through phenomenon.

Description

  The present invention relates to a solar cell and a manufacturing method thereof, for example, a back-passivation type solar cell and a manufacturing method thereof.

  A solar cell that is a kind of photoelectric conversion element has a pn junction in which a p-type semiconductor layer that collects positively-charged holes generated by receiving light and an n-type semiconductor layer that collects negatively-charged electrons are bonded. Have. The pn junction is formed, for example, by depositing an n-type impurity diffusion layer (n + layer) on the surface of a p-type silicon substrate.

(Conventional aluminum BSF solar cell 900)
Conventionally, an aluminum BSF (Back Surface Field) type solar cell is known as a kind of solar cell. An aluminum BSF type solar cell is a solar cell provided with a p + layer formed of an aluminum alloy described below on the back surface side opposite to the light receiving surface. Aluminum BSF solar cells are suitable for mass production because they are relatively easy to manufacture. FIG. 14 shows a schematic diagram of a conventional aluminum BSF solar cell 900 (hereinafter simply referred to as solar cell 900).

  As shown in FIG. 14, a main surface electrode 927 and a back surface electrode 928 are provided on the light receiving surface side and the back surface side of the solar cell 900, respectively. The back electrode 928 is provided on a p + layer 929 formed on the back side of a p-type silicon substrate 920 (hereinafter referred to as a silicon substrate 920). It can be said that the back electrode 928 and the p + layer 929 are combined to have a two-layer structure.

  In solar cell 900, an electric field derived from the potential difference between silicon substrate 920 and p + layer 929 is formed at the interface between silicon substrate 920 and p + layer 929 (hereinafter referred to as the p / p + interface). This electric field mainly reflects electrons inside the silicon substrate 920 when electrons and holes generated in the silicon substrate 920 move to the p / p + interface due to the diffusion phenomenon, while selectively transferring holes to p +. Pass into layer 929. In other words, the electric field generated at the p / p + interface spatially separates the hole density distribution and the electron density distribution in the vicinity of the back surface of the solar cell 900.

  Thereby, the probability that holes and electrons recombine in the vicinity of the back surface of solar cell 900 is reduced. Therefore, the electric field at the p / p + interface has the effect of eliminating electrons and reducing carrier loss caused by recombination of holes and electrons. This effect is called a field passivation effect.

(Problems of aluminum BSF solar cell 900)
As described above, relatively high conversion efficiency can be obtained by the electric field generated at the p / p + interface. However, the solar cell 900 has the following problems when viewed from the viewpoint of further increasing the power generation efficiency.

  In the solar cell 900, most of the electrons are reflected inside the silicon substrate 920 by the electric field at the p / p + interface, but a few electrons having a kinetic energy higher than the potential difference between the silicon substrate 920 and the p + layer 929 are , Get over the p / p + interface and penetrate into the p + layer 929. However, in the p + layer 929, highly doped aluminum forms recombination centers, so the density of recombination centers is high. In other words, the p + layer 929 is a layer having poor quality as a semiconductor. Therefore, electrons that have entered the p + layer 929 cause recombination with high probability and disappear in the p + layer 929.

  On the other hand, electrons that have not disappeared in the p + layer 929 reach the interface between the p + layer 929 and the back electrode 928 (hereinafter referred to as the p + / back electrode interface). However, no passivation treatment is performed at the p + / back electrode interface to reduce the rate of recombination of holes and electrons. For this reason, electrons disappear with recombination at a high probability at the p + / back electrode interface. The disappearance of electrons generated as described above causes a loss of the electrical output of the solar cell 900.

  Furthermore, in the solar cell 900, the light absorption rate of the p + layer 929 is high. Therefore, most of the light reaching the back surface side without being subjected to photoelectric conversion on the light incident surface side of the solar cell 900 is absorbed by the p + layer 929. In addition, electron-hole pairs generated when light is absorbed by the p + layer 929 are immediately extinguished by recombination. Therefore, the light absorbed by the p + layer 929 does not contribute to the electrical output of the solar cell 900.

  Also, in general, the p + / back electrode interface is not smooth, and the physical shape is uneven. Therefore, the light reflectance at the p + / back electrode interface is low.

  It is desirable that a part of the light incident on the light receiving surface of the solar cell 900 and reaching the back surface is reflected on the back surface and returns to the inside of the solar cell 900 again to contribute to power generation. However, in the aluminum BSF solar cell 900, as described above, the p + / back electrode interface was reached due to the high light absorption rate of the p + layer 929 and the low light reflectivity of the p + / back electrode interface. There is a problem that light cannot be used effectively for power generation.

(Conventional backside passivation solar cell 800)
Development of the back surface passivation type solar cell has been promoted with the aim of overcoming the above-described problems of the aluminum BSF type solar cell and replacing the aluminum BSF type solar cell. The back surface passivation type solar cell is not designed based on the technical idea of suppressing the recombination speed of carriers unlike the aluminum BSF type solar cell. Instead, backside passivation solar cells are designed based on the technical idea of reducing the frequency of carrier recombination by reducing the density of recombination centers existing near the backside of the solar cell. Yes.

  FIG. 13 shows a schematic diagram of a conventional backside passivation solar cell 800 (hereinafter simply referred to as solar cell 800). As shown in FIG. 13, the solar cell 800 is different from the solar cell 900 in that most of the back surface of the silicon substrate 820 is covered with an insulating back surface passivation film 825. Since the back surface passivation film 825 is an insulator, it is impossible to extract the power of the solar cell from the portion covered with the insulator. For this purpose, a back electrode 828 is provided on the local remaining portion of the back surface of the silicon substrate 820. The back electrode 828 is generally designed to occupy an area of 10% or less of the entire back surface of the silicon substrate 820. Further, a p + layer 829 is formed at a portion where the back electrode 828 and the silicon substrate 820 are in contact with each other.

  (i) inherently present at the interface between the p-type silicon substrate 820 and the back surface passivation film 825 (hereinafter referred to as the p / back surface passivation film interface), and (ii) the cause of recombination of electrons and holes. As a result, the dangling bonds are terminated by the elements in the film of the back surface passivation film 825, so that they are not recombination centers. Therefore, the possibility that holes and electrons are trapped by dangling bonds at the p / backside passivation film interface is reduced. Therefore, many of the holes and electrons that have reached the p / backside passivation film interface are reflected at the interface and return to the silicon substrate 820 side. This effect is called chemical passivation with respect to field passivation on the back surface of the aluminum BSF solar cell 900.

  In the solar cell 800, when the chemical passivation performance is sufficiently increased in the back surface passivation film 825 and the field passivation performance obtained at the p / p + interface on the back surface of the conventional aluminum BSF solar cell 900 can be exceeded, Higher conversion efficiency can be expected. In addition, since the area ratio of the p + layer 829 occupying the back surface is significantly reduced as compared with the conventional aluminum BSF solar cell 900, the amount of light absorbed due to the p + layer 829 is also reduced. As a result, more light and carriers can be used effectively as electrical output, so that the power generation efficiency of solar cell 800 can be improved.

  Furthermore, since the p / backside passivation film interface of the solar cell 800 can be formed in a smoother shape than the p + / backside electrode interface of the solar cell 900, light is transmitted from the back side to the incident surface side. It can be reflected efficiently. As a result, the amount of light returning from the back surface side of the solar cell 800 to the incident surface side is increased, so that the amount of photoelectrically converted light also increases. As a result of the improvement of the electrical and optical characteristics, the conversion efficiency of the solar cell 800 can be improved.

In recent years, research on a back surface passivation film 825 having a composite layer structure (topology) in which an aluminum oxide film (Al ₂ O ₃ film) and a silicon nitride film (SiNx film) are deposited in this order on the back surface of a silicon substrate 820 and Development has become a global trend. Although there are various discussions on the fine points such as the thickness of each layer constituting the composite layer structure, it is desirable that the back surface passivation film 825 preferably has the above topology. There is no room for discussion.

In the back surface passivation film 825 having the above topology, the Al ₂ O ₃ film gives a negative fixed charge to the p / back surface passivation film interface, so that holes in the silicon substrate 820 are attracted to the interface. As a result, both high-density carriers are holes in both the p + layer 829 and the vicinity of the interface. Accordingly, recombination of electrons and holes hardly occurs in the vicinity of the back electrode 828. Conventionally, the recombination loss between electrons and holes in the vicinity of the back electrode is known to be caused by the use of a SiNx film or the like that gives a positive fixed charge to the interface, and has been called a parasite shunting phenomenon. Use of an Al ₂ O ₃ film solves this problem.

However, it is known that the Al ₂ O ₃ film generates a micro film peeling phenomenon called blistering when fired at a high temperature (for example, 700 ° C. to 900 ° C.). As a protective film for preventing this, a SiNx film is generally laminated on an Al ₂ O ₃ film.

(Main surface electrode structure in aluminum BSF solar cell 900 and back surface passivation type solar cell 800)
Since the focus of the present invention is on the back surface of the solar cell, the method for fabricating the back surface should be described in detail. However, the method of manufacturing the solar cell 800, 900 light incident surface electrode (hereinafter referred to as the main surface electrode) has a great relevance to the method of manufacturing the back surface in the present invention, and will be referred to as a preliminary explanation.

  In the solar cell 900 using the p-type silicon substrate 920 and the 800 using the p-type silicon substrate 820, a pn junction between the silicon substrate 920 or 820 and the n + layer is already formed on the light incident surface. Stated. Therefore, since an n + layer exists on the light incident surface, if a metal is deposited on the same layer, main surface electrodes 927 and 827 having a structure of “n + layer / metal” with sufficiently low contact resistance are manufactured. it can.

  However, an insulator layer such as SiNx is formed on the light incident surface on the n + layer for antireflection and surface passivation. Therefore, simply depositing metal on this will not allow the insulator layer to get in the way to get electrical contact between the n + layer and the metal. For this reason, it is only necessary to peel off the SiNx in the portion where the main surface electrodes 928 and 828 are formed, but this takes time.

  Therefore, this problem is solved by a paste mainly composed of glass frit and silver. By printing and baking this paste on SiNx, silver penetrates SiNx which is an insulator layer, and contacts silicon. In this way, the desired electrode shape can be formed on the silicon without partially etching SiNx. This technology is called fire-through technology.

(Back electrode structure in solar cell 900)
The material of the back electrode 928 is a metal, for example, aluminum is used as the material. In order to form the back electrode 928 and the p + layer 929, an aluminum film is deposited on the back surface of the silicon substrate 920, and then the aluminum film and the silicon substrate 920 are baked at a temperature of 700 ° C. or higher. As a result, a diffusion region in which aluminum is diffused is formed in the silicon substrate 920, and an alloy of aluminum and silicon (the above aluminum alloy) is generated. Since aluminum acts as a dopant for supplying holes, a p + layer 929 is formed in the diffusion region. The p + layer 929 is an aluminum alloy layer made of aluminum and silicon. Thus, part of the aluminum film deposited on the silicon substrate 920 contributes to the formation of the p + layer 929. The remaining aluminum film forms the back electrode 928 on the p + layer 929. Thus, if aluminum is used, the said back surface structure can be produced very rationally.

  Specifically, the back surface structure can be completed by solid-coating and baking a conductive paste containing aluminum on the entire back surface of the silicon substrate 920.

(Electrode structure in solar cell 800; back electrode)
Next, the back electrode 828 of the solar cell 800 will be described. As described above, in the solar cell 800 using the p-type silicon substrate 820, the back surface electrode occupies a part of the back surface although most of the back surface is covered with the insulating film called the back surface passivation film 825. In a portion where 828 exists, it is necessary to have an aluminum BSF structure of “p-type silicon substrate 820 / p + silicon layer 829 / metal 828”.

If the back surface passivation film 825 is removed only on the back surface passivation film 825 where the back surface electrode 828 is to be formed, and the aluminum paste is applied and fired only on this portion, the back surface structure can be reasonably manufactured. Since it is necessary to form an aluminum alloy p + layer, the metal paste for forming the back electrode 828 is preferably an aluminum paste.
(Problems in manufacturing process of back electrode 828 of solar cell 800)
As described above, in the conventional aluminum BSF solar cell 900, since the entire back surface is an electrode, the back surface structure can be easily completed by solid-coating and baking the aluminum paste on the entire surface of the silicon substrate 920. I was able to.

  However, in the above-described back surface passivation type solar cell 800, the back surface is not so easy to manufacture. First, the back surface passivation film 825 needs to be deposited, and second, the back surface electrode 828 needs to be locally produced. Since the aluminum paste necessary for the production of the back electrode 828 does not fire through the SiNx film used as a part of the back surface passivation film 825, the aluminum paste is printed on the back surface passivation film 825 where the back electrode 828 is desired to be produced. Even when firing, the back electrode 828 is not formed because the SiNx film interferes. That is, the fire-through method used for the main surface electrode 827 is not valid. However, a silver paste with fire-through performance cannot be used because it does not form a p + layer 829 by alloying with silicon.

  Due to such a background, conventionally, in the backside passivation type solar cell, prior to the backside electrode formation, first, the backside passivation film of the part where the backside electrode is to be formed is removed to expose the silicon, and then the silicon substrate exposed part. The aluminum paste is partially printed and fired.

  According to this method, an electrode having a back surface structure of “p-type silicon substrate / p + silicon layer / aluminum” can be produced only on the silicon exposed portion, so that the local electrode required for the back surface passivation type solar cell can be obtained. Can be produced.

  At present, the passivation film removal method proposed in the mass production line is a laser heating method. In this method, the back surface passivation film is partially heated with laser light to remove the back surface passivation film and open a hole. However, the laser processing apparatus has a complicated mechanism and is expensive. Moreover, there is a photolithography method as a traditional method that has been used in a process of forming an opening pattern in a semiconductor element. This method is costly and labor-intensive over the laser heating method.

  As described above, the production of the back electrode using a method such as a photolithography method or a laser heating method requires many man-hours and requires expensive equipment and equipment, which increases the cost of solar cells. It has become.

  In view of this, with the aim of simplifying the process of forming a partial opening in the back surface passivation film, a technique for making the back surface passivation film fire-through using a paste containing aluminum has been searched.

  For example, Patent Document 1 described below describes that a metal such as silver is mixed in an aluminum paste to cause the aluminum paste to acquire a fire-through performance for a SiNx film.

  Patent Document 2 listed below describes that a fire-through performance with respect to a SiNx film appears in a paste composition produced by mixing a dispersant containing an anionic site into a paste containing aluminum powder. Has been.

Special table 2010-538466 gazette (December 9, 2010 release) JP 2013-8759 A (published January 10, 2013)

However, the technique described in Patent Document 1 has a problem that it is necessary to use a special and expensive heavy metal such as silver as a material added to the aluminum paste. This causes a high price for the aluminum paste. Further, Patent Document 1 does not show the fire-through performance of the aluminum paste for the Al ₂ O ₃ film, which has become a global trend in recent years. Therefore, the fire-through performance of the aluminum paste for the back surface passivation film having a composite layer structure composed of the Al ₂ O ₃ film and the SiNx film is unknown.

Further, the technique described in Patent Document 2 has a problem that it is necessary to use a special dispersant as a material for the paste composition. Further, Patent Document 2 discloses a fire-through experiment of the paste composition for the SiNx film, but does not disclose an experimental result indicating the fire-through performance of the paste composition for the Al ₂ O ₃ film. Therefore, it is unclear whether the paste composition described in the cited document 2 also exhibits sufficient fire-through performance for a back surface passivation film having a composite layer structure composed of an Al ₂ O ₃ film and a SiNx film. Furthermore, Patent Document 2 does not describe an experimental result demonstrating the performance of a back surface passivation type solar cell produced using the paste composition.

  This invention is made | formed in view of said subject, The objective is to provide the solar cell etc. provided with the electrode which does not contain a special material.

  In order to solve the above problems, a solar cell according to one embodiment of the present invention includes a p-type semiconductor substrate in which a diffusion layer in which impurities are diffused is formed, and a passivation film formed on the surface of the p-type semiconductor substrate. And an electrode that penetrates the passivation film and is electrically connected to the diffusion layer. The electrode contains aluminum as a material, and the diffusion layer includes the impurity. As described above, aluminum contained in the electrode is diffused, and the passivation film has a laminated film structure including an aluminum oxide film and a silicon nitride film, and the refractive index of the silicon nitride film is 2.4 or more and 3.1 or less.

  According to one embodiment of the present invention, it is possible to provide a solar cell including an electrode that does not include a special material.

It is a schematic diagram which shows the structure of the solar cell which concerns on one Embodiment of this invention. It is process drawing which shows the manufacturing method of the solar cell shown in FIG. It is another process drawing which shows the manufacturing method of the solar cell shown in FIG. It is a table | surface which shows the measurement result of the short circuit current density of the solar cell sample shown in FIG. 1, and the conventional solar cell sample, an open circuit voltage, a fill factor, and conversion efficiency. It is a graph which shows each spectral sensitivity characteristic of the sample of the solar cell shown in FIG. 1, and the sample of the conventional solar cell. It is a table | surface which shows the detailed component ratio of the aluminum paste which inventors used in experiment. (A) is the microscope picture of the sample which inventors produced by experiment, (b) is the enlarged view. (A) is the microscope picture of the other sample which inventors produced by experiment, (b) is the enlarged view. It is a microscope picture of the other sample which inventors produced by experiment. It is an experimental result which shows the fire-through performance of the other sample which inventors produced by experiment. It is process drawing which shows the preparation processes of the conventional back surface passivation type solar cell. It is process drawing which shows the preparation process of the conventional aluminum BSF type solar cell. It is a schematic diagram which shows the structure of the conventional back surface passivation type solar cell. It is a schematic diagram which shows the structure of the conventional aluminum BSF type | mold solar cell. (A) is a table | surface which shows the component composition of the aluminum paste which does not contain glass frit, (b) is a table | surface which shows the component composition of the aluminum paste containing glass frit.

Embodiment 1
Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS. 1 to 10 and FIG. 15.

(Outline of the present invention)
The inventors have found that a general aluminum paste (main component: aluminum, glass frit) used in a conventional aluminum BSF solar cell has the following characteristics.

Characteristics 1. The Al ₂ O ₃ film can be fired through. That is, the aluminum paste has a fire-through performance with respect to the Al ₂ O ₃ film.

  Characteristic 2. A SiNx film having a high refractive index (refractive index of 2.4 or more) can be fired through.

  The inventors have also found that the SiNx film has the following characteristics.

  Characteristic 3. When the SiNx film having a high refractive index (refractive index of 2.4 or more) is directly deposited on the surface of the p-type silicon substrate, it does not function as a passivation film. That is, the high refractive index SiNx film / p-type silicon interface has low passivation performance.

From the above characteristics 1 to 3, the inventors believe that the aluminum paste can fire through the back surface passivation film having a composite film structure of “Al ₂ O ₃ film / high refractive index SiNx film”. I guessed. Furthermore, the passivation film interface with which the silicon substrate is in direct contact is not the high refractive index SiNx film / p-type silicon interface but the Al ₂ O ₃ film / p-type silicon interface, so that the passivation performance is also ensured. Here, the composite film structure of “Al ₂ O ₃ film / high refractive index SiNx film” means that an Al ₂ O ₃ film is deposited as a first layer on the surface of a substrate, and then a high refractive index (refractive index). 2.4 or more) is a structure in which a SiNx film is deposited as a second layer. The substrate may be a semiconductor substrate other than the silicon substrate. In this structure, the Al ₂ O ₃ film deposited directly on the surface of the substrate exhibits sufficient passivation performance for the substrate. In addition, this structure is consistent with the topology of the back surface passivation type solar cell that has been widely studied worldwide as described above. An experiment conducted by the inventors for demonstrating the above characteristics 1 and 2 and the results thereof will be described later.

(Configuration of solar cell 100)
Below, the structure of the solar cell 100 which concerns on this embodiment is demonstrated using FIG. FIG. 1 is a schematic diagram showing the configuration of the solar cell 100. As shown in FIG. 1, a solar cell 100 includes a p-type silicon substrate (p-type semiconductor substrate) 20 (hereinafter referred to as a silicon substrate 20), an antireflection film 24, a back surface passivation film (passivation film) 25, an aluminum film. 26, a main surface electrode 27, and a back surface electrode (electrode) 28.

  The silicon substrate 20 is made of p-type silicon. That is, the conductivity type of the silicon substrate 20 is p-type. A micro pyramid structure 21 having a height of several μm is formed on the upper surface of the silicon substrate 20 (the surface corresponding to the light receiving surface of the solar cell 100). The micro pyramid structure 21 suppresses reflection of light at the light receiving surface of the solar cell 100. Thereby, since the light quantity which injects into the solar cell 100 increases, the light quantity photoelectrically converted within a solar cell increases.

  The antireflection film 24 is formed so as to cover the upper surface of the silicon substrate 20 on which the micro pyramid structure 21 is formed. The antireflection film 24 has a function as a passivation film that protects the light receiving surface of the solar cell 100 in addition to the function of suppressing sunlight from being reflected by the light receiving surface of the solar cell 100. An n + layer 23 is formed between the antireflection film 24 and the silicon substrate 20. The n + layer 23 is formed by thermally diffusing phosphorus on the upper surface of the silicon substrate 20.

The back surface passivation film 25 covers most of the back surface of the silicon substrate 20. The back surface passivation film 25 has a laminated film structure in which an Al ₂ O ₃ film (aluminum oxide film) and a SiNx film (silicon nitride film) are laminated on the silicon substrate 20 in this order. Hereinafter, this structure may be referred to as “Al ₂ O ₃ film / SiNx film”. The structure of the back surface passivation film 25 is not limited to the “Al ₂ O ₃ film / SiNx film” structure, and may have a laminated structure including the Al ₂ O ₃ film and the SiNx film.

  Main surface electrode 27 is provided on the light receiving surface of solar cell 100. The main surface electrode 27 penetrates the antireflection film 24 and reaches the n + layer 23, and the main surface electrode 27 and the n + layer 23 are electrically connected.

  The back electrode 28 is provided on a part of the back surface of the silicon substrate 20. The back electrode 28 is formed using aluminum as a main material. A p + layer (diffusion layer) 29 is formed at a portion where the back electrode 28 and the silicon substrate 20 are in contact with each other by diffusing aluminum constituting the back electrode 28 into the silicon substrate 20. The p + layer 29 is a layer made of an aluminum alloy.

(Method for manufacturing solar cell 100)
Below, the manufacturing method of the solar cell 100 is demonstrated using FIG. 2 and FIG. 2 and 3 are process diagrams showing a method for manufacturing the solar cell 100. The manufacturing method of the solar cell 100 is comprised by S1-S9. Refer to FIG. 2 in description of S1-S4. Refer to FIG. 3 in the description of S5 to S9. In addition, the manufacturing method demonstrated here is only an example, and the manufacturing method of the solar cell 100 is not limited to this.

  First, the silicon substrate 20 (longitudinal and lateral 10 cm × 10 cm, thickness 200 μm, resistivity 1 Ωcm, material is p-type polycrystalline silicon (p-Si)) is cleaned using an RCA cleaning method developed by RCA. . Here, the p-type polycrystalline silicon used as the material of the silicon substrate 20 is obtained, for example, by adding a trace amount of a trivalent element such as boron, aluminum, or gallium to the polycrystalline silicon substrate. Alternatively, the silicon substrate 20 may be manufactured using single crystal silicon.

  In the method for manufacturing the solar cell 100, first, texture etching is performed on the silicon substrate 20 using a mixed solution of NaOH aqueous solution and isopropyl alcohol at a liquid temperature of about 90 ° C. (S1). Thereby, as shown in FIG. 2, a micro pyramid structure 21 (a micro uneven structure) having a height of several μm is formed on the surface of the silicon substrate 20. Note that a method used for performing texture etching may be, for example, a reactive ion etching method.

Second, by placing the silicon substrate 20 in a diffusion furnace filled with a high-temperature gas containing POCl ₃ (phosphoryl chloride), phosphorus is diffused into the silicon substrate 20 by a thermal diffusion phenomenon (S2). In S2, for example, n-type silicon layers 22 and 23 having a thickness of 1.0 μm and an impurity concentration of 1.2 × 10 ²⁰ cm ⁻³ are formed on the front and back surfaces of the silicon substrate 20. During the vapor deposition process, the temperature of the silicon substrate 20 and the temperature of the diffusion furnace are set to 850 ° C., for example. The phosphorus diffusion time is set to 10 minutes, for example.

In addition to the vapor phase diffusion method using POCl ₃ described above, the method of diffusing phosphorus on the front and back surfaces of the silicon substrate 20 may be a coating diffusion method using P ₂ O ₅ , or phosphorus ions may be directly applied to the silicon substrate. Alternatively, an ion implantation method for diffusing may be used.

Third, the antireflection film 24 is deposited on the light incident surface of the silicon substrate 20 by using a general-purpose parallel plate type plasma CVD (Chemical Vapor Deposition) method (S3). The antireflection film 24 is made of, for example, SiNx. The thickness of the antireflection film 24 deposited may be 80 nm. The deposition conditions of the antireflection film 24 are as follows: gas flow rate: SiH ₄ / NH ₃ / N ₂ = 25/50/300 SCCM (Standard Cubic Centimeter per Minutes), pressure: 100 Pa, RF (Radio Frequency) power density: 0.086 W / cm ² , substrate temperature: 450 ° C.

The antireflection film 24 may be made of, for example, Al ₂ O ₃ , silicon oxide, or titanium oxide instead of the above-described SiNx. Even when the antireflection film 24 is made of any of these materials, in addition to the antireflection function, the antireflection film 24 also has a function of an inactivating film that provides a passivation effect.

  However, when the silicon substrate 20 is made of polycrystalline silicon, the antireflection film 24 is preferably made of SiN containing hydrogen from the viewpoint of improving the conversion efficiency of the solar cell 100.

  Further, when the antireflection film 24 is deposited on the silicon substrate 20, various CVD methods such as a catalytic CVD method, an atmospheric pressure CVD method, a reduced pressure CVD method, or a photo CVD method can be used instead of the plasma CVD method described above. The method may be used. Alternatively, a PVD (Physical Vapor Deposition) method such as a vacuum evaporation method or a sputtering method may be used. In the case of forming the antireflection film 24 using SiNx as a material, it is preferable to use a plasma CVD method from the viewpoint of easy control of the film thickness.

  Fourth, after applying a protective tape to the light incident surface of the silicon substrate 20, the silicon substrate 20 is placed in a solution of nitric acid and hydrofluoric acid (for example, a solution of nitric acid: hydrofluoric acid = 3: 1) for a predetermined time (for example, Immerse (4 minutes) (S4). As a result, the n-type silicon layer (that is, the n + layer 23) on the light incident surface of the silicon substrate 20 remains, while the n-type silicon layer on the back surface is removed, and the back surface of the p-type silicon substrate 20 is exposed. Thereafter, the protective tape is peeled off from the light incident surface of the silicon substrate 20, the silicon substrate 20 is immersed in hydrofluoric acid, and the oxide film deposited on the back surface of the silicon substrate 20 is removed by the RCA method.

Fifth, an Al ₂ O ₃ film is deposited on the back surface of the silicon substrate 20 by using an ALD (Atomic Layer Deposition) method. Trimethylaluminum (TMA) can be used as the source gas, and ozone can be used as the oxidizing gas. The thickness of the deposited Al ₂ O ₃ film may be 30 nm, for example. Further, the temperature of the silicon substrate 20 at the time of deposition may be 175 ° C., for example. Subsequently, a high refractive index SiNx film having a refractive index of 2.4 or more is deposited on the Al ₂ O ₃ film by using parallel plate type plasma CVD (S5).

The thickness of the high refractive index SiNx film to be deposited may be 100 nm, for example. The deposition conditions for the high refractive index SiNx film are, for example, gas flow rate: SiH ₄ / NH ₃ / N ₂ = 50/25/300 SCCM, pressure: 100 Pa, RF power density: 0.086 W / cm ² , substrate temperature 450 ° C. It may be. In S5, the back surface passivation film 25 having a composite film structure of “Al ₂ O ₃ film / high refractive index SiNx film” is formed.

  Sixth, an aluminum paste is printed in the form of dots on the back surface passivation film 25 using a screen printer (S6; printing step). For example, the dot diameter may be 100 μm and the pitch may be 1 mm.

  Seventh, the intermediate product is annealed using an electrode firing furnace (S7; firing step). The furnace temperature may be fixed at 810 ° C. and the heating time may be set to 12 seconds. By annealing, the aluminum paste fires through the back surface passivation film 25. As a result, the aluminum paste penetrates the back surface passivation film 25 and diffuses into the silicon substrate 20 as p-type impurities. Thereby, the p + layer 29 is formed.

  Thus, in S7, since the p + layer 29 and the back electrode 28 are produced using the fire-through technique, a process of providing an opening in the back passivation film 25 is performed as in the production process of the back electrode 828 in the solar cell 800. Not necessary. Therefore, the manufacturing process of the back electrode 28 in the solar cell 100 is greatly simplified as compared with that of the solar cell 800.

  Eighth, an aluminum film 26 is deposited on the back surface passivation film 25 by vacuum evaporation (S8). The film thickness of the aluminum film 26 may be 2 μm, for example.

  Ninth, after the conductive paste is printed on the antireflection film 24 on the light incident surface side, the intermediate product is annealed (S9). As a result, the conductive paste fires through the antireflection film 24 and reaches the n-type silicon layer 23. The cured conductive paste becomes the main surface electrode 27 electrically connected to the n + layer 23.

  The material of the main surface electrode 27 is not particularly limited, and for example, aluminum, silver, titanium, palladium, gold, or the like may be used. These materials are conventionally used in the technical field of solar cells. However, the material of the main surface electrode 27 is most preferably a mixture of silver and glass frit from the viewpoint of high performance of fire-through the antireflection film 24. Further, the method used when forming the main surface electrode 27 is not limited to the method using the fire-through technique, and may be, for example, a screen printing method or a vacuum deposition method. From the viewpoint of improving productivity and reducing the manufacturing cost, the screen printing method is preferable.

  Thus, the solar cell 100 is completed.

(Performance evaluation of solar cell 100)
Inventors conducted the experiment which produced the sample of the solar cell 100 and measured the conversion efficiency etc. of the sample, in order to evaluate the performance of the solar cell 100. FIG. Moreover, in order to compare with the performance of the solar cell 100, each sample of the conventional back surface passivation type solar cell 800 (see FIG. 13) and the conventional aluminum BSF type solar cell 900 (see FIG. 14) is also produced. The conversion efficiency of the sample was also measured. In addition, the manufacturing method of the solar cells 800 and 900 is described as a supplement at the end of this embodiment.

  Below, the performance of the solar cell 100 is evaluated from the measurement results of the short circuit current density, the open circuit voltage, the fill factor, the conversion efficiency, and the spectral sensitivity efficiency of each sample of the solar cell 100, the solar cell 800, and the solar cell 900.

(1. Short circuit current density, open circuit voltage, fill factor, and conversion efficiency)
FIG. 4 shows measurement results of short-circuit current density Jsc, open-circuit voltage Voc, fill factor FF, and conversion efficiency Eff of solar cell 100 and conventional solar cells 800 and 900. In FIG. 4, the measured values of solar cell 100 and solar cell 800 are normalized with the corresponding measured values of conventional aluminum BSF solar cell 900.

  All measured values were measured under AM (air mass) 1.5 conditions. Here, the AM1.5 condition is based on the distance that sunlight passes through the atmosphere when the incident angle of sunlight entering the atmosphere is 90 degrees (directly above). The incident condition (incident angle 41.8 degrees) is 1.5 times the value.

  FIG. 4 shows that the conversion efficiency Eff of the solar cell 100 is improved by about 3% as compared with the conversion efficiency Eff of the conventional aluminum BSF solar cell 900. In addition, the conversion efficiency Eff of the conventional back surface passivation type solar cell 800 is improved by about 4% compared to the conversion efficiency Eff of the aluminum BSF type solar cell 900. Therefore, the conversion efficiency Eff of the solar cell 100 is approximately 1% lower than the conversion efficiency Eff of the conventional back surface passivation type solar cell 800. However, as described above, the method for manufacturing the solar cell 100 does not include the step of opening the back surface passivation film 25, so that the solar cell 100 is manufactured by a process that is significantly simplified as compared with the solar cell 800. The In view of this point, the solar cell 100 has a high inventive step as a whole.

(2. Spectral sensitivity characteristics)
In FIG. 5, each spectral sensitivity characteristic of the solar cell 100, the solar cell 900, and the solar cell 800 is shown. Each spectral sensitivity characteristic was measured under the conditions of AM1.5.

  From the graph of FIG. 5, in the long wavelength region of 1000 nm or more, the spectral sensitivity characteristics of the solar cell 100 and the solar cell 800 are both improved by about 10% to about 20% compared to the spectral sensitivity characteristic of the solar cell 900. ing. Moreover, it turns out that there is no big difference between the long wavelength spectral sensitivity characteristic of the solar cell 100, and that of the solar cell 800. Therefore, it is considered that the improvement in spectral sensitivity of the solar cell 100 and the solar cell 800 in the long wavelength region is a result brought about by the effect of the back surface passivation structure, that is, the effect of reducing the density of recombination centers on the back surface.

  Therefore, it can be said that the solar cell 100 sufficiently retains the effect of the back surface passivation structure even though its manufacturing method is dramatically simplified as compared with the conventional back surface passivation type solar cell 800.

(Demonstration of characteristics 1 and 2; fire-through performance of aluminum paste)
Here, an experiment conducted by the inventors to verify the above-described characteristic 1 and characteristic 2 and the result thereof will be described. The inventors produced a sample X1 in which an Al ₂ O ₃ film was deposited on a silicon substrate and a sample Y1 in which a SiNx film having a refractive index of 2.5 was deposited on a silicon substrate, and an aluminum paste for each sample The fire-through performance was measured. FIG. 6 shows a detailed component ratio of the aluminum paste used in the experiment. In FIG. 6, the amount indicated beside each substance name represents a weight ratio. The amount of aluminum contained as a main component in the aluminum paste is 65% by weight or more and 75% by weight or less.

  Furthermore, in order to compare the fire-through performance of the aluminum paste with respect to the sample Y1, the inventors also produced a sample Z1 in which a SiNx film having a refractive index of 2.0 is deposited on a silicon substrate. Fire-through performance was also measured. Here, since the SiNx film having a refractive index of about 2.0 has a high passivation performance with respect to the surface of the silicon substrate, it is generally used as an antireflection film or a surface passivation film on the light receiving surface side of the solar cell. Below, the preparation procedure of each sample and the measurement result of the fire-through performance of each sample are shown.

(1. Demonstration of characteristic 1; fire-through performance for Al ₂ O ₃ film)
In the manufacturing process of the sample X1, first, an Al ₂ O ₃ film was deposited on a silicon substrate by using an ALD method. Trimethylaluminum (TMA) was used as the source gas, and ozone was used as the oxidizing gas. The temperature of the silicon substrate during deposition was set to 175 ° C. After the deposition process, the thickness of the Al ₂ O ₃ film deposited on the silicon substrate was 30 nm.

Next, an aluminum paste was printed in a dot shape on the Al ₂ O ₃ film. The dot diameter was set to 150 μm and the pitch was set to 1 mm. Thus, an intermediate product in which an Al ₂ O ₃ film and an aluminum paste were deposited on a silicon substrate was completed.

  Subsequently, the intermediate product was fired in a mixed atmosphere of nitrogen and oxygen. The temperature of the firing furnace was set to 810 ° C., and the firing time was set to 12 seconds. Thus, sample X1 was completed. Then, the metal part (back surface electrode) formed in the part which printed the aluminum paste was removed from the sample X1 using hydrochloric acid. And the sample X1 which removed the metal part was observed with the microscope.

FIGS. 7A and 7B show photographs of the sample X1 observed through a microscope. 7A and FIG. 7B which is an enlarged view thereof, it can be seen that a portion MP1 having a metallic luster is formed on the print mark of the aluminum paste. This is evidence that aluminum penetrates the Al ₂ O ₃ film and an alloy layer of silicon and aluminum is formed in this portion. From the above experimental results, it was found that the aluminum paste can fire through the Al ₂ O ₃ film.

Furthermore, the inventors used a 2 μm thick aluminum vapor-deposited film for the purpose of investigating the fire-through performance of an aluminum paste containing no glass frit such as glass powder (PbO) or silica powder (SiO ₂ ). An experiment similar to the above was performed. As a result, the aluminum deposited film was able to fire through the Al ₂ O ₃ film. This is aluminum, it plays an essential role that respect firing through the Al ₂ O ₃ film, and, if the thickness of the aluminum deposited on the Al ₂ O ₃ film is 2μm or more, fire This means that a slew can occur.

Here, the component composition table | surface of the aluminum paste which does not contain glass frit is shown to (a) of FIG. Of the components of the aluminum paste shown in the figure, diethylene glycol monobutyl ether and diethylene glycol monobutyl ether acetate evaporate at 150 ° C. and ethyl cellulose evaporate at 350 ° C., respectively. Therefore, when an aluminum paste having a component ratio shown in the above component composition table is deposited on the substrate and the Al ₂ O ₃ film, only aluminum remains on the substrate at 700 ° C. or higher where fire-through occurs. This situation is equivalent to the situation at the time of the experiment conducted by depositing an aluminum film on the Al ₂ O ₃ film. Therefore, it can be predicted from the results of this experiment that the aluminum paste containing no glass frit exhibits a fire-through performance with respect to the Al ₂ O ₃ film. In addition, when said experiment is performed using the said aluminum paste, the density | concentration of the aluminum in the back surface electrode part after baking will be 100% (100 weight%).

By the way, the component ratio of the aluminum paste can be changed so that the concentration of aluminum is decreased and the glass frit is increased instead. At this time, the concentrations of ethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, and ethyl cellulose are not changed. Since the glass frit remains in the back electrode portion even after firing, when the Al ₂ O ₃ film is fire-through using the aluminum paste whose component ratio is changed as described above, the glass frit in the electrode portion after firing is While the concentration is high, the concentration of aluminum is low. Specifically, the concentration of aluminum in the electrode part after firing is expressed as follows.
(Concentration of aluminum in the electrode part after firing)
= (Aluminum concentration in aluminum paste)
/ (Aluminum concentration in aluminum paste + glass frit concentration in aluminum paste)
However, in general, an Al ₂ O ₃ film is fired through using an aluminum paste in which the glass frit components, that is, the concentrations of glass powder (PbO) and silica powder (SiO ₂ ) both exceed 5%. In this case, there is a problem that the formation of an alloy layer of aluminum and silicon becomes insufficient. Therefore, an aluminum paste in which the concentrations of glass powder (PbO) and silica powder (SiO ₂ ) both exceed 5% is not adopted as an aluminum paste for making the Al ₂ O ₃ film fire-through.

FIG. 15B shows a component composition table of an aluminum paste in which the concentrations of glass powder (PbO) and silica powder (SiO ₂ ) are both exactly 5%. When the aluminum paste having this component composition is employed, the aluminum concentration in the back electrode portion after firing is calculated based on the above formula, and is 85% (85% by weight). Since the aluminum concentration does not fall below this value (85%) in the back electrode portion after firing, this value is the lower limit of the aluminum concentration in the back electrode portion after firing. Therefore, when the Al ₂ O ₃ film is fired through the aluminum paste, the aluminum concentration in the back electrode portion after firing becomes a value between 85 wt% and 100 wt%.

(2. Demonstration of characteristic 2; Fire-through performance for SiNx film having a refractive index of 2.5)
In the manufacturing process of the sample Y1, first, a SiNx film was deposited on a silicon substrate by using a plasma CVD method. The temperature of the silicon substrate during deposition was set to 450 ° C., the RF power density was set to 0.086 W / cm ² , the RF frequency was set to 13.56 Mhz, and the pressure was set to 100 Pa. In addition, the flow rate ratio of NH ₃ gas and SiH ₄ gas, which is the raw material gas of the SiNx film, was controlled to NH ₃ : SiN ₄ = 1: 2 so that the refractive index of the SiNx film was 2.5. After the deposition process, the thickness of the SiNx film deposited on the silicon substrate was 200 nm.

Thus, the refractive index of the SiNx film can be controlled by the flow rate ratio of the NH ₃ gas and the SiH ₄ gas that are the source gas of the SiNx film.

  Next, in the same manner as the baking treatment in the production process of the sample X1, an aluminum paste was printed in a dot shape on the SiNx film, and then the intermediate product was baked. Thus, sample Y1 was completed. Then, the metal part formed in the part which printed the aluminum paste was removed from the sample Y1 using hydrochloric acid. And sample Y1 which removed the metal part was observed with the microscope.

  FIGS. 8A and 8B show photographs of the sample Y1 observed through a microscope. 8 (a) and FIG. 8 (b) which is an enlarged view thereof, it can be seen that the portion MP2 having a metallic luster is formed on the print mark of the aluminum paste. This is evidence that aluminum penetrates the SiNx film and an alloy layer of silicon and aluminum is formed in this portion. From the above, it was found that the aluminum paste can fire through a SiNx film having a refractive index of 2.5.

  Furthermore, the inventors conducted an experiment similar to the above using an aluminum deposited film having a thickness of 2 μm for the purpose of examining the fire-through performance of an aluminum paste containing no glass frit. As a result, the aluminum deposited film was able to fire through a SiNx film having a refractive index of 2.5. This means that aluminum plays an essential role for the fire through of SiNx film with a refractive index of 2.5, and the thickness of the aluminum deposited on the SiNx film with a refractive index of 2.5 is 2 μm. This means that fire-through can occur.

  As described above, since the aluminum deposited film is 100% aluminum, it can be considered to be equivalent to “aluminum paste not including glass frit”. From this experimental result, it can be predicted that an aluminum paste containing no glass frit exhibits a fire-through performance with respect to a SiNx film having a refractive index of 2.5.

Further, when the SiNx film is fire-through into the aluminum paste, it can be considered that the same argument as in the case where the Al ₂ O ₃ film is fire-through is established with respect to the aluminum concentration in the back electrode portion after firing. Accordingly, even when a SiNx film having a refractive index of 2.5 is fired through the aluminum paste, the lower limit value of the aluminum concentration in the back electrode portion after firing is 85%, and the upper limit value is 100%.

(3. Sample Z1; Fire-through performance for SiNx film having a refractive index of 2.0)
In the manufacturing process of the sample Z1, first, a SiNx film was deposited on a silicon substrate by using a plasma CVD method. The temperature of the silicon substrate during deposition was set to 450 ° C., the RF power density was set to 0.086 W / cm ² , the RF frequency was set to 13.56 Mhz, and the pressure was set to 100 Pa. These deposition conditions are the same as those for the SiNx film having a refractive index of 2.5. The flow rate ratio of NH ₃ gas and SiH ₄ gas was set to NH ₃ : SiN ₄ = 2: 1 so that the refractive index of the SiNx film was 2.0.

  Next, an opening was provided in the SiNx film using a photolithography method to expose the silicon substrate. Subsequently, an aluminum paste was printed on the region of the SiNx film including the exposed opening, and then the intermediate product was baked. Thus, the sample Z1 was completed. Then, the metal part formed in the part which printed the aluminum paste was removed from the sample Z1 using hydrochloric acid. And sample Z1 which removed the metal part was observed with the microscope.

  FIG. 9 shows a photograph of sample Z1 observed through a microscope. In FIG. 9, in the SiNx film, a region HL where the aluminum paste is applied during the printing process is surrounded by a dotted line. Further, the region UD at the center of the region HL is a portion where an opening is provided in the SiNx film using a photolithography method. In the region UD, the silicon substrate is exposed on the print mark of the aluminum paste, and an alloy layer of silicon and aluminum is formed. Here, if the aluminum paste fires through the SiNx film during the firing process, the SiNx film must disappear from the entire area where the aluminum paste is applied, that is, the area HL surrounded by the dotted line. However, as can be seen from FIG. 9, in the region HL surrounded by the dotted line, the SiNx film remains in the region outside the opening, and even the SiNx film in the region is not even damaged. I can't. From the above, it is impossible for the aluminum paste to fire through a SiNx film having a refractive index of 2.0, and if a backside passivation solar cell is produced using this film, the opening of the SiNx film is It turned out to be essential.

(4. Other experiments)
In addition to the experiment using the sample X1, the sample Y1, and the sample Z1, the inventors have made a fire of aluminum paste for each SiNx film having a refractive index of 2.3, 2.4, 2.7, and 3.1. Experiments to investigate through performance were also conducted.

  FIG. 10 shows the results of an experiment relating to the sample of the SiNx film having each refractive index. In FIG. 10, a circle indicates that the fire-through performance is high, a triangle indicates that the fire-through performance is slightly recognized, and a cross indicates that the fire-through performance is low. FIG. 10 shows that the aluminum paste cannot fire through a SiNx film having a refractive index of less than 2.4. On the other hand, the aluminum paste can fire through a SiNx film having a refractive index of 2.4 or higher, particularly a refractive index of 2.5 or higher. Therefore, the refractive index of the SiNx film is preferably 2.4 or more, and more preferably 2.5 or more.

  The inventors determined that it was not worth studying the fire-through performance of the aluminum paste for the SiNx film having a refractive index exceeding 3.1, and did not conduct an experiment. The reason is that when the refractive index of the SiNx film is too large, the SiNx film has a problem in terms of the function as the back surface passivation film of the solar cell. More specifically, if the refractive index of the SiNx film is too large, the amount of light absorption inside the SiNx film increases, and the reflectance of light in the SiNx film decreases. The sunlight absorbed by the SiNx film and the sunlight not reflected by the SiNx film do not contribute to the electrical output of the solar cell, so that the refractive index of the SiNx film is too large, This is undesirable from a point of view.

(Supplementary: Manufacturing process of conventional solar cells 800 and 900)
In the following, as a supplement, manufacturing processes of the conventional solar cell 800 and the solar cell 900 will be described. The manufacturing process of the solar cell 800 and the solar cell 900 is the same as the manufacturing process of the solar cell 100, and the steps S1 to S4 are the same, and the steps corresponding to S5 and subsequent steps are different.

(1. Manufacturing process of conventional backside passivation solar cell 800)
A manufacturing process of a conventional back surface passivation type solar cell 800 (hereinafter referred to as a solar cell 800) will be described with reference to FIG. FIG. 11 is a process diagram showing a manufacturing process of the solar cell 800. The manufacturing process of the solar cell 800 is different in the following first and second differences when compared with the manufacturing process of the solar cell 100 described above. Other steps in the manufacturing process of the solar cell 800 are the same as the corresponding steps in the manufacturing process of the solar cell 100.

First difference: The manufacturing process of the solar cell 800 is different from the manufacturing method of the solar cell 100 in the step corresponding to S5 (see FIG. 3), that is, in the step of forming the back surface passivation film 825. The production method of the second layer is different. That is, in the manufacturing process of the solar cell 800, the same Al ₂ O ₃ as the solar cell 100 is deposited on the first layer that is in direct contact with the silicon substrate 820 as the back surface passivation film 825, and SiNx having a refractive index of 2.0 is deposited thereon. By depositing, a composite film structure of “Al ₂ O ₃ film / SiNx film” is produced. Unlike the second layer high refractive index SiNx (refractive index 2.5) used in the solar cell 100, the refractive index 2.0 SiNx film used in the solar cell 800 is used as a passivation film of the solar cell. Generally used. The deposition process of the second layer SiNx is performed using a general-purpose parallel plate plasma CVD method. The conditions at the time of deposition are as follows. Gas flow rate: SiH ₄ / NH ₃ / N ₂ = 25/50/300 SCCM, pressure: 100 Pa, RF power density: 0.086 W / cm ² , substrate temperature: 450 ° C. Film thickness: 100 nm.

  Second difference: Furthermore, the manufacturing process of the solar cell 800 is different from the manufacturing method of the solar cell 100 in the steps corresponding to S6 to S8 (see FIG. 3), that is, the step of manufacturing the back electrode 828. . Since the SiNx film having a refractive index of 2.0 does not have a fire-through performance with respect to an aluminum paste, the back electrode 828 of the solar cell 800 cannot be manufactured using the fire-through technique. Therefore, in the manufacturing process of the solar cell 800, in the step corresponding to S6 of the solar cell 100, an opening 825a is provided in the back surface passivation film 825 using a photolithography method (S106), and then the back electrode is formed in the opening 825a. 828 is provided (S107). A p + layer 829 is formed at a portion where the back electrode 828 and the silicon substrate 820 are in contact with each other by a diffusion phenomenon.

  The process of providing an opening in the back surface passivation film using the photolithography method as in S106 is most conventionally and routinely performed when attempting to manufacture a back surface passivation cell in the laboratory. However, since this process requires many man-hours and requires expensive equipment and equipment, it is impractical to carry out the solar cell 800 in mass production.

  In the manufacturing process of the solar cell 800, the intermediate product is baked after S107 (S108). Thereby, the intermediate product equivalent to the intermediate product after implementing S7 (refer FIG. 3) in the manufacturing process of the solar cell 100 is completed. Thereafter, an aluminum film 826 is formed on the back surface passivation film 825 (S109), and finally a main surface electrode 827 is formed on the light receiving surface side of the solar cell 800 (S110). S109 and S110 are the same steps as S8 to S9 in the method for manufacturing solar cell 100. Thus, the solar cell 800 is completed.

(2. Manufacturing process of conventional aluminum BSF solar cell 900)
Here, a manufacturing process of a conventional aluminum BSF solar cell 900 (hereinafter referred to as solar cell 900) will be described with reference to FIG. FIG. 12 is a process diagram showing a manufacturing process of the solar cell 900. The manufacturing method of the solar cell 900 is different from the manufacturing method of the solar cell 100 in the following points.

  Difference: The manufacturing process of the solar cell 900 does not include the step of S5 in the manufacturing method of the solar cell 100, that is, the step of forming the back surface passivation film. In the method for manufacturing the solar cell 100, after S4 (see FIG. 2), an aluminum paste (a back electrode 928 in a paste state) is applied to the entire back surface of the silicon substrate 920 without forming a back surface passivation film (S205). ). Thereafter, the intermediate product is annealed (S206). As a result, a p + layer 929 is formed between the silicon substrate 920 and the back electrode 928. Thus, the back surface of the solar cell 900 is completed. Finally, the main surface electrode 927 is formed on the light receiving surface side of the solar cell 900 by the same method as the method for forming the main surface electrode 27 of the solar cell 100 (S207). Thus, the solar cell 900 is completed.

[Embodiment 2]
In the embodiment, in S <b> 8 (see FIG. 3) of the method for manufacturing the solar cell 100, the aluminum film 26 is formed on the back surface passivation film 25 by using a vacuum evaporation method. On the other hand, in the present embodiment, in the step corresponding to S8, the aluminum film 26 is formed on the back surface passivation film 25 using a sputtering method.

  Compared with the vacuum vapor deposition method, the sputtering method (i) has a larger energy of particles as a raw material of the aluminum film 26, and therefore has a larger particle diffusion force to the back surface passivation film 25. (ii) Controls the deposition time. Only has the advantage that the film thickness can be controlled with high accuracy. In addition, (iii) even if the back surface passivation film 25 has a large area, the aluminum film 26 having a uniform film thickness can be formed on the entire back surface passivation film 25. .

[Embodiment 3]
In the second embodiment, the method of forming the aluminum film 26 on the back surface passivation film 25 in order to connect the plurality of back surface electrodes 28 arranged in a dotted pattern (see FIG. 7A) has been described. . On the other hand, in the present embodiment, in the step corresponding to S <b> 8 of the method for manufacturing the solar cell 100, the metal foil is bonded onto the back surface passivation film 25 using an adhesive. Here, when the back surface passivation film 25 and the metal foil are bonded, it is not necessary to apply an adhesive material to the entire surface of the back surface passivation film 25. For example, the adhesive material is placed near the four corners of the surface of the back surface passivation film 25. May be attached. The back surface passivation film 25 (solar cell 100) and the metal foil are firmly pressed later in a laminating step of a modularization process for modularizing the solar cell 1.

  The method of this embodiment has the advantage that the burden of equipment management is reduced because a vacuum process that is indispensable in vacuum deposition and sputtering is not required. In addition, there is a merit that the electrical resistance of the back electrode 28 is lowered.

[Summary]
A solar cell (100) according to aspect 1 of the present invention is formed on a p-type semiconductor substrate (silicon substrate 20) on which a diffusion layer (p + layer 29) in which impurities are diffused is formed, and on the surface of the p-type semiconductor substrate. A passivation film (back surface passivation film 25) and an electrode (back surface electrode 28) that penetrates the passivation film and is electrically connected to the diffusion layer. Aluminum is contained, aluminum contained in the electrode is diffused as the impurity in the diffusion layer, and the passivation film includes an aluminum oxide film (Al ₂ O ₃ film) and a silicon nitride film (SiNx). The silicon nitride film has a refractive index of 2.4 or more and 3.1 or less.

  According to said structure, the refractive index of a silicon nitride film is 2.4 or more and 3.1 or less. As demonstrated by the inventors, the aluminum contained in the back electrode plays an important role in not only the aluminum oxide film but also the silicon nitride film having a refractive index of 2.4 or more.

  Therefore, the electrode of the solar cell having the above-described configuration can be produced, for example, by firing an intermediate product formed by printing a paste containing aluminum on a passivation film. That is, the electrode of the solar cell having the above-described configuration can be manufactured without performing the step of opening the passivation film and providing the electrode in the opening, unlike the electrode of the conventional solar cell. Therefore, the solar cell having the above-described configuration can be manufactured by a manufacturing method greatly simplified as compared with the conventional solar cell.

  The solar cell (100) according to the second aspect of the present invention may be configured such that, in the first aspect, the aluminum oxide film is in contact with the surface of the p-type semiconductor substrate (silicon substrate 20).

  According to the above configuration, since the aluminum oxide film gives a negative fixed charge to the interface between the p-type semiconductor substrate and the passivation film, the parasite shunting phenomenon that occurs between the passivation film interface and the electrode is prevented. Occurrence is suppressed. As a result, it is possible to reduce the deterioration of the characteristics of the solar cell caused by the parasite shunting phenomenon.

  The solar cell (100) according to Aspect 3 of the present invention is the configuration according to Aspect 1 or 2, wherein the electrode (back electrode 28) contains 85% by weight or more and 100% by weight or less of aluminum. May be.

  In the above configuration, when the electrode that penetrates the passivation film and is electrically connected to the diffusion layer is formed of a material containing aluminum as a main component, a firing step is generally performed. In the configuration in which the electrode contains 85 wt% or more and 100 wt% or less of aluminum, a sufficient amount of aluminum penetrates the passivation film and diffuses into the semiconductor substrate by the baking process. Thereby, a diffusion layer sufficiently containing aluminum can be formed.

  When the electrode contains 85 wt% or more and less than 100 wt% of aluminum, an aluminum paste mainly composed of aluminum and glass frit can be selected as the material. On the other hand, when the electrode contains 100% by weight of aluminum, an aluminum paste containing no glass frit is selected as the material.

  In the solar cell (100) according to Aspect 4 of the present invention, in any one of Aspects 1 to 3, the conductivity type of the diffusion layer (p + layer 29) may be p-type.

  According to the above configuration, since the aluminum oxide film included in the passivation film gives a negative fixed charge to the interface between the p-type semiconductor substrate and the passivation film, electrons and holes generated in the p-type semiconductor substrate Of these, holes are attracted to the interface. As a result, both high-density carriers are holes in both the p-type diffusion layer and in the vicinity of the interface. Thereby, recombination of electrons and holes hardly occurs in the vicinity of the electrode provided through the passivation film, so that the photoelectric conversion efficiency can be improved.

  The manufacturing method of the solar cell (100) which concerns on aspect 5 of this invention is a manufacturing method of the solar cell which concerns on aspect 1-4, Comprising: The paste for forming the said electrode containing aluminum on the said passivation film is provided. A printing step for printing, and an intermediate product generated by the printing step are fired, thereby causing the passivation film to fire through the paste and electrically connecting the paste and the p-type semiconductor substrate. And a firing step.

  In the solar cell according to each of the above embodiments, the refractive index of the silicon nitride film is 2.4 or more and 3.1 or less. As demonstrated by the inventors, an electrode paste containing aluminum can fire through not only an aluminum oxide film but also a silicon nitride film having a refractive index of 2.4 or more.

  Therefore, the electrode of the solar cell having the above-described configuration can be manufactured without performing the process of opening the passivation film and providing the electrode in the opening, unlike the electrode of the conventional solar cell. Therefore, according to the manufacturing method of the solar cell (100) which concerns on aspect 5, the manufacturing method remarkably simplified compared with the manufacturing method of the conventional solar cell can be provided.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention. Furthermore, a new technical feature can be formed by combining the technical means disclosed in each embodiment.

  The present invention can be used for solar cells.

100 solar cell 20 p-type silicon substrate (p-type semiconductor substrate)
25 Back surface passivation film (passivation film)
28 Back electrode (electrode)
29 p + layer (diffusion layer)

Claims (5)

A p-type semiconductor substrate on which a diffusion layer in which impurities are diffused is formed;
A passivation film formed on the surface of the p-type semiconductor substrate;
An electrode that penetrates the passivation film and is electrically connected to the diffusion layer,
The electrode contains aluminum as a material,
In the diffusion layer, aluminum contained in the electrode is diffused as the impurity,
The passivation film has a laminated film structure including an aluminum oxide film and a silicon nitride film,
The solar cell according to claim 1, wherein the silicon nitride film has a refractive index of 2.4 or more and 3.1 or less. The solar cell according to claim 1, wherein the aluminum oxide film is in contact with the surface of the p-type semiconductor substrate. The solar cell according to claim 1 or 2, wherein the electrode contains 85 wt% or more and 100 wt% or less of aluminum. The solar cell according to any one of claims 1 to 3, wherein the conductivity type of the diffusion layer is p-type. It is a manufacturing method of the solar cell of any one of Claims 1-4,
A printing step of printing a paste for forming the electrode containing aluminum on the passivation film;
Firing the intermediate product generated by the printing step, thereby causing the paste to fire through the passivation film and electrically connecting the paste and the p-type semiconductor substrate. A method for manufacturing a solar cell.

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