JP2020186454A - Artificial photosynthetic cell - Google Patents

Abstract

An artificial photosynthesis cell using a solar cell capable of improving the conversion efficiency of artificial photosynthesis is provided. A dual-junction thin-film solar cell 10 having a bandgap of 1.1 to 2.0 eV/1.8 to 2.4 eV, a positive electrode 12 as an oxidation reaction electrode, and a negative electrode 14 as a reduction reaction electrode. is an artificial photosynthesis cell 1 comprising: [Selection drawing] Fig. 5

Description

The present invention relates to an artificial photosynthesis cell.

Using solar energy, water (H ₂ O) to hydrogen (H ₂ ), water (H ₂ O) and carbon dioxide (CO ₂ ) to carbon monoxide (CO), formic acid (HCOOH), methanol (CH ₃ ) Research on artificial photosynthesis that synthesizes OH) and the like is extremely active. In the latter case, H ₂ O is oxidized to supply electrons and protons to CO ₂ . The oxidation potential of _{H 2} O in the vicinity of pH 7 0.82 V, the reduction potential is -0.41V (both NHE) (see Non-Patent Document 1). The reduction potentials from CO ₂ to CO, HCOOH, and CH ₃ OH are −0.53V, −0.61V, and −0.38V, respectively. Therefore, the potential difference between the oxidation potential and the reduction potential is 1.20 to 1.43 V.

Material research for artificial photosynthesis began with a semiconductor photocatalyst whose upper end of the valence band is shallower than 0.82 V and whose lower end of the conduction band is deeper than the reduction potential. However, since only a small part of the solar spectrum can be used due to the wide bandgap, instead of a single photocatalytic material, the positive electrode and negative electrode of a solar cell based on a semiconductor pn junction are used as catalysts and reductions for oxidation reaction. Fig. 2 of Non-Patent Document 2 carrying a catalyst for the reaction. 3 (B), Fig. Research on integrated artificial photosynthesis cells as shown in 2 (A) is expanding (see Non-Patent Documents 2 to 4). However, in this case, since the element is immersed in water, as will be described later (see FIG. 2), a loss due to light absorption of water occurs. As shown in Non-Patent Document 5, one method of avoiding this is to package the solar cell and the one filled with the electrolytic solution while the oxide electrode and the reducing electrode are opposed to each other, including the electrical wiring. This is a type artificial photosynthesis cell (see Non-Patent Document 5).

As described above, the potential difference between the oxidation potential and the reduction potential is 1.20 to 1.43 V, and an overvoltage is required to actually cause the synthesis reaction. Therefore, the operating voltage is about 2.0 to 2.4 V. Is required (see FIG. 1). Since a multi-junction solar cell has a plurality of elements connected in series, a high voltage can be obtained. In Non-Patent Documents 2 to 5, a 3-junction amorphous silicon solar cell is used. However, since the conversion efficiency of the 3-junction amorphous silicon-based solar cell is at most 14% (see Non-Patent Document 6), the conversion efficiency of artificial photosynthesis using this is at most about 1/2 of this.

S. N. Habisreutinger, L. Schmidt-Mende, and J. K. Stolarczyk, Angeandte Chemie Int. Ed. 52, 7372 (2013). S. Y. Reece, J. A. Hamel, K. Sung, T. D. Jarvi, A. J. Esswein, J. J. H. Pijpers, and D. G. Nocera, Science 334, 645 (2011). T. Arai, S. Sato, and T. Morikawa, Energy Environ. Sci. 8, 1998 (2015). S. Sato, T. Arai, and T. Morikawa, Nanotechnology 29, 034001 (2018). J.-P. Becker, B. Turan, V. Smirnov, K. Welter, F. Urbain, J. Wolff, S. Haas, and F. Finger, J. Mater. Chem. A 5, 4818 (2017). M. A. Green, Y. Hishikawa, E. D. Dunlop, D. H. Levi, J. Hohl-Ebinger, M. Yoshita, A. W. Y. Ho-Baillie, Prog. Photovolt: Res. Appl. 27, 3 (2018). W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961). L. C. Hirst and N. J. Ekins-Daukes, Prog. Photovolt. Res. Appl. 19, 286 (2011). P. Wurfel, J. Phys. C 15, 3967 (1982). National Renewable Energy Laboratory Home Page "Reference Solar Spectral Irradiance: Air Mass 1.5" S. Kedenburg, M. Vieweg, T. Gissibl, and H. Giessen, Opt. Mat. Express 2, 1588 (2012). F. Urbain, V. Smirnov, J.-P. Becker, A. Lambertz, F. Yang, J. Ziegler, B. Kaiser, W. J, U. Rau, and F. Finger, Energy Environ. Sci. 9 , 145 (2016). G. E. Eperon, et al., Science 354, 861 (2016). B. Zhao, et al., Adv. Mater. 29, 1604744 (2017). J. H. Noh, et al., Nano Lett. 23, 1764 (2013). G. E. Eperon, et al., Energy Environ, Sci, 7, 982 (2014). J. Lopez-Garcia, et al., Thin Solid Films 517, 2240 (2009). J. Lopez-Garcia, et al., Solar Energy Mater. Solar Cells 94, 1263 (2010). J.-C. Park, et al., Opt. Mater. Express 6, 3541 (2016). M. Morihama, et al., Jpn. J. Appl. Phys 53, 04ER09 (2014). M. Umehara, et al., Solar Energy Mater. Solar Cells 134, 1 (2015). M. Umehara, et al., J. Alloys Compd. 689, 713 (2016). X. Zheng, et al., J. Alloys Compd. 738, 484 (2018).

An object of the present invention is to provide an artificial photosynthesis cell using a solar cell, which can improve the conversion efficiency of artificial photosynthesis.

The present invention includes a two-junction thin-film solar cell having a bandgap of 1.1 to 2.0 eV / 1.8 to 2.4 eV, a positive electrode for an oxidation reaction, and a negative electrode for a reduction reaction. , An artificial photosynthesis cell.

In the artificial photosynthesis cell, the two-junction thin-film solar cell, bottom _{cell, (FA 1-x MA 1} -x 'Cs x + x') (Pb 1-y Sn y) I, 0 ≦ x + x '≦ 1,0 ≦ It has a composition of y ≦ 1, and the top cell is (FA _1-x MA _1- x'Cs _{x + x'} ) (I _1-z Br _z ) ₃ , 0 ≦ x + x'≦ 1, 0.4 ≦ z ≦ It is preferable to have a photoelectric conversion layer composed of an organic-inorganic hybrid perovskite having the composition of 1.

In the artificial photosynthesis cell, in the two-junction thin-film solar cell, the bottom cell has a composition of Cu (In _1-x Ga _x ) Se ₂ , 0 ≦ x ≦ 1, and the top cell has a Cu (In _{1-x). 'Al x') Se} 2, preferably has a photoelectric conversion layer configured to include a CIGS-based compound having a composition of 0.9 ≦ x '≦ 1.

In the artificial photosynthesis cell, the two-junction thin-film solar cell, the bottom cell _has a composition of _{Cu 2 ZnSn (S 1-x} Se x) 4, 0 ≦ x ≦ 1, the top _cell, Cu 2 Zn (Sn _{1-x 'Ge x')} S 4, it is preferable to have a photoelectric conversion layer configured to include a CGTS based compound having a composition of 0.6 ≦ x '≦ 1.

In the artificial photosynthesis cell, the two-junction thin-film solar cell, the bottom cell _has a composition of _{Cu 2 Zn (Sn 1-x} Ge x) S 4, 0 ≦ x ≦ 0.8, the top cell, Cu ₂ It is preferable to have a photoelectric conversion layer composed of Zn (Sn _1- x'Ge _x' ) S ₄ , 0.6 ≦ x'≦ 1, but containing a CGTS-based compound having a composition of x ′ <x.

In the artificial photosynthesis cell, the two-junction thin-film solar cell, bottom _{cell, Cu 2 Zn (Sn 1-} x Ge x) Se 4, has a composition of 0.3 ≦ x ≦ 1.0, the top cell, It is preferable to have a photoelectric conversion layer composed of Cu ₂ Zn (Sn _1- x'Ge _x' ) S ₄ and a CGTS-based compound having a composition of 0.6 ≦ x'≦ 1.

In the artificial photosynthesis cell, the two-junction thin-film solar cell, bottom _{cell, Cu 2 (Sn 1-x} Ge x) S 3, having a composition of 0.3 ≦ x ≦ 1.0, the top cell, Cu It is preferable to have a photoelectric conversion layer composed of ₂ Zn (Sn _1- x'Ge _x' ) S ₄ , a CGTS-based compound having a composition of 0.6 ≦ x'≦ 1.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide an artificial photosynthesis cell using a solar cell, which can improve the conversion efficiency.

(A) Current density-potential curve of IrOx anode / Pt, (b) Current density-potential curve of Ru complex + carbon paper cathode / Pt, (c) Current-voltage curve of IrOx anode / Ru complex + carbon paper cathode It is a graph which shows. It is a graph which shows (a) the absorption coefficient of water, (b) the light transmittance up to a depth of 1 cm and 10 cm. It is a figure which shows the influence of the band gap (ε _g1 , ε _g2 ) of the top cell and the bottom cell which affect the characteristic of a thin film polycrystalline two-junction element. It is a graph which shows the characteristic of a thin film polycrystalline two-junction element. (A) to (j) are the results of the bandgap elements shown in Table 7, respectively. It is a figure which shows the schematic structure of an example of the facing type artificial photosynthesis cell which concerns on embodiment of this invention. It is a figure which shows the schematic structure of an example of the integrated artificial photosynthesis cell which concerns on embodiment of this invention. It is a figure which shows the schematic structure of an example of the integrated artificial photosynthesis cell using the mutual penetration electrode which concerns on embodiment of this invention.

Embodiments of the present invention will be described below. The present embodiment is an example of carrying out the present invention, and the present invention is not limited to the present embodiment.

The artificial photosynthesis cell according to the embodiment of the present invention has a two-junction thin-film solar cell having a bandgap of 1.1 to 2.0 eV / 1.8 to 2.4 eV, a positive electrode as an electrode for oxidation reaction, and a reduction reaction. A negative electrode, which is an electrode for use, is provided.

Since the artificial photosynthesis cell according to the present embodiment can take out a large current while securing the potential difference required for the artificial photosynthesis reaction, the conversion efficiency of artificial photosynthesis is improved. This is because, by constructing the solar cell using a material having an appropriate bandgap, the operating point when combined with the oxide electrode and the reducing electrode roughly matches the maximum output operating point of the solar cell.

In artificial photosynthesis cell according to the present embodiment, 2-junction thin-film solar cell, bottom _{cell, (FA 1-x MA 1} -x 'Cs x + x') (Pb 1-y Sn y) I, 0 ≦ x + x '≦ 1 , 0 ≦ y ≦ 1 (bandgap 1.2 to 1.8 eV), and the top cell is (FA _1-x MA _1- x'Cs _{x + x'} ) (I _1-z Br _z ) ₃ It is preferable to have a photoelectric conversion layer composed of an organic-inorganic hybrid perovskite having a composition of 0 ≦ x + x ′ ≦ 1, 0.4 ≦ z ≦ 1 (bandgap 1.8 to 2.3 eV) (composition). For reference, see Non-Patent Documents 13 to 16).

In the artificial photosynthesis cell according to the present embodiment, in the two-junction thin-film solar cell, the bottom cell has a composition of Cu (In _1-x Ga _x ) Se ₂ and 0 ≦ x ≦ 1 (band gap 1.0 to 1.6 eV). The top cell contains a CIGS-based compound having a composition of Cu (In _1- x'Al _x' ) Se ₂ and 0.9 ≦ x'≦ 1 (bandgap 1.8 to 2.0 eV). It is preferable to have a photoelectric conversion layer composed of (see Non-Patent Documents 17 to 19 for the composition).

In artificial photosynthesis cell according to the present embodiment, 2-junction thin-film solar cell, bottom _{cell, Cu 2 ZnSn (S 1-} x Se x) 4, 0 composition ≦ x ≦ 1 (band gap 1.3~1.5eV ), And the top cell has a composition of Cu ₂ Zn (Sn _1- x'Ge _x' ) S ₄ , 0.6 ≦ x'≦ 1 (band gap 1.8 to 2.1 eV). It is preferable to have a photoelectric conversion layer composed of a compound (see Non-Patent Documents 20 to 23 for the composition).

In the artificial photosynthesis cell, the two-junction thin-film solar cell, bottom _{cell, Cu 2 Zn (Sn 1-} x Ge x) S 4, 0 ≦ composition of x ≦ 0.8 (bandgap 1.5~2.0eV ), And the top cell is Cu ₂ Zn (Sn _1- x'Ge _x' ) S ₄ , 0.6 ≤ x'≤ 1, but the composition of x'<x (bandgap 1.8 to 2.). It is preferable to have a photoelectric conversion layer composed of a CGTS-based compound having 1 eV) (see Non-Patent Documents 20 to 23 for the composition).

In the artificial photosynthesis cell, in the two-junction thin-film solar cell, the bottom cell has a composition of Cu ₂ Zn (Sn _1-x Ge _x ) Se ₄ , 0.3 ≦ x ≦ 1.0 (bandgap 1.1 to 1). .35 eV), and the top cell has a composition of Cu ₂ Zn (Sn _1- x'Ge _x' ) S ₄ , 0.6 ≦ x'≦ 1 (bandgap 1.8 to 2.1 eV). It is preferable to have a photoelectric conversion layer composed of a CGTS-based compound (see Non-Patent Documents 20 to 23 for the composition).

In the artificial photosynthesis cell, the two-junction thin-film solar cell, bottom _{cell, Cu 2 (Sn 1-x} Ge x) S 3, 0.3 ≦ x ≦ 1.0 composition (bandgap 1.1-1. CGTS having 7 eV) and a top cell having a composition of Cu ₂ Zn (Sn _1- x'Ge _x' ) S ₄ , 0.6 ≦ x'≦ 1 (bandgap 1.8 to 2.1 eV) It is preferable to have a photoelectric conversion layer composed of a system compound (see Non-Patent Documents 20 to 23 for the composition).

The structure of the artificial photosynthesis cell according to the present embodiment includes the above-mentioned two-junction thin-film solar cell having a bandgap of 1.1 to 2.0 eV / 1.8 to 2.4 eV, a positive electrode as an electrode for oxidation reaction, and a reduction reaction. It is not particularly limited as long as it includes a negative electrode which is a working electrode. The artificial photosynthesis cell according to the present embodiment is, for example, the above-mentioned two-junction thin-film solar cell having a band gap of 1.1 to 2.0 eV / 1.8 to 2.4 eV; a positive electrode as an electrode for oxidation reaction, and a reduction reaction. A negative electrode, which is an electrode for use, and an electrochemical reaction apparatus having the negative electrode;

In the artificial photosynthesis cell according to the present embodiment, for example, as shown in FIG. 5, an electrolytic solution is provided in a flow path 16 between a positive electrode (anode electrode) 12 and a negative electrode (cathode electrode) 14 facing each other in an accommodating portion 18. This is an opposed type artificial photosynthetic cell 1 in which an electrochemical reaction apparatus satisfying the above requirements and a two-junction thin film solar cell 10 arranged adjacent to a negative electrode (cathode electrode) 14 are packaged including electrical wiring. As shown in FIG. 6, the artificial photosynthesis cell according to the present embodiment is, for example, an integrated artificial photosynthesis cell having a positive electrode (anode electrode) 12 and a negative electrode (cathode electrode) 14 on both sides of a two-junction thin-film solar cell 10. It may be set to 3 and may be used by immersing it in the electrolytic solution in the container. Further, in the artificial photosynthesis cell according to the present embodiment, for example, as shown in FIG. 7, a mutual penetration electrode between a positive electrode (anode electrode) 12 and a negative electrode (cathode electrode) 14 is provided on one side of a two-junction thin film solar cell 10. Then, it is in the electrolytic solution (water) in the container 20 or on the electrolytic solution (water) surface (for example, the cell is floating on the electrolytic solution (water) surface, and the two-junction thin film solar cell 10 is in the electrolytic solution (water). Although it is not immersed, the positive electrode (anode electrode) 12 and the negative electrode (cathode electrode) 14 may be in contact with the electrolytic solution (water)) as the artificial photosynthesis cell 5.

The artificial photosynthesis cell 1 of FIG. 5 will be described below as an example. The electrochemical reaction apparatus of the artificial photosynthesis cell 1 is arranged in the accommodating portion 18 at a position where, for example, a positive electrode (anode electrode) 12 and a negative electrode (cathode electrode) 14, which are plate-shaped members, are separated from each other and face each other. Between the anode electrode 12 and the cathode electrode 14, there is a flow path 16 through which an electrolytic solution containing a reaction substrate such as carbon dioxide flows. An electric wire or the like is connected to the anode electrode 12 and the cathode electrode 14, and the potential is maintained at an appropriate potential.

The electrochemical reactor functions by introducing an electrolytic solution containing a reaction substrate such as carbon dioxide into the flow path 16 between the anode electrode 12 and the cathode electrode 14. In the anode electrode 12, for example, water (H ₂ O) is oxidized to obtain oxygen (1 / ₂ O ₂ ). In the cathode electrode 14, for example, carbon dioxide is reduced to produce formic acid (HOOOH) and the like.

The negative electrode (cathode electrode) 14, which is an electrode for a reduction reaction, is an electrode used for reducing a substance by a reduction reaction. The cathode electrode 14 is composed of, for example, a conductive layer and a conductor layer that are sequentially formed on the substrate.

The substrate is a member that structurally supports the electrodes. The material of the substrate is not particularly limited, and examples thereof include a glass substrate. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but for example, silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin ( Sn), palladium (Pd), lead (Pb) and the like can be mentioned. The semiconductor used as the substrate is not particularly limited, but is, for example, titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), zinc oxide (ZnO). , Tantalum oxide (Ta ₂ O ₅ ) and the like. When the substrate contains a metal or a semiconductor, it is preferable to form an insulating layer between the conductive layer and the conductor layer and the substrate. The insulating layer is not particularly limited, and examples thereof include semiconductor oxides, nitrides, and resins. The metal substrate and the conductive layer and the conductive layer may be directly electrically connected to each other.

The substrate may be composed of a carbon material because it has a high specific surface area and the like. The carbon material is not particularly limited, and examples thereof include carbon black, graphene, fullerene, carbon nanotube, carbon cloth, and carbon paper.

The conductive layer is provided to effectively collect current at the cathode electrode 14. The conductive layer is not particularly limited, and examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, considering thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

The conductor layer is composed of a conductor containing a material having a reduction catalyst function. The conductor can be composed of a material including a carbon material (C). It is preferable that the size of the carbon material structure alone is 1 nm or more and 1 μm or less. Examples of the carbon material include at least one of carbon nanotubes, graphene and graphite. For graphene and graphite, the size is preferably 1 nm or more and 1 μm or less. If it is a carbon nanotube, it is preferable that the diameter is 1 nm or more and 40 nm or less. The conductor can be formed, for example, by applying a carbon material mixed with a liquid such as ethanol by spraying and heating the conductor. Instead of spraying, it may be applied by spin coating. Alternatively, the solution may be directly dropped, dried and applied without using spin coating.

As a material having a reduction catalyst function, a complex catalyst or the like can be used. The complex catalyst is preferably, for example, a ruthenium complex. Complex catalysts include, for example, [Ru {4,4'-di (1-H-1-pyrrolipropyl carbonate) -2,2'-bipyridine} (CO) (MeCN) Cl ₂ ], [Ru {4,4'. -Di (1-H-1-pyridypropyl carbonate) -2,2'-bipyridine} (CO) ₂ Cl ₂ ], [Ru {4,4'-di (1-H-1-pyridyl carbonate) -2, 2'-bipyridine} (CO) ₂ ] _n , [Ru {4,4'-di (1-H-1-pyrrolipropyl carbonate) -2,2'-bipyridine} (CO) (CH ₃ CN) Cl ₂ ] And so on.

Modification with a complex catalyst can be produced, for example, by applying a solution of the complex in an acetonitrile (MeCN) solution onto the conductor of the conductor layer. Further, the modification with a complex catalyst can also be performed by an electrolytic polymerization method. For example, an electrode of a conductor in a conductor layer is used as a working electrode, a glass substrate coated with fluorine-containing tin oxide (FTO) is used as a counter electrode, and an Ag / Ag ⁺ electrode is used as a reference electrode, and Ag / is used in an electrolytic solution containing a complex catalyst. A cathode current is passed through the Ag ⁺ electrode so that it becomes a negative voltage, and then an anode current is passed through the Ag / Ag ⁺ electrode so that it becomes a positive potential, so that the conductor of the conductor layer is covered with a complex catalyst. Can be modified. For the solution of the electrolyte, for example, acetonitrile (MeCN) can be used, and for the electrolyte, for example, Tetrabutylammioniumperchlate (TBAP) can be used.

The conductor layer thus formed is supported, coated or affixed on the conductive layer constituting the cathode electrode 14. As a result, the cathode electrode 14 having the conductive layer and the conductive layer in this order is formed on the substrate.

The positive electrode (anode electrode) 12, which is an electrode for an oxidation reaction, is an electrode used for oxidizing a substance by an oxidation reaction. The anode electrode 12 is composed of, for example, a conductive layer and an oxidation catalyst layer which are sequentially formed on the substrate.

The substrate is a member that structurally supports the electrodes. The substrate can be made of the same material as the substrate used for the cathode electrode 14.

The conductive layer is provided to effectively collect current at the anode electrode 12. The conductive layer is not particularly limited, and examples thereof include indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, considering thermal and chemical stability, it is preferable to use fluorine-doped tin oxide (FTO).

The oxidation catalyst layer is composed of a material having an oxidation catalyst function. Examples of the material having an oxidation catalyst function include a material containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of the conductive layer as a nanocolloidal solution (see T. Arai et.al, Energy Environ. Sci 8, 1998 (2015)).

For example, iridium oxide (IrOx) nanocolloids are synthesized. Next, a yellow solution adjusted to pH 13 by adding a 10 wt% sodium hydroxide (NaOH) aqueous solution to 50 mL of a 2 mM potassium iridium (IV) chloride (K ₂ IrCl ₆ ) aqueous solution was prepared at 90 ° C. using a hot stirrer. Heat for 20 minutes. The blue solution thus obtained is cooled with ice water for 1 hour. Then, 3M nitric acid (HNO ₃ ) is added dropwise to the cooled solution (20 mL) to adjust the pH to 1, and the mixture is stirred for 80 minutes to obtain a nanocolloidal aqueous solution of iridium oxide (IrOx). Further, a 1.5 wt% NaOH aqueous solution (1 to 2 mL) is added dropwise to this solution to adjust the pH to 12. The nanocolloidal aqueous solution of iridium oxide (IrOx) thus obtained is applied onto the conductive layer at pH 12, and held in a drying oven at 60 ° C. for 40 minutes for drying. After drying, the precipitated salt can be washed with ultrapure water to form an anode electrode. The application and drying of the nanocolloidal aqueous solution of iridium oxide (IrOx) may be repeated a plurality of times.

Examples of the reaction substrate contained in the electrolytic solution include carbon compounds, and for example, carbon dioxide (CO ₂ ). Further, the electrolytic solution is preferably a phosphate buffered aqueous solution or a boric acid buffered aqueous solution. In a specific configuration example, for example, a tank of a carbon dioxide (CO ₂ ) saturated phosphate buffer solution is provided, and this solution is supplied to the surfaces of the negative electrode and the positive electrode by a pump, and formic acid (HCOOH) and oxygen generated by the reduction reaction are generated. (O ₂ ) etc. are collected in an external fuel tank.

The accommodating portion 18 is a member that supports the anode electrode 12 and the cathode electrode 14 and constitutes a flow path 16 through which the electrolytic solution flows. The housing 18 is made of a material having the mechanical strength required to form the electrochemical reactor as a cell. For example, the accommodating portion can be made of metal, plastic, or the like.

A separator may be provided between the anode electrode 12 and the cathode electrode 14. The separator may be a material that separates liquid and gas and allows protons to move, and is not particularly limited. For example, Nafion (registered trademark), which is a solid polymer electrolyte membrane, may be used. it can.

By connecting the two-junction thin-film solar cell between the anode electrode 12 and the cathode electrode 14, an appropriate bias voltage may be applied. An artificial photosynthetic cell comprising an electrochemical reactor by using a solar cell as a means of applying a bias voltage, and a solar cell that generates power to be supplied to the anode electrode 12 and the cathode electrode 14 of the electrochemical reactor. can do. The two-junction thin-film solar cell 10 can be arranged adjacent to the anode electrode 12 and the cathode electrode 14 of the electrochemical reactor, for example. For example, as shown in FIG. 5, a 2-junction thin-film solar cell 10 is arranged on the back surface of the cathode electrode 14 of the electrochemical reactor, the positive electrode of the 2-junction thin-film solar cell 10 is connected to the anode electrode 12, and the 2-junction thin-film solar cell 10 is connected. The negative electrode of the battery 10 may be connected to the cathode electrode 14.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

Using the experimental results of the current-voltage characteristics of the anode electrode / cathode electrode and the current-voltage characteristic models of the 2-junction thin-film solar cell (Example) and the 3-junction amorphous silicon-based solar cell (Comparative Example), these are combined. The operating current when used on or under water was calculated, and the optimum value of the band gap of the solar cell material was obtained. At this time, the AM1.5G solar spectrum was used, and light absorption by water was taken into consideration in the case of underwater use.

In the opposed cell, the current density of the amorphous silicon-based 3-junction element (operating current 6.5 mA / cm ^{2 at} a voltage of 2.0 V, the same applies hereinafter) is compared with the 2-junction thin-film solar cell (bandgap 1.42 / 1. When 96 eV and an operating current of 13.6 mA / cm ² ) were used, a larger operating current was obtained.

In the integrated cell, the current density of the amorphous silicon-based 3-junction element (operating current 6.4, 5.9 mA / cm ^{2 at} a water depth of 1,10 cm and a voltage of 2.0 V, respectively, the same applies hereinafter) is compared with the 2-junction thin-film solar cell. Use a battery (band gap 1.42 / 1.97 eV at water depth 1 cm, operating current 13.5 mA / cm ² ; band gap 1.42 / 2.01 eV at water depth 10 cm, operating current 12.7 mA / cm ² ). And a larger operating current was obtained.

The details of the calculation method and the result are as follows.

A solar cell using an organic-inorganic hybrid perovskite, CdS, CuInSe ₂ (CIGS system) or the like has obtained a high conversion efficiency of about 20%. Therefore, assuming that an artificial photosynthesis cell is constructed using a two-junction solar cell using these materials, an appropriate band gap of the solar cell material and a reaction current obtained at that time were determined.

[Method of calculation]
As mentioned above, a voltage of about 2.0 to 2.4 V is required for the operation of artificial photosynthesis. An example is shown in FIG. FIGS. 1 (a) and 1 (b) were measured in a combination of IrOx anode and Ru complex + carbon paper + carbon nanotube cathode with Pt counter electrode and bubbling CO ₂ in 0.4 M phosphate buffer electrolytic solution, respectively. It is a current density-potential curve. FIG. 1 (c) shows the result of converting these into a current density (J) -voltage (V) curve when both are connected via a DC power supply. Using this, a configuration of a light charge separation element is obtained so that a voltage as large as possible can be obtained in the range of 2.0 to 2.4 V. In all of the cells of Non-Patent Documents 2, 3 and 5, it is assumed that the area of the solar cell and the anode / cathode are the same. The case where these are not equal will be described at the end.

The JV of an ideal single-junction solar cell is represented by the following equation (1) (see Non-Patent Documents 7 and 8).

(1)

Here, ε _g is the band gap of the light absorbing material in the element, n _sun (ε) is the number of photons per unit photon energy of sunlight, and T is the temperature. q, h, c, and k _B are elementary charges, Planck's constant, speed of light in vacuum, and Boltzmann's constant, respectively. The first term on the right side represents the generation of carriers due to light absorption, and it is assumed that all photons with energies above the bandgap are absorbed and lead to carrier generation. When a voltage is applied to the element, it operates as an LED and thus emits light, which is a loss for the purpose of photovoltaic power generation. Here, Planck's law, which expresses radiation from a blackbody in a thermal equilibrium state, is extended to the case of semiconductors and voltage application, and generalized Planck's law is used (see Non-Patent Document 9). The power density P of the element is P = J × V, and the ratio of the maximum value of P as a function of V to the incident light energy is the conversion efficiency η. Since only the loss due to light emission (radiative recombination) that cannot be eliminated in principle from the power generation mechanism is considered in the equation (1), the conversion efficiency obtained from this equation is called "radiative limit". ..

In reality, there are various losses other than radiation recombination, and even photons with higher energies than the bandgap are not partially absorbed by the generation layer (reflection and transmission). Realistic effects that reduce the characteristics of these devices are incorporated as follows. Bose-Einstein distribution function in equation (1) is approximated by a Boltzmann distribution function, further use of the relation of k _{B T} << ε _g, and are approximated by the equation (1) This expression (2) ..

(2)

The following equation (3) is used to represent a realistic device operation.

(3)

A in the first term on the right side of the equation (3) indicates that the number of absorbed photons is smaller than in the ideal case. B in the second term is a proportional coefficient representing the magnitude of the recombination loss including the non-radiative process. _Tv is a virtual temperature that determines the voltage dependence of the recombination loss, and usually corresponds to the n value indicating the characteristics of the diode. The characteristics of the two-junction element are based on the assumption that the band gaps of the bottom cell and top cell are ε _g1 and ε _g2 , respectively.
(4a)
(4b)
More demanded.

To represent the characteristics of the thin film solar cell, the formula (3) was calculated by substituting _{A = 0.92, B = 5,} T v = 500K to (4). The standard conditions of AM1.5G and 100mW / cm ² were used for the sunlight spectrum and intensity (see Non-Patent Document 10). The calculation results are summarized in Table 1 together with the characteristics of the device having the highest conversion efficiency in the world (see Non-Patent Document 6). Since it was confirmed that they coincide roughly, this A, B, and calculating the characteristic of the two-junction solar cell using a T _v. Table 1, the maximum value and properties (each column top) (see Non-Patent Document 6) at that time of the conversion efficiency of a thin-film solar cells, Equation (3) (A = 0.92, B = 5, T v = 500K) shows a comparison of the characteristics (lower part of each column).

Fig. of Non-Patent Document 2. 3 (B), Fig. Since the integrated cell such as 2 (A) is used in water as described above, the absorption of sunlight by water must be considered. FIG. 2A is an absorption spectrum of water (see Non-Patent Document 11). Visible light with a wavelength of 700 nm or less is hardly absorbed, but the effect of absorption becomes remarkable at 700 nm or more. The transmittance that penetrates water and reaches the integrated cell changes depending on the water depth. Here, assuming that a small element is installed in a shallow place as much as possible, and a large element of about 1 m is installed so that it is below the water surface even if it is slightly tilted or receives wind, the water depth is 1 cm, respectively. Set to 10 cm, and multiply the ² spectra of AM1.5G, 100 mW / cm by the transmittance of FIG. 2 (b) (light transmittance up to a water depth of 1 cm and a water depth of 10 cm) to obtain n in equations (3) and (4). Used for _sun (ε).

In the case of amorphous silicon-based 3-junction elements used for integrated cells and opposed cells (see Non-Patent Documents 1 to 5), the difference between the actually obtained characteristics and the characteristics of "radiative limit" is further increased. large. Fig. 12 of Non-Patent Document 12 describes the characteristics of an amorphous silicon / amorphous silicon / microcrystalline silicon 3-junction element having a maximum output voltage of more than 2 V. It is shown in 4 and 6. In this case, since the diffusion length of the carrier is shorter than that of the crystal, the thickness of each junction is restricted. As a result, the light absorption rate on the long wavelength side gradually decreases, so that the quantum efficiency also decreases. Originally, the formula of JV characteristic that expresses the characteristic of each junction should be derived, and the influence of light absorption of water should be calculated based on it. However, in the case of an amorphous solar cell, the effect of carrier drift caused by the potential difference formed by the pin structure is large, so that the JV characteristic cannot be expressed by the equation (3). On the other hand, Fig. If the element having the quantum efficiency spectrum shown in 4 is used as it is in water, only the current of the bottom cell is reduced due to the influence of the long wavelength component of the incident light being absorbed by water, so that the current matching condition is not satisfied and the efficiency is improved. Although it decreases significantly, this decrease can be easily eliminated by adjusting the thickness of the top and bottom cells. Therefore, the current value J when installed on water is determined by Fig. Read from the JV characteristics shown in 6, and in the case of underwater, the light transmittance of water in FIG. 2 (b) and Fig. The number of absorbed photons was obtained from the quantum efficiency spectrum of 4, and an approximate value was obtained assuming that J was proportional to this.

[Results and Discussion]
The calculation results of conversion efficiency and current value and the optimum value of the band gap are shown below. The installation state of the element, which is used for the facing cell and is not affected by the light absorption of water, is called on the water or at a depth of 0 cm in order to show the comparison with the case of the integrated cell installed in the water. To do.

<Comparison example>
(Amorphous silicon-based 3-junction element)
Table 2 summarizes the characteristics obtained when the amorphous silicon-based 3-junction element is optimized. Conversion efficiency η (ratio of maximum power density P to incident light intensity. Since the incident light energy is 100 mW / cm ² , the value of η (%) directly matches the maximum value of P (mW / cm ² ). The current density J at a voltage V = 2.0, 2.2, 2.4 V, which is a measure of the performance of the photocharge separation element for artificial photosynthesis, is shown. The blanks in Table 2 indicate that the current density J is negative.

The conversion efficiency η is η = 13.6%. Although the current density J in V = 2V is J = 6.5mA / cm ^2, reduced to V = 2.2V In J = 2.9mA / cm ^2, it is impossible to obtain a further V = 2.4V ..

<Example>
(2-junction thin-film solar cell)
Next, consider the conditions for obtaining V = 2.0 to 2.4V using a two-junction element. FIG. 3 shows the calculation results of the two-junction element using A = 0.92, B = 5, Tv = 500K shown above. FIG. 3 shows the influence of the band gaps (ε _g1 , ε _g2 ) of the top cell and the bottom cell on the characteristics of the thin film polycrystalline two-junction device. The contour lines are drawn every 1% of conversion efficiency (η) or 1 mA / cm ^{2 of} current density (J), and the values of η and J of the innermost contour lines are shown in the figure. FIG. 4 shows the characteristics of the thin film polycrystalline two-junction device. (A) to (j) are the results of the bandgap elements shown in Table 3, respectively. Table 3 shows the characteristics of the thin film polycrystalline 2-junction device. The bandgap was optimized so that the value of the * item in the table was maximized. The blanks in Table 3 indicate that the current density J is negative.

In FIG. 3A, the conversion efficiency η of the element on the water is shown as a function of the band gaps ε _g1 and ε _g2 of the bottom cell and the top cell. The contour ridges near the diagonals in the figure are the current matching conditions for the bottom cell and top cell, that is, the conditions under which the generated current densities match, or approximately the conditions under which the number of photons absorbed by both matches. Since the lower right region is ε _g1 > ε _g2 , all the incident light is absorbed by the top cell and does not reach the bottom cell, so no power is generated. The maximum value of η is η = 28.7%, which is obtained when ε _g1 = 1.12 eV and ε _g2 = 1.74 V (Table 3). As can be seen from the JV curve (FIG. 4A) at this time, the short-circuit current density is 19.7 mA / cm ² .

The result of calculating the current density J at V = 2.0 to 2.4 V when there is no influence of water absorption on the assumption that it is used for a facing cell as in Non-Patent Document 5 is shown in FIG. 3 (b). ~ (D). Table 3 summarizes the combinations of band gaps that maximize J for each V, and J and power density P at that time. The JV curves for each are shown in FIG. In FIGS. 3B to 3D, when the open end voltage is less than 2V, it is displayed as J = 0. In the region where ε _g2 is large, J does not depend on ε _g1 because J is limited by ε _g2 . The lower left is the region where the open end voltage is less than V = 2.0 to 2.4V. The current density J at V = 2.0V is maximized than when ε _g1 = 1.42 eV and ε _g2 = 1.96 V, that is, when η shown above is optimized to be maximum. When it is 0.2 to 0.3 eV larger, the maximum value is J = 13.6 mA / cm ² . This is more than twice the value of 6.5 mA / cm ² when an amorphous silicon-based 3-junction element is used. The conversion efficiency of the solar cell at this time is 27.2%, which is lower than the value (28.7%) when optimized to maximize the conversion efficiency shown in FIG. 3 (a). , It can be said that this is more suitable when used for artificial photosynthesis reaction. The calculation results of J for V = 2.2 and 2.4V are shown in FIGS. 3 (c) and 3 (d), respectively. The maximum values of J are J = 11.6 and 9.9 mA / cm ² , respectively, and they do not decrease as much as in the case of the amorphous silicon-based 3-junction element, so the superiority compared to the amorphous silicon-based 3-junction element is greater. ..

Fig. of Non-Patent Document 2. 3 (B), Fig. The results when the water depth is 1 cm are shown in FIGS. 3 (e) to 3 (g), Table 3 and FIG. 4, assuming that the cell is used for an integrated cell such as 2 (A). As shown in Table 3, the bandgap of the bottom cell optimized for water use is 1.42 / 1.65 eV (873 to 751 nm), so that the effect of water absorption is small even if it is used as it is. .. Therefore, the optimum value of the band gap and its characteristics when the water depth is 1 cm are almost the same, and excellent characteristics are maintained as compared with the amorphous silicon-based 3-junction element.

The results when the water depth is 10 cm are shown in FIGS. 3 (h) to (j), Table 3 and FIG. In this case, since the influence of absorption of long-wavelength light by water appears, the current matching conditions change as compared with the case of water and water depth of 1 cm, the optimum value of the band gap is large, and J is small. However, since the amount of decrease in J (ratio of 10 cm / 0 cm) is smaller than that of the amorphous silicon-based 3-junction element, it can be said that the superiority is further increased.

So far, we have examined the case where the area of the solar cell and the anode / cathode are the same. The area ratio of the two can be changed according to the activity of the anode / cathode, and FIG. 7 shows an integrated artificial photosynthetic cell using a mutual penetration electrode. As shown in FIG. 7, the anode / on one side of the solar cell. Mutual penetration electrodes of the cathodes may be provided and may be in water or on the surface of the water (the cell is floating on the surface of the water and the solar cell is not immersed in the water, but the anode and the cathode are in contact with the water). In this case as well, the requirements for the potential difference between the anode and the cathode are the same as when the areas are the same. Therefore, by using a 2-junction thin-film solar cell having an appropriate bandgap instead of the 3-junction amorphous silicon solar cell, it is large. The operating current can be obtained.

As described above, it was found that an artificial photosynthesis cell using a solar cell, which can improve the conversion efficiency of artificial photosynthesis, can be obtained from the examples.

1,3,5 Artificial photosynthesis cell, 102-junction thin-film solar cell, 12 positive electrode (anode electrode), 14 negative electrode (cathode electrode), 16 flow paths, 18 accommodating parts, 20 containers.

Claims (7)

It is characterized by including a two-junction thin-film solar cell having a band gap of 1.1 to 2.0 eV / 1.8 to 2.4 eV, a positive electrode which is an electrode for an oxidation reaction, and a negative electrode which is an electrode for a reduction reaction. Artificial photosynthesis cell. The artificial photosynthesis cell according to claim 1.
The two-junction thin-film solar cell is
The bottom cell _has a composition of _{(FA 1-x MA 1-} x 'Cs x + x') (Pb 1-y Sn y) I, 0 ≦ x + x '≦ 1,0 ≦ y ≦ 1,
The top cell is an organic-inorganic hybrid perovskite having a composition of (FA _1-x MA _1- x'Cs _{x + x'} ) (I _1-z Br _z ) ₃ , 0 ≦ x + x'≦ 1, 0.4 ≦ z ≦ 1. An artificial photosynthesis cell characterized by having a photoelectric conversion layer comprising the above. The artificial photosynthesis cell according to claim 1.
The two-junction thin-film solar cell is
The bottom cell has a composition of Cu (In _1-x Ga _x ) Se ₂ , 0 ≦ x ≦ 1.
The artificial top cell is characterized by having a photoelectric conversion layer composed of a CIGS-based compound having a composition of Cu (In _1- x'Al _x' ) Se ₂ and 0.9 ≦ x'≦ 1. Photosynthesis cell. The artificial photosynthesis cell according to claim 1.
The two-junction thin-film solar cell is
Bottom _{cell, Cu 2 ZnSn (S 1-} x Se x) 4, 0 has the composition ≦ x ≦ 1,
The top cell is characterized by having a photoelectric conversion layer composed of a CGTS-based compound having a composition of Cu ₂ Zn (Sn _1- x'Gex _' ) S ₄ and 0.6 ≦ x'≦ 1. Artificial photosynthesis cell. The artificial photosynthesis cell according to claim 1.
The two-junction thin-film solar cell is
The bottom cell has a composition of Cu ₂ Zn (Sn _1-x Ge _x ) S ₄ , 0 ≦ x ≦ 0.8.
The top cell is a photoelectric conversion layer composed of Cu ₂ Zn (Sn _1- x'Ge _x' ) S ₄ , 0.6 ≤ x'≤ 1, but containing a CGTS-based compound having a composition of x'<x. An artificial photosynthetic cell characterized by having. The artificial photosynthesis cell according to claim 1.
The two-junction thin-film solar cell is
The bottom cell has a composition of Cu ₂ Zn (Sn _1-x Ge _x ) Se ₄ , 0.3 ≦ x ≦ 1.0.
The top cell is characterized by having a photoelectric conversion layer composed of a CGTS-based compound having a composition of Cu ₂ Zn (Sn _1- x'Gex _' ) S ₄ and 0.6 ≦ x'≦ 1. Artificial photosynthesis cell. The artificial photosynthesis cell according to claim 1.
The two-junction thin-film solar cell is
The bottom cell has a composition of Cu ₂ (Sn _1-x Ge _x ) S ₃ , 0.3 ≦ x ≦ 1.0.
The top cell is characterized by having a photoelectric conversion layer composed of a CGTS-based compound having a composition of Cu ₂ Zn (Sn _1- x'Gex _' ) S ₄ and 0.6 ≦ x'≦ 1. Artificial photosynthesis cell.