JP5359672B2 - Light scattering film for solar cell and solar cell using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light scattering film for a solar cell having a high light utilization and improving a light-utilization dependency by an angle of incidence and the solar cell using the light scattering film. <P>SOLUTION: The light scattering film for the solar cell has at least two layers of light scattering layers containing scatterers in matrices. In the light scattering film for the solar cell, (1) the film surface of a first light scattering layer on the solar-light incident side is flattened, and (2) a total light transmittance on the first light scattering layer of a light incident from the normal direction to the film surface is 70% or greater in the first light scattering layer. In the light scattering film for the solar cell, (3) the total light transmittance on a second light scattering layer of the light incident from the normal direction to the film surface is 60% or less in the second light scattering layer installed on the far side from the solar-light incident side to the first light scattering layer, and (4) the total light transmittance on the light scattering layer to the light incident at an angle of 45&deg; to the normal of the film surface of the first and second light scattering layers satisfies a specified relational expression. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a light scattering film for a solar cell having high light utilization efficiency and improved light utilization efficiency dependency depending on an incident angle, and a solar cell using the same.

  In recent years, solar cells have been actively researched as alternative energy for petroleum energy.

  Examples of solar cells include single crystal silicon solar cells, polycrystalline silicon solar cells, thin film silicon solar cells, compound semiconductor solar cells, and dye-sensitized solar cells. Further, the thin film silicon solar cell includes an amorphous silicon solar cell and a microcrystalline silicon solar cell. Furthermore, there are a tandem solar cell in which an amorphous silicon solar cell and a microcrystalline silicon solar cell are stacked, and a triple solar cell in which three solar cells are stacked.

  In particular, the thin-film silicon solar cell has abundant constituent materials on the earth, is easy to increase in area, can be deposited thinly, has a low consumption of constituent materials, It attracts attention with its outstanding features such as high technical maturity.

  For solar cell members, improving photoelectric conversion efficiency is an important issue. For this purpose, a structure called a “texture structure” consisting of an uneven surface structure is provided to increase the film thickness of the photoelectric conversion layer. In order to increase the amount of light absorbed, the path length of light passing through the photoelectric conversion layer is increased by scattering the light.

  However, when a photoelectric conversion layer is formed on the texture structure, many defects are induced in the photoelectric conversion layer (semiconductor film), which may deteriorate the conversion efficiency or increase the film thickness of the photoelectric conversion layer. Problems such as increased costs and reduced production speed remain.

  On the other hand, Patent Document 1 and Patent Document 2 show a solution for a light scattering film that uses a scattering layer and does not cause defects in a semiconductor layer formed thereon and has a high light confinement effect. ing. However, the light confinement efficiency is not sufficient even in the literature method, and further improvement in photoelectric conversion efficiency is demanded.

  In addition, when the solar cell is fixedly installed, the incident angle to the solar cell changes depending on the installation location, time, season, etc., and thus the amount of power generation generally changes. In order to solve this problem, there are devices that automatically turn to the optimal direction in order to obtain the maximum amount of power generation, but there are problems such as damage to the device itself and limited installation location It cannot withstand typical use.

JP 2009-16554 A JP 2009-16555 A

  The present invention has been made in view of the above problems, and a solution to that problem is a light scattering film for a solar cell that has high light use efficiency and improved dependency on light use efficiency depending on an incident angle, and the same. It is to provide a solar cell.

  The above-mentioned problem according to the present invention is solved by the following means.

1. A light scattering film for a solar cell having at least two light scattering layers each containing a scatterer in a matrix, wherein (1) the film surface of the first light scattering layer on the sunlight incident side is flat; 2) The first light scattering layer has a total light transmittance at the first light scattering layer of light incident from the normal direction to the film surface of 70% or more, and (3) the first light scattering layer. The second light scattering layer installed on the side far from the sunlight incident side with respect to the light scattering layer of the first light transmission layer transmits the light incident on the film surface from the normal direction in the second light scattering layer. And (4) total light transmission in the first light scattering layer with respect to light incident at an angle of 45 ° with respect to the normal of the film surface of the first light scattering layer. T1 (45 °), the total light in the second light scattering layer with respect to light incident at an angle of 45 ° with respect to the normal of the film surface of the second light scattering layer The transmittance is taken as T2 (45 °), the T1 (45 °) and T2 (45 °) is less than the following formula (I), the AND (first light-scattering layer and the second light-scattering layer The total light transmittance (through) is 70% or more, and the total light transmittance (through the second light scattering layer and the first light scattering layer) is 60% or less. A light scattering film for solar cells.
Formula (I): T1 (45 °)> T2 (45 °)
2. 2. The solar cell scattering film according to claim 1, wherein the scatterer in the matrix has a refractive index different from that of the matrix.

3. In any two light scattering layers constituting the light scattering film for solar cell, the haze value of the first light scattering layer disposed at a position far from the photoelectric conversion layer is Hz1 (%), a position close to the photoelectric conversion layer The first term or the second term is characterized in that when the haze value of the second light-scattering layer disposed at is set to Hz2 (%), the Hz1 and Hz2 satisfy a relationship of Hz1 <Hz2. Light scattering film for solar cells.
4). ΔN1xy is the refractive index difference between the matrix and the scatterer in a plane parallel to the film surface of the matrix in the first light scattering layer, and the refractive index difference between the matrix and the scatterer in the normal direction to the film surface. ΔN1z is a difference in refractive index between the matrix and the scatterer in a plane parallel to the film surface of the matrix in the second light scattering layer, and ΔN2xy is a difference in refractive index between the matrix and the scatterer in the normal direction to the film surface. The light scattering film for a solar cell according to any one of Items 1 to 3, wherein the following formulas (II) and (III) are satisfied when ΔN2z is set:
Formula (II): ΔN1xy <ΔN1z
Formula (III): ΔN2xy> ΔN2z
[However, ΔN1xy, ΔN2xy, ΔN1z, and ΔN2z in the formulas (II) and (III) are defined by the following relational expressions.
ΔN1xy = | N1xy (matrix) −N1xy (domain) |
ΔN2xy = | N2xy (matrix) −N2xy (domain) |
ΔN1z = | N1z (matrix) −N1z (domain) |
ΔN2z = | N2z (matrix) −N2z (domain) |
here,
N1xy (domain): average refractive index in the plane of the scatterer in the first light scattering layer
N2xy (domain): average refractive index in the plane of the scatterer in the second light scattering layer
N1xy (matrix): average refractive index in the plane of the matrix in the first light scattering layer
N2xy (matrix): average refractive index in the plane of the matrix in the second light scattering layer
N1z (domain): average refractive index of the scatterer in the normal direction in the first light scattering layer
N2z (domain): average refractive index of the scatterer in the normal direction in the second light scattering layer
N1z (matrix): average refractive index of the matrix in the normal direction in the first light scattering layer
N2z (matrix): average refractive index of the matrix in the normal direction in the second light scattering layer]
5. The light scattering film for a solar cell according to item 4, wherein the ΔN1xy is smaller than ΔN1z.
6). The average particle diameter in a plane parallel to the film surface of the scatterer in the first light scattering layer is R1xy, the average particle diameter in the direction perpendicular to the film surface is R1z, and the scattering in the second light scattering layer. R2xy is an average particle diameter in a plane parallel to the film surface of the body, and R2z is an average particle diameter in a direction perpendicular to the film surface, and R1xy, R1z, R2xy, and R2z are expressed by the following formula (IV ) And (V) are satisfied, The solar cell light-scattering film according to any one of the first to fourth items.
Formula (IV): R1xy <R1z
Formula (V): R2xy> R2z
7). 7. The solar cell light scattering film according to claim 6, wherein the scatterer in the first light scattering layer has a needle shape, and the scatterer in the second light scattering layer has a disk shape. .
8). It is a solar cell which has a photoelectric converting layer, Comprising: The light-scattering film for solar cells as described in any one of 1st term | claim to 7th term | arrangement is arrange | positioned to the surface of the said photoelectric converting layer at least in the light incident side. A solar cell characterized by

  By the above means of the present invention, it is possible to provide a light scattering film for a solar cell having high light utilization efficiency and improved dependency on light utilization efficiency depending on an incident angle, and a solar cell using the same.

Cross-sectional schematic diagram showing an example of a solar cell Conceptual diagram showing a method for measuring the total light transmittance at an incident angle of 45 ° with respect to the normal direction. Cross-sectional schematic diagram showing a pin-type photoelectric conversion layer as an example of the photoelectric conversion layer

The light-scattering film for solar cells of the present invention is a light-scattering film for solar cells having at least two light-scattering layers containing a scatterer in a matrix. (1) First light on the sunlight incident side The film surface of the scattering layer is flat. (2) The first light scattering layer has a total light transmittance of 70 incident on the first light scattering layer of light incident from the normal direction to the film surface. (3) the second light scattering layer disposed on the side farther from the sunlight incident side than the first light scattering layer is the light incident on the film surface from the normal direction. The total light transmittance in the second light scattering layer is 60% or less, and (4) the light incident on the light incident at an angle of 45 ° with respect to the normal of the film surface of the first light scattering layer. The total light transmittance of the first light scattering layer is T1 (45 °), and the light incident on the light incident at a 45 ° angle with respect to the normal of the film surface of the second light scattering layer. That the total light transmittance in the second light-scattering layer when the T2 (45 °), the T1 (45 °) and T2 (45 °) is less than the formula (I), the AND (the The total light transmittance (through the light scattering layer of 1 and the second light scattering layer) is 70% or more, and the total light (through the second light scattering layer and the first light scattering layer) A light scattering film for solar cells, wherein the transmittance (back) is 60% or less . This feature is a technical feature common to the inventions according to claims 1 to 8 .

As an embodiment of the present invention, from the viewpoint of manifesting the effects of the present invention, it is preferable that the scatterer in the matrix has a refractive index different from that of the matrix, and any two of the light scattering films for solar cells are formed. In the two light scattering layers, the haze value of the first light scattering layer disposed at a position far from the photoelectric conversion layer is Hz 1 (%), and the haze value of the second light scattering layer disposed at a position close to the photoelectric conversion layer Is set to Hz2 (%), it is preferable that Hz1 and Hz2 satisfy the relationship of Hz1 <Hz2. Further, the difference in refractive index between the matrix and the scatterer in the plane parallel to the film surface of the matrix in the first light scattering layer is ΔN1xy, and the refractive index of the matrix and scatterer in the normal direction to the film surface. The difference is ΔN1z, the refractive index difference between the matrix and the scatterer in the plane parallel to the film surface of the matrix in the second light scattering layer is ΔN2xy, and the refraction of the matrix and scatterer in the normal direction to the film surface. When the rate difference is ΔN2z, it is preferable to satisfy the above formulas (II) and (III). The ΔN1xy is preferably smaller than ΔN1z.

Further, the average particle diameter in a plane parallel to the film surface of the scatterer in the first light scattering layer is R1xy, the average particle diameter in the direction perpendicular to the film surface is R1z, and the second light scattering layer R2xy is an average particle diameter in a plane parallel to the film surface of the scatterer, and R2z is an average particle diameter in a direction perpendicular to the film surface, and R1xy, R1z, R2xy, and R2z are expressed by the above formula ( It is preferable to satisfy IV) and (V). Moreover, it is preferable that the scatterer in a said 1st light-scattering layer is acicular, and the scatterer in a said 2nd light-scattering layer is a disk shape.

  The solar cell light-scattering film of the present invention can be suitably used in a solar cell having a photoelectric conversion layer in such a manner that it is disposed on at least the surface on the light incident side of the photoelectric conversion layer.

  In the present application, “matrix” refers to a substrate or base (matrix) that functions as a medium containing and holding a domain. In the present invention, various resins described later are used as a matrix. A “domain” refers to an individual minute region (light scatterer) made of a substance having a refractive index different from that of a matrix and having a function of scattering light. In the present invention, various inorganic particles or organic particles described later are used as domains.

  Hereinafter, the present invention, its components, and modes and modes for carrying out the present invention will be described in detail.

(Basic configuration of solar cell with light scattering film)
The light scattering film for solar cell of the present invention can be suitably used for various types of solar cells. In FIG. 1, the cross-sectional schematic diagram of an example of the fundamental structure of the solar cell provided with the light-scattering film for solar cells of this invention is shown. A solar cell 1 shown in FIG. 1 has a light scattering film (light scattering film for solar cell) 3, an upper electrode layer 4, a photoelectric conversion layer 5, and a lower electrode layer 6 on one surface of a substrate (transparent substrate) 2. Is a basic structure in which are stacked. Hereinafter, each component will be described.

(substrate)
The substrate receives light from one light capturing surface, and a light scattering film is bonded to the other surface.

  It is preferable that a board | substrate consists of a transparent material in the wavelength range which a photoelectric converting layer converts. This is because light is not lost in the substrate and conversion efficiency in the photoelectric conversion layer can be improved. Examples of the transparent material include inorganic materials such as glass and quartz, resins such as transparent polyimide and heat-resistant polycarbonate, and composite materials in which fillers are introduced into various resins.

  The substrate is preferably made of a heat resistant material because it is exposed to heat when the photoelectric conversion element is manufactured.

  Since solar cells are often used outdoors, the surface of the substrate may be heated by sunlight. Therefore, it is advantageous that the material is excellent in heat resistance and weather resistance.

(Light scattering layer)
The light-scattering film of the present invention has a plurality of light-scattering layers, and is composed of at least two different layers, and has a structure of three or more layers or a layer structure including an intermediate layer that does not cause scattering between the light-scattering layers. I do not care. At least the scattering film includes two light scattering layers. In each light scattering layer, a matrix resin and a scatterer are dispersed. In the present invention, when any two of the light scattering layers are selected, the side closer to the photoelectric conversion layer is defined as the second layer, and the layer farther from the photoelectric conversion layer is defined as the first layer. To do.

  The matrix resin and the scattering particles contained in the light scattering film are preferably made of a transparent material from the viewpoint of transmitting light to the photoelectric conversion element without loss and increasing the photoelectric conversion efficiency.

  The light scattering film of the present invention is provided on at least one surface of the substrate, and may be provided on both surfaces, or the substrate itself may be a light scattering layer.

  In the case where the light scattering film is on the surface layer, it is preferably composed of a material having weather resistance.

  From the viewpoint of weather resistance, the light scattering film is preferably disposed on the side of the weather resistant substrate having high planarity on which the photoelectric conversion element is formed. At that time, in order to maintain the planarity of the layer adjacent to or in the vicinity of the light scattering layer, the surface needs to be flat.

  In the present application, “the surface is flat” means that the arithmetic average roughness Ra of the surface is 10 μm or less. Ra is preferably 5 μm or less, more preferably 2 μm or less, and even more preferably 1 μm or less. When the surface of the scattering film is flat, a high photoelectric conversion efficiency is achieved with few unintended defects when producing a layer in the vicinity of the light scattering layer (for example, an upper electrode layer or a photoelectric conversion layer). be able to. The surface roughness Ra can be measured with a commercially available stylus type surface shape measuring instrument (for example, trade name “Dektak 8” manufactured by ULVAC, Inc.).

(Scattering layer transmittance)
The light-scattering film for solar cells of the present invention is a light-scattering film for solar cells having at least two light-scattering layers containing a scatterer in a matrix. (1) First light on the sunlight incident side The film surface of the scattering layer is flat. (2) The first light scattering layer has a total light transmittance of 70 incident on the first light scattering layer of light incident from the normal direction to the film surface. (3) the second light scattering layer disposed on the side farther from the sunlight incident side than the first light scattering layer is the light incident on the film surface from the normal direction. The total light transmittance in the second light scattering layer is 60% or less, and (4) the light incident on the light incident at an angle of 45 ° with respect to the normal of the film surface of the first light scattering layer. The total light transmittance of the first light scattering layer is T1 (45 °), and the light incident on the light incident at a 45 ° angle with respect to the normal of the film surface of the second light scattering layer. That the total light transmittance in the second light-scattering layer when the T2 (45 °), the T1 (45 °) and T2 (45 °) is satisfied the following formula (I), and (a The total light transmittance (through the light scattering layer of 1 and the second light scattering layer) is 70% or more, and the total light (through the second light scattering layer and the first light scattering layer) The transmittance (back) is 60% or less .
Formula (I): T1 (45 °)> T2 (45 °)
The “first surface” in the present application is a surface on the light capturing side where incident light first passes through the light scattering film, and the “second surface” means that incident light is transmitted through the light scattering film. It is a surface corresponding to the side toward the photoelectric conversion layer. The total light transmittance can be measured with a commercially available haze meter (for example, NDH2000 manufactured by Nippon Denshoku Industries Co., Ltd.) according to JIS-K7361.

  In the present invention, the total light transmittance at an incident angle of 45 ° with respect to the normal direction is a value measured according to FIG. A quartz prism having an angle of 45 ° is set on the sample so that light can be transmitted by glycerin without passing through an air layer. A collimated light source using a halogen light source is prepared on the prism side, and light is incident perpendicular to the prism surface. With respect to the light that passes through the prism and the sample in this order and is inclined toward the opposite side of the light source with respect to the sample, the amount of light is measured using an integrating sphere. In the configuration of FIG. 2, light can be transmitted through the sample at approximately 45 ° with respect to the normal direction of the film surface.

  For the calculation of the total light transmittance, the value measured with the integrating sphere in the measurement without the sample and the prism was set to 100, and a comparison was made with the relative value.

  As an achievement means for satisfying T1 (45 °)> T2 (45 °), two or more layers having different characteristics with respect to light may be stacked. Specifically, light advances by, for example, dispersing a non-spherical domain such as a plate-like particle or a rod-like particle in a matrix, or dispersing a true spherical domain in a matrix having a different three-dimensional refractive index. Optical properties that cannot be achieved with a single layer can be obtained by preparing layers having different scattering characteristics depending on the angle and forming these layers in a laminated structure.

  The mechanism of increasing the photoelectric conversion efficiency by using the light scattering film of the present invention is considered as follows. The light perpendicularly incident on the solar cell arrives with a transmittance of 70% or more. On the other hand, the vertically reflected light is relatively low and has a transmittance of 60% or less. Light incident from the outside of the solar battery cell reaches the photoelectric conversion layer with sufficiently high transmittance and is simultaneously scattered by the light scattering film, thereby increasing the optical path length in the photoelectric conversion layer and increasing the photoelectric conversion efficiency. On the other hand, the light traveling in the vertical direction out of the light reflected without being absorbed by the photoelectric conversion layer has a low transmittance in the light scattering layer, so the direction is changed again to the photoelectric conversion cell direction, or the solar battery cell In response to total reflection at the air interface, the light is led to the photoelectric conversion cell again to increase the light utilization efficiency.

  The condition for total reflection at the solar cell / air interface is determined according to Snell's law and is about 42 ° when the refractive index of the solar cell interface material is 1.5. That is, with respect to light incident within 42 °, light that is emitted from the solar cell to the outside and reaches the interface at an angle larger than that is subjected to total reflection and used again for photoelectric conversion.

  In the present invention, the expression (1) represents the relationship of light transmittance near the critical angle, and satisfying the expression (1) provides the above-mentioned light reuse effect even near the critical angle. Is shown. That is, by satisfying the conditions of the present invention, light reuse occurs in all the light emitted from the solar cell within the critical angle, and the light utilization efficiency can be increased.

  Usually, in a single-layer light scattering film, if the light scattering film is substantially flat, a difference in transmittance between the front and back surfaces cannot be achieved. In the present invention, light scattering films having different transmittances are achieved by combining two light scattering layers having different characteristics with respect to light and further defining the order.

  In the present invention, when the haze value of the second light scattering layer is Hz2 (%) and the haze value of the first light scattering layer is Hz1 (%), it is preferable that the relationship is Hz1 <Hz2.

  Haze is a measure representing the degree of scattered light in the total light transmittance. That is, the greater the haze, the higher the probability that light will travel in a direction different from the travel. Light incident on the solar cell of the light beam undergoes small scattering when passing through the first light scattering layer, but the traveling direction does not have a very wide distribution. Subsequently, since the second light scattering layer receives large scattering, the traveling direction has a wide distribution. Therefore, the incident light travels a long path length in the photoelectric conversion layer, and the light use efficiency increases.

  Next, the light within the critical angle among the light incident from the photoelectric conversion layer side, that is, the light that is emitted outside the solar battery cell will be considered. The light transmitted through the second light scattering layer receives large scattering and has a wide angular distribution. This means that the probability of maintaining the angle without being affected by the light that is going to be inclined in the vertical direction is reduced. Next, light having a large distribution is incident on the first light scattering layer. In the second light scattering layer, since the probability of non-perpendicular light increases, the optical path length when passing through the second light scattering layer becomes relatively long and the ratio of receiving large scattering increases. It is considered that the light utilization efficiency increases when more light travels outside the critical angle at the interface and is led to the photoelectric conversion layer again. When the effects of the present invention are expressed by mathematical formulas, it is considered that the effect is similar to the fact that the matrix AB is operated in the order of the matrix transformation and the same effect does not appear when the matrix BA is operated in the order of the matrix BA.

In the present invention, the difference in refractive index between the matrix and the scatterer in the plane parallel to the film surface of the matrix in the first light scattering layer is ΔN1xy, and the refraction of the matrix and turbulent body in the normal direction to the film surface When the rate difference is ΔN1z, it is preferable to satisfy the following formula (II).
Formula (II): ΔN1xy <ΔN1z
Although the refractive index may vary depending on the wavelength, it is preferable to satisfy this relationship in the entire wavelength region where the photoelectric conversion layer can convert light, but at least one wavelength in the wavelength region to be photoelectrically converted is represented by the formula (II) It is preferable to satisfy the following condition.

Note that ΔN1xy and ΔN1z in the formula (II) are defined by the following relational expressions.
ΔN1xy = | N1xy (matrix) −N1xy (domain) |
ΔN1z = | N1z (matrix) −N1z (domain) |
Here, an xy axis perpendicular to the surface of the light scattering film and a z axis perpendicular to the surface of the light scattering film are virtually set.

Where
N1xy (domain): average refractive index in the plane of the scatterer (in the xy plane) in the first light scattering layer N1xy (matrix): average refraction in the plane of the matrix in the first light scattering layer (in the xy plane) Rate N1z (domain): Average refractive index N1z (matrix) of the scatterer in the normal direction (z-axis direction) in the first light-scattering layer: In the normal direction (z-axis direction) in the first light-scattering layer Represents the average refractive index of the matrix.

  Formula (II) represents the relationship regarding the difference in refractive index between the domain and the scatterer in the first layer.

  Since light has an electric field oscillation in a direction perpendicular to the traveling direction, the light oscillating in the z-axis direction is scattered according to the refractive index difference in the xy plane of the domain and the scatterer. Further, light traveling in the xy plane (light traveling in the horizontal direction on the film surface) is scattered according to the domain in the z-axis direction and the refractive index of the scatterer. In the layer satisfying the relationship of the formula (II), ΔN1xy is smaller than ΔN1z, so that scattering with respect to light traveling in the z-axis direction (perpendicular to the film surface) is relatively light traveling in the film surface parallel direction. Is smaller than the scattering.

In the present invention, the difference in refractive index between the matrix and the scatterer in the plane parallel to the film surface of the matrix in the second light scattering layer is ΔN2xy, and the refraction of the matrix and scatterer in the direction normal to the film surface When the rate difference is ΔN2z, it is preferable to satisfy the following formula (III).

Formula (III): ΔN2xy> ΔN2z
Although the refractive index may vary depending on the wavelength, it is preferable to satisfy this relationship in the entire wavelength region where the photoelectric conversion layer can convert light, but at least one wavelength in the wavelength region to be photoelectrically converted is represented by the formula (III) It is preferable to satisfy the following condition.

Note that ΔN2xy and ΔN2z in the formula (III) are defined by the following relational expressions.
ΔN2xy = | N2xy (matrix) −N2xy (domain) |
ΔN2z = | N2z (matrix) −N2z (domain) |
Here, an xy axis perpendicular to the surface of the light scattering film and a z axis on the vertical plane of the light scattering film are virtually set.

Where
N2xy (domain): average refractive index in the plane of the scatterer (in the xy plane) in the second light scattering layer N2xy (matrix): average refraction in the plane of the matrix in the second light scattering layer (in the xy plane) Ratio N2z (domain): Average refractive index N2z (matrix) of the scatterer in the normal direction (z-axis direction) in the second light-scattering layer: In the normal direction (z-axis direction) in the second light-scattering layer Represents the average refractive index of the matrix.

  Similarly to the formula (II), the formula (III) also represents the relationship regarding the difference in the refractive index between the domain and the scatterer in the second layer.

  In the layer satisfying the relationship of the formula (III), ΔN2z is smaller than ΔN2xy, and therefore, scattering with respect to light traveling in the xy plane direction (horizontal to the film surface) is relatively light traveling in the film surface perpendicular direction. Is smaller than the scattering.

  If the light scattering layer has the first and second scattering layers satisfying the conditions of the formulas (II) and (III) of the present invention, the mechanism for increasing the light utilization efficiency works to increase the light utilization efficiency. . In addition, in the first scattering layer, since the light traveling in the direction close to the film surface direction is greatly scattered, the light traveling in the direction perpendicular to the film due to scattering when entering the second light scattering layer. It has been estimated that the probability of the increase in light efficiency has led to an improvement in the dependency of light use efficiency due to the season and time.

In the present invention, it is preferable that the formula (IV) is satisfied when the average particle diameter in a plane parallel to the film surface of the scatterer in the first light scattering layer is R1xy and the average particle diameter in the direction perpendicular to the film is R1z. .
Formula (IV): R1xy < R1z
In the scattering film, an xy axis orthogonal to each other in the film surface direction and a z axis in a direction perpendicular to the film surface are virtually set.

In formula (IV),
R1xy: Average particle size R1z of the scatterer cross section in the xy plane: Z direction length of the scatterer in a cross section obtained by cutting the scatterer in parallel to the z plane.

  Formula (IV) indicates that the average particle diameter in the cross section on the xy plane is smaller than the length in the z direction of the scatterer in the z plane.

The shape of the specific scatterers, such as ellipsoidal shape with a long axis in the z axis direction is considered. In a scattering layer having such a scatterer, scattering is small for light traveling in the z-axis direction, and scattering for light traveling in the film surface direction is large.

In the present invention, when the average particle diameter in the plane parallel to the film surface of the scatterer in the second light scattering layer is R2xy and the average particle diameter in the film vertical direction is R2z, the formula (IV) is satisfied. preferable.
Formula (V): R2xy > R2z
In the scattering film, an xy axis orthogonal to each other in the film surface direction and a z axis in a direction perpendicular to the film surface are virtually set.

In formula (V),
R2xy: Average particle size R2z of the scatterer cross section in the xy plane: Z direction length of the scatterer in a cross section obtained by cutting the scatterer in parallel to the z plane.

  The expression (V) represents that the average particle diameter in the cross section on the xy plane is larger than the z-direction length of the scatterer in the z plane.

The shape of the specific scatterers, such as ellipsoidal shape having a minor axis in the z axis direction is considered. In a scattering layer having such a scatterer, scattering is large for light traveling in the z-axis direction, and scattering for light traveling in the film surface direction is small .

(Matrix material)
In the present application, the “matrix” refers to a substrate or base (matrix) that functions as a medium containing and holding a light scatterer (domain).

  The matrix used in the present invention is preferably made of a material that is substantially transparent in the visible light region. Specifically, for example, cellulose ester resins such as triacetyl cellulose, cellulose acetate propionate, cellulose acetate butyrate, polymethyl methacrylate resin, polycarbonate resin, polystyrene resin, cycloolefin polymer, crosslinked polyethylene resin, polychlorinated resin Amorphous thermoplastic resins such as vinyl resin, polyarylate resin, polyphenylene ether resin, modified polyphenylene ether resin, polyetherimide resin, polysulfone resin, polyether ketone resin, polyethylene terephthalate, polyethylene resin, polypropylene resin, polybutylene terephthalate Crystalline thermoplastic resins such as aromatic polyester resins, polyacetal resins, and polyamide resins can be used.

  Further, an ultraviolet curable acrylic urethane resin, polyester acrylate resin, epoxy acrylate resin, polyol acrylate resin, epoxy resin, or the like can be used.

  Furthermore, it is also preferable to add nanoparticles such as titanium oxide and zirconium oxide for the purpose of adjusting the refractive index.

(Light scatterer)
Examples of the material of the light scattering particles constituting the light scattering film 2 include acrylic particles, styrene acrylic particles and cross-linked products thereof, melamine resin particles (melamine-formalin condensate particles, benzoguanamine-melamine-formaldehyde condensate particles), Examples include benzoguanamine-formaldehyde condensate particles and silicone resin particles. PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy resin), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PVDF (polyfluorovinylidene), and ETFE (ethylene-tetrafluoroethylene copolymer) Fluorine-containing polymer particles such as can be used. In addition, metal oxides such as silica, alumina, and titanium oxide can be used as the inorganic scatterer.

(Manufacturing method)
A light scattering film is prepared by preparing a solution in which scattering particles are dispersed in a matrix resin, and applying this solution to a wet coating such as spin coating, roll coating, spraying, bar coating, die coating, flow coating or dipping. It can be formed using a process. The solution can contain additives such as an organic solvent, a dispersion aid, a leveling agent, and a coupling agent. Alternatively, a UV curing or thermosetting resin may be used as the matrix resin, and the film may be cured after being formed by the above method. In the present invention, the light scattering film is composed of a plurality of light scattering layers. For the production, it is also preferable to produce a laminate by combining a plurality of methods described above.

  When the photoelectric conversion layer is formed of an amorphous silicon layer, heat treatment is performed in the manufacturing process, and when the solar cell is used outdoors, the substrate surface may be heated by sunlight. Is desirable. It is preferable to have heat resistance at 230 to 250 ° C. for 3 hours.

  Further, the light scattering film desirably has a thermal expansion coefficient close to that of the adjacent layer. For example, when Si is used as the photoelectric conversion layer, it is desirable that the light scattering film be smaller than 60 ppm / ° C.

  Furthermore, it is desirable that the light scattering film has ultraviolet resistance by adding an ultraviolet absorber or including an ultraviolet absorbing functional layer. This is because when the solar cell is used outdoors, the substrate surface is exposed to ultraviolet rays by sunlight. An ultraviolet absorption functional layer may be provided outside the light scattering film. For example, an ultraviolet absorption functional layer may be provided between the light scattering film and the substrate.

(Upper electrode)
The material for the upper electrode layer is preferably a transparent conductive oxide from the viewpoint of transmitting light to the photoelectric conversion layer without loss.

For example, indium oxide (In ₂ O ₃ ), tin oxide (SnO ₂ ), zinc oxide (ZnO), cadmium oxide (CdO), indium cadmium oxide (CdIn ₂ O ₄ ), cadmium tin oxide (Cd ₂ SnO ₄ ), Examples include zinc tin oxide (Zn ₂ SnO ₄ ) and indium zinc oxide (In—Zn—O).

  Further, impurities may be added (dope) to the transparent conductive oxide. For example, indium oxide is doped with tin (Sn), molybdenum (Mo), titanium (Ti), tin oxide is doped with antimony (Sb) and fluorine (F), zinc oxide is doped with indium, aluminum, gallium ( Ga) can be doped.

In particular, tin oxide doped with fluorine and zinc oxide doped with gallium or aluminum are preferred. This is because, in the next photoelectric conversion layer forming step, the upper electrode layer is exposed to hydrogen plasma, and therefore a material resistant to a reducing atmosphere such as F-doped SnO ₂ or Ga-doped or Al-doped ZnO is preferable.

  Examples of the method for producing the upper electrode layer include, but are not limited to, a sputtering method, a vapor deposition method, a CVD (Chemical Vapor Deposition) method, and a sol-gel method.

(Photoelectric conversion layer)
As an example of the photoelectric conversion layer, a pin type photoelectric conversion layer is shown in FIG. The photoelectric conversion layer 5 is formed by stacking a p-type thin film silicon layer 51, an i-type thin film silicon layer 52, and an n-type thin film silicon layer 53 in the order shown in FIG.

  The lower electrode layer 6 is formed by laminating a lower transparent conductive electrode 61 and a metal electrode 62 in the order shown in FIG.

  The structure in which the substrate 2, the upper electrode layer 4, the photoelectric conversion layer 5, and the lower electrode layer 6 are laminated in this order is referred to as “super straight type” in the following description.

  Either a crystalline or amorphous silicon may be used as a material constituting the photoelectric conversion layer. Alternatively, a material containing 30% or more of silicon such as crystalline and amorphous silicon carbide or silicon germanium may be used.

  For the formation of the photoelectric conversion layer, a manufacturing method such as a plasma CVD method, a photo CVD method, or a hot-wire CVD method is used. In particular, it is preferable to use a plasma CVD method. This is because the plasma CVD method can form a film with good film quality with high productivity.

  In the process of forming the photoelectric conversion layer, the p-type thin film silicon layer is first formed by doping p-type dopants such as boron, gallium, and aluminum.

Examples of the raw material used for the p-type doping include a boron source such as diborane, B (CH ₃ ) ₃ , and BF ₃ , a gallium source such as trimethyl gallium and triethyl gallium, and an aluminum source such as trimethylaluminum. Examples thereof include triethylaluminum, but are not limited to these materials.

  Further, by doping carbon when forming the p-type thin film silicon layer, the band gap of the p-type thin film silicon layer can be increased, and more light can be guided into the i layer, and more carriers can be introduced. It is good also as a structure which can produce | generate. The raw material carbon source in this case can be exemplified by methane.

  The i-type thin film silicon layer is formed without doping with a dopant after the p-type thin film silicon layer is formed.

  The n-type thin film silicon layer is formed by forming an i-type thin film silicon layer and then doping phosphorus or the like as an n-type dopant.

(Lower electrode)
As an example of the lower electrode layer, a lower transparent conductive electrode and a metal electrode are sequentially stacked.

  Examples of the method for forming the lower electrode layer include, but are not limited to, sputtering, vapor deposition, CVD (Chemical Vapor Deposition), and sol-gel.

  The material of the lower transparent conductive electrode may be any material that is conductive and made of a transparent oxide.

For example, indium oxide (In ₂ O ₃ ), tin oxide (SnO ₂ ), zinc oxide (ZnO), cadmium oxide (CdO), indium cadmium oxide (CdIn ₂ O ₄ ), cadmium tin oxide (Cd ₂ SnO ₄ ), Examples include zinc tin oxide (Zn ₂ SnO ₄ ) and indium zinc oxide (In—Zn—O).

  Further, an impurity may be added (doped) to the above oxide. For example, indium oxide is doped with tin (Sn), molybdenum (Mo), titanium (Ti), tin oxide is doped with antimony (Sb) and fluorine (F), zinc oxide is doped with indium, aluminum, gallium ( Ga) can be doped.

  The material of the metal electrode is preferably a metal such as Al, Au, Ag, Cu, Pt, or Cr, and particularly preferably Al. This is because Al has specular reflectivity and can confine light in the element and can efficiently extract carriers generated in the photoelectric conversion layer. Moreover, the alloy containing the metal mentioned above may be sufficient.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

Example 1
(Preparation of solar cells for evaluation)
A ZnO: Ga (Ga: 5 mass%) ceramic target was sputtered on a Corning 1737 glass substrate using a DC magnetron sputtering apparatus, and a gallium-doped zinc oxide transparent conductive film was formed to a thickness of 200 nm to form an upper electrode layer.

  A p-type amorphous silicon carbide layer having a thickness of 18 nm, an i-type amorphous silicon layer having a thickness of 280 nm, and an n-type amorphous silicon layer having a thickness of 25 nm are formed in this order on the gallium-doped zinc oxide transparent conductive film by a plasma CVD apparatus. A photoelectric conversion layer was formed.

  Using a DC magnetron sputtering apparatus, a ZnO: Ga (Ga: 5 mass%) ceramic target was sputtered to form a gallium-doped zinc oxide transparent conductive film having a film thickness of 15 nm to form a lower transparent electrode.

  Finally, a metal electrode made of aluminum having a film thickness of 100 nm is formed on the n-type amorphous silicon layer by a vacuum deposition apparatus, and a lower electrode made of a lower transparent electrode and a metal electrode is formed. A solar cell for evaluation was produced.

(Production of light scattering film)
(Preparation of light scattering layer 1)
Using PEN (average refractive index 1.64 Tg = 120 ° C.) as a matrix resin and spherical alumina (average refractive index 1.74 particle size 500 nm) as a scatterer, a film was formed by a melt extrusion method, and sequentially biaxial. The light scattering layer 1 was prepared by adjusting the birefringence of the matrix resin by the stretching method. The temperature during biaxial stretching is set to 110 ° C. and is 1. for each of the melt film forming direction (hereinafter also referred to as MD direction) and the direction perpendicular to the film forming direction (hereinafter also referred to as TD direction). Set to 5 times. The finished film thickness was 10 μm, and the amount of scatterers mixed in the matrix resin (100% by mass) was set to 2%.

  The refractive index of the obtained matrix resin is (Nx, Ny, Nz) = (1.69, 1.69, 1.54) when the x and y axes are in the film plane direction and the film thickness direction is the z axis. there were.

  The adjustment of the refractive index of the matrix resin can be adjusted by using the fact that it increases in the stretching direction by increasing the stretching ratio and decreases in the non-stretching direction. Further, as the temperature during stretching is lower than Tg, the change in refractive index at the same stretch ratio tends to increase, and these can be adjusted in combination. Further, by performing annealing after the end of stretching, the refractive index can be made isotropic, and voids generated in the vicinity of the scatterer can be eliminated by performing this treatment.

(Preparation of light scattering layer 2)
Using PEN (average refractive index 1.64 Tg = 120 ° C.) as a matrix resin and spherical cross-linked polystyrene particles (average refractive index 1.59, particle size 1 μm) as a scatterer, light is obtained by melt extrusion and sequential biaxial stretching. The scattering layer 2 was produced. The temperature during biaxial stretching is set to 110 ° C. and is 1. for each of the melt film forming direction (hereinafter also referred to as MD direction) and the direction perpendicular to the film forming direction (hereinafter also referred to as TD direction). Set to 5 times. The finished film thickness was 10 μm, and the amount of scatterers mixed in the matrix resin (100 mass%) was set to 2.5 mass%.

  The refractive index of the obtained matrix resin is (Nx, Ny, Nz) = (1.69, 1.69, 1.54) when the x and y axes are in the film plane direction and the film thickness direction is the z axis. there were.

(Preparation of light scattering film) (Sample 1)
The light scattering layers 1 and 2 were adhered with a transparent double-sided tape (film thickness: 5 μm, Sekisui Chemical Co., Ltd. # 5511) to obtain a laminated solar cell light scattering film (Sample 1).

(Preparation of solar cells for evaluation)
The solar cell for evaluation was produced by adhering to the glass substrate of the light scattering film sample using the transparent double-sided tape.

  Samples 2 to 5 were prepared using the same method as described above.

(Evaluation method)
(Measurement of total light transmittance)
The total light transmittance was measured using a haze meter (Nippon Denshoku Industries Co., Ltd. NDH2000) according to JIS-K7361.

  In the measurement, the light scattering layer is front (measured with the light entering the solar cell in the same direction as the light from the outside), and back (light from the solar cell to the outside). The measurement was performed twice in such a manner that the light is incident and inclined in the same direction as the light is inclined.

  The haze values obtained through this measurement were set to Hz1 and Hz2, respectively.

(Measurement of haze value)
For the measurement of haze, it was measured using a haze meter (Nippon Denshoku Industries Co., Ltd. NDH2000) according to JIS-K7361. The light scattering film is composed of a plurality of light scattering films. When light travels from the outside of the solar battery cell to the photoelectric conversion layer, the haze value of the layer on the side close to the outside is Hz1, and the layer on the side close to the photoelectric conversion layer The haze value was set to Hz2.

(Measurement of oblique incident total light transmittance (T1 (45 °), T2 (45 °)))
A collimated light source using a halogen lamp as a light source, an isosceles triangular quartz prism having angles of 45 °, 45 °, and 90 °, grease, a sample, and an integrating sphere were prepared.

As shown in FIG. 2 , a measurement system was prepared so that the collimated light was transmitted in the order of prism, grease, sample, and integrating sphere. The light was set so that it was incident from the direction perpendicular to the prism surface when entering the prism, and the side of the prism sandwiched at an angle of 45 ° was directed to the sample surface. In order to prevent an air layer from entering between the prism and the sample, it was made to adhere with grease.

  The intensity of light obliquely incident on the integrating sphere from the light source in the absence of prism, grease, and sample was set to 100, and T1 (45 °) and T2 (45 °) of the sample were measured and compared based on this.

  Here, T1 (45 °) is a measurement value when the sample is arranged so that light enters the photoelectric conversion layer from the outside of the solar cell, and T2 (45 °) is the photoelectric conversion layer side of the solar cell. It is a measured value in the direction where light goes from the solar cell to the outside of the solar cell.

(Measurement of flatness)
Using a stylus type surface shape measuring instrument (Dektak 8 manufactured by ULVAC, Inc.), the surface shape was measured to determine Ra. When the Ra on the front and back of the sample were both 10 μm or less, it was assumed to be substantially flat.

(Measurement of light utilization efficiency)
In an environment of 25 ° C., conversion efficiency was measured with a solar simulator under conditions of AM 1.5 and 100 mW / cm ² .

(Measurement of light utilization efficiency at oblique incidence)
In the measurement system for the light utilization efficiency, the conversion efficiency was measured by measuring the solar cell in a state where it was inclined by 45 °.

  The results are summarized in Table 1 for the obtained samples 1 to 5.

  As is clear from the results in Table 1, Samples 1 and 2 of the present invention have high conversion efficiency and good results under any conditions. The light scattering layer was composed of a plurality of layers, and the light utilization efficiency was satisfactory by adjusting the transmittance and haze to the desired ranges. Sample 3 (comparative example) had a low conversion efficiency when incident from the direction of 45 ° with respect to the normal of the light scattering layer.

(Example 2)
(Preparation of light scattering layer 1)
Using PEN (average refractive index 1.64 Tg = 120 ° C.) as a matrix resin and spherical alumina (average refractive index 1.74 particle size 500 nm) as a scatterer, a film was formed by a melt extrusion method, and sequentially biaxial. The light scattering layer 1 was prepared by adjusting the birefringence of the matrix resin by the stretching method. The temperature during biaxial stretching is set to 110 ° C., and 1 for each of the melt film forming direction (hereinafter also referred to as MD direction) and the direction perpendicular to the film forming direction (hereinafter also referred to as TD direction). Set to 5 times. The finished film thickness was 10 μm, and the amount of scatterers mixed in the matrix resin (100 mass%) was set to 1.5%.

  The refractive index of the obtained matrix resin is (Nx, Ny, Nz) = (1.69, 1.69, 1.54) when the x and y axes are in the film plane direction and the film thickness direction is the z axis. there were.

  The adjustment of the refractive index of the matrix resin can be adjusted by using the fact that it increases in the stretching direction by increasing the stretching ratio and decreases in the non-stretching direction. Further, as the temperature during stretching is lower than Tg, the change in refractive index at the same stretch ratio tends to increase, and these can be adjusted in combination. Further, by performing annealing after the end of stretching, the refractive index can be made isotropic, and voids generated in the vicinity of the scatterer can be eliminated by performing this treatment.

(Preparation of light scattering layer 2)
Using PEN (average refractive index 1.64 Tg = 120 ° C.) as a matrix resin and spherical cross-linked polystyrene particles (average refractive index 1.59, particle size 1 μm) as a scatterer, light is obtained by melt extrusion and sequential biaxial stretching. The scattering layer 2 was produced. The temperature during biaxial stretching is set to 110 ° C. and is 1. for each of the melt film forming direction (hereinafter also referred to as MD direction) and the direction perpendicular to the film forming direction (hereinafter also referred to as TD direction). Set to 5 times. The finished film thickness was 10 μm, and the amount of scatterers mixed with the matrix resin (100 mass%) was set to 3.2 mass%.

  The refractive index of the obtained matrix resin is (Nx, Ny, Nz) = (1.69, 1.69, 1.54) when the x and y axes are in the film plane direction and the film thickness direction is the z axis. there were.

(Preparation of light scattering film (sample 6))
The light scattering layers 1 and 2 were adhered with a transparent double-sided tape (film thickness: 5 μm, Sekisui Chemical Co., Ltd. # 5511) to obtain a laminated solar cell light scattering film (sample 6).

  Table 2 shows the evaluation results of the obtained sample 6 and the sample 1 of Example 1.

  Sample 6 in which the haze value of each layer was adjusted by adjusting the amount of scatterers in the light scattering layer showed good light conversion efficiency.

(Example 3)
(Production of light scattering film)
(Preparation of light scattering layer 1)
Using PEN (average refractive index 1.64 Tg = 120 ° C.) as a matrix resin and spherical alumina (average refractive index 1.74 particle size 600 nm) as a scatterer, a film was formed by a melt extrusion method, and sequentially biaxial. The light scattering layer 1 was prepared by adjusting the birefringence of the matrix resin by the stretching method. The temperature during biaxial stretching is set to 110 ° C. and is 1. for each of the melt film forming direction (hereinafter also referred to as MD direction) and the direction perpendicular to the film forming direction (hereinafter also referred to as TD direction). Set to 5 times. The finished film thickness was 10 μm, and the amount of scatterers mixed in the matrix resin (100% by mass) was set to 2.2%.

  The refractive index of the obtained matrix resin is (Nx, Ny, Nz) = (1.69, 1.69, 1.54) when the x and y axes are in the film plane direction and the film thickness direction is the z axis. there were.

  The adjustment of the refractive index of the matrix resin can be adjusted by using the fact that it increases in the stretching direction by increasing the stretching ratio and decreases in the non-stretching direction. Further, as the temperature during stretching is lower than Tg, the change in refractive index at the same stretch ratio tends to increase, and these can be adjusted in combination. Further, by performing annealing after the end of stretching, the refractive index can be made isotropic, and voids generated in the vicinity of the scatterer can be eliminated by performing this treatment.

(Preparation of light scattering layer 2)
Using PEN (average refractive index 1.64 Tg = 120 ° C.) as a matrix resin and spherical cross-linked polystyrene particles (average refractive index 1.59, particle size 1 μm) as a scatterer, light is obtained by melt extrusion and sequential biaxial stretching. The scattering layer 2 was produced. The temperature during biaxial stretching is set to 110 ° C. and is 1. for each of the melt film forming direction (hereinafter also referred to as MD direction) and the direction perpendicular to the film forming direction (hereinafter also referred to as TD direction). Set to 5 times. The finished film thickness was 10 μm, and the amount of scatterers mixed in the matrix resin (100 mass%) was set to 2.5 mass%.

  The refractive index of the obtained matrix resin is (Nx, Ny, Nz) = (1.69, 1.69, 1.54) when the x and y axes are in the film plane direction and the film thickness direction is the z axis. there were.

(Preparation of light scattering film (sample 12))
The light scattering layers 1 and 2 were bonded with a transparent double-sided tape (film thickness: 5 μm, # 5511 manufactured by Sekisui Chemical Co., Ltd.) to obtain a laminated solar cell light scattering film (sample 12).

(Preparation of light scattering film (sample 11))
When the sample 12 was prepared, the light scattering film (sample 11) was formed using the scattering layer which was set to a finished film thickness of 10 μm without performing sequential biaxial stretching of the second light scattering layer and further annealed at 130 ° C. Obtained. The matrix resin at this time did not have anisotropy (Nx, Ny, Nz) = (1.64, 1.64, 1.64).

(Preparation of light scattering film (sample 10))
When preparing the sample 12, the finished film thickness is set to 10 μm without performing sequential biaxial stretching of the first light scattering layer, and the light scattering film (sample 10) is formed using the scattering layer annealed at 130 ° C. Obtained. The matrix resin at this time did not have anisotropy (Nx, Ny, Nz) = (1.64, 1.64, 1.64).

(Preparation of light scattering film (sample 9))
When the sample 12 was produced, the first and second light scattering layers were not sequentially biaxially stretched, the finished film thickness was set to 10 μm, and a light scattering film (sample) was formed using a scattering layer annealed at 130 ° C. 9) was obtained. The matrix resin at this time did not have anisotropy (Nx, Ny, Nz) = (1.64, 1.64, 1.64).

  The evaluation results of Samples 9 to 12 are summarized in Table 3.

  Sample light scattering films 10 to 12 of the invention achieved good light conversion efficiency. In particular, since the sample 12 satisfactorily exhibited the optical confinement function of the present invention in relation to the refractive index, the result was satisfactory at both the front and oblique incidence angles.

Example 4
(Production of light scattering film)
(Preparation of light scattering layer 2)
Polystyrene (average refractive index 1.7 Tg = 95 ° C.) increased in refractive index by mixing titanium oxide nanoparticles as a matrix resin, and disc-shaped alumina particles (average refractive index 1.74 thickness (R2z) = 60 nm as scatterers) Using the diameter (R2xy) = 300 nm, a film was formed by the melt extrusion method, and the birefringence of the matrix resin was adjusted by the sequential biaxial stretching method to prepare the light scattering layer 2. The temperature during biaxial stretching is set to 82 ° C., and is 1. for each of the melt film forming direction (hereinafter also referred to as MD direction) and the direction perpendicular to the film forming direction (hereinafter also referred to as TD direction). Set to 4 times. The finished film thickness was 10 μm, and the amount of scatterers mixed in the matrix resin (100 mass%) was set to 3.3%.

  The refractive index of the obtained matrix resin is (Nx, Ny, Nz) = (1.69, 1.69, 1.72) when the x and y axes are in the film plane direction and the film thickness direction is the z axis. there were.

  The adjustment of the average refractive index of the matrix resin is determined by the relationship between the volume% ratio of the titanium oxide nanoparticles and the polystyrene resin and the refractive index. Polystyrene resin is a resin that develops negative birefringence. When stretching is performed, the refractive index increases in the direction perpendicular to the stretching direction, and the refractive index in the stretching direction can be adjusted by using the decrease. Further, as the temperature during stretching is lower than Tg, the change in refractive index at the same stretch ratio tends to increase, and these can be adjusted in combination. Further, by performing annealing after the end of stretching, the refractive index can be made isotropic, and voids generated in the vicinity of the scatterer can be eliminated by performing this treatment.

(Preparation of light scattering film (sample 13))
On the light scattering layer 2 obtained above, a mixture of a UV curable resin (refractive index 1.58) and acicular calcium carbonate was applied and then oriented vertically by a magnetic field, followed by curing with ultraviolet rays. The film thickness of the light scattering film (sample 13) was set to 12 μm. The acicular calcium carbonate is (Nxy, Nz) = (1.68, 1.53) and (R 1 xy, R 1 z) = (0.5 μm, 2 μm).

  The light scattering film (sample 13) obtained by the above method was attached to a solar cell with a transparent double-sided adhesive tape in the same manner as in Example 1 and evaluated. Moreover, the result of having performed evaluation by reversing the front and back of the bonding of the sample 13 was evaluated as a sample 14.

  The results are shown in Table 4.

  As is clear from Table 4, the light scattering layer of the present invention satisfying the formulas (IV) and (V) showed good efficiency.

  Based on the above results, the means of the present invention can provide a solar cell light-scattering film having high light use efficiency and improved light use efficiency dependency depending on the incident angle, and a solar cell using the same. I understand.

1 Solar cell 2 Substrate (transparent substrate)
2a Light extraction surface 2b Surface opposite to light capturing surface 2a 3 Light scattering film (light scattering film for solar cell)
3A First light scattering layer 3B Second light scattering layer 3a Flat surface 4 Upper electrode layer 5 Photoelectric conversion layer 6 Lower electrode layer 7 Prism 8 Sample 9 Integration (sphere)
23 Optical member (Optical member for solar cell)
51 p-type thin film silicon layer 52 i-type thin film silicon layer 53 n-type thin film silicon layer 61 lower transparent conductive electrode 62 metal electrode P light

Claims (8)

A light scattering film for a solar cell having at least two light scattering layers each containing a scatterer in a matrix, wherein (1) the film surface of the first light scattering layer on the sunlight incident side is flat; 2) The first light scattering layer has a total light transmittance at the first light scattering layer of light incident from the normal direction to the film surface of 70% or more, and (3) the first light scattering layer. The second light scattering layer installed on the side far from the sunlight incident side with respect to the light scattering layer of the first light transmission layer transmits the light incident on the film surface from the normal direction in the second light scattering layer. And (4) total light transmission in the first light scattering layer with respect to light incident at an angle of 45 ° with respect to the normal of the film surface of the first light scattering layer. T1 (45 °), the total light in the second light scattering layer with respect to light incident at an angle of 45 ° with respect to the normal of the film surface of the second light scattering layer The transmittance is taken as T2 (45 °), the T1 (45 °) and T2 (45 °) is less than the following formula (I), the AND (first light-scattering layer and the second light-scattering layer The total light transmittance (through) is 70% or more, and the total light transmittance (through the second light scattering layer and the first light scattering layer) is 60% or less. A light scattering film for solar cells.
Formula (I): T1 (45 °)> T2 (45 °) The scattering film for a solar cell according to claim 1, wherein the scatterer in the matrix has a refractive index different from that of the matrix. In any two light scattering layers constituting the light scattering film for solar cell, the haze value of the first light scattering layer disposed at a position far from the photoelectric conversion layer is Hz1 (%), a position close to the photoelectric conversion layer 3. The haze value of the second light-scattering layer disposed at is 2 (%), and the Hz1 and Hz2 satisfy a relationship of Hz1 <Hz2. Light scattering film for solar cells. ΔN1xy is the refractive index difference between the matrix and the scatterer in a plane parallel to the film surface of the matrix in the first light scattering layer, and the refractive index difference between the matrix and the scatterer in the normal direction to the film surface. ΔN1z is a difference in refractive index between the matrix and the scatterer in a plane parallel to the film surface of the matrix in the second light scattering layer, and ΔN2xy is a difference in refractive index between the matrix and the scatterer in the normal direction to the film surface. The light scattering film for solar cell according to any one of claims 1 to 3, wherein the following formulas (II) and (III) are satisfied when ΔN2z is set.
Formula (II): ΔN1xy <ΔN1z
Formula (III): ΔN2xy> ΔN2z
[However, ΔN1xy, ΔN2xy, ΔN1z, and ΔN2z in the formulas (II) and (III) are defined by the following relational expressions.
ΔN1xy = | N1xy (matrix) −N1xy (domain) |
ΔN2xy = | N2xy (matrix) −N2xy (domain) |
ΔN1z = | N1z (matrix) −N1z (domain) |
ΔN2z = | N2z (matrix) −N2z (domain) |
here,
N1xy (domain): average refractive index in the plane of the scatterer in the first light scattering layer
N2xy (domain): average refractive index in the plane of the scatterer in the second light scattering layer
N1xy (matrix): average refractive index in the plane of the matrix in the first light scattering layer
N2xy (matrix): average refractive index in the plane of the matrix in the second light scattering layer
N1z (domain): average refractive index of the scatterer in the normal direction in the first light scattering layer
N2z (domain): average refractive index of the scatterer in the normal direction in the second light scattering layer
N1z (matrix): average refractive index of the matrix in the normal direction in the first light scattering layer
N2z (matrix): average refractive index of the matrix in the normal direction in the second light scattering layer] The light scattering film for solar cell according to claim 4, wherein the ΔN1xy is smaller than ΔN1z. The average particle diameter in a plane parallel to the film surface of the scatterer in the first light scattering layer is R1xy, the average particle diameter in the direction perpendicular to the film surface is R1z, and the scattering in the second light scattering layer. R2xy is an average particle diameter in a plane parallel to the film surface of the body, and R2z is an average particle diameter in a direction perpendicular to the film surface, and R1xy, R1z, R2xy, and R2z are expressed by the following formula (IV ) And (V) are satisfied. The light scattering film for solar cells according to any one of claims 1 to 4, wherein:
Formula (IV): R1xy <R1z
Formula (V): R2xy> R2z The light scattering film for a solar cell according to claim 6, wherein the scatterer in the first light scattering layer has a needle shape, and the scatterer in the second light scattering layer has a disk shape. . It is a solar cell which has a photoelectric converting layer, Comprising: The light-scattering film for solar cells as described in any one of Claim 1 to 7 is arrange | positioned to the surface at the side where the light injects at least of the said photoelectric converting layer. A solar cell characterized by

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