JP2000012879A - Transparent electrode for photoelectric converter elements and photoelectric converter element using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a low-resistance and low-reflectivity transparent electrode by using a three-layer transparent electrode having an Ag-based thin film sandwiched between oxide thin films for a transparent electrode on a photoelectric conversion layer. SOLUTION: A transparent electrode 16 formed on a photoelectric conversion layer 12 has a three-layer structure having an Ag-based thin film 14 sandwiched between a lower and upper oxide thin films 13, 15 made of a mixed oxide composed of two or more kinds of metal oxides, the Ag-based thin film 14 is of an Ag alloy contg. at least either Au or Cu 3 at.% or less, and the lower oxide thin film 13 contacting the photoelectric conversion layer 12 is formed thicker than the upper oxide thin film 15. Thus a low-resistance and low- reflectivity transparent electrode can be formed from such three-layer transparent electrode 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transparent electrode formed on a photoelectric conversion layer which constitutes a photoelectric conversion element such as a solar cell, and further relates to a photoelectric conversion element using the transparent electrode.

[0002]

2. Description of the Related Art At present, fossil fuels such as petroleum and coal are expected to be depleted in a few decades when calculated based on recoverable reserves. In addition, the gas (carbon dioxide, hydrogen sulfide, etc.) emitted from fossil fuel consumption is polluted,
This can cause problems such as global warming. As means for solving these problems, a photoelectric conversion element such as a solar cell that converts light into electric energy has attracted attention.
That is, since the photoelectric conversion element can convert almost infinite sunlight into electric energy, it can be said that the photoelectric conversion element is a pollution-free and clean energy source.

[0003] At present, efforts are being made to increase the photoelectric conversion efficiency of photoelectric conversion elements such as solar cells, to improve materials and manufacturing processes, to supply them at low cost, and to reduce power generation costs. Development is underway to spread it to the home.

[0004] As a measure for increasing the photoelectric conversion efficiency of the photoelectric conversion element, improvement of the material and structure of a semiconductor used for the photoelectric conversion layer constituting the photoelectric conversion element can be mentioned. Improving the configuration of the entire photoelectric conversion element and making effective use of light incident on the photoelectric conversion element can also be said to be effective as a measure for increasing the photoelectric conversion efficiency. That is, conventionally, a part of light incident on the photoelectric conversion element is reflected from the photoelectric conversion element and is not used (converted). For this reason, the structure of the photoelectric conversion element is a structure having an effect of confining light incident on the photoelectric conversion element in the element, such as a structure called a texture structure, or an anti-reflection film is formed to reflect reflected light. By reducing, the light incident on the photoelectric conversion element can be effectively used, and the photoelectric conversion efficiency can be increased. As described above, various studies are currently being conducted on a method of effectively using incident light.

Here, a photoelectric conversion layer may be mentioned as an element constituting the photoelectric conversion element, and a transparent electrode may be formed on the photoelectric conversion layer in some cases. One of the roles of the transparent electrode is to transmit electricity converted from light by the photoelectric conversion layer. As a material of the transparent electrode, ITO (a mixed oxide composed of tin oxide and indium oxide), SnO ₂ (tin oxide), Zn
O (zinc oxide) and a mixture of zinc oxide and aluminum (Al) are exemplified. The transparent electrode has a low resistance (for example, 5Ω / □) in order to prevent a decrease in the transmission rate when transmitting electricity.
Degree) is often required. However, in order to make the resistance low with the above-mentioned materials, it is necessary to form a transparent electrode as thick as, for example, about 1 μm. When the thickness of the transparent electrode is formed to be large, the transmittance of the transparent electrode is reduced, the light entering the photoelectric conversion layer is reduced, and the conversion efficiency is reduced. In addition, the production cost is increased, and the productivity is reduced. And other problems. In addition, the thickness of the transparent electrode increases the internal stress, causing film peeling and cracking.

When it is required to increase the transmittance of the transparent electrode in order to increase the light incident on the photoelectric conversion layer and increase the conversion efficiency of the photoelectric conversion element, the optical thickness of the transparent electrode (n × d, n is the refractive index of the transparent electrode, d is the thickness of the transparent electrode) to λ / 4 (λ is the wavelength at which the conversion efficiency of the photoelectric conversion element is the highest among the wavelengths of light incident on the photoelectric conversion element). It can be said that setting to be effective is effective. Incidentally, when the material of the transparent electrode is ITO (n = 1.9), it can be said that it is necessary to form the film with a film thickness (d) of about 70 nm in order to obtain a high transmittance.

However, the optical thickness of the transparent electrode (nxd)
Is formed at λ / 4, the thickness of the film becomes thin, so that the resistance value of the transparent electrode becomes high, for example, about 30 Ω / □ to 40 Ω / □, which causes a problem that the transmission efficiency is lowered and the conversion efficiency is rather lowered. It is.

It is a well-known fact that a film having a low specific resistance can be obtained if the film is formed at a temperature of 200 ° C. or higher when the film is formed from the above-mentioned materials such as ITO. Therefore, when a transparent electrode is formed on the photoelectric conversion layer, the photoelectric conversion element may be heated to 200 ° C. or higher to form a film. However, in a photoelectric conversion element such as a thin-film solar cell using a substrate (substrate) having no heat resistance such as a resin film, the substrate (substrate) undergoes deformation such as carbonization, discoloration, and softening when heated. Film formation by heating at 200 ° C. or higher could not be performed. For this reason, in a photoelectric conversion element using a substrate (substrate) having no heat resistance, sufficient heating cannot be performed during film formation, and the transparent electrode formed on the photoelectric conversion layer has high specific resistance and high resistance. It was difficult to obtain a photoelectric conversion element having good conversion efficiency.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems.
Constituting a photoelectric conversion element such as a solar cell, in the transparent electrode formed on the photoelectric conversion layer, a transparent electrode having a low resistance and a low reflectance was obtained, and further, a transparent electrode with improved conversion efficiency was formed. It is intended to obtain a photoelectric conversion element.

[0010]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems. As a result, a transparent electrode having a three-layer structure in which a silver-based thin film is sandwiched between oxide thin films and formed on a photoelectric conversion layer was proposed by the present inventors in Japanese Patent Application Laid-Open No. 8-262466. It has been found that the above-mentioned problem can be solved by using this method, and the present invention has been accomplished. That is, in claim 1 of the present invention, in the transparent electrode formed on the photoelectric conversion layer, the transparent electrode has a three-layer structure in which a silver-based thin film is sandwiched between a lower oxide thin film and an upper oxide thin film. The oxide thin film is a mixed oxide composed of two or more metal oxides, and the silver thin film is silver containing at least one of Au (gold) or Cu (copper) at 3 at% (atomic percent) or less. A transparent electrode for a photoelectric conversion element, characterized in that the thickness of the lower oxide thin film in contact with the photoelectric conversion layer is formed larger than the thickness of the upper oxide thin film.

A transparent electrode having a three-layer structure in which a silver-based thin film is sandwiched between oxide thin films proposed by the present inventors can realize an electrode having low reflectance and low resistance. However, with such a configuration, it can be said that the silver in the silver-based thin film as the intermediate layer is likely to move and cause migration, and the durability of the transparent electrode is poor. For this reason, the silver alloy used as the silver-based thin film is a silver alloy containing 3 at% (atomic percent) or less of at least one of gold and copper, and the oxide thin film is made of a mixture of two or more metal oxides. If oxide is used, silver can be stabilized and durability can be improved. Further, since the refractive index of the photoelectric conversion layer surface serving as a support for the transparent electrode is high,
In order to suppress the reflection component on the surface of the photoelectric conversion layer, it is proposed that the thickness of the lower oxide thin film is formed to be larger than the thickness of the upper oxide thin film.

The present inventors have also studied the thickness of each layer constituting the transparent electrode, which can lower the reflectance of the transparent electrode. As a result, the optical thickness of the lower oxide thin film (n × d, a value obtained by multiplying the refractive index n by the thickness d)
To 150 to 350, the optical thickness of the silver-based thin film (nxd)
Is set to 0.8 to 3 and the optical thickness (nxd) of the upper oxide thin film is set to a range of 60 to 150, the reflectance of the transparent electrode can be reduced. It was obtained from the standpoint of empirical and empirical suggestions. That is, in the invention according to claim 2, the optical thickness (nxd) of the lower oxide thin film is 150 to 350, the optical thickness (nxd) of the silver-based thin film is 0.8 to 3, and In addition, the transparent electrode for a photoelectric conversion element is characterized in that the optical film thickness (nxd) of the upper oxide thin film is 60 to 150.

Next, a semiconductor such as a-Si (amorphous silicon), which constitutes the photoelectric conversion layer and is in contact with the transparent electrode having the three-layer structure, has a high refractive index (n) of, for example, 3 or more. For this reason, it can be said that a higher refractive index of the oxide thin film can ensure a higher transmittance. The present inventors formed a three-layer transparent electrode on the photoelectric conversion layer made of a-Si by changing the refractive index of the oxide thin film, and examined changes in the reflectance of each of the electrodes. As shown in FIG. 3 below, if the refractive index of the oxide thin film is 1.9 or more,
It is empirically found that the reflectance can be reduced in a wide wavelength range, that is, the transmittance can be increased, and this is proposed. Here, in FIG. 3, a curve F indicates a spectral reflectance when the refractive index of the oxide thin film is 1.7, and similarly, a curve G indicates a refractive index of the oxide thin film is 1.8. The spectral reflectance at the time, curve H is the spectral reflectance when the refractive index of the oxide thin film is 1.9, and the curve I is the refractive index of the oxide thin film at 2.0.
And the curve J indicates the spectral reflectance when the refractive index of the oxide thin film is 2.1. As shown in FIG. 3, when the refractive index of the oxide thin film is higher than 1.9, the refractive index of the oxide thin film is 90 in the wavelength range of 500 to 700 nm where the amount of sunlight is large.
% Or more can be secured. In FIG. 3,
When the refractive index (n) of the oxide thin film changes, the optical thickness (nxd) also changes, but all the optical thicknesses fall within the scope of claim 2 described above.

That is, the invention according to claim 3 is a transparent electrode characterized in that the refractive index of the oxide thin film is higher than 1.9.

Next, in order to suppress the movement of silver which easily causes grain boundary diffusion, the oxide thin film needs to be an oxide thin film of a dense microcrystal having at least very small crystal grains. It is desirable that the material be amorphous without a boundary (amorphous).

That is, a fourth aspect of the present invention is a transparent electrode for a photoelectric conversion element, wherein the base material of the oxide thin film is an amorphous or fine crystal mixed oxide containing indium oxide. It is what it was.

Here, as an additive material for increasing the refractive index of the mixed oxide containing indium oxide as a base material, an oxide of a metal having a large atomic weight such as titanium oxide, cerium oxide, tantalum oxide, and zirconium oxide may be mentioned. Can be
As a result of the study, the present inventors have found that cerium oxide is most preferable as a material for increasing the refractive index of a mixed oxide containing indium oxide as a base material. That is, the transparent electrode may be subjected to an etching process to obtain a predetermined shape. However, if a mixed oxide of indium oxide and cerium oxide is used, it is possible to have both an appropriate etching property and a high refractive index. That's why.

That is, the invention according to claim 5 provides a transparent electrode for a photoelectric conversion element, characterized in that an oxide having a higher refractive index than an oxide used as a base material of the oxide thin film is added to the oxide thin film. 7. The transparent electrode for a photoelectric conversion element according to claim 6, wherein the oxide thin film is composed of a mixed oxide thin film containing indium oxide as a base and to which cerium oxide is added. It is what it was.

The refractive index of cerium oxide in a bulk state is about 2.5, and the refractive index of indium oxide serving as a base material of the mixed oxide is slightly higher than 1.9.
The refractive index of the mixed oxide increases according to the amount of cerium oxide added to the mixed oxide. Here, it can be said that the amount of cerium oxide added may be adjusted according to the purpose of use of the transparent electrode. However, a few at% (atomic percent, not counting oxygen, counting only metal elements) in indium oxide
When a sputtering target is formed by adding a small amount of cerium oxide, when forming a mixed oxide thin film by sputtering or the like using a sputtering target, it is possible to secure a stable sputtering rate without depending on the amount of oxygen introduced. The following effects occur, for example,
It can be said that the addition amount of cerium oxide may be adjusted within the range of 1 to 40 at% (atomic percent, not counting oxygen, but counting only metal elements). In the case of indium oxide or ITO, the sputtering rate tends to decrease as the amount of introduced oxygen increases, so the sputtering rate is checked every time the amount of introduced oxygen is changed, and the film thickness to be formed is set. There is a need.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. <Embodiment 1> As shown in FIG. 1, a photoelectric conversion element 17 according to the present embodiment 1 comprises a plastic film 10, a lower silver electrode 11, a photoelectric conversion layer 12 using a-Si, and a transparent electrode.
16 are sequentially stacked. The transparent electrode 16 is, in order from the surface in contact with the photoelectric conversion layer 12, a lower oxide thin film layer 13, a silver-based thin film layer.
14 and an upper oxide thin film layer 15 in a three-layer structure. In the first embodiment, the thickness of the lower oxide thin film layer 13 is 120 nm, and the thickness of the silver-based thin film layer 14 is 10 nm.
m and the thickness of the upper oxide thin film layer 15 were set to 40 nm.

The oxide thin film layer (lower oxide thin film layer 13)
The upper oxide thin film layer 15) was formed of a mixed oxide composed of indium oxide and cerium oxide. The composition of the mixed oxide was 66.7 at% (atomic percent) for indium oxide and 33.3 at% (atomic percent) for cerium oxide. Note that the above ratio is a ratio of a metal element which does not count an oxygen element.

The silver-based thin film 14 is composed of silver (Ag) and gold (A
u) and copper (Cu), and the composition is as follows: silver 98.5 at% (atomic percent), gold 1.0 a
The atomic ratio was t% (atomic percent) and copper 0.5 at% (atomic percent).

Next, the transparent electrode 16 according to the first embodiment is
Was formed by the manufacturing process described below. First, after the lower silver electrode 11 and the photoelectric conversion layer 12 were formed on the plastic film 10, the plastic film 10 was put into a sputtering device, and then the device was evacuated. When the degree of vacuum in the apparatus reached 5 × 10 ⁻⁴ Pa, Ar (argon) gas and O ₂ (oxygen) gas were introduced, and the gas pressure in the apparatus was set to 0.4 Pa. At this time, the O ₂ (oxygen) gas was adjusted to be 1.5% of the total gas amount.

Next, a lower oxide thin film layer 13 is formed on the surface of the photoelectric conversion layer 12 on the plastic film 10 by applying a discharge voltage to the mixed oxide target having the above-mentioned composition and placed in the apparatus. did.

When the formation of the lower oxide thin film layer 13 was completed, the discharge and the introduction of gas were stopped. Thereafter, when the inside of the apparatus was evacuated to 5 × 10 ⁻⁴ Pa, Ar (argon) gas was introduced, and the gas pressure in the apparatus was adjusted to 0.4 Pa. Next, a silver-based thin film layer 14 was formed on the lower oxide thin film layer 13 by applying a discharge voltage to a silver alloy target having the above-described composition and placed in the apparatus.

When the formation of the silver-based thin film layer 14 was completed, the discharge and the introduction of gas were stopped. Thereafter, the inside of the apparatus is evacuated again, and when the degree of vacuum in the apparatus reaches 5 × 10 ⁻⁴ Pa, Ar (argon) gas and O ₂ (oxygen) gas are introduced, and the gas pressure in the apparatus is reduced. 0.4 Pa. Also at this time, the O ₂ (oxygen) gas was adjusted to be 1.5% of the total gas amount.

Next, an upper oxide thin film layer 15 is formed on the silver-based thin film layer 14 by applying a discharge voltage to the mixed oxide target having the above-mentioned composition and placed in the apparatus. Thus, a transparent electrode 16 having a three-layer structure related to 1 was obtained.

The above-described series of film forming steps are performed without heating the plastic film 10 in the apparatus, and the film formation is continuously performed while maintaining the degree of vacuum in the apparatus at the above-mentioned value. went.

The curve A in FIG. 2 shows the spectral reflectance of the photoelectric conversion element 17 on which the transparent electrode 16 having the three-layer structure obtained in Example 1 is formed. As is clear from FIG. 2, the reflectance when the transparent electrode 16 of Example 1 was formed (curve A).
Is 1 in a wide wavelength range from 500 nm to 700 nm.
It is 0% or less, and the resistance value is 5 Ω / □, which is a low-resistance transparent electrode. In FIG. 2, the vertical axis represents the reflectance (%) and the horizontal axis represents the wavelength (nm). For reference, conventional ITO (a mixed oxide composed of tin oxide and indium oxide) is used. The spectral reflectance of the formed single-layer transparent electrode is also shown. The dashed line B in FIG. 2 indicates the spectral reflectance when the film thickness is 2300 Å of ITO.
The dashed line C indicates the ITO having a thickness of 10,000 Å.
The two-dot chain line D indicates the spectral reflectance when the thickness of the ITO is 700 angstroms.

<Embodiment 2> In Embodiment 2, as in Embodiment 1 described above, a plastic film 10
A lower silver electrode 11, a photoelectric conversion layer 12 made of a-Si, and
The transparent electrodes 16 having a three-layer structure were stacked to form a photoelectric conversion element 17. Further, in the same manner as in Example 1, the lower oxide thin film layer 13, the silver-based thin film layer 14, and the upper oxide thin film layer 15 were sequentially laminated from the surface in contact with the photoelectric conversion layer 12, thereby forming a transparent electrode 16. .

In Example 2, mixed oxides composed of indium oxide, cerium oxide, and zinc oxide were used for the oxide thin film layers (the lower oxide thin film layer 13 and the upper oxide thin film layer 15). Its composition is 68.1 indium oxide.
at% (atomic percent), 14.3 at% cerium oxide
(Atomic percent) and zinc oxide at 17.6 at% (atomic percent). Note that the above ratio is a ratio of a metal element which does not count an oxygen element.

The silver-based thin film layer 14 is formed of a silver alloy in which silver (Ag) is mixed with gold (Au) and copper (Cu), and has a composition of 98.5 at% (atomic percent) of silver and gold.
The atomic ratio was 1.0 at% (atomic percent) and 0.5 at% (atomic percent) copper.

Next, the transparent electrode 16 according to the second embodiment
Was formed by the manufacturing process described below. First, after the lower silver electrode 11 and the photoelectric conversion layer 12 were formed on the plastic film 10, the plastic film 10 was put into a sputtering device, and then the device was evacuated. When the degree of vacuum in the apparatus reached 5 × 10 ⁻⁴ Pa, Ar (argon) gas was introduced, and the gas pressure in the apparatus was adjusted to 0.4 Pa.

In the second embodiment, the lower oxide thin film layer 13, the silver-based thin film layer 14, and the upper oxide thin film layer 15 flow through the plastic film 10 conveyed at a predetermined speed in the sputtering apparatus. It is formed continuously in operation. Each target (mixed oxide target and silver alloy target) placed beforehand in the sputtering device
Will not contaminate other targets during each deposition step.
A predetermined interval is set in the sputtering apparatus, and the apparatus is placed in a shielded state. Further, sufficient exhaust was performed to prevent unnecessary contamination of the target and the plastic film 10. In the second embodiment, at the time of forming the lower oxide thin film layer 13 and the upper oxide thin film layer 15, 1.5% oxygen gas was introduced into the apparatus in addition to Ar (argon) gas. In forming the silver-based thin film layer 14, only Ar (argon) gas was introduced. During the sputtering, the plastic film 10 was not heated.

In the second embodiment, the transparent electrode 16 having a three-layer structure is formed on the plastic film 10 (that is, the photoelectric conversion layer 12) by the above-described method to form the photoelectric conversion element 17, and then the photoelectric conversion element 17 is formed. 17 was subjected to a baking treatment of heating at 200 ° C. for one hour. Thereby, finally, the thickness of the lower oxide thin film layer 13 is 120 nm, and the thickness of the silver-based thin film layer 14 is 10 nm.
A transparent electrode 16 having a three-layer structure in which the thickness of the upper oxide thin film layer 15 was 40 nm was obtained.

When the reflectance of the transparent electrode 16 according to the second embodiment was measured, it was found that the reflectance was 10 at a wavelength of 500 nm to 700 nm.
% Or less. Further, when the resistance value was measured, it was as low as 5 Ω / □.

[0037]

The present invention constitutes a photoelectric conversion element such as a solar cell.
In the transparent electrode formed on the photoelectric conversion layer, ITO, S
A conventional transparent electrode made of nO ₂ , ZnO, a mixture of zinc oxide and aluminum, and the like, has to be thickened in order to reduce the resistance, in which case the reflectance increases and the photoelectric conversion element Conversion efficiency was reduced. Further, when the transparent electrode is made thin in order to improve the conversion efficiency of the photoelectric conversion element, the resistance value of the transparent electrode becomes high and a desired low resistance cannot be obtained.

However, the transparent electrode of the present invention having the above-mentioned three-layer structure has a low reflectance, and has a film thickness of, for example, about 1/5 that of a conventionally used transparent electrode.
A low resistance of about Ω / □ can be obtained. That is, by forming the transparent electrode of the present invention on the photoelectric conversion layer, the efficiency of light utilization and the efficiency of converted electric transport are increased, and a photoelectric conversion element with improved conversion efficiency can be obtained.

[0039]

[Brief description of the drawings]

FIG. 1 is an explanatory cross-sectional view showing one embodiment of a transparent electrode for a photoelectric conversion element of the present invention.

FIG. 2 is a graph showing an example of spectral reflectance obtained by a transparent electrode for a photoelectric conversion element of the present invention and a conventional transparent electrode.

FIG. 3 is a graph showing a change in reflectance when the refractive index of an oxide thin film is changed in a transparent electrode having a three-layer structure.

[Explanation of symbols]

 10 Plastic film 11 Lower silver electrode 12 Photoelectric conversion layer 13 Lower oxide thin film layer 14 Silver-based thin film layer 15 Upper oxide thin film layer 16 Transparent electrode 17 Photoelectric conversion element

Claims (7)

[Claims] 1. A transparent electrode formed on a photoelectric conversion layer constituting a photoelectric conversion element, wherein the transparent electrode is a three-layer structure in which a silver-based thin film is sandwiched between a lower oxide thin film and an upper oxide thin film. In addition, the oxide thin film is a mixed oxide composed of two or more metal oxides, and the silver-based thin film contains at least one of Au (gold) or Cu (copper) at 3 at% (atomic percent) or less. A transparent electrode for a photoelectric conversion element, comprising a silver alloy, wherein a thickness of a lower oxide thin film in contact with a photoelectric conversion layer is formed to be larger than a thickness of an upper oxide thin film. 2. The lower oxide thin film has an optical thickness (nxd) of 150 to 350, the silver-based thin film has an optical thickness (nxd) of 0.8 to 3, and the upper oxide thin film has an optical thickness (nxd) of 0.8 to 3. Optical thickness of thin film (nx
The transparent electrode for a photoelectric conversion element according to claim 1, wherein d) is set to 60 to 150. 3. The transparent electrode for a photoelectric conversion element according to claim 1, wherein the refractive index of the oxide thin film is higher than 1.9. 4. The photoelectric conversion element according to claim 1, wherein the base material of the oxide thin film is an amorphous or microcrystalline mixed oxide containing indium oxide. Transparent electrode. 5. The photoelectric conversion element according to claim 1, wherein an oxide having a higher refractive index than an oxide used as a base material of the oxide thin film is added to the oxide thin film. For transparent electrodes. 6. The oxide thin film according to claim 1, wherein the oxide thin film is composed of a mixed oxide thin film containing indium oxide as a base material and to which cerium oxide is added. Transparent electrode for photoelectric conversion element. 7. A photoelectric conversion element having a structure in which at least a photoelectric conversion layer and a transparent electrode are laminated on a base material, wherein the transparent electrode formed on the photoelectric conversion layer is the photoelectric conversion element according to any one of claims 1 to 6. A photoelectric conversion element comprising a transparent electrode for an element.