JP2013229506A - Solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solar cell excellent in photoelectric conversion efficiency.SOLUTION: The solar cell is formed by laminating a back electrode, a photoelectric conversion layer, a buffer layer, and a transparent conductive layer on a substrate. The buffer layer is an amorphous oxide layer containing In, Ga, and Zn and is preferably formed by a sputtering method or a coating method.

Description

  The present invention relates to a solar cell, and more particularly to a compound thin film solar cell using an amorphous oxide layer as a buffer layer.

  A compound thin film solar cell represented by CIGS or CZTS is expected to have high performance due to the material characteristics of the compound semiconductor layer to be a photoelectric conversion layer, and cost reduction can be expected by reducing the thickness of the compound semiconductor layer to the order of several μm. Therefore, in recent years, development of compound thin film solar cells has been actively promoted.

  The compound thin film solar cell is configured by laminating a back electrode, a p-type photoelectric conversion layer, a buffer layer, a high-resistance buffer layer, and a transparent conductive film in this order. Non-Patent Document 1 describes that a solar cell excellent in photoelectric conversion efficiency can be obtained by using a cadmium sulfide (CdS) layer as a buffer layer.

One of the roles of the buffer layer is to form a pn homojunction. When a buffer layer made of cadmium sulfide (CdS) is formed by a CBD (chemical bath deposition) method, a solar cell having excellent photoelectric conversion efficiency can be provided. However, C ²⁺ ions are present in the vicinity of the surface of the CIGS layer as V _cu ( In order to form a large amount of Cd _cu (Cd present at the copper site) that is replaced with a hole from which copper has been removed and serves as a donor, the vicinity of the surface of the CIGS layer becomes an n-type conductive layer. Thereby, a pn homojunction is formed near the surface of the CIGS layer.

  Another role of the buffer layer is to prevent the occurrence of a shunt path at the interface between the p-type photoelectric conversion layer and the transparent conductive film. However, when the buffer layer is formed by the CBD method, the surface of the buffer layer becomes rough. Therefore, there is a portion where the CdS layer becomes thin near the crystal grain boundary, and there is a shunt path at the interface between the p-type photoelectric conversion layer and the CdS layer. Can happen. Therefore, the formation of a shunt path at the interface between the CIGS layer and the CdS layer is prevented by forming a high resistance buffer layer such as an i-ZnO layer on the buffer layer.

  However, the band gap energy of CdS is 2.4 eV. Therefore, the buffer layer made of CdS absorbs light energy corresponding to the band gap energy of CdS (light having a wavelength near 517 nm). Therefore, in a solar cell including a buffer layer made of CdS, a light absorption loss occurs in a short wavelength region, which causes a reduction in short circuit current.

In addition, Cd-free is required in the RoHS Directive regarding the use of hazardous substances. Therefore, a Cd free buffer layer (hereinafter simply referred to as “Cd free buffer layer”) having a larger band gap energy than CdS, which is an alternative to CdS, has been intensively studied. As a study of such a Cd free buffer layer, for example, development of a Zn compound system such as ZnS (O, OH) or Zn (O, S), and an In compound system such as In (S, O, OH), etc. Development is active. For example, Patent Document 1 discloses that a chemically and transparently sulfur-containing zinc mixed crystal compound semiconductor thin film from a solution is used as a light-absorbing layer having a p-type conductivity type Cu-III-VI group ₂ chalcopyrite structure. It is described that a thin film solar cell with high conversion efficiency is produced by growing stably on a semiconductor thin film to be provided.

Japanese Patent No. 3249342 (JP-A-8-330614)

Ingrid Repins, et. Al ′19 .9% -efficient ZnO / CdS / CuInGaSe2 Solar Cell with 81.2% Fill Factor ’PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS Prog. Photovolt: Res. Appl. 2008; 16: 235-239.

  However, Cd free buffer layers such as ZnS (O, OH) or Zn (O, S) that are currently being studied are lattice mismatched to the photoelectric conversion layer. For this reason, a large number of defects exist at the interface between the photoelectric conversion layer and the buffer layer, which causes an increase in carrier recombination at the interface. This causes a decrease in photoelectric conversion efficiency.

  This invention is made | formed in view of this point, The place made into the objective is to provide the solar cell from which high photoelectric conversion efficiency is obtained.

  The solar cell of the present invention is formed by laminating a back electrode, a photoelectric conversion layer, a buffer layer, and a transparent conductive layer on a substrate. The buffer layer is an amorphous oxide layer containing In, Ga, and Zn.

The carrier concentration of the buffer layer is preferably 5 × 10 ¹⁸ cm ⁻³ or less, and more preferably 5 × 10 ¹⁵ cm ⁻³ or less. The buffer layer is preferably formed by sputtering or coating.

  The main component of the photoelectric conversion layer is a p-type compound semiconductor material having a chalcopyrite type crystal structure composed of at least one of Cu and Ag, at least one of Al, Ga and In, and at least one of S, Se and Te, Alternatively, it is preferably a p-type compound semiconductor having a stannite type crystal structure or a kesterite type crystal structure made of Zn, Sn, at least one of Cu and Ag, and at least one of S, Se, and Te. The photoelectric conversion layer is more preferably made of a p-type compound semiconductor material having a chalcopyrite crystal structure composed of Cu, at least one of Ga and In, and at least one of S and Se.

  In the present invention, a solar cell excellent in photoelectric conversion efficiency can be provided.

It is sectional drawing which shows an example of a structure of the solar cell of this invention. It is the observation image which observed the surface of the buffer layer in Example 1 of this invention with AFM (Atomic Force Microscope).

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings of the present invention, the same reference numerals represent the same or corresponding parts. In addition, dimensional relationships such as length, width, thickness, and depth are changed as appropriate for clarity and simplification of the drawings, and do not represent actual dimensional relationships.

<Configuration of solar cell>
FIG. 1 is a cross-sectional view showing an example of the configuration of the solar cell of the present invention. In the solar cell 10 shown in FIG. 1, a back electrode 12, a photoelectric conversion layer 13, a buffer layer 14, and a transparent conductive film 15 are sequentially stacked on a substrate 11. In the solar cell of the present invention, the surface electrode 16 is preferably provided on the transparent conductive film 15. Hereinafter, the configuration of the solar cell 10 will be described.

<Board>
The substrate 11 is a substrate on which the back electrode 12 and the like are stacked, and is preferably a glass substrate such as blue plate glass, a metal substrate such as a stainless plate, or a resin substrate such as a polyimide film.

<Back electrode>
The back electrode 12 is preferably formed on the substrate 11 and made of a metal having high corrosion resistance and high melting point such as Mo or Ti. The back electrode 12 is preferably formed by sputtering or the like using these metals as targets, and the thickness of the back electrode 12 is not particularly limited and is preferably about 200 to 2000 nm.

<Photoelectric conversion layer>
The photoelectric conversion layer 13 is a layer that is formed on the back electrode 12 and generates a charge by light absorption. Although it does not restrict | limit especially as a main component of the photoelectric converting layer 13, From a viewpoint that high photoelectric conversion efficiency is obtained, at least 1 of Cu and Ag, at least 1 of Al, Ga, and In, S, Se, and Te It may be a p-type compound semiconductor material having a chalcopyrite type crystal structure composed of at least one of Zn, Sn, at least one of Cu and Ag, and at least one of S, Se and Te It may be a p-type compound semiconductor material having a stannite type crystal structure or a kesterite type crystal structure. As the main component of the photoelectric conversion layer 13, the p-type compound semiconductor material having the chalcopyrite type crystal structure, the p-type compound semiconductor material having the stannite type crystal structure, or the p-type compound semiconductor material having the kesterite type crystal structure is used. Thus, a solar cell having a high photoelectric conversion efficiency with a thin film can be obtained. Here, the main component of the photoelectric conversion layer 13 means a material whose content in the photoelectric conversion layer 13 is 50 mass% or more. The content of the main component of the photoelectric conversion layer 13 in the photoelectric conversion layer 13 is preferably 60% by mass or more, more preferably 70% by mass or more, and further preferably 80% by mass.

Examples of the p-type compound semiconductor material having a chalcopyrite type crystal structure include CuAlS ₂ , CuGaS ₂ , CuInS ₂ , CuAlSe ₂ , CuGaSe ₂ , AgAlS ₂ , AgGaS ₂ , AgInS ₂ , AgAlSe ₂ , AgGaSe ₂ , AgInSe ₂ , AgAlTe ₂ , AgGaTe ₂ , AgInTe ₂ , Cu (In, Al) Se ₂ , Cu (In, Ga) (S, Se) ₂ , Cu _1-z In _1-x Ga _x Se _2-y S _y (wherein , 0 ≦ x ≦ 1, 0 ≦ y ≦ 2, 0 ≦ z ≦ 1) (CIGS), Ag (In, Ga) Se ₂ , Ag (In, Ga) (S, Se) _{2 and the} like. it can.

Examples of the p-type compound semiconductor having a stannite type crystal structure or a kesterite type crystal structure include Cu ₂ ZnSnS ₄ , Cu ₂ ZnSnSe ₄ , and Cu ₂ ZnSn (S, Se) ₄ .

  More preferably, the photoelectric conversion layer 13 is made of a p-type compound semiconductor material having a chalcopyrite crystal structure composed of Cu, at least one of Ga and In, and at least one of S and Se.

The thickness of the photoelectric conversion layer 13 is not particularly limited, and is preferably about 1 to 3 μm.
The configuration of the photoelectric conversion layer 13 is not particularly limited. The photoelectric conversion layer 13 may be a single layer made of a single material, may be a single layer containing two or more kinds of materials, or is configured by laminating layers made of different materials. Also good. When the photoelectric conversion layer 13 has a laminated structure, the total thickness of the photoelectric conversion layer 13 is preferably about 1 to 3 μm.

  Such a method for forming the photoelectric conversion layer 13 may be a method using a gas phase raw material such as a multi-source deposition method, a selenization method, or a sputtering method, or a nanoparticle coating method, an electrodeposition method, or the like. Alternatively, a forming method using a raw material in a liquid phase state such as a method or a solution method may be used.

<Buffer layer>
The buffer layer 14 is an amorphous oxide layer formed on the photoelectric conversion layer 13 and containing In, Ga, and Zn. Here, the amorphous oxide layer containing In, Ga, and Zn is amorphous and has a strong ionic bond, and thus becomes a film with few defects. Since the buffer layer 14 composed of a film with few defects is formed on the photoelectric conversion layer 13 as described above, generation of defects and the like at the interface between the photoelectric conversion layer 13 and the buffer layer 14 is suppressed, and thus a good bonding interface. Is formed. That is, it is possible to prevent a decrease in photoelectric conversion efficiency of the solar cell that is a product.

  In addition, since the band gap energy of the amorphous oxide layer containing In, Ga, and Zn is about 3 eV or more, it is larger than the band gap energy of CdS (about 2.4 eV). Therefore, in the solar cell shown in FIG. 1, since the occurrence of light absorption loss in the short wavelength region can be prevented, the short circuit current can be prevented from being lowered. Also by this, a solar cell having high photoelectric conversion efficiency can be provided.

  Furthermore, since the buffer layer 14 is an amorphous oxide layer containing In, Ga, and Zn, it is a Cd-free buffer layer. Therefore, it is possible to provide a solar cell that complies with the RoHS directive regarding the use of harmful substances.

Since the amorphous oxide layer containing In, Ga, and Zn contains moderate Ga, it has carrier concentration stability. Therefore, it is easy to maintain the carrier concentration in the amorphous oxide layer at a carrier concentration that can prevent generation of a shunt path at the interface between the photoelectric conversion layer 13 and the buffer layer 14. Specifically, the carrier concentration in the buffer layer 14 can be controlled to preferably 5 × 10 ¹⁸ cm ⁻³ or less, and more preferably 5 × 10 ¹⁵ cm ⁻³ or less. Thereby, generation | occurrence | production of the shunt path | pass in the interface of the photoelectric converting layer 13 and the transparent conductive film 15 can be prevented more. The carrier concentration of the buffer layer 14 can be measured, for example, by measuring the Hall effect.

  In addition to In, Ga and Zn, the buffer layer 14 includes Be, Na, Mg, Al, Si, Ca, Sc, Ti, V, Fe, Co, Ni, Ge, As, Y, Zr, Nb, Mo, Elements such as Sn, Sb, Ba, Hf, Ta, W, and Pb may be included as additive elements. For example, the buffer layer 14 includes In—Ga—Zn—O (atomic ratio In: Ga: Zn = 1: 1: 1, atomic ratio In: Ga: Zn = 1: 2: 1, or atomic ratio In: Ga: An amorphous oxide layer made of Zn = 1: 3: 1 or the like is preferable. The buffer layer 14 may be a single layer made of one of these materials, a single layer containing two or more of these materials, or from different materials. The layers may be laminated, or layers having different carrier concentrations may be laminated.

  The thickness of the buffer layer 14 is not particularly limited, but is preferably 5 nm or more and 200 nm or less. If the thickness of the buffer layer 14 is less than 5 nm, an amorphous oxide layer containing In, Ga, and Zn may not function as the buffer layer. On the other hand, if the thickness of the buffer layer 14 exceeds 200 nm, there may be a problem that the short circuit current decreases. When the buffer layer 14 has a stacked structure, the entire thickness of the buffer layer 14 is preferably 5 nm or more and 200 nm or less.

<Transparent conductive film>
The transparent conductive film 15 is formed on the buffer layer 14 and functions as an electrode that captures light and flows with carriers generated in the photoelectric conversion layer 13 paired with the back electrode 12. Therefore, the material which comprises the transparent conductive film 15 will not be restrict | limited especially if it has a light transmittance and electroconductivity. For example, the transparent conductive film 15 is preferably a zinc oxide film to which a group III element is added as a dopant, and more preferably formed by sputtering using a target such as Al ₂ O ₃ —ZnO. is there. Here, as a group III element as a dopant, for example, any of B, Al, Ga, and In can be used.

<Surface electrode>
The surface electrode 16 is formed on the transparent conductive film 15. The material of the surface electrode 16 is not particularly limited, but is preferably a conductive material. For example, the surface electrode 16 is preferably made of Al or Ag. Although the formation method of the surface electrode 16 is not specifically limited, It is preferable that it is a vacuum evaporation method, especially a resistance heating evaporation method.

As described above, the solar cell 10 illustrated in FIG. 1 is described as an example of the solar cell of the present invention, but the solar cell of the present invention is not limited to the configuration illustrated in FIG. For example, the solar cell of the present invention may not include the surface electrode 16. Moreover, the solar cell of this invention may be provided with at least 1 of a cover glass, a protective film, an antireflection film, a sealing film, etc. as needed. Here, the antireflection film is preferably made of MgF ₂ or Al ₂ O ₃ , and the sealing film is preferably made of EVA (Ethylene-Vinyl Acetate, ethylene vinyl acetate) resin or epoxy resin.

  Moreover, in the solar cell 10 shown in FIG. 1, each material of the board | substrate 11, the back surface electrode 12, the photoelectric converting layer 13, the transparent conductive film 15, and the surface electrode 16 is not limited to the said material.

  The buffer layer of the present invention is applicable not only to single-junction solar cells but also to multi-junction solar cells. A single junction cell is a solar cell provided with one photoelectric conversion layer like the solar cell 10 shown in FIG. A multi-junction solar cell is a solar cell including two or more photoelectric conversion layers.

<Method for manufacturing solar cell>
The solar cell 10 shown in FIG. 1 can be manufactured according to the method shown below.

  A back electrode 12 and a photoelectric conversion layer 13 are sequentially formed on the substrate 11. The back electrode 12 is preferably formed by sputtering, for example. Further, the method for forming the photoelectric conversion layer 13 may be a method using a gas phase raw material such as a multi-source deposition method, a selenization method, or a sputtering method, or a nanoparticle coating method or an electrodeposition method. Alternatively, a formation method using a raw material in a liquid phase state such as a solution method may be used.

  An amorphous oxide layer (buffer layer 14) containing In, Ga, and Zn is formed over the photoelectric conversion layer 13. After the buffer layer 14 is formed, a transparent conductive film 15 made of zinc oxide or the like is formed on the buffer layer 14 by, for example, sputtering, and the surface electrode 16 is formed on the transparent conductive film 15 by, for example, vacuum evaporation. . In this way, the solar cell 10 shown in FIG. 1 is manufactured.

  Hereinafter, a method for forming the buffer layer 14 will be described. The method for forming the buffer layer 14 is not particularly limited as long as it can form an amorphous layer. For example, the buffer layer 14 is formed by sputtering such as DC (Direct Current) sputtering or RF (Radio Frequency) sputtering, CVD (Chemical Vapor Deposition), PLD (Pulsed Laser Deposition), MBE (Molecular Beam Epitaxy). Alternatively, it can be formed by using a known thin film forming method represented by an IBS (Ion Beam Sputtering) method or the like.

As a material source used in the thin film formation method, an oxide containing one or more of In, Ga and Zn may be used, or a metal body containing one or more of In, Ga and Zn may be used. Also good. As the oxide containing In, Ga, or Zn alone, In ₂ O ₃ , Ga ₂ O ₃ , ZnO, or the like can be used. As the oxide containing a plurality of types of In, Ga, and Zn, InGaZnO, InGaO, or the like can be used.

  Regardless of whether the material source is an oxide or a metal body, the amorphous oxide layer (buffer layer 14) can be formed on the photoelectric conversion layer 13 in a vacuum, a rare gas, or oxygen. A method of forming an amorphous oxide layer on the photoelectric conversion layer 13 may be used, or the buffer layer 14 is formed on the photoelectric conversion layer 13 while oxidizing a metal in a gas atmosphere containing oxygen. It may be a method.

  In order to form an amorphous oxide layer having a low carrier concentration, it is preferable to suppress the generation of conduction electrons that are the origin of the n-type semiconductor as much as possible. When the material source is an In—Ga—Zn—O system, it is known that conduction electrons are generated due to oxygen deficiency. From this viewpoint, it is preferable to form the amorphous oxide layer so that oxygen vacancies are not generated as much as possible in the amorphous oxide layer. Specifically, the amorphous oxide layer may be formed using a material source that is less prone to oxygen vacancies, and the gas atmosphere at the time of formation is controlled so as to suppress the occurrence of oxygen vacancies during the formation. May be. When the gas atmosphere at the time of formation does not contain oxygen gas, it is preferable to control the gas atmosphere at the time of heat treatment after the formation so as to suppress the generation of oxygen vacancies during the heat treatment after formation.

More specifically, in the case of suppressing the generation of oxygen vacancies in the heat treatment during or after the formation by controlling the gas atmosphere during the heat treatment during or after the formation, It is preferable to control the oxygen flow rate. Thereby, an amorphous oxide layer having a carrier concentration of preferably 5 × 10 ¹⁸ cm ⁻³ or less can be formed, more preferably an amorphous oxide having a carrier concentration of 5 × 10 ¹⁵ cm ⁻³ or less. An oxide layer can be formed.

  Among the thin film forming methods, the buffer layer 14 is preferably formed by sputtering. As a result, the surface of the amorphous oxide layer containing In, Ga, and Zn becomes flat even at the atomic level (see FIG. 2 described later), so that a shunt path is formed at the interface between the photoelectric conversion layer 13 and the buffer layer 14. The possibility of occurrence can be significantly reduced. Therefore, a solar cell with reduced generation of shunt paths can be provided without separately forming a high resistance buffer layer on the buffer layer 14. Hereinafter, a specific method for forming the buffer layer 14 by sputtering will be described.

  As the sputtering target, a metal target containing In, Ga and Zn alone or in plural kinds, or a metal oxide target containing In, Ga and Zn alone or in plural kinds can be used. As the metal oxide target containing In or Ga and Zn alone or in plural kinds, the above oxides can be used.

  A substrate formed up to the photoelectric conversion layer 13 (hereinafter referred to as “substrate with the photoelectric conversion layer 13”) is fixed at a predetermined location in the sputtering apparatus, and a rare gas represented by Ar gas, an oxygen gas, or a rare gas, Both oxygen gas and the oxygen gas are introduced into the sputtering apparatus.

In the case where the buffer layer 14 is formed in a mixed gas atmosphere of oxygen gas and rare gas, oxygen gas of 3 sccm (standard cc / min) or more is supplied into the film forming apparatus, and after the buffer layer 14 is formed. It is preferable to perform the heat treatment. In other words, it is preferable that the amount of introduction (volume) of the rare gas is 4% or more with respect to the total introduction amount (volume) including the rare gas, and the heat treatment after the formation of the buffer layer 14 is performed. Thereby, an amorphous oxide layer having a carrier concentration of 5 × 10 ¹⁸ cm ⁻³ or less can be formed.

More preferably, oxygen gas of 6 sccm or more is supplied into the film forming apparatus to form the buffer layer 14 and perform heat treatment after the formation. In other words, it is more preferable to perform the heat treatment after the formation of the buffer layer 14 by setting the introduction amount (volume) of the rare gas to 12% or more with respect to the total introduction amount (volume) including the rare gas. Thereby, an amorphous oxide layer having a carrier concentration of 5 × 10 ¹⁵ cm ⁻³ or less can be formed.

  Next, the target material is knocked out by supplying DC power or RF power to the sputtering target. Thereby, an amorphous oxide layer (buffer layer 14) containing In, Ga, and Zn is formed on the photoelectric conversion layer 13.

  At this time, the power supplied to the target depends on the type of the target and the like, but is preferably 50 W or more and 1000 W or less.

  Moreover, it is preferable to heat the substrate with the photoelectric conversion layer 13 as necessary. Thereby, the reactivity of oxygen gas and a target can be improved. The heating temperature of the substrate with the photoelectric conversion layer 13 is preferably determined according to the heat resistance of the substrate to be applied.

  For the formed amorphous oxide layer, in a vacuum, an atmosphere containing a rare gas, or an oxygen gas, as necessary, for the purpose of suppressing oxygen vacancies or increasing the material uniformity in the thin film. You may heat-process in the atmosphere containing. The heat treatment may be performed in an atmosphere in which the amorphous oxide layer is formed, or may be performed in an atmosphere different from the atmosphere in which the amorphous oxide layer is formed. For example, when an amorphous oxide layer is formed by introducing only a rare gas into the sputtering apparatus, an oxygen gas is introduced into the sputtering apparatus after the amorphous oxide layer is formed on the photoelectric conversion layer 13. The heat treatment is preferably performed in an atmosphere containing oxygen gas.

  As a method for forming the buffer layer 14, it is preferable to use a coating method in which the buffer layer 14 is formed by coating an IGZO precursor other than the above-described thin film forming method. As a method for applying the IGZO precursor, for example, a screen printing method, a spin coating method, a spray method, a roll coating method, a curtain method, an offset printing method, an ink jet method, or the like can be used.

  More specifically, an IGZO precursor is applied on the photoelectric conversion layer 13 and heated (for example, 200 ° C.), and the solvent is evaporated to form a coating film. Here, the application of the IGZO precursor and the evaporation of the solvent by heating may be repeated. For example, after sintering at 450 ° C., oxygen annealing is performed at 180 ° C., for example. Thereby, the buffer layer 14 is formed.

  A passivation step may be added before and after the oxygen annealing treatment step. Examples of the passivation step include a method of immersing in a solution such as an ammonium sulfide solution or a cyan solution, or a method of performing a heat treatment in a gas atmosphere such as hydrogen gas, hydrogen fluoride gas, hydrogen bromide gas, or hydrogen phosphide gas. Can be used.

  Moreover, as a method of evaporating the solvent by heating the IGZO precursor, for example, a method of heating with a hot plate or oven, or a method of heating with a hot plate or oven, an organic solvent and A method for promoting the decomposition and synthesis reaction of the precursor can be used.

  The light source used for the light irradiation treatment includes output light from an excimer laser, output light from a YAG laser, output light from an argon laser, visible light, ultraviolet light, far ultraviolet light, output light from a low pressure or high pressure mercury lamp, Output light from a hydrogen lamp, discharge light of a rare gas, or the like can be used.

  Even in the thin film formation by the coating method, the surface of the amorphous oxide layer containing In, Ga, and Zn is flattened, so that a shunt path may occur at the interface between the photoelectric conversion layer 13 and the buffer layer 14. Can be significantly reduced. Therefore, a solar cell with reduced generation of shunt paths can be provided without separately forming a high resistance buffer layer on the buffer layer 14.

  In addition, each formation method of the back surface electrode 12, the photoelectric converting layer 13, the transparent conductive film 15, and the surface electrode 16 is not limited to the said method, The formation method in a liquid phase may be sufficient, and formation in a gaseous phase It may be a method. Each forming method may be any method as long as it is a method suitable for forming each layer.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated in detail, this invention is not limited to these.

<Example 1>
In Example 1, a CIGS thin film solar cell having the configuration shown in FIG. 1 was produced. Specifically, an Mo back electrode and a CIGS photoelectric conversion layer are sequentially formed on an SLG (soda-lime glass) substrate, and an In—Ga—Zn—O buffer layer is formed on the CIGS photoelectric conversion layer. (Thickness is 50 nm). Further, on the In—Ga—Zn—O buffer layer, a zinc oxide film (thickness: 200 nm) doped with Al as a dopant was formed as a transparent conductive layer, and finally the Al layer was formed as a surface electrode.

A CIGS photoelectric conversion layer was formed by a nanoparticle coating method. Specifically, first, a nanoparticle (having a diameter of about 10 nm) consisting of a core part made of CuInGaSe ₂ and a ligand part surrounding the core part and made of n-octane selenol was prepared. . Next, the nanoparticles were added to anhydrous toluene solvent and stirred for 30 minutes. This prepared the anhydrous toluene solution whose nanoparticle density | concentration is 10 mass%.

  Subsequently, a coating process and a temporary baking process were sequentially performed. First, after the prepared anhydrous toluene solution was dropped on the SLG substrate, the anhydrous toluene solvent was simply dried by allowing it to stand for 15 minutes in a nitrogen atmosphere. Next, the anhydrous toluene solvent was completely removed by heating the SLG substrate on a hot plate at 120 ° C. for 15 minutes (the anhydrous toluene solvent was completely dried). Thereafter, in a nitrogen atmosphere, the SLG substrate was heated at 300 ° C. for 15 minutes on a hot plate to carry out a temporary baking treatment. This coating process and pre-baking process were repeated a plurality of times to repeat this coating-pre-baking process a plurality of times, and finally, heating was performed at 450 ° C. for 30 minutes. Thereby, the baking process was complete | finished and the CIGS photoelectric converting layer was obtained. When the CIGS photoelectric conversion layer formed in this way was visually confirmed with a transmission microscope, it was confirmed that the CIGS photoelectric conversion layer was a good film without cracks. The thickness of the CIGS photoelectric conversion layer was about 2 μm.

A buffer layer was formed by DC magnetron sputtering. In—Ga—Zn—O (In: Ga: Zn = 1: 1: 1 (atomic ratio)) was used as the target, and Ar and O ₂ were used as the deposition gas. During the film formation, the argon (Ar) flow rate was adjusted to 47 sccm and the oxygen (O ₂ ) flow rate was adjusted to 3 sccm to form an amorphous oxide layer containing In, Ga, and Zn. When the Hall effect was measured for the formed buffer layer, the carrier concentration in the buffer layer was 4 × 10 ¹⁸ cm ⁻³ .

  FIG. 2 is an atomic force microscope (AFM) observation image of the surface of the In—Ga—Zn—O buffer layer formed on the CIGS photoelectric conversion layer. The average surface roughness on the surface of the buffer layer was 0.18 nm, and it was found that the surface of the buffer layer had atomic level flatness.

The compound thin film solar cell thus manufactured was irradiated with AM1.5G pseudo-sunlight and measured for cell characteristics. The shunt resistance was 2 × 10 ² Ωcm ² and the photoelectric conversion efficiency was 0. 53%.

<Example 2>
A CIGS thin film solar cell of Example 2 was fabricated in the same manner as in Example 1 except for the conditions for forming the buffer layer.

  Specifically, an Mo back electrode and a CIGS photoelectric conversion layer were sequentially formed on the SLG substrate, and an In—Ga—Zn—O buffer layer (thickness: 50 nm) was formed on the CIGS photoelectric conversion layer. Further, a zinc oxide film (thickness: 200 nm) doped with Al as a dopant was formed as a transparent conductive layer on the buffer layer, and finally an Al layer was formed as a surface electrode.

A buffer layer was formed by DC magnetron sputtering. In—Ga—Zn—O (In: Ga: Zn = 1: 1: 1 (atomic ratio)) was used as the target, and Ar and O ₂ were used as the deposition gas. An amorphous oxide containing In, Ga and Zn having a low carrier concentration is formed by adjusting the argon (Ar) flow rate during film formation to 44 sccm and adjusting the oxygen (O ₂ ) flow rate to 6 sccm to form a buffer layer. A physical layer was formed. When the Hall effect was measured for the formed buffer layer, the carrier concentration in the buffer layer was 4 × 10 ¹⁵ cm ⁻³ .

  When the surface of the formed In—Ga—Zn—O buffer layer was observed using an atomic force microscope, the surface of the buffer layer had flatness at the atomic level as in Example 1. I understood.

The compound thin film solar cell thus manufactured was irradiated with AM1.5G pseudo-sunlight and measured for cell characteristics. The shunt resistance was 4 × 10 ² Ωcm ² and the photoelectric conversion efficiency was 0. 66%. This is because the generation of shunt paths can be prevented more than in Example 1, and the photoelectric conversion efficiency is improved.

<Example 3>
A CIGS thin film solar cell of Example 3 was fabricated in the same manner as in Example 1 except for the method for forming the buffer layer.

  Specifically, an Mo back electrode and a CIGS photoelectric conversion layer were sequentially formed on the SLG substrate, and an In—Ga—Zn—O buffer layer (thickness: 50 nm) was formed on the CIGS photoelectric conversion layer. Further, a zinc oxide film (thickness: 200 nm) doped with Al as a dopant was formed as a transparent conductive layer on the buffer layer, and finally an Al layer was formed as a surface electrode.

A buffer layer was formed by a coating method. An IGZO precursor was applied on the CIGS photoelectric conversion layer, heated in a nitrogen atmosphere at 200 ° C. to evaporate the solvent, and a coating film was formed. Thereafter, annealing was performed at a temperature of 450 ° C. in a nitrogen atmosphere. Thereafter, oxygen annealing was performed at 180 ° C. Thereby, a buffer layer was formed. When the Hall effect was measured for the formed buffer layer, the carrier concentration in the buffer layer was 9 × 10 ¹⁷ cm ⁻³ .

  When the surface of the formed In—Ga—Zn—O buffer layer was observed using an atomic force microscope, the surface of the buffer layer had flatness at the atomic level as in Examples 1 and 2. I found out.

  When the cell characteristics were measured by irradiating AM1.5G pseudo-sunlight to the compound thin film solar cell thus manufactured, the shunt resistance and the photoelectric conversion efficiency equivalent to those of Example 1 were obtained.

<Comparative Example 1>
A CIGS thin film solar cell of Comparative Example 1 was produced in the same manner as in Example 1 except for the material for forming the buffer layer and the conditions for forming the buffer layer.

  Specifically, a Mo back electrode and a CIGS photoelectric conversion layer were sequentially formed on the SLG substrate. The CIGS photoelectric conversion layer was formed by the nanoparticle coating method as in Examples 1 and 2.

Next, ZnS (O, OH) was formed as a buffer layer on the CIGS photoelectric conversion layer by the CBD method. Specifically, the substrate on which the CIGS photoelectric conversion layer was formed was immersed in an aqueous solution of ZnSO ₄ —NH ₄ OH-thiourea and heated to 80 ° C. Thereby, a ZnS (O, OH) thin film was formed on the CIGS photoelectric conversion layer. The surface of the formed ZnS (O, OH) thin film was rough, and it was confirmed that the ZnS (O, OH) layer was thin.

  Thereafter, a zinc oxide film (thickness 50 nm) is formed as a high resistance buffer layer on the buffer layer, and a zinc oxide film (thickness 200 nm) doped with Al as a dopant is transparent on the high resistance buffer layer. A conductive layer was formed, and finally an Al layer was formed as a surface electrode.

  When the cell characteristics were measured by irradiating the solar cell manufactured in this manner with AM1.5G pseudo-sunlight, the photoelectric conversion efficiency was 0.31%. Compared to Examples 1 to 3 above, it is believed that the photoelectric conversion efficiency has decreased due to the open circuit voltage and the fill factor decrease associated with the defect.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  DESCRIPTION OF SYMBOLS 10 Solar cell, 11 board | substrate, 12 back surface electrode, 13 photoelectric converting layer, 14 buffer layer, 15 transparent conductive film, 16 surface electrode.

Claims (7)

A solar cell in which a back electrode, a photoelectric conversion layer, a buffer layer, and a transparent conductive layer are laminated on a substrate,
The said buffer layer is a solar cell which is an amorphous oxide layer containing In, Ga, and Zn. The solar cell according to claim 1, wherein the buffer layer has a carrier concentration of 5 × 10 ¹⁸ cm ⁻³ or less. The solar cell according to claim 2, wherein the buffer layer has a carrier concentration of 5 × 10 ¹⁵ cm ⁻³ or less.   The solar cell according to claim 3, wherein the buffer layer is formed by a sputtering method.   The solar cell according to claim 3, wherein the buffer layer is formed by a coating method. The main component of the photoelectric conversion layer is:
A p-type compound semiconductor material having a chalcopyrite type crystal structure comprising at least one of Cu and Ag, at least one of Al, Ga and In, and at least one of S, Se and Te, or
6. A p-type compound semiconductor material having a stannite-type crystal structure or a kesterite-type crystal structure comprising Zn, Sn, at least one of Cu and Ag, and at least one of S, Se, and Te. A solar cell according to the above.   The photoelectric conversion layer is made of a p-type compound semiconductor material having a chalcopyrite type crystal structure composed of Cu, at least one of Ga and In, and at least one of S and Se. Solar cells.