JP2002280590A - Multi-junction type thin-film solar battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-efficiency multi-junction type thin-film solar battery including a crystalline semiconductor layer which has a sufficient optical confinement effect and is reduced in defective density. SOLUTION: The multi-junction type thin-film solar battery comprises at least a first transparent conductive layer, an amorphous or crystalline silicon photoelectric transfer layer, a second transparent conductive layer, and a crystalline silicon photoelectric transfer layer, which are stack on a substrate in this order. The amorphous silicon photoelectric transfer layer-side surface of the first transparent conductive layer is formed of a plurality of holes each of which the surface has an unevenness, and the crystalline silicon photoelectric transfer layer-side surface of the second transparent conductive layer is formed of a plurality of holes each of which the surface has an unevenness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a multi-junction thin-film solar cell and a method of manufacturing the same, and more particularly, to a multi-junction thin-film solar cell having high photoelectric conversion efficiency and a method of manufacturing the same.

[0002]

2. Description of the Related Art Solar cells are attracting attention as an alternative energy source for fossil fuels such as petroleum, which is concerned about future supply and demand and has a problem of carbon dioxide emission causing global warming. In this solar cell, a pn junction is used for a photoelectric conversion layer that converts light energy into electric power, and silicon is generally most often used as a semiconductor constituting the pn junction. Among silicon, single crystal silicon is preferably used in terms of photoelectric conversion efficiency. However, there are problems of raw material supply, large area, and low cost.

On the other hand, a thin film solar cell using amorphous silicon as a photoelectric conversion layer has been put to practical use as a material advantageous for realizing a large area and a low cost. Furthermore, in order to realize a solar cell that has both high and stable photoelectric conversion efficiency at the level of a single-crystal silicon solar cell and large area and low cost at the level of an amorphous silicon solar cell, a photoelectric conversion layer of crystalline silicon is required. It is being considered for use. In particular, a thin-film solar cell (hereinafter, referred to as a crystalline silicon thin-film solar cell) formed of a crystalline silicon thin film using a thin-film forming technique by a chemical vapor deposition method (hereinafter, referred to as a CVD method) similar to the case of amorphous silicon. Do) is attracting attention.

For example, Japanese Unexamined Patent Publication No. Hei 1-289173 discloses that a photoelectric conversion element using amorphous silicon as an active layer and a photoelectric conversion element using polycrystalline silicon having an energy gap smaller than that of amorphous silicon as an active layer are laminated. A multi-junction thin-film solar cell formed by such a method has been proposed. Such a thin-film solar cell adopts a configuration in which sunlight is incident from the photoelectric conversion element side using amorphous silicon as an active layer, so that solar energy can be used more efficiently than a single-junction type. There is an advantage. Further, by connecting a plurality of photoelectric conversion elements in series, a high open-circuit voltage can be obtained, and the amorphous silicon layer as an active layer can be thinned, so that a Steebler-Wronsk
The deterioration with time of the photoelectric conversion efficiency caused by the i effect can be suppressed,
There are various advantages that an amorphous silicon layer and a crystalline silicon layer can be manufactured by the same apparatus, and research and development are being actively conducted as means for achieving both high efficiency and low cost.

Incidentally, one of the important technologies for realizing a highly efficient thin-film solar cell is a light confinement effect. The light confinement effect means that the surface of the transparent conductive layer or metal layer in contact with the photoelectric conversion layer is made uneven, and light is scattered at the interface to extend the optical path length and increase the amount of light absorbed by the photoelectric conversion layer. It is to let.

For example, Japanese Patent No. 1681183 or Japanese Patent No. 2862174 discloses a highly efficient solar cell substrate by defining the particle size and the size of irregularities of a transparent conductive layer formed on a glass substrate. It is stated that it can be obtained.

The improvement in photoelectric conversion efficiency due to the light confinement effect is as follows.
It has an effect of reducing the thickness of the photoelectric conversion layer. This allows
In particular, in the case of an amorphous silicon solar cell, light degradation can be suppressed. Also, several μ to several tens of times larger than amorphous silicon due to light absorption characteristics.
In the case of a crystalline silicon solar cell requiring a thickness on the order of m, the film formation time can be significantly reduced. That is, it is possible to improve all of the high efficiency, stabilization, and cost reduction, which are major issues for practical use of a thin film solar cell, by optical confinement.

[0008]

However, the photoelectric conversion efficiency of the crystalline silicon thin-film solar cell has been limited to the photoelectric conversion efficiency of the amorphous silicon solar cell. Only at the same level as efficiency.

H. Yamamoto et al., PVSEC-
11, Sapporo, Japan, 1999, when a microcrystalline silicon layer is formed by a plasma CVD method on an Asahi-U substrate in which tin oxide having microscopic unevenness is laminated on a glass substrate surface, the surface of the tin oxide is It was reported that silicon crystal grains grow preferentially in the vertical direction, and a large number of defects occur when crystal grains having different crystal orientations grown from different uneven surfaces collide with each other. Such a defect becomes a recombination center of carriers, and thus significantly degrades the photoelectric conversion efficiency.

[0010] Also, H. Yamamoto et al. Reported that when zinc oxide was further thickened on tin oxide having surface irregularities to reduce the degree of irregularities, silicon crystal grains in a direction perpendicular to the surface of zinc oxide were similar to the case of tin oxide. It was also reported that the crystal grains that grew from different uneven surfaces due to growth collided with each other, but because the orientation difference between them was small, the number of generated defects was reduced. In other words, it is clear that the defects in the crystalline silicon thin film can be reduced by reducing the surface irregularities of the substrate as much as possible.

Eliminating or reducing surface irregularities in this way has a contradictory result with respect to techniques for obtaining highly efficient thin-film solar cells by improving the light confinement effect.

Further, Japanese Patent Application Laid-Open No. H10-117006,
JP-A-10-294481, JP-A-11-214
No. 728, JP-A-11-266027, JP-A-2000-58892, and the like, a lower photoelectric conversion element having a photoelectric conversion layer made of a crystalline silicon layer on a back electrode having an uneven surface is formed. A multi-junction thin-film solar cell has been proposed in which this crystalline silicon layer has a (110) preferred crystal orientation plane parallel to the substrate surface. However, all of these solar cells have a substrate-type element structure in which light enters from the photoelectric conversion element side.In a superstrate-type element structure in which light enters from the substrate side using a transparent substrate, a crystalline silicon thin film is used. A suitable concavo-convex structure that achieves both a reduction in defect density and a light confinement effect has not been found.

The present invention has been made in view of the above problems, and provides a highly efficient multi-junction thin-film solar cell having a crystalline semiconductor layer with a reduced defect density while having a sufficient light confinement effect. The purpose is to do.

[0014]

According to the present invention, at least a first transparent conductive layer, an amorphous or crystalline photoelectric conversion layer, a second transparent conductive layer, and a crystalline photoelectric conversion layer are formed on a substrate in this order. A plurality of holes are formed on the surface of the first transparent conductive layer on the amorphous or crystalline photoelectric conversion layer side and on the surface of the second transparent conductive layer on the crystalline photoelectric conversion layer side, respectively. And a multi-junction thin-film solar cell having irregularities formed on the surface of the hole. According to the present invention, in manufacturing the thin-film solar cell, the surface of the substrate and / or the first transparent conductive layer and / or the surface of the second transparent conductive layer is etched, or the first transparent conductive layer and / or A method for manufacturing a thin-film solar cell in which a plurality of holes are formed on the surfaces of the first transparent conductive layer and the second transparent conductive layer by forming the second transparent conductive layer so that holes are formed on the surface thereof is provided. Provided.

[0015]

DETAILED DESCRIPTION OF THE INVENTION The multi-junction thin-film solar cell of the present invention comprises at least a first transparent conductive layer, an amorphous or crystalline photoelectric conversion layer, a second transparent conductive layer and a crystalline photoelectric conversion layer on a substrate. Are stacked in this order. Note that a third transparent conductive layer and a crystalline photoelectric conversion layer, a fourth transparent conductive layer and a crystalline photoelectric conversion layer, and the like are further laminated on the second transparent conductive layer and the crystalline photoelectric conversion layer. You may.

The substrate that can be used in the solar cell of the present invention is not particularly limited as long as it supports and reinforces the entire solar cell, and further has heat resistance (for example, about 200 ° C.). Is preferred. Further, those which can be used for a superstrate type solar cell are preferable. For example, glass; polyimide, PET, PEN,
A heat-resistant polymer film such as PES or Teflon (registered trademark); a metal such as stainless steel (SUS) or an aluminum plate, a ceramic, or the like can be used alone or in a laminated structure. Especially, it is preferable that it has heat resistance and is transparent. It is appropriate that these substrates have a thickness having appropriate strength and weight, for example, a thickness of about 0.1 to 30 mm. In addition, an insulating film, a conductive film, a buffer layer, or the like, or a combination thereof may be formed according to the usage of the substrate.

The substrate has a surface, preferably a first substrate, which will be described later.
A plurality of holes may be formed on the transparent conductive layer side. Also, irregularities may be formed on the surface of this hole. The shape and size of these holes and unevenness are preferably set so that holes and unevenness can be generated on the photoelectric conversion layer side of the first and / or second transparent conductive layer as described later. It can be appropriately adjusted according to the thickness, material, and the like of the first and second transparent conductive layers described later. Specifically, a substantially cuboid, a rectangular parallelepiped, a cylinder, a cone, a sphere, a hemisphere or the like or a composite shape thereof are mentioned, and the diameter of the hole is 200 to 20.
About 00 nm, the depth of the hole is about 50 to 1200 nm, and the height difference of the unevenness is about 10 to 300 nm.

The unevenness may be formed on the surface of the substrate other than the holes where the holes are formed. In this case, the height difference of the unevenness is about 10 to 300 nm. In the above,
The ratio of the depth of the hole to the diameter of the hole and the ratio of the size of the unevenness to the distance between the unevenness are, for example, about 0.05 to 3, preferably about 0.1 to 2. From another viewpoint, the number density of the holes is about 0.1 to 5 holes / μm ² , preferably about 0.5 to ² holes / μm ² . The method for forming holes or irregularities on the surface of the substrate can be performed in the same manner as the method for forming holes or irregularities on the surface of the transparent conductive layer, as described later. Among them, the processing by the sand blast method is appropriate.

The first transparent conductive layer formed on the substrate is not particularly limited, and may be, for example, SnO ₂ ,
It can be formed of a single layer or a laminated layer of a transparent conductive material such as InO ₃ , ZnO, and ITO. Among them, ZnO having high plasma resistance is preferable. The first transparent conductive layer may contain an impurity from the viewpoint of reducing the resistivity. The impurities in this case include group III elements such as gallium and aluminum. The concentration is, for example,
5 × 10 ^{20 to} 5 × 10 ²¹ ions / cm ³ . The thickness of the first transparent conductive layer is, for example, about 0.1 nm to 2 μm. These can be formed on a substrate by a sputtering method, a vacuum evaporation method, an EB evaporation method, a normal pressure CVD method, a low pressure CVD method, a sol-gel method, an electrodeposition method, or the like. Among them, the sputtering method is preferred because the transmittance and resistivity of the transparent conductive layer can be easily controlled to those suitable for a multi-junction thin-film solar cell.

The first transparent conductive layer has a plurality of holes formed on the surface on the side of the photoelectric conversion layer described later. Further, it is preferable that unevenness is further formed on the surface of the hole. These holes and irregularities are not particularly limited as long as the number, size, shape, depth, and the like can generate a state of scattering or reflection of sunlight suitable for the light absorption characteristics of the photoelectric conversion layer. For example, a material that can produce a sufficient light scattering effect not only at a middle wavelength of about 450 to 650 nm in the center of the solar spectrum but also at a longer wavelength (for example, about 700 to 1200 nm) is preferable. . Specifically, a substantially cubic, rectangular parallelepiped, cylinder, cone,
A sphere, a hemisphere or the like or a composite shape thereof may be mentioned, and the diameter of the hole is about 200 to 2000 nm and the depth of the hole is
About 0 nm, and the height difference of the unevenness is about 10 to 300 nm. In the case where both holes and unevenness are formed on the surface of the first transparent conductive layer, a sufficient state of scattering or reflecting sunlight can be obtained, and furthermore, light absorption characteristics with high photoelectric conversion efficiency and defect density can be obtained. This is more preferable because both reductions can be achieved.

The surface of the first transparent conductive layer on which the holes are formed may have irregularities on the surface other than the holes. In this case, the height difference of the unevenness is about 10 to 300 nm. In the above, the ratio of the depth of the hole to the diameter of the hole and the ratio of the unevenness size to the unevenness interval are, for example, 0.05 to
About 3 and preferably about 0.1 to 2 are mentioned. From another viewpoint, the number density of holes is 0.1 to 5 holes / μ.
m ² , preferably about 0.5 to 2 pieces / μm ² .

The transparent conductive layer is preferably oriented with respect to the substrate surface. Here, being oriented with respect to the substrate surface means that the diffraction peak for a specific surface in X-ray diffraction is 60% or more, more preferably 70% or more, of the sum of the integrated intensities of all diffraction peaks. I do. This eliminates the influence of plane orientation dependence due to processing such as etching on the transparent conductive layer, and makes uniform the holes and irregularities formed on the surface of the transparent conductive layer on the side of the photoelectric conversion layer. Can be made uniform.
Further, with the improvement of the orientation, the depth of the hole formed after the etching treatment is increased, so that long-wavelength light having a wavelength of 700 nm or more can be effectively absorbed. For example, when zinc oxide is used for the transparent conductive layer, the integrated intensity of the (0001) diffraction peak obtained by the X-ray diffraction method is 70% or more of the sum of the integrated intensity of all diffraction peaks. Is more preferable in terms of electrical and optical characteristics.

As a method of forming the plurality of holes and the irregularities on the surface of the first transparent conductive layer as described above, a chemical method of treating the surface of the first transparent conductive layer with an etchant or the like,
Physical method of irradiating the surface of the transparent conductive layer with ions, plasma, and the like, and furthermore, a method of setting conditions for producing the transparent conductive layer by a magnetron sputtering method so that holes and irregularities are naturally generated, as described above. As described above, there are various methods such as a method of generating holes and irregularities in the first transparent conductive layer formed thereon by performing etching or machining on the substrate itself.

The chemical method is preferable because the surface shape of the transparent conductive layer can be easily controlled by appropriately changing the type, concentration, etching time and the like of the etchant. In the case of treatment with an etchant, a method of immersing the first transparent conductive layer in the etchant is preferable in consideration of reduction in manufacturing cost. Examples of the etchant include one or a mixture of two or more acid solutions such as hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, acetic acid, formic acid, hydrogen peroxide, and perchloric acid. Among them, hydrochloric acid and acetic acid are preferable. These solutions can be used, for example, at a concentration of about 0.05 to 5% by weight. In particular, in the case of a relatively weak acid such as acetic acid, it is preferable to use a concentration of about 0.15 to 5% by weight. Also, as an alkaline solution, sodium hydroxide, ammonia, potassium hydroxide, calcium hydroxide,
One type or a mixture of two or more types such as sodium hydroxide is exemplified. Of these, sodium hydroxide is preferred. These solutions are preferably used at a concentration of about 1 to 10% by weight. In the physical method, for example, holes or irregularities can be formed on the surface of the first transparent conductive layer by controlling the type of ion or plasma, generation method, kinetic energy, and the like.

Further, in the method of controlling the film forming conditions, the film forming conditions are controlled, for example, as described in Japanese Patent No. 28620174. Thereby, the crystal grain size and orientation of the first transparent conductive layer can be controlled, and holes or irregularities can be formed on the surface of the first transparent conductive layer.
In a method of forming irregularities on a substrate, for example, Japanese Patent Application Laid-Open
No. 70294 discloses a sand blast method. The above method may be performed alone or in combination of two or more.

The amorphous or crystalline photoelectric conversion layer formed on the first transparent conductive layer is usually formed by a pin junction. Here, "amorphous" means a crystalline component called amorphous unless otherwise noted, and "crystalline" means, in addition to polycrystal and single crystal, unless otherwise noted. And all crystal states such as crystal components called so-called microcrystals or microcrystals. In the photoelectric conversion layer, at least the i-layer may be entirely crystalline, but it is preferable that at least the i-layer is entirely amorphous. i layer, in particular, the material constituting the amorphous layer is an element such as silicon semiconductor, a silicon alloy (e.g., Si _x Ge _1-x or the Si _x C _1-x carbon is added, germanium is added Alloyed silicon such as silicon to which other impurities are added). The p-layer and the n-layer constituting the photoelectric conversion layer may be entirely or partially crystalline layers, or may be entirely amorphous layers. As the material forming the p-layer and the n-layer, the same material as the material forming the crystalline layer can be used.

The p-layer constituting the photoelectric conversion layer is a layer containing a group III element (for example, boron, aluminum, germanium, indium, titanium, etc.). The p-layer may be a single layer, or may be formed by a stacked layer having a different or gradually changing group III element concentration. Examples of the group III element concentration include about 0.01 to 8 atomic%. The thickness of the p-layer is, for example, about 1 to 200 nm. The p-layer can be formed by any method capable of forming a crystalline layer or an amorphous layer to have a p-type conductivity. As a method for forming an amorphous layer or a crystalline layer, a CVD method is typically given. As the CVD method here, normal pressure CVD, low pressure CVD, plasma CVD, EC
Any of R plasma CVD, high temperature CVD, low temperature CVD, etc. may be used. Among them, a method using a high frequency in a frequency band from RF to VHF, an ECR plasma CVD method, a combination thereof, and the like are preferable. For example, when the plasma CVD method is used, the condition is a frequency of 10 to 200.
MHz, a power of several W to several kW, a chamber pressure of about 0.1 to 20 Torr, and a substrate temperature of about room temperature to about 600 ° C.

The silicon-containing gas used when forming the amorphous layer or the crystalline layer is, for example, Si
H ₄ , Si ₂ H ₆ , SiF ₄ , SiH ₂ Cl ₂ , SiCl _{4 and the} like can be mentioned. The silicon-containing gas is generally used as a diluting gas together with an inert gas such as H ₂ , Ar, He, Ne, and Xe. Among them, H ₂ gas is preferable. It is appropriate that the mixing ratio of the silicon-containing gas and the diluent gas is constant or changed, for example, about 1: 1 to 1: 100 by volume. As the doping gas, a gas containing a group III element arbitrarily, for example, B ₂ H ₆
Etc. may be used. In this case, the silicon-containing gas and III
The mixing ratio with the gas containing a group element can be appropriately adjusted according to the size of a film forming apparatus such as CVD, the concentration of a group III element to be obtained, and the like, while being constant or changed, for example, The capacity ratio can be about 1: 0.001 to 1: 1. The doping of the group III element may be performed simultaneously with the formation of the silicon layer as described above, but after the formation of the silicon layer, ion implantation, surface treatment of the silicon layer, solid phase diffusion, or the like is performed. Is also good. Optionally,
A fluorine-containing gas may be added to a silicon-containing gas or the like. Examples of the fluorine-containing gas include F ₂ , SiF ₄ ,
SiH ₂ F _{2 and the} like. In this case, the amount of the fluorine-containing gas used is, for example, about 0.01 to 10 times that of the hydrogen gas.

The i-layer is a layer which does not substantially exhibit p-type and n-type conductivity types, but as long as the photoelectric conversion function is not impaired.
It may have a very weak p-type or n-type conductivity. The i-layer can be formed, for example, by a plasma CVD method. The film thickness in that case is, for example, 0.1
About 10 to about 10 μm. The gas used is III
Except for not containing a group element, those substantially the same as the p-layer can be used similarly. The film forming conditions are as follows:
Substantially the same as the p-layer.

The n-layer is not particularly limited as long as it is an n-layer usually used for a pin junction of a solar cell.
Examples of the impurity serving as a donor include phosphorus, arsenic, and antimony, and the impurity concentration is 10 ¹⁸ to 10 ^20.
cm ^-3 . The thickness of the n-layer is, for example, 10
About 100 nm.

The p-layer, the i-layer, and the n-layer may be formed as layers containing or not containing an amorphous component by separately setting film forming conditions. It is preferable to form the film continuously while changing the flow rate of the used gas and arbitrarily changing other conditions. Thereby, it can be formed as an integral layer without interruption of deposition from the p layer to the n layer.

In addition, the total thickness of the photoelectric conversion layer adjacent to the first transparent conductive layer is determined by the thickness of the hole in the first transparent conductive layer in order to obtain uniform characteristics and to suppress leakage due to pinholes and the like. The average height difference of the unevenness formed on the surface is preferably at least 1 time or more, and the average height difference of the unevenness is 4 times or less in order to maintain the size of the hole necessary for expressing the light scattering effect of the hole. Preferably, there is.

The second transparent conductive layer can be selected from among the first transparent conductive layers. Above all, a layer made of a transparent oxide is preferable, and a layer made of zinc oxide is more preferable from the viewpoint that inexpensive and stable photoelectric conversion efficiency can be obtained. The thickness of the second transparent conductive layer is set to 0.1.
About 5 to 50 nm.

The second transparent conductive layer has a plurality of holes formed on the surface on the side of the crystalline photoelectric conversion layer described later. Further, irregularities may be further formed on the surface of the hole. These holes and irregularities can cause a state of scattering or reflection of sunlight suitable for the light absorption characteristics of the photoelectric conversion layer, and a number suitable for reducing the defect density of the crystalline photoelectric conversion layer laminated thereon. The size, shape, depth, etc., are not particularly limited. Specifically, a shape such as a substantially cuboid, a rectangular parallelepiped, a cylinder, a cone, a sphere, a hemisphere, or a composite shape thereof may be mentioned. The diameter of the hole is about 200 to 2000 nm, preferably about 400 to 1200 nm, and the depth of the hole is 50-1200
nm, preferably about 100 to 800 nm, and the height difference of the unevenness is about 10 to 300 nm, preferably 20 to 20 nm.
About 0 nm. The ratio of the depth of the hole to the diameter of the hole and the ratio of the size of the unevenness to the distance between the unevenness are, for example, about 0.05 to 3, preferably about 0.1 to 2. Further, from another viewpoint, the number density of the holes is 0.1%.
About 1 to 5 pieces / μm ² , preferably 0.5 to 2 pieces / μm ²
Degree. In order to prevent defects from occurring in the crystalline photoelectric conversion layer, the height difference between the surface irregularities formed on the surface of the second transparent conductive layer and the surface irregularities formed on the surface of the first transparent conductive layer are determined. And the ratio of the size of the unevenness to the distance between the unevenness is preferably about 0.1 to 1.

Further, in order to generate a good light scattering or reflection state, irregularities may be formed on the surface other than the holes on the surface of the second transparent conductive layer in which the holes are formed. In this case, the height difference of the unevenness is about 10 to 300 nm, preferably 2 to 300 nm.
About 0 to 200 nm. Further, the ratio of the size of the unevenness to the interval of the unevenness or the number density of the holes is substantially the same as the above. Thereby, a sufficient light scattering effect can be generated even for sunlight incident on the crystalline photoelectric conversion layer. Further, the light scattering effect varies in size depending on the uneven shape, but the generation of the effect is not limited.
The surface of the second transparent conductive layer on the side of the first transparent conductive layer has holes and irregularities affected by the surface shape of the first transparent conductive layer. By changing the surface shape by selecting the conditions for forming the second transparent conductive layer, holes and unevenness suitable for the crystalline photoelectric conversion layer can be formed as described above.

The crystalline photoelectric conversion layer formed on the second transparent conductive layer is usually formed by a pin junction.
In this case, the crystalline photoelectric conversion layer only needs to include at least the i-layer entirely or partially containing crystalline material. Except for being crystalline, it can be formed of substantially the same material, film thickness, and the like as the amorphous photoelectric conversion layer by the same method as the method of forming the silicon photoelectric conversion layer or a method similar thereto.

The total film thickness of the crystalline silicon photoelectric conversion layer is determined by the unevenness formed on the surface of the hole of the second transparent conductive layer in order to obtain uniform characteristics and to suppress leakage due to pinholes and the like. The average height difference is preferably equal to or more than the average height difference. In order to maintain the size of the hole required for expressing the light scattering effect of the hole, it is appropriate that the average height difference is not more than 10 times the average height difference of the unevenness. Specifically, the thickness is about 0.2 to 10 μm. In addition, the crystalline silicon photoelectric conversion layer has i
The layer has the integrated intensity I _{220 of the} (220) X-ray diffraction peak and (11
1) Ratio of integrated intensity I ₁₁₁ of X-ray diffraction peak I ₂₂₀ / I ₁₁₁
Is preferably 5 or more.

The thin-film solar cell of the present invention may be either a superstrate type in which light is incident from the substrate side or a substrate type in which light is incident from the photoelectric conversion layer side. A set of a transparent electrode, an amorphous silicon layer, a transparent conductive layer and a crystalline silicon layer is formed in this order, or a plurality of sets are formed in parallel or in series. Further, a conductive layer is formed on the crystalline silicon layer. It is preferable to have a structure from which light is incident. Here, the conductive layer may be made of a conductive material that can be generally used as an electrode material in addition to the transparent conductive layer. Further, one or more of a buffer layer, an intermediate layer, a conductive layer, an insulating layer, a protective layer, and the like may be optionally provided between the substrate and the p layer, the i layer, and the n layer. Further, such a multi-junction thin-film solar cell may be combined with a plurality of or thin-film solar cells having other structures to constitute a solar cell module.

In the multi-junction thin-film solar cell of the present invention, when the third transparent conductive layer and the photoelectric conversion layer, the fourth transparent conductive layer and the photoelectric conversion layer are formed, the third transparent conductive layer is formed. And the like can be substantially the same as the first and second transparent conductive layers. The third transparent conductive layer and the like may have holes and irregularities on all surfaces, Holes and irregularities may be formed only in the layer. The third photoelectric conversion layer or the like may be a crystalline or amorphous photoelectric conversion layer, but is preferably a crystalline photoelectric conversion layer, and may be substantially the same as the above-mentioned photoelectric conversion layer. Hereinafter, examples of the multi-junction thin-film solar cell of the present invention will be described in detail.

Embodiment 1 FIG. 1 shows a super straight type multi-junction thin film solar cell in this embodiment. This thin-film solar cell has an amorphous silicon photoelectric conversion layer 12, a second transparent conductive layer 13, and a crystal on a thin-film solar cell substrate 11 composed of a glass plate 11a and a first transparent conductive layer 11b formed thereon. The quality silicon photoelectric conversion layer 14, the back surface reflection layer 15, and the back surface electrode 16 are laminated in this order. On the first transparent conductive layer 11b and the second transparent conductive layer 13, a large number of holes that do not reach the substantially spherical glass plate 11a are formed. The amorphous silicon photoelectric conversion layer 12 includes a p-type amorphous silicon layer 12a, an i-type amorphous silicon layer 12b, and an n-type amorphous silicon layer 12a.
The crystalline silicon photoelectric conversion layer 14 is composed of a p-type crystalline silicon layer 14a, an i-type crystalline silicon layer 14b, and an n-type crystalline silicon layer 14c.

Such a multi-junction thin-film solar cell can be formed as follows. First, on a glass plate 11a having a smooth surface, a magnetron sputtering method was used at a substrate temperature of 150 ° C. and a film forming pressure of 0.3 Torr.
The first transparent conductive layer 11b is made of zinc oxide having a thickness of 1200n.
m, and the substrate 11 for thin film solar cells was obtained. The first transparent conductive layer 11b contains gallium of about 1 × 10 ²¹ cm ⁻³ . As a result, the sheet resistance of the obtained first transparent conductive layer 11b was 6Ω / □, and the wavelength was 8
The transmittance for 00 nm light was 80%. Also,
When X-ray diffraction was performed on the first transparent conductive layer 11b, the integrated intensity of the (0001) diffraction peak was 75% of the sum of the integrated intensity of all the diffraction peaks.

Subsequently, the surface of the first transparent conductive layer 11b was etched. After immersing the substrate 11 in a 1% by weight acetic acid aqueous solution at a liquid temperature of 25 ° C. for 200 seconds, the surface of the substrate 11 was sufficiently washed with pure water. When the surface shape of the first transparent conductive layer 11b after etching was observed with a scanning electron microscope, it did not reach the glass plate 11a and had a diameter of 200 to 160 on the surface.
It was found that a large number of substantially circular holes of about 0 nm were formed. In order to examine the surface shape of the hole and the periphery in detail, the surface shape was observed with an atomic force microscope. FIG. 2 schematically shows the surface shape of the transparent conductive layer 11b measured by an atomic force microscope. In FIG. 2, the transparent conductive layer 11b
The depth 21 of one of the holes formed in the surface of
The distribution was about 0 nm, and the ratio of the hole depth 21 to the hole diameter 22 was approximately in the range of 0.1 to 1.
Further, irregularities are formed on the surface of the hole, the size of the irregularities (the height difference of the irregularities) 23 is distributed about 10 to 300 nm, and the interval 24 between the irregularities is 100 to 900 n.
m. The ratio of the unevenness size 23 to the unevenness interval 24 was approximately in the range of 0.1 to 1. Also, the size of the irregularities formed on the portion other than the hole surface was 20 nm or less.

On the substrate 11 thus obtained, high-frequency plasma CV was applied using a high-frequency plasma CVD apparatus.
According to the D method, a p-type amorphous silicon layer 12 having a thickness of 20 nm is formed.
a, an amorphous silicon photoelectric conversion layer 12 was produced by sequentially laminating an i-type amorphous silicon layer 12b having a thickness of 300 nm and an n-type amorphous silicon layer 12c having a thickness of 30 nm.
The substrate temperature during film formation was 200 ° C. for each layer.
At the time of forming the p-type amorphous silicon layer 12a, a material obtained by diluting SiH ₄ gas with H ₂ gas at a flow ratio of 5 times is used as a source gas, and B ₂ H ₆ gas is further converted to SiH ₄ gas.
A film was formed by adding 0.01% to the gas flow rate. Also,
The i-type amorphous silicon layer 12b is formed by using the same source gas as the p-type amorphous silicon layer 12a,
Further, in the case of (c), the PH ₃ gas is added to the SiH ₄ gas flow rate at 0.1%.
01% was used.

The substrate 11 obtained from the plasma CVD device
Was once taken out, and zinc oxide having a thickness of 5 nm was formed again as the second transparent conductive layer 13 by magnetron sputtering under the same conditions as the first transparent conductive layer 11b. When the surface shape of the obtained second transparent conductive layer 13 was observed with a scanning electron microscope, the diameter at the surface was 200 to 1400 n.
It was found that a large number of substantially circular holes of about m were formed. The surface shape of the second transparent conductive layer 13 was measured with an atomic force microscope in order to examine the surface shape of the hole and the periphery in detail. The depth of the holes on the surface of the second transparent conductive layer 13 was distributed in the range of about 80 to 1000 nm, and the ratio of the depth of the holes to the diameter of the holes was approximately in the range of 0.1 to 1. Further, irregularities are also formed on the surface of the hole, and the size of the irregularities is smaller than the irregularities formed on the surface of the first transparent conductive layer and is distributed in about 10 to 240 nm, and the interval between the irregularities is 100 to 900 nm. It was about. Further, the size of the irregularities formed on the portion other than the hole surface was 10 nm or less. The unevenness size 23 relative to the unevenness interval 24
Was in the range of approximately 0.1-1. Table 1 shows the surface shape of the second transparent conductive layer 13.

The second process is again performed by the high frequency plasma CVD method.
A 20-nm-thick p-type crystalline silicon layer 14a, a 2-μm-thick i-type crystalline silicon layer 14b on the transparent conductive layer 13,
The crystalline silicon photoelectric conversion layer 14 was manufactured by sequentially laminating an n-type crystalline silicon layer 14c having a thickness of 30 nm. The substrate temperature during film formation was 200 ° C. for each layer. When the p-type crystalline silicon layer 14a is formed, SiH ₄
A gas obtained by diluting a gas with H ₂ gas having a flow ratio of 50 times is used as a source gas, and B ₂ H ₆ gas is
The silicon layer was sufficiently crystallized by adding 0.01% to the flow rate of the iH ₄ gas to form a film. Also, i
The p-type crystalline silicon layer 14b is a p-type crystalline silicon layer 1
The same source gas as in FIG. 4a is applied to the n-type crystalline silicon layer 14c.
Further reduces the PH ₃ gas to 0.04 with respect to the SiH ₄ gas flow rate.
1% was used.

After taking out from the plasma CVD apparatus, the obtained photoelectric conversion layer 14 was subjected to an X-ray diffraction method. As a result, the integrated intensity I _{220 of} the (220) X-ray diffraction peak and (11)
1) Ratio of integrated intensity I ₁₁₁ of X-ray diffraction peak I ₂₂₀ / I ₁₁₁
Was 3. Here, the actually obtained X-ray diffraction peak is not information of the i-type crystalline silicon layer alone in the crystalline silicon photoelectric conversion layer, but the p-type crystalline silicon layer is compared with the i-type crystalline silicon layer. In addition, since the thickness of the n-type crystalline silicon layer is very small, the thickness may be considered to reflect the crystal orientation of the i-type crystalline silicon layer. Table 1 shows the orientation of the crystalline silicon photoelectric conversion layer.

Thereafter, zinc oxide was formed to a thickness of 50 nm as the back reflection layer 15 by magnetron sputtering, and silver was formed to a thickness of 50 as the back electrode 16 by electron beam evaporation.
A super straight multi-junction thin-film solar cell formed at 0 nm and having light incident from the glass plate 11a side was completed. The AM1.5 (10
Table 2 shows the current-voltage characteristics under the irradiation conditions of 0 mW / cm ² ).

Embodiment 2 When etching the surface of the first transparent conductive layer 11b, the substrate 1
A multi-junction thin-film solar cell was produced in the same manner as in Example 1, except that the time for immersing No. 1 in an acetic acid aqueous solution was set to 280 seconds. After forming the amorphous silicon photoelectric conversion layer 12 and the second transparent conductive layer 13 in the same manner as in Example 1, before forming the crystalline silicon photoelectric conversion layer 14, the surface shape of the second transparent conductive layer 13 is changed. Observation with a scanning electron microscope revealed that a large number of substantially circular holes having a diameter of about 400 to 1400 nm on the surface were formed. The number density was about 1 / μm ² .

In the same manner as in Example 1, in order to examine the surface shape of the hole and the periphery in detail, the surface shape was measured with an atomic force microscope. FIG. 3 shows the surface shape of the second transparent conductive layer 13. The hole depth 31 had a distribution of about 100 to 700 nm, and the ratio of the average hole depth 31 to the average hole diameter 32 was approximately in the range of 0.1 to 1. Also, irregularities were formed on the surface of the hole and on the surface where the hole was not formed, and the size 33 of the irregularity (the difference in height of the irregularity) had a distribution of about 20 to 200 nm (except for the hole). Surface: about 20 to 40 nm). Further, the interval 34 between the irregularities was about 200 to 800 nm. The ratio of the unevenness size 33 to the unevenness interval 34 is approximately 0.1 to
1 range. By increasing the etching time of the first transparent conductive layer 11b compared to the case of Example 1, the shapes of the holes and irregularities formed on the surface of the first transparent conductive layer 11b were made more uniform. It is considered that the shapes of the holes and the irregularities on the surface of the transparent conductive layer 13 are more uniform.

Furthermore, when the crystalline silicon photoelectric conversion layer 14 was formed and then subjected to X-ray diffraction, it was found that (220) X
The ratio I ₂₂₀ / I ₁₁₁ of the line integral intensity of a diffraction peak I ₂₂₀ (111) integrated intensity of the X-ray diffraction peaks I ₁₁₁ is 3, were equivalent to those of the first embodiment. Table 1 shows the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectric conversion layer. The AM1.5 (100 mW / c)
m ² ) Table 2 shows current-voltage characteristics under irradiation conditions.

Example 3 In the same manner as in Example 2 except that the material used in forming the i-type crystalline silicon layer 14b was diluted with H ₂ gas having a flow rate ratio of 30 times the SiH ₄ gas. A multi-junction thin-film solar cell was fabricated. When the X-ray diffraction method was performed after the formation of the crystalline silicon photoelectric conversion layer 14, the ratio I ₂₂₀ / I ₁₁₁ of the integrated intensity I ₂₂₀ of the ( ₂₂₀ ) X-ray diffraction peak and the integrated intensity I ₁₁₁ of the (111) X-ray diffraction peak was obtained. Was 5.5. Table 1 shows the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectric conversion layer 14. The current-voltage characteristics of the multi-junction thin-film solar cell under AM1.5 (100 mW / cm ² ) irradiation conditions are shown in Table 1. Are shown in Table 2.

Comparative Example 1 As an example using the prior art, a glass plate (1
1a) as a first transparent conductive layer formed thereon.
A multi-junction thin-film solar cell in the same manner as in Example 1 except that tin oxide formed by atmospheric pressure CVD and zinc oxide formed thereon by magnetron sputtering were coated thereon and etching was not performed. Was prepared.

Tin oxide was prepared according to the method disclosed in Japanese Patent No. 2862174. That is, a glass plate having a silicon oxide film formed on its surface is heated to 600 ° C., and SnCl ₄ , water, methanol, and hydrofluoric acid diluted with N ₂ gas are sprayed on the surface, so that the average height of the irregularities is increased. Was 150 nm and the average interval was 180 nm, to produce tin oxide having a surface unevenness. This tin oxide has an average thickness of 600 nm, a sheet resistance of 10 Ω / □, and a wavelength of 80.
The transmittance for light of 0 nm was 78%. Zinc oxide is provided to prevent a reduction reaction of tin oxide due to hydrogen plasma generated during the formation of the crystalline silicon layer, and its thickness is as thin as 30 nm. Do not give.

After forming the amorphous silicon photoelectric conversion layer and the second transparent conductive layer in the same manner as in Example 1, before forming the crystalline silicon photoelectric conversion layer 14, the surface shape was observed with a scanning electron microscope. As a result, the average height of the irregularities was 100 n.
m, the average interval is 150 nm, the height difference of the unevenness with respect to the interval of the unevenness is in the range of about 0.5 to 2, and the substantially circular hole seen in the case of Example 1 or 2 was not seen. .

Further, after the crystalline silicon photoelectric conversion layer 14 was formed, X-ray diffraction was carried out.
The ratio I ₂₂₀ / I ₁₁₁ of the line integral intensity of a diffraction peak I ₂₂₀ (111) integrated intensity of the X-ray diffraction peaks I ₁₁₁ was 1.5. With respect to the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectric conversion layer, Table 1 shows current-voltage characteristics of the multi-junction thin-film solar cell under AM1.5 (100 mW / cm ² ) irradiation conditions. It is shown in Table 2.

Comparative Example 2 The difference between the etching of the surface of the transparent conductive layer 11b and the substrate 11
Except that the time of immersion in the aqueous acetic acid solution was changed to 80 seconds.
A multi-junction thin-film solar cell was produced in the same manner as in Example 8. Table 1 shows the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectrically sensitive layer 14. The current-voltage characteristics of this multi-junction thin-film battery under the irradiation conditions of AM1.5 (100 mW / cm ² ). Are shown in Table 2.

Comparative Examples 3 and 4 A multi-junction thin-film solar cell was produced in the same manner as in Example 1 except that the thickness of the i-type amorphous silicon layer 12b was changed to 100 nm and 500 nm. Table 1 shows the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectric conversion layer 14.
Table 2 shows current-voltage characteristics under irradiation conditions of (100 mW / cm ² ).

Example 4 A thin-film solar cell was produced in the same manner as in Example 1 except that the second transparent conductive layer was laminated under a condition different from that of the first transparent conductive layer 11b to a set thickness of 30 nm. The film forming conditions for the second transparent conductive layer were different from the film forming conditions of Example 1, and the film forming pressure was 3 m.
Torr. When the X-ray diffraction method was performed on the transparent conductive layer formed on the glass substrate under the film forming conditions, the transparent conductive layer was in a random orientation. After forming the amorphous silicon photoelectric conversion layer 12 and the second transparent conductive layer 13 in the same manner as in Example 1,
Before forming the crystalline silicon photoelectric conversion layer 14, the surface shape of the second transparent conductive layer 13 was observed with a scanning electron microscope.

As a result, unlike the first embodiment, the unevenness is formed on the entire surface of the second transparent conductive layer 13, and the height of the unevenness is 2
Distribution of about 0 to 280 nm (surfaces other than holes: 20 to 50)
nm). This is presumably because the transparent conductive layer formed under the random alignment condition grew in a stripe shape, and thus irregularities were formed on the entire surface. Table 1 shows the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectric conversion layer 14.
The AM1.5 (100 m) of this multi-junction thin-film solar cell
W / cm ² ) Table 2 shows current-voltage characteristics under irradiation conditions.
Shown in

Example 5 As shown in FIG. 4, instead of a glass plate having a smooth surface, a substrate 11 for a thin-film solar cell was formed using a glass substrate 11a having a processed surface shape. Film thickness 1
A multi-junction thin-film solar cell was produced in the same manner as in Example 1, except that the substrate 11 on which the 000 nm transparent conductive layer 11b was formed was used. The surface of the glass plate 11a was processed by sandblasting using abrasive grains made of alumina having an average particle diameter of 1 μm. The sandblasting conditions were as follows: the injection pressure was set at about 3-4 kg / cm ² , the injection distance was set at about 8 cm, the injection angle was 90 °, and the speed of the processing table was 25 mm / cm ^2.
And the injection amount were set at about 50 g / min. Thereafter, a multi-junction thin-film solar cell was produced in the same manner as in Example 1.

After forming the amorphous silicon photoelectric conversion layer 12 and the second transparent conductive layer 13 in the same manner as in Example 1, before forming the crystalline silicon photoelectric conversion layer 14, the second transparent conductive layer When the surface shape was observed with a scanning electron microscope, it was found that approximately circular holes having a diameter of about 400 to 1000 nm on the surface were formed at a number density of about 1 / μm ² . The hole depth was distributed on the order of 100-700 nm, and the ratio of the hole depth to the hole diameter was in the range of about 0.1-1. Also, irregularities are formed on the surface of the hole and on the surface where the hole is not formed, and the size of the irregularity is distributed in about 30 to 120 nm.
The unevenness had a distribution of about 100 to 400 nm. The ratio of the size of the unevenness to the interval of the unevenness is about 0.
It was in the range of 1-1. Table 1 shows the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectric conversion layer 14.
The AM1.5 (100 m) of this multi-junction thin-film solar cell
W / cm ² ) Table 2 shows current-voltage characteristics under irradiation conditions.
Shown in

Comparative Example 5 A multi-junction thin-film solar cell was produced in the same manner as in Example 5, except for the processing conditions for the surface of the glass plate 11a. Glass plate 11a
The surface shape was processed by sandblasting using abrasive grains made of alumina having an average particle diameter of 20 μm. The sandblasting condition is that the injection pressure is 3-4 kg / cm.
Set the injection distance to about ² and the injection distance to about 8 cm, and set the injection angle to 9
0 °, processing table speed 250 mm / min, injection amount 5
It was set to about 0 g / min. After forming the amorphous silicon photoelectric conversion layer 12 and the second transparent conductive layer 13 in the same manner as in Example 1, before forming the crystalline silicon photoelectric conversion layer 14, the second
When the surface shape of the transparent conductive layer 13 was observed with a scanning electron microscope, it was found that approximately circular holes having a diameter of about 800 to 3000 nm on the surface were formed with a number density of about 0.3 / μm ^2. Do you get it.

The hole depth 31 is distributed in the range of about 700 to 2000 nm, and the ratio of the hole depth to the hole diameter is:
It was in the range of about 0.5-2. Also, irregularities are formed on the surface of the holes and on the surface where no holes are formed, and the size of the irregularities is distributed about 150 to 500 nm, and the interval between the irregularities is about 200 to 800 nm. Was. The ratio of the unevenness size to the unevenness interval was in the range of about 0.5 to 2. Table 1 shows the surface shape of the second transparent conductive layer 13 and the orientation of the crystalline silicon photoelectric conversion layer 14.
Table 2 shows current-voltage characteristics under irradiation conditions of (100 mW / cm ² ).

[0064]

[Table 1]

[0065]

[Table 2]

Examples will be described below in detail with reference to Tables 1 and 2. Table 1 shows the substrate forming conditions such as etching time,
Table 2 shows the surface shape of the second transparent conductive layer and the orientation of the crystalline silicon photoelectric conversion layer, and Table 2 shows current-voltage characteristics of the multi-junction thin-film solar cell.

When the surface shapes of the second transparent conductive layers in Examples 1 and 2 and Comparative Example 1 using a conventional substrate are compared, in Comparative Example 1, irregularities having an average of 100 nm are formed on the second transparent conductive layer. On the other hand, in Examples 1 and 2, holes larger in diameter than the depth of the comparative example 1 and larger than the depth were formed, and further, unevenness smaller than the holes was formed on the surface of these holes. Table 2 shows the characteristics of the multi-junction thin-film solar cell formed by laminating the crystalline silicon photoelectric conversion layer on the second transparent conductive layers having different surface shapes. Comparing the current-voltage characteristics of the multi-junction thin-film solar cells in Examples 1 and 2 and Comparative Example 1, the open-circuit voltage of Comparative Example 1 was lower than that of Examples 1 and 2, and the orientation of the crystalline silicon photoelectric conversion layer was Comparative Example 1
In the surface shape of the second transparent conductive layer described above, it is considered that the orientation of the crystalline silicon photoelectric conversion layer formed thereon is reduced and defects are introduced.

In Examples 1 and 2, the open circuit voltage was lower than that in Comparative Example 1.
Not only is it higher, but also the short-circuit current is equal to or higher than that of Comparative Example 1, so that by forming a crystalline silicon photoelectric conversion layer on the second transparent conductive layer having the surface structure of Examples 1 and 2, It is possible to prevent the introduction of the defect density into the crystalline silicon photoelectric conversion layer due to the unevenness of the substrate while maintaining the confinement effect and the reduction of the reflectance.

Further, in the second embodiment, the short-circuit current is higher than in the first embodiment. Comparison between Example 1 and Example 2 shows that the diameter, depth, and size of the unevenness on the hole surface did not change so much. In Example 2, unevenness of 10 nm or more was formed in addition to the hole surface. It is considered that the high light confinement effect is caused by the presence of the light.

In Example 3, a multi-junction thin-film solar cell having a crystalline silicon photoelectric conversion layer having an orientation I ₂₂₀ / I ₁₁₁ of 5.5 was formed. This solar cell has higher open-circuit voltage and form factor than the multi-junction thin-film solar cell in which the orientation I ₂₂₀ / I ₁₁₁ of the crystalline silicon photoelectric conversion layer in Examples 1 and 2 is 3. From this, it is considered that the orientation I ₂₂₀ / I _{111 of the} crystalline silicon photoelectric conversion layer is preferably 5 or more.

Further, Comparative Examples 2 and 5 show the case where the diameter, the depth and the size of the unevenness of the hole on the surface of the second transparent conductive layer are smaller and larger than those of Examples 1 and 2. In Comparative Example 2, since the holes and the irregularities were too small, the optical confinement effect did not occur, and the short-circuit current was reduced. In Comparative Example 5, not only the open voltage and the shape factor but also the orientation of the crystalline silicon photoelectric conversion layer was reduced. Therefore, it is considered that defects are introduced into the crystalline silicon photoelectric conversion layer due to excessively large diameters, depths, and irregularities of the holes, and the open-circuit voltage and the shape factor are reduced.

Therefore, the crystalline silicon photoelectric conversion layer of the multi-junction thin-film solar cell has a hole diameter, depth, and unevenness suitable for achieving both a reduction in defect density and a high light confinement effect. Hole diameter is 200nm or more, 20
00 nm or less, the depth of the hole is 50 nm or more,
It is considered that the size of the irregularities on the surface of the hole and the surface other than the hole is preferably in the range of 10 nm or more and 300 nm or less in the range of 1200 nm or less. Although the surface shape of the second transparent conductive layer has been described above, since the amorphous silicon photoelectric conversion layer is formed on the first transparent conductive layer,
Defects are not introduced into the photoelectric conversion layer due to unevenness,
The surface of the first transparent conductive layer can be formed with larger irregularities than the second transparent conductive layer to further enhance the light confinement effect.

On the other hand, in Comparative Examples 3 and 4, the thickness of the i-type amorphous silicon layer was increased from 100 nm to 500 nm to reduce the unevenness of the hole surface. It is considered that the unevenness of the surface of the second transparent conductive layer becomes smaller than the unevenness of the first transparent conductive layer by stacking the amorphous silicon photoelectric conversion layer on the first transparent conductive layer under the conditions for forming the photoelectric conversion layer.

Therefore, in all the embodiments in this specification, the irregularities of the second transparent conductive layer are smaller than the irregularities of the first transparent conductive layer, so that the defect density of the crystalline silicon photoelectric conversion layer can be reduced and the light confinement effect can be high. It has a structure suitable for both. Further, there is an appropriate range for the film thickness of the amorphous silicon photoelectric conversion layer. Example 1 and Comparative Example 3
In Nos. 1 to 4, multi-junction thin-film solar cells having different amorphous silicon photoelectric conversion layer thicknesses are formed. In Comparative Example 3, although the value of the short-circuit current hardly changed, the values of the open-circuit voltage and the shape factor were reduced, and the amorphous silicon photoelectric conversion layer was too thin. It is considered that a defect has occurred and a short circuit has occurred. In Comparative Example 4, the values of the open-circuit voltage and the shape factor hardly changed, but the value of the short-circuit current was reduced. It is considered that the structure of the surface of the transparent oxide layer is insufficient to produce a light confinement effect. Therefore, there is an appropriate thickness of the amorphous silicon photoelectric conversion layer with respect to the height of the unevenness of the first transparent conductive layer, and the thickness of the amorphous silicon photoelectric conversion layer is an average of the unevenness of the first transparent conductive layer. It is considered preferable that the height difference is not less than 1 time and not more than 4 times.

In the fourth and fifth embodiments, the first transparent conductive layer and the first transparent conductive layer having the same holes and irregularities as those of the second embodiment are formed by controlling the formation conditions of the second transparent conductive layer and sandblasting the glass substrate. Forming a second transparent conductive layer,
A multi-junction thin-film solar cell having current-voltage characteristics equivalent to that of the second embodiment is formed. For this reason, the surface shape of the transparent conductive layer is important for forming a multi-junction thin-film solar cell exhibiting high photoelectric conversion efficiency, and the method for forming holes and irregularities is not limited to the etching of the transparent conductive layer in Examples 1 to 3. The present invention is not limited thereto, and it is considered that control of the conditions for forming the transparent conductive layer, sandblasting of the glass substrate, and a combination thereof are effective.

[0076]

According to the present invention, a first transparent conductive layer, an amorphous silicon photoelectric conversion layer, a second transparent conductive layer, and a crystalline silicon photoelectric conversion layer are formed in this order on a transparent substrate,
A plurality of holes are formed on the surface of the first transparent conductive layer on the side of the amorphous silicon photoelectric conversion layer and the surface of the second transparent conductive layer on the side of the crystalline silicon photoelectric conversion layer, and irregularities are formed on the surface of the holes. Therefore, it is possible to generate a scattering or reflection state of sunlight suitable for the light absorption characteristics of amorphous silicon and crystalline silicon, thereby increasing the amount of light absorption due to the light confinement effect and the defect density in the crystalline silicon layer. And a multi-junction thin-film solar cell having high photoelectric conversion efficiency can be obtained.

In particular, the difference in height of the surface of the hole formed in the second transparent conductive layer is determined by the difference in height of the surface of the hole formed in the surface of the first transparent conductive layer. When the height difference is smaller than the height difference of the formed hole surface, a photoelectric conversion layer made of crystalline silicon with fewer defects can be obtained.

The diameter of the hole formed on the surface of the second transparent conductive layer is in the range of 200 nm or more and 2000 nm or less, and the depth of the hole is in the range of 50 nm or more and 1200 nm or less. When the height difference of a certain unevenness is in the range of 10 nm or more and 200 nm or less, not only medium wavelength light in the 450 to 650 nm region, which is the center wavelength of the solar spectrum, but also sufficient light for light having a longer wavelength. A light scattering effect can be created.

Further, when irregularities are formed on the surface of the second transparent conductive layer on which the holes are formed other than the holes, and when the height difference of the irregularities is in the range of 10 nm or more and 300 nm or more, light The confinement effect can be exerted more strongly.

When the second transparent conductive layer contains zinc oxide, it is possible to obtain a high conversion efficiency stably at a low cost, and it is possible to obtain a crystal with a photoelectric conversion layer made of amorphous silicon. Light scattering and reflection at the reverse junction with the photoelectric conversion layer made of porous silicon can be used.

Further, the amorphous silicon photoelectric conversion layer
1 1 times or more of the average height difference of the unevenness of the hole surface of the transparent conductive layer,
When it is four times or less, uniform characteristics can be obtained, and leakage due to pinholes and the like can be suppressed. In addition, (22) of the crystalline silicon photoelectric conversion layer
0) When the ratio I ₂₂₀ / I ₁₁₁ of the integrated intensity I ₂₂₀ of the X-ray diffraction peak to the integrated intensity I ₁₁₁ of the (111) X-ray diffraction peak is 5 or more, a photoelectric conversion layer with very few defects is obtained. And a thin-film solar cell having higher photoelectric conversion efficiency can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a main part showing one embodiment of a multi-junction thin-film solar cell (Example 1) of the present invention.

FIG. 2 is a schematic diagram for explaining a surface shape of a transparent conductive layer in FIG.

FIG. 3 is a schematic diagram for explaining a surface shape of a transparent conductive layer in a multi-junction thin-film solar cell (Example 2) of the present invention.

FIG. 4 is a schematic sectional view of a main part showing a multi-junction thin-film solar cell (Example 5) of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Substrate 11a ... Glass plate 11b ... 1st transparent conductive layer 12 ... Amorphous silicon photoelectric conversion layer 12a ... P-type amorphous silicon layer 12b ... i-type amorphous silicon layer 12c ... n-type amorphous silicon layer DESCRIPTION OF SYMBOLS 13 ... 2nd transparent conductive layer 14 ... crystalline silicon photoelectric conversion layer 14a ... p-type crystalline silicon layer 14b ... i-type crystalline silicon layer 14c ... n-type crystalline silicon layer 15 ... backside reflective layer 16 ... backside electrode 21, 31: Hole depth 22, 32: Hole diameter 23, 33: Height difference of unevenness 24, 34: Spacing of unevenness

Claims (11)

[Claims] 1. At least a first transparent conductive layer on a substrate,
An amorphous or crystalline photoelectric conversion layer, a second transparent conductive layer and a crystalline photoelectric conversion layer are laminated in this order, and the surface of the first transparent conductive layer on the amorphous or crystalline photoelectric conversion layer side and the second A multi-junction thin-film solar cell, wherein a plurality of holes are formed on the surface of the transparent conductive layer on the side of the crystalline photoelectric conversion layer, and irregularities are formed on the surface of the holes. 2. The multi-junction thin-film solar cell according to claim 1, wherein a plurality of holes are formed in a surface of the substrate, and the surface of the holes has irregularities. 3. The method according to claim 1, wherein a difference in height of the surface of the hole formed on the surface of the second transparent conductive layer is smaller than a difference in height of the surface of the hole formed on the surface of the first transparent conductive layer. Multi-junction thin-film solar cells. 4. The diameter of a hole formed in the surface of the first and / or second transparent conductive layer is in a range of 200 nm or more and 2000 nm or less, and the depth of the hole is 50 nm or more and 1200 n.
m or less, and the height difference of the unevenness on the surface of the hole is 10 nm or more and 300 nm or less.
3. The multi-junction thin-film solar cell according to any one of 3. 5. An unevenness is formed on a surface of the first and / or second transparent conductive layer on which a hole is formed, except for the hole.
The multi-junction thin-film solar cell according to any one of claims 1 to 4, wherein a height difference of the unevenness is in a range of 10 nm or more and 300 nm or less. 6. The multi-junction thin-film solar cell according to claim 1, wherein the first and / or second transparent conductive layers are mainly formed of zinc oxide. 7. The photoelectric conversion layer adjacent to the first transparent conductive layer is formed so as to have a thickness of 1 to 4 times the average height difference of the irregularities on the hole surface of the first transparent conductive layer. 7. The multi-junction thin-film solar cell according to any one of 1 to 6. 8. The integrated intensity I ₂₂₀ of the (220) X-ray diffraction peak of the i-layer, wherein the crystalline photoelectric conversion layer comprises a pin junction.
(111) the ratio of the integrated intensity I ₁₁₁ of the X-ray diffraction peaks I ₂₂₀
The multi-junction thin-film solar cell according to any one of claims 1 to 7, wherein / I ₁₁₁ is 5 or more. 9. The multi-junction thin-film solar cell according to claim 1, wherein the first transparent conductive layer is oriented with respect to the substrate surface. 10. A substrate and / or a first thin-film solar cell according to claim 1, wherein
A method of manufacturing a thin-film solar cell, comprising forming a plurality of holes in the surfaces of the first transparent conductive layer and the second transparent conductive layer by etching the surface of the transparent conductive layer and / or the second transparent conductive layer. . 11. When manufacturing the thin-film solar cell according to any one of claims 1 to 9, the first transparent conductive layer and / or the second transparent conductive layer are formed such that holes are formed in the surface thereof. Forming a plurality of holes in the surfaces of the first transparent conductive layer and the second transparent conductive layer.