Organic photodetector and photodetector

Abstract

This organic light receiving element is provided with an organic light receiving layer that includes a plurality of organic semiconductor molecules. Each of the plurality of organic semiconductor molecules has formed therein, due to irradiation of light, an excited state that enables inverse intersystem crossing from the highest excited triplet state to the lowest excited singlet state.

Description

Technical Field

[0001] The present disclosure relates to an organic light-receiving element and a light-receiving device.

Background Art

[0002] In general, a bulk heterojunction is employed for a photoelectric conversion layer made of an organic semiconductor material (see, for example, Patent Documents 1 and 2). In such a photoelectric conversion layer, excited molecules diffused near the interface of the bulk heterojunction escape from Coulomb binding due to the energy difference between donor molecules and acceptor molecules, enabling charge separation.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] W However, in the photoelectric conversion layer as described above, since it is necessary to increase the energy difference between donor molecules and acceptor molecules, the open-circuit voltage decreases, and as a result, charge separation may become insufficient. Further, depending on the mixing method of donor molecules and acceptor molecules, the area of the interface of the bulk heterojunction may become small, and as a result, charge separation may become insufficient.

[0005] An object of the present disclosure is to provide an organic light-receiving element capable of obtaining sufficient charge separation and a light-receiving device including such an organic light-receiving element.

Means for Solving the Problems

[0006] One aspect of the present disclosure is an organic photodetector comprising an organic photodetector layer containing a plurality of organic semiconductor molecules, each of which is a molecule in which an excited state is formed upon irradiation with light, in which reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state is possible.

[0007] In this organic photodetector, irradiation with light creates an excited state in each of the multiple organic semiconductor molecules that allows for reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state. In other words, irradiation with light creates an excited state in each individual organic semiconductor molecule that allows for reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state. As a result, for each of the multiple organic semiconductor molecules, in addition to direct charge separation from the lowest excited singlet state, charge separation from the lowest excited triplet state, which has a longer lifetime than the lowest excited singlet state (hereinafter referred to as "direct charge separation from the lowest excited triplet state"), and / or charge separation from the lowest excited singlet state, which uses the lowest excited triplet state, which has a longer lifetime than the lowest excited singlet state, as a temporary refuge from deactivation (hereinafter referred to as "charge separation from the lowest excited singlet state via the lowest excited triplet state"). Therefore, sufficient charge separation can be obtained with this organic photodetector.

[0008] In one aspect of the present disclosure, the difference between the energy of the lowest excited singlet state and the energy of the lowest excited triplet state at an absolute temperature of 77K may be less than 0.3 eV in each of the multiple organic semiconductor molecules. This allows each organic semiconductor molecule to function as a molecule in which an excited state is formed that allows for reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state upon irradiation with light.

[0009] In one aspect of the present disclosure, in each of the multiple organic semiconductor molecules, the intersystem crossover rate constant from the lowest excited singlet state to the lowest excited triplet state may be greater than the fluorescence rate constant from the lowest excited singlet state to the ground state. This allows for direct charge separation from the lowest excited triplet state and / or charge separation from the lowest excited singlet state via the lowest excited triplet state, while suppressing deactivation from the lowest excited singlet state due to luminescence.

[0010] In one aspect of the present disclosure, in each of the multiple organic semiconductor molecules, the intersystem crossover rate constant from the lowest excited singlet state to the lowest excited triplet state may be greater than the inverse intersystem crossover rate constant from the lowest excited triplet state to the lowest excited singlet state. This makes it possible to extend the lifetime of the lowest excited triplet state and improve the photoelectric conversion efficiency.

[0011] In one aspect of the present disclosure, in an organic photodetector, the intersystem crossover rate constant from the lowest excited singlet state to the lowest excited triplet state may be at least twice the inverse intersystem crossover rate constant from the lowest excited triplet state to the lowest excited singlet state in each of the multiple organic semiconductor molecules. This allows for a longer lifetime of the lowest excited triplet state and improves the photoelectric conversion efficiency.

[0012] In one aspect of the present disclosure, in an organic photodetector, the inverse intersystem crossover rate constant from the lowest excited triplet state to the lowest excited singlet state is 1 × 10⁻¹⁰ in each of the multiple organic semiconductor molecules. 7 (sec -1 ) or less may also be used. This allows for a longer lifetime of the lowest excited triplet state, thereby improving the photoelectric conversion efficiency.

[0013] In one aspect of the present disclosure, the dipole moment of each of the multiple organic semiconductor molecules may be greater than 0D. This makes it possible to reduce the energy required for charge separation.

[0014] In one aspect of the present disclosure, the organic photodetector further comprises a plurality of host molecules, and the energy of the lowest excited triplet state at an absolute temperature of 77K in each of the plurality of host molecules may be higher than the energy of the lowest excited triplet state at an absolute temperature of 77K in each of the plurality of organic semiconductor molecules. This makes it possible to suppress the decrease in photoelectric conversion efficiency due to the influence of the plurality of host molecules.

[0015] An organic photodetector according to one aspect of this disclosure may further include a first electrode disposed on one side of the organic photodetector layer and a second electrode disposed on the other side of the organic photodetector layer. This makes it possible to detect current generated due to charge separation.

[0016] In one aspect of the organic photodetector of this disclosure, at least one of the first electrode and the second electrode may be light-transmitting to light irradiated onto the organic photodetector layer. This ensures that light is reliably incident on the organic photodetector layer.

[0017] An organic photodetector according to one aspect of this disclosure may further include a charge blocking layer disposed at one of a first position between the organic photodetector layer and the first electrode, and a second position between the organic photodetector layer and the second electrode. When the charge blocking layer is disposed at the first position between the organic photodetector layer and the first electrode, for example, when a voltage is applied between the first electrode and the second electrode such that an electric field in the direction that causes charge separation is generated in the organic photodetector layer, and charge separation occurs in the organic photodetector layer due to the incidence of light, one of the electrons and holes generated by the charge separation is prevented from moving to the first electrode by the charge blocking layer and remains in the organic photodetector layer, while the other of the electrons and holes generated by the charge separation moves to the second electrode. After a certain amount of time has elapsed in this state, for example, when a voltage is applied between the first electrode and the second electrode such that an electric field in the direction opposite to the direction that causes charge separation is generated in the organic photodetector layer, the other of the electrons and holes moves from the second electrode to the organic photodetector layer and couples with the one of the electrons and holes that remained in the organic photodetector layer. Furthermore, if a charge blocking layer is positioned at a second location between the organic photodetector and the second electrode, for example, when a voltage is applied between the first and second electrodes such that an electric field in the direction that causes charge separation is generated in the organic photodetector, and charge separation occurs in the organic photodetector due to the incidence of light, one of the electrons and holes generated by the charge separation will remain in the organic photodetector because its movement to the second electrode is suppressed by the charge blocking layer, while the other of the electrons and holes generated by the charge separation will move to the first electrode. After a certain amount of time has elapsed in this state, for example, if a voltage is applied between the first and second electrodes such that an electric field in the direction opposite to the direction that causes charge separation is generated in the organic photodetector, the other of the electrons and holes will move from the first electrode to the organic photodetector and combine with the one of the electrons and holes that remained in the organic photodetector. In either case, when one of the electrons and holes that has moved to the organic photodetector layer combines with the other of the electrons and holes that remained in the organic photodetector layer, an electric current is generated. By detecting this current, the organic photodetector can be made to function as an optical memory element.

[0018] An organic photodetector according to one aspect of this disclosure may further include a charge transport layer disposed at the other of the first and second positions. This allows for efficient transfer of electrons and holes from the organic photodetector layer to the first or second electrode. It also allows for efficient transfer of electrons and holes from the first or second electrode to the organic photodetector layer.

[0019] A light-receiving device according to one aspect of the present disclosure comprises the organic photodetector and a control unit electrically connected to the organic photodetector, wherein the control unit adjusts the potential difference between the first electrode and the second electrode during the light-receiving period so that an electric field in a direction that causes charge separation is generated in the organic photodetector layer, and detects a current.

[0020] This light-receiving device enables more efficient charge separation in the organic light-receiving layer, allowing for more efficient detection of current during the light-receiving period.

[0021] A light-receiving device according to one aspect of the present disclosure comprises the above-mentioned organic photodetector and a control unit electrically connected to the organic photodetector, wherein the control unit adjusts the potential difference between the first electrode and the second electrode during the light-receiving period so that an electric field in the direction that causes charge separation is generated in the organic photodetector layer, and during the application period after the holding period has elapsed from the light-receiving period, it applies a voltage between the first electrode and the second electrode so that an electric field in the direction opposite to the direction that causes charge separation is generated in the organic photodetector layer, and detects a current.

[0022] This light-receiving device enables more efficient charge separation in the organic light-receiving layer, allowing for more efficient detection of current after the retention period has elapsed since the light-receiving period. [Effects of the Invention]

[0023] This disclosure makes it possible to provide an organic photodetector that can achieve sufficient charge separation, and a photodetector equipped with such an organic photodetector. [Brief explanation of the drawing]

[0024] [Figure 1] This is a diagram showing the configuration of the light receiving device according to the first embodiment. [Figure 2] Figure 1 shows the energy diagram of the organic photodetector layer. [Figure 3] This is the energy diagram of the organic photodetector layer in the comparative example. [Figure 4] This graph shows the voltage dependence of the quantum efficiency of the organic photodetector layer in the example containing multiple TADF molecules, and the organic photodetector layer in the comparative example containing multiple fluorescent molecules. [Figure 5] This graph shows the voltage dependence of the quantum efficiency of the organic photodetector layer in an example containing 100% of multiple TADF molecules, an organic photodetector layer in an example containing 50% of multiple TADF molecules, and an organic photodetector layer in an example containing 20% ​​of multiple TADF molecules. [Figure 6] This graph shows the voltage dependence of the quantum efficiency of the organic photodetector layer in an example containing TPA-DCPP as multiple TADF molecules, the organic photodetector layer in an example containing 4CzIPN as multiple TADF molecules, and the organic photodetector layer in an example containing 4CzTPN as multiple TADF molecules. [Figure 7] This is a diagram showing the configuration of the light receiving device according to the second embodiment. [Figure 8] Figure 7 shows the energy diagram of the organic photodetector. [Figure 9] This graph shows the time variation of the light emission intensity of the organic photodetector in the example. [Figure 10] This graph shows the relationship between the retention period and normalized emission intensity for the organic photodetector in the example. [Figure 11] This is a photograph showing the light emission state of the organic photodetector in the example. [Figure 12] This graph shows the relationship between the position and light emission intensity of the organic photodetector in the example. [Figure 13] This is an energy diagram of a modified organic photodetector. [Figure 14] This is a diagram showing the configuration of a modified light-receiving device. [Modes for carrying out the invention]

[0025] Embodiments of this disclosure will be described in detail below with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions are omitted. [First Embodiment]

[0026] Figure 1 is a configuration diagram of the light-receiving device 1 of the first embodiment. As shown in Figure 1, the light-receiving device 1 includes an organic light-receiving element 2. The organic light-receiving element 2 includes a support substrate 20, a first electrode 21, a second electrode 22, a first buffer layer 23, a second buffer layer 24, and an organic light-receiving layer 25. In the organic light-receiving element 2, the first electrode 21, the first buffer layer 23, the organic light-receiving layer 25, the second buffer layer 24, and the second electrode 22 are stacked on the support substrate 20 in this order from the support substrate 20 side. In other words, in the organic light-receiving element 2, the first electrode 21 is located on one side of the organic light-receiving layer 25 (the side facing the support substrate 20), and the second electrode 22 is located on the other side of the organic light-receiving layer 25 (the side opposite to the support substrate 20).

[0027] The support substrate 20 is light-transmitting to light irradiated onto the organic light-receiving layer 25 (i.e., light of the object to be received). The support substrate 20 is, for example, a glass substrate, a flexible thin glass film, a resin film, etc. As the glass substrate, for example, a quartz glass substrate, a synthetic quartz glass substrate, a Pyrex® glass substrate, etc., can be used. As the resin film, for example, a PET film, a PEN film, a polyimide film, etc., can be used. When the support substrate 20 is a resin film, it is preferable that the resin film has water vapor barrier properties, and from the viewpoint of extending the lifespan, the resin film should be 10 -3 (g / m 2 It is more preferable that the film has a water vapor permeability of less than or equal to (day). As a resin film having water vapor barrier properties (water vapor barrier film), for example, a film on which an SiO2 film or a SiN film is formed on the surface can be used.

[0028] The first electrode 21 is light-transmitting to light irradiated onto the organic photodetector layer 25. For example, the material of the first electrode 21 can be indium tin oxide, indium oxide, tin oxide, zinc oxide, etc. The second electrode 22 does not need to be light-transmitting to light irradiated onto the organic photodetector layer 25. For the material of the second electrode 22, aluminum, vanadium, gold, silver, platinum, iron, cobalt, carbon, nickel, tungsten, palladium, magnesium, calcium, tin, lead, titanium, yttrium, lithium, ruthenium, manganese, or alloys containing these metals can be used.

[0029] Furthermore, a buffer layer may be introduced on at least one surface of the first electrode 21 and the second electrode 22 to adjust the work function and charge injection barrier of the electrodes. The first buffer layer 23 is light-transmitting to light irradiated onto the organic photodetector layer 25. For example, when the first electrode 21 is used as the anode, the material of the first buffer layer 23 can be, for example, a metal oxide such as molybdenum oxide, tungsten oxide, nickel oxide, or vanadium oxide, or a conductive polymer represented by PEDOT:PSS. The second buffer layer 24 does not need to be light-transmitting to light irradiated onto the organic photodetector layer 25. For example, when the second electrode 22 is used as the cathode, the material of the second buffer layer 24 can be, 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; or alkaline earth metal compounds such as magnesium fluoride and calcium fluoride. If the first electrode 21 is light-transmitting to light irradiated onto the organic photodetector layer 25, then at least the first buffer layer 23 must be light-transmitting to light irradiated onto the organic photodetector layer 25. If the second electrode 22 is light-transmitting to light irradiated onto the organic photodetector layer 25, then at least the second buffer layer 24 must be light-transmitting to light irradiated onto the organic photodetector layer 25. In that case, the film thickness of the second buffer layer 24 is preferably, for example, 50 nm or less.

[0030] The organic light-receiving layer 25 contains multiple organic semiconductor molecules 25a and multiple host molecules 25b. The multiple organic semiconductor molecules 25a are composed of organic semiconductor molecules of the same type, and the multiple host molecules 25b are composed of host molecules of the same type. In other words, the organic light-receiving layer 25 contains one type of organic semiconductor molecule 25a and one type of host molecule 25b. Charge separation occurs in the organic light-receiving layer 25 when light transmitted through the support substrate 20, the first electrode 21, and the first buffer layer 23 is irradiated (details will be described later).

[0031] The organic photodetector 2 configured as described above is manufactured as follows, for example. First, a first electrode 21 made of indium tin oxide is formed on the main surface of one side of a support substrate 20, which is a glass substrate. The thickness of the first electrode 21 is, for example, about 100 nm. Next, a first buffer layer 23 is formed on the first electrode 21 by vacuum deposition of molybdenum oxide. The thickness of the first buffer layer 23 is, for example, about 30 nm. Next, an organic photodetector layer 25 is formed on the first buffer layer 23 by vacuum deposition of TPA-DCPP (multiple organic semiconductor molecules 25a) and CBP (multiple host molecules 25b) from different deposition sources. The thickness of the organic photodetector layer 25 is, for example, about 100 nm. The mass ratio of CBP to TPA-DCPP in the organic photodetector layer 25 is, for example, 50:50. Next, a second buffer layer 24 is formed on the organic photodetector layer 25 by vacuum deposition of titanium oxide. The thickness of the second buffer layer 24 is, for example, about 10 nm. Finally, a second electrode 22 made of aluminum is formed on the second buffer layer 24. The thickness of the second electrode 22 is, for example, about 100 nm.

[0032] Furthermore, a first buffer layer 23, which is light-transmitting to light irradiated onto the organic photodetector layer 25, does not necessarily have to be placed between the first electrode 21 and the organic photodetector layer 25. A second buffer layer 24 does not necessarily have to be placed between the second electrode 22 and the organic photodetector layer 25. When the first electrode 21 functions as an anode and the second electrode 22 functions as a cathode, a hole transport layer or electron blocking layer may be placed between the first electrode 21 and the organic photodetector layer 25. When the first electrode 21 functions as an anode and the second electrode 22 functions as a cathode, an electron transport layer or hole blocking layer may be placed between the second electrode 22 and the organic photodetector layer 25. When the first electrode 21 functions as a cathode and the second electrode 22 functions as an anode, an electron transport layer or hole blocking layer may be placed between the first electrode 21 and the organic photodetector layer 25. When the first electrode 21 functions as the cathode and the second electrode 22 functions as the anode, a hole transport layer or an electron blocking layer may be placed between the second electrode 22 and the organic photodetector layer 25. When the first electrode 21 functions as the cathode and the second electrode 22 functions as the anode, the material used in the first buffer layer 23 and the material used in the second buffer layer 24 may be swapped.

[0033] As materials for the hole transport layer or electron blocking 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, indrocarbazole derivatives, isoindole derivatives, acene derivatives, fluorene derivatives, fluorenone derivatives, polyvinylcarbazole, polymer materials with aromatic amines introduced into the main chain or side chains, oligomers, polysilanes, conductive polymers, etc., can be used. As materials for the electron transport layer or hole blocking layer, for example, nitrogen-containing aromatic heterocyclic derivatives, dibenzofuran derivatives, dibenzothiophene derivatives, silole derivatives, aromatic hydrocarbon ring derivatives, etc., can be used.

[0034] In the organic photodetector 2, the layers positioned on the second electrode 22 side relative to the organic photodetector layer 25 (such as the second electrode 22 and the second buffer layer 24) may be light-transmitting to light irradiated onto the organic photodetector layer 25. In other words, the organic photodetector 2 may be configured such that charge separation occurs in the organic photodetector layer 25 when light transmitted through the layers positioned on the second electrode 22 side relative to the organic photodetector layer 25 is irradiated onto the organic photodetector layer 25. In that case, the layers positioned on the first electrode 21 side relative to the organic photodetector layer 25 (such as the support substrate 20, the first electrode 21, and the first buffer layer 23) do not need to be light-transmitting to light irradiated onto the organic photodetector layer 25. When light is incident on the organic photodetector layer 25 from the second electrode 22 side, the second electrode 22 may be made of a metal such as indium tin oxide, indium oxide, tin oxide, zinc oxide, aluminum, vanadium, gold, silver, platinum, iron, cobalt, carbon, nickel, tungsten, palladium, magnesium, calcium, tin, lead, titanium, yttrium, lithium, ruthenium, manganese, or an alloy containing these metals, and is formed such that, for example, it has a transmittance of 10% or more, a sheet resistance of several hundred Ω / sq or less, and a film thickness of 5 nm to 1000 nm (preferably 5 nm to 100 nm). When light is incident on the organic photodetector layer 25 from the second electrode 22 side, the material of the support substrate 20 can be, for example, a silicon substrate, a sapphire substrate, etc. Based on the above, in the organic photodetector 2, it is sufficient that at least one of the first electrode 21 and the second electrode 22 has light transmittance to light irradiated onto the organic photodetector layer 25.

[0035] The light receiving device 1 further includes a control unit 3. The control unit 3 is electrically connected to the organic light receiving element 2. During the light receiving period, the control unit 3 applies a voltage between the first electrode 21 and the second electrode 22 so that an electric field in the direction causing charge separation is generated in the organic light receiving layer 25, and detects a current. The light receiving period is a period during which the light to be received is irradiated onto the organic light receiving layer 25, for example, a period of 0.1 ns or more and 10 s or less. As an example, when the first electrode 21 functions as an anode and the second electrode 22 functions as a cathode, the control unit 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 negative with reference to the potential of the second electrode 22, 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 reference to the potential of the first electrode 21. Further, when the first electrode 21 functions as a cathode and the second electrode 22 functions as an anode, the control unit 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 reference to the potential of the second electrode 22, 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 reference to the potential of the first electrode 21. The control unit 3 is constituted by, for example, a voltage application circuit, a current reading circuit, and the like. [[ID=~]] [[ID=~]]

[0036] [[ID=~]] FIG. 2 is an energy diagram of the organic light receiving layer 25 shown in FIG. 1. More specifically, FIG. 2 is an energy diagram of the organic light receiving layer 25 in which an excited state is formed by light irradiation. In FIG. 2, S0 indicates the ground state, S1 indicates the lowest excited singlet state, and T1 indicates the lowest excited triplet state. k S r is the fluorescence rate constant from the lowest excited singlet state S1 to the ground state S0, and k S nr is the non-radiative deactivation rate constant from the lowest excited singlet state S1 to the ground state S0. k T r is the phosphorescence rate constant from the lowest excited triplet state T1 to the ground state S0, and k T nr is the non-radiative deactivation rate constant from the lowest excited triplet state T1 to the ground state S0. k ISCk is the intersystem crossover rate constant from the lowest excited singlet state S1 to the lowest excited triplet state T1, and RISC This is the inverse intersystem crossover rate constant from the lowest excited triplet state T1 to the lowest excited singlet state S1.

[0037] As shown in Figure 2, each of the multiple organic semiconductor molecules 25a is a molecule in which, upon irradiation with light, an excited state is formed in which reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1 is possible. In this case, intersystem crossing from the lowest excited singlet state S1 to the lowest excited triplet state T1 is also possible in each of the multiple organic semiconductor molecules 25a. Preferably, each of the multiple organic semiconductor molecules 25a contains a donor functional group and an acceptor functional group.

[0038] Furthermore, "reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1" includes not only "reverse intersystem crossing in which an excited molecule moves from the lowest excited triplet state T1 to the lowest excited singlet state S1 without going through another energy level," but also "reverse intersystem crossing in which an excited molecule moves from the lowest excited triplet state T1 to the lowest excited singlet state S1 via another energy level (e.g., a higher energy level)." Similarly, "intersystem crossing from the lowest excited singlet state S1 to the lowest excited triplet state T1" includes not only "intersystem crossing in which an excited molecule moves from the lowest excited singlet state S1 to the lowest excited triplet state T1 without going through another energy level," but also "intersystem crossing in which an excited molecule moves from the lowest excited singlet state S1 to the lowest excited triplet state T1 via another energy level (e.g., a higher energy level)."

[0039] In each of the multiple organic semiconductor molecules 25a, the difference between the energy of the lowest excited singlet state S1 and the energy of the lowest excited triplet state T1 at an absolute temperature of 77K is less than 0.3 eV. Although not shown in Figure 2, the energy of the lowest excited triplet state at an absolute temperature of 77K in each of the multiple host molecules 25b is higher than the energy of the lowest excited triplet state T1 at an absolute temperature of 77K in each of the multiple organic semiconductor molecules 25a.

[0040] Energy level E is the energy of the lowest excited singlet state S1. S1 The following is how it is determined. First, a film consisting of multiple organic semiconductor molecules 25a is formed on a Si wafer. When the film is formed by vapor deposition, the film thickness is preferably 50 nm to 100 nm. When the film is formed by spin coating, the film thickness can be about 30 nm. Next, the fluorescence spectrum of the film consisting of multiple organic semiconductor molecules 25a is measured at room temperature (absolute temperature 300 K). For the fluorescence spectrum measurement, an LED, tungsten lamp, or deuterium lamp is used as the excitation light source, and a multi-channel spectrometer (Hamamatsu Photonics C10027) is used as the detector. Next, a tangent is drawn to the rising edge on the short-wavelength side of the emission spectrum, where the vertical axis is light intensity and the horizontal axis is wavelength, and the wavelength value λ at the intersection of the tangent and the horizontal axis is determined. edge Determine the (nm) and the energy level E from the following equation (1). S1 We seek. E S1 (eV) = 1239.85 / λ edge …(1)

[0041] Furthermore, the tangent drawn to the rising edge on the short-wavelength side of the fluorescence spectrum is the tangent with the maximum slope among the tangents drawn at each point in the fluorescence spectrum from the short-wavelength side to the maximum value of the fluorescence spectrum appearing at the shortest wavelength side. A maximum value having a peak intensity of 10% or less of the maximum peak intensity of the fluorescence spectrum is not considered the "maximum value of the fluorescence spectrum appearing at the shortest wavelength side" as described above.

[0042] Energy level E is the energy of the lowest excited triplet state T1 at an absolute temperature of 77K. T1The following is how it is determined: First, a Si wafer (as described above) on which a film made of multiple organic semiconductor molecules 25a is formed is cooled to an absolute temperature of 77K using a cryostat (OptistatDN, manufactured by Oxford). Next, in the measurement of the phosphorescence spectrum, the phosphorescence spectrum of the film made of multiple organic semiconductor molecules 25a is measured using the same excitation light source and detector as in the measurement of the fluorescence spectrum described above. Then, a tangent is drawn to the rising edge on the short wavelength side of the phosphorescence spectrum, where the vertical axis is light intensity and the horizontal axis is wavelength, and the wavelength value λ at the intersection of the tangent and the horizontal axis is measured. edge Determine the (nm) and the energy level E from the following equation (2). T1 We seek. E T1 (eV) = 1239.85 / λ edge …(2)

[0043] Furthermore, the tangent drawn to the rising edge on the short-wavelength side of the phosphorescence spectrum is the tangent with the maximum slope among the tangents drawn at each point in the phosphorescence spectrum from the short-wavelength side to the maximum value of the phosphorescence spectrum appearing at the shortest wavelength side. A maximum value having a peak intensity of 10% or less of the maximum peak intensity of the phosphorescence spectrum is not considered the "maximum value of the phosphorescence spectrum appearing at the shortest wavelength side" as described above.

[0044] In each of the multiple organic semiconductor molecules 25a, the intersystem crossover rate constant k from the lowest excited singlet state S1 to the lowest excited triplet state T1 is ISC The fluorescence rate constant k is the fluorescence rate constant from the lowest excited singlet state S1 to the ground state S0. S r It is larger than. In each of the multiple organic semiconductor molecules 25a, the intersystem crossover rate constant k from the lowest excited singlet state S1 to the lowest excited triplet state T1 is ISC This is the inverse intersystem crossover rate constant k from the lowest excited triplet state T1 to the lowest excited singlet state S1. RISC It is larger than. In each of the multiple organic semiconductor molecules 25a, the intersystem crossover rate constant k from the lowest excited singlet state S1 to the lowest excited triplet state T1 is ISC This is the inverse intersystem crossover rate constant k from the lowest excited triplet state T1 to the lowest excited singlet state S1.RISC It is more than twice as much. In each of the multiple organic semiconductor molecules 25a, the inverse intersystem crossover rate constant k from the lowest excited triplet state T1 to the lowest excited singlet state S1 is RISC is 1 × 10 7 (sec -1 ) are as follows:

[0045] The fluorescence rate constant k from the lowest excited singlet state S1 to the ground state S0. S r The intersystem crossover rate constant k from the lowest excited singlet state S1 to the lowest excited triplet state T1. ISC , and the inverse intersystem crossover rate constant k from the lowest excited triplet state T1 to the lowest excited singlet state S1. RISC The following is how it is determined. First, a film consisting of multiple organic semiconductor molecules 25a is formed on a quartz substrate. When the film is formed by vapor deposition, the film thickness is preferably 50 nm to 100 nm. When the film is formed by spin coating, the film thickness can be about 30 nm. Next, the emission quantum yield (number of emitted photons / number of absorbed photons) of the film consisting of multiple organic semiconductor molecules 25a is measured at room temperature (300 K) using an absolute PL quantum yield analyzer (C11347, Hamamatsu Photonics). In this measurement, the film consisting of multiple organic semiconductor molecules 25a is excited with light in the absorption wavelength band, and the emission quantum yield is calculated from the emission spectrum detected under conditions where the absorbance ("Abs" in the analyzer) is 0.1 to 0.9, and the average value of three measurements is taken as Φ.

[0046] Next, the fluorescence lifetime of a film composed of multiple organic semiconductor molecules 25a is measured using a compact fluorescence lifetime analyzer (Hamamatsu Photonics C11367). In this measurement, the film composed of multiple organic semiconductor molecules 25a is excited with light in the absorption wavelength band, and the peak wavelength of the emission spectrum obtained when measuring the emission quantum yield is used as the detection wavelength. All measurements are performed with an integration count such that the peak count is 10,000 or more. In a film composed of multiple organic semiconductor molecules 25a, the prompt emission decay component (τ) is emitted from the lowest excited singlet state S1 without passing through the lowest excited triplet state T1. p) and the intersystem crossing rate from the lowest excited singlet state S1 to the lowest excited triplet state T1, and the delayed emission decay component (τ) emitted after the reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1. d At least two components are observed, but since it is necessary to measure them within time ranges appropriate to their respective time scales, the measurement is carried out using the following procedure.

[0047] First, the τ observed in a film composed of multiple organic semiconductor molecules 25a d The fluorescence lifetime is measured within the measurable time range (sec). A reasonable τ d In calculating this, it is necessary to observe the time-resolved emission waveform of the excitation light. Therefore, a Spectralon (standard diffuse reflector) is used as the measurement target, and the instrument response function (IRF) is measured under the same measurement conditions (excitation wavelength, repetition frequency, time range) as the fluorescence lifetime measurement described above. Subsequently, a 1st-order "tail fit" (single-component exponential function fitting) is performed on the emission decay curve of the molecule measured earlier in the measuring instrument, and τ d The (sec) is calculated. For the calculation range when performing tail fitting, measurements are taken over a time range that is sufficiently wider than the excitation pulse width obtained in the IRF measurement. The calculation is performed in a time region where the signal-to-noise ratio of emission intensity can be ensured (a region where the average value of the emission intensity of 21 points, including 10 points before and after the time of interest, is 5 counts or more greater than the dark level), after the time when the emission decay curve of the prompt component has sufficiently decayed or when the excitation pulse light has decayed to the dark level, whichever is later.

[0048] Next, τ p To calculate τ, a film composed of multiple organic semiconductor molecules 25a is measured in a time range (e.g., 1 microsecond) that ensures sufficient time resolution of the luminescence decay curve of the prompt component and also includes the luminescence decay curve of the delay component. Subsequently, the IRF is measured under the same conditions with Spectralon as the measurement target, and the previously determined τ is calculated. d Using τ as a fixed parameter, we perform multi-component exponential function fitting, p(sec) The ratio of the prompt and delay components is calculated. Note that the prompt component may include two or more luminescence decay processes due to intermolecular interactions, etc. In such cases, fitting is performed with three or more components, including both the prompt and delay components. However, any luminescence decay that does not go through reverse intersystem crossing is defined and calculated as a prompt component. The fitting method follows the method described in the C11367 manual.

[0049] Based on the above measurements, the prompt component Φ of the luminescence quantum yield was found. p and delay component Φ d (Φ=Φ p +Φ d ), as well as the prompt component τ of the fluorescence lifetime p and delay component τ d Therefore, the following equations (3) to (7) give various rate constants (k S r , k ISC , k RISC Calculate (however, non-radiative deactivation from the lowest excited singlet state S1 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)

[0050] Furthermore, if an ideal molecular design is achieved for the light-receiving material, k S r ga k ISCIt is expected that the relative values ​​will be significantly lower, resulting in a decrease in the luminescence intensity of the prompt component and an increase in T1 generation efficiency. Under these conditions, k RISC The non-radiative deactivation rate constant k from T1 is small. T nr ga k RISC If it is significantly smaller than this, a very long emission lifetime is observed. Under such conditions, it may not be possible to obtain a sufficient S / N ratio for calculating the rate constant in fluorescence lifetime measurement, but this is due to convergence to ideal conditions (k RISC This phenomenon can be interpreted as →0). Furthermore, in polar organic semiconductor molecules with a dipole moment of 0D or greater, excited molecules formed by photoexcitation in the film may spontaneously dissociate into charge without contributing to luminescence. As a result, this component may not be observed as luminescence, and a sufficient signal-to-noise ratio (S / N) for calculating the rate constant may not be obtained in fluorescence lifetime measurements.

[0051] The dipole moments (μ) of each of the multiple organic semiconductor molecules 25a are greater than 0D. The dipole moment is a value that can be calculated by quantum chemical calculations, and is generally calculated using calculation methods called the Hartree-Fock (HF) method or density functional theory (DFT) method. Among these, the most commonly used condition (combination of functional and basis function) is B3LYP / 6-31(d).

[0052] Examples of organic semiconductor molecules 25a that satisfy the above conditions are TADF molecules such as TPA-DCPP, 4CzIPN, and 4CzTPN. TADF molecules are known in the field of luminescence, but they can be used as organic semiconductor molecules 25a that satisfy the above conditions. Organic semiconductor molecules 25a are designed by quantum chemical calculations or the like to satisfy the above conditions and are produced by organic synthesis. Examples of host molecules 25b that satisfy the above conditions are CBP, mCBP, T2T, mCP, PPT, DPEPO, etc.

[0053] In the organic photodetector layer 25 configured as described above, irradiation with light creates an excited state within each organic semiconductor molecule 25a that allows for reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1. As a result, in each of the multiple organic semiconductor molecules 25a, in addition to direct charge separation from the lowest excited singlet state S1, charge separation from the lowest excited triplet state T1, which has a longer lifetime than the lowest excited singlet state S1 (hereinafter referred to as "direct charge separation from the lowest excited triplet state T1"), and / or charge separation from the lowest excited singlet state S1, which uses the lowest excited triplet state T1, which has a longer lifetime than the lowest excited singlet state S1, as a temporary refuge from deactivation (hereinafter referred to as "charge separation from the lowest excited singlet state S1 via the lowest excited triplet state T1").

[0054] Figure 3 is the energy diagram of the comparative organic photodetector. The comparative organic photodetector shown in Figure 3 contains multiple organic semiconductor molecules 25c. The organic semiconductor molecules 25c are fluorescent molecules (e.g., Alq3). In the comparative organic photodetector shown in Figure 3, the difference between the energy of the lowest excited singlet state S1 and the energy of the lowest excited triplet state T1 at an absolute temperature of 77K is large, and even when light is irradiated, intersystem crossing from the lowest excited singlet state S1 to the lowest excited triplet state T1 does not occur within each individual organic semiconductor molecule 25c. Therefore, neither direct charge separation from the lowest excited triplet state T1 nor charge separation from the lowest excited singlet state S1 via the lowest excited triplet state T1 naturally occurs. Consequently, in the comparative organic photodetector shown in Figure 3, the dissociation effect on charge decreases, and the components lost as light and heat become large, resulting in the comparative organic photodetector shown in Figure 3 not functioning as an effective photodetector.

[0055] As explained above, in the organic photodetector 2, irradiation with light creates an excited state in each of the multiple organic semiconductor molecules 25a that allows for reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1. In other words, irradiation with light creates an excited state in each of the multiple organic semiconductor molecules 25a that allows for reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1. As a result, for each of the multiple organic semiconductor molecules 25a, in addition to direct charge separation from the lowest excited singlet state S1, direct charge separation from the lowest excited triplet state T1 and / or charge separation from the lowest excited singlet state S1 via the lowest excited triplet state T1 becomes possible. Therefore, sufficient charge separation can be obtained with the organic photodetector 2.

[0056] Unlike photoelectric conversion layers employing bulk heterojunctions, in organic photodetector 2, irradiation with light creates excited states within individual organic semiconductor molecules 25a that allow for reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1. Therefore, it is easier to obtain a higher open-circuit voltage compared to photoelectric conversion layers employing bulk heterojunctions. Furthermore, in photoelectric conversion layers employing bulk heterojunctions, it is necessary to mix donor molecules and acceptor molecules to prevent the interface area of ​​the bulk heterojunction from becoming too small, but such constraints are unnecessary in organic photodetector 2.

[0057] In the organic photodetector 2, the difference between the energy of the lowest excited singlet state S1 and the energy of the lowest excited triplet state T1 at an absolute temperature of 77K is less than 0.3 eV for each of the multiple organic semiconductor molecules 25a. This allows each organic semiconductor molecule 25a to function as a molecule in which an excited state is formed that allows for reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1 upon irradiation with light.

[0058] In the organic photodetector 2, in each of the multiple organic semiconductor molecules 25a, the intersystem crossover rate constant k from the lowest excited singlet state S1 to the lowest excited triplet state T1 is ISC However, the fluorescence rate constant k from the lowest excited singlet state S1 to the ground state S0 is Sr is greater than. This enables direct charge separation from the lowest excited triplet state T1 and / or charge separation from the lowest excited singlet state S1 via the lowest excited triplet state T1 while suppressing deactivation from the lowest excited singlet state S1 by luminescence.

[0059] In the organic light-receiving element 2, in each of the plurality of organic semiconductor molecules 25a, the intersystem crossing rate constant k from the lowest excited singlet state S1 to the lowest excited triplet state T1 ISC is greater than the reverse intersystem crossing rate constant k from the lowest excited triplet state T1 to the lowest excited singlet state S1 RISC is greater than. This can lengthen the lifetime of the lowest excited triplet state T1 and improve the photoelectric conversion efficiency.

[0060] In the organic light-receiving element 2, in each of the plurality of organic semiconductor molecules 25a, the intersystem crossing rate constant k from the lowest excited singlet state S1 to the lowest excited triplet state T1 ISC is greater than the reverse intersystem crossing rate constant k from the lowest excited triplet state T1 to the lowest excited singlet state S1 RISC is twice or more that of. This can lengthen the lifetime of the lowest excited triplet state T1 and improve the photoelectric conversion efficiency.

[0061] In the organic light-receiving element 2, in each of the plurality of organic semiconductor molecules 25a, the reverse intersystem crossing rate constant k from the lowest excited triplet state T1 to the lowest excited singlet state S1 RISC is 1×10 7 (sec -1 ) or less. This can lengthen the lifetime of the lowest excited triplet state T1 and improve the photoelectric conversion efficiency.

[0062] In the organic light-receiving element 2, the dipole moment of each of the plurality of organic semiconductor molecules 25a is greater than 0D. This can reduce the energy for charge separation.

[0063] In the organic photodetector 2, the energy of the lowest excited triplet state T1 at an absolute temperature of 77K in each of the multiple host molecules 25b is higher than the energy of the lowest excited triplet state T1 at an absolute temperature of 77K in each of the multiple organic semiconductor molecules 25a. This suppresses the transition of excited molecules from the lowest excited triplet state T1 of the organic semiconductor molecule 25a to the lowest excited triplet state T1 of the host molecule 25b, and also suppresses the host molecule 25b in the lowest excited triplet state T1 becoming a loss path when the excited state is reformed by the recombination of charges generated by charge separation. In other words, it is possible to suppress the decrease in photoelectric conversion efficiency due to the influence of multiple host molecules 25b.

[0064] In the organic photodetector 2, the first electrode 21 is positioned on one side of the organic photodetector layer 25, and the second electrode 22 is positioned on the other side of the organic photodetector layer 25. This makes it possible to detect the current generated due to charge separation.

[0065] In the organic photodetector 2, the first electrode 21 is light-transmitting to light irradiated onto the organic photodetector layer 25. This ensures that light can be reliably incident on the organic photodetector layer 25.

[0066] In the light-receiving device 1, the control unit 3 applies a voltage between the first electrode 21 and the second electrode 22 during the light-receiving period so that an electric field in the direction that causes charge separation is generated in the organic light-receiving layer 25, and detects the current. As a result, more efficient charge separation is possible in the organic light-receiving layer 25, and the current can be detected more efficiently during the light-receiving period.

[0067] Figure 4 is a graph showing the voltage dependence of quantum efficiency for the organic photodetector layer of the example containing multiple TADF molecules and the organic photodetector layer of the comparative example containing multiple fluorescent molecules. In Figure 4, "quantum efficiency" is the ratio of "number of electrons detected from the organic photodetector layer" to "number of photons absorbed by the organic photodetector layer" (the same applies hereafter). "Voltage" is the voltage applied to the organic photodetector layer when light is irradiated, and a negative value is the value that generates an electric field in the organic photodetector layer in a direction that causes charge separation (the same applies hereafter). As shown in Figure 4, the organic photodetector layer of the example containing multiple TADF molecules showed higher quantum efficiency compared to the organic photodetector layer of the comparative example containing multiple fluorescent molecules.

[0068] Figure 5 is a graph showing the voltage dependence of quantum efficiency for three organic photodetectors: one containing 100% of multiple TADF molecules, one containing 50% of multiple TADF molecules, and one containing 20% ​​of multiple TADF molecules. Here, the proportion of multiple TADF molecules is the mass ratio to the multiple host molecules contained in the organic photodetector. As shown in Figure 5, sufficient quantum efficiency was obtained for all organic photodetectors. In this case, the TADF molecules were TPA-DCPP and the host molecules were CBP. In this case, a higher proportion of multiple TADF molecules resulted in higher quantum efficiency.

[0069] Figure 6 is a graph showing the voltage dependence of quantum efficiency for organic photodetectors in examples containing TPA-DCPP as multiple TADF molecules, organic photodetectors in examples containing 4CzIPN as multiple TADF molecules, and organic photodetectors in examples containing 4CzTPN as multiple TADF molecules. As shown in Figure 6, sufficient quantum efficiency was obtained for all organic photodetectors. The dipole moment of TPA-DCPP was 13.05D, the dipole moment of 4CzIPN was 3.85D, and the dipole moment of 4CzTPN was 0D. Here, a larger dipole moment resulted in higher quantum efficiency. [Second Embodiment]

[0070] Figure 7 is a configuration diagram of the photodetector 1A of the second embodiment. As shown in Figure 7, the photodetector 1A includes an organic photodetector 2A. The organic photodetector 2A differs from the organic photodetector 2 of the first embodiment mainly in that a charge transport layer 26 is located at a first position between the organic photodetector layer 25 and the first electrode 21, and a charge blocking layer 27 is located at a second position between the organic photodetector layer 25 and the second electrode 22. The following will mainly describe the differences from the organic photodetector 2 of the first embodiment, and will omit the description of the points that are the same as the organic photodetector 2 of the first embodiment.

[0071] When the first electrode 21 functions as the anode and the second electrode 22 functions as the cathode, the charge transport layer 26 is a hole transport layer. In other words, the charge transport layer 26 is a layer that facilitates the movement of holes from the organic photodetector layer 25 to the first electrode 21, and the movement of holes from the first electrode 21 to the organic photodetector layer 25. Like the first electrode 21, the charge transport layer 26 is light-transmitting to light irradiated onto the organic photodetector layer 25. The charge transport layer 26 is made of hole transport materials such as triazole derivatives, imidazole derivatives, carbazole derivatives (CBP, Tris-PCz, etc.), indolocarbazole derivatives, polyarylalkane derivatives, pyrazoline derivatives and pyrazolone derivatives, phenylenediamine derivatives, arylamine derivatives, diphenylamine derivatives (NPD, etc.), triphenylamine derivatives (TAPC, TPT1, etc.), aminosubstituted chalcone derivatives, oxazole derivatives, styrylanthracene derivatives, fluorenone derivatives, hydrazone derivatives, stilbene derivatives, silazane derivatives, aniline copolymers, and conductive polymer oligomers. As an example, the charge transport layer 26 has a thickness such that the distance between the first electrode 21 and the charge blocking layer 27 is 400 nm or less.

[0072] When the first electrode 21 functions as the anode and the second electrode 22 functions as the cathode, the charge blocking layer 27 is a layer that prevents the movement of electrons from the organic photodetector layer 25 to the second electrode 22. The charge blocking layer 27 is made of electron transport materials such as triazine derivatives (T2T, etc.), phenanthroline derivatives (Bphen, BCP, etc.), benzimidazole derivatives (TPBi, etc.), nitro-substituted fluorene derivatives, diphenylquinone derivatives, thiopyrandioxide derivatives, carbodiimide, phreolenylidenemethane derivatives, triphenylene derivatives, anthraquinodimethane and anthrone derivatives, oxadiazole derivatives, thiadiazole derivatives, quinoxaline derivatives, and chelate complexes (BAlq, etc.). When the charge blocking layer 27 is an electron blocking layer that prevents the movement of electrons, the hole transport materials mentioned above can also be used as materials for the charge blocking layer 27.

[0073] The light receiving device 1A further comprises a control unit 3A. The control unit 3A is electrically connected to the organic photodetector 2A. During the light receiving period, the control unit 3A applies a voltage between the first electrode 21 and the second electrode 22 so that an electric field in the direction that causes charge separation is generated in the organic photodetector layer 25. The light receiving period is the period during which the light to be received is irradiated onto the organic photodetector layer 25, for example, a period of 0.1 ns to 10 s. During the application period after the holding period has elapsed from the light receiving period, the control unit 3A applies a voltage (pulse voltage) between the first electrode 21 and the second electrode 22 so that an electric field in the direction opposite to the direction that causes charge separation is generated in the organic photodetector layer 25, and detects a current (displacement current). The holding period is the period from the end of the light receiving period to the start of the application period, for example, a period of 100 ns to 100 days. The application period is the period during which a voltage is applied between the first electrode 21 and the second electrode 22 such that an electric field opposite to the direction that causes charge separation is generated in the organic photodetector layer 25, for example, a period of 100 ns to 100 μs. Current detection is performed for any period from the start of the application period (for example, a period of 100 ns to 100 μs).

[0074] When the first electrode 21 functions as the anode and the second electrode 22 functions as the cathode, the control unit 3A can generate an electric field in the organic photodetector layer 25 that causes charge separation by applying a voltage between the first electrode 21 and the second electrode 22 such that the potential of the first electrode 21 becomes negative with respect to the potential of the second electrode 22, or the potential of the second electrode 22 becomes positive with respect to the potential of the first electrode 21. Alternatively, when the first electrode 21 functions as the anode and the second electrode 22 functions as the cathode, the control unit 3A can generate an electric field in the organic photodetector layer 25 that causes charge separation by applying a voltage between the first electrode 21 and the second electrode 22 such that the potential of the first electrode 21 becomes positive with respect to the potential of the second electrode 22, or the potential of the second electrode 22 becomes negative with respect to the potential of the first electrode 21.

[0075] As shown in Figure 8(a), during the light-receiving period, when light to be received is irradiated onto the organic photodetector 25 with a voltage applied between the first electrode 21 and the second electrode 22 such that an electric field in the direction that causes charge separation is generated in the organic photodetector 25, charge separation occurs in the organic photodetector 25, and the electrons generated by charge separation remain in the organic photodetector 25 because their movement to the second electrode 22 is suppressed by the charge-blocking layer 27, while the holes generated by charge separation move to the first electrode 21. As shown in Figure 8(b), during the retention period, the state in which the electrons generated by charge separation remain in the organic photodetector 25 is maintained. As shown in Figure 8(c), during the application period after the light-receiving period has elapsed, when a voltage is applied between the first electrode 21 and the second electrode 22 such that an electric field opposite to the direction that causes charge separation is generated in the organic light-receiving layer 25, holes move from the first electrode 21 to the organic light-receiving layer 25 and combine with electrons that remained in the organic light-receiving layer 25. The control unit 3A detects the current generated when the holes that have moved to the organic light-receiving layer 25 combine with electrons that remained in the organic light-receiving layer 25. Note that the charge transport layer 26 is not shown in Figures 8(a), (b), and (c).

[0076] The organic photodetector 2A configured as described above can also achieve sufficient charge separation, similar to the organic photodetector 2 in the first embodiment.

[0077] In the organic photodetector 2A, the charge blocking layer 27 is positioned at a second location between the organic photodetector 25 and the second electrode 22. As a result, for example, when a voltage is applied between the first electrode 21 and the second electrode 22 such that an electric field in the direction that causes charge separation is generated in the organic photodetector 25, and charge separation occurs in the organic photodetector 25 due to the incidence of light, the electrons generated by the charge separation are prevented from moving to the second electrode 22 by the charge blocking layer 27 and remain in the organic photodetector 25, while the holes generated by the charge separation move to the first electrode 21. After a certain amount of time has elapsed in this state, for example, when a voltage is applied between the first electrode 21 and the second electrode 22 such that an electric field in the direction opposite to the direction that causes charge separation is generated in the organic photodetector 25, the holes move from the first electrode 21 to the organic photodetector 25 and combine with the electrons that remained in the organic photodetector 25. When holes that have moved to the organic photodetector layer 25 combine with electrons that remained in the organic photodetector layer 25, an electric current is generated. By detecting this current, the organic photodetector element 2A can be made to function as an optical memory element.

[0078] In the organic photodetector 2A, the charge transport layer 26 is positioned at a first location between the organic photodetector layer 25 and the first electrode 21. This allows for efficient movement of holes from the organic photodetector layer 25 to the first electrode 21, and also allows for efficient movement of holes from the first electrode 21 to the organic photodetector layer 25.

[0079] In the light receiving device 1A, the control unit 3A applies a voltage between the first electrode 21 and the second electrode 22 during the light receiving period so that an electric field in the direction that causes charge separation is generated in the organic light receiving layer 25. During the application period after the holding period has elapsed since the light receiving period, the control unit 3A applies a voltage between the first electrode 21 and the second electrode 22 so that an electric field in the direction opposite to the direction that causes charge separation is generated in the organic light receiving layer 25, and detects the current. As a result, more efficient charge separation is possible in the organic light receiving layer 25, and the current can be detected more efficiently after the holding period has elapsed since the light receiving period.

[0080] Figures 9(a) and (b) are graphs showing the time change in light emission intensity for the organic photodetector of the example. In this example, an organic photodetector 2A was prepared, in which the first electrode 21 is made of ITO, the second electrode 22 is made of Al, the organic photodetector layer 25 is made of TPA-DCPP (organic light emitter) and mCBP (host material), the charge transport layer 26 is made of TAPC, and the charge blocking layer 27 is made of T2T. A voltage of -2V was applied to the first electrode 21 with the second electrode 22 as the reference during a light reception period of 500 μs, and a voltage of +10V was applied to the first electrode 21 with the second electrode 22 as the reference during a 10 μs application period after the holding period had elapsed from the light reception period. As a result, luminescence was observed after 0 μs, the start time of the applied voltage, in both the case where the retention period was 0.1 s (Figure 9(a)) and the case where the retention period was 3 days (Figure 9(b)). This luminescence is due to holes that moved to the organic photodetector layer 25 when a voltage of +10 V was applied to the first electrode 21 with the second electrode 22 as the reference, and these holes combined with electrons that remained in the organic photodetector layer 25. In other words, this luminescence indicates that electrons generated by charge separation remained in the organic photodetector layer 25 in both the case where the retention period was 0.1 s (Figure 9(a)) and the case where the retention period was 3 days (Figure 9(b)). Note that such luminescence is not essential for the organic photodetector element 2A, but it is acceptable for it to occur.

[0081] By varying the holding period and normalizing the emission intensity of the light emitted after 0 μs, which is the start time of the applied period, the results shown in Figure 10 were obtained. Figure 10 is a graph showing the relationship between the holding period and the normalized emission intensity for the organic photodetector of the example. In this graph, it can be seen that in all holding periods, indicated by the black circles, the electrons generated by charge separation remained in the organic photodetector layer 25.

[0082] Figures 11(a), (b), (c), and (d) are photographs showing the light emission state of the organic photodetector in the example. Here, the same organic photodetector 2A used in the example was used to obtain the results shown in Figures 9(a) and (b). The difference between Figures 11(a), (b), (c), and (d) is the retention period, which is 1s, 10s, 100s, and 1000s, respectively. Figure 12 is a graph showing the relationship between the position and light emission intensity of the organic photodetector in the example. The horizontal axis of the graph shown in Figure 12 is the position along the center line of each photograph in Figures 11(a), (b), (c), and (d). Thus, it can be seen that in all retention periods, the electrons generated by charge separation remained in a certain region of the organic photodetector layer 25 (i.e., were spatially retained).

[0083] The organic photodetector 2A described above is configured such that the first electrode 21 functions as the anode and the second electrode 22 functions as the cathode. However, the first electrode 21 may function as the cathode and the second electrode 22 may function as the anode. In that case, a charge blocking layer 27 is placed at a first position between the organic photodetector layer 25 and the first electrode 21, and a charge transport layer 26 is placed at a second position between the organic photodetector layer 25 and the second electrode 22. When the first electrode 21 functions as the cathode and the second electrode 22 functions as the anode, the charge blocking layer 27 is a layer that prevents the movement of holes from the organic photodetector layer 25 to the first electrode 21. The charge blocking layer 27, like the first electrode 21, is light-transmitting to light irradiated onto the organic photodetector layer 25. When the charge blocking layer 27 is a hole-blocking layer that prevents the movement of holes, the electron transport material described above can be used as the material for the charge blocking layer 27. Furthermore, when the first electrode 21 functions as a cathode and the second electrode 22 functions as an anode, the charge transport layer 26 is an electron transport layer. In other words, the charge transport layer 26 is a layer that facilitates the movement of electrons from the organic photodetector layer 25 to the second electrode 22, and from the second electrode 22 to the organic photodetector layer 25. When the charge transport layer 26 is an electron transport layer that facilitates the movement of electrons, the electron transport material described above can be used as the material for the charge transport layer 26.

[0084] When the first electrode 21 functions as the cathode and the second electrode 22 functions as the anode, as shown in Figure 13(a), during the light-receiving period, a voltage is applied between the first electrode 21 and the second electrode 22 such that an electric field in the direction that causes charge separation is generated in the organic photodetector 25. When light to be received is irradiated onto the organic photodetector 25, charge separation occurs in the organic photodetector 25. The holes generated by the charge separation are prevented from moving to the first electrode 21 by the charge-blocking layer 27 and remain in the organic photodetector 25, while the electrons generated by the charge separation move to the second electrode 22. As shown in Figure 13(b), during the retention period, the state in which the holes generated by the charge separation remain in the organic photodetector 25 is maintained. As shown in Figure 13(c), during the application period after the light-receiving period has elapsed, when a voltage is applied between the first electrode 21 and the second electrode 22 such that an electric field opposite to the direction that causes charge separation is generated in the organic photodetector layer 25, electrons move from the second electrode 22 to the organic photodetector layer 25 and combine with holes that remained in the organic photodetector layer 25. The control unit 3A detects the current generated when the electrons that have moved to the organic photodetector layer 25 combine with holes that remained in the organic photodetector layer 25. Note that the charge transport layer 26 is not shown in Figures 13(a), (b), and (c).

[0085] When the first electrode 21 functions as the cathode and the second electrode 22 functions as the anode, the control unit 3A can generate an electric field in the organic photodetector layer 25 that causes charge separation by applying a voltage between the first electrode 21 and the second electrode 22 such that the potential of the first electrode 21 becomes positive with respect to the potential of the second electrode 22, or the potential of the second electrode 22 becomes negative with respect to the potential of the first electrode 21. Alternatively, when the first electrode 21 functions as the cathode and the second electrode 22 functions as the anode, the control unit 3A can generate an electric field in the organic photodetector layer 25 that causes charge separation by applying a voltage between the first electrode 21 and the second electrode 22 such that the potential of the first electrode 21 becomes negative with respect to the potential of the second electrode 22, or the potential of the second electrode 22 becomes positive with respect to the potential of the first electrode 21.

[0086] Furthermore, in the light-receiving device 1A, the control unit 3A detects the displacement current by applying a pulse voltage between the first electrode 21 and the second electrode 22. Since the displacement current flows if a capacitive component exists, a constant amount of displacement current flows in the organic photodetector 2A having the first electrode 21 and the second electrode 22, regardless of the presence or absence of accumulated charge. Therefore, the control unit 3A either acquires the differential current value or extracts the signal component of the current value by using a low-pass circuit (such as connecting a capacitor). When acquiring the differential current value, the control unit 3A may acquire the differential current value based on the "current value when light is irradiated" and the "current value when light is not irradiated" acquired by a single organic photodetector 2A. Alternatively, the control unit 3A may acquire the differential current value based on the "current value when light is irradiated" acquired by the light-receiving organic photodetector 2A and the "current value when light is not irradiated" acquired by the reference organic photodetector 2A. The reference organic photodetector 2A is an organic photodetector 2A that has been modified to prevent light from entering the organic photodetector layer 25 by a light-shielding film or package. If multiple light-receiving organic photodetectors 2A are provided, it is sufficient to provide at least one reference organic photodetector 2A. However, in that case, if a reference organic photodetector 2A is placed in each area, the influence of variations in the "current value when light is not irradiated" due to differences in areas can be suppressed.

[0087] Furthermore, the organic photodetector 2A does not necessarily have to have a charge transport layer 26. Instead of the charge transport layer 26, the organic photodetector 2A may have a buffer layer that tunnels at a voltage above a certain level. In that case, it is possible to observe the difference in current amount depending on whether or not charge is stored.

[0088] Furthermore, in the organic photodetector 2A, a voltage does not need to be applied between the first electrode 21 and the second electrode 22 during at least one of the light-receiving period and the holding period. Even if the potential difference between the first electrode 21 and the second electrode 22 is 0V during the light-receiving period and the holding period, an internal electric field is generated in the organic photodetector layer 25 due to the difference between the work function of the first electrode 21 and the work function of the second electrode 22, and the polarization of the organic semiconductor molecule 25a, thus enabling charge separation and the retention of one of the electrons and holes generated by charge separation. However, during the light-receiving period, it is necessary to extract the other of the electrons and holes generated by charge separation from the organic photodetector layer 25, so the organic photodetector 2A needs to be connected to a power supply (control unit 3A) regardless of whether a voltage is applied between the first electrode 21 and the second electrode 22. During the holding period, the organic photodetector 2A may be in an electrically floating state in order to maintain the retention of one of the electrons and holes generated by charge separation.

[0089] During the light-receiving period, the greater the voltage applied between the first electrode 21 and the second electrode 22, such that an electric field is generated in the organic light-receiving layer 25 in a direction that causes charge separation, the greater the increase in either electrons or holes generated by charge separation. However, if a voltage is continuously applied between the first electrode 21 and the second electrode 22 during the holding period, the amount of electrons or holes generated by charge separation may decrease due to the effects of leakage current, etc. Therefore, when prioritizing the detection of a large current, it is preferable to increase the voltage applied between the first electrode 21 and the second electrode 22, and conversely, when prioritizing securing a long holding time, it is preferable to decrease the voltage applied between the first electrode 21 and the second electrode 22. [Differentiation]

[0090] This disclosure is not limited to the first and second embodiments described above. For example, the plurality of organic semiconductor molecules 25a contained in the organic photodetector layer 25 may be composed of multiple types of organic semiconductor molecules. The plurality of host molecules 25b contained in the organic photodetector layer 25 may be composed of multiple types of host molecules. The organic photodetector layer 25 may not contain the plurality of host molecules 25b.

[0091] Each of the multiple organic semiconductor molecules 25a does not need to satisfy all of the above conditions, as long as each of the multiple organic semiconductor molecules 25a can form an excited state in which reverse intersystem crossing from the lowest excited triplet state T1 to the lowest excited singlet state S1 is possible upon irradiation with light.

[0092] In the organic photodetector 2 of the first embodiment and the organic photodetector 2A of the second embodiment, a voltage may be applied between the first electrode 21 and the second electrode 22 during the light-receiving period so that an electric field in the direction that causes charge separation is generated in the organic photodetector layer 25, or the potential difference between the first electrode 21 and the second electrode 22 may be set to 0V. In other words, each control unit 3,3A should adjust the potential difference between the first electrode 21 and the second electrode 22 during the light-receiving period so that an electric field in the direction that causes charge separation is generated in the organic photodetector layer 25. In the organic photodetector 2 of the first embodiment and the organic photodetector 2A of the second embodiment, a plurality of organic photodetector layers 25 may be arranged in one or two dimensions on the support substrate 20. In the organic photodetector 2A of the second embodiment, as described in Figures 11(a), (b), (c), and (d) and the experimental results in Figure 12, the electrons generated by charge separation remain in a certain region of the organic photodetector layer 25 (i.e., are spatially retained), so the organic photodetector 2A of the second embodiment can be applied to an image sensor. As an example, as shown in Figures 14(a) and (b), an image sensor may be configured by electrically connecting each of the multiple organic photodetectors 2A to each of the multiple readout circuits 51 arranged in one or two dimensions on a circuit board 50. In the image sensor shown in Figure 14(a), the second electrode 22 of each organic photodetector 2A is electrically connected to each readout circuit 51, and the charge blocking layer 27, organic photodetector layer 25, and first electrode 21 of each organic photodetector 2A are continuous across the multiple organic photodetectors 2A. In the image sensor shown in Figure 14(b), the second electrode 22 of each organic photodetector 2A is electrically connected to each readout circuit 51, and the charge blocking layer 27, organic photodetector layer 25, and first electrode 21 of each organic photodetector 2A are separated by an insulating layer 52. [Explanation of symbols]

[0093] 1,1A...Photodetector, 2,2A...Organic photodetector, 3,3A...Control unit, 21...First electrode, 22...Second electrode, 25...Organic photodetector layer, 25a...Organic semiconductor molecule, 25b...Host molecule, 26...Charge transport layer, 27...Charge blocking layer.

Claims

1. An organic photodetector, The organic light-receiving element comprises a control unit electrically connected to the organic light-receiving element, The aforementioned organic photodetector is An organic photodetector containing multiple organic semiconductor molecules, A first electrode disposed on one side of the organic light-receiving layer, A second electrode is disposed on the other side of the organic light-receiving layer, The system comprises a charge blocking layer disposed at one of the following locations: a first position between the organic light-receiving layer and the first electrode, and a second position between the organic light-receiving layer and the second electrode. Each of the aforementioned plurality of organic semiconductor molecules is a molecule in which, upon irradiation with light, an excited state is formed in which reverse intersystem crossing from the lowest excited triplet state to the lowest excited singlet state is possible. The control unit, During the light-receiving period, the potential difference between the first electrode and the second electrode is adjusted so that an electric field in a direction that causes charge separation is generated in the organic light-receiving layer. A light-receiving device that detects a current by applying a voltage between the first electrode and the second electrode during the application period after the holding period has elapsed since the light-receiving period, such that an electric field in the organic light-receiving layer is generated in the direction opposite to the direction that causes charge separation.

2. The photodetector according to claim 1, wherein in each of the plurality of organic semiconductor molecules, the difference between the energy of the lowest excited singlet state and the energy of the lowest excited triplet state at an absolute temperature of 77 K is less than 0.3 eV.

3. The photodetector according to claim 1 or 2, wherein in each of the plurality of organic semiconductor molecules, the intersystem crossover rate constant from the lowest excited singlet state to the lowest excited triplet state is greater than the fluorescence rate constant from the lowest excited singlet state to the ground state.

4. The photodetector according to any one of claims 1 to 3, wherein in each of the plurality of organic semiconductor molecules, the intersystem crossover rate constant from the lowest excited singlet state to the lowest excited triplet state is greater than the inverse intersystem crossover rate constant from the lowest excited triplet state to the lowest excited singlet state.

5. The photodetector according to claim 4, wherein in each of the plurality of organic semiconductor molecules, the intersystem crossover rate constant from the lowest excited singlet state to the lowest excited triplet state is at least twice the inverse intersystem crossover rate constant from the lowest excited triplet state to the lowest excited singlet state.

6. In each of the plurality of organic semiconductor molecules, the inverse intersystem crossover rate constant from the lowest excited triplet state to the lowest excited singlet state is 1 × 10 ７ (sec －１ The light receiving device according to claim 4 or 5, wherein the device is as follows:

7. The photodetector according to any one of claims 1 to 6, wherein the dipole moment of each of the plurality of organic semiconductor molecules is greater than 0D.

8. The organic light-receiving layer further contains a plurality of host molecules, The photodetector according to any one of claims 1 to 7, wherein the energy of the lowest excited triplet state at an absolute temperature of 77 K in each of the plurality of host molecules is higher than the energy of the lowest excited triplet state at an absolute temperature of 77 K in each of the plurality of organic semiconductor molecules.

9. The light receiving device according to any one of claims 1 to 8, wherein at least one of the first electrode and the second electrode is light-transmitting to light irradiated onto the organic light receiving layer.

10. The photodetector according to any one of claims 1 to 9, wherein the organic photodetector further comprises a charge transport layer disposed at the other of the first and second positions.

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