WO2024176803A1

WO2024176803A1 - Transparent electrically conductive film, substrate having transparent electrically conductive film, and photoelectric conversion element

Info

Publication number: WO2024176803A1
Application number: PCT/JP2024/003815
Authority: WO
Inventors: 崇鯉田; 均齋; 卓矢松井
Original assignee: 国立研究開発法人産業技術総合研究所
Priority date: 2023-02-22
Filing date: 2024-02-06
Publication date: 2024-08-29

Abstract

The purpose of the present invention is to provide a transparent electrically conductive film which can be formed at a low temperature, has high transparency and high electrical conductivity, and has a low indium content or does not contain indium. This transparent electrically conductive film contains a metal oxide whose primary component is amorphous tin oxide. Among metal elements that constitute the metal oxide, the amount of Sn is 85 atom% or more and the amount of In is 4 atom% or less. The transparent electrically conductive film has a resistivity of 2×10-3 Ω·cm or less.

Description

TRANSPARENT CONDUCTIVE FILM, SUBSTRATE WITH TRANSPARENT CONDUCTIVE FILM, AND PHOTOELECTRIC CONVERSION ELEMENT

The present invention relates to a transparent conductive film, a substrate with a transparent conductive film, and a photoelectric conversion element.

_In2O3 -based materials such as ITO (tin-doped indium oxide) are widely known as transparent conductive films for light-receiving transparent electrodes of optoelectronic devices such as displays and solar cells (see, for example, Patent Document 1) _{. In2O3} _- _based materials have the advantages of being capable of being formed into films by low-temperature processes and of having both high transparency and electrical conductivity.

However, since indium is a rare metal and very expensive, it is being considered to replace _In2O3 _- based materials with other materials. As a transparent conductive film that is relatively inexpensive and can be formed at low temperatures, a film made of ZnO (zinc oxide) is known (for example, Patent Document 2). However, a transparent conductive film mainly made of zinc oxide has low moisture resistance and chemical resistance. Therefore, it has been difficult to replace a transparent conductive film mainly made of zinc oxide with a transparent conductive film made of ITO or the like.

On the other hand, films containing SnO ₂ (tin dioxide), such as FTO (fluorine-doped tin oxide) and ATO (antimony-doped tin oxide), are also known as transparent conductive films that are highly transparent and have excellent stability and chemical resistance.

International Publication No. 2017/057556 JP 2012-117903 A

Here, FTO and ATO are polycrystalline, and the conductivity is increased by forming the film at a high temperature and improving the crystallinity. Therefore, the FTO film and ATO film need to be formed at about 500°C by thermal CVD, or at 400°C to 500°C by sputtering. However, film formation at a high temperature exceeding 400°C is likely to cause thermal damage to the substrate on which the film is laminated and each layer of the optoelectronic device. Therefore, it has been difficult to use FTO and ATO as the transparent conductive film of the optoelectronic device. Therefore, in the current situation, it has been necessary to use a transparent conductive film of In ₂ O ₃ system mainly composed of rare indium.

The present invention has been made in consideration of the above problems. The present invention aims to provide a transparent conductive film that can be formed at low temperatures, has high transparency and high conductivity, and further contains a small amount of indium or does not contain indium, as well as a transparent conductive film-coated substrate and a photoelectric conversion element that contain the same.

One embodiment of the present invention provides a transparent conductive film comprising a metal oxide mainly composed of amorphous tin oxide, in which, among metal elements constituting the metal oxide, the amount of Sn is 85 atomic % or more and the amount of In is 4 atomic % or less, and the film has a resistivity of 2×10 ⁻³ Ω·cm or less.

One embodiment of the present invention also provides a substrate with a transparent conductive film, comprising a substrate and the above-described transparent conductive film disposed on the substrate.

Furthermore, one embodiment of the present invention provides a photoelectric conversion element having a photoelectric conversion layer, a first electrode including at least one conductive film arranged adjacent to the photoelectric conversion layer, and a second electrode including at least one conductive film arranged adjacent to the photoelectric conversion layer, in which at least one of the first electrode and the second electrode includes the transparent conductive film.

The present invention provides a transparent conductive film that can be formed at low temperatures, has high transparency and high conductivity, and contains little or no indium. The transparent conductive film can also be applied to various photoelectric conversion elements such as solar cells.

FIG. 1A is a graph for explaining the correlation between the valence of Sn in a transparent conductive film and the extinction coefficient k, and FIG. 1B is a graph for explaining the correlation between the valence of Sn in a transparent conductive film and the absorption coefficient α. FIG. 2 is a graph showing the relationship between the depth from the surface of the transparent conductive film and the hydrogen atom concentration in the transparent conductive film. FIG. 3 is a schematic diagram of a reactive plasma deposition apparatus capable of forming a transparent conductive film according to one embodiment of the present invention. FIG. 4 is a schematic diagram showing the structure of a Si heterojunction solar cell. FIG. 5 is a schematic diagram showing the structure of a perovskite solar cell. FIG. 6 is a graph showing the extinction coefficient k and refractive index n for each wavelength of the amorphous SnO ₂ films produced in Examples 1-1 and 1-2 and the a-In ₂ O ₃ :H film produced in Reference Example 1. FIG. 7A is a graph showing the transmission spectrum and the reflection spectrum of the PET substrate only, the PET substrate and the transparent conductive film (Example 1-3), and the PET substrate with _SiO2 and the transparent conductive film (Example 1-4). FIG. 7B is a graph showing the transmission spectrum and the reflection spectrum of the glass substrate only, and the glass substrate and the transparent conductive film (Example 1-3, Example 1-4). FIG. 8 shows the external quantum efficiency spectra of the front junction Si heterojunction solar cells fabricated in Examples 2-1 and 2-2, and Reference Example 2. FIG. 9 is a graph showing the current-voltage characteristics of the rear junction Si heterojunction solar cells fabricated in Examples 3-1 and 3-2, and Reference Example 3. FIG. 10 is a graph showing the current-voltage characteristics of the rear junction Si heterojunction solar cells fabricated in Example 3-2a and Reference Example 3. FIG. 11 is a graph showing the series resistance values of the Si heterojunction solar cells fabricated in each example.

In this specification, a numerical range indicated with "~" means a numerical range including the numbers written before and after "~".

1. Transparent conductive film The transparent conductive film of the present invention contains a metal oxide mainly composed of amorphous tin oxide, and among the metal elements constituting the metal oxide, the amount of Sn is 85 atomic % or more, the amount of In is 4 atomic % or less, and the resistivity is 2×10 ⁻³ Ω·cm or less. In this specification, the tin oxide in the transparent conductive film is "amorphous" means that when the X-ray diffraction intensity (XRD intensity) is measured, a broad peak derived from the amorphous structure is dominant and a peak derived from the crystal is not significantly confirmed. In other words, when a transmission electron microscope image (TEM image) is observed, the amorphous structure is dominant and the film may have a degree of microcrystalline nature such that crystalline structures are scattered in the amorphous layer. In addition, the transparent conductive film may partially contain components other than the above metal oxide, for example, may be doped with fluorine, within a range that does not impair the object and effect of the present invention, but it is preferable that 85 mass % or more of the transparent conductive film is the above metal oxide, and it is more preferable that the transparent conductive film is made of the above metal oxide.

As described above, polycrystalline FTO (SnO ₂ :F) and ATO (SnO ₂ :Sb) are known to have high transparency and high conductivity. However, it is difficult to form a film of these at low temperatures, making it difficult to put them to practical use. In response to this, the inventors of the present invention conducted extensive research and found that a film mainly containing amorphous tin oxide can not only be formed at low temperatures, but also has excellent transparency and conductivity. The reason for this is believed to be as follows.

The conduction band of tin oxide (especially SnO ₂ ) is mainly composed of Sn5s orbitals. The Sn5s orbitals have a large spatial spread of electrons, and the spherical s orbitals overlap each other. Therefore, even if the bond angle fluctuates due to the amorphous structure, the overlap between the orbitals does not decrease as much as in the case of crystalline materials, and it is considered that high mobility of electrons is realized. In fact, the effective mass of electrons in a film made of amorphous SnO ₂ is 0.35 m ₀ , which is not significantly different from the effective mass of electrons in a film made of crystalline SnO _2. Therefore, it is considered that the carrier conduction path is not easily affected by the amorphous structure, and the amorphous tin oxide (especially SnO ₂ ) realizes high transparency and high conductivity similar to those of crystalline SnO ₂ .

The transparent conductive film of the present invention also has the advantage of having good moisture resistance.

Here, from the viewpoint of realizing high transparency and high conductivity of the transparent conductive film, it is sufficient that 85 atomic % or more of all metal elements constituting the metal oxide is Sn, but the amount of Sn is more preferably 90 atomic % or more, and even more preferably 95 atomic % or more. Furthermore, the metal oxide may contain metal elements other than Sn within a range that does not impair the purpose and effect of the present invention, but the total amount is preferably 15 atomic % or less, more preferably 10 atomic % or less, and even more preferably 5 atomic % or less, of all metal elements constituting the metal oxide.

Examples of metal elements other than Sn contained in the metal oxide include In, Zn, Cd, Nb, Ta, B, Ga, Ba, Mo, Pb, Rb, Re, Sb, W, Ce, Cs, Dy, Er, Ge, Hf, Ho, La, Lu, Nd, Pr, Sc, Si, Sm, Tb, V, Y, Al, Ti, Zr, etc., and among these, Zn, Cd, B, Ga, Si, Ge, Pb, Sb, V, Nb, Ta, Mo, W, and Ce are preferred. The metal oxide may contain only one of these, or may contain two or more. When the metal oxide further contains a metal element other than Sn, the processability of the material for forming the transparent conductive film is improved, and depending on the type of metal element, the density, transparency, and conductivity of the transparent conductive film are further improved.

However, the amount of In relative to the total amount of metal elements constituting the metal oxide is 4 atomic % or less, preferably 3 atomic % or less, preferably 0.09 atomic % or less, and more preferably substantially none. The lower the amount of In, the lower the cost of the transparent conductive film. In addition, if the metal oxide (transparent conductive film) contains In, which is a specified chemical substance, all work involving the handling of the transparent conductive film becomes subject to regulation. In contrast, if the metal oxide does not contain In, it is not subject to regulation, and there is an advantage in that workability is improved. The amount of each metal element constituting the metal oxide can be identified by, for example, confirming the composition of the transparent conductive film using ICP analysis.

In addition, in the transparent conductive film (metal oxide), it is preferable that Sn is mainly contained in a tetravalent (Sn ⁴⁺ ) state. Sn can have a mixed valence of Sn ⁴⁺ and Sn ²⁺ , but as the amount of Sn ²⁺ in the transparent conductive film increases, the number of acceptor-type defects increases. As a result, both the carrier concentration and the mobility decrease, and the resistivity tends to increase, as shown in Table 1 below. Although it is difficult to directly measure the valence of Sn in the transparent conductive film, there is a correlation between the valence of Sn and the extinction coefficient k or absorption coefficient α of the transparent conductive film at a wavelength of 420 nm or more and 500 nm or less. Therefore, the valence of Sn in the transparent conductive film can be evaluated by checking the extinction coefficient k or absorption coefficient α of the transparent conductive film. FIG. 1A shows the extinction coefficient k at a wavelength of 200 nm or more and 1200 nm or less when the ratio of the amount of Sn ⁴⁺ to the amount of Sn ²⁺ is changed. FIG. 1B shows the absorption coefficient α at wavelengths of 400 nm or more and 600 nm or less when the ratio of the amount of Sn ⁴⁺ to the amount of Sn ²⁺ is changed.

As shown in FIG. 1A, in the transparent conductive film mainly containing Sn ⁴⁺ , the extinction coefficient k is very small in the wavelength range of 420 nm to 500 nm. In contrast, when the proportion of Sn ²⁺ in the transparent conductive film increases, the extinction coefficient k in the wavelength range of 420 nm to 500 nm increases. Therefore, the maximum value (maximum extinction coefficient) of the extinction coefficient k in the wavelength range of 420 nm to 500 nm of the transparent conductive film is preferably 0.033 or less, more preferably 0.025 or less, and even more preferably 0.017 or less. When the maximum extinction coefficient in the wavelength range of 420 nm to 500 nm is 0.033 or less, it can be said that the amount of tetravalent Sn in the transparent conductive film is sufficiently large, and high transparency and high conductivity are more easily achieved. The extinction coefficient k can be obtained by measuring the reflected light of light with a wavelength of 200 nm to 1200 nm using a spectroscopic ellipsometry device and determining the change in the polarization state of the incident light and the reflected light.

On the other hand, as shown in FIG. 1B, in the transparent conductive film mainly containing Sn ⁴⁺ , the absorption coefficient α in the wavelength range of 420 nm to 500 nm is very small. In contrast, when the proportion of Sn ²⁺ in the transparent conductive film increases, the absorption coefficient α in the wavelength range of 420 nm to 500 nm increases. The maximum value (maximum absorption coefficient) of the absorption coefficient α in the wavelength range of 420 nm to 500 nm of the transparent conductive film is preferably 1×10 ⁴ cm ⁻¹ or less, more preferably 7.5×10 ³ cm ⁻¹ or less, and even more preferably 5×10 ³ cm ⁻¹ or less. When the maximum absorption coefficient α in the wavelength range of 420 nm to 500 nm is 1×10 ⁴ cm ⁻¹ or less, it can be said that the amount of tetravalent Sn in the transparent conductive film is sufficiently large, and high transparency and high conductivity are more easily achieved. The absorption coefficient α can be determined from the absorption spectrum obtained by measuring the transmittance and reflectance of light having a wavelength of 200 nm or more and 1200 nm or less using a spectrophotometer.

In addition, there are no limitations on the method for adjusting the extinction coefficient k or absorption coefficient α in the transparent conductive film, but they can be adjusted by depositing the transparent conductive film in an atmosphere containing a sufficient amount of oxygen, reducing the partial pressure of water vapor remaining in the deposition chamber, or by heating the film to a temperature of about 200°C or less.

Here, the resistivity of the transparent conductive film may be 2×10 ⁻³ Ω·cm or less, preferably 1.5×10 ⁻³ Ω·cm or less, and more preferably 1×10 ⁻³ Ω·cm or less. If the resistivity of the transparent conductive film is 2×10 ⁻³ Ω·cm or less, the transparent conductive film can be used for various applications, such as a transparent electrode of a photoelectric conversion element. The resistivity can be determined by a Loresta (low resistivity meter).

In addition, according to the inventors' intensive research, it has become clear that in order to reduce the resistivity of a transparent conductive film, it is preferable for the film density of the transparent conductive film to be as high as possible. The following four samples were produced using a method similar to that shown in Example 1-1 described below. The composition, film density, and resistivity at this time are shown in Table 1 below. The average composition was analyzed using the Rutherford backscattering spectrometry (RBS) method, and the resistivity was measured using a Loresta (low resistivity meter). Furthermore, the film density was calculated from the areal density determined by the RBS method and the film thickness determined by spectroscopic ellipsometry.

As shown in Table 1, the resistivity decreases as the film density of the transparent conductive film increases. In view of the results, in order to make the resistivity of the transparent conductive film 2.0×10 ⁻³ or less, it is preferable that the film density analyzed by the Rutherford backscattering spectrometry (RBS) method is 5.6 g/cm ³ or more, more preferably 6.3 g/cm ³ or more, and even more preferably 6.4 g/cm ³ or more. It is also possible to calculate the film density by the X-ray reflectometry (XRR) method. However, the film composition is assumed in the XRR method to calculate the film density. Therefore, it is not yet clear which of the RBS method and the XRR method is closer to the true value, since there is no standard sample that is a thin film and has a known film density. Therefore, although the above results are considered to be values that include an error from the true value, it is clear that a higher film density of the transparent conductive film is preferable, and it can be said that when measuring the film density by the RBS method, it is preferable that the film density is equal to or higher than the above value.

Furthermore, through intensive research by the inventors, it has become clear that in order to reduce the resistivity of the transparent conductive film, it is preferable to control the hydrogen atom concentration in the transparent conductive film as measured by secondary ion mass spectrometry (SIMS) and adjust the hydrogen atom concentration to a low level. It can also be said that a lower hydrogen atom concentration results in a denser film, resulting in a film with a higher film density.

The following five SnO ₂ films (transparent conductive films) were prepared in the same manner as in Examples 1-1 and 1-2 described later. The relationship between the depth of the transparent conductive film and the hydrogen atom concentration when the hydrogen atom concentration was measured from one surface of these transparent conductive films by SIMS is shown in FIG. 2. It is clear from FIG. 2 that hydrogen atoms tend to accumulate on the surface of the transparent conductive film and on the interface between the transparent conductive film and another film (here, a glass substrate), resulting in a higher concentration than inside. In addition, in the SIMS analysis, the element is pushed in by the sputtering ions, resulting in a profile that spreads deeper. Therefore, the concentration of hydrogen atoms in the interior of these transparent conductive films (regions 10 nm or more inward from both surfaces (or interfaces)) was identified. The results are shown in Table 2 below. In addition, the resistivity of each transparent conductive film was measured using a Loresta (low resistivity meter). The results are shown in Table 2 below.

As shown in the above table, the lower the hydrogen atom concentration inside the transparent conductive film, the smaller the resistivity. From the viewpoint of making the resistivity 2.0×10 ⁻³ or less, it can be said that the concentration of hydrogen atoms in the region 10 nm or more inside from each of both surfaces of the transparent conductive film is preferably 8×10 ²¹ atoms/cm ³ or less, more preferably 7×10 ²¹ atoms/cm ³ or less, and even more preferably 6×10 ²¹ atoms/cm ³ or less.

The thickness of the transparent conductive film is appropriately selected depending on the application. When the transparent conductive film is used as a transparent electrode on the light receiving side or the light emitting side of a photoelectric conversion element described below, the thickness is preferably 1 nm or more and 5000 nm or less, and more preferably 10 nm or more and 1000 nm or less. When the thickness of the transparent conductive film is within this range, it becomes easier to obtain the high transparency and high conductivity required for a transparent electrode.

The method for producing the transparent conductive film is not particularly limited as long as the above-mentioned composition and resistivity are satisfied, and the film can be formed by, for example, a film formation method using reactive plasma deposition or a sputtering method. A film formation method using reactive plasma deposition is described below, but the method for producing the transparent conductive film is not limited to this method. In the examples described later, a method for forming a transparent conductive film by magnetron sputtering is also shown.

Film formation using the reactive plasma deposition method can be performed, for example, by a reactive plasma deposition apparatus 100 shown in the schematic diagram of FIG. 3. However, the configuration of the reactive plasma deposition apparatus 100 is not limited to this configuration.

The reactive plasma deposition apparatus 100 has a hearth section 10 for holding the material at a predetermined temperature, a plasma gun 20 for generating a plasma beam 21 and a plasma 22, a plasma beam controller 30 for directing the plasma beam 21 generated from the plasma gun 20 to the material in the hearth section 10, and a chamber 40 for housing these. When forming the above-mentioned transparent conductive film on the substrate 1 by the reactive plasma deposition method, the material is housed in the hearth section 10, and the substrate 1 is placed at a predetermined position. The inside of the chamber 40 is adjusted to a predetermined atmosphere and a predetermined pressure. Then, the plasma beam 21 and the plasma 22 are generated from the plasma gun 20. The plasma beam 21 emitted from the plasma gun 20 is guided to the hearth section 10 by the plasma beam controller 30 and perpendicularly enters the material in the hearth section 10. The material heated by irradiation with the plasma beam 21 sublimes and is ionized in the plasma 22. The ionized material 11 then reaches the substrate 1 in an activated state. This forms the above-mentioned transparent conductive film on the substrate 1.

Here, when forming a transparent conductive film by the reactive plasma deposition method, the material to be accommodated in the hearth unit 10 is preferably a sintered body having a composition substantially similar to that of the transparent conductive film, a sintered body made of a metal constituting the transparent conductive film, a metal oxide including a suboxide of the metal, or a mixture thereof. The sintered body may be amorphous or crystalline. The sintered body is obtained by mixing _SnO2 , SnO, or Sn with other metals or metal oxides as necessary, and sintering the mixture by a known method such as a normal pressure sintering method or a hot press method.

In order to make the tin oxide in the transparent conductor (metal oxide) amorphous, it is preferable to maintain the temperature of the substrate 1 at 300°C or less. The temperature of the substrate 1 may be around room temperature, in particular when not intentionally heated. It may also be intentionally cooled to around 0°C.

From the viewpoint of film formation efficiency, the pressure in the chamber is preferably 0.01 Pa to 10 Pa, and more preferably 0.1 Pa to 1 Pa. The atmosphere in the chamber 40 may be an inert gas atmosphere such as nitrogen or argon, but in order to facilitate the conversion of Sn to tetravalent (Sn ⁴⁺ ), it is preferable to introduce oxygen into the atmosphere, and the oxygen partial pressure is more preferably 0.01 Pa to 10 Pa, and even more preferably 0.1 Pa to 1 Pa. When the oxygen partial pressure is within this range, it becomes easier to obtain a transparent conductive film having the above-mentioned extinction coefficient and absorption coefficient.

Furthermore, after the film is formed by the reactive plasma deposition method, it is preferable to perform a heat treatment at 20°C or more and 400°C or less, and preferably 100°C or more and 300°C or less. The heat treatment time is preferably 0.1 seconds or more and 24 hours or less, and more preferably 0.1 seconds or more and 1 hour or less. When the annealing treatment is performed, structural relaxation occurs, and the obtained transparent conductive film tends to become stable and the conductivity tends to become even lower.

2. Substrate with transparent conductive film The substrate with transparent conductive film of the present invention only needs to have a substrate and the transparent conductive film disposed on the substrate, and the shape of the substrate and the thickness of the transparent conductive film are not particularly limited. The transparent conductive film may be disposed on the entire surface of the substrate, or may be disposed only on a partial region of the substrate. Furthermore, the substrate may include a configuration other than the substrate and the transparent conductive film. For example, an arbitrary layer (e.g., an arbitrary conductive film other than the above, a barrier film, etc.) may be present between the substrate and the transparent conductive film, or on the transparent conductive film. The arbitrary layer may be a known layer.

The material of the substrate may be an inorganic material such as glass, but may also be a resin material. That is, the substrate may be a resin film. The substrate may also be composed of multiple layers. The shape of the substrate is not particularly limited, and may be flat or may have a three-dimensional shape. Furthermore, the optical transparency of the substrate is appropriately selected depending on the application, and may or may not be optically transparent. The substrate may also be flexible.

The transparent conductive film described above has high transparency and conductivity. Furthermore, the transparent conductive film can be formed at a relatively low temperature (e.g., 300°C or less). Therefore, substrates made of various materials can be used as the substrate.

Examples of the structure of the substrate with the transparent conductive film include a laminated structure including a substrate/barrier film/the transparent metal film, a laminated structure including a substrate/another conductive film/the transparent conductive film, and a laminated structure including a substrate/the transparent conductive film/the other conductive film.

　Applications of the substrate with the transparent conductive film are not limited to photoelectric conversion elements described below, and examples include, but are not limited to, various photodetection elements, displays, wearable devices, thin film transistors (TFTs), transparent heaters, infrared communication devices, infrared sensors, heat ray reflecting materials, electromagnetic wave blocking materials, antistatic agents, etc.

3. Photoelectric conversion element The above-mentioned transparent conductive film can be used for either one or both of the first and second electrodes of a photoelectric conversion element having a photoelectric conversion layer, a first electrode including at least one conductive film arranged adjacent to the photoelectric conversion layer, and a second electrode including at least one conductive film arranged adjacent to the photoelectric conversion layer. The first electrode and the second electrode may be composed of multiple layers. In this case, any layer constituting the first electrode or any layer constituting the second electrode may be the above-mentioned transparent conductive film. In addition, at this time, two or more layers may be the above-mentioned transparent conductive film. Depending on the type of photoelectric conversion element, the layer corresponding to the first electrode layer or the second electrode layer may be called an electron transport layer, a carrier selection layer, an n-type buffer layer, a (conductive) cap layer, etc., but the above-mentioned transparent conductive film can also be used for these layers.

In this specification, the term "photoelectric conversion element" refers to an element that converts light energy into electrical energy, or an element that converts electrical energy into light energy. Examples of photoelectric conversion elements include solar cells, organic EL elements, light-emitting diodes, laser diodes, etc. Below, we will explain the case where the photoelectric conversion element is a solar cell, but the above-mentioned transparent conductive film can also be used for the transparent electrode or metal electrode on the light-emitting side of an organic EL element, etc.

In addition, there are no particular limitations on the type of solar cell that can use the transparent conductive film, and it can be used with any type of solar cell known in the art. Several solar cell configurations are shown below, but the configurations of solar cells that can use the transparent conductive film described above are not limited to these.

(Si heterojunction solar cell)
An example of the structure of a Si heterojunction solar cell is shown in Fig. 4. The Si heterojunction solar cell (hereinafter also simply referred to as a "solar cell") 200 has a structure in which a photoelectric conversion layer 130 is sandwiched between a first electrode 131 and a second electrode 132. In the solar cell 200, light is incident from the first electrode 131 side.

In the solar cell 200, the first electrode 131 is composed of the light-receiving transparent electrode 125 and the grid electrode 126, and the second electrode 132 is composed of the backside transparent electrode 127 and the metal electrode 128. The above-mentioned transparent conductive film can be used for either or both of the light-receiving transparent electrode 125 and the backside transparent electrode 127. When the above-mentioned transparent conductive film is used for only one of them, the other can be a known transparent conductive film such as ITO. From the viewpoint of reducing the cost of the solar cell 200, it is preferable to use the above-mentioned transparent conductive film for both the light-receiving transparent electrode 125 and the backside transparent electrode 127. In addition, either or both of the light-receiving transparent electrode 125 and the backside transparent electrode 127 may be a laminate of multiple conductive films. In this case, the above-mentioned transparent conductive film may be used for any of the layers constituting the light-receiving transparent electrode 125 and the backside transparent electrode 127. For example, a laminate of the transparent conductive film and the ITO film may be used as the light-receiving transparent electrode 125 or the back transparent electrode 127. By using the transparent conductive film as part of the light-receiving transparent electrode 125 or the back transparent electrode 127, there is an advantage that the amount of ITO (especially In) used can be reduced. The grid electrode 126 and the metal electrode 128 are similar to the electrodes of known solar cells, and are electrodes made of Ag, Cu, composite metals, etc. In addition, the metal electrode 128 is formed on the entire surface in FIG. 4, but it may be in a grid shape like the grid electrode 126. In that case, a bifacial solar cell can be formed in which light incident from the second electrode 132 side also contributes to power generation.

On the other hand, the photoelectric conversion layer 130 has a structure in which an n-type single crystal silicon layer 120 is sandwiched between a p-type semiconductor layer 122 and an n-type semiconductor layer 124. Between the n-type single crystal silicon layer 120 and the p-type semiconductor layer 122, and between the n-type single crystal silicon layer 120 and the n-type semiconductor layer 124, i-type semiconductor layers 121 and 123 are disposed, respectively.

The order of arrangement of the layers of the photoelectric conversion layer 130 may be reversed, and the n-type semiconductor layer may be arranged on the light receiving surface side and the p-type semiconductor layer may be arranged on the back surface side. As shown in FIG. 4, a structure in which a p-i-n junction structure consisting of a p-type semiconductor layer 122, an i-type semiconductor layer 121, and an n-type single crystal silicon layer 120 is arranged on the light receiving surface side is usually called a front junction type, and a structure in which a p-i-n junction structure is arranged on the back surface side is called a rear junction type. The solar cell of the present invention may be a front junction type or a rear junction type. In addition, in this specification, the photoelectric conversion layer 130 is described as having an n-type single crystal silicon layer 120 as an example, but the photoelectric conversion layer 130 may have a structure in which the n-type single crystal silicon layer 120 is replaced with a p-type single crystal silicon layer. Furthermore, the p-type semiconductor layer and the n-type semiconductor layer may each be arranged in a comb-tooth shape on the back surface side. In this case, either a p-type semiconductor layer or an n-type semiconductor layer may be disposed on the light-receiving surface side, or neither of these may be disposed on the light-receiving surface side.

The n-type single crystal silicon layer 120 of the photoelectric conversion layer 130 is similar to the n-type single crystal silicon layer of a known solar cell, and is a layer made of n-type single crystal silicon into which n-type impurities such as phosphorus (P) have been introduced. Here, the light-receiving surface side and the back surface side of the n-type single crystal silicon layer 120 may be flat surfaces composed of (100) planes, but it is preferable that a random pyramid texture structure composed of silicon (111) facets is formed on one or both surfaces, and it is more preferable that a random pyramid texture structure is formed on both surfaces. When a random pyramid texture structure is formed on each of the light-receiving surface side and the back surface side of the n-type single crystal silicon layer 120, light incident on the n-type single crystal silicon layer 120 is less likely to be reflected on the surface, and further, the incident light is less likely to be emitted due to the light trapping effect. Therefore, the photoelectric conversion efficiency of the solar cell 200 is likely to be increased.

On the other hand, examples of the p-type semiconductor layer 122 include a layer made of p-type hydrogen-containing amorphous silicon (also referred to as "(p)a-Si:H" in this specification) into which p-type impurities such as boron (B) have been introduced. However, when the above-mentioned transparent conductive film and the p-type semiconductor layer 122 are arranged adjacent to each other, it is preferable that the p-type semiconductor layer 122 is a p-type microcrystalline silicon (also referred to as "(p)nc-Si:H" in this specification) layer containing p-type microcrystalline silicon in a hydrogenated amorphous silicon layer. Alternatively, the p-type semiconductor layer 122 may be an alloy layer of p-type microcrystalline silicon, for example, a p-type microcrystalline silicon oxide (also referred to as "(p)nc-SiO _x :H" in this specification) layer in which oxygen has been added to the hydrogenated amorphous silicon layer in the middle. The higher the oxygen concentration of the microcrystalline silicon oxide, the more transparent it becomes. On the other hand, since the higher the oxygen concentration, the lower the electrical conductivity, it is necessary to select an appropriate range of oxygen concentration, and x is preferably 0.1 to 1.5. The microcrystalline silicon phase contained in these layers is composed of minute silicon crystallites, and the crystal size is preferably on the order of nanometers. As will be described in detail in the examples below, when the above-mentioned transparent conductive film and the (p)a-Si:H film are arranged adjacent to each other, the series resistance of the solar cell 200 may increase. As a result, a decrease in the fill factor may be observed compared to when an ITO film is used as the transparent electrode (here, the light-receiving side transparent electrode 125). In contrast, when the above-mentioned transparent conductive film and the (p)nc-Si:H film or the (p)nc-SiO _x :H film are arranged adjacent to each other, the series resistance does not increase, and electrical characteristics that are comparable to those when ITO is used as the transparent electrode are obtained. The reason why such good electrical characteristics are obtained is that the carrier concentration can be increased in the (p)nc-Si:H or (p)nc-SiO _x :H film compared to the (p)a-Si:H film. As a result, it is believed that the thickness of the depletion layer generated at the interface between the transparent conductive film and the p-type layer becomes smaller, enabling carrier movement due to the tunnel effect and reducing resistance.

Examples of the n-type semiconductor layer 124 include a layer made of hydrogen-containing amorphous silicon (also referred to as "(n) a-Si:H" in this specification) into which n-type impurities such as phosphorus (P) have been introduced. However, when the above-mentioned transparent conductive film and the n-type semiconductor layer 124 are disposed adjacent to each other, it is preferable that the n-type semiconductor layer 124 is an n-type microcrystalline silicon (also referred to as "(n) nc-Si:H" in this specification) layer containing n-type microcrystalline silicon in a hydrogenated amorphous silicon layer. Alternatively, the n-type semiconductor layer 124 may be an alloy layer of n-type microcrystalline silicon, for example, an n-type microcrystalline silicon oxide (also referred to as "(n) nc-SiO _x :H" in this specification) layer in which oxygen is added to a hydrogenated amorphous silicon layer in n-type microcrystalline silicon. The higher the oxygen concentration of the microcrystalline silicon oxide, the more transparent it becomes. On the other hand, since the higher the oxygen concentration, the lower the electrical conductivity, it is necessary to select an appropriate range of oxygen concentration, and x is preferably 0.1 to 1.5. The microcrystalline silicon phase contained in these layers is composed of minute silicon crystallites, and the crystal size is preferably on the order of nanometers. As will be described in detail in the examples below, even when the above-mentioned transparent conductive film and the (n) a-Si:H film are arranged adjacent to each other, the series resistance of the solar cell 200 may increase. In contrast, when the above-mentioned transparent conductive film and the (n) nc-Si:H film or the (n) nc-SiO _x :H film are arranged adjacent to each other, the series resistance does not increase, and electrical characteristics that are comparable to those when ITO is used as a transparent electrode can be obtained.

The i-type semiconductor layers 121 and 123 are similar to the i-type semiconductor layers of known solar cells, and are, for example, layers made of intrinsic amorphous silicon with added hydrogen (also referred to as "(i) a-Si:H" in this specification).

In the solar cell 200, light incident from the grid electrode 126 side is incident on the n-type single crystal silicon layer 120 through the transparent electrode 125, the p-type semiconductor layer 122, and the i-type semiconductor layer 121. Of the light incident on the n-type single crystal silicon layer 120, the light energy larger than the band gap of silicon excites the n-type crystalline silicon, forming electron-hole pairs. The electrons (e ^- ) move to the metal electrode 128 side. On the other hand, the holes (h ⁺ ) move to the grid electrode 126 side, and the solar cell 200 operates.

The manufacturing method of the solar cell 200 is not particularly limited. For example, the photoelectric conversion layer 130 is formed by a known method, and the light-receiving transparent electrode 125 and the back transparent electrode 127 are formed on the photoelectric conversion layer 130. When an ITO film is formed as the light-receiving transparent electrode 125 or the back transparent electrode 127, it can be formed by a sputtering method or the like. On the other hand, when the above-mentioned transparent conductive film is formed as the light-receiving transparent electrode 125 or the back transparent electrode 127, it can be formed by the above-mentioned reactive plasma deposition method or the like. As described above, the transparent conductive film containing amorphous SnO ₂ can be formed at a relatively low temperature (for example, 300° C. or less). Therefore, even if it is laminated on the photoelectric conversion layer 130, there is an advantage that each layer in the photoelectric conversion layer 130 is unlikely to deteriorate. After the light-receiving transparent electrode 125 and the back transparent electrode 127 are formed, the above-mentioned grid electrode 126 and metal electrode 128 are formed by a known method.

(Perovskite solar cells)
The transparent conductive film described above can also be applied to, for example, perovskite solar cells. An example of the structure of a perovskite solar cell is shown in FIG. 5. The perovskite solar cell 400 has a structure in which a substrate/first electrode/first buffer layer/light absorbing (halide-based perovskite material) layer/second buffer layer/second electrode are laminated in this order. Either the first buffer layer or the second buffer layer functions as a hole transport layer, and the other functions as an electron transport layer. In order to achieve a better conversion efficiency, it is desirable for the perovskite solar cell to have these layers, but this is not necessarily required in the embodiment, and either or both of these may not be provided. In addition, both or one of the first buffer layer and the second buffer layer may have a structure in which different materials are laminated. The perovskite solar cell may receive light from the second electrode or from the substrate side. The above-mentioned transparent conductive film can be used for either or both of the first electrode and the second electrode of the perovskite solar cell.

The first electrode and the second electrode may each be a laminate of multiple conductive films, in which case the transparent conductive film may be used for one of the layers. When the first electrode and the second electrode are a laminate of multiple conductive films, the first electrode and the second electrode may have a two-layer structure of the transparent conductive film and an ITO film, or a three-layer structure of the transparent conductive film/ITO film/transparent conductive film. Conventional conductive layers and counter electrodes are generally made of an ITO film, but using the transparent conductive film as part of the film has the advantage of reducing the amount of ITO (especially In) used.

The transparent conductive film described above can be formed at a relatively low temperature. Therefore, when the transparent conductive film is used as a conductive layer, not only substrates made of inorganic materials such as glass plates, but also substrates or films made of resins can be used as substrates. The materials of each layer of the perovskite type are the same as the materials of each layer of known perovskite type solar cells.

(Other solar cells)
The transparent conductive film can be used for the electrodes of solar cells having any structure, such as solar cells other than the above-mentioned Si heterojunction solar cells and perovskite solar cells, for example, TOPCon (Tunnel Oxide Passivated Contact) type solar cells, CdTe solar cells, I-III-VI ₂ group compound solar cells represented by CuInSe ₂ , I ₂ -II-IV-VI ₄ group compound solar cells represented by Cu ₂ ZnSnS ₄ , I ₂ -IV-VI ₃ group compound solar cells represented by Cu ₂ SnS ₃ , I ₂ -VI group compound solar cells represented by Cu ₂ S, II-VI group compound solar cells represented by SnS, solar cells with a multi-junction structure combining the above-mentioned perovskite solar cells and Si-based solar cells, etc. The TOPCon type solar cell here has a structure in which the above-mentioned transparent conductive film is laminated on a semiconductor layer on a tunnel oxide film. In any embodiment, the above-mentioned transparent conductive film may be used as a single layer as an electrode, or a laminate of a known conductive film such as an ITO film and the above-mentioned transparent conductive film may be used as an electrode.

The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited by these in any way.

1. Preparation of Transparent Conductive Film The optical properties and conductivity of the transparent conductive film of the present invention were compared with those of a conventional transparent conductive film (In ₂ O ₃ film) and evaluated by the following method.

(Example 1-1)
A transparent conductive film was prepared by the following reactive plasma deposition method.
As the substrate, a Si substrate (dimensions 30 mm x 30 mm) with a thermal oxide film (thickness 50 nm) and an alkali-free glass substrate (XG manufactured by Corning) (dimensions 100 mm x 100 mm) were prepared and fixed at a predetermined position of the reactive plasma deposition apparatus 100 shown in FIG. 3. In addition, a _SnO2 sintered body, a material for a transparent conductive film, was prepared and stored in the hearth part 10 of the reactive plasma deposition apparatus 100. The temperature of the substrate 1 in the chamber 40 was unheated (room temperature). Furthermore, Ar and _O2 gas were introduced into the chamber 40 of the reactive plasma deposition apparatus 100, and the pressure in the chamber 40 was adjusted to 0.4 Pa (oxygen partial pressure 0.3 Pa). Then, the plasma beam 21 was emitted from the plasma gun 20, and the position of the plasma beam 21 was adjusted by the plasma beam controller 30 so that the plasma beam 21 was perpendicularly incident on the material. At this time, the discharge current value was set to 150 mA. The film was formed under these conditions for 0.7 minutes, and a transparent conductive film ( _SnO2 film) with a thickness of 70 nm was obtained.

When the X-ray diffraction intensity (XRD intensity) of the produced transparent conductive film was measured under the following conditions, no peak was confirmed, and it was confirmed that _SnO2 was amorphous.
(Measurement conditions)
Equipment: RIGAKU SmartLab
X-ray: Cu Kα ray Output: 9kW

Furthermore, when the composition of the produced transparent conductive film was confirmed by ICP analysis, the only constituent metal element was Sn, and the amount of In was below the lower limit of quantification. Furthermore, when the resistivity was measured by a Loresta (low resistivity meter), it was 1.2×10 ⁻³ Ω·cm.

The reflected light of light with wavelengths of 200 nm to 1200 nm was measured for the transparent conductive film on the thermally-oxidized Si substrate using a spectroscopic ellipsometry device. The change in the polarization state of the incident light and reflected light, and the maximum extinction coefficient k of light with wavelengths of 420 nm to 500 nm were confirmed, and the maximum extinction coefficient k in that range was found to be 0.018. Furthermore, the refractive index n of the transparent conductive film for wavelengths of 420 nm to 500 nm was 2.09 to 2.17.

Furthermore, the transmission spectrum and reflection spectrum of light having a wavelength of 220 nm or more and 2500 nm or less were measured using a spectrophotometer for the transparent conductive film formed on an XG glass substrate manufactured by Corning Inc. From the obtained spectrum, the maximum absorption coefficient α of light having a wavelength of 420 nm or more and 500 nm or less was confirmed, and the maximum absorption coefficient α in this range was 3219 cm ^-1 .

In addition, the transparent conductive film was placed in a thermo-hygrostat at a temperature of 85° C. and a humidity of 85% for 1000 hours, and the resistivity was measured in the same manner as above, and it was found to be 1.1×10 ⁻³ Ω·cm, confirming that the moisture resistance was also excellent. In addition, for comparison, a similar test was also performed on an FTO-coated glass substrate (type-VU manufactured by AGC Fabritech Co., Ltd.) with a resistivity of 1.1×10 ⁻³ Ω·cm, and the resistivity after the test was 1.1×10 ⁻³ Ω·cm. In other words, it was confirmed that the transparent conductive film (SnO ₂ film) has moisture resistance comparable to that of an FTO film.

(Example 1-2)
An amorphous _SnO2 film (transparent conductive film) having a thickness of 70 nm was prepared on a substrate by the same reactive plasma deposition method as in Example 1-1, and the transparent conductive film was annealed for 0.5 hours at 200° C. in a nitrogen atmosphere. The resistivity of the transparent conductive film after the annealing treatment was measured by a Loresta (low resistivity meter) and found to be 9.1× ¹⁰⁻⁴ Ω·cm.

Furthermore, the reflected light of light having a wavelength of 200 nm or more and 1200 nm or less was measured for the transparent conductive film on the thermally oxidized Si substrate after the annealing treatment by a spectroscopic ellipsometry device. The change in the polarization state of the incident light and the reflected light, and the maximum extinction coefficient k of light having a wavelength of 420 nm or more and 500 nm or less were confirmed, and the maximum extinction coefficient k in the said range was 0.021. Furthermore, the refractive index n of the transparent conductive film in the wavelength range of 420 nm or more and 500 nm or less was 2.08 to 2.16. The transmission spectrum and reflection spectrum of light having a wavelength of 220 nm or more and 2500 nm or less were measured by a spectrophotometer for the transparent conductive film formed on the Corning XG glass substrate. The maximum absorption coefficient α of light having a wavelength of 420 nm or more and 500 nm or less was confirmed from the obtained spectrum, and the maximum absorption coefficient α in the said range was 5740 cm ^-1 .

Furthermore, when the transparent conductive film was placed in a thermo-hygrostat at a temperature of 85° C. and a humidity of 85% for 1000 hours and the resistivity was measured in the same manner as above, it was 9.6×10 ⁻⁴ Ω·cm, confirming that the film also had excellent moisture resistance.

(Reference Example 1)
An amorphous hydrogen-containing In ₂ O ₃ (a-In ₂ O ₃ :H) film (transparent conductive film) was produced by sputtering on a substrate consisting of a Si substrate (dimensions 50 mm x 50 mm) with a thermal oxide film (thickness 50 nm). The thickness of the a-In ₂ O ₃ :H film was 70 nm. The resistivity of the transparent conductive film was measured with a Loresta (low resistivity meter) and found to be 5.4 x 10 ^-4 Ω·cm.

The reflected light of the transparent conductive film (a-In ₂ O ₃ :H film) with wavelengths of 200 nm to 1200 nm was measured using a spectroscopic ellipsometry device, and the extinction coefficient k in the range was determined from the change in the polarization state of the incident light and the reflected light. Furthermore, the refractive index n in the range was also determined.

(evaluation)
Fig. 6 shows the extinction coefficient k and refractive index n at wavelengths of 200 nm or more and 1200 nm or less for each transparent conductive film produced in Examples 1-1 and 1-2 and Reference Example 1. As shown in Fig. 6, the amorphous SnO ₂ films produced in Examples 1-1 and 1-2 have almost the same extinction coefficient k and refractive index n as the amorphous In ₂ O ₃ :H film of Reference Example 1, and the transparent conductive film of the present invention has excellent optical properties. Also, as described above, the resistivity of the transparent conductive films produced in Examples 1-1 and 1-2 was 1.2 x 10 ^-3 Ω cm or less, and all of them showed excellent conductivity.

(Examples 1 to 3)
As the substrates, a PET (polyethylene terephthalate) substrate (dimensions 100 mm x 100 mm) and an alkali-free glass substrate (XG manufactured by Corning Incorporated) (dimensions 100 mm x 100 mm) were prepared. An amorphous SnO ₂ film (transparent conductive film) having a thickness of 70 nm was produced on the substrate by reactive plasma deposition in the same manner as in Example 1-1. The resistivity of the obtained transparent conductive film was measured by a Loresta (low resistivity meter) and found to be 1.27 x 10 ^-3 Ω·cm on the PET substrate and 1.37 x 10 ^-3 Ω·cm on the glass substrate.

(Examples 1 to 4)
As the substrates, a PET substrate with _SiO2 (dimensions 100 mm x 100 mm) and an alkali-free glass substrate (XG manufactured by Corning Incorporated) (dimensions 100 mm x 100 mm) were prepared. An amorphous _SnO2 film (transparent conductive film) with a thickness of 70 nm was produced on the substrate by reactive plasma deposition in the same manner as in Example 1-1. The resistivity of the obtained transparent conductive film was measured by Loresta (low resistivity meter) to find that the resistivity was 1.34 x ^10-3 Ω·cm for the PET substrate with _SiO2 and 1.34 x ^10-3 Ω·cm for the glass substrate.

(evaluation)
For each transparent conductive film-attached substrate, the transmission spectrum and reflection spectrum of light having a wavelength of 220 nm or more and 2500 nm or less were measured. FIG. 7A shows the transmission spectrum and reflection spectrum of the PET substrate only, the PET substrate and the transparent conductive film (Example 1-3), and the PET substrate with SiO ₂ and the transparent conductive film (Example 1-4). FIG. 7B shows the transmission spectrum and reflection spectrum of the glass substrate only, the glass substrate and the transparent conductive film (Example _1-3 ), and the glass substrate and the transparent conductive film (Example 1-4). As shown in FIG. 7A and FIG. 7B, it was possible to form a desired transparent conductive film (a-SnO ₂ film) on the PET substrate and the PET substrate with SiO 2 by the reactive plasma deposition method without damaging the substrate. In addition, as described above, the resistivity of the transparent conductive films prepared in Examples 1-1 and 1-2 was 1.37×10 ⁻³ Ω·cm or less, and both showed excellent conductivity.

(Examples 1-5 to 1-9)
A transparent conductive film was obtained in the same manner as in Example 1-1, except that the material of the transparent conductive film was a sintered body of a metal oxide shown in Table 3 below. The resistivity of the obtained transparent conductive film is also shown in Table 3. Furthermore, the resistivity when the transparent conductive film was annealed in a nitrogen atmosphere at 250° C. for 0.5 hours is also shown in Table 3.

(Examples 1 to 10)
A transparent conductive film was obtained in the same manner as in Example 1-1, except that the material of the transparent conductive film was a sintered body of a metal oxide shown in Table 3 below. The resistivity of the obtained transparent conductive film is also shown in Table 3. Furthermore, the resistivity when the transparent conductive film was annealed in a nitrogen atmosphere at 200° C. for 0.5 hours is also shown in Table 3.

(evaluation)
As shown in Table 3 above, even when the metal oxide contained 3 atomic % of an element other than Sn (Zn, In, or Ga), the resistivity could be reduced to 2×10 ⁻³ Ω·cm or less. Furthermore, in all transparent conductive films, the resistivity was reduced by annealing (Examples 1-5 to 1-8). Note that when the metal oxide contained 3 atomic % of W as a metal element, the resistivity exceeded 2×10 ⁻³ without annealing, but the resistivity could be reduced to 2×10 ⁻³ Ω·cm or less by annealing.

Similarly, when the metal oxide contained 15 atomic % of an element other than Sn (Ga) as a metal element, the resistivity exceeded 2×10 ⁻³ without annealing, but the resistivity could be reduced to 2×10 ⁻³ Ω·cm or less by annealing (Example 1-10).

2. Fabrication of solar cells (Fabrication of front junction Si heterojunction solar cells)
A front-junction Si heterojunction solar cell was fabricated using the transparent conductive film of the present invention as the light-receiving transparent electrode and/or the back-side transparent electrode, and the external quantum efficiency of the solar cell was compared and evaluated with the external quantum efficiency of a conventional Si heterojunction solar cell using an ITO film as the light-receiving transparent electrode and the back-side transparent electrode.

(Example 2-1)
Preparation of photoelectric conversion layer An n-type single crystal silicon substrate having both surfaces of (100) plane, a thickness of 280 μm, and a resistivity of 2 Ωcm was prepared. The surface of the n-type single crystal silicon substrate was wet etched with a solution mainly composed of KOH to form a random texture structure consisting of (111) facets on both sides of the n-type single crystal silicon substrate.
Next, the natural oxide films on both sides of the n-type single crystal silicon substrate were removed with dilute hydrofluoric acid. Then, an i-type a-Si:H layer (thickness of about 5 nm) and an n-type a-Si:H layer (thickness of about 7 nm) were fabricated on the back side of the n-type single crystal silicon substrate by plasma-assisted chemical vapor deposition (PECVD). Furthermore, an i-type a-Si:H layer (thickness of about 5 nm) and a p-type a-Si:H layer (thickness of about 5 nm) were fabricated on the light-receiving surface side of the n-type single crystal silicon substrate by PECVD. As a result, a photoelectric conversion layer was obtained in which a p-type a-Si:H layer/i-type a-Si:H layer/n-type single crystal silicon layer/i-type a-Si:H layer/n-type a-Si:H layer were stacked in this order from the light-receiving surface side.

- Preparation of the first electrode and the second electrode Transparent electrodes made of amorphous _SnO2 film were formed on both sides of the photoelectric conversion layer in the same manner as in Example 1-1 described above. The thickness of each was set to 75 nm. Furthermore, a grid electrode (width 100 μm, thickness 2 μm) made of Ag was formed on the transparent conductive film (light-receiving side transparent electrode) on the light-receiving side by a sputtering method. A metal electrode made of Ag was formed on the entire surface of the transparent conductive film (on the back transparent electrode) on the back side by a sputtering method. After that, an annealing treatment was performed at 160 ° C. to obtain a front junction Si heterojunction solar cell.

(Example 2-2)
A front junction Si heterojunction solar cell was obtained in the same manner as in Example 2-1 above, except that the transparent electrode on the light-receiving surface side was an amorphous _SnO2 film (thickness 75 nm) and the transparent electrode on the back surface side was an ITO film (thickness 75 nm, prepared by a sputtering method).

(Reference Example 2)
A front junction Si heterojunction solar cell was obtained in the same manner as in Example 2-1 above, except that the transparent electrodes on the light-receiving surface side and the back surface side were each an ITO film (75 nm thick, prepared by sputtering).

(evaluation)
The external quantum efficiency spectra of the front junction Si heterojunction solar cells obtained in Examples 2-1 and 2-2, and Reference Example 2 are shown in Figure 8. As shown in Figure 8, the external quantum efficiency of the solar cell of Example 2-1 in which the transparent electrodes on the light receiving surface side/back surface side were amorphous _SnO2 film/amorphous _SnO2 film, and the solar cell of Example 2-2 in which the transparent electrodes on the light receiving surface side/back surface side were amorphous _SnO2 film/ITO film, were comparable to the external quantum efficiency of the conventional solar cell (Reference Example 2).

3. Fabrication of solar cells (Fabrication of rear junction Si heterojunction solar cells)
A rear junction Si heterojunction solar cell was fabricated using the transparent conductive film of the present invention as the light-receiving transparent electrode and/or the back-side transparent electrode. The current-voltage characteristics of the solar cell were compared and evaluated with those of a conventional rear junction Si heterojunction solar cell using an ITO film as the light-receiving transparent electrode and the back-side transparent electrode.

In addition, a rear junction Si heterojunction solar cell was also fabricated in which the type of semiconductor layer adjacent to the transparent conductive film (a- _SnO2 film) of the present invention was changed, and the current-voltage characteristics of the solar cell were compared with those of a conventional rear junction Si heterojunction solar cell.

(Example 3-1)
A rear junction Si heterojunction solar cell was fabricated by carrying out the same steps as in Example 2-1, except that the photoelectric conversion layer was formed in the following order from the light-receiving surface side: n-type a-Si:H layer/i-type a-Si:H layer/n-type single crystal silicon layer/i-type a-Si:H layer/p-type a-Si:H layer.

(Example 3-2)
A rear junction Si heterojunction solar cell was obtained in the same manner as in Example 3-1 above, except that the transparent electrode on the light-receiving surface side was an amorphous _SnO2 film (thickness 75 nm) and the transparent electrode on the back surface side was an ITO film (thickness 75 nm, prepared by a sputtering method).

(Reference Example 3)
A rear junction Si heterojunction solar cell was obtained in the same manner as in Example 3-1, except that the transparent electrodes on the light-receiving surface side and the back surface side were each an ITO film (75 nm thick, prepared by sputtering).

(Example 3-2a)
A rear junction Si heterojunction solar cell was obtained in the same manner as in Example 3-2, except that the n-type a-Si:H film of the photoelectric conversion layer was changed to an n-type nc-SiOx:H film (thickness: 10 nm).

(evaluation)
9 shows the current-voltage characteristics of the rear junction Si heterojunction solar cells obtained in Examples 3-1 and 3-2, and Reference Example 3. The solar cells of Examples 3-1 and 3-2, in which the transparent conductive film of the present invention was used for the light-receiving transparent electrode and/or the back-side transparent electrode, showed good current-voltage characteristics, but compared with the conventional solar cell (Reference Example 3), the fill factor (FF) decreased with an increase in series resistance.

Therefore, a solar cell (Example 3-2a) in which the n-type semiconductor layer adjacent to the transparent conductive film of the present invention was changed from an a-Si:H layer to an n-type nc-SiOx:H layer was similarly evaluated. The current-voltage characteristics of the solar cell of Example 3-2a and the current-voltage characteristics of the solar cell of Reference Example 3 are shown in FIG. 10. As shown in FIG. 10, when the layer adjacent to the transparent conductive film (a- _SnO2 film) of the present invention was an n-type nc-SiOx:H layer, the fill factor (FF) did not decrease, and current-voltage characteristics very close to those of the conventional solar cell (Reference Example 3) were obtained.

4. Preparation of contact resistance evaluation sample In response to the above results, in order to clarify the cause of the decrease in the fill factor (FF), the contact resistance of the n-type and p-type contact structures was evaluated by the TLM (transmission line measurement) method. The n-type and p-type contact resistance evaluation samples had the same layer structure as the light incident side of the rear junction and front junction Si heterojunction solar cells, respectively. That is, the n-type contact resistance evaluation sample was composed of Ag/transparent conductive film/n-type semiconductor layer/i-type a-Si:H layer/n-type crystalline silicon layer, and the series resistance of the light incident side n-type contact structure of the rear junction Si heterojunction solar cell was evaluated by the n-type contact resistance evaluation sample. The p-type contact resistance evaluation sample was composed of Ag/transparent conductive film/p-type semiconductor layer/i-type a-Si:H layer/p-type crystalline silicon layer, and the series resistance of the light incident side p-type contact structure of the front junction Si heterojunction solar cell was evaluated by the p-type contact resistance evaluation sample. The contact resistance evaluation sample and the solar cell differ in two ways: the laminated portion of Ag/transparent conductive film serving as an electrode is patterned into a rectangular shape, and the p-type contact resistance evaluation sample uses p-type crystalline silicon. For the p-type contact structure of the front junction Si hetero-type solar cell (configuration of Example 2-2), a contact resistance evaluation sample (Example 2-2a below) was prepared by changing the type of semiconductor layer adjacent to the transparent conductive film (a-SnO ₂ film) of the present invention, and the contact resistance value was evaluated. Furthermore, for the p-type contact structure of the solar cell using a conventional ITO film (Reference Examples 2 and 3), a contact resistance evaluation sample (Reference Examples 2a and 3a below) was prepared by changing the type of semiconductor layer adjacent to the ITO film, and the contact resistance value was confirmed.

(Example 2-2a)
A p-type contact resistance evaluation sample was obtained having the same structure as in Example 2-2, except that the p-type a-Si:H layer of the photoelectric conversion layer was changed to a p-type nc-Si:H layer (thickness: 20 nm).

(Reference Example 2a)
A p-type contact resistance evaluation sample was obtained having the same structure as in Reference Example 2, except that the p-type a-Si:H layer of the photoelectric conversion layer was changed to a p-type nc-Si:H layer (thickness: 20 nm).

(Reference Example 3a)
An n-type contact resistance evaluation sample was obtained having the same structure as in Reference Example 3, except that the n-type a-Si:H layer of the photoelectric conversion layer was changed to an n-type nc-SiOx:H layer (thickness 10 nm).

(evaluation)
11 shows the contact resistance value of each n-type contact structure (Ag/transparent conductive film/n-type semiconductor layer/i-type a-Si:H layer/n-type crystalline silicon layer) corresponding to the contact structure on the light incident side of the rear junction Si heterojunction solar cells produced in Example 3-2a, Reference Example 3a, Example 3-2, and Reference Example 3. Similarly, FIG. 11 shows the contact resistance value of each p-type contact structure (Ag/transparent conductive film/p-type semiconductor layer/i-type a-Si:H layer/p-type crystalline silicon layer) corresponding to the contact structure on the light incident side of the front junction Si heterojunction solar cells obtained in Example 2-2a, Reference Example 2a, Example 2-2, and Reference Example 2. As is clear from Fig. 11, in both the front junction type (p-type contact on the light incident side) and rear junction type (n-type contact on the light incident side) configurations, the series resistance was significantly reduced by changing the semiconductor layer adjacent to the transparent conductive film (a- _SnO2 film) of the present invention from an a-Si:H layer to an nc-Si:H layer or an nc- _SiOx :H layer (Examples 3-2a and 2-2a). Furthermore, by adopting such a configuration, it was possible to achieve a series resistance value equivalent to or lower than that of conventional solar cells (Reference Examples 3 and 2).

5. Preparation of transparent conductive film by magnetron sputtering (Example)
A transparent conductive film was prepared by the following method using the RF magnetron sputtering method. First, an alkali-free glass substrate (XG manufactured by Corning) (dimensions 50 mm x 50 mm) was prepared as a substrate. Furthermore, a 3-inch φ _SnO2 sintered body was prepared as a target, and the substrate temperature was unheated (room temperature). Ar gas and _O2 gas were introduced into the sputtering device, the oxygen flow ratio was set to 0.25% or 0.375%, and the chamber pressure was set to 0.5 Pa. Then, at a sputtering input power of 100 W, film formation was performed for 20 minutes each, and a transparent conductive film ( _SnO2 film) with a thickness of 70 nm was obtained. The obtained transparent conductive film was annealed at 200 ° C for 0.5 hours in a nitrogen atmosphere.

The X-ray diffraction intensity (XRD intensity) of the prepared transparent conductive film was measured in the same manner as in Example 1-1. Although a diffraction peak due to SnO ₂ of the rutile structure was slightly observed in each thin film, it was confirmed that the amorphous structure was predominant. In addition, the resistivity of the transparent conductive film was measured by Loresta (low resistivity meter). The resistivity of the transparent conductive film prepared at an oxygen flow rate ratio of 0.25% before annealing was 4.7×10 ⁻³ Ω·cm, and the resistivity after annealing was 2.0×10 ⁻³ Ω·cm. On the other hand, the resistivity of the transparent conductive film prepared at an oxygen flow rate ratio of 0.375% before annealing was 7.0×10 ⁻³ Ω·cm, and the resistivity after annealing was 1.9×10 ⁻³ Ω·cm. In addition, the composition of the prepared transparent conductive film was confirmed by ICP analysis in the same manner as in Example 1-1, and the constituent metal element was only Sn, and In was below the lower limit of quantification. That is, a transparent conductive film ( _SnO2 film) was formed by magnetron sputtering and then annealed, thereby obtaining a transparent conductive film containing a metal oxide (among the metal elements, Sn is 85 atomic % or more and In is 4 atomic % or less) mainly composed of amorphous tin oxide and having a resistivity of 2× ^10-3 Ω·cm or less.

The transmission spectrum and reflection spectrum of light having a wavelength of 220 nm or more and 2500 nm or less were measured using a spectrophotometer for each transparent conductive film that had been annealed. The maximum absorption coefficient α of light having a wavelength of 420 nm or more and 500 nm or less was confirmed from the obtained reflection spectrum. The maximum absorption coefficient α of the transparent conductive film fabricated with an oxygen flow ratio of 0.25% was 1059 cm ^-1 , and the maximum absorption coefficient α of the transparent conductive film fabricated with an oxygen flow ratio of 0.375% was 2585 cm ^-1 .

6. Preparation of substrate with transparent conductive film A
(Example a1)
A PI (polyimide) substrate (thickness 125 μm, dimensions 100 mm × 100 mm) was prepared as a substrate. A transparent conductive film mainly composed of amorphous SnO ₂ was then produced on the PI substrate by reactive plasma deposition under the same conditions as in Example 1-1. The sheet resistance of the obtained transparent conductive film was measured by a Loresta (low resistivity meter). The film thickness, sheet resistance, and specific resistance at this time are shown in Table 4. Although not shown in Table 4, when the composition of the transparent conductive film was confirmed by ICP analysis in the same manner as in Example 1-1, the only constituent metal element was Sn, and In was below the lower limit of quantification.

(Example a2)
The substrate with the transparent conductive film produced in Example (a1) was annealed for 0.5 hours at 250° C. The sheet resistance and specific resistance at this time are shown in Table 4.

(Examples a3, a5, a7, and a9)
On a PI substrate, a reactive plasma deposition method was used to deposit an In ₂ O ₃ :Ce,H transparent conductive film, which is one of the In ₂ O ₃ -based transparent conductive films and is doped with Ce and H. Thereafter, a transparent conductive film mainly composed of amorphous SnO ₂ was prepared in the same manner as in Example a1. The film thickness, sheet resistance, and resistivity of each transparent conductive film are shown in Table 4. The resistivity shown in Table 4 is the resistivity (reference data) when it is assumed that the electrical properties of each transparent conductive film are uniform in the film thickness direction. Although not shown in Table 4, the only metal element constituting the transparent conductive film mainly composed of amorphous SnO ₂ is Sn, and In was below the lower limit of quantification. Furthermore, although not shown in Table 4, it is clear from the resistivity of Example a1 that the resistivity of only the transparent conductive film mainly composed of amorphous SnO ₂ is 2×10 ⁻³ Ω·cm or less.

(Examples a4, a6, a8, and a10)
The transparent conductive film-attached substrates produced in Examples a3, a5, a7, and a9 were each annealed for 0.5 hours at 250° C. The sheet resistance and specific resistance at this time are shown in Table 4.

(result)
As shown in Table 4 above, when a transparent conductive film mainly composed of amorphous _SnO2 was laminated on _an _In2O3 :Ce,H transparent conductive film, the sheet resistance of the substrate with the transparent conductive film monotonically decreased by increasing the ratio of the _In2O3 :Ce,H transparent conductive film (Examples _a3 to a10). In other words, it was confirmed that a very useful substrate with a transparent conductive film can _be obtained by further laminating the transparent conductive film ( _amorphous _SnO2 film) of the present invention on a substrate having a known transparent conductive film (In2O3:Ce,H film).

7. Preparation of substrate with transparent conductive film B
(Examples b1, b3, b5, and b7)
A PI (polyimide) substrate (thickness 125 μm, dimensions 100 mm×100 mm) was prepared as the substrate. Then, under the same conditions as in Example 1-1, a transparent conductive film mainly composed of amorphous SnO ₂ was prepared by reactive plasma deposition. Next, an In ₂ O ₃ :Ce,H transparent conductive film was deposited on the transparent conductive film (amorphous SnO ₂ ) by reactive plasma deposition, as in Example a3. Further, under the same conditions as in Example 1-1, a transparent conductive film mainly composed of amorphous SnO ₂ was prepared by reactive plasma deposition. The film thickness, sheet resistance, and resistivity of each transparent conductive film are shown in Table 5. The resistivity shown in Table 5 is the resistivity (reference data) when it is assumed that the electrical properties of each transparent conductive film are uniform in the film thickness direction. In addition, although not shown in Table 5, the metal element constituting the transparent conductive film mainly composed of amorphous SnO ₂ is only Sn, and In is below the lower limit of quantification. Furthermore, although not shown in Table 5, it is clear from the resistivity of the above-mentioned Example a1 that the resistivity of only the transparent conductive film mainly composed of amorphous SnO ₂ is 2×10 ⁻³ Ω·cm or less.

(Examples b2, b4, b6, and b8)
The transparent conductive film-attached substrates produced in Examples b1, b3, b5, and b7 were annealed for 0.5 hours at 250° C. The sheet resistance and specific resistance at this time are shown in Table 5.

(result)
As shown in Table 5, when a transparent conductive film mainly composed of amorphous _SnO2 / _In2O3 :Ce,H film/transparent conductive film mainly composed of amorphous _SnO2 were laminated, the sheet resistance of the substrate with the transparent conductive film monotonically decreased _by increasing the ratio of _the _In2O3 :Ce,H film. In other words, it was confirmed that a very useful substrate with a transparent conductive film can be obtained by laminating the transparent conductive film of the present invention ( _amorphous _SnO2 film) and a known transparent conductive film ( _In2O3 :Ce,H film).

This application claims priority from Japanese Patent Application No. 2023-026205, filed February 22, 2023. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

The present invention provides a transparent conductive film that can be formed at low temperatures, has high transparency and high conductivity, and contains a small amount of indium. This transparent conductive film can be used in various devices as a substitute for conventional ITO films.

REFERENCE SIGNS LIST 1 Substrate 10 Hearth section 11 Material 20 Plasma gun 21 Plasma beam 22 Plasma 30 Plasma controller 40 Chamber 100 Reactive plasma deposition device 120 n-type single crystal silicon layer 121, 123 i-type semiconductor layer 122 p-type semiconductor layer 124 n-type semiconductor layer 125 Light-receiving transparent electrode 126 Grid electrode 127 Back transparent electrode 128 Metal electrode 130 Photoelectric conversion layer 131 First electrode 132 Second electrode 200 Si heterojunction solar cell 400 Perovskite solar cell

Claims

The metal oxide includes amorphous tin oxide as a main component,
The transparent conductive film, among the metal elements constituting the metal oxide, has an amount of Sn of 85 atomic % or more and an amount of In of 4 atomic % or less, and has a specific resistance of 2×10 −3 Ω·cm or less.
The maximum absorption coefficient at a wavelength of 420 nm or more and 500 nm or less is 1×10 4 cm −1 or less.
The transparent conductive film according to claim 1 .
At least one selected from the group consisting of In, Zn, Cd, Nb, Ta, B, Ga, Ba, Mo, Pb, Rb, Re, Sb, W, Ce, Cs, Dy, Er, Ge, Hf, Ho, La, Lu, Nd, Pr, Sc, Si, Sm, Tb, V, Y, Al, Ti, Zr, and Si;
The transparent conductive film according to claim 1 .
The concentration of hydrogen atoms in the regions 10 nm or more inward from each of the two surfaces, as measured by secondary ion mass spectrometry, is 8× 10 atoms/cm or less.
The transparent conductive film according to claim 1 .
A substrate;
The transparent conductive film according to any one of claims 1 to 4, which is disposed on the substrate;
A substrate with a transparent conductive film comprising:
The substrate is a resin film.
The transparent conductive film-attached substrate according to claim 5 .
A photoelectric conversion layer;
a first electrode including at least one conductive film disposed adjacent to the photoelectric conversion layer;
a second electrode including at least one conductive film disposed adjacent to the photoelectric conversion layer;
having
At least one of the first electrode and the second electrode comprises the transparent conductive film according to any one of claims 1 to 4.
Photoelectric conversion element.
the photoelectric conversion element is a solar cell,
The photoelectric conversion layer includes a single crystal silicon layer doped with n-type or p-type, a p-type semiconductor layer disposed on one side of the single crystal silicon layer, and an n-type semiconductor layer disposed on the other side or the same side of the single crystal silicon layer.
The photoelectric conversion element according to claim 7 .
the p-type semiconductor layer is a p-type microcrystalline silicon layer or an alloy layer thereof, and/or the n-type semiconductor layer is an n-type microcrystalline silicon layer or an alloy layer thereof;
the transparent conductive film is disposed adjacent to the p-type microcrystalline silicon layer or its alloy layer and/or the n-type microcrystalline silicon layer or its alloy layer;
The photoelectric conversion element according to claim 8 .