JPS609279B2 - Optical driving method for thin film EL elements - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a three-layer thin film EL element that exhibits a hysteresis phenomenon in the characteristic of luminance versus applied voltage, and is particularly characterized in writing and erasing by light.

Z added with Mn etc. as an active substance forming a luminescent center
It has been reported that in a so-called three-layer structure thin film EL device in which an nS:Mn thin film is sandwiched between Y203 insulating films, there is a hysteresis phenomenon in the emission luminance vs. applied voltage characteristic.

By utilizing this hysteresis phenomenon, the EL element can be operated as a type of memory function element, and information can be arbitrarily written, memorized, and erased based on the magnitude of the luminance of the element. The present invention relates to a method for driving such a three-layer thin film EL element, and is particularly characterized by a writing and erasing method using light. Below, along with an explanation of the hysteresis phenomenon, already proposed driving methods will be described to facilitate understanding of the method of the present invention, which will be described later.

Figure 1 is a cross-sectional view showing the structure of a ZnS:Mn thin film light emitting device with a three-layer structure, in which 1 is a ZnS thin film layer doped with Mn or the like as an active substance forming a luminescent center, 2 and 3 are Y203
4 is a transparent electrode, 5 is a back electrode such as AI, and 6 is a glass substrate.

As is clear from this figure, this light-emitting element has a ZnS thin film layer 1 sandwiched between transparent insulating thin films 2 and 3, and the material or manufacturing conditions of the ZnS thin film layer 1 or transparent insulating thin films 2 and 3 are different from each other. By making appropriate settings, it is possible to cause a hysteresis phenomenon in the luminance of this element. FIG. 2 is a diagram showing the luminance versus applied voltage characteristics of the above ZnS thin film light emitting device, where the horizontal axis represents the amplitude (peak value) V of the applied AC exposure pressure pulse, and the vertical axis represents the luminance B. It is showing.

As is clear from Fig. 2, the luminance of the device as the voltage of the applied pulse increases (shown by curve A) and the luminance of the device as the voltage decreases (shown as the curve) There is a significant hysteresis phenomenon between. Therefore, we will now select a voltage value Vs (maintenance voltage) at a point where the difference between the minimum luminance Be when the voltage increases and the maximum luminance Bw when the voltage decreases is sufficiently large, and use the AC pulse Kutagabi s with this as the amplitude. When driving the element, the element maintains the luminance within the range of Bw depending on the pulse train Ps. That is, when the amplitude of the AC pulse train Ps is instantaneously modulated and a high write voltage yw is momentarily applied to the element, the element emits light at the instantaneous brightness Vw', and then emit light at the write brightness Bw with the next AC sustaining pulse. After settling down, this brightness Bw is maintained by applying the AC sustaining pulse thereafter. On the other hand, if the amplitude of the alternating current sustaining pulse train Ps that maintains the write state in this way is further modulated and a sufficiently low erase voltage Ve is momentarily applied to the element, the element will pass through the instantaneous erase luminance Be' point and then ``At the next AC sustaining pulse train s, the erase brightness Be is reached and this brightness step is maintained to perform erasure and memory of the erased state.

Further, in the above writing and erasing, if the writing or erasing voltages Vw and Ve are arbitrarily selected, an intermediate brightness between the brightness Bw and the brightness can be obtained. What has been described above is a method for driving a ZnS thin film EL element that has already been proposed, and by doing so, the element can be provided with a kind of memory function.

Furthermore, since such a ZnS thin film light emitting element is sensitive to external irradiation light, a method for optically driving this light emitting element is known.

FIG. 3 is a time chart related to the optical driving method,
Figure A shows the applied voltage waveform applied to the element, and the figure in the figure shows the luminance luminance waveform.

In both figures, the time t on the horizontal axis is shown in correspondence with each other.
The optical driving method will be described below with reference to this time chart. As shown in the figure, when an AC pulse s with an amplitude (peak value) of the sustaining voltage Vs and a rest period of the pulse is applied to the element, Before the period T, writing by voltage or light was not performed, and the brightness Be in the erased state was maintained by the alternating current sustaining pulse Ibaraki s.

When this element is irradiated with light from the outside in synchronization with the pulse period of the alternating current sustaining pulse blade s at Kijo-cho 2, the luminance brightness B rises in response to the incident light energy, and becomes complete after sufficient light irradiation. The brightness reaches Bw. Even after the light irradiation is stopped, this written state is maintained for the application period of the maintenance pulse blade s (period L). Next, as shown in period T4, when this element is irradiated with light from the outside in synchronization with the rest period of the above-mentioned pulse train Ps, the luminance brightness B decreases in accordance with the incident light energy, and after sufficient light irradiation, settles into a completely erased state, that is, a state with luminance Be. Even after the light irradiation is stopped, this erased state is maintained during the application period of the maintenance pulse blade fertilizer s (period T5). Therefore, the above-mentioned operation of performing light irradiation in synchronization with the rest period of the pulse train Ps is effective for erasing. It can be considered manipulation. Furthermore, at the time of optical writing and optical erasing in periods m2 and T4, the writing and erasing effects correspond to the wavelength of the irradiated light and the amount of incident light, that is, the magnitude of the optical energy. The brightness caused by erasing can take any value from brightness Bw to brightness Be by appropriately selecting the incident light energy, making it possible to write and erase halftones. In this writing and erasing process, the element may be irradiated with light not only during one pulse period and one pause period, but also over several pulse periods and a pause period. It is understood that the optical writing described above is performed by the optical polarization effect of the ZnS layer, and the optical erasing is performed by the optical relaxation effect of the ZnS layer.

Incidentally, the characteristics of the photosensitivity of the thin film EL element with respect to the wavelength of the irradiated light are as shown in FIG. 4, with the maximum sensitivity peaking at 3500A.

Therefore, it can be seen that the optimum light source is one that includes a large amount of wavelength of 3500A. As this light source, for example, a tungsten lamp is used because it is easy to drive and easy to handle, and a xenon lamp is an example of a flash lamp. Their emission wavelength characteristics are shown in FIG. From this figure, it can be seen that the xenon lamp containing more 3500Δ is more suitable for optical writing and erasing of EL elements than the tungsten lamp. Furthermore, it is impossible to emit light at the right timing as shown in FIG. 3 with a tungsten lamp that has a slow response. This is because the sustain driving frequency of the EL element requires 100 days or more, so it is very difficult to turn on and off the tungsten lamp within 1 hsec. For it,
Xenon flash lamps can emit light for several milliseconds or less, depending on the capacity of the charging capacitor.
It is possible to emit light in sufficient timing with the sustaining AC pulse as shown in FIG. Furthermore, in terms of intensity, a xenon flash lamp is much stronger than a tungsten lamp and is suitable for optical writing and erasing of EL elements. In other words, regarding the slow response of tungsten lamps, for example, when moving the light source position during optical writing to draw a line diagram, etc., the slow response causes writing to be made in the wrong position (or written in the required position). (e.g., not being allowed to enter). In addition, during optical erasing, if light is irradiated from the pulse rest period to the pulse period, both optical writing and optical erasing operations are performed, and optical writing takes priority.
As a result, a write operation will be performed. In order to prevent this, it is also proposed to change the duty so that the pulse pause period becomes particularly long during the latter optical erasing. However, such switching of the AC pulse train has drawbacks such as complicating the circuit configuration. The flash lamp described above can solve the above-mentioned problems, and by simply controlling the lighting timing of the flash lamp according to optical writing and optical erasing, optical writing and erasing can be performed by applying a constant AC pulse train. Both optical erasing can be performed, and there is no need to change the duty.

FIG. 6 shows an example of a drive circuit for optical writing and optical erasing of an EL element using a flash lamp.

FIG. 6 shows the overall circuit and sustaining pulse generation circuit, and FIG. 7 shows the flash lamp lighting circuit. The time chart is shown in FIG. In FIG. 6, Vs is a sustaining voltage power supply applied to the EL element, EL is the thin film EL element, T, - T4 are switching transistors, D, . . . 2 are diodes. VI in FIG. 7 is a power source for lighting the flash lamp. C is a capacitor for flash lamp lighting, M is mutual inductance for generating a high voltage trigger, T
takes the charging timing of capacitor C from the control circuit,
This is a switch transistor for charging C. This T can be a resistor if the lamp has a long light emission interval, but it is expressed by the following formula:
When emitting light at 0Hz or higher, rapid charging is required, so a transistor switch must be used. The control circuit provides a sustain pulse timing to be applied to both ends of the EL, and at the same time provides a flash lamp lighting timing trigger and a flash lamp capacitor charging pulse. The situation will be explained with reference to FIG. When the signals S, , S2, S3, and S4 are applied at the timing shown in FIG. 8, the voltage shown in the figure is applied to both ends of the EL element, and the EL element with memory enters the sustain pulse application state. 6th
The diodes D, D2 shown in the figure function to completely fix the potential across EL to GND when the transistors T2 and T4 are on. For example, signals S, .
After S4 is "1" and a voltage is applied to the EL element Vs, when the signal S, is set to "0" and the transistor T, is turned off, there is still charge left in the EL element (EL
element can be thought of as a kind of capacitor), then the signal S2
When becomes "1" and the transistor T2 is turned on, the electrode on the side of the transistor T, (or T2) of the EL element is connected to GND, but due to the accumulated charge, the electrode of the transistor T3 (or T4) of the EL element is connected to GND. The potential of the side electrode is G
Trying to be below ND. At that time, the current flows through the diode D2, and the transistor L (or T4) of the EL element flows through the diode D2.
) side is fixed at GND without becoming lower than GND.
Diode D similarly prevents the opposite electrode from going below GND. Even if the diodes D,, D2 are removed, the resistance R,
(or R2), a discharge loop is formed through the base collector of the transistor, but by passing through the resistor R, (or R2), the fixed state of both ends of the EL element is better than when connected to GND with only a diode. do not have. When performing optical writing, a trigger is applied in synchronization with the timing at which the signal S or S3 becomes "1", the lamp is turned on when a sustain pulse is applied, and optical writing is performed. At this time, the lamp emission width may be wider than the applied sustaining pulse width, but when comparing writing and erasing using an EL element, writing is performed more sensitively, so Even if there is an erasing time as shown below, writing is almost always given priority, so even if the lamp emit light during the GND period, there is almost no erasing effect.
If a trigger is applied when both S and S3 are "0", that is, when S2 and S4 are simultaneously "1", light irradiation is performed when the voltage across both ends of the EL element is 0, and the polarization is relaxed. Erasure is performed. The charging pulse is applied immediately after the trigger pulse so as not to overlap with the trigger pulse. The upper limit of the repetition frequency of light emission depends on the rating of the flash lamp and the charging speed of this charging switch T (FIG. 7). In the example, the EL drive frequency is 500Hz, V
s=200VVL=250Vc=lOAFM turns ratio 8
0:1, the number of times the flash lamp was emitted was 40Hz, and the charging pulse was 3Hz. As described above, by driving the EL element with the circuit shown in Fig. 6, and lighting the flash lamp with the circuit shown in Fig. 7 to irradiate the EL element with light, the E
Optical writing and erasing to the L element can be performed freely.

If the switch T in Fig. 7 is turned on only once, it will only operate once, but if it is turned on continuously (several tens of Hz) and a lamp trigger is generated during that time, continuous lamp emission will occur. If the light source is made into a handy type like a pen, handwritten figures, characters, etc. can be written and erased on the EL element.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional diagram of the thin-film EL device of the present invention, FIG. 2 is a voltage-luminance characteristic diagram of the thin-film EL device, FIG. Figure 3 shows the relationship between AC pulse train Ps, light irradiation, and luminance, Figure 4 shows the photosensitivity characteristics of the thin-film L element, Figure 5 shows the spectral characteristics of the tungsten lamp and xenon lamp, and Figure 6 shows the main picture. FIG. 7 is a driving circuit diagram of an EL element and a canon lamp according to the invention, FIG. 7 is a driving circuit diagram of a xenon lamp, and FIG. 8 is a time chart explaining the operation of the circuit of FIG. 6. EL is a thin film EL element, K is a xenon lamp, T, T, ~
T4 is a transistor, and C is a capacitor. Figure 1 Figure 2 Figure 3 Figure 4 Figure 6 Figure 7 Figure 8

Claims (1)

[Scope of Claims] 1. A thin film EL element having a hysteresis phenomenon in the applied voltage and luminance characteristics is provided with a voltage value at a point where the difference between the minimum luminance when the voltage increases and the maximum luminance when the voltage decreases is sufficiently large. means for applying an alternating current pulse train having an AC pulse train; and means for irradiating the thin film EL element with light from a flash lamp as a light source;
A control means for selectively lighting the flash lamp in synchronization with a pulse period or a pulse rest period of the AC pulse train in response to optical writing and optical erasing. . 2. The optical drive method for a thin film EL element as set forth in claim 1, wherein the flash lamp is a xenon lamp.