The present invention provides a seleniumn electrophotographic photoreceptor comprising a laminate of a conductive base, a carrier transportaion layer consisting of amorphous selenium or an amorphous Se-Te alloy, a carrier generation layer consisting of an amorphous Se-Te alloy containing 20 to 50 wt % of Te, and an overcoat layer composed of two layers consisting of Se-As alloys having different arsenic concentrations and different thickenesses. In one embodiment of the invention, the lower overcoat layer contains 2-10% by weight arsenic while the upper overcoat layer contains 10-30% by weight arsenic. In another embodiment of the invention, the thickness of the upper overcoat layer is greater than that of the lower overcoat layer but not more than 8 .mu.m.

BACKGROUND OF THE INVENTION 
The present invention relates to a selenium electrophotographic laminate 
photoreceptor which is used in ordinary copying machines and optical 
printers having light-emitting diodes, laser diodes, gas lasers, or the 
like as light sources. 
In making hard copies, optical printers have been widely used owing to high 
copying speeds and good image quality. The wavelength of light from the 
light source of such an optical printer is in the range of 660 to 800 nm, 
namely, a long wavelength range. An electrophotographic photoreceptor with 
a charge generating layer made of a high-concentration Te-Se alloy 
containing 20 to 50 wt % of tellurium so as to have excellent 
electrophotographic properties in a long wavelength range is disclosed in 
applicants' Japanese Patent Laid-Open No. 278858/1986, and has been put to 
practical use. 
The printing durability of such a photoreceptor having a carrier generating 
layer of a high-concentration Te-Se alloy is determined by the overcoat 
layer. It is known that in order to enhance the printing durability, a 
selenium arsenic alloy with an increased arsenic concentration is used for 
the overcoat layer. However, when the arsenic concentration is increased, 
the thermal expansion coefficient of the overcoat layer is reduced, as 
shown in FIG. 2, so that the difference in the thermal expansion 
coefficient between the surface and the base layer of a Te-Se alloy or a 
carrier transportation layer is increased, thereby causing cracking. To 
prevent this, it is necessary to reduce the thickness of the overcoat 
layer, which decreases printing durability. To solve this problem, 
Japanese Patent Laid-Open No. 278858/1986 proposes that the overcoat layer 
have a two-layer structure and that the layer adjacent to the base layer 
have a lower arsenic concentration so as to serve as a buffer layer for 
the difference in the thermal expansion coefficient. 
The object of the present invention is to enhance the effect of the 
above-described structure and to provide a selenium electrophotographic 
photoreceptor having long wavelength sensitivity, heat resistance, and 
improved printing durability. 
SUMMARY OF THE INVENTION 
To achieve this aim, the present invention provides a selenium 
electrophotographic photoreceptor comprising a laminate of a conductive 
base, a carrier transportation layer consisting of amorphous selenium or 
an amorphous Se-Te alloy, a carrier generation layer consisting of an 
amorphous Se-Te alloy containing 20 to 50 wt % of Te, and an overcoat 
layer composed of two layers consisting of Se-As alloys having different 
arsenic concentrations and different thicknesses. In one embodiment of the 
invention, the lower overcoat layer contains 2-10% by weight arsenic while 
the upper overcoat layer contains 10-30% by weight arsenic. In another 
embodiment of the invention, the lower overcoat layer has a lower arsenic 
content than the upper overcoat layer, and the thickness of the upper 
overcoat layer is greater than that of the lower overcoat layer but not 
more than 8 .mu.m.

DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention, the overcoat layer of a function 
separation type selenium electrophotographic photoreceptor is composed of 
an upper overcoat layer having a high arsenic concentration and a lower 
overcoat layer having a low arsenic concentration, the thicknesses of the 
upper and lower overcoat layers being different. These features prevent 
cracking caused by a difference in the thermal expansion coefficients of 
the upper overcoat layer and the base layer. It is possible to enhance the 
printing durability of the photoreceptor by making the upper overcoat 
layer thicker than the lower overcoat layer, but not more than 8 .mu.m to 
avoid deterioration in printing quality. 
By increasing the arsenic concentration to 10-30% by weight in the upper 
overcoat layer and 2-10% by weight in the lower overcoat layer, the upper 
overcoat layer may have a smaller thickness than the lower overcoat layer, 
thereby enhancing the printing durability as well as the printing quality. 
The following non-limiting examples are designed to further illustrate the 
claimed invention. 
EXAMPLE 1 
FIGS. 1(a) and (b) are sectional views of a first embodiment of a 
photoreceptor according to the present invention and a first comparative 
example of a photo-receptor, respectively. They were prepared as follows: 
an aluminum cylinder having a diameter of 120 mm was washed and mounted on 
the support shaft of an evaporation apparatus. While maintaining the 
temperature of the conductive base (1) at about 70.degree. C., the 
apparatus was evacuated to 1.times.10.sup.-5 Torr. The evaporation source 
containing pure selenium was heated to about 300.degree. C., thereby 
depositing a carrier transportation layer (2) having a thickness of about 
50 .mu.m. Thereafter, by flash deposition, a carrier generation layer (3) 
of 44 wt % Te-Se alloy was deposited to a thickness of about 0.5 .mu.m, a 
lower overcoat layer (4) of 1.5 wt % As-Se alloy was next deposited to a 
thickness of about 2 .mu.m, and finally an upper overcoat layer (5) of 4 
wt % As-Se alloy was deposited to a thickness of about 3 .mu.m in the case 
of the first embodiment shown in FIG. 1(a), and about 1 .mu.m in the case 
of the first comparative example shown in FIG. 1(b). The conditions for 
the flash deposition were as follows: The temperature of the support shaft 
was 60.degree. C., the pressure was 1.times.10.sup.-5 Torr, and the 
temperature of the evaporation source was 350.degree. C. 
As a second embodiment, a photoreceptor in which the thickness of the upper 
overcoat layer (5) was about 6 .mu.m, and as a third embodiment, a 
photoreceptor in which the thickness of the upper overcoat layer (5) was 
about 8 .mu.m, were produced, as shown in FIGS. 3 and 4, respectively. 
Both the materials and thicknesses of the base (1), the charge 
transportation layer (2), and the lower overcoat layer (4) were the same 
as those of the first embodiment and the first comparative example. The 
evaporating conditions for each layer including the upper overcoat layer 
(5) were also the same. 
The repetitive properties, printing durabilities and external appearances 
of these photoreceptors were compared. As to the repetitive properties, 
the reduction in charging, which causes photographic fog in printing, and 
the rise in the residual potential, which lowers the printing density, 
were evaluated. All the photoreceptors were at the same level in the 
reduction in charging. The residual potential had a tendency to increase 
as the thickness of the upper overcoat layer (5) became larger, as 
indicated by the value after 250 cycles in FIG. 5. When the thickness of 
the upper overcoat layer (5) exceeded 8 .mu.m, the residual potential 
became 100 V or more, resulting in a reduction in the printing density. 
In order to evaluate the printing durability, after printing had been made 
on 50,000 sheets of A4 paper by using a laser diode printer of a reversal 
development system, the thickness of the upper overcoat layer (5) was 
measured. The results are shown in FIG. 6. The larger the original 
thickness of the upper overcoat layer (5), the larger the thickness of the 
residual upper layer, in other words, the longer the printing life. These 
evaluations are collectively shown in Table 1, in which O denotes 
superior, .DELTA. denotes acceptable and X denotes inferior. 
TABLE 1 
______________________________________ 
Print- 
External 
Repetitive Properties 
ing 
Appear- 
Reduction Residual Dura- Evalu- 
ance In charge Potential 
bility 
ation 
______________________________________ 
First .largecircle. 
.largecircle. 
.largecircle. 
.DELTA. 
.largecircle. 
Embodiment 
Second .largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
.largecircle. 
Embodiment 
Third .largecircle. 
.largecircle. 
.DELTA. 
.largecircle. 
.DELTA. 
Embodiment 
First .largecircle. 
.largecircle. 
.largecircle. 
X X 
Comparative 
Example 
______________________________________ 
EXAMPLE 2 
Two photoreceptors (fourth and fifth embodiments below) in accordance with 
the claimed invention were prepared as follows. An aluminum cylinder 
having a diameter of 80 mm was cleaned and installed on the support shaft 
of an evaporation apparatus as a conductive base. While maintaining the 
temperature of the conductive base at about 60.degree. C., the apparatus 
was evacuated to 1.times.10.sup.-5 Torr. The evaporation source containing 
pure selenium was heated to about 300.degree. C., and a carrier 
transportation layer having a thickness of about 60 .mu.m was deposited on 
the conductive base. Next, by flash deposition, a carrier generation layer 
comprising a Te-Se alloy containing 46% by weight Te was deposited 
thereon. Finally, the lower overcoat layer and upper overcoat layer of a 
surface protective layer were deposited on the carrier generation layer. 
In the fourth embodiment, the lower overcoat layer comprised an As-Se alloy 
containing about 4% by weight As and had a thickness of about 2 .mu.m, 
while the upper overcoat layer comprised an As-Se alloy containing about 
15% by weight As and had a thickness of about 1 .mu.m. 
In the fifth embodiment, the lower overcoat layer comprised an As-Se alloy 
containing 40% by weight As and was about 2 .mu.m thick. The upper 
overcoat layer of this photoreceptor comprised an As-Se alloy containing 
25% by weight As and was about 1 .mu.m thick. The lower and upper overcoat 
layers were formed by evaporation at a temperature of 60.degree. C. 
For comparison, three further comparative examples (second, third and 
fourth comparative examples) were prepared. The second comparative example 
was prepared in the same manner as the fourth and fifth embodiments above, 
however, the upper overcoat layer in the second comparative example 
contained 35% by weight As. 
The third comparative example was also prepared in the same manner, 
however, the lower overcoat layer contained 2% by weight As and the upper 
overcoat layer contained 5% by weight As. 
Finally, the fourth comparative example was also prepared in the same 
manner, however, the lower overcoat layer contained 2% by weight As and 
had a thickness of about 4 .mu.m and the upper overcoat layer contained 
about 5% by weight As and had a thickness of about 2 .mu.m. 
The fourth and fifth embodiments and the second, third and fourth 
comparative examples were compared by measuring their surface hardness 
(Vickers hardness meter) to evaluate their printing proofness, and by 
examining their external appearances (the photoreceptors were checked for 
cracks after standing at 25.degree. C.-45.degree. C. for 1000 hours). The 
results of these comparisons are shown in Table 2. Again, 0 denotes 
superior and X denotes inferior. 
TABLE 2 
______________________________________ 
Estimated 
Film print proof 
Surface thick- sheet no. External 
hardness 
ness (.times. 10,000 
appearance 
(kg/mm.sup.2) 
(.mu.m) sheets 25.degree. C. 
45.degree. C. 
______________________________________ 
Fourth 60 3 15 0 0 
Embodiment 
Fifth 80 3 20 0 0 
Embodiment 
Second 130 3 50 0 X 
Comparative 
Example 
Third 40 3 10 0 0 
Comparative 
Example 
Fourth 40 6 15 0 0 
Comparative 
Example 
______________________________________ 
These results indicate that although surface hardness is enhanced and the 
printing proofness is improved by increasing the As content in the upper 
overcoat layer, when the As content exceeds 30% by weight, cracks are 
generated at high temperatures. On the other hand, when the upper overcoat 
layer contains less than 10% by weight As, electric resistance rises and 
the luster of the photoreceptor surface vanishes. 
Applicants have found that photoreceptors according to the claimed 
invention have virtually the same initial electric characteristics and 
printing characteristics as conventional photoreceptors employing higher 
amounts of As. Yet, the generation of cracks is avoided with applicants' 
two-layer surface protective layer containing different amounts of As.