Note: Descriptions are shown in the official language in which they were submitted.
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PHOTORECEPTOR CONSTRUCTION
Field of the Invention
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The present invention relates to novel electro-
photographic imaging systems and particularly to novel
electrophotographic photoreceptors. These photoreceptors
comprise a conductive substrate, an inorganic barrier-
charge transport layer, and a photoconductive insulative
layer.
Background of the Invention
In the art of electrophotography, and particu-
larly xerography, it is well known to coat a conductive
substrate, such as an electrically conductive aluminum
drum or aluminized polymeric sheeting, with a photoconduc-
tive insulating layer to form a composite, layered, imag-
ing article. The surface of the layered imaging structure
is then uniformly electrostatically charged and exposed to
a pattern of activating electromagnetic radiation, such as
light. m e charge is selectively~d~issipated in the
illuminated areas o~ the photoco~nductive insulator, thus
leaving an electrostatic charge image in the non-
illuminated areas. The electrostatic charge image can
then be developed by a number of means to form a visible
image. If desired, the developed image may be fixed or
made permanent on the photoconductive insulator surface.
Alternatively, the~developed image, in the form of
electrostatically adhèred toner powders or liquids, may be
transferred to paper or some other material and
subsequently affixed by some suitable means. This may be
done, for example,~by attracting fusible ~toner particles
to the charged areas, then transferring and fusing the
imagewise distributed ~particles to another surface.
The conductive substrate utilized in such
electrophotographic systems usually comprises a metal such
as brass, aluminum, gold, platinum, st~eel or the like and
may be of any convenient thickness, rigid or flexible, and
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in the form of a sheet, web or cylinder. This substrate
may also comprise such materials as metallized paper and
plastic sheets, conductive polymers, or glass coated with
a thin conductive coating. In all cases, it is usually
preferred that the support member be strong enough to
permit a certain amount of handling. In some instances,
an interfacial blocking layer for at least one type of
charge carrier is utilized between the base electrode and
the photoconductive insulator.
Typical photoconductive insulating materials
useful in electrophotography include: (1) inorganic
crystalline photoconductors such as cadmium sulfide,
cadmium sulfoselenide, cadmium selenide, zinc sulfide,
zinc oxide, and mixtures thereof, (2) inorganic
photoconductive glasses such as amorphous selenium,
selenium alloys, and selenium-arsenic, and (3) organic
photoconductors such as phthalocyanine pigments and
polyvinyl carbazole with or without additive materials
which extend its spectral sensitivity.
The surface potentiaI is of the utmost
importance in the development of an electrostatic charge
image. For greatest development latitude, the contrast
potential (Vc) resulting from different levels of exposure
should be as large as possible. The contrast potential
~5 (Vc) can be expressed by the equation:
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VC = l~G
where ~a is the change in surface charge density upon
exposure to imaging radiation and C is the capacitance per
unit area of the photoreceptor.
One prior art method o~ decreasing C and hence
increasing Vc has been to simply increase the photo-
conductive insulator thickness. However, the low charge
;~ carrier mobility in photoconductive insulators used in
electrophotographic devices somewhat limits the useful
thickness one can employ to decrease C. If the thickness
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is increased too much, the system will not have a useful
discharge speed. In systems where the thickness can be
increased somewhat to decrease C, then the increased
thickness requirement also restricts the physical
characteristics, such as flexibility and adhesion of the
photoconductor to the final plate, drum or belt. Thus, to
improve potential contrast in such syskems, an electrical-
ly active transport overlayer on the photoconductor has
been used as, ~or example, in U.S. Patent No. 3,928,034.
For xerographic use, this construction requires that the
overlayer be substantially transparent and non-absorbing
in the particular imaging radiation wavelength region. In
additiont even though the overlayer is substantially
transparent, as increasingly thicker layers are required,
adsorption and scattering due to included particles and
partial crystallization become significant and have a
detrimental effect upon the sensitivity of the device and
the quality of the copies produced.
me xerographic apparatus disclosed in U.S.
Patent No. 3,684,368 shows the use of photoreceptor
constructions which bear some similarities to the
constructions of the present invention. The reference
shows the use of anodic, porous aluminum oxide layers
between the metal layer and photoconductive insulator
layer in order to improve the adhesion therebetween. me
photoconductive insulative layers tend to be thick to
provide decreased capacitance, with the preferred
thickness range being 10-15 micrometers. The porous
aluminum oxide layer shown in Example 3 is believed to0 have a thickness o about 0.17 micrometers.
e xerographic photoreceptor shown in Example 3
of U.S. Patent No. 2,901,3~8 discIoses an aluminum sub-
strate with a 100 Angstrom ~approximately 0.01 micro-
meters) coating of aluminum oxide and a twenty micrometer
coating of a vitreous selenium photoconductive insulator
layer.
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The present invention is the barrier-charge transport layer is at
least 0.15 micrometers thick, the pore diameters of the porous zone are be-
tween 0.007 and 0.040 micrometers, the center-to-center spacing of the pores
is from 0.010 to 0.400 micrometers, and the photoconductive insulator layer is
less than 1.0 micrometers thick.
More particularly, the photoreceptor is a novel two-layered photo-
receptor structure comprised of a thin layer of photoconductive insulator depo-
sited on an adjacent, relatively thick, porous anodized aluminum barrier layer/
charge transport layer to produce an improved electrophotographic device. It
was surprisingly found that the relatively thick porous oxide layer sandwiched
between the conductive substrate and the photoconductive insulator also per-
formed as a charge transport layer. ~Ioreover, it was found that surface elec-
trical potential enhancement was achieved and that this was directly propor-
tional to the porous charge transport oxide layer thickness. Because of this
novel construction, a low cost electrophotographic device can be produced
which has improved imaging contrast, a low background in the developed images,
a high recycle rate, long life, and the capability of producing excellent
coples.
The invention wlll now be described in greater detail with reference
to the accompanying drawing which is a sectional view of the electrophotographic
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~device according to this invention.
The novel two-layered photoreceptor structure to provide an improved
electrophotographic device can best be understood by reference to the drawing
in conjunction with the following discussion. The Figure illustrates a photo-
receptor 10 according to this invention. Substrate 12 is an electrically con-
ductive substrate which is capable of lending physical support to the structure
shown. It
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may be comprised of a substantially thick metallic sheet,
aluminum drum blanks, metal or conductive polymer coated
sheets, conductive particle filled polymeric sheets, or
the like or a composite metal coating on a sufficiently
rigid dielectric substrate. The metal may be selected
from such materials as aluminum, brass, steel, silver, or
the like. If it is desired to discharge the device by
flooding radiation from the substrate side, then it is
understood that a combination of materials must be
selected to render substrate 12 sufficiently transparent
to the flooding radiation.
Layer 14 is a unique barrier layer/charge
transport layer according to this invention which is
produced by the anodization of aluminum. Layer 14 has
pores 16 in the aluminum oxide layer. An added asset of
layer 14 is the barrier layer 18 lying adjacent to the
metal surface in which no pores exist. This barrier layer
18 performs as a blocking layer for both positive charges
~holes) and negative charges (electrons).
Layer 20 is a photoconductive insulative film.
Useful photoconductive insulative materials include:
(1) inorganic crystAlline photoconductors such as cadmium
sulfide, cadmium sulfoselenide, cadmium selenide, zinc
sulfide, zinc oxide, and mixtures thereof, (2) inorganic
photoconductive glasses such as amorphous selenium alloys,
and (3) organic photoconductors. It is preferable that
the photoconductive insulative layer _ be capable of
blocking appropriate (i.e., negative or positive) charges
at the free surface.
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Detailed Descri~ on of the Invention
The conductive substrate used in the practice of
the present invention may, as is well known in the art, be
any conductive substrate. It may comprise a metal layer,
a metal coating on a substrate such as a polymeric resin,
a conductive polymer, a coating of a conductive polymer on
a non-conductive polymeric resin, or the like. The
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substrate may be rigid or flexible, transparent or opaque,
and may be in the shape of a cylinder, a sheet, an endless
belt, or various other designs.
The photoconductive insulator layer may be any
S photoconductive insulator layer as known in the art which
is less than 2.0 and preferably less than 1.0 micrometers
thick. The composition of the photoconductive insulator
layer is not critical to the practice of the present inven-
tion and may be selected from amongst any of the known
materials in the art such as (1) inorganic crystalline
photoconductors such as cadmium sulfide, cadmium sulfosel-
enide, cadmium selenide, zinc sulfide, ~inc oxide, and
mixtures thereof, (2) inorganic photoconductive glasses
such as amorphous selenium, selenium alloys, and
selenium-arsenic ~e.g., Ar2Se3), and (3) organic
photoconductors such as phthalocyanine pigments and
polyvinyl carbazole and its derivatives with or without
additive materials which extend its spectral sensitivity.
As long as the layer provides photoconductive and insula-
tive properties, it may be as thin as it can be made.Usually it will not be thinner than 0.05 micrometers,
preferably it is at least 0.10 micrometers, and more
preferably 0.15 micrometers to 0.8 micrometers. The upper
limit on thickness is necessary to achieve the charge
contrast enhancement of the structure of the present
invention.
The barrier-charge transport layer performs
uniquely wlthin the structure of the present invention.
The two zones of this single layer performs as both a
30~ blocking or barrier layer or positive charges (holes) and
as a`charge transport layer when ~a negative charge
(electrons) is photoact~ively released from the
photoconductive charge generating layer. The layer is
produced by the~anodization of aluminum. Anodization in
certain environments generates a porous aluminum oxide
layer. This layer preferably may be from about 0.15 to 25
micrometers thick. The pore diameter~ and the
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center-to-center spacing bet~een pores :is not critical to the practice of the
present invention and varies because of changes in processing conditions during
anodization such as temperature, electrolyte concentration, etc. Pore dia-
meters on the order of 0.007 to 0.040 micrometers and average center-to-center
spacing of from 0.010 to 0.400 are common. It is preferred that the average
pore diameters be between 0.008 and 0.030 micrometers and that the center-to-
center spacing be between an average of 0.010 and 0.080 micrometers or between
0,020 and 0.060 micrometers. The most preferred ranges are 0.010 to 0.020
~and specifically 0.012~ micrometers for the pore size and 0.025 to 0.040
~and specifically 0.033) micrometers for center-to-center spacing of the pores.
The barrier layer portion of the aluminum oxide layer, the non-porous area be
tween the conductive substrate and the pores is usually between 0.003 and 0.05
micrometers, and is preferably between 0.006 and 0.03 micrometers. Typical
pore-forming electrolytes which are used to anodize aluminum are selected from
15% sulfuric acid, 2% oxalic acid, 4% phosphorlc acid, and 3% chromic acid.
One of the most complete discussions of th0 process of anodization and the
effects of parameter changes in the process on the characteristics of the
aluminum oxide is to be found m "Anodic Oxide Films on Aluminum", J.W. Diggle,
T.C. Downie, and C.W. Goulding, Rutherford College of Technology, Newcastle
upon Tyne, England, a paper received July 29, 1968.
The structure of the present invention operates by first receiving
an induced charge on the photoconductive insulator surface. The sensitized
device is then imaged with imaging radia-
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tion. Light is absorbed by the photoconductive layer, creating
electron-hole pairs. The holes and electrons are separated under
the applied electric field. The electrons are injected into and
transported through barrier layer/charge transport layer
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and the holes are transported to the surface of photo-
conductive insulative layer, thereby imagewise discharging
the device where light strikes in proportion to the
integrated amount of light which is absorbed. In the
regions where radiation does not impinge upon the device,
the charge distribution remains substantially the same as
before the imaging step. The imaging step is now complete
and the electrostatic latent charge image has been formed.
The electrostatic charge is then developed with
toner to form a toner image on the electrophotographic
drum. Excellent copy quality results when copies are made
by transferring the toner image and subsequent toner
images to plain paper. Added permanence is introduced in
the transferred toner image i~ it i5 heat-fused or
pressure-fused to the paper. The photoconductive
insulator surface is then easily discharged and cleaned by
conventional means. As previously mentioned, if it is
desired to discharge by radiation from the underside, then
substrate must be sufficiently transparent to the flooding
radiation.
Having described in general the embodiment of
this invention for electrophotography, some specific
examples will now be given.
EXAMPLE 1
The photoconductive insulative layer consisted
of 0.5 ~m sputter deposited cadmium sulfide (CdS) on
commercially available ~lzak~ aluminum (Type 1) which has
porous aluminum oxide on one face thereof. A 5 cm by 5 cm
substrate was prepared by removing the protective
adhesive-backed paper layer and cleaning the exposed
al~minum oxide surface by immersing it in successive
ultrasonic baths of acetone, trichloroethylene (bath 1)
and trichloroethylene (bath 2), followed by rinses of
trichloroethylene, methanol and acetone. The substrate
was then blow dried in a stream of N2 gas. The aluminum
oxide layer on the commerically available Alzak~ aluminum
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was about 5 ~m thick. The substrate was then Placed into a
Randex~ RF sputter deposition vacuum system and coated
with about 0.5 ~m of sputter-deposited CdS in the
following manner.
me substrate was placed on a 6.3 cm by 8.8 cm
aluminum heater block containing a resistive heating
element and a calibrated resistive temperature sensitive
element. me heater block was separated from the
water-cooled J-arm anode platform of a Randex~ sputter
module by a 5 cm by 5 cm by l mm thick piece of quartz.
The heater block, quartz and anode table were thermally
linked by applying a thin layer of high-vacuum silicone
grease to each element. Also, the substrate was joined to
the heater block with silicone grease to ensure that the
temperature of the substrate was nearly the same as that
measured at the heater block. The distance from the
substrate to the hot pressed CdS target was about 5 cm.
The heater block was heated to 150C and the
temperature was held constant to within 5C throughout the
deposition. A premixed gas consisting of 6% H2S and 94%
Ar was admitted to the vacuum chamber at a rate of 20 std
ml/min. me pumping speed was adjusted by use of a
throttle valve located between the vacuum chamber and the
diffuslon pump until the pressure in the vacuum chamber
was stabilized at 2.S mT.
The non-functional properties of the novel
photoreceptor produced according to this invention were
then measured. me surface was charged negatively by
passing a single corona wire across the surface several
times at a distance of about l cm. The surface voltage
was measured with a Monroe electrostatic voltmeter using a
transparent probe and recorded on a chart recorder. me
photoreceptor described above could be charged to 220
volts. The time required to discharge in the dark to
one-half that value (llO volts) was two minutes. When
exposed to monochromatic light of 4~0 nm, 1~ ergs/cm2 were
required to discharge the surface from 220 V to 110 V.
EXAMPLE 2
An anodization cell was fabricated from PVC
plastic to accommodate 15 cm by 8 cm substrates and
yielded substrates which were uniformly anodized over a
12.5 cm by 7.5 cm area. The cell was fabricated with
three slots at each end which held the anode (aluminum
substrate) and two cathodes fixed. The cathodes were 2.5
cm on either side of the anode. The electrolyte consisted
of 15% concentrated H2SO4 and 85% deionized distilled
water. The electrolyte was continuously circulated
through about 6 meters of 1/4 inch plastic tubing which
was immersed in a water bath for the purpose of cooling
the electrolyte. Current was passed from the anode to
both cathodes at a fixed rate which was recorded along
lS with the voltage between the cathodes and the anode, the
time span of the anodization, and the temperature of the
electrolyte. The anodization parameters for this example
were:
Substrate 75 ~ thick aluminum foil which
was 99.99% pure ~i.e., 1199
alumin~m foil)
Current 2.5 amps
Voltage 11.5 volts
Temperature 19C
Time 4.2 minutes
The thickness of the anodized layer is known to be
proportional to the product of the current and time for a
given substraté material and electrolyte temperature.
Typically, 32 amp-min/ft2 will yield 1 ~m of oxide
thickness. Since both~ sides of the substrate are
anodized, both sides are counted in the area.
In this Example, therefore, about a 1.5 ~m thick
oxide film was produced. Upon removal from the electro-
lyte, the substrate was immediately rinsed in running tap
water ~ollowed by a rinse in deionized distilled water and
in isopropyl alcohol and blown dry with N2 gas.
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A 5 cm by 5 cm piece was cut from this substrate
and placed in the Model 3140 Randex~ RF sputter deposition
unit of Example 1. A layer about 0.5 ~m thick of CdS was
then deposited onto this substrate with the ~ollowing
parameters:
RF power 300 W
Gas pressure 2.5 mT
Gas flow 20 std ml/min
Gas composition 6% H2S, 94% Ar
Substrate temp. 132C
Deposition time 8 minutes
The resulting photoreceptor could be charged to
-250 V. More than two minutes were required to discharge
the surface voltage to ~125 V in the dark. A miximum of
125 V contrast between exposed and unexposed regions was
observed. A 'chree second exposure to room light (about 30
ergs/cm2) was required to obtain half of this contrast.
; ~ EXAMPLE 3
A barrier layer/charge transport layer about 5
m thick was prepared on 1199 aluminum as in Example 2. A
photoconductive insulator layer consisting of about 0.24
m thick cadmium sulfide was deposited on layer 18 as in
Example 2, however, the sputtering ga~ composition was
pure argon.
25 : The resulting photoreceptor could be charged to
240 V, the dark decay to~-120 V required about 12
seconds, and~a voltage contrast of 40 volts was observed.
Again, a~three second exposure to room lights (~30
ergs/cm2) was required to~obtain half of this contrast.
EXAMPLE 4
Using resistive heating techniques, a 0.25 ~m
thick photoconductive insulative layer comprised of a 94%
Se, 6% Te alloy~, was vacuum deposited on the commercially
~ available Alzak~ substrate prepared as in Example 1.
: ~ 35 ~owever, on~-half of the aluminum oxide barrier layer/
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charge transport layer was chemically stripped from ~he
substrate prior to the deposition of the photoconductive
insulative SeTe layer. The resulting photoreceptor could
be charged to -140 V where layer remained, but to only -20
V where the layer was stripped off. The voltage contrast
and exposure to one-half contrast were similarly effected
by the presence of the layer, i.e., -80 V to -20 V and 70
ergs/cm2 to 20 ergs/cm2, respectively.
To demonstrate that the barrier layer/charge
transport layer of this invention produces no advantage
and, in fact, is undesirable, for thicker photoconductive
insulative layers, a layer 40 ~m thick of 94% Se, 6% Te
alloy was deposited as above on the stripped and
unstripped commercial Alzak~ suhstrates. When charged
negatively, the voltage acceptance was increased from -425
for the stripped portion to -780 V for the anodized
portion, however the voltage contrast was decreased from
60 V to zero. When charged positively, the voltage
acceptance was reduced slightly from 560 V to 460 V and
the voltage contrast was reduced from 560 V for the
stripped portion to 380 V for the anodi ed portion.
EXAMPLE S
1 ~m of As2Se3 was deposited using resistive
heating techni~ues onto a commercially available Alzak~
substrate, half of which was stripped of the oxide layer.
The voltage acceptance was +113 V when charged positively,
and -120 V when charged negatively for the anodized
portion and +18 V, -27 for the stripped portion. The
corresponding voltage contrast upon exposure was also
increased for the anodized portion to +35, -20 from ~18,
-15 volts when respectively charged positively and
negatively.
In contrast to this when a thick layer (15 ~m)
of Ar2Se3 was deposited onto a similar substrate the
voltage contrast was reduced to ~ 0 volts for the
anodized portion from ~75, -8 volts for the stripped
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portion even though voltage acceptance was increased to +305, -365
from +75, -115 volts. This example shows that the voltage contrast
is enhanced by the anodized aluminum barrier-charge transport layer
of the present invention when used with relatively thin photo-
conductive insulator layers. Conversely, it is surprising that the
voltage contrast is not enhanced and is in fact reduced with
relatively thick (i.e., ~5 micrometers) photoconductive insulator
layers.
EXAMPLE 6
A photoreceptor ~as prepared by coating a 1 ~m thick
coating of Perylene Red onto the aluminum oxide coated substrate of
Example 1. This resulted in a 1.5 ~m thick anodized aluminum
substrate which was compared to a similar coating on stripped
aluminum. The resultlng photoreceptor could be charged to -171 V
compared to -72 V on stripped aluminum. The voltage contrast
compared 167 V to 72 ~.
EXAMPLE 7
A barrier layer/charge transport layer about 2 micrometers
thick was prepared on 1100 aluminum as in Example 2 using 4% phos-
phoric acid as the electrolyte. The anodizing conditions were:
Substrate 100 micrometers thick aluminum
foil which was 99% pure (i.e.,
1100 aluminum foil)
Current 0.7 amps
Voltage 100 volts
Temperature 22C
Time 18 minutes
The resulting oxide layer was similar to that in Example 2 except
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that the pore diameter was approximately 0.03 micrometers and the
center-to-center spacing was approximately 0.28 micrometers.
The resulting photoreceptor could be charged to -230
volts, the dark decay to -115 volts was greater than two minutes,
and a voltage contrast of 127 volts was observed. An exposure of
47 ergs/cm2 was required to obtain half of this contrast.
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