Patent Publication Number: US-2020287148-A1

Title: Photoelectric conversion element and method for producing the same

Description:
TECHNICAL FIELD 
     The present invention relates to a photoelectric conversion element such as a photodetector, and a method for producing the same. 
     BACKGROUND ART 
     For example, a photoelectric conversion element is a device that is highly useful from the viewpoint of energy saving and a reduction in the emission amount of carbon dioxide, and therefore attention is attracted to the photoelectric conversion element. 
     The photoelectric conversion element is an element comprising at least a pair of electrodes composed of an anode and a cathode, and an active layer provided between the pair of electrodes. In a photoelectric conversion element, one of electrodes is made of a transparent or semi-transparent material, and from a side of the electrode that is transparent or semi-transparent, light enters an organic active layer. By the energy (hν) light that enters the organic active layer, charges (holes and electrons) are generated in the organic active layer, the generated holes move toward the anode, and the electrons move toward the cathode. The charges that have reached the anode and the cathode are extracted outside the element. 
     For example, a photoelectric conversion element is used as a photodetector. A photoelectric conversion element that is used as a photodetector is used with a voltage applied, and converts light entering the element and detects the light as a current. However, even when light does not enter, a weak current flows through the photoelectric conversion element. This current is known as a dark current, and causes a reduction in precision of light detection. 
     For a reduction in dark current, for example, a study in which a relation between the thickness of an active layer and dark current is examined is known (see Non Patent Literature 1). 
     RELATED ART DOCUMENTS 
     Non Patent Literature 
     Non Patent Literature 1: Laser Photonics Rev. 8, No. 6, 924-932 
     DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
     However, a conventional photoelectric conversion element, and particularly a photodetector has a problem in which the detectivity (hereinafter referred to as “D*”) is insufficient. Therefore, improvement of detectivity in the photoelectric conversion element is required. 
     Means for Solving Problem 
     In order to solve the aforementioned problem, the present inventors have intensively studied, and as a result, found that when the absorption peak wavelength of a p-type semiconductor material in an active layer is within a predetermined range and the thickness of the active layer is within a predetermined range, the detectivity of a photoelectric conversion element can be improved. Thus, the present invention has been completed. Specifically, the present invention provides the following [1] to [13]. 
     [1] A photoelectric conversion element comprising: 
     an anode; 
     a cathode; and 
     an active layer provided between the anode and the cathode, wherein 
     the active layer contains a p-type semiconductor material being a polymer compound having an absorption peak wavelength of 800 nm or more, and an n-type semiconductor material, and 
     the active layer has a thickness of 300 nm or more and less than 600 nm. 
     [2] The photoelectric conversion element according to above [1], wherein the absorption peak wavelength of the p-type semiconductor material is 900 nm or more and 2,000 nm or less.
 
[3] The photoelectric conversion element according to above [1] or [2], wherein the thickness of the active layer is 350 nm or more and 550 nm or less.
 
[4] The photoelectric conversion element according to any one of above [1] to [3], wherein the n-type semiconductor material is a fullerene derivative.
 
[5] The photoelectric conversion element according to above [4], wherein the n-type semiconductor material is C60PCBM.
 
[6] The photoelectric conversion element according to any one of above [1] to [5], wherein the p-type semiconductor material is a polymer compound having a constitutional unit containing a thiophene skeleton.
 
[7] The photoelectric conversion element according to any one of above [1] to [6], wherein the photoelectric conversion element is a photodetector.
 
[8] An image sensor comprising the photoelectric conversion element according to above [7].
 
[9] A fingerprint authentication device comprising the photoelectric conversion element according to above [7].
 
[10] A method for producing a photoelectric conversion element comprising an anode, a cathode, and an active layer provided between the anode and the cathode, comprising:
 
     forming the active layer comprising a step (i) of applying an ink containing a p-type semiconductor material being a polymer compound having an absorption peak wavelength of 800 nm or more, an n-type semiconductor material, and a solvent to a subject to be applied, to obtain a coating film, and a step (ii) of removing the solvent from the coating film, wherein 
     the active layer has a thickness of 300 nm or more and less than 600 nm. 
     [11] The method for producing a photoelectric conversion element according to above [10], wherein the n-type semiconductor material is a fullerene derivative.
 
[12] The method for producing a photoelectric conversion element according to above [10], wherein the n-type semiconductor material is C60PCBM.
 
[13] The method for producing a photoelectric conversion element according to any one of above [10] to [12], wherein the p-type semiconductor material is a polymer compound having a constitutional unit containing a thiophene skeleton.
 
     Effect of the Invention 
     According to the photoelectric conversion element of the present invention, the detectivity can be effectively improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view schematically illustrating a cut end face of a photoelectric conversion element. 
         FIG. 2  is a view schematically illustrating a configuration example of an image detection portion. 
         FIG. 3  is a view schematically illustrating a configuration example of a fingerprint detection portion. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a photoelectric conversion element according to embodiments of the present invention will be described with reference to the drawings. The drawings only illustrate schematic shapes, sizes, and arrangements of constituent elements enough to understand the present invention. The present invention is not limited by the following description, and each constituent element can be appropriately changed without departing from the scope of the present invention. A configuration according to the embodiments of the present invention is not necessarily limited to the arrangements illustrated in the drawings during production or use. 
     [1. Photoelectric Conversion Element] 
     A photoelectric conversion element according to an embodiment is a photoelectric conversion element including an anode, a cathode, and an active layer provided between the anode and the cathode, wherein the active layer contains a p-type semiconductor material that is a polymer compound having an absorption peak wavelength of 800 nm or more, and an n-type semiconductor material, and the active layer has a thickness of 300 nm or more and less than 600 nm. 
     Herein, an acceptable configuration example of the photoelectric conversion element of the present embodiment will be described.  FIG. 1  is a view schematically illustrating a cut end face of the photoelectric conversion element of the embodiment. 
     As illustrated in  FIG. 1 , for example, a photoelectric conversion element  10  of the embodiment is provided on a supporting substrate  11 . The photoelectric conversion element  10  includes an anode  12  provided in contact with the supporting substrate  11 , a hole transport layer  13  provided in contact with the anode  12 , an active layer  14  provided in contact with the hole transport layer  13 , an electron transport layer  15  provided in contact with the active layer  14 , and a cathode  16  provided in contact with the electron transport layer  15 . In this configuration example, the photoelectric conversion element further includes a sealing substrate  17  provided in contact with the cathode  16 . Hereinafter, constituent elements that may be included in the photoelectric conversion element of the embodiment will be specifically described. 
     (Substrate) 
     The photoelectric conversion element is generally formed on a substrate. On this substrate, an electrode including a cathode and an anode is generally formed. A material for the substrate is not particularly limited as long as it is a material that is not chemically changed during formation of a layer containing particularly an organic compound. Examples of the material for the substrate may include a glass, a plastic, a polymer film, and a silicone. When the substrate is an opaque substrate, it is preferable that an electrode on a side opposite to an electrode provided on a side of the opaque substrate (i.e., an electrode on a side away from the substrate) be a transparent or semi-transparent electrode. 
     (Electrode) 
     Examples of a material for the transparent or semi-transparent electrode may include a conductive metal oxide film and a semi-transparent metal thin film. Specific examples thereof may include conductive materials such as indium oxide, zinc oxide, tin oxide, and composites thereof including indium tin oxide (ITO), indium zinc oxide (IZO), and NESA, gold, platinum, silver, and copper. It is preferable that the material for the transparent or semi-transparent electrode be ITO, IZO, or tin oxide. For the electrode, a transparent conductive film formed from an organic compound such as a polyaniline or a derivative thereof or a polythiophene or a derivative thereof as a material may be used. The transparent or semi-transparent electrode may be an anode or a cathode. 
     When one of electrodes is transparent or semi-transparent, the other may be a low light transmitting electrode. Examples of a material for the low light transmitting electrode may include a metal and a conductive polymer. Specific examples of the material for the low light transmitting electrode may include metals such as lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, aluminum, scandium, vanadium, zinc, yttrium, indium, cerium, samarium, europium, terbium, and ytterbium, alloys of two or more of the metals, alloys of one or more of the metals and one or more types of metal selected from the group consisting of gold, silver, platinum, copper, manganese, titanium, cobalt, nickel, tungsten, and tin, graphite, a graphite intercalation compound, a polyaniline and a derivative thereof, and a polythiophene and a derivative thereof. Examples of the alloys may include a magnesium-silver alloy, a magnesium-indium alloy, a magnesium-aluminum alloy, an indium-silver alloy, a lithium-aluminum alloy, a lithium-magnesium alloy, a lithium-indium alloy, and a calcium-aluminum alloy. 
     As a method for forming an electrode, any suitable and conventionally known formation method may be used. Examples of the method for forming an electrode may include a vacuum evaporation method, a sputtering method, an ion plating method, and a plating method. 
     (Active Layer) 
     The active layer contains a p-type semiconductor material (electron donor compound) and an n-type semiconductor material (electron acceptor compound). 
     In the embodiment, the active layer contains a polymer compound having an absorption peak wavelength of 800 nm or more as a p-type semiconductor material. 
     Herein, the “absorption peak wavelength” is a parameter specified by the absorption peak of an absorption spectrum measured at a predetermined wavelength range, and is the wavelength of an absorption peak in which the absorbance is the highest among absorption peaks of absorption spectra. 
     The absorption peak wavelength of the polymer compound that is the p-type semiconductor material is preferably 800 nm or more, more preferably 900 nm or more and 2,000 nm or less, and further preferably 1,000 nm or more and 1,800 nm or less. 
     Whether a material is a p-type semiconductor material or an n-type semiconductor material can be relatively determined from HOMO and LUMO energy levels of a selected compound. 
     Details of suitable p-type semiconductor material and n-type semiconductor material will be described below. 
     From the viewpoint of improving the detectivity especially in a photodetector, the thickness of the active layer is preferably 300 nm or more and less than 600 nm, more preferably 350 nm or more and 550 nm or less, and further preferably 400 nm or more and 550 nm or less. 
     For example, the thickness of the active layer can be measured by a contact profiler or an electron microscope. Examples of the contact profiler may include Dektak8 (manufactured by Veeco Instruments Inc.). Examples of the electron microscope may include a field emission-scanning electron microscope S-4800 (manufactured by Hitachi, Lid.). 
     When a polymer compound having an absorption peak wavelength of 800 nm or more is used as a p-type semiconductor material for the active layer and the thickness of the active layer is 300 nm or more and less than 600 nm, the external quantum efficiency (EQE) can be further increased, the dark current can be further decreased, and the detectivity can be improved. 
     Herein, EQE is specifically a value of a ratio (%) of electrons extracted to the outside of the photoelectric conversion element among generated electrons to photons absorbed by the photoelectric conversion element. 
     (Interlayer) 
     As illustrated in  FIG. 1 , the photoelectric conversion element may include an additional interlayer such as a charge transport layer (electron transport layer, hole transport layer, electron injection layer, hole injection layer) as a further constituent element for improving properties such as photoelectric conversion efficiency. 
     As a material used for such an interlayer, any suitable and conventionally known material can be used. Examples of the material for the interlayer may include a halide and an oxide of alkali metal or alkaline earth metal, such as lithium fluoride. 
     Examples of the material used for the interlayer may include fine particles of inorganic semiconductors such as titanium oxide, and a mixture (PEDOT:PSS) of poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(4-styrene sulfonate) (PSS). 
     As illustrated in  FIG. 1 , the photoelectric conversion element may include a hole transport layer between the anode and the active layer. The hole transport layer has a function of transporting holes from the active layer to the electrode. 
     The hole transport layer provided in contact with the anode may be especially referred to as hole injection layer. The hole transport layer (hole injection layer) provided in contact with the anode has a function of promoting injection of holes into the anode. The hole transport layer (hole injection layer) may be in contact with the active layer. 
     The hole transport layer contains a hole-transporting material. Examples of the hole-transporting material may include a polythiophene and a derivative thereof, an aromatic amine compound, a polymer compound containing a constitutional unit having an aromatic amine residue, CuSCN, CuI, NiO, and molybdenum oxide (MoO 3 ). 
     As illustrated in  FIG. 1 , the photoelectric conversion element may include an electron transport layer between the cathode and the active layer. The electron transport layer has a function of transporting electrons from the active layer to the cathode. The electron transport layer may be in contact with the cathode. The electron transport layer may be in contact with the active layer. 
     The electron transport layer contains an electron-transporting material. Examples of the electron-transporting material may include nanoparticles of zinc oxide, nanoparticles of gallium-doped zinc oxide, nanoparticles of aluminum-doped zinc oxide, polyethylenimine, polyethylenimine ethoxylated, and PFN-P2. 
     The interlayer can be formed by a coating method that is the same as a method for producing an active layer as described below. 
     (Sealing Layer) 
     The photoelectric conversion element may include a sealing layer. For example, the sealing layer can be provided on a side of an electrode far from the substrate. The sealing layer may be formed from a material having properties of blocking a moisture content (water vapor barrier properties) or properties of blocking oxygen (oxygen barrier properties). 
     (Use of Photoelectric Conversion Element) 
     Under irradiation with light, the photoelectric conversion element of the embodiment can generate a photovoltaic power between electrodes, and act as a solar cell. A plurality of solar cells that are integrated can be used as a thin film solar cell module. 
     When the photoelectric conversion element of the embodiment is irradiated with light from a side of a transparent or semi-transparent electrode with a voltage applied between electrodes, a photocurrent flows, and the photoelectric conversion element can act as a photodetector (photosensor). A plurality of photosensors that are integrated can be also used as an image sensor. 
     (Use Example of Photoelectric Conversion Element) 
     The photoelectric conversion element according to the embodiment of the present invention as described above can be suitably used for a detection portion provided in various electronic devices such as a workstation, a personal computer, a portable information terminal, an access management system, a digital camera, and a medical device. 
     The photoelectric conversion element (photodetector) of the present invention can be suitably used, for example, for an image detection portion (image sensor) for a solid imaging device such as an X-ray imaging device or a CMOS image sensor, a detection portion of detecting given characteristics of a part of a living body such as a fingerprint detection portion, a face detection portion, a vein detection portion, or an iris detection portion, or a detection portion of an optical biosensor such as a pulse oximeter, which is provided in the electronic device exemplified above. 
     Hereinafter, configuration examples of the image detection portion for a solid imaging device and a fingerprint detection portion for a biometric authorization device (fingerprint authentication device) among the detection portions for which the photoelectric conversion element according to the embodiment of the present invention can be suitably used will be described with reference to the drawings. 
     (Image Detection Portion) 
       FIG. 2  is a view schematically illustrating the configuration example of the image detection portion for a solid imaging device. 
     An image device portion  1  includes a CMOS transistor substrate  20 , an interlayer insulating layer  30  provided so as to cover the CMOS transistor substrate  20 , the photoelectric conversion element  10  according to the embodiment of the present invention that is provided on the interlayer insulating layer  30 , an interlayer wiring portion  32  that is provided so as to pass through the interlayer insulating layer  30  and electrically connects the CMOS transistor substrate  20  to the photoelectric conversion element  10 , a sealing layer  40  provided so as to cover the photoelectric conversion element  10 , and a color filter  50  provided on the sealing layer  40 . 
     The CMOS transistor substrate  20  has any suitable and conventionally known configuration in an aspect corresponding to a design. 
     The CMOS transistor substrate  20  includes a function element such as a CMOS transistor circuit (MOS transistor circuit) for achieving various functions, including a transistor, a capacitor, or the like that is formed within the thickness of the substrate. 
     Examples of the function element may include floating diffusion, a reset transistor, an output transistor, and a selection transistor. 
     In the CMOS transistor substrate  20 , a signal reading circuit or the like is formed using such a function element, a wiring, and the like. 
     The interlayer insulating layer  30  can be made from any suitable and conventionally known insulating material such as silicon oxide or an insulating resin. For example, the interlayer wiring portion  32  can be made from any suitable and conventionally known conductive material (wiring material) such as copper or tungsten. For example, the interlayer wiring portion  32  may be a wiring inside a hole that is formed at the same time as formation of a wiring layer, or a buried plug formed separately from the wiring layer. 
     The sealing layer  40  can be made from any suitable and conventionally known material as long as penetration of harmful substance that may functionally deteriorate the photoelectric conversion element  10 , such as oxygen or water, can be prevented or suppressed. The sealing layer  40  may be the sealing substrate  17  described above. 
     As the color filter  50 , for example, a primary color filter that is made from any suitable and conventionally known material and corresponds to a design of the image device portion  1  can be used. As the color filter  50 , a complementary color filter having a thickness smaller than that of the primary color filter can be used. 
     As the complementary color filter, for example, a color filter in which three types of colors (yellow, cyan, magenta), (yellow, cyan, transparent), (yellow, transparent, magenta), or (transparent, cyan, magenta) are combined can be used. These color filters can be optionally and suitably arranged so as to correspond to a design of the photoelectric conversion element  10  and the CMOS transistor substrate  20  as long as a color image data can be produced. 
     Light received by the photoelectric conversion element  10  through the color filter  50  is converted to an electrical signal corresponding to the amount of light received by the photoelectric conversion element  10 , and sent through the electrode outside the photoelectric conversion element  10  as a received signal, that is, an electrical signal corresponding to an imaging subject. 
     The received signal sent from the photoelectric conversion element  10  is then put through the interlayer wiring portion  32  into the CMOS transistor substrate  20 , read by the signal reading circuit formed in the CMOS transistor substrate  20 , and processed by any further, suitable, and conventionally known function portion that is not illustrated. Thus, an image data based on the imaging subject is produced. 
     (Fingerprint Detection Portion) 
       FIG. 3  is a view schematically illustrating the configuration example of the fingerprint detection portion. 
     A display device  2  of a portable information terminal includes a fingerprint detection portion  100  including the photoelectric conversion element  10  according to the embodiment of the present invention as a main constituent element, and a display panel portion  200  that is provided on the fingerprint detection portion  100  and displays a predetermined image. 
     In this configuration example, the fingerprint detection portion  100  is provided at a region that substantially matches a display region  200   a  of the display panel portion  200 . That is, the display panel portion  200  is integrally layered on the fingerprint detection portion  100 . 
     When fingerprint detection is performed for only a part of the display region  200   a,  the fingerprint detection portion  100  may be provided so as to correspond to only the part of the display region. 
     The fingerprint detection portion  100  includes the photoelectric conversion element  10  according to the embodiment of the present invention as a function portion that performs an essential function. The fingerprint detection element  100  may include any suitable and conventionally known member such as a protection film, a supporting substrate, a sealing substrate, a sealing member, a barrier film, a band-pass filter, or an infrared radiation-cutting filter, in an embodiment corresponding to such a design that desired characteristics are obtained. 
     For the fingerprint detection portion  100 , a configuration of the image detection portion described can be utilized. 
     The photoelectric conversion element  10  may be included as any embodiment in the display region  200   a.  For example, a plurality of photoelectric conversion elements  10  may be arranged in a matrix. 
     As described above, the photoelectric conversion element  10  is provided on the supporting substrate  11  or a sealing substrate, and an electrode (anode or cathode) is provided on the supporting substrate  11 , for example, in a matrix. 
     Light received by the photoelectric conversion element  10  is converted to an electrical signal corresponding to the amount of light received by the photoelectric conversion element  10 , and sent through the electrode outside the photoelectric conversion element  10  as a received signal, that is, an electrical signal corresponding to an imaged fingerprint. 
     In this configuration example, the display panel portion  200  is configured as an organic electroluminescent display panel (organic EL display panel) including a touch sensor panel. The display panel portion  200  may be configured, for example, as a display panel having any suitable and conventionally known configuration such as a liquid-crystal display panel including a light source such as a backlight, instead of the organic EL display panel. 
     The display panel portion  200  is provided on the fingerprint detection portion  100  described above. The display panel portion  200  includes an organic electroluminescent element (organic EL element)  220  as a function portion that performs an essential function. The display panel portion  200  may further include any suitable and conventionally known substrate such as a glass substrate (a supporting substrate  210  or a sealing substrate  240 ), and any suitable and conventionally known member such as a sealing member, a barrier film, a polarizing plate such as a circularly polarizing plate, or a touch sensor panel  230 , in an embodiment corresponding to desired characteristics. 
     In the configuration example described above, the organic EL element  220  is used as a light source of a pixel in the display region  200   a,  and further used as a light source for imaging a fingerprint in the fingerprint detection portion  100 . 
     Herein, a motion of the fingerprint detection portion  100  will be simply described. 
     During execution of fingerprint authentication, the fingerprint detection portion  100  detects a fingerprint using light emitted from the organic EL element  220  of the display panel portion  200 . Specifically, light emitted from the organic EL element  220  is sent through a constituent element provided between the organic EL element  220  and the photoelectric conversion element  10  of the fingerprint detection portion  100 , and reflected from the skin of the fingertip (the surface of the finger) disposed in contact with the surface of the display panel portion  200  within the display region  200   a.  At least a part of the light reflected from the surface of the finger is sent through the constituent element provided, received by the photoelectric conversion element  10 , and converted to an electrical signal corresponding to the amount of light received of the photoelectric conversion element  10 . From the converted electrical signal, image information about the fingerprint of the surface of the finger is made. 
     The portable information terminal including the display device  2  compares the obtained image information with fingerprint data for fingerprint authentication recorded in advance and executes fingerprint authentication at any suitable and conventionally known steps. 
     [2. Method for Producing Photoelectric Conversion Element] 
     A method for producing the photoelectric conversion element of the embodiment is not particularly limited. The photoelectric conversion element can be produced by a formation method suitable for a material selected for formation of each constituent element. 
     Since the active layer that is a main constituent element in the photoelectric conversion element of the embodiment is a bulk heterojunction type, the active layer can be produced by a coating method using an ink. 
     The method for producing a photoelectric conversion element is a method for producing a photoelectric conversion element including an anode, a cathode, and an active layer provided between the anode and the cathode, the method including a step of forming the active layer including a step (i) of applying an ink containing a p-type semiconductor material that is a polymer compound having an absorption peak wavelength of 800 nm or more, an n-type semiconductor material, and a solvent to a subject to be applied, to obtain a coating film, and a step (ii) of removing the solvent from the coating film, wherein the active layer has a thickness of 300 nm or more and less than 600 nm. 
     Hereinafter, the steps (i) and (ii) included in a method of forming the active layer that is a main constituent element in the photoelectric conversion element of the present invention will be described. 
     (Step (i)) 
     As a method for applying an ink to a subject to be applied, any suitable coating method can be used. As the coating method, a slit coating method, a knife coating method, a spin coating method, a microgravure-coating method, a gravure coating method, a bar coating method, an ink jet printing method, a nozzle coating method, or a capillary coating method is preferred, a slit coating method, a spin coating method, a capillary coating method, or a bar-coating method is more preferred, and a slit coating method or a spin coating method is further preferred. 
     An ink for formation of the active layer is applied to a subject to be applied according to the photoelectric conversion element and the method for producing the same. The ink for formation of the active layer may be applied to a functional layer that is in the photoelectric conversion element and can include the active layer in a step of producing the photoelectric conversion element. Therefore, the subject coated with the ink for formation of the active layer varies depending on a layer structure of the photoelectric conversion element to be produced and a layer formation order. For example, when the photoelectric conversion element has a layer structure of substrate/anode/hole transport layer/active layer/electron transport layer/cathode and a layer described on a more left side is formed early, the subject coated with the ink is a hole transport layer. For example, when the photoelectric conversion element has a layer structure of substrate/cathode/electron transport layer/active layer/hole transport layer/anode and a layer described on a more left side is formed early, the subject coated with the ink is an electron transport layer. 
     (Step (ii)) 
     As a method for removing a solvent from the coating film of the ink, that is, a method for removing a solvent from the coating film to form a solidified film, any suitable method can be used. Examples of the method for removing a solvent may include drying methods such as a method for directly heating a solvent by a hot plate, a hot air drying method, a drying method by infrared radiation heating, a drying method by flash lamp annealing, and a drying method under reduced pressure. 
     In addition to the steps (i) and (ii), the step of forming the active layer may include another step without impairing the objects and effects of the present invention. 
     The method for producing a photoelectric conversion element may be a method for producing a photoelectric conversion element including a plurality of active layers or a method in which the steps (i) and (ii) are repeated a plurality of times. 
     The active layer according to the embodiment is formed at a predetermined thickness as described above. 
     In the step of forming an active layer, the thickness of the active layer can be adjusted, for example, by changing the amount of the solvent in the total amount of the ink. For example, when the thickness of the active layer is adjusted so as to increase the thickness of the active layer, the amount of the solvent can be decreased to adjust the thickness of the active layer to a suitable thickness. When the thickness of the active layer is adjusted so as to decrease the thickness of the active layer, the amount of the solvent can be increased to adjust the thickness of the active layer to a suitable thickness. 
     When the active layer is formed especially by a spin coating method, the rotation speed (rotation number per predetermined time) can be changed to appropriately adjust the thickness of the active layer. Specifically, when the rotation speed is increased, the thickness of the active layer can be adjusted so as to decrease the thickness of the active layer. When the rotation speed is decreased, the thickness of the active layer can be adjusted so as to increase the thickness of the active layer. 
     (Ink) 
     The ink may be a solution or a dispersion liquid such as a dispersion liquid, an emulsion (emulsion liquid), or a suspension (suspension liquid). The ink of the embodiment is an ink for formation of the active layer, contains the p-type semiconductor material, the n-type semiconductor material, and a first solvent, and may contain a second solvent if desired. Hereinafter, a component for the ink will be described. 
     Herein, terms used commonly in the following description will be firstly described. 
     A “polymer compound” means a polymer that has a molecular weight distribution and a polystyrene-equivalent number average molecular weight of 1×10 3  or more and 1×10 8  or less. A constitutional unit included in the polymer compound is in total 100% by mole. 
     A “constitutional unit” means a unit of which the number present in the polymer compound is one or more. 
     A “hydrogen atom” may be a light hydrogen atom or a heavy hydrogen atom. 
     A “halogen atom” includes a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. 
     “Optionally having a substituent” includes aspects where all hydrogen atoms constituting a compound or group are unsubstituted and a part or all of one or more hydrogen atoms are substituted with a substituent. 
     An “alkyl group” may be linear, branched, or cyclic unless otherwise specified. The number of carbon atoms of a linear alkyl group is usually 1 to 50, preferably 1 to 30, and more preferably 1 to 20, not including the number of carbon atoms of a substituent. The number of carbon atoms of a branched or cyclic alkyl group is usually 3 to 50, preferably 3 to 30, and more preferably 4 to 20, not including the number of carbon atoms of a substituent. 
     The alkyl group optionally has a substituent. Specific examples of the alkyl group may include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, an isoamyl group, a 2-ethylbutyl group, an n-hexyl group, a cyclohexyl group, an n-heptyl group, a cyclohexylmethyl group, a cyclohexylethyl group, an n-octyl group, a 2-ethylhexyl group, a 3-n-propylheptyl group, an adamantyl group, an n-decyl group, a 3,7-dimethyloctyl group, a 2-ethyloctyl group, a 2-n-hexyl-decyl group, an n-dodecyl group, a tetradecyl group, a hexadecyl group, an octadecyl group, and an eicosyl group. Specific examples of an alkyl group having a substituent may include a trifluoromethyl group, a pentafluoroethyl group, a perfluorobutyl group, a perfluorohexyl group, a perfluorooctyl group, a 3-phenylpropyl group, a 3-(4-methylphenyl) propyl group, a 3-(3,5-di-n-hexylphenyl)propyl group, and a 6-ethyloxyhexyl group. 
     An “aryl group” means an atomic group derived by removal of one hydrogen atom directly bonded to a carbon atom constituting a ring from an aromatic hydrocarbon optionally having a substituent. 
     The aryl group optionally has a substituent. Specific examples of the aryl group may include a phenyl group, a 1-naphthyl group, a 2-naphthyl group, a 1-anthracenyl group, a 2-anthracenyl group, a 9-anthracenyl group, a 1-pyrenyl group, a 2-pyrenyl group, a 4-pyrenyl group, a 2-fluorenyl group, a 3-fluorenyl group, a 4-fluorenyl group, a 2-phenylphenyl group, a 3-phenylphenyl group, a 4-phenylphenyl group, and groups in which the groups have a substituent such as an alkyl group, an alkoxy group, an aryl group, or a fluorine atom. 
     An “alkoxy group” may be linear, branched, or cyclic. The number of carbon atoms of a linear alkoxy group is usually 1 to 40, and preferably 1 to 10, not including the number of carbon atoms of a substituent. The number of carbon atoms of a branched or cyclic alkoxy group is usually 3 to 40, and preferably 4 to 10, not including the number of carbon atoms of a substituent. 
     The alkoxy group optionally has a substituent. Specific examples of the alkoxy group may include a methoxy group, an ethoxy group, an n-propyloxy group, an isopropyloxy group, an n-butyloxy group, an isobutyloxy group, a tert-butyloxy group, an n-pentyloxy group, an n-hexyloxy group, a cyclohexyloxy group, an n-heptyloxy group, an n-octyloxy group, a 2-ethylhexyloxy group, an n-nonyloxy group, an n-decyloxy group, a 3,7-dimethyloctyloxy group, and a lauryloxy group. 
     The number of carbon atoms of an “aryloxy group” is usually 6 to 60, and preferably 6 to 48, not including the number of carbon atoms of a substituent. 
     The aryloxy group optionally has a substituent. Specific examples of the aryloxy group may include a phenoxy group, a 1-naphthyloxy group, a 2-naphthyloxy group, a 1-anthracenyloxy group, a 9-anthracenyloxy group, a 1-pyrenyloxy group, and groups in which the groups have a substituent such as an alkyl group, an alkoxy group, or a fluorine atom. 
     An “alkylthio group” may be linear, branched, or cyclic. The number of carbon atoms of a linear alkylthio group is usually 1 to 40, and preferably 1 to 10, not including the number of carbon atoms of a substituent. The number of carbon atoms of a branched or cyclic alkylthio group is usually 3 to 40, and preferably 4 to 10, not including the number of carbon atoms of a substituent. 
     The alkylthio group optionally has a substituent. Specific examples of the alkylthio group may include a methylthio group, an ethylthio group, a propylthio group, an isopropylthio group, a butylthio group, an isobutylthio group, a tert-butylthio group, a pentylthio group, a hexylthio group, a cyclohexylthio group, a heptylthio group, an octylthio group, a 2-ethylhexylthio group, a nonylthio group, a decylthio group, a 3,7-dimethyloctylthio group, a laurylthio group, and a trifluoromethylthio group. 
     The number of carbon atoms of an “arylthio group” is usually 6 to 60, and preferably 6 to 48, not including the number of carbon atoms of a substituent. 
     The arylthio group optionally has a substituent. Examples of the arylthio group may include a phenylthio group, a C1-C12 alkyloxyphenylthio group (“C1-C12” means that the number of carbon atoms of a group described after that is 1 to 12. Hereinafter as the same), a C1-C12 alkylphenylthio group, a 1-naphthylthio group, a 2-naphthylthio group, and a pentafluorophenylthio group. 
     A “p-valent heterocyclic group” (p is an integer of 1 or more) means an atomic group derived by removal of p hydrogen atoms among hydrogen atoms directly bonded to a carbon atom or a heteroatom constituting a ring from a heterocyclic compound optionally having a substituent. Among p-valent heterocyclic groups, a “p-valent aromatic heterocyclic group” is preferred. The “p-valent aromatic heterocyclic group” means an atomic group derived by removal of p hydrogen atoms among hydrogen atoms directly bonded to a carbon atom or a heteroatom constituting a ring from an aromatic heterocyclic compound optionally having a substituent. 
     Examples of a substituent that may be in a heterocyclic compound may include a halogen atom, an alkyl group, an aryl group, an alkoxy group, an aryloxy group, an alkylthio group, an arylthio group, a monovalent heterocyclic group, a substituted amino group, an acyl group, an imine residue, an amido group, an acid imido group, a substituted oxycarbonyl group, an alkenyl group, an alkynyl group, a cyano group, and a nitro group. 
     The aromatic heterocyclic compound includes a compound in which an aromatic ring is fused with a heterocycle not exhibiting aromaticity in addition to a compound in which a heterocycle by itself exhibits aromaticity. 
     Specific examples of the compound in which a heterocycle exhibits aromaticity by itself among aromatic heterocyclic compounds may include oxadiazole, thiadiazole, thiazole, oxazole, thiophene, pyrrole, phosphol, furan, pyridine, pyrazine, pyrimidine, triazine, pyridazine, quinoline, isoquinoline, carbazole, and dibenzophosphol. 
     Specific examples of the compound in which an aromatic ring is fused with a heterocycle not exhibiting aromaticity among aromatic heterocyclic compounds may include phenoxazine, phenothiazine, dibenzoborole, dibenzosilole, and benzopyran. 
     The number of carbon atoms of a monovalent heterocyclic group is usually 2 to 60, and preferably 4 to 20, not including the number of carbon atoms of a substituent. 
     The monovalent heterocyclic group optionally has a substituent. Specific examples of the monovalent heterocyclic group may include a thienyl group, a pyrrolyl group, a furyl group, a pyridyl group, a piperidyl group, a quinolyl group, an isoquinolyl group, a pyrimidinyl group, a triazinyl group, and groups in which the groups have a substituent such as an alkyl group or an alkoxy group. 
     A “substituted amino group” means an amino group having a substituent. Examples of a substituent that may be in the substituted amino group may include an alkyl group, an aryl group, and a monovalent heterocyclic group. It is preferable that the substituent be an alkyl group, an aryl group, or a monovalent heterocyclic group. The number of carbon atoms of the substituted amino group is usually 2 to 30. 
     Examples of the substituted amino group may include dialkylamino groups such as a dimethylamino group and a diethylamino group, and diarylamino groups such as a diphenylamino group, a bis(4-methylphenyl)amino group, a bis(4-tert-butylphenyl)amino group, and a bis(3,5-di-tert-butylphenyl) amino group. 
     The number of carbon atoms of an “acyl group” is usually 2 to 20, and preferably 2 to 18. Specific examples of the acyl group may include an acetyl group, a propionyl group, a butyryl group, an isobutyryl group, a pivaloyl group, a benzoyl group, a trifluoroacetyl group, and a pentafluorobenzoyl group. 
     An “imine residue” means an atomic group derived by removal of one hydrogen atom directly bonded to a carbon atom or a nitrogen atom constituting a carbon atom-nitrogen atom double bond from an imine compound. The “imine compound” means an organic compound having a carbon atom-nitrogen atom double bond in the molecule. Examples of the imine compound may include an aldimine, a ketimine, and a compound in which a hydrogen atom bonded to a nitrogen atom constituting a carbon atom-nitrogen atom double bond in an aldimine is substituted with an alkyl group or the like. 
     The number of carbon atoms of the imine residue is usually 2 to 20, and preferably 2 to 18. Examples of the imine residue may include groups represented by the following structural formulae. 
     
       
         
         
             
             
         
       
     
     An “amido group” means an atomic group derived by removal of one hydrogen atom bonded to a nitrogen atom from an amide. The number of carbon atoms of the amido group is usually 1 to 20, and preferably 1 to 18. Specific examples of the amido group may include a formamido group, an acetamido group, a propioamido group, a butyramido group, a benzamido group, a trifluoroacetamido group, a pentafluorobenzamido group, a diformamido group, a diacetamido group, a dipropioamido group, a dibutyramido group, a dibenzamido group, a ditrifuloroacetamido group, and a dipentafluorobenzamido group. 
     An “acid imido group” means an atomic group derived by removal of one hydrogen atom bonded to a nitrogen atom from an acid imide. The number of carbon atoms of the acid imido group is usually 4 to 20. Specific examples of the acid imido group may include groups represented by the following structural formulae. 
     
       
         
         
             
             
         
       
     
     A “substituted oxycarbonyl group” means a group represented by R′—O—(C═O)—. Herein, R′ is an alkyl group, an aryl group, an arylalkyl group, or a monovalent heterocyclic group. 
     The number of carbon atoms of the substituted oxycarbonyl group is usually about 2 to 60, and preferably 2 to 48. 
     Specific examples of the substituted oxycarbonyl group may include a methoxycarbonyl group, an ethoxycarbonyl group, a propoxycarbonyl group, an isopropoxycarbonyl group, a butoxycarbonyl group, an isobutoxycarbonyl group, a tert-butoxycarbonyl group, a pentyloxycarbonyl group, a hexyloxycarbonyl group, a cyclohexyloxycarbonyl group, a heptyloxycarbonyl group, an octyloxycarbonyl group, a 2-ethylhexyloxycarbonyl group, a nonyloxycarbonyl group, a decyloxycarbonyl group, a 3,7-dimethyloctyloxycarbonyl group, a dodecyloxycarbonyl group, a trifluoromethoxycarbonyl group, a pentafluoroethoxycarbonyl group, a perfluorobutoxycarbonyl group, a perfluorohexyloxycarbonyl group, a perfluorooctyloxycarbonyl group, a phenoxycarbonyl group, a naphthoxycarbonyl group, and a pyridyloxycarbonyl group. 
     An “alkenyl group” may be linear, branched, or cyclic. The number of carbon atoms of a linear alkenyl group is usually 2 to 30, and preferably 3 to 20, not including the number of carbon atoms of a substituent. The number of carbon atoms of a branched or cyclic alkenyl group is usually 3 to 30, and preferably 4 to 20, not including the number of carbon atoms of a substituent. 
     The alkenyl group optionally has a substituent. 
     Specific examples of the alkenyl group may include a vinyl group, a 1-propenyl group, a 2-propenyl group, a 2-butenyl group, a 3-butenyl group, a 3-pentenyl group, a 4-pentenyl group, a 1-hexenyl group, a 5-hexenyl group, a 7-octenyl group, and groups in which the groups have a substituent such as an alkyl group or an alkoxy group. 
     An “alkynyl group” may be linear, branched, or cyclic. The number of carbon atoms of a linear alkenyl group is usually 2 to 20, and preferably 3 to 20, not including the number of carbon atoms of a substituent. The number of carbon atoms of a branched or cyclic alkenyl group is usually 4 to 30, and preferably 4 to 20, not including the number of carbon atoms of a substituent. 
     The alkynyl group optionally has a substituent. Specific examples of the alkynyl group may include an ethynyl group, a 1-propynyl group, a 2-propynyl group, a 2-butynyl group, a 3-butynyl group, a 3-pentynyl group, a 4-pentynyl group, a 1-hexynyl group, a 5-hexynyl group, and groups in which the groups have a substituent such as an alkyl group or an alkoxy group. 
     (p-type Semiconductor Material) 
     The p-type semiconductor material of the photoelectric conversion element of the embodiment is a polymer compound having a predetermined polystyrene-equivalent weight average molecular weight. 
     Herein, the polystyrene-equivalent weight average molecular weight means a weight average molecular weight calculated by gel permeation chromatography (GPC) using a standard sample of polystyrene. 
     From the viewpoint of especially improving the solubility in a solvent, the polystyrene-equivalent weight average molecular weight of the p-type semiconductor material is preferably 3,000 or more and 500,000 or less. 
     Examples of the p-type semiconductor material that is a polymer compound may include a polyvinyl carbazole and a derivative thereof, a polysilane and a derivative thereof a polysiloxane derivative having an aromatic amine structure in a side chain or a main chain, a polyaniline and a derivative thereof, a polythiophene and a derivative thereof, a polypyrrole and a derivative thereof, a polyphenylenevinylene and a derivative thereof, a polythienylenevinylene and a derivative thereof, and a polyfluorene and a derivative thereof. 
     It is preferable that the p-type semiconductor material is a polymer compound having a constitutional unit represented by the following Formula (I) and/or a constitutional unit represented by the following Formula (II). 
     
       
         
         
             
             
         
       
     
     In Formula (I), Ar 1  and Ar 2  are a trivalent aromatic heterocyclic group, and Z is each of groups represented by the following Formulae (Z-1) to (Z-7). 
       [Chemical Formula 4] 
       —Ar 3 —  (II)
 
     In Formula (II), Ar 3  is a divalent aromatic heterocyclic group. 
     
       
         
         
             
             
         
       
     
     In Formulae (Z-1) to (Z-7), R is a hydrogen atom, a halogen atom, an alkyl group, an aryl group, an alkoxy group, an aryloxy group, an alkylthio group, an arylthio group, a monovalent heterocyclic group, a substituted amino group, an acyl group, an imine residue, an amido group, an acid imido group, a substituted oxycarbonyl group, an alkenyl group, an alkynyl group, a cyano group, or a nitro group. When each of Formulae (Z-1) to (Z-7) has two Rs, the two Rs may be the same as or different from each other. 
     It is preferable that the constitutional unit represented by Formula (I) is a constitutional unit represented by the following Formula (I-1). 
     
       
         
         
             
             
         
       
     
     In Formula (I-1), Z has the same meaning as described above. 
     Examples of the constitutional unit represented by Formula (I-1) may include constitutional units represented by the following Formulae (501) to (505). 
     
       
         
         
             
             
         
       
     
     In Formulae (501) to (505), R has the same meaning as described above. When there are two Rs, the two Rs may be the same as or different from each other. 
     The number of carbon atoms of the divalent aromatic heterocyclic group represented by Ar 3  is usually 2 to 60, preferably 4 to 60, and more preferably 4 to 20. The divalent aromatic heterocyclic group represented by Ar 3  optionally has a substituent. Examples of the substituent that may be in the divalent aromatic heterocyclic group represented by Ar 3  may include a halogen atom, an alkyl group, an aryl group, an alkoxy group, an aryloxy group, an alkylthio group, an arylthio group, a monovalent heterocyclic group, a substituted amino group, an acyl group, an imine residue, an amido group, an acid imido group, a substituted oxycarbonyl group, an alkenyl group, an alkynyl group, a cyano group, and a nitro group. 
     Examples of the divalent aromatic heterocyclic group represented by Ar 3  may include groups represented by the following Formulae (101) to (185). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae (101) to (185), R has the same meaning as described above. When there are a plurality of Rs, the Rs may be the same as or different from each other. 
     It is preferable that the constitutional unit represented by Formula (II) be each of constitutional units represented by the following Formulae (II-1) to (II-6). 
     
       
         
         
             
             
         
       
     
     In Formulae (II-1) to (II-6), X 1  and X 2  are each independently an oxygen atom or a sulfur atom, and R has the same meaning as described above. When there are a plurality of Rs, the Rs may be the same as or different from each other. 
     From the viewpoint of availability of raw material compounds, it is preferable that all X 2  and X 2  in 
     Formulae (II-1) to (II-6) be a sulfur atom. 
     It is preferable that the p-type semiconductor material be a polymer compound having a constitutional unit containing a thiophene skeleton. 
     The polymer compound that is the p-type semiconductor material may contain two or more types of constitutional units represented by Formula (I), or contain two or more types of constitutional units represented by Formula (II). 
     In order to improve the solubility in the solvent, the polymer compound that is the p-type semiconductor material may contain a constitutional unit represented by the following Formula (III). 
       [Chemical Formula 13] 
       —Ar 4 —  (III)
 
     In Formula (III), Ar 4  is an arylene group. 
     The arylene group represented by Ar 4  means an atomic group derived by removal of two hydrogen atoms from an aromatic hydrocarbon optionally having a substituent. The aromatic hydrocarbon also contains a compound having a fused ring and a compound in which two or more rings selected from the group consisting of an independent benzene ring and a fused ring are bonded directly or via a divalent group such as a vinylene group. 
     Examples of a substituent that may be in the aromatic hydrocarbon may include substituents that are the same as those exemplified as the substituent that may be in the heterocyclic compound. 
     The number of carbon atoms of the arylene group except for the substituent is usually 6 to 60, and preferably 6 to 20. The number of carbon atoms of the arylene group including the substituent is usually 6 to 100. 
     Examples of the arylene group may include a phenylene group (e.g., the following Formulae 1 to 3), a naphthalene-diyl group (e.g., the following Formulae 4 to 13), an anthracene-diyl group (e.g., the following Formulae 14 to 19), a biphenyl-diyl group (e.g., the following Formulae 20 to 25), a terphenyl-diyl group (e.g., the following Formulae 26 to 28), a fused ring compound group (e.g., the following Formulae 29 to 35), a fluorene-diyl group (e.g., the following Formulae 36 to 38), and a benzofluorene-diyl group (e.g., the following Formulae 39 to 46). 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In Formulae 1 to 46, R that is the substituent has the same meaning as described above. When there are a plurality of Rs, the Rs may be the same as or different from each other. 
     The constitutional unit constituting the polymer compound that is the p-type semiconductor material may be a constitutional unit in which two or more constitutional units that are two or more types of constitutional units selected from the constitutional unit represented by 
     Formula (I), the constitutional unit represented by Formula (II), and the constitutional unit represented by Formula (III) are connected in combination. 
     When the polymer compound that is the p-type semiconductor material contains the constitutional unit represented by Formula (I) and/or the constitutional unit represented by Formula (II), the total amount of the constitutional unit represented by Formula (I) and the constitutional unit represented by Formula (II) is usually 20 to 100% by mole relative to an amount of all constitutional units contained in the polymer compound of 100% by mole. The total amount is preferably 40 to 100% by mole, and more preferably 50 to 100% by mole since the charge transportation of the p-type semiconductor material can be improved. 
     Specific examples of the polymer compound that is the p-type semiconductor material may include polymer compounds represented by the following Formulae P-1 to P-3. 
     
       
         
         
             
             
         
       
     
     The ink may contain only one type of the p-type semiconductor material, or a combination of two or more types of the p-type semiconductor materials at an optional ratio. 
     (n-type Semiconductor Material) 
     The n-type semiconductor material may be a low molecular compound or a polymer compound. 
     Examples of the n-type semiconductor material (electron acceptor compound) that is the low molecular compound may include an oxadiazole derivative, an anthraquinodimethane and a derivative thereof, a benzoquinone and a derivative thereof, a naphthoquinone and a derivative thereof, an anthraquinone and a derivative thereof, a tetracyanoanthraquinodimethane and a derivative thereof, a fluorenone derivative, a diphenyldicyanoethylene and a derivative thereof, a diphenoquinone derivative, metal complexes of 8-hydroxyquinoline and a derivative thereof, fullerenes such as C 60  fullerene and a derivative thereof, and a phenanthrene derivative such as bathocuproine. 
     Examples of the n-type semiconductor material (electron acceptor compound) that is the polymer compound may include a polyvinylcarbazole and a derivative thereof, a polysilane and a derivative thereof, a polysiloxane derivative having an aromatic amine structure in a side chain or a main chain, a polyaniline and a derivative thereof, a polythiophene and a derivative thereof, a polypyrrole and a derivative thereof, a polyphenylenevinylene and a derivative thereof, a polythienylenevinylene and a derivative thereof, a polyquinoline and a derivative thereof, a polyquinoxaline and a derivative thereof, and a polyfluorene and a derivative thereof. 
     The n-type semiconductor material is preferably one or more selected from a fullerene and a fullerene derivative, and more preferably a fullerene derivative. 
     Examples of the fullerene may include a C 60  fullerene, a C 70  fullerene, a C 76  fullerene, a C 78  fullerene, and a C 84  fullerene. Examples of the fullerene derivative may include derivatives of the fullerenes. The fullerene derivative means a compound in which at least a part of a fullerene is modified. 
     Examples of the fullerene derivative may include compounds represented by the following Formulae (N-1) to (N-4). 
     
       
         
         
             
             
         
       
     
     In Formulae (N-1) to (N-4), R a  is an alkyl group, an aryl group, a monovalent heterocyclic group, or a group having an ester structure. A plurality of R a s may be the same as or different from each other. 
     R b  is an alkyl group or an aryl group. A plurality of R b s may be the same as or different from each other. 
     Examples of the group having an ester structure represented by R a  may include a group represented by the following Formula (19). 
     
       
         
         
             
             
         
       
     
     In Formula (19), u1 is an integer of 1 to 6, and u2 is an integer of 0 to 6. R c  is an alkyl group, an aryl group, or a monovalent heterocyclic group. 
     Examples of the C 60  fullerene derivative may include the following compounds. 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     Examples of the C 70  fullerene derivative may include the following compounds. 
     
       
         
         
             
             
         
       
     
     Specific examples of the fullerene derivative may include [6,6]-phenyl-C61 butyric acid methyl ester (C60PCBM), [6,6]-phenyl-C71 butyric acid methyl ester (C70PCBM), [6,6]-phenyl-C85 butyric acid methyl ester (C84PCBM), and [6,6]-thienyl-C61 butyric acid methyl ester. 
     The ink may contain only one type of the n-type semiconductor material, or a combination of two or more types of the n-type semiconductor materials at an optional ratio. 
     (Ratio by Weight (p/n Ratio) of p-type Semiconductor Material and n-type Semiconductor Material) 
     The ratio by weight (p-type semiconductor material/n-type semiconductor material) of the p-type semiconductor material and the n-type semiconductor material in the ink is preferably within a range of 9/1 to 1/9, and more preferably within a range of 5/1 to 1/5. From the viewpoint of making the junction length between a phase of the p-type semiconductor material and a phase of the n-type semiconductor material within a suitable range when the photoelectric conversion element is especially a photodetector, the ratio by weight is particularly preferably within a range of 3/1 to 1/3. 
     (First Solvent) 
     The solvent may be selected in consideration of solubilities of the selected p-type semiconductor material and n-type semiconductor material in the solvent and properties (boiling point, etc.) corresponding to a drying condition during formation of the active layer. 
     It is preferable that the first solvent be an aromatic hydrocarbon (hereinafter simply referred to as aromatic hydrocarbon) optionally having a substituent (e.g., alkyl group or halogen atom) or a halogenated alkyl solvent. 
     It is preferable that the first solvent be selected in consideration of solubilities of the selected p-type semiconductor material and n-type semiconductor material. 
     Examples of the aromatic hydrocarbon that is the first solvent may include toluene, xylene (e.g., o-xylene, m-xylene, and p-xylene), trimethylbenzene (e.g., mesitylene and 1,2,4-trimethylbenzene (pseudocumene)), butylbenzene (e.g., n-butylbenzene, sec-butylbenzene, and tert-butylbenzene), methylnaphthalene (e.g., 1-methylnaphthalene), tetralin, indane, chlorobenzene, and dichlorobenzene (o-dichlorobenzene). 
     Examples of the halogenated alkyl solvent that is the first solvent may include chloroform. 
     The first solvent may include only one type of the aromatic hydrocarbon or two or more types of the aromatic hydrocarbons. It is preferable that the first solvent include only one type of the aromatic hydrocarbon. 
     It is preferable that the first solvent contain one or more selected from the group consisting of toluene, o-xylene, m-xylene, p-xylene, mesitylene, pseudocumene, n-butylbenzene, sec-butylbenzene, tert-butylbenzene, methylnaphthalene, tetralin, indane, chlorobenzene, o-dichlorobenzene, and chloroform. 
     (Second Solvent) 
     From the viewpoint of enhancing the solubility of the n-type semiconductor material and improving the detectivity, it is preferable that the second solvent be a solvent especially selected. Examples of the second solvent may include ketone solvents such as acetone, methyl ethyl ketone, cyclohexanone, acetophenone, and propiophenone, ester solvents such as ethyl acetate, butyl acetate, phenyl acetate, ethyl cellosolve acetate, methyl benzoate, butyl benzoate, and benzyl benzoate, and aromatic carbon solvents such as o-dichlorobenzene. 
     (Combination of First Solvent with Second Solvent). 
     Examples of a combination of the first solvent with the second solvent may include combinations shown in the Table 1 below. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 First solvent 
                 Second solvent 
               
               
                   
                   
               
             
            
               
                   
                 pseudocumene 
                 propiophenone 
               
               
                   
                 chloroform 
                 o-dichlorobenzene 
               
               
                   
                   
               
            
           
         
       
     
     (Ratio by Weight of First Solvent to Second Solvent) 
     From the viewpoint of improving the solubility of the p-type semiconductor material and the n-type semiconductor material, it is preferable that the ratio by weight (first solvent/second solvent) of the first solvent to the second solvent be within a range of 85/15 to 99/1. 
     (Total Percentage by Weight of First Solvent and Second Solvent in Ink) 
     From the viewpoint of improving the solubility of the p-type semiconductor material and the n-type semiconductor material, the total weight of the first solvent and the second solvent in the ink is preferably 90% by weight or more, more preferably 92% by weight or more, and further preferably 95% by weight or more when the total weight of the ink is 100% by weight. From the viewpoint of increasing the concentrations of the p-type semiconductor material and the n-type semiconductor material in the ink and facilitating formation of a film having equal to or more than predetermined thickness, it is preferably 99.9% by weight or less. 
     (Optional Solvent) 
     The ink may contain an optional solvent other than the first solvent and the second solvent. When the total weight of all the solvents in the ink is 100% by weight, the content of the optional solvent is preferably 5% by weight or less, more preferably 3% by weight or less, and further preferably 1% by weight or less. It is preferable that the optional solvent be a solvent having a boiling point higher than that of the second solvent. 
     (Optional Component) 
     In addition to the first solvent, the second solvent, the p-type semiconductor material, and the n-type semiconductor material, the ink may contain optional components such as an ultraviolet light absorber, an antioxidant, a sensitizer for sensitizing a function of generating a charge by absorbed light, and a light stabilizer for enhancing stability to ultraviolet light without impairing the objects and effects of the present invention. 
     (Concentration of p-type Semiconductor Material and n-type Semiconductor Material in Ink) 
     The total concentration of the p-type semiconductor material and the n-type semiconductor material in the ink may be any suitable concentration depending on the required thickness of the active layer. The total concentration of the p-type semiconductor material and the n-type semiconductor material is preferably 0.01% by weight or more and 20% by weight or less, more preferably 0.01% by weight or more and 10% by weight or less, further preferably 0.01% by weight or more and 5% by weight or less, and particularly preferably 0.1% by weight or more and 5% by weight or less. 
     In the ink, the p-type semiconductor material and the n-type semiconductor material may be dissolved or dispersed. It is preferable that at least a part of the p-type semiconductor material and the n-type semiconductor material be dissolved, and it is more preferable that all the p-type semiconductor material and the n-type semiconductor material be dissolved. 
     (Preparation of Ink) 
     The ink can be prepared by a known method. For example, the ink can be prepared by a method in which the first solvent and the second solvent are mixed to prepare a mixed solvent, and the p-type semiconductor material and the n-type semiconductor material are added to the mixed solvent, a method in which the p-type semiconductor material is added to the first solvent, the n-type semiconductor material is added to the second solvent, and the first solvent and the second solvent that contain each of the materials are mixed, or the like. 
     The first solvent and the second solvent and the p-type semiconductor material and the n-type semiconductor material may be mixed with heating at a temperature equal to or lower than the boiling points of the solvents. 
     The first solvent and the second solvent and the p-type semiconductor material and the n-type semiconductor material are mixed, and the obtained mixture is then filtered through a filter. Thus, the obtained filtrate may be used as the ink. As the filter, for example, a filter made of a fluorine resin such as polytetrafluoroethylene (PTFE) can be used. 
     EXAMPLES 
     Hereinafter, Examples will be shown to describe the present invention in more details. The present invention is not limited to Examples described below. 
     In Examples, a p-type semiconductor material (electron donor compound) illustrated in Table 2 below were used, and as an n-type semiconductor material (electron acceptor compound), C60PCBM was used. 
     In measurement of absorption peak wavelength, a spectrophotometer (e.g., ultraviolet-visible-near-infrared spectrophotometer JASCO-V670 manufactured by JASCO Corporation) that was operated in a visible infrared spectrophotometer ultraviolet, visible, and near-infrared wavelength region was used. 
     In measurement of absorption peak wavelength, the absorption spectrum of a substrate used for measurement was measured. As the substrate, a glass substrate was used. A solution containing a compound to be measured or a molten material containing the compound was applied to the glass substrate, to form a thin film having a thickness of 100 nm and containing the compound to be measured. Subsequently, the absorption spectrum of a layered body including the obtained thin film and the substrate was measured. The difference between the absorption spectrum of the layered body of the thin film and the substrate and the absorption spectrum of the substrate was considered to be the absorption spectrum of the thin film. 
     In the absorption spectrum of the obtained thin film, that was, the absorption spectrum shown by plotting the absorbance of the compound on a longitudinal axis and the wavelength on a horizontal axis, a value corresponding to the wavelength corresponding to the absorption peak in which the absorbance was the highest was “absorption peak wavelength.” 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 absorption 
                   
               
               
                   
                   
                 peak 
                   
               
               
                   
                 Number 
                 wavelength 
                 Chemical structure 
               
               
                   
               
             
            
               
                 n-type semi- con- ductor material 
                 P-1 
                 921 nm 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
                 P-2 
                 841 nm 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
                 P-3 
                 801 nm 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
                 P-4 
                 571 nm 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                   
                 P-5 
                 512 nm 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     As a polymer compound P-1 that was a p-type semiconductor material, PDPP3T (trade name, available from Lumtec) was commercially obtained and used. 
     A polymer compound P-2 that was a p-type semiconductor material was synthesized with reference to a method described in WO 2011/052709, and used. 
     A polymer compound P-3 that was a p-type semiconductor material was synthesized with reference to a method described in WO 2013/051676, and used. 
     As a polymer compound P-4 that was a p-type semiconductor material, PCDTBT (trade name, available from Lumtec) was commercially obtained and used. 
     As a polymer compound P-5 that was a p-type semiconductor material, poly(3-hexylthiophene-2,5-diyl) (trade name, available from Sigma-Aldrich) was used. 
     Example 1 
     Preparation and Evaluation of Photoelectric Conversion Element 
     A glass substrate on which a thin film (anode) of ITO with a thickness of 150 nm was formed by a sputtering method was prepared, and subjected to an ozone UV treatment as a surface treatment. 
     A suspension in which poly(3,4-ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT/PSS) were dissolved in water (Clevios P VP AI4083, available from Heraeus) was then filtered through a filter having a pore diameter of 0.45 μm. The suspension after the filtration was applied to the thin film of ITO of the glass substrate at a thickness of 40 nm by a spin coating method to form a coating film. 
     Subsequently, the glass substrate having the coating film was dried in air on a hot plate at 200° C. for 10 minutes, to form a hole transport layer. 
     Next, the polymer P-1 and C60PCBM (trade name: E100, available from Frontier Carbon Corporation) were mixed at a ratio by weight of 1:1.5, and the mixture was added to a mixed solvent (pseudocumene:propiophenone=9:1 (ratio by weight)) of pseudocumene that was a first solvent and propiophenone that was a second solvent, and stirred at 80° C. for 14 hours, to prepare an ink (I-1). 
     The ink (I-1) was applied to the glass substrate having the hole transport layer by a spin coating method, to obtain a coating film, and the coating film was dried on a hot plate heated to 70° C. for 5 minutes to form an active layer as a solidified film. The thickness of the active layer after drying was approximately 350 nm. The thickness of the active layer described in Examples of the specification was measured by Dektak8 (manufactured by Veeco Instruments Inc.). 
     Next, a 45% by weight dispersion liquid of zinc oxide nanoparticles (particle diameter: 20 to 30 nm) in isopropanol (2-propanol) (HTD-711Z, available from Tayca Corporation) was diluted with 3-pentanol in an amount of 10-fold parts by weight of the isopropanol dispersion liquid, to prepare a coating liquid. The obtained coating liquid was applied to the active layer at a thickness of 40 nm by a spin coating method, and the glass substrate having the coating film was dried under a nitrogen gas atmosphere to form an electron transport layer. 
     Subsequently, a silver (Ag) layer was formed at a thickness of approximately 80 nm as a cathode on the formed electron transport layer in a resistive heating vapor deposition device. 
     A UV-curing sealant was then applied to the periphery of the layered body, bonded to the glass substrate that was a sealing substrate, and irradiated with UV light. The layered body was thus sealed to obtain a photoelectric conversion element (photodetector). The planar shape as viewed in a thickness direction of the obtained photoelectric conversion element was a 2 mm×2 mm square. 
     The applied voltage was set to −5 V. The external quantum efficiency (EQE) and dark current at this applied voltage were measured by a solar simulator (CEP-2000, manufactured by Bunkoukeiki Co., Ltd.) and a semiconductor parameter analyzer (Agilent Technology B1500A, manufactured by Agilent Technologies), respectively. 
     While a voltage of −5 V was applied to the photoelectric conversion element, the photoelectric conversion element was irradiated with light having a fixed number of photons (1.0×10 16 ) at every 10 nm at a wavelength range of 300 nm to 1,200 nm, and the current value of generated current was measured for EQE. The spectrum of EQE at a wavelength of 300 nm to 1,200 nm was determined by a known procedure. Among the obtained data points at every 10 nm, the measured value at a wavelength (Xmax) that was the nearest the absorption peak wavelength was the measured value of EQE (%). 
     From the obtained measured value, the detectivity (D*) (Jones) at an applied voltage of −5 V was calculated by a computational expression represented by the following 
     Expression. 
     
       
         
           
             
               
                 
                   
                     D 
                     * 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             λ 
                             max 
                           
                            
                           
                             / 
                           
                            
                           1240 
                         
                         ) 
                       
                       × 
                       
                         ( 
                         
                           E 
                            
                           Q 
                            
                           E 
                         
                         ) 
                       
                     
                     
                       
                         2 
                          
                         e 
                          
                         
                           J 
                           d 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Expression 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Expression, EQE is an external quantum efficiency and represents EQE in λ max , e is an electron quantum, and Jd is a dark current density. The results are shown in Table 3 below. 
     Examples 2 to 4 and Comparative Examples 1 to 5 
     A photoelectric conversion element (photodetector) was prepared in the same manner as in Example 1 except that the thickness of an active layer was changed as shown in Table 3, and the photoelectric conversion element was evaluated in the same manner as in Example 1. The results are shown in Table 3. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Thickness of 
                   
                   
                   
               
               
                   
                 the active 
                 EQE 
                 Dark current 
                 D* 
               
               
                   
                 layer (nm) 
                 (%) 
                 (A/cm 2 ) 
                 (Jones) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 350 
                 35 
                 8.5 × 10 −7   
                 4.9 × 10 11   
               
               
                 Example 2 
                 400 
                 34 
                 3.5 × 10 −7   
                 7.6 × 10 11   
               
               
                 Example 3 
                 450 
                 25 
                 9.5 × 10 −8   
                 1.1 × 10 12   
               
               
                 Example 4 
                 500 
                 21 
                 8.6 × 10 −8   
                 9.3 × 10 11   
               
               
                 Comparative 
                 100 
                 55 
                 5.7 × 10 −5   
                 9.4 × 10 10   
               
               
                 example 1 
                   
                   
                   
                   
               
               
                 Comparative 
                 150 
                 42 
                 1.3 × 10 −5   
                 1.5 × 10 11   
               
               
                 example 2 
                   
                   
                   
                   
               
               
                 Comparative 
                 200 
                 38 
                 6.7 × 10 −6   
                 1.9 × 10 11   
               
               
                 example 3 
                   
                   
                   
                   
               
               
                 Comparative 
                 250 
                 34 
                 4.1 × 10 −6   
                 2.2 × 10 11   
               
               
                 example 4 
                   
                   
                   
                   
               
               
                 Comparative 
                 600 
                 7 
                 1.3 × 10 −7   
                 2.6 × 10 11   
               
               
                 example 5 
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     In the photoelectric conversion elements of Examples 1 to 4, the absorption peak wavelength of the p-type semiconductor material was less than 800 nm, and D* was higher than those of the photoelectric conversion elements of Comparative Examples 1 to 5 in which the requirement for the thickness of the active layer was not satisfied. 
     In general, as the absorption peak wavelength of the p-type semiconductor material is larger, EQE tends to be decreased. In the photoelectric conversion elements of Examples 1 to 4, the thickness of the active layer was 350 nm to 500 nm, and the absorption peak wavelength of the p-type semiconductor material was 800 nm or more (900 nm). At that time, a decrease in EQE was suppressed, and D* was significantly increased. 
     Example 5 
     A glass substrate on which a thin film (cathode) of ITO with a thickness of 150 nm was formed by a sputtering method was prepared, and subjected to a UV ozone treatment as a surface treatment. 
     Next, a 45% by weight dispersion liquid of zinc oxide nanoparticles (particle diameter: 20 to 30 nm) in isopropanol (HTD-711Z, available from Tayca Corporation) was diluted with 3-pentanol in an amount of 10-fold parts by weight of the dispersion liquid, to prepare a coating liquid. The coating liquid was applied to the thin film of ITO of the glass substrate at a thickness of 40 nm by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 150° C. for 10 minutes, to form an electron transport layer as a solidified film. The polymer compound P-2 that was a p-type semiconductor material and C60PCBM (trade name: E100, available from Frontier Carbon Corporation) that was an n-type semiconductor material were mixed at a ratio by weight of 1:2, a mixed solvent (chloroform:o-dichlorobenzene=9:1 (ratio by weight)) of chloroform and o-dichlorobenzene was added, and the mixture was stirred at 80° C. for 4 hours to prepare an ink (I-2). The ink (I-2) was applied to the electron transport layer by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 70° C. for 5 minutes, to form an active layer as a solidified film. The thickness of the active layer after drying was approximately 350 nm. 
     Subsequently, a molybdenum oxide layer that was a hole transport layer was formed on the active layer at a thickness of approximately 15 nm in a resistive heating vapor deposition device. 
     A silver (Ag) layer was then formed on the molybdenum oxide layer at a thickness of approximately 80 nm as an electrode (anode). 
     Next, a UV-curing sealant was applied to the periphery of the layered body, bonded to the glass substrate, and irradiated with UV light. The layered body was thus sealed to obtain a photoelectric conversion element (photodetector). The planar shape as viewed in a thickness direction of the obtained photoelectric conversion element was a 2 mm×2 mm square. 
     The applied voltage was set to −5 V. EQE and dark current at this applied voltage were measured by a solar simulator (CEP-2000, manufactured by Bunkoukeiki Co., Ltd.) and a semiconductor parameter analyzer (Agilent echnology B1500A, manufactured by Agilent Technologies), respectively. Subsequently, the detectivity (D*) at an applied voltage of −5 V was calculated in the same manner as in Example 1. The results are shown in Table 4. 
     Examples 6 and 7 and Comparative Examples 6 to 9 
     A photoelectric conversion element was prepared in the same manner as in Example 5 except that the thickness of an active layer was changed as shown in Table 4, and the photoelectric conversion element was evaluated in the same manner as in Example 5. The results are shown in Table 4. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 Thickness of the 
                 EQE 
                 Dark current 
                 D* 
               
               
                   
                 active layer (nm) 
                 (%) 
                 (A/cm 2 ) 
                 (Jones) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 5 
                 350 
                 34 
                 3.9 × 10 −4   
                 2.1 × 10 10   
               
               
                 Example 6 
                 450 
                 31 
                 3.3 × 10 −4   
                 2.0 × 10 10   
               
               
                 Example 7 
                 580 
                 27 
                 2.7 × 10 −4   
                 2.0 × 10 10   
               
               
                 Comparative 
                 150 
                 45 
                 8.9 × 10 −3   
                 5.7 × 10 9    
               
               
                 example 6 
                   
                   
                   
                   
               
               
                 Comparative 
                 250 
                 37 
                 4.0 × 10 −3   
                 7.0 × 10 9    
               
               
                 example 7 
                   
                   
                   
                   
               
               
                 Comparative 
                 650 
                 14 
                 2.0 × 10 −4   
                 1.2 × 10 10   
               
               
                 example 8 
                   
                   
                   
                   
               
               
                 Comparative 
                 700 
                 10 
                 1.7 × 10 −4   
                 9.1 × 10 9    
               
               
                 example 9 
               
               
                   
               
            
           
         
       
     
     In the photoelectric conversion elements of Examples 5 to 7, the absorption peak wavelength of the p-type semiconductor material was less than 800 nm, and D* was higher than those of the photoelectric conversion elements of Comparative Examples 6 to 9 in which the requirement for the thickness of the active layer was not satisfied. 
     In the photoelectric conversion elements of Examples 5 to 7, the thickness of the active layer was 350 nm to 580 nm, and the absorption peak wavelength of the p-type semiconductor material was 800 nm or more (870 nm). At that time, a decrease in EQE was suppressed, and D* was significantly increased. 
     Example 8 
     A glass substrate on which a thin film (cathode) of ITO with a thickness of 150 nm was formed by a sputtering method was prepared, and subjected to a UV ozone treatment as a surface treatment. 
     Next, a 45% by weight dispersion liquid of zinc oxide nanoparticles (particle diameter: 20 to 30 nm) in isopropanol (HTD-711Z, available from Tayca Corporation) was diluted with 3-pentanol in an amount of 10-fold parts by weight of the dispersion liquid, to prepare a coating liquid. The coating liquid was applied to the thin film of 
     ITO of the glass substrate at a thickness of 40 nm by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 150° C. for 10 minutes, to form an electron transport layer as a solidified film. The polymer compound P-3 that was a p-type semiconductor material and C60PCBM (trade name: E100, available from Frontier Carbon Corporation) that was an n-type semiconductor material were mixed at a ratio by weight of 1:1.5, a mixed solvent (pseudocumene:propiophenone=9:1 (ratio by weight)) of pseudocumene and propiophenone was added, and the mixture was stirred at 60° C. for 12 hours to prepare an ink (1-3). The ink (1-3) was applied to the electron transport layer by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 70° C. for 5 minutes, to form an active layer as a solidified film. The thickness of the active layer after drying was approximately 310 nm. 
     Subsequently, a molybdenum oxide layer that was a hole transport layer was formed on the active layer at a thickness of approximately 15 nm in a resistive heating vapor deposition device. 
     A silver (Ag) layer was then formed on the molybdenum oxide layer at a thickness of approximately 80 nm as an electrode (anode). 
     Next, a UV-curing sealant was applied to the periphery of the layered body, bonded to the glass substrate, and irradiated with UV light. The layered body was thus sealed to obtain a photoelectric conversion element (photodetector). The planar shape as viewed in a thickness direction of the obtained photoelectric conversion element was a 2 mm×2 mm square. 
     The applied voltage was set to −2 V. EQE and dark current at this applied voltage were measured by a solar simulator (CEP-2000, manufactured by Bunkoukeiki Co., Ltd.) and a semiconductor parameter analyzer (Agilent Technology B1500A, manufactured by Agilent Technologies), respectively. Subsequently, the detectivity (D*) at an applied voltage of −2 V was calculated in the same manner as in Example 1. The results are shown in Table 5. 
     Examples 9 to 11 and Comparative Examples 10 to 12 
     A photoelectric conversion element was prepared in the same manner as in Example 8 except that the thickness of an active layer was changed as shown in Table 5, and the photoelectric conversion element was evaluated in the same manner as in Example 8. The results are shown in Table 5. 
     In the photoelectric conversion elements of Examples 8 to 11 in which the thickness of the active layer was 310 nm to 550 nm and the absorption peak wavelength of the p-type semiconductor material was 800 nm or more, a decrease in EQE was suppressed and D* was significantly increased as compared with the photoelectric conversion elements of Comparative Examples 10 to 12 in which the requirement for the thickness of the active layer was not satisfied. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                 Thickness of the 
                 EQE 
                 Dark current 
                 D* 
               
               
                   
                 active layer (nm) 
                 (%) 
                 (A/cm 2 ) 
                 (Jones) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 8 
                 310 
                 58 
                 9.0 × 10 −7   
                 7.0 × 10 13   
               
               
                 Example 9 
                 400 
                 56 
                 7.2 × 10 −7   
                 7.5 × 10 13   
               
               
                 Example 10 
                 500 
                 55 
                 6.0 × 10 −7   
                 8.1 × 10 13   
               
               
                 Example 11 
                 550 
                 49 
                 5.1 × 10 −7   
                 7.8 × 10 13   
               
               
                 Comparative 
                 100 
                 67 
                 5.5 × 10 −6   
                 3.3 × 10 13   
               
               
                 example 10 
                   
                   
                   
                   
               
               
                 Comparative 
                 250 
                 64 
                 2.9 × 10 −6   
                 4.3 × 10 13   
               
               
                 example 11 
                   
                   
                   
                   
               
               
                 Comparative 
                 700 
                 26 
                 4.2 × 10 −7   
                 4.6 × 10 13   
               
               
                 example 12 
               
               
                   
               
            
           
         
       
     
     Comparative Example 13 
     A glass substrate on which a thin film of ITO with a thickness of 150 nm was formed by a sputtering method was prepared, and subjected to a UV ozone treatment as a surface treatment. 
     Next, a 45% by weight dispersion liquid of zinc oxide nanoparticles (particle diameter: 20 to 30 nm) in isopropanol (HTD-711Z, available from Tayca Corporation) was diluted with 3-pentanol in an amount of 10-fold parts by weight of the isopropanol dispersion liquid, to prepare a coating liquid. The obtained coating liquid was applied to the thin film of ITO of the glass substrate at a thickness of 40 nm by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 150° C. for 10 minutes, to form an electron transport layer as a solidified film. 
     Subsequently, the polymer compound P-5 that was a p-type semiconductor material and C60PCBM (trade name: E100, available from Frontier Carbon Corporation) that was an n-type semiconductor material were mixed at a ratio by weight of 1:1, o-dichlorobenzene was added, and the mixture was stirred at 80° C. for 4 hours to prepare an ink (I-5). 
     The ink (I-5) was applied to the electron transport layer by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 150° C. in a glove box under a nitrogen gas atmosphere for 10 minutes, to form an active layer as a solidified film. 
     The thickness of the active layer after drying was approximately 170 nm. Subsequently, a molybdenum oxide layer was formed on the active layer at a thickness of approximately 15 nm as a hole transport layer in a resistive heating vapor deposition device. 
     A silver (Ag) layer was then formed at a thickness of approximately 80 nm as an electrode (anode). Next, a UV-curing sealant was applied to the periphery of a formed layered body, bonded to the glass substrate that was a sealing substrate, and irradiated with UV light. The layered body was thus sealed to obtain a photoelectric conversion element. The planar shape as viewed in a thickness direction of the obtained photoelectric conversion element was a 2 mm×2 mm square. 
     The applied voltage was set to −2 V. EQE and dark current at this applied voltage were measured by a solar simulator (CEP-2000, manufactured by Bunkoukeiki Co., Ltd.) and a semiconductor parameter analyzer (Agilent Technology B1500A, manufactured by Agilent Technologies), respectively. Subsequently, the detectivity (D*) at an applied voltage of −2 V was calculated in the same manner as in Example 1. The results are shown in Table 6. 
     Comparative Examples 14 to 16 
     A photoelectric conversion element was prepared in the same manner as in Comparative Example 13 except that the thickness of an active layer was changed as shown in Table 6, and the photoelectric conversion element was evaluated in the same manner as in Comparative Example 13. The results are shown in Table 6. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                 Thickness of the 
                 EQE 
                 Dark current 
                 D* 
               
               
                   
                 active layer (nm) 
                 (%) 
                 (A/cm 2 ) 
                 (Jones) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative 
                 170 
                 61 
                 2.2 × 10 −2   
                 1.3 × 10 10   
               
               
                 example 13 
                   
                   
                   
                   
               
               
                 Comparative 
                 450 
                 35 
                 5.7 × 10 −3   
                 1.4 × 10 10   
               
               
                 example 14 
                   
                   
                   
                   
               
               
                 Comparative 
                 550 
                 21 
                 2.5 × 10 −3   
                 1.3 × 10 10   
               
               
                 example 15 
                   
                   
                   
                   
               
               
                 Comparative 
                 650 
                 15 
                 1.2 × 10 −3   
                 1.4 × 10 10   
               
               
                 example 16 
               
               
                   
               
            
           
         
       
     
     The photoelectric conversion elements of Comparative Examples 13 to 16 had approximately the same D* regardless of the thickness of the active layer. 
     Comparative Example 17 
     A glass substrate on which a thin film of ITO was formed at a thickness of 150 nm by a sputtering method was subjected to a UV ozone treatment as a surface treatment. 
     Next, a 45% by weight dispersion liquid of zinc oxide nanoparticles (particle diameter: 20 to 30 nm) in isopropanol (HTD-711Z, available from Tayca Corporation) was diluted with 3-pentanol in an amount of 10-fold parts by weight of the isopropanol dispersion liquid, to prepare a coating liquid. 
     The obtained coating liquid was applied to the thin film of ITO of the glass substrate at a thickness of 40 nm by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 150° C. for 10 minutes, to form an electron transport layer as a solidified film. 
     Subsequently, the polymer compound P-4 that was a p-type semiconductor material and C60PCBM (trade name: E100, available from Frontier Carbon Corporation) that was an n-type semiconductor material were mixed at a ratio by weight of 1:2, o-dichlorobenzene was added, and the mixture was stirred at 80° C. for 4 hours to prepare an ink (I-4). 
     The ink (I-4) was applied to the electron transport layer by a spin coating method, to form a coating film, and the coating film was dried on a hot plate heated to 70° C. for 5 minutes, to form an active layer as a solidified film. 
     The thickness of the active layer after drying was approximately 250 nm. Subsequently, a molybdenum oxide layer was formed on the active layer at a thickness of approximately 15 nm as a hole transport layer in a resistive heating vapor deposition device. A silver (Ag) layer was then formed at a thickness of approximately 80 nm as an anode. 
     Next, a UV-curing sealant was applied to the periphery of a formed layered body, bonded to the glass substrate that was a sealing substrate, and irradiated with UV light. The layered body was thus sealed to obtain a photoelectric conversion element. The planar shape as viewed in a thickness direction of the obtained photoelectric conversion element was a 2 mm×2 mm square. 
     The applied voltage was set to −5 V. EQE and dark current at this applied voltage were measured by a solar simulator (CEP-2000, manufactured by Bunkoukeiki Co., Ltd.) and a semiconductor parameter analyzer (Agilent Technology B1500A, manufactured by Agilent Technologies), respectively. Subsequently, the detectivity (D*) at an applied voltage of -5 V was calculated in the same manner as in Example 1. The results are shown in Table 7. 
     Comparative Example 18 
     A photoelectric conversion element was prepared in the same manner as in Comparative Example 17 except that the thickness of an active layer was changed as shown in Table 7, and the photoelectric conversion element was evaluated in the same manner as in Comparative Example 17. The results are shown in Table 7. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                 Thickness of the 
                 EQE 
                 Dark current 
                 D* 
               
               
                   
                 active layer (nm) 
                 (%) 
                 (A/cm 2 ) 
                 (Jones) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Comparative 
                 250 
                 55 
                 1.1 × 10 −2   
                 4.3 × 10 9   
               
               
                 example 17 
                   
                   
                   
                   
               
               
                 Comparative 
                 350 
                 60 
                 9.2 × 10 −3   
                 5.1 × 10 9   
               
               
                 example 18 
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     The photoelectric conversion elements of Comparative Examples 17 and 18 had approximately the same D* regardless of the thickness of the active layer. 
     EXPLANATIONS OF LETTERS OR NUMERALS 
     
         
           1  Image detection portion 
           2  Display device 
           10  Photoelectric conversion element 
           11 ,  210  Supporting substrate 
           12  Anode 
           13  Hole transport layer 
           14  Active layer 
           15  Electron transport layer 
           16  Cathode 
           17 ,  240  Sealing substrate 
           20  CMOS transistor substrate 
           30  Interlayer insulating layer 
           32  Interlayer wiring portion 
           40  Sealing layer 
           50  Color filter 
           100  Fingerprint detection portion 
           200  Display panel portion 
           200   a  Display region 
           220  Organic EL element 
           230  Touch sensor panel