Photoelectric conversion element and its manufacturing method

Abstract

To provide a photoelectric conversion element with excellent photoelectric conversion efficiency and low manufacturing cost.SOLUTION: A photoelectric conversion element according to the present invention includes an electron-transporting layer, a light-absorbing layer provided on the electron-transporting layer, and a hole-transporting layer, and the light-absorbing layer has a porous structure in which a plurality of needle crystals of a compound having a perovskite crystal structure intersect, the hole-transporting layer is provided on the light-absorbing layer and in the pores of the light-absorbing layer, and the light-absorbing layer includes a first region in which the surface of the needle crystal is covered with the hole-transporting layer, and a second region where the surface of the needle crystal is not covered with the hole-transporting layer, and the second region is arranged between the first region and the electron-transporting layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a photoelectric conversion element and a method for manufacturing the same. [Background technology]

[0002] Photoelectric conversion elements are used in, for example, optical sensors, copiers, solar cells, etc. Among these, solar cells are becoming increasingly popular as a representative method of utilizing renewable energy. Solar cells using inorganic photoelectric conversion elements (for example, silicon-based solar cells, CIGS-based solar cells, and CdTe-based solar cells) are widely used. On the other hand, solar cells using organic photoelectric conversion elements (for example, organic thin-film solar cells and dye-sensitized solar cells) are also being considered as solar cells. Solar cells using such organic photoelectric conversion elements can be manufactured by coating processes, which may reduce manufacturing costs. Therefore, solar cells using organic photoelectric conversion elements are expected to be the next generation of solar cells.

[0003] In recent years, organic photoelectric conversion elements have been studied that use a compound having a perovskite crystal structure (hereinafter sometimes referred to as a perovskite compound) in a light absorption layer. For example, the photoelectric conversion element described in Patent Document 1 has a thin layer of a photosensitive material having a perovskite crystal structure and a thin layer of a conductive material made of carbon nanotubes stacked together. Patent Document 2 discloses a photoelectric conversion element that contains a photosensitive material having a perovskite crystal structure and a binder resin. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2014-072327 [Patent Document 2] Japanese Patent Application Publication No. 2020-167329 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the crystalline form (crystal shape) of perovskite compounds is easily affected by moisture (humidity) present in the manufacturing environment. Specifically, when a photoelectric conversion device using a perovskite compound is formed in an air atmosphere (especially an air atmosphere with a humidity of 50% RH or higher), the moisture causes multiple needle-like crystals of the perovskite compound to grow in an intertwined manner, forming a porous light-absorbing layer. When a hole-transport layer is formed on such a porous light-absorbing layer by coating, the coating liquid containing the hole-transport material penetrates into the pores (voids) of the porous light-absorbing layer, resulting in a short circuit between the hole-transport layer and the electron-transport layer, resulting in a decrease in photoelectric conversion efficiency. In particular, as shown in Patent Document 1, when a conductive material (more specifically, a hole-transport material) with excellent conductivity such as carbon nanotubes is used, the decrease in photoelectric conversion efficiency due to the short circuit is significant.

[0006] For this reason, photoelectric conversion elements using perovskite compounds are generally manufactured in an environment with as low humidity as possible (e.g., in a glove box). In such an environment, the perovskite compound grows into plate-like crystals, which can suppress the decrease in photoelectric conversion efficiency due to the above-mentioned short circuit. However, manufacturing photoelectric conversion elements using perovskite compounds in such an environment increases manufacturing costs. Therefore, the inventors proposed to avoid an increase in manufacturing costs by forming a light absorbing layer in an atmospheric environment while ensuring insulation between the hole transport layer and the electron transport layer by including a binder resin together with the hole transport material in the hole transport layer (see Patent Document 2). However, the binder resin contained in the hole transport layer increases the electrical resistance between the hole transport material and the perovskite compound and the electrical resistance of the hole transport layer, making it difficult to increase the photoelectric conversion efficiency of the photoelectric conversion element. The present invention has been made in view of the above circumstances, and provides a photoelectric conversion element that is excellent in photoelectric conversion efficiency and low in manufacturing cost, and a method for manufacturing the same. [Means for solving the problem]

[0007] The present invention provides a photoelectric conversion element comprising an electron transport layer, a light absorption layer provided on the electron transport layer, and a hole transport layer, wherein the light absorption layer has a porous structure in which a plurality of needle-shaped crystals of a compound having a perovskite crystal structure intertwine, the hole transport layer is provided on the light absorption layer and in the pores of the light absorption layer, and the light absorption layer has a first region in which the surfaces of the needle-shaped crystals are covered with the hole transport layer and a second region in which the surfaces of the needle-shaped crystals are not covered with the hole transport layer, and the second region is disposed between the first region and the electron transport layer. [Effects of the Invention]

[0008] The light absorption layer included in the photoelectric conversion element of the present invention has a porous structure in which a plurality of needle-like crystals of a compound having a perovskite crystal structure intertwine, making it possible to produce the photoelectric conversion element in an atmospheric environment, thereby reducing production costs. The second region of the light absorbing layer included in the photoelectric conversion element of the present invention is disposed between the first region and the electron transport layer, thereby suppressing short circuits caused by contact between the electron transport layer and the hole transport layer, thereby enabling the photoelectric conversion element of the present invention to have excellent photoelectric conversion characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view of a photoelectric conversion element according to one embodiment of the present invention. [Figure 2] 1(a) to 1(d) are schematic cross-sectional views of a photoelectric conversion element according to one embodiment of the present invention. [Figure 3] FIG. 1 is a diagram showing a basic unit cell of the crystal structure of a perovskite compound. [Figure 4] 1 is a photograph of a light absorption layer of a photoelectric conversion element. DETAILED DESCRIPTION OF THE INVENTION

[0010] The photoelectric conversion element of the present invention comprises an electron transport layer, a light absorption layer provided on the electron transport layer, and a hole transport layer, wherein the light absorption layer has a porous structure in which a plurality of needle-shaped crystals of a compound having a perovskite crystal structure intertwine, the hole transport layer is provided on the light absorption layer and within the pores of the light absorption layer, and the light absorption layer has a first region in which the surfaces of the needle-shaped crystals are covered with the hole transport layer and a second region in which the surfaces of the needle-shaped crystals are not covered with the hole transport layer, and the second region is disposed between the first region and the electron transport layer.

[0011] The proportion of the first region in the light absorbing layer is preferably 70% to 90%, more preferably 80% to 85%. This prevents the hole transport material contained in the hole transport layer from contacting the electron transport layer, and increases the contact area between the needle-like crystals of the perovskite compound and the hole transport layer, allowing holes generated in the light absorbing layer to be transferred smoothly and efficiently to the second conductive layer. The hole transport layer preferably contains carbon nanotubes as a hole transport material and a resin as a binder, and the proportion of carbon nanotubes in the hole transport layer is preferably 30% by mass to 60% by mass, which allows holes generated in the light absorbing layer to be smoothly and efficiently transferred to the second conductive layer, and also allows the coating liquid for the hole transport layer to have an appropriate viscosity, making it easy to form the second region between the first region and the electron transport layer.

[0012] The present invention also provides a method for manufacturing a photoelectric conversion element, which includes a coating step of applying a coating liquid containing a solvent and a hole transport material onto a light-absorbing layer having a porous structure in which a plurality of needle-shaped crystals of a compound having a perovskite crystal structure intertwine, wherein the viscosity of the coating liquid is 3 mPa s or more and 50 mPa s or less. This manufacturing method makes it possible to easily form a second region between the first region and the electron transport layer.

[0013] The coating step preferably includes the steps of supplying a coating liquid onto the light-absorbing layer, leaving the light-absorbing layer standing after the coating liquid has been supplied, and rotating the light-absorbing layer after the standing to spread the coating liquid by centrifugal force, and the time for leaving the light-absorbing layer standing is preferably from 0.1 seconds to 10 seconds, which allows the second region to be easily formed between the first region and the electron transport layer. The coating step preferably includes a step of supplying the coating liquid onto the rotating light absorbing layer, which makes it possible to easily form the second region between the first region and the electron transport layer.

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings and the following description.

[0015] FIG. 1 is a schematic cross-sectional view of the photoelectric conversion element of this embodiment. The photoelectric conversion element 20 of this embodiment comprises an electron transport layer 4, a light absorption layer 7 provided on the electron transport layer 4, and a hole transport layer 8. The light absorption layer 7 has a porous structure in which a plurality of needle-shaped crystals 12 of a compound having a perovskite crystal structure intersect. The hole transport layer 8 is provided on the light absorption layer 7 and in the pores 13 of the light absorption layer 7. The light absorption layer 7 has a first region 9 in which the surfaces of the needle-shaped crystals 12 are covered with the hole transport layer 8, and a second region 10 in which the surfaces of the needle-shaped crystals 12 are not covered with the hole transport layer 8. The second region 10 is disposed between the first region 9 and the electron transport layer 4.

[0016] The method for manufacturing the photoelectric conversion element 20 of this embodiment includes an electron transport layer forming step of forming an electron transport layer 4 containing an electron transport material, a light absorbing layer forming step of forming a light absorbing layer 7 on the electron transport layer 4, and a hole transport layer forming step of forming a hole transport layer 8 containing a hole transport material on the light absorbing layer 7. The light absorbing layer 7 contains a plurality of needle-like crystals 12 of a perovskite compound. The hole transport layer 8 can contain a binder resin (e.g., polyvinyl butyral resin) and a hole transport material (e.g., carbon nanotubes).

[0017] The light-absorbing layer 7 has a porous region in which needle-shaped crystals of a perovskite compound intertwine. During the hole-transport layer formation process, if the hole-transport layer coating solution penetrates the porous region of the light-absorbing layer 7 and the hole-transport layer 8 is formed in the pores (voids) of the porous region of the light-absorbing layer 7 (on the surfaces of the needle-shaped crystals), the electron-transport material and the hole-transport material contained in the electron-transport layer 4 may approach or come into contact with each other, reducing the electrical resistance between the conductive layers (i.e., between the first conductive layer 3 and the second conductive layer 11), potentially causing a short circuit. However, the inventors have found that the hole-transport layer 8 can be prevented from shorting between the conductive layers by incorporating a binder resin in addition to the hole-transport material. This binder resin fills the gaps between the hole-transport material, thereby providing the hole-transport material with appropriate insulating properties. Even when the hole-transport layer 8 is formed in the pores of the porous region of the light-absorbing layer 7, short circuits between the conductive layers can be suppressed. However, since it is difficult to achieve both of the contradictory properties of electrical conductivity between the needle-like crystals 12 of the perovskite compound and the hole transport layer 8 and insulation between the hole transport layer 8 and the electron transport layer 4, adjustments are difficult, and although the photoelectric conversion efficiency of the photoelectric conversion element 20 can be improved to some extent, there are limitations.

[0018] Therefore, the inventors focused on the process in which the coating liquid for the hole transport layer penetrates into the light absorbing layer 7, which has a porous region formed by the intersection of multiple needle-like crystals 12 of a perovskite compound. They discovered that by stopping the penetration of the coating liquid before it reaches the interface between the light absorbing layer 7 and the electron transport layer 4, an air layer (second region 10) can be formed in the porous region. By maintaining insulation between the electron transport layer 4 and the hole transport layer 8 in this second region 10, they succeeded in further improving the photoelectric conversion efficiency.

[0019] When the hole transport layer coating liquid that has permeated into the porous region of the light absorbing layer 7 is dried, a hole transport layer 8 is formed on the needle-like crystals 12 of the perovskite compound within the pores of the light absorbing layer 7. The inventors of the present application have discovered that the proportion of the first region 9 in the light absorbing layer 7 where the hole transport layer 8 is formed can be controlled by adjusting the degree of permeation of the hole transport layer coating liquid. In conventional technology, the hole transport layer coating liquid is allowed to completely permeate the light absorbing layer 7, so the entire light absorbing layer 7 becomes the first region 9, as shown in Figure 2(d).

[0020] 2(a) to 2(c), by adjusting the degree of penetration of the coating solution for the hole transport layer into the light-absorbing layer 7 by, for example, rotating the spin coater, it is possible to form a first region 9 in which the hole transport layer 8 is formed in the light-absorbing layer 7, which is located near the outer surface of the light-absorbing layer 7, and a second region 10 in which the hole transport layer 8 is not formed in the light-absorbing layer 7, which is located near the interface between the electron transport layer 4 and the light-absorbing layer 7. By locating such a second region 10 between the electron transport layer 8 and the first region 9, it is possible to prevent the hole transport material contained in the hole transport layer 8 from contacting the electron transport layer 8, and thus to prevent short circuits through the electron transport layer 8 and the hole transport layer 8.

[0021] The ratio of the first region 9 to the second region 10 in the light-absorbing layer 7 can be controlled by adjusting the penetration degree of the hole-transport coating liquid, as shown in Figures 2(a) to 2(c). When using a spin coater, the penetration degree of the coating liquid can be controlled by adjusting the time it is left standing after the coating liquid is dropped onto the light-absorbing layer 7 and before rotation begins. That is, if the time (standing time) between dropping the coating liquid and starting rotation is short, or if the coating liquid is dropped onto the light-absorbing layer 7 after rotation has already begun, the state shown in Figure 2(a) will be one in which penetration is not very advanced. As the standing time increases and the penetration of the coating liquid progresses, the state will progress as shown in Figures 2(b) and 2(c), and ultimately a composite layer will be formed in which a hole-transport layer 8 is incorporated into the light-absorbing layer 7, as shown in Figure 2(d).

[0022] <First embodiment: photoelectric conversion element> The first embodiment relates to a photoelectric conversion element 20. The photoelectric conversion element 20 according to this embodiment includes an electron transport layer 4, a hole transport layer 8, and a light absorbing layer 7. The light absorbing layer 7 is disposed between the electron transport layer 4 and the hole transport layer 8. The light absorbing layer 7 contains a plurality of needle-like crystals 12 of a perovskite compound. The hole transport layer 8 may contain a hole transport material (e.g., carbon nanotubes) and a binder resin (e.g., polyvinyl butyral resin). The hole transport layer 8 does not necessarily contain a binder resin.

[0023] The photoelectric conversion element 20 shown in FIG. 1 includes, in order from one side, a substrate 2, a first conductive layer 3, an electron transport layer 4, a light absorption layer 7, a hole transport layer 8, and a second conductive layer 11. The electron transport layer 4 can have a two-layer structure including a dense electron transport layer 5 on the first conductive layer 3 side and a porous electron transport layer 6 on the light absorption layer 7 side. However, the electron transport layer 4 may have a single-layer structure including only the dense electron transport layer 5. When the photoelectric conversion element 20 is in use, light (e.g., sunlight) is irradiated onto the surface on the substrate 2 side. However, when the photoelectric conversion element 20 is in use, light may be irradiated onto the surface on the second conductive layer 11 side.

[0024] The light absorption layer 7 has a porous structure in which a plurality of needle-like crystals 12 of a perovskite compound intertwine. The hole transport layer 8 may contain a hole transport material (e.g., carbon nanotubes) and a binder resin (e.g., polyvinyl butyral resin). The photoelectric conversion element 20 according to this embodiment having such a configuration has the following first, second, and third advantages.

[0025] The first advantage will be explained. As mentioned above, the present invention overcomes the conventional difficulty of achieving both electrical conductivity and insulation. Specifically, the second region 10, where the hole transport layer 8 is not formed, is formed in the porous region of the light absorption layer 7, preventing the hole transport material from contacting the electron transport layer 4, maintaining insulation and preventing a decrease in photoelectric conversion efficiency due to short circuits. Meanwhile, the hole transport material maintains good charge transfer while maintaining a large contact area with the needle-like crystals 12 of the perovskite compound, thereby further improving photoelectric conversion efficiency.

[0026] The second advantage will now be described. As explained in the first advantage, the photoelectric conversion element 20 according to this embodiment can suppress short circuits between conductive layers even when the light absorption layer 7 has a porous structure in which a plurality of needle-like crystals 12 of a perovskite compound intersect. This eliminates the need to manufacture plate-like crystals of a perovskite compound in an environment with as low humidity as possible (for example, inside a glove box). Therefore, the photoelectric conversion element 20 according to this embodiment can be manufactured in an air atmosphere, thereby reducing manufacturing costs.

[0027] The third advantage will now be described. When a coating liquid for a hole transport layer containing a binder resin penetrates into the pores (voids) of the porous structure of the light absorption layer 7, the binder resin is interposed between the needle-like crystals 12 of the perovskite compound, and the photoelectric conversion element 20 has flexibility. Therefore, even if the photoelectric conversion element 20 is subjected to an impact, cracks and the like are unlikely to occur. Furthermore, if the base 2 is, for example, a flexible substrate, the photoelectric conversion element 20 can also be formed into a curved shape, which increases the degree of freedom in the shape of the photoelectric conversion element 20. The first, second, and third advantages have been described above.

[0028] [Base] The shape of the base 2 may be, for example, a plate, a film, or a cylinder. When light is irradiated onto the surface of the photoelectric conversion element 20 facing the base 2, the base 2 is transparent. In this case, examples of the material for the base 2 include transparent glass (more specifically, soda-lime glass, alkali-free glass, etc.) and heat-resistant transparent resin. When light is irradiated onto the surface of the photoelectric conversion element 20 facing the second conductive layer 11, the base 2 may be opaque. In this case, examples of the material for the base 2 include aluminum, nickel, chromium, magnesium, iron, tin, titanium, gold, silver, copper, tungsten, alloys thereof (e.g., stainless steel), and ceramic.

[0029] [First conductive layer] The first conductive layer 3 corresponds to the cathode of the photoelectric conversion element 20. Examples of materials constituting the first conductive layer 3 include transparent conductive materials and non-transparent conductive materials. Examples of transparent conductive materials include copper iodide (CuI), indium tin oxide (ITO), tin oxide (SnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), indium zinc oxide (IZO), and gallium-doped zinc oxide (GZO). Examples of non-transparent conductive materials include sodium, sodium-potassium alloy, lithium, magnesium, aluminum, a magnesium-silver mixture, a magnesium-indium mixture, an aluminum-lithium alloy, an aluminum-aluminum oxide mixture (Al / AlO), and an aluminum-lithium fluoride mixture (Al / LiF). The thickness of the first conductive layer 3 is not particularly limited, and may be any thickness that can exhibit desired properties (e.g., electron transport and transparency).

[0030] [Electron transport layer] The electron transport layer 4 is a layer that transports electrons generated in the light absorbing layer 7 by photoexcitation to the first conductive layer 3. For this reason, the electron transport layer 4 preferably contains a material that easily transfers electrons generated in the light absorbing layer 7 to the first conductive layer 3. The electron transport layer 4 contains, for example, titanium oxide as the electron transport material. When sintered at high temperatures, the content of titanium oxide in the electron transport layer 4 is close to 100 mass %, with almost no organic components remaining. When the electron transport layer 4 is produced at low temperatures, the electron transport layer 4 may further contain components other than titanium oxide. Furthermore, the electron transport layer 4 may contain an organic electron transport material.

[0031] The electron transport layer 4 may include, for example, a dense titanium oxide layer 5 (dense electron transport layer 5) and a porous titanium oxide layer 6 (porous electron transport layer 6) that is a porous layer. Alternatively, the electron transport layer 4 may be a single dense titanium oxide layer 5. The dense titanium oxide layer 5 and the porous titanium oxide layer 6 will be described below.

[0032] (Dense titanium oxide layer) The dense titanium oxide layer 5 has a lower porosity than the porous titanium oxide layer 6. Therefore, even if the light-absorbing material (perovskite compound) used to form the light-absorbing layer 7 during the production of the photoelectric conversion element 20 passes through the porous titanium oxide layer 6, it is difficult for it to penetrate into the dense titanium oxide layer 5. Therefore, the photoelectric conversion element 20 includes the dense titanium oxide layer 5, which suppresses contact between the light-absorbing material and the first conductive layer 3. Furthermore, the photoelectric conversion element 20 includes the dense titanium oxide layer 5, which suppresses contact between the first conductive layer 3 and the second conductive layer 11, which causes a decrease in electromotive force. When the electron transport layer 4 includes two layers, the dense titanium oxide layer 5 and the porous titanium oxide layer 6, the film thickness of the dense titanium oxide layer 5 is preferably 5 nm to 200 nm, more preferably 10 nm to 100 nm. When the electron transport layer 4 is a single dense titanium oxide layer 5, the thickness of the dense titanium oxide layer 5 is preferably 200 nm or more and 1000 nm or less, which is thicker than when two layers are included.

[0033] (Porous titanium oxide layer) The photoelectric conversion element 20 includes the porous titanium oxide layer 6, which offers the following advantages. The porous titanium oxide layer 6 has a higher porosity than the dense titanium oxide layer 5. Therefore, during the manufacture of the photoelectric conversion element 20, the raw material solution of the perovskite compound used to form the light-absorbing layer 7 penetrates the pores of the porous titanium oxide layer 6, increasing the contact area between the light-absorbing layer 7 and the electron transport layer 4. This allows electrons generated by photoexcitation in the light-absorbing layer 7 to efficiently move to the electron transport layer 4. Furthermore, the surface roughness of the porous titanium oxide layer 6 is greater than that of the dense titanium oxide layer 5. When the light-absorbing layer 7 is formed on a porous titanium oxide layer 6 with a large surface roughness, the needle-like crystals 12 of the perovskite compound do not grow excessively and have an appropriate size. The voids in the porous region formed by the needle-like crystals 12 have an appropriate size that prevents excessive penetration of the coating solution for the hole transport layer, further suppressing short-circuiting between the conductive layers. The thickness of the porous titanium oxide layer 6 is preferably 100 nm or more and 2,0000 nm or less, and more preferably 200 nm or more and 1,500 nm or less.

[0034] [Light absorbing layer] The light absorbing layer 7 is a layer that absorbs light incident on the photoelectric conversion element 20 and generates electrons and holes. Specifically, when light is incident on the light absorbing layer 7, low-energy electrons contained in the light absorbing material are photoexcited, generating high-energy electrons and holes. The generated electrons move to the electron transport layer 4. The generated holes move to the hole transport layer 8. This movement of electrons and holes causes charge separation. The light-absorbing layer 7 has a porous structure in which a plurality of needle-shaped crystals 12 of a perovskite compound, which is a light-absorbing material, intertwine. The light-absorbing layer 7 also has a first region 9 and a second region 10. The first region 9 is a region in which the surfaces of the needle-shaped crystals 12 in the pores of the light-absorbing layer 7 are covered with the hole transport layer 8. The first region 9 is a region in which the surfaces of the needle-shaped crystals 12 are covered with the hole transport layer 8 as a result of the hole transport layer coating liquid being filled into the pores of the light-absorbing layer 7 and drying during the manufacturing process. For example, the first region 9 can be a region in which 10% or more of the surfaces of the needle-shaped crystals 12 are covered with the hole transport layer 8. The second regions 10 are regions in the pores of the light-absorbing layer 7 where the surfaces of the needle-like crystals 12 are not covered with the hole transport layer 8. The second regions 10 are also regions in the pores of the light-absorbing layer 7 where the hole transport layer coating liquid was not filled in during the manufacturing process. For example, the second regions 10 can be regions where 95% or more of the surfaces of the needle-like crystals 12 are not covered with the hole transport layer 8. There does not need to be a clear boundary between the first region 9 and the second region 10, and a region in which the surface of the needle-like crystals 12 is partially covered with the hole transport layer 8 (for example, a region in which 5% to 10% of the surface of the needle-like crystals 12 is covered with the hole transport layer 8) may be formed between the first region 9 and the second region 10. Furthermore, the first region 9 and the second region 10 do not need to be layers having a constant thickness. Whether the second region 10 has been formed can be confirmed by the filling rate of the coating liquid for the hole transport layer in the light absorbing layer 7. For example, if the filling rate of the coating liquid for the hole transport layer is 100%, it is considered that the second region 10 has not been formed. For example, if the filling rate of the coating liquid for the hole transport layer is 99% or less, it is considered that the second region 10 has been formed. The second region 10 is disposed between the electron transport layer 4 and the first region 9. This prevents the hole transport material contained in the hole transport layer 8 from coming into contact with the electron transport layer 4, and prevents a short circuit between the first conductive layer 3 and the second conductive layer 11 via the hole transport layer 8 and the electron transport layer 4.

[0035] The proportion of the first region 9 in the light absorbing layer 7 is preferably 70% or more and 90% or less, and more preferably 80% or more and 85% or less. This prevents the hole transport material contained in the hole transport layer 8 from contacting the electron transport layer 4, and increases the contact area between the needle-like crystals 12 of the perovskite compound and the hole transport layer 8, allowing holes generated in the light absorbing layer 7 to be transferred smoothly and efficiently to the second conductive layer 11.

[0036] (Perovskite compounds) The perovskite compound is a compound having a perovskite-type crystal structure. The light-absorbing layer 7 includes a plurality of needle-like crystals 12 of the perovskite compound. In this specification, the needle-like crystals 12 refer to crystals of the perovskite compound having a ratio of the long axis length to the short axis length (aspect ratio) of 5 or more.

[0037] The long axis length of the needle-shaped crystals 12 of the perovskite compound is preferably 5 μm or more and 50 μm or less, more preferably 7 μm or more and 20 μm or less, and even more preferably 7 μm or more and 15 μm or less. The aspect ratio of the needle-shaped crystals 12 of the perovskite compound is preferably 5 or more and 20 or less, more preferably 5 or more and 15 or less, and even more preferably 5 or more and 12 or less. When the long axis length and aspect ratio of the needle-shaped crystals 12 of the perovskite compound are within the above-mentioned ranges, the coating liquid for the hole transport layer can adequately penetrate into the voids in the porous region formed by the needle-shaped crystals 12 of the perovskite compound. In this specification, the long axis length and aspect ratio of the needle-like crystals 12 of the perovskite compound refer to the arithmetic mean values ​​of the long axis length and the arithmetic mean values ​​of the aspect ratio of the needle-like crystals 12 of the perovskite compound, respectively, and these arithmetic mean values ​​are measured by the method described later in this specification.

[0038] The shape and size of the needle-like crystals 12 of the perovskite compound can be changed by the following methods. For example, the higher the humidity of the manufacturing environment, the larger the aspect ratio of the needle-like crystals 12 of the perovskite compound. Also, the higher the moisture content of the materials used in manufacturing, the larger the aspect ratio of the needle-like crystals 12 of the perovskite compound. Furthermore, when needle-like crystals of the perovskite compound are formed on a surface with few irregularities, the long and short axis lengths of the needle-like crystals 12 of the perovskite compound become shorter.

[0039] From the viewpoint of improving photoelectric conversion efficiency, the perovskite compound is preferably a compound represented by the general formula: ABX3 (1) (hereinafter, sometimes referred to as perovskite compound (1)). In general formula (1), A is an organic molecule, B is a metal atom, and X is a halogen atom. In general formula (1), the three Xs may represent the same halogen atom or different halogen atoms.

[0040] The perovskite compound (1) is an organic-inorganic hybrid compound. An organic-inorganic hybrid compound is a compound composed of an inorganic material and an organic material. A photoelectric conversion element 20 using the perovskite compound (1), which is an organic-inorganic hybrid compound, is also called an organic-inorganic hybrid photoelectric conversion element. Figure 3 shows the basic unit cell of the cubic crystal structure of the perovskite compound (1). This basic unit cell has an organic molecule A located at each vertex, a metal atom B located at the body center, and a halogen atom X located at each face center.

[0041] The presence of a cubic fundamental unit cell in a light-absorbing material can be confirmed by X-ray diffraction. Specifically, a light-absorbing layer 7 containing a light-absorbing material is prepared on a glass plate, the light-absorbing layer 7 is collected in powder form, and the diffraction pattern of the collected powdered light-absorbing layer 7 (light-absorbing material) is measured using a powder X-ray diffractometer. Alternatively, the light-absorbing layer 7 is collected in powder form from a photoelectric conversion element 20, and the diffraction pattern of the collected powdered light-absorbing layer 7 (light-absorbing material) is measured using a powder X-ray diffractometer.

[0042] In the general formula (1), examples of the organic molecule represented by A include alkylamine, alkylammonium, and nitrogen-containing heterocyclic compounds. In the perovskite compound (1), the organic molecule represented by A may be only one type of organic molecule, or may be two or more types of organic molecules. Examples of alkylamines include methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, trimethylamine, triethylamine, tripropylamine, tributylamine, tripentylamine, trihexylamine, ethylmethylamine, methylpropylamine, butylmethylamine, methylpentylamine, hexylmethylamine, ethylpropylamine, and ethylbutylamine.

[0043] The alkylammonium is an ionized product of the alkylamine. Examples of the alkylammonium include methylammonium (CH3NH3), ethylammonium, propylammonium, butylammonium, pentylammonium, hexylammonium, dimethylammonium, diethylammonium, dipropylammonium, dibutylammonium, dipentylammonium, dihexylammonium, trimethylammonium, triethylammonium, tripropylammonium, tributylammonium, tripentylammonium, trihexylammonium, ethylmethylammonium, methylpropylammonium, butylmethylammonium, methylpentylammonium, hexylmethylammonium, ethylpropylammonium, and ethylbutylammonium.

[0044] Examples of the nitrogen-containing heterocyclic compound include imidazole, azole, pyrrole, aziridine, azirine, azetidine, azeto, azole, imidazoline, and carbazole. The nitrogen-containing heterocyclic compound may be an ionized compound. As the ionized nitrogen-containing heterocyclic compound, phenethylammonium is preferred.

[0045] The organic molecule represented by A is preferably methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, methylammonium, ethylammonium, propylammonium, butylammonium, pentylammonium, hexylammonium, or phenethylammonium, more preferably methylamine, ethylamine, propylamine, methylammonium, ethylammonium, or propylammonium, and even more preferably methylammonium.

[0046] In general formula (1), examples of the metal atom represented by B include lead, tin, zinc, titanium, antimony, bismuth, nickel, iron, cobalt, silver, copper, gallium, germanium, magnesium, calcium, indium, aluminum, manganese, chromium, molybdenum, and europium. In the perovskite compound (1), the metal atom represented by B may be a single type of metal atom or two or more types of metal atoms. From the viewpoint of improving the light absorption characteristics and charge generation characteristics of the light absorption layer 7, the metal atom represented by B is preferably a lead atom.

[0047] In the general formula (1), examples of the halogen atom represented by X include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. In the perovskite compound (1), the halogen atom represented by X may be one type of halogen atom or two or more types of halogen atoms. From the viewpoint of narrowing the energy band gap of the perovskite compound (1), the halogen atom represented by X is preferably an iodine atom. Specifically, of the three Xs, it is preferable that at least one X represents an iodine atom, and it is more preferable that all three Xs represent iodine atoms.

[0048] As the perovskite compound (1), a compound represented by the general formula "CH3NH3PbX3 (wherein X represents a halogen atom)" is preferred, and CH3NH3PbI3 is more preferred. By using a compound represented by the general formula "CH3NH3PbX3" (particularly CH3NH3PbI3) as the perovskite compound (1), electrons and holes can be generated more efficiently in the light absorption layer 7, and as a result, the photoelectric conversion efficiency of the photoelectric conversion element 20 can be further improved.

[0049] [Hole transport layer] The hole transport layer 8 is a layer that captures holes generated in the light absorbing layer 7 and transports them to the second conductive layer 11, which is an anode. The hole transport layer 8 is formed on the light absorbing layer 7, and is also formed on the needle-like crystals 12 of the perovskite compound in the pores of the first region 9 of the light absorbing layer 7. The thickness of the hole transport layer 8 is preferably 20 nm to 2,000 nm, more preferably 200 nm to 600 nm. By setting the thickness of the hole transport layer 8 to 20 nm to 2,000 nm, holes generated in the light absorption layer 7 can be transferred smoothly and efficiently to the second conductive layer 11. The hole transport layer 8 can contain a hole transport material and a binder resin, or it does not have to contain a binder resin. The hole transport material may be an organic charge transport material such as spiro-MeOTAD having the following structure or an inorganic charge transport material such as carbon nanotubes.

[0050] [ka]

[0051] (carbon nanotubes) Examples of carbon nanotubes used as hole transport materials include single-walled carbon nanotubes and multi-walled carbon nanotubes, with multi-walled carbon nanotubes being preferred. The carbon nanotube content in the hole transport layer 8 is preferably 30% by mass to 60% by mass, and more preferably 40% by mass to 50% by mass. If the carbon nanotube content is 30% by mass or less, the conductivity is poor, making it difficult to extract photocurrent. On the other hand, if the carbon nanotube content exceeds 60 mass%, the resin component becomes less, the viscosity of the coating liquid for the hole transport layer becomes lower, and the carbon nanotubes contained in the coating liquid rapidly penetrate into the light absorbing layer 7 and reach the interface between the light absorbing layer 7 and the electron transport layer 4, which tends to cause a short circuit between the two conductive layers and make it difficult to extract photocurrent. When the ratio is within the above range, the binder resin appropriately increases the viscosity of the coating liquid, making it possible to control the penetration rate of the coating liquid into the light absorbing layer 7, facilitating the formation of the second region 10, and suppressing short circuits between the conductive layers. When the hole transport layer 8 does not contain a binder resin, the viscosity of the coating solution for the hole transport layer can be increased by increasing the ratio of carbon nanotubes to the solvent in the coating solution for the hole transport layer. Alternatively, the viscosity of the coating solution may be increased by using a thickener or the like.

[0052] (Organic charge transport material) The hole transport layer 8 may contain an organic charge transport material as a hole transport material instead of carbon nanotubes as a hole transport material. In this case, the hole transport layer 8 may contain the organic charge transport material as a hole transport material and a binder resin. Even when light is incident from the second conductive layer 11 side, the hole transport layer 8 is almost transparent to visible light, so there is no limit to the content as in the case of carbon nanotubes. When the hole transport layer 8 does not contain a binder resin, the viscosity of the coating solution for the hole transport layer can be increased by increasing the ratio of the organic charge transport material to the solvent in the coating solution for the hole transport layer. Alternatively, the viscosity of the coating solution may be increased by using a thickener or the like. The ratio MC / MR of the mass MC of the organic charge transport material to the mass MR of the binder resin is preferably 4 or more and 15 or less, more preferably 5 or more and 10 or less. The organic charge transport material has lower conductivity than carbon nanotubes, so it is necessary to include more of it. The organic charge transport material is, for example, a compound represented by the structural formula shown in Chemical Formula 1.

[0053] (binder resin) Resins that can be dissolved in the solvent of the coating solution for the hole transport layer are preferred, such as polyvinyl butyral resins, cellulose resins, and polycarbonate resins.

[0054] [Second conductive layer] The second conductive layer 11 corresponds to the anode of the photoelectric conversion element 20. Examples of materials constituting the second conductive layer 11 include metals, transparent conductive inorganic materials, conductive particles, and conductive polymers (particularly, transparent conductive polymers). Examples of metals include gold, silver, and platinum. Examples of transparent conductive inorganic materials include copper iodide (CuI), indium tin oxide (ITO), tin oxide (SnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), indium zinc oxide (IZO), and gallium-doped zinc oxide (GZO). Examples of conductive particles include silver nanowires and carbon nanofibers. Examples of transparent conductive polymers include a polymer containing poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT / PSS).

[0055] When light is incident from the second conductive layer 11 side of the photoelectric conversion element 20, the second conductive layer 11 is preferably transparent or semi-transparent, more preferably transparent, so that the incident light can reach the light absorbing layer 7. The material constituting the transparent or semi-transparent second conductive layer 11 is preferably a transparent conductive inorganic material or a transparent conductive polymer. The film thickness of the second conductive layer 11 is preferably 50 nm or more and 1,000 nm or less, more preferably 100 nm or more and 300 nm or less.

[0056] [others] The photoelectric conversion element 20, which is an example of a photoelectric conversion element according to this embodiment, has been described above with reference to Fig. 1. However, the photoelectric conversion element according to this embodiment is not limited to the photoelectric conversion element 20, and can be modified in the following respects, for example.

[0057] The photoelectric conversion element according to this embodiment may further include a surface layer on the second conductive layer. The surface layer is a layer that suppresses deterioration of the inside of the photoelectric conversion element due to moisture and oxygen in the air. The surface layer also protects the outer surface of the photoelectric conversion element from impacts and scratches during use. A material that has high gas barrier properties is preferred as a material for the surface layer. The surface layer can be formed using, for example, a resin composition, a shrink film, a wrap film, or a clear paint. On the other hand, a photoelectric conversion element that is housed in a sealed container and used preferably does not include a surface layer. When light is incident on the surface layer side of the photoelectric conversion element, the surface layer is preferably transparent or translucent, and more preferably transparent.

[0058] The electron transport layer of the photoelectric conversion element according to this embodiment may not contain titanium oxide. For example, the electron transport layer may include a dense electron transport layer made of a material other than titanium oxide and a porous electron transport layer made of a material other than titanium oxide. The electron transport layer may be a single layer or a multilayer structure of three or more layers.

[0059] The photoelectric conversion element according to this embodiment may not include a base, a first conductive layer, and a second conductive layer. That is, in the photoelectric conversion element, the base, the first conductive layer, and the second conductive layer may each be omitted. Furthermore, when the photoelectric conversion element according to this embodiment includes a base, the base may be conductive. In this case, the base also functions as the first conductive layer.

[0060] <Second embodiment: photoelectric conversion element manufacturing method> The second embodiment relates to a method for manufacturing a photoelectric conversion element 20. The method for manufacturing a photoelectric conversion element 20 according to this embodiment includes an electron transport layer forming step, a light absorbing layer forming step, and a hole transport layer forming step. In the electron transport layer forming step, an electron transport layer 4 containing an electron transport material is formed. In the light absorbing layer forming step, a light absorbing layer 7 is formed on the electron transport layer 4. In the hole transport layer forming step, a hole transport layer 8 containing a hole transport material is formed on the light absorbing layer 7. The light absorbing layer 7 contains a plurality of needle-shaped crystals 12 of a perovskite compound. The hole transport layer 8 may contain a binder resin and a hole transport material. The photoelectric conversion element 20 obtained by the manufacturing method according to this embodiment is, for example, the photoelectric conversion element 20 according to the first embodiment. For the same reasons as those described in the first embodiment, the photoelectric conversion element 20 obtained by the manufacturing method according to this embodiment has excellent photoelectric conversion efficiency and can reduce manufacturing costs.

[0061] The method for manufacturing the photoelectric conversion element 20 includes, for example, a laminate preparation step of preparing a laminate including a base 2 and a first conductive layer 3; an electron transport layer formation step of forming an electron transport layer 4 containing an electron transport material on the first conductive layer 3 in the laminate; a light absorbing layer formation step of forming a light absorbing layer 7 on the electron transport layer 4; a hole transport layer formation step of forming a hole transport layer 8 containing a hole transport material on the light absorbing layer 7 and in the pores of the light absorbing layer 7; and a second conductive layer formation step of forming a second conductive layer 11 on the hole transport layer 8.

[0062] [Laminate preparation process] In this step, a laminate including a base 2 and a first conductive layer 3 is prepared. The laminate is obtained, for example, by forming the first conductive layer 3 on the base 2. Methods for forming the first conductive layer 3 on the base 2 include, for example, vacuum deposition, sputtering, and plating.

[0063] [Electron transport layer formation process] In this step, an electron transport layer 4 containing an electron transport material is formed on the first conductive layer 3 in the laminate. Specifically, in the case of producing the photoelectric conversion element 20 shown in Fig. 1, this step includes a dense titanium oxide layer forming step and a porous titanium oxide layer forming step.

[0064] (Dense titanium oxide layer formation process) In this step, a dense titanium oxide layer 5 is formed on the first conductive layer 3 of the laminate. For example, a method for forming the dense titanium oxide layer 5 on the first conductive layer 3 includes applying a coating liquid for forming a dense titanium oxide layer containing a titanium chelate compound onto the first conductive layer 3, followed by baking. For example, methods for applying the coating liquid for forming a dense titanium oxide layer onto the first conductive layer 3 include spin coating, screen printing, casting, dip coating, roll coating, slot die coating, spray pyrolysis, and aerosol deposition. After baking, the formed dense titanium oxide layer 5 may be immersed in an aqueous titanium tetrachloride solution. This treatment can increase the density of the dense titanium oxide layer 5.

[0065] Examples of the solvent for the coating solution for a dense titanium oxide layer include alcohol (particularly, 1-butanol). Examples of the titanium chelate compound contained in the coating solution for a dense titanium oxide layer include a compound having an acetoacetic ester chelate group and a compound having a β-diketone chelate group.

[0066] The compound having an acetoacetic ester chelate group is not particularly limited, but examples thereof include diisopropoxytitanium bis(methylacetoacetate), diisopropoxytitanium bis(ethylacetoacetate), diisopropoxytitanium bis(propylacetoacetate), diisopropoxytitanium bis(butylacetoacetate), dibutoxytitanium bis(methylacetoacetate), dibutoxytitanium bis(ethylacetoacetate), triisoprop ... Examples of suitable acetoacetates include titanium triisopropoxy(methyl acetoacetate), titanium triisopropoxy(ethyl acetoacetate), titanium tributoxy(methyl acetoacetate), titanium tributoxy(ethyl acetoacetate), titanium triisopropoxy(methyl acetoacetate), titanium tributoxy(ethyl acetoacetate), titanium triisopropoxy(methyl acetoacetate), titanium triisopropoxy(ethyl acetoacetate), titanium triisopropoxy(methyl acetoacetate), titanium triisopropoxy(ethyl acetoacetate), titanium triisopropoxy(methyl acetoacetate), titanium triisopropoxy(ethyl acetoacetate).

[0067] The compound having a β-diketone chelate group is not particularly limited, and examples thereof include diisopropoxytitanium bis(acetylacetonate), diisopropoxytitanium bis(2,4-heptanedionate), dibutoxytitanium bis(acetylacetonate), dibutoxytitanium bis(2,4-heptanedionate), triisopropoxytitanium(acetylacetonate), triisopropoxytitanium(2,4-heptanedionate), tributoxytitanium(acetylacetonate), tributoxytitanium(2,4-heptanedionate), isopropoxytitanium tri(acetylacetonate), isopropoxytitanium tri(2,4-heptanedionate), isobutoxytitanium tri(acetylacetonate), and isobutoxytitanium tri(2,4-heptanedionate).

[0068] The titanium chelate compound is preferably a compound having an acetoacetic ester chelate group, and more preferably diisopropoxytitanium bis(acetylacetonate). As the titanium chelate compound, commercially available products such as the "TYZOR (registered trademark) AA" series manufactured by DuPont may be used.

[0069] (Porous titanium oxide layer forming process) In this step, a porous titanium oxide layer 6 is formed on the dense titanium oxide layer 5. Examples of methods for forming the porous titanium oxide layer 6 include a method in which a titanium oxide-containing coating liquid for a porous titanium oxide layer is applied to the dense titanium oxide layer 5, followed by firing. The coating liquid for a porous titanium oxide layer may further contain, for example, a solvent and an organic binder. When the coating liquid for a porous titanium oxide layer contains an organic binder, the organic binder is removed by firing. Examples of methods for applying the coating liquid for a porous titanium oxide layer to the dense titanium oxide layer 5 include spin coating, screen printing, casting, dip coating, roll coating, slot die coating, spray pyrolysis, and aerosol deposition.

[0070] The pore size and porosity of the porous titanium oxide layer 6 can be adjusted, for example, by the particle size of the titanium oxide particles contained in the coating liquid for the porous titanium oxide layer, and the type and content of the organic binder.

[0071] The titanium oxide contained in the coating solution for forming a porous titanium oxide layer is not particularly limited, but examples thereof include anatase-type titanium oxide. The coating solution for forming a porous titanium oxide layer can be prepared, for example, by dispersing titanium oxide particles (more specifically, "AEROXIDE (registered trademark) TiO2P25" manufactured by Nippon Aerosil Co., Ltd., etc.) in alcohol (more specifically, ethanol, etc.). The coating solution for forming a porous titanium oxide layer can also be prepared, for example, by diluting titanium oxide paste (more specifically, "PST-18NR" manufactured by JGC Catalysts and Chemicals Co., Ltd., etc.) with alcohol (more specifically, ethanol, etc.).

[0072] When the coating liquid for the porous titanium oxide layer contains an organic binder, the organic binder is preferably ethyl cellulose or an acrylic resin. Acrylic resins have excellent low-temperature decomposability, and even when baked at low temperatures, organic matter is unlikely to remain in the porous titanium oxide layer 6. Preferred acrylic resins are those that decompose at a temperature of about 300°C. Examples of acrylic resins include polymers of at least one (meth)acrylic monomer. Examples of (meth)acrylic monomers include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, tert-butyl (meth)acrylate, isobutyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isobornyl (meth)acrylate, n-stearyl (meth)acrylate, benzyl (meth)acrylate, and (meth)acrylic monomers having a polyoxyalkylene structure.

[0073] [Light absorption layer formation process] In this step, a light-absorbing layer 7 is formed on the electron transport layer 4 (on the porous titanium oxide layer 6 in the case of a two-layer structure, or on the dense titanium oxide layer 5 in the case of a single-layer structure). The light-absorbing layer 7 may also be formed in the pores of the porous titanium oxide layer 6. The light-absorbing layer 7 is a porous layer having a porous structure in which a plurality of needle-like crystals 12 of a perovskite compound intersect. The needle-like crystals 12 of a perovskite compound can be produced more stably and with a higher yield than plate-like crystals of a perovskite compound.

[0074] When the perovskite compound is perovskite compound (1), the light absorbing layer 7 can be formed by, for example, the following one-stage method or two-stage method. In the one-step method, a solution containing a compound represented by the general formula "AX" (hereinafter referred to as compound (AX)) and a solution containing a compound represented by the general formula "BX2" (hereinafter referred to as compound (BX2)) are mixed to obtain a mixed solution. A, B, and X in the general formulas "AX" and "BX2" are synonymous with A, B, and X in the general formula (1), respectively. This mixed solution is applied to the electron transport layer 4 and dried to form a porous layer containing the perovskite compound (1) represented by the general formula "ABX3". Examples of methods for applying the mixed solution to the electron transport layer 4 include dip coating, roll coating, spin coating, and slot die coating.

[0075] In the two-stage method, a solution containing compound (BX2) is applied onto the electron transport layer 4 to form a coating film. A solution containing compound (AX) is applied onto this coating film, and compound (BX2) and compound (AX) are reacted in the coating film. The coating film is then dried to form a porous layer containing perovskite compound (1) represented by the general formula "ABX3." Examples of methods for applying a solution containing compound (BX2) onto the electron transport layer 4 and a solution containing compound (AX) onto the coating film include dip coating, roll coating, spin coating, and slot die coating.

[0076] When the light absorbing layer 7 is formed in an air atmosphere (normal environment), needle-like crystals 12 of the perovskite compound (1) are formed due to the influence of moisture, regardless of whether the one-stage method or the two-stage method is used. Since the light absorbing layer 7 can be formed by coating in an air atmosphere, the manufacturing method according to this embodiment allows the photoelectric conversion element 20 to be manufactured at low cost. In the light absorbing layer forming step, in addition to the above-mentioned method, for example, a coating liquid for forming a light absorbing layer containing a perovskite compound and a binder resin may be applied onto the electron transport layer 4.

[0077] [Hole transport layer formation process] In this step, a hole transport layer 8 containing a hole transport material is formed on the light absorbing layer 7 and in the pores of the first region 9 of the light absorbing layer 7. More specifically, the hole transport layer 8 is formed by applying a coating liquid for a hole transport layer onto the light absorbing layer 7 to form a coating film, and then drying the coating film. The coating liquid for a hole transport layer contains, for example, a polyvinyl butyral resin, carbon nanotubes as a hole transport material, and a solvent.

[0078] The solvent contained in the coating liquid for the hole transport layer is preferably an organic solvent, more preferably an organic solvent that does not dissolve the perovskite compound contained in the light-absorbing layer 7 but dissolves the binder resin, and even more preferably isopropyl alcohol, toluene, or chlorobenzene. Using a solvent that does not dissolve the light-absorbing layer 7 allows the structure of the needle-like crystals 12 of the perovskite compound in the light-absorbing layer 7 to be favorably maintained. Using a solvent that dissolves the binder resin allows the dissolved binder resin to coat the surfaces of the needle-like crystals 12 thinly and uniformly with the carbon nanotubes, thereby imparting appropriate insulating properties to the carbon nanotubes. The content of the carbon nanotubes in the coating liquid for the hole transport layer is, for example, 0.5% by mass to 5.0% by mass. The content of the binder resin in the coating liquid for the hole transport layer is, for example, 0.5% by mass to 5.0% by mass.

[0079] When the coating liquid for the hole transport layer contains an organic charge transport material as the hole transport material, the ratio MC / MR of the mass MC of the organic charge transport material to the mass MR of the binder resin is preferably 4 or more and 15 or less, more preferably 5 or more and 10 or less. Because organic charge transport materials have lower conductivity than carbon nanotubes, the hole transport layer 8 needs to contain a higher proportion of the organic charge transport material.

[0080] The hole transport layer coating solution is prepared by dispersing carbon nanotubes and a binder resin in a solvent. A homogenizer or an ultrasonic dispersing device is used for the dispersion. Since the carbon nanotubes are broken when a strong shear force is applied, it is preferable to set gentle dispersion conditions. In the case of an organic charge transport material, since it is dissolved in a solvent, a stirring device such as a magnetic stirrer is used instead of a dispersing device.

[0081] The viscosity of the coating solution for the hole transport layer is preferably 3 mPa·s or more and 50 mPa·s or less. If the viscosity is less than 3 mPa·s, the coating solution will penetrate the light-absorbing layer 7 too quickly and reach the interface between the light-absorbing layer 7 and the electron transport layer 4, making it difficult to control the penetration level to form the second region 10. On the other hand, if the viscosity is greater than 50 mPa·s, the coating solution will penetrate the light-absorbing layer 7 too slowly, requiring a long coating process and reducing productivity. The viscosity of the coating solution for the hole transport layer is more preferably 5 mPa·s or more and 10 mPa·s or less. This is because a filling rate of 80% to 85% can be achieved in the shortest possible time. The viscosity of the coating solution for the hole transport layer can be controlled, for example, by adjusting the proportion of the binder resin in the coating solution, adjusting the proportion of the hole transport layer in the coating solution, or adding a thickener to the coating solution.

[0082] Examples of methods for applying the hole transport layer coating solution include dip coating, spray coating, slide hopper coating, roll coating, and spin coating. The hole transport layer coating solution is preferably applied by spin coating because it is necessary to control the degree of penetration of the hole transport layer coating solution into the light absorbing layer 4, which has a porous structure in which needle-like crystals 12 of the perovskite compound intertwine.

[0083] When the hole transport layer coating liquid is applied onto the light absorbing layer 7 by spin coating, the application can be performed, for example, by the following steps. First, an intermediate product having the light-absorbing layer 7 formed thereon is fixed near the center of rotation on the stage of a spin coater, and a hole transport layer coating solution is dropped onto the light-absorbing layer 7. The intermediate product is then left to stand for a predetermined time to allow the coating solution to penetrate into the light-absorbing layer 7, after which the spin coater stage is rotated. The centrifugal force caused by this rotation causes the coating solution to spread in the direction of the centrifugal force (away from the center of rotation), and excess coating solution is removed. In this way, a coating film of the hole transport layer coating solution is formed on the light-absorbing layer 7 and in the pores of the first region 9 of the light-absorbing layer 7. The coating film is then dried to form a hole transport layer 8 on the light-absorbing layer 7 and in the pores of the first region 9 of the light-absorbing layer 7. The rotation speed of the spin coater can be, for example, 1500 rpm or more and 5000 rpm or less.

[0084] The penetration degree of the hole transport layer coating liquid into the light absorbing layer 7 can be controlled by changing the time (resting time) from when the hole transport layer coating liquid is dropped onto the light absorbing layer 7 until the start of rotation of the intermediate product (light absorbing layer 7). Here, the hole transport coating liquid has an appropriate viscosity (for example, 3 mPa s or more and 50 mPa s or less). If the rotation start time is delayed (the standing time is increased, for example, 30 seconds or more), the coating liquid penetrates deeper into the light absorbing layer 7 and reaches the interface between the light absorbing layer 7 and the electron transport layer 4.

[0085] If the rotation start time is shortened (the standing time is shortened, for example, to 10 seconds or less), the intermediate product (light-absorbing layer 7) rotates before the coating liquid has fully penetrated into the light-absorbing layer 7, and the coating liquid is removed by centrifugal force. Therefore, the penetration of the coating liquid into the light-absorbing layer 7 does not proceed further, and the start of rotation can stop the penetration of the coating liquid. As a result, a region in the light-absorbing layer 7 where the coating liquid has penetrated (from the upper surface of the light-absorbing layer 7 to the penetration tip) and a region where the coating liquid has not penetrated (from the interface between the light-absorbing layer 7 and the electron transport layer 4 to the penetration tip, an air layer) can be formed. When the coating liquid is then dried, a hole transport layer 8 is formed to cover the surface of the needle-shaped crystals 12 in the region of the light-absorbing layer 7 where the coating liquid has penetrated (from the upper surface of the light-absorbing layer 7 to the penetration tip, first region 9). However, in the region where the coating liquid has not penetrated (from the interface between the light-absorbing layer 7 and the electron transport layer 4 to the penetration tip, second region 10), the surface of the needle-shaped crystals 12 is not covered by the hole transport layer 8. By forming such a second region 10 between the electron transport layer 4 and the first region 9, contact between the electron transport layer 4 and the hole transport layer 8 can be prevented, and short circuits can be suppressed.

[0086] The total weight of the light-absorbing layer 7 and the hole-transporting layer 8 can be calculated, for example, by subtracting the weight of the first intermediate product after the formation of the electron-transporting layer 4 and before the formation of the light-absorbing layer 7 from the weight of the second intermediate product after the formation of the hole-transporting layer 8 and before the formation of the second conductive layer 11. Because the weight of the light-absorbing layer 7 of each sample is substantially the same, the difference in weight between each sample (weight of the second intermediate product - weight of the first intermediate product) represents the difference in weight of the hole-transporting layer 8, and corresponds to the difference in the penetration degree of the coating liquid for the hole-transporting layer into the light-absorbing layer 7.

[0087] If the rotation start time is delayed (the standing time is extended, for example, 30 seconds or longer), the hole transport layer coating liquid reaches the interface between the light absorbing layer 7 and the electron transport layer 4. Further extension of the standing time prevents further penetration of the coating liquid into the light absorbing layer 7, resulting in a smaller weight difference (weight of the second intermediate product minus weight of the first intermediate product). The density of the (light absorbing layer + hole transport layer) can be calculated from this weight difference (weight of the second intermediate product minus weight of the first intermediate product), the measured film thickness (total film thickness of the light absorbing layer and the hole transport layer), and the sample size (length × width). This density corresponds to a 100% filling rate of the hole transport layer coating liquid into the light absorbing layer 7. Furthermore, the filling rates can be calculated from the densities of the (light absorbing layer + hole transport layer) of other samples, assuming this density as a 100% filling rate.

[0088] If the rotation start time is shortened (the standing time is shortened, for example, to 10 seconds or less), a region in the light-absorbing layer 7 where the hole-transport-layer coating liquid has not penetrated (a second region 10 from the interface between the light-absorbing layer 7 and the electron transport layer 4 to the penetration tip) is formed, and the density of (light-absorbing layer + hole transport layer) becomes smaller and the filling rate also becomes lower compared to when the filling rate is 100%.

[0089] Whether the hole transport coating liquid has reached the interface between the light absorption layer 7 and the electron transport layer 4 can be confirmed by observing the sample or intermediate product from the glass substrate (base 2) side. That is, because the transparent electrode (first conductive layer 3) and the electron transport layer 4 are transparent, needle-like crystals 12 of the perovskite compound, which are nearly black, can be observed from the glass substrate side. If the hole transport layer coating liquid containing carbon nanotubes as the hole transport material has reached the interface between the light absorption layer 7 and the electron transport layer 4, an even darker black color can be observed. If the hole transport layer coating liquid containing an organic charge transport material as the hole transport material has reached the interface between the light absorption layer 7 and the electron transport layer 4, a yellowish black color can be observed because the organic charge transport material is yellow.

[0090] When applying the hole transport layer coating liquid onto the light absorbing layer 7 by spin coating, the coating can also be performed by the following steps, for example. First, an intermediate product with the light-absorbing layer 7 formed thereon is fixed near the center of rotation on the stage of a spin coater, and the spin coater stage is rotated. Then, a hole transport layer coating solution is dropped onto the rotating light-absorbing layer 7. The dropped coating solution spreads in the direction of centrifugal force (away from the center of rotation) while penetrating the light-absorbing layer 7, and excess coating solution is removed. This allows the formation of a region in the light-absorbing layer 7 where the coating solution has permeated (from the upper surface of the light-absorbing layer 7 to the permeation tip) and a region where the coating solution has not permeated (an air layer from the interface between the light-absorbing layer 7 and the electron transport layer 4 to the permeation tip). The coating solution is then dried, forming a hole transport layer 8 covering the surface of the needle-shaped crystals 12 in the region of the light-absorbing layer 7 where the coating solution has permeated (first region 9, from the upper surface of the light-absorbing layer 7 to the permeation tip). However, in the region where the coating liquid has not permeated (the second region 10, from the interface between the light absorbing layer 7 and the electron transport layer 4 to the permeation tip), the surfaces of the needle-like crystals 12 are not covered with the hole transport layer 8. By forming such a second region 10 between the electron transport layer 4 and the first region 9, contact between the electron transport layer 4 and the hole transport layer 8 can be prevented, and short-circuiting can be suppressed. Even when the hole transport layer 8 is prepared in this manner, the density and packing ratio can be calculated by the above-described method. When the hole transport layer coating liquid is applied onto the light absorbing layer 7 in this manner, the penetration of the coating liquid can be controlled, for example, by adjusting the rotation speed of the spin coater and the spin coating time (the time from when the coating liquid is dropped until the rotation of the stage is stopped).

[0091] [Second conductive layer formation process] In this step, the second conductive layer 11 is formed on the hole transport layer 8. The method for forming the second conductive layer 11 on the hole transport layer 8 is not particularly limited, and the same method as the method for forming the first conductive layer 3 (more specifically, vacuum deposition, sputtering, plating, etc.) can be used.

[0092] [others] 1 has been described as an example of the method for manufacturing the photoelectric conversion element 20 according to this embodiment. However, the method for manufacturing the photoelectric conversion element according to this embodiment is not limited to the above-described manufacturing method, and can be modified in the following respects, for example.

[0093] The method for manufacturing the photoelectric conversion element 20 according to this embodiment may further include a surface layer forming step of forming a surface layer on the second conductive layer 11. Moreover, the method for manufacturing the photoelectric conversion element 20 according to this embodiment may not include the laminate preparation step and the second conductive layer forming step. Furthermore, in the electron transport layer forming step, the electron transport layer may be formed by a method other than the above-mentioned dense titanium oxide layer forming step and porous titanium oxide layer forming step.

[0094] <Photoelectric conversion element manufacturing experiment> Samples A-1 to A-7, B-1, and B-2 shown in Table 1 were produced by the following method. Each sample (photoelectric conversion element) was produced in an environment with a temperature of 25°C and a humidity of 60%RH. Table 1 shows the resin / hole transport material (weight ratio) for each sample, the viscosity of the coating solution for the hole transport layer, the standing time (seconds) or spin coating time (seconds), the total weight of the light absorption layer and hole transport layer, the total film thickness of the light absorption layer and hole transport layer, the calculated film thickness, and the filling factor.

[0095] The terms used in Table 1 have the following meanings: The "resting time" indicates the time from when the hole transport layer coating solution was dropped onto the light absorbing layer until the spin coater started rotating (Samples A-1, A-4 to A-7, B-1, B-2). The "spin coating time" refers to the time from dropping the hole transport layer coating solution onto the light absorbing layer to stopping the rotation of the spin coater in the preparation of samples (A-2, A-3) in which the hole transport layer coating solution was dropped onto the rotating light absorbing layer. The "calculated film thickness" is the film thickness of the (light absorbing layer + hole transport layer) calculated from the density of the (light absorbing layer + hole transport layer) of Sample B-1, the size of each sample (2.5 cm length × 2.5 cm width), and the total weight of the (light absorbing layer + hole transport layer) of each sample. Note that in the production of Sample B-1, it is assumed that the coating liquid for the hole transport layer was filled into all of the pores of the light absorbing layer. The "filling ratio" is the ratio of the area of ​​the light-absorbing layer filled with the coating liquid for the hole transport layer, and was calculated using the formula: (calculated film thickness) / (total film thickness of the light-absorbing layer and the hole transport layer), where (total film thickness of the light-absorbing layer and the hole transport layer) is the measured film thickness of each sample. Other terms will be explained as appropriate in the following description.

[0096] [Table 1]

[0097] [Sample B-1] (Laminate preparation process) A transparent glass plate (Sigma-Aldrich, plate thickness: 2.2 mm) on which fluorine-doped tin oxide was vapor-deposited was cut to a size of 25 mm wide and 25 mm long. This prepared a laminate comprising a substrate (transparent glass plate) and a first conductive layer (vapor-deposited film of fluorine-doped tin oxide). This laminate was subjected to ultrasonic cleaning treatment in ethanol (10 minutes) and UV cleaning treatment (15 minutes).

[0098] (Dense titanium oxide layer formation process) A 1-butanol solution (manufactured by Sigma-Aldrich) containing a titanium chelate compound, diisopropoxytitanium bis(acetylacetonate), at a concentration of 75% by mass was diluted with 1-butanol. This resulted in a coating solution for a dense titanium oxide layer with a titanium chelate compound concentration of 0.02 mol / L. The coating solution for a dense titanium oxide layer was applied onto the first conductive layer of the laminate by spin coating, and the resultant was heated at 450°C for 15 minutes. This resulted in a dense titanium oxide layer with a thickness of 50 nm being formed on the first conductive layer.

[0099] (Porous titanium oxide layer forming process) A coating solution for a porous titanium oxide layer was prepared by diluting 1 g of a titanium oxide paste containing titanium oxide and ethanol ("PST-18NR" manufactured by JGC Catalysts and Chemicals Co., Ltd.) with 2.5 g of ethanol. The coating solution for a porous titanium oxide layer was applied to the dense titanium oxide layer by spin coating, and then baked at 450°C for 1 hour. This resulted in a porous titanium oxide layer with a thickness of 250 nm being formed on the dense titanium oxide layer. At this point, the thickness of the titanium oxide layer and the weight of the first intermediate product including the glass substrate were measured.

[0100] (Light absorption layer forming process) The light-absorbing layer was formed under atmospheric conditions. 922 mg of PbI2 (Tokyo Chemical Industry Co., Ltd.) and 318 mg of CH3NH3I (Tokyo Chemical Industry Co., Ltd.) were dissolved in 1.076 mL of N,N-dimethylformamide (DMF) with heating. The molar ratio of PbI2 to CH3NH3I was 1:1. This resulted in a mixed solution with a solid content of 55% by mass. The intermediate product after the porous titanium oxide layer was formed was rotated at 5,000 rpm for 30 seconds by spin coating, and the mixed solution was applied onto the porous titanium oxide layer. Two drops of toluene were added to the coated intermediate product, and the coating liquid film changed from yellow to black. This confirmed the formation of a perovskite compound (CH3NH3PbI3). The coated liquid film was then dried at 100°C for 60 minutes. This resulted in a 500 nm-thick light-absorbing layer containing the perovskite compound being formed on the porous titanium oxide layer. This light-absorbing layer has a porous structure in which a plurality of needle-like crystals of a compound having a perovskite crystal structure intertwine (see "Observation of the light-absorbing layer" below).

[0101] (Hole transport layer formation process) Using an ultrasonic disperser, 0.2 g of carbon nanotubes (multi-walled carbon nanotubes, "Multi-walled Carbon Nanotubes" manufactured by Tokyo Chemical Industry Co., Ltd.) (hole transport material) and polyvinyl butyral resin ("S-LEC BL-S" (registered trademark) manufactured by Sekisui Chemical Co., Ltd., weight average molecular weight: 2.3 × 10) were mixed. 40.2 g of the resin and hole transport material were dispersed in 12.21 mL of isopropyl alcohol (resin / hole transport material = 1 / 1) (weight ratio). In this way, a coating solution for the hole transport layer was prepared. The viscosity of the coating solution for the hole transport layer was measured and found to be 5 mPa s.

[0102] The hole transport layer coating solution prepared using spin coating was then applied to the light absorbing layer. Specifically, the hole transport layer coating solution was dropped onto the light absorbing layer, and the intermediate product was allowed to stand for 60 seconds (standing time), after which the intermediate product was rotated at 2000 rpm for 20 seconds to form a coating film. During the standing time, the hole transport layer coating solution penetrated deep into the porous light absorbing layer. By rotating the intermediate product, the hole transport layer coating solution spread in the direction of centrifugal force (away from the center of rotation), and excess hole transport layer coating solution was removed. Therefore, the pores in the light absorbing layer can be considered to have been filled with the hole transport layer coating solution at a filling rate of 100%. The coating film formed by applying the hole transport layer coating solution was dried at 100°C for 30 minutes to remove the organic solvent (isopropyl alcohol), thereby forming a hole transport layer on the light absorbing layer and in the pores of the light absorbing layer.

[0103] At this point, the weight of the second intermediate product was measured. The difference between the weight of the second intermediate product and the weight of the first intermediate product was the total weight of the light absorption layer and the hole transport layer. This total weight is shown in Table 1. The total thickness of the light absorbing layer and the hole transporting layer was also measured. The measured total thickness is shown in Table 1. The density of the (light absorbing layer + hole transporting layer) was calculated from the total weight of the light absorbing layer and the hole transporting layer, the measured total film thickness, and the size of the light absorbing layer (25 mm length × 25 mm width). The density was 4.01 g / cm 3 In this intermediate product, the pores in the light absorbing layer can be considered to have been filled with the coating liquid for the hole transport layer at a filling rate of 100%, so the filling rate of sample B-1 was considered to be 100%, and this density was used to calculate the filling rates of the other samples.

[0104] (Second conductive layer formation process) A second conductive layer was formed on the hole transport layer by vacuum deposition. The second conductive layer was a gold-deposited film with a thickness of 150 nm, a width of 5 mm, and a length of 5 mm, and was formed as an anode. This resulted in a sample (photoelectric conversion element) B-1, which included a substrate, a first conductive layer, an electron transport layer (specifically, a dense titanium oxide layer and a porous titanium oxide layer), a light absorption layer, a hole transport layer, and a second conductive layer.

[0105] [Sample B-2] In the spin-coating process for forming the hole transport layer, the coating solution for the hole transport layer was dropped onto the light absorbing layer, and the intermediate product was left standing for 30 seconds (standing time). The intermediate product was then spun at 2000 rpm for 20 seconds to remove excess coating solution for the hole transport layer. Except for this, Sample B-2 was produced in the same manner as Sample B-1. The calculated thickness of the sample B-1 (light absorbing layer + hole transporting layer) was calculated using its density, and the calculated thickness was almost the same as the measured thickness. This suggests that the hole transporting layer coating solution reaches the electron transporting layer when the sample is left standing for 30 seconds or longer, and the penetration of the hole transporting layer coating solution into the light absorbing layer is almost complete.

[0106] [Sample A-1] In the spin-coating process for forming the hole transport layer, the hole transport layer coating solution was dropped onto the light absorbing layer, and the intermediate product was then left to stand for 3 seconds (resting time). The intermediate product was then spun at 2000 rpm for 20 seconds to remove excess hole transport layer coating solution. During the resting time, the coating solution penetrated the porous light absorbing layer, and by rotating the intermediate product, the coating solution spread in the direction of centrifugal force, presumably removing the excess coating solution. In this sample, the short resting time of 3 seconds likely resulted in the formation of a region of the light absorbing layer adjacent to the electron transport layer where the hole transport layer coating solution had not penetrated. Other than this, Sample A-1 was produced using the same method as Sample B-1.

[0107] [Sample A-2] During the spin-coating process for forming the hole transport layer, the intermediate product was rotated (2000 rpm, 20 seconds) before the hole transport layer coating solution was dispensed. The hole transport layer coating solution was dispensed onto the light-absorbing layer for 2 seconds (spin-coating time: 3 seconds) starting 3 seconds before the rotation ended, forming a coating film. In this sample, the coating solution was dispensed onto the rotating light-absorbing layer, which likely resulted in simultaneous penetration of the coating solution into the porous light-absorbing layer and spreading of the coating solution in the direction of centrifugal force. This likely resulted in the formation of a region of the light-absorbing layer adjacent to the electron transport layer where the hole transport layer coating solution had not penetrated. Except for this, Sample A-2 was prepared using the same method as Sample B-1.

[0108] [Sample A-3] During the spin-coating process for forming the hole transport layer, the intermediate product was rotated (2000 rpm, 20 seconds) before the hole transport layer coating solution was dispensed. The hole transport layer coating solution was dispensed onto the light-absorbing layer for 2 seconds (spin-coating time: 10 seconds) starting 10 seconds before the rotation ended, forming a coating film. In this sample, the coating solution was dispensed onto the rotating light-absorbing layer, which likely resulted in simultaneous penetration of the coating solution into the porous light-absorbing layer and spreading of the coating solution in the direction of centrifugal force. This likely resulted in the formation of a region of the light-absorbing layer adjacent to the electron transport layer where the hole transport layer coating solution had not penetrated. Furthermore, because the spin-coating time was longer than that of Sample A-2, the penetration depth of the coating solution into the light-absorbing layer was shallower than that of Sample A-2. Other than this, Sample A-3 was prepared using the same method as Sample B-1.

[0109] [Sample A-4] During the spin-coating process for forming the hole transport layer, the hole transport layer coating solution was dropped onto the light absorbing layer, and the intermediate product was then left to stand for 5 seconds (resting time). The intermediate product was then spun at 2000 rpm for 20 seconds to remove excess hole transport layer coating solution. During the resting time, the coating solution penetrated the porous light absorbing layer, and by rotating the intermediate product, the coating solution spread in the direction of centrifugal force, presumably removing the excess coating solution. In this sample, the short resting time of 5 seconds likely resulted in the formation of a region of the light absorbing layer adjacent to the electron transport layer where the hole transport layer coating solution had not penetrated. Except for this, Sample A-4 was produced using the same method as Sample B-1.

[0110] [Sample A-5] In the spin-coating process for forming the hole transport layer, the hole transport layer coating solution was dropped onto the light absorbing layer, and the intermediate product was then left to stand for 10 seconds (resting time). The intermediate product was then spun at 2000 rpm for 20 seconds to remove excess hole transport layer coating solution. During the resting time, the coating solution penetrated the porous light absorbing layer, and by rotating the intermediate product, the coating solution spread in the direction of centrifugal force, presumably removing the excess coating solution. In this sample, the short resting time of 10 seconds likely resulted in the formation of a region of the light absorbing layer adjacent to the electron transport layer where the hole transport layer coating solution had not penetrated. Except for this, Sample A-5 was produced using the same method as Sample B-1.

[0111] [Sample A-6] In the step of preparing the coating solution for the hole transport layer, 0.2 g of carbon nanotubes (multi-walled carbon nanotubes, "Multi-walled Carbon Nanotubes" manufactured by Tokyo Chemical Industry Co., Ltd.) and 0.2 g of polyvinyl butyral resin ("S-LEC BL-S" manufactured by Sekisui Chemical Co., Ltd., weight average molecular weight: 2.3 × 10) were mixed using an ultrasonic disperser. 4 0.4 g of the resin and hole transport material were dispersed in 12.21 mL of isopropyl alcohol (resin / hole transport material = 2 / 1) (weight ratio). In this way, a coating solution for the hole transport layer was prepared. The viscosity of the coating solution for the hole transport layer was measured and found to be 60 mPa s.

[0112] The hole transport layer coating solution was then applied onto the light-absorbing layer using spin coating. Specifically, the hole transport layer coating solution was dropped onto the light-absorbing layer, and the intermediate product was allowed to stand for 300 seconds (resting time). The intermediate product was then spun at 2000 rpm for 20 seconds to remove excess hole transport layer coating solution. During the resting time, the coating solution penetrated the porous light-absorbing layer. By rotating the intermediate product, the coating solution spread in the direction of centrifugal force, presumably removing excess coating solution. In this sample, the high viscosity of the hole transport coating solution likely resulted in the formation of a region of the light-absorbing layer adjacent to the electron transport layer where the hole transport layer coating solution had not penetrated. Except for these differences, Sample A-6 was manufactured using the same method as Sample B-1.

[0113] [Sample A-7] In the step of preparing the coating solution for the hole transport layer, 0.2 g of carbon nanotubes (multi-walled carbon nanotubes, "Multi-walled Carbon Nanotubes" manufactured by Tokyo Chemical Industry Co., Ltd.) and 0.2 g of polyvinyl butyral resin ("S-LEC BL-S" manufactured by Sekisui Chemical Co., Ltd., weight average molecular weight: 2.3 × 10) were mixed using an ultrasonic disperser. 4 0.1 g of resin and hole transport material were dispersed in 18.321 mL of isopropyl alcohol (resin / hole transport material = 1 / 2) (weight ratio). In this way, a coating solution for the hole transport layer was prepared. The viscosity of the coating solution for the hole transport layer was measured and found to be 2 mPa s.

[0114] The hole transport layer coating solution was then applied onto the light-absorbing layer using spin coating. Specifically, the hole transport layer coating solution was dropped onto the light-absorbing layer, and the intermediate product was allowed to stand for 3 seconds (resting time). The intermediate product was then spun at 2000 rpm for 20 seconds to remove excess hole transport layer coating solution. During the resting time, the coating solution penetrated into the porous light-absorbing layer. By rotating the intermediate product, the coating solution spread in the direction of centrifugal force, presumably removing excess coating solution. In this sample, the short resting time of 3 seconds likely resulted in the formation of a region of the light-absorbing layer adjacent to the electron transport layer where the hole transport layer coating solution had not penetrated. Furthermore, due to the low viscosity of the hole transport layer coating solution in this sample, the coating solution likely penetrated deeper into the light-absorbing layer than in Sample A-1. Except for these differences, Sample A-7 was manufactured using the same method as Sample B-1.

[0115] <Observation of the light absorption layer> After the light-absorbing layer formation step and before the hole-transporting layer formation step, the surface of the light-absorbing layer of Sample A-1 was observed at 2000x magnification using an optical microscope (Keyence Corporation's "Digital Microscope VHX"). Figure 4 shows a photograph of the surface of the light-absorbing layer of Sample A-1. The scale bar shown in the photograph of Figure 4 indicates a length of 10.00 μm. From the photograph shown in Figure 4, it was confirmed that the light-absorbing layer of Sample A-1 was composed of multiple needle-like crystals of the perovskite compound, and that porous regions were formed by the intersection of multiple needle-like crystals of the perovskite compound.

[0116] <Measurement of the long axis length and aspect ratio of needle-like crystals of perovskite compounds> After the light-absorbing layer formation step and before the hole-transporting layer formation step, the surface of the light-absorbing layer of Sample A-1 was observed at 2000x magnification using an optical microscope (Keyence Corporation's "Digital Microscope VHX"). The major axis length and aspect ratio of any 20 of the needle-shaped crystals of perovskite compound found on the surface of the light-absorbing layer were measured, and the sum of the 20 measurements was divided by the number of measurements (20) to determine the arithmetic mean. For the multiple needle-shaped crystals of perovskite compound contained in the light-absorbing layer of Sample A-1, the arithmetic mean of the major axis length was 7 μm, and the arithmetic mean of the aspect ratio was 5.

[0117] <Photoelectric conversion element output measurement> The short-circuit current, open-circuit voltage, fill factor, and photoelectric conversion efficiency of each of Samples A-1 to A-7, B-1, and B-2 were measured using a solar simulator (manufactured by Wacom Electronics Co., Ltd.). A black metal mask with a hole of 5 mm width x 5 mm length, the same size as the electrode area, was placed over the photoelectric conversion element to eliminate the influence of scattered light. The photoelectric conversion element (each sample) was then connected to the solar simulator so that the second conductive layer on the surface layer side of the photoelectric conversion element served as the anode and the first conductive layer on the substrate side served as the cathode. The 100 mW / cm intensity was obtained by passing the light from a xenon lamp through an air mass filter (Nikon Corporation, "AM-1.5"). 2 The photoelectric conversion element (each sample) was irradiated with simulated sunlight. The current-voltage characteristics of the photoelectric conversion element when irradiated were measured, and a current-voltage curve was obtained. From the current-voltage curve, the short-circuit current (J sc ), open circuit voltage (V oc The short-circuit current, open-circuit voltage, fill factor (FF), and photoelectric conversion efficiency (η) were calculated. The higher the short-circuit current, open-circuit voltage, fill factor, and photoelectric conversion efficiency, the better the photoelectric conversion element. The results are shown in Table 2.

[0118] [Table 2]

[0119] As shown in Table 2, samples A-1 to A-7 had better short-circuit current, open-circuit voltage, fill factor, and conversion efficiency than samples B-1 and B-2. This is thought to be because samples A-1 to A-7 have excellent photoelectric conversion properties because the regions in the pores of the light absorption layer where the hole transport layer is not formed are formed in the vicinity of the electron transport layer. Furthermore, among the samples A-1 to A-7, the conversion efficiencies of the samples A-1 to A-3 were particularly good. In addition, sample A-6 had a high viscosity due to the high resin content in the coating solution for the hole transport layer, which slowed the penetration rate of the coating solution into the light absorption layer and made it time-consuming to fabricate the photoelectric conversion element, resulting in a low overall evaluation. In addition, sample A-7 had a low viscosity due to the small amount of resin contained in the coating solution for the hole transport layer, which resulted in the coating solution permeating into the light absorption layer quickly and making it prone to failure in the fabrication of the photoelectric conversion element, resulting in a low overall evaluation.

[0120] Furthermore, samples B-1 and B-2 were inferior to samples A-1 to A-5 in short-circuit current, open-circuit voltage, fill factor, and photoelectric conversion efficiency. This is thought to be because, in samples B-1 and B-2, when the hole transport layer was formed, the carbon nanotubes, which are the hole transport material, did not remain in the voids in the porous region of the light absorption layer but penetrated to the interface with the electron transport layer, resulting in partial short-circuiting between the conductive layers. [Industrial Applicability]

[0121] The photoelectric conversion element according to the present invention can be used in photovoltaic power generation systems such as mega solar systems, solar cells, power sources for small portable devices, and the like. [Explanation of symbols]

[0122] 2: Substrate 3: First conductive layer 4: Electron transport layer 5: Dense electron transport layer (dense titanium oxide layer) 6: Porous electron transport layer (porous titanium oxide layer) 7: Light absorption layer 8: Hole transport layer 9: First region 10: Second region 11: Second conductive layer 12: Needle-shaped crystals 13: Pores 20: Photoelectric conversion element

Claims

1. an electron transport layer, a light absorbing layer provided on the electron transport layer, and a hole transport layer; the light absorbing layer has a porous structure having a plurality of needle-like crystals of a perovskite compound; the hole transport layer is provided on the light absorbing layer and in the pores of the light absorbing layer; the light absorbing layer has a first region where the surfaces of the needle-like crystals are covered with the hole transport layer and a second region where the surfaces of the needle-like crystals are not covered with the hole transport layer; A photoelectric conversion element, wherein the proportion of the first region in the light absorption layer is 69.8% or more and 98.8% or less.

2. The photoelectric conversion element according to claim 1 , wherein the proportion of the first region in the light absorption layer is 70% or more and 90% or less.

3. 2. The photoelectric conversion element according to claim 1, wherein the proportion of the first region in the light absorption layer is 80% or more and 85% or less.

4. the hole transport layer contains carbon nanotubes as a hole transport material and a resin as a binder, 4. The photoelectric conversion element according to claim 1, wherein the content of the carbon nanotubes in the hole transport layer is 30% by mass or more and 60% by mass or less.

5. a coating step of coating a coating liquid containing a solvent and a hole transport material onto the light absorbing layer, 4. The method for producing a photoelectric conversion element according to claim 1, wherein the viscosity of the coating liquid is 3 mPa·s or more and 50 mPa·s or less.

6. A method for manufacturing a photoelectric conversion element as described in Claim 5, wherein the viscosity of the coating liquid is 5 mPa·s or more and 10 mPa·s or less.

7. the coating step includes a step of supplying the coating liquid onto the light absorbing layer, a step of leaving the light absorbing layer standing after supplying the coating liquid onto the light absorbing layer, and a step of rotating the light absorbing layer after leaving the light absorbing layer standing to spread the coating liquid by centrifugal force, The method for producing a photoelectric conversion element according to claim 6 , wherein the time for leaving the light absorbing layer standing is from 0.1 seconds to 10 seconds.

8. A method for manufacturing a photoelectric conversion element as described in Claim 7, wherein the rotation speed of the light absorption layer in the coating process is 1500 rpm or more and 5000 rpm or less.

9. The method for manufacturing a photoelectric conversion element according to claim 6 , wherein the coating step includes a step of supplying the coating liquid onto the rotating light absorption layer.

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