JP7215321B2 - artificial photosynthetic cell - Google Patents

Description

The present invention relates to artificial photosynthetic cells.

Hydrogen (H2) from water ( _H2O ) using solar energy, carbon monoxide (CO), formic _acid (HCOOH), methanol ( _CH3 ) from water ( _H2O ) and carbon dioxide ( _CO2 ) OH) and the like have been actively studied in artificial photosynthesis. In the latter case, _H2O is oxidized to provide electrons and protons to _CO2 . Around pH 7, H ₂ O has an oxidation potential of 0.82 V and a reduction potential of −0.41 V (both NHE) (see Non-Patent Document 1). The reduction potentials from CO ₂ to CO, HCOOH and CH ₃ OH are −0.53 V, −0.61 V and −0.38 V, respectively. Therefore, the potential difference between the oxidation potential and the reduction potential is 1.20-1.43V.

Materials research for artificial photosynthesis began with semiconductor photocatalysts with valence band tops shallower than 0.82 V and conduction band bottoms deeper than the reduction potential. However, since only a small portion of the solar spectrum can be utilized due to its wide bandgap, single photocatalyst materials have been proposed for the positive and negative electrodes of solar cells based on semiconductor pn junctions, respectively. FIG. 3(B), Fig. 3 of Non-Patent Document 3. Research on an integrated artificial photosynthetic cell as shown in 2(A) is expanding (see Non-Patent Documents 2 to 4). However, in this case, since the device is immersed in water, loss occurs due to light absorption by water, as will be described later (see FIG. 2). As shown in Non-Patent Document 5, one method of avoiding this problem is to package an oxidation electrode and a reduction electrode, which are filled with an electrolytic solution while facing each other, and a solar cell, including electrical wiring. It 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 necessary to actually cause the synthesis reaction, so the operating voltage is about 2.0 to 2.4 V. is required (see Figure 1). A multi-junction solar cell has a plurality of elements connected in series, so a high voltage can be obtained. Non-Patent Documents 2 to 5 use a 3-junction amorphous silicon solar cell. However, since the conversion efficiency of a 3-junction amorphous silicon solar cell is 14% at maximum (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 homepage "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 that can improve the conversion efficiency of artificial photosynthesis.

The present invention comprises a two-junction thin-film solar cell with a bandgap of 1.1 to 2.0 eV/1.8 to 2.4 eV, a positive electrode as an oxidation reaction electrode, and a negative electrode as a reduction reaction electrode. , an artificial photosynthetic cell with an operating voltage of 2.0-2.4V .

In the artificial photosynthesis cell, the bottom cell of the two-junction thin-film solar cell has (FA _1-x MA _1-x′ Cs _x+x′ ) (Pb _1-y Sn _y )I, 0≦x+x′≦1, 0≦ 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 comprising an organic-inorganic hybrid perovskite having a composition of 1.

In the artificial photosynthesis cell, the two-junction thin-film solar cell has a bottom cell of Cu(In _1-x Ga _x )Se ₂ having a composition of 0≦x≦1, and a top cell of Cu(In _1-x It is preferable to have a photoelectric conversion layer comprising a CIGS compound having a composition of _' Al _x' )Se ₂ and 0.9≦x'≦1.

In the artificial photosynthesis cell, the two-junction thin-film solar cell has a bottom cell of Cu ₂ ZnSn(S _1-x Se _x ) ₄ with a composition of 0≦x≦1, and a top cell of Cu ₂ Zn(Sn _1−x′ Ge _x′ )S ₄ and a photoelectric conversion layer containing a CGTS-based compound having a composition of 0.6≦x′≦1.

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

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

ADVANTAGE OF THE INVENTION By this invention, the artificial-photosynthesis cell using a solar cell which can improve conversion efficiency can be provided.

(a) IrOx anode/Pt current density-potential curve, (b) Ru complex + carbon paper cathode/Pt current density-potential curve, (c) IrOx anode/Ru complex + carbon paper cathode current-voltage curve. It is a graph showing. 1 is a graph showing (a) absorption coefficient of water and (b) light transmittance up to 1 cm and 10 cm water depth. FIG. 3 is a diagram showing the influence of top cell and bottom cell band gaps (ε _g1 , ε _g2 ) on characteristics of a thin-film polycrystal two-junction device. 4 is a graph showing characteristics of a thin film polycrystalline two-junction device; (a) to (j) are the results of devices with the bandgap shown in Table 7, respectively. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematic structure of an example of the facing type artificial-photosynthesis cell which concerns on embodiment of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematic structure of an example of the integrated artificial-photosynthesis cell which concerns on embodiment of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematic structure of an example of the integrated artificial-photosynthesis cell using the interpenetrating electrode which concerns on embodiment of this invention.

An embodiment of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

The artificial photosynthesis cell according to the embodiment of the present invention includes a two-junction thin-film solar cell with 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

The artificial-photosynthesis cell according to the present embodiment can extract a large current while ensuring the potential difference necessary for the artificial-photosynthesis reaction, so that the conversion efficiency of artificial-photosynthesis is improved. This is because by constructing the solar cell using materials with an appropriate bandgap, the operating point in combination with the oxidizing and reducing electrodes approximately matches the maximum power operating point of the solar cell.

In the artificial photosynthesis cell according to this embodiment, the bottom cell of the two-junction thin-film solar cell has (FA _1-x MA _1-x' Cs _x+x' ) (Pb _1-y Sn _y )I, 0 ≤ x + x' ≤ 1 , 0≦y≦1 (bandgap 1.2-1.8 eV), and the top cell is (FA _1−x MA _1−x′ Cs _x+x′ )(I _1−z Br _z ) ₃ , 0≦x+x′≦1, and 0.4≦z≦1 (bandgap: 1.8 to 2.3 eV). See Non-Patent Documents 13-16).

In the artificial photosynthesis cell according to the present embodiment, the bottom cell of the two-junction thin-film solar cell is Cu(In _1-x Ga _x )Se ₂ and has a composition of 0≦x≦1 (bandgap 1.0 to 1.6 eV). and the top cell contains Cu(In _1-x′ Al _x′ ) Se ₂ and a CIGS compound having a composition of 0.9≦x′≦1 (bandgap 1.8 to 2.0 eV) (For the composition, see Non-Patent Documents 17 to 19).

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

In the artificial photosynthesis cell, the two-junction thin-film solar cell has a bottom cell composed of Cu ₂ Zn(Sn _1-x Ge _x )S ₄ and a composition of 0≦x≦0.8 (bandgap 1.5 to 2.0 eV ), and the top cell is Cu ₂ Zn(Sn _1−x′ Ge _x′ )S ₄ , with a composition of 0.6≦x′≦1, where x′<x (bandgap 1.8-2. 1 eV) of the photoelectric conversion layer (see Non-Patent Documents 20 to 23 for the composition).

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

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

The configuration of the artificial photosynthesis cell according to the present embodiment includes the 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 oxidation reaction electrode, and a reduction reaction. It is not particularly limited as long as it includes a negative electrode that is an electrode for use. The artificial photosynthesis cell according to the present embodiment includes, for example, the 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 that is an electrode for oxidation reaction; an electrochemical reactor having a negative electrode that is an electrode for the reaction;

In the artificial photosynthesis cell according to the present embodiment, for example, as shown in FIG. and a two-junction thin-film solar cell 10 arranged adjacent to a negative electrode (cathode electrode) 14 are packaged together with electrical wiring. The artificial photosynthetic cell according to the present embodiment is, for example, an integrated artificial photosynthetic cell comprising 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, respectively, as shown in FIG. 3, and may be used by being immersed in the electrolytic solution in the container. Further, in the artificial photosynthesis cell according to the present embodiment, for example, as shown in FIG. The two-junction thin-film solar cell 10 is suspended in the electrolyte (water) in the container 20 or on the electrolyte (water) surface (for example, the cell floats on the electrolyte (water) surface, and the two-junction thin-film solar cell 10 is in the electrolyte (water). Although not immersed, the positive electrode (anode electrode) 12 and the negative electrode (cathode electrode) 14 are in contact with the electrolytic solution (water)).

The artificial photosynthesis cell 1 of FIG. 5 will be described below as an example. In the electrochemical reaction device of the artificial photosynthesis cell 1, for example, a positive electrode (anode electrode) 12 and a negative electrode (cathode electrode) 14, which are, for example, plate-shaped members, are arranged in a housing portion 18 at a position facing each other with a gap, Between the anode electrode 12 and the cathode electrode 14, there is provided a channel 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 to maintain an appropriate potential.

The electrochemical reaction device functions by introducing an electrolytic solution containing a reaction substrate such as carbon dioxide into the channel 16 between the anode electrode 12 and the cathode electrode 14 . At the anode electrode 12, for example, water ( _H2O ) is oxidized to obtain oxygen (1/ _2O2 ). At the cathode electrode 14, for example, carbon dioxide is reduced to produce formic acid (HOOOH) or the like.

A negative electrode (cathode electrode) 14, which is a reduction reaction electrode, is an electrode used for reducing a substance by a reduction reaction. The cathode electrode 14 includes, for example, a conductive layer and a conductive 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, but examples thereof include a glass substrate. The substrate may comprise, for example, metals or semiconductors. The metal used as the substrate is not particularly limited, but examples include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin ( Sn), palladium (Pd), lead (Pb), and the like. The semiconductor used as the substrate is not particularly limited, but examples include titanium oxide (TiO ₂ ), tin oxide (SnO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), and zinc oxide (ZnO). , tantalum oxide (Ta ₂ O ₅ ), and the like. When the substrate contains a metal or semiconductor, it is preferable to form a conductive layer and an insulating layer between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. A configuration in which the metal substrate, the conductive layer, and the conductive layer are electrically directly connected may be employed.

The substrate may be configured to contain a carbon material in terms of having a high specific surface area. Examples of the carbon material include, but are not limited to, carbon black, graphene, fullerene, carbon nanotube, carbon cloth, carbon paper, and the like.

The conductive layer is provided to effectively collect current at the cathode electrode 14 . The conductive layer is not particularly limited, but 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 includes a conductor containing a material having a reduction catalytic function. The conductor may comprise a material containing carbon material (C). It is preferable that the size of a single carbon material structure is 1 nm or more and 1 μm or less. Carbon materials include, for example, at least one of carbon nanotubes, graphene, and graphite. Graphene and graphite preferably have a size of 1 nm or more and 1 μm or less. Carbon nanotubes preferably have a diameter of 1 nm or more and 40 nm or less. The conductor can be formed, for example, by spraying and heating a carbon material mixed with a liquid such as ethanol. It may be applied by spin coating instead of spraying. Alternatively, the solution may be directly dropped and dried without using spin coating.

A complex catalyst or the like can be used as the material having a reduction catalytic function. The complex catalyst is preferably a ruthenium complex, for example. Complex catalysts include, for example, [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl ₂ ], [Ru{4,4′ -di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO) _2Cl2 ], [Ru _{ 4,4'-di(1-H-1-pyrrolypropyl carbonate)-2, 2′-bipyridine}(CO) ₂ ] _n , [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(CH ₃ CN)Cl ₂ ] etc.

Modification with a complex catalyst can be produced, for example, by dissolving a complex in an acetonitrile (MeCN) solution and applying it on the conductor of the conductor layer. Modification with a complex catalyst can also be performed by an electropolymerization method. For example, a conductive electrode of a conductive 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. After a cathodic current is passed so that the Ag ⁺ electrode has a negative voltage, an anodic current is passed so that the Ag/Ag ⁺ electrode has a positive potential. can be modified. For the electrolyte solution, for example, acetonitrile (MeCN) can be used, and for the electrolyte, for example, Tetrabutylammonium perchlorate (TBAP) can be used.

The conductive layer formed in this manner is carried, applied or adhered onto the conductive layer constituting the cathode electrode 14 . Thereby, a cathode electrode 14 having a conductive layer and a conductive layer in order is formed on the substrate.

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

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

The conductive layer is provided to effectively collect current at the anode electrode 12 . The conductive layer is not particularly limited, but 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 includes a material having an oxidation catalyst function. Examples of materials having an oxidation catalytic function include materials 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 10 wt% sodium hydroxide (NaOH) aqueous solution to 50 mL of 2 mM potassium iridate (IV) chloride (K ₂ IrCl ₆ ) aqueous solution was stirred at 90° C. using a hot stirrer. Heat for 20 minutes. The resulting blue solution 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 nanocolloid aqueous solution of iridium oxide (IrOx). Furthermore, 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) obtained in this way is applied on the conductive layer at pH 12 and dried by being held at 60° C. for 40 minutes in a drying oven. After drying, the deposited salt can be washed with ultrapure water to form an anode electrode. Note that the application and drying of the iridium oxide (IrOx) nanocolloid aqueous solution may be repeated multiple times.

The reaction substrate contained in the electrolytic solution includes, for example, carbon compounds, such as carbon dioxide (CO ₂ ). Further, the electrolytic solution is preferably an aqueous phosphate buffer solution or an aqueous borate buffer solution. In a specific configuration example, for example, a tank of carbon dioxide (CO ₂ ) saturated phosphate buffer solution is provided, 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 (O ₂ ) and the like are recovered in an external fuel tank.

The housing portion 18 is a member that supports the anode electrode 12 and the cathode electrode 14 and constitutes the flow path 16 through which the electrolytic solution flows. The housing portion 18 is made of a material having the mechanical strength necessary for forming the electrochemical reaction device as a cell. For example, the housing 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 is not particularly limited as long as it is a material that separates a liquid and a gas and allows protons to move. 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 photosynthesis cell comprising an electrochemical reactor and a solar cell for generating power supplied to the anode electrode 12 and the cathode electrode 14 of the electrochemical reactor by using a solar cell as a means of applying a bias voltage. can do. A two-junction thin-film solar cell 10 can be placed, for example, adjacent to an anode electrode 12 and a cathode electrode 14 of an electrochemical reactor. For example, as shown in FIG. The negative electrode of battery 10 may be connected to cathode electrode 14 .

EXAMPLES 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 characteristics model of the two-junction thin-film solar cell (example) and the three-junction amorphous silicon solar cell (comparative example), these were combined. The operating current when used on water or in water was calculated, and the optimum value of the bandgap of the solar cell material was obtained. At this time, the AM1.5G sunlight spectrum was used, and light absorption by water was taken into consideration in the case of underwater use.

Compared to the current density of the amorphous silicon-based three-junction device (operating current of 6.5 mA/cm ² at a voltage of 2.0 V, the same shall apply hereinafter), the facing type cell has a two-junction thin-film solar cell (bandgap of 1.42/1. 96 eV, operating current 13.6 mA/cm ² ), a higher operating current was obtained.

Compared to the current density of the amorphous silicon-based three-junction device (operating currents of 6.4 and 5.9 mA/cm ² at water depths of 1 and 10 cm and a voltage of 2.0 V, the same applies hereinafter), the two-junction thin-film solar cell A battery (bandgap 1.42/1.97 eV, operating current 13.5 mA/cm ² at 1 cm water depth; bandgap 1.42/2.01 eV, operating current 12.7 mA/cm ² at 10 cm water depth) is used. , a higher operating current was obtained.

Details of the calculation method and results are as follows.

Solar cells using organic-inorganic hybrid perovskite, CdS, CuInSe ₂ (CIGS system), etc. have achieved high conversion efficiencies of around 20%. Therefore, assuming that a two-junction solar cell using these materials is used to construct an artificial photosynthesis cell, the appropriate bandgap of the solar cell material and the reaction current obtained at that time were obtained.

[Method of calculation]
As mentioned above, a voltage of about 2.0-2.4 V is required for the operation of artificial photosynthesis. An example is shown in FIG. Figures 1(a) and 1(b) show an IrOx anode, a Ru complex + carbon paper + carbon nanotube cathode, respectively, combined with a Pt counter electrode, and measured in a 0.4 M phosphate buffer electrolyte with _CO2 bubbling. It is a current density-potential curve. FIG. 1(c) is the result of converting these into a current density (J)-voltage (V) curve when both are connected via a DC power supply. Using this, the configuration of the photoelectric charge separation device is determined so as to obtain as large a current as possible within a voltage range of 2.0 to 2.4V. In any of the cells of Non-Patent Documents 2, 3, and 5, the premise is that the areas of the solar cell and the anode/cathode are equal. The unequal case is discussed last.

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

（１）

(1)

Here, ε _g is the bandgap of the light absorbing material in the device, n _sun (ε) is the number of photons per unit photon energy of sunlight, and T is the temperature. q, h, c, k _B are the elementary charge, Planck's constant, the speed of light in vacuum, and Boltzmann's constant, respectively. The first term on the right side represents carrier generation due to light absorption, and it is assumed that all photons having energy equal to or higher than the bandgap are absorbed and lead to carrier generation. The second term indicates that when a voltage is applied to the element, it operates as an LED and emits light, which is a loss for the purpose of photovoltaic power generation. Here, a generalized Planck's law is used, which is an extension of Planck's law representing radiation from a black body in thermal equilibrium to the case of semiconductors and voltage application (see Non-Patent Document 9). The power density P of the device 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. Equation (1) only considers the loss due to light emission (radiative recombination), which cannot be eliminated in principle from the power generation mechanism, so the conversion efficiency obtained from this equation is called the "radiative limit". .

In reality, there are various losses other than radiative recombination, and even photons with energy higher than the bandgap are partially not absorbed by the power generation layer (reflection and transmission). The realistic effects that degrade the performance of these devices are taken into account as follows. By approximating the Bose-Einstein distribution function in equation (1) with the Boltzmann distribution function and using the relationship k _B T <<ε _g , equation (1) is approximated by the following equation (2) .

（２）

(2)

The following expression (3) is used to represent the operation of a realistic device.

（３）

(3)

（４ａ）

（４ｂ）
より求められる。 A in the first term on the right side of 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 recombination loss including non-radiative processes. _Tv is a virtual temperature that determines the voltage dependence of recombination loss, and usually corresponds to the n value that indicates diode characteristics. The characteristics of the two-junction device are as follows, where ε _g1 and ε _g2 are the band gaps of the bottom cell and the top cell, respectively.

(4a)

(4b)
more sought after.

In order to express the characteristics of the thin-film solar cell, calculations were performed by substituting A=0.92, B=5, and T _v =500K into the equations (3) and (4). Standard conditions of AM 1.5 G, 100 mW/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 element with the world's highest conversion efficiency (see Non-Patent Document 6). Since it was confirmed that both values roughly matched, the characteristics of the two-junction solar cell were calculated using these A, B, and _Tv . Table 1 shows the maximum value of the conversion efficiency of the thin film solar cell and the characteristics at that time (upper column of each column) (see Non-Patent Document 6), and formula (3) (A = 0.92, B = 5, T _v = 500K), the comparison of the characteristics (lower part of each column) is shown.

Fig. of non-patent document 2. 3(B), Fig. 3 of Non-Patent Document 3. Since an integrated cell such as 2(A) is used in water as described above, the absorption of sunlight by water must be considered. FIG. 2(a) 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 significant at wavelengths of 700 nm or more. The permeability through water to reach the integrated cell varies with water depth. Here, it is assumed that a small element is installed in a shallow place as much as possible, and a case that a large element of about 1 m is installed so that it is under the water surface even if it is tilted slightly or is exposed to the wind. 10 cm, and the AM 1.5 G, 100 mW / cm ² spectrum is multiplied by the transmittance of FIG. used for _sun (ε).

In the case of amorphous silicon-based three-junction devices that have been used in integrated cells and facing cells (see Non-Patent Documents 1 to 5), the difference between the characteristics actually obtained and the characteristics of the "radiative limit" is even greater. big. FIG. 4,6. In this case, the thickness of each junction is constrained by the short diffusion length of carriers compared to the crystal. As a result, the light absorptance on the long wavelength side gradually decreases, so the quantum efficiency also decreases. Originally, a JV characteristic formula representing the characteristics of each junction should be derived, and the influence of light absorption by water should be calculated based on this formula. 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 the JV characteristic cannot be expressed by Equation (3). On the other hand, FIG. If a device having the quantum efficiency spectrum shown in 4 is used in water as it is, the long wavelength component of the incident light is absorbed by the water, and only the current in the bottom cell is reduced. Although it drops significantly, this drop can be easily eliminated by adjusting the thickness of the top and bottom cells. Therefore, the current value J when installed on the water is shown in FIG. 6, and for the case in water, the optical transmittance of water in FIG. The number of absorbed photons was obtained from the quantum efficiency spectrum of 4, and an approximate value was obtained by assuming that J is proportional to this.

[Results and Discussion]
The calculation results of the conversion efficiency and current value, and the optimum value of the bandgap are shown below. In order to compare the installation state of the element used in the facing type cell and not affected by the light absorption of water with the case of the integrated cell installed in water, it is called above the water or at a depth of 0 cm. do.

<Comparative example>
(Amorphous silicon-based 3-junction device)
Table 2 summarizes the characteristics obtained by optimizing the amorphous silicon-based three-junction device. Conversion efficiency η (the ratio of the maximum power density P to the incident light intensity. Since the incident light energy is 100 mW/cm ² , the numerical value of η (%) matches the maximum value of P (mW/cm ² ) as it is. ), and the current density J at voltages V = 2.0, 2.2, and 2.4 V, which is a measure of the performance of the photocharge separation device for artificial photosynthesis. A blank in Table 2 indicates that the current density J becomes negative.

The conversion efficiency η is η=13.6%. Although the current density J at V = 2 V is J = 6.5 mA/ ^cm2 , it decreases to J = 2.9 mA/ ^cm2 at V = 2.2 V, and V = 2.4 V cannot be obtained. .

<Example>
(Two-junction thin-film solar cell)
Next, let us consider the conditions for obtaining V=2.0 to 2.4 V using a two-junction device. FIG. 3 shows the calculation results of a two-junction device using A=0.92, B=5, and Tv=500K. FIG. 3 shows the effect of the band gaps (ε _g1 , ε _g2 ) of the top and bottom cells on the properties of the thin-film polycrystal two-junction device. Contour lines are drawn for each conversion efficiency (η) of 1% or current density (J) of 1 mA/cm ² , and the values of η and J of the innermost contour line 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 devices with the bandgap shown in Table 3, respectively. Table 3 shows the properties of the thin film polycrystalline two-junction device. The bandgap was optimized to maximize the value of the item marked with * in the table. A blank column in Table 3 indicates that the current density J becomes negative.

FIG. 3(a) shows the conversion efficiency η of the device above water as a function of the band gaps ε _g1 and ε _g2 of the bottom and top cells. The contour ridge near the diagonal in the figure is the current matching condition for the bottom and top cells, i.e., the condition where the generated current densities match, approximately, the conditions where the number of photons absorbed by both match. Since ε _g1 >ε _g2 in the lower right region, all the incident light is absorbed by the top cell and does not reach the bottom cell, so no power is generated. The highest value of η is η=28.7%, obtained when ε _g1 =1.12 eV and ε _g2 =1.74 V (Table 3). As can be seen from the JV curve (FIG. 4(a)) at this time, the short-circuit current density is 19.7 mA/cm ² .

FIG. 3(b) shows the result of calculating the current density J at V = 2.0 to 2.4 V when there is no influence of light absorption of water, assuming that it is used in a facing type cell such as Non-Patent Document 5. ~ (d). Table 3 summarizes the combination of band gaps that maximizes J for each V, and J and power density P at that time. The JV curves for each are shown in FIG. In FIGS. 3(b) to 3(d), when the open-circuit voltage is less than 2V, J=0 is displayed. The reason J does not depend on ε _g1 in the region where ε _g2 is large is that J is limited by ε _g2 . The lower left is a region where the open end voltage is less than V=2.0 to 2.4V. The maximum current density J at V=2.0 V is ε _g1 =1.42 eV and ε _g2 =1.96 V, i. 0.2 to 0.3 eV, and the maximum value is J=13.6 mA/cm ² . This is more than twice the value of 6.5 mA/cm ² in the case of using an amorphous silicon-based three-junction device. The conversion efficiency of the solar cell at this time is 27.2%, which is lower than the value (28.7%) obtained when the conversion efficiency shown in FIG. , it can be said that this is more suitable for use in artificial photosynthesis reactions. 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, which do not decrease as much as in the case of amorphous silicon-based three-junction devices, so the advantage over amorphous silicon-based three-junction devices is greater. .

Fig. of non-patent document 2. 3(B), Fig. 3 of Non-Patent Document 3. 3(e)-(g), Table 3, and FIG. As shown in Table 3, the band gap of the bottom cell optimized for use on water is 1.42/1.65 eV (873 to 751 nm), so even if it is used as it is, the effect of light absorption in water is small. . Therefore, the optimum value of the bandgap and its characteristics at a water depth of 1 cm are almost unchanged, and the characteristics superior to those of the amorphous silicon three-junction device are maintained.

The results at 10 cm water depth are shown in FIGS. 3(h)-(j), Table 3, and FIG. In this case, since the effect of long-wavelength light absorption by water appears, the current matching conditions change compared to the case above water and at a depth of 1 cm, the optimal value of the bandgap 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 three-junction device, it can be said that the superiority is even greater.

So far, the case where the areas of the solar cell and the anode/cathode are equal has been considered. The area ratio of both is changed according to the activity of the anode/cathode, and FIG. 7 shows an integrated artificial photosynthesis cell using interpenetrating electrodes. Cathode interpenetrating electrodes may be provided, either in water or on the surface of the water (where the cell floats on the surface of the water and the solar cell is not immersed in water, the anode and cathode are in contact with the water). Again, since the requirements for the anode-cathode potential difference are the same as for the equal area case, a larger 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 according to the examples.

1, 3, 5 artificial photosynthesis cell, 10 two-junction thin-film solar cell, 12 positive electrode (anode electrode), 14 negative electrode (cathode electrode), 16 channel, 18 container, 20 container.

Claims (7)

A two-junction thin-film solar cell with a bandgap of 1.1 to 2.0 eV/1.8 to 2.4 eV, a positive electrode as an oxidation reaction electrode, and a negative electrode as a reduction reaction electrode ,
An artificial photosynthetic cell characterized by an operating voltage of 2.0-2.4V . The artificial photosynthetic cell of 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;
Organic-inorganic hybrid perovskite whose top cell has 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 photosynthetic cell comprising a photoelectric conversion layer comprising: The artificial photosynthetic cell of 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 top cell has a photoelectric conversion layer comprising Cu(In _1-x' Al _x' ) Se ₂ and a CIGS compound having a composition of 0.9≦x'≦1. photosynthetic cell. The artificial photosynthetic cell of claim 1,
The two-junction thin-film solar cell is
the bottom cell has a composition of Cu ₂ ZnSn(S _1-x Se _x ) ₄ , 0≦x≦1;
The top cell is characterized by having a photoelectric conversion layer comprising Cu ₂ Zn(Sn _1-x' Ge _x' )S ₄ and a CGTS-based compound having a composition of 0.6≦x'≦1. artificial photosynthetic cell. The artificial photosynthetic cell of claim 1,
The two-junction thin-film solar cell is
a bottom cell having a composition of Cu ₂ Zn(Sn _1-x Ge _x )S ₄ , 0≦x≦0.8;
A photoelectric conversion layer in which the top cell contains a CGTS-based compound having a composition of Cu ₂ Zn(Sn _1-x' Ge _x' )S ₄ , 0.6≦x'≦1, where x'<x. An artificial photosynthetic cell characterized by having The artificial photosynthetic cell of 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 comprising Cu ₂ Zn(Sn _1-x' Ge _x' )S ₄ and a CGTS-based compound having a composition of 0.6≦x'≦1. artificial photosynthetic cell. The artificial photosynthetic cell of 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 comprising Cu ₂ Zn(Sn _1-x' Ge _x' )S ₄ and a CGTS-based compound having a composition of 0.6≦x'≦1. artificial photosynthetic cell.

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