JP2014241372A - Photoelectric conversion element and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion element having excellent light absorptivity and charge transportability and capable of exhibiting high photoelectric conversion efficiency without deteriorating current collection efficiency even when a film thickness of a photoelectric conversion layer is thickened so that enough light can be absorbed.SOLUTION: A photoelectric conversion element 10 comprises a photoelectric conversion layer 4 including an organic semiconductor between a positive electrode 2 and a negative electrode 5. An electrode of the positive electrode 2 or the negative electrode 5 has light permeability and a periodical concave-convex structure. The period of the periodical concave-convex structure is 200 to 300 nm. The height of a convex portion of the periodical concave-convex structure is 120 to 220 nm. The distance from an apex of the convex portion of the periodical concave-convex structure to the other facing electrode is 40 to 170 nm.

Description

  The present invention relates to a photoelectric conversion element in which one of a positive electrode and a negative electrode is light transmissive and has a periodic uneven structure, and a method for manufacturing the photoelectric conversion element.

  In recent years, the spread of solar cells as a clean energy source is expanding. Types of solar cells include silicon-based solar cells such as single crystal silicon and amorphous silicon, and inorganic compound-based thin-film solar cells such as GaAs, CIGS (copper / indium / gallium / selenium-containing compound) and CdTe. Is a problem. A factor of high cost is a process in which a semiconductor thin film must be manufactured under high vacuum and high temperature. Thus, attention has been paid to organic thin-film solar cells using organic semiconductors, which are expected to simplify the manufacturing process.

  Since the organic semiconductor thin film can be formed by a coating method or a printing method, it is expected that the manufacturing process can be simplified and the power generation cost can be reduced. In addition, since lightweight and flexible elements and modules can be manufactured, it has excellent portability and has the potential to be used even in areas where electrical infrastructure is not established. Furthermore, since the organic semiconductor can control the light absorption band by molecular design, it can provide a solar cell with various colors and excellent design. However, although these advantages can be expected, further improvement in photoelectric conversion efficiency is required for practical use of the organic thin film solar cell.

  In the photoelectric conversion element, light energy incident from a light-transmitting electrode is absorbed by the photoelectric active layer, and excitons in which holes and electrons are combined are generated. The photoelectric conversion layer is usually composed of a mixture of an electron-donating compound (electron-donating organic semiconductor) and an electron-accepting compound (electron-accepting organic semiconductor), and when excitons reach these interfaces, Due to the difference in LUMO energy and HOMO energy, holes and electrons are separated, and charges (holes and electrons) that can move independently are generated. The generated holes move to the positive electrode, and the electrons move to the negative electrode, so that they can be taken out as electric energy. Since the exciton diffusion distance is only about 10 nm, excitons that cannot reach the interface between the electron-donating organic semiconductor and the electron-accepting organic semiconductor are deactivated and cannot be extracted as electric energy. Therefore, a bulk heterojunction in which an electron-donating organic semiconductor and an electron-accepting organic semiconductor are phase-separated on a nanoscale has been actively studied in the photoelectric conversion layer.

  In order to improve the photoelectric conversion efficiency of the organic thin film solar cell, a photoelectric conversion layer excellent in light absorption ability and charge transport ability is necessary. Increasing the film thickness of the photoelectric conversion layer to improve the light absorption capability increases the distance that the holes and electrons generated by charge separation move to the electrode, increasing the electrical resistance and moving the charge. High photoelectric conversion efficiency cannot be obtained by recombination. In addition, when the photoelectric conversion layer is thinned in order to shorten the charge movement distance, the incident light cannot be sufficiently absorbed. Therefore, it is difficult to satisfy both characteristics of light absorption ability and charge transport ability, and the photoelectric conversion layer has a thickness of about 100 nm.

  In order to improve charge transport performance, a photoelectric conversion element having an ordered structure photoelectric conversion layer in which an electron-donating organic semiconductor and an electron-accepting organic semiconductor are mutually intruded has been proposed (Patent Document 1). Compared with a bulk heterojunction structure, which is a disordered mixed system, the charge transport ability is enhanced because a regular charge transport path is formed. However, in order to effectively separate the absorbed light, an electron-donating organic semiconductor Therefore, it is necessary to reduce the phase separation size of the electron-accepting organic semiconductor to an exciton diffusion length of about 10 nm. However, in the manufacturing method disclosed in Patent Document 1, there is a limit to the size of phase separation to be formed, and it is difficult to form an ideal size of about 10 nm, and thus the expected effect cannot be exhibited.

  On the other hand, there has been proposed a method for increasing the charge collection efficiency by using an electrode having a fine uneven structure. For example, a structure is disclosed in which electrodes of opposing protrusions are arranged so as to be inserted between each other in a photoelectric conversion layer (Patent Document 2). If both the positive electrode and the negative electrode can form a structure with a fine concavo-convex structure, the distance traveled by both holes and electrons will be shortened and the current collection efficiency will be improved, but the manufacturing method shown is practical. Therefore, practical application is difficult. In addition, a photoelectric conversion element having a transparent nanostructure electrode is disclosed (Patent Document 3). As a method for forming a nanostructure on a plate electrode, a method is shown in which a seed crystal is formed and the nanostructure is grown from a solution containing raw material ions on the plate electrode. Since the nanostructure electrode obtained by such a method has a needle-like and non-regular nanorod structure, a buffer layer such as a photoelectric conversion layer, a hole transport layer, or an electron transport layer is densely formed on the nanostructure electrode. It is difficult to form. Therefore, the energy loss at the time of taking out an electric charge from a photoelectric converting layer to an electrode becomes large, and high photoelectric conversion efficiency cannot be obtained.

WO2012-002463 Publication JP 2008-218702 A JP 2012-195512 A

  As described above, although several photoelectric conversion elements having a fine concavo-convex structure in the photoelectric conversion layer or electrode are known, the practicality is poor, and there are still problems in improving the photoelectric conversion efficiency. Therefore, the present invention does not decrease the current collection efficiency even if the film thickness of the photoelectric conversion layer is increased so that sufficient light can be absorbed, and has both excellent light absorption ability and charge transport ability, and high photoelectric conversion efficiency. It aims at providing the photoelectric conversion element which expresses, and its manufacturing method.

  As a result of diligent research to solve the above problems, the present inventors have found that fine irregularities are formed so that the charge transfer distance is shortened even if the film thickness of the photoelectric conversion layer is increased so that sufficient light can be absorbed. It has been found that high photoelectric conversion efficiency can be obtained by designing the pattern.

  The photoelectric conversion element according to claim 1, which has been made to achieve the above object, is a photoelectric conversion element having a photoelectric conversion layer containing an organic semiconductor between a positive electrode and a negative electrode. And either one of the positive electrode and the negative electrode is light transmissive and has a periodic concavo-convex structure, the period of the periodic concavo-convex structure is 200 to 300 nm, and the convex part height of the periodic concavo-convex structure Is 120 to 220 nm, and the distance from the top of the convex portion of the periodic concavo-convex structure to the opposite electrode is 40 to 170 nm.

  Similarly, the photoelectric conversion element according to claim 2 is the photoelectric conversion element according to claim 1, wherein the convex portion occupation ratio of the periodic uneven structure is 5 to 30% by volume.

  The photoelectric conversion device according to claim 3 is the photoelectric conversion device according to claim 1, wherein the convex shape of the periodic concavo-convex structure is a cylinder or a truncated cone, and the upper side diameter is 30 to 150 nm and the bottom side The diameter is 50 to 300 nm, and the upper side diameter does not exceed the bottom side diameter.

  A photoelectric conversion element according to a fourth aspect is the photoelectric conversion element according to any one of the first to third aspects, wherein the electrode having the periodic concavo-convex structure is indium tin oxide.

  The photoelectric conversion element according to claim 5 is the one described in any one of claims 1 to 4, wherein the photoelectric conversion layer is a mixture of an electron donating organic semiconductor and an electron accepting organic semiconductor. It is characterized by that.

  A photoelectric conversion device according to claim 6 is the photoelectric conversion device according to claim 5, wherein the electron-donating organic semiconductor is thiophene, fluorene, carbazole, dibenzosilole, dibenzogermole, benzodithiophene, and diketo. It is a π-electron conjugated polymer containing a monomer unit having at least one heterocyclic skeleton selected from pyrrolopyrrole.

  A photoelectric conversion element according to a seventh aspect is the photoelectric conversion element according to the fifth aspect, wherein the electron-accepting organic semiconductor is a fullerene derivative.

  The photoelectric conversion element according to claim 8 is the photoelectric conversion element according to any one of claims 1 to 7, wherein the hole transport material includes a hole transport material between the photoelectric conversion layer and the positive electrode. It has a layer.

  The method for producing a photoelectric conversion element according to claim 9, which has been made to achieve the above object, includes a photoelectric conversion layer containing an organic semiconductor between a positive electrode and a negative electrode. A method for manufacturing a photoelectric conversion element having a hole transport layer between the photoelectric conversion layer and the positive electrode, wherein the hole transport is dissolved on the surface of the positive electrode having a periodic uneven structure. A material is applied to form the hole transport layer.

  In the photoelectric conversion element of the present invention, at least one of the positive electrode and the negative electrode is light transmissive and has a periodic uneven structure, and even if the film thickness of the photoelectric conversion layer is increased so that sufficient light can be absorbed. Since the current collection efficiency does not decrease, excellent photoelectric conversion efficiency can be exhibited.

  According to the method for producing a photoelectric conversion element of the present invention, since a dense hole transport layer can be formed in a periodic concavo-convex structure, energy loss generated between the electrode and the photoelectric conversion layer can be suppressed, and high photoelectric conversion can be achieved. A photoelectric conversion element having efficiency can be provided.

It is a schematic cross section which shows one typical Example of the photoelectric conversion element to which this invention is applied. It is a schematic cross section showing the gap | interval part and upper remaining film in a photoelectric converting layer of the photoelectric conversion element to which this invention is applied. It is a scanning transmission electron micrograph of the cross section of the photoelectric conversion element in Example 1 which applies this invention. It is a scanning transmission electron micrograph of the cross section of the photoelectric conversion element in Example 2 to which this invention is applied.

  Although embodiments of the present invention will be described, the scope of the present invention is not limited to these embodiments.

  As shown in FIG. 1, a photoelectric conversion element 10 according to an example of the present invention is formed on a substrate 1 and has a photoelectric conversion layer 4 between a pair of positive electrode 2 and negative electrode 5. In this example, the positive electrode 2 is light transmissive and has a periodic uneven structure. The layer existing between the positive electrode 2 and the photoelectric conversion layer 4 is a hole transport layer 3 which is an arbitrary layer.

  The photoelectric conversion element 10 is usually formed on the substrate 1. The substrate 1 may be any substrate that does not change when an electrode is formed and an organic layer is formed. Examples of the material of the substrate 1 include inorganic materials such as alkali-free glass, quartz glass, and silicon, and organic materials such as polyester, polycarbonate, polyolefin, polyamide, polyimide, polyphenylene sulfide, polyparaxylene, epoxy resin, and fluorine resin. A film or plate produced by any method can be used. The substrate 1 may be on the positive electrode 2 side or on the negative electrode 5 side.

  The photoelectric conversion element of the present invention is characterized in that either one of the positive electrode and the negative electrode has light transmittance and has a periodic uneven structure. Examples of the transparent or translucent electrode material having optical transparency include a conductive metal oxide film. Specifically, indium oxide, zinc oxide, tin oxide, and their composites, indium tin oxide (ITO), fluorine tin oxide (FTO), antimony tin oxide, indium zinc oxide (IZO), Gallium / Zinc / Oxide (GZO), Aluminum / Zinc / Oxide (AZO), Antimony / Zinc / Oxide films are used, especially ITO, FTO, IZO, GZO, AZO and the like are preferable.

  The other electrode that does not have the periodic uneven structure may or may not have optical transparency. As an electrode material that does not have optical transparency, a known metal, a conductive polymer, or the like can be used. For example, metals such as platinum, gold, silver, aluminum, chromium, nickel, copper, titanium, magnesium, calcium, barium, sodium, or alloys thereof can be used.

  In this specification, the “periodic concavo-convex structure” means a regular repeating structure of convex portions and concave portions. For example, it means a structure in which cylindrical convex portions are arranged at the same interval, or a structure in which convex portions and concave portions are arranged in a line-and-space form.

  In the photoelectric conversion element of the present invention, the photoelectric conversion layer 4 existing between the positive electrode 2 and the negative electrode 5 includes a gap portion 6 filled in a gap portion of a periodic concavo-convex structure of one electrode, and a convex vertex. To the other electrode facing each other (hereinafter referred to as the upper remaining film). The schematic diagram is shown in FIG. As shown in FIG. 2, the total volume of the gap 6 and the upper remaining film 7 becomes the volume of the photoelectric conversion layer 4, and this determines the amount of light absorbed by the photoelectric conversion layer 4. By selecting such a shape, the volume of the photoelectric conversion layer per unit area becomes larger than that of the 100 nm-thick photoelectric conversion layer formed on the plate electrode, and sufficient light can be absorbed. Therefore, light energy that cannot be used due to reflection is reduced, and the light use efficiency of the photoelectric conversion element is increased. In addition, since the photoelectric conversion element 10 of the present invention has a short moving distance of charges (holes in the example of FIG. 1) that collect current on the electrode having a periodic concavo-convex structure, even if the volume of the photoelectric conversion layer is increased, Increase in electric resistance and recombination of charges during movement can be suppressed, and high current collection efficiency can be obtained.

  The movement distance of the charge can be simply calculated by the following method. The photoelectric conversion layer is divided into cube-shaped elements having an equal volume of 10,000 or more, and the shortest distance from the center of each element to the electrode surface is calculated. For all the elements, the shortest distance is calculated in the same manner, and the average moving distance can be calculated by taking the average value. In the case of a photoelectric conversion element in which a photoelectric conversion layer having a thickness of 100 nm is formed on a conventional flat plate electrode, the average moving distance of charges is 50 nm for both holes and electrons. In order for the photoelectric conversion element to absorb light sufficiently, the photoelectric conversion layer needs a film thickness of 200 to 350 nm, and the average distance between holes and electrons is 100 to 175 nm. In the case of using a flat plate electrode, the average moving distance of electric charges is increased by the increase in the film thickness of the photoelectric conversion layer, but the average moving distance can be shortened by using an electrode having a periodic uneven structure.

  The period L of the periodic concavo-convex structure needs to be 200 to 300 nm. If the period L is small, the occupancy ratio of the convex portion is increased. Therefore, the volume of the photoelectric conversion layer 4 filled in the gap portion is reduced, and the upper remaining film 7 needs to be thickened to absorb sufficient light. In this case, the average moving distance of electric charges becomes long, and high current collection efficiency cannot be obtained. When the period L is large, the average movement distance of the charges generated in the photoelectric conversion layer 4 filled in the gap portion becomes long, and high current collection efficiency cannot be obtained.

  The convex height of the periodic concavo-convex structure needs to be 120 nm to 220 nm. When the height of the convex portion is low, the volume of the photoelectric conversion layer 4 filled in the gap portion becomes small, and the upper remaining film 7 needs to be thick in order to absorb sufficient light. In this case, the average moving distance of electric charges becomes long, and high current collection efficiency cannot be obtained. When the height of the convex portion is higher than 220 nm, the average moving distance of the electric charge collected to the electrode (negative electrode in the example of FIG. 1) facing the electrode having the periodic concavo-convex structure is increased, and thus the current collection effect is reduced. .

  The distance from the apex of the convex portion of the periodic concavo-convex structure to the opposing electrode needs to be 40 to 170 nm, and preferably 70 to 160 nm. If the distance is less than 40 nm, the upper remaining film 7 is thin, and both electrodes are likely to come into contact with each other. It is difficult to electrically short-circuit and operate as a photoelectric conversion element. If the distance is larger than 170 nm, the average moving distance of the charge becomes long, and high current collection efficiency cannot be obtained.

  5-30 volume% is preferable and, as for the convex part occupation rate of a periodic uneven structure, 15-30 volume% is more preferable. Here, the “projection occupancy ratio” refers to the occupancy ratio of the periodic concavo-convex structure (positive electrode protrusion 8) with respect to the gap 6 of the photoelectric conversion layer. When the convex portion occupancy is smaller than 5% by volume, the electric resistance of the electrode having the periodic concavo-convex structure is increased, and the current collection efficiency may be lowered. Further, if the convex portion occupancy is larger than 30% by volume, the volume of the photoelectric conversion layer filled in the gap portion becomes small, and the upper residual film needs to be thickened to absorb sufficient light. In some cases, the average travel distance becomes longer and the current collection efficiency decreases.

  In the photoelectric conversion element of the present invention, the convex shape of the periodic concavo-convex structure is preferably a cylinder or a truncated cone, and the upper side diameter is preferably 30 to 150 nm, and the bottom side diameter is more preferably 50 to 300 nm. However, the upper side diameter is equal to or smaller than the bottom side diameter.

  When the convex shape is a columnar shape, the diameter of the column is preferably in the range of 30 to 150 nm, more preferably in the range of 70 to 110 nm. When the diameter is smaller than 30 nm, it is difficult to densely form a buffer layer such as a photoelectric conversion layer, a hole transport layer, or an electron transport layer on an electrode having a periodic concavo-convex structure, and from the photoelectric conversion layer to the electrode Energy loss at the time of taking out electric charge becomes large, and high current collection efficiency cannot be obtained. When the diameter is larger than 150 nm, the period is in the range of 200 to 300 nm, so that the convex portion occupancy increases and the photoelectric conversion layer needs to thicken the upper remaining film in order to absorb sufficient light. There is. In this case, the average moving distance of electric charges becomes long, and high current collection efficiency cannot be obtained.

  When the convex shape is a truncated cone, the upper side diameter of the truncated cone is preferably in the range of 30 to 150 nm, and more preferably in the range of 40 to 100 nm. Moreover, as a base diameter, the range of 50-300 nm is preferable, and the range of 150-300 nm is more preferable.

  In the photoelectric conversion element 10 of the present invention, the electrode having a periodic concavo-convex structure is preferably indium tin oxide. Since indium tin oxide has high optical transparency and low electrical resistance, energy loss can be reduced at the electrode portion having the periodic concavo-convex structure.

  The photoelectric conversion element 10 of the present invention has a photoelectric conversion layer 4 containing an organic semiconductor between a positive electrode 2 and a negative electrode 5. The photoelectric conversion layer 4 is preferably composed of a mixture of an electron donating organic semiconductor and an electron accepting organic semiconductor. In the present invention, a bulk heterojunction structure of an electron donating organic semiconductor and an electron accepting organic semiconductor is formed. It is preferable that

  The electron donating organic semiconductor used for the photoelectric conversion layer 4 may be a low molecular compound or a high molecular compound. Examples of the low molecular weight compound include phthalocyanine, metal phthalocyanine, porphyrin, metal porphyrin, oligothiophene, tetracene, pentacene, and rubrene. As a high molecular compound, a monomer having at least one heterocyclic skeleton selected from thiophene, fluorene, carbazole, dibenzosilole, dibenzogermole, benzodithiophene, diketopyrrolopyrrole and derivatives thereof as part of the chemical structure And a π-electron conjugated polymer containing a body unit. Among these, a π-electron conjugated polymer including a monomer unit having a heterocyclic skeleton including at least one thiophene ring as a part of the chemical structure is preferable from the viewpoint of photoelectric conversion efficiency. Examples of the heterocyclic skeleton include cyclopentadithiophene, thienopyrrole, dithienopyrrole, dithienosilole, dithienogermol, benzodithiophene, naphthodithiophene, and derivatives thereof. These may have a substituent in the main chain skeleton for the purpose of controlling solubility and polarity. These polymer compounds may be either homopolymers, random or block copolymers, and their molecular chains may be linear, branched, physically or chemically cross-linked. Among these, the electron-donating organic semiconductor is preferably a polymer compound from the viewpoint of application to a coating process, and includes a monomer unit having a heterocyclic skeleton including at least one thiophene ring as part of the chemical structure. A π-electron conjugated polymer is more preferable.

The electron-accepting organic semiconductor used for the photoelectric conversion layer 4 will not be specifically limited if it is an organic material which has electron-accepting property. Examples of the electron-accepting organic semiconductor include 1,4,5,8-naphthalene tetracarboxylic dianhydride, 3,4,9,10-perylene tetracarboxylic dianhydride, N, N′-dioctyl-3,4, 9,10-naphthyltetracarboxydiimide, 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole, 2,5-di (1-naphthyl) -1 Oxazole derivatives such as 1,3,4-oxadiazole, triazole derivatives such as 3- (4-biphenylyl) -4-phenyl-5- (4-t-butylphenyl) -1,2,4-triazole, phenanthroline derivatives , _{C 60} or _{C 70} fullerene derivatives, carbon nanotubes, poly -p- phenylene vinylene-based polymer cyano group to Off to derivatives (CN-PPV) and the like. These may be used alone or in combination of two or more. Among these, fullerene derivatives are preferably used from the viewpoint of an n-type semiconductor that is stable and excellent in carrier mobility.

Fullerene derivatives suitably used as the electron-accepting organic semiconductor include unsubstituted ones such as C ₆₀ , C ₇₀ , C ₇₆ , C ₇₈ , C ₈₂ , C ₈₄ , C ₉₀ , C ₉₄ , and [6 , 6] -phenyl C ₆₁ butyric acid methyl ester (PC ₆₁ BM), [5,6] -phenyl C ₆₁ butyric acid methyl ester, [6,6] -phenyl C ₆₁ butyric acid n-butyl ester, [6,6] -phenyl C ₆₁ butyric acid i-butyl ester, [6,6] -phenyl C ₆₁ butyric acid hexyl ester, [6,6] -phenyl C ₆₁ butyric acid dodecyl ester, [6 6] - diphenyl _{C 62} bis (butyric acid methyl _{ester) (bis-PC 62 BM)} , [6,6] Phenyl _{C 71} butyric acid methyl ester _{(PC 71 BM), [6,6} ] - diphenyl _{C 72} bis (butyric acid methyl _{ester) (bis-PC 72 BM)} , indene _{C 60} - monoadduct, indene _{C 60} - bis-adduct, indene C _{70 -} monoadduct, indene C _{70 -} and substituted derivatives including bis adduct thereof. Among _{these, C 60} or _{C 70} fullerene derivatives are particularly preferably used.

The photoelectric conversion element of the present invention has a charge that is collected on the counter electrode (in FIG. 1) as compared to the movement distance of the charge (hole in the example of FIG. 1) that is collected on the electrode having the periodic uneven structure. In the example, the movement distance of electrons) becomes longer. It is effective to use an electrode having a periodic concavo-convex structure on the side of collecting charges having a smaller charge mobility between the electron-donating organic semiconductor and the electron-accepting organic semiconductor used in the photoelectric conversion layer. For example, the charge mobility of poly (3-hexylthiophene-2,5-diyl as an electron-donating organic semiconductor and [6,6] -phenyl C ₆₁ butyric acid methyl ester as an electron-accepting organic semiconductor is 0, respectively. It is disclosed in WO2012-101241 that it is 0.006 cm ² / Vs and 0.02 cm ² / Vs, and when using a bulk heterojunction structure layer made of these two materials for a photoelectric conversion layer, a method of collecting holes That is, the positive electrode is preferably an electrode having a periodic uneven structure.

  The film thickness of the photoelectric conversion layer 4 is not particularly limited, but is preferably 200 to 350 nm when converted to an electrode having no periodic concavo-convex structure (flat electrode), and preferably 240 to 300 nm. More preferably. When the film thickness is within this range, the effect of the electrode having a periodic concavo-convex structure can be most exhibited, and the photoelectric conversion efficiency is easily improved. If the film thickness is larger than this, even if an electrode having a periodic concavo-convex structure is used, the distance of charge movement increases, so that the electrical resistance increases and the effects of the present invention may not be obtained.

  The photoelectric conversion element 10 of the present invention may have a hole transport layer 3 between the photoelectric conversion layer 4 and the positive electrode 2. The hole transport layer 3 is made of a hole transport material, and functions as a buffer layer that efficiently extracts holes generated in the photoelectric conversion layer 4 to the positive electrode 2 side. By inserting the hole transport layer 3, the energy barrier between the HOMO level of the electron donating material in the photoelectric conversion layer and the Fermi level (work function) of the positive electrode is reduced, so that the positive electrode 2 has the effect of reducing the electrical resistance of the holes moving to 2. Thereby, the photoelectric conversion element 10 can exhibit more excellent photoelectric conversion efficiency.

The material for forming the hole transport layer 3 is not particularly limited as long as it has p-type semiconductor characteristics, but a polythiophene polymer, a polyaniline polymer, a poly-p-phenylene vinylene polymer, a polyfluorene polymer. Conductive polymers such as polymers; low molecular organic compounds exhibiting p-type semiconductor properties such as phthalocyanine derivatives (H ₂ Pc, CuPc, ZnPc, etc.), porphyrin derivatives; transition metal oxides such as molybdenum oxide, tungsten oxide, vanadium oxide The product is preferably used. In particular, polyethylenedioxythiophene (PEDOT), which is a polythiophene polymer, or PEDOT to which polystyrene sulfonate (PSS) is added is preferably used. The thickness of the hole transport layer 3 is preferably 1 nm to 30 nm, and more preferably 5 nm to 20 nm. When the hole transport layer 3 is formed on the surface of the electrode having the periodic uneven structure, the hole transport layer 3 is formed with a uniform thickness on the surface of the periodic uneven structure, and similarly has a layer having the periodic uneven structure. It is preferable.

  When the electrode having the periodic uneven structure is a negative electrode, it is preferable that an electron transport layer made of an electron transport material is formed between the negative electrode having the periodic uneven structure and the photoelectric conversion layer. Materials for forming the electron transport layer include alkali metal halides such as sodium fluoride and cesium fluoride, alkaline earth metal halogen compounds such as calcium fluoride, carbonates such as cesium carbonate, titanium oxide and zinc oxide. An inorganic n-type semiconductor can be mentioned. The thickness of the electron transport layer is preferably 0.5 nm to 30 nm, and more preferably 2 to 20 nm.

  The manufacturing method of the photoelectric conversion element of this invention is demonstrated. A method for manufacturing an electrode having a periodic concavo-convex structure is not particularly limited, and a method of etching an opening portion of a periodic pattern mask provided on a flat plate electrode with a thermoplastic resin such as polystyrene can be used. The period of the periodic concavo-convex structure can be changed according to the shape of the mask. Further, the height of the convex portion of the periodic concavo-convex structure can be changed by controlling the etching time. The thermoplastic resin can be removed by oxygen plasma treatment or sputtering treatment.

  A nanoimprint method will be described in detail as an example of a method for manufacturing an electrode having a periodic uneven structure. First, a polystyrene thin film having a thickness of about 200 nm is formed on the surface of a light transmissive flat electrode substrate made of indium, tin, oxide or the like. Next, a columnar pillar made of polystyrene having a height of about 200 nm is formed on the surface of indium tin oxide by nanoimprinting using a silicon mold having a fine pattern shape. Next, the polystyrene layer remaining in the recesses is removed by oxygen plasma treatment to form a cylindrical pillar (hereinafter referred to as a nanomask) having a height of about 170 nm. The obtained substrate is introduced into a reactive gas such as a halogen gas, the nanomask opening on the indium tin oxide is dry-etched, and then the nanomask remaining on the indium tin oxide is oxygenated. By removing by plasma treatment, indium tin oxide having a periodic concavo-convex structure is produced.

  The photoelectric conversion layer 4 is manufactured by dissolving the electron-donating organic semiconductor and the electron-accepting organic semiconductor in a solvent, forming an organic semiconductor composition solution, and applying the solution onto an electrode having a periodic uneven structure. be able to. The coating method is not particularly limited. For example, dip coating method, spray coating method, ink jet method, aerosol jet method, spin coating method, bead coating method, wire bar coating method, blade coating method, roller coating method, Methods such as a curtain coating method, a slit die coater method, a gravure coater method, a slit reverse coater method, a micro gravure method, and a comma coater method can be employed.

Solvents for dissolving the organic semiconductor include tetrahydrofuran, 1,2-dichloroethane, cyclohexane, chloroform, bromoform, benzene, toluene, o-xylene, chlorobenzene, bromobenzene, iodobenzene, o-dichlorobenzene, anisole, methoxybenzene, Examples include chlorobenzene and pyridine.
These solvents may be used alone or in combination of two or more. Since the solvent hardly evaporates at the bottom of the gaps in the periodic uneven structure, if the difference in solubility between the electron-donating organic semiconductor and the electron-accepting organic semiconductor is large, the higher solubility segregates at the bottom of the gap in the drying process. Cheap. Therefore, o-dichlorobenzene, chlorobenzene, bromobenzene, iodobenzene, chloroform and mixtures thereof having high solubility for each of the electron-donating organic semiconductor and the electron-accepting organic semiconductor are preferable. More preferably, o-dichlorobenzene, chlorobenzene, and a mixture thereof having the highest solubility for each of the electron-donating organic semiconductor and the electron-accepting organic semiconductor are used.

  In the above step, the organic semiconductor composition solution may contain a high-boiling compound as an additive in addition to the electron-donating organic semiconductor and the electron-accepting organic semiconductor. In the process of forming a photoelectric conversion layer by containing a high-boiling compound, a fine and continuous phase separation structure of an electron-donating organic semiconductor and an electron-accepting organic semiconductor is formed, so that photoelectric conversion is excellent in photoelectric conversion efficiency. The layer 4 can be obtained.

  As high boiling point compounds, octanedithiol (boiling point: 270 ° C.), dibromooctane (boiling point: 272 ° C.), diiodooctane (boiling point: 327 ° C.), diiodohexane (boiling point: 142 ° C. [10 mmHg]), diiodobutane (boiling point) : 125 ° C. [12 mmHg]), diethylene glycol diethyl ether (boiling point: 162 ° C.), N-methyl-2-pyrrolidone (boiling point: 229 ° C.), 1- or 2-chloronaphthalene (boiling point: 256 ° C.), etc. . Among these, from the viewpoint of obtaining a photoelectric conversion element having excellent photoelectric conversion efficiency, octanedithiol, dibromooctane, diiodooctane, 1- or 2-chloronaphthalene is preferably used.

  When the photoelectric conversion layer 4 is formed, heat or solvent annealing may be performed as necessary. By performing the annealing treatment, the crystallinity of the material of the photoelectric conversion layer 4 and the phase separation structure between the electron donating organic semiconductor and the electron accepting organic semiconductor can be changed, and an element having excellent photoelectric conversion characteristics can be obtained.

  Thermal annealing is performed by holding the substrate 1 on which the photoelectric conversion layer 4 is formed at a desired temperature. Thermal annealing may be performed under reduced pressure or in an inert gas atmosphere, and a preferable temperature is 40 ° C to 200 ° C, more preferably 70 ° C to 150 ° C. If the temperature is low, sufficient effects cannot be obtained, and if the temperature is too high, the photoelectric conversion layer 4 is oxidized and / or decomposed, and sufficient photoelectric conversion characteristics cannot be obtained. The solvent annealing is performed by holding the substrate 1 on which the photoelectric conversion layer 4 is formed in a solvent atmosphere for a desired time. The solvent used for annealing at this time is not particularly limited, but is preferably a good solvent for the photoelectric conversion layer 4.

  When the photoelectric conversion element of the present invention has a hole transport layer, the hole transport layer is preferably formed by dissolving the hole transport material in a solvent and applying it. As the coating method, any of the coating methods described in the method for forming a photoelectric conversion layer can be employed. However, when the positive electrode is an electrode having a periodic concavo-convex structure, if the concentration is too low, it is difficult to uniformly cover the electrode having the periodic concavo-convex structure, and if the concentration is too high, It is preferable to apply at a suitable concentration because the photoelectric conversion layer cannot be filled. Specifically, the concentration of the hole transport material is preferably 0.05 to 1.0% by weight, more preferably 0.1 to 0.5% by weight. Moreover, as a solvent, a C1-C10 aliphatic alcohol, water, these mixed solvents, etc. can be used according to the wettability of the electrode which has a periodic uneven structure. These solvents may be used singly or in combination of two or more. From the viewpoint of wettability with the surface of the electrode having a periodic concavo-convex structure, a mixed solvent of water and 1-propanol is used. preferable.

  Moreover, when the photoelectric conversion element of this invention has an electron carrying layer, the formation method of an electron carrying layer is not specifically limited, It can form using a vacuum evaporation method, sputtering method, the apply | coating method, etc.

  A method for forming the electrode facing the electrode having the periodic uneven structure is not particularly limited, and a vacuum deposition method, a sputtering method, a coating method, or the like can be used.

  The photoelectric conversion element of the present invention having an electrode having a light-transmitting and periodic concavo-convex structure is excellent in photoelectric conversion efficiency, and can be applied to various photosensors including solar cells.

  Examples of the present invention will be described in detail below, but the scope of the present invention is not limited to these examples. In addition, evaluation in an Example was performed as follows.

[Measurement of film thickness of photoelectric conversion layer]
The film thickness of the photoelectric conversion layer of an Example and a comparative example was measured on the conditions of a stylus pressure: 3 mg and a measurement range: 50 kÅ using a contact-type step meter (DEKTAK8: manufactured by Veeco).

[Measurement of photoelectric conversion efficiency]
Photoelectric conversion efficiencies of the photoelectric conversion elements prepared in Examples and Comparative Examples are solar simulator (OTENTO-SUNII: manufactured by Spectrometer Co., Ltd.) and source meter (KEITHLEY2400: manufactured by KEITHLEY), irradiation spectrum is AM1.5, irradiation The intensity was measured at 100 mW / cm ² . The irradiation intensity at the time of measurement was adjusted to be a solar cell evaluation standard using a photodiode (BS-520, manufactured by Spectrometer Co., Ltd.). At the time of measurement, an irradiation light mask having the same area as the light receiving area of the photoelectric conversion element was worn to eliminate excessive light incidence.

[Image evaluation of cross section of photoelectric conversion element]
The cross section of the photoelectric conversion element using the electrode having a periodic uneven structure was observed by the following method. Using a focused ion beam method (Hitachi High Technology Co., Ltd .: FB-2000) while cooling a thin film slice in the cross-sectional direction of the photoelectric conversion element to −100 ° C. or lower with liquid nitrogen, ions of 80 to 5200 pA are accelerating at 30 kV. It produced with the electric current. The prepared thin film slice had a thickness of 50 to 100 nm. About the produced thin film slice, the scanning transmission electron micrograph of the cross section of a photoelectric conversion element was obtained by implementing by STEM method (Hitachi High Technology company_made: S5500) at the acceleration voltage of 30 kV.

Example 1
A polystyrene pillar (nanomask) having a height of 170 nm was formed by a nanoimprinting method on a substrate (manufactured by Geomat Co., Ltd.) on which 500 nm of indium tin oxide was formed on a 0.7 mm glass. This substrate was introduced into a reactive ion etching apparatus, and the opening portion of the nanomask was etched to form a recess structure having a depth of about 120 nm. Next, the nanomask was removed by oxygen plasma treatment. Furthermore, the obtained substrate was introduced into an ion beam sputtering apparatus, and surface contamination was removed by ion beam sputtering treatment of the electrode surface on which the periodic uneven structure was formed. In the obtained electrode, the convex shape of the periodic concavo-convex structure was a truncated cone. The period of the electrodes was 300 nm, the height of the convex part was 122 nm, the upper side diameter of the truncated cone was 45 nm, and the bottom side diameter was 240 nm. The obtained substrate was subjected to ultrasonic cleaning for 10 minutes with semi-clean (Furuuchi Chemical Co., Ltd.), ultrapure water, acetone and isopropanol, dried, and then dried for 20 minutes using a UV-O ₃ cleaner (manufactured by Filgen). Ozone cleaned.

  A solvent in which water and 1-propanol are mixed at a ratio of 5: 2 (volume ratio) is added to commercially available PEDOT: PSS (manufactured by Clevios) to prepare a PEDOT: PSS solution having a solid content concentration of 0.3% by weight. did. The cleaned ITO substrate was spin-coated at 4000 rpm for 60 seconds. After the substrate on which PEDOT: PSS was formed was introduced into a glove box in a nitrogen atmosphere, heat treatment was performed at 150 ° C. for 10 minutes.

Subsequently, commercially available poly (3-hexylthiophene-2,5-diyl) (manufactured by Aldrich) as an electron-donating organic semiconductor and [6,6] -phenyl C ₆₁ butyric acid methyl ester as an electron-accepting organic semiconductor. (Made by Frontiacabon Co., Ltd.) at a weight ratio of 60:40, and a chlorobenzene solution was added under a nitrogen atmosphere to prepare a solid content concentration of 4.0% by weight, followed by stirring at 40 ° C. for 3 hours. Then, it filtered with the filter made from a 0.45 micrometer polytetrafluoroethylene, and obtained the uniform solution. The prepared solution was spin-coated at 920 rpm for 120 seconds on the buffer layer on the ITO substrate under a nitrogen atmosphere. After vacuum drying for 3 hours, thermal annealing was performed at 150 ° C. for 30 minutes.

Subsequently, the substrate was introduced into a resistance heating vacuum deposition apparatus, and 0.5 nm of lithium fluoride (purity 99.99%) was vacuum deposited under a reduced pressure condition of 5.0 × 10 ⁻⁵ Pa. A transport layer was formed. Furthermore, 80 nm aluminum (purity 99.99%) was vacuum-deposited to produce a photoelectric conversion element. Subsequently, a UV curable resin was applied to a glass sealing cap, bonded to the produced photoelectric conversion element, and then irradiated with a UV lamp to seal the photoelectric conversion element.

The power generation area of the produced photoelectric conversion element was 0.04 cm ² . In addition, from the result of cross-sectional observation of the photoelectric conversion element shown in FIG. 3, the distance from the convex vertex of the positive electrode 2 that is an electrode having a periodic concavo-convex structure to the negative electrode 5 that is an opposite electrode is 153 nm. The rate was 20.5% by volume.

  From FIG. 3, when the volume of the photoelectric conversion layer in the gap portion of the periodic concavo-convex structure and the photoelectric conversion layer of the upper remaining film are totaled, the photoelectric conversion layer 4 having a volume corresponding to a film thickness of 250 nm (film thickness converted to flat) is formed. It has been confirmed that. In addition, the calculation method of the film thickness (film thickness converted to flat) of the photoelectric conversion layer formed in the gap portion of the periodic concavo-convex structure is as follows. It was calculated | required by the method of multiplying.

  As a result of measuring the photoelectric conversion efficiency of the produced photoelectric conversion element, the photoelectric conversion efficiency was 3.84%. In the photoelectric conversion element obtained in Example 1, the light absorption amount of the photoelectric conversion layer is about 1.14 times that of the 100 nm-thick photoelectric conversion element (the following comparative example) formed on the plate electrode. This was calculated by optical calculation. The amount of short-circuit current of the photoelectric conversion element obtained in Example 1 was about 1.16 times that of the following comparative example, and the effect of increasing the film thickness of the photoelectric conversion layer was obtained. Moreover, the average moving distances of holes and electrons of the photoelectric conversion element manufactured in Example 1 were 91 nm and 125 nm.

  Moreover, it can confirm that the obtained PEDOT: PSS layer of thickness 20nm is formed between the electrode 2 which has a periodic uneven structure, and the photoelectric converting layer 4 in the obtained photoelectric converting element. The ability to form the hole transport layer 3 densely in this way is necessary to obtain high current collection efficiency. In the case of the needle-shaped electrode having a non-regular nanorod structure shown in Patent Document 3, it is difficult to form a dense hole transport layer.

(Example 2)
A recess structure having a depth of about 180 nm was formed by doubling the time of the reactive ion etching process of Example 1. Otherwise, a photoelectric conversion element was produced in the same manner as in Example 1. The period of the electrodes was 300 nm, the height of the convex portion was 185 nm, the top side diameter of the truncated cone was 65 nm, and the base side diameter was 269 nm.

  From the result of cross-sectional observation of the photoelectric conversion element shown in FIG. 4, the distance from the top of the convex portion of the positive electrode 2 that is an electrode having a periodic concavo-convex structure to the negative electrode 5 that is an opposite electrode is 116 nm, and the convex portion occupation ratio is It was 27.4% by volume. When the volumes of the photoelectric conversion layer in the gap portion of the periodic concavo-convex structure and the photoelectric conversion layer of the upper remaining film are summed up in the same manner as in Example 1, a photoelectric having a volume corresponding to a film thickness of 250 nm (film thickness in flat conversion) is obtained. It was confirmed that the conversion layer 4 was formed.

  As a result of measuring the photoelectric conversion efficiency of the produced photoelectric conversion element, the photoelectric conversion efficiency was 3.70%. In the photoelectric conversion element obtained in Example 2, the light absorption amount of the photoelectric conversion layer is about 1.14 times that of the 100 nm-thick photoelectric conversion element (the following comparative example) formed on the plate electrode. This was calculated by optical calculation. The short circuit current amount of the photoelectric conversion element obtained in Example 2 was about 1.14 times that of the comparative example, and the effect of increasing the film thickness of the photoelectric conversion layer was obtained. Moreover, the average moving distances of holes and electrons of the photoelectric conversion element produced in Example 2 were 72 nm and 128 nm.

(Comparative example)
A substrate (manufactured by Geomat Co., Ltd.) on which 300 nm of indium tin oxide is formed on a 0.7 mm glass was ultrasonically cleaned with semicoclean (manufactured by Furuuchi Chemical Co., Ltd.), ultrapure water, acetone and isopropanol for 10 minutes. After drying, ozone cleaning was performed using a UV-O ₃ cleaner (manufactured by Philgen) for 20 minutes.

  On the cleaned ITO substrate, commercially available PEDOT: PSS (manufactured by Clevios) was spin-coated at 6000 rpm for 60 seconds. After the substrate on which PEDOT: PSS was formed was introduced into a glove box in a nitrogen atmosphere, heat treatment was performed at 150 ° C. for 10 minutes. Subsequently, the solution for the photoelectric conversion layer prepared in Example 1 was spin-coated at 2500 rpm for 30 seconds, and thermal annealing was performed at 150 ° C. for 30 minutes using a hot plate.

Commercially available poly (3-hexylthiophene-2,5-diyl) (manufactured by Aldrich) and [6,6] -phenyl C ₆₁ butyric acid methyl ester (manufactured by Frontiacabon) as an electron-accepting organic semiconductor Were mixed at a weight ratio of 60:40, and a chlorobenzene solution was added under a nitrogen atmosphere to adjust the solid content concentration to 4.0% by weight. After stirring at 40 ° C. for 3 hours, 0.45 μm polytetrafluoro Filtration through an ethylene filter gave a uniform solution. The prepared solution was spin-coated at 5400 rpm for 30 seconds on the buffer layer on the ITO substrate under a nitrogen atmosphere. After vacuum drying for 3 hours, thermal annealing was performed at 150 ° C. for 30 minutes. The film thickness of the obtained photoelectric conversion layer was 100 nm. Otherwise, a photoelectric conversion element was produced in the same manner as in Example 1. As a result of measuring the photoelectric conversion efficiency of the produced photoelectric conversion element, the photoelectric conversion efficiency was 3.50%.

  From the results of Examples 1 and 2 and the comparative example, the photoelectric conversion element of the present invention has a positive electrode having a light-transmitting periodic concavo-convex structure. In general, the film thickness of the comparative example is 100 nm because the incident light can be sufficiently absorbed by increasing the volume of the photoelectric conversion layer. It has been clarified that superior conversion efficiency can be exhibited as compared with typical photoelectric conversion elements.

  DESCRIPTION OF SYMBOLS 1 is a board | substrate, 2 is the positive electrode in which the periodic uneven | corrugated structure was formed, 3 is a positive hole transport layer, 4 is a photoelectric converting layer, 5 is a negative electrode, 6 is a gap | interval part in a photoelectric converting layer, 7 is in a photoelectric converting layer 8 is a convex part of the positive electrode, 10 is a photoelectric conversion element, and L is a period of the periodic uneven structure.

Claims (9)

  A photoelectric conversion element having a photoelectric conversion layer containing an organic semiconductor between a positive electrode and a negative electrode, wherein one of the positive electrode and the negative electrode is light transmissive and has a periodic uneven structure The period of the periodic concavo-convex structure is 200 to 300 nm, the height of the convex part of the periodic concavo-convex structure is 120 to 220 nm, and the distance from the vertex of the convex part of the periodic concavo-convex structure to the opposite electrode is 40 to A photoelectric conversion element having a thickness of 170 nm.   The photoelectric conversion element according to claim 1, wherein a convex portion occupation ratio of the periodic concavo-convex structure is 5 to 30% by volume.   The convex shape of the periodic concavo-convex structure is a cylinder or a truncated cone, the upper side diameter is 30 to 150 nm and the bottom side diameter is 50 to 300 nm, and the upper side diameter does not exceed the bottom side diameter. Or the photoelectric conversion element of 2.   The photoelectric conversion element according to any one of claims 1 to 3, wherein the electrode having the periodic concavo-convex structure is indium tin oxide.   The photoelectric conversion element according to claim 1, wherein the photoelectric conversion layer is a mixture of an electron donating organic semiconductor and an electron accepting organic semiconductor.   The electron-donating organic semiconductor includes a π-electron conjugated heavy containing a monomer unit having at least one heterocyclic skeleton selected from thiophene, fluorene, carbazole, dibenzosilole, dibenzogermole, benzodithiophene and diketopyrrolopyrrole. The photoelectric conversion element according to claim 5, wherein the photoelectric conversion element is a combination.   The photoelectric conversion element according to claim 5, wherein the electron-accepting organic semiconductor is a fullerene derivative.   The photoelectric conversion element according to claim 1, further comprising a hole transport layer containing a hole transport material between the photoelectric conversion layer and the positive electrode.   A method for producing a photoelectric conversion element having a photoelectric conversion layer containing an organic semiconductor between a positive electrode and a negative electrode, and having a hole transport layer between the photoelectric conversion layer and the positive electrode, A method for producing a photoelectric conversion element, comprising: applying a dissolved hole transport material on a surface of the positive electrode having a periodic uneven structure to form the hole transport layer.