Hole transport material and solar cell using hole transport material

Abstract

To provide: a hole transport material that exhibits excellent hole transport properties stably and sustainably in the absence of a dopant substance; the hole transport material at a low cost; and a solar cell having high battery performance and high durability.SOLUTION: A hole transport material has a structure represented by the general formula (1) in the figure. In the general formula (1), the D moieties are represented by the structure of the same group (I). The solar cell employs the hole transport material.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to a hole transport material and a solar cell using the hole transport material. [Background technology]

[0002] Solar cells generally use elements such as silicon, compound semiconductors, and organic semiconductors, but hybrid solar cells (perovskite solar cells) are attracting attention because of their high light-harvesting ability and the ability to be thin-filmed and low-cost.

[0003] Non-Patent Document 1 discloses a perovskite solar cell comprising a glass substrate with a transparent conductive film, a blocking layer made of a dense titanium dioxide (TiO2) film, a power generation layer formed by layering lead bromide (PbBr2), lead iodide (PbI2), methylamine hydrobromide (CH5N·HBr: hereinafter sometimes abbreviated as "MABr"), and formamidine hydroiodide (CH4N2·HI: hereinafter sometimes abbreviated as "FAI"), which are perovskite compounds that are excited by light to generate electrons, on porous titanium dioxide (TiO2), a hole transport layer, and electrodes.

[0004] This power generation layer is a mixed halide (FAPbI3) 0.85 (MAPbBr3) 0.15 It is formed as a perovskite layer.

[0005] Conventionally, the hole transport material commonly used in the hole transport layer is 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (commonly referred to as "spiro-OMeTAD"; hereinafter, this name will be used), which is represented by the following general formula (A) and is described in Non-Patent Document 2. Crystals of perovskite compounds absorb light, generating electrons and holes. Holes are transported to the counter electrode by the hole transport material, and electrons move to the photoelectrode, and this cycle is repeated to generate electricity.

[0006] [ka]

[0007] Although spiro-OMeTAD is commercially available and easily available, it requires numerous synthesis and purification steps, making it inherently difficult to reduce cost. Furthermore, its hole mobility is relatively low, and a dopant such as lithium bis(trifluoromethanesulfonyl)imide (hereinafter sometimes referred to as "LiTFSI") is required for efficient and stable function as a hole transport material. Furthermore, while spiro-OMeTAD can exhibit high initial performance when incorporated into solar cells as a hole transport material, its durability remains an issue. Therefore, research is underway to develop hole transport materials that can be provided at low cost and provide efficient and stable hole transport.

[0008] Therefore, Non-Patent Document 1 reports a novel hole-transporting material, a compound called LD29, represented by the following general formula (B), which has a carbazole ring in the acceptor (hereinafter referred to as "A") moiety and a triphenyldimethoxyamino group in the donor (hereinafter referred to as "D") moiety, linked in a donor-acceptor-donor (hereinafter referred to as "DAD") structure. In solar cells using LD29 as a hole-transporting material, the addition of 4-tert-butylpyridine (tBP) and a cobalt complex (FK209) as dopants resulted in a conversion efficiency of 18.0%, comparable to that of spiro-OMeTAD. However, the dopant-free conversion efficiency of solar cells using LD29 as a hole-transporting material was 14.29%. Therefore, LD29 requires the addition of a dopant to function efficiently and stably as a hole-transporting material.

[0009] [ka]

[0010] Furthermore, Non-Patent Document 3 reports a compound called YN1, which is represented by the following general formula (C), and Non-Patent Document 4 reports a compound called PTZ2, which is represented by the following general formula (D). Both compounds have a DAD structure, and, like LD29, function efficiently and stably as hole transport materials by adding a dopant substance.

[0011] [ka]

[0012] [ka]

[0013] Thus, much research is being conducted on small-molecular-weight organic materials, such as hole-transport materials with a DAD structure. The inventors of the present application also reported in Patent Document 1 a small-molecular-weight organic material called DHCF-3, which is represented by the following general formula (E). DHCF-3 exhibits stable hole-transport properties when doped with a dopant, and functions well as a hole-transport material for perovskite solar cells. In other words, DHCF-3 alone does not function as a good hole-transport material; the addition of a dopant is required to exhibit hole-transport properties.

[0014] [ka]

[0015] Furthermore, Non-Patent Document 5 reports that when an anthracene-arylamine type compound is used as a hole transport material in the presence of a dopant substance, the efficiency and stability are improved compared to when spiro-OMeTAD is used in the same manner. Furthermore, Non-Patent Document 6 reports that a branched methoxydiphenylamine-substituted fluorene type compound can be prepared more cheaply than spiro-OMeTAD and exhibits high hole transport performance in the presence of a dopant substance.

[0016] Commonly used oxidizing agents, also known as dopant substances, include 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (TPFB), lithium bis(trifluoromethane)sulfonimide (LiTFSI), AgTFSI, tributyl phosphate (tBP), and cobalt complex (FK209).

[0017] As described above, all hole transport materials reported in the prior art require the addition of a dopant substance to exhibit their hole transport properties. However, dopant substances are generally expensive reagents, which results in increased solar cell fabrication costs. It is also known that the electric field generated by solar cell operation can cause the dopant substance to migrate within the hole transport layer, resulting in a biased distribution of the dopant within the hole transport layer. Furthermore, the dopant substance migrates to the perovskite layer adjacent to the hole transport layer and to the interface between the perovskite layer and the adjacent electron transport layer (blocking layer). This migration of the dopant substance significantly degrades solar cell performance. [Prior art documents] [Patent documents]

[0018] [Patent Document 1] Patent Publication No. 2023-46046 [Non-patent literature]

[0019] [Non-Patent Document 1] Xuepeng Liu et al., “A star-shaped carbazole-based hole-transporting material with triphenylamine side arms for perovskite solar cells,” J.Mater.Chem.C., 2018, 6, 12912-12918 (DOI:10.1039 / c8tc04191a) [Non-patent document 2] Michael Saliba et al., “How to Make over 20% Efficient Perovskite Solar Cells in Regular (nip) and Inverted (pin) Architectures,” Chem. Mater. 2018, 30, 13, 4193-4201(DOI:10.1021 / acs.chemmater.8b00136 [Non-patent document 3] Peng Xu et al., “DAD-Typed Hole Transport Materials for Efficient Perovskite Solar Cells: Tuning Photovoltaic Properties via the Acceptor Group”, ACS Appl. Mater. Interfaces 2018, 10, 23 (DOI:10.1021 / acsami.8b04003) [Non-patent document 4] Roberto Grisorio et al., “Molecular Tailoring of Phenothiazine-Based Hole-Transporting Materials for High-Performing Perovskite Solar Cells,” ACS Energy Lett. 2017, 2, 5, 1029-1034 (DOI:10.21 / acsenergylett.7b00054) [Non-patent document 5] Xuepeng Liu et al., “Anthracene-arylamine hole transporting materials for perovskite solar cells”, Chemical Communications, 2017, 53, 9958-9961 (DOI: 10.1039 / C7CC03444J) [Non-patent document 6] Tadas Malinauskas et al., “Branched methoxydiphenylamine-substituted fluorene derivatives as hole transporting materials for high-performance perovskite solar cells”, Energy and Environmental Science, 2016, 9, 1681-1686 (DOI: 10.1039 / C5EE03911H) Summary of the Invention [Problem to be solved by the invention]

[0020] In view of the above-mentioned problems of the conventional technology, there remains a need for a hole transport material that stably and continuously exhibits excellent hole transport properties without the addition of a dopant substance. There is also a need for a hole transport material that is inexpensive. There is also a need for a solar cell with high cell performance that is inexpensive. [Means for solving the problem]

[0021] The hole transport material according to the present invention has the following characteristic features: Having a structure represented by the following general formula (1): [ka] [In general formula (1), the three D portions are regions having the same structure, The D moiety is represented by the structure of group (I) below: [ka] (In group (I), R a1 and R a2 independently, -C m H 2m+1 (where m is an integer of 1 or 2), -C(C m H 2m+1 )(C n H 2n+1 )(C p H 2p+1 ) (wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4), and —CHC(C m H 2m+1 )(C n H 2n+1 )(C p H 2p+1 ) (wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4), R b1 and R b2 are independently -H, R c1 and R c2 independently, -C m H 2m+1 (wherein m is an integer selected from 0 to 4), -OC m H 2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (where m is an integer selected from 0 to 2) R d1 and R d2 independently, -C m H 2m+1 (wherein m is an integer selected from 0 to 4), -OC m H 2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (where m is an integer selected from 0 to 2) R e1 and R e2 are independently -H, R f is -H or -CH3, R g is -H or -CH3. ] point.

[0022] This structure provides a hole transport material with excellent hole transport properties, capable of stably and efficiently capturing and transporting holes. Specifically, the hole transport material of this structure has a structure in which three units of a stilbene structure having a p-amino group are introduced to the nitrogen atom of the general formula (1). It can also be said to have a structure in which a styryl group having a p-amino group is introduced to the p-position of each phenyl group of triphenylamine. The introduction of three units of a stilbene structure, which is a π-conjugated system as a whole, extends π-conjugation throughout the molecule. This further smooths the intramolecular π-conjugation, allowing for appropriate control of the intramolecular push-pull effect and improving intramolecular and intermolecular charge transfer properties. Therefore, the hole transport material of this structure can function as a hole transport material without the addition of a dopant, thereby eliminating the problem of decreased solar cell performance due to the dopant and providing stable solar cells over a long period of time. Furthermore, the absence of expensive dopant materials reduces the cost of solar cell fabrication.

[0023] In this hole-transport material, a secondary amino group is introduced at the p-position of the terminal benzene ring of the three-unit stilbene structure. The introduction of such a bulky substituent suppresses cis-trans isomerization and cyclization reactions of the stilbene structure, and the hole-transport material with this structure can maintain stable π-conjugation, allowing it to exhibit excellent hole-transport properties stably over a long period of time and improving the durability of solar cells.

[0024] The hole transport material of this configuration exhibits almost no light absorption in the visible light range and only weak light absorption in the 400 to 450 nm range. Therefore, solar cells using this hole transport material can minimize light absorption loss due to the hole transport material, and are expected to have high photoelectric conversion efficiency.

[0025] The hole transport material of this configuration also has excellent solubility in solvents, which allows it to be applied using wet processes such as spin coating, facilitating the formation of a hole transport layer for a solar cell.

[0026] Furthermore, the hole transport material of the present invention can be synthesized in a short time with few synthetic steps using inexpensive raw materials, and a high-purity product can be obtained through a simple purification process. Therefore, expensive reagents and complicated processes are not required, and the costs required for synthesis and purification can be reduced, making it possible to provide an inexpensive, high-performance hole transport material.

[0027] Furthermore, the hole transport material of this configuration allows smooth intramolecular and intermolecular hole movement, which is expected to suppress molecular oxidation and improve durability. Furthermore, because the hole transport material of this configuration has improved hydrophobicity, a hole transport layer formed using the hole transport material of this configuration can also function as a protective film for the perovskite layer, which decomposes in water, resulting in the advantage of improved durability of the solar cell.

[0028] As described above, the hole transport material of the present invention has excellent properties and can be suitably used as a hole transport material for solar cells. When the hole transport material of the present invention is used in the hole transport layer of a perovskite solar cell or the like, excellent cell performance was demonstrated in initial experiments. [Brief explanation of the drawings]

[0029] [Figure 1] FIG. 1 is a schematic cross-sectional view of a perovskite solar cell. [Figure 2] FIG. 1 is a perspective view from above of a perovskite solar cell. [Figure 3] FIG. 1 is an explanatory diagram of power generation in a perovskite solar cell. [Figure 4] FIG. 1 is an explanatory diagram showing a procedure for fabricating a perovskite solar cell. [Figure 5]FIG. 1 shows the synthesis schemes of the hole transport material (MSTPA-1) prepared in Example 1 and the hole transport material (MSTPA-7) prepared in Example 4. [Figure 6] FIG. 2 shows the results of 1H NMR analysis confirming the synthesis of the hole transport material (MSTPA-1) prepared in Example 1. [Figure 7] FIG. 1 shows the results of mass spectrometry confirming the synthesis of the hole transport material (MSTPA-1) prepared in Example 1. [Figure 8] FIG. 1 shows the results of high performance liquid chromatography analysis confirming the synthesis of the hole transport material (MSTPA-1) prepared in Example 1. [Figure 9] FIG. 2 shows the results of ultraviolet-visible-near-infrared spectroscopic analysis confirming the synthesis of the hole transport material (MSTPA-1) prepared in Example 1. [Figure 10] 1 is a graph showing the results of cell performance evaluation of solar cells investigated in Example 3, showing the transition of photoelectric conversion efficiency of each solar cell investigated. [Figure 11] FIG. 1 shows the results of H NMR analysis (a), mass spectrometry (b), HPLC analysis (c), and UV-Vis-NIR spectroscopy (d) confirming the synthesis of the hole transport material (MSTPA-7) prepared in Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0030] An embodiment of a perovskite solar cell 10 (an example of a solar cell 10; hereinafter referred to as "solar cell 10") using a hole transport material according to the present invention will be described below with reference to the drawings. In this embodiment, as an example of solar cell 10, a solar cell 10 configured with a perovskite layer 44 made of an organic and inorganic hybrid compound will be described. However, the present invention is not limited to the following embodiment, and various modifications are possible without departing from the spirit of the present invention.

[0031] (Basic configuration of solar cell 10 according to this embodiment) As shown in FIGS. 1 and 2 , a solar cell 10 according to this embodiment includes a substrate 2 having a transparent substrate 21 and a transparent conductive film 22; a blocking layer 3 provided on the substrate 2, which transfers electrons to the transparent conductive film 22 and separates the hole transport layer 5 from the transparent conductive film 22 to prevent recombination of electrons and holes (reverse electron transfer); a power generation layer 4 provided on the blocking layer 3 and formed by stacking a perovskite layer 44, which generates electrons upon excitation by light, on a porous semiconductor 41; and a hole transport layer 5 provided on the power generation layer 4 and through which holes generated in the perovskite layer 44 pass. The solar cell 10 also includes an electrode 6 provided on the surface of the blocking layer 3, which includes a photoelectrode 61 that emits electrons through the transparent conductive film 22, and a counter electrode 62 that receives electrons and is provided on the surface of the hole transport layer 5. The arrangement of the electrode 6 is not particularly limited as long as electrons can be transferred; for example, the photoelectrode 61 may be formed by connecting a conductor to the transparent conductive film 22. In order to improve the durability of the solar cell 10, the counter electrode 62 may be protected by a transparent substrate 21 or the like.

[0032] The transparent substrate 21 is made of a light-transmitting material. For example, a transparent glass substrate, a frosted semi-transparent glass substrate, a transparent resin substrate, etc. The transparent conductive film 22 can be made of, for example, fluorine-doped tin oxide (FTO), tin oxide (TO), tin-doped indium oxide (ITO), zinc oxide (ZnO), aluminum-doped zinc oxide (AZO), etc.

[0033] Metal oxides such as titanium dioxide (TiO), zinc oxide (ZnO), niobium oxide (NbO), tin dioxide (SnO), and aluminum oxide (AlO) are suitable for the blocking layer 3 and the porous semiconductor 41. A sintered body of titanium dioxide (TiO) is particularly preferable, as it provides a large surface area for laminating the perovskite layer 44. The blocking layer 3 extends partially into the transparent conductive film 22, thereby defining the transparent conductive film 22. The blocking layer 3 also includes a dense insulating layer 31 that allows electrons to pass through in the lamination direction but restricts their lateral movement. In other words, electrons entering from the blocking layer 3 smoothly move in the lamination direction of the transparent conductive film 22 and are supplied to the photoelectrode 61. At the same time, the insulating layer 31 prevents electrons from moving to the counter electrode 62, preventing short-circuiting.

[0034] The perovskite layer 44 is produced by reacting a compound composed of lead and a halogen element X (PbX2, X = halogen element) with methylammonium iodide (CH3NH3I, hereinafter sometimes abbreviated as "MAI"). Specifically, a solution 42 containing lead and a halogen element X is infiltrated into the pores of a porous semiconductor 41, dried, and then immersed in a mixed solution 43 of MAI, whereby crystals of the perovskite compound (CH3NH3PbI3 when X = I) that forms the perovskite layer 44 are rapidly produced. The halogen element X can be iodine, bromine, or chlorine, but iodine is preferred due to its high morphological stability. Alternatively, a mixed cation-mixed halide (FAPbI3) using MABr and 0.2 M lead bromide (PbBr2), or FAI and lead iodide (PbI2) can be used. 1-x (MAPbBr3) x ) can also be used. For example, (FAPbI3) 0.85 (MAPbBr3) 0.15 etc. can be suitably used.

[0035] A hole transport material, which will be described later, is used for the hole transport layer 5. For the electrode 6, for example, a simple metal or alloy of a metal such as gold, platinum, silver, or copper, or an oxide conductor such as fluorine-doped tin oxide (FTO) or tin-doped indium oxide (ITO) can be used.

[0036] Here, the principle of how solar cell 10 generates electricity will be explained with reference to Figure 3. When light such as sunlight or room light is incident on transparent substrate 21, this incident light passes through substrate 2 and blocking layer 3 without being absorbed much, and most of it reaches power generation layer 4. When the incident light that has reached power generation layer 4 is irradiated onto perovskite layer 44, this perovskite layer 44 absorbs the light energy and becomes excited. When this excitation raises the energy level of perovskite layer 44 to a predetermined level higher than the conduction band potential of the metal oxide that is porous semiconductor 41, electrons are injected from perovskite layer 44 into porous semiconductor 41. The injected electrons pass through blocking layer 3 and are collected by photoelectrode 61.

[0037] Meanwhile, holes generated in the perovskite layer 44 reach the counter electrode 62 via the hole transport layer 5, where they recombine with electrons that have passed through the external load 7. In other words, a potential gradient is generated between the photoelectrode 61 and the counter electrode 62, and power can be supplied by connecting the external load 7 between the two electrodes.

[0038] (Procedure for producing solar cell 10 according to this embodiment) The procedure for fabricating the solar cell 10 according to this embodiment will be described with reference to FIG. 4. However, the present invention is not limited to the following embodiment, and various modifications are possible without departing from the spirit of the invention. The solar cell can be fabricated with reference to known techniques such as Michael Saliba et al., "Correction to 'How to Make over 20% Efficient Perovskite Solar Cells in Regular (nip) and Inverted (pin) Architectures," Chem. Mater., 2018, 30, 4193-4218.

[0039] First, a transparent conductive film 22 is formed on a transparent substrate 21 to prepare the substrate 2. The transparent conductive film 22 is laminated on the transparent substrate 21 by, for example, chemical vapor deposition (CVD) or sputtering. Next, laser scribing is performed to partially remove the transparent conductive film 22, forming recesses 221 for the insulating layer 31, followed by cleaning. Next, a blocking layer 3 is formed over the entire surface of the substrate 2 by, for example, atomic layer deposition (ALD) or spray pyrolysis (SPD). The blocking layer 3 is preferably formed as a dense TiO layer. Next, a porous semiconductor 41, which is a nanoparticle sintered layer, is formed near the center of the masked substrate 2 and blocking layer 3. It is preferably formed as a porous layer of TiO (p-TiO). This porous semiconductor 41 is formed by diluting a nanoparticle paste with a solvent, applying and drying it by, for example, spin coating at a rotation speed of 4000 rpm to 6000 rpm, and then removing the mask and heating it at 450°C to 550°C to form a sintered layer.

[0040] For example, a PbI2 solution 42 in N,N-dimethylformamide is prepared and dropped onto the porous semiconductor 41. After that, the solution is permeated into the pores (p-TiO2) and excess solution is removed by spin coating at a rotation speed of, for example, 5000 rpm to 8000 rpm. The solution is then dried at 60°C to 120°C (preferably 70°C to 90°C) to form a PbI2 layer.

[0041] The porous semiconductor 41, which contains the substrate 2, blocking layer 3, and lead iodide impregnated therein, is immersed in an isopropyl alcohol solution 43 of MAI (CH3NH3I) (2-20 mg / ml) at 0°C to 80°C (preferably at room temperature) (MAI immersion method). The PbI2 reacts with MAI to form a perovskite compound [(CH3NH3)PbI3(MAPbI3)] as a perovskite layer 44 inside and on top of the pores of the porous semiconductor 41. The porous semiconductor 41 is then rinsed with pure isopropyl alcohol and dried at 60°C to 120°C (preferably 70°C to 100°C). The mixed cation-mixed halide ((FAPbI3) 1-x (MAPbBr3) x)-based perovskite compounds can also be prepared in a similar manner.

[0042] The hole transport material according to this embodiment is prepared as a 60 to 90 mg / ml chlorobenzene solution. The solution is dropped onto the perovskite layer 44, excess solution is removed by spin coating, and the solution is dried to form the hole transport layer 5. The hole transport material according to this embodiment functions stably as a hole transport material without the addition of a dopant substance, so the addition of a dopant substance is not necessary. However, this does not preclude the addition of additives such as a dopant substance as needed. Therefore, a dopant substance such as TPFB may be added. If TPFB is added, the TPFB content of the hole transport material is preferably 0.01 to 100% by weight, and more preferably 0.1 to 50% by weight. For example, the hole transport material according to this embodiment is weighed out to a concentration of 30 mM, and TPFB equivalent to 10% by weight is added. The hole transport layer 5 can be formed using this solution dissolved in chlorobenzene.

[0043] The steps from forming the PbI2 layer to forming the hole transport layer 5 are preferably carried out in a dry nitrogen atmosphere such as in a glove box. Finally, a thin film of gold or the like is deposited on the surfaces of the blocking layer 3 and the hole transport layer 5 by vacuum deposition or the like to form an electrode 6.

[0044] In the above-described fabrication procedure, the crystal growth of the perovskite compound forming the perovskite layer 44 is controlled in two steps, but this process may be performed in one step. For example, a perovskite ((CH3NH3)PbI3) solution is infiltrated into the pores of the porous semiconductor 41 by spin coating. Toluene is added dropwise during spinning to precipitate microcrystals and give the surface a mirror finish (poor solvent precipitation method). Subsequently, the hole transport layer 5 may be formed using the hole transport material according to this embodiment.

[0045] (Hole transport material according to this embodiment) A suitable example of the hole transport material according to this embodiment is a compound represented by the following general formula (1).

[0046] [ka]

[0047] The hole transport material according to this embodiment has a structure in which three D moieties are introduced to a nitrogen atom, and each D moiety is preferably configured as the same group.

[0048] The D portion preferably has the structure of the following group (I). Therefore, the hole transport material according to this embodiment has a structure in which three units of a stilbene structure having a p-amino group are introduced to the nitrogen atom of general formula (1). It can also be said to have a structure in which a styryl group having a p-amino group is introduced to the p-position of each phenyl group of triphenylamine. The wavy line in group (I) indicates the bonding position with the nitrogen atom in general formula (1).

[0049] [ka]

[0050] R in group (I) a1 , R a2 , R b1 , R b2 , R c1 , R c2 , R d1 , R d2 , R e1 , R e2 , R f and R g is defined as follows:

[0051] In group (I), R a1 and R a2 independently, -C m H 2m+1 (where m is an integer of 1 or 2), -C(C m H 2m+1 )(C n H 2n+1 )(Cp H 2p+1 ) (wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4), and —CHC(C m H 2m+1 )(C n H 2n+1 )(C p H 2p+1 ) (wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4). R a1 and R a2 may be the same or different.

[0052] R a1 and R a2 -C is one of the options m H 2m+1 is an alkyl group having a chain length of 1 or 2 carbon atoms. By this definition, it is a straight chain alkyl group having a chain length of 1 or 2, specifically a methyl or ethyl group. Thus, R a1 and R a2 When the option is a straight chain alkyl group, it is limited to a methyl group or an ethyl group.

[0053] R a1 and R a2 -C(C m H 2m+1 )(C n H 2n+1 )(C p H 2p+1 ) and -CHC(C m H 2m+1 )(C n H 2n+1 )(C p H 2p+1 ) is a branched alkyl group. According to this definition, the main chain is a linear alkyl group having 2 to 6 carbon atoms, specifically, an n-ethyl group, an n-butyl group, an n-propyl group, an n-pentyl group, or an n-hexyl group. The side chains to the main chain are alkyl groups having 1 to 4 carbon atoms.

[0054] Unless otherwise specified, the alkyl group serving as a side chain may be straight-chain or branched. Specific examples of alkyl groups having 1 to 4 carbon atoms include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, s-butyl, t-butyl, n-pentyl, isopentyl, neopentyl, t-pentyl, and s-pentyl groups. The position and number of side chains to be introduced are not particularly limited, as long as they satisfy the above definition.

[0055] In group (I), R a1 and R a2 is preferably a methyl group, an isopropyl group, or a 2-ethylhexyl group, and preferably R a1 and R a2 are constituted as the same group.

[0056] Thus, the stilbene structure in group (I) has a structure in which a p-amino group is introduced into the terminal benzene ring, and the amino group is substituted with the N on the amino group by the above R a1 and R a2 is introduced into the stilbene structure of group (I), which is configured as a secondary amino group into which a relatively bulky substituent has been introduced.

[0057] Here, the stilbene constituting group (I) exists as trans and cis stereoisomers, and in this embodiment, the trans isomer is preferred. However, when excited by UV irradiation or other factors, a trans-cis isomerization reaction occurs, resulting in a mixed state of both isomers. Furthermore, it is known that the cis isomer can undergo a cyclization reaction in the presence of an oxidizing agent such as iodine, forming a phenanthrene ring. Such isomerization and cyclization reactions weaken the π-conjugated system, resulting in a decrease in hole transport properties. On the other hand, as described above, by introducing a relatively bulky group into the terminal benzene ring of the stilbene structure in group (I), the trans-cis isomerization and cyclization reaction of the stilbene structure can be suppressed, thereby maintaining the preferred trans isomer.

[0058] In group (I), R b1 and Rb2 are independently -H.

[0059] In group (I), R c1 and R c2 independently, -C m H 2m+1 (wherein m is an integer selected from 0 to 4), -OC m H 2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (wherein m is an integer selected from 0 to 2). In group (I), R c1 and R c2 may be the same or different.

[0060] R c1 and R c2 -C is one of the options m H 2m+1 In this case, when m=0, R c1 and R c2 is -H. Also, when m=1 to 4, R c1 and R c2 is an alkyl group having a chain length of 1 to 4 carbon atoms, the detailed definition of the alkyl group being as described above.

[0061] R c1 and R c2 -OC is one of the options m H 2m+1 is an alkoxy group having a chain length of 1 to 4 carbon atoms. An alkoxy group is a group in which an alkyl group is bonded to an oxygen atom. The detailed definition of the alkyl group is as described above.

[0062] R c1 and R c2 -C is one of the options m F 2m+1 In this case, when m=0, R c1 and R c2 is -F. Also, when m=1 or 2, R c1 and R c2is a fluoroalkyl group, and is a perfluoroalkyl group in which all the hydrogen atoms in the alkyl group are replaced with fluorine atoms. c1 and R c2 Choices include -CF3, -CF2CF3.

[0063] In group (I), R c1 and R c2 is preferably configured as -H. Alternatively, R c1 and R c2 is preferably independently -C m H 2m+1 (wherein m is an integer selected from 1 to 4), -OC m H 2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (where m is an integer selected from 0 to 2). c1 and R c2 are preferably configured as the same group.

[0064] In group (I), R d1 and R d2 independently, -C m H 2m+1 (wherein m is an integer selected from 0 to 4), -OC m H 2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (wherein m is an integer selected from 0 to 2). In group (I), R d1 and R d2 may be the same or different.

[0065] R d1 and R d2 Among the options, -C m H 2m+1 , -OC m H 2m+1 , and -C m F 2m+1 The definition of R c1 and Rc2 It is as defined in

[0066] In group (I), R d1 and R d2 is preferably —H, and R d1 and R d2 are preferably configured as the same group.

[0067] where R c1 and R c2 and R d1 and R d2 Although the options for R are the same, these substituents are independently selected and may be the same or different. c1 , R c2 , R d1 , R d2 Some or all of these may be the same, or all may be different.

[0068] In group (I), R e1 and R e2 are independently -H.

[0069] In group (I), R f is -H or -CH3. f is preferably —H.

[0070] In group (I), R g is -H or -CH3. g is preferably —H.

[0071] Also, R f and R g is located on the double bond connecting the two benzene rings of the stilbene in group (I), and therefore -H on the double bond may or may not be substituted with a methyl group.

[0072] where R f and R g The options are the same, but these substituents may be the same or different.

[0073] [Suitable Examples of Hole Transport Materials According to the Present Embodiment] Suitable examples of compounds of the hole transport material according to this embodiment are summarized in the table below.

[0074] [Table 1]

[0075] The structures of compounds that are suitable examples of the hole transport material according to this embodiment are described below. A compound designated MSTPA-1, which is a compound of general formula (2), has a structure in which a p-dimethylaminostyryl group is introduced at the p-position of each phenyl group of a triphenylamine. A compound designated MSTPA-7, which is a compound of general formula (3), has a structure in which a p-di(isopropyl)aminostyryl group is introduced at the p-position of each phenyl group of a triphenylamine. A compound designated MSTPA-6, which is a compound of general formula (4), has a structure in which a p-di(2-ethylhexyl)aminostyryl group is introduced at the p-position of each phenyl group of a triphenylamine.

[0076] [Method for synthesizing hole transport material according to this embodiment] The hole transport material according to this embodiment can be easily synthesized by referring to the synthesis method described in Example 1. Note that Example 1 shows an example of a synthesis method for MSTPA-1, which is a suitable example of the hole transport material according to this embodiment, but the synthesis is not limited thereto, and other suitable methods can also be used.

[0077] The hole transport material of this embodiment has a structure in which three stilbene units having a p-amino group are introduced to the nitrogen atom of the general formula (1). It can also be said to have a structure in which a styryl group having a p-amino group is introduced to the p-position of each phenyl group of triphenylamine. The hole transport material of this embodiment having such a structure can be synthesized, for example, by reacting tris(4-halogenated phenyl)amine, in which the p-position of each phenyl group of triphenylamine is substituted with a halogen, with 4-vinylaniline, in which the amino group is substituted with a desired substituent and, if necessary, further substituents are introduced, to form a carbon-carbon bond between the phenyl group of the triphenylamine and the terminal of the double bond of the 4-vinylaniline. For this synthesis, a suitable method is the Heck reaction, which reacts an aryl halide with a terminal olefin in the presence of a palladium catalyst and a base to synthesize a substituted olefin. [Example]

[0078] The present invention will be described in detail with reference to the following examples.

[0079] Example 1: Preparation of hole transport material MSTPA-1 The hole transport material prepared in this example is designated "MSTPA-1," represented by the general formula (2) above. MSTPA-1 is a compound having a structure in which a p-dimethylaminostyryl group is introduced at the p-position of each phenyl group of triphenylamine. The preparation of "MSTPA-1" is explained based on Figure 5, which summarizes the synthesis scheme.

[0080] (Synthetic Procedure) [Step] Synthesis of MSTPA-1 Tris(4-bromophenyl)amine 1 (0.4 g, 0.82 mmol), N,N-dimethyl-4-vinylaniline 2 (0.44 g, 2.98 mmol), triethylamine (3.02 g, 29.87 mmol), and 6.6 mL of N,N-dimethylformamide (DMF) were added to a 100 mL Schlenk reaction tube under an argon atmosphere. The reaction mixture was degassed with argon for 10 minutes. Palladium(II) acetate (Pd(OAC)2) (0.02 g, 0.07 mmol) and tri(o-tolyl)phosphine (0.06 g, 0.18 mmol) were then added, and the reaction was carried out at 100 °C for 72 hours. After completion of the reaction, the reaction mixture was extracted with dichloromethane (DCM). After drying over anhydrous sodium sulfate, the solvent was evaporated under reduced pressure. The resulting crude mixture was purified twice by column chromatography on neutral alumina using DCM / hexane (4:6, v / v) as the eluent to give a yellow crude product, which was then further purified by recrystallization with methanol (MeOH) / DCM to give MSTPA-1 as a yellow solid compound (0.38 g, yield 67.25%).

[0081] (confirmation) For the obtained MSTPA-1, 1 The synthesis and purification of DHCF-32 were confirmed by H NMR analysis, mass spectrometry (hereinafter sometimes referred to as "Mass"), high-performance liquid chromatography analysis (hereinafter sometimes referred to as "HPLC"), and ultraviolet-visible-near-infrared spectroscopy (hereinafter sometimes referred to as "UV-Vis") analysis. 1 The results of the 1 H NMR analysis are shown in Figure 7, the results of the mass analysis in Figure 7, the results of the high performance liquid chromatography analysis in Figure 8, and the results of the ultraviolet-visible-near infrared spectroscopic analysis in Figure 9.

[0082] Example 2: Fabrication of solar cell 10 Hereinafter, an example of fabricating a solar cell 10 will be described as Example 3. The solar cell 10 can be fabricated with reference to known techniques such as those described in Chem. Mater., 2018, 30, 4193-4218, as mentioned above.

[0083] As an example, a fluorine-doped tin oxide (FTO) glass (23 mm × 14 mm × 1.6 mm) was used as the transparent substrate 21. This FTO glass was laser scribed to form recesses 221 by removing the FTO, and then thoroughly cleaned. Next, a TiO2 dense film was formed as the blocking layer 3 using the spray pyrolysis method (SPD method). Furthermore, a solution of commercially available TiO2 nanoparticle paste diluted with ethanol was dropped onto the substrate, which was then applied by spin coating and dried at 100°C. After drying, the substrate was baked at 450°C to form a TiO2 nanoparticle layer that would become the porous semiconductor 41. After baking, the substrate was cooled to room temperature.

[0084] Next, a perovskite precursor solution for forming the perovskite layer 44 was prepared. First, FAI (CH(NH2)2I) was weighed out, and a 1.7 M PbI2 / dimethylformamide (DMF) + dimethyl sulfoxide (DMSO) solution that had been prepared in advance was poured into the weighed FAI to prepare a FAPbI3 solution. Next, MAI (CH3NH3I) was weighed out, and a 1.7 M PbBr2 / dimethylformamide (DMF) + dimethyl sulfoxide (DMSO) solution that had been prepared in advance was poured into the weighed MAI to prepare a MAPbBr3 solution.

[0085] Finally, the FAPbI3 solution, the MAPbBr3 solution, and a previously prepared 1.7 M CsI / dimethyl sulfoxide (DMSO) solution were mixed to prepare a precursor solution for spin coating.

[0086] The perovskite layer 44 was formed by dropping the precursor solution onto the TiO2 porous film and applying it by spin coating. After application was completed, the film was heated at 100°C for 60 minutes and then cooled to room temperature.

[0087] Next, a hole transport layer 5 was prepared. In this example, the hole transport layer 5 was prepared without adding a dopant. The hole transport material used was MSTPA-1 synthesized in Example 1 above. The hole transport material was weighed out to a final concentration of 30 mM, and the weighed hole transport material was dissolved in chlorobenzene to prepare a hole transport material solution. The hole transport layer 5 was formed by dropping the prepared hole transport material solution onto the perovskite film and forming it by spin coating.

[0088] As a comparative example, a solar cell 10 was also fabricated using a hole transport layer 5 prepared by adding a dopant. TPFB represented by the following general formula (F) was used as the dopant. For the hole transport layer 5 used in the comparative example, the hole transport material was weighed out to a final concentration of 30 mM, and TPFB equivalent to 10% by weight of the hole transport material was weighed out and added to the hole transport material. A mixture of the hole transport material and TPFB was dissolved in chlorobenzene to prepare a hole transport material solution. The hole transport layer 5 was formed by dropping the hole transport material solution onto a perovskite film and using a spin coating method.

[0089] [ka]

[0090] After the hole transport layer 5 was formed, a gold film of about 100 nm was formed by vacuum deposition, which was then removed and used as the electrode 6, which was then laminated.

[0091] Example 3: Evaluation of cell performance of solar cell 10 In this example, the cell performance of solar cell 10 was evaluated. Here, we investigated solar cell 10 using MSTPA-1 synthesized in Example 1 as the hole transport material, a solar cell 10 with a hole transport layer 5 prepared without adding a dopant substance, and a solar cell 10 (comparative example) with a hole transport layer 5 prepared with adding a dopant substance. Note that solar cell 10 was unsealed and stored in a dark place under a dry atmosphere.

[0092] Figure 10 shows the results of cell performance evaluation of the solar cell 10. In this example, the change over time in the photoelectric conversion efficiency (PCE (%)) of the fabricated solar cell 10 was evaluated. The efficiency was calculated by Voc x Jsc x FF / incident light intensity. Voc is the open circuit voltage [V], and Jsc is the short circuit current density [mA / cm 2 ], FF is the fill factor, and the maximum output P max It is calculated by dividing by (Voc × Jsc).

[0093] The results in Figure 10 show that the solar cell 10 of the example, which used a hole transport layer 5 prepared without adding a dopant substance, exhibited higher efficiency than the solar cell 10 of the comparative example, which used a hole transport layer 5 prepared with adding a dopant substance. Furthermore, the solar cell 10 of the example continued to maintain high efficiency after performance improvement through aging for up to 7 days. The solar cell 10 of the comparative example maintained efficiency for about 20 days after aging for up to 7 days, but then its efficiency decreased. These results confirm that MSTPA-1 can operate stably and sustainably as a hole transport material even without the addition of a dopant substance, and that its hole transport performance is also excellent.

[0094] MSTPA-1 contains three stilbene units, which are π-conjugated throughout the molecule, extending the π-conjugation throughout the entire molecule. Therefore, MSTPA-1 can function as a hole-transporting material without the addition of a dopant. This eliminates the problem of reduced solar cell performance due to the dopant, and is believed to have enabled the provision of solar cell 10, which is stable over a long period of time. Furthermore, a dialkylamino group is introduced into the terminal benzene ring of the three stilbene units in MSTPA-1. The introduction of a bulky substituent such as a dialkylamino group suppresses cis-trans isomerization and cyclization of the stilbene structure, allowing MSTPA-1 to maintain stable π-conjugation, which is believed to have also enabled the provision of solar cell 10, which is stable over a long period of time.

[0095] Furthermore, MSTPA-1 of Example 1 has good solubility in solvents, and after dissolving MSTPA-1 in an appropriate solvent, the hole transport layer 5 can be formed by a wet process such as a spin coating method, which has the advantage of simplifying the fabrication of the solar cell 10. Furthermore, the hole transport layer 5 formed using MSTPA-1 as a hole transport material can also function as a protective film for the perovskite layer 44, which decomposes in water, thereby improving the durability of the solar cell 10. Furthermore, since no dopant substance is added during the fabrication of the solar cell 10, the fabrication cost of the solar cell 10 can be reduced.

[0096] Example 4: Synthesis of various hole transport materials Various hole transport materials can be synthesized in the same manner as in Example 1. For example, a hole transport material called "MSTPA-7" represented by general formula (3) was synthesized in the same manner as in Example 1, except that N,N-dimethyl-4-vinylaniline 2 was replaced with N,N-di(isopropyl)-4-vinylaniline 3 in step 1 of Example 1, as summarized in the synthesis scheme in Figure 5.

[0097] Figure 11 shows the MSTPA-7 1 The results of H NMR analysis, mass spectrometry, high-performance liquid chromatography analysis, and ultraviolet-visible-near-infrared spectroscopy analysis are shown.

[0098] Similarly, a hole transport material called "MSTPA-6" represented by general formula (4) can be synthesized.

[0099] Like MSTPA-1, MSTPA-7 and MSTPA-6 have extended π-conjugation throughout the molecule, and these compounds are expected to have properties similar to those of MSTPA-1.

[0100] In the above embodiment, the following configurations are envisioned.

[0101] (1) A hole transport material having a structure represented by the following general formula (1): [ka] [In general formula (1), the three D portions are regions having the same structure, The D moiety is represented by the structure of group (I) below: [ka] (In group (I), R a1 and R a2 independently, -C m H 2m+1 (where m is an integer of 1 or 2), -C(C m H 2m+1 )(C n H 2n+1 )(C p H 2p+1 ) (wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4), and —CHC(C m H 2m+1 )(C n H 2n+1 )(C p H 2p+1 ) (wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4), R b1 and R b2 are independently -H, R c1 and R c2 independently, -C m H 2m+1 (wherein m is an integer selected from 0 to 4), -OC m H 2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (where m is an integer selected from 0 to 2) R d1 and R d2 independently, -C m H 2m+1 (wherein m is an integer selected from 0 to 4), -OC m H2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (where m is an integer selected from 0 to 2) R e1 and R e2 are independently -H, R f is -H or -CH3, R g is -H or -CH3.

[0102] According to this embodiment, a hole transport material having excellent hole transport properties, capable of stably and efficiently capturing and transporting holes, can be provided. Specifically, the hole transport material according to this embodiment has a structure in which three units of a stilbene structure having a p-amino group are introduced to the nitrogen atom of the general formula (1). It can also be said to have a structure in which a styryl group having a p-amino group is introduced to the p-position of each phenyl group of triphenylamine. Because three units of a stilbene structure, which is an entire π-conjugated system, are introduced, π-conjugation extends throughout the molecule. This further smooths the intramolecular π-conjugation, allowing for appropriate control of the intramolecular push-pull effect and improving intramolecular and intermolecular charge transfer characteristics. Therefore, the hole transport material according to this embodiment can function as a hole transport material without the addition of a dopant substance. This eliminates the problem of decreased solar cell performance due to the dopant substance, allowing for the provision of a solar cell 10 that is stable over a long period of time. Furthermore, the absence of an expensive dopant substance reduces the production cost of the solar cell 10.

[0103] In the hole transport material according to this embodiment, a secondary amino group is introduced at the p-position of the terminal benzene ring of the three-unit stilbene structure. Introduction of such a bulky substituent suppresses cis-trans isomerization and cyclization reactions of the stilbene structure. Furthermore, the hole transport material according to this embodiment can maintain stable π-conjugation, thereby exhibiting excellent hole transport properties stably over a long period of time and improving the durability of the solar cell 10.

[0104] The hole transport material according to this embodiment exhibits almost no light absorption in the visible light region and only weak light absorption in the 400 to 450 nm region. Therefore, a solar cell 10 using the hole transport material according to this embodiment can minimize light absorption loss due to the hole transport material, and is expected to have high photoelectric conversion efficiency.

[0105] The hole transport material according to this embodiment also has excellent solubility in solvents, which allows for the use of wet processes such as spin coating, facilitating the formation of the hole transport layer 5 of the solar cell 10.

[0106] Furthermore, the hole transport material according to this embodiment can be synthesized using inexpensive raw materials with a few synthesis steps in a short time, and a high-purity product can be provided through a simple purification step. Therefore, expensive reagents and complicated steps are not required, and the costs required for synthesis and purification can be reduced, making it possible to provide an inexpensive, high-performance hole transport material.

[0107] Furthermore, the hole transport material according to this embodiment allows smooth intramolecular and intermolecular hole movement, which is expected to suppress molecular oxidation and improve durability. Furthermore, because the hole transport material according to this embodiment has improved hydrophobicity, the hole transport layer 5 formed using the hole transport material according to this embodiment can also function as a protective film for the perovskite layer 44, which decomposes in the presence of water, resulting in improved durability of the solar cell 10.

[0108] As described above, the hole transport material according to this embodiment has excellent properties and can be suitably used as a hole transport material for the solar cell 10. When the hole transport material according to this embodiment is used in the hole transport layer 5 of a perovskite solar cell or the like, excellent cell performance was demonstrated in initial experiments.

[0109] (2) In the hole transport material of (1), R c1 and R c2 independently, -C m H 2m+1 (wherein m is an integer selected from 1 to 4), -OC m H 2m+1 (wherein m is an integer selected from 1 to 4), and -C m F 2m+1 (where m is an integer selected from 0 to 2).

[0110] In the hole-transport material according to this embodiment, the introduction of a substituent at the o-position of the terminal benzene ring of the three-unit stilbene structure allows for appropriate control of the intramolecular push-pull effect and improved intramolecular and intermolecular charge transfer characteristics. Therefore, the hole-transport material according to this embodiment can function as a hole-transport material without the addition of a dopant. This eliminates the problem of reduced solar cell performance due to the dopant, and allows for the provision of a solar cell 10 that is stable over a long period of time. Furthermore, the introduction of a substituent at the o-position further suppresses cis-trans isomerization and cyclization reactions of the stilbene structure. This allows the hole-transport material according to this embodiment to maintain stable π-conjugation and therefore exhibit excellent hole-transport characteristics stably over a long period of time.

[0111] (3) In the hole transport material of (1), R a1 and R a2 is preferably a methyl group.

[0112] In the hole-transport material according to this embodiment, by introducing a dialkylamino group into the p-position of the terminal benzene ring of the three-unit stilbene structure, the intramolecular push-pull effect can be appropriately controlled, improving intramolecular and intermolecular charge transfer characteristics. Therefore, the hole-transport material according to this embodiment can function as a hole-transport material without the addition of a dopant substance. This eliminates the problem of reduced solar cell performance due to the dopant substance, and allows for the provision of a solar cell 10 that is stable over a long period of time. Furthermore, by introducing a dialkylamino group into the p-position, cis-trans isomerization and cyclization reactions of the stilbene structure are further suppressed. This allows the hole-transport material according to this embodiment to maintain stable π-conjugation, thereby enabling it to exhibit excellent hole-transport characteristics stably over a long period of time.

[0113] (4) In the hole transport material of (1), it is preferable that the hole transport material is represented by the following general formula (2). [ka]

[0114] The hole transport material according to this embodiment is constructed by introducing a three-unit stilbene structure in which a dialkylamino group is introduced at the p-position of the terminal benzene ring to the nitrogen atom, thereby enabling appropriate control of the intramolecular push-pull effect and improving intramolecular and intermolecular charge transfer characteristics. Therefore, the hole transport material according to this embodiment can function as a hole transport material without the addition of a dopant. This eliminates the problem of reduced solar cell performance due to the dopant, enabling the provision of a stable solar cell 10 over a long period of time. Furthermore, the introduction of a dialkylamino group at the p-position of the terminal benzene ring of the stilbene structure further suppresses cis-trans isomerization and cyclization reactions of the stilbene structure. This allows the hole transport material according to this embodiment to maintain stable π-conjugation and thereby exhibit excellent hole transport properties over a long period of time.

[0115] (5) A solar cell 10 using the hole transport material of (1), comprising: a substrate 2 having a transparent conductive film 22; a blocking layer 3 that transfers electrons to the transparent conductive film 22 and prevents reverse electron transfer; a power generation layer 4 formed by stacking a perovskite layer 44 that is excited by light to generate the electrons on a porous semiconductor 41; and a hole transport layer 5 through which holes generated from the perovskite layer 44 pass and which contains the hole transport material; and an electrode 6 comprising a photoelectrode 61 that emits the electrons via the transparent conductive film 22 and a counter electrode 62 provided on the surface of the hole transport layer 5.

[0116] The solar cell 10 according to this embodiment includes a hole transport layer 5 formed using the hole transport material according to this embodiment, which has the above-described excellent properties. Specifically, the hole transport material according to this embodiment has excellent hole transport properties, enabling it to stably and efficiently capture and transport holes even in the absence of a dopant substance. In particular, the hole transport material according to this embodiment exhibits a large intramolecular push-pull effect, exhibits excellent intramolecular and intermolecular charge transfer properties, and thus exhibits excellent hole transport properties. Furthermore, as described above, the hole transport material according to this embodiment has excellent light absorption properties and also excellent durability. Therefore, by using a hole transport material with excellent properties as the hole transport layer 5, the solar cell 10 according to this embodiment can improve the photoelectric conversion efficiency, thereby improving cell performance and further improving the durability of the solar cell 10.

[0117] Furthermore, when conventional spiro-OMeTAD and LD29, as described in the [Prior Art] section above, are used as hole transport materials, it is common to add a dopant substance that improves hole transport efficiency. However, dopant substances are generally expensive reagents, resulting in an increase in the cost of fabricating the solar cell 10. It is known that the electric field generated by the operation of the solar cell 10 causes the dopant substance to migrate within the hole transport layer 5, resulting in a biased distribution of the dopant within the hole transport layer 5. Furthermore, the dopant substance migrates to the perovskite layer 44 adjacent to the hole transport layer 5 and to the interface between the perovskite layer 44 and the adjacent electron transport layer (blocking layer 3). Such migration of the dopant substance significantly deteriorates the solar cell's performance and poses durability challenges. The solar cell 10 according to this embodiment does not require the addition of a dopant substance, thereby reducing the fabrication cost of the solar cell 10 and improving its durability.

[0118] Furthermore, as described above, the hole transport material according to this embodiment can be synthesized inexpensively and simply without requiring expensive reagents or complicated processes. By using the cost-effective hole transport material according to this embodiment, it is possible to reduce the price of the solar cell 10 according to this embodiment itself.

[0119] [Other embodiments] In the above embodiment, a perovskite solar cell 10 is shown as an example of a solar cell 10 using the hole transport material according to this embodiment, but this hole transport material may also be used in devices using OPV (organic thin film solar cell), organic EL, organic semiconductors, etc. [Industrial Applicability]

[0120] The present invention can be used as a hole transport material for use in perovskite solar cells 10 and the like, which include perovskite compounds formed from organic and inorganic hybrid compounds. [Explanation of symbols]

[0121] 2 boards 3 Blocking Layer 4 Power generation layer 5. Hole transport layer 6 electrodes 10. Solar Cells 11 Laminate 21 Transparent substrate 22 Transparent conductive film 41 Porous Semiconductors 44 Perovskite Layer 61 Photoelectrode 62 Counter electrode

Claims

1. A hole transport material having a structure represented by the following general formula (1): general formula 【Chemistry 1】 [In general formula (1), three D moieties are regions having the same structure, The D moiety is represented by the structure of group (I) below: 【Chemistry 2】 (In group (I), R ａ１ and R ａ２ are independently -C ｍ H ２ｍ＋１ (wherein m is an integer of 1 or 2), -C(C ｍ H ２ｍ＋１ ) (C ｎ H ２ｎ＋１ ) (C ｐ H ２ｐ＋１ ) wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4, and -CH ２ C (C ｍ H ２ｍ＋１ ) (C ｎ H ２ｎ＋１ ) (C ｐ H ２ｐ＋１ ) wherein m is an integer selected from 0 to 4, n is an integer selected from 1 to 4, and p is an integer selected from 1 to 4; R ｂ１ and R ｂ２ are independently —H; R ｃ１ and R ｃ２ are independently -C ｍ H ２ｍ＋１ (wherein m is an integer selected from 0 to 4), —O—C ｍ H ２ｍ＋１ (wherein m is an integer selected from 1 to 4), and -C ｍ F ２ｍ＋１ (wherein m is an integer selected from 0 to 2) R ｄ１ and R ｄ２ are independently -C ｍ H ２ｍ＋１ (wherein m is an integer selected from 0 to 4), —O—C ｍ H ２ｍ＋１ (wherein m is an integer selected from 1 to 4), and -C ｍ F ２ｍ＋１ (wherein m is an integer selected from 0 to 2) R ｅ１ and R ｅ２ are independently —H; R ｆ is -H or -CH ３ and R ｇ is -H or -CH ３ It is.

2. R ｃ１ and R ｃ２ are independently -C ｍ H ２ｍ＋１ (wherein m is an integer selected from 1 to 4), —O—C ｍ H ２ｍ＋１ (wherein m is an integer selected from 1 to 4), and -C ｍ F ２ｍ＋１ 2. The hole transport material of claim 1, wherein m is an integer selected from 0 to 2.

3. R ａ１ and R ａ２ The hole transport material according to claim 1 , wherein is a methyl group.

4. The hole transport material according to claim 1 , represented by the following general formula (2): 【Transformation 3】

5. A solar cell using the hole transport material according to any one of claims 1 to 4, a substrate having a transparent conductive film; a blocking layer that transfers electrons to the transparent conductive film and prevents reverse electron transfer; a power generation layer formed by laminating a perovskite layer that is excited by light to generate the electrons on a porous semiconductor; a hole transport layer through which holes generated from the perovskite layer pass and which contains the hole transport material; and a laminate in this order. a photoelectrode that emits the electrons through the transparent conductive film, and an electrode that is composed of a counter electrode provided on a surface of the hole transport layer.

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