Exposure head and electrophotographic device

Abstract

The present disclosure provides an exposure head comprising a plurality of light-emitting elements which, during exposure of an electrophotographic device (1), has a sufficiently small irradiation area, and has sufficient irradiance using a simple configuration. The exposure head (12) includes tandem light-emitting elements having a plurality of light-emitting layers arranged so as to overlap, and emits light corresponding to an electrostatic latent image facing an electrophotographic photoreceptor (10). The irradiance on the surface of the electrophotographic photoreceptor (10) per element of the light emitting elements is 0.10 μW / dot or more.

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] An exposure head arranged for each electrophotographic photosensitive member is known as an exposure device for an in-line electrophotographic device that can replace a laser scanning light source device and achieve miniaturization and quietness. 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) or inorganic light-emitting diodes (LEDs)) are arranged with high resolution along the rotation axis direction of the electrophotographic photosensitive member on a substrate arranged on the outer periphery of the electrophotographic photosensitive member along the rotation axis direction of the electrophotographic photosensitive member (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 Unexamined Patent Publication No. 11-198433 Japanese Unexamined Patent 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] The exposure head described in Patent Document 2 achieves sufficient irradiance by concentrating light from multiple circumferentially arranged light sources (OLEDs) into a single circumferential spot. However, in electrophotographic devices that require high-speed, high-resolution image printing, it is difficult to achieve alignment of the exposure head so that the light from multiple OLEDs is concentrated into a single circumferential spot and such an optical system is aligned along the rotation axis. Furthermore, such an exposure head requires high parallelism with the rotation axis of the electrophotographic photosensitive member, which requires extremely high machining precision. Furthermore, as described below, incoherent light emitted from OLEDs or LEDs, unlike coherent laser light, is difficult to achieve by narrowing the spot size because the irradiation area is limited to a value equal to or larger than the element size.

[0008] As such, the conventional technology leaves room for further study from the perspective of achieving a simple configuration for exposure in an electrophotographic device, a sufficiently small irradiation area of ​​one spot (spot size), and sufficient irradiance using an exposure head consisting of multiple light-emitting elements.

[0009] An aspect of the present disclosure aims to provide a technology for achieving exposure in an electrophotographic device with a simple configuration, a sufficiently small irradiation area, and sufficient irradiance using an exposure head consisting of multiple light-emitting elements.

[0010] In order to solve the above 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, the plurality of light-emitting elements including light-emitting elements each having a pair of a light-emitting layer and an electrical functional layer overlapping the light-emitting layer stacked between a pair of electrode layers, and the irradiance per element of the light-emitting elements on the surface of the electrophotographic photosensitive member is 0.10 μW / dot or more, where "dot" refers to the irradiation area of ​​one spot and "W / dot" refers to the radiant flux per spot.

[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, in exposure of an electrophotographic device, it is possible to realize a sufficiently small irradiation area and sufficient irradiance with an exposure head including a plurality of light-emitting elements with a simple configuration.

[0013] FIG. 1 is a diagram schematically showing the configuration of an electrophotographic device according to embodiment 1 of the present disclosure. FIG. 2 is a perspective view schematically showing the configuration of an exposure head according to embodiment 1 of the present disclosure. FIG. 3 is a diagram schematically showing 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 showing the layer configuration of a light source according to embodiment 1 of the present disclosure. FIG. 5 is a diagram schematically showing the layer configuration of a light source according to embodiment 1 of the present disclosure. FIG. 6 is a flowchart showing 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 showing the layer configuration of a light-emitting element according to embodiment 2 of the present disclosure. FIG. 8 is a diagram schematically showing the layer configuration of a light-emitting element according to embodiment 3 of the present disclosure. FIG. 9 is a diagram schematically showing the layer configuration of a light-emitting element according to embodiment 4 of the present disclosure. FIG. 10 is a diagram schematically showing the layer configuration of a light-emitting element according to embodiment 5 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 includes two or more light-emitting layers that emit 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 elements included therein may have the same shape in plan view, for example, a circular shape, e.g., a perfect circle with a diameter of 10 μm. A plurality of light sources 122, i.e., a plurality of light-emitting elements, are arranged in the direction of the rotation axis of the drum-shaped electrophotographic photoreceptor 10, and are arranged in a number of 31,200 pieces at a pitch of 10.58 μm (31,200 dots in the direction of the rotation axis of the electrophotographic photoreceptor 10) so as to achieve, for example, 2,400 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 10 μm is condensed onto a circular area having a diameter of 10 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] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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 40 on the TFT layer 23, across the edge cover film 24 and the sealing layer 25.

[0033] 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.

[0034] 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.

[0035] The TFT layer 23 includes a pixel circuit that controls the light emission of the light emitting element 40 .

[0036] The edge cover film 24 has insulating properties and covers the edge of the anode layer 31 of the light emitting element 40. 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.

[0037] The sealing layer 25 covers the light-emitting element 40 and prevents foreign substances such as water and oxygen from penetrating into the light-emitting element 40. The sealing layer 25 is composed of, for example, two inorganic sealing films and an organic film formed between them.

[0038] The external functional layer 26 is a layer that adds various functions to the light source 122, such as optical control or surface protection.

[0039] [Light-emitting element] 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, the light-emitting element may also be a light-emitting diode (QLED) element that uses quantum dots in its light-emitting layer, which are excited by an electric field or carrier injection to emit light.

[0040] 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 of the quantum dots, the emission wavelength can be varied to, for example, 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), or may be 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.

[0041] Examples of quantum dot core materials include Si, Ge, GaAs, GaP, GaInP, InP, InAs, InSb, InN, GaSb, CdTe, CdS, CdSe, CdSeTe, CdZnTe, CdMnTe, ZnTe, ZnS, ZnSe, ZnSeTe, and CuInS ２ , CuInSe ２ , CuGaS ２ , CuGaSe ２ , HgS, HgSe, PbS, PbSe, ternary mixed crystals thereof, and quaternary mixed crystals thereof. Examples of materials for the shell of the quantum dots include CdS, ZnS, CdSSe, CdTeSe, CdSTe, ZnSSe, ZnSTe, ZnTeSe, AlGaAs, and AlP.

[0042] Examples of combinations of quantum dot core and shell materials include CdSe / CdS, GaAs / AlGaAs, InP / ZnS, ZnSe / ZnS, and CIGS / ZnS.

[0043] 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 as a solid solution on the surface of the core. The quantum dots may also include doped nanoparticles.

[0044] An "electrical functional layer" is a layer having the 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 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.

[0045] 5 schematically illustrates an example of the layer structure of a light-emitting element 40 according to this embodiment. The light-emitting element 40 includes 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, stacked in this order. The light-emitting element 40 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 40, the anode layer 31 functions as an anode, and the cathode layer 35 functions as a cathode.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] Each layer of the light-emitting element 40 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.

[0050] 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.

[0051] 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.

[0052] 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 40 may be the same as or different from that of the p-type first charge generation layer 414.

[0053] The hole transport layers (first hole transport layer 322 and second hole transport layer 415) may be made of an organic hole transport material, such as a triarylamine-based organic compound. The specific material of the first hole transport layer 322 may be the same as or different from that of the second hole transport layer 415.

[0054] The electron blocking layers (first electron blocking layer 323 and second electron blocking layer 416) may also be made of an organic hole transport material, similar to the hole transport layer. The material of the electron blocking layer may be the same as or different from that of the hole transport layer. The specific material of the first electron blocking layer 323 may be the same as or different from that of the second electron blocking layer 416.

[0055] The light-emitting layers (first light-emitting layer 33 and second light-emitting layer 42) are layers that emit light of a predetermined color by the injection and recombination of carriers consisting of holes and electrons. Typically, one light-emitting layer consists of a single organic layer made of a light-emitting material, but it may also be configured as a laminate structure of two or more layers having two or more different light-emitting functions. For example, the light-emitting layer may have a laminate structure in which two or more functional layers corresponding to two or more functions for light emission, such as an immediate light-emitting layer and a delayed light-emitting layer, are stacked, and the entire structure exhibits the function of the light-emitting layer. Furthermore, the light-emitting layer may contain inorganic quantum dots instead of an organic light-emitting material.

[0056] In the present disclosure, a plurality of light-emitting layers are disposed between the anode layer and the cathode layer in the stacking direction. The number of light-emitting layers stacked in the stacking direction is not limited, but is preferably two or more from the viewpoints of increasing light-emitting efficiency and extending the device life, and more preferably three or more from the viewpoint of ensuring sufficient irradiance on the electrophotographic photosensitive member. On the other hand, the number of light-emitting layers stacked in the stacking direction is preferably 10 or less, more preferably 9 or less, from the viewpoints of suppressing an increase in driving voltage and realizing a withstand voltage of a driver corresponding to the driving voltage.

[0057] The thickness of each light-emitting layer in the stacking direction can be determined as a thickness appropriate for the purpose by evaluating the carrier injection properties and / or carrier transport properties between each layer in the light-emitting element through simulation and conducting demonstration experiments based on the evaluation.

[0058] The wavelength of the light emitted by the light-emitting layer may be any wavelength that can sensitize the electrophotographic photoreceptor 10 to form an electrostatic latent image when the light emitted from the light-emitting element 40 is used. The wavelengths of the light emitted 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 emitted by the multiple light sources 122 when viewed in a plan view may also be the same or different within the above-mentioned range.

[0059] The light-emitting layer may be composed of only a single light-emitting material, but it may also be a host-guest light-emitting layer containing a host compound and a guest compound. A host-guest light-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 a light-emitting layer containing such a guest compound, the fluorescence of the host compound is completely lost, and instead, strong light emission matching 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, a host-guest light-emitting layer can emit light from the guest compound with higher quantum efficiency.

[0060] 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.

[0061] Furthermore, doping the emissive 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 emissive layer but also improves the device life due to an increased carrier recombination probability. As a result, host-guest emissive layers achieve further improvements in the luminous efficiency of the emissive layer and the device life of the light-emitting device.

[0062] Furthermore, a phosphorescent dopant can be used as the guest compound. When a phosphorescent dopant is used as the guest compound, light can be extracted from triplet excitons, thereby increasing the quantum efficiency three times that of a fluorescent dopant. Furthermore, by utilizing intersystem crossing, which is the inversion of the spin from singlet excitons to triplet excitons, theoretically, all generated excitons can emit "phosphorescence" from triplet excitons. When a phosphorescent dopant is used as the guest compound, the quantum efficiency can be increased up to four times compared to when a fluorescent dopant is used. Examples of phosphorescent dopants include iridium complexes and palladium-based complexes.

[0063] Examples of guest compounds other than fluorescent dopants and phosphorescent dopants include TADF and hyperfluorescent materials.

[0064] Various known examples of host-guest emitting layer materials can be used. Examples of host compounds include known emitting layer materials for each color.

[0065] 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. Examples of phosphorescent dopants include Ir(ppy). ３ , Ir(thpy) ３ , Ir(t5m-thpy) ３ , Ir(t-5CF ３ -py) ３ , Ir(t-5t-py) ３ , Ir(mt-5mt-py) ３ , Ir(btpy) ３ , Ir(tflpy) ３ , Ir(piq) ３ , Ir(tiq) ３ , Ir(fliq) ３ , FIrpic, FIr6, ppy, tpy, bzq, thp, op, bo, bt, bon, αbsn, btp, ppo, C6, pq, β-bsn, and ppz.

[0066] The hole blocking layers (first hole blocking layer 411 and second hole blocking layer 431) may be composed of an organic electron transport material, such as an oxadiazole-based compound or a phenanthroline-based compound. The material of the hole blocking layer may contain lithium quinoline (Liq) in addition to the electron transport material. The specific material of the first hole blocking layer 411 may be the same as or different from that of the second hole blocking layer 431.

[0067] The electron transport layers (first electron transport layer 412 and second electron transport layer 432), like the hole blocking layer, may be made of the organic electron transport material described above. The material of the electron transport layer may be the same as or different from that of the hole blocking layer. The specific material of the first electron transport layer 412 may be the same as or different from that of the second electron transport layer 432. Lithium quinoline may be included in addition to the electron transport material.

[0068] The charge generation layers (n-type first charge generation layer 413 and p-type first charge generation layer 414) are disposed between two adjacent light-emitting layers 33, 42 in the stacking direction. The charge generation layers are layers that generate one or both of electrons and holes. The charges (charges or carriers) generated in the charge generation layers are supplied to the light-emitting layers located on the anode layer side and the cathode layer side in the stacking direction. The charge generation layers can be made of known charge-generation materials that exhibit the above-mentioned functions.

[0069] The charge generation layer may be composed of an electron generation layer that generates electrons and a hole generation layer that generates holes. Examples of electron generation layers include n-type charge generation layers, and examples of hole generation layers include p-type charge generation layers. When holes are supplied from the anode layer and electrons are supplied from the cathode layer, the n-type charge generation layer generates electrons and the p-type charge generation layer generates holes.

[0070] The n-type charge generation layer may be composed of, for example, an organic electron transport material and a material containing Yb (ytterbium) or Li (lithium), an inorganic metal material added in a range of 5 to 20% and acting as an electron donor material. Examples of organic electron transport materials include oxadiazole-based compounds and phenanthroline-based compounds. Development is also underway to use organic materials for both the electron transport material and the electron donor material in the n-type charge generation layer. Examples of organic electron donor materials include BUPH1, BPen, p-MeO-Phen, and p-NMe. ２ -Phen, and p-Pyrrd-Phen.

[0071] The p-type charge generation layer may be composed of an organic hole transport material and an organic electron accepting material (hole supply material) added in a range of 1 to 10%. Known triarylamine organic compounds may be used as the organic hole transport material. An example of an organic electron accepting material is tetracyanoquinodimethane tetrafluoride (TCNQ-4F). For the p-type charge generation layer, a layer having sufficient hole generation capability may be realized using all organic materials, such as the hole transport material and electron accepting material described above.

[0072] 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. Similarly to the n-type first charge generation layer 413, the electron injection layer 343 may 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).

[0073] 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.

[0074] The hole injection layer and the electron injection layer may be disposed corresponding to the electrode layer, and are usually disposed adjacent to each electrode layer in the stacking direction. The hole transport layer, the electron blocking layer, the light-emitting layer, the hole blocking layer, the electron transport layer, and the charge generation layer may be disposed in a stacking direction of the light-emitting element.

[0075] [Method for Manufacturing Light-Emitting Element] Next, an example of a method for manufacturing the light-emitting element 40 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 40 in this embodiment.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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 hole transport material is deposited to a predetermined thickness. Here, a fine metal mask is used to evaporate the material to the second thickness.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] All of the light emitting elements 40 included in the light source 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 40 may be different from the layer structure (the layer structure of the electrical functional layer) of other light emitting elements 40 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 40. Such partially different layer structures can be achieved by vapor deposition through a mask.

[0093] [Major Effects] The laser light used for exposure in conventional electrophotographic devices is coherent light generated by stimulated emission from the resonator structure of a laser diode. Coherent light refers to light whose light wave amplitude and phase are consistent, has high temporal and spatial coherence, and is difficult to diffuse. Therefore, with coherent light, the focused light diameter (spot size) can be reduced to the diffraction limit without having to consider the concept of light source size. A typical example of coherent light is laser light generated by stimulated emission.

[0094] In contrast, the light emitted from a typical LED, OLED, or QLED is incoherent light. Incoherent light, also known as spontaneous emission, refers to light waves with random amplitude and phase. Most light other than laser light is incoherent. The condensable spot size of incoherent light is significantly affected by the size of the light source. In other words, as is clear from the concept of étendue, the spot size of incoherent light cannot be made smaller than the light source size (more precisely, the size of the light-emitting area of ​​the light source) without wasting the light emitted from the light source; the spot size must be equal to or larger than the light source size (device size). Therefore, the minimum size that can connect all the light rays emitted from an incoherent light source is the same size as the light source. In other words, it is impossible to condense all of the luminous flux emitted from an incoherent light source into an area smaller than the size of the light source. Therefore, in order for an incoherent light source with a finite size to emit higher-power light, the size of the light source must be increased. The more emitted light is utilized, the larger the condensed spot size becomes. Therefore, in order to print high-resolution images using electrophotography, it is necessary to achieve both high output of light emitted from each light source and small enough to accommodate the formation of high-resolution images by reducing the element size (light-emitting area) of each light-emitting element consisting of multiple incoherent light sources.

[0095] Here, light-emitting elements made of OLEDs or QLEDs are suitable as light sources having a minute light-emitting region. However, when an exposure head is constructed by arranging miniaturized OLEDs or QLEDs with high precision as an exposure light source, the individual element size is small, so the light quantity (radiant flux) per pixel and the corresponding irradiance on the electrophotographic photoreceptor are reduced. When a high current is passed through a miniaturized OLED or QLED to increase the light quantity or irradiance, the element life is shortened. As a result, printing unevenness occurs over time, and ultimately the service life of the electrophotographic device is shortened. If the element size of the OLED or QLED is increased to a certain extent to achieve the desired light quantity or irradiance, the emitted light may be incoherent, resulting in insufficient definition of the formed electrophotographic image. Thus, when an OLED or QLED is used as an exposure light source, there is a trade-off between high definition and long life, and the prior art leaves room for further consideration in terms of miniaturization of the light-emitting region, light quantity, irradiance, efficiency, and life.

[0096] 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 described above. The exposure head 12 includes the substrate 121, light source 122, and equal-magnification lens 123 described above. The light source 122 includes a tandem light-emitting element 40 in which a plurality of sets (stacks) of light-emitting layers 33, 42 and electrical functional layers overlying the light-emitting layers are stacked between a pair of electrode layers. The irradiance of each light-emitting element 40 on the electrophotographic photosensitive member 10 via the equal-magnification lens 123 is 0.10 μW / dot or more.

[0097] In this manner, in this embodiment, by tandemly arranging stacks included in the OLED or QLED, which are the light emitting elements 40 in the light source 122, it is possible to increase the amount of light and the irradiance on the electrophotographic photosensitive member while keeping the light emitting area of ​​the exposure light source small in the exposure head 12. This makes it possible to achieve high definition in the electrophotographic apparatus 1. Furthermore, by tandemly arranging stacks, it is possible to increase the amount of light, for example, by two times, three times, or more, without changing the current density, thereby extending the life of the light source 122.

[0098] In this embodiment, the size of the light-emitting region of the light-emitting element 40 when viewed in plan (for example, the diameter when the shape of the light-emitting region of the light-emitting element 40 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.

[0099] The shape of the light-emitting region of the light-emitting element 40 in plan view may be a circle, such as a perfect circle, or 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" can 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. For example, to achieve 2400 dpi, an element pitch of 10.58 μm or less is required. From these perspectives, the size of the light-emitting region of the light-emitting element 40 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 40 may be 9 μm or less, or 8 μm or less. From the above viewpoint, the ratio of the size of the light-emitting region to the element pitch in the light-emitting element 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.

[0100] 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.

[0101] Furthermore, in this embodiment, when a host-guest type light-emitting layer is used as the light-emitting layer, effective utilization of carriers can be realized, thereby improving the light-emitting efficiency and lifespan of the light-emitting elements included in the exposure head.

[0102] As mentioned above, the use of a phosphorescent dopant allows for a further increase in quantum efficiency compared to the use of a fluorescent dopant. However, complexes containing platinum-group elements such as iridium or palladium, which are used as phosphorescent dopants, are very expensive even in small amounts due to the limited production of platinum-group elements and uneven distribution of their sources, and stable supply may be difficult. This embodiment can achieve high luminous efficiency and improved element life of the exposure head, and therefore can reduce the amount of phosphorescent dopant used, which is advantageous from the perspectives of cost reduction and economic security.

[0103] In this manner, in this embodiment, by using a light-emitting element 40 having a plurality of light-emitting layers with minute light-emitting regions and electrical functional layers between a pair of electrodes, high irradiance can be obtained despite the minute light-emitting elements, and optical axis alignment, which is required when using a plurality of light-emitting elements individually in the past, can be eliminated. Therefore, in the exposure of an electrophotographic device, a simple configuration can be achieved with an exposure head consisting of a plurality of light-emitting elements, with a sufficiently small irradiation area and sufficient irradiance.

[0104] If the irradiance on the electrophotographic photosensitive member 10 is not sufficient with a light-emitting element having a two-layer stack as in the above embodiment, the number of layers of the stack may be increased until the required irradiance is obtained.

[0105] 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.

[0106] When stacks in the light-emitting device 40 are arranged in tandem, the balance (carrier balance) of electrons and holes supplied to the light-emitting layers included in each stack may become unbalanced. There are two cases where the carrier supply is unbalanced. The first case is when the same amount of electrons and holes is supplied to each light-emitting layer, forming electron-hole pairs (excitons) without generating excess carriers, but there is a shortage or excess of excitons in one of the light-emitting layers. The other case is when not only there is a shortage or excess of excitons in one of the light-emitting layers, but the balance of electron and hole supply to at least one of the light-emitting layers is disrupted, resulting in the formation of excess carriers other than excitons. In other words, carrier imbalances include a shortage or excess of exciton generation in one of the light-emitting layers, and an imbalance in the amount of electrons and / or holes supplied to one of the multiple light-emitting layers formed between a pair of anode and cathode layers. These cases are collectively referred to as carrier imbalances in tandem light-emitting devices. This can result in reduced luminous efficiency and device life.

[0107] Therefore, the present inventors have introduced a new design concept for a light-emitting device having multiple light-emitting layers. By varying the volume of each light-emitting layer depending on the state of carriers supplied to each light-emitting layer, the carrier balance between the multiple light-emitting layers is achieved and the problem of imbalance in carrier supply is resolved. This makes it possible to resolve the imbalance in carrier balance. As a result, the present disclosure realizes a tandem light-emitting device that can reduce current consumption and driving voltage, and improve light-emitting efficiency and reliability. In the following embodiments, methods for changing the volume of the light-emitting layer are described, including changing the thickness of the light-emitting layer and changing the area of ​​the light-emitting layer in a planar view.

[0108] [Embodiment 2] Fig. 7 schematically shows the layer structure of a light-emitting device in this embodiment. The light-emitting device 50 shown in Fig. 7 has substantially the same layer structure as the light-emitting device 40 shown in Embodiment 1, 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 1, except that the second light-emitting layer 42 is replaced with the second light-emitting layer 52.

[0109] 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).

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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 reduced current consumption and drive voltage and improved light-emitting efficiency compared to the first embodiment described above.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] [Embodiment 3] Fig. 8 schematically shows the layer structure of a light-emitting device according to this embodiment. The light-emitting device 60 shown in Fig. 8 has substantially the same layer structure as the light-emitting device 50 according to the second embodiment, except that it further includes a third light-emitting layer and an electrical functional layer therefor.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] Therefore, the exposure head having a plurality of tandem light emitting elements 60 of this embodiment is even more effective in terms of reducing power consumption.

[0127] 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.

[0128] [Embodiment 4] Fig. 9 schematically shows the layer structure of a light-emitting device in this embodiment. The light-emitting device 70 shown in Fig. 9 has substantially the same layer structure as the light-emitting device 40 shown in Embodiment 1, except for the volume (area in plan view in this embodiment) of the first light-emitting layer. The first stack 70A has substantially the same structure as the first stack 40A in Embodiment 1, except that the first light-emitting layer 33 is replaced with a first light-emitting layer 71.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] [Embodiment 5] The layer structure of a light-emitting device according to this embodiment is schematically shown in Fig. 10. The light-emitting device 80 shown in Fig. 10 has substantially the same layer structure as the light-emitting device 70 according to the above-described embodiment 4, except that the area of ​​the second light-emitting layer is smaller and the light-emitting device 80 further includes a third light-emitting layer and an electrical functional layer therefor.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] 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.

[0146] Therefore, the exposure head having a plurality of tandem light emitting elements 80 of this embodiment is even more effective in terms of realizing higher definition in electrophotographic devices.

[0147] 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.

[0148] 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.

[0149] 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.

[0150] 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.

[0151] Example 1 Tandem light-emitting devices having two red light-emitting layers (emission wavelength: 640 nm) were fabricated under the following three conditions, i) to iii) below: In 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

[0152] The radiant flux was measured for each of the tandem light-emitting elements under conditions i to iii. 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.

[0153] Furthermore, the device life, driving voltage, and current efficiency were measured for each of the tandem light-emitting devices under conditions i to iii.

[0154] 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. This was 1.7 times, 1.85 times, and 1.55 times that of a single light-emitting element (described below) under condition i, condition ii, and condition iii, respectively.

[0155] The current efficiency (=radiant flux / driving current) of the tandem light-emitting element was 0.45 W / A under condition i, 0.49 W / A under condition ii, and 0.41 W / A under condition iii. This was 1.7 times, 1.86 times, and 1.55 times that of a single light-emitting element (described below).

[0156] Furthermore, the element life of the tandem light emitting element was 2.0 times longer under condition i, 2.1 times longer under condition ii, and 1.9 times longer under condition iii than that of a single light emitting element described below.

[0157] Furthermore, the driving voltage of the tandem light emitting element was 1.9 times, 1.94 times, and 2.0 times under condition i, condition ii, and condition iii, respectively, relative to that of a single light emitting element described below.

[0158] Comparative Example 1 A comparative single light-emitting device having a single red light-emitting layer was fabricated. The layer structure of the single light-emitting device was as follows: (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

[0159] As in Example 1, the maximum light intensity of the single light-emitting element was measured, and the element life, driving voltage, and current efficiency of the single light-emitting element were also measured.

[0160] As a result, the radiant flux of the single light-emitting element was 0.065 μW. The current efficiency of the single light-emitting element was 0.264 W / A. After the single light-emitting element of Comparative Example 1 was combined with a 1x1 lens, the irradiance on the electrophotographic photosensitive member was approximately 0.05 μW. This irradiance value is unsuitable for printing high-resolution images of 1200 dpi or more.

[0161] In this way, by using a tandem light-emitting element having multiple light-emitting layers, it is possible to reduce the size of the light-emitting area in the light source of the exposure head while ensuring the radiant flux required for high-speed printing, thereby realizing an exposure head for electrophotographic devices that can handle high speeds and high resolution.

[0162] 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 31 Anode layer 32 First electric functional layer 33, 71 First light-emitting layer 35 Cathode layer 40, 50, 60, 70, 80 Light-emitting element 40A, 70A First stack 40B First charge generation layer 40C, 50C, 80C Second stack 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 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 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 343 Electron injection layer X Arrow indicating first direction in the present disclosure Y Arrow indicating second direction in the present disclosure

Claims

1. An exposure head arranged 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 the plurality of light-emitting elements include light-emitting elements in which a set of a light-emitting layer and an electric functional layer overlapping the light-emitting layer are stacked between a pair of electrode layers, and the irradiance per element of the light-emitting elements on the surface of the electrophotographic photosensitive member is 0.10 μW / dot or more.

2. The exposure head according to claim 1, wherein the size of each of said light emitting elements in plan view is 10 μm or less.

3. An exposure head according to claim 1 or 2, wherein the two overlapping light-emitting layers have different volumes.

4. The exposure head according to claim 3, wherein the thicknesses of the two overlapping light-emitting layers are different from each other.

5. An exposure head according to claim 3 or 4, wherein the areas of the two overlapping light emitting layers are different from each other when viewed in plan.

6. 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 5 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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