Note: Descriptions are shown in the official language in which they were submitted.
" ~ 1 6~gl~ ,
Background o~ the ~nvention
In conventional electropho-toyraphy, a pho-t~-
conduc-tive surface is charyed in the dark and then subjected
to a light image of the document or pho-tograph which is to
be reproduced, generatlng a latent electros-tatic image
corresponding to the original document or photograph. The
latent electros-tatic image is then made visible by toning
with electroscopic particles. The most widely used photo-
conductor in electrophotographic machines is a vitreous or,
more commonly called, amorphous selenium. Its sensitivity,
however, is chiefly in the ranges of blue to yellow, and
the gray scale is such that copies of photographs are very
poor. Furthermore, a selenium photoconductor cannot be
heated to a temperature of over 80C wi-thout losing its
electrophotographic properties~ The chief disadvantage
of a selenium photoconductor is that it wears rapidly in an
electrophotographic machine and must be replaced after use
for between ten thousand and one hundred thousand copies.
Addition of tellurium, arsenic and other dopants to
amorphous selenium is known in the art to effect some
improvement on these properties but substantial improvement
is desirable particularly in the wear characteristics.
Cadmium sulphide has a hardness of between 3
and 3.5 on Moh's scale of hardness. Its spectral response,
when properly formed and doped, is across the entire
visible range from blue to red. It has a higher effective
quantum efficiency--that is the ability to convert light
into charge--from twice to ten times that of selenium. Its
X 1- "~
mab/,c
I
light discharge ch~racter~stic is such -tha-t it produces an
excellent gray scale, enabling it to make excellent re-
productions of photographs.
A photoconductor, in an electrophotographic
process, is mounted on a conductive substrate and charged
by a charging corona. The corona ionizes the air. This
ionized air acts ~s one plate of a capacitor, the other
plate being the conductive substrate, Since a photo-
condactor is a dielectric in the dark, the charge from
the corona sticks to the surface of the photoconductor.
This induces a charge of opposite polarity on the
conductive substrate. The time it takes to tone a latent
electrostatic image on the surface of a photoconductor
~, .
is dependent on the vol-tage to which it may be charged.
The prior art has recognized the advantages of
~ polycrystalline cadmium sulphide as a photocondutor.
Unfortunately, the prior art (Kuehnle 3,884,787~ has been
unable to form a photoconductor oE cadmium sulphid~ of
adequate thickness to create a sufficiently high voltage
at the surface with the required charge densities. This
means that the development of a latent electrostatic
' irnage produced on the surface of the photoconductor will
take an inordinately long time for practical use. When
it is attempted to make a photoconductor of cadmium
X - 2 -
mab/J~
I J ~
sulphide thick, ik flakes Erom the conductive substrate or
cracks.
Corrsin 3,151,982 attempted to overcome the
shor-t life of a vitreous selenium photoconduc-tor by using
cadmium sulphide particles in a glass binder. Lane
3,510,298 also discloses a cadmium sulphide photoconductor
in a glass binder. We have found that glass-bound cad-
mium sulphide does not produce a commercially usable
electrophotographic photoconductor. The latent electro-
static images, when developed, were full of spots which
spolled the images.
The prior art had developed two techniques for
obtaining cadmium sulphide photoconductors without the
use of binders. The first of these is described by Hill
et al in Patent 3,148,084. The prior art, in respect
of obtaining photoconductive films, is discussed in this
patent, and the disadvantages of the evaporation process,
the chemical deposition process, and the vapox-reaction
process are pointed out. Hill et al disclose the for-
mation of photoconductive films by spraylng reagents on
a heated substrate. Their ~ilms inclucle sulphides of
many metals, as well as sulphoselenides of cadmium,
cobalt, and indium. The photoconductive films were formed
on an insulating substrate. Co-inventor Chamberlin
further described the method in the Journal of the Electro-
chemical Society, Vol. 113, pages 86-89, in an article
X - 3
mab/JC
~ 3 6 ~
written with J. S. Skarman in 1966. The films were not
intended to be used for electrophotography, but ra-ther
in the manuEacture o thin-film solar cells. 1'hese photo~
voltaic converters were formed by a thin film of copper
sulphide ~0.1 ~) together with a thin film o~ cadmium
sulphide ~
Another method of forming thin-film photoconduc-
tors is by spuitering. This process i5 described by
Kuehnle in Patent 3,884,787. Films having a thickness
up to .5 ~ (5000 A) were formed. These films were trans-
parent to yellow light and were excellent photoconductors.
A charged photoconductor in the dark is ana-
logous to a charged capacitor in which the photoconductor
I is the insulating or dielectric medium. In order to
achieve a rapid development, a high voltage is necessary
to attract toner particles from a large distance. In
the case o~ cadmium sulphide, the corona charge is nega-
tive, so the charge of the toner particles of the deve-
loper is positive. The speed at which the charged toner
particles in the developing liquid moves to the laten-t
electrostatic image on the photoconductor is a function
of the voltage of the latent electrostatic image. The
higher the vo].tage, the speedier will be the development.
With a thin-film photoconductor, a surface charge density
above a certain value cannot be maintained, and the
excess charge is transported
- 4 -
mab/~c
1 1 ~56 ~ Z
across -the dielectric. The voltage generated at the maximuTn
charge level is proportional to the thickness of the photo-
conductive layer and inversely proportional to the dielectric
constan-tn Thus in order to provide the high voltayes
re~uired for fast toning we seek to increase the charge
density accepted by the surface, and to make the film as
thick as possible. When we attempted to form a thicker
film bv carrying on the pyrolytic formation of cadmium
sulphide from aqueous solutions of reagents to a greater
extentr the film flaked off the conductive substrate.
Shattuck et al U.S~ Patent No. 3,676,210 discloses
a recognition of the defects in Hill et al 3,148,084, for
use as an electrophotographic pho-toconductor, and attempts
to overcome these disadvantages of a thin film by using a
resin binder~ In Shattuck et al, the inventors use an aqueous
emulsion of polyvinyl acetate in the method disclosed by Hill
et al and obtain a resin-bound cadmium sulphide photoconductor.
There is no disclosure of the use of zinc or of copper as
dopants. Reference is made, in Shattuck et al, to Middleton
et al U.S. Patent Nos. 3,1~1,006 and 3,121,007, hoth of
which disclose a photosensitive layer consisting o an
inorganic photoconductive powder dispersed in a resin binder.
Shattuck et al merely use the Hill et al process to manu-
facture the Middleton et al products. Shattuck et al set
forth that the photoconductive compounds which are formed
by their method are not crystalline, but are, instead,
amorphous.
Cadium su]phide, as an electrophotographic
photoconductor, has several disadvantages. First, it has a
--5
cb/~
~ ~ 6.~61 ~
memory; that is, aEter charyiny and exposiny to a liyht
image to fo~m a latent electrostatic image, developiny that
image, and transferring it to a carrier .sheet, the latent
image still remains on the photoconductor. Stated othe~wise,
the decay time in the presence of light is too slight, so
that offsetting occurs. Another disadvantage of a cadmi.um
sulphide electrophotographic photoconductor is fatigue;
that is, as the photoconductor is used and reused, the
maximum voltage to which it can be charged becomes less and
less. Cadmium sulphide, however, can retain a much higher
charge density than tellurium-doped selenium. Cadmium
sulphide has a hi~her photosensitivity than selenium.
-5a~
ch/~-
~ J 6~ 2
riel~! oE th~ ~nvcnkion
Our inventlon rela~e~ ~o an irnproved el~ctrophoto-
graphi c photoconduc~or havi.llcJ a spectral sensitivlty across
the ent;re visible range and a hL~h wear resist~nce~
Descripl:ion of thc Priox ~rt
The prior art, whic}l has been discussed above
in connec~ion with the backgroulld of the invention, is:
Hill et al ........ ........... ~.. Patent 3,148~084
Corrsin .~...................... ... Patent 3,151,9~2
Lane ........................... ... Patcnt 3,510,298
Shattuck et al..... ........~....... Patent 3,676,210
Kuehnle ~.......... ......~......... Patent 3,884,787
Chamberlin et ~1 .. ..........~..... Journal o the Electro-
chemical Society,
Volume 113, pages 86~89
None of the prior art teaches a c~dmium sulphlde
electrophotographic phvtoconductor havin~ a thickness o~ a~ ;
least threc ml-xons.
.. ~
Marlor et al Patent 3,75~,g85 discloses a proc~s
of making a sintered photoconductor comprising cadmium
sulphide doped with copper and chlorine.
Cnamberlin et al disclose the use of cadmium
acetate and thiourca or ~tN dime~h-,71 thiourea. Tl~
reference also shows the doping o~ a cadmium sulphide f'lm
~ wi~h copper~ as well aS ~h~ ~act that, with cadmium acet~te
;~ 6
_ ~ _ . . .
~ ~ 6561 ~
as a starting material, crystallites smaller than 400 A
in size are formed.
_ummary of the Invention
In generaI, our invention contemplates an elec
trophotographic pho-toconductor comprising, essentially,
cadmium sulphide having a thickness of three microns or
more, formed by spray pyrolysis of an aqueous solution o~
cadmium acetate and thiourea. A minor amount of zinc
sulphide is codeposited by the addition of zinc acetate
in the reagent solution to raise the acceptance potential
of the photoconductor. A minor amount of copper is used
as the dopant to improve the spectral response, to reduce
memory which causes offsetting when in use, and to reduce
fatigue. The copper dop,ant is codeposited by adding
copper acetate to the solution being sprayed in the ~yrolytic
spray process. The spraying takes place on a heated metal
surface such as steel or aluminum~ preferabl~ chromium or
cadmium plated for adhesion and corxosion control. Irhe
spray pyrolysis takes place in three stages. In the first
stage, an amount of lead acetate is added to the spraying
solution to make an appropriate contact layer and to eli-
minate the white spo-ts on the black parts of the copy. In
the second stage, the lead acetate is eliminated and
chlorine is added in the form of cadmium chloride. The
chlorine acts as a dopant to assist the transport of photo-
-- 7 --
mab/~
~:16561Z
generated charge to the substrate as well as to assist in
the removal of memory and fatiyue. Ik also increases the
number of carriers. rrhis stage of photoconductor ~orma-
tion is carried on as long as feasible to increase the
thickness of the photoconductor and hence enable it to
accept a higher level of charge than theretofore possible
with cadmium sulphide photoconductors. The first part
of the process is carried on to produce a layer having a
thickness of about 3,000 A. The second layer, which is
. a charge transport layer, is carried on until it has a
thickness of about 17,50C A. The solution is then changed
to remove the chlorine dopant, so.that the.solution com-
prises only cadmium acetate, copper aceta-te, and zinc
acetate with thiourea. The formation of the third layer,
, . . .
which is the light-absorbing portion of the photoconductor,
is carried on until it has a thickness of about 9,500 A,
sufficient to absorb almost all the light incident on the
surface. The finished photoconductor can be charged to a
very high level and has an excellent li.ght response which
enables it to produce a high-contrast image haviny a super-
ior gray scale. The photoconductor has a ~ery long shelf
life which is not affected by temperature or humidity.
The layers may be formed on any suitable sub-
strate, preferably on a cylinder which-is to be used i.n
an electrophotographic machine. The cylinder is rotated
while being heated by means of a radiant heat element to a t~perature of
- 8 -
mab~
3 ~ 6 ~
between 125C and 200C (~ 25C.)~ measured at the surface
of the drum~ The solutions are sprayed at the rate of about
300 cc. per hourO
The spraying takes place in the presence of the
atmosphere, which contains oxygen. The oxygen appears to
be adsorbed on the surface of the crystals. It is advan-
tageous because it apparently increases the resistivity of
the cadmium sulphide film.
According to one specific aspect of fhe invention
there is provided an electrophotographic photoconductor in-
cluding in combination a conductive substrate having formed
thereon a layer of a homogeneous alloy of metal sulphides at
least three microns thick, the layer comprising a major amount
of cadmium sulphide and a minor amount of zinc sulphide.
: ,,
mab/ ,S
S 6 ~ ~
~s~r iL~l LOIl 0~ l(' D~ rl~s
In the acc ompan~i~7 clraiJi.ngs, which form part o:c
~he instar,i: speci.ication ancl ~,Jhich are tc: be read in
- col~junc~io~ therewith, ancl in ~ich likc xeference nu,-nerals
a~e used t~o lndicat~ li.];e paxts in the various v ie~rs:
FIGUi~E 1 i.s a diagxammat:ic view, drawn on an
enlarg~d scale~ showing a ~ragment o:e our improved
p~otoconcl~ctor .
FIGURE: 2 is a diagrammatic view o:E an appaxa-~us
--10--
1:165612
capable of manufacturing the photoconductor shown in
FIGUR~ 1.
FIGURE 3 is a sectional view, taken alon~ the
line 3-3 of FIGUR~ 2.
FIGURE 4 is a photomicrograph of crystals exposed
from a bulk of our improved cadmium sulphide photoconductor,
photographed on a magnification of twenty thousand times.
FIGURE S is a view, similar to FIGURE 4, of
another portion of a bulk of our improved cadmium sulphide
photoconductor, photographed on a magnification of fifty
thousand times.
escription of the Preferred Embodiment
More particularly, in forming our improved
electrophotographic photoconductor, we employ a spray
pyrolysis process which is known in the art. The apparatus
for forming the photoconductor is shown in FIGURE 2, in
which a metal drum 2, formed of aluminum or mild steel,
is plated with chromium or cadmium. It is thorouyhly
cleaned before startiny the process, first with nitric
acid, then with water, and then with household detergent,
until no oil or grease is present. The presence of oil
on the surface of the drum can be detected by the break
test; that is, a drop of water will break into an even film
on the surface when it is completely oil-free. After this,
the surface is rinsed with deionized water and then with
J~ - 11 -
mab/l ~
3~;5~l.2
isopropyl alcohol to clean off the water. In the many
photoconductors which we made, we employed a chromium-
plated drum for corrosion control. We have found, however,
that a bette:r bond is created wi-th cadmium plating.
The drum 2 is mounted on a pair of fixtures 4
and 6 into.which the drum may be fitted by friction, as can
readily be seen by reference to FIGURE 2 of the drawings.
The fixtures 4 and 6 are provided with flanges 8 and 10
which engage two pairs of rotary saddles 12 and 14, shown
in FIGURES 2 and 3. The saddles are mounted on a pair of
shafts 16 and 18 which are carried by two pairs of pedestals
20 and 22. The shaft 16 is driven by a prime mover such
as an electric motor 24 supplied with voltage through
conductors 26 and 28. The shaft 16 carries a drive pulley
30 which drives a pulley 32 through a bel-t 34. A shaft 36
is mounted in a fixture 38 for rotation with pulley 32.
It carries a double helical screw 40 adapted to reciprocate
an atomizing head 42 back and forth along the fixture 38.
Attached to the atomizing head 42 we pOSitiOll a pair oE
flexible hoses 44 and 46. The hose 44 is connected to a
source of compressed air (.not shown) having a pressure in
the order of twenty pounds per square inch. The hose 46
communicates with the aqueous reagent solutions which are
used successively to obtain the three differing cadmium
sulphide compositions forming the improved electro-
photographic photoconductor. The reagent solutions may
be fed by gravity or by air pressure, or in any other
- 12 -
1/
mab/~~
S~12
appropriate manner known to the art. I'he rate of flow is
governed by a valve (not shown) positioned between the
reagent-solution supply and the atomizing head 42 and is
controlled to form a spray, at the rate of 300 cc. or less
per hour, of reagent for contact with the drum 2. A
resistance heating element 48 is positioned in the interior
of the rotating drum 2. Current flows ~rom the conductor
28, connected to the source of potential, through armature
50 of a relay, through conductor 52, through the heating
element 48, through conductor 54 to complete the circuit
through conduclor 26 to the source of potential. A pyro-
meter 56 is positioned to sense the temperature on the
surface of the drum 2 being coated. I-t is set to a tem~
perature between 130C. and 180C. If the temperature
becomes too high, a windin~ 58 of the relay opens the
circuit by lifting armature 50. When the temperature cools
to within the desired range, the winding 58 is de-energized
and the armature 50 again energize~. the heating element 48.
It is to be understood that any appropriate pyromet~r ]cnown
to the art, such as a thermistor, may be employed. The
average temperature at the surface of the drum is maintained
at about 150C.
In attempting to produce a cadmium sulphide elec-
trophotographic photoconductor, we experimented for about
three years and coated in the order of five hundred test
drums before determining the best mode of carrying out
our invention. We soon learned that most cadmium su]phide
- 13 -
mab/Y
~ 3 6~6 1. 2
photoconductors, while having photoconduc-tive properkie~,
would not be operative in the electrophotographic mode to
produce an ~cceptable electropho-tographic image when used
in electrophotographic machines. Cadmium sulphide has a
natural hardness and, hence, greatly improved abrasion
resistance over vitreous selenium. Our improved cadmium
sulphide photoconductor was ahle to produce over a million
copies~ as compared with not more than one hundred thousand
copies for vitreous selenium, when used in a conventional
plain-paper photocopying machine. Cadmium sulphide photo-
conductive films having su~ficient th ckness could not be
formed by spray pyrolysis. If it was attempted to make
the film too thick, it would flake from the metal substrate.
A thin film would give rise to only a small voltage level.
Furthermore, the dark decay was too high, so that it would
take several passes under one corona to charge the photo-
conductor to the maximum level permitted by the thin layer
of cadmium sulphide. Attempts to raise the voltage level
would cause the cadmium sulphide photoconductor to break
down. Moreover, cadmium sulphide had a memory; that is,
after imagewise exposure, development, and printing on the
carrier sheet, the latent image still remained on the photo-
conductor. The decay time in the light was too slow. We
also found that, after using a cadmium sulphide photo-
conductor for a while, the maximum voltage to which it
could be charged became less and less. We made numerous
experiments to improve this situation each of which led us
mab/.,w
3 ~656~2
to the presen-t invention, step by step.
Cadmium sulphide i.s generally less sensi-tive to
red light. The addition o~ copper, as is known to the art,
sensitized cadmium sulphide to red liyht. We found that
the addition of copper also reduced fatigue and memory,
and the resultant electro~hotographic photoconductor was
rendered sensitive across the whole spectrum, including
the red area.
~ good photoconductor for use in electrophoto-
graphic machines must be able to accept a voltage
sufficiently high, especially when developed ~y electro-
phoresis with toner particles suspend~d in an insulating
carrier liquid, so that development will take place rapidly.
This is a function of both the thickness of -the photo-
conductor and its dark resistance. We found that the
incorporation of zinc, in the form of zinc sulphide, enabled
~he photoconductor to be charged to a higher volta~e
In making photoconductor-coated drums containiny
no zinc, the charge level was not high enough to make ~or
rapid development. Furthermore, the contrast b~tween the
most highly exposed areas and those lesser exposed suffered.
The addition of zinc made an enormous difference. The
addition of zinc, however, makes the photoconductor less
sensitive, especially to red, so that there is a limit~
readily determined by the color response, to which zinc
can be added.
15 -
mab/~
1:~656~
One o~ the disadvantages o cadmium sulphide, as
pointed out above, is i-ts higher dark decay; that is, it
is not as good an insulator in the dark as is required by
a good and prac~ically usable electrophotographic pho-to-
conductor. We have found that manufacture of cadmium
sulphide by spra~ pyrolysis from the acetate greatly
increases the dielectric properties of the photoconductor
in the dark.
The interface between the conductive substrate
and the cadmium sulphide photoconductor is important. There
musk he an appropriate rectifying electrical contact at
this interface. We ha~e found that by making additions
to the contact layer--that is, the layer o~ the photo~
,conductor in contact with the conductive substrat,e---
specifically with lead, -the proper electrical contact is
obtained. Other t~in~s being equal, we found that cadmium
sulphide electrophotographic photoconductors doped with
copper and doped with chlorine, and containing zinc sulphide,
still were unsatisfactory because light spots on the black
parts of the copy appeared. A~ter many e~periments, we
determined this could be caused by improper electrical
contact between the photoconductor and the substrate~ By
adding an amount of lead we increased the conducti~ity of
the contact layer and the spotting was eliminated.
We determined the amount of lead empirically by
addin~ more and more until the spots got larger and larger
- 16 -
mab/~
~ 1 6 rS ~
and an optimum value was found to obviate the spo-ts. We
did not determine the maximum amoun-t of lead which could
be used without deleterious effects.
In the prior art, oxygen was considered a
disadvantaye. In Hill et al 3/1~8,084, Column 4, beginning
at line 62, the inventors indicate that none of the elements
in the photoresponsive film are derived from the substrate
or the surrounding atmosphere. The inventors further
point out, in Column 7, beginning at line 65, that post
film formation they subject the photoconductor to
t~mperatures of between ~00F. and 1,200F. This, of course,
would drive off any adsorbed oxygen. Kuehnle 3,884,787
forms a fil~ by sputtering in an atmosphere of inert gas
such as argon.
: :
We have discovered, by subjecting our finished
electrophotographic photoconductor to analysis by ~uyer
spectroscopy, that there is a low amount oE oxygen present.
We are unable to determine the nature of the o~ygen, but
we believe it is adsorbed on the bounc1ari.es oE the crystals
which form the photoconductor. ~he oxygen increases the
resistivity of cadmium sul.phide, which usually has free
electrons inside its crystals. Our hypothesis is that
oxygen, adsorbed on the crystals' surface, attracts the
spare electrons from the i.nterior of the crystals and
positions them on the surface of the crystals, making the
crystals a better insulator in the dark. The oxygen is
mab/ ~
~3~)5~;~2
` p.rescnt in t-he ~aLcr use~ lo mclkc t~le aque~ou~ ~o:Lu~ Jn~ o
the rcac3ents and i.s ~l~o prosenk ;.n t:h~ ~1;mosph~r~
While wc have made a yreat many cad~nium sulphide-
coated electxo~hotograp~lic drum~, the ~est. mode ~Je have
found o carxyinc3 out our invention is this: ~le first
~orm an aqueous sol~tion of a major amount o cadmi~n
acetate and a minor amount of coppcr acetate, le~d acetate,
and zinc acet~tc. ~ s~arate solution o~ thiourea may be
formed. Since the solut ion o~ metal acetates and i:he
aqueous solution o~ thiourea will react slowly at room
temperature, they may be introduced from separate contain~rs
into the atomizing head 42 through hose 4G. The reaction
is so slow, how~ver, that this is not necess~ry, and a
single solut.ion may be made as follo~s.
60LUTION I
Thiourca -- .008 molar
Cadmi.um Acet~te -- .006 molar
Copper ~cetate -- .00~012 mol~r
%i.nc ~ceta~e -~ ~0006 molar
Lead Acetate -- .00022 molar
One li~er oE the a~ove solution is ormed and is
sprayed at the rate o~ about 300 cc. per hour upon the
drum 2 which is b~incJ rot~ated at ~bout sevent~en re~lutions
- per minute~ It is to he notedt by ref~rence ~o FIGURE 2
that the atomi~in~ heacl 42 rcciprocates while th~ drùm 2
~ 18-
~l7 2; OCo7~0n
~ 3 ~ 2
is rotatin~. The rate o~ reciprocation is be~ween four and
five cycles per minute. :[~ the spra~ is directed at one
portion of the drum too lonyj it will cool it locally
below the pyrolytic deposition temperature. It will be
noted that there is an excess of thiourea in the above
solution. This is used in order to drive the reaction to
completion, since the law of mass action requires an
excess o~ the driving component to compél -the reaction
in the desired direction. At a spray rate of 3no cc. per
hour, it will take 3-1/3 hours to lay down the first layer,
which will have a thickness in the order of .3 microns.
The constant change of the spray position, in respect of
the substrate, produces an extremely uniform layer which
is extremely smooth. No binder is used, and the cadmium
.
sulphide region is formed having elongated crystallites
about 300 A or 400 A in diameter. We subjected the bulk
of the composite layer to a process of etching with a beam
- of argon ions. This process is known ~o the art as "argon
sputter etching". FIGURE 4 is a photomicrograph show:iny
the crystalline structure of our cadmium sulphide electro-
photographic photoconductor, ma~nified twenty thousancl
diameters. FIGURE 5 is a view similar to ~'IGURE 4, in
which the crystalline structure is revealed in a photo-
micrograph magnified fifty thousand diameters. This cry-
stalline structure prevailed throughout -the composite
photoconductor, the formation of which is herein described.
- 19 -
mab~ ~
`` ~ 1 f~5~:1 2
.'~.i ncc i t: i.c, n(~c~ r t~ n ~;~ L~ trtjL3~ 0CJXcl~1if.
photoconcluc1~o.r havc~ sufficjently rapid ligh~ deca~r and be
03~ sufficienl: t~lick~less to ~cc~pt a high enough charge~ ~,e
forme~cl a di~fe~ent aqucous solution to pro~uce a di~fere~t
rcgiol~ o OUL co~osite electrophotographic photocorlductor.
This solution is as follows:
S~I~ION lI
Thiourea -~ .008 molar
Cadmium Acetate -- O0055 molar
Coppcr Acetate -- .00.012 molar
Zinc Acetate -- . 00û6 mo:l.ar
Cadmium Chloride -- .00031 molar
It will be observed t~at the metal salts o
cac~nium, zinc, and copper, used by Hill et al, are chlorides.
We have found that the use o~ chloride salts, instead o~
acetate salt~, produce.s cadmium ~ulphide whic~ has ~ lo~
dark resi.~tance and will not accept a suffici.ently hi~h
charge to ma];e ~or rapid dcvelopment o~ the l~ten~ electro
static irnag~. We use an amount: of caamium chloride as a
20 dopant, so the resiaual potential is greatly reduced for
a given f ool~-candle quan~i~y o:~ light. Stated otherwise,
the light decay time of the region formed by SOLUTIO~ II
is greatly decreasecl. While the region of our improved
compo.site electrnp~lotographic p~otoconductor ~ormîng the
contact l~yer is thin ~ . 3 mi.crons~, the region o~ our
--20--
, ~, , .
~ ~ 656:12
pllOtOCOn(lUCLo:r Eor~ wit:h t~le chlorine d~Jp~llt, the charge
t:XanSI~Ort ILI~e~ iS In~lCl(e (:0 ~I~lVe a tlliCkne~;~3 in the orde~
of 1~75 microns. ~ro accomp~ish this, we ~mploy a ~l.utio~
having a volume of si.x liters and continue tlle spray
pyrol~sis Eor an ad~litional eighteen hollrs or more.
~ le ncxJc ~orm thc light-absorbing region o~ our
cadmi.um sulphid~ cl~ctrop~lotoc~rapllic photoconduc~or~ This
- is accomplished by Eormin~ an aqueous solution as follows:
SOI.~.~ON III
Thiourca -- .008 molar
Cadmium Acetate -~ ~006 molar
Copper Aceta~e - .00012 molar
Zinc Acetate -- .0006 molar
The solution is formed in a quantity of about three li-texs0
and it ta~;es about ten hours to ~orm a layer llnder ~e same
process condltions as the othcr layers descri~ecl above.
The l.i~ht~ sorb.illg layor wi.lL have a thickness o~ about
.. 95 micron.s .
Our improved composite electrophotographic
: ~0 pllo~oconductor, thus ~ormecl, is shown diagrar~mati~ally in
FIGUP~E 1. A conduc~ive substrate 100 i.s formed of any
- appropriate metal, such as mild steel or a~uminumO
contact r~gi~n or layer 102 has a thic~ness o~ 3~000 A and
comprises, cssclltially, a major amoun~ o cadmium sulp~i.de0
2S a minor arnoun~ o~ zinc~ su~pllide~ and a .subs~an~lal amount
. . , ~
~ 21
.~.
` 1~6~61~
of lead sulphide, the layer being doped wi~h copper. It
will be obse.rved that all of the la~ers or regions of our
composlte photoconductor comprise, essentlally, a major
amount of cadmium sulphide with a mino~ amount of zinc
sulphide. All of the layers are doped with copper. A
charge transport layer 104 is doped with copper and
chlorine and has a thickness of 17,500 A. A liyht-absorbing
region or layer 106 comprises a major amount of cadmium
sulphide and a minor amount of zinc sulphidel the layer
being doped with copper~
It will be understood that we have found it
advantageous, in order to produce a practical electro-
photographic photoconductor, that the composite photo-
; .conductor have a thickness of at least three microns. It
will be understood, of course, that the contact layer may
be made thinner, since its main function is to form an
appropriate electrical contact between the con~uctive
substrate and the composite photoconductor. It is
unnecessary to make it too thick. The charge transport
20 layer alds greatly in contributing to the desired thick-
ness of the composite photoconductor. A thick photo-
conductor is necessary to increase the level of potential
to which the composite photoconductor can be charged.
The amount of chlorine in the charge transport layer was
determined empirically.
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1 ~ 6~ 2
Furthermore, cadmium chloride has a pronounced
effect on dark decay, whieh of eourse is related to the
rate of eharging. If the eharges leak ofE while charging,
the rate o charyin~ is correspondingly reduced. The
effect of eadmium ehloride is to inerease the dieleetrie
properties of the composite photoconauctor in this region.
We performed many tests, varying the amount of chIorine
in SOLUTION II. We have found that a 25-percent shift
[+] in the eoneen-tration does not appear to make too much
differenee. Too little chlorine increases the eharging
time, and too mucll reduees the dielectrie properties.
All of the reyions of our eomposite photoeonduetor
show the presence of oxyyen, which appears to increase the
.
resistivity of the eadmium sulphide.
We have found that our improved eomposite photo-
eonduetor aecepts a eharge as high as 300 volts; and the
eharye density is much hi~her, in our composite cadmium
sulphide electrophotoyraphic photoconductor, than selenium
doped with tellurium.
We eould detect no demarcation between adjacent
zones or layers of our eomposite photoconductor~ Its dark
resistanee was between 1012 and 1016 ohm-centimeters.'
This is markedly different from photovoltaic cells, which
require very hiyh conductance.
23 -
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I ~ B ~
We have found tha-t our composite photoconcluctor,
having a thickness of between three and four microns,
opexates well in the electrophotographic mode. Theore-t-
ically, a thicker photoconductor would be be-tter, but we
have found that after a thickness of six microns is reached,
cracks develop. A thin photoconductor canno-t accept a
voltage level high enough to be practical; that is, a
thin photoconductor will require a longer time in developing
the latent electrostatic image to which it has been sub-
jected àfter charging.
It will be observed that we have described our
invention in respect of thiourea as the sulphur-bearing
reagent and only metal salts of cadmium. We have tried
.
other sulphur compounds, such as N,N-dimethyl thiourea,
but prefer thiourea. The dimethyl and diethyl thioureas
do not produce the results as well as thiourea, but are
usable. A selenourea is less suitable than thiou~ea.
Though it is usable, it is more difficult to handle; it
is less chemically stahle and it tends to depo~lt elemental
selenium, it decomposes rapidly by itself in room light.
Copper ls useful in the liyht-absorbing layer
or zone, since it extends the spectral response to the
lower wave length so that our finished electrophotographic
photoconductor ls sensitive over the entire visible range
from blue to red.
- 2A -
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6 ~ ~,
The production of our cadmium sulphide photo~
conductor from acetate salts, instead of chloride salts,
of cadmium and zinc is very important. Only in the ~rans-
port layer do we use a minor amount of cadmium chloride to
ensure the presencè of chlorine. We have disco~ered that
the use of cadmium sulphide, derived by spray pyrolysis
from the chloride, produces a photoconductor which has a
rapid dark decay; that is, its resistance or i~s dielectric
strength in the dark is not sufficiently great to be used
in the electrophotographic mode. The elimination of excess
chlorine by using the acetate salts solved the problem.
The introduction of chlorine in the charge
transport layer enables us to utilize a sufficiently
thick photoconductor to receive a charge as high as 250 to
350 volts, which ensures rapid development with a liquid-
carried toner. We have produced satisfactory electro-
photographic photoconductors of composite zones, according
to our invention, having a thickness between three micron~
and six microns. After a thickness o six microns has
been reached, cracks will develop through the film.
It will he seen that we have accomplished the
objects of our invention. We have provided a cadmium
sulphide electrophotographic photoconductor having improved
wear characteristics. A conductive drum bearing our
improved photoconductor was tested in a simulated office
photocopier mode and experienced over a million copies
- 25 -
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~ 1 6~
without significank wear. Our sensitivity or ef~ective
quantum efficiency, is two to ten times greater than
selenium. While selenium is sensitive only in ~he ranges
from blue to yellow, our photoconductor is sensikive across
the entire visible range from blue to red. Our photo-
conductor produces copies having photographic ~uality--
that is, having a gray scale sufficiently graduated so
that photographic reproductions are made from photographic
originals. A latent electrostatic image is produced,
with our photoconductor, which can be readily and rapidly
toned with a liquid toner. The main fault of cadmium
sulphide photoconductors -- that is, the presence of
memory -- has been substantially eliminated, and a latent
electrostatic image will decay rapidly in light. ~his
enables us to produce copies at a high speed. A high-
speed photocopying maGhin-e can be made with our photoconductor,
since a thick layer of photoconductive material may be
carried ~y a conductive substrate, enabling us to charge
to a hiyher potential than the thin cadmium sulphide
photoconductors of the prior art. Continued use of our
photoconductor fails to develop significant fatigue.
A selenium photoconductor cannot be heated to
over 80C. without deleterious effects. Our improved
photoconductor is unaffected by normal climatic values
and has an apparently infinite storage life. ~he micro-
crystalline structure and the presence of oxygen, in our
- 26 -
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:
~ ~ 8 ~
photoconductor, gives our composite photoconductor a
sufficiently high insulating property in the dark ~- that
is, a low dark decay -- so that a very satisfacto.ry
latent electrostatic image can be formed frorn a single
exposu~e.
It will be understood that certain features and
subcombinations are of utility and may be employed without
reference to other features and subcombinations. This
is contemplated by and is within -the scope of our claims.
It is fùrther obvious that various changes may be made in
details within the scope of our claims without departing
from the spirit of our inyention. It is 7 therefore, to
be understood -that our invention is not to be limited to
*he specific details shown and described.
~ Iaving thus described our invention, what we
claim is:
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