Patent Publication Number: US-2022238832-A1

Title: Organic modulation element and modulation device

Description:
TECHNICAL FIELD 
     The present disclosure relates to an organic modulation element and a modulation device. 
     BACKGROUND 
     A solar cell including a wavelength conversion film and a photoelectric conversion element is described in International Publication WO 2013/054818. An organic fluorescent substance that converts light in a short wavelength region into light in a long wavelength region is used for the wavelength conversion film described in International Publication WO 2013/054818. 
     SUMMARY 
     The wavelength conversion film described in International Publication WO 2013/054818 only has a certain function of converting light in a short wavelength region into light in a long wavelength region, and it is difficult to modulate input light in correspondence with a purpose. 
     An object of the present disclosure is to provide an organic modulation element capable of modulating input light in correspondence with a purpose and a modulation device including the organic modulation element. 
     According to an aspect of the present disclosure, there is provided an organic modulation element including: an organic modulation layer containing a plurality of organic semiconductor molecules; a first electrode having a light-transmitting property with respect to input light to the organic modulation layer, and disposed on one side of the organic modulation layer; and a second electrode disposed on the other side of the organic modulation layer. Each of the plurality of organic semiconductor molecules is a molecule in which an excited state enabling reverse intersystem crossing from a lowest excited triplet state to a lowest excited singlet state in each of the plurality of organic semiconductor molecules is formed due to irradiation with the input light. In each of the plurality of organic semiconductor molecules, an intersystem crossing rate constant from the lowest excited singlet state to the lowest excited triplet state is greater than a reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state. In each of the plurality of organic semiconductor molecules, the reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state is greater than a non-radiative deactivation rate constant from the lowest excited triplet state to a ground state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of a modulation device according to a first embodiment. 
         FIG. 2  is an energy diagram of an organic modulation layer illustrated in  FIG. 1 . 
         FIGS. 3A and 3B  are configuration diagrams illustrating an operation of the modulation device illustrated in  FIG. 1 . 
         FIG. 4  is an energy diagram of an organic modulation layer according to a comparative example. 
         FIG. 5  is a graph showing voltage dependence of a quantum efficiency with respect to the organic modulation layer that includes a plurality of TADF molecules according to an example, and the organic modulation layer that includes a plurality of fluorescent molecules according to the comparative example. 
         FIGS. 6A and 6B  are configuration diagrams illustrating an operation of a modulation device according to a second embodiment. 
         FIG. 7  is a configuration diagram of a system for verifying a function of the modulation device according to the second embodiment. 
         FIGS. 8A and 8B  are graphs showing verification results relating to the function of the modulation device according to the second embodiment. 
         FIGS. 9A and 9B  are configuration diagrams illustrating an operation of a modulation device according to a third embodiment. 
         FIG. 10  is a configuration diagram of a system for verifying a function of the modulation device according to the third embodiment. 
         FIGS. 11A and 11B  are graphs showing verification results relating to the function of the modulation device according to the third embodiment. 
         FIG. 12  is a configuration diagram illustrating an operation of a modulation device according to a fourth embodiment. 
         FIG. 13  is a graph showing verification results relating to the function of the modulation device according to the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Note that, in the respective drawings, the same reference numeral will be given to the same or equivalent portion, and redundant description will be omitted. 
     First Embodiment 
     As illustrated in  FIG. 1 , a modulation device  1 A according to a first embodiment includes an organic modulation element  2 A. The organic modulation element  2 A according to the first embodiment includes an organic modulation layer  20 , a first electrode  21 , and a second electrode  22 . The first electrode  21  is disposed on one side (one side in a thickness direction of the organic modulation layer  20 ) of the organic modulation layer  20 . The second electrode  22  is disposed on the other side (side opposite to the one side in the thickness direction of the organic modulation layer  20 ) of the organic modulation layer  20 . As an example, the second electrode  22 , the organic modulation layer  20 , and the first electrode  21  are stacked on a support substrate having a light-transmitting property (not illustrated) in this order. 
     The first electrode  21  has a light-transmitting property with respect to input light L 1  to the organic modulation layer  20 . The second electrode  22  has a light-transmitting property with respect to output light L 2  from the organic modulation layer  20 . As a material of an electrode that functions as a positive electrode between the first electrode  21  and the second electrode  22 , for example, indium tin oxide, indium oxide, tin oxide, zinc oxide, and the like can be used. As a material of an electrode that functions as a negative electrode between the first electrode  21  and the second electrode  22 , for example, aluminum, vanadium, gold, silver, platinum, iron, cobalt, carbon, nickel, tungsten, palladium, magnesium, calcium, tin, lead, titanium, yttrium, lithium, ruthenium, manganese, alloys containing these metals, and the like can be used. As an example, in the case of an oxide type, the thickness of each of the first electrode  21  and the second electrode  22  is, for example, approximately 100 nm because transparency is high and resistance is high. In the case of a metal type, the thickness is approximately 10 nm because resistance is low and transparency is low. 
     Note that, a buffer layer (not illustrated) configured to adjust a work function of an electrode and/or a charge injection barrier may be formed on at least one surface between a surface of the first electrode  21  on the organic modulation layer  20  side and a surface of the second electrode  22  on the organic modulation layer  20  side. It is necessary for the buffer layer on the first electrode  21  side to have a light-transmitting property with respect to the input light L 1 . It is necessary for the buffer layer on the second electrode  22  side to have a light-transmitting property with respect to the output light L 2 . As a material of a buffer layer on an electrode side that functions as a positive electrode between the first electrode  21  and the second electrode  22 , for example, metal oxides such as molybdenum oxide, tungsten oxide, nickel oxide, and vanadium oxide, a conductive polymer represented by PEDOT:PSS, and the like can be used. As a material of a buffer layer on an electrode side that functions as a negative electrode between the first electrode  21  and the second electrode  22 , for example, metals such as magnesium, silver, calcium, titanium, zinc, cesium, and lithium, alloys containing these metals, metal oxides such as titanium oxide and zinc oxide, alkali metal compounds such as lithium fluoride, sodium fluoride, potassium fluoride, and cesium fluoride, alkali-earth metal compounds such as magnesium fluoride and calcium fluoride, and the like can be used. Since the buffer layers are necessary to have a light-transmitting property with respect to the input light L 1  or the output light L 2 , for example, the thickness of the buffer layers is preferably 50 nm or less. 
     In addition, a hole transport layer or an electron block layer may be disposed between an electrode that functions as a positive electrode between the first electrode  21  and the second electrode  22 , and the organic modulation layer  20 . An electron transport layer or a hole block layer may be disposed between an electrode that functions as a negative electrode between the first electrode  21  and the second electrode  22 , and the organic modulation layer  20 . A layer on the first electrode  21  side among the layers is necessary to have a light-transmitting property with respect to the input light L 1 . A layer on the second electrode  22  side among the layers is necessary to have a light-transmitting property with respect to the output light L 2 . As a material of the hole transport layer or the electron block layer, for example, metal oxides, porphyrin derivatives, phthalocyanine derivatives, oxazole derivatives, oxadiazole derivatives, triazole derivatives, imidazole derivatives, pyrazoline derivatives, pyrazolone derivatives, phenylenediamine derivatives, hydrazone derivatives, stilbene derivatives, polyarylalkane derivatives, triarylamine derivatives, carbazole derivatives, indolocarbazole derivatives, isoindole derivatives, acene-based derivatives, fluorene derivatives, fluorenone derivatives, polyvinylcarbazoles, polymer materials with aromatic amine introduced into a main chain or a side chain, oligomers, polysilanes, conductive polymers, and the like can be used. As a material of the electron transport layer or the hole block layer, for example, nitrogen-containing aromatic heterocyclic derivatives, dibenzofuran derivatives, dibenzothiophene derivatives, silole derivatives, aromatic hydrocarbon ring derivatives, and the like can be used. 
     The organic modulation layer  20  contains a plurality of organic semiconductor molecules  20   a  and a plurality of host molecules  20   b . The plurality of organic semiconductor molecules  20   a  are constituted by the same kind of organic semiconductor molecules, and the plurality of host molecules  20   b  are constituted by the same kind of host molecules. That is, the organic modulation layer  20  contains one kind of organic semiconductor molecules  20   a  and one kind of host molecules  20   b . In the organic modulation layer  20 , charge separation occurs due to irradiation with the input light L 1  (details thereof will be described later). As an example, the organic modulation layer  20  is formed on the second electrode  22  formed on the support substrate by vacuum evaporation of TPA-DCPP (the plurality of organic semiconductor molecules  20   a ) and CBP (the plurality of host molecules  20   b ) from evaporation sources different from each other. For example, the thickness of the organic modulation layer  20  is approximately 100 nm. For example, a mass ratio between the CBP and TPA-DCPP in the organic modulation layer  20  is 50:50. 
     The modulation device  1 A further includes a controller  3 . The controller  3  is electrically connected to the organic modulation element  2 A. The controller  3  applies a voltage between the first electrode  21  and the second electrode  22  so that an electric field in a direction of causing charge separation occurs in the organic modulation layer  20 , and modulates the voltage. As an example, in a case where the first electrode  21  is caused to function as a positive electrode and the second electrode  22  is caused to function as a negative electrode, the controller  3  may apply a voltage between the first electrode  21  and the second electrode  22  so that a potential of the first electrode  21  becomes negative with a potential of the second electrode  22  set as a reference, or may apply a voltage between the first electrode  21  and the second electrode  22  so that the potential of the second electrode  22  becomes positive with the potential of the first electrode  21  set as a reference. In addition, in a case where the first electrode  21  is caused to function as a negative electrode and the second electrode  22  is caused to function as a positive electrode, the controller  3  may apply a voltage between the first electrode  21  and the second electrode  22  so that the potential of the first electrode  21  becomes positive with the potential of the second electrode  22  set as a reference, or may apply a voltage between the first electrode  21  and the second electrode  22  so that the potential of the second electrode  22  becomes negative with the potential of the first electrode  21  set as a reference. For example, the controller  3  is constituted by a voltage application circuit, a current read-out circuit, or the like. 
       FIG. 2  is an energy diagram of the organic modulation layer  20  illustrated in  FIG. 1 . More specifically,  FIG. 2  is an energy diagram of the organic modulation layer  20  in which an excited state is formed due to irradiation with the input light L 1 . In  FIG. 2 , S 0  represents a ground state, S 1  represents a lowest excited singlet state, T 1  represents a lowest excited triplet state. k s   r  represents a fluorescence rate constant from the lowest excited singlet state S 1  to the ground state S 0 , k s   nr  represents a non-radiative deactivation rate constant from the lowest excited singlet state S 1  to the ground state S 0 . k T   r  represents a phosphorescence rate constant from the lowest excited triplet state T 1  to the ground state S 0 , k T   nr  represents a non-radiative deactivation rate constant from the lowest excited triplet state T 1  to the ground state S 0 . k ISC  represents an intersystem crossing rate constant from the lowest excited singlet state S 1  to the lowest excited triplet state T 1 , and k RISC  represents a reverse intersystem crossing rate constant from the lowest excited triplet state T 1  to the lowest excited singlet state S 1 . 
     As illustrated in  FIG. 2 , each of the plurality of organic semiconductor molecules  20   a  is a molecule in which an excited state enabling reverse intersystem crossing from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  in each of the plurality of organic semiconductor molecules  20   a  is formed due to irradiation with the input light L 1 . In this case, intersystem crossing from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  is possible in each of the plurality of organic semiconductor molecules  20   a . Each of the plurality of organic semiconductor molecules  20   a  preferably includes a donor functional group, and an acceptor functional group. 
     Note that, the reverse intersystem crossing from “lowest excited triplet state T 1  to lowest excited singlet state S 1 ” includes not only “reverse intersystem crossing in which an excited molecule transitions from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  without through another energy level”, but also “reverse intersystem crossing in which an excited molecule transitions from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  through another energy level (for example, a high-order energy level or the like)”. Similarly, “intersystem crossing from the lowest excited singlet state S 1  to the lowest excited triplet state T 1 ” includes not only “intersystem crossing in which an excited molecule transitions from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  without through another energy level”, but also “intersystem crossing in which an excited molecule transitions from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  through another energy level (for example, a high-order energy level or the like)”. 
     In each of the plurality of organic semiconductor molecules  20   a , a difference between energy of the lowest excited singlet state S 1  and energy of the lowest excited triplet state T 1  at an absolute temperature of 77 K is less than 0.3 eV. Note that, although not illustrated in  FIG. 2 , energy of a lowest excited triplet state at an absolute temperature of 77 K in each of the plurality of host molecules  20   b  is higher than energy of a lowest excited triplet state T 1  at an absolute temperature of 77 K in each of the plurality of organic semiconductor molecules  20   a.    
     An energy level E S1  that is energy of the lowest excited singlet state S 1  is obtained as follows. First, a film composed of the plurality of organic semiconductor molecules  20   a  is formed on an Si wafer. In the case of performing film formation by an evaporation method, a film thickness is preferably 50 nm to 100 nm. In the case of performing film formation by a spin coating method, a film thickness may be approximately 30 nm. Next, a fluorescence spectrum of the film composed of the plurality of organic semiconductor molecules  20   a  is measured at normal temperature (absolute temperature of 300 K). In measurement of the fluorescence spectrum, an LED, a tungsten lamp, or a deuterium lamp is used as an excitation light source, and a multi-channel spectroscope (C10027, manufactured by HAMAMATSU PHOTONICS K.K.) is used as a detector. Next, a tangent line is drawn with respect to short-wavelength side rising of a light-emission spectrum in which the vertical axis represents optical intensity and the horizontal axis represents a wavelength, a wavelength value λ edge  (nm) of an intersection between the tangent line and the horizontal axis is obtained, and the energy level E S1  is obtained from the following Expression (1). 
         E   S1  (eV)=1239.85/λ edge    (1)
 
     Note that, the tangent line drawn with respect to the short-wavelength side rising of the fluorescence spectrum is a tangent line in which an inclination becomes a maximum value among tangent lines drawn at respective points of the fluorescence spectrum in a range from the short wavelength side of the fluorescence spectrum to a maximum value of the fluorescence spectrum appearing on the shortest wavelength side. A maximum value having peak intensity that is 10% or less of maximum peak intensity of the fluorescence spectrum does not become “the maximum value of the fluorescence spectrum appearing on the shortest wavelength side”. 
     An energy level E T1  that is energy of the lowest excited triplet state T 1  at an absolute temperature of 77 K is obtained as follows. First, an Si wafer (described above) on which the film composed of the plurality of organic semiconductor molecules  20   a  is formed is cooled down to the absolute temperature of 77 K by a cryostat (Optistat DN manufactured by Oxford Instruments). Next, in measurement of a phosphorescence spectrum, the same excitation light source and detector as in the measurement of the fluorescence spectrum are used, and a phosphorescence spectrum of the film composed of the plurality of organic semiconductor molecules  20   a  is measured. Next, a tangent line is drawn with respect to short-wavelength side rising of a phosphorescence spectrum in which the vertical axis represents optical intensity and the horizontal axis represents a wavelength, a wavelength value λ edge  (nm) of an intersection between the tangent line and the horizontal axis is obtained, and the energy level E T1  is obtained from the following Expression (2). 
         E   T1  (eV)=1239.85/λ edge    (2)
 
     Note that, the tangent line drawn with respect to the short-wavelength side rising of the phosphorescence spectrum is a tangent line in which an inclination becomes a maximum value among tangent lines drawn at respective points of the phosphorescence spectrum in a range from the short wavelength side of the phosphorescence spectrum to a maximum value of the phosphorescence spectrum appearing on the shortest wavelength side. A maximum value having peak intensity that is 10% or less of maximum peak intensity of the phosphorescence spectrum does not become “the maximum value of the phosphorescence spectrum appearing on the shortest wavelength side”. 
     In each of the plurality of organic semiconductor molecules  20   a , the intersystem crossing rate constant k ISC  from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  is greater than the fluorescence rate constant k S   r  from the lowest excited singlet state S 1  to the ground state S 0 . In each of the plurality of organic semiconductor molecules  20   a , the intersystem crossing rate constant k ISC  from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  is greater than the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1 . In each of the plurality of organic semiconductor molecules  20   a , the intersystem crossing rate constant k ISC  from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  is two or more times the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1 . In each of the plurality of organic semiconductor molecules  20   a , the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  is 1×10 7  (sec −1 ) or less. In each of the plurality of organic semiconductor molecules  20   a , the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  is greater than the non-radiative deactivation rate constant k T   nr  from the lowest excited triplet state T 1  to the ground state S 0 . 
     The fluorescence rate constant k S   r  from the lowest excited singlet state S 1  to the ground state S 0 , the intersystem crossing rate constant k ISC  from the lowest excited singlet state S 1  to the lowest excited triplet state T 1 , and the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  are obtained as follows. First, a film composed of the plurality of organic semiconductor molecules  20   a  is formed on a quartz substrate. In the case of performing film formation by an evaporation method, a film thickness is preferably 50 to 100 nm. In the case of performing film formation by a spin coating method, the film thickness may be approximately 30 nm. Next, a light-emission quantum yield (the number of light-emitting photons/the number of light-absorbing photons) of the film composed of the plurality of organic semiconductor molecules  20   a  is measured at normal temperature (300 K) by using an absolute PL quantum yield measuring instrument (C11347, manufactured by HAMAMATSU PHOTONICS K.K.). In the measurement, excitation is performed with light in an absorption wavelength band of the film composed of the plurality of organic semiconductor molecules  20   a , a light-emission quantum yield is calculated from a light-emission spectrum detected under a condition in which absorbance (“Abs” in the measuring instrument) becomes 0.1 to 0.9, and an average value in measurement performed three times is set as “Φ”. 
     Next, a fluorescence lifetime of the film composed of the plurality of organic semiconductor molecules  20   a  is measured by using a small-sized fluorescence lifetime measuring instrument (C11367, manufactured by HAMAMATSU PHOTONICS K.K.). In the measurement, excitation is performed with light in the absorption wavelength band of the film composed of the plurality of organic semiconductor molecules  20   a , a peak wavelength of a light-emission spectrum obtained in measurement of a light-emission quantum yield is set as “detection wavelength”. Note that, all measurements are performed in the number of times of integration so that a peak count becomes 10,000 or greater. In the film composed of the plurality of organic semiconductor molecules  20   a , among a lifetime of prompt fluorescence component (τ p ) that emits light from the lowest excited singlet state S 1  without through the lowest excited triplet state T 1 , an intersystem crossing rate from the lowest excited singlet state S 1  to the lowest excited triplet state T 1 , and a lifetime of delay fluorescence component (τ d ) that emits light after through reverse intersystem crossing from the lowest excited triplet state T 1  to the lowest excited singlet state S 1 , at least two components are measured. However, it is necessary to perform measurement in a time range suitable for each time scale, and thus measurement is performed in the following procedure. 
     First, a fluorescence lifetime is measured in a time range in which τ d  (sec) can be measured in the film composed of the plurality of organic semiconductor molecules  20   a . Since it is necessary to view a time-resolved light-emission waveform of excitation light when calculating reasonable τ d , Spectron (standard diffusion reflection plate) is set as a measurement target, and an instrument response function (IRF) is measured under the same measurement conditions (an excitation wavelength, a repetition frequency, and a time range) as in the measurement of the fluorescence lifetime. Next, 1 st  order “Tail fit” (exponential function fitting of a single component) in a measurement instrument is performed with respect to a previously measured molecular light-emission decay curve, and τ d  (sec) is calculated. With regard to a calculation range when performing the Tail fit, after performing measurement in a time range sufficiently wider than an excitation pulse obtained in the IRF measurement, calculation is performed in a time region capable of obtaining S/N of light-emission intensity (a region in which an average value of light-emission intensity of 21 points including 10 point before and after time of interest is greater by 5 counts with respect to a dark level) after a late time between time when a light-emission decay curve of a prompt component is sufficiently attenuated or after time when excitation pulse light is attenuated to a dark level. 
     Next, in order to calculate τ p , measurement on the film composed of the plurality of organic semiconductor molecules  20   a  is performed in a time range (for example, one microsecond) in which time resolution of the light-emission curve of the prompt component can be sufficiently secured, and a light-emission decay curve of a delay component is also included. Next, the IRF is measured under the same conditions with Spectron set as a measurement target, a multi-component exponential function fitting is performed with previously obtained τ p  set as a fixed parameter, and τ p  (sec), and the prompt and delay components are calculated. Note that, with regard to the prompt component, a light-emission decay process of two or more components may be included due to intermolecular interaction. In this case, fitting is performed with three or more components including the prompt and delay components, but light-emission decay without through reverse intersystem crossing is defined as a prompt component and is calculated. A fitting method conforms to a method described in manual of C11367. 
     Since a prompt component Φ p  and a delay component Φ d  of the light-emission quantum yield (Φ=Φ p +Φ d ), and a prompt component τ p  and a delay component τ d  of the fluorescence lifetime are obtained through the above-described measurement, various rate constants (k S   r , k ISC , k RISC , and k T   nr ) are calculated by the following Expressions (3) to (8) (however, non-radiative deactivation from the lowest excited singlet state S 1  is ignored). 
       τ p =1 /k   p    (3)
 
       τ d =1 /k   d    (4)
 
       k S   r =Φ p k p    (5)
 
         k   ISC =(1−Φ p ) k   p    (6)
 
         k   RISC =( k   p   k   d Φ d )/( k   ISC Φ p )   (7)
 
         k   T   nr   =k   d −Φ p   k   RISC    (8)
 
     Note that, in a case where an ideal molecular design as a modulation material is realized, k S   r  is expected to be significantly lower than k ISC , and as a result, light-emission intensity of the prompt component is attenuated, and T 1  generation efficiency increases. Under this condition, in a case where k RISC  is small and a non-radiative deactivation rate constant k T   nr  from T 1  is significantly smaller than k RISC , a very long light-emission lifetime is observed. In the case of this condition, sufficient S/N for rate constant calculation may not be obtained in fluorescence lifetime measurement, but this is a phenomenon that can be regarded as convergence to ideal conditions (k RISC →0). In addition, in a polar organic semiconductor molecules having a dipole moment of 0 D or greater, since a phenomenon in which excited molecules formed by photoexcitation in a film does not contribute to light-emission and is spontaneously decomposed to charges may occur, there is a possibility that the components are not measured as light-emission, and sufficient S/N for rate constant calculation may not be obtained in the fluorescence lifetime measurement. 
     A dipole moment (μ) of each of the plurality of organic semiconductor molecules  20   a  is greater than 0 D. The dipole moment is a value that can be calculated by quantum chemistry calculation, and is a value that is typically calculated by a calculation method called a Hartree-Fock (HF) method or a density functional theory (DFT) method. Among these, the most commonly used condition (a combination of the functional theory and a basis function) is B3LYP/6-31(d). 
     Examples of the organic semiconductor molecules  20   a  that satisfy the above-described conditions include TADF molecule such as TPA-DCPP, 4CzIPN, and 4CzTPN. The TADF molecule is a molecule that is known in a light-emission field, but can be used as the organic semiconductor molecules  20   a  that satisfy the above-described conditions. The organic semiconductor molecules  20   a  is designed by quantum chemistry calculation or the like so as to satisfy the above-described conditions, and is generated by organic synthesis. Note that, examples of the host molecules  20   b  that satisfy the above-described conditions include CBP, mCBP, T2T, mCP, PPT, DPEPO, and the like. 
     In the organic modulation layer  20  configured as described above, an excited state enabling reverse intersystem crossing from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  is formed in each of the plurality of organic semiconductor molecules  20   a  due to irradiation with the input light L 1 . According to this, in each of the plurality of organic semiconductor molecules  20   a , in addition to high-speed charge separation observed until the lowest excited singlet state S 1  is mitigated, charge separation from the lowest excited triplet state T 1  having a longer lifetime in comparison to the lowest excited singlet state S 1  (hereinafter, referred to as “direct charge separation from lowest excited triplet state T 1 ”), and/or charge separation from the lowest excited singlet state S 1  in which the lowest excited triplet state T 1  having a longer lifetime in comparison to the lowest excited singlet state S 1  is set as a temporary shelter from deactivation (hereinafter, referred to as “charge separation from the lowest excited singlet state S 1  through the lowest excited triplet state T 1 ”) occur. 
     In the modulation device  1 A, characteristics of the organic modulation layer  20  are used, and the input light L 1  is modulated as follows. As illustrated in  FIG. 3A , in a state in which a voltage is not applied between the first electrode  21  and the second electrode  22  by the controller  3 , and an electric field in a direction of causing charge separation does not occur in the organic modulation layer  20 , when the input light L 1  is incident to the organic modulation layer  20  through the first electrode  21 , a light beam La included in the input light L 1  is absorbed to the organic modulation layer  20 , and a light beam Lb is emitted to the outside due to spontaneous emission through the second electrode  22 . At this time, a light beam Lc included in the input light L 1  is not absorbed to the organic modulation layer  20  and is emitted to the outside through the second electrode  22 . Accordingly, in this case, output light L 2  including the light beam Lb and the light beam Lc is obtained. Note that, the light beam La, the light beam Lb, and the light beam Lc are light beams of wavelength bands different from each other. 
     As illustrated in  FIG. 3B , in a state in which a voltage is applied between the first electrode  21  and the second electrode  22  by the controller  3 , and an electric field in a direction of causing charge separation occurs in the organic modulation layer  20 , when the input light L 1  is incident to the organic modulation layer  20  through the first electrode  21 , the light beam La included in the input light L 1  is absorbed to the organic modulation layer  20 , but molecules in an excited state are attenuated due to charge separation, and thus the light beam Lb (refer to  FIG. 3A ) is not generated. At this time, the light beam Lc included in the input light L 1  is not absorbed to the organic modulation layer  20 , and is emitted to the outside through the second electrode  22 . Accordingly, in this case, output light L 2  that does not include the light beam Lb and includes the light beam Lc is obtained. Note that, when increasing a value (absolute value) of the voltage that causes an electric field in a direction of causing charge separation to occur in the organic modulation layer  20 , a ratio of the light beam Lb included in the output light L 2  can be reduced, and when decreasing the value of the voltage, a ratio of the light beam Lb included in the output light L 2  can be increased. 
     As described above, in the organic modulation element  2 A, an excited state enabling reverse intersystem crossing from the lowest excited triplet state T 1  the lowest excited singlet state S 1  is formed in each of the plurality of organic semiconductor molecules  20   a  due to irradiation with the input light L 1  transmitted through the first electrode  21 . According to this, with respect to each of the plurality of organic semiconductor molecules  20   a , in addition to high-speed charge separation observed until the lowest excited singlet state S 1  is mitigated, direct charge separation from the lowest excited triplet state T 1 , and/or charge separation from the lowest excited singlet state S 1  through the lowest excited triplet state T 1  become possible. Here, in each of the plurality of organic semiconductor molecules  20   a , since the intersystem crossing rate constant k ISC  from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  is greater than the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1 , the lifetime of the lowest excited triplet state T 1  is lengthened, and thus sufficient charge separation is likely to be obtained. Accordingly, for example, when a voltage is applied between the first electrode  21  and the second electrode  22  under irradiation with the input light L 1  to cause an electric field in a direction of causing charge separation to occur in the organic modulation layer  20 , an excited state in which deactivation (spontaneous emission) is to originally occur according to light-emission is converted into a charge separation state, and thus attenuation is possible. On the other hand, in each of the plurality of organic semiconductor molecules  20   a , since the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  is greater than the non-radiative deactivation rate constant k T   nr  from the lowest excited triplet state T 1  to the ground state S 0 , deactivation due to heat is less likely to occur, and light-emission is likely to be obtained. Accordingly, for example, when a voltage is not applied between the first electrode  21  and the second electrode  22 , and an electric field in a direction of causing charge separation is not caused to occur in the organic modulation layer  20 , light-emission due to spontaneous emission can be promoted. As described above, according to the organic modulation element  2 A, the input light L 1  can be modulated in correspondence with a purpose. 
     According to the organic modulation element  2 A, in each of the plurality of organic semiconductor molecules  20   a , a difference between energy of the lowest excited singlet state S 1  and energy of the lowest excited triplet state T 1  at an absolute temperature of 77 K is less than 0.3 eV. According to this, the individual organic semiconductor molecules  20   a  can be caused to function as molecules in which an excited state enabling reverse intersystem crossing from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  is formed due to irradiation with the input light L 1 . 
     According to the organic modulation element  2 A, in each of the plurality of organic semiconductor molecules  20   a , the intersystem crossing rate constant k ISC  from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  is greater than the fluorescence rate constant k S   r  from the lowest excited singlet state S 1  to the ground state S 0 . According to this, direct charge separation from the lowest excited triplet state T 1  and/or charge separation from the lowest excited singlet state S 1  through the lowest excited triplet state T 1  become possible while suppressing deactivation from the lowest excited singlet state S 1  due to light-emission. 
     According to the organic modulation element  2 A, in each of the plurality of organic semiconductor molecules  20   a , the intersystem crossing rate constant k ISC  from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  is two or times the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1 . According to this, the lifetime of the lowest excited triplet state T 1  is lengthened, and thus photoelectric conversion efficiency can be improved. 
     According to the organic modulation element  2 A, in each of the plurality of organic semiconductor molecules  20   a , the reverse intersystem crossing rate constant k RISC  from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  is 1×10 7  (sec −1 ) or less. According to this, the lifetime of the lowest excited triplet state T 1  is lengthened, and thus photoelectric conversion efficiency can be improved. 
     According to the organic modulation element  2 A, a dipole moment of each of the plurality of organic semiconductor molecules  20   a  is greater than 0 D. According to this, energy for charge separation can be reduced. 
     According to the organic modulation element  2 A, energy of the lowest excited triplet state T 1  at an absolute temperature of 77 K in each of the plurality of host molecules  20   b  is higher than energy of the lowest excited triplet state T 1  at an absolute temperature of 77 K in each of the plurality of organic semiconductor molecules  20   a . According to this, transition of excited molecules from the lowest excited triplet state T 1  of the organic semiconductor molecules  20   a  to the lowest excited triplet state T 1  of the host molecules  20   b , and a loss path of the host molecules  20   b  in the lowest excited triplet state T 1  in a case where an excited state is reformed due to recoupling of charges generated by charge separation are suppressed. That is, it is possible to suppress a decrease in photoelectric conversion efficiency due to an influence of the plurality of host molecules  20   b.    
     In the organic modulation element  2 A, the second electrode  22  has light-transmitting property with respect to the output light L 2  from the organic modulation layer  20 . According to this, the modulated output light L 2  can be allowed to be emitted to a side opposite to an incident side of the input light L 1 . 
     In the modulation device  1 A, the controller  3  modulates a voltage that is applied between the first electrode  21  and the second electrode  22 . According to this, the input light L 1  can be modulated in correspondence with a purpose. 
       FIG. 4  is an energy diagram of an organic modulation layer according to a comparative example. The organic modulation layer according to the comparative example illustrated in  FIG. 4  includes a plurality of organic semiconductor molecules  20   c . The organic semiconductor molecules  20   c  are fluorescent molecules (for example, Alq3 or the like). In the organic modulation layer according to the comparative example illustrated in  FIG. 4 , a difference between the energy of the lowest excited singlet state S 1  and the energy of the lowest excited triplet state T 1  at an absolute temperature of 77 K is large, and even when being irradiated with light, intersystem crossing from the lowest excited singlet state S 1  to the lowest excited triplet state T 1  does not occur in the individual organic semiconductor molecules  20   c . Therefore, neither direct charge separation from the lowest excited triplet state T 1  nor charge separation from the lowest excited singlet state S 1  through the lowest excited triplet state T 1  occurs. Accordingly, in the organic modulation layer according to the comparative example illustrated in  FIG. 4 , a dissociation effect to charges decreases, and components which are lost as light or heat are increased. As a result, the organic modulation layer according to the comparative example illustrated in  FIG. 4  does not function as an effective modulation layer. 
       FIG. 5  is a graph showing voltage dependence of a quantum efficiency with respect to the organic light-receiving layer that includes a plurality of TADF molecules according to the example, and the organic light-receiving layer that includes a plurality of fluorescent molecules according to the comparative example. In  FIG. 5 , “quantum efficiency” is a ratio of “the number of electrons detected from the organic light-receiving layer” to “the number of photons absorbed by the organic light-receiving layer”. In addition, “voltage” is a voltage that is applied to the organic light-receiving layer at the time of irradiation with light, and a negative value is a value that causes an electric field in a direction of causing charge separation to occur in the organic light-receiving layer (the same shall apply hereinafter). As shown in  FIG. 5 , in the organic light-receiving layer including the plurality of TADF molecules according to the example, the quantum efficiency becomes higher and a variation rate of the quantum efficiency with respect to a voltage becomes larger in comparison to the organic light-receiving layer including the plurality of fluorescent molecules according to the comparative example. This represents that the organic light-receiving layer according to the example functions as an effective modulation layer. 
     Second Embodiment 
     As illustrated in  FIGS. 6A and 6B , a modulation device  1 B according to a second embodiment is different from the modulation device  1 A according to the first embodiment in that an organic modulation element  2 B includes a light-receiving layer  23 . The organic modulation element  2 B according to the second embodiment includes the organic modulation layer  20 , the first electrode  21  disposed on one side of the organic modulation layer  20 , the second electrode  22  disposed on the other side of the organic modulation layer  20 , and the light-receiving layer  23  disposed on the other side of the second electrode  22 . The light-receiving layer  23  receives output light L 2 . For example, the light-receiving layer  23  constitutes a PD, and charges generated by reception of the output light L 2  are detected as a current signal from an electrode (not illustrated) that is electrically connected to the light-receiving layer  23 . 
     According to the organic modulation element  2 B configured as described above, input light L 1  is modulated in correspondence with a purpose, and the output light L 2  modulated in correspondence with the purpose can be detected. 
     Specifically, as illustrated in  FIG. 6A , in a state in which a voltage is not applied between the first electrode  21  and the second electrode  22  by the controller  3 , and an electric field in a direction of causing charge separation does not occur in the organic modulation layer  20 , when the input light L 1  is incident to the organic modulation layer  20  through the first electrode  21 , the light beam La included in the input light L 1  is absorbed to the organic modulation layer  20  and light-emission due to charge separation spontaneous emission in the organic modulation layer  20  occurs, and a light beam Lb is incident to the light-receiving layer  23  through the second electrode  22 . At this time, a light beam Lc included in the input light L 1  is not absorbed to the organic modulation layer  20  and is incident to the light-receiving layer  23  through the second electrode  22 . Accordingly, in this case, output light L 2  including the light beam Lb and the light beam Lc is detected. 
     As illustrated in  FIG. 6B , in a state in which a voltage is applied between the first electrode  21  and the second electrode  22  by the controller  3 , and an electric field in a direction of causing charge separation occurs in the organic modulation layer  20 , when the input light L 1  is incident to the organic modulation layer  20  through the first electrode  21 , the light beam La included in the input light L 1  is absorbed to the organic modulation layer  20 , and thus charge separation occurs in the organic modulation layer  20 , but light-emission due to spontaneous emission does not occur, and the light beam Lb (refer to  FIG. 6A ) is not generated. At this time, the light beam Lc included in the input light L 1  is not absorbed to the organic modulation layer  20 , and is incident to the light-receiving layer  23  through the second electrode  22 . Accordingly, in this case, the output light L 2  that does not include the light beam Lb and includes the light beam Lc is detected. In this manner, in a state in which a voltage is applied between the first electrode  21  and the second electrode  22 , the light beam La included in the input light L 1  can be detected by the organic modulation layer  20 , and the light beam Lc included in the input light L 1  can be detected by the light-receiving layer  23 . Note that, when increasing a value (absolute value) of the voltage that causes an electric field in a direction causing charge separation to occur in the organic modulation layer  20 , a ratio of the light beam Lb included in the output light L 2  can be reduced, and when decreasing the value of the voltage, a ratio of the light beam Lb included in the output light L 2  can be increased. 
       FIG. 7  is a configuration diagram of a system S 1  configured to verify a function of a modulation device  1 B according to the second embodiment. In the system S 1 , an element in which the organic modulation layer  20  containing TPA-DCPP (the plurality of organic semiconductor molecules  20   a ) and CBP (the plurality of host molecules  20   b ) in a mass ratio of 50:50 is disposed between the first electrode  21  and the second electrode  22  is used as the organic modulation element  2 B. As a material of the first electrode  21 , a material having a light-transmitting property with respect to the input light L 1  and the output light L 2  is used. As a material of the second electrode  22 , a material that reflects the input light L 1  and the output light L 2  is used. In the system S 1 , the input light L 1  emitted from an LED  4  is incident to the organic modulation layer  20  through the first electrode  21 . In addition, the output light L 2  emitted from the organic modulation layer  20  to the outside through the first electrode  21  is incident to an SiPD 6 through an optical fiber  5 . 
     The following results are obtained through verification in the system S 1 . As shown in  FIG. 8A , a current value detected by the SiPD 6 in a state in which a voltage is not applied between the first electrode  21  and the second electrode  22  is greater than a current value detected by the SiPD 6 in a state in which a voltage is applied between the first electrode  21  and the second electrode  22 . In contrast, as shown in  FIG. 8B , a current value detected by the organic modulation layer  20  in a state in which a voltage is applied between the first electrode  21  and the second electrode  22  is greater than a current value detected by the organic modulation layer  20  in a state in which a voltage is not applied between the first electrode  21  and the second electrode  22 . The results represent that the modulation device  1 B according to the second embodiment appropriately functions. 
     Third Embodiment 
     As illustrated in  FIGS. 9A and 9B , a modulation device  1 C according to a third embodiment is different from the modulation device  1 A according to the first embodiment in that an organic modulation element  2 C includes a light-emitting layer  24 . The organic modulation element  2 C according to the third embodiment includes the organic modulation layer  20 , the first electrode  21  disposed on one side of the organic modulation layer  20 , the second electrode  22  disposed on the other side of the organic modulation layer  20 , and the light-emitting layer  24  disposed on one side of the first electrode  21 . The light-emitting layer  24  emits input light L 1 . For example, the light-emitting layer  24  constitutes an LED. 
     According to the organic modulation element  2 C configured as described above, input light L 1  is modulated in correspondence with a purpose, and output light L 2  modulated in correspondence with the purpose can be emitted. 
     Specifically, as illustrated in  FIG. 9A , in a state in which a voltage is not applied between the first electrode  21  and the second electrode  22  by the controller  3 , and an electric field in a direction of causing charge separation does not occur in the organic modulation layer  20 , when the input light L 1  is emitted from the light-emitting layer  24  and is incident to the organic modulation layer  20  through the first electrode  21 , a partial light beam L 11  in the input light L 1  is absorbed to the organic modulation layer  20 , and thus light-emission due to charge separation and spontaneous emission occurs in the organic modulation layer  20 , and a light beam Lb is emitted to the outside through the second electrode  22 . At this time, the remaining light beam L 12  in the input light L 1  is not absorbed to the organic modulation layer  20 , and is emitted to the outside through the second electrode  22 . Accordingly, in this case, the output light L 2  including a light beam Lb and the light beam L 12  is emitted to the outside. 
     As illustrated in  FIG. 9B , in a state in which a voltage is applied between the first electrode  21  and the second electrode  22  by the controller  3 , and an electric field in a direction of causing charge separation occurs in the organic modulation layer  20 , when the input light L 1  is emitted from the light-emitting layer  24  and the input light L 1  is incident to the organic modulation layer  20  through the first electrode  21 , the partial light beam L 11  in the input light L 1  is absorbed to the organic modulation layer  20 , and thus charge separation occurs in the organic modulation layer  20 , but light-emission due to spontaneous emission does not occur, and the light beam Lb (refer to  FIG. 9A ) does not occur. At this time, the remaining light beam L 12  in the input light L 1  is not absorbed to the organic modulation layer  20 , and is emitted to the outside through the second electrode  22 . Accordingly, in this case, output light L 2  that does not include the light beam Lb and includes the light beam L 12  is emitted to the outside. Note that, when increasing a value (absolute value) of the voltage that causes an electric field in a direction of causing charge separation to occur in the organic modulation layer  20 , a ratio of the light beam Lb included in the output light L 2  can be reduced, and when decreasing the value of the voltage, a ratio of the light beam Lb included in the output light L 2  can be increased. 
       FIG. 10  is a configuration diagram of a system S 2  configured to verify a function of the modulation device  1 C according to the third embodiment. In the system S 2 , an element in which the organic modulation layer  20  containing TPA-DCPP (the plurality of organic semiconductor molecules  20   a ) and CBP (the plurality of host molecules  20   b ) in a mass ratio of 50:50 is disposed between the first electrode  21  and the second electrode  22  is used as the organic modulation element  2 C. As a material of the first electrode  21 , a material having a light-transmitting property with respect to the input light L 1  and the output light L 2  is used. As a material of the second electrode  22 , a material that reflects the input light L 1  and the output light L 2  is used. In the system S 2 , the input light L 1  emitted from an LED  4  is incident to the organic modulation layer  20  through the first electrode  21 . In addition, the output light L 2  emitted from the organic modulation layer  20  to the outside through the first electrode  21  is incident to a spectrometer  7  through the optical fiber  5 . 
     The following results are obtained through verification in the system S 2 .  FIG. 11A  is a graph showing a spectrum of the output light L 2  detected by the spectrometer  7  in a case where blue input light L 1  is emitted from the LED  4 . As shown in  FIG. 11A , when increasing a value (absolute value) of the voltage that causes an electric field in a direction of causing charge separation to occur in the organic modulation layer  20 , a ratio of a red light beam included in the output light L 2  decreases.  FIG. 11B  is a graph showing a spectrum of the output light L 2  detected by the spectrometer  7  in a case where green input light L 1  is emitted from the LED  4 . As shown in  FIG. 11B , when increasing a value (absolute value) of the voltage that causes an electric field in a direction of causing charge separation to occur in the organic modulation layer  20 , a ratio of a red light beam included in the output light L 2  decreases. The results represent that the modulation device  1 C according to the third embodiment appropriately functions. 
     Fourth Embodiment 
     As illustrated in  FIG. 12 , a modulation device  1 D according to a fourth embodiment is different from the modulation device  1 A according to the first embodiment in that the second electrode  22  in an organic modulation element  2 D does not have a light-transmitting property with respect to input light L 1  and output light L 2 . In the organic modulation element  2 D according to the fourth embodiment, the first electrode  21  has a light-transmitting property with respect to the input light L 1  and the output light L 2 . According to this, modulated output light L 2  can be emitted to an incident side of the input light L 1 . 
     In the modulation device  1 D, the controller  3  applies an arbitrary pulse voltage between the first electrode  21  and the second electrode  22 . In a state in which a pulse voltage is applied between the first electrode  21  and the second electrode  22 , when the input light L 1  that is CW light is incident from the outside to the organic modulation layer  20  through the first electrode  21 , the output light L 2  that is pulse light having a pulse waveform corresponding to the pulse voltage is emitted from the organic modulation layer  20  to the outside through the first electrode  21 . 
       FIG. 13  is a graph showing verification results relating to a function of the modulation device  1 D according to the fourth embodiment. As shown in  FIG. 13 , when a typical OLED is electrically excited (electrically driven), in the case of high-speed modulation, a waveform of output light is distorted. In contrast, in the modulation device  1 D (the organic modulation layer  20  according to this embodiment), the organic semiconductor molecules  20   a  in an excited state are forcibly dissociated into charges and an afterglow component is suppressed, and thus the output light L 2  having light-emission intensity in a rectangular wave shape can be obtained. Note that, so as to suppress the afterglow component, it is preferable to dispose a hole transport layer having a HOMO level shallower than a HOMO level of the organic modulation layer  20  between an electrode that functions as a positive electrode between the first electrode  21  and the second electrode  22 , and the organic modulation layer  20 . In addition, so as to suppress the afterglow component, it is preferable to dispose an electron transport layer having a LUMO level shallower than a LUMO level of the organic modulation layer  20  between an electrode that functions as a negative electrode between the first electrode  21  and the second electrode  22 , and the organic modulation layer  20 . With regard to arrangement of the hole transport layer and arrangement of the electron transport layer, any one configuration may be embodied or both configurations may be embodied. In addition, any one or both of a molecule capable of being used in the hole transport layer and a molecule capable of being used in the electron transport layer may be contained in the light-emitting layer, and the light-emitting layer may have a structure in which a layer containing the molecule capable of being used in the hole transport layer and a layer containing the molecule capable of being used in the electron transport layer are laminated. 
     MODIFICATION EXAMPLE 
     The present disclosure is not limited to the above-described embodiments. In the above-described embodiments, the plurality of organic semiconductor molecules  20   a  contained in the organic modulation layer  20  may be constituted by a plurality of kinds of organic semiconductor molecules. In the above-described embodiments, the plurality of host molecules  20   b  contained in the organic modulation layer  20  may be constituted by a plurality of kinds of host molecules. In the above-described embodiments, the organic modulation layer  20  may not contain the plurality of host molecules  20   b.    
     In the above-described embodiments, each of the plurality of organic semiconductor molecules  20   a  may not satisfy the above-described various conditions as long as a molecule in which an excited state enabling the reverse intersystem crossing from the lowest excited triplet state T 1  to the lowest excited singlet state S 1  in each of the plurality of organic semiconductor molecules  20   a  is formed due to irradiation with the input light L 1 . 
     In the first embodiment, the first electrode  21  may have a light-transmitting property with respect to the input light L 1  and the output light L 2 , and the second electrode  22  may not have a light-transmitting property with respect to the input light L 1  and the output light L 2 . In the fourth embodiment, the first electrode  21  may have a light-transmitting property with respect to the input light L 1 , and the second electrode  22  may have a light-transmitting property with respect to the output light L 2 . In the above-described embodiments, a plurality of the organic modulation layer  20  may be one-directionally arranged or two-directionally arranged. 
     An organic modulation element according an aspect of the present disclosure includes: an organic modulation layer containing a plurality of organic semiconductor molecules; a first electrode having a light-transmitting property with respect to input light to the organic modulation layer, and disposed on one side of the organic modulation layer; and a second electrode disposed on the other side of the organic modulation layer. Each of the plurality of organic semiconductor molecules is a molecule in which an excited state enabling reverse intersystem crossing from a lowest excited triplet state to a lowest excited singlet state in each of the plurality of organic semiconductor molecules is formed due to irradiation with the input light. In each of the plurality of organic semiconductor molecules, an intersystem crossing rate constant from the lowest excited singlet state to the lowest excited triplet state is greater than a reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state. In each of the plurality of organic semiconductor molecules, the reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state is greater than a non-radiative deactivation rate constant from the lowest excited triplet state to a ground state. 
     In the organic modulation element, an excited state enabling reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state is formed in each of the plurality of organic semiconductor molecules due to irradiation with the input light transmitted through the first electrode. According to this, in each of the plurality of organic semiconductor molecules, in addition to high-speed charge separation observed until the lowest excited singlet state is mitigated, charge separation from the lowest excited triplet state having a longer lifetime in comparison to the lowest excited singlet state (hereinafter, referred to as “direct charge separation from lowest excited triplet state”), and/or charge separation from the lowest excited singlet state in which the lowest excited triplet state having a longer lifetime in comparison to the lowest excited singlet state is set as a temporary shelter from deactivation (hereinafter, referred to as charge separation from the lowest excited singlet state through the lowest excited triplet state) become possible. Here, in each of the plurality of organic semiconductor molecules, an intersystem crossing rate constant from the lowest excited singlet state to the lowest excited triplet state is greater than a reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state, and thus the lifetime of the lowest excited triplet state is lengthened, and thus sufficient charge separation is likely to be obtained. Accordingly, for example, when a voltage is applied between the first electrode and the second electrode under irradiation with the input light to cause an electric field in a direction of causing charge separation to occur in the organic modulation layer, an excited state in which deactivation (spontaneous emission) is to originally occur according to light-emission is converted into a charge separation state, and thus attenuation is possible. On the other hand, in each of the plurality of organic semiconductor molecules, since the reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state is greater than the non-radiative deactivation rate constant from the lowest excited triplet state to the ground state, deactivation due to heat is less likely to occur, and light-emission is likely to be obtained. Accordingly, for example, when a voltage is not applied between the first electrode and the second electrode, and an electric field in a direction of causing charge separation is not caused to occur in the organic modulation layer, light-emission due to spontaneous emission can be promoted. As described above, according to the organic modulation element, the input light can be modulated in correspondence with a purpose. 
     In the organic modulation element according to the aspect of the present disclosure, in each of the plurality of organic semiconductor molecules, a difference between energy of the lowest excited singlet state and energy of the lowest excited triplet state at an absolute temperature of 77 K may be less than 0.3 eV. According to this, the individual organic semiconductor molecules can be caused to function as molecules in which an excited state enabling reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state is formed due to irradiation with the input light. 
     In the organic modulation element according to the aspect of the present disclosure, in each of the plurality of organic semiconductor molecules, the intersystem crossing rate constant from the lowest excited singlet state to the lowest excited triplet state may be greater than a fluorescence rate constant from the lowest excited singlet state to the ground state. According to this, direct charge separation from the lowest excited triplet state and/or charge separation from the lowest excited singlet state through the lowest excited triplet state become possible while suppressing deactivation from the lowest excited singlet state due to light-emission. 
     In the organic modulation element according to the aspect of the present disclosure, in each of the plurality of organic semiconductor molecules, the intersystem crossing rate constant from the lowest excited singlet state to the lowest excited triplet state may be two or more times the reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state. According to this, the lifetime of the lowest excited triple state is lengthened, and thus photoelectric conversion efficiency can be improved. 
     In the organic modulation element according to the aspect of the present disclosure, in each of the plurality of organic semiconductor molecules, the reverse intersystem crossing rate constant from the lowest excited triplet state to the lowest excited singlet state may be 1×10 7  (sec −1 ) or less. According to this, the lifetime of the lowest excited triple state is lengthened, and thus photoelectric conversion efficiency can be improved. 
     In the organic modulation element according to the aspect of the present disclosure, a dipole moment of each of the plurality of organic semiconductor molecules may be greater than 0 D. According to this, energy for charge separation can be reduced. 
     In the organic modulation element according to the aspect of the present disclosure, the organic modulation layer may further contain a plurality of host molecules, and energy of the lowest excited triplet state at an absolute temperature of 77 K in each of the plurality of host molecules may be higher than energy of a lowest excited triplet state at an absolute temperature of 77 K in each of the plurality of organic semiconductor molecules. According to this, it is possible to suppress a decrease in photoelectric conversion efficiency due to an influence of the plurality of host molecules. 
     In the organic modulation element according to the aspect of the present disclosure, the second electrode may have a light-transmitting property with respect to output light from the organic modulation layer. According to this, modulated output light can be emitted to a side opposite to an incident side of the input light. 
     In the organic modulation element according to the aspect of the present disclosure, the first electrode may have a light-transmitting property with respect to output light from the organic modulation layer. According to this, modulated output light can be emitted to the incident side of the input light. 
     An organic modulation element according to another aspect of the present disclosure includes: an organic modulation layer containing a plurality of organic semiconductor molecules; a first electrode having a light-transmitting property with respect to input light to the organic modulation layer, and disposed on one side of the organic modulation layer; a second electrode having a light-transmitting property with respect to output light from the organic modulation layer, and disposed on the other side of the organic modulation layer; a light-receiving layer disposed on the other side of the second electrode and configured to receive the output light. Each of the plurality of organic semiconductor molecules is a molecule in which an excited state enabling reverse intersystem crossing from a lowest excited triplet state to a lowest excited singlet state in each of the plurality of organic semiconductor molecules is formed due to irradiation with the input light. 
     According to the individual organic modulation element, input light is modulated in correspondence with a purpose, and output light modulated in correspondence with the purpose can be detected. 
     An organic modulation element according to still another aspect of the present disclosure includes: an organic modulation layer containing a plurality of organic semiconductor molecules; a first electrode having a light-transmitting property with respect to input light to the organic modulation layer, and disposed on one side of the organic modulation layer; a second electrode having a light-transmitting property with respect to output light from the organic modulation layer, and disposed on the other side of the organic modulation layer; and a light-emitting layer disposed on the one side of the first electrode, and configured to emit the input light. Each of the plurality of organic semiconductor molecules is a molecule in which an excited state enabling reverse intersystem crossing from a lowest excited triplet state to a lowest excited singlet state in each of the plurality of organic semiconductor molecules is formed due to irradiation with the input light. 
     According to the organic modulation element, input light is modulated in correspondence with a purpose, and output light modulated in correspondence with the purpose can be emitted. 
     A modulation device according to still another aspect of the present disclosure includes: an organic modulation layer containing a plurality of organic semiconductor molecules; a first electrode having a light-transmitting property with respect to input light to the organic modulation layer, and disposed on one side of the organic modulation layer; a second electrode disposed on the other side of the organic modulation layer; and a controller configured to modulate a voltage to be applied between the first electrode and the second electrode. Each of the plurality of organic semiconductor molecules is a molecule in which an excited state enabling reverse intersystem crossing from a lowest excited triplet state to a lowest excited singlet state in each of the plurality of organic semiconductor molecules is formed due to irradiation with the input light. 
     According to the modulation device, input light can be modulated in correspondence with a purpose. 
     According to the present disclosure, it is possible to provide an organic modulation element capable of modulating input light in correspondence with a purpose, and a modulation device including the organic modulation element.