Hole transport layer, solar cell element, and solar cell module

Abstract

This hole transport layer comprises: a base material having a triphenylamine skeleton and an electron-attracting group in the molecule thereof; and a dopant, wherein the electron-attracting group is bonded to a para-position of a benzene ring belonging to the triphenylamine skeleton.

Description

Hole transport layer, solar cell element and solar cell module CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Application No. 2024-62744 (filed April 9, 2024), the entire disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates to a hole transport layer, a solar cell element, and a solar cell module.

[0003] Conventionally, solar cell elements have been proposed in which a negative electrode, an electron transport layer, a photoelectric conversion layer, a hole transport layer, and a positive electrode are stacked in this order (for example, Patent Documents 1 and 2).

[0004] JP 2009-252407 A International Publication No. 2022 / 244336 A

[0005] A hole transport layer, a solar cell element, and a solar cell module are disclosed.

[0006] In one embodiment, the hole transport layer comprises a host material and a dopant. The host material has a triphenylamine skeleton and an electron-withdrawing group in its molecule. The electron-withdrawing group is bonded to the para position of a benzene ring belonging to the triphenylamine skeleton.

[0007] In one embodiment, the solar cell element includes a positive electrode, a negative electrode, a photoelectric conversion layer, and a hole transport layer. The photoelectric conversion layer is located between the positive electrode and the negative electrode and generates electricity based on light. The hole transport layer is located between the positive electrode and the photoelectric conversion layer.

[0008] In one embodiment, the solar cell module includes a plurality of solar cell elements.

[0009] FIG. 1 is a perspective view schematically illustrating a portion of a solar cell element according to a first embodiment. FIG. 2 is a top view schematically illustrating a portion of a solar cell element according to the first embodiment. FIG. 3 is an end view of the solar cell element according to the first embodiment, taken along the line III-III in FIG. 2 . FIG. 4 is a diagram schematically illustrating an example of an electron-withdrawing group bonded to a benzene ring. FIG. 5 is a diagram schematically illustrating an example of the range of a dopant present in a triphenylamine skeleton. FIG. 6 is a diagram schematically illustrating an example of the range of a dopant present in a Spiro-OMeTAD derivative. FIG. 7 is a bar graph illustrating an example of the sheet resistance of a hole transport portion. FIG. 8 is a bar graph illustrating an example of the sheet resistance of a base material of a hole transport portion. FIG. 9 is a diagram schematically illustrating an example of the range of a dopant present in a Spiro-OMeTAD derivative. FIG. 10 is a cross-sectional view schematically illustrating an example of a portion of a solar cell element according to a third embodiment. FIG. 11 is a cross-sectional view schematically illustrating an example of a solar cell module according to a fourth embodiment.

[0010] Devices including a hole transport layer are known. A specific example of such a device is a solar cell element. The open-circuit voltage of a solar cell element depends on the HOMO level of the hole transport layer. Specifically, the deeper the HOMO level, the higher the open-circuit voltage. On the other hand, a decrease in the conductivity of the hole transport layer reduces the short-circuit current of the solar cell element.

[0011] Therefore, the inventors of the present disclosure have created a technology that can achieve both deepening of the HOMO level and improvement of the conductivity of the hole transport layer. Hereinafter, first to fifth embodiments will be described with reference to the drawings.

[0012] In the drawings, parts having the same or similar configurations and functions are denoted by the same reference numerals, and duplicate explanations will be omitted in the following description. The drawings are schematic illustrations. The drawings may include a right-handed XYZ coordinate system.

[0013] <1. First embodiment> <1-1. Solar cell element> A solar cell element 10, which is an example of a device including a hole transport part 1031 (described below) according to a first embodiment, will be described with reference to Figs. 1 to 3. Fig. 1 shows a part of the solar cell element 10 according to the first embodiment. Fig. 2 is a top view of a part of the solar cell element 10 according to the first embodiment. Fig. 3 is a cross-sectional view taken along line III-III in Fig. 2. Figs. 1 to 3 do not show the entire solar cell element 10, but show a part obtained by cutting out a part of the solar cell element 10.

[0014] As shown in Fig. 1, the solar cell element 10 has a light-receiving surface F1 that mainly receives light and a back surface F2 located on the opposite side of the light-receiving surface F1. In the first embodiment, the light-receiving surface F1 faces the +Z direction, and the back surface F2 faces the -Z direction. For example, the +Z direction may be set to face the sun at its zenith moment. Fig. 2 shows a portion of the solar cell element 10 as viewed from the light-receiving surface F1 side.

[0015] As shown in Fig. 3, the solar cell element 10 may include a first electrode unit 101, a diffusion reduction unit 102, a solar cell unit 103, a second electrode unit 104, and a substrate unit 105. Also, as shown in Fig. 3, the solar cell unit 103 may include a hole transport unit 1031, a photoelectric conversion unit 1032, and an electron transport unit 1033. In the first embodiment, the second electrode unit 104, the electron transport unit 1033, the photoelectric conversion unit 1032, the hole transport unit 1031, the diffusion reduction unit 102, and the first electrode unit 101 are stacked on the substrate unit 105 in this order.

[0016] Although not shown, an anti-reflection film may be positioned on the surface of the solar cell module 1 (described later). For example, an insulating film made of silicon nitride or the like is used as the anti-reflection film. Furthermore, although not shown, a passivation film may be positioned between the first electrode unit 101, the diffusion reduction unit 102, or the solar cell unit 103 and the anti-reflection film. For example, a thin film made of an oxide such as aluminum oxide or a nitride is used as the passivation film.

[0017] By connecting a plurality of solar cell elements 10 together, the plurality of solar cell elements 10 can form a solar cell module 1. For example, by connecting a plurality of solar cell elements 10, a solar cell module 1 having a size of about 1 m square can be produced. Furthermore, by connecting a plurality of solar cell modules 1 together, the plurality of solar cell modules 1 can form a solar cell string. By connecting a plurality of solar cell strings together, the plurality of solar cell strings can form a solar cell array.

[0018] Next, a description will be given of the parts included in the solar cell element 10. For ease of explanation, each part will be described below in order starting from the substrate part 105.

[0019] <1-1-1. Substrate portion 105> The substrate portion 105 is a base that forms the portion included in the solar cell element 10 (such as the solar cell portion 103). The material of the substrate portion 105 may be, for example, glass, plastic such as acrylic or polycarbonate, or metal such as stainless steel. The shape of the substrate portion 105 may be, for example, a flat plate, a sheet, or a film. The thickness of the substrate portion 105 may be, for example, about 0.01 millimeters (mm) to 5 mm. The substrate portion 105 may also be simply referred to as a support substrate.

[0020] <1-1-2. Second electrode unit 104> The second electrode unit 104 is located on the substrate unit 105. The second electrode unit 104 may also be simply referred to as a second electrode. Alternatively, the second electrode unit 104 may also be referred to as a second electrode layer. The second electrode unit 104 can collect carriers generated by photoelectric conversion in response to light irradiation of the solar cell unit 103 (described later). The second electrode unit 104 can, for example, function as an electrode (also referred to as a negative electrode) that collects electrons as carriers. As a material for the second electrode unit 104, for example, a metal with excellent conductivity such as silver (Ag), gold (Au), copper (Cu), titanium (Ti), indium (In), or tin (Sn) may be used.

[0021] The second electrode unit 104 may be made of, for example, a transparent conductive oxide (TCO) that is transparent to light in a specific wavelength range. The second electrode unit 104 may have a thickness of, for example, about 10 nanometers (nm) to 1000 nm. The second electrode unit 104 may be formed on the substrate unit 105 by a vacuum process such as sputtering.

[0022] Examples of TCO include indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), boron-doped zinc oxide (BZO), gallium-doped zinc oxide (GZO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO), titanium-doped indium oxide (ITiO), and indium zinc oxide (ITiO). Indium Gallium Zinc Oxide (IZO), Indium Gallium Zinc Oxide (IGZO), Tantalum-doped Tin Oxide (Ta-doped Tin Oxide: SnO ２ : Ta), niobium-doped tin oxide (Nb-doped tin oxide: SnO ２ : Nb), tungsten-doped tin oxide (W-doped tin oxide: SnO ２ : W), molybdenum-doped tin oxide (Mo-doped tin oxide: SnO ２ : Mo), fluorine-doped tin oxide (F-doped tin oxide: SnO ２:F), hydrogen-doped indium oxide (Hydrogen-doped Indium Oxide:IOH), etc., are not particularly limited. The transparent conductive oxide film may be a laminated film having a plurality of films, and in addition to the above oxides, a film of tin oxide or the like may be contained in the laminated film. The dopant for the tin oxide or the like film may be one or more selected from the group consisting of In, silicon (Si), germanium (Ge), Ti, Cu, antimony (Sb), Nb, F, Ta, W, Mo, bromine (Br), iodine (I), chlorine (Cl), etc., are not particularly limited.

[0023] <1-1-3. Solar Cell Unit 103> The solar cell unit 103 is located between the first electrode unit 101 and the second electrode unit 104. Specifically, the solar cell unit 103 is located on the second electrode unit 104. The -Z direction surface of the solar cell unit 103 may be in contact with the +Z direction surface of the second electrode unit 104. The solar cell unit 103 may also be called a power generation layer. The solar cell unit 103 converts light incident from the outside (e.g., sunlight) into electricity. For example, the solar cell unit 103 may generate carriers by photoelectric conversion in response to irradiation with light. The carriers include at least one of electrons and holes. The solar cell unit 103 has a light-receiving surface F1.

[0024] 3 , the solar cell section 103 may include a hole transport section 1031, a photoelectric conversion section 1032, and an electron transport section 1033. The hole transport section 1031 may also be called a hole transport layer. The photoelectric conversion section 1032 may also be called a photoelectric conversion layer. The electron transport section 1033 may also be called an electron transport layer.

[0025] The photoelectric conversion section 1032 is located between the first electrode section 101 and the second electrode section 104, the hole transport section 1031 is located between the first electrode section 101 and the photoelectric conversion section 1032, and the electron transport section 1033 is located between the photoelectric conversion section 1032 and the second electrode section 104. In the first embodiment, the electron transport section 1033, the photoelectric conversion section 1032, and the hole transport section 1031 are stacked in this order on the second electrode section 104. As shown in FIG. 3 , the +Z direction surface of the second electrode section 104 may be in contact with the −Z direction surface of the electron transport section 1033, the +Z direction surface of the electron transport section 1033 may be in contact with the −Z direction surface of the photoelectric conversion section 1032, and the +Z direction surface of the photoelectric conversion section 1032 may be in contact with the −Z direction surface of the hole transport section 1031. A p-type semiconductor is used for the hole transport section 1031, an intrinsic semiconductor (i.e., an i-type semiconductor) is used for the photoelectric conversion section 1032, and an n-type semiconductor is used for the electron transport section 1033. In this case, the hole transport section 1031, the photoelectric conversion section 1032, and the electron transport section 1033 form a PIN junction region. The PIN junction region can generate electricity through photoelectric conversion in response to light irradiation. Note that the photoelectric conversion section 1032 may be a p-type semiconductor or an n-type semiconductor.

[0026] In the first embodiment, the solar cell unit 103 will be described as a perovskite solar cell. However, this is merely an example, and other types of solar cells may be applied. For example, the solar cell may be an inorganic solar cell or an organic solar cell. The inorganic solar cell may be a silicon solar cell or a compound solar cell. The organic solar cell may be a dye-sensitized solar cell or an organic thin-film solar cell. Furthermore, for example, the solar cell may be a crystalline solar cell or a thin-film solar cell. The crystalline solar cell may be a silicon solar cell or a compound semiconductor solar cell such as a CIGS (Cu, In, Ga, Se) solar cell. The thin-film solar cell may be a perovskite solar cell, a dye-sensitized solar cell, an organic thin-film solar cell, or the like.

[0027] Next, a description will be given of each part included in the solar cell section 103. For ease of explanation, each part will be described below in order starting from the electron transport section 1033.

[0028] The electron transporting section 1033 is located on the second electrode section 104. For example, a semiconductor made of an inorganic material (also referred to as an inorganic semiconductor) having a higher electrical resistance than the second electrode section 104 may be used for the electron transporting section 1033.

[0029] In the first embodiment, an n-type semiconductor is used as the inorganic semiconductor material. The electron transport unit 1033 functions as a so-called hole blocking layer or electron transport layer (ETL). The electron transport layer, for example, collects and outputs electrons.

[0030] As the n-type semiconductor, in addition to inorganic semiconductors, semiconductors made of organic materials may be used. For example, the n-type semiconductor may be [6,6]-phenyl-C-61-methyl butyrate (PCBM), C60, or an oxide semiconductor layer. For example, the oxide semiconductor layer may be titanium(IV) oxide (TiO ２ ), zinc oxide (ZnO), indium(III) oxide (In ２ O ３ ), tin(IV) oxide (SnO ２ ), or magnesium oxide (MgO) may be applied.

[0031] The photoelectric conversion section 1032 is located on the electron transport section 1033. This photoelectric conversion section 1032 can absorb light that has passed through the diffusion reduction section 102 and the hole transport section 1031, which will be described later. In the first embodiment, for example, an i-type semiconductor is applied to the photoelectric conversion section 1032. For example, a semiconductor having a perovskite structure (also referred to as a perovskite semiconductor) may be applied to the i-type semiconductor. The perovskite semiconductor may include, for example, a halide-based organic-inorganic perovskite semiconductor. The halide-based organic-inorganic perovskite semiconductor may be, for example, an ABX ３ A is a semiconductor having a perovskite structure with the following composition: ３ NH ３ ), formamidinium (CH(NH ２ ) ２), cesium (Cs), rubidium (Rb), or potassium (K) ions are applied to B, for example, one or more ions of lead (Pb) or tin (Sn). X, for example, one or more ions of iodine (I), bromine (Br), or chlorine (Cl) are applied to X. Specifically, ABX ３ The semiconductor having a perovskite structure with the composition is, for example, CH ３ NH ３ PbI ３ or (CH(NH ２ ) ２ ,Cs)Pb(I,Br) ３ The photoelectric conversion unit 1032 may be configured of an organic perovskite such as SiO 2 or SiO 3 . The organic perovskite may be formed, for example, by applying a first source liquid onto the electron transport unit 1033 and drying the applied liquid. Here, the organic perovskite is a thin film having crystallinity. The first source liquid may be generated, for example, by dissolving the source materials, alkylamine halide and lead halide, in a solvent. The thickness of the photoelectric conversion unit 1032 may be, for example, approximately 100 nm to 2000 nm.

[0032] The hole transport section 1031 is located on the photoelectric conversion section 1032. The surface of the hole transport section 1031 opposite to the photoelectric conversion section 1032 may be the light-receiving surface F1. A p-type semiconductor is applied to the hole transport section 1031. The hole transport section 1031 functions as a so-called electron blocking layer or hole transport layer (HTL). The hole transport layer, for example, collects and outputs holes.

[0033] The hole transporting part 1031 may have an electron-withdrawing group in the molecule, which can deepen the HOMO level of the hole transporting part 1031.

[0034] <1-1-4. Diffusion reduction unit 102> The diffusion reduction unit 102 is located on the solar cell unit 103. In other words, the diffusion reduction unit 102 is located on the light-receiving surface F1 of the solar cell unit 103. In further other words, the diffusion reduction unit 102 is located between the first electrode unit 101 and the solar cell unit 103.

[0035] A conductive inorganic material may be used as the material of the diffusion reduction portion 102. For example, a conductive oxide or a conductive nitride may be used. The diffusion reduction portion 102 may be formed on the hole transport portion 1031 by, for example, a vacuum process. The vacuum process may be sputtering, chemical vapor deposition (CVD), vacuum evaporation, atomic layer deposition (ALD), or the like. The thickness of the diffusion reduction portion 102 may be, for example, approximately 1 nm to 100 nm.

[0036] The diffusion reduction section 102 functions as a diffusion barrier film. That is, the diffusion reduction section 102 reduces at least one of the amount of metal migration from the first electrode section 101 to the hole transport section 1031 and the amount of metal migration from the hole transport section 1031 to the first electrode section 101. This reduces the possibility of deterioration of the hole transport section 1031 or the photoelectric conversion section 1032. Therefore, it is possible to reduce the possibility of deterioration in the conversion efficiency of the solar cell element 10.

[0037] <1-1-5. First electrode unit 101> The first electrode unit 101 is located on the diffusion reduction unit 102. The first electrode unit 101 can collect carriers generated by photoelectric conversion in response to light irradiation of the photoelectric conversion unit 1032. The first electrode unit 101 can, for example, function as an electrode (also referred to as a positive electrode) that collects holes as carriers. The first electrode unit 101 may also, for example, function as a collecting electrode.

[0038] The first electrode portion 101 may be made of a metal with excellent conductivity, such as Ag, Au, Cu, Ti, In, or Sn. The first electrode portion 101 may be formed on the diffusion reduction portion 102 by a vacuum process, such as sputtering. The average thickness of the first electrode portion 101 is not particularly limited, but may be in the range of 1 μm or more and 50 μm or less, for example.

[0039] The first electrode unit 101 may be formed, for example, by applying a metal paste as a coating liquid by screen printing or the like, followed by drying and solidifying the metal paste. The metal paste may be prepared, for example, by adding conductive particles with high light reflectance to a binder such as a translucent resin. Here, an epoxy resin or the like may be used as the translucent resin. The particles contained in the metal paste may be metal particles such as Cu, Al, Ni, and alloys of Zn and Ag. In this case, the first electrode unit 101 may contain a large number of conductive particles, and the conductivity of the first electrode unit 101 may be ensured by the large number of particles. The first electrode unit 101 may be, for example, layered.

[0040] The shape of the first electrode portion 101 is not limited to the shape shown in Figures 1 to 3 of the first embodiment. The shapes of the first electrode portion 101 and the diffusion reduction portion 102 may be changed as appropriate to improve carrier collection efficiency. The shapes of the first electrode portion 101 and the diffusion reduction portion 102 may be a so-called comb electrode structure, or may be a structure combining a so-called bus bar electrode and a so-called finger electrode.

[0041] Furthermore, the solar cell element 10 does not need to include the diffusion reduction portion 102. In this structure, the surface of the first electrode portion 101 in the −Z direction can come into contact with the surface of the hole transport portion 1031 in the +Z direction.

[0042] For example, a lead wire is electrically connected to each of the first electrode unit 101 and the second electrode unit 104. Specifically, for example, a first lead wire is electrically connected to the first electrode unit 101, and a second lead wire is connected to the second electrode unit 104. Each lead wire can be joined to each of the first electrode unit 101 and the second electrode unit 104 by, for example, soldering. Here, for example, the output obtained by photoelectric conversion in the solar cell element 10 can be extracted by the first lead wire and the second lead wire.

[0043] <1-2. Composition of Hole Transporting Section> Next, the composition of the hole transporting section 1031 will be described in detail.

[0044] The electron-withdrawing group is a substituent that has high electron-withdrawing properties, for example, in the inductive effect (also called the I effect). Examples of the electron-withdrawing group include a halogeno group, a hydroxyl (OH) group, an alkoxy (OR) group, an amino (NH ２ ) group, NR ２ group, NR ３ group, formyl (CHO) group, carbonyl (COR) group, carboxy (COOH) group, COOR group, COHN ２ group, cyano (CN) group, nitro (NO ２ ) group, sulfo (SO ３ H) group, sulfonyl (S(O) ２ The halogeno group may be at least one of a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. The electron-withdrawing group bonded to the benzene ring may be a substituent having high electron-withdrawing properties in the mesomeric effect (also called M effect). For example, a halogeno group, a formyl group, a carbonyl group, a carboxy group, a COOR group, a COHN ... ２ At least one of a group, a cyano group and a nitro group may be used as the electron-withdrawing group.

[0045] The hole transport unit 1031 may include a host material and a dopant. The host material may be made of an organic semiconductor. The dopant is a p-type dopant and may be made of, for example, an organic material.

[0046] <1-2-1. Base Material> The base material may include a benzene ring, and as a more specific example, may include a triphenylamine skeleton. The triphenylamine skeleton has a structure in which three benzene rings are bonded to a nitrogen atom. As a specific example, the base material may include PTAA (Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]), MeOTTVT (4-((E)-4-(bis(4-((E)-4-(bis(4-methoxyphenyl)amino)styryl)phenyl)amino)styryl)-N,N-bis(4-methoxyphenyl)aniline), or a derivative of PTAA or MeOTTVT. The base material may further include a spiro skeleton. Specifically, the base material may include Spiro-OMeTAD (2,2',7,7'-Tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene), Spiro-TTB (2,2',7,7'-Tetrakis(di-p-tolylamino)-9,9'-spirobi[fluorene]), or a derivative of Spiro-OMeTAD or Spiro-TTB.

[0047] The electron-withdrawing group is bonded to a benzene ring in the molecule of the base material. FIG. 4 is a schematic diagram showing an example of an electron-withdrawing group (fluorine atom in the figure) bonded to a benzene ring. The electron-withdrawing group attracts electrons from the atomic group including the nitrogen atom and the benzene ring with greater force, thereby stabilizing the positive charge of the nitrogen atom. This allows the HOMO level of the base material to be deepened. The open-circuit voltage of the solar cell element 10 corresponds to the difference between the energy level of the lowest unoccupied molecular orbital (LUMO) of the electron transport unit 1033 and the HOMO level of the hole transport unit 1031. Therefore, by deepening the HOMO level of the hole transport unit 1031, the open-circuit voltage of the solar cell element 10 can be increased.

[0048] For example, any of the materials shown in formulas EF1 to EF7 can be used as the base material.

[0049]

[0050]

[0051]

[0052]

[0053]

[0054]

[0055]

[0056] For example, in formula EF1, one triphenylamine skeleton in the unit structure has one electron-withdrawing group bonded to the para position of one benzene ring, and no electron-withdrawing groups bonded to the other two benzene rings. In formula EF2, two triphenylamine skeletons in the unit structure have one electron-withdrawing group bonded to the para position of one benzene ring, and no electron-withdrawing groups bonded to the other two benzene rings.

[0057] In Formulae EF3 to EF7, an electron-withdrawing group (for example, a fluorine atom) is bonded to the para position of at least one of the two outer benzene rings of the triphenylamine skeleton that do not belong to the spirobifluorene skeleton.

[0058] Although any of the above specific examples may be applied to the electron-withdrawing group, a fluorine atom may also be applied. Fluorine atoms have high electronegativity and a strong ability to attract electrons, which can deepen the HOMO level of the hole transport part 1031. This can more effectively improve the open-circuit voltage of the solar cell element 10.

[0059] 2. Second Embodiment In the second embodiment, the position of the electron-withdrawing group for improving the conductivity of the hole transport section 1031 will be described. In the second embodiment, the solar cell element 10 can also be applied as a device including the hole transport section 1031. However, in the second embodiment, it is assumed that the hole transport section 1031 has one or more triphenylamine skeletons in its molecule. One or more electron-withdrawing groups are bonded to at least one of the benzene rings of the triphenylamine skeleton.

[0060] <2-1. Position of Electron-Withdrawing Group on Benzene Ring> In the second embodiment, the hole transport unit 1031 includes, in its molecule, an electron-withdrawing group bonded to the para-position of a benzene ring belonging to a triphenylamine skeleton. Specific examples include any of the materials represented by formulas EF1 to EF7 shown in the first embodiment. As represented by formulas EF1 to EF7, the fluorine atom, which is the electron-withdrawing group, is located at the para-position of the benzene ring. In other words, the electron-withdrawing group is bonded to the benzene ring at the para-position relative to the nitrogen atom.

[0061] Because the para position is the farthest from the nitrogen atom among the carbon atoms on the benzene ring, the electron-withdrawing group bonds to the benzene ring at the farthest position from the nitrogen atom. Therefore, the host material is less likely to cause steric hindrance to the dopant. Therefore, each dopant molecule can exist closer to the host material molecule. Here, as an example, we will explain the case where a dopant of formula EF8 (i.e., BCF) is used as the dopant. Figure 5 is a diagram schematically illustrating an example of the existence range R1 of BCF relative to a triphenylamine skeleton. The existence range R1 shown in Figure 5 is a schematic representation of the range of the molecular structure of BCF. In BCF, three benzene rings are bonded to the boron atom, so the existence range R1 is schematically represented by the overlapping range of three circles centered on each vertex of an equilateral triangle. The existence range R1 of this BCF can exist in an inverted triangular position relative to the triphenylamine skeleton of the host material. As shown in Figure 5, the para positions of the three benzene rings in the triphenylamine skeleton do not overlap with the existence range R1. Therefore, the para-position is less likely to act as a steric hindrance to the BCF. Therefore, the BCF molecules can be closer to the host material molecules. Because the BCF molecules are located closer to the host material molecules, the BCF can more easily exchange electrons and holes with the host material. Therefore, the dopant efficiency in the hole transport region 1031 can be increased, and the conductivity of the hole transport region 1031 can be improved. In other words, the sheet resistance of the hole transport region 1031 can be reduced.

[0062] FIG. 6 is a schematic diagram illustrating an example of the dopant presence range R1 for Spiro-OMeTAD. In FIG. 6, two BCF presence ranges R1 are shown for Spiro-OMeTAD. The two presence ranges R1 are located near the nitrogen atoms located diagonally. The diagonal angle here refers to the diagonal corners when the positions of the four nitrogen atoms in Spiro-OMeTAD are considered as the vertices of an imaginary square. The two presence ranges R1 are located in an inverted triangle relationship. As shown in FIG. 6, the para positions of each outer benzene ring do not overlap with either of the two presence ranges R1. Therefore, the para positions are unlikely to act as steric hindrance to the BCF. This reduces the sheet resistance of the hole transport region 1031.

[0063] 7 is a bar graph showing an example of the sheet resistance of the hole transport section 1031. Bar graph A1 shows the sheet resistance of the hole transport section 1031 including a base material of formula EF3 in which the electron-withdrawing group is located at the para position and a dopant of formula EF8. Bar graph A2 shows the sheet resistance of the hole transport section 1031 including a base material of formula EF9 (described below) in which the electron-withdrawing group is located at the meta position and a dopant of formula EF8. In each of bar graphs A1 and A2, the content ratio (e.g., volume ratio) of the dopant relative to the hole transport section 1031 was 20%.

[0064]

[0065] The material having the formula EF8 may also be called Spiro-mF (N2,N2',N7,N7'-Tetrakis(3-fluoro-4-)-N2,N2',N7,N7'-tetrakis(4-methoxyphenyl)-9,9'-spirobi"fluorene"-2,2',7,7'-tetraamine).

[0066] As shown in FIG. 7, the sheet resistance of the hole transport section 1031 in which the electron-withdrawing group is located at the para position is 6.00×10 ８ Ω / sq., and the sheet resistance of the hole transport portion 1031 in which the electron-withdrawing group is located at the meta position is 11.7×10 ８ Ω / sq.

[0067] 8 is a bar graph showing an example of the sheet resistance of a base material. That is, FIG. 8 shows the sheet resistance of the base material alone, not the hole transport portion 1031 including the base material and the dopant. Bar graph B1 shows the sheet resistance of the base material of formula EF3, and bar graph B2 shows the sheet resistance of the base material of formula EF9. As shown in FIG. 8, the sheet resistance of the base material in which the electron-withdrawing group is located at the para position is 9.90×10 １２ Ω / sq., and the sheet resistance of the base material in which the electron-withdrawing group is located at the meta position is 6.80×10 １２ Ω / sq.

[0068] As can be seen from Figure 8, the sheet resistance of a base material in which the electron-withdrawing group is located at the para position is higher than the sheet resistance of a base material in which the electron-withdrawing group is located at the meta position. However, as can be seen from Figure 7, the relationship between the sheet resistances is reversed in the hole transport section 1031 including a base material and a dopant. That is, the sheet resistance of the hole transport section 1031 including a base material located at the para position is lower than the sheet resistance of the hole transport section 1031 including a base material located at the meta position. This is thought to be because the electron-withdrawing group located at the meta position causes steric hindrance, resulting in lower dopant efficiency and higher sheet resistance of the hole transport section 1031. Conversely, electron-withdrawing groups located at the para position are less likely to cause steric hindrance, and can reduce the sheet resistance of the hole transport section 1031.

[0069] As described above, it is possible to achieve both a deepening of the HOMO level of the hole transport section 1031 due to the electron-withdrawing group being bonded to the benzene ring and an improvement in the conductivity of the hole transport section 1031 due to the electron-withdrawing group being located at the para position. In other words, it is possible to achieve both improvements in the open-circuit voltage and short-circuit current of the solar cell element 10. Therefore, it is possible to more effectively improve the power generation efficiency of the solar cell element 10.

[0070] It is not necessary that the electron-withdrawing group be bonded to the para position in all of the multiple molecules contained in the base material. In some molecules, the electron-withdrawing group may not be bonded to the para position but may be bonded to a position other than the para position. In the base material of the hole transport section 1031, the proportion of molecules having the electron-withdrawing group bonded to the para position may be, for example, 85% or more, 90% or more, 95% or more, or 98% or more. This proportion may be purity. If this proportion is 95% or more, the sheet resistance of the hole transport section 1031 can be effectively reduced.

[0071] The electron-withdrawing group may be bonded only to the para position of the benzene ring. In this case, steric hindrance can be more reliably reduced, and the sheet resistance of the hole transport section 1031 can be more reliably reduced. On the other hand, when multiple electron-withdrawing groups are bonded to one benzene ring, electron-withdrawing groups can be bonded to both the para position and a position other than the para position. Even in this case, the sheet resistance of the hole transport section 1031 can be reduced compared to a structure in which the number of electron-withdrawing groups is the same but no electron-withdrawing groups are located at the para position. Furthermore, in this case, since the number of electron-withdrawing groups is increased, the HOMO level of the hole transport section 1031 can be deepened, and the open-circuit voltage of the solar cell element 10 can be increased.

[0072] The electron-withdrawing group may be a single atom such as a halogeno group. In this case, the electron-withdrawing group has a small molecular size, so that the electron-withdrawing group is less likely to cause steric hindrance between the base material and the dopant, and the dopant can be positioned closer to the base material. This allows the sheet resistance of the hole transport region 1031 to be reduced more effectively.

[0073] <2-2. Relationship between Bonding Destination of Electron-Withdrawing Group and Conductivity> In each of Formulae EF3 to EF6, four electron-withdrawing groups (here, fluorine atoms) are located. That is, the number of electron-withdrawing groups in Formulae EF3 to EF6 is the same. Furthermore, in each of Formulae EF3 to EF6, the position of the electron-withdrawing group on the benzene ring is the same as in each of Formulae EF3 to EF6, i.e., the electron-withdrawing group is located on the outermost side in each of Formulae EF3 to EF6. Therefore, the electron-withdrawing properties of the hole transport portion 1031 are comparable. However, the molecules of Formulae EF3 to EF6 differ from one another in terms of which benzene ring the electron-withdrawing group is bonded to. The conductivity of the hole transport portion 1031 differs depending on the bonding destination of the electron-withdrawing group. First, specific examples of bonding destinations of the electron-withdrawing groups will be described in detail below.

[0074] Hereinafter, in the derivatives of formulas EF1 to EF7, the upper fluorene skeleton is referred to as the first fluorene skeleton, the lower fluorene skeleton is referred to as the second fluorene skeleton, and the two triphenylamine skeletons having a benzene ring belonging to the first fluorene skeleton are referred to as the first triphenylamine skeleton and the second triphenylamine skeleton, respectively, and the two triphenylamine skeletons having a benzene ring belonging to the second fluorene skeleton are referred to as the third triphenylamine skeleton and the fourth triphenylamine skeleton, respectively.

[0075] In formula EF3, in each of the first to fourth triphenylamine skeletons, an electron-withdrawing group is bonded to one of the two outer benzene rings. Specifically, one electron-withdrawing group is bonded to one of the outer benzene rings. Furthermore, no electron-withdrawing group is bonded to the other outer benzene ring, but a substituent (e.g., a methoxy group) that has higher electron-donating properties than the electron-withdrawing group is bonded to the other outer benzene ring. Here, for example, the electron-donating M effect (positive mesomeric effect) can be applied as an index showing electron-donating properties. The electron-donating M effect can also be called the mesomeric effect. Examples of highly electron-donating substituents include an O (oxygen atom) group, an amino (NH ２) group, alkoxy (OR) (hereinafter, R represents a hydrocarbon) group, N-acylamido (NHCOR) group, and OCOR group can be applied. The electron donating properties of these substituents increase in this order. In other words, the O group has the highest electron donating property. Hereinafter, a highly electron donating substituent will also be referred to as an electron donating group. In Formula EF3, in each of the first to fourth triphenylamine skeletons, one electron donating group is bonded to the outer benzene ring of the other. The electron withdrawing group and the electron donating group are each bonded to the para position of the benzene ring.

[0076] In Formula EF4, in each of the first triphenylamine skeleton and the third triphenylamine skeleton, an electron-withdrawing group is bonded to each of the two outer benzene rings. Specifically, one electron-withdrawing group is bonded to each of the two outer benzene rings. Furthermore, in each of the second triphenylamine skeleton and the fourth triphenylamine skeleton, no electron-withdrawing group is bonded to the benzene ring, and an electron-donating group is bonded to each of the two outer benzene rings. Specifically, one electron-donating group is bonded to each of the two outer benzene rings. In Formula EF4, the electron-withdrawing group and the electron-donating group are also bonded to the para-position of the benzene ring.

[0077] In other words, in each of the two triphenylamine skeletons located on the first side (left side of the paper in Formula EF4) with respect to the spiro skeleton, one electron-withdrawing group is bonded to the para-position of each of the two outer benzene rings, and in the two triphenylamine skeletons located on the second side opposite the first side with respect to the spiro skeleton, no electron-withdrawing group is bonded to the benzene ring, but an electron-donating group is bonded to the para-position of each of the outer benzene rings.

[0078] The spiro skeleton can rotate around the carbon atom located at its center. Specifically, the first fluorene skeleton and the second fluorene skeleton can rotate around the carbon atom. Therefore, the pair of the first triphenylamine skeleton and the second triphenylamine skeleton located above the carbon atom and the pair of the third triphenylamine skeleton and the fourth triphenylamine skeleton located below the carbon atom can rotate around the carbon atom. Therefore, the hole transport portion 1031 includes a plurality of molecules in which the relative positions of the pair of the first triphenylamine skeleton and the second triphenylamine skeleton and the pair of the third triphenylamine skeleton and the fourth triphenylamine skeleton are different. For example, referring to FIG. 9 , the hole transport portion 1031 also includes molecules in which electron-withdrawing groups are bonded to two triphenylamine skeletons located diagonally opposite the carbon atom at the rotation center, and electron-donating groups are bonded to two triphenylamine skeletons located diagonally opposite the first diagonal.

[0079] In Formula EF5, in each of the first triphenylamine skeleton and the second triphenylamine skeleton, an electron-withdrawing group is bonded to each of the two outer benzene rings. Specifically, one electron-withdrawing group is bonded to each of the two outer benzene rings. In each of the third triphenylamine skeleton and the fourth triphenylamine skeleton, no electron-withdrawing group is bonded to the benzene ring, and an electron-donating group is bonded to each of the two outer benzene rings. Specifically, one electron-donating group is bonded to each of the two outer benzene rings. In Formula EF5, the electron-withdrawing group and the electron-donating group are also bonded to the para-positions of the benzene rings.

[0080] In other words, one of two triphenylamine skeletons having a benzene ring belonging to the same fluorene skeleton has an electron-withdrawing group bonded to it. As a specific example, an electron-withdrawing group is bonded to the para-position of each of the two outer benzene rings of the triphenylamine skeleton. The other triphenylamine skeleton has no electron-withdrawing group bonded to each of the two outer benzene rings, and an electron-donating group is bonded to each of the para-positions of the two benzene rings.

[0081] In Formula EF6, an electron-withdrawing group is bonded to each of the two outer benzene rings of the first triphenylamine skeleton. Specifically, one electron-withdrawing group is bonded to each of the two outer benzene rings. In each of the second and third triphenylamine skeletons, an electron-withdrawing group is bonded to one outer benzene ring and an electron-donating group is bonded to the other benzene ring. Specifically, one electron-withdrawing group is bonded to the benzene ring closer to the first triphenylamine skeleton and one electron-donating group is bonded to the benzene ring farther from the first triphenylamine skeleton. An electron-donating group is bonded to each of the two outer benzene rings of the fourth triphenylamine skeleton. Specifically, one electron-donating group is bonded to each of the two outer benzene rings. In Formula EF6, the electron-withdrawing group and the electron-donating group are also bonded to the para-position of the benzene ring.

[0082] Here, an imaginary diagonal line will be introduced for explanation. This diagonal line is an imaginary line passing through two of the four nitrogen atoms located diagonally opposite each other and the central carbon atom of the spiro skeleton. In Formula EF6, one electron-withdrawing group is bonded to the para-position of each of the four benzene rings located on the third side of the imaginary line (diagonally upward to the left on the paper in Formula EF6). In Formula EF6, no electron-withdrawing group is bonded to the four benzene rings on the fourth side, opposite the third side of the imaginary line, and one electron-donating group is bonded to the para-position of each of the four benzene rings.

[0083] It was found that the sheet resistance of the base material of each Spiro-OMeTAD derivative and the hole transport portion 1031 containing a dopant differed depending on the bonding site of the electron-withdrawing group of the base material. Specifically, the sheet resistance of the hole transport portion 1031 of formula EF3 was the highest, the sheet resistance of the hole transport portion 1031 of formula EF5 was lower than the sheet resistance of the hole transport portion 1031 of formula EF3, the sheet resistance of the hole transport portion 1031 of formula EF6 was lower than the sheet resistance of the hole transport portion 1031 of formula EF5, and the sheet resistance of the hole transport portion 1031 of formula EF4 was lower than the sheet resistance of the hole transport portion 1031 of formula EF6. Conversely, the conductivity of the hole transport portion 1031 increased in the order of formula EF4, formula EF6, formula EF5, and formula EF3. The reason for this can be considered as follows.

[0084] First, since the electron-donating group has a higher electron-donating property than the electron-withdrawing group, the repulsion of the electron-donating group against the dopant is smaller than the repulsion of the electron-withdrawing group against the dopant. This is because, when the electron-donating property is high, for example, electrons (specifically, unshared electron pairs) are delocalized, and the electron repulsion force of the substituent is reduced. Therefore, the repulsion force between the part where the electron-donating group is located and the dopant is relatively small.

[0085] Also, as shown in FIG. 9, two dopants can be present at diagonal positions relative to the matrix of the Spiro-OMeTAD derivative.

[0086] From these viewpoints, it is believed that differences occur in the conductivity of the hole transport section 1031 in formulas EF4 to EF7. Specifically, as shown in FIG. 9 , in the hole transport section 1031 having a molecule of formula EF4, there is a base molecule in which two triphenylamine skeletons having only electron-donating groups are positioned diagonally. This allows the two dopants to be located closer to the molecule. Therefore, electrons and holes can easily move between the dopant and the molecule, reducing the sheet resistance of the hole transport section 1031.

[0087] In Formula EF6, the fourth triphenylamine skeleton does not have an electron-withdrawing group bonded thereto, but has an electron-donating group bonded thereto. Furthermore, in the second triphenylamine skeleton and the third triphenylamine skeleton, an electron-donating group is bonded to a benzene ring close to the fourth triphenylamine skeleton. In other words, electron-donating groups with low repulsive force are concentrated near the fourth triphenylamine skeleton. This allows one dopant to be located closer to the fourth triphenylamine skeleton. Therefore, the sheet resistance of the hole transporting portion 1031 of Formula EF6 can be reduced next to that of the hole transporting portion 1031 of Formula EF4.

[0088] In Formula EF5, the third triphenylamine skeleton and the fourth triphenylamine skeleton are not bound to an electron-withdrawing group, but are bound to an electron-donating group. Therefore, one dopant can be located closer to the third triphenylamine skeleton or the fourth triphenylamine skeleton. However, an electron-withdrawing group is bound to the first triphenylamine skeleton and the second triphenylamine skeleton adjacent to the third triphenylamine skeleton and the fourth triphenylamine skeleton, respectively. Therefore, compared to Formula EF6, the dopant is more likely to be repelled by the host material. In other words, the sheet resistance of the hole transporting portion 1031 of Formula EF5 can be reduced next to the hole transporting portion 1031 of Formula EF6.

[0089] In Formula EF3, an electron-withdrawing group is located in each of the four triphenylamine skeletons. Therefore, the dopant is likely to be repelled by each of the four triphenylamine skeletons. Therefore, the sheet resistance of the hole transporting portion 1031 of Formula EF3 is the highest.

[0090] The inventors prepared hole transport sections 1031 and measured their sheet resistances. Specifically, the inventors prepared three hole transport sections 1031 including base materials of Spiro-OMeTAD, formula EF3, and formula EF7, and measured their sheet resistances. A dopant of formula EF8 was used as the dopant. The content (e.g., volume ratio) of the dopant relative to the hole transport section 1031 was 20%. The sheet resistance of the hole transport section 1031 of formula EF3 was 6.00×10 ８ The sheet resistance of the hole transport part 1031 of Spiro-OMeTAD was 3.42×10 ８ Ω / sq, which was lower than the sheet resistance of the hole transporting portion 1031 of formula EF3. This is thought to be because in Spiro-OMeTAD, electron-withdrawing groups are not bonded to all benzene rings, and electron-donating groups are bonded instead. In addition, the sheet resistance of the hole transporting portion 1031 of formula EF7 was 1.59×10 １０Ω / sq, which was larger than the sheet resistance of the hole transport moiety of Formula EF3. This is thought to be because in Formula EF7, electron-donating groups are not bonded to all benzene rings, and electron-withdrawing groups are bonded to all outer benzene rings.

[0091] 3. Third Embodiment In the third embodiment, another aspect of the solar cell element to which the hole transport section 1031 is applied will be described. Differences from the first embodiment will be mainly described below. The solar cell element 50 according to the fourth embodiment is a multi-junction solar cell, a tandem solar cell, a stacked solar cell, a stacked solar cell, or the like. More specifically, the solar cell element of the multi-junction solar cell according to the third embodiment is a solar cell element in which thin-film solar cells are joined together. The combination of thin-film solar cells may be a combination of perovskite solar cells, or a combination of a perovskite solar cell and another thin-film solar cell such as a silicon-based thin-film solar cell. For example, a dye-sensitized solar cell or an organic thin-film solar cell may be applied as one of the thin-film solar cells.

[0092] 10 is a cross-sectional view perpendicular to the longitudinal direction of the first electrode unit 101 when the light-receiving surface F1 of the solar cell element 50 according to the third embodiment is viewed from above. The solar cell element 50 according to the third embodiment further includes a second solar cell unit 108 in addition to the first solar cell unit 107. In the third embodiment, the second electrode unit 104, the second solar cell unit 108, the first solar cell unit 107, the diffusion reduction unit 102, and the first electrode unit 101 are stacked on the substrate unit 105 in this order. The diffusion reduction unit 102 does not necessarily have to be provided.

[0093] The first solar cell unit 107 converts externally incident light (e.g., sunlight) into electricity. For example, the solar cell unit 103 may generate carriers through photoelectric conversion in response to light irradiation. The carriers include at least one of electrons and holes. The first solar cell unit 107 may be a perovskite solar cell, or other types of solar cells may be applied. For example, the solar cell type may be an inorganic solar cell or an organic solar cell. The inorganic solar cell may be a silicon solar cell or a compound solar cell. The organic solar cell may be a dye-sensitized solar cell or an organic thin-film solar cell. The first solar cell unit 107 may include a hole transport unit 1031, a photoelectric conversion unit 1032, and an electron transport unit 1033, as in the first embodiment.

[0094] Here, the material constituting the first solar cell unit 107 is translucent to light having an absorption wavelength of the second solar cell unit 108. By employing this structure, light having an absorption wavelength of the second solar cell unit 108 can be incident on the second solar cell unit 108.

[0095] The second solar cell unit 108 converts light (e.g., sunlight) incident from the outside into electric power. The second solar cell unit 108 has an absorption wavelength different from the absorption wavelength of the first solar cell unit 107. The absorption wavelength of the second solar cell unit 108 may be longer than the absorption wavelength of the first solar cell unit 107. The absorption wavelength may include not only one wavelength but also a certain wavelength band. For example, the absorption wavelength may be a wavelength band such as the visible light region, or may be a wavelength band from a first wavelength to a second wavelength.

[0096] The second solar cell unit 108 may be a perovskite solar cell, or other types of solar cells may be applied. For example, the solar cell type may be an inorganic solar cell or an organic solar cell. The inorganic solar cell may be a silicon solar cell or a compound solar cell. The organic solar cell may be a dye-sensitized solar cell or an organic thin-film solar cell. The second solar cell unit 108 may include a hole transport unit 1031, a photoelectric conversion unit 1032, and an electron transport unit 1033, as in the first embodiment.

[0097] By adopting this configuration, the plurality of solar cell units 103, such as the first solar cell unit 107 and the second solar cell unit 108, are stacked together, so that the plurality of solar cell units 103 are connected in series. This allows the output power of the solar cell element 50 to be increased, thereby improving the conversion efficiency of the solar cell element 50.

[0098] 10 , the solar cell element 50 according to the third embodiment may further include a buffer portion (not shown) between the first solar cell portion 107 and the second solar cell portion 108. The buffer portion is used to connect the first solar cell portion 107 and the second solar cell portion 108. The buffer portion is translucent to the absorption wavelength of the second solar cell portion 108.

[0099] 10 illustrates a two-layer multi-junction solar cell element, but the solar cell element 50 is not limited to this. For example, the solar cell element 50 may be a two-layer or more multi-junction solar cell element. Furthermore, although the above description shows that light is incident from the side opposite to the substrate portion 105, light may be incident from the substrate portion 105. In this case, the substrate portion 105 is translucent. Light passes through the substrate portion 105, and a portion of the light is absorbed by the second solar cell portion 108. Then, the light that has passed through the second solar cell portion 108 is absorbed by the first solar cell portion 107.

[0100] Furthermore, at least one of the first solar cell unit 107 and the second solar cell unit 108 may function as a support plate (i.e., the substrate unit 105). In this case, the substrate unit 105 is unnecessary. For example, if the second solar cell unit 108 is a crystalline silicon solar cell, the second solar cell unit 108 can function as a support plate for the first solar cell unit 107. The first solar cell unit 107 may be, for example, a perovskite solar cell.

[0101] 4. Fourth Embodiment In a solar cell module 1, as shown in FIG. 11 , for example, a plurality of solar cell elements 10 are formed on a single substrate 105. In other words, the plurality of solar cell elements 10 share the substrate 105. In this case, the substrate 105 can support the plurality of solar cell elements 10 and protect the plurality of solar cell elements 10. For example, a flat plate having a rectangular surface is used as the substrate 105. For example, glass or a resin such as acrylic or polycarbonate is used as the material of the substrate 105. For example, a material with high light transmittance such as white plate glass, tempered glass, or heat reflective glass can be used as the glass.

[0102] The plurality of solar cell elements 10 are arranged in a plane along the +X direction as a first direction. Here, "arranged in a plane" means that each solar cell element 10 is positioned along a virtual or actual plane and the plurality of solar cell elements 10 are arranged in a line. In the fourth embodiment, the plurality of solar cell elements 10 are arranged on the substrate portion 105 along the surface of the substrate portion 105.

[0103] More specifically, for example, the plurality of solar cell elements 10 may include five solar cell elements 10 arranged along the first direction (+X direction) on the substrate portion 105. The five solar cell elements 10 include, for example, a first solar cell element 111, a second solar cell element 112, a third solar cell element 113, a fourth solar cell element 114, and a fifth solar cell element 115 arranged in order in the +X direction. In other words, the plurality of solar cell elements 10 includes an n-th solar cell element 11n (n is a natural number from 1 to 5).

[0104] In the fourth embodiment, each solar cell element 10 has a rectangular shape with its longitudinal direction in the +Y direction. Each solar cell element 10 has a first electrode portion 101, a diffusion reduction portion 102, a solar cell portion 103, and a second electrode portion 104.

[0105] In the fourth embodiment, five second electrode units 104 are arranged in a planar arrangement in the +X direction on the substrate unit 105. Here, the second electrode unit 104 of the mth solar cell element 11m (m is a natural number from 1 to 4) and the second electrode unit 104 of the (m+1)th solar cell element 11(m+1) are arranged with a gap (also referred to as a first gap) G1 sandwiched between them. For example, the second electrode unit 104 of the first solar cell element 111 and the second electrode unit 104 of the second solar cell element 112 are arranged with a gap (also referred to as a first gap) G1 sandwiched between them. Each first gap G1 has a longitudinal direction along the +Y direction. Furthermore, a first groove P1 is present, with the substrate unit 105 serving as the bottom surface and the two opposing end faces of the two second electrode units 104 sandwiching the first gap G1 as side surfaces.

[0106] In the fourth embodiment, five first electrode units 101 are arranged in order in a planar manner in the +X direction. Here, the first electrode unit 101 of the mth solar cell element 11m and the first electrode unit 101 of the (m+1)th solar cell element 11(m+1) are arranged with a gap (also referred to as a second gap) G2 sandwiched between them. For example, the first electrode unit 101 of the first solar cell element 111 and the first electrode unit 101 of the second solar cell element 112 are arranged with a gap (second gap) G2 sandwiched between them. Each second gap G2 has a longitudinal direction along the +Y direction, for example. A third groove P3 is also formed with the second electrode unit 104 as its bottom surface. In each solar cell element 10, the second electrode unit 104 protrudes in the +X direction more than the first electrode unit 101. From another perspective, the first gap G1 is located at a position offset in the first direction (+X direction) from the second gap G2.

[0107] The connection portion 12 electrically connects two adjacent solar cell elements 10 among the plurality of solar cell elements 10 in series. In the fourth embodiment, the mth connection portion 12m electrically connects the mth solar cell element 11m and the (m+1)th solar cell element 11(m+1). For example, the first connection portion 121 electrically connects the first solar cell element 111 and the second solar cell element 112. More specifically, the mth connection portion 12m electrically connects the second electrode portion 104 of the mth solar cell element 11m and the first electrode portion 101 of the (m+1)th solar cell element 11(m+1). For example, the first connection portion 121 electrically connects the second electrode portion 104 of the first solar cell element 111 and the first electrode portion 101 of the second solar cell element 112. This allows the plurality of solar cell elements 10 to be electrically connected in series.

[0108] Furthermore, the connection portions 12 are located between the solar cell portions 103 in the +X direction. From another perspective, there is a second groove P2 having the solar cell portions 103 as both side surfaces and the -Z direction surface of the second electrode portion 104 as its bottom surface. The second groove P2 has a longitudinal direction along the +Y direction. Then, for example, the connection portions 12 are provided in the second groove P2. In this case, the connection portions 12 may be formed by filling the second groove P2 with the first electrode portion 101.

[0109] In the first solar cell element 111, the first electrode unit 101 has a first protrusion 101e that protrudes in the −X direction further than the diffusion reduction unit 102, the solar cell unit 103, and the second electrode unit 104. In the fifth solar cell element 115, the second electrode unit 104 has a second protrusion 104e that protrudes in the +X direction further than the first electrode unit 101, the diffusion reduction unit 102, and the solar cell unit 103. A first conductor W1 for positive output is electrically connected to the first protrusion 101e. A second conductor W2 for negative output is electrically connected to the second protrusion 104e.

[0110] <5. Other Devices> In the above examples, the solar cell element 10 and the solar cell element 10 are described as examples of devices including the hole transport section 1031, but the device is not necessarily limited to this. The device according to this embodiment may be various devices including a semiconductor section that functions as a photoelectric conversion section that converts light to electricity and a hole transport section 1031 located on the semiconductor section. For example, an organic LED (Light Emitting Diode) or an organic EL (Electro Luminescence) display may be applied as the device. Such a device includes a photoelectric conversion section that converts electricity to light and emits light, and a hole transport section 1031 located on the photoelectric conversion section. Alternatively, the device according to this embodiment may be an organic transistor. The organic transistor also includes a semiconductor section and a hole transport section 1031 located on the semiconductor section.

[0111] As described above, the hole transport section 1031, which is a hole transport layer, and the solar cell element and solar cell module including the hole transport section 1031 have been described in detail. However, the above description is merely an example in all respects, and this disclosure is not limited thereto. Furthermore, the various examples described above can be applied in combination as long as they are not mutually contradictory. It is understood that countless examples not illustrated can be envisioned without departing from the scope of this disclosure.

[0112] This disclosure includes the following:

[0113] In one embodiment, (1) the hole transport layer may comprise a base material having a triphenylamine skeleton and an electron-withdrawing group in its molecule, and a dopant, and the electron-withdrawing group may be bonded to the para position of a benzene ring belonging to the triphenylamine skeleton.

[0114] (2) In the hole transport layer of (1) above, the electron-withdrawing group can be bonded only to the para position of the benzene ring.

[0115] (3) In the hole transport layer of (1) or (2), the base material may have a derivative of Spiro-OMeTAD having four of the triphenylamine skeletons in its molecules, and the electron-withdrawing group may be bonded to each of two triphenylamine skeletons located at a first diagonal among the four triphenylamine skeletons, and a substituent having a higher electron-donating property than the electron-withdrawing group may be bonded to each of two triphenylamine skeletons located at a second diagonal different from the first diagonal among the four triphenylamine skeletons.

[0116] (4) The hole transport layer according to any one of (1) to (3), wherein the base material may comprise a derivative of Spiro-OMeTAD having four triphenylamine skeletons in its molecules, and in the derivative, the electron-withdrawing group may be bonded to each of two outer benzene rings of a first triphenylamine skeleton belonging to a first fluorene skeleton, the electron-withdrawing group and a substituent having a higher electron-donating property than the electron-withdrawing group may be bonded to each of two outer benzene rings of a second triphenylamine skeleton belonging to the first fluorene skeleton, respectively, the substituents may be bonded to each of the two outer benzene rings of a third triphenylamine skeleton belonging to the second fluorene skeleton, and the electron-withdrawing group and the substituent may be bonded to each of two outer benzene rings of a fourth triphenylamine skeleton belonging to the second fluorene skeleton, respectively.

[0117] (5) A hole transport layer according to any one of (1) to (3), wherein the base material may have a derivative of Spiro-OMeTAD having four triphenylamine skeletons in its molecules, and in the derivative, the electron-withdrawing group may be bonded to each of the two outer benzene rings in each of the two triphenylamine skeletons belonging to the first fluorene skeleton, and a substituent having a higher electron-donating property than the electron-withdrawing group may be bonded to each of the two outer benzene rings in each of the two triphenylamine skeletons belonging to the second fluorene skeleton.

[0118] (6) The solar cell element may include a positive electrode, a negative electrode, a photoelectric conversion layer located between the positive electrode and the negative electrode and generating electricity based on light, and a hole transport layer selected from any one of (1) to (5) above located between the positive electrode and the photoelectric conversion layer.

[0119] (7) The solar cell module may include a plurality of solar cell elements according to (6) above.

[0120] REFERENCE SIGNS LIST 1 Solar cell module 10 Solar cell element 101 Positive electrode (first electrode portion) 104 Negative electrode (second electrode portion) 1032 Photoelectric conversion layer (photoelectric conversion portion) 1031 Hole transport layer (hole transport portion)

Claims

1. A hole transport layer comprising: a base material having a triphenylamine skeleton and an electron-withdrawing group in its molecule; and a dopant, wherein the electron-withdrawing group is bonded to the para position of a benzene ring belonging to the triphenylamine skeleton.

2. A hole transport layer according to claim 1, wherein said electron-withdrawing group is bonded only to the para position of said benzene ring.

3. A hole transport layer according to claim 1 or 2, wherein the base material comprises molecules of a derivative of Spiro-OMeTAD having four of the triphenylamine skeletons, the electron-withdrawing group is bonded to each of two of the four triphenylamine skeletons located at a first diagonal, and a substituent having a higher electron-donating property than the electron-withdrawing group is bonded to each of two of the four triphenylamine skeletons located at a second diagonal different from the first diagonal.

4. A hole transport layer according to any one of claims 1 to 3, wherein the base material comprises a derivative of Spiro-OMeTAD having four triphenylamine skeletons in its molecules, wherein the derivative has the electron-withdrawing group bonded to each of the two outer benzene rings of a first triphenylamine skeleton belonging to a first fluorene skeleton, and the electron-withdrawing group and a substituent having a higher electron-donating property than the electron-withdrawing group bonded to each of the two outer benzene rings of a second triphenylamine skeleton belonging to the first fluorene skeleton, respectively; the substituents bonded to each of the two outer benzene rings of a third triphenylamine skeleton belonging to the second fluorene skeleton, and the electron-withdrawing group and the substituents bonded to each of the two outer benzene rings of a fourth triphenylamine skeleton belonging to the second fluorene skeleton, respectively.

5. A hole transport layer according to any one of claims 1 to 3, wherein the base material comprises a derivative of Spiro-OMeTAD having four triphenylamine skeletons in its molecules, wherein in the derivative, the electron-withdrawing group is bonded to each of the two outer benzene rings in each of two triphenylamine skeletons belonging to a first fluorene skeleton, and a substituent having a higher electron-donating property than the electron-withdrawing group is bonded to each of the two outer benzene rings in each of two triphenylamine skeletons belonging to a second fluorene skeleton.

6. A solar cell element comprising: a positive electrode; a negative electrode; a photoelectric conversion layer located between the positive electrode and the negative electrode and generating electricity based on light; and a hole transport layer according to any one of claims 1 to 5 located between the positive electrode and the photoelectric conversion layer.

7. A solar cell module comprising a plurality of solar cell elements according to claim 6.

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