JP7416372B2 - Copper arsenate compound, copper arsenate compound for solar cell materials, and method for producing copper arsenate compound - Google Patents

Description

The present invention relates to a copper arsenate compound, a copper arsenate compound for use in solar cell materials, and a method for producing a copper arsenate compound.

In the smelting of sulfide ores such as copper, zinc, lead, nickel, and cobalt, multiple elements that are highly toxic and have a high environmental impact are separated and recovered as by-products. Among them, arsenic used to have large uses and markets commensurate with its production volume, such as pesticides, glass decolorizers, pigments, and preservatives, but in recent years, all markets have shrunk due to its strong toxicity. It is either stored as an insoluble compound such as iron or calcium arsenate, stabilized in glassy slag, buried, used as aggregate for concrete, or finally disposed of.

However, either method requires a large amount of processing costs and is not a permanent solution because of space restrictions for storage and burial.

In order to solve the above problems, there is a need to develop a new functional material containing arsenic, which is a smelting byproduct, and studies have been made on materials containing arsenic.

For example, Non-Patent Document 1 shows the theoretical limit conversion efficiency of a single junction solar cell for a plurality of compounds. Among these, Cu ₃ AsS ₄ (Enargite), which contains arsenic, shows the same conversion efficiency as Si, CdTe, CuInSe ₂ and GaAs, which are widely known as solar cell materials, and is promising as a solar cell material. It shows that. Solar cells have a large market size among semiconductor materials, and are a promising industrial application field for arsenic.

Non-patent Document 2 describes similar compounds of Enargite, such as CuSbS ₂ (Chalcostibite), Ag ₅ SbS ₄ (Stephanite), PbCuSbS ₃ (Bournonite), Pb ₄ FeSb ₆ S ₁₄ (Jamesonite), and Pb ₉ A. s ₄ S ₁₅ (Gratonite ), but Non-Patent Document 3 similarly discloses Cu ₃ SbSe ₄ -Cu ₃ SbS ₄ , which is a similar compound to Enargite, and judging from the direct transition of each, it can be used as a semiconductor for solar cells. It has been shown that there is.

Adv.Mter.Optics Electronics, Vol.5, Issue 6, p.289 (1995) Conference Record of the IEEE Photovoltaic Specialists Conference, p.2774 (2016) Appl. Phys. Lett. Vol.98, Issue 26, p.261911 (2011)

However, the materials disclosed in Non-Patent Documents 1 to 3 are all sulfides, selenides, or double salts thereof, and have low stability. For this reason, for example, when used as a solar cell material and the solar cell deteriorates over time after being used in a natural environment for a long period of time, when it is damaged by external force, or when the solar cell is subjected to a recycling process, etc. , they disassembled and could not be used for a long period of time, and care had to be taken when handling them for recycling.

Since a stable material is required so that it can be used continuously over a long period of time, a chemically stable arsenic-containing compound has been sought.

In view of the above problems of the prior art, one aspect of the present invention aims to provide a chemically stable arsenic-containing compound.

In one aspect of the present invention to solve the above problems,
A copper arsenate compound having the chemical formula: Cu ₃ AsO ₄ is provided.

According to one aspect of the present invention, a chemically stable arsenic-containing compound can be provided.

FIG. 2 is a diagram showing a band diagram and density of states of Cu ₃ AsS ₄ . It is a band diagram _{of Cu3AsO4} , and a diagram showing the density of states. It is a figure showing _the optical absorption spectrum _{of Cu3AsO4} and _Cu3AsS4 .

A copper arsenate compound according to an embodiment of the present disclosure (hereinafter referred to as "this embodiment") will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is indicated by the scope of the claims, and is intended to include all changes within the meaning and scope equivalent to the scope of the claims.
[Copper arsenate compound]
An example of the structure of the copper arsenate compound of this embodiment will be described below.

The copper arsenate compound of this embodiment is represented by the chemical formula: Cu ₃ AsO ₄ .

The copper arsenate compound of this embodiment can be represented by the chemical formula: Cu ₃ AsO ₄ as described above. As is clear from the chemical formula, the copper arsenate compound of this embodiment has components such as sulfide ions, selenide ions, telluride ions, arsenide ions (As ^3- ), etc., which have a risk of decomposition and elution due to oxidation. Does not contain certain ingredients. That is, it is a stable substance that has high oxidation resistance, and even if it is oxidized, some of its components will not be decomposed or eluted. Therefore, although it is a compound containing arsenic, it is a chemically stable compound and has chemical properties suitable as, for example, a solar cell material that can be used outdoors for a long period of time.

As described above, the copper arsenate compound of the present embodiment exhibits particularly high performance when used as a solar cell material, and is a copper arsenate compound for solar cell materials represented by the chemical formula: Cu ₃ AsO ₄ It can be done. That is, it can be suitably used as a copper arsenate compound for solar cell materials.

The crystal structure of the copper arsenate compound of the present embodiment is not particularly limited, but may have, for example, an Enargite structure.

Here, in order to evaluate the band structure, which is important as a solar cell material, band diagrams were created for each of the existing Cu ₃ AsS ₄ and Cu ₃ AsO ₄ , which is the copper arsenate compound of this embodiment, by first-principles calculation. , a comparative evaluation was conducted.

FIGS. 1(A) to 1(C) show band diagrams and state density diagrams of Cu ₃ AsS ₄ . Figure 1 (A) shows the band structure, Figure 1 (B) shows the total state density and the partial state density of each atom, and Figure 1 (C) shows the partial state density of the orbits of Cu, As, and S. are shown respectively.

Further, FIGS. 2(A) to 2(C) show band diagrams and diagrams representing the density of states of Cu ₃ AsO ₄ . Figure 2 (A) shows the band structure, Figure 2 (B) shows the total state density and the partial state density of each atom, and Figure 2 (C) shows the partial state density of the orbits of Cu, As, and O. are shown respectively. Note that calculations are performed assuming that Cu ₃ AsS ₄ and Cu ₃ AsO ₄ have an Enargite structure.

As a result of band structure evaluation, the bottom band of the conduction band of Cu ₃ AsO ₄ mainly consists of Cu 4s orbital, its energy is in the range of 0 eV to 3 eV, the dispersion is large, and the effective mass of electrons (m _e * /m ₀ ) is as small as 0.50. Such characteristics are due to the d ¹⁰ s ⁰ electron configuration of zinc oxide (ZnO; m _e */m ₀ =0.2) and tin oxide (SnO ₂ ; m _e */m ₀ =0.3). This is similar to an oxide semiconductor composed of cations of p-block elements, and it can be seen that Cu ₃ AsO ₄ is a promising compound as an n-type semiconductor.

The bottom conduction band of Cu ₃ AsS ₄ , an existing semiconductor, mainly consists of the S 3p orbital, with almost no contribution from the Cu 4s orbital or the As 4s orbital, and the energy is in a narrow range of 0 eV to 1 eV and is dispersed. is small. This characteristic is completely different from that of ZnO and SnO ₂ and well supports the fact that Cu ₃ AsS ₄ with n-type conductivity has not been reported.

On the other hand, the upper part of the valence band of Cu ₃ AsO ₄ mainly consisted of Cu 3d orbitals, the dispersion was not so large, and the effective mass of holes (m _h */m ₀ ) was 2.5. This means that when compared with Cu ₂ O (m _h */m ₀ =0.69), which similarly contains monovalent Cu whose upper valence band consists of Cu 3d orbitals, the effective mass of holes in Cu ₃ AsO ₄ is Although not small, α-CuGaO ₂ (m _h */m ₀ =0.4 to 1.4), which exhibits p-type conductivity, and β-CuGaO ₄ (m _h */m ₀ =1.7 to 5. 1) and Cu ₃ AsS ₄ (m _h */m ₀ =1.1). From this, it was confirmed that there is a high possibility that conductivity will be developed when holes are injected into the valence band.

From the above first-principles calculation results, it can be said that Cu ₃ AsO ₄ is a material that can exhibit both n-type and p-type conductivity, and is a semiconductor that can form a homo pn junction consisting only of Cu ₃ AsO ₄ . It can be said that it is a material. That is, it was confirmed that Cu ₃ AsO ₄ is suitably used as a semiconductor material.

The Eg (band gap) of Cu ₃ AsO ₄ determined by the above calculation is 0.125 eV, but Eg is usually estimated to be small in calculations using the GGA (Generalized Gradient Approximation) method used in this calculation.

Based on the fact that the calculated Eg of Cu ₃ AsS ₄ is 0.162 eV and the experimentally determined Eg of Cu ₃ AsS ₄ is 1.28 eV, the actual Eg of Cu ₃ AsO ₄ is 1 eV. Estimated to be around.

The top of the valence band of Cu ₃ AsO ₄ is located at the X point in the reciprocal lattice space, and the bottom of the conduction band is located at the Γ point, so Cu ₃ AsO ₄ is an indirect transition type semiconductor. In general, light absorption near the energy of the band gap of indirect transition semiconductors is not so strong, but Cu ₃ AsO ₄ has a large density of states in the valence band and a direct gap (valence band Γ point - conduction band Γ point). Since the difference between the band gap (point) and the indirect gap (valance band point

FIG. 3 shows the results of a calculated comparison of the optical absorption spectra of Cu ₃ AsO ₄ and Cu ₃ AsS ₄ . The absorption rise near the band edge of Cu ₃ AsO ₄ is steeper than that of Si (x-Si in the figure), which is an indirect transition type semiconductor, and the absorption coefficient is higher than that of existing thin-film solar cell materials such as CdTe and GaAs. It can be confirmed that it is as large as that of a direct transition type semiconductor. From this, it was confirmed that a semiconductor using a copper arsenate compound represented by the chemical formula: Cu ₃ AsO ₄ can be suitably used as a light absorption layer of a thin film solar cell, that is, as a solar cell material.
[Method for producing copper arsenate compound]
Next, a method for producing a copper arsenate compound according to the present embodiment will be explained. Note that since the copper arsenate compound described above can be produced by the method for producing a copper arsenate compound of this embodiment, some of the matters already explained will be omitted.

The method for producing a copper arsenate compound of the present embodiment relates to a method for producing a copper arsenate compound represented by the chemical formula: Cu ₃ AsO ₄ and can include the following steps.
A mixing step of preparing a mixed solution by mixing an aqueous solution containing a copper (I) compound and an aqueous solution containing arsenate ions.
A recovery process that separates and collects the precipitate generated in the mixed solution.

Since the copper arsenate compound described above is a chemically stable compound, it is expected that it can be used for various applications such as semiconductor applications, and the present invention particularly shows suitable characteristics as a compound semiconductor for solar cells. The inventors of 2003 and 2007 investigated the manufacturing method.

As a copper arsenate, Cu ₃ (AsO ₄ ) _2, which is a Cu(II) compound, is widely known. Both Cu ²⁺ and AsO ₄ ^3- , which are the constituent components of this compound, are the highest valence ions of Cu and As, and redox reactions do not proceed with each other, so the high temperature of CuO and As ₂ O ₅ Whether it is a direct reaction or synthesis from an aqueous solution of a compound containing Cu ²⁺ and AsO ₄ ^3- , the reaction can proceed easily and quantitatively.

However, the copper arsenate compound represented by the chemical formula: Cu ₃ AsO ₄ is a compound consisting of reduced Cu ⁺ and oxidized AsO ₄ ^3- , so Cu ₂ O and As ₂ O ₅ are mixed together. When heated, a mutual redox reaction proceeds and CuO and As ₂ O ₃ are produced, making it difficult to synthesize the target substance.

Therefore, the inventors of the present invention conducted extensive studies and found that by mixing an aqueous solution containing a copper (I) compound and an aqueous solution containing arsenate ions and carrying out wet synthesis, the chemical formula: Cu It was discovered that a copper arsenate compound represented by ₃ AsO ₄ could be obtained, and the present invention was completed.

Each step of the method for producing a copper arsenate compound of this embodiment will be described below.
(Mixing process)
In the mixing step, a mixed solution is prepared by mixing an aqueous solution containing a copper (I) compound and an aqueous solution containing arsenate ions, and the copper ions and arsenate ions are reacted to form a precipitate of the copper arsenate compound. Obtainable.

In this way, in the mixing process, by reacting the copper (I) compound with arsenate ions, the oxidation of copper in the copper (I) compound used as a raw material and the reduction of arsenate are suppressed, and the target substance is suppressed. It becomes possible to more reliably obtain a copper arsenate compound that is

The aqueous solution containing the copper (I) compound to be subjected to the mixing step preferably contains a copper (I) compound having the same valence as the copper in the target copper arsenate compound, that is, a monovalent copper compound. As the aqueous solution containing the copper (I) compound, it is preferable to use, for example, an aqueous solution containing copper (I) halide and a complex forming agent.

Copper (I) halides are usually chemically stable, but are difficult to dissolve in water, so by adding a complexing agent, an aqueous solution containing the copper (I) compound is prepared. This is because it is possible.

Copper(I) halides include copper(I) fluoride, copper(I) chloride, copper(I) bromide, and copper(I) iodide, but copper(I) fluoride has particularly low solubility. , difficult to form complexes. Therefore, the copper(I) halide is more preferably one or more selected from copper(I) chloride, copper(I) bromide, and copper(I) iodide, and especially copper(I) chloride. The copper(I) halide is more preferably copper(I) chloride because it is easily available and is less susceptible to redox reactions.

Further, as the complex forming agent, a compound capable of causing a complex forming reaction with copper (I) halide and increasing the solubility of copper (I) halide can be suitably used. Examples of such complexing agents include alkali halides, aqueous ammonia, cyanides, water-soluble amines, and organic complexing agents such as EDTA and carboxylic acids. However, if the complex is too stable, the reactivity with arsenate ions may be reduced, so although it is possible to increase the solubility of copper halides, the formation of complexes that lead to the formation of complexes with low complex stability constants is possible. It is preferable to use an agent. Further, it is preferable that the complex forming agent does not react with arsenate ions to form a precipitate.

From this point of view, an alkali halide can be preferably used as the complex forming agent. Among the alkali halides, when ease of availability is emphasized, one or more selected from lithium chloride, sodium chloride, potassium chloride, ammonium chloride, etc. can be more preferably used, and sodium chloride can be even more preferably used.

When preparing an aqueous solution containing a copper (I) compound as described above as an aqueous solution containing copper (I) halide and a complex forming agent, the complex formation reaction with copper (I) halide is sufficiently advanced; In order to dissolve the complex-forming agent, it is preferable to adjust the amount and concentration of the complex-forming agent added. In this case, the concentration of the complex forming agent is preferably 1 mol/L or more, and more preferably 3 mol/L or more, from the viewpoint of allowing the complex formation reaction to proceed sufficiently. The upper limit of the concentration of the complex-forming agent is not particularly limited, but when an alkali halide is used as the complex-forming agent, it can be set to, for example, 10 mol/L or less from the viewpoint of its solubility.

The aqueous solution containing arsenate ions to be subjected to the mixing step is preferably an aqueous solution containing a normal salt of arsenate, which is a salt that does not contain H ⁺ . By using a normal arsenate salt, it is possible to suppress the generation of by-products such as hydrogen arsenate and quantitatively produce a copper(I) arsenate compound. As the normal salt of arsenate, an alkali normal salt is more preferable in consideration of solubility in water. As the alkaline normal salt of arsenate, for example, one or more selected from sodium arsenate, potassium arsenate, lithium arsenate, rubidium arsenate, cesium arsenate, etc. can be preferably used. In particular, from the viewpoint of high solubility in water and easy availability, one or more types selected from sodium arsenate and potassium arsenate can be more preferably used as the alkali normal salt of arsenate. Particularly, potassium arsenate can be more preferably used because of its high solubility.

The respective concentrations of the aqueous solution containing the copper compound and the aqueous solution containing arsenate ions to be subjected to the mixing step are not particularly limited. However, the higher the concentration and the faster the mixing, the finer the product becomes, and the lower the concentration and the gradual or continuous mixing, the coarser the product becomes. Therefore, it can be selected depending on the particle size required for the product.
(Collection process)
In the recovery step, the copper arsenate compound, which is a precipitate generated in the mixed solution, can be separated and recovered.

The method for separating the copper arsenate compound from the mixed solution is not particularly limited, and can be separated using various solid-liquid separation means such as a suction filter or a filter press. Moreover, the collected precipitate can also be washed with pure water, if necessary.

Since the precipitate recovered in the recovery step contains water, the method for producing a copper arsenate compound of this embodiment includes a drying step of drying the recovered precipitate to reduce the moisture content as necessary. It is also possible to have more.

The present invention will be described below with reference to specific examples, but the present invention is not limited to these examples.
[Example 1]
(Mixing process)
In an aqueous solution (Cl: 6.8 mol/L) made by adding water to 400 g of sodium chloride, which is an alkali halide, as a complex forming agent, 50 g (0.0 g) of copper (I) chloride, which is a copper (I) halide, is added. 5 mol) was added thereto, and stirred and mixed for 1 hour to dissolve. The residue was removed by filtration to prepare an aqueous solution containing a copper(I) compound.

Next, water was added to 80 g (0.17 mol) of potassium arsenate dodecahydrate, which is a normal salt of an alkali, to make 500 mL, and an aqueous solution containing arsenate ions was separately prepared.

Then, the aqueous solution containing arsenate ions was gradually added dropwise to the aqueous solution containing the copper(I) compound over 10 minutes, and stirred for 1 hour to prepare a mixed solution.
(Collection process)
The precipitate formed in the mixed solution was separated and collected by filtration.
(Washing process, drying process)
The collected precipitate was suction-washed twice with 300 mL of deionized water, and then dried in a vacuum desiccator, and 53.4 g of white precipitate was collected as a dry product.

The reagents used were all special grade reagents manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.

After collection, the washed and dried precipitate was analyzed using an X-ray analyzer (model: RINT2500, manufactured by Rigaku Co., Ltd.), and as a result, it was confirmed that the precipitate was almost pure Cu ₃ AsO ₄ having an Enargite structure.

Although the obtained precipitate was left in the air for several days, there was no change in the XRD pattern, confirming that it was a stable material.

Moreover, as a result of analyzing the diffuse reflection spectrum of the powder of the above-mentioned precipitate with a spectrophotometer (model: U4000, manufactured by Hitachi, Ltd.), a value of Eg (band gap) of 1.02 eV was obtained.

Furthermore, as a result of measuring the thermoelectromotive force of a green compact obtained by uniaxially pressing the precipitate at a pressure of 100 MPa (self-made device), the Seebeck coefficient was positive, and the precipitated powder exhibited p-type conductivity. These results confirmed that Cu ₃ AsO ₄ has excellent properties as a compound semiconductor for solar cells.
[Comparative example 1]
21.5 mg of copper (I) oxide (Cu ₂ O, 0.15 mmol, 99.5% product, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 11.5 mg of diarsenic pentoxide (As ₂ O ₅ , 0 05 mmol, 99.9% product, manufactured by Strem Inc.) in a nitrogen-filled glove box to obtain a raw material mixture.

Next, three samples were prepared by enclosing the above raw material mixture in a quartz glass tube in a degassed manner, and each sample was fired at 600°C, 800°C, and 1000°C for 6 hours.

As a result of analyzing the product obtained after calcination with an X-ray analyzer (model: RINT2500, manufactured by Rigaku Co., Ltd.), in addition to the raw material Cu ₂ O, only peaks of Cu ₃ (AsO ₄ ) ₂ and As ₂ O ₃ were observed. It was confirmed that the redox reaction progressed between Cu(I) and As(V) and the target Cu ₃ AsO ₄ was not produced.

Claims (3)

A copper arsenate compound represented by the chemical formula: Cu ₃ AsO ₄ . A copper arsenate compound for solar cell materials represented by the chemical formula: Cu ₃ AsO ₄ . a mixing step of preparing a mixed solution by mixing an aqueous solution containing a copper (I) compound and an aqueous solution containing arsenate ions;
a recovery step of separating and recovering the precipitate generated in the mixed solution;
has
As the aqueous solution containing the copper (I) compound, an aqueous solution containing copper (I) halide and a complex forming agent,
Using an aqueous solution containing a normal alkali salt as the arsenate ion-containing aqueous solution,
The copper (I) halide is one or more types selected from copper (I) chloride, copper (I) bromide, and copper (I) iodide,
the complex forming agent is an alkali halide,
A method for producing a copper arsenate compound represented by the chemical formula: Cu ₃ AsO ₄ , wherein the alkali normal salt is potassium arsenate.

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