CA1056042A - Trigonal selenium composite photoreceptor and method - Google Patents

Abstract

TRIGONAL SELENIUM COMPOSITE PHOTORECEPTOR AND METHOD

ABSTRACT OF THE DISCLOSURE
A method of making a photosensitive member which comprises forming a dispersion of finely divided vitreous selenium particles in a liquid organic resin solution, coating said solution onto a supporting substrate and drying to form a photoinjecting binder layer comprising vitreous selenium particles contained in an organic resin matrix.
The member is then heated to an elevated temperature for a time sufficient to convert the vitreous selenium to the crystalline trigonal form, followed by cooling the member to room temperature.

Description

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BACKGROUND OF THE INVENTION
This invention relates in general to xerography and more specifically to a method of fabricating a photoreceptor device comprising finely divided trigonal selenium particles contained in a photoconductive binder lay~r.
In the art of xerography, a xerographic plate con-taining a photoconductive insulating layer is first given a uniform electrostatic charge in order to sensitize the surface of the photoconductive layer. The plate is then exposed to an image of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind the latent electrostatic image in the nonilluminated areas.
The latent electrostatic image may be developed and made visible by depositing finely divided electroscopic marking particles on the surface of the photoconductive layer. This concept was originally described by C. Carlson in U. S. Patent

2,297,691 issued October 6, 1942 and is further amplified and described by many related patents in the field.
Conventional xerographic plates or drums usually comprise a photoconductive insulating layer overlaying a con-ductive support, A photoconductive material which has had wide use as a reusable photoconductor in commercial xerography comprises vitreous or amorphous selenium. Vitreous selenium in essence comprises super cooled selenium liquid and may read-ily be formed by vacuum evaporation by cooling the liquid or vapor so suddenly that crystals of selenium do not have time to form. Although vitreous selenium has had wide acceptance for commercial use in xerography, its spectral response is limited largely to the blue-green portion of the electromagnetic spectrum ~ ' . . .

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which is below about 5200 Angstrom Units. In addition, the preparation of vitreous selenium by vacuum deposition requires a significant capital expenditure for vacuum coating apparatus and closely controlled process parameters are re~uired in order to obtain a photoconductive layer having the desired electrical characteristics. In general, one requirement of a photo-conductor, such as vitreous selenium, is that its resistivity should drop at least several orders of magnitude in the presence of activating radiation or light in comparison to its resistivity in the dark. Also, the photoconductive layer should be able to support a significant electrical potential in the absence of radiation.
Selenium also exists in a crystalline form known as trigonal or hexagonal selenium which is well known to the semiconductor art for use in the manufacture of selenium rectifiers. In the crystalline trigonal form, the structure of the selenium consists of helical chains of selenium atoms which are parallel to each other along the crystallographic c-axis. Trigonal selenium is not normally used in xerography as a homogeneous photoconductive layer because of its relat-ively high electrical conductivity in the dark, although in some instances trigonal selenium can be used in binder structures where trigonal selenium particles are dispersed in a matrix of another material such as an electrically insulating resin, an electrically active organic material, or a photoconductor such as vitreous selenium.
U.S. Patents 2,739,079, P. Keck, issued March 20, 1956 and 3,692,521, W. von Grable, issued September 19, 1972 both describe photosensitive members utilizing small amDunts of crystalline h ~ go~ (tri~l) selenium oont~d in pre~x~nantly vitreous selenium matrices. In addition, copendin~ Canadian , . , , . . . ~, .. .. . . ..

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Patent Application Serial No. 894,169, filed May 14, 1968 describes a special form of red-hexagonal selenium suitable for use in binder structure in which finely divided red-hexagonal selenium particles are contained in a resin binder matrix.
Although trigonal selenium exhibits a wider spectral response and is more thermally stable than vitreous selenium, as stated above, trigonal selenium is not normally used in xero-graphy because of its relatively high electrical conductivity in the dark. However, imaging members which are able to use hexagonal selenium would have advantages over those using ~itre-ous selenium with regard to improved spectral response. Further, the use of trigonal selenium in xerographic members, especially in the binder form, would provide greater ease in the manufacture of the photoconductive device in that the expensive vacuum coating apparatus required for forming vitreous selenium would not be necessary in-forming a binder layer containing trigonal selenium particles. Binder layers are also inherently more flexible than evaporated layers. In addition, solvent coated binder layers can adhere more tenaciously to substrates than vacuum evaporation layers.
SUMM~RY OF THE IN~7ENTION
In accordance with one aspect of this invention there is provided a method of making a photosensitive me~ber compris-ing: (a) forming a dispersion of finely divided vitreous selenium particles in a liquid organic resin solution; (b) coating said solution onto a supporting substrate and drying to form a photo-injecting binder layer comprising vitreous selenium particles contained in an organic resin matrix; and (c~ heating said member to an elevated temperature for a time sufficient to convert the vitreous selenium to the crystalline trigonal form.
In accordance with another aspect of this invention there is provided an imaging device which comprises a supporting substrate having thereon a layer of electrically active material, -4~

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a photoconductive binder layer overlaying said active layer, with said binder layer comprising finely divided particles of trigonal selenium dispersed in an organic resin matrix.
By way of added explanation, the present invention in one of its aspects is directed to a method of fabricating a photoreceptor device including a binder layer which contains crystalline trigonal selenium particles. More specifically, the method comprises dispersing finely divided amorphous or vitreous selenium particles in a resin solution, and coating said solution onto a supporting substrate. The solution is dried to form a photoconductive binder layer comprising vitreous selenium part-icles dispersed in an organic resin matrix. The vitreous selen-ium is then converted to the trigonal form in situ by heating to an elevated temperature for a time sufficient to effect convers-ion of vitreous selenium to the crystalline trigonal form followed by carefully cooling the device to room temperature.
The device of the present invention may be used in structures in which the photoinjecting trigonal selenium binder layer is used in conjunction with a contiguous layer of active organic ;
material such as poly-n-vinyl carbazole or poly-vinyl pyrene.
BRIEF DES RIPTION OF THE DRAWINGS
Figures 1-5 represent five different embodiments of imaging structures suitable for using particulate trigonal selenium of the present invention.
Figure 6 illustrates apparatus suitable for thermally converting vitreous selenium particles to trigonal selenium.
DETAILED DESCRIPTION OF THE DRAWINGS
The particulate trigonal selenium binder layers of the present invention can be utilized in a variety of imaging structures, illustrated more clearly by Figures 1-5 of the -5~

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drawings. Figure 1 comprises an imaging member 10 having a conductive substrate 11 overcoated with a binder photogenerating layer 12 comprising trigonal selenium particles 13 dispersed in a matrix 14 which usually comprises an electrically active polymer such as poly-n-vinyl carbazole (PVK), polyvinyl pyrene (PVP), or 2,4,7-trinitro-9-fluorenone (TNF); or a combination of PVK or PVP, with TNF or sImilar compounds. A transport layer 15 overlays the photogenerating binder layer and comprises an electrically active material such as polyvinyl carbazole, polyvinyl pyrene, PVK or PVP with TNF. Under the influence of an electrical field, such polymers are capable of transporting a photoinjected charge from the photogenerating layer and hence are referred to as active polymers. A satisfactory thickness for the binder photoinjecting layer is from about 0.5 to 6 microns. The thickness of the active transport layer is from about 5 to 10~ microns, but thicknesses outside this range can also be used. ; ~ -The imaging member 20 of Figure 2 is similar to the member of Figure 1 except that substrate 21 is overcoated with photoinjecting layer 22 comprising the same trigonal selenium particles 23 in substantial particle-to-particle contact con-tained in an electrical insulating resin matrix 24 such as a silicone resin or polyester. Alternatively, the insulating resin may be replaced with an electrically active material of the type described for Figure 1 above. The transport overlayer 25 is electrically active and identical or equivalent to the materials described for the transport layer 15 of Figure 1. In operation, the imaglng members of Figures 1 and 2 are normally ' .
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uniformly electrostatically charged and then imaged by exposure to a pattern of light to which the top transport layer is sub-stantially nonabsorbing or transparent. Charge carriers are generated by the photogenerating layer, and injected into and transported through the transport layer to selectively dis-charge a surface charge on top of the transport layer.
The imaging members 30 and 40 of Figures 3 and 4, respecti~ely, are directed to alternative embodiments of Figures 1 and 2, respectively, in which the photogenerating layer is contained on top of the transport layer. More specifically, in Figure 3, conductive substrate 31 is over~
coated with layer of active organic material 32 which contains -a top binder layer 33 comprising trigonal selenium particles 34 contained in an electrically active matrix 35. Similarly, Figure 4 is an alternative embodiment of Figure 3 in which conductive substrate 41 overlayed with a transport layer 42, contains a binder layer 43 in which the trigonal selenium particles 44 are in substantial particle-to-particle contact and contained in a matrix of electrically insulating material 2Q 45 or an active material such as PVK, PVP, PVK or PVP and TNF. In operation the imaging members of Figures 3 and 4 are uniformly electrostatically charged to a given polarity and thenimaged with light to which the top photogenerating layer is absorbing. The charge carriersgenerated by the top layer are injected into and transported through the middle transport layer, while an opposite charge dissipated the electrostatic charge at the surface of the top layer. In this case, the transport layer need not be transparent to light since most of the light is absorbed in the generator layer.
In another embodiment o~ the present invention, - , ~ ~ ' . ... .

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illustrated in Figure 5, imaging member 50 comprises a single binder layer 52 formed on conductive substrate 51. Binder layer 52 comprises a relatively small amount of photoconductive trigonal selenium 53 contained in an electrically active matrix 54 which may comprise a material such as polyvinyl carbazole or polyvinyl pyrene. These materials may be used in combination with other materials such as TNF in order to improve the cycling characteristics of the imaging member.
In thermally converting in situ the vitreous selenium particles in the binder layer to the trigonal form, any suitable apparatus or technique may be used. In one --embodiment, the imaging member is placed in a stainless steel ampul 60 illustrated in Figure 6. The ampul is made of stainless steel and comprises a body portion 61 having a removable cover 62 at the top. In order to insure a proper seal between the cover and body a resilient O-ring 63 is placed in the contact area between the ampul cover and body.
A vacuum port 64 is located at the top of the ampul through the cover and comprises Kovar seal 66 connected to a Pyrex ~ -(trade mark for a heat resistant glass) glass tube 65.
This vacuum port is connected to a suitable source of vacuum (not shown). In operation, each imaging member in which the .: ....
vitreous selenium particles are to be converted to the trigonal form is placed in the ampul and the ampul evacuated -to a suitable pressure in the vicinity of about 1 x 10 4 Torr and sealed. The selenium in each plate is then ~hermally ~- ---converted to the trigonal form by heating to an elevated temperature for a particular time and then cooled to room temperature.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
-The following examples further specifically define the present invention with respect to a method of thermally . , lOS1~042 converting particulate vitreous selenium contained in a binder layer to the trigonal form. The percentages are by weight unless otherwise indicated. Examples are intended to illus-trate various preferred embodiments of the present invention.
EXAMPLE
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Sixteen imaging members having the structure illus-trated in Figure 4 of the drawings are made by the following method. P~K transport layers approximately 13 microns in thickness are coated onto 16 strips of 0.006 inch thick 10 anodized ball-gained aluminum from a 9% by weight solution of -PVK dissolved in chloroform. These layers are heated under vacuum at 180C for about 24 hours, after which time they were overcoated with approximately 2 micron generator layers containing 50~ vitreous selenium particles by volume dispersed in P~K. The vitreous selenium is obtained in pellet form from Canadian Copper Refiners Limited and is about 99.999%
pure. It is ground with a morter and pestle and put through a 100 mesh sieve prior to use. The samples are further dried for 4 hours under vacuum at 40C. At this temperature, the vitreous selenium remains in the vitreous form. After drying under vacuum at 180C for 24 hours, PVK transport layers have been found to contain less than 0.01% by weight solvent as determined by gas chromatography. The generator layers are prepared by adding 0.378 grams of vitreous selenium having a size distribution of 1 to 70 microns and an average particle size of about 14 microns to 0.10 gram of PVX in 5 ml. of CHC13, and 40 grams of 1/8 inch diameter sheet shot. These mixtures are then milled for 4 hours resulting in a further reduction in selenium particle 30 size after which time they are coated onto the PVX transport ~ ~-layerswith a Bird Applicator. The PVX used in these experiments ..
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was obtained from BASF and had a molecular weight average of 2.1 x 106.
The seleniwm in each plate is thermally converted to the trigonal form under a variety of conditions. A total of sixteen plates in all were prepared. Two plates were prepared in 8 sets under each of th~ following conditions.
DATA TABLE

Cooling Time (From Anneal-ing Temperature Annealing to Room Temperature ( C) Time Temperature) Set 1 (2 plates) 125 1 hr. _ 90 min.
Set 2 (2 plates) 180 1 hr. ~ 90 min.
Set 3 (2 plates~ 125 16 hrs. ~ 90 min.
Set 4 (2 plates) 180 16 hrs. ~,90 min.
- Set 5 (2 plates) 125 1 hr. ~,3.5 min.
Set 6 (2 plates) 180 1 hr. ~v3-5 min.
Set 7 (2 plates) 125 16 hrs. ~3.5 min.
Set 8 (2 plates) i80 16 hrs. ~3.5 min.

~ he data were evaluated statistically by the Yates analysis technique, and a statistically significant temperature-annealing time interaction was observed. Samples annealed for 1 hour at 125 had an average photodischarge rate of 155 volts/sec.
under the experimental condi~ions used while samples annealed for 16 hours at 125C and for 1 hour at 180C had average photospeeds of 405 volts/sec. and 485 volts/sec., respectively, -when measured at a field of 45 volts/micron.
The preparation temperature was found to have a statistically significant effect on dark decay. Samples heated at 180C had an average dark discharge rate of 27 volts/sec.
under the experimental conditions used. Samples prepared at 125C had an average dark decay of 80 volts/sec. These --1 0-- ' ' ' ' lOS6;042 measurements were also made at a field of 45 volts/micron.
Several factors were found to have a statistically significant effect on charge acceptance values: For example, temperature was found to be an important factor. Samples prepared at 180C, on the average, charged to 39 volts/micron under the experimental conditions used, whereas samples prepared at 125C charged to an average field of 28 volts/
micron.
In addition, annealing time, under certain condi-10 tions was found to be a significant variable. At 125C, -samples annealed for 16 hours charged to an average field of 35 volts/micron under the experimental conditions used, while samples annealed for 1 hour at this temperature charged to only 20 volts/micron.
Data from other experiments suggest that cooling the samples from preparation temperature to room temperature in about 45 seconds or less may have a detrimental effect on charge acceptance.
The statistically significant trends observed in these experiments indicate that, on the average, samples prepared at about 180C for about 1 hour exhibit higher photodischarge rates than samples prepared under the othex conditions. Samples prèpared at about 125C for about 16 hours discharge almost as well. Also, samples prepared at about 180C, on the average, exhibit the lowest dark dis-charge rates. In addition, samples prepared at about 180C
as well as samples prepared at about 125C for about 16 --hours, on the average, charge to higher fields than samples prepared under the other conditions.
With regard to the thermal conversion of vitreous selenium particles in a binder layer to the trigonal form, ,' : :

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10~;042 the conversion temperature, time and temperature, and the cooling rate appear to be significant factors which must be carefully controlled. In general, the conversion temperature between about 125C to 180C have been found satisfactory and cooling rates less than about 45 seconds from the conversion temperature down to room temperature have under certain conditions been found unsatisfactory.
Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are also intended to be within the scope of this invention.

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of making a photosensitive member comprising:
(a) forming a dispersion of finely divided vitreous selenium particles in a liquid organic resin solution; (b) coating said solution onto a supporting substrate and drying to form a photo-injecting binder layer comprising vitreous selenium particles contained in an organic resin matrix; and (c) heating said member to an elevated temperature for a time sufficient to convert the vitreous selenium to the crystalline trigonal form.

2. The method of claim 1 in which the member is heated to a temperature of about 170°C to 190°C for about 30 minutes to 2 hours.

3. The method of claim 1 in which the member is heated to a temperature of about 110°C to 140°C for about 8 to 24 hours.

4. The method of claim 1 in which the member, after heat-ing, is cooled to room temperature in a time greater than about 45 seconds.

5. The method of claim 1 in which the member is heated to a temperature of 170°C to 190°C for 30 minutes to 2 hours or to a temperature of 110°C to 140°C for 8 to 24 hours to convert the vitreous selenium to the crystalline trigonal form and wherein the member, after heating, is cooled to a room temperature in a time greater than about 45 seconds.

6. The method of claim 1 or claim 5 in which a layer of active organic material is formed over the binder layer after step (b).

7. The method of claim 1 or claim 5 in which a layer of active organic material is formed over the substrate prior to step (a).