JP3775316B2 - Driving method of EL element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
In the present invention, a driving voltage is applied to both ends of an EL element to emit light. E The present invention relates to a driving method of an L element.
[0002]
[Problems to be solved by the invention]
For example, a display composed of EL elements using ZnS: Mn in the light emitting layer is disclosed in Japanese Patent Laid-Open No. 9-54566. However, since the EL element has a single amber display color, its display power is poor. Accordingly, development of displays using EL elements having other luminescent colors is underway.
[0003]
On the other hand, among printers, there is an LED printer having a printer head using an LED as a light source, but the printing quality is not good due to large variation in light emission of the LED. In view of this, a printer head in which an EL element using ZnS: Mn in the light emitting layer instead of the LED is used as a light source is disclosed in JP-A-5-221019.
[0004]
However, as shown in FIG. 16, 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.
[0005]
That is, FIG. 17A shows a case where circular dots and elliptical dots are printed every other dot, and FIG. 17B shows an example where these dots are printed every other dot. As shown in FIG. 17B, dots are overlapped when dots extending in an elliptical shape are printed every other dot. In order to avoid such a situation, the printing speed or resolution must be lowered after all.
[0006]
As a means for solving such a problem, for example, it is conceivable to use, as a light source for a printer head, an EL element having a light emitting layer made of a material having a short emission fall time characteristic. However, the above characteristics cause a problem of insufficient optical power. That is, human vision senses brightness according to the time integration of the amount of light, so if the fall time is short, the time integration value of the light emission amount becomes small and the light power is reduced.
[0007]
Further, if the light emission fall time of the EL element having a light emission color other than the above-described amber is short, the shortage of light power similarly becomes a problem. That is, when the light power of the EL element is insufficient, it cannot be employed in the display substantially, and variations in the emission color cannot be increased.
[0008]
The present invention has been made in view of the above circumstances, and an object of the present invention is to ensure sufficient optical power when using an EL element having a short emission fall time. E The object is to provide a method for driving an L element.
[0009]
[Means for Solving the Problems]
EL according to claim 1 Device driving method According to , E The L element is caused to emit light a plurality of times within one drive period, which is a period for selecting one of the scan electrodes as a scan voltage application target. The That is, even if an EL element has a short light emission fall time, the time integral value of the light emission amount can be increased by emitting light a plurality of times continuously within one drive period. Obtainable. Therefore, the element can be effectively applied to an application that needs to use an EL element having the above characteristics for some reason.
[0010]
EL according to claim 2 Device driving method According to the present invention, the light emission center material constituting the light emitting layer of the EL element is Ce or Eu. Because Even when the light emission fall time is short, higher light emission power can be obtained.
[0011]
EL according to claim 3 Device driving method According to the above, the base material constituting the light emitting layer of the EL element is SrS. That is, when the luminescent center material is Ce or Eu, the host material is selected and combined in this way. 2 The characteristics described in (1) can be obtained better.
[0012]
EL according to claim 4 Device driving method According to 1 First and second drive voltages having different polarities within the drive period are alternately output to both ends of the EL element. The That is, some EL elements emit light as the applied voltage changes in a predetermined voltage range. Therefore , E An AC voltage is continuously applied to both ends of the L element, and the EL element having the above characteristics can continuously emit light to obtain a sufficient light emission power.
[0013]
EL according to claim 5 Device driving method According to 1 The number of outputs of the first drive voltage and the second drive voltage in the drive period is different. To The 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.
[0014]
Therefore, as in claim 5 By Since the drive voltage is output an odd number of times, the voltage application polarity 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.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
A first embodiment in which the present invention is applied to a dot matrix type EL display device will be described below with reference to FIGS. FIG. 3 is a functional block diagram showing the overall electrical configuration. The control circuit 3 sends a scanning side driver (driving circuit) 4 and a data side driver (driving circuit) 5 for driving the EL display 2 composed of the EL elements 1 (see FIG. 4) based on data given from the outside. A control signal is output.
[0025]
FIG. 4 shows a more detailed electrical configuration centering on the EL display 2, the scanning side driver 4 and the data side driver 5. As shown in FIG. The EL display 2 is configured as a simple dot matrix display by arranging the scanning electrodes 9 and the data electrodes 10 in a matrix and arranging the EL elements 1 represented by capacitor symbols in a matrix in the intersecting region. The odd-numbered scan electrodes 9 are used as scan electrodes 9o, and the even-numbered scan electrodes 9 are used as scan electrodes 9e.
[0026]
The scan electrode drive circuit 11o is a push-pull type drive circuit, includes a P-channel FET 12a and an N-channel FET 12b connected to the scan electrode 9o, and applies a scan voltage to the scan electrode 9o based on a signal given from the drive circuit 13. It is designed to be applied. Parasitic diodes 12c and 12d are formed between the source and drain of the FETs 12a and 12b, respectively, so that the scan electrode 9o is set to a desired reference voltage.
[0027]
The scan electrode drive circuit 11e has the same configuration, and includes a P-channel FET 14a, an N-channel FET 14b, and a drive circuit 15, and applies a scan voltage to the scan electrode 9e. Similarly, the data electrode drive circuit 16 includes a P-channel FET 17a, an N-channel FET 17b, and a drive circuit 18, and applies a data voltage to the data electrode 10.
[0028]
The scan electrode drive circuits 11o and 11e are provided with scan voltage supply circuits 19a and 19b. The scanning voltage supply circuit 19a includes switching elements 20 and 21, and a DC voltage (writing voltage) Vr (driving voltage) or a ground voltage is applied to the P-channel FET source in the scanning electrode driving circuits 11o and 11e according to the on / off state thereof. This is supplied to the side common line L1. Similarly, the scanning voltage supply circuit 19b includes switching elements 22 and 23, and a DC voltage (−Vr + Vm) (driving voltage) is supplied to the N-channel FET source side in the scanning electrode driving circuits 11o and 11e in accordance with the on / off state thereof. It is supplied to the common line L2.
[0029]
The data electrode driving circuit 16 is provided with a data voltage supply circuit 24 for supplying a DC voltage (modulation voltage) Vm to the P-channel FET source side common line of the data electrode driving circuit 16 and an N-channel FET. A ground voltage is supplied to the source side common line.
[0030]
In the above configuration, the one including the scan electrode drive circuits 11o and 11e and the scan voltage supply circuits 19a and 19b corresponds to the scan side driver 4, and the one including the data electrode drive circuit 16 and the data voltage supply circuit 24 is the data. This corresponds to the side driver 5.
[0031]
The structure of the EL element 1 will be described with reference to FIG. (A) is a top view of EL element 1, (b) is a figure showing typically an AA section of (a). In the EL element 1, a first electrode 52 (scanning electrode 9), a first insulating layer 53, a light emitting layer 54, a second insulating layer 55, and a second electrode 56 (data electrode 10) are provided on a glass substrate 51 which is an insulating substrate. The first electrode 52, the first insulating layer 53, the second insulating layer 55, and the second electrode 56 are composed of a material having translucency at least on the light extraction side (display side). Has been.
[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. In practice, there are more EL elements 1 than those shown in FIG.
[0033]
For example, the first electrode 52 is an ITO (Indium Tin Oxide) film, and the first insulating layer 53 is Al. ₂ O ₃ Layer and titanium oxide TiO ₂ Al with alternating layers ₂ O ₃ / TiO ₂ A laminated structure film (hereinafter referred to as an ATO film), the light emitting layer 54 is an SrS: Ce film, the second insulating layer 55 is an ATO film similar to the first insulating layer 53, and the second electrode 56 is an Al film.
[0034]
A manufacturing process of the EL element 1 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 layer 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) ₂ O) with Al ₂ O ₃ Form 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, AlCl ₃ Is introduced into the reactor with argon (Ar) carrier gas for 1 second, and then AlCl in the reactor is introduced. ₃ Purge enough to evacuate the gas. By repeating this cycle, Al having a predetermined film thickness ₂ O ₃ Form a layer.
[0036]
In the subsequent second step, titanium tetrachloride (TiCl ₄ ), Water (H ₂ A titanium oxide layer is formed using O). That is, TiCl as in the first step. ₄ Is introduced into the reactor with Ar carrier gas for 1 second, and TiCl in the reactor is then introduced. ₄ Purge enough to evacuate the gas. Next, H ₂ Similarly, after introducing O into the reactor with Ar carrier gas for 1 second, H in the reactor ₂ Purge 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. ₂ O ₃ / TiO ₂ A laminated structure film is formed, and the first insulating layer 53 is formed. Al ₂ O ₃ Layer, TiO ₂ Each layer is 30 nm thick, with a film thickness per layer of 5 nm. Al ₂ O ₃ / TiO ₂ The uppermost layer and the lowermost layer of the laminated structure film are made of Al. ₂ O ₃ Layer, TiO ₂ Any 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, the film does not function as an insulator. 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, an SrS: Ce (strontium sulfide: cerium) layer containing SrS as a base material and Ce added as a light emission center is formed on the first insulating layer 53 as the light emitting layer 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. At this time, since a predetermined amount of sulfur may not be contained in the film, it is preferable to add a sulfur content such as hydrogen sulfide during the film formation. The film thickness of the SrS: Ce layer 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 EL display 2, and 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.
[0039]
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 1 constituting the EL display 2 can be produced. As described above, when the EL display 2 is configured by using the EL element 1 using the SrS: Ce layer as the light emitting layer 54, the light emitting display color is blue.
[0040]
It should be noted that “Japan Display '86 242-245” describes which of the base material and the light emission center material constituting the light emitting layer affects the light emission fall time. SrS is preferable as the base material, and compatibility with the base material is good when Ce is the luminescent center material. Other combinations are conceivable, but the above combination was selected because of the ease of producing pellets for vapor deposition.
[0041]
Next, the operation of the present embodiment will be described with reference to FIGS. Details of the basic operations of the scanning side driver 4 and the data side driver 5 are disclosed in, for example, Japanese Patent Laid-Open No. 9-54566, but in this embodiment, the basic operations are modified and the EL is performed. The element 1 is driven.
[0042]
First, the drive system disclosed in Japanese Patent Laid-Open No. 9-54566 will be outlined with reference to FIG. In order to cause the EL element 1 of the EL display 2 to emit light, an alternating pulse voltage needs to be applied between the scanning electrode 9 and the data electrode 10. For this reason, a pulse voltage whose polarity is reversed positively and negatively for each field is created and driven for each scanning line. In the positive field, after the reference voltage of each electrode is set to an offset voltage Vm of about 45V, a voltage (scanning voltage) Vr of about 210V is sequentially applied to the scanning electrode 9. Further, the other scan electrodes 9 to which the voltage Vr is not applied are set in a floating state.
[0043]
On the data electrode 10 side, the data electrode 10 connected to the EL element 1 that emits light is set to the ground voltage (display voltage), so that a voltage Vr equal to or higher than the light emission threshold is applied to both ends of the EL element. To emit light. Further, by keeping the voltage of the data electrode 10 to which the EL element 1 that is not allowed to emit light remains Vm, the voltage applied to both ends of the EL element 1 becomes (Vr−Vm), which is below the threshold value. Therefore, the EL element 1 does not emit light. Thereafter, the charge accumulated in each EL element is discharged to return to the initial state.
[0044]
On the other hand, in the negative field, the same operation as in the positive field is performed by inverting the polarity of the applied voltage. At this time, the reference voltage of each electrode becomes the ground voltage. Then, scanning is performed by applying a voltage (−Vr + Vm) to the scanning electrode 9, and on the data electrode 10 side, the voltage of the data electrode 10 to which the EL element 1 that emits light is connected is set to Vm, contrary to the normal field. The data electrode 10 connected to the EL element 1 that does not emit light is set to the ground voltage. Then, the voltage applied to both ends of the EL element 1 that emits light becomes −Vr, and the EL element 1 emits light. Further, since the voltage applied to both ends of the EL element 1 that does not emit light remains (−Vr + Vm), the EL element 1 does not emit light below the threshold value.
The two-cycle display operation is completed by driving the positive and negative fields as described above, and the EL display 2 displays the screen by repeating this operation.
[0045]
On the other hand, since the EL element 1 in this example uses the SrS: Ce layer as the light emitting layer 54, the light emission fall time is extremely short. For example, the light emission fall time of the ZnS: Mn-EL element shown in FIG. 17 is about 5 milliseconds, whereas the EL element 1 is about several microseconds. The “light emission fall time” referred to here is the light intensity from “0.9” to “0.1” when the maximum value of the light intensity is “1” as shown in FIG. (Time B in FIG. 2). The vertical axis in FIG. 2 is unitless because it changes depending on the measurement conditions, and is represented by a relative value.
[0046]
Therefore, even if the driving method shown in FIG. 4 is applied as it is and the EL element 1 emits light at intervals of m seconds (time A in FIG. 2), the optical power becomes extremely low and cannot be put into practical use. It is. That is, human vision senses brightness according to the time integration of the amount of light, so that if the light emission fall time is short, the time integration value of the light emission amount becomes small and the light power decreases.
[0047]
Therefore, in this embodiment, as shown in FIG. 1A, the control circuit 3 performs drive control so that the EL element 1 emits light a plurality of times within one scanning period (driving period). Here, “one scanning period” means that one EL element emits light once in one cycle in FIG. 4 (= 1 field, one scanning period, which corresponds to time A in FIG. 2). This corresponds to time (corresponding to time C in FIG. 2).
[0048]
That is, within one scanning period (time C), the scanning voltage (Vr, −Vr + Vm) is switched and applied a plurality of times so that the polarity of the voltage Vr 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 the voltage is applied (actually, light is emitted twice at the rise and fall of the applied voltage waveform. In FIG. In the human vision, it is recognized that light is continuously emitted within one scanning period (time C).
[0049]
Here, the number of times of voltage application is set to an odd number. For example, FIG. 1B shows the case of even number of times. When the 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.
[0050]
FIG. 1C shows a pattern in which the polarity of the last applied voltage 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 optical power unless it is AC driven, this pattern also has a problem.
[0051]
As shown in FIG. 1A, 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.
[0052]
As described above, according to the present embodiment, the EL display 2 is configured by arranging a plurality of EL elements 1 in a matrix, and the control circuit 3 is arranged at a position where the scanning electrode 9 and the data electrode 10 intersect. The EL element 1 is made to emit light a plurality of times within one scanning period. Therefore, even in the EL display 2 using the EL element 1 with a short emission fall time, the required optical power can be sufficiently secured and the display quality can be improved by increasing the time integral value of the emission amount. . In addition, the EL display 2 having a blue emission color that has not existed in the past can be put into practical use, and color variations can be increased.
[0053]
Originally, it would be ideal if an EL element having a light emission fall time corresponding to the required light emission power could be used. However, in practice, an element having such ideal characteristics cannot always be formed as intended. Therefore, if the driving method as in this embodiment is adopted, an EL element having a light emission fall time shorter than at least the time required by the product specifications can be formed. It is possible to adjust so that the light emission power can be appropriately obtained.
[0054]
Further, by setting Ce as the luminescent center material constituting the luminescent layer 54 of the EL element 1, higher light emission power can be obtained, and since the base material is SrS, better characteristics can be obtained by the combination of both. be able to. Further, since the light emitting layer 54 is sandwiched between the first electrode 52 (scanning electrode 9) and the second electrode 56 (data electrode 10) via the insulating layers 53 and 55, the EL display 2 is required. It is easy to ensure uniform characteristics when configured in a shape. In addition, since the EL element 1 is a voltage-driven thin film EL element, heat generation is less likely to be a problem as compared with the current-driven organic EL element, so that the design is facilitated.
[0055]
Further, since the data electrode 10 is made of a metal (Al) having a low resistance value, the data electrode 10 can be formed thin. Further, since the rounding of the signal waveform transmitted by the data electrode 10 is also reduced, for example, the rising response (period D) of light emission can be increased.
[0056]
Further, the control circuit 3 alternately outputs scanning voltages having different polarities to the scanning electrode 9 within one scanning period, and outputs the both voltages differently, that is, the scanning voltage is output an odd number. Therefore, the polarity of the voltage applied to both ends of the EL element 1 can be continuously reversed by cooperating with the output voltage control on the data electrode 10 side. In addition, since the polarity of the voltage applied at the end of each scanning period can also be reversed every time, the EL element 1 can emit light well every scanning period, and the characteristics of the EL element 1 fluctuate. It is possible to extend the life by suppressing the above.
[0057]
(Second embodiment)
7 to 12 show a second embodiment in which the present invention is applied to a printer head of an optical printer. The same parts as those in the first embodiment are denoted by the same reference numerals and the description thereof is omitted. Only will be described. FIG. 7 schematically shows the main structure of the optical printer.
[0058]
The photosensitive drum 31 rotates in the clockwise direction in FIG. 7. First, the charging unit 32 charges the surface with negative charges. Subsequently, exposure according to print image data is performed by the EL element array 33 and the Selfoc lens 34 shown in FIG. A combination of the EL element array 33 and the SELFOC lens 34 is a printer head 60. 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.
[0059]
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.
[0060]
FIG. 8 is a perspective view showing the printer head 60 and the photosensitive drum 31 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. 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.
[0061]
FIG. 9 shows an EL element array 33 configured by using the EL element 1 in the first embodiment. The glass substrate 51 is configured to also serve as the substrate of the EL element array 33. Further, the first electrode 52 (scanning electrode 9) has only one first electrode 52, since a large number of EL elements 1 may be arranged in a line as a light source of an optical printer.
[0062]
In addition, on the glass substrate 51, a control circuit 42, a scanning side driver (driving circuit) 43, a data side driver (driving circuit) 44, and an external connection terminal 45 for electrical connection with the control circuit on the optical printer main body side. Etc. are installed. The control circuit 42 causes the EL element 1 to emit light by the same driving method as in the first embodiment.
[0063]
Here, the timing of the drive signal suitable 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.
[0064]
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. 1 coincides with the paper feeding speed of the optical printer, the period A is set to 706 μs. In order to set the resolution to 600 dpi, it is necessary to set the width of the scanning electrodes 9 and the arrangement interval of the data electrodes 10 to 42 μm.
[0065]
The voltage application period C shown in the figure can be arbitrarily determined within 706 μs, 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.
[0066]
The applied voltage may be set so that the light emission threshold voltage of the EL element 1 and the light power necessary for forming a latent image on the photosensitive drum 31 can be obtained. I just need it.
[0067]
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 (period B) of the EL element 1 is too long, as shown in FIG. 16B, the shape of the light (dots) projected on the surface of the photoreceptor 31 becomes vertically long, and the printing speed As the speed increases, dots are connected. When the period C is determined, if the period B is set within (AC), dots are not connected as shown in FIG.
[0068]
In the EL element 1, since SrS: Ce is used for the light emitting layer 54, the light emission fall time is extremely short. Although this time seems to change to some extent by changing the stoichiometric composition of the light emitting layer 54, the time that the inventors of the present invention made as a trial is 0.5 μsec.
[0069]
Here, FIG. 11A shows a drive voltage waveform with a pulse width of 5 μs applied to the EL element 1, and FIG. 11B detects light emission of the EL element 1 with a photomultiplier tube, and the detected voltage. An example of observing this waveform with an oscilloscope is shown below. It can be seen that the EL element 1 emits light at each of the rise and fall of the drive voltage waveform. However, when the light emission fall time is 0.5 μsec, it is not possible to obtain the optical power necessary for forming a latent image on the photosensitive drum 31.
[0070]
Therefore, as shown in FIG. 10A, the EL element 1 is continuously driven 71 times within the period C of 100 μsec. That is, a positive voltage is applied 36 times and a negative voltage is applied 35 times. In this case, the drive voltage pulse width is about 1.4 μsec. FIG. 10A shows a driving voltage waveform observed with an oscilloscope, and is an example in which a voltage waveform based on light emission of the EL element 1 is observed with an oscilloscope in the same manner as FIG. 10B and FIG. Accordingly, the EL element 1 emits light continuously in the voltage application period C as shown in FIG. 10B, and the optical power necessary for the light source of the printer head 60 is ensured.
[0071]
Here, for comparison, FIG. 12 shows a configuration example of the printer head 100 using LEDs. The LED unit 101 is obtained by cutting a plurality of LED elements 102 formed on a silicon substrate as a unit. A plurality of the LED units 101 are prepared and arranged on the printed circuit board 103, and the driver 104 and the LED element 102 on each unit 101 are wired one by one.
[0072]
That is, as compared with the EL element array 33 in the second embodiment, it takes time to arrange the plurality of LED units 101 on the printed circuit board 103 and connect the wiring to the driver 104, and the boundary characteristics between the adjacent LED units 101. It also takes time to adjust. On the other hand, when the EL element array 33 of the second embodiment is used, the assemblability is improved and the labor required for adjustment is not required.
[0073]
As described above, according to the second embodiment, the EL element 1 is arranged in a line at a position where one scanning electrode 9 and a plurality of data electrodes 10 intersect, and is used as a light source of an optical printer. Since the array 33 is configured, the optical power required as a light source can be secured even when the EL element 1 having a short emission fall time is used. If the EL element array 33 is incorporated into an optical printer, the printing speed or resolution of the optical printer can be improved.
[0074]
(Third embodiment)
13 and 14 show a third embodiment of the present invention, and only the parts different from the second embodiment will be described. In the third embodiment, the EL element 1 is applied to the EL element array 33A as in the second embodiment. As shown in FIG. 13, a capacitor 46 is interposed between the scanning driver 43 and the scanning electrode 9. Is inserted. Other configurations are the same as those of the second embodiment.
[0075]
Next, the operation of the third embodiment will be described with reference to FIG. When the driving voltage shown in FIG. 14A is output due to the coupling between the scanning driver 43 and the scanning electrode 9 by the capacitor 46, the scanning electrode 9 is applied to the scanning electrode 9 as shown in FIG. 14B. A differential waveform having sharp peaks at the rising and falling edges of the drive voltage waveform is output. Since the EL element 1 emits light both at the rising edge and the falling edge of the applied voltage, it is possible to emit light four times in total if the driving voltage is output once.
[0076]
As described above, according to the third embodiment, since the scanning driver 43 and the scanning electrode 9 are coupled by the capacitor 46, the output frequency of the scanning voltage can be lowered.
[0077]
(Fourth embodiment)
FIG. 15 shows a fourth embodiment of the present invention, and only different portions from the second embodiment will be described. The fourth embodiment is a case where the EL element is applied to the printer head as in the second embodiment, but the structure of the electrodes is different. In order to simplify the explanation, it is assumed that 15 EL elements 1 are arranged in a line. When configured similarly to the EL element array 33 of the second embodiment, the EL element 1 may be formed at the intersection of one scanning electrode 9 and 15 data electrodes 10.
[0078]
In the fourth embodiment, as shown in FIG. 15, the EL element 1 is formed at the intersection of 3 (= m) scanning electrodes 47 and 5 (= n) data electrodes 48. That is, the five data electrodes 48 are formed in a crank shape by being bent 180 degrees at the lower end side and the upper end side in FIG. Then, each of the three scanning electrodes 47 (1) to 47 (3) is arranged linearly so as to intersect the five data electrodes 48 once.
[0079]
According to the fourth embodiment configured as described above, the number of necessary driver outputs is (3 + 5 =) 8, but in the case of the EL element array 33 of the second embodiment, (1 + 15 =) 16. Become.
[0080]
Therefore, with this configuration, the number of necessary driver outputs can be reduced. If a separate drive source is required inside the driver according to the number of output terminals, the printer head can be reduced in size. In this case, the merit increases as the number of EL elements 1 to be arranged increases.
In the configuration of the fourth embodiment, there are three scanning electrodes 47, but it goes without saying that these are controlled by the control circuit so as to simultaneously output scanning voltages in each scanning period.
[0081]
The present invention is not limited to the embodiments described above and shown in the drawings, and the following modifications or expansions are possible.
In the first embodiment, the scanning electrode driving circuits 11e and 11o may be integrated so that the scanning cycle is halved.
As long as the printing speed defined in the second embodiment is realized, the light emission falling time of the EL element may be 350 μsec or less. That is, as described above, the light emission fall time of the EL element required to satisfy the printing speed evaluated as sufficiently high at the current product level is about 700 μsec. In the present invention, since the EL element emits light a plurality of times in one scanning cycle, the required optical power can be ensured even with an element whose emission fall time is half or less of 700 μsec. Alternatively, adjustment can be performed to obtain an appropriate optical power.
[0082]
In addition, when the printing speed set for the optical printer differs according to the individual design, the light emission fall time that is optimum for each case may be used.
Further, when the present invention is applied to other than the printer head, the present invention can be applied even if the light emission fall time exceeds 350 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 individual design.
In the first embodiment, the scanning driver 4 may be configured as a three-level output circuit of a positive scanning voltage, a negative scanning voltage, and a ground level by adopting a two-stage push-pull configuration. In this case, voltage switching performed in the scanning voltage supply circuits 19a and 19b becomes unnecessary.
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.
[0083]
When the characteristic variation of the EL element does not become a problem, the drive voltage may be printed in the pattern shown in FIG.
Similar control may be performed by alternately changing the polarity of the data voltage (drive voltage).
The printer head of the second embodiment or the like is not only applied to the optical printer, but can also be applied to a copying machine or a facsimile machine using the same electrophotographic technology.
The present invention is not limited to an EL display or a printer head, and can be applied to any device that emits light from an EL element having a short emission fall time.
[Brief description of the drawings]
FIGS. 1A and 1B show a first embodiment when the present invention is applied to an EL display, in which FIG. 1A shows a driving voltage application pattern for an EL element in this embodiment, and FIG. 1B shows an even number of times of applying a driving voltage. The pattern (No. 1) and (c) in the case show the pattern (No. 2) when the number of times of applying the drive voltage is an even number.
2A is a drive voltage waveform, and FIG. 2B is a diagram showing light emission intensity of an EL element.
FIG. 3 is a functional block diagram showing the overall electrical configuration.
FIG. 4 is a diagram showing a more detailed electrical configuration centering on an EL display, a scanning side driver, and a data side driver;
FIG. 5 is a timing chart for explaining a driving method disclosed in Japanese Patent Laid-Open No. 9-54566.
6A is a plan view of an EL element, and FIG. 6B is a diagram schematically showing an AA cross section of FIG.
FIG. 7 is a diagram schematically illustrating a main structure of an optical printer according to a second embodiment when the present invention is applied to a printer head of the optical printer.
FIG. 8 is a perspective view with the printer head and photosensitive drum as the center.
FIG. 9 is a diagram showing a part of a printer head configured using an EL element array.
10A is a diagram showing an example of a driving voltage waveform actually applied to an EL element, and FIG. 10B is a diagram showing an example in which light emission of the EL element is detected with a photomultiplier tube and the detected voltage waveform is observed with an oscilloscope.
11A is a single drive voltage waveform applied to an EL element, and FIG. 11B is a diagram showing a light emission voltage waveform of the EL element.
FIG. 12 is a diagram illustrating a configuration example of a printer head using LEDs.
FIG. 13 is a view corresponding to FIG. 9 showing a third embodiment of the present invention.
14A is a waveform of a driving voltage, and FIG. 14B is a diagram illustrating a voltage waveform applied to an EL element.
FIG. 15 is a view corresponding to FIG. 9 showing a fourth embodiment of the present invention.
FIG. 16 shows the prior art, and is a diagram corresponding to FIG. 2 for an EL element using ZnS: Mn in the light emitting layer.
FIG. 17A is a diagram showing an example in which circular dots and elliptical dots are printed every other dot, and FIG. 17B is a diagram showing an example in which those dots are printed every other dot.
[Explanation of symbols]
1 is an EL element, 2 is an EL display, 3 is a control circuit, 4 is a scanning side driver (driving circuit), 5 is a data side driver (driving circuit), 9 is a scanning electrode, 10 is a data electrode, and 33 and 33A are printers Head, 42 is a control circuit, 43 is a scanning side driver (driving circuit), 44 is a data side driver (driving circuit), 46 is a capacitor, 47 is a scanning electrode, 48 is a data electrode, 53 is a first insulating film, 54 is The light emitting layer, 55 is a second insulating film, and 56 is a second electrode (metal electrode).

Claims (5)

A method of emitting light from an EL element formed at a position where a plurality of scan electrodes and data electrodes intersect ,
A method of driving an EL element, wherein the EL element is caused to emit light a plurality of times in one driving period, which is a period for selecting one of the scanning electrodes as a scanning voltage application target. 2. The method of driving an EL element according to claim 1, wherein the light emission center material constituting the light emitting layer of the EL element is Ce or Eu. 3. The method of driving an EL element according to claim 2, wherein the base material constituting the light emitting layer of the EL element is SrS. 4. The driving of an EL element according to claim 1, wherein first and second driving voltages having different polarities are alternately applied to both ends of the EL element within the one driving period. Method. 5. The method of driving an EL element according to claim 4, wherein the number of outputs of the first drive voltage and the second drive voltage in the one drive period is controlled to be different.

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