US 3791826 A
An electrophotographic plate comprising in successive layers: (a) a conductive substrate, (b) a barrier layer, (3) an inorganic charge generating layer at least 0.15 microns thick, and (d) an organic charge transport layer from 3 to 20 microns thick comprising at least about 20 percent by weight of 2, 4, 7-trinitro-9-fluorenone.
Description (OCR text may contain errors)
United States Patent 1 Cherry et al.
[ ELECTROPHOTOGRAPHIC PLATE  Inventors: Albert J. Cherry, Morgan Hill;
Robert R. Neiman; Meredith D. Shattuck, both of San Jose; William J. Weiche, Los Gatos, all of Calif.
 Assignee: International Business Machines Corporation, Armonk, N.Y.
22 Filed: Jan. 24, 1972 21 Appl. No.: 220,471
Related [1.8. Application Data  Continuation-impart of Ser. No. 33,531, April 30,
 US. Cl 96/L5, 117/215, 117/218, 96/l.8
51 Int. Cl. G03g 5/04, 003 5/08  Field of Search 96/1.5l.8; 117/218  References Cited UNITED STATES PATENTS 2,901,348 8/1959 'Dessauer et al. 96/l. 5
[451 Feb. 12, 1974 3,598,582 8/1971 Herrick et al 96/l.5 3,484,237 12/1969 Shattuck et a1. 96/l.5 3,344,001 7/1968 Makino 96/l.5 3,408,189 l0/l968 Mammino 96/l.5 3,379,527 4/1968 Corrsin et al. 96/1 .5 3,573,906 4/1971 Goffe 96/1.8 3,l 13,022 12/1963 Cassiers et al.. 96/l.5 X 3,256,089 6/1966 Clark et a1. 96/1.5 3,634,079 1/1972 Champ 96/l.5
Primary ExaminerCharles E. Van Horn Attorney, Agent, or Firm.l0seph G. Walsh [5 7 ABSTRACT An electrophotographic plate comprising in successive layers: (a) a conductive substrate, (b) a barrier layer, (3) an inorganic charge generating layer at least 0.l5 microns thick, and (d) an organic charge transport layer from 3 to 20 microns thick comprising at least about 20 percent by weight of 2, 4, 7-trinitr0-9- fluorenone.
1 Claim, No Drawings ELECTROPHOTOGRAPHIC PLATE The present application is a continuation-in-part of our co-pending application, Ser. No. 33,531, filed Apr. 30, 1970 and now abandoned.
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is concerned with electrophotographic plates. In particular, it is concerned with electrophotographic plates which combine the desirable photosensitivity of inorganic photoconductive materials with the desirable surface film properties of organic polymeric materials.
2. Description of the Prior Art There have in the past been attempts to combine the advantageous features of inorganic photoconductive materials with organic materials. U.S. Pat. Nos. 3,121,006 and 3,121,007 describe the use of inorganic photoconductive materials dispersed in organic resin binders. U.S. Pat. No. 3,312,547 discloses an electrophotographic plate comprising successive layers of various organic and inorganic materials, as does U.S. Pat. No. 2,901,348. To the present time, however, there has been no commercial use of a plate. simultaneously combining the advantages of organic and inorganic materials.
SUMMARY OF THE INVENTION The present invention provides a solution to the problem of taking advantage of the photosensitivity of inorganic photoconductive materials and the surface film properties of organic materials. Plates made according to the present invention require: (a) a conductive substrate, (b) a barrier layer, (c) an inorganic charge generating layer, (d) an organic charge transport layer. Each component will be discussed in order below.
No particular type of conductive substrate is required. Any of the many materials which have been used in the past is satisfactory. Many are well-known in the art. These include, for example, aluminum, metal covered organic materials, and the like. The selection of the particular substrate will be determined by the mode in which it is desired to use the photographic plate. For example, the substrate may be rigid and selfsupporting or, alternatively, it may be flexible, or arranged in the shape of a cylindrical drum or an endless belt. The particular type of substrate is not a critical feature of the present invention.
A barrier layer is required in the electrophotographic plates of the present invention. The function of .the barrier layer is to prevent unwanted charge injection and to maintain a suitable charge acceptance. The exact process by which the barrier layer functions is not clearly understood. It is known, however, that insulating materials, particularly metallic oxides, have been found to function well as barrier layers in the present system. It is probable that tunneling of electronic charge permits the plate to be discharged when exposed to light.
Barrier layers suitable for use in the present invention may be prepared in many different ways. A preferred method involves simple thermal oxidation of aluminum substrate. (Exposure to the atmosphere of aluminum plate may be sufficient to bring about the formation of a suitable barrier layer by the natural formation of an adherent film of aluminum oxide.) Alternatively either aluminum or aluminized plastic such as polyethylene terephthalate may be anodized to form a barrier layer of aluminum oxide. Still another acceptable method is the vapor phase hydrolysis of a metal halide, for example, SiCl AlCl or TiCl to yield the corresponding metal oxide. The barrier layer may also be organic, for example a polyamide or polyurethane layer. In cases where brass is used as the conducting substrate, oxidation of the brass provides a satisfactory barrier layer. In general, the thickness of the barrier layer is less than about 0.2 microns, with a thickness of 0.17 microns of aluminum oxide being most preferred. a
The next essential component of the present invention is an inorganic photoconductive layer which provides the electronic charge generation necessary to perform the electrophotographic discharge. The most preferred method for preparing these inorganic photoconductive layers is by in situ formation using a spray technique. (Such aspray technique is discussed in detail in U.S. Pat. No. 3,148,084.)
In general, the most preferred inorganic photoconductive materials may be described as II-IV compounds, i.e., metals from Group II and nonmetals from Group VI of the Periodic Chart. Specifically, the sulfines, selenides and tellurides of cadmium and zinc are preferred. By use of spray techniques, uniform layers are obtained. The layers appear to be amorphous rather than crystalline.
It is an essential feature of the present invention that the inorganic charge generating layer be at least 0.15
' microns thick. The theoretical explanation of this is not understood, but it may be stated as an empirically established fact that unless the charge-generating layer is at least 0.15 microns thick, insufficient charge is generated. In general, the optimum thickness for a single layer of inorganic charge generating material is approximately 0.3 microns. It is generally preferable that the thickness of the charge generating layer be less than about one micron.
In a particularly preferred variation of the present invention, the inorganic charge generating layer is comprised of multiple strata. In the formation of such multiple strata, various combinations of Cd, Zn, S, Se, and Te are represented. A very large number of permutations and combinations are possible. Vitreous selenium and alloys thereof may also be used. The use of more than one layer of charge generating material introduces a very desirable element of flexibility in the design of the plates. 7
The most preferred embodiment of the present invention involves the use of a double layer of the charge generating material; the first layer (that closest to the barrier layer) is cadmium sulfoselenide, which may be represented by the formula CdS Se This layer is approximately 0.15 microns thick. The second layer is CdS Se which is approximately 0.25 microns thick.
The final required element of the present invention is the organic charge transport layer, which must be between 3 and 20 microns thick, and preferably between 10 and 15 microns. This layer functions to provide a high resistivity surface, to maintain electrostatic charge, to transport electronic charge from the charge generating layer, and to provide a surface with film properties suitable for the development of an electrophotographic image. Many techniques may be used to apply this layer. The preferred technique is that of doctorcoating with a solution. The requirement that good surface film properties be possessed by this layer dictates the use of an organic polymeric material. Many such materials are known to the prior art. What is extremely surprising, however, is the empirically established fact that adequate charge transport is obtained when, and only when, this organic layer comprises at least about 20% by weight of the compound 2, 4, 7-trinitro-9-fluorenone. (This compound may be represented below by the abbreviation TNF.)
Unless the organic charge transport is at least 3 microns thick, it will not accept sufficient voltage to be useful. When it is thicker than 20 microns, detrimental residual voltage is encountered.
The compound TNF by itself may be used as the organic charge transport layer when applied in the melted state and allowed to solidify. The TNF forms a charge transport layer which has excellent electrophotographic properties. It is, however, not desirable for commercial use since it is fairly glass-like in its brittleness. It is preferred that the TNF be used in combination with a resin binder.
The prior art is very familiar with resin binders. There is no particular requirement for the resin binder in the present invention except that it be chemically compatible with the TNF. The resin binder may be either photoconductive per se, or not photoconductive. The electrophotographic plates of the present invention operate with both types of binders. In another variation of the present invention, dyes may be added to the TNF-resin material to reduce residual potential and to give additional photosensitivity. Such photosensitizing dyes are well-known in the art, and are taught, for example, in US. Pat. Nos. 3,037,861, 3,169,060 and 3,287,l l 3. Care, however, should always be taken that the charge transport layer does not become so light absorbing that light will not reach the charge generating layer. Sulfur-containing materials such as thiourea or thioacetamide, etc., may also be used in place of the dyes. The exact function of these additives is not known, but'it may relate to the electronic compatibility between the organic and inorganic layers. Purity, particularly the purity of the TNF, is also sometimes of importance in achieving optimum electrophotographic performance. The particular resin binder with which the best results have been obtained to date is polyethylene terephthalate. This material is available commercially from DuPont under the trademark Mylar 49000.
In a most preferred method of using the plates of the present invention, the incident light is filtered to remove wavelengths that would be absorbed by the transport layer, i.e. wavelengths below 4300 Angstroms are removed by the use of appropriate filters. When the plates are used in this mode, fatigue effects are greatly reduced.
The electrophotographic plates of the present invention may be used in any of the conventional modes of electrophotography, several variations of which are well-known in the art.
The following Examples are given solely for the purpose of illustration and are not to be deemed limitations on the present invention, many variations of which will occur to those skilled in the art, without departing from the spirit or scope thereof.
barrier EXAMPLE I An aluminum offset plate, 6 mils thick, and 6 1% inches X 1 1 inches in size was heated for five minutes at 565C; thus a barrier layer (A1 0 was formed on a conductive substrate. A layer of CdSSe was deposited on the barrier layer-substrate by spraying 1.25ml/inch of a 50 percent (by volume) H 0, 50 percent (by volume) ethylene glycol solution 0.02M in Cd++ (as Cd(CI-I COO) 0.02M in S (as (NH C%), and 0.02M in Se (as(CH N-C=Se-NH at a substrate temperature of 290F. A layer of CdSe was then deposited on top of the CdSSe similarly by spraying 1.0ml/inch of the above-mentioned Cd and Se solutions at a substrate temperature of 290F. (Gravimetric measurements in single-layer CdSe deposited by this technique indicate that the thickness is about 0.3 The organic transport layer was deposited by doctorblading (wet gap thickness 7.6 mils), a tetrahydrofuran (THF) solution of one part (by weight) TNF to one part (by weight) Mylar adhesive 49000 with 0.1 wt. cryptocyanine and 2 drops of Cd-200 silicone oil. The plate is then air-dried and cured for fifteen minutes at 95C.
In the rotating disc electrometer results were: Light decay T 0.25 sec. (1.47 neutral density filter and opal glass). (T /2 represents the time required under illumination to reach one-half the voltage acceptance prior to illumination.)
Voltage Acceptance +670 volts.
Note that this speed is very fast, i.e., about ten times as fast as commercially used materials.
This plate was run on a cycling robot and gave excellent copies, with very good resolution and density.
EXAMPLE II A plate of pure aluminum was anodized by the constant-current method to 125 volts at 50ma to form the layer on a conductive substrate. This was coated with CdSSe and CdSe as above; in addition, the organic transport layer was applied in the same manner as above.
Rotating disc electrometer results here were:
T k 0.15 sec. (1.47 NDF 0.6.)
Voltage Acceptance +620 volts.
Note that this speed is extremely fast, about 20 times as fast as commercially used material.
EXAMPLE III A barrier layer conductive substrate was prepared as in Example I. This layer was coated with a single layer of CdSe by spraying a 1.0ml/inch deposition, as in the above examples. (Thickness about 0.3,u., see Example I.) The TNF-49000-cryptocyanine overcoat was then applied and treated as above. Results were:
T k 0.45 sec. (1.47 NDF 06) Voltage Acceptance +640 volts.
EXAMPLE IV Another plate was made according to the procedure of Example I. This plate was mounted in a cycling robot in the humidity room; robot copy and electrophotographic parameters were obtained for 24 hours at percent RH. and F. Results were:
AV Initial 295 Volts Example V An aluminum-laminated nylon paper (available from DuPont under the trademark Nomex) material was used as a conductive support. This is an example of a flexible substrate, which is highly desirable for systems applications. The barrier layer was formed by anodization to 75 volts at 50 ma. The charge-generating layers 1 and charge-transport layer were applied as in Example I. Results were:
T V: 0.63 sec. (1.47 NDF Voltage Acceptance 640 Volts EXAMPLE VI A plate was formed in the same manner as in Example I except that the organic transport layer was formed by doctor-blading (wet-gap thickness 5.0 mils) a THF solution similar to Example I except that a 2 percent solution of thiourea was used to replace the dye. A control without the thiourea was also made at the same time. Rotating disc electrometer results were:
Sample with thiourea: T V2 0.43 sec. (1.47 NDF 06) Voltage Acceptance +370 Volts Control: T ii 1.2 sec. (1.47 NDF 00) Voltage Acceptance +330 Volts. Residual potential was markedly reduced also with the thiourea sample.
EXAMPLE VI] A plate was made similar to Example Vl except that a 2 percent solution of thioacetamide was used in place of the thiourea. A control was also made. Rotating disc electrometer results were:
Sample with Thioacetamide': T k 35 sec. (1.47 NDF 06) Voltage Acceptance +420 Volts Control: T A 1.2 sec. (L47 NDF 0G) Voltage Acceptance +330 Volts. Residual potential was also markedly reduced with use of the thioacetamide material.
EXAMPLE Vlll A photoconductive plate was prepared by applying an organic layer (about 15 microns thick) consisting of 10 parts, TNF, 10 parts Bakelite polyketone-252, (a polyketone manufactured by Union Carbide Plastics Company) and parts tetrahydrofuran. The remainder of the plate was as in Example I.
Light decay T k 0.30 seconds Voltage Acceptance +680 volts Example 1x A photoconductive plate was prepared by applying an organic layer (about 10 microns thick) consisting of 10 parts poly-4-vinyldibenzofuran, 16 parts TNF and 100 parts tetrahydrofuran on an inorganic layer approximately 0.3 microns thick of CdSe. A thermally grown oxide of aluminum was used as the barrier layer.
V Light decay T 0.80 seconds 0 1 Voltage Acceptance +670 volts EXAMPLE X A photoconductive plate was prepared by applying an organic layer (about 15 microns thick) consisting of 10 parts TNF, 40 parts DuPont Mylar adhesive 49,000,
250 parts tetrahydrofuran, and 0.005 parts cryptocyanine dye. Other parts of the plate were as in Example 7 polyethylene terephthalate.