JP2013084921A - Photoelectric conversion element and solar cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow a photoelectric conversion element to obtain high photoelectric conversion efficiency.SOLUTION: In a photoelectric conversion element 1, a lower electrode 40, a photoelectric conversion semiconductor layer 50, and an upper electrode 80 are sequentially laminated on a substrate 10. A main component of the photoelectric conversion semiconductor layer 1 is at least one compound semiconductor of a chalcopyrite structure, which comprises a group Ib element, a group IIIb element, and a group VIb element. The photoelectric conversion semiconductor layer 50 contains lithium ions or potassium ions, and sodium ions.

Description

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

  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 Document 1). Conventionally, Na is diffused into the photoelectric conversion layer using a soda-lime glass substrate containing Na.

  However, when an alkali-free glass substrate that does not contain Na, a metal substrate, a polymer substrate, a ceramic substrate, or the like is used as a solar cell substrate, sodium cannot be supplied from the substrate, and conversion efficiency does not increase. There is a problem. 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 2 discloses applying alkali metal silicate, specifically sodium silicate, by liquid phase application. Patent Document 3 discloses that an anodized substrate is brought into contact with a sodium hydroxide aqueous solution and doped with sodium. Furthermore, Patent Document 4 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.

  By the way, it is known that lithium, potassium, and cesium, which are alkali metals other than sodium, are less in amount that can be added to CIGS than sodium, and even if added, the effect of improving photoelectric conversion efficiency is low ( Non-Patent Documents 1 and 2).

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

Thin Solid Films, 361, p.9-p.16 (2000) MA contreras, et. Al, “Conference Record of the Twenty Sixth IEEE Photovoltaic Specialists Conference”, 2000, p.359-p.362, titled “On the role of Na and modifications to CIGS absorber materials using high MF precursor layers” "

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.
This invention is made | formed in view of the said situation, and it aims at providing the photoelectric conversion element which can obtain high photoelectric conversion efficiency, and a solar cell using the same.

The photoelectric conversion element of the present invention is a photoelectric conversion element in which a lower electrode, a photoelectric conversion semiconductor layer, and an upper electrode are sequentially stacked on a substrate, and the main components of the photoelectric conversion semiconductor layer are an Ib group element and an IIIb group element And at least one chalcopyrite structure compound semiconductor comprising a group VIb element, wherein the photoelectric conversion semiconductor layer contains lithium ions or potassium ions and sodium ions.
Each content of lithium ions or potassium ions and sodium ions contained in the photoelectric conversion semiconductor layer is preferably 1 × 10 ¹⁵ atoms / cm ³ or more. Furthermore, the content is preferably 1 × 10 ¹⁷ atoms / cm ³ or more.

The lithium ions or potassium ions and sodium ions contained in the photoelectric conversion semiconductor layer are preferably supplied from an alkali supply layer formed between the substrate and the lower electrode.
The main component of the photoelectric conversion semiconductor layer is at least one type Ib group element selected from the group consisting of Cu and Ag, and at least one type IIIb group element selected from the group consisting of Al, Ga and In; It is preferably at least one compound semiconductor comprising at least one VIb group element selected from the group consisting of S, Se, and Te.

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 solar cell of the present invention comprises the above photoelectric conversion element.

  The photoelectric conversion element of the present invention can improve the power generation efficiency of the photoelectric conversion element by including lithium ions or potassium ions in addition to sodium ions in the photoelectric conversion semiconductor layer. The mechanism of action by which lithium ions or potassium ions are combined with sodium ions is not necessarily clear. However, according to the elemental analysis in the photoelectric conversion semiconductor layer, the photoelectric conversion semiconductor layer containing lithium ions or potassium ions in addition to sodium ions and the photoelectric conversion semiconductor layer containing only sodium ions are included in the photoelectric conversion semiconductor layer. The amount of sodium added is almost the same. From this, although the influence on the physical properties and characteristics of the photoelectric conversion semiconductor layer is not clear, it is presumed that the presence of lithium ions or potassium ions contributes to the improvement of photoelectric conversion efficiency. This was first discovered by the inventor.

It is a schematic sectional drawing which shows one Embodiment of the photoelectric conversion element of this invention. It is a schematic sectional drawing which shows another embodiment of the photoelectric conversion element of this invention.

  The photoelectric conversion element of the present invention will be described with reference to the drawings. 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 anodic oxide film 20 formed by anodic oxidation on the substrate 10, an alkali supply layer 30, a lower electrode 40, and a hole / electron pair formed by light absorption. The generated photoelectric conversion semiconductor layer 50, the buffer layer 60, the translucent conductive layer (transparent electrode) 70, and the upper electrode (grid electrode) 80 are sequentially laminated.

  1 shows the photoelectric conversion element in which the anodic oxide film 20 formed by anodic oxidation and the alkali supply layer 30 are formed on the substrate 10, but as shown in FIG. In addition, an embodiment in which the alkali supply layer 30 is formed may be used (in FIG. 2, the same components as those in FIG. 1 are given the same numbers). 1 and 2 show an embodiment in which the alkali supply layer 30 is provided, lithium ions or potassium ions and sodium ions contained in the photoelectric conversion semiconductor layer are formed during the formation of the photoelectric conversion semiconductor layer. When co-evaporation or the like is performed and it is not necessary to supply from the alkali supply layer 30, it is not necessary to provide the alkali supply layer 30.

The photoelectric conversion semiconductor layer 50 is a compound semiconductor having at least one chalcopyrite structure in which the main component is a photoelectric conversion semiconductor layer composed mainly of an Ib group element, an IIIb group element, and a VIb group element. Here, the main component means a component of 20% by mass or more.
The main component of the photoelectric conversion semiconductor layer 50 is:
At least one group Ib element selected from the group consisting of Cu and Ag;
At least one group IIIb element selected from the group consisting of Al, Ga and In;
It is preferably at least one compound semiconductor comprising at least one VIb group element selected from the group consisting of S, Se, and Te.

As the compound semiconductor,
CuAlS ₂ , CuGaS ₂ , CuInS ₂ ,
CuAlSe ₂ , CuGaSe ₂ , CuInSe ₂ (CIS),
AgAlS ₂ , AgGaS ₂ , AgInS ₂ ,
AgAlSe ₂ , AgGaSe ₂ , AgInSe ₂ ,
AgAlTe ₂ , AgGaTe ₂ , AgInTe ₂ ,
_{Cu (In 1-x Ga x} ) Se 2 (CIGS), Cu (In 1-x Al x) Se 2, Cu (In 1-x Ga x) (S, Se) 2,
_{Ag (In 1-x Ga x} ) Se 2, and _{Ag (In 1-x Ga x} ) (S, Se) 2 , and the like.

The photoelectric conversion semiconductor layer 50 particularly preferably includes CuInSe ₂ (CIS) and / or Cu (In, Ga) Se ₂ (CIGS) in which Ga is dissolved. CIS and CIGS are semiconductors having a chalcopyrite crystal structure, have high light absorption, and high energy conversion efficiency has been reported. Moreover, there is little degradation of efficiency by light irradiation etc. and it is excellent in durability.
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 method for forming the photoelectric conversion semiconductor layer is not particularly limited. For example, film formation of a CI (G) S-based photoelectric conversion semiconductor layer containing Cu, In, (Ga), S is as follows: 1) multi-source co-evaporation method, 2) selenization method, 3) sputtering method, 4 The hybrid sputtering method and the 5) mechanochemical process method are known.

1) Multi-source co-evaporation methods include three-stage method (JRTuttle et.al, Mat. Res. Soc. Symp. Proc., Vol.426 (1996) p.143., Etc.) and EC group co-evaporation method. (L. Stolt et al .: Proc. 13th ECPVSEC (1995, Nice) 1451. etc.).
In the former three-stage method, In, Ga, and Se are first co-deposited at a substrate temperature of 300 ° C. in a high vacuum, and then heated to 500 to 560 ° C., and Cu and Se are co-evaporated. In this method, Ga and Se are further vapor-deposited. The latter EC group simultaneous vapor deposition method is a method in which Cu-excess CIGS is vapor-deposited in the early stage of vapor deposition and In-rich CIGS is vapor-deposited in the latter half.

In order to improve the crystallinity of the CIGS film, as a method of improving the above method,
a) a method using ionized Ga (H. Miyazaki, et.al, phys.stat.sol. (a), Vol.203 (2006) p.2603, etc.),
b) Method of using cracked Se (68th Japan Society of Applied Physics Academic Lecture Proceedings (Autumn 2007, Hokkaido Institute of Technology) 7P-L-6 etc.),
c) Method using radicalized Se (Proceedings of the 54th Japan Society of Applied Physics (Aoyama Gakuin University) 29P-ZW-10 etc.)
d) A method using a photoexcitation process (the 54th Japan Society of Applied Physics Academic Lecture Proceedings (Spring 2007 Aoyama Gakuin University) 29P-ZW-14 etc.) is known.

  2) The selenization method is also called a two-step method. First, a metal precursor of a laminated film such as a Cu layer / In layer or a (Cu—Ga) layer / In layer is formed by sputtering, vapor deposition, or electrodeposition. In this method, a selenium compound such as Cu (In1-xGax) Se2 is produced by a thermal diffusion reaction by forming a film and heating it in selenium vapor or hydrogen selenide to about 450 to 550 ° C. This method is called a vapor phase selenization method. In addition, there is a solid-phase selenization method in which solid-phase selenium is deposited on a metal precursor film and selenized by a solid-phase diffusion reaction using the solid-phase selenium as a selenium source.

  In the selenization method, in order to avoid the rapid volume expansion that occurs during selenization, a method of previously mixing selenium in a metal precursor film at a certain ratio (T. Nakada et.al, Solar Energy Materials and Solar Cells 35 (1994) 204-214, etc.), and multilayer precursor film with selenium sandwiched between thin metal layers (for example, Cu layer / In layer / Se layer ... stacked with Cu layer / In layer / Se layer) (T. Nakada et.al, Proc. Of 10th European Photovoltaic Solar Energy Conference (1991) 887-890. Etc.) is known.

  In addition, as a method for forming a graded band gap CIGS film, a Cu—Ga alloy film is first deposited, an In film is deposited thereon, and when this is selenized, natural thermal diffusion is used to form Ga. There is a method in which the concentration is inclined in the film thickness direction (K. Kushiya et.al, Tech.Digest 9th Photovoltaic Science and Engineering Conf. Miyazaki, 1996 (Intn. PVSEC-9, Tokyo, 1996) p.149.).

3) As a sputtering method, a method using CuInSe ₂ polycrystal as a target, a two-source sputtering method using Cu ₂ Se and In ₂ Se ₃ as a target and using a mixed gas of H ₂ Se / Ar as a sputtering gas (JHErmer, et. al., Proc. 18th IEEE Photovoltaic Specialists Conf. (1985) 1655-1658., etc.) al, Jpn.J.Appl.Phys.32 (1993) L1169-L1172.).

  4) As the hybrid sputtering method, in the sputtering method described above, Cu and In metal are DC sputtering, and only Se is vapor deposition (T. Nakada, et.al., Jpn.Appl.Phys.34 ( 1995) 4715-4721.

  5) In the mechanochemical process method, raw materials corresponding to the CIGS composition are put into a planetary ball mill container, and the raw materials are mixed by mechanical energy to obtain CIGS powder, which is then applied onto the substrate by screen printing and annealed. To obtain a CIGS film (T. Wada et.al, Phys.stat.sol. (A), Vol.203 (2006) p2593, etc.).

  Examples of other CIGS film formation methods include screen printing, proximity sublimation, MOCVD, and spray (wet film formation). For example, a fine particle film containing a group Ib element, a group IIIb element, and a group VIb element is formed on a substrate by a screen printing method (wet film forming method) or a spray method (wet film forming method), and then pyrolyzed ( At this time, a crystal having a desired composition can be obtained by performing a thermal decomposition treatment in a VIb group element atmosphere (JP-A-9-74065, JP-A-9-74213, etc.).

The photoelectric conversion semiconductor layer 50 contains sodium ions and lithium ions or potassium ions. Lithium ions and potassium ions may contain both. Each content of lithium ion, potassium ion and sodium ion contained in the photoelectric conversion semiconductor layer is preferably 1 × 10 ¹⁵ atoms / cm ³ or more. More preferably, the content of sodium ions is 1 × 10 ¹⁸ atoms / cm ³ to 1 × 10 ²¹ atoms / cm ³ , more preferably 2 × 10 ¹⁸ atoms / cm ³ to 1 × 10 ²¹ atoms / cm ³ , Is preferably 5 × 10 ¹⁸ atoms / cm ³ to 1 × 10 ²¹ atoms / cm ³ . The lithium ion or potassium ion content is 1 × 10 ¹⁵ atoms / cm ^{3 to} 5 × 10 ¹⁹ atoms / cm ³ , more preferably 1 × 10 ¹⁶ atoms / cm ^{3 to} 5 × 10 ¹⁹ atoms / cm ³ , 1 × 10 ¹⁷ atoms / cm ^{3 to} 5 × 10 ¹⁹ atoms / cm ³ , particularly 1 × 10 ¹⁸ atoms / cm ^{3 to} 5 × 10 ¹⁹ atoms / cm ³ are preferable.

  Lithium ions or potassium ions and sodium ions contained in the photoelectric conversion semiconductor layer may be co-evaporated at the time of film formation of the photoelectric conversion semiconductor layer, or fluoride such as NaF, LiF, KF, etc. on the Mo electrode in advance. It may be evaporated. In addition, a photoelectric conversion semiconductor layer is formed on a Mo film containing Na, Li, or K, or a compound containing Na, Li, or K is deposited and post-annealed after the photoelectric conversion semiconductor layer is formed. Good. Alternatively, the lower electrode 40 may be transmitted through the alkali supply layer 30 and diffused. Note that the above-described methods are examples of means for adding sodium, lithium, or potassium into the CIGS-based photoelectric conversion semiconductor layer, and are not limited thereto. Hereinafter, several embodiments described above will be described. As described above, there are various methods for forming the CIGS, and the method for adding sodium and lithium or potassium can be appropriately selected according to the method for forming the CIGS.

  First, a method of transmitting and diffusing the lower electrode 40 from the alkali supply layer 30 will be described. The alkali supply layer 30 is preferably an alkali metal silicate layer, and the alkali metal silicate layer preferably contains sodium silicate and lithium silicate or potassium silicate. The alkali metal silicate layer is preferably provided by coating.

  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. In order for the photoelectric conversion semiconductor layer 50 to contain sodium ions, lithium ions or potassium ions of 1 × 10 ¹⁵ atoms / cm ³ or more, the concentration of the coating solution, the film thickness of the coating film to be formed, and the coating described later It can carry out by adjusting later heat processing temperature suitably, for example, as a film thickness of the coating film to form, it is preferable to adjust in the range of 10 nm-1 micrometer. 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, the substrate 10 used in the photoelectric conversion element of the present invention is a clad substrate in which aluminum and a different metal are combined and an anodized film is formed on the aluminum surface. preferable. 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.

  The lithium ions or potassium ions and sodium ions contained in the alkali metal silicate layer 30 are heated to several hundred degrees C. after the photoelectric conversion semiconductor layer 50 is provided. The ions are sufficiently supplied, and these ions are diffused into the photoelectric conversion layer via the lower electrode 40 which is the upper layer, so that the photoelectric conversion semiconductor layer 50 contains lithium ions or potassium ions and sodium ions. .

  Other methods for adding sodium and lithium or potassium include the following methods. 1) After forming a sodium and lithium or potassium-containing compound layer on the back electrode, and after forming a CIGS precursor layer, heat treatment is performed in a selenium atmosphere or hydrogen selenide atmosphere, so that sodium and lithium in CIGS Alternatively, a method of adding potassium, 2) forming a CIGS precursor layer on the back electrode, and forming a sodium and lithium or potassium-containing compound layer on the precursor layer, followed by a selenium atmosphere or a hydrogen selenide atmosphere There is a method of adding sodium and lithium or potassium into CIGS by heat treatment.

  1) and 2) In both methods, when a precursor layer is heat-treated in a selenium atmosphere or a hydrogen selenide atmosphere to form a CIGS film, sodium and lithium or a potassium-containing compound that is present below or above the precursor layer From the layer, it is characterized by adding sodium and lithium or potassium into CIGS by thermal diffusion. The heat treatment temperature is preferably about 450 to 550 ° C. These techniques have the feature that the composition adjustment of CIGS is relatively easy. As in the present invention, when sodium and lithium or potassium have particularly preferable concentration ranges, by using these techniques, by controlling the amount of sodium and lithium or potassium-containing compound relative to the precursor layer, The addition amount of sodium and lithium or potassium can be adjusted. The selenization may be sulfurization. In this case, a Cu (In, Ga) S compound to which sodium and lithium or potassium are added is generated. In addition, a Cu (In, Ga) (S, Se) compound may be formed by a film forming method combining selenization and sulfurization.

  Examples of the method for forming the sodium and lithium or potassium-containing compound layer include the following. First, a sodium and lithium-containing aqueous solution or a sodium and potassium-containing aqueous solution is brought into contact with the back electrode in the method 1) and the CIGS precursor layer in the method 2). Form a layer. These aqueous solutions are prepared by adding a lithium compound or a potassium compound to a sodium metal-containing aqueous solution selected from sodium tetraborate 10 hydrate, sodium sulfide 9 hydrate, sodium aluminum sulfate 12 hydrate, sodium molybdate and the like. What is necessary is just to add. The sodium-containing aqueous solution may be sodium nitrate, sulfate, phosphate, silicate, chloride, bromide, iodide and the like. As the lithium compound or potassium compound, lithium or potassium nitrate, sulfate, phosphate, silicate, fluoride, chloride, bromide, iodide or the like is used.

More _{specifically, Na 2 B 4 O 7 ·} 10H 2 O as a sodium _{compound, Na 2 S · 9H 2 O} , Na 2 SeO 3 · 5H 2 O, Na 2 TeO 3 · 5H 2 O, Na 2 SO 3 7H ₂ O, AlNa (SO ₄ ) ₂ · 12H ₂ O, NaCl,
Examples of potassium compounds include K ₂ TeO ₃ .3H ₂ O, K ₂ Al ₂ O ₄ .3H ₂ O, AlK (SO ₄ ) ₂ · 12H ₂ O, KOH, KF, K ₂ SeO ₃ , K ₂ TeO ₃ , KCl. K ₂ [CuCl ₄ (H ₂ O) ₂ ], KBr, KBH ₄ , K ₂ S ₂ O ₃ .nH ₂ O, K ₂ S ₂ O ₅ , K ₂ S ₂ O ₇ , KF · 2H ₂ O, KF · HF, K ₂ TiF ₆ , K ₂ B ₄ O ₇ · 4H ₂ O, KHSO ₄ , KI,
Examples of the lithium compound include Li ₂ B ₄ O ₇ .3H ₂ O, Li ₂ B ₄ O ₇ , LiCl, LiBr · H ₂ O, LiF, and Li ₂ SO ₄ · H ₂ O.

  The sodium and lithium or potassium-containing compound layer reacts with the precursor and disappears apparently. Therefore, CIGS may contain other elements contained in the compound layer, but there is no particular influence on the performance. Examples of other elements contained in CIGS include aluminum, sulfur, molybdenum, and boron.

  The second is a method of forming a lithium and potassium-containing compound layer by vapor deposition or the like. As the sodium source, lithium source, and potassium source, fluoride, chloride, bromide, iodide and the like are used.

Furthermore, a plurality of addition methods may be combined, or different addition methods may be used in combination for each additive element. For example, when soda lime glass is used for the substrate, sodium ions diffuse from the substrate during CIGS film formation, and sodium can be added to CIGS. Therefore, a lithium or potassium-containing compound layer is formed on the back electrode or CIGS precursor layer and heat-treated in a selenium atmosphere, thereby diffusing sodium from the soda lime glass substrate and lithium or potassium to lithium or potassium. By diffusing from the containing compound layer, a CIGS layer to which sodium and lithium or potassium are added can be formed.
As a method for adding the sodium-containing compound and the lithium-containing compound or the potassium-containing compound, the method described in Non-Patent Document 1 can be applied.

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 light weight, a metal substrate is 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) p. 1286” that moisture is retained inside a solid. As is 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 layers other than the above of the photoelectric conversion element of the present invention can be appropriately provided by a known method.
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 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 of this invention can be preferably used for a solar cell etc. 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 photoelectric conversion element of the present invention will be described in more detail with reference to examples.

(Preparation of coating solution)
Lithium source (Nissan Chemical, trade name: lithium silicate 45), potassium source (Fuji Chemical, trade name: 2K potassium silicate), sodium source (Showa Chemical, trade name: No. 1 sodium silicate, No. 3 sodium silicate and Tosoh Sangyo Co., Ltd., trade name: No. 4 sodium silicate) and water were mixed at a mass ratio shown in Table 1 to prepare a coating solution. In addition, the mass ratio of SiO ₂ / Li ₂ O / water of the lithium silicate 45 is 20.1% / 2.3% / 77.7%, and the mass ratio of SiO ₂ / K ₂ O / water of potassium 2 is It was 20.9% / 9.0% / 70.1%, and the mass ratio of SiO ₂ / Na ₂ O / water content of the sodium source is shown in Table 2.

(Examples 1-4 and Comparative Examples 1-3)
The coating solution prepared above was applied onto a 3 cm square alkali-free glass substrate as shown in Table 3, and 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. 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.

(Examples 5 and 6)
On a 3 cm square non-alkali glass substrate, Mo was formed to a thickness of 800 nm by DC sputtering. The coating liquid A or B shown in Table 3 was applied and dried at 100 ° C. Thereafter, a precursor composed of a Cu—Ga alloy layer and an In layer was formed by DC sputtering. Furthermore, the substrate was heat-treated in a H ₂ Se atmosphere at a maximum temperature of 500 ° C. to form a CIGS thin film having a thickness of about 1.8 μm. 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.

(Examples 7 and 8)
On a 3 cm square non-alkali glass substrate, Mo was formed to a thickness of 800 nm by DC sputtering. A precursor made of a Cu—Ga alloy layer and an In layer was formed by DC sputtering. Then, the coating liquid A or B shown in Table 3 was applied and dried at 100 ° C. Furthermore, the substrate was heat-treated in a H ₂ Se atmosphere at a maximum temperature of 500 ° C. to form a CIGS thin film having a thickness of about 1.8 μm. 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.

(Measurement of alkali metal ion concentration)
About the photoelectric conversion element of an Example and a comparative example, the alkali metal ion density | concentration of the photoelectric converting layer (CIGS layer) was measured. The 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. The alkali metal ion concentration in the photoelectric conversion layer (CIGS layer) has a distribution in the thickness direction, but is integrated to derive an average value, and this average value is used for evaluation of the alkali metal ions.

  Table 4 shows the measurement results of the types of coating solutions, sodium concentration, lithium or potassium concentration, and power generation efficiency in Examples and Comparative Examples.

  As shown in Table 4, the power generation efficiency was low in Comparative Examples 2 and 3 that did not contain Na ions and contained only Li ions or K ions. On the other hand, in Examples 1 to 8 containing Li ions or K ions in addition to Na ions, the power generation efficiency was significantly increased. In the example containing Li ion or K ion in addition to Na ion and Comparative Example 1 containing only Na ion but not Li ion or K ion, the concentration is almost the same when comparing only Na ion, The presence of lithium ions or K ions greatly contributes to the improvement of photoelectric conversion efficiency even though the content of Li ions or K ions is lower than that of Na ions and the amount of Li ions or K ions is very small. I understand.

1 Photoelectric Conversion Element 10 Substrate 20 Anodized Film 30 Alkali Supply 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 (10)

  A photoelectric conversion element in which a lower electrode, a photoelectric conversion semiconductor layer, and an upper electrode are sequentially stacked on a substrate, wherein a main component of the photoelectric conversion semiconductor layer is composed of at least an Ib group element, an IIIb group element, and a VIb group element A photoelectric conversion element, which is a compound semiconductor having one kind of chalcopyrite structure, wherein the photoelectric conversion semiconductor layer contains lithium ions or potassium ions and sodium ions. 2. The photoelectric conversion element according to claim 1, wherein each content of lithium ions or potassium ions and sodium ions contained in the photoelectric conversion semiconductor layer is 1 × 10 ¹⁵ atoms / cm ³ or more. The photoelectric conversion element according to claim 2, wherein the content is 1 × 10 ¹⁷ atoms / cm ³ or more.   The lithium ion or potassium ion and sodium ion contained in the photoelectric conversion semiconductor layer are supplied from an alkali supply layer formed between the substrate and the lower electrode. The photoelectric conversion element according to 1, 2 or 3. The main component of the photoelectric conversion semiconductor layer is
At least one group Ib element selected from the group consisting of Cu and Ag;
At least one group IIIb element selected from the group consisting of Al, Ga and In;
5. The photoelectric conversion element according to claim 1, wherein the photoelectric conversion element is at least one compound semiconductor comprising at least one VIb group element selected from the group consisting of S, Se, and Te. .   The photoelectric conversion element according to claim 1, wherein the substrate is a metal substrate.   The photoelectric conversion element according to claim 6, wherein an anodized aluminum film is formed on the surface of the metal substrate.   8. The photoelectric conversion element according to claim 7, 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 according to claim 8, wherein the anodized aluminum film is a porous type anodized aluminum film, and the porous type anodized aluminum film has a compressive stress.   A solar cell comprising the photoelectric conversion element according to claim 1.