text: (a) Field of the Invention
  The present invention relates to an element for use in electrophotography, and in more detail relates to a multilayer-type electrophotographic element comprising a charge generating layer containing a substance which generates a charge carrier when exposed to light radiation (which will hereinafter be referred to as a charge generating substance) and a charge transfer layer containing a substance which receives the charge carrier generated by the charge-generating layer and transfers said charge carrier (which will hereinafter be referred to as a charge transfer substance).
  (b) Description of the Prior Art
  Typical elements for use in electrophotography include inorganic systems using either selenium and its alloy or one comprising dispersing a dye-sensitized zinc oxide in a binder resin, and an organic system using a charge transfer complex comprising 2,4,7-trinitro-9-fluorenone (which will be referred to hereinafter as TNF.) and poly-N-vinylcarbozole (which will hereinafter be referred to as PVK.). However, the fact is that these electrophotographic elements are possessed of various merits as well as various demerits. For instance, the selenium electrophotographic element used widely now is defective in that its manufacture is difficult, the manufacturing cost is expensive, it is difficult to work said element into a belt due to lack of flexibility, and additionally, attention must be paid to its handling because it is sensitive to heat and mechanical impact. The zinc oxide electrophotographic element is inexpensive to manufacture cost because it can be made by coating a substrate, with a low cost zinc oxide but is defective in mechanical properties such as surface smoothness, hardness, tensile strength, abrasion resistance and the like. Therefore, said zinc oxide electrophotographic element encounters various problems, such as durability and the like, when used repeatedly as the electrophotographic in a common paper copying machine. The electrophotographic element using the charge transfer complex comprising TNF and PVK is inferior in sensitivity compared to other elements, and is unsuitable as the electrophotographic element for use in a high speed copying machine.
  In recent years, wide investigations have been carried out in order to eliminate the drawbacks inherent in these electrophotographic elements. As a result, various organic system electrophotographic elements have been proposed. Among them, a multilayer-type electrophotographic element, which comprises a charge generating layer made by forming a thin film of an organic pigment on an electrically conductive substance and a charge transfer layer, superposed on said charge generating layer, which consists essentially of a charge transfer substance, is attracting public attention. As the electrophotographic element for use in the plain paper copying machine, said multilayer type electrophotographic element is normally superior in sensitivity, stable in chargeability and the like, as compared with usual organic system electrophotographic elements.
  As usual multilayer type electrophotographic elements of this sort, the following ones are well known, that is (1) one having used a perylene derivative in the charge generating layer and an oxadiazole derivative in the charge transfer layer see U.S. Pat. No. 3,871,882), (2) one comprising a charge generating layer formed by coating Chloro Dian Blue by using an organic amine as a solvent and the charge transfer layer using a pyrazoline derivative therein (see Japanese Laid Open Patent Application No. 55643/1977 and Japanese Laid Open Patent Application No. 72231/1977), and (3) one comprising a charge generating layer formed by coating a dispersion prepared by dispersing a triphenylamine system trisazo pigment (see Japanese Laid Open Patent Application No. 132347/1978) for instance in a dispersion medium of tetrahydrofuran or the like, and the charge transfer layer using 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole or TNF.
  However, the fact is that the usual multilayer-type electrophotographic elements of this sort possess various merits as well as various demerits.
  For instance, the electrophotographic element using a perylene derivative and an oxadiazole derivative therein as shown above in (1) surely raises no question of practical use, but is inferior in sensitivity when used in a high-speed copying machine. Further, in this electrophotographic element, a perylene derivative, a charge generating substance controlling the spectral sensitivity of this electrophotographic element, is not absorptive to the overall visible light region. Therefore, this electrophotographic element is defective in that it is unsuitable for use in a color copying machine.
  The electrophotographic element using Chloro Dian Blue and a pyrazoline derivative therein, shown above in (2), is observed to have a comparatively superior sensitivity according to our experiments, but is defective because in its preparation it is necessary to use an organic amine (for instance, ethylenediamine) which is generally difficult to handle, as coating solvent for forming the charge generating layer.
  The electrophotographic element shown in (3) above is the one which we proposed. This electrophotographic element is advantageous in that is charge generating layer can be formed with ease by coating a pigment dispersion, obtained by dispersing fine pigment particles in an organic solvent (if needed, a binder resin may be added) onto a substrate, but is somewhat inferior in sensitivity. Due to this, this electrophotographic element is insufficient as the one for use in a high-speed copying machine.
  In recent years the demand for an electrophotographic element for use in a laser printer has been rising, in particular, development of an electrophotographic element which is highly sensitive to the wavelength region of semiconductor laser is in demand. However, it is a fact that the above-mentioned electrophotographic elements can not be put to practical use because said elements are extremely low in sensitivity to the semi-conductor laser.
  The mechanism for a multilayer-type electrophotographic element of this sort forming an electrostatic latent image is considered to consist of the following when the charged electrophotographic element is exposed to radiation of light, the light passes through the transparent charge transfer layer and is absorbed by the charge generating substance contained in the charge generating layer. The charge generating substance, having absorbed the light, generates a charge carrier. This charge carrier is injected into the charge transfer layer, transfers through the charge transfer layer along the electric field having been caused by charging and neutralizes the charge present on the surface of the electrophotographic element to thereby form an electrostatic latent image. Accordingly, the charge generating substance used in the electrophotographic element of this sort is demanded to generate the charge carrier efficiently when exposed to radiation of light for image formation.
  The charge transfer substance is required to be transparent to the light used, to be capable of maintaining a predetermined charge potential and to be capable of promptly transferring the charge carrier generated by the charge generating substance.