Image forming apparatus

Abstract

To provide an image forming apparatus that can achieve high image quality.SOLUTION: An image forming apparatus has an exposure light source that has an organic light emitting device, and a photoreceptor that receives light emitted from the organic light emitting device. When the intensity of light emission from the organic light emitting device at the end of supply of current to the organic light emitting device is defined as L0, a period from the end of supply of current to the organic light emitting device to when the organic light emitting device is turned off as T1, and a period from the start of supply of current to the organic light emitting device to the end of supply of current to the organic light emitting device as T2, T2 is larger than T1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Image forming apparatuses using electrophotography are widely used as copiers, facsimile machines, and printers. Such image forming apparatuses are equipped with an exposure light source having a light-emitting unit. The light-emitting elements include those using light-emitting diodes (LEDs) and those using organic light-emitting diodes (OLEDs). A photoconductor drum (hereinafter sometimes referred to as a "photoconductor") using an organic electrophotographic photoconductor (OPC) is exposed to light emitted from these light-emitting elements, and an image corresponding to the latent image formed on the photoconductor drum is printed on recording paper.

[0003] Patent Document 1 describes an image forming apparatus equipped with an exposure light source that uses an organic light-emitting element (hereinafter, sometimes referred to as an "organic electroluminescence element" or an "organic EL element"). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-128040 Summary of the Invention [Problem to be solved by the invention]

[0005] Organic light-emitting elements have the property of continuing to emit light even after the supply of current has ceased, and the emission intensity attenuates over time. Therefore, when using an organic light-emitting element as an exposure light source, it is necessary to take into consideration the light emitted from the organic light-emitting element after the supply of current has ceased (hereinafter, sometimes referred to as "afterglow").

[0006] However, Patent Document 1 does not suggest the relationship between the exposure time of an organic light-emitting element (hereinafter sometimes referred to as an "organic electroluminescence element" or "organic EL element") to a photosensitive body and the time from when the supply of current to the organic light-emitting element ends until it goes out of light, which poses an issue with image quality.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an image forming apparatus that can achieve high image quality. [Means for solving the problem]

[0008] One aspect of the image forming apparatus according to the present invention comprises an exposure light source having an organic light-emitting element and a photoreceptor that receives light emitted from the organic light-emitting element, and is characterized in that, when the light emission intensity from the organic light-emitting element at the end of supplying current to the organic light-emitting element is L0, the period from the end of supplying current to the organic light-emitting element to the organic light-emitting element until the organic light-emitting element becomes extinguished is T1, and the period from the start of supplying current to the organic light-emitting element to the end of supplying current to the organic light-emitting element is T2, T1 and T2 satisfy the relationship of formula (a). (a) T1 <T2

[0009] Another aspect of the image forming apparatus according to the present invention is characterized in that it comprises an exposure light source having an organic light-emitting element and a photosensitive member that receives light emitted from the organic light-emitting element, and is characterized in that, when the light emission intensity from the organic light-emitting element at the end of supplying current to the organic light-emitting element is L0, the period from the end of supplying current to the organic light-emitting element to the organic light-emitting element until the organic light-emitting element becomes extinguished is T1, and the period from the end of supplying current to the organic light-emitting element to the start of supplying current to the organic light-emitting element is T3, T1 and T3 satisfy the relationship of formula (c). (c) T1 <T3 [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an image forming apparatus that can achieve high image quality. [Brief explanation of the drawings]

[0011] [Figure 1] 1A is a schematic diagram showing an example of an image forming apparatus according to an embodiment of the present invention, FIG. 1B is a schematic diagram showing an example of an exposure light source of an image forming apparatus according to an embodiment of the present invention, and FIG. 1C is a schematic diagram showing an example of an exposure light source of an image forming apparatus according to an embodiment of the present invention. [Figure 2] 3A to 3C are schematic diagrams illustrating the operation of the image forming apparatus according to the present embodiment. [Figure 3] 1 is a schematic diagram showing one form of a light-emitting device according to an embodiment of the present invention. [Figure 4] 1A is a schematic perspective view of a head substrate, FIG. 1B is a diagram showing an arrangement of a plurality of light-emitting elements provided on the head substrate, and FIG. 1C is an enlarged view of a portion of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] In this specification, the luminous intensity from the organic light-emitting element at the end of the supply of current to the organic light-emitting element is defined as L0, the period from the end of the supply of current to the organic light-emitting element to the organic light-emitting element becoming extinguished is defined as T1, the period from the start of the supply of current to the organic light-emitting element to the end of the supply of current to the organic light-emitting element is defined as T2, and the period from the end of the supply of current to the organic light-emitting element to the start of the supply of current to the organic light-emitting element is defined as T3.

[0013] The quenching of an organic light-emitting element refers to a state in which the light-emitting intensity is reduced to such an extent that the organic light-emitting element is unable to form a latent image on a photoreceptor. For example, the quenching of an organic light-emitting element may be the timing when the light-emitting intensity becomes L0 / 2, L0 / e, or L0 / 10, where e is Napier's constant.

[0014] In this specification, the host material is the compound with the largest mass ratio among the compounds constituting the light-emitting layer. The guest material is a compound with a mass ratio smaller than that of the host among the compounds constituting the light-emitting layer, and is a compound that is mainly responsible for emitting light. The assist material is a compound with a mass ratio smaller than that of the host material among the compounds constituting the light-emitting layer, and assists the emission of the guest material. The assist material is also called a second host. The host material can also be called a first compound, and the assist material can also be called a second compound.

[0015] <Image forming device> An image forming apparatus according to this embodiment will be described with reference to the drawings.

[0016] FIG. 1(a) is a schematic diagram illustrating an example of an image forming apparatus according to an embodiment of the present invention. The image forming apparatus 40 is an electrophotographic image forming apparatus and includes a photoconductor 27, an exposure light source 28, a charging unit 30, a developing unit 31, a transfer unit 32, a transport roller 33, and a fixing unit 35. The photoconductor 27 and the exposure light source 28 are arranged facing each other. Light 29 is emitted from the exposure light source 28, forming an electrostatic latent image on the surface of the photoconductor 27. The exposure light source 28 includes one or more organic light-emitting elements. More specifically, the photoconductor 27 receives the light 29 emitted from the organic light-emitting elements of the exposure light source 28, forming an electrostatic latent image on the surface of the photoconductor 27. The exposure light source 28 may further include a lens array. The exposure light source 28 may include multiple light-emitting element rows or a single light-emitting element row. The developing unit 31 includes toner, etc. The charging unit 30 charges the photoconductor 27. The transfer unit 32 transfers the developed image to a recording medium 34. The transport roller 33 transports the recording medium 34. The recording medium 34 is, for example, paper. The fixing unit 35 fixes the image formed on the recording medium 34.

[0017] FIG. 1(b) and FIG. 1(c) are diagrams showing the exposure light source 28, and are schematic diagrams showing a state in which a plurality of light-emitting portions 36 are arranged on a long substrate. Arrow 37 is a direction parallel to the axis of the photoreceptor, and represents the column direction in which the light-emitting portions 36 having organic light-emitting elements are arranged. This column direction is the same as the direction of the axis about which the photoreceptor 27 rotates. This direction can also be called the major axis direction of the photoreceptor 27. FIG. 1(b) shows a form in which the light-emitting portions 36 are arranged along the major axis direction of the photoreceptor 27. FIG. 1(c) shows a form different from FIG. 1(b), in which the light-emitting portions 36 are alternately arranged in the column direction in each of the first column and the second column. The first column and the second column are arranged at different positions in the row direction. In the first column, a plurality of light-emitting portions 36 are arranged at intervals. The second column has light-emitting portions 36 at positions corresponding to the intervals between the light-emitting portions 36 in the first column. That is, in the row direction as well, a plurality of light-emitting portions 36 are arranged at intervals. The arrangement in FIG. 1(c) can also be described as, for example, a state of being arranged in a grid pattern, a state of being arranged in a staggered grid, or a checkered pattern.

[0018] The image forming apparatus according to the present embodiment has the following characteristics (a) or (b). (a) The organic light-emitting element satisfies T1 < T2 (b) The organic light-emitting element satisfies T1 < T3

[0019] Hereinafter, these will be described.

[0020] (a) The organic light-emitting element satisfies T1 < T2 The image forming apparatus according to the present embodiment has a configuration in which T2 is longer than T1. By having this characteristic, the image forming apparatus according to the present embodiment can achieve high image quality.

[0021] The image forming apparatus according to this embodiment will be described in detail with reference to FIG. 2. In FIG. 2, the timing at which the emission intensity of the organic light-emitting element reaches L0 / e is defined as the timing at which the organic light-emitting element is extinguished. In FIG. 2, t1 and t1' are the timings at which current is supplied to the organic light-emitting element, t2 and t2' are the timings at which the supply of current to the organic light-emitting element ends, and t3 and t3' are the timings at which the organic light-emitting element is extinguished. Here, the period from t1 to t2 can be referred to as the first emission period, and the period from t1' to t2' can be referred to as the second emission period, and hereafter, can be referred to as the third emission period, the fourth emission period, etc. Furthermore, T1 is the period from t2 to t3 or the period from t2' to t3', and T2 is the period from t1 to t2 or the period from t1' to t2'. In this specification, T1 is sometimes referred to as the afterglow life.

[0022] The organic light-emitting element shown in FIG. 2 is an embodiment in which light of the same emission intensity is emitted during the first and second emission periods, but this is not limiting. The emission intensity may be different during the first and second emission periods. Specifically, the emission intensity during the first emission period may be higher than the emission intensity during the second emission period, and the emission intensity during the first emission period may be lower than the emission intensity during the second emission period. Alternatively, the emission intensity may change during the first (second) emission period.

[0023] The image forming apparatus according to this embodiment selects organic light-emitting elements (pixels) to emit light based on image data to be formed, and exposes the light to the rotating photoconductor 27, thereby forming a latent image. At this time, after the supply of current to the organic light-emitting elements is stopped, residual light is irradiated onto the photoconductor 27, which may result in the formation of an unintended latent image on the photoconductor 27. Therefore, the organic light-emitting elements included in the image forming apparatus according to this embodiment have T1 shorter than T2, thereby reducing the occurrence of vertical stripes, ghosts, and the like. This feature makes it possible to suppress the formation of unintended latent images on the photoconductor 27, thereby enabling the image forming apparatus according to this embodiment to achieve high image quality.

[0024] The image forming apparatus according to the present embodiment preferably has a value of T2 / T1 greater than 1.0 and 10 or more, more preferably 20 or more, and particularly preferably 100 or more. The value of T2 / T1 may be 1000 or less, may be 500 or less, and may be 1000 or less. Specifically, 1.0 < T2 / T1 ≤ 100 may be satisfied, 1.0 < T2 / T1 ≤ 500 may be satisfied, and 1.0 < T2 / T1 ≤ 1000 may be satisfied. Further, T1 may be 10 μs or less, may be 2 μs or less, may be 1 μs or less, may be less than 1 μs, may be 50 ns or less, and may be 10 ns or less.

[0025] (b) The organic light emitting element satisfies T1 < T3 The image forming apparatus according to the present embodiment has a configuration in which T3 is longer than T1. By having this configuration, the image forming apparatus according to the present embodiment can achieve high image quality.

[0026] The image forming apparatus according to the present embodiment will be specifically described with reference to FIG. 2. In FIG. 2, the timing when the light emission intensity of the organic light emitting element becomes L0 / e is set as the timing when the organic light emitting element is extinguished. In FIG. 2, t1 and t '1 are the timings when current is supplied to the organic light emitting element, t2 and t '2 are the timings when the supply of current to the organic light emitting element ends, and t3 and t '3 are the timings when the organic light emitting element is extinguished. Here, the period from t1 to t2 may be referred to as the first light emission period, and the period from t '1 to t '2 may be referred to as the second light emission period, and thereafter, the third light emission period, the fourth light emission period, and the like can be referred to. Further, T1 is the period from t2 to t3 or the period from t '2 to t '3, and T3 is the period from t2 to t '1 or the period from t '2 to the start of current supply to the organic light emitting element in the third light emission period. In this specification, T1 may be referred to as the afterglow lifetime, and T3 may be referred to as the light emission interval period.

[0027] <s Note that the organic light-emitting device described in FIG. 2 is in an embodiment where light of the same emission intensity is irradiated during the first emission period and the second emission period, but it is not limited thereto. The emission intensity may be different between the first emission period and the second emission period. Specifically, the emission intensity in the first emission period may be higher than the emission intensity in the second emission period, and the emission intensity in the first emission period may be lower than the emission intensity in the second emission period. Also, during the first (second) emission period, the emission intensity may change.

[0028] The image forming apparatus according to the present embodiment selects an organic light-emitting device (pixel) that emits light based on the image data to be formed, and forms a latent image by exposing it on the rotating photoreceptor 27. At this time, there may be a case where an unintended latent image is formed on the photoreceptor 27 due to afterglow being irradiated to the photoreceptor 27 after the current supply to the organic light-emitting device ends. Specifically, the formation of a latent image on the photoreceptor 27 due to the afterglow from the organic light-emitting device may lead to the occurrence of vertical streaks, ghosts, etc. Therefore, the organic light-emitting device included in the image forming apparatus according to the present embodiment is characterized in that T1 is shorter than T3. By having this feature, the formation of an unintended latent image on the photoreceptor 27 can be suppressed, so the image forming apparatus according to the present embodiment can achieve high image quality.

[0029] In the image forming apparatus according to the present embodiment, the value of T3 / T1 is preferably greater than 1.0 and 5.0 or more, more preferably 10 or more, still more preferably 15 or more, still more preferably 20 or more, and particularly preferably 50 or more. Also, 1.0 < T3 / T1 ≤ 5 may be satisfied, 1.0 < T3 / T1 ≤ 15 may be satisfied, 1.0 < T3 / T1 ≤ 20 may be satisfied, and 1.0 < T3 / T1 ≤ 50 may be satisfied. Also, T1 may be 10 μs or less, 2 μs or less, 1 μs or less, less than 1 μs, 50 ns or less, or 10 ns or less.

[0030] Note that an image forming apparatus according to another embodiment may have both features (a) and (b). By having both features (a) and (b), higher image quality can be achieved.

[0031] The image forming apparatus according to the present embodiment may further include the following configurations. When the image forming apparatus includes these configurations, it may include any one of the following configurations (c) to (g), or may include multiple configurations from (c) to (g). (c) The organic light-emitting element has a top-emission configuration. (d) The organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits fluorescence. (e) The organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits phosphorescence. (f) The organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits delayed fluorescence. (g) Multiple pixels of organic light-emitting elements are arranged two-dimensionally at high density.

[0032] These configurations will be described below.

[0033] (c) The organic light-emitting element has a top-emission configuration. The image forming apparatus according to this embodiment preferably has a top emission configuration. This is because the top emission configuration has superior light extraction efficiency compared to the bottom emission configuration. The bottom emission configuration is a light emission configuration in which light is extracted from the side where the pixel circuits described below are provided. The top emission configuration is a light emission configuration in which light is extracted from the side opposite to the side where the pixel circuits described below are provided.

[0034] In this embodiment, it is further preferable that the organic light-emitting element has an interference structure. Specifically, when the organic light-emitting element has, from the substrate side, a reflective layer, a first electrode, an emitting layer, and a second electrode in this order, the optical distance between the reflective electrode and the emitting layer is an optical distance that intensifies the emission wavelength. In this case, a layer made of an insulating material or a transparent material such as ITO (indium tin oxide) or IZO (indium zinc oxide) may be disposed between the reflective layer and the first electrode. In addition, when the organic light-emitting element has, from the substrate side, a first electrode, an emitting layer, and a second electrode in this order, it is preferable that the light emitted from the emitting layer can be intensified between the first electrode and the second electrode. In this case, it is preferable that the first electrode can reflect the light emitted from the emitting layer.

[0035] (d) The organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits fluorescence. In the image forming apparatus according to this embodiment, the organic light emitting element preferably has a light emitting layer containing a light emitting material, and the light emitting material preferably emits fluorescence.

[0036] It is generally known that light emission from a singlet excited state has a particularly short afterglow lifetime. As described above, light emission from a singlet excited state is preferable because a short afterglow lifetime enables high image quality to be achieved. Specifically, the light emission from a singlet excited state is preferably fluorescent light emission.

[0037] Furthermore, since the image forming apparatus according to this embodiment may emit light with high emission intensity, the light-emitting material is required to be stable during repeated light emission processes. Therefore, the fluorescent light-emitting material is preferably a light-emitting material having a condensed polycyclic hydrocarbon skeleton, specifically, preferably having a perylene skeleton, and more preferably a perylene skeleton. Among these, the fluorescent light-emitting material is more preferably a hydrocarbon compound. This is because hydrocarbon compounds have high bond stability and can suppress deterioration during repeated light emission. Specific examples of fluorescent light-emitting materials include, but are not limited to, the exemplary compounds RD1 to RD6 described below.

[0038] From another perspective, in the light-emitting material, the freely rotatable single bond is preferably a carbon-carbon bond, more preferably an sp2 carbon-sp2 carbon bond, more preferably all of the freely rotatable single bonds are carbon-carbon bonds, and even more preferably all of the freely rotatable single bonds are sp2 carbon-sp2 carbon bonds. In this specification, a freely rotatable single bond refers to a bond in which unit A and unit B are not fused together, when the single bond between unit A and unit B is represented by "AB." Units A and B may be atoms such as carbon atoms or nitrogen atoms, or molecules such as benzene or carbazole. Table 1 shows the bond energies of each bond.

[0039] [Table 1]

[0040] The bond energy of F1 and F2, which have carbon-nitrogen bonds, is 3.9 eV. On the other hand, the bond energy of F3, which has a freely rotatable carbon-carbon bond, is 4.5 eV, and the bond energy of F4, which has a freely rotatable sp2 carbon bond, is 5.0 eV. Therefore, when the freely rotatable single bond is a carbon-carbon bond, it is preferable because it is a skeleton that is difficult to decompose. Among carbon-carbon bonds, bonds between sp2 carbons have particularly high bond energy, so a skeleton in which the freely rotatable single bond is an sp2 carbon-sp2 carbon bond is even more preferable because it is more difficult to decompose.

[0041] In the image forming apparatus according to this embodiment, the light-emitting layer of the organic light-emitting element may include a first organic compound different from the fluorescent light-emitting material. The first organic compound may have a minimum excited singlet energy higher than that of the fluorescent light-emitting material. The first organic compound preferably has a condensed polycyclic hydrocarbon skeleton, specifically, a naphthalene skeleton, an anthracene skeleton, a phenanthrene skeleton, a fluorene skeleton, a pyrene skeleton, a triphenylene skeleton, a chrysene skeleton, a tetracene skeleton, or a perylene skeleton. In the first organic compound, the freely rotatable single bond is preferably a carbon-carbon bond, more preferably an sp2 carbon-sp2 carbon bond, more preferably all the freely rotatable single bonds are carbon-carbon bonds, and more preferably all the freely rotatable single bonds are sp2 carbon-sp2 carbon bonds.

[0042] Furthermore, in the image forming apparatus according to this embodiment, the light-emitting layer of the organic light-emitting element may contain a delayed fluorescent material different from the fluorescent light-emitting material. Specifically, the delayed fluorescent material is a compound in which the difference between the minimum excited singlet energy and the minimum excited triplet energy of the delayed fluorescent material is 0.20 eV or less. The use of the delayed fluorescent material enables reverse intersystem crossing of excitons from the triplet state to the singlet state. The minimum excited singlet energy of the delayed fluorescent material may be higher than the minimum excited singlet energy of the fluorescent light-emitting material. Furthermore, when the light-emitting layer contains a delayed fluorescent material, it may further contain the first organic compound described above. This configuration is called a TADF Assisted Fluorescence (TAF) configuration, and exhibits higher luminous efficiency than general fluorescent light.

[0043] In this specification, the light-emitting material that emits fluorescence may also contain light-emitting components other than fluorescence. For example, in addition to fluorescence, delayed fluorescence and phosphorescence may also be contained. However, the amount of fluorescence must be greater than the other light-emitting components. The same applies to light-emitting materials that emit phosphorescence and light-emitting materials that emit delayed fluorescence.

[0044] (e) The organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits phosphorescence. In the image forming apparatus according to this embodiment, the organic light emitting element preferably has a light emitting layer containing a light emitting material, and the light emitting material preferably emits phosphorescence.

[0045] Generally, excitons generated by electrical energy are distributed between triplet and singlet states in a ratio of 3:1, and it is known that light emission from the triplet state (phosphorescence) has higher luminous efficiency than light emission from the singlet state (fluorescence). Therefore, the image forming apparatus according to this embodiment is preferable because it has excellent luminous efficiency.

[0046] The phosphorescent light-emitting material may be a compound having a metal complex, preferably an iridium complex or a platinum complex. The ligand of the metal complex may be a substituted or unsubstituted phenyl-isoquinoline ligand, a substituted or unsubstituted phenyl-quinoline ligand, a substituted or unsubstituted phenyl-benzoisoquinoline ligand, or a substituted or unsubstituted phenyl-naphthoisoquinoline ligand. Specific examples include, but are not limited to, the exemplary compounds RD9 to RD16 described below.

[0047] In the image forming apparatus according to this embodiment, the light-emitting layer of the organic light-emitting element may include a second organic compound different from the phosphorescent light-emitting material. The second organic compound may be a compound having a lowest excited triplet energy higher than that of the phosphorescent light-emitting material. The second organic compound is preferably a compound having a fused polycyclic hydrocarbon skeleton which may have a substituent or a heterocyclic skeleton which may have a substituent. From another perspective, the freely rotatable single bond of the second organic compound is preferably a carbon-carbon bond, more preferably an sp2 carbon-sp2 carbon bond, more preferably all of the freely rotatable single bonds are carbon-carbon bonds, and more preferably all of the freely rotatable single bonds are sp2 carbon-sp2 carbon bonds.

[0048] The condensed polycyclic hydrocarbon skeleton may be a skeleton having 10 to 25 carbon atoms, and specific examples thereof include a naphthalene skeleton, a fluorene skeleton, an anthracene skeleton, a phenanthrene skeleton, a pyrene skeleton, a chrysene skeleton, a triphenylene skeleton, a tetracene skeleton, a fluoranthene skeleton, and a perylene skeleton.

[0049] The heterocyclic skeleton may be a skeleton having 3 to 30 carbon atoms, or may be a skeleton having 3 to 18 carbon atoms, and examples thereof include a dibenzofuran skeleton, a dibenzothiophene skeleton, a xanthone skeleton, a thioxanthone skeleton, a carbazole skeleton, an indolocarbazole skeleton, and a triazine skeleton.

[0050] The second organic compound is preferably a compound having a triphenylene skeleton, a xanthone skeleton, or an indolocarbazole skeleton. These highly planar skeletons enhance charge mobility, which is expected to result in a lower driving voltage for the organic light-emitting device.

[0051] (f) The organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits delayed fluorescence. In the image forming apparatus according to this embodiment, the organic light-emitting element preferably has a light-emitting layer containing a light-emitting material that emits delayed fluorescence. Here, delayed fluorescence refers to fluorescence emitted by reverse intersystem crossing of excitons or fluorescence emitted by triplet-triplet fusion (TTF).

[0052] In the image forming device according to this embodiment, the light-emitting layer contains a delayed fluorescent material. The delayed fluorescent material is a compound in which the difference between the minimum excited singlet energy and the minimum excited triplet energy of the delayed fluorescent material is 0.20 eV or less. The use of the delayed fluorescent material enables reverse intersystem crossing of excitons from the triplet state to the singlet state. In this case, the light-emitting layer may further contain the first organic compound described in (d), and the minimum excited singlet energy of the first organic compound may be higher than the minimum excited singlet energy of the delayed fluorescent material.

[0053] It is generally known that excitons generated by electrical energy are distributed between triplet and singlet states in a ratio of 3:1. The presence of a delayed fluorescent material allows excitons distributed in triplet states to undergo reverse intersystem crossing to the singlet state, thereby enabling the organic light-emitting device to exhibit higher luminous efficiency. Therefore, the image-forming device according to this embodiment is preferable due to its excellent luminous efficiency.

[0054] The image forming apparatus according to this embodiment may also have a light-emitting layer using TTF. TTF is a structure in which triplet excitons collide with each other to generate higher-energy excitons, which are then used to emit light from a singlet state. The use of TTF is preferable because it allows triplet excitons to be used for fluorescent emission, resulting in an organic light-emitting element with excellent luminous efficiency.

[0055] (g) Multiple pixels of organic light-emitting elements are arranged two-dimensionally at high density. The image forming apparatus according to this embodiment comprises a plurality of pixels arranged two-dimensionally in the direction of rotation of the photoreceptor and the direction of the rotation axis of the photoreceptor. This allows organic light-emitting elements to be densely packed two-dimensionally, improving the light emission intensity during exposure. Specifically, the organic light-emitting elements may have a resolution of 600 dpi or more, 1200 dpi or more, 2400 dpi or more, or 4800 dpi or more.

[0056] When the light emission intensity is improved, the photosensitive layer of the photoreceptor is efficiently excited, and the charge is quickly transferred to the charge transport material, which is an excellent mechanism, so that residual charge can be suppressed and deterioration of image quality can be suppressed.

[0057] The configuration of the exposure light source 28 of the image forming apparatus according to this embodiment will be described in more detail below.

[0058] (First embodiment) 3 is a schematic diagram showing one form of a light-emitting device included in an image forming apparatus according to this embodiment. The light-emitting device according to this embodiment has a rectangular shape with long sides parallel to a first direction and short sides parallel to a direction intersecting the first direction, and the first direction may be, for example, a direction along the rotation axis of a photosensitive member of the image forming apparatus.

[0059] The substrate 1701 has a polygonal shape, and an example of a rectangular substrate 1701 will be described here. In this specification, the direction of the long sides of the rectangular substrate 1701 is referred to as the first direction, and the direction of the short sides perpendicular to the long side is referred to as the second direction. In addition, the polygon in this specification also includes shapes with rounded corners. A moisture-resistant ring 1700 is disposed on the rectangular substrate 1701, and serves to suppress and prevent moisture from penetrating into the light-emitting device. The moisture-resistant ring 1700 can be, for example, a guard ring formed of a wiring layer.

[0060] A light-emitting region 1702, a contact region 1703, pads 1704, and a circuit 1706 are arranged inside the moisture-resistant ring 1700. In this embodiment, the contact region 1703 includes a first contact region 1703_1, a second contact region 1703_2, and a third contact region 1703_3. The pads 1704 include a first pad 1704_1, a second pad 1704_2, and a third pad 1704_3.

[0061] Circuit 1706 is a part of the circuitry for driving each light-emitting device, and specific examples include, but are not limited to, an input protection circuit, an input circuit into which data for each drive is input, and a logic circuit for processing data.

[0062] In the light-emitting region 1702, light-emitting elements EL are arranged in rows and columns. The light-emitting elements EL have organic light-emitting elements having the characteristics of the present invention. The contact region 1703 is an area where wiring electrically connected to a common electrode of the light-emitting elements EL is arranged. The pad 104_1 electrically connects the contact region 1703 to an external element.

[0063] The outer periphery of the moisture-resistant ring 1700 may include a plurality of recessed portions, which can be used as contact areas for contacting ribs that are part of a mask for vapor deposition in a film-forming process, for example.

[0064] As described above, each of the plurality of light-emitting elements EL arranged in a matrix in the light-emitting region 1702 is composed of a light-emitting layer and a first electrode and a second electrode that sandwich the light-emitting layer. In this embodiment, an example is shown in which the first electrode is an independent electrode provided for each light-emitting element EL, and the second electrode is a common electrode provided in common to all the light-emitting elements EL.

[0065] For example, in the case where light-emitting region 1702 has four rows of light-emitting elements EL, the initial positions of the light-emitting elements EL in the first row and the second row may be shifted in the X direction by ¼ of the X-direction dimension of the light-emitting elements EL, as illustrated in Fig. 3. In the case of n rows, where n is an integer of 2 or greater, the initial positions of the light-emitting elements EL in the first row and the second row may be shifted in the X direction by 1 / n of the X-direction dimension of the light-emitting elements EL. Such a configuration is advantageous for improving resolution.

[0066] The contact region 1703 is an area adjacent to the light emitting region 1702 of the substrate 1701, and is disposed inside the moisture-resistant ring 1700. At least one of the first contact region 1703, the pad 104, and the circuit 1706, together with the recess of the moisture-resistant ring 1700, may be disposed between the light emitting region 102 and one long side end of the substrate 1701, and may be disposed in series in the long side direction.

[0067] In this way, by providing the contact region 1703, the pad region 1704, the circuit 1706, etc. at the same position in the short side direction, the length of the light emitting device in the short side direction can be reduced, making it possible to miniaturize the light emitting device.

[0068] The light-emitting device of this embodiment has multiple contact regions 1703 between the common electrode of the light-emitting element EL and the power supply wiring along the long side edge of the light-emitting device. If the common electrode is made of, for example, a transparent electrode material with relatively high electrical resistance, the amount of voltage drop in the long axis direction may be large. Therefore, the voltage applied to each OLED varies depending on the distance from the contact region to which the potential is supplied. This may result in differences in actual light emission brightness (light emission intensity) between OLEDs to which a voltage is applied to emit light with the same brightness (light emission intensity), which may cause shading or the like. By having multiple contact regions 1703 in the long axis direction as in this embodiment, the voltage drop of the common electrode in the long axis direction can be suppressed, thereby preventing shading and the like.

[0069] (Second embodiment) In this embodiment, an example in which a light emitting device is used in a head substrate 1800 of an exposure head of an image forming apparatus will be described with reference to FIG.

[0070] Fig. 4(a) is a schematic perspective view of the head substrate 1800. Fig. 4(b) shows the arrangement of a plurality of light-emitting elements EL provided on the head substrate 1800, and Fig. 4(c) shows an enlarged view of a part of Fig. 4(b).

[0071] An LED chip 1803 is mounted on the head substrate 1800. As the LED chip 1803, for example, the light emitting device described in the first embodiment can be used.

[0072] 4(a), an LED chip 1803 is provided on one surface of a head substrate 1800, and a long flexible flat cable (FFC) connector 1807 is provided on the other surface. The one surface of the head substrate 1800 here refers to the surface (upper surface, front surface) on which the LED chip 1803 is provided. The other surface of the substrate refers to the surface (lower surface, back surface) opposite to the side on which the LED chip 1803 is provided.

[0073] The FFC connector 1807 is attached to the other surface (bottom surface, back surface) of the head substrate 1800 so that its longitudinal direction follows the longitudinal direction of the head substrate 1800. The long FFC connector 1807 is provided to input a control signal (drive signal) from a control circuit section of the main body of the image forming apparatus, and the control signal is transferred to each LED chip 1803. The LED chips 1803 are driven (to emit light or turn off) by the control signal input to the head substrate 1800.

[0074] The LED chips 1803 mounted on the head substrate 1800 will now be described. As shown in FIGS. 4(b) and 4(c), a plurality of light-emitting elements EL are arranged on one surface of the head substrate 1800. For example, LED chips 1803-1 to 1803-29 (29 pieces) are arranged. FIG. 4(b) shows LED chips 1803_1, 1803_13, 1803_14, 1803_15, 1803_16, and 1803_29 as examples. Each of the LED chips 1803-1 to 1803-29 has a plurality of light-emitting elements EL arranged in its longitudinal direction, and for example, 516 light-emitting elements EL are arranged.

[0075] The center-to-center distance k2 between adjacent light-emitting elements EL in the longitudinal direction of the LED chip 1803 corresponds to the resolution of the image forming device. For example, when the resolution of the image forming device in this embodiment is 1200 dpi, the light-emitting elements EL are arranged so that the center-to-center distance k2 between adjacent light-emitting elements EL in the longitudinal direction of the LED chips 1803-1 to 1803-29 is 21.16 μm. Therefore, the exposure range of the exposure head in this embodiment is approximately 314 mm.

[0076] The photosensitive layer of the photosensitive drum is formed with a width of 314 mm or more. Since the long side of A4 size recording paper and the short side of A3 size recording paper are 297 mm, the exposure head of this embodiment has an exposure range that can form images on A4 size recording paper and A3 size recording paper. Note that while Figure 4 shows an example in which multiple light-emitting elements EL are arranged in the longitudinal direction, light-emitting elements EL may be arranged in the lateral direction as well as the longitudinal direction.

[0077] A plurality of LED chips 1803-1 to 1803-29 are arranged in the axial direction of the photosensitive drum. Specifically, the LED chips 1803-1 to 1803-29 are alternately arranged in two rows along the axial direction of the photosensitive drum. That is, as shown in FIG. 4(b), counting from the left, odd-numbered LED chips 1803-1, 1803-3, ... 1803-29 are mounted in a row in the longitudinal direction of the substrate 1800. Also, counting from the left, even-numbered LED chips 1803-2, 1803-4, ... 1803-28 are mounted in a row in the longitudinal direction of the substrate 1800. The LED chips 1803 are arranged in this manner.

[0078] 4(c), the center-to-center distance k1 of the light-emitting elements EL can be made equal to the center-to-center distance k2 of the light-emitting elements EL in the longitudinal direction of the LED chip 1803. Here, the center-to-center distance k1 of the light-emitting elements EL refers to the center-to-center distance between the light-emitting elements EL arranged at one end of the LED chip 1803_13 and the other end of the LED chip 1803_14. The center-to-center distance k2 of the light-emitting elements EL refers to the center-to-center distance k2 between adjacent light-emitting elements EL in the LED chip 1803_14.

[0079] In other words, the center-to-center distance k1 between adjacent light-emitting elements EL arranged on one end of an LED chip 1803 and the other end of another LED chip 1803 can be made equal to the center-to-center distance k2 between adjacent light-emitting elements EL on one LED chip 1803.

[0080] In this embodiment, the light-emitting element EL is an organic light-emitting element that is a current-driven light-emitting element. The organic light-emitting elements are arranged in a line along the main scanning direction (the axial direction of the photosensitive drum 2) on a TFT (Thin Film Transistor) substrate, for example, and are electrically connected in parallel by power supply wiring that is also provided along the main scanning direction.

[0081] When the light-emitting device is used in an exposure head, linear exposure is performed, and therefore the ratio of the longitudinal direction (first direction X) to the lateral direction (second direction Y) of the shape of the light-emitting region 1702 is larger than when the light-emitting device is used in a display device, etc. Similarly, the ratio of the longitudinal direction (first direction X) to the lateral direction (third direction Y) of the shape of the LED chip substrate is also larger.

[0082] Specifically, for example, the length of the long side of the LED chip (or light-emitting region 1702) is at least 5 times the length of the short side of the LED chip (or light-emitting region 1702), and may be at least 10 times the length. For example, the length of the long side of the LED chip (or light-emitting region 1702) can be at least 20 times the length of the short side of the LED chip (or light-emitting region 1702).

[0083] The length of the long side of the LED chip is determined by the axial length of the photosensitive drum 2, the number of LED chips arranged in the axial direction, and the arrangement of the LED chips 1803. The length of the short side of the LED chip 1803 is determined in the direction perpendicular to the axis of the photosensitive drum by whether the light-emitting element EL is arranged in the light-emitting region 1702 and the arrangement of the pads 1704 and contact region 1703.

[0084] Furthermore, the organic layer 66 may be configured to have a light-emitting layer that emits red light in consideration of the wavelength dependency of the photosensitivity of the photosensitive drum 2.

[0085] The LED chip 1803 may have a color filter, which can improve print quality by absorbing stray light from unintended directions without reducing the amount of normal light incident on the photosensitive drum.

[0086] <Organic light-emitting element> The organic light-emitting device will be described in detail below, but is not limited to these.

[0087] Specific examples of the organic light-emitting device of this embodiment include a multilayer device structure in which electrode layers and organic compound layers shown in (A) to (F) below are sequentially stacked on a substrate. In any device structure, the organic compound layers always include a light-emitting layer containing a light-emitting material. (A) Anode / Emitting layer / Cathode (B) Anode / hole transport layer / light-emitting layer / electron transport layer / cathode (C) Anode / hole transport layer / light-emitting layer / electron transport layer / electron injection layer / cathode (D) Anode / hole injection layer / hole transport layer / light-emitting layer / electron transport layer / cathode (E) Anode / hole injection layer / hole transport layer / light-emitting layer / electron transport layer / electron injection layer / cathode (F) Anode / hole transport layer / electron blocking layer / light-emitting layer / hole blocking layer / electron transport layer / cathode

[0088] However, these examples of device configurations are merely basic device configurations and are not limited to these. For example, various layer configurations can be adopted, such as providing an insulating layer, an adhesive layer, or an interference layer at the interface between the electrode and the organic compound layer, configuring the electron transport layer or hole transport layer from two layers with different ionization potentials, or configuring the light-emitting layer from two layers made of different light-emitting materials.

[0089] In the organic light-emitting device according to this embodiment, conventionally known low-molecular-weight and high-molecular-weight hole-injecting or hole-transporting compounds, host compounds, light-emitting compounds, electron-injecting or electron-transporting compounds, etc. may be used together as needed. Examples of these compounds are listed below.

[0090] As the hole injection transport material, a material with high hole mobility is preferred, which facilitates the injection of holes from the anode and transports the injected holes to the light-emitting layer. Furthermore, a material with a high glass transition temperature is preferred to suppress deterioration of film quality, such as crystallization, in the organic light-emitting device. Examples of low-molecular-weight and high-molecular-weight materials with hole injection transport properties include triarylamine derivatives, arylcarbazole derivatives, phenylenediamine derivatives, stilbene derivatives, phthalocyanine derivatives, porphyrin derivatives, poly(vinylcarbazole), poly(thiophene), and other conductive polymers. Furthermore, the above-mentioned hole injection transport materials are also suitable for use in electron blocking layers. Specific examples of compounds usable as hole injection transport materials are listed below, but the present invention is not limited to these.

[0091] [ka]

[0092] Examples of light-emitting materials that primarily contribute to light-emitting function include fused ring compounds (e.g., fluorene derivatives, naphthalene derivatives, pyrene derivatives, perylene derivatives, tetracene derivatives, anthracene derivatives, rubrene, etc.), quinacridone derivatives, coumarin derivatives, stilbene derivatives, organoaluminum complexes such as tris(8-quinolinolato)aluminum, iridium complexes, platinum complexes, rhenium complexes, copper complexes, europium complexes, ruthenium complexes, and polymer derivatives such as poly(phenylenevinylene) derivatives, poly(fluorene) derivatives, and poly(phenylene) derivatives. The concentration of the light-emitting material is preferably 0.01% to 20% by weight, and more preferably 0.1% to 10% by weight, based on the total weight of the light-emitting layer. Specific examples of compounds that can be used as light-emitting materials are listed below, but are not limited to these.

[0093] [ka]

[0094] [ka]

[0095] Specific examples of the host or assist contained in the light-emitting layer include, but are not limited to, aromatic hydrocarbon compounds or derivatives thereof, carbazole derivatives, azine derivatives, xanthone derivatives, dibenzofuran derivatives, dibenzothiophene derivatives, organic aluminum complexes such as tris(8-quinolinolato)aluminum, and organic beryllium complexes. Specific examples are shown below.

[0096] [ka]

[0097] The electron transport material can be arbitrarily selected from those capable of transporting electrons injected from the cathode to the light-emitting layer, and is selected taking into consideration the balance with the hole mobility of the hole transport material. Examples of materials having electron transport properties include oxadiazole derivatives, oxazole derivatives, pyrazine derivatives, triazole derivatives, triazine derivatives, quinoline derivatives, quinoxaline derivatives, phenanthroline derivatives, organoaluminum complexes, and fused ring compounds (e.g., fluorene derivatives, naphthalene derivatives, chrysene derivatives, anthracene derivatives, etc.). Furthermore, the above electron transport materials are also suitable for use in hole-blocking layers. Specific examples of compounds used as electron transport materials are listed below, but of course, the present invention is not limited to these. Specific examples are listed below.

[0098] [ka]

[0099] The electron injection material can be selected from those that allow easy electron injection from the cathode, taking into consideration the balance with hole injection properties, etc. Examples of organic compounds include n-type dopants and reducing dopants. Examples include compounds containing alkali metals such as lithium fluoride, lithium complexes such as lithium quinolinol, benzimidazolidene derivatives, imidazolidene derivatives, fulvalene derivatives, and acridine derivatives. They can also be used in combination with the above-mentioned electron transport materials.

[0100] Examples of the light-emitting material that emits light from a singlet excited state in the present invention include compounds represented by BD1 to BD8, BD10, GD1 to GD9, and RD1 to RD8, but are not limited to these compounds.

[0101] Examples of phosphorescent materials that emit light from a triplet excited state in the present invention include compounds represented by BD9, GD10 to GD18, and RD9 to RD16, but are not limited to these compounds.

[0102] The TADF material in the present invention is, for example, a compound represented by EM35 to EM38, but is not limited to these compounds.

[0103] <More detailed configuration of organic light-emitting element> Next, the organic light-emitting device according to this embodiment will be described in more detail. The organic light-emitting device is provided by forming an insulating layer, a first electrode, an organic compound layer, and a second electrode on a substrate. A protective layer, a color filter, a microlens, etc. may be provided on the second electrode. When a color filter is provided, a planarizing layer may be provided between the protective layer and the color filter. The planarizing layer may be made of acrylic resin or the like. The same applies when a planarizing layer is provided between the color filter and the microlens.

[0104] [substrate] Examples of the substrate include quartz, glass, a silicon wafer, a resin, and a metal. Furthermore, the substrate may be provided with a switching element such as a transistor and wiring, and an insulating layer thereon. Any material can be used for the insulating layer, as long as it can form contact holes so that wiring can be formed between the first electrode and the insulating layer, and can ensure insulation from wiring that is not connected. For example, resins such as polyimide, silicon oxide, silicon nitride, etc. can be used.

[0105] [electrode] A pair of electrodes can be used. The pair of electrodes may be an anode and a cathode. When an electric field is applied in the direction in which the organic light-emitting element emits light, the electrode with a higher potential is the anode, and the other is the cathode. It can also be said that the electrode that supplies holes to the light-emitting layer is the anode, and the electrode that supplies electrons is the cathode.

[0106] The anode material should have as high a work function as possible. Examples include simple metals such as gold, platinum, silver, copper, nickel, palladium, cobalt, selenium, vanadium, and tungsten, mixtures containing these metals, alloys of these metals, and metal oxides such as tin oxide, zinc oxide, indium oxide, indium tin oxide (ITO), and zinc indium oxide. Conductive polymers such as polyaniline, polypyrrole, and polythiophene can also be used.

[0107] These electrode materials may be used alone or in combination of two or more. The anode may be composed of one layer or multiple layers.

[0108] When used as a reflective electrode, for example, chromium, aluminum, silver, titanium, tungsten, molybdenum, or alloys or laminates thereof can be used. The above materials can also function as a reflective film without functioning as an electrode. When used as a transparent electrode, transparent conductive oxide layers such as indium tin oxide (ITO) and indium zinc oxide can be used, but are not limited to these. Photolithography techniques can be used to form the electrode.

[0109] On the other hand, materials with a low work function are preferred for the cathode. Examples include alkali metals such as lithium, alkaline earth metals such as calcium, and metals such as aluminum, titanium, manganese, silver, lead, and chromium, as well as mixtures containing these metals. Alternatively, alloys combining these metals can be used. For example, magnesium-silver, aluminum-lithium, aluminum-magnesium, silver-copper, and zinc-silver can be used. Metal oxides such as indium tin oxide (ITO) can also be used. These electrode materials can be used alone or in combination. The cathode can have either a single-layer or multi-layer structure. Among these, silver is preferred, and a silver alloy is even more preferred to reduce silver aggregation. The alloy ratio is not critical as long as silver aggregation can be reduced. For example, the silver:other metal ratio can be 1:1, 3:1, or the like.

[0110] The cathode may be a top-emission element using an oxide conductive layer such as ITO, or a bottom-emission element using a reflective electrode such as aluminum (Al), and is not particularly limited. The method for forming the cathode is not particularly limited, but DC and AC sputtering methods are more preferred because they provide good film coverage and make it easier to reduce resistance.

[0111] [Organic compound layer] The organic compound layer has at least a light-emitting layer, and in addition to the light-emitting layer, may have a hole injection layer, a hole transport layer, an electron blocking layer on the anode side, and a hole blocking layer, an electron transport layer, an electron injection layer, etc., selected as needed on the cathode side. The organic compound layer is mainly composed of organic compounds, but may also contain inorganic atoms or inorganic compounds. For example, it may contain copper, lithium, magnesium, aluminum, iridium, platinum, molybdenum, zinc, etc.

[0112] The organic compound layer constituting the organic light-emitting device according to one embodiment of the present invention can be formed by dry processes such as vacuum deposition, ionization deposition, sputtering, plasma, etc. Alternatively to the dry process, a wet process can be used in which the compound is dissolved in an appropriate solvent and a layer is formed by a known coating method (for example, spin coating, dipping, casting, LB method, inkjet method, etc.).

[0113] Here, when a layer is formed by a vacuum deposition method or a solution coating method, crystallization is unlikely to occur and the layer has excellent stability over time. When a film is formed by a coating method, the film can be formed by combining with an appropriate binder resin.

[0114] Examples of the binder resin include, but are not limited to, polyvinylcarbazole resin, polycarbonate resin, polyester resin, ABS resin, acrylic resin, polyimide resin, phenol resin, epoxy resin, silicone resin, and urea resin.

[0115] These binder resins may be used singly or in combination as homopolymers or copolymers, and may further contain known additives such as plasticizers, antioxidants, and ultraviolet absorbers, if necessary.

[0116] [Charge generation layer] The charge generation layer may be formed by forming an n / p junction with a p layer and an n layer, or by using a p-doped layer or an n-doped layer. For example, when only a P / N junction is formed, it is preferable to provide an electron injection layer to further inject electrons extracted by the P layer into the electron transport layer. An n-doped layer / p-doped layer may also be used. The present invention is not limited to any particular combination of charge generation layers.

[0117] The p-doped layer may be made by mixing Lewis acids such as HAT-CN and molybdenum oxide, which have high electron-withdrawing properties, with aromatic amine compounds. HAT-CN and molybdenum oxide are n-type materials, which means that they can be stacked by withdrawing electrons. The p-doped layer or the np junction consisting of a stack of n-type material and p-type material is treated as a p-charge generation layer.

[0118] The n-doped layer is preferably made of a material with a low work function, such as an alkali metal or alkaline earth metal, which is easy to dope with electrons. In this invention, the n-doped layer is treated as an n-charge generation layer. Alternatively, an electron injection layer may be formed by combining a thin film of a reducing metal such as LiF, forming an n-type layer and a p-type layer to generate charges.

[0119] Examples of n dopants that can be used include alkali metals such as Li, Na, K, Rb, and Cs, alkaline earth metals such as Mg, Ca, Sr, and Ba, and rare earth metals such as Yb, as well as compounds thereof (alkali metal compounds (including oxides such as lithium oxide, halides, and carbonates such as lithium carbonate and cesium carbonate), alkaline earth metal compounds (including oxides, halides, and carbonates), and rare earth metal compounds (including oxides, halides, and carbonates).

[0120] [Protective layer] A protective layer may be provided on the second electrode. For example, by adhering glass with a moisture absorbent on the second electrode, the infiltration of water and other contaminants into the organic compound layer can be reduced, thereby reducing the occurrence of display defects. In another embodiment, a passivation film such as silicon nitride may be provided on the second electrode to reduce the infiltration of water and other contaminants into the organic compound layer. For example, after forming the second electrode, the second electrode may be transferred to another chamber without breaking the vacuum, and a 2 μm-thick silicon nitride film may be formed by CVD to serve as a protective layer. A protective layer may be provided using atomic layer deposition (ALD) after the CVD film formation. The material of the film formed by ALD is not limited, and may be silicon nitride, silicon oxide, aluminum oxide, or the like. Silicon nitride may be further formed on the film formed by ALD by CVD. The film formed by ALD may have a thickness smaller than that of the film formed by CVD. Specifically, the thickness may be 50% or less, or even 10% or less.

[0121] [Color Filter] A color filter may be provided on the protective layer. For example, a color filter taking into consideration the size of the organic light-emitting element may be provided on a separate substrate, and the substrate on which the organic light-emitting element is provided may be bonded to the color filter. Alternatively, a color filter may be patterned on the protective layer described above using photolithography technology. The color filter may be made of a polymer.

[0122] [Planarization layer] A planarization layer may be provided between the color filter and the protective layer. The planarization layer is provided for the purpose of reducing the unevenness of the underlying layer. It may also be called a material resin layer without limiting its purpose. The planarization layer may be composed of an organic compound, and may be either a low molecular weight or a high molecular weight, but a high molecular weight is preferred.

[0123] The planarizing layer may be provided above or below the color filter, and may be made of the same or different materials, such as polyvinyl carbazole resin, polycarbonate resin, polyester resin, ABS resin, acrylic resin, polyimide resin, phenol resin, epoxy resin, silicone resin, and urea resin.

[0124] [Microlens] An organic light-emitting element or a light-emitting device having an organic light-emitting element may have an optical component such as a microlens on its light-emitting side. The microlens may be made of acrylic resin, epoxy resin, or the like. The microlens may be intended to increase the amount of light extracted from the organic light-emitting element or light-emitting device or to control the direction of the extracted light. The microlens may have a hemispherical shape. When the microlens has a hemispherical shape, among the tangents to the hemisphere, there is a tangent that is parallel to the insulating layer, and the point of contact between this tangent and the hemisphere is the vertex of the microlens. The vertex of the microlens can be determined in the same way in any cross-sectional view. In other words, among the tangents to the semicircle of the microlens in the cross-sectional view, there is a tangent that is parallel to the insulating layer, and the point of contact between this tangent and the semicircle is the vertex of the microlens.

[0125] It is also possible to define the midpoint of a microlens. In the cross section of the microlens, a line segment is imagined from the point where an arc shape ends to the point where another arc shape ends, and the midpoint of this line segment can be called the midpoint of the microlens. The cross section for determining the vertex and midpoint may be a cross section perpendicular to the insulating layer.

[0126] [Counter substrate] An opposing substrate may be provided on the planarization layer. The opposing substrate is called an opposing substrate because it is provided at a position corresponding to the aforementioned substrate. The constituent material of the opposing substrate may be the same as that of the aforementioned substrate. When the aforementioned substrate is defined as a first substrate, the opposing substrate may be a second substrate.

[0127] [Pixel circuit] A light-emitting device having organic light-emitting elements may have a pixel circuit connected to the organic light-emitting elements. The pixel circuit may be an active matrix type that controls the emission of a plurality of organic light-emitting elements independently. The active matrix type circuit may be voltage-programmed or current-programmed. The drive circuit has a pixel circuit for each pixel. The pixel circuit may have an organic light-emitting element, a transistor that controls the emission luminance (emission intensity) of the organic light-emitting element, a transistor that controls the emission timing, a capacitor that holds the gate voltage of the transistor that controls the emission luminance (emission intensity), and a transistor for connecting to GND without going through the light-emitting element.

[0128] The light-emitting device has a display area and a peripheral area arranged around the display area. The display area has a pixel circuit, and the peripheral area has a display control circuit. The mobility of a transistor constituting the pixel circuit may be smaller than the mobility of a transistor constituting the display control circuit. The slope of the current-voltage characteristics of the transistor constituting the pixel circuit may be smaller than the slope of the current-voltage characteristics of the transistor constituting the display control circuit. The slope of the current-voltage characteristics can be measured by the so-called Vg-Ig characteristics. The transistor constituting the pixel circuit is a transistor connected to an organic light-emitting element.

[0129] [Pixels] A light emitting device having an organic light emitting element may have a plurality of pixels.

[0130] The pixel emits light from an area called the pixel aperture. This area is the same as the first area. The pixel aperture may be 15 μm or less, or 5 μm or more. More specifically, it may be 11 μm, 9.5 μm, 7.4 μm, 6.4 μm, etc.

[0131] The pixels may be arranged in a known manner in a plan view. For example, they may be arranged in a stripe arrangement, a delta arrangement, a pentile arrangement, or a Bayer arrangement. The shape may be, for example, a rectangle, a quadrilateral such as a diamond, or a hexagon. Of course, a shape that is close to a rectangle, rather than an exact shape, is included in the rectangle. The shape of the subpixels and the pixel arrangement may be combined.

[0132] The photoconductor 27 of the image forming apparatus according to this embodiment will be described in detail below.

[0133] <Photosensitive layer> The photosensitive layer of a photosensitive drum is mainly classified into (1) a multi-layer type photosensitive layer and (2) a single-layer type photosensitive layer. (1) A multi-layer type photosensitive layer is a photosensitive layer having a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material. (2) A single-layer type photosensitive layer is a photosensitive layer containing both a charge generation material and a charge transport material.

[0134] (1) Laminated photosensitive layer The laminated photosensitive layer has a charge generating layer and a charge transport layer.

[0135] (1-1) Charge generation layer The charge generating layer preferably contains a charge generating material and a resin.

[0136] Examples of charge-generating materials include azo pigments, perylene pigments, polycyclic quinone pigments, indigo pigments, and phthalocyanine pigments. Among these, azo pigments and phthalocyanine pigments are preferred. Among phthalocyanine pigments, oxytitanium phthalocyanine pigments, chlorogallium phthalocyanine pigments, and hydroxygallium phthalocyanine pigments are preferred.

[0137] The content of the charge generating material in the charge generating layer is preferably 40% by mass or more and 85% by mass or less, and more preferably 60% by mass or more and 80% by mass or less, based on the total mass of the charge generating layer.

[0138] Examples of the resin include polyester resin, polycarbonate resin, polyvinyl acetal resin, polyvinyl butyral resin, acrylic resin, silicone resin, epoxy resin, melamine resin, polyurethane resin, phenol resin, polyvinyl alcohol resin, cellulose resin, polystyrene resin, polyvinyl acetate resin, polyvinyl chloride resin, etc. Among these, polyvinyl butyral resin is more preferred.

[0139] The charge generating layer may further contain additives such as antioxidants and ultraviolet absorbers, etc. Specific examples include hindered phenol compounds, hindered amine compounds, sulfur compounds, phosphorus compounds, and benzophenone compounds.

[0140] The charge generating layer can be formed by preparing a coating solution for the charge generating layer containing the above-mentioned materials and solvent, forming a coating film of this on the undercoat layer, and drying it. Examples of the solvent used in the coating solution include alcohol-based solvents, sulfoxide-based solvents, ketone-based solvents, ether-based solvents, ester-based solvents, and aromatic hydrocarbon-based solvents.

[0141] The thickness of the charge generating layer is preferably 0.1 μm or more and 1 μm or less, and more preferably 0.15 μm or more and 0.4 μm or less.

[0142] (1-2) Charge transport layer The charge transport layer preferably contains a charge transport material and a binder material.

[0143] When a protective layer, which will be described later, is not provided, the charge transport layer becomes the surface layer of the photosensitive drum.

[0144] Examples of the charge transport material include polycyclic aromatic compounds, heterocyclic compounds, hydrazone compounds, styryl compounds, enamine compounds, triarylamine compounds, and resins having groups derived from these materials. Among these, triarylamine compounds are preferred.

[0145] The content of the charge transport material in the charge transport layer is preferably 25% by mass to 70% by mass, and more preferably 30% by mass to 55% by mass, based on the total mass of the charge transport layer.

[0146] As the binding material, a thermoplastic resin (hereinafter also referred to as "resin") is used.

[0147] Examples of thermoplastic resins include polyester resins, polycarbonate resins, acrylic resins, and polystyrene resins. Among these, polycarbonate resins and polyester resins are preferred. As the polyester resin, polyarylate resins are particularly preferred.

[0148] The content ratio (mass ratio) of the charge transport material to the resin is preferably 4:10 to 20:10, and more preferably 5:10 to 12:10.

[0149] The charge transport layer may also contain additives such as antioxidants, ultraviolet absorbers, plasticizers, leveling agents, etc. Specific examples of such additives include hindered phenol compounds, hindered amine compounds, sulfur compounds, phosphorus compounds, benzophenone compounds, siloxane-modified resins, silicone oils, polystyrene resin particles, polyethylene resin particles, and boron nitride particles.

[0150] The charge transport layer can be formed by preparing a coating solution for the charge transport layer containing the above-mentioned materials and solvent, forming a coating film of this on the charge generation layer, and drying it. Examples of solvents used in the coating solution include alcohol-based solvents, ketone-based solvents, ether-based solvents, ester-based solvents, and aromatic hydrocarbon-based solvents. Among these solvents, ether-based solvents and aromatic hydrocarbon-based solvents are preferred.

[0151] The thickness of the charge transport layer is preferably from 5 μm to 50 μm, more preferably from 8 μm to 40 μm, and particularly preferably from 10 μm to 30 μm.

[0152] (2) Single-layer photosensitive layer The single-layer photosensitive layer can be formed by preparing a coating solution for the photosensitive layer containing a charge generating material, a charge transport material, a resin, and a solvent, forming the coating on the undercoat layer, and drying the coating. The charge generating material, charge transport material, and resin are the same as those exemplified in "(1) Multilayer Photosensitive Layer" above.

[0153] <Protective layer> In the present disclosure, a protective layer may be provided on the photosensitive layer, which can improve durability.

[0154] When a protective layer is provided, the protective layer becomes the surface layer of the photosensitive drum.

[0155] The protective layer may be formed as a cured film by polymerizing a composition containing, for example, a monomer having a polymerizable functional group, which is a raw material for the binder material. Examples of the reaction include thermal polymerization, photopolymerization, and radiation polymerization. Examples of the polymerizable functional group possessed by the monomer having the polymerizable functional group include an isocyanate group, a blocked isocyanate group, a methylol group, an alkylmethylol group, an epoxy group, a metal alkoxyl group, a hydroxyl group, an amino group, a carboxyl group, a thiol group, a carboxylic anhydride group, and a group containing a carbon-carbon double bond. Examples of the group containing a carbon-carbon double bond include an acryloyl group and a methacryloyl group. A monomer having charge transport capability may be used as the monomer having the polymerizable functional group.

[0156] Here, the cured product of the monomer having a polymerizable functional group is the binder material of the protective layer.

[0157] As the monomer having a polymerizable functional group, it is preferable to use a hole transporting compound having a chain-polymerizable functional group.

[0158] The hole transporting compound having a chain-polymerizable functional group is more preferably a compound represented by the following formula (CT-1) or (CT-2).

[0159] [ka]

[0160] In the formula (CT-1), Ar11 to Ar13 each independently represent a substituted aryl group or an unsubstituted aryl group. The substituent that the substituted aryl group may have is an alkyl group having 1 to 6 carbon atoms, or a monovalent functional group represented by any one of the following formulae (P-1) to (P-3). However, the compound represented by the formula (CT-1) has at least one monovalent functional group represented by any one of the following formulae (P-1) to (P-3).

[0161] [ka]

[0162] In the formula (CT-2), Ar21 to Ar24 each independently represent a substituted aryl group or an unsubstituted aryl group, and Ar25 represents a substituted arylene group or an unsubstituted arylene group. The substituent that the substituted aryl group may have is an alkyl group having from 1 to 6 carbon atoms, or a monovalent functional group represented by any of the following formulae (P-1) to (P-3). The substituent that the substituted arylene group may have is an alkyl group having from 1 to 6 carbon atoms, or a monovalent functional group represented by any of the following formulae (P-1) to (P-3). However, the compound represented by the formula (CT-2) has at least one monovalent functional group represented by any of the following formulae (P-1) to (P-3).

[0163] [ka]

[0164] In the formula (P-1), Z11 represents a single bond or an alkylene group having 1 to 6 carbon atoms, and X11 represents a hydrogen atom or a methyl group.

[0165] [ka]

[0166] In the formula (P-2), Z21 represents a single bond or an alkylene group having 1 to 6 carbon atoms.

[0167] [ka]

[0168] In the formula (P-3), Z31 represents a single bond or an alkylene group having 1 to 6 carbon atoms.

[0169] The protective layer may contain fluorine atom-containing resin particles, which can improve the abrasion resistance of the protective layer.

[0170] Examples of resins contained in the fluorine atom-containing resin particles include polytetrafluoroethylene resin, polychlorotrifluoroethylene resin, polytetrafluoroethylenepropylene resin, polyvinyl fluoride resin, polyvinylidene fluoride resin, and polydichlorodifluoroethylene resin. It is also preferable to use particles containing multiple types of the above resins. Among the above, from the viewpoint of improving dispersibility, it is more preferable that the fluorine atom-containing resin particles be polytetrafluoroethylene (PTFE) resin.

[0171] In cross-sectional observation of the surface layer, the fluorine atom-containing resin particles preferably have an arithmetic mean of the major axes of the primary particles (average primary particle size) measured from a secondary electron image taken with a scanning electron microscope of 150 nm to 300 nm, from the viewpoints of improving dispersibility and suppressing potential fluctuations.More preferably, the fluorine atom-containing resin particles have an average primary particle size of 180 nm to 250 nm.

[0172] The content of the fluorine atom-containing resin particles in the protective layer is preferably from 5 to 40% by mass, more preferably from 25 to 35% by mass, based on the total mass of the protective layer.

[0173] The protective layer may also contain additives such as antioxidants, ultraviolet absorbers, plasticizers, leveling agents, etc. Specific examples include hindered phenol compounds, hindered amine compounds, sulfur compounds, phosphorus compounds, benzophenone compounds, siloxane-modified resins, silicone oils, polystyrene resin particles, polyethylene resin particles, and boron nitride particles.

[0174] The protective layer can be formed by preparing a coating solution for the protective layer containing the above-mentioned materials and solvent, forming a coating film of this on the photosensitive layer, and drying and / or curing it. Examples of the solvent used in the coating solution include alcohol-based solvents, ketone-based solvents, ether-based solvents, sulfoxide-based solvents, ester-based solvents, and aromatic hydrocarbon-based solvents.

[0175] The thickness of the protective layer is preferably 0.50 μm or more and 10 μm or less, and more preferably 1 μm or more and 7 μm or less.

[0176] <Surface treatment of photosensitive drum> In the present disclosure, the surface of the photosensitive drum may be processed. By performing the surface processing, the behavior of the cleaning means (cleaning blade) that comes into contact with the photosensitive drum can be further stabilized. Examples of surface processing methods include a method in which a mold having convex portions is pressed against the surface of the photosensitive drum to transfer the shape, a method in which an uneven shape is imparted by mechanical polishing, or a method in which powder is collided with the surface of the photosensitive drum to roughen the surface. In this way, by providing concave or convex portions on the surface layer of the photosensitive drum, the behavior of the cleaning means that comes into contact with the photosensitive drum can be further stabilized.

[0177] The recesses or protrusions may be formed over the entire surface of the photosensitive drum or may be formed on only a part of the surface of the photosensitive drum. When the recesses or protrusions are formed on only a part of the surface of the photosensitive drum, it is preferable that the recesses or protrusions are formed over at least the entire area of ​​contact with the cleaning means (cleaning blade).

[0178] When forming recesses, a mold having protrusions corresponding to the recesses is pressed against the surface of the photosensitive drum to transfer the shape, thereby forming the recesses on the surface of the photosensitive drum. [Example]

[0179] The present invention will be described in more detail below using examples, but the present invention is not limited thereto.

[0180] In this example, a top-emission stacked organic EL device was fabricated in which an anode, a hole injection layer, a hole transport layer, an emitting layer, a first electron transport layer, a second electron transport layer, an electron injection layer, a cathode, a first protective layer, a second protective layer, and a third protective layer were sequentially formed on a substrate. The emitting layer was formed from a red emitting material and a host material.

[0181] A 40 nm titanium (Ti) film was deposited as an anode on a silicon substrate with a wiring layer stacked on it by sputtering, and then patterned using photolithography to form a Ti pixel array with square pixel apertures and wiring, each 20 μm on a side.

[0182] Next, the UV / ozone-cleaned silicon substrate and the material were placed in a vacuum deposition apparatus, and 1.0 × 10 -4 Pa(1×10 -6 The chamber was evacuated to a pressure of 1000 psi (1 Torr). The thicknesses of the hole injection layer and hole injection layer were then adjusted to create an interference structure that intensifies the emission wavelength. Next, an emission layer containing a red light-emitting material was formed. Next, a first electron transport layer, a second electron transport layer, and an electron injection layer were formed in this order to create an interference structure that intensifies the emission wavelength. A cathode was then formed. Furthermore, a silicon nitride film was formed by CVD as the first protective layer, an aluminum oxide film was formed by ALD as the second protective layer, and a silicon nitride film was formed by CVD as the third protective layer, in this order, to produce an organic light-emitting device.

[0183] <Afterglow Life and Exposure Time> The afterglow life of the organic light-emitting device in each example was measured.

[0184] The decay time was measured and calculated by exciting the material with a pulsed laser and measuring the time it took for the emission intensity to decay to 1 / e of the initial value.

[0185] The exposure time was evaluated using patterns of 1 μsec and 20 μsec.

[0186] <Image quality evaluation> Organic light-emitting diode (OLED) chips fabricated on a silicon substrate were cut and 17 chips were attached to a PCB substrate. The PCB substrate with these OLED chips was prepared and placed in a housing with a lens array attached to it to create a print head.

[0187] The prepared print head was installed in a modified Canon copier (product name: imageRUNNER iR-ADVC5051) and image quality was evaluated.

[0188] Next, the electrophotographic photoreceptor was installed in the station of the magenta process cartridge. A test chart with an image ratio of 5% was printed on 30,000 sheets of A4 portrait size paper. After that, a halftone image was printed and the image quality (presence or absence of vertical streaks and ghosting) was evaluated as follows. The results are shown in Table 2. A: No abnormalities such as vertical streaks or ghosting are observed. B: Vertical lines and ghosts are visible when observed under magnification.

[0189] [Table 2]

[0190] As shown in Table 2, the configurations described in Examples 1 to 4 exhibited high image quality after outputting 30,000 sheets. It is believed that the configurations described in Examples 1 to 4 exhibited high image quality because the exposure time was longer than the afterglow life, and therefore the influence of the afterglow irradiated onto the photosensitive member was small.

[0191] On the other hand, the configurations described in Comparative Examples 1 and 2 showed low image quality after outputting 30,000 sheets. This is thought to be because the configurations described in Comparative Examples 1 and 2 have a longer afterglow life than the exposure time, and therefore an unintended latent image was formed on the photosensitive drum due to the afterglow irradiated onto the photosensitive drum.

[0192] In this way, the afterglow life of the organic light emitting element is shorter than the exposure time to the photosensitive member, so that an image forming apparatus that exhibits high image quality can be obtained.

[0193] As described above, by using a configuration in which the afterglow lifetime is shorter than the exposure time as in the present invention, it is possible to suppress afterglow and provide an image forming apparatus with excellent image quality. Also, by using a configuration in which the afterglow lifetime is shorter than the light emission interval, it is possible to suppress afterglow and provide an image forming apparatus with excellent image quality.

[0194] The present invention can also have the following configuration.

[0195] (Configuration 1) an exposure light source having an organic light-emitting element; and a photoreceptor that receives light emitted from the organic light-emitting element; When the luminescence intensity from the organic light-emitting element at the end of the supply of current to the organic light-emitting element is L0, the period from the end of the supply of current to the organic light-emitting element to the time when the organic light-emitting element becomes extinct is T1, and the period from the start of the supply of current to the organic light-emitting element to the time when the supply of current to the organic light-emitting element is ended is T2, An image forming apparatus, wherein T1 and T2 satisfy the relationship of formula (a). (a) T1 <T2

[0196] (Configuration 2) 2. The image forming apparatus according to configuration 1, wherein T1 and T2 satisfy the relationship of formula (b). (b) T2 / T1>1.0

[0197] (Configuration 3) When the period from the end of supplying current to the organic light-emitting element to the start of supplying current to the organic light-emitting element is T3, 3. The image forming apparatus according to configuration 1 or 2, wherein T1 and T3 satisfy the relationship of formula (c). (c) T1 <T3

[0198] (Configuration 4) an exposure light source having an organic light-emitting element; and a photoreceptor that receives light emitted from the organic light-emitting element; When the light emission intensity from the organic light emitting element at the end of the supply of current to the organic light emitting element is L0, the period from the end of the supply of current to the organic light emitting element to the time when the organic light emitting element becomes extinct is T1, and the period from the end of the supply of current to the organic light emitting element to the start of the supply of current to the organic light emitting element is T3, An image forming apparatus, wherein T1 and T3 satisfy the relationship of formula (c). (c) T1 <T3

[0199] (Configuration 5) 5. The image forming apparatus according to any one of configurations 1 to 4, wherein the organic light-emitting element has a top-emission configuration.

[0200] (Configuration 6) 6. The image forming apparatus according to any one of configurations 1 to 5, wherein the organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits fluorescent light.

[0201] (Configuration 7) 6. The image forming apparatus according to any one of configurations 1 to 5, wherein the organic light-emitting element has a light-emitting layer containing a light-emitting material, and the light-emitting material emits phosphorescence.

[0202] (Configuration 8) 7. The image forming apparatus according to claim 6, wherein the light-emitting material has a condensed polycyclic hydrocarbon skeleton.

[0203] (Configuration 9) 9. The image forming apparatus according to configuration 8, wherein the light-emitting material is a hydrocarbon compound.

[0204] (Configuration 10) 10. The image forming apparatus according to any one of Configurations 6 to 9, wherein the light-emitting layer contains a compound different from the light-emitting material, and the compound has a condensed polycyclic hydrocarbon skeleton.

[0205] (Configuration 11) 11. The image forming apparatus according to claim 10, wherein the condensed polycyclic hydrocarbon skeleton is a naphthalene skeleton, an anthracene skeleton, a phenanthrene skeleton, a fluorene skeleton, a pyrene skeleton, a triphenylene skeleton, a chrysene skeleton, a tetracene skeleton, or a perylene skeleton.

[0206] (Configuration 12) 8. The image forming apparatus according to claim 7, wherein the light-emitting material is a metal complex.

[0207] (Configuration 13) 13. The image forming apparatus according to claim 12, wherein the light-emitting material is an iridium complex or a platinum complex.

[0208] (Configuration 14) 7. The image forming apparatus according to claim 6, wherein the fluorescence is delayed fluorescence.

[0209] (Configuration 15) 7. The image-forming device according to claim 6, wherein the light-emitting layer further comprises a delayed fluorescent material, and the lowest excited singlet energy of the delayed fluorescent material is higher than the lowest excited singlet energy of the light-emitting material.

[0210] (Configuration 16) 16. The image forming apparatus according to claim 15, wherein the light-emitting layer further comprises an organic compound different from the delayed fluorescent material, and the lowest excited singlet energy of the organic compound is higher than the lowest excited singlet energy of the light-emitting material.

[0211] (Configuration 17) 17. The image forming apparatus according to any one of configurations 1 to 16, wherein T1 is less than 1 μsec.

[0212] (Configuration 18) 18. The image forming apparatus according to any one of configurations 1 to 17, wherein the organic light-emitting element has an interference structure.

[0213] (Configuration 19) the organic light-emitting element has a reflective layer, a first electrode, a light-emitting layer, and a second electrode; 19. The image forming apparatus according to Structure 18, wherein light emitted from the light emitting layer can be intensified between the reflective layer and the light emitting layer.

[0214] (Configuration 20) the organic light-emitting element has a first electrode, a light-emitting layer, and a second electrode; 19. The image forming apparatus according to Structure 18, wherein light emitted from the light-emitting layer can be intensified between the first electrode and the second electrode. [Explanation of symbols]

[0215] 27 Photoreceptor 28 Exposure light source

Claims

1. an exposure light source having an organic light-emitting element; and a photoreceptor that receives light emitted from the organic light-emitting element; The light emission intensity from the organic light emitting element when the supply of current to the organic light emitting element is terminated is L ０ When the period from the end of supplying current to the organic light-emitting element to the time when the organic light-emitting element is extinguished is T1, and the period from the start of supplying current to the organic light-emitting element to the time when the supply of current to the organic light-emitting element is ended is T2, An image forming apparatus, wherein T1 and T2 satisfy the relationship of formula (a). (a) T1<T2

2. 2. The image forming apparatus according to claim 1, wherein T1 and T2 satisfy the relationship of formula (b). (b) T2 / T1>1.0

3. When the period from the end of supplying current to the organic light-emitting element to the start of supplying current to the organic light-emitting element is T3, 3. The image forming apparatus according to claim 1, wherein T1 and T3 satisfy the relationship of formula (c). (c) T1<T3

4. an exposure light source having an organic light-emitting element; and a photoreceptor that receives light emitted from the organic light-emitting element; The light emission intensity from the organic light emitting element when the supply of current to the organic light emitting element is terminated is L ０ When the period from the end of supplying current to the organic light-emitting element to the time when the organic light-emitting element becomes extinguished is T1, and the period from the end of supplying current to the organic light-emitting element to the start of supplying current to the organic light-emitting element is T3, The image forming apparatus is characterized in that T1 and T3 satisfy the relationship of formula (c). (c) T1<T3

5. 5. The image forming apparatus according to claim 1, wherein the organic light emitting element has a top emission structure.

6. 5. The image forming apparatus according to claim 1, wherein the organic light emitting element has a light emitting layer containing a light emitting material, and the light emitting material emits fluorescent light.

7. 5. The image forming apparatus according to claim 1, wherein the organic light emitting element has a light emitting layer containing a light emitting material, and the light emitting material emits phosphorescence.

8. 7. The image forming apparatus according to claim 6, wherein the light-emitting material has a condensed polycyclic hydrocarbon skeleton.

9. 9. The image forming apparatus according to claim 8, wherein the light-emitting material is a hydrocarbon compound.

10. 8. The image forming apparatus according to claim 6, wherein the light emitting layer contains a compound different from the light emitting material, and the compound has a condensed polycyclic hydrocarbon skeleton.

11. 11. The image forming apparatus according to claim 10, wherein the condensed polycyclic hydrocarbon skeleton is a naphthalene skeleton, an anthracene skeleton, a phenanthrene skeleton, a fluorene skeleton, a pyrene skeleton, a triphenylene skeleton, a chrysene skeleton, a tetracene skeleton, or a perylene skeleton.

12. 8. The image forming apparatus according to claim 7, wherein the light-emitting material is a metal complex.

13. 13. The image forming apparatus according to claim 12, wherein the light-emitting material is an iridium complex or a platinum complex.

14. 7. The image forming apparatus according to claim 6, wherein the fluorescent light is delayed fluorescent light.

15. 7. The image-forming apparatus according to claim 6, wherein the light-emitting layer further comprises a delayed fluorescent material, and the lowest excited singlet energy of the delayed fluorescent material is higher than the lowest excited singlet energy of the light-emitting material.

16. 16. The image forming apparatus according to claim 15, wherein the light-emitting layer further comprises an organic compound different from the delayed fluorescent material, and the lowest excited singlet energy of the organic compound is higher than the lowest excited singlet energy of the light-emitting material.

17. 5. The image forming apparatus according to claim 1, wherein the time T1 is less than 1 microsecond.

18. 5. The image forming apparatus according to claim 1, wherein the organic light emitting element has an interference structure.

19. the organic light-emitting element has a reflective layer, a first electrode, a light-emitting layer, and a second electrode; 19. The image forming apparatus according to claim 18, wherein light emitted from the light emitting layer can be intensified between the reflective layer and the light emitting layer.

20. The organic light-emitting element has a first electrode, a light-emitting layer, and a second electrode, 19. The image forming apparatus according to claim 18, wherein light emitted from the light emitting layer can be intensified between the first electrode and the second electrode.

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