JP2001156307A - Material for forming optical absorption layer of solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a material capable of forming an optical absorption layer of a solar cell capable of manufacturing at a low cost with a high conversion efficiency and having a small initial deterioration. SOLUTION: The material for forming the optical absorption layer of the solar cell comprises at least one type of oxide selected from an oxide having a perovskite crystal structure represented by formula: an R2CuO4 (wherein R is a rare earth element) and an oxide having a perovskite crystal structure represented by formula: an XZ2Cu3O6 (wherein X is yttrium, lanthanum or praseodymium, and Z is an alkali earth metal).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a material for forming a light absorbing layer of a solar cell and a solar cell having a light absorbing layer formed from the material.

[0002]

2. Description of the Related Art The current solar cell is a photovoltaic power generation technology and application (September 1999,
Mon, JSAP Catalog Number: AP9
9133), silicon is mainly used as the light absorbing layer, and its conversion efficiency is about 15%. And in order to achieve such conversion efficiency,
The impurity concentration in silicon needs to be 10 ^-6 or less as a ratio of the number of atoms, and enormous energy is required for manufacturing.

[0003] Solar cells using silicon as a light absorbing layer include those using amorphous silicon and those using single crystal or polycrystalline silicon. Of these,
A solar cell using amorphous silicon has a drawback that initial deterioration of energy conversion efficiency occurs by about 20%. A solar cell using single crystal or polycrystalline silicon requires a heating step of 1000 ° C. or more to form a light absorption layer. It requires a complicated process that consumes a large amount of energy, requires a light absorbing layer having a thickness of about 0.3 mm or more, and consumes a large amount of semiconductor material.

[0004]

SUMMARY OF THE INVENTION The main object of the present invention is to:
It is an object of the present invention to provide a material which can form a light absorbing layer of a solar cell which has high conversion efficiency, can be manufactured at low cost, and has little initial deterioration.

[0005]

The present inventors have conducted intensive studies in view of the problems of the prior art as described above. As a result, the light absorbing layer formed of a specific oxide having a perovskite type crystal structure has been developed. It has the property that it can be manufactured at low cost, has good light absorption, and can travel a long distance without disappearing electrons and holes formed at that time.
It has been found that a solar cell having high energy conversion efficiency can be manufactured by utilizing this characteristic, and the present invention has been completed.

That is, the present invention provides the following material for forming a light absorbing layer of a solar cell, and a solar cell. 1. An oxide having a perovskite crystal structure represented by a general formula: R ₂ CuO ₄ (where R is a rare earth element), and a general formula: XZ ₂ Cu ₃ O ₆ (where X is yttrium and lanthanum) Or a material for forming a light absorption layer of a solar cell, comprising at least one oxide selected from oxides having a perovskite crystal structure represented by praseodymium, and Z is an alkaline earth metal. 2. Item 2. The material for forming a light absorbing layer according to Item 1, wherein the band gap energy is in the range of 1.5 to 2.3 eV. 3. Item 3. A solar cell having a light absorption layer formed of the material for forming a light absorption layer according to item 1 or 2. 4. The maximum light absorption coefficient of the light absorbing layer in the visible region is κ (c
m ^-1 ), the thickness L (cm) of the light absorbing layer is
Item 4. The solar cell according to the above item 3, wherein 0.5 / κ <L <5 / κ. 5. When the electrical conductivity σ in the thickness direction of the light absorbing layer is σ <10 ⁻²
The solar cell according to the above item 3 or 4, which is ((Ωcm) ⁻¹ ). 6. Item 6. The solar cell according to any one of Items 3 to 5, wherein an ac surface of the oxide having a perovskite crystal structure is oriented in a thickness direction of the light absorbing layer. 7. Item 7. The solar cell according to any one of Items 3 to 6, wherein the light absorption layer is formed by a sputtering method. 8. Item 7. The solar cell according to any one of Items 3 to 6, wherein the light absorption layer is formed by a laser aberration method. 9. Item 7. The solar cell according to any one of Items 3 to 6, wherein the light absorption layer is formed by a metal complex salt pyrolysis method. 10. The above items 3 to 6, wherein the light absorption layer is formed by a method of compressing a fine powder of an oxide having a perovskite crystal structure on the electrode or a method of applying a paste containing the fine powder to the electrode and drying the paste. The solar cell according to any one of the above. 11. Item 11. The solar cell according to any one of Items 3 to 10, wherein a light reflection layer is formed on a side of the light absorption layer opposite to the incident light.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The material for forming a light absorbing layer of a solar cell according to the present invention is an oxide having a perovskite crystal structure represented by the general formula: R ₂ CuO ₄ (where R is a rare earth element). And an oxide having a perovskite-type crystal structure represented by the general formula: XZ ₂ Cu ₃ O ₆ (where X is yttrium, lanthanum, or praseodymium, and Z is an alkaline earth metal). And at least one oxide.

In the above general formula, as the rare earth element represented by R, for example, La, Pr, Nd, Sm, E
u, Gd and the like can be exemplified, and as the alkaline earth metal represented by Z, Ba, Sr, Ca and the like can be exemplified. These oxides can be used alone or in combination of two or more.

When a light absorbing layer of a solar cell is formed using an oxide having such a perovskite type crystal structure, the efficiency of energy conversion can be increased for the following reasons.

That is, in the oxide having a perovskite type crystal structure used in the present invention (hereinafter referred to as “perovskite type oxide”), one of the ac planes is composed of a CuO ₂ plane. Here, Cu and O are actually Cu
²⁺ and O ²⁻ , and as shown in the schematic diagram of FIG.
Since the 3d orbital of u ²⁺ and the 2p orbital of O ²⁻ constitute a hybrid orbital, the mobility of electrons in the CuO ₂ plane is very large. In addition, the absorption coefficient κ of light having an electric field vector parallel to the CuO ₂ surface is also very large at about κ = 10 ⁴ cm ⁻¹ .
Here, the light absorption coefficient κ is between the incident intensity I ₀ of the light with the ratio and the optical path length L of the transmitted light intensity I _t, the formula: I _t / I ₀ = e
xp (−κL). For this,
The perovskite oxide has a characteristic that it absorbs light well and can move a long distance without annihilation of generated electrons and holes. By utilizing this characteristic, a solar cell having a thin light absorption layer can be manufactured, and the electrons and holes generated therein reach the electrodes without disappearing. For this reason, a solar cell with high energy conversion efficiency can be manufactured.

The above-mentioned perovskite oxide has a band gap between 1.5 eV and 2.3 eV, and a light absorption layer material of a conventional solar cell has a band gap of about 1.1 eV. Compared to
It has a very large band gap. For this reason, the light absorption layer formed from the perovskite oxide has a thickness of 20%.
% And high energy conversion efficiency
When the conventional light absorbing layer material is used, the energy conversion efficiency is higher than when the energy conversion efficiency is at most about 15%.

In particular, the above general formula: XZ_TwoCu_ThreeO₆so
Among the oxides represented, YBa_TwoCu _ThreeO₆about,
The band gap becomes 1.4 to 1.6 eV,
It is a material that always has excellent energy conversion efficiency.

The solar cell of the present invention has a light absorbing layer formed of the above-described perovskite oxide and includes a structure in which a lower electrode, a light absorbing layer, and a transparent electrode are sequentially formed. Such a solar cell includes, in addition to the above-described solar cell formed only of the lower electrode, the light absorbing layer, and the transparent electrode, a light absorbing layer and a transparent electrode on a transparent electrode of a glass substrate on which the transparent electrode is formed. And a solar cell having a structure in which a lower electrode is formed on a substrate such as plastics or ceramics, and a light absorbing layer and a transparent electrode layer are sequentially laminated on the lower electrode.

In the solar cell of the present invention, components other than the light absorbing layer may be the same as those of a known solar cell.
For the transparent electrode and the lower electrode, known materials conventionally used in solar cells may be used, and metallic materials,
Various materials such as semiconductors can be used.

A method for forming a light absorbing layer made of a perovskite oxide represented by the above general formula is as follows.
There is no particular limitation, and a known oxide layer forming method can be applied.

For example, according to a method in which a perovskite oxide formed by a sintering method is pulverized and a light absorbing layer is formed by using the obtained oxide powder, the light absorbing layer having a small amount of impurities can be obtained by a simple method. Is very advantageous since In this method, for example, first, an oxide containing a constituent element of a target perovskite oxide is used as a raw material, and these raw materials are mixed so as to have a mixing ratio corresponding to the target substance, and then fired. To synthesize a perovskite-type oxide, and then the average particle size is 1 to 5 μm.
Pulverize to a degree to produce an oxide powder. The conditions for synthesizing the perovskite oxide are not particularly limited,
For example, by firing at about 900 to 1100 ° C. in an oxidizing atmosphere such as air or a reducing atmosphere such as a hydrogen gas atmosphere, a target perovskite oxide can be formed. Thereafter, the obtained oxide powder is compressed on the lower electrode, or a transparent resin such as polymethyl methacrylate or polycarbonate is added as a binder to the oxide powder and applied as a paste to the lower electrode, followed by drying. Thereby, a light absorption layer can be formed.

In addition, various methods such as a sputtering method, a laser average method, a molecular beam epitaxy (MBE) method, a spray method, a sol-gel method, a metal complex salt pyrolysis method, an evaporation method, and a chemical vapor deposition (CVD) method are used. Light absorbing layer can be formed.

Among these, the metal complex salt pyrolysis method refers to a method in which a carboxylate, a fatty acid salt, a chelate compound, a diketone, an acetylacetonate complex or the like containing a metal component constituting a target perovskite oxide is used as a raw material. In this method, a perovskite-type oxide layer is formed by mixing the raw material compounds, applying the mixture to the lower electrode, and then thermally decomposing the mixture. In particular, the method of using a fatty acid salt as a raw material is a method known as a metal soap firing method, which is applied and then fired at a temperature of about 300 ° C. or more in air to obtain a target perovskite type. An oxide layer can be formed.

In each of the other methods, a desired perovskite oxide layer can be formed according to known conditions.

Although the thickness of the light absorbing layer is not particularly limited, in order to efficiently absorb sunlight, the maximum light absorption coefficient of the light absorbing layer in the visible region is κ (cm ⁻¹ ). In this case, the thickness L (cm) of the light absorbing layer is 0.5 / κ <L
It is preferable to be within the range of <5 / κ.

It is preferable that the light absorbing layer does not exhibit conductivity under darkness. Specifically, the electrical conductivity σ in the thickness direction of the light absorbing layer is σ <10 ^-2 (( Ωc
m) ^-1 ) are preferred. Such a light-absorbing layer that does not exhibit conductivity in the dark can be formed by reducing the impurity concentration as much as possible in each of the above-described manufacturing methods. For example, when a perovskite-type oxide is formed by a firing method, an oxide having an electrical conductivity of σ <10 ⁻² ((Ωcm) ⁻¹ ) in the dark is formed by firing in a reducing atmosphere. can do. In such a perovskite oxide that does not exhibit conductivity under darkness, carriers are generated only at the time of light irradiation and move in the CuO ₂ plane. At this time, the mobility of the carriers in the CuO ₂ plane is very high as seen in superconductivity, and the impurities are small.
The carrier once generated reaches the electrode with almost no deactivation. Thereby, a solar cell with high photoelectric conversion efficiency can be realized.

The light absorbing layer is formed on the ac surface (Cu surface) of a perovskite oxide for efficient transport of electrons and holes.
(O ₂ plane) is preferably oriented in the film thickness direction. The light-absorbing layer having such a structure can be formed by molecular beam epitaxy (MB
The oxide film can be formed by forming an oxide film by E) and then performing a heat treatment. In addition, after using a large single crystal of perovskite oxide and bringing it into close contact with the lower electrode,
It can also be formed by a method of shaving the surface by ion milling.

In the solar cell of the present invention, it is particularly preferable to provide a light reflecting layer on the side opposite to the incident light of the light absorbing layer, that is, on the lower electrode side of the light absorbing layer. By providing the light reflection layer, light passes through the light absorption layer a plurality of times, and the energy conversion efficiency can be further increased. As the reflective layer, for example, an aluminum layer, a silver layer, or the like can be used. The thickness of the light absorbing layer is preferably 100 nm or more in order to efficiently reflect light,
More preferably, it is about 0 nm or more.

The light reflecting layer is formed between the lower electrode and the light absorbing layer, or when the lower electrode is formed of a transparent electrode, the light reflecting layer is formed between the lower electrode and the substrate. Can be. Further, when the lower electrode is formed of a metal such as aluminum or silver, these metals can also serve as the lower electrode and the light reflection layer.

[0025]

The solar cell of the present invention uses a specific perovskite oxide as the light absorbing layer, and has a higher light absorption rate than the conventional solar cell using silicon as the light absorbing layer. Since the lifetime of generated electrons and holes is long and the mobility is large, light energy can be efficiently converted to electric energy.

The perovskite-type oxide can be easily produced with a sufficiently small amount of impurities by a simple method such as firing in clean air, so that it can be formed at a low production cost. In addition, since it is stable in air and has high mechanical strength, there is an advantage that deterioration due to use is small.

[0027]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. Example 1 A fine powder obtained by mixing La ₂ O ₃ and CuO at a molar ratio of La and Cu of 2: 1 was mixed in a dry air atmosphere with 100 powder.
It was baked at 0 ° C. for 1 hour. Furthermore, this is added to 10% by volume of hydrogen
La ₂ CuO ₄ after reduction for 3 hours at 135 ° C. in nitrogen containing
I got The appearance of the obtained oxide was a black powder, and as a result of X-ray diffraction spectrum measurement, the crystal structure was found to be a perovskite containing one layer of CuO ₂ planes in the unit cell as shown in the schematic diagram of FIG. It had a type crystal structure (single-layer perovskite).

This oxide was pulverized to have an average diameter of about 2 μm. When the electric conductivity σ of this powder was measured, it was about 10 ⁻⁵ (Ωcm) ⁻¹ .

Using the thus obtained La ₂ CuO ₄ oxide powder as a material for forming a light absorbing layer, a solar cell having a structure shown in a sectional view in FIG. 3 was produced.

That is, a silver plate (about 1 m thick) as a light reflecting layer
m) 2 and n-type transparent conductive film (transparent electrode) (thickness: 50 n)
m) is bonded to the glass substrate (thickness: 1 mm) 4 on which the 3 is formed by an adhesive at 5 cm intervals using the polyimide film 1 having a thickness of 2 μm as a spacer, and La is placed between the silver plate 2 and the glass substrate 4. _A solar cell was manufactured by sandwiching ₂ CuO ₄ oxide powder and pressing strongly to form the light absorbing layer 5. About 1% by weight of polymethyl methacrylate was added as a binder to the La ₂ CuO ₄ oxide powder. The silver plate 2 also serves as a lower electrode.

The light absorption layer thus formed has an absorption peak wavelength (energy gap position) at about 2 electron volts (wavelength: about 610 nm) in the visible region, and the extinction coefficient κ at this time is κ = 10 ⁴ cm ⁻ It was about ¹ . Therefore,
At a film thickness of 2 microns, approximately 85% of the light is absorbed, which is convenient. Further, the remaining 15% of the light is reflected by the metal layer and passes through the perovskite layer again, so that most of the light is absorbed and used to generate electrons and holes.

By irradiating the solar cell with sunlight, the electrons and holes generated in the La ₂ CuO ₄ layer quickly propagate to the cathode and anode according to the electric field gradient, and a voltage of about 1 volt is applied. showed that. As shown in FIG.
The lead wire 6 is joined to the transparent electrode 3 and the silver plate (light reflection layer) 2 using a silver paste, and an electric resistance (load) 7 is applied between the lead wires.
By connecting the power, power could be extracted and high energy conversion efficiency could be obtained.

In this case, the conductive surface (a made of CuO ₂₎
The orientation of (−c plane) is random and the efficiency is 2 / compared to the case where the CuO ₂ plane crystal planes are completely aligned in the film thickness direction.
It was estimated that it had dropped to about 3. Example 2 A La ₂ CuO ₄ film was produced by a metal soap firing method as described below.

That is, a solution in which copper naphthenate is dissolved in an organic solvent (Kishida Chemical) (containing 5 parts by weight of copper metal with respect to 100 parts by weight of the organic solvent) and a solution in which lanthanum octylate is dissolved in a toluene solution ( Brand name Nikka Octix Lantern, Nippon Chemical Industry Co., Ltd.) (Toluene 1
(A lanthanum amount of 7 parts by weight with respect to 00 parts by weight) was mixed at a molar ratio of La to Cu of 2: 1 and mixed well to prepare a mixed solution.

This mixture was applied on a clean silver plate, and then fired at 900 ° C. for 1 hour in a clean dry air atmosphere and taken out. At this time, the sample on the silver plate was blackened. By measuring the X-ray diffraction spectrum of this sample, it was confirmed that La ₂ CuO ₄ having a perovskite crystal structure was generated.

When the firing temperature is 800 ° C., La_TwoCuO_Four
Requires at least 5 hours of firing to produce
Was. If the firing temperature is lower than 800 ° C., La_TwoC
uO_FourIn addition to other substances, which are considered to be carbonates,
Was. In firing at 500 ° C. or less, La _TwoCuO_FourDo not generate
won. Even at a firing temperature of 1000 ° C, La_TwoCuO_FourIs generated
did.

When a large amount of the mixed solution is applied on the silver plate at once, the combustion reaction occurs violently and the sample is scattered. Therefore, in the case of a silver plate of 5 cm square, the mixed solution is applied repeatedly about 0.5 cc each and fired. It was effective to repeat.

In this case, when the firing was repeated seven times, the formed film became very dense.

The obtained film was pressed with a quartz substrate provided with a transparent electrode, and further, electrodes were provided on a silver plate and a quartz substrate, thereby producing a solar cell. This method has an advantage that a thin and relatively dense light absorbing layer can be easily obtained.

When light irradiation was performed on the obtained solar cell in the same manner as in Example 1, an electromotive force was observed.

The above-mentioned calcined components are further heated in a reducing atmosphere, for example, in an atmosphere in which hydrogen is mixed with about 5 to 10% by volume of nitrogen at about 135 ° C. for about 3 to 5 hours. As a result, a solar cell having higher specific resistance and higher electromotive force was obtained. This is La ₂ CuO ₄
It is presumed that the chemical equivalent was approached. Example 3 A YBa ₂ Cu ₃ O ₆ film was formed by a sputtering method according to the following method.

That is, a reactive magnetron sputtering method was performed at room temperature in an atmosphere in which oxygen was mixed with 10% of oxygen, using YBa ₂ Cu ₃ O ₆ as a target.
The distance between the silver substrate and the target was about 4 cm, and the energy density of the plasma on the target was kept at 5 to 10 W / cm ² .

A solar cell was manufactured in the same manner as in Example 1 except that the formed YBa ₂ Cu ₃ O ₆ film having a perovskite crystal structure was used as a light absorbing layer. The obtained solar cell generated an electromotive force by light irradiation in the same manner as in Example 1.

However, in this case, the conduction surface (ac-plane made of CuO ₂ ) tended to be parallel to the film, and the electron transport efficiency was not good. Example 4 YBa was obtained by a laser ablation method using the following method.
The ₂ Cu ₃ O ₆ film was produced.

That is, a YBa ₂ Cu ₃ O ₆ film was formed on a silver substrate placed in vacuum by irradiating a pulsed ArF laser with YBa ₂ Cu ₃ O ₆ as a target. The energy density of the laser was about 1 J / cm ² / pulse, the repetition frequency was 4 Hz, and the pulse width was 20 ns.

A solar cell was manufactured in the same manner as in Example 1 using the YBa ₂ Cu ₃ O ₆ film having a perovskite crystal structure manufactured as described above as a light absorbing layer. The obtained solar cell generated an electromotive force by light irradiation in the same manner as in Example 1.

In this case, the conductive surface (a made of CuO ₂₎
The orientation of (−c plane) is random and the efficiency is 2 / compared to the case where the CuO ₂ plane crystal planes are completely aligned in the film thickness direction.
It was estimated that it had dropped to about 3.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a hybrid orbital of a 3d orbital of Cu ²⁺ and a 2p orbital of O ^{2− in} a perovskite oxide.

FIG. 2 is a schematic diagram of a crystal structure of La ₂ CuO ₄ manufactured in Example 1.

FIG. 3 is a cross-sectional view of the solar cell manufactured in Example 1.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Polyimide film, 2 ... Silver plate (light reflection layer) 3 ... Transparent electrode, 4 ... Glass substrate 5 ... Light absorption layer, 6 ... Lead wire 7 ... Electric resistance (load)

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Eiichi Hanamura 2180-9 Ariake, Hotaka-cho, Minamiazumi-gun, Nagano F-term (reference) 5F051 HA15

Claims (11)

[Claims] An oxide having a perovskite-type crystal structure represented by the general formula: R ₂ CuO ₄ (where R is a rare earth element); and a general formula: XZ ₂ Cu ₃ O ₆ (wherein, X is yttrium, lanthanum or praseodymium, and Z is an alkaline earth metal) for forming a light absorption layer of a solar cell comprising at least one oxide selected from oxides having a perovskite crystal structure. material. 2. The band gap energy is 1.5-2.
2. The material for forming a light absorbing layer according to claim 1, which is in a range of 3 eV. 3. A solar cell having a light absorbing layer formed from the material for forming a light absorbing layer according to claim 1. 4. When the maximum light absorption coefficient in the visible region of the light absorbing layer is κ (cm ⁻¹ ), the thickness L (c) of the light absorbing layer
The solar cell according to claim 3, wherein m) is in the range of 0.5 / κ <L <5 / κ. 5. The electric conductivity σ in the thickness direction of the light absorbing layer is σ
<10^-2((Ωcm) ^-15. The method according to claim 3, wherein
Solar cell. 6. The oxide having a perovskite crystal structure, wherein the ac plane is oriented in the thickness direction of the light absorbing layer.
A solar cell according to any one of claims 1 to 5. 7. The solar cell according to claim 3, wherein the light absorbing layer is formed by a sputtering method. 8. The solar cell according to claim 3, wherein the light absorbing layer is formed by a laser aberration method. 9. The solar cell according to claim 3, wherein the light absorbing layer is formed by a metal complex salt pyrolysis method. 10. The light-absorbing layer is formed by a method of compressing a fine powder of an oxide having a perovskite crystal structure on an electrode or a method of applying a paste containing the fine powder to an electrode and drying the paste. The solar cell according to claim 3. 11. The solar cell according to claim 3, wherein a light reflection layer is formed on a side of the light absorption layer opposite to the incident light.