Photoelectric conversion element and method for manufacturing photoelectric conversion layer

Abstract

To provide a photoelectric conversion element having a power storage function.SOLUTION: The photoelectric conversion element includes two electrodes, and a photoelectric conversion layer formed between the two electrodes and containing an n-type semiconductor material and a p-type semiconductor material containing carbon nanotubes having p-type semiconductor characteristics. The photoelectric conversion layer has a storage function of storing electrons and holes generated in the photoelectric conversion layer. The photoelectric conversion layer has a stacked structure in which a p-type semiconductor layer containing the p-type semiconductor material and an n-type semiconductor layer containing the n-type semiconductor material are stacked, or a bulk heterojunction structure.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] Generally, a photoelectric conversion element has a photoelectric conversion layer composed of a p-type semiconductor material and an n-type semiconductor material between an anode and a cathode (see, for example, Patent Document 1). When light is incident on the photoelectric conversion layer, excitons are generated, generating electrons and holes. The electrons move to the cathode and the holes move to the anode, causing a current to flow in an external circuit connected to the photoelectric conversion element. When light irradiation of the photoelectric conversion layer is stopped, electrons and holes are no longer generated in the photoelectric conversion layer, and a current no longer flows to the external circuit. In other words, conventional photoelectric conversion elements could only pass a current while light irradiation of the photoelectric conversion element continued. Therefore, in order to continue supplying a current to the photoelectric conversion element after light irradiation of the photoelectric conversion element has stopped, an external battery has been connected to the photoelectric conversion element, and after light irradiation of the photoelectric conversion element has stopped, a current has been supplied from the external battery (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6005785 [Patent Document 2] Japanese Patent Publication No. 2022-168820 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when an external battery is connected to the photoelectric conversion element, the photoelectric conversion element and the external battery are separate, which results in an increase in the size of the device.

[0005] In view of the above circumstances, the present invention provides a photoelectric conversion element (first photoelectric conversion element) having an electricity storage function and a method for manufacturing a photoelectric conversion layer of the photoelectric conversion element (first photoelectric conversion element). It also provides a photoelectric conversion element (second photoelectric conversion element) having characteristics different from those of the above photoelectric conversion element and a method for manufacturing a photoelectric conversion layer of the photoelectric conversion element (second photoelectric conversion element). [Means for solving the problem]

[0006] The photoelectric conversion element of the present invention is characterized by comprising two electrodes and a photoelectric conversion layer formed between the two electrodes and including an n-type semiconductor material and a p-type semiconductor material containing carbon nanotubes having p-type semiconductor properties.

[0007] In the photoelectric conversion element of the present invention, the photoelectric conversion layer has a function of storing electrons and holes generated in the photoelectric conversion layer.

[0008] In the photoelectric conversion element of the present invention, when the storage function restricts the movement of electrons and holes generated in the photoelectric conversion layer from the photoelectric conversion layer to the outside, and the storage region is defined as the region in the photoelectric conversion layer where the presence of electrons and holes is permitted, the region of the photoelectric conversion layer containing the carbon nanotubes includes the storage region.

[0009] In the photoelectric conversion element of the present invention, the n-type semiconductor material contains titanium oxide (TiO2).

[0010] In the photoelectric conversion element of the present invention, the n-type semiconductor material contains a titanium compound containing chlorine.

[0011] In the photoelectric conversion element of the present invention, the photoelectric conversion layer has a bulk heterojunction structure.

[0012] In the photoelectric conversion element of the present invention, the photoelectric conversion layer has a laminated structure in which a p-type semiconductor layer containing the p-type semiconductor material and an n-type semiconductor layer containing the n-type semiconductor material are laminated.

[0013] In addition, the method for manufacturing a photoelectric conversion layer of the present invention is characterized by comprising a film-forming process for forming a coating of a mixed liquid containing carbon nanotubes and titanium tetrachloride, and a heating process for heating the coating of the mixed liquid formed in the film-forming process to form a photoelectric conversion layer with a bulk heterojunction structure.

[0014] In the method for producing a photoelectric conversion layer of the present invention, the heating step is characterized in that an n-type semiconductor material containing at least titanium oxide (TiO2) is produced by heating titanium tetrachloride.

[0015] In the method for producing a photoelectric conversion layer of the present invention, the photoelectric conversion layer that has undergone the heating step has a function of storing electrons and holes generated in the photoelectric conversion layer.

[0016] The method for manufacturing a photoelectric conversion layer of the present invention is characterized by comprising a film-forming step of forming a film of a mixed liquid containing an n-type semiconductor material and carbon nanotubes, and a heating step of heating the film of the mixed liquid formed in the film-forming step to form a photoelectric conversion layer with a bulk heterojunction structure.

[0017] In the method for producing a photoelectric conversion layer of the present invention, the photoelectric conversion layer that has undergone the heating step has a function of storing electrons and holes generated in the photoelectric conversion layer.

[0018] Furthermore, the method for manufacturing a photoelectric conversion layer of the present invention is a method for manufacturing a photoelectric conversion layer having a stacked structure in which a p-type semiconductor layer containing a p-type semiconductor material and an n-type semiconductor layer containing an n-type semiconductor material are stacked, and is characterized in that it comprises an n-type semiconductor layer formation step of forming the n-type semiconductor layer from the n-type semiconductor material, and a p-type semiconductor layer formation step of forming the p-type semiconductor layer from the p-type semiconductor material containing carbon nanotubes, and the stacked structure is formed by the n-type semiconductor layer formation step and the p-type semiconductor layer formation step.

[0019] In the method for producing a photoelectric conversion layer of the present invention, the photoelectric conversion layer has a function of storing electrons and holes generated in the photoelectric conversion layer. [Effects of the Invention]

[0020] The photoelectric conversion element of the present invention can provide the excellent effect of being able to store electricity. [Brief explanation of the drawings]

[0021] [Figure 1] 1A is a schematic cross-sectional view of a photoelectric conversion element according to an embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of a modified example of the photoelectric conversion element according to an embodiment of the present invention. [Figure 2] 1A is a schematic cross-sectional view of a photoelectric conversion layer of a photoelectric conversion element according to a first embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of a modified example of the electricity storage region of the photoelectric conversion layer of a photoelectric conversion element according to the first embodiment of the present invention. [Figure 3] 1A is a scanning electron microscope (SEM) image of the junction (boundary) between the n-type semiconductor layer and the p-type semiconductor layer of the photoelectric conversion element according to the first embodiment of the present invention, observed at a magnification of 100,000 times. 1B is a scanning electron microscope (SEM) image of the surface of the cathode of the photoelectric conversion element according to the first embodiment of the present invention, observed at a magnification of 100,000 times. [Figure 4]1A is an enlarged cross-sectional view of the photoelectric conversion layer of the photoelectric conversion element according to the first embodiment of the present invention, showing a state in which the convex portions are fitted into the concave portions so that the entire convex surfaces of the convex portions and the entire concave surfaces of the concave portions are in contact with each other. 1B is an enlarged cross-sectional view of the photoelectric conversion layer of the photoelectric conversion element according to the first embodiment of the present invention, showing a state in which the convex portions are fitted into the concave portions so that the part of the convex surface of the convex portion and the part of the concave surface of the concave portion are in contact with each other. [Figure 5] 1 is a flowchart showing a flow of manufacturing a photoelectric conversion element (layered structure) according to the first embodiment of the present invention. [Figure 6] Graph (A) shows the change in the current-voltage characteristic curve of the photoelectric conversion element of Example 1 of the present invention over time during light irradiation, and graph (B) shows the change in the current-voltage characteristic curve of the photoelectric conversion element of Example 1 of the present invention over time after light irradiation has ended. [Figure 7] 1 is a current-voltage characteristic curve of a photoelectric conversion element of a comparative example in Example 1 of the present invention. [Figure 8] 1A is a schematic cross-sectional view of a photoelectric conversion element (bulk heterojunction structure) according to a second embodiment of the present invention, and FIG. 1B is a schematic cross-sectional view of a modified example of the electricity storage region of the photoelectric conversion layer of the photoelectric conversion element according to the second embodiment of the present invention. [Figure 9] 10 is a flowchart showing the flow of manufacturing a photoelectric conversion element (bulk heterojunction structure) according to a second embodiment of the present invention. [Figure 10] 10 is a graph showing changes in current-voltage characteristic curves of photoelectric conversion elements (first to fifth photoelectric conversion elements) in Example 2 of the present invention when irradiated with light. [Figure 11] 10 is a graph showing changes in the current-voltage characteristic curve of the photoelectric conversion element (third photoelectric conversion element) in Example 2 of the present invention for each elapsed time after the end of light irradiation. DETAILED DESCRIPTION OF THE INVENTION

[0022] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Figures 1 to 9 show an example of an embodiment of the present invention, and in the drawings, parts with the same reference numerals represent the same items.

[0023] First Embodiment A photoelectric conversion element 1 according to a first embodiment of the present invention will be described with reference to FIGS. 1 and 2. As shown in FIG. 1(A), the photoelectric conversion element 1 according to this embodiment includes an anode (first electrode) 2, a cathode (second electrode) 3, and a photoelectric conversion layer 4. The photoelectric conversion element 1 has the cathode (second electrode) 3, the photoelectric conversion layer 4, and the anode (first electrode) 2 stacked in this order on one surface of a substrate 5. The direction in which the layers of the photoelectric conversion element 1 are stacked is defined as the stacking direction K. The stacking direction K is parallel to the thickness direction of the photoelectric conversion layer 4. The cathode may be referred to as the first electrode, and the anode may be referred to as the second electrode.

[0024] The photoelectric conversion element 1 in this embodiment is assumed to be a thin-film photoelectric conversion element having a film thickness of, for example, about 5 to 50 μm, but is not limited to this and may be a photoelectric conversion element having a film thickness in another range.

[0025] <Substrate> The substrate 5 is, for example, a plate-like member made of a transparent material having insulating properties. The substrate 5 may be made of a deformable material (for example, a flexible material), or may be made of a non-deformable material. Specific examples of materials for the substrate 5 include glass such as borosilicate glass, white plate glass, and quartz glass, and insulating materials such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), and polyethersulfone (PES). The thickness of the substrate 5 is not particularly limited, but may be, for example, in the range of 100 μm to 100 mm.

[0026] <Anode> The anode 2 may be made of at least a conductive material. Examples of materials for the anode 2 include conductive polymer compounds, metals such as platinum, gold, silver, copper, and aluminum, and mixtures of these, but are not limited thereto. Other materials may also be used. Examples of conductive polymer compounds include PEDOT-PSS, but are not limited thereto. Examples include polythiophene, polyaniline, polypyrrole, and derivatives thereof. PEDOT-PSS is an abbreviation for a composite of poly(3,4-ethylenedioxythiophene) (PEDOT), a substituted polythiophene containing ions with added polyanions, and polystyrene sulfonic acid (PSS).

[0027] The anode 2 may be a single layer or a laminate of multiple materials. The anode 2 may be made of a material that is conductive and optically transparent. In this case, the anode 2 may be made of any of the materials for the cathode 3 described below.

[0028] <Cathode> The cathode 3 is preferably made of a material that is conductive and optically transparent. Examples of optically transparent materials include transparent materials. Specific examples of materials that can be used to form the cathode 3 include conductive metal oxides such as tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide (ATO), aluminum-doped zinc oxide (AZO), and gallium-doped zinc oxide (GZO).

[0029] The cathode 3 may be a single layer or a laminate of multiple materials. The cathode 3 may be made of the material of the anode 2 described above.

[0030] <Photoelectric conversion layer> The photoelectric conversion layer 4 generates electrons and holes due to light incident from the outside. And the photoelectric conversion layer 4 is formed between the anode 2 and the cathode 3. As shown in Fig. 1(A), when light is incident on the photoelectric conversion element 1, excitons are generated in the photoelectric conversion layer 4, and electrons and holes are generated. The photoelectric conversion layer 4 as described above contains a p-type semiconductor material and an n-type semiconductor material.

[0031] <p-type semiconductor material> The p-type semiconductor material in the photoelectric conversion layer 4 includes a carbon allotrope having p-type semiconductor characteristics (hereinafter referred to as a p-type carbon allotrope). Examples of the p-type carbon allotrope include carbon nanotubes. The carbon nanotube may be a single-layer one (single-wall carbon nanotube) or a multi-layer one (multi-wall nanotube).

[0032] In addition to the above p-type carbon allotrope, the p-type semiconductor material in the photoelectric conversion layer 4 may contain other electron-donating compounds. Examples of other electron-donating compounds include p-type organic semiconductor materials such as polyvinylcarbazole and its derivatives, polysilane and its derivatives, polysiloxane derivatives having an aromatic amine in the side chain or main chain, polyaniline and its derivatives, polythiophene and its derivatives (for example, P3HT (poly(3-hexylthiophene))), polypyrrole and its derivatives, polyphenylene vinylene and its derivatives, polythienylene vinylene and its derivatives, polyfluorene and its derivatives. When an inevitable component (inevitable impurity) that is inadvertently contained in the process of providing a mixture with the above p-type carbon allotrope is contained, such a material is also included in the p-type semiconductor material.

[0033] Even when other electron-donating compounds are included, in order to improve the power storage function (power storage characteristics) described later, it is preferable that the p-type semiconductor material mainly consists of a p-type carbon allotrope (for example, carbon nanotubes). In order to better improve the power storage characteristics described later, when the entire p-type semiconductor material is 100% by mass, the amount of the p-type carbon allotrope in the p-type semiconductor material is preferably 85% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more.

[0034] <n-type semiconductor material> The n-type semiconductor material in the photoelectric conversion layer 4 is selected in consideration of the energy difference from the above p-type carbon allotrope. Examples of such an n-type semiconductor material include products generated when an aqueous titanium tetrachloride solution of a predetermined concentration is heated and fired at a predetermined temperature for a predetermined time. The product contains titanium dioxide (TiO2) as the main component, but may further contain a precursor of titanium dioxide (TiO2) and inevitable components (inevitable impurities). Examples of the precursor of titanium dioxide (TiO2) include TiO (2-X) (0 ≦ X < 2), and a titanium compound containing chlorine (TiCl Y O (Z-0.5Y) )(0 < Y < 4, 0 < Z ≦ 2, 0 < (Z - 0.5Y) ≦ 2) may be included. Incidentally, titanium dioxide (TiO2) may also be referred to as titanium dioxide. When the entire product is 100% by mass, the ratio of titanium dioxide (TiO2) is preferably 95% by mass or more, and the ratio of the precursor of titanium dioxide (TiO2) is preferably 5% by mass or less. Also, the product may have a ratio of titanium dioxide (TiO2) of 90% by mass or more and a ratio of the precursor of titanium dioxide (TiO2) of 10% by mass or less. Further, titanium dioxide (TiO2) as the above product preferably has a crystal structure. Also, the precursor of titanium dioxide (TiO2) preferably has a crystal structure.

[0035] The n-type semiconductor material in the photoelectric conversion layer 4 may be, for example, an oxide semiconductor (e.g., zinc oxide) and other electron-accepting compounds, or a mixture of at least two of the above. Even in this case, if the product or the mixture contains unavoidable components (unavoidable impurities) that are unintentionally contained in the process of providing the mixture, the material is also included in the n-type semiconductor material.

[0036] <Photoelectric conversion layer structure> 2(A) and 2(B), the photoelectric conversion layer 4 in this embodiment has a stacked structure in which a p-type semiconductor layer 41 containing a p-type semiconductor material and an n-type semiconductor layer 42 containing an n-type semiconductor material are stacked in stacking direction K (thickness direction of the photoelectric conversion layer 4). An example of this stacked structure is a planar heterojunction structure in which the p-type semiconductor layer 41 and the n-type semiconductor layer 42 are joined in a planar manner. In the photoelectric conversion layer 4 having this stacked structure, the boundary surface where the p-type semiconductor layer 41 and the n-type semiconductor layer 42 come into contact forms a pn junction interface 43. When light strikes the pn junction interface 43, charge separation occurs there, generating electrons and holes.

[0037] The photoelectric conversion layer 4 having a laminated structure has a storage function of storing (accumulating) electricity (electrons and holes generated by charge separation) generated in the photoelectric conversion layer 4. The storage function restricts the movement of electrons and holes generated by charge separation caused by light irradiation in the photoelectric conversion layer 4 from the photoelectric conversion layer 4 to the outside. If the region included in the photoelectric conversion layer 4 in which the presence of electrons and holes generated by charge separation is permitted is defined as a storage region 44, this storage function restricts the operation of the electrons and holes generated by charge separation within the storage region 44 so as not to move to the outside through the anode 2 and the cathode 3.

[0038] The electricity storage region 44 may be configured to have a p-type-side electricity storage region 44A included in the p-type semiconductor layer 41 and an n-type-side electricity storage region 44B included in the n-type semiconductor layer 42. Holes generated by charge separation are stored (retained) in the p-type-side electricity storage region 44A and are restricted from moving out of the p-type-side electricity storage region 44A. Electrons generated by charge separation are stored (retained) in the n-type-side electricity storage region 44B and are restricted from moving out of the n-type-side electricity storage region 44B.

[0039] The p-type side storage region 44A is considered to be composed of a region (hereinafter referred to as a p-type carbon allotrope region) where p-type carbon allotropes gather (aggregate) in the p-type semiconductor layer 41. When the p-type carbon allotropes contain carbon nanotubes, the carbon nanotubes are distributed in a fibrous, dispersed state in the region (hereinafter referred to as a carbon nanotube region) where the carbon nanotubes are contained.

[0040] 2(A), the p-type-side charge storage region 44A (p-type carbon allotrope region: the same applies hereinafter) may extend over the entire region of the p-type semiconductor layer 41. Alternatively, as shown in FIG. 2(B), the p-type-side charge storage region 44A may extend over a portion of the p-type semiconductor layer 41. In this case, a plurality of p-type-side charge storage regions 44A may be formed in the p-type semiconductor layer 41 (see FIG. 2(B)), or only one p-type-side charge storage region 44A may be formed.

[0041] The n-type side charge storage region 44B may extend over the entire region of the n-type semiconductor layer 42. Alternatively, as shown in Fig. 2(B), the n-type side charge storage region 44B may extend over a portion of the n-type semiconductor layer 42. In this case, a plurality of n-type side charge storage regions 44B may be formed in the n-type semiconductor layer 42 (see Fig. 2(B)), or only one n-type side charge storage region 44B may be formed.

[0042] That is, the electricity storage region 44 has one of four modes in which the p-type side electricity storage region 44A extends over the entire region or a portion of the region of the p-type semiconductor layer 41, and the n-type side electricity storage region 44B extends over the entire region or a portion of the region of the n-type semiconductor layer 42. Furthermore, the number of electricity storage regions 44 (p-type side electricity storage regions 44A and n-type side electricity storage regions 44B) in Fig. 2(B) is just one example, and fewer or more electricity storage regions 44 may be distributed.

[0043] It is assumed that the p-type side electricity storage region 44A and the n-type side electricity storage region 44B are continuous (see the left side region in FIG. 2(B)). In this case, as shown in FIG. 2(B), the electricity storage region 44 will be in a state in which it straddles the p-type semiconductor layer 41 and the n-type semiconductor layer 42. The p-type side electricity storage region 44A and the n-type side electricity storage region 44B may also be separate and not continuous (see the right side region in FIG. 2(B)).

[0044] As long as light irradiation continues, charge separation continues at pn junction interface 43, and therefore, due to the action of the charge storage function, charge continues to be stored in charge storage region 44 until the maximum charge storage capacity is reached. When light irradiation stops, charge separation stops at pn junction interface 43, and therefore, electrons and holes move from charge storage region 44 to the outside of photoelectric conversion layer 4 (charge storage region 44) over time, which is called discharging.

[0045] FIG. 3(A) shows a scanning electron microscope (SEM) image of the junction surface 42A of the n-type semiconductor layer 42 with the p-type semiconductor layer 41 (see FIGS. 2(A) and 2(B)) observed at a magnification of 100,000 times. FIG. 3(B) shows a scanning electron microscope (SEM) image of the surface of the cathode 3 of the cathode-attached substrate 5 before the n-type semiconductor layer 42 is provided, observed at a magnification of 100,000 times. Comparing the SEM images of FIGS. 3(A) and 3(B) reveals that multiple granular white spots are uniformly distributed on the junction surface 42A of the n-type semiconductor layer 42. The granular white spots are convex portions 45 that extend from the flat portion 42B of the surface of the n-type semiconductor layer 42 along the stacking direction K toward the p-type semiconductor layer 41, as shown in FIG. 4(A). That is, when the junction surface 42A of the n-type semiconductor layer 42 with the p-type semiconductor layer 41 is observed using a scanning electron microscope (SEM) from the side facing the junction surface 42A, a plurality of minute protrusions 45, which appear as white spots in the SEM image, are formed on the junction surface 42A of the n-type semiconductor layer 42 with the p-type semiconductor layer 41. In the region of the junction surface 42A of the n-type semiconductor layer 42 shown in the SEM image of FIG. 3(A), the protrusions 45 (the white dots constituting the white spots) are distributed approximately uniformly, but this is not limitative and the protrusions 45 may be distributed unevenly. Note that the plurality of protrusions 45 may include protrusions of various heights and widths, or may include protrusions of approximately the same height or width.

[0046] The p-type semiconductor layer 41 is bonded to (in contact with) the n-type semiconductor layer 42 at a bonding surface 41A of the p-type semiconductor layer 41. The p-type semiconductor layer 41 has recesses 46 that fit (engage) with the protrusions 45 of the n-type semiconductor layer 42, as shown in FIG. 4A. The p-type semiconductor layer 41 is engaged (in contact with) the n-type semiconductor layer 42 in a state where the recesses 46 fit into the protrusions 45 and cover the protrusions 45 (in an engaged state). In order to ensure a large contact area at the bonding surfaces 42A, 41A of the n-type semiconductor layer 42 and the p-type semiconductor layer 41, it is preferable to provide a large region where the recesses 46 fit into the protrusions 45 so that the entire convex surfaces of the protrusions 45 and the entire concave surfaces of the recesses 46 are in contact with each other, as shown in FIG. 4A. As shown in Figure 4(B), there may be an area where the recess 46 fits into the convex portion 45, with part of the convex surface of the convex portion 45 in contact with part of the concave surface of the concave portion 46, and the remaining part of the convex surface of the convex portion 45 and the remaining part of the concave surface of the concave portion 46 not in contact.

[0047] When light strikes pn junction interface 43 where junction surfaces 41A and 42A come into contact, electrons and holes are generated there. Compared to the contact area of ​​the virtual n-type semiconductor layer and the virtual p-type semiconductor layer where pn junction interface 43 is smooth, the contact area between junction surface 42A of n-type semiconductor layer 42 and junction surface 41A of p-type semiconductor layer 41 in this embodiment is larger. Therefore, the photoelectric conversion element 1 in this embodiment generates a larger number of electrons and holes when struck by light. As a result, the photoelectric conversion element 1 in this embodiment has better power storage characteristics than a photoelectric conversion element having a virtual n-type semiconductor layer and a virtual p-type semiconductor layer.

[0048] In the present invention, the n-type semiconductor layer 42 does not necessarily have to have the protrusion 45. Such a structure is also included in the scope of the present invention.

[0049] <Method of manufacturing photoelectric conversion element> Next, an example of a method for manufacturing the photoelectric conversion element 1 according to this embodiment will be described with reference to FIG.

[0050] <Cathode formation process> First, a cathode 3 is formed on a substrate 5 (cathode formation step: step S100). Specifically, for example, a film of the material described in the above <cathode> is provided (cathode-side film formation step). More specifically, for example, the material described in the above <cathode> is made into a paste form and printed on the substrate 5 by a printing device to provide the film. Note that the film of the material described in the above <cathode> may be provided on the substrate 5 by a sputtering method, a vapor deposition method, or other film formation methods.

[0051] Then, a heat treatment is performed on the substrate 5 on which the film is formed at a predetermined temperature (for example, 130 °C) for a predetermined time (for example, 10 minutes) (cathode-side heating step). Thereby, the cathode 3 is formed on the substrate 5.

[0052] <n-type semiconductor layer formation step> Subsequently, an n-type semiconductor layer 42 is formed on the cathode 3 (n-type semiconductor layer formation step). Specifically, first, a film of an aqueous titanium tetrachloride solution is formed on the cathode 3 (n-type semiconductor layer-side film formation step: step S101). More specifically, for example, at least the cathode 3 side of the substrate 5 is immersed in an aqueous titanium tetrachloride solution diluted to a predetermined concentration for a predetermined time (for example, 10 minutes to 30 minutes). Thereby, a film of the aqueous titanium tetrachloride solution is formed on the cathode 3 side of the substrate 5. Note that the film of the aqueous titanium tetrachloride solution may be provided on the substrate 5 by a spin coating method, or may be provided by printing or other film formation methods.

[0053] <n-type semiconductor layer-side heating step> The substrate 5 on which the film of the aqueous titanium tetrachloride solution is formed is dried in an atmosphere at a predetermined temperature for a predetermined time. That is, the film of the aqueous titanium tetrachloride solution is heated to evaporate (remove) moisture from the film of the aqueous titanium tetrachloride solution (first heating step: step S102). Note that the first heating step may be regarded as a moisture removal step in the sense of evaporating (removing) moisture from the film of the aqueous titanium tetrachloride solution.

[0054] Incidentally, the predetermined temperature in the first heating step (moisture removal step) may be any temperature (boiling point of water) that can evaporate (remove) moisture from the coating of the titanium tetrachloride aqueous solution, and is preferably, for example, in the range of 70°C to 100°C, and more preferably, in the range of 80°C to 90°C. Furthermore, the predetermined temperature in the first heating step (moisture removal step) may be constant for a predetermined time, or may be varied. Furthermore, the predetermined time in the first heating step (moisture removal step) may be any time that can evaporate (remove) moisture from the coating of the titanium tetrachloride aqueous solution. For example, when the predetermined temperature is 80°C, the predetermined time in this step is preferably about 60 minutes.

[0055] Furthermore, even if another heating step is performed between the first heating step and the second heating step (baking step), if the heating step evaporates (removes) moisture from the titanium tetrachloride aqueous solution coating, the heating step may also be considered to be included in the moisture removal step. In other words, the moisture removal step may be composed of multiple heating steps.

[0056] At heating temperatures below 200°C, titanium dioxide (TiO2) having a crystalline structure or a precursor of titanium dioxide (TiO2) having a crystalline structure is hardly produced from the titanium tetrachloride aqueous solution. In this state, the electron transport property is extremely low, so charge-separated electrons are hardly transported to the electrode. On the other hand, titanium dioxide (TiO2) having a crystalline structure has a high electron transport property. Therefore, in order to produce titanium dioxide (TiO2) having a crystalline structure from the titanium tetrachloride aqueous solution, at least the coating side of the substrate 5 that has undergone the first heating step (moisture removal step) is heated and baked at a predetermined temperature (e.g., 500°C) for a predetermined time (e.g., 45 minutes) (second heating step (baking step): step S103). In other words, the coating on the substrate 5 from which moisture has been removed is baked on the cathode 3 of the substrate 5. The second heating step may be considered a baking step, in that it means heating and baking at least the coating side of the substrate 5 that has undergone the moisture removal step for a predetermined time.

[0057] During the firing process, the coating from which moisture has been removed undergoes a chemical reaction such as oxidation, and the predetermined product already described is produced on the cathode 3 of the substrate 5. Then, during the firing process, the predetermined product forms the n-type semiconductor layer 42, and at the same time, a plurality of protrusions 45 are formed on the bonding surface 42A of the n-type semiconductor layer 42. To reiterate, the predetermined product contains titanium oxide (TiO2) as the main component, and may also contain a precursor of titanium oxide (TiO2) and unavoidable components (unavoidable impurities) depending on the firing conditions.

[0058] The predetermined temperature (baking temperature) in the second heating step (baking step) may be any temperature at which the moisture-removed coating can be baked onto the substrate 5, and is preferably, for example, 450°C to 550°C. If the predetermined temperature (baking temperature) exceeds 450°C, titanium oxide (TiO2) and titanium oxide (TiO2) precursors will definitely have an anatase crystal structure, while if the predetermined temperature (baking temperature) exceeds 550°C, the resistivity of the cathode 3 may increase. Incidentally, if the predetermined temperature (baking temperature) is 500°C to 550°C, it is presumed that titanium oxide (TiO2) and titanium oxide (TiO2) precursors will include not only those with an anatase crystal structure but also those with a rutile crystal structure.

[0059] Furthermore, even if another heating step is performed after the second heating step, if that heating step is for baking the coating, that heating step may also be considered to be included in the baking step. In other words, the baking step may be composed of multiple heating steps.

[0060] For ease of explanation, the first heating step (moisture removal step) and the second heating step (baking step) are collectively defined as the n-type semiconductor layer side heating step in this embodiment. The reason for dividing the n-type semiconductor layer side heating step into two steps is that if the first heating step (moisture removal step) is omitted and the second heating step (baking step) is performed, the n-type semiconductor layer 42 may be heated at a high temperature due to thermal expansion, which could cause cracks to form in the n-type semiconductor layer 42. By dividing the n-type semiconductor layer side heating step into two steps as in this embodiment, it is possible to provide an n-type semiconductor layer 42 that is free of cracks and has a plurality of fine protrusions 45 distributed on the bonding surface 42A. The titanium tetrachloride aqueous solution may further contain an electron-accepting compound, which is the n-type semiconductor material described above.

[0061] Furthermore, an additional step whose main purpose is different from that of providing the n-type semiconductor layer 42 may be performed before the first heating step (moisture removal step), between the first heating step (moisture removal step) and the second heating step (baking step), or after the second heating step (baking step). Although the additional step is not the main purpose, it may or may not ultimately achieve the purpose of the moisture removal step or the baking step. As in the method for manufacturing a photoelectric conversion element of this embodiment, as long as at least the first heating step and the second heating step are included, the n-type semiconductor layer 42 can be provided by performing moisture removal and baking.

[0062] In a conventional method for manufacturing a dye-sensitized photoelectric conversion element, an existing semiconductor layer (titanium oxide layer) is treated with an aqueous titanium tetrachloride solution in order to improve the bonding (necking) between semiconductor particles in the semiconductor layer (titanium oxide layer). On the other hand, in the method for manufacturing a photoelectric conversion element in this embodiment, an existing semiconductor layer is not treated with an aqueous titanium tetrachloride solution, but the aqueous titanium tetrachloride solution is used as a main material (precursor) to form the semiconductor layer (n-type semiconductor layer 42) itself. In other words, the two manufacturing methods are completely different, as one (the conventional method for manufacturing a dye-sensitized photoelectric conversion element) uses an aqueous titanium tetrachloride solution in the improvement treatment of the semiconductor layer, and the other (the method for manufacturing a photoelectric conversion element in this embodiment) uses an aqueous titanium tetrachloride solution in the treatment to form the semiconductor layer itself.

[0063] Furthermore, in the method for manufacturing a photoelectric conversion device according to the present embodiment, convex portions 45 are formed in the n-type semiconductor layer 42, thereby increasing the contact area between the n-type semiconductor layer 42 and the p-type semiconductor layer 41, and increasing the amounts of electrons and holes generated when light hits. That is, the method for manufacturing a photoelectric conversion device according to the present embodiment is premised on this laminated structure. On the other hand, a conventional dye-sensitized photoelectric conversion device generates electricity by causing electron movement between a semiconductor layer and an electrolytic solution when light hits, which is completely different from the power generation principle in this laminated structure.

[0064] Note that the n-type semiconductor layer 42 may be provided by other processing methods as long as the above product having a plurality of convex portions 45 can be formed on the cathode 3 using an aqueous titanium tetrachloride solution diluted to a predetermined concentration.

[0065] Incidentally, although it is repetitive, the n-type semiconductor layer 42 does not necessarily need to have a plurality of convex portions 45. Also, in the film formation step on the n-type semiconductor layer side, instead of an aqueous titanium tetrachloride solution, a film may be formed using an aqueous solution containing an n-type semiconductor material as described in <n-type semiconductor material>, and the treatment described in <heating step on the n-type semiconductor layer side> may be performed on the film. Examples of the n-type semiconductor material include an oxide semiconductor (e.g., zinc oxide) and other electron-accepting compounds.

[0066] <p-type semiconductor layer formation step> Next, a p-type semiconductor layer 41 is formed on the n-type semiconductor layer 42 (p-type semiconductor layer forming step). Specifically, first, a film of a p-type semiconductor material is formed on the n-type semiconductor layer 42 (p-type semiconductor layer side film forming step: step S104). Here, as the p-type semiconductor material, carbon nanotubes which are p-type carbon allotropes (having p-type semiconductor characteristics) are used. For example, a carbon nanotube dispersion liquid in which carbon nanotubes are dispersed in a dispersion medium is applied onto the n-type semiconductor layer 42, and a film of the carbon nanotube dispersion liquid is provided on the n-type semiconductor layer 42 by the spin coating method. In the film, the carbon nanotubes are in a sufficiently dispersed state. The dispersion medium is preferably, for example, pure water or an organic solvent such as dimethyl sulfoxide (DMSO) or ethylene glycol, but other ones may also be used. Note that the film of the carbon nanotube dispersion liquid may be provided by dipping at least the n-type semiconductor layer 42 side of the substrate 5 in the carbon nanotube dispersion liquid, similar to the case of the n-type semiconductor layer side film forming step, or may be provided on the n-type semiconductor layer 42 by printing or other film forming methods.

[0067] Then, a heat treatment is performed on the substrate 5 on which the film of the carbon nanotube dispersion liquid is formed at a predetermined temperature (for example, 100°C) for a predetermined time (for example, 10 minutes) (p-type semiconductor layer side heating step: step S105). Thereby, the p-type semiconductor layer 41 laminated on the n-type semiconductor layer 42 is completed. In addition to the carbon nanotubes as the main component, the p-type semiconductor layer 41 may contain inevitable components (inevitable impurities) that are unavoidably included unintentionally in the process of providing the p-type semiconductor layer 41 by the <p-type semiconductor layer forming step>. When the p-type semiconductor layer 41 is formed using the carbon nanotube dispersion liquid, the photoelectric conversion layer 4 is provided with a power storage function. Note that the p-type semiconductor layer side heating step may be divided into two steps (moisture removal step, firing step) similar to the case of the above n-type semiconductor layer side heating step. In this case, the predetermined temperature (firing temperature) in the step corresponding to the moisture removal step may be the same as that in the n-type semiconductor layer side heating step. Also, when the p-type semiconductor layer 41 is provided using other single materials or mixed materials described in the <p-type semiconductor material>, the above description is applicable.

[0068] Finally, the anode 2 is formed on the p-type semiconductor layer 41 (anode formation step: step S106). Specifically, first, a coating made of the material described in the above <anode> is provided on the p-type semiconductor layer 41 (anode-side film formation step). More specifically, for example, the material described in the above <anode> is made into a paste and printed on the substrate 5 using a printing device to provide the coating. Note that the material described in the above <anode> may be used to provide a coating on the p-type semiconductor layer 41 by sputtering, vapor deposition, or other film formation methods.

[0069] Then, the substrate 5 on which the coating has been formed is subjected to a heat treatment at a predetermined temperature (for example, 130°C) for a predetermined time (for example, 10 minutes) (anode-side heating step). As a result, the anode 2 is formed on the p-type semiconductor layer 41. Through the above steps, the photoelectric conversion element 1 is completed. This photoelectric conversion element 1 is a thin film. Note that the order in the above method for manufacturing the photoelectric conversion element 1 may be reversed, and the anode 2 (anode formation step), p-type semiconductor layer 41 (p-type semiconductor layer formation step), n-type semiconductor layer 42 (n-type semiconductor layer formation step), and cathode 3 (cathode formation step) may be formed in this order.

[0070] <Modification> 1(B), a modified example of the photoelectric conversion element 1 in this embodiment will be described. As shown in FIG. 1(B), the photoelectric conversion element 1 in this modified example is configured by adding an electron transport layer 6 and a hole transport layer 7 to the photoelectric conversion element 1 in this embodiment.

[0071] The electron transport layer 6 has a function of efficiently transporting electrons generated in the photoelectric conversion layer 4 to the cathode 3. The electron transport layer 6 is provided between the cathode 3 and the photoelectric conversion layer 4 (n-type semiconductor layer 42). The material constituting the electron transport layer 6 is not particularly limited, but examples thereof include titanium (IV) isopropoxide, and n-type semiconductors having electron transport properties, such as fullerene and derivatives thereof.

[0072] The hole transport layer 7 efficiently transports holes generated in the photoelectric conversion layer 4 to the anode 2. The hole transport layer 7 is preferably formed of a material with high hole mobility. The hole transport layer 7 is provided between the anode 2 and the photoelectric conversion layer 4 (p-type semiconductor layer 41). The material constituting the hole transport layer is not particularly limited, and examples thereof include low molecular weight compounds such as aromatic cyclic acid anhydrides typified by NTCDA, and high molecular weight compounds such as known conductive polymers typified by PEDOT-PSS and poly(3,4-ethylenedioxy)thiophene.

[0073] When the electron transport layer 6 and the hole transport layer 7 are provided in the photoelectric conversion element 1, the electron transport layer 6 and the hole transport layer 7 may be provided by stacking corresponding materials at corresponding positions using a film formation method including sputtering, vapor deposition, spin coating, printing, etc.

[0074] The photoelectric conversion element 1 and the method for manufacturing the photoelectric conversion element 1 in which only one of the electron transport layer 6 and the hole transport layer 7 is provided are also included in the scope of the present invention. [Example]

[0075] Next, the inventors of the present application fabricated a photoelectric conversion element as Example 1 of the present invention and two types of photoelectric conversion elements as comparative examples, and conducted an experiment to evaluate the current-voltage characteristics. The photoelectric conversion element as Example 1 was constructed by depositing a photoelectric conversion layer 4 and an anode 2 in this order on a glass substrate (manufactured by Asahi Glass Co., Ltd.) with an FTO (fluorine-doped tin oxide transparent conductive film). The FTO portion of the glass substrate with FTO constitutes the cathode, and the glass substrate portion constitutes the substrate.

[0076] In preparing the photoelectric conversion element of Example 1, an immersion solution was prepared by adjusting a titanium tetrachloride aqueous solution (manufactured by Osaka Titanium Technologies) to 40 mM (molar) using pure water as the solvent, and the FTO-attached glass substrate was immersed in the immersion solution for 10 minutes. The FTO-attached glass substrate was then placed in an 80°C atmosphere for 60 minutes to evaporate (remove) the water content of the immersion solution. The FTO-attached glass substrate was then baked at 500°C for 45 minutes. This resulted in an n-type semiconductor layer 42 with multiple fine protrusions on the bonding surface. The SEM image in Figure 3(A) was taken of the surface of the n-type semiconductor layer 42 of the photoelectric conversion element of this example.

[0077] Next, 1 wt % of carbon nanotubes (manufactured by Thomas Swan & Co. Ltd.) were mixed and dispersed in pure water as a dispersion medium to prepare a carbon nanotube dispersion. In the carbon nanotube dispersion, the carbon nanotubes are sufficiently dispersed. The carbon nanotube dispersion is then applied as a coating liquid onto n-type semiconductor layer 42, and a coating of the carbon nanotube dispersion is formed on n-type semiconductor layer 42 by spin coating. The coating is then subjected to a heat treatment in which it is heated at 100°C for 10 minutes. This completes p-type semiconductor layer 41, and photoelectric conversion layer 4 is completed.

[0078] Next, the anode 2 is screen-printed on the p-type semiconductor layer 41 using a conductive paste (Clevios (registered trademark) S V3 Stab), and then a heat treatment is performed by heating at 130° C. for 10 minutes. This completes the photoelectric conversion element of Example 1.

[0079] The two types of photoelectric conversion elements used as comparative examples differ in the configuration of the p-type semiconductor layer 41 from that of the photoelectric conversion element of this Example 1, but the other layers, such as the n-type semiconductor layer 42, were prepared in the same manner as the photoelectric conversion element of this Example 1.

[0080] The p-type semiconductor layer 41 in the first comparative photoelectric conversion element (hereinafter referred to as the first comparative photoelectric conversion element) was prepared as follows: A copper iodide (I) solution was prepared by adding 0.5 (wt%) copper iodide (I) (CuI) to N,N-dimethylformamide (DMF), and the copper iodide (I) solution was added dropwise to the n-type semiconductor layer 42, followed by spin coating at 2000 rpm for 30 seconds. The glass substrate with FTO was then baked at 100°C for 10 minutes. This completed the p-type semiconductor layer 41 of the first comparative photoelectric conversion element.

[0081] The p-type semiconductor layer 41 in the second comparative photoelectric conversion element (second comparative photoelectric conversion element) is prepared as follows: A P3HT solution is prepared by adding 0.5 (wt%) P3HT (poly(3-hexylthiophene)) to chlorobenzene as a solvent, and the P3HT solution is dropped onto the n-type semiconductor layer 42, followed by spin coating at 2000 rpm for 30 seconds. The glass substrate with FTO is then baked at 100°C for 10 minutes. This completes the p-type semiconductor layer 41 of the second comparative photoelectric conversion element.

[0082] A solar simulator (XES-4051 manufactured by Minae Electric Mfg. Co., Ltd.) was used to measure the current-voltage characteristics of the photoelectric conversion element of this Example 1, the first comparative example photoelectric conversion element, and the second comparative example photoelectric conversion element fabricated as described above. The resulting current-voltage characteristics of the photoelectric conversion element of this Example 1, the first comparative example photoelectric conversion element, and the second comparative example photoelectric conversion element are shown in Figures 6(A), (B), and 7.

[0083] As shown in FIG. 6(A), when comparing the current-voltage characteristics of the photoelectric conversion element of Example 1 at the time when light irradiation of the photoelectric conversion element of Example 1 begins with the current-voltage characteristics of the photoelectric conversion element of Example 1 three minutes after the start of light irradiation, the output current of the latter increases more than that of the former as the voltage increases. On the other hand, as shown in FIG. 7, the current-voltage characteristics of the first comparative example photoelectric conversion element and the second comparative example photoelectric conversion element did not change with the elapsed time after the start of light irradiation. From this, it can be said that it was confirmed that the photoelectric conversion element of Example 1 has a power storage function, and electrons and holes generated by charge separation due to light irradiation are stored in the photoelectric conversion layer (power storage region), but that the first comparative example photoelectric conversion element and the second comparative example photoelectric conversion element do not have a power storage function.

[0084] Furthermore, as shown in Figure 6(A), the current-voltage characteristics of the photoelectric conversion element of this Example 1 5 or 10 minutes after the start of light irradiation are not significantly different from those 3 minutes after the start of light irradiation, so it can be inferred that the maximum storage capacity that can be stored in the photoelectric conversion layer (storage region) of the photoelectric conversion element of this Example is reached 3 minutes after the start of light irradiation.

[0085] Furthermore, as shown in FIG. 6(B), when the photoelectric conversion element of Example 1 is irradiated with light for 10 minutes and then the light irradiation is stopped, the current density per voltage in the current-voltage characteristics of the photoelectric conversion element of Example 1 gradually decreases over time. This is presumably because the electrons and holes stored in the photoelectric conversion layer (storage region) of the photoelectric conversion element of Example 1 are released (discharged) from the photoelectric conversion layer (storage region). The graph shown in FIG. 6(B) also supports the existence of a storage region in the photoelectric conversion element of Example 1. As a result, it can be said that the current-voltage characteristics of the photoelectric conversion element of Example 1 represent the storage characteristics.

[0086] The major difference between the photoelectric conversion element of this Example 1 and the photoelectric conversion element of the first and second comparative examples is the material of the p-type semiconductor layer 41. When the n-type semiconductor layer 42 was produced using an aqueous solution of titanium tetrachloride, the above experiment confirmed that the photoelectric conversion element had a power storage function when the material of the p-type semiconductor layer 41 of the photoelectric conversion element was carbon nanotubes, which are carbon allotropes having p-type semiconductor properties.

[0087] Second Embodiment A photoelectric conversion element 1 according to a second embodiment of the present invention will be described with reference to FIG. 8 . As shown in FIG. 8 , the photoelectric conversion element 1 according to this embodiment has a photoelectric conversion layer 4 with a bulk heterojunction structure (hereinafter, the same applies). More specifically, the bulk heterojunction structure is formed of a mixture in which n-type and p-type semiconductor materials are randomly mixed. The bulk heterojunction structure according to this embodiment includes a plurality of p-type semiconductor regions 47 made of p-type semiconductor materials and a plurality of n-type semiconductor regions 48 made of n-type semiconductor materials. The p-type semiconductor regions 47 and the n-type semiconductor regions 48 are phase-separated and bonded not only in the thickness direction of the photoelectric conversion layer 4 but also in a direction perpendicular to the thickness direction of the photoelectric conversion layer 4 and / or in a direction inclined toward the thickness direction of the photoelectric conversion layer 4. As a result, the bulk heterojunction structure has many p-n junction interfaces throughout the entire layer compared to the stacked structure according to the first embodiment. Therefore, many of the excitons generated by light absorption can reach the p-n junction interfaces, thereby improving the efficiency of charge separation.

[0088] In this embodiment, the n-type semiconductor material includes one or more types of materials similar to those in the first embodiment. In this embodiment, the p-type semiconductor material includes a carbon allotrope having p-type semiconductor properties, as in the first embodiment. The carbon allotrope is preferably a carbon nanotube. The carbon nanotube may be a single-walled one (single-walled carbon nanotube) or a multi-walled one (multi-walled nanotube). Furthermore, the p-type semiconductor material may include one or more types of p-type semiconductor materials different from the carbon allotrope having p-type semiconductor properties.

[0089] The photoelectric conversion layer 4 of the bulk heterojunction structure in this embodiment also has a power storage function. As in the photoelectric conversion element of the first embodiment, the power storage function in the photoelectric conversion element of this embodiment restricts the movement of electrons and holes generated by charge separation in the photoelectric conversion layer 4 due to light irradiation from the photoelectric conversion layer 4 to the outside. The electrons and holes generated by charge separation are restricted in their operation within the power storage region 44 so as not to move to the outside through the anode 2 and the cathode 3. The power storage region 44 is defined in the same way as in the first embodiment. The photoelectric conversion layer 4 (power storage region 44) of this embodiment also has a p-type power storage region 44A and an n-type power storage region 44B, as shown in FIGS. 8(A) and 8(B). 8(A), the photoelectric conversion layer 4 (electricity storage region 44) of this embodiment may take any one of four forms, in which the p-type-side electricity storage region 44A extends over the entire region or a portion of the p-type semiconductor region 47, and the n-type-side electricity storage region 44B extends over the entire region or a portion of the n-type semiconductor region 48. Other descriptions of the p-type-side electricity storage region 44A and the n-type-side electricity storage region 44B in this embodiment can be applied by appropriately replacing "p-type semiconductor layer 41" with "p-type semiconductor region 47" and "n-type semiconductor layer 42" with "n-type semiconductor region 48" in the description of <Photoelectric Conversion Layer Structure> of the first embodiment. Furthermore, the number of electricity storage regions 44 (p-type-side electricity storage region 44A and n-type-side electricity storage region 44B) in FIG. 8(B) is merely an example, and fewer or more electricity storage regions 44 may be distributed. Furthermore, as in the first embodiment, in the photoelectric conversion element 1 of this embodiment, both or one of the electron transport layer 6 and the hole transport layer 7 may be provided adjacent to the photoelectric conversion layer 4 (see Figure 1(B)).

[0090] <Method of manufacturing photoelectric conversion element according to second embodiment> Next, an example of a method for manufacturing the photoelectric conversion element 1 in this embodiment will be described with reference to Fig. 9. As shown in Fig. 9, in the method for manufacturing the photoelectric conversion element 1 in this embodiment, the cathode forming step (step S100), heating step, and anode forming step (step S106) are the same as those in the first embodiment, and therefore the same descriptions can be applied as much as possible.

[0091] When the photoelectric conversion layer 4 has a bulk heterojunction structure, in the photoelectric conversion layer formation step (step S107) for providing the photoelectric conversion layer 4, a coating of a mixed solution containing titanium tetrachloride, which is an n-type semiconductor material or a precursor of the n-type semiconductor material, and a p-type carbon allotrope (e.g., carbon nanotubes), may be formed on an adjacent layer (e.g., the cathode 3 or the anode 2) (film formation step). The mixed solution is prepared by adding titanium tetrachloride (a titanium tetrachloride aqueous solution), which is an n-type semiconductor material or a precursor of the n-type semiconductor material, and a p-type carbon allotrope (e.g., carbon nanotubes) to a solvent or dispersion medium and mixing them. Note that the carbon nanotubes are preferably sufficiently dispersed in the mixed solution. Examples of the solvent or dispersion medium include pure water, a liquid mainly composed of pure water, and organic solvents such as dimethyl sulfoxide (DMSO) and ethylene glycol. The mixed solution may further contain different types of p-type semiconductor materials, n-type semiconductor materials, or precursors of n-type semiconductor materials. That is, the above-mentioned mixed solution may contain one or more types of p-type semiconductor materials and n-type semiconductor materials.

[0092] The next heating step may be divided into two steps, as shown in FIG. 9, similar to the first heating step (step S102: moisture removal step) and the second heating step (step S103: baking step) in the first embodiment, or the moisture removal step and baking step may be performed in one step. However, the predetermined temperature (baking temperature) in the baking step is preferably determined from the viewpoint of preventing an increase in the resistivity of the crystal structure and other parts of the n-type semiconductor material and the p-type semiconductor material. Furthermore, after the above heating step, the titanium tetrachloride in the mixed solution becomes a product containing titanium oxide (TiO) as a main component. The product may further contain a titanium oxide (TiO) precursor. Titanium oxide (TiO) and titanium oxide (TiO) precursors include those having a crystalline structure. Therefore, the n-type semiconductor material of the n-type semiconductor region 48 of the photoelectric conversion layer 4 in this embodiment contains titanium oxide (TiO). In either case, the substrate with the cathode 3 on which the coating is formed is subjected to a heat treatment in the heating step, and a photoelectric conversion layer 4 having a bulk heterojunction structure is formed on the adjacent layer (e.g., the cathode 3 or the anode 2). The photoelectric conversion layer 4 may contain inevitable components (unavoidable impurities) that are unintentionally contained in the mixed solution during the heating step. The order of the above-described method for manufacturing the photoelectric conversion element 1 may be reversed, and the anode 2 (anode formation step), the photoelectric conversion layer 4 (photoelectric conversion layer formation step), and the cathode 3 (cathode formation step) may be formed in this order. Furthermore, even when both or either of the electron transport layer 6 and the hole transport layer 7 are provided adjacent to the photoelectric conversion layer 4 in the photoelectric conversion element 1 of this embodiment, they are also formed using the same manufacturing method as in the first embodiment. [Example]

[0093] Next, the inventors of the present application fabricated five types of photoelectric conversion elements as Example 2 of the second embodiment of the present invention, and conducted an experiment to evaluate their current-voltage characteristics. Each photoelectric conversion element of Example 2 was fabricated by sequentially depositing a photoelectric conversion layer 4 and an anode 2 on a glass substrate (manufactured by Asahi Glass Co., Ltd.) with an FTO (fluorine-doped tin oxide) transparent conductive film, using the photoelectric conversion layer formation step and anode formation step described in <Method for manufacturing a photoelectric conversion element in the second embodiment>. The FTO portion of the glass substrate with FTO constitutes the cathode 3, and the glass substrate portion constitutes the substrate.

[0094] For ease of explanation, the photoelectric conversion elements in Example 2 will be referred to as the first to fifth photoelectric conversion elements below. The mixed solution used to form the photoelectric conversion layers of the first to fifth photoelectric conversion elements contains a titanium tetrachloride aqueous solution (manufactured by Osaka Titanium Technologies; the same applies hereinafter) and carbon nanotubes (manufactured by Thomas Swan & Co. Ltd.; the same applies hereinafter). The carbon nanotubes are sufficiently dispersed in the mixed solution. In the first photoelectric conversion element, the titanium tetrachloride aqueous solution is contained at 1.04 wt% based on the solvent primarily composed of pure water, and the carbon nanotubes are contained at 1 wt% based on the mixture of the solvent and the titanium tetrachloride aqueous solution. In the second photoelectric conversion element, the titanium tetrachloride aqueous solution is contained at 2.07 wt% based on the solvent primarily composed of pure water, and the carbon nanotubes are contained at 1 wt% based on the mixture of the solvent and the titanium tetrachloride aqueous solution. In the third photoelectric conversion element, the titanium tetrachloride aqueous solution is contained at 4.125 (wt%) based on the solvent mainly composed of pure water, and the carbon nanotubes are contained at 1 (wt%) based on the mixture of the solvent and the titanium tetrachloride aqueous solution. In the fourth photoelectric conversion element, the titanium tetrachloride aqueous solution is contained at 8.25 (wt%) based on the solvent mainly composed of pure water, and the carbon nanotubes are contained at 1 (wt%) based on the mixture of the solvent and the titanium tetrachloride aqueous solution. In the fifth photoelectric conversion element, the titanium tetrachloride aqueous solution is contained at 16.5 (wt%) based on the solvent mainly composed of pure water, and the carbon nanotubes are contained at 1 (wt%) based on the mixture of the solvent and the titanium tetrachloride aqueous solution.

[0095] Each of the above mixed solutions is applied as a coating solution onto each FTO-attached glass substrate, and a coating of the mixed solution is formed on each FTO-attached glass substrate by spin coating. The coating is then subjected to a heat treatment at 450°C for 30 minutes. This completes the photoelectric conversion layer 4 of each of the first to fifth photoelectric conversion elements.

[0096] Next, an anode 2 is screen-printed on each of the photoelectric conversion layers of the first to fifth photoelectric conversion elements using a conductive paste (Clevios (registered trademark) S V3 Stab), and then a heat treatment is performed by heating at 130° C. for 10 minutes. This completes the first to fifth photoelectric conversion elements of Example 2.

[0097] The current-voltage characteristics of the first to fifth photoelectric conversion elements of this example fabricated as described above were measured using a solar simulator (XES-4051 manufactured by Minae Electric Mfg. Co., Ltd.) The resulting current-voltage characteristics of the first to fifth photoelectric conversion elements of this example are shown in FIG.

[0098] The current-voltage characteristics of the first to fifth photoelectric conversion elements shown in FIG. 10 show that they have predetermined power generation characteristics. That is, when the voltage of each of the first to fifth photoelectric conversion elements is 0 (V), the short-circuit current of each of the first to fifth photoelectric conversion elements is 0 (mA / cm 2 ), and when the voltage increases, the current density of each of the first to fifth photoelectric conversion elements decreases until the current density reaches 0 (mA / cm 2 ), the open circuit voltage of each of the first to fifth photoelectric conversion elements has a value exceeding 0 (V). In other words, the first to fifth photoelectric conversion elements have predetermined power generation characteristics.

[0099] Focusing on the current density in the current-voltage characteristics of the first to fifth photoelectric conversion elements, the current density increases in the order of the first photoelectric conversion element, the second photoelectric conversion element, and the third photoelectric conversion element, and decreases in the order of the third photoelectric conversion element, the fourth photoelectric conversion element, and the fifth photoelectric conversion element. It can be estimated that the short-circuit current becomes maximum when the concentration of the titanium tetrachloride aqueous solution in the mixed solution relative to pure water as the solvent is anywhere in the range of 1.7 to 6.75 (wt%).

[0100] Furthermore, when focusing on the open-circuit voltage, the open-circuit voltage increases in the order of the first photoelectric conversion element, the second photoelectric conversion element, the third photoelectric conversion element, the fourth photoelectric conversion element, and the fifth photoelectric conversion element. Therefore, it can be inferred that the open-circuit voltage can be increased by increasing the concentration of the titanium tetrachloride aqueous solution in the mixed solution.

[0101] Furthermore, the third photoelectric conversion element was irradiated with light for 10 minutes, and the current-voltage characteristics of the third photoelectric conversion element at each elapsed time after the light irradiation was stopped are shown in Figure 11. As is clear from Figure 11, the current density of the third photoelectric conversion element changes with time. This indicates the characteristic of electrons and holes stored in the photoelectric conversion layer (storage region) being released (discharged) from the photoelectric conversion layer (storage region). In other words, it can be said that the third photoelectric conversion element has a storage characteristic (storage function).

[0102] <Comparison of Results Between Example 1 of the First Embodiment and Example 2 of the Second Embodiment> The results of Example 1 of the first embodiment and Example 2 of the second embodiment are compared. When comparing the two in terms of power generation characteristics, the photoelectric conversion element of Example 2 of the second embodiment has different power generation characteristics depending on the concentration of the titanium tetrachloride aqueous solution. On the other hand, the photoelectric conversion element of Example 1 of the first embodiment has a short-circuit current and open-circuit voltage of approximately 0, and therefore is presumed to have lower power generation characteristics than the photoelectric conversion element of Example 2 of the second embodiment. Furthermore, when comparing the two in terms of power storage characteristics, the photoelectric conversion element of Example 1 of the first embodiment clearly has higher power storage characteristics (power storage function) than the photoelectric conversion element of Example 2 of the second embodiment. In other words, the characteristics of the photoelectric conversion elements of Example 1 of the first embodiment and Example 2 of the second embodiment are different.

[0103] Considering the above results, it is presumed that the maximum power storage capacity of the power storage region 44 depends on the volume of a single p-type-side power storage region 44A. In other words, even if the total volume of the p-type-side power storage regions 44A is the same, it is presumed that a power storage region 44 in which a small number of single p-type-side power storage regions 44A each with a large volume are distributed will have a larger maximum power storage capacity than a power storage region 44 in which a large number of single p-type-side power storage regions 44A each with a small volume are distributed.

[0104] The single p-type-side power storage region 44A in Example 1 of the first embodiment corresponds to the entire region of the p-type semiconductor layer 41, while the p-type-side power storage region 44A in Example 2 of the second embodiment is divided into multiple p-type semiconductor regions 47. For this reason, if the total volume of the p-type-side power storage regions 44A is the same, it is presumed that the photoelectric conversion element in Example 1 of the first embodiment will have a larger maximum power storage capacity than the photoelectric conversion element in Example 2 of the second embodiment. This point is clear from the experiments in Examples 1 and 2.

[0105] As the maximum storage capacity increases, the number of electrons and holes retained in the photoelectric conversion layer 4 increases. As a result, the current that can be extracted to the outside decreases, and the power generation characteristics deteriorate. On the other hand, when the maximum storage capacity is small, once the number of electrons and holes whose movement is restricted reaches saturation, the remaining electrons and holes reach the cathode and anode, respectively, and a current flows. From this perspective, the photoelectric conversion element of Example 2 of the second embodiment, which has a small maximum storage capacity, can extract more current to the outside than the photoelectric conversion element of Example 1 of the first embodiment, which has a large maximum storage capacity, and therefore has better power generation characteristics.

[0106] As described above, when comparing the photoelectric conversion element (stacked structure) of the first embodiment with the photoelectric conversion element (bulk heterojunction structure) of the second embodiment, it was confirmed that the former has better power storage characteristics than the latter, and the latter has better power generation characteristics than the former.

[0107] The photoelectric conversion element of the present invention and the method for manufacturing the photoelectric conversion element are not limited to the above-described embodiments, and it goes without saying that various modifications can be made within the scope of the present invention without departing from the gist of the present invention. [Explanation of symbols]

[0108] 1 Photoelectric conversion element 2 Anode 3 cathode 4 Photoelectric conversion layer 5. Substrate 6 Electron transport layer 7. Hole transport layer 41 n-type semiconductor layer 41A n-type semiconductor layer junction 42 p-type semiconductor layer 42A p-type semiconductor layer junction 43 pn junction interface 44 Energy storage area 44A p-type side storage area 44B n-type side storage area 45 Convex part 46 Recess 47 p-type semiconductor region 48 n-type semiconductor region

Claims

1. Two electrodes; a photoelectric conversion layer formed between the two electrodes and including an n-type semiconductor material and a p-type semiconductor material including carbon nanotubes having p-type semiconductor properties; characterized in that it comprises Photoelectric conversion element.

2. The photoelectric conversion layer has a function of storing electrons and holes generated in the photoelectric conversion layer. The photoelectric conversion element according to claim 1 .

3. When the electricity storage function restricts the movement of electrons and holes generated in the photoelectric conversion layer from the photoelectric conversion layer to the outside, and the region in the photoelectric conversion layer where the presence of electrons and holes is permitted is defined as an electricity storage region, the region of the photoelectric conversion layer containing the carbon nanotubes includes the electricity storage region. The photoelectric conversion element according to claim 2 .

4. The n-type semiconductor material includes titanium oxide (TiO ２ ) The photoelectric conversion element according to claim 1 .

5. The n-type semiconductor material includes a titanium compound containing chlorine. The photoelectric conversion element according to claim 4 .

6. The photoelectric conversion layer has a bulk heterojunction structure. The photoelectric conversion element according to any one of claims 1 to 5.

7. the photoelectric conversion layer has a laminated structure in which a p-type semiconductor layer containing the p-type semiconductor material and an n-type semiconductor layer containing the n-type semiconductor material are laminated. The photoelectric conversion element according to any one of claims 1 to 5.

8. a film-forming step of forming a coating of a mixed solution containing carbon nanotubes having p-type semiconductor properties and titanium tetrachloride; a heating step of heating the coating of the mixed solution formed in the film forming step to form a photoelectric conversion layer with a bulk heterojunction structure; characterized in that it comprises A method for manufacturing a photoelectric conversion layer.

9. In the heating step, at least titanium oxide (TiO ２ ) ) n-type semiconductor material is produced. The method for producing a photoelectric conversion layer according to claim 8 .

10. The photoelectric conversion layer that has undergone the heating step has a storage function of storing electrons and holes generated in the photoelectric conversion layer. The method for producing a photoelectric conversion layer according to claim 8 or 9.

11. a film-forming step of forming a film of a mixed solution containing an n-type semiconductor material and carbon nanotubes; a heating step of heating the coating of the mixed solution formed in the film forming step to form a photoelectric conversion layer with a bulk heterojunction structure; characterized in that it comprises A method for manufacturing a photoelectric conversion layer.

12. The photoelectric conversion layer that has undergone the heating step has a storage function of storing electrons and holes generated in the photoelectric conversion layer. The method for producing a photoelectric conversion layer according to claim 11 .

13. A method for producing a photoelectric conversion layer having a stacked structure in which a p-type semiconductor layer containing a p-type semiconductor material and an n-type semiconductor layer containing an n-type semiconductor material are stacked, comprising: an n-type semiconductor layer forming step of forming the n-type semiconductor layer using the n-type semiconductor material; a p-type semiconductor layer forming step of forming the p-type semiconductor layer using the p-type semiconductor material containing carbon nanotubes; Equipped with the stacked structure is formed by the n-type semiconductor layer forming step and the p-type semiconductor layer forming step. A method for manufacturing a photoelectric conversion layer.

14. The photoelectric conversion layer has a function of storing electrons and holes generated in the photoelectric conversion layer. The method for producing a photoelectric conversion layer according to claim 13 .

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