Hole transport materials and solar cells using the hole transport materials

Abstract

To provide: a hole transport material stably exhibiting excellent hole transport properties; an inexpensive hole transport material; and an inexpensive solar cell with high battery performance.SOLUTION: The hole transport material is a compound having a donor-acceptor-donor (D-A-D) structure and represented by the general formula (1) in the figure. The solar cell employs the hole transport material.SELECTED DRAWING: None

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, 2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)-9,9'-spirobifluorene (commonly known as "spiro-OMeTAD"; hereinafter, this name will be used) shown in the following general formula (A) has been widely used as the hole transport material for the hole transport layer. When crystals of perovskite compounds absorb light, electrons and holes are generated. The holes are transported to the counter electrode by the hole transport material, and the electrons move to the photoelectrode, and this cycle is repeated to generate electricity.

[0006] [ka]

[0007] However, as reported in Non-Patent Document 1, spiro-OMeTAD has problems such as high costs due to the large number of synthesis and purification steps required, relatively low hole mobility, and the need for a dopant such as a cobalt complex to function efficiently and stably as a hole transport material. Therefore, research is being conducted on hole transport materials that can be provided at low cost and exhibit efficient and stable hole transport effects. Non-Patent Document 1 reports a novel hole transport material, a compound called LD29, shown in the following general formula (B), in which the acceptor (hereinafter referred to as "A") moiety has a carbazole ring and the donor (hereinafter referred to as "D") moiety has a triphenyldimethoxyamino group linked in a donor-acceptor-donor (hereinafter referred to as "DAD") structure. In solar cells using LD29 as a hole transport material, the conversion efficiency was 14.29% without dopants. However, by adding lithium bis(trifluoromethanesulfonyl)imide (hereinafter sometimes referred to as "LiTFSI"), 4-tert-butylpyridine (TBP), and a cobalt complex (FK209) as dopants, the conversion efficiency improved to 18.0%, achieving a conversion efficiency comparable to that of spiro-OMeTAD.

[0008] [ka]

[0009] In LD29, a carbazole ring is introduced into the A moiety. The unpaired electron of the heteroatom in the carbazole ring is thought to have an undesirable effect on electron-withdrawing properties. This reduces the hole transport ability within the LD29 molecule, making it difficult to achieve sufficient solar cell efficiency in solar cells using LD29 as a hole transport material. Furthermore, these solar cells have durability issues, such as a decrease in solar cell efficiency over time. This is thought to be due to the diffusion of lithium ions contained in LiTFSI, which is added as a dopant to the hole transport material, out of the hole transport layer, and the strong hygroscopicity of LiTFSI itself.

[0010] Furthermore, Non-Patent Document 2 reports a compound called YN1, which is represented by the following general formula (C), and Non-Patent Document 3 reports a compound called PTZ2, which is represented by the following general formula (D). Both compounds have a DAD-type structure, but suffer from the same problems as those of LD29.

[0011] [ka]

[0012] [ka] [Prior art documents] [Non-patent literature]

[0013] [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] 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 3] 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) Summary of the Invention [Problem to be solved by the invention]

[0014] In view of the above-mentioned problems of the conventional technology, there is a need to provide a hole transport material that stably exhibits excellent hole transport properties. There is also a need to provide a hole transport material at low cost. There is also a need to provide a solar cell with high cell performance at low cost. [Means for solving the problem]

[0015] The present inventors conducted extensive research to solve the above-mentioned problems and discovered the following: The inventors constructed a hole-transport material having a DAD structure, in which the A moiety contains an electron-accepting group such as a fluorene ring composed of carbon and hydrogen atoms, and a bis(4-methoxyphenyl) group is introduced at the 9-position of the fluorene ring. The D moiety contains an electron-donating group such as a 4-(bis(4-methoxyphenyl)amino)phenyl group or a 3,6-bis[N,N-bis(4-methoxyphenyl)amino]-9H-carbazol-9-yl group at the 2- and 7-positions of the fluorene ring. The inventors confirmed that the hole-transport material thus constructed, in which a bis(4-methoxyphenyl) group is introduced at the 9-position of the fluorene ring, is stabilized by a stacked arrangement of planar aromatic rings (referred to as "π-π stacking"), and exhibits excellent hole-transport properties. It was also confirmed that hole transport materials constructed by introducing a highly electron-donating group into the D portion exhibit a large intramolecular push-pull effect, improving the intramolecular charge transfer characteristics and thereby exhibiting excellent hole transport properties. Furthermore, when the hole transport material is used in the hole transport layer of a perovskite solar cell or the like, the hole transport material exhibits excellent hole transport properties and shows excellent cell performance in initial experiments. Based on these findings, the present invention has been completed.

[0016] The present invention relates to a hole transport material, and is characterized in that it is a compound represented by the following general formula (1). [ka] [In the general formula (1), R A teeth 、 [ka] and base (III ) Leave, V and Z are 0, R B represents the following group (I): [ka] (X and Y are 0 and , R 1 -H, -C n H 2n+1 , -OC n H 2n+1 , and -SC n H 2n+1 where n=an integer from 1 to 8; R 3 -H, -C n H 2n+1 , -OC n H 2n+1 , and -SC n H 2n+1 where n=an integer from 1 to 8; In addition, R 1 and R 3 cannot be simultaneously -H, R 2a1 , R 2a2 , R 2b1 , and R 2b2 is independently selected from -H, -F, and -CF, where R 2a1 and R 2a2 At least one of them is -H and R 2b1 and R 2b2 At least one of the groups is -H.)

[0017] The above-described 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 DAD structure, in which the A moiety contains an electron-accepting group, such as a fluorene ring composed of carbon and hydrogen atoms. A bis(4-methoxyphenyl) group is introduced at the 9-position of the fluorene ring, and D moieties containing electron-donating groups, such as a 4-(bis(4-methoxyphenyl)amino)phenyl group or a 3,6-bis[N,N-bis(4-methoxyphenyl)amino]-9H-carbazol-9-yl group, are introduced at the 2- and 7-positions of the fluorene ring. It has been confirmed that hole transport materials with bis(4-methoxyphenyl) groups introduced at the 9-position of the fluorene ring are stabilized by a stacked arrangement of planar aromatic rings (referred to as "π-π stacking"), and exhibit excellent hole transport properties. Furthermore, hole transport materials constructed by introducing a highly electron-donating group into the D moiety in this way exhibit a stronger push-pull effect within the molecule, improving the intramolecular charge transfer characteristics. As a result, electrons move more smoothly within the molecule, resulting in clearer HOMO-LUMO charge separation. In other words, hole transport materials with this configuration have electrons concentrated in the D moiety at the HOMO and HOMO-1 levels, and electrons concentrated in the A moiety at the LUMO level. The improved intramolecular charge transfer characteristics allow the material to exhibit excellent hole transport properties.

[0018] Furthermore, the hole transport material of this configuration has a deep HOMO energy level, which allows appropriate adjustment of the alignment with the HOMO level of the perovskite, thereby improving the hole transport properties and facilitating the improvement of solar cell efficiency.

[0019] This hole transport material exhibits almost no light absorption in the visible light range, and its light absorption intensity in the range of up to 450 nm is weaker than that of the commonly used spiro-OMeTAD. 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.

[0020] The hole transport material of this embodiment also has excellent solubility in solvents. Therefore, wet processes such as spin coating can be used to easily form the hole transport layer of a solar cell. Furthermore, since the hole transport material of this embodiment is soluble in nonpolar solvents, it is possible to fabricate the hole transport layer of a solar cell using inexpensive, environmentally friendly solvents. Meanwhile, conventionally widely used spiro-OMeTAD typically uses chlorobenzene as the solvent when fabricating the hole transport layer. However, chlorobenzene contains chlorine atoms in the molecule and is expensive, which increases the cost of post-processing equipment, including solvent disposal, and also poses significant safety and environmental risks due to exhaust gas leaks.

[0021] 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.

[0022] As described above, the hole transport material of the present configuration has excellent properties and can be suitably used as a hole transport material for solar cells.

[0023] Another characteristic feature of the hole transport material of the present invention is that the R B teeth , B S(4- Alkoxy Phenyl)amine Nomoto It is at that point.

[0024] According to the above-mentioned configuration, the group introduced as the D portion is a group having high electron donating ability. Ruby S(4- Alkoxy Phenyl)amine NomotoBy including the compound containing the compound, the push-pull effect within the molecule of the hole transport material can be appropriately controlled, and the intramolecular charge transfer properties can be further improved. This makes it possible to provide a hole transport material having excellent hole transport properties, in particular, capable of stably and efficiently capturing and transferring holes.

[0025] Another characteristic feature of the hole transport material of the present invention is that the donor portion of the donor-acceptor-donor (DAD) structure has a donor-acceptor-donor (D'-A'-D') structure.

[0026] The above-described configuration provides a hole transport material having a DAD structure, in which each D moiety in the DAD structure has a D'-A'-D' structure. Due to the high electron-donating properties of the D moieties and their interaction with the A moieties, the hole transport material exhibits a large intramolecular push-pull effect, improving the intramolecular charge transport properties. The improved intramolecular charge transport properties allow the material to exhibit particularly excellent hole transport properties.

[0028] According to the above configuration, the A' section Ka By using a rubazole ring, it is possible to provide a hole transport material that can exhibit particularly excellent hole transport properties.

[0029] Another characteristic feature of the hole transport material of the present invention is that it is a compound represented by the following general formula (2). [ka]

[0030] According to the above configuration, the hole transport material has a DAD structure, and each D moiety in the DAD structure is a 3,6-bis[N,N-bis(4-methoxyphenyl)amino]-9H-carbazol-9-yl group having a D'-A'-D' structure. This allows the high electron donating properties of the D moieties and their interaction with the A moieties to appropriately control the push-pull effect within the molecule of the hole transport material, further improving the intramolecular charge transfer properties. This makes it possible to provide a hole transport material with excellent hole transport properties, particularly capable of stably and efficiently capturing and transporting holes.

[0033] The present invention also includes an invention related to a solar cell. A solar cell according to the present invention is characterized in that it comprises a laminate in which the following layers are laminated in this order: 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, on a porous semiconductor, a perovskite layer that is excited by light to generate the electrons; and a hole transport layer through which holes generated from the perovskite layer pass and which contains the hole transport material described above; and an electrode composed of a photoelectrode that releases the electrons via the transparent conductive film and a counter electrode provided on the surface of the hole transport layer.

[0034] By using a hole transport material with excellent hole transport properties in the hole transport layer of a solar cell as in this configuration, the photoelectric conversion efficiency can be improved, and the cell performance of the solar cell can be improved. Furthermore, by using a hole transport material that can be synthesized inexpensively and easily and that can achieve excellent hole transport properties in the hole transport layer of a solar cell, it is also possible to reduce the price of the solar cell itself.

[0035] Another characteristic feature of the solar cell of the present invention is that the hole transport material further contains 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate.

[0036] When conventional spiro-OMeTAD and LD29, as described in the [Prior Art] section above, are used as hole transport materials, LiTFSI is typically added as a dopant to improve hole transport efficiency. However, due to factors such as the lithium ions contained in LiTFSI diffusing out of the hole transport layer and the strong hygroscopicity of LiTFSI itself, the solar cell efficiency decreases over time, resulting in issues with durability. In the solar cell of the present configuration, the durability of the solar cell can be improved by adding 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (hereinafter sometimes referred to as "TPFB") instead of LiTFSI. This is thought to be due to the large molecular size of the cation moiety of TPFB, which makes it difficult to diffuse, and the lack of hygroscopicity of TPFB itself. [Brief explanation of the drawings]

[0037] [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 a synthesis scheme of a hole transport material (DHCF-9) prepared in Comparative Example 1. [Figure 6] FIG. 1 shows the results of 1H NMR analysis confirming the synthesis of the hole transport material (DHCF-9) prepared in Comparative Example 1. [Figure 7] FIG. 1 shows the results of mass spectrometry confirming the synthesis of the hole transport material (DHCF-9) prepared in Comparative Example 1. [Figure 8] FIG. 2 shows the results of ultraviolet-visible-near-infrared spectroscopic analysis confirming the synthesis of the hole transport material (DHCF-9) prepared in Comparative Example 1. [Figure 9]FIG. 1 shows a synthesis scheme of a hole transport material (DHCF-7) prepared in Comparative Example 2. [Figure 10] FIG. 1 shows the results of 1H NMR analysis confirming the synthesis of the hole transport material (DHCF-7) prepared in Comparative Example 2. [Figure 11] FIG. 1 shows the results of mass spectrometry confirming the synthesis of a hole transport material (DHCF-7) prepared in Comparative Example 2. [Figure 12] FIG. 1 shows the results of ultraviolet-visible-near-infrared spectroscopic analysis confirming the synthesis of the hole transport material (DHCF-7) prepared in Comparative Example 2. [Figure 13] FIG. 1 shows a synthesis scheme of the hole transport material (DHCF-8) prepared in Example 1. [Figure 14] FIG. 2 shows the results of 1H NMR analysis confirming the synthesis of the hole transport material (DHCF-8) prepared in Example 1. [Figure 15] FIG. 1 shows the results of mass spectrometry confirming the synthesis of the hole transport material (DHCF-8) prepared in Example 2. [Figure 16] FIG. 1 shows the results of ultraviolet-visible-near-infrared spectroscopic analysis confirming the synthesis of the hole transport material (DHCF-8) prepared in Example 2. [Figure 17] FIG. 1 shows a synthesis scheme of a hole transport material (DHCF-10) prepared in Example 2. [Figure 18] FIG. 1 shows the results of 1H NMR analysis confirming the synthesis of the hole transport material (DHCF-10) prepared in Example 2. [Figure 19] FIG. 1 shows the results of mass spectrometry confirming the synthesis of the hole transport material (DHCF-10) prepared in Example 2. [Figure 20] FIG. 1 shows the results of ultraviolet-visible-near-infrared spectroscopic analysis confirming the synthesis of the hole transport material (DHCF-10) prepared in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0038] Below, an embodiment of a perovskite solar cell 10 (an example of a solar cell; hereinafter referred to as "solar cell 10") using a hole transport material according to an embodiment of the present invention will be described with reference to the drawings. The solar cell 10 is configured to include a perovskite layer 44 made of an organic and inorganic hybrid compound. However, the solar cell is not limited to the following embodiment, and various modifications are possible without departing from the spirit of the invention.

[0039] (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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] (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.

[0047] 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 at 450°C to 550°C to form a sintered layer.

[0048] 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.

[0049] 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). PbI2 and MAI react to form a perovskite compound [(CH3NH3)PbI3(MAPbI3)] as a perovskite layer 44 inside and on 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.

[0050] The hole transport material according to this embodiment is prepared as a 60 to 90 mg / ml chlorobenzene solution, for example. 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. An additive such as TPFB may be added to the hole transport material. When 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 to the hole transport material. The hole transport layer 5 can be formed using this solution dissolved in chlorobenzene.

[0051] 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.

[0052] 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.

[0053] (Hole transport material according to this embodiment) The hole transport material according to this embodiment has a DAD structure, and a suitable example thereof is a compound represented by the following general formula (1):

[0054] [ka]

[0055] An electron-accepting group is introduced into the A portion, which is preferably configured as a fluorene ring. The fluorene ring is composed of carbon atoms and hydrogen atoms and does not contain heteroatoms such as nitrogen atoms. The fluorene ring is linked to the D portion at the 2- and 7-positions, respectively. The D portions linked to the 2- and 7-positions on the fluorene ring are configured as the same group. Furthermore, a bis(4-methoxyphenyl) group is introduced into the 9-position on the fluorene ring.

[0056] The D portion is an electron-donating group, and is represented by R A corresponds to the area of ​​R A is preferably configured as the following group (I), group (II), group (III), or group (IV).

[0057] [ka] [ka] [ka] [ka]

[0058] Group (I) is R B The group (I) R B is preferably a group having the following structure: [ka]

[0059] In the group (I), X and Y are independently an integer of 0, 1, or 2. Therefore, the group (I) includes a diphenylamino group (X=0, Y=0), a 4-(diphenylamino)phenyl group (X=1, Y=0), a 4'-(diphenylamino)[1,1'-biphenyl]-4-yl group (X=2, Y=0), a 5-(diphenylamino)thiophen-2-yl group (X=0, Y=1), a 5'-(diphenylamino)[2,2'-bithiophen]-5-yl group (X=0, Y=2), a 5-[4-(diphenylamino)phenyl]phenyl group (X=0, Y=0), a 5-[4-(diphenylamino)phenyl]phenyl group (X=1 ... and 5'-[4'-(diphenylamino)phenyl][1,1'-biphenyl]-4-yl]thiophen-2-yl group (X=1, Y=1), 5'-[4-(diphenylamino)phenyl][2,2'-bithiophene]-5-yl group (X=1, Y=2), and 5'-[4'-(diphenylamino)[1,1'-biphenyl]-4-yl][2,2'-bithiophene]-5-yl group (X=2, Y=2).

[0060] In group (I), R 1 -H, -C n H 2n+1 , -OC n H 2n+1 , and -SC n H 2n+1 where n=an integer from 1 to 8; and R 3 -H, -C n H 2n+1 , -OC n H 2n+1 , and -SC n H 2n+1 where n is an integer from 1 to 8, and R 1 and R 3 may be the same or different, but cannot be both -H. 1 and R 3 teeth 、 At least one of them is -C n H 2n+1 , -OC n H 2n+1 , and -SC n H 2n+1 is selected from.

[0061] R 1 and R 3 -C in n H 2n+1 is an alkyl group, and may be straight-chain or branched. There is no particular limitation on the chain length as long as n is in the range of 1 to 8. Specific examples include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, s-butyl, t-butyl, n-pentyl, isopentyl, neopentyl, t-pentyl, s-pentyl, 2-methylbutyl, 1-ethylpropyl, 2-ethylpropyl, n-hexyl, isohexyl, neohexyl, t-hexyl, 2,2-dimethylbutyl, 2-methylpentyl, 3-methylpentyl, 1-ethylbutyl, 2-ethylbutyl, 1-propylpropyl, n-heptyl, isoheptyl, s-heptyl, t-heptyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 1-methylhexyl, 2 Examples of the alkyl group include 1-methylhexyl group, 3-methylhexyl group, 4-methylhexyl group, 1-ethylpentyl group, 2-ethylpentyl group, 3-ethylpentyl group, 1-propylbutyl group, 2-propylbutyl group, n-octyl group, isooctyl group, t-octyl group, neooctyl group, 2,2-dimethylhexyl group, 3,3-dimethylhexyl group, 4,4-dimethylhexyl group, 1-methylheptyl group, 2-methylheptyl group, 3-methylheptyl group, 4-methylheptyl group, 5-methylheptyl group, 1-ethylhexyl group, 2-ethylhexyl group, 3-ethylhexyl group, 4-ethylhexyl group, 1-propylpentyl group, 2-propylpentyl group, and 3-propylpentyl group.

[0062] In group (I), R 2a1 , R 2a2 , R 2b1 , and R 2b2 is independently selected from -H, -F, and -CF; R 2a1 , R 2a2 , R 2b1 , and R 2b2 may be the same or different, but R 2a1 and R2a2 At least one of them is -H and R 2b1 and R 2b2 At least one of the groups is -H. Therefore, R 2a1 , R 2a2 , R 2b1 , and R 2b2 When -F or -CF3 is introduced as R 2a1 and R 2a2 Either one of and / or R 2b1 and R 2b2 It is introduced into one of the following.

[0063] Group (I) R B is preferably a 4-(bis(4-alkoxyphenyl)amino)phenyl group, and particularly preferably a 4-(bis(4-methoxyphenyl)amino)phenyl group, but is not limited thereto. A But R B When constituted as group (I) which is preferably R A is a 4-(bis(4-methoxyphenyl)amino)phenyl group, but is not limited thereto.

[0064] In groups (II), (III), and (IV), V and Z are independently selected from 0, 1, and 2. Thus, R A is group (II), group (III), or group (IV), a secondary amino group, -N(R B ) 2 units, 3rd and 6th place are R B or a carbazol-9-yl group substituted with R B The phenothiazin-10-yl group substituted with the above is introduced at the 2- and 7-positions of the fluorene ring directly (V=0, Z=0), or via a benzene ring (V=1, Z=0), a biphenyl ring (V=2, Z=0), a thiophene ring (V=0, Z=1), a bithiophene ring (V=0, Z=2), a benzene ring and a thiophene ring (V=1, Z=1), a biphenyl ring and a thiophene ring (V=2, Z=1), a benzene ring and a bithiophene ring (V=1, Z=2), or a biphenyl ring and a bithiophene ring (V=2, Z=2).

[0065] Also, R A is group (III) or group (IV), the hole transport material of this embodiment has a DAD type structure as shown in chemical formula model (1), and further, the D portion itself has a D'-A'-D' type structure. [ka]

[0066] A' in the D'-A'-D' type structure is a group with high electron-withdrawing properties, and is preferably a phenothiazine ring, a carbazole ring, or a benzothiadiazole group. A carbazole ring is particularly preferred. Furthermore, the D' portion in the D'-A'-D' type structure is preferably the above-mentioned R B (Group (I)). The A and D parts in the DAD type structure are connected adjacent to the A part via the nitrogen atom of the A' part of the D'-A'-D' type structure that constitutes the D part. Therefore, R A is configured as the D moiety having a D'-A'-D' type structure, it is preferably a 3,6-bis[N,N-bis(4-alkoxyphenyl)amino]-9H-carbazol-9-yl group, and particularly preferably a 3,6-bis[N,N-bis(4-methoxyphenyl)amino]-9H-carbazol-9-yl group, but is not limited thereto.

[0067] [Suitable Examples of Hole Transport Materials According to the Present Embodiment] One suitable example of the hole transport material according to this embodiment is a compound represented by the following general formula (2): Hereinafter, the hole transport material of general formula (2) may be referred to as "DHCF-8".

[0068] [ka]

[0069] The structure of DHCF-8, one of the preferred examples of the hole transport material according to this embodiment, will be described in detail. DHCF-8 has a DAD structure, and the D moiety in the DAD structure has a D'-A'-D' structure. The A moiety is configured as a fluorene ring, and a bis(4-methoxyphenyl) group is introduced at the 9-position on the fluorene ring. D moieties with a D'-A'-D' structure are introduced at the 2-position and the 7-position on the fluorene ring, respectively. Each D moiety contains N 3 ,N 3 ,N 6 ,N 6 -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine-9-yl group is introduced.

[0070] Another suitable example of the hole transport material according to this embodiment is a compound represented by the following general formula (3): Hereinafter, the hole transport material of general formula (3) may be referred to as "DHCF-10".

[0071] [ka]

[0072] The structure of DHCF-10, one of the preferred examples of the hole transport material according to this embodiment, will be described in detail. DHCF-10 has a DAD structure. The A moiety is configured as a fluorene ring, and a bis(4-methoxyphenyl) group is introduced at the 9-position on the fluorene ring. D moieties are introduced at the 2- and 7-positions on the fluorene ring, respectively. Each D moiety contains a 4-(bis(4-methoxyphenyl)amino)phenyl group.

[0073] [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 methods described in the following Examples 1 and 2. Note that Examples 1 and 2 show examples of synthesis methods for DHCF-8 and DHCF-10, which are suitable examples of the hole transport material according to this embodiment, respectively, but the synthesis is not limited to these, and other suitable methods can also be used.

[0074] A suitable example of the hole transport material according to this embodiment has a DAD structure, in which the A moiety contains a fluorene ring consisting of carbon and hydrogen atoms. A bis(4-methoxyphenyl) group is introduced at the 9-position of the fluorene ring. Preferably, the D moiety can further have a D'-A'-D' structure. In this case, the A' moiety is configured as an electron-accepting group such as a carbazole ring or a phenothiazine ring. During synthesis, groups constituting the D moiety can be introduced at the 2- and 7-positions of the fluorene ring using cross-coupling or other techniques known in the art. Subsequently, the reaction intermediate can be synthesized by allylation with 4-methoxybenzene, such as 1-bromo-4-methoxybenzene, in which the 1-position, where the fluorene ring is to be introduced, is substituted with a halogen atom such as a bromine atom.

[0075] DHCF-8, which is a preferred example of the above, has a bis(4-methoxyphenyl) group introduced at the 9-position on the fluorene ring constituting the A moiety, and also has N-substituted fluorene groups at the 2- and 7-positions on the fluorene ring as the D moiety. 3 ,N 3 ,N 6 ,N 6 DHCF-8 is a fluorene ring constituting the A moiety in which the 2- and 7-positions, which are the positions for introducing the D moiety, on the fluorene ring constituting the A moiety are substituted with halogen atoms such as bromine atoms, and N-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine-9-yl group is introduced. 3 ,N 3 ,N 6 ,N 6The resulting intermediate is cross-coupled with a secondary amine such as -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine using a Buchwald-Hartwig amination reaction, and the resulting intermediate is allylated with a 4-methoxybenzene, such as 1-bromo-4-methoxybenzene, in which the 1-position, which is the position for introduction into the fluorene ring, is substituted with a halogen atom such as a bromine atom.

[0076] DHCF-10, the preferred example mentioned above, has a bis(4-methoxyphenyl) group introduced at the 9-position on the fluorene ring constituting the A moiety, and 4-(bis(4-methoxyphenyl)amino)phenyl groups introduced as D moieties at the 2- and 7-positions on the fluorene ring. DHCF-10 can be synthesized by cross-coupling fluorene, in which the 2- and 7-positions on the fluorene ring constituting the A moiety, are substituted with halogen atoms such as bromine atoms, with an organoboron compound, such as 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaboran-2-yl)phenyl)aniline, constituting the D moiety, using Suzuki-Miyaura coupling. The resulting reaction intermediate can be synthesized by allylation with 4-methoxybenzene, such as 1-bromo-4-methoxybenzene, in which the 1-position, the position for introduction into the fluorene ring, is substituted with a halogen atom such as bromine atoms.

[0077] [Characteristics of the hole transport material according to this embodiment] The hole transport material according to this embodiment has excellent hole transport properties that enable it to stably and efficiently capture and transport holes.

[0078] Specifically, the hole transport material according to this embodiment has a DAD structure. The A moiety contains an electron-accepting group, such as a fluorene ring composed of carbon and hydrogen atoms. A bis(4-methoxyphenyl) group is introduced at the 9-position of the fluorene ring. Furthermore, the D moiety contains an electron-donating group, such as a 4-(bis(4-methoxyphenyl)amino)phenyl group or a 3,6-bis[N,N-bis(4-methoxyphenyl)amino]-9H-carbazol-9-yl group, at the 2- and 7-positions of the fluorene ring. It has been confirmed that hole transport materials with a bis(4-methoxyphenyl) group introduced at the 9-position of the fluorene ring are stabilized by a stacked arrangement of planar aromatic rings (referred to as "π-π stacking") and exhibit excellent hole transport properties. Furthermore, hole transport materials with a highly electron-donating group introduced at the D moiety exhibit a strong intramolecular push-pull effect, improving intramolecular charge transport properties. As a result, electrons move smoothly within the molecule, and the HOMO-LUMO charge separation becomes clearer. That is, in the hole transport material according to this embodiment, electrons are concentrated in the D portion at the HOMO and HOMO-1 levels, and electrons are concentrated in the A portion at the LUMO level. In this way, the improved charge transfer characteristics within the molecule enable the material to exhibit excellent hole transport properties.

[0079] The hole transport material according to this embodiment has a DAD structure, and preferably further has a D'-A'-D' structure as the D moiety. By using a 3,6-bis[N,N-bis(4-methoxyphenyl)amino]-9H-carbazol-9-yl group or the like having such a D'-A'-D' structure as the D moiety, the high electron donating properties of the D moiety and the interaction with the A moiety can appropriately control the intramolecular push-pull effect of the hole transport material, further improving the intramolecular charge transfer properties. This makes it possible to provide a hole transport material with excellent hole transport properties, particularly capable of stably and efficiently capturing and transporting holes.

[0080] Furthermore, since the hole transport material according to this embodiment has a deep HOMO energy level, the arrangement with the HOMO level of the perovskite can be appropriately adjusted, which has the advantage of improving the hole transport properties and facilitating the improvement of solar cell efficiency.

[0081] The hole transport material according to this embodiment exhibits almost no light absorption in the visible light region, and its light absorption intensity in the region of up to 450 nm is weaker than that of the general-purpose spiro-OMeTAD described in the prior art section. 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.

[0082] The hole transport material according to this embodiment has excellent solubility in solvents, and thus can be applied to a wet process such as a spin-coating method, facilitating the formation of the hole transport layer 5 of the solar cell 10. Furthermore, the hole transport material according to this embodiment is soluble in nonpolar solvents, and therefore the hole transport layer 5 of the solar cell 10 can be formed using an inexpensive, environmentally friendly solvent. Meanwhile, when using conventional spiro-OMeTAD, chlorobenzene is typically used as the solvent for the formation of the hole transport layer 5. However, chlorobenzene contains chlorine in the molecule and is expensive, resulting in high costs for post-processing equipment, including solvent disposal, and also presenting significant safety and environmental impacts due to exhaust gas leaks.

[0083] Furthermore, the hole transport material according to this embodiment can be synthesized using inexpensive raw materials in a short time with fewer synthesis steps, 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.

[0084] As described above, the hole transport material of this embodiment has excellent properties and can be suitably used as a hole transport material for the solar cell 10.

[0085] [Characteristics of the solar cell 10 according to this embodiment] 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 excellent properties as described above. Specifically, the hole transport material according to this embodiment has excellent hole transport properties, capable of stably and efficiently capturing and transporting holes. In particular, the hole transport material according to this embodiment has a large intramolecular push-pull effect, exhibits excellent intramolecular charge transfer properties, and can therefore exhibit excellent hole transport properties. Furthermore, as described above, the hole transport material according to this embodiment has excellent light absorption properties. Therefore, by using a hole transport material with excellent properties as the hole transport layer 5, the solar cell 10 according to this embodiment has improved photoelectric conversion efficiency and, ultimately, improved cell performance.

[0086] Furthermore, when conventional spiro-OMeTAD and LD29, as described in the [Prior Art] section above, are used as hole transport materials, LiTFSI is typically added as a dopant with the function of improving hole transport efficiency. However, due to factors such as the lithium ions contained in LiTFSI diffusing out of the hole transport layer and the strong hygroscopicity of LiTFSI itself, there are issues with durability, such as a decrease in solar cell efficiency over time. In the solar cell of the present configuration, the durability of the solar cell can be improved by adding TPFB instead of LiTFSI. This is thought to be due to the large molecular size of the cation moiety of TPFB, which makes it difficult to diffuse, and the lack of hygroscopicity of TPFB itself.

[0087] 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.

[0088] [Other embodiments] In the above embodiment, a perovskite solar cell is shown as an example of the 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. [Example]

[0089] The present invention will be described in detail with reference to the following examples.

[0090] (Comparative Example 1) Preparation of hole transport material (DHCF-9) The hole transport material prepared as Comparative Example 1 was designated "DHCF-9" and represented by the following general formula (4). The D moiety contained N at the 2-position and the 7-position on the fluorene ring constituting the A moiety. 3 ,N 3 ,N 6 ,N 6 This compound has a -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine-9-yl group introduced, but no substituent at the 9-position on the fluorene ring. The synthesis scheme is summarized in Figure 5. Details of the synthesis and purification were similar to those in Example 1 below.

[0091] (synthesis) Step a: N 3 ,N 3 ,N 6 ,N 6 DHCF-9 was synthesized by the Buchwald-Hartwig amination reaction of -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine (1) with 2,7-dibromo-9H-fluorene (2).

[0092] (Purification and confirmation) The obtained crude product was purified according to a standard method. 1The synthesis and purification of DHCF-9 were confirmed by H NMR analysis, mass spectrometry (hereinafter sometimes referred to as "Mass"), and ultraviolet-visible-near-infrared spectroscopy (hereinafter sometimes referred to as "UV-Vis"). 1 The results of the 1 H NMR analysis are shown in Figure 7, the results of the mass analysis are shown in Figure 8, and the results of the ultraviolet-visible-near-infrared spectroscopic analysis are shown in Figure 8.

[0093] (Comparative Example 2) Preparation of hole transport material (DHCF-7) The hole transport material prepared as Comparative Example 2 was designated "DHCF-7" and represented by the following general formula (5). The D moiety contained N at the 2-position and the 7-position on the fluorene ring constituting the A moiety. 3 ,N 3 ,N 6 ,N 6 This compound has a -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine-9-yl group introduced therein, and a bis(4-(bis(4-methoxyphenyl)amino)phenyl) group introduced at the 9-position on the fluorene ring constituting the A moiety. The synthesis scheme will be explained based on FIG. 9. Details of the synthesis and purification were as described in Example 1 below.

[0094] (synthesis) Step b: DHCF-9 synthesized in Comparative Example 1 was allylated with 4-bromo-N,N-bis(4-methoxyphenyl)aniline (3) to synthesize DHCF-7.

[0095] (Purification and confirmation) The obtained crude product was purified according to a standard method. 1 The synthesis and purification of DHCF-7 were confirmed by H NMR analysis, mass spectrometry, and UV-Vis-NIR spectroscopy. 1 The results of the 1 H NMR analysis are shown in FIG. 11, the results of the mass analysis are shown in FIG. 12, and the results of the ultraviolet-visible-near-infrared spectroscopic analysis are shown in FIG.

[0096] (Example 1) Preparation of hole transport material (DHCF-8) The hole transport material prepared in this example is referred to as "DHCF-8" and is represented by the following general formula (2). The D moiety contains N at the 2-position and 7-position on the fluorene ring constituting the A moiety. 3 ,N 3 ,N 6 ,N 6 This compound has a -tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine-9-yl group introduced, as well as a bis(4-methoxyphenyl) group introduced at the 9-position on the fluorene ring that constitutes the A portion. The synthesis scheme will be explained based on Figure 13.

[0097] DHCF-8 was synthesized by acylation of DHCF-9 obtained in Comparative Example 1 with 1-bromo-methoxybenzene (4), and purified according to a standard method. The synthesis and purification of DHCF-8 are described in detail below.

[0098] (synthesis) Process c: In a 50 mL round-bottom flask, add 9,9´-(9H-fluorene-2,7-diyl)bis(N 3 ,N 3 ,N 6 ,N 6 1-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine (DHCF-9) (0.18 g, 0.128 mmol), 1-bromo-4-methoxybenzene (4) (0.055 g, 0.28 mmol), and potassium tert-butoxide (KOtBu) (0.43 g, 0.34 mmol) were added and dissolved in 25 mL of dry toluene. The reaction mixture was degassed under an argon atmosphere for 30 minutes, and Pd(dba) (0.011 g, 0.019 mmol) and triphenylphosphine (TPP) (0.01 g, 0.038 mmol) were added. The reaction mixture was then heated to reflux at 110 °C for 24 hours under an argon atmosphere. After the reaction was completed, the reaction mixture was cooled to room temperature and extracted with dichloromethane (hereinafter sometimes referred to as "DCM"). It was subsequently washed with water and brine solution, and the organic phase was separated and concentrated to give the crude product.

[0099] (Purification and confirmation) The obtained crude product was purified by column chromatography (silica gel: 100-200 mesh) using methanol:DCM=1:99 as an eluent to obtain DHCF-8 as a yellow solid (yield: 0.14 g, yield: 67.6%). 1 The synthesis and purification of DHCF-3 were confirmed by H NMR analysis, mass spectrometry, and UV-Vis-NIR spectroscopy. 1 The results of the 1 H NMR analysis are shown in FIG. 15, the results of the mass analysis are shown in FIG. 16, and the results of the ultraviolet-visible-near-infrared spectroscopic analysis are shown in FIG.

[0100] Example 2: Preparation of hole transport material (DHCF-10) The hole transport material prepared in Example 2 is designated "DHCF-10" and is represented by the following general formula (5). This is a compound in which 4-(bis(4-methoxyphenyl)amino)phenyl groups are introduced as D moieties at the 2- and 7-positions of the fluorene ring constituting the A moiety, and a bis(4-methoxyphenyl) group is introduced at the 9-position of the fluorene ring constituting the A moiety. The synthesis scheme is explained based on Figure 17. Details of the synthesis and purification were similar to those in Example 1 above.

[0101] (synthesis) Process d: 4,4´-(9H-fluorene-2,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (6) was synthesized by Suzuki-Miyaura coupling of 2,7-dibromo-9H-fluorene (2) with 4-methoxy-N-(4-methoxyphenyl)-N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaboran-2-yl)phenyl)aniline (5).

[0102] Process e: DHCF-10 was synthesized by allylation of 4,4'-(9H-fluorene-2,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (6) synthesized in step d with 1-bromo-4-methoxybenzene (4).

[0103] (Purification and confirmation) The obtained crude product was purified by a standard method in accordance with Example 1. Subsequently, DHCF-10 was subjected to the following purification. 1 The synthesis and purification of DHCF-10 were confirmed by H NMR analysis, mass spectrometry, and UV-Vis-NIR spectroscopy. 1 The results of the 1 H NMR analysis are shown in FIG. 19, the results of the mass analysis are shown in FIG. 20, and the results of the ultraviolet-visible-near-infrared spectroscopic analysis are shown in FIG.

[0104] The structures of the hole transport materials synthesized in Comparative Examples 1 and 2 and Examples 1 and 2 are summarized below. [ka]

[0105] Example 3: 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.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] Next, a hole transport layer 5 was prepared. The hole transport materials used were DHCF-9 synthesized in Comparative Example 1, DHCF-7 synthesized in Comparative Example 1, DHCF-8 synthesized in Example 1, and DHCF-10 synthesized in Example 2. Each hole transport material solution was weighed out to a final concentration of 30 mM, and TPFB equivalent to 10 wt% of that was weighed and added to the hole transport material. A hole transport material solution was prepared by dissolving a mixture of the hole transport material and TPFB in chlorobenzene. Note that the cobalt complex (FK209), which is commonly used as a dopant, was not added. The hole transport layer 5 was formed by dropping the hole transport material solution onto the perovskite film and forming it by spin coating.

[0111] 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.

[0112] (Example 3) Evaluation of cell performance of solar cell (10) In this example, the cell performance of solar cell 10 was evaluated. Here, solar cells (10) using DHCF-9 synthesized in Comparative Example 1 above, DHCF-7 synthesized in Comparative Example 2 above, DHCF-8 synthesized in Example 1 above, and DHCF-10 synthesized in Example 2 above as hole transport materials were investigated.

[0113] DHCF-9 (general formula (4)) synthesized in Comparative Example 1, DHCF-7 (general formula (5)) synthesized in Comparative Example 2, DHCF-8 (general formula (2)) synthesized in Example 1, and DHCF-10 (general formula (4)) synthesized in Example 2 all have a DAD type structure. Furthermore, the D moiety of DHCF-9 in Comparative Example 1, DHCF-7 in Comparative Example 2, and DHCF-8 in Example 1 has a D'-A'-D' type structure. DHCF-8 in Example 1 contains N as the D moiety. 3 ,N 3 ,N 6 ,N 6 DHCF-9 of Comparative Example 1 and DHCF-7 of Comparative Example 2 have N-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine-9-yl groups introduced at the 2- and 7-positions of the fluorene ring, respectively, and a bis(4-methoxyphenyl) group introduced at the 9-position of the fluorene ring constituting the A moiety. 3 ,N 3 ,N 6 ,N 6 DHCF-10 in Example 2, like DHCF-8, has a bis(4-methoxyphenyl) group introduced at the 9-position on the fluorene ring constituting the A moiety, while DHCF-9 has no substituent at the 9-position on the fluorene ring constituting the A moiety, and Comparative Example 2 has a bis(4-(bis(4-methoxyphenyl)amino)phenyl) group introduced at the 9-position on the fluorene ring constituting the A moiety. DHCF-10 in Example 2, like DHCF-8, has a bis(4-methoxyphenyl) group introduced at the 9-position on the fluorene ring constituting the A moiety, while 4-(bis(4-methoxyphenyl)amino)phenyl groups are introduced at the 2-position and 7-position on the fluorene ring constituting the D moiety.

[0114] Table 1 shows the results of the cell performance evaluation of solar cell 10. Table 1 summarizes the parameters of solar cell 10: short-circuit current density (hereinafter sometimes referred to as "Jsc"), open-circuit voltage (hereinafter sometimes referred to as "Voc"), fill factor (hereinafter sometimes referred to as "FF"), and conversion efficiency. FF is the maximum output P at the point where the product of current and voltage is maximum. max is calculated by dividing by (Voc×Jsc), and the conversion efficiency is calculated by (Voc×Jsc×FF÷incident light intensity).

[0115] [Table 1]

[0116] The results in Table 1 show that the conversion efficiencies of DHCF-8 in Example 1, DHCF-10 in Example 2, DHCF-9 in Comparative Example 1, and DHCF-7 in Comparative Example 2 were in the 12-19% range, with DHCF-8 in Example 1 exhibiting the highest solar cell performance. Comparison of DHCF-8 in Example 1, DHCF-9 in Comparative Example 1, and DHCF-7 in Comparative Example 2 revealed that differences in the substituent introduced at the 9-position of the fluorene ring affect cell performance. Differences in the D moiety affect the intramolecular stacking effect and solubility of the hole transport material. For example, DHCF-9 had problems with solubility, and DHCF-7's bulky molecule likely contributed to its reduced solar cell performance. On the other hand, the introduction of a bis(4-methoxyphenyl) group at the 9-position of the fluorene ring, as in DHCF-8 in Example 1, may have provided a favorable balance between the solubility and stacking effect of the molecule, resulting in improved solar cell performance. In addition, DHCF-10 of Example 2, which shares the same 9-position on the fluorene ring as DHCF-8 of Example 1 and has 4-(bis(4-methoxyphenyl)amino)phenyl groups introduced at the 2- and 7-positions on the fluorene ring, also had good battery performance.

[0117] It was also found that adding TPFB can lead to the construction of highly durable solar cells. The addition of TPFB has the advantage that it can be done simply by dissolving it in a solvent together with the hole transport material. [Industrial Applicability]

[0118] The present invention can be used as a hole transport material for use in perovskite solar cells and the like, which comprises a perovskite compound formed from an organic and inorganic hybrid compound. [Explanation of symbols]

[0119] 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 donor-acceptor-donor (DAD) structure and being a compound represented by the following general formula (1): 【Chemistry 1】 [In general formula (1), R Ａ teeth, 【Chemistry 4】 and In group (III), V and Z are 0; R Ｂ represents the following group (I): 【Transformation 6】 (X and Y are 0, R １ is -H, -C ｎ H ２ｎ＋１ , —O—C ｎ H ２ｎ＋１ , and -SC ｎ H ２ｎ＋１ where n=an integer from 1 to 8; R ３ is -H, -C ｎ H ２ｎ＋１ , —O—C ｎ H ２ｎ＋１ , and -SC ｎ H ２ｎ＋１ where n=an integer from 1 to 8; In addition, R １ and R ３ cannot be simultaneously -H, R ２ａ１ , R ２ａ２ , R ２ｂ１ , and R ２ｂ２ are independently —H, —F, and —CF ３ where R ２ａ１ and R ２ａ２ At least one of R is —H, ２ｂ１ and Ｒ２ｂ２ At least one of the groups is —H.

2. The R Ｂ 2. The hole transport material of claim 1, wherein is a bis(4-alkoxyphenyl)amino group.

3. 3. The hole transport material according to claim 1, wherein the donor moiety of the donor-acceptor-donor (DAD) structure has a donor-acceptor-donor (D'-A'-D') structure.

4. 3. The hole transport material according to claim 1, which is a compound represented by the following general formula (2): 【Transformation 7】

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.

6. 6. The solar cell of claim 5, wherein the hole transport material further comprises 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate.

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