Exposure head and electrophotographic device

Abstract

The present disclosure provides a technology for realizing, by means of an exposure head using a light-emitting element comprising a plurality of OLEDs, a sufficient response speed of exposure light enabling high-speed printing in exposure of an electrophotographic device (1). An exposure head (12) faces an electrophotographic photoreceptor (10) and emits thereto light corresponding to an electrostatic latent image. The exposure head (12) includes a light-emitting element. A light-emitting layer of the light-emitting element is a fluorescent light-emitting layer containing a host compound and a fluorescent dopant which is a guest compound.

Description

Exposure head and electrophotographic device

[0001] The present disclosure relates to an exposure head and an electrophotographic apparatus.

[0002] Copiers, printers, and other devices that utilize the electrophotographic process are collectively referred to as electrophotographic devices. For the purpose of forming full-color images and high-speed printing, electrophotographic devices employ a configuration equipped with multiple electrophotographic photoreceptors, known as an inline or tandem configuration. The inline (tandem) configuration is an electrophotographic device configuration in which each toner color has its own configuration for achieving the process from forming an electrostatic latent image by exposure to transferring a toner image. Conventional electrophotographic device exposure devices use a laser scanning light source device consisting of a laser light source and a polygon mirror. However, inline electrophotographic devices with laser scanning light source devices require further consideration in terms of miniaturization and quietness.

[0003] As an exposure device for an in-line electrophotographic device that can replace a laser scanning light source device and achieve miniaturization and quietness, an exposure head arranged for each electrophotographic photosensitive member is known. The exposure head is a light source device for exposing an electrophotographic photosensitive member to light, in which a plurality of small light sources (e.g., light-emitting elements made of organic light-emitting diodes (OLEDs)) are arranged with high precision along the rotation axis direction on a substrate arranged on the outer periphery of the electrophotographic photosensitive member along the rotation axis direction (see, for example, Patent Document 1).

[0004] Further, an exposure head for an electrophotographic device is known in which a plurality of (n pieces, n≧2) OLEDs and a plurality of ball lenses are arranged in the circumferential direction on a curved substrate that is arranged along the outer peripheral surface of an electrophotographic photosensitive member, and the light emitted from each OLED is all converged toward a single point in the circumferential direction of the electrophotographic photosensitive member (see, for example, Patent Document 2).

[0005] Japanese Patent Publication No. 11-198433 Japanese Patent Application Publication No. 2005-047011

[0006] The exposure head using organic electroluminescence (OEL) light-emitting elements (OLED) described in Patent Document 1 has difficulty in achieving the same small spot size and high irradiance as a laser scanning light source device using a conventional laser light source. Therefore, it is difficult to achieve high resolution in printed images. Furthermore, the exposure head using OLED described in Patent Document 1 has the problem of a short light source life.

[0007] In contrast, the exposure head described in Patent Document 2 achieves sufficient irradiance by concentrating light from multiple light sources (OLEDs) arranged in the circumferential direction into one spot in the circumferential direction. Thus, in the prior art, high-speed electrophotographic printing using an exposure head having OLEDs is achieved by increasing the amount of exposure light (irradiance).

[0008] On the other hand, realizing high-speed printing in electrophotography also requires a high response speed of the light source of the exposure light. The aforementioned patent documents do not describe the realization of such a high response speed of the light source using OLEDs. Therefore, the prior art leaves room for further study from the perspective of realizing a sufficient response speed of the exposure light that enables high-speed printing in the exposure of an electrophotographic device using an exposure head that uses light-emitting elements consisting of multiple OLEDs.

[0009] An object of one aspect of the present disclosure is to provide a technique for realizing a sufficient response speed of exposure light that enables high-speed printing in exposure of an electrophotographic device using an exposure head that uses a plurality of light-emitting elements.

[0010] In order to solve the above-mentioned problems, an exposure head according to one aspect of the present disclosure is an exposure head arranged opposite the surface of a drum-shaped electrophotographic photosensitive member, and includes a plurality of light-emitting elements arranged side by side in a first direction and emitting light corresponding to an electrostatic latent image to be formed on the surface of the electrophotographic photosensitive member in a second direction intersecting the first direction, wherein each of the plurality of light-emitting elements comprises a set of a light-emitting layer and an electrical functional layer overlapping the light-emitting layer, which is arranged between a pair of electrode layers, and the light-emitting layer is a fluorescent light-emitting layer containing a host compound and a fluorescent dopant which is a guest compound.

[0011] In order to solve the above-mentioned problems, an electrophotographic apparatus according to one aspect of the present disclosure includes a drum-shaped electrophotographic photosensitive member, a charging device that charges the electrophotographic photosensitive member, the above-mentioned exposure head that irradiates the charged electrophotographic photosensitive member with light to form an electrostatic latent image, a developing device that develops the electrostatic latent image formed on the surface of the electrophotographic photosensitive member with toner, a transfer device that transfers the toner image formed on the surface of the electrophotographic photosensitive member onto a recording medium, and a fixing device that fixes the toner image transferred onto the recording medium to the recording medium.

[0012] According to one aspect of the present disclosure, an exposure head using a plurality of light-emitting elements can achieve a sufficient response speed of exposure light that enables high-speed printing in exposure of an electrophotographic apparatus.

[0013] FIG. 1 is a diagram schematically illustrating a configuration of an electrophotographic device according to embodiment 1 of the present disclosure. FIG. 2 is a perspective view schematically illustrating a configuration of an exposure head according to embodiment 1 of the present disclosure. FIG. 3 is a diagram schematically illustrating an example of the arrangement of light sources in an exposure head according to embodiment 1 of the present disclosure. FIG. 4 is a diagram schematically illustrating the layer structure of a light source according to embodiment 1 of the present disclosure. FIG. 5 is a diagram schematically illustrating the layer structure of a light source according to embodiment 1 of the present disclosure. FIG. 6 is a flowchart illustrating an example of a method for manufacturing a light emitting element according to embodiment 1 of the present disclosure. FIG. 7 is a diagram schematically illustrating the layer structure of a light source according to embodiment 2 of the present disclosure. FIG. 8 is a flowchart illustrating an example of a method for manufacturing a light emitting element according to embodiment 2 of the present disclosure. FIG. 9 is a diagram schematically illustrating the layer structure of a light emitting element according to embodiment 3 of the present disclosure. FIG. 10 is a diagram schematically illustrating the layer structure of a light emitting element according to embodiment 4 of the present disclosure. FIG. 11 is a diagram schematically illustrating the layer structure of a light emitting element according to embodiment 5 of the present disclosure. FIG. 12 is a diagram schematically illustrating the layer structure of a light emitting element according to embodiment 6 of the present disclosure.

[0014] 1 is a diagram schematically illustrating the configuration of an electrophotographic device according to embodiment 1 of the present disclosure. In the diagram of the electrophotographic device, the color of the toner (cyan (C), magenta (M), yellow (Y), and black (K)) may be displayed after the reference numeral to indicate that the configuration corresponds to a specific toner.

[0015] As shown in FIG. 1 , the electrophotographic apparatus 1 includes an electrophotographic photoreceptor 10, a charging device 11, an exposure head 12, a developing device 13, a transfer device 14, a conveying device 15, and a fixing device 16. The electrophotographic apparatus 1 is a full-color image-producing electrophotographic apparatus capable of forming toner images of each color, cyan (C), magenta (M), yellow (Y), and black (K). The electrophotographic apparatus 1 is also an in-line electrophotographic apparatus having four sets of the electrophotographic photoreceptor 10, the charging device 11, the exposure head 12, the developing device 13, and the transfer device 14. Each set is arranged along the conveying direction of a recording medium 17, and is configured to form a toner image of each color.

[0016] The electrophotographic photoreceptor 10 is drum-shaped and has a photosensitive layer formed on the outer peripheral wall surface. The photosensitive layer is made of, for example, an organic photoconductor (OPC) material. Various known photosensitive materials for electrophotographic photoreceptors can be used as the material for the photosensitive layer of the electrophotographic photoreceptor 10.

[0017] The charging device 11 charges the electrophotographic photoreceptor 10 before exposure and adjusts the potential of the photosensitive layer. The charging device 11 is, for example, a corona discharge charging device that charges the electrophotographic photoreceptor 10 by corona discharge, but may also be another charging device such as a charging roller.

[0018] [Exposure Head] The exposure head 12 is disposed opposite the surface of the electrophotographic photoreceptor 10. Fig. 2 schematically shows the configuration of the exposure head 12 of this embodiment. The exposure head 12 is arranged such that a substrate 121, a light source 122, and an equal-magnification lens 123 are stacked in this order. Fig. 3 also schematically shows an example of the arrangement of the light sources 122 in the exposure head 12 of this embodiment. In this example, a plurality of light sources 122 are arranged in a row on the substrate 121.

[0019] In the drawing, arrow X indicates a first direction, and arrow Y indicates a second direction. Arrow X is an arrow extending from the plane of FIG. 1 toward the back, and the first direction represented by arrow X is the longitudinal direction of the substrate 121 of the exposure head 12, and in the electrophotographic apparatus 1, it is a direction along the rotation axis of the electrophotographic photosensitive member 10. Arrow Y is an arrow extending from one main surface of the substrate 121 in a direction perpendicular to the main surface, and the second direction represented by arrow Y is the emission direction of light from each light source 122 of the exposure head 12, and is a direction along the optical axis of each light source 122. In the electrophotographic apparatus 1, the second direction is a direction in which the light source 122 of the exposure head 12 faces the electrophotographic photosensitive member 10.

[0020] The substrate 121 extends along the rotation axis direction of the electrophotographic photosensitive member 10. When viewed from above, the substrate 121 has an elongated rectangular shape. The substrate 121 is disposed at a position a specific distance away from the outer circumferential surface of the electrophotographic photosensitive member 10 so that the central axis of the shape when viewed from above is aligned with the rotation axis direction of the drum-shaped electrophotographic photosensitive member 10.

[0021] Each light source 122 includes one light-emitting element. Each light-emitting element is, for example, an OLED, and includes a light-emitting layer that emits light with a wavelength of, for example, 640 nm so as to match the sensitivity spectrum of the electrophotographic photoreceptor 10. A more detailed configuration of the light source 122 will be described later. The light source 122 and the light-emitting region of the light-emitting element included therein may have the same shape in a plan view, for example, a circle, for example, a perfect circle with a diameter of 15 μm. A plurality of light sources 122 are arranged in the direction of the rotation axis of the drum-shaped electrophotographic photoreceptor 10, and are arranged in a number of 15,600 pieces at a pitch of 21.17 μm (15,600 dots in the direction of the rotation axis of the electrophotographic photoreceptor 10) so as to achieve, for example, 1,200 dpi.

[0022] The arrangement of the light sources 122 (light-emitting elements) can be set appropriately within a range that allows exposure of the electrophotographic photosensitive member. For example, the light-emitting elements may be arranged in a line along the first direction as shown in the figure, or may be arranged in a staggered arrangement extending along the first direction.

[0023] The equal-magnification lens 123 condenses the light from the light source 122 to an equal-magnification size on the surface of the electrophotographic photosensitive member 10. That is, the light from the circular light-emitting element having a diameter of 15 μm is condensed onto a circular area having a diameter of 15 μm on the surface of the electrophotographic photosensitive member 10.

[0024] The minimum irradiance per element of the light-emitting element of the light source 122 on the electrophotographic photoreceptor 10 through the 1x1 lens 123 is 0.10 μW / dot. The irradiance per element of the light-emitting element through the 1x1 lens 123 is measured using an optical power meter installed at approximately the same distance as the irradiation position on the electrophotographic photoreceptor 10. The irradiance per element of the light-emitting element of the light source 122 may be 0.10 μW / dot or more, and from the viewpoint of increasing the exposure sensitivity of the electrophotographic device, the higher the irradiance, the more preferable. On the other hand, from the viewpoint of the design of the light-emitting element, the ease of obtaining materials for the light-emitting element, and the viewpoint that the effect of a sufficiently high irradiance will plateau, the irradiance may be 1.00 μW / dot or less.

[0025] Furthermore, in this embodiment, the irradiance can be increased by increasing the current flowing through the light-emitting element or by employing a tandem structure for the light-emitting element (a light-emitting element employing a tandem structure is also referred to as a "tandem light-emitting element"), as described below. In a tandem light-emitting element, by increasing the number of stages in the stack including the light-emitting layer, it is possible to increase the amount of light emitted by the light-emitting element without increasing the amount of current flowing through each light-emitting layer even when the applied voltage is increased. Therefore, employing a tandem light-emitting element is preferable from the viewpoint of achieving both suppression of degradation of the light-emitting layer and an increase in the amount of light emitted.

[0026] The exposure head 12 includes a driver (not shown), which controls the light emission of the light source 122 so that each light-emitting element of the light source 122 sequentially emits light corresponding to the electrostatic latent image to be formed on the electrophotographic photoreceptor 10. If multiple discrete inorganic light-emitting diodes (LEDs) are used as the light source of the exposure head, a separate external driver must be installed to control the light emission state of each inorganic LED light source, which can result in a complex and large-sized exposure head configuration. Furthermore, it is difficult to further miniaturize the inorganic LED light source itself and to package it at a high density. In contrast, when OLEDs are used as in the present disclosure, a simple exposure head 12 can be realized with minimal external drivers by adopting a configuration similar to that of a TFT (thin film transistor) circuit integrated with the light-emitting elements in an organic EL display. Similarly, a simple exposure head 12 can be realized with minimal external drivers by using quantum dot light-emitting diodes (QLEDs) having a light-emitting layer containing quantum dots instead of the organic light-emitting layer of an OLED as the light-emitting element.

[0027] The developing device 13 has, for example, a toner container that stores toner of each color, and a developing roller that is rotatably disposed at the opening of the toner container. The toner carried by the developing roller is transferred from the developing roller to the surface of the electrophotographic photoreceptor 10 in accordance with the electrostatic latent image on the electrophotographic photoreceptor 10, and a toner image in accordance with the electrostatic latent image is formed on the surface of the electrophotographic photoreceptor 10.

[0028] The transfer device 14 forms an electric field so as to transfer the toner image on the surface of the electrophotographic photosensitive member 10 to a recording medium 17. The transfer device 14 is, for example, a corona discharge charging device.

[0029] The conveying device 15 conveys the recording medium 17 toward the fixing device 16 while sandwiching it with each of the four electrophotographic photosensitive members 10 at a transfer nip portion (a portion where the transfer device 14 faces the electrophotographic photosensitive members 10). The conveying device 15 has an endless conveying belt 151, and conveying rollers 152 and 153 that stretch the conveying belt 151 and rotate in a direction toward the fixing device 16. The transfer device 14 is disposed inside the endless track formed by the conveying belt 151, and applies a voltage to the electrophotographic photosensitive members 10 via the conveying belt 151.

[0030] The fixing device 16 is composed of, for example, a heating roller 161 and a pressure roller 162. The heating roller 161 and the pressure roller 162 heat and pressurize a recording medium carrying an unfixed toner image introduced into the portion where they face each other (a fixing nip portion), thereby melting the toner of the toner image and fixing it to the recording medium 17. In this way, the electrophotographic apparatus 1 forms a full-color image fixed on the recording medium 17.

[0031] The recording medium 17 is a sheet-like member such as plain paper that bears the toner image of the electrophotographic photoreceptor 10 and can be supported as a fixed toner image by the fixing device 16. The recording medium 17 is sequentially transported by the transport device 15 to the transfer nip portions of the four electrophotographic photoreceptors 10. The recording medium 17 bears toner images of specific colors that each electrophotographic photoreceptor 10 has, superimposed on one another.

[0032] Each set of the electrophotographic photosensitive member 10, the charging device 11, the exposure head 12, the developing device 13, and the transfer device 14 may further have other configurations than those described above, as long as the effects of this embodiment can be obtained. Examples of such other configurations include a cleaning device for removing residual toner on the electrophotographic photosensitive member 10 after transfer, and a static eliminator for eliminating electrostatic history of the electrophotographic photosensitive member 10 after transfer. Furthermore, some or all of each set (for example, the electrophotographic photosensitive member 10, the exposure head 12, the developing device 13, and, if necessary, the cleaning device, etc.) may constitute a process cartridge that is integrally held in a specific positional relationship by a holding member.

[0033] 4 shows a schematic diagram of the layer structure of the light source 122 in this embodiment. The light source 122 has a layered structure in which a substrate 21, a buffer layer 22, a TFT layer 23 including pixel circuits, an edge cover film 24, a sealing layer 25, and an external functional layer 26 are stacked in this order. The light source 122 has a light-emitting element 30 on the TFT layer 23, across the edge cover film 24 and the sealing layer 25.

[0034] The substrate 21 may be a rigid substrate made of glass, metal, hard resin, or the like, or may be a flexible substrate mainly composed of polyimide resin, etc. However, from the viewpoint of strictly controlling the size of the exposure pattern formed on the electrophotographic photoreceptor 10, it is preferable that the distance between the electrophotographic photoreceptor 10 and the light source 122 is constant. From this viewpoint, it is sometimes preferable that the substrate 21 is rigid. The substrate 21 may be the substrate 121.

[0035] The buffer layer 22 is made of a material that prevents the intrusion of foreign substances such as water and oxygen, for example, an inorganic insulating layer.

[0036] The TFT layer 23 includes a pixel circuit that controls the light emission of the light emitting element 30 .

[0037] The edge cover film 24 has insulating properties and covers the edge of the anode layer 31 of the light emitting element 30. The edge cover film 24 is formed, for example, by applying an organic material such as polyimide or acrylic resin and then patterning it by photolithography.

[0038] The sealing layer 25 covers the light-emitting element 30 and prevents foreign substances such as water and oxygen from penetrating into the light-emitting element 30. The sealing layer 25 is composed of, for example, two inorganic sealing films and an organic film formed between them.

[0039] The external functional layer 26 is a layer that adds various functions to the light source 122, such as optical control or surface protection.

[0040] [Light-emitting element] In the following embodiments, when layers with the same name exist in the same light-emitting element, they are designated by an ordinal number such as first or second from the anode layer 31 side, and are designated by a different reference number as necessary.

[0041] The light-emitting element 30 has a layered structure in which an anode layer 31, a first electrical functional layer 32, a light-emitting layer 33, a second electrical functional layer 34, and a cathode layer 35 are stacked in this order. Note that, although an organic light-emitting diode (OLED) element will be described as an example of the light-emitting element, as will be described later, the light-emitting element may also be a light-emitting diode (QLED) element that uses quantum dots in the light-emitting layer.

[0042] An "electrical functional layer" is a layer having a function of controlling the movement of electrons or holes to the light-emitting layer. Examples of electrical functional layers for controlling the movement of electrons include electronic functional layers. Examples of electronic functional layers include electron injection layers and electron transport layers. Examples of electrical functional layers for controlling the movement of holes include hole functional layers. Examples of hole functional layers include hole injection layers and hole transport layers. The first electrical functional layer 32 is mainly composed of layers for controlling the movement of holes to the light-emitting layer 33. The second electrical functional layer 34 is mainly composed of layers for controlling the movement of electrons to the light-emitting layer 33.

[0043] 5 schematically shows an example of the layer structure of the light-emitting element 30 in this embodiment. The light-emitting element 30 is configured by stacking an anode layer 31, a hole injection layer 321, a hole transport layer 322, an electron blocking layer 323, a light-emitting layer 33, a hole blocking layer 341, an electron transport layer 342, an electron injection layer 343, and a cathode layer 35 in this order. The light-emitting element 30 is a top-emission type (a structure in which light is extracted from the upper side, i.e., the cathode layer 35 side). In the light-emitting element 30, the anode layer 31 functions as an anode, and the cathode layer 35 functions as a cathode.

[0044] The first electrical functional layer 32 is composed of a hole injection layer 321, a hole transport layer 322, and an electron blocking layer 323. The second electrical functional layer 34 is composed of a hole blocking layer 341, an electron transport layer 342, and an electron injection layer 343.

[0045] Each layer of the light-emitting element 30 will be described below. The anode layer 31 is electrically conductive. Furthermore, the anode layer 31 has optical properties, for example, of reflecting part of visible light and transmitting the rest. Typically, the anode layer 31 includes both an electrode material that reflects visible light and an electrode material that transmits visible light.

[0046] From the viewpoint of enhancing hole injection properties, a material with a relatively high work function (e.g., a material with a work function of 4.5 eV or more) is preferably used as the material for the anode layer 31. Examples of electrode materials with a high work function include Pt (5.65 eV), Ir (5.25 eV), Ni (5.2 eV), Au (5.15 eV), Pd (5.15 eV), and indium tin oxide (In—Sn—O). Examples of electrode materials that reflect visible light include metal materials such as Al, Mg, Li, Ag, Pd, and Cu, as well as alloys of these metal materials (e.g., APC (Ag—Pd—Cu) alloy, etc.). Examples of electrode materials that transmit visible light include thin films of transparent metal oxides (e.g., indium tin oxide, indium zinc oxide (In—Zn—O), and indium gallium zinc oxide (In—Ga—Zn—O)), thin films made of metal materials such as Al, Mg, and Ag, and nanowires (NW) made of these metal materials. Electrode materials that transmit visible light are also used in the cathode layer of top-emission light-emitting devices, which will be described later.

[0047] Among transparent metal oxides, indium tin oxide has a relatively high work function of 4.6 to 5.0 eV, and is therefore suitable for use as the material for the anode layer 31. Furthermore, for the anode layer 31, a laminate (for example, indium tin oxide / Ag) in which indium tin oxide is formed on the surface of a metal material may be used, with the aim of improving the conductivity as an electrode layer or adding the function of reflecting visible light.

[0048] The hole injection layer 321 is disposed adjacent to the anode layer 31, for example. The hole injection layer 321 may be composed of a hole transport material and an electron acceptor material. These materials may be the same organic materials as those described below for the p-type first charge generation layer 414. The specific material of the hole injection layer 321 in the light-emitting element 30 may be the same as or different from that of the p-type first charge generation layer 414.

[0049] The hole transport layer 322 may be made of an organic hole transport material, for example, a triarylamine organic compound.

[0050] Like the hole transport layer, the electron blocking layer 323 may be made of an organic hole transport material. The material of the electron blocking layer 323 may be the same as or different from that of the hole transport layer 322.

[0051] The light-emitting layer 33 is a layer that emits light of a predetermined color by injection and recombination of carriers consisting of holes and electrons. The light-emitting layer 33 is a fluorescent light-emitting layer. The fluorescent light-emitting layer contains a host compound and a fluorescent dopant, which is a guest compound.

[0052] The fluorescent-emitting layer may have a laminated structure of two or more layers having two or more different light-emitting functions. For example, the fluorescent-emitting layer may have a laminated structure in which two or more functional layers corresponding to two or more functions for fluorescent emission are stacked one on top of the other, and the layered structure as a whole exhibits the function of the light-emitting layer.

[0053] The wavelength of the light emitted by the fluorescent light-emitting layer may be any wavelength that can sensitize the electrophotographic photoreceptor 10 to form an electrostatic latent image when emitted from the light-emitting elements 30. The wavelengths of the light irradiated by different light-emitting layers in the stacking direction may be the same or different within the above-mentioned range. Furthermore, the wavelengths of the light irradiated by the light-emitting layers of the multiple light-emitting elements 30 included in the light source 122 may also be the same or different within the above-mentioned range.

[0054] In this embodiment, the fluorescent-emitting layer is a host-guest emitting layer containing a host compound and a guest compound. A host-guest emitting layer contains a small amount (e.g., about 0.1 to several mol %) of a fluorescent dopant or the like as a guest compound in a solid medium that is a host compound. In an emitting layer containing such a guest compound, the fluorescence of the host compound is completely lost, and instead, strong light emission that matches the fluorescent spectrum of the guest compound is obtained. This is because the excitation energy of the host compound is transferred to the guest compound. Due to this transfer of excitation energy, the host-guest emitting layer can emit light from the guest compound with higher quantum efficiency.

[0055] The host-guest emitting layer can be formed by doping a host compound with a guest compound, which can be achieved by a co-evaporation method using multiple evaporation sources.

[0056] Doping the light-emitting layer with a fluorescent dopant dramatically improves the device life of the light-emitting device. This is because the fluorescent dopant, which is a guest compound, functions as a carrier (electron or hole) trap in the solid medium of the host compound, becoming a carrier recombination center and directly generating excitons in the solid medium. The process by which these generated excitons relax to the ground state is called the deactivation process. The deactivation process can be classified into a non-radiative process (thermal deactivation) and a radiative process (emission). Of these, electroluminescence (EL) is the phenomenon in which light is emitted by the radiative process. The guest compound's function as a carrier trap not only improves the quantum efficiency of the fluorescent-emitting layer but also improves the device life due to an increased carrier recombination rate. As a result, a host-guest fluorescent-emitting layer can further improve the luminous efficiency of the light-emitting layer and the device life of the fluorescent light-emitting device.

[0057] Various known examples of host-guest fluorescent-emitting layer materials can be used. Examples of host compounds include known light-emitting layer materials for each color.

[0058] Examples of fluorescent dopants include pyrene-based compounds, anthracene-based compounds, perylene, BCzVBi, ADN, TBP, DPT, Coumarin6, C545T, PMDFB, quinacridone, rubrene, BTX, ABTX, DCM1, DCM2, DCJTB, DCJMTB, and TDPF.

[0059] The energy release process when a substance returns from an excited state to a ground state may involve luminescence such as fluorescence, phosphorescence, or delayed fluorescence. The decay time of luminescence when a substance is instantaneously excited by ultrashort pulsed light is called the luminescence lifetime (also called the "fluorescence lifetime"). The luminescence lifetime of a fluorescent-emitting layer is the time it takes for a light-emitting material (guest compound) in the fluorescent-emitting layer to be excited, emit a photon, and return to the ground state. This luminescence lifetime can be measured using a known luminescence lifetime measurement device that utilizes a photon counting method such as time-correlated single photon counting. The luminescence lifetime of the fluorescent-emitting layer may be any length that allows for exposure for high-speed printing in an electrophotographic device, and may be, for example, 50 nanoseconds or less, 20 nanoseconds or less, or 10 nanoseconds or less. The luminescence lifetimes of the fluorescent-emitting layers of the individual light-emitting elements 30 of the light source 122 may be the same or different as long as the above-described exposure can be achieved. The luminescence lifetime of the fluorescent-emitting layer may be, for example, 1 nanosecond or more, as long as the above-described exposure can be achieved. Generally, the luminescence lifetime refers to the decay time during photoexcitation, but in the present disclosure, the decay time during injection and excitation of carriers is also referred to as the luminescence lifetime.

[0060] The emission lifetime of the fluorescent light-emitting layer is on the order of nanoseconds. Therefore, by using an OLED having a fluorescent light-emitting layer in an exposure head, an emission lifetime of 50 nanoseconds or less can be achieved, thereby realizing high-speed exposure that corresponds to high-speed printing in electrophotographic devices.

[0061] The light-emitting layer may contain inorganic quantum dots instead of a fluorescent dopant. Quantum dots are semiconductor particles with a particle size of approximately 50 nm or less (e.g., approximately 2 nm to 30 nm). Because their composition is derived from semiconductor materials, they are sometimes referred to as semiconductor nanoparticles. By adjusting the particle size or composition, the quantum dots can vary their emission wavelength, for example, to any wavelength from visible light to near-infrared light (wavelengths of 380 to 2500 nm), or any wavelength from red light to a portion of near-infrared light (wavelengths of 620 to 1500 nm). The shape of the quantum dots is not limited. For example, the shape of the quantum dots may be a spherical three-dimensional shape (circular cross-sectional shape), a polygonal cross-sectional shape, a rod-shaped three-dimensional shape, a branch-shaped three-dimensional shape, or a three-dimensional shape with an uneven surface, or a combination thereof.

[0062] The quantum dots may be formed of only a core, or may have a core-shell structure including a core and a shell. The shell may be formed in a solid solution state on the surface of the core. The quantum dots may also include doped nanoparticles. Examples of combinations of core and shell materials for quantum dots include CdSe / CdS, GaAs / AlGaAs, InP / ZnS, ZnSe / ZnS, and CIGS / ZnS.

[0063] When the light-emitting layer is a QLED, the decay time of the excited quantum dots is the luminescence lifetime. The luminescence lifetime of the QLED luminescence layer can be measured in the same manner as the luminescence lifetime of the fluorescent luminescence layer. The luminescence lifetime of a luminescence layer containing quantum dots having GaAs, InP, CdSe, ZnSe, etc. in the core is 0.1 nanoseconds to 10 nanoseconds. Therefore, even when a QLED is used in an exposure head, a luminescence lifetime of 50 nanoseconds or less can be achieved, thereby realizing high-speed exposure.

[0064] In this manner, the present disclosure allows the light-emitting layer of the light-emitting element to be appropriately determined within a range that allows for a light-emitting lifetime that enables desired high-speed image formation. Therefore, the light-emitting layer of the light-emitting element in the present disclosure may be a host-guest light-emitting layer containing a host compound in an OLED and a fluorescent dopant as a guest compound, or may be a QLED light-emitting layer.

[0065] The hole blocking layer 341 may be made of an organic electron transport material, such as an oxadiazole-based compound or a phenanthroline-based compound. The material of the hole blocking layer 341 may contain lithium quinoline (Liq) in addition to the electron transport material.

[0066] The electron transport layer 342 may be made of the organic electron transport material described above, similar to the hole blocking layer 341. The material of the electron transport layer may be the same as or different from that of the hole blocking layer 341. The electron transport layer may contain lithium quinoline in addition to the electron transport material.

[0067] The electron injection layer 343 is disposed adjacent to the cathode layer 35, for example. The electron injection layer 343 may be made of an electron transport material. Examples of electron transport materials that make up the electron injection layer 343 include lithium fluoride (LiF), which is an inorganic material. The electron injection layer 343 may also be made of an organic material such as an oxadiazole-based compound or a phenanthroline-based compound doped with a metal material (e.g., Li or Yb).

[0068] The cathode layer 35 is disposed opposite the anode layer 31 in the stacking direction. The cathode layer 35 is an electrode layer for supplying electrons to each layer constituting the light-emitting element. The cathode layer 35 has, for example, electrical conductivity and transparency to visible light. A material with a relatively small work function is preferably used as the material for the cathode layer 35, for example, from the viewpoint of enhancing electron injection properties. Examples of electrode materials constituting the cathode layer 35 include metal materials such as alkali metals, alkaline earth metals, and Al, alloys containing these, and nanowires (e.g., Ag nanowires). Examples of alloys include an alloy of Mg and Ag, and Al doped with a small amount of Li.

[0069] The hole injection layer and the electron injection layer may be disposed corresponding to the electrode layers, and are usually disposed adjacent to the electrode layers in the stacking direction.

[0070] [Method for Manufacturing Light-Emitting Element] Next, an example of a method for manufacturing the light-emitting element 30 will be described with reference to Fig. 6. Fig. 6 is a flowchart showing an example of a method for manufacturing the light-emitting element 30 in this embodiment.

[0071] In step S1, the anode layer 31 is formed on the TFT layer 23. Specifically, an Ag layer and an indium tin oxide layer are formed in this order by sputtering.

[0072] In step S2, a hole injection layer 321 is formed on the anode layer 31. Specifically, the hole transport material and the electron acceptor material are co-deposited at a predetermined deposition rate by adjusting the deposition temperature and deposition time so that these materials are deposited to a predetermined thickness and ratio. Here, a deposited film is formed uniformly over the entire surface of the workpiece without using a fine metal mask.

[0073] In step S3, the hole transport layer 322 is formed on the hole injection layer 321. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. Here, the evaporated film is formed without using a fine metal mask.

[0074] In step S4, the electron blocking layer 323 is formed on the hole transport layer 322. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to evaporate the material to a first thickness.

[0075] In step S5, the light-emitting layer 33 is formed on the electron blocking layer 323. Specifically, the co-evaporation of the host compound and the guest compound (dopant) is performed at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined film thickness and guest compound concentration (dopant concentration). Here, a fine metal mask is used to precisely control the thickness and guest compound concentration during evaporation. Note that the light-emitting layer 33 may be formed using a resist mask formed by photolithography instead of the fine metal mask.

[0076] In step S6, the hole blocking layer 341 is formed on the light-emitting layer 33. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined film thickness. In this step, the evaporated film is formed without using a fine metal mask.

[0077] In step S7, the electron transport layer 342 is formed on the hole blocking layer 341. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. The evaporation may be performed by evaporating only the electron transport material, or by co-evaporating the electron transport material and lithium quinoline. In this step, the evaporated film is formed without using a fine metal mask.

[0078] In step S8, the electron injection layer 343 is formed on the electron transport layer 342. Specifically, lithium fluoride is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a layer having a predetermined thickness is deposited. In this step, the evaporated film is formed without using a fine metal mask.

[0079] In step S9, the cathode layer 35 is formed on the electron injection layer 343. Specifically, for example, a magnesium-silver alloy thin film is formed by vapor deposition.

[0080] All of the light-emitting elements 30 in the light sources 122 may have the same layer structure, but the layer structure (for example, the layer structure of the electrical functional layer) of some of the light-emitting elements 30 may be different from the layer structure (the layer structure of the electrical functional layer) of other light-emitting elements 30 as long as the effects of this embodiment can be obtained. For example, the types and thicknesses of the layers in the electrical functional layer may be different for some of the light-emitting elements 30. Such partially different layer structures can be achieved by vapor deposition using a mask.

[0081] [Major Effects] In order to improve the printing speed of an electrophotographic apparatus, it is necessary for the light source for exposure to be able to be modulated (on / off) at high speed. For example, in order to print 28 sheets per minute using an A4-sized recording medium in an electrophotographic apparatus, the modulation speed of the light source must be at least several tens of MHz, for example, 50 MHz (=20 nanoseconds) for 1200 dpi and 100 MHz (=10 nanoseconds) for 2400 dpi.

[0082] Conventionally, the exposure device of an electrophotographic device uses a laser scanning light source system including a laser light source and a polygon mirror for scanning the laser light. This laser light source typically uses an InGaAs / GaAs laser diode made of inorganic semiconductor material oscillating at a wavelength of 780 nm. This light source can be easily modulated at several tens of GHz (on the order of picoseconds). Inorganic LEDs can also be modulated on the order of nanoseconds. Therefore, with light sources using inorganic materials, the modulation speed is rarely a limiting factor in printing speed. On the other hand, the use of a laser scanning optical system in the exposure device of an inline electrophotographic device limits the miniaturization of the electrophotographic device and leaves room for improvement in quietness. Furthermore, as described below, there is a limit to how much discrete inorganic LEDs can be further miniaturized, making it difficult to print high-resolution images.

[0083] Adopting an exposure head in which miniaturizable OLEDs are arranged at high resolution on a substrate in the exposure device of an electrophotographic device is desirable because it is expected to realize a smaller, quieter, and higher-resolution electrophotographic device. On the other hand, there is still room for further study in this exposure head from the perspective of achieving modulation speeds equivalent to those of conventional laser light sources or inorganic LEDs, thereby increasing printing speeds. For example, the emission lifetime of red phosphorescent OLEDs, which are considered as alternatives to the above-mentioned laser light sources, is approximately 1 microsecond or longer. On the other hand, it is said that a dot clock frequency of 20 MHz (= 0.05 microseconds) is required to print 21 sheets of A4 paper per minute at 600 dpi. Therefore, in order to adopt OLEDs in the exposure device of an electrophotographic device, consideration must be given to making them compatible with high-speed printing.

[0084] In this embodiment, the electrophotographic apparatus 1 has an exposure head 12 as an exposure device that irradiates light onto the charged electrophotographic photosensitive member 10 to form an electrostatic latent image, for the electrophotographic photosensitive member 10, charging device 11, developing device 13, transfer device 14, and fixing device 16. The exposure head 12 includes the substrate 121, light source 122, and equal-magnification lens 123, and the light source 122 includes a light-emitting element 30, and the light-emitting layer 33 in the light-emitting element 30 is a fluorescent light-emitting layer.

[0085] The emission lifetime of the fluorescent light-emitting layer is on the order of nanoseconds. This makes it possible to significantly improve the modulation speed of the light source 122, thereby enabling high-speed exposure that corresponds to high-speed printing in the electrophotographic apparatus. Therefore, the electrophotographic apparatus 1 of this embodiment can achieve high-speed printing similar to that achieved when using a conventional inorganic material-based exposure device (a laser scanning light source device or an exposure head using an inorganic LED).

[0086] Furthermore, as described above, the fluorescent light-emitting layer can effectively utilize the carrier. Therefore, in this embodiment, in addition to high-speed printing, it is possible to improve the luminous efficiency and reliability of the light-emitting elements included in the exposure head.

[0087] In this embodiment, the irradiance per element of the light-emitting element 30 on the electrophotographic photosensitive member 10 via the equal-magnification lens 123 is 0.10 μW / dot or more. Therefore, this embodiment is suitable for exposure in the electrophotographic apparatus 1 from the viewpoint of realizing a simple configuration, a sufficiently small irradiation area, and a sufficient amount of light using minute light-emitting elements.

[0088] Furthermore, in this embodiment, the size of the light-emitting region of the light-emitting element 30 when viewed in plan (for example, the diameter when the shape of the light-emitting region of the light-emitting element 30 when viewed in plan is a perfect circle) can be 10 μm or less. Therefore, this embodiment is even more effective from the perspective of realizing an exposure head equipped with multiple light-emitting elements that can emit a high amount of light in an extremely small area.

[0089] The shape of the light-emitting region of the light-emitting element 30 in plan view may be a circle such as a perfect circle, a polygon such as a triangle or a square (rectangle), or a polygon with rounded corners. From the perspective of realizing high-resolution electrophotography, a small element pitch (repeating unit) of each light-emitting element 40 is preferable. However, from the perspective of achieving a light intensity sufficient for electrophotography, a large size (diameter or length of one side, etc.) of the light-emitting region of the light-emitting element 40 is preferable. The "element pitch" may be expressed as the distance from one end of the light-emitting region of one light-emitting element to the other end of the light-emitting region of two adjacent light-emitting elements. From these perspectives, the size of the light-emitting region of the light-emitting element 30 may be 1 μm or more, or 2 μm or more. From the above perspective, the size of the light-emitting region of the light-emitting element 30 may be 12 μm or less, or 10 μm or less. From the above perspective, the ratio of the size of the light-emitting region of the light-emitting element to the element pitch is preferably 0.7 or more, and more preferably 0.8 or more. It is difficult to achieve both the above-mentioned miniaturization of the element pitch and the size of the light-emitting region with conventional discrete inorganic LEDs.

[0090] As described above, according to this embodiment, a technique is provided in which an exposure head using a plurality of light emitting elements is used to achieve a sufficient response speed of exposure light that enables high-speed printing in exposure of an electrophotographic apparatus.

[0091] Other embodiments of the present disclosure will be described below. In the following description of the embodiments, for the sake of convenience, the same explanation as in the above-described embodiment will not be repeated, and the same reference numerals will be used to designate components having the same functions as those described in the above-described embodiment, and the explanations thereof will not be repeated.

[0092] [Embodiment 2] Figure 7 shows a schematic diagram of the layer structure of a light-emitting element in this embodiment. This embodiment differs from the previously described embodiment 1 in that a light source 222 is used instead of the light source 122. The light source 222 differs from the previously described embodiment 1 in that a light-emitting element 40 is used instead of the light-emitting element 30. The light-emitting element 40 has substantially the same layer structure as the light-emitting element 30 in embodiment 1, except that it is a so-called tandem light-emitting element in which multiple sets of a light-emitting layer and an electrical functional layer overlapping the light-emitting layer are stacked between a pair of electrode layers. In embodiment 2, the same components as those in embodiment 1 are denoted by the same reference numerals.

[0093] 7 , the light source 222 has a light-emitting element 40, and the light-emitting element 40 has a layered structure in which an anode layer 31, a first electrical functional layer 32, a first light-emitting layer 33, a second electrical functional layer 41, a second light-emitting layer 42, a third electrical functional layer 43, and a cathode layer 35 are stacked in this order. Note that, although an organic light-emitting diode (OLED) element will be described as an example of the light-emitting element in this embodiment as well, the light-emitting element may be a quantum dot light-emitting diode (QLED) element having quantum dots in its light-emitting layer that are excited by an electric field or carrier injection to emit light.

[0094] The first electrical functional layer 32 is mainly composed of a layer for controlling the movement of holes to the first light-emitting layer 33. The second electrical functional layer 41 is mainly composed of a layer for controlling the movement of electrons to the first light-emitting layer 33 and a layer for controlling the movement of holes to the second light-emitting layer 42. The third electrical functional layer 43 is mainly composed of a layer for controlling the movement of electrons to the second light-emitting layer 42.

[0095] 8 schematically shows an example of the layer structure of the light-emitting element 40 according to this embodiment. The light-emitting element 40 is a top-emission type, and is configured by stacking an anode layer 31, a hole injection layer 321, a first hole transport layer 322, a first electron blocking layer 323, a first light-emitting layer 33, a first hole blocking layer 411, a first electron transport layer 412, an n-type first charge generation layer 413, a p-type first charge generation layer 414, a second hole transport layer 415, a second electron blocking layer 416, a second light-emitting layer 42, a second hole blocking layer 431, a second electron transport layer 432, an electron injection layer 343, and a cathode layer 35 in this order.

[0096] The first electrical functional layer 32 is composed of a hole injection layer 321, a first hole transport layer 322, and a first electron blocking layer 323. The second electrical functional layer 41 is composed of a first hole blocking layer 411, a first electron transport layer 412, an n-type first charge generation layer 413, a p-type first charge generation layer 414, a second hole transport layer 415, and a second electron blocking layer 416. The third electrical functional layer 43 is composed of a second hole blocking layer 431, a second electron transport layer 432, and an electron injection layer 343.

[0097] The assembly of layers from the hole injection layer 321 to the first electron transport layer 412 is a layer that functions mainly for the emission of light from the first light-emitting layer 33. A first stack 40A represents this assembly of layers. The n-type first charge generation layer (electron generation layer) 413 and the p-type first charge generation layer (hole generation layer) 414 are layers that function to generate charges in the light-emitting element 40. A first charge generation layer 40B represents this assembly of layers. The assembly of layers from the second hole transport layer 415 to the electron injection layer 343 is a layer that functions mainly for the emission of light from the second light-emitting layer 42. A second stack 40C represents this assembly of layers.

[0098] In this way, the stacks (first stack 40A and second stack 40C) may include an electrical functional layer in addition to one light-emitting layer. In this embodiment, multiple stacks may be arranged between the anode layer and the cathode layer in the stacking direction. In the present disclosure, the anode layer, the cathode layer, and the charge generation layer are not included in the stack. The thickness of the stack is determined so that the amount of light emitted from the light-emitting layer is a theoretical value or a value close to the theoretical value. Note that the thickness of the stack is calculated as the sum of the thicknesses of the light-emitting layers and the electrical functional layers in the stack. However, among the light-emitting layers and the electrical functional layers, layers with very small thicknesses (e.g., layers with a thickness of less than 1 nm) may be ignored when calculating the thickness of the stack.

[0099] FIG. 9 is a flowchart illustrating an example of a method for manufacturing the light-emitting element 40 according to the present embodiment. The light-emitting element 40 is manufactured by using the layer configuration of the light-emitting element 30 according to Embodiment 1 as a first stack, then forming a charge generation layer, and then forming a second stack in the same manner as the first stack. Therefore, the flowchart for the example of manufacturing the light-emitting element 40 is the same as that of Embodiment 1 up to step S7, and the flowchart of FIG. 9 is substantially the same as the flowchart of Embodiment 1 described above, except that steps (S8 to S14) for forming the charge generation layer and the second stack are included between steps S7 and S8 of Embodiment 1.

[0100] In step S1, the anode layer 31 is formed on the TFT layer 23. Specifically, an Ag layer and an indium tin oxide layer are formed in this order by sputtering.

[0101] In step S2, a hole injection layer 321 is formed on the anode layer 31. Specifically, the hole transport material and the electron acceptor material are co-deposited at a predetermined deposition rate by adjusting the deposition temperature and deposition time so that these materials are deposited to a predetermined thickness and ratio. Here, a deposited film is formed uniformly over the entire surface of the workpiece without using a fine metal mask.

[0102] In step S3, the first hole transport layer 322 is formed on the hole injection layer 321. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. Here, the evaporated film is formed without using a fine metal mask.

[0103] In step S4, the first electron blocking layer 323 is formed on the first hole transport layer 322. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to evaporate the material to a first thickness.

[0104] In step S5, the first light-emitting layer 33 is formed on the first electron blocking layer 323. Specifically, the co-evaporation of a host compound and a guest compound (dopant) is performed at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a predetermined film thickness and guest compound concentration (dopant concentration) are formed. Here, a fine metal mask is used to precisely control the thickness and guest compound concentration during evaporation. Note that the first light-emitting layer 33 may be formed using a resist mask formed by photolithography instead of a fine metal mask.

[0105] In step S6, the first hole blocking layer 411 is formed on the first light-emitting layer 33. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. Here, the evaporated film is formed without using a fine metal mask.

[0106] In step S7, the first electron transport layer 412 is formed on the first hole blocking layer 411. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a predetermined film thickness is formed. The evaporation may be the evaporation of only the electron transport material, or the co-evaporation of the electron transport material and lithium quinoline. In this step, the evaporated film is formed without using a fine metal mask.

[0107] In step S8, the n-type first charge generation layer 413 is formed on the first electron transport layer 412. Specifically, co-evaporation of an organic electron transport material and an inorganic metal material, i.e., Yb or Li, which is an electron donor material, is performed at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layers are laminated to a predetermined thickness and ratio. Here, the evaporated film is formed without using a fine metal mask.

[0108] In step S9, a p-type first charge generation layer 414 is formed on the n-type first charge generation layer 413. Specifically, an organic hole transport material and an organic electron acceptor material are co-deposited at a predetermined deposition rate by adjusting the deposition temperature and deposition time so that the layers are laminated to a predetermined thickness and ratio. Here, a deposited film is formed without using a fine metal mask.

[0109] In step S10, the second hole transport layer 415 is formed on the p-type first charge generation layer 414. Specifically, the hole transport material is vapor-deposited at a predetermined vapor deposition rate by adjusting the vapor deposition temperature and vapor deposition time so that the layer is deposited to a predetermined thickness. Here, the vapor-deposited film is formed without using a fine metal mask.

[0110] In step S11, the second electron blocking layer 416 is formed on the second hole transport layer 415. Specifically, the hole transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. Here, a fine metal mask is used to form the evaporated film.

[0111] In step S12, the second light-emitting layer 42 is formed on the second electron blocking layer 416. Specifically, the co-evaporation of a host compound and a guest compound (dopant) is performed at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a predetermined film thickness and guest compound concentration (dopant concentration) are obtained. Here, a fine metal mask is used to precisely control the thickness and guest compound concentration during evaporation. In this embodiment, the second light-emitting layer 42 is evaporated so that its thickness is substantially the same as that of the first light-emitting layer 33. Note that the second light-emitting layer 42 may be formed using a resist mask formed by photolithography instead of a fine metal mask.

[0112] In step S13, the second hole blocking layer 431 is formed on the second light-emitting layer 42. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. Here, the evaporated film is formed without using a fine metal mask.

[0113] In step S14, the second electron transport layer 432 is formed on the second hole blocking layer 431. Specifically, the electron transport material is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that the layer is deposited to a predetermined thickness. Only the electron transport material may be evaporated, or the electron transport material and lithium quinoline may be co-evaporated. In this step, the evaporated film is formed without using a fine metal mask.

[0114] In step S15, the electron injection layer 343 is formed on the second electron transport layer 432. Specifically, lithium fluoride is evaporated at a predetermined evaporation rate by adjusting the evaporation temperature and evaporation time so that a layer having a predetermined thickness is formed. Here, the evaporated film is formed without using a fine metal mask.

[0115] In step S16, the cathode layer 35 is formed on the electron injection layer 343. Specifically, for example, a magnesium-silver alloy thin film is formed by vapor deposition.

[0116] In this embodiment, by tandemly arranging the OLEDs, which are the light-emitting elements 40 in the light source 222, it is possible to increase the amount of light and the irradiance on the electrophotographic photosensitive member while keeping the light-emitting area in the exposure head 12 small. This makes it possible to achieve high definition in the electrophotographic apparatus 1. Furthermore, by tandemly arranging the OLEDs, it is possible to increase the amount of light, for example, by two times, three times, or more, without changing the current value or current density flowing through the light-emitting elements 40, thereby making it possible to extend the life of the light source 222.

[0117] Furthermore, the light-emitting element 40 of this embodiment is preferably configured as a top-emission light-emitting element. In a top-emission light-emitting element, the light extraction efficiency can be increased by utilizing the microcavity effect. Therefore, from the viewpoint of adjusting the optical path length, one of the layers of the electrical functional layer may be thickened. The light-emitting element 40 is a so-called tandem light-emitting element having multiple light-emitting layers and their corresponding electrical functional layers in the stacking direction. Therefore, the electrical functional layer does not need to be thickened to adjust the distance between the electrode layers. Therefore, this embodiment is even more effective from the viewpoint of reducing the consumption of functionally unnecessary materials.

[0118] [Embodiment 3] Fig. 10 schematically shows the layer structure of a light-emitting device in this embodiment. The light-emitting device 50 shown in Fig. 10 has substantially the same layer structure as the light-emitting device 40 shown in Embodiment 2, except for the volume (thickness in this embodiment) of the second light-emitting layer. The second stack 50C has substantially the same structure as the second stack 40C in Embodiment 2, except that the second light-emitting layer 42 is replaced with the second light-emitting layer 52.

[0119] The thickness of the second light-emitting layer 52 of the light-emitting element 50 is thinner than the thickness of the first light-emitting layer 33. For example, when the thickness of the first light-emitting layer 33 is 1, the thickness of the second light-emitting layer 52 is 0.1 to 0.9. When the thickness of the first light-emitting layer 33 is 1, the thickness of the second light-emitting layer 52 is preferably 0.1 or more, and more preferably 0.3 or more, from the viewpoints of extending the element life and reducing excess carriers (holes), as described below. On the other hand, the thickness is preferably 0.9 or less, and more preferably 0.8 or less, from the viewpoints of achieving both improved luminous efficiency and reduced driving voltage (power consumption).

[0120] The thickness of the light-emitting layer in this embodiment can be determined, for example, by the following method. That is, the applied voltage, luminous efficiency, or device life of a light-emitting device having light-emitting layers with the same thickness in the stacking direction is measured and used as a reference value. Meanwhile, a light-emitting device is fabricated in which the thickness of the second light-emitting layer is thinner than that of the first light-emitting layer, and the applied voltage, luminous efficiency, or device life is measured. Based on the obtained measured values ​​and the reference value, an appropriate thickness according to the purpose is determined.

[0121] Alternatively, the thickness of the light-emitting layer in this embodiment can be determined as a thickness according to the purpose, for example, by evaluating the carrier injection property and / or carrier transport property between each layer in the light-emitting element by simulation and conducting a demonstration experiment based on the evaluation.

[0122] In the light-emitting element 50 of this embodiment, the second light-emitting layer 52 formed on the cathode layer 35 side is thinner than the first light-emitting layer 33 formed on the anode layer 31 side. As a result, holes are supplied to the second light-emitting layer 52 in an amount commensurate with the thickness of the second light-emitting layer 52. Therefore, electron-hole pairs (excitons) are generated in the second light-emitting layer 52 without generating excess holes that cannot combine with electrons. Furthermore, the first light-emitting layer 33 generates an amount of electron-hole pairs (excitons) substantially equal to the theoretical value. Therefore, each of the light-emitting layers 33, 52 stacked in the stacking direction emits light with a luminance corresponding to its thickness. Furthermore, the generation of excess holes can be prevented in each light-emitting layer. Therefore, in this embodiment, further reductions in current consumption and, therefore, power consumption can be further reduced.

[0123] As described above, in this embodiment, the tandem light-emitting element 50 has multiple light-emitting layers, and by changing the configuration (thickness) of each light-emitting layer, a carrier balance is achieved between the multiple light-emitting layers, and the problem of imbalance in carrier supply is solved. As a result, in this embodiment, an appropriate amount and balance of excitons are generated in each light-emitting layer in the stacking direction. Therefore, the exposure head having multiple tandem light-emitting elements of this embodiment can achieve further reductions in current consumption and drive voltage, and improvement in light-emitting efficiency, compared to the first embodiment described above.

[0124] In a top-emission light-emitting element, generally, if there is even a slight defect in the sealing layer 25 formed on the cathode layer 35, oxygen or moisture may penetrate and deteriorate the cathode layer 35, reducing the electron injection property, and as a result, the amount of electrons supplied to the second light-emitting layer 52 directly below the cathode layer 35 may decrease. However, by anticipating such a case and forming the thickness of the second light-emitting layer 52 on the cathode layer 35 side thinner than the theoretical value from the initial design stage, it is possible to prevent the generation of excess holes in the second light-emitting layer 52 due to deterioration of the cathode layer 35.

[0125] Furthermore, in general, when LiF is used for the electron injection layer 343 in a light-emitting element, there is a risk of thermal damage to a layer of an organic material that has been previously deposited. This is because the deposition of an electric functional layer containing an inorganic material such as Yb, not limited to LiF, is generally carried out at a higher temperature than the deposition of an emissive layer and an electric functional layer made of an organic material, due to the high melting point of the inorganic material. From this perspective, it is preferable that each layer constituting the light-emitting element is made only of organic materials. Examples of organic electron injection materials that can have sufficient properties when combined with a cathode layer made of aluminum (Al) or the like, which can form a vapor deposition layer at a relatively low temperature, include BUPH1, BPen, p-MeO-Phen, and p-NMe. ２ The electron injection ability of electron injection layers using these organic materials tends to be inferior to that of electron injection layers using inorganic materials such as LiF.

[0126] Given this technical background, if an electron injection layer made of an organic material is disposed adjacent to the cathode layer 35 in this embodiment, the amount of electrons supplied to the light-emitting layer (second light-emitting layer 52) formed on the cathode layer 35 side may be insufficient. In such cases, the thickness of the second light-emitting layer 52 on the cathode layer 35 side can be made thinner than the theoretical value to reduce the effect of the reduced electron supply, thereby fully utilizing the advantages of adopting a tandem light-emitting device structure. That is, by setting the thickness of each light-emitting layer in the stacking direction as described above, the balance (carrier balance) between the electrons and / or holes supplied from the charge generation layer and the holes and / or electrons supplied from each electrode layer is optimized. Therefore, in this embodiment, the generation of excess carriers is suppressed, resulting in a tandem light-emitting device that is excellent in terms of reduced power consumption.

[0127] In addition, in this embodiment, the thickness of each light-emitting layer in the stacking direction can be set within the range of the ratio described above, and therefore, it is preferable from the viewpoint of improving the light-emitting efficiency of the light-emitting device to set the thickness of the stack according to the thickness of the light-emitting layer set in this manner. From this viewpoint, in this embodiment, the ratio of the thickness of the light-emitting layer in the stack to the thickness of the stack in the stacking direction is preferably 0.05 or more and preferably 0.35 or less.

[0128] [Embodiment 4] Fig. 11 shows a schematic diagram of the layer structure of a light-emitting device according to this embodiment. The light-emitting device 60 shown in Fig. 11 has substantially the same layer structure as the light-emitting device 50 according to the third embodiment, except that it further includes a third light-emitting layer and an electrical functional layer therefor.

[0129] The light-emitting element 60 has a layer configuration in which an anode layer 31, a first stack 40A, a first charge generation layer 40B, a second stack 50C, a second charge generation layer 60D, a third stack 60E, and a cathode layer 35 are stacked in this order. More specifically, compared to the light-emitting element 50, the light-emitting element 60 has a third electric functional layer 61 instead of the third electric functional layer 43, and further has a third light-emitting layer 62 and a fourth electric functional layer 63.

[0130] The third electrical functional layer 61 is mainly composed of a layer for controlling the movement of electrons to the second light-emitting layer 52 and a layer for controlling the movement of holes to the third light-emitting layer 62. The third electrical functional layer 61 is composed of, for example, a second hole blocking layer, a second electron transport layer, an n-type second charge generation layer, a p-type second charge generation layer, a third hole transport layer, and a third electron blocking layer stacked in this order.

[0131] The second charge generation layer 60D in the third electrical functional layer 61 is composed of, for example, the above-mentioned n-type second charge generation layer and p-type second charge generation layer.

[0132] Except for its thickness, the third light-emitting layer 62 has substantially the same structure as the first light-emitting layer 33 and the second light-emitting layer 52. The thickness of the third light-emitting layer 62 is thinner than the thickness of the second light-emitting layer 52, and for example, when the thickness of the second light-emitting layer 52 is 1, the thickness of the third light-emitting layer 62 is 0.1 to 0.9.

[0133] The fourth electrical functional layer 63 is mainly composed of a layer for controlling the movement of electrons to the third light-emitting layer 62. The fourth electrical functional layer 63 is composed of, for example, a third hole-blocking layer, a third electron-transporting layer, and an electron-injecting layer 343.

[0134] Thus, the light-emitting element 60 is a tandem light-emitting element having three light-emitting layers, and in the light-emitting element 60, the thicknesses of two of the three light-emitting layers that are adjacent to each other and overlap in the stacking direction are different from each other.

[0135] As with the light-emitting element 50 described above, the light-emitting element 60 can optimize the balance (carrier balance) between the electrons and / or holes supplied from the charge generation layer and the holes and / or electrons supplied from each electrode layer by appropriately setting the thickness of each light-emitting layer in the stacking direction as described above. Thus, like the light-emitting element 50, the light-emitting element 60 can also suppress the generation of excess carriers and become a tandem light-emitting element that is excellent in terms of reducing power consumption.

[0136] Therefore, the exposure head having a plurality of tandem light emitting elements 60 of this embodiment is even more effective in terms of realizing a reduction in power consumption.

[0137] Furthermore, the light-emitting element 60 is suitable for adjusting the carrier balance in one stack when, as a result of adjusting the carrier balance in two adjacent stacks out of three stacks by a method other than controlling the thickness of the light-emitting layer, a good carrier balance cannot be obtained in the remaining stack. Examples of the configuration of such a light-emitting element include a configuration in which the thicknesses of corresponding electrical functional layers in the first stack and the second stack are different, a configuration in which the materials of corresponding electrical functional layers in the first stack and the second stack are different, and a configuration in which the material of the first light-emitting layer included in the first stack is different from the material of the second light-emitting layer included in the second stack, and a configuration in which the thickness ratio of the light-emitting layers at least between the second stack and the third stack is the aforementioned thickness ratio.

[0138] [Embodiment 5] Fig. 12 schematically shows the layer structure of a light-emitting device in this embodiment. The light-emitting device 70 shown in Fig. 12 has substantially the same layer structure as the light-emitting device 40 shown in Embodiment 2, except for the volume of the first light-emitting layer (in this embodiment, the area in a plan view). The first stack 70A has substantially the same structure as the first stack 40A in Embodiment 2, except for the first light-emitting layer 33 being replaced with a first light-emitting layer 71.

[0139] The area of ​​the first light-emitting layer 71 of the light-emitting element 70 is smaller than the area of ​​the second light-emitting layer 42. The "area" of the light-emitting layer refers to the area of ​​each light-emitting layer when viewed from the side from which light is emitted (the cathode layer 35 side). For example, the area of ​​the first light-emitting layer 71 is 10 to 90% of the area of ​​the second light-emitting layer 42. From the viewpoint of increasing light-emitting efficiency, the area of ​​the first light-emitting layer 71 is preferably 10% or more of the area of ​​the second light-emitting layer 42, more preferably 20% or more, and even more preferably 30% or more. On the other hand, from the viewpoint of suppressing the generation of excess carriers and reducing power consumption, the area is preferably smaller than 1, more preferably 90% or less, and even more preferably 80% or less of the area of ​​the second light-emitting layer 42.

[0140] The area of ​​the light-emitting layer in this embodiment can be determined, for example, by the following method. That is, the applied voltage, luminous efficiency, or device life of a light-emitting device having a light-emitting layer with the same area in the stacking direction is measured and used as a reference value. Meanwhile, a light-emitting device having a smaller area of ​​the first light-emitting layer is fabricated, and the applied voltage, luminous efficiency, or device life is measured. Based on the obtained measured values ​​and the reference value, an appropriate area according to the purpose is determined.

[0141] Alternatively, the area of ​​the light-emitting layer in this embodiment can be determined as an area according to the purpose, for example, by evaluating the carrier injection property and / or carrier transport property between each layer in the light-emitting element by simulation and conducting a demonstration experiment based on the evaluation.

[0142] Thus, the light emitting element 70 is a tandem light emitting element having two light emitting layers, and the two overlapping light emitting layers have different areas when viewed in plan.

[0143] In general, in a tandem light-emitting device in which multiple light-emitting layers are arranged in the stacking direction, when a charge generation layer is interposed between the light-emitting layers, optimizing the supply of carriers (electrons and / or holes) in the light-emitting layer on one side of the charge generation layer in the stacking direction may result in the supply of electrons and / or holes in the light-emitting layer on the other side being deviated from the optimal value, which may result in an imbalance in the carrier balance between the multiple light-emitting layers in the same light-emitting device.

[0144] In the light-emitting element 70, the area of ​​the first light-emitting layer 71 is smaller and the area of ​​the second light-emitting layer 42 is larger. Therefore, when the amount of carriers (electrons and / or holes) supplied to the second light-emitting layer 42 is greater than the amount of carriers (electrons and / or holes) supplied to the first light-emitting layer 71, the volume of the second light-emitting layer 42, where the carrier supply becomes excessive, is larger than that of the first light-emitting layer 71 in the light-emitting element 70. Therefore, in the light-emitting element 70, the carrier balance of all light-emitting layers in the stacking direction is optimized.

[0145] Therefore, in the light-emitting element 70, generation of excess carriers that cannot contribute to light emission can be suppressed, and an appropriate amount of excitons is generated in both the first light-emitting layer 71 and the second light-emitting layer 42. Therefore, the light-emitting element 70 can emit light with high efficiency and high brightness.

[0146] Therefore, the exposure head having a plurality of tandem light emitting elements 70 of this embodiment is even more effective from the viewpoint of realizing higher definition in electrophotographic devices.

[0147] [Embodiment 6] The layer structure of a light-emitting device according to this embodiment is schematically shown in Fig. 13. The light-emitting device 80 shown in Fig. 13 has substantially the same layer structure as the light-emitting device 70 according to the above-described embodiment 5, except that the area of ​​the second light-emitting layer is smaller and a third light-emitting layer and an electrical functional layer therefor are further provided.

[0148] The light-emitting element 80 has a layer configuration in which an anode layer 31, a first stack 70A, a first charge generation layer 40B, a second stack 80C, a second charge generation layer 80D, a third stack 80E, and a cathode layer 35 are stacked in this order. More specifically, compared to the light-emitting element 70, the light-emitting element 80 has a third electric functional layer 83 instead of the third electric functional layer 43, and further has a third light-emitting layer 84 and a fourth electric functional layer 85.

[0149] The third electrical functional layer 83 is mainly composed of a layer for controlling the movement of electrons to the second light-emitting layer 82 and a layer for controlling the movement of holes to the third light-emitting layer 84. The third electrical functional layer 83 is composed of, for example, a second hole blocking layer, a second electron transport layer, an n-type second charge generation layer, a p-type second charge generation layer, a third hole transport layer, and a third electron blocking layer stacked in this order.

[0150] The second charge generation layer 80D in the third electrical functional layer 83 is composed of, for example, the above-mentioned n-type second charge generation layer and p-type second charge generation layer.

[0151] The fourth electrical functional layer 85 is mainly composed of a layer for controlling the movement of electrons to the third light-emitting layer 84. The fourth electrical functional layer 85 is composed of, for example, a third hole-blocking layer, a third electron-transporting layer, and an electron-injecting layer 343.

[0152] The third light-emitting layer 84 has substantially the same configuration as the second light-emitting layer 42 in the light-emitting element 70. For example, the area of ​​the second light-emitting layer 82 is 10 to 90% of the area of ​​the third light-emitting layer 84, and the area of ​​the first light-emitting layer 71 is 10 to 90% of the area of ​​the second light-emitting layer 82.

[0153] In this way, the light-emitting element 80 is a tandem light-emitting element having three light-emitting layers, and in the light-emitting element 80, of the three light-emitting layers, two light-emitting layers that are adjacent to each other and overlap each other in the stacking direction have different areas when viewed in a plane.

[0154] In the light-emitting element 80, similarly to the light-emitting element 70, the area of ​​the second light-emitting layer 82 is smaller than the area of ​​the third light-emitting layer 84, and the area of ​​the first light-emitting layer 71 is smaller than the area of ​​the second light-emitting layer 82. Therefore, when the amount of carriers (electrons and / or holes) supplied to the light-emitting layer on the cathode layer 35 side is greater than the amount of carriers (electrons and / or holes) supplied to the light-emitting layer on the anode layer 31 side, in the light-emitting element 80, the volume of the light-emitting layer on the cathode layer 35 side, where the carrier supply is excessive, is larger than that of the light-emitting layer on the anode layer 31 side. Therefore, in the light-emitting element 80, the carrier balance of all light-emitting layers in the stacking direction is optimized.

[0155] Therefore, in the light-emitting element 80, as in the light-emitting element 70, generation of excess carriers that cannot contribute to light emission can be suppressed, and an appropriate amount of excitons is generated in each light-emitting layer, thereby enabling the light-emitting element 80 to emit light with high efficiency and brightness.

[0156] Therefore, the exposure head having a plurality of tandem light emitting elements 80 of this embodiment is even more effective from the viewpoint of realizing higher definition in electrophotographic devices.

[0157] Furthermore, the light-emitting element 80 is suitable for adjusting the carrier balance in one stack when, as a result of adjusting the carrier balance in two adjacent stacks out of three stacks by a method other than controlling the thickness of the light-emitting layer, a good carrier balance cannot be obtained in the remaining stack. Examples of the configuration of such a light-emitting element include a configuration in which the thicknesses of corresponding electrical functional layers in the first stack and the second stack are different, a configuration in which the materials of corresponding electrical functional layers in the first stack and the second stack are different, and a configuration in which the material of the first light-emitting layer included in the first stack is different from the material of the second light-emitting layer included in the second stack, and a configuration in which the relationship in the area of ​​the light-emitting layer between at least the second stack and the third stack satisfies the above-mentioned area ratio.

[0158] Other Embodiments From the viewpoint of optimizing the carrier balance of all light-emitting layers in the stacking direction, in the light-emitting element of the present disclosure, the thickness of the light-emitting layer on the cathode layer 35 side in the stacking direction may be thicker than that of the light-emitting layer on the anode layer 31 side. Furthermore, from the viewpoint of optimizing the carrier balance of all light-emitting layers in the stacking direction, in the light-emitting element of the present disclosure, the area of ​​the light-emitting layer on the cathode layer 35 side in the stacking direction may be smaller than that of the light-emitting layer on the anode layer 31 side. Even with this configuration, it is possible to optimize the carrier balance of all light-emitting layers in the stacking direction depending on the situation, suppress the generation of excess carriers that cannot contribute to light emission, and generate an appropriate amount of excitons in each light-emitting layer. Therefore, even with the above configuration, highly efficient and high-brightness light emission may be possible.

[0159] The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

[0160] According to the present disclosure, an exposure head including multiple OLEDs can be used to achieve a simple configuration for exposure in an electrophotographic device, a sufficiently small irradiation area, and sufficient irradiance. The present disclosure is expected to contribute to the realization of high-speed, high-resolution electrophotographic technology and to bring about revolutionary changes in the configuration of electrophotographic devices. The technology of the present disclosure is expected to contribute to the achievement of, for example, Goal 12 "Responsible Consumption and Production" and Goal 9 "Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation" of the Sustainable Development Goals (SDGs) proposed by the United Nations.

[0161] Example 1 A single light-emitting device was fabricated having a single red light-emitting layer (emission wavelength: 640 nm, light-emitting region: perfect circle with a diameter of 15 μm). The layer structure of the single light-emitting device is as follows: The red light-emitting layer is a fluorescent light-emitting layer, and contains a dibenzoperiflanthene derivative (TDPF) as a guest compound and Alq as a host compound. ３ (Layer structure) Cathode layer / first stack (electron transport layer / hole blocking layer / red light-emitting layer (thickness: 35 nm) / electron blocking layer / hole transport layer / hole injection layer) / anode layer (Thickness ratio) Ratio of the thickness of the red light-emitting layer (35 nm) to the thickness of the first stack (260 nm): 0.135

[0162] The emission lifetime and radiant flux of the single light-emitting element were measured. The emission lifetime was measured using a fluorescence lifetime measurement device equipped with a spectrometer and a streak camera. If the emission lifetime is 20 nanoseconds or less, it can be said that there is no practical problem in use as an exposure head for high-speed electrophotography. Furthermore, the radiant flux was measured by directly facing the light-emitting region of the light-emitting element to the light-receiving surface of an optical power meter using a Si photodiode. If the radiant flux is 0.1 μW or more, it can be said that there is no practical problem in use as an exposure head for high-speed electrophotography. Furthermore, the element lifetime (reliability), driving voltage, and current efficiency of the single light-emitting element were measured.

[0163] As a result, the emission lifetime of the single light-emitting element was 5 nanoseconds, the radiant flux was 0.146 μW, and the current efficiency of the single light-emitting element was 0.264 W / A.

[0164] Comparative Example 1 A comparative single light-emitting device was fabricated in the same manner as in Example 1, except that the red light-emitting layer was an emitting layer containing a phosphorescent dopant used in ordinary organic EL displays. The red light-emitting layer in the comparative single light-emitting device was a phosphorescent emitting layer containing Btp2Ir(acac) with benzothienylpyridine as a ligand as a guest compound. (Layer structure) Cathode layer / first stack (electron transport layer / hole blocking layer / red light-emitting layer (thickness: 35 nm) / electron blocking layer / hole transport layer / hole injection layer) / anode layer (Thickness ratio) Ratio of the thickness of the red light-emitting layer (35 nm) to the thickness of the first stack (260 nm): 0.135

[0165] The emission lifetime and radiant flux of the comparative single light-emitting element were measured, and the element lifetime, driving voltage, and current efficiency were also measured.

[0166] As a result, the luminous lifetime of the comparative single light-emitting element was 4000 nanoseconds, and the radiant flux was 0.2 μW. The current efficiency of the single light-emitting element was 0.362 W / A. As can be seen from these results, the comparative single light-emitting element, which uses a phosphorescent dopant equivalent to that used in conventional red OLEDs, has a high radiant flux and current efficiency, but its luminous lifetime is more than 100 times longer than that of the light-emitting element of Example 1, which uses a fluorescent dopant, and afterglow remains even after the light is electrically switched from the on state to the off state. Therefore, although it is suitable for display or lighting applications, it is unsuitable as a light source for an exposure head in high-speed electrophotography.

[0167] Example 2 Tandem light-emitting devices having two red light-emitting layers were fabricated under three conditions, i) to iii) below. The red light-emitting layer was a fluorescent light-emitting layer containing a dibenzoperiflanthene derivative (TDPF) as a guest compound and Alq as a host compound. ３In conditions i to iii, the light-emitting region was a perfect circle with a diameter of 10 μm. <Condition i> (Layer structure) Cathode layer / second stack (electron injection layer / electron transport layer / hole blocking layer / second red light-emitting layer (thickness: 35 nm) / hole transport layer / hole injection layer) / charge generation layer / first stack (electron transport layer / hole blocking layer / first red light-emitting layer (thickness: 35 nm) / electron blocking layer / hole transport layer / hole injection layer) / anode layer (Thickness ratio) Ratio of thickness of second red light-emitting layer (35 nm) to thickness of second stack (160 nm): 0.219 Ratio of thickness of first red light-emitting layer (35 nm) to thickness of first stack (260 nm): 0.135 <Condition ii> (Layer structure) Same as condition i (Thickness ratio) Total thickness of electrical functional layers in second stack layer: 135 nm Ratio of thickness of second red light-emitting layer (35 nm) to thickness of second stack layer (170 nm): 0.206 Other than that, same as condition i <Condition iii> (Layer structure) Same as that of condition i (Thickness ratio) Total thickness of the electrical functional layers in the second stack layer: 145 nm Ratio of the thickness of the second red-emitting layer (35 nm) to the thickness of the second stack layer (180 nm): 0.194 Other than that, same as condition i

[0168] The luminous lifetime, radiant flux, device lifetime (reliability), and driving voltage were measured for each of the tandem light-emitting devices under conditions i to iii.

[0169] The emission lifetime of the tandem light-emitting element was 5 nanoseconds under conditions i, ii, and iii. This value is substantially the same as that of the single light-emitting element of Example 1.

[0170] The radiant flux of the tandem light-emitting element was 0.11 μW under condition i, 0.12 μW under condition ii, and 0.1 μW under condition iii. The area of ​​the light-emitting region of Example 2 was significantly reduced to 1 / 2.25 times (=0.44 times) that of Example 1, but the radiant flux was 0.75 times under condition i, 0.82 times under condition ii, and 0.68 times under condition iii compared to that of the single light-emitting element of Example 1, all of which were 0.1 μW or more, and therefore were at a level that would not pose a problem in practical use as an exposure head for electrophotography capable of high-speed printing.

[0171] Furthermore, the element life of the tandem light emitting element was 2.0 times longer than that of the single light emitting element of Example 1 under condition i, 2.1 times longer under condition ii, and 1.9 times longer under condition iii.

[0172] Furthermore, the driving voltage of the tandem light emitting element was 1.9 times that of the single light emitting element of Example 1 under condition i, 1.94 times under condition ii, and 2.0 times under condition iii.

[0173] In this way, by using a tandem light-emitting element having multiple fluorescent light-emitting layers, it is possible to make the light-emitting area smaller while still maintaining a short light-emitting life that is suitable for high-speed printing, thereby realizing an exposure head for electrophotographic devices that can handle high speeds and high resolution.

[0174] REFERENCE SIGNS LIST 1 Electrophotographic apparatus 10 Electrophotographic photosensitive member 11 Charging device 12 Exposure head 13 Developing device 14 Transfer device 15 Conveying device 16 Fixing device 17 Recording medium 21, 121 Substrate 22 Buffer layer 23 TFT layer 24 Edge cover film 25 Sealing layer 26 External functional layer 30, 40, 50, 60, 70, 80 Light-emitting element 31 Anode layer 32 First electric functional layer 33, 71 First light-emitting layer 35 Cathode layer 40A, 70A First stack 40B First charge generation layer 40C, 50C, 80C Second stack 34, 41 Second electric functional layer 42, 52, 82 Second light-emitting layer 43 Third electric functional layer 60D, 80D Second charge generation layer 60E, 80E Third stack 61, 83 Third electric functional layer 62, 84 Third light-emitting layer 63, 85 Fourth electric functional layer 122, 222 Light source 123 Equal magnification lens 151 Conveyor belt 152, 153 Conveyor roller 161 Heating roller 162 Pressure roller 321 Hole injection layer 322 (First) hole transport layer 323 (First) electron blocking layer 341 Hole blocking layer 342 Electron transport layer 343 Electron injection layer 411 First hole blocking layer 412 First electron transport layer 413 First n-type charge generation layer 414 First p-type charge generation layer 415 Second hole transport layer 416 Second electron blocking layer 431 Second hole blocking layer 432 Second electron transport layer X Arrow indicating first direction in the present disclosure Y Arrow indicating second direction in the present disclosure

Claims

1. An exposure head disposed opposite the surface of a drum-shaped electrophotographic photosensitive member, comprising a plurality of light-emitting elements arranged in a line in a first direction and emitting light corresponding to an electrostatic latent image to be formed on the surface of the electrophotographic photosensitive member in a second direction intersecting the first direction, wherein each of the plurality of light-emitting elements comprises a set of a light-emitting layer and an electrical functional layer overlapping the light-emitting layer, which is disposed between a pair of electrode layers, and the light-emitting layer is a light-emitting layer containing a host compound and a fluorescent dopant which is a guest compound.

2. An exposure head according to claim 1, wherein the light emitting element has a light emission lifetime of 50 nanoseconds or less.

3. The exposure head according to claim 1 or 2, wherein the irradiance per element of said light-emitting element on the surface of said electrophotographic photosensitive member is 0.10 μW / dot or more.

4. An exposure head according to any one of claims 1 to 3, wherein the size of each of said light emitting elements in plan view is 10 μm or less.

5. An exposure head according to any one of claims 1 to 4, wherein in the light-emitting element, a plurality of sets of the light-emitting layer and the electrical functional layer overlapping therewith are stacked between a pair of the electrode layers.

6. An exposure head according to claim 5, wherein the two overlapping light-emitting layers have different volumes.

7. The exposure head according to claim 6, wherein the thicknesses of the two overlapping light-emitting layers are different from each other.

8. An exposure head according to claim 6 or 7, wherein the two overlapping light emitting layers have different areas when viewed in a plane.

9. An electrophotographic apparatus comprising: a drum-shaped electrophotographic photosensitive member; a charging device that charges the electrophotographic photosensitive member; an exposure head according to any one of claims 1 to 8 that forms an electrostatic latent image by irradiating light onto the charged electrophotographic photosensitive member; a developing device that develops the electrostatic latent image formed on the surface of the electrophotographic photosensitive member with toner; a transfer device that transfers the toner image formed on the surface of the electrophotographic photosensitive member onto a recording medium; and a fixing device that fixes the toner image transferred onto the recording medium onto the recording medium.

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