JP2011165839A - Method of manufacturing light absorbing layer of thin film solar cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a composition for a light absorbing layer of a thin film solar cell, capable of improving crystallinity of the light absorbing layer, and facilitating preparing a more stable aqueous solution of the composition for the light absorbing layer. <P>SOLUTION: The aqueous solution which contains at least copper compound, zinc compound, tin compound, and acidic compound but does not contain chlorine compound, is coated and dried on a base material and then sulfurized to provide the light absorbing layer of a thin film solar cell. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing a light absorption layer of a thin film solar cell.

In recent years, research and development of solar cells has been energetically performed around the world as a trump card to prevent global warming. The semiconductor used in solar cells, monocrystalline Si, polycrystalline Si, amorphous _{Si, GaAs, InP, CdTe,} CuIn 1-x Ga x Se 2 (CIGS), Cu 2 ZnSnS 4 (CZTS) such as is known Yes. The current mainstream is Si crystal solar cells, but Si is an indirect transition semiconductor, so it has a low light absorption rate. Therefore, a thick film is required. There are concerns. Therefore, development of a thin film solar cell advantageous for cost reduction is underway. Among them, chalcopyrite type semiconductors typified by CIGS and CZTS have a large light absorption coefficient, so that they can be thinned and are advantageous for cost reduction. CIGS-based thin-film solar cells have already been commercialized, but they contain toxic Se as a constituent element and rare element In, which causes resource constraints and disposal problems during mass production. There is a fear. On the other hand, CZTS-based thin-film solar cells are not yet commercialized at the development stage, but they are free from toxic elements and are composed only of materials that are abundant in the crust. Recently, it has begun to attract attention as one of the solar cell candidates.

CZTS has a band gap of about 1.5 eV, a value very close to the optimum value of the solar cell light absorption layer, and has a light absorption coefficient of 10 ⁴ cm ⁻¹ or more and a large value comparable to CIGS. Therefore, it is extremely promising as a light absorption layer for thin film solar cells. A PN bond of a semiconductor to be a solar cell can be produced by laminating an interface layer and a window layer on this CZTS.

  Since this CZTS is generally produced by an apparatus under vacuum such as sputtering, the apparatus is very expensive, and at the same time, there is a problem that it takes time for vacuuming or sputtering. On the other hand, for example, a method for producing a light absorbing layer by applying a composition for a light absorbing layer in the air by a method such as a spray method or a coating method, drying it, and sulfiding it has been proposed (for example, patents). Reference 1). With this method, CZTS can be produced with a simple apparatus and process.

JP 2007-269589 A

  In order to easily produce a CZTS-based thin film solar cell at low cost, it has been studied to apply the light absorbing layer composition by a simple method such as a spray method or a coating method. However, in the method described in Patent Document 1, since a chlorine compound is used for the composition for the light absorption layer, chlorine remaining after the sulfurization may adversely affect the light absorption layer, and conversion efficiency may be deteriorated. was there. In addition, since the chlorine compound has strong activity, it easily reacts with other compounds, and it is difficult to produce a stable composition for a light absorption layer. For this reason, there is a problem that the production process becomes complicated, such as addition of a solvent other than water and temperature adjustment.

  In view of this situation, the present invention improves the crystallinity of the light absorption layer by removing chlorine compounds that are highly reactive with other compounds, and further includes an acidic oxide, thereby further stabilizing light absorption. It aims at providing the manufacturing method of the composition for light absorption layers of the thin film solar cell which made it possible to produce easily the aqueous solution of the composition for layers.

  That is, the present invention applies an aqueous solution containing at least a copper compound, a zinc compound, a tin compound and an acidic compound and not containing a chlorine compound on a substrate and drying it, and heating it in an atmosphere containing hydrogen sulfide or sulfur atoms. By doing this, a light absorption layer is obtained, and the method for producing a light absorption layer of a thin film solar cell.

  By using the method of the present invention, by removing chlorine compounds that are highly reactive with other compounds, the crystallinity of the light absorption layer is improved, and further by adding an acidic compound, a more stable for the light absorption layer An aqueous solution of the composition can be easily prepared. In addition, since it is unnecessary to add a solvent other than water, it is easy to reuse the solvent, and a product with a low environmental load can be produced.

  The composition for light absorption layers used by manufacture for the light absorption layer of the thin film solar cell of this invention is an aqueous solution which contains a copper compound, a zinc compound, a tin compound, and an acidic compound at least, and does not contain a chlorine compound. The light absorption layer of a thin film solar cell can be produced by applying and drying this on a substrate and heating it in an atmosphere containing hydrogen sulfide or sulfur atoms.

  The composition for light absorption layers used by manufacture of the light absorption layer of the thin film solar cell of this invention does not contain a chlorine compound, but contains each compound of copper, zinc, and tin. Copper compounds include copper nitrate (II), copper sulfate (II), copper acetate (II), copper hydroxide (II), copper oxide (II), cuprous oxide, copper pyrophosphate, copper carbonate, rhodan copper , Copper bromide, copper bromide soda, potassium bromide, etc., and zinc compounds include zinc nitrate, zinc sulfate, zinc acetate, zinc hydroxide, zinc oxide, zinc bromide, zinc carbonate, Zinc oxide, zinc pyrophosphate, etc., and tin compounds include tin (II) sulfate, tin acetate, stannous pyrophosphate, sodium stannate, potassium stannate, metastannic acid, stannous oxide, Examples thereof include, but are not limited to, tantalum and the like, as long as it is not a chlorine compound. Moreover, these may have a hydrate.

  Examples of the acidic compound used in the composition for the light absorption layer of the thin film solar cell of the present invention include inorganic acids such as nitric acid, sulfuric acid, phosphoric acid, boric acid, formic acid, acetic acid, citric acid, oxalic acid, lactic acid, propionic acid, Examples thereof include organic acids such as butyric acid, malic acid, succinic acid, aconitic acid, amino acids, L-ascorbic acid, fumaric acid, and tartaric acid. Preferably, it is an organic acid. More preferred is citric acid.

  The solvent of the composition for a light absorption layer used in the production of the light absorption layer of the thin film solar cell of the present invention is preferably only water, but a solvent other than water may be appropriately added depending on the situation. . When only water is used, the solvent can be easily reused, and a product with a low environmental load can be produced.

  As a method for preparing the composition for a light absorption layer, it is preferable to add a tin compound to a mixed solution in which a copper compound, a zinc compound, and an acidic compound are first mixed and then dissolved by adding water. By doing so, the solubility of the solute of the composition for light absorption layers improves. Moreover, when an insoluble component generate | occur | produces, a solubility improves by increasing an acidic compound. Further, the manufacturing process can be performed at room temperature, and it is not necessary to adjust the temperature.

  The composition for a light absorption layer of the thin film solar cell of the present invention does not contain a chlorine compound. Chlorine compounds are generally soluble in water, but because of their high reactivity, they easily react with other compounds. As a result, the solution stability of the composition for the light absorption layer is deteriorated, so that it can be stabilized. Therefore, further studies such as solvent addition and temperature adjustment were necessary. As a result of eliminating the chlorine compound, it was found that the solubility in water deteriorates, but the addition of an acidic compound improves the solubility and also increases the stability of the composition for the light absorption layer. In addition, since a solvent other than water is unnecessary, it is easy to reuse the solvent, and a light absorption layer of a thin-film solar cell with a low environmental load can be produced.

  The light absorption layer of CZTS obtained by the production of the composition for the light absorption layer of the thin-film solar cell of the present invention has low power generation efficiency in the stoichiometric composition, but if the Cu is slightly reduced compared to the stoichiometric composition, High conversion efficiency. In the present invention, CZTS includes not only a compound having a stoichiometric composition, but also all non-stoichiometric compounds exhibiting relatively high conversion efficiency, or all compounds containing Cu, Zn, Sn, and S as a main component. Is included.

  The conversion efficiency is obtained from the product of the open circuit voltage Voc, the short circuit current Isc, which is an intercept of the graph of the voltage V and the current I, and the fill factor FF at the time of light irradiation. It shows that the performance is high.

  In order to obtain high conversion efficiency, it is preferable that the light absorption layer has an atomic ratio and a Cu / (Zn + Sn) ratio of 0.69 to 0.99. However, when sulfiding the composition for the light absorption layer, zinc which is a component of the composition is likely to be scattered, and there is a difference between the atomic ratio obtained with the composition for light absorption and the atomic ratio obtained after sulfidation. Is expected. Therefore, it is preferable to set the atomic ratio of Cu / (Zn + Sn) in advance to be small. Specifically, the Cu / (Zn + Sn) of the composition for a light absorption layer is preferably in the range of 0.3 to 0.9.

  Next, the manufacturing method of the composition for light absorption layers of the thin film solar cell of this invention is demonstrated in detail. However, the following method is an example, and the material used in each step may be appropriately changed within the scope of the present invention.

  First, the substrate can be selected from, for example, silicon wafers, glasses, ceramics, gallium arsenide, organic substrates, and inorganic substrates, but is not limited thereto. Examples of organic substrates include glass-based copper-clad laminates such as glass cloth / epoxy copper-clad laminates, composite copper-clad laminates such as glass nonwoven fabrics / epoxy copper-clad laminates, polyetherimide resin substrates, polyethers Examples thereof include heat-resistant / thermoplastic substrates such as ketone resin substrates, polyimide resin substrates, and polysulfone resin substrates, and flexible substrates such as polyester copper-clad film substrates and polyimide copper-clad film substrates. Examples of inorganic substrates include ceramic substrates such as alumina substrates, aluminum nitride substrates, and silicon carbide substrates, and metal substrates such as aluminum base substrates and iron base substrates. Among these, it is preferable to use an inexpensive glass substrate, and it is preferable to use alkali-free glass.

  Next, a lower electrode is formed on the substrate. The lower electrode on the substrate takes out the current generated in the light absorption layer. For the lower electrode, a material having high electrical conductivity and good adhesion to the substrate is used. Specific examples of the material used for the electrode include Mo, Al, Ag, Cr, Ta, Cu, Ti, Ni, Mo alloy, Al alloy, Ag alloy, In—Sn—O, and In—Zn—O. is there. In particular, Mo is suitable as a material for the lower electrode because of its high adhesion to glass. The thickness of the lower electrode is preferably 0.5 to 2 μm.

  The method for forming the lower electrode is not particularly limited, and an optimum method is selected according to the materials of the substrate and the lower electrode. Specific examples of the method for forming the lower electrode include a sputtering method, a vacuum deposition method using an electron beam heating method, a vacuum deposition method using a resistance heating method, and a pulse laser deposition method (PLD) method.

  Next, the composition for light absorption layers described in this specification is applied to the lower electrode on the substrate. As a method for applying the composition for the light absorption layer, there are methods such as coating using a spinner and spray coating. Moreover, although a coating film thickness changes with application methods and solid content concentration of a composition, it is normally applied so that the film thickness after drying may be 0.1 to 5 μm. This film thickness is substantially maintained after sulfidation.

  Next, the solvent is removed from the applied thin film of the light absorbing layer composition. Examples of the method for removing the solvent include oven drying, hot plate, heat drying with infrared rays, vacuum drying, and the like. Heat drying is preferably performed in the range of 70 ° C. to 350 ° C. for 1 minute to 10 minutes.

  After removing the solvent, the thin film of the composition for the light absorption layer is sulfurized. Sulfurization is usually performed in a range of 450 ° C. to 560 ° C. in a nitrogen atmosphere containing 1 to 20% by volume of hydrogen sulfide. The sulfurization time is preferably 30 minutes to 5 hours. When it becomes higher than 560 ° C., the base material may be melted when it is alkali glass. In addition, when sulfur is used as a method for sulfurization, there are a method in which sulfur is placed in a glass container and heated with a burner or the like, and a method in which resistance heating is used. In addition, when using a sulfur compound, thiourea, sodium thiosulfate, thiosulfuric acid, or the like is included in the composition for the light absorption layer, and sulfur in the compound is generated by heating the substrate, ultraviolet rays, or the like. There are methods to use.

Next, an interface layer is laminated on the light absorption layer obtained by sulfiding the thin film of the composition for light absorption layer. The interface layer is for improving the conversion efficiency by improving the connection between the light absorption layer and the window layer. For the interface layer, a semiconductor that has high resistance and transmits most of visible light to near infrared light is used. Examples of the material used for the interface layer include CdS, ZnO, Zn _1-x Mg _x O, and In ₂ S ₃ . Among these, CdS is particularly suitable as a buffer layer. The thickness of the interface layer is preferably 0.01 to 1 μm.

  The formation method of the interface layer is not particularly limited, and an optimum method is selected according to the composition of the interface layer. For example, when CdS is used as the interface layer, the CdS layer is formed by a method (chemical bath film formation (CBD) method) in which thiourea or the like is dissolved in an aqueous solution in which metal ions are dissolved, and the substrate is immersed in the solution. be able to.

Next, a window layer is laminated on the obtained interface layer. The window layer is for extracting light and reaching light to the light absorption layer at the same time. For the window layer, a semiconductor that has a low resistance and transmits most of visible light to near infrared light is used. Examples of the material used for the window layer include ZnO: Al, ZnO: B, In—Sn—O, and In—Zn—O. Among these, ZnO: Al is particularly suitable as the window layer. The thickness of the window layer is preferably 0.1 to 2 μm.
The method for forming the window layer is not particularly limited, and an optimal method is selected according to the material of the window layer. Specific examples of the method for forming the window layer include sputtering, electron beam heating vacuum deposition, resistance heating vacuum deposition, pulsed laser deposition (PLD), spray, spinner, and the like. .

  Next, an upper electrode is formed on the obtained window layer. The upper electrode is used to efficiently extract the current collected in the window layer to the outside, and since the light needs to reach the light absorption layer, it is usually formed in a comb shape. Examples of the material of the upper electrode include Al, Cu, Ag, Au, and alloys containing any one or more of these. Specific examples of such an alloy include an Al—Ti alloy, an Al—Mg alloy, an Al—Ni alloy, a Cu—Ti alloy, a Cu—Sn alloy, a Cu—Zn alloy, a Cu—Au alloy, and Ag. -Ti alloy, Ag-Sn alloy, Ag-Zn alloy, Ag-Au alloy, and the like. The thickness of the electrode is preferably 0.01 to 2 μm.

  The method for forming the upper electrode is not particularly limited, and an optimum method is selected according to the material of the upper portion. Specific examples of the method for forming the window layer include a sputtering method, a vacuum vapor deposition method using an electron beam heating method, a vacuum vapor deposition method using a resistance heating method, and a pulse laser deposition method (PLD) method. Usually, in order to make a solar cell into an element, dozens of upper electrodes are formed using a mask having dozens of comb-shaped patterns.

Elements constituting the solar cell, such as the above-mentioned substrate, lower electrode, light absorption layer, interface layer, window layer, and additional layer other than the upper electrode include an adhesive layer, a light scattering layer, an antireflection layer, etc. There is.
The adhesive layer is for enhancing the adhesion between the base material and the lower electrode, and can be formed as necessary. For example, when a glass substrate is used as the substrate and Mo is used as the lower electrode, it is preferable to use Ti, Cr, Ni, W, or an alloy containing any one or more of these for the adhesive layer.

The light scattering layer is for reflecting incident light and increasing the light absorption efficiency in the light absorption layer, and can be formed as necessary. There are a light scattering layer provided on the upper electrode side from the light absorption layer and a light scattering layer provided on the substrate side from the light absorption layer.
The antireflection layer is for reducing the amount of incident light reflected by the window layer and increasing the light absorption efficiency of the light absorption layer, and can be formed as necessary. For the antireflection layer, for example, a transparent body having a refractive index smaller than that of the window layer, an aggregate composed of transparent particles having a diameter sufficiently smaller than the wavelength of sunlight, and sufficiently inside the wavelength of sunlight. It is preferable to use a space having a small diameter.

  Next, the obtained solar cell is made into an element. An element is formed by dividing so that one upper electrode enters one grid and the window layer, the interface layer, and the light absorption layer are completely separated. Further, the window layer, the interface layer, and the light absorption layer are scraped off at a place where no element is formed on the substrate, the lower electrode is exposed, and a silver paste is applied to the spot. Using the lower electrode to which the Ag paste is applied and the electrode of the upper electrode formed into an element, current-voltage characteristics can be evaluated by a four-terminal method at the time of light irradiation, and conversion efficiency can be obtained.

As a specific calculation method of the conversion efficiency, the short-circuit current Isc (mA) and the open-circuit voltage Voc, which are intercepts of the graph of the IV characteristics obtained using an AM1.5 solar simulator with an incident intensity of 100 mW / cm ^2. The conversion efficiency η = Isc / S × Voc × FF / 100 × 100 can be obtained from (V), FF which is a fill factor, and element area S (cm ² ). Here, the curve factor can be obtained by dividing the product of Vmax and Imax, which is the maximum output point, by the product of Isc and Voc.

  Examples of the present invention will be described below, but the present invention is not limited to these examples.

Example 1
First, after mixing 0.8 g of copper (II) nitrate trihydrate, 1 g of zinc nitrate hexahydrate and 0.4 g of citric acid, 2 g of water was added and dissolved, and tin (II) sulfate was added to the solution. 0.42g was added and the composition for light absorption layers was produced. Next, the composition for light absorption layers was apply | coated using the spinner on the alkali free glass substrate coated with the molybdenum film | membrane used as a lower electrode, and it dried at 200 degreeC for 4 minutes using the hotplate. The thin film of the composition for light absorption layers obtained was subjected to sulfurization treatment at 500 ° C. for 1 hour in an atmosphere of 5% by volume hydrogen sulfide + nitrogen to produce a light absorption layer. Next, a cadmium sulfide film was formed on the light absorption layer by the CBD method. The thickness of CdS is 0.5 μm or less. Further, an Al-doped ZnO film was formed as a CdS film by a spin method. A total of 16 comb-shaped electrodes made of an Al film were formed on the Al-doped ZnO film by sputtering. As a 4 mm × 4 mm grid in which the comb-shaped electrode completely enters, the window layer, the interface layer, and the light absorption layer were removed, and the Mo electrode film was exposed to form a solar cell panel element. Furthermore, in order to improve the electrical conductivity of the electrode, a silver paste was applied to the exposed Mo electrode surface. The conversion efficiency was measured using a four-terminal method with a Mo film as a lower electrode and a comb-shaped Ag as an upper electrode. Among the 16 obtained elements, the maximum one was defined as the conversion efficiency of the composition. As a result, 0.64% was obtained. Moreover, when the cross section of the solar cell panel made into an element was observed by SEM, the thickness of the light absorption layer was 0.9 μm.

Examples 2, 3, 4
A composition for a light absorption layer was prepared in the same manner as in Example 1 except that the rotation speed of the spinner was changed and the thickness of the light absorption layer was changed. Was evaluated. The results are shown in Table 1.

Examples 5 and 6
A composition for a light absorption layer was prepared in the same manner as in Example 1 except that the type of the acidic compound in the composition for the light absorption layer was changed, and the conversion efficiency and the evaluation of the thickness of the light absorption layer were performed according to the methods described above. Went. The results are shown in Table 1.

Examples 7 and 8
A composition for a light absorption layer was prepared in the same manner as in Example 1 except that the amount ratio of the metal compound used for the preparation of the composition for the light absorption layer was changed. The thickness was evaluated. The results are shown in Table 1.

Examples 9, 10, 11
A composition for the light absorption layer was prepared in the same manner as in Example 1 except that the combination of the metal compounds used for the preparation of the composition for the light absorption layer was changed. The thickness was evaluated. The results are shown in Table 1.

Examples 12 and 13
A composition for a light absorption layer was prepared in the same manner as in Example 1 except that the conditions for the sulfurization temperature of the composition for the light absorption layer were changed, and the conversion efficiency and evaluation of the thickness of the light absorption layer were performed according to the methods described above. Went. The results are shown in Table 1.

Comparative Example 1
A composition for a light absorption layer was prepared in the same manner as in Example 1 except that it did not contain citric acid, but the evaluation was not performed because the metal compound for the light absorption layer became insoluble.

Comparative Example 2
A composition for a light absorption layer was prepared in the same manner as in Example 1 except that the copper compound and the tin compound were as shown in Table 2, but the evaluation was performed because the metal compound for the light absorption layer became insoluble. There wasn't.

Comparative Example 3
A composition for a light absorbing layer was prepared in the same manner as in Example 1 except that the copper compound and the tin compound were as shown in Table 2, and the conversion efficiency and the thickness of the light absorbing layer were evaluated according to the methods described above. It was. The results are shown in Table 2.

Claims (3)

An aqueous solution containing at least a copper compound, a zinc compound, a tin compound, and an acidic compound and not containing a chlorine compound is applied to a substrate and dried, and then sulfidized to obtain a light absorption layer of a thin film solar cell. Manufacturing method of light absorption layer of thin film solar cell. The method according to claim 1, wherein the acidic compound is an organic acid. The production method according to claim 1, wherein the acidic compound is citric acid.