JPH05265232A - Electrophotographic photoreceptor - Google Patents

Abstract

PURPOSE:To provide an electrophotographic photoreceptor having high sensitivity and small residual electric potential. CONSTITUTION:In an electrophotographic photoreceptor where at least electric charge generating layers 2 and an electric charge transporting layer one laminated on an electrically conductive carrier 1, the electrophotographic photoreceptor is formed in a structure where these electric charge generating layers 2 are laminated in two or more layers and is constituted in such a way that concentration of an electric charge generating substance in the electric charge generating layers 2 becomes gradually large toward the electric charge transporting layer 3 side from the farthest side from the electric charge transporting layer 3.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to electrophotographic photoreceptors,
The present invention relates to an electrophotographic photoreceptor having high sensitivity and a small residual potential.

[0002]

2. Description of the Related Art Conventionally, there has been proposed a photoreceptor having a charge generating layer composed of two layers each containing a charge generating substance having a different spectral absorption region (Japanese Patent Laid-Open No. 63-3894).
2).

The purpose of this photoreceptor is to provide a so-called panchromatic photoreceptor.

[0004]

SUMMARY OF THE INVENTION The present invention is intended to provide an electrophotographic photoreceptor having a very high sensitivity and a small residual potential.

[0005]

The constitution of the present invention for solving the above-mentioned problems is an electrophotographic photoreceptor as set forth in the claims.

That is, according to the present invention, (1) an electrophotographic photosensitive member comprising a conductive support and at least a charge generation layer and a charge transport layer laminated on the conductive support, the charge generation layer comprising at least two layers. On the electrophotographic photosensitive member and (2) the conductive support, which is a generating layer and in which the concentration of the charge generating substance in the charge generating layer gradually increases from the farthest side from the charge transporting layer to the charge transporting layer side. In the electrophotographic photosensitive member having at least a charge generation layer and a charge transport layer laminated, the charge generation layer is a single layer, and the concentration of the charge generation substance in the charge generation layer is continuously changed, An electrophotographic photosensitive member that increases in order from the farthest side from the transport layer toward the charge transport layer side, and (3) Charge generation in the portion of the charge generation layer in (1) and (2) above that is in contact with the charge transport layer. Substance concentration Of 80% by weight or more, and the concentration of the charge transport substance in the portion of the charge transport layer in any one of the above (1) to (3) in contact with the charge generation layer is 40% by weight or more. An electrophotographic photoreceptor is provided.

In the laminated organic electrophotographic photoreceptor, high sensitivity is an important point. In order to increase the sensitivity of the photoconductor, two methods of increasing quantum efficiency or mobility can be considered.
The present inventors have paid attention to the former and have made repeated studies.

As a result, it was clarified that in the carrier generation process of the organic electrophotographic photosensitive member, the initial stage of carrier generation occurs by the electron transfer reaction between the charge generating substance and the charge transporting substance. That is, it was found that the contact between the charge generating substance and the charge transporting substance plays an important role in the carrier generating and injecting processes, and the larger the contact amount between the charge generating substance and the charge transporting substance, the higher the carrier generating and injecting efficiency. ..

That is, a structure (form) in which the contact amount of the charge-generating substance and the charge-transporting substance becomes large at the interface between the charge-generating layer and the charge-transporting layer when the laminated photoreceptor is produced.
It was found that what is done is one condition for increasing the sensitivity.

In the conventional photoconductor, it is common knowledge that the charge generation layer contains a binder resin in order to improve the adhesion of the charge generation layer or to reduce the fatigue of the photoconductor during repeated use. Has become an effective means.

On the other hand, according to the study by the present inventor, it has been clarified that the binder resin, which is an effective means, inhibits the contact between the charge generating substance and the charge transporting substance.

That is, it was found that the contact area between the charge generating substance and the charge transporting substance was small because the binder resin was present in the charge generating layer, and the electron transfer reaction was not carried out efficiently.

In order to carry out the electron transfer reaction efficiently, it suffices that the charge generating layer does not contain a binder resin, but in that case, the adhesiveness between the charge generating layer and the lower layer is also deteriorated. Or, there is still a problem that the fatigue of the photoconductor increases when it is repeatedly used.

Therefore, as a result of studying a method of increasing the electron transfer reaction efficiency (increasing the sensitivity) while improving the above problems, as a result, the charge generation layer has a structure of two or more layers, From the farthest side to the charge transport layer side, the concentration of the charge generating substance is gradually increased, or the charge generating layer is a single layer and the farthest side from the charge transporting layer is the charge transporting layer side. The concentration of the charge generating substance increases continuously toward (to increase the concentration gradient)
The inventors have found that the present invention can be achieved by using the structure, and have completed the present invention.

Next, a method of sequentially changing the charge generating substance in the charge generating layer will be described.

1) Method of changing stepwisely Two or more kinds of charge generating layer coating liquids having different charge generating substance concentrations (charge generating substance / binder resin ratio) are used, and the layers are successively coated. At this time, a charge generation layer coating liquid having a concentration of the charge generation substance that increases in order from the side farthest from the charge transport layer toward the charge transport layer side is used.

Further, in a photoreceptor having a structure in which a charge generating layer and a charge transporting layer are laminated in this order, a vacuum thin film forming method (vapor deposition method) can be used for the uppermost charge generating layer.

2) Method of Continuously Changing The concentration of the charge-generating substance (charge-generating substance / binder resin ratio) is changed, and two or more types of coating liquids for the charge-generating layer are used before the lower layer is dried. Overcoat one after another.

Also in this case, a charge generation layer coating solution having a concentration of the charge generation substance that is gradually increased from the side farthest from the charge transport layer toward the charge transport layer side is used.

In both cases 1) and 2), the concentration of the charge-generating substance in the charge-generating layer in contact with the charge-transporting layer is important,
It is important that the concentration of the charge generating substance is 80% by weight or more. Preferably, in the charge generation layer in contact with the charge transport layer, the structure is made of only the charge generation substance.

The concentration of the charge transport material in the charge transport layer in contact with the charge generation layer is also a major factor in determining the amount of contact between the charge generation material and the charge transport material. If the concentration of the charge-transporting substance in the charge-transporting layer in contact with the charge-generating layer is too low, the electron-transfer reaction still fails, resulting in problems such as deterioration of sensitivity and generation of residual potential. The concentration of the charge transport material in the charge transport layer in contact with the charge generation layer is at least 40% by weight.
The above is necessary, and it is preferable to use only the charge transport material. Here, in the case of a photoreceptor having a charge transport layer as the uppermost layer, if the concentration of the charge transport substance in the outermost charge transport layer is too high, problems such as film abrasion during repeated use occur. In this case, the charge transport layer may have a multi-layer structure, and the concentration of the charge transport substance in the outermost charge transport layer may be reduced within a range where there is no problem in electrical characteristics.

The method of forming the charge transport layer in this case may be the same as the method of forming the charge generating layer described above.

As described above, the object of the present invention is achieved, and the present invention has been completed.

Next, the photoreceptor of the present invention will be specifically described with reference to the drawings.

FIG. 1 is a cross-sectional view showing a constitutional example of the photoconductor of the present invention, in which a charge generating layer 2 and a charge transporting layer 3 having a multilayer structure are laminated on a conductive support 1.

FIG. 2 is a sectional view showing another example of the structure, in which the charge generating layer 2 having a multi-layer structure is laminated on the charge transporting layer 3.

FIG. 3 is a cross-sectional view showing still another structural example, in which a protective layer 4 is provided on the charge transport layer 3.

FIG. 4 is a cross-sectional view showing still another structural example, in which an intermediate layer 5 is provided between the conductive support 1 and the charge generation layer 2.

The conductive support 1 has a volume resistance of 10 ¹⁰
Those exhibiting conductivity of Ωcm or less, for example, metals such as aluminum, nickel, chromium, nichrome, copper, silver, gold and platinum, oxides such as tin oxide and indium oxide,
Film-shaped or cylindrical plastic, paper, etc. coated by vapor deposition or sputtering, or
Aluminum, aluminum alloy, nickel, stainless steel plates, etc. I. , I. I. ， Extrusion 、
It is possible to use a tube or the like which has been made into a raw tube by a method such as drawing and then surface-treated by cutting, superfinishing, polishing or the like.

Next, the charge generation layer 2 will be described. The charge generation layer 2 has a multi-layer structure (at least two layers or more) having different charge generation substance concentrations, is a layer containing a charge generation substance as a main component, and a binder resin may be used if necessary. As the binder resin, polyamide, polyurethane, epoxy resin, polyketone, polycarbonate, silicone resin, acrylic resin, polyvinyl butyral,
Polyvinyl formal, polyvinyl ketone, polystyrene, poly-N-vinylcarbazole, polyacrylamide and the like are used. These binder resins can be used alone or as a mixture of two or more kinds.

The thickness of this charge generation layer is 0.05 to 5 μm.
In the case of multiple layers, the total thickness is appropriately about 10 μm or less.

A known material can be used as the charge generating substance. For example, metal phthalocyanine, phthalocyanine pigments such as metal-free phthalocyanine, azurenium salt pigment, squaric acid methine pigment, azo pigment having a carbazole skeleton, azo pigment having a triphenylamine skeleton, azo pigment having a diphenylamine skeleton,
Azo pigments having a dibenzothiophene skeleton, azo pigments having an oxadiazole skeleton, azo pigments having a fluorenone skeleton, azo pigments having a bisstilbene skeleton, azo pigments having a distyryloxadiazole skeleton, azo having a distyrylcarbazole skeleton Pigments, perylene pigments, anthraquinone pigments or polycyclic quinone pigments, quinone imine pigments, diphenylmethane and triphenylmethane pigments, benzoquinone and naphthoquinone pigments, cyanine and azomethine pigments, indigoid pigments, bisbenzimidazole pigments, etc. Can be mentioned. These charge generating substances can be used alone or as a mixture of two or more kinds.

Next, the charge transport layer 3 will be described.

The charge transport layer 3 is dissolved or dispersed in an appropriate solvent containing a charge transport material and a binder resin, and is applied with this solution.
It can be formed by drying. The appropriate thickness of this charge transport layer is about 10 to 50 μm.

The charge transport material includes a hole transport material and an electron transport material.

Examples of the electron-transporting substance include chloranil, bromoanil, tetracyanoethylene, tetracyanoquinone dimethane, 2,4,7-trinitro-9-
Fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2,4,5,7-tetranitroxanthone, 2,4,8-trinitrothioxanthone, 2,6,6
Examples thereof include electron-accepting substances such as 8-trinitro-4H-indeno [1,2-b] thiophen-4-one and 1,3,7-trinitrodibenzothiophenone-5,5-dioxide. These electron transport materials may be used alone or
It can be used as a mixture of two or more species.

As the hole-transporting substance, the electron-donating substances shown below and the like can be mentioned, which are preferably used. For example, poly-N-vinylcarbazole and its derivative, poly-γ-carbazolylethylglutamate and its derivative, pyrene-formaldehyde condensate and its derivative, polyvinylpyrene, polyvinylphenanthrene, oxazole derivative, oxadiazole derivative, imidazole derivative, Triphenylamine derivative, 9- (p-diethylaminostyryl) anthracene, 1,1-bis-
(4-dibenzylaminophenyl) propane, styrylanthracene, styrylpyrazoline, phenylhydrazones, α-phenylstilbene derivatives, thiazole derivatives, triazole derivatives, phenazine derivatives, acridine derivatives, benzofuran derivatives, benzimidazole derivatives, thiophene derivatives, etc. Can be mentioned. These hole transport materials can be used alone or as a mixture of two or more kinds.

As the binder resin used in the charge transport layer 3, polycarbonate (bisphenol A type, bisphenol Z type), polyester, methacrylic resin, acrylic resin, polyethylene, vinyl chloride, vinyl acetate, polystyrene, phenol resin, epoxy resin,
Polyurethane, polyvinylidene chloride, alkyd resin,
Silicone resin, polyvinyl carbazole, polyvinyl butyral, polyvinyl formal, polyacrylate, polyacrylamide, phenoxy resin and the like are used. These binders can be used alone or as a mixture of two or more kinds.

The intermediate layer 5 provided between the conductive support 1 and the charge generation layer 2 is provided for the purpose of improving the adhesiveness, and its material is SiO, Al ₂ O ₃ , a silane coupling agent, An inorganic material such as a titanium coupling agent or a chromium coupling agent, a polyamide resin, an alcohol-soluble polyamide resin, a water-soluble polyvinyl butyral, a polyvinyl butyral, or a binder resin having good adhesiveness such as PVA is used. In addition, a binder resin having ZnO, TiO ₂ , ZnS or the like dispersed therein can be used. As the method of forming the intermediate layer, a method such as sputtering or vapor deposition is used when the inorganic material is used alone, and a usual coating method is used when the organic material is used. The thickness of the intermediate layer is appropriately 5 μm or less.

The protective layer 4 is provided for the purpose of protecting the surface of the photoreceptor, and the materials used for this are ABS resin and AC.
S resin, olefin-vinyl monomer copolymer, chlorinated polyether, allyl resin, polyacetal, polyamide, polyamideimide, polyacrylate, polyallylsulfone, polybutylene, polybutylene terephthalate, polycarbonate, polyethersulfone, polyethylene, polyethylene terephthalate, Polyimide, acrylic resin, polymethylpentene, polypropylene, polyphenylene oxide, polysulfone, polystyrene,
Examples thereof include AS resin, butadiene-styrene copolymer, polyurethane, polyvinyl chloride, epoxy resin, and the like, and among these, a cured product of a curable material and a curing agent.
In addition to the above, the protective layer may be a fluororesin such as polytetrafluoroethylene, a silicone resin, or a dispersion of an inorganic material such as titanium oxide, tin oxide, or potassium titanate in these resins for the purpose of improving wear resistance. It can be added. As a method for forming the protective layer, a usual coating method is adopted. It is suitable that the protective layer has a thickness of about 0.5 to 10 μm.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples.

The amount (parts) of each component used in the examples
Represents all parts by weight.

[0043]

【Example】

Example 1 On a polyethylene terephthalate film on which aluminum was vapor-deposited, a coating solution for charge generation layer 1 having the following composition, a coating solution for charge transport layer 2 and a coating solution for charge transport layer were sequentially applied and dried, respectively. A 2 μm-thick charge generation layer 1, a 0.1 μm-thick charge generation layer 2, and a 20 μm-thick charge transport layer were formed to prepare an electrophotographic photoreceptor of the present invention.

Charge Generating Layer 1 Coating Solution 5 parts of charge generating substance having the following structure

[0045]

[Chemical 1]

Polyvinyl butyral resin (Denka Butyral # 4000), Denki Kagaku Kogyo Co., Ltd. 2 parts, cyclohexanone 250 parts, 2-butanone 80 parts, charge generation layer 2 coating liquid, charge generation material having the following structure: 5 parts

[0047]

[Chemical 2]

Cyclohexanone 250 parts 2-butanone 80 parts Charge transport layer coating liquid 10 parts Charge transport material having the following structure

[0049]

[Chemical 3]

Polycarbonate (GE company: Lexan L-141) 10 parts Methylene chloride 100 parts Example 2 A polyethylene terephthalate film provided with a Hastelloy conductive layer was coated with an undercoat layer coating solution having the following composition and dried to generate a charge. The layer-forming coating solution 1 was applied, and the charge-generating layer coating solution 2 was applied by a spray method before it was dried to form the charge-generating layer according to claim 2 of the present invention. Further, a charge transport layer coating liquid is applied and dried thereon to form an undercoat layer of 0.3 μm, a charge generation layer of 0.2 μm, and a charge transport layer of 25 μm, to prepare an electrophotographic photoreceptor of the present invention. did.

Undercoat layer coating liquid Combustible nylon (Toray: Alamine CM8000) 3 parts Methanol 70 parts Butanol 30 parts Charge generating layer 1 coating liquid Charge generating substance with the following structure 3 parts

[0052]

[Chemical 4]

Polyester (Toyobo: Byron 200) 1 part Cyclohexanone 200 parts Tetrahydrofuran 200 parts Charge transport layer 2 coating liquid 5 parts charge transport material having the following structure

[0054]

[Chemical 5]

Polyester (Toyobo: Byron 200) 0.5 part Cyclohexanone 200 parts Tetrahydrofuran 200 parts Charge generation layer coating liquid 9 parts Charge generation substance having the following structure

[0056]

[Chemical 6]

Polycarbonate (Teijin Kasei: Panlite K-1300) 10 parts Tetrahydrofuran 150 parts Example 3 A charge generation layer 1 coating liquid having the following composition and charge generation on an Al plate (JIS 1080) having a thickness of 0.2 mm. Layer 2 coating liquid, charge generation layer 3 coating liquid, charge transport layer coating liquid are sequentially applied and dried to form 0.1 μm charge generation layer 1, 0.1 μm charge generation layer 2 and 0.05 μm, respectively. Charge generation layer 3, 22 μm
The charge transport layer of 1 was formed to prepare the electrophotographic photosensitive member of the present invention.

Charge Generating Layer 1 Coating Solution 1 part of the charge generating substance having the following structure

[0059]

[Chemical 7]

Polyvinyl butyral (UCC: XYHL) 1 part Cyclohexanone 500 parts Charge generation layer 2 coating liquid 2 parts charge generation substance having the following structure

[0061]

[Chemical 8]

Polyvinyl butyral (UCC: XYHL) 1 part Cyclohexanone 500 parts Charge generation layer 3 coating liquid 10 parts charge generation substance having the following structure

[0063]

[Chemical 9]

Polyvinyl butyral (UCC: XYHL) 1 part Cyclohexanone 500 parts Charge transport layer coating liquid 8 parts Charge transport material having the following structure

[0065]

[Chemical 10]

Polyarylate (Unitika: U polymer U-100) 10 parts Tetrahydrofuran 20 parts Example 4 A charge transport layer coating solution having the following composition and a charge generating layer 1 coating solution having the following composition on the same support as in Example 1. Charge generation layer 2 coating solution sequentially,
After coating and drying, 23μm charge generation layer, 0.1μm
A charge generation layer 1 having a thickness of m and a charge generation layer 2 having a thickness of 0.2 μm were formed to prepare an electrophotographic photoreceptor of the present invention.

Charge Transport Layer Coating Liquid 10 parts of charge generating substance having the following structure

[0068]

[Chemical 11]

Polycarbonate (A type, molecular weight 25000) 10 parts Methylene chloride 100 parts Charge generation layer 1 coating liquid Charge generation substance having the following structure 6 parts

[0070]

[Chemical 12]

Cyclohexanone 300 parts Tetrahydrofuran 100 parts Charge generation layer 2 coating liquid 3.5 parts of charge generation material having the following structure

[0072]

[Chemical 13]

Phenoxy resin (UCC: VYHH) 1 part Cyclohexanone 300 parts Tetrahydrofuran 100 parts Example 5 A charge generating layer 1 coating liquid, a charge generating layer 2 coating liquid having the following composition on a polyethylene terephthalate film vapor-deposited with aluminum, The charge transport layer 1 coating liquid and the charge transport layer 2 coating liquid are sequentially applied and dried to form 0.1 μm charge generation layer 1, 0.15 μm charge generation layer, 25 μm charge transport layer 1 and 3 μm charge, respectively. The transport layer 2 was formed to prepare the electrophotographic photosensitive member of the present invention.

5 parts of the charge generating substance having the following structure

[0075]

[Chemical 14]

Polyvinyl butyral resin (Sekisui Chemical Co., Ltd .: S-REC BL-1) 2 parts Cyclohexanone 200 parts 2-butanone 100 parts Charge generation layer 2 coating liquid 5 parts charge generation substance having the following structure

[0077]

[Chemical 15]

Polyvinyl butyral resin (Sekisui Chemical Co., Ltd .: S-REC BL-1) 0.5 part Cyclohexanone 200 parts 2-butanone 100 parts Charge transport layer 1 coating liquid 7 parts charge transport material having the following structure

[0079]

[Chemical 16]

Polycarbonate (Z type molecular weight 40,000) 10 parts Methylene chloride 150 parts Charge transport layer 2 coating liquid 0.5 part charge transport material having the following structure

[0081]

[Chemical 17]

Polycarbonate (Z-type molecular weight 40,000) 10 parts Methylene chloride 100 parts Comparative Example 1 The same procedure as in Example 1 was carried out except that only the charge generation layer 1 was used.

Comparative Example 2 Except that the charge generation layer in Example 2 was formed by using only the coating solution for the charge generation layer 1, the same preparation was carried out.

Comparative Example 3 The same procedure as in Example 3 was carried out except that only the charge generating layer 2 was used as the charge generating layer.

Comparative Example 4 Except that only the charge generation layer 2 was used as the charge generation layer in Example 4, the same preparation was carried out.

Comparative Example 5 Except that only the charge generation layer 1 was used as the charge generation layer in Example 5, the same preparation was carried out.

Comparative Example 6 The same procedure as in Example 5 was carried out except that the coating solution for the charge transport layer 1 in Example 5 was changed to the following composition.

Charge Transport Layer 1 Coating Solution 5 parts of charge transport material having the following structure

[0089]

[Chemical 18]

Polycarbonate (Z type molecular weight 40,000) 10 parts Methylene chloride 150 parts The photoconductors prepared as described above were measured for the characteristics of each photoconductor using an electrostatic copying paper tester (Kawaguchi Electric Co., Ltd .: SP-428 type). Was evaluated as.

First, -6.3 kV (or 6.3 kV)
V) discharge voltage, corona discharge for 20 seconds, and then dark decay to obtain surface potential of -800V or 800V.
When it became, it was irradiated with 4.5 lux of tungsten light.

At this time, the surface potential is -400 V or 4
Exposure required to reach 00V E ₄₀₀ (lux · se
c), the potential V ₃₀ (V) 30 seconds after the irradiation with the tungsten light was measured.

[0093]

[Table 1]

Comparative Example 7 The same procedure as in Example 1 was carried out except that the charge generating layer in Example 1 was formed as a single layer having a thickness of 0.3 μm by using only the charge generating layer coating liquid 2. ..

Comparative Example 8 The same procedure as in Example 3 was carried out except that the charge generating layer in Example 3 was formed as a single layer of 0.25 μm by using only the charge generating layer coating liquid 3. ..

The electrostatic characteristics of the photoconductors of Example 3 and Comparative Example 8 produced as described above were measured for 30 minutes under the following conditions after the electrostatic characteristics were measured by using an electrostatic copying paper test apparatus. Forced fatigue was performed, and the electrostatic characteristics after fatigue were measured in the same manner as above.

Discharge voltage: -6.3 kV Exposure intensity: 50 lux The results are shown in Table 2.

[0098]

[Table 2]

The photoconductors of Example 1 and Comparative Example 7 were joined to each other by belt bonding to prepare a photoconductor for mounting.

The photoconductors for both the photoconductors were copied to each other by the copying machine Myricopy M10.
Then, the running test was performed on 5000 sheets.

As a result, the photoconductor of Example 1 formed a normal image even after the 5000th sheet, but the photoconductor of Comparative Example 7 showed a decrease in image density due to a decrease in charging property.

[0102]

According to the present invention, a highly sensitive photosensitive member can be obtained.

A photoreceptor having a small residual potential can be obtained.

That is, it is possible to prevent background stains in the positive-positive phenomenon, image density reduction in negative-positive development, and image defects such as black streaks.

[Brief description of drawings]

[Figure 1]

[Fig. 2]

[Figure 3]

FIG. 4 is a schematic cross-sectional view showing the configuration of a specific example of the electrophotographic photosensitive member of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Conductive support 2 Multilayer charge generation layer 3 Charge transport layer 4 Protective layer 5 Intermediate layer

Claims (4)

[Claims] 1. An electrophotographic photosensitive member comprising at least a charge generation layer and a charge transport layer laminated on a conductive support, wherein the charge generation layer is a charge generation layer comprising at least two layers, and An electrophotographic photosensitive member characterized in that the concentration of the charge-generating substance in the charge-generating layer gradually increases from the side farthest from the charge-transporting layer toward the charge-transporting layer side. 2. An electrophotographic photosensitive member comprising at least a charge generation layer and a charge transport layer laminated on a conductive support, wherein the charge generation layer is a single layer, and the charge generation in the charge generation layer is An electrophotographic photosensitive member characterized in that the substance concentration is continuously changed, and is gradually increased from the side farthest from the charge transport layer toward the charge transport layer side. 3. The electrophotographic photosensitive member according to claim 1, wherein the concentration of the charge generating substance in the portion of the charge generating layer in contact with the charge transporting layer is 80% by weight or more. 4. The electrophotographic photosensitive member according to claim 1, wherein a concentration of the charge transport substance in a portion of the charge transport layer which is in contact with the charge generation layer is 40% by weight or more. ..