Layered Electrophotographic Photosensitive Element Having Hydrazone Charge Transport Material

1. Field of the Invention

The present invention relates to an electrophotographic photosensitive element having on a conductive support thereof a photosensitive layer composed in combination of a carrier-generating phase with a carrier-transporting phase.

2. Description of the Prior Art

There have been widely known electrophotographic photosensitive elements which have a photosensitive layer comprised principally of such inorganic photoconductive materials as selenium, zinc oxide, cadmium sulfide and the like. However, these elements cannot necessarily be satisfactory in respect of the stability to heat and durability, and besides have problems in the handling and the manufacture due to their toxic character.

On the other hand, electrophotographic photosensitive elements having a photosensitive layer comprised principally of organic photoconductive compounds have in recent years focused the spotlight of attention upon such various advantages thereof as the relative ease of manufacture, inexpensiveness, ease of handling and the like, and in addition they are generally superior in the stability to heat to those having selenium as the sensitive element thereof. Among such organic photoconductive compounds, poly-N-vinyl carbazole is best known. There has already been made practical reality an electrophotographic photosensitive element having a photosensitive layer comprised mainly of a charge-transfer complex formed from said poly-N-vinyl carbazole and a Lewis acid such as 2,4,7-trinitro-9-fluorenone, while there has been known an electrophotographic photosensitive element having a function-separated photosensitive layer of either the multilayered type or dispersed type constituted so that carrier-generating function and carrier-transporting function may be performed in different material phases respectively; for example, an electrophotographic photosensitive element having a photosensitive layer composed in combination of a carrier-generating layer of amorphous selenium thin layer with a carrier-transporting layer of poly-N-vinyl carbazole is now available for practical use.

However, a layer comprising poly-N-vinyl carbazole has so little flexibility that said layer is hard and fragile, tending to cause cracks and peeling off of the layer, thereby causing the photosensitive element to become less durable, while when a plasticizer is added to improve such defects, the residual electric potential at the time when provided in the electrophotographic process becomes high, and as the element is repeatedly used, the residual electric potential becomes accumulated, thereby gradually producing fog on the reproduced image.

In addition, low molecular weight organic photoconductive compounds are desirable in that they, for lack of film formability, are generally used in combination with a binder, so that, by selecting the kind and composition ratio of binders to be used, the physical or electrophotographic characteristics thereof may be controlled to some extent. However, the number of kinds of organic photoconductive compounds having high compatibility with binders is limited as understood from the fact that only a few are applied to the composition of photosensitive layers for electrophotographic photosensitive element.

For example, the 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole as described in U.S. Pat. No. 3,189,447 has low compatibility with binder that is in general suitably used as materials for the photosensitive layer of an electrophotographic photosensitive element, so that when the material is used to form a photosensitive layer by mixing with such a binder as polyester, polycarbonate in the proportion required for obtaining desirable electrophotographic characteristics, crystals of the oxadiazole come to deposit at a temperature of above 50.degree. C., thereby resulting in the deterioration of electrophotographic characteristics such as the charge acceptance, photosensitivity, etc., while the diaryl alkane derivatives described in U.S. Pat. No. 3,820,989 have no problems in respect of the compatibility with binders, but have little stability to light, so that when applied to the composition of the photosensitive layer of an electrophotographic photosensitive element intended for use in the repetitive transfer type electrophotographic process in which charging-exposure cycle is repeatedly effected, the photosensitivity of the photosensitive layer becomes gradually lowered.

The p-dimethylaminobenzaldehyde diphenyl hydrazone described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 59143/1979 and No. 150128/1979 has good compatibility with binders and enables to provide an electrophotographic photosensitive element having relatively good initial characteristics, but said photosensitive element is lacking in durability because as it is repeatedly used, its photosensitivity becomes gradually lowered and the residual electric potential becomes accumulated.

As has been mentioned, it is the fact that there are still not found any organic photoconductive compounds having practically suitable characteristics for an electrophotographic photosensitive element.

It is an object of the present invention to provide an electrophotographic photosensitive element having a carrier-transporting phase composed of organic photoconductive compounds excellent in compatibility with binder, stable to heat and light, and excellent in carrier transporting ability.

It is another object of the present invention to provide an electrophotographic photosensitive element having a photosensitive layer with a high layer strength and excellent in the stability even when repeatedly used.

It is still another object of the present invention to provide an electrophotographic photosensitive element having a high photosensitivity and small residual electric potential.

It is a further object of the present invention to provide a highly durable electrophotographic photosensitive element which, when used as a repetitive transfer type electrophotographic photosensitive element with which a charging-exposure-development-transfer cycle is repeated, is hardly deteriorated with exhaustion after repetitious use and retains stable characteristics for an extended period of time.

As a result of our study to attain the above objects, we have found that the use of specific hydrazone compounds as carrier-transporting materials enables to attain the objects, and thus we have accomplished the present invention.

The foregoing objects are attainable by providing a conductive support thereon with a photosensitive layer comprised in combination of a carrier-generating phase with a carrier-transporting phase comprising a hydrazone compound having formula ›A! ##STR2## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 each is an aryl group which may have a substituent or substituents, and Ar is an arylene group which may have a substituent or substituents.

FIGS. 1 through 6 are the respective sectional views of the construction examples of the electrophotographic photosensitive element of the present invention, and

FIG. 7 shows the characteristic curves of an exemplified electrophotographic photosensitive element of the present invention and of a comparison example therewith.

In the present invention, utilizing the excellent carrier-transporting ability of hydrazone compounds having formula ›A!, the compound may be used as a carrier-transporting phase component material in the photosensitive layer of the so-called function-separated type photosensitive element wherein the generating and transporting of carrier are carried out separately by different materials respectively, whereby an electrophotographic photosensitive element may be produced which is excellent in respect of the physical properties of the layer and electrophotographic image forming characteristics such as charge acceptance, photosensitivity, residual electric potential and the like, and which is not only hardly deteriorated with exhaustion even when repeatedly used but the said characteristics do not change and is thus capable of displaying stable characteristics.

In the above formula ›A!, a preferable aryl group is a phenyl group or a substituted phenyl group, and a preferable arylene group is a phenylene group or a naphthylene group. Above groups may have a substituent or substituents which is optional. Preferable substituent is an alkyl group such as a methyl group, an ethyl group, most preferable a lower alkyl group, or a halogen atom such as chlorine, bromine.

Hydrazone compounds having formula ›A! which are useful in the present invention include, for example, those having the following formulas: ##STR3##

The above-enumerated hydrazone compounds may be readily synthesized in known manners. For example, the N,N-diaryl hydrazine and N,N-diaryl aminoarylaldehyde having the following formulas ›B! and ›C! respectively are subjected to dehydration-condensation reaction in a solvent such as alcohol in the presence of an acid catalyst, whereby a hydrazone compound having formula ›A! may be obtained. ##STR4##

Subsequently, a typical synthesis process of a hydrazone compound applicable to the present invention is illustrated in detail below:

Eleven grams of N,N-diphenyl hydrazine hydrochloride (0.05 mol) and 13.6 g of N,N-diphenyl aminobenzaldehyde (0.05 mol) are dissolved in 200 ml of ethanol, and the solution is refluxed by heating for a period of two hours. The deposited crystals are filtered, washed with methanol, and then dried, and thereafter recrystallized from a mixture of methyl-ethyl ketone and ethanol, whereby Exemplified Compound (1) is obtained. Yield: 18.3 g, 83.2% Melting Point: 166.0.degree.-167.0.degree. C.

The above-mentioned hydrazone compound may be used not merely in single but in a mixture of two or more kinds thereof, and furthermore in a mixture of one or more kinds thereof with one or more kinds of different carrier-transporting materials, and in the latter case, there is the possibility to form a charge-transfer complex on both the foregoing hydrazone compounds and said different carrier-transporting materials, whereby it may become possible to control arbitrarily the transfer degree of a carrier of a specific sign in said carrier-transporting phase as well as to effectively lose the internal trap. This carrier-transporting phase may at need contain a plasticizer, residual electric potential reducing agent, exhaustion-reducing agent and the like. Since hydrazone compounds used in the present invention have in themselves no film formability, the combined use with various binders is required to form a carrier-transporting phase for composing a photosensitive layer.

As binders mentioned herein above, although discretional materials may be used, most preferably usable are high molecular polymers capable of forming hydrophobic, highly dielectric and insulating film. Such high molecular polymers may include, for example, compounds enumerated below, but those applicable to the present invention are not limited thereto:

(1) Polycarbonate,

(2) Polyester,

(3) Methacrylic resin,

(4) Acrylic resin,

(5) Polyvinyl chloride,

(6) Polyvinylidene chloride,

(7) Polystyrene,

(8) Polyvinyl acetate,

(9) Styrene-butadiene copolymer,

(10) Vinylidene chloride-acrylonitrile copolymer,

(11) Vinyl chloride-vinyl acetate copolymer,

(12) Vinyl chloride-vinyl acetate-maleic anhydride copolymer,

(13) Silicone resin,

(14) Silicone-alkyd resin,

(15) Phenol-formaldehyde resin,

(16) Styrene-alkyd resin,

(17) Poly-N-vinyl carbazole.

These binder materials may be used either in single or in a mixture of two or more kinds.

The afore-mentioned carrier-transporting phase may effectively compose the photosensitive layer of an electrophotographic photosensitive element by the combination thereof with a carrier generating phase comprised of discretional one selected from among various carrier-generating materials which absorb light to produce carriers.

Suitably usable carrier-generating materials include, for example, those enumerated below:

(1) Selenium, selenium alloys,

(2) Inorganic photoconductive compounds such as CdS, CdSe, CdSSe, HgS,

(3) Perylene pigments such as perylene tetracarboxylic anhydride, perylene tetracarboxylic diimide,

(4) Indigoid pigments,

(5) Polycyclic quinones such as anthraquinones, pyrenequinones, flavanthrones, anthanthrones,

(6) Bisbenzimidazole pigments,

(7) Quinacrydone pigments,

(8) Azo dyes such as monoazo dye, bisazo dye, trisazo dye,

(9) Indanthrone dyes,

(10) Squarylium dyes,

(11) Phthalocyanine pigments such as none-metallic phthalocyanine, metallic phthalocyanine,

(12) Charge-transfer complex formed from electron-donating materials such as poly-N-vinyl carbazole and electron-accepting materials such as trinitrofluorenone, and

(13) Co-crystalline complex formed from pyrilium salt or thiapyrilium salt and polycarbonate.

These carrier-generating materials may be used in single or in combination of two ro more, and when making the form of a layer in the case where carrier-generating materials used in single have no layer formability, there may be applied thereto one arbitrarily selected from binder materials similar to those used for the carrier-transporting phase.

The construction of the electrophotographic photosensitive element of the present invention is illustrated below:

In the present invention, as shown in FIG. 1 and FIG. 2, a conductive support 1 is provided thereon with a photosensitive layer 4 composed of a carrier-generating layer 2 containing as the principal component thereof a carrier-generating material and a carrier-transporting layer 3 containing as the principal component thereof an aforementioned hydrazone compound. As shown in FIG. 3 and FIG. 4, photosensitive layer 4 may also be provided through an intermediate layer 5 on conductive support 1. When photosensitive layer 4 is thus composed of two layers a most excellent electrophotographic characteristics-having electrophotographic photosensitive element can be obtained. And in the present invention, as shown in FIG. 5 and FIG. 6, conductive support 1 may also be provided directly thereon or through intermediate layer 5 with photosensitive layer 4 composed of carrier-generating phase 7 prepared by dispersing a carrier-generating material in the powder form into layer 6 of the carrier-transporting phase containing as the principal component thereof the foregoing carrier-transporting material.

In the case where photosensitive layer is composed of two layers as described above, which of carrier-generating layer 2 and carrier-transporting layer 3 should come above depends on the selection of charging polarity; that is, in the case of negative charge type photosensitive layer, it is advantageous that carrier-transporting layer 3 come above because the hydrazone compound in said carrier-transporting layer 3 is a material having a high transportability to positive hole.

Carrier-generating layer 2 of photosensitive layer 4 which is composed of two layers may be formed directly on either conductive support 1 or carrier-transporting layer 3, or at need on an intermediate layer provided such as an adhesive layer, barrier layer, etc., in the manner of

(1) method of vacuum deposition,

(2) method of coating a solution of a carrier-generating material dissolved in an appropriate solvent, or

(3) method of coating a dispersion liquid obtained in the manner that a carrier-generating material is made in the fine particle form in a dispersion medium by means of a ball mill or homomixer and then at need dispersedly mixed with a binder.

The thickness of the thus formed carrier-generating layer 2 is preferably from 0.01 to 5 microns, most preferably from 0.05 to 3 microns, while the thickness of carrier-transporting layer, though allowed to be changed at need, is preferably normally from 5 to 30 microns.

The proportion of the components composing carrier-transporting layer 3 is preferably one part by weight of a carrier-transporting material composed principally of the foregoing hydrazone compound to 0.8-4 parts by weight of a binder. However, in the case of forming photosensitive layer 4 in which a carrier-generating material in the powder form is dispersed, the proportion should preferably by within the range of not more than 5 parts by weight of the binder per part by weight of the carrier-generating material. Further, in the case of composing carrier-generating layer 2 as of the dispersion type with the use of a binder, the proportion should likewise be within the range of not more than 5 parts by weight of the carrier-generating material per part by weight of the binder.

In addition, as conductive support 1 for use in composing the electrophotographic photosensitive element there may be used a metallic plate or paper or plastic film provided with conductivity by being coated, vacuum-deposited or laminated with a conductive compound such as, a conductive polymer, an indium oxide or with a thin metallic layer such as, aluminum, palladium, gold. As intermediate layer 5 such as an adhesive layer, barrier layer there may be used organic polymer such as gelatin, casein, starch, polyvinyl alcohol, vinyl acetate, ethyl cellulose, carboxy-methyl cellulose or aluminum as well as high molecular polymers usable as the foregoing binder.

The electrophotographic photosensitive element of the present invention, which is composed as has been described above, as apparent from the hereinafter described examples, is excellent in charging, photosensitivity, and image formability characteristics, and particularly in the durability that it hardly becomes deteriorated with exhaustion even when used in the repetitive type electrophotographic process.

Examples of the present invention are illustrated in detail below, but the embodiment of the present invention is not limited thereto.

A conductive support made of polyester film laminated with an aluminum foil was provided thereon with an intermediate layer with the thickness of 0.05 micron made of vinyl chloride-vinyl acetate-maleic acid anhydride copolymer "S-lec MF-10" (manufactured by Sekisui Kagaku Co., Ltd.). On this was coated to form a carrier-generating layer so that the coated amount after drying becomes 0.2 g/m.sup.2 a solution prepared by dissolving 1 part by weight of chlorodian blue having the formula ##STR5## into a mixture of ethylenediamine, n-butylamine and tetrahydrofuran mixed in the proportion of 1.2:1.0:2.2. Subsequently, a solution prepared by dissolving 7.5 parts by weight of Exemplified compound (2) and 10 parts by weight of a polycarbonate "Panlite L-1250" (manufactured by Teijin Chemical Industry Co., Ltd.) into 85 parts by weight of 1,2-dichloroethane was coated on the foregoing product so that the coated layer is 9 microns thick after drying to form a carrier-transporting layer, whereby an electrophotographic photosensitive element of the present invention was produced.

The obtained electrophotographic photosensitive element was evaluated on the electrophotographic characteristics thereof in the dynamic system with the use of an electrostatic paper Analyser Model SP-428 (manufactured by Kawaguchi Electric Works Co., Ltd.); that is, surface potential V.sub.A when the surface of the photosensitive layer of the foregoing element was charged for 5 seconds at the charging voltage of -6.0 KV; surface potential V.sub.I after being left in the dark for the subsequent 5 seconds; subsequently with a tungsten lamp light controlled so that the illuminance thereof on the surface of the photosensitive layer becomes 35 luxes the exposure (half-decay exposure) E1/2 required for reducing surface potential V.sub.I to one second; and surface potential (residual potential) V.sub.R after being exposed with the exposure of 30 lux. sec. were found by measurements respectively. The same measurements were repeated 10 times in succession respectively. The results obtained are shown in Table 1.

As apparent from the table, even in the tenth measurements the results show nearly the same characteristics as those in the first.

An electrophotographic photosensitive element for control was prepared in the same manner as in Example 1 with the exception that for the formation of a carrier-transporting layer, N,N-dimethylaminobenzaldehydediphenyl hydrazone was used which have the formula ##STR6## in place of the exemplified compound (2), and the resulting product was subjected to similar measurements. The results are shown in Table 2.

As apparent from the results, in the 10th operation the respective characteristics are found out to be significantly deteriorated.

An electrophotographic photosensitive element was produced in the same manner as in Example 1 with the exception that a carrier-generating layer with the thickness of 0.5 micron was formed by the vacuum deposition of dibromoanthanthrone "Monolite Red 2Y" (C.I. No. 59300, manufactured by I.C.I), and a liquid prepared by dissolving 7.5 parts by weight of exemplified compound (1) and 10 parts by weight of a polycarbonate "Panlite L-1250" (manufactured by Teijin Chemical Industry Co., Ltd.) into 85 parts by weight of 1,2-dichloroethane was coated so that the layer thickness becomes 8 microns after drying to form a carrier-transporting layer. The obtained electrophotographic photosensitive element was also subjected to measurements similar to those in Example 1, and further was applied to an electrophotographic copier U-BiX 2000R (manufactured by Konishiroku Photo Ind. Co., Ltd.) to be subjected to 10,000-time repeated charging and exposure operations for the purpose of a durability test, upon completion of which the same measurements were made again on the photosensitive element. The results are shown in Table 3.

As apparent from the results, changes in the respective characteristics are very small even after the durability test of 10,000-time operations.

An electrophotographic photosensitive element for control was produced in the same manner as in Example 2 with the exception that for the formation of a carrier-transporting layer, N,N-diethylaminobenzaldehyde-diphenyl hydrazone having the formula ##STR7## was used in place of the exemplified compound (1), and measurements similar to those in Example 2 were made on the resulting photosensitive element. The results obtained are shown in Table 4.

As apparent from the above table, the results show such significantly deteriorated characteristics that photosensitivity is not only reduced but the residual potential is increased after the durability test of 10,000-time operations.

And each sample of the electrophotographic photosensitive element of Example 2 and control electrophotographic photosensitive element in Control 2 was subjected to repetitive cycles of charging, exposure and cleaning processes by the application of an electrophotographic copier U-BiX 2000R (manufactured by Konishiroku Photo Industry Co., Ltd.). In order to determine the degree of deterioration from electrical exhaustion (durability) in the processes, each exposure amount of 6 lux. sec. was applied to the surface of the light-sensitive layer, and how the surface potential of the photosensitive element obtained at this time changes according to the repetition of the cycle of the foregoing processes was examined, then the results as shown in FIG. 7 were obtained wherein curve I is the result of the electrophotographic photosensitive element in Example 2, while curve II shows the result of the control electrophotographic photosensitive element in Control 2. As apparent from these curves the electrophotographic photosensitive element of the present invention is so satisfactory that the change in the surface potential after exposure is small even when repeatedly used, whereas in the control electrophotographic photosensitive element the surface potential thereof when subjected to a given exposure tends to gradually rise as the repetition of its use goes on. This is because the photosensitive layer becomes deteriorated with exhaustion as it is repeatedly used and the residual potential becomes accumulated to gradually lower the photosensitivity thereof. Such a tendency is undesirable because it causes fog on the reproduced image.

An electrophotographic photosensitive element of the present invention was produced in the same manner as in Example 1 with the exception that a carrier-generating layer was formed by coating a dispersion liquid so that the thickness becomes 1 micron after drying, said dispersion liquid being obtained by adding 1 part by weight of a bisazo compound having the formula ##STR8## and 1 part by weight of a polyester "Vylon 200" (manufactured by Toyo Spinning Co., Ltd.) to 80 parts by weight of 1,2-dichloroethane and dispersing the mixture by means of a ball mill, while a carrier-transporting layer was formed by coating a liquid so that the thickness becomes 14 microns after drying, said liquid being prepared by dissolving 2 parts by weight of exemplified compound (3) and 3 parts by weight of a polyester "Vylon 200" into 50 parts by weight of 1,2-dichloroethane.

The thus produced electrophotographic photosensitive element was subjected to a copying test, using U-BiX 2000R (manufactured by Konishiroku Photo Industry Co., Ltd.), then a clearly copied image having a satisfactory contrast and faithful to the original was obtained. Further the same light-sensitive body was subjected to a 5,000-time repeated copying test without changing the given exposure, then there were obtained quality image copies as good as the initial one.

On a conductive support produced by the vacuum deposition of aluminum on a polyester film a carrier-generating layer with the thickness of 0.5 micron was formed by the vacuum deposition of N,N'-dimethyl perylene-3,4,9,10-tetracarboxylic acid imide "Paliogen Maroon 3920" (C.I. No. 71130, manufactured by BASF). On this was subsequently formed a carrier-transport layer by coating a coating liquid so that the thickness thereof becomes 12 microns after drying, said liquid being prepared by dissolving 7.5 parts by weight of exemplified compound (4), 9.5 parts by weight of a polycarbonate "Jupilon S-1000" (manufactured by Mitsubishi Gas Chemicals Co., Lnc.), and 0.5 part by weight of methacrylic resin "Acrypet" (manufactured by Mitsubishi Rayon Co., Ltd.) into 85 parts by weight of 1,2-dichloroethane, whereby an electrophotographic photosensitive element of the present invention was produced.

The electrophotographic photosensitive element thus obtained was subjected to measurements similar to those in Example 1, then E1/2 was 4.1 lux. sec. and residual potential V.sub.R was O(V).

An electrophotographic photosensitive element of the present invention was produced in the same manner as in Example 4 with the exception that exemplified compound (5) was used in place of the exemplified compound (4), and was subjected to measurements similar to those in Example 1, then E1/2 was 4.3 lux. sec.