Solar cell

Abstract

A solar cell 10 according to the present disclosure comprises a first electrode 2, a photoelectric conversion layer 4, a hole transport layer 5, and a second electrode 6. The photoelectric conversion layer 4 contains a perovskite compound. The hole transport layer 5 contains at least one selected from the group consisting of an organic boron compound and a metal complex, and at least one selected from the group consisting of a compound A represented by formula (1) and a compound B represented by formula (2). R in formula (1) and formula (2) is a substituent including at least one selected from the group consisting of elemental nitrogen, elemental oxygen, elemental phosphorus, and elemental sulfur.

Description

solar cells

[0001] This disclosure relates to solar cells.

[0002] In recent years, research and development of perovskite solar cells has been progressing, using perovskite crystals represented by the compositional formula ABX3 (where A is a monovalent cation, B is a divalent cation, and X is a halogen anion) and similar structures (hereinafter referred to as "perovskite compounds") as photoelectric conversion materials. Various efforts are being made to improve the photoelectric conversion characteristics of perovskite solar cells, such as photoelectric conversion efficiency and durability (for example, Non-Patent Documents 1 and 2). Generally, in a perovskite solar cell, a photoelectric conversion layer containing a perovskite compound and a hole transport layer are arranged between a first electrode and a second electrode.

[0003] Examples of hole transport materials include poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (hereinafter sometimes abbreviated as "PTAA") and 2,2',7,7'-Tetrakis[N,N-di-p-methylphenylamino]-9,9'-spirobifluorene (hereinafter sometimes abbreviated as "Spiro-MeOTAD").

[0004] As a dopant to the hole transport layer containing the above-mentioned hole transport material, metal complexes such as Lithium bis(trifluoromethanesulfonyl)imide (hereinafter sometimes abbreviated as "Li-TFSI") may be used.

[0005] Furthermore, organoboron compounds that act as Lewis acids, such as tris(pentafluorophenyl)borane (hereinafter sometimes abbreviated as TPFPB), are sometimes used as dopants to improve the hole transport properties of the hole transport materials mentioned above.

[0006] James A. Dawson et al., ACS Energy Letters, 2017, vol. 2, pp. 1818-1824. Luyan Zhang, et al. , “Tris(pentafluorophenyl)borane-water complex doped Spiro-TTB for high-efficiency and stable perovskite solar "cells", J. Mater. Chem. A, 2023, 11, 11866-11873.

[0007] Non-patent document 1 reports that Li ions in Li-TFSI affect the photoelectric conversion characteristics of solar cells. Specifically, it has been shown that Li ions in Li-TFSI diffuse into the photoelectric conversion layer containing the perovskite compound through grain boundaries toward the electron transport layer, becoming metallic lithium, and that the perovskite compound reacts with the metallic lithium and decomposes, affecting the photoelectric conversion characteristics.

[0008] Non-patent document 2 reports that TPFPB alone does not have sufficiently high dopant activity, and that adding other additives improves the dopant effect on hole transport materials, resulting in high photoelectric conversion efficiency. Non-patent document 2 proposes water as one of the other additives.

[0009] This disclosure aims to improve the photoelectric conversion characteristics of perovskite solar cells.

[0010] The solar cell of this disclosure comprises a first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode, wherein the photoelectric conversion layer comprises a perovskite compound, and the hole transport layer comprises at least one selected from the group consisting of organoboron compounds and metal complexes, and at least one selected from the group consisting of compound A represented by the following formula (1) and compound B represented by the following formula (2), In formulas (1) and (2), R is a substituent that includes at least one element selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur.

[0011] This disclosure can improve the photoelectric conversion characteristics of perovskite solar cells.

[0012] Figure 1 is a schematic cross-sectional view showing a first example of a solar cell according to an embodiment. Figure 2 is a schematic cross-sectional view showing a second example of a solar cell according to an embodiment. Figure 3 is a schematic cross-sectional view showing a third example of a solar cell according to an embodiment. Figure 4 is a schematic cross-sectional view showing a fourth example of a solar cell according to an embodiment.

[0013] <Knowledge forming the basis of this disclosure> As described in the background technology section above, conventionally, metal complexes such as Li-TFSI and organoboron compounds have been proposed as dopants to improve the hole transport properties of hole transport materials.

[0014] The hole transport layer in a solar cell includes, for example, a hole transport material and a supporting salt material. For example, Li-TFSI may be used as this supporting salt material. Li-TFSI, for instance, is an oxidizing agent and acts as a p-type dopant. However, Li-TFSI has small Li ions, which are cations, and is easily thermally diffused through grain boundaries into the perovskite layer, which is the photoelectric conversion layer. When Li-TFSI diffuses into the photoelectric conversion layer, the built-in electric field within the photoelectric conversion layer weakens, reducing the photoelectric conversion efficiency. Furthermore, as reported in Non-Patent Document 1, Li ions that have thermally diffused into the photoelectric conversion layer are converted into valence-zero metallic lithium by high-energy electrons generated by photocatalysis. The perovskite compound then reacts with metallic lithium and decomposes. In other words, when metal ions from the supporting salt material contained in the hole transport layer thermally diffuse into the photoelectric conversion layer, it degrades the photoelectric conversion layer and reduces the photoelectric conversion efficiency.

[0015] When organoboron compounds such as TPFPB are used as dopants to improve the hole transport properties of hole transport materials, such organoboron compounds do not have sufficient ability to stabilize partially positively charged hole transport materials, and therefore cannot sufficiently improve the photoelectric conversion efficiency of solar cells. For example, TPFPB has three benzene rings attached around a boron atom, and it is thought that its ability to stabilize partially positively charged hole transport materials is low due to steric hindrance.

[0016] As described above, conventional perovskite solar cells had room for improvement in their photoelectric conversion characteristics.

[0017] <Embodiments of the Disclosure> [First Embodiment] [Solar Cell] A solar cell according to the first embodiment of the Disclosure comprises a first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode. The solar cell according to this embodiment may, for example, comprise the first electrode, the photoelectric conversion layer, the hole transport layer, and the second electrode in this order. The solar cell according to this embodiment may further comprise an electron transport layer between the first electrode and the photoelectric conversion layer.

[0018] The photoelectric conversion layer contains a perovskite compound.

[0019] The hole transport layer includes at least one selected from the group consisting of organoboron compounds and metal complexes, and at least one selected from the group consisting of compound A represented by the following formula (1) and compound B represented by the following formula (2). Here, R in formulas (1) and (2) above is a substituent that includes at least one element selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur.

[0020] In the first embodiment, at least one compound selected from the group consisting of compound A and compound B, which is included in the hole transport layer, will be referred to as compound X1.

[0021] If the hole transport layer contains, for example, an organoboron compound, the inclusion of compound X1 together with the organoboron compound in the hole transport layer effectively stabilizes the partially positively charged hole transport material, thereby improving the hole transport properties of the hole transport material. As a result, the solar cell according to this embodiment can improve its photoelectric conversion characteristics (e.g., photoelectric conversion efficiency).

[0022] If the hole transport layer includes, for example, a metal complex, the inclusion of compound X1 together with the metal complex in the hole transport layer reduces the amount of metal ions (e.g., Li ions) contained in the metal complex that diffuse from the hole transport layer to the photoelectric conversion layer. As a result, the solar cell according to this embodiment can improve the photoelectric conversion characteristics (e.g., light resistance) compared to conventional solar cells equipped with a hole transport layer that does not contain compound X1.

[0023] Furthermore, when the solar cell according to this embodiment contains a metal complex in the hole transport layer, it can maintain the initial efficiency of conventional solar cells, that is, it can have an initial efficiency at the same level as or better than conventional solar cells, thereby improving the photoelectric conversion characteristics.

[0024] Furthermore, the solar cell according to this embodiment can have heat resistance comparable to that of conventional solar cells. Therefore, the solar cell according to this embodiment can improve photoelectric conversion characteristics by improving light resistance while maintaining heat resistance comparable to that of conventional solar cells.

[0025] In this specification, photoelectric conversion characteristics mean at least one of the following characteristics: photoelectric conversion efficiency, initial efficiency, light resistance, and heat resistance.

[0026] In the solar cell according to this embodiment, the hole transport layer contains the compound X1, so that the oxoammonium cation in compound A represented by formula (1) accepts electrons from the hole transport material and changes into a radical molecule having a nitroxyl group represented by formula (2). That is, compound A accepts electrons from the hole transport material and changes into compound B. Furthermore, the substituent R contains at least one element selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur, and the unpaired electrons of this element can stabilize the partially positively charged hole transport material. As a result, the solar cell according to this embodiment can improve photoelectric conversion efficiency and photoelectric conversion characteristics compared to conventional solar cells equipped with a hole transport layer that does not contain compound X1.

[0027] For example, in the solar cell according to the present embodiment, it is preferable that the compound X1 contained in the hole transport layer contains compound A, and may further contain compound B obtained by changing compound A upon receiving an electron.

[0028] Compound B is a TEMPO derivative in which the 4-position of 2,2,6,6-tetramethylpiperidine 1-oxyl (hereinafter may be abbreviated as "TEMPO") is substituted with a substituent R. Compound A is a compound obtained by oxidizing the above TEMPO derivative.

[0029] The substituent R in compound X1 may be a substituent containing at least one selected from the group consisting of a nitrogen element and an oxygen element. Thereby, the photoelectric conversion characteristics of the solar cell according to the present embodiment can be further improved.

[0030] The substituent R in compound X1 may be a substituent containing at least one selected from the group consisting of a hydroxy group, an ether group, an ester group, an amino group, and an amide group. Thereby, the photoelectric conversion characteristics of the solar cell according to the present embodiment can be further improved.

[0031] The substituent R in compound X1 may be a hydroxy group, a primary amino group, a methoxy group, an acetamido group, or a benzoyloxy group. Thereby, the photoelectric conversion characteristics of the solar cell according to the present embodiment can be further improved.

[0032] The substituent R in compound X1 may be represented by the following formula (3). Thereby, the photoelectric conversion characteristics of the solar cell according to the present embodiment can be further improved.

[0033] For example, compound X1 may contain compound A, and the compound A may be represented by the following formula (4). Note that in the compound represented by the following formula (4), the substituent R is an acetamido group, and therefore the compound may be hereinafter referred to as "acetamido TEMPO".

[0034] Compound X1 may further contain a compound corresponding to compound B, in which the oxoammonium cation of acetamide TEMPO accepts electrons from a hole transport material and is transformed into a radical molecule having a nitroxyl group.

[0035] Compound X1 contains compound A, which may be represented by the following formula (5). Since the substituent R in the compound represented by the following formula (5) is a hydroxyl group, this compound may hereafter be referred to as "hydroxyTEMPO".

[0036] Compound X1 may further contain a compound corresponding to compound B, in which the oxoammonium cation of hydroxytempo accepts electrons from a hole transport material and is transformed into a radical molecule having a nitroxyl group.

[0037] Compound X1 contains compound A, which may be represented by the following formula (6). Since the substituent R in the compound represented by the following formula (6) is a methoxy group, this compound may hereafter be referred to as "methoxyTEMPO".

[0038] Compound X1 may further contain a compound corresponding to compound B, in which the oxoammonium cation of methoxyTEMPO accepts electrons from a hole transport material and is transformed into a radical molecule having a nitroxyl group.

[0039] Compound X1 contains compound A, which may be represented by the following formula (7). In the compound represented by the following formula (7), the substituent R is a benzoyloxy group, and therefore this compound may be referred to as "benzoyloxyTEMPO" or "BzO-TEMPO" below.

[0040] Compound X1 may further contain a compound corresponding to compound B, in which the oxoammonium cation of benzoyloxyTEMPO accepts electrons from a hole transport material and is transformed into a radical molecule having a nitroxyl group.

[0041] Compound X1 contains compound A, which may be represented by the following formula (8). Since the substituent R in the compound represented by the following formula (8) is an amino group, this compound may hereafter be referred to as "aminoTEMPO".

[0042] Compound X1 may further contain a compound corresponding to compound B, in which the oxoammonium cation of aminoTEMPO accepts electrons from a hole transport material and is transformed into a radical molecule having a nitroxyl group.

[0043] Compound X1 contains compound A, which may be represented by the following formula (9). Hereinafter, the compound represented by the following formula (9) may be referred to as "Seb-TEMPO".

[0044] Compound X1 may further contain a compound corresponding to compound B, in which the oxoammonium cation of Seb-TEMPO accepts electrons from a hole transport material and is transformed into a radical molecule having a nitroxyl group.

[0045] When the hole transport layer contains the organoboron compound, it is desirable that the organoboron compound contained in the hole transport layer has Lewis acidity. Examples of organoboron compounds having Lewis acidity include TPFPB, bis(pentafluorophenyl)borane, and triphenylborane. By including the organoboron compound, the hole transport material can be oxidized more efficiently, and therefore the photoelectric conversion characteristics of the solar cell of this embodiment can be improved. To further improve the photoelectric conversion efficiency, it is desirable that the organoboron compound contains TPFPB. The organoboron compound is preferably not a metal complex. That is, organoboron compounds that correspond to metal complexes are preferably excluded from the organoboron compound contained in the hole transport layer.

[0046] If the hole transport layer contains the metal complex, the metal complex contained in the hole transport layer may include at least one selected from the group consisting of TFSI anions, tetrafluoroborate anions, and hexafluorophosphate anions. The metal complex containing the anion improves the initial efficiency of the solar cell. Furthermore, by including the metal complex containing the anion, the metal ions of the metal complex can be efficiently immobilized within the hole transport layer, so that the solar cell of this embodiment can have improved light resistance. To further improve initial efficiency and light resistance, it is desirable that the hole transport layer contains a metal complex containing a TFSI anion.

[0047] The metal complex contained in the hole transport layer may contain a cation of at least one metal selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, iron, cobalt, nickel, copper, silver, and zinc. This can improve the initial efficiency.

[0048] The hole transport layer described above may contain at least one selected from the group consisting of pyridine, n-methylpyrrolidone, and 4-terto-butylpyridine (hereinafter sometimes abbreviated as "tBP"). These materials are usually added to the hole transport layer as dispersion materials to homogeneously disperse the metal complex contained in the hole transport layer. When the dispersion material coordinates to the cations of the metal complex, the anions of the metal complex coordinate to the hole transport material, making it easier to dopage the material with holes. This can increase the initial efficiency. To further improve the initial efficiency, it is preferable that the dispersion material contains tBP.

[0049] The hole transport layer described above uses PTAA, Spiro-MeOTAD, poly(3-hexylthiophene-2,5-diyl) (hereinafter sometimes abbreviated as "P3HT"), 4,4′,4″-tris[9,9-dimethyl-2-fluorenyl(4-methoxy-phenyl)amino]triphenylamine (hereinafter sometimes abbreviated as "MeO-TFATA"), polyaniline, and poly[2 It may include at least one selected from the group consisting of ,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)) (hereinafter sometimes abbreviated as "PCPDTBT") and poly(3,4-ethylenedioxythiophene) (hereinafter sometimes abbreviated as "PEDOT").

[0050] A hole transport layer containing the above-mentioned substance as a hole transport material can improve the initial efficiency of a solar cell. To further improve initial efficiency and heat resistance, it is desirable to include PTAA as the hole transport material.

[0051] The solar cell of this embodiment will be described in detail below with reference to the drawings.

[0052] Figure 1 is a schematic cross-sectional view showing a first example of a solar cell according to this embodiment.

[0053] The solar cell 10 shown in Figure 1 comprises a substrate 1, a first electrode 2, an electron transport layer 3, a photoelectric conversion layer 4, a hole transport layer 5, and a second electrode 6 in this order. The solar cell 10 does not necessarily have a substrate 1. The solar cell 10 does not necessarily have an electron transport layer 3.

[0054] When light is shone on the solar cell 10, the photoelectric conversion layer 4 absorbs the light, generating excited electrons and holes. These excited electrons move to the first electrode 2 through the electron transport layer 3. Meanwhile, the holes generated in the photoelectric conversion layer 4 move to the second electrode 6 through the hole transport layer 5. As a result, the solar cell 10 can extract current from the first electrode 2, which acts as the negative electrode, and the second electrode 6, which acts as the positive electrode.

[0055] The solar cell 10 can be manufactured, for example, by the following method.

[0056] First, a first electrode 2 is formed on the surface of the substrate 1 by chemical vapor deposition, sputtering, solution coating, atomic layer deposition, or the like. Next, an electron transport layer 3 is formed on the first electrode 2 by chemical vapor deposition, sputtering, solution coating, atomic layer deposition, or the like. Next, a photoelectric conversion layer 4 is formed on the electron transport layer 3. For example, a perovskite compound may be cut to a predetermined thickness to form the photoelectric conversion layer 4 and placed on the electron transport layer 3. Next, a hole transport layer 5 is formed on the photoelectric conversion layer 4 by chemical vapor deposition, sputtering, solution coating, atomic layer deposition, or the like. Next, a second electrode 6 is formed on the hole transport layer 5 by chemical vapor deposition, sputtering, solution coating, atomic layer deposition, or the like. By doing so, a solar cell 10 can be obtained.

[0057] In the solar cell of this embodiment, an electron transport layer may be formed on the first electrode 2 by a solution coating method of nanoparticle dispersion or the like. In this case, the electron transport layer will have a porous structure. Figure 2 is a schematic cross-sectional view showing a second example of a solar cell according to the embodiment. As shown in Figure 2, the solar cell 20 of the second example includes an electron transport layer 21 having a porous structure.

[0058] Figure 3 is a schematic cross-sectional view showing a third example of a solar cell according to an embodiment. As shown in the third example solar cell 30 in Figure 3, an electron transport layer 31 may be formed on the first electrode 2 by chemical vapor deposition, sputtering, solution coating, atomic layer deposition, etc., and an electron transport layer 32 may be formed on the electron transport layer 31 by solution coating of a nanoparticle dispersion, etc. In this case, the electron transport layer 32 will have a porous structure.

[0059] Figure 4 is a schematic cross-sectional view showing a fourth example of a solar cell according to the embodiment. As with the solar cell 40 of the fourth example, the solar cell according to this embodiment does not necessarily have an electron transport layer (i.e., electron transport layer 3, or electron transport layers 31 and 32).

[0060] The following provides a detailed explanation of each component of a solar cell.

[0061] (Substrate 1) Substrate 1 is an incidental component. Substrate 1 plays the role of holding each layer of the solar cell. Substrate 1 can be formed from a transparent material. For example, a glass substrate or a plastic substrate can be used as substrate 1. A plastic substrate may be, for example, a plastic film. As materials for such a plastic film, polyimide, polyethylene terephthalate, polyethylene naphthalate, cycloolefin polymer, etc. can be used. When using a plastic substrate, it is preferable to form a barrier layer that has weather resistance. Also, if the second electrode 6 is translucent, the material of substrate 1 may be a non-translucent material. For example, a metal, ceramics, or a resin material with low translucency can be used as the material of substrate 1. If the first electrode 2 has sufficient strength, each layer can be held by the first electrode 2, so substrate 1 may not be necessary.

[0062] (First Electrode 2) The first electrode 2 is conductive. If the solar cell does not have an electron transport layer 3, the first electrode 2 is made of a material that does not form ohmic contact with the photoelectric conversion layer 4. Furthermore, the first electrode 2 has blocking properties for holes from the photoelectric conversion layer 4. Blocking properties for holes from the photoelectric conversion layer 4 mean that it allows only electrons generated in the photoelectric conversion layer 4 to pass through, and not holes to pass through. A material having such properties is a material whose Fermi energy is higher than the energy at the top of the valence band of the photoelectric conversion layer 4. The above material may also be a material whose Fermi energy is higher than the Fermi energy of the photoelectric conversion layer 4. Aluminum is a specific example of such a material. If the solar cell has an electron transport layer 3 between the first electrode 2 and the photoelectric conversion layer 4, the first electrode 2 does not need to have the property of blocking holes moving from the photoelectric conversion layer 4. The first electrode 2 may be made of a material that can form ohmic contact with the photoelectric conversion layer 4.

[0063] The first electrode 2 is translucent. For example, it transmits light from the visible region to the near-infrared region. The first electrode 2 can be formed using, for example, a transparent and conductive metal oxide and / or metal nitride. Examples of such materials include titanium oxide doped with at least one selected from the group consisting of lithium, magnesium, niobium, and fluorine; gallium oxide doped with at least one selected from the group consisting of tin and silicon; gallium nitride doped with at least one selected from the group consisting of silicon and oxygen; tin oxide doped with at least one selected from the group consisting of antimony and fluorine; zinc oxide doped with at least one selected from the group consisting of boron, aluminum, gallium, and indium; indium-tin composite oxide; or composites thereof.

[0064] Furthermore, the first electrode 2 can be formed using an opaque material with a light-transmitting pattern. Examples of light-transmitting patterns include linear, wavy, grid-like, or perforated metal-like patterns with numerous fine through-holes arranged regularly or irregularly. When the first electrode 2 has such patterns, light can pass through areas where there is no electrode material. Examples of opaque electrode materials include platinum, gold, silver, copper, aluminum, rhodium, indium, titanium, iron, nickel, tin, zinc, or alloys containing any of these. Conductive carbon materials can also be used.

[0065] The light transmittance of the first electrode 2 may be, for example, 50% or more, or 80% or more. The wavelength of the light to be transmitted depends on the absorption wavelength of the photoelectric conversion layer 4. The thickness of the first electrode 2 is, for example, in the range of 1 nm to 1000 nm.

[0066] (Electron transport layer 3) The electron transport layer 3 includes a semiconductor. The electron transport layer 3 may be a semiconductor with a band gap of 3.0 eV or more and 3.8 eV or less. By forming the electron transport layer 3 with a semiconductor with a band gap of 3.0 eV or more, visible light and infrared light can be transmitted to the photoelectric conversion layer 4. An example of a semiconductor is an inorganic n-type semiconductor.

[0067] Examples of inorganic n-type semiconductors include metal oxides, metal nitrides, and perovskite oxides. More specific examples include TiO2 (rutile type 3.0 eV, anatase type 3.2 eV), SnO2 (3.6 eV), Nb2O5 (3.4 eV), GaN (3.4 eV), SrTiO3 (3.2 eV), and CaTiO3 (3.4 eV). Here, the numbers in parentheses indicate typical band gap values.

[0068] The electron transport layer 3 may include multiple layers made of different materials and structures (for example, electron transport layers 31 and 32). For example, by making the electron transport layer 3 a porous structure, or by arranging a porous structure on the photoelectric conversion layer 4 side of the electron transport layer 3, the photoelectric conversion layer 4 can be easily formed. The material of the photoelectric conversion layer 4 penetrates into the voids of the porous structure, and the porous structure acts as a scaffold for the photoelectric conversion layer 4. Therefore, the material of the photoelectric conversion layer 4 is less likely to be repelled or aggregated on the surface of the electron transport layer 3 having a porous structure. Thus, the photoelectric conversion layer 4 can be easily formed as a uniform film. Examples of materials for forming the porous structure include porous bodies in which insulating or semiconductor particles are linked together. Examples of porous body particles with electron transport properties include titanium oxide, tin oxide, and niobium oxide particles. On the other hand, when porous particles with poor electron transport properties, such as aluminum oxide or silicon oxide particles, are used, they only function as a scaffold for forming the photoelectric conversion layer 4, and the effect of an electron transport layer 3 cannot be obtained. An electron transport layer 3 having a porous structure is formed, for example, by coating a nanomolecular dispersion of a porous material.

[0069] The surface roughness of the electron transport layer 3 having a porous structure may be 10 or more, or 100 or more, given by the surface roughness coefficient calculated as effective area / projected area. Projected area is the area of ​​the shadow cast behind an object when it is illuminated from directly in front. Effective area is the actual surface area of ​​the object. The effective area can be calculated from the volume obtained from the projected area and thickness of the object, and from the specific surface area and bulk density of the materials constituting the object. Specific surface area is measured, for example, by the nitrogen adsorption method.

[0070] The voids in the porous electron transport layer 3 are connected from one main surface to the other main surface of the electron transport layer 3. This allows the material of the photoelectric conversion layer 4 to fill the voids in the electron transport layer 3.

[0071] Since the electron transport layer 3 has a porous structure, light scattering occurs, and an effect of increasing the optical path length of light passing through the photoelectric conversion layer 4 is also expected. It is predicted that when the optical path length increases, the amount of electrons and holes generated in the photoelectric conversion layer 4 increases.

[0072] (Photoelectric Conversion Layer 4) The photoelectric conversion layer 4 contains a perovskite compound represented by the composition formula ABX₃. A is a monovalent cation. Examples of monovalent cations include monovalent cations such as alkali metal cations and organic cations. More specifically, methylammonium cation (MA + or CH₃NH₃ + ), formamidinium cation (FA + or HC(NH₂)₂ + ), ethylammonium cation (CH₃CH₂NH₃ + ), guanidinium cation (CH₆N₃ + ), potassium cation (K + ), cesium cation (Cs + ), and rubidium cation (Rb + ), and the like. B is a divalent lead cation (Pb 2+ ) and a tin cation (Sn 2+ ). X is a monovalent anion such as a halogen anion. Each of the A, B, and X sites may be occupied by a plurality of types of ions.

[0073] The thickness of the photoelectric conversion layer 4 is, for example, 50 nm or more and 10 μm or less. The photoelectric conversion layer 4 can be formed using a solution coating method, a printing method, a vapor deposition method, a sputtering method, or the like. The photoelectric conversion layer 4 may be formed by cutting out a perovskite compound.

[0074] The photoelectric conversion layer 4 may mainly contain a perovskite compound represented by the compositional formula ABX3. Here, "the photoelectric conversion layer 4 mainly contains a perovskite compound represented by the compositional formula ABX3" means that the photoelectric conversion layer 4 contains 60% by mass or more of the perovskite compound represented by the compositional formula ABX3. The photoelectric conversion layer 4 may contain 95% by mass or more of the perovskite compound represented by the compositional formula ABX3. The photoelectric conversion layer 4 may consist of a perovskite compound represented by the compositional formula ABX3. The photoelectric conversion layer 4 only needs to contain a perovskite compound represented by the compositional formula ABX3, and may contain defects or impurities.

[0075] The photoelectric conversion layer 4 may further contain other compounds different from the perovskite compound represented by the compositional formula ABX3. Examples of other different compounds include compounds having a Ruddlesden-Popper type layered perovskite structure.

[0076] (Hole transport layer 5) As described above, the hole transport layer 5 includes at least one selected from the group consisting of organoboron compounds and metal complexes, and compound X1. The organoboron compounds, metal complexes, and compound X1 are as described above.

[0077] The organoboron compound is preferably added to the hole transport material in an amount of 1% to 50% by molar ratio. However, if the hole transport material is a polymer, the organoboron compound is preferably added such that the molar ratio of the organoboron compound is 1% to 50% relative to the monomer units of the polymer of the hole transport material. If the organoboron compound is within this concentration range, sufficient hole transport characteristics can be exhibited, enabling highly efficient photoelectric conversion. Furthermore, compound X1 is preferably added to the hole transport material in an amount of 1% to 50% by molar ratio. However, if the hole transport material is a polymer, compound X1 is preferably added such that the molar ratio of compound X1 is 1% to 50% relative to the monomer units of the polymer of the hole transport material. If compound X1 is within this concentration range, sufficient hole transport characteristics can be exhibited, enabling highly efficient photoelectric conversion. Note that the amount of compound X1 is the sum of the amounts of compound A and compound B.

[0078] It is preferable that the metal complex is added to the hole transport material in an amount of 1% to 50% by molar ratio. However, if the hole transport material is a polymer, it is preferable that the metal complex is added in such a way that the molar ratio of the metal complex to the monomer units of the polymer hole transport material is 1% to 50%. If the metal complex is within this concentration range, sufficient hole transport properties can be exhibited, and photoelectric conversion can be performed with high efficiency.

[0079] Compound X1 is preferably added to the hole transport material in an amount of 1% to 50% by molar ratio. However, if the hole transport material is a polymer, it is preferable that compound X1 is added such that the molar ratio of compound X1 to the monomer units of the polymer hole transport material is 1% to 50%. If compound X1 is within this concentration range, sufficient hole transport characteristics can be exhibited, and photoelectric conversion can be performed with high efficiency. Note that the amount of compound X1 is the sum of the amounts of compound A and compound B.

[0080] The hole transport layer 5 further contains a hole transport material. The hole transport material is a material that transports holes. The hole transport layer 5 is composed of hole transport materials such as organic or inorganic substances.

[0081] Typical examples of organic substances used as hole transport materials include PTAA, Spiro-MeOTAD, P3HT, MeO-TFATA, polyaniline, PCPDTBT, and PEDOT. As described above, the hole transport layer 5 may contain at least one selected from the group consisting of PTAA, Spiro-MeOTAD, P3HT, MeO-TFATA, polyaniline, PCPDTBT, and PEDOT as a hole transport material. Examples of main solvents used for these organic substances include toluene, xylene, mesitylene, dichlorotoluene, chlorobenzene, dichlorobenzene, methoxypyridine, acetonitrile, chloroform, dichloromethane, and propylene glycol monomethyl ether acetate.

[0082] Inorganic semiconductors used as hole transport materials are p-type semiconductors. Examples of inorganic semiconductors include Cu₂O, CuGaO₂, CuSCN, CuI, and NiO. x MoO x These are carbon materials such as V2O5 or graphene oxide. These inorganic semiconductors may be mixed with organic materials. For example, nanoparticles of inorganic semiconductors may be mixed with organic materials such as PTAA.

[0083] The hole transport layer 5 may include multiple layers made of different materials. For example, the hole transport characteristics are improved by stacking multiple layers such that the ionization potential (or HOMO level) of the hole transport layer 5 becomes progressively shallower than that of the photoelectric conversion layer 4.

[0084] The thickness of the hole transport layer 5 may be between 1 nm and 1000 nm, or between 10 nm and 200 nm. Within this range, sufficient hole transport characteristics can be achieved and low resistance can be maintained, thus enabling highly efficient photovoltaic power generation.

[0085] Methods for forming the hole transport layer 5 include coating, printing, and vapor deposition. Examples of coating methods include doctor blade, bar coating, spray, dip coating, and spin coating. An example of a printing method is screen printing. If necessary, the hole transport layer 5 may be prepared by mixing multiple materials and then subjected to pressure or firing. If the material of the hole transport layer 5 is an organic low-molecular-weight substance, it can also be prepared by vacuum vapor deposition.

[0086] The hole transport layer 5 may contain a supporting salt material (i.e., a supporting electrolyte) and a supporting salt dispersion material. The supporting salt material, when added to the hole transport material, has the effect of improving conductivity. The supporting salt dispersion material, when added to the hole transport material together with the supporting salt material, can uniformly disperse the supporting salt material within the hole transport layer, thereby enhancing the conductivity-improving effect of the supporting salt material. In other words, the supporting salt material and the supporting salt dispersion material have the effect of stabilizing holes in the hole transport layer 5, contributing to an improvement in photoelectric conversion efficiency, particularly an improvement in initial efficiency.

[0087] For example, if the hole transport layer 5 contains the above-mentioned metal complex, the metal complex contained in the hole transport layer 5 may function as a supporting salt material. Therefore, the materials described above as metal complexes can be used as supporting salt materials. The supporting salt material may contain, as an anion, at least one anion selected from the group consisting of TFSI, tetrafluoroboric acid, and hexafluorophosphate, and as a cation, at least one metal ion selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, iron, cobalt, nickel, copper, silver, and zinc. An example of such a material is Li-TFSI.

[0088] For example, if the hole transport layer 5 contains compound A, which is a cation, it is desirable that a paired anion be included in order to achieve electrical neutrality, and this anion may function as a supporting salt material. That is, the hole transport layer 5 may contain at least one anion selected from the group consisting of TFSI, bis(fluorosulfonyl)imide anion, (fluorosulfonyl)(trifluoromethanesulfonyl)imide anion, trifluoromethanesulfonic acid anion, 1,1,2,2,3,3,4,4,4-nonafluoro-1-butanesulfonic acid anion, acetate anion, hexafluorophosphate anion, perchlorate anion, tetrafluoroborate anion, dihydrogen phosphate anion, tris(trifluoromethanesulfonyl)methide anion, iodide anion, bromide anion, and chloride anion.

[0089] The hole transport layer 5 may contain a supporting salt material containing a metal cation. As such a metal cation, the hole transport layer 5 may contain at least one metal cation selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, iron, cobalt, nickel, copper, silver, and zinc.

[0090] As the supporting salt dispersion material, a dispersion material that disperses the supporting salt material may be used. As the supporting salt dispersion material, the above-mentioned dispersion material that disperses the metal complex may be used. Therefore, as the supporting salt dispersion material, at least one selected from the group consisting of pyridine, n-methylpyrrolidone, and tBP may be used.

[0091] By including the compound X1 in addition to the support salt material in the hole transport layer 5, the degradation of the photoelectric conversion layer 4 that may occur due to the diffusion of metal ions such as Li ions into the photoelectric conversion layer can be suppressed. Therefore, the light resistance of the solar cell 10 can be improved.

[0092] For example, as a method for manufacturing a solar cell according to this embodiment, a method may be used that includes preparing a precursor for the hole transport layer 5 and obtaining the hole transport layer 5 by reducing the content of the supporting salt dispersion material in the precursor. Here, examples of methods for reducing the content of the supporting salt dispersion material in the precursor for the hole transport layer 5 include a reduced pressure method, a drying method, and a heating method.

[0093] The above manufacturing method may further include drying the obtained hole transport layer. In this case, the drying temperature of the hole transport layer may be lower than the boiling point of the supporting salt dispersion material. According to this method, the supporting salt dispersion material is not completely removed from within the hole transport layer 5, and a minimum amount of supporting salt dispersion material remains so as not to reduce the initial efficiency and heat resistance of the solar cell. Therefore, homogeneous dispersion of the supporting salt dispersion material in the hole transport layer 5 can be more reliably achieved, making it possible to manufacture solar cells with improved initial efficiency and light resistance.

[0094] (Second electrode 6) When the solar cell includes a hole transport layer 5, the second electrode 6 does not need to block electrons from the photoelectric conversion layer 4. That is, the material of the second electrode 6 may be a material that makes ohmic contact with the photoelectric conversion layer 4. Therefore, the second electrode 6 can be formed to be translucent.

[0095] Of the first electrode 2 and the second electrode 6, the electrode on the side into which light is incident only needs to be light-transmitting. Therefore, one of the first electrode 2 and the second electrode 6 does not need to be light-transmitting. In other words, one of the first electrode 2 and the second electrode 6 does not need to be made of a light-transmitting material, nor does it need to have a pattern that includes an opening that transmits light.

[0096] [Additive for Hole Transport Layer] Compound X1 contained in the hole transport layer described in the solar cell according to the first embodiment above is used as an additive for the hole transport layer. That is, the additive for the hole transport layer according to the embodiment of this disclosure includes at least one selected from the group consisting of compound A represented by the following formula (1) and compound B represented by the following formula (2).

[0097] In the hole transport layer additive according to embodiments of the present disclosure, R in formulas (1) and (2) is a substituent comprising at least one element selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur.

[0098] A solar cell in which the hole transport layer additive according to this embodiment is added to the hole transport layer can improve the photoelectric conversion efficiency, as described in detail in the solar cell according to the above embodiment.

[0099] Compound A represented by formula (1) and compound B represented by formula (2) included in the hole transport layer additive according to this embodiment are the same as those described as compound X1 included in the hole transport layer in the description of the solar cell according to the embodiment, so a detailed explanation is omitted here.

[0100] The hole transport layer additive according to this embodiment preferably further contains an organoboron compound. The organoboron compound contained in the hole transport layer additive according to this embodiment is the same as the organoboron compound contained in the hole transport layer described in the description of the solar cell according to the embodiment, so a detailed explanation is omitted here.

[0101] [Second Embodiment] [Solar Cell] A solar cell according to the second embodiment of the present disclosure comprises a first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode. The photoelectric conversion layer comprises a perovskite compound. The hole transport layer comprises compound X2 and a metal complex.

[0102] Compound X2 is represented by the following formula (10): In equation (10), R 1 This is a nitroxyl group containing an oxoammonium cation moiety represented by the following formula (11) or a nitroxyl radical moiety represented by the following formula (12).

[0103] In the above formula (10), R 2 It is an organic group.

[0104] In the solar cell according to this embodiment, the hole transport layer contains compound X2, allowing the oxygen in the ester bond (-COO-) contained in compound X2 to immobilize metal ions within the metal complex. Therefore, the solar cell according to this embodiment can reduce the amount of metal ions (e.g., Li ions) contained in the metal complex that diffuse from the hole transport layer to the photoelectric conversion layer. As a result, the solar cell according to this embodiment can improve light resistance compared to conventional solar cells equipped with a hole transport layer that does not contain compound X2.

[0105] Furthermore, by including a metal complex in the hole transport layer, the solar cell according to this embodiment can maintain its initial efficiency compared to conventional solar cells, that is, it can have an initial efficiency that is the same as or better than conventional solar cells.

[0106] The solar cell according to this embodiment can have heat resistance comparable to that of conventional solar cells. Therefore, the solar cell according to this embodiment can improve light resistance while maintaining heat resistance comparable to that of conventional solar cells.

[0107] The solar cell according to this embodiment may, for example, comprise a first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode in this order. The solar cell according to this embodiment may further comprise an electron transport layer between the first electrode and the photoelectric conversion layer.

[0108] In the above formula (10), R 1 The nitroxyl group may include an oxoammonium cation moiety represented by formula (11). That is, compound X2 has a nitroxyl group containing an oxoammonium cation moiety represented by formula (11) and an organic group R 2 The compound X2 may be a cation having a structure linked by an ester bond (-COO-). By providing a hole transport layer containing such a compound X2, the solar cell according to this embodiment can have improved light resistance.

[0109] In the above formula (10), R 1 The nitroxyl group may be an organic group.

[0110] The nitroxyl group may, for example, have a cyclic structure. That is, compound X2 may contain a cyclic nitroxyl group. By providing a hole transport layer containing compound X2 having such a structure, the solar cell according to this embodiment can have improved light resistance.

[0111] The above compound X2 may also be represented by the following formula (13).

[0112] When compound X2 is represented by formula (13) above, compound X2 is such that the 4-position of "TEMPO", which is 2,2,6,6-tetramethylpiperidine 1-oxyl, is -OCOR 2 This corresponds to an oxoammonium cation derived from a TEMPO derivative, which is produced by the oxidation of a TEMPO derivative substituted with the substituent. In a hole transport layer containing such a compound X2, the hole transport material can be efficiently oxidized.

[0113] The above organic group R in compound X2 2 This can also be expressed by the following formula (14).

[0114] The organic group R represented by the above formula (14) 2 Because it contains a long-chain alkyl group, compound X2 effectively inhibits the penetration of metal ions such as Li ions into the photoelectric conversion layer, and the deformation of the long-chain alkyl group allows for flexible immobilization of metal ions. Furthermore, the organic group R 2 Because it further contains TEMPO and ester bonds (-COO-), its ability to attract and immobilize metal ions is enhanced. This can significantly improve the light resistance of solar cells.

[0115] The above organic group R 2 As the compound X2 represented by the above formula (14), for example, a material represented by the following formula (15) can be mentioned. The material represented by the following formula (15) may be written as "Seb-TEMPO" as described in the first embodiment.

[0116] The above organic group R in compound X2 2This may be an aryl group. The aryl group may be a phenyl group.

[0117] The above organic group R 2 Examples of compound X2 in which is a phenyl group include the material represented by the following formula (16). The material represented by the following formula (16) may be written as "benzoyloxyTEMPO" or "BzO-TEMPO" as described in the first embodiment.

[0118] The metal complex contained in the hole transport layer may include at least one selected from the group consisting of TFSI anions, tetrafluoroborate anions, and hexafluorophosphate anions. The metal complex containing the above anion improves the initial efficiency of the solar cell. Furthermore, by including the metal complex containing the above anion, the metal ions of the metal complex can be efficiently immobilized within the hole transport layer, so that the solar cell of this embodiment can have improved light resistance. To further improve initial efficiency and light resistance, it is desirable that the metal complex containing the TFSI anion be included in the hole transport layer.

[0119] The metal complex contained in the hole transport layer may contain a cation of at least one metal selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, iron, cobalt, nickel, copper, silver, and zinc. This can improve the initial efficiency.

[0120] The hole transport layer described above may contain at least one selected from the group consisting of pyridine, n-methylpyrrolidone, and tBP. These materials are usually added to the hole transport layer as dispersion materials to homogeneously disperse the metal complex contained in the hole transport layer. When the dispersion material coordinates to the cations of the metal complex, the anions of the metal complex coordinate to the hole transport material, making it easier to dopage the material with holes. This can increase the initial efficiency. To further improve the initial efficiency, it is preferable that the dispersion material contains tBP.

[0121] The hole transport layer described above may contain at least one selected from the group consisting of PTAA, Spiro-MeOTAD, P3HT, MeO-TFATA, PCPDTBT, and PEDOT as a hole transport material.

[0122] A hole transport layer containing the above-mentioned substance as a hole transport material can improve the initial efficiency of a solar cell. To further improve initial efficiency and heat resistance, it is desirable to include PTAA as the hole transport material.

[0123] The configuration examples and manufacturing methods of the solar cell of this embodiment are the same as the configuration examples and manufacturing methods of the first to fourth examples described with reference to Figures 1 to 4 in the first embodiment, so details are omitted here. Also, the specific descriptions of each component of the solar cell of this embodiment (except for the description of the "hole transport layer 5") are the same as those described in the first embodiment, so details are omitted here.

[0124] The following is a description of the "hole transport layer 5" in the configuration examples of the solar cell of this embodiment (see Figures 1 to 4).

[0125] (Hole Transport Layer 5) The hole transport layer 5 contains compound X2 and a metal complex. Compound X2 and the metal complex are as described above. Here, it is preferable that compound X2 is added in an amount of 1% to 50% of the hole transport material by molar ratio. However, if the hole transport material is a polymer, it is preferable that compound X2 is added in an amount of 1% to 50% of the monomer units of the polymer of the hole transport material by molar ratio. If compound X2 is within this concentration range, sufficient hole transport characteristics can be exhibited, and photoelectric conversion can be performed with high efficiency. Note that the amount of compound X2 is the sum of the amount of compound A and the amount of compound B. Furthermore, it is preferable that the metal complex is added in an amount of 1% to 50% of the hole transport material by molar ratio. However, if the hole transport material is a polymer, it is preferable that the metal complex is added in an amount of 1% to 50% of the monomer units of the polymer of the hole transport material by molar ratio. Within this concentration range, the metal complex can exhibit sufficient hole transport properties, enabling highly efficient photoelectric conversion.

[0126] The hole transport layer 5 further contains a hole transport material. The hole transport material is a material that transports holes. The hole transport layer 5 is composed of hole transport materials such as organic or inorganic substances.

[0127] Typical examples of organic substances used as hole transport materials include PTAA, Spiro-MeOTAD, P3HT, MeO-TFATA, polyaniline, PCPDTBT, and PEDOT. As described above, the hole transport layer 5 may contain at least one selected from the group consisting of PTAA, Spiro-MeOTAD, P3HT, MeO-TFATA, polyaniline, PCPDTBT, and PEDOT as a hole transport material. Examples of main solvents used for these organic substances include toluene, xylene, mesitylene, dichlorotoluene, chlorobenzene, dichlorobenzene, methoxypyridine, acetonitrile, chloroform, dichloromethane, and propylene glycol monomethyl ether acetate.

[0128] Inorganic semiconductors used as hole transport materials are p-type semiconductors. Examples of inorganic semiconductors include Cu₂O, CuGaO₂, CuSCN, CuI, and NiO. x MoO x These are carbon materials such as V2O5 or graphene oxide. These inorganic semiconductors may be mixed with organic materials. For example, nanoparticles of inorganic semiconductors may be mixed with organic materials such as PTAA.

[0129] The hole transport layer 5 may include multiple layers made of different materials. For example, the hole transport characteristics are improved by stacking multiple layers such that the ionization potential (or HOMO level) of the hole transport layer 5 becomes progressively shallower than that of the photoelectric conversion layer 4.

[0130] The thickness of the hole transport layer 5 may be between 1 nm and 1000 nm, or between 10 nm and 50 nm. Within this range, sufficient hole transport characteristics can be achieved and low resistance can be maintained, thus enabling highly efficient photovoltaic power generation.

[0131] Methods for forming the hole transport layer 5 include coating, printing, and vapor deposition. Examples of coating methods include doctor blade, bar coating, spray, dip coating, and spin coating. An example of a printing method is screen printing. If necessary, the hole transport layer 5 may be prepared by mixing multiple materials and then subjected to pressure or firing. If the material of the hole transport layer 5 is an organic low-molecular-weight substance, it can also be prepared by vacuum vapor deposition.

[0132] The hole transport layer 5 may contain a supporting salt material (i.e., a supporting electrolyte) and a supporting salt dispersion material. The supporting salt material, when added to the hole transport material, has the effect of improving conductivity. The supporting salt dispersion material, when added to the hole transport material together with the supporting salt material, can uniformly disperse the supporting salt material within the hole transport layer, thereby enhancing the conductivity-improving effect of the supporting salt material. In other words, the supporting salt material and the supporting salt dispersion material have the effect of stabilizing holes in the hole transport layer 5, contributing to an improvement in photoelectric conversion efficiency, particularly an improvement in initial efficiency.

[0133] For example, the metal complex contained in the hole transport layer 5 may function as a supporting salt material. Therefore, the materials described above as metal complexes can be used as supporting salt materials. The supporting salt material may contain, as an anion, at least one anion selected from the group consisting of TFSI, tetrafluoroboric acid, and hexafluorophosphate, and as a cation, at least one metal ion selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, iron, cobalt, nickel, copper, silver, and zinc. An example of such a material is Li-TFSI.

[0134] As the supporting salt dispersion material, the above-mentioned dispersion material for dispersing metal complexes may be used. Therefore, as the supporting salt dispersion material, at least one selected from the group consisting of pyridine, n-methylpyrrolidone, and tBP may be used.

[0135] By including the compound X2 in addition to the support salt material in the hole transport layer 5, the degradation of the photoelectric conversion layer 4 that may occur due to the diffusion of metal ions such as Li ions into the photoelectric conversion layer can be suppressed. Therefore, the light resistance of the solar cell 10 can be improved.

[0136] For example, as a method for manufacturing a solar cell according to this embodiment, a method may be used that includes preparing a precursor for the hole transport layer 5 and obtaining the hole transport layer 5 by reducing the content of the supporting salt dispersion material in the precursor. Here, examples of methods for reducing the content of the supporting salt dispersion material in the precursor for the hole transport layer 5 include a reduced pressure method, a drying method, and a heating method.

[0137] The above manufacturing method may further include drying the obtained hole transport layer. In this case, the drying temperature of the hole transport layer may be lower than the boiling point of the supporting salt dispersion material. According to this method, the supporting salt dispersion material is not completely removed from within the hole transport layer 5, and a minimum amount of supporting salt dispersion material remains so as not to reduce the initial efficiency and heat resistance of the solar cell. Therefore, homogeneous dispersion of the supporting salt dispersion material in the hole transport layer 5 can be more reliably achieved, making it possible to manufacture solar cells with improved initial efficiency and light resistance.

[0138] [Additive for Hole Transport Layer] Compound X2 contained in the hole transport layer described in the solar cell according to the above embodiment is used as an additive for the hole transport layer. That is, the additive for the hole transport layer according to the embodiment of this disclosure includes compound X2 represented by the following formula (10). In equation (10), R 1 This is a nitroxyl group containing an oxoammonium cation moiety represented by the following formula (11) or a nitroxyl radical moiety represented by the following formula (12).

[0139] In equation (10), R 2 It is an organic group.

[0140] A solar cell in which the hole transport layer additive according to this embodiment is added to the hole transport layer can have improved light resistance, as described in detail in the solar cell according to the above embodiment.

[0141] The hole transport layer additive according to this embodiment is the same as compound X2 contained in the hole transport layer described in the description of the solar cell according to the embodiment, so a detailed explanation is omitted here.

[0142] The hole transport layer additive according to this embodiment preferably further contains a metal complex. The metal complex contained in the hole transport layer additive according to this embodiment is the same as the metal complex contained in the hole transport layer described in the description of the solar cell according to the embodiment, so a detailed explanation is omitted here.

[0143] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0144] (Technology 1) A first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode are provided, wherein the photoelectric conversion layer contains a perovskite compound, and the hole transport layer contains at least one selected from the group consisting of organoboron compounds and metal complexes, and at least one selected from the group consisting of compound A represented by the following formula (1) and compound B represented by the following formula (2), A solar cell in which R in formulas (1) and (2) is a substituent comprising at least one element selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur.

[0145] In the solar cell of Technology 1, the photoelectric conversion characteristics can be improved by including at least one compound selected from the group consisting of compound A and compound B in the hole transport layer, along with at least one selected from the group consisting of organoboron compounds and metal complexes.

[0146] (Technical 2) The solar cell according to Technical 1, wherein R in formulas (1) and (2) is a substituent comprising at least one element selected from the group consisting of nitrogen and oxygen.

[0147] With the above configuration, the solar cell of technology 2 can further improve its photoelectric conversion characteristics.

[0148] (Technical 3) The solar cell according to Technical 2, wherein R in formulas (1) and (2) is a substituent comprising at least one selected from the group consisting of a hydroxyl group, an ether group, an ester group, an amino group, and an amide group.

[0149] With the above configuration, the solar cell of technology 3 can further improve its photoelectric conversion characteristics.

[0150] (Technical 4) The solar cell according to Technical 3, wherein R in formulas (1) and (2) is a hydroxyl group, a primary amino group, a methoxy group, an acetamide group, or a benzoyloxy group.

[0151] With the above configuration, the solar cell of technology 4 can further improve its photoelectric conversion characteristics.

[0152] (Technical 5) R in formulas (1) and (2) above is expressed by the following formula (3): Solar cell as described in Technology 2.

[0153] With the above configuration, the solar cell of technology 5 can further improve its photoelectric conversion characteristics.

[0154] (Technical 6) The solar cell according to any one of Technical 1 to 5, wherein the organoboron compound comprises tris(pentafluorophenyl)borane.

[0155] With the above configuration, the solar cell of technology 6 can further improve its photoelectric conversion characteristics.

[0156] (Technical 7) The solar cell according to any one of Technical 1 to 6, wherein the metal complex comprises at least one selected from the group consisting of bis(trifluoromethanesulfonyl)imide anion, tetrafluoroborate anion, and hexafluorophosphate anion.

[0157] With the above configuration, the solar cell of technology 7 can further improve its photoelectric conversion characteristics.

[0158] (Technical 8) The solar cell according to any one of Technical 1 to 7, wherein the metal complex comprises a cation of at least one metal selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, iron, cobalt, nickel, copper, silver, and zinc.

[0159] With the above configuration, the solar cell of technology 8 can further improve its photoelectric conversion characteristics.

[0160] (Technical 9) The hole transport layer is made of hole transport material such as poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], 2,2',7,7'-Tetrakis[N,N-di-p-methoxyphenylamino]-9,9'-spirobiflurene, poly(3-hexylthiophene-2,5-diyl), 4,4',4''-tris[9,9-dimethyl-2-flurene(4-me A solar cell according to any one of the technologies 1 to 8, comprising at least one selected from the group consisting of thoxy-phenyl)amino]triphenylamine, polyaniline, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)], and poly(3,4-ethylenedioxythiophene).

[0161] With the above configuration, the solar cell of technology 9 can further improve its photoelectric conversion characteristics.

[0162] (Technical 10) The solar cell according to any one of Technical 1 to 9, wherein the hole transport layer further comprises at least one selected from the group consisting of pyridine, n-methylpyrrolidone, and 4-terto-butylpyridine.

[0163] With the above configuration, the solar cell of technology 10 can further improve its photoelectric conversion characteristics.

[0164] The present disclosure will be described in more detail below with reference to examples and comparative examples. However, the present invention is not limited to the following examples.

[0165] In the examples and comparative examples, solar cells using perovskite compounds were fabricated, and their photoelectric conversion efficiency was evaluated.

[0166] [Examples 1 to 6, Comparative Examples 1 to 3] The configurations of the solar cells in Examples 1 to 6 and Comparative Examples 1 to 3 are as follows. The solar cells in Examples 1 to 6 and Comparative Examples 1 to 3 had the same structure as the solar cell 10 shown in Figure 1. ・Substrate 10: Glass substrate (thickness: 1.3 mm) ・First electrode 2: Fluorine-doped tin oxide film (thickness: 400 nm) ・Electron transport layer 3: Tin oxide (SnO2) (thickness: 40 nm) ・Photoelectric conversion layer 4: Layer containing a perovskite compound (thickness: 500 nm) ・Hole transport layer 5: Layer mainly containing PTAA (thickness: 50 nm) ・Second electrode 6: Indium-tin composite oxide (thickness: 100 nm)

[0167] <Fabrication of Solar Cells> (Example 1) First, a substrate 1 having a transparent conductive layer (i.e., a layer of fluorine-doped tin oxide film) on its surface that functions as the first electrode 2 was prepared. In this example, a glass substrate with a thickness of 1.3 mm was used as the substrate 1.

[0168] As the first electrode 2, a layer of fluorine-doped tin oxide film was formed on the substrate 1 by thermal CVD.

[0169] Next, as the electron transport layer 3, a layer of tin oxide was formed on the first electrode 2 by applying a solution of tin oxide sol (manufactured by Taki Chemical Co., Ltd.) and ultrapure water (manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd.) using a spin-coating method.

[0170] Next, a raw material solution for the photoelectric conversion material was applied by spin coating to form a photoelectric conversion layer 4 containing a perovskite compound. The raw material solution contained 1.49 mol / L of lead iodide (manufactured by Tokyo Chemical Industry Co., Ltd.), 1.29 mol / L of formamidinium iodide (manufactured by GreatCell Solar Co., Ltd.), 0.18 mol / L of cesium iodide (manufactured by Sigma-Aldrich Co., Ltd.), and 0.03 mol / L of rubidium iodide (manufactured by Iwatani Corporation). The solvent for the solution was a mixed solvent of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), N,N-dimethylformamide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and N,N'-dimethylpropylene urea.

[0171] Next, a hole transport layer 5 mainly containing PTAA was formed on the photoelectric conversion layer 4 by applying a raw material solution of the hole transport material using a spin-coating method. Specifically, a layer containing PTAA, TPFPB, and acetamide TEMPO-TFSI was formed. The raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 1.45 g / L of acetamide TEMPO-TFSI. The solvent for the raw material solution was toluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). Here, acetamide TEMPO-TFSI was synthesized by mixing acetamide TEMPO (manufactured by Tokyo Chemical Industries) in acetone (manufactured by Fujifilm Wako Pure Chemical Industries), bis(trifluoromethanesulfonyl)imide (manufactured by Tokyo Chemical Industries), sodium hypochlorite pentahydrate (manufactured by Tokyo Chemical Industries), and sodium hydroxide aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries) in ultrapure water (manufactured by Fujifilm Wako Pure Chemical Industries), followed by isolation and purification.

[0172] Next, a second electrode 6 was formed on the hole transport layer 5 by sputtering to create an indium-tin composite oxide film. In this way, the solar cell of Example 1 was obtained.

[0173] (Example 2) In Example 2, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 1.33 g / L of hydroxyTEMPO-TFSI (manufactured by Okumoto Laboratory). Otherwise, the solar cell of Example 2 was obtained in the same manner as in Example 1.

[0174] (Example 3) In Example 3, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 1.58 g / L of Seb-TEMPO-TFSI (manufactured by Okumoto Laboratory). Otherwise, the solar cell of Example 3 was obtained in the same manner as in Example 1.

[0175] (Example 4) In Example 4, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 1.37 g / L of methoxy-TEMPO-TFSI. Otherwise, the solar cell of Example 4 was obtained in the same manner as in Example 1. Here, the methoxy-TEMPO-TFSI used was synthesized by mixing a solution of methoxy-TEMPO (manufactured by Tokyo Chemical Industry Co., Ltd.) in acetone (manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd.), bis(trifluoromethanesulfonyl)imide (manufactured by Tokyo Chemical Industry Co., Ltd.), sodium hypochlorite pentahydrate (manufactured by Tokyo Chemical Industry Co., Ltd.), and aqueous sodium hydroxide solution (manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd.) in ultrapure water (manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd.), then isolating and purifying the mixture.

[0176] (Example 5) In Example 5, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 1.64 g / L of benzoyloxy-TEMPO-TFSI. Otherwise, the solar cell of Example 5 was obtained in the same manner as in Example 1. Here, the benzoyloxy-TEMPO-TFSI used was synthesized by mixing a solution of benzoyloxy-TEMPO (manufactured by Tokyo Chemical Industry Co., Ltd.) in acetone (manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd.), bis(trifluoromethanesulfonyl)imide (manufactured by Tokyo Chemical Industry Co., Ltd.), sodium hypochlorite pentahydrate (manufactured by Tokyo Chemical Industry Co., Ltd.), and aqueous sodium hydroxide solution (manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd.) in ultrapure water (manufactured by Fujifilm Wako Pure Chemical Industries Co., Ltd.), then isolating and purifying the mixture.

[0177] (Example 6) In Example 6, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 1.33 g / L of aminoTEMPO-TFSI (manufactured by Okumoto Laboratory). Otherwise, the solar cell of Example 6 was obtained in the same manner as in Example 1.

[0178] (Comparative Example 1) In Comparative Example 1, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions) and 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.). In other words, compound X1 was not contained in the raw material solution. Otherwise, the solar cell of Comparative Example 1 was obtained in the same manner as in Example 1.

[0179] (Comparative Example 2) In Comparative Example 2, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 0.84 g / L of bis(trifluoromethanesulfonyl)imide lithium. Otherwise, the solar cell of Comparative Example 2 was obtained in the same manner as in Example 1.

[0180] (Comparative Example 3) In Comparative Example 3, the raw material solution for the layer mainly containing PTAA contained 12 g / L of PTAA (manufactured by Kyocera Document Solutions), 3.88 g / L of TPFPB (manufactured by Tokyo Chemical Industry Co., Ltd.), and 1.28 g / L of TEMPO-TFSI (manufactured by Okumoto Laboratory). Otherwise, the solar cell of Comparative Example 3 was obtained in the same manner as in Example 1.

[0181] <Measurement of Photoelectric Conversion Efficiency> The photoelectric conversion efficiency of the solar cells obtained in Examples 1 to 6 and Comparative Examples 1 to 3 was measured.

[0182] The current-voltage characteristics of solar cells under light irradiation were measured using an electrochemical analyzer (ALS660B, BAS) and a simulated solar light source. Before measurement, the light intensity of the simulated solar light source was set to 1 Sun (100 mW / cm²) using a standard cell. 2 The calibration was performed at a voltage sweep rate of 1 mV / s. No pre-adjustments such as irradiation with light intensity of 1 Sun or prolonged application of forward bias were performed before the start of the measurement. To fix the effective area and reduce the effect of scattered light, the aperture was set to 0.1 cm. 2 With the solar cell masked with a black mask, light was irradiated from the mask / substrate side. Output measurements were performed at room temperature under dry air conditions (<2% RH).

[0183]

[0184] As shown in Table 1, the solar cells of Examples 1 to 6, which had hole transport layers to which compounds corresponding to compound X1 as additives, namely acetamide-TEMPO-TFSI, hydroxy-TEMPO-TFSI, Seb-TEMPO-TFSI, methoxy-TEMPO-TFSI, benzoyloxy-TEMPO-TFSI, and amino-TEMPO-TFSI, showed a significant improvement in photoelectric conversion efficiency compared to the solar cell of Comparative Example 1, which had a hole transport layer without additives. Furthermore, the solar cell of Comparative Example 2, which had a hole transport layer to which Li-TFSI was added as an additive, did not show any improvement in photoelectric conversion efficiency compared to the solar cell of Comparative Example 1. In other words, it was confirmed that the improvement in photoelectric conversion efficiency observed in Examples 1 to 6 was not due to the effect of the TFSI anion, but rather to the effect of the TEMPO derivative cation of compound A added as compound X1. Furthermore, the solar cell of Comparative Example 3, which had a hole transport layer to which TEMPO-TFSI was added as an additive, showed less improvement in photoelectric conversion efficiency compared to Comparative Example 1 compared to Examples 1 to 6. In other words, it was confirmed that the improvement in photoelectric conversion efficiency was greater when a TEMPO derivative cation having a substituent R containing a nitrogen or oxygen element, as in Examples 1 to 6, was added, rather than a TEMPO cation, as in Comparative Example 3.

[0185] From the above results, it was confirmed that a solar cell comprising a first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode, wherein the photoelectric conversion layer contains a perovskite compound and the hole transport layer contains an organoboron compound and compound X1, can achieve excellent photoelectric conversion efficiency.

[0186] [Examples 7 to 9, Comparative Examples 4 to 5] The configurations of the solar cells in Examples 7 to 9 and Comparative Examples 4 to 5 are as follows. The solar cells in Examples 7 to 9 and Comparative Examples 4 to 5 had the same structure as the solar cell 10 shown in Figure 1. ・Substrate 10: Glass substrate (thickness: 1.3 mm) ・First electrode 2: Fluorine-doped tin oxide film (thickness: 400 nm) ・Electron transport layer 3: Tin oxide (SnO2) (thickness: 40 nm) ・Photoelectric conversion layer 4: Layer containing a perovskite compound (thickness: 500 nm) ・Hole transport layer 5: Layer mainly containing PTAA (thickness: 50 nm) ・Second electrode 6: Indium-tin composite oxide (thickness: 100 nm)

[0187] <Fabrication of Solar Cell> (Example 7) First, a substrate 1 having a transparent conductive layer (i.e., a layer of fluorine-doped tin oxide film) on its surface that functions as the first electrode 2 was prepared. In this example, a glass substrate with a thickness of 1.3 mm was used as the substrate 1.

[0188] As the first electrode 2, a layer of fluorine-doped tin oxide film was formed on the substrate 1 by thermal CVD.

[0189] Next, as the electron transport layer 3, a porous tin oxide layer was formed on the first electrode 2 by applying a solution of a mixture of SnO2 colloidal dispersion and ultrapure water (manufactured by Fujifilm Wako Pure Chemical Industries) using a spin-coating method.

[0190] Next, a raw material solution for the photoelectric conversion material was applied by spin coating to form a photoelectric conversion layer 4 containing a perovskite compound. The raw material solution contained 1.0 mol / L of lead(II) iodide (manufactured by Tokyo Chemical Industry Co., Ltd.), 0.85 mol / L of formamidinium iodide (manufactured by GreatCell Solar Co., Ltd.), 0.12 mol / L of cesium iodide (a rare metal), and 0.03 mol / L of rubidium iodide (a rare metal). The solvent of the solution was a mixed solvent of dimethyl sulfoxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and N,N-dimethylformamide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). The mixing ratio of dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) in this mixed solvent (DMSO:DMF) was 1:4 by volume.

[0191] Next, a hole transport layer 5 mainly containing PTAA was formed on the photoelectric conversion layer 4 by spin coating a raw material solution of the hole transport material. Specifically, a layer was formed containing PTAA, as well as additives Li-TFSI (manufactured by Tokyo Chemical Industry Co., Ltd.), tBP (manufactured by Sigma-Aldrich Co., Ltd.), and Seb-TEMPO-TFSI (manufactured by Okumoto Laboratory Co., Ltd.). PTAA was included as the hole transport material, tBP as the supporting salt dispersion material, Li-TFSI as the metal complex (supporting salt material), and Seb-TEMPO-TFSI as compound X1. The solvent for the raw material solution of the layer mainly containing PTAA was toluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.). The raw material solution contained 14 g / L of PTAA, 11.3 g / L of tBP, 5.1 g / L of Li-TFSI, and 1.0 g / L of Seb-TEMPO-TFSI (manufactured by Okumoto Research Institute).

[0192] Next, a second electrode 6 was formed on the hole transport layer 5 by sputtering to create an indium-tin composite oxide film. In this way, the solar cell of Example 1 was obtained.

[0193] (Example 8) In Example 8, the raw material solution for the layer mainly containing PTAA contained 14 g / L of PTAA, 11.3 g / L of tBP, 5.1 g / L of Li-TFSI, and 0.5 g / L of BzO-TEMPO-TFSI (manufactured by Okumoto Laboratory). Otherwise, the solar cell of Example 8 was obtained in the same manner as in Example 7.

[0194] (Example 9) In Example 9, the raw material solution for the layer mainly containing PTAA contained 14 g / L of PTAA, 11.3 g / L of tBP, 5.1 g / L of Li-TFSI, and 1.0 g / L of BzO-TEMPO-TFSI. Otherwise, the solar cell of Example 9 was obtained in the same manner as in Example 7.

[0195] (Comparative Examples 4 and 5) In Comparative Examples 4 and 5, the raw material solution for the layer mainly containing PTAA contained 14 g / L of PTAA, 11.3 g / L of tBP, and 5.1 g / L of Li-TFSI. That is, compound X1 was not contained in the formed hole transport layer. Otherwise, the solar cells of Comparative Examples 4 and 5 were obtained in the same manner as in Example 7. The solar cells of Comparative Examples 4 and 5 had the same configuration.

[0196] <Measurement of Photoelectric Conversion Efficiency> The photoelectric conversion efficiency of the solar cells obtained in Examples 7 to 9 and Comparative Examples 4 to 5 was measured.

[0197] The current-voltage characteristics of solar cells under light irradiation were measured using an electrochemical analyzer (ALS660B, BAS) and a simulated solar light source. Before measurement, the light intensity of the simulated solar light source was set to 1 Sun (100 mW / cm²) using a standard cell. 2 The calibration was performed at a voltage sweep rate of 1 mV / s. No pre-adjustments such as irradiation with light intensity of 1 Sun or prolonged application of forward bias were performed before the start of the measurement. To fix the effective area and reduce the effect of scattered light, the aperture was set to 0.1 cm. 2 With the solar cell masked with a black mask, light was irradiated from the mask / substrate side. Output measurements were performed at room temperature under dry air conditions (<2% RH).

[0198] Table 2 shows the initial efficiency, efficiency after the lightfastness test (470 hours), and efficiency after the heat resistance test (806 hours) for Example 7 and Comparative Example 4. The lightfastness test was performed with the solar cell in an open circuit state after sealing it in a nitrogen atmosphere using sealing resin, getter material, and cover glass, and then irradiating it with light at 50°C for 1 Sun. The heat resistance test was performed by leaving the solar cell undisturbed in a drying oven (in the dark) set to 85°C.

[0199]

[0200] As shown in Table 2, the solar cell of Example 7, which had a hole transport layer doped with compound X1 (Seb-TEMPO), showed significantly improved light resistance compared to the solar cell of Comparative Example 4, which had a hole transport layer without compound X1. Furthermore, the solar cell of Example 7 had superior heat resistance compared to the solar cell of Comparative Example 4.

[0201] Table 3 shows the initial efficiency, efficiency after the lightfastness test (308 hours), and efficiency after the heat resistance test (806 hours) for Examples 8 to 9 and Comparative Example 5. The lightfastness test was performed with the solar cell in an open circuit state after sealing it in a nitrogen atmosphere using sealing resin, getter material, and cover glass, and then irradiating it with light at 50°C for 1 Sun. The heat resistance test was performed by leaving the solar cell undisturbed in a drying oven set to 85°C (in the dark).

[0202]

[0203] As shown in Table 3, the solar cells of Examples 8 and 9, which had a hole transport layer in which BzO-TEMPO was added as compound X1, showed significantly improved light resistance compared to the solar cell of Comparative Example 5, which had a hole transport layer without compound X1. Furthermore, the solar cells of Examples 8 and 9 had heat resistance comparable to that of the solar cell of Comparative Example 5.

[0204] From the above results, it was confirmed that a solar cell comprising a first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode, wherein the photoelectric conversion layer contains a perovskite compound and the hole transport layer contains compound X1 and a metal complex, can achieve excellent light resistance.

[0205] This disclosure can be applied to solar cells where high efficiency is required, and therefore has extremely high potential for industrial use.

Claims

1. The device comprises a first electrode, a photoelectric conversion layer, a hole transport layer, and a second electrode, wherein the photoelectric conversion layer contains a perovskite compound, and the hole transport layer contains at least one selected from the group consisting of organoboron compounds and metal complexes, and at least one selected from the group consisting of compound A represented by the following formula (1) and compound B represented by the following formula (2), A solar cell in which R in formulas (1) and (2) is a substituent comprising at least one element selected from the group consisting of nitrogen, oxygen, phosphorus, and sulfur.

2. The solar cell according to claim 1, wherein R in formulas (1) and (2) is a substituent comprising at least one element selected from the group consisting of nitrogen and oxygen.

3. The solar cell according to claim 2, wherein R in formula (1) and formula (2) is a substituent comprising at least one selected from the group consisting of a hydroxyl group, an ether group, an ester group, an amino group, and an amide group.

4. The solar cell according to claim 3, wherein R in formula (1) and formula (2) is a hydroxyl group, a primary amino group, a methoxy group, an acetamide group, or a benzoyloxy group.

5. R in the above formula (1) and the above formula (2) is represented by the following formula (3), The solar cell according to claim 2.

6. The solar cell according to claim 1, wherein the organoboron compound comprises tris(pentafluorophenyl)borane.

7. The solar cell according to claim 1, wherein the metal complex comprises at least one selected from the group consisting of bis(trifluoromethanesulfonyl)imide anion, tetrafluoroborate anion, and hexafluorophosphate anion.

8. The solar cell according to claim 1, wherein the metal complex comprises a cation of at least one metal selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, scandium, yttrium, iron, cobalt, nickel, copper, silver, and zinc.

9. The hole transport layer is made of the following hole transport material: poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], 2,2',7,7'-Tetrakis[N,N-di-p-methoxyphenylamino]-9,9'-spirobiflurene, poly(3-hexylthiophene-2,5-diyl), 4,4',4''-tris[9,9-dimethyl-2-flurene(4-me A solar cell according to claim 1, comprising at least one selected from the group consisting of thoxy-phenyl)amino]triphenylamine, polyaniline, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)], and poly(3,4-ethylenedioxythiophene).

10. The solar cell according to claim 1, wherein the hole transport layer further comprises at least one selected from the group consisting of pyridine, n-methylpyrrolidone, and 4-terto-butylpyridine.

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