JP3747901B2 - EL element drive device, EL element, printer head, and EL element drive control method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a driving device for driving an EL element including a main light emitting layer made of a light emitting material having a characteristic that the light emission falling time is 700 μsec or less, an EL element driven by the driving device, and the EL The present invention relates to a printer head configured using elements and a driving method of EL elements.
[0002]
[Prior art]
Printers are broadly classified into page printers that are mainly used for office use and expected to achieve high-speed processing, and inkjet printers that are mainly used for home use and are expected to be low-cost.
[0003]
There are two types of page printers: a laser printer, which has a large optical system constituting the printer head but is relatively low cost, and an optical printer, which has a small optical system but is relatively expensive. An optical printer uses a light source to expose a photosensitive member, and a plurality of LEDs arranged in a row is generally used as the light source (LED printer).
[0004]
The LED printer is expensive because the structure of the printer head is complicated. The printer head is configured, for example, by arranging several tens of LED chips in which about 100 LEDs are collectively manufactured. In addition, it is necessary to select chips with uniform characteristics in order to suppress variations in power at the portion corresponding to the joint between the LED chips, and further, the displacement of the chip mounting position must be within several hundreds of microns. Under such conditions, the production yield of the printer head has to be deteriorated.
[0005]
For example, if a plurality of LEDs (thousands to tens of thousands) can be created at once, the manufacturing cost of the printer head can be reduced, but this is extremely difficult in practice because of material constraints. is there. Therefore, a technique using an EL element as a light source of an optical printer head is disclosed in Japanese Patent Laid-Open No. 5-221019.
[0006]
In this prior art, EL elements arranged in an array are used as a light source, and an erecting equal-magnification image is formed on a latent image drum through a microlens array. It is disclosed that ZnS: Mn is used as a light emitting layer material of an EL element.
[0007]
[Problems to be solved by the invention]
However, the EL element using ZnS: Mn in the light emitting layer has a long rise time of several milliseconds while the rise time of light emission is several μs. For this reason, in a printer head using the above-described EL element as a light source, when the printing speed increases, the print dots extend in the paper feed direction.
[0008]
That is, FIG. 10A shows a case where a circular dot is printed by one dot, and FIG. 10B shows an example where an elliptical dot is printed every other dot. Thus, in (b), when every other dot is printed, the dots overlap. In order to avoid such a situation, the printing speed or resolution must be lowered after all.
[0009]
Conventionally, EL elements are often used for displays. Since the human eye cannot recognize a light emission change of 10 milliseconds or less, the light emission scanning period of the display is not set so fast. Therefore, no problem occurred even when a display was constructed using the above-described EL element.
[0010]
[Problems to be solved by the invention]
Therefore, the inventor of the present invention creates an EL element in which the rise and fall times of the light emission are extremely short on the order of μ seconds by using SrS: Ce (strontium sulfide: cerium) for the light emitting layer, and is used for the printer head. Thus, an optical printer capable of printing at high speed was considered (Japanese Patent Application 2002-190368).
[0011]
Here, a constant scanning voltage is applied to the EL element at every driving cycle by the scanning electrode, and light emission (ON, printing) and non-light emission (OFF, non-printing) are controlled by the presence or absence of the data voltage applied to the data electrode. Assume a case. The voltage for causing the EL element to emit light at a level that can be printed by the optical printer is around 200 V, but the driver on the data electrode side is configured to perform a logic determination to determine the output of the data voltage in accordance with the display data signal given from the outside. It is necessary to construct a circuit, and it is expensive to cope with a high breakdown voltage.
[0012]
That is, it is desirable that the withstand voltage of the data electrode side driver is limited to about 40V to 60V. For this purpose, for example, the scanning voltage is set to 180V, the data voltage is set to 40V, and both voltage levels are set asymmetrically. However, with such a configuration, the terminal voltage difference applied to the EL element when it is ON and OFF is only about 40 V. With this voltage difference, the EL element emits light with substantially low power even when it is OFF. I have to be in a state of being.
[0013]
As a result, when an EL element using SrS: Ce for the light emitting layer is used in the printer head, even if the EL element has a terminal voltage difference of about 40 V when ON and OFF, the printer Therefore, it is necessary to have a dynamic range (printing contrast) of the light emission power so that the printing and non-printing can be satisfactorily determined.
[0014]
Here, FIG. 11 shows a difference in light emission power when the voltage applied to the EL element in which the light emitting layer is composed of ZnS: Mn (zinc sulfide: manganese) and SrS: Ce is changed. Conventionally used ZnS: Mn has a sufficient dynamic range even with a terminal voltage difference of about 40 V, but SrS: Ce has a relatively small dynamic range, which may cause a practical problem.
[0015]
The present invention has been made in view of the above circumstances, and an object of the present invention is to appropriately cause an EL element having a light emitting layer composed of a light emitting material layer having a short light emission fall time to emit light appropriately according to the purpose of use. Another object of the present invention is to provide a driving device capable of driving, an EL element driven by the driving device, a printer head configured using the EL element, and a driving method of the EL element.
[0016]
[Means for Solving the Problems]
  According to the EL element driving apparatus of claim 1, the control circuit includes a main light emitting layer made of a light emitting material having a characteristic that the light emission falling time is 700 μsec or less, and a sub-light emitting material made of ZnS: Mn. An EL element having a structure in which a layer is sandwiched between electrodes via an insulating film,For periodically emitting lightLight is emitted a plurality of times within one driving period.
[0017]
That is, a conventional light emitting material using ZnS: Mn has a sufficient dynamic range of light emission power even when the applied terminal voltage difference is small, but the light emission fall time is low. EL elements made of a light-emitting material with extremely short characteristics such that is less than 700 μs tend to have a reduced dynamic range. As a result of the experiment, the inventor confirmed that the dynamic range exhibits intermediate characteristics between the two when the light emitting layer is formed together.
[0018]
Therefore, when an EL element is applied to an application that requires high light emission response, a light emitting material with a short light emission fall time is selected as the main light emitting layer, and the breakdown voltage of the drive driver IC cannot be set large. However, the dynamic range of the light emission power can be further increased by providing a sub light emitting layer whose light emitting material is ZnS: Mn.
[0019]
In addition, since the light emission fall time is short, it means that the optical power is small. Therefore, depending on the composition of the light emitting layer, the fall time is too short, and the EL element does not necessarily satisfy the optical power required in the application. There is a case. Therefore, with respect to such an EL element, if the light is emitted a plurality of times within one driving period, the time integration value of the light emission amount can be increased, and adjustment can be made so as to obtain a necessary light power. .
[0020]
  AlsoWhen the EL element is configured as an AC drive type, the control circuit causes the applied voltage polarity to alternately change within one drive period to emit light, and the number of changes is an odd number. That is, since the EL element has characteristics as a ferroelectric substance, the polarization state is maintained inside the element even when the application of voltage is stopped. In order to stabilize the characteristics of the EL element, it is not preferable that the polarization state maintained during the voltage non-application period is biased to one polarity.
[0021]
  Therefore,the aboveWith this configuration, the polarity of the applied voltage is alternately inverted, and the polarity of the drive voltage applied at the end of each drive period also changes each time. Therefore, by applying an AC voltage to the EL element, light emission can be performed satisfactorily for each driving period, and the lifetime of the element can be extended.
[0022]
  Claim2According to the described EL element, the claim1An EL element suitable for the driving mode of the mounted driving apparatus can be obtained.
  Claim3According to the described EL element, the light emitting layer is formed by arranging the sub light emitting layer on the upper layer and the lower layer of the main light emitting layer. With such a configuration, the symmetry of the element structure is improved, and the characteristics of the element can be stabilized by reducing the change with time.
[0023]
  Claim4According to the EL element described, the dielectric constant of the insulating film is 30 or more. That is, when such a material having a high relative dielectric constant is used, the capacitance of the EL element increases, and accordingly, the light emission output of the EL element can be improved.
[0024]
  Claim5According to the described printer head, the claims2Thru4The EL element described in any one of the above is used as a light source. That is, as described above, if an EL element that has a high light emission response characteristic and can secure a large dynamic range of light emission power with a relatively small driving voltage difference is used for the printer head, even if the printing speed is increased, the paper Since it is possible to prevent the printing dots from extending in the feeding direction as much as possible, printing can be performed at a higher speed. And claims1By being driven by the mounted driving device, it is possible to obtain a dynamic range of optical power necessary for forming and printing a latent image on the photosensitive member.
[0025]
  Claim6According to the described printer head, the SELFOC lens that collects the light output from the EL element and forms a latent image on the photosensitive member is provided. That is, if a Selfoc lens, which is a kind of microlens array, is used, the printer head can be configured at low cost.
[0026]
Further, by providing the EL element with the main light emitting layer and the sub light emitting layer, the wavelength of light emitted by each layer is strictly different. Since the SELFOC lens has chromatic aberration, if the adjustment is made so that the emission wavelength of the main emission layer converges on the surface of the photoreceptor, the emission wavelength of the secondary emission layer converges on the surface of the photoreceptor due to chromatic aberration. No longer. Accordingly, it is possible to reduce the influence of light emission by the sub-light emitting layer which is not originally provided for the purpose of using the light emission.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
A first embodiment in which the present invention is applied to an optical printer will be described below with reference to FIGS. FIG. 3 schematically shows the main structure of the optical printer 30.
[0028]
The photosensitive drum (photosensitive member) 31 rotates in the clockwise direction in FIG. 3. First, the charging unit 32 charges the surface with negative charges. Subsequently, exposure according to the print image data is performed by the EL element array 33 and the SELFOC lens 34. The potential of the exposed portion of the photosensitive drum 31 is increased to form an electrostatic latent image. Next, toner is attached to the charged portion in the developing unit 35 to form a toner image.
[0029]
The toner image formed on the surface of the photosensitive drum 31 is transferred to the paper 37 by the transfer unit 36, and the transferred image is fixed to the paper 37 by the fixing unit 38. Thereafter, the photosensitive drum 31 is neutralized by the neutralization unit 39, and toner is further cleaned by the cleaning unit 40.
[0030]
FIG. 4 is a perspective view showing the EL element array 33 and the Selfoc lens 34 as the center. The EL element array 33 is configured as a linear light source, and the SELFOC lens 34 is configured as a microlens array. A combination of the EL element array 33 and the SELFOC lens 34 is a printer head 60. The light emitted from the EL element array 33 is condensed by the Selfoc lens 34 and projected onto the surface of the photosensitive drum 31 as an erecting equal-magnification image.
[0031]
Next, the structure of the EL element array 33 will be described with reference to FIG. FIG. 1A is a plan view of the EL element 1, and FIG. 1B is a diagram schematically showing an AA cross section of FIG. In the EL element 1, a first electrode 52 (scanning electrode), a first insulating film 53, a light emitting layer 54, a second insulating film 55, and a second electrode 56 (data electrode) are sequentially stacked on a glass substrate 51 that is an insulating substrate. Of the first electrode 52, the first insulating film 53, the second insulating film 55, and the second electrode 56, at least the light extraction side (display side) is made of a light-transmitting material. Yes.
[0032]
The EL element 1 refers to a light emitting portion between one second electrode 56 and the first electrode 52 in the above configuration. Further, although only eleven EL elements 1 are shown in FIG. 1, there are actually a larger number of EL elements 1.
[0033]
For example, the first electrode 52 is an ITO (Indium Tin Oxide) film, and the first insulating film 53 is an Al film.₂O₃Layer and titanium oxide TiO₂Al with alternating layers₂O₃/ TiO₂A laminated structure film (hereinafter referred to as an ATO film), the second insulating film 55 is an ATO film similar to the first insulating film 53, and the second electrode 56 is an Al film. The light emitting layer 54 has a two-layer structure of a main light emitting layer 54A made of SrS: Ce as a light emitting material and a sub light emitting layer 54B made of ZnS: Mn as a light emitting material.
[0034]
A manufacturing process of the EL element array 33 will be described below. An optically transparent ITO film is formed on the glass substrate 51 as the first electrode 52 by sputtering. Since the transmittance of the ITO film is 70% or more and a large number of elements are formed for one first electrode 52, the film thickness is set to 250 nm or more so that the sheet resistance is 10Ω / □ or less.
[0035]
On the first electrode 52, an ATO film is formed as a first insulating film 53 by an ALE (Atomic Layer Epitaxy) method. That is, in the first step, aluminum trichloride (AlCl) is used as a source gas of aluminum (Al).₃), Water (H) as a source gas of oxygen (O)₂O) with Al₂O₃Form a layer. In the ALE method, since a film is formed one atomic layer at a time, source gases are supplied alternately. Therefore, in this case, AlCl₃Is introduced into the reactor with argon (Ar) carrier gas for 1 second, and then AlCl in the reactor is introduced.₃Purge enough to evacuate the gas. By repeating this cycle, Al having a predetermined film thickness₂O₃Form a layer.
[0036]
In the subsequent second step, titanium tetrachloride (TiCl₄), Water (H₂A titanium oxide layer is formed using O). That is, TiCl as in the first step.₄Is introduced into the reactor with Ar carrier gas for 1 second, and TiCl in the reactor is then introduced.₄Purge enough to evacuate the gas. Next, H₂Similarly, after introducing O into the reactor with Ar carrier gas for 1 second, H in the reactor₂Purge enough to evacuate O gas. By repeating this cycle, a titanium oxide layer having a predetermined film thickness is formed.
[0037]
Then, by repeating the first and second steps described above, Al having a predetermined film pressure is obtained.₂O₃/ TiO₂A laminated structure film is formed, and a first insulating film 53 is formed. Al₂O₃Layer, TiO₂Each layer is 30 nm thick, with a film thickness per layer of 5 nm. Al₂O₃/ TiO₂The uppermost layer and the lowermost layer of the laminated structure film are made of Al.₂O₃Layer, TiO₂Any of the layers may be used. When a film is formed on the atomic layer order by the ALE method, if the film thickness per layer is less than 0.5 nm, it does not function as an insulator, and if the film thickness exceeds 100 nm, the withstand voltage improvement effect due to the laminated structure decreases. Resulting in. Therefore, the film thickness per layer may be set in the range of 0.5 nm to 100 nm, and preferably in the range of 1 nm to 10 nm.
[0038]
Next, SrS (strontium sulfide) is used as a base material on the first insulating film 53, and the main light emitting layer 54A made of SrS: Ce to which Ce (cerium) is added as an emission center is used as a base material. Then, the sub-light-emitting layers 54B made of ZnS: Mn with Mn (manganese) added as the light emission center are formed as the light-emitting layers 54 by vapor deposition. That is, a vapor deposition pellet having a predetermined stoichiometric composition is prepared, and an electron beam is irradiated thereon to form a film.
[0039]
The film thickness of the sub light emitting layer 54B may be about 100 to 1000 nm. This film thickness is an element that determines the size of the dynamic range of the optical power required for the EL element 1, and is appropriately set as necessary. Regarding the main light emitting layer 54A, since a predetermined amount of sulfur may not be included in the film, it is preferable to add a sulfur content such as hydrogen sulfide during the film formation. The film thickness may be set to about 500 nm to 2000 nm. The selection of the film thickness may be determined according to the specifications of the printer head 60. When the thickness is less than 500 nm, there are many areas that do not contribute to light emission, and the light emission efficiency is extremely reduced. On the other hand, if the thickness is greater than 2000 nm, the stress increases and there is a risk of film peeling or cracking.
[0040]
In this way, the production process can be stabilized by disposing the sub-light emitting layer 54B on the upper layer of the main light emitting layer 54A. That is, since SrS: Ce has a property of being easily dissolved in water, it is possible to prevent the main light emitting layer 54A from coming into contact with moisture by the upper side light emitting layer 54B. Therefore, when forming the light emitting layer 54, it is desirable to perform the continuous treatment while maintaining a vacuum.
[0041]
Then, the second insulating film 55 is formed by the ALE method by the same process as the first insulating film 53, and finally, the Al film is formed by the sputtering method as the second electrode 56. As described above, the EL element array 33 constituting the printer head 60 can be produced. The insulating films 53 and 55 are made of a material having a relative dielectric constant of at least 30 or more.
[0042]
In order to set the resolution of the printer to 600 dpi, it is necessary to set the width of the scanning electrodes and the arrangement interval of the data electrodes to 42.3 μm. The EL element 1 configured as described above emits light with sufficient intensity to form a latent image on the photosensitive drum 31 when a terminal voltage of approximately 200 V is applied.
[0043]
FIG. 2 is a diagram showing the arrangement of the EL element array 33, the control circuit, and the like. The glass substrate 51 is configured to also serve as the substrate of the EL element array 33. In addition, since the first electrode 52 may be a light source of an optical printer, a large number of EL elements 1 may be arranged in a line, so that only one first electrode 52 is formed.
[0044]
In addition, on the glass substrate 51, a control circuit 42, a scanning side driver 43, a data side driver 44, an external connection terminal 45 for electrically connecting with a control circuit on the optical printer main body side, and the like are mounted. Then, the control circuit 42 outputs drive control signals to the drivers 43 and 44 to cause the EL element 1 to emit light.
[0045]
Next, the reason why the light emitting layer 54 has a two-layer structure of the main light emitting layer 54A and the sub light emitting layer 54B will be described. First, characteristics required for using the EL element 1 as a light source of a printer head are as follows.
(1) High-speed response corresponding to the printing speed specification of the printer.
{Circle around (2)} Optical power sufficient to form a latent image on the photosensitive drum 31.
(3) The dynamic range of the optical power obtained within the withstand voltage range of the data side driver 44 is such that a clear difference (contrast) can be obtained as to whether or not a latent image is formed on the photosensitive drum 31.
[0046]
<About <1 >>
The timing of the driving signal appropriate for operating the printer head 60 is specifically defined. First, the printing speed required for the optical printer is estimated. The light emission fall time of the ZnS: Mn-EL element used in the above-mentioned JP-A-5-221019 is about 5 milliseconds. In this case, since the maximum scanning frequency is 200 Hz, it takes about 1 minute to print one A3 sheet with a resolution of 600 dpi (dot per inch). This is too slow and it is difficult to say that the printing speed can withstand practical use.
[0047]
Therefore, the printing speed evaluated to be sufficiently high at the current product level is defined as the speed at which 8 sheets of A3 paper are printed per minute at a resolution of 600 dpi. In this case, the light emission fall time of the EL element defined by the necessary printer head scanning cycle is approximately 706 μs. Since the period A shown in FIG. 6 coincides with the paper feed speed of the optical printer, the period A is set to 706 μs.
[0048]
The voltage application period (drive period) C shown in the figure can be arbitrarily determined within 706 microseconds, but the printer configuration is preferably set to 2 to 75% of the period A. If it is less than 2%, it is difficult to ensure a sufficient contrast (light emission power) for printing, and if it exceeds 75%, it is difficult to ensure the tolerance of the optical system design.
[0049]
Here, the voltage application period C is set to 100 μsec, which is about 14% of the period A. If the light emission fall time of the EL element 1 is too long, as shown in FIG. 10B, the light shape (dots) projected on the surface of the photosensitive drum 31 becomes vertically long, and if the printing speed is increased, the dots are increased. Will be connected. When the period C is determined, if the light emission fall time is set within (AC), dots are not connected as shown in FIG.
[0050]
Since the EL element 1 uses the SrS: Ce layer as the main light emitting layer 54A, the light emission fall time is extremely short. Here, the “light emission fall time” is defined as the time until the light intensity decreases from 90% to 10% when the maximum value of the light intensity is 100%. For example, the light emission fall time of the ZnS: Mn-EL element is about 5 milliseconds, whereas the EL element 1 is about several microseconds. Although this time seems to change to some extent by changing the stoichiometric composition of the main light emitting layer 54A, the prototype of the present invention is 5 μs.
[0051]
In order to obtain such characteristics, SrS is preferable as a base material, and if the luminescent center material is Ce for the base material, the compatibility is good. Other combinations are conceivable, but the above combination was selected because of the ease of producing pellets for vapor deposition. Further, although the emission color changes depending on the selection of the emission center material, if it is visible light, the photosensitive agent on the photosensitive drum 31 can form a latent image.
[0052]
<About (2)>
Further, when a rectangular wave voltage is applied, the EL element 1 emits light at the rising edge and the falling edge of the applied voltage waveform, and the light emission rapidly falls (see FIG. 8C). Accordingly, when the voltage application period C is set to 100 μsec, even if a rectangular wave voltage having a pulse width of 100 μsec is simply applied to the EL element 1, the optical power becomes extremely low and a latent image is formed on the photosensitive drum 31. The optical power required to do this cannot be obtained. As will be described later, the influence of the sub-light-emitting layer 54B on the optical power is small, and the characteristics of the main light-emitting layer 54A are dominant.
[0053]
Therefore, in this embodiment, as shown in FIG. 6A, the control circuit 42 performs drive control so that the EL element 1 emits light a plurality of times (for example, 11 times) within one scanning period (driving period). That is, in one scanning period (time C), the scanning voltage is switched and applied a plurality of times so that the polarity of the voltage applied to both ends of the EL element 1 is alternately inverted. Then, the EL element 1 emits light according to the number of times of voltage application (actually, light is emitted twice for one voltage application as shown in FIG. 8C).
[0054]
Here, the number of times of voltage application is set to an odd number. For example, FIG. 6B shows the case of even number of times. When application is first started with positive polarity, the end is ended with negative polarity, and this state is repeated. Since the EL element 1 has characteristics as a ferroelectric substance, the polarization state is maintained inside the element even when the application of voltage is stopped. In order to stabilize the characteristics of the EL element 1, it is not preferable that the polarization state maintained during the voltage non-application period is biased to one polarity.
[0055]
FIG. 6C shows a pattern in which the polarity of the voltage applied last is reversed every time after the number of times of voltage application is an even number. In this case, at the beginning of the next scanning period. The polarity of the applied voltage becomes the same as the last time. Since the EL element 1 does not emit light with sufficient power unless it is driven by alternating current, this pattern also has a problem.
[0056]
As shown in FIG. 6A, if the number of times of voltage application is an odd number, the polarity of the applied voltage can be continuously inverted, and the polarity of the voltage applied at the end of each scanning period is also inverted every time. Can be made.
[0057]
<About (3)>
The reason (3) becomes a problem is as described in [Problems to be solved by the invention]. FIG. 5 shows a scanning voltage waveform (b) applied to the scanning electrode, a data voltage waveform (c) applied to the data electrode, and a terminal voltage waveform (a) applied to both ends of the EL element 1. It is.
[0058]
That is, the voltage for causing the EL element 1 to emit light at a level printable by the optical printer is around 200V. The driver 44 on the data electrode side needs to be configured to determine the output of the data voltage in accordance with the display data signal given from the control circuit 42. Since it is costly to cope with a high breakdown voltage, the driver The breakdown voltage is kept at about 40V to 60V. As a result, the scanning voltage is set to 180 V (Vth, see FIG. 5B), and the data voltage is set to 40 V (see FIG. 5C).
[0059]
Therefore, when ON, 180 + 40 = 220 (V) is applied to both ends of the EL element 1, and the EL element 1 enters a light emitting state (see FIG. 5A). Further, when OFF, only the scanning voltage of 180 V is applied to both ends of the EL element 1, so that the EL element 1 enters a non-light emitting state.
[0060]
That is, even in the latter non-light-emitting state, since the voltage difference from the light-emitting state is only 40V, it is necessary to be in a non-light-emitting state due to the drop of 40V, or a practically sufficient non-light-emitting state. That is, in the case of application to the printer head 60, even if the non-light emitting state is physically a light emitting state, there is no practical problem if a latent image is not formed on the photosensitive drum 31 by the light emission. Absent.
[0061]
In the present embodiment, the light emitting layer 54 of the EL element 1 includes a main light emitting layer 54A having a fast light emission fall time but a narrow optical power dynamic range, and a sub light emitting layer 54B having a light emission fall time but a wide dynamic range. The problem was solved by comprising two layers. That is, by configuring the light emitting layer 54 in such a manner, the dynamic range of ON (printing) / OFF (non-printing) becomes an intermediate characteristic between the two, and a range having no practical problem can be obtained. If the film thickness ratio between the main light emitting layer 54A and the sub light emitting layer 54B is set to about 1: 1 to 4: 1, the characteristics shown in FIG. 7 can be obtained.
[0062]
<4> Effect of sub-light emitting layer 54B on other light emitting characteristics>
As shown in FIG. 6, the main light emitting layer 54A shows a tendency that the light emission starting voltage decreases when a plurality of pulses are applied within one driving period, but the light emission starting voltage of the sub light emitting layer 54B hardly changes. Therefore, the light emission of the sub light emitting layer 54B can be suppressed by the voltage difference.
[0063]
Further, FIG. 8 shows the same rectangular wave voltage (a) for the EL element (b) in which the light emitting layer is composed only of the ZnS: Mn film and the EL element (c) in which the light emitting layer is composed only of the SrS: Ce film. ) Is a light intensity waveform when light is emitted. When the intensity peak of the ZnS: Mn—EL element is “1”, the relative intensity peak of the SrS: Ce—EL element is “20”. Therefore, even when configured like the EL element 1 in this embodiment, the influence of light emission by the sub-light emitting layer 54B is extremely small.
[0064]
Furthermore, the emission wavelength of the ZnS: Mn-EL element is 580 nm, and the emission wavelength of the SrS: Ce-EL element is 480 nm. The printer head 60 includes the Selfoc lens 34. Since the Selfoc lens 34 has chromatic aberration, adjustment can be performed so that the emission wavelength of the main light emitting layer 54A converges on the surface of the photosensitive drum 31. For example, the emission wavelength of the sub-light emitting layer 54B does not converge on the surface of the photosensitive drum 31 due to chromatic aberration.
[0065]
As described above, according to the present example, the main light emitting layer 54A made of a light emitting material having the characteristic that the light emission fall time is about 5 μsec and the sub light emitting layer 54B made of ZnS: Mn are used as the insulating film 53. , 55 to constitute the EL element 1 having a structure sandwiched between the electrodes 52, 56. That is, in order to apply the EL element 1 to an application such as the printer head 60 that requires a high light emission response, a light emitting material having a short light emission fall time is selected for the main light emitting layer 54A, and the driver 44 Even when the breakdown voltage cannot be set large, the dynamic range of the light emission power can be increased by providing the sub light emitting layer 54B together.
[0066]
Since the control circuit 42 alternately outputs drive voltages having different polarities to both ends of the EL element 1 within one drive period to cause the EL element 1 to emit light a plurality of times, the time integration value of the light emission amount is increased. The EL element 1 can be adjusted so that the optical power necessary for forming a latent image on the photosensitive drum 31 can be obtained. In addition, since the polarity of the voltage applied to both ends of the EL element 1 can be continuously reversed, light emission can be performed satisfactorily for each scanning period.
[0067]
Furthermore, since the number of times of output of the drive voltage is an odd number, the polarity of the voltage applied at the end of each drive period can be reversed each time, and the characteristic variation of the EL element 1 is suppressed and the life is prolonged. be able to. Moreover, since the dielectric constants of the insulating films 53 and 55 are 30 or more, the capacitance of the EL element 1 can be increased, and the light emission output of the EL element 1 can be improved accordingly.
[0068]
Further, according to the present embodiment, the printer head 60 used as the light source of the optical printer 30 is configured by using the EL element array 33 in which the EL elements 1 are arranged in a line. By incorporating it into the printer 30, the printing speed or resolution of the optical printer 30 can be improved.
[0069]
Further, since the light emitted from the EL element array 33 is condensed by the Selfoc lens 34 and projected onto the photosensitive drum 31, the printer head 60 can be configured at low cost. Further, by providing the main light emitting layer 54A and the sub light emitting layer 54B of the EL element 1, even when the light emission wavelengths of the respective layers are different, the light emission wavelength of the main light emitting layer 54A is obtained by utilizing the chromatic aberration of the Selfoc lens 34. Is adjusted so as to converge on the surface of the photosensitive drum 31, it is possible to reduce the influence of light emission by the sub-light emitting layer 54B.
[0070]
(Second embodiment)
FIG. 9 shows a second embodiment of the present invention. The same parts as those of the first embodiment are denoted by the same reference numerals and the description thereof is omitted. Only different parts will be described below. The EL element 71 in the second embodiment is configured by disposing the sub-light emitting layer 54C between the insulating film 53 and the main light emitting layer 54A of the EL element 1 in the first embodiment. The thickness of the sub light emitting layer 54C is the same as that of the sub light emitting layer 54B.
[0071]
As described above, by arranging the sub-light emitting layers 54B and 54C in the upper layer and the lower layer of the main light emitting layer 54A, the production process can be stabilized. Further, by arranging the sub light emitting layer 54C on the lower layer side, the symmetry becomes good, and the change with time of the light emission characteristics in the EL element 71 becomes smaller.
[0072]
  The present invention is not limited to the embodiments described above and shown in the drawings, and the following modifications or expansions are possible.
  The EL element in the present invention may be one having a light emission fall time of 700 μsec or less. That is, as described above, when applied to an optical printer, the EL element emission fall time required to satisfy the printing speed evaluated to be sufficiently high at the current product level is about 700 μsec. .
  The number of times of application of the drive voltage to the EL element may be appropriately set according to the light emission fall time and the specifications required by the individual design (required optical power).
  Eu (europium) may be used instead of Ce as the light emission center material of the light emitting layer. Further, ZnS may be used as the base material instead of SrS. Alternatively, any material other than these materials may be used as long as the necessary light emission fall time can be obtained..
  The printer head of the present invention is not only applied to an optical printer, but can also be applied to a copying machine, a facsimile machine, and the like that use similar electrophotographic technology.
[Brief description of the drawings]
FIG. 1 is a first embodiment of the present invention, (a) is a plan view of an EL element, and (b) is a diagram schematically showing an AA cross section of (a).
FIG. 2 is a diagram showing an arrangement of an EL element array, a control circuit, and the like.
FIG. 3 is a diagram schematically showing a main part structure of an optical printer.
FIG. 4 is a perspective view showing an EL element array and a Selfoc lens as a center.
5A is a diagram illustrating a terminal voltage waveform applied to both ends of an EL element, FIG. 5B is a diagram illustrating a scan voltage waveform applied to a scan electrode, and FIG. 5C is a diagram illustrating a data voltage waveform applied to a data electrode.
6A is a drive voltage application pattern for an EL element in the present embodiment, FIG. 6B is a pattern when the drive voltage is applied evenly (part 1), and FIG. 6C is a drive voltage application frequency. Is a diagram that shows the pattern (No. 2) in principle when I is even
FIG. 7 is a diagram showing applied voltage on the horizontal axis and optical power on the vertical axis for EL elements having different light emitting layer materials.
8A is a rectangular wave voltage, FIG. 8B is a light intensity waveform when the voltage of FIG. 8A is applied to an EL element having a light emitting layer composed of only a ZnS: Mn film, and FIG. 8C is a light emission. The figure which shows the light intensity waveform at the time of applying the same voltage to the EL element which comprised the layer only by the SrS: Ce film | membrane.
FIG. 9 is a view corresponding to FIG. 1 (b) showing a second embodiment of the present invention.
10A and 10B show a prior art, in which FIG. 10A shows an example in which one dot is printed with a circular dot and an elliptical dot, and FIG. 10B shows an example in which these dots are printed every other dot.
11 is a view corresponding to FIG.
[Explanation of symbols]
1 is an EL element, 30 is an optical printer, 31 is a photosensitive drum (photoconductor), 33 is an EL element array, 34 is a Selfoc lens, 42 is a control circuit, 43 is a scanning side driver, 44 is a data side driver, and 53 is The first insulating film, 54 is the light emitting layer, 54A is the main light emitting layer, 54B and 54C are the sub light emitting layers, 55 is the second insulating film, 56 is the second electrode, 57 is the ceramic substrate, 60 is the printer head, 71 is the EL An element is shown.

Claims (7)

An EL having a structure in which a main light-emitting layer made of a light-emitting material having a characteristic that the light emission fall time is 700 μsec or less and a sub-light-emitting layer made of ZnS: Mn as a light-emitting material are sandwiched between electrodes through an insulating film A control circuit for controlling the element to emit light a plurality of times within one drive period for periodically emitting light ;
When the EL element is configured as an AC drive type,
The control circuit is configured to cause the EL element to emit light by alternately changing the applied voltage polarity within the one driving period, and to change the number of times to an odd number. Device drive device. An EL element driven by the driving device according to claim 1. 3. The EL device according to claim 2 , wherein the light emitting layer is formed by arranging a sub light emitting layer in an upper layer and a lower layer of the main light emitting layer . 4. The EL element according to claim 2, wherein the dielectric constant of the insulating film is 30 or more . A printer head comprising the EL element according to claim 2 as a light source. 6. The printer head according to claim 5, further comprising a selfoc lens for condensing the light output from the EL element to form a latent image on the photosensitive member . An EL having a structure in which a main light emitting layer made of a light emitting material having a characteristic that the light emission falling time is 700 μsec or less and a sub light emitting layer made of ZnS: Mn as a light emitting material are sandwiched between electrodes through an insulating film A driving method for driving an element,
When the EL element is configured as an AC drive type,
A method for driving an EL element, wherein the EL element is caused to emit light a plurality of times by alternately changing the polarity of an applied voltage within one driving period for periodically emitting light, and the number of changes is odd. .

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