JP2010111649A - Quinoxaline compound, electron-donating organic material produced by using the same, material for photovoltaic element and photovoltaic element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photovoltaic element having high photovoltaic conversion efficiency. <P>SOLUTION: There is provided a quinoxaline compound satisfying requirements that (a) it contains a quinoxaline skeleton and an oligothiophene skeleton, (b) it has a band gap (Eg) of ≤1.8 eV and (c) the level of highest occupied molecular orbital (HOMO) is ≤-4.8 eV. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a quinoxaline compound, an electron-donating organic material using the compound, a material for a photovoltaic device, and a photovoltaic device.

  Solar cells are attracting attention as an environmentally friendly electrical energy source and an influential energy source for increasing energy problems. Currently, inorganic materials such as single crystal silicon, polycrystalline silicon, amorphous silicon, and compound semiconductors are used as semiconductor materials for photovoltaic elements of solar cells. However, solar cells manufactured using inorganic semiconductors have not been widely used in ordinary households because of high costs compared with power generation methods such as thermal power generation and nuclear power generation. The high cost factor is mainly in the process of manufacturing a semiconductor thin film under vacuum and high temperature. Therefore, organic solar cells using organic semiconductors and organic dyes such as conjugated polymers and organic crystals are being studied as semiconductor materials expected to simplify the manufacturing process.

  However, an organic solar cell using a conjugated polymer or the like has the biggest problem that the photoelectric conversion efficiency is lower than that of a conventional solar cell using an inorganic semiconductor, and has not yet been put into practical use. The photoelectric conversion efficiency of organic solar cells using conventional conjugated polymers is mainly due to the low solar absorption efficiency and the excitons that are difficult to separate the electrons and holes generated by sunlight. This is because a state is formed and a trap for trapping carriers (electrons and holes) is easily formed, so that the generated carriers are easily trapped in the trap and the mobility of carriers is slow.

  Conventional photoelectric conversion elements using organic semiconductors can be generally classified into the following element configurations at present. A Schottky type that joins an electron-donating organic material (p-type organic semiconductor) and a metal having a low work function, and an electron-accepting organic material (n-type organic semiconductor) and an electron-donating organic material (p-type organic semiconductor). Heterojunction type. In these elements, only the organic layer (about several molecular layers) at the junction contributes to the photocurrent generation, so that the photoelectric conversion efficiency is low, and its improvement is a problem.

As a method for improving photoelectric conversion efficiency, a bulk heterojunction in which an electron-accepting organic material (n-type organic semiconductor) and an electron-donating organic material (p-type organic semiconductor) are mixed to increase the bonding surface contributing to photoelectric conversion There is a type (for example, see Non-Patent Document 1). Among them, the conjugated polymer used as the electron donating organic material (p-type organic semiconductor), a fullerene or a carbon nanotube, such as other C ₆₀ of the conductive polymer having the semiconductor characteristics of the n-type as the electron accepting organic material The used photoelectric conversion material is reported (for example, refer nonpatent literatures 2-3 and patent documents 1-2).

In addition, in order to efficiently absorb radiant energy over a wide range of the solar spectrum, a photoelectric conversion material using an organic semiconductor in which an electron-donating group and an electron-withdrawing group are introduced into the main chain to reduce the band gap has been reported. (For example, refer nonpatent literature 4). As this electron-donating group, a thiophene skeleton has been studied vigorously, and as an electron-withdrawing group, a benzothiadiazole skeleton and a quinoxaline skeleton have been vigorously studied (see, for example, Non-Patent Document 5 and Patent Documents 3 to 6). However, sufficient photoelectric conversion efficiency has not been obtained.
J. et al. J. et al. M.M. Halls, C.I. A. Walsh, N .; C. Greenham, E.I. A. Marsegla, R.M. H. Frind, S.M. C. Moratti, A.M. B. Homes, "Nature", 1995, 376, 498 E. Kymakis, G .; A. J. et al. Amaruntunga, "Applied Physics Letters" (USA), 2002, 80, 112. G. Yu, J. et al. Gao, J .; C. Hummelen, F.M. Wudl, A.W. J. et al. Heeger, "Science", 1995, 270, 1789 E. Bundgaard, F.M. C. Krebs, "Solar Energy Materials & Solar Cells", 2007, 91, 954. X. Li, W. Zeng, Y. et al. Zhang, Q.H. Hou, W.H. Yang, Y. et al. Cao, “European Polymer Journal”, 2005, 41, 2923 JP 2003-347565 A (Claims 1 and 3) JP 2004-165474 A (Claims 1 and 3) JP-T-2006-502981 (Claim 1) JP 2003-104976 A (Claims 1 and 9) JP 2006-222429 A (Claim 7) JP 2004-534863 A (Claim 1)

  As described above, all of the conventional organic solar cells have a problem of low photoelectric conversion efficiency. An object of this invention is to provide a photovoltaic device with high photoelectric conversion efficiency.

  That is, the present invention includes (a) a quinoxaline skeleton and an oligothiophene skeleton, (b) a band gap (Eg) of 1.8 eV or less, and (c) a highest occupied molecular orbital (HOMO) level of −4. A quinoxaline compound having a voltage of 0.8 eV or less, an electron donating organic material, a material for a photovoltaic device, and a photovoltaic device using the same.

  ADVANTAGE OF THE INVENTION According to this invention, the photovoltaic device with high photoelectric conversion efficiency which can make high short circuit current density (Isc) and high open circuit voltage (Voc) compatible can be provided.

The quinoxaline compound of the present invention includes (a) a quinoxaline skeleton and an oligothiophene skeleton, (b) a band gap (Eg) of 1.8 eV or less, and (c) a highest occupied molecular orbital (HOMO) level. It is -4.8 eV or less. Hereinafter, the quinoxaline compound of the present invention will be described.

  The quinoxaline compound of the present invention includes (a) a quinoxaline skeleton and an oligothiophene skeleton. Here, the oligothiophene skeleton refers to a skeleton in which two or more thiophene rings are linked by a conjugated bond. By having both a quinoxaline skeleton and an oligothiophene skeleton, (1) a deep highest occupied molecular orbital (HOMO) level, (2) a wide absorption wavelength region, and (3) a high carrier mobility are compatible at a high level. It becomes possible. In particular, when used as a material for a photovoltaic device, in order to obtain a high short-circuit current (Isc), a wider light absorption wavelength region is preferable. From this viewpoint, the number of thiophene rings constituting the oligothiophene skeleton is 3 The number is preferably at least 4, and more preferably at least 4. On the other hand, in order to obtain a high open circuit voltage (Voc) in a photovoltaic device, it is effective to deepen the highest occupied molecular orbital (HOMO) level of the electron donating organic material. The number of thiophene rings constituting the skeleton is preferably 12 or less, and more preferably 8 or less. In order to widen the light absorption wavelength region and at the same time secure a high carrier transport capability as an organic semiconductor, this thiophene ring is located at positions 2 and 5 (carbon adjacent to the sulfur atom contained in the thiophene ring). It is preferable.

  The quinoxaline compound of the present invention has (b) a band gap (Eg) of 1.8 eV or less. The band gap is the energy difference between the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level. By setting this to 1.8 eV or less, the light absorption efficiency is increased. Can do. In particular, when used as a material for a photovoltaic device, if the band gap exceeds 1.8 eV, the solar light absorption efficiency decreases, and thus the photoelectric conversion efficiency decreases. On the other hand, if the band gap is too small, the open circuit voltage (Voc) may be lowered. From this viewpoint, the band gap is preferably 1.8 eV or less and 1.2 eV or more, and 1.8 eV or less and 1.4 eV or more. More preferably. Here, the band gap of the quinoxaline compound is obtained from the light absorption edge of the thin film formed from the quinoxaline compound using the following formula.

Eg (eV) = 1240 / light absorption edge wavelength of thin film (nm)
The method for forming the thin film used here is not particularly limited. Usually, the thin film is dissolved in an organic solvent such as chloroform, tetrahydrofuran, or chlorobenzene, and the thin film is formed on the glass substrate by a wet coating method such as spin coating. The thickness of the thin film is preferably in the range of 5 nm to 500 nm, preferably in the range of 5 nm to 500 nm because the absorbance is too low if it is too thin, and the absorbance may be too high if it is too thick. More preferably.

  In order to reduce the band gap to 1.8 eV or less, it is easy to cause intramolecular charge transfer (CT), that is, the quinoxaline skeleton that is an electron-withdrawing group and the oligothiophene skeleton that is an electron-donating group are alternately arranged. It is effective to do. In addition, minimizing the twisting between the thiophene rings constituting the oligothiophene is also effective in reducing the band gap to 1.8 eV or less. For this purpose, it is effective that at least one of two adjacent substituents of two adjacent thiophene rings is a hydrogen atom having a small steric hindrance.

  Further, the quinoxaline compound of the present invention is effective to deepen the highest occupied molecular orbital (HOMO) level in order to obtain a high open circuit voltage (Voc) as described above. Specifically, (c) As described above, it is important that the voltage be −4.8 eV or less. When the highest occupied molecular orbital (HOMO) level exceeds −4.8 eV, the open-circuit voltage (Voc) decreases, and the photoelectric conversion efficiency decreases. On the other hand, if the highest occupied molecular orbital (HOMO) level is too low, electrons and holes generated by exciton charge separation may recombine and disappear on the electron-accepting organic material side. Therefore, the highest occupied molecular orbital (HOMO) level is preferably −4.8 eV or lower and −6.1 eV or higher, and more preferably −4.8 eV or lower and −5.5 eV or higher. Here, the highest occupied molecular orbital (HOMO) level of the quinoxaline compound is obtained by forming a thin film from the quinoxaline compound and measuring the photoelectron spectrum. The method for forming the thin film used here is not particularly limited, but the above-described method can be usually used. If the thickness of the thin film is too thin, it is affected by the underlying substrate, and if it is too thick, the film thickness may be uneven and may interfere with the measurement. Therefore, it is usually preferably in the range of 5 nm to 500 nm. More preferably, it is in the range of 10 nm to 200 nm. In addition, -4.8 eV or less shows that the absolute value is 4.8 or more like -4.9 eV and -5.0 eV.

  In order to reduce the HOMO to −4.8 eV or less, it is important that both the number of oligothiophene skeletons which are electron donating groups and the number of thiophene rings contained in the oligothiophene skeleton are not too many. Specifically, the ratio of the quinoxaline skeleton that is an electron-withdrawing group to the oligothiophene skeleton that is an electron-donating group is preferably 1: 1 to 1: 2, and is included in one oligothiophene skeleton. The number of thiophene rings is preferably 12 or less.

  As the quinoxaline compound having the above-described characteristics, a compound having a structure represented by the following general formula (1) is preferable. Compared to the case where the quinoxaline skeleton and the oligothiophene skeleton are randomly arranged, the light absorption efficiency in the long wavelength region is improved by arranging the oligothiophene skeleton-quinoxaline skeleton-oligothiophene skeleton regularly as shown in the formula (1). improves.

R ^{1 to} R ²⁰ may be the same or different and are selected from hydrogen, an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and a halogen. A represents a divalent arylene group having a 6-membered ring structure or a divalent heteroarylene group having a 6-membered ring structure. m is 0 or 1. n represents a range of 1 to 1000.

  Here, the alkyl group is, for example, a saturated aliphatic hydrocarbon such as methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, decyl group, undecyl group, dodecyl group. The group may be linear, branched or cyclic, and may be unsubstituted or substituted. Examples of the substituent when substituted include the following alkoxy groups, aryl groups, heteroaryl groups, and halogens. The number of carbon atoms in the alkyl group is preferably 2 or more from the viewpoint of processability, and is preferably 20 or less in order to further increase the light absorption efficiency. The alkoxy group refers to an aliphatic hydrocarbon group through an ether bond such as a methoxy group, an ethoxy group, a propoxy group, or a butoxy group, and the aliphatic hydrocarbon group may be unsubstituted or substituted. The number of carbon atoms of the alkoxy group is preferably 2 or more from the viewpoint of processability, and is preferably 20 or less in order to further increase the light absorption efficiency. Examples of the substituent when substituted include the following aryl groups, heteroaryl groups, and halogens. The aryl group is an aromatic hydrocarbon group such as a phenyl group, a naphthyl group, a biphenyl group, a phenanthryl group, an anthryl group, a terphenyl group, a pyrenyl group, a fluorenyl group, and a perylenyl group, which is substituted even if it is unsubstituted. It may be done. The number of carbon atoms of the aryl group is preferably 6 or more from the viewpoint of processability. Moreover, 30 or less are preferable. Examples of the substituent in the case of substitution include the above alkyl group, the following heteroaryl group, and halogen. The heteroaryl group is, for example, thienyl group, furyl group, pyrrolyl group, imidazolyl group, pyrazolyl group, oxazolyl group, pyridyl group, pyrazyl group, pyrimidyl group, quinolinyl group, isoquinolyl group, quinoxalyl group, acridinyl group, indolyl group. , Carbazolyl group, benzofuran group, dibenzofuran group, benzothiophene group, dibenzothiophene group, benzodithiophene group, silole group, benzosilol group, dibenzosilole group, etc. It can be replaced or replaced. Examples of the substituent in the case of substitution include the above alkyl group, aryl group, and the following halogen. Halogen is any one of fluorine, chlorine, bromine and iodine.

In addition, the divalent arylene group having a 6-membered ring structure represents a divalent (two bonding sites) aromatic hydrocarbon group, which may be unsubstituted or substituted. Examples of the substituent in the case of substitution include the above alkyl group, heteroaryl group, and halogen. Preferable specific examples of the divalent arylene group having a 6-membered ring structure include divalent groups such as a phenylene group, a naphthylene group, a biphenylene group, a phenanthrylene group, an anthrylene group, a terphenylene group, a pyrenylene group, a fluorenylene group, and a peryleneylene group. Groups. Moreover, the divalent heteroarylene group having a 6-membered ring structure represents a divalent heteroaromatic ring group, which may be unsubstituted or substituted. Preferable specific examples of the divalent heteroarylene group having a 6-membered ring structure include a pyridylene group, a pyrazylene group, a quinolinylene group, an isoquinolylene group, a quinoxalylene group, an acridinylene group, an indolenylene group, a carbazolylene group, a benzofuran group, a dibenzofuran group, Examples thereof include a divalent heteroaromatic ring group such as a benzothiophene group, a dibenzothiophene group, a benzodithiophene group, a benzosilole group, and a dibenzosilole group. Among these, a fluorenylene group and a dibenzosilole group are preferable from the viewpoint of ensuring solubility because two soluble substituents such as an alkyl group can be easily introduced. R ^{1 to} R ¹⁸ in the compound represented by the general formula (1) are preferably any one of hydrogen, an alkyl group, and an alkoxy group from the viewpoint of ensuring ease of synthesis and solubility. R ¹⁹ and R ²⁰ are preferably an alkyl group or an aryl group from the viewpoint of ease of synthesis and stability of the compound.

N in the general formula (1) indicates the degree of polymerization of the quinoxaline compound and is in the range of 1 to 1000. In view of ease of film formation, the quinoxaline compound is preferably soluble in a solvent. From this viewpoint, n is preferably about 1 or more and 500 or less. The degree of polymerization can be determined from the weight average molecular weight. The weight average molecular weight can be determined by measuring using GPC (gel permeation chromatography) and converting to a polystyrene standard sample. Moreover, in order to make it soluble in a solvent as mentioned above, it is preferable that at least one of R ^{1 to} R ²⁰ is an alkyl group or an alkoxy group. When the molecular weight of the quinoxaline compound is small, if the number of alkyl groups or alkoxy groups is too large, the glass transition temperature and the melting point are lowered and the heat resistance of the thin film is lowered. On the contrary, when the molecular weight of the quinoxaline compound is large, if the number of alkyl groups or alkoxy groups is too small, the solubility is lowered and it is difficult to form a thin film. Therefore, when n in the general formula (1) is less than 10 1 or more, more preferably an alkyl group or an alkoxy group of R ¹ to R ²⁰ is 8 or less 1 or more, the general formula (1) In the case where n is 10 or more, it is more preferable that R ^{1 to} R ²⁰ have 4 or more alkyl groups or alkoxy groups.

  M in the general formula (1) is 0 or 1, but when n is 2 or more, m = 1 is preferable. This is because even if A in the general formula (1) has a 6-membered ring structure, the main chain is twisted due to its steric hindrance so that conjugation is not easily expanded. This is because the highest occupied molecular orbital (HOMO) level can be kept deep.

  Examples of the quinoxaline compound having the structure represented by the general formula (1) include the following structures. In the following structure, n is in the range of 1 to 1000.

  The quinoxaline compound having a structure represented by the general formula (1) is, for example, Macromolecules 2000, 33, 9277-9288, Journal of American Chemical Society (Journal). of American Chemical Society) 2006, 128, 10992-10993. For example, 5,5′-dibromo-2,2′-bithiophene and a 3-alkylthiophene-2-boronic acid derivative are reacted by a Suzuki coupling method using a palladium catalyst, and the resulting quarterthiophene compound is lithiated. And boronate esterification, and the resulting product and dibromodiphenylquinoxaline are further reacted by the Suzuki coupling method using a palladium catalyst.

The electron donating organic material of the present invention contains the quinoxaline compound of the present invention. It may consist only of the quinoxaline compound or may contain other compounds. Examples of other compounds include polythiophene polymers, poly-p-phenylene vinylene polymers, poly-p-phenylene polymers, polyfluorene polymers, polypyrrole polymers, polyaniline polymers, polyacetylene polymers. Conjugated polymers such as polymers, polythienylene vinylene polymers, phthalocyanine derivatives such as H ₂ phthalocyanine (H ₂ Pc), copper phthalocyanine (CuPc), zinc phthalocyanine (ZnPc), porphyrin derivatives, N, N′— Diphenyl-N, N′-di (3-methylphenyl) -4,4′-diphenyl-1,1′-diamine (TPD), N, N′-dinaphthyl-N, N′-diphenyl-4,4 ′ -Triarylamine derivatives such as diphenyl-1,1'-diamine (NPD), 4,4'-di (carbazol-9-yl) bif Carbazole derivatives such as sulfonyl (CBP), oligothiophene derivatives (terthiophene, quarter thiophene, sexithiophene, octyl thiophene etc.) and the low-molecular organic compound such.

  The electron-donating organic material of the present invention can be used for photovoltaic device materials, organic electroluminescent device materials, organic transistor materials, wavelength conversion device materials, organic laser materials, and the like. It can be preferably used as an element material.

  Since the quinoxaline compound of the present invention exhibits an electron donating property (p-type semiconductor characteristics), it must be combined with an electron-accepting organic material (n-type organic semiconductor) to obtain higher photoelectric conversion efficiency when used in a photovoltaic device. Is preferred. The material for a photovoltaic device of the present invention includes an electron donating organic material containing the quinoxaline compound and an electron accepting organic material.

The electron-accepting organic material used in the present invention is an organic material exhibiting n-type semiconductor characteristics. For example, 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), 3,4,9,10- Perylenetetracarboxylic dianhydride (PTCDA), 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI), N, N′-dioctyl-3,4,9,10-naphthyltetracarboxydiimide (PTCDI) -C8H), 2- (4-biphenylyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (PBD), 2,5-di (1-naphthyl) -1,3 , 4-oxadiazole (BND) and other oxazole derivatives, 3- (4-biphenylyl) -4-phenyl-5- (4-t- Triazole derivatives such as butylphenyl) -1,2,4-triazole (TAZ), phenanthroline derivatives, phosphine oxide derivatives, fullerene compounds (C ₆₀ , C ₇₀ , C ₇₆ , C ₇₈ , C ₈₂ , C ₈₄ , C ₉₀ , as unsubstituted, including C _94, [6,6] - phenyl C61 butyric acid methyl ester ([6,6] -PCBM), [ 5,6] - phenyl C61 butyric acid methyl ester ([ 5,6] -PCBM), [6,6] -phenyl C61 butyric acid hexyl ester ([6,6] -PCBH), [6,6] -phenyl C61 butyric acid dodecyl ester ([6,6] -PCBD), phenyl C71 butyric acid methyl ester (PC ₇₀ BM), phenyl C85 butyric acid methyl ester (PC ₈₄ BM, etc.), carbon nanotubes (CNT), and derivatives in which a cyano group is introduced into a poly-p-phenylene vinylene polymer (CN-PPV). Among these, fullerene compounds are preferably used because of their high charge separation speed and electron transfer speed. Among the fullerene compounds, C ₇₀ derivatives (such as the PC 70 _BM) because excellent light absorption characteristics, it is possible to obtain a relatively high photoelectric conversion efficiency, more preferred.

  In the photovoltaic device material of the present invention, the content ratio (weight fraction) of the electron-donating organic material and the electron-accepting organic material of the present invention is not particularly limited, but an electron-donating organic material: an electron-accepting organic material The weight fraction is preferably in the range of 1 to 99:99 to 1, more preferably in the range of 10 to 90:90 to 10, still more preferably in the range of 20 to 60:80 to 40. . The electron donating organic material and the electron accepting organic material of the present invention are preferably used as a mixture. Although it does not specifically limit as a mixing method, After adding to a solvent in a desired ratio, the method of making it melt | dissolve in a solvent combining 1 type or multiple types of methods, such as a heating, stirring, and ultrasonic irradiation, is mentioned. . As will be described later, when the photovoltaic element material forms a single organic semiconductor layer, the above-described content ratios of the electron-donating organic material and the electron-accepting organic material of the present invention contained in the single layer are included. When the organic semiconductor layer has a laminated structure of two or more layers, it means the content ratio of the electron donating organic material and the electron accepting organic material of the present invention in the whole organic semiconductor layer.

  In order to further improve the photoelectric conversion efficiency, it is preferable to remove impurities that can trap carriers as much as possible. In the present invention, the method for removing impurities of the above-described quinoxaline compound, the electron-donating organic material containing the compound, and the electron-accepting organic material is not particularly limited, but column chromatography, recrystallization, sublimation, reprecipitation Method, Soxhlet extraction method, molecular weight fractionation method by GPC (gel permeation chromatography), filtration method, ion exchange method, chelate method and the like can be used. In general, a column chromatography method, a recrystallization method, and a sublimation method are preferably used for purification of a low molecular weight organic material. On the other hand, for the purification of high molecular weight compounds, when removing low molecular weight components, reprecipitation, Soxhlet extraction, or molecular weight fractionation by GPC (gel permeation chromatography) is preferably used to remove metal components. In such a case, a reprecipitation method, a chelate method, or an ion exchange method is preferably used. A plurality of these methods may be combined.

  Next, the photovoltaic element of the present invention will be described. The photovoltaic device of the present invention has at least a positive electrode and a negative electrode, and includes the photovoltaic device material of the present invention between them. FIG. 1 is a schematic view showing an example of the photovoltaic element of the present invention. In FIG. 1, reference numeral 1 is a substrate, reference numeral 2 is a positive electrode, reference numeral 3 is an organic semiconductor layer containing the photovoltaic element material of the present invention, and reference numeral 4 is a negative electrode.

  The organic semiconductor layer 3 contains the photovoltaic element material of the present invention. That is, the electron donating organic material and the electron accepting organic material of the present invention are included. These materials may be mixed or laminated. When mixed, the electron-donating organic material and the electron-accepting organic material of the present invention are compatible or phase-separated at the molecular level. The domain size of this phase separation structure is not particularly limited, but is usually 1 nm or more and 50 nm or less. When laminated, the layer having an electron-donating organic material exhibiting p-type semiconductor characteristics is preferably on the positive electrode side, and the layer having an electron-accepting organic material exhibiting n-type semiconductor characteristics is preferably on the negative electrode side. An example of the photovoltaic element in the case where the organic semiconductor layer 3 is laminated is shown in FIG. Reference numeral 5 denotes a layer having a quinoxaline compound having a structure represented by the general formula (1), and reference numeral 6 denotes a layer having an electron-accepting organic material. The organic semiconductor layer preferably has a thickness of 5 nm to 500 nm, more preferably 30 nm to 300 nm. When laminated, the layer having the electron donating organic material of the present invention preferably has a thickness of 1 nm to 400 nm, more preferably 15 nm to 150 nm.

  In addition, the organic semiconductor layer 3 may contain an electron-donating organic material (p-type organic semiconductor) other than the quinoxaline compound of the present invention and an electron-accepting organic material. Examples of the electron donating organic material (p-type organic semiconductor) used here include those exemplified above as other compounds of the electron donating organic material.

In the photovoltaic device of the present invention, it is preferable that either the positive electrode 2 or the negative electrode 4 has light transmittance. The light transmittance of the electrode is not particularly limited as long as incident light reaches the organic semiconductor layer 3 and an electromotive force is generated. Here, the light permeability in the present invention is a value determined by [transmitted light intensity (W ^{/ m} 2) ^/ incident light intensity ^{(W / m 2)] ×} 100 (%). The thickness of the electrode may be in a range having light transparency and conductivity, and is preferably 20 nm to 300 nm, although it varies depending on the electrode material. The other electrode is not necessarily light-transmitting as long as it has conductivity, and the thickness is not particularly limited.

  As an electrode material, it is preferable to use a conductive material having a high work function for one electrode and a conductive material having a low work function for the other electrode. An electrode using a conductive material having a large work function is a positive electrode. Conductive materials with a large work function include metals such as gold, platinum, chromium and nickel, transparent metal oxides such as indium and tin, composite metal oxides (indium tin oxide (ITO), indium Zinc oxide (IZO) or the like is preferably used. Here, the conductive material used for the positive electrode 2 is preferably one that is in ohmic contact with the organic semiconductor layer 3. Furthermore, when a hole transport layer described later is used, it is preferable that the conductive material used for the positive electrode 2 is in ohmic contact with the hole transport layer.

  An electrode using a conductive material having a low work function serves as a negative electrode. As the conductive material having a low work function, alkali metal or alkaline earth metal, specifically, lithium, magnesium, or calcium is used. Tin, silver, and aluminum are also preferably used. Furthermore, an electrode made of an alloy made of the above metal or a laminate of the above metal is also preferably used. Further, it is possible to improve the extraction current by introducing a metal fluoride such as lithium fluoride or cesium fluoride into the interface between the negative electrode 4 and the electron transport layer. Here, the conductive material used for the negative electrode 4 is preferably one that is in ohmic contact with the organic semiconductor layer 3. Furthermore, when an electron transport layer described later is used, it is preferable that the conductive material used for the negative electrode 4 is in ohmic contact with the electron transport layer.

  The substrate 1 is a substrate on which an electrode material and an organic semiconductor layer can be laminated according to the type and application of the photoelectric conversion material, for example, inorganic materials such as alkali-free glass and quartz glass, polyester, polycarbonate, polyolefin, polyamide, polyimide, polyphenylene A film or plate produced by an arbitrary method from an organic material such as sulfide, polyparaxylene, epoxy resin or fluorine resin can be used. When light is incident from the substrate side and used, it is preferable that each substrate described above has a light transmittance of about 80%.

In the present invention, a hole transport layer may be provided between the positive electrode 2 and the organic semiconductor layer 3. As a material for forming the hole transport layer, polythiophene-based polymers, poly -p- phenylene vinylene-based polymer, or a conductive polymer such as polyfluorene-based polymer, a phthalocyanine derivative (H 2 _Pc, CuPc, ZnPc, etc. ), Low molecular organic compounds exhibiting p-type semiconductor properties such as porphyrin derivatives are 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 is preferably 5 nm to 600 nm, more preferably 30 nm to 200 nm.

  In the photovoltaic device of the present invention, an electron transport layer may be provided between the organic semiconductor layer 3 and the negative electrode 4. The material for forming the electron transport layer is not particularly limited, but the above-described electron-accepting organic materials (NTCDA, PTCDA, PTCDI-C8H, oxazole derivatives, triazole derivatives, phenanthroline derivatives, phosphine oxide derivatives, fullerene compounds, An organic material exhibiting n-type semiconductor characteristics such as CNT and CN-PPV is preferably used. The thickness of the electron transport layer is preferably 5 nm to 600 nm, more preferably 30 nm to 200 nm.

  In the photovoltaic device of the present invention, two or more organic semiconductor layers may be stacked (tandemized) via one or more intermediate electrodes to form a series junction. For example, a laminated structure of substrate / positive electrode / first organic semiconductor layer / intermediate electrode / second organic semiconductor layer / negative electrode can be given. By laminating in this way, the open circuit voltage can be improved. Note that the hole transport layer described above may be provided between the positive electrode and the first organic semiconductor layer and between the intermediate electrode and the second organic semiconductor layer, and between the first organic semiconductor layer and the intermediate electrode. The hole transport layer described above may be provided between the second organic semiconductor layer and the negative electrode.

In the case of such a laminated structure, at least one layer of the organic semiconductor layer contains the photovoltaic device material of the present invention, and the other layer does not reduce the short-circuit current. It preferably contains an electron donating organic material having a different band gap. Examples of such an electron-donating organic material include the polythiophene polymer, poly-p-phenylene vinylene polymer, poly-p-phenylene polymer, polyfluorene polymer, polypyrrole polymer, polyaniline described above. Conjugated polymers such as polymer, polyacetylene polymer, polythienylene vinylene polymer, phthalocyanine derivatives such as H ₂ phthalocyanine (H ₂ Pc), copper phthalocyanine (CuPc), zinc phthalocyanine (ZnPc), porphyrin Derivatives, N, N′-diphenyl-N, N′-di (3-methylphenyl) -4,4′-diphenyl-1,1′-diamine (TPD), N, N′-dinaphthyl-N, N ′ -Triarylamine derivatives such as diphenyl-4,4'-diphenyl-1,1'-diamine (NPD), 4,4'-di (cal Tetrazole-9-yl) carbazole derivatives such as biphenyl (CBP), oligothiophene derivatives (terthiophene, quarter thiophene, sexithiophene include low molecular organic compound of octyl thiophene etc.) and the like. In addition, the material for the intermediate electrode used here is preferably a material having high conductivity, for example, the above-mentioned metals such as gold, platinum, chromium, nickel, lithium, magnesium, calcium, tin, silver, aluminum, and transparent Metal oxides such as indium and tin, composite metal oxides (indium tin oxide (ITO), indium zinc oxide (IZO), etc.), alloys composed of the above metals and laminates of the above metals, polyethylene Examples include dioxythiophene (PEDOT) and those obtained by adding polystyrene sulfonate (PSS) to PEDOT. The intermediate electrode preferably has a light transmission property, but even a material such as a metal having a low light transmission property can often ensure a sufficient light transmission property by reducing the film thickness.

  Next, the manufacturing method of the photovoltaic element of this invention is demonstrated. A transparent electrode such as ITO (corresponding to a positive electrode in this case) is formed on the substrate by sputtering or the like. Next, the photoelectric conversion element material including the quinoxaline compound of the present invention and the electron-accepting organic material is dissolved in a solvent to form a solution, which is applied on the transparent electrode to form an organic semiconductor layer. The solvent used at this time is preferably an organic solvent. For example, methanol, ethanol, butanol, toluene, xylene, o-chlorophenol, acetone, ethyl acetate, ethylene glycol, tetrahydrofuran, dichloromethane, chloroform, dichloroethane, chlorobenzene, dichlorobenzene, Examples include chlorobenzene, chloronaphthalene, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, and γ-butyrolactone. Two or more of these may be used. Moreover, the photoelectric conversion efficiency can be further improved by containing a fluorous solvent (an organic solvent having one or more fluorine atoms in the molecule). Examples of such a fluorous solvent include benzotrifluoride, hexafluorobenzene, 1,1,1,3,3,3-hexafluoro-2-propanol, perfluorotoluene, perfluorodecalin and the like. More preferably, benzotrifluoride is used. The content of the fluorous solvent is preferably 0.01 to 20% by volume, more preferably 0.1 to 5% by volume with respect to the total amount of solvent.

  When an organic semiconductor layer is formed by mixing the electron-donating organic material and the electron-accepting organic material of the present invention, the electron-donating organic material and the electron-accepting organic material of the present invention are added to the solvent in a desired ratio. Then, the solution is dissolved using a method such as heating, stirring, and ultrasonic irradiation to form a solution, which is applied on the transparent electrode. In this case, the photoelectric conversion efficiency of the photovoltaic element can be improved by using a mixture of two or more solvents. This is presumably because the electron-donating organic material and the electron-accepting organic material undergo phase separation at the nano level, and a carrier path that forms a path for electrons and holes is formed. As the solvent to be combined, an optimal combination type can be selected depending on the types of the electron donating organic material and the electron accepting organic material to be used. When the electron donating organic material of the present invention is used, chloroform and chlorobenzene are mentioned as preferable solvents to be combined among the above. In this case, the mixing volume ratio of each solvent is preferably in the range of chloroform: chlorobenzene = 5: 95 to 95: 5, more preferably in the range of chloroform: chlorobenzene = 10: 90 to 90:10. When the organic semiconductor layer is formed by laminating the electron-donating organic material and the electron-accepting organic material of the present invention, for example, the electron-donating organic material is applied by applying a solution of the electron-donating organic material of the present invention. After forming the layer, a solution of the electron-accepting organic material is applied to form the layer. Here, when the electron-donating organic material and the electron-accepting organic material of the present invention are low molecular weight substances having a molecular weight of about 1000 or less, it is also possible to form a layer using a vapor deposition method.

  For organic semiconductor layer formation, spin coating, blade coating, slit die coating, screen printing coating, bar coater coating, mold coating, printing transfer method, dip pulling method, ink jet method, spray method, vacuum deposition method, etc. This method may be used, and the formation method may be selected according to the characteristics of the organic semiconductor layer to be obtained, such as film thickness control and orientation control. For example, when spin coating is performed, the electron donating organic material of the present invention and the electron accepting organic material have a concentration of 1 to 20 g / l (the electron donating organic material, the electron accepting organic material and the solvent of the present invention). The weight of the electron-donating organic material and the electron-accepting organic material of the present invention with respect to the volume of the solution containing selenium is preferable, and a homogeneous organic semiconductor layer having a thickness of 5 to 200 nm can be obtained by using this concentration. Can do. In order to remove the solvent, the formed organic semiconductor layer may be subjected to an annealing treatment under reduced pressure or in an inert atmosphere (in a nitrogen or argon atmosphere). A preferable temperature for the annealing treatment is 40 ° C to 300 ° C, more preferably 50 ° C to 200 ° C. Further, by performing the annealing process, the effective area where the stacked layers permeate and contact each other at the interface increases, and the short-circuit current can be increased. This annealing treatment may be performed after the formation of the negative electrode.

  Next, a metal electrode such as Al (corresponding to a negative electrode in this case) is formed on the organic semiconductor layer by vacuum deposition or sputtering. When the metal electrode is vacuum-deposited using a low molecular organic material for the electron transport layer, it is preferable that the metal electrode is continuously formed while maintaining the vacuum.

  When a hole transport layer is provided between the positive electrode and the organic semiconductor layer, a desired p-type organic semiconductor material (such as PEDOT) is applied on the positive electrode by spin coating, bar coating, blade casting, or the like. Then, the solvent is removed using a vacuum thermostat or a hot plate to form a hole transport layer. In the case of using a low molecular organic material such as a phthalocyanine derivative or a porphyrin derivative, it is also possible to apply a vacuum vapor deposition method using a vacuum vapor deposition machine.

When an electron transport layer is provided between the organic semiconductor layer and the negative electrode, a desired n-type organic semiconductor material (fullerene derivative, etc.) is applied onto the organic semiconductor layer by spin coating, bar coating, blade casting, or spraying. After coating with, for example, a solvent is removed using a vacuum thermostat or a hot plate to form an electron transport layer. When using a low molecular organic material such as a phenanthroline derivative or C _60, it is also possible to apply a vacuum deposition method using a vacuum deposition machine.

  The photovoltaic element of the present invention can be applied to various photoelectric conversion devices using a photoelectric conversion function, an optical rectification function, and the like. For example, it is useful for photovoltaic cells (such as solar cells), electronic elements (such as optical sensors, optical switches, and phototransistors), optical recording materials (such as optical memories), and the like.

Hereinafter, the present invention will be described more specifically based on examples. In addition, this invention is not limited to the following Example. Of the compounds used in the examples and the like, those using abbreviations are shown below.
ITO: indium tin oxide PEDOT: polyethylene dioxythiophene PSS: polystyrene sulfonate PC ₇₀ BM: phenyl C71 butyric acid methyl ester Eg: band gap HOMO: highest occupied molecular orbital Isc: short circuit current density Voc: open circuit voltage FF: fill Factor η: Photoelectric conversion efficiency For the ¹ H-NMR measurement, an FT-NMR apparatus (JEOL JNM-EX270 manufactured by JEOL Ltd.) was used. Moreover, the average molecular weight (number average molecular weight, weight average molecular weight) was calculated by an absolute calibration curve method using a GPC apparatus (manufactured by TOSOH Co., Ltd., to which chloroform was fed, high-speed GPC apparatus HLC-8220 GPC). The degree of polymerization n was calculated by the following formula.
Degree of polymerization n = [(weight average molecular weight) / (molecular weight of one monomer unit)]
The light absorption edge wavelength is an ultraviolet-visible absorption spectrum (measurement wavelength) of a thin film formed on glass with a thickness of about 60 nm using a U-3010 spectrophotometer manufactured by Hitachi, Ltd. (Range: 300-900 nm). The band gap (Eg) was calculated from the light absorption edge wavelength by the above formula. The thin film was formed by spin coating using chloroform as a solvent.

  In addition, the highest occupied molecular orbital (HOMO) level was determined by using a surface analyzer (in-air ultraviolet photoelectron spectrometer AC-1 type, manufactured by Riken Kikai Co., Ltd.) for a thin film formed on ITO glass to a thickness of about 60 nm. ). The thin film was formed by spin coating using chloroform as a solvent.

Synthesis example 1
Compound A-1 was synthesized by the method shown in Formula 1.

  Compound (1-a) (manufactured by Tokyo Chemical Industry Co., Ltd.) 4.3 g and bromine (manufactured by Wako Pure Chemical Industries, Ltd.) 10 g are added to 150 ml of 48% hydrobromic acid (manufactured by Wako Pure Chemical Industries, Ltd.). In addition, the mixture was stirred at 120 ° C. for 3 hours. After cooling to room temperature, the precipitated solid was filtered through a glass filter and washed with 1000 ml of water and 100 ml of acetone. The obtained solid was vacuum-dried at 60 ° C. to obtain 6.72 g of compound (1-b).

5.56 g of the above compound (1-b) is added to 180 ml of ethanol (manufactured by Wako Pure Chemical Industries, Ltd.), and 13.2 g of NaBH ₄ (manufactured by Wako Pure Chemical Industries, Ltd.) is added at 5 ° C. in a nitrogen atmosphere. After that, the mixture was stirred at room temperature for 2 days. After distilling off the solvent, 500 ml of water was added, and the solid was collected by filtration and washed with 1000 ml of water. The obtained solid was dissolved in 200 ml of diethyl ether, washed with 300 ml of water, and dried over magnesium sulfate. The solvent was distilled off to obtain 2.37 g of compound (1-c).

  2.37 g of the above compound (1-c) and 1.87 g of benzyl (manufactured by Wako Pure Chemical Industries, Ltd.) are added to 80 ml of chloroform, and methanesulfonic acid (manufactured by Wako Pure Chemical Industries, Ltd.) 3 is added in a nitrogen atmosphere. After adding the drops, the mixture was heated to reflux for 11 hours. The resulting solution was washed with an aqueous sodium bicarbonate solution and then dried over magnesium sulfate. The resulting solution was purified by column chromatography (filler: silica gel, eluent: chloroform) and washed with methanol to obtain 3.72 g of compound (1-d).

  8. 10.2 g of compound (1-e) (manufactured by Tokyo Chemical Industry Co., Ltd.) is dissolved in 100 ml of dimethylformamide (manufactured by Kishida Chemical Co., Ltd.), and N-bromosuccinimide (manufactured by Wako Pure Chemical Industries, Ltd.) 24 g was added, and the mixture was stirred at room temperature for 3 hours under a nitrogen atmosphere. 200 ml of water, 200 ml of n-hexane and 200 ml of dichloromethane were added to the resulting solution, and the organic layer was separated, washed with 200 ml of water, and then dried over magnesium sulfate. The resulting solution was purified by column chromatography (filler: silica gel, eluent: hexane) to obtain 14.4 g of compound (1-f).

  14.2 g of the above compound (1-f) was dissolved in 200 ml of tetrahydrofuran (manufactured by Wako Pure Chemical Industries, Ltd.) and cooled to -80 ° C. After adding 35 ml of n-butyllithium 1.6M hexane solution (made by Wako Pure Chemical Industries, Ltd.), it heated up to -50 degreeC and cooled again to -80 degreeC. 13.6 ml of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (manufactured by Wako Pure Chemical Industries, Ltd.) was added, the temperature was raised to room temperature, and 4 in a nitrogen atmosphere. Stir for hours. 200 ml of 1N aqueous ammonium chloride and 200 ml of ethyl acetate were added to the resulting solution, and the organic layer was separated, washed with 200 ml of water, and then dried over magnesium sulfate. The resulting solution was purified by column chromatography (filler: silica gel, eluent: hexane / dichloromethane) to obtain 14.83 g of compound (1-g).

  14.83 g of the above compound (1-g) and 6.78 g of 5,5′-dibromo-2,2′-bithiophene (manufactured by Tokyo Chemical Industry Co., Ltd.) were added to dimethylformamide (manufactured by Wako Pure Chemical Industries, Ltd.). ) In addition to 200 ml, 26.6 g of potassium phosphate (manufactured by Wako Pure Chemical Industries, Ltd.) and 1.7 g of [bis (diphenylphosphino) ferrocene] dichloropalladium (manufactured by Aldrich) were added at 100 ° C. under a nitrogen atmosphere. For 4 hours. To the resulting solution, 500 ml of water and 300 ml of ethyl acetate were added, the organic layer was separated, washed with 500 ml of water, and dried over magnesium sulfate. The resulting solution was purified by column chromatography (filler: silica gel, eluent: hexane) to obtain 4.53 g of compound (1-h).

  The above compound (1-h) 4.53 g was dissolved in 40 ml of tetrahydrofuran (manufactured by Wako Pure Chemical Industries, Ltd.) and cooled to -80 ° C. After adding 6.1 ml of n-butyllithium 1.6M hexane solution (made by Wako Pure Chemical Industries, Ltd.), the temperature was raised to −5 ° C. and cooled to −80 ° C. Add 2.3 ml of 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (manufactured by Wako Pure Chemical Industries, Ltd.), raise the temperature to room temperature, and add 2 under nitrogen atmosphere. Stir for hours. To the resulting solution, 150 ml of 1N aqueous ammonium chloride solution and 200 ml of ethyl acetate were added, the organic layer was separated, washed with 200 ml of water, and dried over magnesium sulfate. The resulting solution was purified by column chromatography (filler: silica gel, eluent: dichloromethane / hexane) to obtain 2.31 g of compound (1-i).

200 mg of the above compound (1-d) and 615 mg of the above compound (1-i) are added to 5 ml of dimethylformamide, and 1.15 g of potassium phosphate (manufactured by Wako Pure Chemical Industries, Ltd.) under a nitrogen atmosphere, [bis 18 mg of (diphenylphosphino) ferrocene] dichloropalladium (manufactured by Aldrich) was added and stirred at 100 ° C. for 10 hours. 100 ml of water and 100 ml of chloroform were added to the resulting solution, and the organic layer was separated, washed with 300 ml of water, and dried over magnesium sulfate. The resulting solution was purified by column chromatography (filler: silica gel, eluent: dichloromethane / hexane) to obtain 400 mg of compound A-1. The ¹ H-NMR measurement result of Compound A-1 is shown.
¹ H-NMR (CDCl ₂ , ppm): 8.12 (s, 2H), 7.82-7.77 (m, 6H), 7.43-7.41 (m, 6H), 7.20- 7.12 (m, 8H), 7.05 (d, 2H), 6.96 (d, 2H), 2.84 (m, 8H), 1.78-1.61 (m, 8H), 1 .29 (m, 40H), 0.88 (m, 12H)
Compound A-1 had a light absorption edge wavelength of 697 nm, a band gap (Eg) of 1.78 eV, and a highest occupied molecular orbital (HOMO) level of -4.97 eV.

Synthesis example 2
Compound A-2 was synthesized by the method shown in Formula 2.

  372 mg of Compound A-1 of Synthesis Example 1 was dissolved in 10 ml of chloroform, and 96 mg of dimethylformamide (2 ml) solution of N-bromosuccinimide (manufactured by Wako Pure Chemical Industries, Ltd.) was added at 5 ° C. to 10 ° C. for 5 hours. Stir. 100 ml of water and 100 ml of dichloromethane were added to the resulting solution, and the organic layer was separated, washed with 200 ml of water, and dried over magnesium sulfate. The resulting solution was purified by column chromatography (filler: silica gel, eluent: dichloromethane / hexane) to obtain 281 mg of compound (2-a).

  281 mg of the above compound (2-a) and 91 mg of the compound (2-b) (manufactured by Aldrich) were dissolved in 20 ml of toluene. 6 ml of water, 500 mg of potassium carbonate, tetrakis (triphenylphosphine) palladium (0) (manufactured by Tokyo Chemical Industry Co., Ltd.) 11 mg, and 1 drop of Aliquat336 (manufactured by Aldrich) were added thereto, and the solution was added at 10 ° C. under nitrogen atmosphere at 100 ° C. Stir for hours. Next, 5 drops of bromobenzene (manufactured by Tokyo Chemical Industry Co., Ltd.) was added and stirred at 100 ° C. for 30 minutes. 200 ml of methanol was added to the resulting solution, and the generated solid was collected by filtration and washed with methanol, water, methanol, acetone, and hexane in this order. The obtained solid was dissolved in 200 ml of chloroform, passed through a silica gel short column (eluent: chloroform), concentrated to dryness, and then washed with methanol to obtain 200 mg of compound A-2. The weight average molecular weight was 7818, the number average molecular weight was 3740, and the degree of polymerization n was 4.4. The light absorption edge wavelength was 712 nm, the band gap (Eg) was 1.74 eV, and the highest occupied molecular orbital (HOMO) level was −5.10 eV.

  Table 1 shows various physical properties (light absorption edge wavelength, band gap (Eg), highest occupied molecular orbital (HOMO) level) of the compounds A-1 to A-2 and compounds B-1 to B-2 described later. Shown in

Example 1
1 mg of the above A-1 and 4 mg of PC ₇₀ BM (manufactured by Science Laboratories) are added to a sample bottle containing 0.25 ml of chlorobenzene, and an ultrasonic cleaner (US-2 manufactured by Inoue Seieido Co., Ltd.) The solution A was obtained by ultrasonic irradiation for 30 minutes in an output of 120 W).

A glass substrate on which an ITO transparent conductive layer serving as a positive electrode having a thickness of 120 nm was deposited by sputtering was cut into 38 mm × 46 mm, and then ITO was patterned into a 38 mm × 13 mm rectangular shape by photolithography. The obtained substrate was subjected to ultrasonic cleaning for 10 minutes with an alkali cleaning solution (“Semico Clean” EL56 (trade name), manufactured by Furuuchi Chemical Co., Ltd.), and then washed with ultrapure water. After this substrate was UV / ozone treated for 30 minutes, a PEDOT: PSS aqueous solution (0.8% by weight of PEDOT, 0.5% by weight of PPS) serving as a hole transport layer was formed on the substrate to a thickness of 60 nm by spin coating. did. After heating and drying at 200 ° C. for 5 minutes using a hot plate, the above solution A was dropped onto the PEDOT: PSS layer, and an organic semiconductor layer having a thickness of 100 nm was formed by spin coating. Thereafter, the substrate on which the organic semiconductor layer is formed and the cathode mask are placed in a vacuum evaporation apparatus, and the vacuum is exhausted again until the degree of vacuum in the apparatus becomes 1 × 10 ⁻³ Pa or less. An aluminum layer was deposited to a thickness of 80 nm. As described above, a photovoltaic device having an area where the stripe-shaped ITO layer intersects with the aluminum layer was 5 mm × 5 mm was produced.

The positive and negative electrodes of the photovoltaic device thus fabricated were connected to a picoammeter / voltage source 4140B manufactured by Hewlett-Packard Co., and simulated sunlight (from Yamashita Denso Co., Ltd., simplified) from the ITO layer side in the atmosphere. Type solar simulator YSS-E40, spectrum shape: AM1.5, intensity: 100 mW / cm ² ), and the current value was measured when the applied voltage was changed from −1V to + 2V. At this time, the short-circuit current density (value of the current density when the applied voltage is 0 V) is 5.75 mA / cm ² , the open circuit voltage (value of the applied voltage when the current density is 0) is 0.81 V, and the fill factor (FF) was 0.34, and the photoelectric conversion efficiency calculated from these values was 1.58%. The fill factor and photoelectric conversion efficiency were calculated by the following equations.
Fill factor = JVmax / (Short-circuit current density × Open-circuit voltage)
(Here, JVmax is the product of the current density and the applied voltage at the point where the product of the current density and the applied voltage is maximum when the applied voltage is between 0 V and the open circuit voltage value.)
Photoelectric conversion efficiency = [(short circuit current density × open circuit voltage × fill factor) / pseudo sunlight intensity (100 mW / cm ² )] × 100 (%)
The fill factor and photoelectric conversion efficiency in the following examples and comparative examples were all calculated by the above formula.

Example 2
A photovoltaic device was produced in the same manner as in Example 1 except that A-2 was used in place of A-1, and current-voltage characteristics were measured. At this time, the short-circuit current density was 6.96 mA / cm ² , the open-circuit voltage was 0.75 V, the fill factor (FF) was 0.422, and the photoelectric conversion efficiency calculated from these values was 2.20%. .

Comparative Example 1
A photovoltaic device was produced in the same manner as in Example 1 except that the following B-1 (weight average molecular weight: 2630, number average molecular weight: 1604, polymerization degree n: 3.8) was used instead of A-1. Then, the current-voltage characteristics were measured. The short-circuit current density at this time was 3.57 mA / cm ² , the open-circuit voltage was 0.90 V, the fill factor (FF) was 0.28, and the photoelectric conversion efficiency calculated from these values was 0.90%. .

Comparative Example 2
A photovoltaic device was produced in the same manner as in Example 1 except that B-2 (weight average molecular weight: 5860, number average molecular weight: 3120, polymerization degree n: 4.3) was used instead of A-1. Then, the current-voltage characteristics were measured. At this time, the short-circuit current density was 4.31 mA / cm ² , the open-circuit voltage was 0.88 V, the fill factor (FF) was 0.30, and the photoelectric conversion efficiency calculated from these values was 1.14%. .

  The results of Examples 1-2 and Comparative Examples 1-2 are summarized in Table 2.

The schematic diagram which showed the one aspect | mode of the photovoltaic device of this invention. The schematic diagram which showed another aspect of the photovoltaic element of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 2 Positive electrode 3 Organic semiconductor layer 4 Negative electrode 5 Layer having a quinoxaline compound having a structure represented by the general formula (1) 6 Layer having an electron-accepting organic material

Claims (7)

(A) including a quinoxaline skeleton and an oligothiophene skeleton, (b) a band gap (Eg) of 1.8 eV or less, and (c) a highest occupied molecular orbital (HOMO) level of -4.8 eV or less. Quinoxaline compounds. The quinoxaline skeleton and the oligothiophene skeleton are alternately covalently bonded, the ratio of the quinoxaline skeleton: oligothiophene skeleton is in the range of 1: 1 to 1: 2, and the number of thiophene rings contained in one oligothiophene skeleton is The quinoxaline compound according to claim 1, wherein the number is 3 or more and 12 or less. The quinoxaline compound according to claim 2, which has a structure represented by the general formula (1).

(R ^{1 to} R ²⁰ may be the same or different and are selected from hydrogen, an alkyl group, an alkoxy group, an aryl group, a heteroaryl group, and halogen. A is a divalent arylene group having a 6-membered ring structure. Or a divalent heteroarylene group having a 6-membered ring structure, m is 0 or 1. n represents a range of 1 to 1000.) An electron-donating organic material comprising the quinoxaline compound according to claim 1. A material for a photovoltaic device comprising an electron-accepting organic material and the electron-donating organic material according to claim 4. The photovoltaic element material according to claim 5, wherein the electron-accepting organic material is a fullerene compound. A photovoltaic element having at least a positive electrode and a negative electrode, wherein the photovoltaic element comprises the material for a photovoltaic element according to claim 5 or 6 between the negative electrode and the positive electrode.

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