JPH0513510B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to an electrophotographic photoreceptor. [Prior Art] Various 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, in this type of photoreceptor, sufficient sensitivity has not yet been obtained. 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 functionally separated photoreceptors, 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) Carrier movement in the charge transport layer It is expressed qualitatively as a comprehensive evaluation such as efficiency (drift mobility).
Therefore, in order to improve sensitivity, it is necessary to increase the intensity of the 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. . now,
Assuming the intensity of irradiated light is constant, it is thought that it is unlikely that organic photoconductive compounds known so far can 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, the mobility ranges over a wide range of 10 ^-9 to 10 ^-4 cm ² /V.sec, and the mobility of organic photoconductive materials generally used in electrophotography is 10 ^-7 to ^{10 -6} cm ² /V.sec.
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 carrier injection efficiency 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 spectroscopy, 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. 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 macromolecules, it is thought that there is a limit to the computer's calculation time when performing molecular orbital calculations. Therefore, the present inventors focused on various cyanine dyes that are used as charge-generating materials and sensitizers for silver salts. Unlike pigments, dyes are soluble in solvents, making it possible to measure oxidation potential in a solution state. Furthermore, commonly used charge transport materials also include
It is possible to measure oxidation potential 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. Furthermore, the present inventors have noticed that it is effective to include a charge transport material in the charge generation layer in order to efficiently move optical carriers generated in the charge generation layer. Based on the above studies, we measured the oxidation potential of the charge transport material and the sensitivity of the laminated photoreceptor for the cyanine dye, which is the charge generation material, and found the optimal combination of the charge generation material and the charge transport material. This has led to the present invention. [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 containing a cyanine dye and a charge transporting material and a charge transporting layer containing a charge transporting material on a conductive support, the charge transporting layer containing a charge transporting material. The oxidation potential of the cyanine dye contained in the charge transport layer is in the range of 0.35 to 0.50 volts, the oxidation potential of the charge transport material contained in the charge transport layer is in the range of 0.60 volts or less, and the charge transport material contained in the charge generation layer is This is achieved by the electrophotographic photoreceptor of the present invention, which has an oxidation potential greater than that of the charge transport material contained in the charge transport layer. [Specific Description and Examples of the Invention] First, the oxidation potential of the charge transport material (referred to as CT-1) contained in the charge generation layer is higher than that of the charge transport material (referred to as CT-2) in the charge transport layer. The mechanism by which the characteristics of the photoreceptor are improved when the oxidation potential is higher than the oxidation potential of .) will be explained. This mechanism can be considered as follows. Of the photocarriers generated in the charge generation layer, electrons move between cyanine dyes, while holes move between CT-1 molecules. When holes are injected from the charge generation layer into the charge transport layer, the oxidation potential of CT-1 increases.
It seems reasonable that holes can be injected efficiently only when the oxidation potential is higher than the oxidation potential of CT-2. If the above relationship is not satisfied, the sensitivity will be reduced as will be described in detail later in Examples. This is thought to be due to the fact that the holes act as an energy barrier. From the above considerations, it has been found that it is necessary to include a charge transport material with a higher oxidation potential than CT-2 in the charge generation layer, and by this, the layer thickness of the charge generation layer can be reduced. For example, even when the thickness is 1 micron or more, carrier traps in the charge generation layer can be reduced. Therefore, in the case of a positively charged photoreceptor in which a charge generation layer is laminated on a charge transport layer, the charge generation layer can be applied thickly. Next, regarding cyanine dye, which is a charge generating material, it is necessary that the oxidation potential is in the range of 0.35 to 0.50 volts. When the oxidation potential is lower than 0.35 volts, the dark decay of the laminated photoreceptor increases 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
It turned out that it needed to be in the range of 0.60 volts or less. It has been found that when the oxidation potential is greater than 0.60V, 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 has been 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 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, and the value of the oxidation potential is expressed as the peak value of the first oxidation wave (E ^ox ). was used. The oxidation potential used for the charge generation layer in the present invention is 0.35.
Cyanine dyes in the range of ~0.50 volts include, for example, compounds having the compound numbers, structural formulas, and oxidation potentials shown below. Next, in the present invention, a charge transport material (CT
-2) includes, for example, the compounds shown below. On the other hand, the charge transport material (CT-1) to be contained in the charge generation layer in the present invention is a charge transport material CT-1.
One can choose from a wide range of charge transport materials with oxidation potentials greater than 2. As long as the above relationship is satisfied, compounds selected from the group of compounds exemplified in the case of CT-2 can be used, and furthermore, compounds No., structural formula, and oxidation potential shown below that have an oxidation potential of 0.6V or more can be used. Compounds such as these can also be mentioned. Note that the charge transport materials CT1 and CT-2 are as follows:
It is also possible to use the compound groups used in the Examples described below. The charge generation layer used in the present invention includes one or more cyanine dyes as the charge generation substance used in the present invention and one or more charge transport substances CT-1.
It can be formed by dispersing the above seeds in a suitable binder and coating it on a substrate, or it can 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 variety of insulating resins, as well as 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, fatty acid 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 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 CT-2 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 themselves conductive can be used, such as aluminum, aluminum alloy, copper, zinc, stainless steel, vanadium, molybdenum, chromium, titanium, nickel, indium, gold, and platinum. 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 undercoat layer is made of casein, polyvinyl alcohol, nitrocellulose, ethylene-acrylic acid copolymer polyamide (nylon 6, nylon 66, nylon 610,
It can be formed from 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 widely used not only in electrophotographic copying machines but also in electrophotographic applications such as laser printers and CRT printers. Further, the organic photoconductor of 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 64 Eight types of cyanine dyes as charge generating materials according to the present invention and eight types of charge transporting 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, compounds No. (64) to (71) exemplified below
A 20% monochlorobenzene solution containing 5 g of each of the charge transport materials and a styrene-acrylic resin (trade name: Nippon Steel Chemical MS-200) as a binder at a weight ratio of 0.95:1 was added to the charge generating material. A charge transport layer was formed by coating on the layer so that the film thickness after drying was 16 microns. The 64 types of electrophotographic photoreceptors prepared in this way were tested using an electrostatic photographic paper testing device Model SP- manufactured by Kawaguchi Electric Co., Ltd.
428 using the dynamic method, hold it for 1 second in a dark place, and then charge it with 4
It was exposed to light for a second and the charging characteristics were examined. As for the charging characteristics, we measured the surface potential and the amount of exposure (E1/2) (lux sec) required to attenuate the potential (V ₀ = 600 volts) to 1/2 when dark decayed for 1 second. . Also, residual potential after 15lux・sec exposure (V _R )
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 preferred range for reference in the present invention, 〓 means that the dark decay is large (200 volts or more), and - means that there is no sensitivity. It represents. From the results in Table 1, as shown by the thick line frame,
The oxidation potential of the cyanine dye of the charge generating material is 0.35~
The oxidation potential of the charge transport material in the charge transport layer is within the range of 0.50V and is within the range of 0.60V.The laminated photoreceptor using a combination of each compound has a low residual potential and high sensitivity. I understand. Next, in the laminated photoreceptor of Test Example 1-64, Compound No. (66) was used as the charge transport material in the charge transport layer.
An experiment containing 1) was conducted. For example, CT
Using Compound No. (66) as -1 and Compounds No. (1) to (8) as cyanine dyes, the weight ratio of the compounds constituting the charge generation layer (various cyanine dyes): (CT-1):
(Butyral resin) = 3:3:2 was applied onto an aluminum sheet to form a charge generation layer having a film thickness of 0.2 microns after drying. Next, charge transport material (CT-2) of compound No. (66)
5g and styrene-acrylic resin (trade name Nippon Steel Chemical MS-200) as a binder.
A charge transport layer was formed by coating a 20% by weight solution of monochlorobenzene mixed at a weight ratio of 0.95:1 on the charge generation layer so that the film thickness after drying was 16 microns, and then using the method described above. Photoconductor characteristics E1/2
(lux·sec) and V _R (volts) were measured, and the results shown in Table 2 were obtained.

[Table] In this table, the range surrounded by a thick frame is the preferred range for reference in the present invention, 〓 means that the dark decay is large (200 volts or more), and - means that there is no sensitivity. It represents. By comparing Table 2 and Table 1, we can see that the photoreceptors that were highly sensitive in Table 1 were also highly sensitive in Table 2, and the photoreceptors that were low in sensitivity in Table 1 were shown in Table 2. However, it can be seen that the sensitivity is low. Examples using materials other than Compound No. (66) as CT-1 and CT-2 will be described later, but from the above results, the charge-transporting material (CT-1) is included in the charge generation layer. Even in this case, the oxidation potential of the cyanine dye used as the charge-generating material is 0.35~
It can be seen that the sensitivity is high according to the range of 0.50V. Test Examples 73-121 A photoreceptor was prepared by combining seven types of compounds selected from the above compound Nos. (64)-(67) and (69)-(71) as charge transport materials CT-1 and CT-2, respectively. Created. Note that cyanine dye of Compound No. (4) was used as the charge generating material. First, 5 g of cyanine dye of Compound No. (4), 2 g of butyral resin (degree of butyralization 63 mol%), and various charge transport materials of Compound No. (64) to (67), (69) to (71), or 5g of each type with isopropyl alcohol 95
After sand mill dispersion with the solution dissolved in ml,
It was coated onto an aluminum sheet to form a charge generation layer with a dry film thickness of 0.2 microns. Next, 5 g of any one of Compound No. (64) to (67), (69) to (71) and a styrene-acrylic resin as a binder (product name: Nippon Steel Chemical MS-200) A charge transport layer was formed by coating a 20% by weight solution of monochlorobenzene, which was obtained by mixing the above and the like in a weight ratio of 0.95:1, on the charge generation layer so that the film thickness after drying was 16 microns. As described above, 7 types of charge transport materials (CT-1) were used in the charge generation layer, 7 types of charge transport materials (CT-2) were used in the charge transport layer, and 49 types of these were combined. A laminated photoreceptor was created. Regarding the photoreceptor thus prepared, E1/2 (lux・
sec) and V _R (volts) were measured, and the results are summarized in Table 3.

【table】

〔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. An electrophotographic photoreceptor having a charge generation layer containing a cyanine dye and a charge transport material and a charge transport layer containing the charge transport material on a conductive support, the cyanine dye contained in the charge generation layer The oxidation potential of the charge transport material is in the range of 0.35 to 0.50 volts, and the oxidation potential of the charge transport material contained in the charge transport layer is 0.60 volts.
volt or less, and the oxidation potential of the charge transport material contained in the charge generation layer is greater than the oxidation potential of the charge transport material contained in the charge transport layer. body.