JP5412376B2 - Solar cell - Google Patents

Description

  The present invention relates to a solar cell provided with an antireflection film for preventing reflection on the surface of a substrate.

  In a solar cell, a technique for preventing reflection on the light receiving surface is indispensable in order to suppress reflection loss on the light receiving surface and increase conversion efficiency. As a conventional antireflection technique, a configuration is known in which a transparent thin film having a refractive index lower than that of a substrate surface constituting a solar cell is used as an antireflection film. In addition, a technique is known in which a texture structure is formed on the surface of a semiconductor substrate constituting a solar cell so that light reflected on the surface at the time of incidence is re-incident into the base material to reduce the reflectance.

Zhao et al. , IEEE Transactions on Electron Devices 1991, 38, 1925-1934.

  Although the conventional antireflection film exhibits antireflection performance at a specific wavelength, there is a problem that the antireflection performance is drastically reduced in a region other than the specific wavelength. For this reason, in order to obtain an antireflection effect in a wide wavelength range, it is necessary to form a multilayer antireflection film by multilayering antireflection films having different refractive indexes, and an increase in cost is inevitable.

  Further, when a texture structure is formed on the surface of the semiconductor substrate, the structure of the surface of the semiconductor substrate is disturbed, the surface area of the semiconductor substrate is increased, and the photoelectric conversion performance of the solar cell is decreased. For example, when a texture structure is provided on the surface of a semiconductor substrate, the recombination speed of carriers in the surface layer of the semiconductor substrate increases, so that the conductivity of the semiconductor substrate decreases, and consequently the photoelectric conversion performance of the solar cell decreases.

  The present invention has been made in view of these problems, and an object thereof is to provide a solar cell provided with an antireflection film that exhibits excellent antireflection performance in a wider wavelength region.

  One embodiment of the present invention is a solar cell. The solar cell includes a semiconductor substrate, a transparent film having an average refractive index lower than that of the semiconductor substrate provided on the light receiving surface of the semiconductor substrate, and a plurality of metal nanoparticles provided in a two-dimensional array on the transparent film. And an average particle size of the metal nanoparticles is in a range of 100 nm or more and less than 200 nm.

  According to the solar cell of the above aspect, an antireflection effect can be obtained in a wide wavelength range, and consequently the solar cell characteristics can be improved.

  In the solar cell of the above aspect, the upper limit of the particle diameter of the metal nanoparticles may be 200 nm. Moreover, the metal nanoparticles may be made of Au, Ag, Al, Cu, Li, Na, K, or an alloy of these metals. Further, the metal nanoparticles may be made of Ag or an alloy mainly composed of Ag. Further, when the average particle diameter of the metal nanoparticles is D, the film thickness d of the transparent film may be in a range represented by the following formula.

また、透明膜の平均屈折率が１．２以上１．８以下であってもよい。

Further, the average refractive index of the transparent film may be 1.2 or more and 1.8 or less.

  A combination of the above-described elements as appropriate can also be included in the scope of the invention for which patent protection is sought by this patent application.

  According to the present invention, a solar cell provided with an antireflection film that exhibits excellent antireflection performance in a wider wavelength region is provided.

FIG. 1A is a schematic cross-sectional view illustrating a configuration of a solar cell having an antireflection film according to an embodiment. FIG. 1B is a plan view showing a state of arrangement of metal nanoparticles when the semiconductor substrate is viewed in plan. It is process sectional drawing which shows the preparation methods of the reflection preventing film which concerns on embodiment. It is a graph which shows the relationship between the particle diameter of a metal nanoparticle, and the maximum short circuit current value in the case of the optimal transparent film thickness in each particle diameter. It is a graph which shows the relationship between the refractive index of a transparent film, and the maximum short circuit current value in the optimal transparent film thickness in each transparent film refractive index. It is a graph which shows the relationship between a short circuit current and the film thickness of a transparent film. It is a graph which shows the relationship between the thickness of a transparent film | membrane and the average particle diameter of a metal nanoparticle which can aim at the improvement of a short circuit current.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same reference numerals are given to the same components, and the description will be omitted as appropriate.

  FIG. 1A is a schematic cross-sectional view showing a configuration of solar cell 10 having antireflection film 30 according to the embodiment. FIG. 1B is a plan view showing the arrangement of the metal nanoparticles 34 when the semiconductor substrate is viewed in plan. FIG. 1A corresponds to a cross-sectional view taken along the line AA in FIG. As shown in FIG. 1A, the solar cell 10 includes a semiconductor substrate 20 and an antireflection film 30.

  The semiconductor substrate 20 has a pn junction in which a p-type semiconductor and an n-type semiconductor are joined, and light energy from the sun is converted into electrical energy by the photovoltaic effect of the pn junction. A direct current can be taken out of the solar cell 10 by attaching electrodes (not shown) to the n-type semiconductor and the p-type semiconductor, respectively. In the present embodiment, the semiconductor substrate 20 is a single crystal Si substrate (refractive index 3.67 (wavelength 1000 nm)) and has a pn junction that is well-known as a solar cell composed of a group IV semiconductor substrate.

  The antireflection film 30 is provided on the main surface of the semiconductor substrate 20 on the light receiving surface side of the solar cell 10. In other words, one main surface of the antireflection film 30 is a contact surface with the main surface of the semiconductor substrate 20. In the present embodiment, the antireflection film 30 includes a transparent film 32 and metal nanoparticles 34, and one main surface of the transparent film 32 is used as the contact surface described above.

The transparent film 32 has transparency in the wavelength region of light received by the solar cell 10, in other words, in the wavelength region of light that prevents reflection. The transparent film 32 has a lower average refractive index than that of the semiconductor substrate 20 and is preferably 1.2 or more and 1.8 or less. Examples of the transparent material forming the transparent film 32 include SiO ₂ and TiO ₂ .

  The plurality of metal nanoparticles 34 are two-dimensionally arranged on the transparent film 32. In other words, the plurality of metal nanoparticles 34 are scattered in a two-dimensional array on the transparent film 32. The plurality of metal nanoparticles 34 are separated from the main surface on the light receiving surface side of the semiconductor substrate 20 by a transparent film 32 at a certain distance. Therefore, the distance between the bottom of the metal nanoparticles 34 and the main surface on the light receiving surface side of the semiconductor substrate 20 is equal to the film thickness d of the transparent film 32.

  The film thickness d of the transparent film 32 is preferably in the range represented by the following formula, where D is the average particle diameter of the metal nanoparticles 34.

  The material of the metal nanoparticles 34 is not particularly limited as long as it is a metal material, but Froehlich (the alphabetical expression “oe” is the German word “Oh umlaut”) (Bohren and Huffman, Absorption and Scattering of Light by Small Particles) , Wiley, 1983) where the resonance wavelength is close to the wavelength of the light that prevents reflection, such as Au, Ag, Al, Cu, Li, Na, K or alloys of these metals. Ag is particularly preferred.

  The shape of the metal nanoparticles 34 is not particularly limited, and examples thereof include a hemispherical shape, a cylindrical shape, and a prismatic shape. The average D of the particle diameters M of the metal nanoparticles 34 when the semiconductor substrate 20 is viewed in plan is preferably 100 nm or more and 200 nm or less. In addition, the upper limit of the particle diameter M of the metal nanoparticles 34 when the semiconductor substrate 20 is viewed in plan is 200 nm, and it is preferable that metal nanoparticles having a particle diameter exceeding 200 nm are not substantially present. The height H of the metal nanoparticles 34 when the main surface of the transparent film 32 opposite to the semiconductor substrate 20 (exposed surface of the transparent film 32) is used as a reference surface is, for example, 100 nm.

  When the solar cell 10 according to the embodiment described above is outlined, a plurality of average particle diameters of 100 nm to 200 nm and / or the upper limit of the particle diameter is 200 nm on the transparent film 32 provided on the light receiving surface of the solar cell 10. The metal nanoparticles 34 are two-dimensionally arranged, and the plurality of metal nanoparticles 34 are separated from the surface of the semiconductor substrate 20 by a predetermined thickness of the transparent film 32. With this configuration, an antireflection effect can be obtained in a wide wavelength range of 400 to 1000 nm, and as a result, the characteristics of the solar cell can be improved.

  Further, since the contact surface of the transparent film 32 is smooth and the main surface of the semiconductor substrate 20 that is in contact with the contact surface is also smooth, it is not necessary to increase the surface area of the light receiving surface of the semiconductor substrate 20 more than necessary. For this reason, it is suppressed that a carrier recombines in the surface layer of the semiconductor substrate 20, and by extension, the photoelectric conversion performance of a solar cell can be aimed at.

  Further, by arranging the plurality of metal nanoparticles 34 two-dimensionally on the transparent film 32, the distance between the contact surface of the transparent film 32 and the metal nanoparticles 34 can be kept constant throughout the antireflection film 30. In-plane variation in reflection performance in the antireflection film 30 can be suppressed.

(Preparation method of antireflection film)
FIG. 2 is a process cross-sectional view illustrating a method for manufacturing an antireflection film provided in the solar cell according to the embodiment. A method for manufacturing the antireflection film according to the embodiment will be described with reference to FIGS.

  First, as shown in FIG. 2A, a transparent film 32 having a predetermined thickness is formed on the incident surface of a semiconductor substrate 20 made of a single crystal Si substrate. In this embodiment mode, a single crystal Si substrate is used as the semiconductor substrate 20, but the present invention is not limited to this, and the substrate can be formed on any substrate as long as it is flat. For example, a polycrystalline Si substrate, a glass, quartz, or sapphire substrate on which some thin film is formed can be used.

The method for forming the transparent film 32 is not particularly limited. For example, when the semiconductor substrate 20 is a single crystal Si substrate, a method of thermally oxidizing the surface of the semiconductor substrate 20 can be used. In addition, there is a method in which a transparent material such as SiO ₂ or TiO ₂ is applied to the semiconductor substrate 20 by spin coating. In this case, the film thickness of the transparent film 32 can be adjusted by the number of rotations during spin coating and the solution concentration of the coating solution.

  Next, as shown in FIG. 2B, a mask 40 is formed on the transparent film 32. In the mask 40, a plurality of openings 42 are formed so that the metal nanoparticle formation region on the transparent film 32 is exposed. For example, after anodizing the surface of the aluminum substrate, the mask 40 removes the aluminum substrate other than the anodized surface (alumina barrier layer) and forms a through hole in the alumina barrier layer using a phosphoric acid solution. Can be produced. In addition, the mask 40 can be made of a resist in which a predetermined opening is patterned. By using a resist as the mask 40, the metal nanoparticles can be regularly arranged two-dimensionally.

  Next, as shown in FIG. 2C, Au, Ag, Al, Cu, or an alloy of these metals is deposited by vacuum evaporation toward the transparent film 32 through the mask 40. The metal particles pass through the opening 42 provided in the mask 40 and are selectively deposited on the transparent film 32 in the opening 42. Thereby, metal nanoparticles 34 are formed in the opening 42, and a plurality of metal nanoparticles 34 are two-dimensionally arranged on the transparent film 32. The particle diameter M of the metal nanoparticles 34 when the semiconductor substrate 20 is viewed in plan is defined by the diameter of the opening 42 provided in the mask 40. In other words, the particle diameter M of the metal nanoparticles 34 does not exceed the diameter of the opening 42 provided in the mask 40, and the diameter of the opening 42 provided in the mask 40 is equal to the particle diameter M of the metal nanoparticles 34. It becomes the upper limit of.

  When the mask 40 is formed using an alumina barrier layer, the size of the opening 42 is proportional to the applied voltage during aluminum anodic oxidation. For example, when 40 V is applied to the aluminum substrate with 0.3 mol / l oxalic acid electrolyte, the diameter of the opening 42 is about 50 nm and the diameter of the metal nanoparticles 34 is also about 50 nm. Further, when 120 V is ignited on the aluminum substrate with 0.4 mol / l malonic acid electrolyte, the diameter of the opening 42 is about 150 nm and the diameter of the metal nanoparticles 34 is also about 150 nm. In addition, the height of the metal nanoparticles 34 when the surface of the transparent film 32 is the reference surface can be controlled by changing the time of vacuum deposition. When the time of vacuum vapor deposition is short, the spherical surface is a hemispherical surface facing upward, and when the time of vacuum vapor deposition is sufficiently long, the shape is cylindrical or prismatic.

  Next, as shown in FIG. 2D, the mask 40 is removed, and the antireflection film 30 including the transparent film 32 and the metal nanoparticles 23 is obtained.

  Through the steps described above, the antireflection film 30 can be easily formed on the semiconductor substrate 20.

Example 1
A sample simulating a solar cell having the antireflection film of Example 1 was produced under the following conditions.
<Preparation of transparent film>
A Si substrate ((100) surface, p-type, resistivity 3 to 5 Ωcm) having a single-side polished surface of 300 μm thickness was prepared. The polished surface of this Si substrate was oxidized by thermal oxidation at 900 ° C. for 60 minutes to form a SiO ₂ layer having a thickness of 50 nm as a transparent film.

<Production of metal nanoparticles>
Through an alumina mask provided with an opening having an opening diameter of 100 nm, Au is vacuum-deposited on the transparent film formed as described above, and 2 metal nanoparticles made of hemispherical Au having a particle diameter of 100 nm in plan view are obtained. A dimensional array was formed. The coverage with the metal nanoparticles with respect to the area of the transparent film was about 30%.

(Comparative Example 1)
A sample simulating a solar cell having the antireflection film of Comparative Example 1 is the same as Example 1 except that metal nanoparticles are not formed.

  For the samples of Example 1 and Comparative Example 1, the reflectance of the substrate surface on the side where the antireflection film was formed was measured using a spectrophotometer U4000 manufactured by Hitachi High-Tech. The average reflectance from a wavelength of 400 nm to a wavelength of 1000 nm was 27.3% in Comparative Example 1, but in Example 1, it was confirmed to decrease to 18.3%.

(Example 2)
The solar cell having the antireflection film of Example 2 was fabricated using a double-side polished Si substrate having a thickness of 100 μm and a resistivity of 1 to 10 Ωcm. That is, first, an emitter layer is formed by doping phosphorus on the surface at 900 ° C. using a diffusion furnace. Next, the surface phosphosilicate glass layer was removed with hydrofluoric acid. Thereafter, the end face of the substrate was removed with a mixed solution of hydrofluoric acid, nitric acid and acetic acid in order to remove the short circuit of the pn junction. Thereafter, a silver aluminum electrode was formed on the back side by sputtering. Then, an antireflection layer was produced on the surface side in the same procedure as in Example 1 except that the metal nanoparticle material was Ag.

(Comparative Example 2)
The solar cell having the antireflection film of Comparative Example 2 is the same as Example 2 except that the metal nanoparticles are not formed.

The short-circuit current under 1 SUN of the samples of Example 2 and Comparative Example 2 was measured. The short-circuit current of the sample of Comparative Example 2 was 29.2 mA / cm ² . On the other hand, the short-circuit current of the sample of Example 2 is 31.4 mA / cm ² , and it was confirmed that the antireflection effect by the metal nanoparticles brings about the improvement of the solar cell characteristics.

(Example 3)
The solar cell having the antireflection film of Example 3 is the same as Example 2 except that the thickness of the Si substrate is 150 μm, the thickness of the transparent film is 60 nm, and the material of the metal nanoparticles is Ag. Produced by the procedure.

Example 4
The solar cell having the antireflection film of Example 4 is the same as Example 2 except that the thickness of the Si substrate is 150 μm and the thickness of the transparent film is 60 nm. The material of the metal nanoparticles is Au It is.

The short-circuit current under 1 SUN of the samples of Example 3 and Example 4 was measured. As a result, in the sample of Example 3, the short-circuit current is 33.7 mA / cm ² , whereas in the sample of Example 4, the short-circuit current is 31.9 mA / cm ² , which improves the solar cell characteristics. It was confirmed that Ag is more preferable as a material for the metal nanoparticles to be formed.

(Example 5)
In the solar cell having the antireflection film of Example 5, the thickness of the Si substrate is 200 μm, a SiN layer having a thickness of 60 nm is formed as a transparent film by plasma CVD, and the metal nanoparticles are hemispherical with a particle diameter of 150 nm. It was produced in the same procedure as in Example 2 except that Ag was used.

(Example 6)
The solar cell having the antireflection film of Example 6 is the same as Example 5 except that the transparent film is made of SiO ₂ produced by thermal oxidation.

The short-circuit current under 1 SUN of the samples of Example 5 and Example 6 was measured. As a result, in the sample of Example 5, the short-circuit current is 32.1 mA / cm ² , whereas in the sample of Example 6, the short-circuit current is 34.3 mA / cm ² , which improves the solar cell characteristics. It was confirmed that SiO ₂ is more preferable as a material for the transparent film to be formed.

(Relationship between particle size and optical properties of metal nanoparticles)
A SiO ₂ layer having a thickness of 10 to 150 nm was formed as a transparent film (low refractive index layer) on a 100 μm thick Si substrate, and metal nanoparticles composed of hemispherical hemispherical silver were formed on the transparent film. Regarding the structure, the optical properties when the particle size of the metal nanoparticles was changed were calculated using Lumerical's FDTD solutions. In addition, the maximum short circuit current value was used as a value which shows an optical characteristic. FIG. 3 is a graph showing the relationship between the particle diameter of the metal nanoparticles and the maximum short-circuit current value at the optimum transparent film thickness at each particle diameter. As shown in FIG. 3, it was confirmed that when the particle diameter of the metal nanoparticles was larger than 200 nm, the solar cell characteristics were extremely deteriorated, and good solar cell characteristics were obtained when the particle diameter was in the range of 100 to 200 nm.

(Relationship between refractive index and optical properties of transparent film)
A structure in which a transparent film (low refractive index layer) having a thickness of 10 to 150 nm is formed on a Si substrate having a thickness of 100 μm, and hemispherical silver nanoparticles and gold nanoparticles having a particle diameter of 100 nm are formed on the transparent film. The optical characteristics when the refractive index of the transparent film was changed in the range of 1.0 to 2.5 were calculated using Lumerical's FDTD solutions. In addition, the maximum short circuit current value was used as a value which shows an optical characteristic. FIG. 4 is a graph showing the relationship between the refractive index of the transparent film and the maximum short-circuit current value at the optimum transparent film thickness at each transparent film refractive index. As shown in FIG. 4, it can be seen that the maximum short-circuit current value suddenly decreases when the refractive index of the transparent film exceeds 1.8 in both cases of silver nanoparticles and gold nanoparticles. Further, when the refractive index of the transparent film is lower than 1.2, the characteristics are similarly lowered, which is not desirable.

(Relationship between transparent film thickness and optical properties 1)
Regarding a structure in which a SiO ₂ layer is formed as a transparent film (low refractive index layer) on a 100 μm thick Si substrate, and metal nanoparticles made of hemispherical silver having a particle diameter of 100 nm are formed on the transparent film. The short-circuit current was measured when the thickness was changed. FIG. 5 is a graph showing the relationship between the short-circuit current and the film thickness of the transparent film. As shown in FIG. 5, it can be seen that the short-circuit current is maximized when the thickness of the transparent film is about 50 nm under the condition that the particle diameter of the metal nanoparticles is fixed.

(Relationship between transparent film thickness and optical properties 2)
Regarding a structure in which a SiO ₂ layer is formed as a transparent film (low refractive index layer) on a 100 μm thick Si substrate and metal nanoparticles made of hemispherical silver are formed on the transparent film, the thickness of the transparent film and the metal The short-circuit current was measured when both the nanoparticle particle sizes were changed. FIG. 6 is a graph showing the relationship between the thickness of the transparent film and the particle diameter of the metal nanoparticles, which can improve the short-circuit current. When the range where the short-circuit current exceeds 30 mA / cm ² was determined, it was found that the region surrounded by the curve A and the curve B shown in FIG. A curve A shown in FIG. 6 is a curve that becomes d = −0.0052D ² + 1.7346D-45.883, where D is the average particle diameter of the metal nanoparticles and d is the thickness of the transparent film. Moreover, the curve B shown in FIG. 6 is a curve which becomes d = -0.0054D ² -1.3215D + 108.92. That is, when the particle diameter of the metal nanoparticles is D and the film thickness of the transparent film is d, the characteristics of the solar cell are remarkably improved when the film thickness d of the transparent film is in the range represented by the following formula. Was found.

  The present invention is not limited to the above-described embodiments, and various modifications such as design changes can be added based on the knowledge of those skilled in the art. The form can also be included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 10 Solar cell, 20 Semiconductor substrate, 30 Antireflection film, 32 Transparent film, 34 Metal nanoparticle, 40 Mask

Claims (6)

A semiconductor substrate;
A transparent film having an average refractive index lower than that of the semiconductor substrate, provided on the light receiving surface of the semiconductor substrate;
A plurality of metal nanoparticles provided two-dimensionally on the transparent film;
With
The average particle diameter of a metal nanoparticle is the range of 100 nm or more and less than 200 nm, The solar cell characterized by the above-mentioned.   The solar cell according to claim 1, wherein the upper limit of the particle diameter of the metal nanoparticles is 200 nm.   The solar cell according to claim 1 or 2, wherein the metal nanoparticles are made of Au, Ag, Al, Cu, Li, Na, K, or an alloy of these metals.   The solar cell according to claim 1, wherein the metal nanoparticles are made of Ag or an alloy mainly composed of Ag. 5. The sun according to claim 1, wherein when the average particle diameter of the metal nanoparticles is D, the film thickness d of the transparent film is in a range represented by the following formula. battery.

  6. The solar cell according to claim 1, wherein the transparent film has an average refractive index of 1.2 or more and 1.8 or less.

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