JPH0513509B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to an electrophotographic photoreceptor. [Prior Art] Various laminated electrophotographic photoreceptors (hereinafter sometimes referred to as laminated photoreceptors) in which a photosensitive layer is functionally separated into a charge generation layer and a charge transport layer are known. However, sufficient sensitivity has not yet been obtained in this type of photoreceptor. In other words, the reason why the sensitivity of the laminated photoreceptor does not improve is that there are many carrier traps generated by exposure in the charge generation material and charge transport material, and the holes and electrons generated by light irradiation are not efficiently used. It is believed that carriers cannot move well 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 a functionally separated 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) Charge transport layer It is qualitatively expressed as a comprehensive evaluation of the efficiency with which the carrier moves through the vehicle (drift mobility). Therefore, in order to improve sensitivity, it is necessary to increase the intensity of the light irradiated to the photoreceptor, the absorption coefficient and quantum efficiency of the charge generating material, the carrier injection efficiency, the drift mobility in the charge transport layer, etc. be. Now, if the intensity of irradiated light is constant, it is unlikely that the organic photoconductive compounds known so far will be able to dramatically increase the extinction coefficient or quantum efficiency. There is. 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. There is a range of 10 ^-9 to 10 ^-4 cm ² /
The mobility of organic photoconductive materials generally used in electrophotography is 10 ^-7 .
~10 ^-6 cm ² /V·sec, and at present no organic photoconductive material with a value higher than this has yet been found. Therefore, the present inventors focused on the fact that the efficiency of carrier injection from the charge generation layer to the charge transport layer is an important factor that affects sensitivity, and as a result of measuring the oxidation potential of many organic materials, the sensitivity It was found that there is a correlation between the oxidation potential and the oxidation potential. By the way, there is a report 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, in IEEE Trans magazine, IA-17. roll,
It is described on 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, there are methods using mass spectra, methods using photoelectron spectroscopy, methods that create charge transfer complexes and use their absorption spectra, methods that measure oxidation potential as a substitute physical property value, and methods that calculate using molecular orbital method. be. 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. The experimental value of
Alternatively, it can be said that, as far as the present invention is aware, there are no reports regarding calculated values. The reason for this is that the pigment is 1
It is conceivable that the fact that properties appear as a molecular assembly (aggregate) rather than in a molecularly dispersed state may be an experimental problem. Furthermore, since pigments are generally large molecules, it is thought that there are limitations such as computational time on the computer when performing molecular orbital calculations. Therefore, the present inventors focused on various cyanine dyes that are used as sensitizers for silver salts as charge-generating materials. Unlike pigments, dyes are soluble in solvents, making it possible to measure oxidation potential in a 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, we measured the oxidation potential of the cyanine dye, which is a charge-generating material, and the charge-transporting material, and the sensitivity of the laminated photoreceptor.
As a result, the optimum combination of a charge generating material and a charge transporting material was found, and the present invention was achieved. [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 an electrophotographic photoreceptor having a charge generation layer and a charge transport layer on a conductive support, wherein the charge generation layer comprises a layer containing a cyanine dye having an oxidation potential of 0.35 to 0.50 volts as a charge generation material. is achieved by the electrophotographic photoreceptor of the present invention, wherein the charge transport layer comprises a layer containing a charge transport material having an oxidation potential of 0.70 volts or less. [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. The peak value of 1-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 was found that the sensitivity is high only when the oxidation potential of both materials falls within a certain range of values. . First, it was found that the oxidation potential of cyanine dye, which is a charge-generating material, must be in the range of 0.35 to 0.50 volts. Oxidation potential is 0.35
If it is smaller than volt, the dark decay of the laminated photoreceptor will increase regardless of the type of charge transport material. Furthermore, it was found that when the oxidation potential of the cyanine dye exceeds 0.50 volts, the sensitivity of the laminated photoreceptor decreases significantly, regardless of the type of charge transport material. This is thought to be because when the oxidation potential of the cyanine dye increases, the dye itself absorbs in the ultraviolet region and no longer has sensitivity in the visible region. On the other hand, for charge transport materials, the oxidation potential is 0.70
It turns out that it needs to be in the range below volts. It has been found that when the oxidation potential is greater than 0.70 volts, the sensitivity of the laminated photoreceptor deteriorates significantly, regardless of the type of cyanine dye used as the charge-generating material. From the above results, it was found that in order to obtain a highly sensitive multilayer photoreceptor, there is an optimum range for the oxidation potential values of the charge generating material and the charge transporting material as described above. 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. Based on the above experimental results and considerations, the present invention has revealed for the first time a correlation between the combination of a charge generating material and a charge transporting material and sensitivity. The oxidation potential used for the charge generation layer in the present invention is 0.35.
Examples of cyanine dyes in the range of ~0.50 volt include compounds having the following compound numbers, structural formulas, and oxidation potentials. Next, in the present invention, the oxidation potential used for the charge transport layer is
Charge transport materials in the range below 0.70V include:
For example, the compounds shown below can be mentioned. The charge generation layer used in the present invention can be formed by dispersing one or more cyanine dyes as the charge generation substance used in the present invention in a suitable binder and coating the mixture. It can also be obtained by forming a vapor deposited film using a vacuum evaporation device. Binders that can be used to form the charge generating layer by coating can be selected from a wide range of insulating resins, as well as organic photoconductive polymers such as poly-N-vinylcarbazole, polyvinylanthracene, and polyvinylpyrene. You can choose from. 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 suitably 80% by weight or less, preferably 40% by weight or less.
Organic solvents used during coating include alcohols such as methanol, ethanol, and isopropanol, ketones such as acetone, methyl ethyl ketone, and cycloexanone, and amides such as N,N-dimethylformamide and N,N-dimethylacetamide. , sulfoxides such as dimethyl sulfoxide, ethers such as tetrahydrofuran, dioxane, and ethylene glycol monomethyl ether, esters such as methyl acetate and ethyl acetate, and aliphatics such as chloroform, methylene chloride, dichloroethylene, carbon tetrachloride, and trichloroethylene. Halogenated hydrocarbons or aromatics such as benzene, 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 and generates a large number of charge carriers, and that the generated charge carriers are not deactivated by recombination or trapping, and the charge transport layer This is due to the need to inject. The charge transport layer used in the present invention can be formed by forming a film using one or more of the charge transport substances 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.
As the substrate having the conductive layer, materials that are conductive themselves such as aluminum, aluminum alloy, copper, zinc, stainless steel, panadium, molybdenum, chromium, titanium, nickel, indium, gold, and platinum can be used. In addition, plastics (e.g., polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, (acrylic resin, polyethylene fluoride, 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 subbing 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 and the charge transporting material used in the present invention can be used not only in the above-mentioned electrophotographic photoreceptor but also in solar cells and optical sensors. Solar cells can be prepared, for example, by sandwiching the aforementioned organic photoconductors with indium oxide and aluminum. EXAMPLES Next, the present invention will be explained with reference to Examples, but the present invention is not limited to these in any way. Examples 1 to 64 Eight types of cyanine dyes as charge generating materials according to the present invention and eight types of charge transport materials according to the present invention exemplified above were used in combination with each other, and 64 types were obtained. An electrophotographic photoreceptor was fabricated. That is, first, 5 g of each of the cyanine dyes of Compound No. A charge generation layer was formed by coating on a sheet and having a thickness of 0.1 micron after drying. Next, 5 g of each of the charge transport materials of Compounds Nos. (18) to (25) exemplified above and a styrene-acrylic resin (trade name: Nippon Steel Chemical MS) as a binder were added.
-200) at a weight ratio of 0.95:1, and a 20% by weight solution of monochlorobenzene is applied onto the charge generation layer to a dry film thickness of 16 microns to form a charge transport layer. did. The 64 types of electrophotographic photoreceptors created in this way were tested using an electrostatic copying paper tester Model SP- manufactured by Kawaguchi Electric Co., Ltd.
428 using a dynamic method to charge a negative corona, hold it for 1 second in a dark place, and then
It was exposed to light for 4 seconds at lux, and its charging characteristics were examined. As for the charging characteristics, measure the surface position and the exposure amount (E _1/2 ) (lux・sec) required to attenuate the potential (V _O = 600 volts) to 1/2 when dark decayed for 1 second. did. The residual potential (V _R ) after exposure at 1.5 lux·sec was also measured. The results of the photoreceptor obtained by the above method are summarized in Table 1. The numbers in [ ] in Table 1 indicate V _R (volts).

[Table] In this table, the area surrounded by a thick frame is the electrophotographic photoreceptor within the scope of the present invention, * indicates large dark decay (200 volts or more), and - indicates no sensitivity. It represents. From the results in Table 1, the oxidation potential of the cyanine dye of the charge generating material is in the range of 0.35 to 0.50 volts,
It can be seen that the electrophotographic photoreceptor of the present invention using a combination of the respective compounds in which the oxidation potential of the charge transport material is in a range exceeding 0.70 volts has a low residual potential and high sensitivity. Example 65 The charge-generating material used in this example was the above-mentioned exemplified compound No. (4), and the charge-transporting material was the above-mentioned exemplified compound No. (20), and was photosensitive in the same manner as in Examples 1 to 64. Created a body. However, in this example, a charge transport layer was first formed on an aluminum sheet, and then a charge generation layer was laminated on the charge transport layer, and the positive corona charging characteristics were measured. The results are shown in Table 2.

[Table] Table 2 also shows the characteristics of the charged photoreceptors made of the same material combinations shown in Examples 1 to 64. The results of this example show that regardless of the order in which the charge generation layer and the charge transport layer are stacked, a highly sensitive photoreceptor can be obtained by using a combination of materials that meet the scope of the claims. Example 66 5 g of the cyanine dye of Exemplified Compound No. (9) and 2 g of butyral resin (degree of butyralization 63 mol%) were dispersed in a sand mill with a solution of 95 ml of isopropyl alcohol, then coated on an aluminum sheet and dried. A charge generation layer with a subsequent thickness of 0.2 microns was formed. Next, a solution prepared by dissolving 5 g of the charge transport material of Exemplary Compound No. (26) and 5 g of polyarylate resin (condensation polymer of bisphenol A and terephthalic acid-isophthalic acid) in 70 ml of tetrahydrofuran was dried on the charge generating layer. It was coated to a final film thickness of 15 microns and dried to form a charge transport layer. Example 1 Charging characteristics of the photoreceptor thus prepared
~64 was measured using the same method as above, and the following results were obtained.

〔Effect 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. In an electrophotographic photoreceptor having a charge generation layer and a charge transport layer on a conductive support, the charge generation layer is composed of a layer containing a cyanine dye having an oxidation potential of 0.35 to 0.50 volts as a charge generation material, and An electrophotographic photoreceptor, wherein the transport layer comprises a layer containing a charge transport material having an oxidation potential of 0.70 volts or less.