JPH0513512B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an electrophotographic photoreceptor, and more particularly to a laminated electrophotographic photoreceptor in which a photosensitive layer is functionally separated into a charge generation layer and a charge transport layer. [Prior Art] Laminated electrophotographic photoreceptors in which a photosensitive layer is functionally separated into a charge generation layer and a charge transport layer are known.
However, this type of laminated photoreceptor has not yet achieved sufficient sensitivity. The cause of this is that there are many carrier traps generated by exposure in the charge generating material and charge transporting material.
Possible reasons include that holes and electrons generated by light irradiation cannot move efficiently, and that carriers are not efficiently injected from the charge generation layer to the charge transport layer. Sensitivity is one of the most important characteristics of a photoreceptor, but in the case of the aforementioned laminated electrophotographic photoreceptor, the sensitivity is generally determined by: (1) the amount of light that reaches the charge generation layer (light intensity); , extinction coefficient), (2) Carrier generation efficiency in the charge generation layer (carrier generation quantum efficiency), (3) Carrier injection efficiency from the charge generation layer to the charge transport layer (ionization potential, oxidation potential, etc.), (4 ) It is qualitatively expressed as a comprehensive evaluation of the efficiency of carrier movement in the charge transport layer (drift mobility).
Therefore, in order to improve sensitivity, it is necessary to increase the intensity of light irradiated to the photoreceptor, the extinction coefficient and quantum efficiency of the charge generating material, the carrier injection efficiency, the drift mobility in the charge transport layer, etc. It is.
Assuming that the intensity of irradiated light is constant, it is unlikely that organic photoconductive compounds known so far can be expected to dramatically increase the extinction coefficient or quantum efficiency. On the other hand, the hole mobility of various organic photoconductive compounds has been measured using a charge transporting material alone or a system in which it is molecularly dispersed in an insulating polymer, and the values vary depending on the measurer. However, 10 ^-9 to 10 ^-4 cm ² /V・sec
The mobility of organic photoconductive materials generally used in electrophotography is 10 ^-7 to 10 ^-6.
The current value is approximately cm ² /V·sec, and no organic material with a value higher than this is known yet. Therefore, the present inventors focused on the carrier injection efficiency from the charge generation layer to the charge transport layer as one of the factors that can improve the sensitivity, and as a result of measuring the oxidation potential of many organic materials, the sensitivity It was discovered that there is a correlation between the charge transport material and the oxidation potential, and that sensitivity can be improved by optimizing the oxidation potential of the charge transport material. It has been reported that there is a correlation between the effective injection of carriers generated in the charge generation layer into the charge transport layer and the ionization potential of the charge transport material. For example, IEEE Trans. IA-17
Volume, page 382 (published in 1981). Ionization potential, considered the most important factor in carrier injection efficiency, is measured in a variety of ways. For example, methods using mass spectroscopy, methods using photoelectron spectroscopy, creating charge transfer complexes,
There are methods that use the absorption spectrum, methods that measure the oxidation potential as a substitute physical property value, and methods that calculate using the molecular orbital method. However, the methods described above have so far only been able to determine the ionization potential of relatively low-molecular compounds such as charge-transporting materials, and the ionization potential of macromolecules such as pigments that are commonly used as charge-generating materials has not been determined. As far as the inventors know, there are no reports on experimental or calculated values of . The reason for this is thought to be that the pigment exhibits characteristics as a molecular assembly (aggregate) rather than as a single molecule dispersed state, which poses an experimental problem. Furthermore, since pigments are generally large molecules, it is thought that there is a limit to the computational time required by a computer when performing molecular orbital calculations. Therefore, the present inventors focused on a pigment soluble in a solvent as a charge-generating material, and measured the oxidation potential in the solution state. Furthermore, the oxidation potential of commonly used charge transport materials can also be measured in a solution state.
Since ionization potential and oxidation potential are generally considered to be in a proportional relationship, it is thought that oxidation potential can be a substitute characteristic for ionization potential. Therefore, the present invention was achieved by measuring the oxidation potential of the charge-generating material and the charge-transporting material and the sensitivity of a laminated photoreceptor using them, and finding the optimal combination of the charge-generating material and the charge-transporting material. . [Objective and Summary of the Invention] An object of the present invention is to provide an electrophotographic photoreceptor with outstandingly high sensitivity by using a newly selected combination of a charge-generating material and a charge-transporting material. There is a particular thing. The above object is to have a charge generation layer containing an azulene compound having the following structural formula [] and a charge transport layer containing a hydrazone compound having an oxidation potential in the range of 0.55 to 0.70 volts on a conductive support. This is achieved by the laminated electrophotographic photoreceptor of the present invention, which is characterized by the following. The azulene compound represented by the above structural formula [] has panchromatic sensitivity characteristics over a wide wavelength range of 400 nm to 900 nm. [Specific Description and Examples of the Invention] The oxidation potential used in the present invention is measured using acetonitrile as a solvent, tetraethylammonium perchlorate as a supporting electrolyte, and a saturated calomel electrode as an electrode. 1st
The peak value of the oxidation wave (E ^ox ) was used. Conventionally, it has been confirmed that there is a strong correlation between sensitivity and the ionization potential of the charge transport material, and it is believed that the smaller the ionization potential of the charge transport material, the higher the sensitivity. However, by measuring the oxidation potential of not only the charge-transporting material but also the charge-generating material, as was done in the present invention, it is possible to obtain high sensitivity only when the oxidation potential of both materials falls within a certain range of values. found. First, the oxidation potential of the azulene compound of the structural formula [], which is a charge-generating material, is 0.84V. On the other hand, it has been found that the oxidation potential of the hydrazone compound, which is a charge transporting material, must be in the range of 0.55 to 0.70V. As will be specifically described in Examples, it has been found that when the voltage is smaller than 0.55V, the dark decay increases, and when the voltage is larger than 0.70V, the residual potential increases, resulting in low sensitivity. From the above results, in order to obtain a highly sensitive laminated photoreceptor, the above-mentioned optimum range exists for the oxidation potential values of the charge generating material and the charge transporting material. Only when this condition is met, holes generated in the charge generation layer by light irradiation can be efficiently injected into the charge transport layer without being affected by the energy barrier at the interface between the charge generation layer and the charge transport layer. This is a reasonable result. The oxidation potential used in the charge transport layer in the present invention is 0.55.
Hydrazone compounds in the range of ~0.70 volts are
In addition to those used in the Examples described below, examples include compounds having the compound numbers, structural formulas, and oxidation potentials shown below. The charge generation layer used in the present invention is formed by dispersing the azulene compound having the above structural formula as the charge generation material used in the present invention in a suitable binder, and coating this on a substrate. It can also be obtained by forming a vapor deposited film using a vacuum evaporation apparatus. Binders that can be used to form the charge generating layer by coating can be selected from a wide range of insulating resins, and organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene, and polyvinylpyrene. You can choose. Preferably, polyvinyl butyral, polyarylate (condensation polymer of bisphenol A and phthalic acid, etc.), polycarbonate, polyester, phenoxy resin, polyvinyl acetate, acrylic resin, polyacrylamide resin, polyamide, polyvinylpyridine, cellulose resin, urethane Examples include insulating resins such as resin, epoxy resin, casein, polyvinyl alcohol, and polyvinylpyrrolidone. The resin contained in the charge generation layer is 80
Weight % or less, preferably 40 weight % or less is suitable. Organic solvents used during coating include alcohols such as methanol, ethanol and isopropanol, ketones such as acetone, methyl ethyl ketone and cyclohexanone, amides such as N,N-dimethylformamide and N,N-dimethylacetamide, dimethyl Sulfoxides such as sulfoxide, tetrahydrofuran, dioxane,
Ethers such as ethylene glycol monomethyl ether, esters such as methyl acetate and ethyl acetate, aliphatic halogenated hydrocarbons such as chloroform, methylene chloride, dichloroethylene, carbon tetrachloride, trichlorethylene, or benzene,
Aromatics such as toluene, xylene, ligroin, monochlorobenzene, dichlorobenzene, etc. can be used. Coating can be carried out using coating methods such as dip coating, spray coating, spinner coating, bead coating, Meyer bar coating, blade coating, roller coating, and curtain coating. The charge generation layer contains as much of the organic photoconductor as possible in order to obtain sufficient absorbance and is preferably a thin film layer, for example less than 5 microns, in order to shorten the range of the generated charge carriers. teeth
A thin film layer having a thickness of 0.01 micron to 1 micron is preferable. This means that most of the incident light is absorbed by the charge generation layer, generating many charge carriers, and that the generated charge carriers are not deactivated by recombination or trapping, but are transferred to the charge transport layer. This is due to the need for injection. The charge transport layer used in the present invention can be formed by forming a film using one or more of the charge transport materials used in the present invention. When the charge transport material does not have film-forming properties,
A film can be formed by selecting an appropriate binder. Resins that can be used as binders are:
For example, insulating resins such as acrylic resin polyarylate, polyester, polycarbonate, polystyrene, acrylonitrile-styrene copolymer, acrylonitrile-butadiene, copolymer, polyvinyl butyral, polyvinyl formal, polysulfone, polyacrylamide, polyamide, chlorinated rubber, or poly-N- Organic photoconductive polymers such as vinylcarbazole, polyvinylanthracene, polyvinylpyrene and the like may be mentioned. Since the charge transport layer has a limit in its ability to transport charge carriers, it cannot be made thicker than necessary. Typically it is between 5 microns and 30 microns, with a preferred range between 8 microns and 20 microns. When forming the charge transport layer by coating, an appropriate coating method as described above can be used. A photosensitive layer having such a laminated structure of a charge generation layer and a charge transport layer in an arbitrary layer order is provided on a conductive support comprising, for example, a base having a conductive layer. Examples of the substrate having a conductive layer include those whose substrate itself is conductive, such as aluminum, aluminum alloy, copper, zinc, stainless steel, vanadium, molybdenum, chromium, titanium, nickel, indium,
Gold, platinum, etc. can be used, as well as aluminum, aluminum alloy, indium oxide,
Plastics (e.g., polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, acrylic resin, polyethylene fluoride, etc.) having a layer formed by vacuum evaporation of tin oxide, indium oxide-tin oxide alloy, etc.
A substrate in which conductive particles (e.g., carbon black, silver particles, etc.) are coated on plastic together with a suitable binder, a substrate in which plastic or paper is impregnated with conductive particles, a plastic containing a conductive polymer, etc. can be used. . A subbing layer having barrier and adhesive functions can also be provided between the conductive layer and the photosensitive layer. The undercoat layer is made of casein, polyvinyl alcohol, nitrocellulose, ethylene-acrylic acid copolymer, polyamide (nylon 6, nylon 66, nylon
610, copolymerized nylon, alkoxymethylated nylon, etc.), polyurethane, gelatin, aluminum oxide, etc. The thickness of the undercoat layer is 0.1 micron to 5 micron.
Preferably, 0.5 micron to 3 micron is appropriate. The electrophotographic photoreceptor provided by the present invention can be used not only in electrophotographic copying machines, but also in a wide range of electrophotographic applications such as laser printers and CRT printers. Further, the charge generating material used in the present invention can also be used in solar cells and optical sensors in addition to the above-mentioned electrophotographic photoreceptor. Solar cells can be prepared, for example, by sandwiching the aforementioned organic photoconductors with indium oxide and aluminum. Next, the present invention will be explained with reference to Examples, but the present invention is not limited thereto in any way. Test Examples 1 to 16 5 g of the azulene compound as the charge generating material used in this example and 2 g of butyral resin (degree of butyralization 63 mol%) were mixed with 95 ml of isopropyl alcohol.
After dispersing it in a sand mill with a solution dissolved in , it was coated on an aluminum sheet to form a charge generation layer with a dry film thickness of 0.2 microns. Next, various charge transport material compounds used in this example
The structural formulas and oxidation potentials of Nos. (1) to (8) are shown below. Therefore, 5 g of any one of the charge transport materials of Compounds Nos. (1) to (8) above and a styrene-acrylic resin (trade name: Nippon Steel Chemical MS-
A charge transport layer was formed by coating a 20% by weight solution of monochlorobenzene (200) in a weight ratio of 0.95:1 on the charge generation layer so that the film thickness after drying was 16 microns. . In this way, eight types of negatively charged laminated photoreceptors were created. Subsequently, a charge transport layer was first formed on the aluminum sheet, and then a charge generation layer was laminated on the charge transport layer to produce eight types of positive charging laminated photoreceptors. In the manner described above, 16 types of laminated electrophotographic photoreceptors were produced. The 16 types of electrophotographic photoreceptors prepared in this way were dynamically corona charged using an electrostatic copying paper testing device Model SP-428 manufactured by Kawaguchi Electric Co., Ltd., and after being held in a dark place for 1 second, the illuminance was
It was exposed to light at 5 lux for 4 seconds and its charging characteristics were examined. As for the charging characteristics, the surface potential and the exposure amount (E1/2) required to attenuate the potential (V ₀ =±600 volts) to 1/2 when dark decayed for 1 second were measured. or,
The residual potential (V _R ) after 15 lux·sec exposure was also measured.
The results of the photoreceptor obtained by the above method are summarized in Table 1. In Table 1, the numbers in brackets indicate V _R (volts).

[Table] In this table, the range surrounded by a thick frame 〓〓 is the characteristic of the laminated electrophotographic photoreceptor of the present invention, and 〓 indicates that the dark decay is large (200 V or more).
The results revealed that the multilayer electrophotographic photoreceptor of the present invention, in which the oxidation potential of the hydrazone compound of the charge transport material is in the range of 0.55 to 0.70 V, has low dark decay and high sensitivity. Moreover, this is true;
It can be seen that this applies to both negative and laminated photoreceptors. [Effects of the Invention] According to the present invention, by optimizing the combination of a charge generating material and a charge transporting material, an electrophotographic photoreceptor with higher sensitivity can be provided even in a laminated type photoreceptor.

Claims (1)

[Claims] 1. A charge generation layer containing an azulene compound having the following structural formula [] on a conductive support, and a charge transport layer containing a hydrazone compound having an oxidation potential in the range of 0.55 to 0.70 volts. A laminated electrophotographic photoreceptor comprising: