1. Field of the Invention
This invention relates to an electrophotographic photoconductor and to an image formation method. More particularly, this invention relates to an electrophotographic photoconductor and an image formation method that are employed in copying machines, printers and the like.
2. Prior Arts
The electrophotographic technology invented by C. F. Carlson provides images having high spontaneousness, high quality and high preservation property. Therefore, this technology has been used widely not only in the field of copying machines but also in the fields of various printers and facsimiles, and has become wide spread. This electrophotographic technology is fundamentally an image formation process that comprises uniform charging of a photoconductor, formation of an electrostatic latent image by image exposure, development of the latent image by a toner, transfer of the toner image to sheets of paper (through an intermediate transfer member in some cases), and fixation.
The material of the photoconductor that constitutes the gist of the electrophotographic technology has shifted in recent years from the conventional inorganic materials such as selenium, arsenic-selenium alloys, cadmium sulfide, zinc oxide, etc, to organic materials because they are pollution-free, can easily form films and can be produced easily.
A laminate type photoconductor, which is produced by laminating a charge generation layer and a charge transfer layer, has become predominant at present and is mass-produced because it has a higher sensitivity, has a wide selection range of materials, has high safety and high productivity by coating or the like, and is relatively advantageous from the aspect of the production cost.
Means capable of digitally forming the images have been limited in the past to laser printers and LED printers, as output machines of word processors and personal computers, and a part of color laser copying machines. Recently, however, the digital image formation technology has made a rapid progress in the field of ordinary copying machines in which analog image formation has been predominant, so as to acquire the image having higher image quality, to store the input image and to execute free edition of the image.
Performance required for the photoconductor to be employed for such digital image formation is mainly as follows.
(1) The photoconductor has sensitivity to light of a long wavelength. PA0 (2) A dark potential must be kept always constant. PA0 (3) Interference between incident light and reflected light must be prevented. PA0 (1) when computer information is directly handled, electric signals of the information are converted to optical signals and are then inputted to the photoconductor. When the information is inputted from an original manuscript, the information is read as optical information, is then converted to a digital electric signal, is again converted to an optical signal and is thereafter inputted to the photoconductor. In either case, the digital signal is inputted as the optical signal to the photoconductor. A laser beam or an LED beam has been mainly used for such an optical input. The oscillation wavelength of the optical input that has gained the widest application at present is near infrared-rays of 780 nm or light of a long wavelength of 650 nm. Therefore, the photoconductor used for the digital image formation must be sensitive to such light of a long wavelength. PA0 (2) When digital image formation is carried out, a so-called "inversion development system" which causes a toner to adhere to a portion to which light are irradiated and forms the image is employed in many cases so as to utilize effectively light and to improve resolution. In this inversion development system, a non-exposed portion (dark potential) becomes a white base and an exposed portion (bright potential) becomes a black base (image line). Unlike a normal development system, the inversion development system is free from the occurrence of a so-called "photographic fog" (the occurrence of black spots on the white base) even when the bright potential rises. When the dark potential drops, however, the fog occurs. Therefore, the photoconductor used for the digital image formation must keep the dark potential always constant. A Scorotron charger is ordinarily used as a charger. PA0 (3) Laser is used to write digitally the image onto the photoconductor. Such a coherent ray of light is likely to invite interference. The incident light and the reflected light from a conductive support interfere with one another in the photoconductor, generating a bright and dark fringe pattern (moire) on the image. Therefore, the interference between the incident light and the reflected light must be prevented. Japanese Patent No. 1,929,859, for example, proposes to coarsen the support surface, and Japanese Patent No. 1,932,365 proposes to sandwich an opaque intermediate layer.
In other words,
A wide variety of materials have been examined so far to constitute such a photoconductor. Among them, phthalocyanine compounds can be synthesized relatively easily and most of them are sensitive to light of a long wavelength. Therefore, they have been examined widely and put into practical application. For example, Japanese Patent No. 2,073,696 discloses a photoconductor using titanyl phthalocyanine, Japanese Patent Laid-Open No. SHO59-155851 discloses a photoconductor using .beta. type indium phthalocyanine, Japanese Patent Publication No. SHO56-17657 discloses a photoconductor using X type non-metallic phthalocyanine, and Japanese Patent Laid-Open No. SHO61-28557 discloses a photoconductor using vanadyl phthalocyanine.
Digital copying machines, in general, create an intermediate tone by modulating pulse width. More concretely, they produce the laser output by ON/OFF two values, divide one dot of the oscillation time into 256, for example, and generate gradation by changing the output in 256 stages from 256/256 as the maximum output to 1/256 as the minimum output. When this ratio (that is referred to as "duty") is small, exposure energy is low and a high potential is held. When the ratio is greater, on the other hand, exposure energy is great and the potential drops to a low level, giving contrast of the potential.
The photoconductor using the phthalocyanine compound that satisfies these three requirements as the charge generation material has high sensitivity to light of a long wavelength. As a result, the potential drops greatly to light having a low duty and small exposure energy, but the potential does not drop sufficiently to light having a high duty and large exposure energy. Therefore, the contrast on the low potential side cannot be acquired sufficiently.
This phenomenon results from the following fact. The ionization potential of the charge generation layers using phthalocyanine, in general, is smaller than the ionization potential of the charge transfer layer, that is, about 5.2 to about 5.6 eV. The difference of their ionization potentials functions as a barrier to the injection of holes from the charge generation layer to the charge transfer layer. This phenomenon becomes remarkable particularly when the potential of a photosensitive layer is low and the potential gradient is small. In this case, it becomes difficult to secure the potential contrast.
The ionization potential of titanyl phthalocyanine (oxytitanium phthalocyanine) varies depending on the crystal condition, and is within the range of 5.2 to 5.4 eV. It is 5.2 to 5.3 eV (in Japanese Patent Laid-Open No. SHO6-250421), 5.22 eV (in Japanese Patent Laid-Open No. HEI7-319179) and 5.35 V (in Japanese Patent Laid-Open No. HEI10-319613).
As to injection of the holes from the charge generation layer to the charge transfer layer, no injection barrier exists and the injection of the holes can be done more smoothly if the ionization potential of the charge generation layer is greater than that of the charge transfer layer. However, since the ionization potential of titanyl phthalocyanine used conventionally is small, the injection of the holes into the charge transfer layer cannot be made smoothly. In consequence, the charge that cannot be injected remains in the charge generation layer on the lower potential side and the potential does not drop sufficiently. Therefore, the contrast of the potential cannot be made great, a beautiful intermediate tone cannot be obtained.
In view of the background described above, titanyl phthalocyanine having a large ionization potential has been desired so that the injection of the holes can be made smoothly from the charge generation layer to the charge transfer layer.