An electrophotographic element comprising an electrically conductive substrate and a monolayer or multilayer type photo-sensitive layer, superposed thereon, containing as an effective ingredient a disazo pigment having the general formula (I): ##STR1## wherein Ar.sup.1 and Ar.sup.2 each represent a phenylene group. However, when Ar.sup.1 is 1,4-phenylene group Ar.sup.2 is 1,2- or 1,3-phenylene group; when Ar.sup.1 is 1,3-phenylene group Ar.sup.2 may be any one of 1,2-, 1,3- and 1,4-phenylene groups; and when Ar.sup.1 is 1,2-phenylene group Ar.sup.2 is 1,3-phenylene group. X represents aromatic rings such as benzene ring, naphthalene ring and the like, hetero rings such as indole ring, carbazole ring, benzofuran ring and the like or their substitution products; Ar.sup.3 represents aromatic rings such as benzene ring, naphthalene ring and the like, hetero rings such as dibenzofuran ring and the like or their substitution products; and R represents a hydrogen atom, a lower alkyl group, phenyl group or its substitution product.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates in general to an electrophotographic element, and in 
particular to a novel electrophotographic element which comprises a 
photosensitive layer containing as an effective ingredient a disazo 
pigment having the general formula (I): 
##STR2## 
(wherein Ar.sup.1 and Ar.sup.2 can each represent a phenylene group. 
However, when Ar.sup.1 is 1,4-phenylene group Ar.sup.2 is 1,2- or 
1,3-phenylene group; when Ar.sup.1 is 1,3-phenylene group Ar.sup.2 may be 
any one of 1,2-, 1,3- and 1,4-phenylene groups; and when Ar.sup.1 is 
1,2-phenylene group Ar.sup.2 is 1,3-phenylene group. X can represent 
aromatic rings such as benzene ring, naphthalene ring and the like, hetero 
rings such as indole ring, carbazole ring, benzofuran ring and the like or 
their substitution products; Ar.sup.3 can represent aromatic rings such as 
benzene ring, naphthalene ring and the like, hetero rings such as 
dibenzofuran ring and the like or their substitution products; and R can 
represent a hydrogen atom, a lower alkyl group, phenyl group or its 
substitution product.) 
(2) Description of the Prior Art 
As light-sensitive materials which have long been utilized in 
electrophotographic elements, there are widely known inorganic 
light-sensitive materials such as amorphous selenium, selenium alloys, 
calcium sulfide, zinc oxide, etc., poly-N-vinyl carbazole and its 
derivatives. 
It is known that among them, the amorphous selenium and selenium alloys are 
universally put to practical use because of the fact that they possess 
exceedingly superior characteristics indispensable for electrophotographic 
elements. However, the amorphous selenium is observed to be defective in 
that its light sensitive wavelength region is limited to the blue region 
and is scarcely sensitive to the red region. 
Various kinds of methods have hitherto been proposed in order to widen the 
sensitivity to the long wave regions. However, the fact is that the 
sensitivity of the amorphous selenium to the long wave light is not 
improved to such a degree as intended by those methods because the 
selection of the light sensitive wavelength region is subjected to various 
limitations. In the case of utilizing zinc oxide or cadmium sulfide as 
light-sensitive material, it is needed to add various kinds of sensitizers 
in order that said zinc oxide or calcium sulfide may serve for practical 
purposes, because the light sensitive wavelength region of these materials 
is also limited. 
While the poly-N-vinyl carbazole, universally known as organic 
photoconductive material, is abundant in superior abilities such as 
transparency, film forming ability, flexibility, hole transportability and 
the like, it is observed to be defective in that it is itself scarcely 
sensitive to 400 to 700 nm visible light wave length. 
As a photosensitive element which has successfully overcome the above 
mentioned deficiencies, Japanese Patent Publication No. 10496/1975 
discloses one utilizing therein a charge transfer complex consisting of 
poly-N-vinyl carbazole and 2,4,7-trinitrofluorenone. 
In Japanese Patent Publication Nos. 5349/1970, 3168/1974, 14914/1975, 
10982/1976, etc. are further discussed photosensitive elements of the type 
which comprises, in successive layers; a charge carrier generating layer 
formed of amorphous selenium or selenium alloy and a charge transfer 
layer, each layer being designed to play a part allotted thereto. 
Still further, the following different photosensitive elements of the type 
which comprises, in successive layers, a charge carrier generating layer 
including a different kind of pigment and a charge transfer layer, have 
hitherto been developed. U.S. Pat. No. 3,837,851 discloses a 
photosensitive element which comprises a charge carrier generating layer 
and a charge transfer layer including at least one tri-arylpyrazoline. In 
U.S. Pat. No. 3,850,630 is disclosed a photosensitive element which 
comprises a transparent charge transfer layer and a charge carrier 
generating layer including an indigo dye. U.S. Pat. No. 3,871,882 
discloses a photosensitive element which comprises a charge carrier 
generating layer including a perylene pigment derivative and a charge 
transfer layer including a condensate of 3-bromopylene and formaldehyde. 
Japanese Laid-open Patent Application No. 133445/1978 discloses a 
photosensitive element which comprises a transparent charge transfer layer 
and a charge carrier generating layer including a disazo pigment having 
astilbene skeleton. Some of the above mentioned photosensitive elements 
have already been commercially available. However, the fact is that there 
have not been developed yet photosensitive elements capable of satisfying 
various proposed properties in a satisfactory degree. 
SUMMARY OF THE INVENTION 
The inventor has carried out various studies to thereby discover that a 
compound having the general formula referred to below, when utilized in 
electrophotographic elements, works effectively as the charge carrier 
generating material or photoconductive material. The present invention has 
been completed on the basis of this discovery. 
It is therefore a primary object of this invention to provide a charge 
carrier generating material which has a flat sensitivity to the visible 
light region, in particular the short wavelength region and thereby 
provide an electrophotographic element which is capable of exhibiting a 
superior reproducibility in the visible light region irrespective of 
various kinds of light sources to be employed in electrophotographic 
copying machines. It is another object of this invention to provide an 
electrophotographic element containing as the effective ingredient a 
charge carrier generating material capable of exhibiting a higher 
sensitivity to the visible light region, in particular the short 
wavelength region as compared with conventional charge carrier generating 
materials. 
In other words, the electrophotographic element according to the present 
invention is characterized in that it comprises an electrically conductive 
substrate and a photosensitive layer, superposed thereon, containing as an 
effective ingredient a disazo pigment having the general formula: 
##STR3## 
(wherein Ar.sup.1 and Ar.sup.2 can each represent a phenylene group. 
However, when Ar.sup.1 is 1,4-phenylene group Ar.sup.2 is 1,2- or 
1,3-phenylene group; when Ar.sup.1 is 1,3-phenylene group Ar.sup.2 may be 
any one of 1,2-, 1,3- and 1,4-phenylene groups; and when Ar.sup.1 is 
1,2-phenylene group Ar.sup.2 is 1,3-penylene group. X can represent 
aromatic rings such as benzene ring, naphthalene ring and the like, hetero 
rings such as indole ring, carbazole ring, benzofuran ring and the like or 
their substitution products; Ar.sup.3 can represent aromatic rings such as 
benzene ring, naphthalene ring and the like, hetero rings such as 
dibenzofuran ring and the like or their substitution products; and R can 
represent a hydrogen atom, a lower alkyl group, phenyl group or its 
substitution product.) 
The disazo compounds having the above mentioned general formula (I) may be 
classified into six types according to the bonding modes of Ar.sup.1 with 
Ar.sup.2. These modes may be represented by the following general formulas 
(II) to (VII), wherein 
##STR4## 
The disazo compounds having the above mentioned general formula (I) 
suitably used in the present invention can be specifically represented by 
the following structural formulas in accordance with the aforesaid types: 
(1) Disazo pigments having the general formula (II) 
__________________________________________________________________________ 
Pig- 
ment 
No. Structural formula 
__________________________________________________________________________ 
##STR5## 
##STR6## 
##STR7## 
##STR8## 
##STR9## 
##STR10## 
##STR11## 
##STR12## 
##STR13## 
##STR14## 
##STR15## 
##STR16## 
##STR17## 
##STR18## 
##STR19## 
##STR20## 
##STR21## 
##STR22## 
##STR23## 
##STR24## 
##STR25## 
##STR26## 
##STR27## 
##STR28## 
##STR29## 
##STR30## 
##STR31## 
##STR32## 
##STR33## 
##STR34## 
##STR35## 
##STR36## 
##STR37## 
##STR38## 
##STR39## 
##STR40## 
##STR41## 
##STR42## 
##STR43## 
##STR44## 
##STR45## 
##STR46## 
##STR47## 
##STR48## 
##STR49## 
##STR50## 
##STR51## 
##STR52## 
##STR53## 
##STR54## 
##STR55## 
##STR56## 
##STR57## 
##STR58## 
##STR59## 
##STR60## 
(2) Disazo pigments having the general formula (III) 
__________________________________________________________________________ 
Pig- 
ment 
No. Structural formula 
__________________________________________________________________________ 
##STR61## 
##STR62## 
##STR63## 
##STR64## 
##STR65## 
##STR66## 
##STR67## 
##STR68## 
##STR69## 
##STR70## 
##STR71## 
##STR72## 
##STR73## 
##STR74## 
##STR75## 
##STR76## 
##STR77## 
##STR78## 
##STR79## 
##STR80## 
##STR81## 
##STR82## 
##STR83## 
##STR84## 
##STR85## 
##STR86## 
##STR87## 
##STR88## 
##STR89## 
##STR90## 
##STR91## 
##STR92## 
##STR93## 
##STR94## 
##STR95## 
##STR96## 
##STR97## 
##STR98## 
##STR99## 
##STR100## 
##STR101## 
##STR102## 
##STR103## 
##STR104## 
##STR105## 
##STR106## 
##STR107## 
##STR108## 
(3) Disazo pigments having the general formula (IV) 
__________________________________________________________________________ 
Pig- 
ment 
No. Structural formula 
__________________________________________________________________________ 
##STR109## 
##STR110## 
##STR111## 
##STR112## 
##STR113## 
##STR114## 
##STR115## 
##STR116## 
##STR117## 
##STR118## 
##STR119## 
##STR120## 
##STR121## 
##STR122## 
##STR123## 
##STR124## 
##STR125## 
##STR126## 
##STR127## 
##STR128## 
##STR129## 
##STR130## 
##STR131## 
##STR132## 
##STR133## 
##STR134## 
##STR135## 
##STR136## 
##STR137## 
##STR138## 
##STR139## 
##STR140## 
##STR141## 
##STR142## 
##STR143## 
##STR144## 
##STR145## 
##STR146## 
##STR147## 
##STR148## 
##STR149## 
##STR150## 
##STR151## 
##STR152## 
##STR153## 
##STR154## 
##STR155## 
##STR156## 
__________________________________________________________________________ 
(4) Disazo pigments having the general formula (V) 
__________________________________________________________________________ 
Pig- 
ment 
No. 
Structural formula 
__________________________________________________________________________ 
##STR157## 
##STR158## 
##STR159## 
##STR160## 
##STR161## 
##STR162## 
##STR163## 
##STR164## 
##STR165## 
##STR166## 
##STR167## 
##STR168## 
##STR169## 
##STR170## 
##STR171## 
##STR172## 
##STR173## 
##STR174## 
##STR175## 
##STR176## 
##STR177## 
##STR178## 
##STR179## 
##STR180## 
##STR181## 
##STR182## 
##STR183## 
##STR184## 
##STR185## 
##STR186## 
##STR187## 
##STR188## 
##STR189## 
##STR190## 
##STR191## 
##STR192## 
##STR193## 
##STR194## 
##STR195## 
##STR196## 
##STR197## 
##STR198## 
##STR199## 
##STR200## 
##STR201## 
##STR202## 
##STR203## 
##STR204## 
__________________________________________________________________________ 
(5) Disazo pigments having the general formula (VI) 
__________________________________________________________________________ 
Pigment 
No. Structural formula 
__________________________________________________________________________ 
##STR205## 
##STR206## 
##STR207## 
##STR208## 
##STR209## 
##STR210## 
##STR211## 
##STR212## 
##STR213## 
##STR214## 
##STR215## 
##STR216## 
##STR217## 
##STR218## 
##STR219## 
##STR220## 
##STR221## 
##STR222## 
##STR223## 
##STR224## 
##STR225## 
##STR226## 
##STR227## 
##STR228## 
##STR229## 
##STR230## 
##STR231## 
##STR232## 
##STR233## 
##STR234## 
##STR235## 
##STR236## 
##STR237## 
##STR238## 
##STR239## 
##STR240## 
##STR241## 
##STR242## 
##STR243## 
##STR244## 
##STR245## 
##STR246## 
##STR247## 
##STR248## 
##STR249## 
##STR250## 
##STR251## 
##STR252## 
__________________________________________________________________________ 
(6) Disazo pigments having the general formula (VII) 
__________________________________________________________________________ 
Pigment 
No. Structural formula 
__________________________________________________________________________ 
##STR253## 
##STR254## 
##STR255## 
##STR256## 
##STR257## 
##STR258## 
##STR259## 
##STR260## 
##STR261## 
##STR262## 
##STR263## 
##STR264## 
##STR265## 
##STR266## 
##STR267## 
##STR268## 
##STR269## 
##STR270## 
##STR271## 
##STR272## 
##STR273## 
##STR274## 
##STR275## 
##STR276## 
##STR277## 
##STR278## 
##STR279## 
##STR280## 
##STR281## 
##STR282## 
##STR283## 
##STR284## 
##STR285## 
##STR286## 
##STR287## 
##STR288## 
##STR289## 
##STR290## 
##STR291## 
##STR292## 
##STR293## 
##STR294## 
##STR295## 
##STR296## 
##STR297## 
##STR298## 
##STR299## 
##STR300## 
__________________________________________________________________________ 
The above enumerated disazo pigments can be manufactured readily by 
diazotizing Diamino distyryl benzene to isolate tetrazonium salt and 
subjecting it to the coupling reaction in a suitable organic solvent, for 
instance, such as N,N-dimethyl formamide and in the presence of a coupler 
corresponding to each of the above enumerated disazo pigments and alkali. 
Preparation Example 1 (Tetrazonium difluoroborate) 
28.0 g of 1,4-bis(3-aminostyryl)benzene were added to a dilute hydrochloric 
acid solution consisting of 1200 ml of water and 60 ml of concentrated 
hydrochloric acid. The resulting mixture was stirred at 60.degree. C. for 
30 minutes and then cooled to 0.degree. C. In succession, a solution 
obtained by dissolving 13.0 g of sodium nitrite in 40 ml of water was 
dropped therein at a temperature ranging from 0.degree. to 3.degree. C. 
over 40 minutes. Subsequently, the same was stirred for 30 minutes at the 
same temperature, and then a very small quantity of insoluble matter as 
filtered out therefrom. 50 ml of a 42% borofluoric acid were added to this 
filtrate. The thus separated crystals were filtered out, washed with water 
and dried to obtain 42.8 g (yield: 91.0%) of tetrazoniumdifluoroborate 
fulvescent crystals. This crystal was observed to have a decomposition 
point of 74.degree. C. or more. And, referring to its infrared absorption 
spectrum taken by KBr tablet method, there were observed the absorption 
zone caused by N.sub.2 at the wave number of 2260 cm.sup.-1 and that 
caused by the trans-CH.dbd.CH-- at the wave number of 965 cm.sup.-1. 
Preparation Example 2-6 
The same procedure as Preparation Example 1 was followed with the exception 
that the diamino compounds as enumerated in the following Table A were 
employed in place of the 1,4-bis(3-aminostyryl) benzene to thereby obtain 
tetrazonium difluoroborates. The obtained results are as shown in Table A. 
TABLE A 
__________________________________________________________________________ 
Tetrazonium difluoroborate 
Prepa- Decompo- 
ration sition 
Example Appear- 
Yield 
point IR spectrum (KBr) 
cm.sup.-1 
No. Diamino compound ance (%) (.degree.C.) 
.nu.N.sub.2 
.delta.t-CHCH 
__________________________________________________________________________ 
##STR301## yellow 
95.0 
105 2240 960 
3 
##STR302## orange 
92.0 
83 2230 960 
4 
##STR303## yellow 
94.0 
92 2240 965 
5 
##STR304## yellow 
91.8 
70 2250 965 
5 
##STR305## brownish yellow 
90.2 
70 2260 960 
__________________________________________________________________________ 
Preparation Example 7 (Aforesaid No. II-1 disazo pigment) 
3.0 g of the tetrazonium salt obtained according to Preparation Example 1 
and 3.3 g of 2-hydroxy-3-anilide naphthoate serving as a coupler were 
dissolved in 390 ml of cooled N,N-dimethylformamide. A solution consisting 
of 1.9 g of sodium acetate and 20 ml of water was dropped in the former 
solution at a temperature ranging from 4.degree. to 8.degree. C. for 1 
hour, and thereafter the same was stirred at room temperature for about 3 
hours. Then, precipitates were filtered out, washed three times with 300 
ml of water, and further washed seven times with 300 ml of 
N,N-dimethylformamide. The thus obtained red crystalline product was dried 
at 70.degree. C. under a reduced pressure of 2 mmHg, thereby obtaining 4.7 
g (yield: 91%) of No. II-1 disazo pigment. This pigment was observed to 
have a melting point of 250.degree. C. or more. 
The obtained elementary analysis results are as shown below. 
______________________________________ 
Calculated value (%) 
Measured value (%) 
______________________________________ 
C 78.11 77.95 
H 4.69 4.51 
N 9.76 9.61 
______________________________________ 
With reference to the infrared absorption spectrum taken by KBr tablet 
method there were observed an absorption zone caused by secondary amide at 
the wave number of 1680 cm.sup.-1 and that caused by the trans-CH.dbd.CH-- 
at the wave number of 955 cm.sup.-1. 
Preparation Examples 8-12 
The same procedure as Preparation Example 7 with the exception that the 
tetrazonium salts obtained according to Preparation Examples 2-6 were 
employed in place of the tetrazonium salt employed in Preparation Example 
7 was repeated to obtain disazo pigments of this invention. The obtained 
results are as shown in Table B. 
TABLE B 
__________________________________________________________________________ 
Prepa- 
ration 
Disazo Melting 
Elementary analysis.sup.a 
Example 
pigment 
Appear- 
Yield 
point 
(%) IR spectrum (KBr) cm.sup.-1 
No. No. ance (%) (.degree.C.) 
Cobtd. 
Hobtd. 
Nobtd. 
.nu.c = 0 
.delta.t-CH.dbd.CH-- 
__________________________________________________________________________ 
8 III-1 
reddish 
89.0 
&gt;250 77.85 
4.70 
9.75 
1680 960 
purple 
9 IV-1 dark 91.0 
&gt;250 77.82 
4.78 
9.72 
1675 960 
red 
10 V-1 dark 93.0 
&gt;250 77.90 
4.56 
9.55 
1680 950 
red 
11 VI-1 red 75.6 
&gt;250 77.85 
4.70 
9.52 
1670 955 
12 VII-1 
orange 
91.0 
300 77.88 
4.61 
9.51 
1680 960 
red (dec.) 
__________________________________________________________________________ 
.sup.a The values calculated with disazo pigments III1.about. VII-1 are 
all the same. 
Ccalcd. = 78.11%, Hcalcd. = 4.69%, Ncalcd. = 9.76% 
The electrophotographic elements according to the present invention, which 
contain the above enumerated disazo pigments, make take the forms as 
illustrated in FIGS. 1 to 3 depending on the application modes of these 
pigments. 
The element illustrated in FIG. 1 comprises, in successive layers; an 
electrically conductive substrate 1, a charge carrier generating layer 5 
consisting essentially of a disazo pigment 3 and a charge transfer layer 6 
consisting essentially of a charge transfer material. 
In the element illustrated in FIG. 1, said element is exposed to light in 
an imagewise manner and said light passes through the charge transfer 
layer and reaches the charge carrier generating layer 5 to thereby permit 
the disazo pigment 3 at the light struck portions of the element to 
generate charge carriers. The charges generated in the charge carrier 
generating layer are injected into the charge transfer layer 6 and are 
transported therethrough. Accordingly, the element of the present 
invention may be said to have a mechanism so designed that the generation 
of charge carriers required for light decay is made by the disazo pigment 
and the transfer of charges is made by the charge transfer layer. In this 
regard, it is to be understood that in this instance the relative position 
of the charge carrier generating layer to the charge transfer layer may be 
inverted. 
The element illustrated in FIG. 2 comprises an electrically conductive 
substrate 1 and a photosensitive layer 2', superposed thereon, consisting 
essentially of a disazo pigment 3 and a charge transfer medium 4, said 
medium consisting of a charge transfer material and an insulating binder. 
The disazo pigment referred to herein is likewise a charge carrier 
generating material. 
The element illustrated in FIG. 3 comprises an electrically conductive 
substrate 1 and a photosensitive layer 2", superposed thereon, consisting 
essentially of a disazo pigment 3 (which is used herein as a 
photoconductive material) and an insulating binder. 
The element illustrated in FIG. 1 can be prepared by vacuum depositing the 
disazo pigment onto the electrically conductive substrate in accordance 
with the vacuum deposition methods disclosed in U.S. Pat. Nos. 3,973,959; 
3,996,049; etc., or by coating, if need be, a dispersion, which is 
obtained by dispersing fine disazo pigment particles in a suitable solvent 
with a binder dissolved therein, onto the electrically conductive 
substrate and drying, or by coating a solution containing a charge 
transfer material and a binder onto the electrically conductive substrate 
after surface finishing or film-thickness controlling has been completed 
by buffing as disclosed, for instance, in Japanese Laid-open Patent 
Application No. 90827/1976 or the like and drying. 
The element illustrated in FIG. 2 can be prepared by dispersing fine disazo 
pigment particles in a solution having dissolved therein a charge transfer 
material and a binder, coating the resulting dispersion onto an 
electrically conductive substrate and drying. 
The element illustrated in FIG. 3 can be prepared by dispersing fine disazo 
pigment particles in a binder solution, coating the resulting dispersion 
onto an electrically conductive substrate and drying. 
In this connection, it is to be understood that in each case the disazo 
pigment to be utilized in the present invention is pulverized by means of 
a ball mill or the like so that the particle size thereof may be 5.mu. or 
less, preferably 2.mu. or less. The coating is effected by means of the 
conventional methods, for instance, such as doctor blade coating, dip 
coating, wire bar coating or the like. 
Reference will be made to the thickness of the photosensitive layer. In the 
case of the element illustrated in FIG. 1, the thickness of the charge 
carrier generating layer is between 0.01.mu. to 5.mu., preferably between 
0.05.mu. to 2.mu.. When this thickness is less than 0.01.mu. the 
generation of charge carriers is insufficient, while when said thickness 
is over 5.mu. the residual potential is too high to be put to practical 
use. Referring to the thickness of the charge transfer layer, it is 
between 3.mu. to 50.mu., preferably between 5.mu. to 20.mu.. When this 
thickness is less than 3.mu. the quantity of electricity charged is 
insufficient, while when said thickness is more than 50.mu. the residual 
potential is too high to be put to practical use. The percentage of the 
disazo pigment contained in the charge carrier generating layer 5 
desirably is 30% by weight or more, preferably 50% by weight or more. The 
percentage of the charge transfer material contained in the charge 
transfer layer is between 10 to 95% by weight, preferably between 30 to 
90% by weight. When the percentage of the charge transfer material 
occupying the charge transfer layer is less than 10% by weight the 
transfer of charges scarcely takes place, and on the other hand when said 
percentage is more than 95% by weight the element can not be put to 
practical use because the mechanical strength of the film of the 
photosensitive layer is extremely deteriorated. 
In the element illustrated in FIG. 2, the thickness of the photosensitive 
layer 2' is between 3 and 50.mu., preferably between 5 and 20.mu., and the 
percentage of the disazo pigment 3 contained in the photosensitive layer 
2' is less than 50% by weight, preferably 20% by weight or less. 
In the element illustrated in FIG. 3, the thickness of the photosensitive 
layer is between about 3.mu. to 50.mu., preferably between 5 to 20.mu., 
and the percentage of the disazo pigment contained in the photosensitive 
layer is between 30 to 70% by weight relative to the photosensitive layer, 
preferably about 50% by weight. 
In this regard, it is to be understood that plasticizers may be employed in 
conjunction with binders in the preparation of each one of elements 
illustrated in FIGS. 1 to 3. 
As the electrically conductive substrates suitably used for the elements of 
the present invention there can be enumerated metal plates of aluminum, 
copper, zinc, etc.; plastic sheets or film such as polyester or the having 
deposited thereon electrically conductive substance like aluminum, 
SnO.sub.2, etc.; or electroconductively treated papers, etc. 
As the binders suitably used for the present invention there can be 
enumerated condensation resins such as polyamide resins, polyurethane 
resins, polyester resins, epoxy resins, polyketone resins, polycarbonate 
resins, etc.; vinyl polymers such as polyvinylketone, polystyrene, 
poly-N-vinylcarbazole, polyacrylamide, etc.; and the like. However, it is 
to be undersood that any insulating and adhesive resins may be employed in 
the present invention. 
As the plasticizers suitable for the present invention there can be 
enumerated halogenized paraffin, polychlorobiphenyl, dimethylnaphthalene, 
dibutyl phthalate, etc. Silicone oil and the like may be added thereto in 
order to improve the surface smoothness of the element. 
As particular preferred charge transfer materials, furthermore, there can 
be enumerated high molecular ones, namely, vinyl polymers such as 
poly-N-vinyl carbazole, polyvinyl indoloquinoxaline, 
polyvinyldibenzothiophene, polyvinyl anthracene, polyvinyl acridine, etc. 
and condensation resins, such as bromopyrene.about.formaldehyde resin, 
ethylcarbazole.about.formaldehyde resin, etc.; and low molecular ones 
(monomers) such as 2,4,7-trinitro-9-fluoroenone, 
2,6,8-trinitro-4H-indeno[1,2-b]thiophene-4-one, 
2,8-dinitrodibenzothiophene, 1,3,7-trinitrodibenzothiophene-5,5-dioxide, 
1,3,7,9-tetranitrobenzo[c]cinnoline-5-oxide, 2,4,8-trinitrothioxanthone, 
1-bromopyrene, N-ethylcarbazole, 2-phenylindole, 2-phenylnaphthalene, 
2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, 
2,5-bis(4-diethylaminophenyl)-1,3,4-triazole, 
1-phenyl-3-(4-diethylaminostyryl)-5-(4-diethylaminophenyl) pyrazoline, 
2-phenyl-4-(4-diethylaminophenyl)-5-phenyloxazole, triphenylamine, 
tris(4-diethylaminophenyl)methane, 
3,6-bis(dibenzylamino)-9-ethylcarbazole, 4,4'-bis(dibenzylamino)diphenylme 
thane, 4,4'-bis(dibenzylamino)diphenylether, 
1,1-bis(4-dibenzylaminophenyl)propane, 
2-(.alpha.-naphthyl)-5-(4-diethylaminophenyl)-1,3,4-oxadiazole, 
2-styryl-5-(3-N-ethylcarbazolyl)-1,3,4-oxadiazole, 
2-(4-methoxyphenyl)-5-(3-N-ethylcarbazolyl)-1,3,4-oxadiazole, 
2-(4-diethylaminophenyl)-5-(3-N-ethylcarbazolyl)-1,3,4-oxadiazole, 
9-(4-diethylaminostyryl)anthracene, 9-(4 -diethylaminostyryl)anthracene, 
.alpha.-(9-anthryl)-.beta.-(3-N-ethylcarbazolyl)ethylene, 
5-methyl-2-(4-diethylaminostyryl)benzoxazole, 
9-(4-dibenzylaminobenzylidene)fluorene, 
N-ethyl-3-(9-fluorenylidene)carbazole, 
2,6-bis(4-diethylaminostyryl)pyrydine, 
methylphenylhydrazono-3-methylidene-9-ethylcarbazole, 
methylphenylhydrazono-4-methylidene-N,N-diethylaniline, etc. These charge 
transfer materials may be used singly or in combination with two or more 
members. The most preferable charge transfer material varies depending on 
the kinds of disazo pigments used. Although the reason is unknown, the 
most preferable electrophotographic element is in fact manufactured by 
combining a certain kind of disazo pigment with a certain kind of charge 
transfer material. 
In this context, it is to be noted that in each of the electrophotograhic 
elements obtained as aforesaid an adhesive layer or a barrier layer may be 
interposed between the electrically conductive substrate and the 
photosensitive layer as occasion demands. The preferred materials for this 
adhesive or barrier layer include polyamide, nitrocellulose, aluminum 
oxide and the like, and the preferable thickness of said layer is 1.mu. or 
less. 
The reproduction using the element of the present invention is achieved by 
electrifying the surface of the photosensitive layer of the element, 
exposing the same to light, then developing and, if needed, transferring 
to paper or the like. The element according to this invention, which is 
rich in flexibility and is highly sensitive to the visible light region, 
in particular the short wavelength region, can exhibit a superior 
reproducibility in the visible light region irrespective of various kinds 
of light sources to be employed in electrophotographic copying machines, 
and further the element according to this invention, which as a matter of 
course can dispense with a filter for cutting off the long wavelength 
light, is advantageous in that it can enhance the sensitivity of the 
copying system as a whole and so forth.