JP6035122B2 - Photoelectric conversion element and method for producing buffer layer of photoelectric conversion element - Google Patents

Description

  The present invention relates to a photoelectric conversion element used for solar cells, CCD sensors, and the like, a method for manufacturing a buffer layer of the photoelectric conversion element, and a method for manufacturing the photoelectric conversion element.

A photoelectric conversion element including a photoelectric conversion layer and an electrode connected to the photoelectric conversion layer is used for applications such as solar cells. Conventionally, in the case of solar cells, Si-based solar cells using bulk single crystal Si or polycrystalline Si, or thin-film amorphous Si have been mainstream, but research and development of Si-independent compound semiconductor solar cells has been made. ing. Known compound semiconductor solar cells include bulk systems such as GaAs systems and thin film systems such as CIS or CIGS systems composed of group Ib elements, group IIIb elements, and group VIb elements. CI (G) S is represented by the general formula _{Cu 1-z In 1-x} Ga x Se 2-y S y ( where, 0 ≦ x ≦ 1,0 ≦ y ≦ 2,0 ≦ z ≦ 1) When x = 0, it is a CIS system, and when x> 0, it is a CIGS system. In this specification, CIS and CIGS are collectively described as “CI (G) S”.

  In conventional thin film photoelectric conversion elements such as CI (G) S, a CdS buffer layer is generally provided between a photoelectric conversion layer and a translucent conductive layer formed thereon. In such a system, the buffer layer is usually formed by a chemical bath deposition method (CBD method).

  In recent years, Cd-free buffer layers have been studied in consideration of environmental loads. As the main component of the Cd-free buffer layer, zinc-based compounds such as Zn (S, O) have been studied.

  Patent Documents 1 and 2 propose a method for manufacturing a photoelectric conversion element including a Zn (S, O) -based buffer layer.

JP 2011-119964 A JP 2012-18953 A

  However, the present inventors have studied that the photoelectric conversion efficiency of a solar cell obtained by integrating CIGS photoelectric conversion elements to which a conventional Zn-based compound buffer layer is applied is lower than expected from a single cell. It was revealed. The present inventors have found that a photoelectric conversion element including a buffer layer made of a Zn-based compound has a very large variation in characteristics between cells. For this reason, a large number of solar cells (for example, a plurality of single cells connected in series) In the submodule), it has been found that the expected performance may not be exhibited due to the influence of a cell having low characteristics.

  The Zn-based compound buffer layer is less uniform than the CdS-based buffer layer, and thus it is considered that the variation between cells is large. Therefore, in developing a photoelectric conversion element having a Zn-based compound buffer layer, it is important to improve the photoelectric conversion efficiency and reduce the characteristic variation between cells. Note that it is preferable that the characteristic variation between cells is small, not only in the case of a Zn-based compound buffer layer, but also in the case where a buffer layer made of a compound of any composition is provided.

  This invention is made | formed in view of the said situation, and it aims at providing the photoelectric conversion element provided with the buffer layer which can suppress the characteristic dispersion | variation between elements.

  Another object of the present invention is to provide a method for manufacturing a buffer layer capable of suppressing variations in device characteristics, and further to provide a method for manufacturing a photoelectric conversion element to which the method for manufacturing the buffer layer is applied. .

The photoelectric conversion element of the present invention comprises, on a substrate, a lower electrode layer, a photoelectric conversion layer mainly composed of a compound semiconductor having at least one chalcopyrite structure composed of an Ib group element, an IIIb group element, and a VIb group element; A photoelectric conversion element in which a buffer layer and a translucent conductive layer are laminated,
The buffer layer is made of a compound containing an alkali metal,
The alkali metal is at least one of lithium, sodium and potassium;
The alkali metal content in the above compound is 0.1 at% or more and 5 at% or less.

  The compound constituting the buffer layer preferably contains at least one metal selected from the group consisting of Cd, Zn, In, and Sn, sulfur, and oxygen.

  Furthermore, it is preferable that the metal is any one of Zn, In, and Sn, particularly Zn.

  The alkali metal content is preferably 0.5 at% or more and 2.5 at% or less.

  The thickness of the buffer layer is preferably 1 nm or more and 100 nm or less.

The group Ib element is at least one selected from the group consisting of Cu and Ag;
The group IIIb element is at least one selected from the group consisting of Al, Ga and In;
The VIb group element is preferably at least one selected from the group consisting of S, Se, and Te.

  The solar cell of the present invention is characterized in that a plurality of the photoelectric conversion elements of the present invention are integrated.

The method for producing a buffer layer according to the present invention includes a photoelectric conversion mainly comprising a lower electrode layer and a compound semiconductor having at least one chalcopyrite structure composed of a group Ib element, a group IIIb element, and a group VIb element on a substrate. A method of manufacturing a buffer layer in a photoelectric conversion element in which a layer, a buffer layer, and a light-transmitting conductive layer are laminated,
Prepare a reaction solution for chemical bath deposition containing a predetermined metal ion, thiourea, 1M or more alkali metal ion,
The buffer layer is deposited on the photoelectric conversion layer by bringing at least the surface of the photoelectric conversion layer into contact with the reaction solution of the buffer film-forming substrate in which the lower electrode layer and the photoelectric conversion layer are laminated in this order on the substrate. It is characterized by.

  The predetermined metal is preferably at least one of Cd, Zn, In, and Sn, and more preferably Zn, In, or Sn. Particularly preferred is Zn.

  The alkali metal is preferably at least one of sodium, potassium, and lithium.

The method for producing a photoelectric conversion element of the present invention comprises a photoelectric conversion element comprising, as a main component, a lower electrode layer, and a compound semiconductor having at least one chalcopyrite structure composed of a group Ib element, a group IIIb element, and a group VIb element on a substrate. In the method for manufacturing a photoelectric conversion element in which a conversion layer, a buffer layer, and a light-transmitting conductive layer are laminated,
The buffer layer is formed by the buffer layer manufacturing method of the present invention.

  In the photoelectric conversion element of the present invention, an alkali metal composed of sodium, potassium and / or lithium is contained at 0.1 at% or more and 5 at% as a buffer layer provided on a photoelectric conversion layer mainly composed of a compound semiconductor having a chalcopyrite structure. %, It has excellent in-plane uniformity of characteristics. An element including a buffer layer containing alkali metal at 0.1 at% or more and 5 at% or less is a photoelectric conversion element having a uniform characteristic with small variation in characteristics between photoelectric conversion elements formed simultaneously on the same substrate. Can be obtained.

  Since the buffer layer contains an alkali metal, the buffer layer can be used as an alkali metal supply layer to the photoelectric conversion layer, and the photoelectric conversion efficiency is improved by adding the alkali metal to the photoelectric conversion layer. Can be obtained without increasing the number of steps.

  Since the solar cell of the present invention can be provided with highly uniform photoelectric conversion elements, it suppresses a decrease in photoelectric conversion efficiency due to the influence of elements having poor characteristics, which is a problem when the uniformity between the photoelectric conversion elements is low. As a result, high photoelectric conversion efficiency can be obtained.

Schematic sectional view showing a layer structure of a photoelectric conversion element according to an embodiment of the present invention Schematic sectional view showing an example of an anodized substrate applicable to the photoelectric conversion element of the present invention Schematic plan view of an integrated solar cell according to an embodiment of the present invention Partial enlarged cross-sectional view of an integrated solar cell The figure which shows the compositional analysis result of the depth direction of the sample of Example 3

  Hereinafter, embodiments of the present invention will be described in detail.

  With reference to drawings, the structure of the photoelectric conversion element of one Embodiment which concerns on this invention, and its manufacturing method are demonstrated. FIG. 1 is a cross-sectional view illustrating a schematic configuration of a photoelectric conversion element. In order to facilitate visual recognition, the scale of each component in the figure is appropriately different from the actual one.

The photoelectric conversion element 1 has a photoelectric conversion element composed mainly of a compound semiconductor having at least one chalcopyrite structure composed of a lower electrode layer (back electrode) 20, a group Ib element, a group IIIb element, and a group VIb element on a substrate 10. The conversion layer 30, the buffer layer 40, a translucent conductive layer (transparent electrode) 60 as an upper electrode layer, and a grid electrode 70 are sequentially stacked. It consists of a compound containing at a ratio of 1 at% or more and 5 at% or less, and the alkali metal is at least one of lithium, sodium and potassium.
A so-called window layer may be provided between the buffer layer 40 and the translucent conductive layer 60.

  The buffer layer 40 is provided for the purpose of preventing recombination of photogenerated carriers, band discontinuous matching, lattice matching, surface irregularity coverage of the photoelectric conversion layer, and mitigating damage during the formation of the light-transmitting conductive layer. Layer.

  In the present embodiment, the compound constituting the buffer layer 40 is a compound containing at least one metal selected from the group consisting of Cd, Zn, In, and Sn (which may include inevitable impurities), sulfur, and oxygen. is there. The metal is more effective when it is Zn, In or Sn. In particular, when the metal is Zn, that is, when the buffer layer is a Zn-based compound, the effect of the present invention is particularly great.

  When the metal is Zn, the main component constituting the buffer layer 40 is Zn (S, O) or Zn (S, O, OH). The description of Zn (S, O) and Zn (S, O, OH) means a mixed crystal of zinc sulfide and zinc oxide and a mixed crystal of zinc sulfide, zinc oxide and zinc hydroxide. In addition, zinc oxide and zinc hydroxide may be present without forming a mixed crystal because they are amorphous. In this case, the buffer layer contains an alkali metal element means that the buffer layer is composed of a compound in which an alkali metal element is added to Zn (S, O) and Zn (S, O, OH) as main components. Means that When the metal is Cd, In, Sn, the compound is obtained by replacing Zn in the above chemical formula with Cd, In, Sn.

  The content of the alkali metal in the compound constituting the buffer layer 40 is not less than 0.1 at% and not more than 5 at%. The present inventors have found that when the content is 0.1 at% or more, the in-plane uniformity of the buffer layer is high, and the photoelectric conversion element including such a buffer layer has extremely small variation in characteristics between cells. (See Examples below). In addition, when forming a buffer layer by the CBD method, it is difficult to add an alkali metal exceeding 5 at% in the buffer layer. The alkali metal content in the buffer layer is more preferably 0.5 at% or more and 2.5 at% or less.

In addition, the compound which comprises a buffer layer may contain 2 or more types of alkali metals. When two or more kinds of alkali metals are included, the total content is 0.1 at% or more and 5 at% or less, and more preferably 0.5 at% or more and 1.5 at% or less.
The composition of the buffer layer can be specified, for example, by measurement by X-ray photoelectron spectroscopy (XPS), and the alkali metal content can be determined from the measurement result.

  Although the mechanism of the effect that the variation in photoelectric conversion efficiency between elements is reduced by including an alkali metal in the buffer layer is not clear, the band structure of the buffer layer is changed by including the alkali metal. And it is thought that the effect which suppresses the recombination of the carrier generate | occur | produced in the photoelectric converting layer has arisen.

  Moreover, the characteristic improvement of the photoelectric converting layer by the alkali metal supply to the photoelectric converting layer which consists of a CIGS layer can also be anticipated by including an alkali metal in a buffer layer. It has been conventionally known that photoelectric conversion efficiency is improved by adding an alkali metal to the CIGS layer. As a means for adding alkali metal to the CIGS layer, a glass substrate such as soda lime glass containing an alkali metal or a substrate provided with an alkali metal supply layer is used. A method of adding an alkali metal to the surface (JP-A-2011-233874) or a method of forming an alkali metal-containing CIGS by forming an alkali metal-containing compound on the CIGS surface after forming a CIGS precursor (International Publication) No. 2005/109525 pamphlet) is known. In a method other than using a glass substrate such as soda lime glass containing an alkali metal in advance, it is necessary to increase the number of steps for supplying the alkali metal to the CIGS layer. In the present invention, since a buffer layer containing an alkali metal is formed, the buffer layer can also serve as an alkali metal supply layer to the CIGS layer. That is, by performing heat treatment at an appropriate temperature after forming the buffer layer, the alkali metal diffuses into the CIGS layer, and the photoelectric conversion efficiency can be improved.

  The buffer layer 40 has a thickness of 1 nm or more and 1 μm or less, preferably 1 nm or more and 100 nm or less, more preferably 5 nm or more, and particularly preferably 10 nm or more.

  The buffer layer 40 preferably has a crystalline part and an amorphous part. The molar ratio of sulfur atoms to the total number of moles of sulfur atoms and oxygen atoms in the buffer layer 40 is preferably 0.2 to 0.8. The molar ratio is more preferably 0.4 to 0.5.

  Further, the conductivity type of the buffer layer 40 is not particularly limited, but is preferably n-type.

  Below, the manufacturing method of the buffer layer 40 is demonstrated. The compound may contain inevitable impurities.

<Film formation process>
A CBD method is used for forming the buffer layer.
The “CBD method” is a general formula [M (L) _i ] ^{m +} Ｍ M ^{n +} + iL (wherein, M: metal element, L: ligand, m, n, i: each represents a positive number). This is a method for precipitating crystals on a substrate at a moderate rate in a stable environment by setting the supersaturation condition by an equilibrium as expressed.

  As the reaction solution for chemical bath deposition, a solution containing a predetermined metal ion, thiourea, and 1M or more alkali metal ions is used. Hereinafter, the preferable composition of the reaction solution for chemical bath deposition will be described. Hereinafter, the case where the predetermined metal is zinc will be described, but the same applies to the case of cadmium, indium, and tin.

The Zn source preferably contains at least one selected from the group consisting of zinc sulfate, zinc acetate, zinc nitrate, zinc citrate, and hydrates thereof. The concentration is not particularly limited and is preferably 0.001 to 0.5M.
The concentration of thiourea is not particularly limited and is preferably 0.01 to 1.0M.

  Furthermore, 1M or more of alkali metal ions are contained. Here, the alkali metal source preferably contains trisodium citrate, tripotassium citrate, trilithium citrate and / or a hydrate thereof. Sodium is taken into the buffer layer when trisodium citrate is used, potassium is used when tripotassium citrate is used, and lithium is used when trilithium citrate is used.

  The trisodium citrate, tripotassium citrate and trilithium citrate also serve as components that function as complexing agents, etc., and have been added to the reaction solution during buffer layer formation by chemical bath deposition. It has been done. For example, Patent Document 1 describes that the concentration of trisodium citrate in the reaction solution is preferably 0.001 to 0.25M. In the present invention, since the concentration of alkali metal ions in the reaction solution is 1M or more, the concentration of trisodium citrate is 0.34M or more. In order to sufficiently contain the alkali metal in the buffer layer, it is necessary to set the concentration to this level. In the preferred concentration range of conventional trisodium citrate, the concentration of sodium ions is at most about 0.75 M, and it is difficult to obtain a buffer layer having a sodium content of 0.1 at% or more.

The reaction solution preferably contains an ammonium salt such as NH ₄ OH that functions as a pH adjuster or the like. The ammonium salt is also a component that functions as a complexing agent or the like. The concentration of the ammonium salt is preferably 0.001 to 0.40M. Thereby, pH can be adjusted and the solubility and supersaturation degree of a metal ion can be adjusted. If the ammonium salt concentration is in the range of 0.001 to 0.40 M, the reaction rate is fast, and film formation is carried out at a practical production rate without providing a fine particle layer formation step before the film formation step. If the concentration of the ammonium salt that can be produced exceeds 0.40 M, the reaction rate becomes slow, and it is necessary to devise such as adding a fine particle layer before the film forming step. The concentration of the ammonium salt is preferably 0.01 to 0.30M.

The pH of the reaction solution before starting the reaction is 9.0 to 12.0.
When the pH of the reaction solution before the start of reaction is less than 9.0, the decomposition reaction of the component (S) such as thiourea does not proceed, or even if it proceeds, the precipitation reaction does not proceed. The decomposition reaction of thiourea is as follows. The decomposition reaction of thiourea is described in Journal of the Electrochemical Society, 141, 205-210 (1994) and Journal of Crystal Growth 299, 136-141 (2007).
SC (NH ₂ ) ₂ + OH ⁻ ⇔ SH ⁻ + CH ₂ N ₂ + H ₂ O
SH ⁻ + OH ⁻ ⇔ S ² + + H ₂ O
When the pH of the reaction solution before the start of the reaction exceeds 12.0, the effect that the component (N) that also functions as a complexing agent or the like makes a stable solution increases, and the precipitation reaction does not proceed or proceeds. It will be very slow. The pH of the reaction solution before starting the reaction is preferably 9.5 to 11.5.

  In the reaction solution used in the present invention, the concentration of the component (N) is 0.001 to 0.40 M, and if it is such a concentration, special pH adjustment such as using a pH adjusting agent other than the component (N) is performed. Even if not, the pH of the reaction solution before starting the reaction usually falls within the range of 9.0 to 12.0.

  The pH after completion of the reaction of the reaction solution is not particularly limited. The pH of the reaction solution after completion of the reaction is preferably 7.5 to 11.0. When the pH of the reaction solution after completion of the reaction is less than 7.5, it means that the reaction does not proceed, and it is meaningless when considering efficient production. In addition, if there is such a pH drop in a system containing ammonia that has a buffering effect, it is highly possible that ammonia has volatilized excessively in the heating process, and it is considered that manufacturing improvements are necessary. It is done. When the pH of the reaction solution after completion of the reaction is more than 11.0, the decomposition of thiourea is promoted, but since most of the zinc ions are stabilized as ammonium complexes, the progress of the precipitation reaction may be remarkably slowed. The pH of the reaction solution after completion of the reaction is more preferably 9.5 to 10.5.

  In the reaction solution used in the present invention, the pH of the reaction solution after starting the reaction is usually in the range of 7.5 to 11.0 without special pH adjustment such as using a pH adjusting agent other than the component (N). Inside.

  The reaction temperature is 70 to 95 ° C. When the reaction temperature is less than 70 ° C., the reaction rate becomes slow, and the thin film does not grow, or even if the thin film is grown, it is difficult to obtain a desired thickness at a practical reaction rate. When the reaction temperature exceeds 95 ° C., generation of bubbles and the like increases in the reaction solution, which adheres to the film surface and makes it difficult to grow a flat and uniform film. Furthermore, when the reaction is carried out in an open system, a concentration change due to evaporation of the solvent or the like occurs, making it difficult to maintain stable thin film deposition conditions. The reaction temperature is preferably 80 to 90 ° C.

  The reaction time is not particularly limited. In the present invention, the reaction can be carried out at a practical reaction rate without providing a fine particle layer. Although the reaction time depends on the reaction temperature, for example, the base can be satisfactorily covered in 10 to 90 minutes, and a layer having a sufficient thickness as a buffer layer can be formed.

The reaction solution used in the present invention is aqueous, and the pH of the reaction solution is not a strong acid condition. Although the pH of the reaction solution may be 11.0 to 12.0, the reaction can be carried out even under a mild pH condition of less than 11.0, so that the alkali such as a metal capable of forming a complex ion with a hydroxide ion is used. Anodization mainly composed of Al ₂ O ₃ on at least one surface side of an Al base material mainly composed of Al, which can be applied as a substrate including a metal that is easily dissolved in a solvent, for example, a flexible substrate An anodized substrate having a film formed thereon, Al ₂ O ₃ on at least one surface side of a composite base material in which an Al material containing Al as a main component is composited on at least one surface side of an Fe material containing Fe as a main component An anodized substrate having an anodic oxide film mainly composed of Fe, and at least one of a base material having an Al film mainly composed of Al formed on at least one surface side of an Fe material mainly composed of Fe to mainly composed of _Al 2 _{O 3} on the side Even when using the electrode oxide film anodized substrate of the anodized substrate and the like are formed such, there is no possibility of damage to the substrate can be formed with high uniformity buffer layer.

  In addition, after the film formation process, at least the buffer layer is annealed at a temperature lower than the heat resistant temperature of the substrate. An annealing treatment temperature of 150 ° C. or higher is effective for improving photoelectric conversion efficiency. The method for the annealing treatment is not particularly limited, and may be heated in a heater or a dryer, or may be light annealing such as laser annealing or flash lamp annealing.

  The photoelectric conversion element of the present invention is not particularly limited with respect to the configuration other than the buffer layer. Below, the preferable example about each component other than a buffer layer is demonstrated in order.

(substrate)
The substrate 10 is not particularly limited, and a glass substrate, a metal substrate such as stainless steel having an insulating film formed on the surface, and Al ₂ O ₃ is mainly used on at least one surface side of an Al base material mainly composed of Al. An anodized substrate on which an anodized film as a component is formed, at least one surface side of a composite base material in which an Al material containing Al as a main component is combined with at least one surface side of an Fe material containing Fe as a main component An anodized substrate on which an anodized film mainly composed of Al ₂ O ₃ is formed, and an Al film mainly composed of Al formed on at least one surface side of an Fe material mainly composed of Fe An anodized substrate in which an anodized film mainly composed of Al ₂ O ₃ is formed on at least one surface side of the material, a resin substrate such as polyimide, and the like can be used.

  Since production by a roll-to-roll process is possible, a flexible substrate such as a metal substrate having an insulating film formed on the surface, an anodized substrate, and a resin substrate is preferable.

In consideration of thermal expansion coefficient, heat resistance, substrate insulation, etc., an anodized film mainly composed of Al ₂ O ₃ was formed on at least one surface side of an Al base composed mainly of Al. The main component is Al ₂ O ₃ on at least one surface side of a composite base material in which an Al material containing Al as a main component is combined with at least one surface side of an Fe material containing Fe as a main component. An anodized substrate on which an anodic oxide film is formed, and Al on at least one surface side of a base material on which an Al film mainly composed of Al is formed on at least one surface side of an Fe material containing Fe as a main component An anodized substrate selected from the group consisting of an anodized substrate on which an anodized film composed mainly of ₂ O ₃ is formed is particularly preferred.

The left figure of FIG. 2 is a board | substrate with which the anodic oxide film 12 was formed in both surfaces of the Al base material 11, and the right figure of FIG. It is shown in. The anodic oxide film 12 is a film containing Al ₂ O ₃ as a main component.
In order to suppress the warpage of the substrate due to the difference in thermal expansion coefficient between Al and Al ₂ O ₃ and the film peeling due to this in the device manufacturing process, as shown in the left diagram of FIG. It is preferable that the anodic oxide film 12 is formed on both sides of the film.

  Further, a soda lime glass (SLG) layer may be provided on the anodized film of the anodized substrate. By providing the soda lime glass layer, Na can be diffused in the photoelectric conversion layer. When the photoelectric conversion layer contains Na, the photoelectric conversion efficiency can be further improved.

  Further, a soda lime glass (SLG) layer may be provided on the substrate. By providing the soda lime glass layer, Na can be diffused in the photoelectric conversion layer. When the photoelectric conversion layer contains Na, the photoelectric conversion efficiency can be further improved.

(Lower electrode)
The main component of the lower electrode (back electrode) 20 is not particularly limited, and Mo, Cr, W, and combinations thereof are preferable, and Mo is particularly preferable. The film thickness of the lower electrode (back electrode) 20 is not limited and is preferably about 200 to 1000 nm. The lower electrode 20 can be formed on the substrate by sputtering, for example.

(Photoelectric conversion layer)
The main component of the photoelectric conversion layer 30 is not particularly limited and is preferably a compound semiconductor having at least one chalcopyrite structure because high photoelectric conversion efficiency is obtained. The Ib group element, the IIIb group element, and the VIb group More preferably, it is at least one compound semiconductor composed of an element.

  As a main component of the photoelectric conversion layer 30, at least one type Ib group element selected from the group consisting of Cu and Ag, and at least one type IIIb group element selected from the group consisting of Al, Ga, and In, It is preferably at least one compound semiconductor comprising at least one VIb group element selected from the group consisting of S, Se, and Te.

Examples of the compound semiconductor 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 , AgGaTe ₂ , AgGaTe2 _{in, Al) Se 2, Cu} (in, Ga) (S, Se) 2, Cu in _{1-z in 1-x Ga} x Se 2-y S y ( wherein, 0 ≦ x ≦ 1,0 ≦ y ≦ 2, 0 ≦ z ≦ 1) (CI (G) S), Ag (In, Ga) Se ₂ , Ag (In, Ga) (S, Se) _{2 and the} like.

_{Further, Cu 2 ZnSnS 4, Cu 2} ZnSnSe 4, Cu 2 ZnSn (S, Se) may be ₄ or the like.

  The film thickness of the photoelectric conversion layer 30 is not particularly limited, and is preferably 1.0 to 4.0 μm, particularly preferably 1.5 to 3.5 μm.

  The method for forming the photoelectric conversion layer 30 is not particularly limited, and can be formed by a vacuum deposition method, a sputtering method, an MOCVD method, or the like.

(Window layer)
As described above, an insulating layer may be provided as a window layer between the buffer layer 40 and the translucent conductive layer 60. This insulating layer is an intermediate layer that captures light and inhibits recombination of photoexcited electrons and holes, thereby contributing to improvement in power generation efficiency. The composition of the insulating layer is not particularly limited, but i-ZnO or the like is preferable. The film thickness is not particularly limited, preferably 10 nm to 2 μm, and more preferably 15 to 200 nm. The film forming method is not particularly limited, but a sputtering method or an MOCVD method is suitable. On the other hand, when the buffer layer 40 is manufactured by the liquid phase method, it is also preferable to use the liquid phase method in order to simplify the manufacturing process.

(Translucent conductive layer)
The translucent conductive layer (transparent electrode) 60 is a layer that captures light and functions as an upper electrode that is paired with the lower electrode 20 and through which a current generated in the photoelectric conversion layer 30 flows. The composition of the translucent conductive layer 60 is not particularly limited, and n-ZnO such as ZnO: Al is preferable. The film thickness of the translucent conductive layer 60 is not particularly limited, and is preferably 50 nm to 2 μm. The film forming method of the translucent conductive layer 60 is not particularly limited, but as with the window layer, a sputtering method, an MOCVD method, a reactive plasma deposition method, an ion plating method, and the like are suitable. On the other hand, it is also preferable to use a liquid phase method in order to simplify the manufacturing process.

(Grid electrode)
The grid electrode 70 is an electrode used to extract electric power from the element, and the main component thereof is not particularly limited, and examples thereof include Al. The film thickness of the grid electrode 70 is not particularly limited, and is preferably 0.1 to 3 μm.

  The photoelectric conversion element 1 of this embodiment is configured as described above.

  If the manufacturing method of the photoelectric conversion element 1 forms a buffer layer using the manufacturing method of the said buffer layer, the formation method about another layer will not have a restriction | limiting in particular, The various demonstrated in the description part of each said layer. A film formation method can be applied.

  The photoelectric conversion element 1 can be preferably used for a solar cell or the like. While integrating the photoelectric conversion element 1, a cover glass, a protective film, etc. can be attached as needed and it can be set as a solar cell.

  FIG. 3 shows a plan view of a solar cell 2 in which a plurality of photoelectric conversion elements 1 are integrated, and FIG. 4 is an enlarged cross-sectional view of a part of FIG.

  As shown in FIG. 3 and FIG. 4, the solar cell 2 is a photoelectric conversion element in which a lower electrode 20, a photoelectric conversion layer 30, a buffer layer 40, and a translucent conductive layer 60 are stacked on a single substrate 10. A number of cells 1 are connected in series.

  After the lower electrode 20 is formed on the substrate 10, the electrode layer is scribed to form the first scribe line 61, and the photoelectric conversion layer 30 and the buffer layer 40 are sequentially formed on the first scribe line 61 and penetrate therethrough. Then, a scribe line 62 reaching the surface of the electrode layer is formed, a translucent conductive layer (upper electrode) 60 is laminated, and a number of scribe lines 64 extending from the translucent conductive layer 60 to the surface of the lower electrode 20 are formed. The solar cell 2 in which the cells 1 are integrated can be obtained. Adjacent cells are separated by a scribe line 64, and adjacent cells are connected in series by a translucent conductive layer material embedded in the scribe line 62.

  Thus, the solar cell in which a large number of photoelectric conversion elements 1 are connected in series has an efficiency that is expected from the average photoelectric conversion efficiency of the cells due to the influence of the characteristics of the low-efficiency cells in the large number of photoelectric conversion elements. In the past, it was not always possible. However, the photoelectric conversion element of the present invention can achieve the efficiency expected from the integrated solar cell from the efficiency obtained by a single cell, since the fluctuation of the efficiency between each element is suppressed small, It is possible to obtain a solar cell with higher photoelectric conversion efficiency which is more stable than before.

"Design changes"
The present invention is not limited to the above-described embodiment, and the design can be changed as appropriate without departing from the spirit of the present invention.

  Examples and comparative examples according to the present invention will be described.

<Substrate to photoelectric conversion layer>
The following substrates 1 and 2 were prepared as substrates.
Substrate 1: Soda lime glass (SLG) substrate.
Substrate 2: 100 μm-thick stainless steel (SUS) -30 μm-thick Al composite base material is anodized so that an anodized aluminum substrate (AAO) is formed on the Al surface. A soda lime glass (SLG) layer is provided on the AAO surface of the anodized substrate. Board. The thickness of each layer of the substrate 2 is SUS (100 μm), Al (20 μm), AAO (10 μm), and SLG (0.2 μm).

A Mo lower electrode having a thickness of 0.8 μm is formed on the substrate 1 or the substrate 2 by sputtering, and a Cu (In) film having a thickness of 1.8 μm is formed on the Mo lower electrode as a photoelectric conversion layer using a three-step method. _{A 0.7} Ga _0.3 ) Se ₂ layer was deposited. This is referred to as a buffer film forming substrate.

<Surface treatment>
A reaction vessel containing a 10% aqueous solution of KCN was prepared, and the surface of the CIGS layer, which is the outermost surface of the buffer film formation substrate, was immersed at room temperature for 3 minutes to remove impurities from the CIGS layer surface. After taking out, it fully washed with water.

  After the formation of the Mo electrode and the CIGS layer and the surface treatment using the substrate 1 or the substrate 2, a buffer layer was formed on the surface of the CIGS layer under the conditions of the following Examples 1 to 10 and Comparative Examples 1 to 3.

Example 1
Substrate 1 was used.
Zinc sulfate aqueous solution (0.18 [M]) as aqueous solution (I), thiourea aqueous solution (thiourea 0.30 [M]) as aqueous solution (II), and trisodium citrate aqueous solution (2.4 as aqueous solution (III)) [M]) and aqueous ammonia (0.3 [M]) were prepared as an aqueous solution (IV). Next, among these aqueous solutions, I, II and III are mixed in the same volume, zinc sulfate 0.06 [M], thiourea 0.10 [M], trisodium citrate 0.8 [M]. A mixed solution to be obtained was completed, and this mixed solution and 0.3 [M] aqueous ammonia were mixed in equal volumes to obtain a reaction solution 1. When mixing the aqueous solutions (I) to (IV), the aqueous solution (IV) was added last. In order to obtain a transparent reaction solution, it is important to add the aqueous solution (IV) last. Reaction liquid 1 is zinc sulfate 0.03 [M], thiourea 0.05 [M], trisodium citrate 0.4 [M], and ammonia 0.15 [M]. The alkali metal in the reaction solution 1 is sodium, and its concentration is 1.2M. The pH of the obtained reaction liquid 1 was 10.4.

Next, Zn (S, O) and / or Zn (S, O, OH) is applied to the surface of the outermost photoelectric conversion layer (CIGS) of the buffer film formation substrate by the CBD method using the reaction solution 1. A buffer layer having a main component was formed. Specifically, the substrate for buffer film formation was immersed in 500 mL of a reaction solution adjusted to 90 ° C. for 120 minutes, then the substrate was taken out, and the surface was sufficiently washed with pure water, and then dried at room temperature. In the step of immersing the substrate in the reaction solution, the substrate was placed so that the substrate surface was perpendicular to the bottom surface of the reaction solution container.
After the above buffer layer was formed, the buffer layer was formed by annealing at 200 ° C. for 1 hour.

(Example 2)
A buffer layer was formed in the same manner as in Example 1 except that the substrate 2 was used instead of the substrate 1.

Example 3
A buffer layer was formed in the same manner as in Example 1 except that the time for immersing the buffer film-forming substrate in the reaction solution (reaction time) was 60 minutes.

Example 4
A buffer layer was formed in the same manner as in Example 3 except that the substrate 2 was used instead of the substrate 1.

(Example 5)
A buffer layer was formed in the same manner as in Example 1 except that the time (reaction time) for immersing the buffer film-forming substrate in the reaction solution was 90 minutes.

(Example 6)
A buffer layer was formed in the same manner as in Example 5 except that the substrate 2 was used instead of the substrate 1.

(Example 7)
Reaction solution 2 comprising zinc sulfate 0.03 [M], thiourea 0.05 [M], tripotassium citrate 0.4 [M], and ammonia 0.15 [M] was prepared.
A buffer layer was formed in the same manner as in Example 1 except that this reaction solution 2 was used.

(Example 8)
A buffer layer was formed in the same manner as in Example 7 except that the substrate 2 was used instead of the substrate 1.

Example 9
Reaction solution 3 consisting of zinc sulfate 0.03 [M], thiourea 0.05 [M], trilithium citrate 0.4 [M], and ammonia 0.15 [M] was prepared. A buffer layer was formed in the same manner as in Example 1 except that this reaction solution 3 was used.

(Example 10)
A buffer layer was formed in the same manner as in Example 9 except that the substrate 2 was used instead of the substrate 1.

(Comparative Example 1)
Reaction liquid 4 consisting of zinc sulfate 0.16 [M], thiourea 0.6 [M], and ammonia 7.5 [M] was prepared. That is, the reaction solution 4 of Comparative Example 1 does not contain alkali metal ions.
The buffer layer was formed in the same manner as in Example 1 except that the reaction solution 4 was used, the temperature of the reaction solution was 80 ° C., and the time (reaction time) for immersing the buffer film-forming substrate in the reaction solution was 40 minutes. Formed.

(Comparative Example 2)
Reaction liquid 5 consisting of zinc acetate 0.025 [M], thiourea 0.4 [M], and ammonia 2.5 [M] was prepared. The reaction sources 1 to 4 are different from the Zn source, and the reaction solution 5 does not contain alkali metal ions.
The buffer layer was formed in the same manner as in Example 1 except that the reaction solution 5 was used, the temperature of the reaction solution was 80 ° C., and the time (reaction time) for immersing the buffer film-forming substrate in the reaction solution was 180 minutes. Formed.

(Comparative Example 3)
Reaction liquid 6 comprising zinc sulfate 0.03 [M], thiourea 0.05 [M], trisodium citrate 0.03 [M], and ammonia 0.15 [M] was prepared. The alkali metal ion concentration in the reaction solution 6 is 0.09 [M].
A buffer layer was formed in the same manner as in Example 3 except that the reaction solution 6 was used.

Table 1 shows a summary of the buffer layer deposition conditions for each of the examples and comparative examples.

<Evaluation of composition of buffer layer>
A CIGS substrate cut in advance to a 1 cm square (with a CIGS layer formed on the substrate) was formed up to the buffer layer under the same conditions as in Examples 1 to 10 and Comparative Examples 1 to 3. The composition analysis of the obtained samples was performed. For the measurement, a buffer layer was formed, and composition analysis in the depth direction was performed by X-ray photoelectron spectroscopy (XPS). As an example, the results of Example 3 are shown in FIG. FIG. 5 shows the etching depth direction from the buffer layer surface on the horizontal axis and the concentration of each element on the vertical axis. The concentration shown in FIG. 5 is the concentration of each element with respect to the total content of O, C, Na, S, Zn, Cu, Ga, In, and Se that are elements constituting the sample. Although measurement is performed for all of these elements, FIG. 5 shows only O, C, S, Zn, and Cu.

The XPS measurement with etching was performed under the following conditions. In FIG. 5, the horizontal axis indicates the etching depth [nm] obtained from the sputtering time (minutes). The conversion from the sputtering time to the etching depth can be calculated based on the etching rate calculated using a SiO ₂ film (substrate is a Si wafer) whose thickness is known as a standard sample. The thickness of the buffer layer was calculated based on the position at which the concentration was halved with respect to the maximum concentration of the S component in the buffer layer.

XPS analyzer: Quantera SXM manufactured by PHI X-ray photoelectron spectrometer
X-ray source: Single crystal spectroscopy AlKα ray Output / analysis area: 25W / φ100μm
Pass Energy: Depth direction analysis Narrow Scan-280.0eV (0.25eV / Step)
Geometry: θ = 45 ° (θ: Angle between sample surface and detector)
Ar + etching conditions Acceleration voltage: 1 kV, raster size: 2 x 2 mm, rate: about 2.6 nm / min (for SiO ₂ )

The thickness of the buffer layer and the distribution of each component in the buffer layer are obtained by XPS measurement as shown in FIG. As a result, it was confirmed that both S and O exist in the buffer layer. Further, the buffer layer thickness was obtained by calculation based on the position where the concentration becomes half the maximum concentration of S. The thickness for each example and comparative example is shown in Table 2, respectively. The type of alkali metal can be identified from the peak position of the XPS spectrum. Further, since the thickness of the buffer layer is obtained by the above-described method, the average concentration of the alkali metal in the buffer layer was calculated from the concentration distribution of the alkali metal in the etching depth direction.
It was also confirmed that Zn was present even at a position where S was no longer detected. This is probably because Zn ²⁺ diffused in the CIGS layer in the CBD process.

  Further, the ratio of the S component in the buffer layer is shown in Table 2 as S / (S + O) value. The S / (S + O) value was 0.51 to 0.55 in Examples 1 to 8, and there was almost no difference, and there was a variation of 0.42 to 0.6 in Comparative Examples 1 to 3. However, it can be said that the composition of the buffer layer contains S, O or S, O, and OH, regardless of the conditions of Examples 1 to 8 and Comparative Examples 1 to 3.

<Preparation of photoelectric conversion element-continued>
On the buffer layer produced in Examples 1-10 and Comparative Examples 1-3, the translucent conductive film which consists of an Al dope conductive zinc oxide thin film with a film thickness of 300 nm was formed into a film with sputtering method. Next, an Al electrode was formed as a grid electrode by a vapor deposition method to produce a photoelectric conversion element (single cell solar cell).
In addition, eight cells were produced for each example and comparative example. Eight cells in each example were obtained by the same simultaneous process which was produced by forming each layer on a single substrate and then cutting it out to a predetermined size.

<Evaluation of photoelectric conversion efficiency>
About the photoelectric conversion element (each 8 cells) of the obtained Examples and Comparative Examples, using a solar simulator, Air Mass (AM) = 1.5, under the condition using 100 mW / cm ² pseudo sunlight, The photoelectric conversion efficiency was measured. The photoelectric conversion efficiency was measured after 30 minutes of light irradiation.

The obtained results are shown in Table 2. In Table 2, the maximum value, the average value, and the minimum value of the photoelectric conversion efficiency of 8 cells for each example and comparative example are shown, and the coefficient of variation obtained therefrom is also shown. The variation coefficient is a value (in this case, expressed as a percentage) obtained by dividing the standard deviation of the photoelectric conversion efficiency of 8 cells by the average value of the photoelectric conversion efficiency of 8 cells.

  As shown in Table 2, for the examples of the present invention, the coefficient of variation was 12% or less, and except for Example 5, all could be suppressed to 10% or less. Considering that all of the comparative examples have a variation coefficient of 20% or more, it is clear that the embodiment of the present invention has a high variation coefficient suppression effect. In Comparative Examples 1 and 2 in which a buffer layer was formed using a reaction solution that did not contain an alkali metal source, no alkali metal was detected in the buffer layer, and their coefficient of variation was very large. When the alkali metal is contained in the buffer layer in an amount of 0.7 at% or more as shown in Examples 1 to 8, the coefficient of variation is significantly smaller than that in Comparative Example 3 in which the content is 0.05 at%. It has become. Even if any of sodium, potassium and lithium was added as the alkali metal, the effect of increasing the uniformity was obtained.

  In the above, an example in which the buffer layer is made of a Zn-based compound has been shown. However, even if the buffer layer is made of a Cd-based compound, an In-based compound, or an Sn-based compound, an alkali metal is similarly included. As a result, it is considered that the band structure of the buffer layer changes and the effect of suppressing the recombination of carriers generated in the photoelectric conversion layer is produced, so that the effect of improving the uniformity of the photoelectric conversion characteristics can be obtained. .

DESCRIPTION OF SYMBOLS 1 Photoelectric conversion element 2 Integrated solar cell 10 Board | substrate 11 Base material 12 Anodized film 20 Lower electrode (back surface electrode)
30 Photoelectric conversion layer 40 Buffer layer 60 Upper electrode (translucent conductive layer)
70 Grid electrode

Claims (9)

On the substrate, a lower electrode layer, a photoelectric conversion layer mainly composed of a compound semiconductor having at least one chalcopyrite structure composed of a group Ib element, a group IIIb element, and a group VIb element, a buffer layer, and a light transmitting property A photoelectric conversion element in which a conductive layer is laminated,
The buffer layer is made of a compound containing an alkali metal,
The alkali metal is at least one of lithium, sodium and potassium;
Content of the said alkali metal in the said compound is 0.1 at% or more and 5 at% or less, The photoelectric conversion element characterized by the above-mentioned.   The photoelectric conversion device according to claim 1, wherein the compound constituting the buffer layer contains at least one metal selected from the group consisting of Cd, Zn, In, and Sn, sulfur, and oxygen.   The photoelectric conversion element according to claim 2, wherein the metal is Zn.   4. The photoelectric conversion device according to claim 1, wherein the content of the alkali metal is 0.5 at% or more and 2.5 at% or less.   5. The photoelectric conversion element according to claim 1, wherein the buffer layer has a thickness of 1 nm or more and 100 nm or less. The Ib group element is at least one selected from the group consisting of Cu and Ag;
The group IIIb element is at least one selected from the group consisting of Al, Ga and In;
6. The photoelectric conversion element according to claim 1, wherein the VIb group element is at least one selected from the group consisting of S, Se, and Te.   A solar cell comprising a plurality of integrated photoelectric conversion elements according to claim 1. On the substrate, a lower electrode layer, a photoelectric conversion layer mainly composed of a compound semiconductor having at least one chalcopyrite structure composed of a group Ib element, a group IIIb element, and a group VIb element, a buffer layer, and a light transmitting property A method for producing a buffer layer in a photoelectric conversion element in which a conductive layer is laminated,
Prepare a reaction solution for chemical bath deposition containing Zn, In or Sn ions, thiourea, 1M or more alkali metal ions,
By bringing at least the surface of the photoelectric conversion layer into contact with the reaction solution of the buffer film forming substrate in which the lower electrode layer and the photoelectric conversion layer are laminated in this order on the substrate, the reaction liquid is formed on the photoelectric conversion layer. A method for producing a buffer layer, comprising depositing the buffer layer. On the substrate, a lower electrode layer, a photoelectric conversion layer mainly composed of a compound semiconductor having at least one chalcopyrite structure composed of a group Ib element, a group IIIb element, and a group VIb element, a buffer layer, and a light transmitting property In the method for producing a photoelectric conversion element in which a conductive layer is laminated,
The said buffer layer is formed with the manufacturing method of the buffer layer of Claim 8, The manufacturing method of the photoelectric conversion element characterized by the above-mentioned.

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