JP2005136081A - Method for manufacturing solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a solar cell wherein reduction of reflection factor and improvement of curvilinear factor can be simultaneously attained by a simple method. <P>SOLUTION: In the method for manufacturing a solar cell, the following processes are installed. (1) After a first conduction type polycrystalline semiconductor substrate 1 is etched with alkali solution, treatment is performed with acid solution, and an uneven surface is formed at least on a light receiving surface of the substrate 1. (2) Continuously, a pn junction is formed on the substrate 1 by forming a second conduction type dopant layer 5 on the light receiving surface. (3) After that, electrodes 13, 15 are formed on a surface and a back of the light receiving surface, respectively. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a solar cell, and more particularly to a method for manufacturing a solar cell including a step of forming an uneven surface for reducing reflection on the surface of a polycrystalline semiconductor substrate.

  In a solar cell, in order to improve the conversion efficiency, a technique is generally used in which a large number of irregularities are formed on the surface of the substrate and light reflected by the substrate surface is reincident to reduce loss due to reflection.

  In the case of manufacturing a solar cell using a single crystal silicon substrate having a (100) plane crystal orientation, a method of forming an uneven surface using etching with an alkali solution such as a sodium hydroxide solution is known. This utilizes the fact that the etching rate with the alkaline solution is different between the (100) plane and the (111) plane.

In addition, a method of performing isotropic etching on a polycrystalline silicon substrate using an acid solution containing hydrofluoric acid and nitric acid is known (see, for example, Non-Patent Document 1).
Hiroshi Inada, 1 other person, "Isotropic texture etching of polycrystalline Si surface with acid solution", "10th High Efficiency Solar Cell and Photovoltaic System Workshop Gifu (2002) Proceedings", The Institute of Electrical Engineers, etc. , November 2002, p74-77

  When etching using an alkaline solution is performed on a polycrystalline substrate containing crystal grains of various crystal orientations, irregularities are formed on the crystal grains with the (100) plane exposed on the surface. Since no unevenness is sufficiently formed, the effect of reducing the reflectance cannot be sufficiently obtained.

  Etching using an acid solution can reduce the reflectivity, but does not show much improvement in the fill factor.

  This invention is made | formed in view of such a situation, and provides the manufacturing method of a solar cell which can achieve the reduction | decrease of reflectance and improvement of a fill factor by a simple method collectively. .

  In the method for producing a solar cell of the present invention, (1) after etching the first conductive type polycrystalline semiconductor substrate with an alkaline solution, an uneven surface is formed on at least the light receiving surface of the substrate by treating with an acid solution, (2) Next, a pn junction is formed on the substrate by forming a second conductivity type impurity layer on the light receiving surface, and (3) after that, an electrode is formed on the light receiving surface and the back surface.

  The inventor can achieve this in the case of using either one of both the reflectance and the fill factor by causing the alkaline solution and the acid solution to act on the polycrystalline semiconductor substrate in this order. The present inventors have found that it is possible to produce a solar cell having excellent characteristics that has not been achieved.

  According to the manufacturing method of the present invention, it is possible to manufacture a solar cell having a lower reflectance on the surface than the conventional method using etching with an alkaline solution.

  According to the production method of the present invention, it is possible to produce a solar cell having a larger fill factor than the conventional method using etching with an acid solution.

  As described above, according to the production method of the present invention, both the reflectance and the fill factor could not be achieved when either one of the alkaline solution and the acid solution was used. A solar cell having both characteristics can be manufactured. Therefore, according to the manufacturing method of the present invention, a solar cell with high photoelectric conversion efficiency can be manufactured.

  In the manufacturing method of the present invention, since etching using an alkaline solution and an acid solution, that is, wet etching is used, it is possible to easily form unevenness compared to a method using dry etching or a method of mechanically forming unevenness. it can.

  In the method for producing a solar cell of the present invention, (1) after etching the first conductive type polycrystalline semiconductor substrate with an alkaline solution, an uneven surface is formed on at least the light receiving surface of the substrate by treating with an acid solution, (2) Next, a pn junction is formed on the substrate by forming a second conductivity type impurity layer on the light receiving surface, and (3) after that, an electrode is formed on the light receiving surface and the back surface.

  First, the step (1), that is, the step of forming an uneven surface on at least the light receiving surface of the substrate by etching the first conductive type polycrystalline semiconductor substrate with an alkaline solution and then treating with an acid solution will be described. To do.

  As the polycrystalline semiconductor substrate, those generally used for manufacturing solar cells can be used. For example, a polycrystalline silicon substrate can be used. The polycrystalline silicon substrate can be manufactured by slicing a silicon block prepared by cooling and solidifying molten silicon to a thickness of, for example, about 100 to 500 μm, preferably about 280 to 300 μm.

The first conductive type polycrystalline semiconductor substrate refers to a p-type or n-type polycrystalline semiconductor substrate. The carrier concentration can be, for example, about 10 ¹⁴ to 10 ¹⁸ cm ⁻³ .

  As the alkaline solution, one capable of etching the substrate is used. Specifically, sodium hydroxide, potassium hydroxide, calcium hydroxide, or the like can be used. The concentration can be about 10 to 70 wt%, preferably about 20 to 50 wt%, and more preferably about 30 to 50 wt%. The liquid temperature can be about 20-100 degreeC, Preferably, it is about 70-100 degreeC, More preferably, it can be about 90-100 degreeC. This is because the etching rate is high at about 70 to 100 ° C., and the etching rate is higher at about 90 to 100 ° C.

  An organic solvent such as about 0.1 to 20% isopropyl alcohol may be further added to the alkaline solution. Thereby, the size of the unevenness can be controlled.

  Etching with an alkaline solution can be performed, for example, by immersing the substrate in an alkaline solution. Moreover, you may carry out by spraying an alkaline solution, rotating the said board | substrate.

  Etching is preferably performed to such an extent that slice damage that occurs on the surface of the substrate during slicing can be removed. For example, the average etching depth can be 7 to 21 μm, preferably 10 to 14 μm. Here, the average etching depth means the etching weight per unit area divided by the density of the substrate. The etching time can be about 1 to 60 minutes, preferably about 4 to 10 minutes.

  As the acid solution, an aqueous solution containing hydrofluoric acid and nitric acid is preferably used. The acid solution is, for example, about 20 to 80 wt%, preferably about 40 to 60 wt% hydrofluoric acid aqueous solution, about 30 to 90 wt%, preferably about 50 to 70 wt% nitric acid aqueous solution, and water. It can be prepared by mixing in proportions. As for the mixing ratio, for example, the ratio of hydrofluoric acid: nitric acid: water can be about 1:20:14 to 1:20:27, preferably 1:20:21 in volume ratio.

  The acid solution may contain an acetic acid aqueous solution of about 50 to 100 wt%, preferably about 90 to 100 wt%, instead of water. The mixing ratio is, for example, volume ratio, and the ratio of acetic acid: hydrofluoric acid: nitric acid is about 14: 1: 20 to 21: 1: 20, preferably 21: 1: 20.

  The treatment with an acid solution means, for example, immersing the substrate in an acid solution, or spraying an acid solution while rotating the substrate.

  It is only necessary that irregularities are formed on at least the light receiving surface of the substrate, and irregularities may be formed on the light receiving surface and the light receiving back surface.

  Next, the step (2), that is, the step of forming a pn junction on the substrate by forming a second conductivity type impurity layer on the light receiving surface will be described.

  The second conductivity type impurity layer refers to an impurity layer having a conductivity type different from that of the substrate. That is, when the substrate is p-type, the impurity layer is n-type, and when the substrate is n-type, the impurity layer is p-type. The impurity layer can be formed by doping the substrate with a second conductivity type impurity.

  The doping of the n-type impurity can be performed, for example, by applying PSG (Phosphosilicate glasses) or the like by a method such as spin flow and then heating at 850 to 900 ° C. for about 10 to 20 minutes.

Further, instead of PSG, another solid layer source or dopant liquid containing a group V element such as arsenic (As) or phosphorus (P) may be used. Further, doping may be performed by injecting a group V element into the substrate by a method such as ion implantation and diffusing at 900 ° C. for about 5 to 20 minutes.

  The doping of the p-type impurity is performed, for example, by applying BSG (Boron silicate glasses) or the like by a method such as spin flow and then heating at 800 to 950 ° C. for about 10 to 30 minutes. Alternatively, a group III element such as boron (B) can be implanted into the substrate by a method such as ion implantation and diffused at 800 to 950 ° C. for about 5 to 20 minutes.

  For example, a semiconductor layer containing a second conductivity type impurity may be newly formed on the light receiving surface of the substrate. The semiconductor layer can be formed by a method such as CVD, for example.

  A pn junction is formed on the substrate by forming the second conductivity type impurity layer on the light receiving surface of the substrate.

  The method for manufacturing a solar cell of the present invention may further include a step of forming an antireflection film on the light receiving surface of the substrate. Thereby, a solar cell with a still lower reflectance can be manufactured. As the antireflection film, for example, an insulating film such as a silicon oxide film or a silicon nitride film can be used, and the film thickness can be set to a film thickness that can reduce the reflectance. The antireflection film can be formed by a CVD method, a vapor deposition method, or the like.

  Moreover, the manufacturing method of the solar cell of this invention may be equipped with the process of forming a high concentration 1st conductivity type impurity layer in the surface opposite to the surface in which the 2nd conductivity type impurity layer was formed. Accordingly, the solar cell has a BSF structure, and the potential barrier formed by the high-concentration first conductivity type impurity layer can reduce the recombination loss of the generated carriers, thereby improving the performance of the solar cell. be able to.

  The high-concentration first conductivity type impurity layer is formed, for example, by printing aluminum and drying it, then performing heat treatment at a temperature of about 600 ° C. to 800 ° C., and diffusing the aluminum by heat treatment to form an alloy. be able to. Preferably, the second conductivity type impurity layer is formed on the light receiving surface, and the high concentration first conductivity type impurity layer is formed on the light receiving back surface.

  Next, the step (3), that is, the step of forming electrodes on the light receiving surface and the back surface will be described.

  The electrode can be formed by, for example, forming a silver paste by printing and drying, followed by heat treatment at about 600 ° C. to 700 ° C. In addition, a metal such as Au, Pt, Al, Ni, Ti or an alloy thereof is formed by vapor deposition or sputtering before forming the antireflection film or after forming the opening. May be.

  When an antireflection film is formed on the light receiving surface, an electrode can be formed by printing a silver paste on the antireflection film and then performing a heat treatment. In this case, the silver paste fires through the antireflection film and is electrically connected to the substrate by heat treatment. Alternatively, an opening may be formed in the antireflection film, and an electrode may be formed in the opening.

  From another point of view, the present invention has an uneven surface comprising a step of forming an uneven surface on the surface of a polycrystalline semiconductor substrate by etching the polycrystalline semiconductor substrate with an alkaline solution and then treating with an acid solution. A method for manufacturing a polycrystalline semiconductor substrate is provided.

  By using a polycrystalline semiconductor substrate having a concavo-convex surface manufactured by the manufacturing method of the present invention, when either one of an alkaline solution and an acid solution is used in both reflectance and fill factor A solar cell having excellent characteristics that could not be achieved can be manufactured.

  FIGS. 1-6 shows the manufacturing method of the photovoltaic cell in Example 1 of this invention.

  First, the p-type polycrystalline silicon substrate 1 as the first conductive type polycrystalline semiconductor substrate shown in FIG. 1 is immersed in a sodium hydroxide (NaOH) aqueous solution having a liquid temperature of 100 ° C. and a concentration of 49 wt% for 4 to 10 minutes. As a result, the surface of the p-type polycrystalline silicon substrate 1 was etched.

  Next, it was immersed for 6 minutes in the acid solution containing water, nitric acid, and hydrofluoric acid. The acid solution is a mixture of 49 wt% hydrofluoric acid aqueous solution, 60 wt% nitric acid aqueous solution, and water so that the ratio of hydrofluoric acid: nitric acid: water is, for example, 1:20:21. It was prepared by. Thereby, the uneven surface 3 is formed on the surface of the p-type polycrystalline silicon substrate 1 (FIG. 2).

Next, as a solid layer source including an n-type diffusion source, PSG (Phospho Silicate Glasses) is applied to the surface of the p-type polycrystalline silicon substrate 1 by spin flow, and heat treatment is performed at 900 ° C. for 10 minutes in a diffusion furnace. As a result, an n ⁺ layer 5 as a second conductivity type impurity layer was formed on the light receiving surface of the p-type polycrystalline silicon substrate 1 to form a pn junction.

  Further, the solid layer source was removed, and a silicon nitride film 7 having a thickness of 80 nm was deposited as an antireflection film (FIG. 3). In this case, the film thickness should be selected so that surface reflection is minimized.

Next, the aluminum 9 is printed on the back surface of the light-receiving surface, dried, and then subjected to heat treatment at 700 ° C. for 5 minutes. The aluminum 9 is diffused and alloyed by the heat treatment, so A p ⁺ layer 11 having a higher concentration was formed (FIG. 4).

  Next, the back surface silver electrode 13 is printed on the light receiving back surface of the p-type polycrystalline silicon substrate 1, and the surface silver electrode 15 is printed on the antireflection film 7 on the light receiving surface side, dried, and then heat treated at 600 ° C. Thus, electrodes 13 and 15 were formed on the light receiving surface and the back surface (FIG. 5).

  Next, the front surface silver electrode 15 and the back surface silver electrode 13 were covered with the solder 17, and the manufacture of the solar cell by the manufacturing method of the present invention was completed (FIG. 6).

Measurement Results FIG. 7 is an electron micrograph (magnification of 10,000 times) of the surface of the polycrystalline silicon substrate 1 after being etched for 4 minutes with an alkaline solution and then immersed in an acid solution for 6 minutes. According to this photograph, it can be seen that a good uneven surface 3 is formed on the surface of the polycrystalline silicon substrate 1 by the method of the present invention.

  FIG. 8 is a graph showing the relationship between the average etching depth and the fill factor. The average etching depth was changed by changing the etching time in the alkaline solution.

  According to this graph, it can be seen that the larger the average etching depth is, the larger the curve factor is. When the average etching depth is 10 μm or more, the curve factor becomes a good value. In order to set the average etching depth to 10 μm, the etching time in the alkaline solution may be about 4 minutes.

  FIG. 9 is a graph showing the relationship between the average etching depth and the carrier lifetime. The average etching depth was changed by changing the etching time in the alkaline solution. For the measurement of the carrier lifetime, the μ-PCD method was used and a chemical passivation treatment using an iodine ethanol solution was performed.

  According to this graph, it can be seen that the larger the average etching depth is, the shorter the carrier lifetime is, and when the average etching depth is 14 μm or less, the carrier lifetime is a good value. In order to set the average etching depth to 14 μm, the etching time in the alkaline solution may be about 4 minutes.

  FIG. 10 is a diagram showing the relationship between the presence or absence of acid treatment or the composition of the acid solution and the reflectance. The measurement was performed on the substrate etched with an alkaline solution so that the average etching depth was 10 μm.

  21 is no acid treatment, and 23, 25, and 27 correspond to the case where the ratio of hydrofluoric acid: nitric acid: water is 1: 20: 9, 1:20:14, 1:20:21 by volume ratio, respectively. To do. According to this figure, when the ratio of hydrofluoric acid: nitric acid: water is 1:20:21 in volume ratio, the effect of reducing the reflectance is the greatest. Compared to the case without acid treatment, a reflectance reduction of about 5% at maximum is observed in the wavelength range of 400 nm to 600 nm.

  FIG. 11 is a graph showing the relationship between voltage and current under AM 1.5 (standard sunlight spectrum). It measured about the solar cell using the board | substrate etched with the alkaline solution so that an average etching depth might be 10 micrometers.

The short circuit current density was 31.7 mA / cm ² and the fill factor was 0.763. According to the method using only the acid solution shown in Non-Patent Document 1, the curve factor was about 0.5, and it can be seen that according to the method of the present invention, a solar cell having a large curve factor can be manufactured. .

  As an acid solution, a 99 wt% acetic acid aqueous solution, a 49 wt% hydrofluoric acid aqueous solution, and a 60 wt% nitric acid aqueous solution were mixed so that the volume ratio of acetic acid: hydrofluoric acid: nitric acid was 21: 1: 20. A solar cell was manufactured in the same manner as in Example 1 except that the prepared material was used.

Measurement Result FIG. 12 is a diagram showing the relationship between the presence or absence of acid treatment and the reflectance. The measurement was performed on the substrate etched with an alkaline solution so that the average etching depth was 10 μm.

  31 is no acid treatment, and 33 is a volume ratio of acetic acid: hydrofluoric acid: nitric acid corresponding to 21: 1: 20. According to this figure, compared with the case without acid treatment, when acid treatment is performed, the reflectance is reduced by about 5% at maximum in the wavelength range of 400 nm to 600 nm.

  FIG. 13 is a graph showing the relationship between voltage and current under AM 1.5 (standard sunlight spectrum). It measured about the solar cell using the board | substrate etched with the alkaline solution so that an average etching depth might be 10 micrometers.

The short-circuit current density was 31.8 mA / cm ² and the fill factor was 0.754. As in the case of Example 1, it can be seen that the present invention can produce a solar cell with a large fill factor.

It is a side view of the p-type polycrystalline semiconductor substrate which shows the manufacturing method of the solar cell of Example 1 of this invention. It is the FIG. 1 equivalent view which shows the different state of the manufacturing method of the solar cell of Example 1 of this invention. It is FIG. 1 equivalent view which shows the further different state of the manufacturing method of the solar cell of Example 1 of this invention. It is FIG. 1 equivalent view which shows the further different state of the manufacturing method of the solar cell of Example 1 of this invention. It is FIG. 1 equivalent view which shows the further different state of the manufacturing method of the solar cell of Example 1 of this invention. It is FIG. 1 equivalent view which shows the further different state of the manufacturing method of the solar cell of Example 1 of this invention. 1 is an electron micrograph (magnification 10,000 times) of the surface of a polycrystalline silicon substrate after treatment with an acid solution in Example 1. 6 is a graph showing the relationship between the average etching depth and the fill factor for Example 1. 4 is a graph showing the relationship between average etching depth and carrier lifetime for Example 1. It is a figure which shows the relationship between the presence or absence of an acid treatment, or the composition of an acid solution, and a reflectance about Example 1. FIG. 3 is a graph showing the relationship between voltage and current of a solar cell manufactured by the manufacturing method of Example 1. It is a figure which shows the relationship between the presence or absence of an acid treatment, and a reflectance about Example 2. FIG. 6 is a graph showing a relationship between voltage and current of a solar cell manufactured by the manufacturing method of Example 2.

Explanation of symbols

1 p-type polycrystalline semiconductor substrate 3 uneven surface 5 n ⁺ layer 7 antireflection film 9 aluminum 11 p ⁺ layer 13 light receiving back surface silver electrode 15 light receiving surface silver electrode 17 solder 21 no acid treatment 23 hydrofluoric acid: nitric acid: water = 1: 20: 9
25 hydrofluoric acid: nitric acid: water = 1: 20: 14
27 Hydrofluoric acid: Nitric acid: Water = 1: 20: 21
31 No acid treatment 33 Acetic acid: hydrofluoric acid: nitric acid = 21: 1: 20

Claims (7)

(1) The first conductive type polycrystalline semiconductor substrate is etched with an alkaline solution and then treated with an acid solution to form an uneven surface on at least the light receiving surface of the substrate. (2) Next, a second surface is formed on the light receiving surface. A method of manufacturing a solar cell, comprising: forming a pn junction on the substrate by forming a conductive impurity layer; and (3) forming an electrode on the light receiving surface and the back surface after an appropriate time. The method for manufacturing a solar cell according to claim 1, wherein the step of etching the substrate with an alkaline solution is a step of etching the substrate with an alkaline solution such that an average etching depth thereof is 10 μm or more. The method for manufacturing a solar cell according to claim 1, wherein the step of etching the substrate with an alkaline solution is a step of etching the substrate with an alkaline solution so that an average etching depth thereof is 14 μm or less.   The step of etching the substrate with an alkaline solution is a step of etching the substrate with an alkaline solution having a concentration of 20 to 50 wt% at 70 ° C to 100 ° C. Battery manufacturing method.   The method for manufacturing a solar cell according to any one of claims 1 to 4, wherein the acid solution is an aqueous solution containing hydrofluoric acid and nitric acid.   The method for manufacturing a solar cell according to claim 5, wherein a ratio of hydrofluoric acid: nitric acid: water is 1:20:21 by volume. A method for manufacturing a polycrystalline semiconductor substrate having a concavo-convex surface, comprising a step of forming a concavo-convex surface on a surface of a polycrystalline semiconductor substrate by etching the polycrystalline semiconductor substrate with an alkaline solution and then treating the polycrystalline semiconductor substrate with an acid solution.

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