Hydrazone containing charge transport element and photoconductive process of using same
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to electrophotographic reproduction, and in particular to electrophotographic reproduction utilizing hydrazone materials of the composition; ##STR2## as the active material in the p-type charge transport layer of a multi-layer photoconductor system. 
     2. Description of the Prior Art 
     Electrophotographic processes and materials, such as xerography, are of course well known. Fundamentally, such processes involve the formation of a uniform electrostatic charge upon a normally insulating plate or element under &#34;dark&#34; conditions. Thereafter, the element is exposed to light in an imagewise manner to render the light struck portions of the element conductive thereby permitting the electrostatic charge to be conducted from the surface of the element. Thus a latent image in the form of charged surface areas are formed on the portions of the surface not struck by light. The latent electrostatic image on the surface of the element is then typically developed by exposure to an oppositely charged powder, i.e., a toner which is held to the charged portion of the element by the affinity of the toner for the opposite charge. The discharged portion of the element displays no such affinity for the toner. The thus formed image of toner is thereafter transferred to another surface, such as paper, and adhered thereto by, for instance, pressure sensitive, heat sensitive, etc, adhesives admixed with the toner. 
     A particularly useful electrophotographic element is that in which a charge generation layer, which is responsive to actinic radiation to generate electron-hole pairs, is utilized in conjunction with a p-type charge transport layer adjacent thereto. Numerous charge generation layers responsive to selected actinic radiation are known. The charge transport layer is not responsive to actinic radiation under the operating conditions, but serves to transport the positive charge from the charge generation layer, to, depending upon the particular system involved, the surface of the negatively charged transport layer at which the image is formed, or alternatively, to a conductor in a positively charged system. U.S. Pat. No. 3,837,851  discusses an electrophotographic plate utilizing as the active material in the charge transport layer a tri-aryl pyrazoline compound. 
     Hydrazones of a differing nature than those with which the instant invention is concerned have been employed in photoconductors essentially as a material responsive to actinic radiation. U.S. Pat. No. 3,717,462, issued Feb. 20, 1973, discloses such use of a hydrazone compound. Other similar uses of hydrazone compounds, in general are to be found in U.S. Pat. No. 3,765,884. 
     In summary, the prior art has recognized the use of charge transport layers in conjunction with distinct charge generating layers, but has not suggested the use of hydrazones in general, and particularly the hydrazones of the instant invention, for use as the active material in a charge transport layer. On the other hand, hydrazones differing from the specific hydrazones of the instant invention have been employed as light responsive materials as opposed to charge transport materials. 
     SUMMARY OF THE INVENTION 
     The present invention, which provides a surprising and heretofore unavailable improvement in multilayer electrophotographic elements, comprises an element having, as its fundamental parts, a conductor, a charge generation layer, which is substantially conventional in nature, and a novel p-type charge transport layer adjacent thereto including, as the active material, a hydrazone having the general formula; ##STR3## A particularly preferred charge transport material is p-diethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR4## Other preferred charge transport materials are; o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR5## o-methyl-p-diethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR6## o-methyl-p-dimethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR7## p-dipropylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR8## p-diethylaminobenzaldehyde-(benzylphenylhydrazone), i.e., ##STR9## p-dibutylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR10## p-dimethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR11## 
     Multilayer electrophotographic elements are, in general, known. The charge generation layer, which may be organic or inorganic, is responsive to actinic radiation falling thereupon to generate an electron-hole pair. While the charge generation layer may be self-supporting, preferably a pliant support such as a polymeric film having a metalized surface is employed. Biaxially oriented polyethylene terephthalate is a preferred pliant support. As discussed above, the charge generation layer must be in electrical communication with a conductor to facilitate selective discharge of the element. Again with regard to the preferred but conventional aspect of the instant invention, an aluminized Mylar (polyethylene terephthalate film) is conveniently employed with the aluminum constituting the conducting layer. The charge generation layer is preferably formed on the support and in contact with the conducting layer. While not critical, the charge generation layer is generally between 0.05 microns to 0.20 microns thick. Inorganic charge generating materials include selenium, and tellurium. Organic charge generation materials including disazo compounds such as those described in U.S. patent application Ser. No. 129,635, now abandoned, and U.S. patent application Ser. No. 648,610 are generally operable. Useful results are obtainable with charge generating materials comprising methine dye derived from squaric acid. Materials of this type are discussed in U.S. patent application Ser. No. 323,678. 
     Chlorodiane Blue, methyl squarylium and hydroxy squarylium are particularly preferred charge generating materials. 
     More specifically, these preferred materials are; 4-4&#34;[-(3,3&#39;-dichloro-4,4&#39;-biphenylylene)bis(azo)]-bis[3-hydroxy-2-naphthanilide], i.e., ##STR12## 2,4-bis-(2-methyl-4-dimethylaminophenyl)-1,3-cyclobutadienediylium-1,3-diolate, i.e., ##STR13## 2,4-bis-(2-hydroxy-4-dimethylaminophenyl)-1,3-cyclobutadienediylium-1,3-diolate, i.e., ##STR14## but will, for convenience, be hereafter identified as Chlorodiane Blue, methyl squarylium and hydroxy squarylium, respectively. 
     In summary, a broad range of inorganic and organic charge generation materials are operable with the charge transport material of the instant invention. However, since the charge transport material must, in most embodiments of the invention, be substantially transparent to the actinic radiation which activates the charge generation material, it is preferred that the charge generation material be responsive to actinic radiation in the visible light and longer wave lengths, i.e., longer than 3900 angstroms. This requirement is of concern in the preferred embodiment wherein the charge transport material is interposed between the charge generation material and the source of actinic radiation, i.e., as in a negative charging system. However, in a positive charging system, the charge generation material may be directly exposed to the actinic radiation and the charge transport material interposed between the charge generation material and the conductor. In the latter case, charge generation materials and actinic radiation sources operating at shorter than visible light wave lengths are suitable for use with the charge transport material of the instant invention. 
     In the preferred embodiment of the instant invention in which organic charge generation materials are employed, such materials are, as is conventional, coated onto the metalized support utilizing, for instance, meniscus coating, doctor blade coating or dip coating. Preferably, an adhesive layer is provided on the support to aid in bonding the charge generation layer thereto. Polyester resins are a preferred adhesive layer. 
     The novel charge transport layer according to the instant invention is preferably coated onto the charge generation layer to form the top or exposed layer of the element. Preferably, the charge transport layer is between 7 microns and 35 microns thick; but may be thicker, and operably may be less than 7 microns, i.e., 5 microns thick. Though the following discussion will primarily address this preferred embodiment, it is to be understood that, with regard to positive charging systems, the charge transport layer can be interposed between the charge generation layer and the support in the manner illustrated in the drawing and discussed below. 
     The active material of the p-type charge transport layer of the instant invention is a hydrazone of the generic structure; ##STR15## A particularly preferred charge transport material is p-diethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR16## Other preferred charge transport materials are; o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR17## o-methyl-p-diethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR18## o-methyl-p-dimethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR19## p-dipropylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR20## p-diethylaminobenzaldehyde-(benzylphenylhydrazone), i.e., ##STR21## p-dibutylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR22## p-dimethylaminobenzaldehyde-(diphenylhydrazone), i.e., ##STR23## In use, the hydrazone material is admixed with a binder in an organic solvent, coated onto the charge generation layer and dried in a forced air oven. While numerous polymeric binders will be apparent to those skilled in the art, typical binders include polycarbonate resins, i.e., M-60 available from Mobay Chemical Company, polyester resins such as PE-200 available from Goodyear, and acrylic resins such as A-11 available from Rohm and Haas. Various other resins are also operable as will be demonstrated below. The resins, which may be used singularly or in combination, are admixed with an organic solvent or solvents, preferably tetrahydrofuran and toluene, though other appropriate solvents will be apparent to those skilled in the art. 
     Various other constituents for lubrication, stability, enhanced adhesion, coating quality, etc., may be included in the charge transport layer to accomplish purposes evident to those skilled in the art. For instance, a silicon oil, such as that available under the trademark DC-200 from Dow Corning, is included in the charge transport layer solution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the Drawing: 
     FIG. 1 is a simplified sectional view of the charge generation and charge transport layers of the preferred embodiment of the instant invention illustrating the response to exposure of a negatively charged element to actinic radiation; 
     FIG. 2 is a view similar to that of FIG. 1 illustrating the resulting negative charge on the element surface; 
     FIG. 3 is a view similar to that of FIG. 1 illustrating a positive charging element; and 
     FIG. 4 is a view similar to that of FIG. 2 illustrating the resulting positive charge on the surface of the positive charged element. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Turning now to the drawing, wherein like components and constituents are designated by like reference numerals throughout the various figures, a multilayer electrophotographic element is depicted in FIG. 1 and generally designated by the reference numeral 10. 
     Element 10 includes charge generation layer 12 and charge transport layer 14. As illustrated, a negative charge exists on the surface of charge transport layer 14. A positive charge is provided adjacent charge generation layer 12, i.e., in a conducting layer (not shown). Actinic radiation 16 is shown passing through charge transport layer 14 at area 18 and inducing charge generation layer 12 to produce the electron-hole pair charges. The hole is attracted to the negative charge on the surface of charge transport layer 14. Thus, as shown in FIG. 2, the hole is injected into and travels through charge transport layer 14 to discharge area 18. Charge transport layer 14 is essentially an insulating material relative to the negative charge thereon. Thus, localized discharge is maintained at area 18. The electron, of course, is attracted to the positive charge at the conducting layer (not shown). 
     A similar result is illustrated in FIG. 3 and FIG. 4. However, element 10&#39;, while including the same layers, is arranged differently. Charge generation layer 12 is positively charged and exposed directly to actinic radiation 16. Charge transport layer 14 is interposed between charge generation layer 12 and the negative charge, usually carried at a conductive layer (not shown). Again actinic radiation 16 develops electron-hole pair charges. Area 18 of charge generation layer 12 is discharged by the electrons while the corresponding holes pass through charge transport layer 14 in response to the attraction of the negative charges. Element 10&#39; has the advantage of not requiring that actinic radiation 16 pass through charge transport layer 14, but charge generation layer 12 is not protected. 
     Other embodiments are contemplated but not illustrated. For instance, element 10 of FIG. 1 could be exposed to actinic radiation from the opposite side, i.e., through the conducting layer. 
     Detailed Examples of the Invention 
     The illustrative examples below are provided to permit those skilled in the art to practice the preferred embodiment of the instant invention as well as to illustrate the operable variations in the invention. However, it is not contemplated that the illustrative examples will extend to all operable combinations, or specify the various alternative components apparent to those skilled in the art. 
     EXAMPLE 1 
     A support appropriate for the instant invention was prepared by coating an aluminized Mylar (duPont Trademark for polyethylene terephthalate) substrate with a solution of polyester resin (PE 200 available from Goodyear) dissolved in tetrahydrofuran:toluene solvent system in a 9:1 ratio (0.7% to 1.4% solids, weight:weight). The polyester coating was meniscus coated and dried in a forced air oven. Chlorodiane Blue (0.73% solids by weight) was then dissolved in 1.2:1.0:2.2 (by weight) mixture of ethylenediamine, n-butylamine and tetrahydrofuran. Silicon oil (available under the Trademark DC-200 from Dow Corning) was then added in the amount of 2.3% by weight relative to the Chlorodiane Blue. The resulting solution was meniscus coated onto the polyester coated substrate, and the resulting coated substrate dried in a forced air oven. Thus a relatively conventional Chlorodiane Blue charge generation layer was produced on an again conventional polyester support. 
     The novel charge transport layer of the instant invention was formed by admixing a polycarbonate resin binder (M-60 available from Mobay Chemical Company) in the amount of 7.65 grams, a polyester resin (PE-200 available from Goodyear) in the amount of 3.60 grams, and an acrylic resin (A-11 available from Rohm and Haas) in the amount of 2.25 grams in 86.5 grams of tetrahydrofuran and toluene solvent, the solvents being present in a ratio of approximately 9:1 by weight. A preferred hydrazone according to the instant invention, i.e., p-diethylaminobenzaldehyde-(diphenylhydrazone) was then added in the amount of 9.0 grams in conjunction with 0.02 grams of silicon oil (DC-200). Additional tetrahydrofuran may then be added to adjust the viscosity to that appropriate for the chosen coating technique. In the instant example, the resulting solution was meniscus coated onto the charge generation layer as formed above and the entire film again dried in a forced air oven to form a multilayered electrophotographic element. The electrophotographic element was tested by first charging the surface thereof to -870 volts in the dark, exposing the charged electrophotographic element to light typical of that utilized in commercial electrophotographic apparatus under various light intensity conditions, and determining the light intensity necessary to discharge the element to a voltage of -150 volts after 454 milliseconds under such conditions. It was determined that the element of the instant example required 1.10 microjoules per square centimeter for such discharge. Such value is indicative of excellent hole transport. Electrophotographic elements essentially identical to that of the instant examples were tested in commercially designed copying equipment and provided excellent results as to charge transport, resistance to toner filming, physical resistance to wear, long-term stability of electrical and physical properties, and low temperature operation. 
     EXAMPLE 2a-f 
     Multilayered electrophotographic elements similar to those of example 1 were prepared with varied transport layer resins in differing amounts. 
     
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Binder Resins                                                             
Example  M-60 (gms) PE-200 (gms)                                          
                                A-11 (gms)                                
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2a       13.5       0           0                                         
2b       0          13.5        0                                         
2c       9.0        2.25        2.25                                      
2d       10.12      2.25        1.13                                      
2e       9.90       3.60        0                                         
2f       7.65       2.25        3.60                                      
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     Testing in the manner set forth in Example 1 yielded the following results. 
     
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        Discharge                  Exposure                               
        Response                   Energy                                 
        Time       Dark    Discharge                                      
                                   (microjoules/                          
Example (milliseconds)                                                    
                   Voltage Voltage centimeter.sup.2)                      
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2a      454        -870    -150    1.38                                   
2b      454        -870    -190    1.34                                   
2c      454        -870    -150    1.10                                   
2d      454        -870    -150    1.15                                   
2e      454        -870    -150    1.10                                   
2f      454        -870    -150    1.03                                   
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     EXAMPLE 3 
     A multilayered electrophotographic element similar to that of Example 1 was prepared with the exception that a transport layer solution containing 14.5 grams of acrylic resin (A-11) as the sole binder and 14.5 grams of p-diethylaminobenzaldehyde-(diphenylhydrazone) was employed. When tested as set forth in Example 1, 3.0 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 volts at a discharge response time of 454 milliseconds. 
     EXAMPLE 4 
     A multilayered electrophotographic element similar to that of Example 1 was prepared with the exception that the acrylic resin employed was B-50, a proprietary resin available from Rohm and Haas, in place of the A-11 acrylic resin. When tested as set forth in Example 1, 1.16 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 at a discharge response time of 454 milliseconds. 
     EXAMPLE 5a-e 
     Multilayered electrophotographic elements similar to that of Example 2e were prepared with the exception that the following polyester resins were substituted in the same amount for the PE200 polyester resin. 
     
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Example       Polyester                                                   
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5a            PE222        (Goodyear)                                     
5b            49000        (duPont)                                       
5c            PE207        (Goodyear)                                     
5d            VPE5545      (Goodyear)                                     
5e            PE307        (Goodyear)                                     
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     Results substantially similar to those of Example 2e were obtained in each instance. 
     EXAMPLE 6a-k 
     Multilayered electrophotographic elements similar to that of Example 1 were prepared with the exception that the initial adhesive coatings were prepared with the following resins in place ot the polyester (PE200) in similar amounts. Each element was charged to -870 volts and discharged to -150 volts in 146 milliseconds. The indicated light energy in microjoules per square centimeter was required. 
     
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Example                                                                   
       Trademark Resin         Exposure Energy                            
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6a     PE 222    polyester     1.14                                       
6b     PE 207    polyester     1.28                                       
6c     49000     polyester     1.28                                       
6d     A-11      acrylic       1.34                                       
6e     B-66      acrylic       1.51                                       
6f     M-60      polycarbonate 1.48                                       
6g               polysulfone   1.36                                       
6h     15/95S    formvar       1.28                                       
6i     B-72A     butvar        1.22                                       
6j     B-76      formvar       1.26                                       
6k               polyvinyl carbazole                                      
                               1.23                                       
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     EXAMPLE 7a and 7b 
     Multilayer electrophotographic elements similar to that of example 2e were prepared with the exception that 5.78 grams of p-diethylaminobenzaldehyde-(diphenylhydrazone) were substituted in the transport layer solution in Example 7a, and 7.27 grams were similarly substituted in Example 7b. When tested under the same discharge voltage and discharge response time as in Example 1, it was found that the element of Example 7a required 1.4 microjoules per square centimeter of light energy and that of Example 7b required 1.3 microjoules per square centimeter of light energy. 
     EXAMPLE 8 
     A multilayered electrophotographic element similar to that of Example 2a was prepared with the exception that 13.5 grams of p-diethylaminobenzaldehyde-(diphenylhydrazone) was employed in the charge transport layer solution. When tested as set forth in Example 1, 1.37 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 volts at a discharge response time of 146 milliseconds. 
     EXAMPLE 9 
     A multilayered electrophotographic element similar to that of Example 2awas prepared with the exception that 20.25 grams of p-diethylaminobenzaldehyde-(diphenylhydrazone) was employed in the charge transport layer solution. When tested as set forth in Example 1, 1.37 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 volts at a discharge response time of 146 milliseconds. 
     EXAMPLE 10 a-d 
     Multilayered electrophotographic elements were prepared similar to that of Example 2a with the exception that the following alternative hydrazone compounds were employed in the same quantity in the charge transport solution. 
     EXAMPLE 
     10a--o-methyl-p-dimethylaminobenzaldehyde-(diphenylhydrazone) 
     10b--o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone) 
     10c--o-methyl-p-diethylaminobenzaldehyde-(diphenylhydrazone) 
     10d--p-dimethylaminobenzaldehyde-(diphenylhydrazone) 
     The following results were obtained. 
     
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       Discharge                   Exposure                               
       Response                    Energy                                 
       Time       Dark     Discharge                                      
                                   (microjoules/                          
Example                                                                   
       (milliseconds)                                                     
                  Voltage  Voltage centimeter.sup.2)                      
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10a    146        -800     -190    1.71                                   
10b    146        -800     -190    1.24                                   
10c    146        -800     -190    1.64                                   
10d    146        -800     -190    1.65                                   
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     EXAMPLE 11a-c 
     Multilayered electrophotographic elements were prepared similar to that of Example 2a with the exception that 13.5 grams of the following hydrazones were employed in the transport layer solution. 
     Example 
     11a--o-methyl-p-dimethylaminobenzaldehyde-(diphenylhydrazone) 
     11b--o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone) 
     11c--o-methyl-p-diethylaminobenzaldehyde-(diphenylhydrazone) 
     The following results were obtained. 
     
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       Discharge                   Exposure                               
       Response                    Energy                                 
       Time       Dark     Discharge                                      
                                   (microjoules/                          
Example                                                                   
       (milliseconds)                                                     
                  Voltage  Voltage centimeter.sup.2)                      
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11a    146        -870     -150    1.56                                   
11b    146        -870     -150    1.21                                   
11c    146        -870     -150    1.60                                   
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     EXAMPLE 12a-c 
     Multilayered electrophotographic elements were prepared similar to that of Example 1 with the exception that the transport layer solution contained 6.75 grams of polyester resin (PE200), 6.75 grams of polycarbonate resin (M60) and 13.5 grams of the following hydrazone compounds; 
     Example 
     12a--p-dimethylaminobenzaldehyde-(diphenylhydrazone) 
     12b--p-dipropylaminobenzaldehyde-(diphenylhydrazone) 
     12c--p-dibutylaminobenzaldehyde-(diphenylhydrazone) 
     The following results were obtained. 
     
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       Discharge                   Exposure                               
       Response                    Energy                                 
       Time       Dark     Discharge                                      
                                   (microjoules/                          
Example                                                                   
       (milliseconds)                                                     
                  Voltage  Voltage centimeter.sup.2)                      
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12a    146        -800     -190    1.81                                   
12b    146        -800     -190     .92                                   
12c    146        -800     -190    1.51                                   
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     EXAMPLE 13 
     In a manner generally similar to that of Example 1, hydroxy squarylium in the amount of 1 gram in a solvent mixture of 1 milliliter of ethylenediamine, 5 milliliters propylamine, and 24 milliliters of tetrahydrofuran was meniscus coated on an aluminized polyester substrate (Mylar) to form a charge generation layer and dried. A novel transport layer in accord with the instant invention was formed by meniscus coating a solution of polycarbonate resin (M60) in the amount of 8.12 grams and 8.12 grams of p-diethylaminobenzaladehyde-(diphenylhydrazone) in a 9:1 mixture of tetrahydrofuran and toluene on the coated support and drying to form a multilayered electrophotographic element. When tested as set forth in Example 1, 1.40 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 volts at a discharge response time of 146 milliseconds. 
     EXAMPLE 14 
     A multilayered electrophotographic element similar to that of Example 13 was prepared with the exception that o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone) was employed in the transport layer solution. When tested as set forth in Example 1, 1.02 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 volts at a discharge response time of 146 milliseconds. 
     EXAMPLE 15 
     A multilayered electrophotographic element similar to that of Example 13 was prepared with the exception that the charge generation layer solution contained 0.85 grams of hydroxy squarylium and 0.15 grams of methyl squarylium. When tested as set forth in Example 1, 0.86 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 volts at a discharge response time of 146 milliseconds. 
     EXAMPLE 16 
     A multilayered electrophotographic element similar to that of Example 13 was prepared with the exception that the charge generation solution contained 0.85 grams of hydroxy squarylium and 0.15 grams of methyl squarylium, and the charge transport layer solution contained 8.12 grams of polycarbonate resin (M60) and 5.42 grams of p-diethylaminobenzaldehyde-(diphenylhydrazone). When tested as set forth in Example 1, 1.10 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -870 volts to -150 volts at a discharge response time of 146 milliseconds. 
     EXAMPLE 17 
     A multilayered electrophotographic element was prepared by coating onto a charge generation layer (formed of vacuum deposited selenium and tellurium) a charge transport layer from a solution of 6.75 grams of polyester resin (PE200), 6.75 grams of polycarbonate resin (M 60) and 13.5 grams of p-diethylaminobenzaldehyde-(diphenylhydrazone). When tested as set forth in Example 1, 2.0 microjoules per square centimeter of light energy were required to discharge the element from a dark voltage of -800 volts to -300 volts at a discharge response time of 454 milliseconds. 
     From the above examples, it is apparent that the p-type charge transport system of the instant invention is operable with varying types of resin binders as well as a substantial number of hydrozone compounds of the designated type. Both organic and inorganic charge generation layers are suitable for use with the charge generation layer of the instant invention, and various combinations of solvents, polymeric binders etc. may be employed as is known in the art. When used in relatively high concentrations, certain of the hydrazones display a tendency to crystallize, thereby degrading the charge transport function. However, when lesser amounts are used, operable results are obtainable. Such adjustment will be readily accomplished by those skilled in the art. 
     The electrophotographic elements utilizing the charge transport layer in accord with the instant invention display an excellent balance between sensitivity, particularly at low temperatures, adhesion to adjacent layers and resistance to physical wear again at varying temperatures. The elements have been found to age well and display remarkable resistance to toner filming. 
     Although in view of the wide usage to which the present invention can be put, only limited embodiments of the invention have been described for purposes of illustration, it is, however, anticipated that various changes and modifications will be apparent to those skilled in the art, and that such changes and modifications may be made without departing from the scope of the invention as defined by the following claims.