JP2014053420A - Solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solar cell including a structure capable of inhibiting carrier recombination at a heterointerface of a CIGS photoelectric conversion layer and an InGaZnO amorphous oxide semiconductor layer.SOLUTION: A solar cell comprises: a CIGS photoelectric conversion layer; and two and more amorphous oxide semiconductor layers which are arranged on the CIGS photoelectric conversion layer and contain In, Ga and Zn and have carrier concentrations different from each other. The amorphous oxide semiconductor layer having the lower carrier concentration among the amorphous oxide semiconductor layers having different carrier concentrations from each other is arranged closer to the photoelectric conversion layer than the amorphous oxide semiconductor layer having the higher carrier concentration.

Description

  The present invention relates to a solar cell, and particularly to a solar cell including an amorphous oxide semiconductor layer containing In, Ga, and Zn.

  A compound thin film solar cell represented by CIGS or CZTS can be expected to have high performance due to the material characteristics of the compound semiconductor layer to be a photoelectric conversion layer and cost reduction by thinning 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 cells currently in circulation are mainly 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. Yes.

  On the other hand, a solar cell structure in which an n-type transparent conductive film is laminated directly on a p-type photoelectric conversion layer without using a buffer layer has been proposed (for example, Non-Patent Document 1 and Patent Document 1). Non-Patent Document 1 describes a structure (CIGS / ZnO / Al-doped ZnO) in which a zinc oxide layer is directly formed on a CIGS photoelectric conversion layer by electrodeposition or sputtering. It is also described that conversion efficiency can be obtained.

The material forming the transparent conductive film is not limited to a polycrystalline oxide semiconductor such as ZnO, but is described as an oxide semiconductor such as In—Ga—Zn—O (hereinafter referred to as “InGaZnO amorphous oxide semiconductor”). ). An InGaZnO amorphous oxide semiconductor has a large band gap energy of about 3 eV or more, and has a higher electron mobility (> 10 cm ² / Vs) than ZnO even when deposited at room temperature. For these reasons, InGaZnO amorphous oxide semiconductors are attracting attention as materials for channel layers of TFTs that drive electronic paper, liquid crystal panels, organic ELs, and the like.

An InGaZnO amorphous oxide semiconductor, which is a wide gap material (a material having a large band gap energy), is an n-type window that improves open-circuit voltage and reduces optical absorption loss in, for example, a heterojunction solar cell with a p-type silicon substrate. It is also attractive as a material constituting the layer. Patent Document 1 proposes a photoelectric device including a p-type semiconductor substrate and an n-type transparent amorphous oxide semiconductor layer provided on the p-type semiconductor substrate. The n-type transparent amorphous oxide semiconductor layer described in Patent Document 1 mainly includes zinc oxide (ZnO), a mixture of tin oxide and zinc oxide (ZnO—SnO ₂ mixture) or zinc oxide. And a mixture of indium oxide (ZnO—In ₂ O ₃ mixture), and further, aluminum, gallium, indium, boron, yttrium, scandium, fluorine, vanadium, silicon, germanium, zirconium, hafnium, nitrogen and Contains at least one of beryllium.

JP 2009-283886 A

D. Gal, et. Al 'Electrochemical deposition of zinc oxide films from non-aqueous solution: a new buffer / window process for thin film solar cells', Thin Solid Films 361-362 (2000) 79-83 Alex Zunger, et. Al ‘Intrinsic DX Centers in Ternary Chalcopyrite Semiconductors’ PHYSICAL REVIEW LETTERS 100, 016401 (2008)

Carrier trapping is likely to occur in the layer and at the interface of a layer made of a p-type compound semiconductor material having a chalcopyrite type crystal structure containing Ga, Cu, In, and at least one of S and Se. FIG. 1 is a graph showing the density of states when Ga / (In + Ga) is changed in CIGS. According to Non-Patent Document 2, the point defect Ga _cu captures two electrons to become Ga _cu ²⁺ , thereby forming a deep level called a DX center as shown in FIG. It is described that it is trapped. That is, when Ga / (In + Ga) in CIGS is increased, the band gap energy of the CIGS is increased, while carrier traps are easily trapped in the layer and at the interface.

  In InGaZnO amorphous oxide semiconductors, it is known that conduction electrons are generated due to oxygen vacancies, while defect levels are formed in the band gap due to oxygen vacancies. Therefore, in a layer made of a conductive InGaZnO amorphous oxide semiconductor (hereinafter referred to as “InGaZnO amorphous oxide semiconductor layer”), the carrier concentration is high and many defect levels are formed. Carrier traps easily in the layer and at the interface.

  Therefore, a photoelectric conversion layer (hereinafter referred to as “CIGS photoelectric conversion layer”) containing a p-type compound semiconductor material having a chalcopyrite type crystal structure containing Ga, Cu, In, and at least one of S and Se. If a heterojunction is formed by directly forming a conductive InGaZnO amorphous oxide semiconductor layer having a high carrier concentration, the increase in carrier recombination at the heterointerface and a decrease in photoelectric conversion efficiency are described. cause.

  The present invention has been made in view of this point, and the object of the present invention is to suppress carrier recombination at the heterointerface between the CIGS photoelectric conversion layer and the conductive InGaZnO amorphous oxide semiconductor layer. It is providing the solar cell provided with the simple structure.

  The solar cell of the present invention is provided on a photoelectric conversion layer including a p-type compound semiconductor material having a chalcopyrite type crystal structure composed of at least one of Cu, Ga, In, S, and Se, and the photoelectric conversion layer. And two or more amorphous oxide semiconductor layers containing In, Ga, and Zn and having different carrier concentrations. The carrier concentration in the amorphous oxide semiconductor layer provided on the photoelectric conversion layer side is lower than the carrier concentration in the amorphous oxide semiconductor layer provided far from the photoelectric conversion layer.

  Ga / (In + Ga) in the photoelectric conversion layer is preferably 0.3 or more. The thickness of the amorphous oxide semiconductor layer provided on the photoelectric conversion layer side is preferably 10 nm or less. Any of the amorphous oxide semiconductor layers having different carrier concentrations is preferably formed by a sputtering method.

  By providing another InGaZnO amorphous oxide semiconductor layer having a carrier concentration lower than that of the InGaZnO amorphous oxide semiconductor layer between the CIGS photoelectric conversion layer and the conductive InGaZnO amorphous oxide semiconductor layer. A heterointerface is formed by the CIGS photoelectric conversion layer and the InGaZnO amorphous oxide semiconductor layer having a low carrier concentration. Therefore, since carrier recombination at the heterointerface is suppressed, the photoelectric conversion efficiency is improved in the solar cell of the present invention.

It is a graph which shows the density of states when Ga / (In + Ga) is changed in CIGS. It is sectional drawing which shows an example of a structure of the solar cell of this invention. It is a graph which shows the relationship between Ga / (In + Ga) in CIGS, and its forbidden bandwidth Eg. It is a graph which shows the relationship between the forbidden bandwidth of CIGS, and the conversion efficiency of the solar cell containing the CIGS.

  Hereinafter, the solar cell 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. 2 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. 2, a back electrode 12, a photoelectric conversion layer 13, and an amorphous oxide semiconductor layer 14 having a low carrier concentration (referred to simply as “amorphous oxide semiconductor layer 14”) are formed on a substrate 11. The amorphous oxide semiconductor layer 15 having a high carrier concentration (may be simply referred to as “amorphous oxide semiconductor layer 15”), and the surface electrode 16 are sequentially stacked. 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 laminated, and is preferably a glass substrate made of blue plate glass, a metal substrate such as a stainless plate, or a resin substrate such as a polyimide film. The thickness of the substrate 11 is not particularly limited.

<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 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. The main component of the photoelectric conversion layer 13 is preferably a p-type compound semiconductor material having a chalcopyrite crystal structure composed of Cu, Ga, In, and at least one of S and Se. If the p-type compound semiconductor material containing Ga as the main component of the photoelectric conversion layer 13 is easily trapped by the DX center. However, in the solar cell 10 of the present invention, the amorphous oxide semiconductor layer 14 having a low carrier concentration is provided closer to the photoelectric conversion layer 13 than the amorphous oxide semiconductor layer 15 having a high carrier concentration. Carrier recombination at the interface can be suppressed, and thus the photoelectric conversion efficiency can be improved. Therefore, if the main component of the photoelectric conversion layer 13 is the p-type compound semiconductor material, the effect of the present invention is exhibited.

  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 or more.

  FIG. 3 is a graph showing the relationship between Ga / (In + Ga) and its forbidden bandwidth Eg in CIGS. FIG. 4 is a graph showing the relationship between CIGS forbidden bandwidth and the conversion efficiency of a solar cell including a photoelectric conversion layer made of CIGS. When Ga / (In + Ga) in CIGS is changed as shown in FIG. 3, the band gap energy can be changed in the range of about 1.0 eV to 1.7 eV. As shown in FIG. 4, in an ideal pn junction solar cell, the conversion efficiency becomes maximum when the band gap energy is around 1.4 eV. Therefore, by controlling Ga / (In + Ga) in CIGS, it is possible to produce CIGS having an optimum band gap energy, and thus improve the conversion efficiency of a solar cell including a photoelectric conversion layer made of CIGS. be able to.

  However, when the band gap energy of CIGS exceeds about 1.2 eV, that is, when Ga / (In + Ga) is 0.3 or more, the photoelectric conversion efficiency of the solar cell including the photoelectric conversion layer made of CIGS decreases. Has been reported. The reason is that when Ga / (In + Ga) is 0.3 or more, carrier traps by the DX center are remarkably increased, and therefore, carrier traps in the photoelectric conversion layer made of CIGS and at the interface are increased. Therefore, a CIGS photoelectric conversion layer having a large Ga / (In + Ga), in particular, a heterointerface is formed by a photoelectric conversion layer made of CIGS having Ga / (In + Ga) of 0.3 or more and a conductive InGaZnO amorphous oxide semiconductor layer. In this case, the effect of the present invention is more exhibited.

  The method for measuring Ga / (In + Ga) in the p-type compound semiconductor material is not particularly limited, and examples thereof include secondary ion mass spectrometry, Auger electron spectroscopy, and energy dispersive X-ray spectroscopy.

  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.

  The thickness of the photoelectric conversion layer 13 is not particularly limited, and is preferably about 1 to 3 μm. 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.

<Amorphous oxide semiconductor layers with different carrier concentrations>
The amorphous oxide semiconductor layer 14 having a low carrier concentration is formed on the photoelectric conversion layer 13, and the amorphous oxide semiconductor layer 15 having a high carrier concentration is formed on the amorphous oxide semiconductor layer 14 having a low carrier concentration. Is formed. Both the amorphous oxide semiconductor layer 14 having a low carrier concentration and the amorphous oxide semiconductor layer 15 having a high carrier concentration are amorphous oxide semiconductor layers containing In, Ga, and Zn (InGaZnO amorphous). Oxide semiconductor layer). The carrier concentration in the amorphous oxide semiconductor layer 14 having a low carrier concentration is lower than the carrier concentration in the amorphous oxide semiconductor layer 15 having a high carrier concentration.

  In an InGaZnO amorphous oxide semiconductor, it is known that conduction electrons are generated due to oxygen vacancies, and it is known that defect levels are formed in the band gap due to oxygen vacancies. On the other hand, if the carrier concentration in the InGaZnO amorphous oxide semiconductor is low, it means that there are few oxygen vacancies in the InGaZnO amorphous oxide semiconductor, so that it is possible to prevent the formation of defect levels in the band gap. Therefore, if such an amorphous oxide semiconductor layer 14 having a low carrier concentration is provided between the photoelectric conversion layer 13 and the amorphous oxide semiconductor layer 15 having a high carrier concentration, an InGaZnO amorphous oxide semiconductor is provided. A layer having less oxygen vacancies than the amorphous oxide semiconductor layer 15 having a high carrier concentration can be provided between the photoelectric conversion layer 13 and the amorphous oxide semiconductor layer 15 having a high carrier concentration. Therefore, since carrier recombination at the heterointerface can be suppressed, the photoelectric conversion efficiency of the resulting solar cell 10 is improved.

The carrier concentration in the amorphous oxide semiconductor layer 14 having a low carrier concentration is preferably 5 × 10 ¹⁸ cm ⁻³ or less, more preferably 5 × 10 ¹⁵ cm ⁻³ or less. When the carrier concentration in the amorphous oxide semiconductor layer 14 with low carrier concentration exceeds 5 × 10 ¹⁸ cm ⁻³ , the amorphous oxide semiconductor layer 14 with low carrier concentration and the amorphous oxide semiconductor with high carrier concentration Differentiating from the layer 15 becomes difficult, and the effect obtained by providing the amorphous oxide semiconductor layer 14 having a low carrier concentration may not be sufficiently obtained.

The carrier concentration in the amorphous oxide semiconductor layer 15 having a high carrier concentration is preferably 1 × 10 ¹⁹ cm ⁻³ or more, more preferably 5 × 10 ¹⁹ cm ⁻³ or more. When the carrier concentration in the amorphous oxide semiconductor layer 15 having a high carrier concentration is lower than 1 × 10 ¹⁹ cm ⁻³ , the short circuit current may be decreased due to the decrease in conductivity.

  The carrier concentration means the concentration of conduction electrons or holes, depends on impurities and defects, and is not determined only by the amount of doped conductive impurities. That is, the carrier concentration in the amorphous oxide semiconductor layer 14 having a low carrier concentration is not determined only by the amount of the conductive impurity doped in the amorphous oxide semiconductor layer 14 having a low carrier concentration, but is an amorphous material having a high carrier concentration. The carrier concentration in the oxide semiconductor layer 15 is not determined only by the amount of conductive impurities doped in the amorphous oxide semiconductor layer 15 having a high carrier concentration.

  The carrier concentration can be obtained by using Hall effect measurement (measurement by Van der Pauw method). The Hall effect is a phenomenon in which an electromotive force V appears when an electric field is generated in a direction perpendicular to both the current direction and the magnetic field direction when a magnetic field is applied in a direction perpendicular to the direction in which the current flows. The film thickness used for calculating the carrier concentration can be measured by, for example, an atomic force microscope (AFM). The carrier concentration may be calculated based on the result of the voltage-capacitance characteristics of the solar cell (sometimes referred to as “CV characteristics”, where CV is an abbreviation for Capacity-Voltage).

The InGaZnO amorphous oxide semiconductor constituting the amorphous oxide semiconductor layer 14 having a low carrier concentration and the InGaZnO amorphous oxide semiconductor constituting the amorphous oxide semiconductor layer 15 having a high carrier concentration are each in appropriate amounts. It is preferable that Ga is contained. Ga ³⁺ has a strong binding energy with oxygen. Therefore, if the InGaZnO amorphous oxide semiconductor contains an appropriate amount of Ga, excessive oxygen vacancies can be suppressed. As a result, since the InGaZnO amorphous oxide semiconductor constituting each of the amorphous oxide semiconductor layers 14 and 15 has carrier concentration stability, the amorphous oxide semiconductor layers 14 and 15 with stable carrier concentration are obtained. Can be formed.

  The amorphous oxide semiconductor layer 14 with a low carrier concentration and the amorphous oxide semiconductor layer 15 with a high carrier concentration are made of Be, Na, Mg, Al, Si, Ca, Sc, Ti, in addition to In, Ga, and Zn. , V, Fe, Co, Ni, Ge, As, Y, Zr, Nb, Mo, Sn, Sb, Ba, Hf, Ta, W, and Pb may be included as an additive element. . The amorphous oxide semiconductor layer 14 having a low carrier concentration and the amorphous oxide semiconductor layer 15 having a high carrier concentration may each be a single layer made of a single material, or two materials having different compositions. A single layer containing more than one species may be used, or layers made of materials having different compositions may be laminated.

  The thickness of the amorphous oxide semiconductor layer 14 having a low carrier concentration is preferably 10 nm or less. Thereby, the fall of a short circuit current can be prevented.

  The thickness of the amorphous oxide semiconductor layer 15 having a high carrier concentration is not particularly limited, but is preferably 10 nm to 1.5 μm.

<Surface electrode>
The surface electrode 16 is formed on the amorphous oxide semiconductor layer 15 having a high carrier concentration. The material of the surface electrode 16 is not particularly limited as long as it is a conductive material, and is preferably made of, for example, Al or Ag. The surface electrode 16 is formed by, for example, resistance heating or electron beam vacuum deposition using Al metal or Ag metal, sputtering, MOCVD (Metal Organic Chemical Vapor Deposition), or MBE (Molecular Beam Epitaxy). It may be formed, or may be formed by applying (screen printing) a conductive paste containing aluminum powder or silver powder and then baking. For the patterning of the surface electrode 16, for example, an existing method such as an etching method using a photo process, a lift-off method, or a deposition method using a metal mask can be used.

  The surface electrode 16 may be an electrode formed by a vacuum deposition method or the like and stacked in the order of Ti / Pd / Ag.

The surface electrode 16 may be a transparent electrode made of ITO formed on the entire upper surface of the amorphous oxide semiconductor layer 15 having a high carrier concentration, and a group III element is added as a dopant to impart conductivity. A conductive zinc oxide film may be used. As the dopant, any of B, Al, and Ga can be used. The surface electrode 16 having such a configuration is preferably formed by sputtering using In ₂ O ₃ —SnO ₂ or Al ₂ O ₃ —ZnO as a target.

  The surface electrode 16 may be an electrode formed by laminating a transparent electrode and a metal electrode. In other words, the solar cell of the present invention may have a back electrode type structure (back contact structure) having no electrode on the light receiving surface. The surface electrode 16 may be provided on a transparent electrode made of ITO or the like.

  As described above, the solar cell 10 illustrated in FIG. 2 is described as an example of the solar cell of the present invention. However, in the solar cell of the present invention, the amorphous oxide semiconductor layer 14 having a low carrier concentration is not a high carrier concentration. What is necessary is just to be provided in the photoelectric converting layer 13 side rather than the crystalline oxide semiconductor layer 15. FIG. Therefore, the amorphous oxide semiconductor layer 14 having a low carrier concentration has one or more layers (for example, an amorphous oxide semiconductor layer having a lower carrier concentration than the amorphous oxide semiconductor layer 14) sandwiched between It may be provided on the conversion layer 13. The amorphous oxide semiconductor layer 15 having a high carrier concentration has one or more layers (for example, a carrier concentration higher than the carrier density of the amorphous oxide semiconductor layer 14 and lower than the carrier density of the amorphous oxide semiconductor layer 15). The amorphous oxide semiconductor layer 14 may be provided over the amorphous oxide semiconductor layer 14 having a low carrier concentration. The surface electrode 16 has an amorphous oxide semiconductor layer 15 having a high carrier concentration across one or more layers (for example, an amorphous oxide semiconductor layer having a higher carrier concentration than the amorphous oxide semiconductor layer 15). It may be provided above. In addition to the amorphous oxide semiconductor layer 15 having a high carrier concentration, an amorphous oxide semiconductor layer having an arbitrary carrier concentration is provided between the amorphous oxide semiconductor layer 14 having a low carrier concentration and the surface electrode 16. One or more layers may be provided.

The solar cell of the present invention is not limited to the configuration shown 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.

  In the solar cell 10 shown in FIG. 2, each material of the board | substrate 11, the back surface electrode 12, and the surface electrode 16 is not limited to the said material. In the solar cell 10 shown in FIG. 2, the photoelectric conversion layer 13 and the amorphous oxide semiconductor layers 14 and 15 may contain materials other than the above materials.

  The amorphous oxide semiconductor layers 14 and 15 in the present invention can be applied not only to a single junction cell but also to a multi-junction solar cell. 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]
An example of the manufacturing method of the solar cell 10 shown in FIG. 2 is shown below.

  First, the back electrode 12 and the photoelectric conversion layer 13 are sequentially formed on the substrate 11. The back electrode 12 is preferably formed by, for example, sputtering using a metal material constituting the back electrode 12 as a target. The photoelectric conversion layer 13 may be formed using a gas phase raw material such as a multi-source deposition method, a selenization method, or a sputtering method, or a liquid such as a nanoparticle coating method, an electrodeposition method, or a solution method. You may form using the raw material of a phase state.

  Next, an amorphous oxide semiconductor layer 14 having a low carrier concentration, an amorphous oxide semiconductor layer 15 having a high carrier concentration, and a transparent electrode made of ITO are sequentially formed on the photoelectric conversion layer 13. Thereafter, the surface electrode 16 is formed by vacuum vapor deposition (particularly resistance heating vapor deposition). An annealing process (for example, 280 ° C.) may be performed on the solar cell thus obtained.

  Hereinafter, a method for forming the amorphous oxide semiconductor layers 14 and 15 will be described. The method for forming the amorphous oxide semiconductor layers 14 and 15 is not particularly limited as long as it is a method capable of forming an InGaZnO amorphous oxide semiconductor layer. For example, DC (Direct Current) sputtering or RF (Radio) Known thin film forming methods represented by sputtering methods such as frequency (sputtering) methods, chemical vapor deposition (CVD) methods, pulsed laser deposition (PLD) methods, molecular beam epitaxy (MBE) methods, and ion beam sputtering (IBS) methods Can be mentioned.

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.

  As long as an InGaZnO amorphous oxide semiconductor layer with few defect levels can be formed as the amorphous oxide semiconductor layer 14 with a low carrier concentration, the amorphous oxide semiconductor layer 14, using a material source with the same composition, 15 may be formed, or the amorphous oxide semiconductor layers 14 and 15 may be formed using material sources having different compositions. In addition, regardless of whether an oxide or a metal body is used as the material source, the amorphous oxide semiconductor layers 14 and 15 may be sequentially formed in a vacuum, a rare gas, or oxygen. Alternatively, the amorphous oxide semiconductor layers 14 and 15 may be sequentially formed while oxidizing the metal in a gas atmosphere containing oxygen. However, it is preferable to form the amorphous oxide semiconductor layers 14 and 15 according to the following conditions.

  When the material source is an InGaZnO system, conduction electrons are generated due to oxygen vacancies. Therefore, when forming the amorphous oxide semiconductor layer 14 with 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. Specifically, the amorphous oxide semiconductor layer 14 may be formed using a material source in which oxygen vacancies are less likely to occur, and generation of oxygen vacancies is suppressed when the amorphous oxide semiconductor layer 14 is formed. As described above, the gas atmosphere during formation may be controlled. 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.

  In the case where the material source is used, if the amorphous oxide semiconductor layer 14 with few defect levels can be formed, the amorphous oxide semiconductor layer 14 and the amorphous oxide semiconductor layer 15 are formed using the material source having the same composition. Alternatively, the amorphous oxide semiconductor layer 14 and the amorphous oxide semiconductor layer 15 may be formed using different material sources. For example, the amorphous oxide semiconductor layer 14 and the amorphous oxide semiconductor layer 15 are formed using InGaZnO (In: Ga: Zn = 1: 1: 1 (atomic ratio)) having the same composition. May be. Alternatively, the amorphous oxide semiconductor layer 14 is formed using InGaZnO (In: Ga: Zn = 1: 1: 1 (atomic ratio)), while InGaZnO (In: Ga: Zn = 1: 2: 1 ( The amorphous oxide semiconductor layer 15 may be formed using InGaZnO (In: Ga: Zn = 1: 3: 1 (atomic ratio)). In general, an oxide semiconductor containing Ga easily forms a defect level. For this reason, when the amorphous oxide semiconductor layer 15 is formed using InGaZnO having a high Ga content as a material source, the effect of the present invention is more exhibited.

In the case of suppressing the generation of oxygen deficiency in the heat treatment during or after the formation by controlling the gas atmosphere during the heat treatment during or after the formation, the oxygen flow rate during the heat treatment during or after the formation is controlled. Is preferred. For example, the formation of the amorphous oxide semiconductor layer 14 having a low carrier concentration or the carrier concentration in a mixed gas atmosphere having a higher oxygen gas content than when the amorphous oxide semiconductor layer 15 having a high carrier concentration is formed. It is preferable to perform heat treatment after formation of the low-amorphous amorphous oxide semiconductor layer 14. When the amorphous oxide semiconductor layer 14 having a low carrier concentration is formed in a mixed gas atmosphere of oxygen gas and rare gas, the amount of oxygen gas introduced is 6 volumes with respect to the total amount introduced including the rare gas. It is preferable to perform formation of the amorphous oxide semiconductor layer 14 and heat treatment after the formation of the amorphous oxide semiconductor layer 14 or more. The amount of oxygen gas introduced is 12% by volume or more with respect to the total amount of introduction including the rare gas. More preferably, the physical semiconductor layer 14 is formed and heat treatment is performed after the formation. Thereby, the amorphous oxide semiconductor layer 14 having a carrier concentration of 5 × 10 ¹⁸ cm ⁻³ or less, more preferably 5 × 10 ¹⁵ cm ⁻³ or less is formed. Such control is preferably performed even when the amorphous oxide semiconductor layer 14 is formed using a material source in which oxygen vacancies are unlikely to occur.

  When the amorphous oxide semiconductor layer 15 having a high carrier concentration is formed, the composition of the gas atmosphere at the time of formation is not particularly limited. It is preferable to perform the formation of the amorphous oxide semiconductor layer 15 and the heat treatment after the formation with the introduction amount of the oxygen gas being 2% by volume or less with respect to the total introduction amount including the rare gas, in an atmosphere containing only the rare gas. More preferably, the amorphous oxide semiconductor layer 15 is formed and heat treatment is performed after the formation. The heat treatment after the formation of the amorphous oxide semiconductor layer 15 having a high carrier concentration is optional.

  Among the thin film formation methods, it is preferable to form the amorphous oxide semiconductor layers 14 and 15 by sputtering. As a result, the amorphous oxide semiconductor layers 14 and 15 that are flat at the atomic level and have a nano-order thickness are formed. Hereinafter, a specific method for forming the amorphous oxide semiconductor layers 14 and 15 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.

  Next, the target material is knocked out by supplying DC power or RF power to the sputtering target. Thereby, an amorphous oxide semiconductor layer 14 having a low carrier concentration and an amorphous oxide semiconductor layer 15 having a high carrier concentration are sequentially formed on the photoelectric conversion layer 13. At this time, when the amorphous oxide semiconductor layers 14 and 15 are formed using a target having the same composition, the gas atmosphere during the formation of the amorphous oxide semiconductor layer 14 or during the heat treatment after the formation is changed. It is preferable to control as described above. Further, the amorphous oxide semiconductor layer 14 is formed using a target made of InGaZnO (In: Ga: Zn = 1: 1: 1 (atomic ratio)), and InGaZnO (In: Ga: Zn = 1: 1). The amorphous oxide semiconductor layer 15 may be formed using a target of 2: 1 (atomic ratio) or InGaZnO (In: Ga: Zn = 1: 3: 1 (atomic ratio)).

  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.

  For the formed amorphous oxide layer, particularly for the amorphous oxide semiconductor layer 14 having a low carrier concentration, for the purpose of suppressing oxygen vacancies or improving the uniformity of the material in the thin film. If necessary, the heat treatment may be performed in a vacuum, in an atmosphere containing a rare gas, or in an atmosphere containing an oxygen gas. The heat treatment may be performed in an atmosphere in which the amorphous oxide semiconductor layer 14 having a low carrier concentration is formed, or may be performed in an atmosphere different from the atmosphere in which the amorphous oxide semiconductor layer 14 having a low carrier concentration is formed. You can do it. For example, when the amorphous oxide semiconductor layer 14 having a low carrier concentration is formed by introducing only a rare gas into the sputtering apparatus, the amorphous oxide semiconductor layer 14 having a low carrier concentration is formed on the photoelectric conversion layer 13. After the formation, oxygen gas is preferably introduced into the sputtering apparatus and heat treatment is performed in an atmosphere containing oxygen gas.

  In addition, each formation method of the back surface electrode 12, the photoelectric converting layer 13, the amorphous oxide semiconductor layers 14 and 15, and the surface electrode 16 is not limited to the said method, The formation method in a liquid phase may be sufficient. Alternatively, a formation method in a gas phase may be used. 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. 2 was manufactured. Specifically, an Mo back electrode and a CIGS photoelectric conversion layer were sequentially formed on an SLG (soda-lime glass) substrate. On the CIGS photoelectric conversion layer, an amorphous oxide semiconductor layer with a low carrier concentration (thickness 9 nm), an amorphous oxide semiconductor layer with a high carrier concentration (thickness 25 nm), and a transparent electrode (thickness) made of ITO Were formed in order. Thereafter, a surface electrode made of Al was formed on the transparent 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, the prepared anhydrous toluene solution was dropped on the SLG substrate, and then allowed to stand for 15 minutes in a nitrogen atmosphere. Thereby, the anhydrous toluene solvent was simply dried. 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. After repeating this coating process and temporary baking process a plurality of times, this coating-temporary baking process was repeated 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 whose thickness is about 2 micrometers was obtained. When Ga / (In + Ga) in the CIGS photoelectric conversion layer was determined by secondary ion mass spectrometry, it was 0.31.

An amorphous oxide semiconductor layer having a low carrier concentration and an amorphous oxide semiconductor layer having a high carrier concentration were formed by DC magnetron sputtering. InGaZnO (In: Ga: Zn = 1: 1: 1 (atomic ratio)) was used as a target, and Ar and O ₂ were used as gases during film formation.

When forming an amorphous oxide semiconductor layer having a low carrier concentration, the flow rate of argon (Ar) gas was set to 44 sccm, and the flow rate of oxygen (O ₂ ) gas was adjusted to 6 sccm. When the Hall effect was measured for the formed amorphous oxide semiconductor layer, the carrier concentration in the amorphous oxide semiconductor layer having a low carrier concentration was 4 × 10 ¹⁵ cm ⁻³ .

When forming an amorphous oxide semiconductor layer with a high carrier concentration, the flow rate of argon (Ar) gas was 49 sccm, and the flow rate of oxygen (O ₂ ) gas was adjusted to 1 sccm. When the Hall effect was measured for the formed amorphous oxide semiconductor layer, the carrier concentration in the amorphous oxide semiconductor layer having a high carrier concentration was 2 × 10 ¹⁹ cm ⁻³ .

  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 0.63%.

<Example 2>
A solar cell of Example 2 was manufactured in the same manner as in Example 1 except that Ga / (In + Ga) was 0.20.

  Specifically, an Mo back electrode and a CIGS photoelectric conversion layer were sequentially formed on an SLG (soda-lime glass) substrate. On the CIGS photoelectric conversion layer, an amorphous oxide semiconductor layer with a low carrier concentration (thickness 9 nm), an amorphous oxide semiconductor layer with a high carrier concentration (thickness 25 nm), and a transparent electrode (thickness) made of ITO Were formed in order. Thereafter, a surface electrode made of Al was formed on the transparent 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, the prepared anhydrous toluene solution was dropped on the SLG substrate, and then allowed to stand for 15 minutes in a nitrogen atmosphere. Thereby, the anhydrous toluene solvent was simply dried. 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. After repeating this coating process and temporary baking process a plurality of times, this coating-temporary baking process was repeated 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 whose thickness is about 2 micrometers was obtained. When Ga / (In + Ga) in the CIGS photoelectric conversion layer was determined by secondary ion mass spectrometry, it was 0.20.

An amorphous oxide semiconductor layer having a low carrier concentration and an amorphous oxide semiconductor layer having a high carrier concentration were formed by DC magnetron sputtering. InGaZnO (In: Ga: Zn = 1: 1: 1 (atomic ratio)) was used as a target, and Ar and O ₂ were used as gases during film formation.

When forming an amorphous oxide semiconductor layer having a low carrier concentration, the flow rate of argon (Ar) gas was set to 44 sccm, and the flow rate of oxygen (O ₂ ) gas was adjusted to 6 sccm. When the Hall effect was measured for the formed amorphous oxide semiconductor layer, the carrier concentration in the amorphous oxide semiconductor layer having a low carrier concentration was 4 × 10 ¹⁵ cm ⁻³ .

When forming an amorphous oxide semiconductor layer with a high carrier concentration, the flow rate of argon (Ar) gas was 49 sccm, and the flow rate of oxygen (O ₂ ) gas was adjusted to 1 sccm. When the Hall effect was measured for the formed amorphous oxide semiconductor layer, the carrier concentration in the amorphous oxide semiconductor layer having a high carrier concentration was 2 × 10 ¹⁹ cm ⁻³ .

  When the cell characteristics were measured by irradiating the solar cell thus produced with AM1.5G pseudo-sunlight, the photoelectric conversion efficiency was 0.56%.

<Example 3>
A solar cell of Example 3 was manufactured in the same manner as in Example 1 except that an amorphous oxide semiconductor layer having a low carrier concentration and a thickness of 18 nm was formed.

  Specifically, an Mo back electrode and a CIGS photoelectric conversion layer were sequentially formed on an SLG (soda-lime glass) substrate. On the CIGS photoelectric conversion layer, an amorphous oxide semiconductor layer with a low carrier concentration (thickness: 18 nm), an amorphous oxide semiconductor layer with a high carrier concentration (thickness: 25 nm), and a transparent electrode (thickness) made of ITO Were formed in order. Thereafter, a surface electrode made of Al was formed on the transparent 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, the prepared anhydrous toluene solution was dropped on the SLG substrate, and then allowed to stand for 15 minutes in a nitrogen atmosphere. Thereby, the anhydrous toluene solvent was simply dried. 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. After repeating this coating process and temporary baking process a plurality of times, this coating-temporary baking process was repeated 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 whose thickness is about 2 micrometers was obtained. When Ga / (In + Ga) in the CIGS photoelectric conversion layer was determined by secondary ion mass spectrometry, it was 0.31.

An amorphous oxide semiconductor layer having a low carrier concentration and an amorphous oxide semiconductor layer having a high carrier concentration were formed by DC magnetron sputtering. InGaZnO (In: Ga: Zn = 1: 1: 1 (atomic ratio)) was used as a target, and Ar and O ₂ were used as gases during film formation.

When forming an amorphous oxide semiconductor layer having a low carrier concentration, the flow rate of argon (Ar) gas was set to 44 sccm, and the flow rate of oxygen (O ₂ ) gas was adjusted to 6 sccm. When the Hall effect was measured for the formed amorphous oxide semiconductor layer, the carrier concentration in the amorphous oxide semiconductor layer having a low carrier concentration was 4 × 10 ¹⁵ cm ⁻³ .

When forming an amorphous oxide semiconductor layer with a high carrier concentration, the flow rate of argon (Ar) gas was 49 sccm, and the flow rate of oxygen (O ₂ ) gas was adjusted to 1 sccm. When the Hall effect was measured for the formed amorphous oxide semiconductor layer, the carrier concentration in the amorphous oxide semiconductor layer having a high carrier concentration was 2 × 10 ¹⁹ cm ⁻³ .

  When the cell characteristics were measured by irradiating the solar cell thus manufactured with AM1.5G pseudo-sunlight, the photoelectric conversion efficiency was 0.51%.

<Comparative Example 1>
A solar cell of Comparative Example 1 was produced in the same manner as in Example 1 except that the solar cell was produced without forming an amorphous oxide semiconductor layer having a low carrier concentration.

  Specifically, a Mo back electrode and a CIGS photoelectric conversion layer were sequentially formed on the SLG substrate. On the CIGS photoelectric conversion layer, an amorphous oxide semiconductor layer (thickness: 25 nm) having a high carrier concentration and a transparent electrode (thickness: 100 nm) made of ITO were sequentially formed. Thereafter, a surface electrode made of Al was formed on the transparent electrode.

An amorphous oxide semiconductor layer having a high carrier concentration was formed by DC magnetron sputtering. InGaZnO (In: Ga: Zn = 1: 1: 1 (atomic ratio)) was used as a target, and Ar and O ₂ were used as gases during film formation. The flow rate of argon gas was 49 sccm, and the flow rate of oxygen gas was adjusted to 1 sccm. When the Hall effect was measured for the formed amorphous oxide semiconductor layer, the carrier concentration in the amorphous oxide semiconductor layer having a high carrier concentration was 2 × 10 ¹⁹ cm ⁻³ .

  When the cell characteristics were measured by irradiating the solar cell thus manufactured with AM1.5G pseudo-sunlight, the photoelectric conversion efficiency was 0.30%.

<Discussion>
The photoelectric conversion efficiency of the solar cells of Examples 1 to 3 was higher than the photoelectric conversion efficiency of the solar cell of Comparative Example 1. The following can be considered as the reason. In the solar cells of Examples 1 to 3, since the amorphous oxide semiconductor layer having a low carrier concentration is provided between the CIGS photoelectric conversion layer and the amorphous oxide semiconductor layer having a high carrier concentration, the heterointerface Carrier recombination in is suppressed. On the other hand, in the solar cell of Comparative Example 1, since the amorphous oxide semiconductor layer having a low carrier concentration is not provided between the CIGS photoelectric conversion layer and the amorphous oxide semiconductor layer having a high carrier concentration, the heterointerface The carrier recombination in was not suppressed, and thus the photoelectric conversion efficiency was lowered.

  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 Substrate, 12 Back electrode, 13 Photoelectric conversion layer, 14 Amorphous oxide semiconductor layer with low carrier concentration, 15 Amorphous oxide semiconductor layer with high carrier concentration, 16 Surface electrode

Claims (4)

A photoelectric conversion layer including a p-type compound semiconductor material having a chalcopyrite type crystal structure composed of Cu, Ga, In, and at least one of S and Se;
A solar cell provided on the photoelectric conversion layer, comprising two or more amorphous oxide semiconductor layers including In, Ga, and Zn and having different carrier concentrations,
The solar cell in which the carrier concentration in the amorphous oxide semiconductor layer provided on the photoelectric conversion layer side is lower than the carrier concentration in the amorphous oxide semiconductor layer provided at a position far from the photoelectric conversion layer.   The solar cell according to claim 1, wherein Ga / (In + Ga) in the photoelectric conversion layer is 0.3 or more.   The solar cell according to claim 1 or 2, wherein a thickness of the amorphous oxide semiconductor layer provided on the photoelectric conversion layer side is 10 nm or less.   The solar cell according to claim 1, wherein the amorphous oxide semiconductor layers having different carrier concentrations are all formed by a sputtering method.

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