JP6868957B2 - Photoelectric conversion element - Google Patents

Description

The present invention relates to a photoelectric conversion element.

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

Japanese Patent No. 5379767

By the way, when the double-sided light-receiving solar cell module composed of the double-sided light-receiving solar cell receives light on both the light-receiving surface and the back surface, it is installed at an angle considering the balance of the outputs of both sides. Not installed in consideration of maximum output. Further, both sides of the double-sided light receiving type solar cell module are covered with glass so that sunlight can be received from both sides. Therefore, for example, even if the light 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 of the light transmitted through the back surface cannot be expected. Further, by flattening the uneven structure on the back surface side of the double-sided light receiving type solar cell as much as possible, the conversion efficiency can be improved to some extent by the effect of the reflected light on the back surface. However, as the back surface is flattened, the adhesive strength between the silicon substrate and the electrodes on the back surface side decreases, and the reliability of the double-sided light-receiving solar cell decreases.

The present invention provides a double-sided light receiving type photoelectric conversion element that injects light from both the front surface and the back surface of a semiconductor substrate, and can improve the reflection efficiency of the back surface while ensuring a certain degree of reliability. With the goal.

The photoelectric conversion element according to one embodiment of the present invention is formed on a semiconductor substrate having a first conductive type and a surface of the semiconductor substrate on which light is incident, and is formed on a second conductive type opposite to the first conductive type. A semiconductor layer having a mold, a first electrode formed in contact with the semiconductor layer on the surface of the semiconductor substrate, and a back surface of the semiconductor substrate arranged on the opposite side of the surface and incident with light. The semiconductor substrate is provided with the formed second electrode, and the front surface and the back surface of the semiconductor substrate have an uneven shape, and the cross section of the uneven shape on the back surface has an arc shape.

According to the embodiment of the present invention, in a double-sided light receiving type photoelectric conversion element in which light is incident from both sides of a semiconductor substrate, it is possible to improve the reflection efficiency of the back surface while ensuring a certain degree 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 the cross-sectional structure of the photoelectric conversion element shown in FIG. FIG. 3 is a flow showing a manufacturing process of the photoelectric conversion element according to the first embodiment. FIG. 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 showing a state in which a texture structure is formed on both sides of the substrate shown in FIG. 4A. FIG. 4C is a cross-sectional view showing the pn junction forming step shown in FIG. FIG. 4D is a cross-sectional view showing the back etching process shown in FIG. FIG. 4E is a cross-sectional view showing the Ion Implantation step and the silicon oxide film forming step shown in FIG. 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 forming step shown in FIG. FIG. 5A is a table showing the results of measuring the characteristics of the photoelectric conversion element according to the first embodiment and Comparative Examples 1 and 2. FIG. 5B is a photographic image of the plane and the cross section of the back surface of the semiconductor substrate having different amounts of scraping 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 each photoelectric conversion element according to the first embodiment and the second embodiment. FIG. 8 is a table showing the results of measuring the characteristics of each photoelectric conversion element according to the first embodiment and the third embodiment. FIG. 9 is a table showing the results of measuring the characteristics of each photoelectric conversion element according to the first embodiment, the second embodiment, and the fourth embodiment. FIG. 10 is a cross-sectional view schematically showing the cross-sectional structure of the photoelectric conversion element according to the fifth embodiment.

The photoelectric conversion element according to one embodiment of the present invention is formed on a semiconductor substrate having a first conductive type and a surface of the semiconductor substrate on which light is incident, and is formed on a second conductive type opposite to the first conductive type. A semiconductor layer having a mold, a first electrode formed in contact with the semiconductor layer on the surface of the semiconductor substrate, and a back surface of the semiconductor substrate arranged on the opposite side of the surface and incident with light. The semiconductor substrate is provided with the formed second electrode, and 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 (first configuration).

According to the first configuration, in the photoelectric conversion element having an uneven shape on the front surface and the back surface of the semiconductor substrate, the uneven cross section of the back surface of the semiconductor substrate has an arc shape. Therefore, the reflected light from the back surface can be efficiently taken in as compared with the case where the uneven shape of the back surface of the semiconductor substrate is not arcuate. Further, the second electrode is less likely to be peeled off as compared with the case where the uneven shape is not formed on the back surface, and a certain degree of reliability can be ensured.

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

According to the second configuration, since the concave portion on the back surface has an arc shape, the reflected light can be efficiently taken into the back surface as compared with the case where this configuration is not provided.

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

According to the third configuration, in the photoelectric conversion element having the uneven shape on the front surface and the back surface of the semiconductor substrate, the surface area of the uneven shape on the back surface is smaller than the surface area of the uneven shape on the front surface, so that the uneven shape on the back surface is the uneven shape on the front surface. It is gentler than the shape. Therefore, the reflected light from the back surface can be efficiently taken in as compared with the case where the surface area of the uneven shape of the back surface is the same as that of the front surface. Further, since the concavo-convex shape is formed on the back surface, the second electrode is less likely to be peeled off as compared with the case where the concavo-convex shape is not formed on the back surface, and a certain degree of reliability can be ensured.

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

According to the fourth configuration, as compared with the case where the surface area ratio of the uneven shape on the back surface is 1.11 or less or 1.90 or more, the incident on the back surface is incident while ensuring a certain adhesive strength of the second electrode on the back surface. Efficiency can be improved.

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

According to the fifth configuration, as 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 fire-through through the antireflection film and the second electrode is appropriately placed on the back surface of the semiconductor substrate. It can be connected and the conversion efficiency can be improved.

In any of the first to fifth configurations, the occupancy rate 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 light received on the back surface of the semiconductor substrate can be improved and the current can be efficiently collected as compared with the case where the occupancy rate of the second electrode is 9.0% or more.

In any of the first to sixth configurations, an impurity diffusion layer having the first conductive type, which is formed in contact with the back surface of the semiconductor substrate and connected to the second electrode, may be further provided. (7th configuration).

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

In any 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, it is possible to reduce the disappearance due to carrier recombination on at least one surface of the front surface and the back surface of the semiconductor substrate.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the 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 constituent members are omitted. Further, the dimensional ratio between the constituent members shown in each figure does not necessarily indicate the 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]
FIG. 1 (a) 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. 1 (b) is a diagram showing the back surface side of the photoelectric conversion element of the present invention. It is a figure which represented the plane of. FIG. 2 is a cross-sectional view schematically showing a cut surface of the photoelectric conversion element shown in FIGS. 1A and 1B in the Y-axis direction. Hereinafter, the configuration of the photoelectric conversion element will be described with reference to FIGS. 1 and 2.

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 formed 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 on the side opposite to the light receiving surface (Z-axis negative direction side). The texture is formed. As shown in FIG. 2, the concave-convex shape 100 on the light receiving surface has a pyramid-shaped (non-arc-shaped) cross section, whereas the concave-convex shape 110 on the back surface has an arc-shaped cross section.

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 film 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 film 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 film 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.

As shown in FIG. 1A, the electrode 16 has a bus bar electrode 161 and a finger electrode 162. The electrode 16 contains, for example, a metal such as silver or aluminum, and as shown in FIG. 2, is formed so as to penetrate the antireflection film 14 and be in contact with the p-type semiconductor layer 12.

The electrode 17 has a bus bar electrode 171 and a finger electrode 172 as shown in FIG. 1 (b). The electrode 17 contains, for example, a metal such as silver, and is formed so as to penetrate the antireflection film 15 and be in contact with the back surface electric field layer 13 as shown in FIG.

(Production method)
Next, a method of manufacturing the photoelectric conversion element 1 will be described. 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 each process shown in FIG. Hereinafter, each step will be specifically described with reference to FIGS. 3 and 4A to 4G.

In the texture structure forming step of step S1 of FIG. 3, first, ^{a semiconductor substrate 10 having a size of 15.6 cm × 15.6 cm (243.36 cm 2} ) is prepared (see FIG. 4A). The flat surface 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 with an alkaline etching solution. Specifically, an alkaline solution obtained by adding an additive to a 2.5% potassium hydroxide aqueous solution is used as the etching solution, the alkaline solution is maintained at 80 ° C., and the semiconductor substrate 10 is immersed for about 20 minutes. Then, the semiconductor substrate 10 is neutralized with an acid solution and thoroughly washed with pure water and warm water. As a result, a semiconductor substrate 10a having a concavo-convex shape 100 having a pyramid structure is formed on the entire substrate (see FIG. 4B).

Subsequently, in the pn junction forming step of step S2, boron tribromide (BBr ₃ ) is used as a diffusion material, and the semiconductor substrate 10a is set in a quartz tube furnace set at 950 ° C. or higher. The semiconductor substrates 10a may be set one by one in the quartz tube furnace, or two semiconductor substrates 10a may be set in a stack. When two sheets are stacked and set, the overlapped surface is the back surface side. Then, the semiconductor substrate 10a is heat-treated in a quartz tube furnace for about 30 minutes to diffuse boron in the gas phase 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 described above, and a coating diffusion method in which a coating liquid containing boron (B) is applied to the light receiving surface of the semiconductor substrate 10a and heat-treated may be used. Then, boron ions may be implanted into the surface of the semiconductor substrate 10a by using the ion implantation method. As a result, 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 / □.

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 hydrofluoric acid after the formation of the p-type semiconductor layer 12. Remove with an aqueous solution.

Next, in the back etching step of step S3, the p-type semiconductor layer 12 formed on the back surface of the semiconductor substrate 10a is removed. Specifically, the light receiving surface of the semiconductor substrate 10a is protected from the etching solution by covering it with pure water or the like, and the back surface of the semiconductor substrate 10a is immersed in a mixed solution of a hydrofluoric acid solution and a nitric acid solution. To carry. 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 me. 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 removed, and the uneven shape 110 is formed on the back surface of the semiconductor substrate 11 ( See FIG. 4D). That is, the concave-convex shape 110 formed on the back surface of the semiconductor substrate 11 has the apex portion of the pyramid structure of the concave-convex shape 100 cut off, has a lower kurtosis than the concave-convex shape 100, and has an arc-shaped cross section. In the present embodiment, the amount of shaving of the uneven 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. You may.

Therefore, the concave-convex shape 110 formed on the back surface has smoother irregularities than the concave-convex shape 100 formed on the light receiving surface, so that the surface area of the back surface on which the concave-convex shape 110 is formed is the light receiving surface on which the concave-convex shape 100 is formed. It is smaller than the surface area of the surface. Further, when the flat area of one surface of the semiconductor substrate 11 is S, the surface area when the concave-convex shape is formed is X, and the ratio of the surface area X to the flat 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 step of step S4, phosphorus (P) is implanted into the back surface side of the semiconductor substrate 11 by the ion implantation method. In this example, ion implantation was performed under the conditions that the acceleration energy was 10 keV and the dose amount was 3.2 × 10 ¹⁵ cm- ^2. Although this condition differs depending on the annealing temperature at the time of forming the silicon oxide film in the next step, a condition may be arbitrarily selected so that the backside electric field layer 13 has an optimum sheet resistance value.

Then, after driving phosphorus (P) into the back surface side of the semiconductor substrate 11, the semiconductor substrate 11 is SC1 washed with a mixed solution of hydrogen peroxide water and hydrogen peroxide water, and then HF is washed with dilute hydrofluoric acid. Perform cleaning.

Subsequently, in the silicon oxide film forming step of step S5, the washed semiconductor substrate 11 is thermally oxidized. In this example, thermal oxidation treatment was performed at about 850 ° C. for 30 minutes. As a result, phosphorus (P) driven into the back surface of the semiconductor substrate 11 is activated to form the back surface electric field layer (BSF layer) 13, and the silicon oxide film 14a is formed on the light receiving surface and the back surface side of the semiconductor substrate 11, respectively. , 15a are formed (see FIG. 4E). The sheet resistance of the back surface electric field layer 13 at this time is 40 Ω / □. As a method for forming the back surface electric field layer 13, in addition to the above method, _{a vapor phase diffusion method using phosphoryl chloride (POCl 3} ) or a coating diffusion method using a PSG solution may be used. 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 step of step S6, the 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 silane gas and ammonia gas by the plasma CVD method. .. After that, the silicon nitride film 15b is formed on the silicon oxide film 15a formed on the back surface of the semiconductor substrate 11. As a result, 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. The antireflection film 15 is formed by laminating the above (see FIG. 4F).

In this example, the total film thicknesses of the antireflection films 14 and 15 in which the silicon oxide film and the silicon nitride film on the light receiving surface and the back surface are combined are about 85 nm and about 100 nm, respectively. In this example, the silicon nitride film having the same composition ratio is 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, an organic solvent, or 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, an organic solvent and the like and not containing aluminum is printed.

Then, the conductive paste is printed on both sides of the semiconductor substrate 11, dried, and fired at 700 ° C. or higher. 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. As a result, 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). The method of forming the electrodes 16 and 17 is not limited to the above method. For example, the antireflection films 14 and 15 may be patterned using an etching paste or a laser to provide an opening, and the conductive paste may be printed on the openings to fire the electrodes 16 and 17.

In this example, as shown in FIG. 1A, the electrode 16 is composed of three busbar 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, as shown in FIG. 1B, the electrode 17 is composed of three bus bar electrodes 171 and 70 finger electrodes 172. The width of the bus bar electrode 171 is about 2.5 mm, the width of the finger electrode 172 is about 0.1 mm, and the height of the bus bar electrode 171 and the finger electrode 172 is about 0.025 mm.

Here, the transport speed of the semiconductor substrate 11 is changed, and the amount of scraping of the texture structure on the back surface is 0.05 g, 0.15 g, 0.25 g, 0.50 g, and the surface area ratio Y on the back surface is different from each other. FIG. 5A shows a schematic cross section of the back surface of the photoelectric conversion element having each surface area ratio and the results of measuring its characteristics. Further, in FIG. 5A, as a comparative example of the photoelectric conversion element according to the present embodiment, a schematic cross section of the back surface of the photoelectric conversion element of Comparative Examples 1 and 2 and the result of measuring the characteristics thereof are also shown. The characteristic values excluding the scraping amount, the back surface area ratio, and the adhesive strength in FIG. 5A are values based on each characteristic value of Comparative Example 1 (1.000).

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

(Comparative Example 1)
First, the uneven shape 100 is formed on both sides of the semiconductor substrate by performing the texture structure forming step of step S1 on both sides of the semiconductor substrate as in the first embodiment as in the first embodiment. After that, a silicon oxide film is formed on one of the back surfaces by using, for example, a normal pressure CVD method. Then, as in the first embodiment, the pn junction forming step of step S2 is performed to form a p-type semiconductor layer on the light receiving surface of the semiconductor substrate. Then, the semiconductor substrate is immersed in the hydrofluoric acid aqueous solution to remove the silicon oxide film formed on the back surface and the BSG layer formed on the p-type semiconductor layer.

Next, using the same conditions as in the first embodiment, the Ion Implantation step of step S4 is performed, phosphorus (P) is implanted on the back surface side of the semiconductor substrate, SC1 cleaning and HF cleaning are performed, and then step S5. A silicon oxide film forming step is performed to form a silicon oxide film on the front surface of the semiconductor substrate and a back surface electric field layer (BSF layer) 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 the same sheet resistance of 40Ω / □ as in the first embodiment, the conditions of the thermal oxidation treatment in the silicon oxide film forming step were set to about 850 ° C. for 20 minutes. The time of the thermal oxidation treatment was shortened as compared with the first embodiment.

Next, as in the first embodiment, the antireflection film forming step of step S6 is performed to form a silicon nitride film on the silicon oxide film on the light receiving surface and the back surface of the semiconductor substrate, thereby forming the light receiving 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 has not undergone the back etching step of step S3, the concave-convex 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 of forming the photoelectric conversion element of Comparative Example 2 will be described. First, the steps S1 and S2 are performed on the same semiconductor substrate as in the first embodiment as in the first embodiment. Then, in the back etching step of step S3 in the first embodiment, etching was performed using an alkaline solution instead of the mixed solution of the hydrofluoric acid solution and the nitric acid solution as the etching solution. The alkaline solution is, for example, an aqueous solution of sodium hydroxide. In this example, a 20% concentration sodium hydroxide aqueous solution was maintained at about 70 ° C., the light receiving surface of the semiconductor substrate was protected with an acid-resistant and alkali-resistant film and immersed, and then neutralized with an acid solution and washed. At this time, the amount of scraping on the back surface of the semiconductor substrate was about 0.65 g. Then, after the back etching step, each step of steps S4 to S7 was performed in the same manner 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 in order to more clearly confirm the effect of the reflected light on the back surface side of the photoelectric conversion element, the electrical characteristic is not affected on the gold-plated stage. A high-reflection film or the like was attached in the range for measurement. Further, for the adhesive strength, for example, an interconnector coated with a solder film on a copper foil having a width of 1.5 mm and a thickness of 0.2 mm is connected on the bus bar electrode on the back surface of the photoelectric conversion element, and the interconnector is connected in the opposite direction by 180 °. The photoelectric conversion element and the interconnector were forcibly peeled off by pulling to, and the force at that time was measured as the adhesive strength.

The shaving amount of 0.05 g of the photoelectric conversion element 1 of the first embodiment in FIG. 5A is the minimum shaving amount for removing the p-type semiconductor layer 12, and the shaving amounts of 0.15 g, 0.25 g, and 0. 50 g is the amount of scraping for the purpose of flattening the back surface of the semiconductor substrate 11.

Here, for reference, a photographic image of a semiconductor substrate having a scraped amount of 0.057 g and 0.510 g on the back surface and a plane and a cross section of the back surface of the semiconductor substrate on which the back surface has not been scraped is shown in FIG. 5B. (A) and (b) of FIG. 5B are a plane and a cross section of the back surface of the semiconductor substrate whose back surface is not cut. (C) and (d) of FIG. 5B are a flat surface and a cross section of the back surface of the semiconductor substrate having a scraping amount of 0.057 g on the back surface. (E) and (f) of FIG. 5B are a plane and a cross section of the back surface of the semiconductor substrate having a scraping amount of 0.510 g on the back surface. As shown in FIG. 5B (b), when the back surface is not scraped, the uneven shape of the pyramid structure is formed on the back surface, but when the back surface is scraped, as shown in FIG. 5B (d). In addition, the apex of the pyramid structure is shaved to reduce the kurtosis, and the apex portion becomes a rounded uneven shape. Then, when the amount of scraping is 0.510 g, as shown in FIG. 5B (f), the kurtosis becomes smaller than that in FIG. 5B (d), and the height difference of the unevenness becomes further smaller. Therefore, as the amount of scraping on the back surface increases, the kurtosis of the concave-convex shape becomes smaller, the concave-convex shape on the back surface approaches flattening, and each cross section of the concave-convex-shaped concave portion and the convex portion on the back surface has an arc shape. That is, as shown in FIGS. 5B (d) and 5B, at least the cross section of the concave-convex shape on the back surface of the semiconductor substrate has a plurality of arcuate shapes. The plurality of arcs are arranged in the in-plane direction on the back surface of the semiconductor substrate. Further, the plurality of arcs are arranged so as to be continuous.

As shown in FIG. 5A, the back surface area ratio Y of the semiconductor substrate 11 when the scraping amounts are 0.05 g, 0.15 g, 0.25 g, and 0.50 g is 1.84, 1.77, 1.53, 1. It was .21. Further, since the photoelectric conversion element of Comparative Example 1 was not subjected to the back etching step, the surface area ratio Y of the light receiving surface and the back surface was 1.90. In the photoelectric conversion element of Comparative Example 2, the amount of scraping on the back surface by the back etching step was 0.65 g, and the back surface area ratio Y was 1.11.

FIG. 6A is a graph showing the relationship between the back surface area ratio Y shown in FIG. 5A and the short-circuit current density Jsc, and FIG. 6B is the back surface area ratio Y shown in FIG. 5A and the conversion efficiency η. It is a graph which shows the relationship with. 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. 6B, the conversion efficiency η changes 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 it comes to Comparative Example 2) and 1.90 (Comparative Example 1), it decreases sharply. That is, when the back surface area ratio Y is larger than 1.11 and less than 1.90, the reflected light on the back surface is efficiently taken in, and the conversion efficiency η is improved.

On the other hand, as shown in FIG. 5A, it can be seen that the adhesive strength improves as the back surface area ratio Y increases. Specifically, the adhesive strength of the first embodiment is smaller than that of Comparative Example 1, but a constant adhesive strength of a predetermined reference value of 1 N / mm or more is guaranteed. On the other hand, in Comparative Example 2, since the back surface is flatter than that in the first embodiment, the contact area between the electrode on the back surface side and the electric field layer on the back surface is further reduced, which is about 1/2 of that of Comparative Example 1. Adhesive strength is reduced.

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

[Embodiment 2]
In the present embodiment, as in the first embodiment, the semiconductor substrate 10 is subjected to each of the steps S1 to S7, and the two photoelectric conversion elements 1a having a total film thickness different from that of 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 scraping on the back surface of the semiconductor substrate 11 is 0.05 g. Further, 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 step of 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 having a scraping amount of 0.05 g. In FIG. 7, each characteristic value excluding the film thickness and the 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 thickness of the antireflection film 15 is 80 nm, the short-circuit current density Jsc and the open circuit voltage Voc decrease, but 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 film thickness of the antireflection film 15 on the back surface is increased to 120 nm, the fire-through of the antireflection film 15 becomes insufficient when forming the electrode 17 on the back surface, and the electrode 17 and the electric field layer 13 on the back surface are appropriately connected. It is not connected and the conversion efficiency η is reduced. Therefore, considering the fire-through of the antireflection film 15, the total film 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, each step of steps S1 to S7 similar to that of the first embodiment is performed, and the amount of scraping 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 1 g in which the structure of the electrode 17 on the back surface side is different from that of the photoelectric conversion element 1 were produced.

The electrode 17 on the back surface of the photoelectric conversion element 1 of the first embodiment is composed of three bus bar electrodes 171 and 70 finger electrodes 172, and the width of the bus bar electrode 171 is about 2.5 mm and the width of the finger electrodes 172. 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 electrodes 172 is the photoelectric conversion element 1. different. Hereinafter, the differences from the photoelectric conversion element 1 will be specifically described.

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

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

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

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

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

FIG. 8 shows the measurement results of the electrical characteristics of the photoelectric conversion element 1 and the photoelectric conversion elements 1c to 1 g in which the amount of scraping 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 surface electrode occupancy rate (%) 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. Further, in FIG. 8, each characteristic value excluding the finger electrode width, the number of finger electrodes, and the back surface electrode occupancy 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 surface electrode occupancy rate is 9.0% or more, the amount of light received 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 surface electrode occupancy rate is less than 9.0%, the resistance of the finger electrode 172 increases, the curve 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 surface electrode occupancy rate is less than 9.0%.

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

FIG. 9 shows the photoelectric conversion element 1 of the first embodiment in which the amount of scraping on the back surface is 0.05 g and the total film thickness of the antireflection film 15 is 100 nm, and the total film thickness of the antireflection film 15 is 120 nm. The measurement results of the characteristics of the photoelectric conversion element 1b of the second embodiment and the photoelectric conversion element 1h of the present embodiment are shown. The electrodes 17 on the back surface side of the photoelectric conversion elements 1, 1b, and 1h are 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. It is composed of 172. Further, 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 by using the same glass frit as in the first embodiment has a total film thickness of the antireflection film 15 as compared with the photoelectric conversion element 1 of the first embodiment. Therefore, the fire-through of the antireflection film 15 when forming the electrode 17 is not sufficient, and the conversion efficiency η is lowered. Similar to the photoelectric conversion element 1b, the photoelectric conversion element 1h has a total film thickness of the antireflection film 15 of 120 nm, but the electrode 17 is formed by using a glass frit that easily fires through. Therefore, the antireflection film 15 was sufficiently fire-through, a high curve factor FF could be maintained, and the conversion efficiency η could be improved. 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 easily fires through the antireflection film 15 as the material of the electrode 17.

[Embodiment 5]
In the above-described first to fourth embodiments, _{an example in which boron tribromide (BBr 3} ) is diffused on the light receiving surface of the semiconductor substrate 11 to form the p-type semiconductor layer 12 has been described, but the p-type conductive type is provided. However, 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, and the like. It is composed of any one 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 forming step of step S2, for example, H ₂ gas, SiH ₄ gas, and diborane (B ₂ H ₆ ) gas are used as reaction gases by the plasma CVD method, and the p-type amorphous semiconductor is used. The layer may be formed on the light receiving surface of the semiconductor substrate 11.

Further, as shown in FIG. 10, a passivation film 21a 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 21a, and a 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. With such a configuration, the passivation property on the light receiving surface and the back surface can be improved, and the conversion efficiency can be further improved. The passivation films 21a and 21b are amorphous semiconductor films that are substantially intrinsic and contain 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 is composed of either amorphous silicon oxide, i-type amorphous silicon carbon oxide, or the like. In this case, after the texture structure forming step of step S1, for example, by the plasma CVD method, silane gas and hydrogen gas are used as reaction gases, and the passivation films 21a and 21b are formed on the light receiving surface and the back surface, respectively. Good.

[Modification example]
It should be considered that the embodiments disclosed this time are exemplary in all respects and not restrictive. The scope of the present invention is shown by the scope of claims rather than the description of the embodiment described above, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims. An example of the modification is shown below.

As the photoelectric conversion element in the above-described first to fifth embodiments, an example in which an n-type single crystal silicon substrate is used as the semiconductor substrate 11 has been described, but a p-type single crystal silicon substrate may be used. In this case, the n-type semiconductor layer may be formed on the light receiving surface side of the semiconductor substrate 11, and the back surface electric field layer 13 containing boron (B) may be formed on the back surface side.

1,1a to 1i ... Photoelectric conversion element, 10,10a, 11 ... Semiconductor substrate, 12 ... p-type semiconductor layer, 13 ... Backside electric field layer (impurity diffusion layer), 14,15 ... Antireflection film, 16,17 ... electrode, 161,171 ... bus bar electrode, 162,172 ... finger electrode, 21a, 21b ... passion film, 22 ... p-type amorphous semiconductor Layer, 23 ... n-type amorphous semiconductor layer, 100, 110 ... Concavo-convex shape

Claims (8)

A semiconductor substrate including a portion having a first conductive type and further including a semiconductor layer having a second conductive type opposite to the first conductive type on a surface on which light is incident.
A first electrode formed in contact with the semiconductor layer on the surface of the semiconductor substrate,
The semiconductor substrate includes a second electrode arranged on the opposite side of the front surface and formed on the back surface on which light is incident.
The front surface and the back surface of the semiconductor substrate have an uneven shape.
A photoelectric conversion element in which the cross section of the region from the portion where the height of the uneven shape on the back surface is the highest to the portion where the height is the lowest is an arc shape which is convex outward. A semiconductor substrate having a first conductive type and
A semiconductor layer arranged outside the surface of the semiconductor substrate on which light is incident and having a second conductive type opposite to the first conductive type,
A first electrode formed in contact with the semiconductor layer on a surface of the semiconductor layer opposite to the semiconductor substrate,
The semiconductor substrate includes a second electrode arranged on the opposite side of the front surface and formed on the back surface on which light is incident.
The front surface and the back surface of the semiconductor substrate have an uneven shape.
A photoelectric conversion element in which the cross section of the region from the portion where the height of the uneven shape on the back surface is the highest to the portion where the height is the lowest is an arc shape which is convex outward. The photoelectric conversion element according to claim 1 or 2, wherein the surface area of the uneven shape on the back surface is smaller than the surface area of the uneven shape on the front surface. When the surface area of the back surface is the area X and the flat area of the back surface is the area S, the surface area ratio Y (= X / S) indicating the ratio of the area X to the area S on the back surface is larger than 1.11. The photoelectric conversion element according to any one of claims 1 to 3, which is less than 1.90. Further provided with an antireflection film provided on the back surface side of the semiconductor substrate,
The second electrode penetrates the antireflection film and is provided on the back surface of the semiconductor substrate.
The photoelectric conversion element according to claim 1, wherein the antireflection film has a film thickness of 100 nm or more and less than 120 nm. The photoelectric conversion element according to any one of claims 1 to 5, wherein the occupancy rate of the second electrode on the back surface of the semiconductor substrate is less than 9.0%. The photoelectric conversion element according to claim 1 or 5, wherein the semiconductor substrate includes an impurity diffusion layer having the first conductive type connected to the second electrode on the back surface side. The photoelectric conversion element according to claim 2, 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.

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