JPS60142355A - Stabilized electrostatic charging method - Google Patents

Abstract

PURPOSE:To prevent memory after-image development without decreasing the amount of charging by providing a detrapping process wherein a conductive flexible member applied with a bias voltage is brought into contact between a cleaning process and a charging process. CONSTITUTION:Residual charges on a photosensitive drum 11 after toner transfer are removed by the front-surface exposure of a discharging lamp 21 and residual toner is removed by a cleaning mechanism 22, so that the photosensitive drum 11 has optical fatigue of order which can not be ignored. When detrapping is carried out previously by applying a bias voltage before the charging, surface charges are not neutralized in the earliest charging, so the optical fatigue is prevented completely. For the purpose, the bias voltage is applied from a conductive rubber roller 31 connected to one electrode of a power source 30 which is grounded at the other electrode. The level of the bias voltage depends up the kinds of the light source in use and an amorphous silicone photosensitive body and is generally 90-400V.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for stabilizing charging, and more specifically, when forming an image by electrophotography using an amorphous silicon-based photoconductor layer, This invention relates to a method for stabilizing the surface potential of body layers to a certain level.

An amorphous silicon-based photoconductor layer has high surface hardness, is sensitive to light on the -long wavelength side, and has good sensitivity itself, so it is attracting attention as a photoreceptor for electrophotography.

However, according to research conducted by researchers such as Zero Inventor, although amorphous silicon has the above-mentioned excellent properties, it has the problem of relatively large optical fatigue during high-speed copying. For example, during a normal copying cycle, charging, exposure,
When the various operations of development, transfer, and cleaning are repeated on a photosensitive layer, in the case of a selenium photosensitive layer, the amount of charge after the second time is only about 0.5 to 3% lower than the amount of charge from the first time, and this is due to optical fatigue. Although the effect is almost negligible, in the case of amorphous silicon, the amount of charge after the second time is as much as 5 to 20% lower than the amount of charge at the first time. For this reason, when the outer circumference of the drum is longer than the length of the copy, there is a drawback that the density of the second and subsequent images is reduced to an extent that can be visually discerned compared to the first image.

Furthermore, when the length of the outer circumference of the drum is shorter than the length of the copy, there is a drawback that density differences occur in the same copy or a memory afterimage phenomenon occurs. Here, the memory afterimage phenomenon refers to, for example, when using a document like the one shown in Figure 1-a, the distance between the text area 1゛ and the evening area 2 degrees! This refers to a phenomenon that occurs in the copy shown in FIG. 1 as an image in which the character portion 1 overlaps the black solid portion 2 when the length coincides with the outer circumferential length of the drum. The reason why this phenomenon occurs is that in the first rotation of the drum, the character 3 part of the character part 1 is not exposed, but the back shadow 4 part is exposed, so the character 3
There is a difference in the degree of optical fatigue of the photoreceptor in each part of the back shadow 4, and as a result, even if the photoreceptor is charged in order to form a latent image of the black solid area 2 at this position on the surface of the photoreceptor in the second rotation of the drum. , the amount of charge on the part 5 that was a character last time is maintained at the same potential as the first time, but the amount of charge on the part 6 that was a shadow last time decreases due to the influence of optical fatigue, and the amount of charge on the part 5 that was a character last time decreases due to the effect of optical fatigue. It is thought that there is a difference in concentration between the two.

Recently, as a means of preventing such optical fatigue, it has been proposed to use light of 600 nm or less as a light source to remove static electricity, as disclosed in, for example, Japanese Patent Laid-Open No. 58-62659. According to this method, photofatigue occurs due to repeated exposure, resulting in a decrease in the amount of charge, but this is maintained at a constant level, and since light of 60'Onm or less is used, even if photofatigue occurs, the amount of charge remains low. It is stabilized at the level of the amount of charge. However, although this proposal also achieves a high level of stabilization and is generally good in actual use, it is currently not possible to completely prevent optical fatigue. For this reason, a method that can completely prevent optical fatigue is desired.

Therefore, the object of the present invention is to increase the surface charging potential of the amorphous silicon photoconductor layer in the first step and in the second and subsequent steps.
The object of the present invention is to propose a method that can adjust the charge amount to substantially the same level without reducing it in any way and prevent the memory afterimage phenomenon caused by the difference in optical fatigue.

According to the present invention, an image forming method is provided in which an electrophotographic photoreceptor having an amorphous silicon-based photoconductor layer on a conductive substrate is charged by repeating the steps of one-image exposure, development, transfer, and cleaning. A stabilizing charging method is provided, which includes a detrapping step in which a conductive flexible member to which a bias voltage is applied is brought into contact between the cleaning step and the charging step.

In FIG. 2 for explaining the stabilizing charging method of the present invention, an amorphous silicon-based photoconductor layer is provided on the surface of a metal drum that is driven and rotated to constitute a photosensitive drum 11. Around this drum are a main charging corona charger 13; an image exposure mechanism consisting of a lamp I4, a document support transparent plate 15, and an optical system 16; a developing mechanism 18 having toner 17; an I-chi transfer corona charger 19; A separation corona charger 20; a static elimination lamp 2; and a cleaning mechanism 22 are provided in this order.

First, the photoreceptor drum 11 is charged with a constant polarity using the corona charger 13. Next, the original 23 to be copied is illuminated by the lamp 14, and the photosensitive drum 11 is exposed to a light beam image of the original through the optical system 16, thereby forming an electrostatic latent image corresponding to the original image. This electrostatic latent image is developed with toner 17 by a developing mechanism 18. The transfer paper 24 is supplied so as to be in contact with the drum surface at the position of the toner transfer charger 19, and a corona charge with the same polarity as the electrostatic latent image is performed from the back side of the transfer paper 24 to transfer the toner image onto the transfer paper 24. Make a transcription.

The transfer paper 24 on which the toner image has been transferred is electrostatically peeled off from the drum by the separation corona charger 20 and sent to a processing area such as a fixing area (not shown).

After the toner has been transferred, the photoreceptor drum 11 is exposed to light from the front side by the charge eliminating pump 2 to erase residual charges, and then the cleaning mechanism 22 removes the residual toner.

As already mentioned, the photosensitive drum 11 having the amorphous silicon photosensitive layer on its surface used in the present invention exhibits a non-negligible level of optical fatigue, and the charged potential of the photosensitive layer after exposure is different from that of the exposed photosensitive layer. The charge potential of the photosensitive layer is reduced by up to 20% compared to the charged potential of the photosensitive layer without the photosensitive layer.

The present inventors have discovered that the photofatigue of amorphous silicon-based photoconductors is
Some of the photocarriers generated in the photoconductor layer during exposure are trapped and remain in the photoconductor layer, and during the next charging, these are released by the action of the electric field created by the surface charge, neutralizing the surface charge. It is inferred that this is a phenomenon in which charging characteristics deteriorate. Therefore, after exposure or irradiation with static eliminating light but before the next main charging, a bias voltage is applied to the surface of the photoreceptor drum whose drum base is grounded from a power source whose one electrode is grounded to remove the photo from the trap. When a sufficient electric field is applied to release carriers, the trapped photocarriers in the photoconductor layer are de-tramped by the action of this electric field, and the tramped photocarriers are erased. This is what we discovered.

FIG. 3 is a diagram showing the wavelength dependence of optical fatigue measured using monochromatic light with a light intensity of 28.0 μW/cm2, where the horizontal axis is the wavelength at the time of exposure of the photosensitive layer, and the vertical axis is the surface It shows the amount of decrease in potential or the degree of decrease (light fatigue rate %). Referring to Fig. 3, the optical fatigue of amorphous silicon greatly depends on the wavelength of light, with maximum fatigue occurring at a wavelength of 725 nm, and almost no fatigue occurring for light with a wavelength of 600 nm or less. You can see that it is not shown. Therefore, in the above-mentioned known method, light with a wavelength of 600 nm or less is used as exposure and neutralizing light. The reason for this is that the wavelength is 6
For light of 00nm or less, absorption of light occurs on the surface of the photoreceptor, and photocarriers are mainly generated from the surface of the photosensitive layer, so the proportion of photocarriers trapped is small, whereas for long wavelength light, absorption of light occurs throughout the photosensitive layer. This is because photocarriers are generated within the volume (bulk) of the photosensitive layer and the amount of trapped milk increases (bulk effect).

However, when a wavelength of 600 nm or less is used, the amount of photocarriers trapped is small, and if the next charging is used as it is, the amount of charging will decrease somewhat, even if it does not pose a practical problem, and photo-induced fatigue will completely occur. did not prevent it.

On the other hand, according to the present invention, if a bias voltage is applied before charging to remove the traps, the surface charge can no longer be neutralized during charging, so that optical fatigue can be completely prevented.

Moreover, according to the present invention, since the trapping is caused by the electric field strength of the applied voltage, it is easier to detramp the photocarriers trapped on the surface of the photoreceptor than the carriers trapped in the bulk. In this sense, it is more advantageous to use a light source with a wavelength of 600 nm or less, but by increasing the electric field strength, it is also possible to detrap photocarriers in the bulk of the photosensitive layer, making it possible to use long wavelength light. can be used,
It has the advantage of being applicable to more light sources, including the use of semiconductor lasers.

In the present invention, any means known per se can be used as a means for applying a bias voltage to the surface of the photoreceptor as long as it is capable of applying a bias voltage uniformly over the length of the photoreceptor drum. .

For example, in FIG.
A bias voltage is applied by a conductive rubber roller 31 connected to the other electrode of 30. In addition to this rubber roller, conductive members such as conductive rubber blades, conductive brushes made of carbon or metal fibers, etc. that do not apply strong mechanical stress to the photoreceptor when rubbed against the surface of the photoreceptor may be used. Flexible members can also be used.

The amount of bias voltage applied to the surface of the photoreceptor varies depending on the type of light source used and the amorphous silicon photoreceptor, but generally 90 to 400V, particularly 200 to 400V may be applied. The type of bias voltage may be either positive or negative, and may be an alternating current bias.

Furthermore, as a means that can replace the bias voltage application means, A
A method using a C corona charger is also considered, but it generates ozone etc. during discharge, which deteriorates the surface of the photoreceptor and causes image deletion, that is, lateral leakage development of the electrostatic latent image formed on the surface of the photoreceptor. It's also undesirable.

As the amorphous silicon-based photoconductor layer, any known material can be used, for example, amorphous silicon deposited on the substrate by plasma decomposition of silane gas, etc.
This material may be doped with hydrogen, halogen, etc., and further doped with an element of group 1 or group 2 of the periodic table, such as poron or phosphorus.

The physical properties of a typical amorphous silicon photoreceptor are dark electrical conductivity <1g-12Ω-'・cm-', activation energy <0.85eV, and photoconductivity>10-7Ω-1・cm-
', the optical band gap is 1.7 to 1.9 eV,
Further, the amount of bonded hydrogen is 10 to 20 atomic %, and the dielectric constant of the film is in the range of 11.5 to 12.5.

The invention is illustrated by the following example.

Example 1 Using an electrophotographic copying apparatus having the configuration shown in FIG. ~ Experiment No. 4 was conducted. The photoreceptor drum is an a-8i:H drum (90φ) with a film thickness of 20 μm.
was installed.

The manuscript used in this series of experiments was Δ3 size, and as shown in Figure 1-a, the first half had a solid black character area 3 with a reflection density of 1.5, and the second half had a solid black character area 3 with a reflection density of 1.5. It has a halftone black solid portion 2 with a reflection density of 0.8. This first
The value of 1 in the original document based on FIG.

Table 1 □ The irregular densities of each part of the copy made from this manuscript using the above electrophotographic copying device are (A) (corresponding to character part 3 of the manuscript), (B) (corresponding to 6 in the figure), and ( C) (5 in the diagram
The results are shown in Table 2.

From this result, in the case of Experiment No. 1 and No. 2, it was found that the copy (
There is no density difference between portions B) and (C), and the memory afterimage phenomenon has disappeared. Moreover, when a halogen lamp with a long wavelength light component is used as the static elimination light (Nol)
There is almost no difference between the case where a green lamp that cant long wavelength light of 600 nm or more is used (No. 2), and it was found that the detrapping process of the present invention is effective even when long wavelength light is included. On the other hand, when no voltage was applied using the conductive rubber roller (No. 3 and No. 4), there was a difference in density in the parts (B) and (C) during copying, and a clear memory afterimage phenomenon occurred. It was alive.

Example 2 Voltage V applied to the conductive rubber roller and (B
) and (C), an experiment was conducted under the conditions of the light source and the static eliminating light of Example No. 2. A graph of this result is shown in FIG.

From this result, it was found that if the limit concentration of 0.05 at which a concentration difference cannot be visually discerned is taken into consideration, an effective applied voltage of 90 .mu. or more becomes an effective applied voltage.

At this time, when the applied voltage is 200μ or more, Δ■D becomes almost constant, and there is little need to apply more voltage, and a
-3i: Considering the amount of charge due to the main charge of H, 400
I was able to consider the area around ■ to be the upper limit.

Example 3 A conductive brush was installed in place of the conductive rubber roller in Experiment No. 2 of Example 1, and when copying was performed by applying an AC bias instead of a DC bias, ΔIDfJ<0.04
An AC applied voltage of 120 μm was required to achieve the following.

In this case, it was sufficient for the AC frequency range to be several H2 or more, preferably 10H2 or more.

Example 4 In the experiment No. 2 of Example 1, copying was performed under all the same conditions except that the voltage applied to the conductive rubber roller 31 was -200V, and the reflection density of each part was as follows.

(A) 1.38 (B) 0.71 (C) 0.72 From this, it was found that similar results can be obtained even when a negative bias voltage is applied.

In other words, the photocarriers trapped in the photosensitive layer are attracted to the electric field caused by the applied voltage, and the side opposite to that in Example 1 is
In other words, it is thought that it moved to the substrate and disappeared.

[Brief explanation of the drawing]

FIG. 1-a is a diagram showing a manuscript used to investigate the memory afterimage phenomenon, FIG. A schematic diagram showing a copying apparatus, FIG. 3 is a graph showing the wavelength dependence of optical fatigue, and FIG. 4 is a graph showing the relationship between the applied bias voltage and the density difference of the copy caused by optical fatigue. be. In the figure, the printed numeral 11 represents the a-3i:H drum, 13 the main charger, 14 the light source lamp, 19 the transfer charger, 21 the neutralizing light, and 31 the bias voltage applying means. Patent applicant Sanda Kogyo Co., Ltd. IR1 Fig. α 5 (C) IR2 Fig. 0 Fig. 3 shihide ball (n town Fig. 4 θ lθθ ?ρθ 3ρθ 4θθ hill ppυ quality and (V)

Claims (4)

[Claims] (1) An electrophotographic photoreceptor having an amorphous silicon-based photoconductor layer on a conductive substrate is charged and exposed to three images. In an image forming method in which development, transfer, and cleaning steps are repeated each time, a delipping/wrapping step is performed by bringing a conductive flexible member to which a bias voltage is applied into contact between the cleaning step and the charging step. A stabilizing charging method characterized by providing: (2) The method according to claim 1, wherein the conductive flexible member is a conductive rubber roller. (3) The method according to claim 1, wherein the bias voltage is a positive or negative DC bias. (4) The method according to claim 1, wherein the bias voltage is an alternating current bias.