JP2012227504A - Substrate for photoelectric conversion element with molybdenum electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a substrate for a photoelectric conversion element which can efficiently diffuse an alkali metal ion to a photoelectric conversion semiconductor layer, and can enhance the photoelectric conversion efficiency of a photoelectric conversion element.SOLUTION: The substrate for the photoelectric conversion element has an alkali metal silicate layer 30 including lithium silicate or potassium silicate, and sodium silicate on a substrate 10.

Description

  The present invention relates to a photoelectric conversion element suitable for applications such as a solar cell using the photoelectric conversion element substrate and the photoelectric conversion element substrate.

  A photoelectric conversion element having a laminated structure of a lower electrode (back electrode), a photoelectric conversion layer that generates current by light absorption, and an upper electrode (transparent electrode) on a substrate is used for applications such as solar cells. Conventionally, in solar cells, Si-based solar cells using bulk single-crystal Si or polycrystalline Si, or thin-film amorphous Si have been mainstream, but in recent years, research and development of compound semiconductor-based solar cells that do not depend on Si have been conducted. Has been made. As a compound semiconductor solar cell, a thin film system such as a CIS (Cu—In—Se) system or a CIGS (Cu—In—Ga—Se) system composed of a group Ib element, a group IIIb element, and a group VIb element has a light absorption rate. It is known that the photoelectric conversion efficiency is high.

  In photoelectric conversion elements such as CIS or CIGS, it is known that the alkalinity, preferably Na, is diffused into the photoelectric conversion layer, whereby the crystallinity of the photoelectric conversion layer is improved and the photoelectric conversion efficiency is improved. (Patent Documents 1 and 2). Conventionally, Na is diffused into the photoelectric conversion layer using a soda-lime glass substrate containing Na.

  However, when a metal substrate, a polymer substrate, a ceramic substrate, or the like is used as a solar cell substrate, there is a problem that conversion efficiency does not increase because sodium cannot be supplied from the substrate. Therefore, when using a substrate that does not contain sodium, an alkali supply layer is provided by a liquid phase method, sodium is introduced by co-evaporation with CIGS, or Mo-Na is provided as an electrode. Yes. For example, Patent Document 3 discloses applying alkali metal silicate, specifically sodium silicate, by liquid phase application. Patent Document 4 discloses that an anodized substrate is brought into contact with a sodium hydroxide aqueous solution and doped with sodium. Further, Patent Document 5 describes that a silicon oxide film is formed on a stainless steel substrate by a sol-gel method, and an insulating layer is further formed of a material containing Na.

Japanese Patent No. 2922465 JP 11-312817 A JP 2009-267332 A JP 2010-232427 A JP 2004-158511 A

  As described above, conventionally, when forming the photoelectric conversion semiconductor layer, the knowledge that the power generation efficiency is improved by diffusing sodium from the sodium supply layer into the photoelectric conversion semiconductor layer was very common, It was found that even if a sodium supply layer was provided, the power generation efficiency would not be higher than expected. The cause is that impurities are generated because sodium reacts with the molybdenum of the electrode, and in the manufacturing process of the integrated solar cell, it is necessary to perform water washing after the scribing after the electrode film formation. It was found that sodium was eluted by this washing, leading to loss.

  The present invention has been made in view of the above circumstances, and is capable of efficiently diffusing alkali metal ions into the photoelectric conversion semiconductor layer, and can increase the photoelectric conversion efficiency of the photoelectric conversion element. An object of the present invention is to provide an element substrate and a photoelectric conversion element using the photoelectric conversion element substrate.

The substrate for a photoelectric conversion element of the present invention has an alkali metal silicate layer containing an alkali metal silicate (wherein the alkali metal is other than sodium) and sodium silicate on the substrate. It is.
The alkali metal silicate is preferably lithium silicate or potassium silicate.
The molar ratio of lithium or potassium to silicon in the alkali metal silicate layer is preferably 0.001 or more and 1 or less.
The sum of the molar ratio of lithium or potassium to silicon and the molar ratio of sodium to silicon is preferably 1 or less.
The alkali metal silicate layer preferably contains boron or phosphorus.
The thickness of the alkali metal silicate layer is preferably 2 μm or less.

The substrate is preferably a metal substrate.
It is preferable that an anodized aluminum film is formed on the surface of the metal substrate.
The metal substrate is preferably a clad material in which one or both surfaces of aluminum, stainless steel, or steel plate are integrated with an aluminum plate.
The anodized aluminum film is a porous anodized aluminum film, and the porous anodized aluminum film preferably has a compressive stress.
The photoelectric conversion element of the present invention is formed on the photoelectric conversion element substrate described above.

  The substrate for a photoelectric conversion element of the present invention has an alkali metal silicate layer containing an alkali metal silicate (wherein the alkali metal is other than sodium) and sodium silicate, and sodium and another alkali metal are used in combination. As a result, even if an electrode made of molybdenum is formed on the alkali metal silicate layer, it is possible to suppress the generation of impurities due to the reaction between sodium and molybdenum and the elution of sodium by washing with water. The sodium of the alkali metal silicate layer can be efficiently diffused into the photoelectric conversion semiconductor layer, and the power generation efficiency of the photoelectric conversion element can be improved.

  The mechanism of action by which sodium and another alkali metal are combined is not always clear, but other alkali metals, especially lithium and potassium, are less hygroscopic than sodium and When lithium or potassium is contained in the metal silicate layer, the moisture contained in the alkali metal silicate layer is absolutely reduced, so that the oxidation reaction caused by the moisture is less likely to occur. It is presumed that sodium elution is reduced by washing with water.

It is a schematic sectional drawing which shows one Embodiment of the photoelectric conversion element using the board | substrate for photoelectric conversion elements of this invention. It is a schematic sectional drawing which shows one Embodiment of the photoelectric conversion element using the board | substrate for photoelectric conversion elements of another aspect of this invention. It is an electron micrograph of the CIGS crystal of Example 4 and Comparative Examples 1 and 5.

The substrate for photoelectric conversion element of the present invention will be described in detail.
The alkali metal silicate layer in the substrate for a photoelectric conversion element of the present invention contains an alkali metal silicate (wherein the alkali metal is other than sodium) and sodium silicate. The alkali metal silicate (however, the alkali metal is other than sodium) may include a plurality of types.
The alkali metal silicate (the alkali metal is other than sodium) is particularly preferably lithium silicate or potassium silicate, and the lithium silicate and potassium silicate may contain both.

  When the alkali metal silicate is lithium silicate or potassium silicate, the molar ratio of lithium or potassium to silicon in the alkali metal silicate layer is preferably 0.001 or more and more preferably 1 or less. 01 or more and 1 or less, more preferably 0.02 or more and 1 or less, and particularly preferably 0.05 or more and 0.5 or less. Silicon is the total silicon contained in the alkali metal silicate layer (including silicon derived from sodium silicate). When both lithium silicate and potassium silicate are included, lithium silicate derived, potassium silicate derived , And the molar ratio of the sum of lithium and potassium to silicon from sodium silicate. When the molar ratio of lithium or potassium to silicon is larger than 1, lithium or potassium becomes too much and it becomes difficult to solidify as a silicate. On the other hand, when the molar ratio of lithium or potassium to silicon is less than 0.001, the effect of addition cannot be obtained due to too little lithium or potassium, and the photoelectric conversion efficiency of the photoelectric conversion element does not increase.

  When the alkali metal silicate is lithium silicate or potassium silicate, the molar ratio of lithium or potassium to the total silicon contained in the alkali metal silicate layer and the sodium to the total silicon contained in the alkali metal silicate layer Is preferably 1 or less, more preferably 0.8 or less. If lithium or potassium is not contained, the insulating property is lowered, and, in addition, impurities are formed when a molybdenum film generally used for the back electrode is formed, so that the power generation efficiency is lowered. This is presumed to be due to the high water absorption of sodium. On the other hand, power generation efficiency cannot be increased only with lithium or potassium. In addition, when the sum of the molar ratio of lithium or potassium to silicon and the molar ratio of sodium to silicon is greater than 1, it is difficult to solidify as a silicate, and the amount of silicon is small, resulting in reduced adhesion to the substrate. To do.

  Known methods for producing sodium silicate, lithium silicate, and potassium silicate include wet methods and dry methods. Silicon oxide is dissolved in sodium hydroxide, lithium hydroxide, and potassium hydroxide, respectively. Can be produced. In addition, alkali metal silicates having various molar ratios are commercially available and can be used.

As sodium silicate, lithium silicate, and potassium silicate, various molar ratios of sodium silicate, lithium silicate, and potassium silicate are commercially available. The SiO ₂ / A ₂ O (A: alkali metal) molar ratio is often used as an index indicating the ratio of silicon and alkali metal. For example, as lithium silicate, there are lithium silicate 35, lithium silicate 45, lithium silicate 75, etc. manufactured by Nissan Chemical Industries, Ltd. As potassium silicate, No. 1 potassium silicate, No. 2 potassium silicate and the like are commercially available.

  As sodium silicate, sodium orthosilicate, sodium metasilicate, No. 1 sodium silicate, No. 2 sodium silicate, No. 3 sodium silicate, No. 4 sodium silicate, etc. are known, and the molar ratio of silicon is up to several tens. Elevated high mol sodium silicate is also commercially available.

  By mixing the sodium silicate, lithium silicate, and potassium silicate with water at an arbitrary ratio, a solution having an arbitrary concentration can be obtained. The ratio of lithium or potassium to silicon can be changed by mixing these alkali metal silicates, and can also be changed by mixing various sodium silicates in an arbitrary ratio. By changing the amount of water added, the viscosity of the coating solution can be adjusted to determine appropriate coating conditions. There is no particular limitation on the method for applying the coating liquid on the substrate, and for example, a doctor blade method, a wire bar method, a gravure method, a spray method, a dip coating method, a spin coating method, a capillary coating method, or the like may be used. it can.

  Note that the lithium silicate, potassium silicate, and sodium silicate of the alkali metal silicate layer do not necessarily need to have lithium silicate, potassium silicate, and sodium silicate as the supply source during production. For example, when the alkali metal silicate layer contains lithium silicate and sodium silicate, lithium silicate and sodium hydroxide, or lithium hydroxide and sodium silicate, and the alkali metal silicate layer is potassium silicate. And sodium silicate, potassium silicate and sodium silicate, or potassium silicate and sodium hydroxide can be mixed with water at an arbitrary ratio, respectively, and lithium silicate and sodium silicate or An alkali metal silicate layer comprising potassium silicate and sodium silicate can be made. Moreover, you may add lithium salt, potassium salt, and sodium salt as a supply source, respectively. For example, nitrates, sulfates, acetates, phosphates, chlorides, bromides, iodides and the like are used.

  Coating solutions for alkali metal silicates other than lithium silicate, potassium silicate, and sodium silicate include desired alkali metal nitrates, sulfates, acetates, phosphates, chlorides, bromides, iodides, etc. It can be easily obtained by adding to a sodium silicate solution.

  A compound containing boron or a compound containing phosphorus may be added to the aqueous alkali metal silicate solution. By adding these, the suitability for Mo film formation and the power generation efficiency can be further improved. Although details are not necessarily clear, the addition of boron or phosphorus to the alkali metal silicate changes the microstructure of the glass and improves the stability of the alkali metal ions in the glass. It is presumed that the release of alkali metal ions is suppressed, the suitability of Mo film formation is improved, and the power generation efficiency is improved.

Preferred examples of the boron source include borates such as boric acid and sodium tetraborate.
Phosphoric acid, peroxophosphoric acid, phosphonic acid, phosphinic acid, diphosphoric acid, triphosphoric acid, polyphosphoric acid, cyclo-triphosphoric acid, cyclo-tetraphosphoric acid, diphosphonic acid, and their salts, For example, lithium phosphate, sodium phosphate, potassium phosphate, lithium hydrogen phosphate, ammonium phosphate, sodium hydrogen phosphate, calcium hydrogen phosphate, ammonium hydrogen phosphate, lithium dihydrogen phosphate, phosphoric acid Preferred examples include sodium dihydrogen, calcium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium pyrophosphate, sodium triphosphate and the like.

  An alkali metal silicate layer can be prepared by applying a coating solution on a substrate and then performing a heat treatment. When the inventors measured the dehydration temperature using the techniques of thermogravimetric analysis and temperature-programmed degassing analysis, it was found that dehydration occurred at about 200 ° C to 300 ° C. A temperature lower than 200 ° C. is not preferable because the coating solution cannot be sufficiently dried and an alkali metal silicate layer having high water resistance is not formed. In addition, in the heat treatment at 300 ° C. or lower, the alkali metal silicate layer has a large amount of residual moisture and reacts with carbon dioxide in the atmosphere to form impurities such as carbonate on the surface, or sodium molybdate during Mo electrode sputtering. Or other problems occur. Accordingly, the heat treatment temperature is preferably 200 ° C. or higher, more preferably 300 ° C. or higher, and particularly preferably 400 ° C. or higher.

Since the heat treatment is performed at a higher temperature, it is preferable to use a clad substrate in which aluminum and a different metal are combined and an anodized film is formed on the aluminum surface as the substrate used in the present invention. As will be described later, the clad substrate is known to have high heat resistance without cracking of the anodized film even at a high temperature of 400 ° C. or higher. It is also known that compressive stress can be applied to the anodized film by heat-treating the substrate at 300 ° C. or higher in advance, heat resistance can be further improved, and long-term reliability of insulation can be ensured. By performing this treatment after the application of the alkali metal silicate layer, it is possible to perform both the heat treatment necessary for dehydration of the alkali metal silicate layer and the heat treatment necessary for increasing the compressive stress of the anodized film.
On the other hand, a temperature exceeding 600 ° C. is not preferable because it exceeds the glass transition temperature of the alkali metal silicate.

  The thickness of the alkali metal silicate layer after the heat treatment is 0.01 to 2 μm, preferably 0.05 to 1.5 μm, more preferably 0.1 to 1 μm. If the thickness of the alkali metal silicate layer is greater than 2 μm, the amount of shrinkage of the alkali metal silicate during the heat treatment increases and cracks are likely to occur, which is not preferable.

  A photoelectric conversion element using the photoelectric conversion element substrate of the present invention will be described. FIG. 1 is a schematic cross-sectional view showing an embodiment of a photoelectric conversion element. In addition, in order to make it easy to visually recognize, the scales and the like of each component are appropriately changed from actual ones. As shown in FIG. 1, the photoelectric conversion element 1 includes an anodized film 20 formed by anodic oxidation, an alkali metal silicate layer 30, a lower electrode 40, and holes / holes by light absorption. A photoelectric conversion semiconductor layer 50 that generates electron pairs, a buffer layer 60, a translucent conductive layer (transparent electrode) 70, and an upper electrode (grid electrode) 80 are sequentially stacked. 1 shows a photoelectric conversion element in which an anodized film 20 formed by anodization and an alkali metal silicate layer 30 are formed on a substrate 10, but as shown in FIG. An embodiment in which an alkali metal silicate layer 30 is formed on the substrate 10 may be used (in FIG. 2, components equivalent to those in FIG. 1 are given the same numbers). .

As the substrate 10, a ceramic substrate (such as non-alkali glass, quartz glass, or alumina), a metal substrate (such as stainless steel, titanium foil, or silicon), or a polymer substrate (such as polyimide) can be used. From the viewpoint of heat resistance and lightness, a metal substrate is particularly preferable. In particular, a material in which a metal oxide film generated on the surface of a metal substrate by anodic oxidation becomes an insulator can be used. Specifically, at least selected from aluminum (Al), iron (Fe), zirconium (Zr), titanium (Ti), magnesium (Mg), copper (Cu), niobium (Nb) and tantalum (Ta). A substrate containing one metal or an alloy of the above metals is preferred. In particular, a clad material in which one or both surfaces of aluminum, stainless steel, or steel plate are integrated with an aluminum plate is more preferable from the viewpoint of easy formation of anodization and high durability. In the case of an integrated clad material with both surfaces sandwiched between aluminum plates, it is possible to suppress substrate warpage due to the difference in thermal expansion coefficient between aluminum and the oxide film (Al ₂ O ₃ ), and film peeling due to this. Therefore, it is more preferable.

  For the substrate, cleaning treatment / polishing smoothing treatment, if necessary, for example, a degreasing step for removing the adhering rolling oil, a desmut treatment step for dissolving the smut on the surface of the aluminum plate, and roughening the surface of the aluminum plate It is preferable to use one subjected to a roughening treatment step.

  The anodic oxide film 20 formed by anodic oxidation is obtained by forming an insulating oxide film having a plurality of pores by anodic oxidation, thereby ensuring high insulation. Anodization can be performed by immersing the substrate 10 as an anode in an electrolyte together with a cathode, and applying a voltage between the anode and the cathode. Carbon, aluminum, or the like is used as the cathode.

The anodizing conditions are not particularly limited depending on the type of electrolyte used. Conditions include, for example, an electrolyte concentration of 0.1 to 2 mol / L, a liquid temperature of 5 to 80 ° C., a current density of 0.005 to 0.60 A / cm ² , a voltage of 1 to 200 V, and an electrolysis time of 3 to 500 minutes. Is appropriate. The electrolyte is not particularly limited, and an acidic electrolytic solution containing one or more acids such as sulfuric acid, phosphoric acid, chromic acid, oxalic acid, malonic acid, sulfamic acid, benzenesulfonic acid, and amidosulfonic acid is preferable. Used. When such an electrolyte is used, an electrolyte concentration of 0.2 to 1 mol / L, a liquid temperature of 10 to 80 ° C., a current density of 0.05 to 0.30 A / cm ² , and a voltage of 30 to 150 V are preferable.

  The anodic oxide film is preferably composed of a barrier layer portion and a porous layer portion, and the porous layer portion has a compressive strain at room temperature. In general, since the barrier layer has compressive stress and the porous layer has tensile stress, it is known that the whole anodic oxide film becomes tensile stress in a thick film of several μm or more. On the other hand, when the above-described clad material is used and, for example, a heat treatment described below is performed, a porous layer having a compressive stress can be produced. Therefore, even if the film thickness is several μm or more, the entire anodic oxide film can be subjected to compressive stress, no cracking occurs due to the difference in thermal expansion during film formation, and long-term reliability near room temperature is excellent. Insulating film can be obtained.

In this case, the magnitude of the compressive strain is preferably 0.01% or more, more preferably 0.05% or more, and particularly preferably 0.10% or more. Moreover, it is preferable that it is 0.25% or less.
When the compressive strain is less than 0.01%, although it is compressive strain, it is insufficient and the effect of crack resistance cannot be obtained. Therefore, when the final product is subjected to bending strain, undergoes a temperature cycle over a long period of time, or receives impact or stress from the outside, cracks occur in the anodized film formed as an insulating layer, resulting in insulating properties. Leading to a decline.

On the other hand, if the compressive strain is too large, the anodic oxide film is peeled off or a strong compressive strain is applied to the anodic oxide film, resulting in cracks, rising of the anodic oxide film, lowering the flatness, and peeling. As a result, the insulating property is critically reduced. Therefore, the compressive strain is preferably 0.25% or less.
The Young's modulus of the anodic oxide film is known to be about 50 to 150 GPa. Therefore, the magnitude of the compressive stress is preferably about 5 to 300 MPa.

  Heat treatment may be performed after the anodizing treatment. By performing the heat treatment, compressive stress is applied to the anodized film, and crack resistance is increased. Therefore, heat resistance and insulation reliability are improved, and the metal substrate with an insulating layer can be more suitably used. The heat treatment temperature is preferably 150 ° C. or higher. When the above clad material is used, heat treatment at 300 ° C. or higher is preferable. By performing the heat treatment in advance, the amount of water contained in the porous anodic oxide film can be reduced, and the insulation can be improved.

  In a conventional substrate made of only aluminum, when heat treatment is performed at 300 ° C. or higher, the aluminum softens and loses its function as a substrate, or anodization occurs due to the difference in thermal expansion coefficient between aluminum and the anodized film. Although there was a problem that the film was cracked and the insulating property was lost, the use of a clad material made of a metal different from aluminum makes it possible to heat at a temperature of 300 ° C. or higher.

  An anodized film is an oxide film formed in an aqueous solution, and it is described in, for example, “Chemistry Letters Vol. 34, No. 9, (2005) p1286” that moisture is retained inside a solid. As known. From the solid-state NMR measurement of the anodic oxide film as in this document, it was found that the amount of water (OH group) inside the solid of the anodic oxide film decreased when heat-treated at 100 ° C. or higher, particularly at 200 ° C. or higher. is there. Therefore, it is presumed that the combined state of Al—O and Al—OH changes due to heating, and stress relaxation (annealing effect) occurs.

  Further, from the measurement of the amount of dehydration of the anodic oxide film by the inventors, it has been clarified that most dehydration occurs from room temperature to about 300 ° C. When an anodic oxide film is to be used as an insulating film, the greater the amount of moisture contained, the lower the insulating property. Therefore, performing heat treatment at 300 ° C. or higher is extremely effective from the viewpoint of improving the insulating property. By using a clad material of aluminum and a different metal as a base material and combining with a heat treatment at 300 ° C. or higher, an annealing effect can be effectively expressed, and a high compressive strain and a low water content that cannot be achieved by the prior art can be realized. This makes it possible to provide a photoelectric conversion element substrate with higher insulation reliability.

From the viewpoint of electrical insulation, the anodic oxide film preferably has a thickness of 3 to 50 μm. By having a film thickness of 3 μm or more, it is possible to achieve both insulation, heat resistance during film formation by having compressive stress at room temperature, and long-term reliability.
The film thickness is preferably 5 μm or more and 30 μm or less, and particularly preferably 5 μm or more and 20 μm or less.

  If the film thickness is extremely thin, there is a possibility that damage due to electrical insulation and mechanical shock during handling cannot be prevented. In addition, the insulation and heat resistance are drastically lowered, and deterioration with time is also increased. This is because the influence of the unevenness on the surface of the anodic oxide film becomes relatively large due to the thin film thickness, the crack becomes the starting point of cracks, and the anodic oxidation originates from metal impurities contained in the aluminum. The effect of metal deposits, intermetallic compounds, metal oxides, and voids in the film is relatively large, resulting in a decrease in insulation, and breakage when the anodized film is impacted or stressed from the outside. This is because cracks are likely to occur. As a result, when the anodic oxide film is less than 3 μm, the insulating property is lowered, so that it is not suitable for use as a flexible heat-resistant substrate or for production by roll-to-roll.

  On the other hand, an excessively thick film thickness is not preferable because flexibility is lowered and cost and time required for anodization are increased. In addition, bending resistance and thermal strain resistance are reduced. The cause of the decrease in bending resistance is that when the anodized film is bent, the tensile stress at the interface between the surface and the aluminum differs, so the stress distribution in the cross-sectional direction increases and local stress concentration occurs. This is presumed to be easier. The cause of the decrease in thermal strain resistance is that when a tensile stress is applied to the anodized film due to the thermal expansion of the base material, a greater stress is applied to the interface with aluminum, and the stress distribution in the cross-sectional direction increases, resulting in local stress. It is estimated that this is because stress concentration tends to occur. As a result, when the anodic oxide film exceeds 50 μm, bending resistance and thermal strain resistance are lowered, so that it is not suitable for use as a flexible heat-resistant substrate or roll-to-roll production. Also, the insulation reliability is lowered.

  The component of the lower electrode (back electrode) 40 is not particularly limited, and Mo, Cr, W, and combinations thereof are preferable, and Mo or the like is particularly preferable. The film thickness of the lower electrode (back electrode) 40 is not limited and is preferably about 200 to 1000 nm.

  The photoelectric conversion semiconductor layer 50 is a compound semiconductor-based photoelectric conversion semiconductor layer, and is not particularly limited as a main component (the main component means a component of 20% by mass or more), and high photoelectric conversion efficiency is obtained. A compound semiconductor, a compound semiconductor having a chalcopyrite structure, or a compound semiconductor having a defect stannite structure can be preferably used.

As a chalcogen compound (compound containing S, Se, Te),
II-VI compounds: ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, etc.
I-III-VI ₂ group _{compounds: CuInSe 2, CuGaSe 2, Cu} (In, Ga) Se 2, CuInS 2, CuGaSe 2, Cu (In, Ga) (S, Se) 2 , etc.,
Preferable examples include I-III ₃ -VI ₅ group compounds: Cull ₃ Se ₅ , CuGa ₃ Se ₅ , Cu (ln, Ga) ₃ Se _{5 and the} like.

As a compound semiconductor having a chalcopyrite structure and a defect stannite structure,
I-III-VI ₂ group _{compounds: CuInSe 2, CuGaSe 2, Cu} (In, Ga) Se 2, CuInS 2, CuGaSe 2, Cu (In, Ga) (S Se) 2 , etc.,
Preferred examples include I-III ₃ -VI ₅ group compounds: CuIn ₃ Se ₅ , CuGa ₃ Se ₅ , Cu (In, Ga) ₃ Se _{5 and the} like.
In the above description, (In, Ga) and (S, Se) are (In _1-x Ga _x ) and (S _1-y Se _y ) (where x = 0 to 1, y = 0-1).

The method for forming the photoelectric conversion semiconductor layer is not particularly limited. For example, a CI (G) S-based photoelectric conversion semiconductor layer containing Cu, In, (Ga), and S can be formed using a method such as a selenization method or a multi-source evaporation method.
The film thickness of the photoelectric conversion semiconductor layer 50 is not particularly limited, and is preferably 1.0 to 3.0 μm, particularly preferably 1.5 to 2.0 μm.

  The buffer layer 60 is not particularly limited, but CdS, ZnS, Zn (S, O) and / or Zn (S, O, OH), SnS, Sn (S, O) and / or Sn (S, O, OH). A metal sulfide containing at least one metal element selected from the group consisting of Cd, Zn, Sn, and In, such as InS, In (S, O) and / or In (S, O, OH). Is preferred. The thickness of the buffer layer 40 is preferably 10 nm to 2 μm, and more preferably 15 to 200 nm.

The translucent conductive layer (transparent electrode) 70 is a layer that captures light and functions as an electrode that is paired with the lower electrode 40 and through which the current generated in the photoelectric conversion layer 50 flows. The composition of the translucent conductive layer 70 is not particularly limited, and n-ZnO such as ZnO: Al is preferable. The film thickness of the translucent conductive layer 70 is not particularly limited, and is preferably 50 nm to 2 μm.
The upper electrode (grid electrode) 80 is not particularly limited, and examples thereof include Al. The film thickness of the upper electrode 80 is not particularly limited and is preferably 0.1 to 3 μm.

The photoelectric conversion element substrate of the present invention can be preferably used for solar cells and the like. If necessary, a cover glass, a protective film, or the like can be attached to the photoelectric conversion element 1 to form a solar cell.
Hereinafter, the substrate for a photoelectric conversion element of the present invention will be described in more detail with reference to examples.

(Preparation of substrate)
3 cm square non-alkali glass substrate (used in Examples 1, 7 and 32), SUS430 substrate (thickness 100 μm, Examples 2, 5, 8, 25, 27, Comparative Examples 1, 5, 6, 8) Use) and an anodized aluminum substrate (used in Examples other than the above and Comparative Examples 2, 3, 4, and 7) were prepared. The anodized aluminum substrate was produced by the following method. A clad material made of 30 μm thick aluminum and 100 μm thick SUS430 was anodized under a constant voltage condition of 40 V using an oxalic acid electrolytic solution to prepare a substrate on which 10 μm anodized aluminum was formed.

(Preparation of coating solution and examples and comparative examples)
Lithium supply source (manufactured by Nissan Chemical, trade names: lithium silicate 45, lithium silicate 75, lithium hydroxide), potassium supply source (manufactured by Fuji Chemical, trade names: 1K potassium silicate, 2K potassium silicate), sodium supply source ( Showa Chemical Co., Ltd., trade name: No. 1 sodium silicate, No. 3 sodium silicate, and Toso Sangyo, trade name: No. 4 sodium silicate, high-mol sodium silicate, sodium hydroxide) and water are shown in Table 1. 3 and 3, and boron or sodium tetraborate as a boron supply source and phosphoric acid as a phosphoric acid source were added at a mass ratio shown in Table 2 to prepare a coating solution. The coating solution was dropped on the substrate, and an alkali metal silicate layer was formed by spin coating. Thereafter, the substrate was heat-treated at 450 ° C. for 30 minutes. After the heat treatment, Mo was formed to a thickness of 800 nm on the substrate by DC sputtering.

(Mo film formability evaluation)
The amount of impurities on the Mo surface on the alkali metal silicate layer formed above was observed using an optical microscope to observe the amount of foreign matter. According to the number of foreign matter per 1 mm square, Table 3 shows “◯” when no foreign matter is observed, “△” when it is 1 or more and less than 10, and “×” when it is 10,000 or more. It was.

(Production of solar cells)
A CIGS solar cell was formed on the Mo electrode. In this embodiment, granular raw materials of high-purity copper and indium (purity 99.9999%), high-purity Ga (purity 99.999%), and high-purity Se (purity 99.999%) were used as the evaporation source. . A chromel-alumel thermocouple was used as a substrate temperature monitor. After the main vacuum chamber is evacuated to 10 ⁻⁶ Torr (1.3 × 10 ⁻³ Pa), the deposition rate from each evaporation source is controlled, and the film thickness is about 530 ° C. under the film forming conditions. A 1.8 μm CIGS thin film was formed. Subsequently, a CdS thin film of about 90 nm was deposited as a buffer layer by a solution growth method, and a ZnO: A1 film of a transparent conductive film was formed thereon with a thickness of 0.6 μm by a DC sputtering method. Finally, an Al grid electrode was formed as an upper electrode by a vapor deposition method to produce a solar battery cell.

(Measurement of power generation efficiency)
The solar cell thus produced (area 0.5 cm ² ) was irradiated with simulated sunlight of Air Mass (AM) = 1.5 and 100 mW / cm ² to measure energy conversion efficiency. About the photoelectric conversion element of an Example and a comparative example, 8 samples were produced, respectively. For each photoelectric conversion element, the photoelectric conversion efficiency was measured under the above conditions, and the maximum value among them was defined as the conversion efficiency of the photoelectric conversion element of each example and comparative example. In addition, the coefficient of variation (a value obtained by dividing the standard deviation of 8 cells by the average value) was evaluated as a variation in cell efficiency.

(Measurement of sodium concentration)
About the photoelectric conversion element of an Example and a comparative example, the sodium concentration of the photoelectric converting layer (CIGS layer) was measured. This sodium concentration was measured using SIMS (secondary ion mass spectrometer). The primary ion species for measurement was Cs ⁺ and the acceleration voltage was 5.0 kV. Although the sodium concentration in the photoelectric conversion layer (CIGS layer) has a distribution in the thickness direction, it was integrated to derive an average value, and this average value was used for evaluation of the sodium concentration.

  Tables 1, 2 and 3 show the results of measurement of Mo film suitability, sodium concentration, and power generation efficiency together with the formulation of the coating solution. The mass ratio of the sodium source, lithium source and potassium source used in the examples and comparative examples Tables 4, 5 and 6 show. The molar ratios in Tables 1, 2 and 3 are converted from this mass ratio to molar ratio (so that sodium hydroxide and lithium hydroxide are solid). In addition, the sum of the molar ratio in Table 1 is a value calculated before rounding off the three decimal places of the molar ratio of lithium or potassium to silicon and the molar ratio of sodium to silicon.

  As shown in Table 1, Examples 1 to 10 in which an alkali metal silicate layer containing lithium silicate or potassium silicate and sodium silicate was provided are alkali metal silicate layers made only of sodium silicate. As compared with Comparative Examples 5 to 8 which are, power generation efficiency nearly doubled was obtained. In any of these cases, the sodium concentration is almost the same level, and it is considered that sodium diffusion into CIGS is sufficiently performed. However, Mo in Comparative Examples 5 to 8 has a high impurity peak ratio, which is attributed to this. It is thought that the power generation efficiency was low.

  On the other hand, in Comparative Examples 1 and 2 which are alkali metal silicate layers made only of lithium silicate, and in Comparative Examples 3 and 4 which are alkali metal silicate layers made only of potassium silicate, the film forming suitability of Mo was good. However, since it did not contain sodium silicate, the power generation efficiency was about 30% lower than that of the example on average. Since sodium is not present in the alkali metal silicate layer, it is considered that sodium was not diffused into CIGS and the efficiency was not improved. This shows that the power generation efficiency can be greatly increased by using lithium silicate or potassium silicate in combination with sodium silicate.

  In Example 11, sodium hydroxide was used as a sodium supply source instead of sodium silicate, and lithium silicate was used in combination. In Examples 12 and 13, lithium hydroxide was used as a lithium supply source instead of lithium silicate, and sodium silicate was used in combination. Even in this case, the obtained alkali metal silicate is a mixture of sodium silicate and lithium silicate according to the present invention. However, in Example 11, compared with Examples 1 to 10, film forming suitability and power generation efficiency were slightly low. This is presumably because the sum of the molar ratio of lithium to silicon and the molar ratio of sodium to silicon is greater than one. Moreover, compared with Examples 1 to 10, Example 13 was slightly inferior in power generation efficiency. This is considered because the molar ratio of lithium to silicon is larger than 1.

  FIG. 2 is an electron micrograph of CIGS crystals of Example 4 and Comparative Examples 1 and 5. As is apparent from the electron micrographs of the three CIGS crystals, in Example 4, which is the substrate for photoelectric conversion elements of the present invention, the grain size of the CIGS crystal is larger than that of Comparative Example 1, and the effect of adding an alkali is confirmed. It was. Comparative Example 5 also has a large particle size and an alkali addition effect, but the conversion efficiency is low because the suitability of Mo film formation is low and impurities are formed at the Mo / CIGS interface.

Examples 14-19 change the addition amount of a sodium supply source, a lithium supply source, and a potassium supply source. From these examples, the molar ratio of lithium or potassium to silicon is in the range of 1 or less, and the sum of the molar ratio of lithium to silicon and the molar ratio of sodium to silicon is also 1 or less. You can see that
In Examples 23 to 33 shown in Table 2, the alkali metal silicate layer further contains boron or phosphorus. At first glance, it seems that there is no great difference in power generation efficiency as compared with Examples not containing boron or phosphorus. As can be seen, since the coefficient of variation is small, all of the fabricated cells have high power generation efficiency, and it can be seen that the power generation efficiency of the entire fabricated cell is increased.

1 Photoelectric Conversion Element 10 Substrate 20 Anodized Film 30 Alkali Metal Silicate Layer 40 Lower Electrode (Back Electrode)
50 Photoelectric conversion semiconductor layer 60 Buffer layer 70 Translucent conductive layer (transparent electrode)
80 Upper electrode (grid electrode)

Claims (11)

  A substrate for a photoelectric conversion element, comprising an alkali metal silicate layer containing an alkali metal silicate (wherein the alkali metal is other than sodium) and sodium silicate on the substrate.   2. The substrate for a photoelectric conversion element according to claim 1, wherein the alkali metal silicate is lithium silicate or potassium silicate.   The photoelectric conversion element substrate according to claim 2, wherein a molar ratio of the lithium or the potassium to silicon in the alkali metal silicate layer is 0.001 or more and 1 or less.   4. The photoelectric conversion element substrate according to claim 3, wherein a sum of a molar ratio of the lithium or the potassium to the silicon and a molar ratio of the sodium to the silicon is 1 or less. 5.   The substrate for a photoelectric conversion element according to claim 1, wherein the alkali metal silicate layer contains boron or phosphorus.   The substrate for a photoelectric conversion element according to claim 1, wherein the alkali metal silicate layer has a thickness of 2 μm or less.   The substrate for a photoelectric conversion element according to claim 1, wherein the substrate is a metal substrate.   8. The photoelectric conversion element substrate according to claim 7, wherein an anodized aluminum film is formed on the surface of the metal substrate.   9. The photoelectric conversion element substrate according to claim 8, wherein the metal substrate is a clad material in which one or both surfaces of aluminum, stainless steel, or steel plate are integrated with an aluminum plate.   The photoelectric conversion element substrate according to claim 9, wherein the anodized aluminum film is a porous type anodized aluminum film, and the porous type anodized aluminum film has a compressive stress.   A photoelectric conversion element formed on the photoelectric conversion element substrate according to claim 1.