JP2017107964A - Photoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a double-sided light-receiving photoelectric conversion element, where light is incident from the front and rear surfaces of a semiconductor substrate, and capable of improving the incident efficiency of light to the rear surface while ensuring certain reliability.SOLUTION: A photoelectric conversion element 1 includes a semiconductor substrate 11 having a first conductivity type, a semiconductor layer 12 formed on the front surface of the semiconductor substrate 11 on the Z axis positive direction side, and having a second conductivity type opposite to the first conductivity type, a first electrode 16 formed in contact with the semiconductor layer 12 on the surface of the semiconductor substrate 11, and a second electrode 17 formed on the rear surface of the semiconductor substrate 11 on the Z axis negative direction side. The rear surface of the semiconductor substrate 11 has an irregular shape 110 of arcuate cross section.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a photoelectric conversion element.

  Patent Document 1 below discloses a pn junction type double-sided light receiving solar cell in which a texture structure is formed on the light receiving surface and the back surface of a silicon substrate. In this double-sided light receiving solar cell, the texture structure is uniformly formed on the entire surface of the silicon substrate, and the texture structure on the light receiving surface and the back surface has substantially the same shape.

Japanese Patent No. 5379767

  By the way, the double-sided light-receiving solar cell module comprising the double-sided light-receiving solar cells is installed at an angle that takes into account the balance of the output on both sides when receiving light on both the light-receiving surface and the back surface. It is not installed in consideration of the maximum output. The double-sided light receiving solar cell module is covered with glass so that sunlight can be received from both sides. Therefore, for example, even if it is installed in consideration of the maximum output of the light receiving surface, it is difficult to reflect the light transmitted through the back surface and return it to the cell, and the effect of the reflected light cannot be expected. Further, by flattening the uneven structure on the back surface side of the double-sided light receiving solar cell, the conversion efficiency can be improved to some extent by the effect of reflected light on the back surface. However, as the back surface is flattened, the adhesive strength between the silicon substrate and the electrode on the back surface decreases, and the reliability of the double-sided light receiving solar cell decreases.

  The present invention relates to a double-sided light receiving photoelectric conversion element in which light is incident from both the front and back surfaces of a semiconductor substrate, and a photoelectric conversion element capable of improving light incident efficiency on the back surface while ensuring a certain level of reliability. The purpose is to provide.

  A photoelectric conversion element in one embodiment of the present invention is formed on a semiconductor substrate having a first conductivity type and a surface on which light is incident on the semiconductor substrate, and a second conductivity opposite to the first conductivity type. A semiconductor layer having a mold; a first electrode formed in contact with the semiconductor layer on the front surface of the semiconductor substrate; and a rear surface disposed on the opposite side of the surface of the semiconductor substrate to which light is incident. A second electrode formed, wherein the front surface and the back surface of the semiconductor substrate have a concavo-convex shape, and the cross-section of the concavo-convex shape on the back surface has an arc shape.

  According to the embodiment of the present invention, in a double-sided light receiving photoelectric conversion element in which light is incident from both sides of a semiconductor substrate, it is possible to improve light incident efficiency on the back surface while ensuring a certain level of reliability. .

FIG. 1A is a plan view schematically showing a light receiving surface of the photoelectric conversion element according to the first embodiment. FIG. 1B is a plan view schematically showing the back surface of the photoelectric conversion element according to the first embodiment. FIG. 2 is a cross-sectional view schematically showing a cross-sectional structure of the photoelectric conversion element shown in FIG. FIG. 3 is a flow showing the manufacturing process of the photoelectric conversion element according to the first embodiment. 4A is a cross-sectional view showing a substrate used in the texture structure forming step shown in FIG. FIG. 4B is a cross-sectional view illustrating a state in which texture structures are formed on both surfaces of the substrate illustrated in FIG. 4A. FIG. 4C is a cross-sectional view showing the pn junction formation step shown in FIG. FIG. 4D is a cross-sectional view showing the back etching step shown in FIG. 4E is a cross-sectional view showing the Ion Implantation process and the silicon oxide film forming process shown in FIG. 4F is a cross-sectional view showing the antireflection film forming step shown in FIG. FIG. 4G is a cross-sectional view showing the electrode formation step shown in FIG. FIG. 5A is a table showing results of measuring characteristics of the photoelectric conversion element according to Embodiment 1 and Comparative Examples 1 and 2. FIG. 5B is a photographic image of a plane and a cross section of the back surface of a semiconductor substrate having a different amount of cutting on the back surface. FIG. 6A is a graph showing the relationship between the back surface area ratio and the short-circuit current density shown in FIG. 5A. FIG. 6B is a graph showing the relationship between the back surface area ratio and the conversion efficiency shown in FIG. 5A. FIG. 7 is a table showing the results of measuring the characteristics of the photoelectric conversion elements according to the first and second embodiments. FIG. 8 is a table showing the results of measuring the characteristics of the photoelectric conversion elements according to the first and third embodiments. FIG. 9 is a table showing the results of measuring the characteristics of the photoelectric conversion elements according to the first embodiment, the second embodiment, and the fourth embodiment. FIG. 10 is a cross-sectional view schematically showing a cross-sectional structure of the photoelectric conversion element according to the fifth embodiment.

  A photoelectric conversion element in one embodiment of the present invention is formed on a semiconductor substrate having a first conductivity type and a surface on which light is incident on the semiconductor substrate, and a second conductivity opposite to the first conductivity type. A semiconductor layer having a mold; a first electrode formed in contact with the semiconductor layer on the front surface of the semiconductor substrate; and a rear surface disposed on the opposite side of the surface of the semiconductor substrate to which light is incident. A second electrode formed, wherein the front surface and the back surface of the semiconductor substrate have a concavo-convex shape, and a cross-section of the concavo-convex shape on the back surface has an arc shape (first configuration).

  According to the first configuration, in the photoelectric conversion element having the uneven shape on the front surface and the back surface of the semiconductor substrate, the uneven surface of the back surface of the semiconductor substrate has an arc shape. Therefore, compared with the case where the uneven shape on the back surface of the semiconductor substrate is not an arc shape, the reflected light can be efficiently taken into the back surface. Moreover, compared with the case where uneven | corrugated shape is not formed in the back surface, a 2nd electrode is hard to peel, and fixed reliability can be ensured.

  In the first configuration, at least the cross section of the concave portion of the concave-convex shape on the back surface may have an arc shape (second configuration).

  According to the 2nd structure, since the recessed part in a back surface has an arc shape, compared with the case where this structure is not provided, reflected light can be taken in into a back surface efficiently.

  A photoelectric conversion element in one embodiment of the present invention is formed on a semiconductor substrate having a first conductivity type and a surface on which light is incident on the semiconductor substrate, and a second conductivity opposite to the first conductivity type. A semiconductor layer having a mold; a first electrode formed in contact with the semiconductor layer on the front surface of the semiconductor substrate; and a back surface of the semiconductor substrate opposite to the front surface on which light is incident. A surface of the semiconductor substrate having a concavo-convex shape, and a surface area of the concavo-convex shape on the back surface is smaller than a surface area of the concavo-convex shape on the front surface (first 3 configuration).

  According to the third configuration, in the photoelectric conversion element having the concavo-convex shape on the front surface and the back surface of the semiconductor substrate, the concavo-convex shape surface area on the back surface is smaller than the concavo-convex shape surface area on the front surface. It is gentler than the shape. Therefore, compared with the case where the uneven surface area of the back surface is equivalent to that of the front surface, the reflected light can be efficiently captured on the back surface. In addition, since the concavo-convex shape is formed on the back surface, the second electrode is less likely to peel compared to the case where the concavo-convex shape is not formed on the back surface, and certain reliability can be ensured.

  In any one of the first to third configurations, when the surface area of one surface of the semiconductor substrate having the concavo-convex shape is an area X, and the plane area of the semiconductor substrate is an area S, the area S on the back surface is The surface area ratio Y (= X / S) indicating the ratio of the area X may be greater than 1.11 and less than 1.90 (fourth configuration).

  According to the fourth configuration, compared with the case where the surface area ratio of the concavo-convex shape on the back surface is 1.11 or less, or 1.90 or more, while maintaining a certain adhesive strength of the second electrode on the back surface, incidence on the back surface Efficiency can be improved.

  In any of the first to fourth configurations, the semiconductor substrate further includes an antireflection film formed so as to be in contact with the back surface of the semiconductor substrate, and the second electrode penetrates the antireflection film and is formed on the back surface of the semiconductor substrate. The antireflection film may have a thickness of 100 nm or more and less than 120 nm (fifth configuration).

  According to the fifth configuration, compared with the case where the film thickness of the antireflection film is less than 100 nm or 120 nm or more, the second electrode is fired through the antireflection film so that the second electrode is appropriately placed on the back surface of the semiconductor substrate. It is possible to connect, and the conversion efficiency can be improved.

  In any of the first to fifth configurations, the occupation ratio of the second electrode on the back surface of the semiconductor substrate may be less than 9.0% (sixth configuration).

  According to the sixth configuration, the amount of received light on the back surface of the semiconductor substrate can be improved and current can be collected efficiently compared to the case where the occupation ratio of the second electrode is 9.0% or more.

  In any one of the first to sixth configurations, the semiconductor substrate may further include an impurity diffusion layer having the first conductivity type formed in contact with the back surface of the semiconductor substrate and connected to the second electrode. (Seventh configuration).

  According to the sixth configuration, the carrier collection efficiency can be improved by the impurity diffusion layer.

  In any one of the first to seventh configurations, a passivation film formed so as to be in contact with at least one of the front surface and the back surface of the semiconductor substrate may be further provided (eighth configuration).

  According to the eighth configuration, annihilation due to carrier recombination on at least one of the front surface and the back surface of the semiconductor substrate can be reduced.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated. In addition, in order to make the explanation easy to understand, in the drawings referred to below, the configuration is shown in a simplified or schematic manner, or some components are omitted. Further, the dimensional ratio between the constituent members shown in each drawing does not necessarily indicate an actual dimensional ratio.

  In the present invention, the semiconductor layer formed on the surface of the semiconductor substrate includes both the semiconductor layer formed in the semiconductor substrate by diffusing impurities from the surface of the semiconductor substrate and the semiconductor layer formed on the surface of the semiconductor substrate. It is a waste.

[Embodiment 1]
1A is a diagram schematically showing a plane on the light receiving surface (front surface) side of the photoelectric conversion element of the present invention, and FIG. 1B is a back surface side of the photoelectric conversion element of the present invention. It is the figure which represented the plane of this. FIG. 2 is a cross-sectional view schematically showing a cut surface in the Y-axis direction of the photoelectric conversion element shown in FIGS. Hereinafter, the configuration of the photoelectric conversion element will be described with reference to FIGS.

  As shown in FIG. 2, the photoelectric conversion element 1 includes a semiconductor substrate 11, a p-type semiconductor layer 12, a back surface electric field layer 13, antireflection films 14 and 15, and electrodes 16 and 17.

  The semiconductor substrate 11 is made of, for example, an n-type single crystal silicon substrate. The semiconductor substrate 11 has a texture having a concavo-convex shape 100 on the entire light-receiving surface arranged on the Z-axis positive direction side, and has a concavo-convex shape 110 on the entire back surface opposite to the light-receiving surface (Z-axis negative direction side). A texture is formed. As shown in FIG. 2, the uneven shape 100 on the light receiving surface has a pyramid shape (non-arc shape), whereas the uneven shape 110 on the back surface has an arc shape.

  The p-type semiconductor layer 12 is formed in contact with the light receiving surface of the semiconductor substrate 11. The p-type semiconductor layer 12 contains boron and has a thickness of about 0.6 μm.

  The back surface electric field layer 13 is formed in contact with the back surface of the semiconductor substrate 11. The back surface electric field layer 13 contains phosphorus and has a film thickness of about 0.5 μm.

  The antireflection film 14 is formed in contact with the p-type semiconductor layer 12. The antireflection film 14 is formed by laminating a silicon oxide film and a silicon nitride film, and has a thickness of about 85 nm. The silicon oxide film is formed in contact with the p-type semiconductor layer 12, and the silicon nitride film is formed in contact with the silicon oxide film.

  The antireflection film 15 is formed in contact with the back surface electric field layer 13. The antireflection film 15 is formed by laminating a silicon oxide film and a silicon nitride film, and has a thickness of about 100 nm. The silicon oxide film is formed in contact with the back surface electric field layer 13, and the silicon nitride film is formed in contact with the silicon oxide film.

  The electrode 16 includes a bus bar electrode 161 and a finger electrode 162 as shown in FIG. The electrode 16 includes, for example, a metal such as silver or aluminum, and is formed in contact with the p-type semiconductor layer 12 through the antireflection film 14 as shown in FIG.

  The electrode 17 includes a bus bar electrode 171 and a finger electrode 172 as shown in FIG. The electrode 17 includes, for example, a metal such as silver, and is formed in contact with the back surface electric field layer 13 through the antireflection film 15 as shown in FIG.

(Production method)
Next, the manufacturing method of the photoelectric conversion element 1 is demonstrated. FIG. 3 is a flow showing a manufacturing process of the photoelectric conversion element 1, and FIGS. 4A to 4G are cross-sectional views schematically showing the processes shown in FIG. Hereinafter, each process is demonstrated concretely using FIG. 3 and FIG.

In the texture structure forming process in step S1 of FIG. 3, first, a 15.6 cm × 15.6 cm (243.36 cm ² ) semiconductor substrate 10 is prepared (see FIG. 4A). The planar shape and area of the semiconductor substrate 10 are not limited to this. Then, the light receiving surface and the back surface of the semiconductor substrate 10 are wet etched using an alkaline etching solution. Specifically, an alkaline solution obtained by adding an additive to a 2.5% potassium hydroxide aqueous solution is used as an etching solution, and the alkaline solution is kept at 80 ° C., and the semiconductor substrate 10 is immersed for about 20 minutes. Thereafter, the semiconductor substrate 10 is neutralized with an acid solution and sufficiently washed with pure water and warm water. Thereby, the semiconductor substrate 10a in which the concavo-convex shape 100 having the pyramid structure is formed on the entire substrate is formed (see FIG. 4B).

Subsequently, in the pn junction formation step of Step S2, the semiconductor substrate 10a is set in a quartz tube furnace set to 950 ° C. or higher using boron tribromide (BBr ₃ ) as a diffusion material. In the quartz tube furnace, the semiconductor substrates 10a may be set one by one, or two may be stacked and set. When two sheets are stacked and set, the overlapped surface is the back side. Then, the semiconductor substrate 10a is heat-treated for about 30 minutes in a quartz tube furnace, and boron is vapor-phase diffused over the entire surface of the semiconductor substrate 10a. The method for forming the p-type semiconductor layer 12 is not limited to the vapor phase diffusion method, and a coating diffusion method in which a coating solution containing boron (B) is applied to the light receiving surface of the semiconductor substrate 10a and heat-treated may be used. Alternatively, boron ions may be implanted into the surface of the semiconductor substrate 10a using an ion implantation method. Thereby, the p-type semiconductor layer 12 is formed on the entire surface of the semiconductor substrate 10a (see FIG. 4C). The sheet resistance at this time is about 60Ω / □.

  In addition, since a glass layer (BSG: Boron Silicate Glass) is formed on the p-type semiconductor layer 12 when the p-type semiconductor layer 12 is formed, the BSG layer is converted to hydrofluoric acid after the p-type semiconductor layer 12 is formed. Remove with aqueous solution.

  Next, in the back etching process of step S3, the p-type semiconductor layer 12 formed on the back surface of the semiconductor substrate 10a is removed. Specifically, the semiconductor substrate 10a is protected from the etching solution by covering the light receiving surface of the semiconductor substrate 10a with pure water or the like, and the back surface of the semiconductor substrate 10a is immersed in a mixed solution of hydrofluoric acid solution and nitric acid solution. Transport. In this example, the mixing ratio of the mixed solution is, for example, hydrofluoric acid: nitric acid: pure water = 1: 7: 3. Then, after etching with the mixed solution, the porous silicon layer (stain layer) formed on the semiconductor substrate 10a is removed with a thin alkaline solution, neutralized again with an acid solution (hydrofluoric acid aqueous solution, etc.), washed with water and dried. Let As a result, the p-type semiconductor layer 12 formed on the back surface of the semiconductor substrate 10a is removed, the uneven shape 100 on the back surface of the semiconductor substrate 10a is scraped, and the uneven shape 110 is formed on the back surface of the semiconductor substrate 11 ( (See FIG. 4D). That is, the concavo-convex shape 110 formed on the back surface of the semiconductor substrate 11 has the apex portion of the pyramidal structure of the concavo-convex shape 100 cut away, has a kurtosis smaller than the concavo-convex shape 100, and has a cross-section of an arc shape. In this embodiment, the amount of shaving of the concavo-convex shape 100 on the back surface is adjusted by controlling the transport speed of the semiconductor substrate 11, but the amount of shaving is adjusted by adjusting the mixing ratio of hydrofluoric acid and nitric acid. May be.

  Accordingly, since the uneven shape 110 formed on the back surface is smoother than the uneven shape 100 formed on the light receiving surface, the surface area of the back surface on which the uneven shape 110 is formed is the light receiving surface on which the uneven shape 100 is formed. It is smaller than the surface area of the surface. Further, when the plane area of one surface of the semiconductor substrate 11 is S, the surface area when the irregular shape is formed is X, and the ratio of the surface area X to the plane area S is the surface area ratio Y (= X / S). The surface area ratio Y of the back surface of the semiconductor substrate 11 is smaller than the surface area ratio Y of the light receiving surface.

Next, in the Ion Implantation process of step S4, phosphorus (P) is implanted into the back surface side of the semiconductor substrate 11 by ion implantation. In this example, ion implantation was performed using the conditions where the acceleration energy was 10 keV and the dose was 3.2 × 10 ¹⁵ cm ⁻² . This condition varies depending on the annealing temperature at the time of forming the silicon oxide film in the next step, but a condition that allows the back surface electric field layer 13 to have an optimum sheet resistance value may be arbitrarily selected.

  Then, after implanting phosphorus (P) into the back surface side of the semiconductor substrate 11, the semiconductor substrate 11 is SC1 cleaned using a mixed solution of ammonia hydrogen water and hydrogen peroxide solution, and then HF using dilute hydrofluoric acid. Wash.

Subsequently, the cleaned semiconductor substrate 11 is thermally oxidized in the silicon oxide film forming step in step S5. In this example, thermal oxidation treatment was performed at about 850 ° C. for 30 minutes. As a result, phosphorus (P) implanted on the back surface of the semiconductor substrate 11 is activated to form a back surface field layer (BSF layer) 13 and silicon oxide films 14a on the light receiving surface and the back surface side of the semiconductor substrate 11, respectively. 15a are formed (see FIG. 4E). At this time, the sheet resistance of the back surface electric field layer 13 is 40Ω / □. As a method of forming the back surface electric field layer 13, a vapor phase diffusion method using phosphoryl chloride (POCl ₃ ) or a coating diffusion method using a PSG solution may be used in addition to the above method. When these methods are used, it is necessary to protect the light receiving surface of the semiconductor substrate 11.

  Next, in the antireflection film forming process in step S6, a silicon nitride film 14b is formed on the silicon oxide film 14a formed on the light receiving surface of the semiconductor substrate 11 by using a silane gas and an ammonia gas by a plasma CVD method. . Thereafter, a silicon nitride film 15 b is formed on the silicon oxide film 15 a formed on the back surface of the semiconductor substrate 11. Thus, the antireflection film 14 in which the silicon oxide film 14a and the silicon nitride film 14b are laminated is formed on the light receiving surface of the semiconductor substrate 11, and the silicon oxide film 15a and the silicon nitride film 15b are formed on the back surface of the semiconductor substrate 11. Is formed (see FIG. 4F).

  In this example, the total film thickness of the antireflection films 14 and 15 including the silicon oxide film and the silicon nitride film on the light receiving surface and the back surface is about 85 nm and about 100 nm, respectively. In this example, the silicon nitride films having the same composition ratio are formed on the light receiving surface and the back surface, but silicon nitride films having different composition ratios may be formed.

  Subsequently, in the electrode forming step of step S7, a conductive paste containing silver is printed on the antireflection films 14 and 15 by, for example, a screen printing method. In this example, a conductive paste containing silver powder, aluminum powder, glass frit, resin, organic solvent and the like is printed on the antireflection film 14 on the light receiving surface side of the semiconductor substrate 11. On the other hand, on the antireflection film 15 on the back surface side of the semiconductor substrate 11, a conductive paste containing silver powder, glass frit, resin, organic solvent, etc. and not containing aluminum is printed.

  And after printing an electrically conductive paste on both surfaces of the semiconductor substrate 11, it is made to dry and baked at 700 degreeC or more. The conductive paste printed on the light receiving surface of the semiconductor substrate 11 by firing passes through the antireflection film 14 and is electrically connected to the p-type semiconductor layer 12. Further, the conductive paste printed on the back surface of the semiconductor substrate 11 fires through the antireflection film 15 and is electrically connected to the back surface electric field layer 13. Thereby, the electrode 16 is formed on the light receiving surface of the semiconductor substrate 11, and the electrode 17 is formed on the back surface (see FIG. 4G). In addition, the method of forming the electrodes 16 and 17 is not restricted to the said method. For example, the antireflection films 14 and 15 may be patterned using an etching paste or a laser to provide openings, and a conductive paste may be printed thereon to fire the electrodes 16 and 17.

  In this example, as shown in FIG. 1A, the electrode 16 includes three bus bar electrodes 161 and 100 finger electrodes 162. The width of the bus bar electrode 161 is about 1.5 mm, the width of the finger electrode 162 is about 0.05 mm, and the height of the bus bar electrode 161 and the finger electrode 162 is about 0.02 mm. On the other hand, the electrode 17 includes three bus bar electrodes 171 and 70 finger electrodes 172 as shown in FIG. The bus bar electrode 171 has a width of about 2.5 mm, the finger electrode 172 has a width of about 0.1 mm, and the bus bar electrode 171 and the finger electrode 172 have a height of about 0.025 mm.

  Here, by changing the transport speed of the semiconductor substrate 11, the photoelectric conversion elements having different surface area ratios Y on the back surface with 0.05 g, 0.15 g, 0.25 g, and 0.50 g on the back surface texture structure shaving amounts, respectively. FIG. 5A shows a schematic cross section of the back surface of the photoelectric conversion element manufactured and measured for each surface area ratio, and results of measuring the characteristics thereof. Moreover, in FIG. 5A, the comparative example of the photoelectric conversion element which concerns on this Embodiment is shown together with the result of having measured the schematic cross section of the back surface of the photoelectric conversion element of Comparative Examples 1 and 2, and its characteristic. In addition, each characteristic value except the amount of shaving, back surface area ratio, and adhesive strength in FIG. 5A is a value based on each characteristic value of Comparative Example 1 (1.000).

  Comparative Example 1 is an example of a photoelectric conversion element having a larger surface area ratio Y on the back surface than that of the photoelectric conversion element 1 of Embodiment 1, and Comparative Example 2 is a surface area of the back surface of the photoelectric conversion element 1 of Embodiment 1. This is an example of a photoelectric conversion element having a small ratio Y. The photoelectric conversion elements of Comparative Examples 1 and 2 are formed as follows.

(Comparative Example 1)
First, the concavo-convex shape 100 is formed on both surfaces of the semiconductor substrate by performing the texture structure forming step of step S1 on both surfaces of the semiconductor substrate similar to the first embodiment, as in the first embodiment. Thereafter, a silicon oxide film is formed on one surface serving as the back surface by using, for example, an atmospheric pressure CVD method. Then, in the same manner as in the first embodiment, the pn junction forming process in step S2 is performed to form a p-type semiconductor layer on the light receiving surface of the semiconductor substrate. Thereafter, the semiconductor substrate is immersed in an aqueous hydrofluoric acid solution, and the silicon oxide film formed on the back surface and the BSG layer formed on the p-type semiconductor layer are removed.

  Next, using the same conditions as in the first embodiment, the Ion Implantation process of Step S4 is performed, phosphorus (P) is implanted into the back side of the semiconductor substrate, SC1 cleaning and HF cleaning are performed, and then Step S5 is performed. A silicon oxide film forming step is performed to form a silicon oxide film on the surface of the semiconductor substrate, and a back surface field layer (BSF layer) is formed on the back surface of the semiconductor substrate. In Comparative Example 1, in order to set the sheet resistance of the silicon oxide film to 40 Ω / □, which is the same as that of the first embodiment, the condition of the thermal oxidation treatment in the silicon oxide film forming process is about 850 ° C. for 20 minutes. The thermal oxidation treatment time was shorter than that in the first embodiment.

  Next, in the same manner as in the first embodiment, the antireflection film forming step of step S6 is performed, and a silicon nitride film is formed on the silicon oxide film on the light receiving surface and the back surface of the semiconductor substrate. An antireflection film composed of a silicon oxide film and a silicon nitride film is formed on the back surface. The film thickness of the antireflection film on the light receiving surface and the back surface of Comparative Example 1 is about 85 nm. Finally, as in the first embodiment, the electrode forming step of step S7 is performed to form electrodes composed of bus bar electrodes and finger electrodes on the light receiving surface side and the back surface side of the semiconductor substrate, respectively.

  Since the photoelectric conversion element of Comparative Example 1 does not perform the back etching process of Step S <b> 3, the uneven shape 100 is formed on the light receiving surface and the back surface of the photoelectric conversion element of Comparative Example 1.

(Comparative Example 2)
Next, a method for forming the photoelectric conversion element of Comparative Example 2 will be described. First, steps S1 and S2 are performed on the same semiconductor substrate as in the first embodiment, as in the first embodiment. Thereafter, in the back etching process of Step S3 in Embodiment 1, etching was performed using an alkaline solution instead of a mixed solution of a hydrofluoric acid solution and a nitric acid solution as an etching solution. The alkaline solution is, for example, an aqueous sodium hydroxide solution. In this example, a 20% aqueous sodium hydroxide solution was maintained at about 70 ° C., and the light receiving surface of the semiconductor substrate was immersed in an acid and alkali resistant film, and then neutralized and washed with an acid solution. At this time, the amount of cutting of the back surface of the semiconductor substrate was about 0.65 g. Then, after the back etching process, each of the steps S4 to S7 was performed as in the first embodiment.

  Each back surface area ratio shown in FIG. 5A was measured using a laser microscope (wavelength: 408 nm) and a measuring instrument of LEXT OLS3000 manufactured by OLYMPUS. In addition, each electrical characteristic was evaluated using a solar simulator, and the effect of reflected light on the back side of the photoelectric conversion element is more clearly confirmed, so that the gold-plated stage does not affect the electrical characteristics. Measurement was performed by attaching a highly reflective film or the like in the range. In addition, the adhesive strength is, for example, connecting an interconnector solder-coated on a copper foil having a width of 1.5 mm and a thickness of 0.2 mm on the bus bar electrode on the back surface of the photoelectric conversion element, and the interconnector is rotated 180 ° in the reverse direction. Then, the photoelectric conversion element and the interconnector were forcibly separated from each other, and the force at that time was measured as the adhesive strength.

  In FIG. 5A, the amount of cutting 0.05 g of the photoelectric conversion element 1 of Embodiment 1 is the minimum amount of cutting for removing the p-type semiconductor layer 12, and the amount of cutting is 0.15 g, 0.25 g,. 50 g is a shaving amount for the purpose of further flattening the back surface of the semiconductor substrate 11.

  Here, for reference, FIG. 5B shows a photographic image of a plane and a cross section of the back surface of a semiconductor substrate having a back surface shaving amount of 0.057 g and 0.510 g and a semiconductor substrate whose back surface is not shaved. 5A and 5B are a plan view and a cross section of the back surface of the semiconductor substrate whose back surface is not cut. (C) and (d) in FIG. 5B are a plan view and a cross section of the back surface of the semiconductor substrate having a back surface shaving amount of 0.057 g. (E) and (f) in FIG. 5B are a plan view and a cross section of the back surface of the semiconductor substrate having a shaving amount of 0.510 g on the back surface. As shown in (b) of FIG. 5B, when the back surface is not shaved, the uneven shape of the pyramid structure is formed on the back surface, but when the back surface is shaved, as shown in (d) of FIG. 5B. In addition, the vertices of the pyramid structure are shaved to reduce the kurtosis, and the apex portion has a rounded uneven shape. When the shaving amount is 0.510 g, as shown in (f) of FIG. 5B, the kurtosis becomes smaller than that of (d) of FIG. Accordingly, the greater the amount of shaving on the back surface, the smaller the kurtosis of the concavo-convex shape, the concavo-convex shape on the back surface approaches flattening, and each cross section of the concavo-convex concave portion and convex portion on the back surface has an arc shape. That is, as shown in (d) and (f) of FIG. 5B, at least the concave section of the concave-convex shape on the back surface of the semiconductor substrate has a plurality of arc shapes. The plurality of arcs are arranged in the in-plane direction on the back surface of the semiconductor substrate. Further, the plurality of arc shapes are arranged to be continuous.

  As shown in FIG. 5A, the back surface area ratio Y of the semiconductor substrate 11 when the shaving amounts are 0.05 g, 0.15 g, 0.25 g, and 0.50 g are 1.84, 1.77, 1.53, 1 .21. Moreover, since the photoelectric conversion element of Comparative Example 1 did not perform the back etching process, the surface area ratio Y between the light receiving surface and the back surface was 1.90. The photoelectric conversion element of Comparative Example 2 had a back surface scraping amount of 0.65 g and a back surface area ratio Y of 1.11.

  6A is a graph showing the relationship between the back surface area ratio Y and the short-circuit current density Jsc shown in FIG. 5A, and FIG. 6B is the back surface area ratio Y and the conversion efficiency η shown in FIG. 5A. It is a graph which shows the relationship. As shown in FIGS. 5A and 6A, the short-circuit current density Jsc gradually decreases when the back surface area ratio Y is between 1.11 and 1.84, and the back surface area ratio Y is 1.90 (comparison). In Example 1), it drops sharply. Further, as shown in FIG. 6 (b), the conversion efficiency η transitions at substantially the same value when the back surface area ratio Y is between 1.21 and 1.84, but the surface area ratio Y is 1.11 ( When compared with Comparative Example 2) and 1.90 (Comparative Example 1), the value decreases sharply. That is, when the back surface area ratio Y is greater than 1.11 and less than 1.90, the reflected light on the back surface is efficiently captured and the conversion efficiency η is improved.

  On the other hand, as shown to FIG. 5A, it turns out that adhesive strength improves, so that the back surface area ratio Y becomes large. Specifically, the adhesive strength of Embodiment 1 is smaller than that of Comparative Example 1, but a certain adhesive strength of a predetermined reference value of 1 N / mm or more is ensured. On the other hand, since the back surface of Comparative Example 2 is flattened more than that of Embodiment 1, the contact area between the back-side electrode and the back surface electric field layer is further reduced, which is about ½ of Comparative Example 1. The adhesive strength has decreased.

  Therefore, in order to improve the conversion efficiency while ensuring a certain adhesive strength, the back surface area ratio Y is preferably greater than 1.11 and less than 1.90.

[Embodiment 2]
In the present embodiment, as in the first embodiment, each process of steps S1 to S7 is performed on the semiconductor substrate 10, and two photoelectric conversion elements 1a having different total film thickness from the antireflection film 15 of the first embodiment, 1b was formed. The photoelectric conversion elements 1a and 1b are subjected to the back etching step (S3) so that the amount of cutting of the back surface of the semiconductor substrate 11 is 0.05 g. In the photoelectric conversion elements 1a and 1b, the total film thickness of the antireflection film 15 on the back surface is about 80 nm and about 120 nm, respectively, by adjusting the transport speed of the semiconductor substrate 11 in the antireflection film forming process in step S6. It has become.

  FIG. 7 shows the measurement results of the characteristics of the photoelectric conversion element 1 and the photoelectric conversion elements 1a and 1b with a shaving amount of 0.05 g. In FIG. 7, each characteristic value excluding the film thickness and adhesive strength of the back surface antireflection film is a value when the photoelectric conversion element 1 is used as a reference (1.000). As shown in FIG. 7, when the total film thickness of the antireflection film 15 is 80 nm, the short-circuit current density Jsc and the open circuit voltage Voc are reduced. However, when the total film thickness of the antireflection film 15 is 100 nm or more, the short-circuit current density Jsc is reduced. And the open circuit voltage Voc is improved. However, if the total thickness of the back surface antireflection film 15 is increased to 120 nm, the fire-through of the antireflection film 15 becomes insufficient when the back surface electrode 17 is formed, and the electrode 17 and the back surface electric field layer 13 are appropriately formed. Not connected, the conversion efficiency η is reduced. Therefore, considering the fire-through of the antireflection film 15, the total thickness of the antireflection film 15 on the back surface of the photoelectric conversion element is preferably 100 nm or more and less than 120 nm.

[Embodiment 3]
In the present embodiment, the same steps S1 to S7 as in the first embodiment are performed, and the amount of shaving is 0.05 g and the total film thickness of the antireflection film 15 on the back surface side is 100 nm as in the photoelectric conversion element 1. Five photoelectric conversion elements 1c to 1g having different configurations of the electrode 17 on the back surface side from the photoelectric conversion element 1 were produced.

  The back surface electrode 17 in the photoelectric conversion element 1 according to the first embodiment includes three bus bar electrodes 171 and 70 finger electrodes 172. The bus bar electrode 171 has a width of about 2.5 mm and the finger electrode 172 has a width. Is about 0.1 mm. In the photoelectric conversion elements 1c to 1g, the number of bus bar electrodes 171 and the width of the bus bar electrode 171 are the same as those of the photoelectric conversion element 1, but at least one of the number of finger electrodes 172 and the width of the finger electrode 172 is the same as that of the photoelectric conversion element 1. Different. Hereinafter, differences from the photoelectric conversion element 1 will be specifically described.

  In the photoelectric conversion element 1c, the number of finger electrodes 172 is 70, and the width of the finger electrodes 172 is 0.07 mm.

  In the photoelectric conversion element 1d, the number of finger electrodes 172 is 60, and the width of the finger electrodes 172 is 0.07 mm.

  In the photoelectric conversion element 1e, the number of finger electrodes 172 is 100, and the width of the finger electrodes 172 is 0.07 mm.

  In the photoelectric conversion element 1f, the number of finger electrodes 172 is 120, and the width of the finger electrodes 172 is 0.07 mm.

  The number of finger electrodes 172 in the photoelectric conversion element 1g is 70, and the width of the finger electrodes 172 is 0.04 mm.

  FIG. 8 shows measurement results of electrical characteristics of the photoelectric conversion element 1 and the photoelectric conversion elements 1c to 1g in which the amount of shaving on the back surface is 0.05 g and the total film thickness of the antireflection film 15 on the back surface is 100 nm. In FIG. 8, the back electrode occupation ratio (%) is the ratio of the area of the electrode 17 on the back surface to the surface area S of the back surface when the uneven shape is not formed. In FIG. 8, each characteristic value excluding the finger electrode width, the number of finger electrodes, and the back electrode occupation ratio is a value based on each characteristic value of the photoelectric conversion element 1 (1.000).

  As shown in FIG. 8, in the case of the photoelectric conversion elements 1, 1e, and 1f in which the back electrode occupation ratio is 9.0% or more, the amount of received light on the back surface is reduced and the conversion efficiency η is 1.000 or less. . On the other hand, in the case of the photoelectric conversion elements 1c, 1d, and 1g in which the back electrode occupation ratio is less than 9.0%, the resistance of the finger electrode 172 increases and the fill factor FF decreases, but the conversion efficiency η improves. Therefore, as shown in FIGS. 8 and 9, in order to improve the conversion efficiency η, it is preferable to form the electrode 17 so that the back electrode occupation ratio is less than 9.0%.

[Embodiment 4]
In the present embodiment, steps S1 to S6 similar to those in the first embodiment are performed, and the photoelectric conversion element 1h using the material of the electrode 17 different from that in the first embodiment is formed in the electrode forming process in step S7. . In the first embodiment, a conductive paste containing glass frit is used in forming the electrode 17. However, in this embodiment, the type of glass frit that is easier to fire through than in the first embodiment is changed. In this case, a glass frit having a lower melting point than that of Embodiment 1 may be used as the glass frit. In the present embodiment, etching is performed so that the amount of shaving in the back etching step (S3) is 0.05 g, and reflection is performed so that the total thickness of the back surface in the antireflection film forming step (S6) is 120 nm. A prevention film was formed.

  In FIG. 9, the total thickness of the antireflection film 15 and the photoelectric conversion element 1 of Embodiment 1 in which the amount of shaving on the back surface is 0.05 g and the total thickness of the antireflection film 15 is 100 nm and the antireflection film 15 is 120 nm. The measurement result of each characteristic of the photoelectric conversion element 1b of Embodiment 2 and the photoelectric conversion element 1h of this Embodiment is shown. Each of the electrodes 17 on the back side of the photoelectric conversion elements 1, 1 b, and 1 h has three bus bar electrodes 171 having a width of about 2.5 mm and 70 finger electrodes having a width of about 0.07 mm. 172. In FIG. 9, each characteristic value excluding the back surface antireflection film is a value based on each characteristic value of the photoelectric conversion element 1 (1.000).

  As shown in FIG. 9, the photoelectric conversion element 1b in which the electrode 17 is formed using the same glass frit as in the first embodiment has a total film thickness of the antireflection film 15 that is greater than that of the photoelectric conversion element 1 in the first embodiment. Therefore, the fire-through of the antireflection film 15 when the electrode 17 is formed is not sufficient, and the conversion efficiency η is reduced. As in the photoelectric conversion element 1b, the photoelectric conversion element 1h has a total thickness of the antireflection film 15 of 120 nm, but the electrode 17 is formed using a glass frit that is easy to fire through. Therefore, the anti-reflective film 15 is sufficiently fire-through, can maintain a high fill factor FF, and can improve the conversion efficiency η. Therefore, when the total film thickness of the antireflection film 15 is 100 nm or more, it is preferable to use a glass frit that can easily fire through the antireflection film 15 as the material of the electrode 17.

[Embodiment 5]
In the first to fourth embodiments described above, the example in which the p-type semiconductor layer 12 is formed by diffusing boron tribromide (BBr ₃ ) on the light receiving surface of the semiconductor substrate 11 has been described. Alternatively, a p-type amorphous semiconductor layer containing hydrogen may be formed. The p-type amorphous semiconductor layer includes, for example, p-type amorphous silicon, p-type amorphous silicon germanium, p-type amorphous germanium, p-type amorphous silicon carbide, p-type amorphous silicon nitride, It consists of any of p-type amorphous silicon oxide, p-type amorphous silicon oxynitride, p-type amorphous silicon carbon oxide, and the like.

In this case, in the pn junction formation process in step S2, for example, by a plasma CVD method, H ₂ gas, SiH ₄ gas, and diborane (B ₂ H ₆ ) gas are used as reaction gases to form a p-type amorphous semiconductor. A layer may be formed on the light receiving surface of the semiconductor substrate 11.

  In addition, as shown in FIG. 10, a passivation film 21 a is formed on the light receiving surface of the semiconductor substrate 11, a p-type amorphous semiconductor layer 22 is formed on the passivation film 21 a, and the passivation is formed on the back surface of the semiconductor substrate 11. The film 21b may be formed, and the n-type amorphous semiconductor layer 23 may be formed on the passivation film 21b. By comprising in this way, the passivation property in a light-receiving surface and a back surface improves, and conversion efficiency can be improved more. The passivation films 21a and 21b are substantially intrinsic and amorphous semiconductor films containing hydrogen. The passivation films 21a and 21b are, for example, i-type amorphous silicon, i-type amorphous silicon germanium, i-type amorphous germanium, i-type amorphous silicon carbide, i-type amorphous silicon nitride, i-type It consists of either amorphous silicon oxide or i-type amorphous silicon carbon oxide. In this case, after the texture structure forming step of step S1, the passivation films 21a and 21b are formed on the light-receiving surface and the back surface, respectively, by using, for example, plasma CVD, using silane gas and hydrogen gas as reaction gases. Good.

[Modification]
The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description of the embodiments but by the scope of claims for patent, and is intended to include meanings equivalent to the scope of claims for patent and all modifications within the scope. The modification is shown below.

  Although the photoelectric conversion element in Embodiment 1-5 mentioned above demonstrated the example which uses an n-type single crystal silicon substrate as the semiconductor substrate 11, you may make it use a p-type single crystal silicon substrate. In this case, an n-type semiconductor layer may be formed on the light receiving surface side of the semiconductor substrate 11 and a back surface electric field layer 13 containing boron (B) may be formed on the back surface side.

  DESCRIPTION OF SYMBOLS 1,1a-1i ... Photoelectric conversion element 10,10a, 11 ... Semiconductor substrate, 12 ... P-type semiconductor layer, 13 ... Back surface electric field layer (impurity diffusion layer), 14, 15, ... Antireflection film, 16, 17 ... electrode, 161, 171 ... busbar electrode, 162, 172 ... finger electrode, 21a, 21b ... passivation film, 22 ... p-type amorphous semiconductor Layer, 23... N-type amorphous semiconductor layer, 100, 110...

Claims (8)

A semiconductor substrate having a first conductivity type;
A semiconductor layer formed on a surface of the semiconductor substrate on which light is incident and having a second conductivity type opposite to the first conductivity type;
A first electrode formed in contact with the semiconductor layer on the surface of the semiconductor substrate;
The semiconductor substrate, the second electrode disposed on the opposite side of the front surface and formed on the back surface on which light is incident, and
The front surface and the back surface of the semiconductor substrate have an uneven shape,
The concavo-convex cross section on the back surface has an arc shape.   The photoelectric conversion element according to claim 1, wherein at least the concave section of the concave-convex shape on the back surface has an arc shape. A semiconductor substrate having a first conductivity type;
A semiconductor layer formed on a surface of the semiconductor substrate on which light is incident and having a second conductivity type opposite to the first conductivity type;
A first electrode formed in contact with the semiconductor layer on the surface of the semiconductor substrate;
A second electrode formed on a back surface of the semiconductor substrate opposite to the front surface, on which light is incident;
The front surface and the back surface of the semiconductor substrate have an uneven shape,
The photoelectric conversion element, wherein the uneven surface area on the back surface is smaller than the uneven surface area on the surface.   The surface area indicating the ratio of the area X to the area S on the back surface, where area X is the surface area of the one surface having the concavo-convex shape of the semiconductor substrate and area S is the plane area of the one surface of the semiconductor substrate. The photoelectric conversion element according to claim 1, wherein the ratio Y (= X / S) is greater than 1.11 and less than 1.90. Further comprising an antireflection film formed so as to be in contact with the back surface of the semiconductor substrate;
The second electrode is provided on the back surface of the semiconductor substrate through the antireflection film,
5. The photoelectric conversion element according to claim 1, wherein a thickness of the antireflection film is 100 nm or more and less than 120 nm.   6. The photoelectric conversion element according to claim 1, wherein an occupation ratio of the second electrode on the back surface of the semiconductor substrate is less than 9.0%.   The photoelectric conversion according to claim 1, further comprising an impurity diffusion layer having the first conductivity type, which is formed in contact with the back surface of the semiconductor substrate and connected to the second electrode. element.   The photoelectric conversion element according to claim 1, further comprising a passivation film formed so as to be in contact with at least one of the front surface and the back surface of the semiconductor substrate.