JP2015138959A - Photovoltaic device and photovoltaic device manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve a highly efficient solar battery which limits a surface recombination rate of a p-type diffusion junction layer in an n-type substrate solar battery effectively and at low cost.SOLUTION: A photovoltaic device which includes an n-type semiconductor substrate 101 having a p-type diffusion layer 102 where a p-type impurity element is diffused on a first surface 101A and uses the p-type diffusion layer 102 as a light receiving surface comprises: a protection film 103 which contains on the p-type diffusion layer 102, the same element with the p-type impurity and contains an oxide film obtained by oxidation of the semiconductor substrate; an anti-reflection film 104 formed on the protection film 103; and a first electrode 105 which pierces the protection film 103 and the anti-reflection film 104 to be electrically connected to the p-type diffusion layer 102.

Description

  The present invention relates to a photovoltaic device and a method for manufacturing the photovoltaic device, and more particularly to an inexpensive and high-output photovoltaic device that effectively suppresses surface recombination of a p-type diffusion layer.

  Conventionally, for example, in a solar cell, in order to enhance its performance, each of the three basic functions is efficiently executed by taking incident light into the inside, converting it into electric energy, and taking out this electric energy to the outside. Ingenuity is made.

  In particular, in recent years, there has been remarkable progress in high-efficiency in practical mass production and production technology. Taking a single-crystal silicon solar cell as an example, a structure and manufacturing process that can realize low cost and productivity are 19%. Each company has been able to achieve production that exceeds efficiency. On the other hand, it also means that the actual situation is approaching the theoretical limit, and the problem in actually aiming at higher efficiency tends to be limited to a small number of difficult factors.

  One of the directions for increasing the efficiency is to make the substrate n-type. In a purely theoretical comparison, the use of p-type substrates is more suitable for higher efficiency, and should inherently contain contradictions. In consideration of realistic problems such as these, studies have been increasing in recent years as a direction to be reviewed again.

Table 2008-065918

  However, although high efficiency is achieved, the cost and productivity cannot be ignored according to the above-described conventional technology in consideration of the background as described above. In order to achieve high efficiency while ensuring these, it is appropriate to follow as much as possible the production process of the conventional p-type substrate solar cell that is widely spread.

  Considering the structure of a conventional typical p-type substrate solar cell, as a p-layer formation method other than the initial substrate impurities, a method of utilizing an Al—Si alloy for the formation of the BSF layer on the back side is widely spread. ing. However, in the general structure of an n-type substrate solar cell, since the p-type diffusion junction layer is naturally formed on the light receiving surface side, this method is not possible, and thermal diffusion such as boron (B; boron) is mainly used. To form a diffusion layer.

  Although boron diffusion itself differs in the materials used and the detailed conditions, basic parts such as methods and devices can be treated in the same way as phosphorus diffusion. Phosphorus diffusion is a method that is also widely used as a method for manufacturing p-type substrate solar cells.

  However, boron diffusion tends to increase (deteriorate) the surface recombination rate, and is an obstacle to higher efficiency. The method of cleaning with a hydrofluoric acid aqueous solution after the diffusion treatment is a standard process called phosphorus glass removal in phosphorus diffusion, but in the case of boron diffusion, after a corresponding cleaning process, as described later There is a risk that the characteristics will deteriorate significantly.

  The use of a silicon nitride film (SiN film) as an insulating film or an antireflection film is a technique that is widely used particularly in the structure and manufacturing method of solar cells made of p-type substrates, but solar cells made of n-type substrates. In this case, there are the following disadvantages. In other words, when boron diffusion is used as the bonding layer, the SiN film is formed directly on the boron diffusion layer, and the fixed charges formed at the interface during the formation of the SiN film are regenerated on the surface of both interfaces. This will affect the direction of increasing the coupling speed. That is, as a problem before the cleaning method, the direct contact between the SiN film and the boron diffusion layer becomes a significant impediment to improving the characteristics.

  On the other hand, as shown in Patent Document 1, forming an oxide film between the two is effective as a method for suppressing the surface recombination rate, and it is possible to increase the efficiency. However, this increases the number of processes and manufacturing equipment, and is not a desirable direction for cost reduction and high productivity.

  Boron diffusion tends to increase the surface recombination rate, and is an obstacle to higher efficiency. In addition, as described above, even in the production of conventional p-type substrate solar cells, there is a background in which cells with an efficiency exceeding 19% are being obtained. In order to achieve high efficiency of the desired n-type substrate solar cell, it is required to suppress the surface recombination rate of the diffusion layer by an inexpensive and effective method.

  The present invention has been made in view of the above, and suppresses the surface recombination rate of the p-type diffusion bonding layer in an n-type substrate solar cell at low cost and effectively makes a highly efficient solar cell sufficiently inexpensive. The purpose is to obtain.

  In order to solve the above-described problems and achieve the object, the present invention includes an n-type semiconductor substrate having a p-type diffusion layer in which a p-type impurity element is diffused on a first surface, and the p-type diffusion layer. In a photovoltaic device that uses as a light receiving surface, a protective film containing the same element as the p-type impurity element on the p-type diffusion layer and including an oxide film obtained by oxidation of the semiconductor substrate, and penetrating the protective film And a first electrode electrically connected to the p-type diffusion layer.

  According to the present invention, an oxide film containing the same element as the p-type impurity contained in the p-type diffusion layer, such as a BSG layer (boron silicate glass) which is a reaction product of boron diffusion, is grown under appropriate conditions. By giving the property similar to an oxide film and remaining it, the surface recombination speed of the p-type diffusion layer is suppressed, and an effect is obtained that a highly efficient n-type substrate solar cell can be obtained by an inexpensive means. .

FIG. 1 is a cross-sectional view schematically showing the configuration of the solar cell according to the first embodiment of the present invention. 2A to 2E are manufacturing process diagrams of the solar cell. FIGS. 3A and 3B are manufacturing process diagrams of the solar cell. FIG. 4 is a flowchart showing manufacturing steps of the solar cell. FIG. 5 is a comparative diagram showing the results of measuring the open circuit voltage of the solar cell of the embodiment of the present invention and the comparative example. FIG. 6 is a comparison diagram showing the results of measuring the spectral sensitivity of the solar cells of the embodiments and comparative examples of the present invention. FIG. 7 is a diagram schematically illustrating the configuration of the solar cell according to the second embodiment of the present invention, in which (a) is a cross-sectional view and (b) is an enlarged cross-sectional view of a main part of the solar cell.

  Embodiments of a photovoltaic device and a manufacturing method thereof according to the present invention will be described below in detail with reference to the drawings. In addition, this invention is not limited to the following description, In the range which does not deviate from the summary of this invention, it can change suitably. In the drawings shown below, the scale of each member may be different from the actual scale for easy understanding. The same applies between the drawings. Further, even a plan view may be hatched to make the drawing easy to see.

Embodiment 1 FIG.
FIG. 1 is a cross-sectional view schematically showing the configuration of the solar cell 1 according to the first embodiment of the present invention. 2 (a) to 2 (e) and FIGS. 3 (a) and 3 (b) are manufacturing process diagrams of the solar cell 1, and FIG. 4 is a flowchart showing the manufacturing process of the solar cell 1. FIG. A shows the light-receiving surface side of the solar cell 1, and B shows the back surface side. The solar cell 1 of Embodiment 1 includes an n-type semiconductor substrate 101 having a p-type diffusion layer 102 in which a p-type impurity element is diffused on the first surface 101A side which is a light receiving surface, and a p-type diffusion layer 102. A BSG film as a protective film 103 made of a mixed film of the same element as the p-type impurity element and silicon oxide, an antireflection film 104 formed on the protective film 103, the protective film 103, and the antireflection film A first electrode 105 penetrating through 104 and electrically connected to the p-type diffusion layer 102; and a second electrode 109 formed on a second surface 101B opposite to the light receiving surface of the semiconductor substrate 101. ing. The second surface 101B is also provided with a back surface insulating film 107 and a back surface reflective film 108 via a BSF (Back Surface Field) layer 106 made of an n-type diffusion layer.

  The inventors of the present invention have been made paying attention to the fact that the oxide film effectively works to suppress the surface recombination of the p-type diffusion layer 102 formed of the boron diffusion layer. When boron is diffused, a BSG layer (boron silicate glass) is formed as a reaction product, and this BSG layer is used as a protective film having properties similar to an oxide film depending on the formation conditions through the study on the above problems. Thus, it was found that the surface recombination of the boron diffusion layer can be suppressed by removing this rather than removing it.

  That is, the solar cell 1 of the first embodiment is a semiconductor substrate 101 constituting a solar cell substrate made of n-type single crystal silicon having a photoelectric conversion function, as shown in FIG. The first surface 101A is a p-type diffusion layer 102 forming a junction, and the opposite second surface (back surface) 101B is a BSF layer 106 made of an n-type diffusion layer. On the surface of the p-type diffusion layer 102, a protective film 103 made of a BSG film and an antireflection film 104 formed thereon to prevent reflection of incident light on the light receiving surface are formed. A light receiving surface side electrode as the first electrode 105 is formed on the first surface 101A, which is the surface on the light receiving surface side of the semiconductor substrate 101, so as to penetrate the protective film 103 and the antireflection film 104. . On the BSF layer 106 made of the n-type diffusion layer on the second surface 101B which is the back surface, a back surface insulating film 107 and a back surface reflecting film 108 are sequentially stacked, and further, the second surface which is the back surface of the semiconductor substrate 101. In 101B, a back-side electrode that is the second electrode 109 is provided so as to penetrate the back-side insulating film 107 and the back-side reflecting film.

  Although omitted in FIG. 1, the surface of the semiconductor substrate 101 (p-type diffusion layer 102) on the first surface 101A side that is the light receiving surface and the surface of the second surface 101B that is the back surface are finely uneven as texture structures. Is formed with high density. The minute unevenness has a function of confining light by changing the angle of reflected light particularly on the light receiving surface, suppressing substantial reflectivity through a plurality of reflections.

  In the solar cell 1 configured as described above, sunlight is applied to the pn junction (interface between the semiconductor substrate 101 and the p-type diffusion layer 102) of the semiconductor substrate 101 from the first surface A side that is the light receiving surface of the solar cell 1. When done, it generates holes and electrons. The generated electrons move toward the semiconductor substrate 101 made of n-type silicon and the holes move toward the p-type diffusion layer 102 by the electric field of the pn junction. As a result, an excess of electrons in the semiconductor substrate 101 made of n-type silicon and holes in the p-type diffusion layer 102 result in the generation of photovoltaic power. This photovoltaic power is generated in a direction in which the pn junction is forward-biased, and the first electrode 105 on the light receiving surface (first surface 101A) side connected to the p-type diffusion layer 102 becomes a positive pole, and the BSF layer 106 is The second electrode 109 on the back surface side connected to the semiconductor substrate 101 made of n-type silicon via the negative electrode becomes a negative pole, and a current flows to an external circuit (not shown).

  Next, a method for manufacturing the solar cell according to the first embodiment will be described. First, for example, an n-type single crystal silicon substrate having a thickness of several hundred μm is prepared as the semiconductor substrate 101 (FIG. 2A). Since the n-type single crystal silicon substrate is manufactured by slicing an ingot formed by cooling and solidifying molten silicon with a wire saw, damage on the surface remains on the surface. Therefore, the n-type single crystal silicon substrate is etched near the surface of the n-type single crystal silicon substrate by etching the surface by immersing the surface in an acid or heated alkaline solution, for example, an aqueous sodium hydroxide solution. Remove the damage area that exists in the. For example, the surface is removed by a thickness of 10 μm to 20 μm with an alkaline solution such as sodium hydroxide or potassium hydroxide of several wt% to 20 wt%. Here, as an n-type single crystal silicon substrate used for the semiconductor substrate 101, an n-type single crystal silicon substrate having a specific resistance of 0.1Ω · cm to 5Ω · cm and having a (100) plane orientation will be described as an example. .

  Following the removal of damage, an additive that promotes anisotropic etching such as IPA (isopropyl alcohol) was added to the same alkali low concentration solution, such as several wt% sodium hydroxide or potassium hydroxide. Perform anisotropic etching with solution. By this anisotropic etching, minute uneven portions 101T having a substantially quadrangular pyramid shape are formed on the light receiving surface side and back surface side of the n-type single crystal silicon substrate so that the silicon (111) surface is exposed, thereby forming a texture structure. (Step S10, FIG. 2B). That is, the texture structure is formed on the front and back surfaces of the n-type single crystal silicon substrate by wet etching (alkali texture method) using an alkaline solution.

Next, a pn junction is formed in the semiconductor substrate 101. That is, a group III element such as boron (B) is diffused into the semiconductor substrate 101 to form the p-type diffusion layer 102 having a thickness of several hundred nm. Here, a pn junction is formed by diffusing boron by thermal diffusion to form a p layer with respect to an n-type single crystal silicon substrate having a texture structure on the surface. In this diffusion step, an n-type single crystal silicon substrate is heated at a high temperature of, for example, 800 ° C. to 1100 ° C. by a vapor phase diffusion method in a mixed gas atmosphere of, for example, boron tribromide (BBr ₃ ) gas, nitrogen gas, and oxygen gas. The p-type diffusion layer 102 in which boron (B) is diffused is uniformly formed in the surface layer of the n-type single crystal silicon substrate by sufficient thermal diffusion (step S20). Good electrical characteristics of the solar cell can be obtained when the sheet resistance range of the p-type diffusion layer 102 formed on the surface of the semiconductor substrate 101 is about 30Ω / □ to 80Ω / □.

  Then, it is further exposed to an atmosphere containing oxygen or water vapor, and a protective film 103 made of an oxide layer is formed by enclosing a reaction product adhering to the p-type diffusion layer 102 when the p-type diffusion layer 102 is formed (step S30). FIG. 2 (c)).

Here, the p-type diffusion layer 102 and the protective film 103 are formed on the entire surface of the semiconductor substrate 101. Therefore, the first surface 101A and the second surface 101B of the semiconductor substrate 101 are in an electrically connected state. Therefore, in order to cut off this electrical connection, the end surface region of the semiconductor substrate 101 and the second surface 101B side are etched by dry etching, for example.
(Step S40, FIG. 2D).

Next, in order to improve the photoelectric conversion efficiency, a silicon nitride film is formed with a uniform thickness as an antireflection film 104 on one surface of the first surface 101A on the light receiving surface side of the semiconductor substrate 101 (step S50). The film thickness and refractive index of the antireflection film 104 are set to values that most suppress light reflection. The antireflection film 104 is formed by using, for example, a plasma CVD method, using a mixed gas of silane (SiH ₄ ) gas and ammonia (NH ₃ ) gas as a raw material, and at 300 ° C. or higher and under reduced pressure. A silicon nitride film is formed as 104. The refractive index is, for example, about 2.0 to 2.2, and the optimum antireflection film thickness is, for example, 70 nm to 90 nm.

  In addition, as the antireflection film 104, a silicon oxide film, a titanium oxide film, or two or more films having different refractive indexes may be stacked. In addition to the plasma CVD method, the antireflection film 104 may be formed by vapor deposition, thermal CVD, or the like. It should be noted that the antireflection film 104 formed in this way is an insulator, and simply forming the first electrode 105 thereon does not act as a solar battery cell.

Next, phosphorus oxychloride (POCl ₃ ), phosphoric acid, etc. are diffused by thermal diffusion on the second surface 101B side, which is the back surface of the semiconductor substrate 101 made of an n-type single crystal silicon substrate, to form an n ⁺ -type BSF layer. 106 is formed (step S60, FIG. 2E). In the case as described above, the protective film 103 and the antireflection film 104 function as a mask material, so that phosphorus is not diffused or has a very small amount of diffusion that does not affect the first surface side. Therefore, it can be expressed that the n ⁺ -type BSF layer 106 can be formed only on the second surface side.

The structure of the n ⁺ -type BSF layer 106 is not particularly limited because it is not a part directly related to the feature of the present embodiment. However, from the viewpoint of improving the photoelectric conversion efficiency, a single structure has an impurity peak concentration of 3 × 10 ¹⁹ to 2 × 10 ²⁰ cm ⁻³ and an impurity diffusion depth of 0.4 to 0.8 μm. The sheet resistance is preferably in the range of 40 to 80Ω / □.

Alternatively, as in the selective emitter structure in the light-receiving surface side impurity diffusion layer of the p-type silicon solar cell, the region near the back side electrode described later has a low sheet resistance, and the other regions have a high sheet resistance, depending on the region. You may form by dividing. In this case, the low sheet resistance portion is preferably in the range of 20 to 60Ω / □, and the high sheet resistance portion is preferably in the range of 80 to 200Ω / □. A silicon oxide film (phosphor silicate glass: PSG film or another name: phosphorus glass) containing phosphorus (P: phosphor) on the surface of the n type single crystal silicon substrate immediately after the formation of the n ⁺ type BSF layer 106. ) Is formed and removed using hydrofluoric acid or the like.

Next, a back surface insulating film 107 made of a silicon nitride film (SiN film) is formed on the second surface 101B on the back surface side of the semiconductor substrate 101 (step S70, FIG. 3A). The back surface insulating film 107 made of a silicon nitride film (SiN film) having a refractive index of 1.9 to 2.2 and a thickness of 60 nm to 300 nm is applied to the silicon surface exposed on the back surface side of the semiconductor substrate 101 by, for example, plasma CVD. Is deposited. By forming such a back surface insulating film 107, the carrier recombination speed on the back surface of the semiconductor substrate 101 can be suppressed, and a sufficient back surface interface can be realized for high output. The back surface insulating film 107 is provided over the entire back surface of the n-type silicon substrate. In addition to the silicon nitride film (SiN film) described above, a silicon oxide film (SiO ₂ film), a titanium oxide film (TiO ₂ ), or these It may be made of an insulating film made of a laminated film or a mixed film.

As described above, the formation of the p-type diffusion layer 102 on the light-receiving surface side, the n ⁺ -type BSF layer 106 on the back surface side, and the antireflection film 104 and the back surface insulating film 107 thereon are different from each other. You can use the method. Depending on the characteristics of each method to be selected, the formation order may be appropriately selected, and the formation may be performed in an order other than that described above.

  Next, in the back surface insulating film 107, an opening 107a is formed in a region where a back silver paste for forming the second electrode 109 made of silver is applied (step S80, FIG. 3B). The opening 107a is formed using, for example, a laser or an etching paste. Note that this step is not necessary if the paste used can fire through the back insulating film 107. The opening 107a is formed in a comb pattern, for example.

  Next, electrodes are formed by screen printing. First, the first electrode 105 on the light receiving surface side is manufactured (before firing). That is, an electrode material paste including glass frit in the shape of the front grid electrode and the front bus electrode on the antireflection film 104 which is the light receiving surface of the semiconductor substrate 101, the main component of the metal component being silver, After applying a paste containing aluminum (hereinafter sometimes referred to as an aluminum electrode paste) by screen printing, the aluminum electrode paste is dried (step S90). If a good electrical connection can be obtained, the inclusion of aluminum is not essential, and a silver-based paste similar to the back surface paste described below may be used.

  Next, a back silver paste, which is an electrode material paste, is applied to the shape of the second electrode 109, which is a back electrode, by screen printing on the back side of the semiconductor substrate 101 and dried (step S100, FIG. 3C). Here, the back silver paste is printed by filling the opening 107 a provided in the back insulating film 107.

  Thereafter, the electrode paste on the front surface and the back surface of the semiconductor substrate 101 is simultaneously fired at, for example, 600 ° C. to 900 ° C., so that the glass material contained in the aluminum-based electrode paste on the first surface 101A side of the semiconductor substrate 101 is used. While the antireflection film 104 is melted, the aluminum material comes into contact with silicon and resolidifies. As a result, a front grid electrode and a front bus electrode as the first electrode 105 on the light receiving surface side are obtained, and conduction between the first electrode 105 and the silicon of the semiconductor substrate 101 is ensured (step S110). Such a process is called a fire-through method.

Further, the back silver paste also reacts with the silicon of the semiconductor substrate 101 to obtain the second electrode 109 as the back electrode, and a good contact is formed with the BSF layer 106 made of the n-type diffusion layer. Further, a BSF layer made of a higher concentration n ⁺ diffusion layer may be formed immediately below the back electrode (described later in Embodiment 2). Further, the silver material of the silver paste comes into contact with silicon and re-solidifies to obtain the second electrode 109 composed of the back silver electrode.

  Next, a back silver sputtering film may be formed as the back surface reflection film 108 on the back surface insulating film 107 on the back surface of the semiconductor substrate 101 by sputtering. Note that there is no particular problem even if silver is sputtered onto the second electrode 109 which is a back silver electrode.

  By performing the above steps, the solar cell 1 according to the present embodiment shown in FIG. 1 is obtained. Note that the order of arrangement of the paste, which is an electrode material, on the semiconductor substrate 101 may be switched between the light receiving surface side and the back surface side.

  As described above, the oxide film effectively works to suppress the surface recombination of the p-type diffusion layer 102 made of the boron diffusion layer. When boron is diffused, a BSG layer (boron silicate glass) is formed as a reaction product, and this BSG layer is used as a protective film having properties similar to an oxide film depending on the formation conditions through the study on the above problems. I can do it. Therefore, the surface recombination of the boron diffusion layer is suppressed by not removing the BSG layer which is a reaction product of the boron diffusion but rather leaving it.

Analysis of the BSG layer with the above characteristics confirmed the presence of 1% boron (B; boron) relative to silicon and the distribution of characteristics seen in the bond energy distribution of B ₂ O ₃ (diboron trioxide). It was done. From this result, it can be said that the aforementioned BSG layer is a silicon oxide film (SiO ₂ ) containing 1% of B ₂ O ₃ . If such a BSG layer is used, it can function as a protective film that suppresses the surface recombination rate. Here, 1% B ₂ O ₃ in a silicon oxide film containing 1% B ₂ O ₃ may be slightly increased or decreased within a range of error, and B including approximately 1/2 to 2 times increase / decrease. Any material containing ₂ O ₃ may be used.

  It should be noted that similar effects can be obtained when the boron content is in the range of 0.1 to 10%, more preferably in the range of 0.5 to 2%, from various experimental results in which the boron content is changed. all right. If it exceeds 10%, the silicide of silicon and boron found at the interface between the silicon and the general BSG layer increases, and this adverse effect cannot be ignored.

  Silicide is a solid different from silicon and has unique physical properties such as a thermal expansion coefficient. In addition, since it is basically a crystalline solid, it has almost no flexibility like a glassy substance. Especially when it exists in direct contact with a silicon semiconductor, it can react with the silicon semiconductor crystal depending on heating and other loads. It causes strain and stress, and also causes frequent crystal defects. For this reason, the surface recombination rate at the contact interface is increased, and the function as a protective film for suppressing the surface recombination rate is greatly reduced. If the boron content exceeds 10%, this effect cannot be ignored.

  On the other hand, if it is less than 0.1%, the function as a boron supply source is lowered, the carrier speed is lowered, and in order to compensate for this, the processing temperature is further increased and the time is increased, resulting in the characteristics and production of solar cells. In the worst case, the supply of boron does not progress and the pn junction cannot be formed. When the boron content is in the range of 0.1 to 10%, more desirably 0.5 to 2%, a solar cell having a low surface recombination rate and good characteristics is provided with higher productivity. It becomes possible.

  In order to impart the above properties to the BSG layer, it is desirable that the BSG layer is grown in the range of 5 nm to 20 nm in terms of thickness after being exposed to an oxidizing atmosphere after the original diffusion process. If the film thickness of the BSG layer is less than 5 nm, the effect of suppressing the surface recombination rate is insufficient. On the other hand, if the film thickness of the BSG layer is 20 nm or more, the treatment time becomes long and the risk increases from the viewpoint of productivity. As the time increases, the concentration distribution of the boron diffusion layer itself changes, which increases the risk of deterioration of characteristics.

  As the p-type impurity other than boron, aluminum or gallium can be used. Similarly to the above, a silicon oxide film containing an oxide of another group III element such as aluminum oxide or gallium oxide can be used. Similar effects can be obtained.

  In order to promote the growth of the BSG layer effectively, it is desirable that the oxidizing atmosphere is a water vapor-containing atmosphere. Although a desired structure can be obtained even in a simple oxygen-containing atmosphere, an appropriate range of conditions is limited because the basic growth rate is slow. Higher temperature is also mentioned as one of the directions, but this has substantially the same effect as longer time, so the risk that the concentration distribution will change similarly increases, and eventually the condition range is limited. Connected.

  For the growth of the BSG layer, it is also possible to add the diffusion furnace and the oxidation furnace in succession, but the diffusion furnace and the oxidation furnace are devices that can share the basic structure and properties. By switching the gas and the substrate temperature in this apparatus, the substrate can be processed almost continuously without being exposed to the atmosphere, and the processing can be performed in a shorter time. By performing the continuous treatment, it is possible to prevent the intake of impurities, dust, and the like in the atmosphere, and to provide a more reliable semiconductor device.

However, especially when using an atmosphere containing water vapor, boron tribromide (BBr ₃ ), which is a raw material for boron diffusion, and water vapor (H ₂ O) have the property of reacting directly, so they are mixed carelessly without being thermally decomposed. Therefore, it is desirable to add certain considerations to safety risks.

  More specifically, it is more appropriate to avoid the simultaneous processing of both, and to perform a two-stage process in which boron diffusion and steam oxidation are separated. As described above, each process may be executed by an independent apparatus, or in the case where the same equipment is continuously used, a stage of performing atmosphere replacement with nitrogen or the like may be interposed between the two.

As described above, the BSG layer on the light receiving surface side is not removed but rather left, which contributes to higher efficiency of the solar cell. However, this is not the case with the opposite surface side (back surface side). This is not particularly necessary when the BSG layer and the p-type diffusion layer are formed only on the light-receiving surface side. However, when the BSG layer and the p-type diffusion layer are formed on the entire exposed surface as described above, in step S40, In view of the fact that an n ⁺ -type BSF (Back Surface Field) layer is formed as described above, it is desirable to remove not only the BSG layer but also the boron diffusion layer itself on the back surface side, more preferably on the side surface and the back surface side. It is desirable that

  Specifically, in addition to the above-described dry etching (step S40 in the first embodiment), using a single-sided etching apparatus, the BSG layer and the boron diffusion layer only on the back surface side or only on the end surface and the back surface side are mixed with hydrofluoric acid aqueous solution or It is desirable to etch away properly with a mixture of hydrofluoric acid. Alternatively, the light-receiving surface side may be protected with a resist or acid-resistant resin and then immersed in an etching solution such as a fluorinated nitric acid solution to remove the BSG layer and the boron diffusion layer on the end surface and the back surface side.

  Alternatively, when the antireflection film described later includes a silicon nitride film, both sides are put into an appropriate etching tank using this layer as a mask material, while the light receiving surface side is protected by the silicon nitride film, The method of removing the BSG layer and the boron diffusion layer is also effective particularly from the viewpoint of cost reduction since it can be used including a widely used general etching apparatus.

  In contrast to the solar cell 1 configured as described above, the conventional solar cell has no protective film 103 made of a BSG layer due to the structure of the solar cell 1 in FIG. The film 104 is formed. The other structure is basically the same as that of the solar cell 1 of FIG.

  In order to make the effect of the protective film 103 made of the BSG layer more remarkable, the following two types of solar cells of comparative examples were prepared. In contrast to the prototype (Type-A) according to the solar cell of the present embodiment, one is obtained by removing the BSG layer immediately after the diffusion treatment and forming an antireflection film (Type-B), and the other. After the diffusion treatment, the oxidation treatment shown in the solar cell of this embodiment is continued, and then the grown BSG layer is intentionally removed and washed with a hydrofluoric acid aqueous solution, and an antireflection film is formed thereon. Formed (Type-C).

  A total of three types of characteristic comparisons are shown in FIGS. The characteristics in which the BSG layer according to this embodiment is left (Type-A) is clearly improved as compared with the two types (Type-B, C) without the BSG layer. From the comparison between the open-circuit voltage (Voc; FIG. 5) and the short wavelength sensitivity of the spectral sensitivity (FIG. 6), it can be seen that the improvement at the extreme surface portion of light absorption greatly contributes.

  In FIG. 6, the horizontal axis represents the wavelength. In addition, an impossible value (> 100%) is seen in a part of the spectral sensitivity result (FIG. 6), and it cannot be denied that the accuracy of this result is reduced due to the influence of noise. However, even if it is subtracted, it can be said that the difference between Type-A and Type-B, C is obvious. Furthermore, not only Type-B but also the characteristics of Type-C are poor, and it can be seen that the removal of the BSG layer is a decisive factor for the deterioration of the characteristics. Paradoxically, the presence of the BSG layer is the surface recombination. It shows that it functions effectively to suppress the speed.

Embodiment 2. FIG.
Fig.7 (a) is sectional drawing which shows typically the structure of the solar cell concerning Embodiment 2 of this invention. FIG.7 (b) is a principal part expanded sectional view of the solar cell. In the solar cell 1 of the second embodiment, the BSF layer 106B on the back side is formed only around the second electrode 109 by diffusion from the second electrode 109 formed of a conductive paste containing antimony. It is a thing.

Here, the conductive paste containing antimony reacts with the silicon of the semiconductor substrate 101 to obtain the second electrode 109, and the BSF layer 106B made of an n ⁺ diffusion layer is formed immediately below the second electrode 109. . When the conductive paste further contains silver, the silver material comes into contact with silicon and re-solidifies to obtain the second electrode 109 made of the back silver electrode.

  The other points are the same as those of the solar cell of the first embodiment, and the same portions are denoted by the same reference numerals, but description thereof is omitted here. The second electrode may have a two-layer structure of a first layer containing a group V element and a low-resistance second layer for forming a BSF layer. The second electrode may be formed using a paste containing a group V element such as bismuth in addition to antimony.

  In each of the first and second embodiments, the antireflection film 104 is formed. However, the antireflection film 104 as a single structure film is not necessarily required. For example, a structure in which the composition of the antireflection film 104 is continuously changed to change the refractive index, or a structure in which an antireflection structure is formed by a multilayer film can be appropriately modified.

  In the above embodiment, a protective film made of a mixed film of the same element as the p-type impurity element and silicon oxide is used on the p-type diffusion layer. However, the impurity element forming the p-type diffusion layer is boron. It is applicable not only when it contains a plurality of elements including other group III elements such as aluminum. It suffices that at least the first electrode contains at least one element of impurity elements forming the p-type diffusion layer.

  As described above, the photovoltaic device according to the present invention and the method for manufacturing the photovoltaic device according to the present invention appropriately grow the surface layer in the p-type diffusion layer such as the boron diffusion layer by appropriately growing the oxide layer containing the p-type impurity such as the BSG layer. It is possible to effectively reduce the bonding speed at a low cost. Therefore, it is possible to contribute to both high efficiency and low cost of a solar cell using an n-type substrate.

  DESCRIPTION OF SYMBOLS 1 Solar cell, 101 Semiconductor substrate, 101A 1st surface (light-receiving surface), 101B 2nd surface (back surface), 102 p-type diffused layer, 103 protective film, 104 Antireflection film, 105 1st electrode, 106 BSF Layer, 106B BSF layer, 107 back insulating film, 108 back reflecting film, 109 second electrode.

Claims (15)

An n-type semiconductor substrate having a p-type diffusion layer in which a p-type impurity element is diffused on the first surface side which is a light receiving surface;
A protective film containing the same element as the p-type impurity element on the p-type diffusion layer and including an oxide film obtained by oxidation of the semiconductor substrate;
A first electrode penetrating the protective film and electrically connected to the p-type diffusion layer;
A photovoltaic device comprising: a second electrode formed on a second surface facing the light receiving surface of the semiconductor substrate. An antireflection film formed on the protective film;
2. The photovoltaic device according to claim 1, wherein the first electrode penetrates the protective film and the antireflection film and is electrically connected to the p-type diffusion layer.   The photovoltaic device according to claim 2, wherein the second surface is also provided with a back surface insulating film and a back surface reflecting film via an n-type diffusion layer.   4. The photovoltaic device according to claim 1, wherein the p-type impurity element is boron, and the protective film is a BSG layer. 5.   The photovoltaic device according to claim 4, wherein the antireflection film is a silicon nitride film.   6. The photovoltaic device according to claim 4, wherein the BSG layer has a boron content of 0.1 to 10%.   The photovoltaic device according to claim 6, wherein the BSG layer has a boron content of 0.5 to 2%.   The photovoltaic device according to any one of claims 4 to 7, wherein the BSG layer has a thickness in a range of 5 nm to 20 nm.   The photovoltaic device according to any one of claims 1 to 8, wherein the p-type impurity element is aluminum, and the protective film is an aluminum oxide layer.   The photovoltaic device according to any one of claims 1 to 8, wherein the p-type impurity element is gallium, and the protective film is a gallium oxide layer. diffusing a p-type impurity element on the first surface side of the n-type semiconductor substrate to form a p-type diffusion layer;
After the step of forming the p-type diffusion layer, it is subsequently exposed to an atmosphere containing oxygen or water vapor, and oxidized to protect the reaction product adhering to the p-type diffusion layer during the formation of the p-type diffusion layer. An oxidation step to form a film;
Forming a first electrode that penetrates the protective film and is electrically connected to the p-type diffusion layer;
Forming a second electrode on a second surface opposite to the first surface of the semiconductor substrate. A method for manufacturing a photovoltaic device, comprising: Forming an antireflection film on the protective film,
12. The photovoltaic device according to claim 11, wherein the step of forming the first electrode includes a step of passing through the protective film and the antireflection film and electrically connecting to the p-type diffusion layer. A method for manufacturing a power device. The step of forming the p-type diffusion layer is a step of forming the p-type diffusion layer on both surfaces of the n-type semiconductor substrate.
A step of removing the protective film and the p-type diffusion layer from the second surface while holding the protective film and the p-type diffusion layer on the first surface after continuously performing the oxidation step. The method for manufacturing a photovoltaic device according to claim 12, comprising: The step of forming the p-type diffusion layer is a vapor phase diffusion step using boron oxide,
The method for manufacturing a photovoltaic device according to claim 11, wherein the oxidation step is a step of forming a BSG layer.   The method of manufacturing a photovoltaic device according to claim 12, wherein the antireflection film is a silicon nitride film.

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