JP2014120590A - Organic photoelectric conversion element, and organic thin-film solar battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion element having an improved photoelectric conversion efficiency and an excellent economical efficiency by modifying a conventional art without employing a novel material nor a tandem structure, and to provide an organic solar battery having the photoelectric conversion element.SOLUTION: A photoelectric conversion element comprises: a P-type organic semiconductor layer; an N-type organic semiconductor layer structure forming a PN junction surface between the N-type organic semiconductor layer structure and the P-type organic semiconductor layer; a first electrode electrically connected with the P-type organic semiconductor layer; and a second electrode electrically connected with the N-type organic semiconductor layer structure. The N-type organic semiconductor layer structure comprises a lamination structure in which an N-type organic semiconductor layer consisting of a fullerene derivative and an N-type organic semiconductor layer having a different light absorption region from the fullerene derivative are laminated. The layer located at the second electrode side in the lamination structure is an N-type organic semiconductor layer having the light absorption region in a visible light region or a near-infrared region and having an absorption coefficient of 4 or more at a film thickness of 1 μm to perform a strong light absorption. Further, an exciton block layer is provided between the N-type organic semiconductor layer structure and the second electrode.

Description

  The present invention relates to a photoelectric conversion element capable of efficiently converting incident light useful as a material for an organic semiconductor thin film solar cell into electricity, and an organic thin film solar cell using the photoelectric conversion element.

  The energy problem is now one of the most important issues that science should solve, and the large-scale development of non-depleting renewable energy is important. Photovoltaic power generation is attracting attention as clean energy, but a significant reduction in power generation cost is an urgent issue. In particular, organic semiconductor thin-film solar cells can be produced on substrates such as plastic films by an energy-saving process, so they are considered to be a powerful solution for reducing the cost of solar power generation. It has been. Improvement of conversion efficiency is mentioned as an important subject of an organic semiconductor thin film solar cell.

  Organic semiconductor thin film solar cells (hereinafter also referred to as “organic solar cells” or “organic thin film solar cells”) are lightweight and flexible, and can be attached to curved surfaces such as roofs, walls, and windows. Therefore, it becomes possible to use sunlight that could not be used with conventional inorganic solar cells. In addition, since roll-to-roll enables mass production by printing, vacuum deposition, etc., it is possible to reduce the cost of manufacturing solar power generation elements, which is expected to greatly reduce solar power generation costs. Yes.

  Regarding the photoelectric conversion element constituting the organic solar cell, it has been proposed to use an organic material instead of an inorganic material such as silicon, gallium arsenide, or cadmium sulfide (see Non-Patent Document 1, for example). However, since organic materials have a narrow light absorption range, the current situation is that they do not reach the conventional silicon solar cells in terms of power generation efficiency. Therefore, a structure in which a plurality of cells having different light absorption regions are tandemd (in-line lamination) and new materials are being developed. In addition, an organic solar cell in which an organic semiconductor layer is formed by co-evaporation with a perylene derivative having a wide light absorption region and a large absorption and having an excellent semiconductor characteristic as an N-type organic semiconductor layer (Patent Document) 1) In addition, a solar cell using a perylene derivative having a fluorophenylalkyl group in which a fluorine group is substituted with fluorine has been reported (Patent Document 2). In addition, an organic solar cell produced by vacuum deposition using a fullerene derivative and a phthalocyanine derivative has been reported. In this technique, the exciton lifetime of fullerene is extremely longer than that of phthalocyanine, and the fullerene exciton generated inside the fullerene diffuses to the PN junction surface and separates charges, thereby contributing to a high current (Patent Document 3).

  Moreover, the solar cell which raises a photoelectric conversion efficiency by multilayering the organic-semiconductor layer of an organic solar cell is reported (patent document 4). In this organic solar cell, in the P-type organic semiconductor layer in which a plurality of organic semiconductor layers are stacked, the organic semiconductor layer on the side close to the PN junction surface has a small energy gap and is positioned on the side far from the PN junction surface. With a structure with a large energy gap. And it is described that the following effect is acquired by setting it as such a structure. When light is incident on the PN junction surface, the light energy absorbed by the organic semiconductor layer located on the side far from the PN junction surface moves to the organic semiconductor layer located on the side closer to the PN junction surface as excitation energy. An excited state can be formed. In addition, light which is not absorbed by the organic semiconductor layer located on the side far from the PN junction surface and is incident on the organic semiconductor layer located on the side close to the PN junction surface forms an excited state in the organic semiconductor layer and takes The excited state is essentially the same as the excited state formed by the excitation energy moved from the adjacent organic semiconductor layer, and charge separation occurs efficiently in the vicinity of the PN junction surface, so that a large current can be taken out.

JP 2002-076391 A Special table 2011-519350 gazette Japanese Patent Application Laid-Open No. 09-074216 JP 2010-141268 A

C. W. Tang, [Applied Physics Letters], Vol. 48, p183

  However, the perylene derivative used in the above-described conventional technology is an excellent electron transporting material and has a wide light absorption range in visible light and a large extinction coefficient, but efficiently improves charge separation at the PN junction interface. Therefore, it is difficult to use the organic solar cell for which high conversion efficiency is required. In addition, the fullerene derivative used in the above-described conventional technology is more expensive than other organic semiconductor materials, and a device using the fullerene derivative as a constituent material causes an increase in manufacturing cost. Furthermore, in addition to this, there is a problem that light absorption in the visible light region is small due to the symmetry of fullerene, and sunlight cannot be used effectively. In addition, cell tandemization means that if the generated currents cannot be matched in a plurality of cells, the cell with the smallest generated current defines the generated current of the entire solar cell, and a stable current cannot be obtained. There is a problem of bringing about a phenomenon.

Further, the solar cell having improved efficiency by multilayered organic semiconductor layer of the organic solar cell in the above-described prior art, although the characteristics of the solar cell is improved by a multilayer of P-type organic semiconductor layer, C ₆₀ as fullerene Since the organic semiconductor layer made of is used as an N-type organic semiconductor layer, according to the study by the present inventors, there are the following problems. That is, since the organic semiconductor layer made of C ₆₀ as the fullerene is used as the N-type organic semiconductor layer, there is a problem that light absorption in the visible light region is small and sunlight cannot be used effectively. Furthermore, not only this point, but also because expensive fullerene is used for the N-type organic semiconductor layer, it leads to high cost of the organic solar cell element and is suitable for enabling the provision of a general-purpose organic solar cell at low cost. However, there is a problem that the economic efficiency, which is an important requirement for promoting industrial use, is not satisfied. The organic solar cell of the C ₇₀ is N-type organic semiconductor layer as a fullerene, charge separation occurs efficiently by having the light absorption in the visible light region, can be used as an organic solar high efficiency, C ₇₀ Has a large practical problem that it is difficult to produce in large quantities, and the solar cell becomes more expensive than when C ₆₀ is used.

  Here, the operation of the organic solar cell will be briefly described. First, when light is applied to the organic solar cell, the P-type semiconductor layer made of an electron donor mainly constituting the organic photoelectric conversion element absorbs the light. Exciton is generated. The generated excitons move to the PN junction interface, and electrons move from the P-type organic semiconductor layer to the N-type organic semiconductor layer, which is an electron acceptor, and a charge separation state occurs. Then, the holes generated in the electron donor move to the transparent electrode substrate side, and the electrons generated in the electron acceptor flow to the other electrode, whereby a current flows to the external circuit, thereby forming a solar cell.

  However, the present inventors have attempted to further increase the efficiency of the organic photoelectric conversion element exhibiting the above-described action, increase its practicality, and make it industrially usable. The organic semiconductor layer also has a large light absorption region, and it has been recognized that it is necessary to efficiently generate excitons by the absorbed light and to perform charge separation at the PN junction interface.

  Therefore, in view of the above-mentioned problems of the prior art, the object of the present invention is to improve the prior art without adopting a new material that causes high cost of the solar cell or adopting a tandem structure. Another object of the present invention is to provide a photoelectric conversion element that is excellent in economic efficiency and can improve photoelectric conversion efficiency, and an organic semiconductor thin film solar cell including the photoelectric conversion element.

  The above object is achieved by the present invention described below. That is, the present invention is electrically connected to a P-type organic semiconductor layer, an N-type organic semiconductor layer structure that forms a PN junction surface between the P-type organic semiconductor layer, and the P-type organic semiconductor layer. A photoelectric conversion element having a first electrode and a second electrode electrically connected to the N-type organic semiconductor layer structure, wherein the N-type organic semiconductor layer structure is an N-type organic semiconductor layer made of a fullerene derivative. And at least one N-type organic semiconductor layer having a light absorption range different from that of the fullerene derivative, and the layer located on the second electrode side in the stacked structure has a visible light region. Alternatively, it is an N-type organic semiconductor layer having a light absorption region in the near-infrared region and having a strong light absorption with an absorption coefficient larger than 4 at a film thickness of 1 μm, and further, the N-type organic semiconductor layer structure and the second electrode With exciton blocking layer between It becomes possible to provide a photoelectric conversion device characterized.

  The following are mentioned as a preferable form of the photoelectric conversion element of the said invention. The HOMO level of the N-type organic semiconductor layer located on the second electrode side is equal to or lower than the HOMO level of the N-type organic semiconductor layer made of the fullerene derivative located on the PN junction surface, and located on the second electrode side. The photoelectric conversion element whose LUMO level of the said N type organic-semiconductor layer is below the LUMO level of the N-type organic-semiconductor layer which consists of a fullerene derivative located in a PN junction surface. The photoelectric conversion element in which the exciton blocking layer is a layer made of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline).

The film thickness of the N-type organic semiconductor layer, which is the layer located on the second electrode side, is 5 nm to 50 nm in thickness of the N-type organic semiconductor layer made of the fullerene derivative, constituting the N-type organic semiconductor structure. 5 nm to 50 nm, the total film thickness of these layers in the N-type organic semiconductor layer structure is in the range of 20 nm to 100 nm, and the short-circuit current in pseudo sunlight (AM1.5G) 100 mW / m ² A photoelectric conversion element having a value J _SC of 2.0 mA / cm ² or more. The photoelectric conversion element whose thickness of the N type organic semiconductor layer which consists of said fullerene derivative is 5 nm to 15 nm.

Ａ：芳香族環、ヘテロ芳香族環、または、これらの誘導体のいずれかを表す。
Ｙ：アルキル基、または、ヘテロアルキル基を表すが、Ｙはなくてもよい。 The photoelectric conversion element whose layer located in the 2nd electrode side is an N-type organic-semiconductor layer which consists of a perylene derivative. A photoelectric conversion element in which an N-type organic semiconductor layer made of the perylene derivative is made of a perylene tetracarboxylic acid diimide derivative having a conjugated cyclic structure in a side chain represented by the following formula.

A: represents an aromatic ring, a heteroaromatic ring, or a derivative thereof.
Y: represents an alkyl group or a heteroalkyl group, but Y may not be present.

A photoelectric conversion element in which an N-type organic semiconductor layer made of the perylene derivative is made of N, N′-diphenylperylenetetracarboxylic acid diimide represented by the following formula.

  As another embodiment of the present invention, there is provided an organic semiconductor thin film solar cell comprising any one of the photoelectric conversion elements described above.

  According to the present invention, without adopting a new material that leads to high cost of solar cells or adopting a tandem structure, by improving the configuration of the N-type organic semiconductor layer that is an electron acceptor in the prior art, Also in the N-type organic semiconductor layer, it has a large light absorption region, efficiently generates excitons by absorbed light, enables efficient charge separation at the PN junction interface, and is excellent in economic efficiency. A photoelectric conversion element capable of improving the photoelectric conversion efficiency and an organic semiconductor thin film solar cell including the photoelectric conversion element can be provided.

  More specifically, a P-type organic semiconductor layer, an N-type organic semiconductor layer structure forming a PN junction surface between the P-type organic semiconductor layer, and a first electrically connected to the P-type organic semiconductor layer. In a photoelectric conversion element having an electrode and a second electrode electrically connected to the N-type organic semiconductor layer, the N-type organic semiconductor layer structure is composed of an N-type organic semiconductor layer made of a fullerene derivative, and a fullerene derivative. A layered structure in which at least one N-type organic semiconductor layer having a different light absorption range is stacked, and a layer located on the second electrode side in the layered structure has a light absorption range in the visible light region or near infrared region. The photoelectric conversion element can stably obtain a large current with a high light conversion efficiency by being composed of an N-type organic semiconductor layer having a light absorption coefficient greater than 4 at a film thickness of 1 μm. It will be a thing.

  Moreover, the photoelectric conversion element of the present invention has an N-type organic semiconductor layer structure in which the HOMO level of the N-type organic semiconductor layer located on the second electrode side is made of a fullerene derivative located on the PN junction surface. The LUMO level of the N-type organic semiconductor layer made of a fullerene derivative located on the PN junction surface is equal to or lower than the HOMO level of the layer and the LUMO level of the N-type organic semiconductor layer located on the second electrode side By configuring so as to be the following or the same level, excitons generated in each N-type organic semiconductor layer can be efficiently transported to the PN junction interface.

  In addition, the photoelectric conversion element of the present invention has a stacked structure in which organic thin films having a thickness smaller than the wavelength of light are stacked in multiple layers. A part of the light incident from the transparent electrode reaches the second electrode without being absorbed by the organic semiconductor layer, and is reflected by the metal surface of the second electrode. The incident light and the reflected light from the electrode cause interference of light, resulting in a region where many photons exist and a region where photons exist, and so-called standing waves are generated. In the photoelectric conversion element of the present invention, by defining the film thickness of the N-type organic semiconductor layer, an organic semiconductor is formed on the antinode of the standing wave of the light absorption region wavelength of the material forming the organic semiconductor layer (portion where electron density is high). A layer can be provided, sunlight can be used effectively, and conversion efficiency can be improved.

  Moreover, according to the preferable form of the photoelectric conversion element of the present invention, the N-type organic semiconductor layer structure is visible by forming the N-type organic semiconductor layer formed on the second electrode side with a perylene derivative. The probability of generating excitons in the optical region is further increased, and the generated excitons move to the N-type organic semiconductor layer made of stacked fullerenes, so that charge separation is more efficiently performed at the PN junction interface. Furthermore, by using a perylene derivative for the N-type organic semiconductor layer, the N-type organic semiconductor layer formed of fullerene can be thinned, so that an efficient and low-cost photoelectric conversion element can be realized. Become.

It is a schematic diagram which shows the photoelectric conversion element structure of this invention. It is a schematic diagram which shows an example of the photoelectric conversion element of this invention. It is a figure which shows an example of the energy level of each material used for formation of the photoelectric conversion element of this invention. It is a figure which shows the light absorption characteristic of each material used for formation of the photoelectric conversion element of this invention. It is a figure which shows the difference in the photoelectric conversion characteristic of an element when the film thickness of the layer (perylene layer) located in the 2nd electrode side which comprises the photoelectric conversion element of this invention is changed. It is a figure which shows the difference in the light absorption characteristic of an element when the film thickness of the layer (perylene layer) located in the 2nd electrode side which comprises the photoelectric conversion element of this invention is changed. It is a figure which shows the difference of the photoelectric conversion characteristic of the photoelectric conversion element of this invention in which the layer which consists of fullerene derivatives is provided, and the element which does not provide this layer. 6 is a diagram illustrating photoelectric conversion characteristics of Examples 1 to 4 and Comparative Example 1. FIG. It is a figure which shows the difference in the standing wave calculated in 380 nm and 630 nm produced by the difference in the film thickness of the layer (perylene layer) located in the 2nd electrode side which comprises the photoelectric conversion element of this invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail. However, the present invention is not limited to the following embodiments, and can be carried out without departing from the gist of the present invention. In addition, the value measured with the ionization potential measuring apparatus can be used for the HOMO (Highest Occupied Molecular Orbital) level of each organic material used in the present invention, and the LUMO (Lowest Unoccupied Molecular Orbital) level was measured. It can be calculated by the sum of the long wavelength end in the UV-visible absorption spectrum and the HOMO level.

<Photoelectric conversion element>
Hereinafter, the photoelectric conversion element of the present invention will be described. The photoelectric conversion element of the present invention is electrically connected to a P-type organic semiconductor layer, an N-type organic semiconductor layer structure forming a PN junction surface between the P-type organic semiconductor layer, and the P-type organic semiconductor layer. And a second electrode electrically connected to the N-type organic semiconductor layer structure, the N-type organic semiconductor layer structure comprising an N-type organic semiconductor layer made of a fullerene derivative, and a fullerene A layered structure in which at least one N-type organic semiconductor layer having a light absorption range different from that of the derivative is stacked, and the layer located on the second electrode side in the layered structure is a visible light region or a near infrared region. It is an N-type organic semiconductor layer having a light absorption region in the region and having a strong light absorption with an absorption coefficient larger than 4 at a film thickness of 1 μm.

  The photoelectric conversion element of the present invention has a laminated structure of transparent electrode / (buffer layer) / P-type organic semiconductor layer / N-type organic semiconductor layer structure in which at least two layers are laminated / exciton block layer. For this reason, the photoelectric conversion element of this invention is producible by a simple lamination process. More specifically, although an example is shown in FIG. 1, the stacked structure of the photoelectric conversion element of the present invention has a transparent electrode (first electrode) 21 with high light transmittance on a transparent substrate 11, and if necessary. A buffer layer 41, a P-type organic semiconductor layer 31 made of an electron donor (donor material), an N-type organic semiconductor layer 32 made of a fullerene derivative that is an organic electron acceptor (acceptor material), a visible light or near-infrared region. And an N-type organic semiconductor layer 33 made of an organic electron acceptor (acceptor material) having an absorption at the substrate, an exciton block layer 42 and the second electrode 12.

  As described above, the N-type organic semiconductor stack forming the photoelectric conversion element of the present invention includes the N-type organic semiconductor layer 32 made of fullerene, and the N-type organic semiconductor layer 33 having absorption in the visible light or near-infrared region. It is necessary to have a laminated structure including The inventors of the present invention have such a laminated structure and further provided an exciton blocking layer, so that an N-type organic semiconductor layer also has a large light absorption region and efficiently generates excitons by absorbed light. The present inventors have found that a photoelectric conversion element capable of increasing the efficiency of light conversion compared to the prior art by performing charge separation at the PN junction interface can be achieved.

(N-type organic semiconductor layer structure in which at least two layers are laminated)
The N-type organic semiconductor laminate 32 constituting the photoelectric conversion element of the present invention needs to be formed of a fullerene derivative. Examples of fullerenes include C ₆₀ , C ₇₀ , C ₈₄ , PCBM (phenyl C ₆₁ butyric acid methyl ester), SIMEF (silylmethyl fullerene), and the like. Among these fullerenes, C ₆₀ is easily available, has advantages such as is advantageous in cost. However, in the present invention, C ₆₀ , C ₇₀ , etc. can be used without particular limitation.

  The N-type organic semiconductor stack 33 constituting the photoelectric conversion element of the present invention is formed of an N-type semiconductor material as described below. Examples of the N-type organic semiconductor material include Alq3 ((tris (8-hydroxyquinolinol) aluminum) complex), naphthalenetetracarboxylic anhydride (NTCDA), dialkylnaphthalenetetracarboxylic anhydride diimide (NTCDI), perylene, perylene. Tetracarboxylic anhydride (PTCDA), dialkylperylenetetracarboxylic acid diimide (PTCDI), perylenebisbenzimidazole (PTCBI), dialkylanthraquinone, fluorenylidenemethane, tetracyanoethyllene, fluorenone, diphenoquinone oxadiazole, anthrone, Thiopyran dioxide, diphenoquinone, benzoquinone, malononitrile, dinitrobenzene, nitroanthraquinone, pentacene fluoride, phthalocyanine fluorine Things, fluorinated alkylated oligothiophene, fullerenes and carbon nanotubes, and, derivatives thereof.

  Examples of the method for forming the N-type organic semiconductor layer made of the above materials include vacuum processes such as vacuum deposition, sputtering, and CVD, or methods such as coating. Moreover, although the film thickness of the N-type organic semiconductor layer formed by such a method varies depending on the material used, it is preferably about 1 nm to 100 nm, for example. That is, it is preferable that the thickness is less than 1 nm because the light incident on the photovoltaic element cannot be sufficiently absorbed and transmitted, and defects and defects are generated in the thin film, so that a sufficient effect cannot be obtained. Absent. Moreover, when exceeding 100 nm, since the series resistance in a photovoltaic device increases, the performance of a photoelectric conversion element falls and the problem that a manufacturing cost and material cost become high arises, and it is unpreferable. According to the study by the present inventors, the film thickness of each N-type organic semiconductor layer is more preferably 5 nm to 50 nm. Further, according to the study by the present inventors, in order to obtain the effect of the present invention more remarkably, an N-type organic semiconductor layer 32 made of fullerene constituting the N-type organic semiconductor layer structure constituting the present invention, It is preferable that the thickness of the layer in which the N-type organic semiconductor layer 33 having absorption in the visible light or near infrared region is stacked is in the range of 20 nm to 100 nm. This point will be described later.

  The N-type organic semiconductor laminate 33 constituting the photoelectric conversion element of the present invention has a large light absorption region in the visible light region or the near infrared region, and has a light absorption coefficient of 4 or more in the organic semiconductor layer (thickness 1 μm). Therefore, even if it is a 10 nm thin film, 99% or more of light can be absorbed, and photoelectric conversion efficiency can be improved.

  As a particularly preferable material for forming the N-type organic semiconductor laminate 33 constituting the photoelectric conversion element of the present invention, perylenetetracarboxylic acid represented by dimethylperylenetetracarboxylic acid diimide (Me-PTCDI) is used for the following reasons. Derivatives. As a more preferable material, the imide group of perylenetetracarboxylic acid diimide is substituted with a substituent including an aromatic group such as a phenyl group, a biphenyl group, a naphthyl group, a tolyl group, and a xylyl group, a heteroaromatic ring, and a cyclohexyl group. Perylenetetracarboxylic acid derivatives. These derivatives may be directly bonded to the imide group, but may be bonded via an alkyl group such as a methylene group, an ethylene group or a propylene group, or a group containing a hetero atom. Furthermore, N, N'-diphenylperylenetetracarboxylic acid diimide is mentioned as a preferable material.

  In the present invention, the organic semiconductor layer located on the second electrode side of the N-type organic semiconductor layer structure formed by the perylene tetracarboxylic acid derivative as described above has a large absorption region in the visible light region and is visible. Light can be photoelectrically converted. Furthermore, in the case of an organic semiconductor layer formed using a perylenetetracarboxylic acid derivative having a substituent containing an aromatic group, a heteroaromatic ring, or a cyclohexyl group, the π stack between molecules is strong due to the effect of these substituents. Thus, a wide absorption region from the visible light region to the near infrared region can be provided, and the wavelength region in which light incident on the photoelectric conversion element can be photoelectrically converted can be expanded. For example, an organic semiconductor layer (thickness 1 μm) formed by vacuum deposition of N, N′-diphenylperylenetetracarboxylic acid diimide has an extinction coefficient of 7.2 near 550 nm and is 99% or more even with a 10 nm thin film. Light can be absorbed and the photoelectric conversion efficiency can be increased.

The N-type organic semiconductor layer structure constituting the photoelectric conversion element of the present invention forms a laminated structure as described above. However, in the two adjacent organic semiconductor layers, the organic layer located on the side far from the PN junction surface is formed. A configuration in which the LUMO (minimum unoccupied orbit) level of the semiconductor layer is equal to or lower than the LUMO level of the organic semiconductor layer made of fullerene close to the PN junction interface may be adopted. Moreover, if the LUMO level of the organic semiconductor layer located far from the PN junction surface and the LUMO level of the organic semiconductor layer close to the PN junction are lower, the internal resistance in the photoelectric conversion element can be reduced. For example, the energy level of a perylene tetracarboxylic acid derivative suitable as a material for forming an N-type organic semiconductor layer constituting the photoelectric conversion element of the present invention is HOMO level −6 of fullerene C ₆₀ that can be used as a material for the layer used in combination with the layer. Compared to .2 eV, LUMO level -4.5 eV, HOMO level is around -6.2 to -6.6 eV, LUMO level is around -4.5 to 4.6 eV, and electrons generated at the PN interface are more efficient Can be moved to.

  The N-type organic semiconductor layer 32 made of fullerene having the N-type organic semiconductor layer structure constituting the photoelectric conversion element of the present invention, and the N-type organic semiconductor on the second electrode side having a large absorption region in the visible light or near infrared region The arrangement of the layer 33 is such that the N-type organic semiconductor layer 32 is formed at a position 5 nm to 100 nm away from the second electrode, and the thickness of the layer is 5 nm to 80 nm, so that light incident on the photoelectric conversion element can be converted into an organic semiconductor. Light that becomes a standing wave can be absorbed by reaching the second electrode without being absorbed by the layer and being reflected by the electrode. When light having a wavelength of 380 nm, which is a fullerene absorption region, is reflected by the second electrode and becomes a standing wave, an antinode of the standing wave is formed at a position of 20 to 60 nm from the second electrode. By installing the N-type organic semiconductor layer made of fullerene at the antinodes of the standing wave, light can be efficiently absorbed and more excitons can be generated (see FIG. 9).

  In the photovoltaic device of the present invention, for example, the thickness of the N-type semiconductor layer on the second electrode side is set to 5 nm to 50 nm, and the thickness of the N-type organic semiconductor layer made of fullerene is set to 5 nm to 50 nm. It can be set as the photoelectric conversion element which utilizes light efficiently. However, since the fullerene material is more expensive than the perylene derivative material, the ratio of the film thickness of the N-type organic semiconductor layer 32 made of fullerene to the N-type organic semiconductor layer 33 provided on the second electrode side (film thickness of 32: 33 The film thickness) is preferably 1: 1 to 1:10.

  It is more preferable that each layer of the N-type organic semiconductor layer structure made of the material as described above has a thickness in the range of 5 to 50 nm. That is, if the thickness is less than 5 nm, light that is not sufficiently absorbed is transmitted, and the photoelectric conversion characteristics are deteriorated. For the following reasons, it is not very effective to increase the thickness beyond 50 nm. That is, the exciton diffusion length of the excitons generated in the organic semiconductor layer (distance that the excitons can move stably) is about 20 to 30 nm. Therefore, the thickness of the organic semiconductor layer is the exciton diffusion length. Although it is preferable that the thickness be larger than that, absorption of light occurs, but charge separation cannot be performed, so that the photoelectric characteristics are deteriorated. Further, by setting the film thickness of the N-type organic semiconductor layer on the second electrode side to 5 to 50 nm, an antinode of the standing wave of the fullerene absorption wavelength caused by the reflection of the second electrode is present in the fullerene layer. It becomes an organic thin-film solar cell which can utilize the light of an area | region efficiently.

(P-type organic semiconductor layer)
The material for forming the P-type organic semiconductor layer 31 constituting the photovoltaic device of the present invention is not particularly limited as long as it has an electron donating property. For example, compounds such as phthalocyanines, aromatic tertiary amines, acenes, thiophene and selenophene derivatives, derivatives in which functional groups such as hydroxyl groups, alkyl groups, amino groups, and methyl groups are introduced into these compounds, Polymers of compounds such as aromatic tertiary amines, phenylene, vinylene, thiophene, fluorene, carbazole, vinylcarbazole, pyrrole, etc., or functional groups such as hydroxyl groups, alkyl groups, amino groups, and methyl groups are introduced into these compounds (Co) polymers of the resulting derivatives. Particularly preferable P-type organic semiconductor layers in the present invention include, for example, thiophene oligomers such as poly-3hexylthiophene, metal phthalocyanines such as metal-free phthalocyanine, nickel phthalocyanine, copper phthalocyanine, tin phthalocyanine, and lead phthalocyanine, BP3T (2,5 (Thiophene / phenylene) co-oligomers such as -bis (4-biphenylyl) -2,2 ': 5', 2 "-terthiophene).

  Examples of the method for forming the P-type organic semiconductor layer made of the above-described materials include vacuum processes such as vacuum deposition, sputtering, and CVD, or methods such as coating. The P-type organic semiconductor layer constituting the present invention is formed by the method as described above, and the film thickness varies depending on the material used, for example, about 5 to 100 nm, more preferably 5 to 50 nm. Preferably there is. If the thickness is less than 5 nm, the light incident on the photovoltaic device of the present invention cannot be sufficiently absorbed and transmitted, and defects or defects may occur in the thin film. Is not preferable because it may not be obtained. Moreover, when exceeding 100 nm, the series resistance in a photovoltaic device increases, and while the performance of a photoelectric conversion element falls, the problem that manufacturing cost and material cost become high will arise. The P-type organic semiconductor layer constituting the photovoltaic device of the present invention can have a multilayer structure as necessary. In the P-type organic semiconductor layer, it is preferable that the HOMO (maximum occupied orbit) level of the organic semiconductor layer located far from the PN junction surface is equal to or higher than the HOMO level of the organic semiconductor layer close to the PN junction. If comprised in this way, a hole can be taken out smoothly to a 1st electrode.

(First electrode)
The first electrode constituting the photovoltaic device of the present invention is a conductive material having a work function capable of ohmic contact with the P-type organic semiconductor layer, and is required to transmit light. Therefore, as the first electrode, for example, a light-transmitting conductive material such as ITO (Indium Tin Oxide), In ₂ O ₃ , SnO ₂ , or ZnO is used. Furthermore, as the first electrode, Sn-doped indium oxide (In ₂ O ₃ : Sn), F-doped tin oxide (SnO ₂ : F), Sb-doped tin oxide (SnO ₂ : Sb), Al-doped zinc oxide (ZnO: The light-transmitting conductive material may be doped with an impurity such as Al) or Ga-doped zinc oxide (ZnO: Ga).

(Buffer layer)
In the photovoltaic device of the present invention, a buffer layer may be formed between the first electrode 21 and the P-type organic semiconductor layer as necessary. The buffer layer can be used without any problem as long as it is a material that transports holes. Although the buffer layer is not particularly limited, it is formed by a vapor deposition method or a printing method. Therefore, by providing the buffer layer, it is possible to form a smooth layer that compensates for the unevenness of the first electrode on which the buffer layer is laminated. As a material for forming the buffer layer, for example, a p-type semiconductor material that can be used for the organic semiconductor layer described above can be used. For example, what is normally used as a hole transport material, such as phthalocyanine, naphthalocyanine, oxadiazole, triphenylamine, porphyrin, triazole, imidazole, or derivatives thereof, can be used. In addition, for example, poly-3,4-ethylenedioxythiophene / polystyrene sulfonate (PEDOT / PSS, trade name: CLEVIOS P) (hereinafter abbreviated as PEDOT: PSS), which is commercially available as a buffer layer forming material, is also available. Can be used. Since the buffer layer formed by PEDOT / PSS lowers the contact resistance between the organic semiconductor layer and the first electrode and has an energy level of about 5 eV, holes can be efficiently collected and transferred to the electrode. Moreover, it is excellent in transparency, and the stable photoelectric conversion element which compensates the defect | defect on the electrode surface can be formed.

(Second electrode)
The second electrode constituting the photovoltaic device of the present invention is required to be a conductive material having a work function capable of ohmic contact with the formed N-type organic semiconductor layer. Specific examples of such a conductive material include metals such as gold, platinum, silver, copper, aluminum, nickel, rhodium, and indium, alloys thereof, the above translucent conductive material, and carbon.

  Examples of the method for forming the electrode used in the photoelectric conversion element of the present invention include vacuum processes such as vacuum deposition, sputtering, and CVD, or coating. The thickness of the electrode formed by these methods varies depending on the material used, but is preferably about 10 nm to 1,000 nm, for example. Note that when the thickness is less than 10 nm, the transistor element may not operate. When the thickness exceeds 1,000 nm, the manufacturing cost and material cost increase.

(Exciton block layer)
The photoelectric conversion element of the present invention needs to include an exciton block layer between the N-type organic semiconductor layer structure and the second electrode. This exciton blocking layer functions as a hole blocking layer or the like, controls the flow of carriers (holes, electrons), and allows carriers to reach the electrodes efficiently. According to the study by the present inventors, in particular, an exciton blocking layer made of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) (hereinafter abbreviated as BCP) Although the LUMO level is almost the same as that of the perylene tetracarboxylic acid derivative and the HOMO level is higher than that of the perylene tetracarboxylic acid derivative, electrons are extracted to the aluminum electrode. This is presumably because energy levels are formed in the band gap when the BCP thin film layer is formed, and electrons are transported to the aluminum electrode via the in-band levels. According to the study of the present inventors, the photoelectric conversion element of the present invention is listed below by providing the exciton block layer as described above between the N-type organic semiconductor layer structure and the second electrode. It will be an excellent one that has achieved the effect. In the photoelectric conversion element of the present invention, by forming the exciton block layer as described above, the excitons can not only efficiently reach the PN junction interface, but also suppress the diffusion of the organic semiconductor layer of the atoms forming the electrode. It will be something that can be done. Specifically, the exciton blocking layer can suppress diffusion of the second electrode material, for example, metal atoms into the organic semiconductor layer. On the other hand, a photoelectric conversion element that does not have an exciton block layer gradually decreases in conversion efficiency and cannot be used as a photoelectric conversion element. Therefore, providing an exciton block layer is an important practical requirement. .

  Examples of the method for forming the exciton block layer used in manufacturing the photoelectric conversion element of the present invention include vacuum processes such as vacuum deposition, sputtering, and CVD, or methods such as coating. The exciton blocking layer is formed by such a method, but the thickness of the exciton blocking layer varies depending on the material used, but is preferably about 0.5 nm to 50 nm, for example. In addition, when the thickness is less than 0.5 nm, defects or defects may occur in the thin film, and the sufficient effect may not be obtained. Moreover, when exceeding 50 nm, while the series resistance in a photovoltaic device increases, the performance of a photoelectric conversion element tends to fall, and the problem that manufacturing cost and material cost become high arises, and it is not preferable.

<Organic semiconductor thin film solar cell provided with photoelectric conversion element>
The photoelectric conversion element of the present invention described above can be formed into an organic semiconductor thin film solar cell by forming wiring according to a conventional method. The organic semiconductor thin film solar cell of the present invention formed in this way can absorb light from the visible light region to the near infrared region, and has excellent photoelectric conversion characteristics such as a short circuit current value and an open voltage value. Will be useful to show. This point will be described later.

  The present invention will be described in more detail with reference to examples and comparative examples. First, each material used will be described.

(Organic thin film materials)
As an organic thin film material for forming the P-type semiconductor layer, BP3T (2,5-bis (4-biphenylyl) -2,2 ′: 5 ′, 2 ″ -terthiophene) (abbreviated as BP3T) is prepared, and N as the organic thin film material for forming the type organic semiconductor layer, fullerene (C ₆₀ substantially), N, was prepared N'- diphenyl-perylenetetracarboxylic diimide (PTCDI-ph substantially). In addition, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (abbreviated as BCP) was prepared as a material for forming the exciton blocking layer. All the materials described above were purified by sublimation.

(Absorption characteristics of organic thin film materials)
The organic thin film material (BP3T, C ₆₀ , PTCDI-ph) prepared for the semiconductor layer prepared above is deposited on each quartz substrate by vacuum vapor deposition to form an organic thin film layer having a thickness of 1 μm, and UV-visible spectroscopy Ultraviolet-visible absorption characteristics were measured with a photometer. The results are shown in FIG. Although as shown in FIG. 4, the absorption band of BP3T and C ₆₀ are mainly 400nm or less, PTCDI-ph is strong light absorption characteristics in 600nm wavelength range from 400nm (absorption peak: 568 nm, 538 nm, 485 nm, 458 nm )showed that. The absorption coefficient of PTCDI-ph is 1 or more from the ultraviolet region to 620 nm, 1 or more from 410 to 590 nm, and more than 4 from 420 to 580 nm. It was confirmed that the material can produce an N-type organic semiconductor layer that is effective as a layer located on the second electrode side constituting the N-type organic semiconductor layer structure defined in the present invention and has a strong light absorption. .

(Energy level of organic thin film materials)
About each organic thin film material prepared above, the HOMO level was determined from the value measured with the photoelectron spectrometer (AC-3: Riken Keiki make). The LUMO level was calculated by the sum of the long wavelength end in the measured UV-visible absorption spectrum and the level of the HOMO level. The HOMO level and LUMO level of each organic thin film material are BP3T (HOMO: -5.28 eV, LUMO: -2.92 eV), C ₆₀ (HOMO: -6.2 eV, LUMO: -4.5 eV), respectively. , PTCDI-ph (HOMO: -6.57 eV, LUMO: -4.55 eV), BCP (HOMO: -6.7 eV, LUMO: -3.2 eV).

In FIG. 3, the energy level of each material in the layer structure of the photoelectric conversion element of the Example mentioned later was shown. In PTCDI-ph a is N-type organic semiconductor layer, an exciton generated by light absorption, caused excitons move to N-type organic semiconductor layer made of C _60, electrons in the PN junction interface consisting BP3T and C ₆₀ And charge separation into holes. P-type organic semiconductor layer made of BT3T, and excitons generated in each layer of the N-type organic semiconductor layer made of C ₆₀ is also moved to the PN junction interface, the charge separation. As the photoelectric conversion element of the present invention, HOMO of N-type organic semiconductor layer for forming the second electrode side (Al side shown in FIG. 3), the LUMO, N-type organic semiconductor layer made of C ₆₀ to be laminated therewith By making it equal to or smaller than HOMO and LUMO, charge separated electrons can flow to the second electrode without any problem.

(Element fabrication method)
A glass substrate with ITO (surface resistance value: 15Ω / square, ITO film thickness: 150 nm) was washed, and each layer and electrode were formed on the substrate to produce a photovoltaic device. Specifically, first, on a glass substrate with ITO, as a buffer layer, PEDOT: PSS [poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) trade name: Clevios P AI 4083, H.R. C. Starks] (hereinafter abbreviated as PEDOT: PSS) was formed by spin coating so as to have a film thickness of 30 nm. Further, an organic semiconductor layer and an exciton block layer are formed thereon by a vacuum deposition method (vacuum degree: <2.0 × 10 ⁻⁴ , deposition rate: 0.05 nm / s), and finally, a vacuum deposition method ( An aluminum electrode layer (film thickness: 150 nm) was formed on the exciton block layer at a deposition rate of 1 nm / s) to obtain a photoelectric conversion element. The effective area of the element was 6 mm ² .

(Evaluation of organic thin film solar cells)
Evaluation of the photoelectric power generation element was performed under an inert gas atmosphere, including the one obtained above. Specifically, artificial sunlight AM1.5G (100 mW / cm ² ) is irradiated, power generation characteristics are measured, and short circuit current value (J _SC ), open circuit voltage (V _OC ), and fill factor (FF) are evaluated. did. Moreover, the spectral sensitivity characteristic (IPCE) measured the photocurrent value by irradiating the photoelectric conversion element with monochromatic light separated by a monochromator.

<Example 1>
The film thicknesses described below are all dry film thickness or vapor deposition film thickness. The photovoltaic device of Example 1 having the structure shown in FIG. 2 was obtained by the device fabrication method described above. First, a BP3T thin film (10 nm) which is a P-type organic semiconductor layer is formed on a substrate with ITO coated with PEDOT: PSS (30 nm) as a buffer layer, and the following two layers are laminated thereon. An N-type organic semiconductor layer structure was formed. Specifically, a C ₆₀ thin film (40 nm) that is an N-type semiconductor layer made of a fullerene derivative was formed, and then a PTCDI-ph thin film (10 nm) that was an N-type organic semiconductor layer was formed. Further, a BCP thin film (6 nm) as an exciton block layer was laminated by a vacuum deposition method, and finally an aluminum electrode was formed with a thickness of 150 nm to obtain a photovoltaic device of Example 1. The characteristics of the organic semiconductor thin-film solar cell using the photoelectric conversion element of Example 1 obtained as described above are as follows: short-circuit current value (J _SC ): 2.4 mA / cm ² , open circuit voltage (V _OC ): It was 0.54 V, fill factor (FF): 0.65. The organic thin-film solar cell of Example 1 was excellent in long-term stability and had a slight decrease in photoelectric conversion characteristics during long-term use.

<Examples 2 to 4>
Example Practically the same as Example 1 except that the thickness of the PTCDI-ph thin film, which is an N-type organic semiconductor layer constituting the photovoltaic device of Example 1, was changed to 20 nm, 30 nm, and 40 nm, respectively. 2, The photoelectric conversion element of Example 3 and Example 4 was produced, respectively.

About the organic solar cell using each photoelectric conversion element obtained above, it evaluated similarly to the case of Example 1. FIG. As a result, the characteristics of the organic thin-film solar cell using the photoelectric conversion element of Example 2 were as follows: short-circuit current value (J _SC ): 2.8 mA / cm ² , open circuit voltage (V _OC ): 0.55 V, fill factor (FF): It was 0.65. The characteristics of the organic thin-film solar cell using the photoelectric conversion element of Example 3 are as follows: short-circuit current value (J _SC ): 3.4 mA / cm ² , open circuit voltage (V _OC ): 0.57 V, fill factor ( FF): 0.63. The characteristics of the organic thin-film solar cell using the photoelectric conversion element of Example 4 are as follows: short-circuit current value (J _SC ): 3.2 mA / cm ² , open circuit voltage (V _OC ): 0.56 V, fill factor ( FF): 0.60.

  FIG. 8 shows an organic thin film solar cell using the photoelectric conversion element of Example 1 described above, an organic thin film solar cell of Comparative Example 1 using a photoelectric conversion element that does not form a PTCDI-ph thin film layer, which will be described later, and the above examples. Using the photoelectric conversion characteristic values obtained in the organic thin film solar cells using 2 to 4 photoelectric conversion elements, the relationship between the thickness of the PTCDI-ph thin film which is an N-type organic semiconductor layer and the photoelectric conversion characteristics was shown. As a result, it was confirmed that the photoelectric conversion characteristics such as the short-circuit current value greatly differ depending on the thickness of the PTCDI-ph thin film. 9A shows an organic thin film solar cell using the photoelectric conversion element of Example 4 (PTCDI-ph thin film layer thickness = 40 nm), and FIG. 9B shows the photoelectric conversion element of Example 1 (PTCDI−). It is a figure which shows the standing wave calculated in 380 nm and 630 nm of the organic thin film solar cell using ph thin film layer thickness = 10nm.

<Example 5>
In the same manner as the photoelectric conversion element manufacturing method performed in Example 1, a BP3T thin film (10 nm), which is a P-type organic semiconductor layer, and an N-type semiconductor layer are formed on a substrate with ITO coated with PEDOT: PSS (30 nm). Next, a C ₆₀ thin film (10 nm), an N-type organic semiconductor layer PTCDI-ph thin film (30 nm), and an exciton blocking layer BCP thin film (6 nm) are laminated by vacuum deposition. An aluminum electrode was formed with a thickness of 150 nm to obtain a photovoltaic device of this example. Thus, the characteristic of the organic thin-film solar cell using the photoelectric conversion element of Example 5 obtained was a short circuit current value (J _SC ): 2.4 mA / cm ² .

<Comparative Example 1>
The photoelectric conversion element of Comparative Example 1 was prepared in the same manner except that the PTCDI-ph thin film layer was not formed by the photoelectric conversion element manufacturing method performed in Example 1. The characteristics of the organic thin-film solar cell using the photoelectric conversion element of Comparative Example 1 thus obtained are as follows: short-circuit current value (J _SC ): 1.9 mA / cm ² , open circuit voltage (V _OC ): 0.51 V The fill factor (FF) was 0.64, and it was confirmed that the photoelectric conversion characteristics of the solar cell using the photoelectric conversion element of the example of the present invention were superior.

<Comparative example 2>
The photoelectric conversion element of Comparative Example 2 having no P-type organic semiconductor layer was prepared in the same manner except that the BP3T thin film layer was not formed by the photoelectric conversion element manufacturing method performed in Example 3. The characteristic of the organic thin-film solar cell using the photoelectric conversion element of Comparative Example 2 obtained in this way is a short-circuit current value (J _SC ): 0.4 mA / cm ² , and the photoelectric conversion element of the example of the present invention The superiority of the photoelectric conversion characteristics of the solar cell using this was confirmed.

<Comparative Example 3>
In method for manufacturing a photoelectric conversion element was carried out in Example 5, except not forming the C ₆₀ thin layer in the same manner to prepare a photoelectric conversion element of Comparative Example 3 is not C ₆₀ thin layer. The characteristics of the organic thin-film solar cell using the photoelectric conversion element of Comparative Example 3 thus obtained are short-circuit current value (J _SC ): 0.33 mA / cm ² , and the photoelectric conversion element of the example of the present invention The superiority of the photoelectric conversion characteristics of the solar cell using this was confirmed.

<Comparative example 4>
In the method for producing a photoelectric conversion element performed in Example 1, the PTCDI-ph thin film, which is an N-type organic semiconductor layer, is changed to a dimethylperyleneperylenetetracarboxylic acid diimide (Me-PTCDI) layer, and an exciton blocking layer is further formed. A photoelectric conversion element of Comparative Example 4 having no exciton blocking layer was produced in the same manner except that it was not formed. The characteristics of the organic thin film solar cell using the photoelectric conversion element of Comparative Example 4 obtained in this manner were a short circuit current value (J _SC ): 0.5 mA / cm ² or less. From this, the superiority of the photoelectric conversion characteristics of the solar cell using the photoelectric conversion element of the example of the present invention could be confirmed.

11: Transparent substrate 12: Second electrode 21: Transparent electrode (first electrode)
31: P-type organic semiconductor layer made of electron donor (donor material) 32: N-type organic semiconductor layer made of fullerene derivative 33: N-type organic semiconductor made of organic electron acceptor having absorption in the visible light or near infrared region Layer 41: Buffer layer 42 provided as necessary: Exciton block layer

Claims (9)

A P-type organic semiconductor layer; an N-type organic semiconductor layer structure forming a PN junction surface between the P-type organic semiconductor layer; a first electrode electrically connected to the P-type organic semiconductor layer; A photoelectric conversion element having a second electrode electrically connected to the N-type organic semiconductor layer structure,
The N-type organic semiconductor layer structure has a laminated structure in which an N-type organic semiconductor layer made of a fullerene derivative and at least one N-type organic semiconductor layer having a different light absorption range from the fullerene derivative are laminated, and The layer located on the second electrode side in the laminated structure has a light absorption region in the visible light region or the near infrared region, and an N-type organic semiconductor having a strong light absorption with an absorption coefficient greater than 4 at a film thickness of 1 μm. Layer,
Furthermore, the exciton block layer is provided between the said N-type organic-semiconductor layer structure and the said 2nd electrode, The photoelectric conversion element characterized by the above-mentioned.   The HOMO level of the N-type organic semiconductor layer located on the second electrode side is equal to or lower than the HOMO level of the N-type organic semiconductor layer made of the fullerene derivative located on the PN junction surface, and located on the second electrode side. 2. The photoelectric conversion element according to claim 1, wherein the LUMO level of the N-type organic semiconductor layer is equal to or lower than the LUMO level of the N-type organic semiconductor layer made of a fullerene derivative located on the PN junction surface.   The photoelectric conversion element according to claim 1 or 2, wherein the exciton blocking layer is a layer made of BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline). The film thickness of the N-type organic semiconductor layer, which is the layer located on the second electrode side, is 5 nm to 50 nm in thickness of the N-type organic semiconductor layer made of the fullerene derivative, constituting the N-type organic semiconductor structure. 5 nm to 50 nm, the total film thickness of these layers in the N-type organic semiconductor layer structure is in the range of 20 nm to 100 nm, and the short-circuit current in pseudo sunlight (AM1.5G) 100 mW / m ² The photoelectric conversion element according to claim 1, wherein the value J _SC is 2.0 mA / cm ² or more.   The photoelectric conversion element according to claim 4, wherein the N-type organic semiconductor layer made of the fullerene derivative has a thickness of 5 nm to 15 nm.   The photoelectric conversion element according to claim 1, wherein the layer located on the second electrode side is an N-type organic semiconductor layer made of a perylene derivative. The photoelectric conversion element according to claim 6, wherein the N-type organic semiconductor layer made of the perylene derivative is made of a perylene tetracarboxylic acid diimide derivative having a conjugated cyclic structure in a side chain represented by the following formula.

A: represents an aromatic ring, a heteroaromatic ring, or a derivative thereof.
Y: represents an alkyl group or a heteroalkyl group, but Y may not be present. The photoelectric conversion element according to claim 6, wherein the N-type organic semiconductor layer made of the perylene derivative is made of N, N′-diphenylperylenetetracarboxylic acid diimide represented by the following formula.

  An organic thin-film solar cell comprising the photoelectric conversion element according to claim 1.

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