JP2014082430A - Method of manufacturing solar cell element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a solar cell element, capable of improving power generation conversion efficiency.SOLUTION: The method of manufacturing a solar cell element comprises: a semiconductor substrate preparing step of preparing a semiconductor substrate of a first conductivity type; a texture structure forming step of forming a texture structure on a surface of the semiconductor substrate; and a second conductivity type layer forming step of forming a second conductivity type layer of a conductivity type opposite to the first conductivity type on the surface of the semiconductor substrate having the texture structure formed. The method of manufacturing a solar cell element also comprises a nitric acid cleaning step of cleaning the surface of the semiconductor substrate using a nitric acid at least either before the texture structure forming step or between the texture structure forming step and the second conductivity type layer forming step.

Description

  The present invention relates to a method for manufacturing a solar cell element using a semiconductor substrate.

  A solar cell element is a photoelectric conversion element that converts incident solar energy into electrical energy. The solar cell element is classified into a crystal (bulk) system or a thin film system depending on its form, or a silicon system, an inorganic compound system, an organic compound system, or the like depending on its material. Among these, the solar cell element which is currently mainstream in the market is a crystalline silicon solar cell element using a silicon substrate.

  Silicon substrates used for crystalline silicon solar cell elements include single crystal silicon substrates and polycrystalline silicon substrates. In either case, a silicon block of a desired shape is mechanically used using a wire saw device or the like. In general, a thin plate-like substrate is obtained by slicing.

  In the slicing process, a damage layer into which many crystal defects are introduced due to mechanical damage at the time of cutting is formed on the surface of the silicon substrate, and is contaminated by metal impurities contained in a metal wire used for cutting. . These metal impurities are diffused into the substrate in the subsequent process and become recombination centers of carriers, which causes a decrease in photoelectric conversion efficiency of the solar cell element.

  Various silicon substrate cleaning methods have been proposed so far. For example, in RCA cleaning, a silicon substrate is cleaned using a mixed solution of hydrochloric acid and a hydrogen peroxide solution to remove metal impurities on the substrate surface by etching. Note that RCA cleaning is generally used in the manufacture of semiconductor devices such as LSI.

  The following Patent Document 1 describes RCA cleaning using hydrochloric acid and an aqueous hydrogen peroxide solution.

JP 2009-290013 A

  Usually, in RCA cleaning, the substrate is cleaned by maintaining a mixed solution of hydrochloric acid and hydrogen peroxide solution at a temperature of 50 ° C. or higher. At this time, since the chemical component is easily vaporized, it is necessary to frequently replenish or replace the solution in order to keep the composition of the solution at a constant ratio. For this reason, there is a concern about an increase in the environmental load associated with the disposal of the chemical solution, and there is a concern about an increase in costs related to the chemical solution, the chemical solution treatment, and the equipment.

  Further, in the crystalline silicon solar cell, a fine uneven shape for reducing the reflectance of light incident on the silicon substrate surface is formed. For example, when the RCA cleaning is performed after the formation of the uneven structure, the uneven shape may change, resulting in a surface shape that is not sufficiently effective in reducing the reflectance.

The present invention has been made in view of the above problems, and is for forming a texture structure having a fine uneven shape for reducing reflectance and a pn junction on the first main surface side of a first conductivity type semiconductor substrate. In the manufacturing method of the solar cell element having the second semiconductor layer, there is provided a substrate cleaning method that effectively removes metal impurities adhering to the surface of the semiconductor substrate and does not impair the surface irregularity shape for reducing the reflectance. For the purpose.

  The method for manufacturing a solar cell element according to the present invention includes a semiconductor substrate preparation step of preparing a first conductivity type semiconductor substrate, a texture structure forming step of forming a texture structure on the surface of the semiconductor substrate, and forming the texture structure A method for manufacturing a solar cell element, comprising: a second conductivity type layer forming step of forming a second conductivity type layer having a conductivity type opposite to the first conductivity type on the surface of the semiconductor substrate. There is a nitric acid cleaning step of cleaning the surface of the semiconductor substrate with nitric acid before the structure forming step and at least one of the texture structure forming step and the second conductivity type layer forming step.

  According to the method for manufacturing a solar cell element described above, a semiconductor substrate that can remove metal impurities existing on the surface of the semiconductor substrate with a simple, low-cost and low chemical use amount and is effective in maintaining the texture structure. A cleaning method can be provided.

It is a figure which shows typically an example of the solar cell element which concerns on one Embodiment of this invention, and is the top view seen from the light-receiving surface (surface) side of a solar cell element. It is the top view seen from the non-light-receiving surface (back surface) side of the solar cell element shown in FIG. It is the KK sectional view taken on the line in FIG. It is a process flow figure for explaining a manufacturing method of a solar cell element concerning the present invention.

  Hereinafter, the manufacturing method of the solar cell element concerning one Embodiment of this invention is demonstrated in detail, referring drawings. In the drawings, parts having similar configurations and functions are denoted by the same reference numerals. Further, the drawings are schematically shown, and the sizes of the components and the positional relationships of the components in the drawings are not accurately shown.

<Solar cell element>
As shown in FIGS. 1 to 3, the solar cell element 10 includes a first surface 9 a that is a light receiving surface on which light is incident and a second surface 9 b that is a non-light receiving surface corresponding to the back surface of the first surface 9 a. Have.

  As shown in FIG. 3, the solar cell element 10 includes a semiconductor substrate 1 including a first semiconductor layer 2 and a second semiconductor layer 3, and an antireflection film 4 provided on the first surface 9 a side of the semiconductor substrate 1. And. The first semiconductor layer 2 is a one-conductivity-type semiconductor layer, and the second semiconductor layer 3 is provided on the first surface 9a side of the first semiconductor layer 2 and forms a pn junction with the first semiconductor layer 2. It is a conductive semiconductor layer.

  Further, the solar cell element 10 includes a first electrode 5 provided on the first surface 9 a of the semiconductor substrate 1 and a second electrode 6 provided on the second surface 9 b of the semiconductor substrate 1.

As the semiconductor substrate 1, a single crystal silicon substrate or a polycrystalline silicon substrate having a predetermined dopant element of about 1 × 10 ^{15 to} 5 × 10 ¹⁷ atoms / cm ³ and exhibiting one conductivity type (for example, p-type) is used. The crystalline silicon substrate is preferably used.

  The antireflection film 4 plays a role of reducing the reflectance of light in a desired wavelength region and increasing the amount of light incident on the solar cell element, thereby increasing the amount of photogenerated carriers and improving the photoelectric conversion efficiency. The antireflection film 4 can be formed using, for example, one or more materials selected from silicon nitride, silicon oxide, and the like. Further, the antireflection film 4 such as a silicon nitride film formed by the plasma CVD method can also have a function as a passivation film.

A BSF (Back Surface Field) region 7 has a role of reducing a decrease in efficiency due to carrier recombination in the vicinity of the second surface 9b of the semiconductor substrate 1, and an internal electric field is formed on the second surface 9b side of the semiconductor substrate 1. Is formed. The BSF region 7 has the same conductivity type as that of the semiconductor substrate 1, but the BSF region 7 originally has a carrier concentration higher than the concentration of majority carriers contained in the semiconductor substrate 1. If p-type is exhibited, the BSF region 7 is a p + type semiconductor region having a higher dopant element concentration. In the BSF region 7, for example, a dopant element such as boron or aluminum is diffused on the second surface 9 b side, so that the concentration of these dopant elements is 1 × 10 ^{18 to} 5 × 10 ²¹ atoms /
may be formed such that the cm ^3.

  As shown in FIG. 1, the first electrode 5 includes a first output extraction electrode 5a and a plurality of linear first current collecting electrodes 5b, and may further include an auxiliary electrode 5c. At least a part of the first output extraction electrode 5a intersects the first current collecting electrode 5b. The first output extraction electrode 5a has a width of about 1 to 3 mm, for example. The 1st current collection electrode 5b is a linear form thinner than the 1st output extraction electrode 5a, The width | variety is about 50-200 micrometers, for example. The plurality of linear first current collecting electrodes 5b are provided with an interval of about 1 to 3 mm. The auxiliary electrode 5c is an electrode for connecting ends of the plurality of first current collecting electrodes 5b, and the width of the auxiliary electrode 5c is approximately the same as the width of the first current collecting electrode 5b. The thickness of the first electrode 5 is about 10 to 40 μm.

  The second electrode 6 includes a second output extraction electrode 6a and a second current collecting electrode 6b. The thickness of the second electrode 6 is, for example, about 1 to 10 μm, and is formed on substantially the entire surface of the semiconductor substrate 1 on the second surface 9b side. The second electrode 6 is electrically connected to the semiconductor substrate 1 through the BSF region 7.

<Method for producing solar cell element>
Next, the manufacturing method of the solar cell element mentioned above is demonstrated. First, a basic manufacturing method of the semiconductor substrate 1 will be described.

  In the present embodiment, when the semiconductor substrate 1 is obtained by performing a cutting process such as a slicing process, the processes shown in P1 to P4 in FIG. 4 are performed. That is, the manufacturing method of the solar cell element of the present embodiment includes a semiconductor substrate preparation step (P1) for preparing the first conductivity type semiconductor substrate 1 having a damaged layer generated by cutting on the surface, and the semiconductor substrate 1 Damage layer removing step (P2) for removing the damaged layer, texture structure forming step (P3) for forming the texture structure on the surface of the semiconductor substrate 1 from which the damaged layer has been removed, and the surface of the semiconductor substrate 1 on which the texture structure has been formed And a second conductivity type layer forming step (P4) for forming a second conductivity type layer having a conductivity type opposite to that of the first conductivity type. Further, in at least one of the damage layer removing step (P2) and the texture structure forming step (P3) and at least one of the texture structure forming step (P3) and the second conductivity type layer forming step (P4), the damage layer A nitric acid cleaning step is performed in which the surface of the semiconductor substrate 1 from which the metal is removed or the surface of the semiconductor substrate 1 on which the texture structure is formed is cleaned using nitric acid.

  However, when cutting processing such as slicing is not taken into consideration, the semiconductor substrate preparation step (P1) may be a step of merely preparing a first conductivity type semiconductor substrate. Further, in this case, the damage layer removing step (P2) is unnecessary, and before the texture structure forming step (P3) and between the texture structure forming step (P3) and the second conductivity type layer forming step (P4). At least one is subjected to a nitric acid cleaning step of cleaning the surface of the semiconductor substrate 1 with nitric acid.

  Subsequent to the nitric acid cleaning step, a hydrofluoric acid cleaning step of cleaning the surface of the semiconductor substrate 1 cleaned with nitric acid with hydrofluoric acid may be further performed.

  In the damaged layer removing step (P2), the damaged layer may be removed by etching using an alkaline aqueous solution.

  In the texture structure forming step (P3), the texture structure may be formed by reactive ion etching.

  In the second conductivity type layer forming step (P4), the second conductivity type layer may be formed by a thermal diffusion method.

  Furthermore, in the nitric acid cleaning step, nitric acid having a nitric acid concentration of 10% by mass to 70% by mass and a temperature of 20 ° C. or more and 80 ° C. or less may be used, but the nitric acid concentration is more preferably 14% by mass or more and 35% by mass. Nitric acid having a temperature of 35 ° C. or higher and 50 ° C. or lower may be used.

  Next, the above manufacturing method will be specifically described.

  As shown in FIG. 4, the general manufacturing method of the solar cell element using a silicon substrate is as follows.

  In the semiconductor substrate preparation step (P1), a plate-like semiconductor substrate 1 exhibiting a first conductivity type having a desired shape and size is prepared.

  In the damaged layer removing step (P2), the damaged layer formed on the surface of the semiconductor substrate 1 at the time of cutting such as slicing is removed with an acid aqueous solution or an alkali aqueous solution.

  In the texture structure forming step (P3), a texture structure having a fine uneven shape for reducing the reflectance is formed on one main surface side of the semiconductor substrate 1.

  In the second conductivity type layer forming step (P4), a second conductivity type semiconductor layer is formed on one main surface side of the semiconductor substrate 1.

  In the antireflection film forming step (P5), an antireflection film is formed on one main surface side of the semiconductor substrate 1.

  In the electrode forming step (P6), a collecting electrode and an output extraction electrode are formed.

  Next, the manufacturing method of the solar cell element which is the semiconductor device which concerns on this embodiment is demonstrated in detail, referring FIGS.

  First, the semiconductor substrate preparation process (P1) which prepares the semiconductor substrate 1 which comprises the solar cell element 10 is implemented. The semiconductor substrate 1 is obtained by cutting a silicon block obtained by processing a single crystal or polycrystalline silicon ingot having one conductivity type into a desired shape into a large number of plate substrates using a wire saw device or the like. The single crystal silicon ingot is manufactured by a CZ (Czochralski) method or the like, and the polycrystalline silicon ingot is manufactured by a cast method or the like. In the present embodiment, an example in which a p-type polycrystalline silicon substrate is used as the semiconductor substrate 1 will be described. In order to make the silicon substrate exhibit p-type, boron or gallium, for example, may be added as a dopant element during the production of the silicon ingot.

  When a silicon block is manufactured by cutting a silicon block using a wire saw device or the like, a damaged layer into which many crystal defects are introduced is formed on the surface of the silicon substrate due to mechanical damage during cutting. Therefore, a damaged layer removing step (P2) is performed in which the damaged layer is removed by etching with an acidic or alkaline solution. To remove the damaged layer, the damaged layer formed on the surface of the silicon substrate may be etched using an alkaline aqueous solution such as a sodium hydroxide aqueous solution or a potassium hydroxide aqueous solution, or an acidic aqueous solution such as a mixed acid containing hydrofluoric acid and nitric acid. . The semiconductor substrate 1 may be manufactured without being cut by a method of directly manufacturing a substrate from a melt. When the semiconductor substrate 1 manufactured by a manufacturing method in which there is no such mechanical cutting process and a damaged layer is not formed, the damaged layer removing process (P2) can be omitted.

  In order to improve the photoelectric conversion efficiency, it is important to efficiently take incident sunlight into the solar cell element 10. Therefore, a texture structure forming step (P3) for forming a texture structure having a minute uneven structure on the surface of the semiconductor substrate 1 is performed. By forming a texture structure on the surface of the semiconductor substrate 1, the reflectance of sunlight incident on the solar cell element 10 can be reduced, and the characteristics of the solar cell element 10 are improved. As a method for forming such a texture structure, a wet etching method using a solution obtained by adding isopropyl alcohol to an alkaline solution such as an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution or a mixed acid containing nitric acid and hydrofluoric acid, RIE (Reactive Ion) Etching) and other dry etching methods.

  The damage layer removal step (P2) and the texture structure formation step (P3) can be carried out as separate steps, but depending on the etching method, conditions, etc., it is also possible to form a texture shape while removing the damage layer. is there.

  For example, damage layer removal and texture shape formation can be performed by immersing the semiconductor substrate 1 in a mixed acid containing nitric acid and hydrofluoric acid for 20 to 240 seconds. In order to adjust the etching rate and texture shape, water, acetic acid, sulfuric acid or phosphoric acid may be added to the mixed acid, or physical roughening treatment such as sand blasting or metal ions may be performed before dipping in the mixed acid. You may perform the porous process which immerses the semiconductor substrate 1 in the aqueous solution containing this.

  Further, a second conductivity type layer forming step (P4) for forming the n-type second semiconductor layer 3 in the surface layer on the first surface 9a side in the semiconductor substrate 1 is performed. If a p-type silicon substrate is used as the semiconductor substrate 1, the second semiconductor layer 3 is formed so as to exhibit an n-type conductivity type. The n-type two-conductivity semiconductor layer 3 is formed, for example, by diffusing a dopant element such as phosphorus on the first surface 9a side in the semiconductor substrate 1.

Such a second semiconductor layer 3 has a coating thermal diffusion method in which P ₂ O ₅ in a paste state is applied to the surface of the semiconductor substrate 1 for thermal diffusion, and phosphorus oxychloride (POCl ₃ ) in a gas state is a diffusion source. The gas phase thermal diffusion method is used. The second semiconductor layer 3 is formed with a sheet resistance of about 60 to 150Ω / □ over a depth of about 0.1 to 1 μm from the surface of the semiconductor substrate 1. When the second semiconductor layer 3 is formed on the second surface 9b side, only the second surface 9b side is removed by etching to expose the p-type conductivity type region on the second surface 9b. For example, the second semiconductor layer 3 may be removed by immersing only the second surface 9b side of the semiconductor substrate 1 in a hydrofluoric acid solution. Further, the phosphorus glass adhering to the surface of the semiconductor substrate 1 when forming the second semiconductor layer 3 is removed by etching with hydrofluoric acid or the like.

  Usually, in the thermal diffusion method, the semiconductor substrate 1 is kept at a high temperature of 700 ° C. or higher. At this time, if the surface of the semiconductor substrate 1 is contaminated with metal impurities, the metal impurities diffuse into the semiconductor substrate 1. Such a metal impurity acts as a recombination center of carriers generated by light absorption, and thus causes deterioration in the characteristics of the solar cell element.

  Therefore, in the present invention, after the semiconductor substrate preparation step (P1), the first period before the texture structure forming step (P3), or after the texture structure forming step (P3) and before the second conductivity type layer forming step (P4). In at least one of the second periods, a nitric acid cleaning step of cleaning the semiconductor substrate with nitric acid is performed to remove metal impurities.

  When a damaged layer is formed by machining or the like on the surface of the semiconductor substrate 1 prepared in the semiconductor substrate preparation step (P1), it is between the damage layer removal step (P2) and the texture structure formation step (P3). In at least one of the third period and the second period, a nitric acid cleaning step (however, after using nitric acid, cleaning with pure water) may be performed.

  Hydrofluoric acid cleaning may be performed after the nitric acid cleaning, and the oxide film formed during the nitric acid cleaning may be removed. In addition, an additive for improving the cleaning effect may be appropriately added to the nitric acid solution used in the nitric acid cleaning step. However, when a chemical solution having a property of etching an oxide film such as hydrofluoric acid is added, the surface of the silicon substrate 1 is Since etching proceeds, it is not suitable as a chemical solution used in this embodiment.

  The temperature of the chemical solution used for the nitric acid cleaning may be 20 ° C. or more and 80 ° C. or less, and the nitric acid concentration may be 10% by mass or more and 70% by mass or less. In particular, when the chemical temperature is 35 ° C. or more and 50 ° C. or less and the nitric acid concentration is 14% by mass or more and 35% by mass or less, it has a sufficient cleaning effect and a texture shape with a low reflectance is ensured uniformly in the substrate surface. Can be formed.

  In the case of dry etching using the RIE method performed in the texture structure forming step (P3), a fine texture structure is formed on the surface of the semiconductor substrate 1 as compared with wet etching using, for example, hydrofluoric acid or an alkaline aqueous solution. Since metal impurities are likely to remain on the surface of the semiconductor substrate 1, the cleaning according to the present embodiment is particularly effective.

  Further, an antireflection film forming step (P5) for forming the antireflection film 4 on the second semiconductor layer 3 is performed. The antireflection film 4 is made of an insulating film such as a silicon oxide film or a silicon nitride film, or a laminated film of these films. Regarding the refractive index and thickness of the antireflection film 4, for example, in a crystalline silicon solar cell element, the refractive index is preferably about 1.5 to 2.5, and the thickness is preferably about 500 to 1200 mm.

  The antireflection film 4 can be produced by a chemical vapor deposition method such as a plasma CVD method or a thermal CVD method, or a physical vapor deposition method such as a sputtering method or a vapor deposition method. The silicon nitride film produced by the plasma CVD method has a passivation effect that inactivates dangling bonds at the silicon substrate surface or crystal grain boundaries by terminating with hydrogen atoms, and the antireflection film 4 of the solar cell element 10. Can be suitably used.

When a silicon nitride film is formed using a plasma CVD method, the reaction chamber of the plasma CVD apparatus is set to about 500 ° C., and a mixed gas of silane (SiH ₄ ) and ammonia (NH ₃ ) is diluted with nitrogen gas or hydrogen gas. Then, the reaction gas is supplied into the reaction chamber. Then, the reactive gas is converted into plasma by glow discharge decomposition and deposited on the surface of the silicon oxide film, whereby a silicon nitride film can be formed.

Next, an electrode forming step (P6) for forming the first electrode 5 and the second electrode 6 is performed. The first electrode 5 is manufactured using a silver paste containing a metal powder made of, for example, silver or the like, an organic vehicle, and glass frit. This silver paste is applied to the first surface 9a side of the semiconductor substrate 1 using a screen printing method or the like, and then baked at a maximum temperature of 600 to 900 ° C. for several tens of seconds to several tens of minutes.

  As a result, the first electrode 5 that breaks through the antireflection film 4 is formed on the semiconductor substrate 1. The second electrode 6 includes a second output extraction electrode 6a and a second collector electrode 6b. First, an aluminum paste containing aluminum powder and an organic vehicle is applied to substantially the entire surface of the semiconductor substrate 1 on the second surface 9b side. As the coating method, a screen printing method or the like can be used.

  By baking the semiconductor substrate 1 in a baking furnace at a maximum temperature of 600 to 850 ° C. for several tens of seconds to several tens of minutes, an aluminum layer serving as the second current collecting electrode 6b of the second electrode 6 is formed. At this time, aluminum diffuses into the silicon substrate, so that the BSF region 7 is formed on the second surface 9 b side of the semiconductor substrate 1.

  The second output extraction electrode 6a is produced using a silver paste containing metal powder made of, for example, silver powder, an organic vehicle, and glass frit. The second output extraction electrode 6a is formed by applying this silver paste in a desired shape using a screen printing method and thereafter baking it at a maximum temperature of 600 to 850 ° C. for several tens of seconds to several tens of minutes. The The silver paste is applied at a position in contact with a part of the aluminum paste (the aluminum layer), so that a part of the second output extraction electrode 6a and the second collector electrode 6b overlap to make electrical contact. Can be obtained.

  The solar cell element 10 can be produced by performing the above steps. In addition, the manufacturing method of the solar cell element 10 is not limited to embodiment mentioned above, It cannot be overemphasized that correction and a change can be added within the scope of the present invention. For example, in the second conductivity type layer forming step, the second semiconductor layer 3 may be formed by plasma CVD or the like, or a region of the second semiconductor layer 3 that is in contact with the first electrode 5 is highly concentrated. Alternatively, a selective emitter structure doped with may be used. Both the first electrode 5 and the second electrode 6 may be electrodes having a back contact structure formed on the second main surface 9b side. The first electrode 5 and the second electrode 6 may be formed by sputtering or vapor deposition. It may be formed.

  Hereinafter, examples in which the above-described embodiment is more specific will be described.

  First, as the semiconductor substrate 1, a plurality of p-type polycrystalline silicon substrates having 150 mm □, a thickness of about 200 μm, and a specific resistance of 1 Ωcm were prepared (P1). These polycrystalline silicon substrates are obtained by processing a polycrystalline silicon ingot manufactured by a casting method by doping boron so as to exhibit a p-type conductivity, and then processing silicon carbide abrasive grains and stainless steel wires. What was produced by slicing with a multi-wire saw of loose abrasive system using

  Next, the damaged layer was removed by wet etching with a sodium hydroxide aqueous solution having a temperature of about 80 ° C. and a concentration of 25% by etching about 40 μm on both sides of the polycrystalline silicon substrate (P2).

  Further, a texture structure was formed on the first surface 9a side of the polycrystalline silicon substrate by using the RIE (Reactive Ion Etching) method (P3). Here, in the RIE apparatus, while flowing chlorine, oxygen, and sulfur hexafluoride at a flow ratio of 1: 5: 5, the reaction pressure was set to 7 Pa, and plasma was generated by applying RF power to perform etching for a predetermined time. . Thereby, a fine texture structure was formed on the first surface 9 a side of the semiconductor substrate 1.

  Next, phosphorus atoms were diffused by a gas thermal diffusion method using phosphorus oxychloride as a source on the polycrystalline silicon substrate to form an n-type second semiconductor layer 3 having a sheet resistance of about 90Ω / □ (P4). The second semiconductor layer 3 formed on the second surface 9b side was removed with a hydrofluoric acid solution, and then the phosphorous glass remaining on the second semiconductor layer 3 was removed with a hydrofluoric acid solution.

Then, an antireflection film 4 made of a silicon nitride film was formed by plasma CVD using silane (SiH ₄ ) and ammonia (NH ₃ ) as raw material gases (P5). The silicon nitride film had a refractive index of about 2.0 and a film thickness of about 700 mm.

  Here, as shown in the condition column of Table 1 below, the sample No. In other sample No. 1 with respect to the solar cell element using the semiconductor substrate in which the nitric acid cleaning process was not performed at all. The characteristic comparison with 2-6 was performed. Sample No. In sample No. 2, sample no. 3, the sample No. 4, the sample No. In sample No. 5, sample no. In No. 6, nitric acid was washed between the P2 step and the P3 step and between the P3 step and the P4 step (however, after using nitric acid, washing with pure water). The conditions of nitric acid in the nitric acid cleaning were a temperature of 20 ° C., a nitric acid concentration of 35 mass%, and a treatment time of 20 seconds. The nitric acid used was a commercially available EL grade.

  Then, a silver paste, which is a conductive paste, is applied to the linear pattern of the first electrode 5 as shown in FIG. 1 on the first surface 9a side, and an aluminum paste, which is a conductive paste, is applied to the second surface 9b side. It was applied to. Thereafter, the patterns of the conductive paste were baked to form the first electrode 5, the second collector electrode 6b, and the BSF region 7 as shown in FIG. The first electrode 5 penetrated the antireflection film 4 by firing (fire-through), and was in electrical contact with the semiconductor substrate 1. Further, the second output extraction electrode 6a was formed by applying a silver paste on the second surface 9b side to the pattern of the second output extraction electrode as shown in FIG.

  In this way, sample no. 1 to 6 solar cell elements were produced.

Sample No. About each of 1-6, the solar cell element output characteristic (photoelectric conversion efficiency) was measured and evaluated. In addition, the measurement of these characteristics was performed on the conditions of irradiation of AM (Air Mass) 1.5 and 100 mW / cm < ² > based on JISC8913. The results are shown in Table 1. For comparison of characteristics, sample No. The value normalized with the photoelectric conversion efficiency of 1 as 1.000 was described as the photoelectric conversion efficiency.

Sample No. Sample No. 1-6 6 has the highest photoelectric conversion efficiency. 3,
Sample No. 4, Sample No. 2, Sample No. 5, Sample No. The order was 1.

  From these results, when a solar cell element is manufactured using a semiconductor substrate that has been subjected to a nitric acid cleaning process in at least one of the P2 process and the P3 process and between the P3 process and the P4 process, the photoelectric conversion efficiency is improved. Was confirmed.

Table 2 shows the results of metal impurity analysis after the damage layer removal step (P2) depending on whether or not nitric acid was washed. The analysis was performed by the ICP-MS method, and a comparison was made between when the nitric acid cleaning was performed after the damaged layer removal step (Sample No. 3) and when it was not (Sample No. 1). Analysis sample No. Is a region 1 μm from the surface layer of the semiconductor substrate 1 and includes a surface natural oxide film. Note that “AE + N” in the table represents “A × 10 ^{+ N} ”.

  From Table 2, it is considered that the concentration of Fe is reduced by one digit and the concentration of Cu is reduced by two digits by nitric acid cleaning, and the solar cell characteristics are improved by removing metal impurities on the surface of the semiconductor substrate 1 by nitric acid cleaning. .

  From the above results, it is considered that a part of the metal impurities in the damaged layer is reattached to the surface of the semiconductor substrate 1 in the damaged layer removing step (P2) after removing the metal impurities by nitric acid cleaning.

  Further, when nitric acid cleaning is performed after the damage layer removing step (P2), the metal impurities derived from the damaged layer attached to the semiconductor substrate surface during the P2 step and the alkali metal derived from the etching solution in the damaged layer removing step are effectively removed. be able to. Then, the oxide film having a uniform film quality and film thickness is formed on the surface of the semiconductor substrate 10 by the nitric acid cleaning with fewer defects than the natural oxide film, and is listed in the subsequent texture structure portion forming step (P3). The uniformity of the uneven shape is improved.

  Even if the nitric acid cleaning is performed after the texture structure forming step (P3), the shape of the texture structure hardly changes before and after the nitric acid cleaning step, so that the characteristics of the solar cell element do not deteriorate. Further, by performing the nitric acid cleaning step immediately before the thermal diffusion step processed at a high temperature, diffusion of metal impurities into the semiconductor element 10 during the second conductivity type layer forming step (P4) is effectively reduced. be able to. Further, by forming an oxide film having a uniform film quality and thickness on the surface of the semiconductor substrate 1 by the nitric acid cleaning process, a more uniform phosphorous glass is formed than in the prior art in the subsequent second conductivity type layer forming process (P4). It is considered that the uniformity of the dopant concentration and thickness of the second semiconductor layer is improved.

  Further, when nitric acid cleaning is performed after the second conductivity type layer forming step (P4), the cleaning is performed after the metal impurities on the surface of the semiconductor substrate 1 are diffused into the solar cell element 10 during thermal diffusion. It is considered that the effect of removing impurities is insufficient.

If the nitric acid cleaning process increases, the manufacturing cost and the tact increase. Therefore, if nitric acid cleaning is performed in a plurality of processes, the semiconductor substrate preparation process (P1) and the texture structure forming process (P It is desirable to implement in the 1st period between P3) and the 2nd period of a texture structure formation process (P3) and a 2nd conductivity type layer formation process (P4).

  Next, in order to compare the processing conditions and other chemicals, Table 3 shows the damage layer removal step (P2), that is, between the damage layer removal step (P2) and the texture structure formation step (P3). The nitric acid was washed under the conditions of temperature, concentration (mass%) and treatment time of the nitric acid shown. In the same manner as in 1-6, the solar cell element sample No. The result of producing 7-26 and measuring and comparing the photoelectric conversion efficiency will be described. In addition, the notation “N” in Table 3 and Table 4 described later indicates that the effect is small or ineffective, “G” indicates that the effect is effective, and “VG” indicates that the effect is remarkable. Respectively.

As shown in Table 3, Sample No. As shown in FIG. 7, when the temperature of nitric acid was 20 ° C., the photoelectric conversion efficiency was 1.002 when the concentration was 7%, and the effect was small.

  Sample No. As in 12, 13, 16, 17, 25, and 26, when the chemical temperature is 35 ° C. or more and 50 ° C. or less and the nitric acid concentration is 14% by mass or more and 35% by mass or less, stable nitric acid cleaning treatment is possible and solar cell It was confirmed that variations in the reflectance of the element surface within the element surface and between elements were small. And said sample No. In 12, 13, 16, 17, 25, and 26, the photoelectric conversion efficiency was 1.012 or more, so that it has a stable and sufficient cleaning effect and a texture shape with a low reflectance is in the substrate surface. It was found that it can be formed uniformly and reliably.

  Next, various types of cleaning liquids were used to change the sample No. In the same manner as in 1-6, the solar cell element sample No. 27-34 will be described and the photoelectric conversion efficiency measured and compared will be described with reference to Table 4.

  Sample No. In No. 27, after the nitric acid treatment in the nitric acid cleaning step, the oxide film formed during the nitric acid cleaning was removed by treating the substrate with hydrofluoric acid having a concentration of 15% by mass.

  As a result, as shown in Table 4, by washing with nitric acid after washing with nitric acid (washing with pure water after using hydrofluoric acid), metal impurities in the oxide film are further removed by etching, and the solar cell element It seems that the metal impurity concentration in the medium has been improved.

  Further, a mixed liquid of hydrofluoric acid and sulfuric acid (sample No. 28-30), a mixed liquid of hydrofluoric acid and hydrogen peroxide (sample No. 31-32), and sulfuric acid (sample No. 33-34). As a result of washing using each of them, no particular improvement effect was observed in the mixed solution of hydrofluoric acid and sulfuric acid (sample No. 28-30). The mixed solution of hydrofluoric acid and hydrogen peroxide (sample No. 31-32) and sulfuric acid (sample No. 33-34) have a small improvement effect compared with nitric acid cleaning (sample No. 27). As a result.

  In the damage layer removing step (P2) and the texture structure forming step (P3), hydrofluoric acid (50 mass%) and nitric acid (70 mass%) are used instead of damage layer removal using an aqueous sodium hydroxide solution and texture formation by the RIE method. ) And water were mixed at a ratio of 1: 4: 2 in a volume ratio of a sample that was subjected to removal of the damaged layer and texture formation at the same time. Comparison was made between those subjected to nitric acid cleaning under the conditions of a temperature of 20 ° C., a nitric acid concentration of 35 mass%, and a treatment time of 20 seconds before the layer formation step (P4), and the photoelectric conversion efficiency was improved to 1.004 times. Thus, it was found that even when texture formation is performed using a mixed acid containing nitric acid, the characteristics are further improved by appropriately performing the nitric acid cleaning step.

1: Semiconductor substrate 2: 1st semiconductor layer 3: 2nd semiconductor layer 4: Antireflection film 5: 1st electrode 5a: 1st output extraction electrode 5b: 1st current collection electrode 5c: Auxiliary electrode 6: 2nd electrode 6a : Second output extraction electrode 6b: second collector electrode 7: BSF region 9a: first surface 9b: second surface 10: solar cell element

Claims (9)

A semiconductor substrate preparation step of preparing a first conductivity type semiconductor substrate;
A texture structure forming step of forming a texture structure on the surface of the semiconductor substrate;
A method for manufacturing a solar cell element, comprising: a second conductivity type layer forming step of forming a second conductivity type layer having a conductivity type opposite to the first conductivity type on the surface of the semiconductor substrate on which the texture structure is formed. There,
A solar cell element having a nitric acid cleaning step of cleaning the surface of the semiconductor substrate with nitric acid before the texture structure forming step and at least one of the texture structure forming step and the second conductivity type layer forming step Manufacturing method. The semiconductor substrate preparation step is a step of preparing a first conductivity type semiconductor substrate having a damaged layer produced by cutting on the surface,
Between the semiconductor substrate preparation step and the texture structure formation step, further comprising a damage layer removal step of removing the damage layer of the semiconductor substrate,
2. The sun according to claim 1, wherein the nitric acid cleaning step is provided at least between the damage layer removing step and the texture structure forming step and between the texture structure forming step and the second conductivity type layer forming step. A battery element manufacturing method.   The method for producing a solar cell element according to claim 2, wherein, in the damage layer removing step, the damage layer is removed by etching using an alkaline aqueous solution.   The method for manufacturing a solar cell element according to claim 2 or 3, wherein nitric acid having a nitric acid concentration of 14% by mass to 35% by mass and a temperature of 35 ° C to 50 ° C is used in the nitric acid cleaning step.   The method for manufacturing a solar cell element according to claim 1, comprising the nitric acid cleaning step between the texture structure forming step and the second conductivity type layer forming step.   The method for manufacturing a solar cell element according to claim 1, wherein a silicon substrate is used as the semiconductor substrate.   The method for manufacturing a solar cell element according to any one of claims 1 to 6, further comprising a hydrofluoric acid cleaning step of cleaning the surface of the semiconductor substrate with hydrofluoric acid following the nitric acid cleaning step.   The method for manufacturing a solar cell element according to claim 1, wherein the texture structure is formed by reactive ion etching in the texture structure forming step.   The method for manufacturing a solar cell element according to any one of claims 1 to 8, wherein, in the second conductive type layer forming step, the second conductive type layer is formed by a thermal diffusion method.

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